# #

\* Perovskite # La2


sis. In: Xinhe B, Yide X, editors. *Studies in Surface Science and Catalysis*. Volume 147: Elsevier; 2004. p. 319-24.


[20] Pecchi G, Jiliberto MG, Delgado EJ, Cadús LE, Fierro JLG. Effect of B-site cation on the catalytic activity of La1−xCaxBO3 (B = Fe, Ni) perovskite-type oxides for toluene combustion. *J Chem Technol Biotechnol*. 2011;86(8):1067-73.

sis. In: Xinhe B, Yide X, editors. *Studies in Surface Science and Catalysis*. Volume 147:

[8] Escalona N, Fuentealba S, Pecchi G. Fischer–Tropsch synthesis over LaFe1−xCoxO3 perovskites from a simulated biosyngas feed. *Appl Catalysis A: General*. 2010

[9] Yu L, Diao G, Ye F, Sun M, Zhou J, Li Y, et al. Promoting Effect of Ce in Ce/OMS-2 Catalyst for Catalytic Combustion of Dimethyl Ether. *Catal Lett*. 2011

[10] Liang JJ, Weng H-S. Catalytic properties of lanthanum strontium transition metal ox‐ ides (La1-xSrxBO3; B = manganese, iron, cobalt, nickel) for toluene oxidation. *Indus*

[11] Teraoka Y, Nii H, Kagawa S, Jansson K, Nygren M. Influence of the simultaneous substitution of Cu and Ru in the perovskite-type (La,Sr)MO3 (M Al, Mn, Fe, Co) on the catalytic activity for CO oxidation and CO–NO reactions. *Appl Catalys A: General*.

[12] Wu Y, Yu T, Dou B-s, Wang C-x, Xie X-f, Yu Z-l, et al. A comparative study on perov‐ skite-type mixed oxide catalysts A′xA1 − xBO3 − λ (A′ = Ca, Sr, A = La, B = Mn, Fe,

[13] Tejuca LG, Fierro JLG, Tascón JMD. Structure and Reactivity of Perovskite-Type Ox‐ ides. In: D.D. Eley HP, Paul BW, editors. *Advances in Catalysis*. Volume 36: Academic

[14] Kharton VV, Viskup AP, Naumovich EN, Tikhonovich VN. Oxygen permeability of

[15] Rao CNR, Gopalakrishnan J. *New Directions in Solid State Chemistry*. Cambridge Uni‐

[16] Yu Z, Gao L, Yuan S, Wu Y. Solid defect structure and catalytic activity of perovskitetype catalysts La1-SrNiO3-[small lambda] and La1-1.333ThNiO3-[small lambda]. *J*

[17] Popa M, Frantti J, Kakihana M. Lanthanum ferrite LaFeO3+d nanopowders obtained by the polymerizable complex method. *Solid State Ionics*. 2002 12/2/;154–155:437-45.

[18] Sumathi R, Johnson K, Viswanathan B, Varadarajan TK. Selective oxidation and de‐ hydrogenation of benzyl alcohol on ABB′O3 (A=Ba, B=Pb, Ce, Ti and B′=Bi, Cu, Sb) type perovskite oxides-temperature programmed reduction studies. *Appl Catalysis A:*

[19] Pecchi G, Reyes P, Zamora R, Campos C, Cadús LE, Barbero BP. Effect of the prepa‐ ration method on the catalytic activity of La1−xCaxFeO3 perovskite-type oxides. *Cat‐*

LaFe1−xNixO3−δ solid solutions. *Mater Res Bull*. 1999 10/8/;34(8):1311-7.

Elsevier; 2004. p. 319-24.

538 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

6/15/;381(1–2):253-60.

2011/01/01;141(1):111-9. English.

2000 3/13/;194–195:35-41.

Press; 1989. p. 237-328.

versity Press; 1997.

*Chem Soc Faraday Trans*. 1992;88(21):3245-9.

*General*. 1998 8/24/;172(1):15-22.

*alysis Today*. 2008 4//;133–135:420-7.

*Engin Chem Res*. 1993 1993/11/01;32(11):2563-72.

Co) for NH3 oxidation. *J Catalys*. 1989 11//;120(1):88-107.


[45] Mikkola J-P, Sjöholm R, Salmi T, Mäki-Arvela P. Xylose hydrogenation: kinetic and NMR studies of the reaction mechanisms. *Catalysis Today*. 1999 1/27/;48(1–4):73-81.

[33] Pecchi G, Dinamarca R, Campos CM, Garcia X, Jimenez R, Fierro JLG. Soot oxidation on silver-substituted LaMn0.9Co0.1O3 perovskites. *Indus Engin Chem Res*. 2014

[34] Bui VN, Laurenti D, Afanasiev P, Geantet C. Hydrodeoxygenation of guaiacol with CoMo catalysts. Part I: Promoting effect of cobalt on HDO selectivity and activity.

[35] Laurent E, Delmon B. Study of the hydrodeoxygenation of carbonyl, carylic and guaiacyl groups over sulfided CoMo/γ-Al2O3 and NiMo/γ-Al2O3 catalysts: I. Cata‐

[36] Sepúlveda C, Escalona N, García R, Laurenti D, Vrinat M. Hydrodeoxygenation and hydrodesulfurization co-processing over ReS2 supported catalysts. *Catalysis Today*.

[37] Gutierrez A, Kaila RK, Honkela ML, Slioor R, Krause AOI. Hydrodeoxygenation of guaiacol on noble metal catalysts. *Catalysis Today*. 2009 10/15/;147(3–4):239-46.

[38] Lee CR, Yoon JS, Suh Y-W, Choi J-W, Ha J-M, Suh DJ, et al. Catalytic roles of metals and supports on hydrodeoxygenation of lignin monomer guaiacol. *Catalys Commun*.

[39] Nimmanwudipong T, Runnebaum RC, Block DE, Gates BC. Catalytic conversion of guaiacol catalyzed by platinum supported on alumina: Reaction network including

[40] Escalona N, Aranzaez W, Leiva K, Martínez N, Pecchi G. Ni nanoparticles prepared from Ce substituted LaNiO3 for the guaiacol conversion. *Appl Catalys A: General*.

[41] Simonetti DA, Kunkes EL, Dumesic JA. Gas-phase conversion of glycerol to synthe‐ sis gas over carbon-supported platinum and platinum–rhenium catalysts. *J Catalys*.

[42] Dieuzeide ML, Jobbagy M, Amadeo N. Glycerol steam reforming over Ni/γ-Al2O3 catalysts, modified with Mg(II). Effect of Mg (II) content. *Catalys Today*. 2013

[43] Iriondo A, Güemez MB, Barrio VL, Cambra JF, Arias PL, Sánchez-Sánchez MC, et al. Glycerol conversion into H2 by steam reforming over Ni and PtNi catalysts support‐ ed on MgO modified γ-Al2O3. In: E.M. Gaigneaux MDSHPAJJAM, Ruiz P, editors.

[44] Franchini CA, Aranzaez W, Duarte de Farias AM, Pecchi G, Fraga MA. Ce-substitut‐ ed LaNiO3 mixed oxides as catalyst precursors for glycerol steam reforming. *Appl*

*Studies in Surface Science and Catalysis*. Volume 175: Elsevier; 2010. p. 449-52.

*Catalys B: Environmental*. 2014 4/5/;147:193-202.

hydrodeoxygenation reactions. *Energy Fuels*. 2011 2011/08/18;25(8):3417-27.

lytic reaction schemes. *Appl Catalys A: General*. 1994 2/17/;109(1):77-96.

*Appl Catalys B: Environmental*. 2011 1/14/;101(3–4):239-45.

2014/06/18;53(24):10090-6.

540 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

2012 11/15/;195(1):101-5.

2012 1/5/;17:54-8.

2014 7/5/;481:1-10.

9/15/;213:50-7.

2007 4/25/;247(2):298-306.


## **Improvement of Catalytic Performance of Perovskites by Partial Substitution of Cations and Supporting on High Surface Area Materials**

Fabio Souza Toniolo and Martin Schmal

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61279

## **Abstract**

In this chapter, we present two relevant strategies to improve the activity and selectivity of perovskite-mixed oxides ABO3 in heterogeneous catalytic reactions such as the oxida‐ tion of hydrocarbons, soot combustion and CO selective oxidation, for which the surface sites and lattice oxygen species play important roles for the chemical transformations. Be‐ sides, we focus on synthesis of higher alcohols, partial oxidation of methane, oxidative reforming of diesel and dry reforming of methane for which the perovskite is a precursor that leads to a dispersed metal active phase over an oxide matrix. But which strategies are we talking about? First, the partial substitution of cations A and B by different elements, which change atomic distances, causes unit cell distortions, stabilizes multiple oxidation states or induces cationic or anionic vacancies within the lattice. And all these new fea‐ tures perturb the solid reactivity by changing the reaction mechanism on the catalyst sur‐ face. Thus, appropriate cations substitutions may lead to better catalysts. The second strategy comprises supporting the perovskite, which usually presents low surface area, on high surface area materials to maximize the exposed surface sites.

**Keywords:** Catalysis, Partial substitution, Vacancy, Monolith

## **1. Introduction**

Perovskite-type oxides with ABO3 structure have attracted significant interest in many areas of solid-state chemistry, including catalysis. Several ABO3 formulations have demonstrated success, especially in total oxidation and partial oxidation of hydrocarbons, carbon monoxide oxidation, alkenes hydrogenation, alkanes hydrogenolysis, alcohols synthesis, dry reforming and water gas shift reaction. However, these materials present some limitations for broader

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

application in catalysis such as low activity and stability in certain experimental conditions, low metal dispersion when the perovskite structure collapses to form a supported-type catalyst B/AOx or even low surface area resulting from high calcination temperature.

To overcome these limitations aiming to improve the catalytic activity, selectivity and stability of perovskite-type oxides, one can design these materials by substituting partially and properly the cations A and B or even supporting the perovskite on high surface area materials (porous oxides, monolith-type structure). By substituting the original A and B cations, one can control the extent of substitution and decide for an appropriate cation that will bring significant structural changes, such as lattice distortions, stabilization of multiple oxidation states or generation of cationic and anionic vacancies, all having as a direct consequence the change in catalytic activity. Upon spreading the perovskite on a support, one can choose the best matrix to accommodate the oxide particles and expose the largest amount of active sites in order to improve the catalytic performance.

In this sense, we discuss in the next pages some recent advances that apply perovskite-type oxides for reactions of interest in catalysis and how the above strategies impact on the performance of the catalysts. All visited literature was critically analyzed and here we present well-structured discussions and hypothesis based on extensive experiments and precise characterization techniques.

## **2. Partial substitution of cations as a strategy to enhance catalytic performance**

Perovskite-type oxides ABO3 can be properly modified by the partial substitution of atoms at A and/or B-sites, creating isostructural A1-xAxB1-yByO3 crystals (or AA'BB'O3 for simplici‐ ty), which may stabilize unusual oxidation states of the B component, induce structural distortions and create cationic or anionic vacancies. The nature and extent of the dopant that substitutes A and B elements impact on the physicochemical properties and catalytic activity of the material [1].

From the catalysis point of view, a solid containing surface and lattice defects presents more structural and electronic instability, which is compensated by the interaction with molecules from gas phase through chemical reactions that break and form bonds. Defects are welcome in catalysis, and the partial substitution of cations within the perovskite lattice tends to enhance the defects density of the crystal structure.

Perovskite oxides may also be considered as catalytic precursors for generating highly active metals dispersed onto a matrix, B/AOx, that can be obtained *in situ* by reducing the precursor ABO3 [2–4]. The partial substitution of cations can increase the dispersion of B in reducing treatments and avoid metal sintering during reaction conditions. In this sense, high metal dispersion from perovskite oxides emerges as an interesting alternative to conventional metal supported catalysts.

For instance, several lanthanum-containing perovskites have been considered promising for different applications such as soot combustion in automotive exhaust treatments, oxidation of volatile organic compounds (VOCs) in air pollution control and reforming or partial oxidation of hydrocarbons for hydrogen production [5]. Though perovskites have a remarkable activity and some desired features such as low-cost synthesis and high thermal stability, their indus‐ trial application is still missing due to drawbacks related to the low surface area and low density of structural defects, which both can be improved by selecting an appropriate synthesis method to (i) insert different cations into the crystal lattice and (ii) spread the oxide phase over a porous support.

## **2.1. Perovskites for oxidation reactions**

application in catalysis such as low activity and stability in certain experimental conditions, low metal dispersion when the perovskite structure collapses to form a supported-type catalyst

To overcome these limitations aiming to improve the catalytic activity, selectivity and stability of perovskite-type oxides, one can design these materials by substituting partially and properly the cations A and B or even supporting the perovskite on high surface area materials (porous oxides, monolith-type structure). By substituting the original A and B cations, one can control the extent of substitution and decide for an appropriate cation that will bring significant structural changes, such as lattice distortions, stabilization of multiple oxidation states or generation of cationic and anionic vacancies, all having as a direct consequence the change in catalytic activity. Upon spreading the perovskite on a support, one can choose the best matrix to accommodate the oxide particles and expose the largest amount of active sites in order to

In this sense, we discuss in the next pages some recent advances that apply perovskite-type oxides for reactions of interest in catalysis and how the above strategies impact on the performance of the catalysts. All visited literature was critically analyzed and here we present well-structured discussions and hypothesis based on extensive experiments and precise

Perovskite-type oxides ABO3 can be properly modified by the partial substitution of atoms at A and/or B-sites, creating isostructural A1-xAxB1-yByO3 crystals (or AA'BB'O3 for simplici‐ ty), which may stabilize unusual oxidation states of the B component, induce structural distortions and create cationic or anionic vacancies. The nature and extent of the dopant that substitutes A and B elements impact on the physicochemical properties and catalytic

From the catalysis point of view, a solid containing surface and lattice defects presents more structural and electronic instability, which is compensated by the interaction with molecules from gas phase through chemical reactions that break and form bonds. Defects are welcome in catalysis, and the partial substitution of cations within the perovskite lattice tends to enhance

Perovskite oxides may also be considered as catalytic precursors for generating highly active metals dispersed onto a matrix, B/AOx, that can be obtained *in situ* by reducing the precursor ABO3 [2–4]. The partial substitution of cations can increase the dispersion of B in reducing treatments and avoid metal sintering during reaction conditions. In this sense, high metal dispersion from perovskite oxides emerges as an interesting alternative to conventional metal

**2. Partial substitution of cations as a strategy to enhance catalytic**

B/AOx or even low surface area resulting from high calcination temperature.

544 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

improve the catalytic performance.

characterization techniques.

activity of the material [1].

supported catalysts.

the defects density of the crystal structure.

**performance**

An interesting study about the effect of partial substitution of single-site (A or B) and dual-site (A and B) in the LaCoO3 perovskite on the activity for toluene oxidation and methane oxidation was presented in ref. [5]. This investigation focused onto the effect of single- and dual-site modifications, while most of the researches explain only single-site substitution. The oxidation reactions on perovskite-type oxides usually take place through different mechanisms, either involving weakly chemisorbed oxygen (i.e., α-O in the toluene oxidation, for instance) denoted as suprafacial mechanism, or comprising surface lattice oxygen species (i.e., α'-O in the methane oxidation) denoted as intrafacial mechanism. The latter is directly related to the mobility and amount of oxygen species within the crystal lattice. The following catalysts were prepared by continuous flow hydrothermal synthesis method [5]: LaCoO3 (ABO3), LaCa‐ CoO3 (AA'BO3), LaCoMgO3 (ABB'O3), and LaCaCoMgO3 (AA'BB'O3). After heat treatments at 700, 800 and 900°C, these materials presented mostly the perovskite phase, but some traces of La2O3, Co3O4 and MgO (depending on the catalyst) and the specific surface area was lower than 30 m2 /g. Authors evidenced the calcium substitution by verifying distortions in the angle α of the rhombohedral structure of LaCaCoO3, since a significant contraction of the unit cell should not be expected due to the similar ionic radii of Ca2+ (0.100 nm) and La3+ (0.103 nm). On the other hand, magnesium substitution led to more severe angular distortions and unit cell expansion, because Mg2+ has ionic radius (0.072 nm) larger than Co3+ (0.063 nm), as well as the oxidation state of Co3+ changed to Co4+ to guarantee the structural electroneutrality, as revealed by X-ray photoelectron spectroscopy (XPS).

By applying those materials in the oxidation of 500 ppm toluene (in the absence and presence of water 3 vol.%), which is believed to take place through the suprafacial mechanism, the activity sequence was LaCaCoMgO3 > LaCoMgO3> LaCoO3> LaCaCoO3. According to XPS measurements [5], the insertion of Ca2+ into the perovskite lattice slightly increased the amount of oxygen species chemisorbed on the surface (O– and O2 – , denoted as Oads) relating to lattice oxygen (Olat). However, Mg2+ insertion had a more relevant impact and practically doubled Oads/Olat ratio, suggesting that Mg2+ (with valence lower than Co3+ and La3+) generated more oxygen vacancies than Ca2+ to ensure charge compensation. As a consequence, the surface vacancies were occupied by oxygen species from gas phase. According to the authors, these findings agreed with the temperature-programmed desorption of oxygen (O2-TPD) which showed that Mg2+-substituted materials released larger amount of weakly chemisorbed oxygen (α-O), which were adsorbed on surface oxygen vacancies. Since toluene oxidation makes use of surface oxygen (suprafacial mechanism), we should expect LaCaCoMgO3 and LaCoMgO<sup>3</sup> (which contain this type of oxygen) to present higher activity.

On the other hand, Ca2+ insertion decreased the catalyst reducibility (shifting reduction peaks to higher temperature in H2-TPR experiments), which explained higher activity of LaCoO<sup>3</sup> instead of LaCaCoO3, since the non-substituted perovskite would release surface lattice oxygen easier to oxidize toluene than LaCaCoO3. Furthermore, the authors estimated the activation energy for the different catalysts in the temperature range of 140–280°C and found out that the dual-insertion of Ca2+ and Mg2+ into the LaCoO3 perovskite induced the activation energy to decrease from 143 kJ/mol (LaCoO3) to 34 kJ/mol (LaCaCoMgO3), which is comparable or even smaller than some noble metals applied in this suprafacial oxidation process (31–37 kJ/mol on Au/LaCoO3 [6]; 86 kJ/mol on Pt/Al2O3 [7].

For the oxidation of 1 vol.% CH4 (in the absence and presence of water 3 vol.%), which is believed to occur by an intrafacial mechanism, ref. [5] found the sequence of the catalytic performance LaCoMgO3 > LaCaCoMgO3> LaCoO3> LaCaCoO<sup>3</sup> to agree with the sequence of surface lattice oxygen (α'-O) released in O2-TPD measurements, i.e., 19.9 > 13.6 > 6.2 > 2.7 μmol O2/g, confirming the important role of surface lattice oxygen in this type of reaction. Calcium substitution had a negative impact on the generation of surface lattice oxygen (decreasing α'- O which releases during O2-TPD). Differently, magnesium substitution increased considerably surface lattice oxygen species, which participates in methane oxidation reactions (intrafacial mechanism). The estimated activation energy within the temperature range 390–450°C reduced from 152 kJ/mol on LaCoO3 to 80 kJ/mol on LaCoMgO3, which is similar to noble metal-based catalysts (BaZrMO3, M = Rh, Pd, Pt, Mn, Ni, Co, activation energy = 72–106 kJ/ mol) [8].

Thus, the dual-site substitution of Ca2+ and Mg2+ was beneficial for suprafacial oxidation processes by increasing oxygen species chemisorbed on the surface oxygen vacancies, but had a less beneficial effect on intrafacial oxidation processes, for which only the single substitution of Co3+ by Mg2+ is highly indicated to increase the amount of surface lattice oxygen.

The work of Liu *et al.* [9] brings insights on the role of the partial substitution of La3+ by Ce4+ into La1–*x*Ce*x*MnO3 (0 < *x* < 0.1) which is a very active perovskite for the oxidation of volatile organic compounds (VOCs). The enhancement in activity is usually explained in terms of oxygen excess in the lattice, cationic vacancies, structural defects and the presence of multiple Mn oxidation states (Mn3+/Mn4+). Inserting cerium into the perovskite structure is a great strategy due to the Ce3+/Ce4+ redox behavior, which along with Mn3+/Mn4+ may increase oxygen transfer within the lattice. These researchers found that the activity of La1–*x*Ce*x*MnO<sup>3</sup> perov‐ skites (all presenting specific area approximately 25 m<sup>2</sup> /g) in the oxidation of 1,000 ppm benzene at the temperature range of 100–450°C correlated directly with the extent of cerium substitution (*x* value), *i.e.*, the higher the cerium content, the higher the benzene conversion according to the sequence *x* = 0 < 0.025 ≈ 0.05 < 0.075 < 0.1. But how to explain the promoter effect of cerium on the catalytic oxidation of benzene?

Firstly, authors confirmed cerium insertion into the perovskite lattice (using X-ray diffraction measurements, XRD) by verifying absence of segregated CeO2 phase, presence of lattice distortions and the decrease of La1–*x*Ce*x*MnO3 crystalline domain size as a function of cerium content. That should be expected since the higher Ce4+ coordination with their surrounding oxygen atoms (within the same crystal plan) than trivalent La3+ tends to inhibit crystal growth, resulting in smaller crystal size.

(α-O), which were adsorbed on surface oxygen vacancies. Since toluene oxidation makes use of surface oxygen (suprafacial mechanism), we should expect LaCaCoMgO3 and LaCoMgO<sup>3</sup>

On the other hand, Ca2+ insertion decreased the catalyst reducibility (shifting reduction peaks to higher temperature in H2-TPR experiments), which explained higher activity of LaCoO<sup>3</sup> instead of LaCaCoO3, since the non-substituted perovskite would release surface lattice oxygen easier to oxidize toluene than LaCaCoO3. Furthermore, the authors estimated the activation energy for the different catalysts in the temperature range of 140–280°C and found out that the dual-insertion of Ca2+ and Mg2+ into the LaCoO3 perovskite induced the activation energy to decrease from 143 kJ/mol (LaCoO3) to 34 kJ/mol (LaCaCoMgO3), which is comparable or even smaller than some noble metals applied in this suprafacial oxidation process (31–37

For the oxidation of 1 vol.% CH4 (in the absence and presence of water 3 vol.%), which is believed to occur by an intrafacial mechanism, ref. [5] found the sequence of the catalytic performance LaCoMgO3 > LaCaCoMgO3> LaCoO3> LaCaCoO<sup>3</sup> to agree with the sequence of surface lattice oxygen (α'-O) released in O2-TPD measurements, i.e., 19.9 > 13.6 > 6.2 > 2.7 μmol O2/g, confirming the important role of surface lattice oxygen in this type of reaction. Calcium substitution had a negative impact on the generation of surface lattice oxygen (decreasing α'- O which releases during O2-TPD). Differently, magnesium substitution increased considerably surface lattice oxygen species, which participates in methane oxidation reactions (intrafacial mechanism). The estimated activation energy within the temperature range 390–450°C reduced from 152 kJ/mol on LaCoO3 to 80 kJ/mol on LaCoMgO3, which is similar to noble metal-based catalysts (BaZrMO3, M = Rh, Pd, Pt, Mn, Ni, Co, activation energy = 72–106 kJ/

Thus, the dual-site substitution of Ca2+ and Mg2+ was beneficial for suprafacial oxidation processes by increasing oxygen species chemisorbed on the surface oxygen vacancies, but had a less beneficial effect on intrafacial oxidation processes, for which only the single substitution

The work of Liu *et al.* [9] brings insights on the role of the partial substitution of La3+ by Ce4+ into La1–*x*Ce*x*MnO3 (0 < *x* < 0.1) which is a very active perovskite for the oxidation of volatile organic compounds (VOCs). The enhancement in activity is usually explained in terms of oxygen excess in the lattice, cationic vacancies, structural defects and the presence of multiple Mn oxidation states (Mn3+/Mn4+). Inserting cerium into the perovskite structure is a great strategy due to the Ce3+/Ce4+ redox behavior, which along with Mn3+/Mn4+ may increase oxygen transfer within the lattice. These researchers found that the activity of La1–*x*Ce*x*MnO<sup>3</sup> perov‐

benzene at the temperature range of 100–450°C correlated directly with the extent of cerium substitution (*x* value), *i.e.*, the higher the cerium content, the higher the benzene conversion according to the sequence *x* = 0 < 0.025 ≈ 0.05 < 0.075 < 0.1. But how to explain the promoter

/g) in the oxidation of 1,000 ppm

of Co3+ by Mg2+ is highly indicated to increase the amount of surface lattice oxygen.

skites (all presenting specific area approximately 25 m<sup>2</sup>

effect of cerium on the catalytic oxidation of benzene?

(which contain this type of oxygen) to present higher activity.

546 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

kJ/mol on Au/LaCoO3 [6]; 86 kJ/mol on Pt/Al2O3 [7].

mol) [8].

Then, XPS measurements revealed the existence of Ce3+ and Ce4+ in the perovskites with higher cerium content (*x* = 0.05; 0.075; 0.10), as well as an increasing Mn4+/Mn3+ ratio and decreasing Oads/Olat ratio as a function of cerium substitution, apparently unexpected. Ce4+ substitution should induce Mn to have lower oxidation states, or at least decrease Mn4+/Mn3+ ratio as a charge compensation mechanism. Similarly, in the presence of Ce4+ and Mn4+, the Oads/Olatratio should increase, representing more oxygen species adsorbed on the surface to compensate the positive cationic charges, but the opposite occurred. According to [9], these findings indicated that cerium substitution created non-stoichiometric Ce-Mn-O compounds on the perovskite surface, and plenty of oxygen was accommodated into the lattice decreasing Oads/Olat ratio as the cerium content increased. Releasable oxygen from lattice justifies the higher activity of cerium-substituted perovskites in the benzene oxidation, which is believed to involve oxygen species supplied by the reducible oxide structure. The H2-TPR results supported this hypoth‐ esis by showing that cerium substitution increased the reducibility, specially at the tempera‐ ture range of 300–580°C, suggesting that Ce4+ increased the amount of oxygen species within the lattice (to reach structural neutrality) and also their diffusion from bulk to the surface.

The reactivity of substituted lanthanum cobaltite perovskites in the carbon oxidation (a solid– solid–gas catalytic reaction) has also been studied [10]. Changes in the LaCoO3 perovskite structure and the existence of gradient in the cobalt valence from bulk to surface were evaluated as a function of partial substitution of La3+ by Sr2+ and temperature reaction. LaCoO3 and La0.5Sr0.5CoO3 were prepared by spray pyrolysis method (specific surface area < 14 m2 /g), and then ground with carbon at catalyst:carbon = 9:1 ratio (w/w) to give a tight contact. O2-TPD measurements revealed that the maximum oxidation rate of carbon occurred at 650°C in the absence of catalyst, but LaCoO3 perovskite reduced this temperature to 540°C, and then Sr2+ substitution (La0.5Sr0.5CoO3) led to the lowest temperature 510°C. CO and CO2 were the byproducts in the carbon oxidation. The direct oxidation (absence of catalyst) led to similar amounts of CO and CO2 (50%/50%), but LaCoO3 and LaSrCoO3 were more selective to CO2, 89% and 94%, respectively. To explain the improvement in the activity and selectivity, the authors investigated the cations oxidation state and composition of the catalytic systems by using EELS (electron energy loss spectroscopy, which is a bulk sensitive analysis) and XPS (surface sensitive), before, at an intermediate temperature, and after the carbon oxidation process. The conclusions are illustrated in Figure 1, which shows a catalyst–carbon particle model for the catalytic phenomenon.

The partial substitution of La3+ by Sr2+ created plenty of oxygen vacancies within the perovskite lattice (as a charge compensator mechanism), which also caused the cobalt oxidation state to decrease from Co3+ (in LaCoO3) to values between +2.5 and +2.7 (in La0.5Sr0.5CoO3). Authors [10] estimated that the as-prepared Sr-substituted perovskite (La0.5Sr0.5CoO3) presented 50%Co3+ and 50%Co2+ (Figure 1a), which was consistent with the unit cell expansion due to the higher ionic radius of Co2+ (0.074 nm) related to Co3+ (0.063 nm). Surprisingly, the tight contact between carbon and perovskite at room temperature immediately decreased Co3+/Co2+ ratio in the bulk, as well as enhanced Co2+ (60–70%) on the surface (Figure 1b). This partial reduction of cobalt in the bulk and especially on the catalyst surface suggests oxygen mobility from bulk to surface and a remarkable surface reactivity for carbon oxidation, even at room temperature, which agreed with the increase of carbonate species on the surface, as seen by XPS. The partial substitution of La3+ by Sr2+ created plenty of oxygen vacancies within the perovskite lattice (as a charge compensator mechanism), which also caused the cobalt oxidation state to decrease from Co3+ (in LaCoO3) to values between +2.5 and +2.7 (in La0.5Sr0.5CoO3). Authors [10] estimated that the as-prepared Sr-substituted perovskite (La0.5Sr0.5CoO3) presented 50%Co3+ and 50%Co2+ (Figure 2a), which was consistent with the unit cell expansion due to the higher ionic radius of Co2+ (0.074 nm) related to Co3+ (0.063 nm). Surprisingly, the tight contact between carbon and perovskite at room temperature immediately decreased Co3+/Co2+ ratio in the bulk, as well as enhanced Co2+ (60–70%) on the surface (Figure 2b). This partial reduction of cobalt in the bulk and especially on the catalyst surface suggests oxygen mobility from bulk to surface and a remarkable surface reactivity for carbon oxidation, even at room temperature, which

agreed with the increase of carbonate species on the surface, as seen by XPS.

structural neutrality) and also their diffusion from bulk to the surface.

shows a catalyst–carbon particle model for the catalytic phenomenon.

Then, XPS measurements revealed the existence of Ce3+ and Ce4+ in the perovskites with higher cerium content (*x* = 0.05; 0.075; 0.10), as well as an increasing Mn4+/Mn3+ ratio and decreasing Oads/Olat ratio as a function of cerium substitution, apparently unexpected. Ce4+ substitution should induce Mn to have lower oxidation states, or at least decrease Mn4+/Mn3+ ratio as a charge compensation mechanism. Similarly, in the presence of Ce4+ and Mn4+, the Oads/Olat ratio should increase, representing more oxygen species adsorbed on the surface to compensate the positive cationic charges, but the opposite occurred. According to [9], these findings indicated that cerium substitution created non-stoichiometric Ce-Mn-O compounds on the perovskite surface, and plenty of oxygen was accommodated into the lattice decreasing Oads/Olat ratio as cerium content increased. Releasable oxygen from lattice justifies the higher activity of cerium-substituted perovskites in the benzene oxidation, which is believed to involve oxygen species supplied by the reducible oxide structure. The H2- TPR results supported this hypothesis by showing that cerium substitution increased the reducibility, specially at the temperature range of 300–580°C, suggesting that Ce4+ increased the amount of oxygen species within the lattice (to reach

The reactivity of substituted lanthanum cobaltites perovskite in the carbon oxidation (a solid–solid–gas catalytic reaction) has also been studied [10]. Changes in the LaCoO3 perovskite structure and the existence of gradient in the cobalt valence from bulk to surface were evaluated as a function of partial substitution of La3+ by Sr2+ and temperature reaction. LaCoO3 and La0.5Sr0.5CoO3 were prepared by spray pyrolysis method (specific surface area < 14 m2/g), and then ground with carbon at 9:1 ratio (w/w) to give a tight contact. O2-TPD measurements revealed that the maximum oxidation rate of carbon occurred at 650°C in the absence of catalyst, but LaCoO3 perovskite reduced this temperature to 540°C, and then Sr2+ substitution (La0.5Sr0.5CoO3) led to the lowest temperature 510°C. CO and CO2 were the byproducts in the carbon oxidation. The direct oxidation (absence of catalyst) led to similar amounts of CO and CO2 (50%/50%), but LaCoO3 and LaSrCoO3 were more selective to CO2, 89% and 94%, respectively. To explain the improvement in the activity and selectivity, the authors investigated the cations oxidation state and composition of the catalytic systems by using EELS

intermediate temperature, and after the carbon oxidation process. The conclusions are illustrated in Figure 2, which

Figure 2. Scheme of cobalt oxidation states distribution within the perovskite La0.5Sr0.5CoO3 as a consequence of the interaction and reaction with carbon particulates: (a) perovskite as prepared (without carbon); (b) after grinding carbon and catalyst at room temperature; (c) heating treatment at 400°C in the presence of 0.5 Torr O2; (d) final state of the perovskite after removal of carbon by prolonged thermal oxidation. The expressed percentages of cobalt were deduced from EELS calculations. Adapted from [10], copyright 2008 by Elsevier Inc. Adapted with permission. **Figure 1.** Scheme of cobalt oxidation states distribution within the perovskite La0.5Sr0.5CoO3 as a consequence of the interaction and reaction with carbon particulates: (a) perovskite as prepared (without carbon); (b) after grinding car‐ bon and catalyst at room temperature; (c) heating treatment at 400°C in the presence of 0.5 Torr O2; (d) final state of the perovskite after removal of carbon by prolonged thermal oxidation. The expressed percentages of cobalt were deduced from EELS calculations. Reprinted from reference [10], copyright (2008) with permission from Elsevier Inc.

As depicted in Figure 1c, by increasing the temperature reaction in the presence of oxygen, the carbon oxidation activity enhanced because O2– species migrated from bulk to surface promoting surface carbonates elimination (as proved by XPS). For this reason, the overall oxidation state of cobalt decreased by the end of reaction and the temperature for the maximum oxidation rate of carbon reduced by 140°C (from 650°C in the absence of catalyst to 510°C on LaSrCoO3). In the presence of oxidative atmosphere, carbon particulates on the catalyst surface were oxidized by O2 from gas phase and also by lattice O2– anions leading to the consumption of lattice oxygen, which decreased the overall oxidation state of cobalt. After complete carbon oxidation, the perovskite surface presented abundance of Co3+ (90–95%) while bulk showed oxygen deficit (Co2+-rich) as shown in Figure 1d, due to the diffusion of oxygen species through the structural vacancies towards surface or re-establishment of the surface lattice with oxygen from gas phase. The overall reaction (1) represents the interaction between the perovskite surface and carbon particles, summarizing the phenomenon in Figure 1 ([Vo] means an anionic vacancy):

Improvement of Catalytic Performance of Perovskites by Partial Substitution of Cations and Supporting on... http://dx.doi.org/10.5772/61279 549

$$\left(\text{Co}^{\text{+}}-\text{O}^{\text{-}}-\text{Co}^{\text{+}}\right) + \text{C} \rightarrow \left(\text{Co}^{\text{2+}}-\left[\text{V}\_{o}\right]-\text{Co}^{\text{2+}}\right) + \text{CO}\_{\text{x}} \tag{1}$$

Therefore, Sr2+ substitution into the perovskite lattice created oxygen vacancies, increasing cobalt reducibility and enhancing the mobility of lattice oxygen species, which were an additional source of oxygen for the carbon oxidation towards CO2. But the partial substitution of A and/or B sites into the ABO3 mixed oxide may lead to consequences other than the generation of anionic vacancies and stabilization of unusual oxidation states, such as the stabilization of the crystal lattice avoiding the segregation of undesired phases (which worsen the catalytic performance), as reported in the reference [11] for LaCoO3 substituted by K+ and Mg2+ applied for the soot oxidation.

La1–*x*K*x*Co1–*y*Mg*y*O3 perovskites (*x* = 0 – 0.4; e *y* = 0 – 0.2) prepared by citric acid complexation [11] showed an increasing segregation of Co3O4 as La3+ was substituted by K+ (no Mg substi‐ tution then). The phase segregation can be explained because of the larger ionic radius of K+ (0.133 nm) than La3+ (0.103 nm), which caused severe lattice distortions and unstable occupancy of the lanthanum site by K+ leading to cobalt to segregate as Co3O4. However, the increasing Mg2+ substitution into the perovskite containing the higher amount of K+ (*i.e.*, *x* = 0.4: La0.6K0.4Co1–*y*Mg*y*O3) led to a gradual diminishment of segregated Co3O4, since the larger ionic radius of Mg2+ (0.072 nm) than Co3+ (0.063 nm) gave more stability for the crystal lattice avoiding cobalt segregation. This hypothesis was confirmed by XRD, FTIR, TEM and EDS results which evidenced Co3O4 phase decreasing as a function of Mg substitution. By replacing both cations K+ and Mg2+ into the perovskite, Co4+ was detected by XPS, as a charge compen‐ sator mechanism. According to ref. [11], the activity of the perovskites in the oxidation of carbon (using synthetic air 20 vol.% O2/N2 and tight catalyst/carbon contact) was established as a function of the temperature for the maximum oxidation rate (*T <sup>m</sup>*). The non-catalyzed reaction (absence of catalyst) presented *T <sup>m</sup>* = 590°C, but the perovskite LaCoO3 led to *T <sup>m</sup>* = 448°C, which gradually decreased to *T <sup>m</sup>* = 370°C with increasing substitution of La3+ by K+ within the perovskite. By inserting Mg2+ into the B site of La0.6K0.4Co1–*y*Mg*y*O3, the structure stability increased and cobalt segregation decreased (ensuring the existence of anionic vacancies useful for oxygen mobility) and as a consequence, the lowest *T <sup>m</sup>* = 359°C was reached on La0.6K0.4Co0.9Mg0.1O3. Therefore, the improvement in the carbon oxidation followed exactly the increasing amount of K+ up to *x* = 0.4 (into A site) and increasing amount of Mg2+ up to *y* = 0.1 (into B site), which is very consistent with the increasing reducibility of the perovskites (and redox properties) as a function of the partial substitution of K+ and Mg2+ (as evidenced by H2-TPR measurements). In particular, Mg2+ stabilized the crystal lattice and reduced cobalt segregation. That ensured the integrity of the perovskite oxide structure and the mobility of lattice oxygen towards surface which enhanced carbon particulate oxidation.

### **2.2. Perovskites for CO selective oxidation**

and 50%Co2+ (Figure 1a), which was consistent with the unit cell expansion due to the higher ionic radius of Co2+ (0.074 nm) related to Co3+ (0.063 nm). Surprisingly, the tight contact between carbon and perovskite at room temperature immediately decreased Co3+/Co2+ ratio in the bulk, as well as enhanced Co2+ (60–70%) on the surface (Figure 1b). This partial reduction of cobalt in the bulk and especially on the catalyst surface suggests oxygen mobility from bulk to surface and a remarkable surface reactivity for carbon oxidation, even at room temperature, which

Then, XPS measurements revealed the existence of Ce3+ and Ce4+ in the perovskites with higher cerium content (*x* = 0.05; 0.075; 0.10), as well as an increasing Mn4+/Mn3+ ratio and decreasing Oads/Olat ratio as a function of cerium substitution, apparently unexpected. Ce4+ substitution should induce Mn to have lower oxidation states, or at least decrease Mn4+/Mn3+ ratio as a charge compensation mechanism. Similarly, in the presence of Ce4+ and Mn4+, the Oads/Olat ratio should increase, representing more oxygen species adsorbed on the surface to compensate the positive cationic charges, but the opposite occurred. According to [9], these findings indicated that cerium substitution created non-stoichiometric Ce-Mn-O compounds on the perovskite surface, and plenty of oxygen was accommodated into the lattice decreasing Oads/Olat ratio as cerium content increased. Releasable oxygen from lattice justifies the higher activity of cerium-substituted perovskites in the benzene oxidation, which is believed to involve oxygen species supplied by the reducible oxide structure. The H2- TPR results supported this hypothesis by showing that cerium substitution increased the reducibility, specially at the temperature range of 300–580°C, suggesting that Ce4+ increased the amount of oxygen species within the lattice (to reach

The reactivity of substituted lanthanum cobaltites perovskite in the carbon oxidation (a solid–solid–gas catalytic reaction) has also been studied [10]. Changes in the LaCoO3 perovskite structure and the existence of gradient in the cobalt valence from bulk to surface were evaluated as a function of partial substitution of La3+ by Sr2+ and temperature reaction. LaCoO3 and La0.5Sr0.5CoO3 were prepared by spray pyrolysis method (specific surface area < 14 m2/g), and then ground with carbon at 9:1 ratio (w/w) to give a tight contact. O2-TPD measurements revealed that the maximum oxidation rate of carbon occurred at 650°C in the absence of catalyst, but LaCoO3 perovskite reduced this temperature to 540°C, and then Sr2+ substitution (La0.5Sr0.5CoO3) led to the lowest temperature 510°C. CO and CO2 were the byproducts in the carbon oxidation. The direct oxidation (absence of catalyst) led to similar amounts of CO and CO2 (50%/50%), but LaCoO3 and LaSrCoO3 were more selective to CO2, 89% and 94%, respectively. To explain the improvement in the activity and selectivity, the authors investigated the cations oxidation state and composition of the catalytic systems by using EELS (electron energy loss spectroscopy, which is a bulk sensitive analysis) and XPS (surface sensitive), before, at an intermediate temperature, and after the carbon oxidation process. The conclusions are illustrated in Figure 2, which

The partial substitution of La3+ by Sr2+ created plenty of oxygen vacancies within the perovskite lattice (as a charge compensator mechanism), which also caused the cobalt oxidation state to decrease from Co3+ (in LaCoO3) to values between +2.5 and +2.7 (in La0.5Sr0.5CoO3). Authors [10] estimated that the as-prepared Sr-substituted perovskite (La0.5Sr0.5CoO3) presented 50%Co3+ and 50%Co2+ (Figure 2a), which was consistent with the unit cell expansion due to the higher ionic radius of Co2+ (0.074 nm) related to Co3+ (0.063 nm). Surprisingly, the tight contact between carbon and perovskite at room temperature immediately decreased Co3+/Co2+ ratio in the bulk, as well as enhanced Co2+ (60–70%) on the surface (Figure 2b). This partial reduction of cobalt in the bulk and especially on the catalyst surface suggests oxygen mobility from bulk to surface and a remarkable surface reactivity for carbon oxidation, even at room temperature, which

Figure 2. Scheme of cobalt oxidation states distribution within the perovskite La0.5Sr0.5CoO3 as a consequence of the interaction and reaction with carbon particulates: (a) perovskite as prepared (without carbon); (b) after grinding carbon and catalyst at room temperature; (c) heating treatment at 400°C in the presence of 0.5 Torr O2; (d) final state of the perovskite after removal of carbon by prolonged thermal oxidation. The expressed percentages of cobalt were deduced from EELS calculations. Adapted from [10], copyright 2008 by Elsevier Inc.

**Figure 1.** Scheme of cobalt oxidation states distribution within the perovskite La0.5Sr0.5CoO3 as a consequence of the interaction and reaction with carbon particulates: (a) perovskite as prepared (without carbon); (b) after grinding car‐ bon and catalyst at room temperature; (c) heating treatment at 400°C in the presence of 0.5 Torr O2; (d) final state of the perovskite after removal of carbon by prolonged thermal oxidation. The expressed percentages of cobalt were deduced

As depicted in Figure 1c, by increasing the temperature reaction in the presence of oxygen, the carbon oxidation activity enhanced because O2– species migrated from bulk to surface promoting surface carbonates elimination (as proved by XPS). For this reason, the overall oxidation state of cobalt decreased by the end of reaction and the temperature for the maximum oxidation rate of carbon reduced by 140°C (from 650°C in the absence of catalyst to 510°C on LaSrCoO3). In the presence of oxidative atmosphere, carbon particulates on the catalyst surface were oxidized by O2 from gas phase and also by lattice O2– anions leading to the consumption of lattice oxygen, which decreased the overall oxidation state of cobalt. After complete carbon oxidation, the perovskite surface presented abundance of Co3+ (90–95%) while bulk showed oxygen deficit (Co2+-rich) as shown in Figure 1d, due to the diffusion of oxygen species through the structural vacancies towards surface or re-establishment of the surface lattice with oxygen from gas phase. The overall reaction (1) represents the interaction between the perovskite surface and carbon particles, summarizing the phenomenon in Figure 1 ([Vo] means an anionic

from EELS calculations. Reprinted from reference [10], copyright (2008) with permission from Elsevier Inc.

agreed with the increase of carbonate species on the surface, as seen by XPS.

Adapted with permission.

vacancy):

agreed with the increase of carbonate species on the surface, as seen by XPS.

shows a catalyst–carbon particle model for the catalytic phenomenon.

548 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

structural neutrality) and also their diffusion from bulk to the surface.

Carbon monoxide selective oxidation reaction (CO-SELOX) is an interesting and economic approach to remove CO from H2-rich gas streams, but a selective catalyst should avoid H2 consumption since CO and H2 oxidation reactions compete each other. Pt-supported catalysts are typically used because CO adsorbs more strongly on Pt surface than H2. Other classes of CO selective oxidation catalysts comprise non-noble metals such as Ag, Au and also oxides, e.g., CuO-CeO2, CeO2, Cu-LaO2-CeO2 as well as cobalt-based oxides. Literature reports bulk and supported cobalt oxides applied for SELOX, but rarely cobalt-based perovskites. The main constraint for cobalt oxides application is their low stability in H2-rich stream and consequent metallic cobalt formation on the catalyst surface which catalyzes undesirable reactions such as H2 oxidation and methanation. For instance, CoOx/ZrO2 showed high CO conversion at 175°C, but above this temperature H2 oxidation took place, and methanation reaction occurred above 250°C [12].

For this reason, a suitable catalyst for CO selective oxidation should not only be able to keep the oxide structure during the process but also allow oxygen mobility through the crystal lattice in order to enhance the CO oxidation rates towards CO2 on the surface active sites. Cobaltcontaining perovskites seem to satisfy all these requirements.

Magalhães *et al.* [13] applied Ce-substituted LaCoO3 perovskites in the CO-SELOX reaction in H2-rich feedstream (1% CO, 1% O2, 60% H2, He balance, vol.%) and found out the new structures containing cerium increased the resistance to undesirable reactions and did not damage the high selectivity to CO2 presented by the non-substituted perovskite. They obtained single phase La1-xCexCoO3 perovskites (*x* = 0; 0.05 and 0.10) with low surface area (< 15 m2 /g) and verified, by Rietveld refinements, that the Ce4+ substitution into La3+ sites decreased both cell parameters of the rhombohedral unit cell and crystalline domain sizes, since the ionic radius of Ce4+ is smaller than La3+, exactly as verified by ref. [9] for LaCeMnO3. Figure 2 illustrates the effect of cerium insertion into the perovskite on the CO conversion to CO2 and H2O byproducts during temperature-programmed reactions: region 1 evidenced the selective CO oxidation in which CO and O2 were exclusively converted to CO2, and one can note that the presence of cerium affected the reaction kinetics shifting CO conversion to higher temper‐ atures. In region 2 the undesired H2 oxidation gained importance leading to water formation, and finally methanation reaction occurred at higher temperatures at region 3 (leading to CH4 and H2O formation). Though cerium substitution shifted the range of temperature for CO-SELOX reaction up to 240°C, all the undesired reactions also shifted to higher temperature.

Authors [13] affirmed that Ce4+ changed some Co3+ to Co2+ to keep the charge neutrality within the structure, consequently decreasing the amount of active Co3+ sites on the surface and decreasing the activity for CO conversion (for this reason, La0.90Ce0.10CoO3 showed CO conversions at higher temperature than LaCoO3 in Figure 2). However, the formation of cationic vacancies into A and B sites as a second mechanism to keep the charge neutrality would stabilize the entity Co3+/O2 on the surface, ensuring high CO2 selectivity for ceriumsubstituted perovskites. At temperatures higher than 350°C, Co*<sup>n</sup>*<sup>+</sup> species coexist with metallic Co0 which leads to undesirable methanation reaction. This hypothesis was supported by H2- TPR experiments that showed perovskite reduction above 300°C (i.e., Treduction > 300°C). These results clearly evidenced that Co3+ as part of a perovskite structure is more resistant to reduction when compared to typical supported cobalt catalysts, such as CoO*x*/CeO2 (Treduction > 275°C) [14], CoO*x*/ZrO2 (Treduction > 250°C) [12] and Co3O4/CeZrO2 (Treduction > 200°C) [15].

CO conversion to higher temperatures. In region 2 the undesired H2 oxidation gained importance leading to water formation, and finally methanation reaction occurred at higher temperatures at region 3 (leading to CH4 and H2O formation). Though cerium substitution shifted the range of temperature for CO-SELOX reaction up to 240°C, all the Improvement of Catalytic Performance of Perovskites by Partial Substitution of Cations and Supporting on... http://dx.doi.org/10.5772/61279 551

Magalhães *et al.* [13] applied Ce-substituted LaCoO3 perovskites in the CO-SELOX reaction in H2-rich feedstream (1% CO, 1% O2, 60% H2, He balance, vol.%) and found out the new structures containing cerium increased the resistance to undesirable reactions and did not damage the high selectivity to CO2 presented by the non-substituted perovskite. They obtained single phase La1?xCexCoO3 perovskites (*x* = 0; 0.05 and 0.10) with low surface area (< 15 m2/g) and verified, by Rietveld refinements, that the Ce4+ substitution into La3+ sites decreased both cell parameters of the rhombohedral unit cell and crystalline domain sizes, since the ionic radius of Ce4+ is smaller than La3+, exactly as verified by ref. [9] for LaCeMnO3. Figure 3 illustrates the effect of cerium insertion into the perovskite on the CO conversion to CO2 and H2O byproducts during temperature-programmed reactions: region 1 evidenced the selective CO oxidation in which CO and O2 were exclusively converted to CO2, and one can note that the presence of cerium affected the reaction kinetics shifting

undesired reactions also shifted to higher temperature.

are typically used because CO adsorbs more strongly on Pt surface than H2. Other classes of CO selective oxidation catalysts comprise non-noble metals such as Ag, Au and also oxides, e.g., CuO-CeO2, CeO2, Cu-LaO2-CeO2 as well as cobalt-based oxides. Literature reports bulk and supported cobalt oxides applied for SELOX, but rarely cobalt-based perovskites. The main constraint for cobalt oxides application is their low stability in H2-rich stream and consequent metallic cobalt formation on the catalyst surface which catalyzes undesirable reactions such as H2 oxidation and methanation. For instance, CoOx/ZrO2 showed high CO conversion at 175°C, but above this temperature H2 oxidation took place, and methanation reaction occurred

For this reason, a suitable catalyst for CO selective oxidation should not only be able to keep the oxide structure during the process but also allow oxygen mobility through the crystal lattice in order to enhance the CO oxidation rates towards CO2 on the surface active sites. Cobalt-

Magalhães *et al.* [13] applied Ce-substituted LaCoO3 perovskites in the CO-SELOX reaction in H2-rich feedstream (1% CO, 1% O2, 60% H2, He balance, vol.%) and found out the new structures containing cerium increased the resistance to undesirable reactions and did not damage the high selectivity to CO2 presented by the non-substituted perovskite. They obtained single phase La1-xCexCoO3 perovskites (*x* = 0; 0.05 and 0.10) with low surface area (< 15 m2

and verified, by Rietveld refinements, that the Ce4+ substitution into La3+ sites decreased both cell parameters of the rhombohedral unit cell and crystalline domain sizes, since the ionic radius of Ce4+ is smaller than La3+, exactly as verified by ref. [9] for LaCeMnO3. Figure 2 illustrates the effect of cerium insertion into the perovskite on the CO conversion to CO2 and H2O byproducts during temperature-programmed reactions: region 1 evidenced the selective CO oxidation in which CO and O2 were exclusively converted to CO2, and one can note that the presence of cerium affected the reaction kinetics shifting CO conversion to higher temper‐ atures. In region 2 the undesired H2 oxidation gained importance leading to water formation, and finally methanation reaction occurred at higher temperatures at region 3 (leading to CH4 and H2O formation). Though cerium substitution shifted the range of temperature for CO-SELOX reaction up to 240°C, all the undesired reactions also shifted to higher temperature.

Authors [13] affirmed that Ce4+ changed some Co3+ to Co2+ to keep the charge neutrality within the structure, consequently decreasing the amount of active Co3+ sites on the surface and decreasing the activity for CO conversion (for this reason, La0.90Ce0.10CoO3 showed CO conversions at higher temperature than LaCoO3 in Figure 2). However, the formation of cationic vacancies into A and B sites as a second mechanism to keep the charge neutrality would stabilize the entity Co3+/O2 on the surface, ensuring high CO2 selectivity for cerium-

Co0 which leads to undesirable methanation reaction. This hypothesis was supported by H2- TPR experiments that showed perovskite reduction above 300°C (i.e., Treduction > 300°C). These results clearly evidenced that Co3+ as part of a perovskite structure is more resistant to reduction when compared to typical supported cobalt catalysts, such as CoO*x*/CeO2 (Treduction > 275°C) [14], CoO*x*/ZrO2 (Treduction > 250°C) [12] and Co3O4/CeZrO2 (Treduction > 200°C) [15].

/g)

species coexist with metallic

containing perovskites seem to satisfy all these requirements.

550 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

substituted perovskites. At temperatures higher than 350°C, Co*<sup>n</sup>*<sup>+</sup>

above 250°C [12].

Figure 3. Effect of temperature and cerium substitution on compounds distribution in the CO-SELOX reaction for (a) LaCoO3 and (b) La0.90Ce0.10CoO3 (1% CO, 1% O2, 60% H2, He balance, vol.%). Adapted from [13], copyright 2010 by Elsevier B.V. Adapted with permission. **Figure 2.** Effect of temperature and cerium substitution on compounds distribution in the CO-SELOX reaction for (a) LaCoO3 and (b) La0.90Ce0.10CoO3 (1% CO, 1% O2, 60% H2, He balance, vol.%). Reprinted from reference [13], copyright (2010) with permission from Elsevier B.V.

Schmal *et al*. [16] investigated the effect of strontium insertion into La1-*x*Sr*x*CoO3 (*x* = 0; 0.05) on the catalytic performance in the CO-SELOX reaction. Differently from cerium, Sr2+ as a cationic dopant is expected to increase cobalt oxidation state and/or create oxygen vacancies within the crystal lattice, which could enhance oxygen mobility and provide lattice oxygen for CO oxidation on the surface. These researchers found out evidences for complete Sr2+ substitution into the rhombohedral crystalline structure of the perovskite (by means of XRD and Raman spectroscopy), which led to distortions and expansion of the unit cell volume, since the ionic radius of Sr2+ (0.132 nm) is larger than La3+. XPS indicated that Sr2+ did not modify the cobalt oxidation state (Co3+) in La0.95Sr0.05CoO3, suggesting that anionic vacancies have been created within the lattice as a charge compensator. This hypothesis was supported by the enhancement in both activity to CO conversion and selectivity to CO2 after Sr2+ substitution: the temperature to convert 50% of CO decreased from 170°C (LaCoO3) to 155°C (La0.95Sr0.05CoO3), and selectivity to CO2 (at 29% isoconversion of CO) increased slightly from 87% to 91%. Interestingly La0.95Sr0.05CoO3 was more stable than LaCoO3 on stream at 170°C, presenting constant CO conversion 75% for 48 h, while the non-substituted perovskite showed decreasing CO conversion from ca. 75% to 45% at the same conditions. Authors [13] affirmed that Ce4+ changed some Co3+ to Co2+ to keep the charge neutrality within the structure, consequently decreasing the amount of active Co3+ sites on the surface and decreasing the activity for CO conversion (for this reason, La0.90Ce0.10CoO3 showed CO conversions at higher temperature than LaCoO3 in Figure 3). However, the formation of cationic vacancies into A and B sites as a second mechanism to keep the charge neutrality would stabilize the entity Co3+/O2 on the surface, ensuring high CO2 selectivity for cerium-substituted perovskites. At temperatures higher than 350°C, Co*n*+ species coexist with metallic Co0 which leads to undesirable methanation reaction. This hypothesis was supported by H2-TPR experiments that showed perovskite reduction above 300°C (i.e., Treduction > 300°C). These results clearly evidenced that Co3+ as part of a perovskite structure is more resistant to reduction when compared to typical supported cobalt catalysts, such as CoO*x*/CeO2 (Treduction > 275°C) [14], CoO*x*/ZrO2 (Treduction > 250°C) [12] and Co3O4/CeZrO2 (Treduction > 200°C) [15]. Schmal *et al*. [16] investigated the effect of strontium insertion into La1?*x*Sr*x*CoO3 (*x* = 0; 0.05) on the catalytic performance in the CO-SELOX reaction. Differently from cerium, Sr2+ as a cationic dopant is expected to increase cobalt oxidation state and/or create oxygen vacancies within the crystal lattice, which could enhance oxygen mobility and provide lattice oxygen for CO oxidation on the surface. These researchers found out evidences for complete Sr2+ substitution into the rhombohedral crystalline structure of the perovskite (by means of XRD and Raman spectroscopy), which led to distortions and expansion of the unit cell volume, since the ionic radius of Sr2+ (0.132 nm) is larger than La3+. XPS indicated that Sr2+ did not modify the cobalt oxidation state (Co3+) in La0.95Sr0.05CoO3, suggesting that anionic vacancies have been

How to explain that? The dopant Sr2+ caused distortions in the lattice due to its size and oxidation state, giving rise to anionic (oxygen) vacancies, which initially neutralized the structure charge but then enhanced the oxygen transfer through the lattice, i.e., from bulk to surface during reaction (indeed, the full redox process also comprises the re-establishment of lattice oxygen from oxygen coming from gas phase). As a consequence, this oxygen supplying from the lattice enhanced the CO oxidation on the catalyst surface. created within the lattice as a charge compensator. This hypothesis was supported by the enhancement in both activity to CO conversion and selectivity to CO2 after Sr2+ substitution: the temperature to convert 50% of CO decreased from 170°C (LaCoO3) to 155°C (La0.95Sr0.05CoO3), and selectivity to CO2 (at 29% isoconversion of CO) increased slightly from 87% to 91%. Interestingly La0.95Sr0.05CoO3 was more stable than LaCoO3 on stream at 170°C, presenting constant CO conversion 75% for 48 h, while the non-substituted perovskite showed decreasing CO conversion from ca. 75% to 45% at the same conditions.

A short communication [17] highlighted the influence of cerium substitution in La1-*x*Ce*x*NiO3 (*x* = 0; 0.05; 0.1) on the catalysts properties and performance in the CO-SELOX reaction (2.5% CO, 5% O2, 33% H2 and N2 balance, vol.%). The surface area of the catalysts with rhombohedral structure did not change with cerium addition, which usually takes place only at Ce content above *x* > 0.4 simultaneously to the appearance of segregated CeO*x* phases [18]. CO conversion was higher for the cerium-doped perovskites than for LaNiO3 at the entire range of 150–230°C, How to explain that? The dopant Sr2+ caused distortions in the lattice due to its size and oxidation state, giving rise to anionic (oxygen) vacancies, which initially neutralized the structure charge but then enhanced the oxygen transfer through the lattice, i.e., from bulk to surface during reaction (indeed, the full redox process also comprises the reestablishment of lattice oxygen from oxygen coming from gas phase). As a consequence, this oxygen supplying from the lattice enhanced the CO oxidation on the catalyst surface.

and undesirable H2 oxidation took place only above 230°C (H2 conversion < 12%). The authors discussed the promoter effect of cerium in terms of (i) very active surface Ce3+/Ce4+ redox cations that directly adsorbs and activates O2, (ii) stabilization of reduced Ni*<sup>n</sup>*<sup>+</sup> species (*n* < 3), not discarding (iii) the enhanced lattice oxygen mobility from bulk to surface supplying oxygen for the CO oxidation.

Literature reports that CO oxidation over perovskites proceeds through a suprafacial mecha‐ nism involving weakly chemisorbed oxygen. The following mechanistic proposal has been suggested by Tascon *et al.* [19] to explain CO oxidation on LaCoO3: molecular oxygen chemi‐ sorbs on Co2+ cations as an O2 – anion, dissociating to form atomic oxygen (O– ) on the cobalt sites. Simultaneously, CO chemisorbs on surface oxide ions yielding a labile species that interacts with adsorbed atomic oxygen, producing carbonates (the rate-determining step) which decompose towards CO2 and oxygen.

$$\text{O}\_{2(g)} \rightarrow \text{O}\_{2}^{-}\text{(ad)} \rightarrow 2\text{O}^{-}\_{(\text{ad})} \tag{2}$$

$$\text{CO}\_{\text{(g)}} \rightarrow \text{CO}\_{\text{(aq)}} \tag{3}$$

$$\text{CO}\_{\text{(ad)}} + \text{2O}^{\cdot}\_{\text{(ad)}} \rightarrow \text{CO}\_{3}^{2-} \text{ (atle/mitting step)}\tag{4}$$

$$\text{CO}\_3^{2-} \text{(ad)} \rightarrow \text{CO}\_{2(ad)} + \text{O}^{2-} \text{(ad)} \rightarrow \text{CO}\_{2(g)} + \text{O}^{2-} \text{(ad)}\tag{5}$$

Though the mentioned mechanism (equations (2)–(5)) does not clarify about the role of the lattice oxygen within the elementary steps, we may infer that those mobile oxygen species, specially the surface lattice oxygen, participate more easily in the redox process and contribute in some extent into the elementary step (4).

## **2.3. Perovskite as a catalytic precursor**

Considering only geometric factors, the Goldschmidt tolerance factor (*t*) for perovskites indicates that lanthanum, which is the largest lanthanide ion in the series, leads to the most stable perovskite structure [20]. However, the thermal stability of ABO3 perovskites strongly depends on cations at both positions A and B. Particularly, the thermal stability in a reducing atmosphere such as H2, during reduction treatments or redox cycles, has been the focus of research because the metal B can reach high degree of dispersion over an AO*x* matrix when Figure 2 these treatments. Considering the importance of dispersed metals in heterogeneous catalysis, the reduction or reduction–oxidation of perovskites under controlled conditions and atmosphere may offer a promising methodology for the preparation of highly active and dispersed catalysts [20]. In this sense, we can face a perovskite-type oxide as a catalytic precursor for a dispersed metal catalyst.

Copper-substituted materials have been widely investigated in the literature, not only in perovskite oxides, since copper may modify the carbon formation mechanism avoiding coke deposition. Tien-Thao *et al.* [21] verified that replacing cobalt by copper into LaCo1–*x*Cu*x*O3 perovskites (0 < *x* < 0.6; specific surface area 10–60 m2 /g) increased the reducibility and caused strong cobalt–copper interaction that enhanced metallic dispersion of cobalt and prevented metal sintering. The main catalytic consequence of Cu2+ substitution was the notable selectivity to higher alcohols in the syngas conversion (H2/CO = 2 diluted to 20 vol.% in He, 1000 psi, 275°C) by using the perovskites partially reduced in H2. According to these authors, X-ray diffraction of the as-prepared materials proved the copper substitution due to the clear structure distortions, which affected the redox properties and stability of these mixed oxides. Temperature-programmed desorption of O2 showed large amount of oxygen releasing from Cu2+-substituted perovskites, reflecting a high concentration of surface oxygen vacancies (Co3+ substituted by Cu2+ creates anionic vacancies for charge compensation). The partially reduced LaCo1–*x*Cu*x*O3 perovskites were tested in the syngas conversion and yielded a homologous series of linear hydrocarbons (from methane to undecane) and a set of linear primary alcohols with the chain growth probability values ranging from 0.34 to 0.42.

and undesirable H2 oxidation took place only above 230°C (H2 conversion < 12%). The authors discussed the promoter effect of cerium in terms of (i) very active surface Ce3+/Ce4+ redox

not discarding (iii) the enhanced lattice oxygen mobility from bulk to surface supplying oxygen

Literature reports that CO oxidation over perovskites proceeds through a suprafacial mecha‐ nism involving weakly chemisorbed oxygen. The following mechanistic proposal has been suggested by Tascon *et al.* [19] to explain CO oxidation on LaCoO3: molecular oxygen chemi‐

sites. Simultaneously, CO chemisorbs on surface oxide ions yielding a labile species that interacts with adsorbed atomic oxygen, producing carbonates (the rate-determining step)

( ) ( ) ( ) ( ) ( ) 2– 2– 2–

Though the mentioned mechanism (equations (2)–(5)) does not clarify about the role of the lattice oxygen within the elementary steps, we may infer that those mobile oxygen species, specially the surface lattice oxygen, participate more easily in the redox process and contribute

Considering only geometric factors, the Goldschmidt tolerance factor (*t*) for perovskites indicates that lanthanum, which is the largest lanthanide ion in the series, leads to the most stable perovskite structure [20]. However, the thermal stability of ABO3 perovskites strongly depends on cations at both positions A and B. Particularly, the thermal stability in a reducing atmosphere such as H2, during reduction treatments or redox cycles, has been the focus of research because the metal B can reach high degree of dispersion over an AO*x* matrix when Figure 2 these treatments. Considering the importance of dispersed metals in heterogeneous catalysis, the reduction or reduction–oxidation of perovskites under controlled conditions and atmosphere may offer a promising methodology for the preparation of highly active and dispersed catalysts [20]. In this sense, we can face a perovskite-type oxide as a catalytic

anion, dissociating to form atomic oxygen (O–

( ) ( ) ( ) – – O O 2O 2 g ® ® <sup>2</sup> ad ad (2)

ad ad <sup>3</sup> ad CO + 2O CO rate-limiting step ® (4)

<sup>3</sup> ad 2 ad ad 2 g ad CO CO + O CO + O ® ® (5)

(g) (ad) CO CO ® (3)

species (*n* < 3),

) on the cobalt

cations that directly adsorbs and activates O2, (ii) stabilization of reduced Ni*<sup>n</sup>*<sup>+</sup>

552 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

( ) ( ) ( ) – 2–

for the CO oxidation.

sorbs on Co2+ cations as an O2 –

which decompose towards CO2 and oxygen.

in some extent into the elementary step (4).

**2.3. Perovskite as a catalytic precursor**

precursor for a dispersed metal catalyst.

The authors of ref. [21] expressed they did not expect to have higher alcohols on the cobaltbased perovskites, since cobalt is indeed known as a good Fischer-Tropsch catalyst (forming higher hydrocarbons) but not active for higher alcohols. However, the basic properties of the amorphous matrix La2O3 catalyzed or promoted in some extent the synthesis of these alcohols (Fischer-Tropsch metal catalysts can yield alcohols when promoted with a base). And at the reaction conditions, dispersed metallic cobalt and copper species were expect‐ ed to coexist over intermediate oxides (La2Co0.75Cu0.25O4±δ and La2O3) as evidenced by XRD for samples partially reduced in H2. Furthermore, Tien-Thao *et al.* [21] emphasized the selectivity to higher alcohols showed to be dependent on the content of intra-lattice copper in the perovskite. The selectivity to alcohols (36.5–49.5%) and productivity (a maximum at 49.6 mg/(gcat.h) for LaCo0.7Cu0.3O3) indicated that by increasing copper content, the selectiv‐ ity to alcohol (C2 <sup>+</sup> OH) also increased simultaneously with a decrease in the selectivity to methane and C2 + hydrocarbon. Those authors stated that a uniform distribution of the metallic cobalt–copper on the catalyst surface, as well as a high metal dispersion, seem to be the key for the synthesis of higher alcohols, but they did not prove that copper increased the cobalt dispersion. However, this kind of evidence was discussed in ref. [22] in the reduction of cobalt-based perovskites substituted with copper.

LaCoO3 and LaCo0.8Cu0.2O3 perovskite oxides with low surface area (< 10 m2 /g) were obtained by the polymerizable complex route, which is based on the polyesterification between citric acid and ethylene glycol in solution containing the soluble precursor anions. Then, the authors of ref. [22] confirmed the partial substitution of copper due to the increase in cell parameters caused by the larger ionic radius of the hexacoordinated Cu2+ and also due to the absence of segregated copper phases. By applying these perovskites in the partial oxidation of methane CH4/O2/He = 2/1/37 and 5/1/64, they verified a dynamic structural transformation in which the perovskite collapsed towards lanthanum-based matrix and metallic cobalt and copper species. The perovskites showed comparable performance up to 650°C, but remarkable differences in reduction of cobalt-based perovskites substituted with copper.

between 600 and 900°C in which LaCoO3 favored CH4 dissociation leaving carbon on the surface, while the copper-substituted perovskite was more efficient to produce syngas and suppress carbon deposition (thermogravimetry showed up to 18.5 times less carbon on LaCo0.8Cu0.2O3 than on LaCoO3). In this study, addition of copper increased the reducibility of the perovskite, decreased the temperature to obtain syngas and inhibited carbon deposition, and authors attributed that to a higher cobalt dispersion caused by copper. And how did they prove that? cell parameters caused by the larger ionic radius of the hexacoordinated Cu2+ and also due to the absence of segregated copper phases. By applying these perovskites in the partial oxidation of methane CH4/O2/He = 2/1/37 and 5/1/64, they verified a dynamic structural transformation in which the perovskite collapsed towards lanthanum-based matrix and metallic cobalt and copper species. The perovskites showed comparable performance up to 650°C, but remarkable differences in between 600 and 900ºC in which LaCoO3 favored CH4 dissociation leaving carbon on the surface, while the copper-substituted perovskite was more efficient to produce syngas and suppress carbon deposition (thermogravimetry showed up to 18.5 times less carbon on LaCo0.8Cu0.2O3 than on LaCoO3). In this study, addition of copper increased the

alcohols (Fischer-Tropsch metal catalysts can yield alcohols when promoted with a base). And at the reaction conditions, dispersed metallic cobalt and copper species were expected to coexist over intermediate oxides (La2Co0.75Cu0.25O4±δ and La2O3) as evidenced by XRD for samples partially reduced in H2. Furthermore, Tien-Thao *et al.* [21] emphasized the selectivity to higher alcohols showed to be dependent on the content of intra-lattice copper in the perovskite. The selectivity to alcohols (36.5–49.5%) and productivity (a maximum at 49.6 mg/(gcat.h) for LaCo0.7Cu0.3O3) indicated that by increasing copper content, selectivity to alcohol (C2+OH) also increased simultaneously to a decrease in the selectivity to methane and C2+ hydrocarbon. Those authors stated that a homogeneous distribution of the metallic cobalt–copper on the catalyst surface, as well as high metal dispersion, seem to be the key for the synthesis of higher alcohols, but they did not prove that copper increased the cobalt dispersion. However, this kind of evidence was discussed in ref. [22] in the

LaCoO3 and LaCo0.8Cu0.2O3 perovskite oxides with low surface area (< 10 m2/g) were obtained by the polymerizable

soluble precursor anions. Then, the authors of ref. [22] confirmed the partial substitution of copper due to the increase in

According to their results, the presence of copper doubled cobalt dispersion and metal specific area, *S <sup>m</sup>*, (*S <sup>m</sup>* = 6.2 m2 /gCo and 12.1 m2 /gCo for LaCoO3 and LaCo0.8Cu0.2O3, respectively) since the amount of H2 able to chemisorb on the surface doubled on the sample with Cu as seen in Figure 3 during H2 chemisorption measurements. However, H2 does not chemisorb on copper at such conditions [23], and therefore this finding suggests a higher cobalt dispersion promoted by Cu due to a strong cobalt–copper interaction as proposed by Tien-Thao *et al.* [21] for similar materials. Therefore, copper may be a very important dopant for perovskites applied in reactions with hydrocarbons. Differently from catalytic systems involving transition metals (iron, nickel or cobalt), copper does not tend to catalyze Fisher-Tropsch reactions or processes involving carbonium ion chemistry due to its low activity for breaking C–O bonds or forming C–C bonds, which avoids both wax formation in CO/H2 reactions and coke formation from hydrocarbons [24]. reducibility of the perovskite, decreased the temperature to obtain syngas and inhibited carbon deposition, and authors attributed that to a higher cobalt dispersion caused by copper. And how did they prove that? According to their results, the presence of copper doubled cobalt dispersion and metal specific area, *Sm*, (*Sm* = 6.2 m2/gCo and 12.1 m2/gCo for LaCoO3 and LaCo0.8Cu0.2O3, respectively) since the amount of H2 able to chemisorb on the surface doubled on the sample with Cu as seen in Figure 4 during H2 chemisorption measurements. However, copper does not chemisorb H2 at such conditions [23], and therefore this finding suggests a higher cobalt dispersion promoted by Cu due to a strong cobalt–copper interaction as proposed by Tien-Thao *et al.* [21] for similar materials. Therefore, copper may be a very important dopant for perovskites applied in reactions with hydrocarbons. Differently from catalytic systems involving transition metals (iron, nickel or cobalt), copper does not tend to catalyze Fisher-Tropsch reactions or processes involving carbonium ion chemistry due to its low activity for breaking C–O bonds or forming C–C bonds, which avoids both wax formation in CO/H2 reactions and coke formation from hydrocarbons [24].

Figure 4. Hydrogen adsorption on (a) LaCoO3 and (b) LaCo0.8Cu0.2O3 after complete reduction of the perovskite structure. Symbols: (?,?) total H2 isotherm; (?,?) reversible H2 isotherm. In the literature, total H2 uptake is reported to be used for quantifying chemisorbed H2 over cobalt, since the process is activated at 150°C [4, 25–27]. Extrapolation of the straight-line portion of the total adsorption isotherm to zero pressure gives the chemisorbed H2 over cobalt, and full symbols represent the data used to calculate dispersion and metal specific area. Approximately double H2-uptake can be observed for copper-substituted perovskite. Adapted from [22], copyright 2012 by Elsevier B.V. **Figure 3.** Hydrogen adsorption on (a) LaCoO3 and (b) LaCo0.8Cu0.2O3 after complete reduction of the perovskite struc‐ ture. Symbols: (■,□) total H<sup>2</sup> isotherm; (●,○) reversible H<sup>2</sup> isotherm. In the literature, total H2 uptake is reported to be used for quantifying chemisorbed H2 over cobalt, since the process is activated at 150°C [4, 25–27]. Extrapolation of the straight-line portion of the total adsorption isotherm to zero pressure gives the chemisorbed H2 over cobalt, and full symbols represent the data used to calculate dispersion and metal specific area. Approximately double H2-uptake can be observed for copper-substituted perovskite. Reprinted from reference [22], copyright (2012) with permission from Elsevier B.V.

Reforming and partial oxidation of hydrocarbons aiming syngas production still find the deactivation as the main challenge to be overcome. Noble metals such as Rh, Ru, Pd, Pt and Ir are very active and more stable against coke deposition and metal sintering than Ni, but the latter is highly available and presents a lower price, which makes Ni more Reforming and partial oxidation of hydrocarbons aiming syngas production still find the deactivation as the main challenge to be overcome. Noble metals such as Rh, Ru, Pd, Pt and Ir are very active and more stable against coke deposition and metal sintering than Ni, but the latter is highly available and presents a lower price, which makes Ni more appropriate for

increase dispersion of B but also be an active phase.

appropriate for industrial applications. By considering perovskite-type oxides ABO3, nickel can occupy the B position and also be partially substituted by another metal in order to obtain a structure more stable, resistant to carbon deposition and more active (ABB'O3). As discussed previously, the treatment under reducing atmosphere may cause a collapse of the oxide structure, leading the B and B' metals to be dispersed on the surface of AOx. The dopant B' may contribute to

Adapted with permission.

cell parameters caused by the larger ionic radius of the hexacoordinated Cu2+ and also due to the absence of segregated copper phases. By applying these perovskites in the partial oxidation of methane CH4/O2/He = 2/1/37 and 5/1/64, they verified a dynamic structural transformation in which the perovskite collapsed towards lanthanum-based matrix and metallic cobalt and copper species. The perovskites showed comparable performance up to 650°C, but remarkable differences in between 600 and 900ºC in which LaCoO3 favored CH4 dissociation leaving carbon on the surface, while the copper-substituted perovskite was more efficient to produce syngas and suppress carbon deposition (thermogravimetry industrial applications. By considering perovskite-type oxides ABO3, nickel can occupy the B position and also be partially substituted by another metal in order to obtain a structure more stable, resistant to carbon deposition and more active (ABB'O3). As discussed previously, the treatment under reducing atmosphere may cause a collapse of the oxide structure, leading the B and B' metals to be dispersed on the surface of AOx. The dopant B' may contribute to increase dispersion of B, but B' can also be an active phase.

between 600 and 900°C in which LaCoO3 favored CH4 dissociation leaving carbon on the surface, while the copper-substituted perovskite was more efficient to produce syngas and suppress carbon deposition (thermogravimetry showed up to 18.5 times less carbon on LaCo0.8Cu0.2O3 than on LaCoO3). In this study, addition of copper increased the reducibility of the perovskite, decreased the temperature to obtain syngas and inhibited carbon deposition, and authors attributed that to a higher cobalt dispersion caused by copper. And how did they

reduction of cobalt-based perovskites substituted with copper.

According to their results, the presence of copper doubled cobalt dispersion and metal specific

attributed that to a higher cobalt dispersion caused by copper. And how did they prove that?

the amount of H2 able to chemisorb on the surface doubled on the sample with Cu as seen in Figure 3 during H2 chemisorption measurements. However, H2 does not chemisorb on copper at such conditions [23], and therefore this finding suggests a higher cobalt dispersion promoted by Cu due to a strong cobalt–copper interaction as proposed by Tien-Thao *et al.* [21] for similar materials. Therefore, copper may be a very important dopant for perovskites applied in reactions with hydrocarbons. Differently from catalytic systems involving transition metals (iron, nickel or cobalt), copper does not tend to catalyze Fisher-Tropsch reactions or processes involving carbonium ion chemistry due to its low activity for breaking C–O bonds or forming C–C bonds, which avoids both wax formation in CO/H2 reactions and coke formation from

both wax formation in CO/H2 reactions and coke formation from hydrocarbons [24].

**Figure 3.** Hydrogen adsorption on (a) LaCoO3 and (b) LaCo0.8Cu0.2O3 after complete reduction of the perovskite struc‐ ture. Symbols: (■,□) total H<sup>2</sup> isotherm; (●,○) reversible H<sup>2</sup> isotherm. In the literature, total H2 uptake is reported to be used for quantifying chemisorbed H2 over cobalt, since the process is activated at 150°C [4, 25–27]. Extrapolation of the straight-line portion of the total adsorption isotherm to zero pressure gives the chemisorbed H2 over cobalt, and full symbols represent the data used to calculate dispersion and metal specific area. Approximately double H2-uptake can be observed for copper-substituted perovskite. Reprinted from reference [22], copyright (2012) with permission from

Reforming and partial oxidation of hydrocarbons aiming syngas production still find the deactivation as the main challenge to be overcome. Noble metals such as Rh, Ru, Pd, Pt and Ir are very active and more stable against coke deposition and metal sintering than Ni, but the latter is highly available and presents a lower price, which makes Ni more appropriate for

/gCo for LaCoO3 and LaCo0.8Cu0.2O3, respectively) since

appropriate for industrial applications. By considering perovskite-type oxides ABO3, nickel can occupy the B position and also be partially substituted by another metal in order to obtain a structure more stable, resistant to carbon deposition and more active (ABB'O3). As discussed previously, the treatment under reducing atmosphere may cause a collapse of the oxide structure, leading the B and B' metals to be dispersed on the surface of AOx. The dopant B' may contribute to

alcohols (Fischer-Tropsch metal catalysts can yield alcohols when promoted with a base). And at the reaction conditions, dispersed metallic cobalt and copper species were expected to coexist over intermediate oxides (La2Co0.75Cu0.25O4±δ and La2O3) as evidenced by XRD for samples partially reduced in H2. Furthermore, Tien-Thao *et al.* [21] emphasized the selectivity to higher alcohols showed to be dependent on the content of intra-lattice copper in the perovskite. The selectivity to alcohols (36.5–49.5%) and productivity (a maximum at 49.6 mg/(gcat.h) for LaCo0.7Cu0.3O3) indicated that by increasing copper content, selectivity to alcohol (C2+OH) also increased simultaneously to a decrease in the selectivity to methane and C2+ hydrocarbon. Those authors stated that a homogeneous distribution of the metallic cobalt–copper on the catalyst surface, as well as high metal dispersion, seem to be the key for the synthesis of higher alcohols, but they did not prove that copper increased the cobalt dispersion. However, this kind of evidence was discussed in ref. [22] in the

/gCo and 12.1 m2

554 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

prove that?

area, *S <sup>m</sup>*, (*S <sup>m</sup>* = 6.2 m2

hydrocarbons [24].

Adapted with permission.

Elsevier B.V.

increase dispersion of B but also be an active phase.

showed up to 18.5 times less carbon on LaCo0.8Cu0.2O3 than on LaCoO3). In this study, addition of copper increased the reducibility of the perovskite, decreased the temperature to obtain syngas and inhibited carbon deposition, and authors According to their results, the presence of copper doubled cobalt dispersion and metal specific area, *Sm*, (*Sm* = 6.2 m2/gCo and 12.1 m2/gCo for LaCoO3 and LaCo0.8Cu0.2O3, respectively) since the amount of H2 able to chemisorb on the surface doubled on the sample with Cu as seen in Figure 4 during H2 chemisorption measurements. However, copper does not Ruthenium-substituted LaCoO3 perovskites were investigated as catalytic precursors in the oxidative reforming of diesel for hydrogen production [28] and authors concluded that Co substitution by Ru (i) increased the material reducibility, (ii) led to smaller particle size of La2O3 and metallic Co0 phases and (iii) increased the amount of Ru atoms on the catalyst surface, affecting directly the catalytic behavior: the greater Co0 + Ru0 exposition and higher extension of the La2O2CO3 phase derived from the perovskite structure, the higher the activity and stability of the catalysts.

chemisorb H2 at such conditions [23], and therefore this finding suggests a higher cobalt dispersion promoted by Cu due to a strong cobalt–copper interaction as proposed by Tien-Thao *et al.* [21] for similar materials. Therefore, copper may be a very important dopant for perovskites applied in reactions with hydrocarbons. Differently from catalytic systems involving transition metals (iron, nickel or cobalt), copper does not tend to catalyze Fisher-Tropsch reactions or processes involving carbonium ion chemistry due to its low activity for breaking C–O bonds or forming C–C bonds, which avoids The effect of replacing nickel by zinc in LaNi1–*x*Zn*x*O3 perovskites (0 < *x* < 1) on the methane dry reforming (CH4/CO2/He = 1/1/1, 750°C) was studied in ref. [29], showing that, for *x* ≤ 0.4, the partial substituted oxides were highly active towards syngas production. Particular‐ ly, LaNi0.8Zn0.2O3 presented the highest CH4 and CO2 conversions and also the highest resistance to carbon deposition among all the catalysts. Characterization before and after catalytic evaluation showed decomposition of the rhombohedral structure of the perov‐ skite towards metallic Ni0 and hexagonal La2O3 phases (for high Zn content, ZnO and La2NiO4 phases also emerged). Authors found out that the partial substitution of Ni by Zn in LaNiO3 increased the reduction temperature of the perovskite, suggesting a more stable structure under reaction conditions which could avoid Ni sintering. For this reason LaNi1– *<sup>x</sup>*Zn*x*O3 samples (*x* ≤ 0.4) presented activity and stability improvements in the methane dry reforming, opposite to the lower activity shown by LaNiO3 and Zn high-content perov‐ skites (the latter containing insufficient active nickel phase). Other investigators claim that LaNiO3-derived catalysts form oxycarbonate species such as La2O2CO3 during methane dry reforming, due to CO2 adsorption on La2O3, and these species electronically stabilize nickel particles inhibiting metal sintering [30].

Figure 4. Hydrogen adsorption on (a) LaCoO3 and (b) LaCo0.8Cu0.2O3 after complete reduction of the perovskite structure. Symbols: (?,?) total H2 isotherm; (?,?) reversible H2 isotherm. In the literature, total H2 uptake is reported to be used for quantifying chemisorbed H2 over cobalt, since the process is activated at 150°C [4, 25–27]. Extrapolation of the straight-line portion of the total adsorption isotherm to zero Therefore, the above discussion comprises the application of perovskites for important catalytic reactions and points out the efforts made to better comprehend the role of the active sites on perovskite-type oxides. Investigations focus on elucidating why the *substituting cations* enhance the activity and stability of this type of oxide structure or how they modify the characteristics of the supported metal catalysts obtained from the precursor perovskite under reducing treatments.

pressure gives the chemisorbed H2 over cobalt, and full symbols represent the data used to calculate dispersion and metal specific area. Approximately double H2-uptake can be observed for copper-substituted perovskite. Adapted from [22], copyright 2012 by Elsevier B.V. Reforming and partial oxidation of hydrocarbons aiming syngas production still find the deactivation as the main challenge to be overcome. Noble metals such as Rh, Ru, Pd, Pt and Ir are very active and more stable against coke deposition and metal sintering than Ni, but the latter is highly available and presents a lower price, which makes Ni more More examples are found in details in the literature: [31] studied LaSrCoO3 perovskites and concluded that Sr substitution could increase the amount of chemisorbed oxygen species over the perovskite, improving the catalytic performance in the toluene oxidation. Refs. [32] and [33] showed that by substituting La3+ by Ca2+ into LaFeO3 lattice, iron valence changed from Fe3+ to Fe4+, improving the catalytic performance in oxidation reactions. Similar substitution of La3+ by Ca2+ into LaCoO3 increased the surface oxygen vacancy density, yielding higher catalytic activities in the propane oxidation [34]. A series of B site substitutions over LaCoO3 perovskite showed that Mn2+, Fe2+, Ni2+, Cu2+ dopants could enhance CO oxidation [35]. All these interesting researches illustrate the relevance of perovskites as catalytic precursors or as active oxides for direct application in heterogeneous catalysis. Efforts on *in situ* studies are a strong tendency and will bring important insights about the physicochemical properties and dynamic transformations of the perovskites under relevant conditions in catalysis.

## **3. Supporting perovskites on high surface materials as a strategy to enhance catalytic performance**

Bulk perovskites prepared via conventional procedures exhibit rather surface area lower than 30 m2 /g [36], which strongly limits the application of these materials as catalysts. The La‐ CoO3 reported in [37] showed high CO oxidation activity, but authors observed a decrease in the catalytic activity for samples synthesized at high temperatures, which was ascribed to the abrupt loss of surface area. An alternative to overcome this drawback is to support perovskites on traditional porous solid matrices such as Al2O3 and SiO2 in order to spread the perovskite particles and increase the exposed active sites.

LaCoO3 perovskite (low surface area < 10 m2 /g) has been supported on alumina, *x*%LaCoO3/γ-Al2O3 (*x* = 10, 20, 40 wt.%) by using a physical mixture and thermal treatment [38], leading to materials with higher surface area: 146, 131 and 96 m2 /g, respectively. As the perovskite load increased on alumina (164 m2 /g), the overall surface area and mesopore volume decreased due to blocking of support pores. The 40%LaCoO3/γ-Al2O3 catalyst presented the best performance in the CO-SELOX reaction showing the lowest temperature for 50%CO conversion T50%CO = 168°C, while bulk LaCoO3 had intermediate activity with T50%CO = 240°C. The higher effective exposition of the perovskite active phase over the alumina support explains the catalytic performance. The authors estimated the metal cobalt surface from H2-chemisorption experi‐ ments as an indirect measurement of the oxide perovskite dispersion on the alumina surface (since the active catalyst in the CO-SELOX reaction is indeed the oxide perovskite and not the reduced structure, a H2-reduction pretreatment was exclusively performed only before H2 chemisorption measurements, and not before the catalytic tests). The supported 40%LaCoO3/ γ-Al2O3 perovskite had ca. twice as many metallic area than LaCoO3 (13.5 and 6.2 m2 /gCo, respectively), which suggests that the particles of oxide perovskite over the alumina-support also presented approximately twice exposed surface area than the bulk perovskite. For this reason, 40%LaCoO3/γ-Al2O3 showed higher CO conversion. Besides, the catalyst with higher load of perovskite showed the highest selectivity to CO2 (>75%) at the temperature range of 100–170°C, above that the H2 oxidation side reaction also took place decreasing CO2 selectivity for all the catalysts. By comparing the performance of 40%LaCoO3/γ-Al2O3 with noble metalbased catalysts, this supported perovskite showed higher selectivity to CO2 (i.e*.*, 75% at T50%CO = 168°C) than 1wt% Pt/Al2O3 (46%) evaluated in the CO-SELOX at similar conditions of space velocity and isoconversion [39]. The physical mixture between support and perovskite and thermal treatment as described in ref. [38], allowed a tight contact between those phases, but not a uniform spreading of the perovskite over the alumina as revealed by electron microscopy and energy-dispersive X-ray spectroscopy (EDS). In this sense, new methodolo‐ gies should be investigated in order to improve the formation of smaller perovskite particles and their uniform distribution on the support. Simple methods such as precipitation, copre‐ cipitation and wet impregnation should be considered.

perovskite showed that Mn2+, Fe2+, Ni2+, Cu2+ dopants could enhance CO oxidation [35]. All these interesting researches illustrate the relevance of perovskites as catalytic precursors or as active oxides for direct application in heterogeneous catalysis. Efforts on *in situ* studies are a strong tendency and will bring important insights about the physicochemical properties and

**3. Supporting perovskites on high surface materials as a strategy to enhance**

Bulk perovskites prepared via conventional procedures exhibit rather surface area lower than

Al2O3 (*x* = 10, 20, 40 wt.%) by using a physical mixture and thermal treatment [38], leading to

to blocking of support pores. The 40%LaCoO3/γ-Al2O3 catalyst presented the best performance in the CO-SELOX reaction showing the lowest temperature for 50%CO conversion T50%CO = 168°C, while bulk LaCoO3 had intermediate activity with T50%CO = 240°C. The higher effective exposition of the perovskite active phase over the alumina support explains the catalytic performance. The authors estimated the metal cobalt surface from H2-chemisorption experi‐ ments as an indirect measurement of the oxide perovskite dispersion on the alumina surface (since the active catalyst in the CO-SELOX reaction is indeed the oxide perovskite and not the reduced structure, a H2-reduction pretreatment was exclusively performed only before H2 chemisorption measurements, and not before the catalytic tests). The supported 40%LaCoO3/ γ-Al2O3 perovskite had ca. twice as many metallic area than LaCoO3 (13.5 and 6.2 m2

respectively), which suggests that the particles of oxide perovskite over the alumina-support also presented approximately twice exposed surface area than the bulk perovskite. For this reason, 40%LaCoO3/γ-Al2O3 showed higher CO conversion. Besides, the catalyst with higher load of perovskite showed the highest selectivity to CO2 (>75%) at the temperature range of 100–170°C, above that the H2 oxidation side reaction also took place decreasing CO2 selectivity for all the catalysts. By comparing the performance of 40%LaCoO3/γ-Al2O3 with noble metalbased catalysts, this supported perovskite showed higher selectivity to CO2 (i.e*.*, 75% at T50%CO = 168°C) than 1wt% Pt/Al2O3 (46%) evaluated in the CO-SELOX at similar conditions of space velocity and isoconversion [39]. The physical mixture between support and perovskite and thermal treatment as described in ref. [38], allowed a tight contact between those phases, but not a uniform spreading of the perovskite over the alumina as revealed by electron microscopy and energy-dispersive X-ray spectroscopy (EDS). In this sense, new methodolo‐

/g [36], which strongly limits the application of these materials as catalysts. The La‐ CoO3 reported in [37] showed high CO oxidation activity, but authors observed a decrease in the catalytic activity for samples synthesized at high temperatures, which was ascribed to the abrupt loss of surface area. An alternative to overcome this drawback is to support perovskites on traditional porous solid matrices such as Al2O3 and SiO2 in order to spread the perovskite

/g) has been supported on alumina, *x*%LaCoO3/γ-

/g), the overall surface area and mesopore volume decreased due

/g, respectively. As the perovskite load

/gCo,

dynamic transformations of the perovskites under relevant conditions in catalysis.

**catalytic performance**

increased on alumina (164 m2

particles and increase the exposed active sites. LaCoO3 perovskite (low surface area < 10 m2

materials with higher surface area: 146, 131 and 96 m2

556 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

30 m2

Once again, these findings indicate that supported perovskites may be a very good alternative for CO-SELOX reaction. Other investigations also took advantage of supporting perovskite oxides on Al2O3-La2O3 [40], Ce1–*x*Zr*x*O2 [41], ZrO2 [42] and MCM-41 molecular sieve [43] to enhance the catalytic performance by increasing the number of exposed perovskite active sites.

Also attempts to support perovskites onto a monolithic structure have been reported, onto a metallic [44–46] or ceramic one [35, 46–48]. The ceramic monoliths are usually made of synthetic cordierite, which is wash-coated with γ-Al2O3 to increase the contact area between gas phase and solid surface. Monolith, by itself, presents several advantages over conventional catalysts, such as good mass transfer, easier product separation, good thermal and mechanical properties, easy scale up and low pressure drop, making this structure particularly suitable for high gas space velocities. Compared to conventional powdered perovskites used in fixedbed reactors, monolith-supported perovskites have some unquestionable advantages: the pressure drop remains very low for high-cell density monoliths, and the desired extent of conversion is obtained with a smaller amount of perovskite.

Brackmann *et al.* [47] coated the cordierite substrate with γ-Al2O3 by repeating six times the wash-coating procedure and obtained 10 wt.% alumina well dispersed on the cordierite. The alumina had high adherence, since negligible mass loss was verified after the alumina/ cordierite pieces underwent ultrasonic bath for 30 min. Then, LaCoO3 synthesized by Pechini route was deposited on?γ-Al2O3/cordierite by successive dip coatings, in which 11 immersion cycles led to 10 wt.% mass increment on the support. This technique was efficient for depositing LaCoO3 perovskite on γ-Al2O3/cordierite by avoiding formation of other oxides and by promoting a good adherence of the perovskite phase, since only 3.5% mass loss was verified after pieces underwent ultrasonic bath for 30 min. The pH of the perovskite suspension in the presence of γ-Al2O3/cordierite was properly controlled to increase electrostatic repulsion among particles and avoid agglomeration. This procedure tends to promote a uniform distribution of the perovskite on the γ-Al2O3/cordierite. Figure 4 shows pieces of the cordierite structure coated with γ-Al2O3 and LaCoO3/γ-Al2O3 calcined at 500 and 700°C, respectively.

LaCoO3/γ-Al2O3/cordierite was active for syngas production in the partial oxidation of methane (800°C; CH4/O2 = 4/1; 1 atm; W/F = 6.67 × 10–5 gcat.min.cm–3) yielding the following byproducts distribution: 42, 54, 3.5% to H2, CO, CO2, respectively, at 36% CO conversion for 30 h. The H2/CO ratio lower than unit was ascribed to the reverse water–gas shift and dry reforming of methane reactions taking place at those conditions [47]. The high stability was discussed in terms of (i) basic properties of La2O3 (which originated from the perovskite collapse during activation in H2), (ii) stable and dispersed cobalt particles onto La2O3/Al2O3/ cordierite, (iii) dispersion of metal cobalt on the top of carbon nanotubes (which grow up at reaction conditions). The monolith structure was pointed out as a promising support to increase the exposition of the perovskite phase, showing excellent adherence properties for γ-Al2O3 and LaCoO3.

conversion is obtained with a smaller amount of perovskite.

and wet impregnation should be considered.

active sites.

temperature range of 100–170°C, above that the H2 oxidation side reaction also took place decreasing CO2 selectivity for all the catalysts. By comparing the performance of 40%LaCoO3/-Al2O3 with noble metal-based catalysts, this supported perovskite showed higher selectivity to CO2 (i.e*.*, 75% at T50%CO = 168°C) than 1wt% Pt/Al2O3 (46%) evaluated in the CO-SELOX at similar conditions of space velocity and isoconversion [39]. The physical mixture between support and perovskite and thermal treatment as described in ref. [38], allowed a tight contact between those phases, but not a uniform spreading of the perovskite over the alumina as revealed by electron microscopy and energy-dispersive X-ray spectroscopy (EDS). In this sense, new methodologies should be investigated in order to improve the formation of smaller perovskite particles and their uniform distribution on the support. Simple methods such as precipitation, coprecipitation

Once again, these findings indicate that supported perovskites may be a very good alternative for CO-SELOX reaction. Other investigations also took advantage of supporting perovskite oxides on Al2O3-La2O3 [40], Ce1–*x*Zr*x*O2 [41], ZrO2 [42] and MCM-41 molecular sieve [43] to enhance the catalytic performance by increasing the number of exposed perovskite

Also attempts to support perovskites onto a monolithic structure have been reported, onto a metallic [44–46] or ceramic one [35, 46–48]. The ceramic monoliths are usually made of synthetic cordierite, which is wash-coated with -Al2O3 to increase the contact area between gas phase and solid surface. Monolith, by itself, presents several advantages over conventional catalysts, such as good mass transfer, easier product separation, good thermal and mechanical properties, easy scale up and low pressure drop, making this structure particularly suitable for high gas space velocities. Compared to conventional powdered perovskites used in fixed-bed reactors, monolith-supported perovskites have some unquestionable advantages: the pressure drop remains very low for high-cell density monoliths, and the desired extent of

Brackmann *et al.* [47] coated the cordierite substrate with -Al2O3 by repeating six times the wash-coating procedure and obtained 10 wt.% alumina well dispersed on the cordierite. The alumina had high adherence, since negligible mass loss was verified after the alumina/cordierite pieces underwent ultrasonic bath for 30 min. Then, LaCoO3 synthesized by Pechini route was deposited on-Al2O3/cordierite by successive dip coatings, in which 11 immersion cycles led to 10 wt.% mass increment on the support. This technique was efficient for depositing LaCoO3 perovskite on -Al2O3/cordierite by avoiding formation of other oxides and by promoting a good adherence of the perovskite phase, since only 3.5% mass loss

procedure tends to promote a uniform distribution of the perovskite on the -Al2O3/cordierite. Figure 5 shows pieces of

the cordierite structure coated with -Al2O3 and LaCoO3/-Al2O3 calcined at 500 and 700°C, respectively.

Figure 5. Pieces of honeycomb-type cordierite structure approximately 1.5 cm length, 0.8 cm wide, 0.65 cm high, cell density of 400 cell/in2. (a) -Al2O3/cordierite calcined at 500°C and (b) LaCoO3/-Al2O3/cordierite calcined at 700°C. Wash-coating and dip coating procedures were efficient to load alumina and perovskite phase onto the honeycomb-type cordierite. The perovskite LaCoO3 was previously prepared by Pechini route. Adapted from [47], copyright 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. Adapted with permission. **Figure 4.** Pieces of honeycomb-type cordierite structure approximately 1.5 cm length, 0.8 cm wide, 0.65 cm high, cell density of 400 cell/in2 . (a) γ-Al2O3/cordierite calcined at 500°C and (b) LaCoO3/γ-Al2O3/cordierite calcined at 700°C. Wash-coating and dip coating procedures were efficient to load alumina and perovskite phase onto the honeycombtype cordierite. The perovskite LaCoO3 was previously prepared by Pechini route. Reprinted from reference [47], copy‐ right (2014) with permission from Hydrogen Energy Publications LLC and Elsevier Ltd.

An important progress on the synthesis of supported perovskites on monolith structure has been reported by ref. [48]. Initially, three different catalysts based on typical LaCoO3 perovskite were obtained as (i) powder LaCoO3 prepared by citrate method, (ii) LaCoO3/monolith prepared by traditional wash-coating and (iii) LaCoO3/monolith directly synthesized by a modified citrate route, all calcined at 650°C/6 h and then tested in the oxidation of CO and hydrocarbons (environmental applications) as a model reaction to validate the direct synthesis, which showed to be simple, sustainable, fast and reproducible. Results confirmed the expected formation of crystalline LaCoO3 for the powdered sample (low surface area 9.8 m2 /g) and also over the cordierite (which presented 21 m2 /g) for both methods (ii) and (iii). By supporting the perovskite on the cordierite using the traditional wash-coating, a minimum of eight cycles were mandatory to achieve a uniform coating, since only afterwards the XPS revealed the cordierite species to disappear from the surface spectra. SEM images also confirmed that eight cycles were necessary to completely cover the cordierite and form a grain-type morphology. Activity measurements indicated that more than eight cycles (in the wash-coating procedure) did not lead to further significant increase of catalytic activity. Therefore, we can learn from wash-coating method that a significant number of cycles is required as well as lots of labor and time in order to ensure reproducibility and a uniform coating (which is a requisite for high activity). LaCoO3/-Al2O3/cordierite was active for syngas production in the partial oxidation of methane (800°C; CH4/O2 = 4/1; 1 atm; W/F = 6.67 × 10–5 gcat.min.cm–3) yielding the following byproducts distribution: 42, 54, 3.5% to H2, CO, CO2, respectively, at 36% CO conversion for 30 h. The H2/CO ratio lower than unit was ascribed to the reverse water–gas shift

The direct synthesis of the perovskite on the bare monolith (no γ-Alumina supported in advance) was carried out in a single step by immersing the monolith pieces in a solution containing La3+ and Co2+ precursors, the piece was then calcined [48]. XPS measurements proved the interaction between the cordierite and the deposited perovskite, and SEM images indicated a layer of porous perovskite on the cordierite, and this layer can be modulated by using a precursor solution with appropriate concentration and by controlling the immersion time. The directly synthesized LaCoO3/monolith proved to be as effective as the best washcoated catalyst (obtained after 8 cycles) in the oxidation of CO, C3H6 and C3H8 (environmental applications) leading to similar performances. Thus, we can learn that the direct method is simpler, faster, reproducible and does not deal with organic solvents, making it very attractive for industrial application through a sustainable process.

## **4. Conclusion**

This chapter presented two strategies to overcome some limitations of perovskite-type oxides in catalysis: (1) the partial substitution of cations, which stabilizes unusual oxidation states of the metal components and creates anionic or cationic vacancies within the perovskite lattice. This is interesting for oxidation reactions, since vacancies can increase oxygen mobility from lattice to surface, increasing the catalytic activity of oxidation or improving the selectivity to products of interest. If the perovskite is a precursor, the partial substitution of cations can increase the reducibility and metal dispersion of the catalyst. The strategy (2) comprises to support perovskites on porous materials like a monolith or a conventional high surface area material to increase the number of exposed perovskite active sites.

## **Nomenclature**

An important progress on the synthesis of supported perovskites on monolith structure has been reported by ref. [48]. Initially, three different catalysts based on typical LaCoO3 perovskite were obtained as (i) powder LaCoO3 prepared by citrate method, (ii) LaCoO3/monolith prepared by traditional wash-coating and (iii) LaCoO3/monolith directly synthesized by a modified citrate route, all calcined at 650°C/6 h and then tested in the oxidation of CO and hydrocarbons (environmental applications) as a model reaction to validate the direct synthesis, which showed to be simple, sustainable, fast and reproducible. Results confirmed the expected

**Figure 4.** Pieces of honeycomb-type cordierite structure approximately 1.5 cm length, 0.8 cm wide, 0.65 cm high, cell

Wash-coating and dip coating procedures were efficient to load alumina and perovskite phase onto the honeycombtype cordierite. The perovskite LaCoO3 was previously prepared by Pechini route. Reprinted from reference [47], copy‐

Figure 5. Pieces of honeycomb-type cordierite structure approximately 1.5 cm length, 0.8 cm wide, 0.65 cm high, cell density of 400 cell/in2. (a) -Al2O3/cordierite calcined at 500°C and (b) LaCoO3/-Al2O3/cordierite calcined at 700°C. Wash-coating and dip coating procedures were efficient to load alumina and perovskite phase onto the honeycomb-type cordierite. The perovskite LaCoO3 was previously prepared by Pechini route. Adapted from [47], copyright 2014, Hydrogen Energy Publications, LLC. Published by Elsevier

. (a) γ-Al2O3/cordierite calcined at 500°C and (b) LaCoO3/γ-Al2O3/cordierite calcined at 700°C.

LaCoO3/-Al2O3/cordierite was active for syngas production in the partial oxidation of methane (800°C; CH4/O2 = 4/1; 1 atm; W/F = 6.67 × 10–5 gcat.min.cm–3) yielding the following byproducts distribution: 42, 54, 3.5% to H2, CO, CO2, respectively, at 36% CO conversion for 30 h. The H2/CO ratio lower than unit was ascribed to the reverse water–gas shift

perovskite on the cordierite using the traditional wash-coating, a minimum of eight cycles were mandatory to achieve a uniform coating, since only afterwards the XPS revealed the cordierite species to disappear from the surface spectra. SEM images also confirmed that eight cycles were necessary to completely cover the cordierite and form a grain-type morphology. Activity measurements indicated that more than eight cycles (in the wash-coating procedure) did not lead to further significant increase of catalytic activity. Therefore, we can learn from wash-coating method that a significant number of cycles is required as well as lots of labor and time in order to ensure reproducibility and a uniform coating (which is a requisite for high

The direct synthesis of the perovskite on the bare monolith (no γ-Alumina supported in advance) was carried out in a single step by immersing the monolith pieces in a solution containing La3+ and Co2+ precursors, the piece was then calcined [48]. XPS measurements

/g) and also

/g) for both methods (ii) and (iii). By supporting the

temperature range of 100–170°C, above that the H2 oxidation side reaction also took place decreasing CO2 selectivity for all the catalysts. By comparing the performance of 40%LaCoO3/-Al2O3 with noble metal-based catalysts, this supported perovskite showed higher selectivity to CO2 (i.e*.*, 75% at T50%CO = 168°C) than 1wt% Pt/Al2O3 (46%) evaluated in the CO-SELOX at similar conditions of space velocity and isoconversion [39]. The physical mixture between support and perovskite and thermal treatment as described in ref. [38], allowed a tight contact between those phases, but not a uniform spreading of the perovskite over the alumina as revealed by electron microscopy and energy-dispersive X-ray spectroscopy (EDS). In this sense, new methodologies should be investigated in order to improve the formation of smaller perovskite particles and their uniform distribution on the support. Simple methods such as precipitation, coprecipitation

Once again, these findings indicate that supported perovskites may be a very good alternative for CO-SELOX reaction. Other investigations also took advantage of supporting perovskite oxides on Al2O3-La2O3 [40], Ce1–*x*Zr*x*O2 [41], ZrO2 [42] and MCM-41 molecular sieve [43] to enhance the catalytic performance by increasing the number of exposed perovskite

Also attempts to support perovskites onto a monolithic structure have been reported, onto a metallic [44–46] or ceramic one [35, 46–48]. The ceramic monoliths are usually made of synthetic cordierite, which is wash-coated with -Al2O3 to increase the contact area between gas phase and solid surface. Monolith, by itself, presents several advantages over conventional catalysts, such as good mass transfer, easier product separation, good thermal and mechanical properties, easy scale up and low pressure drop, making this structure particularly suitable for high gas space velocities. Compared to conventional powdered perovskites used in fixed-bed reactors, monolith-supported perovskites have some unquestionable advantages: the pressure drop remains very low for high-cell density monoliths, and the desired extent of

Brackmann *et al.* [47] coated the cordierite substrate with -Al2O3 by repeating six times the wash-coating procedure and obtained 10 wt.% alumina well dispersed on the cordierite. The alumina had high adherence, since negligible mass loss was verified after the alumina/cordierite pieces underwent ultrasonic bath for 30 min. Then, LaCoO3 synthesized by Pechini route was deposited on-Al2O3/cordierite by successive dip coatings, in which 11 immersion cycles led to 10 wt.% mass increment on the support. This technique was efficient for depositing LaCoO3 perovskite on -Al2O3/cordierite by avoiding formation of other oxides and by promoting a good adherence of the perovskite phase, since only 3.5% mass loss was verified after pieces underwent ultrasonic bath for 30 min. The pH of the perovskite suspension in the presence of - Al2O3/cordierite was properly controlled to increase electrostatic repulsion among particles and avoid agglomeration. This procedure tends to promote a uniform distribution of the perovskite on the -Al2O3/cordierite. Figure 5 shows pieces of

the cordierite structure coated with -Al2O3 and LaCoO3/-Al2O3 calcined at 500 and 700°C, respectively.

formation of crystalline LaCoO3 for the powdered sample (low surface area 9.8 m2

right (2014) with permission from Hydrogen Energy Publications LLC and Elsevier Ltd.

over the cordierite (which presented 21 m2

Ltd. Adapted with permission.

density of 400 cell/in2

and wet impregnation should be considered.

conversion is obtained with a smaller amount of perovskite.

558 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

active sites.

activity).

CO-SELOX: carbon monoxide selective oxidation reaction EDS: energy-dispersive X-ray spectroscopy EELS: electron energy loss spectroscopy FTIR: Fourier transform infrared spectroscopy H2-TPR: temperature-programmed reduction (by H2) O2-TPD: temperature-programmed desorption of oxygen SEM: scanning electron microscopy TEM: transmission electron microscopy VOCs: volatile organic compounds XPS: X-ray photoelectron spectroscopy XRD: X-ray diffraction

## **Author details**

Fabio Souza Toniolo\* and Martin Schmal\*

\*Address all correspondence to: toniolo@peq.coppe.ufrj.br; schmal@peq.coppe.ufrj.br

Chemical Engineering Program – COPPE, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, Brazil

## **References**


[10] Hueso JL, Caballero A, Ocaña M, González-Elipe AR. Reactivity of lanthanum substi‐ tuted cobaltites toward carbon particles. J Catal 2008;257:334–44. DOI:10.1016/j.jcat. 2008.05.012

**Author details**

Janeiro, RJ, Brazil

**References**

Fabio Souza Toniolo\*

and Martin Schmal\*

560 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

\*Address all correspondence to: toniolo@peq.coppe.ufrj.br; schmal@peq.coppe.ufrj.br

Chemical Engineering Program – COPPE, Federal University of Rio de Janeiro, Rio de

Catal B 1998;16:119–26. DOI:10.1016/S0926-3373(97)00065-9

2005;107–108:106–13. DOI:10.1016/j.cattod.2005.07.073

2012;197:236–42. DOI:10.1016/j.cattod.2012.08.034

gy 2015;19:60–8. DOI:10.1016/j.partic.2014.07.001

10.1016/j.apcatb.2015.02.006

10.1016/j.cej.2013.05.070

[4] Echchahed B, Kaliaguine S, Alamdari H. Well Dispersed Co0

Appl Catal A 1999;180:163–73. DOI:10.1016/S0926-860X(98)00343-3

[1] Ferri D, Forni L. Methane combustion on some perovskite-like mixed oxides. Appl

[2] Provendier H, Petit C, Estournès C, Libs S, Kiennemann A. Stabilisation of active nickel catalysts in partial oxidation of methane to synthesis gas by iron addition.

[3] Goldwasser MR, Rivas ME, Lugo ML, Pietri E, Pérez-Zurita J, Cubeiro ML, Griboval-Constant A, Leclercq G. Combined methane reforming in presence of CO2 and O2 over LaFe1-xCoxO3 mixed-oxide perovskites as catalysts precursors. Catal Today

Perovskite. Int J Chem Reactor Eng 2006;4:A29. DOI:10.2202/1542-6580.1332

[5] Zhang J, Tan D, Meng Q, Weng X, Wu Z. Structural modification of LaCoO3 perov‐ skite for oxidation reactions: the synergistic effect of Ca2+ and Mg2+ co-substitution on phase formation and catalytic performance. Appl Catal B 2015;172–173:18–26. DOI:

[6] Li X, Dai H, Deng J, Liu Y, et al. Au/3DOM LaCoO3: high-performance catalysts for the oxidation of carbon monoxide and toluene. Chem Eng J 2013;228:965–75. DOI:

[7] Łojewska J, Kołodziej A, Dynarowicz-Łątka P,Wesełucha-Birczyńska A. Engineering and chemical aspects of the preparation of microstructured cobalt catalyst for VOC

[8] Gallucci K, Villa P, Groppi G, Usberti N, Marra G. Catalytic combustion of methane on BaZr(1-x)MexO3 perovskites synthesised by a modified citrate method. Catal Today

[9] Liu G, Li J, Yang K, Tang W, et al. Effects of cerium incorporation on the catalytic oxidation of benzene over flame-made perovskite La1-xCexMnO3 catalysts. Particuolo‐

combustion. Catal Today 2005;101:81–91. DOI:10.1016/j.cattod.2005.01.005

by Reduction of LaCoO3


[35] Yan X, Huang Q, Li B, Xu X, et al. Catalytic performance of LaCo0.5M0.5O3 (M = Mn, Cr, Fe, Ni, Cu) perovskite-type oxides and LaCo0.5Mn0.5O3 supported on cordierite for CO oxidation. J Indust Eng Chem 2013;19:561–65. DOI:10.1016/j.jiec.2012.09.026

[23] Figueiredo JL, Ribeiro FR. Catálise Heterogênea. 1st ed. Fundação Calouste Gulben‐

[24] Twigg MV, Spencer MS. Deactivation of copper metal catalysts for methanol decom‐ position, methanol steam reforming and methanol synthesis. Top Catal 2003;22:191–

[25] Reuel RC, Bartholomew CH. The stoichiometries of H2 and CO adsorptions on co‐ balt: effects of support and preparation. J Catal 1984;85:63–77. DOI:

[26] Xiong J, Borg Ø, Blekkan E A, Holmen A. Hydrogen chemisorption on rhenium-pro‐ moted γ-alumina supported cobalt catalysts. Catal Commun 2008;9:2327–30. DOI:

[27] Silva RRCM, Schmal M, Frety R, Dalmon JA. Effect of the support on the fischer– tropsch synthesis with Co/Nb2O5 catalysts. J Chem Soc, Faraday Transac

[28] Mota N, Navarro RM, Alvarez-Galvan MC, Al-Zahrani SM, Fierro JLG. Hydrogen production by reforming of diesel fuel over catalysts derived from LaCo1-xRuxO3 per‐ ovskites: effect of the partial substitution of Co by Ru (x = 0.01-0.1). J Power Sources

[29] Moradi GR, Rahmanzadeh M, Khosravian F. The effects of partial substitution of Ni by Zn in LaNiO3 perovskite catalyst for methane dry reforming. J CO2 Utilization

[30] Su YJ, Pan KL, Chang MB. Modifying perovskite-type oxide catalyst LaNiO3 with Ce for carbon dioxide reforming of methane. Int J Hydrogen Energy 2014;39:4917–25.

[31] Pereñíguez R, Hueso JL, Gaillard F, Holgado JP, Caballero A. Study of oxygen reac‐ tivity in La1-xSrxCoO3?δ perovskites for total oxidation of toluene. Catal Lett

[32] Barbero BP, Gamboa JA, Cadús LE. Synthesis and characterisation of La1-xCaxFeO3 perovskite-type oxide catalysts for total oxidation of volatile organic compounds.

[33] Pecchi G, Jiliberto MG, Delgado EJ, Cadús LE, Fierro JLG. Effect of B-site cation on the catalytic activity of La1-xCaxBO3 (B = Fe, Ni) perovskite-type oxides for toluene combustion. J Chem Technol Biotechnol 2011;86:1067–73. DOI:10.1002/jctb.2611

[34] Merino NA, Barbero BP, Grange P, Cadús L E. La1-xCaxCoO3 perovskite-type oxides: preparation, characterisation, stability, and catalytic potentiality for the total oxida‐

tion of propane. J Catal 2005;231:232–44. DOI:10.1016/j.jcat.2005.01.003

kian. Lisbon/Portugal; 1987. 347 p. (pp. 166–168).

562 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

203. DOI:10.1023/A:1023567718303

10.1016/0021-9517(84)90110-6

10.1016/j.catcom.2008.05.017

1993;89:3975–80. DOI: 10.1039/FT9938903975

2014;6:7–11. DOI:10.1016/j.jcou.2014.02.001

2012;142:408–16. DOI:10.1007/s10562-012-0799-z

Appl Catal B 2006;65:21–30. DOI:10.1016/j.apcatb.2005.11.018

DOI:10.1016/j.ijhydene.2014.01.077

2011;196:9087–95. DOI:10.1016/j.jpowsour.2010.11.143


## **Copper-based Perovskite Design and Its Performance in CO2 Hydrogenation to Methanol**

Feng Li, Haijuan Zhan, Ning Zhao and Fukui Xiao

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61520

## **Abstract**

[47] Brackmann R, Perez CAC, Schmal M. LaCoO3 perovskite on ceramic monoliths: pre and post reaction analyzes of the partial oxidation of methane. Int J Hydrogen Ener‐

[48] Guiotto M, Pacella M, Perin G, Iovino A, et al. Washcoating vs. direct synthesis of LaCoO3 on monoliths for environmental applications. Appl Catal A 2015;499:146–57.

gy 2014;39:13991–4007. DOI:10.1016/j.ijhydene.2014.07.027

DOI:10.1016/j.apcata.2015.04.013

564 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

Three series of perovskite-type catalysts, i.e., La–M–Mn–Cu–O (M = Mg, Y, Zn, Ce), La–M–Cu–Zn–O (M = Ce, Mg, Zr, Y), and La–Mn–Zn–Cu–O, were designed and ap‐ plied in CO2 hydrogenation to methanol. The materials were characterized by XRD, N2-adsorption, N2O-adsorption, ICP-OES, XPS, and TPD techniques. Perovskite struc‐ tures were observed and the ''metal on oxide'' could be realized via reduction. Upon the introduction of the fourth elements, more structure defects, smaller particles, high‐ er Cu dispersion, larger amount of hydrogen desorption at low temperature, and more amount of basic sites were obtained. The selectivity for methanol and the TOF values were higher for the catalysts derived from perovskite-type precursors. The catalytic performance was related to Cuα+ and/or Cu0 species, low-temperature H2 adsorption on the unit, and the weak basic sites.

**Keywords:** Perovskite, Copper, CO2, Methanol, Hydrogenation

## **1. Introduction**

Perovskite-type oxides have received significant attention because of their important electric, magnetic, ferromagnetic, pyroelectric, and piezoelectric properties [1,2]. Recently, much attention has been paid to perovskite-type oxides as catalysts due to their high activity and thermal stability. For a typical ABO3 perovskite, A-site is a larger rare earth and/or alkaline earth cation and B-site is a smaller transition metal cation. In such structure, the A-site keeps the structure and the B-site provides the catalytic activity site. B-site cations could be reduced to well-dispersed metallic species supported on the A-site cations oxide, which leads to ideal catalyst precursors for many reactions that involve metal as active sites [3,4]. Besides, perov‐ skite-type A2BO4 mixed oxides with the K2NiF4 structure, consisting of alternating layers of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABO3 perovskite and AO rock salt, have also been studied [5], which exhibit variable oxygen stoichiometry. The replacement of A-site and/or B-site cations by other metal cations often results in the formation of crystal microstrain and adjustable activity [6].

CO2 is the main greenhouse gas, and various strategies have been implemented to reduce its concentration [7-10]. An important CO2 utilization is the hydrogenation to methanol, which is considered as the most valuable product since it can be used as solvent, alternative fuel, and raw material, and it can be converted to olefins, aromatics, or gasoline derived from traditional petrochemical processes [11,12].

The synthesis of methanol over Cu/ZnO-type catalysts has been studied for many years. However, several important problems still remain open, such as the working oxidation of copper and the reaction mechanism [13-15]. In addition, the low activity and stability of catalysts, which are partly attributed to Cu sintering accelerated by the presence of the byproduct, water vapor, create major barriers for practical application [16]. It is found that the catalysts with higher Cu dispersion, easier reduction property, and better adsorption proper‐ ties for relative gases could achieve better catalytic performance for methanol synthesis [16].

Few work on the application of Cu-based perovskite-type oxides for CO2 hydrogenation has been investigated. In the present work, three series of perovskite-type-based catalysts were prepared and tested for CO2 hydrogenation to methanol, and the relationship between physical–chemical property and catalytic performance was discussed.

## **2. Catalyst preparation**

The perovskite-type oxides were prepared by sol-gel method using citric acid as complexing agent. The precursor salts were La(NO3)3. nH2O; Mn(NO3)2, 50% solution; Cu(NO3)2. 3H2O; Mg(NO3)2. 6H2O; Y(NO3)3. 6H2O; Zn(NO3)2. 6H2O; Ce(NO3)3. 6H2O; ZrO(NO3)2.2H2O. Ade‐ quate amounts of the precursor salts along with citric acid were dissolved in deionized water at a molar ratio of 2:1 (metal cations: citric acid). The solution was heated to 353 K to remove the water, and then the temperature was increased to 423 K until ignition. The resulting powder was finally calcined under air at 673 K for 2 h and then at 1073 K for 4 h. The three series of catalysts were: (1) For doped La–M-Mn–Cu–O based (M= Mg, Y, Zn, Ce) perovskite materials, the ratio for La, M, Mn, Cu is 0.8: 0.2: 0.5: 0.5. The La–Mn–Cu–O catalyst and Mg, Y, Zn, Ce doping catalysts were then denoted as P, Mg–P, Y–P, Zn–P, and Ce–P, respectively. (2) A series of La–M-Cu–Zn–O (M= Ce, Mg, Zr, Y) based perovskite-type catalysts, i.e., LaCu0.7Zn0.3Ox, La0.8Ce0.2Cu0.7Zn0.3Ox, La0.8Mg0.2Cu0.7Zn0.3Ox, La0.8Zr0.2Cu0.7Zn0.3Ox and La0.8Y0.2Cu0.7Zn0.3Ox samples were prepared, of which the subscripts were the nominal composition. The catalysts were then denoted as LCZ-173, LCCZ-8273, LMCZ-8273, LZCZ-8273 and LYCZ-8273, respec‐ tively. (3) The LaZn0.4Cu0.6Oy, LaMn0.1Zn0.3Cu0.6Oy, LaMn0.2Zn0.2Cu0.6Oy, LaMn0.3Zn0.1Cu0.6Oy, and LaMn0.4 Cu0.6Oy samples were prepared, of which the subscripts were the nominal composition. The catalysts were denoted as LZC-046, LMZC-136, LMZC-226, LMZC-316, and LMC-406, respectively.

## **3. Results and discussion**

the nominal values.

SBET (m2 g -1)

> 6.5 5.4 11.3 4.1 7.2

a

**3.1.2 The XPS investigations**

Dispersio n (%)<sup>a</sup>

> - 0.9 3.8 0.7 -

b Subscripts came from ICP results.

reproduced by permission of The Royal Society of Chemistry).

SCu (m2 g -1)

> - 1.2 4.6 0.9 -

Calculated from N2O dissociative adsorption.

Sample s

> P Mg-P Y-P Zn-P Ce-P

ABO3 perovskite and AO rock salt, have also been studied [5], which exhibit variable oxygen stoichiometry. The replacement of A-site and/or B-site cations by other metal cations often

CO2 is the main greenhouse gas, and various strategies have been implemented to reduce its concentration [7-10]. An important CO2 utilization is the hydrogenation to methanol, which is considered as the most valuable product since it can be used as solvent, alternative fuel, and raw material, and it can be converted to olefins, aromatics, or gasoline derived from traditional

The synthesis of methanol over Cu/ZnO-type catalysts has been studied for many years. However, several important problems still remain open, such as the working oxidation of copper and the reaction mechanism [13-15]. In addition, the low activity and stability of catalysts, which are partly attributed to Cu sintering accelerated by the presence of the byproduct, water vapor, create major barriers for practical application [16]. It is found that the catalysts with higher Cu dispersion, easier reduction property, and better adsorption proper‐ ties for relative gases could achieve better catalytic performance for methanol synthesis [16].

Few work on the application of Cu-based perovskite-type oxides for CO2 hydrogenation has been investigated. In the present work, three series of perovskite-type-based catalysts were prepared and tested for CO2 hydrogenation to methanol, and the relationship between

The perovskite-type oxides were prepared by sol-gel method using citric acid as complexing agent. The precursor salts were La(NO3)3. nH2O; Mn(NO3)2, 50% solution; Cu(NO3)2. 3H2O; Mg(NO3)2. 6H2O; Y(NO3)3. 6H2O; Zn(NO3)2. 6H2O; Ce(NO3)3. 6H2O; ZrO(NO3)2.2H2O. Ade‐ quate amounts of the precursor salts along with citric acid were dissolved in deionized water at a molar ratio of 2:1 (metal cations: citric acid). The solution was heated to 353 K to remove the water, and then the temperature was increased to 423 K until ignition. The resulting powder was finally calcined under air at 673 K for 2 h and then at 1073 K for 4 h. The three series of catalysts were: (1) For doped La–M-Mn–Cu–O based (M= Mg, Y, Zn, Ce) perovskite materials, the ratio for La, M, Mn, Cu is 0.8: 0.2: 0.5: 0.5. The La–Mn–Cu–O catalyst and Mg, Y, Zn, Ce doping catalysts were then denoted as P, Mg–P, Y–P, Zn–P, and Ce–P, respectively. (2) A series of La–M-Cu–Zn–O (M= Ce, Mg, Zr, Y) based perovskite-type catalysts, i.e., LaCu0.7Zn0.3Ox, La0.8Ce0.2Cu0.7Zn0.3Ox, La0.8Mg0.2Cu0.7Zn0.3Ox, La0.8Zr0.2Cu0.7Zn0.3Ox and La0.8Y0.2Cu0.7Zn0.3Ox samples were prepared, of which the subscripts were the nominal composition. The catalysts were then denoted as LCZ-173, LCCZ-8273, LMCZ-8273, LZCZ-8273 and LYCZ-8273, respec‐ tively. (3) The LaZn0.4Cu0.6Oy, LaMn0.1Zn0.3Cu0.6Oy, LaMn0.2Zn0.2Cu0.6Oy, LaMn0.3Zn0.1Cu0.6Oy, and LaMn0.4 Cu0.6Oy samples were prepared, of which the subscripts were the nominal composition. The catalysts were denoted as LZC-046, LMZC-136, LMZC-226, LMZC-316, and

results in the formation of crystal microstrain and adjustable activity [6].

566 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

physical–chemical property and catalytic performance was discussed.

petrochemical processes [11,12].

**2. Catalyst preparation**

LMC-406, respectively.

## **3.1. Performance of the La–M-Mn–Cu–O (M = Mg, Y, Zn, Ce) based perovskite precursors**

## *3.1.1. Textural and structural properties*

As shown in Figure 1a [17], for the La–M-Mn–Cu–O (M = Mg, Y, Zn, Ce) based perovskite precursors, the LaMnO3 (JCPDS # 75-0440) are the main phase. The diffraction peak at about 2θ=32.5° shift towards higher values when the fourth elements were doped. Small peaks at 2θ=35.6°and 38.9°assigned to CuO (JCPDS # 89-5899) appear in the doped samples but not in the P sample. In all diffraction patterns, no phase that ascribes to Mg, Y, or Zn is observed, while a new phase ascribed to CeO2 is found for the sample of Ce–P, which demonstrates that it is difficult for all the Ce to enter the perovskite lattice, which agrees with the conclusion by Weng et al. [18].

For all reduced samples (Figure 1b), LaMnO3 phase is still the main phase, which reveals that the reduction process does not destroy the perovskite structure. Meanwhile, the CuO phase disappears and the Cu phase emerges.

Figure 3.1.1 XRD patterns of the calcined (a) and reduced (b) perovskite-type catalysts: (□) LaMnO3; (●) CuO; (♦) CeO2; (\*) Cu (taken from ref.17, reproduced by permission of The Royal **Figure 1.** XRD patterns of the calcined (a) and reduced (b) perovskite-type catalysts: (□) LaMnO3; (●) CuO; (♦) CeO2; (\*) Cu (taken from ref.17, reproduced by permission of The Royal Society of Chemistry).

Society of Chemistry). The physicochemical properties of the calcined perovskite-type catalysts are summarized in Table 3.1.1 [17]. Low specific surface area for perovskite-type oxides is common. For this series samples, the largest one is only 11.3 m2 g -1 for Y–P and the lowest one is only 4.1 m2 g -1 for the Zn–P. The exposed Cu surface area and the Cu dispersion measured by N2O adsorption technique are also low, which even cannot be measured for both P and Ce–P. Y–P possesses the largest Cu The physicochemical properties of the calcined perovskite-type catalysts are summarized in Table 1 [17]. Low specific surface area for perovskite-type oxides is common. For this series samples, the largest one is only 11.3 m2 g-1 for Y–P and the lowest one is only 4.1 m2 g-1 for the Zn–P. The exposed Cu surface area and the Cu dispersion measured by N2O adsorption technique are also low, which even cannot be measured for both P and Ce–P. Y–P possesses the largest Cu surface area and the Cu dispersion by comparison. The lower copper surface area may not be favorable for the conversion of CO2 to methanol [19]. The ICP results show that the experimental lanthanum amount is lower than the theoretical value, and other element amounts are similar to the nominal values.

surface area and the Cu dispersion by comparison. The lower copper surface area may not be

favorable for the conversion of CO2 to methanol [19]. The ICP results show that the experimental

lanthanum amount is lower than the theoretical value, and other elements amounts are similar to

Table 3.1.1 The physiochemical properties of the perovskite-type catalysts (taken from ref.17,

The XPS results presented in Table 3.1.2 [17] show that lanthanum ions of the reduced

perovskite-type catalysts are present in the trivalent form because the La3d5/2 peak is close to the

value of pure lanthana at 834.4 eV [20]. However, the BE of La3d5/2 of P sample is higher than the

other samples, which implies the increasing of the electron cloud density around La ions for the

doped samples. It may due to the fourth elements affect the transfer of the electrons of La to O,

since O has the highest electronegativity value among all elements [21]. For O1s, the binding energy at around 528.9–-529.1 eV is assigned to the lattice oxygen (O2-) [21,22] and the binding

Elemental composition (ICP-OES)<sup>b</sup>

**Comment [Nomita6]:** AQ: please check latter part of sentence "The exposed Cu…" for

**Comment [Nomita7]:** AQ: please check sentence "It may due to…" for clarity.

clarity.

La0.84Mn0.51Cu0.50 La0.67Mg0.22Mn0.49Cu0.50 La0.67Y0.23Mn0.47Cu0.50 La0.67 Zn0.18Mn0.50Cu0.50 La0.68 Ce0.19Mn0.49Cu0.50


a Calculated from N2O dissociative adsorption.

b Subscripts came from ICP results.

**Table 1.** The physiochemical properties of the perovskite-type catalysts (taken from ref.17, reproduced by permission of The Royal Society of Chemistry).

## *3.1.2. The XPS investigations*

The XPS results presented in Table 2 [17] show that lanthanum ions of the reduced perovskitetype catalysts are present in the trivalent form because the La3d5/2 peak is close to the value of pure lanthana at 834.4 eV [20]. However, the BE of La3d5/2 of P sample is higher than the other samples, which implies the increasing of the electron cloud density around La ions for the doped samples. It may due to the fourth elements affect the transfer of the electrons of La to O, since O has the highest electronegativity value among all elements [21]. For O1s, the binding energy at around 528.9–529.1 eV is assigned to the lattice oxygen (O2-) [21,22] and the binding energy at around 530.8–533.0 eV is ascribed to the adsorbed oxygen species (Oad) in the surface, which contains hydroxyl (OH- ), carbonate species (CO3 2-), and molecular water. The binding energy decreases after the fourth element except Ce adding, which indicates that there are more electrons around oxygen. It is likely that the fourth components transfer the electronic to the oxygen. The presence of surface adsorbed oxygen species suggests the formation of oxygen vacancies in the defected oxides [23], which is favorable for the activation of the catalyst. The Oad/O2- ratio is increased for the doped samples, which implies the improvement of catalysis activity. The binding energy values of Mn2p3/2 for the perovskite-type catalysts are located at 641.3 eV–642.2 eV. The peak positons of level of MnO, Mn2O3, and MnO2 are 640.6, 641.9, and 642.2 eV, respectively. The values are very similar, and the mean oxidation state of Mn ions at the surface layers is extremely difficult to detect by XPS, as reported in other studies [24,25]. However, the previous reports suggested that the BE difference between Mn2p3/2 and O1s emissions increases with about 0.6–0.7 eV for the change of the oxidation state between Mn3+ and Mn4+. As shown in Table 2, the BE difference is in the range of 112.3–113.0 eV, i.e., increasing with 0.7 eV, which means a change of the Mn4+/Mn3+ ratio for the perovskite-type catalysts [26,27].

Since the binding energy of the Cu2p3/2 band in the metal (932.6 eV) and in Cu+ (932.4 eV) is almost same, they can be distinguished by different kinetic energy of the Auger Cu LMM line position in Cu0 (918.6 eV), Cu+ (916.7 eV), or in Cu2+ (917.9 eV) [19,28]. The Auger electron spectroscopies of Cu LMM of reduced samples are shown in Figure 2 [17]. The profiles are convoluted into two peaks. It can be seen that the majority of the copper species exist as Cu+ for all samples, which is in accordance with the report of Jia et al. [29]. The weak Cu0 peak could be the explanation for the immeasurable of exposed Cu0 for P and Ce–P (Table 1).

**Samples**

P Mg-P Y-P Zn-P Ce-P

a

b

**SBET (m2 g-1)**

568 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

6.5 5.4 11.3 4.1 7.2

Calculated from N2O dissociative adsorption.

Subscripts came from ICP results.

of The Royal Society of Chemistry).

*3.1.2. The XPS investigations*

which contains hydroxyl (OH-

catalysts [26,27].

position in Cu0

(918.6 eV), Cu+

**Dispersion (%)a**

> - 0.9 3.8 0.7 -

**SCu (m2 g-1)**

> - 1.2 4.6 0.9 -

**Table 1.** The physiochemical properties of the perovskite-type catalysts (taken from ref.17, reproduced by permission

The XPS results presented in Table 2 [17] show that lanthanum ions of the reduced perovskitetype catalysts are present in the trivalent form because the La3d5/2 peak is close to the value of pure lanthana at 834.4 eV [20]. However, the BE of La3d5/2 of P sample is higher than the other samples, which implies the increasing of the electron cloud density around La ions for the doped samples. It may due to the fourth elements affect the transfer of the electrons of La to O, since O has the highest electronegativity value among all elements [21]. For O1s, the binding energy at around 528.9–529.1 eV is assigned to the lattice oxygen (O2-) [21,22] and the binding energy at around 530.8–533.0 eV is ascribed to the adsorbed oxygen species (Oad) in the surface,

), carbonate species (CO3

energy decreases after the fourth element except Ce adding, which indicates that there are more electrons around oxygen. It is likely that the fourth components transfer the electronic to the oxygen. The presence of surface adsorbed oxygen species suggests the formation of oxygen vacancies in the defected oxides [23], which is favorable for the activation of the catalyst. The Oad/O2- ratio is increased for the doped samples, which implies the improvement of catalysis activity. The binding energy values of Mn2p3/2 for the perovskite-type catalysts are located at 641.3 eV–642.2 eV. The peak positons of level of MnO, Mn2O3, and MnO2 are 640.6, 641.9, and 642.2 eV, respectively. The values are very similar, and the mean oxidation state of Mn ions at the surface layers is extremely difficult to detect by XPS, as reported in other studies [24,25]. However, the previous reports suggested that the BE difference between Mn2p3/2 and O1s emissions increases with about 0.6–0.7 eV for the change of the oxidation state between Mn3+ and Mn4+. As shown in Table 2, the BE difference is in the range of 112.3–113.0 eV, i.e., increasing with 0.7 eV, which means a change of the Mn4+/Mn3+ ratio for the perovskite-type

Since the binding energy of the Cu2p3/2 band in the metal (932.6 eV) and in Cu+ (932.4 eV) is almost same, they can be distinguished by different kinetic energy of the Auger Cu LMM line

spectroscopies of Cu LMM of reduced samples are shown in Figure 2 [17]. The profiles are convoluted into two peaks. It can be seen that the majority of the copper species exist as Cu+

(916.7 eV), or in Cu2+ (917.9 eV) [19,28]. The Auger electron

**Elemental composition (ICP-OES)b**

La0.84Mn0.51Cu0.50 La0.67Mg0.22Mn0.49Cu0.50 La0.67Y0.23Mn0.47Cu0.50 La0.67 Zn0.18Mn0.50Cu0.50 La0.68 Ce0.19Mn0.49Cu0.50

2-), and molecular water. The binding



**Table 2.** The binding energy of La, Mn, O, and the ratio of different oxygen species (taken from ref. 17, reproduced by permission of The Royal Society of Chemistry).

**Figure 2.** Cu LMM Auger electron spectroscopy of (a) P; (b) Mg–P; (c) Y–P; (d) Zn–P; (e) Ce–P samples after reduction (taken from ref. 17, reproduced by permission of The Royal Society of Chemistry).

## *3.1.3. H2-TPD and CO2-TPD analysis*

The H2 desorption over the prereduced materials on the unit surface area below 523 K (test temperature) increases apparently with the addition of the fourth element, as shown in Table 3 [17].

Two CO2 desorption peaks are observed for all samples (Figure 3), which are denoted as peak α and peak β [17]. The peak α at around 400 K could be assigned to weak basic sites and the peak β at around 600 K could be assigned to strong basic sites. With the introduction of the fourth components, the peak α shifts to higher temperature, while the peak β shifts to lower temperature, which indicate the increase of the weak basic sites' strength but the decrease of the strong basic sites' strength. The strength for the weak basic sites of the catalysts increases in the order of: P < Ce–P < Y–P < Mg–P < Zn–P. The amount of the basic sites is also changed with the fourth element doping. The quantitative analysis for the CO2-TPD based on the relative area of the profiles is listed in Table 3, in which the P sample is assigned as 1.00. Both the weak basic sites and the strong basic sites increase due to the alkalinity of Mg for Mg–P. For Y–P, the amount of total basic sites and strong basic sites improved remarkably with the amount of weak basic sites' decreasing. Moreover, the amount of the weak basic sites increases, but the amount of the strong basic sites and total basic sites decrease for Zn–P and Ce–P samples.

**Figure 3.** CO2-TPD curves of the catalysts (taken from ref. 17, reproduced by permission of The Royal Society of Chem‐ istry).


a The amount of basicity of P is assigned as 1.00 to compare with other samples and the values in parentheses are the desorption temperature (K).

**Table 3.** The H2-TPD and CO2-TPD data (taken from ref. 17, reproduced by permission of The Royal Society of Chemistry).

## *3.1.4. Catalytic performance*

*3.1.3. H2-TPD and CO2-TPD analysis*

570 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

3 [17].

samples.

istry).

The H2 desorption over the prereduced materials on the unit surface area below 523 K (test temperature) increases apparently with the addition of the fourth element, as shown in Table

Two CO2 desorption peaks are observed for all samples (Figure 3), which are denoted as peak α and peak β [17]. The peak α at around 400 K could be assigned to weak basic sites and the peak β at around 600 K could be assigned to strong basic sites. With the introduction of the fourth components, the peak α shifts to higher temperature, while the peak β shifts to lower temperature, which indicate the increase of the weak basic sites' strength but the decrease of the strong basic sites' strength. The strength for the weak basic sites of the catalysts increases in the order of: P < Ce–P < Y–P < Mg–P < Zn–P. The amount of the basic sites is also changed with the fourth element doping. The quantitative analysis for the CO2-TPD based on the relative area of the profiles is listed in Table 3, in which the P sample is assigned as 1.00. Both the weak basic sites and the strong basic sites increase due to the alkalinity of Mg for Mg–P. For Y–P, the amount of total basic sites and strong basic sites improved remarkably with the amount of weak basic sites' decreasing. Moreover, the amount of the weak basic sites increases, but the amount of the strong basic sites and total basic sites decrease for Zn–P and Ce–P

**Figure 3.** CO2-TPD curves of the catalysts (taken from ref. 17, reproduced by permission of The Royal Society of Chem‐

The catalytic performances of the catalysts for CO2 hydrogenation to methanol are summarized in Table 4 [17]. Both the CO2 conversion and methanol selectivity are improved when the fourth element is added. With the introduction of Zn, the activity increased greatly, which might be due to the fact that the active site is Cuδ+–O–Zn2+ [31,32]. However, the activity improvement is slight for Ce-P. The relationship between the CO2 conversion and the amount of H2 desorption on unit surface area below 523 K (Table 3) is shown in Figure 4 [17]. It can be seen that the more the H2 that is desorbed on the unit area, the more the CO2 that is converted. Lower CO2 conversion may result from lower copper content as well as lower surface area of copper in the system. The results of Figure 5 [17] show that the trend of the weak basic sites' strength and methanol selectivity is similar, which indicates their dependency.


Reaction conditions: n(H2)/n(CO2)=3:1, T=523 K, P=5.0 MPa, GHSV=4000 h-1.

**Table 4.** The performance for methanol synthesis from CO2 hydrogenation over the reduced catalysts (taken from ref. 17, reproduced by permission of The Royal Society of Chemistry).

**Figure 4.** Relationship between the CO2 conversion and the amount of H2 desorbed on unit surface area below 523 K (taken from ref. 17, reproduced by permission of The Royal Society of Chemistry).

**Figure 5.** Relationship between the selectivity for methanol and the strength of the weak basic sites of the catalysts (taken from ref.17, reproduced by permission of The Royal Society of Chemistry).

## **3.2. Performance of the La-M-Cu-Zn-O (M = Ce, Mg, Zr, Y) based perovskite precursors**

## *3.2.1. Textural and structural properties*

**Figure 4.** Relationship between the CO2 conversion and the amount of H2 desorbed on unit surface area below 523 K

**Figure 5.** Relationship between the selectivity for methanol and the strength of the weak basic sites of the catalysts

(taken from ref. 17, reproduced by permission of The Royal Society of Chemistry).

572 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

(taken from ref.17, reproduced by permission of The Royal Society of Chemistry).

Figure 6 [33] shows that the orthorhombic perovskite structure (A2BO4) with high degree of crystallinity of the La2CuO4 (JCPDS # 82-2142) is the main phase and two small peaks at 2θ=35.6˚ and 38.9˚ ascribed to the CuO phase (JCPDS # 89-5899) present in all samples. The weak peak at 2θ=36.3˚ attributed to the ZnO phase (JCPDS # 80-0075) appears in all samples except LMCZ-8273, which implies that Mg seems to have a special effect on the structure. The phase of Ce7O12 and La2Zr2O7 appear in the sample of LCCZ-8273 and LZCZ-8273, respectively, while there are no new phases containing Mg and Y appearing in the sample of LMCZ-8273 and LYCZ-8273.

**Figure 6.** XRD patterns of the perovskite-type catalysts: (□)La2CuO4; (\*)CuO; (♦)La2Zr2O7; (●)ZnO; (∇)Ce7O12 (taken from ref. 33, reproduced by permission of Elsevier B.V.)

The crystallographic parameters of the prepared materials were calculated by employing leastsquares refinement, assuming an orthorhombic crystal system for the samples, and the results are listed in Table 5 [33]. A certain degree of changes of the lattice parameters occured after the fourth component was introduced. The lattice parameters a, b, and c were lower than those of LCZ-173, which can be attributed to the shrinkage of the La2CuO4 due to the introduction of the fourth elements. The mean grain size of La2CuO4 calculated by the Scherrer equation shows that the particles size of the La2CuO4 decreased remarkably for LCCZ-8273, LZCZ-8273, and LMCZ-8273, but slightly for LYCZ-8273. The physicochemical properties of the calcined catalysts are summarized in Table 6 [33]. The BET surface area for all calcined samples are rather low (SBET < 3 m2 g-1), which is common for perovskite-type of materials [4]. It can be seen that the highest specific surface area is just only 2.3 m<sup>2</sup> g-1 for LZCZ-8273. Moreover, the tendency of the exposed Cu surface area and the Cu dispersion measured by N2O adsorption is the same for the materials. The LZCZ-8273 shows the highest Cu surface area (SCu) and the best dispersion of copper (DCu). The physicochemical properties of LYCZ-8273 are similar to that of the LCZ-173, which indicates that the influence of the Y doping is negligible.


**Table 5.** The lattice parameters of the perovskite-type catalysts (taken from ref. 33, reproduced by permission of Elsevier B.V.).


a Calculated from N2O dissociative adsorption.

**Table 6.** The physiochemical properties of the perovskite-type catalysts (taken from ref. 33, reproduced by permission of Elsevier B.V.).

## *3.2.2. XPS investigations*

The reduced perovskite-type catalysts are analyzed by XPS, and the binding energies (BE) of La3d5/2 and Zn2p3/2 are presented in Table 7 [33]. According to the literature, La3d5/2 features in perovskite structure are located at 837.5 and 834.3 eV [20,34] which are close to the values of pure lanthana at 837.8 and 834.4 eV, indicating that lanthanum ions are present in the trivalent form. A slight shift in the La3d5/2 binding energy is observed upon introduction of the fourth elements and the values are in the range of 837.86–838.01 eV and 834.06–834.36 eV, respectively. Small differences may relate to the changes in crystal structure and/or electronic structure. In addition, small changes are also observed for the binding energy of Zn at around 1021.7 eV for different samples. The Auger electron spectroscopies of Cu LMM of reduced samples are shown in Figure 7 [33]. A broad peak appears in the range of 915.0 eV–920.0 eV for all samples, and it is hard to distinguish the Cu+ , Cu2+, and Cu0 apparently. However, the peaks at around 918.6 eV attributed to Cu0 are distinct for all samples. A new peak appears at around 911.2– 914.3 eV, lower than that of Cu+ , which is defined as peak α, implying that a special Cu species Cuα+ exists in the perovskite system. The presence of Cu+ may accelerate the reduction of CO2 to CO (RWGS) [34],while the Cuα+ (not Cu2+, Cu+ , Cu0 ) plays an important role for the methanol synthesis from CO2/H2 [31,35].

tendency of the exposed Cu surface area and the Cu dispersion measured by N2O adsorption is the same for the materials. The LZCZ-8273 shows the highest Cu surface area (SCu) and the best dispersion of copper (DCu). The physicochemical properties of LYCZ-8273 are similar to

that of the LCZ-173, which indicates that the influence of the Y doping is negligible.

574 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

**Table 5.** The lattice parameters of the perovskite-type catalysts (taken from ref. 33, reproduced by permission of

**g-1) Dispersiona**

**Table 6.** The physiochemical properties of the perovskite-type catalysts (taken from ref. 33, reproduced by permission

The reduced perovskite-type catalysts are analyzed by XPS, and the binding energies (BE) of La3d5/2 and Zn2p3/2 are presented in Table 7 [33]. According to the literature, La3d5/2 features in perovskite structure are located at 837.5 and 834.3 eV [20,34] which are close to the values of pure lanthana at 837.8 and 834.4 eV, indicating that lanthanum ions are present in the trivalent form. A slight shift in the La3d5/2 binding energy is observed upon introduction of the fourth elements and the values are in the range of 837.86–838.01 eV and 834.06–834.36 eV, respectively. Small differences may relate to the changes in crystal structure and/or electronic structure. In addition, small changes are also observed for the binding energy of Zn at around 1021.7 eV for different samples. The Auger electron spectroscopies of Cu LMM of reduced samples are shown in Figure 7 [33]. A broad peak appears in the range of 915.0 eV–920.0 eV for all samples,

LCZ-173 0.7 5.3 3.4 LCCZ-8273 1.3 8.5 5.9 LMCZ-8273 1.2 8.5 6.2 LZCZ-8273 2.3 8.6 6.5 LYCZ-8273 0.7 4.5 3.2

 **(%) SCu (m2**

, Cu2+, and Cu0 apparently. However, the peaks at around

**g-1)**

Elsevier B.V.).

a

of Elsevier B.V.).

*3.2.2. XPS investigations*

**Samples SBET (m2**

Calculated from N2O dissociative adsorption.

and it is hard to distinguish the Cu+

The X-ray photoelectron spectroscopies of the fourth elements in the reduced samples suggest that both Ce3+ and Ce4+ exist in the LCCZ-8273 and the +4 oxidation state is predominant. The result agrees with the XRD analysis. The Zr in the LZCZ-8273 sample exists in the phase of La2Zr2O7.


**Figure 7.** Cu LMM Auger electron spectroscopy of (a) LCZ-173; (b) LCCZ-8273; (c) LMCZ8273; (d) LZCZ-8273; (e) LYCZ-8273 samples after reduce (taken from ref. 33, reproduced by permission of Elsevier B.V.).

## *3.2.3. Catalytic performance*

The catalytic performance for La-M-Cu-Zn-O (M = Ce, Mg, Zr, Y) catalysts are listed in Table 8 [33]. The LMCZ-8273 shows the highest CO2 conversion and the maximum yield of methanol despite the lowest selectivity among all the samples. The LMCZ 8273 shows the highest methanol selectivity and the LCCZ-8273 shows moderate CO2 conversion and methanol selectivity. The lowest CO2 conversion and less improvement for methanol selectivity are observed for LYCZ-8273. The varying of the CO2 conversion had the same tendency as the surface area of copper (Figure 8 [33]), indicating more surface copper existing in the catalysts may lead to higher activity, i.e., Cu0 is the active site for CO2 hydrogenation to methanol [10,16,19,22,35]. It is noteworthy that all catalysts show promising CH3OH selectivity, espe‐ cially for LMCZ-8273. The order of the selectivity to CH3OH is as follows: LMCZ-8273 > LCCZ-8273 > LYCZ-8273 > LCZ173 > LZCZ-8273. The relationship between CH3OH selectivity and the Cuα+ Auger peaks is shown in Figure 9 [33]. It can be seen that Cuα+ had a strong effect on the selectivity for methanol: the lower the binding energy of the peak α, the higher is the CH3OH selectivity. Cu<sup>+</sup> is favorable for the reduction of CO2 to CO (RWGS), so it can be derived that the farther away from 916.6 eV (the binding energy of Cu<sup>+</sup> in Cu LMM) for the peak α, the higher the CH3OH selectivity that can be obtained. As discussed above, doping of Mg leads to the proper oxide state of copper, which results in the best selectivity for methanol. For LCCZ-8273 and LYCZ-8273, Ce and Y substitute La in the A-site with the same charge (+3) and similar ionic radius, which produces more defects in the perovskite structure that causes the special oxide state for copper species. With the special structure of La2CuO4 perovskite, the high dispersed copper species can be realized and stronger physical and electric interaction between the copper and other metal oxides can be obtained, which may lead to the formation and stabilization of the copper species with special valence [23]. However, for the LZCZ-8273, the formation of lanthanum zirconium pyrochlore has little influence on the perovskite structure but a great influence on the content of La2CuO4 perovskite, which may lead to the lowest selectivity of methanol. The turnover frequency (TOF), which represents the number of CO2 molecules hydrogenated in a unit site per second (s-1), is calculated from the exposed copper surface area for the perovskite-type catalysts (Table 8 [33]). The TOF values of the perovskite-type catalysts were very high compared with other catalytic systems [22,35], indicating the better efficiency for copper atoms on perovskite-type catalysts.


Reaction conditions: n(H2)/n(CO2)=3:1, T=523 K, P=5.0 MPa, GHSV=3600 h-1.

**Table 8.** Catalytic performance for methanol synthesis from CO2 hydrogenation over the reduced catalysts (taken from ref. 33, reproduced by permission of Elsevier B.V.).

*3.2.3. Catalytic performance*

CH3OH selectivity. Cu<sup>+</sup>

may lead to higher activity, i.e., Cu0

that the farther away from 916.6 eV (the binding energy of Cu<sup>+</sup>

576 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

The catalytic performance for La-M-Cu-Zn-O (M = Ce, Mg, Zr, Y) catalysts are listed in Table 8 [33]. The LMCZ-8273 shows the highest CO2 conversion and the maximum yield of methanol despite the lowest selectivity among all the samples. The LMCZ 8273 shows the highest methanol selectivity and the LCCZ-8273 shows moderate CO2 conversion and methanol selectivity. The lowest CO2 conversion and less improvement for methanol selectivity are observed for LYCZ-8273. The varying of the CO2 conversion had the same tendency as the surface area of copper (Figure 8 [33]), indicating more surface copper existing in the catalysts

[10,16,19,22,35]. It is noteworthy that all catalysts show promising CH3OH selectivity, espe‐ cially for LMCZ-8273. The order of the selectivity to CH3OH is as follows: LMCZ-8273 > LCCZ-8273 > LYCZ-8273 > LCZ173 > LZCZ-8273. The relationship between CH3OH selectivity and the Cuα+ Auger peaks is shown in Figure 9 [33]. It can be seen that Cuα+ had a strong effect on the selectivity for methanol: the lower the binding energy of the peak α, the higher is the

higher the CH3OH selectivity that can be obtained. As discussed above, doping of Mg leads to the proper oxide state of copper, which results in the best selectivity for methanol. For LCCZ-8273 and LYCZ-8273, Ce and Y substitute La in the A-site with the same charge (+3) and similar ionic radius, which produces more defects in the perovskite structure that causes the special oxide state for copper species. With the special structure of La2CuO4 perovskite, the high dispersed copper species can be realized and stronger physical and electric interaction between the copper and other metal oxides can be obtained, which may lead to the formation and stabilization of the copper species with special valence [23]. However, for the LZCZ-8273, the formation of lanthanum zirconium pyrochlore has little influence on the perovskite structure but a great influence on the content of La2CuO4 perovskite, which may lead to the lowest selectivity of methanol. The turnover frequency (TOF), which represents the number of CO2 molecules hydrogenated in a unit site per second (s-1), is calculated from the exposed copper surface area for the perovskite-type catalysts (Table 8 [33]). The TOF values of the perovskite-type catalysts were very high compared with other catalytic systems [22,35],

indicating the better efficiency for copper atoms on perovskite-type catalysts.

**Table 8.** Catalytic performance for methanol synthesis from CO2 hydrogenation over the reduced catalysts (taken from

Reaction conditions: n(H2)/n(CO2)=3:1, T=523 K, P=5.0 MPa, GHSV=3600 h-1.

ref. 33, reproduced by permission of Elsevier B.V.).

is the active site for CO2 hydrogenation to methanol

in Cu LMM) for the peak α, the

is favorable for the reduction of CO2 to CO (RWGS), so it can be derived

**Figure 8.** Relationship between copper surface area and CO2 conversion (taken from ref. 33, reproduced by permission of Elsevier B.V.).

**Figure 9.** Relationship between methanol selectivity and the binding energy of Cuα+ (taken from ref. 33, reproduced by permission of Elsevier B.V.).

## **3.3. Performance of the La–Mn–Zn–Cu–O based perovskite precursors**

### *3.3.1. Textural and structural properties*

The XRD patterns of the fresh and reduced perovskites are presented in Figure 10a and b, respectively [36]. La2CuO4 perovskite-like structure can be observed for all fresh samples. LaMnO3 phase emerges and the La2CuO4 phase transfers from tetragonal (JPCDS 81-2450) to orthorhombic (JPCDS 81-0872) as the manganese is introduced, which indicates that the manganese introduction distorts the structure of the La2CuO4. With the increasing of the manganese amount, the intensity of the La2CuO4 phase decreases while that of the LaMnO3 phase increases, which implies that the formation of LaMnO3 is easier than that of La2CuO4. This phenomenon reveals that the structure of LaMnO3 is more stable than La2CuO4. Small peaks for both CuO and ZnO can also be observed except LMC-046, which indicates the perovskite structure has certain tolerance for the involved elements for this perovskite-type system. Moreover, the peak intensity of the separated CuO decreases when the value of Mn/Zn decreases, which means the formation of LaMnO3 can lead to the separation of copper from the La2CuO4 perovskite structure. For the reduced sample (Figure 10b), the La2CuO4 perovskite structure disappears and the metallic copper and La2O3 is observed, which indicates that the ''metal-on-oxide'' can be attained. The appearance of La0.974Mn0.974O3 phase reveals that the reduction progress can result in defects rather than destruction for the Mn-based perov‐ skite. crystallites. The change of lattice parameters of the samples implies that interaction between the involved elements might be different. The BET specific surface area along with the exposed surface copper area and the copper dispersion measured by N2O adsorption are summarized in Table 3.3.2 [36]. The specific surface area for all samples is low, which is common for perovskite‐type materials [37]. LMC‐406 possesses the largest specific surface area (SBET), the exposed surface copper area (SCu) as well as the copper dispersion (DCu), while the LZC‐046 sample shows the lowest SCu and DCu, which indicate that the existence of LaMnO3 perovskite structure is favorable for increasing the surface copper area due to the extension of the space structure for the samples with manganese (Table 3.3.1).

Figure 10. X‐ray patterns of the fresh (a) and reduced (b) catalysts: (o) tetragonal La2CuO4; ( ) orthorhombic La2CuO4; (Ñ) CuO; (¨) ZnO; (§) LaMnO3; (©) La2O3; (∙) La0.974Mn0.974O3; (\*) Cu (taken from ref. 36, reproduced by permission of Springer Science+Business Media). **Figure 10.** X-ray patterns of the fresh (a) and reduced (b) catalysts: (o) tetragonal La2CuO4; (□) orthorhombic La2CuO4; (▽) CuO; (♦) ZnO; (♣) LaMnO3; (♥) La2O3; (∙) La0.974Mn0.974O3; (\*) Cu (taken from ref. 36, reproduced by permission of Springer Science+Business Media).

The crystallographic parameters of the prepared materials were calculated by employing leastsquares refinement and the results are listed in Table 9 [36]. It can be found that the axes are elongated for the four element samples, which means that doping may strut the perovskite structure. The size of La2CuO4 crystallites becomes smaller with the introduction of manga‐ nese. The LaMnO3 phase changes from cubic to orthorhombic structure as zinc is introduced. Moreover, the LMZC-136 and LZC-046 possess the smallest LaMnO3 and the largest La2CuO4

Table 3.3.1 The lattice parameters of the perovskite‐type samples (taken from ref. 36, reproduced by

**SCu (m2 g‐**

Table 3.3.2 The physiochemical properties of the perovskite‐type catalysts (taken from ref. 36,

The XPS results of the reduced perovskite‐type catalysts are listed in Table 3.3.3 [36]. The values of

**1)**

2.3 2.7 2.8 3.0 4.5

permission of Springer Science+Business Media).

**Dispersiona**

<sup>a</sup> Calculated from N2O dissociative adsorption.

reproduced by permission of Springer Science+Business Media).

**(%)**

1.9 2.3 2.6 2.7 4.2

Samples **SBET (m2 g‐ 1)**

**3.3.2 XPS investigations**

2.4 1.1 1.4 1.2 2.5

LZC‐046 LMZC‐ 136 LMZC‐ 226 LMZC‐ 316

LMC‐406

crystallites among all the samples, which implies that the abundant zinc species can improve the formation of small LaMnO3 and large La2CuO4 crystallites. The change of lattice parameters of the samples implies that interaction between the involved elements might be different.

The BET specific surface area along with the exposed surface copper area and the copper dispersion measured by N2O adsorption are summarized in Table 10 [36]. The specific surface area for all samples is low, which is common for perovskite-type materials [37]. LMC-406 possesses the largest specific surface area (SBET), the exposed surface copper area (SCu) as well as the copper dispersion (DCu), while the LZC-046 sample shows the lowest SCu and DCu, which indicate that the existence of LaMnO3 perovskite structure is favorable for increasing the surface copper area due to the extension of the space structure for the samples with manganese (Table 9).


**Table 9.** The lattice parameters of the perovskite-type samples (taken from ref. 36, reproduced by permission of Springer Science+Business Media).


Calculated from N2O dissociative adsorption.

**Table 10.** The physiochemical properties of the perovskite-type catalysts (taken from ref. 36, reproduced by permission of Springer Science+Business Media).

## *3.3.2. XPS investigations*

**3.3. Performance of the La–Mn–Zn–Cu–O based perovskite precursors**

578 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

The XRD patterns of the fresh and reduced perovskites are presented in Figure 10a and b, respectively [36]. La2CuO4 perovskite-like structure can be observed for all fresh samples. LaMnO3 phase emerges and the La2CuO4 phase transfers from tetragonal (JPCDS 81-2450) to orthorhombic (JPCDS 81-0872) as the manganese is introduced, which indicates that the manganese introduction distorts the structure of the La2CuO4. With the increasing of the manganese amount, the intensity of the La2CuO4 phase decreases while that of the LaMnO3 phase increases, which implies that the formation of LaMnO3 is easier than that of La2CuO4. This phenomenon reveals that the structure of LaMnO3 is more stable than La2CuO4. Small peaks for both CuO and ZnO can also be observed except LMC-046, which indicates the perovskite structure has certain tolerance for the involved elements for this perovskite-type system. Moreover, the peak intensity of the separated CuO decreases when the value of Mn/Zn decreases, which means the formation of LaMnO3 can lead to the separation of copper from the La2CuO4 perovskite structure. For the reduced sample (Figure 10b), the La2CuO4 perovskite structure disappears and the metallic copper and La2O3 is observed, which indicates that the ''metal-on-oxide'' can be attained. The appearance of La0.974Mn0.974O3 phase reveals that the reduction progress can result in defects rather than destruction for the Mn-based perov‐

crystallites. The change of lattice parameters of the samples implies that interaction between the involved elements

The BET specific surface area along with the exposed surface copper area and the copper dispersion measured by N2O adsorption are summarized in Table 3.3.2 [36]. The specific surface area for all samples is low, which is common for perovskite‐type materials [37]. LMC‐406 possesses the largest specific surface area (SBET), the exposed surface copper area (SCu) as well as the copper dispersion (DCu), while the LZC‐046 sample shows the lowest SCu and DCu, which indicate that the existence of LaMnO3 perovskite structure is favorable for increasing the surface copper area due to the extension of

Figure 10. X‐ray patterns of the fresh (a) and reduced (b) catalysts: (o) tetragonal La2CuO4; ( ) orthorhombic La2CuO4; (Ñ) CuO; (¨) ZnO; (§) LaMnO3; (©) La2O3; (∙) La0.974Mn0.974O3; (\*) Cu (taken from ref. 36, reproduced by permission of Springer Science+Business Media).

**Figure 10.** X-ray patterns of the fresh (a) and reduced (b) catalysts: (o) tetragonal La2CuO4; (□) orthorhombic La2CuO4; (▽) CuO; (♦) ZnO; (♣) LaMnO3; (♥) La2O3; (∙) La0.974Mn0.974O3; (\*) Cu (taken from ref. 36, reproduced by permission of

The crystallographic parameters of the prepared materials were calculated by employing leastsquares refinement and the results are listed in Table 9 [36]. It can be found that the axes are elongated for the four element samples, which means that doping may strut the perovskite structure. The size of La2CuO4 crystallites becomes smaller with the introduction of manga‐ nese. The LaMnO3 phase changes from cubic to orthorhombic structure as zinc is introduced. Moreover, the LMZC-136 and LZC-046 possess the smallest LaMnO3 and the largest La2CuO4

Table 3.3.1 The lattice parameters of the perovskite‐type samples (taken from ref. 36, reproduced by

**SCu (m2 g‐**

Table 3.3.2 The physiochemical properties of the perovskite‐type catalysts (taken from ref. 36,

The XPS results of the reduced perovskite‐type catalysts are listed in Table 3.3.3 [36]. The values of

**1)**

2.3 2.7 2.8 3.0 4.5

*3.3.1. Textural and structural properties*

the space structure for the samples with manganese (Table 3.3.1).

permission of Springer Science+Business Media).

Springer Science+Business Media).

**Dispersiona**

<sup>a</sup> Calculated from N2O dissociative adsorption.

reproduced by permission of Springer Science+Business Media).

**(%)**

1.9 2.3 2.6 2.7 4.2

Samples **SBET (m2 g‐ 1)**

**3.3.2 XPS investigations**

2.4 1.1 1.4 1.2 2.5

LZC‐046 LMZC‐ 136 LMZC‐ 226 LMZC‐ 316

LMC‐406

skite.

might be different.

The XPS results of the reduced perovskite-type catalysts are listed in Table 11 [36]. The values of La3d5/2 binding energy (BE) are located in the range of 834.2–834.6 eV, demonstrating that La ions are in the trivalent form for all samples. Small changes of Zn (around 1021.7 eV) and Mn (around 642.0 eV) BE may relate to the small distortions in electronic structure and/or crystal structure. For the O1s patterns, the peak at around 528.2–529.3 eV can be attributed to the oxygen ions in the crystal lattice (O2-) and the peak at around 531.1–531.5 eV can be assigned to the adsorbed oxygen species (Oad) derived from the defects or oxygen vacancies in the structure [38]. The O1s BE shifts to lower value with the decreasing of the Mn/Zn ratio, which suggests the increasing of electron cloud density around O element. The value of Oad/O2- is max for the LZC-046, which decreases for the Mn containing samples, indicating that the LaMnO3 could reduce the structural defects.

For this series catalyst, the binding energies of Cu2p3/2 are lower than that for the copper oxide (933.0 eV) apparently, indicating that the Cu atoms are not in the simple copper oxides form. Figure 11 [36] shows the Auger electron spectroscopies of Cu LMM for the reduced samples. A broad peak can be observed, which can then be deconvoluted into three peaks. The peak at around 916.5 and 919.0 eV matched with kinetic binding energy of Cu+ and Cu0 within the error limit, respectively. However, a new peak at around 911.2–913.2 eV is observed which may be ascribed to the Cuα+. According to literatures and our works, Cuα+ can be appeared in perovskite-type system.


**Table 11.** XPS data of the perovskite-type catalysts (taken from ref. 36, reproduced by permission of Springer Science +Business Media).

## *3.3.3. Catalytic performance*

The performance of the La–Mn–Zn–Cu–O based perovskite catalysts for methanol synthesis from CO2 hydrogenation is shown in Table 12 [36]. The LMC-406 shows the worst performance despite the largest surface area and exposed copper surface area. The LZC- 046, which also contains three metal elements, but Zn instead of Mn, shows a moderate catalytic performance. It is well-known that the site of Cu+ -O-Zn2+ favors the adsorption of hydrogen that can transport to the bulk copper species via spillover [20,39]. So the lack of the site of Cu+ -O-Zn2+ may be the reason for the poor catalytic performance of LMC-406. Moreover, the TOFCu value increases sharply upon Zn introduction, which verifies that the copper sites are not the only active sites

structure. For the O1s patterns, the peak at around 528.2–529.3 eV can be attributed to the oxygen ions in the crystal lattice (O2-) and the peak at around 531.1–531.5 eV can be assigned to the adsorbed oxygen species (Oad) derived from the defects or oxygen vacancies in the structure [38]. The O1s BE shifts to lower value with the decreasing of the Mn/Zn ratio, which suggests the increasing of electron cloud density around O element. The value of Oad/O2- is max for the LZC-046, which decreases for the Mn containing samples, indicating that the

For this series catalyst, the binding energies of Cu2p3/2 are lower than that for the copper oxide (933.0 eV) apparently, indicating that the Cu atoms are not in the simple copper oxides form. Figure 11 [36] shows the Auger electron spectroscopies of Cu LMM for the reduced samples. A broad peak can be observed, which can then be deconvoluted into three peaks. The peak at

error limit, respectively. However, a new peak at around 911.2–913.2 eV is observed which may be ascribed to the Cuα+. According to literatures and our works, Cuα+ can be appeared in

LZC-046 834.6 932.2 - 1021.4 528.2 1.33

LMZC-136 834.3 932.4 642.0 1021.8 528.7 1.29

LMZC-226 834.2 932.5 641.9 1021.6 528.8 1.25

LMZC-316 834.3 932.7 642.0 1021.6 529.2 1.18

LMC-406 834.5 932.6 642.0 - 529.3 1.13

**Table 11.** XPS data of the perovskite-type catalysts (taken from ref. 36, reproduced by permission of Springer Science

The performance of the La–Mn–Zn–Cu–O based perovskite catalysts for methanol synthesis from CO2 hydrogenation is shown in Table 12 [36]. The LMC-406 shows the worst performance despite the largest surface area and exposed copper surface area. The LZC- 046, which also contains three metal elements, but Zn instead of Mn, shows a moderate catalytic performance.

reason for the poor catalytic performance of LMC-406. Moreover, the TOFCu value increases sharply upon Zn introduction, which verifies that the copper sites are not the only active sites

to the bulk copper species via spillover [20,39]. So the lack of the site of Cu+

**La3d5/2 Cu2p3/2 Mn2p3/2 Zn2p3/2 O1s Oad/O2-**


and Cu0

531.1

531.3

531.3

531.4

531.5


within the

around 916.5 and 919.0 eV matched with kinetic binding energy of Cu+

**Samples Binding energy (eV)**

LaMnO3 could reduce the structural defects.

580 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

perovskite-type system.

+Business Media).

*3.3.3. Catalytic performance*

It is well-known that the site of Cu+

**Figure 11.** Cu LMM Auger electron spectroscopy of the reduced perovskite-type catalysts (taken from ref. 36, repro‐ duced by permission of Springer Science+Business Media).

for the reaction [40]. With more reducible copper species and high TOFCu values, the fourcomponent samples show a good catalytic performance. The ratio of both Cuα+ and Cu0 species to the total copper species (calculated from the Auger spectroscopy (Figure 11)) shows the same change tendency with the CO2 conversion (Figure 12) [36], indicating that both Cuα+ and Cu0 species could be the active sites for the conversion of CO2. In addition, with the change of the ratio for Mn/Zn, the synergy between copper and the other components might vary, and then the reduction state of copper species (Cuα+ and Cu0 ) changes. The four-component samples with two kinds of perovskites show better methanol selectivity, which implies that the strong synergy for different elements and different perovskite phases are significant for the improvement of the catalytic performance. In addition, it is also found that the lower the BE of the Cuα+, the higher is the CH3OH selectivity (Figure 13) [36].


a The ratio value was calculated from the Auger spectroscopy (Figure 11).

Reaction conditions: n(H2)/n(CO2) = 3:1, T = 543 K, P = 5.0 MPa, GHSV = 3800 h-1.

**Table 12.** The catalytic performance for methanol synthesis from CO2 hydrogenation over the reduced catalysts (taken from ref. 36, reproduced by permission of Springer Science+Business Media).

**Figure 12.** Relationship between CO2 conversion and ratio value of (Cuα++Cu<sup>0</sup> )/CuTotal (taken from ref.36, reproduced by permission of Springer Science+Business Media).

**Figure 13.** Relationship between methanol selectivity and binding energy of Cuα+ (taken from ref. 36, reproduced by permission of Springer Science+Business Media).

## **4. Conclusions**

**Figure 12.** Relationship between CO2 conversion and ratio value of (Cuα++Cu<sup>0</sup>

582 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

**Figure 13.** Relationship between methanol selectivity and binding energy of Cuα+ (taken from ref. 36, reproduced by

by permission of Springer Science+Business Media).

permission of Springer Science+Business Media).

)/CuTotal (taken from ref.36, reproduced

Three series of catalysts derived from perovskite-type precursors were prepared by solgel method, which were applied in the CO2 hydrogenation to methanol. The conclusions are as follows:


## **Acknowledgements**

This work was financially supported by the Key Science and Technology Program of Shanxi Province, China (MD2014-10), the National Key Technology Research and Development Program of the Ministry of Science and Technology (2013BAC11B00), and the Natural Science Foundation of China (21343012).

## **Author details**

Feng Li1\*, Haijuan Zhan2 , Ning Zhao1,3\* and Fukui Xiao1,3

\*Address all correspondence to: lifeng2729@sxicc.ac.cn; zhaoning@sxicc.ac.cn

1 State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, People's Republic of China

2 School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, People's Republic of China

3 National Engineering Research Center for Coal-based Synthesis, People's Republic of China

## **References**


[10] Peters M., K°ohler B., Kuckshinrichs W., Leitner W. Chemical technologies for ex‐ ploiting and recycling carbon dioxide into the value chain. *ChemSusChem*. 2011; 4: 1216-1240. DOI: 10.1002/cssc.201000447

**Author details**

Republic of China

**References**

China

Feng Li1\*, Haijuan Zhan2

of Sciences, Taiyuan, People's Republic of China

584 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

DOI: 10.1016/s0921-4534(02)02500-5

DOI: 10.2298/jsc0610049z

[6] Riza F., Ftikos C. Influence of A-

10:2000-2003.10.1016/j.catcom.2009.07.017

2006; 84: 739-763. DOI: 10.1205/cherd05049

*Chem Rev*. 2001; 101: 1981-2017. DOI: 10.1021/cr980129f

, Ning Zhao1,3\* and Fukui Xiao1,3

1 State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy

2 School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, People's

[1] Tejuca L.G, Fierro J.L.G, Tascón J.M.D. Structure and Reactivity of Perovskite-Type

[2] Sun X.F., Komiya S., Ando Y. Anomalous damping of phonon thermal transport in lightly Y- or Eu-doped La2CuO4 single crystals. *Physica C*. 2003; 388-389: 355-356.

[3] Jia L.s., Gao J., Fang W.p., Li Q.b. Carbon dioxide hydrogenation to methanol over the pre-reduced LaCr0.5Cu0.5O3 catalyst. *Catalys Commun*. 2009; 10: 2000-2003. DOI:

[4] Pen˜a M.A., Fierro J.L.G. Chemical structures and performance of perovskite oxides.

[5] Zhong H., Zeng R. Structure of LaSrMO4(M = Mn, Fe, Co, Ni, Cu) and their catalytic properties in the total oxidation of hexane. *J Serbian Chem Soc*. 2006; 71: 1049-1059.

La2CoO4±δ. *J Eur Cera Soc*. 2007; 27: 571-573. DOI: 10.1016/j.jeurceramsoc.2006.04.069

[7] Li B., Duan Y., Luebke D., Morreale B. Advances in CO2 capture technology: A patent review. *Appl Energy*. 2013; 102: 1439-1447. DOI: 10.1016/j.apenergy.2012.09.009

[8] Steeneveldt R., Berger B., Torp T.A. CO2 capture and storage. *Chem Engin Res Design*.

[9] Yu K.M., Curcic I., Gabriel J., Tsang S.C. Recent advances in CO2 capture and utiliza‐

tion. *ChemSusChem*. 2008; 1: 893-899. DOI: 10.1002/cssc.200800169

site doping on the properties of the system

and B-

Oxides. *Adv Catalys*. 1989; 36: 237-328. DOI: 10.1016/s0360-0564(08)60019-x

3 National Engineering Research Center for Coal-based Synthesis, People's Republic of

\*Address all correspondence to: lifeng2729@sxicc.ac.cn; zhaoning@sxicc.ac.cn


of Cu/ZnO/ZrO2 catalysts for the methanol synthesis from CO2. *Appl Catalys A*. 2003; 249: 129-138. DOI: 10.1016/s0926-860x(03)00191-1


perovskite-type precursors. *J Power Sources*. 2014; 251: 113-121. DOI: 10.1016/j.jpows‐ our.2013.11.037

[34] Maluf S.S., Nascente P.A.P., Afonso C.R.M., Assaf E.M. Study of La2−xCaxCuO4 perov‐ skites for the low temperature water gas shift reaction. *Appl Catalys A*. 2012; 413-414: 85-93. DOI: 10.1016/j.apcata.2011.10.047

of Cu/ZnO/ZrO2 catalysts for the methanol synthesis from CO2. *Appl Catalys A*. 2003;

[23] Li Z.q., Meng M., Zha Y.q., Dai F.f., Hu T.d., Xie Y.n., Zhang J. Highly efficient multi‐ functional dually-substituted perovskite catalysts La1−xKxCo1−yCuyO3−δ used for soot combustion, NOx storage and simultaneous NOx-soot removal. *Appl Catalys B*. 2012;

[24] Rubio-Marcos F., Quesada A., García M.A., Banares M.A., Fierro J.L.G, Martin-Gon‐ zalez M.S., Costa-Kramer J.L., Fernandez J.F. Some clues about the interphase reac‐ tion between ZnO and MnO2 oxides. *J Solid State Chem*. 2009; 182: 1211-1216. DOI:

[25] Kenji Tabata Y.H., Suzuki E. XPS studies on the oxygen species of LaMn1-xCuxO3+λ.

[26] Batis N.H., Delichere P., Batis H. Physicochemical and catalytic properties in meth‐ ane combustion of La1−xCaxMnO3±y (0≤x≤1; −0.04≤y≤0.24) perovskite-type oxide. *Appl*

[27] Najjar H., Batis H. La–Mn perovskite-type oxide prepared by combustion method: Catalytic activity in ethanol oxidation. *Appl Catalys A*. 2010; 383: 192-201. DOI:

[28] Gao P., Li F., Xiao F.K., Zhao N., Sun N.N, Wei W., Zhong L.S, Sun Y.H. Preparation and activity of Cu/Zn/Al/Zr catalysts via hydrotalcite-containing precursors for methanol synthesis from CO2 hydrogenation. *Catalys Sci Technol*. 2012; 2: 1447. DOI:

[29] Jia L.s., Gao J., Fang W.p., Li Q.b. Influence of copper content on structural features and performance of pre-reduced LaMn1-xCuxO3 (0≤x<1) catalysts for methanol syn‐ thesis from CO2/H2. *J Rare Earths*. 2010; 28: 747-751. DOI: 10.1016/

[30] Aykut Y., Parsons G.N, Pourdeyhimi B., Khan S.A. Synthesis of mixed ceramic MgxZn1-xO nanofibers via Mg2+ doping using sol-gel electrospinning. *Langmuir*. 2013;

[31] Arena F., Italiano G., Barbera K., Bordiga S., Bonura G., Spadaro L., Frusteri F. Solidstate interactions, adsorption sites and functionality of Cu-ZnO/ZrO2 catalysts in the CO2 hydrogenation to CH3OH. *Appl Catalys A*. 2008; 350: 16-23. DOI: 10.1016/j.apcata.

[32] Miller B.J.A., Martin-luengo M.A., Vong M.S.W., Wang Y., Self V.A., Chapmanc S.M, Sermon P.A. Junctions between CuOx and ZnOy in sensors for CO and catalysts for

[33] Zhan H.j., Li F., Gao P., Zhao N., Xiao F.k., Wei W., Sun Y.h. Methanol synthesis from CO2 hydrogenation over La-M-Cu-Zn-O(M=Y, Ce, Mg, Zr) catalysts derived from

CO hydrogenation. *J Mater Chem*. 1997; 7: 2155-2160. DOI: 10.1039/a700747g

*Appl Catalys A*. 1998; 170: 245–254. DOI: 10.1016/j.molstruc.2014.04.065

*Catalys A*. 2005; 282: 173-180. DOI: 10.1016/j.apcata.2004.12.009

249: 129-138. DOI: 10.1016/s0926-860x(03)00191-1

586 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

121-122: 65-74. DOI: 10.1016/j.apcatb.2012.03.022

10.1016/j.jssc.2009.02.009

10.1016/j.apcata.2010.05.048

10.1039/c2cy00481j

s1002-0721(09)60193-9

2008.07.028

29: 4159-4166. DOI: 10.1021/la400281c


## **Designing Perovskite Oxides for Solid Oxide Fuel Cells**

Idoia Ruiz de Larramendi, Nagore Ortiz-Vitoriano, Isaen B. Dzul-Bautista and Teófilo Rojo

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61304

## **Abstract**

Perovskite-type oxides with the general formula ABO3 have been widely studied and are utilized in a large range of applications due to their tremendous versatility. In particular, the high stability of the perovskite structure compared to other crystal arrangements and its ability, given the correct selection of A and B cations, to maintain a large oxygen vacancy concentration makes it a good candidate as electrode in solid oxide fuel cell (SOFC) applications. Utilizing this novel structure allows the engineer‐ ing of advanced, effective electrolytes for such devices. This review details the development of current state-of-the-art perovskite-type oxides for solid oxide fuel cell (SOFC) applications.

**Keywords:** SOFC, Nanostructure

## **1. Introduction**

The development of new energy technologies has become important with the present situation of increasing energy demand, rising energy prices, and reinforcement of countermeasures for global warming and its detrimental climatological, ecological, and sociological effects [1]. The supplies of fossil fuels are constantly decreasing and some believe that we have reached the peak for oil production. A reappearing problem in the energy field is the conversion from available to usable form. This concerns the conversion of chemical energy in the form of fossil resources and derivatives, such as hydrogen and alcohols, into electrical energy. It has become increasingly important to reduce the losses associated with the applied conversion techniques partly due to industrialization and technological progress. Traditional conversion to electrical energy is by gas turbine, steam turbine, or reciprocating engine driving a generator, where the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Carnot cycle sets a limit to the efficiency [2]. A fuel cell provides an alternative, whereby electrical energy can be made available with small losses, and no Carnot limit [3]. The effluents are in principle water, heat and CO2 if the fuel is clean.

The energy situation has caused a push for sustainable energy technologies. Here, fuel cells play an important role in the renewable energy sector because of their highly efficient energy conversion and their especially high reliability. During the last decades, fuel cells have attracted much attention because of their potential for producing electricity more efficiently than conventional power generation like heat engines, which normally are limited by the Carnot cycle. The discovery of the fuel cell is ascribed to Sir William Grove (1839), demon‐ strating the reversibility of electrolytic water separation [4]. In 1899, Nernst contributed by demonstrating that certain oxides attained remarkably high electrical conductivity by doping with other oxides [5]. In the late twentieth century, the development accelerated and the interest in fuel cell technology increased. Today, there are five different types of fuel cells, all named after their electrolyte materials:


Depending on the electrolyte, each fuel cell has certain characteristics such as operation temperature, electric efficiency, and demands for fuel composition (Table 1).


**Table 1.** Most important characteristics for different types of fuel cells

Low-temperature fuel cells (AFC, SPFC, and PAFC) have potential for the propulsion of cars, where a short heating time is needed and the efficiency has to be compared with about 20% for a combustion engine, whereas high-temperature fuel cells (MCFC and SOFC) are suitable for continuous power and heat production, where the cell temperature can be maintained. The high-temperature fuel cells have higher efficiency and are more tolerant to the choice of fuel compared to the low-temperature fuel cells [3].

## **2. What is a solid oxide fuel cell?**

Carnot cycle sets a limit to the efficiency [2]. A fuel cell provides an alternative, whereby electrical energy can be made available with small losses, and no Carnot limit [3]. The effluents

The energy situation has caused a push for sustainable energy technologies. Here, fuel cells play an important role in the renewable energy sector because of their highly efficient energy conversion and their especially high reliability. During the last decades, fuel cells have attracted much attention because of their potential for producing electricity more efficiently than conventional power generation like heat engines, which normally are limited by the Carnot cycle. The discovery of the fuel cell is ascribed to Sir William Grove (1839), demon‐ strating the reversibility of electrolytic water separation [4]. In 1899, Nernst contributed by demonstrating that certain oxides attained remarkably high electrical conductivity by doping with other oxides [5]. In the late twentieth century, the development accelerated and the interest in fuel cell technology increased. Today, there are five different types of fuel cells, all

Depending on the electrolyte, each fuel cell has certain characteristics such as operation

temperature, electric efficiency, and demands for fuel composition (Table 1).

**Fuel Cell AFC PEMFC PAFC MCFC SOFC**

Charge carrier OH– H+ H+ CO3 2– O2–

Electrolyte KOH Polymer H3PO4 Li2CO3+K2 CO3 ZrO2+Y2O3

*T*oper.(°C) 65%–220 60%–130 150%–220 650 700%–1000 Electrical efficiency 45%–60% 40%–60% 35%–40% 45%–60% 45%–60%

Fuel Pure H2 Pure H2 Co-free H2 H2, CO, CH4 H2, CO, CH4, NH3

Low-temperature fuel cells (AFC, SPFC, and PAFC) have potential for the propulsion of cars, where a short heating time is needed and the efficiency has to be compared with about 20% for a combustion engine, whereas high-temperature fuel cells (MCFC and SOFC) are suitable for continuous power and heat production, where the cell temperature can be maintained. The high-temperature fuel cells have higher efficiency and are more tolerant to the choice of fuel

are in principle water, heat and CO2 if the fuel is clean.

590 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

named after their electrolyte materials:

**•** Phosphoric acid fuel cells (PAFC)

**•** Solid oxide fuel cells (SOFC)

**•** Molten carbonate fuel cells (MCFC)

**•** Polymer exchange membrane fuel cells (PEMFC)

**Table 1.** Most important characteristics for different types of fuel cells

compared to the low-temperature fuel cells [3].

**•** Alkaline fuel cells (AFC)

Solid oxide fuel cells (SOFC) are electrochemical devices which convert chemical energy directly into electrical energy with high efficiency and low environmental impact and are expected to become the emerging technology for stationary power production [6].

SOFCs are considered by many researchers to be the most suitable for generating electricity from hydrocarbons because of their simplicity, efficiency, and ability to tolerate some degree of impurities. There are many advantages of SOFC with respect to other fuel cells. Some of the main advantages are [7]:


However, there are some drawbacks related to the choice of materials in relation to costs. There are roughly two design types, tubular and flat plate. For the tubular cell, the material problems are less, but fabrication costs are high, whereas for the flat plate design, fabrication costs are less but more materials problems arise [8].

The operating principle of the solid oxide fuel cells is illustrated in Figure 1.

SOFCs consist of three layers of functional materials: the anode, the electrolyte and the cathode. The anode is where the oxidation reaction takes place, and the cathode is where the reduction takes place. The cathode is fed with air or oxygen while the anode is fed with fuel gas. At the cathode, oxygen molecules are reduced to oxygen ions (½ O2 (g) + 2 e– → O2–). The oxygen ions are conducted through the electrolyte to the anode. At the anode, oxygen ions oxidize the fuel gas and forms water and carbon dioxide, while the resulting free electrons are transported via the external circuit back to the cathode (H2 (g) + O2– → H2O (g) + 2 e– ).

To limit cathode overpotential, the traditional SOFC with yttria-stabilized zirconia (YSZ) electrolyte and strontium-doped lanthanum manganite (LSM) cathodes operate at relatively high temperatures (800°C–1000°C). These high temperatures encourage cell degradation increasing cell, stack, and system maintenance. In addition, even higher temperatures are required for fabrication, encouraging electrode–electrolyte reactions, often forming undesir‐ able, insulating secondary phases and consequently increasing cell resistance. Lowering the operating and fabrication temperature to 400°C–800°C could reduce both cell degradation and manufacturing costs. Cells which operate in this temperature range are known as intermediate temperature SOFCs (IT-SOFC). The advancement of this technology, limited by the high temperatures required, can be overcome by reducing operating temperatures, thus increasing theoretical efficiency. In order to operate efficiently at these reduced temperatures and to develop the next generation of hydrogen-related energy devices, new materials are required and the utilized processing routes must be optimized [6,9].

Among the new generation of materials, those with ABO3-type perovskite structures stand out due to their great versatility. Through the correct choice of A and B site cations and the introduction of dopants, it is possible to obtain a large variety of materials with a wide range of properties and applications. This chapter will cover a selection of key materials developed for use in SOFC devices, their advantages and disadvantages and the optimization strategies published so far.

## **3. Evolution of perovskite-type structured materials as electrodes in SOFCs**

The general formula unit of the perovskite is ABO3, where A is a larger cation with a coordi‐ nation number of 12 and B is a smaller cation with a coordination number of 6 (Figure 2). The large A cations can be rare earth, alkaline earth, alkali, and others [10] whereas the smaller B site accommodates many transition metals. These elements are typically the source of elec‐ tronic conductivity. Cations of a rather wide range of ionic radii and valence are able to enter one or another site in the perovskite structure, exhibiting versatile physical and chemical properties and, thus, high concentrations of oxide vacancies and high ionic conductivity may be achieved [11]. This ability to hold a large content of oxygen vacancies makes them good candidates as electrodes in SOFC applications.

**Figure 2.** Atomic structure and oxygen transport in mixed conducting perovskites. On the left, the positions of the ions in a cubic structure. On the right, corner-sharing BO6 octahedra and oxygen vacancy migration path.

3

The B cations are located in the corners of the cube and the A cation occupies the center. The oxygen is placed at the centers of the twelve cube edges, giving corner-shared strings of BO6 octahedra. The A cation has the same size as the oxide ion, while the B cation is smaller. Depending on the type of cations, it is possible to create oxygen vacancies in the structure. These oxygen vacancies move along the structure giving rise to ionic conduction (Figure 2).

The composition and the microstructure of electrode materials greatly influence the perform‐ ance of SOFCs. The high temperature and the reducing or oxidizing atmospheres limit the choice of these materials.

## **3.1. Cathode materials**

3

To limit cathode overpotential, the traditional SOFC with yttria-stabilized zirconia (YSZ) electrolyte and strontium-doped lanthanum manganite (LSM) cathodes operate at relatively high temperatures (800°C–1000°C). These high temperatures encourage cell degradation increasing cell, stack, and system maintenance. In addition, even higher temperatures are required for fabrication, encouraging electrode–electrolyte reactions, often forming undesir‐ able, insulating secondary phases and consequently increasing cell resistance. Lowering the operating and fabrication temperature to 400°C–800°C could reduce both cell degradation and manufacturing costs. Cells which operate in this temperature range are known as intermediate temperature SOFCs (IT-SOFC). The advancement of this technology, limited by the high temperatures required, can be overcome by reducing operating temperatures, thus increasing theoretical efficiency. In order to operate efficiently at these reduced temperatures and to develop the next generation of hydrogen-related energy devices, new materials are required

Among the new generation of materials, those with ABO3-type perovskite structures stand out due to their great versatility. Through the correct choice of A and B site cations and the introduction of dopants, it is possible to obtain a large variety of materials with a wide range of properties and applications. This chapter will cover a selection of key materials developed for use in SOFC devices, their advantages and disadvantages and the optimization strategies

**3. Evolution of perovskite-type structured materials as electrodes in SOFCs**

The general formula unit of the perovskite is ABO3, where A is a larger cation with a coordi‐ nation number of 12 and B is a smaller cation with a coordination number of 6 (Figure 2). The large A cations can be rare earth, alkaline earth, alkali, and others [10] whereas the smaller B site accommodates many transition metals. These elements are typically the source of elec‐ tronic conductivity. Cations of a rather wide range of ionic radii and valence are able to enter one or another site in the perovskite structure, exhibiting versatile physical and chemical properties and, thus, high concentrations of oxide vacancies and high ionic conductivity may be achieved [11]. This ability to hold a large content of oxygen vacancies makes them good

**Figure 2.** Atomic structure and oxygen transport in mixed conducting perovskites. On the left, the positions of the ions

in a cubic structure. On the right, corner-sharing BO6 octahedra and oxygen vacancy migration path.

and the utilized processing routes must be optimized [6,9].

592 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

candidates as electrodes in SOFC applications.

published so far.

One of the major research efforts in SOFC technology is directed towards decreasing the cell operating temperature. The best performance of the electrolyte, anode, and cathode materials typically is seen at an operating temperature above 800°C. This makes the use of expensive alloys as interconnectors and current collectors necessary. Decreasing the operating tempera‐ ture would make it possible to use soft steel alloys with a remarkable decrease of the initial cost of the device. It is necessary, however, to develop new materials which exhibit good performance at lower temperatures in terms of conductivity, and chemical and mechanical compatibility. The widely used materials also need to be optimized in order to minimize technical problems at elevated temperatures. These problems are often associated with a mismatch between the thermal expansion coefficients of the electrolyte and cathode or as a consequence of oxygen loss from the cathode material.

The role of the cathode as the active site for the electrochemical reduction of oxygen is an important parameter to take into account in the material's design. SOFCs operate at high temperatures for long periods of time and, therefore, some requirements must be fulfilled [12]: (i) high electronic conductivity (σ > 100 S cm–1); (ii) a thermal expansion coefficient (TEC) match with other SOFC components; (iii) chemical compatibility with the electrolyte and interconnect materials; (iv) adequate porosity to allow mass transport of oxygen; (v) high thermal stability; (vi) high catalytic activity for the oxygen reduction reaction (ORR); and (vii) low cost.

The cathodic reaction is quite complex and compromises a number of single steps such as diffusion, adsorption, dissociation, ionization, and finally, incorporation of oxygen into the electrolyte [13]. Oxygen molecules adsorb on the surface where they form partially reduced ionic/atomic species. These electroactive species must be transported along surfaces, interfaces, or inside the bulk of the electrode material to the electrolyte, where they are fully incorporated as O2–.

The cathode materials can be classified into two groups: mainly, electronic conductors (with poor ionic conduction) and mixed ionic-electronic conductors (MIECs). Depending on the type of material, the conduction mechanism in the cathode will be different (Figure 3). In a poor ionic conductor, adsorption, dissociation, and diffusion of oxygen between the gas phase, electrode, and electrolyte [14] occurs through the triple phase boundary (TPB). In the MIECs, the conduction mechanism also occurs through the double phase boundary (DPB) [15,16]. The mixed conducting nature of the oxide ensures that electrochemical reactions occur at the MIEC/ gas double phase boundary (DPB).

**Figure 3.** Schematic representation of conduction mechanisms (DPB: double phase boundary; TPB: triple phase boun‐ dary).

The deposition technique is also an important parameter which influences the electrochemical behaviour. Conduction in porous electrodes occurs through the three-phase boundary (TPB), whereas in MIECs, this also occurs through the DPB. In the case of dense electrodes, there is no direct contact between the cathode, electrolyte, and gas (hence, no TPB conduction exists). In this case, the oxygen reduction reaction occurs anywhere on the cathode surface, forming oxide anions which diffuse into the bulk of the electrode material towards the electrolyte. The material must present mixed ionic and electronic conduction [17].

La1-*x*Sr*x*MnO3-*δ* (LSM) and La1-*x*Sr*x*CoO3-*δ* (LSC) have been widely used as cathodes in SOFC. LSM has high electronic conductivity due to the introduction of Sr2+ on the La3+ site which will be charge-compensated by the Mn (Mn3+–Mn4+), making it a good cathode at high temperatures (1000°C). At intermediate temperatures (500°C–700°C), however, a large increase in the cell overpotential (from <1 to 2000 ohm cm2 ) is observed, which hinders its use as a cathode [18]. One of the main limitations of LSM is its low ionic conductivity (~10–7 S/cm at 900°C), which is related to the Mn charge compensation where oxygen vacancies are not generated. The ease with which oxygen vacancies form is strongly influenced by the Sr content [19]. The influence of the cation vacancy on the transport properties of these materials has led to a study of the defects in these materials, such as LMO, and the effect of substitutions on the lanthanide site [20]. DeSouza et al. found that in both rhombic and rhombohedral structures, nonstoichiom‐ etry leads to the formation of vacancies on both cation positions, presenting a tendency toward the formation of vacancies [21]. These details are essential in understanding the behaviour of the materials and the degradation processes which can lead to the formation of insulating phases affecting the cathode performance. To obtain better performance from dense or porous LSM on YSZ electrolytes, a two-layer cathode is necessary—a 0.3 mm porous layer and in between a dense film of YSZ (~1 μm) [22]. This structure combines the best properties of both types of cathode in a single process and with more favorable properties than when using only the porous cathode.

Several authors have investigated other rare earths in the lanthanide position such as Pr, Sm, or Nd in order to improve the performance of manganites and avoid problems associated with the formation of the pyrochlore-insulating phases at the LSM/YSZ interface [23]. By substitut‐ ing La with Sm, new materials with lower energetic barriers for adsorption and diffusion of oxygen species have been obtained [24]. In addition, they present excellent compatibility with YSZ and electronic conductivity well above that required in a SOFC (100 S/cm) [25]. The most promising electrode materials are Pr0.7Sr0.3MnO3-*δ* and Nd0.7Sr0.3MnO3 in terms of thermal expansion coefficient, reactivity, and conductivity [26]. The study of the compatibility of (Ln, Sr)MnO3-*δ* with electrolytes has been limited to YSZ due to its widespread use. The performance of Nd1-*x*Sr*x*MnO3-*δ* with gadolinia-doped ceria electrolyte (most suitable for use in IT-SOFC), however, revealed no reaction between the components at the interface after prolonged hightemperature treatments [27]. The same authors indicated that Pr0.5Sr0.5MnO3-*δ* shows very promising properties with an electrical conductivity of 226 S/cm at 500°C [28]. Most studies on manganites have focused on doping the lanthanide site with Sr. In terms of chemical compatibility and electrical conductivity, however, the use of calcium as a dopant can offer very promising results, as in the case of Pr0.7Ca0.3MnO3-*δ*, which does not react with the electrolyte and has a maximum conductivity of 266 S/cm [29].

mixed conducting nature of the oxide ensures that electrochemical reactions occur at the MIEC/

**Figure 3.** Schematic representation of conduction mechanisms (DPB: double phase boundary; TPB: triple phase boun‐

The deposition technique is also an important parameter which influences the electrochemical behaviour. Conduction in porous electrodes occurs through the three-phase boundary (TPB), whereas in MIECs, this also occurs through the DPB. In the case of dense electrodes, there is no direct contact between the cathode, electrolyte, and gas (hence, no TPB conduction exists). In this case, the oxygen reduction reaction occurs anywhere on the cathode surface, forming oxide anions which diffuse into the bulk of the electrode material towards the electrolyte. The

La1-*x*Sr*x*MnO3-*δ* (LSM) and La1-*x*Sr*x*CoO3-*δ* (LSC) have been widely used as cathodes in SOFC. LSM has high electronic conductivity due to the introduction of Sr2+ on the La3+ site which will be charge-compensated by the Mn (Mn3+–Mn4+), making it a good cathode at high temperatures (1000°C). At intermediate temperatures (500°C–700°C), however, a large increase in the cell

One of the main limitations of LSM is its low ionic conductivity (~10–7 S/cm at 900°C), which is related to the Mn charge compensation where oxygen vacancies are not generated. The ease with which oxygen vacancies form is strongly influenced by the Sr content [19]. The influence of the cation vacancy on the transport properties of these materials has led to a study of the defects in these materials, such as LMO, and the effect of substitutions on the lanthanide site [20]. DeSouza et al. found that in both rhombic and rhombohedral structures, nonstoichiom‐ etry leads to the formation of vacancies on both cation positions, presenting a tendency toward the formation of vacancies [21]. These details are essential in understanding the behaviour of the materials and the degradation processes which can lead to the formation of insulating phases affecting the cathode performance. To obtain better performance from dense or porous LSM on YSZ electrolytes, a two-layer cathode is necessary—a 0.3 mm porous layer and in between a dense film of YSZ (~1 μm) [22]. This structure combines the best properties of both types of cathode in a single process and with more favorable properties than when using only

Several authors have investigated other rare earths in the lanthanide position such as Pr, Sm, or Nd in order to improve the performance of manganites and avoid problems associated with

) is observed, which hinders its use as a cathode [18].

material must present mixed ionic and electronic conduction [17].

overpotential (from <1 to 2000 ohm cm2

the porous cathode.

gas double phase boundary (DPB).

594 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

dary).

While looking for alternative materials to manganites, lanthanum cobaltites doped with bivalent metals such as Ca or Sr emerged [30]. These materials exhibit high conductivity values when 30% or 40% of Ca or Sr are added, respectively. In addition, the substitution of La by other lanthanides such as Pr and Sm has also lead to promising results. Despite the good results that these cobaltites exhibit, other drawbacks such as their high TECs and the appearance of secondary phases, limit their application.

In order to use the beneficial properties of these cobaltites, these materials are doped with Fe. Although, the addition of Fe promotes the decrease of the conductivity of the orthoferrites, lower TEC values are also achieved. Among these cobalto-ferrites, the Ba0.5Sr0.5Co0.8Fe0.2O3 material stands out for SOFC cathode application. Shao et al. reported excellent performance at intermediate temperatures, proving that the orthoferrites are an interesting alternative for IT-SOFCs [30]. At the same time, the use of La1-*x*Sr*x*Fe1-*y*Co*y*O3 has been widely investigated [31– 33]. These cobalt-based cathodes exhibit higher electrocatalytic performances than those of the conventional LSM cathodes. Unfortunately, they have high TEC, high cost of cobalt, and easy evaporation and reduction of cobalt [34,35].

Rare earth orthoferrites with perovskite-type structure are interesting materials for many electronic applications because of their mixed conductivity [36,37]. A site–doped rare earth orthoferrite compounds have been studied as candidates to replace manganites and cobaltite cathodes due to its high catalytic activity and mixed ionic and electronic conductivities at reduced temperature (*σ* total = 100 S/cm at 600°C–800°C) [35,38]. Lanthanum is a com‐ mon element and key component of most SOFC, where several of its oxides provide the necessary electronic conductivity and high catalytic activity for oxygen reduction and some, in addition, combine ionic and electronic conductivity [39]. The choice of using Pr as the rare earth element is due to its peculiarity to show 3+ and 4+ oxidation states, which might induce interesting electrical properties [35]. Some of the Pr-doped orthoferrites display good performance as cathode materials in high-temperature solid oxide fuel cells because of their mixed, electronic and ionic, conductivity [40]. In particular, Pr0.8Sr0.2Fe0.8Ni0.2O3−*<sup>δ</sup>* presents

low electrical resistivity and good oxygen ion conductivity [41]. Fe-based perovskite oxides have also attracted much attention as possible alternatives to cobaltites due to their interesting transport properties. When the Fe fraction is higher than 0.5, the materials exhibit its highest electronic conductivity as observed for the LaNi0.2Fe0.8O3 phase in which a conductivity of 135 S/cm, at 800°C, was obtained [42,43]. Calcium is another effective dopant for the A site, with low cost. For example, Pr0.4Ca0.6Fe0.8Ni0.2O3 material exhibits an ASR value of 0.09 ohm cm2 at 850°C [35]. Taking into account the low ASR value and the electrical conductivity of this cathode (at temperatures above 600°C, which is over 100 S/ cm), this material is a promising cathode for IT-SOFC applications. Pr1-*x*M*x*MnO3 (M = Ca, Sr) have also been studied where Ca-doped PrMnO3 phases exhibit better performance than those doped with Sr. Ca-doped materials show higher electrical conductivity, lower cathodic overpotential and more similar thermal expansion coefficient [44]. Moreover, calcium could be a good candidate because of the similarity of its ionic radius with La3+ giving rise to higher stability than that of strontium-substituted phases [45].

While a large number of studies concerning cathode materials report the presence of secondary phases, they are often left unidentified [46] and are typically considered undesirable and associated with insulating phases that give rise to low conductivity values and poor perform‐ ance [47]. Conversely, actual research in SOFC cathodes is focused on the development of composite materials, as the inherent requirements are so wide-ranging that no single material is capable of fulfilling every aspect [48]. The creation of composite cathodes is a good way to enhance the cathodic performance. They are composed of a solid electrolyte and an electronic conducting electrocatalytic material as in the La0.6Ca0.4Fe0.8Ni0.2O3-*δ*/samarium-doped ceria (SDC) symmetrical cells reported in a previous work [49]. The investigated composite cathodes include Sm0.5Sr0.5CoO3 (SSC)-Ce0.8Sm0.2O1.9 (SDC), La0.6Sr0.4Co0.2Fe0.8O3 (LSCF)-SDC, LSCF-Ce0.8Gd0.2O2 (CGO), and Ba0.5Sr0.5Co0.2Fe0.8O3 (BSCF)-SDC, all of which exhibit low polarization resistance at 600°C [50,51]. The electrochemical performance of these cathodes is generally governed by triple phase boundary (TPB) kinetics, mass transport, and ohmic drop. The primary disadvantages of utilizing composite materials arise from the need for two independ‐ ent materials; this increases both production complexity and the possibility of undesirable phases forming through inter-reaction of the component phases. Recently, the La1 *<sup>x</sup>*Ca*x*Fe0.8Ni0.2O3-*<sup>δ</sup>* (LCFN) system has been proposed and applied as a cathode featuring highly competitive performance [48]. Contrary to expectations, the formation of perovskite brown‐ millerite pseudo-composites resulted in a clear enhancement of the electrochemical perform‐ ance and excellent thermomechanical compatibility with conventional electrolytes. Furthermore, new insights are gained into material surface properties controlling oxygen reduction processes.

The development of double perovskite materials such as GdBaCo2O5+*x* [52], PrBaCo2O5+*x* [53], SmBaCo2O5+*δ* [54], or NdBaCo2O5+*δ* [55] have also been investigated for IT-SOFCs. Their ORR activity is higher than many single perovskites [56]. They present high thermal expansion coefficients, however, which make them incompatible with the electrolytes developed so far. Table 2 summarizes the most widely study cathode materials for SOFC.


**Table 2.** The main properties of SOFC cathodes with both single and double perovskite structures

In recent years, advancements in cathode material properties have been made where a variety of perovskite oxides with a wide range of properties have been investigated. There are still, however, many chemistries to investigate which may yield superior performance in the future.

## **3.2. Anode materials**

low electrical resistivity and good oxygen ion conductivity [41]. Fe-based perovskite oxides have also attracted much attention as possible alternatives to cobaltites due to their interesting transport properties. When the Fe fraction is higher than 0.5, the materials exhibit its highest electronic conductivity as observed for the LaNi0.2Fe0.8O3 phase in which a conductivity of 135 S/cm, at 800°C, was obtained [42,43]. Calcium is another effective dopant for the A site, with low cost. For example, Pr0.4Ca0.6Fe0.8Ni0.2O3 material exhibits an ASR value of 0.09 ohm cm2 at 850°C [35]. Taking into account the low ASR value and the electrical conductivity of this cathode (at temperatures above 600°C, which is over 100 S/ cm), this material is a promising cathode for IT-SOFC applications. Pr1-*x*M*x*MnO3 (M = Ca, Sr) have also been studied where Ca-doped PrMnO3 phases exhibit better performance than those doped with Sr. Ca-doped materials show higher electrical conductivity, lower cathodic overpotential and more similar thermal expansion coefficient [44]. Moreover, calcium could be a good candidate because of the similarity of its ionic radius with La3+ giving rise to

While a large number of studies concerning cathode materials report the presence of secondary phases, they are often left unidentified [46] and are typically considered undesirable and associated with insulating phases that give rise to low conductivity values and poor perform‐ ance [47]. Conversely, actual research in SOFC cathodes is focused on the development of composite materials, as the inherent requirements are so wide-ranging that no single material is capable of fulfilling every aspect [48]. The creation of composite cathodes is a good way to enhance the cathodic performance. They are composed of a solid electrolyte and an electronic conducting electrocatalytic material as in the La0.6Ca0.4Fe0.8Ni0.2O3-*δ*/samarium-doped ceria (SDC) symmetrical cells reported in a previous work [49]. The investigated composite cathodes include Sm0.5Sr0.5CoO3 (SSC)-Ce0.8Sm0.2O1.9 (SDC), La0.6Sr0.4Co0.2Fe0.8O3 (LSCF)-SDC, LSCF-Ce0.8Gd0.2O2 (CGO), and Ba0.5Sr0.5Co0.2Fe0.8O3 (BSCF)-SDC, all of which exhibit low polarization resistance at 600°C [50,51]. The electrochemical performance of these cathodes is generally governed by triple phase boundary (TPB) kinetics, mass transport, and ohmic drop. The primary disadvantages of utilizing composite materials arise from the need for two independ‐ ent materials; this increases both production complexity and the possibility of undesirable phases forming through inter-reaction of the component phases. Recently, the La1 *<sup>x</sup>*Ca*x*Fe0.8Ni0.2O3-*<sup>δ</sup>* (LCFN) system has been proposed and applied as a cathode featuring highly competitive performance [48]. Contrary to expectations, the formation of perovskite brown‐ millerite pseudo-composites resulted in a clear enhancement of the electrochemical perform‐ ance and excellent thermomechanical compatibility with conventional electrolytes. Furthermore, new insights are gained into material surface properties controlling oxygen

The development of double perovskite materials such as GdBaCo2O5+*x* [52], PrBaCo2O5+*x* [53], SmBaCo2O5+*δ* [54], or NdBaCo2O5+*δ* [55] have also been investigated for IT-SOFCs. Their ORR activity is higher than many single perovskites [56]. They present high thermal expansion coefficients, however, which make them incompatible with the electrolytes developed so far.

Table 2 summarizes the most widely study cathode materials for SOFC.

higher stability than that of strontium-substituted phases [45].

596 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

reduction processes.

The requirements for a SOFC anode are quite similar to those for cathodes (changing the oxidizing atmosphere by a reducing environment), including: (i) proper electronic conductiv‐ ity; (ii) thermomechanical and chemical compatibility with the electrolyte and interconnect materials; (iii) adequate porosity to allow gas transport to the reaction sites; and (iv) low cost. High ionic and electronic conductivity over a wide pO2 range and high surface oxygen exchange kinetics and good catalytic properties for the anode reactions are also desirable properties for a good anode material. Nickel-YSZ cermet has been widely used in the SOFC anode. This material fulfills most requirements: Ni exhibits good catalytic activity for the dissociation of hydrogen while remaining stable at operating conditions and YSZ provides structural support for the Ni particles and inhibits their coarsening while matching the thermal expansion properties of the rest of the cell components [87]. Areas where this anode compo‐ sition is lacking, however, include poor redox stability, low sulphur and carbon deposition tolerance when hydrocarbon fuels are used, and the tendency for nickel to agglomerate after prolonged operation [88]. In the search for alternative anode materials that are capable of withstanding sulphur contamination and carbon deposition, oxides with perovskite structure have drawn considerable attention [87]. Several authors have studied anode materials based on the perovskite structure as very promising alternatives [89,90]. Among the numerous materials with perovskite structure, SrTiO3-type titanates have received a great deal of attention due to their high stability under reducing atmospheres and high temperatures [91]. This material, however, suffers from poor electronic conductivity, but this can be overcome through the introduction of appropriate dopants in the structure [92]. Marina et al. studied the effect of La doping in SrTiO3 with a clear effect seen from the substitution of the Sr and the oxygen partial pressure on the total electrical conductivity with values as high as 500 S/cm at 500°C for the La0.3Sr0.7TiO3 phase [93]. Furthermore, doping with Sr in the A site and Nb in the B site, it was possible to increase the conductivity to 10 S/cm; however, these materials exhibited a poor ionic conductivity [94]. Ti has also been substituted by other cations such as Al, Fe, Ga, Mg, Mn, or Sc, affecting significant changes on the redox properties of the material and conductivities [95]. By adding multivalent cations such as Mn or Fe, reduction or oxidation occurs in these cations in preference to or together with Ti, leading to a general decrease in the presence of Ti (III), resulting in a decrease in conductivity [96]. The effect of Co has also been analyzed, observing that after reduction with H2, the segregation of Co nanoparticles occurs on the anode surface, which favors the oxidation of the fuel, thus reducing the resistance under anodic polarization [97]. To improve the behaviour of titanates, various dopants have been introduced on the A site (Y, La, Ce) [98,99]. The introduction of Ce3+ leads to decomposition into a variety of phases due to its ready oxidation in air. Some Ce-rich phases migrate to the grain boundaries, which result in an increase in the catalytic properties of up to an order of magnitude [100]. It is particularly interesting to note the results obtained by Morales-Ruiz et al., where (La,Sr)(Ti,M)O3 (M = Ga, Mn) anodes exhibited comparable performance to those of conventional Ni-YSZ anodes [101]. Recently, AMoO3 perovskite types (A = Ca, Sr, and Ba), containing Mo4+ ions have received interest for their potential use as anodes [102]. While these materials have high electrical conductivities (104 S/cm) they also present problems associated with the diffusion of oxide ions [102]. This limitation can be overcome by doping with other metals such as Fe or Cr on the Mo site [103,104]. Co is another suitable dopant which also creates oxygen vacancies, supplying sufficient ionic transport making a material with excellent catalytic properties for the oxidation of hydrogen [105]. Another alternative is to create composites with materials which have high ionic conductivity [106]. In this regard, the most commonly used materials are Y0.08Zr0.92O2 (YSZ), La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM), and Gd0.1Ce0.9O2 (GDC), which have been traditionally used as electrolytes, facilitating greater thermal, mechanical, and chemical compatibility with the anode composite. From these systems, a new material with the general formula Sr2Fe1.5Mo0.5O6 (SFMO) and perovskite-type structure has been developed, which features suitable mixed ionic-electronic conductivity and interesting catalytic activity [107]. It exhibits great stability when H2, fuel or natural gas is used and high tolerance to sulfides [108]. Recently, Sutirakun et al. developed a theoretical model for analyzing the electrochemical oxidation of H2 on the surface of this material [109]. One strategy for optimizing the material is to increase either the Mo content or add a small amount of active transition metal such as Ni in order to reduce the energy required for the formation of vacancies on the surface of SFMO. The other family of materials which has been widely studied are the chromites (LaCrO3). These materials initially developed as interconnectors for SOFCs, which will be discussed later, because of their high stability in both oxidizing and reducing atmospheres and low tendency to accumulate carbon [110]. Their low catalytic activity, however, makes their use with some fuels such as methane impossible. To solve this problem, the effect of different dopants on the properties of this material (Ca and Sr on the A site and Mg, Mn, Fe, Co, and Ni in the B site) has been studied [111]. (La, Sr) (Cr, Fe)O3 is the best candidate due to its high redox stability, high conductivity, and electrochemical activity [111]. In addition, this material works well as a catalyst for methane [112] and has a thermal expansion coefficient very similar to YSZ and LSGM [113]. Fowler et al. have also studied these materials, finding that the La0.6Sr0.4Cr0.4Fe0.6O3-*δ* composition presents the best performance and highest stability [114]. In recent years, a new generation of anode materials with the double perovskite structure, such as Sr2MgMoO6 (SMMO) [115], PrBaM2O5 (M = Co, Mn) [116,117], and La4SrTi5O17 [118] have been developed. These materials have very interesting properties which permit the use of multiple fuels.

anode. This material fulfills most requirements: Ni exhibits good catalytic activity for the dissociation of hydrogen while remaining stable at operating conditions and YSZ provides structural support for the Ni particles and inhibits their coarsening while matching the thermal expansion properties of the rest of the cell components [87]. Areas where this anode compo‐ sition is lacking, however, include poor redox stability, low sulphur and carbon deposition tolerance when hydrocarbon fuels are used, and the tendency for nickel to agglomerate after prolonged operation [88]. In the search for alternative anode materials that are capable of withstanding sulphur contamination and carbon deposition, oxides with perovskite structure have drawn considerable attention [87]. Several authors have studied anode materials based on the perovskite structure as very promising alternatives [89,90]. Among the numerous materials with perovskite structure, SrTiO3-type titanates have received a great deal of attention due to their high stability under reducing atmospheres and high temperatures [91]. This material, however, suffers from poor electronic conductivity, but this can be overcome through the introduction of appropriate dopants in the structure [92]. Marina et al. studied the effect of La doping in SrTiO3 with a clear effect seen from the substitution of the Sr and the oxygen partial pressure on the total electrical conductivity with values as high as 500 S/cm at 500°C for the La0.3Sr0.7TiO3 phase [93]. Furthermore, doping with Sr in the A site and Nb in the B site, it was possible to increase the conductivity to 10 S/cm; however, these materials exhibited a poor ionic conductivity [94]. Ti has also been substituted by other cations such as Al, Fe, Ga, Mg, Mn, or Sc, affecting significant changes on the redox properties of the material and conductivities [95]. By adding multivalent cations such as Mn or Fe, reduction or oxidation occurs in these cations in preference to or together with Ti, leading to a general decrease in the presence of Ti (III), resulting in a decrease in conductivity [96]. The effect of Co has also been analyzed, observing that after reduction with H2, the segregation of Co nanoparticles occurs on the anode surface, which favors the oxidation of the fuel, thus reducing the resistance under anodic polarization [97]. To improve the behaviour of titanates, various dopants have been introduced on the A site (Y, La, Ce) [98,99]. The introduction of Ce3+ leads to decomposition into a variety of phases due to its ready oxidation in air. Some Ce-rich phases migrate to the grain boundaries, which result in an increase in the catalytic properties of up to an order of magnitude [100]. It is particularly interesting to note the results obtained by Morales-Ruiz et al., where (La,Sr)(Ti,M)O3 (M = Ga, Mn) anodes exhibited comparable performance to those of conventional Ni-YSZ anodes [101]. Recently, AMoO3 perovskite types (A = Ca, Sr, and Ba), containing Mo4+ ions have received interest for their potential use as anodes [102]. While these

598 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

with the diffusion of oxide ions [102]. This limitation can be overcome by doping with other metals such as Fe or Cr on the Mo site [103,104]. Co is another suitable dopant which also creates oxygen vacancies, supplying sufficient ionic transport making a material with excellent catalytic properties for the oxidation of hydrogen [105]. Another alternative is to create composites with materials which have high ionic conductivity [106]. In this regard, the most commonly used materials are Y0.08Zr0.92O2 (YSZ), La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM), and Gd0.1Ce0.9O2 (GDC), which have been traditionally used as electrolytes, facilitating greater thermal, mechanical, and chemical compatibility with the anode composite. From these systems, a new material with the general formula Sr2Fe1.5Mo0.5O6 (SFMO) and perovskite-type structure has been developed, which features suitable mixed ionic-electronic conductivity and interesting catalytic activity [107]. It exhibits great stability when H2, fuel or natural gas is used

S/cm) they also present problems associated

materials have high electrical conductivities (104

## **4. Perovskite oxides as electrolytes and interconnectors for SOFC**

Electrolyte ceramic materials must meet the following requirements: high ionic conductivity and low electronic conductivity; the charge carrier must either be from the oxidizer (O2) or fuel (H2), i.e. O2– or protons [119]. To date, SOFC electrolytes are, in most commercial cases, 8 mol % yttria stabilized zirconia (8-YSZ) [120]. This material belongs to the group of fluorite-type solid solutions and up to now is the O2– ion conductor most used as electrolyte material for SOFC due to its high ionic conductivity and thermomechanical stability. Doping ZrO2 with Y2O3 has two main functions. First, to stabilize the cubic, fluorite phase, otherwise only stable at elevated temperatures, and second, to compensate the insertion of the trivalent Y3+ ions by oxygen vacancies in the zirconia lattice, giving rise to an enhancement of the oxygen ion conductivity. Generally, yttria-doped zirconia with 8 mol% Y2O3 exhibit an ionic conductivity higher than 0.1 S/cm at 1000°C and an electrical conductivity lower than 10–4 S/cm [121].

An important aspect to be considered is the chemical stability of the candidate materials at cell operation conditions, presenting high ionic conductivity (>0.1 S/cm) at intermediate temper‐ atures and thermal expansion coefficients similar to the components of the cell [119]. The decrease in operating temperature can lead to an increase in chemical stability and cell lifetime, as well as a reduction in manufacturing costs; however, in the case of YSZ, cell performance decreases at lower temperatures due to the thermally activated ionic conductivity [122].

In order to obtain materials with improved properties at temperatures below 800°C, two families with perovskite type structure: gallates and cerates have been developed [123]. Most studies focus on Sr-doped LaGaO3, but other dopants such as Ba have also been investigated [124]. Phase segregation occurs when Ba is used due to its large size compared to Sr which forms a mixture of LaBaGa3O7 and LaGaO3 resulting in poor conductivities [125]. The ionic conductivity of this material can be increased by introducing Mg in the perovskite B site due to the introduction of compensating defects in the structure [126,127]. It has also been shown that the presence of a stoichiometry deficient in Ga may result in an increase in the concen‐ tration of oxygen vacancies [128]. In fact, this material is stable in CO-rich atmospheres, which allows its use as an electrolyte in direct coal solid oxide fuel cells (DC-SOFCs) [129]. Another strategy to increase the ionic conductivity of the material is through the production of composites with doped ceria electrolytes [130]. Through this method, it has been possible to optimize conduction through the grain boundaries, resulting in up to 10 times higher conduc‐ tivity than ceria electrolytes at 500°C. In summary, the cells employing (La,Sr)(Ga,Mg)O3-*<sup>δ</sup>* electrolytes have high power densities at 800°C, which makes them excellent candidates for IT-SOFC electrolytes [131,132].

The other family of materials with perovskite structures is based on BaZrO3 and BaCeO3, which traditionally have been developed as proton-conducting oxides, especially the cerates [133]. The production method, the temperatures employed in the process and the type and concen‐ tration of the dopant will affect proton conduction in the material. The cerates allow the introduction of precious metals such as Pd, which facilitates oxygen mobility, although the best results were obtained by using metal nanoparticles and by decorating the surface of the perovskite which acts as active catalysts [134]. Other dopants, both divalent and trivalent (Y, Yb, Gd, Sm, Nd, and La) have been employed [135–137]. This makes it possible to design materials resistant to reducing atmospheres with high conductivity and thermal and chemical stability in IT-SOFC operating conditions, even at temperatures as low as 500°C.

SOFC interconnect material requirements are as follows: (i) electronic conductivity > 100 S/cm; (ii) ionic transport number < 0.01 to avoid chemical shortcut permeation; (iii) gas tight; (iv) tolerate both reducing (H2) and oxidizing (air/O2) atmospheres; (v) be compatible with anode and cathode electrode materials (TEC and chemistry); and (vi) mechanical strength. LaCrO3 chromites are the most widely used SOFC interconnects and, doped with other elements, their properties are shown to be improved [138]. The dopants most widely employed in the A site of the perovskite are Sr, Mg, and Ca, with dissolution limits of 50% [139], 15% [140], and 50% [141], respectively. Furthermore, the B site is also doped with Co or Fe, in order to limit the Cr content as much as possible due to its volatility [142]. Doped lanthanum chromites seem ideal materials for use as interconnects as they are highly stable in both oxidizing and reducing atmospheres and, at SOFC operating temperatures, do not react with the other cell compo‐ nents. Regarding thermal compatibility, through appropriate doping, it is possible to tailor the thermal expansion coefficient to that of the other components of the cell. LaCrO3 is a p-type conductor that, upon divalent cation substitution on the La site, is seen to charge compensate by a valence change in the Cr (Cr3+–Cr4+), accompanied by an increase in the electronic conductivity of the material. The main problem associated with these compounds is their sinterability, as at high temperatures and under high oxygen pressure, volatilization of chromium oxide can occur. To avoid this, various strategies such as more reactive synthesis methods at lower temperatures, the introduction of cationic vacancies, and new fabrication techniques (including microwave sintering, freeze drying with EDTA, and more) have been analyzed [143].

## **5. Nanostructured perovskites for improving solid oxide fuel cells**

forms a mixture of LaBaGa3O7 and LaGaO3 resulting in poor conductivities [125]. The ionic conductivity of this material can be increased by introducing Mg in the perovskite B site due to the introduction of compensating defects in the structure [126,127]. It has also been shown that the presence of a stoichiometry deficient in Ga may result in an increase in the concen‐ tration of oxygen vacancies [128]. In fact, this material is stable in CO-rich atmospheres, which allows its use as an electrolyte in direct coal solid oxide fuel cells (DC-SOFCs) [129]. Another strategy to increase the ionic conductivity of the material is through the production of composites with doped ceria electrolytes [130]. Through this method, it has been possible to optimize conduction through the grain boundaries, resulting in up to 10 times higher conduc‐ tivity than ceria electrolytes at 500°C. In summary, the cells employing (La,Sr)(Ga,Mg)O3-*<sup>δ</sup>* electrolytes have high power densities at 800°C, which makes them excellent candidates for

600 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

The other family of materials with perovskite structures is based on BaZrO3 and BaCeO3, which traditionally have been developed as proton-conducting oxides, especially the cerates [133]. The production method, the temperatures employed in the process and the type and concen‐ tration of the dopant will affect proton conduction in the material. The cerates allow the introduction of precious metals such as Pd, which facilitates oxygen mobility, although the best results were obtained by using metal nanoparticles and by decorating the surface of the perovskite which acts as active catalysts [134]. Other dopants, both divalent and trivalent (Y, Yb, Gd, Sm, Nd, and La) have been employed [135–137]. This makes it possible to design materials resistant to reducing atmospheres with high conductivity and thermal and chemical

SOFC interconnect material requirements are as follows: (i) electronic conductivity > 100 S/cm; (ii) ionic transport number < 0.01 to avoid chemical shortcut permeation; (iii) gas tight; (iv) tolerate both reducing (H2) and oxidizing (air/O2) atmospheres; (v) be compatible with anode and cathode electrode materials (TEC and chemistry); and (vi) mechanical strength. LaCrO3 chromites are the most widely used SOFC interconnects and, doped with other elements, their properties are shown to be improved [138]. The dopants most widely employed in the A site of the perovskite are Sr, Mg, and Ca, with dissolution limits of 50% [139], 15% [140], and 50% [141], respectively. Furthermore, the B site is also doped with Co or Fe, in order to limit the Cr content as much as possible due to its volatility [142]. Doped lanthanum chromites seem ideal materials for use as interconnects as they are highly stable in both oxidizing and reducing atmospheres and, at SOFC operating temperatures, do not react with the other cell compo‐ nents. Regarding thermal compatibility, through appropriate doping, it is possible to tailor the thermal expansion coefficient to that of the other components of the cell. LaCrO3 is a p-type conductor that, upon divalent cation substitution on the La site, is seen to charge compensate by a valence change in the Cr (Cr3+–Cr4+), accompanied by an increase in the electronic conductivity of the material. The main problem associated with these compounds is their sinterability, as at high temperatures and under high oxygen pressure, volatilization of chromium oxide can occur. To avoid this, various strategies such as more reactive synthesis methods at lower temperatures, the introduction of cationic vacancies, and new fabrication techniques (including microwave sintering, freeze drying with EDTA, and more) have been

stability in IT-SOFC operating conditions, even at temperatures as low as 500°C.

IT-SOFC electrolytes [131,132].

analyzed [143].

Nowadays, the research in solid oxide fuel cells (SOFCs) is focused on lowering the operating temperature below 800°C in order to overcome problems such as the ageing of the materials. Concurrently, lowering the operating temperature has the detrimental consequences of decreasing the rate of surface reactions and bulk diffusion in the cathode, giving rise to a worsening of the cell performance. Surface exchange and ionic conduction must therefore be improved to maximize the yield of the cathode reaction. This improvement can be carried out not only by new material selection but also by the detailed control of the microstructure [144]. As stated before, a common strategy for improving the electrochemical performance of the electrodes is the fabrication of composite materials, combining ionic and mixed ionic electronic conducting materials, where the ionic conductor is homogeneously distributed [49]. Using composites, the ionic conductivity across the electrode will be enhanced and, simultaneously, a higher thermomechanical compatibility with the electrolyte can be achieved. The catalytic activity of the material is associated with its microstructure, so if the latter is improved, the active surface area will be increased, and a higher electrochemical durability will be obtained [145]. A significant number of studies have also been conducted to increase active surface area via microstructural control: the use of organic materials as pore formers, template materials such as colloidal crystals, meshes, foams or microfibers, glassy carbon microspheres, or membrane-based templates [146]. These methods look for higher catalytic activities for electrode reactions and lower annealing temperatures due to the numerous active sites and large surface areas. The same goal can be achieved using a new tool that has been developed during the last two decades: nanotechnology [147].

Recently, nanotechnology has been shown to overturn many established theories in a wide range of scientific fields, often with highly desirable properties [148]. This has naturally resulted in a great deal of interest from both scientific and industrial communities in the properties of nanostructured materials. This interest arises from new and unexpected behav‐ iour when compared with bulk materials. Enhancements have been reported in electrical and ionic conductivity, chemical reactivity, and other properties. The shift from bulk to nanoma‐ terials is therefore a promising approach in the development of new advanced technologies capable of achieving higher performance and low environmental impact.

Nanomaterials have typically been considered for low-temperature devices, as high temper‐ atures could promote coarsening and therefore loss of the desired properties. This has generally kept nanomaterials from application in solid oxide fuel cells (SOFCs), but constant material advances have led to the decrease in their operational temperature. These so-called intermediate temperature SOFCs (IT-SOFCs), operating at 500°C–850°C, allow new possibili‐ ties for the use of nanomaterials.

The high surface area to volume ratio inherent in nanomaterials provides a large active area for SOFC electrodes. Nanomaterials are defined as a range of materials where at least one dimension is below 100 nm, resulting in nanostructures in zero- to three-dimensions. Nano‐ particles, 0-D in nature, are well suited for use in composite electrodes, 1-D nanostructures, including nanotubes and nanowires, are thought to operate as promising electrodes, and thin films, 2-D in nature are of interest for application in micro-SOFCs (μ-SOFCs) and interlayers. Although the electrical properties of these nanostructures have been analyzed as independent 0- or 1-dimensional structures, the 2- or 3-dimesional behavior is more frequently reported, as it provides reliable results.

When working with SOFCs, there are two main approaches to develop nanomaterials with improved properties: optimizing existing composites by transforming them to nanocompo‐ sites and exploring novel nanostructured materials with high mixed ionic and electronic conductivity. In order to fabricate nanocomposites, wet impregnation/infiltration has been gaining increasing attention in recent years [149]. The infiltration process consists of placing a drop of a metal salt solution, with the chemistry required to give rise to the MIEC electrode material after decomposition, on top of a porous material (typically the ionic conductor) [150].

In parallel, MIEC nanostructured materials have also been developed in order to find further optimization of current state-of-the-art electrode materials. This kind of material features one dimension below 100 nm, giving rise to different structures such as nanoparticles, nanotubes, nanofibers, etc. There are several techniques for obtaining these unique phase nanomaterials with enhanced active areas and lower particle sizes with one of the most commonly used being via templating. Only some examples utilizing this procedure will be shown, although a large number of articles have been published on this topic. However, the excellent review by Ruiz-Morales et al. is recommended for those interested in a more detailed description of this method [146]. There are just a few materials able to act as templates because they must fulfill requirements such as removability, compatibility with the process conditions, wettability with the network forming precursor solution and a narrow particle size distribution to achieve optimal packing. Some of the most interesting templates are the organic polymer spheres of polycarbonate (PC), polystyrene (PS), and polymethyl methacrylate (PMMA) [151]. It has been demonstrated that templated porosity is maintained and highly influences electrochemical behavior, presenting an effective means of enhancing the triple phase boundary (TPB), and thus improving cell performance [152]. Another interesting method to obtain economical nanostructures is based on the use of carbon nanotubes (CNTs) as particle growth controller templates [153,154]. The growth controller material must fulfill the following two character‐ istics for the desired application: thermal stability and either an ease of removal or sufficient electrical properties to form a composite material. This facile and economical route allows to synthesize perovskite nanoparticles with grain sizes as small as 16 nm and surface areas of 151 m2 /g, improving electrochemical performance of the electrode by approximately one order of magnitude [153].

Of all possible nanostructures, nanotube-shaped materials seem to exhibit the most interesting improvements to electrode performance [155]. Although inorganic nanowires have been synthesized by several methods such as hydrothermal reaction [156], vapor transport [157], and electrospinning [158], the complexity of perovskite nanotube synthesis has resulted in the template-assisted synthesises becoming the most employed method. There are currently several membranes with different properties which can be used as templates, with anodized alumina (AAO) membranes being the most common [159]. The most important advantage of these membranes is their thermal stability, which allows the control of the morphology at high temperatures. Their high cost and the problems associated with their anodization do, however, limit their use. These membranes are classified in the literature as hard membranes while there is another group of templates known as soft membranes, which are primarily polymeric in nature. While not costly, they decompose between 200°C and 300°C, which means that the morphology cannot be completely controlled at higher synthesis temperatures. There are several types of polymeric membranes, with polycarbonates being the most commonly used [160]. The use of the pore wetting technique with polycarbonate membranes as templates and subsequent freeze-drying allows the fabrication of highly ordered three-dimensional nano‐ structures [161]. The electrospinning technique has also been used for the production of MIEC lanthanum strontium cobalt orthoferrite nanofibers [162]. This nanofiber-based cathode architecture is highly stable at intermediate temperatures (600°C–800°C) and provides continuous pathways for charge transport throughout the cathode.

Finally, there is a third generation of materials consisting of nanostructured nanocomposites. An improvement in oxygen reduction reaction (ORR) activity was reported when LSM nanoparticles were loaded on a porous YSZ framework [163–166]. Also, electrocatalytic nanoparticles could be produced in oxide anodes for solid oxide fuel cell (SOFC) by an exsolution method, i.e., by incorporating metals into a perovskite oxide phase in air followed by the reduction of the perovskite oxide [167]. The improvement in the performance of the cell by using these nanocomposites lies in the extension of the TPB to the newly generated surfaces [168]. Templates such as polycarbonate membranes have also been used for the production of composite nanotubes with 20 nm wall thicknesses [169]. With this strategy, a clear decrease in polarization resistance of the electrode is observed, giving rise to higher efficiencies at temperatures as low as 700°C. Recently, an electrolyte-supported SOFC was fabricated with all-nanocomposite components and operated below 600°C [170]. The highly active nanocom‐ posite electrodes and easily sintered nanocomposite electrolyte allow an in situ low-temper‐ ature sintering while preserving the microstructure and electrochemical performance stability upon thermal cycling.

The performance of intermediate temperature SOFCs can be improved by engineering the electrode architecture on the nanoscale. Lowering the temperature facilitates the use of nanotechnology in synthesizing new nanostructured materials in which parameters such as porosity, the distribution of generated pores, and surface area can be closely controlled. These parameters have a significant influence on the performance of the materials used for energy conversion and storage, which means that these methods are an important starting point for the design and optimization of these types of energy devices.

## **6. Conclusions**

films, 2-D in nature are of interest for application in micro-SOFCs (μ-SOFCs) and interlayers. Although the electrical properties of these nanostructures have been analyzed as independent 0- or 1-dimensional structures, the 2- or 3-dimesional behavior is more frequently reported, as

When working with SOFCs, there are two main approaches to develop nanomaterials with improved properties: optimizing existing composites by transforming them to nanocompo‐ sites and exploring novel nanostructured materials with high mixed ionic and electronic conductivity. In order to fabricate nanocomposites, wet impregnation/infiltration has been gaining increasing attention in recent years [149]. The infiltration process consists of placing a drop of a metal salt solution, with the chemistry required to give rise to the MIEC electrode material after decomposition, on top of a porous material (typically the ionic conductor) [150].

In parallel, MIEC nanostructured materials have also been developed in order to find further optimization of current state-of-the-art electrode materials. This kind of material features one dimension below 100 nm, giving rise to different structures such as nanoparticles, nanotubes, nanofibers, etc. There are several techniques for obtaining these unique phase nanomaterials with enhanced active areas and lower particle sizes with one of the most commonly used being via templating. Only some examples utilizing this procedure will be shown, although a large number of articles have been published on this topic. However, the excellent review by Ruiz-Morales et al. is recommended for those interested in a more detailed description of this method [146]. There are just a few materials able to act as templates because they must fulfill requirements such as removability, compatibility with the process conditions, wettability with the network forming precursor solution and a narrow particle size distribution to achieve optimal packing. Some of the most interesting templates are the organic polymer spheres of polycarbonate (PC), polystyrene (PS), and polymethyl methacrylate (PMMA) [151]. It has been demonstrated that templated porosity is maintained and highly influences electrochemical behavior, presenting an effective means of enhancing the triple phase boundary (TPB), and thus improving cell performance [152]. Another interesting method to obtain economical nanostructures is based on the use of carbon nanotubes (CNTs) as particle growth controller templates [153,154]. The growth controller material must fulfill the following two character‐ istics for the desired application: thermal stability and either an ease of removal or sufficient electrical properties to form a composite material. This facile and economical route allows to synthesize perovskite nanoparticles with grain sizes as small as 16 nm and surface areas of 151

/g, improving electrochemical performance of the electrode by approximately one order of

Of all possible nanostructures, nanotube-shaped materials seem to exhibit the most interesting improvements to electrode performance [155]. Although inorganic nanowires have been synthesized by several methods such as hydrothermal reaction [156], vapor transport [157], and electrospinning [158], the complexity of perovskite nanotube synthesis has resulted in the template-assisted synthesises becoming the most employed method. There are currently several membranes with different properties which can be used as templates, with anodized alumina (AAO) membranes being the most common [159]. The most important advantage of these membranes is their thermal stability, which allows the control of the morphology at high

it provides reliable results.

602 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

m2

magnitude [153].

The great versatility of the perovskite structure allows for different doping to obtain a variety of properties, which is a key feature in the development of materials for solid oxide fuel cells (SOFCs). This chapter not only briefly introduces the foundation and operation of SOFCs but also shows the evolution of perovskite materials for device components. Among the electro‐ lytes, BaCeO3-based cerates exhibit the highest ionic conduction at intermediate temperatures while chromites doped with Sr find potential as interconnector materials. By introducing Fe in the Cr site, these chromites become interesting alternatives to the traditional NiO anode, in addition to the new trend of double perovskite-type materials such as Sr2MgMoO6, which present very promising results for anode application. It is in the area of SOFC cathodes in which the most progress has been made. Initially, in the 1960s, doped manganese perovskites (La1-*x*Sr*x*MnO3—LSM) were used as cathodes, but the low ionic conduction forced the devel‐ opment of new materials based on Co and/or Fe-containing perovskites. In addition, they have mixed ionic-electronic conductivity, allowing the active electrode area to extend across the surface. This leaves Ba0.5,Sr0.5,Co0.8,Fe0.2O3 perovskite as the most promising cathode material, although as in the case of anodes, double perovskites are also alternative materials with interesting properties.

Finally, it is worth mentioning the importance of not only the material composition but also its structure, morphology and porosity. It has been found that a higher catalytic electrode area results in a significant improvement in the electrochemical system efficiency. Reducing operating temperatures of the SOFC has allowed nanotechnology to become a useful tool for the development of future generations of materials for IT-SOFCs.

## **Acknowledgements**

This work has been partially financed by the Ministerio de Educación y Ciencia under project MAT2013-41128-R and by the Eusko Jaurlaritza/Gobierno Vasco under project IT-570-13. N. Ortiz-Vitoriano acknowledges a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007– 2013).

## **Author details**

Idoia Ruiz de Larramendi1 , Nagore Ortiz-Vitoriano2 , Isaen B. Dzul-Bautista1,3 and Teófilo Rojo1,2

\*Address all correspondence to: idoia.ruizdelarramendi@ehu.es

1 Departamento de Química Inorgánica, Universidad del País Vasco UPV/EHU, Bilbao, Spain

2 CIC energiGUNE, Parque Tecnológico de Álava, Miñano, Spain

3 Centro de Innovación, Investigación y Desarrollo en Ingeniería y Tecnología, PIIT Monterrey, CP, Apodaca, Nuevo León, Mexico

## **References**

lytes, BaCeO3-based cerates exhibit the highest ionic conduction at intermediate temperatures while chromites doped with Sr find potential as interconnector materials. By introducing Fe in the Cr site, these chromites become interesting alternatives to the traditional NiO anode, in addition to the new trend of double perovskite-type materials such as Sr2MgMoO6, which present very promising results for anode application. It is in the area of SOFC cathodes in which the most progress has been made. Initially, in the 1960s, doped manganese perovskites (La1-*x*Sr*x*MnO3—LSM) were used as cathodes, but the low ionic conduction forced the devel‐ opment of new materials based on Co and/or Fe-containing perovskites. In addition, they have mixed ionic-electronic conductivity, allowing the active electrode area to extend across the surface. This leaves Ba0.5,Sr0.5,Co0.8,Fe0.2O3 perovskite as the most promising cathode material, although as in the case of anodes, double perovskites are also alternative materials with

Finally, it is worth mentioning the importance of not only the material composition but also its structure, morphology and porosity. It has been found that a higher catalytic electrode area results in a significant improvement in the electrochemical system efficiency. Reducing operating temperatures of the SOFC has allowed nanotechnology to become a useful tool for

This work has been partially financed by the Ministerio de Educación y Ciencia under project MAT2013-41128-R and by the Eusko Jaurlaritza/Gobierno Vasco under project IT-570-13. N. Ortiz-Vitoriano acknowledges a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–

1 Departamento de Química Inorgánica, Universidad del País Vasco UPV/EHU, Bilbao,

3 Centro de Innovación, Investigación y Desarrollo en Ingeniería y Tecnología, PIIT

, Isaen B. Dzul-Bautista1,3 and

, Nagore Ortiz-Vitoriano2

\*Address all correspondence to: idoia.ruizdelarramendi@ehu.es

2 CIC energiGUNE, Parque Tecnológico de Álava, Miñano, Spain

Monterrey, CP, Apodaca, Nuevo León, Mexico

the development of future generations of materials for IT-SOFCs.

604 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

interesting properties.

**Acknowledgements**

2013).

**Author details**

Teófilo Rojo1,2

Spain

Idoia Ruiz de Larramendi1


[29] Rim HR, Jeung SK, Jung E, Lee JS. Characteristics of Pr1−xMxMnO3 (*M* = Ca, Sr) as cathode material in solid oxide fuel cells. Mater. Chem. Phys. 1998;52:54–59.

[15] Ciucci F, Chueh WC, Goodwin DG, Haile SM. Surface reaction and transport in mixed conductors with electrochemically-active surfaces: a 2-D numerical study of

[16] Muñoz-García AB, Ritzmann AM, Pavone M, Keith JA, Carter EA. Oxygen transport in perovskite-type solid oxide fuel cell materials: insights from quantum mechanics.

[17] Ruiz de Larramendi I, Vivès S, Ortiz-Vitoriano N, Ruiz de Larramendi JI, Arriortua MI, Rojo T. La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 thin films obtained by pulsed laser ablation: effect of the substrate on the electrochemical behaviour. Solid State Ionics. 2011;192:584–

[19] Pavone M, Muñoz-García AB, Ritzmann AM, Carter EA. First-principles study of lanthanum strontium manganite: insights into electronic structure and oxygen va‐

[20] Takeda, Y.; Sakaki, Y.; Ichikawa, T.; Imanishi, N.; Yamamoto, O.; Mori, M.; Mori, N. y Abe, T. Stability of La1−xAxMnO3−z (A=Ca, Sr) as cathode materials for solid oxide fuel

[21] DeSouza RA, Islam MS, Ivers-Tiffee EJ. Formation and migration of cation defects in

[22] Herle JV, McEvoy AJ, Thampi KR. A study on the La1−xSrxMnO3 oxygen cathode.

[23] Skinner SJ. Recent advances in perovskite-type materials for solid oxide fuel cell

[24] Choi Y, Choi MC, Liu M. Rational design of novel cathode materials in solid oxide fuel cells using first-principles simulations. J. Power Sources. 2010;195:1441–1445. [25] Wen TL, Tu H, Xu Z, Yamamoto O. A study of (Pr,Nd,Sm)SrMnO cathode materials

[26] Sakaki Y, Takeda Y, Kato A, Imanishi N, Yamamoto O, Hattori M, Iio M, Esaki Y. Ln1−xSrxMnO3 (Ln = Pr, Nd, Sm and Gd) as the cathode material for solid oxide fuel

[27] Kostogloudis GC, Ftikos C. Characterization of Nd1-xSrxMnO3±δ SOFC cathode materi‐

[28] Kostogloudis GC, Vasihkos N, Ftikos C. Preparation and characterization of Pr1 xSrxMnO3 ± δ (*x* = 0, 0.15, 0.3, 0.4, 0.5) as a potential SOFC cathode material operating at intermediate temperatures (500–700°C). J. Eur. Ceram. Soc. 1997;17:1513–1521.

[18] Jacobson AJ. Materials for solid oxide fuel cells. Chem. Mater. 2010;22:660–674.

cancy formation. J. Phys. Chem. C. 2014;118:13346–13356.

the perovskite oxide LaMnO3. Mater. Chem. 1999;9:1621–1627.

for solid oxide fuel cell. Solid State Ionics. 1999;121:25–30.

ceria. Phys. Chem. Chem. Phys., 2011;13:2121–2135.

Acc. Chem. Res. 2014;47:3340–3348.

606 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

cells. Solid State Ionics. 1994;72:257–264.

Electrochim. Acta. 1996;41:1447–1454.

Cathodes. Int. J. Inorg. Mater. 2001;3:113–121.

cells. Solid State Ionics. 1999;118:187–194.

als. J. Eur. Ceram. Soc. 1999;19:497–505.

590.


material for intermediate-temperature solid oxide fuel cells. J. Power Sources. 2009;194:704–711

[55] Zhao L, He BB, Xun ZQ, Wang H, Peng RR, Meng GY, Liu XQ. Characterization and evaluation of NdBaCo2O5+δ cathode for proton-conducting solid oxide fuel cells. Int. J. Hydrogen Energ. 2010;35:753–756.

[42] Chiba R, Yoshimura F, Sakurai Y. An investigation of LaNi1−xFexO3 as a cathode ma‐

[43] Ruiz de Larramendi I, Ortiz N, López-Antón R, Ruiz de Larramendi JI, Rojo T. Struc‐ ture and impedance spectroscopy of La0.6Ca0.4Fe0.8Ni0.2O3−δ thin films grown by pulsed

[44] El-Himri A, Marrero-López D, Ruiz-Morales JC, Peña-Martínez J, Núñez P. Structur‐ al and electrochemical characterisation of Pr0.7Ca0.3Cr1−yMnyO3−δ as symmetrical solid

[45] Ortiz-Vitoriano N, Ruiz de Larramendi I, Ruiz de Larramendi JI, Arriortua MI, Rojo T. Synthesis and electrochemical performance of La0.6Ca0.4Fe1−xNixO3 (*x* = 0.1, 0.2, 0.3)

[46] Taguchi H, Masunaga Y, Hirota K, Yamaguchi O, Synthesis of perovskite-type (La1−xCax)FeO3 (0 ≤ *x* ≤ 0.2) at low temperature. Mater Res. Bull. 2005;40:773–780.

[47] Montini T, Bevilacqua M, Fonda E, Casulla MF, Lee S, Tavagnacco C, Gorte RJ, For‐ nasiero P. Relationship between electrical behavior and structural characteristics in

[48] Ortiz-Vitoriano N, Ruiz de Larramendi I, Cook SN, Burriel M, Aguadero A, Kilner JA, Rojo T. The formation of performance enhancing pseudo-composites in the high‐ ly active La1–xCaxFe0.8Ni0.2O3 system for IT-SOFC Application. Adv. Funct. Mater.

[49] Ortiz-Vitoriano N, Ruiz de Larramendi I, Ruiz de Larramendi JI, Arriortua MI, Rojo T. Optimization of La0.6Ca0.4Fe0.8Ni0.2O3–Ce0.8Sm0.2O2 composite cathodes for inter‐ mediate-temperature solid oxide fuel cells. J. Power Sources. 2011;196:4332–4336.

[50] Dusastre V, Kilner JA. Optimisation of composite cathodes for intermediate tempera‐

[51] Wang K, Ran R, Zhou W, Gu H, Shao Z, Ahn J. Properties and performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ + Sm0.2Ce0.8O1.9 composite cathode. J. Power Sources.

[52] Tarancón A, Morata A, Dezanneau G, Skinner SJ, Kilner JA, Estradé S, Hernández-Ramírez F, Peiró F, Morante JR. GdBaCo2O5+x layered perovskite as an intermediate

[53] Frontera C, García-Muñoz JL, Castaño O, Ritter C, Caneiro A. The effect of oxygen disorder on magnetic properties of PrBaCo2O5.50layered cobaltite. J. Phys.: Condens.

[54] Kim JH, Kim YM, Connor PA, Irvine JTS, Bae J, Zhou WZCS. Structural, thermal and electrochemical properties of layered perovskite SmBaCo2O5+d, a potential cathode

temperature solid oxide fuel cell cathode. J. Power Sources. 2007;174:255–263.

ture SOFC applications. Solid State Ionics. 1999;126:163–174.

material for solid oxide fuel cell cathode. J. Power Sources. 2009;192;63–69.

Sr-doped LaNi0.6Fe0.4O3−δ mixed oxides. Chem. Mater. 2009;21:1768–1774.

terial for solid oxide fuel cells. Solid State Ionics. 1999;124:281–288.

laser deposition. J. Power Sources. 2007;171:747–753.

608 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

2013;23:5131–5139.

2008;179:60–68.

Matter. 2008;20:104228(8)

oxide fuel cell electrodes. J. Power Sources. 2009;188:230–237.


[80] Meng F, Xia T, Wang J, Shi Z, Zhao H. Praseodymium-deficiency Pr0.94BaCo2O6-δ dou‐ ble perovskite: A promising high performance cathode material for intermediatetemperature solid oxide fuel cells. J. Power Sources. 2015;293:741–750.

[67] Chiba R, Yoshimura F, Sakurai Y. Properties of La1-ySryNi1-xFexO3 as a cathode material for a low-temperature operating SOFC. Solid State Ionics. 2002;152–153:575–

[68] Zhou Q, Xu L, Guo Y, Jia D, Li Y, Wei WCJ. La0.6Sr0.4Fe0.8Cu0.2O3−δ perovskite oxide as

[69] Meng X, Lü S, Ji Y, Wei T, Zhang Y. Characterization of Pr1−xSrxCo0.8Fe0.2O3−δ (0.2≤ *x* ≤0.6) cathode materials for intermediate-temperature solid oxide fuel cells. J. Power

[70] Hashimoto S, Kammer K, Poulsen FW, Mogensen M. Conductivity and electrochemi‐ cal characterization of PrFe1−xNixO3−δ at high temperature. J. Alloys Compd.

[71] Rebello J, Vashook V, Trots D, Guth U. Thermal stability, oxygen non-stoichiometry, electrical conductivity and diffusion characteristics of PrNi0.4Fe0.6O3−δ, a potential

[72] Guo YQ, Yin YM, Tong Z, Yin JW, Xiong MW, Ma ZF. Impact of synthesis technique on the structure and electrochemical characteristics of Pr0.6Sr0.4Co0.2Fe0.8O3−δ (PSCF)

[73] Kostogloudis GC, Ftikos C. Crystal structure, thermal expansion and electrical con‐ ductivity of Pr1−xSrxCo0.2Fe0.8O3−δ (0≤ *x* ≤0.5). Solid State Ionics. 2000;135:537–541.

[74] Chang CL, Hsu CS, Hwang BH. Unique porous thick Sm0.5Sr0.5CoO3 solid oxide fuel cell cathode films prepared by spray pyrolysis. J. Power Sources. 2008;179:734–738.

[75] Yang S, He T, He Q. Sm0.5Sr0.5CoO3 cathode material from glycine-nitrate process: Formation, characterization, and application in LaGaO3-based solid oxide fuel cells. J.

[76] Wei B, Lü Z, Huang X, Liu M, Li N, Su W. Synthesis, electrical and electrochemical properties of Ba0.5Sr0.5Zn0.2Fe0.8O3−δ perovskite oxide for IT-SOFC cathode. J. Power

[77] Lee S, Lim Y, Lee EA, Hwang HJ, Moon J-W. Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and La0.6Ba0.4Co0.2Fe0.8O3−δ (LBCF) cathodes prepared by combined citrate-EDTA method

[78] Wei B, Lü Z, Huang X, Miao J, Sha X, Xin X, et al. Crystal structure, thermal expan‐ sion and electrical conductivity of perovskite oxides BaxSr1−xCo0.8Fe0.2O3−δ (0.3≤ *x* ≤0.7).

[79] Chen D, Ran R, Zhang K, Wang J, Shao Z. Intermediate-temperature electrochemical performance of a polycrystalline PrBaCo2O5+δ cathode on samarium-doped ceria elec‐

cathode material for IT-SOFCs. J. Power Sources. 2011;196:3705–3712.

cathode material. Solid State Ionics. 2011;193:18–22.

for IT-SOFCs. J. Power Sources. 2006;157:848–854.

J. Eur. Ceram. Soc. 2006;26:2827–2832.

trolyte. J. Power Sources. 2009;188:96–105.

Alloys Compd. 2008;450:400–404.

Sources. 2008;176:1–8.

cathode for IT-SOFC. Int. J. Hydrogen Energ. 2012;37:11963–11968.

582.

Sources. 2008;183:581–585.

610 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

2007;428:256–261.


[109] Suthirakun S, Ammal SC, Muñoz-García AB, Xiao G, Chen F, zur Loye HC, Carter EA, Heyden A. Theoretical investigation of H2 oxidation on the Sr2Fe1.5Mo0.5O6 (001) perovskite surface under anodic solid oxide fuel cell conditions. J. Am. Chem. Soc. 2014;136:8374–8386.

[95] Canales-Vázquez J, Ruiz-Morales JC, Irvine JTS, Zhou W. Sc-substituted oxygen ex‐ cess titanates as fuel electrodes for SOFCs. J. Electrochem. Soc. 2005;152:A1458–

[96] Miller DN, Irvine JTS. B site doping of lanthanum strontium titanate for solid oxide

[97] Cui SH, Li JH, Zhou XW, Wang GY, Luo JL, Chuang KT, Bai Y, Qiao LJ. Cobalt dop‐ ed LaSrTiO3−δ as an anode catalyst: effect of Co nanoparticle precipitation on SOFCs

[98] Ma Q, Tietz F. Comparison of Y and La-substituted SrTiO3 as the anode materials for

[99] Park BH, Choi GM. Ex-solution of Ni nanoparticles in a La0.2Sr0.8Ti1−xNixO3−δ alterna‐

[100] Périllat-Merceroz C, Gauthier G, Roussel P, Huvé M, Gélin P, Vannier RN. Synthesis and study of a Ce-doped La/Sr titanate for solid oxide fuel cell anode operating di‐

[101] Ruiz-Morales JC, Canales-Vázquez J, Savaniu C, Marrero-López D, Zhou W, Irvine JTS. Disruption of extended defects in solid oxide fuel cell anodes for methane oxida‐

[102] Chamberland BL, Danielson PS. Alkaline-earth vanadium (IV) oxides having the

[103] Martínez-Coronado R, Alonso JA, Aguadero A, Fernández-Díaz MT. Optimized en‐ ergy conversion efficiency in solid-oxide fuel cells implementing SrMo1−xFexO3−δ per‐

[104] Martínez-Coronado R, Alonso JA, Aguadero A, Fernández-Díaz MT. New SrMo1−xCrxO3−δ perovskites as anodes in solid-oxide fuel cells. Int. J. Hydrogen Energ.

[105] Martínez-Coronado R, Alonso JA, Fernández-Díaz MT. SrMo0.9Co0.1O3−δ: A potential anode for intermediate-temperature solid-oxide fuel cells (IT-SOFC). J. Power Sour‐

[106] Du Z, Zhao H, Yang C, Shen Y, Yan C, Zhang Y. Optimization of strontium molyb‐ date based composite anode for solid oxide fuel cells. J. Power Sources. 2015;274:568–

[107] Zheng K, Świerczek K, Bratek J, Klimkowicz A. Cation-ordered perovskite-type anode and cathode materials for solid oxide fuel cells. Solid State Ionics.

[108] Liu Q, Dong X, Xiao G, Zhao F, Chen F. A novel electrode material for symmetrical

tive anode for solid oxide fuel cell. Solid State Ionics. 2014;262:345–348.

operating on H2S-containing hydrogen. J. Mater. Chem. A. 2013;1:9689–9696.

fuel cell anodes. J. Power Sources. 2011;196:7323–7327.

SOFCs. Solid State Ionics. 2012;225:108–112.

612 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

tion. Nature 2006;439:568–571.

2014;39:4067–4073.

ces. 2014;258:76–82.

2014;262:354–358.

SOFCs. Adv. Mater. 2010;22:5478−5482.

574.

rectly on methane. Chem. Mater. 2011;23:1539–1550.

AVO3 composition. J. Solid State Chem. 1971;3:243–247.

ovskites as anodes. J. Power Sources. 2012;208:153–158.

A1465.


[135] Stevenson DA, Jiang N, Buchanan RM, Henn FEG. Characterization of Gd, Yb and Nd doped barium cerates as proton conductors. Solid State Ionics. 1993;62:279–285.

[121] Chen XJ, Khor KA, Chan SH, Yu LG. Influence of microstructure on the ionic con‐ ductivity of yttria-stabilized zirconia electrolyte. Mater. Sci. Eng. A 2002;A335:246–

[122] Vlasov AN, Perfiliev MV. Ageing of ZrO2-based solid electrolytes. Solid State Ionics

[123] Slater PR, Irvine JTS, Ishihara T, Takita Y. The structure of the oxide ion conductor La0.9Sr0.1Ga0.8Mg0.2O2.85 by powder neutron diffraction. Solid State Ionics.

[124] Kim S, Chun MC, Lee KT, Lee HL. Oxygen-ion conductivity of BaO- and MgO-dop‐

[125] Sood K, Singh K, Pandey OP. Structural and electrical behavior of Ba-doped LaGaO3

[126] Kurumada M, Hara H, Munakata F, Iguchi E. Electric conductions in La0.9Sr0.1GaO3−δ

[127] Mineshige A, Izutsu J, Nakamura M, Nigaki K, Abe J, Kobune M, Fujii S, Yazawa T. Introduction of *A* site deficiency into La0.6Sr0.4Co0.2Fe0.8O3–δ and its effect on structure

[128] Ahamd-Khanlou A, Tietz F, Stöver D. Material properties of La0.8Sr0.2Ga0.9+xMg0.1O3−δ

[129] Zhang L, Xiao J, Xie Y, Tang Y, Liu J, Liu M. Behavior of strontium- and magnesiumdoped gallate electrolyte in direct carbon solid oxide fuel cells. J. Alloys Compd.

[130] Wang XP, Zhou DF, Yang GC, Sun SC, Li ZH, Fu H, Meng J. Nonstoichiometric (La0.95Sr0.05)xGa0.9Mg0.1O3−δ electrolytes and Ce0.8Nd0.2O1.9–(La0.95Sr0.05)xGa0.9Mg0.1O3−δ composite electrolytes for solid oxide fuel cells. Int. J. Hydrogen Energ. 2014;39:1005–

[131] Feng M, Goodenough JB, Huang K, Milliken C. Fuel cells with doped lanthanum gal‐

[132] Lo Faro M, Aricò AS. Electrochemical behaviour of an all-perovskite-based inter‐ mediate temperature solid oxide fuel cell. Int. J. Hydrogen Energ. 2013;38:14773–

[133] Yamazaki Y, Blanc F, Okuyama Y, Buannic L, Lucio-Vega JC, Grey CP, Haile SM. Proton trapping in yttrium-doped barium zirconate. Nat. Mater. 2013;12:647–651.

[134] Liu Y, Ran R, Li S, Jiao Y, Tade MO, Shao Z. Significant performance enhancement of yttrium-doped barium cerate proton conductor as electrolyte for solid oxide fuel cells

through a Pd ingress–egress approach. J. Power Sources. 2014;257:308–318.

ed LaGaO3 electrolytes. J. Power Sources. 2001;93:279–284.

614 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

composite electrolyte. J. Renew. Sust. Energy. 2014;6:063112.

and La0.9Sr0.1Ga0.9Mg0.1O3−δ. Solid State Ionics. 2005;176:245–251.

as a function of Ga content. Solid State Ionics. 2000;135:543–547.

and conductivity. Solid State Ionics. 2005;176:1145–1149.

late electrolyte. J. Power Sources. 1996;63:47–51.

252

1987;25:245–253.

1998;107:319–323.

2014;608:272–277.

1013

14778


[160] Boehme M, Ionescu E, Fu G, Ensinger W. Room temperature synthesis of indium tin oxide nanotubes with high precision wall thickness by electroless deposition. J. Nanotech. 2011;2:119–126.

[148] Jimenez de Aberasturi D, Serrano-Montes AB, Liz-Marzán LM. Modern applications of plasmonic nanoparticles: from energy to health. Adv. Optical Mater. 2015;3:602–

[149] Jiang SP. A review of wet impregnation—An alternative method for the fabrication of high performance and nano-structured electrodes of solid oxide fuel cells. Mater.

[150] Liu Z, Liu B, Ding D, Liu M, Chen F, Xia C. Fabrication and modification of solid ox‐ ide fuel cell anodes via wet impregnation/infiltration technique. J. Power Sources.

[151] Marrero-López D, Ruiz-Morales JC, Peña-Martínez J, Canales-Vázquez J, Núñez P. Preparation of thin layer materials with macroporous microstructure for SOFC appli‐

[152] Pinedo R, Ruiz de Larramendi I, Gil de Muro I, Insausti M, Ruiz de Larramendi JI, Arriortua MI, Rojo T. Influence of colloidal templates on the impedance spectroscop‐ ic behaviour of Pr0.7Sr0.3Fe0.8Ni0. or solid oxide fuel cell applications. Solid State Ionics.

[153] Pinedo R, Ruiz de Larramendi I, Jimenez de Aberasturi D, Gil de Muro I, Aguayo AT, Ruiz de Larramendi JI, Rojo T. A straightforward synthesis of carbon nanotube– perovskite composites for solid oxide fuel cells. J. Mater. Chem. 2011;21:10273–10276.

[154] Pinedo R, Ruiz de Larramendi I, Khavrus VO, Jimenez de Aberasturi D, Ruiz de Lar‐ ramendi JI, Ritscheld M, Leonhardt A, Rojo T. Microstructural improvements of the gradient composite material Pr0.6Sr0.4Fe0.8Co0.2O3/Ce0.8Sm0.2O1.9 by employing vertically

[155] Bellino MG, Sacanell JG, Lamas DG, Leyva AG, Walsöe de Reca NE. High-perform‐ ance solid-oxide fuel cell cathodes based on cobaltite nanotubes. J. Am. Chem. Soc.

[156] Liu J, Wang X, Peng Q, Li Y. Vanadium pentoxide nanobelts: highly selective and

[157] Kuo TJ, Huang MH. Gold-catalyzed low-temperature growth of cadmium oxide

[158] Ostermann R, Li D, Yin Y, McCann JT, Xia Y. V2O5 Nanorods on TiO2 Nanofibers: A new class of hierarchical nanostructures enabled by electrospinning and calcination.

[159] Kwon CW, Son JW, Lee JH, Kim HM, Lee HW, Kim KB. High-performance microsolid oxide fuel cells fabricated on nanoporous anodic aluminum oxide templates.

nanowires by vapor transport. J. Phys. Chem. B 2006;110:13717–13721.

aligned carbon nanotubes. Int. J. Hydrogen Energ. 2014;39:4074–4080.

stable ethanol sensor materials. Adv. Mater. 2005;17:764–767.

617.

Sci. Eng. A. 2006;418:199–210.

616 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

cations. J. Solid State Chem. 2008;181:685–692.

2013;237:243–259.

2011;192:235–240.

2007;129:3066–3067.

Nano Lett. 2006;6:1297–1302.

Adv. Funct. Mater. 2011;21:1154–1159.


## **Chapter 21**

## **Perovskites Used in Fuel Cells**

Diego Pereira Tarragó, Berta Moreno, Eva Chinarro and Vânia Caldas de Sousa

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61465

### **Abstract**

Fuel cells are devices for energy generation with very high theoretical efficiency. Many researches were been carried out in the last few decades in order to develop reliable fuel cells. Solid oxide fuel cells (SOFC) and polymeric exchange membrane fuel cells (PEMFC) are those with more potential for commercial use. Specially for SOFC cathodes, many perovskites have been proposed as potential materials for this application. Nevertheless, other components of SOFC, such as the electrolytes, anodes and interconnects, have also been targeted with potential perovskites. More recently, the use of perovskites in PEMFC has also been proposed and studied. As many perovskite compositions can be used in SOFC components, some of the most important are discussed in this chapter and some recent works in perovskites for PEMFC are also referred. As a whole, in this chapter, the reader will find the relationship between the properties of perovskites with their compo‐ sitions and the main effects of dopant agents regarding the utilization of these materials in different components of SOFC and in electrodes of PEMFC.

**Keywords:** SOFC, IT-SOFC, PEMFC, Nonstoichiometric compounds

## **1. Introduction**

Fuel cells are devices that convert the chemical energy of a fuel directly into electrical energy and heat. The most common fuel is H2, but other hydrocarbon compounds such as methanol, methane, natural gas, ethanol or others can also be used. A single cell is composed of three main components: anode, cathode and electrolyte. For the effective use of fuel cells, single cells must be interconnected to increase the power production, which requires the use of two more components: interconnects, for the serial electrical connection, and sealants, for the hermetic sealing of the set. The electrodes are permeated by the gases, fuel in the anode and oxygen (air) in the cathode, and they catalyse electrochemical reactions through electron capitation or

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conduction from or to the reactive sites; the electrolyte, an electrical insulator, promotes ionic conductivity. Figure 1 shows a general schematic drawing of a fuel cell operation. The residual water (if pure H2 is used) can be produced in the anode or in the cathode and it will depend of the nature of the electrolyte. If the electrolyte is a cationic conductor, the water will form in the cathode and, conversely, if it is an anionic conductor, the water will be formed in the anode.

**Figure 1.** General operation scheme of a fuel cell running with H2.

In general, a fuel cell works similarly to a battery; however, its energy is not stored in its electrodes, so there is no need for recharging because there is a continuous supply of fuel in the anode and oxidants (air) in the cathode. The electrical work provided by the electrochem‐ ical reactions does not consume the cell's components, which keeps on converting the chemical energy into electricity and heat while its electrodes are supplied with the gases.

There are different types of fuel cells and they are named according to the material used in the electrolyte. Besides the materials used in their components, the fuel cells also differ in other important aspects such as efficiency and operation temperature. In Table 1, the main charac‐ teristics of fuel cells are summarized.


**Table 1.** Fuel cell types and their characteristics

Among all the fuel cells, the SOFC is the one where materials with a perovskite structure are the most applied and studied. Except for the sealant, all other components of the SOFC can potentially consist of perovskite ceramics. The most common materials found in the single SOFC configuration are yttria-stabilized zirconia (YSZ) in the electrolyte, Ni/YSZ cermet in the anode and strontium-doped lanthanum manganite (LSM) in the cathode. As can be seen, only the cathode is composed of a perovskite material. However, several scientific researchers have attempted to substitute the materials of the electrolyte and the anode with perovskites. In the case of the electrolyte, their main goal is to increase the ionic conductivity at lower tempera‐ tures and, for the anode, the aim is to include hydrocarbons as potential fuels, since Ni is often poisoned by fuels containing carbon. For the interconnects, the perovskite materials used in this component were gradually substituted with stainless steel, a much cheaper material that is capable of supporting the cell; recently, perovskite-coated stainless steel has arisen as a potential interconnect material. Also, a new group of perovskites has received attention for proton-conducting SOFC, where the electrolyte works with cationic conduction, although these cells have not yet achieved a performance similar to the conventional oxygen-conducting cells.

As can be seen, the materials used in SOFC must fit many requirements in terms of electro‐ chemical properties. Besides, chemical and physical compatibility must exist between the materials for each component. As SOFC runs in high temperature, a chemical decomposition or a chemical reaction cannot occur during its operation and the chosen materials must have a similar thermal expansion coefficient in order to avoid the formation of cracks during thermal cycling. The high operation temperature of SOFC brings some advantages: increasing the activity of the electrodes and the conductivity of the components and favours ing the kinetics of the electrochemical reactions and gaseous exchange. However, reducing their operation temperature also has its advantages: decreased densification and thermal stress and, more importantly, diversifying the materials used in their components, since many perovskites are unstable at high temperatures. SOFCs that work below 800°C can be referred to as intermediate temperature SOFC (IT-SOFC).

Recent researches indicate that besides SOFC components, perovskite materials also have potential for application in PEMFC electrodes. However, this study still is very incipient. In the coming sessions, perovskites used in the SOFC components have been addressed sepa‐ rately and according to the components where it can be applied. Then, in one session, perov‐ skites for use in PEMFC electrodes were discussed.

## **2. Cathodes for SOFC**

conduction from or to the reactive sites; the electrolyte, an electrical insulator, promotes ionic conductivity. Figure 1 shows a general schematic drawing of a fuel cell operation. The residual water (if pure H2 is used) can be produced in the anode or in the cathode and it will depend of the nature of the electrolyte. If the electrolyte is a cationic conductor, the water will form in the cathode and, conversely, if it is an anionic conductor, the water will be formed in the anode.

In general, a fuel cell works similarly to a battery; however, its energy is not stored in its electrodes, so there is no need for recharging because there is a continuous supply of fuel in the anode and oxidants (air) in the cathode. The electrical work provided by the electrochem‐ ical reactions does not consume the cell's components, which keeps on converting the chemical

There are different types of fuel cells and they are named according to the material used in the electrolyte. Besides the materials used in their components, the fuel cells also differ in other important aspects such as efficiency and operation temperature. In Table 1, the main charac‐

> **Polymeric Exchange Membrane (PEMFC)**

hydroxide Phosphoric acid Polymer Molten carbonates Dense ceramics

50-120°C 180-210°C 60-100°C 550-650°C 550-1000°C

**Molten Carbonate (MCFC)**

**Solid Oxide (SOFC)**

energy into electricity and heat while its electrodes are supplied with the gases.

**(PAFC)**

*Transported ion* OH- H+ H+ CO3 2- O2- *Expected efficiency* 35-55% 35-45% 35-45% 45-55% 40-60%

**Figure 1.** General operation scheme of a fuel cell running with H2.

620 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

teristics of fuel cells are summarized.

**Table 1.** Fuel cell types and their characteristics

*Electrolyte*

*Operation Temperature*

**Fuel cell type Alkaline (AFC) Phosphoric Acid**

Potassium

The cathode, in an SOFC, is the interface between the electrolyte and the oxygen, its main functions are conduct electrons to the reactive sites and catalyze the reduction of the O2 molecules. Therefore, the material applied in this component must have specific properties in the operation temperature such as electronic conductivity, catalytic activity, chemical and physical compatibility with the electrolyte and interconnect, as well as a porous and stable microstructure. The choice of material depends mainly on the chemical composition of the electrolyte and the operating temperature.

As said, LSM is the most common material used in SOFC cathode due to its properties and compatibility with YSZ at high temperatures. Pure lanthanum manganite (LaMnO3), with A3+B3+O3 stoichiometry, is an intrinsic p-type semiconductor where the presence of cationic vacancies, primarily in the A sites, induces an oxygen nonstoichiometry. These vacancies are occupied by oxygen anions which lead to an oxygen excess that, in turn, causes the appearance of Mn4+ species in order to maintain the overall neutrality. The Mn4+/Mn3+ ratio can be increased by doping both A and B sites, adding electronic holes in the structure and, hence, increasing the electronic conductivity. The most common is to dope the A site with bivalent cations such as Sr2+, establishing the general formula La1–*x*Sr*x*MnO3–*δ*, according to the molar amount of strontium occupying lanthanum sites.

It is important to note that by doping the perovskites, different properties are influenced at the same time and, for application in SOFC, some of them can be disadvantageous. According to Shuk et al., doping A sites of lanthanum manganite improves its electronic conductivity and reaches its maximum at the composition La0.45Sr0.55MnO3–*δ*, Sr2+ is at 55% mol or *x* = 55, although this property also depends on the oxygen partial pressure. However, at this amount, the thermal expansion coefficient is too high, as observed by Florio et al., causing a physical mismatch with YSZ electrolytes. In amounts until 15% of strontium, there is a stabilization of the perovskite against YSZ, as reported by Yokokawa et al. but, despite this, the most common composition found in the literature is La0.7Sr0.3MnO3–*δ*. Doping the B sites increases the electronic conductivity and also the ionic conductivity, which is positive for the cathode performance; however, Tai et al. observed negative effects on the thermal expansion coeffi‐ cient, derailing its use with YSZ electrolytes.

In the case of IT-SOFC, the temperature decrease impairs the performance of LSM, mostly because of the loss in catalytic activity. One alternative is to mix LSM with a fraction of YSZ, forming a composite cathode which can lead to better cathode/electrolyte adhesion and to an increase in the reactive sites. Mogensen et al. tested LSM/YSZ composite cathodes and showed an improvement in the catalytic activity. However, yet more promising is the use of mixed ionic electronic conductor (MIEC) perovskites. The use of this kind of material increases the amount of reactive sites, compensating the kinetic losses at temperatures below 800°C.

Doped lanthanum ferrites MIEC perovskites are a common example where the kinetic losses in the cathode, due to the low temperature, are compensated by an increase in the active reduction area. In the LSM perovskite, the B site doping with Co ions significantly increases the oxygen diffusion, and the substitution of Mn by Fe can enhance surface exchange processes. Based on this improvement, strontium- and cobalt-doped lanthanum ferrites (LSCF), with a general formula of La1–*x*Sr*x*Co1–*y*Fe*y*O3–*δ*, have excellent electrochemical properties for use in IT-SOFC. However, the thermal expansion coefficient of LSCF is not compatible with YSZ electrolytes, hence these perovskites can only be used with compatibility layers or with other compounds as electrolytes. In an attempt to overcome the thermal expansion incompatibility with YSZ electrolytes, Ko et al. used a composite cathode consisting of LSCF and GDC (gadolinea-doped ceria), another common electrolyte material, and achieved good overall performance, with a decrease of cathode polarization, for many hours of cell operation. However, other authors who tested cells with LSCF cathodes showed voltage loss with time, probably due to the thermal decomposition of this perovskite. Different amounts of dopants can be found in the literature and they report the high electronic and ionic conductivity of these compounds; they also show high activity for the O2 reduction reaction.

microstructure. The choice of material depends mainly on the chemical composition of the

As said, LSM is the most common material used in SOFC cathode due to its properties and compatibility with YSZ at high temperatures. Pure lanthanum manganite (LaMnO3), with A3+B3+O3 stoichiometry, is an intrinsic p-type semiconductor where the presence of cationic vacancies, primarily in the A sites, induces an oxygen nonstoichiometry. These vacancies are occupied by oxygen anions which lead to an oxygen excess that, in turn, causes the appearance of Mn4+ species in order to maintain the overall neutrality. The Mn4+/Mn3+ ratio can be increased by doping both A and B sites, adding electronic holes in the structure and, hence, increasing the electronic conductivity. The most common is to dope the A site with bivalent cations such as Sr2+, establishing the general formula La1–*x*Sr*x*MnO3–*δ*, according to the molar amount of

It is important to note that by doping the perovskites, different properties are influenced at the same time and, for application in SOFC, some of them can be disadvantageous. According to Shuk et al., doping A sites of lanthanum manganite improves its electronic conductivity and reaches its maximum at the composition La0.45Sr0.55MnO3–*δ*, Sr2+ is at 55% mol or *x* = 55, although this property also depends on the oxygen partial pressure. However, at this amount, the thermal expansion coefficient is too high, as observed by Florio et al., causing a physical mismatch with YSZ electrolytes. In amounts until 15% of strontium, there is a stabilization of the perovskite against YSZ, as reported by Yokokawa et al. but, despite this, the most common composition found in the literature is La0.7Sr0.3MnO3–*δ*. Doping the B sites increases the electronic conductivity and also the ionic conductivity, which is positive for the cathode performance; however, Tai et al. observed negative effects on the thermal expansion coeffi‐

In the case of IT-SOFC, the temperature decrease impairs the performance of LSM, mostly because of the loss in catalytic activity. One alternative is to mix LSM with a fraction of YSZ, forming a composite cathode which can lead to better cathode/electrolyte adhesion and to an increase in the reactive sites. Mogensen et al. tested LSM/YSZ composite cathodes and showed an improvement in the catalytic activity. However, yet more promising is the use of mixed ionic electronic conductor (MIEC) perovskites. The use of this kind of material increases the amount of reactive sites, compensating the kinetic losses at temperatures below 800°C.

Doped lanthanum ferrites MIEC perovskites are a common example where the kinetic losses in the cathode, due to the low temperature, are compensated by an increase in the active reduction area. In the LSM perovskite, the B site doping with Co ions significantly increases the oxygen diffusion, and the substitution of Mn by Fe can enhance surface exchange processes. Based on this improvement, strontium- and cobalt-doped lanthanum ferrites (LSCF), with a general formula of La1–*x*Sr*x*Co1–*y*Fe*y*O3–*δ*, have excellent electrochemical properties for use in IT-SOFC. However, the thermal expansion coefficient of LSCF is not compatible with YSZ electrolytes, hence these perovskites can only be used with compatibility layers or with other compounds as electrolytes. In an attempt to overcome the thermal expansion incompatibility with YSZ electrolytes, Ko et al. used a composite cathode consisting of LSCF and GDC (gadolinea-doped ceria), another common electrolyte material, and achieved good overall

electrolyte and the operating temperature.

622 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

strontium occupying lanthanum sites.

cient, derailing its use with YSZ electrolytes.

Other compounds such as layered or double perovskites are also MIECs and potential materials for SOFC cathodes. These perovskites can have a structure coordinated by 4 or 5 oxygen atoms, and are typically A2BO4+*δ* or AA'B2O5± *δ*. Some typical compositions of layered perovskites are La2NiO4+*δ* and La2CoO4+*δ*, or with three cations GdBaCo2O5± *δ* and PrBaCo2O5± *<sup>δ</sup>*. An interesting aspect of these materials is that they do not need to be heavily substituted to promote high oxygen diffusion. This characteristic is owed to its structure, where oxygen vacancies are concentrated in a plan between layers formed by cations, leading to anisotropic ion conduction due to a decrease in the activation energy for vacancy migration. Tarancón et al. compiled some values and showed that the oxygen self-diffusion coefficient at 500°C of La2NiO4+*<sup>δ</sup>* is 3.3 × 10–9 cm²/s, which is one order of magnitude higher than a typical LSCF/GDC composite cathode. The cation-ordered structure also plays a role in the oxygen surface exchange, which becomes a very important factor in cathodes with good oxygen diffusion, such as these.

Studies have also been carried out in order to determine the doping in layered perovskites; however, an undesirable increase in the thermal expansion coefficient was also observed. According to Kim et al., partial substitution with strontium to form NdBa1–*x*Sr*x*Co2O5+*<sup>δ</sup>* increases the specific area resistance of the cathodes and, relatively with pure layered perovskite, the doped layered perovskites showed a higher cathodic polarization due to the oxygen disorder introduced by dopant cations.

Cathodes for proton-conducting SOFC are MIEC perovskites based mainly on barium and iron and a very common general composition is Ba1–*x*Sr*x*Co1–*y*Fe*y*O3–*δ*. The ideal cathode for this kind of SOFC should conduct protons simultaneously with oxygen and electrons. Cathodes with Co dopant cations exhibit excellent performance; however, it was found that these cobaltcontaining perovskites usually have problems with thermal expansion, undesired reduction and evaporation of Co. Hence, other cations are proposed, such as Ni or Nb, in order to eliminate the Co from these compositions. Another approach, as proposed by Zhang et al., is to modify the proton-conducting electrolytes, such as BaCe0.8Sm0.2O3–*δ*, introducing transitional elements, such as Fe. However, despite of good cationic and anionic conduction, these modified cathodes present low electrical conduction and poor catalytic activity, and need to be further improved. All compositions of cathodes for proton conducting SOFC are commonly applied with electrolytes based on cerium oxide or even with other perovskites composition, that were addressed in the Electrolytes session.

Besides the composition, the final microstructure of the cathode is of great importance. It has been demonstrated that modifications in the microstructure of the cathode can have great impact on the overall fuel cell performance. The optimum cathode porous microstructure should enhance the gas flow and have a high surface area. It also must own a minimal mechanical resistance in order to prevent cracking and collapsing of the component.

## **3. Interconnects for SOFC**

Interconnects provide an electrical connection between cathode of one individual cell to the anode of the adjacent cell in a SOFC stack and ensure a physical barrier between the reducing atmosphere (at the anode) and the oxidizing atmospheres (at the cathode). Therefore, the material to be used as interconnect has to present a series of important characteristics, which considerably reduces the candidate material for this component. High electrical conductivity, with values higher than 1 S/cm or, in terms of area-specific resistance, values below 0.1 Ω.cm² are required with no ionic conductivity. Chemical stability at the operation temperatures and in reducing and oxidant atmospheres, considering that atomic interdiffusion can be a recurrent problem and reaction with sealant materials can also destabilize the perovskite. Thermal expansion compatible with anode and cathode. Enough mechanical strength to bear the load of the stack and support other components. Finally, in the case of the utilization of the heat generated by the SOFC in cogeneration, a thermal conductivity of at least 5 W/mK is necessary.

The most common perovskite compositions for interconnects are based on lanthanum chromite (LaCrO3). In its structure, larger-sized cations, such as Ca2+ and Sr2+, can substitute for La3+ while smaller cations, such as Ni2+, Cu2+ or Al3+, can replace Cr3+. Mahato et al. dem‐ onstrated that the divalent cations, such as Ca2+ or Sr2+, increase the conductivity of LaCrO3. The divalent cations will act as acceptor dopants when residing at the trivalent (La3+ or Cr3+) sites. Thus, in order to maintain charge neutrality, holes are created as a charge compensating defect, which consequently leads to p-type conductivity. Similarly, Fergus et al. attributed the addition of trivalent cations, such as Al3+, to an increase in the p-type conductivity, but in this case, occurring mainly due to an increase in carrier mobility. It may be noted that the solubility of the divalent cations decreases upon increasing *p*O2.

As with other perovskites, the dopant type and amount must be carefully chosen, because they often modify thermal expansion and chemical stability at the same time as electrical conduc‐ tivity. For example, pure LaCrO3 has a thermal expansion coefficient of 9.5e-6 K–1 and an electrical conductivity of 1 S/cm at 1000°C. With the addition of 20% of cobalt in the B sites, these values increase to 14.6e-6 K–1 and 15 S/cm, respectively. Whereas the addition of 10% of magnesium in the A sites keeps the thermal expansion stable with an increase in the conduc‐ tivity of up to 3 S/cm. Another issue related to doped LaCrO3 is its poor sinterability in air, which Anderson et al. attributed to the high vapor pressure of volatile chromium components. Hence, the use of CaO, for example, creates a liquid phase during sintering and enhances the sinterability of the (La,Ca)CrO3 compound.

In the case of cells operating below 700°C, there is the possibility of using metallic intercon‐ nects, which brings great advantages. With metallic interconnects, the other components can be deposited as thin films layers which can diminish costs with raw material. Besides, it can lead to better mechanical strength with more efficient accommodation of thermal tensions during heating and cooling. However, even alloys with high antioxidant capacity are proven to suffer in extreme conditions where interconnects are employed, so a ceramic coating with conventional high-temperature interconnect materials is necessary. Brylewski et al. coated a ferritic stainless steel with a La0.8Sr0.2CrO3 ceramic film and compared the oxidation rate with an uncoated sample in wet atmosphere. In this study, it was shown that oxidation rates could be decreased with the use of conducting films; however, the authors also observed that chromium oxide could segregate from the perovskite to form undesired phases. Usually, the coating materials are perovskites, but recently, some spinel compositions have also been proposed.

## **4. Anodes for SOFC**

**3. Interconnects for SOFC**

624 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

of the divalent cations decreases upon increasing *p*O2.

sinterability of the (La,Ca)CrO3 compound.

Interconnects provide an electrical connection between cathode of one individual cell to the anode of the adjacent cell in a SOFC stack and ensure a physical barrier between the reducing atmosphere (at the anode) and the oxidizing atmospheres (at the cathode). Therefore, the material to be used as interconnect has to present a series of important characteristics, which considerably reduces the candidate material for this component. High electrical conductivity, with values higher than 1 S/cm or, in terms of area-specific resistance, values below 0.1 Ω.cm² are required with no ionic conductivity. Chemical stability at the operation temperatures and in reducing and oxidant atmospheres, considering that atomic interdiffusion can be a recurrent problem and reaction with sealant materials can also destabilize the perovskite. Thermal expansion compatible with anode and cathode. Enough mechanical strength to bear the load of the stack and support other components. Finally, in the case of the utilization of the heat generated by the SOFC in cogeneration, a thermal conductivity of at least 5 W/mK is necessary. The most common perovskite compositions for interconnects are based on lanthanum chromite (LaCrO3). In its structure, larger-sized cations, such as Ca2+ and Sr2+, can substitute for La3+ while smaller cations, such as Ni2+, Cu2+ or Al3+, can replace Cr3+. Mahato et al. dem‐ onstrated that the divalent cations, such as Ca2+ or Sr2+, increase the conductivity of LaCrO3. The divalent cations will act as acceptor dopants when residing at the trivalent (La3+ or Cr3+) sites. Thus, in order to maintain charge neutrality, holes are created as a charge compensating defect, which consequently leads to p-type conductivity. Similarly, Fergus et al. attributed the addition of trivalent cations, such as Al3+, to an increase in the p-type conductivity, but in this case, occurring mainly due to an increase in carrier mobility. It may be noted that the solubility

As with other perovskites, the dopant type and amount must be carefully chosen, because they often modify thermal expansion and chemical stability at the same time as electrical conduc‐ tivity. For example, pure LaCrO3 has a thermal expansion coefficient of 9.5e-6 K–1 and an electrical conductivity of 1 S/cm at 1000°C. With the addition of 20% of cobalt in the B sites, these values increase to 14.6e-6 K–1 and 15 S/cm, respectively. Whereas the addition of 10% of magnesium in the A sites keeps the thermal expansion stable with an increase in the conduc‐ tivity of up to 3 S/cm. Another issue related to doped LaCrO3 is its poor sinterability in air, which Anderson et al. attributed to the high vapor pressure of volatile chromium components. Hence, the use of CaO, for example, creates a liquid phase during sintering and enhances the

In the case of cells operating below 700°C, there is the possibility of using metallic intercon‐ nects, which brings great advantages. With metallic interconnects, the other components can be deposited as thin films layers which can diminish costs with raw material. Besides, it can lead to better mechanical strength with more efficient accommodation of thermal tensions during heating and cooling. However, even alloys with high antioxidant capacity are proven to suffer in extreme conditions where interconnects are employed, so a ceramic coating with conventional high-temperature interconnect materials is necessary. Brylewski et al. coated a ferritic stainless steel with a La0.8Sr0.2CrO3 ceramic film and compared the oxidation rate with

The efficient use of hydrocarbons as fuels for SOFC is one of the most relevant issues concern‐ ing its current development, and their utilization depends mainly on the characteristics of the anode. Conventional SOFC anodes work only when pure H2 is used as fuel; otherwise, the deposition of carbon in the catalyst surface poisons the cell and rapidly compromises its performance. Therefore, new materials are proposed in order to promote the direct oxidation of hydrocarbons, or even an internal reforming, considering the longevity of the SOFC lifespan. Many of these new materials are perovskites.

One potential perovskite for use in anodes is based on lanthanum chromite, a typical inter‐ connect composition. Doping this compound on A and B sites enhances its activity towards methane oxidation. The presence of chromium at the B sites also improves redox stability and tolerance to sulphur; however, it costs the compound a decrease in the total conductivity. The use of strontium and manganese dopants helps to maintain the compound's stability and a typical composition is La0.75Sr0.25Cr0.5Mn0.5O3–*δ* (LSCM). These dopant cations also enhance ionic/electronic conductivity and catalytic activity. The conductivity of LSCM can reach 38 S/ cm at 900°C in air, but in the presence of H2, this value can decrease to 1.5 S/cm at the same temperature.

The overall performance of LSCM anodes are compatible with the conventional Ni/YSZ cermets, showing better performance when methane is used as fuel. Also, if the fuel has sulphur impurities (H2S), LSCM is more tolerant than the conventional cermet anodes. However, Zha et al. demonstrated the formation of sulphides within a few days when LSCM anodes operate with a 10%H2S containing H2 fuel. The authors also demonstrate that an increase in the Cr content on the perovskite lattice increases the number of impurity phases.

The use of cerium cations in the A sites of the LSCM lattice is also studied, with an expectation that increasing the catalytic activity would also increase the open circuit voltage (OCV) in cells fuelled with methane. The substitution with Ru in the B sites was studied due to its high catalytic activity in the steam reforming reaction with very good stability, but its application is limited by its very high cost.

Another proposed perovskite anode is based on lanthanum-doped strontium titanate, which shows good electrical conductivity in reducing atmospheres and reliable dimensional and chemical stability. Besides, it has good capacity at high temperatures to operate with CH4 fuel without the detecting of carbon deposits formation and with high OCV, this can be a key factor in the SOFC fuel flexibility. According to Ruiz-Morales et al., anodes with the substitution of Ti by Mn and Ga cations can put strontium titanates on par with conventional Ni-YSZ cermets when the cells are operating with H2 fuel.

In order to enhance its electrocatalytic performance, a B site doping must be carried out and the choice of dopant must influence the redox properties and the conductivity. A potential B site dopant is chromium, for example, La0.3Sr0.7Ti0.8Cr0.2O3–*δ* (LSTC). This composition has excellent stability and electrical conductivity; however, its catalytic activity is still very low, in order to be used as an SOFC anode, it is necessary to incorporate some catalytic materials.

## **5. Electrolytes for SOFC**

In recent years, most of the efforts in SOFC development have been focussed on intermediate temperature solid oxide fuel cells (IT-SOFCs). The majority of them use a fluorite-structured oxide, CGO and/or YSZ in the electrolyte. In 1971, Takahashi et al. discovered that perovskitestructured materials possessed oxide ion conductivity; in 1992, Goodenough et al. reported that LaGaO3 exhibited important ionic conductivity that was improved with different dopants being modified with strontium and magnesium in La and Ga sites (LSGM), respectively, showed higher oxide ion conductivity. However, these kinds of perovskites can only work under certain operating conditions, as they are not stable at low oxygen pressures. In CO and CO2 atmosphere, they form carbonates; in reducing atmospheres, there is a Ga depletion along the grain boundaries degrading the material; and at high temperatures, they show an impor‐ tant solubility of Al, Ni, Co oxides. Therefore, this material (LSGM) is considered a good candidate as electrolyte for intermediate temperature SOFCs (IT-SOFCs). Since the 1970s, LnAlO3 perovskites have been studied mainly taking into account their lower cost and high reduction and volatilization stability with respect to ceria and lanthanum galates; however, they exhibit problems of high electronic conductivity at high oxygen partial pressures and poor sinterability why they are constrained as an additive to composite solid electrolytes.

Most recently, there have been studies about ceramic materials that present protonic conduc‐ tivity, and they are being employed as electrolytes in SOFC. In this respect, the first studies were presented by Iwahara et al. about SrCeO3 materials. Some years later, the studies were directed mainly towards BaCeO3 and BaZrO3 perovskites due to of their higher proton conductivity, which was improved by Y doping (BCY and BZY, respectively). It is known that these barium cerate electrolytes exhibit both oxide ion and protonic conduction depending on the working temperature, changing from protonic to oxide ion transport when temperature are varied from 600°C to 1,000°C, whereas this behaviour has not been observed in strontium cerates. However, against this high conductivity is the very low chemical stability of BaCeO3 materials, while Y-doped BaZrO3 with a little lower proton conductivity exhibit a great chemical stability, with the problem of resistive grain boundaries. Nevertheless, Y-doped BaZrO3 could be more appropriate for its application in SOFCs.

For SOFC development, relatively high ionic conductivity of solid electrolytes is not the only requirement; an enhancement of durability is also needed. To improve both aspects, many works have been carried out, some propose to decrease the thickness of the electrolyte or using the named composite electrolytes (e.g., BCY and a molten salt phase).

## **6. Perovskites used in PEMFC**

Ti by Mn and Ga cations can put strontium titanates on par with conventional Ni-YSZ cermets

In order to enhance its electrocatalytic performance, a B site doping must be carried out and the choice of dopant must influence the redox properties and the conductivity. A potential B site dopant is chromium, for example, La0.3Sr0.7Ti0.8Cr0.2O3–*δ* (LSTC). This composition has excellent stability and electrical conductivity; however, its catalytic activity is still very low, in order to be used as an SOFC anode, it is necessary to incorporate some catalytic materials.

In recent years, most of the efforts in SOFC development have been focussed on intermediate temperature solid oxide fuel cells (IT-SOFCs). The majority of them use a fluorite-structured oxide, CGO and/or YSZ in the electrolyte. In 1971, Takahashi et al. discovered that perovskitestructured materials possessed oxide ion conductivity; in 1992, Goodenough et al. reported that LaGaO3 exhibited important ionic conductivity that was improved with different dopants being modified with strontium and magnesium in La and Ga sites (LSGM), respectively, showed higher oxide ion conductivity. However, these kinds of perovskites can only work under certain operating conditions, as they are not stable at low oxygen pressures. In CO and CO2 atmosphere, they form carbonates; in reducing atmospheres, there is a Ga depletion along the grain boundaries degrading the material; and at high temperatures, they show an impor‐ tant solubility of Al, Ni, Co oxides. Therefore, this material (LSGM) is considered a good candidate as electrolyte for intermediate temperature SOFCs (IT-SOFCs). Since the 1970s, LnAlO3 perovskites have been studied mainly taking into account their lower cost and high reduction and volatilization stability with respect to ceria and lanthanum galates; however, they exhibit problems of high electronic conductivity at high oxygen partial pressures and poor sinterability why they are constrained as an additive to composite solid electrolytes.

Most recently, there have been studies about ceramic materials that present protonic conduc‐ tivity, and they are being employed as electrolytes in SOFC. In this respect, the first studies were presented by Iwahara et al. about SrCeO3 materials. Some years later, the studies were directed mainly towards BaCeO3 and BaZrO3 perovskites due to of their higher proton conductivity, which was improved by Y doping (BCY and BZY, respectively). It is known that these barium cerate electrolytes exhibit both oxide ion and protonic conduction depending on the working temperature, changing from protonic to oxide ion transport when temperature are varied from 600°C to 1,000°C, whereas this behaviour has not been observed in strontium cerates. However, against this high conductivity is the very low chemical stability of BaCeO3 materials, while Y-doped BaZrO3 with a little lower proton conductivity exhibit a great chemical stability, with the problem of resistive grain boundaries. Nevertheless, Y-doped

For SOFC development, relatively high ionic conductivity of solid electrolytes is not the only requirement; an enhancement of durability is also needed. To improve both aspects, many

BaZrO3 could be more appropriate for its application in SOFCs.

when the cells are operating with H2 fuel.

626 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

**5. Electrolytes for SOFC**

From a catalytic point of view, the use of perovskites has been principally reserved for hightemperature fuel cells due to the improved kinetics of reactions in electrodes with temperature. Nevertheless, the possible use of ABO3 structures as catalysts in the replacement of noble metals is an idea that has been proposed in the early 1990s, based on the mixed oxide-ion/ electronic conductivity and the low cost of these materials.

These structures are a potential alternative to Pt or Au in different air cathode-electrochemical cells, where the main drawbacks to the commercial viability of those devices are the low activity of oxygen-reduction and oxygen-evolution reactions (ORR and OER), respectively. In the proton exchange fuel cell field, most of the published materials with ABO3 structures are directed towards the ORR. In alkaline solutions, they should exhibit comparable activities to those platinum-based metal catalysts, and this activity is supposed to be directly related with the electronic configuration of their surface cations. Among the different materials studied, lanthanum is the most common A cation because, combined with other transition metals, it has demonstrated ORR activity. B site substitutions with certain transition-metal elements should impact ORR rates (e.g., with Mn, Co and Ni) and should enhance the chemical and electrochemical stability of the perovskites (e.g., Cr and Fe) in alkaline solutions. Among lanthanum-based perovskites, LaCoO3 has revealed itself to be interesting due to its large ORR current density and the positive shift shown in the onset potential. In addition, LaNiO3-based oxides with B cation substitution, such as LaNi1−*<sup>x</sup>*Fe*x*O3 (*x* = 0–0.2) are promising materials in alkaline media, with an improved catalytic activity related with the increase in the valence state of Ni with the B site substitution. On this basis, perovskites have been proposed to work in temperature ranging from 60°C to 200°C, such as cathodes in alkaline fuel cells. In this media, a 2-electron pathway mechanism has been described where the HO2– created is further reduced. Good performance has been reported with different compositions in the La1–*x*A ´*x*BO3 system with *A*´= Ca, Sr and *B* = Co, Ni and Mn.

ABO3 oxides are also an alternative to platinum in high temperature protonic exchange fuel cells (HT-PEMFC); in these devices, new membranes based on polybenzimidazole (PBI) impregnated by H3PO4 allows for an increase in the operation temperature to 130°C–200°C. The increase in the operation temperature gives a chance to Pt-free catalysts, where perovskites show improved chemical resistance under these temperatures. With this aim, LaMnO3 and LaSrMnO3 have been prepared by combustion synthesis with high electrical conductivity at 200°C and improved resistance towards H3PO4 corrosion.

Although perovskites have been proposed for noble metal replacement mainly on the cathode side of fuel cells, there are strong proposals that consider its use for the oxidation of alcohols in the anode of direct methanol fuel cells (DMFCs) working at 60°C–80°C. White y Sammells proposed, in a pioneer work, perovskite electrocatalysts in, among others, SrMO3 (M = Ru,Pd), SmCoO3 and SrRuMO3 (M = Pt, Pd) systems. They demonstrated activity towards direct methanol oxidation during cyclic voltammetry measurements that gave methanol oxidation currents up to 28 mA/cm² at 0.45 V vs. SCE. Following these results, SrRuO3 was prepared by the combustion method as a high specific surface area catalyst with performance comparable to PtRu towards MeOH oxidation at potentials ranging from 0.25 to 0.35 V; nevertheless, the authors proposed the addition of Pt to enhance its catalytic activity. Similarly, strontiumsubstituted lanthanum cobaltite and copper-based perovskite nanoparticles were synthesized by the sol–gel method. Although the methanol oxidation onset potential was 0.03 V lower for LSCo than that for LSCu, an improved electrocatalytic activity for LSCu was found. This was attributed to the higher methanol adsorption capability of Cu ions and to the oxygen ion (O2−) transport into the proximity of adsorbed methanol oxidation that facilitates the formation of intermediates at the reaction site.

Finally, perovskites are being reconsidered as anode catalysts in alkaline DMFC. In contrast to acid electrolytes, the use of an alkaline media improves the kinetics of the reaction while much less expensive catalysts, particularly oxides, could potentially be used. With this idea, La2−*<sup>x</sup>*SrxNiO4 (0 ≤ *x* ≤ 1) was prepared using the citric acid sol–gel route producing higher current densities and negligible poisoning by the methanol oxidation products than those already reported on other perovskite oxides.

## **7. Synthesis and processing of perovskites for fuel cells**

There are several different methods to synthetize perovskites with suitable properties and morphology for use in fuel cells and the method chosen to obtain these materials can influence the final performance of the cell. The mixture of oxides through reaction-sintering was used in the beginnings of the obtaining of oxides with multiple cations, such as perovskites for application in fuel cells and. In some cases, it is investigated until nowadays. However, some disadvantages like compositional heterogeneity, grain growth and, in the case of electrodes, low specific surface area, led to the development and utilization of new methods.

Many of these novel methods were based on a chemical route, where a solution containing the desired cations is used in order to obtain the final multiple oxide. The use of the coprecipitation method can enhance the oxidation catalysis of cathode materials; however, it can be more difficult to carry out because of external influences during the precipitation reaction. Sol–gel is a very common method to synthesize perovskites and it has a relative ease of control and requires lower temperatures of crystallization, allowing the obtaining of a single-phase homogeneous microstructure.

Drip pyrolysis can produce electrode materials with a high concentration of surface reaction sites, but with a poor surface area. One of the most applied methods is the solution combustion synthesis, due to its simplicity and quickness to produce fine powders and the possibility to vary some parameters in order to modify the final morphology.

The processing of materials is one of the great challenges in the fabrication of SOFC devices and many different processing methods were applied and reported in order to fabricate SOFC components. The use of expensive methods such as RF sputtering were carried out at the end of the last century; however, more recently, researchers and industries in this field have a great tendency of choosing simpler and less expensive techniques. Methods like screen printing, tape casting and dip coating, which are based on the preparation of a suspension containing a previously synthesized powder, are common methods to fabricate electrodes and electrolytes for SOFC and allow the control of the microstructure, which is a very important factor since electrodes must be porous and electrolytes, dense.

SmCoO3 and SrRuMO3 (M = Pt, Pd) systems. They demonstrated activity towards direct methanol oxidation during cyclic voltammetry measurements that gave methanol oxidation currents up to 28 mA/cm² at 0.45 V vs. SCE. Following these results, SrRuO3 was prepared by the combustion method as a high specific surface area catalyst with performance comparable to PtRu towards MeOH oxidation at potentials ranging from 0.25 to 0.35 V; nevertheless, the authors proposed the addition of Pt to enhance its catalytic activity. Similarly, strontiumsubstituted lanthanum cobaltite and copper-based perovskite nanoparticles were synthesized by the sol–gel method. Although the methanol oxidation onset potential was 0.03 V lower for LSCo than that for LSCu, an improved electrocatalytic activity for LSCu was found. This was attributed to the higher methanol adsorption capability of Cu ions and to the oxygen ion (O2−) transport into the proximity of adsorbed methanol oxidation that facilitates the formation of

Finally, perovskites are being reconsidered as anode catalysts in alkaline DMFC. In contrast to acid electrolytes, the use of an alkaline media improves the kinetics of the reaction while much less expensive catalysts, particularly oxides, could potentially be used. With this idea, La2−*<sup>x</sup>*SrxNiO4 (0 ≤ *x* ≤ 1) was prepared using the citric acid sol–gel route producing higher current densities and negligible poisoning by the methanol oxidation products than those

There are several different methods to synthetize perovskites with suitable properties and morphology for use in fuel cells and the method chosen to obtain these materials can influence the final performance of the cell. The mixture of oxides through reaction-sintering was used in the beginnings of the obtaining of oxides with multiple cations, such as perovskites for application in fuel cells and. In some cases, it is investigated until nowadays. However, some disadvantages like compositional heterogeneity, grain growth and, in the case of electrodes,

Many of these novel methods were based on a chemical route, where a solution containing the desired cations is used in order to obtain the final multiple oxide. The use of the coprecipitation method can enhance the oxidation catalysis of cathode materials; however, it can be more difficult to carry out because of external influences during the precipitation reaction. Sol–gel is a very common method to synthesize perovskites and it has a relative ease of control and requires lower temperatures of crystallization, allowing the obtaining of a single-phase

Drip pyrolysis can produce electrode materials with a high concentration of surface reaction sites, but with a poor surface area. One of the most applied methods is the solution combustion synthesis, due to its simplicity and quickness to produce fine powders and the possibility to

The processing of materials is one of the great challenges in the fabrication of SOFC devices and many different processing methods were applied and reported in order to fabricate SOFC

low specific surface area, led to the development and utilization of new methods.

intermediates at the reaction site.

homogeneous microstructure.

already reported on other perovskite oxides.

628 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

**7. Synthesis and processing of perovskites for fuel cells**

vary some parameters in order to modify the final morphology.

Figure 2 shows two scanning electron micrographs of porous thin LSM films fabricated on YSZ substrates by dip coating. The LSM powders were obtained by combustion synthesis and different solvents (a and b) were used to prepare the dispersions. Even with sintering at the same temperature, it is possible to observe that slightly different microstructures were obtained, but a strong influence on the thickness of films can be seen.

**Figure 2.** LSM films obtained on YSZ substrates by dip coating with different solutions.

Another approach to fabricate SOFC components is based on wet chemical techniques, where a solution is prepared and the perovskite crystallization reaction takes place on another SOFC component. The adaptation of sol–gel with dip coating or spray pyrolysis, for example, makes it possible to obtain the perovskite *in situ*. That is, already in a form of a final SOFC component instead of obtaining the powder and then coating another surface with it. These are very promising techniques because they can also reduce the processing temperature in some cases.

## **8. Final considerations**

Ceramic perovskite-type oxides have great potential for utilization in efficient energy conver‐ sion devices such as fuel cells, especially in SOFC and PEMFC. The research and development on PEMFC is more recent and, until now, has been restricted to electrodes whereas in SOFC it has been shown that the perovskites, together with processing, will definitely play a key role towards its commercialization. The vast variety of compositions obtained with doping elements in nonstoichiometric amounts allows the modification of properties in a wide range. Very often, the use of such elements interferes in more than one property and it is common to have deleterious effects in the overall component performance.

In SOFCs, many perovskite compositions have already been tested, particularly for cathodes, and the majority are based on one or more of these cations: La, Mn, Fe, Sr Cr, Co, Ni, among others. The catalytic activity and the mixed ionic and electronic conduction are the most sought characteristics for use in the electrodes while for electrolytes only the ionic conduction is desirable, whereas for interconnects, electronic conduction is the one. Of course, besides electrochemical properties, the compositions must have chemical and physical compatibility with each other when used in a fuel cell. This means that they should not react or decompose to form undesirable phases and the thermal expansion coefficient of all components must be similar in order to avoid the formation or propagation of cracks during operation and/or thermal cycling.

## **Author details**

Diego Pereira Tarragó1\*, Berta Moreno2 , Eva Chinarro2 and Vânia Caldas de Sousa1


2 Instituto de Cerámica y Vidrio CSIC, Madrid, Spain

## **References**

[1] EG&G Technical Services: Science and Applications International Corporation. Fuel Cell Handbook. 6th ed. United States of America: U.S. Department of Energy; 2002.

[2] Andújar J.M., Segura F.. Fuel cells: History and updating. A walk along two centu‐ ries. Renewable and Sutainable Energy Reviews. 2009;13:2309–2322. DOI: 10.1016/ j.rser.2009.03.015

instead of obtaining the powder and then coating another surface with it. These are very promising techniques because they can also reduce the processing temperature in some cases.

Ceramic perovskite-type oxides have great potential for utilization in efficient energy conver‐ sion devices such as fuel cells, especially in SOFC and PEMFC. The research and development on PEMFC is more recent and, until now, has been restricted to electrodes whereas in SOFC it has been shown that the perovskites, together with processing, will definitely play a key role towards its commercialization. The vast variety of compositions obtained with doping elements in nonstoichiometric amounts allows the modification of properties in a wide range. Very often, the use of such elements interferes in more than one property and it is common to

In SOFCs, many perovskite compositions have already been tested, particularly for cathodes, and the majority are based on one or more of these cations: La, Mn, Fe, Sr Cr, Co, Ni, among others. The catalytic activity and the mixed ionic and electronic conduction are the most sought characteristics for use in the electrodes while for electrolytes only the ionic conduction is desirable, whereas for interconnects, electronic conduction is the one. Of course, besides electrochemical properties, the compositions must have chemical and physical compatibility with each other when used in a fuel cell. This means that they should not react or decompose to form undesirable phases and the thermal expansion coefficient of all components must be similar in order to avoid the formation or propagation of cracks during operation and/or

, Eva Chinarro2

[1] EG&G Technical Services: Science and Applications International Corporation. Fuel Cell Handbook. 6th ed. United States of America: U.S. Department of Energy; 2002.

and Vânia Caldas de Sousa1

have deleterious effects in the overall component performance.

630 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

**8. Final considerations**

thermal cycling.

**Author details**

**References**

Diego Pereira Tarragó1\*, Berta Moreno2

\*Address all correspondence to: diego.tarrago@ufrgs.br

2 Instituto de Cerámica y Vidrio CSIC, Madrid, Spain

1 Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil


for SOFCs. International Journal of Hydrogen Energy. 2013;38:1068–1073. DOI: 10.1016/j.ijhydene.2012.10.099

[26] Onuma S., Miyoshi S., Yashiro K., Kaimai A., Kawamura K., Nigara Y., et al.. Phase stability of La1–xCaxCrO3–δ in oxidizing atmosphere. Journal of Solid State Chemistry. 2003;170:68–74.

[15] Siebert E., Roux C., Boréave A., Gaillard F., Vernoux P.. Oxido-reduction properties of La0.7Sr0.3Co0.8Fe0.2O3–δ perovskite oxide catalyst. Solid State Ionics. 2011;183:40–47.

[16] Ko H.J., Myung J., Lee J., Hyun S., Chung J.S.. Synthesis and evaluation of (La0.6Sr0.4) (Co0.2Fe0.8)O3 (LSCF) - Y0.08Zr0.92O1.96 (YSZ) - Gd0.1Ce0.9O2–δ (GDC) dual composite cath‐ odes for high performance and durability. International Journal of Hydrogen Energy.

[17] DiGiuseppe G., Sun L.. Electrochemical performance of a solid oxide fuel cell with an LSCF cathode under different oxygen concentrations. International Journal of Hydro‐

[18] Tarancón A., Peña-Martínez J., Marrero-López D., Morata A., Ruiz-Morales J.C., Nu‐ ñez P.. Stability, chemical compatibility and electrochemical performance of GdBa‐ Co2O5+x layered perovskite as a cathode for intermediate temperature solid oxide fuel

[19] Kim J.H., Irvine J.T.S.. Characterization of NdBa1–xSrxCo2O5+δ (*x* = 0 and 0.5) as cath‐ ode materials for IT-SOFC. International Journal of Hydrogen Energy. 2012;37:5920–

[20] Huang B., Wang S.R., Liu R.Z., Ye X.F., Nie H.W., Sun X.F., Wen T.L.. Performance of La0.75Sr0.25Cr0.5Mn0.5O3–δ perovskite-structure anode material at lanthanum gallate elec‐ trolyte for IT-SOFC running on ethanol fuel. Journal of Power Sources. 2007;167:39–

[21] Wenyi T., Qin Z., Han Y., Xiufang Z., Hongyi L.. Deactivation of anode catalyst La0.75Sr0.25Cr0.5Mn0.5O3±δ in SOFC with fuel containing hydrogen sulfur. The role of lat‐ tice oxygen. International Journal of Hydrogen Energy. 2012;37:7398–7404. DOI:

[22] Zha S., Tsang P., Cheng Z., Liu M. Electrical properties and sulphur tolerance of La0.75Sr0.25Cr1–x MnxO3 under anodic conditions. Journal of Solid State Chemistry.

[23] Sauvet A.-L., Fouletier J., Gaillard F., Primet M.. Surface properties and physico‐ chemical characterizations of a new type of anode material, La1−xSrxCr1−yRuyO3−δ, for a solid oxide fuel cell under methane at intermediate temperature. Journal of Catalysis.

[24] Ruiz-Morales J.C., Canales-Vázquez J., Savaniu C., Marrero-López D., Zhou W., Ir‐ vine J.T.S.. Disruption of extended defects in solid oxide fuel cell anodes for methane.

[25] Du Z., Zhao H., Zhou X., Xie Z., Zhang C.. Electrical conductivity and cell perform‐ ance of La0.7Sr0.7Ti1–xCrxO3- perosvkite oxides used as anode and interconnect material

cells. Solid State Ionics. 2008;179:2372–2378. DOI: 10.1016/j.ssi.2008.09.016

2012;37:17209–17216. DOI: 10.1016/j.ijhydene.2012.08.099

5929. DOI: 10.1016/j.ijhydene.2011.12.150

46. DOI: 10.1016/j.jpowsour.2007.02.022

2005;178:1844–1850. DOI: 10.1016/j.jssc.2005.03.027

Nature. 2006;439:568–571. DOI: 10.1038/nature04438

2002;209:25–34. DOI: 10.1006/jcat.2002.3588

10.1016/j.ijhydene.2012.02.008

gen Energy. 2011;36:5076–5087. DOI: 10.1016/j.ijhydene.2011.01.017

DOI: 10.1016/j.ssi.2010.11.012

632 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications


[50] Suntivich J., May K.J., Gasteiger H.A, Goodenough J.B., Shao-Horn Y.. A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles. Sci‐ ence. 2011;334:1383–1385. DOI: 10.1126/science.1212858

[37] Feng M., Goodenough J.B.. A superior oxide-ion electrolyte. European Journal of Sol‐

[38] Zhao Y., Xia C., Jia L., Wang Z., Li H., Yu J., Li Y.. Recent progress on solid oxide fuel cell: lowering temperature and utilizing non-hydrogen fuels. International Journal of Hydrogen Energy. 2013;38(36):16498–16517. DOI: 10.1016/j.ijhydene.2013.07.077

[39] Yokokawa H., Sakai N., Horita T., Yamaji K.. Recent developments in solid oxide fuel cell materials. Fuel Cells. 2001;1(2):117–131. DOI:

[40] Thangadurai V., Weppner W.. Recent progress in solid oxide an lithium ion conduct‐ ing electroytes research. Ionics. 2006;12(1):81–92. DOI: 10.1007/s11581-006-0013-7

[41] Karton V.V., Marques F.M.B., Atkinson A.. Transport properties of solid oxide elec‐ trolyte ceramics: a brief review. Solid State Ionics. 2004;174(1–4):135–149. DOI:

[42] Gauckler L.J., Beckel D., Buergler B.E., Jud E., Muecke U.P., Prestat M., et al.. Solid oxide fuel cells: systems and materials Chimia. 2004;58(12):837–850. DOI:

[43] Iwahara H., Uchida H., Tanaka S.. Studies on solid electrolyte gas cells with hightemperature-type proton conductor and oxide ion conductor. Solid State Ionics.

[44] Bi L., Traversa E.. Synthesis strategies for imporving the performance of doped-BaZ‐ rO3 materials in solid oxide fuel cell applications. Journal of Materials Research.

[45] Bonanos N., Knight K.S., Ellis B.. Perovskite solid electrolyte: structure, transport properties and fuel cell applications. Solid State Ionics. 1995;79:161–170. DOI:

[46] Fabbri E., Pergolesi D., Traversa E.. Materials challenges toward proton-conducting oxide fuel cells: a critical review. Chemical Society Reviews. 2010;39: 4355–4369. DOI:

[47] Santiso J., Burriel M.. Deposition and characterization of epitaxial oxide thin films for SOFCs. Journal of Solid State Electrochemistry. 2011;15(5):985–1006. DOI: 10.1007/

[48] Jiang S.P.. Development of lanthanum strontium manganite perovskite cathode ma‐ terials of solid oxide fuel cells: a review. Journal of Materials Science. 2008;43(21):

[49] Christensen J., Albertus P., Sanchez-Carrera R.S., Lohmann T., Kozinsky B., Liedtke R., et al.. A critical review of Li/air batteries. Journal of the Electrochemical Society.

id State and Inorganic Chemistry. 1994;31(8–9):663–672.

634 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

10.1002/1615-6854(200107)1:2<117::AID-FUCE117>3.0.CO;2-Y

1983;11(2):109–115. DOI: 10.1016/0167-2738(83)90047-4

2014;29(1):1–15. DOI: 10.1557/jmr.2013.205

6799–6833. DOI: 10.1007/s10853-008-2966-6

2012;159(2):1–30. DOI: 10.1149/2.086202jes

10.1016/j.ssi.2004.06.015

10.2533/000942904777677047

10.1016/0167-2738(95)00056-C

10.1039/b902343g

s10008-010-1214-6


bifunctional air electrodes. Crystal Engineering. 2002;5(3):449–447. DOI: 10.1016/ S1463-0184(02)00056-4


[75] Arendt E., Maione A., Klisinska A., Sanz O., Montes M., Suarez S., et al.. Structura‐ tion of LaMnO3 perovskite catalysts on ceramic and metallic monoliths: Physicochemical characterisation and catalytic activity in methane combustion. Applied Catalysis A: General. 2008;339:1–14. DOI: 10.1016/j.apcata.2008.01.016

bifunctional air electrodes. Crystal Engineering. 2002;5(3):449–447. DOI: 10.1016/

[63] Villaseca L., Moreno B., Chinarro E.. Perovskites based on La(Sr)-Mn-O system as electrocatalyst in PEM fuel cell of high temperature. International Journal of Hydro‐

[64] White J.H., Sammells A.F.. Perovskite anode electrocatalysis for direct methanol fuel cells. Journal of the Electrochemical Society. 1993;140(8):2167–2177. DOI:

[65] Deshpande K., Mukasyan A., Varma A.. High throughout evaluation of perovskitebased anode catalysts for direct methanol fuel cells. Journal of Power Sources.

[66] Yu H.C., Fung K.Z., Chang W.L.. Syntheses of perovskite oxides nanoparticles La1−xSrxMO3−δ (M = Co and Cu) as anode electrocatalyst for direct methanol fuel cell.

[67] Singh R.N., Sharma T., Singh A., Anindita, Mishra D., Tiwari S.K.. Perovskite-type La2−xSrxNiO4 (0≤ *x* ≤1) as active anode materials for methanol oxidation in alkaline solutions. Electrochimica Acta. 2008;53:2322–2330. DOI: 10.1016/j.electacta.

[68] Raghuveer V., Viswanathan B.. Can La2–xSrxCuO4 be used as anodes for direct metha‐ nol fuel cells? Fuel. 2002;81:219–2197. DOI:10.1016/S0016-2361(02)00167-9

[69] Bell R.J., Millar G.J., Drennan J.. Influence of synthesis route on the catalytic proper‐

[70] Gaoke Z., Ying L., Xia Y., Yanping W., Shixi O., Hangxing L.. Comparison of synthe‐ sis methods, crystal structure and characterization of strontium cobaltite powders. Materials Chemistry and Physics. 2006;99:88–95. DOI: 10.1016/j.matchemphys.

[71] Ghosh A., Sahu A.K., Gulnar A.K., Suri A.K.. Synthesis and characterization of lan‐ thanum strontium manganite. Scripta Materialia. 2005;52:1305–1309. DOI: 10.1016/

[72] Tarragó D.P., Malfatti C.F., Sousa V.C.. Influence of fuel on morphology of LSM powders obtained by solution combustion synthesis. Powder Technology.

[73] Bebelis S., Kotsionopoulus N., Mai A., Tietz F.. Electrochemical characterization of perovskite-based SOFC cathodes. Journal of Applied Electrochemistry. 2007;37:15–

[74] Gharbage B., Mandier F., Lauret H., Roux C., Pagnier T.. Electrical properties of

Electrochimica Acta. 2004;50(2–3):811–816. DOI: 10.1073/pnas.1210315109

gen Energy. 2012;37(8):7161–7170. DOI: 10.1016/j.ijhydene.2011.12.142

2006;158:60–68. DOI: 10.1016/j.jpowsour.2005.09.025

ties of La1–xSrxMnO3. Solid State Ionics. 2000;131:211–220.

2015;269:481–487. DOI: 10.1016/j.powtec.2014.09.037

La0.5Sr0.5MnO3 thin films. Solid State Ionics. 1995;82:85–94.

S1463-0184(02)00056-4

636 Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

10.1149/1.2220791

2007.09.047

2005.09.078

j.scriptamat.2005.02.020

20. DOI: 10.1007/s10800-006-9215-y


## *Edited by Likun Pan and Guang Zhu*

The book summarizes the current state of the know-how in the field of perovskite materials: synthesis, characterization, properties, and applications. Most chapters include a review on the actual knowledge and cutting-edge research results. Thus, this book is an essential source of reference for scientists with research fields in energy, physics, chemistry and materials. It is also a suitable reading material for graduate students.

Photo by auipuistock / DollarPhoto

Perovskite Materials - Synthesis, Characterisation, Properties, and Applications

Perovskite Materials

Synthesis, Characterisation,

Properties, and Applications

*Edited by Likun Pan and Guang Zhu*