**3. Perovskites as photocatalytic**

photocatalysis and so in the present proposal we develop the strontium doped neodymium manganites nanocomposites within perovskite like structure as photocatalysis and studying its performance and so the main goals are; −synthesis new perovskite materials enhanced the photocatalysis performance applying the obtained results for solar energy utilizations.

Metal oxide photocatalysis is based on metal oxide like titanium dioxide as light-activated catalysts [7]. Three types of materials are used in the degradation of organic matter which has

**Figure 3.** Photo-degradation of AO in the presence of CeO<sup>2</sup> **1** nanoparticles [5].

**Figure 4.** Schematic representation; top light with energy higher than band gap leads to charge separation, with electron reducing a donor (usually oxygen) and hole oxidizing a donor (usually water); summary of processes occurring. Image

based on Bahnemann (2004) [7].

132 Nanocomposites - Recent Evolutions

ABO3 perovskites are very essential family of oxide materials because they possess very interesting physical and chemical properties. These unusual properties may lead to use these materials in potential applications. The corner-shared octahedral BO<sup>6</sup> lattice site in these materials play very important role in transfer of oxygen and electrons easily and may lead to nonstoichiometry of oxygen [8–23]. Moreover, the mixed valence states of the transition metal at B-site are also important term in such perovskite-type oxides, which affect their activity. Nevertheless a lot of applications depend on the A and B cations in the ABO3 perovskites, such as electrocatalysts for O2 evolution [8–10], catalysts [11, 12], photo/electro- catalysts for hydrogen production and pollutants degradation [13–19] and electrode material used in fuel cells [13]. The synthesis of perovskite materials could be done using different methods such as solid state reaction [24–28], chemical co-precipitation [29–33], sol–gel [34–38]. In each method there are parameters to play with in order to improve the properties of the required materials. A lot of perovskite oxides have been synthesized such as tantalate [39–43], titanate [14, 44–50], ferrite, [51, 52] vanadium-and niobium-based perovskites [53–56], and manganites [57, 58] and they have shown visible light photocatalytic activity as a result of their unique electronic properties and crystal structures [59]. The reduced band-gap energy values in the doped alkaline rare-earth transition metal perovskite-like structure oxides focus more attention because this property enhances the separation of charge carriers (photogenerated electrons and holes) [60]. Intensive studies have been done on these materials because of the capability of tuning their electrical and optical properties, indicating a control of their rational design structure by substitutions of cationic in ABO3 pervoskite [61, 62]. Therefore, we can say that the perovskite compounds are one of the promising structure that are adapting the bandgap values to harvest visible-light absorption and the potentials of band edge to tailor the needs of particular photocatalysis.

Furthermore, the lattice distortion existed in the rare earth transition metal perovskite compounds strongly affects the separation of photogenerated charge carriers [59, 63, 64]. The distortion in the bond angles resulted from both; metal-ligand or the metal-ligand-metal into perovskite framework are significantly related to their charge carriers and band gap values [65–67]. The crystallinity, phase structure, size, and surface area affect the efficiency of photocatalysts. Consequently, control of the shape of perovskites and the size and crystal phase is essential and significant parameter for assessing their phase-dependent photoactivity and promoting perovskites-based driven visible light photocatalysts. According to Abdel-Latif et al. [66], Nd0.6Sr0.4MnO<sup>3</sup> was studied as superior photocatalyst under visible light, different modifications of perovskite Nd0.6Sr0.4MnO<sup>3</sup> to get high harvesting of photons and enhancing the migration and separation of the photogenerated charge carriers through the photocatalytic reaction [61–65]. For the first time, the impact on phase structures and photocatalytic efficiencies under visible light of the annealed Nd0.6Sr0.4MnO<sup>3</sup> perovskite which prepared by sol–gel method in the presence of polyethylene glycol and citric acid was studied by Abdel-Latif et al. [66], and the Nd0.6Sr0.4MnO<sup>3</sup> perovskite annealed at 500°C was found to be a superior photocatalyst than that annealed at 800, 1000 and 1150°C. Nd0.6Sr0.4MnO<sup>3</sup> semiconductor has a narrow band gap energy values ranged from 2 to 2.98 eV, which we can control its value by changing its annealing temperatures. Charge carriers created by absorbing visible light (photogenerated electrons and holes) depend on the excitation by this visible light. The hole, which photogenerated in the valence band reacts either with the adsorbed **−**OH ions or H<sup>2</sup> O onto the surface of NSMO producing OH•. On the other side, the electron, which photogenerated in the conduction band reduces O<sup>2</sup> to get O2 •**−** give rising to other oxidative O<sup>2</sup> species (i.e., OH• and H<sup>2</sup> O2 ). The photocatalytic efficiencies of the Nd0.6Sr0.4MnO<sup>3</sup> nanocomposites were evaluated in Ref. [66] for the MB photodegradation, where they calculated the MB photodecomposition under visible light illumination by recording absorption spectra. They found that MB is negligible at the photolysis and it is stable after visible light illumination for 3 h. Furthermore, there is a slight decrease in MB concentration as a result of adsorption onto Nd0.6Sr0.4MnO<sup>3</sup> surface when it is suspended with MB solution in dark as shown in **Figure 5**. The observed MB absorption bands at λ = 663 and 291 nm gradually decreased upon boosting illumination times.

As it is clear from the photocatalytic performance of the Nd0.6Sr0.4MnO<sup>3</sup> perovskite, the crystalline size (55 nm), which depends on the annealed temperature (500°C). The mixed perovskite structure Nd0.6Sr0.4MnO<sup>3</sup> (26.18% orthorhombic "Orth" and 73.82% monoclinic "Mon") obtained at annealing temperature 500°C is a superior photocatalyst candidate than that of Nd0.6Sr0.4MnO<sup>3</sup> perovskite obtained at annealing temperature 1150°C and with mixed structure (82.22% cubic "Cub" and 17.78% orthorhombic "Orth" phases). The observed photo degradation was 100% by the annealing temperate 500°C of the Nd0.6Sr0.4MnO<sup>3</sup> perovskite [66]. However, as a result of the increase in the annealing temperature to 1150°C, reduction in the photocatalytic efficiency was observed to be 60%. Looking at the effect of the annealing temperature in Nd0.6Sr0.4MnO<sup>3</sup> perovskite according to Abdel-Latif et al., [66], the overall photodegradation rate of the sample annealed at 500°C is significantly 3-times higher than that of the other sample, which annealed at 1150°C. The superiority of the neodymium strontium doped manganite, which annealed at 500°C is attributed to the mixed crystallographic structure with double phases (Mon/Orth) framework, high crystallinity, and the Mn-O polyhedron distortion. From this work on can say that key factors for the high photocatalytic activity of the obtained neodymium strontium doped manganite with annealing temperature 500°C are the high visible-light absorption, lattice distortion and narrow band gap.

Another example of the rare earth manganites is the non-stoichiometric perovskites; La<sup>1</sup>−<sup>x</sup> Srx MnO<sup>3</sup>−δ (x = 0.35, 0.50, 0.65, 0.80) series, which was examined by Antoine Demont and Stéphane Abanades [67] in the context of solar-driven two-step thermo-chemical dissociation of CO<sup>2</sup> . All the performance characterization measurements such as X-ray diffraction and thermochemical characterizations were carried out in order to the evaluation of the redox activity of these materials toward the thermal reduction under inert atmosphere followed by the re-oxidation process and carbon oxide generation from CO<sup>2</sup> . They found that, the control of introducing strontium into lantanium manganite allowed tuning the redox thermodynamics within the series. The high activity observed toward both thermal reduction and CO<sup>2</sup> dissociation occurred. As a result of analysis of experimental measurements they found that the La0.50Sr0.50MnO<sup>3</sup>−*<sup>δ</sup>* composition is a promising candidate for thermochemical CO<sup>2</sup> splitting **Figures 6** and **7**.

Maximum production of carbon oxide is reached in the range of 270 μmol g−<sup>1</sup>

for nano Nd0.6Sr0.4MnO<sup>3</sup>

values and the percentage of the monoclinic phase (b) [66].

erties suitable for efficient solar-driven thermochemical CO<sup>2</sup>

nanostructured perovskite-like structure La1–*<sup>x</sup>*

**Figure 5.** Optical bandgap energy *Eg*

bandgap energy *Eg*

carbon dioxide splitting step with an optimal temperature of re-oxidation 1050°C (thermal reduction performed under Argon gas at 1400°C), in spite of the re-oxidation yield limitation "50%". The evolution of the manganese oxidation state reveal partial re-oxidation of Mn3+ into Mn4+, thus the activation of Mn4+/Mn3+ redox pair in the perovskites was confirmed. They concluded that the mixed valence perovskites have clear potential for displaying redox prop-

Perovskite Strontium Doped Rare Earth Manganites Nanocomposites and Their Photocatalytic Performances

Oxygen diffusion and desorption in oxides have been developed for slightly defective and well crystallized bulky materials in Ref. [68]. The relation between nanostructure and the change of the mechanism of oxygen mobility has been studied in this work. Temperature programmed oxygen desorption and thermogravimetric analysis applied to study some

A*x*

MnO3±*<sup>δ</sup>*

during the

dissociation [67].

perovskite annealed at 500°C (a), relation between the

http://dx.doi.org/10.5772/intechopen.79479

135

samples (A = Sr. and Ce, 20–60 nm

Perovskite Strontium Doped Rare Earth Manganites Nanocomposites and Their Photocatalytic Performances http://dx.doi.org/10.5772/intechopen.79479 135

method in the presence of polyethylene glycol and citric acid was studied by Abdel-Latif et al.

band gap energy values ranged from 2 to 2.98 eV, which we can control its value by changing its annealing temperatures. Charge carriers created by absorbing visible light (photogenerated electrons and holes) depend on the excitation by this visible light. The hole, which photogen-

NSMO producing OH•. On the other side, the electron, which photogenerated in the conduc-

•**−** give rising to other oxidative O<sup>2</sup>

[66] for the MB photodegradation, where they calculated the MB photodecomposition under visible light illumination by recording absorption spectra. They found that MB is negligible at the photolysis and it is stable after visible light illumination for 3 h. Furthermore, there is a

it is suspended with MB solution in dark as shown in **Figure 5**. The observed MB absorption

crystalline size (55 nm), which depends on the annealed temperature (500°C). The mixed

"Mon") obtained at annealing temperature 500°C is a superior photocatalyst candidate than

structure (82.22% cubic "Cub" and 17.78% orthorhombic "Orth" phases). The observed photo

[66]. However, as a result of the increase in the annealing temperature to 1150°C, reduction in the photocatalytic efficiency was observed to be 60%. Looking at the effect of the annealing

todegradation rate of the sample annealed at 500°C is significantly 3-times higher than that of the other sample, which annealed at 1150°C. The superiority of the neodymium strontium doped manganite, which annealed at 500°C is attributed to the mixed crystallographic structure with double phases (Mon/Orth) framework, high crystallinity, and the Mn-O polyhedron distortion. From this work on can say that key factors for the high photocatalytic activity of the obtained neodymium strontium doped manganite with annealing temperature 500°C are

Another example of the rare earth manganites is the non-stoichiometric perovskites; La<sup>1</sup>−<sup>x</sup>

the performance characterization measurements such as X-ray diffraction and thermochemical characterizations were carried out in order to the evaluation of the redox activity of these materials toward the thermal reduction under inert atmosphere followed by the re-oxidation process

into lantanium manganite allowed tuning the redox thermodynamics within the series. The

result of analysis of experimental measurements they found that the La0.50Sr0.50MnO<sup>3</sup>−*<sup>δ</sup>*

Abanades [67] in the context of solar-driven two-step thermo-chemical dissociation of CO<sup>2</sup>

MnO<sup>3</sup>−δ (x = 0.35, 0.50, 0.65, 0.80) series, which was examined by Antoine Demont and Stéphane

alyst than that annealed at 800, 1000 and 1150°C. Nd0.6Sr0.4MnO<sup>3</sup>

to get O2

perovskite structure Nd0.6Sr0.4MnO<sup>3</sup>

The photocatalytic efficiencies of the Nd0.6Sr0.4MnO<sup>3</sup>

erated in the valence band reacts either with the adsorbed **−**OH ions or H<sup>2</sup>

slight decrease in MB concentration as a result of adsorption onto Nd0.6Sr0.4MnO<sup>3</sup>

As it is clear from the photocatalytic performance of the Nd0.6Sr0.4MnO<sup>3</sup>

bands at λ = 663 and 291 nm gradually decreased upon boosting illumination times.

degradation was 100% by the annealing temperate 500°C of the Nd0.6Sr0.4MnO<sup>3</sup>

the high visible-light absorption, lattice distortion and narrow band gap.

high activity observed toward both thermal reduction and CO<sup>2</sup>

tion is a promising candidate for thermochemical CO<sup>2</sup>

perovskite annealed at 500°C was found to be a superior photocat-

semiconductor has a narrow

species (i.e., OH• and H<sup>2</sup>

nanocomposites were evaluated in Ref.

(26.18% orthorhombic "Orth" and 73.82% monoclinic

perovskite obtained at annealing temperature 1150°C and with mixed

perovskite according to Abdel-Latif et al., [66], the overall pho-

. They found that, the control of introducing strontium

splitting **Figures 6** and **7**.

dissociation occurred. As a

O onto the surface of

O2 ).

surface when

perovskite, the

perovskite

. All

composi-

[66], and the Nd0.6Sr0.4MnO<sup>3</sup>

134 Nanocomposites - Recent Evolutions

tion band reduces O<sup>2</sup>

that of Nd0.6Sr0.4MnO<sup>3</sup>

Srx

temperature in Nd0.6Sr0.4MnO<sup>3</sup>

and carbon oxide generation from CO<sup>2</sup>

**Figure 5.** Optical bandgap energy *Eg* for nano Nd0.6Sr0.4MnO<sup>3</sup> perovskite annealed at 500°C (a), relation between the bandgap energy *Eg* values and the percentage of the monoclinic phase (b) [66].

Maximum production of carbon oxide is reached in the range of 270 μmol g−<sup>1</sup> during the carbon dioxide splitting step with an optimal temperature of re-oxidation 1050°C (thermal reduction performed under Argon gas at 1400°C), in spite of the re-oxidation yield limitation "50%". The evolution of the manganese oxidation state reveal partial re-oxidation of Mn3+ into Mn4+, thus the activation of Mn4+/Mn3+ redox pair in the perovskites was confirmed. They concluded that the mixed valence perovskites have clear potential for displaying redox properties suitable for efficient solar-driven thermochemical CO<sup>2</sup> dissociation [67].

Oxygen diffusion and desorption in oxides have been developed for slightly defective and well crystallized bulky materials in Ref. [68]. The relation between nanostructure and the change of the mechanism of oxygen mobility has been studied in this work. Temperature programmed oxygen desorption and thermogravimetric analysis applied to study some nanostructured perovskite-like structure La1–*<sup>x</sup>* A*x* MnO3±*<sup>δ</sup>* samples (A = Sr. and Ce, 20–60 nm

of nanostructured photosensitizers, for example, plasmonic metal nanostructures, quantum dots, and carbon nanostructures engaged with the wide-bandgap in transition metal oxides that allow us to design a new visible-light active photocatalysts [4]. The implied mechanisms of the nanocomposite photocatalysts, for example, the charge separation inducing light and the visible-light photocatalytic reaction procedure in environmental treatment besides solar

Perovskite Strontium Doped Rare Earth Manganites Nanocomposites and Their Photocatalytic Performances

The rare earth manganites as well as the rare earth cobalt with perovskite-like structure (the rare earth like; lanthanum, praseodymium, or neodymium) are studied in Ref. [69], where they found that these materials are active catalysts for the oxidation of carbon monoxide. Comparing initial activity and lifetime in crushed single crystals of these composites and the commercial platinum catalysts showed its good performance. Therefore, one can say that these materials are considered as a promising alternate for platinum in devices for the catalytic treatment of auto exhaust.

troscopy was reported recently [70]. They reported the effect of temperature on Raman spectra and they showed the shift in the phonon frequency of most intense modes in dysprosium chro-

tion with the spin–phonon coupling. The impedance spectroscopy described in this work implied the anomalies in the dielectric constant *dependent on* temperature near the magnetic transitions

lets. Furthermore, UV–Vis absorption spectroscopy has been measured beside the photocatalytic

enough for the photocatalytic activity application. The efficient photocatalytic activity of DyCrO3 nanoplatelets are described in this work, where degrading value was 65% for 8 h irradiation [70]. Three-way catalysts (TWC) were introduced more than 40 years ago and the development of a sustainable TWC still remains an important subject owing to the increasingly stringent emission regulations together with the price and scarcity of precious metals [71]. Perovskitetype oxides are alternatives to the conventionally used TWC compositions and it is suitable for a wide range of automotive applications, ranging from TWC to diesel oxidation catalysts (DOC). The interest in these catalysts has been renewed because of the catalyst regenerability of perovskite-based TWC concept. Principally, it is applicable to other catalytic processes and there is possibility to reduce the amounts of critical elements, such as valuable metals without

Studying catalysts *in situ* is of high interest for understanding their surface structure and elec-

were found to be active for the oxygen evolution reaction (OER) from water splitting as a result of electro-catalytic water splitting. X-ray absorption near-edge spectroscopy (XANES), at the Mn L- and O K-edges, was measured and analyzed in Ref. [72], besides measuring the X-ray photoemission spectroscopy (XPS) of the O 1s and Ca 2p states. Both measurements were carried out under the following conditions; in water vapor under positive applied bias, in ultra-high vacuum and at room temperature [72]. According to the research in Ref. [72] under the oxidizing conditions of the OER a reduced Mn2+ species is generated at the catalyst surface and the Mn valence shift is accompanied by the formation of surface oxygen vacancies.

tronic states in operation [72]. The epitaxial manganite perovskite thin films (Pr<sup>1</sup>−*<sup>x</sup>*

point that may lead to postulate possible weak magnetoelectric coupling in DyCrO3

). The change in Raman line-width is observed, which is an indication to its correla-

nanoplatelets. The band gap deduced from the optical absorp-

nanoplatelets and this energy is considered as a good

) nanoplatelets by Raman spec-

http://dx.doi.org/10.5772/intechopen.79479

137

nanoplate-

Ca*<sup>x</sup>* MnO<sup>3</sup> )

fuel generation fields, are also presented [10].

mate (DyCrO3

activity measurement for DyCrO3

tion spectrum was ∼2.8 eV for DyCrO3

industriously lowering the catalytic performance.

The phonon-mode assignment of dysprosium chromate (DyCrO3

**Figure 6.** Solar-driven two-step thermochemical dissociation of CO<sup>2</sup> in La1-xSr<sup>x</sup> MnO3-<sup>δ</sup> [67].

**Figure 7.** Three-way catalytic converter TWC [71].

particle size) [53]. Depending on the temperature range and oxygen depletion of the material different rate-determining steps have been identified. Particularly, oxygen diffusion was demonstrated at low temperature and defect concentration, whereas the oxygen recombination at the surface seems is controlled at high temperature. However, the lower activation energy is responsible for the oxygen recombination step.

Utilizing the sunlight efficiently for solar energy conversion, the research on visible-light active photocatalysts attracted a lot of interest [4]. The photosensitization of transition metal oxides is a promising approach for achieving effective visible light photocatalysis. The world of nanostructured photosensitizers, for example, plasmonic metal nanostructures, quantum dots, and carbon nanostructures engaged with the wide-bandgap in transition metal oxides that allow us to design a new visible-light active photocatalysts [4]. The implied mechanisms of the nanocomposite photocatalysts, for example, the charge separation inducing light and the visible-light photocatalytic reaction procedure in environmental treatment besides solar fuel generation fields, are also presented [10].

The rare earth manganites as well as the rare earth cobalt with perovskite-like structure (the rare earth like; lanthanum, praseodymium, or neodymium) are studied in Ref. [69], where they found that these materials are active catalysts for the oxidation of carbon monoxide. Comparing initial activity and lifetime in crushed single crystals of these composites and the commercial platinum catalysts showed its good performance. Therefore, one can say that these materials are considered as a promising alternate for platinum in devices for the catalytic treatment of auto exhaust.

The phonon-mode assignment of dysprosium chromate (DyCrO3 ) nanoplatelets by Raman spectroscopy was reported recently [70]. They reported the effect of temperature on Raman spectra and they showed the shift in the phonon frequency of most intense modes in dysprosium chromate (DyCrO3 ). The change in Raman line-width is observed, which is an indication to its correlation with the spin–phonon coupling. The impedance spectroscopy described in this work implied the anomalies in the dielectric constant *dependent on* temperature near the magnetic transitions point that may lead to postulate possible weak magnetoelectric coupling in DyCrO3 nanoplatelets. Furthermore, UV–Vis absorption spectroscopy has been measured beside the photocatalytic activity measurement for DyCrO3 nanoplatelets. The band gap deduced from the optical absorption spectrum was ∼2.8 eV for DyCrO3 nanoplatelets and this energy is considered as a good enough for the photocatalytic activity application. The efficient photocatalytic activity of DyCrO3 nanoplatelets are described in this work, where degrading value was 65% for 8 h irradiation [70].

Three-way catalysts (TWC) were introduced more than 40 years ago and the development of a sustainable TWC still remains an important subject owing to the increasingly stringent emission regulations together with the price and scarcity of precious metals [71]. Perovskitetype oxides are alternatives to the conventionally used TWC compositions and it is suitable for a wide range of automotive applications, ranging from TWC to diesel oxidation catalysts (DOC). The interest in these catalysts has been renewed because of the catalyst regenerability of perovskite-based TWC concept. Principally, it is applicable to other catalytic processes and there is possibility to reduce the amounts of critical elements, such as valuable metals without industriously lowering the catalytic performance.

Studying catalysts *in situ* is of high interest for understanding their surface structure and electronic states in operation [72]. The epitaxial manganite perovskite thin films (Pr<sup>1</sup>−*<sup>x</sup>* Ca*<sup>x</sup>* MnO<sup>3</sup> ) were found to be active for the oxygen evolution reaction (OER) from water splitting as a result of electro-catalytic water splitting. X-ray absorption near-edge spectroscopy (XANES), at the Mn L- and O K-edges, was measured and analyzed in Ref. [72], besides measuring the X-ray photoemission spectroscopy (XPS) of the O 1s and Ca 2p states. Both measurements were carried out under the following conditions; in water vapor under positive applied bias, in ultra-high vacuum and at room temperature [72]. According to the research in Ref. [72] under the oxidizing conditions of the OER a reduced Mn2+ species is generated at the catalyst surface and the Mn valence shift is accompanied by the formation of surface oxygen vacancies.

particle size) [53]. Depending on the temperature range and oxygen depletion of the material different rate-determining steps have been identified. Particularly, oxygen diffusion was demonstrated at low temperature and defect concentration, whereas the oxygen recombination at the surface seems is controlled at high temperature. However, the lower activation

in La1-xSr<sup>x</sup>

MnO3-<sup>δ</sup> [67].

Utilizing the sunlight efficiently for solar energy conversion, the research on visible-light active photocatalysts attracted a lot of interest [4]. The photosensitization of transition metal oxides is a promising approach for achieving effective visible light photocatalysis. The world

energy is responsible for the oxygen recombination step.

**Figure 6.** Solar-driven two-step thermochemical dissociation of CO<sup>2</sup>

136 Nanocomposites - Recent Evolutions

**Figure 7.** Three-way catalytic converter TWC [71].

According to Madhavan and Ashok [73], perovskite materials exhibiting proton and oxide ion conductivities have been used for various energy-related applications such as solid oxide fuel cells (SOFCs), hydrogen production, gas sensors, etc. Nowadays, nanoperovskites were synthesized and were studied for catalytic activity and energy-related applications. The mechanism of proton and oxide ion conduction, and some specific properties and behaviors of few nanoperovskites as oxide ion and proton conductors and applications have been reported and discussed in this work [73].

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