**3. Catalytic applications**

#### **3.1. nZVI and supported Fe0 nanocomposites**

## *3.1.1. Catalyzed removal or decomposition of pollutants*

nZVI has been extensively reported to resolve a series of environmental problems, related with destruction, adsorption, precipitation, reduction or oxidation of heavy metals, salt anions, hydrocarbons and halogenated organic pollutants (Fig. 1) [50], leading to their conversion to final non-hazardous products. In these processes, iron nanoparticles have shown high efficiency and practically no damage for the environment because of absence of toxicity. Thus, the efficiencies of *nitrate removal* from aqueous solution by single (TiO2 and nZVI), and composite (nano-TiO2–Fe0 composite, NTFC) system under UV illumination were studied [51]. Among the three systems, both nZVI and NTFC can effectively remove nitrate. However, only NTFC can achieve satisfactory transformation of nitrate to N2. Reactive materials for catalytic *degradation of chlorinated organic compounds* in water at ambient conditions have been prepared on the basis of silica-supported Pd-Fe nanoparticles [52]. Nanoscale Fe-Pd particles were synthesized inside porous silica supports using (NH4)3[Fe(C2O4)3] and [Pd(NH3)4]Cl2 or Pd acetate as reaction precursors. The reduction of these supported precursors using hydrogen led to materials, revealed high activity in the processes of perchloroethene (PCE) degradation and 2-chlorobiphenyl (2-ClBP) dechlorination. It was established that highly dispersed amorphous Fe-Pd bimetallic nanoparticles on silica support possess superior catalytic activity against PCE dechlorination, comparing with the free-standing Fe-Pd nanoparticles. It was also established that the addition of vitamin B12 (it is known to be an effective electron mediator, having strong synergistic effects with nZVI for reductive dehalogenation reactions) can significantly enhance the reductive dechlorination of PCE by nZVI [53]. A remarkable reductive dechlorination of PCE (0.25±0.01 h−1) was observed in nZVI suspension (0.05 g/24 mL) with 0.5 mM vitamin B12 in 6 h, while no significant reductive dechlorination of PCE was observed in the nZVI suspension without vitamin B12. Similar composite material based on deposition of nZVI particles and cyanocobalamine (vitamin B12) on a diatomite matrix was also used for catalytic transformation of PCE and other organic contaminants in water [54]. The composite material rapidly degrades or transforms completely a large spectrum of water contaminants, including halogenated solvents like TCE, PCE, and *cis*-DCE, pesticides like alachlor, atrazine and bromacyl, and common ions like nitrate, within minutes to hours. In a related publication [55], iron nanoparticles were applied for remediation of PCB-contaminated soil, taking into account a maximization of PCB destruction in each treatment stage. The efficiency of PCB destruction during the first step treatment (mixing of soil and iron nanopar‐ ticles in water) can be improved by increasing the water temperature. The PCB destruction efficiency of minimum 95% can be achieved. In air at 300 °C, Fe2O3 is also a good catalyst for remediating PCB-contaminated soils. In addition, photo-Fenton like method using nZVI/UV/ H2O2 was applied [56] for removing total *petroleum hydrocarbons* (TPH) and determining the optimal conditions using Taguchi method. The removal rate in optimal conditions was between 95% and 100%. The nZVI particles can be reused in a magnetic field. This process may enhance the rate of diesel degradation in polluted water and could be used as a pretreatment step for the biological removal of TPH from diesel fuel in the aqueous phase. Among other degradation applications, we note the use of nZVI/AC for catalytic wet peroxide oxidation of phenol [57]. The catalytic activity of phenol degradation was improved by application of nZVI/ AC catalysts compared to that of Fe/AC. For the range 150–1,000 mg/L, the phenol conversion ≥90% can be achieved using these nanocatalysts during 15 min of the reaction in presence of the stoichiometric hydrogen peroxide for complete mineralization. At last, as an example of classic organic synthesis, we emphasize the conversion of synthesis gas to C2 through C4 olefins with up to 60% selectivity by carbon [58], using catalysts which comprise Fe promoted nanoparticles (5–30 nm in diameter) homogeneously dispersed on weakly interactive αalumina or carbon nanofiber supports.

composite (nano-TiO2–Fe0

40 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

alumina or carbon nanofiber supports.

composite, NTFC) system under UV illumination were studied [51].

Among the three systems, both nZVI and NTFC can effectively remove nitrate. However, only NTFC can achieve satisfactory transformation of nitrate to N2. Reactive materials for catalytic *degradation of chlorinated organic compounds* in water at ambient conditions have been prepared on the basis of silica-supported Pd-Fe nanoparticles [52]. Nanoscale Fe-Pd particles were synthesized inside porous silica supports using (NH4)3[Fe(C2O4)3] and [Pd(NH3)4]Cl2 or Pd acetate as reaction precursors. The reduction of these supported precursors using hydrogen led to materials, revealed high activity in the processes of perchloroethene (PCE) degradation and 2-chlorobiphenyl (2-ClBP) dechlorination. It was established that highly dispersed amorphous Fe-Pd bimetallic nanoparticles on silica support possess superior catalytic activity against PCE dechlorination, comparing with the free-standing Fe-Pd nanoparticles. It was also established that the addition of vitamin B12 (it is known to be an effective electron mediator, having strong synergistic effects with nZVI for reductive dehalogenation reactions) can significantly enhance the reductive dechlorination of PCE by nZVI [53]. A remarkable reductive dechlorination of PCE (0.25±0.01 h−1) was observed in nZVI suspension (0.05 g/24 mL) with 0.5 mM vitamin B12 in 6 h, while no significant reductive dechlorination of PCE was observed in the nZVI suspension without vitamin B12. Similar composite material based on deposition of nZVI particles and cyanocobalamine (vitamin B12) on a diatomite matrix was also used for catalytic transformation of PCE and other organic contaminants in water [54]. The composite material rapidly degrades or transforms completely a large spectrum of water contaminants, including halogenated solvents like TCE, PCE, and *cis*-DCE, pesticides like alachlor, atrazine and bromacyl, and common ions like nitrate, within minutes to hours. In a related publication [55], iron nanoparticles were applied for remediation of PCB-contaminated soil, taking into account a maximization of PCB destruction in each treatment stage. The efficiency of PCB destruction during the first step treatment (mixing of soil and iron nanopar‐ ticles in water) can be improved by increasing the water temperature. The PCB destruction efficiency of minimum 95% can be achieved. In air at 300 °C, Fe2O3 is also a good catalyst for remediating PCB-contaminated soils. In addition, photo-Fenton like method using nZVI/UV/ H2O2 was applied [56] for removing total *petroleum hydrocarbons* (TPH) and determining the optimal conditions using Taguchi method. The removal rate in optimal conditions was between 95% and 100%. The nZVI particles can be reused in a magnetic field. This process may enhance the rate of diesel degradation in polluted water and could be used as a pretreatment step for the biological removal of TPH from diesel fuel in the aqueous phase. Among other degradation applications, we note the use of nZVI/AC for catalytic wet peroxide oxidation of phenol [57]. The catalytic activity of phenol degradation was improved by application of nZVI/ AC catalysts compared to that of Fe/AC. For the range 150–1,000 mg/L, the phenol conversion ≥90% can be achieved using these nanocatalysts during 15 min of the reaction in presence of the stoichiometric hydrogen peroxide for complete mineralization. At last, as an example of classic organic synthesis, we emphasize the conversion of synthesis gas to C2 through C4 olefins with up to 60% selectivity by carbon [58], using catalysts which comprise Fe promoted nanoparticles (5–30 nm in diameter) homogeneously dispersed on weakly interactive α-

**Figure 1.** Core–shell structure of nZVI depicting various mechanisms for the removal of metals and chlorinated com‐ pounds. Adapted from Li et al. 2006 with permission.

**Degradation of dyes** with use of nZVI is carried out mainly on carbon-based nanocomposites. Thus, nZVI/activated carbon (nZVI/AC) was investigated as heterogeneous Fenton catalyst in 3D electrode system for methyl orange (MO) degradation [59]. The mineralization of MO was significantly improved by 20–35% compared to 2D AC system at the optimum conditions. A possible mechanism for decolorization and mineralization was proposed, which was attrib‐ uted to the combination of adsorption, anodic oxidation, and Fenton oxidation in 3D nZVI/AC system. As an example of application of CNTs-based system, an ozone catalyst capable of working on acidic solution environments (labeled as CNTs-Fe0 ), which was prepared by immobilizing nZVI onto the surface of MWCNTs and used for decomposition of methylene blue (MB) by formed hydroxyl radicals (HO⋅) [60]. At pH 3, the production of HO⋅ was found to be considerably accelerated in the presence of CNTs-Fe0 about 80 times in comparison with results using plain ozone. In the process of CNTs-Fe0 catalytic ozonation, the CNTs support was analyzed to perform as effective "promoter" allowing the fast surfacemediated reactions, owing to the combination of its surface-active nature, conductivity, and chemical stability. All these and above technologies perfectly fit into advanced water treatment technologies, whose additional representative example is as follows. Thus, the *in situ* synthesis of air-stable nZVI embedded in cellulose fibers led to the assembly of highly reactive magnetic filter papers (membrane nanocomposites) [61]. This nZVI@FP nanocomposite (Fig. 2) showed high activity towards the removal of hexavalent chromium as well as an excellent catalytic ability to convert phenols into catechols, by simple filtration processes of the contaminated water solutions.

**Figure 2.** (a) Optical image showing the 2-step assembly of the magnetic nZVI@FP nanocomposite. (b) Scanning elec‐ tron micrograph of nZVI@FP loaded with 5% in weight of nZVI. (c) TEM micrograph of NZVI entrapped over a cellu‐ lose fibre of FP. The inset shows the particle size distribution, as estimated from TEM (Freq.-nZVI *vs*. nZVI-size), together with the fitting analysis (red-line). (d) TEM micrograph of a magnified region of (c) revealing the intimate interaction between NZVIs and the fiber surface. FP = filter paper. Adapted from Datta et. al. 2014 with permission.

## **3.2. Fe–M nanoalloys, bimetallic NPs and core–shell nanostructures**

**Degradation of pollutants**. Iron-based alloys, core-shell, and bimetallic nanoparticles, especially with noble metals, have been extensively applied in the catalysis and reviewed [62, 63, 64], so we will present in this section only their most representative recently reported examples. As well as nZVI described above, bimetallic Fe-containing nanoparticles (Fe with Pt, Ru, Rh, Ni, Co, Au, Cu, Ag) are used for the catalytic elimination of environmental pollutants. Reactions between them (pollutants and nanoparticles) can be mainly divided in 4 types [65]: a) catalytic replacement reactions for removal of heavy metals, b) hydrodehaloge‐ nation reactions (in case of halogenated hydrocarbons), c) azo and nitro hydrogenation reactions (for azo and nitro) compounds, and d) hydrodeoxygenation reactions (for oxyan‐ ions). In comparison with monometallic iron nanoparticles, the bimetallic iron NPs have considerable capacity to be separated and catalytic ability for degradation of non-biodegrad‐ able pollutants. Among them, Fe-Pt NPs are of an especial interest. Thus, Pt-Fe application as heterogeneous Fenton-like catalysts was reported for hydrogen peroxide decomposition and the decolorization of methylene blue [66]. FePt (and also Fe3O4, see more information below) NPs were prepared and tested as heterogeneous Fenton-like catalysts to evaluate and compare their efficiency toward the decolorization of MB dye in solution. Both FePt and Fe3O4 exhibited high activity toward the MB decolorization reaction though FePt exhibited a reaction rate that was 100 times faster for a 5 ppm catalyst concentration. Both FePt and Fe3O4 NPs are super‐ paramagnetic and thus can be easily separated with a magnet and reused for subsequent catalytic cycles. The same objective was reached using core-shell NPs on the nZVI basis. Thus, a series of nanocomposites consisting of nZVI encapsulated in SiO2 microspheres were applied for the degradation of organic dyes was investigated using MB as the model dye in the presence of H2O2 [67]. The degradation efficiency and apparent rate constant of the degradation reaction were significantly enhanced with increased nZVI encapsulated in SiO2 microspheres, whereas the dosage of H2O2 remarkably promoted degradation rate without affecting degradation efficiency.

It should be also noted that Fe-containing *thin films* can be applied for dye degradation. Thus, 2D nano-TiO2 and Fe-doped nano-TiO2 thin films with large sizes were obtained [68] at low temperature in an aqueous system via molecular self-assembly approach. Degradation of methyl orange solution under action of UV and visible light radiation was applied for evaluation of the photocatalytic activity. The doped iron presence was shown to improve the TiO2 photocatalytic activity. The degradation yields of methyl orange were 98.62% and 89.24%, respectively, under illumination by UV lamp and using visible light. As an example of another degradation process, we note that the longevity and reactivity of nZVI and palladized bimetallic particles (BNP) were evaluated in batch and column experiments for remediation of a trichloroethene (TCE)-contaminated plume within a clayey soil [69]. The particle behavior was found to be severely affected by clay sediments. Results of butch studies testified that TCE degradation in ORR clayey soil corresponds to a pseudo-first-order kinetic model with reaction rate constants (k) of 0.05–0.24 day−1 at varied iron-to-soil ratios. Despite of elevated reactivity in water phase, the BNP were less effective in the site-derived clay sediment resulting calculated TCE removal efficiencies of 98.7% and 19.59%, respectively.

**Figure 2.** (a) Optical image showing the 2-step assembly of the magnetic nZVI@FP nanocomposite. (b) Scanning elec‐ tron micrograph of nZVI@FP loaded with 5% in weight of nZVI. (c) TEM micrograph of NZVI entrapped over a cellu‐ lose fibre of FP. The inset shows the particle size distribution, as estimated from TEM (Freq.-nZVI *vs*. nZVI-size), together with the fitting analysis (red-line). (d) TEM micrograph of a magnified region of (c) revealing the intimate interaction between NZVIs and the fiber surface. FP = filter paper. Adapted from Datta et. al. 2014 with permission.

**Degradation of pollutants**. Iron-based alloys, core-shell, and bimetallic nanoparticles, especially with noble metals, have been extensively applied in the catalysis and reviewed [62, 63, 64], so we will present in this section only their most representative recently reported examples. As well as nZVI described above, bimetallic Fe-containing nanoparticles (Fe with Pt, Ru, Rh, Ni, Co, Au, Cu, Ag) are used for the catalytic elimination of environmental pollutants. Reactions between them (pollutants and nanoparticles) can be mainly divided in 4 types [65]: a) catalytic replacement reactions for removal of heavy metals, b) hydrodehaloge‐ nation reactions (in case of halogenated hydrocarbons), c) azo and nitro hydrogenation reactions (for azo and nitro) compounds, and d) hydrodeoxygenation reactions (for oxyan‐ ions). In comparison with monometallic iron nanoparticles, the bimetallic iron NPs have considerable capacity to be separated and catalytic ability for degradation of non-biodegrad‐ able pollutants. Among them, Fe-Pt NPs are of an especial interest. Thus, Pt-Fe application as heterogeneous Fenton-like catalysts was reported for hydrogen peroxide decomposition and the decolorization of methylene blue [66]. FePt (and also Fe3O4, see more information below) NPs were prepared and tested as heterogeneous Fenton-like catalysts to evaluate and compare their efficiency toward the decolorization of MB dye in solution. Both FePt and Fe3O4 exhibited high activity toward the MB decolorization reaction though FePt exhibited a reaction rate that

**3.2. Fe–M nanoalloys, bimetallic NPs and core–shell nanostructures**

42 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Transformations of ketones** using stabilized nZVI and bimetallic Fe NPs are known. Among these catalytic processes, we note that porous Pt-Fe bimetallic nanocrystals were found to effectively facilitate the manufacturing of 2-propanol from acetone [70]. It was suggested that the high reactivity is strictly related to the interface consist of bimetallic Pt-Fe alloy and the Fe2O3-*x*. As an example of reduction of substituted aromatic ketones to alcohols, iron nanopar‐ ticles (size 9 nm, 14 nm, and 17 nm), stabilized by polyethylene glycol (PEG), carboxymethyl cellulose (CMC), and poly N-vinyl pyrrolidone (PVP), were used as catalysts in the hydroge‐ nation reaction of various substituted aromatic ketones to alcohols with NaBH4 [71]. Fe-PEG NPs were found to act as better catalyst than Fe-CMC NPs and Fe-PVP NPs. The trend in the catalytic activity among metals falls in the line of decreasing size effect of the nanoparticles i.e., the order of the nanoparticle sizes increase as Fe-PEG < Fe-CMC < Fe-PVP. Also, effects of substituents in the aromatic ring of ketones revealed that +I substituents are better catalyzed than –I substituents. In addition, bimetallic ruthenium-iron nanoparticles constitute a mag‐ netically recoverable heterogeneous catalyst for transfer hydrogenation with a pronounced selectivity for ketones over aldehydes and nitro groups [72]. The nanoparticles are recyclable up to five times without significant decrease in activity or leaching.

**Other processes**. An Fe group ternary nanoalloy FeCoNi (NA) catalyst (its synthesis see Fig. 3) enabled selective electrocatalysis towards CO2-free power generation from highly deliver‐ able ethylene glycol (EG) [73]. This FeCoNi nanoalloy catalyst exhibited the highest selectivi‐ ties toward the formation of C2 products and to oxalic acid, *i.e*., 99% and 60%, respectively, at 0.4 V vs. the reversible hydrogen electrode (RHE), without CO2 generation. The key feature was the formation of an atomically mixed FeCoNi alloy to enhance the synergetic effect of the Fe group elements on anti-self-oxidation and selective oxidation of EG to oxalic acid. Reduction processes also required polymetallic particles. Thus, trimetallic core/shell Pd/FePt NPs were applied in oxygen reduction reaction (ORR) catalysis [74]. The uniform FePt shell was formed by controlled nucleation of Fe(CO)5 in the presence of a Pt salt and Pd NPs at designated reaction temperatures.

**Figure 3.** Synthetic scheme for the preparation of a FeCoNi nanoalloy catalyst supported on carbon (FeCoNi/C). Metal‐ lic Fe, Co and Ni form in the presence of polyethylene glycol (PEG) and a carbon support (vulcan) after the addition of an aqueous solution of NaBH4. The metallic species are oxidised spontaneously, with production of an oxide mixture composed of Fe3O4, Co3O4, NiO, and so on is produced. FeCoNi/C was prepared by hydrogen reduction of the oxide mixture. Adapted from Matsumoto et al., 2014 with permission.

Iron-iron oxide core–shell nanoparticles were used as a catalyst for the *hydrogenation of olefins and alkynes* (reaction 3) under mild conditions in ethanol and in an aqueous medium [75]. The system is active in respect of a row of substrates and considerably selective for alkenes and alkynes over aromatic and carbonyl groups. The authors supposed that the presence of an oxide shell does not decrease its activity and provides a certain protection against oxidation by oxygen and water. In addition, highly active and well-defined AuPt nanoalloys, supported on the surface of ellipsoidal Fe@SiO2 nanoparticles, were prepared by a method involving the loading of Pt NPs on the Fe2O3@SiO2 nanocapsules *via* Sn2+ linkage and reduction, then *in situ* fabrication of Au nanoparticles by the galvanic replacement reaction between Au and Pt, and finally calcination and reduction to convert the nonmagnetic Fe2O3 to Fe core with high saturation magnetization [76]. The obtained Fe@SiO2/AuPt samples exhibited a remarkably higher catalytic activity in comparison with the supported monometallic counterparts toward the *reduction of 4-nitrophenol to 4-aminophenol* by NaBH4. The catalyst can be reused for several cycles with convenient magnetic separation.

Hydrogenation of olefin catalyzed by Fe CSNPs.

**Other processes**. An Fe group ternary nanoalloy FeCoNi (NA) catalyst (its synthesis see Fig. 3) enabled selective electrocatalysis towards CO2-free power generation from highly deliver‐ able ethylene glycol (EG) [73]. This FeCoNi nanoalloy catalyst exhibited the highest selectivi‐ ties toward the formation of C2 products and to oxalic acid, *i.e*., 99% and 60%, respectively, at 0.4 V vs. the reversible hydrogen electrode (RHE), without CO2 generation. The key feature was the formation of an atomically mixed FeCoNi alloy to enhance the synergetic effect of the Fe group elements on anti-self-oxidation and selective oxidation of EG to oxalic acid. Reduction processes also required polymetallic particles. Thus, trimetallic core/shell Pd/FePt NPs were applied in oxygen reduction reaction (ORR) catalysis [74]. The uniform FePt shell was formed by controlled nucleation of Fe(CO)5 in the presence of a Pt salt and Pd NPs at designated

44 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 3.** Synthetic scheme for the preparation of a FeCoNi nanoalloy catalyst supported on carbon (FeCoNi/C). Metal‐ lic Fe, Co and Ni form in the presence of polyethylene glycol (PEG) and a carbon support (vulcan) after the addition of an aqueous solution of NaBH4. The metallic species are oxidised spontaneously, with production of an oxide mixture composed of Fe3O4, Co3O4, NiO, and so on is produced. FeCoNi/C was prepared by hydrogen reduction of the oxide

mixture. Adapted from Matsumoto et al., 2014 with permission.

reaction temperatures.

## **3.3. Nano-Fe2O3 phases and their composites**

**Destruction of dangerous substances and pollutants**. As well as Fe(0)-containing nanocom‐ posites, oxidated iron forms are successfully used for degradation of several substances, both inorganic and organic. Thus, the decrease in temperature of *decomposition of ammonium perchlorate* in the presence of nano-ferric oxide was investigated [77] (see also section below on ferrites). It was shown that addition of nanometer-sized ferric oxide leads to a significant decrease in higher decomposition temperature of ammonium perchlorate. The catalytic activity of a colloidal catalyst (based on iron(III) oxides and obtained by hydrolysis followed by peptization of FeCl3⋅6H2O salt in water in the presence of 1% ethanol) in *decomposition of H2O2* was studied [78]. The obtained catalyst is mainly composed of α-Fe2O3 crystals with an admixture of other crystalline structures of iron oxides, as well as carbon-containing com‐ pounds. Its activity with respect to Н2О<sup>2</sup> decomposition varies nonlinearly and nonmonoton‐ ically and its particle size grows starting from 1–3 nm with an increase in the initial concentration of FeCl3⋅6H2O used to synthesize the catalyst. In addition, thermal decomposi‐ tion of silver acetate at 200 °C in the presence of iron oxide microspheres in diphenyl ether led to the formation of iron oxide@Ag core–shell nanoparticles, exhibiting superparamagnetic behavior with a blocking temperature of about 42 K [79]. Their good catalytic activity and magnetic recovery was demonstrated by using two reactions, namely, *reduction of 4-nitrophe‐ nol* and *reduction of methylene blue* in aqueous solution. An especial case is the use of iron oxide / TiO2 nanoparticles [80]. Thus, photocatalytic oxidation with TiO2 nanoparticles (6–20 nm) was investigated as a promising water-treatment process [81, 82]. TiO2 nanoparticles, after UV irradiation, are able to adsorb and degrade a huge variety of organic contaminants present in the environment. For example, in case of *organic arsenic species* (monomethylarsonic [MMA] and dimethylarsinic [DMA] acids), the strong affinity between the TiO2 nanoparticles surface results a covalent bonding between MMA or DMA and the nanoparticle surface via formation of bidentate (AsMMA-Ti 3.32 Å) and monodentate (AsDMA-Ti 3.37 Å) inner-sphere com‐ plexes, respectively. Dopation of TiO2 nanoparticles with Fe3+ ions at 0.1–0.5% may signifi‐ cantly increase the photocatalytic activity. The doped ions act as charge separators of the photoinduced electron–hole pair and enhanced interfacial charge transfers. Finally, an interesting application is known for *adsorption of elemental sulfur*. Thus, photoinduced sulfur desorption from the surfaces of Au Nps loaded on a series on metal oxides, in particular Fe2O3, was studied [83]. Elemental sulfur S8 was selectively adsorbed on the Au Nps surfaces of Au/metal oxides in an atomic state. This phenomenon is applicable to the low temperature cleaning of sulfur-poisoned metal catalysts.

**Organic synthesis**. Free iron oxide(III) NPs and its nanocomposites have found numerous application in organic synthesis. Thus, hematite α-Fe2O3 NPs (diameters in the 7–18 nm range, synthesized by thermolysing a PVA-Fe(OH)3 gel matrix at moderate temperatures) can effectively catalyze the *epoxidation of styrene* with *tert*-butyl hydroperoxide (TBHP) as the terminal oxidant [84]. Iron oxide nanoparticles supported on zirconia were tested in the gasphase *conversion of cyclohexanol to cyclohexanone* in a fixed-bed flow type, Pyrex glass reactor, at 433–463 K [85]. Major detected products were cyclohexanone, cyclohexene and benzene, depending on the used catalyst. Experimental results showed that there was no leaching of metal, and that the catalyst was thus truly heterogeneous. In addition, core-shell Fe2O3/Pt nanoparticles with amorphous iron oxide cores exhibited superior catalytic activity with lower peak potential and enhanced CO2 selectivity toward *methanol electrooxidation* in acidic medium [86]. This catalytic performance may be attributed to the uniform distribution of Pt particles on the amorphous Fe2O3 surface as well as interactions between the Pt particles and amorphous Fe2O3 cores. The catalytic activity of core-shell Fe2O3/Pt nanoparticles first increases and then decreases with decreasing Pt content. These nanomaterials also were found to have much higher structural stability and tolerance to the intermediates of methanol oxidation.

A magnetically separable core–shell iron oxide@nickel (IO@Ni) nanocatalyst, synthesized by reduction of Ni2+ ions in the presence of iron oxide (Fe2+, Fe3+) by a one-pot synthetic route using NaBH4 as a reducing agent and starch as a capping agent, was found to have excellent activity for the *hydrogenation reactions* of aromatic nitro compounds under mild conditions using water as a green solvent (reaction *4*). Excellent chemoselectivity and recyclability up to 30 cycles for the nitro group reduction was demonstrated. Nano propylsulphonated γ-Fe2O3 (NPS-γ-Fe2O3, reaction *5*) was applied as a magnetically recyclable heterogeneous catalyst for the efficient one-pot synthesis of bis(pyrazolyl)methanes in water (reaction *6*) [87]. The catalyst was easily isolated from the reaction mixture by a magnetic bar and reused at least five times without significant degradation in activity. Nanoporous α-Fe2O3 nanoparticles (about 100 nm in size and containing pores <10 nm) were synthesized *via* a hydrothermal method and applied in the catalytic *benzylation of benzene and benzyl chloride* (BC) in the fabrication of diphenylmethane (DPM) [88]. The BC conversion reached 100% in a reaction time of 3 min with 97.76% selectivity to DPM. The nanoporous α-Fe2O3 nanoparticles also have potential applications in other Friedel-Crafts alkylations, especially in large molecular reactions. Another example is related with chemical recycling of PET. Thus, easily recoverable superparamagnetic γ-Fe2O3 nano‐ particles (10.5 nm in size and 147 m<sup>2</sup>**.** g−1 surface area, produced by calcining Fe3O4 nanoparticles prepared by the co-precipitation method) were used as a reusable catalyst for *PET glycolysis* [89]. At 300°C and a 0.05 catalyst/PET weight ratio, the maximum bis(2-hydroxyethlyl) terephthalate (BHET) monomer yielded reached more than 90% in 60 min. The catalyst was reused 10 times, giving almost the same BHET yield each time. In addition, heterogeneous photo-Fenton reaction which utilizes nanosized iron oxides as catalyst for maximizing the activity due to the enhanced physical or chemical properties brought about by the unique structures was described [90].

General scheme for the reduction of various nitroaromatics.

Synthesis of NPS-*γ*-Fe2O3.

concentration of FeCl3⋅6H2O used to synthesize the catalyst. In addition, thermal decomposi‐ tion of silver acetate at 200 °C in the presence of iron oxide microspheres in diphenyl ether led to the formation of iron oxide@Ag core–shell nanoparticles, exhibiting superparamagnetic behavior with a blocking temperature of about 42 K [79]. Their good catalytic activity and magnetic recovery was demonstrated by using two reactions, namely, *reduction of 4-nitrophe‐ nol* and *reduction of methylene blue* in aqueous solution. An especial case is the use of iron oxide / TiO2 nanoparticles [80]. Thus, photocatalytic oxidation with TiO2 nanoparticles (6–20 nm) was investigated as a promising water-treatment process [81, 82]. TiO2 nanoparticles, after UV irradiation, are able to adsorb and degrade a huge variety of organic contaminants present in the environment. For example, in case of *organic arsenic species* (monomethylarsonic [MMA] and dimethylarsinic [DMA] acids), the strong affinity between the TiO2 nanoparticles surface results a covalent bonding between MMA or DMA and the nanoparticle surface via formation of bidentate (AsMMA-Ti 3.32 Å) and monodentate (AsDMA-Ti 3.37 Å) inner-sphere com‐ plexes, respectively. Dopation of TiO2 nanoparticles with Fe3+ ions at 0.1–0.5% may signifi‐ cantly increase the photocatalytic activity. The doped ions act as charge separators of the photoinduced electron–hole pair and enhanced interfacial charge transfers. Finally, an interesting application is known for *adsorption of elemental sulfur*. Thus, photoinduced sulfur desorption from the surfaces of Au Nps loaded on a series on metal oxides, in particular Fe2O3, was studied [83]. Elemental sulfur S8 was selectively adsorbed on the Au Nps surfaces of Au/metal oxides in an atomic state. This phenomenon is applicable to the low temperature

**Organic synthesis**. Free iron oxide(III) NPs and its nanocomposites have found numerous application in organic synthesis. Thus, hematite α-Fe2O3 NPs (diameters in the 7–18 nm range, synthesized by thermolysing a PVA-Fe(OH)3 gel matrix at moderate temperatures) can effectively catalyze the *epoxidation of styrene* with *tert*-butyl hydroperoxide (TBHP) as the terminal oxidant [84]. Iron oxide nanoparticles supported on zirconia were tested in the gasphase *conversion of cyclohexanol to cyclohexanone* in a fixed-bed flow type, Pyrex glass reactor, at 433–463 K [85]. Major detected products were cyclohexanone, cyclohexene and benzene, depending on the used catalyst. Experimental results showed that there was no leaching of metal, and that the catalyst was thus truly heterogeneous. In addition, core-shell Fe2O3/Pt nanoparticles with amorphous iron oxide cores exhibited superior catalytic activity with lower peak potential and enhanced CO2 selectivity toward *methanol electrooxidation* in acidic medium [86]. This catalytic performance may be attributed to the uniform distribution of Pt particles on the amorphous Fe2O3 surface as well as interactions between the Pt particles and amorphous Fe2O3 cores. The catalytic activity of core-shell Fe2O3/Pt nanoparticles first increases and then decreases with decreasing Pt content. These nanomaterials also were found to have much

higher structural stability and tolerance to the intermediates of methanol oxidation.

A magnetically separable core–shell iron oxide@nickel (IO@Ni) nanocatalyst, synthesized by reduction of Ni2+ ions in the presence of iron oxide (Fe2+, Fe3+) by a one-pot synthetic route using NaBH4 as a reducing agent and starch as a capping agent, was found to have excellent activity for the *hydrogenation reactions* of aromatic nitro compounds under mild conditions using water

cleaning of sulfur-poisoned metal catalysts.

46 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

The reaction of benzaldehyde with 1-phenyl-3-methyl-5-pyrazolone.

**Antibacterial activities**. The doping effects of silver(I) and iron(III) on photocatalytic results using TiO2 thin films were studied [91]. Ag and Fe doping and co-doping contents on nanotitania photocatalytic bactericidal films were obtained by sol-gel technique, thus uniting three classic active antibacterial species (Ag, Fe and TiO2). The photocatalytic activity of TiO2 films was confirmed by the sterilizing rate of the *E-coli* in each case. Applying fluorescent light irradiation, the optimal doping amounts of silver(I)/titania and iron(III)/titania were found to be 0.05% and 0.1%, respectively. In addition, a photocatalytic technique using visible light and carbon nanotubes and nano-sized Fe2O3 powder was used to *inhibit pathogenic bacterial growth* in water [92]. It was suggested that after careful design, this system can be used to disinfect drinking water, making it free of pathogenic bacteria.

## **3.4. Nano-Fe3O4 phases and their composites**

Iron(II,III) oxide based nanostructures are slightly less explored in the organic synthesis. Thus, haemin-functionalized magnetic iron(II,III) oxide nanoparticles (Fe3O4/haemin) exhibited pronounced electrocatalytic activity towards *trichloroacetic acid* (TCA) like haemin itself (a linear detection range of 5–80 M and a detection limit of 0.3 M at 60 °C) [93]. This activity towards TCA was affected by detection temperature, which was controlled via electrically heated carbon paste electrodes. The maximal catalytic current was 5.8 times higher at 60 °C than at room temperature (25 °C). A process capable of synthesizing minor fractions of *aromatic hydrocarbons* (benzene, toluene, xylenes, and mesitylene) from CO2 and H2 at modest temper‐ atures (T = 380 to 540°C) employing Fe/Fe3O4 nanoparticles as catalyst was designed [94]. The authors consider this technology as principally compatible with solar heat and hydrogen technology and having the potential to mitigate the impacts of global warming by making use of the existing distribution technology for gasoline. Also, *trisubstituted imidazoles* can be synthesized condensation reaction from 1,2-diketones, aromatic aldehydes, and ammonium acetate (reaction *7*) in high yield in the presence of sulphamic acid functionalized magnetic Fe3O4 nanoparticles (reaction *8*) as a solid acid catalyst under solvent-free classical heating conditions or using microwave irradiation [95].

One-pot synthesis of 2,4,5-trisubstituted imidazoles catalyzed by sulphamic acid functionalized magnetic Fe3O4 nano‐ particles under conventional heating conditions or using microwave irradiation.

Preparation steps for fabricating sulphamic acid functionalized magnetic Fe3O4 nanoparticles.

The reaction of benzaldehyde with 1-phenyl-3-methyl-5-pyrazolone.

48 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

disinfect drinking water, making it free of pathogenic bacteria.

**3.4. Nano-Fe3O4 phases and their composites**

conditions or using microwave irradiation [95].

**Antibacterial activities**. The doping effects of silver(I) and iron(III) on photocatalytic results using TiO2 thin films were studied [91]. Ag and Fe doping and co-doping contents on nanotitania photocatalytic bactericidal films were obtained by sol-gel technique, thus uniting three classic active antibacterial species (Ag, Fe and TiO2). The photocatalytic activity of TiO2 films was confirmed by the sterilizing rate of the *E-coli* in each case. Applying fluorescent light irradiation, the optimal doping amounts of silver(I)/titania and iron(III)/titania were found to be 0.05% and 0.1%, respectively. In addition, a photocatalytic technique using visible light and carbon nanotubes and nano-sized Fe2O3 powder was used to *inhibit pathogenic bacterial growth* in water [92]. It was suggested that after careful design, this system can be used to

Iron(II,III) oxide based nanostructures are slightly less explored in the organic synthesis. Thus, haemin-functionalized magnetic iron(II,III) oxide nanoparticles (Fe3O4/haemin) exhibited pronounced electrocatalytic activity towards *trichloroacetic acid* (TCA) like haemin itself (a linear detection range of 5–80 M and a detection limit of 0.3 M at 60 °C) [93]. This activity towards TCA was affected by detection temperature, which was controlled via electrically heated carbon paste electrodes. The maximal catalytic current was 5.8 times higher at 60 °C than at room temperature (25 °C). A process capable of synthesizing minor fractions of *aromatic hydrocarbons* (benzene, toluene, xylenes, and mesitylene) from CO2 and H2 at modest temper‐ atures (T = 380 to 540°C) employing Fe/Fe3O4 nanoparticles as catalyst was designed [94]. The authors consider this technology as principally compatible with solar heat and hydrogen technology and having the potential to mitigate the impacts of global warming by making use of the existing distribution technology for gasoline. Also, *trisubstituted imidazoles* can be synthesized condensation reaction from 1,2-diketones, aromatic aldehydes, and ammonium acetate (reaction *7*) in high yield in the presence of sulphamic acid functionalized magnetic Fe3O4 nanoparticles (reaction *8*) as a solid acid catalyst under solvent-free classical heating

Among other reactions, we note an efficient one-pot, three-component condensation reaction between 4-hydroxycoumarin, aryl glyoxals, and malononitrile catalyzed by Fe3O4 nanoparti‐ cles, which was carried out for the synthesis of several *dihydropyrano[c]chromenes* [96]. Also, an inexpensive and non-hazardous sulfuric acid functionalized magnetic Fe3O4 nanoparticles (efficiently catalyze one-pot multicomponent condensation of β-naphthol with aromatic and aliphatic aldehydes and amide derivatives (reaction *9*) at 100 °C under solvent-free conditions to afford the corresponding *amidoalkyl naphthols* in excellent yields and in very short reaction times [97]. Silver(0) nanoparticles supported on silica-coated Fe3O4 (synthesis see reactions *10*) serve as an efficient and recyclable heterogeneous catalyst for oxidant-free *dehydrogenation of alcohols* to the corresponding carbonyl compounds [98]. The catalyst can be easily recovered and reused for 8 reaction cycles without considerable loss of activity. At last, a nanocomposite of functionalized Ni(II) complex containing surface of pyridine, methoxysilanyl and amino groups with iron(II,III) oxide, Fe3O4@[-Ni(bpy)2(py-tmos)] was found to be highly efficient green catalyst for the synthesis of a diverse range of 3,4-dihydropyrimidin-2(1H)-ones under solvent free conditions, and in addition it could be easily recovered by a simple magnetic separation and recycled at least 5 times without deterioration in catalytic activity [99].

Preparation of magnetically recoverable heterogeneous nanocatalyst Fe3O4@SiO2-Ag.

### **3.5. Ferrites**

**Cobalt ferrites** having different sizes, from ultrasmall (2 nm) to 50 nm, can be fabricated by distinct techniques [100], mainly co-precipitation method (CPM), sometimes without using any capping agents/surfactants. Thus, the CPM was used to synthesize ultrasmall CoFe2O4 superparamagnetic nanoparticles (SPMNPs, 2–8 nm of an average size and high surface area of 140.9 m2 /g) without any surfactant [101]. Their catalytic activity was verified in the prepa‐ ration of *arylidene barbituric acid derivatives* (reaction *11*) applying CoFe2O4 SPMNPs in aqueous ethanol as a reusable catalyst, which can be magnetically separated from the reaction system. Main advantages of this approach were found to be high yields, short reaction time and high turnover frequency, a clean reaction methodology, and chemoselectivity, among several others. More large-size CoFe2O4 magnetic nanoparticles (25 nm) were used as a catalyst for the *oxidation of various alkenes* in the presence of *tert*-butylhydroperoxide (*t*-BuOOH) with almost quantitative yields (reaction *12*) [102]. It seemed that this heterogeneously catalysis system proceeds by coordination of *t*-BuOOH to the metal (Fe3+ cations) on the surface of the catalyst. The separation of the catalyst from the reaction media was easily achieved with the aid of an external magnet, and the catalyst can be reused several times with no loss of activity. In addition, combination of synthesis techniques can also be used for cobalt ferrite preparation. Thus, synthesis of spinel CoFe2O4 MNPs (average sizes 40–50 nm) was achieved by a combined sonochemical and co-precipitation technique in aqueous medium, also without any surfactant or organic capping agent [103]. These uncapped NPs were utilized directly for *aldol reaction* in ethanol (reaction 13). After the reaction was over, the nanoparticules were compartmented by using an external magnet.

Optimization of reaction conditions using CoFe2O4 nanocatalyst. Yields 40–95%, best results in EtOH.

Oxidation of alkenes using CoFe2O4 catalyst.

Synthesis of amidoalkyl naphthols

50 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**3.5. Ferrites**

of 140.9 m2

Preparation of magnetically recoverable heterogeneous nanocatalyst Fe3O4@SiO2-Ag.

**Cobalt ferrites** having different sizes, from ultrasmall (2 nm) to 50 nm, can be fabricated by distinct techniques [100], mainly co-precipitation method (CPM), sometimes without using any capping agents/surfactants. Thus, the CPM was used to synthesize ultrasmall CoFe2O4 superparamagnetic nanoparticles (SPMNPs, 2–8 nm of an average size and high surface area

ration of *arylidene barbituric acid derivatives* (reaction *11*) applying CoFe2O4 SPMNPs in aqueous ethanol as a reusable catalyst, which can be magnetically separated from the reaction system. Main advantages of this approach were found to be high yields, short reaction time and high turnover frequency, a clean reaction methodology, and chemoselectivity, among several others. More large-size CoFe2O4 magnetic nanoparticles (25 nm) were used as a catalyst for the

/g) without any surfactant [101]. Their catalytic activity was verified in the prepa‐

**Figure 4.** Aldol condensation reaction in presence of cobalt ferrite MNPs.

**Nickel ferrites.** Pure or doped nickel ferrites out of the nano-range size are common and frequently used in several catalytic processes. For instance, high reactivity of NiFe2O4 (111) surfaces (higher that in Fe3O4) is well-known; NiFe2O4 is an effective metal-doped ferrite catalyst in a typical industrial process such as the water-gas shift (WGS) reaction [104]. Similarly, NiFe2O4 was examined as catalyst in photocatalytic water oxidation using [Ru(bpy)3] 2+ as a photosensitizer and S2O8 2− as a sacrificial oxidant [105]. The catalytic activity of NiFe2O4 is comparable to that of a catalyst containing Ir, Ru, or Co in terms of O2 yield and O2 evolution rate under ambient reaction conditions. As an example of non-nano-sized doped ferrites, their catalysts (granules of ~1 mm diameter) of nickel, cobalt and copper, prepared by co-precipitation hydrothermal route and impregnated with palladium, cerium and lanthanum as promoters [106], were tested for carbon monoxide oxidation activities. The catalysis of NiFe2O4 nanoparticles on the *hydrogen storage performances* of magnesium hydride synthesized by high-energy ball milling was studied [107], showing that the initial dehydrogenation temperature of 7 mol.% NiFe2O4-doped MgH2 is 191°C, which is 250°C lower than that of pristine MgH2. The enhancement in the H2 storage performances of MgH2 by adding NiFe2O4 nanoparticles is primarily ascribed to intermetallic Fe7Ni3 and (Fe, Ni) phases during the desorption procedure, which act as the real catalyst in the 7 mol.% NiFe2O4-doped sample. As an example of another application, a magnetic acidic catalyst comprising Preyssler (H14[NaP5W30O110]) heteropoly acid supported on silica coated nickel ferrite nanoparticles (NiFe2O4@SiO2) was investigated for the synthesis of *bis(dihydropyrimidinone)benzene and 3,4*‐ *dihydropyrimidin*‐*2(1H)*‐*ones derivatives* by the Biginelli reaction [108]. With the catalyst, the reactions occurred in less than 1 h with good to excellent yields.

**Copper ferrites.** Non-nanosized range copper ferrites have certain catalytic applications, such as, for example, for CO conversion to CO2 [109]. In a difference of pure nickel ferrites, copper ferrite NPs are applied in organic catalysis in more uniform particle size (mainly about 20 nm). Thus, nano material on the basis of copper ferrite (20 nm) was applied as reusable heteroge‐ neous initiator in the preparation of 1,4-dihydropyridines. The interaction of substituted *aromatic aldehydes, ethyl acetoacetate and ammonium acetate* (reaction *14*) was observed in presence of CuFe2O4 nano powders in ethanol at ambient conditions. The nano catalyst can be magnet‐ ically recovered and reused [110]. The same 20-nm size copper ferrite nano material also was reported as reusable heterogeneous initiator in the synthesis of *β*,*γ*-unsaturated ketones and allylation to acid chlorides in THF at r.t. without any additive/co-catalyst (reactions 15–16) [111]. The notable advantages are less expensive, heterogeneous reusable catalyst; mild reaction conditions, high yields of products, shorter reaction times, no isomerization during the reaction, and easy workup. In addition, 20-nm CuFe2O4 was applied as reusable hetero‐ geneous initiator in the synthesis of α*-aminonitriles* by one-pot reaction of different aldehydes with amines and trimethylsilyl cyanides at r.t. in water as a solvent (reactions *17–18*) [112]. α-Aminonitriles are important in preparing a wide variety of amino acids, amides, diamines, and nitrogen containing heterocycles. In addition, a strategy for the synthesis of *benzoxazoles* from substituted N-(2-halophenyl)benzamides (reaction *19*) was developed [113], where inexpensive, readily available, air-stable, recyclable copper(II) ferrite serves as a nanocatalyst. Also, larger-size cubic copper ferrite CuFe2O4 nanopowders (24–51 nm in size) were synthe‐ sized *via* a hydrothermal route using industrial wastes (ferrous sulfate containing free sulfuric acid ≈10%, 0.01% Zn2+ and 2% silica; copper waste 12.5% Cu, 8.7% Cl- with minor Ni 0.001%) [114]. Study of photocatalytic degradation of the *methylene blue* (MB, C16H18ClN3S) dye using copper ferrite powders showed a good catalytic efficiency (95.9%) at hydrothermal tempera‐ ture 200 °C for hydrothermal time 24 h at pH = 12 due to high surface area (118.4 m<sup>2</sup> /g).

**Nickel ferrites.** Pure or doped nickel ferrites out of the nano-range size are common and frequently used in several catalytic processes. For instance, high reactivity of NiFe2O4 (111) surfaces (higher that in Fe3O4) is well-known; NiFe2O4 is an effective metal-doped ferrite catalyst in a typical industrial process such as the water-gas shift (WGS) reaction [104]. Similarly, NiFe2O4 was examined as catalyst in photocatalytic water oxidation using

of NiFe2O4 is comparable to that of a catalyst containing Ir, Ru, or Co in terms of O2 yield and O2 evolution rate under ambient reaction conditions. As an example of non-nano-sized doped ferrites, their catalysts (granules of ~1 mm diameter) of nickel, cobalt and copper, prepared by co-precipitation hydrothermal route and impregnated with palladium, cerium and lanthanum as promoters [106], were tested for carbon monoxide oxidation activities. The catalysis of NiFe2O4 nanoparticles on the *hydrogen storage performances* of magnesium hydride synthesized by high-energy ball milling was studied [107], showing that the initial dehydrogenation temperature of 7 mol.% NiFe2O4-doped MgH2 is 191°C, which is 250°C lower than that of pristine MgH2. The enhancement in the H2 storage performances of MgH2 by adding NiFe2O4 nanoparticles is primarily ascribed to intermetallic Fe7Ni3 and (Fe, Ni) phases during the desorption procedure, which act as the real catalyst in the 7 mol.% NiFe2O4-doped sample. As an example of another application, a magnetic acidic catalyst comprising Preyssler (H14[NaP5W30O110]) heteropoly acid supported on silica coated nickel ferrite nanoparticles (NiFe2O4@SiO2) was investigated for the synthesis of *bis(dihydropyrimidinone)benzene and 3,4*‐ *dihydropyrimidin*‐*2(1H)*‐*ones derivatives* by the Biginelli reaction [108]. With the catalyst, the

**Copper ferrites.** Non-nanosized range copper ferrites have certain catalytic applications, such as, for example, for CO conversion to CO2 [109]. In a difference of pure nickel ferrites, copper ferrite NPs are applied in organic catalysis in more uniform particle size (mainly about 20 nm). Thus, nano material on the basis of copper ferrite (20 nm) was applied as reusable heteroge‐ neous initiator in the preparation of 1,4-dihydropyridines. The interaction of substituted *aromatic aldehydes, ethyl acetoacetate and ammonium acetate* (reaction *14*) was observed in presence of CuFe2O4 nano powders in ethanol at ambient conditions. The nano catalyst can be magnet‐ ically recovered and reused [110]. The same 20-nm size copper ferrite nano material also was reported as reusable heterogeneous initiator in the synthesis of *β*,*γ*-unsaturated ketones and allylation to acid chlorides in THF at r.t. without any additive/co-catalyst (reactions 15–16) [111]. The notable advantages are less expensive, heterogeneous reusable catalyst; mild reaction conditions, high yields of products, shorter reaction times, no isomerization during the reaction, and easy workup. In addition, 20-nm CuFe2O4 was applied as reusable hetero‐ geneous initiator in the synthesis of α*-aminonitriles* by one-pot reaction of different aldehydes with amines and trimethylsilyl cyanides at r.t. in water as a solvent (reactions *17–18*) [112]. α-Aminonitriles are important in preparing a wide variety of amino acids, amides, diamines, and nitrogen containing heterocycles. In addition, a strategy for the synthesis of *benzoxazoles* from substituted N-(2-halophenyl)benzamides (reaction *19*) was developed [113], where inexpensive, readily available, air-stable, recyclable copper(II) ferrite serves as a nanocatalyst. Also, larger-size cubic copper ferrite CuFe2O4 nanopowders (24–51 nm in size) were synthe‐ sized *via* a hydrothermal route using industrial wastes (ferrous sulfate containing free sulfuric

2− as a sacrificial oxidant [105]. The catalytic activity

[Ru(bpy)3]

2+ as a photosensitizer and S2O8

52 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

reactions occurred in less than 1 h with good to excellent yields.

Catalyst: Copper ferrite (1 mol.%), R = a) Ph, b) 4-Me-*o*-C6H4, c) 4-ClC6H4, d) 4-NO2-C6H4, e) 4-MeC6H4, (f) 3-NO2-C6H4, (g) *n*-C9H9, (h) 2-NO2-C6H4, (i) 2-furyl, (j) 2-Me-*o*-C6H4.

Synthesis of *β*,*γ*-unsaturated ketone using allyl bromide. R = (a) C6H5, (b) 2-ClC6H4, (c) 2-Br, 5-F, C6H3, (d) 2-Br, 5-F, C6H3, (e) furanyl, (f) 5-phenyl, 3-Methyl, 4-Isoxazolyl, (g) 5-(2,5-dichloro)phenyl, 3-methyl, 4-isoxazolyl, (h) (CH3)3C-, (i) C11H23-, (j) C15H31-.

Synthesis of *β*,*γ*-unsaturated ketone using cinnamyl chlorides. Synthesis of *β*,*γ*-unsaturated ketone using cinnamyl chlorides. R = (a) C6H5, (b) 2-ClC6H4, (c) furanyl, (d)-CH(CH3)2.

The synthesis of α-aminonitriles in the presence of nano CuFe2O4 in water as green solvent at r.t.

Suggested mechanism for the synthesis of α-aminonitriles derivatives in presence of acidic nano copper ferrite.

Catalyzed cyclization of N-(2-bromophenyl)benzamide to 2-phenyl-1,3-benzoxazole.

**Zinc ferrite**. Non-nano-sized zinc ferrites (ZnFe2O4) have been used in oxidative organic reactions. Thus, the catalytic behavior for *oxidative conversion of methane and oxidative coupling of methane* was investigated over pure and neodymium substituted zinc ferrites prepared by combustion method [115]. The catalytic activity proved to be strongly related to the oxide structure as well as to the specific defects created by substitution. The pure zinc ferrite (ZnFe2O4) and ZnNd2O4 exhibited high activity for coupling reaction whereas the neodymium substituted ferrites (ZnFe1.75Nd0.25O4, ZnFe1.5Nd0.5O4 and ZnFeNdO4) was low active in this reaction. The order of the catalytic activities expressed as yields to C<sup>2</sup> + were ZnNd2O4 > ZnFe2O4 > ZnFe1.75Nd0.25O4 > ZnFeNdO4 > ZnFe1.5Nd0.5O4.

Analyzing pure zinc ferrite nanocatalysts, we note that mainly ultrasmall particles are currently applied in catalytic purposes. Thus, a nanosized highly ordered mesoporous zinc ferrite (ZF, 7–10 nm in size) was synthesized *via* co-precipitation method, further sulfated

with ammonium sulfate solution to obtain sulfated ZF (SZF) and was used for the synthe‐ sis of nopol by Prins condensation of *β*-pinene and paraformaldehyde (reaction *20*) [116]. 70% *conversion of β-pinene* with 88% selectivity to nopol was observed; the spent catalyst was regenerated and reused successfully up to four cycles with slight loss in catalytic activity. The influence of various reaction parameters such as solvent, reaction temperature, effect of substrate stoichiometry and catalyst loading was investigated. In particular, very low conversion (9%) of *β*-pinene was observed in protic solvents such as methanol (solvent effect); in case of apolar–aprotic solvents such as hexane, ethyl acetate and toluene, increase in the *β*-pinene conversion as well as nopol selectivity was observed. In case of reaction tempera‐ ture, as the temperature increased to 110 °C, *β*-pinene conversion increased to 72% with slight drop in selectivity for nopol (57%). 95 °C was identified as optimal reaction temperature for further studies. At molar ratio of *β*-pinene to paraformaldehyde 1:3 molar ratio, maximum conversion was observed. The *β*-pinene conversion increased with an increase the catalyst loading (0.12 to 0.16 g) without affecting the nopol selectivity.

Prins condensation reaction of *β*-pinene and paraformaldehyde.

The synthesis of α-aminonitriles in the presence of nano CuFe2O4 in water as green solvent at r.t.

54 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Suggested mechanism for the synthesis of α-aminonitriles derivatives in presence of acidic nano copper ferrite.

**Zinc ferrite**. Non-nano-sized zinc ferrites (ZnFe2O4) have been used in oxidative organic reactions. Thus, the catalytic behavior for *oxidative conversion of methane and oxidative coupling of methane* was investigated over pure and neodymium substituted zinc ferrites prepared by combustion method [115]. The catalytic activity proved to be strongly related to the oxide structure as well as to the specific defects created by substitution. The pure zinc ferrite (ZnFe2O4) and ZnNd2O4 exhibited high activity for coupling reaction whereas the neodymium substituted ferrites (ZnFe1.75Nd0.25O4, ZnFe1.5Nd0.5O4 and ZnFeNdO4) was low active in this

Analyzing pure zinc ferrite nanocatalysts, we note that mainly ultrasmall particles are currently applied in catalytic purposes. Thus, a nanosized highly ordered mesoporous zinc ferrite (ZF, 7–10 nm in size) was synthesized *via* co-precipitation method, further sulfated

+

were ZnNd2O4 >

Catalyzed cyclization of N-(2-bromophenyl)benzamide to 2-phenyl-1,3-benzoxazole.

reaction. The order of the catalytic activities expressed as yields to C<sup>2</sup>

ZnFe2O4 > ZnFe1.75Nd0.25O4 > ZnFeNdO4 > ZnFe1.5Nd0.5O4.

**Other simple ferrites**. The catalytic behavior of supported Au NPs for the process of *oxidation of benzyl alcohol* was elucidated in presence of gold nanoparticles [117]. Mg2+ ions, being present in the ferrite structure, led to an improvement of the catalytic activity of supported Au NPs to *ca*. 35% conversion, when an additional base was absent. Modifying the support with addition of MgO, the catalytic activity of supported Au nanoparticles was further improved to *ca.* 50% conversion; however, the catalyst was found to be deactivated in successive recycling tests. As well as nano-Fe2O3 mentioned above in the corresponding section, nano-MnFe2O4 particles (20–30 nm in size), synthesized by co-precipitation phase inversion method and low-temperature combustion method, using MnCl2, FeCl3, Mn(NO3)2, Fe(NO3)3, NaOH and C6H8O7, were applied for thermal *decomposition of ammonium perchlo‐ rate* [118]. The catalytic mechanism was explained by the favorable electron transfer space provided by outer *d* orbit of transition metal ions and the high specific surface absorption effect of MnFe2O4 particles. Manganese ferrite nanoparticles were also applied in the synthesis of *spirooxindoles* (compounds **I-III**) *via* a one-pot and three-component reaction of isatins, malononitrile, and anilinolactones in the presence of a catalytic amount of MnFe2O4 NPs in PEG-400, as a nontoxic, green, and reusable solvent [119].

Selected spirooxindole natural products.

**Mixed-metal or core-shell ferrites.** Ferrites containing 2 metal ions, additionally to iron, are much more widespread in the nano-catalysis; their nanoparticle size can vary in a broad range, from ultrasmall particles (5–8 nm) up to 100 nm or more (in case of supported NPs). Both nanosized and out-of-nano-sized mixed-metal ferrite NPs can be synthesized by a variety of methods, in particular classic sol-gel and co-precipitation methods or microwave heating (MnZnFe2O4 [120]).

*Cobalt-based ferrite nanoparticles*. For cobalt-containing ferrite NPs, as well as for zinc ferrite above, one of important applications is the methanol decomposition to CO and hydrogen. Thus, Cu1-*x*Co*x*Fe2O4 (0<*x*<1, 8–40 nm in size) was applied as a nanodimensional powder for this purpose [121]. The stabilization of the cubic structure with the substitution of copper ions by cobalt in mixed Cu-Co ferrites was observed. Cobalt containing ferrites exhibited higher and more stable catalytic activity and selectivity in *methanol decomposition* to CO and hydrogen in comparison with the CuFe2O4 one. Photocatalytic properties of the cobalt zinc ferrite Co1 *<sup>x</sup>*Zn*x*Fe2O4 (0<*x*<1) nanoparticles (10.5−14.8 nm in size), prepared by a hydrothermal method, were studied on the example of *degradation of methyl blue* in aqueous solution [122]. It was elucidated that the oxidation-reduction potential of methyl blue aqueous solution in presence of the ferrite nano-particles at pH=7 under natural sunlight irradiation was negative and increased with increase in Zn content. The degradation rate of methyl blue also decreases as increase in Zn content in sunlight.

*Nickel-based ferrite nanoparticles*. Similar to cobalt ferrites, several nickel-containing mixed or core-shell ferrites have been reported as nanocatalysts but in more narrow size range (18–50 nm). Thus, a magnetically separable catalyst consisting of ferric hydrogen sulfate (FHS) supported on silica‐coated nickel ferrite nanoparticles (50 nm) was prepared [123]. This catalyst was shown to be an efficient heterogeneous catalyst for the *synthesis of 1,8*‐*dioxodecahydroacri‐ dines* (reaction *21*) under solvent‐free conditions. The catalyst can be recycled several times with no significant loss of catalytic activity.

*Other mixed-metal ferrite nanoparticles*. Ferrite nanoparticles, containing other metals and applied in the catalysis, are represented more chaotically in the available literature. Thus, the spinel ferrites Cu1-*x*Cd*x*[Fe1-*x*Al*x*Cr1-*x*Mn*x*]O4, where 0<*x<*1, having unknown particle size, were

Synthesis of 1,8‐dioxodecahydroacridines in the presence of NiFe2O4@SiO2‐FHS.

prepared by the coprecipitation technique [124]. Catalytic studies using *decomposition of H2O2* as a model reaction between 303 and 343 K for 1–5 h using first order rate law suggested higher catalytic power for the composition *x* = 0 and then it decreases gradually. For the mixed spinel ferrite system Mn1-*x*Cu*x*Fe2O4 (*x* = 0, 0.25, 0.5, 0.75, 1.0), the formation of phase pure spinels with FCC cubic structure with particle size ranging from 5.21 nm to 20 nm was observed at 333 K applying co-precipitation method with MnCl2, Fe(NO3)3 . 9H2O and Cu(NO)3 **.** 3H2O as precur‐ sors [125]. These ferrites were used as catalysts in the *alkylation of aniline*, showing a maximum conversion of 80.5% of aniline with selectivity of 98.6% towards N-methylaniline at 673 K, methanol/aniline molar ratio of 5:1 and weight hour space velocity of 0.2 h-1. It was found that the yield is maximum for CuFe2O4. In addition, the catalytic performance of the ferrites was found to be proportional to surface area as well as acidity.
