Perovskites as Substitutes of Noble Metals for ... - ACS Publications

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Perovskites as Substitutes of Noble Metals for Heterogeneous Catalysis: Dream or Reality Sébastien Royer,†,‡ Daniel Duprez,*,† Fabien Can,† Xavier Courtois,† Catherine Batiot-Dupeyrat,† Said Laassiri,†,‡ and Houshang Alamdari‡ †

Université de Poitiers, CNRS UMR 7285, IC2MP, 4 Rue Michel Brunet, TSA 51106, 86073 Poitiers Cedex, France Department of Mining, Metallurgical and Materials Engineering, Université Laval, Québec, Québec G1V 0A6, Canada

‡

4.3.1. Effect of the Material Composition 4.3.2. Mechanism of CH 4 Oxidation and Relationship with Surface and Bulk Properties 4.4. Volatile Organic Compounds (VOC) Elimination 4.4.1. Alkanes 4.4.2. Alkenes 4.4.3. Aromatics 4.4.4. Alcohols and Other Oxygenates 4.4.5. Comparison Between Hydrocarbons and Oxygenates 4.4.6. Chlorinated Hydrocarbons 4.4.7. Ammonia and Other N Compounds 4.5. Solid−Solid Reaction: Soot Combustion 4.5.1. Soot Combustion in Oxygen or Air 4.5.2. Simultaneous Removal of Soot and NOx 4.5.3. Kinetics and Mechanisms of Soot Oxidation over Perovskites 4.6. Water Depollution 4.6.1. Wet Peroxide Oxidation 4.6.2. Ozonation and Wet Air Oxidation 4.7. Conclusions 5. Perovskite for Reduction Reactions 5.1. NOx Decomposition 5.1.1. Nitrous Oxide Decomposition 5.1.2. Nitric Oxide Decomposition 5.2. NOx Reduction 5.2.1. NOx Reduction in Continuous Processes (SCR and TWC) 5.2.2. NOx Storage Reduction (NSR) 5.3. Conclusions 6. Perovskites in Selective Oxidation Reactions 6.1. Partial Oxidation of Methane (POM) and Hydrocarbons 6.2. Oxidative Coupling of Methane (OCM) 6.3. Selective Oxidation of Ethane to Ethylene 7. Reforming Reactions for Hydrogen and Syngas Production 7.1. Methane Dry Reforming 7.2. Steam Reforming of Methane 7.3. Other Molecules, Including Short Alcohols, Glycerol, and Renewable Molecules 7.3.1. Steam Reforming of Ethanol 7.3.2. Fuel Reforming

CONTENTS 1. Introduction and Background 2. Recent Advances in Perovskite Synthesis: From Large Nano to Small Nano 2.1. Advances in Solid−Solid Processes for NanoPerovskite Synthesis 2.1.1. Reactive Grinding Synthesis 2.1.2. Microwave Crystallization 2.2. Advances in Solution-Mediated Processes 2.2.1. Classical Hydroxyacid Complexation Route 2.2.2. Autocombustion Processes 2.2.3. Nonaqueous Solvothermal Synthesis Route 2.2.4. Mesostructuration: Nanocasting/Hard Templating as an Efficient Approach 2.2.5. Macrostructuration Approaches 2.3. Conclusions 3. Surface and Bulk Characterization 3.1. Redox Properties and Oxygen Mobility of Perovskites 3.1.1. Redox Properties 3.1.2. Oxygen Mobility 3.2. Acid−Base Surface Properties of Perovskites 4. Oxidation Reactions 4.1. General Trends in Oxidation Reactions over Perovskites 4.2. Low-Temperature Oxidation Reaction: CO Oxidation 4.2.1. Effect of the Material Composition 4.2.2. Mechanism of CO Oxidation and Relationship with Surface and Bulk Properties 4.3. High-Temperature Oxidation Reaction: CH4 Oxidation © 2014 American Chemical Society

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10311 Received: January 18, 2014 Published: September 25, 2014

10312 10292

dx.doi.org/10.1021/cr500032a | Chem. Rev. 2014, 114, 10292−10368

Chemical Reviews 7.4. Conclusions 8. Carbon Nanotube Synthesis as New Catalytic Application 9. General Conclusions and Perspectives Author Information Corresponding Author Notes Biographies Acknowledgments References

Review

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Table 1. List of Cations with Their Respective Radii, Found in ABO3-Type Perovskite5

10353 10355 10355 10355 10355 10355 10357 10357

dodecahedral A site

1. INTRODUCTION AND BACKGROUND Perovskites can be described by the general formula ABO3, A and B being two cations and O being an oxygen anion. They represent probably the most studied mixed-oxide system in the field of heterogeneous catalysis. The first catalytic studies on perovskites were published at the beginning of the 1970s, reporting exceptional catalytic properties in oxidation reactions and NO reduction, suggesting the possibility of replacing platinum-group metals (PGMs) by perovskite in automotive exhaust catalytic convertors.1−4 Today, perovskites are indeed considered as serious alternatives for PGMs in various catalytic applications, mainly due to their ease of synthesis and low cost compared to PGMs and the extraordinary capability of their structure to accommodate a wide range of substituting and doping elements, allowing tailoring their properties to better targeting their applications. In the perovskite structure, the A cation can be a lanthanide, alkaline, or alkaline-earth cation while the B cation is a metallic element from the 3d, 4d, or 5d configuration. Considering the possible valences of the cations and the electroneutrality of the structure, different charge distributions can be encountered in the structure, i.e., AIBIVO3, AIIBIVO3, or AIIIBIIIO3, as in typical examples of NaWO3, (Ca,Ba)TiO3, or LnBO3 (Ln being a trivalent lanthanide and B a transition metal).5 As such, more than 90% of the metallic elements from the periodic table can enter the perovskite structure. Table 1 shows the cations commonly found in perovskite structure. The ideal crystalline unit cell of perovskite is described as cubic from the Pm3m space group, where the A cation is in the center of the cube, formed by the B cations, and the oxygen ions are located in the middle of the edges. The B cation is in octahedral coordination toward oxygen, while the A cation is in dodecahedral coordination.5 One of the advantages of the perovskite structure is thus the possibility to adopt a wide range of different compositions, changing either the A or the B cation or partially substituting each cation by other cations of the same or different valences, resulting in a general formula of A1−xA′xB1−yB′yO3±δ, to adjust its redox and surface properties. The stability of the structure depends directly on the geometrical constraints of octahedral and dodecahedral cavities. To have a stable BO6 octahedron, the Bx+ radii should be higher than 0.51 Å. Insertion of the A cation in the cubooctahedral cavities of the BO6 arrangement leads to distortion of the octahedrons and formation of more stable orthorhombic or rhombohedral structures. The crystal would be stable as long as the tolerance factor, as defined in eq 1, lies between 0.8 and 1. t=

rA + rO 2 (rB + rO)

a

octahedral B site

ion

radius (Å)a

radius (Å)b

ion

Na+ K+ Rb+ Ag+ Ca2+ Sr2+ Ba2+ Pb2+ La3+ Pr3+ Nd2+ Bi3+ Ce4+ Th4+

1.06 1.45 1.61 1.40 1.08 1.23 1.46 1.29 1.22 1.10 1.09 1.07 1.02 1.09

1.32? (IX) 1.60? 1.73 1.30 (VIII) 1.35 1.44 1.60 1.49 1.32? 1.14 (VIII) 1.12(VIII) 1.11(VIII) 0.97(VIII) 1.04(VIII)

Li+ Cu2+ Mg2+ Zn2+ Ti3+ V3+ Cr3+ Mn3+ Fe3+ Co3+ (LS) Co3+ (HS) Ni3+ (LS) Ni3+ (HS) Rh3+ Ti4+ Mn4+ Ru4+ Pt4+ Nb5+ Ta5+ Mo6+ W6+

radius (Å)a

radius (Å)b

0.68 0.72 0.66 0.74 0.76 0.74 0.70 0.66 0.64

0.74 0.73 0.72 0.75 0.67 0.64 0.62 0.65 0.64 0.52 0.61 0.56 0.60 0.66 0.60 0.54 0.62 0.63 0.64 0.64 0.60 0.60

0.63 0.62 0.68 0.68 0.56 0.67 0.65 0.69 0.69 0.62 0.62

From ref 6. bFrom ref 7.

The ideal cubic structure is characterized by a tolerance factor of 1; however, the cubic structure can be preserved if the tolerance factor lies between 0.75 and 1.0.8 If the value of t drops below 0.75, the crystal adopts a hexagonal ilmenite structure (FeTiO3), while for higher t values (1.00 < t < 1.13) the crystal exhibits hexagonal symmetry. Due to the role of the BO6 octahedron on crystal unit cell formation, an octahedral factor (rB/rO) was also introduced, being as important as the tolerance factor.9 The constraints on octahedron formation are, however, not enough to predict the cubic structure formation. Since early reports on this class of materials,1−4 manganeseand cobalt-based perovskites have been reported to present higher catalytic activity for oxidation reactions than do the other transition metal-based perovskites, explaining why most of the published reports have been dealing with these pure or substituted compositions.5,8,10 Substitution of the A cation was found to be efficient to modulate catalytic activity, e.g., rare earth in (La,Pr)CoO3 and (La,Pr,Nd)1−xPbxMnO3 was substituted by lead.2−4 Some of these structures, namely, PrCoO3 and Nd1−xPbxMnO3, were proposed as potential alternatives for Pt in exhaust catalytic converters, mainly due to their hydrothermal and sulfur resistance compared to Pt/Al2O3 catalysts. For example, the catalytic efficiency of (La,Nd)0.7Pb0.3MnO3 is not affected by addition of 2 vol % water in the oxidation feed.4 Nevertheless, perovskites require higher temperatures to have the same efficiency as Pt/Al2O3. For instance, T5 (temperature of 5% conversion) was reported to be 80 and 160 °C, respectively, for Pt/Al2O3 and PrCoO3 at reaction conditions of CO:O2 molar ratio = 2:1, 30 mL min−1, 2 cm3 of catalyst.2 To some extent, similar differences have been reported for T10, T20, and T50. Although perovskites require higher conversion temperatures, these results were encouraging, especially considering their very limited surface

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Table 2. Representative Examples of Perovskite Material and Derived Properties Obtained from Different Synthesis Routes temperature of crystallization

synthesis route

materials and conditions of synthesis

ceramic method

20 different materials LaBO3 (where B = Cr, Mn, Co, Fe, Mn) and La1−xA′xBO3 (where A′ = Sr, Ca, Bi, K, Rb, Na) from oxide precursors 13 different materials of La1−xSr′xCo1−yFeyO3, from nitrate and acetate precursors La1−xA′xMnO3 (where A′ = Pb, Sr) using different precursors (nitrate, chloride, acetate) and different bases (KOH, NH4OH, (NH4)2CO3...)

coprecipitation

complexation

freeze drying

LaBO3 (where B = Fe, Co, Ni, Cr) and La0.85A′0.15CoO3 (where A′ = Ca, Ba, Ce) AMnO3 (where A = La, Sr) and La1−xSrxMnO3 using nitrate precursors and citric acid LaCoO3, surface and textural properties studied with the amount of citric acid used and the calcination temperature La0.7Pb0.3MnO3 + ε.Pt by freeze drying LaBO3 (where B = Fe, Co, Ni, Cr) and La0.85A′0.15CoO3 (where A′ = Ca, Ba, Ce) by freeze-drying A1−xA′xMnO3 (where A = La, Nd and A′ = Sr, K) by freeze drying

area (typically less than 0.1 m2·g−1). In addition, a number of compositions exhibited high stability against inhibitors such as H2O. A short time later, some results were published dealing with perovskite doped by noble metals, aiming at increasing the activity of Sr- and Pb-substituted LaMnO3 materials.11,12 Hightemperature stability is reported for the Sr-doped compounds due to the lower mobility of Sr atoms on the crystal surface compared to that of Pb, making Sr2+ a suitable substituent for lanthanides. However, the contribution of Pt doping on the activity of La0.7Pb0.3MnO3 is limited at low doping contents. Below 570 ppm, no improvement of the initial activity was noticed, and only at higher loading levels, e.g., 1600 and 5500 ppm, a significant increase of activity was observed. Platinum also increases sulfur resistance of the perovskite: no deactivation was observed in the presence of 50 ppm SO2 with a Pt loading as low as 200 ppm. Yao observed similar behavior,13 confirming the real potential of these materials for catalytic reactions. Knowledge on perovskite properties has significantly progressed since these pioneering works, resulting in rapid use of perovskite in a wide variety of catalytic reactions.5,8 Since 2000, a considerable number of papers have been published on new synthesis routes and applications of nanoperovskites. This review is primarily focused on recent advances in development and application of perovskites in heterogeneous catalytic processes. The first part of the review is devoted to advances in the synthesis of nanoscaled perovskite. Surface and bulk properties of perovskites as well as their catalytic properties are thereafter reviewed in the second part.

from 1100 to 1400 °C 850 °C

surface area/m2 g−1 (crystal size/nm)

ref

TbCoO3 < DyCoO3, with EuCoO3 as the optimal perovskite formulation in regards to H2-TPR experiments. Partial substitution of La by Sr (La0.8Sr0.2CoO3−x) gives a less crystallized structure, being easier to reduce (below 200 °C). In addition, a new reduction peak appears at 750 °C, being attributed to decomposition of the SrCO3 impurities, resulting from the citrate synthesis method.152 Nakamura et al.154 studied the related properties of oxygen both in the bulk and on the surface of perovskite-type mixed oxides (La1−xSrxCoO3). The authors concluded that Sr incorporation favors formation of unstable Co4+, together with formation of oxygen vacancies. A similar two-step reduction mechanism was observed for LaNiO3 perovskite: (a) 1 e−/molecule between 150 and 400 °C and (b) 3 e−/molecule between 300 and 650 °C.155−157 This result was supported by the work of Falcon et al.,158 who showed that reduction of Ni 3+/4+ occurs through two consecutive reduction steps (Ni3+/4+ → Ni2+ → Ni0). Note that Crespin et al.151 found a stable intermediate reduction level of 2 e−/molecule, resulting from a three-step LaNiO3 reduction mechanism. The effect of Sr substitution in LnNiO3 (Ln = Pr, Sm, Eu) perovskites was also investigated, leading to similar behavior to those reported for cobalt-containing perovskites. Additionally, Sr-doped samples show the presence of Ni in a high oxidation state (5 atom % of Ni atoms), which facilitates reduction of the structure and leads to an important modification on the catalytic properties.158 Tejuca et al.23 studied the reducibility of ABO3 perovskitetype oxides. For LaFeO3, they reported a reduction of higher than 3 e−/molecule, indicating an excess of O2 in the oxide structure, possibly due to the presence of Fe4+ ions in the structure.24 To the opposite, TPR over AFeO3 perovskites (A = La, Nd, Sm) and LaFe1−xMgxO3 (x = 0.1−0.5), prepared by the citrate method and calcined at 800 °C, evidenced a very small fraction of Fe4+, which reduces to Fe3+.159 The fraction of Fe4+ in the LaFe1−xMgxO3 samples increased with increasing Mg content up to x = 0.2; then it remained nearly constant. In addition, over LaAl1−xFexO3 (0 ≤ x ≤ 1) perovskites, a very low extent of reduction, due to the Fe4+ → Fe3+ process, was suggested by TPR analysis.160 The influence of the A-site cation in AFeO3+x (A = La, Pr, Nd, and Gd) perovskite-type oxides on

the reducibility has been studied by Marti et al.161 They reported that the reduction occurs in two successive steps. However, for LaFeO3 the reduction was not complete. Different results are reported by Tascón et al.162 for reduction of LaFeO3+δ under hydrogen; a first reduction was attributed to reduction of the nonstoichiometric compound to a stoichiometric one, and during the second reduction step Fe3+ is reduced to metallic iron. A multistep reduction mechanism was also observed over LaMnO3-based perovskites.163,164 The first hydrogen consumption peak in the TPR profile is observed at 400 °C, with a shoulder at higher temperature. The baseline was, however, not recovered after the first peak, indicating a not well-resolved multistep reduction process. The first lowtemperature reduction peak can be associated to reduction of Mn4+. The second peak (547−927 °C) is assigned to reduction of Mn3+ to Mn2+.165 Fierro et al.165 reported a stable reduced state around 827 °C (Figure 12), accompanied by the disappearance of the perovskite structure and formation of La2O3 and MnO (eq 5) LaMnO3 + 1/2H 2 → 1/2La 2O3 + MnO + 1/2H 2O (5)

Figure 12. Temperature-programmed reduction (300 Torr H2) of LaMnO3 (heating rate 4 °C min−1).165

Significant differences in redox properties were reported for Sr-substituted manganese-based perovskites.164 Two clear reduction regions are observed at 150−530 and 550−930 °C (Figure 13). The reduction profiles of La1−xSrxMnO3 samples in the 150−530 °C region display two major components, and their intensity and position depend on the degree of substitution. This peak is associated with reduction of Mn4+ concentration, which varies with the Sr2+ substitution degree. The peak (or shoulder) at slightly lower temperatures, whose intensity reaches a maximum for the unsubstituted LaMnO3 sample, can be assigned to removal of the overstoichiometric oxygen, accommodated within the lattice. The second peak (550−930 °C) is assigned to reduction of Mn3+ to Mn2+, as already reported for LaMnO3. Finally, Sr substitution promotes the reducibility of Mn(IV), probably because substitution results in a lower crystallinity of the perovskite, as suggested by Irutsa et al.152 AMnO3 (A = La, Nd, Sm) and Sm1−xSrxMnO3 (x = 0.1, 0.3, 0.5) redox properties were investigated by Ciambelli et al.166 TPR measurements on AMnO3 showed that the perovskites are reduced in two steps both at low and at high temperature 10305


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Figure 14. TPD spectra of O2 (m/z = 32) from sample La0.99Co0.86Fe0.15O3−ó calcined at various temperatures.175

perovskite. The desorption process is accompanied by reduction of the transition metal cations in the perovskite, and the β-oxygen desorption appears to be reversible.175 The nature of α- and β-oxygen, in relation to the structural defect of a wide variety of perovskite-type oxides, has been studied by Teraoka et al.169 The results show that α-oxygen desorption increases with partial substitution of A-site cations, while desorption of β-oxygen corresponds to reduction of Bsite cations to lower valence number. In addition, Bell et al.180 and Nakamura et al.154 demonstrated that the desorption rate of α-O2 increased by increasing Sr2+ content in La1−xSrxCoO3 perovskites, as reported in Figure 15. The amount of desorbed

Figure 13. Temperature-programmed reduction profiles of La1−xSrxMnO3 catalysts.164

related to Mn(IV) → Mn(III) and Mn(III) → Mn(II) reductions, respectively. The onset temperatures were in the order of LaMnO3 > NdMnO3 > SmMnO3. In Sm1−xSrxMnO3, Sr substitution results in formation of Mn(IV), easily reducible to Mn(II), even at low temperature. In the same line, manganese substitution by copper in LaMn1−xCuxO3 was also observed to affect the reducibility of Mn4+ species (increasing the temperature of reduction).163 3.1.2. Oxygen Mobility. The oxygen mobility in perovskites is mainly characterized by temperature-programmed desorption (TPD) experiments and isotopic oxygen exchange analysis. A huge number of articles15,30,154,161,163−176 report related properties of O2, both in the bulk and on the surface of perovskites. It is now established that perovskites can adsorb a large amount of oxygen and desorbs two types of oxygen, α and β.177,178 The low-temperature peak, usually referred to α-O 2 desorption, is ascribed to oxygen species weakly bounded to the surface of perovskite. This first O2 desorption peak is usually observed below 500 °C. The position of the α peak shifts to higher temperatures, and the corresponding amount of oxygen increases first and then decreases with increasing calcination temperature (Figure 14).175 The amount of released oxygen is also small compared to the value obtained by assuming full oxygen surface coverage of the sample (4−6 mmol of O2 m−2 for one surface layer).175 It was proposed that during calcination α species are formed upon oxygen adsorption on the surface with a strength depending on the calcination temperature. As evidenced by XPS analysis, most of the α-oxygen species would be considered as O− or O2− species,175,179 weakly bounded to the transition metal sites on the surface and desorbing at low-to-intermediate temperature. The second desorption peak (β-O) is characterized by a sharp desorption of oxygen at higher temperature (Figure 14). By contrast with α-oxygens, increasing the calcination temperature of perovskite yields an increase of the amount of β-oxygen released. The desorbed β-oxygen, corresponding to more than one monolayer,17 comes primarily from the bulk of the

Figure 15. TPD curves of oxygen from La1−xSrxCoO3 (x = 0−0.6).154

α-O2 for x = 0.4 and 0.6 corresponds to several layers of oxygen, suggesting that, for specific material compositions (mostly those presenting high density of vacancies in the lattice), α-O2 cannot be ascribed only to the surface oxygen species.154 Similar results were obtained by Ferri et al.181 and Nitadori et al.167 over the LaFeO3 system. Partial substitution of La3+ by Ce4+ in LaCoO3 also results in an increase of the amount of desorbed oxygen.167 Regarding La1−xSrxNiO3, Garcia de la Cruz et al.182 demonstrated that partial substitution of La 3+ by Sr 2+ alters the oxide stoichiometry, yielding a mixture of NiII/NiIII oxidation states 10306


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surface: a preadsorbed molecule is displaced by a gas-phase molecule that adsorbs. Then this mechanism does not involve scission of any O−O bond.188 18 O-exchange between 18O2 in the gas phase and oxygen from perovskite-type La1−xSrxCoO3 oxides, as well as 18Oequilibrium in the gas phase, was investigated by Nakamura et al.154 The authors observed that the exchange reaction between gaseous and lattice oxygen occurs under oxygen atmosphere in La1−xSrxCoO3, and 18O diffuses into the bulk at a considerable rate. At low temperature (150 °C), the rates of exchange reaction are not affected for the low Sr substitution degree (for x = 0 or 0.2). On the other hand, Sr incorporation considerably increases the exchange rate at 300 °C from x = 0.2 (La0.8Sr0.2CoO3). For higher Sr content (x = 0.6), the exchange rate is enhanced even at low temperature (i.e., 150 °C). It is established that homoexchange (or equilibration, eq 6) usually proceeds through dissociative adsorption of a dioxygen molecule, even at low temperature. Thus, dissociation of the oxygen molecule at the surface appears much faster than the exchange reaction and is enhanced by Sr substitution. Therefore, the rate-determining step of the exchange reaction is probably not the surface reaction but diffusion of oxygen species in the bulk (O2− species). Finally, Nakamura et al.154 concluded that Sr2+ substitution resulted in an increase of oxygen vacancies, enhancing the oxygen diffusion rate from the surface to the bulk. Ce incorporation in the A position of LaCoO3 perovskites also promotes oxygen mobility, with a continuous increase in equilibration rate with the Ce4+ content. Similar results were obtained by Nitadori et al.167 for substitution of La3+ by Sr2+ in LaFeO3. The authors found that desorption of lattice oxygen or formation of oxygen vacancies becomes easier with increasing Sr 2+ substitution (in La1−xSrxFeO3), making the lattice oxygen more reactive, particularly at x = 0.2. In addition, the capability of dissociating the oxygen molecule varies with x. Isotopic exchange over La1−xSrxFeO3 (x = 0−0.6) showed that isotopic equilibration in the gas phase takes place much more rapidly than that over LaFeO3. In contrast, the isotopic equilibration proceeds very slowly for both x = 0.4 and 0.6. The authors supposed that when La is partially substituted by Sr2+ in the A site of LaFeO3 unstable Fe4+ is formed in the crystal, making oxygen desorption easier. The properties of La1−xSrxMnO3 were investigated by Nitadori et al. in isotopic exchange.168 The rate of isotopic equilibration for La1−xSrxMnO3 (0 ≤ x ≤ 1) was found to vary as follows: x = 0.6 > 0.2 ≈ 1.0 > 0 (Figure 16). However, exchange proceeds very slowly over all samples, and exchange is suggested to be of only 0.7−0.9 layers after 90 min of reaction. These values are much smaller than those obtained for the La1−xAxFeO3 (A = Sr, Ce) or La1−xCexCoO3 systems. Exchange seems thus to be limited only to the oxygen from the surface or the subsurface of the Mn-containing materials even with strontium insertion in the material. However, in La1−xAxMnO3 (A = Ce, Hf: x = 0.1, 0.4), the equilibration rate increased by both Ce and Hf substitutions. It is noticed, in the case of the Ce-containing system, that the exchange rates remarkably increased with increasing substitution degree. The observed rates resulted in 3 and 16 layers of oxygen exchanged after 90 min for x = 0.1 and 0.4, respectively. In this case, the activity is comparable to that of the La1−xCexCoO3 systems.167 This effect was assumed to originate from formation of cation vacancies at the A site upon substitution.

and oxygen vacancies, together with surface enrichment of Srcontaining phases. In addition, the intensity of the O2desorption peak strongly depends on the substitution degree; desorption increases from x = 0.02 to a maximum at x = 0.10 and decreases at higher substitution levels. A rough estimation of the quantity of desorbed oxygen indicates that it exceeds one monolayer, suggesting that a fraction of desorbed oxygen also comes from the bulk. The oxygen isotopic exchange reaction also helps to understand the mechanisms of catalytic oxidation reactions. Using the 16O/18O isotopic exchange technique, Winter183 proposed at least two possible effects of O2 on catalyst activity: (i) the gas may be chemisorbed on the surface, having a direct poisoning effect, and (ii) it may also be slowly incorporated in the lattice, altering the defect structure of the oxide and, thus, affecting the intrinsic reactivity of the catalyst. In fact, the isotopic exchange between 18O2 in the gas phase and 16O in a catalyst involves (1) dissociative adsorption of oxygen with formation of adsorbed atoms or ions, (2) exchange of these atoms or ions with oxygen ions from the oxide, and (3) desorption in the form of the exchanged molecules.184 According to Boreskov184,185 and Novakova186,187 three types of exchange, described by eqs 6−11, could occur on the oxide surfaces. (a) Homoexchange (or equilibration) occurs without appreciable participation of the oxygen from the oxide (eq 6). This reaction occurs according to a mechanism of adsorption−desorption. The rate of exchange of the surface oxygen species from the oxide is negligible. During this homoexchange, the isotopic oxygen fraction in the gas phase remains constant 18

O2(gas) + 16O2(gas) ⇆ 218O16O(gas)

(6)

(b) Simple heteroexchange occurs with participation of only one atom of oxygen from the oxide at each step (eqs 7 and 8). Note that during this simple heteroexchange on oxide surfaces formation of a triatomic surface intermediate (O3)−ads can be considered 18 18

O O(gas) + 16O(s) ⇆ 18O16O(gas) + 18O(s)

18 16

O O(gas) + 16O(s) ⇆ 16O16O(gas) + 18O(s)

(7) (8)

(c) Multiple heteroexchange occurs with participation of two oxygen atoms of the oxide at each step of the exchange following eqs 9−11 18 18

O O(gas) + 216O(s) ⇆ 16O2(gas) + 218O(s)

(9)

18 16

O O(gas) + 218O(s) ⇆ 16O2(gas) + 16O(s) + 18O(s) (10)

18 16

O O(gas) + 218O(s) ⇆ 18O2(gas) + 16O(s) + 18O(s) (11)

For this multiple exchange on the oxide surfaces there are two possible mechanisms: isotopic exchange and “place exchange”. These two mechanisms differ by the oxygen surface intermediates. The isotopic exchange mechanism occurs through an associative mechanism with a four-atomic (O4)−ads surface intermediate. On the contrary, place exchange occurs without requiring a four-atom oxygen intermediate on the 10307


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diffusion, surface−grain boundary−bulk, obviously depend on the crystal network state (density of vacancies and transition metal reducibility) but also on the material microstructure (grain boundary density and accessible surface area). 3.2. Acid−Base Surface Properties of Perovskites

Acid−base properties are also key parameters conditioning the activity of materials for catalytic reactions. In this way, Zou et al.190 showed that calcination of γ-alumina, doped with K+ and La3+, up to 1000 °C results in formation of a perovskite phase (LaAlO3) on the surface of the La2O3/Al2O3 samples. Surface properties were evaluated by microcalorimetric measurements for adsorption of NH3 and CO2. Perovskite phase formation was then claimed to greatly decrease the acid−base surface properties of the catalyst. To the contrary, Natile et al.191 argued that the acidic properties of perovskite-based materials might be modulated over nanostructured LaCoO3 powders. Carbon monoxide adsorption, monitored by IR spectroscopy, revealed a CO absorption band at room temperature (RT) around 2058 cm−1, whatever the [Co/La]nominal ratio (i.e., 0.055 or 1). The authors assigned this feature to the interaction of CO with Lewis acid site centers distributed on the surface of the samples. This contribution disappears at temperatures higher than RT, and new bands in the spectral region 2280− 2400 cm−1, assigned to carbonated species, appear for T ≥ 150 °C. This result can correlate with the high reactivity of the LaCoO3 surface toward CO oxidation at temperatures higher than 150 °C. A similar oxidation capability was observed for the nanocomposite at higher Co/La ratio. Reactivity however decreases at lower Co/La nominal atomic ratio. Surface properties of cobaltites were investigated by Kuhn and Ozkan.192 The authors examined the surface properties of La0.6Sr0.4CoyFe1−yO3−δ (with y = 0.1, 0.2, and 0.3) using methanol as a probe molecule. It was demonstrated that Febased perovskite-type oxides present basic and acidic Lewis surface sites. The amount of Lewis sites is relatively constant and correlated to the Sr content. Substitution of La3+ by Sr2+ into the crystal results in charge balance changes that can generate the acidic sites. On the contrary, the amount of basic sites seems to depend on Co content. This conclusion is inconsistent with the work of Natile et al.191 that showed an acidic character for LaCoO3 powders, induced by lanthanum(III) or cobalt(III), distributed on the surface of the samples.

Figure 16. Isotopic exchange of oxygen over La1−xSrxMnO3 at 300 °C. (A) Equilibration in the gas phase. (B) Exchange between gaseous and lattice oxygen: (a) x = 0, (b) x = 0.2, (c) x = 0.6, (d) x = 1.0.168

Most of the results of isotopic oxygen exchange tend to confirm the important role of the oxidation state of the reducible cation (in the B position) and the role of oxygen vacancies (induced by partial substitution of cations A and/or B) on the oxygen transfer properties of perovskite. These results are entirely complementary with the information issued from the reduction and oxygen desorption experiments. An exceptional result is the perfect correlation that can be obtained between (i) the capacity of oxygen adsorption, (ii) the properties of the material in exchange/equilibration, and (iii) the catalytic activity, as reported by Nitadori et al.168 However, Royer et al.189 evidenced the importance of the material morphostructural properties on the oxygen mobility. Indeed, over materials with comparable chemical compositions, significantly different bulk oxygen mobility can be obtained, this parameter being mainly conditioned by the crystal domain size of the materials. The importance of the microstructure was recently confirmed by Yang et al.123 Oxygen transfer properties in perovskite are then a complex mechanism involving successive steps: (i) adsorption and dissociation of the dioxygen and (ii) diffusion of atoms from the surface into the grain boundaries or directly into the bulk. Rates of oxygen

Figure 17. Catalytic oxidation of H2, CO, and CH4 over LaMnO3 (8.3 m2 g−1) and LaCoO3 (6.2 m2 g−1) perovskite. Reaction conditions: same concentrations of H2, CO, and CH4 (1.5%) + 17.6% O2 + 4.1% CO2 + 1.5% H2O. Temperature ramp: 10 °C min−1. Gas hourly space velocity: 240 000 h−1.195 10308


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Kuhn and Ozkan192 then proposed that surface Co ions, more difficult to oxidize than Fe, generate more surface oxygen vacancies, which allows rapid incorporation of oxygen into the perovskite lattice. The basicity of LaMnO3+y sample (La1−δMn1−δO3) was also studied.193 Basicity was evaluated studying the surface interaction between CO2 and the oxide surface by means of density functional theory calculations and experimental adsorption isotherms in the temperature range of 25−200 °C. Results show that the perovskite surface is heterogeneous, presenting different sites of various strengths for CO 2 adsorption. CO2 adsorption is very weak over the MnO2−terminated surfaces. Oxygen-deficient sites and surface oxygen from the perfect surface are considered as the two main anionic active sites for CO2 adsorption. Strong adsorption occurs on the exposed surface presenting accessible La cations. The strongest adsorption is associated with the Fs center. Finally, adsorption on cationic sites (La or Mn) leads to weakly adsorbed species and consequently weak basic sites.

of Mn3+ to Mn4+, which confers to the catalyst a better activity in methane oxidation.198 However, ESR study by Oliva et al.197 showed that Mn4+ can also arise from a disproportionation mechanism (eq 14), so that electron transfer properties can play a role in oxygen mobility and availability for oxidation reaction 2Mn 3 + → Mn 4 + + Mn 2 +

(14)

Perovskite-like oxides are constituted of small nanoparticles merely agglomerated to form powders, layered materials, or other macrostructures. Two-dimensional defects in these nanoparticles (the so-called “grain boundaries”) were shown to be portholes for fast oxygen diffusion inside the bulk of the material.189 All these aspects of oxidation catalysis over perovskites will be reviewed below. According to the classical distinction between suprafacial and intrafacial mechanisms, lowtemperature oxidation (mainly CO) and high-temperature oxidation (methane) are successively reviewed below. Many volatile organic compounds (VOCs) were also oxidized over perovskite catalysts. The corresponding literature is analyzed in a third part of this section.

4. OXIDATION REACTIONS 4.1. General Trends in Oxidation Reactions over Perovskites

4.2. Low-Temperature Oxidation Reaction: CO Oxidation

4.2.1. Effect of the Material Composition. Catalytic oxidation of carbon monoxide over transition metal oxides was recently reviewed by Royer and Duprez.199 They confirmed that manganite and cobaltite perovskites were very active for CO oxidation. However, partial substitution of Mn or Co (B cation) as well as partial substitution of the A cation (generally a lanthanide) can improve the catalytic activity and material stability.52 The mode of catalyst preparation can also influence the catalyst performance: according to the synthesis route, materials having higher BET area and showing very different redox behavior, oxygen mobility, etc., may be obtained, all these factors having a great impact on the catalytic activity. 4.2.1.1. Manganites. Although there might be confusion with the term “manganite” (a terrestrial rock of composition MnO(OH)), AMnO 3 perovskites will be denoted as manganites and not manganates because of the low oxidation of manganese in these solids ( H2 ≫ CH4 over Pd). The activity is however much higher over the noble metal: CO and H2 are oxidized between 100 and 150 °C and methane oxidation starts at 300−350 °C. Two mechanisms are proposed for oxidation reactions over perovskite catalysts according to the temperature range. At low temperature, a suprafacial mechanism can occur, a mechanism in which only surface oxygen species are involved in the oxidation process. CO oxidation is a typical reaction governed by this mechanism. On LaMnO3.15 (30% Mn4+ and 70% Mn3+), the suprafacial mechanism can be depicted by the following two steps (subscript s for the surface and g for the gas phase) 2Mn 4 + + O2s − + COg → 2Mn 3 + + □s + CO2g

(12)

2Mn 3 + + 1/2O2g → 2Mn 4 + + O2s −

(13)

where □s is an oxygen anion vacancy at the perovskite surface. At higher temperature, an intrafacial mechanism, similar to the Mars and Van Krevelen mechanism, can occur.196 Steps of the suprafacial mechanism (eqs 12 and 13) remain valid, but the oxygen anions can migrate from the bulk to the surface to fill in the oxygen vacancies so that, virtually, all oxygens of the bulk can participate in the oxidation process. The activity of La1−xMxMnO3+δ perovskite catalysts (M = Sr or Ce) in methane oxidation could be correlated to the number of Mn4+ ions and more precisely to the number of −O−Mn3+−O− Mn4+−O− species.197 Substitution of La by Sr favors oxidation 10309


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precursors resulted in segregated oxides or spinels. The sample, containing 30 wt % LaMnO3 dispersed on δ-Al2O3, was found to be the most active for CO oxidation among all the examined catalysts. Very good results were also obtained with LaMnO3 perovskites supported on zirconia.203 The best performance in terms of CO oxidation rate per gram of perovskite was observed for a LaMnO3 loading of 4 wt %. Catalysts with TiO2/ LaSrMnO3 composite nanorod array were prepared by Guo et al.204 using hydrothermal synthesis and the radiofrequency (RF) magnetron sputtering method. TiO2/(La,Sr)MnO3 core− shell nanorod arrays are efficient CO oxidation materials. Improvement of CO activity in these materials is ascribed to the high lattice oxygen mobility in the (La,Sr)MnO3 phase. Lanthanum was also substituted by Ca,73,205 Ce,206,207 Al,208 and Ag209 showing very good performance with cerium as substituent (Figure 18). It was proved that a physical mixture of

Figure 19. Interconnection of Co3+ octahedral sites in LaCoO3 and Co3O4.210

Co3O4. XRD and EXAFS were used to identify both structures and cation environments. Contrasting with the results of Ngamou and Bahlawane, Colonna et al. showed that Co3O4 was much more active at room temperature than LaCoO3, which needs temperatures higher than 130 °C for significant CO conversion. Strontium-substituted LaCoO3 perovskites were investigated by different authors,213−215 all confirming the high activity of La1−xSrxCoO3 perovskites. Isupova et al.213 found two maxima of activity at x = 0.3 and 0.8 for the materials annealed at 1100 °C. Over their samples annealed at 800 °C, Borovskikh et al.214 showed that the catalytic activity increased from x = 0 to 0.5, while Hueso et al.215 investigated only the composition x = 0.5. It is possible to insert an additional LaSr layer to form the pseudoperovskite phase La2−xSrxCoO4 crystallizing in the K2NiF4 structure. This structure is more open with more mobile oxygen than La1−xSrxCoO3.216 Borovskikh et al.214 did not confirm the superiority of the pseudoperovskite phase over the La1−xSrxCoO3 materials, LaSrCoO4 being less active than La0.7Sr0.3CoO3 and La0.5Sr0.5CoO3. The properties of the pseudoperovskite phases La2−xSrxCoO4 were also investigated by Yang et al. and tested for CO and propane oxidation.217,218 The highest activity in CO and propane oxidation was observed for the pseudoperovskite LaSrCoO4.217 Substitutions in A (La) and B positions (Co) were investigated by Li et al. (La1−xSrxCo1−xMnxO3 systems),219 Dai et al. (La0.8Ba0.2Co0.8Bi0.2O2.87),220 and Seyfi et al. (La 0.8 Sr 0.2 Co 0.8 Cu 0.2 O 3 ). 221 The best performances of La1−xSrxCo1−xMnxO3 samples prepared by the citrate route were observed for x = 0.6. Substituting Bi for Co also increased the reactivity of the perovskite.220 In similar conditions (space velocity of 30 000 h−1), light-off temperatures (50% conversion) were shifted to lower temperatures (typically 220 and 180 °C for the nonsubstituted and Bi-substituted perovskites). Partial substitution of Co by Cu also resulted in a beneficial effect on CO conversion with a shift of the light-off temperatures of about 30−40 °C.221 4.2.1.3. Ferrites. LaFeO3 and substituted ferrites are generally found to be less active in CO oxidation than Mn and Co perovskites.199 However, due to the high availability and low price of iron precursors, interest for ferrites remains constant. Fe4+ and Fe3+ are present in the oxidized state in the perovskite, and the reducibility of Fe ions is a prerequisite for a good oxidation activity. Most studies were devoted to the increase of the performances of LaFeO3 by substituting either La or Fe by different cations. La1−xSrxFeO3 perovskites were studied by Yakovleva et al.222 after annealing at 1100 °C. The

Figure 18. CO oxidation over La1−xCexMnO3 perovskite catalysts. Reaction conditions: 1% CO/1% O2 in 98% He (space velocity = 2 × 10−2 m3 s−1 kg−1).206

80% LaMnO3 + 20% CeO2 exhibits very poor results (close to pure LaMnO3), confirming the crucial role of the substitution (insertion of the cerium atoms in the crystal). Beyond x = 0.2, the catalytic activity of perovskites decreases, likely as a result of formation of a nonactive CeO2 phase (not shown in Figure 18). 4.2.1.2. Cobaltites. Nonsubstituted LaCoO3 perovskites were investigated by Taguchi et al.91 and Ngamou et al.210,211 Taguchi et al.91 confirmed the crucial role of the preparation: adding ethylene glycol to citrate precursors led to better performances in CO oxidation. The arrangement of octahedrally coordinated trivalent cobalt cations in Co3O4 spinel and LaCoO3 perovskite was studied by Ngamou and Bahlawane.210 These materials were grown as films on metallic substrate, XRD confirming that pure phases were formed. Differences in catalytic reactivity were explained by the arrangement of Co3+ octahedral sites: they are interconnected through their vertex, sharing one O2− anion in the perovskite, while they are interconnected by their edges, sharing two O2− anions, in the spinel structure (Figure 19). Co3O4 possesses more reducible cations, while LaCoO3 has more abundant and more reactive surface oxygens. This explains why the conversion of CO starts at much lower temperatures on the perovskite. The same authors reported that the activity strongly depends on the film thickness.211 LaCoO3 perovskites were supported on zirconia by Colonna et al.,212 who also compared the activity of bulk LaCoO3 and 10310


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best performance was obtained for x = 0.3 and to a lesser extent for x = 0.8. XRD analyses showed that the most active materials are composed of two distinct phases of different La/Sr ratios, probably favoring oxygen activation. Substitution of La by Ca was studied by Isupova et al.223 After calcination at 900−1100 °C, the authors did not observe the presence of Fe4+ cations in the La1−xCaxFeO3−y series. The rate of CO oxidation was correlated with the TPR peak ascribed to surface or subsurface oxygen species consumption, with a maximum of activity at x = 0.6. AFeO3 (A = La, Nd, Sm) and LaFe1−xMgxO3 perovskites were fully characterized by Porta et al.,224 while their catalytic activity was evaluated by Ciambelli et al.159 The order of activity for CO oxidation is Nd > La > Sm, while substitution of Fe by Mg improves the specific catalytic activity at low Mg content. This is due to a better surface area of the Mgcontaining materials: Mg does not increase the intrinsic activity of LaFeO3 per m2 (Table 7).

production. However, some studies dealt with the activity of nickelates for the CO oxidation reaction. LaNi1−xFexO3 perovskites were investigated by Falcón et al.228 The intrinsic activity was higher for LaNiO3 than for LaFeO3, and this was explained on the basis of the higher capability of Ni3+ ions to adsorb oxygen. Even higher activity was observed over mixed transition metal materials, when Fe3+ partially substitutes Ni3+, with two clear maxima at x = 0.10 and 0.50 correlated with the presence of Ni(III) + Fe(IV) species enhancing oxygen bonding to the surface. The group of Madrid also studied (Ln,Sr)NiO3 perovskites with Ln = Pr, Sm, Eu.158 Materials were prepared by citrate decomposition and treatment under very high O2 pressures (200 bar) at 1000 °C stabilizing Ni3+− Ni4+ mixed valences. Strontium doping of PrNiO3 perovskites increases catalytic activity by a factor 3 (Pr0.95Sr0.05NiO3 compared to PrNiO3). Catalytic properties could be correlated to the higher reducibility of Sr-doped perovskites and the ease of O removal. This confirmed the role of weakly bounded oxygens in oxidation reactions over perovskites, as revealed by low-temperature exoemission of negative charges.229 Zhu et al.230 studied CO oxidation over perovskite-like oxides La2−xSrxMO4 (x = 0, 0.5, 1.0 and M = Ni or Cu) by means of cyclic voltammetry. The catalytic activity could be correlated to the surface area of the redox peaks of the cyclic voltammogram and not to their respective potentials. Substitution of Co by Ni in the B position or Zn by Ni in the A position was investigated by Vaz and Salker in the LaNi1−xCoxO3 system231 and by Shetkar and Salker in the Zn1−xNixMnO3 system.232 There is clear benefit of substituting Ni for Co in the perovskite.231 This is explained by the degree of filling of the orbital eg of the B cation (t62ge0g for Co3+ and t62ge1g for Ni3+), empty levels of the lowest antibonding d orbital being required for good activity in CO oxidation. The intrinsic activity of NiMnO3 is about 2−3 times higher than that of ZnMnO3.232 A synergy effect is observed when Zn is substituted by Ni, the intrinsic activity showing a maximum around x = 0.4. 4.2.1.5. Chromites and Cuprites. LaCr1−xAlxO3 perovskites were prepared by Cordischi et al.233 using citrate precursors. Calcination at 650 °C leads to a mixture of phases (LaCrO4 and La2CrO6) with the presence of Cr3+ and Cr5+ ions, while pure perovskite phases, with chromium in the Cr3+ state, are obtained after calcination at 800 °C. Substitution of Cr by Al is required to confer a significant activity to the materials. La1−xSrxCrO3 perovskites were prepared by Rida et al.234 using solid-state reaction. XPS showed that the materials surface is enriched in Sr with an overoxidation state of chromium (Cr6+). Activity for CO oxidation is maximal for x = 0.1. LaCuO3−δ perovskites were also prepared and characterized by Falcón et al.235 Oxygen-deficient materials are significantly more active for CO oxidation than LaCuO3 with δ close to zero. La2−xSrxCuO4Sy (0 LaCoO3 (T50 = 560 °C) > LaFeO3 (T50 = 625 °C). The higher surface area obtained with manganites (21.8 m2 g−1 for LaMnO3 vs 5.5 m2 g−1 for LaCoO3 and 9.8 m2 g−1 for LaFeO3) may in part explain their better performances. Najjar et al. investigated the combustion technique (glycine−nitrate) for preparing different manganites.103 They showed that increasing the glycine/nitrate ratio led to materials of higher surface areas (28−37 m2 g−1), more active for the CH4 oxidation reaction. Other techniques were developed for preparation of LaMnO3 used in CH4 combustion: oil−water/H2O2 two-phase system249 and microwave-assisted synthesis.83 These catalysts have better performances than those prepared by conventional methods such as the citrate route. Excellent performances of LaMnO3 perovskites were also reported by Zou et al., who prepared their materials by solid−gas reaction from polyvinylpyrrolidone−metal complex as solid precursor.250 The La/Mn stoichiometry ratio can change the catalytic behavior of La−manganites. Three LaxMnyO3 perovskites (with x/y = 0.9, 1.0, or 1.1) were synthesized by Spinicci et al., who showed that a slight deficit in Mn (LaMn0.9O3) led to more active catalysts.251 Choudhary et al.252 showed that hydrothermal treatment in water was able to increase the BET area of LaMnO3 (and LaCoO3). The activity for methane combustion increased, but intrinsic reaction rates (per m2) remained virtually constant for LaMnO3, confirming again the crucial role of the material textural properties. Catalytic performances of manganites could be tuned either by changing the cation in the A position (AMnO3 or La1−xAxMnO3 series) or by substituting part of Mn by another B cation (LaMn1−yByO3 series) or by combining the two strategies (La 1 − x A x Mn 1 − y B y O 3 series). A series of La1−xSrxMnO3 manganites (0 < x < 0.5) was prepared by Ponce et al. by the citrate route. The higher x, the higher the BET area, but the substitution x = 0.2 showed the highest intrinsic activity for methane combustion.164 Ponce et al. attributed this higher activity to the great stability of Mn4+ in La 0.8 Sr 0.2 MnO 3 . Perovskites with similar compositions (La0.9Sr0.1MnO3) were proven to have the highest sulfur resistance, far better than the corresponding cobaltites or ferrites.198 AMnO3 perovskites were investigated by Ciambelli

proposed to account for the CO oxidation reaction (eqs 16−19) k1

O2 + 2s XooY 2Os k −1

K1 =

k1 k −1

(16)

K2 =

k2 k −2

(17)

k2

CO + s XooY COs k −2

k3

COs + Os XooY CO2s k −3

K3 =

k4

CO2s → CO2 + s

k3 k −3

(18) (19)

Carbon dioxide would be adsorbed as carbonates whose decomposition to gaseous CO2 would be the rate-determining step (rate constant k4). C18O/C16O exchange and reaction confirmed that adsorbed CO is in equilibrium with the gas phase. It reacts with surface oxygen species of the perovskite, immediately replaced by O coming from the gas phase. Kinetic derivation of this four-step mechanism led to the following rate expression (eq 20) R = Nk4

K11/2K 2K3PCOPO1/2 2 1 + K11/2PO1/2 2

(20)

where N is the number of sites and PCO and PO2 are the partial pressures of CO and O2 in gas phase. The reaction is first order with respect to CO, while the kinetic order with respect to O2 should be comprised between 0 and 0.5 and tends to zero at high oxygen pressure. The reaction rate then becomes (eq 21) R = Nk4K 2K3PCO

(21)

The apparent activation energy is very low (around 13 kJ mol−1) at high O2 concentration, while it increases to 63 kJ mol−1 in stoichiometric conditions. The adsorption energy of O2 no longer intervenes in the overall activation energy at high O2 concentration. Moreover, there is not a good correlation between the specific reaction rate and the surface area of perovskites. This is indicative for lattice oxygen migration during CO oxidation: obviously, oxygen of inner layers of manganites can intervene in CO oxidation. Hueso et al.215 also found a first-order reaction for CO oxidation over cobaltites. The apparent activation energy changes with temperature: it is close to 82 kJ mol−1 below 320 °C and drops to 40 kJ mol−1 above this temperature. From different experimental observations, Hueso et al. concluded that there is a change of mechanism around 320 °C due to the probable role of bulk diffusion above this temperature. Kinetic data of CO oxidation over LaFeO3/ZrO2 catalysts were modeled by Colonna et al. by a first-order reaction in CO and a quasi-zero-order reaction in O2, present in large excess.225 The apparent activation energy of unsupported oxides was close to 80 kJ mol−1 but decreased for zirconia-supported perovskite: values between 29 and 68 kJ mol−1 were then found, suggesting that the structure of the active site and/or the enthalpy of the CO adsorption change with composition and support in a complex way. 4.3. High-Temperature Oxidation Reaction: CH4 Oxidation

4.3.1. Effect of the Material Composition. Methane oxidation has been widely investigated over perovskites as potential substitutes of noble metals for natural gas combustion.237 Among the noble metals, partially oxidized Pd is the most active catalyst for CH4 oxidation.238,239 However, it 10312


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et al.166 The order of activity in CH4 combustion is LaMnO3 ≈ SmMnO3 > NdMnO3. Partial substitution of Sm by Sr does not improve activity, probably because Sr tends to promote Mn reduction in the SmMnO3 structure. La1−xAxMnO3 manganites with A = Ba, Sr, or Ca were synthesized by Liu et al.253 using Na2CO3−NaOH precipitation techniques. Materials with a surface area of 20 m2 g−1 could be obtained after calcination at 1000 °C. La0.8Ba0.2MnO3 was the most active catalyst of this series for methane oxidation. Controlled particle morphology may be a way to obtain better catalysts. Improved performances of Ba-substituted LaMnO3 materials were obtained by Liang et al.,254 who succeeded in preparing La0.5Ba0.5MnO3 nanocubes, extremely stable for methane combustion at 560 °C. Similar high stability was observed for nanocubes of La0.5Sr0.5MnO3.255 Manganite nanocubes were synthesized by the hydrothermal route at 240 °C described by Spooren et al.256 and extended to different La/Ba ratios by Urban et al.257 Contrary to LaMnO3+ó perovskites, in which δ is close to 0.15, there is no oxygen excess in La0.5Sr0.5MnO3 nanocubes prepared by the hydrothermal route. LaMnO3 perovskites were also modified by substituting La with Ca,258,259 Ce,260,261 Sr + rare earth (Y, Sm, Dy, or Ce),262 or Ag.209,263 Contrasted results with these cations were obtained. Calcium in La1−xCaxMnO3+ó improves both the stability and the activity of LaMnO3+ó with a maximum for x = 0.6258 or 0.7.259 Ca2+ decreases the oxygen excess δ while increasing the percentage of Mn4+ in the perovskite. Batis et al.258 showed that the most active catalyst calcined at 700 °C was La0.4Ca0.6MnO3.03 with a BET area of 16.7 m2 g−1 and a Mn4+ content of 66%. Substitution of La by Ce has a beneficial effect on the thermal stability but generally tends to decrease catalytic activity. Substitution should be limited to 10%, and the best compromise, found by Alifanti et al.,260 was the perovskite La0.9Ce0.1MnO3 prepared by the citrate route. A slight cerium excess with respect to LaMnO3 (LaCe0.1MnO3) can still improve the catalytic performance. Cerium also has a positive effect on the resistance of the perovskite to sulfur (100 ppmv H2S).261 Contrasting with Ce alone, double substitution of La by Sr and Y or Dy significantly improves both stability and catalytic activity. Figure 20 shows the results obtained by Liu et al. with these double-substituted perovskites.262 Substitution by Ag improves both the activity and the thermal resistance of perovskites. Kucharczyk and Tylus209 found an optimal composition for La0.8Ag0.2MnO3. Buchneva et al.263 investigated the 0−0.1 range of substitution and showed that the catalytic activity increased with increasing Ag substitution. Substitution of the B cation (LaMn1−yByO3 series) was also investigated for improving catalytic performance of the perovskites for methane combustion. Interesting results were obtained with Ti,264 Cu,265 Mg,266 and ZrY.267 Substitution of 20% Mn by the other cations seems to be a good compromise between textural and structural effects of the substitution on the catalytic performances. In most cases, substitutions of both A and B cations (La1−xAxMn1−yByO3 series) were studied to cover every latitude in optimizing the catalytic system. Alifanti et al.268,269 and Ahn et al.270 conducted detailed investigations of the substitution of Mn by Co,268,270 Ni and Cu,269 or Mo,271 while the A cation in substitution of La is either Ce268 or Sr.269,270 Li et al.271 also prepared double-perovskites Sr0.2Mg1−xMnxMoO6. Contrasted results were obtained: small or negligible gains in performance were recorded with Co, Ni, or Cu substitution in the B

Figure 20. Methane conversion over double-substituted La0.5RE0.1Sr0.4MnO3 perovskites calcined at 1000 °C. Comparison with La0.6Sr0.4MnO3. Specific surface areas are in the 13−16.5 m2 g−1 range for La−RE−Sr−Mn samples, except for the La−Sr−Mn perovskite (7.8 m2 g−1). Reaction conditions: 3.5%CH4−17% O2 in N2. Space velocity: 20 000 cm3 gcat−1 h−1.262

position, especially with Ce in the A position. The main advantage of Ce seems to confer to the perovskite a moderate sulfur resistance.268 With LaSr perovskites, some 10 °C in T50 can be gained by substituting Mn by 10−20% Co or Ni. The highest activity was obtained for the SrMg0.5Mn0.5MoO6 sample.271 Catalytic activities of these double perovskites seem to correlate with the surface concentration of lattice oxygen and manganese cations and not with their BET surface area. It is clear that the mode of preparation may greatly affect the results, so that definitive conclusions cannot be drawn from these studies. As palladium is very active in methane combustion, several authors tried to take advantage of this specificity to introduce Pd in the B position.272,273 As expected, Pd in its oxide state increases the CH4 oxidation rate. However, detailed characterization reveals that Pd in the B position does not lead to the highest activity. Dispersed PdO at the surface of LaMnO3 (or LaCoO3) is very likely the active site for the oxidation reaction.273 Kucharczyk and Tylus even reported results showing that Pd in the A position (in substitution of La) could be more active than in the B position.272 Certain authors directly dispersed Pd over perovskite surface. Koponen et al. investigated 2%Pd/AMn1−xFexO3 catalysts (A = Ba, La, Pr; x = 0.4, 0.6, 1)274 and reported that methane conversion increases in the order La > Pr > Ba, and the highest resistance to SO2 poisoning is obtained with the 2%Pd/LaMn0.4Fe0.6O3 catalyst. 4.3.1.2. Cobaltites. Very few studies were carried out on bulk LaCoO3 perovskites. Lee et al.32 prepared La cobaltites of 24 m2 g−1 by spray freezing, allowing crystallization at low temperature (500 °C). They showed that their materials are more active than Pt/Al2O3 for CH4 oxidation. Perovskites and hexaaluminates were compared by Ersson et al.275 for methane and biogas combustion. They showed that hexaaluminates are more active than perovskites owing to their very high surface area (an order of magnitude higher than those of perovskites after calcination at 1200 °C). LaCoO3 would be more active than LaMnO3 but suffers from thermal deactivation. A series of LaCoO3 perovskites was prepared by flame-spray pyrolysis by Chiarello et al.,276 who developed a representative model for this technique allowing predicting particle size and surface area (30−70 m2 g−1). Finally, mesostructured LaCoO3 were prepared by nanocasting synthesis from mesoporous silica.277 10313


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La0.2Ce0.8CoO3 for Machin et al.289 Cerium seems to improve oxygen mobility in the perovskite. Atomistic simulation techniques allowed French et al.290 to explain the defect chemistry of the cerium-doped perovskite, the improved anion mobility, and finally the better activity in methane oxidation. Lanthanum cobaltites substituted by Ce were characterized by EPR (electron paramagnetic resonance) and EMR (electron magnetic resonance).174,291 These techniques are extremely sensitive to the presence of crystalline impurities such as Co3O4, La2O3, and CeO2. Oxygen adsorption was accompanied by formation of paramagnetic species. It was proven that pure perovskite was more active than samples containing crystalline impurities. The EMR technique was also applied by Oliva et al.292 to La cobaltites substituted by Pr, Sm, or Tb. Ordóñez et al.293 compared a La0.9Ce0.1CoO3 perovskite with several simple oxides (NiO, CuO, Mn2O3, Cr2O3, Co3O4) for methane oxidation in the presence of SO2. They found that Cr2O3 was slightly more sulfur tolerant than the perovskite and much more tolerant than a Pd/Al2O3 catalyst used as reference. La0.9Ce0.1CoO3 was chosen by Auer and Thyrion294 for a detailed kinetic study on methane oxidation. The main results of this study will be analyzed in the next section. Substitution of La by Ag or Pd was also investigated.295,296 Kucharczyk and Tylus 295 showed that the surface of La0.92Pd0.08CoO3 and La0.9Ag0.1CoO3 was completely upset after reaction, exhibiting a segregation of silver and palladium phases. In spite of these changes, the catalysts kept a good activity for methane oxidation (500 h at 700−750 °C). Buchneva et al. compared a series of LaCoO3-supported Ag catalysts with La1−xAgxCoO3 perovskites (x = 0.05, 0.1, and 0.2). Silver in substitution of lanthanum gives better catalysts than supported ones.296 However, supported Ag catalysts and Ag−perovskites are more active than the nondoped bulk LaCoO3 perovskite. Substitution of Co by other B cations was also explored as a possibility to improve the activity and stability of LaCo1−xBxO3 or La1−ySryCo1−xBxO3 series. In most cases, cobalt was substituted by iron. Methane oxidation was investigated by Szabo et al.297 over LaCo1−xFexO3. The materials were prepared by the reactive grinding technique developed by Kaliaguine et al.175 Initially hydroxylated, the materials loose a large fraction of their OHs by condensation of these hydroxyl groups upon heating. Sample calcination initiates an electron transfer process, leading to reduction of Co3+ when anionic O surface species (β-oxygen) are desorbed.175 By controlling the calcination temperature, it was possible to maintain a high concentration of accessible surface O− species very active in methane combustion.297 An extensive study of the preparation of LaCo1−xFexO3 perovskites by various techniques was carried out by Royer et al.17 They showed that there was no direct relationship between CH4 oxidation rate and catalyst properties such as BET area, oxygen desorption, O mobility, cation reducibility, etc., parameters often invoked to explain perovskite activities in oxidation. Several combined factors should be taken into account in order to predict catalytic activity. Royer et al.48 also investigated the SO2 resistance of LaCo1−xFexO3 perovskites. Two steps of deactivation can be seen on conversion vs reaction time curves. In the first step, only the surface is sulfated and the deactivation is reversible. In the second step, perovskites are sulfated in the bulk, which leads to a profound and irreversible deactivation. Gallium was also used as a substituent of Co in La0.3Sr0.7Co0.8Ga0.2O3−δ perovskites.298,299 Double substitution in the A and B positions were claimed by

Perovskites thus prepared have higher surface area, higher proportion of Con+ cations (n = 3 and 4), and better oxidation activity than classical bulk LaCoO3. A cellulose-templating method of preparation was also developed by Langfeld et al.278 for preparation of ACoO3 cobaltites. The main results are reported in Table 8 for LaCoO3. In general, ACoO3 when A is lanthanide, appears as the more active perovskite and alkalineearth cations give less active catalysts. Table 8. Performances of ACoO3 Perovskites for the Methane Oxidation Reactiona perovskite

Tcalc (°C)

SBET (m2 g−1)

CH4 conversion (%) at 550 °C

LaCoOx SrCoOx NdCoOx PrCoOx SmCoOx BaCoOx CaCoOx

650 850 700 650 650 850 850

4.4 2.4 3.4 6.4 4.6 1.0 1.6

22.5 3.5 17.1 15.9 11.4 2.1 0.7

a

Catalysts are prepared by the cellulose templating method. Reaction conditions: methane excess (20% H4, 10% O2).278

Many studies were devoted to substitution of La by a second A cation (La1−xAxCoO3 series). As for manganites, Sr cations were often used in substitution of La.214,279−281 A low degree of substitution is generally recommended for optimal activity in methane oxidation. For instance, Gao and Wang280 obtained the following ranking of activity for the La 1−x Srx CoO3 perovskites, prepared by the urea decomposition method: x = 0.1 > x = 0.2 > x = 0 or 0.3. Contrary to manganites, cobaltites present a stoichiometry of lattice oxygen inferior to 3, with δ in La1−xSrxCoO3‑ó increasing with x. The increase of charge on Co (more Co4+ is detected when x increases) is not compensated by the default of charge created by Sr2+ insertion. The electrospinning method was used by Chen et al.282 to synthesize nanofibers of perovskites of different composition. Among the samples tested, La0.8Sr0.2CoO3 had the highest activity for methane combustion. This composition seems extremely stable even at 900 °C.281 La1−xSrxCoO3 perovskites were used by Hueso et al.283 for methane conversion in a plasma reactor. Insertion of La0.5Sr0.5CoO3 in the glow zone of the plasma allows increasing the CO2 selectivity up to 90% at 200 °C with a methane conversion of 80%. It is worthy of note that there is no oxidation reaction on this catalyst at 200 °C when the plasma is off, a 50% conversion being reached at 600 °C under these conditions. This is coherent with a recent study performed by Baylet et al.,284 who confirmed that the plasma allows methane oxidation at low temperature but produces mainly CO and H2O. The catalyst is required to oxidize CO into CO2. A comparison between several substituents (A = Sr, Ca, Ba) was performed by Taguchi et al.285 for x = 0.04 and 0.06. No difference in activity was observed between the three perovskites for x = 0.04, while for x = 0.06 the reactivity increased with the ionic radius of the substituent (Ba > Sr > Ca). The authors attributed the better activity of La0.94Ba0.06CoO2.98 to the decrease in the Co−O−Co angle, in agreement with another study, carried out on La1−xNdxCoO3 perovskites.286 Several studies were also devoted to substitution of La by Ce.176,287−289 Partial substitution of La by Ce leads to more active catalysts, however with different maximum activity according to the authors: La0.8Ce0.2CoO3 for Kirchnerova et al.176 and Bosomoiu et al.,288 La0.7Ce0.3CoO3 for Cui et al.,287 10314


Chemical Reviews

Review

Campagnoli et al. for a better activity in CH4 combustion.300 La0.9Ce0.1Fe0.5Co0.5O3±δ and La0.6Sr0.4Fe0.8Co0.2O3−δ prepared by the sol−gel citrate route showed the highest performances in CH4 oxidation with light-off temperatures (T50) of 400 °C. However, the presence of cerium seems to change the mechanism of reaction with a predominance of intrafacial mechanism and quasi-absence of suprafacial reaction. Recently, still more complex substitutions, including Cu for Co and several cations in the A position, were proposed by Deng et al.301 Two series of perovskite-like compounds were tested in methane combustion: NdSrCu1−xCoxO4−δ and Sm1.8Ce0.2Cu1−xCoxO4+δ (x = 0, 0.2, and 0.4). Cu3+ and Cu2+ are present in the Nd series, while copper is in the form of Cu2+ and Cu+ cations in the Sm series. The catalytic activity seems to be correlated to the Cu3+/Cu2+ ratio in the Nd series and to the Cu+/Cu2+ ratio in the Sm series. 4.3.1.3. Ferrites. Due to their relatively low stability, pure AFeO3 were rarely investigated. LaFeO3 and La-substoichiometric materials (La1−εFeO3−1.5ε with ε = 0.1−0.3) were studied by Spinicci et al.302 A moderate increase of La substoichiometry (up to ε = 0.1) leads to catalysts with better catalytic activities. A SmFeO3 series was prepared by a sol−gel route using CTAB as surfactant and tested by Stathopoulos for methane oxidation.303 Significant activity was obtained (T50 around 500 °C) owing to the large BET area of the materials prepared by this technique (20−30 m2 g−1 after calcination at 800 °C). In many studies, La was substituted by Sr to tentatively improve activity and/or s ta bility of LaFeO3.222,304,307 Substitution of La3+ by Sr2+ induces a positive charge on Fe3+ (which tends to be oxidized to Fe4+), enhancing formation of anionic vacancies. The materials can release oxygen when heated in He. It was proven that the amount of desorbed O2 was proportional to the number of Fe4+ stabilized in the perovskite network.308 Oxygen adsorption is easier, and O2− bulk diffusion is increased, which explains the improved activity of substituted perovskites. La-free SrFeO3−δ perovskites were consequently prepared and tested by Falcón et al.309 The most deficient oxide, SrFeO2.74, was the most active catalyst for CH4 oxidation. It shows a complex neutron diffraction diagram corresponding to a superstructure of perovskite in which one-half of the Fe cations are pentacoordinated to oxide anions. Substitution of La by Ce was investigated by Belessi et al.,310 who showed that the maximum activity was obtained for y = 0.3 in the La1−yCeyFeO3 series prepared by nitrate decomposition (1−4 m2 g−1). However, double substitution (La1−x−ySrxCeyFeO3 series) gave more active catalysts with an optimum at x = 0.15 and y = 0.05. Strong synergy effects were observed simply by mixing SrFeO3 and CeO2 with the appearance of Fe5+, detected by Mössbauer spectroscopy. High adsorption energies of O2 (53−211 kJ mol−1) were determined on these materials by analyzing the kinetic data.311 A few studies were finally devoted to substitution of La by Ca.223,312,313 Ciambelli et al.312 and Pecchi et al.313 confirmed that substitution of La by a moderate amount of Ca can improve the oxidation activity by creating anionic vacancies and an overoxidation of Fe3+ to Fe4+. These Fe3+/Fe4+ ion pairs seem to play a determining role in methane oxidation. Substitution of iron in the B position was investigated by several authors. Methane oxidation was carried out by Yaremchenko et al.314 in a membrane reactor made of Sr0.7La0.3Fe0.8Al0.2O3−δ as active material. Kirchnerova and Klvana showed that the double substitution by Al and Mg

may improve the catalyst stability at high temperatures.315 The most active perovskite, La0.85Sr0.15Al0.3Fe0.5Mg0.2O3, keeps a significant surface area after annealing at 700 °C. La(Fe,B)O3 or (La,A)(Fe,B)O3 perovskites, prepared by substitution of Fe by Mg and Ti,316 Mo,317 Mg and Mo,318 or Ga,319 were also developed for different applications of methane oxidation. It is generally observed that a moderate substitution of Fe should be followed to avoid a dramatic decrease of activity. Falcón et al.317 observed that Sr2FeMoO6−δ was the most active double perovskite in the series A2FeMoO6 (A = Ca, Sr, or Ba) with 80% conversion of methane at 530 °C. Finally, some perovskites were doped with palladium, either in substitution of Fe105,320,321 or as Pd/perovskite catalysts.322 It was shown that Pd doping has a stronger effect on ferrites than on other perovskites. However, the benefit of the doping remains limited since light-off temperatures are shifted by 40 °C when LaFeO3 is replaced by LaFe0.95Pd0.05O3.105 Direct deposition of Pd on the perovskite would have a much higher effect on the light-off temperature, even though other supports like CeZrOx are more adequate as support of palladium for oxidation reactions.322 4.3.1.4. Nickelates, Cuprates, and Other Perovskites. La1−xSrxNiO3 (x = 0.00−0.20) mixed oxides have been prepared and tested for methane combustion by Garciá de la Cruz et al.182 The maximum activity was observed for x = 0.10, associated with the largest proportion of oxygen nonstoichiometry and the lowest Sr segregation on the surface of the materials. Perovskite-like cuprates were also investigated in methane combustion by Zhang et al.323,324 and Yuan et al.325 Cuprates are intrinsically less active than manganites and cobaltites for methane oxidation with light-off temperatures between 560 and 700 °C. Their activities are linked to the presence of Cu3+ in the structure. The better reducibility of Cu3+ ions seems to be a key factor for a good O2 activation for methane oxidation. Double perovskites associating Ni and Cu were prepared by Hu et al.,326 who showed that the activity of La2CuNiO6 for methane oxidation was much higher than that of the single perovskite (LaNiO3) while the physical mixture LaNiO3 + LaCuO3 was less active than LaNiO3 at isospace velocity. Chromites were also investigated by Kaddouri et al.,327 while titanates were studied by the group of Forni328−330 and by Popescu et al.331,332 ESR characterization showed the presence of different oxygen species associated to Ti cations, O3−/Ti4+ species having a crucial role in oxidation reactions. Silver seems to be a good substitute for Sr in SrTiO3 titanates. Fabbrini et al. suggested that Ag2+ (evidenced by ESR) is stabilized in the structure when substituted for Sr2+ in the Sr1−xAgxTiO3 perovskite.329 Electric conductivities of BaTiO3 and PbTiO 3 were compared under methane oxidation conditions.332 These perovskites behave as n-type semiconductors in air, while they become p type when CH4 is added. Methane would react through H abstraction by O− species, forming CH3• radicals. Less conventional perovskites were also studied: BaCeO3 cerates,333 zirconates and metaldoped zirconates (LaZr1−xMxO3; M = Pd, Rh, Mn, Ni, Ru, Pt, Co),334,335 and La3.5Ru4O13 ruthenates.336 4.3.1.5. Supported Perovskites and Structured Reactors. For practical use in natural gas catalytic burners and gas turbines, perovskites should be supported and/or deposited on structured monolithic reactors. The main developments reported for this application are summarized in Table 9. Most studies were carried out by Italian groups using either ceramic monoliths or metallic FeCr alloy fibers as structured support for the catalytic material in the burner. LaMnO3 is 10315


10316

cordierite

La-Al2O3 or MgO La−Al2O3

ceramic foam

La2O3

LaAlO3/Al2O3

MgO MgO ZrO2, MgO, CeO2, or ZnO ZrO2 ZrO2

Pd (1−2.5 wt %)/LaMnO3 30% LaMnO3 40%

LaMnO3 30%

LaMnO3 6% LaMnO3 6% LaMnO3 various loading

Pd (0−2%)/LaMnO3 Pd (2%)/LaMnO3

cordierite

La-ZrO2 (75%)− Al2O3 (25%) La−Al2O3

LaMnO3 20%

metallic fibers metallic fibers

metallic (FeCrAl fibers) metallic fibers metallic fibers metallic fibers

cordierite

cordierite cordierite

La−Al2O3 La−Al2O3

LaMnO3 20% LaMnO3 20%


La−Al2O3 La−Al2O3

cordierite


La2O3 La2O3

cordierite

Al2O3-sepiolite or metallic support α-Al2O3-monolith

metallic

monolith

LaMnO3 30% LaMnO3 30%

LaMnO3 30%

La0.9Ce0.1CoO3 2% LaBO3 (B = Fe, Mn, Co) and La2NiO4 2−5% LaMnO3 20−30%

Pt and Pd (0.2%)/LaBO3 (B = Mn, Co, Fe, Ni, Cu) La0.9Ce0.1CoO3 2%

no (direct coating) no (direct coating) Al2O3 or La2O3

MgO MgO La−Ti−Al2O3

SBA15 SBA15

LaCoO3 60% Pd (1%)/LaMn0.4Fe0.6O3 30−40% LaMn1−xMgxO3 6−50% LaCr0.5−xMnxMg0.5O3 6% LaMnO3 (or LaCoO3) 10− 20% LaMnO3 2−12%

LaCoO3 2−30%

LaCoO3 1−2%

La−Al2O3 or NiAl2O4 ZrO2 or La− ZrO2 Ce0.8Zr0.2O2

support (wash coat)

La0.8Sr0.2MnO3 20%

catalyst and perovskite content (wt %) main objective and conclusions

ref

support for Pd is prepared by direct synthesis of LaMnO3·2ZrO2; T50 is lowered by 100 °C with respect to the Pd/ZrO2 catalyst SO2 poisoning study on the catalytic burner described in ref 362; prolonged exposure to SO2 slightly deteriorates the performances; CO and NOx emissions remain very low

362 363

360 361 102

359

356, 357 358

355

353 354

352 243

351

350

348 349

347

346

345

high BET area of LaMnO3/ceramic; satisfying performances at high T; metallic monoliths are more appropriate for low-T combustion manganites and ferrites are well adapted for impregnation over alumina monolith (minimum incorporation of Al ions in the perovskite structure) alumina wash coat is better for promoting high initial activity; lanthana is preferable to alumina as primer wash coat oxide for long-term operation La nitrate is preferable to La acetate for wash coating; more uniform and resistant coating on the monolith wall thermal resistance strongly depends on the nature of the B cation; nickel perovskites are poorly stable; manganites and ferrites are the most stable supported perovskites strong interaction of the perovskite with alumina after calcination at 1100 °C leading to dramatic deactivation; magnesia gives more stable catalysts kinetic modeling including heat and mass transfer; transient behavior during start up and shut down; simulation of the deactivation (up to 1000 °C) extension of the model presented in ref 351 for simulating temperature excursions reaction kinetic over fresh and deactivated catalysts; kinetic order in CH4 (0.8) and activation energy (97 kJ mol−1) do not change with deactivation; cordierite catalysts are much more stable than the unsupported powder perovskite; CO selectivity was very low high- pressure combustion (1−11 bar); coupling with homogeneous combustion allows predicting CH4 conversion, increasing with P effect of CO, H2, and C3H8 on the catalytic combustion of CH4; second fuel added to CH4 improves autoignition of methane; CO has the maximal effect good dispersion and higher activity of the catalyst compared to bulk perovskite; stability is improved; slower but deeper deactivation than with alumina dual-site catalyst; Pd starts ignition at low T (320 °C); oxidation goes on over the perovskite at higher T; nature of gases used in pretreatment affects the catalyst performance the higher the percent of lanthanum used as support, the higher the CH4 conversion; in the burner exhaust gas, NOx remains below 100 ppm and CO below 40 ppm precoating with LaAlO3 allows strong interaction of the perovskite with alumina; at the same power, the catalyst allows significant reduction of CO, HC, and NOx monodimensional modeling of the burner (catalytic and noncatalytic) study of the catalyst microstructure on the metallic FeCrAl fibers; optimization of the deposition comparison of different supports; ZrO2 offers a very good thermal resistance to sintering at 900 °C

342 343 344

340 341

339

338

337

MgO acts as a promoter of methane combustion (up to 17% perovskite) and increases SOx resistance Cr lowers the catalytic activity of the fresh catalyst but reinforces resistance to SOx poisoning LaMnO3 catalysts are more active than LaCoO3 but less resistant to thermal shock

co-impregnation of La and Co nitrates leads to a mixture of LaCoO3 and Co3O4; pure Co catalysts (no La) are slightly more active (due to the Co3O4 activity); La presence improves catalyst stability catalysts prepared by the citrate route are slightly more active than those prepared by nitrate impregnation; supported catalysts are more active than bulk perovskites preparation by a microwave-assisted method gives much better activity than conventional SBA15-supported LaCoO3 catalysts dispersing the perovskite over SBA-15 slows down deactivation and poisoning by SO2

only NiAl2O4-supported perovskites keep a good oxidation activity even after annealing at 1200 °C

Table 9. Supported Perovskites and Structured Reactors for Methane Burners or Gas Turbines

Chemical Reviews Review


366

R=

K 2PCH4(K O2PO2)1/2 1 + (K O2PO2)1/2

+ k1(PCH4)

(22)

The rate equation is based on an Eley−Rideal mechanism between gaseous methane and dissociatively adsorbed oxygen species. K2 corresponds to the kinetics of methane oxidation with surface oxygen (low-temperature process). The second member of the rate equation (k1) gives the contribution of bulk oxygen diffusing to the surface at high temperature. This last process does not depend on the partial pressure of oxygen in the gas phase. At high temperature, the rate of oxygen diffusion becomes significant but the surface reactivity is extremely high: once they have reached the surface, O species react immediately, resulting in rapid depletion of these species. At low temperature, the oxidation rate depends on the partial pressure of oxygen. Adsorption constants are generally small over perovskites, which implies that (KO2PO2)1/2 ≪ 1. The rate equation becomes R = K 2PCH4(K O2PO2)1/2 + k1PCH4

(23)

metallic (FeCrAl)

The kinetics remain of first order for methane over the entire range of temperatures. However, plotting R vs (PO2)1/2 allows determining the contribution of bulk oxygens to the reaction rate. The one-half order with respect to oxygen should be adjusted to fit the experimental data. This first-order kinetics was verified by many authors and can be accepted for methane oxidation over most perovskite catalysts.176,288,297 A detailed kinetic analysis was performed by Auer and Thyrion on a La0.9Ce0.1CoO3 perovskite.294 They considered the following sequence of reaction steps (eqs 24−27). (1) An eventual weak adsorption of methane on the catalyst surface (site “s”) k0

Al2O3

various

k −0

Pd (0−2%)/LaMnO3 various loadings LaFe1−xMgxO3

ref

365 metallic fibers

SO2 poisoning of catalytic burners; comparison between Pd/LaMnO3·2ZrO2 and Pd/CeO2·2ZrO2 catalysts in real conditions (CH4/O2/CO2/ H2O); fresh perovskite catalyst is less active than the CeZr materials but much more stable comparative study of different catalysts: LaMnO3−ZrO2, CeO2−ZrO2, BaCeO3−ZrO2 with or without Pd; perovskite catalysts have the better long-term stability after S exposure; Pd/CeO2−ZrO2 should be better for reducing CO emissions maximum activity for x = 0.5 and 0.3; Mg increases the thermal stability; alumina and FeCrAl modify the redox properties of the perovskite metallic fibers

main objective and conclusions

most often employed, confirming the high activity of this perovskite for methane combustion. An oxide wash-coat layer, MgO or ZrO2, acts as a dispersing support for the perovskite. It improves thermal stability and/or SOx resistance as previously mentioned. 4.3.2. Mechanism of CH4 Oxidation and Relationship with Surface and Bulk Properties. The mobility of active oxygen species in the outer shell of the perovskite crystal is often invoked in the reaction mechanism.52 However, methane oxidation activity is generally correlated to the amount of oxygen being desorbed at low temperature.367 In accordance with the study of Arai et al.,29 Peña and Fierro proposed the following rate equation to describe the kinetics of methane oxidation over perovskites (eq 22)194

CH4 + s XooY CH4s ZrO2

monolith

Review

Pd (0−2%)/LaMnO3

support (wash coat) catalyst and perovskite content (wt %)

Table 9. continued

364

Chemical Reviews

(24)

(2) Nondissociative adsorption of dioxygen followed by its fast transformation into active oxygen species Os* k

k′

k −0

fast

0 1 O2 + s XooY O2s ⎯→ ⎯ O*s

(25)

According to Che and Tench368,369 and Golodets,370 these active oxygen species may be superoxide O2−, peroxide O22−, single-charged oxygen anion O−, and double-charged lattice oxygen anion O2−. 10317


Chemical Reviews

Review

La0.66Sr0.34Co0.2Fe0.8O3.380 A detailed kinetic study was carried out by Song et al.379 over La0.66Sr0.34Ni0.3Co0.7O3. The best fit was obtained with an extended Mars and van Krevelen mechanism leading to the following rate equation (eq 29) including product adsorption constants

(3) Reaction between CH4 (adsorbed or not) and reactive oxygen species forming an intermediate Is, quickly transformed into adsorbed products “Ps” (CO2 and H2O) k

+O*, k ′

2 s 2 CH4s + O*s → Is ⎯⎯⎯⎯⎯⎯⎯→ Ps

fast

(26)

R

(4) Desorption of products (27)

k −3

Depending on the determining step, different scenarios were examined by Auer and Thyrion, leading to multiple rate equations. The best fit was obtained with a Mars and van Krevelen model (steady state of Os* concentration) in which the oxidation rate is controlled by the desorption of products. This mechanism led to the following rate equation (eq 28) R=

k1k 2PO2PCH4 k1PO2 + νk 2PCH4 +

νPk1k 2 PO2PCH4 k3

5kPPP + k OPO0.5(1 + K CO2PCO2 + K H2OPH2O)

(29)

As a rule, propane oxidation is easier than ethane oxidation. It starts above 200 °C, and in most cases, light-off temperatures are observed around 300 °C. Noble metals (Pt, Pd, Rh) show a similar behavior, the rate of n-alkane oxidation increasing rapidly with the number of carbon atoms in the molecule.381,382 Butane oxidation over LaMnO3 (36.6 m2 g−1 after calcination at 600 °C) confirms this tendency with a light-off temperature at 275 °C.383 Spinicci et al.384 compared three perovskites LaBO3 (B = Mn, Co, and Fe) with PdO/Al2O3 catalysts for catalytic combustion of hexane. They found the following activity scale: LaFeO3 > LaCoO3 > LaMnO3 > PdO/Al2O3. Surprisingly, ferrites, less active than manganites and cobaltites for methane oxidation, show higher performances for long-chain alkane oxidation. Szabo et al. prepared a series of LaCo1−xFexO3 perovskites by reactive grinding. Light-off temperatures for hexane oxidation are close to 200 °C for the most active perovskites, not very far from that of Pt catalysts.382 However, these perovskites, prepared by reactive grinding, present exceptional surface and textural properties, allowing their uses for low-temperature oxidation reactions. Kinetic constants and activation energies in hexane oxidation were linked to O2 adsorption energy, varying from 105 to 135 kJ mol−1. It was proven that the low-temperature rate (term in K2, eq 22) was associated with α-oxygen properties, while the high-temperature process involved β-oxygen properties. Finally, heptane oxidation was studied by Fernandes et al.385 over lanthanum nickelate synthesized by the Pecchini method. It seems that the rate of nC7 combustion is linked to the presence of Ni3+ ions. 4.4.2. Alkenes. Alkene combustion studies were exclusively performed with propene as model compound. Oxidation reactions were carried out over manganites386 or manganite− CGO (cerium−gadolinium oxide) electrochemical catalysts,387 cobaltites,388 cuprites,389 or chromites.390,391 Buciuman et al. compared the oxidation of H2 and C3H6 over substituted manganites, La 0.8A 0.2 MnO 3 (A = Sr, Ba, K, Cs) and LaMn0.8B0.2O3 (B = Ni, Zn, Cu).392 The order for H2 oxidation (2% in air) is LaSrMn ≫ LaBaMn > LaMn ≈ LaKMn > LaCsMn (La0.8A0.2MnO3 series) and LaMnCu ≫ LaMn > LaMnNi > LaMnZn. All catalysts have intrinsic reaction rates ranging from 0.15 to 0.26 μmol m−2 s−1 at 400 °C, except for LaMnCu whose rate is significantly higher (0.75 μmol m−2 s−1). The activity of these perovskites for propene oxidation, expressed in terms of light-off temperatures, is given in Table 10. Two perovskites exhibit higher activity than LaMnO3 for propene oxidation: La0.8Sr0.2MnO3 and LaMn0.8Ni0.2O3, owing in part to their greater surface areas. Very basic perovskites (LaKMn and LaCsMn) are poorly active, while LaMnCu which was, by far, the most active catalyst for H2 oxidation, shows only fair performances for propene oxidation. 4.4.3. Aromatics. Except for the report of Einaga et al.,393 which deals with benzene oxidation, all authors have chosen

k3

Ps XooY Pgas

kPPPk OPO0.5

(28)

where ν and νP are the stoichiometric coefficients of O2 (2 with respect to methane) and products (1 for CO2 and 2 for H2O). Eley−Rideal mechanisms were invalidated by Auer and Thyrion because they led to negative values of adsorption energies. The best fit with a Mars and van Krevelen mechanism suggests that feeding of active sites with oxygen species plays a significant role in methane oxidation, which reinforces the idea that O mobility is a key parameter of the reaction. A recent kinetic study of high-pressure CH4, CO, and H2 combustion over LaMnO3 confirmed this approach for methane with some variances on the kinetic parameters.371 4.4. Volatile Organic Compounds (VOC) Elimination

Compared to CO and CH4 oxidation, fewer studies were devoted to VOC elimination over perovskites. It is rather surprising if one consider their very good activity in oxidation reactions. The performance of perovskites in eliminating hydrocarbons (alkanes, alkenes, aromatics), oxygenates, and finally miscellaneous compounds will be successively reviewed below. 4.4.1. Alkanes. Perovskite catalysts were used for oxidation of light alkanes (C2−C7). To our knowledge, oxidation of heavy alkanes (C8+) was not investigated over perovskites. Ethane combustion was studied by Lee et al.372 over La1−xKxMnO3+δ perovskites of 20−26 m2 g−1 and by Alifanti et al.373 over SmCoO3 and PrCoO3 cobaltites. Ethane oxidation can be observed between 300 and 400 °C on these materials. Substitution of La by K in manganites tends to decrease oxygen nonstoichiometry, δ, and ethane oxidation rate. The presence of potassium also promotes the oxidehydrogenation (ODH) of ethane to ethylene. It seems that K hinders formation of superoxide and peroxide species, highly active for total oxidation. Surprisingly, while SmCoO3 was totally selective to CO2 and H2O, PrCoO3 catalyzed ethane oxidehydrogenation to ethylene. Praseodymium stabilizes Co3+ ions and prevents their reduction to Co2+ and Co0 much more favorable to total oxidation of ethane. Propane oxidation was extensively studied on manganites,201,374,375 cobaltites,376 ferrites,377 and nickelates.378 Double-substituted perovskites were also investigated for propane oxidation: La0.66Sr0.34Ni0.3Co0.7O3379 and 10318


Chemical Reviews

Review

of Fe by Zn is more effective for increasing the activity of ferrites, with T50 located below 300 °C for some Fe−Zncontaining materials.408 Phase segregation of ZnO seems to play a decisive role by allowing better oxygen adsorption capacity. As La1−xSrxCoO3 and La1−xSrxMnO3 are reference materials for the toluene oxidation reaction, several authors have investigated the substitution of Fe by Co409,410 or Mn.411 Optimal composition in terms of reactivity was La0.7Sr0.3Co0.8Fe0.2O3,409 synthesized by a sol−gel technique using citric acid−EDTA as complexing agents.410 Cobalt insertion favors formation of Fe4+ ions and highly active O species such as electrophilic O− ions. Similar optimal compositions were obtained by Deng et al. for Fe substitution by Co (La0.6Sr0.4Co0.9Fe0.1O3, with complete conversion of toluene at 245 °C). However, the best Mn−Fe materials (La 0. 6 Sr 0 . 4 Mn 0 . 9 Fe 0 .1 O 3 ) would not be superior to La0.6Sr0.4FeO3. Recently, significant efforts have been devoted to the synthesis of three-dimensionally ordered macroporous (3DOM) ferrites for applications in toluene oxidation: SrFeO3,412 La0.6Sr0.4FeO3,413 Eu0.6Sr0.4FeO3,414 or CoOx/ Eu0.6Sr0.4FeO3.414,415 3DOM ferrites of 22−31 m2 g−1 were obtained with supported cobalt oxide particle, having sizes of 7−11 nm. Light-off temperatures of toluene oxidation close to 250 °C were recorded on these materials. Improved activities were obtained by partial substitution of Fe by Bi (T50 close to 220 °C over La0.6Sr0.4Fe0.8Bi0.2O3).416 4.4.4. Alcohols and Other Oxygenates. Methanol combustion was investigated by Levasseur and Kaliaguine over AMnO3 (A = Y, La, Pr, Sm, Dy)417 and LaBO3 perovskites (B = Co, Mn, Fe)71 prepared by reactive grinding. Methanol reacts easily over these materials: light-off temperatures, close to 150 °C, are generally observed, and total conversion is reached far below 250 °C. The order of activity in the LaBO3 series is LaMnO3 > LaCoO3 ≫ LaFeO3. Cobaltites are more stable than manganites at 250 °C, while the reverse is observed at 400 °C. Activity is clearly linked to α-oxygen species concentration on the surface of the material. Methanol is adsorbed on the perovskite as methoxy species, which react with hydroxyl groups to give formaldehyde. Total combustion is achieved by reaction of α-oxygens with CH2O.71 LaMnO3 and YMnO3 are the most active compounds from the AMnO3 series. They have the highest concentration of α-oxygens and the highest amount of Mn4+ ions.417 Inserting LaCoO3 in MCM-41 mesoporous molecular sieve improves the catalytic activity of the cobaltite.418 This would be due to two conjugated effects: higher dispersion of the LaCoO3 phase and lower oxidation state of cobalt. Mesoporous LaMnO3 perovskites were prepared by Nair et al., who used KIT-6 silica as a hard template.130 Owing to their very high surface area, these materials showed superior activity for methanol oxidation. The general scheme of alcohol oxidation over oxide catalysts involves (i) partial oxidation to aldehyde followed by oxidecarbonylation to shorter aldehydes or (ii) dehydration to alkenes on acidic oxides and their further oxidation to COx and H2O419

Table 10. Performances of La0.8A0.2MnO3 and LaMn0.8B0.2O3 Perovskites for the Propene Oxidation Reactiona La1−xAxMnO3 perovskite

SBET (m2 g−1)

T50 (°C)

LaMn1−yByO3 perovskite

SBET (m2 g−1)

T50 (°C)

LaMnO3 La0.8Sr0.2MnO3 La0.8Ba0.2MnO3 La0.8K0.2MnO3 La0.8Cs0.2MnO3

11.2 21.9 23.1 22.1 14.5

308 279 308 317 404

LaMnO3 LaMn0.8Ni0.2O3 LaMn0.8Zn0.2O3 LaMn0.8Cu0.2O3

11.2 18.6 10.7 10.8

308 297 338 321

a

Reaction conditions: 500 ppm C3H6 + 5% O2 + 5% H2O in He; volume hourly space velocity 60 000 h−1).392

toluene as a model compound of aromatic hydrocarbons. The activity of microcubes of Sr-doped lanthanum manganites was investigated by Deng et al.,394−396 who also compared the activity of La1−xSrxMnO3 and La1−xSrxCoO3 for toluene oxidation.397 Better performances of manganites could be linked to the Mn surface enrichment and to the higher proportion of Mn4+ ions. Cobaltites gave even better performances with catalytic activity following the order LaMnO3.10 (T80% = 295 °C) < LaCoO2.89 (T80% = 246 °C) < La0.6Sr0.4MnO3.03 (T80% = 233 °C) < La0.6Sr0.4CoO2.76 (T80% = 219 °C), the experimental conditions being 1000 ppm toluene, space velocity of 20 000 h−1.397 The effect of grinding on the catalytic activity of La0.8Sr0.2MnO3 for carbon black and toluene oxidation was investigated by Rougier et al.398 Activity could be linked to oxygen nonstoichiometry and lattice disorder in the grain boundaries. Unfortunately, manganites remain very sensitive to SO2 poisoning, and partial substitution of Mn by Cu does not improve the sulfur tolerance.399 La1−xSrxCoO3 cobaltites were used by Worayingyong et al.400 ́ and Pereñiguez et al.401 for toluene oxidation. Catalytic activity seemed well correlated to the extent of α-oxygens desorbing at low temperature, suggesting that toluene oxidation follows a suprafacial mechanism. Supported cobaltites were prepared by Alifanti et al. over CeZrOx oxides402 or ceria-stabilized alumina.403 Adequate preparations allow avoiding perovskite segregation at the support surface or detrimental oxide− support interaction (such as formation of inert CoAl2O4 phase on alumina surface). Good dispersion of cobaltite over CeZrOx supports (up to 20% loading) was confirmed by in situ151Eu Mössbauer investigation using EuCoO3 as model perovskite.404 Supported perovskites are then much more active than their bulk counterparts. LaCoO3/CeZrOx catalysts were also used by Alifanti et al. in a comparative study of benzene and toluene oxidations.405 In terms of oxidation rate per mass unit of perovskite, activity order is 10 times higher for toluene than for benzene. A reverse variation with the perovskite loading was observed for the two aromatics, toluene oxidation being faster on highly loaded perovskites, while better activity was measured on low-loading catalysts for benzene oxidation. Substitution of Co by Ni (LaNi1−yCoyO3−ó series) allows achieving very good performances, as observed for x = 0.5 with total conversion of toluene below 250 °C.406 Pure ferrites do not seem very active for toluene conversion, light-off temperatures being observed around 300−350 °C. Substitution of Fe by Ag increases the reducibility and amount of β-oxygen at the expense of α-oxygen species.407 The catalytic activity being lower on LaAg1−xFexO3 than on LaFeO3, Rioseco et al. concluded that α-oxygens favored the low-temperature oxidation of toluene by a suprafacial mechanism similar to the mechanism of CO oxidation (see eqs 12 and 13). Substitution 10319


propanol, propanal, acetone, ethyl acetate, iC3−acetate, MEK

propane, propene, hexane, benzene, toluene, methyl cyclohexane,

propane, benzene, gasoline toluene, hexane

acetone

acetone, butyl acetate, ethyl acetate

cyclohexane, toluene CH4 toluene CH4, CH3Cl

propanol ethyl acetate ethyl acetate acetone

HC and other VOC

CH4, CO CH4 C3−C4 propane propane hexane

methanol methanol ethanol ethanol ethanol ethanol

oxygenates

LaMnO3 or La0.75Ag0.25MnO3 wash coated La0.8Sr0.2MnO3

La1−xSrxCoO3 La1−xCaxFeO3 LaCoO3/SBA-15 La1−xSrxB1−yB′y O3 (B, B′ = Mn, Co, Fe) GdAlO3

La1−xCex Co1−yFey O3 LaCoO3/ZrO2 La0.6Sr0.4Co0.2Fe0.8O3 La1−xCaxFeO3 LaCo1−yFeyO3 La1−xCaxMnO3

perovskite

main results

ref.

446

447

conversions (VOC individually or mixed) are total below 350 °C; reactivity of VOC decreases as follows: ethanol > ethyl acetate > isopropyl acetate > propanol > MEK > propanal > acetone > propene > methylcyclohexane > toluene > cyclohexane > hexane > benzene > propane; toluene inhibits ethyl acetate oxidation (and conversely), while a promoting effect is observed in the acetone−toluene mixture

445

441 442 443 444

435 436 437 438 439 440

reactivity decreases as follows: acetone > butyl acetate > ethyl acetate > toluene > hexane

very good performance for acetone oxidation; fair results for HC oxidation (especially gasoline)

Ce increases O mobility and reactivity while Fe leads to the adverse effect ZrO2 maintains high-perovskite dispersion and high proportion of Co3+ active in VOC oxidation total conversion below 400 °C for C3−C4 and 250 °C for ethanol; activity order iC4 > nC4 > nC3 Ca increases the proportion of Fe4+, which is beneficial to the catalytic activity small substitution of Co by Fe (y = 0.1) contributes to the increase of the catalyst performance light-off temperatures of 260 °C for nC6 and 180 °C for ethanol; Ca increases Mn oxidation mean valence and decreases the number of cationic vacancies; activity is slightly increased reactivity order: propanol > toluene > cyclohexane; Sr (x = 0.2) increases activity Ca decreases activity for CH4 conversion but increases activity for ester oxidation up to 50% LaCoO3 could be inserted in SBA-15; better dispersion and highly reducible LaCoO3 species improve activity ceramic technology for VOC combustion

Table 11. Comparison of Oxygenate and Hydrocarbon Combustion over Perovskite Catalysts


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Ethanol combustion was investigated over manganites,420 planar LaCoO3,211 Sr- or Ce-doped cobaltites,421 or doublesubstituted perovskites La1−xCaxCo1−yFeyO3.422 Acid−base properties of the perovskite have a pronounced effect on both activity and selectivity in ethanol conversion. La substitution by Sr in LaCoO3 cobaltite increases its basic character and selectivity to acetaldehyde (up to 60% at 200 °C).421 Total combustion is observed only above 260 °C. Substitution of Co by Fe tends to decrease the catalytic activity for total oxidation. This detrimental effect on the CO2 selectivity can be compensated by Ca substitution in the A position.422 The same tendency was observed with manganites around 170 °C: selectivity to acetaldehyde (ACA) is almost total for ethanol conversion below 10%. Above this temperature, ACA selectivity drops abruptly while ethanol reaches full combustion at 210 °C.420 Interesting activities and selectivities were obtained over Ag/La0.6Sr0.4MnO3 catalysts for methanol and ethanol combustion.423 Silver promotes formation of reactive surface oxygen species (O2− and O−) and redox properties of manganese. Contrary to noble metals, noxious intermediates such as formaldehyde are virtually suppressed on the silver-containing perovskite catalyst. Esters are not easily oxidized over noble metals. Promotion by Bi (Pt/Ce0.63Zr0.17Bi0.20O1.90)424 or Cr (Pd/Cr2O3−ZrO2)425 is required to improve catalytic activity. Contrary to HC oxidation, acid−base and redox properties of the material are crucial parameters for ester combustion. This gives perovskites a good opportunity to replace/substitute noble metals in VOC catalysts. Ethyl acetate combustion was investigated over LaMn1−xCuxO3,426 La1−xSrxBO3 (B = Co, Mn; x = 0, 0.4),427 and various ferrite perovskites.428 Lanthanum cobaltites and manganites show comparable activities, significantly improved by Sr doping. The temperatures for total conversion of ethyl acetate are as follows: LaCoO2.91 (26.4 m2 g−1); 230 °C ≈ LaMnO3.12 (32.5 m2 g−1); 235 °C > La0.6Sr0.4MnO3.02 (31.5 m2 g−1); 190 °C > La0.6Sr0.4CoO2.78 (20.2 m2 g−1); 175 °C.427 Strontium insertion increases the Mn4+/Mn3+ and Co3+/Co2+ ratios and thus the Mn and Co reducibility and the number of lattice defects. Ferrites (total conversion by 300 °C428) seem less active than cobaltites and manganites. Acetyl acetate combustion was studied by Pecchi et al. on various perovskites: LaMn1−yCoyO3429,430 and LaFe1−yNiyO3.431 Pure LaMnO3 showed the lowest light-off temperature, essentially because it had the highest surface area (38.6 m2 g−1). The following order for the reaction rates at 227 °C (mmol m−2 h−1) was then established: 429 LaMnO 3 ; 0.060 < LaCoO 3 ; 0.073 < LaMn0.50Co0.50O3; 0.083 < LaMn0.10Co0.90O3; 0.146. A similar trend was observed for ferrites substituted by Ni. The intrinsic activity of these materials, being less than that of cobaltites, was given at 250 °C:431 LaFeO3; 0.010 < LaFe0.70Ni0.30O3; 0.23 < LaNiO3; 0.19. A cooperative effect between nickel oxide (segregated at the surface) and the perovskite phase could explain the excellent performance of LaFe0.70Ni0.30O3. Oxidations of some ketones were also studied: acetone over SmMnOx perovskites432 and methyl ethyl ketone (MEK) over La−transition metal perovskites (Cr, Ni, Co, Mn). 433 Manganites were also doped with potassium. The following order of activity was found by Á lvarez-Galván et al.:433 La0.9K0.1MnO3 > LaCoO3 > LaNiO3 ≈ LaMnO3 ≫ LaCrO3. MEK oxidation produces significant amounts of acetaldehyde and small amounts of methyl vinyl ketone and 2,3-butanedione. All these intermediates disappear at high conversion degrees.

4.4.5. Comparison Between Hydrocarbons and Oxygenates. VOC combustion over various catalysts was reviewed by Li et al.434 These authors stressed two major facts concerning perovskites: (i) their ability to activate oxygen and (ii) their relatively low surface area which could be increased by supporting the mixed oxides. Combustion of hydrocarbons and oxygenates, individually or in combination, was compared in several studies, as summarized in Table 11. Most studies conclude that VOC oxidation occurs via a suprafacial mechanism. The very good activity of perovskites for ester oxidation makes them able to replace noble metals for treatment of indoor air of industrial workshops using large amounts of solvents. 4.4.6. Chlorinated Hydrocarbons. Oxidation of chlorinated hydrocarbons (CHC) can produce HCl (eq 30) or Cl2 (eq 31) ⎛ y Cx HyCl z + ⎜x + ⎝ y− → xCO2 + 2

− z⎞ ⎟O 2 4 ⎠ z H 2O + z HCl

(30)

⎛ y⎞ y z Cx HyCl z + ⎜x + ⎟O2 → xCO2 + H 2O + Cl 2 ⎝ 4⎠ 2 2 (31)

The ratio between HCl and Cl2 is generally controlled by the Deacon equilibrium (eq 32) 2HCl + 1/2O2 ⇄ Cl 2 + H 2O

(32)

This is an exothermic reaction (ΔH° = 58.4 kJ mol−1 at 350 °C). Chlorine is thus produced in higher concentration at low temperature. For environmental reasons, it is preferable to orientate the reaction toward HCl, more easily eliminated by soda washing. Whatever the product, HCl or Cl2, the oxidation catalyst has to work in severe conditions, generally leading to rapid deactivation. Catalyst stability is an essential parameter for VOC oxidation processes involving chlorinated hydrocarbons. Total oxidation of methane, chloromethane, and dichloromethane was studied by Stephan et al. over zirconia-supported lanthanum or didymium (Pr + Nd) manganites.448−450 DiMnO3 showed the highest and PrMnO3 the lowest catalytic activity in the total oxidation of methane and CHC. Partial substitution of La or Di by Sr leads to an enhancement of the catalytic activity in the total oxidation of methane but not in the total oxidation of CHC. The catalytic behavior of LaMnO3, LaCoO3, and La0.84Sr0.16Mn0.67Co0.33O3 perovskites, supported on cordierite monoliths, was studied by Schneider et al. in the total oxidation of chlorinated hydrocarbons.451 Several byproducts (higher chlorinated hydrocarbons, CCl4, CHCl3, C2Cl4, ...) were formed in low temperature ranges depending on the concentration of CHC in the feed. By addition of water, methane or propane diminishes formation of these byproducts. Destruction of chlorinated C1 hydrocarbons (CH2Cl2, CHCl3, and CCl4) was investigated by Sinquin et al. over LaMnO3 and LaCoO3.452,453 Two reaction pathways were identified for destruction of CH2Cl2: hydrolysis (eq 33) and oxidation (eq 34)

10321

CH 2Cl 2 + H 2O → HCHO + 2HCl

(33)

CH 2Cl 2 + O2 → CO2 + 2HCl

(34)


Chemical Reviews

Review

Formaldehyde, formed by reaction 33, is rapidly oxidized to CO2 and never detected. Water has a positive effect on CH2Cl2 destruction, but O2 is necessary for the reaction to occur. By contrast, the presence of O2 is not required for destruction of CCl4, since its hydrolysis leads to the final products (reaction in eq 35) CCl4 + 2H 2O → CO2 + 4HCl

2NH3 + 2O2 → N2O + 3H 2O ΔH °(600°C) = −550 kJ mol−1

Ammonia oxidation was investigated over manganites,462 cobaltites,463 and ferrites.464,465 Five perovskites LaBO3 (B = Mn, Co, Fe, Ni, Cu) were also compared for selective oxidation of NH3 by Isupova et al.466 The reaction should be carried out at high temperatures for kinetic reasons. Light-off temperatures above 600 °C are currently observed over perovskites. Higher NO yields and better stability were obtained over LaMnO3,466 while LaCoO3 would have the lowest selectivity to N2O (far below the values observed over noble metals).463 Increased performances were obtained by Pérez-Ramirez and Vigeland over calcium- and strontium-substituted lanthanum ferrite perovskite membranes.465 NO selectivity up to 98% with no N2O formation coupled to in situ O2 separation would allow these membrane-type catalysts to be implemented into miniaturized plants for on-site production of nitric acid. Similar membrane reactors, based on doubly substituted perovskites (Ba0.5Sr0.5Co0.8Fe0.2O3‑ó), were investigated in detail by Sun et al.467 Lower temperature could favor the route yielding NO due to the weak oxygen bonding strength on the perovskite. Optimized reaction conditions allowed obtaining NO selectivity around 85−90% with this membrane reactor. However, etching of the perovskite by NH3 requires improving the membrane for a more stable process. The reaction mechanism of NH3 oxidation to NO or N2O has been studied over LaCoO3 by Biausque and Schuurman,463 who used TAP experiments with labeled molecules. The most important reaction intermediates would be nitroxyl HNO species. The net reaction is an incorporation of the surface lattice oxygen into the NO molecule, according to a Mars and van Krevelen mechanism. N2O (produced in very low concentration) would be formed by recombination of two nitroxyl species. Selective NH3 oxidation into N2 for application in stationary DeNOx processes has been investigated by Hung over cuprites.468 N2 selectivity is favored at low temperatures, while NO formation is observed above 350 °C. Oxidation of other N compounds was also reported: 1,1-dimethylhydrazine over Ca0.7Sr0.3Fe(Co)O3 or La0.7Sr0.3Fe(Co)O3469 and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) over LaMnO3 or La0.8Sr0.2MnO3.470 Selectivity to NO and N2O increases with the reaction temperature.469 As a rule, the oxidation state of Fe, Co, and Mn as well as the O substoichiometry are important parameters for a better oxidation rate.469,470

(35)

However, a minimum concentration of O2 should be maintained for stabilizing the perovskite, which is destroyed in the presence of water alone. Due to the retention of large amounts of chlorine species by LaCoO3, higher chlorinated HC are formed in the reaction of CH2Cl2 and CHCl3. These byproducts are formed in much lower proportion over LaMnO3. Manganites appear to be more selective to CO2 and more stable than cobaltites.454 Destruction of C2-CHC was also investigated by Sinquin et al. over LaMnO3.455 As for C1-CHC, water plays a significant role and H2O/O2 mixtures are recommended for destruction of these compounds. Oxidation of trichloroethylene (TCE) was investigated by Maghsoodi et al. over lanthanum manganites,456 by Musialik-Piotrowska and Syczewska over La0.5Ag0.5MnO3,457 and by De Paoli and Barresi over ferrites.458 An excess of manganese (LaMn1.2O3)456 and substitution of La by Ag457 allow enhancing TCE oxidation activity. Musialik-Piotrowska and Syczewska also compared TCE oxidation in the presence of toluene and oxygenated compounds (ethanol, ethyl acetate) over La0.5Ag0.5MnO3 and noble-metal catalysts.457 Complex inhibiting effects were recorded over Pt and Pd, while the reactivity of each compound seemed less affected by the presence of other VOCs over the perovskite. Vinyl chloride is widely used for production of poly(vinyl chloride) (PVC). Its high toxicity requires that industrial gaseous effluents be purified to less than 5 ppm CH2CHCl. Zhang et al. reported that vinyl chloride could be oxidized over LaMnO3 or LaB0.2Mn0.8O3 (A = Ni, Co, Fe) catalysts.459 Partial substitution of Mn by B cations increases both O mobility and proportion of Mn4+ and improved activity for vinyl chloride combustion. LaNi0.2Mn0.8O3 is then the most active catalyst, with a 50% conversion at 168 °C and a 90% conversion at 210 °C, compared to 188 and 240 °C, respectively, for LaMnO3. Oxidation of 1,2-dichlorobenzene has been investigated by Poplawski et al. over ABO3 perovskites (A = La, Y, Nd, or Gd; B = Fe, Mn, Cr, or Co).460 YCrO3 is the most active catalyst among the different systems studied. All perovskites retained their crystalline structure under reaction conditions, except LaCoO3: formation of LaOCl and Co3O4 was observed after testing. A positive effect of water in the feed is noted on the oxidation rates. This could be attributed to faster removal of surface Cl− ions from the catalyst in the presence of water. 4.4.7. Ammonia and Other N Compounds. Perovskites were mainly used for selective oxidation of NH3 to NO (eq 36). This reaction is currently carried out over Pt−Rh gauzes in HNO3 production units461 2NH3 +

4.5. Solid−Solid Reaction: Soot Combustion

Today, more and more stringent regulations, limiting soot emissions (in mass and in particulate number) from automotive engines, incinerators, and power plants, are imposed to mobile and stationary sources. Carbonaceous soots, especially ultrafine particles, are suspected to provoke numerous human diseases.471,472 Diesel cars emit much higher amounts of particulates than gasoline engines. They should be now equipped with diesel particulate filters (DPF). Regularly, these DPF should be regenerated by a thermal treatment allowing combustion of the soot deposited on the DPF walls. To decrease the regeneration temperature, a catalyst is generally wash coated on the DPF to accelerate soot combustion. Soot oxidation/combustion is a solid−solid reaction occurring at the junction between the soot particle, the catalyst particle, and the gas phase. Owing to their specific

5 O2 → 2NO + 3H 2O 2

ΔH °(600°C) = −451 kJ mol−1

(37)

(36)

One of the problems encountered with noble metals is their propensity to form N2O (eq 37) 10322


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4.5.1.2. Cobaltites. Liu et al. showed that LaCoO3 was more active than Co3O4 in soot oxidation,488 but partial substitution of La by Sr increases the performance of the cobaltites.489 Co3+ would be partially reduced to Co2+, while a very active oxygen species with low electron density is evidenced by XPS. Zhang et al.490 compared La1−xAxCoO3 (A = Sr or Ce) perovskites with single oxides. Better performance was obtained on the perovskites than on the single oxides and La2O3−Co3O4 mixtures. The catalytic activity of these perovskites was found to follow the trend La0.8Ce0.2CoO3 > La0.9Ce0.1CoO3 > La0.9Sr0.1CoO3 > La0.8Sr0.2CoO3 ≈ LaCoO3. Moreover, less CO was produced during soot oxidation. Dhakad et al.491 confirmed the excellent doping effect of Ce ions on SrCoO3 perovskite activity: the ignition temperature of synthetic soot is observed at 241 °C for Sr0.8Ce0.2CoO3, compared to 300 °C for SrCoO3 and 400 °C for noncatalyzed carbon oxidation. Barium in the A position can also improve the soot oxidation ability of LaCoO3, but it seems to be less efficient than cerium with an ignition temperature of 363 °C for La0.9Ba0.1CoO3 instead of 395 °C for LaCoO3.492 Soot oxidation was investigated by Russo et al. over alkalimetal-substituted LaCoO3.493 Their results (Table 12) show

properties, involving formation and high mobility of reactive oxygen species, perovskites are good candidates for replacing noble metals, generally used in catalyzed DPF. It is now admitted that the soot oxidation rate is much higher in the presence of O2 + NO2 than in O2 alone. Kinetic models should take into account the presence of NO2 in the gases,473,474 while advanced processes involve combined DPF−DeNOx systems.475,476 In this section, soot combustion in the presence of O2 will be first reviewed. In a second part, papers dealing with simultaneous removal of soot and NOx will be examined. 4.5.1. Soot Combustion in Oxygen or Air. Alkali metals, especially K or Li, are often added to the perovskites for facilitating carbon oxidation. It was proven that potassium carbonate, supported on LaMn1−xCuxO3 perovskite, is a remarkable catalyst for coal char combustion.477 Process parameters may also have a great impact on the soot combustion. For instance, loose or tight contact between the soot and the catalyst can change the temperature-programmed oxidation (TPO) profile.478 This point will not be reviewed here for perovskite catalysts. 4.5.1.1. Manganites. Fino et al.109 found that the activity order of simple perovskites for soot combustion is (Tmax is indicated between parentheses) La1−xKxCrO3 (457 °C) < LaCrO3 (503 °C) < LaFeO3 (520 °C) < LaMnO3 (527 °C), which is just the reverse of the intrinsic activity toward methane combustion. Temperatures of the peak maxima of soot combustion are found to be between 450 and 530 °C over perovskites, while a maximum at 650 °C is observed in the absence of catalyst. Manganites are thus not very active for carbon oxidation. Attempts were made to improve soot trapping using LaMnO3 fibers.479 However, better improvements were obtained by doping LaMnO3 with Sr or K. Fujimoto et al. 4 8 0 reported good performance of La1.8Sr1.2Mn2O7. Potassium-doped manganites, with K in substitution of La, were studied by Wang et al.481 and Shimokawa et al.,482 while double-substituted manganites were investigated by Li et al. (LaKMnCoOx perovskites)483 and Shan et al. over La2−xKxMnNiO6 perovskites.484 Substitution of La by K is essential for decreasing the ignition temperature of the soot. La0.8K0.2MnO3 shows the best catalytic performance with ignition temperature around 280 °C instead of 340 °C for LaMnO3.481 Partial substitution of Mn by Co or Ni seems to improve soot oxidation at higher temperatures.483,484 A comprehensive study of La or Mn substitution by different cations was carried out by He et al.485 Two series were investigated: La0.8A0.2MnO3 with A = Cs, K, Ce and LaMn0.8B0.2O3 with B = Cu, V, Fe, Co. The activity enhancement decreased in the order of Cs > K > V > Ce > Co > Cu > Fe, which confirms that substitution of La by alkalimetal ions has the highest impact on soot oxidation. Doggali et al.486 showed that multisubstitution of manganites in the A position could give interesting performance in soot oxidation. Their lanthanum-free catalyst (Pr0.7Sr0.2K0.1MnO3), coated on ceramic foam filter, allowed an ignition temperature below 200 °C and two temperature maxima of combustion at 350 and 500 °C. Microwave-assisted regeneration of the catalytic filter has been proposed to accelerate soot combustion. This way was explored by Zhang-Steenwinkel et al.487 for regeneration of a La0.8Ce0.2MnO3 perovskite-coated ceramic monolith. With an incident power of 200 W, maximum conversion toward CO2 was reached after 2.5 min at 650 °C. Complete regeneration with no CO formation was recorded in less than 8 min.

Table 12. Soot Oxidation over Alkali-Substituted Cobaltites, Aged under Humid Air at 850 °Ca catalyst

BET surface area (m2 g−1)

Ea (kJ mol−1)

T5 (°C)

Tmax (°C)

none LaCoO3 La0.9CoO3 La0.9Na0.1CoO3 La0.9K0.1CoO3 La0.9Rb0.1CoO3

5.9 7.1 4.9 7.2 8.8

157 145 142 136 131 111

470 402 360 358 345 330

560 455 437 427 415 393

a

Oxidation conditions: catalyst-to-carbon ratio = 9 (tight contact); 50 mg of the carbon/catalyst mixture, diluted in 150 mg of silica powder. Heating rate in air (100 mL min−1): 5 °C min−1. T5 is the temperature for a 5% conversion of soot, while Tmax is the temperature at the CO2 peak maximum.493

that alkali-metal effects on soot oxidation are in the following order: Rb > K > Na. It should be noted that La0.9CoO3 is much more active than the stoichiometric perovskite LaCoO3. This is in part due to the lower BET area of LaCoO3 but also to a higher reducibility of La0.9CoO3. A comparison between K/La2O3 and KCo/La2O3 revealed that Co limits carbonate accumulation at the catalyst surface while K is necessary for perovskite formation.494 Several studies were devoted to the improvement of K-doped LaCoO3 perovskites. La0.9K0.1CoO3 fibers of 7.1 m2 g−1, prepared by Gong et al.495 (Figure 21A), showed ignition temperatures at 350−360 °C. Three-dimensionally ordered macroporous La1−xKxCoO3 catalysts with interconnected macropores were prepared by Xu et al. 496 using carboxy-modified colloidal crystal templates. SEM micrographs of these materials can be seen in Figure 21B. These catalysts showed remarkable activity for soot oxidation with T50 below 380 °C (3D-La0.9K0.1CoO3). Improved performance in soot oxidation was obtained by Liu et al.497 by mixing La0.9K0.1CoO3 active component and CeO2 support nanoparticles of comparable size. The highest activity was observed for a Co/Ce atomic ratio of 20. It is however important to avoid formation of LaCeO3 perovskite, which causes an activity loss of the catalyst.498 10323


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Review

Figure 21. (A) SEM picture of La0.9K0.1CoO3 fibers for soot combustion.495 (B) SEM image of 3D macroporous LaCoO3.496

La substoichiometry), Russo et al. found that La0.8Cr0.9Li0.1O3 had the best performance with a peak at 408 °C512 and the lowest activation energy (Ea = 125 kJ mol−1 instead of 147 kJ mol−1 for LaCrO3 and 159 kJ mol−1 for the noncatalytic reaction).513 The very good performance of Li-substituted chromites prompted Fino et al. to investigate several phase c o m p o s i t i o n s . 5 1 4 T h e be s t s i n g l e - p h a s e c a t a l y s t (La0.8Cr0.8Li0.2O3), already active well below 350 °C, could be wash coated on monolithic particulate. Tests in engine bench confirmed the promising results obtained on powder catalysts. A high-throughput approach to the catalytic combustion of diesel soot was carried out with La0.8Cr0.8Li0.2O3 or high-surface area (HSA) CeO2 as reference materials.515 Promising mixed oxides were discovered but apparently not tested in real conditions. Though the LaCrLi perovskite resulted in excellent performance, confirming the critical role of both surface area and oxygen donor character of the catalyst. 4.5.1.5. Titanates. Potassium-doped SrTiO3 catalysts for soot combustion were developed by Ura et al.,516 who published a review on this subject.517 Ignition temperatures of soot decrease in the order SrTiO3 (495 °C) > 4.5%K/Al2O3 (470 °C) > 4.5%K/SrTiO3 (400 °C) > Sr0.8K0.2TiO3 (380 °C). The activity order for soot oxidation seems to correlate with the perovskite basicity rather than with K loading or BET area. Alkali-doped strontium titanates were also investigated by Bialobok et al., who prepared a series of Sr1−xA x TiO 3 perovskites (A = Li, K, Cs; x = 0.05−0.2).518 The better catalysts (Sr0.8K0.2TiO3 and Sr0.8Cs0.2TiO3) led to ignition temperatures of 302−303 °C, while Li-substituted perovskites catalyzed soot combustion at much higher temperatures (ignition at 460 °C). A relationship with oxide basicity was suggested by Bialobok et al.518 4.5.1.6. Other Perovskites. La2−xKxCuO4 cuprites were investigated by Zhu et al.519 The best performance was obtained over La1.8K0.2CuO4 (Tmax at 380 °C), which have the highest proportion of Cu3+ in its structure. Ruthenates were studied by Labhsetwar et al.,520 while Pecchi et al. focused their research on alkaline niobiates.521,522 La3.5Ru4.0O13 showed excellent soot combustion activity (ignition temperature below 200 °C and Tmax around 380 °C), but this perovskite belongs to the category of noble-metal catalysts. Niobiates are less active, but KNbO4 leads to combustion temperatures very close to those reported for conventional LaBO3 perovskites (B = Mn, Co, Fe, Cr). The activity order of niobiates calcined at

Sr-substituted lanthanum cobaltites were also proposed by Prasad et al.499 as an alternative to LaKCoOx perovskites. A series of La1−xSrxCoO3 perovskites was synthesized by citrate− EDTA method with surface areas ranging between 4.4 and 9.3 m2 g−1. The most active catalyst (x = 0.4) has also the highest BET area. In terms of soot combustion activity (T50 at 585 °C), these catalysts seem to be less active than K-doped cobaltites. Partial substitution of Co by Fe also led to interesting properties. Gu et al.500 prepared a series of perovskite-like Ca2FexCo2−xO5 compounds, able to start carbon oxidation below 380 °C (Ca2Fe0.8Co1.2O5). Zhang et al. 501 and Xu et al.502 investigated a series of La1−xKxCo1−yFeyO3 catalysts. Partial substitution of Co by Fe improved the catalytic activity of LaCoO3 but not that of La0.9K0.1CoO3, proving that the presence of potassium is essential for good activity in soot combustion. 4.5.1.3. Ferrites. LaFeO3 perovskites were used by Taniguchi et al.,503 Hwang et al.,504 and Xu et al.505 The method of preparation can change the activity for soot oxidation, even though BET area remains the most important parameter. Taniguchi et al. were able to obtain high surface area LaFeO3 samples by mechanical milling (up to 48 m2 g−1). While the starting unmilled perovskite (0.89 m2 g−1) led to a 50% conversion of soot at 485 °C, a T50 close to 400 °C could be observed on the milled perovskites.503 Partial substitution of La by K allowed remarkable improvement of the activity of LaFeO3 for soot combustion.506,507 Substituting La for Sr is also a good way to improve both activity and stability, but Sr doping seems to be less efficient than K for soot combustion.508 Nevertheless, optimal performance was obtained over La0.2Sr0.8FeO3 and then at a high Sr substitution degree. 4.5.1.4. Chromites. As reported by Fino et al.109 in 2003, chromites are excellent catalysts for soot combustion. Catalyzed particle filters, equipped with LaCrO3, were tested by the group of Torino. They showed good regeneration by soot combustion above 350 °C.509,510 Ifrah et al.511 studied the crystallographic stability of LaCrO3 in O2 or H2 atmosphere. Activities of La1−xAxCrO3 perovskites (A = Na, K, Rb, Li) were obtained by Russo et al.512 A detailed kinetic analysis of soot combustion on the same materials was published later.513 The activity order of the alkali dopant would be Li ≫ Rb > Na ≈ K. Russo et al. also showed that a moderate substoichiometry of La in LaCrO3 had a positive impact on activity: La0.9CrO3 or La0.8CrO3 being equivalent to La0.9Rb0.1CrO3 in terms of soot combustion profiles. Combining the two parameters (alkali substitution and 10324


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650 °C is (Tmax in parentheses): KNbO4 (460 °C) > NaNbO4 (523 °C) > RbNbO4 (612 °C) > LiNbO4 (636 °C).522 4.5.2. Simultaneous Removal of Soot and NOx. As a rule, similar perovskites were used for soot oxidation in the presence of O2 + NO as in O2 alone, with a preference for alkali-doped materials. However, attention was paid to the NO oxidation reaction, NO2 being much more active than NO in the carbon oxidation reaction. Another important parameter is the selectivity of the C + NO2 reaction: for environmental reasons, only N2 should form. Studies dealing with simultaneous removal of soot and NOx are listed in Table 13. 4.5.3. Kinetics and Mechanisms of Soot Oxidation over Perovskites. Fino et al.543 studied the kinetics of soot combustion and NOx removal in isothermal conditions over a layered perovskite (La1.8K0.2Cu0.9V0.1O4). The rates of NOx reduction (RNO) and C combustion (RCO2) were expressed as power-law equations (eqs 38 and 39) n m RNO = k 0e(Ea/ RT )PO2 PNO

immediately adjacent to catalyst particles) to form N2 and NO according to the following reactions (eqs 42 and 43)

The Starink ln

β Tα1.92

553

(39)

(40)

equation

⎛ E ⎞ = −1.0008⎜ a ⎟ + constant ⎝ RTα ⎠

(43)

Heterogeneous catalysts were extensively studied for oxidation of organic pollutants in water treatments.554,555 However, real industrial applications of catalytic wet oxidation (CWO) processes remain scarce due to a lack of stability of solid catalysts in water556 and for a lack of a comprehensive reactor design framework, inherent to the multiphase nature of the CWO reactors.557 Depending on the nature of the oxidant (H2O2, O3, or air), three processes may be considered: catalytic wet peroxide oxidation (CWPO), catalytic wet ozonation (CWO), or catalytic wet air oxidation (CWAO). As iron or copper catalysts are generally used as Fenton-like catalysts in CWPO processes, Fe− or Cu−perovskites were mainly investigated for this application. Other perovskites were also used in ozonation and air oxidations. 4.6.1. Wet Peroxide Oxidation. Phenol degradation by CWPO processes was studied on LaFeO3558 or LaTi1−xCuxO3.559,560 Phenol oxidation was also performed over LaMnO3 perovskites by coupling CWPO and photocatalysis.561 LaFeO3 perovskites were prepared by a glycine/nitrate autocombustion process. Increasing the glycine/NO3− ratio (τ) allows preparing perovskites with higher surface areas and higher Fe2+ contents. Phenol conversion at 30 min amounts to 88% with a TOC abatement at 240 min of 76%.558 Faye et al. also showed that dissolved oxygen appeared as a required cooxidant for the wet peroxide oxidation of phenol in the presence of iron-based perovskite-type oxides: improved performance was obtained in the H2O2/air mixture than in H2O2/N2. Similar results were reported by Sotelo et al. for CWPO of phenol over LaTi1−xCuxO3 catalysts. As the reaction was carried out at higher temperature (80 °C instead of 40 °C in the work of Faye et al.), TOC removal is accelerated (80% after 30 min over LaTi0.44Cu0.61O3). Hydroquinone and catechol are the main intermediates detected in the products of CWPO. A Cu2+ leaching of 22% was observed; this led to a homogeneous/heterogeneous combined reaction. However, the homogeneous reaction, carried out with the same Cu2+ concentration (as that measured after leaching), gave significantly poorer performance. This result proved that the heterogeneous reaction remained predominant. Wet peroxide oxidation of methylene blue, a dye model molecule, was investigated by Moura et al.562 over Fe0/ LaMnO3 and by Magalhães et al.563 over LaMn1−xFexO3 and LaMn0.1−xFe0.90MoxO3 perovskites. LaMnO3 is inactive for the CWPO reaction: it leads to a high rate of hydrogen peroxide decomposition. Substitution of 90% Mn by Fe has a great impact on the reaction with an increased rate of CWPO and a decrease of H2O2 decomposition.563 Molybdenum has no negative effect on H2O2 decomposition but significantly improves methylene blue discoloration. The optimal catalyst for this application would be LaMn0.01Fe0.90Mo0.09O3. 4.6.2. Ozonation and Wet Air Oxidation. Ozone is a strong oxidant. Compared to H2O2, it seems easier to control its decomposition to O2. Catalytic ozonation has been extensively studied by Beltran, Rivas, and co-workers. α-

Rates were determined at different soot conversions between 15% and 90% and at different temperatures, which allowed Fino et al. to calculate kinetic orders and activation energies (Table 14). The kinetic order with respect to O2 is strongly affected by the presence of NO. It decreases with the %C converted, which suggests that O2 adsorption would be favored on residual unburned carbon. The rate of soot combustion is extremely dependent on the NO content in the reaction gas (1.7 < m < 2). The presence of this gas dramatically decreases the activation energy of combustion. The procedure used by Fino et al.543 (integration of N2 and CO2 formation at different temperatures and for various C conversions) gives reliable kinetic data, but it is very time consuming. Most authors determined activation energies from temperature-programmed soot combustion. Two models are generally used for derivation of activation energies from TPO curves. The Ozawa552 equation ⎛ E ⎞ ln β = −1.0518⎜ a ⎟ + constant ⎝ RTα ⎠

C + NO2 → NO + CO → 1/2N2 + CO2

4.6. Water Depollution

(Reaction path of C/NO/O2 )

(Reaction path of C/O2 )

(42)

Direct oxidation of soot by NO2 reforms NO, which should be further reduced to N2.

(38) n′ R CO2 = k′0 e(−E ′ a/ RT )PO2

C + 2NO2 → 2NO + CO2

(41)

where β is the heating rate and Tα is the temperature for a given conversion of soot α. Hernández et al.513 showed that the Starink equation gave Ea values very close to those obtained by more complex models whose solutions can only be obtained using computer algorithms. Several mechanisms of soot oxidation were proposed. They can be summarized in the reaction scheme provided by Liu et al.545 (Figure 22). Oxygen would be adsorbed as superoxide or O− species which react with soot to form carbon oxides. These oxygen species could also react with NO to form nitrates or gaseous NO2. Nitrates could then be decomposed into N2 (or N2O), while NO2 may react with soot particles (not 10325


531

Ti decreased in the order Co > Mn > Fe; excellent performance of La1−xCsxCoO3 catalysts, optimal for x = 0.4 (Ti = 200 °C and Tmax = 300 °C) effect of CO, C3H6, CH4, H2, NOx, SO2, and H2O molecules (0.3% except SO2, 30 ppm and H2O, 10%); soot combustion is slightly promoted by reducing agents in the order H2 ≈ C3H6 > CH4 > CO; NO +NO2 as well as water vapor accelerate soot combustion, while SO2 seems to slow down the rate of carbon oxidation Ti = 350 °C, Tmax = 450 °C; increasing δ (higher O substoichiometry) seems to be beneficial for NOx trapping (and soot combustion) rate of soot combustion increases with x; NOx conversion is maximal for x = 0.25; better performance of K-doped catalyst linked to their high surface coverage by O species (O1s at 528.5 eV) best performance for y = 0.1; partial substitution of Co by Fe increases Oads coverage and Fe4+ content and hence the activity for the NO oxidation reaction and the NOx storage capacity performance increases with x (K content) but decreases with y (Fe content); better catalysts: La0.6K0.4CoO3 (Ti = 283 °C; Tmax = 382 °C) and LaCo0.9Fe0.1O3 (Ti = 309 °C; Tmax = 409 °C) Pd increases the BET area of LaCoO3 (from 5.7 to 14.6 m2 g−1 for y = 0.03); Pd slightly lowers the ignition temperature but significantly improves NOx conversion unsubstituted ferrite LaFeO3 is fairly active (Tmax = 415 °C and max NOx conv = 21%); LaKFeCuO3 (composition not provided) is the best catalyst with Tmax = 359 °C and max NOx conv = 79%; perovskite wash coated on a filter and tested in real conditions (engine bench) gave less good performance (NOx conv is significantly decreased) combination of a fuel born with a perovskite catalyst (physical mixture) improves soot oxidation (Tmax = 410 °C instead of 430 °C over the perovskite alone)

0.5%NO + 4%O2

0, 1, or 10%O2 + various gases added to O2 cat./soot = 20 4%NO + 18%O2 0.2%NO + 5%O2 cat./soot = 5

10326

La2−xKxCuO4 La2−xKxCu1−yVyO4 Ln2−xNaxCuO4, Ln = La, Pr, Nd, Sm, Gd La2−xRbxCuO4 La1.7Rb0.3CuO4−CeO2 composites La0.9K0.1Cr0.9O3−δ + 1 wt % Pt SrTi1−xCuxO3 or MgTi1−xCuxO3 or Cu− titanates Sr0.8K0.2TiO3 or K/SrTiO3 or K−Cu/SrTiO3

La0.7K0.3FeO3 mixed with a Fe/Ce fuel-born catalyst La1−xCexNiO3 La2−xAxNi1−yByO4, A = Sr, Ba; B = Mn, Fe La2Ni1−yCuyO4

La1−xKxCoO3 and LaCo1−yFeyO3 LaCo1−yPdyO3 La1−xAxFe1−yByO3, A = Na, K, Rb; B = Cu

La0.9K0.1Co1−yFeyO3−δ

BaCoO3−ó La1−xKxCoO3

0.05%NO + 5%O2 cat./soot = 4

0.1%NO + air or air alone cat./ soot = 9 0.05%NO + 5%O2 cat./soot = 4

0.2%NO + 5%O2 cat./soot = 5 0.2%NO + 5%O2 cat./soot = 5

0.2%NO + 5%O2 cat./soot = 5 0.18%NO + 4%O2 cat./soot = 9 0.2%NO + 5%O2 cat./soot = 5

0.5%NO + 5%O2 cat./soot = 9

0.5%NO + 5%O2 cat./soot = 9 1%NO + 5%O2 cat./soot = 10

0.05%NO + 8%O2 cat./soot = 3

0.06%NO + 10%O2 cat./soot = 20 0.08%NO + 10%O2 cat./soot = 10 0.5%NO + 5%O2 cat./soot = 15 0.1%NO + 10%O2 cat./soot = 9

530

Ti = 250 °C; Tmax for CO2 and N2 production at 455 °C for LaMn and 410 °C for LaAgMn

548, 549

SrTi0.89Cu0.11O3 is the most active catalyst; partial substitution of Ti by Cu significantly improves soot oxidation activity of SrTiO3 but not that of MgTiO3; surface CuO is the active component of supported Cu−titanates study of potassium stability during soot oxidation; potassium is more active than Cu in fresh catalysts but tends to deactivate (loss of K); Sr0.8K0.2TiO3 is much more stable but less active than K/SrTiO3

547

550

545 546

542 543 544

541

539 540

Ce (x < 0.05) does not change Ti (around 300 °C) but improves the reactivity of C with NO2 (12% C is oxidized by NOx instead of 9% over LaNiO3) best catalyst would be La1.8Sr0.2NiO4 (Ti = 160 °C, Tmax = 300 and 400 °C for production of N2 and CO2, respectively); Ba blocks the reactivity of C with NO2; substitution in the B position (Mn, Fe) shifts Tmax to higher temperatures copper substitution increases the proportion of Ni3+ and O vacancies, which improves NO oxidation activity; best performance is obtained with y = 0.6 (Ti = 264 °C instead of 329 °C for La2NiO4) optimal performances for x = 0.5; two maxima of CO2 production at 370 and 440 °C (instead of 550 °C for La2CuO4) better performance over La1.8K0.2Cu0.9V0.1O4: Tmax = 485 °C (515 °C after 8 cycles) with higher N2 formation (3.8% C oxidized by NOx instead of 1.8% with La2CuO4) on the basis of Tmax of soot oxidation, activity order for Ln2CuO4 samples is La > Pr > Nd ≈ Sm > Gd; partial substitution of La by Na improves soot combustion (optimum for x = 0.7) and NO reduction (optimum for x = 0.3) concentrations of Cu3+ and O vacancies increase with x (from 0 to 0.3); optimal compositions for x = 0.4 (Tmax = 505 °C) or 0.3 (maximal yield of N2) ceria-supported perovskites are much more active for soot combustion; optimal La1.7Rb0.3CuO4/CeO2 molar ratio = 20% (Tmax = 401 °C instead of 508 °C for the unsupported perovskite and improved production of N2) Pt and the presence of NO slightly improve soot oxidation activity of La−K−Cr (Tmax in air = 446 °C with Pt vs 456 °C without Pt; Tmax in air + NO = 432 °C with Pt)

538

537 475

536

535

533 534

532

529

Partial substitution of Mn by Cu improves perovskite crystallinity. Higher amount of Mn4+ in the structure. NOx removal is improved; no NOx and no HC after plasma-assisted reaction Tmax of soot oxidation (tight contact): 372 (LaMnNi) and 433 °C (LaMn) in NO + O2 and 435 (LaMnNi) and 496 °C (LaMn) in O2 alone

LaMnO3 and LaMn0.7Ni0.3O3 LaMnO3 and La0.7Ag0.3MnO3 La1−xAxBO3 A = Ce, Cs, Sr, Ba B = Co, Mn, Fe LaCoO3

La0.8K0.2Cu0.05Mn0.95O3 La0.8K0.2Cu0.05Mn0.95O3

525, 526 527 528

Ti = 275 (x = 0) and 260 °C (x = 0.05); max conversion of NO into N2: 52% (x = 0) and 55% (x = 0.05).

0.25%NO + 5%O2 cat./soot = 20 0.25%NO + 5%O2 0.25%NO + 5%O2 + 0.27%C3H6 plasma assisted oxidation 0.1%NO + 10%O2 or 10%O2 alone; cat./soot = 10 0.2%NO + 5%O2 cat./soot = 10

ref 523 524

main results best performance obtained over La0.75K0.25MnO3; ignition temperature of soot at 215 °C, but significant N2O is formed in NO2+O2: highest performance over La0.8Bi0.2MnO3; ignition temperature by NO2 at 150 °C and O2 at 260 °C; for comparison, Ti(NO2) = 230 °C and Ti(O2) = 260 °C over LaMnO3; Pt addition improves oxidation by O2 but not by NO2

reaction conditions

0.5%NO + 5%O2 cat./soot = 20 0.5%NO2 + 10.5%O2 or 10%O2 alone cat./soot = 10

perovskite

La1−xKxMnO3 LaMnO3, La1−xAxMnO3 (A = Ce or Bi) LaMn1−yFeyO3 La0.8K0.2CuxMn1−xO3

Table 13. Perovskites Used for Simultaneous Removal of Soot and NOxa



Review

Table 14. Kinetic Orders at 420 °C and Activation Energies for the C/NO/O2 and CO/O2 Reactions over La1.8K0.2Cu0.9V0.1O4a

520

C/NO/O2 reaction

C/O2 reaction

%C converted

m

n

n′

15 50 90 Ea and E′a (kJ mol−1)

1.7 1.8 1.98 41

0.72 0.56 0.35

0.37 0.34 0.31 129

Reaction conditions: T = 390−450 °C, NO = 1000−1980 ppm; O2 = 0−8%; cat./soot = 9.543

a

Figure 22. Proposed mechanism of soot oxidation in O2 and NO + O2.545

Ti is the ignition temperature; Tmax is the temperature of the maximal production of CO2.

Ketoacids (such as pyruvic acid) as well as phenolic compounds (such as gallic acid or wastewaters from distillery) were treated at room temperature over a LaTi0.15Cu0.85O3 perovskite.564,566 On pure compounds, optimal performance was obtained at pH = 2, while basic pH would be more convenient for treatment of industrial wastewaters.566 Pharmaceutical compounds can also be treated by catalytic ozonation over the same perovskite catalyst. Diclofenac and α-ethynylestradiol567 or sulfamethoxazole568 were fully oxidized within 10−20 min at 20 °C. However, significant TOC removal (80−90%) needs longer times (2 h) for obtaining clean water. Different advanced oxidation processes for treatment of pyruvic acid were compared by Rivas et al.569 The most efficient process is photocatalytic ozonation combining O3 and UV radiation. Ideally, catalyst concentration (LaTi0.15Cu0.85O3) should be adjusted because the perovskite has two opposite effects: it increases the rate of catalytic ozonation but decreases that of photocatalysis. Combination of H2O2 and photocatalysis also shows an outstanding capacity for pyruvic acid removal, but a relatively high concentration of hydrogen peroxide is required for a significant TOC abatement (50%). Compared to H2O2 and O3, oxygen is a mild oxidant. Catalytic wet air oxidation thus requires higher temperatures, generally between 140 and 240 °C. Moreover, the solubility of oxygen in water being limited, high pressures are needed to obtain sufficient concentrations of O2 in water. These drawbacks are compensated by the lower price of the reactant. CWAO of phenol was investigated by Resini et al.570 over a La0.8Sr0.2Mn0.98O3 perovskite. A phenol conversion of 80% could be reached at 150 °C, 4 bar O2 in less than 3 h. Benzoquinone and hydroquinone are the main intermediates of total oxidation. Oxidation of salicyclic acid was investigated by Yang et al.571 over LaFeO3. Good performance (80% conversion) and minimum Fe leaching (15 ppm) were recorded at 140 °C after 3 h. Higher temperatures are not recommended because salicyclic acid tends to be transformed into less reactive intermediates. Substituted manganites or cobaltites are also good candidates for CWAO of fatty acids.

a

LaRuO3 or La3.5Ru4.0O13

0.15%NO + 10%O2, 0.15%NO2 + 10%O2 or 10%O2 cat./soot =5 NO + 18%O2 or O2 alone cat./ soot = 20

La3.5Ru4.0O13 ruthenate with almost all the ruthenium in the +4 valence state is more active than LaRuO3; activity for soot oxidation is very high in O2 alone (Ti = 150 ° C and Tmax = 375 °C); moderate effect of NO (concentration not given) can be observed (Tmax = 365 °C)

516

ref main results

comparison of diesel and biodiesel soot oxidation; biodiesel soot is more reactive owing to their higher O content (14.2% vs 9.4% O); Ti is lower over Cu−Al2O3, but Tmax is much lower over the perovskite significant improvement of soot ignition in the presence of NO2; over Sr0.8K0.2TiO3, Ti = 380, 360, and 240 °C with O2 alone, NO + O2, and NO2+O2, respectively; K has the highest impact when soot is oxidized in O2 alone

reaction conditions

0.05%NO + 5%O2 cat./soot = 4

perovskite

Sr0.8K0.2Ti0.9Cu0.1O3 or 5% Cu−Al2O3 Sr1−xKxTiO3, 0 < x < 0.5

Table 13. continued

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Oxidation of stearic acid was studied by Royer et al.70 over La1−xAxBO3 perovskites (A = Sr, Ce; B = Mn, Co). The reaction proceeds via a recurrent decarboxylation process leading to shorter acids and carbonates. The oxidation rate is initially extremely fast. However, the perovskites are massively carbonated during the reaction, which limits their reactivity by destruction of the perovskite structure. Low-temperature CWAO was reported for destruction of azo dyes over layered perovskite La4Ni3O10.572 Methyl orange (MO) was used as model compound. Total degradation of MO is obtained in less than 4 h in air at 25 °C. The reaction mechanism would imply an electron transfer from MO to La4Ni3O10, well explained by the unique electronic properties of this material.

oxide decomposition is the adsorption of N2O followed by its decomposition, leading to N2 formation in association with a surface oxygen. In this classical model, oxygen formation occurs through recombination of two adsorbed atomic oxygen species (O*) or through reaction of N2O with adsorbed atomic oxygen species. Over iron-containing zeolites, the kinetics of direct N2O decomposition is consistent with an oxygen transfer redox mechanism, in which N2O simultaneously acts as oxidizing (eq 47) and reducing (eq 48) agent. The rate-determining step is the catalyst reduction by N2O.575

4.7. Conclusions

Owing to their exceptional redox properties (multiple valence states of the cations) coupled to a high oxygen mobility, perovskites can be remarkable oxidation catalysts. Even though cobaltites and manganites (and to a lesser extent, ferrites) dominate this application, the quasi-infinite possibilities to substitute A and B cations allowed us to tune the catalytic properties on demand. However, intrinsic activity of perovskites for most oxidation reactions remains 1 or 2 orders of magnitude lower than that of noble metals. Intensive research was performed to increase surface area (see section 2). Perovskites of 100 m2 g−1 can now compete with noble-metal catalysts having currently metallic surface areas of 1 m2 g−1. Keeping a high surface area in oxidation reactions is however challenging. Higher stability could be obtained by supporting perovskite on other oxides such as zirconia and alumina. Finally, perovskites were proven to be excellent supports of metal catalysts: combining the two active materials (perovskite oxide and metal) gives rise to innovative catalysts in many applications.

N2O + * → N2 + * − O

(47)

N2O + * − O → N2 + O2 + *

(48)

Ivanov et al. studied the reactivity of La1−xSrxMnO3 (with x from 0 to 0.7) for high-temperature (900 °C) N 2 O decomposition.576,577 Sr-doped manganite materials show high activity for oxygen mobility-assisted reaction, which makes them good candidates for N2O decomposition (eqs 44−46). In the N2O decomposition mechanism, oxygen desorption is the rate-determining step. Then a correlation between N2O decomposition rate and surface oxygen binding energy can be established.578−580 In this way, oxygen mobility in layer-structured La1−xSrxMnO3 perovskites (x = 0−0.5) as well as oxygen diffusion through intergrain boundaries, were examined by Ivanov et al.576 by means of steady-state isotopic transient kinetic analysis. The results indicate that introduction of Sr into the perovskite lattice noticeably increases the coefficient of bulk oxygen diffusion. Besides, two different types of oxygen are present in the bulk of the catalyst: fast and slow exchangeable types. This means that in Sr-substituted samples a second faster pathway of isotope transfer in the bulk of the catalyst appears (Figure 23). Increasing the strontium insertion (x > 0.3) leads to an increase of the overall rate of oxygen exchange, due to a greater contribution of the fast pathway of oxygen transfer through vacancies, formed in the perovskite lattice, to compensate for the reduced cation charge. Evident correlation between oxygen mobility and catalytic activity in the

5. PEROVSKITE FOR REDUCTION REACTIONS Great efforts were devoted to development of perovskite catalysts for NO2, NO, or N2O abatement in automotive or stationary depollution processes.573 The sum NO2 + NO is currently referred as “NOx”. The nitrous oxide N2O, not submitted to drastic regulations as NOx, is considered as a nontoxic greenhouse gas. However, its GWP (global warming potential) is 310 times higher than that of CO2, and its abatement is often considered as a priority to avoid erratic climate changes in the next decades. NOx and N2O can be reduced to N2 by direct decomposition or by chemical reduction using reducers such as hydrocarbons, CO, H2, NH3, etc. 5.1. NOx Decomposition

5.1.1. Nitrous Oxide Decomposition. According to the commonly accepted mechanism, N2O decomposition is a sensitive reaction to oxygen mobility (eqs 44−46)574 k1

N2O + * XooY N2O* k −1

k2

N2O* → N2 + O*

(44) (45)

k3

2O* XooY O2 + 2* k −3

(46)

In this multistep mechanism, the asterisk “*” is an active site for N2O adsorption, accepted as a coordinatively unsaturated site (CUS) of oxidic transition metal. The first step for nitrous

Figure 23. Schematic view of the two possible pathways of the gas/ surface (or subsurface)/bulk oxygen exchange.576 10328


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Dacquin et al.50 reported the effect of synthesis route on the catalytic performance of LaCoO3 for N2O decomposition. Different synthesis routes such as complexation, colloidal templating, and reactive grinding were used. The authors showed that structural and textural properties are not the only parameters governing the catalytic performance. Higher performance is obtained with reactive grinding, resulting in materials with the highest specific surface area and the highest density of oxygen vacancies. Palladium impregnation over LaCoO3 resulted in an increase in N2O decomposition rate.582 In the same way, Santiago et al.583 compared LaCoO3 with commercial catalysts, namely, FeZSM-5, Co2AlO 4 and BaFeAl11O19. Catalytic tests were performed under representative flue-gas conditions, including 0.3 mbar N2O, 60 mbar O2, 0.1 mbar NO, 0.14 mbar CO, 0.1 mbar SO2, and 100 mbar H2O in He. In the temperature range of 200−850 °C, in spite of their remarkable performance for N2O abatement in nitric acid plants, mixed-oxide catalysts such as Co2AlO4, LaCoO3, and BaFeAl11O19 show lack of stability: strong sensitivity to water and sulfur dioxide; fast deactivation in the simulated gas mixture. Precisely, bulk sulfate phases were detected over perovskite and hexaaluminate, and it is known that these phases strongly alter catalytic properties of mixed oxides.48 Barium-based perovskites (La 0.8 Ba 0.2 MnO 3 or BaCoxFeyZr1−x−yO3−δ) were also used as possible water-resistant active catalysts.584,585 For instance, Kumar et al.584 studied the Ba substitution and Ag promotion in LaMnO3 perovskite. Both Ba and Ag incorporations have a promotional effect on N2O decomposition rate. Interesting results are obtained with 1 wt % Ag-promoted La0.8Ba0.2MnO3 sample, showing 100% N2O conversion at 550 °C, in the presence of NO and O2. From a mechanistic aspect, N2O decomposition involves surface vacant sites. Modification of Mn4+/Mn3+ ratio in LaMnO3 due to the Ba and Ag substitution as well as the vacancies formation can explain the observed catalytic activity evolutions. Finally, enhancement in water dissociation and nitrous oxide decomposition rate was achieved using an in situ oxygen removal approach in catalytic membrane-type reactor.585 5.1.2. Nitric Oxide Decomposition. Although several methods exist to reduce the NOx, direct decomposition of NO into N2 and O2 (2NO → N2 + O2) is recognized as the “ideal” reaction for NOx removal because it does not require any reducer and is simple and economical. 5.1.2.1. Mechanism, Active Site Structure, and Kinetic Point of View. Figure 26 presents different structures of perovskite(-like) oxides according to the coordination number of the B cation: with T′ (with B−O square), T (with B−O octahedron), and T* (with B−O pyramids) phases. The contribution of different phases to the NO decomposition is complex, while oxygen vacancies in the T* phase favor mobility of lattice oxygen.586 Shin et al.587 suggest that the active sites for NO decomposition are composed of two adjacent oxygen vacancies (route a, Figure 26). Considering LaSrNi1−xAlxO4 with different B-site cations and La2−ySryCuO4 with different crystal phases, it appears that the active sites contain two oxygen vacancies587 but with one lattice oxygen located between the two transition metal sites (route b in Figure 26). Oxygen vacancies are located on the apex of the MO6 octahedron, and lattice oxygen is located between two transition metals (M−O−M). Kinetics and mechanism of NO decomposition over La0.4Sr0.6Mn0.8Ni0.2O3 perovskite-type oxides were investigated by Zhu et al.588 using a power rate law, as presented in eq 49

reaction of N 2 O decomposition was obtained for La1−xSrxMnO3 (Figure 24). LaMnO3 exhibits both the lowest

Figure 24. Direct correlation between catalytic activity of the La1−xSrxMnO3 (x = 0, 0.3, and 0.5) samples in N2O decomposition in the absence (■) or presence (●) of oxygen (900 °C, contact time 5 × 10−4 s) and the diffusion coefficient (▲).576

rate of surface oxygen exchange and the lowest bulk oxygen mobility and is almost inactive for N2O decomposition; while the La1−xSrxMnO3 samples with x = 0 and 0.3 are single phase, the sample with x = 0.5 is multiphase and contained, in addition to simple perovskite, a layer-structured perovskite phase (La1−ySry)2MnO4.577 N2O decomposition rate increases at low Sr substitution degree (x ≤ 0.3), related to fast oxygen mobility caused by the lattice disordering during polymorphic phase transition from hexagonal to cubic. For higher strontium substitution (x > 0.3), formation of layer-structured perovskite (LaSrMnO4) promotes the activity as well. Ishihara et al.581 also studied LaMnO3-based perovskite, with Ba and In as substituting cation for La and Mn, respectively. The temperature dependence of N2O decomposition activity was studied on La0.7Ba0.3Mn0.8In0.2O3, and complete N2O decomposition was observed at 500 °C. However, N2O decomposition rate is affected by the presence of O2 in the gas phase, showing the inhibiting effect of oxygen on reaction rate (Figure 25).

Figure 25. Temperature dependence of N2O decomposition activity on La0.7Ba0.3Mn0.8In0.2O3 catalyst under coexistence of oxygen (N2O, 10%; He, balance; W/F = 3 gcat s cm−3).581 10329


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Figure 26. Crystal structure of A2BO4 compounds with different phases (T′, T, and T*). In the T* phase, possible active sites and reaction routes for NO decomposition is shown. Reproduced from ref 586. Copyright 2007 American Chemical Society.

Figure 27. Possible reactions occurring in the process of NO decomposition.594

Figure 28. Recycle mechanism of NO decomposition over perovskite-like oxides.594

r = k[NO]m [O2 ]n

the square root of the oxygen partial pressure, suggesting that the apparent reaction order of O2 could be taken as −0.5. PO2 and PNO dependence on N2 formation rate was estimated by Iwakuni,589,590 Zhu,588 Ishihara,581 and Tofan et al.591−593 over different perovskites. An almost constant reaction order to NO (∼1.2−1.3) was obtained for La 0.7 Ba 0.3 Mn 0.8 In 0.2 O 3 , 581 Ba0.8La0.2Mn0.8Mg0.2O3,590 and Pt-loaded SrFe0.7Mg0.3O3 catalysts.589 Catalyst composition moderately affects the rate dependence to PNO. Cofeeding O2 and NO also lowered the N2 yield on Ba0.8La0.2Mn0.8Mg0.2O3590 and SrFe0.7Mg0.3O3,589 with reaction orders to O2 of −0.18 and −0.12, respectively. Therefore, the oxygen inhibition effect depends on the catalyst composition, the weakest effect being recorded with Pt-loaded SrFe0.7Mg0.3O3.

(49)

where [NO] and [O2] are, respectively, the partial pressures of NO and O2 in the feed gas and m and n are the apparent reaction orders of NO and O2, respectively. N2 formation rate increased monotonically with the increase of NO partial pressure. The reaction order of NO is similar (m ≈ 1) whatever the reaction temperature, indicating that a high partial pressure of NO facilitates NO decomposition in the temperature range of 650−850 °C. In contrast, the oxygen concentration has a negative effect on NO decomposition rate, as reported by Iwakuni et al.589,590 and Ishihara et al.581 The reaction order toward O2 depends on the temperature, and it increases from n = −0.24 at 650 °C to n = −0.08 at 850 °C. Zhu et al.588 provided a simple model by plotting 1/r as a function of [O2]−1/2. The reaction rate increased linearly with 10330


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Table 15. Apparent Activation Energies and Pre-Exponential Factors for the Constants, Representing CO2 and H2O Inhibition for NO Direct Decomposition over Perovskites (r = kNOPNO/(1 + KPO2 + kinh jPj).591

a

perovskite

(Einh CO2)app (kJ/mol)

Ln Ainh CO2a (bar−1)

(Einh H2O)app (kJ/mol)

Ln Ainh H2Oa (bar−1)

LSMN-28 La0.87Sr0.13Mn0.2Ni0.8O3−δ LSNC La0.66Sr0.34Ni0.3Co0.7O3−δ LSCuF La0.8Sr0.2Cu0.15Fe0.85O3−δ

87 ± 6 78 ± 6 60 ± 7

17.8 ± 1.3 16.1 ± 0.8 13.5 ± 0.5

84 ± 9 49 ± 4 40 ± 3

14.5 ± 1.2 9.2 ± 0.5 8.5 ± 0.8

ln Ainh j = ln kinh j + (Einh j)app/RT, where j = CO2 or H2O.

Tofan et al.592 provided a kinetic study of NO decomposition over La0.87Sr0.13Mn0.2Ni0.8O3−δ, La0.66Sr0.34Ni0.3Co0.7O3−δ, and La0.8Sr0.2Cu0.15Fe0.85O3−δ. The inhibition effect of oxygen strongly depends on the temperature and material composition.593 Different reaction mechanisms, with a possible dual role of oxygen, were proposed. At limited concentration it can act as a reactant, probably by participating in continuous formation of intermediates, possibly dimmer species (N2O4 or even N2O3). O2 can also act as an inhibitor by blocking regeneration of the active sites. To complete these kinetic studies, the mechanism of NO decomposition over perovskite(-like) oxides was explored by Zhu et al.594 Possible reaction steps occurring in the process of NO decomposition are presented in Figure 27. Over LaSrNiO4 and La0.4Sr0.6Mn0.8Ni0.2O3 samples, Zhu et al.594 suggested that NO2 formation mainly occurs on the catalyst surface (step 6 in Figure 27). At low temperature, step 7 is the main route of O2 formation, since the NO2 dissociation reaction cannot occur spontaneously. This route is mainly performed on catalysts having activity for NO decomposition below 494 °C. At high temperatures (T > 600 °C), step 10′ becomes the main route to O2 formation. The decrease of activity in the presence of gaseous oxygen is then ascribed to inhibition of the NO2 dissociation reaction, by way of step 10′. Finally, it was found that the mechanism of NO decomposition over perovskite-like oxides was performed in a “recycle” way. NO2 and O2 are formed through the way of NO(a) +O(a) ↔ NO2(a) and 2NO2(g) ↔ 2NO(g) +O2(g). As illustrated in Figure 28, gaseous NO is first adsorbed and dissociated into N2 and atomic oxygen, which then reacts with another NO molecule to form adsorbed NO2 species. Thereafter, adsorbed NO2 desorbs and dissociates into NO and O2. Finally, the effect of CO2, H2O, and CH4 on nitric oxide decomposition over perovskite-based material (La0.87Sr0.13 Mn0.2Ni 0.8O 3−δ, La 0.66Sr0.34Ni0.3Co0.7O3−δ, and La0.8Sr0.2Cu0.15Fe0.85O3−δ) was studied by Tofan et al.591 The effect of carbon dioxide (0.5−10%) and water (1.6 or 2.5%) was evaluated between 327 and 357 and 450−650 °C, respectively. Both CO2 and H2O inhibit NO decomposition, but inhibition by CO2 is significantly stronger. Similar results were obtained by Teraoka et al.595 over La0.8Sr0.2CoO3 and by Liu et al.596 over Ag/La0.6Ce0.4CoO3. Kinetic parameters for the inhibiting effects of CO2 and H2O over the three materials were determined (Table 15), showing that inhibition increases with temperature (Figure 29).591 Inhibition might result from adsorption of CO2 on specific surface oxygen species, thus reducing the number of adsorption sites required for adsorption of NO. 5.1.2.2. Effect of Material Composition. Ishihara et al.581 studied LaBO3 perovskites for NO direct decomposition, B being a transition metal. They also studied substituted manganites for the same purpose, where La was substituted by Ba, Sr, or Ca, and Mn was substituted by Ga or In (Table 16). Regardless of the material composition, no N2O formation was observed, due to the high reaction temperature. The

Figure 29. Arrhenius plots for the inhibition constants kinh CO2 (full lines) and kinh H2O (dotted lines) of the kinetic models representing the inhibition by CO2 and H2O of nitric oxide decomposition over three perovskites (La0.87Sr0.13Mn0.2Ni0.8O3−δ, La0.66Sr0.34Ni0.3Co0.7O3−δ, and La0.8Sr0.2Cu0.15Fe0.85O3−δ).591

Table 16. NO Direct Decomposition into N2 and O2 over LaMO3 (M = Cr, Fe, Cu, Co, Mn) Catalysta .

catalyst

BET surface area (m2/g)

of NO

into N2

into O2

into N2O

into NO2b

La0.7Ba0.3Cr0.8Ga0.2O3 La0.7Ba0.3Fe0.8Ga0.2O3 La0.7Ba0.3Cu0.8Ga0.2O3 La0.7Ba0.3Co0.8Ga0.2O3 La0.7Ba0.3Mn0.8Ga0.2O3 La0.7Sr0.3Mn0.8In0.2O3 La0.7Ca0.3Mn0.8In0.2O3 La0.7Ba0.3Mn0.8In0.2O3 La0.7Ba0.3Mn0.8Al0.2O3 In2O3

1.9 3.1 1.0 0.9 7.5 4.2 0.8 8.0 7.0 4.5

3.8 48.9 58.4 78.6 92.3 37.5 24.4 96.8 89.7 4.2

4.1 31.3 40.7 54.1 60.2 21.8 12.8 63.7 59.7 1.8

0.1 1.5 3.9 8.9 22.8 1.3 0.5 24.4 14.5 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 17.6 17.7 24.5 32.1 15.7 12.6 33.1 30.0 2.4

Conditions: 15 min of reaction, T = 800 °C, W/F = 3.0 g s cm−3.581. Estimated on material balance of nitrogen. Catalyst, 1 g; 1.0% NO in He; W/F = 3.0 g s cm−3. temperature, 800 °C.

a b

activity of NO decomposition decreased in the order Mn>Co>Cu>Fe ≫ Cr. Among the examined perovskites, Bacontaining formulations exhibit both higher NO conversion and N2 yield, probably due to the higher surface area displayed by the materials. The best catalytic performances were then obtained for La0.7Ba0.3Mn0.8Ga0.2O3 and La0.7Ba0.3Mn0.8In0.2O3. Indium would thus be the best substituting element for Mn. The improved activity for NO decomposition, displayed by the 10331


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presence of such sites can explain the improved activity, measured over Ce-containing materials.

In-containing materials, results from (i) the weakening of oxygen adsorption and (ii) the increase in NO adsorption capacity. In 2007, Iwakuni et al.590 reported the effect of dopants in BaMnO3 perovskites. They reported that the activity in NO decomposition increased in the order Mg > Zr > Fe > Ni > Sn > Ta for the Mn-substituting element and La > Pr for the Basubstituting element.590 The highest N2 yield was then achieved by La substitution in the Ba site and Mg substitution in the Mn site (Ba0.8La0.2Mn0.8Mg0.2O3). By cofeeding H2, N2 yield was greatly improved around 500 °C. This suggests that the surface of the catalyst was covered by strongly adsorbed oxygen, and removal of surface oxygen followed by adsorption of NO might be the most important step for the NO decomposition reaction. La2−xBaxNiO4 (x ≤ 1.2) catalysts were also studied by Zhu et al.597 Materials are composed of BaCO3 and perovskite. BaCO3 phase influences the catalytic activity, since it acts as a NOx storage component. The effect of barium was examined by Hong et al.598 Although Ce−Fe mixed oxides (Ce−Fe(x)) exhibited low activities for direct decomposition of NO into N2 and O2, Ba catalysts supported on these mixed oxides are more active. The highest NO conversion was achieved with 3 wt % BaO/Ce−Fe(0.02). SrFeO3 was also studied for direct decomposition of NO, and its activity was shown to be strongly affected by the dopant (Sr, Mg). 589 The highest N 2 yield was achieved on SrFe0.7Mg0.3O3. When SrFe0.7Mg0.3O3 was loaded with Pt, the N2 yield was further improved. On this catalyst, the yields of N2 and O2 at 850 °C were 56% and 35%, respectively. Zhu et al. published several papers dealing with direct NO decomposition over LaSrNiO4-599 or La2CuO4600-based perovskites. The formulation has been modified, and various kinds of metal substitutions were proposed, i.e., La1−xCexSrNiO4601−603 or La0.4Sr0.6Mn0.8Ni0.2O3.588,604 Substitutions were also investigated over La 2 CuO 4 , Th (La 2−x Th x CuO 4 ), 605 and Sr (La2−xSrxCuO4).600,606 The effect of Sr substitution on the redox properties and catalytic activity of La2−xSrxNiO4 (x = 0.0−1.2)599 indicates that an optimal activity is obtained at x = 0.6. Characterization revealed that this material possesses suitable abilities for both oxidation and reduction, which facilitates the processes of oxygen desorption and NO adsorption. At a temperature below 700 °C, oxygen desorption is the rate-determining step of NO decomposition. These results are similar to those obtained by Zhao et al.,607 who proposed that direct decomposition of NO over perovskite and related mixed oxides follows a redox mechanism. The lower valence metal ions (Ni2+) and disordered oxygen vacancies seem to be the active sites. The oxygen vacancies favor adsorption and activation of NO molecules and increase mobility of lattice oxygen, which is beneficial to regeneration of the active sites. With the increase of the reaction temperature (T > 700 °C), the oxygen desorption is favored and the activity of NO on the reduced site (NO + VO + Ni2+ → NO−−Ni3+) becomes the rate-determining step. In La1−xCexSrNiO4 (0 ≤ x ≤ 0.3) perovskites,601,602 La substitution by Ce allows improving the catalytic activity (with or without oxygen in the feed). At 850 °C, the activity of La0.7Ce0.3SrNiO4 reaches 72% of conversion in the absence of oxygen. Conversion drops however to 42% in the presence of 6.0% O2.602 Active sites are supposed to be located on B-site cations with low oxidation state and oxygen vacancy, described as Ni2+−□−Ni3+. In La1−xCexSrNiO4 materials (0 < x ≤ 0.3), new active sites, Ce3+−O−(Ni2+−□−Ni3+), might be formed (Figure 30). The

Figure 30. Active site description in La1−xCexSrNiO4 active for NO decomposition: (■) oxygen vacancy.602

Compared with La 0.7 Ce 0.3 SrNiO 4 bulk perovskite, 602 La1−xCexSrNiO4/MgO-supported material603 does not present significantly higher activity. Another way to enhance the redox properties of LaSrNiO4 materials is incorporation of Mn in Ni sites.604 Due to the dependence of catalytic activity to oxygen mobility, the activity for NO decomposition significantly increases with the substitution of Ni by Mn but decreases when Mn is completely replaced by Ni. The optimal activity is obtained at x = 0.8 in LaSrMn1−xNixO4. NO decomposition was also studied over La2−xThxCuO4.605 A close correlation between the valence of copper and the activity was observed, the mean Cu valence being decreased with the increase in Th substitution degree. The low valence (Cu+) is more active than high valance (Cu2+), due to the easier oxidation of Cu+ in the redox cycle of Cu+ ↔ Cu2+, resulting from adsorption of NO. Similar results were obtained over La2−xSrxCuO4.600,606 By correlating the solid-state properties and the activity of NO decomposition, it can be concluded that the presence of T* phase (Figure 26) enhances the catalytic activity of La2−xSrxCuO4. The activity of La0.6Ce0.4CoO3 can be strongly enhanced by addition of Ag up to 1 wt % (350−450 °C, NO = 1000 ppm, O2 = 8%, GHSV = 30 000 h−1).596 The beneficial effect of silver is assigned to the presence of Ag+ species on the surface of La0.6Ce0.4CoO3, which partially occupy La3+ sites. Doping by Ag then induces (i) oxidation of Co2+ to Co3+ and (ii) formation of Schottky defects, two parameters favorable to the adsorption− activation of NO molecule. A different approach for NO decomposition consists of the use of the electrocatalytic cell in which perovskite-type oxide can also be used. Hwang et al.608,609 proposed a new type of electrochemical cell, which consists of YSZ and perovskite-type electrodes, LaCoO3 or LaMnO3. The electrocatalytic cell consists of porous electrodes and a solid electrolyte. NOx is decomposed to N2 and O2 at the cathode, and O2 diffuses through the electrolyte to the anode by applying an electrical field. In the absence of oxygen, the LaCoO3 powder was active both in decomposition of NO and in reduction of NO by C2H4. The catalytic activity is completely depressed in the presence of 2% of O2. 5.2. NOx Reduction

The two main processes, namely, NOx storage reduction (NSR) and selective catalytic reduction (SCR), are proposed for NOx abatement in excess of oxygen (lean conditions). The NSR process has largely been studied since 1990s.610−614 Model NOx-trap catalysts usually contain a noble metal (Pt), allowing NO oxidation into NO2, and a basic phase (Ba oxide/carbonate), trapping NO2 as nitrite/ 10332


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formation of different defect structures, in regard to the nature of the B-site cation substituted. Perovskites with wide compositions were investigated for H2SCR reactions. For instance, Rodriguez et al.628 described the synthesis, chemical characteristics, and H2-SCR activity of a palladium-modified BaTiO3 perovskite. Materials were treated under oxidizing and reducing atmosphere. Tanaka and Misono629 showed that such treatments can induce precious metal ions reverse diffusion in and out of the crystal lattice. Then, under oxidizing conditions, almost pure BaTiO3 phase is detected, while under reduced conditions the presence of metallic palladium is identified. Metallic palladium disappears upon reoxidation (static air, 600 °C). However, only a few Pd ions seem to be incorporated into the lattice, the remaining being dispersed as surface PdO over BaTi0.95Pd0.05O3. H2-SCR experiments (conducted with CO2 and H2O added to the feed) show that 85−90% of NOx conversion is reached between 150 and 210 °C with BaTi0.95Pd0.05O3 catalyst, calcined at 500 °C. NOx conversion drops to 60−70% for similar temperatures over samples, calcined at 900 °C, due to alteration of material textural properties. Slightly higher NOx conversions are obtained for Pd-supported on BaTiO3. However, the N2 selectivity is not total, and a part of NOx is converted into N2O. Then palladium incorporation seems to be more appropriate to the NOx reduction reaction under lean conditions. Palladium-containing perovskites were also studied by Twagirashema et al.,630 Dhainaut et al.,631 and Reneme et al.632 Catalytic tests were performed under prereduced samples. Thermal reduction induces a partial reduction of LaCoO3, accompanied with formation of Co0 external species over La2O3.630 Bulk palladium external phase was not detected, suggesting a high dispersion of metallic particle. NO conversion, in excess of O2, is very low over bulk LaCoO3 perovskite (below 5%) over a large temperature range (255− 500 °C). Over the Pd-containing material, NO conversion is complete at 148 °C, with a selectivity of about 63% toward N2, in the absence of CO2 and H2O. However, NO conversion sharply decreases beyond this temperature to reach 25% with a N2 selectivity of about 75% at 207 °C. The low DeNOx activity of LaCoO3 is explained by formation of stable nitrates, which can be removed from the surface in the presence of Pd, leading to Pd−LaCoO3 material being largely more active. Perovskite phase was also evidenced to prevent platinum sintering, leading to more stable catalysts.633 Stathopoulos et al.634 studied the catalytic properties of La0.8Sr0.2FeO3−x, prepared by two different methods, namely, ceramic and surfactant methods (cetyl trimethylammonium bromide, CTAB), leading to significantly different textural properties being obtained for both materials (surfactant method, 33 m2 g−1; ceramic, 3 m2 g−1). The authors suggested that the crystal phase, which is able to adsorb large amounts of oxygen, is SrFeO3−x, and the enhancement of oxygen adsorption can be correlated with the Fe5+ content in SrFeO3−x phase, only detected for the sample prepared with the ceramic method (eqs 56 and 57)

nitrate intermediates. Both precious metals and storage phases are usually supported on a modified alumina substrate.615 Frequent components are rhodium, for its positive effect on NOx reduction into N2 in stoichiometric/rich media, and cerium-based oxides, for their redox behavior, NOx storage capacity, and sulfur resistance.616,617 Among other possible basic storage phases, potassium-based oxides are the most frequentlyly studied.618,619 The NSR catalyst operates in fast lean/rich transients. During the lean step of approximately 1 min, the gas phase is constituted by the standard exhaust gas from the lean burn engine. NO is then oxidized into NO2 over the precious metals and trapped as nitrite/nitrate on the basic components of the catalyst. The “saturated” trap is then regenerated during short incursions in rich media for a few seconds in order to reduce the stored NOx into N2 over the precious metals. In fact, the rich cycles are generated by injecting pulses of fuel, immediately transformed into HC, CO, and H2 on a precatalyst (usually a diesel oxidation catalyst implemented before the NSR system). These rich pulses induce exothermic reactions which favor nitrate desorption and its reduction into nitrogen. Selective catalytic reduction (SCR) is described as the most promising way, due to the large choice of reducers like hydrocarbons, urea, ammonia, hydrogen, or alcohol. In SCR, NOx are continuously reduced. Historically, use of hydrocarbons for selective catalytic reduction of NO (HC-SCR) in oxygen-rich atmospheres620−622 has been studied since the first reports by Iwamoto et al.623 and Held et al.,624 following the patents of Volkswagen625 and Toyota.626 H2 and CO can also act as effective reducers for SCR. Perovskite and related oxides have been studied for NOx abatement from automotive exhaust gas for many years, in part because of their ability to incorporate and combine many chemical elements, like precious metal, or to accommodate B-site cation substitution. In the following section, NSR and SCR DeNOx efficiency of perovskite is presented in regard of the nature of the reducer (CH4, CO, H2, NH3, ...) and gas mixture composition. A section is also included reviewing the NOx reduction in stoichiometric mixtures, related to the three-way catalysis (TWC). 5.2.1. NOx Reduction in Continuous Processes (SCR and TWC). 5.2.1.1. Lean SCR. 5.2.1.1.1. H2-SCR. In 1975, Voorhoeve et al.627 proposed that the H2-SCR mechanism includes reduction of the catalyst. The following mechanism was proposed (eqs 50−55) NO + □ → NOA

(50)

NOA + NO → N2O + OA

(51)

2NOA → 2NA + O2

(52)

NOA + NA → N2O + □

(53)

2NOA → N2 + 2OA

(54)

2OA → O2 + 2□

(55)

where □ represents oxygen vacancy and the subscript A indicates adsorbed species. The authors suggested that reaction depicted in eq 51 is favored at low temperature, while the reaction depicted in eq 54 is favored at higher temperature, when active sites are abundant and the concentration of NOA is lower. Treatment in oxygen-rich or inert atmosphere induces significant modification in catalytic behavior, assigned to

2(SrFe5 +O3.5□■□SrFe5 +O3.5) ↔ 1/2O2 + 2(SrFe5 +O3.5□■□SrFe 4 +O3□)

(56)

or 10333


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Figure 31. NO conversion to N2 on powder La0.8Sr0.2Fe0.9Pd0.1O3: (A) 1000 ppmv NO, 4000 ppmv H2, He balance and (B) 1000 ppmv NO, 10.000 ppmv H2, 5% O2, He balance.108

reaction in lean condition (i.e., in excess of O2), CH4, C3H8, and C3H6 are the most studied reducers. 5.2.1.1.2.1. CH4-SCR. Catalytic reduction of NO, using CH4 as reducer, was widely studied over perovskite.591,634−637 In CH4-SCR, it is assumed that methane reacts with NO and O2, according to the following competitive reactions (eqs 58−60)

2(SrFe5 +O3.5□■□SrFe5 +O3.5) ↔ 1/2O2 + 2(SrFe5 +O3.5□■□SrFe5 +O3□/e−) (57) −

where the symbols □■□ and □/e indicate a kind of electronic ‘‘bridge’’ between the two SrFe5+O3.5 sites and an oxygen vacancy with a trapped electron (F-center), respectively. NO conversion, on perovskite material prepared by ceramic method, is slightly higher at low temperature (20% at 250 °C) but tends to a similar maximal value (60%, in the 350−475 °C) regardless of the synthesis method. On the other hand, the rate in N2 production is clearly different depending on the perovskite synthesis route. N2 production was enhanced for La0.8Sr0.2FeO3−x, prepared by the ceramic method. For instance, at 375 °C, a maximum N2 production rate of 56.4 × 10−3 μmol g−1 s−1, corresponding to NO conversion of 29% and N2 selectivity of 94%, is measured for this sample. B-site substitution in perovskite-type oxides is essential in material design.628,630 Furfori et al.108 prepared A- and Bsubstituted ferrites (LaFeO3, La0.8Sr0.2FeO3, Pd/La0.8Sr0.2FeO3, La0.8Sr0.2Fe0.9Pd0.1O3, La0.7Sr0.2Ce0.1FeO3, Pd/ La0.7Sr0.2Ce0.1FeO3, La0.7Sr0.2Ce0.1Fe0.9Pd0.1O3) by solution combustion synthesis. Specific surface areas between 3.7 and 18.8 m2/g were obtained, and all materials were evaluated in H2-SCR. Results obtained for La0.8Sr0.2Fe0.9Pd0.1O3, the most active material, are presented Figure 31 (in the absence (A) or presence (B) of 5% O2 in the feed gas).108 NO conversion to N2 in the absence of O2 (Figure 31A) shifts to lower temperatures when the space velocity decreases and reaches 100% at 150, 175, and 200 °C for space velocities of 20 000, 30 000, and 40 000 h−1, respectively. Material activity however significantly decreases in the presence of O2: maximal conversion of 75% at 130 °C (GHSV = 20 000 h−1) is obtained. O2 in the feed also induces N2O and NO2 formation. The authors proposed that conversion of NO to N2 depends on (1) kinetic limitations at low temperatures and (2) formation of N2O and NO2 at high temperatures. Over Pd-containing materials, a change in the Fe oxidation state is supposed to be promoted by introduction of Pd, in favor of the NO and H2 chemisorption capacities. Unfortunately, the presence of O2 inhibits oxygen vacancy formation, needed for the H2-SCR reaction (eqs 50−55). 5.2.1.1.2. Hydrocarbon-Selective Catalytic Reduction (HCSCR). Among possible hydrocarbons involved in the HC-SCR

2NO + CH4 + O2 → N2 + CO2 + 2H 2O

(58)

2NO + O2 → 2NO2

(59)

CH4 + 2O2 → CO2 + 2H 2O

(60)

N2O can also form during reaction according to eqs 61 and 62 2NO + CH4 + (3/2)O2 → N2O + CO2 + 2H 2O

(61)

2NO → N2O + 1/2O2

(62)

635

Belessi et al. used La1−x−ySrxCeyFeO3 perovskites for the NO + CH4 + O2 reaction. La0.5Ce0.5FeO3 catalyst is active in NO decomposition (Figure 32), but a noticeable enhancement

Figure 32. Temperature profiles of the rate of N2 formation during NO/He, NO/O2/He, and CH4/NO/O2 “lean-NOx” reactions on La0.5Ce0.5FeO3. CH4 = 0.67 mol %, NO = 0.5 mol %, O2 = 5 mol %, W = 0.5 g, GHSV = 12 000 h−1.635

in NO conversion is achieved by substitution of La by divalent Sr and tetravalent Ce. All catalysts exhibit maximum reaction rates in the 370−400 °C temperature range. The highest reaction rate of about 25% of NO conversion and 93% N2 selectivity was obtained at 400 °C over La0.5Sr0.2Ce0.3FeO3 catalyst. In addition, the presence of 5% O2 in the feed significantly promotes the NO decomposition reaction. Finally, 10334


Chemical Reviews

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Figure 33. Illustration of NO interaction with the mixed-valence SrFeO3−x active sites. Route I: CH4/NO/O2 reaction (Oa, lattice oxygen; Ob, mobile surface oxygen; Oc, high-valence Fe-associated oxygen).634

addition of CH4 in the NO/O2/He feed further enhances the reaction rate. Stathopoulos et al.634 reported the NO/CH4/O2 lean deNOx activities of La0.8Sr0.2FeO3−δ, prepared by ceramic and surfactant methods. At 350 °C, La0.8Sr0.2FeO3−x obtained by the surfactant route exhibited an increase of NO conversion rate by 50% and N2 formation rate by 30%. It was suggested that the SrFeO3−x phase may contribute to an easier oxidation of adsorbed NO to NO2 through mobile surface oxygen atoms. Compared to the adsorbed NO species, the NO2 species are more easily reduced by CH4, as illustrated by Route 1 in Figure 33. The authors suggested that the main parameters controlling catalytic behavior are (i) mobility of surface lattice oxygen, (ii) concentration of oxygen-deficient sites, and (iii) mixed valences of metal cations present in La0.8Sr0.2FeO3−x. 5.2.1.1.2.2. C3H8-SCR. Wu et al.638 studied the NO-selective reduction by propane over La1−xSrxMnO3 (x = 0, 0.1, 0.3, 0.5, 0.7). NO reduction rate is significantly enhanced by Sr addition, the best formulation being La0.7Sr0.3MnO3, calcined at 900 °C. Catalytic behavior is explained by the theory of oxygen vacancies and cationic defects. At low Sr substitution degrees, adjacent O2− anions on the dodecahedron sites become more reactive, possibly leaving a vacancy. If oxygen vacancy is located at the surface of the catalyst, some gas molecules, such as NO, will be adsorbed and activated, contributing to a higher NO reduction rate over Sr-containing materials. Finally, it was demonstrated that C3H8 can act as the reducer for NO reduction below 400 °C. Unfortunately, NO reduction is severely inhibited by O2, especially at high temperatures (Figure 34). NO reduction rate reaches 100% at 560 °C under 0.5% O2 but drops to 0% at the same temperature under 10% O2.

Figure 35. NO reduction activity of AII2BIIWO6 double perovskites for the NO−C3H8−O2 reaction at 600 °C. Flow composition: NO, 0.44%; C3H8, 0.29%; O2, 4.4%; He balance.639

for catalysts with BII = Ca, Mg, and Ni. As it can be seen for AII2CaWO6 and AII2MgWO6, the oxides with Sr in the AII position show higher NO conversion into N2, compared to those with AII = Ba. Then total NO reduction activity is mainly governed by BII cation properties. The standard enthalpy of formation of metal oxide per oxygen atom (−ΔHf°/O-atom) can be used as a parameter of metal−oxygen bond strength. On simple metal-oxide materials it is reported that the catalytic activity for complete oxidation of propylene and the amount of desorbed oxygen increase with the decrease in −ΔHf°/O-atom. Wei et al.639 then proposed a relationship between the total NO conversion and −ΔHf°/O-atom, suggesting that the strength of the BII−O bond or the redox property of the BII ion is of primary importance for the NO reduction activity in the NO−C3H8−O2 reaction. 5.2.1.1.2.3. C3H6-SCR. Compared to propane, much more efforts have been made to improve NO removal using propylene as reducer. At the end of the 1990s, data related to NO + C3H6 + O2 reaction over perovskites showed limited catalytic performance, with a maximal NO conversion to N2 limited at 0.35. On the other hand, a continuous increase in activity is observed with addition of Fe in the La0.6Sr0.4Fe1−yMnyO3±δ structure. Substitution of iron by manganese allows increasing material activity. Since manganese-rich perovskites are strong oxidizing materials, this result suggests that the rate-limiting step in the SCR process could be an oxidation step, i.e., oxidation of nitric oxide to nitrogen dioxide or oxidation of propylene. The maximum NO conversion reaches 34.4% at 325 °C. A mechanism was proposed for oxidation of nitric oxide to nitrogen dioxide before reacting with propylene. Besides, it was also proposed that the inhibiting effect of O2 in the NO

The generated −OC3H6• radical (allylic adspecies) can react with the surrounding α-O2 to form CO2 and water (eq 63) At higher temperature, it is established that NO removal occurs through the nitrate route.58 Nitrate species are formed via oxidation of NO by α-oxygen, according to eq 64 (first step). α-Oxygen surface concentration over copper-doped manganite is enhanced due to the vacancies generated upon Cu substitution

Organonitrogen compounds are proposed as reaction intermediates over manganites (eq 65). They are formed by

reaction between adsorbed hydrocarbons and surface nitrate, and this step is considered as the rate-determining step of the whole NO-SCR process. Then formation of 1-nitropropane (eq 65) involves a reaction between the adsorbed species, formed by reaction depicted in eq 63, and the nitrate species (eq 64) Organonitrogen species are highly reactive in the presence of O2642 and converted into isocyanate (R−NCO) through a cyclic intermediate.58 Finally, C2H5NCO reacts with NO to yield N2 and CO2. The participation of oxygen species (especially α-oxygen) can promote oxidation of the ethyl group into CO2 and H2O and accelerate formation of N2 via coupling of nitrogen atoms (eq 66)

The good performance, achieved over Cu-substituted perovskites, can be ascribed to easy formation of nitrate species. Lower DeNOx efficiency is achieved over LaCo0.8Cu0.2O3: N2 yield reaches about 46% at 500 °C (3000 ppm NO, 3000 ppm C3H6, 1% O2 in helium; space velocity of 55 000 h−1).59 Note that the catalytic performance of the unsubstituted sample is poor, with a maximal conversion of 10336


Chemical Reviews

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Figure 36. Maximum conversion of nitric oxide to nitrogen as a function of x in La1−xSrxMnO3+□ and as a function of y in La0.6Sr0.4Fe1−yMnyO3±□.645

Figure 37. Temperature dependence of NO conversion over 1% Ag/ La0.6Ce0.4CoO3 catalyst under different gas feed compositions. NO = 1000 ppm, C3H6 = 1000 ppm, O2 = 8%, GHSV = 30 000 h−1.596

dissociation at high temperature, together with the limited temperature-dependent equilibrium of the NO to NO2 oxidation, can explain the decrease of NO conversion with temperature.645 The effect of supported metal doping on perovskite activity was also studied.596,646,647 For Au on La−Ce−Mn perovskites,647 a beneficial effect of gold is observed. The catalytic activity increases with Au loading, and the maximal conversion reaches 47% at 360−400 °C. Weng et al.646 suggest that the dispersion and uniformity of the Au distribution on perovskite have an important influence. Silver-doped catalysts were also studied by Zhang et al.596,647 Ag simultaneously catalyzes NO reduction and C3H6 oxidation, and its activity is conditioned by the material redox properties. La0.88Ag0.12FeO3, prepared by reactive grinding, completely converts NO at 500 °C (1000 ppm NO, 3000 ppm C3H6, and 1% O2 at a GHSV of 30 000 h−1; C3H6 conversion = 92%). These values are much higher than the NO conversion of 55% and C3H6 conversion of 45%, obtained over a 3 wt % Ag/Al2O3 reference material. An excess of oxygen (10% O2) leads to a total inhibition of NO conversion over this catalyst. The same increase in O2 content facilitated however the NO reduction over Al2O3. Perovskitetype oxide (La0.6Ce0.4CoO3) and its doped Ag derived (1wt %) also showed activity in the C3H6-SCR.596 NOx conversion into N2 of about 63% is obtained at 400 °C over 1 wt % Ag/ La0.6Ce0.4CoO3 (the optimal silver loading). The feed gas composition effect was also investigated, showing that oxygen is necessary for the reaction (Figure 37). Addition of propylene in the NO/O2/He feed inhibits conversion of NO to some degree, although inhibition is much less than that observed by Buciuman et al.644 On the basis of these results and on the similarity between NO decomposition curves and NOx reductions curves, Liu et al.596 postulated that decomposition of NO is the predominant process even in the presence of propylene. 5.2.1.1.2.4. Effect of O2 on Catalyst Deactivation during C3H6-SCR. The effect of O2 in the gas mixture was studied by Zhang et al.,57−59 Zhong et al.,643 and Wiesniewski et al.648 Over LaMn 1 − x Cu x O 3 , 5 8 LaCo 1 − x Cu x O 3 , 5 9 and LaFe1−x(Cu,Pd)xO3,57 gas-phase oxygen acts as a promoter when its concentration is lower than 1000 ppm, oxygen promoting nitrate formation and organonitrogen compounds transformation. Similar observations were made over noblemetal catalysts with a positive effect of O2 over a larger

concentration range.649 At a concentration higher than 3000 ppm, oxygen acts however as an inhibitor due to combustion of C3H6 by O2. Over La0.8Sr0.2Mn0.5Cu0.5O3, Wisniewski et al.648 showed that the NOx reduction rate is slightly increased by the presence of oxygen up to 0.5% O2 (Figure 38). Without O2, 32% of NOx is reduced. The maximal conversion reached 50% at 0.5% O2. At higher O2 concentrations, NOx conversion stabilized at 38%. On the other hand, Zhang et al.57 reported a maximum N2 yield with 300 ppm O2, but NOx conversion to N2 dramatically decreased when O2 content in the feed overtake was 3000 ppm (Figure 38). The effect of O2 was also studied by Zhong et al.643 over La1−xCaxMnO3. The authors reported remarkable deNOx efficiency at 500 °C without O2. Activity is related to the composition of the materials, especially to the valence state of the Mn ion or lanthanum deficiency in the compound (Figure 39). The authors postulated that both the Mn4+ relative content and the lanthanum deficiency affect the catalytic properties of La1−xCaxMnO3. Deactivation and durability of LaFe0.8Cu0.2O3 perovskite under H2O or SO2 was likewise studied by Zhang et al.53,65 A competitive adsorption between water vapor with O2 and NO molecules at anion vacancies occurs over LaFe0.8Cu0.2O3, leading to a decrease in N2 yield from 77% to 51% with or without 10% H2O (3000 ppm NO, 3000 ppm C3H6, 1% O2, 0− 10% H2O).53 H2O deactivation, fully reversible, is more pronounced at low temperatures (250−500 °C). On the basis of the mechanism previously proposed,58 a deactivation mechanism, involving the filling of low-valence surface sites by water, was proposed (eqs 67 and 68).

10337


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Figure 38. (A) Influence of O2 concentration on C3H6-SCR over La0.8Sr0.2Mn0.5Cu0.5O3. Catalyst: perovskite + Al2O3. Reaction conditions: NOx, 1000 ppm; C3H6, 1000 ppm; GHSV = 4065 h−1; T = 375 °C.648 (B) Effect of O2 concentration on C3H6-SCR over LaFe0.8Cu0.2O3. Conditions: GHSV = 50 000 h−1, T = 400 °C, 3000 ppm C3H6, 3000 ppm NO.57

regeneration needs treatment under reducing conditions (5% H2/He). 5.2.1.2. Stoichiometric DeNOx: Three-Way Catalysis. Automotive three-way catalysis (TWC) was developed in order to treat the three main pollutants from gasoline exhaust gas, namely, carbon monoxide (CO), unburned hydrocarbons (HC), and nitrous oxides (NOx, NO + NO2). CO and HC have to be oxidized into CO2 and H2O, while NOx must be reduced into N2. As a consequence, optimal conversions are obtained with stoichiometric oxidizer/reducer gas composition. In spite of large improvements in engine regulation, fluctuations around the stoichiometry cannot be ruled out. Thus, classical catalysts include also components exhibiting high oxygen storage capacity (OSC), such as ceria-based materials.650−652 In the last decades, efforts have been made to develop new catalysts, still having high OSC. Perovskites were then logically proposed for the CO + NO model reaction, representative reaction for the TWC. 5.2.1.2.1. NO + CO Reaction. Most of the studies devoted to the CO−NO reaction over perovskites were performed on Lacontaining solids (partially substituted by Sr and/or Ce), associated with transition metals such as Mn, Cu, Fe, Co, or Ni. Less conventional formulations were also proposed using other lanthanides, such as dysprosium or ytterbium.653 Synthesis protocols strongly affect the resulting specific surface area, which is reported as a key factor for the catalytic activity,654 as summarized in Table 17. High calcination temperatures (800− 1000 °C) lead to low specific surface areas, generally below 10 m2 g−1. Improved preparation methods such as the complexation method (5−15 m2 g−1),655 the microemulsion method655,656 (10−30 m2 g−1),655,656 nanocasting134 (30−50 m2 g−1), or reactive grinding657 (100 m2 g−1) allow achieving a wide range of surface areas after thermal stabilization. 5.2.1.2.1.1. Mechanism and Kinetic Parameters for the NO + CO Reaction. Mechanism and kinetic parameters were mainly studied by Pomonis et al.656,666 and Zhang et al.61 Catalytic reduction of nitric oxide by carbon monoxide on perovskites has been described to proceed according to the following elementary surface reactions (eqs 69−76)667−669

Figure 39. Variation of the Mn4+ ion relative content, lanthanum deficiency, and largest conversion efficiency of La1−xCaxMnO3 for C3H6-SCR. (▲) conversion efficiency (η); (■), lanthanum deficiency (δ); (●) Mn4+ ion relative content, [Mn4+].643

C3H6 adsorption as well as its oxidation were totally inhibited by water vapor, which strongly decreases the concentration of adsorption sites for C3H6 (Fe3+O−) and O2 (Fe2+□). In addition, H2O inhibits generation of organonitrogen compounds by reducing the number of adsorbed nitrate. All these effects contribute to the reduction of the deNOx efficiency. A lower sensitivity to water vapor was obtained for LaCoO3, a material having abundant anion vacancies compared to LaFeO3 and LaMnO3.53 The resistance to H2O deactivation of lanthanum cobaltite can be further improved by a subsequent Cu substitution.53 Finally, the best water resistance is obtained for Pd−perovskite, ascribed to the special properties of palladium in NO adsorption and C3H6 transformation. The poisoning effect of SO2 on LaFe0.8Cu0.2O3 was also evaluated.53 This material showed high activity: nearly complete NO conversion and more than 80% C3H6 conversion at T > 450 °C (conditions: 3000 ppm NO, 3000 ppm C3H6, 1% O2, GHSV = 50 000 h−1). Two distinct poisoning mechanisms, depending on SO2 feed concentration, were proposed.65 At low SO2 concentrations (≤20 ppm), a reversible poisoning takes place, involving chemisorption of gaseous SO2 on anionic vacancies and formation of minor amounts of sulfite and sulfate. At higher SO2 concentrations (up to 80 ppm), the loss in catalytic activity becomes more severe. Large amounts of sulfate form in the bulk, resulting in destruction of the perovskite structure into La2(SO4)3 and Fe2O3 phases. At this step, catalyst 10338

NOgas → NOads

(69)

NOgas → Nads + Oads

(70)

COgas → COads

(71)


Chemical Reviews

Review

Table 17. Catalytic Activity of Perovskite(-like) Samples in NO Reduction by CO surface area (m2 g−1)

sample/preparation

reaction conditions

LaFeO3 solid-state LaFeO3 citric LaFeO3 evap. decomposition LaFeO3 coprecipitation LaMnO3/Au doped with Ce LaMnO3 citric LaMnO3 coprecipitation LaFeO3 ceramic LaFeO3 ceramic LaMnO3 ceramic LaMnO3 reverse microemulsion LaMnO3 bicontinuous microemulsion LaFeO3 reverse microemulsion LaFeO3 bicontinuous microemulsion LnFeO3 (Ln = La, Pr, Er, Tb, ...)

0.1 g/CO:O2:He = 2:1:17/15 mL min−1 0.5 g/1%CO, 20%O2 in He/100 mL min−1 2000 ppmv NO, 2000 ppmv CO in He 0.3 g/1500 ppm NO, 1500 ppm CO, He 0.5 g/1%NO, 10%O2, He 1%CO, 20%O2, He 12 000 mL h−1 g−1 0.3 g/1500 ppm NO, 1500 ppm CO in He 0.2 g/NO:CO:He = 2:2:96/total flow mL min−1 0.2 g/NO:CO:He = 2:2:96/total flow mL min−1 0.2 g/NO:CO:He = 2:2:96/total flow 100 mL min−1 0.2 g/NO:CO:He = 2:2:96/total flow 100 mL min−1 Id

La1−xSrx(Fe3+/Fe4+)O3±δ La0.9Sr0.1FeO3 La0.8Ce0.2FeO3 La1−x−ySrxCey−FeO3 La2CuO4 La1.5Sr0.5CuO4 LaSrCuO4 LaCo1−xCuxO3 LaMn1−xCuxO3

XNO (%)

XCO (%)

22 7.6 2 2 2 24 12

60 31 62 54 82 74

29 49 29 63 57

658 225 659 660 661 208 661 662 663 308 656 656

Id Id

30 14

400 400

82 92

80 70

656 656

0.05 g/NO:CO:He = 5:10:85/total flow 10 mL min−1 0.2 g/NO:CO:He = 2:2:96/total flow 100 mL min−1 0.2 g/NO:CO:He = 2:2:96/total flow 100 mL min−1 0.2 g/NO:CO:He = 2:2:96/total flow 100 mL min−1 0.25 g/NO:CO:He = 2:2:96/total flow 90 mL min−1 0.5 g/NO:CO:He = 0.4:0.4:99.2/total flow 22.5 mL min−1 0.5 g/NO:CO:He = 0.4:0.4:99.2/total flow 22.5 mL min−1 0.5 g/NO:CO:He = 0.4:0.4:99.2/total flow 22.5 mL min−1 0.05 g/NO:CO:He = 0.3:0.3:99.4/total flow 60 mL min−1 0.05 g/NO:CO:He = 0.3:0.3:99.4/total flow 60 mL min−1

10−50

300

10 (Er) 80 (La)

2 51 61 4 2.5

400 300 300 400 400

30−60 50 80 40 1

2.4

400

60

665

2.6

400

96.8

665, 600

22.4−29.2

400

90−94

100

61

40.6−42.6

400

85−90

100

61

(72)

2·Nads → N2

(73)

NOads + Nads → N2Oads

(74)

N2Oads → N2 + Oads

(75)

2Oads → O2

(76)

80 90 40

82 80 80

ref

400 650 480 400 400 600 400 400 400 400 400 400

COgas + Oads → CO2ads

2 6 2.5 10.5

Treaction (°C)

80

653

29 60 60−85

663 654 654 664, 635 665

CO appears to be more weakly adsorbed than NO on the same sites.61,666 Adsorption occurs on anion vacancies through a coordinative bond without electron transfer (eq 78)

Over LaCo1−xCuxO3 and LaMn1−xCuxO3 perovskites, the proposed mechanism involves dissociation of chemisorbed NO (eq 69) to form N2 and/or N2O (eqs 73 and 74). These reactions resulted in surface site oxidation.61 Continuous reduction of the surface occurs by its reaction with CO to produce CO2 (eq 72). Formation of (mono-, di-) nitrosyl and (monodentate, bidentate, bridging) nitrate species were previously observed over LaCoO3,670 LaMnO3,671 and Cusubstituted materials after NO adsorption.61 NO3− species are evidenced by a NO desorption peak at T > 300 °C, associated with O2 desorption. Besides, NO chemisorption results in the negatively charged form NO−.672 The first step of the NO + CO reaction over manganites is61

CO is thereafter oxidized into CO2 via a suprafacial catalytic process involving surface atomic oxygen (O−) (eq 79)

At low temperatures, dissociation of adsorbed NO species, occurring over reduced sites and yielding N2O and N2, was recognized as the rate-determining step for catalytic reduction of NO by CO, with possible dimeric N2O2 species formation as intermediate. Two parallel reactions can then be proposed, resulting in N2 and N2O as products (eqs 80 and 81) The reaction presented in eq 81 dominates at low temperature and involves cleavage of only one N−O bond, 10339


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Figure 40. Ratio of conversion XNO/XCO over LaSrCuO4 and La2CuO4 at different temperatures.600

while it is not the case for La2CuO4, confirming once again the importance of the ionic vacancies density on the catalytic mechanism and activity. Giannakas et al.656 studied the activation energy for CO and NO for reactions at low temperature (LT; T < 300 °C) and high temperature (HT; T > 300 °C) over LaMnO3 and LaFeO3 solids, prepared using the microemulsion method both in the reverse (r) and in the bicontinuous (b) state, as well as the ceramic method (c). Results are reported in Table 18. Eapp is always higher at LT than at HT for both reactants, and Eapp for NO conversion is systematically lower than that of CO. In addition, the values of Eapp for both NO and CO at LT on LaFeO3 solids are higher than the corresponding values measured for the LaMnO3 solids. This is due to the intrinsic higher activity of Mn(III) cations compared with that of Fe(III) species. Thus, for LaMnO3, the fraction of Mn3+−O−Mn4+ pairs seems to be a critical factor for the tested materials. Substituted La1−xSrx(MnIII/MnIV)O3±δ were also studied by Leontiou et al.666 Strontium substitution decreased catalytic activity. Increasing the Sr content leads to a transition from an oxygen-excess state to an oxygen-deficient one. To compensate for the reduced cation charge, vacancies in the perovskite lattice can be formed upon Sr insertion.576 To explain the lower activity of the oxygen-deficient structure, the authors proposed that part of the oxygen, produced by decomposition of NO (NOads → Nads + Oads, eq 70), remains on the material surface, thus inhibiting all reaction steps requiring anionic vacancies. The variation of activation energies E1 and E2 (for routes A and B, respectively) as a function of Sr content is presented in Figure 41.666 For most of the cases, E1 is larger than E2. The authors concluded that E1 corresponds to the NO decomposition reaction, while E2 corresponds to the CO oxidation reaction. Except for x = 0.8 and 0.9, the E1 and E2 values do not show appreciable changes with x (E1 ≈ 120 kJ mol−1; E2 ≈ 110 kJ mol−1). They are higher than those reported by Giannakas et al.656 for LaMnO3 samples but compared satisfactorily with those reported in the literature for Pt−Rh/Al2O3−CeOx TWC model catalysts for equations presented673,674

making this reaction much easier to occur than the reaction presented in eq 80 (two N−O bond cleavages). Regeneration of active surface anionic vacancies occurs through CO oxidation (eq 79). Then the greater the amount of oxygen vacancies, the greater the amount of adsorbed NO is observed. As a consequence, the activity depends largely on the oxygen desorption properties. These reactions can be expressed as two main routes, depending on the reaction temperature Route A: 2NO + 2CO → N2 + 2CO2

(82)

Route B: 2NO + CO → N2O + CO2

(83)

Route B predominates at low temperatures, while route A occurs at higher temperatures. For instance, for LaMnO3 and LaFeO3 solids, route B is observed at temperatures lower than 250 °C. In the 250−350 °C temperature range, both routes coexist, while only route B occurs above 350 °C.61 For La1−xSrxMnO3 perovskites, the change from route B to route A is observed at temperatures above 350 °C. Only route A is observed at T ≥ 450 °C.666 For La0.8Ce0.2B0.4Mn0.6O3 (B = Cu or Ag) perovskite, He et al.655 observed N2O emission mainly in the temperature range of 150−400 °C (route B). At higher temperatures (i.e., T > 400 °C), neither N2O nor O2 was detected, which indicates that route A also occurs at high temperature. Route B is suspected to be controlled by oxygen generated from decomposition of NO, as described in eq 70. For instance, N2O emission is proposed to occur after a first NO adsorption decomposition (eq 84), where □ is a surface oxygen vacancy.655 Cu − □ − Cu + NO → Cu − O − Cu + Nads

(84)

On the other hand, route A appears to be controlled by the reaction depicted in eq 72.655 A similar mechanism is reported by Zhu et al.594 Finally, to elucidate the catalytic route (A or B) dominating at low or high temperature, many works reported evolution of the ratio of NO on CO conversions (XNO/XCO) with temperature (Figure 40).600 When XNO/XCO = 1, route A predominates, and when XNO/XCO = 2, route B predominates. If 1 < XNO/XCO < 2, both routes coexist simultaneously. Comparison between LaSrCuO4 and La2CuO4 in Figure 40 shows that route A predominates above 400 °C for LaSrCuO4,

2NO + 2CO → 2CO2 + N2

Eapp ≈ 87

673

or 70.0 kJ mol

(85)

−1 674

CO + 1/2O2 → CO2

(86) −1 674

Eapp = 112−119 or 95 kJ mol . Comparable apparent activation energies are reported by Belessi et al.,635 for La1−yCeyFeO3 (y = 0.2, 0.3, 0.5), 10340


Chemical Reviews

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Table 18. Catalytic Properties of the Solids LaMnO3 and LaFeO3 Prepared Via the Reverse (r) and Bicontinuous (b) Microemulsion Technique and the Ceramic (c) Method656 sample

BET area (m2 g−1)

Mn3+ (%)/Mn4+ (%)

Eapp(NO,LT) (kJ mol−1)

Eapp(CO,LT) (kJ mol−1)

Eapp(NO,HT) (kJ mol−1)

Eapp(CO,HT) (kJ mol−1)

LaMnO3 (r) LaMnO3 (b) LaMnO3 (c) LaFeO3 (r) LaFeO3 (b) LaFeO3 (c)

24 12 2.4 30 14 2.0

55.3/44.7 79.7/20.3 83.5/16.5

38.2 34.7 42.0 50.0 53.3 56.2

46.1 44.6 60.9 61.3 55.9 70.2

8.3 16.3 16.7 18.4 15.7 10.5

16.2 26.3 27.5 25.1 23.3 16.5

Table 19. Calculated Apparent Activation Energies for N2, N2O, and CO Conversion at Low Temperature over La− Ce−Sr−Fe−O Perovskite Materials635

Figure 41. E1 (■), E2 (□), ΔHads(NO) (●), and ΔHads(CO) (○) parameters calculated from the simulation method as a function of the degree of substitution x. (Inset) Percentage of perovskite phase vs x.666

Eapp(N2) (kJ mol−1)

Eapp(N2O) (kJ mol−1)

Eapp(CO) (kJ mol−1)

La0.8Ce0.2FeO3 La0.7Ce0.3FeO3 La05Ce0.5FeO3 La0.8Sr0.2FeO3 La0.7Sr0.3FeO3 La0.5Sr0.5FeO3 La0.8Sr0.05Ce0.15FeO3 La0.8Sr0.15Ce0.05FeO3 La0.7Sr0.1Ce0.2FeO3 La0.7Sr0.2Ce0.1FeO3 La0.5Sr0.2Ce0.3FeO3 La0.5Sr0.3Ce0.2FeO3

108.5 135.6 125.9 145.1 141.6 161.0 128.5 138.1 135.0 136.5 134.4 109.2

32.9 28.0 41.2 23.2 41.9 49.7 44.6 53.9 51.4 18.3 62.5 38.6

76.8 70.7 88.0 95.1 79.3 93.1 89.7 86.8 79.5 96.5 68.0 73.1

LaCrO3, LaMnO3, LaFeO3, LaCoO3, LaNiO3, La2NiO4, and La2CuO4. For LaMO3 solids (M = Cr, Mn, Co, and Ni), the activity decreased at temperatures above 650 °C. The opposite was observed for LaFeO3 and La2CuO4. Their activity was even more enhanced when they are supported onto magnesium aluminate support. This effect is important for La2CuO4/ MgAl2O4, while for LaFeO3, it remains limited. In spite of different BET surface areas, the following rank of activity at 300 °C is proposed for bulk perovskite: La2CuO4 ≈ LaFeO3 ≫ LaNiO3 ≈ LaMnO3 ≈ LaCoO3 > LaCrO3.675 Tanabe et al.676 also studied the effect of perovskite composition (LaNiO 3 , LaMnO 3 , La 0,7 Sr 0,3 NiO 3 , and La0,7Sr0,3MnO3) for the NO + CO reaction at 400 and 500 °C. La0,7Sr0,3MnO3 presented the highest activity. Partial substitution of lanthanum by strontium increased the NO conversion and N2 yield, as also observed for Sr2+ and Ce4+ double substitution in La0.5SrxCeyFeOz.635,662 Double-substituted solids, (La1−x−ySrxCeyFeO3 with x + y > 0.3 and y > x) were found to be the most active catalysts compared to the single-substituted ones.662 The SrFeO3−x phase in the samples seems to be mainly responsible for the activity increase, and the amount of SrFeO3−x phase is greatly enhanced by the presence of CeO2. A synergetic effect takes place between the two phases of CeO2 and SrFe5+O3−x, whose coexistence resulted in the maximum enhancement of activity, via alternative oxidation− reduction cycles between the two phases. The properties of La0.5SrxCeyFeOz were also evaluated, with a comparable effect of Sr and Ce insertion on the catalytic activity.635 The turnover frequencies (TOFs, s−1) measured at 300 °C over perovskites revealed values comparable to those of Rh/α-Al2O3 catalyst. The catalytic activity over perovskite is then strongly dependent on the structural defects (mainly oxygen vacancies) and redox (mainly transition metal ion couples) properties. Dai et al.677 observed that generation of oxygen vacancies by A-site

La1−xSrxFeO3 (x = 0.2, 0.3, 0.5), and La1−x−ySrxCeyFeO3 (x/y = 0.05/0.15, 0.15/0.05, 0.1/0.2, 0.2/0.1, 0.2/0.3, and 0.3/0.2) materials (Table 18). The main conclusions drawn by the authors are as follows. (i) NO conversion is favored by the increase of the CeO2 content. (ii) For solids without CeO2, NO conversion is favored by the existence of SrFeO3−x phase at low temperatures (280−440 °C) while it decreases at high temperatures (440−560 °C) Again, two reaction pathways, depending on temperature, were observed. At low temperature (280−440 °C), route B predominates with a NO conversion higher than CO conversion. Apparent activation energies for N2 formation are much higher than those for N2O formation for all materials (Table 19), in accordance with the low N2 selectivity measured at low temperature. Apparent activation energies for CO conversion are found to be lower than the corresponding values for NO: reaction pathway for CO2 formation is energetically more favorable than that for N2 formation. At high temperature, conversion of NO becomes close to that of CO: route A predominates.635 No emission of N2O is observed, suggesting that the following equation may also occur on the material surface (eq 87) N2O + CO → N2 + CO2

perovskite composition

(87)

5.2.1.2.1.2. Activity Dependence with Material Composition. Peter et al.675 compared the catalytic performance of 10341


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Figure 42. (A) NO conversion to N2 on powder La0.8Sr0.2Fe0.9Pd0.1O3: 1000 ppmv NO, 4000 ppmv H2, He balance. (B) NO conversion to N2 in NO + H2 reaction over La0.8Sr0.2Fe0.9Pd0.1O3 deposited on a honeycomb cordierite monolith; GHSV = 30 000 h−1: 100 ppmv NO, 1000 ppmv H2, N2 balance.683

vacancies are involved. Properties of ferrites for the NO + H2 reaction in the absence of O2 were reported by Furfori et al.683 (LaFeO3, La0.8Sr0.2FeO3, Pd/La0.8Sr0.2FeO3, La0.8Sr0.2Fe0.9Pd0.1O3, La0.7Sr0.2Ce0.1FeO3, Pd/ La0.7Sr0.2Ce0.1FeO3, and La0.7Sr0.2Ce0.1Fe0.9Pd0.1O3). The catalysts were tested in the 25−350 °C temperature range, and significant activities were measured in the 150−250 °C region. Among these catalysts, La0.8Sr0.2Fe0.9Pd0.1O3 presents promising performance (Figure 42). Complete conversion of NO to N2 was achieved at 150−200 °C, while a maximal conversion of 75% was achieved at 160 °C when oxygen is added to the reaction flow. The effect of La0.8Sr0.2Fe0.9Pd0.1O3 dispersion on monolith on its catalytic activity was also reported, and an appraisal of a deNOx system based on H2 for light-duty diesel engine vehicles was also presented.684 The authors obtained similar activities, with complete NO conversion to N2 in the absence of oxygen. Iwakuni et al.685 studied the effects of the presence of CO2 and H2 on the NO decomposition over BaMnO3-based perovskite. It appears that N2 yield on Ba0.8La0.2Mn0.2Mg0.2O3 decreased from 70% to 30% with addition of 1% CO2, representing a much larger negative effect than that of O2. Simultaneous feeding of H2 as a reducer is efficient for increasing NO conversion, since H2 helps in removal of strongly bounded nitrate species. It is also efficient for increasing the NO decomposition activity in the presence of CO2. 5.2.2. NOx Storage Reduction (NSR). To decrease CO2 emissions of automotive vehicles, a possible way is the use of engines, working in lean conditions, i.e., with an excess of oxygen. Usual three-way catalysts become less efficient to reduce the nitrogen oxides (NOx) under such conditions. To overcome this problem, a possible technological solution is the NOx storage reduction process, succinctly described at the beginning of section 5.2. NSR catalyst works in transient lean/ rich conditions. It usually contains an alkaline storage phase (such as Ba oxide, carbonate) and a precious metal to ensure fast NO oxidation for storage and an efficient reduction. In addition to the high cost of precious metals, one of the drawbacks of the NSR catalysts is the high sensitivity of barium components to sulfur poisoning, which leads to formation of stable sulfates, detrimental to NOx storages capacities.686,687 The possibility of using perovskites for this application mainly focused on the first step of the NSR process, i.e., NOx storage/ adsorption, with special considerations for sulfur poisoning. In

replacements favors activation of O2 and NOx, while modification of B-site ion oxidation states by isovalent ion substitutions in A and/or B sites promotes the redox process on the catalyst, both of them favoring the NO + CO reaction. Duan et al.678,679 showed that in La1−xSrxCoO3 catalytic activity reached a maximum at x = 0.2.678 B-site substitution, especially when Cu substitutes Co or Mn, can also result in improved catalytic properties.61 Pure cobaltite seems more active than pure manganite: 93% N2 yield and 91% CO conversion at 500 °C vs 76% N2 yield and 76% CO conversion at 500 °C, for LaCoO3 and LaMnO3 respectively.657 The catalytic activity of cobaltite is significantly improved over LaCo0.8Cu0.2O3, with 97% N2 yield and nearly complete CO conversion at 450 °C. Similar improvement is obtained for LaFe1−xCuxO3.60 This improvement was ascribed to the increase in anionic vacancies concentration and the increase in lattice oxygen mobility, due to Cu insertion. Comparable conclusions were drawn by Zhu et al.600,665 LaSrCuO4 showed high activity at T = 400 °C, while no activity was observed on La2CuO4 until T > 500 °C. The activity of iron-substituted perovskite was also measured for La1−xSrxFeO3±δ (x = 0, 0.15, 0.3, 0.4, 0.6, 0.7, 0.8, and 0.9).663 Gradual substitution of La by Sr results in formation of Fe4+, stable in the SrFe4+O3 perovskite structure. The more active materials for the NO + CO reaction are those having the lowest amount of Fe4+. The authors proposed that the increase of the Fe4+ content in the material negatively affects NO adsorption, since adsorption involves B-site cations in reduced valence state. Fe-based perovskites substituted by copper and palladium,60 Pd−(La, Sr)2MnO4680 and Cu/Ru-substituted (La,Sr)MO3 (M = Al, Mn, Fe, Co),681 are other kinds of systems developed for the NO + CO reaction. Great performances at low temperature were reported for LaFe0.97Pd0.03O3 (96% NO conversion and 86% CO conversion at 300 °C). For comparison, similar NO conversion (close to 100%) is achieved at 500 °C over LaFeO3 materials.60 The high catalytic activity of the Pd-containing ferrites is attributed to their outstanding redox properties. The beneficial effect of palladium incorporation was also observed by Karita et al.,680 who reported excellent catalytic performance for Pd/La0.2Sr1.2MnO4 (full NO conversion at 300 °C) that is associated with formation of SrPd3O4 phase. 5.2.1.2.2. H2 + NO Reaction. According to the work of Ferri et al.,682 NO reduction by H2 over perovskites involves a comparable process, as described for H2-SCR, in which oxygen 10342


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Figure 43. Possible pathway for NOx trapping−regeneration process over the BaCoO3−y perovskite.533

firmed.694 A significant improvement of the NOx-trap capacity was observed when this phase was present. NOx storage properties of BaFeO3 and BaCeO3 perovskites were also reported.695 These two materials present much higher NSC than BaSnO3, with a maximum NOx storage at 400 °C. BaFeO3 exhibited the highest NSC after calcination at 750 °C (NSC = 115 μmolNOx/g), whereas the optimal calcination temperature for BaCeO3 was 900 °C (NSC = 57 μmolNOx/g). For comparison, a NSC of 340 μmolNOx/g was obtained over a Pt/BaO/Al2O3 model catalyst under comparable conditions.696 However, both BaFeO3 and BaCeO3 perovskites exhibited very limited surface areas (700 °C, necessary to activate methane). The catalysts are mostly supported metal oxides with basic properties such as alkali metals, alkaline-earth metals, and rare-earth metals.724 It is admitted that methane dehydrogenates at the surface of the catalyst, producing methyl radicals, which can react either at the surface of the solid or in gas phase. Oxygen ions are involved in the abstraction of hydrogen atoms from methane. The most basic oxygen species (O2−), strongly 10346


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(2) CH3• radicals form by reaction between adsorbed O2 and gas-phase methane (Rideal model), (3) CH3• radicals are converted to methylperoxy radical (CH3O2•), which can then produce COx, (4) C2H6 is produced by coupling of CH3• radicals through the third-body (M) interaction. The reaction kinetics of oxidative coupling of methane was also investigated by Mokhtari et al.732,733 over SnBaTiO3. The authors showed the influence of operating parameters such as the methane to oxygen ratio, reaction temperature, and contact time. They concluded that the appropriate reaction conditions were temperature range between 750 and 775 °C, CH4/O2 ratio of 2, and a short contact time of GHSV 100 min−1. Under these experimental conditions, the best C2 yield was 28%. Correlation of the experimental kinetic data was performed with six different kinetic models; the parameters were optimized by genetic algorithms. Experimental data obtained were best described by a Langmuir−Hinshelwood mechanism with the assumption of adsorption of methane, ethane, ethylene, carbon monoxide, and oxygen on the same sites. From an Arrhenius plot, the activation energy for coupling of methane was equal to 33.2 kJ mol−1 with a frequency factor k0 of 2.95 10−3 mol g−1 s−1; Pa(m+n) (with m, n = reaction order). To avoid homogeneous formation of carbon oxides, the OCM reaction was performed in a periodic regime where methane and oxygen were in contact with the catalyst alternatively. High selectivity to C2+ hydrocarbons and good stability were achieved with SrCoO3 and AxSr1−xCoO3 (A = Li, Na, K).734 The authors showed that partial substitution of strontium with alkali metals increases the catalytic activity of the cobalt-based perovskite. The best results were obtained with K0.125Na0.125Sr0.75CoO2.5−x, the C2+ yield reaching 20% during pure CH4 pulse (1.5 min) at 800 °C. According to the reaction mechanisms for OCM reaction, many studies were performed using a membrane reactor. The benefit of oxygen transportation through the membrane in the form of oxygen ions, more selective in C2, was clearly evidenced. Many efforts were made to improve the oxygenpermeable membrane reactor, and the number of studies using perovskite material with many different compositions is very large in the literature. For example, the significant contribution around perovskite-type materials for OCM was reviewed by Yang et al.735

bounded lattice oxygen, are responsible for high C2-hydrocarbons selectivity via formation of methyl radicals according to eq 95 CH4 + O2 − → CH3 + OH− + e−

(95)

725

According to Krylov, the other oxygen species, less strongly bounded to lattice and more oxidizing such as O2−, O22−, or O−, are supposed to favor formation of carbon oxides. Lanthanum oxide-based catalysts exhibited high activity in the OCM reaction due to its high basicity, and different M/La2O3 (M = Ca, Sr, Ba, ...) catalysts were widely studied. Modification of lanthanum oxide with strontium favors the catalytic activity due to an increase in oxygen mobility attributed to formation of anionic vacancies.726 Perovskite materials were recognized as suitable catalysts for oxidative coupling of methane since perovskites are characterized by a high mobility of electrons and ions. Use of perovskite in OCM reaction was first reported by Imai et al.727 The authors showed that LaAlO3 exhibited good activity for C2 hydrocarbon formation at 710 °C under high oxygen pressure with a C2 selectivity of 47% at a methane conversion of 25%. Partial substitution of trivalent La or Al by metals with lower oxidation state, in La1−xMxAlO3 (M = Na, K, Ca, Ba), produces oxygen vacancies that increase the catalytic activity and selectivity to C2-hydrocarbons.728 Baidya et al.729 showed that the activity of SrO−Al2O3 mixed oxides, with double-perovskite crystalline phase, increases with the amount of strontium in the catalyst. The ratio of tetrahedrally to octahedrally coordinated alumina (AlIV/AlVI) increases with increasing Sr content, leading to the presence of less ordered phases. In the noncrystalline phase, the presence of oxygen ion vacancies would result from charge balance (AlIV), which can explain the higher activity in the OCM reaction at high Sr/Al ratio. The catalyst with a Sr/Al ratio of 1.25 showed 29% methane conversion with 62% C2 selectivity (810 °C, CH4/O2 = 108/22, GHSV 26000 L kgcat−1 h−1), performances comparable with those of the well-known catalyst for this reaction: 5%Sr/La2O3. The catalytic performance of XBaSrTiO3 oxides (X = Li, Na, Mg) with perovskite structure was investigated by Fakhroueian et al.730 The best results were obtained with Na-doped perovskite with a maximum C2 yield of 24% at 800 °C (CH4/O2/N2 40/20/40, GHSV 6000 h−1). The authors concluded that lower acidity or higher basicity of the catalyst favor the C2 yield. Kinetic models of the OCM reaction over La0.6Sr0.4Co0.8Fe0.2O3−δ perovskite were proposed by Taheri et al.731 An Eley−Rideal mechanism was successfully used to fit the experimental data in which the first step is reversible adsorption of molecular oxygen at the surface of the catalyst (eq 96) followed by reaction of methane with adsorbed oxygen in the second step (eq 97) O2 + S ↔ O2 ·S

(96)

CH4 + O2 ·S → CH3 + O2 H·S

(97)

CH3 + O2 ↔ CH3O2

(98)

CH3O2 → ... → COx

(99)

2CH3 + M → C2H6 + M

6.3. Selective Oxidation of Ethane to Ethylene

Oxidative dehydrogenation of ethane is of significant importance to produce selectively ethylene and could be an alternative to steam cracking of hydrocarbons, which is still the main industrial technology for producing light olefins.736 High yields in olefin were obtained using platinum-based catalysts in autothermal conditions operated at high space velocity.737 Doping of Pt with tin or copper improved the catalytic performance, with a significant ethylene yield up to 55%. However, the high cost of bimetallic systems and the volatility of active phase at the high temperatures required to perform the reaction are considered as significant drawbacks. Use of perovskite oxides to perform the dehydrogenation of ethane to ethylene was first reported by Yi et al.738 The authors showed that ethane conversion reaches 87% with 43% of selectivity to ethylene using the SrFeO3−δ perovskite under the following conditions: temperature 650 °C, C2H6/O2 1/1, and space velocity 7000 mL h−1 g−1. Dai et al.739 investigated the catalytic performance of the La1.6Sr0.4CuO3.852, La1.6Sr0.4CuO3.855F0.143,

(100)

The reaction steps then involve (1) Oxygen molecule adsorbs reversibly and nondissociatively on the active site of the catalyst (redox model), 10347


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metal from groups 10 and 11, supported over an oxide. Noble metals are well known for their high catalytic activities in reforming reactions, but nickel-based catalysts are also widely used due their lower cost. The mechanism of steam reforming can be generalized to all hydrocarbons. It consists of three steps: (1) dissociative adsorption of water onto the support to obtain hydroxyl species, (2) migration of OH groups at the surface of the support and their transfer to the metal site, (3) adsorption and reaction of the hydrocarbon on the metal site. In the case of ethanol steam reforming, the catalyst has to favor C−C bond rupture but not dehydration of ethanol to produce ethylene, which is a source of coke formation. Therefore, selection of the support plays an important role: acidic support has to be avoided or its acidity has to be neutralized by adding basic species. High activity and stability was obtained by Ni et al.744 using a basic catalyst: Ni/La2O3. The good catalytic performance was attributed to formation of lanthanum oxycarbonate species (La2O2CO3), which could react with carbon deposited to prevent catalyst deactivation. Considering carbon dioxide reforming of methane, a similar mechanism can be proposed replacing water by carbon dioxide. It is well admitted that CO2 is activated at the surface of the support while the hydrocarbon is activated at the metallic surface. Using La2O3 as support, the role of the lanthanum oxycarbonate, as intermediate species, was evidenced by Zhang et al.745 Different studies showed that nickel-based catalysts give good performance in terms of hydrocarbon conversion and selectivity to synthesis gas. However, the main problem of reforming reactions is the low resistance of catalyst to carbon deposition, particularly while using nickel-based materials, due to its sintering. A high dispersion of metal species over the support can reduce coke deposition. The classical method of catalyst preparation by impregnation of an aqueous solution of a metal salt followed by drying, calcination, and/or reduction leads generally to metal species, poorly distributed at the surface of the oxide support. In this context, an interesting concept was developed by Shiozaki et al.,746 based on the use of a well-defined structure such as a BaTi1−xNixO3−δ as precursor of the metallic active phase. After the reduction step, a high dispersion of nickel is obtained resulting in high activity and stability for partial oxidation of methane to synthesis gas. Use of perovskites for CO2 reforming of methane was first reported in 1996 by Slagtern et al.747 and Choudhary et al.748 The following paragraph will present the advance in the use of perovskite structures as precursors of metallic phase for reforming reactions.

and La1.6Sr0.4CuO3.856Cl0.126 perovskites for oxidative dehydrogenation of ethane. They concluded that incorporation of halide ions, with similar ionic radii to O2−, promotes the lattice oxygen mobility. The O− oxygen species desorbing below 600 °C favor ethane complete oxidation, whereas, the O2− lattice oxygens desorbing in the 600−700 °C range are responsible for selective ethane dehydrogenation to ethylene. These authors proposed a mechanism based on a redox cycle of Cu3+/Cu2+ and oxygen (eqs 101−109)740 Cu 3 + − O− − Cu 2 + + C2H6 → Cu 2 + − −Cu 2 + + C2H5 + OH−

(101)

Cu 3 + − O2 − − Cu 2 + + C2H5 → Cu 2 + − −Cu 2 + + C2H5O

(102)

Cu 2 + − − Cu 2 + + 1/2O2 → Cu 3 + − O− − Cu 2 +

(103)

Cu 3 + − O− − Cu 2 + → Cu 3 + − O2 − − Cu 2 + + e

(104)

C2H5O → C2H4 + OH−

(105)

C2H5O + OH− → C2H4 + H 2O + O2 −

(106)

Cu 3 + − O2 − − Cu 2 + + C2H6 + e → Cu 2 + − −Cu 2 + + C2H4 + H 2O

(107)

Cu 2 + − −Cu 2 + + O2 − → Cu 3 + − O2 − − Cu 2 + + e (108)

Cu 3 + − O− − Cu 2 + + C2H5O → Cu 2 + − −Cu 2 + + COx + CH4 + H 2O

(109)

In the mechanism proposed above, a certain amount of both O− and active lattice O2− are required for the redox cycle and the consumed O− and O2− are replaced by the oxygen from the gas phase. Donsi et al.741 investigated the oxidative dehydrogenation of ethane at short contact time over LaMnO3-based honeycomb monoliths. They reported a 55% ethylene yield using an ethane/oxygen ratio of 1.5 at 400 °C, which is higher than that reported in the same experimental conditions over Pt-based catalysts. The authors proposed that formation of ethylene occurs through two different parallel mechanisms: one at the surface of the monolith and the other in the gas phase through radical reactions. LaMnO3 was also used as support for Pt, showing a fine dispersion of Pt and its stabilization at the operation temperature.742,743 The strong interaction between the two active phases, Pt and LaMnO3, favors the catalytic activity, resulting in high ethylene yield: 65% preserved during 100 h of reaction at 900 °C.

7.1. Methane Dry Reforming

Following the results obtained by Zhang et al.,749,750 showing good activity and stability of Ni/La2O3 for carbon dioxide reforming of methane, the perovskite LaNiO3 was tested as catalyst precursor by several authors. Since the active phase is the metal Ni0, activation of the oxide precursor was followed by temperature-programmed reduction (TPR). The TPR profile of LaNiO3 (Figure 45) shows two main peaks: the first in the range 350−500 °C and the second one in the range 500−700 °C.751 At low temperature, reduction of Ni3+ to Ni2+ proceeds leading to formation of the brownmillerite La2Ni2O5 structure, early evidenced by Crespin et al.,752 or of La2NiO4, as shown by

7. REFORMING REACTIONS FOR HYDROGEN AND SYNGAS PRODUCTION Catalytic reforming is the process of choice to produce hydrogen and synthesis gas. Steam reforming of methane is the most important large-scale process to industrially produce hydrogen. Since 1990, many research activities have focused on substitution the fossil reactant by biomass-derived oxygenates such as alcohols or bio-oil components. Reforming reactions are endothermic and require high reaction temperatures (greater than 700 °C). The active catalyst is composed of a 10348


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prepared by coprecipitation: 15 nm against 21 and 56 nm for the catalyst prepared respectively by sol−gel method and impregnation. After 24 h of reaction, the best catalytic activity was achieved with the perovskite prepared by coprecipitation, which can be explained by the higher remaining nickel dispersion. In many published works, the starting perovskites are usually well characterized in terms of surface area, surface composition, and reducibility. However, characterization after the reduction step is often missing, while it is during this procedure that the catalyst is generated. The reduction temperature of LaNiO3 to obtain Ni0 and La2O3 commonly used is 700 °C; the average nickel particle size determined from transmission electron microscopy or X-ray diffraction analysis (using the Scherrer formula) was equal to ∼15 nm according to Rivas et al.,754 Gallego et al.,758 and Alvarez et al.759 The surface composition of the catalyst after the reduction treatment was determined by X-ray photoelectron spectroscopy. The La/Ni molar ratio was equal to 2.9, which is much higher than the expected value showing that there is enrichment in lanthanum at the surface of the catalyst.760 The covering of nickel crystallites by LaOx species in Ni/La2O3 catalysts was suggested by different authors.745,761 The performance of LaNiO3 was compared with a lanthanum oxide-supported nickel catalyst (Ni0/La2O3) with various amounts of nickel, prepared by the wet impregnation method. The reaction was performed using a mixture, CH4/CO2/He of 10/10/80 mL min−1 at 700 °C, after reduction of the catalyst at 700 °C under hydrogen.758 After 15 h of reaction the best catalytic activity is obtained with LaNiO3 as catalyst precursor, methane and carbon dioxide conversions reaching, respectively, 65% and 81%. By increasing the nickel loading on the Ni/ La2O3 catalyst from 1% to 17%, CH4 and CO2 conversions increase but remain lower than those obtained for the perovskite-derived catalyst. However, elemental analysis performed after reaction shows that significant carbon deposition occurs over the 17%Ni/La2O3 catalyst. It was attributed to the presence of large nickel particles (with a size up to 100 nm) after the reduction step that favors carbon formation. The higher CO2 conversion compared to CH4 conversion and the molar ratio H2/CO lower than one were attributed to the occurrence of the reverse water−gas shift reaction (RWGS CO2 + H2 → CO + H2O). The ratio H2/CO is determined thermodynamically as shown by Swaan et al.:762 at low CH4 conversion the reaction between CO2 and H2 is favored giving a low H2/CO ratio, whereas at high conversion the amount of hydrogen converted to H2O by the RWGS reaction is rather low, leading to a H2/CO ratio close to 1. After reaction the phases detected by XRD were metallic nickel and the hexagonal lanthanum oxycarbonate phase La2O2CO3. Catalytic tests performed using the perovskite LaNiO3 without reduction prior to the reaction showed no significant differences in CH4 and CO2 conversion with the reduced material. Characterization performed after a few minutes of reaction proved that the perovskite was reduced under the reactant mixture at a reaction temperature of 700 °C.763 Kinetic studies were conducted to confirm the mechanism for dry methane reforming over LaNiO3 as catalyst precursor. A kinetic model including the presence of two distinct active sitesmetallic nickel and La2O3 supportfits very well the experimental data.760 Moradi et al.764 showed that the same mechanism was obtained for LaNiO3 and Ni/La2O3, despite different rate and absorption constants.

Figure 45. TPR profile of the perovskite LaNiO3 calcined under various conditions of temperature and time.751

in situ XRD.753 The second peak is associated with reduction of Ni2+ to Ni0 according to eqs 110 and 111 La 2Ni 2O5 + 2H 2 → La 2O3 + 2Ni0 + 2H 2O

(110)

La 2NiO4 + H 2 → Ni0 + La 2O3 + H 2O

(111)

Starting from LaNiO3, hydrogen consumption, corresponding to the second reduction peak, should be twice the value of the first peak. In some cases, the value was lower than two. It was interpreted by the presence of amorphous NiO phase (not detected by XRD) besides the LaNiO3 perovskite, this NiO phase being reduced to Ni0 simultaneously with the Ni3+ to Ni2+ reduction from the perovskite structure.754 The presence of the NiO phase was detected by Pereniguez et al. using EXAFS.755 In this study, the lanthanum nickelite was prepared by spray pyrolysis. The presence of nickel, not incorporated in the perovskite structure but present as external NiO, could result from the too low calcination temperature used by the authors (600 °C). The authors proposed an approximation of the proportion of the LaNiO3 and NiO phases from the peak intensities obtained by the Fourier transforms of the EXAFS profiles of the Ni K edge.756 They showed that an increase of the calcination temperature at 800 °C under pure oxygen for 24 h leads to complete disappearance of the external NiO phase, proving that formation of a pure perovskite LaNiO3 is possible as soon as the calcination temperature is sufficiently high. A pure perovskite LaNiO3 is much more active for the dry reforming of methane than a mixture NiO−LaNiO3, which is attributed to the presence of a smaller nickel particle size after the reduction step over the pure LaNiO3 phase. The influence of the method of synthesis of the LaNiO3 on its structural, surface, and textural properties is well established.757 However, at the temperature required to perform reforming reactions (>700 °C) under reductive atmosphere, the perovskite is completely transformed into metallic nickel and lanthanum oxide. It was thus important to verify if the synthesis method of the perovskite can affect the morphology of the “real” catalyst and consequently its activity toward reforming reactions. To answer this question, Rivas et al.754 prepared LaNiO3 perovskites by three different methods: coprecipitation, sol−gel, and impregnation, all materials being calcined at 750 °C for 5 h. They showed that the metallic nickel particle size after the reduction step (10% H2/N2 at 700 °C for 2 h) depends on the preparation method. Lower particle size and higher nickel dispersion was obtained with the catalyst 10349


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peak of reduction is shifted to higher temperatures, resulting from a stronger metal−metal interaction.774 A reduction step at 700 °C completely destroyed the perovskite structure, the only phases present being Ni0, Co0, and La2O3. Formation of a Co− Ni alloy was then suggested by Valderrama et al.,775 based on XRD analysis and cell parameter evaluation. Poor activity was evidenced for LaCoO3 in different studies. Computational calculations revealed that the nickel atom cleaves the C−H bond, while cobalt is not able to activate the CH4 molecule. Despite the low activity displayed by LaCoO3, Valderrama et al.775 showed high catalytic activity for LaNi1−xCoxO3 perovskites for all x values lower than 1. However, data were collected at thermodynamic equilibrium. Sierra et al.774 obtained the best catalytic activity with LaNiO3. The presence of cobalt seems to lead to a decrease of the catalytic activity due to Co−Ni alloy formation. Nevertheless, the stability toward carbon deposition was enhanced in the presence of cobalt. Similar results were obtained by De Araujo et al.776 using a Ru associated with nickel in the perovskite structure in LaNi 1−x Ru x O 3 . The most interesting catalyst was LaNi0.8Ru0.2O3, which, in spite of being less active than LaNiO3, showed the highest resistance against carbon deposition due to the presence of Ru, which is known to be less sensitive to carbon deposit than nickel. A low amount of magnesium or rhodium in the structure can increase the catalytic activity and also the stability of the nickelbased perovskite. The solubility of Mg in the perovskite structure is limited. When x > 0.1 in LaNi1−xMgxO3, phases such as NiO and MgO are evidenced by XRD.774 The presence of magnesium modifies the reduction temperatures of nickel. As the amount of Mg increases, the second reduction peak is shifted to higher temperatures. However, at 700 °C, the temperature at which the reduction is performed before reaction, the perovskite phase is completely reduced into Ni0, MgO, and La2O3. In the presence of rhodium, the behavior of the perovskite during TPR is different; the peaks of reduction are shifted toward lower temperatures since Rh3+ reduces to Rh0 at lower temperature than Ni3+ to Ni2+. It is also proposed that Rh0 crystallites favor the reducibility of nickel cations.754 According to the high activity of niobium in oxidation reactions, niobium-based perovskites were prepared by Alvarez et al.759 and tested in the dry reforming of methane. After calcination, the perovskite structure is maintained for x < 0.3 in LaNi1−xNbxO3. With higher niobium content, the presence of LaNbO4 is detected by XRD. The best catalytic performance was obtained for LaNi0.5Nb0.5O3, attributed to the presence of highly dispersed Ni0 particles interacting strongly with acidic niobium species such as NbO4 and Nb2O3. It was previously presented that the nature of the cation in the A site of the structure influences the structural and redox properties and catalytic activity of the final material. Therefore, much research was performed to examine the influence of lanthanum substitution by different cations. Rynkowski et al.777 showed that a small amount of strontium in the perovskite slightly decreases the catalytic activity but improves the stability of the catalyst. XRD analysis revealed that strontium is not completely incorporated in the La1−xSrxNiO3 perovskite structure for x = 0.25 and 0.5, even after a calcination step at 900 °C. The lower activity with high Sr content was explained by the probable coverage of nickel particles by SrCO3 phase. Using La1−xSrxNi0.4Co0.6O3 as catalyst precursor, Valderrama et al. obtained comparable results, the unsubstituted lanthanum perovskite being the more active catalyst.778 The authors

The La2NiO4 precursor, with the perovskite-like K2NiF4 structure, was also used with the target to obtain a better nickel dispersion than with LaNiO3 after reduction under hydrogen, due to the lower amount of nickel in the mixed oxide precursor. High catalytic activity and low carbon deposition was reported by Gao and co-workers765,766 and Li et al.767 Gallego et al.760 obtained higher CH4 and CO2 conversions with La2NiO4 than with LaNiO3, while Guo et al.768 found that LaNiO3 was more active than La2NiO4. Results obtained by Guo et al.768 can however be explained by the too low reduction temperature used (500 °C under hydrogen), which do not allow complete reduction of the perovskite(-like) materials, particularly for La2NiO4. Using LaNiO3 or La2NiO4 under severe reaction conditions (low amount of catalyst, mixture of CH4/CO2 without dilution gas) resulted in carbon deposition through the Boudouard reaction (eq 112) or decomposition of methane (eq 113) after several hours of reaction 2CO = CO2 + C

(112)

CH4 = C + 2H 2

(113)

Methane decomposition is favored at high temperatures and low pressures, whereas the Boudouard reaction is favored at low temperatures and high pressures. However, due to the reaction temperature used to perform dry reforming of methane, from 700 to 900 °C, both reactions can occur. The nature of carbon deposition was characterized by transmission electron microscopy and Raman spectroscopy. Li et al.767 showed the presence of rope-like nanotubes over La2NiO4, with the outer diameter ranging from 15 to 60 nm and with tips capped by nickel particles. Similar multiwalled carbon nanotubes were observed by Sierra et al.;769 the degree of graphitization of carbon nanotubes was indicated by the relative intensity ratio ID/IG from the Raman spectrum. The value obtained using La2NiO4 as starting material (ID/IG = 1.15) indicated that the carbon nanotubes are well graphitized (ID/IG = 0.051 for highly oriented pyrolytic nanotubes and ID/ IG = 3.56 for CNTs prepared by the catalytic method).770 A considerable advantage of using perovskite-type structures is the possibility of total or partial substitution of A and B cations, which modifies the redox properties of the catalyst and consequently its activity. Provendier et al.771 showed that the presence of iron in LaNixFe1−xO3 does not improve its catalytic activity in dry reforming of methane but strongly promotes its stability: no carbon deposition can be observed after a 250 h of reaction at 800 °C. Formation of a nickel−iron alloy, which prevents surface poisoning by carbon deposition through dilution of the active nickel sites, was proposed by the authors.772 De Lima et al.751 showed that LaNi0.8Fe0.2O3, among the substituted catalyst, was the most active one in the dry reforming of methane. The authors showed that for 0.2 < x < 0.7 in LaNixFe1−xO3 part of the perovskite structure was still present as LaFeO3 after 10 h of reaction at 650 °C. Similar results were reported by Kapokova et al.,773 who attributed the high catalytic activity of the substituted materials to the presence of the Ni−Fe alloy released from the perovskite lattice and stabilized on its surface. Co-substituted nickel perovskites were also used for dry reforming of methane, aiming at better dispersing of the active nickel species at the surface of the support. The presence of cobalt in LaNi1−xCoxO3 strongly modifies the reduction temperatures: as the amount of cobalt increases, the second 10350


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to the presence of different segregated phases as soon as the amount of calcium is greater than 0.1 it is difficult to conclude on the influence of calcium. When x = 0.05, calcium is incorporated in the perovskite structure and XPS analysis revealed an important enrichment of calcium at the surface of material, which could be responsible for the higher activity of the substituted lanthanum-based perovskite. The beneficial effect of the substitution of lanthanum by calcium was evidenced also by Goldwasser et al.783 in La1−xCaxRu0.8Ni0.2O3 perovskites. The authors also studied the influence of the substitution of lanthanum by other lanthanides such as samarium and neodymium. The reaction was performed at 700 °C with a WHSV of 24 L/h/g. The best catalytic performance was obtained with samarium-based perovskite, materials for which no significant carbon deposit can be detected. Zhang et al.784 explained the results by strong interaction between nickel and Sm2O3−La2O3 support, which can prevent aggregation of nickel particles, increasing the resistance to coking. Incorporation of alkali metals in the perovskite La1−xMxAl0.7Ni0.3O3 (M = Li, Na, K) was studied by Khalesi et al.785 The best catalytic performance was obtained with sodium substitution (x = 0.25), while the lowest amount of coke formation was found for lithium-substituted materials (x = 0.2), proving that basic promoters improved the stability of nickel-based catalysts. Finally, in the B position Sutthiumporn et al.786 studied the activity of La0.8Sr0.2Ni0.8M0.2O3 (M = Bi, Co, Cr, Cu, and Fe) for the DRM. They showed that the Cu-substituted Ni catalyst exhibited the highest initial activity, due to the highest accessibility of nickel and the availability of lattice oxygen for C−H activation. However, the stability of the material was poor due to rapid nickel sintering. The presence of abundant lattice oxygen species (originating from copper insertion) however resulted in decreased carbon deposition. Perovskite phase was also used as catalyst support for dry reforming of methane.787 BaTiO3 as support for nickel exhibited higher activity than γ-Al2O3, particularly at high reaction temperature (800 °C). Indeed, γ-Al2O3 undergoes phase transition, while BaTiO3 remains unaltered. Moreover, the higher acidity of γ-Al2O3 is not favorable for absorption of carbon dioxide, and consequently, its activity is lower than BaTiO3. α-Al2O3 was also used as support for La2NiO4 in order to lower the amount of lanthanum in the catalyst, which makes it less expensive.788 During calcination of the material, different phases form such as La3Ni2O6.92 with orthorhombic symmetry and LaAl1−xNixO3 with rhombohedral symmetry. Li et al. reported the use of the spinel phase BaAl2O4.789 The authors suggest that the interaction of nickel species with well-defined structures of La−Ni or La−Al−Ni oxides are responsible for the high catalytic activity and the good stability of the catalysts for the DRM. Finally, La2NiO4 was deposited on a 5 A molecular sieve. This support was chosen for its affinity to CO2 and its high surface area (300 m2 g−1).790 After the reduction step under hydrogen at 500 °C the surface area drops drastically to 157 m2 g−1 due to formation of new phases and/or collapse of 5 A crystallites. The size of the nickel particles generated is small, 9 nm, leading to high catalytic activities for CO2/CH4 reforming. However, carbon deposition proceeds, resulting in significant catalyst deactivation. In order to favor the dispersion of the active species, a mesoporous silica material was used by Gonzales et al.791 The mesoporous material was a molecular sieve developed by INTEVEP, which possesses high surface area (1067 m2 g−1). The average pore size was 25 Å, with a bimodal pore

showed that the presence of Sr does not significantly affect the metallic particle size but promotes reduction of the perovskite at lower temperatures producing Ni0 and Co0 active species highly dispersed on the La2O3 and SrO phases. After reaction at 800 °C, XRD analyses show the presence of Ni0, Co0, La2O2CO3, and SrO phases. During the DRM reaction, formation of very small particles of SrCO3 phase, not detected by XRD, was suggested by the authors. They also proposed that SrCO3 is an important intermediate in the reaction according to La 2O3 + SrCO3 → La 2O2 CO3 + SrO

(114)

La 2O3 + CO2 → La 2O2 CO3

(115) 779

Kinetic measurements performed by Pichas et al. showed that the activation energy of carbon dioxide is low in the presence of strontium. It was attributed to reaction of CO2 with La2O3 and SrO to form La2O2CO3 and SrCO3, leading to higher CO2 conversion and suppressing formation of coke. Indeed, it is proposed by different authors755,742 that lanthanum carbonate plays an important role in the removal of deposited carbon according to C − Ni0La 2O3CO3 → 2CO + Ni0 + La 2O3

(116)

Substitution of lanthanum by cerium or praseodymium was also investigated. For the substituted La1−xCexNiO3 perovskites, the XRD patterns reveal the presence of external NiO for 0.2 < x < 0.4, associated with CeO2 oxide. For the La1−xPrxNiO3 series, the perovskite structure is obtained only for x < 0.4.780 The peaks of reduction in the TPR profiles are shifted to lower temperatures as the substitution degree increases. These results are consistent with a lower stability of the perovskite structure under reducing conditions with decreasing ionic radius of the lanthanide element. The highest catalytic activity and stability were obtained with La0.9Pr0.1NiO3 as catalyst precursor. CH4 and CO2 conversions were, respectively, 49% and 55% after 5 h of reaction at 700 °C using a stoichiometric mixture of CH4 and CO2 without diluent gas. No carbon deposit was observed after 100 h of reaction. Such limited poisoning is attributed to the presence of small nickel particles (6 nm after reduction of La0.9Pr0.1NiO3 against 15 nm for LaNiO3). The high resistance to deactivation was also related to the redox properties of the praseodymium oxide Pr2O3 according to the following reactions Pr2O3 + CO2 → 2PrO2 + CO

(117)

4PrO2 + C → 2Pr2O3 + CO2

(118)

The favorable role of praseodymium in the perovskite structure for dry reforming of methane was confirmed by Kapokova et al.773 using LnFe0.7Ni0.3O3 (Ln = La, Pr, Sm) perovskites. The role of cerium oxide in the stability of the catalyst was elucidated by Lima et al.781 according to the comparable process as for praseodymium oxide: CO2 is dissociated into CO and atomic oxygen which regenerate CeO2. CeO2 acts as an active phase for combustion of the carbon deposit, thus minimizing catalyst deactivation. The influence of the partial substitution of lanthanum by calcium in La1−xCaxNiO3 was studied by Lima et al.782 The perovskite structure was the only compound formed with 0 < x < 0.05, whereas for x > 0.1, La2NiO4, NiO, and CaO were observed in addition to the perovskite phase. The best catalytic activity is obtained with x = 0.3 and 0.5, but the catalysts exhibited the highest amount of carbon deposit (lowest stability). According 10351


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distribution: 55% mesoporosity and 45% microporosity. Impregnation of the support with metal precursors leads to a significant decrease in the surface area (300 m2g−1), attributed to the sintering of the support upon thermal treatment and to complete obstruction of the microporous channels during impregnation. Compared to the unsupported perovskite, the stability of mesoporous-supported perovskite is not good; CH4 and CO2 conversion decreases regularly with time on stream due to carbon deposition. The stability of the catalytic activity was strongly improved with incorporation of the perovskite into a mesoporous SBA-15 silica host support.792 Strong interaction between the perovskite oxide and the SBA-15 support was evidenced by the authors, which can be responsible for the improvement of the catalytic performance of the supported perovskite compared to the unsupported one. Wang et al.793 compared LaNiO3/SBA-15 and LaNiO3/MCM-41. They showed that the perovskite was formed inside the channels of mesoporous structure of support. Higher catalytic activity was obtained with LaNiO3/MCM-41 due to higher nickel dispersion, while LaNiO3/SBA-15 exhibited better longterm stability attributed to the stable silica matrix, restricting the agglomeration of nickel species. The authors showed that the hexagonal mesopores of LaNiO3/SBA-15 remain unaltered after 60 h of reaction at 700 °C, while the mesoporous structure in LaNiO3/MCM-41 collapses.

of the lattice oxygen occurred during the reaction, which is considered to play an important role in promoting oxidation of CHx species adsorbed on metallic nickel. It is important to mention that perovskite materials for the water reforming of methane reaction were widely investigated for solid oxide fuel cell (SOFC) applications. It has been shown that lanthanum−chromium-based perovskites are very stable under highly reducing atmosphere used in SOFC and consequently are particularly interesting for such applications. Many substitutions in the A and B sites of the perovskite were performed in order to stabilize the B cation as well as introduce structural defects, e.g., oxygen vacancies suitable for SOFC. The number of publications in this field is huge and could be the subject of a specific review; consequently, SOFC applications are not treated in this review.

7.2. Steam Reforming of Methane

On the basis of thermodynamic data, ethanol is much more reactive than methane.799,800 Production of hydrogen is significant from 277 °C and increases sharply above 377 °C. The temperature range generally used to perform the reaction is comprised between 500 and 800 °C. The reaction is often accompanied by formation of undesirable byproducts such as carbon monoxide, methane, ethylene, acetaldehyde, and acetone depending on the properties of the metal oxide catalyst. The LaNiO3 perovskite was used as catalyst precursor for steam reforming of ethanol by De Lima et al.801 The authors showed that ethanol was completely converted at 500 °C under a H2O/ethanol ratio of 2 during the first 6 h of reaction. Thereafter, ethanol conversion sharply decreases to 22%. Besides hydrogen, the main products formed were CO2, CO, CH4, and a small amount of acetaldehyde. An increase of the H2O/ethanol molar ratio slightly promotes catalyst stability. Increasing the reaction temperature from 500 to 800 °C or including oxygen in the feed strongly limits carbon deposition due to carbon gasification at higher temperature and/or in the presence of oxygen. Autothermal reforming of ethanol (ATRE) over LaNiO3 was investigated by Chen et al.802 The authors showed that the reduced perovskite was much more active than the conventional impregnated Ni/La2O3 catalyst. Perovskitederived catalyst favors dehydrogenation of ethanol, its decomposition, methane reforming, and water−gas shift reactions due to the presence of well-dispersed nickel particles on lanthanum oxide after reduction. Other perovskites were tested for ATRE such as LaCoO3, LaFeO3, and LaMnO3, but these catalysts exhibited lower activity for hydrogen production in spite of their high oxidative activity, breaking of the C−C bond being not favored. Introduction of iron in nickel-based perovskite favors the catalytic performance in the ATR of ethanol.803 The phases present after the reduction step at 650 °C are the perovskite La(Ni,Fe)O3, Ni0, and the FeNi3 alloy. These species are active for conversion of ethanol, transformation of acetaldehyde and methane, as well as water−gas shift reaction. The yield of hydrogen remains stable during 30 h of reaction: 3 mol of H2/mol of ethanol at 600 °C·11 000 h−1.

7.3. Other Molecules, Including Short Alcohols, Glycerol, and Renewable Molecules

7.3.1. Steam Reforming of Ethanol. Ethanol is an interesting source of energy since it can be produced easily from various biomass sources. Catalytic steam reforming of ethanol is an effective way to obtain hydrogen according to eq 119 C2H5OH + 3H 2O → 2CO2 + 6H 2

Steam reforming (SR) of natural gas is the most effective and developed method for production of hydrogen at relatively low cost with high H2/CO ratio required for the process. Supported nickel catalysts are widely used in SR owing to its low cost in comparison with noble metals and in spite the lower activity. Studies performed by Wei and Iglesia and Zeppieri et al. showed that the overall reaction kinetic is governed by the reactivity of the metal toward the C−H bond breaking, and it is well accepted that methane activation is the rate-determining step.794−796 Efforts have been made to propose suitable catalysts with strict control of the nickel metal formation and a strong interaction between nickel and a basic or weakly acidic support. Provendier et al.797 showed that LaNixFe1−xO3 (0 < x < 1) perovskites are efficient catalysts in SRM. The perovskite is treated under hydrogen prior to the reaction. The best catalytic activity was obtained with 0.2 < x < 0.8. At lower nickel content, the perovskite is not reducible and the catalytic activity is low. After reaction with 0.4 < x < 0.9, the crystalline phases present are LaFeO3, La2O3, NiO, and Ni0. The presence of oxidized nickel was explained by hydroxyl groups present on La2O3 acting as a water reservoir for oxidation. A strong interaction between free nickel and the perovskite structure was evidenced. The nickel metal particle size is lower in the presence of iron compared with LaNiO3. This can explain the good aging behavior of the nickel−iron-based perovskite due to limited coke deposition. The optimal H2O/CH4 ratio is equal to 1 in terms of methane conversion and selectivity to CO and H2. However, more important coke formation occurs at this ratio than at a H2O/CH4 ratio of 3. Various Ni/perovskite catalysts were used by Urasaki et al.798 in the SRM with the target to enhance the resistance to coking thanks to the lattice oxygen availability in these oxygen-ion-conducting oxides. Ni/ LaAlO3 and Ni/SrTiO3 showed higher catalytic activity and resistance to coking than conventional catalyst such as Ni/αAl2O3. The authors showed that the consumption and recovery 10352

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Partial substitution of La3+ by Ce3+ in the A site of the perovskite improved the resistance to carbon deposition during oxidative steam reforming of ethanol, no carbon deposition being observed with La0.9Ce0.1NiO3.804 Reduction of the perovskite leads to formation of La2O3- and CeOx-supported Ni0 particles. The improved resistance toward coke deposition in the presence of a small amount of ceria was attributed to the high concentration of oxygen vacancies and to the presence of nickel crystallites of small size. Increasing the ceria content (x > 0.4) leads to formation of NiO and CeO2 phases, besides the perovskite phase. Consequently, larger Ni0 particles are present after the reduction step, and carbon deposition occurs during the reaction. Natile et al.805 prepared La0.6Sr0.4Co1−yFeyO3−δ perovskites and investigated the influence of the cobalt/iron ratio on the catalytic activity. The catalysts were submitted to a reduction step at 873 K, after which the perovskite phase is mainly preserved, but the reduction treatment results in strontium segregation on the surface and formation of Co0 and Fe0 nanoparticles. The highest catalytic activity was obtained with La0.6Sr0.4Co0.5Fe0.5O3−δ, ethanol conversion being complete above 577 °C. This result was attributed to the presence of Co0 nanoparticles, known to be highly active for the steam reforming of ethanol.806 With a Co/Fe ratio higher than 1, an increase in Co0 nanoparticle density in the material could result in their sintering during reaction, leading to lower catalytic activities. Chen et al.807 showed that during the reduction over La0.7Ca0.3Fe0.7Ni0.3O3 at 700 °C nickel ions migrate out from the perovskite structure in the form of metallic nickel. A reoxidation treatment leads to reincorporation of nickel into the perovskite structure according to the following reaction

the energy needed for the reforming. The conventional catalysts used to perform the reaction are composed of noble (Pt, Rh) or non-noble (Ni,Co) metals deposited at the surface of a support (alumina, metal oxides). The catalysts must be resistant to sintering during long-term operation at high temperature (>800 °C). Thus, perovskites were used as an alternative to conventional catalysts due to their carbon tolerance and thermal stability. Lanthanum cobaltite perovskite (LaCoO3) was successfully tested in oxidative reforming of diesel by Fierro and co-workers.810 Diesel conversion reached 100% at 850 °C at a gas hourly space velocity of 20 000 h−1. Conversion was maintained for 60 h on stream, but hydrogen selectivity decreased to about 40% in the first hours of reaction. The structure of the perovskite LaCoO3 was not stable under the reducing conditions present in the reformer. La2O3 and Co0/CoO phases are observed to form. Similar results were obtained by Mawdsley et al. using LaNiO3 as catalyst precursor for the autothermal reforming of 2,2,4-trimethylpentane.811 However, carbon deposition occurs during reaction on metal particles. Carbon decomposition can be lowered by the partial substitution of cobalt by ruthenium.812 Incorporation of Ru into the perovskite induces modification of the structure, from rhombohedral for LaCoO3 to orthorhombic for LaCo1−xRuxO3. The presence of Ru into the perovskite structure leads to a decrease of the crystallite size of the generated La2O3 and Co0 phases, which can contribute to the lower amount of carbon deposit present after reaction over LaCo1−xRuxO3 as catalyst precursor.813 The LaFeO3, LaCrO3, and LaMnO3 perovskites were not structurally modified during autothermal reforming of isooctane, but these oxides were significantly less active than LaNiO3 and LaCoO3. Further modification of the LaFeO3 perovskite with Pd at the B site improved the catalytic performance but increases the problem of the higher cost of the material thus prepared.814 Addition of cerium in the A site of LaFeO3 significantly increased the resistance to carbon deposition. Improved oxygen mobility generated upon Ce insertion favors carbon oxidation on the surface of the catalyst.815 Mundschau et al.816 showed that the La0.5Sr0.5CoxO3 and La0.5Sr0.5FexO3 perovskites can tolerate 15 ppm by mass sulfur in diesel fuel. They attributed this behavior to the strong affinity of lanthanum and strontium for sulfur, which may protect Fe and Co metallic sites from poisoning until the system is saturated with sulfur.

reduction

La 0.7Ca 0.3Fe0.7Ni 0.3O3 XooooooooY Ni°, CaO/La 0.7Ca 0.3 − xFeO3 oxidation

(120)

Under oxidative steam reforming of ethanol conditions it is proposed that the reduction−oxidation cycle explains the particularly high resistance to coke deactivation of the material. Therefore, oxygen in the feed is helpful since it overcomes the problem of nickel sintering by regulating the redox ability of the perovskite oxide. Perovskites La1−xAxFe1−yNiyO3 (A = Ca, Sr) were used as support of nickel.808 Substitution of lanthanum by strontium or calcium leads to enrichment in oxygen vacancies, but insertion of strontium into the perovskite lowers the nickel dispersion, while the influence of Ca on Ni dispersion is limited. Consequently, a poor activity in steam reforming of ethanol is obtained with Sr-based perovskites. In the presence of calcium, it is proposed that the activation temperature for the lattice oxygen diffusion is decreased, and consequently, the bulk oxygens can participate in coke elimination. Very good stability is obtained for the Ca-substituted catalysts. With LaFe1−yNiyO3supported nickel catalysts, highly dispersed Ni particles are also formed, which confers to the materials very good carbon deposition resistance.809 7.3.2. Fuel Reforming. Steam and autothermal reforming of liquid hydrocarbons are of particular interest to produce hydrogen for use in fuel cells. Steam reforming is supposed to generate higher hydrogen yields than partial oxidation, but the reaction requires continuous energy input due to its endothermicity. Consequently, autothermal reforming, which is a combination of steam reforming and partial oxidation, offers advantages of the two reactions as the oxidation provides

7.4. Conclusions

Perovskite-type oxide has been successfully developed as catalyst precursor for CO2 dry reforming of methane. The well-defined structure of the perovskite producing very small particles well dispersed at the surface of a basic support after reduction treatment provides favorable conditions to limit carbon formation and increases the activity and stability of the catalyst. Partial substitution of the A and/or B sites in the perovskite modifies the oxidation state and improves the structural defects; it plays a crucial role for carbon suppression. Significant progress was made to avoid growth of metal particles during time on stream using a mesoporous SBA-15 host support for the perovskite, without alteration of the hexagonal mesopores of LaNiO3/SBA-15.

8. CARBON NANOTUBE SYNTHESIS AS NEW CATALYTIC APPLICATION Synthesis of carbon nanotubes has attracted much interest since their discovery by Iijima.817 Among the different methods 10353


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Figure 46. TEM micrographs of carbon material obtained using LaNiO3 as catalyst precursor and ethanol as carbon source: reaction temperature (A, B) 700 and (C, D) 500 °C.827

resistance performance than that obtained from La2NiO4. Use of LaNiO3 as catalyst precursor instead of a NiO-supported La 2 O 3 catalyst precursor allows achieving higher CH 4 conversion and maintaining catalytic activity for long reaction times. Indeed, encapsulating carbon species are formed with Nisupported La2O3, resulting in rapid deactivation of the catalyst.822 Sierra et al.823 showed that LaNiO3-derived catalyst was still active after 22 h of reaction at 700 °C, production of MWCNTs reaching 17 g per gram of catalyst and hydrogen production being equal to 63.5 L per gram of catalyst. MWCNTs longer than 20 μm were formed with inner diameters ranging from 5 to 16 nm and outer diameters up to 40 nm. The LaNiO3 catalyst precursor was also used for simultaneous production of hydrogen and carbon nanotubes using ethanol as carbon source.824 Raman characterization showed that, at a given temperature, the nature of the carbon source influences the degree of graphitization of the carbon materials that is better when ethanol is used. It is attributed to the etching effect of decomposed OH radical attacking carbon atoms with a dangling bond.825 However, the inner diameter of the CNTs does not depend on the nature of the carbon source but is correlated with the size of the nickel particles.826 The reaction temperature also has a strong effect on the nature of carbon materials produced. As shown in Figure 46, formation of carbon nanofibers is favored at 500 °C due to the easier

employed for CNTs synthesis, catalytic chemical vapor deposition (CCVD) is well developed because it is a simple and cheap method, allowing production in large quantities. Effective catalysts are composed of active metal particles such as iron, cobalt, or nickel dispersed on a support (Al2O3, SiO2, ...). It was reported that the size of metal particles was very important for CNT growth and can determine the diameter of nanotubes. Therefore, perovskite-type metal oxides appear as attractive catalyst precursors due to the possibility to obtain uniform metallic particles after a reduction step. Liang et al. studied formation of multiwalled carbon nanotubes (MWCNTs) using La2 NiO 4 as catalyst precursor and methane818 and carbon monoxide819 as carbon sources. The presence of rope-like CNTs was evidenced by TEM images with diameters between 25 and 35 nm, which are close to the size of the Ni particles generated by the reduction of the La2NiO4 precursor. The relationship between the size of metallic particles and the size of carbon nanotubes was confirmed by Kuras et al. using LaNiO3 perovskite precursors synthesized by different methods.820 Jiang et al.821 investigated the synthesis of CNTs with different compositions of Ni−La− O catalyst precursors; the CNT yield from LaNiO3 was over twice that obtained from La2NiO4 (56.4 gCNT gNi−1 h−1 against 25.5 gCNT gNi−1 h−1, respectively). The CNTs prepared from LaNiO3 presented larger outer diameter and better oxidation 10354


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solubility of carbon in nickel particles at low temperature according to Inoue et al.,827 while MWCNTs are mainly produced at 700 °C. Liu et al.828 showed that it was possible to produce singlewalled carbon nanotubes (SWNTs) by catalytic decomposition of methane at 1010 °C using LaFeO3 as catalyst precursor. Methane was introduced in the catalytic reactor in a mixture with hydrogen, known to favor growth of tubular nanofilaments with a small internal diameter.829 The diameter of the SWNTs was in a narrow range of 0.8−1.8 mm, attributed to the uniform Fe nanoparticles, obtained in situ by reduction of LaFeO3. The perovskite LaCoO3 was also used to produce carbon nanotubes after reduction at 700 °C, resulting in formation of Co0 supported on La2O3.830 A large amount of carbon nanotubes with uniform diameter were produced from methane or acetylene at 700 °C. The presence of helix-shaped carbon nanotubes was evidenced by the author when C2H2 was used, whereas such a structure was not observed when using methane as carbon source. Thiele et al.831 showed that La0.6Sr0.4CoO3 perovskite structure, as support for iron, was preserved up to 675 °C under reducing conditions. On such materials, multiwalled carbon nanotubes were successfully grown with small diameter at 600 °C. As the temperature increases, the CNT diameter becomes larger and bamboo-like CNTs and fibers with diameters up to 150 nm are observed. At temperatures higher than 675 °C, the perovskite structure is destroyed and Co0 acts as a catalyst for CNTs growth. The metal particles form large particles, leading to growth of larger CNTs. Different substitution in the B site of the perovskite (LaFexMoyMnzO3) was investigated by Moura et al.832 The unsupported perovskite precursor leads to formation of MWCNTs from CH4 between 900 and 1100 °C, while the perovskite, dispersed on Al2O3, allows growth of single- and double-walled NTs. The results were explained by formation of stabilized metallic particles at the surface of the alumina support during the reduction step. The optimum composition of the perovskite for SWNT growth was 25 wt % of LaFe0.9Mo0.08Mn0.02O3 on Al2O3.

techniques of characterization helped scientists to understand the reaction mechanisms. Some solutions were then proposed to overcome the main drawbacks of perovskites in catalysis such as their instability in reducing media or in redox cycling conditions and their high sensitivity to poisons. With these new techniques of preparation, the performance of perovskite catalysts approached those of noble-metal catalysts. However, total replacement of noble metals by perovskite is probably not conceivable in the near future, both perovskites and noble metals having their own advantages. Rather, simultaneous use of perovskites and noble metals may offer a considerable technological advantage. In light of the tremendous research data available in the literature, it seems that perovskites could be considered as a suitable “active” support for noble metals, both allowing reducing the noblemetal loadings for the same performance and increasing their life span. Thus, the future of perovskites in catalysis is more likely to take advantage of special interactions between metals and perovskites to develop hybrid catalysts, as stated by Misono.834 Such evolution in catalytic materials is in line with the present considerations, dealing with development of more sustainable catalytic technologies. Finally, a look at the abundant literature published during the past 10 years reveals that redox properties in oxidation reactions remained the main topic for most research groups. Certainly, in addition to consideration of the redox-specific properties of perovskites, future efforts should also be made to take advantage of their acid−base surface properties. Reactions requiring these types of sites, especially organic reactions for application in green chemistry, are largely understudied until now.

9. GENERAL CONCLUSIONS AND PERSPECTIVES Although many doped oxides may have interesting properties,833 the unique structure of perovskites and the versatility of their composition make them remarkable materials for numerous applications. For five decades considerable developments have been made on perovskite-based materials for their applications in heterogeneous catalysis, and good performances have been reported regarding their capabilities to efficiently catalyze oxidation reactions of CO, methane, VOC, etc. Intrinsic properties of perovskites were fully characterized for these applications: oxygen mobility, ion vacancies, redox/mixed valency properties of cations, etc. Adsorption of gas species such as O2, CO, CH4, NO, NO2, H2O, and CO2 were extensively studied, and detailed models as well as precise descriptions of adsorbed species are now available. However, up to 2000, perovskite materials showed relatively modest textural properties with low surface area, typically in the range of 10−20 m2 g−1. During the past 10 years, great efforts were made to prepare high-surface perovskites, specific surface areas being reached up to 100 m2 g−1 or more. The new synthesis techniques provided the possibilities to increase the performance of the perovskites in heterogeneous catalysis. Meanwhile, new sophisticated

Biographies

AUTHOR INFORMATION Corresponding Author

*Phone: +33-(0)5-49-45-39-98. Fax: +33-(0)5-49-45-34-99. Email: [email protected]. Notes

The authors declare no competing financial interest.

Sebastien Royer obtained his Ph.D. degree in Chemical Engineering in 2004 from the Laval University, Quebec. His research focused on the catalytic properties of nanocrystalline perovskites for oxidation reactions. In 2006, he got a position of Associated Professor at LACCO Poitiers (now IC2MP). His research focuses on the design of size- and pore-controlled active oxides for applications in environment and energy. 10355


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treatment. In 2001, he joined the LACCO Laboratory at the University of Poitiers, France, as Assistant Professor. To date, he continues to work on catalysis for exhaust gas treatment but especially for deNOx applications (Selective Catalytic Reduction and NOx Storage Reduction processes). In 2011, he was promoted to Associate Professor.

Daniel Duprez obtained his Ph.D. degree in 1975 from Nancy Polytechnicum. After a 2-year stay at the Elf Research Center at Solaize (near Lyon), he joined the Laboratory of Catalysis in Organic Chemistry of Poitiers University and CNRS (LACCO) in 1978. He developed several projects on the use of isotopic exchange for measuring oxygen and hydrogen mobility on supported metal catalysts with applications in H2 production from biomass resources, H2 purification, oxidation and deNOx reactions, and water purification processes (CWAO). Oxides having redox functions are more and more used alone or as support of metals in these applications. Catherine Batiot-Dupeyrat received her Ph.D. degree in the synthesis of substitutes for CFCs from the University of Poitiers, France, in 1994. She was an Assistant Professor at the University of Poitiers in 1997 and promoted to Professor in 2012. Her current research interests include the catalytic carbon dioxide reforming of methane, reforming of methane using nonthermal plasmas, and VOC removal by photocatalysis coupled with nonthermal plasmas.

Fabien Can obtained his Ph.D. degree in FCC Gasoline Sulfur Reduction additives in 2004 from Caen University, France. In 2007 he joined the Laboratory of Catalysis in Organic Chemistry of Poitiers University and CNRS (LACCO), now IC2MP, for an Assistant Professor position. His main research topic is focusing in NOx abatement for exhaust gas treatment (Selective Catalytic Reduction and NOx Storage Reduction processes).

Said Laassiri received his Master’s degree in Chemistry in 2010 from the University of Poitiers, France. He obtained his Ph.D. degree in Chemistry in 2014 from both Laval University, Canada, and University of Poitiers, France. His main research focuses on the preparation, characterization, and reactivity of perovskite and hexaaluminate mixed oxides. After his Ph.D., he obtained a postdoctoral position in the Institute of Chemistry of Poitiers (IC2MP). He is working on the

Xavier Courtois obtained his Ph.D. degree in 2000 at the University of LyonFrance on three-way catalysis for gasoline exhaust gas

design and testing of catalytic materials for automotive applications. 10356


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Houshang Alamdari received his M.Sc. degree in 1996 and Ph.D. degree in 2000 from Université Laval, Canada. He pursued research activities at the Hydro-Québec Research Institute, Canada, on the synthesis of nanocrystalline materials for hydrogen storage. He held the process director position at Nanox Inc, Canada, and was involved in development and scale up of a production process for nanostructured perovskite-type materials for automotive catalysts. In 2006, he joined Laval University as Professor at the Department of Mining, Metallurgy and Materials Engineering, Université Laval, Canada. He is currently Director of the Regal-Laval Research Center, where his research activities are focused on the aluminium production process.

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