Rare earth oxides and their supported noble metals in application of environmental catalysis

Journal Pre-proof Rare earth oxides and their supported noble metals in application of environmental catalysis Zhiquan Hou, Wenbo Pei, Xing Zhang, Kunfeng Zhang, Yuxi Liu, Jiguang Deng, Lin Jing, Hongxing Dai PII:

S1002-0721(19)30923-8

DOI:

https://doi.org/10.1016/j.jre.2020.01.011

Reference:

JRE 691

To appear in:

Journal of Rare Earths

Received Date: 14 November 2019 Revised Date:

11 January 2020

Accepted Date: 14 January 2020

Please cite this article as: Hou Z, Pei W, Zhang X, Zhang K, Liu Y, Deng J, Jing L, Dai H, Rare earth oxides and their supported noble metals in application of environmental catalysis, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2020.01.011. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.

Rare earth oxides and their supported noble metals in application of ☆

environmental catalysis

Zhiquan Hou, Wenbo Pei, Xing Zhang, Kunfeng Zhang, Yuxi Liu, Jiguang Deng, Lin Jing, Hongxing Dai* Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of Beijing on Regional Air Pollution Control, Key Laboratory of Advanced Functional Materials, Education Ministry of China, Laboratory of Catalysis Chemistry and Nanoscience, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China1

☆

Foundation item: Project supported by National Natural Science Foundation of China (21677004, 21876006, 21622701), National Natural Science Committee of China−Liaoning Provincial People's Government Joint Fund (U1908204), and Foundation on the Creative Research Team Construction Promotion Project of Beijing Municipal Institutions (IDHT20190503). * Corresponding author. E-mail address: [email protected] (H.X. Dai) Tel. No.: +8610-6739-6118; Fax: +86-10-67391983. 1

Abstract： ： Volatile organic compounds (VOCs), methane, carbon monoxide, soot, automotive exhaust, and nitrogen oxides are harmful to the atmosphere and human health. It is urgent to strictly control their emissions. Heterogeneous catalysis is an effective pathway for the removal of these pollutants, and the critical issue is the development of novel and high-performance catalysts. In this review, we briefly summarize the preparation methods, physicochemical properties, catalytic activities, and related reaction mechanisms for the above pollutants removal of the rare earth oxides, mixed rare earth oxide, rare earth oxide-supported noble metal, and mixed rare earth oxide-supported noble metal catalysts that have been investigated by our group and other researchers. It was found that catalytic performance was associated with the factors, such as specific surface area, pore structure, particle size and dispersion, adsorbed oxygen species concentration, reducibility, reactant activation ability or interaction between metal nanoparticles and support. Furthermore, we also envision the development trend of such a topic in future work. Keywords: volatile organic compound; Atmospheric pollutant; Rare earth oxide; Mixed rare earth oxide; Supported noble metal catalyst; Porous mixed rare earth oxide.

2

Volatile organic compounds (VOCs) are one of the main sources of air pollution, which are from industrial activities, such as motor vehicles, ship building, spray paint, chemical and printing, footwear and pharmaceuticals, rubber and plastics processing, kitchen fume, and etc. VOCs include light hydrocarbons, aromatics, alcohols, ketones, ethers, esters, aldehydes, carboxylic acids, amines, and halogen-containing organic substances. Most of VOCs are toxic, malodorous, and flammable and explosive. Therefore, it is extremely urgent to strictly control the emissions of VOCs. The main methods for eliminating VOCs include physical methods (adsorption, absorption, and membrane separation methods) and chemical methods (catalytic oxidation, incineration, plasma destruction, and photocatalysis), among which catalytic oxidation is currently recognized as the most effective pathway and the key issue is the development of high-performance catalysts. To date, catalytic materials used for the effective removal of VOCs are transition metal oxides, rare earth oxides, mixed metal oxides, and their supported noble metals. The former three catalysts are cheap but show low activities at low temperatures, whereas the last one is expensive but exhibits good low-temperature performance. The commonly used transition metal oxides, rare earth oxides, and mixed metal oxides are usually bulk materials that possess low surface areas, resulting in low activities for VOCs combustion. If these bulk materials were made into specific morphologies or porous structures, their catalytic performance for VOCs combustion would be enhanced greatly. In the past years, a large number of single transition metal oxide, rare earth oxide, mixed metal oxide, and their supported noble metal catalysts have been developed, among which the rare earth oxide-based catalysts show good or excellent activities for some reactions. Recently, some authors have made reviews from different viewpoints on the preparation of the above mentioned catalysts and their activities for the removal of VOCs and other pollutants.1−11 Up to now, there have been reports in the literature on the preparation of a number of rare earth oxides and related mixed oxides and their catalytic applications in the environmental remediation, which include rare earth oxides, mixed rare earth oxide, rare earth oxide-supported noble metal, and mixed rare earth oxide-supported noble metal catalysts for the removal of typical VOCs, methane, carbon monoxide, soot, automotive exhaust, and nitrogen oxides. In this review, we put our focus on the brief summarization on the preparation methods, physicochemical properties, catalytic activities, and reaction mechanisms of these materials in the removal of typical VOCs and other pollutants. It should be noted that catalytic activity is usually expressed by the reaction temperature (Tx%) at which the conversion of a pollutant reached a certain level (for example, 10%, 50%, 90%, 99% or 100%). Generally, the reaction temperature over a catalyst increases with a rise in space velocity (SV). Therefore, the Tx% can be used to compare the activities of various catalysts only at the same SV. The specific reaction rate at a certain reaction temperature can also be used to evaluate the catalytic activity, but the activity of each catalyst must be compared at the same reaction temperature. For the supported noble metal catalysts, turnover frequency (TOF) is usually employed to compare the activities of different catalysts, but the same reaction temperature is also required. Table 1 summarizes the activities, specific reaction rates at a certain temperature, and apparent activation energies (Ea) for various reactions at different SVs of the catalysts reported in the literature. 3

1. Rare earth oxide catalysts Catalytic combustion is an efficient and economic technology to eliminate VOCs. Rare earth oxides with variable oxidation states and oxygen vacancies possess good redox properties and reactive adsorbed oxygen species, which render these materials (especially specifically morphological rare earth oxides) to show catalytic activities for the oxidation of typical VOCs, methane, carbon monoxide, automotive exhaust, and soot. For example, Feng et al.12 prepared the 3D hierarchical CeO2 nanospheres with different compositions (self-assembled by nanoparticles (NPs), nanorods, and nanospheres) via a hydrothermal route. The sample self-assembled by nanospheres with a crystal size of 13 nm showed the best catalytic activity for toluene combustion (T90% = 205 oC at SV = 48000 mL/(g·h)), with a lowest apparent activation energy 62.9 kJ/mol. Moreover, this sample was catalytically stable within 50 h of on-stream reaction, and the partial deactivation induced by water vapor introduction was reversible. The authors believed that the good catalytic performance of the nanospheres-self-assembled CeO2 sample was related to its large surface area (129.1 m2/g) and abundant oxygen vacancies (31.3%). Adopting the hydrothermal method, Ye and coworkers13 prepared the nanorod-like, hollow spherical, and cube-like CeO2 catalysts (Fig. 1). The hollow spherical CeO2 performed the best in toluene oxidation (T90% = 207 oC at SV = 48000 mL/(g·h)) and possessed good long-term catalytic stability and reusability, which was due to its large surface area (130.2 m2/g) and more amount of oxygen vacancies (25.8%). López et al.14 fabricated a series of nanostructured rod- and cube-like CeO2 catalysts via a hydrothermal route at different NaOH concentrations and temperatures. The nanorod-like CeO2 derived from 15 mol/L NaOH aqueous solution at 70 oC showed the best toluene oxidation activity (T90% = 260 oC at SV = 520000 mL/(g·h)) and good thermal stability at 250 oC, which was due to its high oxygen vacancy concentration. Jia and coworkers15 synthesized the mesoporous CeO2 catalysts through pyrolysis of the Ce−MOF precursor. The CeO2 sample obtained after pyrolysis at 350 oC exhibited the best activity in catalyzing the oxidation of toluene (T90% = 223 oC at SV = 20000 mL/(g·h)) and good thermal stability and H2O-resistance. The excellent performance of this sample was ascribed to several factors, such as three-dimensional (3D) penetrating mesoporous channels, large surface area, small average grain size, high Ce3+/Ce4+ and Oads/Olatt contents, large oxygen storage capacity, high oxygen vacancy concentration, good low-temperature reducibility, and more active oxygen species and acid sites. Li et al.16 generated the rod- and cube-like nano-CeO2 catalysts by a hydrothermal method at different temperatures. The nanorod-like CeO2 sample obtained hydrothermally at 100 oC showed better ethanol oxidation activity (T90% = 170 oC at SV = 66000 mL/(g·h)). Based on the characterization results, the authors thought that nanorod-like CeO2 sample possessed a higher content of the surface reactive oxygen species, which was favorable for the activation of O2. Murciano et al.17 obtained the different morphological (NPs, rods, and cubes) CeO2 catalysts after the hydrothermal treatment, and observed that the CeO2 NPs with a high surface area and a small crystallite size performed the best for the oxidation of naphthalene (T90% = 190 oC at SV = 25000 4

mL/(g·h)). Huang et al.18 generated the Eu-doped CeO2 nanosheets on the Ti substrate through the facile anodic electrodeposition and annealing in N2 atmosphere. Compared with the CeO2 nanosheets, the 4 wt% Eu/CeO2 nanosheets exhibited a much higher catalytic activity (T90% = 120 o C at SV = 30000 mL/(g·h)) and better stability for HCHO oxidation, which was attributable to the abundance of surface defects, high surface area (28.2 m2/g), large pore volume (0.22 cm3/g), and strong redox ability. Wang et al.19 examined the effect of CeO2 morphology on methanol oxidation at room temperature in a plasma-catalytic system. Among the CeO2 with the rod-like, particle, and cube-like morphologies prepared by a hydrothermal method, the rod-like CeO2 possessed the highest amount of oxygen vacancies, thus exhibiting the best catalytic activity (methanol conversion = 94.1%, CO2 selectivity = 90.1%, and ozone suppression). In situ Raman results proved that the performance of ozone decomposition was related to the amount of oxygen vacancies in CeO2. Surface oxygen vacancies in the rod-like CeO2 could act as the active sites for ozone decomposition, resulting in a more amount of the reactive oxygen species that could effectively oxidize methanol and increase CO2 selectivity in the plasma-catalytic system. Methanol oxidation concurrently occurred in the discharge region and on the catalyst surface in plasma. In the discharge zone, the intermediate products produced by methanol oxidation were formaldehyde and formic acid, and the final product (CO) was mainly generated via the gas-phase plasma degradation of methanol. On the surface of the catalyst, three methoxy species were initially formed, and then transformed into monodentate and bidentate formates prior to conversion to CO2 in the plasma-catalytic system. CO2 was mainly produced by the deep oxidation of methanol over the catalyst (Fig. 2). Feng et al.20 synthesized the three-dimensionally (3D) hierarchical CeO2 nanospheres (PS), nanorods (RS), and small nanospheres (HS) (Fig. 3) via a hydrothermal route without the use of templates, which primarily exposed (111), (110), and (100) crystal planes, respectively. The characterization results showed differences in surface area, pore size, surface composition, oxygen vacancy amount, and redox property of the three materials. The HS performed the best in catalytic oxidation of toluene (T90% = 205 oC at SV = 48,000 mL/(g h) , Ea = 62.9 kJ/mol, and turnover frequency at 180 oC (TOFOV, which was normalized per oxygen vacancy concentration) was 6 times higher than that over RS). The good catalytic performance of the HS was attributed to its large surface area and hierarchical porous structure, which could provide a more amount of surface oxygen vacancies. In addition, the HS sample also showed an excellent catalytic stability in the presence of water vapor. Catalytic oxidation of toluene was investigated over the hierarchical CeO2 catalysts prepared by a hydrothermal-driven assembly method.21 The structure of the CeO2 catalyst was composed of nanowires-self-assembled hierarchical microspheres. The new hierarchical CeO2 catalyst showed much better activity than the nonporous counterpart derived from the traditional hydrothermal method. More than 90% toluene conversion was achieved over the hierarchical CeO2 catalyst even at a temperature as low as 210 oC and a high SV of 60000 mL/(g·h), and the high catalytic activity was attributed to its high surface area and large amount of surface oxygen vacancies. Using a thermal decomposition method with salt nitrates as precursor, Lu and coworkers22 5

prepared the CeO2 catalysts and investigated their catalytic behaviors in the combustion of trichloroethlyene (TCE). The CeO2 catalyst after calcination at 550 oC was the most active (T90% = 205 oC at SV = 15000 mL/(g·h)), which was attributed to its strong surface basicity, high oxygen mobility, and strong oxygen-supplying ability. TCE conversion declined rapidly after 10 h of reaction at 350 oC and such a deactivation was more pronounced at a lower temperature (200 oC). The authors thought that HCl or Cl2 were strongly adsorbed on the surface of the catalyst and blocked the active sites for TCE combustion. Therefore, it was necessary to remove or transfer rapidly the chlorine or chloride species adsorbed on the catalyst surface, so that the active sites could be exposed. Dai et al.23 generated the CeO2 and phosphate-modified CeO2 nanosheets by the aqueous-phase precipitation and incipient wetness impregnation methods, respectively, and studied their catalytic activities for the oxidation of dichloromethane (DCM). It was found that the higher POx content could prohibit generation of the monochloromethane (MCM) byproduct, and the catalyst stability could also be improved by doping an appropriate amount of POx. The POx-CeO2-0.2 (P/Ce molar ratio = 0.2) was the most promising catalyst for DCM oxidation: The T90% was 300 oC and MCM concentration was decreased to 2% (Fig. 4). The authors concluded that the presence of POx (Brønsted acid sites) could effectively lower amount of the medium-strong basic sites (O2−), accelerate the removal of adsorbed chlorine species on the active redox sites, improve the catalyst stability, and reduce the hydrogen transfer rate and the MCM byproduct formation. González-Rovira et al.24 synthesized the CeO2 nanotubes using a porous alumina membrane as template, and investigated their catalytic performance for CO oxidation. The CeO2 nanotubes showed better catalytic activity than the conventional CeO2 powders, and the light-off temperature (189 oC) over the former was much lower than that (300 oC) over the latter. The authors claimed that the excellent catalytic activity of the nanotube-like CeO2 sample was associated with its small crystal size, nanocrystalline morphology (which exposed a more mount of reactive planes (e.g., (100)), and presence of a significant volume of boundary regions between the nanocrystals. Zhang et al.25 fabricated the CeO2 nanotubes with a porous and hollow structure after calcination of carbon nanotubes. The CeO2 sample calcined at 450 oC with a surface area of 95 m2/g exhibited the highest CO oxidation activity (T100% = 300 oC at SV = 16000 mL/(g·h)) due to its large surface area. Liu et al.26 obtained the CeO2 nanorods (ceria-A and ceria-B) with 100−300 nm in length and 12−20 nm in diameter by the hydrothermal method with CeCl3 and Ce(NO3)3 as cerium precursor, respectively. The ceria-A sample was much more active than the ceria-B sample for CO oxidation, with the specific reaction rate at 160 oC being 0.51 and 0.21 µmol/g, respectively. The larger size oxygen vacancy clusters and a more amount of available reactive oxygen species were favorable for the enhanced catalytic activity of the ceria-A sample. Pan et al.27 synthesized CeO2 with different morphologies using a cetyltrimethyl ammonium bromide (CTAB)-assisted hydrothermal treatment at 100−160 oC. The CeO2 nanoplates with a size of 40 nm showed the best CO oxidation activity (T90% = 302 oC at SV = 60000 mL/(g·h)), in which the exposed active (100) crystal plane played an important role in enhancing the catalytic activity. Working on the single-crystal-like CeO2 hollow nanocubes (average edge length = 120 6

nm and shell thickness = 30 nm) derived solvothermally for CO oxidation, Chen et al.28 found that CO conversion over the CeO2 hollow nanocubes could reach 56% at 270 oC, which was almost 3.5 times higher than that over the commercial CeO2 powders. The authors concluded that the interconnected hollow structure enabled a better contact with the reactants owing to the existence of interior space and penetrable shells, thus giving rise to better catalytic activity. Feng et al.29 prepared the rod-, tube-, and cube-like CeO2 catalysts through a hydrothermal process. The nanorod-like CeO2 (length = 300 nm−1 µm and diameter = 30−40 nm) sample showed the best catalytic activity in CO oxidation (T90% = 228 oC at SV = 12000 mL/(g·h)), which was associated with its more easily reducible oxygen species and higher surface area. Shen and coworkers30 generated the CeO2 nanowires and nanorods by a hydrothermal method and the CeO2 NPs via a precipitation route. The CeO2 nanowires (average diameter = 7 nm and length = 140 nm) showed the best catalytic activity for CO oxidation (T90% = 300 oC at SV = 18000 mL/(g·h)), which was related to its high oxygen storage capacity, good redox property, and a large proportion of the exposed reactive (110) and (100) crystal planes. Using an ionic liquid (i.e., 1-hexadecyl-3-methylimidazolium bromide) as both template and solvent, Li et al.31 synthesized a nearly monodisperse spherical aggregates of CeO2 nanocrystals. The mesoporous CeO2 with a surface area of 227 m2/g was also prepared by simply tuning the amount of the ionic liquid. The spherical aggregates with average diameter of 100–150 nm were composed of CeO2 nanocrystals (3.5 nm in size), which was used as building units to form a 3D open porous structure with a high surface area (119 m2/g). After loading of 5 wt% CuO, the spherical aggregates and mesoporous CeO2 samples exhibited high catalytic activities for CO conversion (T100% = 150 oC at SV = 60000 mL/(g·h)). The high catalytic activities of the two samples were assigned to their reversible Ce4+ ↔ Ce3+ transition in the CeO2 support with intrinsically small crystallite sizes and much more amounts of oxygen vacancies. It can be realized from the above investigations that the CeO2 with nanorod-, nanowire-, nanosphere-, hollow nanosphere-, and cube-like as well as nanoparticle morphologies could be generated using the hydrothermal method in the absence or presence of surfactants. Such specifically morphological rare earth oxides exhibited good catalytic performance for the removal of the mentioned atmosphere pollutants, which were generally associated with their unique morphologies, high surface areas, exposed reactive crystal planes, high oxygen vacancy and adsorbed oxygen species concentrations as well as good lattice oxygen mobility.

2. Mixed rare earth oxide catalysts Mixed rare earth oxides, such as rare earth-based perovskite-type oxides, perovskite-like oxides, and hexaaluminates, contain valence-variable transition-metal ions and a number of oxygen vacancies, hence showing good catalytic activities for the elimination of atmosphere pollutants. For example, Liu et al.32 prepared the 3DOM-structured rhombohedral LaMnO3 materials with nanovoid skeletons using the polymethyl methacrylate (PMMA)-templating 7

method with the assistance of surfactant poly(ethylene glycol) (PEG400) or triblock copolymer (Pluronic P123). The use of PEG400 and P123 could generate LaMnO3 with a high-quality 3DOM structure (Fig. 5), nanovoid skeletons, and a high surface area (37−39 m2/g), while the use of PEG400 alone led to a 3DOM-structured LaMnO3 without nanovoid skeletons. For toluene oxidation, the porous LaMnO3 samples were superior to the bulk counterpart in catalytic performance, with the nanovoid-containing 3DOM LaMnO3 catalyst performing the best (T50% and T90% were 222−232 and 243−253 oC at a SV of 20000 mL/(g·h), respectively). The apparent activation energies (57−62 kJ/mol) obtained over the 3DOM LaMnO3 catalysts were much lower than that (97 kJ/mol) over the bulk LaMnO3 catalyst. The authors believed that the excellent performance of the 3D macroporous LaMnO3 in catalyzing the combustion of toluene might be due to the factors, such as large surface area, high oxygen adspecies concentration, good low-temperature reducibility, and unique nanovoid-containing 3DOM structure of the material. The same group33 prepared the rhombohedrally crystallized 3DOM LaMnO3 with mesoporous skeletons by the PEG- and/or L-lysine-assisted PMMA-templating method. Addition of appropriate amounts of PEG400 and L-lysine was beneficial for generation of the high-quality 3DOM-structured LaMnO3 (denoted as LaMnO3-PL-1, LaMnO3-PL-2, and LaMnO3-PL-3 derived with a PEG400/L-lysine molar ratio of 1.23, 0.61, and 0.31, respectively) with mesoporous skeletons and high surface areas of 32−38 m2/g (Fig. 6). Among all of the LaMnO3 samples, the LaMnO3-PL-2 possessed the largest surface area and the highest contents of surface Mn4+ and adsorbed oxygen species. 3DOM-structured LaMnO3 showed better low-temperature reducibility than bulk LaMnO3, with the LaMnO3-PL-2 sample possessing the best low-temperature reducibility, thus performing the best for toluene oxidation (T50% and T90% were 226 and 249 oC at SV = 20000 mL/(g·h), respectively). The authors concluded that large surface area, high oxygen adspecies content, good low-temperature reducibility, and unique bimodal pore structure were responsible for the good performance of 3DOM-architectured LaMnO3 with mesoporous skeletons for toluene combustion. Ji et al.34 prepared the single-phase orthorhombic 3DOM EuFeO3 (EFO-3DOM, EFO-sucrose-1, EFO-sucrose-2, and EFO-sucrose-3, respectively) using the PMMA-templating method in the absence or presence of sucrose. After a series of characterization, it was found that EuFeO3 possessed a higher surface area, a higher surface oxygen species concentration, and enhanced low-temperature reducibility. Hence, the EuFeO3 samples performed good catalytic activities, and the best performance was achieved over the EFO-sucrose-1 sample for toluene oxidation: T50% and T90% were 312 and 347 oC at SV = 20000 mL/(g·h), respectively, and the apparent activation energies of the 3DOM-structured EFO samples were in the range of 82−97 kJ/mol. The same group35 fabricated the porous LaFeO3 (LFO) catalysts with a surface area of 15−26 m2/g and an orthorhombic crystal structure via a glucose-assisted hydrothermal route. It was found that the LFO-170 sample derived at a hydrothermal temperature of 170 oC possessed the highest surface area and surface oxygen concentration and the best low-temperature reducibility. Among the LFO samples, the LFO-170 sample showed the best performance for toluene combustion, giving the T10%, T50%, and T90% of 180, 250, and 270 oC at SV = 20000 mL/(g·h), respectively. The apparent 8

activation energies of the LFO samples were 50−55 kJ/mol. The authors attributed the good catalytic performance of the LFO-170 sample to its high surface area and surface oxygen concentration and good low-temperature reducibility. Zhao et al.36 prepared the single-phase orthorhombic 3DOM La0.6Sr0.4Fe0.8Bi0.2O3−δ (LSFB) using the surfactant (Pluronic F127, PEG, L-lysine or xylitol)-assisted PMMA-templating method. It was found that the 3DOM LSFB sample showed a higher Fe4+/Fe3+ or Oads/Olatt molar ratio and improved low-temperature reducibility than the bulk counterpart. For toluene oxidation, the LSFB sample derived with xylitol exhibited the best catalytic activity (T50% and T90% were 220 and 242 oC at 20000 mL/(g·h), respectively). The apparent activation energies of the porous LSFB samples were in the range of 46−74 kJ/mol. Zhao et al.37 prepared the 3DOM La0.6Sr0.4FeO3−δ (LSF−F127, LSF−PEG, LSF−PEG−EtOH, and LSF−Lysine) with mesoporous or nanovoid-like skeletons using the surfactant (Pluronic F127, PEG or L-lysine)-assisted PMMA-templating method. The characterization results demonstrated that the LSF samples displayed a 3DOM architecture with a single-phase orthorhombic crystal structure. The nature of surfactant and solvent could influence the pore structure and surface area of the final product. LSF−PEG showed the best catalytic performance for toluene combustion (T10% = 54 oC, T50% = 225 oC, and T90% = 280 oC at SV = 20000 mL/(g·h)). The authors concluded that the excellent catalytic performance of 3DOM-structured LSF was associated with their larger surface areas, higher oxygen adspecies concentrations, better low-temperature reducibility, and high-quality 3DOM structures. Spinicci et al.38 prepared the LaMnO3 and LaCoO3 catalysts by the citric acid-complexing method. Redox titration results showed that the cobalt in LaCoO3 was present exclusively in an oxidation state of 3+, whereas LaMnO3 contained a considerable amount of Mn4+ (35%) in addition to Mn3+. Adsorptive property experiments demonstrated the LaMnO3 surface was the most reactive to the adsorption of VOCs and H2. The role of the adsorbed oxygen was studied by examining electrical conductivity variations of the catalysts during the oxygen adsorption–desorption processes. For VOCs oxidation, the total oxidation reactivity followed the trend: acetone > isopropanol > benzene, and the increase of the oxygen partial pressure was beneficial for total oxidation of acetone. Huang et al.39 generated the La1−xSrxCoO3 (x = 0, 0.2) NPs by a co-precipitation method. Compared to LaCoO3, La0.8Sr0.2CoO3 was much higher in catalytic ability. Total oxidation of VOCs increased in the order of cyclohexane < toluene < propyl alcohol. The T99% of cyclohexane oxidation was 40 oC lower than that of toluene oxidation, which appeared to be determined by the bond strengths of the weakest C−H and C−C bonds. The 100-h stability experiments showed that La1−xSrxCoO3 was highly stable. Working on the La0.8Sr0.2MnO3+x prepared by the citric acid sol−gel method, Blasin-Aubé et al.40 evaluated their catalytic performance for the oxidation of a variety of VOCs, such as propane, propene, hexane, cyclohexane, methylcyclohexane, benzene, toluene, ethanol, propan-2-ol, propanal, acetone, ethyl acetate, and isopropyl acetate (Fig. 7). Total conversion to CO2 and H2O occurred over this catalyst at temperatures below 250 oC. However, some byproducts were detected after the reactor. The authors considered that there was a mixture effect on the composition of the mixed VOCs. The substitution of the A- or B-site cations in the perovskite lattice can lead to modifications on 9

their

redox

behaviors.

Levasseur

and

Kaliaguine41

prepared

the

high-surface-area

La1−yCeyCo1−xFexO3 catalysts via a reactive grinding route, and found that cerium allowed an enhancement in reducibility of the B-site cations in the perovskite structure and an increase in amount of the Oβ species. Hence, the catalytic activity in the oxidation of methanol, methane or carbon monoxide was enhanced by doping of Ce. After investigating the LaFeO3, LaNiO3, and LaFe1−yNiyO3 (y = 0.1, 0.2, 0.3) catalysts derived by the citrate method, Pecchi et al.42 claimed that total insertion of nickel in the LaFeO3 lattice took place at y = 0.1; however, NiO segregation occurred to some extent, specifically at higher substitutions (y > 0.1). For the oxidation of ethanol and acetyl acetate, the catalytic activity expressed as intrinsic activity was found to increase substantially with nickel substitution. These results could be explained in terms of the cooperative effect of LaFe1−yNiyO3 and NiO phases, whose relative concentration determined the oxygen activation ability and hence their reactivity. Kustov et al.43 prepared the LaCoOx NPs incapsulated in the mesoporous matrix of zirconia via decomposition of La−Co glycine complexes. The catalysts were studied by the diffuse reflectance FTIR technique using CO, CD3CN, and CDCl3 as probe molecules to test low-coordinated metal ions. At low temperatures (up to 400 oC), the low-coordinated Co3+ ions were predominant in the LaCoOx NPs, whereas basically Co2+ ions were formed upon increasing the decomposition temperature to 600 oC. The catalysts exhibited a high activity in the abatement of methanol and light hydrocarbons. Zawadzki et al.44 fabricated the rhombohedral La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) catalysts by the combustion (LSCF-C) and reactive grinding (LSCF-G) methods (Fig. 8). The LSCF materials showed good catalytic activity: 100% conversions of C3−C4 and ethanol were achieved below 400 and 260 oC, respectively, and the activity was stable. Based on the characterization results, the authors believed that high catalytic activity of the as-synthesized material was related to generation of readily accessible active sites (electrophilic surface oxygen species) due to the specific composition of surface layer. Adopting the reactive grinding method, Kaliaguine and coworkers45 obtained the LaCo1−xFexO3 catalysts for n-hexane oxidation. All of the catalysts were significantly more active than a reference sample derived from the conventional synthesis pathway with amorphous citrate complex. The activity per unit surface area was found to depend on the grinding condition and calcination temperature. The authors pointed out that the enhanced activities were associated with the high surface area and defect density achieved after the reactive grinding process. Furthermore, the cobalt-rich perovskites were more active than the iron perovskites calcined at the same temperature in terms of the activity per unit surface area. The LaMO3 (M = Co and Mn) catalysts derived from a sol-gel-like method with propionic acid as solvent exhibited good activities for CVOCs combustion.46 The perovskite structure of LaCoO3 decomposed in the same manner as LaOCl and Co3O4, however, the Mn-based perovskite structure was not altered under the same conditions. The stability in perovskite structure of LaMnO3+δ could be explained by formation of an oxygen overstoichiometric phase which was thermodynamically more resistant to chlorination than the stoichiometric LaMnO3 (Fig. 9). For the destruction of CVOCs, two ways (i.e., hydrolysis and oxidation) were involved in the destruction of the chlorinated hydrocarbons: The 10

first one required the acidic sites and was less sensitive to modification of the catalyst surface induced by chlorine than the second one that required the metallic oxidation sites. Hosseini and coworkers47 prepared the nanostructured LaFeO3 and substituted LaZnxFe1−xO3 (x = 0.01−0.3) catalysts by the sol−gel auto-combustion method. The characterization data revealed that the total insertion of zinc into LaFeO3 took place at x ≤ 0.1. However, ZnO segregation occurred to some extent, especially at x > 0.1. Catalytic activity of these perovskite-type oxides for toluene combustion increased substantially with a rise in zinc substitution. The enhanced catalytic activity was attributed to the cooperative effect between LaZnxFe1−xO3 and zinc oxide phases. The relative concentration of these phases determined their oxygen activation ability and reactivity. By employing the electrospinning and calcination processes, Chen and coworkers48 obtained the La1−xCexCoOδ (x = 0−1.0) catalysts and evaluated their activities for the total oxidation of benzene. The characterization results showed that the amount of Ce doping obviously affected the physicochemical and catalytic properties of La1−xCexCoOδ (Fig. 10), and the CeCoOδ sample exhibited the best activity and good thermal durability. The increased activity over the perovskite phase-dominated oxides was ascribed to a larger surface area, while the activity enhancement over the metal oxides mainly resulted from a higher valance of Co and better redox property. Adopting a simple precipitation/decomposition procedure with oxalate precursor, Chen and coworkers49 generated the hierarchical layer-stacking Mn−Ce composite oxides with a mesoporous structure and investigated their activities for the complete oxidation of benzene, toluene, and ethyl acetate. The hierarchical layer-stacking Mn−Ce composite oxides possessed superior physiochemical properties, such as good low-temperature reducibility, high manganese oxidation state, and rich adsorbed surface oxygen species, thus exhibiting a higher catalytic activity as compared with the Mn−Ce composite oxide synthesized via a traditional Na2CO3 route: The T10%, T50%, and T90% were 75, 105, and 160 oC lower than those over the Mn−Ce−Na2CO3-route sample for benzene combustion. The effects of metal atomic ratio in CeO2−MnOx binary oxides on the structure and catalytic behavior of the catalysts were probed. For example, using the hydrothermal method, Wang et al.50 prepared a series of CeO2−MnOx composite oxides for the complete oxidation of benzene. Catalytic activities of the CeO2−MnOx composite oxides were higher than those of pure CeO2 and MnOx for benzene combustion. When the Ce/Mn atomic ratio was 3:7, the catalytic activity reached the highest (T100% = 375 oC). It was found that the MnOx could provide available oxygen species and CeO2 could enhance the oxygen mobility in the main phase. The positron annihilation spectra revealed that the concentration of oxygen vacancies in the CeO2-MnOx composite oxides was higher than that in pure CeO2 or MnOx, which caused the difference in benzene oxidation activity. The Cu-Mn-Ce oxides supported on cordierites with different Cu/Mn/Ce molar ratios were prepared by the in situ sol-gel method without the use of any binders, and their catalytic performance for the combustion of typical VOCs (hexane, cyclohexane, benzene, toluene, xylene, acetone, ethanol, acetaldehyde, ethyl acetate, and methyl methacrylate) was evaluated.51 The well-dispersed mixed oxide NPs were adhered firmly to the cordierite surface. The Cu0.15Mn0.3Ce0.55/cordierite was the most active. 11

Compared with the commercial Pd/Al2O3 sample, Cu0.15Mn0.3Ce0.55/cordierite showed higher activities for the combustion of various types of VOCs, especially for the oxy-derivative compounds (the light off temperatures were below 200 ºC). Working on the MnOx−CeO2 catalysts derived from a urea combustion method for the oxidation of typical VOCs, Delimaris et al.52 found that the Mn-rich mixed oxide (Mn0.75Ce0.25 and Mn0.5Ce0.5) catalysts possessed two CeO2 and Mn3O4 cubic fluorite phases, and only CeO2 phase was present at Mn/(Ce + Mn) atomic ratio <0.5 (formation of a solid solution between Mn2O3 and CeO2). The MnOx-CeO2 catalysts displayed a surface area of 38−59 m2/g, much higher that those of (5–10 m2/g) CeO2 and MnOx. The Mn0.5Ce0.5 sample showed the best catalytic activities (The T100% was 200, 220, and 260 oC, respectively) and good CO2- and H2O-resistance for the oxidation of ethanol, ethyl acetate, and toluene, which was thought to be associated with its better low-temperature reducibility and higher surface area. In the same group,53 they also synthesized the CuO-CeO2 catalysts by the same method. The Cu0.15Ce0.85 performed the best (The T100% was 220, 240, and 260 oC, respectively) and good CO2- and H2O-resistance in the oxidation of ethanol, ethyl acetate, and toluene, which was also attributed to its better low-temperature reducibility and higher surface area (52.9 m2/g). Luo and coworkers54 synthesized a series of CexPr1−xO2−δ catalysts via a sol-gel route, and measured their catalytic properties for CO, CH3OH, and CH4 combustion (Fig. 11). It was shown that only a cubic CeO2 phase was formed at x = 0.8−1.0, CeO2 and Pr6O11 were present at x = 0.3−0.7), and CexPr1−xO2−δ solid solutions were generated at x = 0.80−0.99. The Ce0.9Pr0.1O2−δ sample exhibited the best catalytic activities (T90% = 350 oC for CO oxidation and T90% = 275 oC for CH3OH oxidation), which was related to its high oxygen vacancy concentration. In addition, the Ce0.5Pr0.5O2–δ sample showed the highest activity (T50% = 530 oC and T90% = 600 oC) for methane combustion, which was associated with its best low-temperature reducibility. Using a hard-template method, Zhou et al.55 obtained the CuOx-CeO2 composite oxide catalysts with different Ce/Cu molar ratios, and investigated their catalytic activities for the oxidation of benzene, toluene, xylene, and ethylbenzene. The sample with a Ce/Cu molar ratio of 3 performed the best, and the decreased activity followed the order of ethylbenzene > toluene > xylene > benzene, with the T90% being 225, 240, and 220 oC for toluene, xylene, and ethylbenzene combustion, respectively, and the T60% being 220 oC for benzene combustion. Such an excellent catalytic activity of this sample was associated with its well-ordered and developed mesoporous structure, abundant active oxygen species, and superior reducibility. Cuo et al.56 fabricated a series of MnOx-CeO2 catalysts through an impregnation process, and observed that the MnOx-CeO2-3:1 catalyst exhibited the best activity for benzene combustion (T90% = 244 oC at SV = 5000 mL/(g h)) and good stability in the presence of 90 vol% water vapor in benzene stream, which was due to its good lower-temperature reducibility and abundant active oxygen species. Qin et al.57 prepared a series of CuCeOx nanofiber catalysts with different Cu/(Cu + Ce) molar ratios by an electrospinning method. The authors pointed out that the Cu0.50Ce0.50Ox sample with an average diameter of 295 nm showed the best activity for acetone oxidation (T50% = 190 oC and T90% = 225 oC at SV = 79000 mL/(g·h)). Such a good catalytic activity of Cu0.50Ce0.50Ox was due 12

to its high surface area (74.5 m2/g) and oxygen vacancy concentration (52.43%). Lin et al.58 used the transition metals (M = Mn, Co, Ni, Cu, Fe) as the promoter to synthesize the M−Ce/Al-MSPs catalysts through a spray pyrolysis process, and found that Mn-Ce/Al-MSPs with a Mn/Ce molar ratio of 2:1 was the most active (T90% = 137 oC at SV = 15000 mL/(g·h)) and catalytically durable for acetone oxidation. Doping of Mn improved the reducibility of ceria, made the latter easier to be reduced via the interaction between Mn and Ce oxides, and enhanced the acetone adsorption ability. Working on the Cu-doped CeO2 catalysts derived from the one-step precipitation method, García and coworkers59 pointed out that the Ce0.984Cu0.016 catalyst with a Cu/(Ce + Cu) molar ratio of 0.016 showed the best performance for naphthalene oxidation (T90% = 220 oC at SV = 75000 mL/(g·h)). Compared with the pure CeO2 and CuO samples, the Ce1–xCux sample exhibited an increased activity due to an increased surface oxygen defect concentration and hence an increased surface adsorbed oxygen species. Zhou and coworkers60 synthesized a series of the transition metal-doped cerium (4Ce1M, M = V, Cr, Mn, Fe, Co, Ni, Cu) catalysts by the coprecipitation method, and studied their activities for the deep oxidation of typical CVOCs. It was shown that these transition metal oxides were relatively homogeneous dispersed on CeO2 and some of transition metal ions were incorporated into the fluorite lattice, which contributed to enhancement in stability of the active components, while the vanadium oxide easily aggregated on the surface of ceria. Compared with pure CeO2, the 4Ce1M mixed oxides showed a mesoporous structure (2−80 nm in pore size) and larger surface areas, which were favorable for the adsorption and activation of CVOCs on the internal surface of these catalysts. Moreover, the interaction between CeO2 and MOx could improve mobility of the active oxygen species on the surface and reducibility of the MOx species, which facilitated the destruction of the reactants and byproducts in the CVOCs oxidation process at lower temperatures. By comparing with the T90%, the catalytic activity for TCE destruction decreased in the order of 4Ce1Cu (224 oC) > 4Ce1Cr (232 oC) > 4Ce1Mn (336 oC) > 4Ce1Fe (398 oC) > 4Ce1Co (404 oC) > 4Ce1Ni (441 oC) > 4Ce1V (480 oC) > CeO2 > (550 oC) (Fig. 12). Especially for the 4Ce1Cr catalyst, the stronger interaction between CeO2 and CrOx could increase amount of the Cr6+ species with a stronger oxidizing ability, which avoided coke deposition and improved the chemical resistance to Cl-poisoning due to its preferable redox property. Consequently, the Ce-Cr-containing catalyst exhibited the best catalytic performance for CVOCs oxidation. Moreover, addition of water could give rise to an influence on CVOCs combustion over the 4Ce1Cr catalyst in a complicated way, and a significant “mixture effect” was observed when a given CVOC was destroyed in the presence of water. Water could inhibit TCE and CB combustion, promoted DCM oxidation, but exerted little effect on DCE conversion. The 4Ce1Cr sample also showed good durability for DCE destruction within 100 h of continuous reaction, and the chemical adsorbed Cl species on the surface could be removed above 325 oC. The presence of water or non-chlorinated VOCs could slightly decrease conversion of the CVOCs at lower temperatures due to their competitive adsorption at the active sites, but promote oxidation of the CVOCs at higher temperatures (above 300 oC) because of the contribution to removal of the Cl species from the surface. 13

A series of CeO2−TiO2 mixed oxides with different Ce/Ti molar ratios were prepared by the sol−gel method and evaluated for the oxidation of 1,2-dichloroethane (DCE), a typical model reactant of chlorinated volatile organic compounds (CVOCs).61 Compared to pure CeO2 and TiO2, the CeO2−TiO2 mixed oxide exhibited much higher catalytic activity for DCE removal. The CeTi-14 catalyst with a Ce/Ti molar ratio of 0.25 showed the best activity and its T90% was only 275 oC. The CeO2 and TiOx NPs in the mixed oxides were highly distributed, but there was a strong interaction between CeO2 and TiOx, which obviously improved their redox ability, textural, and acidic properties. The crystallite size of CeO2 was greatly reduced with a rise in TiO2 content, and the lattice distortion and lattice defects of CeO2 were increased. The CeO2-TiO2 mixed oxides with Ce/Ti molar ratio > 1.0 were composed of c-CeO2, Ti2O3, and h-TiO2 NPs, while there were only c-CeO2 and h-TiO2 NPs at Ce/Ti molar ratio = 0.5−0.25. The CeO2-TiO2 mixed oxides with Ce/Ti molar ratios of 0.5−0.25 exhibited much higher catalytic activities and selectivities toward HCl and COx than pure CeO2 or TiO2. Moreover, the CeO2−TiO2 mixed oxides also showed good stability for DCE oxidation either in dry air or in the presence of water and benzene at lower temperatures. The preparation method and calcination temperature can influence the performance of a catalyst. Wang et al. 62 prepared the CeO2-TiO2 catalysts using the different methods. Characterization analysis reveals that the CeO2-TiO2 catalysts derived from the sol-gel and coprecipitation routes exhibited higher surface areas, CeO2 and TiO2 particles were highly dispersed into each other, and the reducibility and mobility of active oxygen species were obviously promoted due to the strong interaction between CeO2 and TiO2. The CeO2-TiO2 catalysts derived from the sol-gel and coprecipitation routes exhibited much higher catalytic activities for the deep oxidation of DCE than the CeTi-DP-500 catalyst prepared by the deposition method, over the former 50 % DCE conversion was achieved at a temperature as low as 224 oC (SV = 15000 h−1) . Zhang et al.63 synthesized the tetragonal La2−xSrxCuO4 (x = 0, 1) single crystallites with microrod-like morphologies via the hydrothermal treatment at 240 oC with PEG or CTAB as the surfactant. Doping Sr2+ to the La2CuO4 lattice could enhance the catalytic activity for methane combustion and the LaSrCuO4 catalyst derived from PEG was the best in activity (T10% = 520 oC and T90% = 765 oC). The authors concluded that the factors, such as adsorbed oxygen species concentration, reducibility, and surface area, governed the catalytic performance of such single-crystalline materials. Yuan et al.64 prepared the 3DOM orthorhombically crystallized La2CuO4 adopting the PMMA-templating strategy. The La2CuO4-1 catalyst derived with PMMA and citric acid possessed a 3DOM structure and a surface area of 46 m2/g, whereas the La2CuO4-2 catalyst prepared with PMMA but without citric acid exhibited a 3D wormhole-like macroporous structure and a surface area of 39 m2/g. The La2CuO4-1 catalyst showed higher surface adsorbed oxygen species concentration and better low-temperature reducibility, as compared with La2CuO4-2 and La2CuO4-Citrate (derived from the conventional citric acid-complexing route) catalysts. For the oxidation of methane, the La2CuO4-1 catalyst showed the best performance (T90% = 672 oC and reaction rate is ca. 40 mmol/(g·h) at CH4/O2 molar ratio of 1/10 and SV of 50000 mL/(g·h)). The authors believed that the excellent catalytic performance of La2CuO4-1 was 14

mainly related to its higher surface area, higher surface adsorbed oxygen species concentration, better low-temperature reducibility, and 3DOM architecture. Wang et al.65 prepared the hexapod mesostructured NPs (Fig. 13) by disassembling an ordered porous La0.6Sr0.4MnO3 derived from the PMMA-templating method. The sample exposed a new crystal facet after disassembling, and it was more active for catalytic methane combustion, exhibiting excellent low-temperature methane oxidation activity (T90% = 438 oC and reaction rate = 4.84 × 10−7 mol/(m2·s)). The first principle calculation results suggested that the fractures, which occurred at weak joints within the 3DOM architecture, afforded a large area of the (001) surface that displayed a reduced energy barrier for hydrogen abstraction, thereby facilitating methane oxidation. Xu et al.66 prepared the 3DOM CeO2-CuO catalysts with different Ce/Cu molar ratios by the hard-templating method. The CeO2-CuO sample at a Ce/Cu molar ratio of 2 exhibited the best catalytic activity for CO oxidation (T100% = 115 oC at SV = 40000 mL/(g·h)), which was related to its large surface area, good low-temperature reducibility, high Ce3+ concentration, and large amount of CO2 desorption. It was reported that Mn-modified hexagonal YbFeO3 (Mn-h-YbFeO3) derived from a solvothermal approach exhibited excellent catalytic performance for the oxidation of CO and C3H6.67 The activity of Mn-h-YbFeO3 was dramatically enhanced by loading of a small amount of Pd or Ru, which could act as promoters in improving the CO or C3H6 adsorption and desorption ability and oxygen release capacity from Mn-h-YbFeO3. It can be seen from the above studies that the 3DOM-structured perovskite-type oxides and perovskite-like oxides could be fabricated using the PMMA-templating method with the assistance of a surfactant (PEG, Pluronic P123, Pluronic F127, sucrose, glucose, L-lysine or xylitol); and the binary rare earth-transition metal oxides could be obtained adopting the citric acid-complexing, co-precipitation, sol−gel, combustion, reactive grinding, electrospinning, hydrothermal, precipitation/decomposition, and hard-templating approaches. Most of the perovskite-type oxides and perovskite-like oxides with a 3DOM architecture showed better catalytic performance than the binary rare earth-transition metal oxides for the removal of atmosphere pollutants, which was related to the ordered porous structures, higher surface areas and oxygen adspecies concentrations, and better reducibility of the former catalysts.

3. Rare earth oxide-supported noble metal catalysts Loading of noble metals on the rare earth metal oxide surface can further improve the catalytic performance for the removal of atmosphere pollutants. For example, Ye and coworkers68 fabricated the Pt-loaded CeO2 nanorods (Pt/CeO2-r), nanoparticles (Pt/CeO2-p), and nanocubes (Pt/CeO2-c) via the hydrothermal and ethylene glycol reduction routes, and investigated their performance in toluene oxidation. They observed a significant support-morphology-dependent catalytic phenomenon, and Pt/CeO2-r performed the best (T50% = 138 oC and T90% = 150 oC at SV = 48000 mL/(g·h)) due to its highest oxygen vacancy concentration and largest oxygen storage capacity. The TOFPt of these catalysts were significantly different, while the TOFOV remained constant. This result suggested that the reaction rate was controlled by the concentration of oxygen vacancies which was dependent on the shape of ceria. Toluene oxidation over Pt/CeO2 15

obeyed the Mars-van Krevelen (MvK) mechanism (Fig. 14), and the surface oxygen vacancies played an important role in promoting the ability to replenish the consumed oxygen species, accelerate the oxygen recycle, and enhance the activity. The authors thought that the as-developed preparation method could generate the highly efficient Pt/CeO2 catalysts for toluene oxidation by tuning the nature of the support, and offer a fundamental understanding on the key role of oxygen vacancies in ceria. Peng et al.69 first fabricated a series of size-controllable Pt NPs through a glycol reduction pathway and then supported them on CeO2 nanorods via an adsorption route. It was observed that the Pt/CeO2 catalysts displayed a significant effect of Pt particle size on catalytic activity for toluene oxidation, in which the Pt/CeO2-1.8 sample performed the best (T50% = 132 oC and T90% = 143 oC at SV = 48000 mL/(g·h)) due to the balance of both Pt dispersion and oxygen vacancy concentration. Since the TOFPt and TOFOV were diverse while the TOFPt·OV was almost identical, both the exposed Pt atoms and oxygen vacancies on the ceria surface were the active sites of controlling the reaction rate. Furthermore, the most active Pt/CeO2 catalyst was rather stable during the toluene oxidation process. Although addition of 5 vol% water vapor to the feed temporarily inhibited the activity of Pt/CeO2-1.8, such a negative effect was eliminated with the rise in reaction temperature to above 155 oC. Yu et al.70 examined the effect of CeO2 morphology (nanorods (r-CeO2), NPs (p-CeO2), and nanocubes (c-CeO2) prepared by the hydrothermal method) in the Ag/CeO2 catalysts with various shapes of CeO2 on activity for the oxidation of HCHO at low temperatures. The Ag NPs were well dispersed on the surface of CeO2. The Ag/r-CeO2, Ag/p-CeO2, and Ag/c-CeO2 samples possessed different oxygen vacancy concentrations, in which more amounts of oxygen vacancies and surface chemisorbed oxygen species were formed in/on the Ag/r-CeO2 sample. There was a synergetic interaction between Ag and CeO2, and loading of Ag NPs could promote generation of the surface chemisorbed oxygen species that were favorable for HCHO oxidation. The characterization results and activity data demonstrated that catalytic property of the CeO2-supported sample was greatly dependent on the morphology of CeO2 that influenced the surface oxygen vacancy concentration and reducibility. The Ag/r-CeO2 sample exhibited the best activity for HCHO oxidation (reaction rate at 100 oC = 7.43 nmol/(m2·s), TOFAg at 100 oC = 0.0071 s−1, and T100% = 110 oC at SV = 84000 h−1). Over the Ag/r-CeO2 catalyst, the main reaction intermediate was methyl formate, which was then further oxidized into H2O and CO2. Li and coworkers71 synthesized the Pd NPs by employing a biogenic method with the cacumen platycladi (CP) leaf extract as the reducing agent, and the Pd NPs were then anchored on a 3D ordered mesoporous CeO2 (i.e., kit-CeO2) support derived from the KIT-6-templating method, thus generating the xPd/kit-CeO2 catalysts. Surface areas of the support and catalysts were in the range of 105−109 m2/g. In benzene oxidation over 0.5Pd/kit-CeO2, the T90% was significantly dropped to 187 oC and the activity was stable within 150 h of on-stream reaction. The density functional theory calculation results indicated that the synergetic action of Pd NPs and kit-CeO2 facilitated the activation of benzene adsorbed on Pd NPs prior to oxidation. Catalytic performance of 0.5Pd/kit-CeO2 correlated well with the features of CP leaf extract, high Pd0 concentration, 16

abundant oxygen adspecies, low-temperature reducibility, and strong interaction between Pd NPs and kit-CeO2. The biogenic method was better than the chemical method in synthesizing the Pd NPs with higher catalytic activity, while the kit-CeO2 was better than the commercial CeO2 as a support. Jiang et al.72 first synthesized two kinds of CeO2 by the hydrothermal (CeO2-h) and sacrificial template (CeO2-s) methods, and then loaded MnOx on CeO2 to obtain the MnOx/CeO2-h and MnOx/CeO2-s catalysts via an impregnation route. The rod-like MnOx/CeO2-s (width = 100−200 nm and length = 2−4 µm) sample exhibited the best activity for ethyl acetate oxidation (T90% = 205 oC at SV = 60000 mL/(g·h)), which was benefited from large surface area (155.9 m2/g) and high oxygen vacancy concentration (23.1%). Konsolakis et al.73 investigated the REO (CeO2-, Gd2O3-, Sm2O3-, and La2O3-) supported Cu catalysts derived from the wet impregnation method for the oxidation of ethyl acetate, and claimed that the best catalytic activity was achieved over the CeO2-supported Cu sample (T100% = 275 oC at SV = 1200000 mL/(g·h)). The H2-TPR results revealed that the Cu/CeO2 sample possessed the lowest reduction temperature (232 oC) and the highest H2 consumption (3.1 mmol/g). The authors attributed the superior performance of Cu/CeO2 to the synergistic effect between copper and ceria, which facilitated reduction of surface oxygen species. Huang et al.74 investigated the morphological and crystal-plane effects of nanoscale ceria on catalytic activity of the Ru/CeO2 catalysts for the combustion of chlorobenzene. They employed the supports of CeO2 nanorods (CeO2-r), nanocubes (CeO2-c), and nano-octahedra (CeO2-o) (which were enclosed by the (110) and (100), (100) and (111) crystal planes, respectively) that were prepared by the wet impregnation method. State and structure of the Ru species were quite dependent on the enclosed various facets. The Ru/CeO2-r sample possessed much more amounts of the Ru4+ species, oxygen vacancies, and Ru−O−Ce bonds than the Ru/CeO2-c and Ru/CeO2-o samples, indicating presence of a stronger interaction between Ru and CeO2-r. The surface oxygen mobility and reducibility decreased in the order of Ru/CeO2-r > Ru/CeO2-c > Ru/CeO2-o, leading to a better catalytic activity of Ru/CeO2-r than those of Ru/CeO2-c and Ru/CeO2-o for chlorobenzene oxidation, with the T10% and T90% being 160 and 280 oC over the Ru/CeO2-r catalyst, respectively. The better performance of Ru/CeO2-r was related to its larger number of Ru−O−Ce bonds, higher Ru4+ content, and better surface oxygen mobility and reducibility. Hence, activity of the Ru/CeO2 catalyst was greatly influenced by the shape and/or crystal plane of CeO2. Recently, our group generated the 3DOM CeO2-supported Au–Pd alloys (xAuPdy/3DOM CeO2, x is the total loading (wt%) of Au and Pd, and y is the Pd/Au molar ratio) using the PMMA-templating and PVA-protected reduction methods.75 It was found that the xAuPdy/3DOM CeO2 samples possessed a good-quality 3DOM architecture, and the noble metal NPs with a size of 3–4 nm were uniformly dispersed on the skeleton surface of 3DOM CeO2. Among all of the samples, 2.85AuPd1.87/3DOM CeO2 exhibited the highest catalytic activity for the combustion of TCE, with the T90% being 415 oC at a SV of 20000 mL/(g·h). Furthermore, the 2.85AuPd1.87/3DOM CeO2 sample showed the lowest apparent activation energy (33 kJ/mol), excellent catalytic stability, and good moisture- and chlorine-tolerance. Alloying of Au with Pd 17

changed the pathway of TCE oxidation and reduced formation of perchloroethylene. The authors thought that the factors, such as the highly dispersed AuPd alloy NPs, high adsorbed oxygen species concentration, good low-temperature reducibility, and strong interaction between AuPd NPs and 3DOM CeO2 as well as the high-quality 3DOM structure and high surface acidity were responsible for the excellent catalytic performance of 2.85AuPd1.87/3DOM CeO2. The same group76 prepared the 3DOM CeO2 and its supported Pd@Co (CoxPd/3DOM CeO2, x (Co/Pd molar ratio) = 2.4−13.6) nanocatalysts using the PMMA-templating and modified PVA-protected reduction methods, respectively. The Pd@Co NPs displayed a core-shell (core: Pd; shell: Co) structure with an average size of 3.5−4.5 nm and were well dispersed on the surface of 3DOM CeO2. The CoxPd/3DOM CeO2 samples exhibited high catalytic performance and super stability for methane combustion, with the Co3.5Pd/3DOM CeO2 sample showing the highest activity (T90% = 480 oC at a SV of 40,000 mL/(g h)) and excellent stability below 800 oC. The apparent activation energies (58−73 kJ/mol) obtained over CoxPd/3DOM CeO2 were much lower than those (104−112 kJ/mol) over Co/3DOM CeO2 and 3DOM CeO2, with the Co3.5Pd/3DOM CeO2 sample possessing the lowest apparent activation energy (58 kJ/mol). The authors concluded that the excellent catalytic performance of Co3.5Pd/3DOM CeO2 was associated with its good ability to adsorb oxygen and methane as well as the unique core-shell structure of CoPd NPs. The preparation method is important in generating a catalyst that shows high performance for certain reactions. For example, ceria-supported Pd NPs synthesized by a conventional deposition−precipitation method were efficient catalysts for vehicle exhaust purification, especially for the diesel oxidation.77 The exhaust removal over the Pd/CeO2 catalysts often undergoes under harsh conditions (e.g., high temperatures up to ca. 750 oC). These conditions caused the Pd NPs to sinter. In addition, carbonate and sulfate species might be formed on the catalyst surface, blocking the active sites. The activity and durability of the Pd/CeO2 catalysts were significantly enhanced after the hydrothermal treatment in 10 vol% H2O/air at 750 oC for 25 h. For example, CO conversion over the hydrothermally treated 2 wt% Pd/CeO2 catalyst could reach 100% at 75 oC and a SV of 120000 mL/(g·h) . In the presence of C3H6, CO and C3H6 conversions reached 100% at 145 and 155 oC, respectively. Even when the catalyst was poisoned by SO2, CO conversion was still 100% at 145 oC. There was a promotional effect due to Pd re-dispersion and surface hydroxyl groups formed after the hydrothermal treatment. A novel ultrasonic-assisted membrane reduction (UAMR)-hydrothermal method was used to prepare the flower-like Pt/CeO2 catalysts that were different in activity as compared those derived from conventional wetness impregnation method.78 The Pt/CeO2 catalysts with a typical 3D hierarchical porous structure fabricated by the novel UAMR-hydrothermal strategy showed a high surface area and metal dispersion; in addition, the strong interaction between Pt NPs and CeO2 improved the thermal stability. Therefore, the UAMR-hydrothermal-derived Pt/CeO2 catalysts exhibited a higher three-way catalytic activity and better thermal stability than the one prepared by the conventional method. Zheng and coworkers79 prepared the Ag catalysts supported on the Pr6O11 nanorods (Pr6O11-NRs) or NPs (Pr6O11-NPs) by the conventional incipient wetness impregnation method. The Ag/Pr6O11-NRs sample showed a higher catalytic activity than the 18

Ag/Pr6O11-NPs sample for CO oxidation at low temperatures. This improved performance might be ascribed to the mesoporous feature and oxygen vacancy of Pr6O11 nanorods as well as the large surface area and homogeneously dispersed Ag species. As a result, CO conversions of 98.7 and 100% over Ag/Pr6O11-NRs were achieved at 210 and 240 oC, respectively, while a temperature of 320 oC was required to obtain a 100% CO conversion over the Ag/Pr6O11-NPs sample. Therefore, Pr6O11-NRs was a preferable support for generating the effective Ag-loaded catalysts for CO oxidation. Active and stable catalysts are highly desired for converting harmful substances (e.g., CO and NOx) in vehicle exhaust to safe gases at low temperatures. For example, Kibis et al.80 studied the influence of Ag−CeO2 interaction on catalytic activity for CO oxidation at low temperatures. The Ag/CeO2 catalysts were prepared by the pulsed laser ablation in liquids (PLA). Although the initial Ag/CeO2 composite did not show activity in CO oxidation below 100 oC, thermal activation in an oxidizing atmosphere above 450 oC led to a significant improvement in low-temperature catalytic activity, which was related to the transition of Ag0 NPs in contact with CeO2 to the ionic Ag+ species. The ionized species were stabilized on the surface of CeO2. The catalyst activated at 450 oC exhibited good stability under the adopted conditions due to the effective reversible transformation of Ag+/CeO2 ↔ Agn0/CeO2, where Agn0 was the small metal clusters on the CeO2 surface. Such a reversible transition was facilitated by the defects on the surface of CeO2. Yang et al.81 adopted a solvent evaporation-induced co-assembly strategy with high-molecular-weight poly(ethylene oxide)-block-polystyrene as the template to prepare the ordered mesoporous CexZr1−xO2 (0 ≤ x ≤ 1). The as-obtained mesoporous CexZr1−xO2 possessed high surface areas (60–100 m2/g) and large pore size (12–15 nm), enabling its great capacity in stably immobilizing Pt NPs (4.0 nm in size) without blocking the pore channels. The Pt/mesoporous Ce0.8Zr0.2O2 catalyst exhibited superior CO oxidation activity and excellent stability, with the T100% being 130 o C due to the rich lattice oxygen vacancies in the support framework. The authors considered that the intrinsic high surface oxygen mobility and well-interconnected pore structure of the Pt/mesoporous CexZr1−xO2 catalysts were responsible for the remarkable catalytic efficiency. Wang et al.82 fabricated the α-Fe2O3 catalysts with different morphologies through a chemical precipitation route and 5 wt% Ag/Fe and 5 wt% Ag/Fe@Ce catalysts by the incipient wetness impregnation method, which were aimed to investigate the crystal plane effects on soot oxidation. It was found that (i) the soot oxidation activity of hematite surface facets followed the order of (113) > (014) ≈ (012), and the electron-rich state of the surface Fe atoms was behind the high content of Ox− and the high activity of Fe2O3 (113) plane; (ii) a polycrystalline CeO2 outer layer significantly improved the oxygen utilization of Fe2O3, and the oxygen delivery from Fe2O3 to CeO2 resulted in fast generation of Ox− (Fig. 15) and hence better soot oxidation activity of Fe2O3@CeO2 than Fe2O3; and (iii) Ag loading facilitated both the utilization of bulk oxygen in ceria and the activation of gas-phase O2, and with the Fe2O3 → CeO2 → Ag tandem delivery of oxygen, Ag loading promoted the Fe2O3@CeO2 catalysts to show a superior activity at low temperatures for soot oxidation. These Ag/Fe2O3@CeO2 catalysts overwhelmed the nano-cubic Ag/CeO2 in catalyst cost and activity, making them very promising for application in catalyzing 19

gasoline particulate filters. Xiong et al.83 generated the efficiently multifunctional core-shell PdAu@CeO2 catalysts supported on 3D ordered meso-macroporous Ce0.3Zr0.7O2 (PdAu@CeO2/3DOMM-CZ) and 3DOMM CZ-supported core-shell PdAu@CeO2 catalysts using the gas bubbling-assisted membrane reduction (GBMR) and GBMR-precipitation (GBMR/P) methods, respectively. The uniform 3DOMM structure with the larger surface area improved the contact efficiency between the reactants (O2, NO, and soot) and the catalyst and increased the active site density of the catalyst. The nanostructure of AuPd (core) and CeO2 layer (shell) with an optimal interface area of the metal-oxide could increase amount of the surface oxygen vacancies that were favorable for adsorption and activation of O2 and NO molecules. Due to the above reasons, the PdAu@CeO2/3DOMM-CZ catalyst exhibited super catalytic performance for soot oxidation. For instance, its TOF at 290 oC was 2.51 h−1, which was more than 5 times than that (0.46 h−1) over 3DOMM-CZ, and the T10%, T50%, and T90% were 276, 363, and 404 oC, respectively. Furthermore, the PdAu@CeO2/3DOMM-CZ catalyst possessed high stability and good SO2-tolerance during the catalytic soot oxidation process. Liu et al.84 prepared the MoFe/Beta@CeO2 core-shell catalysts with the Beta-zeolite-supported nanosized MoFe oxides as the core and CeO2 thin film as the shell (Fig. 16). It was found that the MoFe/Beta@CeO2 catalyst exhibited a higher catalytic activity (90% NO conversion at 300 oC), better thermal stability and H2O- and SO2-tolerance than pure CeO2 or MoFe/Beta for the NH3-SCR reaction, which was ascribed to the interface effect between the core (MoFe/Beta) and shell (CeO2). The chemisorbed oxygen (O2− and/or O−) species and surface area increased after the catalyst was coated by the CeO2 shells. Coating of the CeO2 shells not only increased the acid amount but also enhanced its acid strength, which were beneficial for improvement in NO oxidation during the NH3-SCR process. Furthermore, there was a strong interaction between the iron or molybdenum oxide and CeO2 shells. The CeO2 shells could serve as an effective barrier to inhibit the active metal oxide NPs from aggregation at high temperatures. The CeO2 shells suppressed formation of the ammonium nitrate and sulfate species (which could block the iron active sites) and restrained generation of the iron sulfate species, thus giving rise to a good SO2and H2O-tolerance performance. Hu et al.85 used the atomic layer deposition (ALD) strategy to highly disperse Pt NPs on CeO2 nanorods, and studied their catalytic activities for the NO + CO reaction (Fig. 17). For comparison purposes, Pt NPs-loaded CeO2 nanospheres (Pt/CeO2-NS and PtIWI/CeO2-NS) and CeO2 nanorods (Pt/CeO2-NR) were also fabricated by the ALD and incipient wetness impregnation (IWI) methods. The Pt/CeO2-NS and Pt/CeO2-NR catalysts prepared by the ALD method displayed narrower and more uniform Pt particle size distribution (3−3.4 nm) than the PtIWI/CeO2-NS catalyst prepared by the incipient wetness impregnation method. The characterization results confirmed that the strong interaction between Pt NPs and CeO2 nanorods led to the undermining of the CO-poisoning effect on Pt and activation of oxygen around the interfaces. As a result, complete conversion of NO took place at a lower temperature (200 oC), and the activity over the ALD-derived Pt/CeO2-NR catalyst was greatly enhanced (T50% = 185 oC and T90% = 190 oC). 20

Kobiro and coworkers86 used the porous CeO2 aggregate, SiO2-CeO2 nanocomposite, and TiO2−CeO2 nanocomposite derived solvothermally as supports to prepare supported Ru NPs by the precipitation–deposition method, and investigated their catalytic performance and sintering-resistant capability in the methanation of CO2 with H2. The Ru NPs were highly dispersed on the surface of each support. The low-temperature (150−200 oC) activity and CH4 production of the Ru NPs supported on the CeO2 aggregate and TiO2-CeO2 nanocomposite were higher than those supported on the commercial CeO2 aggregates. A higher CH4 yield (ca. 80%) was achieved over the Ru/porous CeO2 aggregate, Ru/SiO2-CeO2 nanocomposite, and Ru/TiO2−CeO2 nanocomposite, whereas a much lower CH4 yield (55%) was obtained over the Ru/commercial CeO2 at 350 oC. Moreover, the Ru catalysts supported on these as-prepared supports possessed a long-term stability under the adopted reaction conditions. It can be seen from the above reports that there was a significant morphological effect on activity of such materials according to support shapes and catalytic activities of the rare earth oxide-supported noble metal (Ag, Pt, Pd or Ru), core-shelled noble metal (PdAu), core-shelled transition metal (MoFe), core-shelled transition metal−noble metal (CoPd) or transition metal (Cu, Fe or Mo) catalysts.

4. Mixed rare earth oxide-supported noble metal catalysts Tan et al.87 prepared the Ce0.6Zr0.3Y0.1O2 (CZY) nanorods-supported nanosized gold (xAu/CZY, x = 0.4 wt%−4.7 wt%) catalysts using the hydrothermal and polyvinyl pyrolidone (PVP)-protected reduction methods. Among these catalysts, the 4.7Au/CZY catalyst showed the highest activity: the T90% was 60 oC for CO oxidation and 265 °C for toluene oxidation at SV = 20000 mL/(g·h). In addition, the authors also prepared the CZY nanorods-supported gold and palladium alloy (zAuxPdy/CZY; z = 0.80−0.93 wt%; x or y = 0, 1, 2) NPs,88 and found that 0.90Au1Pd2/CZY performed the best: T90% was 218 oC for toluene oxidation at the same SV. The CZY-supported Au−Pd alloy NPs outperformed the supported Au NPs due to a strong interaction between Au and Pd NPs of the former sample. As revealed from Fig. 18, the excellent catalytic performance of 4.7Au/CZY and 0.90Au1Pd2/CZY was associated with its high adsorbed oxygen species concentration, good low-temperature reducibility, and strong interaction between noble metal NPs and CZY nanorods. Tang et al.89 prepared the Pt/MnOx-CeO2 catalysts by a modified coprecipitation method and investigated the complete oxidation of formaldehyde. They claimed that the promotional effect of Pt improved the reducibility of MnOx-CeO2. The Pt/MnOx-CeO2 catalyst derived from a chlorine-free precursor showed extremely high activity and stability after pretreatment in H2 at 200 oC. These authors also prepared the Ag/MnOx-CeO2 catalysts,90 and claimed that addition of the silver species resulted in a significant improvement in activation of the gas-phase oxygen molecules. Accordingly, they proposed a consecutive oxygen transfer mechanism: The oxygen species released from Ag2O decomposition participated in the oxidation of formaldehyde, re-oxidation of Ag to Ag2O was achieved by the oxygen species from MnO2, 21

transformation of the produced Mn2O3 to MnO2 was simultaneously realized by the oxygen species from the oxygen reservoir of CeO2, and the formed Ce2O3 could be re-oxidized into CeO2 by the gas-phase oxygen in the feed stream. Ma et al.91 investigated formaldehyde oxidation over the Au/meso-Co3O4-CeO2 catalysts at room temperature, analyzed surface adsorbed species during the formaldehyde oxidation process, and proposed some key steps involved in the oxidation pathway, active sites, and intermediate species. Formaldehyde oxidation included a microreaction process over the mesoporous Co3O4, Au/meso-Co3O4, and Au/meso-Co3O4-CeO2 catalysts at 25 oC: Co3O4 facets composed mainly of Co3+ cations were the active facets for formaldehyde oxidation, the adsorbed formaldehyde was oxidized into formate over these sites, then a second attack of the surface active oxygen led to generation of the bicarbonate species that finally decomposed into CO2. Matejová et al.92 found that the deposition of platinum on Ce0.5Zr0.5O2 resulted in a significant increase in performance for total oxidation of ethanol, but its activity was lower as compared with the parent Ce0.5Zr0.5O2 material for total oxidation of dichloromethane, which might be due to a lower amount of the acid sites that acted as chemisorption sites for chlorinated compounds. On the other hand, the supported platinum catalysts exhibited significantly enhanced selectivity to CO2 in comparison with the Ce0.5Zr0.5O2 support. Moreover, the positive effect of a higher noble metal loading on catalytic performance was more pronounced for the supported Pt catalysts. Ceria is not as expensive as platinum but it is still expensive as compared with the Al2O3 support. Therefore, by deposition of a small amount of ceria on alumina, a fairly cost-effective catalyst could be generated. Also, it did not possess a surface area as large as Al2O3. After deposition of ceria on a high-surface-area support like alumina, a new support with a high surface area and good redox property was achieved.[93−95] For instance, Abbasi et al.93 prepared the Pt/Al2O3−CeO2 nanocatalysts with a Pt loading of 1 wt% and a ceria loading of 30 wt% via the wet impregnation route, and applied them to catalytic oxidation of VOCs (e.g., benzene, toluene, and xylene (BTX)). Platinum NPs with an average size of 5–20 nm were fairly well dispersed on Al2O3-CeO2. The TPR results demonstrated that by adding 1 wt% platinum to the support, the reducibility was greatly improved. It was found that the as-synthesized nanocatalysts were highly active and able to remove nearly 100% of toluene or xylene and about 85% of benzene. Lakshmanan et al.94 explored propene (1200 ppm) oxidation over the Au/xCeO2/Al2O3 catalysts (x = 1.5 wt%, 3 wt%, 5 wt%, and 10 wt%) in an excessive amount of oxygen to mimic the conditions of VOCs combustion. The authors thought that several factors (e.g., ceria loading, catalyst activation method (i.e., gold oxidation state) influenced the catalytic performance of Au/xCeO2/Al2O3. The presence of ceria, even in low amount, strongly enhanced the catalytic activity as compared with gold on mere alumina. In order to study gold oxidation state, these authors treated the samples in the oxidative and reductive atmospheres, respectively, and found that the reduced samples were more active than the calcined samples for propene oxidation, and activity of the reduced sample increased whereas that of the calcined sample decreased with a rise in ceria loading. These differences were due to the facts that the metallic gold was more active than the oxidized gold and the Au0 on ceria was more active than the Au0 on alumina. Yang et al.95 22

prepared the 3D ordered macro-/mesoporous 26.9 wt% CeO2-Al2O3 (denoted as 3DOM 26.9CeO2-Al2O3)-supported noble metal (xM/3DOM 26.9CeO2-Al2O3, x = 0.27 wt%−0.81 wt%; M = Au, Ag, Pd, and Pt) nanocatalysts using the PMMA-templating and PVP- or polyvinyl alcohol (PVA)-protected reduction methods, respectively. These catalysts displayed a high-quality 3DOM architecture with a bimodal pore (macropore size = 180−200 nm and mesopore size = 4–6 nm) structure and a surface area of 102−108 m2/g, with the noble metal NPs (3–4 nm in size) being uniformly dispersed on the 3DOM 26.9CeO2−Al2O3 surface. The 0.27Pt/3DOM 26.9CeO2−Al2O3 sample performed the best (T90% = 198 oC at SV = 20000 mL/(g·h)) and it possessed the lowest apparent activation energy (47 kJ/mol) for toluene oxidation. The authors concluded that the good catalytic performance was associated with its higher adsorbed oxygen species concentration, better low-temperature reducibility, and stronger interaction between Pt and 3DOM 26.9CeO2−Al2O3 as well as the unique bimodal porous structure. Dai et al.96 investigated catalytic combustion of the chlorinated hydrocarbons over the Ti-doped CeO2-supported RuO2 (Ru/Ti-CeO2) catalysts at lower temperatures, including chlorobenzene (CB), 1,2-dichloroethane (DCE), and TCE. The doping of Ti could obviously improve catalytic activity and stability of the CeO2-based catalyst. The better activity of Ru/Ti-CeO2 was ascribed to exposure of a more amount of oxygen vacancies and high-energy lattice planes CeO2 (110) and (100), and the Cl dissociatively adsorbed at the active sites of CeO2 could be oxidized into Cl2 catalyzed by RuO2 supported on Ti-CeO2 at lower temperatures (e.g., 200 oC). Hou et al.97 prepared the Pt/Ce0.65Zr0.35O2 and Pt-WO3/Ce0.65Zr0.35O2 catalysts adopting the incipient-wetness impregnation strategy and applied these materials to catalytic toluene oxidation. According to the catalytic activity data, they pointed out that Pt-WO3/Ce0.65Zr0.35O2 performed better than Pt/Ce0.65Zr0.35O2. Based on the characterization results, the authors concluded that the good low-temperature reducibility, higher surface adsorbed oxygen concentration, and greater medium strength acidity were responsible for the excellent catalytic activity of Pt−WO3/Ce0.65Zr0.35O2. Previously, our group prepared the xAu/3DOM LaCoO3 (x = 1.54 wt%−7.63 wt%)98 and xAu/3DOM La0.6Sr0.4MnO3 (xAu/3DOM LSMO; x = 3.4 wt%−7.9 wt%)99 catalysts using the PMMA-templating and PVA-protected reduction methods. These samples displayed a 3DOM architecture and a high surface area (Fig. 19). Over 7.63Au/3DOM LaCoO3, the T50% and T90% were 188 and 202 oC for toluene oxidation at SV = 20000 mL/(g·h), and −6 and 42 oC for CO oxidation at SV = 10000 mL/(g·h), respectively. On the other hand, the 6.4Au/LSMO catalyst performed the best, giving the T50% and T90% of 150 and 170 oC for toluene oxidation, and −19 and 3 oC for CO oxidation, respectively. The apparent activation energies obtained over the xAu/3DOM LaCoO3 and xAu/3DOM LSMO samples corresponded to 31−38 and 44−48 kJ/mol for toluene oxidation, respectively. It was concluded that high oxygen adspecies concentration, good low-temperature reducibility, and strong interaction between Au and support were important factors in influencing catalytic performance of the sample. In one of our previously published works,100 we fabricated the 3DOM La0.6Sr0.4MnO3 (LSMO)-supported manganese oxide and gold (yAu/zMnOx/3DOM LSMO; y = 1.76 wt %−6.85 wt %; z = 8 wt%) nanocatalysts by means of in situ PMMA-templating and reduction methods. 23

The MnOx and Au NPs (3.2−3.8 nm in size) were well dispersed on the surface of 3DOM LSMO. Among the as-prepared samples, 5.92Au/8MnOx/3DOM LSMO showed the highest activity for toluene oxidation at a SV of 20000 mL/(g·h): the T50% and T90% were 205 and 220 oC, respectively. It was found that catalytic activity well correlated with the adsorbed oxygen species concentration and low-temperature reducibility. The 3DOM La0.6Sr0.4CoO3 (LSCO) and its supported gold and manganese oxide (yMn3O4−zAu/3DOM LSCO; y = 0.75 wt%−2.50 wt%, z = 2.0 wt%) NPs were prepared by the PMMA-templating, reduction and physical adsorption methods.101 The characterization results showed that the 3DOM LSCO in yMn3O4−zAu/3DOM LSCO with a surface area of 20−24 m2/g displayed a rhombohedral crystal structure, and Mn3O4 (5−12 nm in size) and Au (3−4 nm in size) NPs were highly dispersed on the surface of 3DOM LSCO. The 1.67Mn3O4−2Au/3DOM LSCO sample performed the best for toluene oxidation (T50% = 214 oC ans T90% = 230 oC at SV = 20000 mL/(g·h)). It was observed that introduction of water vapor to the reaction system caused a partial deactivation of the 1.67Mn3O4−2Au/3DOM LSCO sample, and such a deactivation was reversible. The authors concluded that the large surface area, high adsorbed oxygen species concentration, good low-temperature reducibility, and strong interaction between Au or MnOx NPs and support were accountable for the good catalytic performance of 1.67Mn3O4−2Au/3DOM LSCO. In terms of reducibility and lattice oxygen availability, there was a strong correlation between activity and redox property of a catalyst. For example, Carabineiro et al.102 examined the detrimental effect of lanthanide (Gd, La, Pr, Nd or Sm) and/or copper doping on ethyl acetate oxidation, and observed that the catalytic activity decreased in the order of CeO2 ≈ Ce0.5Pr0.5O1.75 > Ce0.5Sm0.5O1.75 > Ce0.5Gd0.5O1.75 > Ce0.5Nd0.5O1.75 > Ce0.5La0.5O1.75; Cu doping improved the catalytic activity, with the best performance (T100% = 290 oC at SV = 60000 mL/(g·h)) being achieved over Cu/CeO2 and Cu/Ce0.5Pr0.5O1.75. Working on the Cu/Ce1−xSmxOδ (x = 0−1) catalysts for ethyl acetate oxidation, Konsolakis et al.103 pointed out that the Cu/CeO2 catalyst showed the best performance (T100% = 260 oC at SV = 60000 mL/(g·h)), and incorporation of samarium in ceria exerted a negative effect on textural and reducible properties of the catalysts, thus decreasing the catalytic activity. Pd NPs supported on lanthanum-based perovskites LaBO3 (B = Co, Mn, Fe, and Ni) were prepared using a wet impregnation method.104 Easiness of chlorobenzene destruction was found to follow the sequence in terms of the T50%: Pd/LaMnO3+δ (243 oC) > Pd/LaFeO3 (270 oC) > Pd/Al2O3 (348 oC) > Pd/LaCoO3 (360 oC) > Pd/LaNiO3 (408 oC). In situ XPS studies on Pd/LaCoO3 revealed that Pd0 was progressively (oxy)chlorined while the perovskite network was reconstructed with generation of LaOCl and Co3O4 in the reactive atmosphere. Jia et al.105,106 prepared the Au/LaCoO3 and Au/LaMnO3 catalysts by the sol-gel and deposition-precipitation (DP) methods. Performance and stability of the catalysts for CO low-temperature oxidation were compared. The results showed that the gold catalysts supported on perovskite oxides exhibited higher activity and stability than the gold catalysts supported on the simple metal oxides. Among these catalysts, 3.60Au/LaCoO3 and 0.84Au/LaMnO3 showed the highest performance, giving the T100% of 90 and 100 oC at a SV of 15000 mL/(g·h), respectively. In the long-time catalytic stability test, a gradual decrease in initial activity during 100 h of the reaction was observed, 24

which could be ascribed to aggregation of the gold NPs and transformation from the oxidized gold to the metallic gold. Arandiyan et al.107 synthesized the xPt/3DOM Ce0.6Zr0.3Y0.1O2 (x = 0.6 wt%, 1.1 wt%, and 1.7 wt%) catalysts via the CTAB/P123-assisted gas-bubbling reduction route. Pt NPs with a size of 2.6−4.2 nm were uniformly dispersed on the surface of 3DOM CZY, and the catalysts displayed a high surface area of 84−94 m2/g. The 1.1Pt/3DOM CZY sample showed an excellent catalytic activity, giving a T90% of 598 oC at SV = 30000 mL/(g·h) and the highest turnover frequency (TOFPt) of 6.98 × 10−3 mol/(molPt·s) for methane combustion at 400 oC. The authors assigned the excellent performance of this sample to its high oxygen adspecies concentration, good low-temperature reducibility, and strong interaction between Pt NPs and CZY as well as large surface area and unique nanovoid walled 3DOM structure (Fig. 20). In addition, yAg/3DOM LSMO (y = 0 wt%, 1.57 wt%, 3.63 wt%, and 5.71 wt%) with high surface areas (38.2−42.7 m2/g) were also prepared by a facile novel reduction method with PMMA colloidal crystal as template in a dimethoxytetraethylene glycol (DMOTEG) solution.108 Among all of the samples, 3.63Ag/3DOM LSMO exhibited the highest activity (the T50% and T90% were 454 and 524 oC, respectively) and the highest turnover frequency (TOFAg) of 1.86 × 10−5 mol/(molAg·s) for methane combustion at 300 oC. Loading of silver could improve the resistance to sulfur poisoning, mainly by increasing the concentration of the acidic Mn4+ ions and weakening the SO2 adsorption on the sample. Moreover, the AuPd/3DOM LSMO catalysts were synthesized with the Au−Pd alloy NPs (2.05−2.35 nm) highly dispersed on the internal walls of macropores.109 As revealed by the in situ DRIFT results, including Au in the bimetallic system accelerated the reaction rate and altered the reaction pathway for methane oxidation by enriching the adsorbed oxygen species and decreasing the bonding strength between the reaction intermediates and the Pd atoms. Eyssler et al.110,111 prepared the Pd-containing perovskite-type oxide (LaFe0.95Pd0.05O3) catalysts by the amorphous citrate method and LaFeO3-supported Pd NPs (Pd/LaFeO3) via a wet impregnation route. The catalysts were tested for methane combustion in the temperature range of 200−900 oC. It was found that Pd/LaFeO3 exhibited high performance for the combustion of methane (T50% = 460 oC), which correlated with a high concentration of the Pd species on the oxide surface. On the contrary, when Pd was incorporated in the lattice of LaFeO3, the corresponding catalyst showed poor activity (similar to that of LaFeO3). The authors utilized the XANES, EXAFS, and other techniques to characterize the chemical state of Pd in various samples, and found that palladium most probably existed in a distorted octahedral coordination, i.e., Pd was fully incorporated in the perovskite structure and substituted the partial Fe, which was confirmed by the EXAFS refinement. On the contrary, the XANES spectrum of Pd/LaFeO3 revealed that Pd existed in a square planar coordination, suggesting that Pd was mainly distributed on the surface of LaFeO3 and was present most likely in the form of PdO. In addition, the authors compared catalytic activities of LaFe0.95Pd0.05O3, 2.00Pd/LaFeO3, and 2.00Pd/Al2O3 for methane combustion at 500 oC under the periodic redox conditions, and found that the continuous reduction−oxidation of Pd occurred over all of the catalysts at each change of the feed 25

composition. However, by this process Pd in LaFe0.95Pd0.05O3 was reversibly emerged on the LaFeO3 surface under the reducing conditions and entered the LaFeO3 structure under the oxidizing conditions. On the contrary, Pd oscillated between the reduced and partially oxidized states in Pd/Al2O3 and Pd/LaFeO3, in which the well-defined Pd NPs were already available. Guo et al.112 synthesized the 3DOM-structured "self-regeneration" LaMn0.97Pd0.03O3 (LMPO) catalysts, carried out a reduction treatment in the temperature range of 300−800 oC, and observed that the 500 oC-treated LMPO sample exhibited the highest catalytic activity with a T50% of 412 oC and an Ea of 51.5 kJ/mol for methane combustion. Among the catalysts investigated for the combustion of VOCs and methane, rare earth-based hexaaluminates have been regarded as an excellent candidate that is expected to solve the problems associated with VOCs and methane combustion. Our group prepared the 3DOM LaMnAl11O19 (LMAO) and its supported Pd NPs (Pd/3DOM LMAO),113 Au–Pd NPs (AuPd/3DOM LMAO),114 and Pd−Pt NPs (PdPt/3DOM LMAO)115 catalysts by the PMMA-templating and PVA-protected reduction methods. It was found that the 0.97Pd/3DOM LMAO, 1.91AuPd1.80/3DOM LMAO, and 1.14Pd2.8Pt/3DOM LMAO samples showed good catalytic performance for methane combustion, with the T90% being 343, 402, and 456 oC at SV = 20000 mL/(g·h)), respectively. The 0.97Pd/3DOM LMAO catalyst was catalytically stable, whereas the 0.98Pd/3DOM Mn2O3 sample was partially deactivated after 50 h of methane oxidation. Introduction of H2O or CO2 to the reaction system resulted in the reversible deactivation of the above samples, but addition of SO2 led to the irreversible deactivation, and doping of Pt to the Pd-based catalyst could improve the H2O-, CO2-, and SO2-resistance performance. The authors believed that the good catalytic activity was related to its good-quality 3DOM structure, highly dispersed noble metal NPs, high adsorbed oxygen species concentration, good low-temperature reducibility, and strong interaction between noble metal NPs and 3DOM LaMnAl11O19. From the above investigations, one can realize that loading of transition metals, noble metals or their alloy NPs could considerably enhance performance of the mixed rare earth oxide-supported catalysts, and activities of the porous materials were much higher than those of their bulk counterparts.

5. Conclusions and perspectives Based on the above reported works, we know that specifically morphological (such as nanorod-, nanowire-, nanosphere-, hollow sphere-, and nanocube-like as well as nanoparticle) rare earth oxides and their mixed oxides could be synthesized using the hydrothermal method with or without the aid of surfactants; and porous rare earth oxides and related mixed oxides (such as perovskite-type oxides, perovskite-like oxides, and hexaaluminates) could be fabricated adopting the polymer microbeads-templating approach in the absence or presence of surfactants. With the protection of PVA or PVP, noble metal NPs with a size of smaller than 5 nm could be generated via reduction of noble metal precursors by a reducing agent (e.g., NaBH4 or EG), and loading the as-synthesized noble metals NPs on the surface of rare earth oxides and their mixed oxides could 26

obtain the supported noble metal nanocatalysts. According to the characterization results and activity data (Table S1 of the Supplementary Data), it can be concluded that activity of the catalyst for the removal of pollutants (e.g., typical VOCs, methane, carbon monoxide, soot, automotive exhaust, and nitrogen oxides) was mainly associated with one or more factors, such as surface area, unique morphology, porous structure, exposed reactive crystal plane, dispersed noble metal NPs, adsorbed oxygen species, lattice oxygen mobility, low-temperature reducibility, reactant activation ability, and interaction between noble metal NPs and support. Although many efforts on the preparation and catalytic applications of rare earth oxides, mixed rare earth oxide, and their supported noble metals in the removal of the atmospheric pollutants have been made, most of these catalytic materials were high in cost and it is difficult to be used in industry. In the future, we need to do the following work: (i) optimization of the catalyst compositions and structures (by selecting more appropriate supports and synthesizing high-surface-area mesoporous catalysts) and development of the catalyst preparation strategies and characterization techniques (so as to obtain high-performance catalysts and establish the structure-performance relationships); (ii) preparation of single-atom noble metal catalysts (which is an effective strategy to reduce the used amount of noble metals); (iii) preparation of hydrophobic and SO2-resistant catalysts (so that their H2O- and SO2-resistance performance can be improved); (iv) preparation of core-shell-structured catalysts (so that their thermal and hydrothermal stability can be enhanced); (v) identification of primary reaction steps in the removal of atmospheric pollutants using in situ characterization techniques and quantum chemical calculations (so that the reaction mechanisms can be clarified); and (vi) coupling of multiple pollutants removal pathways (so that an optimal removal efficiency can be achieved).

Acknowledgements This work receives the support from Scientific Research Base Construction−Science and Technology Creation Platform−National Materials Research Base Construction.

Appendix A. Supplementary data Supplementary data to this https://doi.org/10.1016/j.jre.2020.XX.XXX.

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Fig. 1. SEM and TEM images of nanorod-like CeO2 (a−c), hollow spherical CeO2 (d−f), and cube-like CeO2 (g−i).13

Fig. 2. Plausible major reaction pathways of methanol oxidation in the plasma-catalytic system.19

36

Fig. 3. SEM images of the samples derived hydrothermally at different time. PS: (a) 10 h, (b) 20 h, (c) 24 h; RS: (e) 120 min, (f) 160 min, (g) 200 min; HS: (i) 5 h, (j) 8 h, (k) 10 h; and (d), (h), and (l) are the schematic illustrations of the PS, RS and HS, respectively.20

Fig. 4. Catalytic oxidation stability tests of the pristine CeO2 and POx−CeO2 nanosheets with different POx contents.23

37

(b)

(a)

(c)

100 nm

(e)

(d)

100 nm

1 µm

(f)

200 nm

200 nm

200 nm

Fig. 5. SEM images of LaMnO3–MeOH (a), LaMnO3–PEG (b, c), LaMnO3–PP-1 (d), LaMnO3–PP-2 (e), and LaMnO3–PP-3 (f).

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(a)

(c)

(b)

(d)

100 nm

200 nm

200 nm

(e)

(f)

200 nm

1 µm

100 nm

Fig. 6. SEM images of LaMnO3-MeOH (a), LaMnO3-PEG (b), LaMnO3-PL-1 (c), LaMnO3-PL-2 (d, e), and LaMnO3-PL-3 (f).33

38

Fig. 7. Conversion of hydrocarbons and product yields on La0.8Sr0.2MnO3+x versus reaction temperature ([HC] = 500 ppm, [O2] = 21 vol%, SV = 20000 h−1).40

Fig. 8. SEM and TEM images as well as SAED pattern (inset in b) of LSCF-C (a, b) and LSCF-G (c, d).44

Fig. 9. (a) Ageing test of LaCoO3 and LaMnO3+δ with 1000 ppm of CH2Cl2 (T = 500 oC, 3.3 vol% O2, 1.3 vol% H2O); (b) Ageing test of LaCoO3 (T = 460 oC) and LaMnO3+δ (T = 420 oC) with 500 ppm of CCl4, 3.3 vol% O2, 1.3 vol% H2O.46

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Fig. 10. Benzene conversion as a function of temperature over the La1−xCexCoOδ catalysts.48

Fig. 11. The T90% as a function of Ce/(Ce + Pr) molar ratio in the combustion of CO, CH3OH, and CH4.54

Fig. 12. Catalytic performance for TCE destruction over the catalysts in dry air. (a) TCE conversion; (b) concentration of the byproduct C2Cl4.55

40

Fig. 13. (a, b) FE−HRTEM images of 3DOM LSMO, (c) HAADF−STEM image of 3DOM LSMO, (d−f) FE−HRTEM images of 3D-hm LSMO, (g) 3D topography of 3D-hm LSMO extracted from (h) combined HAADF−STEM−EDS mapping of La, Sr, Mn and O with individual element mapping shown in the insets, and (i) HAADF−STEM image of the intersecting surface of the fractures in 3D-hm LSMO. Scale bars in (a, d, e) are 300 nm, those in (b, f, h) are 20 nm, and those in (c) and (i) are 5 nm.65

Fig. 14. Reaction mechanism of toluene oxidation over the Pt/CeO2 catalysts.69 41

Fig. 15. Mechanism of soot oxidation over the Ag/Fe@Ce catalysts. Black arrows indicate the delivery of oxygen species.82

Fig. 16. Schematic illustration of the formation of MoFe/Beta@CeO2 core-shell catalyst.84

Fig. 17. The proposed reaction mechanism of NO reduction by CO over Pt/CeO2-NR catalyst.85 42

Fig. 18. Toluene consumption rate at 220 oC versus (a) initial H2 consumption rate at 220 oC and (b) Oads/Olatt molar ratio over the as-prepared samples under the conditions of toluene concentration = 1000 ppm, toluene/O2 molar ratio = 1/400, and SV = 20000 mL/(g · h).88

Fig. 19. TEM images and SAED patterns (insets) as well as the Au size distribution of the LSMO and xAu/LSMO samples.99

43

Fig. 20. Schematic illustration of loading Pt NPs on 3DOM CZY.107

44

TOC Atomosphere pollutants

Catalyst

Noble metal/meso-Rare earth oxide

Harmless products

Noble metal/3DOM Rare earth oxide

Heterogeneous catalysis is an effective pathway for the removal of atmosphere pollutants, in which rare earth-based oxides and their supported noble metals show good catalytic performance. Several factors, such as specific surface area, pore structure, particle size and dispersion, adsorbed oxygen species concentration, reducibility, reactant activation ability, and interaction between metal nanoparticles and support, can influence catalytic performance of these materials.

45

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: