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Research progress on the catalytic elimination of atmospheric molecular contaminants over supported metal-oxide catalysts Xiaojiang Yao,ab Changjin Tang,ab Fei Gaoab and Lin Dong*ab Catalytic elimination is an important technique to reduce the emission of atmospheric molecular contaminants (such as CO, NOx, VOCs, HC, and PM, etc.) efficiently. In this field, the supported metal-oxide catalysts have attracted more and more attention in recent years due to their low cost and excellent catalytic performance. It is well known that catalytic performances are significantly dependent on the supports, surface-dispersed components, and the pretreatment of the catalysts. In this work, we present a brief review and propose some perspectives for supported metal-oxide catalysts according to the above-mentioned three aspects. Meanwhile, this paper covers some interesting results about the

Received 30th March 2014, Accepted 23rd May 2014 DOI: 10.1039/c4cy00397g www.rsc.org/catalysis

preparation of supported metal-oxide catalysts and the improvement of their catalytic performances for the elimination of atmospheric molecular contaminants obtained by our research group. Moreover, we propose the concepts of “green integration preparation (GIP)” and “surface synergetic oxygen vacancy (SSOV)” to understand the relationship between the “composition–structure–activity” of the supported metal-oxide catalysts, and further clarify the nature of the catalytic reactions.

Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China. E-mail: [email protected]; Fax: +86 25 83317761; Tel: +86 25 83592290 b Jiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern Analysis, Nanjing University, Nanjing 210093, PR China

pollution is an urgent task. In particular, the control of atmospheric contamination has attracted much attention due to its wide pollution range and strong mobility. According to the composition of pollutants, atmospheric molecular contaminants mainly contain carbon monoxide (CO), nitrogen oxides (e.g., N2O, NO, NO2, and N2O5, etc., hereafter denoted as NO x), volatile organic compounds (VOCs), hydrocarbons (HCs), particulate matter (PM, such as soot), and sulfur compounds (SO2 and H2S, etc.).1–6 Many investigation results have indicated that catalytic elimination is the most efficient approach for handling the

Xiaojiang Yao was born in Chongqing, China, in 1986. He received his bachelor degree of applied chemistry from Chongqing University in 2009. Now, he is studying for a doctor degree of chemistry at Nanjing University under the guidance of Professor Lin Dong. His main research topic is the preparation, characterization and application of supported metal-oxide catalysts and composite oxide catalysts Xiaojiang Yao for the catalytic elimination of atmospheric molecular contaminants.

Changjin Tang was born in Anhui, China, in 1984. He received his PhD degree from the School of Chemistry and Chemical Engineering of Nanjing University in 2011 with supervisor of Professor Lin Dong, and was a postdoctoral fellow at the School of Environment of Nanjing University with Professor Shourong Zheng. He is currently an associate researcher of Nanjing University. His research Changjin Tang interest is preparation of functional materials and their applications in environmental catalysis.

1. Introduction With the rapid development of industry, environmental pollution has become more and more serious in recent years, which is very harmful to human health and the ecological balance. As a result, the treatment of the environmental a

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above-mentioned atmospheric molecular contaminants.7–11 It is well known that the catalyst is the key within this technique, in which supported noble-metal catalysts exhibit excellent catalytic performances. But the scarcity of this resource, its expensive price, low hydrothermal stability, and poor sulfur resistance are major drawbacks for their wide application in the catalytic elimination of atmospheric molecular contaminants.12–15 Therefore, as alternatives to supported noble-metal catalysts, supported metal-oxide catalysts have been widely investigated in recent years due to their excellent catalytic performance and low cost.16–20 It is well known that the supports, additives, and active species are the main components of supported metal-oxide catalysts. Commonly used supports are carbon materials, γ-Al 2O 3, CeO 2, ZrO 2, TiO 2, SiO2, and their mixed oxides, e.g. CeO2, MnOx, CoOx, etc. are usually chosen as the additives due to their variable valence states; whereas the active species mainly focused on are CuO, NiO, MnO x, CoOx, FeOx, etc.21–27 In order to understand the nature of the catalytic reactions, it is necessary to investigate the relationship amongst the “composition–structure–activity” of the supported metal-oxide catalysts at the molecular and atomic levels.28 It has long been considered that the properties of the surface-dispersed components (additives and active species) are always influenced to some extent by the characteristics of the support.29 Therefore, understanding the interaction between the surface-dispersed components and support of the supported metal-oxide catalyst is a key step for exploring the nature of the catalytic reactions, and to provide a valuable scientific basis for the design and preparation of novel, practical, and efficient catalysts for the catalytic elimination of atmospheric molecular contaminants. Furthermore, a lot of investigation results have shown that interactions between each component of the supported metal-oxide catalyst can be adjusted by many factors, which results in different catalytic

performances.30–36 These influencing factors mainly include: (1) the composition, structure, and exposed crystal planes of the supports; (2) the type, loading amount, preparation conditions, and modification of the active species; and (3) the pretreatment of the catalysts, etc. In the present work, we summarize the application of supported metal-oxide catalysts in the catalytic elimination of atmospheric molecular contaminants from the aspect of the support preparation, the loading of surface-dispersed components, and the catalyst pretreatment. We expect to further understand the nature of the catalytic reactions through investigating the relationship between the “composition– structure–activity” of the supported metal-oxide catalysts, and to provide some theoretical basis for the design and preparation of novel, practical, and efficient catalysts for the catalytic elimination of atmospheric molecular contaminants, which is very useful for future investigations.

Fei Gao was born in Jiangsu, China, in 1980. He received his PhD degree in physical chemistry under Professor Lin Dong's guidance in 2008 from Nanjing University. After graduation, he started to work as a faculty member in the Center of Modern Analysis and Jiangsu Key Laboratory of Vehicle Emissions Control, Nanjing University. His research is focused on the synthesis and surface modification Fei Gao of nanomaterials and mesoporous materials, as well as their applications in environmental catalysis (deNOx and photocatalysis, etc.) with in situ characterizations (in situ IR and ex situ XPS, etc.).

Lin Dong was born in Sichuan, China, in 1963. He received his PhD degree from Nanjing University in 1995 under the guidance of Professor Yi Chen, and then was a postdoctoral fellow at Nanjing University with Professor Naiben Min. He has been a professor in the School of Chemistry and Chemical Engineering of Nanjing University since 2003. Now, he is the director of Jiangsu Key Laboratory of VehiLin Dong cle Emissions Control, Nanjing University. He leads a research group of ca. 20 members and his research interest includes the preparation of supported catalysts and functional materials, as well as their applications in environmental catalysis. He has published more than 120 papers and 20 patents in the field of air pollution control.


2. Effect of the support It is recognized that the support, as one of the essential components of the supported metal-oxide catalyst, can influence the corresponding catalytic performance significantly. In recent years, investigation into supports has mainly focused on the different single oxide supports, various mixed oxide supports, as well as special structures and specific exposed crystal planes of the supports. Therefore, in this section, we will discuss the effect of the support by classification, as follows. 2.1. Different single oxide supports In order to screen out the most suitable support for a supported metal-oxide catalyst applied for the catalytic elimination of atmospheric molecular contaminants, researchers have carefully investigated the influence of different single

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oxide supports (such as γ-Al 2O3, CeO2, TiO 2, SiO2, and ZrO 2, etc.) on the corresponding catalytic performances.37–42 Wang et al.43 prepared a series of CoOx/γ-Al2O3, CoOx/SiO2, and CoOx/TiO2 catalysts for CO oxidation, and found that the catalytic performance of CoOx/SiO2 was better than that of the CoOx/γ-Al2O3 and CoOx/TiO2 catalysts (Fig. 1). A possible reason might be that the interaction between CoOx and γ-Al2O3 or TiO2 was too strong, which promoted the formation of CoAl2O4 or CoTiO3 spinel, further resulting in the decrease of surface area, reduction behavior, and catalytic performance. Braun et al.44 compared the catalytic performances of MoO 3/γ-Al 2O 3 and MoO 3/SiO 2 catalysts for soot combustion, and pointed out that molybdenum species on the surface of γ-Al2O3 and SiO2 were in the form of MoO4− and Mo 2O 72−, respectively. They concluded that the excellent catalytic performance of the MoO3/SiO2 catalyst was due to the sufficient contact between the Mo2O72− species and soot. Furthermore, the effect of different single oxide supports on the physicochemical properties and catalytic performances of the supported metal-oxide catalysts was also observed in the complete oxidation of acetone over MnO x supported on Al 2O 3- and ZrO 2-pillared clay catalysts.45 Our research group also carried out some investigations to explore the influence of different single oxide supports on the catalytic performances of the supported metal-oxide catalysts.22,46 A series of CuO/γ-Al2O3, CuO/ZrO2, and CuO/CeO2 catalysts was prepared for NO reduction by CO. And the corresponding catalytic performances showed the following sequence: CuO/CeO 2 > CuO/ZrO2 > CuO/γ-Al2O3, which was not consistent with the BET specific surface area and grain size of the γ-Al2O3, ZrO2, and CeO2 supports (Table 1). Therefore, we gave a reasonable explanation from the coordination structure of copper species to discuss the influence of the support, as follows: on the surface of CeO2, the incorporated Cu2+ species was in an unstable five-coordination structure; on the surface of ZrO2, the Cu2+ species was in an elongated environment; whereas on the surface of γ-Al2O3, the Cu2+ species was in a symmetrical and stable octahedral coordination. The variation

Fig. 1 Catalytic activity of CoO x/γ-Al2O 3, CoOx/SiO 2, and CoO x/TiO2 catalysts for CO oxidation.43


Catalysis Science & Technology Table 1 The BET specific surface area and grain size of γ-Al2O3, ZrO2, and CeO2 supports22

Sample

SBET (m2 g−1)

Grain size (nm)

γ-Al2O3 ZrO2 CeO2

154.3 126.8 68.0

7.11 60.89 34.67

of the coordination structure of the Cu2+ species could affect the reduction behavior significantly, and further resulted in different catalytic performances. It can be seen that the above-mentioned studies are mainly about the supported single metal-oxide catalysts. With the deepening of investigations, researchers began to explore the influence of different single oxide supports on the catalytic performance of supported dual metal-oxide catalysts.40,47,48 Pantaleo et al.40 impregnated CuO–Cr 2O 3 on the surface of γ-Al 2O 3 and SiO 2 to obtain CuO–Cr 2O 3/γ-Al 2O 3 and CuO–Cr2O3/SiO2 catalysts, and evaluated their catalytic performance for CO oxidation. They found that SiO2 was beneficial to the generation of interaction between CuO and Cr 2O 3 to promote the formation of CuCr 2O 4 and CuCrO 2, which resulted in an excellent catalytic performance. However, there was no evidence for such an interaction existing between CuO and Cr 2O3 on the surface of γ-Al2O3. Jin et al.48 reported that the catalytic performance of MnOx–CeO2/TiO2 for the selective catalytic reduction of NO by NH 3 was better than that of MnO x–CeO 2/γ-Al 2O 3 at low temperature (80–150 °C) due to there being more Lewis acid sites. While the temperature was higher than 150 °C, the MnOx–CeO2/γ-Al2O3 catalyst exhibited a superior activity over MnOx–CeO2/TiO2, because the former possessed more Brønsted acid sites, which was beneficial to the oxidation of NO to NO 2 at higher temperature, further promoting the enhancement of the catalytic performance.

2.2. Various mixed oxide supports Many investigation results have indicated that supported metal-oxide catalysts with mixed oxide supports exhibit an excellent catalytic performance compared to the corresponding catalysts with single oxide supports.49–53 For example, Lin et al.50 investigated the physicochemical properties and catalytic performance of the CuO/CeO 2, CuO/Ce 0.7Sn 0.3O 2, and CuO/SnO 2 catalysts for CO oxidation through H 2-TPR, XRD, CO-TPD, and the CO + O2 reaction. They found that the CuO/Ce0.7Sn0.3O2 catalyst showed the optimal catalytic performance because the introduction of Sn4+ into CuO/CeO2 promoted the generation of a synergistic interaction between the dispersed CuO and Ce 0.7Sn 0.3O 2, which further resulted in the reduced CuO/Ce0.7Sn0.3O2 catalyst that could be easily oxidized to supply active oxygen species. Therefore, the investigation of supported metal-oxide catalysts with mixed oxide supports has attracted more attention in recent years. Some researchers have systematically studied the influence of the atomic ratio of the mixed oxide supports on the catalytic performances of the supported metal-oxide


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catalysts for the elimination of atmospheric molecular contaminants.7,13,16,24,54–57 Wang et al.16 adopted XRD, Raman, H2-TPR, XPS and the CO + O2 reaction to investigate the influence of the Ce/Zr ratio on the catalytic performance of CuO/CexZr1−xO2 catalysts for CO oxidation. They concluded that there were three kinds of copper oxide species on the surface of the CexZr1−xO2 support, i.e., highly dispersed CuO, clustered CuO, and crystalline CuO. The Ce/Zr ratio could affect the dispersion behavior of CuO on the surface of the CexZr1−xO2 support, and further influence the catalytic performance of the CuO/Ce xZr 1−xO 2 catalysts because the highly dispersed CuO was the active species for CO oxidation. Ren et al.24 synthesized a series of CuO/CexMn1−xO2 catalysts for propane combustion, and found that CuO/Ce 0.6Mn 0.4O 2 exhibited the best catalytic performance due to the synergistic effect between CuO and Ce 0.6Mn 0.4O 2, as well as the increase of oxygen migration ability and the surface oxygen defect. Liu et al.58 compared the catalytic performances of CuO/TixCe1−xO2 for the selective catalytic reduction of NO by C3H 6. They found that the CuO/Ti 0.9Ce 0.1O 2 catalyst showed the best catalytic activity and N 2 selectivity due to its abundant Lewis acid sites and adsorbed oxygen species on the surface of this catalyst, which provided more active sites for the adsorption and activation of NO. Furthermore, the in situ FT-IR results suggested that the synergistic effect of Cu and Ce was beneficial to the formation of nitrates, oxygenated hydrocarbons, and the key intermediate isocyanate (–NCO species), which promoted the enhancement of the catalytic performance. Recently, Hong et al.59 thoroughly investigated the effect of Fe content on the physicochemical properties and catalytic performance of BaO/Ce xFe 1−xO 2−y catalysts for the direct decomposition of NO. They pointed out that the catalyst with the Fe/(Ce + Fe) ratio of 0.02 showed the optimal catalytic performance due to the most isolated tetrahedral Fe3+ ions and the increase of the concentration of surface oxygen vacancies. Some similar results were obtained in BaO/CexMn1−xO2−y catalysts,60 and it was found that the catalyst with the Mn/(Ce + Mn) ratio of 0.25 exhibited the best catalytic performance. From this, the authors correlated the NO conversion to the concentration of surface oxygen vacancies very well, as shown in Fig. 2. However, not all supported metaloxide catalysts with mixed oxide supports exhibit an enhanced catalytic performance. For example, CuO/Ce1−xSmxOδ catalysts show a lower activity than the CuO/CeO2 catalyst for ethyl acetate oxidation, because the incorporation of Sm3+ leads to the deterioration of textural and redox characteristics, which in turn negatively affects the activity of ethyl acetate oxidation.61 On the other hand, adjusting the types of dopant to prepare various mixed oxide supports and to investigate their influence on the corresponding catalytic performance of the supported metal-oxide catalysts has become a hot topic in the catalytic elimination of atmospheric molecular contaminants during recent years.62–66 Rao et al.64 evaluated the catalytic performances of CuO/CeO2–Al2O3, CuO/CeO2–ZrO2, and CuO/CeO2–SiO2 catalysts for CO oxidation. They reported that the CuO/CeO 2–Al 2O 3 catalyst exhibited the most excellent


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Fig. 2 Effect of the Mn/(Ce + Mn) ratio on I 256/I 456 calculated from the Raman spectra of BaO/Ce xMn 1−xO 2−y catalysts. For comparison, NO conversion (800 °C) and the normalized NO conversion (600 °C) according to the surface oxygen vacancies (I 256/I 456 value) were also plotted in the figure.60

catalytic performance due to the good dispersion and enhanced reduction behavior of the copper oxide species on the surface of the CeO2–Al2O3 support. They also found that the CuO/CeO2– ZrO2 catalyst displayed the highest activity for soot combustion among these catalysts.65 The obtained results indicated that the introduction of Zr4+ into the CuO/CeO2 catalysts benefited the creation of more structural defects, which could accelerate the diffusion of oxygen and induce more surface active oxygen species, further promoting the enhancement of the catalytic performance at low temperature. Bennici et al.66 pointed out that the catalytic performance of CuO/SiO2–ZrO2 was better than that of the CuO/SiO2–Al2O3 and CuO/SiO2–TiO2 catalysts for the selective catalytic reduction of NOx by ethane. They attributed the reason to the SiO2–ZrO2 support possessing the most acid sites, which could enhance the interaction between CuO and the SiO2–ZrO2 support, further resulting in the excellent catalytic performance. Recently, the influence of the atomic ratio and dopant type of the mixed oxide support on the catalytic performance of supported metal-oxide catalysts has also been systematically investigated by our research group.67,68 Firstly, we loaded CuO onto a series of CexZr1−xO2 supports, and evaluated their catalytic performance for NO reduction by CO. The obtained results indicated that CuO/Ce0.8Zr0.2O2 exhibited the best catalytic performance, which could be attributed to the unstable fivecoordination structure of Cu2+ and the synergistic interaction between the copper oxide species and ceria-rich phase support easily promoting the reduction of copper oxide species and surface oxygen species of the support, as well as the activation of the adsorbed NO species. After that, we compared the catalytic performances of CuO/Ce0.67M0.33O2 (M = Zr4+, Ti4+, and Sn4+) for NO reduction by CO, and found that the catalytic performance of CuO/Ce0.67Zr0.33O2 was better than that of the CuO/Ce0.67Ti0.33O2 and CuO/Ce0.67Sn0.33O2 catalysts. A possible reason is that the electronegativity of Ce (1.10) and Zr (1.33) are smaller than that of Ti (1.54), Cu (1.90), and Sn (1.96), meaning that it is easier to attract electrons from cerium and zirconium species to copper species, causing the reduction of Cu2+ to


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form active Cu+/Cu0 species, and further promoting the enhancement of catalytic performance.


2.3. Special structures The special structures of the supports are not only conducive to the dispersion of the active species, but also benefit the activation and diffusion of the reactant molecules. They can promote the enhancement of catalytic performance of the supported metal-oxide catalysts, and have consequently become one of the hot topics in the catalytic elimination of atmospheric molecular contaminants. In 1997, Velev et al.69 developed a new method to prepare the SiO 2 support with highly uniform and well-structured pores of tunable size in the submicrometer region, in which the modified colloidal crystals were used as templates for the polymerization of silica. And then, Holland et al.70 synthesized TiO 2, ZrO 2, and Al 2O 3 materials with periodic threedimensional arrays of macropores from the corresponding metal alkoxides by using latex spheres as templates, and found that the resulting samples could be applied in many fields, such as heterogeneous catalysis. After that, porous materials with well-structured pores of tunable size (especially three-dimensionally ordered macroporous (3DOM) materials and mesoporous materials) were widely investigated, because they are good supports for the preparation of supported metal-oxide catalysts, and have the potential for application in heterogeneous catalysis. For the 3DOM materials, Ji et al.71 fabricated 3DOM and bulk Eu0.6Sr 0.4FeO3 supports by PMMA-templating and citric acidassisted hydrothermal methods, respectively. And then, they loaded Co3O4 onto the surface of these supports to prepare Co 3O 4/3DOM-Eu 0.6Sr 0.4FeO 3 and Co 3O 4/bulk-Eu 0.6Sr 0.4FeO 3 catalysts for the combustion of toluene. The obtained results indicated that the catalytic performance of Co3O4/3DOM-Eu0.6Sr0.4FeO3 was obviously better than that of Co3O4/bulk-Eu0.6Sr0.4FeO3, which was because the formation of the 3DOM structure was beneficial to the increase of specific surface area, as well as the adsorption and diffusion of reactant molecules, further resulting in a higher adsorbed oxygen species concentration and better low-temperature reducibility. Similarly, Li et al.72 also found that Co3O4/3DOM-La0.6Sr0.4CoO3 exhibited an excellent catalytic performance for toluene combustion compared with Co3O4/bulk-La0.6Sr0.4CoO3. This phenomenon could be attributed to the high adsorbed oxygen species concentration, good low-temperature reducibility, and synergistic interaction between Co3O4 and its 3DOM-La0.6Sr0.4CoO3 support, as well as the high-quality 3DOM structure. Furthermore, the promotion effect of the 3DOM structure on the catalytic performance was also observed in the combustion of toluene and methanol over MnOx/3DOM-LaMnO3 catalysts by Liu and co-workers.73 Recently, as a support for the supported metal-oxide catalysts, mesoporous materials have been widely investigated due to their excellent catalytic performances for the elimination of atmospheric molecular contaminants.74–77



Patel et al.75 investigated the catalytic performance of copper oxide supported on different mesoporous silica (SBA-15, MCM-41, MCM-48, and KIT-6) catalysts for NO reduction by CO. They found that CuO/SBA-15 and CuO/MCM-41 catalysts exhibited a higher catalytic activity than CuO/MCM-48 and CuO/KIT-6 samples, which resulted from the good dispersion of CuO and the excellent reduction behavior of the copper oxide species in the channels of the mesoporous SBA-15 and MCM-41. Szegedi et al.32 reported that CuO–FeO x/SBA-15 showed a better catalytic performance for toluene combustion than the CuO–FeOx/SBA-16 catalyst. The reason for this could be attributed to the good dispersion of copper and iron oxides in the mesoporous channels of SBA-15 (whereas they were on the outer surface of SBA-16, and more easily agglomerated) was beneficial for generating a bimetallic phase, and further enhancing the catalytic activity and stability for toluene combustion. Li et al.78 synthesized two kinds of mesoporous SBA-15 materials with different pore diameters; they then loaded cobalt oxide into the corresponding mesoporous channels for benzene combustion. They found that the Co3O4/SBA-15 catalyst with a larger pore diameter exhibited a better catalytic performance due to the good dispersion of the cobalt oxide species. Moreover, materials with tube-like structures have attracted more attention in recent years due to their unique properties and excellent catalytic performance for the elimination of atmospheric molecular contaminants.79–82 Jiang and Song80 pointed out that tuning the surface structures of carbon nanotubes (CNTs) could improve the catalytic performance of Co3O4/CNTs for toluene combustion. In particular, the surface defect structures of CNTs could enhance the redox properties of Co3O4 and increase the ratio between the adsorbed oxygen species and the surface lattice oxygen species, which led to the excellent catalytic performance. Su et al.83 adopted different loading methods to obtain MnO x/CNTs catalysts with MnOx only on the outside surface or both on the inside and outside surfaces of CNTs for the selective catalytic reduction of NO by NH 3. They pointed out that MnOx confined in the channels of the CNTs exhibited an excellent catalytic performance due to the better ability of supplying oxygen and adsorbing NO, which could be related to the electronic interaction between MnO x species and the inner surface of the CNTs. In addition, Ren et al.81 designed a domain-confined macroporous catalyst (Co3O4 nanocrystals anchored on TiO2 nanotubes, denoted as Co3O4/TiO2-NTs) for soot combustion. The obtained results indicated that the Co3O4/TiO2-NTs catalyst exhibited a better catalytic performance than TiO2-powdersupported Co3O4 nanocrystals, which could be attributed to the good reduction behavior and confined macroporous structure of the Co3O4/TiO2-NTs. For supported metal-oxide catalysts with special structured supports, some interesting results were obtained in our recent investigations.84,85 From the viewpoint of maximized utilization of material and environmental protection, we innovatively designed the following route of green integration preparation (GIP). Firstly, polyhydroxy carbohydrate


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compounds (e.g., glucose) underwent a hydrothermal treatment and were then filtered and washed with distilled water to obtain hard templates (residues, such as carbon microspheres) and soft templates (helical carbon chains in the filtrates). Finally, according to the individual unique properties of the hard and soft templates, SiO 2 hollow spheres and novel three-grade porous helical silica tubes were fabricated by utilizing the interaction between silicon sources and templates, respectively, which are displayed in Fig. 3. On the one hand, SiO 2 hollow spheres were synthesized by a sol–gel method using carbon microspheres as hard templates, and then used as a support to prepare the CuO/SiO2 catalyst. We found that the CuO species supported on SiO 2 hollow spheres showed a better catalytic performance for CO oxidation than a commercial, SiO2-supported CuO catalyst, which could be because the unique hollow spherical texture was beneficial to the formation of more active sites and the diffusion of reactant molecules. On the other hand, the novel three-grade porous helical silica tubes were prepared by an ingenious multi-soft-template method, which is a promising kind of support for supported metal-oxide catalysts. We think that the active species supported on first-, second- or thirdgrade pores of the novel three-grade porous helical silica tubes are bound to exhibit different physicochemical properties and catalytic performances. As a result, we plan to load some active species (such as CuO, FeO x, and CoO x, etc.) on this material applied for the catalytic elimination of atmospheric molecular contaminants. We believe that the design and preparation of multi-grade porous materials as the a support for supported metal-oxide catalysts will be a promising investigation direction in the future.

2.4. Specific exposed crystal planes It is well known that supports with different exposed crystal planes lead the supported metal-oxide catalysts to exhibit different physicochemical properties and catalytic performances.

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Therefore, the influence of the support with specific exposed crystal planes on the catalytic performance of the supported metal-oxide catalyst for the elimination of atmospheric molecular contaminants has become a hot topic in recent years.86–89 Zhou et al.86 reported that CuO/CeO 2-nanorods exhibited a better catalytic performance for CO oxidation than CuO/CeO 2-nanoparticles. The reason might be that the CeO 2-nanorods exposed high-energy and more reactive {001} and {110} facets, which were beneficial to the generation of a synergistic interaction between the copper oxide species and ceria. The research group of Zhang88,89 systematically investigated the influence of the support with specific exposed crystal planes on the catalytic performance of the supported metal-oxide catalysts. Firstly, they synthesized a series of MnOx/CeO 2–ZrO2-nanorods, MnOx/CeO2–ZrO2-nanocubes, and MnOx/CeO2–ZrO2-nanopolyhedra catalysts for the selective catalytic reduction of NO by NH 3. They found that MnOx/CeO2–ZrO2-nanorods with {110} and {100} facets exhibited a better catalytic performance than MnOx/CeO2–ZrO2-nanocubes with {100} facets and MnOx/CeO2–ZrO2-nanopolyhedra with {111} and {100} facets, which could be attributed to the large amounts of Mn4+ species, surface adsorbed oxygen, and oxygen vacancies associated with the exposed {110} facets. After that, they further investigated the catalytic performance of MnOx/Ce0.9Zr0.1O2-nanorods for the selective catalytic reduction of NO by NH3 through experimental and theoretical methods. They pointed out that the MnOx/Ce0.9Zr0.1O2-nanorods mainly exposed {110} facets, which benefited the generation of interactions between MnOx and the Ce0.9Zr0.1O2-nanorods, as well as the formation of oxygen vacancies and the active nitrite intermediate (NOO˙), further promoting the enhancement of the catalytic performance. In addition, our research group also carried out some related investigations in this aspect.90,91 Firstly, the influence of the exposed crystal planes of CeO 2 on the catalytic performance of CuO/CeO 2 catalysts for NO reduction by CO was deeply investigated. The obtained results suggested that

Fig. 3 Schematic diagram of green integration preparation (GIP): the residues are carbon microspheres (hard templates) used to prepare the desired metal oxide hollow spheres; the filtrates are rich in the required helical carbon chains (soft templates) applied for the preparation of the helical structure materials.



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CeO2-nanopolyhedra were enclosed by {111} and {100} facets, CeO 2-nanorods mainly exposed {110} and {100} facets, and CeO2-nanocubes displayed only polar {100} facets. The coordination environment of Cu2+ ions was different on the {111}, {110}, and {100} facets of CeO 2, in which the {110} facets were the most active, so CuO/CeO2-nanorods exhibited the best catalytic performance. And then, we further evaluated the catalytic performance of CuO/Ce 0.67Zr 0.33O 2 {110} facets and CuO/Ce0.67Zr0.33O2 {111} facets for CO oxidation. It was noticed that CuO/Ce0.67Zr0.33O2 {110} facets exhibited the optimal catalytic performance. The reason for this might be that the coordination environment of the reduced Cu+ dispersed on the {110} facets was a stable symmetrical octahedral structure, which was conducive to the stabilization of reduced Cu+, and further promoting the enhancement of catalytic performance due to the adsorption and activation of CO on the Cu+ species. Supports with specific exposed crystal planes have been widely investigated in the catalytic elimination of atmospheric molecular contaminants, but their poor thermal stability has seriously limited their practical application. Therefore, how to improve the thermal stability of supports with specific exposed crystal planes will be a key investigation area in the future.

3. Effect of the surface-dispersed components It has been recognized that the dispersion states, valence states, and synergistic interaction of the surface-dispersed components can inevitably affect the catalytic performance of supported metal-oxide catalysts. Therefore, related investigations have attracted more attention in recent years, in which the different types of active species, various loading amounts, different preparation conditions, and the introduction of additives have been adopted to adjust the dispersion states, valence states, and synergistic interaction of the surface-dispersed components, and some interesting results have been obtained. 3.1. Different types of active species Some research results indicate that the inherent properties of the active species can significantly affect the catalytic performance of supported metal-oxide catalysts for the elimination of atmospheric molecular contaminants.92–96 Various metal-oxides (CuO, MnO x, FeO x, V 2O 5, MoO 3, Co 3O 4, NiO, and ZnO) were supported on the surface of γ-Al 2O 3 for toluene combustion.37 It was found that CuO/γ-Al2O3 exhibited the best catalytic performance among these catalysts due to the synergistic interaction between CuO and γ-Al2O3. Bourikas et al.94 prepared a series of V2O5/TiO 2, CrO3/TiO2, MoO3/TiO2, and WO3/TiO2 catalysts for the selective catalytic reduction of NO by NH3. They found that the catalytic performance of these catalysts followed the order of V 2O 5/TiO 2 > CrO 3/TiO 2 > MoO 3/TiO 2 ≥ WO 3/TiO 2. The obtained results demonstrated that the catalytic performance of these catalysts correlated well with the intensity of the



UV-vis diffuse reflectance spectroscopy (UV-vis DRS) absorption band appearing at ca. 400 nm, which was considered as a measurement of the extent of interaction between the active species and support in these catalysts. Doggali et al.97 compared the catalytic performance of FeO x/ZrO 2, Co 3O 4/ZrO 2, NiO/ZrO 2, CuO/ZrO 2, and MnO x/ZrO 2 for CO oxidation and soot combustion. They pointed out that the catalytic performances of these catalysts could be ranked as Co3O4/ZrO2 > MnOx/ZrO2 > CuO/ZrO2 > FeOx/ZrO2 > NiO/ZrO2, in which the optimal catalytic performance of Co3O4/ZrO2 was attributed to the inherent property of the active Co3O4 phase rather than to other reasons. In addition, Leocadio et al.98 reported that MoO3/γ-Al2O3 exhibited a better catalytic performance for soot combustion than the V2O5/γ-Al2O3 catalyst. The reason for this they attributed to the reaction occurring through the formation of carbonate species at the catalyst/soot interface to yield CO2, and the carbonate species on the surface of the MoO3/γ-Al2O3 catalyst could be easily decomposed, which was evidenced by their infrared absorption spectroscopy results of CO adsorption. Our research group also investigated the influence of different types of active species on the catalytic performance of supported metal-oxide catalysts.99 We prepared a series of CeO 2–ZrO 2–Al 2O 3 supported metal-oxide (FeO x, Co 3O 4, NiO, CuO, and MnO x) catalysts for NO reduction by CO. It was noticed that the CuO/CeO 2–ZrO 2–Al 2O 3 catalyst showed the best catalytic performance due to the high dispersion of copper oxide species, low-temperature reducibility, and more surface oxygen vacancies, which resulted from the synergistic interaction between CuO and CeO2–ZrO2–Al2O3. 3.2. Various loading amounts of active species The loading amount of active species can obviously influence the corresponding dispersion states and reduction behavior, which further results in different catalytic performances of supported metal-oxide catalysts. As a result, in recent years many researchers have systematically investigated the effect of varying the active species loading amount.100–105 Xing et al.101 prepared a series of CrO x/γ-Al 2O 3 catalysts with different loading amounts of Cr for benzene combustion, and observed that the catalyst with a Cr loading amount of 8.5 wt% (near the dispersion capacity, 7.5 wt%) exhibited the best catalytic performance due to its lowest apparent activation energy, as shown in Fig. 4. In order to observe the difference between the catalytic performances of these CrOx/γ-Al2O3 catalysts clearly, they also gave the temperatures of 50% (T 50) and 90% (T 90) benzene conversion (Table 2). Patel et al.106 reported that the catalytic performances of CuO/SBA-15 catalysts for NO reduction by CO increased with the increase of CuO loading amount from 4.01 to 8.67 wt%, while they declined with further increase of the CuO loading amount to 10.1 wt% due to the formation of crystalline CuO and Cu 2O at these higher CuO loading amounts. The effect of copper loading amount was also discussed in soot combustion.107 It is well known that the activity of the


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Indeed, there is always a threshold value for the dispersion of metal-oxide on certain supports. In the 1990s, based on the analysis of many experimental results and relevant data reported in the literature, our research group proposed the “Incorporation Model” theory to predict the dispersion capacities of active species on the surface of supports, and to describe the interaction between active species and supports, which was very important for the design and preparation of efficient and practical supported metal-oxide catalysts.108,109 Furthermore, we investigated the influence of copper oxide loading amounts on the catalytic performances of CuO/Ce0.5Zr0.5O2 catalysts for CO oxidation, and found that the catalyst with a copper oxide loading amount at the dispersion capacity (according to the “Incorporation Model” theory) exhibited the best catalytic performance, because the highly dispersed CuO was the main active species of copper-based catalysts for CO oxidation.110

3.3. Different preparation conditions

Fig. 4 (a) Catalytic activity, and (b) Arrhenius plots for CrO x/γ-Al 2O 3 catalysts with different loading amounts of Cr toward benzene combustion.101

Table 2 The values of T50, T90, and apparent activation energy (Ea) of these CrOx/γ-Al2O3 catalysts for benzene oxidation101

Sample

T50 (°C)

T90 (°C)

Ea (kJ mol−1)

γ-Al2O3 1.7CrOx/γ-Al2O3 5.1CrOx/γ-Al2O3 8.5CrOx/γ-Al2O3 10.2CrOx/γ-Al2O3 11.9CrOx/γ-Al2O3 13.6CrOx/γ-Al2O3

425 377 333 310 320 327 330

495 433 387 338 370 375 378

— 97 82 69 84 91 102

CuO/γ-Al 2O 3 catalyst is dependent on the amount of easilyreduced Cu2+ species, where the amount of the most active Cu2+ species increased with the amount of copper loading from 1 to 5 wt% and remained almost a constant for higher copper loading amounts. Therefore, the CuO/γ-Al2O3 catalyst with a copper loading of 5 wt% exhibited the best catalytic performance for soot combustion. All of the above-mentioned results indicate that supported metal-oxide catalysts with the loading amount of active species near the dispersion capacity display their optimal catalytic performance for the elimination of atmospheric molecular contaminants.


It is well known that the loading process of active species is very important for the preparation of supported metal-oxide catalysts. Recently, the influence of the loading method, precursor, and calcination atmosphere of the active species on the catalytic performance of supported metal-oxide catalysts for the elimination of atmospheric molecular contaminants has been investigated systematically.111–115 Ataloglou et al.112 prepared a series of Co3O4/γ-Al2O3 catalysts by pore volume impregnation (pvi), equilibrium deposition filtration (edf), and nitrilotriacetic acid-assisted pore volume impregnation (na-pvi) methods, respectively. They found that the Co3O4/γ-Al2O3 catalyst obtained by the na-pvi method exhibited the best catalytic performance for benzene combustion, which was because the active Co3O4 species was highly dispersed and moderately interacted with the γ-Al2O3 support. Zhang et al.116 evaluated the catalytic performance of MnOx/TiO2 catalysts obtained by traditional impregnation (TI) and ultrasonic impregnation (UI) methods for the selective catalytic reduction of NO by NH 3. They found that the MnO x/TiO 2-UI catalyst displayed a better catalytic performance than the MnOx/TiO2-TI catalyst, which was related to the ultrasonic process obviously improving the dispersion behavior and surface acid property of MnOx on the surface of TiO2, significantly enhancing their synergistic interaction. Harrison et al.117 discussed the influence of cobalt precursors (cobalt nitrate and cobalt acetate) on the catalytic performance of Co 3O 4/CeO 2 catalysts for soot combustion. They reported that the Co 3O 4/CeO 2 catalyst prepared from cobalt acetate showed a better catalytic performance due to smaller crystallite size of Co3O4. Li et al.118 also investigated the influence of different precursors (manganese nitrate and manganese acetate) on the catalytic performance of MnO x/TiO 2 catalysts for the selective catalytic reduction of NO by NH 3. They found that the MnOx/TiO2 catalyst obtained from manganese acetate exhibited a higher activity than that prepared from manganese nitrate, because the Mn species in the former was highly dispersed Mn 2O 3 and possessed a higher


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surface Mn concentration, which was beneficial to the enhancement of catalytic performance, whereas the Mn species in the latter was mainly crystalline MnO 2 accompanied with small amount of manganese nitrate. Moreover, the calcination atmosphere can significantly affect the catalytic performance of supported metal-oxide catalysts. Inaba et al.119 pointed out that CoOx/SiO2 catalysts prepared from cobalt acetate and calcined in N2 and air, respectively, exhibited different catalytic performances for the selective catalytic reduction of NO by C3H6. They found that the catalytic performance of the sample calcined in N2 was better than that of the sample calcined in air since it possessed more highly dispersed surface Co2+ species, which was the main active species for this reaction due to its solid acid properties. Our research group also carried out some related investigations to explore the influence of different preparation conditions on the catalytic performance of supported metal-oxide catalysts.120,121 With regard to the method used for loading the active species, we synthesized a series of CuO/CeO2 catalysts by an impregnation method (IM), a grinding method (GM), and a mechanical mixing method (MMM) for NO reduction by CO. It was noticed that the CuO/CeO2-IM catalyst showed the optimal catalytic performance due to the highest number of surface oxygen vacancies and Cu+ species, as well as the excellent reduction behavior. For the precursors of active species, Cu(NO3)2·3H2O and Cu(CH3COO)2·H2O were chosen as the precursors of copper oxide to prepare CuO/CeO2–ZrO2–Al2O3 catalysts, which were used for NO reduction by CO. The obtained results indicated that the CuO/CeO2–ZrO2–Al2O3 catalyst prepared from Cu(CH3COO)2·H2O exhibited a better catalytic performance due to the presence of Cu+ species and good reducibility (this work has not been published). Furthermore, we prepared a series of CuO–CoO x/γ-Al 2O 3 catalysts from Cu(NO3) 2·3H2O and Co(CH3COO) 2·4H2O, and calcined them in N2 and air atmospheres, respectively, to investigate the influence of calcination atmosphere on the catalytic performance of the CuO–CoOx/γ-Al2O3 catalysts for NO reduction by CO. It could be found that the Co species was in the form of crystalline Co 3O 4 in the CuO–CoO x/γ-Al 2O 3 catalysts calcined in air, whereas it was present as highly dispersed CoO in the CuO–CoOx/γ-Al2O3 catalysts calcined in N2, which was beneficial to the formation of Cu2+–O–Co2+ active species, and further promoted the enhancement of catalytic performance. Based on the above-mentioned results, we believe that the development of novel loading procedures of active species to enhance the catalytic performance of supported metal-oxide catalysts for the elimination of atmospheric molecular contaminants will be an important investigation direction in the future.

3.4. Introduction of additives Many research results have indicated that the introduction of additives (especially the metal-oxides with variable valence states, such as FeOx, CoOx, MnOx, and CeO2, etc.) can significantly enhance the catalytic performance of supported



metal-oxide catalysts for the elimination of atmospheric molecular contaminants.122–126 Todorova et al.124 investigated the effect of CoOx additives on the catalytic performance of the MnO x/SiO 2 catalyst for the combustion of n-hexane and ethyl acetate. They found that the catalytic performance was significantly improved by the introduction of cobalt oxide due to the high mobility of lattice oxygen, the presence of the Mn4+/Mn3+ redox couple, and the predominance of Co2+ on the surface of the MnO x–CoO x/SiO 2 catalyst. Menon et al.127 pointed out that the improved catalytic performance of CuO–CeO2/γ-Al2O3 for toluene combustion compared to the CuO/γ-Al 2O 3 catalyst was attributed to the formation of the Ce xCu 1−xO 2−x solid solution, in which the oxidation of toluene occurred at Cu2+ sites whilst the reduction of oxygen took place at Ce3+ sites. In other words, the redox couples of Ce4+/Ce3+ and Cu2+/Cu+ played a key role in the toluene combustion. Drenchev et al.128 reported that the modification of MnO x/γ-Al 2O 3 by CeO 2 led to a better dispersion of manganese oxide on the MnO x–CeO 2/γ-Al 2O 3 catalyst compared to the MnO x/γ-Al 2O 3 sample, because of the partial disappearance of alumina OH groups and the blocking of part of Al3+ Lewis acid sites, which was beneficial to the enhancement of catalytic performance for many catalytic reactions (including NO direct decomposition, NO reduction by CO, and CO oxidation, etc.). Furthermore, Khristova et al.129 systematically investigated the influence of CeO2 additives on the catalytic performance of the CuO/γ-Al 2O 3 catalyst for NO reduction by CO. They found that not only the introduction of CeO 2 additives improved the catalytic performance of the CuO/γ-Al2O3 catalyst, but also the impregnation sequence of copper and cerium precursors strongly affected the formation of different metaloxide phases and various active sites, further leading to different catalytic performances for NO elimination. It has widely been reported that the introduction of Ca additives can obviously enhance the N 2 selectivity of manganese-based deNOx catalysts, but the mechanism is still unclear. Therefore, Liu et al.130 prepared a series of Ca-modified CeO 2–MnO x/TiO 2 catalysts to investigate the mechanism. In-situ DRIFTS results indicated that the addition of Ca species significantly inhibited the formation of NH on the surface of the catalyst, which limited the reaction between NH and NO to generate N2O. Moreover, Ca additives also decreased the formation of NO2, which inhibited the reaction between NO2 and NH3 to form N2O, and further improved the N2 selectivity of the catalysts. Based on the in-situ DRIFTS results, they proposed a possible mechanism to understand the suppression of Ca additives on N2O formation, as shown in Fig. 5. Recently, our research group also systematically investigated the influence of additives on the catalytic performance of supported metal-oxide catalysts.131–133 Firstly, we prepared a series of Mn2O3-modified CuO/γ-Al2O3 catalysts, and found that Mn2O3 grew epitaxially on the surface of the γ-Al2O3 support, which led to the dispersion capacity of CuO on the surface of the γ-Al2O3 support increasing from 0.75 to 1.10 mmol


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components and support, which is considered to be one of the most efficient approaches for improving the catalytic performance of supported metal-oxide catalysts.


4.1. Atmosphere pretreatment

Fig. 5 The possible mechanism of the suppression of Ca additives on N2O formation.130

Cu2+/100 m2 γ-Al2O3 (obtained from our previously proposed “Incorporation Model” theory), which further enhanced the catalytic performance for NO reduction by CO. And then, we discussed the influence of WO 3 monolayer modification on the physicochemical properties and catalytic performance of CuO/Ce xZr 1−xO 2 catalysts. It could be observed that these CuO/CexZr1−xO2 catalysts showed different reduction properties, adsorption behaviors, and catalytic performances with the variation of Ce/Zr ratio before WO3 monolayer modification. Whereas after WO 3 monolayer modification, they exhibited similar physicochemical properties and catalytic performances that were independent of the variation in Ce/Zr ratio due to the formation of the WO 3 monolayer on the surface of the Ce xZr 1−xO 2 support, which would block the interaction between CuO and the Ce xZr 1−xO 2 support. Furthermore, a possible surface model of CuO dispersed on CexZr1−xO2 and monolayer WO3-modified CexZr1−xO2 supports was proposed, which is presented in Fig. 6. Finally, we explored the influence of the impregnation sequence of copper and manganese precursors on the catalytic performance of CuO–MnO2/CeO2 catalysts for NO reduction by CO. It could be found that the catalysts prepared by a co-impregnation method exhibited a better catalytic performance than the catalysts obtained by a stepwise-impregnation method. The reason for this could be attributed to the co-impregnation method being more conducive to strengthening the interaction amongst the components of the CuO–MnO2/CeO2 catalysts through more sufficient contact, which resulted in good reduction behaviors.

4. Effect of pretreatment Pretreatment of the supported metal-oxide catalysts can adjust the valence- and coordination-states of the active species, as well as the interaction between the surface-dispersed

Atmosphere pretreatment (such as oxidation pretreatment, reduction pretreatment, and reaction atmosphere pretreatment, etc.) of supported metal-oxide catalysts has been investigated exhaustively because it can lead to different catalytic performances.134–138 In particular, it is well known that a reduction pretreatment can result in the formation of coordinately unsaturated cations and suspension bonds on the surface of the supported metal-oxide catalysts, which lead the catalysts to being in unstable states, and further promoting the enhancement of the catalytic performance. Therefore, the influence of the reduction pretreatment on the catalytic performance of supported metal-oxide catalysts for the elimination of atmospheric molecular contaminants has attracted more attention in recent years.36,139–142 Pan et al.36 reported that the reduction pretreatment of CuO/γ-Al 2O 3 catalysts by H 2 and re-oxidation by air (HA) led to a better dispersion of copper species on the surface of γ-Al2O3 and a larger metal area per gram of Cu, both of which were beneficial to the enhancement of catalytic performance of the CuO/γ-Al2O3 catalyst for styrene combustion (Table 3). Yang et al.140 discussed the effect of reduction pretreatment on the catalytic performance of CuO/SBA-15 catalysts for benzene combustion. They found that the reduced catalysts exhibited a better catalytic performance than the unreduced catalysts, which could be attributed to the Cu0 species being more active than Cu2+ species for benzene combustion. Moreover, Boccuzzi et al.142 pointed out that the reduction pretreatment obviously enhanced the catalytic performance of CuO/TiO2 catalysts for NO reduction by CO, which was because the reduced state copper-based catalysts were beneficial to the dissociation of NO (the ratedetermining step). And then, they proposed a possible reaction mechanism to further understand the improvement in catalytic performance, as follows (note that (g) represents the gas phase, and (a) refers to the adsorbed state): CO(g) + Cu0 → Cu0–CO(a)

(1)

2NO(g) + 4Cu0 → 2Cu0–N(a) + 2Cu0–O(a) → N2(g) + 2Cu2+O− Cu2+O− + CO(g) → CO2(g) + 2Cu0

(2) (3)

Table 3 The information of active species dispersion and the temperature of 50% styrene conversion (T50) over CuO/γ-Al2O3 catalysts36

Fig. 6 Possible schematic diagram of surface model of CuO dispersed on CexZr1−xO2 and monolayer WO3-modified CexZr1−xO2 supports.132


Catalyst Metal dispersion (%) Metal surface area (m2 g−1 sample) Metal surface area (m2 g−1 metal) Active particle diameter (nm) T50 (°C)

CuO/γ-Al2O3 3.35 0.75 21.6 31.14 354

CuO/γ-Al2O3–HA 8.75 1.96 56.42 11.92 325


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NO(g) + e−(TiOx) → N(a) + O−(a)

(4)

NO(g) + N(a) → N2O(a)

(5)

N(a) + N(a) → N2(g)

(6)

CO(g) + N(a) → NCO(a)

(7)

N2O(a) → N2(g) + O(a)

(8)

Recently, our research group systematically explored the influence of the reduction pretreatment on the catalytic performance of supported metal-oxide catalysts for NO reduction by CO, and obtained some interesting results.143–146 We deeply investigated the catalytic performance of the 0.6Cu0.3Mn/Al catalyst (representing that the loading amounts of CuO and Mn2O3 were 0.6 mmol Cu2+/100 m2 γ-Al2O3 and 0.3 mmol Mn3+/100 m2 γ-Al2O3, respectively) before and after CO reduction pretreatment for NO reduction by CO. The obtained results indicated that the CO reduction pretreatment enhanced the catalytic performance of the 0.6Cu0.3Mn/Al catalyst remarkably. In order to further understand the reason for the obvious enhancement of the catalytic performance, we gave some reasonable explanations based on our previously proposed “Incorporation Model” theory, as shown in Fig. 7. Firstly, there were three kinds of Cu2+–O–Cu2+, Cu2+–O–Mn3+, and Mn3+–O–Mn3+ species on the surface of the γ-Al2O3, when Cu2+ and Mn3+ were simultaneously dispersed on the surface of γ-Al2O3. And then, XPS and EPR results showed that the

dispersed Cu2+ and Mn3+ were reduced to Cu+ and Mn2+ species after CO reduction pretreatment, which suggested that the CO reduction pretreatment would remove the oxygen species between the surface-dispersed components to generate Cu+– –Cu+, Cu+– –Mn2+, and Mn2+– –Mn2+ species. Furthermore, we defined the oxygen vacancy between different Cu+ and Mn2+ cations as the “surface synergetic oxygen vacancy” (SSOV). Finally, we carried out an in situ FT-IR experiment to explore the role of the SSOV in NO reduction by CO reaction, and therefore proposed a possible reaction mechanism, which is displayed in Fig. 8. In situ FT-IR results indicated that CO adsorbed mainly on the Cu+ species, whereas NO primarily adsorbed on the Mn2+ species, and the SSOV played a key bridging role between Cu+–CO and Mn2+–NO, which led to the remarkable enhancement in the catalytic performance. With the purpose of investigating the universality of the SSOV, we carried out the CO reduction pretreatment on other catalysts and found that the catalytic performances of CuO–CoO/γ-Al 2O 3 and NiO–Mn 2O 3/γ-Al 2O 3 catalysts for NO reduction by CO were also improved obviously after the CO reduction pretreatment.

4.2. Acid pretreatment Many investigation results have indicated that the regeneration of deactivated catalysts, the determination of active sites, and the improvement of catalytic performance could be achieved through acid pretreatment of supported metal-oxide catalysts.147–150 As a result, in recent years acid pretreatment

Fig. 7 The “Incorporation Model” diagram of CuO and Mn 2O3 dispersed on the (110) plane of γ-Al 2O 3 (C-layer) before and after CO reduction pretreatment: (a) 0.6Cu/Al, (b) 0.6Cu0.3Mn/Al, (c) 0.3Mn/Al, (a′) 0.6Cu/Al–CO, (b′) 0.6Cu0.3Mn/Al–CO, and (c′) 0.3Mn/Al–CO.143



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Fig. 8 Possible reaction mechanism of NO reduction by CO over 0.6Cu0.3Mn/Al catalyst after CO reduction pretreatment.143

Perspective

In particular, it is widely reported that the catalytic performance of supported metal-oxide catalysts for deNOx catalysis strongly depends on the amount of NO 2 in the exhaust, whereas the combination of microwave plasma pretreatment and selective catalytic reduction is beneficial to the oxidation of NO to NO 2, which further promotes the enhancement of catalytic performance.154 As a result, our research group has systematically investigated the effect of microwave plasma pretreatment on the catalytic performance of CuO/TiO2 catalysts for NO reduction by CO.155 We found that the microwave plasma pretreatment clearly enhanced the activity and N2 selectivity of the CuO/TiO2 catalysts due to the generation of highly active oxygen species (O˙), which facilitated the oxidation of NO to NO 2, and subsequently adsorbed on the surface of CuO/TiO 2 catalysts as distorted nitrate species. Furthermore, it is necessary to explore new pretreatment techniques to improve the catalytic performance of supported metal-oxide catalysts for the elimination of atmospheric molecular contaminants in the future.

has become an emerging topic for the catalytic elimination of atmospheric molecular contaminants. Kim and Shim147 adopted acid pretreatment of the industrial, deactivated, supported copper-based catalysts by different acid aqueous solutions of HNO3, CH3COOH, H2SO4, HCl, and H3PO4 to regenerate them. They observed that the catalytic performances of these pretreated catalysts for the combustion of benzene, toluene, and o-xylene could be ranked by HNO 3 > CH 3COOH > HCl > H 3PO 4 > H 2SO 4, in which acid pretreatment by HNO 3 exhibited the best effect due to the efficient regeneration of active sites (copper species). Jia et al.149 investigated the catalytic performance of CuO/CexCu1−xO2−δ catalysts for CO oxidation before and after acid pretreatment by HNO3. They concluded that the synergistic effect between the oxygen vacancies in the Ce xCu 1−xO 2−δ solid solution and surface CuO species (which could be removed by HNO 3 pretreatment) was the origin of excellent catalytic performance, because the former promoted the activation of oxygen, and the latter was conducive to CO chemisorption. In addition, Martín et al.151 discussed the influence of different acid pretreatments (H 2SO 4, H 3PO 4, HNO 3, and HCl) on the catalytic performance of CoO x/γ-Al 2O 3 catalysts for the selective catalytic reduction of NO x by CH 4. The obtained results indicated that the catalyst pretreated by H2SO 4 exhibited the best catalytic performance among these pretreated and unpretreated catalysts due to the increase of surface acidity and the stabilization of the active Co2+ species.

5. Conclusions and perspectives

4.3. Other pretreatment

Acknowledgements

The influence of other pretreatments (including hydrothermal aging pretreatment, microwave plasma pretreatment, etc.) on the catalytic performance of supported metal-oxide catalysts for the elimination of atmospheric molecular contaminants was also investigated in recent years.152,153


Supported metal-oxide catalysts have been widely applied for the catalytic elimination of atmospheric molecular contaminants in recent years. It is well known that the support, surface-dispersed component, and pretreatment of the catalyst can significantly affect the catalytic performance of supported metal-oxide catalysts. Therefore, we have carried out a brief review and proposed some perspectives according to the above-mentioned three aspects in the present work. Many researchers have made great efforts to investigate the catalytic performance of supported metal-oxide catalysts for the elimination of atmospheric molecular contaminants, and attempted to clarify the nature of these catalytic reactions. However, the relationship amongst the “composition– structure–activity” is still not very clear. In the future, with the development of material preparation approaches, solid catalyst surface characterization techniques, and theoretical calculation methods, we can synthesize highly thermallystabilized supports with special structures, regular morphologies, and specific exposed crystal planes to simplify the catalyst systems, and further deeply investigate the interaction between surface-dispersed components and supports to understand the relationship amongst the “composition– structure–activity” of supported metal-oxide catalysts for the catalytic elimination of atmospheric molecular contaminants, which can provide a valuable scientific basis for the design and preparation of novel, practical, and efficient catalysts.

The financial supports of the National Natural Science Foundation of China (no. 20573053, 20873060, 20973091, 21273110, and 21203091), the National Basic Research Program of China (973 program, no. 2003CB615804, 2004CB719502, 2009CB623500,


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and 2010CB732300), the Natural Science Foundation of Jiangsu Province (BK2012298), and Jiangsu Province Science and Technology Support Program (Industrial, BE2011167) are gratefully acknowledged. Furthermore, kind assistance of Dr Xi Hong, Lei Zhang, Yuan Cao, Yan Xiong, and Weixin Zou in checking the English and collecting the literatures is greatly appreciated.

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