On the Synergetic Catalytic Effect in Heterogeneous Nanocomposite

Review pubs.acs.org/CR

On the Synergetic Catalytic Effect in Heterogeneous Nanocomposite Catalysts Jianlin Shi* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China; Department of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200233, People’s Republic of China; and National Engineering Research Center for Nanotechnology, 28 East Jiangchuan Road, Shanghai 200241, People’s Republic of China 3.2.1. Synergetic Effect Proposals 3.2.2. Henry and Adol Condensation Reactions by Mesoporous Acid−Base Catalysts 3.2.3. Dye Degradation by Fe-Doped Mesoporous Zeolite 3.2.4. Ammonia Selective Catalytic Oxidation (SCO) to Nitrogen by a Mesostructured CuO/RuO2 Composite 3.3. Degradation Prevention of the Main Catalyst(s) by Secondary Component(s) for Sustained Catalytic Reactions 3.3.1. Synergetic Effect Proposals 3.3.2. H2 Electro-oxidation by Mesostructured Pt/WO3 Composite Catalysts 3.3.3. Methanol Electro-oxidation by Mesostructured Pt/WO3 Composite Catalysts 3.4. Reactant Storage/Release by the Catalytic Assistants in Multireactant Redox Reactions Catalyzed by Tricomponent Catalysts 3.4.1. Synergetic Effect Proposal 3.4.2. Oxygen Storage/Release in Three Way Catalysis for Automobile Exhaust Purification 3.4.3. Promoted Redox Reaction between CO and NO and between NO and C3H6 by a Tricomponent Catalyst with an Oxygen Storage Support 4. Further Discussions: Role of Interface and Mesostructure in Synergetic Catalytic Effects 4.1. Role of Interface Formation and Contact Angle between Components in the Synergetic Catalytic Effects 4.2. Advantages of Mesostructured Composites for the Synergetic Effects 5. Conclusions and Outlook Author Information Corresponding Author Notes Biography Acknowledgments References

CONTENTS 1. Introduction 1.1. Definition of the “Synergetic Catalytic Effect” in Heterogeneous Composite Catalysts 1.2. Scope of the Present Review 2. Synthesis of Nanocomposite Catalysts Focused on Mesostructured Composites with a Crystallized Framework 2.1. General Description of Chemical Routes for Synthesizing Nanocomposite Catalysts 2.2. Soft Templating Routes for the Mesoporous Oxides, Mesostructured Zeolites, and Nanocomposites 2.3. Replication Routes for Crystallized Mesoporous Oxides and Binary Mesoporous Composites 2.4. Template-free Routes for Mesoporous Metal Oxides and Composites 2.5. Postencapsulation Routes for the GuestLoaded Mesoporous Composites 3. Synergetic Catalytic Effects 3.1. One Component Activation by the Other between Two Catalytic Components 3.1.1. Synergetic Effect Proposals 3.1.2. CO Oxidation by Mesoporous CuO/CeO2 Composite Catalysts via Strong Metal− Semiconductor Interaction (SMSI) 3.1.3. Methanol Synthesis by Cu/ZnO-Based Catalysis via Strong Metal−Semiconductor Interaction (SMSI) 3.1.4. Suzuki/Heck C−C Coupling by Mesostructured Pd/NiFe2O4 Composite Catalysts 3.1.5. Selective Oxidation of Organics by Au/ TiO2 and Au/CeO2 Nanocomposite Catalysts 3.1.6. Organics Degradation by Au/TiO2, TiO2/ Carbon, and TiO2/Semiconductor Nanocomposite Photocatalysts 3.2. Successive Catalytic Functioning of Two Components in Multistep Reactions © XXXX American Chemical Society

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dx.doi.org/10.1021/cr3002752 | Chem. Rev. XXXX, XXX, XXX−XXX

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1. INTRODUCTION The advantages of heterogeneous catalysis over homogeneous one, for example, repeatable use of catalysts, easy separation of catalysts from products, etc., have been well-described in standard textbooks of catalysis. Single component heterogeneous catalysts have been extensively investigated, and some of them have been successfully applied in chemical industries.1−8 Recently, research on the single-site heterogeneous catalysts (SSHCs) on atomic and/or molecular scales is becoming increasingly interesting,9−14 in which the active centers are spatially isolated from each other and uniformly distributed over a large surface area of a porous solid, and each site has the same energy of interaction between the site and the incoming reactant. The single-site catalysis constitutes an indispensable basis for the in-depth understanding of the heterogeneous catalytic process on the atomic scale.

our own work on nanocomposite catalysts, we believe that such a synergetic effect does exist in some composite catalystinvolved catalytic reaction processes, and moreover, these synergetic catalytic effects may play an important, or even a decisive role, in the catalytic reactions, such as in the selective catalytic oxidation/reduction reactions, etc.. This review aims to explicate and understand, though rather preliminarily, such cooperations/interactions between/among catalytic components which result in synergetic catalytic effects. In addition to the conventional chemically synthesized nanocomposites,22,23 which show high surface area and other interesting features, mesoporous materials have also attracted great attention in the last two decades thanks to their welldefined pore structures, controlled pore size from 2 to 50 nm, and high surface areas, which are found to be technologically important for a variety of applications, such as in heterogeneous catalysis, adsorption, chemical sensing, electrode materials, transport/storage of fluids and gases, and also some biological areas.30−36 The heterogeneous atoms-doped mesoporous silicas, such as Al-, Ti-, Zr-doped MCM-41 or SBA-15 type mesoporous silicas, were extensively studied and found to be catalytically active in a number of reactions;37−46 however, pure mesoporous silica with an amorphous framework usually shows unsatisfactory thermal and/or hydrothermal stability and negligible catalytic activities. Nevertheless, pure mesoporous silica is a kind of excellent support material for loading/grafting active catalytic species in its mesopore system. A synergetic catalytic effect may work between the loaded/grafted catalytically active groups and the in situ present Si−OH groups or between two different kinds of introduced functional groups on the pore surface of silica. Compared to the amorphous framework of mesoporous silica, mesoporous metal oxides synthesized by a hard templatereplicating method usually have a crystallized structure and exhibit excellent catalytic activities, as reported in many documents. The loading or dispersion of catalytically active guest species into the host mesopore network results in mesostructured composites of a crystallized framework and highly dispersed catalytic species in its mesopore network. Not only the confinement effect on the loaded catalytic guests by the host mesopore network endows the composites with much enhanced catalytic performances, also the interaction between the loaded guests and the crystalline framework of the hosts would play an important role in affecting/determining the catalytic reaction process, as we will propose and briefly discuss in section 2.

1.1. Definition of the “Synergetic Catalytic Effect” in Heterogeneous Composite Catalysts

To achieve more satisfactory catalytic performance for enhanced activity and selectivity and reduced environmentally-unfriendly side effects, multicomponent composite catalysts are the natural choices.15−21 In fact, bicomponent or multicomponent composite catalysts have attracted great attention recently in the heterogeneous catalysis field. These heterogeneous composite catalysts are generally composed of one or more catalytically active components and a functional support, in which the interaction between the catalytic components and the support materials can possibly endow the composite catalysts with much improved catalytic properties, such as significantly enhanced catalytic activity, selectivity for target product(s), chemical stability, and prolonged lifetime. The heterogeneous catalytic performance is largely dependent on the catalyst nanostructures or, in another word, processing technologies, in addition to the intrinsic physical and chemical properties of the constitutive components. Chemically processed nanomaterials, such as those synthesized by sol−gel routes, usually show greatly enhanced catalytic activity thanks to the high surface area and high density of active sites on the surface of nanoparticles, as recently reviewed by Sanchez et al.22 and Debecker et al.23 In addition to this, the more important is the possible synergetic catalytic effect in nanocomposite catalysts which was found to prevail in many nanocomposites mostly synthesized by chemical processes.22−25 The synergetic catalytic effect is here defined as a certain kind of cooperation between different components and/ or active sites in one catalyst, which results in significantly, or even strikingly, enhanced catalytic performances than the arithmetic summation of those by corresponding individual components. Synergetic catalytic effects must be present between the different catalytic components or between the catalytic component(s) and the support(s) when such an enhancement or improvement in catalytic performances, such as catalytic activity, reactant conversion, product selectivity, catalyst durability and lifetime, etc., clearly occurs, as can be found in very recent reviews22−29 in varied composite catalysts. However, since the cooperations/interactions between different catalyst components are usually complicated, the possible synergetic catalytic effects and the underlying mechanisms have not been thoroughly addressed in the literature, though the synergetic effects in multicomponent catalysts have been extensively reported in different types of reaction systems. Nevertheless, from a large amount of literature reports and also

1.2. Scope of the Present Review

In this review, we will first review the syntheses of nanocomposite catalysts very briefly, then mostly the mesoporous metal oxides, especially those with crystallized framework by different methods, and the nanocomposites based on these mesoporous oxides. Subsequently the emphasis will be placed on the catalytic performances of the meso- and nonmesostructured catalytic composites enhanced by the synergetic effects between the loaded/grafted guest components and the host framework/supports. Four kinds of possible synergetic effects and the underlying mechanisms based on solid nanocomposite catalysts will be proposed. To understand the synergetic effects more clearly, catalytic reactions catalyzed by meso- or nonmesostructured nanocomposite catalysts both reported in the literature and of our own investigations will be listed as examples and discussed as to their specific synergetic B


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Figure 1. Schematic drawings of the general physical/chemical processing strategies for the preparation of nanocomposites by (a) “post-deposition” approach of the secondary component (the smaller red particles) on the first major component (larger black ones), and (b) “one-pot” or “one-step” approach starting from the homogeneous precursor solution containing all necessary elements/components, in which the secondary component (very small black dots) can be dispersed/loaded more homogeneously compared to “post-deposition” if suitably processed, in/on the other major component (relatively larger particles).

situ generated water is used.23 Oxygen comes from other oxygen donors (ether, alcohol, alkoxyde, etc.) in the case of the nonhydrolytic sol−gel process where no water is used at all. The basic concept of the chemical processes, as a matter of fact, is the solid formation from a homogeneous solution or sol, by various approaches, such as conventional instant precipitation or coprecipitation, slow homogeneous precipitation, controlled hydrolysis, sol−gel transition, self-assembly on the molecular level, and crystallization, etc., either under (quasi-)equilibrium conditions without quickly removing solvent (water) at ambient or elevated temperatures or under forced conditions such as quick solvent evaporation by spray-drying in heated media. Recently, the synthesis of multicomponent nanomaterials by chemical sol−gel routes has become very attractive, as those composites, especially the mixed oxide-based composites, may exhibit excellent and/or unique properties in, for example, catalysis. In fact, heterogeneous catalysts are commonly those of active components supported on supports, such as common oxides like silica, alumina, and so on. From the viewpoint of synergetic catalytic effects, supported catalysts can also be regarded as multicomponent catalysts if the support can play a certain kind of positive role in the catalytic process. The common preparation of supported heterogeneous catalysts is to deposit/graft/coat, both chemically and physically, active species or their precursors, onto/into the presynthesized supports, followed by post-treatment if necessary. This is the most conventional strategy for the preparation of multicomponent catalysts, especially for supported catalysts. Figure 1a schematically shows such a postdeposition strategy, in which the introduced secondary component is usually dispersed/supported on the first component (i.e., support), but the secondary component may be unable to be thoroughly dispersed/supported, and part of them may exist independently as particles and/or aggregates.

mechanisms in terms of the cooperation/interactions between/ among the catalytic component(s) and/or support(s), including the CO catalytic oxidation by CuO/CeO2; methanol synthesis from syngas by Cu/ZnO; benign C−C coupling chemistry by Pd/NiFe2O4; selective oxidations of propene and alcohol by Au/TiO2 and Au/CeO2; photodegradation of organic pollutants by TiO2-based nanocomposite photocatalysts; ammonia selective catalytic oxidation (SCO) by CuO/RuO2; dye contaminant degradation by Fe-doped mesostructured zeolite; Henry and adol condensation by acid−base catalysts; antipoisoning electrochemical reactions in low temperature proton exchange membrane fuel cells and direct methanol fuel cells catalyzed by Pt/WO3; and three-way redox catalytic conversions of CO/CH/NOx by tricomponent catalysts containing oxygen storage/release components. These proposed synergetic effects and the underlying mechanisms between catalytic component(s) and/or support(s), as we believe, can be equally applied to other kinds of nanostructured composite catalysts.

2. SYNTHESIS OF NANOCOMPOSITE CATALYSTS FOCUSED ON MESOSTRUCTURED COMPOSITES WITH A CRYSTALLIZED FRAMEWORK 2.1. General Description of Chemical Routes for Synthesizing Nanocomposite Catalysts

Nanomaterials, such as nanoparticles, nanowires, thin films containing nanophase(s), etc., can be synthesized by various chemical approaches. The traditional aqueous sol−gel chemistry has been employed for decades for the preparation of oxides based on the formation of oxo-bridges by hydrolysis and polycondensation of molecular precursors under the presence of water,47−53 or by the nonqaueous sol−gel process using in situ generated water instead of the added,54−58 or even by a truly nonhydrolytic process where neither added water nor in C


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mesostructured nanocomposites are the direct soft templating approach for inorganic or organic/inorganic hybrid mesoporous materials (section 2.2) and the direct replication route from the mesoporous hard templates for inorganic mesoporous nanocomposites (section 2.3). In the later, two or more miscible precursors are dissolved together and cast into the mesoporous network of hard templates, and in the following chemical etching and thermal treatments, the minor phase, which is thermodynamically not solid-soluble in the main phase, would segregate from the framework of the main one into the space where the wall of the hard template had been. In this case the composite mesostructure can also be tuned by either regulating the mesoporosity of hard templates or by monitoring the composition of the precursor mixtures and/or the later thermal treatments.

Though such traditional approaches usually suffer from the drawbacks of multistep preparation, dependent dispersivity of active species on processing route, and the properties of supports, however, it allows a very broad selection and combination of both supports (hosts) and loaded species (guests) and the possible elaborate control over the structure and morphology of both the hosts and the loaded guests. An alternative way to synthesize multicomponent catalysts can be called, as a whole, the “one-pot”, or sometimes, “onestep” approach, in which the desired species, mostly their precusors such as inorganic salts and/or metallorganics (e.g., metal alkoxides), are incorporated together in one system, as a homogeneous solution, sol, or dispersed mixture before extensive solid formation, as schematically demonstrated in Figure 1b. The preparation of final products, mostly the composite powders, can be accomplished by different routes, such as coprecipitation and cohydrolysis of different components,23 and instant forced solvent evaporation, such as aerosol processing,22 etc., followed mostly by post-treatments such as thermal decomposition. The “one-pot” synthesis has the most distinguished advantages of simplicity, component homogeneity, and least waste of substances. The initial homogeneous dispersions of different components, for example, their molecular level homogeneity in solutions, can be maintained to a large extent into the final products if suitably processed. Especially in the aerosol processing combined with solution and/or sol−gel chemistry, composite materials with special chemical compositions, dispersion, and structures, which can be hardly obtained under quasi-equilibrium conditions by usual precipitation methods, can be prepared as a result of the fast evaporation, and these composites may show hierarchical structures of porous inorganic or hybrid materials with confined heterogeneous domains and metastable state. However, the one-pot synthesis will inevitably suffer from the difficulties in the facile and precise tuning of morphology, size and size distribution, and component dispersity. However, in this review only the syntheses of mesostructured composites will be further reviewed. The formation of mesostructured composites can be achieved by two approaches, similar to those of common nanocomposites above-mentioned. One is the common postdeposition of catalytically active species into the pore network of presynthesized mesoporous supports with or without a crystallized framework, in which the mesoporous supports can be prepared by either a “soft” or “hard” templating route, or even a template-free method, as will be discussed in sections 2.2, 2.3, and 2.4. Briefly, rigid templates such as carbons, carbonates, and other inorganic materials are usually called “hard” templates while relatively flexible templates such as surfactant micelles and block copolymers are regarded as “soft” templates. The syntheses based on these “hard” or “soft” templates are simply termed as “hardtemplating” and “soft-templating” approaches, respectively. The mesostructure properties, such as surface area, pore volume, pore size, and size distribution, can be tuned by using surfactant templates of varied micelle diameters or hard templates of different porosities or by employing controlled thermal treatments (section 2.4). The effective loading of the catalytically active species into the pore network of the above mesoporous supports determines their catalytic performances to a large extent, which can be accomplished and controlled effectively by using different postencapsulation strategies, as will be discussed in sections 2.4 and 2.5. The other alternative but less commonly used approaches for the synthesis of

2.2. Soft Templating Routes for the Mesoporous Oxides, Mesostructured Zeolites, and Nanocomposites

Mesoporous inorganic oxide materials, in the form of either powder or thin film, with high surface areas, ordered pore structures, finely tunable pore sizes, and flexible wallcompositions have been investigated widely of their chemical synthesis and potential applications in catalysis, adsorption, chemical sensing, electrochemistry, biomedical areas, and so on.30−36,59−61 The controlled pore size of 2−50 nm in diameter of mesoporous materials breaks the pore-size limitation of microporous zeolites (95% and an integrated N2 production of 75% over the entire rich/lean cycle, and later they found that the CuO/Al2O3 catalyst showed extremely low NOx selectivity in SCO of NH3 under fuel-rich conditions, both with and without water added in a wide temperature range (300−500 °C), while Fe-zeolite catalyst showed low NOx yields only under fuel-lean conditions.324 Xie et al.325 reported that the ammonia was mainly oxidized to N2 and N2O at relatively low temperatures (300 °C), possibly following reactions 41 and 42.

catal + O2

catal

catal + O2

catal + O2

NH3 ⎯⎯⎯→ NH3(ads) ⎯⎯⎯⎯⎯⎯⎯⎯→ NH 2 ⎯⎯⎯⎯⎯⎯⎯⎯→ N2H 2 ⎯⎯⎯⎯⎯⎯⎯⎯→ N2 (43) O2

O2

O2

NH 2 → NH → HNO → N2O + NO → NO

(44)

catal

NH (or NH 2) + NO ⎯⎯⎯→ N2 + H 2O

(45)

catal

NH3 + NO ⎯⎯⎯→ N2 + H 2O

(46)

Recently, our group developed a new kind of mesostructured composite catalyst, mesostructured CuO/RuO2 bimetal oxides, which was synthesized by the one-step conanocasting method by using the KIT-6-type mesoporous silica as hard template and Cu(NO3)2·3H2O and RuCl3 as copper and ruthenium sources, respectively. This mesostructured composite had a relatively high BET surface area of ∼100 m2/g (ref 105) and showed a high catalytic activity on ammonia SCO reaction. The mesostructured bicomponent metal oxides showed a 100% ammonia conversion and near 98% N2 selectivity at a temperature of as low as 180 °C, dependent highly on the CuO loading amount in the composite, as can be found in Figure 26. The reactant molecule activation into O‑cus and

4NH3 + 3O2 → 2N2 + 6H 2O

(39)

4NH3 + 4O2 → 2N2O + 6H 2O

(40)

4NH3 + 5O2 → 4NO + 6H 2O

(41)

2N2O + O2 → 4NO

(42)

Figure 26. NH3 oxidation conversion rate and the corresponding selectivity of NH3 oxidation to N2 as a function of CuO content in the mesostructured CuO/RuO2 composite catalysts at 180 °C (Adapted with permission from from ref 105. Copyright 2010 Elsevier).

This indicates that, to eliminate intermediates such as NO, the selective reduction of the nitrogen oxides to N2 is critical during the ammonia SCO reaction over the CuO-based catalyst. During the ammonia SCO reaction, Ramis et al.326 and Amores et al.327 proposed a mechanism for copper- and ironcontaining catalysts, which suggested that NH2 is the common intermediate in both ammonia SCO and nitrogen oxide SCR. That is to say that the nitrogen oxide SCR reaction is in competition with ammonia oxidation through the intermediate amide NH2. At relatively low temperatures, NH3 is oxidized,

NH3‑cus, and the formation of adsorbed NO species (NO‑cus) and N2 on the RuO2 surface are illustrated in Figure 27,105 and a synergetic catalytic effect by successional catalytic functioning of mesoporous RuO2 and CuO in the multistep reactions is also proposed, as schematically shown in Figure 28. In the synergetic catalysis, RuO2 first catalyzes the reaction of ammonia SCO (reactions 43 and 44, or reactions 47−49); subsequently, CuO catalyzes the reaction between NO‑cus and Y


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In such a mesostructured CuO/RuO2 bimetal oxide, ruthenium oxide is the main catalyst component α in the α/ β type composite catalyst. First, RuO2 is a unique kind of metal oxide, and the (110) surface of stoichiometric RuO2 is highly susceptive to NH3 molecules (Figure 27A), which can adsorb and catalyze the reduction/oxidation of NH3 molecules.328 Especially for mesoporous RuO2, the porous surface possesses numerous dangling bonds and exposes coordinatively unsaturated Ru‑cus atoms and, therefore, is chemically very active. The NH3 molecules (NH3‑cus) and/or oxygen atoms (O‑cus) can be adsorbed easily onto these unsaturated Ru‑cus atoms, as illustrated in Figure 27A. The interaction between these adsorbed NH3‑cus molecules and O‑cus atoms would take place, which leads eventually to the formation of N‑cus and H2O according to a previous report via the following reaction (Figure 27B):313,329 NH3 ‐ cus + O‐cus → NH 2 ‐ ads + OHads → N ‐cus + H 2O (47)

These formed N‑cus species may recombine either with each other into N2 or with neighboring O‑cus into intermediate NO‑cus or NxOy‑cus, as shown in Figure 27B, following reactions 48 and 49, respectively: N ‐cus + N ‐cus → N2

(48)

N ‐cus + O‐cus → NO‐cus (NxOy ‐ cus )

(49)

NH3 molecules react with O2 molecules on the surface of mesoporous RuO2 and form intermediates NO‑cus and/or NxOy‑cus; that is, the RuO2 functions first for the NH3 oxidation (component α, catalyst in reaction 47). Reaction 49 is in agreement with reaction 44 that the intermediate products of nitrogen oxides will be generated during the NH3 conversion. Many documents have reported that CuO is catalytically active and shows high selective catalytic performance in the reduction of nitrogen oxides into N2 during ammonia oxidation,323,330−332 which means that the intermediate NO‑cus or NxOy‑cus species generated during NH3 oxidation can be reduced to N2 selectively by CuO following reaction 50, as schematically illustrated in Figures 27 and 28.

Figure 27. Schematic drawings of the lattice structure of stoichiometric RuO2 during the SCO of ammonia: (A) Stick and ball model of the stoichiometric RuO2 (110) surface with adsorbed NH3 molecules and O atoms (O‑cus). O-bridge and Ru‑cus are 2-fold coordinated oxygen atoms and 5-fold coordinated Ru atoms, respectively. O‑cus is the additional oxygen atom, which may be adsorbed on top of Ru‑cus by further exposure to environmental O2. (B) Possible surface reaction processes and the formation of N2 between N‑cus and N‑cus or of NOx between N‑cus and O‑cus (Adapted with permission from ref 105. Copyright 2010 Elsevier).

NH3‑cus (reactions 45 and 46, or reaction 50), which is preformed on a RuO2 surface, into N2.

Figure 28. Schematic illustration of the proposed synergetic catalytic effect between CuO and RuO2: RuO2 first catalyzes the reaction of ammonia SCO (reactions 43 and 44, or reactions 47−49); successively, CuO catalyzes the reaction between NO‑cus and NH3‑cus (reactions 45 and 46, or reaction 50), which are preformed on the RuO2 surface, into N2. Z


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active component nanoparticles. Secondary components should be selected to prevent the occurrence of these unfavorable issues and thus stabilize the structural, compositional, and morphological features of the main components. Taking the poisoning prevention as the first example, this type of catalytic reaction can be expressed by a general reaction formula as eq 51 under the catalysis by α, and usually the initial catalytic activity of α is rather high, and the reaction between reactant A and B is fast in the early stage. During the catalytic reaction, catalyst α can easily interact with one of the impurities (imp) or intermediates (interm) via adsorption or reaction, to form α(imp) or α(interm) (in the following text α(imp) will be used as the representative). Thus, the active sites of catalysts will be covered to a certain extent and its catalytic activity will be consequently decreased, and as a result, the catalytic reaction can be greatly slowed down or even stopped:

NO‐cus (NxOy ‐ cus ) + NH3 ‐ cus(or NH 2 ‐ ad) ⎯⎯⎯→ N2 + H 2O (50)

That is to say that the intermediate NO‑cus or NxOy‑cus species produced during the ammonia SCO reaction can be selectively reductive-catalyzed to N2 by CuO functioning as secondary catalyst component β (i.e., catalyst in reactions 45 and 46). After the reaction following reaction 44, the active (110) surface of RuO2 is exposed again to NH3 and O2 molecules, which can be adsorbed onto the surface of RuO2, making the ammonia further oxidized following reaction 47. The consecutive reactions following reactions 49 and 50, in addition to reaction 48, result in both enhanced conversion of ammonia oxidation and high N2 selectivity on this mesostructured CuO/ RuO2 bimetal oxide catalyst. On the other hand, the mesoporous RuO2 with high surface area and porous structure provides a large amount of catalytic active sites leading to the enhanced ammonia oxidation activity (i.e., high NH3 conversion rate) at relatively low temperature, and the secondary catalytic component CuO highly dispersed in the mesopore network of RuO2 plays an important role in making the intermediates (NOx) generated during NH3 oxidation selectively reduced to N2. The synergetic effect by the successional catalytic functioning of RuO2 and CuO in mesoporous CuO/RuO2 bicomponent catalyst results in both high catalytic activity and high N2 selectivity in the multistep reactions of ammonia SCO. This example of synergetic effect by successional functioning of different components is different from the above two examples, in which the first step is the activation of reactant(s) by adsorption and/or the chemical bonding between the reactants and the catalytic sites, followed by the second step of reaction between activated reactant(s) leading to enhanced conversions, while this example is the successional functioning of components in different reaction steps in a multistep reaction having multiple products, resulting in not only the enhanced conversion but also the largely elevated selectivity to target product.

α

deactivated α(imp)

A + B → ··· ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ ··· reaction slow down or stop (51)

When a secondary component β was introduced forming a composite catalyst with α, the impurity adsorption can be partially or completely prevented by the stronger impurity adsorption on β than on α, or by reaction between the impurity and β (eq 52): β

α(imp) → α + β(imp)

(52)

This process can take place by the transfer/migration of the adsorbed impurity molecules from the surface of α to that of β due to the stronger bonding between the impurity and the component β than between the impurity and the component α. In such a way, the secondary catalytic component β can effectively prevent/eliminate the surface coverage by impurities or intermediates on the main catalytic component α and thus prevent its poisoning by forming β(imp), which can recover to β later by, for example, decomposition. Thus, the active catalytic sites of the catalyst α component can be re-exposed, and the reactant molecules are further adsorbed and catalyzed. As a whole, the composite α/β catalyst will demonstrate enhanced and well-sustained catalytic activity and/or much prolonged lifetime as compared to single component α, as illustrated in eq 53:

3.3. Degradation Prevention of the Main Catalyst(s) by Secondary Component(s) for Sustained Catalytic Reactions

3.3.1. Synergetic Effect Proposals. In this type of synergetic effects, the secondary component does not play a direct activation role to the main component, or successional functioning role, in the catalytic reactions, but an indirect role in preventing the degradation of the main component by varied factors, for example, poisoning of the main component by impurities (or reaction intermediates, or even products), structural change, or decomposition of the main components or considerable surface loss by local overheating, in the reaction systems, which results in significant activity losses in the time courses of reactions. The poisoning effect can be either the active site covering by the impurities/intermediates or undesirable reaction between the catalyst and impurities/ intermediates, and the secondary catalyst component can relieve or completely eliminate the main component poisoning by preferentially adsorbing or reacting with these detrimental impurities prior to their adsorption onto or reaction with the main components. The structural change or decomposition of the main catalytic components may lead to the substantial decreases or even complete loss of the catalytic activity of active sites. The surface loss can be caused by the agglomeration among nanocatalytic particles and/or substantial growth of the

α/β

α(imp)/ β

α / β(imp)

α/β

A + B ⎯⎯⎯→ ··· ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ ··· ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ ··· ⎯⎯⎯→ C or C + D (53)

Where α/β(imp) can be later recovered to α/β by removing the impurities from the surface of β through, for example, thermal or chemical treatments. This type of synergetic effect by poisoning prevention can be well explicated in electrode catalytic reactions involving noble metal catalyst(s). Herein mesoporous tungsten oxide supported Pt catalysts for hydrogen electrocatalytic oxidation will be exampled in low temperature proton exchange membrane fuel cells (PEMFCs) and for methanol electrocatalytic oxidation in direct methanol fuel cells (DMFCs), as detailed in sections 3.3.2 and 3.3.3, respectively. The effects of structural, compositional, and/or morphological changes of catalysts on their deactivation are rather simple and are expressed in eq 54. When a single component catalyst is used, catalyst α could be deactivated due to the activity and/or surface losses by the above factors taking place during the reaction: AA


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structural/compositional/morphological changes

α ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ α(deactivated) (54)

If a secondary component can prevent the occurrence of these unfavorable phenomena, α will be stable in terms of its structure, surface area, and activity; that is, it will not be significantly deactivated: structural/compositional/morphological changes prevented

α /β ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ α /β (undeactivated)

(55)

This kind of synergetic effect is easy to understand and will not be further discussed and exampled in the following. However, it is worthy to point out that such a degradationprevention effect should be performed before reactions take place, because they are usually irreversible. 3.3.2. H2 Electro-oxidation by Mesostructured Pt/WO3 Composite Catalysts. One of the key components that determine the performance and cost of PEMFCs is the electrode catalyst. Pt-based catalyst is widely used as the electrocatalytic material in PEMFCs for its high activity for both electro-oxidation of hydrogen at the anode and reduction of oxygen at the cathode.333,334 Unfortunately, Pt catalyst is susceptible to CO poisoning when using fuel gases containing even a very low amount of CO and will lose its catalytic activity with respect to time, leading to the Pt catalyst poisoning.335 Klaiber et al. reported that the presence of CO at a volume ratio as low as 1 × 10−6 in these fossil fuels would make the Pt catalyst poisoned and lose its catalytic activity.336 To date, one of the most promising poisoning-insensitive anode catalysts for hydrogen oxidation in PEMFCs is PtRu/C bimetallic catalyst, though many kinds of Pt based catalysts have been reported.337−340 Tungsten oxide-based electrocatalytic materials have received considerable attention most recently because of their assistant electrocatalytic effect with Pt on hydrogen/methanol electrooxidation. In this work, a Pt-loaded mesoporous WO3 was prepared which had a relatively low amount of Pt (e.g., 7.5 wt %) and a high BET surface area of ∼80 m2/g,145 and the electro-oxidation properties of hydrogen in acidic solution were studied. Figure 29 gives the SEM and TEM images of the mesostructured composites (Figure 29a−c) and the HRTEM image (Figure 29e), which shows the loading of Pt nanoparticles in the mesoporous structure of WO3.145 The result showed that the secondary catalyst component mesoporous WO3 can effectively prevent the Pt poisoning by CO by its more preferential reaction with CO molecules initially adsorbed on the Pt catalyst surface, making the mesostructured Pt/WO3 composite an electrocatalytically active and long-lasting catalyst.145 From the chronoamperometric tests given in Figure 30, it is not hard to find an apparent decrease of current in 400 s with a standard 20 wt % Pt/C catalyst when CO was introduced (Figure 30B), and only two-third of the current value was retained at the end of the test as compared to that without the CO introduction. In contrast, the 7.5 wt % Pt/ mesoporous WO3 electrocatalyst retained much higher mass current output, which was 89% of that under the absence of CO in 400 s (Figure 27A). This means that the mesostructured electrocatalyst Pt/WO3 has much better CO tolerance than pure Pt/C catalyst when using mesoporous WO3 as catalyst support. Therefore, the high CO tolerance of the mesoporous Pt/WO3 electrocatalyst can be attributed to the secondary component mesoporous WO3, which can effectively remove the

Figure 29. Typical FE-SEM image (a) and TEM images (b, c) of the m-WO3 in the [111] and [100] directions, respectively, with the SAED pattern in the inset in part c, and the EDS pattern of WO3 (d). Part e is the HRTEM image of mesostructured 7.5 wt % Pt/WO3 (Adapted with permission from ref 145. Copyright 2009 American Chemical Society).

adsorbed impurities (-COads) during the CO-containing hydrogen catalytic oxidation.342 The possible reaction mechanism is discussed as follows: For pure Pt catalyst (component α only), the hydrogen catalytic oxidation can be formulated as follows: 2Pt + H 2 → 2Pt‐Hads (56) Pt‐Hads → Pt + H+aq + e‐

(57)

For mesostructured Pt/WO3 composite catalyst (composite catalyst α/β), the secondary component WO3 could form two stable hydrogen tungsten bronzes, H0.18WO3 and H0.35WO3, and/or substoichiometric oxides, WO3−y, by the reaction with hydrogen in the acidic electrolyte:341,342 WO3 + x H+ + x e− → Hx WO3 (0 < x < 1)

(58)

WO3 + 2y H+ + 2ye− → WO3 − y + y H 2O (0 < y < 1) (59)

So the hydrogen oxidation reaction on mesostructured Pt/WO3 composite catalyst could be rewritten as a whole as follows: x Pt‐Hads + WO3 → x Pt + HxWO3

(60)

HxWO3 → x H+aq + WO3 + x e−

(61)

This means that the secondary component of mesoporous WO3 can possibly accelerate the hydrogen oxidation on Pt AB


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Figure 30. Chronoamperometric curves of (A) 7.5 wt % Pt/mesoporous WO3 and (B) 20 wt % Pt/C (E-TEK) in 0.5 M H2SO4 solution purged with pure 100% H2 gas (1) and in 0.5 M H2SO4 solution purged with a gas mixture of 1% CO and 99% H2 (2) for 1 h under a scan rate of 0.05 V·s−1 at 298 K (Adapted with permission from ref 145. Copyright 2009 American Chemical Society). The current output losses are ca. 11% and 33% for 7.5 wt % Pt/mesoporous WO3 (A) and 20 wt % Pt/C (B), respectively, indicating the greatly enhanced CO-tolerance of the former than the later.

Figure 31. Schematic illustration of the proposed synergetic effect in the electro-oxidation of hydrogen by Pt/WO3: The adsorbed COads on Pt (a, b) from a CO-containing H2 inlet is desorbed and removed by WO3 (c) through reaction 66, which leads to enhanced CO-tolerance and the sustained electrocatalytic reaction.

For mesostructured Pt/WO3 composite catalyst, the secondary catalyst component WO3 can react with water to form oxo-species -OHads due to the oxophilic nature of tungsten oxide:342

catalyst by its special nature of hydrogen tungsten bronze formation with hydrogen. In fact, such a reaction promotion effect follows well the type II synergetic catalytic model of successional functioning of Pt (activating H2 molecules by adsorbing them on its surface) and WO3 (H2 co-oxidation) as proposed in section 3.2 and specifically sections 3.2.2 and 3.2.3. Possibly the more important is its role of the CO poisoning prevention of Pt by WO3. When a small amount of CO was introduced in the reaction system, the CO molecules will be strongly adsorbed on the surface of Pt to form Pt-(CO)ads species covering the active sites of Pt, which results in the poisoning of Pt catalyst: Pt + CO → Pt‐COads

−O−W−O−W−O + H 2O → −O−W−OHads +

(65)

The chemisorbed -OHads species on the surface of WO3 will react with the -CO species adsorbed on Pt: −O−W−OHads + Pt‐COads → WO3 + Pt + CO2 + H 2O (66)

(62)

In this way the adsorbed -CO intermediates on Pt catalyst are removed effectively by this secondary catalyst component, mesoporous WO3, leading to the re-exposure of the active sites of Pt. Thus, the reaction of hydrogen electrocatalytic oxidation can take place in a sustained way on the anode composite catalyst Pt/WO3 through such a synergetic effect of CO poisoning prevention. A simple schematic drawing is presented in Figure 31, in which COads on Pt was detached by WO3 from the Pt surface, forming CO2 and H2O, which leads to the enhanced CO tolerance. 3.3.3. Methanol Electro-oxidation by Mesostructured Pt/WO3 Composite Catalysts. Direct methanol fuel cells (DMFCs) are another kind of low temperature fuel cells which have attracted much attention recently as portable energy sources due to the use of the safer and more portable fuel of methanol rather than hydrogen.344 However, the slow oxidation kinetics of methanol and the platinum catalyst “poisoning” by CO is its intrinsic technical drawback, impeding its future

In order to eliminate the -COads intermediate, the Pt catalyst should react with water to form a Pt-(OH)ads species, as reaction 63 shows, which can react with the -CO ads intermediate to form CO2 and make the active sites of Pt catalyst exposed again: Pt + H 2O → Pt‐(OH)ads + H+aq + e−

ads HO−W−O−

(63)

Pt‐(CO)ads + Pt‐(OH)ads → 2Pt + CO2 + H+aq + e− (64)

However, the formation of Pt-(OH)ads species needs a relatively high potential of about 0.6 V and above,343 which may inhibit the hydrogen redox process and become the ratedetermining step. Thus, the Pt catalyst would quickly lose its catalytic activity as long as this adsorbed -COads species cannot be removed in time. AC


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Figure 32. Cyclic voltammetry curves of various catalysts for methanol oxidation in 0.5 M CH3OH + 0.5 M H2SO4 electrolyte, with a scan rate of 50 mV s−1 at 298 K (the arrows indicate the direction of scanning) (Adapted with permission from ref 146. Copyright 2008 American Chemical Society).

Figure 33. Chronoamperometry curves of various catalysts for methanol oxidation measured at 0.65 V (A) and 0.32 V (B), 298 K (Adapted with permission from ref 146. Copyright 2008 American Chemical Society).

showed the highest mass activity (706 mA·mg-Pt−1) among all the catalysts, which was 80% higher than that of the 20 wt % Pt/C (383 mA·mg-Pt−1). The electrochemical activity for methanol oxidation of the commercial 20 wt % PtRu/C catalyst was also measured for comparison, and the result is shown in Figure 32 B. It could be found that the peak currents of m20Pt/WO3 are comparable to and even higher than that of 20 wt % PtRu/C, though PtRu/C has been reported to be a much better anti-CO “poisoning” catalyst for methanol oxidation than Pt/C.146 These indicate that the catalysts of Pt supported on mesoporous WO3 have high electrocatalytic activity toward methanol oxidation. Figure 33 gives the chronoamperometry test results of the mesoporous Pt/WO3 for methanol oxidation, and it can be seen that the activity and stability of the catalysts at 0.65 V follow the following sequence: m-20Pt/WO3 > m-15Pt/WO3 > m-10Pt/WO3 ≈ 20 wt % Pt/C > m-5Pt/WO3 (Figure 33A). The current values of m-10Pt/WO3 and 20 wt %Pt/C at the end of the tests are very close to each other, and the relatively lower electrocatalytic activity of methanol oxidation on 20 wt % Pt/C catalyst is most probably resulted from the CO poisoning of Pt. The mesoporous m-20Pt/WO3 catalyst retained high output current during the methanol oxidation reaction by effectively eliminating the intermediate “-COads” adsorbed on the Pt, which made the active site re-exposed and, thus, prevented Pt catalyst from CO “poisoning”. For comparison, it is clear that the current output of m-20Pt/WO3 is higher than that of 20 wt % PtRu/C at the end of the test, and both of

application because CO species will be inevitably generated in situ during methanol oxidation. Among the Pt-based binary,345−347 ternary,348,349 and even quaternary350,351 metal catalysts, Pt−Ru alloys catalyst is still the most effective catalyst for methanol oxidation and CO tolerance as well352−354 among the reported noble metal anode catalysts. In addition, noble metal catalysts can be prepared into a nanoporous structure for enhanced catalytic activity.355 The most recent advances in catalysts for direct methanol fuel cells can be found in a recent review.356 Alternatively, Pt/WO3 has also been reported to be a promising anode catalyst candidate recently.101,146,357,358 Our group successfully synthesized ordered mesoporous WO3 with a BET surface area of about 80 m2/g by using KIT-6 as hard template and PW12 (12-phosphotungstic acid) as tungsten source, which was then used as a support to load Pt nanoparticles, and the obtained mesostructured Pt/WO3 composite has high electrochemical catalytic activity for methanol oxidation. The electrocatalytic curves of methanol oxidation on mesostructured WO3 and m-Pt/WO3 composites are shown in Figure 32A.146 The pure mesoporous WO3 without loading Pt displayed no electrochemical activity. By contrast, the mesostructured Pt/WO3 composites showed high current density for methanol oxidation with a specific activity sequence as follows: m-20Pt/WO3 > m-15Pt/WO3 > 20 wt % Pt/C > m-10Pt/WO3 > m-5Pt/WO3. The electrocatalytic activity of m-20Pt/WO3 was 55% higher than that of the commercial catalyst 20 wt % Pt/C. The sample m-10Pt/WO3 AD


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71. Finally, the possible reaction acceleration and CO poisoning prevention mechanisms in the electro-oxidation of methanol by mesoporous Pt/WO3 can be expressed in reaction 76:

them, as expected, are much higher than that of 20 wt % Pt/C, though 20 wt % PtRu/C showed considerably higher current than those of m-Pt/WO3 composites and 20 wt % Pt/C at the low potential of 0.32 V (Figure 33 B). Mesoporous WO3 supported Pt catalysts showed enhanced electrochemical catalytic activity for methanol oxidation,146 and the in situ-generated carbonaceous intermediates -CO ads adsorbed on catalyst can be eliminated effectively. In this reaction mechanism, mesoporous WO3 as a secondary catalyst component also played an important role in preventing the main Pt catalyst from being poisoned during the methanol oxidation process. As reported by Laborde et al.,359 methanol oxidation is a complicated chemical reaction. The methanol molecules (CH3OH) are first adsorbed and assembled on the Pt surface and subsequently form intermediates Pt-(CHO)ads and Pt(CO)ads, which will finally react with Pt-(OH)ads to give CO2 and H+. The reaction mechanism is presented in the following chemical reactions, as put forward previously in the literature.360−362 Normal Pt catalysis mechanism: Pt + CH3OH → Pt‐(CH3OH)ads

(67)

Pt‐(CH3OH)ads → Pt‐CH 2(OH)ads + H+aq + e−

(68)

Pt‐(CH3OH)ads + WO3 + H 2O → Pt‐(CO)ads + −O−W−OHads + Hx WO3 → Pt + WO3 + CO2 + H 2O + H+aq + e−

Due to the formation of the bronze phase of WO3 with H+aq, WO3 can promote the dehydration of Pt-(CHxO)ads, and as a result the methanol electro-oxidation is accelerated. Meanwhile, the de-CO reaction of Pt-(CO)ads was promoted by the formation of -OHads on WO3 instead of on Pt due to the oxophilic nature of tungsten oxide; then the intermediate -(CO)ads can be oxidized effectively, leading to diminished poisoning of Pt by CO, that is, enhanced CO tolerance. Additionally, the mesoporous structure of WO3 support with high surface area and the nanosized framework makes a large amount of the active sites of Pt nanoparticles loaded in the mesostructure exposed on the one hand, and on the other hand also results in easy formation of active hydrogen tungsten bronze compound (HxWO3) and hydrated intermediate WO3−OHads during the electrochemical reactions. The secondary component mesoporous WO3 plays an important role both in accelerating the hydrogen/methanol electrooxidation and in preventing the CO poisoning of Pt catalyst during the hydrogen/methanol oxidation reactions.

... Pt‐(CHO)ads → Pt‐(CO)ads + H+aq + e− Pt + H 2O → Pt‐(OH)ads + H

+ aq

+e

−

(69)

3.4. Reactant Storage/Release by the Catalytic Assistants in Multireactant Redox Reactions Catalyzed by Tricomponent Catalysts

(70)

Finally, intermediate species -COads react with -OHads, giving CO2 and more H+:

3.4.1. Synergetic Effect Proposal. This type of synergetic effect can be found in complicated redox reactions involving multicomponent reactants (without oxygen or hydrogen peroxide as the oxidant) in reaction systems, which includes the reduction of some reactant(s) such as oxides (labeled as AxOy, e.g., NOx) and the oxidation of the other reactants such as carbonaceous compound(s) (labeled as B, e.g., CO and hydrocarbon compounds). To achieve a balanced reaction result, that is, relatively high conversions of both oxidation and reduction reactions, the oxygen level as a special kind of reactant or product in the system plays a decisive role in this type of multipurpose redox reaction, and the conversion rates of the reactants are determined by the concentration of oxygen to a large extent. Neither O2-deficient nor O2-rich conditions are desirable; that is, the oxygen concentration must be maintained at a relatively constant level for the high conversion rates in both reduction and oxidation reactions. To reach this goal, a functional catalyst with quick response and high capacity oxygen storage/release capability, in addition to its redox catalytic activity, is important, so the multicomponent catalyst is an indispensable choice. Such a composite catalyst should be composed of three main components, namely those for the oxidation (component α), reduction (component β), and oxygen storage and release (component γ). In the O2-rich condition, the component γ can capture oxygen from its environment, or directly from the surface of reductive catalytic component β via the interface between β and γ, and store it in its lattice:

Pt‐(CO)ads + Pt‐(OH)ads → 2Pt + CO2 + H+aq + e− (71)

Since reaction 70 needs a high potential of 0.6 V and above,343 it becomes a rate determining step; then the oxidation of the intermediate Pt-(CO)ads into CO2 will be greatly inhibited, leading to the serious poisoning of Pt catalyst, which impedes the further adsorption and oxidation of methanol molecules. Therefore, the key is to find a more quick formation mechanism of -(OH) ads . Here mesostructured Pt/WO 3 composites can also be used in methanol electrocatalytic oxidation: x Pt‐(CH3OH)ads + WO3 → x Pt‐CH 2(OH)ads + Hx WO3 (72)

... x Pt‐(CHO)ads + WO3 → x Pt‐(CO)ads + Hx WO3

(73)

And in all: x Pt‐(CH3OH)ads + 4WO3 → x Pt‐(CO)ads + 4Hx WO3 (74)

Hx WO3 → WO3 + x H

+ aq

+ xe

−

(76)

(75)

The presence of secondary component mesoporous WO3 promotes the dehydrogenation and oxidation of methanol. In addition, the improved CO tolerance by WO3 addition can also be achieved by reactions 65 and 66 instead of reactions 70 and

γ + O2 → γ(O) AE

(77) dx.doi.org/10.1021/cr3002752 | Chem. Rev. XXXX, XXX, XXX−XXX

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In the O2-deficient condition, γ(O) releases oxygen into the environment, or directly onto the surface of oxidative catalytic component α from its lattice:

γ(O) → γ + O2

Precious metals, such as Pt, Pd, Rh, etc., are the main catalyst components of TWCs. Among them, Pt and/or Pd are used as the catalyst for HC and CO oxidations, and Rh is for the reduction of NOx to N2.366,367 Thus, either Pt/Pd/Rh or Pt/Rh or Pd/Rh has been extensively used as the three-way catalysts to substantially lower the HC/CO/NOx emissions from gasoline engines. Initially, high specific surface area γ-Al2O3 was adopted as the support for the noble metal nanoparticles to prevent the agglomeration and exaggerated growth of precious metal particles during the high temperature reactions. Even now, γAl2O3 is still used as part of the support due to its high surface area, easy coating onto honeycombs from its slurry, and low cost. For TWCs with only γ-Al2O3 as the support, the oxygen concentration in exhausts should be precisely controlled, which, accordingly, demands an ideal combustion in engine chambers at a strictly fixed ratio of air inlet to fuel injection, called air/fuel (A/F) ratio. This ratio in volume is usually at around 14.6. Under the ideal condition, the following redox reaction can proceed with high conversion rates of the three reactants in a balanced way:368,369

(78)

By the oxygen storage and release (reactions 77 and 78) of the component γ, oxygen concentration might be kept at a balanced level; as a result, both the reduction and oxidation reactions can be accomplished in the same reaction system as follows: α

A xOy → A + O2

(79)

β

B + O2 → Bx Oy

(80)

In all: A xOy + B + O2 + VÖ /[O]o α/β/γ

⎯⎯⎯⎯⎯⎯→ A + Bx Oy + O2 + [O]o /VÖ

(81)

Here VÖ /[O]o are the oxygen vacancies and lattice oxygen in component γ, as the reactant, respectively, under the O2-rich and O2-deficient conditions, respectively, during the redox reaction. Component γ plays a role of oxygen storage or release, respectively, from the environment into the oxygen vacancies or from the lattice oxygen into the environment, to maintain a balanced oxygen concentration for the completion of the redox reaction, corresponding to the above O2-rich and O2-deficient conditions, finally resulting in the formation of lattice oxygen and oxygen vacancies [O]o/VÖ in γ, respectively, in addition to the desired products A and BxOy. One of the typical and also practically important redox catalytic reactions containing both nonoxygen oxidant and reducer is the so-called three-way catalysis (TWC) of the exhaust by gasoline engines, as will be discussed as the first example (section 3.4.2), followed by simpler ones of the redox reactions of CO + NO and NO + C3H6 using tricomponent catalysts containing noble metals and oxygen-storage assistances (section 3.4.3). 3.4.2. Oxygen Storage/Release in Three Way Catalysis for Automobile Exhaust Purification. Three-way catalytic conversions of gasoline engine exhausts HC/CO/NOx is a typical example of such a complicated redox reaction. There are three types of redox reactions taking place at the same time, including the reduction of nitrogen oxide (NOx) and the oxidations of hydrocarbons (HCs) and carbon monoxide (CO).363 The corresponding catalysts are named as three-way catalysts (TWCs), which have been used concurrently almost in all new gasoline engine automobiles. Though the three way catalysis has been investigated for decades, it is still an attractive subject of research presently.364,365 The increasingly strict regulations for the automobile exhaust control demand even-higher conversion rate, lower start-up temperature, lower consumption of precious metals, and higher stability against aging. The special character of the TWCs is its high and stable catalytic activities on the oxidation of CO and HC and the reduction of NOx simultaneously. While the single catalyst component can hardly meet all these requirements, so the TWCs are multicomponent catalysts, which are mainly composed of two kinds of precious metals responsible for catalytic oxidation and reduction, respectively, and the support(s) for oxygen storage/release as well.

NOx + CO → N2 + CO2

(82)

CO + O2 → CO2

(83)

HxCy + O2 → CO2 + H 2O

(84)

In all: NOx + CO + HC + [O2 ] → N2 + H 2O + CO2

(85)

Here [O 2 ] represents the oxygen species of a fixed concentration precisely at [A/F] = 14.6. Unfortunately, the A/F ratio will inevitably deviate from the ideal value in practice, which is dependent on a number of factors, such as atmosphere pressure, oxygen level in air, quality of fuel, etc., leading to the fluctuation of oxygen concentration, that is, incomplete conversions of NO or CO/HC at higher or lower A/F values than the ideal one, respectively. To deal with the fluctuation of oxygen concentration, a certain degree of tolerance to the deviation of A/F ratio from the ideal value, that is, a widened A/F window, must be created in the practical applications. Cerium oxide has been found to be able to take on this function on the basis of its easy valence transition between Ce4+ and Ce3+. Oxygen can be stored or released in/from the CeO2−x lattice respectively under O2-rich or O2-deficient conditions; therefore, this material is usually called the oxygen storage component (OSC): O2 ‐deficient/rich

CeO2 ⇐======⇒ CeO2 − x + [O2 ]

(86)

Here [O2] has the same meaning as in reaction 85. The ionization energy from Ce3+ to Ce4+ is so low that CeO2 functions as an OSC without any change in the crystalline structure. Hence, the special redox conversion capability between Ce3+ and Ce4+ can stabilize the oxygen concentration in the exhausts by oxygen storage/release in a certain fluctuation range of oxygen concentration, which in turn enhances and also balances the three way catalysis.370,371 As ceria nanoparticles are subject to easy growth and agglomeration, some stabilizing agents such as zirconia and rare earth oxides are incorporated into the ceria lattice, forming solid solutions such as ZrxCe1−xO2, which is usually called Ce/ Zr powder. A high amount of zirconia, e.g., 20−80 wt %, can be incorporated in ZrxCe1−xO2, and further heterogeneous ions AF


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Figure 34. Simplified schematic illustration of the proposed synergetic effect by the oxygen storage/release in the three way catalysis of CO, HC, and NOx over a Pd/Rh-loaded ZrxCe1−xO2 catalyst, where oxygen is stored/released into/out of the ZrxCe1−xO2 lattice under O2-rich (a) and O2deficient (b) conditions, respectively, for balanced quick and simultaneous conversions of CO, HC, and NOx (eqs 82−84).

obtained mesostructured nanocomposite catalysts showed excellent catalytic performances for the three-way exhaust conversions with lowered ignition temperatures, enhanced thermal stability, and aging-resistance. The almost completed oxidation conversions of CO and C3H6 could be achieved at 250 and 270 °C (Figure 36A and B) on the fresh mesostructured Pd-Rh/ACZ nanocomposite, respectively, which is very favorable for the quick activation of the catalyst on engine start-up, since the main exhausted gases are CO and CxHy at the start-up stage of gasoline engines. Moreover, the NO reduction was ignited by CO at around 240 °C, and it reached complete conversion at about 350 °C (Figure 36C), indicating the excellent redox catalytic activity of this mesostructured Pd−Rh/ACZ nanocomposite catalyst. Even the aged nanocomposite catalyst (after calcination at 1000 °C for 2 h) showed only a limited increase by 20−40 °C of ignition temperatures in the three-way catalytic reactions, indicating the high aging resistance of mesostructured Pd−Rh/ ACZ composite catalyst. The selective catalytic reduction of NO by C3H6 in the presence of different concentrations of oxygen over the prepared fresh mesostructured Pd−Rh/ACZ nanocomposite catalyst is shown in Figure 36D. The conversion rate of C3H6 increases with the increasing amount of oxygen, and almost 100% NO can be selectively converted into N2 by C3H6 in the absence of oxygen; however, it decreases significantly at increased oxygen level. Nevertheless, even in the presence of excess oxygen (x = 5, 6), as high as 70% NO conversion to N2 can be obtained. In addition to the enhancement of the thermal stability of Ce/Zr powder, the metastable incorporation of alumina in the lattice of Ce/Zr can in the meantime create oxygen vacancies:

can be doped into ZrxCe1−xO2 for enhanced oxygen storage capacity.372−374 Some reports show that not only the CeO2/ ZrO2 ratio but also the ratio of Ce/Zr to γ-Al2O3, with the latter being used as part of the support to increase the dispersivity of Ce/Zr powder and also the precious metal nanoparticles, will affect the thermal stability and the oxygen storage properties. A simple illustration is given in Figure 34 to schematically explicate the synergetic effect in the three way catalysis among CO, HC, and NOx by Pd/Rh-loaded ZrxCe1−xO2 tricomponent catalyst. 3.4.3. Promoted Redox Reaction between CO and NO and between NO and C3H6 by a Tricomponent Catalyst with an Oxygen Storage Support. To further enhance the surface area of Ce/Zr powder, recently, a mesostructured tricomponent Al2O3−CeO2−ZrO2 (ACZ, BET surface area: >180 m2/g) nanocomposite with a small amount of La doping was synthesized in our group by a polycopolymer-assisted selfassembly process.375 In fact, alumina has been found to be capable of metastably entering zirconia lattice during a chemical preparation route though thermodynamically alumina and zirconia are almost not solid-soluble with each other, leading to much increased dispersivity, thermal stability, and surface area, and this Al2O3−CeO2−ZrO2 nanocomposite had uniform wormlike mesoporous structure and very high thermal stability.376 In addition, La2O3 was doped to inhibit the phase transformation of alumina on the other hand. When a small amount of precious metals (Pd, Rh) were homogeneously loaded and dispersed into the pore channels of the mesoporous ACZ nanocomposite, as can be seen in Figure 35 where 1 wt % Pd/Rh (Pd/Rh weight ratio is 8 to 1) was incorporated, the

Al 2O3 → 2Al Cė + 3VÖ

(87)

The pre-existing oxygen vacancies in the Ce/Zr lattice by Al2O3 doping could provide quick diffusion channels for the O2− migration into/from the lattice and its exchange with the environmental oxygen, which will no doubt contribute to the much accelerated oxygen storage/release rate, much enhanced oxygen storage/release capacity, as shown in Figure 37, and lowered ignition temperature (Figure 36). The stick−ball model shown in Figure 38 gives a schematic illustration of the oxygen vacancies and diffusion channels created by Al-doping. So in the case of C3H6 as a typical hydrocarbon compound, the redox reaction under the

Figure 35. Typical TEM images recorded from 1 wt % Pd/Rh (with a Pd/Rh ratio of 8:1) loaded nanocomposite CeO2-ZrO2-Al2O3 catalysts after calcination at 500 °C (A) and 1000 °C (B) (Adapted with permission from ref 375. Copyright 2011 Elsevier). AG


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Figure 36. Temperature-programmed reaction profiles for the oxidation reactions of CO (A) and C3H6 (B) and the redox reactions of NO + CO (C) over the prepared fresh TWC catalyst calcined at 550 °C as well as the aged TWC catalyst at 1000 °C for 2 h. Part D is the NO and C3H6 conversions in the redox reactions of NO + C3H6 + O2 over the prepared fresh TWC catalyst (Adapted with permission from ref 375. Copyright 2011 Elsevier).

Figure 38. Proposed stick and ball models for a stoichiometric CeO2 (110) surface without (A) and with (B) Al-doping which substitutes for part of cerium in the ceria lattice. Figure 37. O2-TPD profiles of the prepared samples Al2O3/CeO2/ ZrO2 (ACZ) and CeO2/ZrO2 (CZ) (Adapted with permission from ref 375. Copyright 2011 Elsevier).

structure of the tricomponent ACZ nanocomposite. Such a mesostructure would lead to the enhanced thermal stability of the loaded noble metal nanoparticles against aggregation and quick growth and to the fast diffusion of oxygen species across the framework of several nanometers in thickness, which is also responsible for the low ignition temperature and high conversion rates, in addition to the oxygen vacancy precreation by Al-doping.

assistance of high performance ACZ OSC support can be expressed as follows: NO + C3H6 + O2 → N2 + CO2 + H 2O

(88)

The above reaction can take place even under excess amount of oxygen in the reaction system thanks to the high oxygen buffering effect of mesoporous ACZ support in the TWC of mesostructured Pd−Rh/ACZ nanocomposites due to its excellent oxygen storage/release properties. Similar to the above mentioned role of the mesostructure in the composite catalysts in sections 3.1−3.3, in the case of TWCs, the noble metal Pd and Rh nanoparticles were highly dispersed and confined into the wormlike mesoporous

4. FURTHER DISCUSSIONS: ROLE OF INTERFACE AND MESOSTRUCTURE IN SYNERGETIC CATALYTIC EFFECTS 4.1. Role of Interface Formation and Contact Angle between Components in the Synergetic Catalytic Effects

The presence of an interface between two catalytic components is the prerequisite for most synergetic catalytic effects. In type I AH


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interface energies, but they can also be affected largely by the catalyst processing history, such as thermal treatments and impurity.

of the synergetic effect, one component is activated by the other via, for example, charge transfer and oxygen vacancy formation across the interface. Oxygen molecules in environments are converted/activated into active oxygen components such as Oads and/or peroxo species responsible for the oxidation of reactants such as CO or organics on the surface of oxide supports, or more probably at the interface between two components (Figures 9, 10, 12, 15, and 17). Similarly, in type II, the close attachment between the two components favors the quick and immediate successional functioning of the two components (Figures 23, 25, and 28), leading to accelerated reactions. Though the interface does not function directly in this case, however, the formation of an interface ensures the firm and close bonding/attachment between components, which is also indispensable for the synergetic catalysis. In types III and IV, the interface acts as the bridge for the free transfer of poisonous species, such as CO, from the catalytically active sites of Pt to the secondary component(s), such as WO3, in the Pt/WO3 catalytic composites for poisoning prevention (Figure 31) or for the free transfer of oxygen species directly from the surface of reductive catalytic components into the lattice of ceria for oxygen storage under oxygen-rich environments, or from the ceria lattice directly onto the surface of oxidative catalytic component for oxygen release under oxygen-deficient environments, for the quick and balanced reduction/oxidation of harmful gasoline engine exhausts (Figure 34). To obtain as high as possible specific interface area in a given composite system of fixed relative contents, one major approach is to get ever higher dispersion of the components, for example, to minimize the particle size of the active phase and to disperse the small-sized main component(s) on/in the support(s) of high surface area, such as mesoporous supports, as discussed in the following section. Besides, the contact angle (θ) between different components can also significantly affect the interface area and consequently the synergetic effects. Only minimal interface area can be obtained at overlow contact angle (e.g., up to zero degree), in which two phases have minimum affinity, i.e., extraordinarily high interface energy between each other. In contrast, very high interface area can be generated in a system where two phases can form an interface of much lower interface energy than their surface energies. A simple calculation shows that the specific interface area in a composite of θ = 150° will be around twofold that at θ = 90° in a fixed system by using a most common two-dimensional arc particle-on-flat-surface model. In the case that only the dispersion effect is considered, the contact angle might be less important because the dispersity of the catalytically active phase is mainly determined by its particle size and size distribution, and also the high dispersion effect is mainly determined by supports for the catalyst(s), though a continuous coating of the active phase(s) on the support surface at θ = 180° will create maximal interface area(s) between the components. However, in a system where the synergetic effect between components is important, there will be an optimal contact angle, i.e., interface area/surface area ratio, between the two phases, to ensure that the one active component can get sufficiently high dispersion while the other component can also play an indispensable role in the synergetic catalytic effect with the help of the interface formed between them under an adequate interface area/surface area ratio. The contact angles between different phases are mainly determined by their intrinsic chemical properties, i.e., relative surface and

4.2. Advantages of Mesostructured Composites for the Synergetic Effects

In this article, proposals and discussions are made based on both the nonmesostructured and the mesostructured composite catalysts, in the latter, the main active species are loaded in the mesopore network of functional/catalytically active mesoporous supports of a well-crystallized framework. Typically, the composite mesostructure ensures the high dispersion of the guest active component(s) in the pore channels of a mesoporous matrix. In addition to this, many other benefits of mesostructures are also apparent, such as the effective inhibition on the exaggerated growth and/or aggregation of the catalytic species confined in the pore channels for the stabilization of the loaded active species, and the much enhanced surface area and interface area as well, as compared with traditional composites which are commonly prepared by loading active species onto the outer surface of much larger support particles. The confinement effect on the catalytic species by the mesostructure makes the catalytic effect much more durable, and the enhanced surface area is usually responsible for the enhanced catalytic activity. Additionally but more importantly, synchronously increased interface area by the enhanced dispersity is highly beneficial to the synergetic effect enhancement, as most of the synergetic effects work at the interface or by making use of the interface. Also interestingly, pores are concave judged from the pore side; therefore, the interface between the loaded species and the porous matrix would also be concave rather than approximately flat in the traditional composites where supports are usually much larger than the loaded ones. This will lead to an interesting phenomenon that, for a unit volume of loaded particles, the specific concave interface area of a porous composite will be significantly higher than the flat one at the same contact angles. For example, when a half-cylindrical nanoparticle is loaded on a flat surface under a contact angle of 90°, the interface area will be 2/π times the convex surface area of the particle (Figure 39a). Comparatively, under the same

Figure 39. Simplified models of catalyst particle loading on the flat surface of a traditional large support (a) and in the pore channels of a mesoporous support (b) under the assumption of a 90° contact angle and a half-cylindrical shape of the loaded catalyst particle in both cases.

contact angle of 90° and the same shape of the nanoparticle loaded in a mesopore, a special condition of half filling of the nanoparticle in a one-dimensional mesopore can be obtained, and such a ratio of interface area to surface area is π/2 (Figure 39b). The specific concave interface area of a porous composite will be π/2 times the flat interface area of traditionally loaded and nonporous-structured composite under the same contact AI


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cussed corresponding to each type of reaction in detail to explicate and support our proposals about the synergetic effects, and schematic illustrations are presented for most specific nanocomposite catalysts and reactions. Some specific issues of the advantages by using mesostructured composite as catalysts and the role of the interface formation and the contact angle between components are also discussed (sections 4.1 and 4.2). As can be found in the literature and also the present review, the most prevailing synergetic catalytic effects can be attributed to type I: One component activation by the other. The activation of components can be achived either by doping heterogeneous ions or valence change of one component (Figures 9, 10, 12, 15, and 17), by promoted/sustained electrohole separation (Figures 19 and 21), or by metal particle charging via the electron donation from the oxide components (Figures 13 and 19). In spite of all the differences in the synergetic processes, typically a charge transfer process between the components is usually included in type I of the synergetic effect. Moreover, most examples in this type can be found in the oxidation/reduction reactions by using a combination of metal (or oxide (1)) and oxide (2), where the active oxygen species such as Oads and/or peroxide species such as Ce−O− O* and Ti−O−O* are created on its surface or at the interface between the components for oxidation reaction (i.e., oxygen reduction), and the metal or oxide (1) might act as the adsorbent/activator for the compounds to be oxidized. In such a composite catalyst, metal ions in oxide (2) should be valencechangeable and oxygen vacancy can thus be generated on the surface and/or in the lattice of oxide (2) for the creaction of active oxygen species under the promotion by the metal/oxide (1), and the metal/oxide (1) should be able to act as electron donor/receptor to donate/attract electrons facilitating the charge transfer and/or valence change to/from the oxide (2). Therefore it can be inferred that oxide (2) could usually be oxide semiconductors such as cerium, zinc, tatinium, manganese, cobalt, copper, tungsten, and iron oxides, etc., and the metal components should be the chemically stable and well-dispersed noble metal nanoparticles, such as Au NPs, and metal ions in oxide (1) should also be highly valencechangeable, such as copper in copper oxides. Type II of the synergetic effects can be actually further divided into two kinds of successional functioning effects of different components. The first one is the reactant activation by the one component/active site via chemical adsorption/ bonding followed by reactive catalysis by the other component/active site. As well-known, the common catalytic process by single component catalysts usually involves an adsorption (activation)catalytic reaction process; in the present case, the adsorption and catalytic steps were respectively undertaken by two cooperative components (active sites) instead of one component, while the second kind is much more complicated, involving successional functioning of different components in different steps in a multistep reaction, and both high conversion of reactants and selectivity to target product can be achieved through an appropriate combination between catalytic components. Type III of the synergetic catalytic effect can be found in a catalytic reaction where degradation of the main component may take place, e.g., poisoning of the main components such as noble metal by reactant(s) or impurities which can firmly cover the active sites of the catalyst, or the structural/compositional/ morphological changes such as phase transition, decomposition, aggregation, and/or growth of nanocatalytic particulates during

angle, the same amount, size, and shape of loaded nanoparticle catalyst in composites. This will lead to a significantly more profound synergetic effect between the loaded guests and the crystallized mesoporous framework in mesostructured composites, than that in nonmesoporous composites, in addition to the dispersity and stability enhancements of the guest components by the confinement effect. All above benefits of porous materials, in addition to the high dispersion of catalytic species alone, will contribute to the enhanced catalytic performance and synergetic effects. In addition to these mentioned above, the tunable pore size of porous composites greatly favors selective catalytic reactions: only those reactants smaller than the pore size can access the catalytically active sites in the pores and only the products smaller than the pore size can be generated significantly in the catalytic reactions. If compared to microporous zeolites where coke formation would frequently happen during reactions involving organic molecules, mesostructured zeolite showed much relieved coke formation due to the much quicker diffusion of reactants/products in the mesopore channels rather than in the microporous network.73,77

5. CONCLUSIONS AND OUTLOOK In this review, we first discussed very briefly the synthesis of nanocomposite catalysts by solution and/or sol−gel chemistry, in which the synthetic strategies are roughly classified into two major approaches: postdeposition and “one-pot” or “one-step” strategies. We then reviewed the syntheses and structural formation of mesostructured nanocomposites by several approaches, such as direct soft-templating (section 2.2), hard templating (section 2.3), and postencapsulation (section 2.5). The main content is on the proposals and discussions of four kinds of possible synergetic catalytic effects in nanocomposite catalysts between different components in bi- or tricomponent composite catalysts with or without mesoporous structure: (1) One component activation by the other between two catalytic components for enhanced catalytic activity (section 3.1); (2) successional catalytic functioning of bicomponents in multistep reactions for enhanced reaction activity and/or selectivity (section 3.2); (3) degradation prevention of the main component(s) by the secondary component(s) for sustained catalytic reactions (section 3.3); and (4) reactant storage/ release by the catalytic assistant(s) in complicated redox reactions (e.g., three-way HC/CO/NO catalytic conversions) (section 3.4). Various types of catalytic reactions, including the CO catalytic oxidation (section 3.1.2), methanol synthesis from syngas (section 3.1.3), C−C couplings (section 3.1.4), selective oxidation of propene and alcohol (section 3.1.5), photocatalytic degradation of organic pollutants (section 3.1.6), Henry and aldol condensation reactions (section 3.2.2), dye degradation by Fe-doped mesoporous zeolite (section 3.2.3), ammonia selective catalytic oxidation (section 3.2.4), antipoisoning of nobel metal electrode catalysts in electrochemical reactions (sections 3.3.2 and 3.3.3), and three-way catalysis of CO/CH/ NOx conversion reactions (sections 3.4.2 and 3.4.3), have been exampled by using bi- or tricomponent nanocomposite catalysts, where different types of synergetic effects between the different components, for example, between loaded guest species and supporting matrices, are believed to play important, or in some cases decisive, roles in the reactions. The underlying synergetic mechanisms in terms of the cooperation/interaction between/among the components and/or supports are disAJ


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been proved or expected to be present between two elements/ metal ions in single phase materials containing biactive or multiple active elements, such as bi- or multicomponent metal alloys like precious−precious and precious−nonprecious metal alloys, heterogeneous atom-doped solid solutions like heterogeneous metal-doped oxides and N-doped oxides, bimetal oxides (oxyacidic salts) like perovskites, pyrochlores, and spinels, or other single phase compounds containing more than one active site. Finally, it should be pointed out that the synergetic effect presented in this review is mainly based on the discussions on nano- or mesoscales where the roles of catalytic nanoparticles are considered basically as a whole. In fact, recently great advances in probing the catalytic mechanisms on the atomic scales, such as in the single-site heterogeneous catalytic mechanisms, have been made, which can give a much deeper understanding about the nature of catalysis. It will be of great interest to combine the current understanding of the essences of catalytic process on atomic/molecular scales with the synergetic effects on nanoscales as being proposed preliminarily in this review article. It is expected that such an integration of the current investigations on different scales will bring a more comprehensive but in the meantime more detailed and profound understanding of the catalytic process, and therefore will be of great significance in searching for and finding novel nanocomposite catalysts of further enhanced activity, selectivity, durability, and environmental compatibility.

the reaction. If a secondary component(s) capable of preventing or relieving these degradations of the main components can be found and introduced, synergetic catalytic effects can be achieved which are also of great importance and significance. Though type IV of the synergetic catalytic effect is rather special and can be only found in complicated redox reactions involving multiple reactants and products, three-way catalysis for the purification of gasoline engine exhausts is a typical example, in which an oxygen storage component such as CeO2based composite oxides plays a decisive role in maintaining a specially designated oxygen concentration suitable for the simultaneous and almost complete conversions of both oxidative and reductive pollutants in a balanced way. Nanocomposite catalysts composed of two types of noble metal nanoparticles respectively responsible for the catalytic oxidation and reduction reactions, and together with oxygen storage supports, have been in application of the three-way catalysis for purifying gasoline engine exhausts for decades, and they will be continuously applied in the future in automobiles using gasoline as the fuel. The proposed synergetic effects should be applicable to a variety of nanocomposite catalysts both with and without mesostructures for heterogeneous catalytic reactions; however, the synergetic effects themselves might be more dependent on the intrinsic atomic scale physical and chemical properties of the components involved than on the composite structure on nanoscale. The role of the mesostructure of the composite catalysts is to make the synergetic effect more significant and profound, and those of nonmesostructured composites have been considered to be less profound than well-mesostructured counterparts due to their lack of pore confinement and dispersion effects for loaded main catalytic component(s). We have discussed the importance and the significant roles of the mesoporous structure and the extensive interface formation between components in enhancing or magnifying the synergetic catalytic effects in sections 4.1 and 4.2. Nevertheless, before considering these detailed structural features on nano- or mesoscales, one should first focus on the compositional design of the composite catalysts, according to, but not limited to, the possible mechanisms of synergetic effects proposed in this review. The two components should either interact with each other for catalyst activation (type I) or help each other in catalyzing the reactions via successional or simultaneous functioning of different components (type II, IV) or via poisoning/degradation prevention (type III) of the main component(s) by the other, or via other routes presently unclear or not included in this review. General synergetic effect proposals are given and discussed in sections 3.1.1, 3.2.1, 3.3.1, and 3.4.1, for the synergetic effects of types I, II, III, and IV, respectively, for the reader’s reference, and probably more to be identified in the future. This paper only presents an introductive study of the synergetic catalytic effects possibly present in the composite catalysts, and the corresponding catalytic mechanisms underlying each type of nanocomposite catalysts are also explored very preliminarily based only on limited reaction examples and literature reports. More work is needed in the future investigation for a more clear understanding of the synergetic effects in the composite catalyst for the inventions of better nanocomposite catalysts of enhanced performances. In addition to nanocomposites where two or more components/phases are involved, similar catalytic effects have

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: 86-21-52412712. Fax: 86-21-52413122. Notes

The authors declare no competing financial interest. Biography

Jianlin Shi received his Bachelor degree from Nanjing University of Technology in 1983, obtained his Ph.D. degree in 1989 at Shanghai Institute of Ceramics, Chinese Academy of Sciences, and has been working at the institute since then. He had once worked on the processing science of advanced ceramics, solid state sintering theory of advanced ceramics, and high temperature reliability of structural ceramics from 1983 to 1998. Presently his main research interest includes the structural design and synthesis of mesoporous materials and mesostructured nanocomposites, and the catalytic and biomedical performances of the materials for applications in environmental protection and nanomedicine. AK


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