Metal-support interaction effects on gold catalysts over reducible oxides

In this chapter, we first introduced the syntheses of heterogeneous gold supported reducible oxide catalysts (Au-MO_x) prepared by chemical routes. Subsequently the emphasis has been on the catalytic performance of various heterogeneous composite catalysts enhanced by the interaction effects between the gold nanoparticles and the host supports, depending on the varied interfacial surfaces among them. Several kinds of possible synergetic/cooperation effects or interaction effects and the corresponding structural properties based on heterogeneous composite catalysts have been proposed. To understand the interaction effects more clearly, catalytic reactions catalyzed by composite catalysts both reported in the literature and the subject of our own investigations have been presented as typical examples and discussed as to their specific behaviors and properties in terms of the interaction effects between the noble metal(s) and support(s). CO oxidation was used as a sensitive probe reaction in all the cases because it is easy to carry out. It is generally agreed that the catalytic activity of gold catalysts depends on the size of the gold particles, but the nature of the supports, the preparation methods, and the activation procedures have also been suggested to play a key role.81 In this chapter, exceptionally high activities for oxidation of CO have been reported for highly dispersed Au catalysts on reducible oxides. It has been proposed that the reducible oxides supply oxygen to form oxidic gold sites exceptionally active in CO oxidation. In addition, positively charged gold particles or negatively charged clusters of a few nanometres have been shown to be catalytically active. Other explanations focus on the charge transfer between the support, particularly negatively charged defects (F centers), and the Au particles. Strain in the Au particle due to the mismatch of the lattices at the interface with the support has also been suggested. Besides, the presence of a metal to non-metal transition in very small, two-dimensional Au particles may exist in the gold catalysts. More importantly, several groups of researchers have pointed to the effect of lowcoordinated sites and the roughness of Au clusters on the reducible oxides. Effects related to the interaction with the support may contribute to a considerably smaller extent compared with the effect of a low-coordination number or gold particle size as reported by Nørskov and co-workers.81 However, the effect of the interaction with the reducible supports is generally much stronger than that with the irreducible supports. Therefore, the latter are generally less reactive than the former, even by a factor of 2-4. This means that effects related to exchange of oxygen between the gold and the support, and effects due to charge transfer from oxygen vacancies to the gold particles can be the main contributor at a given same particle size. Furthermore, support-specific interactions between adsorbates or reaction intermediates on gold close to the perimeter of the support are apparently the main contributors to the activity in gold catalysts at the same level of gold particle size. This chapter presents only an introductive study of the interaction effects possible in composite catalysts, and the corresponding behaviors and properties related to each type of heterogeneous composite catalysts are also explored very preliminarily based on limited reaction examples and literature reports. More work is needed in future investigations to obtain a clearer understanding of the interaction effects in the composite catalysts for the invention of better composite catalysts with enhanced performance. We feel that aspects deserving further attention in the near future are the following: (1) Despite the reported interface control via building up and tuning the metal-metal and metal-oxide interactions, metal-organic interactions of Au-oxide composite catalysts could optimize the catalytic performance of this kind of catalysts. (2) Since the interactions between Au particles and reducible oxide supports usually arise from steric or electronic effects, characterization techniques such as in situ ETEM or IR-CO probe should be applied to accurately recognize the interfacial interactions. (3) Studies on other interfaces, such as metal-hydroxide, bi-metal-oxide, and metal-bi-oxide should be envisaged. (4) A combination of DFT calculations and structural/surface characterization techniques could be used to obtain more insight into the metal-support interaction effects in catalysis.