Ag-CeO₂Interaction And Structure-Activity Relation Of Ag/CeO₂Catalysts

The fundamental understanding of structure-catalytic performance relation of catalysts is of great importance in the design and exploring of novel efficient catalysts. The employed main approach is to study the catalytic surface chemistry of model catalysts with well-defined surface structures. However, there exist the so-called "materials gap" and "pressure gap" between the catalytic surface chemistry study of traditional model catalysts based on single crystals and the real catalytic systems. The rapid development of the synthesis of nanomaterials realizes the controllable preparation of metal and metal oxides nanocrystals with uniform and well-defined structures. Nanocrystals with uniform and well-defined structures consist of novel model catalysts for the catalytic surface chemistry study without the "material gap" and the "pressure gap", meanwhile, they also are nice candidates for active and selective catalysts.In this dissertation, Ag/CeO2catalyst was chosen as the target system, Ag/CeO2model catalysts based on CeO2and Ag nanocrystals have been fabricated and systematically investigated. The Ag-CeO2interaction and the structure-activity relation of Ag/CeO2catalyst were derived, including::1. Ag/CeO2catalysts with various Ag loadings were synthesized via traditional deposition-precipitation method using CeO2powder as support. It was found that the calcination temperature significantly affects the Ag-CeO2interaction structure and catalytic performance of Ag/CeO2catalysts. Ag/CeO2catalysts calcined at200℃contain Ag+dissolved in the CeO2lattice; calcinations at500℃result in the segregation of these dissolved Ag+from the CeO2lattice. Interestingly the segregation of dissolved Ag+from the CeO2lattice leads to the restructuring of Ag nanoparticles supported on CeO2and such a restructuring process depends on the size (Ag loading) of these Ag nanoparticles. The segregation of dissolved Ag+from the CeO2lattice results in the redispersion of supported Ag nanoparticles for Ag/CeO2with low Ag loadings and fine supported Ag nanoparticles but the aggregation of supported Ag nanoparticles for Ag/CeO2with high Ag loadings and large supported Ag nanoparticles. The catalytic activity of Ag/CeO2catalysts correlates positively with the dispersion of supported Ag nanoparticles and the Ag NPs-CeO2interface of Ag/CeO2catalysts is the active structure to catalyze CO oxidation.2. Uniform CeO2nanocrystals with different morphologies including cubic nanocrystals exposing{100} crystal planes, rod-like CeO2nanocrystals exposing{100} and{110} crystal planes were synthesized and employed as the supports to prepare Ag/CeO2catalysts by impregnation method. The shape-dependent interplay between oxygen vacancies concentration/type and Ag-CeO2interaction in Ag/CeO2catalysts and their influence on the catalytic activity of CO oxidation have been successfully elucidated. CeO2nanorods with a high oxygen vacancy concentration and small-sized/large-sized oxygen vacancies can stabilize the partially positively-charged Agn+clusters whereas under the same condition CeO2nanocubes with a low oxygen vacancy concentration and only largely-sized oxygen vacancies can only stabilize Ag nanoparticles; Ag nanoparticles exhibit stronger abilities to activate the lattice oxygen of CeO2and to promote the reducibility of CeO2than Agn+clusters. Ag nanoparticles--CeO2interface with suitable concentration and structure of oxygen vacancies are most active to catalyze CO oxidation. CeO2nanocubes are the suitable support for the preparation of Ag/CeO2with a low Ag loading but active in CO oxidation.3. Uniform Ag nanocrystals with different morphologies including cubic Ag nanocrystals exposing{100} crystal planes, plate-like Ag nanocrystals exposing{111} crystal planes, rod-like Ag nanocrystals exposing{100} and{110} crystal planes and Ag polyhedron exposing a variety of crystal faces were successfully synthesized. The growth mechanisms of Ag nanocubes and nanorods via the EG reduction of AgNO3in the presence of PVP and HCl have been elucidated. A novel hydrothermal method (oxygen-assisted hydrothermal method) was successfully developed for the growth of CeO2nanoparticles on Ag nanocrystals, i.e., the inverse CeO2/Ag model catalysts based on Ag nanocrystals. It was observed that the surface enhanced Raman effects of Ag nanoparticles can be enhanced by the presence of CeO2adparticles. CeO2/Ag inverse catalysts also exhibit nice activity in catalyzing CO oxidation, supporting that the Ag NPs-CeO2interface is the active structure.Above experimental results provide deep insights into the fabrication of metal/oxide model catalysts based on nanocrystals and the metal-oxide support interactions and structure-activity relation of supported catalysts.