Engineering Of Surface And Interface Structures Of CeO₂-based Composites And Their Catalysis Applications

The identification of the active sites of heterogeneous catalysts is essential for better understanding the reaction mechanism and the design of highly efficient catalysts.Due to the complexity of the surface structures of the traditional powder catalysts,the rational identification of the active sites is challenging.Alternatively,uniform nanocrystallites with exposed single facets are promising supports for catalysts,which exhibited strong crystal plane effects during various reactions,and thus provides good opportunity for the distinguish of active sites and the establishment of structure-property relationship.Cerium oxide（CeO₂）has been widely applied in catalysis owing to its unique oxygen storage/release capability（OSC）and excellent redox performance.In this thesis,CeO₂-based composites（e.g.,Pd/CeO₂ and CeO₂-CuO/Cu₂O）with different morphologies and exposed facets were synthesized and tested for CO₂hydrogenation and CO oxidation,aiming to reveal the roles of metal-support interaction on the reaction behaviors.The detailed contents are as follows:1.CeO₂ morphology-dependent Pd-CeO₂ interaction and catalysis in CO₂hydrogenation to formate.Several Pd catalysts supported on CeO₂ with different morphologies（i.e.CeO₂ rods（r-CeO₂）,cubes（c-CeO₂）and polyhedra（p-CeO₂））were tested for the CO₂ hydrogenation into formate.Remarkable morphology-dependent Pd-CeO₂ interaction was observed,which was related to the oxygen vacancies in the CeO₂.The r-CeO₂ with the highest concentration of oxygen vacancy resulted in smaller Pd particles and more positively-charged Pd species in the Pd/r-CeO₂ catalyst.The best performance was obtained over the Pd/p-CeO₂ catalyst,giving a turnover frequency of746 h^-1 at 40 ^oC.The enhanced activity was due to the highest content of metallic Pd species in the catalyst,which was responsible for the facile activation of H₂.Moreover,in situ spectroscopic results revealed that the hydrogenation of the carbonaceous intermediates was the rate-determining step.This work demonstrates oxide morphology engineering as a powerful strategy in developing efficient Pd-based catalysts for CO₂hydrogenation.2.Structure sensitivity of CuO in CO oxidation over CeO₂-CuO/Cu₂O catalysts.Several CeO₂-CuO/Cu₂O nanocomposites with different CuO structures were used to identify the structure sensitivity of CuO in the CeO₂-CuO/Cu₂O catalyzed CO oxidation.The CO oxidation catalyzed by various CuO-CeO₂ interfacial sites involves a typical Mars-van Krevelen mechanism,in which the CuO-CeO₂ interfaces in the CeO₂-CuO/c-Cu₂O（cubes）nanocomposites are more intrinsically active,exhibiting ca.15 k J mol^-1lower activation energy than those in the CeO₂-CuO/o-Cu₂O（octahedra）and CeO₂-CuO/d-Cu₂O（rhombic dodecahedra）nanocomposites at CeO₂ loadings no less than0.75 wt.%.The higher activity is relevant to lower coordinated oxygen ions on CuO/c-Cu₂O surface and thus better CO reactivity for the CeO₂-CuO/c-Cu₂O nanocomposites,which therefore indicates that the active oxygen species on CuO-CeO₂ interface should come from CuO rather than CeO₂.These results demonstrate a successful experimental strategy of using nanocrystal model catalysts to realize an all-chain investigation of heterogeneous catalysis,from fundamental study including active site and reaction mechanism to guiding the structural design of catalysts and,finally,to the realization of highly efficient catalysts.