Support Effect For MO_x（M= Ru, Ir, Sn, Pd） Supported On TiO₂ Catalysts: A Quantum Chemical Study

Supported oxide catalysts are important in industrial application. The interaction between active component and support is found to have an important influence on structure and performance of a supported catalyst. The nature of the interaction is an important and basic research subject in heterogeneous catalysis. In this paper, using density functional theory calculation, we investigated the interaction between different oxide active components（MO-type PdO and MO₂-type RuO₂, IrO₂ and SnO₂ oxides as active components） and the same support（TiO₂（110） as the support）, and its effect on the adsorption of CO, O₂ or H₂ and CO oxidation reaction. The influences of the active component thickness and the presence of oxygen vacancy in support or active component on the support effect were also explored in detail when the active component dispersed as monolayer and multilayer. The main results show that:1. The presence of a support will lead to the formation of interface bonds, charge redistribution, the frontier orbital recombination and the change of the redox ability of surface active sites, which is responsible for the effect on a adsorption process and CO oxidation reaction. For monolayer PdO supported on TiO₂, the results indicate that few charges are transferred between TiO₂ support and PdO, while the presence of TiO₂ support leads to the charge redistribution in PdO. Consequently, the oxidizing ability of the surface Pd active sites（Pdcus） is improved, which leads to H₂ molecular adsorption strengthened on the top of Pdcus compared with that on pure PdO surface. While for monolayer RuO_2-x supported on TiO₂, TiO₂ support leads to the change of relative frontier orbitals and the improved reducibility of surface Ru sites, which results in O₂ adsorption ability strengthened compared with that on pure RuO_2-x surface.2. On the same support, if the active components have the same crystal structure but with different components, the support effect shows a significant difference due to the intrinsic properties differences of the metal ions, such as different redox abilities. For MO_2-x/ TiO₂（M=Ru, Ir, Sn） systems, it is revealed that for defective RuO_2-x films, the TiO₂（110） support and the film thickness have an evident impact on the O₂ adsorbed species. On the contrary, the two factors show little influence for defective IrO_2-x and SnO_2-x films.3. For PdO as an active component, TiO₂ as the support, support effect lead to the change of the most preferred site for CO adsorption, thus altering the reaction pathway of CO oxidation, whereas the support effect has little influence on the activation barrier of CO oxidation. However, for TiO_2-x（TiO₂ with an oxygen vacancy） as the support, the presence of the oxygen vacancy leads to the activation barrier remarkably decresed for CO oxidation compared with that on the pure PdO or PdO/TiO₂ surface. The presence of the oxygen vacancy results in the tuning of CO adsorption strength at the suface active oxygen, which is beneficial for the extraction process of surface oxygen.