A Density Functional Theory Study Of CO Oxidation On Au_x/CeO₂ Catalysts

Today, Au/CeO2 catalyst has caused the majority of attention, but its active sites and catalytic mechanism of CO oxidation on it are still challenged. Here, we firstly calculated the adsorption of O2 molecule on CeO2 (111) surface by DFT+U method. Then the roles of formed oxygen species (peroxide and superoxide) in Aux adsorption on CeO2 (111) surface were studied. Based on the calculated results for Aux/CeO2, the active sites and the mechanism of CO oxidation on this catalyst were investigated. Through this established theoretical model of Aux/CeO2, it can be explained for the electronic storage capacity of ceria, which can provide a scientific basis for the development of high-performance rare earth catalytic materials. The main results obtained are described as follows:1. O2 adsorption on the CeO2 (111) surface containing surface oxygen vacancy or sub-surface oxygen vacancy formed peroxide and superoxide species respectively. Peroxide is more stable than superoxide species. Furthermore, the superoxide can convert to peroxide species under certain conditions, which includes two steps:the migration of sub-surface oxygen vacancy to surface oxygen vacancy and the conversion of superoxide to peroxide, in which the former is the controlling step.2. Aux(x=1-4) clusters adsorption on the CeO2(111) surface and CO adsorption on the Aux/CeO2 catalyst were investigated. The results show that, Au adsorbed on the CeO2 (111) surface with the O vacancies has strong negative charge (Auδ-), which is not favorable for CO adsorption. After peroxide was introduced on CeO2 (111) surface, Auδ- adsorbed was oxidized to result in the reduction of negative charge of Auδ- to near 0 (～Au0), which improved the adsorption/spreading of Au on the ceria surface and CO adsorption on Aux/Ce02 catalyst. Ceria can act as an electrons buffer in the adsorption processes of Au on ceria and CO on Au/ceria.3. The catalytic mechanism of CO oxidation on Au2/CeO2 catalyst was investigated. The results show that, both peroxide and superoxide can oxidize CO. On Au4/CeO2 catalyst, CO oxidation occurs at the interface between Au4 cluster and ceria, that is to say, an interfacial effect.