| Ceria-based catalysts have been widely applied into heterogenous catalysis reactions due to its high oxygen storage and release capacities. The reactivity of ceria alone is relatively low. Therefore, noble metals were often used in the ceria-based catalyst to improve its reactivity. There are two interaction ways between noble metals and ceria. One is the single atom or small clusters dispersed on the ceria surface; the other is doping into the ceria to form solid oxides, which might exist simultaneously in the practical catalysts. It is difficult to investigate the two different metal-ceria interactions only in experiment. In this thesis, we design M/CeO2 and MxCe1-xO2-d catalysts and investigate their structures and electronic properties using density functional theory (DFT) method. The adsorption and reaction behaviors of formaldehyde on the AuxCe1-xO2 catalyst are also tentatively calculated to explore its oxidative mechanism.The adsorption behaviors of four typical noble metals (Au, Pd, Pt, and Rh) on the CeO2(111) surface were systematically investigated using density functional theory method. It was indicated that Au prefers to adsorb on the top site, while Pd and Pt on the O-bridge site. Rh adsorbed on the 3-fold hollow site is the most stable configurations. When the noble metals adsorb on the top site, the adsorption strength order is as follows:Pt>Rh>Pd>Au. The new electronic peaks present between the Ce4f and O2p peaks when Pd, Pt, and Rh atoms adsorb on the CeO2(111) surface; while no peak is found during Au adsorption. It is found that the d electronic peak of Au overlaps with the O2P peaks at -4～-1 eV. According to the density of states (DOS) analysis, when Au adsorbs on the atop site, Pd and Pt on the bridge site, and Rh on the 3-fold hollow site, their interactions with surface oxygen atoms on the CeO2(111) surface are stronger than other configurations, which is in good agreement with the results of Bader charge population.The MxCe1-xO2-d(M=Au, Pd, Pt or Rh) structure and electronic properties were systematically investigated by DFT method. Our results suggest that the structures of CeO2(111) surface change due to the doping of noble metal atoms. The surface oxygen vacancy formation energies of Au, Pd, Pt and Rh are 0.32, 0.41,1.04, and 1.42 eV, respectively, which are much lower than that of the stoichiometric CeO2(111) surface. That is to say, the doping of noble metals promotes the formation of oxygen vacancy, and then improves the reactivity of metal ceria-based catalysts. According to the density of states, the doping peaks appear near the Fermi energy level. The charges of the doped atoms increase as the Au, Pd, Pd, and Pt order. When oxygen vacancy forms, the charges of doping metal atoms are than those of MxCe1-xO2-d without oxygen vacancy.The adsorption and H dissociation were tentatively calculated using density functional theory method. Three stable configurations were obtained when formaldehyde adsorbs on AuxCe1-xO2(111) surface. Their adsorption energies are much higher than that on the stoichiometric CeO2(111) surface, which indicates that the doping Au enhances the adsorption of formaldehyde on ceria-based catalysts. Transition state calculations suggested that the dissociation energy barrier for the first hydrogen of formaldehyde also greatly decrease compared with that on the stoichiometric CeO2(111) surface. This implies that the AuxCe1-xO2(111) slab model has high oxidative reactivity for formaldehyde oxidation, which is well consistent with the experimental results. |
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