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A Density Functional Theory Study about Ceria-based Gold Catalyst

Author: ZhaoYue
Tutor: ZuoBoTao
School: Zhejiang Normal University
Course: Physical and chemical
Keywords: DFT Ceria Gold oxygen species metal doped catalysts metal supported catalysts
CLC: O643.36
Type: Master's thesis
Year: 2012
Downloads: 92
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Abstract


Due to the reversible exchange between Ce4+and Ce3+, Ceria is well-know for its oxygen storage capacity and has been widely applied in heterogenous catalytic reaction. Metal doped or supported ceria-based metal catalysts have higher performance than the pure ceria in catalytic reactions. However, the electronic properties of the interactions between metal and ceria, as well as the possible reaction mechanisms, can not be obtained only through experimental method. In the present thesis, using density functional theory method, the structures and electronic properties of the MxCe1-xO2-δ (M=Au, Pd, Pt, and Rh) modeling catalysts were systematically investigated; then the adsorption and possible activation were compared on Au clusters with and without ceria support; finally, the adsorption and oxidation of formaldehyde on Au-doped CeO2was also studied. The detail results are as follows:We systematically compare the structures and electronic properties of the MxCe1-xO2-δ (M=Au, Pd, Pt, and Rh) modeling catalysts in theory. Two kinds of active oxygen species were obtained by O2filling into or standing aside the surface oxygen vacancy on MxCe1-xO2-δ(111) surface. Using CO as a probe molecule, tridentate and bidentate carbonates form by reaction with the two different oxygen species. Finally, the possible oxidative activities of the four noble metals doped ceria-based catalysts were evaluated by comparing the formation energy of surface oxygen vacancy, as well as the energy differences between carbonate and CO2desorption. Our work will shed light on the design of catalyst with high oxidation performance.The adsorption and activity behaviors of O2on Au clusters with and without CeO2were systematically calculated by density functional theory method. When O2adsorbed on Au(111), only physical adsorption happenedoccurs, while on3Au/CeO2(111) and AU19cluster the adsorption energiesy increased to-0.23and-0.58eV on3Au/Au(111) and AU19cluster, respectively. Furthermore, the adsorption energies strength of O2on these single Au catalysts carrying the postively charged Au models is smaller weaker than those on the neutral ones. When Aux adsorbs on the stoichiometric CeO2(111) surface, electrons of Aux transfer to the support, leading to Aux with positive charge. However, the adsorption energies of O2on AU9and AU3clusters supported on CeO2(111) surface is increased to-1.19and-1.77eV, respectively. The0-0bonds also lengthen with the increase of adsorption energies, indicating that the0-0bond is activated due to the interactions between Aux clusters and ceria support. But the adsorption energies of O2on Au3/CeO2and Aug/CeO2is much higher than the single ones. Then we analyzed the electronic properties Based on the charge density difference (CDD) and spin charge density difference(SCDD) method and make sure the possible peroxide and superoxide oxygen species on the surfacewere analyzed.Finally, we systematically calculated the adsorption and oxidation reaction of formaldehyde on Au dopped CeO2catalyst. Three different adsorbed sites were studied when formaldehyde molecule on the surface of the AuxCe1-xO2. Then the transition states of the CH2O dehydrogenation were calcualted using CINEB method. The possible reaction mechanism of CH2O on AuxCe1-xO2was proposed. The results herein proved that Au doped CeO2has high activity on formaldehyde oxidation.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Chemical kinetics,catalysis > Catalytic > Catalyst
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