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A Density Functional Theory Study of Ceria-based Catalysts

Author: ZhaoYun
Tutor: ZuoBoTao
School: Zhejiang Normal University
Course: Physical and chemical
Keywords: Ceria Density functional theory metal and metal cluster Superoxide peroxide catalytic effects
CLC: O647.31
Type: Master's thesis
Year: 2012
Downloads: 66
Quote: 0
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Abstract


Due to its high oxygen storage and release capacities, ceria-based catalysts have been widely applied into heterogenous catalysis reactions, including the automotive three way conversion, the elimination of volatile organic compounds, and so on. A systematic investigation of the properties of ceria support, active component and their interactions is very important to improve the understanding of the adsorption and reaction behaviors of reactants on ceria-based catalysts. It can also provide valuable information for catalyst design and development.The adsorption behaviors and electronic properties of Sn on the CeO2(111) surface were systematically investigated using density functional theory (DFT) method. Our results suggested that Sn on the hollow site is more stable than that on the top oxygen site at the coverage of0.25ML, while Sn on the top oxygen site is the most stable configurations when the coverage of Sn is equal or higher than0.5ML. Charge density difference (CDD) analysis indicates that electrons transfer from the Sn adatom to the substrate, leading to the reduction of Ce4+to Ce3+ion, which is in agreement with the experimental spectroscopy. The reduction degree of the substrate increases with the Sn coverage, which is well supported by the CDD and spin resolved density of states (DOS) of the most stable xSn/CeO2(111) configurations. The adsorption of Sn can partially activate the surface oxygen of ceria. The tentatively study of a probe molecule CO adsorption on Sn/CeO2(111) surface indicates that CO adsorption is enhanced due to the strong tin-ceria interactions.Based on O2adsorption behaviors on two partially reduced CeO2surface with different sizes, the electronic formation mechanisms of superoxide and peroxide species were systematically investigated by density functional theory method. When CeO2surface is partially reduced, surface oxygen vacancy forms and leads to the reduction from Ce4+to Ce3+ions. If O2adsorbs at the top site of Ce3+ion nearby surface oxygen vacancy, it slips into the vacancy, and a diamagnetic peroxide species forms accompanying with two4f electrons of Ce3+ions feedbacked to the π2p*orbital of O2. Three magnetic superoxide species were obtained only if O2adsorbs at the top site of Ce3+ion apart form the surface oxygen vacancy. Meanwhile, only one Ce3+ion is oxidized to Ce4+due to one4f electron transferred to the π2p*orbital of O2. The migration barrier from O2-(η2-1) to O22-is0.35eV, indicating that superoxide might easily transform into peroxide with the increase of temperature. CO is directly oxidized to CO2by the superoxide without energy barrier; while a carbonate forms when CO reacts with peroxide. A high desorption barrier of the carbonate to form a gas CO2molecule indicates that peroxide species might play the dominant role at relatively high temperature.The adsorption behaviors of typical metals (Pt, Au, Pd, Ag, Ir and Sn) and their polyatom 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, Pt and Ir on the O-bridge site. Ag and Sn adsorbed on the3-fold hollow site are the most stable configurations. When the metals adsorb at the top site, Pt has the strongest interactions with CeO2(111) surface. Au and Pd atoms might easily aggregate to form cluster on the CeO2(111) surface; while Sn is opt to lying on the surface, which is in agreement with experiment data.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > The physical chemistry of surface phenomena > Adsorption > Adsorption theory
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