Kinetic and Morphological Studies of Pd Oxidation in O2-CH4 mixtures

Han, Jinyi

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Kinetic and Morphological Studies of Pd Oxidation in O2-CH4 mixtures

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The oxidation of Pd single crystals: Pd(111), Pd(100) and Pd(110) was studied using Temperature Programmed Desorption (TPD), X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), Low Electron Energy Diffraction (LEED) and Scanning Tunneling Microscopy (STM) as they were subjected to O2 in the pressure range between 1 and 150 Torr at temperatures 600-900 K. The oxygen species formed during oxidation, the oxygen uptake dependence on the sample history, the Pd single crystal surface morphology transformations, and the catalytic methane combustion over Pd single crystals were investigated in detail. The Pd single crystal oxidation proceeded through a three-step mechanism. Namely, (1) oxygen dissociatively adsorbed on Pd surface, forming chemisorbed oxygen and then surface oxide; (2) atomic oxygen diffused through a thin surface oxide layer into Pd metal, forming near surface and bulk oxygen; (3) bulk PdO formed when a critical oxygen concentration was reached in the near surface region. The diffusion of oxygen through thin surface oxide layer into Pd metal decreased in the order: Pd(110)>Pd(100)>Pd(111). The oxygen diffusion coefficient was estimated to be around 10-16 cm2 s-1 at 600 K, with an activation energy of 80 kJ mol-1. Once bulk PdO was formed, the diffusion of oxygen through the bulk oxide layer was the rate-determining step for the palladium oxidation. The diffusion coefficient was equal to 10-18 cm2 s-1 at 600 K and the activation energy was approximately 120 kJ mol-1. The oxygen diffusion through thin surface oxide layer and bulk PdO followed the Mott-Cabrera parabolic diffusion law. The oxygen uptake on Pd single crystals depended on the sample history. The uptake amount increased with the population of the bulk oxygen species, which was achieved by high oxygen exposure at elevated temperatures, for example in 1 Torr O2 at above 820 K. Ar+ sputtering or annealing in vacuum at 1300 K depleted the bulk oxygen. The Pd single crystal surface morphology was determined by the oxidation conditions: O2 pressure, treatment temperature and exposure time. When bulk PdO was formed, the single crystal surface was covered with semi-spherical agglomerates 2-4 nm in size, which tended to aggregate to form a ""cauliflower-like"" superstructure. The single crystal surface area during oxidation, determined by integrating the STM image, experienced three major expansions in consistent with a three-step oxidation mechanism. The surface area on the oxidized single crystals increased in the order: Pd(110)<Pd(100)<Pd(111). Only amorphous PdO was formed during the catalytic CH4 combustion in excess O2 over Pd single crystals. The benchmark turnover rate was determined to be in the range of 0.72-0.9 s-1 on the (111), (100) and (110) surfaces at 160 Torr O2, 16 Torr CH4, 1 Torr H2O and 600 K. The results suggested that CH4 combustion was structure insensitive. The activation period observed for CH4 combustion in which the initial turnover rate was lower than the steady state rate was attributed partially to the slow oxidation of Pd single crystals and partially to the surface area increase during the reaction. Carbon dissolution was observed only during CH4 combustion in excess CH4 but in excess O2.

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