Reversed Hysteresis during CO Oxidation over Pd75Ag25(100)

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• 作者：Vasco R. Fernandes ; Maxime Van den Bossche ; Jan Knudsen ; Mari H. Farstad ; Johan Gustafson ; Hilde J. Venvik ; Henrik Grönbeck ; Anne Borg
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：July 1, 2016
• 年：2016
• 卷：6
• 期：7
• 页码：4154-4161
• 全文大小：485K
• 年卷期：0
• ISSN：2155-5435

文摘

CO oxidation over Pd(100) and Pd₇₅Ag₂₅(100) has been investigated by a combination of near-ambient-pressure X-ray photoelectron spectroscopy, quadrupole mass spectrometry, density functional theory calculations, and microkinetic modeling. For both surfaces, hysteresis is observed in the CO₂ formation during the heating and cooling cycles. Whereas normal hysteresis with light-off temperature higher than extinction temperature is present for Pd(100), reversed hysteresis is observed for Pd₇₅Ag₂₅(100). The reversed hysteresis can be explained by dynamic changes in the surface composition. At the beginning of the heating ramp, the surface is rich in palladium, which gives a CO coverage that poisons the surface until the desorption rate becomes sufficiently high. The thermodynamic preference for an Ag-rich surface in the absence of adsorbates promotes diffusion of Ag from the bulk to the surface as CO desorbs. During the cooling ramp, an appreciable surface coverage is reached at temperatures too low for efficient diffusion of Ag back into the bulk. The high concentration of Ag in the surface leads to a high extinction temperature and, consequently, the reversed hysteresis.