Oxygen Desorption from Polycrystalline Palladium:Thermal Desorption of O_2 from a Chemisorbed Layer of O_(ads) in the Course of the Decomposition of PdO Surface Oxide and in the Release of Oxygen from the Bulk of Palladium

摘要

The desorption of oxygen from polycrystalline palladium (Pd(poly)) was studied using temperatureprogrammed desorption (TPD) at 500-1300 K and the amounts of oxygen absorbed by palladium (n) from 0.05 to 50 monolayers. It was found that the desorption of O_2 from Pd(poly), which occurred from a chemisorbed oxygen layer (O_(ads)), in the release of oxygen from a near-surface metal layer in the course of the decomposition of PdO surface oxide, and in the release of oxygen from the bulk of palladium (Oabs), was governed by repulsive interactions between O_(ads) atoms and the formation and decomposition of O_(ads)-Pd~*-O_(abs) structures (Pd* is a surface palladium atom). At 0 ≤0.5, the repulsive interactions between O_(ads) atoms (ε_(aa)= 10 kJ/mol) resulted in the desorption of O_2 from Pd(poly) at 650-950 K. At 0.5≤ n ≤1.0, the release of inserted oxygen from a near-surface palladium layer occurred during TPD in the course of the migration of (O_(abs) atoms to the surface and the formation-decomposition of O_(ads)-Pd~*-O_(abs) satbrsu -ctures. As a result, the desorption of 02 occurred in accordance with a first-order reaction with a thermal desorption (TD) peak at T_(max)～700 K. At 1.0≤ n≤ 2.0, the decomposition of PdO surface oxide ～occurred at a constant surface coverage with oxygen during TPD in the course of the formation-decomposition of O_(abs)-Pd*-O_(abs) structures. Because of this, the desorption of O_2 occurred in accordance with a zero-order reaction at low temperatures with a TD peak at T_(max) ～ 675 K. At 1.0≤ n≤ 50, oxygen atoms diffused from deep palladium layers in the course of TPD and arrived at the surface at high temperatures. As a result, O_2 was desorbed with a high-temperature TD peak at T> 750 K.