Oxygen dissolution in polycrystalline palladium at O₂ pressures of 0.1 to 100 Pa

Abstract

Oxygen dissolution in polycrystalline palladium Pd(poly) at O₂ pressures ($P_{O_2 }$) of 0.1 to 100 Pa and a temperature of 600 K has been investigated by temperature-programmed desorption. The dissolution process under these conditions includes O₂ chemisorption on the oxide film surface, the insertion of Oads atoms under the oxide layer, and their diffusion into the subsurface layers of palladium. During chemisorption, a structure ensuring that the O_ads coverage of the surface increases with increasing $P_{O_2 }$ forms on the surface of the oxide film. This is favorable for O_ads penetration through the oxide film and increases the amount of absorbed oxygen. The O_ads coverage of the surface calculated via the Langmuir equation at an O₂ desorption activation energy of E _des = 125 kJ/mol correlates with the number of absorbed oxygen monolayers (n). At n ≥ 1, oxygen absorption by Pd(poly) is due to the diffusion of O atoms in the palladium lattice. After the accumulation of 14–18 oxygen monolayers in the subsurface layers of palladium, oxygen absorption practically stops depending on $P_{O_2 }$. Thus, the acceleration of oxygen dissolution in palladium is due to the formation of the surface oxide film and the increase in the O_ads coverage of this film, which facilitates the insertion of O_ads atoms into the subsurface layers of palladium.