Adsorption of oxygen and surface oxide formation on Pd(111) and Pd foil studied with ellipsometry, LEED, AES and XPS

Abstract

The interaction of oxygen with Pd(111) and polycrystalline palladium foil has been studied with ellipsometry, LEED, AES and XPS in the temperature range of 300 to 770 K and pressures up to 1 Pa. Ellipsometry was used to monitor the adsorption of oxygen and gave indication for the formation of a surface oxide at higher temperatures ( T ≥ 470 K) and pressures ( p ≥ 10 −4Pa). The presence of a surface oxide is supported by a complex LEED pattern, ascribed to a square lattice with a = 7.5 ± 0.5Å and domains in six orientations. It was not possible to match this structure with a simple overlayer structure on the (111) plane or with an unreconstructed crystal plane of PdO. XPS measurements on palladium foil, after the same treatment, showed the presence of ≈0.5 ML PdO on the surface. Bulk oxide was not formed. The amount of oxygen on the surface could not be determined with AES because during AES the electron beam easily removed adsorbed oxygen, especially on Pd(111). On palladium foil the oxygen is removed less effectively by the electron beam, which indicates that oxygen is bound more tightly to defects. Bulk palladium oxide, produced by heating the palladium foil in air, was not affected by the electron beam, even at high current densities. The ellipsometric parameters δΔ and δΨ never exceeded 0.40° and 0.08° respectively, both on Pd(111) and palladium foil. This indicates that the diffusion of oxygen is limited to surface layer(s) under the conditions studied. Diffusion to the bulk did not occur.