IDENTIFICATION OF 2 TYPES OF OXIDIZED PALLADIUM ON GAMMA-ALUMINA BY X-RAY PHOTOELECTRON-SPECTROSCOPY

It was demonstrated by X-ray photoelectron spectroscopy (XPS) that oxidized palladium supported on gamma-alumina can exist in two chemically different entities, distinguishable by their observed electron binding energies and by selective reduction. Larger oxidized particles am characterized by a binding energy attributed to PdO. This oxide is reduced to metallic palladium at room temperature in hydrogen, as would be expected for bulk PdO. On the other hand, at low palladium loadings of 0.5 wt.-% and less, the majority of the palladium is present at a binding energy shifted approximately 1.6 eV higher than that observed for PdO. The shift appears to be associated with a metal-support interaction at concentrations where the palladium is highly dispersed. Contrary to that observed at loadings greater than 0.5 wt.-%, this palladium remains mostly oxidized when exposed to hydrogen at room temperature. The results are compared with literature data reported for platinum/gamma-alumina.