The oxidation state of Ge on Pd/C catalysts investigated by XPS

A commercially available 15%-Pd-on-active-carbon catalyst was modified with a Ge sub-monolayer to make it suitable for hydrogenation reactions. Two modified catalysts were prepared via reductive adsorption of Ge4+, the Ge concentrations on the Pd surface being equivalent to 0.18 and 0.71 monolayer, respectively. Information about the valence state of the Ge on the Pd/C catalysts for fresh and hydrogen-treated samples was obtained using X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (AES). The hydrogen-treated catalyst samples were handled in a high-quality glove box directly coupled to the XPS instrument, to prevent re-oxidation of the samples. The high resolution XPS results, as well as the position of the Auger line, the line shape of the Auger line and the Auger parameter, all show that the Ge present in sub-monolayer concentration on the freshly prepared Pd catalyst is present in oxidised form and that it can be reduced completely to the metallic state in a H-2 atmosphere at low (50 degrees C) temperature. (C) 1999 Elsevier Science B.V. All rights reserved.