STRUCTURE AND CATALYTIC ACTIVITY OF ALUMINA-SUPPORTED PT-CO BIMETALLIC CATALYSTS .1. CHARACTERIZATION BY X-RAY PHOTOELECTRON-SPECTROSCOPY

A series of Pt1-xCo(x)/Al2O3 bimetallic catalysts has been studied by X-ray photoelectron spectroscopy following in situ hydrogen reduction of the samples previously calcined in oxygen at 570 K. Platinum was found to be in the zerovalent state for the entire composition range and its dispersion increased with decreasing platinum loading. When combined with platinum, cobalt stayed partly in a hardly reducible cobalt oxide surface phase (denoted by CSP) and partly in the metallic state, whereas without platinum CoO was the predominant form. At low cobalt concentration the variation of the XPS intensity changes of the cobalt 2p3/2 line in the different states was interpreted by assuming that the cobalt surface phase was being covered by platinum. On the other hand, at high cobalt contents saturation of the alumina surface by the cobalt surface phase was concluded from the occurrence of a constant, high amount of cobalt in the surface phase. On the basis of the comparison of surface platinum and cobalt contents first platinum-like properties and then cobalt-like is predicted for these catalysts with increasing cobalt content.