Oxidation state of cerium in cerium-based catalysts investigated by spectroscopic probes

Pd-Ce catalysts, coimpregnated on ..gamma..-alumina, have been prepared, calcined, and reduced according to a standard procedure. The oxidation state of cerium in these catalysts has been studied both by X-ray photoemission spectroscopy (XPS) on the 3d core level and by X-ray absorption spectroscopy (XAS) on the L/sub III/ edge. Both techniques indicate a partial reduction of ceria in these systems. The reduction is more pronounced on the surface than in the bulk. The degree of reduction is influenced by (1) the concentration of cerium, (2) the presence of the transition metal, (3) the nature of the precursor salt of the transition metal, and (4) the experimental conditions of the reduction step. Implications concerning the role of ceria in the activity of transition metal, and (4) the experimental conditions of the reduction step. Implications concerning the role of ceria in the activity of transition metal-ceria/..gamma..-alumina catalysts are discussed, including (1) the promotion of the catalytic activity, (2) the increase of the thermal stability of the support, and (3) the increase and stabilization of the dispersion of the transition metal.