Surface structure of [formula omitted] catalysts studied by X-ray photoelectron spectroscopy

Abstract

X-Ray photoelectron spectroscopic studies of oxidic and sulfided CoOMoO 3 Al 2O 3 catalysts revealed the chemical species, the surface structure of the catalysts, and the promoting effect of Co. It was found from the Mo(3d) Al(2s) and Co(2p) Al(2s) intensity ratios that the surface structure of the oxidic catalysts was highly sensitive to the preparation method. Bilayer structures are proposed for the catalysts prepared by sequential impregnations, while a separate phase structure is suggested to be plausible for the catalysts prepared by a simultaneous impregnation. On sulfidation the surface structure of the CoOMoO 3 Al 2O 3 catalysts was not essentially altered under atmospheric pressure, compared to that of the oxidic precursor catalysts, although Co and Mo were sulfided. The oxidic catalysts mainly consist of Co 3O 4 and pseudo-CoAl 2O 4, the fraction of Co 3O 4 increasing with Co content and depending on the preparation method. On the basis of the observation that both the extent of sintering and the sulfidation degree of Mo are depressed by Co, it is suggested that the stabilization effect of Co (most likely pseudo-CoAl 2O 4) for Mo monolayers is operative during hydrodesulfurizations, thus holding the Mo effective for the reactions.