Surface structure of CoO$z.sbnd;MoO3/Al2O3 catalysts studied by X-ray photoelectron spectroscopy

X-Ray photoelectron spectroscopic studies of oxidic and sulfided CoOMoO3Al2O3 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MoO3Al2O3 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 Co3O4 and pseudo-CoAl2O4, the fraction of Co3O4 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-CoAl2O4) for Mo monolayers is operative during hydrodesulfurizations, thus holding the Mo effective for the reactions.