X-ray photoelectron spectroscopy study of cobalt-molybdenum binary oxide catalysts

Abstract

The surface compositions of cobalt-molybdenum binary oxide catalysts were studied with X-ray photoelectron spectroscopic techniques for the calcined catalysts and for the catalysts exposed to various reactive gases relating to the hydrodesulfurization of thiophene. In the cases of the cobalt-molybdenum catalysts prepared by calcining the mixtures of cobalt nitrate and ammonium paramolybdate, a surface segregation of molybdenum was observed over a wide range of the bulk composition of the catalyst, while a cobalt enrichment was observed in the catalyst with a small cobalt content (< 6 at.%). However, the catalysts prepared by the calcination of the mixed composite oxides, CoO and MoO 3, showed no enrichment of cobalt or molybdenum in the catalyst surface, except for the catalyst containing a small amount of CoO. It was found that the surface composition of the catalyst was changed considerably by treating at 400 °C with 10 Torr of various reactive gases. Hydrogen reduction caused a surface enrichment of molybdenum, accompanied by the reduction of both oxides to MoO 2 and Co metal. Thiophene/H 2 also resulted in a surface segregation of molybdenum and the sulfidation of MoO 2 to MoS 2. H 2S H 2 treatments, on the other hand, produced a drastic segregation of cobalt in the catalyst surface and sulfided the catalyst completely to form MoS 2 and CoS and/or more likely Co 9S 8, together with excess sulfur. A similar behavior was observed for a zinc-molybdenum binary oxide catalyst. It is concluded that the surface compositions of the cobalt-molybdenum catalysts depend strongly on the preparation procedures and on the kinds of gases in contact with the catalysts.