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 2 S 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 9 S 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.
Published Version
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