On the formation of cobalt-molybdenum sulfides in silica-supported hydrotreating model catalysts

Model catalysts, consisting of a conducting substrate with a thin SiO2 layer on top of which the active catalytic phase is deposited by spincoating impregnation, were applied to study the formation of the active CoMoS phase in HDS catalysts. The catalysts thus prepared showed representative activity in the hydrodesulfurization of thiophene, confirming that these models of HDS catalysts are realistic. Combination of the sulfidation behaviour of Co and Mo studied by XPS and activity measurements shows that the key in the formation of the CoMoS phase is the retardation of the sulfidation of Co. Complexing Co to nitrilotriacetic acid complexes retarded the Co sulfidation, resulting in the most active catalyst. Due to the retardation of Co in these catalysts, the sulfidation of Mo precedes that of Co, thereby creating the ideal conditions for CoMoS formation. In the CoMo catalyst without NTA the sulfidation of Co is also retarded due to a Co-Mo interaction. However, the sulfidation of Mo still lags behind that of Co, resulting in less active phase and a lower activity in thiophene HDS.