Activation of unsupported Co$z.sbnd;Mo catalysts in thiophene hydrodesulfurization

A series of unsupported oxidic CoMo catalysts with different mole fractions r = Co(Co + Mo) was prepared by coprecipitation of solutions of (NH4)6Mo7O24 and Co(NO3)2. The calcined catalysts contain a-CoMoO4 (r = 0.5) mixed with MoO3 (r < 0.5) or with Co3O4 (r > 0.5) according to X-ray diffraction (XRD) and electron diffraction. The a-CoMoO4 transforms partially into b-CoMoO4 upon grinding. The higher the cobalt (r) and b-CoMoO4 contents, the higher are the surface area increase and the degree of reduction of calcined catalysts during hydrogen treatment at 673 K. Electron microscopy (EM) data agree well with the surface area increase observed after reduction. X-ray photoelectron spectroscopy (XPS) shows the reduction of molybdenum rather than that of cobalt. Reduced crystalline phases cannot be identified by these techniques. Sulfidation with a mixture of H2/thiophene following reduction caused a drastic drop in surface area but the particle size seen by EM does not increase. Weak oxythiomolybdate XRD bands appeared after slight sulfidation, most XRD signals disappeared after massive sulfidation of samples with r = 0.5. Cobalt promotes sulfidation of molybdenum in the bulk, but the maximum sulfidation degree was about half of the stoichiometric value. XPS shows surface cobalt enrichment, XRD and EM traces of Co9S8 in samples with r = 0.38 and 0.50. A pronounced maximum was observed in initial hydrodesulfurization (HDS) activity and hydrogenation (HYD) selectivity at medium Co content. On used catalysts, this synergism disappeared. We attribute the highest HDS activity of short-living cobalt-oxythiomolybdate(s) formed initially during sulfidation. HYD was promoted by sulfided molybdenum and by less surface Cobalt.