Promoting effect of fluorine on cobalt—molybdenum/ titania hydrodesulfurization catalysts

Abstract

The effect of fluoride incorporation on the titania support on the surface structure of promoted cobalt-molybdenum catalysts and their catalytic activity for thiophene hydrodesulfurization (HDS) has been studied. The relative activity between fluoride-modified and unmodified catalysts was found to be maximum and close to 5 for a fluorine content of 0.8 wt.-%. From high-resolution electron microscopy (HREM) examination of sulphided samples, the increase in HDS could be partly explained by smaller molybdenum sulphide crystallites (increase in the dispersion of the active phase). Diffuse reflectance spectroscopy (DRS) of the oxidic precursors shows that the effect of fluoride incorporation is also to increase the amount of the Mo VI in tetrahedral surroundings and to enhance the part of cobalt involved in the generation of the “CoMoS” active phase (probably cobalt in octahedral symmetry).