Effect of the support on the structure of Mo-based hydrodesulfurization catalysts : activated carbon versus alumina

The structure of oxidic and sulfided Mo catalysts supported on activated carbon was studied by means of X-ray photoelectron spectroscopy (XPS), temperature programmed sulfiding (TPS), and sulfur analysis measurements. In the oxidic state the Mo phase was highly dispersed as isolated or polymerized monolayer species at Mo loadings below 3 wt% and as very tiny three-dimensional particles at higher loadings. Upon sulfiding particle growth took place, although the size of the sulfide particles remained below 4.6 nm even in the sample with the highest Mo loading (14.1 wt%). TPS patterns showed that sulfiding proceeded via a mechanism of O---S substitution reactions and was completed at temperatures below 560 K. In the sulfided catalysts only Mo(IV) was detected by XPS and S/Mo stoichiometries determined by XPS, TPS, and chemical sulfur analysis varied between 1.5 and 2.0, demonstrating that MoS2 was the major phase present after sulfidation. The higher catalytic activity for Mo/C compared to Mo/Al2O3 is explained by differences in the structure of the sulfide phases present and in the interaction between these phases and the respective supports.