The role of Co in sulfidized Co-Mo hydrodesulfurisation catalysts supported on carbon and alumina

The thiophene hydrodesulphurisation activities of cobalt sulphide catalysts supported on activated carbon have been measured in a flow microreactor operating at atmospheric pressure. The cobalt content was varied between 1.3 and 13.3 wt% Co, and the oxidic precursor catalysts were dried by three different procedures. Structural characteristics and the degree of Co dispersion in the oxidic precursor and in the sulphidised state of the catalysts were obtained by X-ray photoelectron spectroscopy. The oxidic cobalt phase present in the precursor catalysts was found to be inhomogeneously dispersed over the carbon carrier surface. Dispersion decreased significantly during catalyst sulphidation. XPS results showed that the cobalt was fully sulphidised, although sulphur was found to be present in excess of the amount corresponding to stoichiometric Co9S8. The catalysts demonstrated very high hydrodesulphurisation activities, which were by far superior to those of corresponding carbon- or aluminasupported molybdenum catalysts. By extrapolation of the inverse activities to zero loading the activities of optimally dispersed cobalt and molybdenum sulphide were determined. Comparison of the activity of carbon-supported Co-Mo with the activities of the optimally dispersed catalyst supported on carbon demonstrated that the so-called promoter effect in sulphidised Co-Mo catalysts can be explained completely by the exceptionally high catalytic activity of cobalt sites and showed that the role of MoS2 in these catalysts is mainly to function as a support for optimally dispersed cobalt ions.