The Sulphided State of Hydrodesulphurisation Catalysts Characterized by XPS

Typical Co or Ni-Mo-σAl2O3 hydrodesulphurisation catalysts are (partly) sulphided in the catalytic conditions and the questions about the structure, the sulphur content of the catalysts, the chemical state of the promotors are subject to controversial results or interpretations in the literature. In this paper, we present some results about the sulphided state of HDS catalysts by using X-ray photoelectron spectroscopy (XPS), a powerful technique to analyze this kind of problem. In particular, the influence of sulphiding conditions on the formation of MoS2 cristallites is shown. By performing experiments under mild sulphiding conditions, the molybdenum monolayer observed in the precursor oxide is not greatly modified. Results on the extent of sulphidation expressed by the fraction of MoIV species in a sulphided environment (Mo) and the stoichiometry Sm/Mo are given. On the other hand, the reactivity towards sulphidation of the promotors (Ni or Co) is highly dependent on the localisation in octahedral or tetrahedral sites on the support or on a surface phase. In general, the octahedral sites are more easily sulphided.