Hydrodesulfurization catalysis by Chevrel phase compounds

Reduced ternary molybdenum sulfides, known as Chevrel phase compounds, were investigated to determine their catalytic activity at 400 °C for thiophene hydrodesulfurization and for 1-butene hydrogenation. Chevrel phase compounds of the general composition M_xMo₆S₈, with M being Ho, Pb, Sn, Ag, In, Cu, Fe, Ni, or Co, were found to have hydrodesulfurization activities comparable to model unpromoted and cobalt-promoted MoS₂ catalysts. The most active catalysts were the "large" cation compounds (Ho, Pb, Sn), and the least active catalysts were the "small" cation compounds (Cu, Fe, Ni, Co). Except for the nickel phase Ni_1.6Mo₆S₈, all Chevrel phase catalysts had hydrogenation activities which were much lower than the model MoS₂ catalysts. Fresh and used catalysts were characterized by X-ray diffraction, laser Raman spectroscopy, and X-ray photoelectron spectroscopy. While the bulk structures were stable under hydrodesulfurization reaction conditions, differences were observed in the stability of the surface molybdenum oxidation state for the specific classes of Chevrel phase catalysts: the oxidation state of the surface molybdenum atoms of the large cation compounds was found to be unchanged after 10 h of thiophene reaction, but some oxidation was apparent for the small cation compounds. The differences in stability can be related to the mobilities of the ternary metal atoms within the Chevrel phase structures.