On the surface sites of MoP/SiO₂ catalyst under sulfiding conditions: IR spectroscopy and catalytic reactivity studies

The surface sites of MoP/SiO₂ catalysts and their evolution under sulfiding conditions were characterized by IR spectroscopy using CO as the probe molecule. The HDS activities of thiophene were measured on the MoP/SiO₂ catalyst that was subjected to different sulfidation and reactivation pretreatments. Cus Mo^δ+ (0<δ≤2) sites are probed on the surface of fresh MoP/SiO₂ by molecularly adsorbed CO, exhibiting a characteristic IR band at 2045 cm^-1. The surface of MoP/SiO₂ is gradually sulfided in HDS reactions, as revealed by the shift of the IR band at 2045 to ca. 2100 cm^-1. Although the surface of a MoP/SiO₂ catalyst becomes partially sulfided, the HDS activity tests show that MoP/SiO₂ is fairly stable in the initial stage of the HDS reaction, providing further evidence that molybdenum phosphide is a promising catalytic material for industrial HDS reactions. Two kinds of surface sulfur species are formed on the sulfided catalyst: reversibly and irreversibly bonded sulfur species. The MoP/SiO₂ catalyst remains stable in the HDS of thiophene because most sulfur species formed under HDS conditions are reversibly bonded on the catalyst surface. A detrimental effect of presulfidation on the HDS activity is observed for the MoP/SiO₂ catalyst treated by H₂S/H₂ at temperatures higher than 623 K, which is ascribed to the formation of a large amount of the irreversibly bonded sulfur species. The irreversibly sulfided catalyst can be completely regenerated by an oxidation and a subsequent reduction under mild conditions.