Abstract
The differences between MoS2, CoMoS and NiMoS HDS catalysts supported on γ-alumina and high SSA titania are investigated based on the results of [35S]DBT HDS experiments. Previous studies of MoS2 and CoMoS are reviewed, discussed and compared with new results for NiMoS. Introduction of Ni or Co to MoS2/Al2O3 catalysts classically yields a significant increase in HDS performance. Irrespective of the promoter, an increase in S0, the number of labile sulfur atoms, is observed. In contrast, kRE, the H2S liberation rate constant, plotted as a function of the Ni/Mo ratio, presents a volcano profile on Ni-promoted catalysts, but kRE reaches a plateau from low Co/Mo ratios on Co-promoted catalysts. The 'TiMoS' phase, which is formed in-situ during HDS on Mo/TiO2 catalysts, promotes sulfur mobility and makes Mo/TiO2 catalysts more active than Mo/Al2O3 catalysts. Nevertheless, CoMo/TiO2 catalysts are less active than CoMo/Al2O3 catalysts because further promotion of 'TiMoS' phase with Co might yield excessive weakening of the metal-sulfur bonds, and/or some Co atoms might be 'lost' in the TiO2 matrix without interacting with MoS2. In contrast, introduction of Ni to Mo/TiO2 catalysts yields significant increases in both kRE and S0. The NiMo/TiO2 catalysts exhibit HDS performances close to those of Al2O3-supported catalysts. Clearly catalytic behavior over Co- and Ni-promoted catalysts is different.
