Comparison by ³⁵S radiotracer methods of hydrodesulfurization behavior for molybdenum, cobalt-molybdenum and nickel-molybdenum catalysts supported on γ-alumina and high specific surface area titania

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.