DFT calculations of unpromoted and promoted MoS2-based hydrodesulfurization catalysts

Self-consistent density functional theory (DFT) is used to study the struct ure and active sites in unpromoted and promoted MoS2-based hydrodesulfuriza tion (HDS) catalysts. A model consisting of single-layer MoS2 chains with a nd without promoter atoms located at the edges is used to represent the str uctures in the catalysts, and full relaxation is allowed to find the lowest energy configurations. The results show that the most favored edge structu res deviate significantly from those considered in the literature and those expected from simple terminations of the bulk MoS2 structures. The calcula tions also show that the promoter atoms prefer to be located at the so-call ed sulfur-terminated ((1) over bar 010) MoS2 edges. Although such structure s have not been considered previously it is found that they are in agreemen t with available structural information from Extended X-Ray Absorption Fine Structure (EXAFS) experiments. Since the creation of sulfur vacancies is b elieved to be the first step for many hydrotreating reactions, the energy r equired to remove sulfur from the different structures has also been calcul ated. Comparison with catalytic activity results for MoS2, Co-Mo-S, Ni-Mo-S , and Fe-Mo-S structures shows that the highest HDS activity is obtained fo r the structures with the lowest metal sulfur binding energy, in general ag reement with the bond energy model (BEM). A more detailed analysis of the s ulfur bonding in promoted MoS2 structures based on a simple LCAO-type model explains the origin of the different promotional behaviors. Finally, the a dsorption of hydrogen on the different structures is discussed. We find hyd rogen adsorption at edge sulfur atoms to be strong, and suggest that the S- edge is partly covered by SH groups during catalysis, (C) 1999 Academic Pre ss.