Oxygen effects on the structure and hydrogenation activity of the MoS2 active site: A mechanism study by DFT calculation

- Analyzed the location of oxygen which directly influenced the MoS2 active sites.
- Analyzed the oxygen effect on basic physicochemical property of MoS2 active sites.
- Analyzed the oxygen effect on fine structure of MoS2 active sites under HDS reaction conditions.
- Analyzed oxygen effects on activities and selectivities of hydrogenation saturation and hydrogenolysis of MoS2 active sites.

Density functional theory (DFT) calculation with numerical atomic functions is used to study oxygen effects on MoS2 hydrotreating active sites. The optimal form of oxygen on active sites is first investigated by experimentation and calculation. Then, the fine structures of a MoS2 nanocluster (S-Mo-S) and an oxygen containing MoS2 nanocluster (O-Mo-S) under reaction conditions, as well as their hydrodesulfurization (HDS) selectivity towards thiophene, are calculated. The results show that bent MoS2 nanoclusters in supported hydrotreating catalysts are more thermodynamically stable when oxygen is substituted for sulfur on the basal plane. Oxygen also reduces the electron density and unoccupied d-orbital energy levels of Mo atoms. The S atoms on the Mo-edge are also more readily substituted by H on O-Mo-S. During thiophene HDS, O-Mo-S has a stronger thiophene adsorption ability than S-Mo-S. Hydrogen transfer has a higher energy requirement along the Mo-edge on O-Mo-S than on S-Mo-S. The hydrogenation saturation activity to CC is lower on the Mo-edge of O-Mo-S than of S-Mo-S, whereas the hydrogenolysis activity at the CS bond is promoted by oxygen. It is predicted that the oxygen can increase the hydrogenolysis selectivity of a MoS2 active nanocluster.

Oxygen effects on MoS2 hydrotreating active sites are investigated by DFT calculation with Numerical atomic functions. The oxygen can substitute for sulfur atoms on the basal 0001 plane of MoS2 nanocluster. The oxygen lowers the molecular orbital energy level of MoS2 nanocluster, resulting to the promotion of hydrogenolysis and weakening of hydrogenation activities on oxygen based MoS2 active sites.59