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Theoretical insight into the rearrangement of sulfur atoms on the Ni- and Cu-doped MoS2 S-edge induced by hydrogen adsorption under HDS reaction conditions

Abstract:

Density functional theory (DFT) calculations and an atomistic thermodynamic approach were used to study the geometric rearrangement of sulfur atoms on the Ni- and Cu-doped MoS<inf>2</inf> S-edge upon hydrogen adsorption. Under HDS conditions, thermodynamically stable hydrogenated structures were identified as SH groups on the undoped S-edge with 100% sulfur coverage, on the Ni-doped S-edge with 50% sulfur coverage and on the Cu-doped S-edge with 25% sulfur coverage. It was found that the rearrangement of the S atoms is essential to reach the most stable state at the edge for the undoped and Ni-doped S-edge. Hydrogen adsorption on the Ni-doped S-edge leads to the greatest amount of S rearrangement (ΔE<inf>Rearrang</inf> = 0.93 eV/H<inf>2</inf>). Our results suggest that under the reaction conditions, the H<inf>2</inf> dissociative adsorption process is strongly coupled to the rearrangement of the sulfur atoms. By examining the differential hydrogen adsorption energy on the most stable edge structures, we found a plausible explanation for the trend in the hydrogenation activity of the doped edges. Our results suggest that Ni enhances the hydrogenation activity of the S-edge by decreasing the S-H bond strength, while Cu poisons it by increasing the S-H bond strength.