Formation and adsorption properties of the bridging sulfur vacancies at the (1¯ 0 1 0) edge of Mo27S(54−x): A theoretical study

The structure and adsorption of Mo27S(54−x) (x = 1–6) clusters have been investigated using density functional theory method. It was found that considerable relaxation occurred at the (1¯ 0 1 0) edge. The activity has been analyzed on the basis of frontier molecular orbital properties. The results suggest that the possible catalytic sites might be situated between two vicinal 4-fold coordinative unsaturated site CUS. The formation of Mo27S(54−x) (x = 4–6) from Mo27S54 is easy in the presence of atomic hydrogen, but difficult under molecular hydrogen. But the situation is different from the case of Mo27S(54−x) formation (x = 1–3) that is facile under both the atomic and molecular hydrogen reagent (under the normal HYD/HDS condition). The adsorption of thiophene at various vacancies on the (1¯ 0 1 0) edge of MoS2 represents that thiophene is unstable to flatly adsorb at the vacancy of Mo27S50. Although uprightly adsorbed thiophene is adsorbed weakly, the flatly adsorbed thiohene is strongly activated when adsorbed between two vicinal 4-fold CUS (9, 10). On the basis of Hirshfeld charge analysis, the donation and back-donation between the thiophene and the substrate might account for the activation of thiophene.

The successive removal of surface sulfur atoms with the formation of coordinatively unsaturated sites (CUS) was studied in details, which have been considered as the active sites for hydrodesulfurization of sulfur-containing compounds. On the basis of the computed enthalpies of vacancy formation for Mo27S(54−x) deduced from Mo27S54, it is found that both H2 and atomic hydrogen are active for Mo27S(54−x) (x = 1–3), while only atomic hydrogen is active for Mo27S(54−x) (x = 4–6). However, CUS of Mo27S(54−x) (x = 4–6) can activate thiophene more strongly than that of Mo27S(54−x) (x = 1–3). Figure optionsDownload as PowerPoint slide