Mechanistic Study of Methanol Synthesis from CO2 and H2 on a Modified Model Mo6S8 Cluster

We report the methanol synthesis from CO2 and H-2 on metal (M = K, Ti, Co, Rh, Ni, and Cu)-modified model Mo6S8 catalyst using density functional theory (DFT). The results show that the catalytic behavior of a Mo6S8 cluster is changed significantly due to the modifiers, via the electron transfer from M to Mo6S8 and therefore the reduction of the Mo cation (ligand effect) and the direct participation of M in the reaction (ensemble effect) to promote some elementary steps. With the most positively charged modifier, the ligand effect in the case of K-Mo6S8 is the most obvious among the systems studied; however, it cannot compete with the ensemble effect, which plays a dominate role in determining activity via the electrostatic attraction in particular to stabilize the CHxOy species adsorbed at the Mo sites of Mo6S8. In comparison, the ligand effect is weaker and the ensemble effect is more important when the other modifiers are used. In addition, the modifiers also vary the optimal reaction pathway for methanol synthesis on Mo6S8, ranging from the reverse water-gas shift (RWGS) + CO hydrogenation as that of Mo6S8 to the formate pathway. Finally, K is able to accelerate the methanol synthesis on Mo6S8 the most, whereas the promotion by Rh is relatively small. Using the modifiers like Ti, Co, Ni, and Cu, the activity of Mo6S8 is decreased instead. The relative stability between *HCOO and *HOCO is identified as a descriptor to capture the variation in mechanism and scales well with the estimated activity. Our study not only provides better understanding of the reaction mechanism and actives on the modified Mo6S8 but also predicts some possible candidates, which can be used as a promoter to facilitate the CH3OH synthesis on Mo sulfides.