Mo₆S₈-based single-metal-atom catalysts for direct methane to methanol conversion

The single atom catalysts have been attracting much attention for catalysis. In this work, the significant influence of single-metal-atom (M = K, Ti, Fe, Co, Ni, Cu, Rh) doping on a Mo₆S₈ cluster was revealed for the direct methane to methanol conversion in water stream using density functional theory calculations. It was found that all single atom dopants help to facilitate the conversion via the steam reforming of methane (SRM). The single Fe atom on Mo₆S₈ (Fe-Mo₆S₈) exhibits the most significant promoting effect, which is followed by Ni, Co, Rh-Mo₆S₈ > K, Ti, Cu-Mo₆S₈ > Mo₆S₈ in a decreasing sequence. The enhanced activity by single atom doping on Mo₆S₈ is mainly associated with the interplay between the ensemble effect via the direct participation of an active M dopant and the site confinement imposed by doping of a single M atom, in tuning the methane conversion and methanol selectivity. It generates the new active center, M, which confines the SRM to occur at the M-Mo bridge sites and facilitates the selective production of methanol. A good single-atom promoter should not only bind ^*OH or ^*O moderately, being strongly enough to help water dissociation and weakly enough to allow the oxidation of methane, but also impose the confinement effect to facilitate the C-O bond association and production of methanol. Our results highlight the importance of the interplay among ligand, ensemble, and confinement effects in promoting the complex SRM over single atom catalysts.