Structural Properties of Gas Phase Molybdenum Sulfide Clusters [Mo3S13]2–, [HMo3S13]−, and [H3Mo3S13]+ as Model Systems of a Promising Hydrogen Evolution Catalyst

Amorphous molybdenum sulfide (MoS x ) is a potent catalyst for the hydrogen evolution reaction (HER). Since mechanistic investigations on amorphous solids are particularly difficult, we use a bottom-up approach and study the [Mo 3 S 13 ] 2- nanocluster and its protonated forms. The mass selected pure [Mo 3 S 13 ] 2- as well as singly and triply protonated [HMo 3 S 13 ] - and [H 3 Mo 3 S 13 ] + ions, respectively, were investigated by a combination of collision induced dissociation (CID) experiments and quantum chemical calculations. A rich variety of H x S y elimination channels was observed, giving insight into the structural flexibility of the clusters. In particular, it was calculated that the observed clusters tend to keep the Mo 3 ring structure found in the bulk and that protons adsorb primarily on terminal disulfide units of the cluster. Mo-H bonds are formed only for quasi-linear species with Mo centers featuring empty coordination sites. Protonation leads to increased cluster stability against CID. The rich variety of CID dissociation products for the triply protonated [H 3 Mo 3 S 13 ] + ion, however, suggests that it has a large degree of structural flexibility, with roaming H/SH moieties, which could be a key feature of MoS x to facilitate HER catalysis via a Volmer-Heyrovsky mechanism.