Hydrogen evolution from water reactions with molybdenum sulfide cluster anions

A study of reactions between a distribution of (MoxSy-)-Mo-98 clusters ranging from sub-sulfide (x similar to y) to hypersulfide (y > 2x) is presented, compared with previous studies on homologous MoxOz- clusters, and analyzed with supporting computational studies of the reaction free energy pathways, giving insights into the molecular scale interactions involved in the electro- or photocatalytically driven hydrogen evolution reaction. First, reactions between MoxSy- clusters with water are approximately an order of magnitude slower than reactions between oxides with water, an effect attributed to longer-range repulsion between the cluster anions and water. However, sequential oxidation of the MoxSy- clusters, which generates oxosulfides and H-2, is observed for several species. Evidence of oxidation of several hypersulfides with H-2 evolution is also observed, indicating that oxidation of S-2 groups is also possible. Based on the computational results on several of the bimetallic species, oxidation of the subsulfide Mo2S2- and MO2S3- clusters is barrierless, while oxidation of the nascent Mo2S2O- cluster formed by oxidation of Mo2S2- is low-barrier. The stoichiometric Mo2S4- +H2O free energy path is predicted to have a modest barrier to the dissociative addition of water, which is consistent with the experimental observation of both oxidation (with H-2 production) and water addition reactions. Calculations on the reaction free energy pathways of the oxosulfide species suggest that the formation of two dihydroxide groups, rather than a thiol and hydroxide group, is favored. Oxidation of the oxides is more thermodynamically favored than oxidation of sulfide analogs. (C) 2018 Elsevier B.V. All rights reserved.