Bond Activation and Hydrogen Evolution from Water through Reactions with M₃S₄ (M = Mo, W) and W₃S₃ Anionic Clusters

Transition metal sulfides (TMS) are being investigated with increased frequency because of their ability to efficiently catalyze the hydrogen evolution reaction. We have studied the trimetallic TMS cluster ions, Mo₃S₄^-, W₃S₄^-, and W₃S₃^-, and probed their efficiency for bond activation and hydrogen evolution from water. These clusters have geometries that are related to the edge sites on bulk MoS₂ surfaces that are known to play a role in hydrogen evolution. Using density functional theory, the electronic structures of these clusters and their chemical reactivity with water have been investigated. The reaction mechanism involves the initial formation of hydroxyl and thiol groups, hydrogen migration to form an intermediate with a metal hydride bond, and finally, combination of a hydride and a proton to eliminate H₂. Using this mechanism, free energy profiles of the reactions of the three metal clusters with water have been constructed. Unlike previous reactivity studies of other related cluster systems, there is no overall energy barrier in the reactions involving the M₃S₄ systems. The energy required for the rate-determining step of the reaction (the initial addition of the cluster by water) is lower than the separated reactants (-0.8 kcal/mol for Mo and -5.1 kcal/mol for W). They confirm the M₃S₄^- cluster's ability to efficiently activate the chemical bonds in water to release H_2. Though the W₃S₃^- cluster is not as efficient at bond activation, it provides insights into the factors that contribute to the success of the M₃S₄ anionic systems in hydrogen evolution.