Elucidating active sites and decomposition mechanisms for oxythiomolybdate clusters (Mo2O2Sx, x = 6;8) as catalyzers for hydrogen evolution reactions

Abstract

We investigated the decomposition mechanism of Mo3S13 (chalcogenides) and Mo2O2Sx(x = 6,8) through Density Functional Theory, in particular Mo2O2Sx. Oxythiomolybdates are more stable than chalcogenides, and decompose through cathodic reactions, while chalcogenides decompose also through anodic reactions. The solvent acts on the polymerization mechanism of Mo2O2S6, blocking or reducing the polymerization between Mo2O2S6 clusters. The electrons’ excess is responsible for the catalyst decomposition and, also induces absorption of hydrogen on terminal oxygen. Moreover, Mo2O2S6 absorbs two hydrogens on each terminal disulfide without absorbing electrons. The Mo2O2S4 can absorb one hydrogen and catalyze hydrogen evolution reactions (HER), however, decomposition still occurs. Ab initio molecular dynamics shows the explicit absorption of hydrogen in Mo2O2S6 by adding electrons to the system, in which hydrogen can be absorbed in a disulfide or in an oxygen terminal. Heyrovsky mechanism tends to dominate HER, and hydrogen absorption can also remove oxygen from Mo2O2S6 and produce water.