Catalytic Activities of Sulfur Atoms in Amorphous Molybdenum Sulfide for the Electrochemical Hydrogen Evolution Reaction

The catalytic activities of sulfur sites in amorphous MoSx for the electrochemical hydrogen evolution reaction (HER) was investigated in aqueous 0.5 M H2SO4 electrolyte. Using X-ray photoelectron spectroscopy and linear sweep voltammetry, we found the turnover frequency for H-2 production to increase linearly with the percentage of S atoms with higher electron binding energies. These S atoms could be apical S2- and/or bridging S-2(2-). To distinguish the catalytic performances of these two types of atoms, we turn to quantum chemical simulations using density functional theory. The apical S2- atoms were found to adsorb H weakly with a Gibbs free energy for atomic H adsorption (Delta G(H)) in excess of +1 eV, and were thus ruled out as reaction sites for HER. In situ Raman spectroscopy of the model [Mo3S13](2-) cluster further demonstrate the higher catalytic reactivity of the bridging S-2(2-) over terminal S-2(2-) (which have lower electron binding energy) for proton reduction.