Molybdenum oxysulfide MoOS2 as a hydrogen evolution catalyst: On the possible role of coordinated oxygen

Abstract

Amorphous molybdenum oxysulfide MoOS2 obtained by acidification of aqueous MoO2S22− solutions was structurally characterized and tested as a catalyst for hydrogen evolution reaction (HER). Raman, XPS and XAS characterizations show that the solid contains oligomers based on the Mo(V) core Mo2O2S4 units, exposing dangling MoO bonds and connected via Mo-S-Mo bridges. MoOS2 demonstrated higher HER activity than reference catalysts including nanocrystalline MoS2, amorphous MoS3 and MoS2/MoO2 composite obtained by thermal decomposition of MoOS2. DFT calculations suggest that introduction of oxygen adjacent to the disulfide bridges in the molybdenum coordination does not change significantly the free energy of hydrogen desorption, as compared with fully sulfided MoxSy clusters. Higher HER catalytic activity of the MoOS2 material was explained by abundance of disulfide S22− moieties able to dissociate hydrogen and by fine dispersion of the material.