Monomeric MoS42--Derived Polymeric Chains with Active Molecular Units for Efficient Hydrogen Evolution Reaction

Molybdenum sulfides have attracted widespread attention as promising nonprecious-metal catalysts for the hydrogen evolution reaction (HER). Since the MoS2 edge was proposed as a major active site, molecular and polymeric analogues to the MoS2 edge have been widely explored as the HER catalysts. In particular, amorphous MoSx coordination polymers have been considered as active HER catalysts because they are rich in unsaturated Mo-S coordination, which is the characteristic of the active MoS2 edge. Herein, we report that the simple monomeric thiomolybdate (MoS42-) could adopt a polymeric chain structure, which exhibited high HER activity; its turnover frequency surpassed those of dimeric [Mo2S12](2-)- and trimeric [Mo3S13](2-)-derived MoSx catalysts. This high HER activity of monomeric MoS42- is attributed to the polymerization of MoS42- anions, generating active molecular analogues that comprise monomeric S2- sites bridging Mo(V) and Mo(W). Density functional theory calculations of possible polymeric chain structures identified the Mo(IV)Mo(V)(2)(S-2(2-))(2)(S2-)(5) unit as the most plausible structure that best matched the experimentally deduced structure. The Gibbs free energy for hydrogen adsorption on the bridging S2- (p-S2-) site in Mo(IV)Mo(V)(2)(S-2(2-))(2)(S2-)(5) was found to be -0.05 eV, which is close to the thermoneutral state. Combined analyses by resonance Raman spectroscopy and extended X-ray absorption fine structure suggested the role of Mo-oxo (Mo=O-x) species to generate the active Mo(V)-(mu-S2-)-Mo(IV) center for effective hydrogen adsorption.