Science 335, 698–702 (2012)

Credit: © 2012 AAAS

In common with many heterogeneous catalysts, the bulk of molybdenum disulfide is relatively inactive compared with the surface, which is effective for industrial hydrodesulfurization reactions. MoS2 nanoparticles have also been proposed as cheaper alternatives to platinum for the photo/electrocatalytic production of hydrogen from water. The active surfaces are believed to feature molybdenum atoms bonded to disulfide units, forming an MoS2 triangle, however, the exact details of the structure and mechanism have proved difficult to fully explore.

Now, a team led by Jeff Long and Chris Chang from the University of California at Berkeley have prepared a molecular analogue of the MoS2 active site, which itself acts as an effective hydrogen-generation catalyst. Based on their previous work in which they used a large pentapyridyl ligand to stabilize a molybdenum-oxo species, they treated a Mo(II) precursor with S8 to form an Mo(IV) disulfide complex. X-ray crystallography revealed a triangular MoS2 group very similar to those believed to be the active sites on the surface of solid MoS2 catalysts.

The molybdenum–sulfur bonds were long enough to suggest single bonds, and the sulfur–sulfur bonds were similar in length to those in S8, which suggests they are single in character too. In aqueous media at pH 3, the disulfide complex acts an electrocatalyst for the reduction of protons from water, whereas the similar oxo complex does not, demonstrating the importance of the disulfide unit. Even without further optimization, the complex's stability and turnover is in excess or comparable (respectively) to hydrogenase enzymes.