Surface Anchoring and Active Sites of [Mo₃S₁₃]^2- Clusters as Co-Catalysts for Photocatalytic Hydrogen Evolution

Achieving light-driven splitting of water with high efficiency remains a challenging task on the way to solar fuel exploration. In this work, to combine the advantages of heterogeneous and homogeneous photosystems, we covalently anchor noble-metal- and carbon-free thiomolybdate [Mo₃S₁₃]^2- clusters onto photoactive metal oxide supports to act as molecular co-catalysts for photocatalytic water splitting. We demonstrate that strong and surface-limited binding of the [Mo₃S₁₃]^2- to the oxide surfaces takes place. The attachment involves the loss of the majority of the terminal S₂ ^2- groups, upon which Mo-O-Ti bonds with the hydroxylated TiO₂ surface are established. The heterogenized [Mo₃S₁₃]^2- clusters are active and stable co-catalysts for the light-driven hydrogen evolution reaction (HER) with performance close to the level of the benchmark Pt. Optimal HER rates are achieved for 2 wt % cluster loadings, which we relate to the accessibility of the TiO₂ surface required for efficient hole scavenging. We further elucidate the active HER sites by applying thermal post-treatments in air and N₂. Our data demonstrate the importance of the trinuclear core of the [Mo₃S₁₃]^2- cluster and suggest bridging S₂ ^2- and vacant coordination sites at the Mo centers as likely HER active sites. This work provides a prime example for the successful heterogenization of an inorganic molecular cluster as a co-catalyst for light-driven HER and gives the incentive to explore other thio(oxo)metalates.