Surface Anchoring and Active Sites of [Mo(3)S(13)](2–) Clusters as Co-Catalysts for Photocatalytic Hydrogen Evolution

[Image: see text] 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 [Mo(3)S(13)](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 [Mo(3)S(13)](2–) to the oxide surfaces takes place. The attachment involves the loss of the majority of the terminal S(2)(2–) groups, upon which Mo–O–Ti bonds with the hydroxylated TiO(2) surface are established. The heterogenized [Mo(3)S(13)](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 TiO(2) surface required for efficient hole scavenging. We further elucidate the active HER sites by applying thermal post-treatments in air and N(2). Our data demonstrate the importance of the trinuclear core of the [Mo(3)S(13)](2–) cluster and suggest bridging S(2)(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.