Making Use of the δ Electrons in K 4 Mo 2 (SO 4 ) 4 for Visible-Light-Induced Photocatalytic Hydrogen Production.

Quadruply bonded dimolybdenum complexes with a σπδ electronic configuration for the ground state have rich metal-centered photochemistry. An earlier study showed that stoichiometric or less amount of molecular hydrogen was produced upon irradiation by ultraviolet light (λ = 254 nm) of KMo(SO) in sulfuric acid solution, which was attributed to the reductive capability of the ππ* excited state. To make use of the δ electrons for visible-light-induced photocatalytic hydrogen evolution, a multicomponent heterogeneous photocatalytic system containing KMo(SO) photosensitizer, TiO electron relay, and MoS cocatalyst is designed and tested. With ascorbic acid added as a sacrificial reagent, irradiation by artificial sunlight (AM 1.5) on the reaction in 5 M HSO has produced 13 400 μmol g of molecular hydrogen (based on the Mo complex), which is 30 times higher than the hydrogen yield obtained from the reaction of bare KMo(SO) with HSO under ultraviolet light irradiation. Further improvement of hydrogen evolution is achieved by addition of oxalic acid, along with an electron donor, which gives an additional 50% increase in H yield. Spectroscopic analyses indicate that, in this case, a junction between the Mo complex and TiO is built by the oxalate bridging ligand, which facilitates charge injection and separation from the Mo core. This Mo-TiO-MoS system has achieved a high hydrogen evolution rate up to 4570 μmol g h. The efficiency of KMo(SO) as a metal-centered photosensitizer is also proved by parallel experiments with a dye chromophore, fluorescein, which presents comparable H yields and hydrogen evolution rates. Most importantly, in this study, detailed analyses illustrate that the photocatalytic cycle with hydrogen gas as an outcome of the reaction is established by involvement of the δδ* excited state generated by visible light irradiation. Therefore, this work shows the potential of quadruply bonded Mo complexes as photosensitizers for photocatalytic hydrogen evolution.