Visible-light-driven hydrogen evolution from water using a noble-metal-free polyoxometalate catalyst

In an effort to address the need to develop hydrolytically more stable, molecular water reduction catalysts (WRCs) amenable to in-depth investigation, we report here one prototype: a tetra-manganese-containing V-centered polyoxotungstate, Na10[Mn4(H2O)2(VW9O34)2] (1). The electronic structure of 1 was elucidated using the DFT approach. Complex 1 is readily prepared by a one-pot procedure in aqueous solution and catalyzes the reduction of water using visible light irradiation (λ = 455 nm) with [Ru(bpy)3]2+ and triethanolamine (TEOA) as a photosensitizer and sacrificial electron donor, respectively. Upon irradiation, the excited state [Ru(bpy)3]2+* is oxidatively quenched by 1, as confirmed by steady-state and time-resolved fluorescence decay studies, to form [Ru(bpy)3]3+ and a reduced form of the catalyst. The [Ru(bpy)3]2+ is rapidly regenerated by reaction with TEOA. The reduced form of the catalyst, 1, reacts with water to generate hydrogen. Isotope labeling experiments demonstrate that the hydrogen comes from water. The stability of the catalyst was assessed using different spectroscopic methods. A mechanism based on experimental results is proposed.