Manganese-Substituted Polyoxometalate as an Effective Shuttle Redox Mediator in Z-Scheme Water Splitting under Visible Light

In the present study, a polyoxometalate is for the first time applied as a shuttle redox in two-step (Z-Scheme) photocatalytic water splitting. Photocatalytic H-2 evolution using a Mn-substituted polyoxometalate [SiW11O39MnII(H2O)](6-) as an electron donor proceeded over a Ru-loaded SrTiO3:Rh photocatalyst under visible light with relatively high selectivity, accompanied by the stoichiometric production of its oxidized form [SiW11O39MnIII(H2O)](5-). Photocatalytic O-2 evolution using the oxidized [SiW11O39MnIII(H2O)](5-) as an electron acceptor proceeded over PtOx-loaded WO3 photocatalyst under visible light with relatively high quantum efficiency and selectivity, whereas the loading of effective PtOx cocatalyst was necessary to facilitate the reduction of polyoxometalate. Finally, a two-step water splitting into H-2 and O-2 was demonstrated under visible light using the couple of Mn-substituted polyoxometalate as shuttle redox between Ru/SrTiO3:Rh and PtOx/WO3 photocatalysts, under mildly acidic conditions with pH approximate to 4.5.