Reduced Phosphomolybdate Hybrids as Efficient Visible-Light Photocatalysts for Cr(VI) Reduction

Photocatalytic reduction of hexavalent chromium [Cr(VI)] is a promising technology approach to highly efficiently and environmentally tackle the problem of Cr(VI) pollution, in which the key challenge is in the development of effective photocatalysts. In this work, highly reduced hourglass-type molybdophosphate hybrids with the formulas [Zn(mbpy)(H2O)2]2[Zn(mbpy)(H2O)]2{Zn[P4Mo6O31H7]2}·9H2O (1), [Na(H2O)2]2[Zn(mbpy)(H2O)]2[Zn(mbpy)(H2O)2]2{Zn[P4Mo6O31H6]2}·15H2O (2), and (H2mbpy){[Zn(mbpy)(H2O)]2[Zn(H2O)]2}{Zn[P4Mo6O31H6]2}·10H2O (3) (mbpy = 4,4′-dimethyl-2,2′-bipyridine) have been hydrothermally synthesized and used as photocatalysts for the reduction of Cr(VI) under mild conditions. Structural analysis showed that the inorganic moieties in crystals 1–3 are composed of a unique 0D single cluster form, a 1D chainlike structure, and a 2D-layered structure, respectively, in which polyanions were constructed by hourglass-type molybdophosphates with one Zn(II) ion as the central metal. These hybrids displayed good performance for the photocatalytic reduction of Cr(VI) by virtue of their wide visible-light adsorption, suitable energy band structures, and specific spatial arrangements of polyanionic species. Among them, hybrid 2 exhibits the best photocatalytic performance with a Cr(VI) reduction conversion rate of almost 94.7% within 180 min of reaction time. The photocatalysis mechanism investigation revealed that highly reduced hourglass-type molybdophosphate clusters can be illuminated by visible light. The photoinduced electrons induced by hourglass-type polyanions can directly reduce Cr(VI) to Cr(III), while the photogenerated holes are used to oxidize the sacrificial agent isopropyl alcohol to acetone. This work provides new guidance for the design and preparation of highly efficient photocatalysts for the reduction of Cr(VI).