Central Metals to Guide the Bandgap of Hourglass-type Polyoxometalate Hybrids as Photocatalyst for the Reduction of Cr(VI)

Visible-light photocatalytic reduction of hexavalent chromium Cr(VI) has become one of the most challenging topics in the field of environmental remediation. The key is to explore a stable and efficient photocatalyst. In this work, a series of highly reduced hourglass-type phosphomolybdate hybrids with molecular formula of (H2bpe)3{M[P4Mo6O31H8]2}·8H2O (M = Na for 1, Ca for 2, Cd for 3, Mn for 4, Zn for 5; bpe = trans-1,2-bi(4-pyridyl)-ethylene) were synthesized by hydrothermal methods and used to reduce Cr(VI) under visible-light conditions. The experimental results showed that hybrids 1–5 were of isomorphic structure and constructed by organic bpe cations and hourglass phosphomolybdate clusters with different metal centers via hydrogen-bonding interactions to extend the structure into supramolecular network. With the excellent redox properties and extensive visible light absorption, hybrids 1–5 displayed good photocatalytic activity for Cr(VI) reduction with the reduction conversion rates of 87.84%, 80.31%, 89.07%, 90.92%, and 92.83% within just 10 W white-light irradiation of 20 min at room temperature. Among them, Zn-centered hybrid 5 showed the best photocatalytic performance and recycle stability. The mechanism study showed that the different central metal M can regulate the band gap of hybrid photocatalysts due to its different electronic property, thus affecting their photocatalytic performance. This work provided a promising way to design efficient polyoxometalate-based photocatalysts via the molecular-level composition modulation strategy.