Two Novel Viologen/Polyoxometalate-Based Compounds for the Efficient Photocatalytic Reduction of Cr (VI) Under Visible Light Irradiation

Abstract

Designing efficient photocatalyst has attracted extensive attention for treating polluted wastewater containing Cr (VI). In this work, two viologen/polyoxometalate (POM)-based compounds were synthesized, [Ni (mimb)₂(H₂O)₄(β-Mo₈O₂₆)]·4H₂O (1) and [Zn (mimb)₂(H₂O)₄(β-Mo₈O₂₆)]·4H₂O (2) (mimb = 1-((3,5-dimethylisoxazol-4-yl)methyl)-[4,4′-bipyridin]-1-ium), through a hydrothermal method and investigated their ability of photocatalytic reduction of Cr (VI). The structures of compounds 1–2 were characterized by single-crystal X-ray diffraction, revealing that both possess a zero-dimensional (0D) structure and can form one-dimensional (1D) chain structures through hydrogen bondings. Photocatalytic experiments demonstrate that compound 1 has superior photocatalytic performance compared to compound 2. Under optimal conditions (pH = 2, 15 mg catalysts, and 10 mg L⁻¹ Cr (VI) concentration), compound 1 can completely reduce Cr (VI) within 20 min, while compound 2 requires 30 min. Compounds 1–2 also show good stability after photocatalytic reaction, and maintain high photocatalytic activity after five reaction cycles. Additionally, the photocatalytic reaction mechanism of compounds 1–2 was studied, and the free radical scavenging experiments show that its main active species are e⁻, h⁺, and ˙O₂⁻. The nitroblue tetrazolium (NBT) experiments further confirm the crucial role of ˙O₂⁻ in the photocatalytic process. The good photocatalytic activity, stability, and recyclability of the viologen/POM-based compounds suggest their potential application in the treatment of wastewater containing Cr (VI).