Boosting Visible Light Photocatalytic Performance of Polyoxometalate/Viologen-Based Crystalline Complexes for Cr(VI) Reduction

In order to continuously remove Cr(VI) from wastewater, it is very important to develop a photocatalyst with stable performance, especially the synergistic two-component system of photosensitive viologen and redox active polyoxometalate with excellent photocatalytic performance. In this study, three new POM/viologen-based crystalline complexes were successfully constructed by a one-pot hydrothermal method, namely, (Hmimb)₂·[SiMo₁₂O₄₀] (1), [Ni(mimb)₂(SiMo₁₂O₄₀)]·1.8H₂O (2), and {[Ni(mimb)₂(H₂O)₃]₂·[Ni₄(H₂O)₂(GeMo₉O₃₄)₂(MoO₂)₂]}·9H₂O (3) (mimb = 1-((3,5-dimethylisoxazol-4-yl)methyl)-[4,4'-bipyridin]-1-ium). Notably, complex 3 features a rarely reported Mo-capped Finke-type POM structure. These complexes were employed as photocatalysts for Cr(VI) reduction. Complex 3 was demonstrated superior charge separation and visible light absorption capabilities in photoelectrochemical tests, which can also be confirmed by the analysis of the minimum hydrogen bond distances and dihedral angles between adjacent unit cells in the crystal structure. Under the optimized conditions, complex 3 achieved 99.80% Cr(VI) reduction within 30 min under visible light. Moreover, complexes 1-3 exhibited strong resistance to ionic interference (anions/cations), making them highly practical for real water matrices applications. After six consecutive cycles, the Cr(VI) removal efficiency showed only 3% fluctuation. Superoxide radicals (•O₂⁻) and photogenerated electrons (e⁻) dominate photocatalysis according to radical trapping experiments. This work provides critical insights for designing POM/viologen-based crystalline materials for efficient Cr(VI) reduction.