Keggin Cluster Modulated Photocatalytic Activity of Aryl Sulfonium Polyoxometalate Hybrids toward Dichromate...

Dichromate (Cr₂O₇^2-) ion having chromium in its +6 oxidation state is a carcinogen and a potential threat to humans and aquatic life. The photocatalytic reduction of toxic Cr(VI) species into less toxic Cr(III) is an important target in heterogeneous catalysis. In this work, the catalytic activities of a series of Keggin cluster-based aryl sulfonium polyoxometalate hybrids, (FPDS)₃[PMo₁₂O₄₀] (1), (FPDS)₃[PW₁₂O₄₀] (2), (FPDS)₄[SiMo₁₂O₄₀] (3), and (FPDS)₄[SiW₁₂O₄₀] (4), toward the photocatalytic reduction of Cr(VI) have been analyzed and compared. Here, we used the aryl sulfonium counterions to modulate the POM cluster's solubility in water and stabilize the photogenerated e^--h⁺ pair on the cluster. All of the hybrids 1-4 catalyzed the reduction of Cr(VI) to Cr(III) under ultraviolet (UV) irradiation, and their photocatalytic efficiencies followed the order hybrid 1 > hybrid 3 > hybrid 2 > hybrid 4, with the rate-constant values of 0.048, 0.0056, 0.0035, and 0.0028 min^-1, respectively. Hybrid 1 with [PMo₁₂O₄₀]^3- Keggin cluster exhibited the best photocatalytic activity in the series yielding a 99% reduction in 120 min. The reasons behind the best photocatalytic activity of hybrid 1 are identified as its low band gap, less charge recombination, and fast photoresponse. The electron-trapping analyses performed using AgNO₃ revealed electrons as the main reactive species responsible for the photocatalytic reduction of Cr(VI). A plausible photocatalytic mechanism has also been proposed based on electron-trapping experiments. The present study shows that aryl sulfonium Keggin hybrids can function as efficient photocatalysts for Cr(VI) reduction, and their catalytic efficiency varies with the nature of the Keggin cluster.