Photocatalytic reduction of Cr(VI) by iron tungstosilicate under visible light

In this study, visible-light-active iron tungstosilicate (FeSiW) was fabricated via solvothermal method. Fe³⁺ was applied as counter ion to precipitate with silicotungstic acid (H₄SiW₁₂O₄₀) in methanol solution. It can be observed from FT-IR spectra that the Keggin structure of SiW₁₂O₄₀^4- was well maintained in FeSiW. Besides, the effect of different initial molar ratios of Fe³⁺ to H₄SiW₁₂O₄₀ was investigated (from 4:1 to 4:4). By photoelectrochemical characterizations, such as photocurrent response and Mott-Schottky (M-S) plots, easiest generation of electron-hole pairs and lowest charge transfer resistance can be observed on FeSiW with initial molar ratio of 4:3. Moreover, FeSiW with 4:3 ratio also displayed highest activity for photocatalytic reduction of Cr(VI), despite of the largest band gap (ca. 2.71 eV). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the optimized FeSiW were estimated to be +0.03 V and 2.74 V vs NHE, respectively, which were thermodynamically feasible for Cr(VI) reduction and H₂O oxidation. Even under acidic condition (pH 3), the optimized photocatalyst displayed good stability in cyclic runs. Therefore, the present study opens a new way for heterogeneous polyoxometalates as efficient photocatalyst for environmental application.