Polyoxometalate-mediated reduction of dichromate under UV irradiation

Reduction of Cr(VI) into Cr(III) was examined under UV light irradiation (λ > 320 nm) in the aerated aqueous solution of four Keggin-type polyoxometalates (POM). All the POMs were effective to initiate the Cr(VI) photoreduction, but they exhibited notably different activity, following the order of H3PW12O40 ≫ H4SiW12O40 > H4GeW12O40 > H3PMo12O40. Whereas the thermodynamics favored Cr(VI) reduction by each reduced POM, the observed order in photoactivity was not in agreement with the one expected from POM redox potentials. Instead, the relative photoactivity among four POMs was exactly the same as those observed for photodegradation of propan-2-ol and textile dye X3B. It was proposed that generation of the reduced-POM was the rate-determining step both for Cr(VI) reduction and for organic oxidation in water.