Experimental Evidence in Favor of an Initial One-Electron-Transfer Process in the Heterogeneous Photocatalytic Reduction of Chromium(VI) over TiO₂

Direct evidence of the involvement of Cr(V) intermediacy in Cr(VI) heterogeneous photocatalytic reduction over TiO2 was obtained by EPR spectroscopy, in the presence and absence of ethylenediaminetetraacetic acid (EDTA). The rate of formation of Cr(V) becomes rapidly inhibited in the absence of a hole scavenger. ofEDTAis found to efficiently suppress the electron-shuttle mechanism by which Cr(VI) reduction is inhibited. Analysis of the data indicates that Cr(V) is formed by a one-electron-transfer process and does not involve Cr(IV) disproportion. It is suggested that Cr(III) formation occurs similarly from Cr(IV) through a charge-transfer mechanism.