Experimental evidence in favor of one-electron-transfer processes in the TiO2 photocatalytic reduction of Cr(VI). Involvement of Cr(V) species

M.I. LITTER; J.J. TESTA, M.A. GRELA

Granada, España

Simposio; XX IUPAC Symposium on Photochemistry; 2004

IUPAC

We have recently probed Cr(V) intermediacy in Cr(VI) heterogeneous photocatalytic reduction over TiO2 by EPR spectroscopy. Irradiation experiments were carried out inside the EPR cavity with a medium-pressure metal halide lamp (300 £ l £ 450 nm). In the presence of EDTA (4) (typical conditions: 1 mM K2Cr2O7, 10 mM EDTA, 0.1 g L-1 TiO2, pH 2.0), Cr (V) species, in the form of aquo- (or peroxo-) and EDTA-complexes were detected. Analysis of the data indicates that Cr(V) is formed by an one-electron-transfer process and that there is not Cr(IV) disproportion. It is suggested that Cr(IV) and Cr(III) are formed similarly from Cr(V) and Cr(IV). In the case of oxalate (5) (typical conditions: 1.6 mM K2Cr2O7, 16 mM oxalic acid, 0.1 g L-1 TiO2, pH 1.5), [CrV(O)(Ox)2]-, [CrV(OH2)(Ox)2]- and [CrV(O)(OH)2(Ox)]- were identified. The kinetic analysis of the temporal evolution of the Cr(V) species indicates, in addition, an effective photocatalytic degradation of Cr(V)-oxalate complexes by cb electrons. Since a Cr(V) species, [Cr(O)(H2O)5], was also detected in the absence of donors, it is suggested that this sequential one-electron-transfer processes may represent a general behaviour for the Cr(VI) reduction over UV-irradiated TiO2 particles. The results explain also other experimental features of the photocatalytic reaction, such as the lack or slight influence of oxygen and the inhibition of the electron-shuttle mechanism in the presence of reducing hole scavengers.