Photoreduction of Cr(VI) using hydroxoaluminiumtricarboxymonoamide phthalocyanine adsorbed on TiO2

Hydroxoaluminiumtricarboxymonoamide phthalocyanine (AlTCPc) adsorbed at different loadings on TiO2 Degussa P-25 was tested for Cr(VI) photocatalytic reduction under visible irradiation in the presence of 4-chlorophenol (4-CP) as sacrificial donor. A rapid reaction takes place in spite of the presumable aggregation of the dye on the TiO2 surface. The removal of Cr(VI) is fairly negligible under visible-light irradiation, either without photocatalyst or in the presence of bare TiO2. The fast capture of conduction band electrons by Cr(VI), which forms a surface complex with TiO2, inhibits the formation of reactive oxygen species in the reductive pathway. This fact and the easier oxidation of 4-CP as compared to AlTCPc hinder the photobleaching of the dye and make feasible Cr(VI) reduction under visible irradiation. The consumption of Cr(VI) follows a pseudo-first order kinetics; the decay constant depends, in the studied range, on the photocatalyst mass, but it is barely affected by dye loading. The presence of 4-CP is essential, but its concentration has no effect on the Cr(VI) decay rate. Oxidation products of 4-CP, such as hydroquinone, catechol or benzoquinone, are not observed. Direct evidence of the one-electron reduction of Cr(VI) to Cr(V) was obtained by EPR spectroscopy using citric acid as Cr(V) trapping agent. In this case, disappearance of Cr(V) also follows a first order decay, but conduction band electrons do not seem to be involved. The fact that oxidation products of 4-CP are not observed is consistent with the fast dark removal of reaction intermediates by Cr(V), proved by EPR.