Nano-titania assisted photoreduction of Cr(VI): The role of the different TiO2 polymorphs

Nanocrystalline TiO2 samples, with controlled phase composition, were obtained by combining reactions in solution with mild hydrothermal or thermal treatments. By using as the starting salt TiCl3, pure phase rutile with crystallite sizes of 6 nm and a BET surface area of 130 m2 g−1 was obtained. The photocatalytic activity of the synthesized samples along with that of several commercially available TiO2 samples, was tested for the photoreduction of Cr(VI). The reaction was performed in TiO2 aqueous slurry in the presence of different sacrificial molecules (formic acid (FA), isopropyl alcohol (IPA) and sodium sulphite (SS)). The pH of the reaction (2.5) was defined on the grounds of the energy level of the TiO2 conduction band and of the reduction potential of the Cr(VI)/Cr(III) couple. The reaction kinetics could be described, for all samples, by a pseudo-first order rate equation.The photocatalytic activity of the anatase–brookite synthesized composite and of the anatase–rutile commercial P25 were found to be the highest (94% at 90 min reaction time). Rutile samples, both synthesized (〈d〉 = 6 nm) and commercial (〈d〉 = 93 nm), showed a lower activity with respect to the other samples (around 80% at 90 min reaction time). This occurrence is discussed also on the grounds of diffuse reflectance spectra.Cr(0) was observed at the surface of TiO2 samples by XPS analyses. Since thermodynamic analysis shows that TiO2 cannot photoreduce Cr(III) to Cr(0), the formation of Cr(0) is attributed to the reducing action of the produced IPA radicals.The different contributions to the global reaction pathway are critically discussed.