Visible light activity of TiO2 for the photoreduction of CCl4 and Cr(VI) in the presence of nonionic surfactant (Brij)

Abstract

We report a novel surfactant–TiO2 photochemical system where TiO2 in surfactant solutions exhibits visible light activities for the first time. In the presence of a simple nonionic surfactant having polyoxyethylene groups (Brij), CCl4 and Cr(VI) can be successfully converted in aqueous TiO2 suspensions under visible light illumination. A visible light induced electron transfer on surfactant–TiO2 reductively degrades CCl4 into Cl− and CO2 or reduces Cr(VI) to Cr(III). As a spectroscopic evidence for this visible light activity, a broad absorption band (320–500nm) is observed in the surfactant–TiO2 solution. It is proposed as a hypothesis that a complex formation between the surfactant functional groups and TiO2 surface is responsible for the weak visible light absorption and the subsequent photoinduced electron transfer to CCl4 or Cr(VI). The visible light induced reduction of both CCl4 and Cr(VI) on TiO2 is strongly dependent on the kind of surfactants and only Brij-series show significant activities. The light absorption and the corresponding photoactivity are well correlated: the greater the absorption band, the higher the reactivity. Platinizing TiO2 particles rapidly decreases both the visible absorption and the visible reactivity since the Pt deposits mask the surface functional groups and consequently inhibit the complexation with the surfactant functional groups. Both the CCl4 dechlorination rate and the visible absorption band increase with the surfactant concentration, which indicates that more surface complexes are formed at higher surfactant concentrations. Although the visible light activity of this surfactant–TiO2 system has been demonstrated for the reduction of CCl4 and Cr(VI) only in this study, other photocatalytic or photoelectrochemical conversion processes that are based on the visible light-induced electron transfer on TiO2 surface might be also carried out using this system as a general method of TiO2 activation.