Photocatalytic performance of Pt-TiO2, Pt-N-TiO2 and Pt-N/F-TiO2 towards simultaneous Cr(VI) reduction/benzoic acid oxidation: Insights into photogenerated charge carrier dynamics and catalyst properties

Abstract Pt-TiO 2 , Pt-N-TiO 2 and Pt-N/F-TiO 2 photocatalysts were prepared with different Pt contents (0.5%, 1.0% and 1.5% wt.) via a sol-gel impregnation method. In all cases, XRD analysis showed mainly (>90%) formation of anatase phase with ca. 10 nm particle diameter and about 10% brookite phase fraction with 6–8 nm particle diameter. UV–vis Diffuse Reflectance Spectroscopy (DRS) showed that visible light photoactivity is influenced by the type of doping atoms. N/F co-doped TiO 2 presented higher visible light absorption than N-doped-TiO 2 . XPS revealed that platinum existed mainly in Pt 2+ and Pt 2+ , Pt 4+ for Pt-TiO 2 and Pt-N(N/F)-TiO 2 catalysts, respectively. Electron Paramagnetic Resonance (EPR) spectroscopy was used to investigate the photodynamics of hole-electron (h + -e − ) pairs. In all cases, the amount of Pt-loading affected decisively the photoinduced e − (both surface and lattice electrons) and simultaneously enhanced the life-time of holes (h + ). The optimal h + /e − balance ratio was detected for 0.5% wt. Pt-loading in all TiO 2 , N-TiO 2 and N/F-TiO 2 materials. The EPR data on the e − /h + ratio was consistent with the observed photocatalytic kinetics towards the simultaneous Cr(VI) reduction and benzoic acid oxidation. Pt showed to improve the visible-light absorption efficiency as well as the e − transfer to Cr(VI). Accordingly, this mechanism explains the non-obvious observed trend for Cr(VI) reduction, i.e. Pt-N-TiO 2  > Pt-N/F-TiO 2  > Pt-TiO 2 .