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

Abstract Pt-TiO2, Pt-N-TiO2 and Pt-N/F-TiO2 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 TiO2 presented higher visible light absorption than N-doped-TiO2. XPS revealed that platinum existed mainly in Pt2+ and Pt2+, Pt4+ for Pt-TiO2 and Pt-N(N/F)-TiO2 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 TiO2, N-TiO2 and N/F-TiO2 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-TiO2 > Pt-N/F-TiO2 > Pt-TiO2.