Preparation, characterization of N–I co-doped TiO2 and catalytic performance toward simultaneous Cr(VI) reduction and benzoic acid oxidation

Abstract

Four N–I co-doped TiO2 catalysts having Ti:N/I molar ratios of 1:1, 1:3, 1:5 and 1:10 were prepared via a sol–gel method using NH4I as N–I dopant precursor. A pure TiO2 (undoped) sample was also prepared by the same method for comparison. The catalysts were evaluated for the simultaneous photocatalytic reduction of Cr(VI) and oxidation of benzoic acid (BA). TiO2 anatase phase was formed for all N–I co-doped catalysts as shown by XRD. UV–vis diffuse reflectance spectra showed that N–I co-doping resulted in increased absorption at visible wavelengths and a decrease of the band gap energy (Eg). The smallest Eg value of 2.34eV was observed for the 1:5 Ti:N/I molar ratio. The structure and photodynamics of the TiO2 catalysts was investigated in detail by Electron Paramagnetic Resonance (EPR) spectroscopy. The EPR data showed: [i] formation of non-photoactive NO centers and photoinduced Nb paramagnetic species as a result of N doping, [ii] photoinduced Ti3+ surface ions, and [iii] formation of surfacial oxygen O2− radical ions and trapped holes TiO4+–O−. The Nb species act upon the narrowing of the band gap and the production of photogenerated electrons, i.e. Ti3+ surface ions. These Ti3+ surface ions have a key role, capturing gas O2 and supporting both reduction and oxidation process. Cr(VI) reduction by the N–I co-doped catalysts followed the trend: TNI5>TNI10>TNI1>TNI3>TiO2[undoped] which correlates with the concentration of Nb species formed and the narrowing of Eg values. Oxidation of benzoic acid (BA) followed a more complex trend as follows: TNI1>TNI3>TNI10>TNI5 which is the reverse of the trend for trapped holes TiO4+–O−. Finally progressive microwave EPR saturation experiments show that Nb species are located deeper in the TiO2 lattice in particles with narrower Eg values.