Photocatalytic activity of N-doped and NaF co-doped TiO2 and reduction of chromium(VI) in aqueous solution: An EPR study

N-doped and NaF co-doped TiO2 catalysts were prepared via a sol-gel method using NH4Cl and NH4F as N and NaF dopant precursors, respectively, having Ti: N and/or F molar ratios of 1: 1, 1: 2 and 1: 3. The catalysts were tested for the photocatalytic reduction of Cr(VI) in the presence of oxalate ions. XRD analysis showed the formation of TiO2 anatase phase in all cases. UV-vis DRS spectra showed that both NaF and N-doping resulted in a decrease in the band gap energy (Eg), at the values of 2.81 eV and 3.01 eV, respectively. Thus, NaF doped TiO2 showed enhanced absorption at visible wavelengths. The structure and photodynamics of the TiO2 catalysts was investigated in detail by electron paramagnetic resonance (EPR) spectroscopy. The EPR data showed that: [i] NO centers, Nb and O2- radicals were formed. In addition, lattice Ti3+ ions were detected in NaF co-doped solids; [ii] the Nb and Ti3+ species were photoactive, while the NO species were non-photoactive. The photocatalytic efficiency for Cr(VI) reduction in the presence of oxalate ions, followed the trend TNF1 > TN1 > TNF2 > TN3 > TN2 > TNF3. Importantly, an apparent correlation between the catalytic efficiency and the concentration of Nb species was revealed by EPR. The location of Nb in the crystal lattice of TiO2 has been assessed also by measuring their microwave saturation parameters P1/2. Electron capturing by O2 and subsequent generation of O2- was favored for N-doped catalysts. In contrast, in NaF co-doped catalysts, O2 could not compete efficiently with Cr(VI) for the photogenerated electrons in energy states below the conduction band of TiO2, resulting in higher reduction efficiency for these catalysts.