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

Authors: A.E. Giannakas, E. Seristatidou, Y. Deligiannakis, I. Konstantinou

Abstract:
N-doped and N–F co-doped TiO₂ catalysts were prepared via a sol–gel method using NH₄Cl and NH₄F as N and N–Fdopantprecursors, respectively,having Ti:Nand/or Fmolar ratios of 1:1, 1:2 and1:3. 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 N–F 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, N–F 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, N_b^• and O₂− • radicals were formed. In addition, lattice Ti³⁺ ions were detected in N–F co-doped solids; [ii] the N_b^• 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 N_b^• species was revealed by EPR. The location of N_b^• in the crystal lattice of TiO₂ has been assessed also by measuring their microwave saturation parameters P_1/2. Electron capturing by O₂ and subsequent generation of O₂− • was favored for N-doped catalysts. In contrast, in N–F 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.

Keywords:
Photocatalysis
N-doped
TiO₂
N–F co-doped
TiO₂ Cr(VI) reduction
EPR N_b^• species
Microwave saturation

Published in: Applied Catalysis B: Environmental (Volumes 132–133, Pages 1-552, 27 March 2013)

Publisher: Elsevier  

ISSN Information: 0926-3373

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

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