Defect structure and evolution mechanism of O2− radical in F-doped V2O5/TiO2 catalysts

Fluorine (F)-doped titania (TiO2) nanoparticles were synthesized by the sol–gel method, which could create more oxygen vacancies on the TiO2 support. The experiments of X-ray diffraction (XRD) and high transmission electron microscopy (TEM) indicated that F-doping could inhibit anatase-to-rutile transition and active component V2O5 had a good dispersion on the support. This paper illustrated the preparation process of the F-doped V2O5/TiO2 catalyst and the generation of stabled superoxide radical anion (O2−) on the surface of the F-doped V2O5/TiO2 catalyst. The characterization and stability of O2− over V2O5/TiO2 was investigated by using electron paramagnetic resonance (EPR) spectroscopy. F-doping could produce more oxygen vacancies on the surface of the catalyst and increase the Ti3+ concentration. UV–vis spectrophotometer was employed to characterize the effect of F-doping on the band gap of the samples. Photoluminescence (PL) spectra strongly confirmed the enhancement of oxygen vacancies by F-doping. The presence of Ti3+ and V4+ in the samples was confirmed by X-ray photoelectron spectroscopy (XPS) spectra. The following mechanism was proposed according to the results from EPR spectra and reaction stoichiometry studies of radical species. Metal oxide catalysts might be viewed as electron donors while the adsorbed oxygen molecules behaved as surface electron acceptors. Over the isolated tetrahedral Ti species, a [Ti3+-OV−] radical pair was formed by F-doping. One possible explanation for the enhancement of selective catalytic reduction (SCR) catalytic activity was that the presence of Ti3+ and V4+ formed by charge compensation, and the subsequently OV− moiety reacted with O2 to form O2−, which could react with NO to yield NO2.