Photocatalytic Oxidation Of Nitrogen Oxides Over Modified Titania Under Visible Light

As nitrogen oxides pose increasingly severe pollution on the environment, their removal by means of photocatalytic oxidation (PCO) has drawn much research attention recently. Because of its mild reaction conditions, low energy costs, and little secondary pollution, the PCO of nitrogen oxides indicates great potential in future application, while this oxidation pathway agrees with nitrogen fixation in nature, and the oxidation product is ready to recycle for use.This study aims to remove NO2 under visible light by photocatalytic oxidation over modified titania. With the analyses of XRD, TEM, UV-vis, XPS, and FTIR results, the response to visible light, effects of element doping, and the mechanism of the PCO of NO2 were investigated.Nitrogen-doped TiO₂ was prepared following the method of hydrolytic precipitation. By introducing nitrogen into the lattice of TiO₂, the band gap of TiO₂ was narrowed, and TiO₂ became responsive to visible light region. Therefore, NO2 was oxidized under visible light, and the removal rates over nitrogen-doped TiO₂ were much higher than that over undoped TiO₂. While calcination temperature and nitrogen doping quantity influence the removal rate, the optimum removal of NO2 was achieved over nitrogen-doped TiO₂ with nitrogen doping quantity of 0.0160mol and calcined at 600℃for 2 hours.Iron-doped TiO₂ was prepared by the adsorption and deposition method. Accommodated into the lattice of TiO₂, Fe3+ substituted Ti4+ in their sites, and caused lattice deformation, leading to the decrease of TiO₂ particle sizes and the increase of specific surface area, both of which had positive effects on the PCO reaction. Furthermore, the doping of Fe3+ also caused TiO₂ responsive to visible light, and the UV-visible absorption spectra of modified TiO₂ showed significant shifts to the visible light region. Under visible light, the optimum removal of NO2 was achieved over iron-doped TiO₂ with 0.2 at % Fe3+ and calcined at 600℃for 2 hours. Nitrogen, iron co-doped TiO₂ inherited the advantages of nitrogen-doped and iron-doped TiO₂. Their crystals grew well, and particle sizes were even and small, which were positive for the PCO reaction. While the UV-visible absorption spectra of TiO₂ extended to the visible light region, higher absorption of visible light was observed, and the PCO reaction was enhanced, with an optimum removal rate of 95%. As the PCO product of NO2, nitrate anions adsorbed onto catalyst surface could be transferred into water phase by rinsing, leaving the catalyst regenerated. Therefore, the PCO of nitrogen oxides over modified TiO₂ exhibits a promising future for application.