Selective photocatalytic oxidation of gaseous ammonia at ppb level over Pt and F modified TiO2

Ammonia (NH3) as an important precursor to form atmospheric fine particles and secondary inorganic aerosols, should be strictly controlled. Photocatalysis has provided a facile and an effective way to eliminate NH3 pollution under mild conditions, whereas the undesirable products, such as NO, NO2 would be generated during the reaction and the mechanism remains unclear. In this study, F or Pt modified TiO2 were explored to reduce the formation of NOx during photocatalytic oxidation of low-concentration NH3, and its photocatalytic activity, selectivity and mechanism of NH3 conversion were systematically studied. Results indicate that surface fluorination on TiO2 contribute to the reduction of noxious NOx, especially for NO2, since the modified TiO2 achieved enhanced adsorption of NH3 and strong electron-trapping ability, which can retard the recombination of photo generated electrons and holes. In addition, the deposition of Pt could further extend the lifetime of the electron hole pairs by strongly capture the electron, and enhance the oxidation of NH3 into nitrates and nitrites species. From the in-situ DRIFT spectroscopy and XPS results, we can deduce that reactive amino radical (center dot NH2) would be formed on TiO2 under photoirradiation after the adsorption of NH3 on Lewis acid cites. The formed center dot NH2 can react with reactive oxygen species in the presence of H2O, and produce NOx and HNOx. By both enhancing the adsorption of NH3 and separation efficiency of electron-hole pairs, the presence of F and Pt modification on the TiO2 changes the photocatalytic pathway of NH3 conversion. The proposed selective oxidation mechanism may offer a novel insight into the photocatalytic oxidation of atmospheric NH3 on other metal oxide with surface modification and can be broadly employed in air pollution control in indoor environments.