Photocatalytic oxidation of gaseous ammonia over fluorinated TiO2 with exposed (001) facets

A surface-fluorinated TiO2 (F-TiO2) catalyst was synthesized by a hydrothermal method using hydrofluoric acid (FIF) solution as a capping agent, and defluorinated TiO2 (D-TiO2) was next obtained by washing the F-TiO2 with NaOH solution. The as-prepared catalysts were tested for the photocatalytic oxidation (PCO) of gaseous NH3 under UV light. The F-TiO2 catalyst exhibited remarkable activity for NH3 removal, about twice as high as that of the commercial catalyst P25. In contrast, D-TiO2 showed an obvious decrease in PCO activity. The catalysts were characterized by X-ray diffractometry (XRD), Brunauer-Emmett-Teller (BET) adsorption analysis, High-resolution Transmission electron microscopy (HR-TEM), and X-ray photoelectron spectroscopy (XPS). The results showed that the surface fluorination process formed the surface equivalent to Ti-F group and also increased the percentage of reactive (0 0 1) facets to about 50%; the surface defluorination removed the fluorine (F) element from the F-TiO2 surface but showed no influence on the percentage of reactive (0 0 1) facets. By comparing the specific activities of the catalysts, we found that both the active (0 0 1) facets and the surface group contributed to the improvement of the PCO activity, while the surface equivalent to Ti-F group plays the dominant role. The equivalent to Ti-F group could retard the recombination of photogenerated electrons and holes, which is possibly the major reason for the excellent activity of the F-TiO2 catalyst. (c) 2014 Elsevier B.V. All rights reserved.