Photocatalytic Oxidation Of Nitrogen Oxides With {001} Facets TiO₂Photocatalyst

Nitrogen oxides （NO_x） are major contributors to air pollution. Photocatalysis isapromising and environmental friendly de-NO_xtechnology. Anatase titanium dioxide（TiO₂） as a good photocatalyst has received much attention, due to its safety, chemicalstability, strong oxidizing power and cheapness. Traditional anatase TiO₂nanocrystalsare dominated by the thermodynamically stable {101} facets other than the morereaetive {001} faeets. Its high photoinduced electron-hole recombination rate and lowquantum efficiency have greatly inhibited the photocatalytic activity. In addition,taking the small BET surface area into account, loading of catalysts onto the supportwas considered. Molecular sieves become a popular support due to their regular pore,UV-vis light transmittance and large surface area. As typical microporous zeolite andmolecular sieves respectively, HZSM-5and MCM-41are studied in this work workas supports for {001}TiO₂loading. In this work, Anatase TiO₂with exposed {001}facets were synthesized using hydrofluoric acid （HF） as a capping agent, and loadedto the HZSM-5and MCM-41molecular sieve respectively. The photocatalyticactivity of samples for NO_xoxidation was investigated under UV light irradiation.A series anatase TiO₂with different percentage of {001} surface weresynthesized by a hydrothermal route. The prepared catalysts were characterized byX-ray diffraction, UV-vis diffuse reflectance spectroscopy, scanning electronmicroscopy, transmission electron microscopy techniques and X-ray photoelectronspectroscopy analysis. The photocatalytic activity of samples for NO_xoxidation wasinvestigated under UV light irradiation. The results showed that all the TiO₂sampleswith {001} facets had anatase crystallite and nanosheet structure, as well as strongultraviolet light absorbance. The exposure rate of {001} facets could reach64.46%, Tionly existed as Ti4+in TiO₂, and F only existed on surface of samples. The resultsshowed that the surface F inhibited NO_xphotocatalytic oxidation. Compared to{101}TiO₂and P25,{001}TiO₂-NaOH washed with NaOH to remove surface F ionshad an obvious photocatalytic activity for NO_xoxidation under UV light irradiation.When the NaOH concentration was0.6M, the catalyst showed the highest photocatalytic activity and F ion content in the sample surface is only0.8%. Thehigher percentage of the {001} facets, the higher photocatalytic activity of the samplewas.HZSM-5and MCM-41hybridized with TiO₂photocatalysts were prepared bysolid state dispersion. The reactivity of the complex photocatalysts with different{001}TiO₂contents on photocatalytic oxidation of NO_xwas investigated. Theprepared catalysts were characterized by X-ray diffraction, UV-vis diffuse reflectancespectroscopy, scanning electron microscopy. It was found that the composite catalysts{001}TiO₂/HZSM-5remained the skeleton structure of zeolite, and{001}TiO₂/MCM-41remained mesoporous state. The composite catalysts with40%{001}TiO₂had the highest photocatalytic activity for NO_xoxidation. Compared with{001}TiO₂/MCM-41, with the increase of NO removal the NO2yield relativelyreduced on {001}TiO₂/HZSM-5.