Photocatalytic oxidation of NOx over TiO2/HZSM-5 catalysts in the presence of water vapor: Effect of hydrophobicity of zeolites

Highlights: ► The photooxidation of NOx over TiO2/HZSM-5 was tested in the presence of water vapor. ► TiO2/HZSM-5 exhibited higher NO conversion and lower NO2 formation than pure TiO2. ► Water vapor related to relative humidity and water pre-adsorption was effect factor. ► TiO2 hybridized with hydrophobic HZSM-5 zeolite retained high reactivity. ► Photocatalytic reactivity of TiO2/HZSM-5 depended on hydrophobicity of zeolites. - Abstract: TiO2 hybridized with HZSM-5 zeolites photocatalysts were prepared by a simple solid state dispersion method. The physicochemical properties of the catalysts were characterized by X-ray diffraction, UV–vis diffuse reflectance and FT-IR spectroscopy. The photocatalytic oxidation of NOx over TiO2/HZSM-5 having different Si/Al ratios was carried out under various levels of humidity and different pre-adsorption times in dark. The TiO2/HZSM-5 composite catalysts exhibited higher NO conversion and lower NO2 formation than pure TiO2. Pre-adsorption with water vapor and the high humidity during the photoreaction were harmful to the reactivity of TiO2 hybridized with hydrophilic HZSM-5 zeolite. However, the photocatalytic reactivity of TiO2 hybridized with hydrophobic zeolite varied little with increase in humidity. The results indicated that the high photocatalytic reactivity of TiO2/HZSM-5 catalysts is largely depended on the hydrophobicity of the zeolites.