Study On Microwave-assisted Preparation Of H₃ PO_x, Ag Modified TiO₂ For Photocatalytic Degradation Of Gaseous Toluene

Substantial effort has been devoted to study the photocatalytic degradation of organic pollutants using nanostructured TiO₂ catalyst due to its high oxidation efficiency, outstanding chemical stability and environmentally friendly nature. However, the efficiency of the photocatalytic degradation reaction is limited by the wide band gap (> 3.2 eV, only absorbing the UV light ofλ<385 nm) and the high recombination rate of photoinduced electrons and holes. At the same time, the practical application of TiO₂ in the field of gas-solid photocatalysis is limited by the weak adsorption capacity. In order to improve the photocatalytic activity, increase the adsorption capacity, and extend the optical absorption to visible light region, in this dissertation, TiO₂ was modified by H3POx and Ag using the method of microwave radiation. The effects of Microwave conditions, precursor of TiO₂, dosage of H3POx or AgNO3 on the structural properties were investigated. These modified TiO₂ nanoparticles were characterized by X-ray diffraction (XRD), fourier transform infrared absorption(FT-IR), UV-Visible diffuse reflectance (UV-Vis), environmental scanning electron microscope (SEM)and transmission electron microscope (TEM). Gaseous toluene was chosen as a model reactant to valuate the photocatalytic activity of the catalysts. Additionally, the photocatlytic degradation mechanism of toluene was given in this dissertation. The effect of surface temperature of catalyst on the photocatalytic degradation of toluene was also investigated. The optimal reaction parameters of photocatalytic degradation of toluene were also studied. The main conclusions are drawn as follows:(1) The TiO₂ photocatalyst with small crystallite size and high crystallization could be prepared by microwave radiation using TiO₂ xerogel as precursor. Increasing temperature, power and pressure or prolonging time of microwave reaction could improve the crystallite of TiO₂, but could inhibit the photocatalytic reaction activity because of the large crystallite size of TiO₂.(2) The dopant phosphate in TiO₂ xerogel could effectively decrease the size of crystal as well as had an inhibition effect on the growth of anatase crystallite. High phosphate content inhibited the photocatalytic activity. In this work, when phosphate/TiO₂ molar ratio was 1:10, the photocatalytic activity of the catalyst attached the best. It was also found that the doping of phosphate could improve the thermal stability of the samples. The catalyst showed a higher UV or visible light absorption ability, and its optical adsorption edge shifted red with H3POx doping. When H3PO2/TiO₂ molar ratio was 1:10, the catalyst showed higher photocatalytic degradation ratio of gaseous toluene (61.6%) under UV light radiation for 3 hours. the photocatalytic intermediates (benzyl alcohol, benzaldehyde and methyl phenol) were strongly adsorbed on the surface of catalyst. There seemed to be a correlation between the accumulation of photocatalytic intermediates on the surface and catalyst deactivation. It was proved that H3PO2 doping in TiO₂ can suppress deactivation of catalyst.(3) It was found that the Ag dopant could inhibit the grain growth of catalyst as well as decreased the temperature of anatase-to-rutile phase transformation, and Ag dopant was favorable to prepare TiO₂ with mixed crystal (anatase, rutile and brookite). However, the size of TiO₂ increased while the Ag/TiO₂ molar ratio exceeded 1:100. The catalyst doped with Ag showed a higher UV or visible light absorption ability, and its optical adsorption edge shifted red. When the Ag/TiO₂ molar ratio was 1:100, the catalyst showed higher photocatalytic degradation ratio of gaseous toluene. It was proved that the presence of both H3PO2 and silver (PO2/Ag/TiO₂/MW) could improve photocatalytic activity of TiO₂.(4) It was proved that hydrogen was produced by photocatalytic decomposition of gaseous toluene over PO2/Ag/TiO₂/MW. Hydrogen can inhibit the photocatalytic degradation ratio of gaseous toluene as well as had an inhibition effect on the deactivation of catalyst. The presence of O3 in the reaction system could increase degradation ratio of gaseous toluene and the hydrogen production simultaneously.(5) It was proved that the photocatalytic degradation ratio of gaseous toluene increased with increasing the surface temperature of catalyst. As the carrier, ACF could increase the surface temperature of catalyst, while the degradation ratio of gaseous toluene decreased using ACF/TiO₂ because of the adsorption competition of toluene between TiO₂ and ACF. The silver paper showed higher scattering property of light, which improve the efficiency of TiO₂ optical adsorption, and improved the photocatalytic degradation ratio of gaseous toluene. The influence of catalyst surface area, wavelength of light and content of TiO₂ on photocatalytic degradation ratio of gaseous toluene were investigated by the orthogonal experiment. In the reactor with a diameter of 9 cm and a highth of 45 cm, the optimal parameters of photocatalytic degradation of toluene were the surface area of catalyst film at 660 cm2, the distance of light at 3 cm and the content of TiO₂ at 4.32 g.