Effect Of Surface Active Sites On TiO₂for Adsorption And Photocatalytic Degradation Of Gaseous Toluene

With the increasingly serious air pollution, extensive research has been carried out to remove the volatile organic compounds (VOCs). Heterogeneous photocatalytic oxidation (PCO) is considered to be an attractive method, as it can decompose VOCs under mild operating conditions. TiO2is the most widely used photocatalyst for degradation of toluene, which is one of the typical VOCs. In this dissertation, the adsorption and photocatalytic degradation of toluene on the pure TiO2and acidic/alkali modified TiO2were studied. This dissertation consists of four chapters.In chapter one, we introduce the related background knowledge and review the research progress. First of all, we introduce the sources and harmful effects of VOCs, and some removal methods. Secondly, the principle and applications of photocatalysis and TiO2photocatalyst are introduced. Thirdly, the research progress in the photocatalytic degradation of VOCs and the studies on the photocatalytic degradation of toluene using in-situ DRIFTS are reviewed. Finally, we illustrate the research contents.In chapter two, the photocatalytic degradation of gaseous toluene on TiO2with weak-bond adsorption analysis using in-situ far infrared spectroscopy is investigated. Combined with comparison experiments and theoretical calculation, it is found that toluene is adsorbed on the hydroxyl groups through the OH…π-electron-type weak interaction on the surface of TiO2with three possible adsorption structures, ortho-, meta-and para-, instead of ipso-adsorption structure. The methyl group of toluene is consumed first during the process of toluene photocatalytic degradation due to the conjugation of the aromatic ring and the function of the charge transfer. Base on these, a reaction route for the photocatalytic degradation of gaseous toluene on TiO2surface is proposed. In addition, the result of this study is consistent with the conclusion that the presence of hydroxyl groups is essential for the photocatalytic degradation of toluene.In chapter three, the effects of surface modification with H2S and NH3on TiO2for the adsorption and photocatalytic degradation of gaseous toluene are studied. The TiO2nanoparticles are prepared by a sol-gel method using Ti(OC4H9)4as the precursor. Acidic/alkali modified TiO2is obtained by treating the as-prepared pure TiO2in H2S or NH3gas streams, respectively. The structures of these photocatalysts are characterized by means of XRD, BET, UV-vis and so on. The adsorption and photocatalytic degradation of gaseous toluene are investigated by using in-situ DRIFTS. Surface modification with H2S is observed to enhance the adsorption of toluene and promote the degradation rate at the start of photocatalytic degradation, while that with NH3inhibits the adsorption of toluene but enhances the photocatalytic activity for the toluene degradation. It is found that the surface modification with H2S causes the dissociative adsorption of H2S on the TiO2surface to satisfy the coordination of the surface Ti4+sites. The formed sulfhydryl groups are favorable for toluene adsorption by the interaction of the aromatic ring with sulfhydryl groups. The surface sulfates positively affect the preservation of surface hydroxyl groups and the separation of photogenerated electron-hole pairs but may negatively affect the regeneration of the surface hydroxyl groups. At the start of the reaction, the high adsorption ability, and inhibition ability of the recombination of electron-hole pairs of surface sulfates dominate the photocatalytic process and enhance the degradation rate. By contrast, in the latter part of the reaction, the inhibition of the regeneration of surface hydroxyl groups and poor generation of O2-·radicals lead to the accumulation of the highly stable intermediates, thus lowering the degradation efficiency. In the case of NH3modification, NH3is strongly adsorbed on Ti4+through the direct H3N…Ti bonding or adsorbed on the surface hydroxyl groups. Steric hindrance interferes the interaction of toluene with NH3or hydroxyl groups, thus decreasing the adsorption of toluene. The large amount of the adsorbed water molecules and hydroxyl groups on the surface might lead to the NH3modified TiO2exhibiting a good photocatalytic activity.In chapter four, we summarize the work of this dissertation and prospect the research which can be carried out subsequently.