Preparation, Structure, Photocatalytic Performace And Mechanism Of Photocatalytic Degradation Of Volatile Organic Compounds (VOCs) Over TiO₂-based Photocatalysts

This dissertation titled"Preparation, structure, photocatalytic performace and mechanism of photocatalytic degradation of volatile organic compounds (VOCs) over TiO₂-based photocatalysts"consists of four chapters. The first chapter gives a literature review. Chapter two describes the photocatalytic degradation mechanism of gaseous formaldehyde over TiO₂ using the in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) and chapter three focuses on the preparation, performance and reaction kinetics of M-TiO₂ (M=Fe, Co, Ag, Cu) for photocatalytic degradation of toluene and o-xylene under visible light irradiation. The last chapter is the study of the effect of Fe doping on the structure and photocatalytic performance of TiO₂ for photocatalytic degradation of gaseous toluene under visible light irradiation. Beside it, the stability and regeneration of photocatalyst is also investigated.Chapter one reviews the literatures on the related backgrounds knowledge and research progresses. At first, a brief introduction to the properties, classes, sources and harmful effects of VOCs is given. The contamination control and treatment methods of VOCs are also summarized. Secondly, the research progresses of photocatalytic degradation of VOCs, especially the TiO₂-based photocatalysts, are introduced. The influencing factors and existing problems are also discussed. Thirdly, the studies on the kinetics and mechanism of photocatalysis reaction in literatures are reviewed. The applications of in-situ DRIFTS are introduced in detail. Finally, the research proposal of this dissertation is presented.In chapter two, the reaction mechsnism of photocalytic degradation of gaseous formaldehyde over TiO₂ is studied by in-situ DRIFTS. It is found that the formaldehyde molecules can be adsorbed on the hydroxyl groups on the TiO₂ surface via hydrogen bonding. With UV irradiation, the adsorbed formaldehyde rapidly converts to the formate species even on the pure TiO₂ at room temperature and atmospheric pressure. In the dry environment, the superoxide radical anion O2-?, formed by adsorbed oxygen reacting with electrons, is suggested to play an important role in the formaldehyde oxidation. The introduction of water vapor provides a large amount of water and hydroxyl groups on the catalyst surface. Oxidation of water and hydroxyl groups by the photogenerated holes produces the very active OH·radicals, which take part in redox reactions and improves significantly the mineralization rate of formaldehyde on the TiO₂ due to its high redox potential (2.8 v).In chapter three, the preparation, performance and reaction kinetics of M-TiO₂ (M=Fe, Co, Ag, Cu) for photocatalytic degradation of gaseous toluene and o-xylene under visible light irradiation are studied. The nanoscale M-TiO₂ with the dosage level of 0.2%, 0.5% and 1.0% are prepared by a so-gel method using glacial acetic acid as chelating agent and Ti(OC4H9)4 as precursor. The catalyst structures are characterized by means of XRD, BET and UV-vis DRS. All the samples are crystallized in pure anatase phase after calcining at 480 oC for 3 h. Compared with the pure TiO₂, the M-TiO₂ series photocatalysts have larger surface area and visible light response. The humidity has a positive effect on the photocatalytic oxidation of toluene and o-xylene over the M-TiO₂. The Fe-TiO₂ shows betst activity among M-TiO₂ samples. The photocatalytic process follows the typical of Langmuir-Hinshelwood model of first-order reaction. The apparent rate constant is calculated according to the kinetic model. Beside it, the reaction pathways of toluene and o-xylene photo-degradation over the photocatalyst is proposed based on the results of in-situ DRIFTS.In chapter four, the effect of doped Fe on the structure and photocatalytic performance of TiO₂, as well as the stability and regeneration of photocatalyst are studied. The synthesized Fe-TiO₂ samples have the nanocrystalline mesoporous-assembled structure with a narrow pore size distribution of 3-5 nm, large pore volume and high surface area. The incorporated Fe3+ substitutes the octahedrally coordinated Ti4+ in the TiO₂ lattice to extend the absorption of TiO₂ to visible light region and promote the formation of electron-hole pair. More importantly, the doping of Fe decreases the recombination rate of electron-hole and increases the separation and transportation efficiency because the Fe3+ incorporated into the TiO₂ lattice can act as an electron-trap agent. The maximum electron-hole separation efficiency arises at Fe/Ti ratio of 0.7%. At higher Fe concentration, the electron-hole separation efficiency decreases remarkably. The reason may be attributed to that the Fe3+ begin to aggregate at high concentration, which acts as the recombination centers for the electrons and holes. The Fe-TiO₂ shows excellent photocatalytic performance for toluene degradation under visible-light irradiation. The optimal Fe/Ti ratio is 0.7%. Partial deactivation of the photocatalytic activity was observed after 20 consecutive reaction runs. From the in-situ DRIFTS experiment, the deactivation can be attributed to the formation of stable intermediates, such as benzaldehyde and benzoic acid, which occupied the active sites on the surface of the photocatalyst. The adsorbed benzaldehyde and benzoic acid could be removed with a heat treatment at 380℃for 3 h and the deactivated photocatalyst can be regenerated completely.