Studies On Gas-Phase Photocatalytic Oxidation Of Benzene Over Titanium Dioxide Based Catalysts

Air pollution by Volatile organic counds (VOCs) has drawn considerable attention in recent years. Among the technologies developed for the treatment of VOCs, the photocatalytic oxidation process is considered to be a promising technology. Aromatic hydrocarbons, especially benzene, are typical pollutants emitted from numerous urban and industrial sources and difficult to degrade due to their stability. VOCs removal from air by photocatalytic oxidation techniques has been paid much more attention to in recently. However, the deactivation of the titanium dioxide catalyst during gas-solid photocatalytic oxidation of air contaminated by VOCs is a serious issue.The decomposition of VOCs has been difficult because of the low conversion and the common deactivation of photocatalyst. Therefore it is crucial to prolong the lifetime of photocatalyst and enhance its photoactivity. In this thesis, a series of novel photocatalysts by modifying TiO₂ with silane coupling agent and loading of TiO₂ onto the fluorescent material Sr2CeO4 and CeO2-ZrO2 were synthesized, and their activity with benzene as the model gas in a batch reactor was investigated. The main contents and results in this thesis can be summarized as follow:1. Nanosized titanium dioxide powder was prepared by Sol– Gel methods, Hydrothermal methods and Hydrolysis methods from tetrabutyl titanate. Sol-Gel method itself is simple and can be easily manipulated .TiO₂ prepared by Sol-Gel method possesses the smallest particle size and the highest specific surface area. TiO₂ is anatase after annealing at 500℃for 2h and is rutile after annealing at 600℃for 2h.The results show that TiO₂ prepared by Sol-Gel method can be excited by visible light, binding energy of Ti2p is the lowest and the conversion of benzene is the highest on TiO₂ prepared by Sol-Gel method, but the lifetime of the catalyst is short. Therefore it is crucial to prolong the lifetime of photocatalyst and enhance its photocatalytic activity.2.Photocatalytic degradation of benzene on pure TiO₂ matches Langmuir-Hinshelwood (L-H) kinetic model,the value of k and K over 0.2g pure TiO₂ are 0.0064mgL-1min-1 and 4.6719 Lmg-1 respectively.The intermediates of photocatalytic oxidation of benzene over TiO₂ are : (1) butylated hydroxytoluene (2) (3-methyl-oxiran-2-yl)-methanol (3) ethyl acetate (4) dibutyl phthalate (5)2,6-bis(1,1-dimethylethyl)-4,4-dimethylycyclohe(6)2,5-cyclohexadiene-1,4,dione,2,6-bis(1,1-dim). It is plausible that at least one of these less-reactive intermediates causes the deactivation of the photocatalyst. The gas-phase photocatalytic oxidation of benzene has different reaction mechanisms under different reaction conditions.3.The photoactivity of different TiO₂ loadings on Sr2CeO4 was investigated. The results show that the highest efficiency of 65.0% is obtained when the TiO₂ loading amount is 1.0%, the photocatalytic activity of TiO₂/Sr2CeO4 is 1.74 times of that of TiO₂ and the lifetime of TiO₂/Sr2CeO4 is 2.0 times of that of TiO₂.The effect of light source, water and Fe3+ doped into TiO₂ on conversion of benzene were investigated. The conversion of benzene under visible light is only 15.3%,the optimum water in the reactor is 50mg/L and the concertration of Fe3+ doped into TiO₂ is 1.0%(atom)4. The characterization results of XRD indicate that TiO₂ loaded on Sr2CeO4 is anatase at 773K firing temperature. TiO₂/Sr2CeO4 absorbs much more visible light than TiO₂ in the visible light region. The XPS spectrum show that the binding energy value of Ti2p3/2 transfers to a lower value,so it is obvious that there is stronger interaction between Sr2CeO4 and TiO₂, which may promote the catalytic activity of TiO₂. Photocatalytic degradation of benzene on 20g TiO₂/Sr2CeO4 matches Langmuir-Hinshelwood (L-H) kinetic model, the value of k and K are 0.0064mgl-1min-1 and 9.2078lmg-1 respectively. The chemical species adsorbed on the surface of deactivated photocatalyst were scanned by a FT-IR spectroscopy, the results show there are C=O bonding and OH bonding on the surface. Qualitative analysis of the chemical species adsorbed on the surface of deactivated TiO₂/Sr2CeO4 catalyst samples was carried on the 6890N GC-5973 MS. The detected compounds included: (1)2,6-bis(1,1-dimethylethyl)-4,4-dimethylycyclohe(2)ethyl acetate (3)2,5-cyclohexadiene-1,4,dione,2,6-bis(1,1-dim) (4) butylated hydroxytoluene (5)(3-methyl-oxiran-2-yl)-methanol. It is plausible that at least one of these less-reactive intermediates causes the deactivation of the photocatalyst. The gas-phase photocatalytic oxidation of benzene has different reaction mechanisms under different reaction conditions.The presence of Sr2CeO4 can prolong the life of electron-hole pairs and inhibit the recombination of electron-hole pairs. The synergy of electron transfer and the stronger absorption in the region 400–850 nm promotes the catalytic activity of TiO₂/Sr2CeO4.5.The FT-IR characterization result indicates that the pure TiO₂ has been modified by silane coupling agent (labeled as M-TiO₂) and pure TiO₂ has been successfully encapsulated by methylmethacrylate(labeled as P-TiO₂). The XPS spectrum show that there are Ti,O,C and Si elements on the surface of the M-TiO₂, there are Ti,O,C elements on the surface of the P-TiO₂.Only Ti2p3/2 was detected on the surface of the P-TiO₂, but both Ti2p3/2 of two catalysts shift to a lower value. Uv-Vis diffuse reflective spectra between M-TiO₂ and TiO₂ have no obvious difference, compared to the UV-vis diffuse reflective spectra of pure TiO₂, spectra of P-TiO₂ appears red-shift and has strong absorption in visible light.6.The photocatalytic activity of M-TiO₂ and P- TiO₂ is more than1.0 times of that of TiO₂ and the lifetime of M-TiO₂ and P- TiO₂ is 2.0 times of that of TiO₂. M-TiO₂ is not stable and can be degraded, while P-TiO₂ is stable.The ester group on theγ-MPS molecule can prolong the life of electron-hole pairs and inhibit the recombination of electron-hole pairs, which promotes the catalytic activity of TiO₂.7.The BET surface areas of TiO₂/CexZr1-xO₂ decrease, the band gap and Ti2p3/2 decrease and then increase as the Zr amount increasing. TiO₂/Ce0.5Zr0.5O₂ has the highest photocatalytic activity. The BET surface areas of TiO₂/Ce0.5-xZr0.5-xMn2xOy decrease, the UV-vis diffuse reflective spectra of TiO₂/Ce0.5-xZr0.5-xMn2xOy appears red-shift and the degree of response to the light increases as the Mn amount increasing. The binding energy value of Ti2p3/2 of TiO₂/Ce0.475Zr0.475Mn0.05Oy transfers to the lowest value, TiO₂/Ce0.475Zr0.475Mn0.05Oy has the highest photocatalytic activity.The BET surface areas of TiO₂/Ce0.5-xZr0.5-xLa2xOy decrease, the particle sizes of TiO₂/Ce0.5-xZr0.5-xLa2xOy increase as the La amount increasing. The Uv-Vis diffuse reflective spectra of TiO₂/Ce0.5-xZr0.5-xLa2xOy appears red-shift, the binding energy value of Ti2p3/2 of TiO₂/Ce0.5-xZr0.5-xLa2xOy transfers to the lowest value, TiO₂/Ce0.5-xZr0.5-xLa2xOy has the highest photocatalytic activity.There is no obvious difference among the BET surface areas of TiO₂/Ce0.5-xZr0.5-xY2xOy as the Y amount increasing. The UV-vis diffuse reflective spectra of TiO₂/Ce0.5-xZr0.5-xY2xOy appear red-shift. The binding energy values of Ti2p3/2 of TiO₂/Ce0.5Zr0.5O₂, TiO₂/Ce0.475Zr0.475Y0.05Oy and TiO₂/Ce0.45Zr0.45Y0.10Oy transfer to the lowest value, TiO₂/Ce0.45Zr0.45Y0.10Oy has the highest photocatalytic activity.The role of CeO₂-ZrO₂ in the photocatalytic reaction has been speculated. The synergy of electron transfer, the stronger absorption in violet regions and shift of binding energy of Ti2p and O (1s) promote the photocatalytic activity of TiO₂/CeO₂-ZrO₂.