多孔性TiO2對CO2光催化還原特性探討研究

Abstract

光催化還原CO2 是現今非常受重視的溫室效應氣體處理技術，藉由光觸媒的電子在 能帶間的躍遷與表面轉移，可將光能轉換為化學能，使CO2 還原為高化學能的有機化 合物，提供清靜能源技術的燃料來源，然而CO2 光還原的產物包括CO、HCOOH、 HCHO、CH4 及CH3OH，其中僅CH3OH 與CH4 為熱值較高的燃料，因此提升光催化還 原CO2 的效率與產物選擇性是眾所努力的目標。許多研究指出TiO2 光觸媒能有效地以 H2O 為電子供給者，將CO2 光催化還原成CH3OH 與CH4，而觸媒的比表面積與微結構 決定光觸媒催化活性與產物選擇性，因此本研究創新地將結合模版溶膠-凝膠法與表面 溶膠-凝膠法製備高活性與高產物選擇性的表面金屬摻雜TiO2 光觸媒。首先以共聚合物 在溶膠溶液中形成的微胞為模版，經凝膠過程形成TiO2 後以鍛燒方式移除模版即可得 到高比表面積的中孔洞TiO2，爾後再以表面溶膠-凝膠法，將不同金屬離子(Si4+、Cu2+、 Fe3+與V5+)摻雜在TiO2 表面數個分子層的位置即形成中孔洞表面摻雜TiO2。此方法製 備出的中孔洞結構能提供大比表面積，同時骨架內的晶粒大小為能表現最大光催化活性 的尺寸(10-14 nm)，而表面摻雜物能降低電荷再結合機會，因此能大幅提昇CO2 光催化 還原效率，另外，表面摻雜物造成的氧缺陷能使H2O 更有效率地產生H 自由基而提高 CH3OH 產率。研究中將比較不同金屬摻雜物對TiO2 對光催化動力影響，同時最佳化光 催化還原系統的操作條件，最後分析中間產物與光觸媒反應前後表面微結構與化學組成 變化，配合比較不同催化條件下CO2 還原速率與產物物種分佈，提出CO2 光催化還原 機制，並釐清不同摻雜物、反應物物種與物種間比例對CO2 光催化反應動力與選擇性 的影響。預期本研究將能針對光催化還原CO2 提出高活性與高選擇性的光觸媒化學組 成與微結構組合，瞭解材料特性控制CO2 光催化還原反應的機制，並建立永續環保的 再生能源系統。

Photocatalytic reduction of CO2 attracts large attention recently because of global warming issue. Photocatalysts have been demonstrated to effectively convert solar energy to chemical energy via electron transitions from valence band to conduction band followed by interfacial charge transfer to reactants. Photocatalysis can reduce inorganic CO2 to organic compounds with higher chemical potentials. Several products including CO、HCOOH、HCHO、CH4 及CH3OH are obtained after photocatalytic reduction of CO2. Among these compounds, only CH3OH and CH4 can be as the fuel for fuel cell because of substantial heat values. TiO2 has been demonstrated to efficiently reduce CO2 with H2O and selectively produce CH3OH. Surface area and microstructures dominate the high efficiency and selectivity of photocatalyts. Previous study anchored Ti4+ within micropores of zeolite to well disperse TiO2 and maintain high surface area for high reduction efficiency of CO2. Moreover, tetrahedrally coordinated Ti4+ is responsible for the selectively formation of CH3OH. However, this method limits the loading of TiO2 as well as its photoactivities. In this study, we prepare advanced porous Mn+/TiO2 (Mn+: Si4+, V5+, Fe3+ and Cu2+) with surface doping of metal ions by a templating sol-gel and a surface sol-gel method. The large surface area, adequate crystallite sizes (10-14 nm) and surface defects of the mesoporous Mn+/TiO2 provide high photoactivities toward CO2 reduction. Moreover, the dopants introduce oxygen vacancies and reduce the coordination numbers of Ti4+ enhance the selectivity to CH3OH. Systematical study of CO2 photoreduction in the presence of different Mn+/TiO2 indicates the different contribution of surface dopants on the photoactivity and selectivity. The physicochemical properties including charge lifetime, interfacial charge transfer rate, bandgaps and adsorption behavior are determined to elucidate the photocatalytic reduction behavior with respect to microstructures. Moreover, the parameters in the photoreduction system are optimized. The surface properties of the pjotocatalyts after photocatalysis are thoroughly examined. Base on these results, we propose the mechanism of CO2 reduction on the surface of TiO2. Results in this study not only demonstrate a novel method for preparation of surface doped TiO2 with high photoactivity and selectivity for CO2 reduction, but also clearly clarify the microstructure controlled photocatalytic reactions. Moreover, we provide an alternative promising way for renewable carbon cycle for sustainable environmental protection.