鋯離子摻雜與金沉積對中孔洞二氧化鈦微結構與光催化還原二氧化碳研究

Abstract

近年來全球暖化的影響日趨嚴重，光催化還原CO2被視為最理想的解決技術之一，其靈感來於自然界植物的光合作用，不但處理溫室氣體的同時，也提供可利用的碳氫化合物做為能源。本研究成功利用蒸發誘導自組裝法(evaporation-induced self-assembly)合成中孔洞鋯離子摻雜二氧化鈦，並另以沉積沉澱法(deposition-precipitation)在材料表面沉積奈米金顆粒。此中孔洞材料以三區段共聚高分子(triblock copolymer)作為孔洞模板，具有高的比表面積(103-217 m2 g-1)及較集中的孔徑分部。分析結果顯示，鋯摻雜濃度決定鋯離子在二氧化鈦結構中的分佈，以致影響二氧化鈦結構與物化特性，當Zr/Ti整體元素比為0.02-0.04時，鋯離子傾向摻雜於TiO2晶粒表面，除提高孔洞熱穩定性外，也使TiO2晶粒由9.6nm增加至11.6 nm，能隙由3.09提高至3.15 eV，然而鋯離子在高濃度時傾向摻雜於內部晶格，除抑制TiO2結晶外，也致使光催化氧化RhB的活性變差，當Zr/Ti比例為0.03與Au負載量為1.0 wt.%時，TiO2有最高的光催化氧化活性。CO2光催化還原實驗以水氣作為還原劑，以批次反應槽中進行，而甲烷為反應唯一的偵測產物。相較於修飾的光觸媒，單純二氧化鈦擁有較高的還原活性，於第一小時可產生0.73 μmole g-1甲烷量，並在第四小時達最高累積量1.03 μmole g-1，隨後甲烷氧化速率提升，於第八小時降低為0.45 μmole g-1。摻雜鋯離子與沈積Au奈米顆粒雖使TiO2初始甲烷產率降低為0.23與0.33 mole g-1，然而卻抑制CH4被氧化的逆反應速率，經8小時反應後，Zr-doped TiO2(Zr/Ti=0.03)與Au-TiO2樣品累積甲烷量可分別達0.81與0.54 μmole g-1。EPR結果顯示電荷能有效於觸媒表面轉移至CO2與H2O，因此產生難還原的中間產物是導致低還原效率的原因，Au奈米顆粒為電荷再結合的媒介，對光催化反應會造成負面影響，而Zr4+摻雜導入的缺陷能階決定其反應活性與逆反應速率。

Photocatalytic reduction of CO2 that mimics natural photosynthesis is a promising technology to both reduces the greenhouse gas emissions and provides alternative energy sources. In this study, mesoporous TiO2 and Zr-doped TiO2 photocatalysts were successfully synthesized using an EISA process. In addition, Au nanoparticles were loaded through a deposition-precipitation (DP) method. These mesostructured materials possess large surface areas of 103-217m2 g-1 and narrow pore size distributions. The concentration of Zr4+ ions determines the distribution of the doped ions in the TiO2 matrix, so as the microstructures and physicochemical properties. When the Zr/Ti ratio was in the range of 0.02-0.04, the Zr4+ ions were doped within the boundaries. As the result, the thermal stability of the porous structure was improved. In addition, the crystallite size of the TiO2 increased from 9.6 to 11.6 nm, and the corresponding bandgap increased from 3.09 to 3.15 eV. When the Zr/Ti ratio was over 0.05, the Zr4+ ions tend to be doped within the TiO2 lattice, thus inhibiting crystallization and photocatalytic activity of the doped TiO2 samples. The TiO2 samples exhibited the highest activity for RhB degradation when the Zr/Ti ratio and Au-loading were 0.03 and 1.0 wt.%, respectively. Photoreduction of CO2 with water vapor was carried out in a batch system. CH4 was the only detectable product in the reduction. The pure TiO2 exhibited the highest activity over the modified samples. It generated 0.45 μmole g-1 CH4 in the first hour, while the Zr-doped TiO2 (Zr/Ti= 0.03) and 1.0 wt.% Au-TiO2 produced 0.23 and 0.33 μmole g-1, respectively. The pure TiO2 reached to the highest CH4 yield of 1.03 μmole g-1 at 4th hour. The yield subsequently reduced to 0.45 μmole g-1 at the 8th hour because of increased reoxidation rate. The reoxidation of CH4 was suppressed by the Zr-doped and Au-loaded TiO2 samples, which resulted in 0.81 and 0.54 μmole g-1 CH4 after 8 hr irradiation. EPR results show that interfacial charge transfer from the catalysts to the adsorbed CO2 and water is prompt. The formation of the intermediates which have high reductive barriers determines the low reduction efficiency. The Au nanoparticles serve as the mediator to promote charge recombination, thus are detrimental for the photocatalytic activity. On the other hand, the impurities energy levels introduced by the doped Zr4+ ions within the bandgap dominate the reductive activity of the doped TiO2 and reoxidation rate of CH4.