二氧化鈦表面特性對光催化還原二氧化碳反應活性探討

Abstract

本研究利用溶膠-凝膠法製備以二氧化鈦(TiO2)為基材的光觸媒,仿照綠色植物 進行人工光合作用還原 CO2 產生 CH4,並釐清材料表面性質(如、氫氧官能基密度與鐵 修飾濃度)與環境因子(如、分散方式以及光強度)對於還原活性之影響。研究結果指 出 Thiele modulus 於本系統中介於 0.012 - 0.019,顯示速率限制步驟為化學反應,而可 忽略掉質傳上的影響。負載方式以直接分散法,具有最佳 CH4 產生量(1.52 μmol g-1), 而對於TiO2經過含水的氧氣前處理後,表面氫氧官能基密度由3.7提升至7.2 #OH nm-2, 初始反應速率則由 0.39 μmol g-1 h-1 提升至 1.19 μmol g-1 h-1,還原活性提升 3.1 倍,來自 於 TiO2 表面經光誘導後變為超親水性質與去除表面積碳所致。另種改善表面性質的方 式,是利用 0.1 at.%鐵表面修飾的 Fe-TiO2 光觸媒,在表面氫氧官能基密度為 11.6 #OH nm-2 時,初始反應速率達到 1.72 μmol g-1 h-1,其量子效率為 6.8%,相較單純 TiO2 光催 化還原活性提升 1.5 倍(1.13 μmol g-1 h-1)。而表面鐵摻雜量達 40 at.%時,最高甲烷產 量(3.35 μmol g-1)相較於 0.1 at.%狀況,提升了 1.9 倍的 CO2 還原活性。而對於釐清電 荷利用率與再結合速率,將光觸媒於不同光強度測試中,得到最佳光強度操作闕值位於 147μW cm-2。

In this study, the photocatalytic behavior of pure and Fe-surface-doped TiO2 photocatalsts for CO2 reduction is carried out under different loading types of the catalysts, humidity, and light intensity to clarify the contributions of doped Fe ions and density of hydroxyl groups to the photocatalytic activity. Direct dispersion of the photocatalysts in the reactor showed the highest CH4 yield for the gaseous system. The Thiele modulus in the range of 0.012-0.019 indicates the surface reaction, rather than mass diffusion, determines the kinetics. Pre-treatment of the catalysts with UV irradiation in the presence of O2 and H2O vapor increased surface density of hydroxyl groups, thus raising the reductive activity by 3.1 times. Incorporation of Fe ion in the surface lattice further improved the reductive activity in the humified CO2 atmosphere by 1.5 times due to enhanced surface acidity, though it reduced the activity a bit in the absence of H2O vapor. The optimal light intensity for the most efficient reduction is 147 μW cm-2. The highest quantum efficiency of 6.8 % was achieved when 0.1 at.% Fe-doped TiO2 was irradiated with UV light under the optimal light intensity and in the presence of water vapor. The highest accumulated CH4 yield of 3.35 μmol g-1 was produced by 40.0 at.% Fe-doped TiO2 after 4-hour irradiation. The lower reduction potential of the holes resulting from the segregated iron oxide clusters retard the oxidation of the products.