摻雜銅離子於二氧化鈦表面對光催化還原二氧化碳特性探討

Abstract

本研究利用溶膠-凝膠法製備表面摻雜銅離子之二氧化鈦光觸媒材料以進行光催化還原CO2產CH4的研究，研究中將分別探討Cu-TiO2的材料性質(包括Cu離子價態及表面摻雜厚度)與光催化系統條件(包括環境氣氛及光源)對光催化活性之影響。研究結果指出1 at.% Cu-TiO2在第一小時的CH4產率（1.28 µmolg-1）是P-25的3.6倍，而CH4產率在10 at.% Cu-TiO2系統中則隨時間緩升，四小時後的總產率是第一小時的10倍，ESCA結果顯示Cu離子在1 at.%及5 at.% Cu-TiO2¬樣品中以Cu+為主，而在10 at.%及20 at.% Cu-TiO2中則以Cu2+為主，兩種不同Cu物種型態造成不同還原動力特性，另外表面摻雜厚度為60 nm時觸媒有最好的CH4產率，低於此厚度，表面Cu離子含量不足以有效抑制電荷再結合，超過此厚度，捕捉的電子不易傳遞至表面而降低電荷利用率。環境氣氛調控下發現，H2O及CO2彼此間處於競爭狀態，讓兩者同時佔據表面反應位置會有最高反應效率。在激發光源上我們發現以UVB光源活化光觸媒在強度為40 μW cm-2時有最好光催化活性，由於電荷利用效率遠低於產生速率，過多表面電荷將導致再結合速率上升，因此CH4產率並不隨著光強度的增強而提升。藉由DRIFT分析還原過程中化學鍵結變化的結果指出，CO2於表面接受電子後先並形成CO2-，接著質子化產生CH4或先形成碳酸鹽類再形成CH4。

In this study, we doped Cu ions into TiO2 surface lattice through a sol-gel method and investigated the photocatalytic behavior of the Cu-TiO2 photocatalysts for reduction of CO2 into CH4. The effects of doping concentrations, doping thicknesses, and operational conditions on the reductive kinetics were examined. In addition, the types of dopants and the surface speciation were characterized to clarify the roles of Cu ions in the reductive kinetics and mechanisms. Results showed that the 1 at.% Cu-TiO2 powders produced 1.28 µmolg-1 CH4 in the first hour, which was 3.6 times higher than the yield by P-25. The 10 at.% Cu-TiO2 sample was less photoactive in the beginning, but progressively generated CH4 with the irradiation time. The total yield after irradiating for four hours was ten times higher than that within the first hour. ESCA analysis indicates that Cu+ ions were predominant in the 1 at.% and 5 at.% Cu-TiO2 samples, while Cu2+ ions were the majority in the 10 at.% and 20 at.% Cu-TiO2 samples. The two types of the Cu ions and their distributions in the surface lattice lead to the different reductive behaviors. The optimal doping thickness for the highest reductive rate was 60 nm. Lower than the thickness, the Cu loading is insufficient to effectively inhibit charge recombination. On the other hand, the trapped charge carriers in the deeper surface are unable to tunnel to the adsorbed species when the doping thickness is higher than the critical value, thus reducing the activity. CO2 and H2O molecules in the reductive system compete to occupy the surface active sites. Simultaneous adsorption of the two reactants performed the highest reductive efficiency. Irradiation with UVB light with 40 μWcm-2 led to the highest CH4 yield. Because the interfacial charge transfer was inefficient, higher light intensity did not enhance the reducing rate. In-situ DRIFT monitoring reveals that CO2 molecules receive electrons to form CO2- species after adsorption. The CO2- species is then converted into CH4 through protonation, or turned into carbonates species followed by transformation toward CH4.