表面修飾TiO2光觸媒應用於氣相與水溶液相光催化還原CO2特性探討

Abstract

本研究成功以溶膠-凝膠法製備表面摻雜二氧化鈦(TiO2)，研究中除探討銅(Cu)、鐵(Fe)以及釩(V)離子對於表面摻雜二氧化鈦(CT,FT,VT)的特性差異(元素比例、元素分布、載子分離效率及表面特性)以及光催化還原二氧化碳(CO2)特性的影響外，並比較氣相及水溶液相還原系統在還原特性上的差異，最後為了改善水溶液相系統的效率，探討系統促進劑—溴化鉀(KBr)與還原態氧化石墨烯(rGO)—對於光催化還原 CO2 效率改善的影 響。元素分析結果顯示金屬離子主要分布在 TiO2 表層晶格中，濃度比例(M/Ti, M=摻雜離子)為 0.71-1.63 %，受金屬離子向外擴散程度的影響，VT 樣品呈現較高表面金屬摻雜量。VT 觸媒在三種摻雜觸媒中具有最好的還原效率，其 CH4 產率(1.3 µ mol/g)在氣相系統中更高於 TiO2 的兩倍，V5+離子在表面抑制載子再結合，且導入較高的表面酸度，是呈現高活性的原因。氣相系統中的還原產物以甲烷(CH4)為主，而水溶液相中則除了 CH4，還有乙烯(C2H4)、乙烷(C2H6)以及丙烯(C3H6)的存在，雖物種較多，然水溶液系統的還原總產率卻不如氣相系統，原因為水中 CO2 濃度僅為氣相系統濃度的一半，降低 CO2 參與還原反應的機率，另外，反應初期生成的產物受到周圍水分子的籠效應影響佔據觸媒表面活性位置，也造成反應速率低下。系統中存在溴離子與 rGO 時，產物生成率均受到抑制，雖然兩者能去除強氧化性的氫氧自由基(‧OH)，但卻都因為影響 CO2 的吸附而造成界面電荷轉移效率降低，因而對系統造成負面的影響。

In this study, Cu-, Fe-, and V-surface-doped TiO2 photocatalysts have been prepared with a sol-gel method for CO2 reduction. The effects of the three types of dopants on surface and physicochemical properties and the photocatalytic activity for CO2 reduction in gaseous and aqueous systems were explicitly characterized and elucidated. In addition, the reduction in the presence of KBr or rGO, which were used as radical scavengers, in the aqueous phase was examined. Elemental analysis results indicate that the doped ions mainly distributed in the surface lattice with the concentrations of 0.71-1.63 at%. Due to low diffusion energy, V ions easily underwent outward migration during calcination and led to the highest accumulation at the surface of the doped TiO2 powders. The V-doped TiO2 exhibited the highest activity over the Cu- and Fe-doped photocatalysts for CO2 reduction. In gaseous phase, its methane yield (1.3 µ mol/g) was even twice as high as that by the pure TiO2 sample. The merits of V ions on the improved activity are mainly associated to inhibited charge recombination and increased quantity of Bronsted sites. While methane was the only detectable product in gaseous phase, short chained hydrocarbons, including methane, ethane, ethylene, and propene, were obtained in the aqueous system. Although more species were produced in aqueous phase, the product yield in the aqueous phase was lower than the gaseous system. The limited activity is ascribed to low solubility of CO2 in water and the block of active sites by products due to solvent-cage effect. In the presence of KBr or rGO, product yields declined. These two species hindered charge transfer between the photocataysts and CO2 molecules, thus causing adverse effect to the system.