二氧化鈦表面摻雜鎢光催化還原二氧化碳特性探討

Abstract

本研究以表面摻雜鎢的二氧化鈦(W-TiO2)進行光催化還原CO2反應，探討材料特性(表面結構、離子價態，元素比例、元素分布、電子轉移)與光催化還原CO2活性，並比較氣相與液相系統的還原特性，最後為了解光催化還原CO2反應機制，而進行光觸媒表面官能基分析。從材料鑑定結果顯示，W均勻分布於TiO2上，並以六價形式存在，在粒徑大小上因W作為不純物在TiO2表面上使鍛燒時顆粒間無法縮合形成大顆粒，並且W能在TiO2表面能隙導入兩個未填滿能階，驅使電子移動至表面並捕捉電子降低電子電洞對再結合機率。以TiO2與W-TiO2進行氣相光催化還原CO2反應，甲烷為主要產物，不同表面摻雜W/Ti比例皆在第一小時有最佳甲烷產率，並且在表面摻雜W/Ti為3.2 at.%W-TiO2時，有最佳產量0.84 μmol/g，是TiO2的2.6倍。液相反應系統中，除主要產物甲烷外，還有次要產物乙烯，另外因CO2水溶解度低、大量水分子阻礙CO2與光觸媒接觸，因此使液相系統總碳量約小於氣相系統的2倍。由EPR與DRIFT的分析中可知，在光催化反應過程中CO2會先形成單牙基與雙牙基碳酸錯合吸附在W-TiO2表面，而光激發電子會從強電負度的W上進行界面轉移至吸附的碳酸根離子，爾後C-O鍵結逐漸被打斷且進行氫化反應，最後因W強電負度的特性使表面C-O斷鍵而生成甲烷。W離子的強電負度促使CO2鍵結於W-TiO2表面上進行電子轉移，並增加表面酸性而益於吸附H2O，而促進光催化還原CO2活性。

In the study, W ion were incorporated into the surface lattice of TiO2 photocatalysts to improve the activity for CO2 reduction. Photocatalytic activity and byproducts in the gaseous and aqueous phases were measured and characterized. Material properties, including microstructures, bandgaps, chemical compositions, chemical states of W ions, and interfacial charge transfer behaviors were characterized. Moreover, photoreduction mechanism of CO2 were proposed based on charge transfer pathways and changes in the surface functional groups during photocatalysis. Sol-gel-derived W-TiO2 contained hexavalent W ions which distributed evenly in surface lattice. The lattice defects retarded crystallization and increased surface areas. In addition, they introduced two unoccupied energy states in the band gap in the surface lattice which promote charge diffusion from the bulk to the surface and trap the charge carriers to suppress recombination. CH4 was the major product in the gas phase reaction system. The W-TiO2 photocatalyst with a surface W/Ti ratio of 0.032 had the highest CH4 yield of 0.84 μmol/g after 4-hour irradiation. The reduction efficiency was 2.6 times higher than that of unmodified TiO2 powders. In the aqueous phase, C2H4 were formed in addition to CH4 as the minor by-product. Due to low solubility of CO2 in water and competitive adsorption of water molecules, total carbon transformation in the aqueous system was 2 times lower than that in the gas phase system. Mechanisms for CO2 reduction were proposed according to EPR and DRIFT analysis. CO2 molecules transformed into m-CO32- and b-CO32- species and then chelated to the W-TiO2 surface for the first step. The carbonate species constantly received electrons through the doped W ions and the H+ ions from water molecules to generate C-H bonds. Due to strong W-O bonding, the last protonation of :CH3 generated CH4. Their high electron negativity not only allows the W6+ ions to strongly bind the carbonate species to facilitate charge transfer, but also increase surface acidity to enhance water adsorption, thus effectively promoting the photocatalytic activity of TiO2 .