運用孔洞光觸媒複合材料光催化還原乙醇胺水溶液中之二氧化碳研究

Abstract

本論文之研究動機在於現今CO2捕獲與再利用技術(Carbon dioxide Capture, Storage and Utilization, CCSU)中，尚未開發出足以改善CO2經乙醇胺化學吸收法捕獲後，從乙醇胺中分離CO2需耗費大量能源之問題，然而乙醇胺又為CCS技術中最普遍被探討之吸收劑，因此本研究擬利用光催化法來開發出能結合CO2吸收及再利用之技術，以解決現今於CCS技術中所面臨之問題。本研究首先探討以乙醇胺作為CO2吸收劑兼光催化還原劑之可行性，並與光催化還原CO2研究中常用之還原劑進行CO2還原效率比較，其目標還原產物為甲烷(Methane, CH¬4)，並同時探討反應後產生的總可燃有機化合物(total combustible organic compounds, TCOCs)之產量。後續並研究實驗光觸媒於不同操作及製備參數下光催化還原CO2之效率比較，以尋求最適操作觸媒與其製備條件，最後評估光觸媒在可見光下反應的可行性及其光利用率。 本研究結果顯示，乙醇胺可以同時作為CO2吸收劑兼光催化還原劑，且相較於文獻中常用之還原劑有更高的CO2還原效率。於首批探討之複合式高比表面積光觸媒(Ti-MCM-41)測試實驗中，結果顯示在不同Si/Ti莫耳比之Ti-MCM-41(X)實驗中，Si/Ti莫耳比為50之Ti-MCM-41(50)有最佳的CO2還原效率，其甲烷產量在經9瓦紫外光源(波長為254nm)照射8小時後可達62.42μmol/g，而一氧化碳產量為27.65μmol/g。然而因Ti-MCM-41的孔徑僅2~3nm，導致Ti-MCM-41之應用受限於只能利用低波長之光源進行反應。 因此本研究再探討具管狀型態之高比表面積光觸媒二氧化鈦奈米管(titania nanotubes, TNTs)，並結合金屬改質法摻雜金屬鉬(Mo)於TNTs表面。其結果顯示，以二次含浸之製備條件下所得之Mo-T-500(pH=3)有最佳的CO2還原效率，其甲烷產量在經8瓦紫外光源(波長為365nm)照射反應6小時後產量可達0.52μmol/g，其一氧化碳與總可燃有機化合物產量分別為10.41μmol/g與13.53μmol/g。並在一般8瓦日光燈的照射下光催化還原CO2亦有其可行性，在反應24小時後總可燃有機化合物已達到10.29μmol/g。 此外從光觸媒特性分析結果發現，鍛燒溫度對於Mo-TNTs的重要性，當鍛燒溫度控制在500℃時，Mo-TNTs的管狀結構崩解轉換成銳鈦礦顆粒，使得表面的Mo6+再一次還原成Mo5+，造成Mo-TNTs表面的Mo產生不同價態間的電荷轉移躍遷作用(intervalence charge transfer, IVCT)，而生成氧空缺位置(oxygen vacancy sites)。研究結果顯示，Mo摻雜以及氧空缺(oxygen vacancies)可能是掌握CO2光催化還原效率的關鍵因素。因此於研究中研擬出在不同鍛燒溫度下Mo-TNTs可能的結構轉變機制，以及光催化還原CO2與Mo-TNTs表面之氧空缺位置的反應機制。

This study intends to propose and study a technology which combines CO2 capture and utilization (CCU) by using ethanolamine (MEA) as the CO2 absorbent and reductant in the solution. To explore the feasibility of ethanolamine as the CO2 absorbent and photocatalytic reducing agent, the CO2 reduction efficiency was compared with commonly used reducing agents in the literature. Photocatalysts with high specific surface area were then prepared and tested under different operating parameters including the light wavelenths from UV to visible light. The CO2 reduction efficiency, photo-reduction quantum efficiency (PQE), and the possible reaction mechanisms were proposed in this study. The innovative results of this study include the prove of MEA to be the best absorbent/reductant as compared to the NaOH and H2O solution. Therefore, MEA solution is employed in this study for studying the photocatalytic reduction of CO2 to form valuable energy source of methane (CH4) and the total combustible organic compounds (TCOCs) at different light spectra of UV light sources (254, 365 nm) and solar concentrator as well. The results showed that methane yields of the modified photocatalysts of Ti-MCM-41(X) and Mo-TNTs were better than the pure TiO2 when irradiated under both UV and visible light sources. The 8 hours test results showed that the best metal photocatalyst at 254nm was Ti-MCM-41(50), which has the methane yield of 62.42μmol/g and the carbon monoxide yield of 27.65μmol/g under 32μW/cm2 light intensity. And the best photocatalyst was Mo-T-500 under 365nm UV light, which material was prepared under pH=3. Its methane production rate was 0.52μmol/g with the light intensity of 63μW/cm2. And the other products of carbon monoxide and TCOCs yields were 10.41μmol/g and 13.53μmol/g, respectively. In addition, the Mo-T-500 was also tested for its long-term (24hrs) stability under visible light condition (fluorescent lamp, 840nm, 8W). The product yield of TCOCs was up to 10.29 µmol/g after 24 hrs. The long-term stability test for photocatalytic reduction of CO2 under visible light proved the feasibility of Mo-TNTs to work in MEA solution. From the analysis of chemical and physical properties of Mo-TNTs, it revealed that the structure of Mo-TNTs was changed with the increase of calcination temperature. For Mo-TNTs calcined at 500 °C, the partial corruption of titanate nanotubes into anatase particles caused the reduction of Mo species from Mo6+ to Mo5+ and produced oxygen vacancies, which resulted in the highest CO2 reduction ability. It was found that the molybdenum structure and oxygen vacancies could be the key factors controlling the photocatalytic reduction efficiency of CO2. Possible structure transformation of Mo-TNTs at different calcination temperatures was inferred. And reaction mechanism for photocatalytic CO2 reduction with oxygen vacancy sites of Mo-TNTs was proposed.