利用純與廢棄石墨製備含鈦石墨烯負載TiO2及其應用於二氧化碳還原為燃料之研究

Abstract

隨著全球氣候變暖問題日益嚴重，溫室氣體的排放越來越引起社會廣泛關注。如何在低能耗條件下，有效利用溫室氣體二氧化碳，並將其轉化為能源是本研究的主要目的。理論上CO2在接受H+ 和電子後可以被還原轉化為甲酸與甲醇等能源產物。但由於CO2極其穩定的分子結構而使得上述反應難以時間，因此為了達到CO2良好的還原效率，反應需要有高效率的催化劑的參與和適合的操作條件。本研究採用純石墨與廢棄石墨製備石墨烯-TiO2作為光催化劑進行CO2還原反應。此外，由於該反應機理與材料本身特性、產物種類以及參與反應的自由基均有相關，本研究將從這三個方面入手，深入探討反應機制與路徑。 為瞭解材料自身特性，本研究採用元素分析儀(EA)，X光螢光光譜(XRF)與能量散射光譜(EDS)對原材料與最終製備之觸媒進行所含成分分析。利用N2等溫吸脫附儀進行材料表面積之分析。利用X光繞射光譜儀對材料晶型結構進行分析。此外，由於材料表面有眾多官能基團，會影響材料表面電性與親水性，從而對材料在液相反應中的效率有一定程度之影響。為瞭解這些官能基團特性，本研究利用傅立葉轉換紅外光譜(FTIR) 和X射線光電子能譜(XPS)對材料表面官能集團進行分析。另外通過穿透式電子顯微鏡(TEM) 對材料進行表觀形態分析之結果顯示，通過純石墨製備之觸媒中碳的主要存在形態是單層石墨烯。反之，在以廢棄物製備之觸媒中則出現多層石墨烯堆疊在一起之情形。光催化材料之光學特性與電子移轉能力則分別利用紫外-可見分光光度計與光電化學測試系統進行分析。 為評估環境因素對還原產率之影響，本實驗亦探討部份環境因素如石墨烯負載量，催化劑投加量，pH值和回收次數等對還原產率之影響。實驗結果顯示，在40%石墨烯-TiO2投加量為0.4 g L-1時，可見光CO2光催化還原反應在中性條件下可以達到最佳產物產率。隨著觸媒投加量的增加，反應效率初始呈現上升趨勢。但是觸媒投加量過多的條件下，會對光產生一定程度的遮蔽作用，進而使產物產率呈現下降趨勢。此外，因為石墨烯負載量與材料本身之電子轉移能力直接相關，所以隨著石墨烯負載量之增加，反應效率亦呈現上升趨勢。此外由於在不同pH條件下，CO2在水中之溶解形態不同，表面所帶電量也不同，在催化反應發生時會與表面帶電的材料產生不同吸附或排斥作用，進而對反應產率產生影響。最後，為了探討自製觸媒材料之穩定性，本研究對回收材料的光還原效能亦進行評估，其結果顯示在回收2次以後，觸媒催化性能基本穩定，仍具有較好之催化能力，適宜回收再利用。 最後，為了整體瞭解該還原反應之反應機理，本研究從最終產物種類與反應產生之自由基兩個方面進行深入探討。實驗結果顯示，利用石墨烯-TiO2對CO2進行光化學還原的主要產物是甲酸和甲醇，其中甲酸的產量持續高於甲醇。而利用電子自旋光譜(ESR)對反應產生自由基進行偵測之結果顯示，在反應進行過程中有一氧化碳自由基生成。結合文獻分析得知一氧化碳自由基通過與氫離子與電子結合反應後，可以產生甲醇。結合最終產物種類與自由基檢測結果，本研究進行之CO2光催化反應之路徑可以歸納總結為CO2 → HCOOH → CH2O → CH3OH 和 HCO3- → ∙CO2- → ∙CO- → CH3OH兩種方式。 基於本研究進行機理研究之成果，採用兩種動力學模型對反應進行模擬：(1) 擬一級反應動力學模型：假設在反應階段，溶解于水中之CO2於不同產物之轉換均符合擬一級反應動力學模型，並以此對反應資料進行擬合。(2) 近似穩態模型：該模型適用於系統中具有不確定濃度中間體的自由基反應。故於本研究中，根據機理分析得出之CO2還原路徑，利用近似穩態模型（PSSH）對中間產物與最終產物進行資料擬合。 綜上所述，本研究成功利用純石墨與廢棄石墨進行石墨烯-TiO2材料之製備，並成功將該材料應用於CO2還原反應，更進一步針對反應機理與動力學進行深入探討。

The continuous increase in concentration of CO2 in the atmosphere, as well as the depletion of fossil fuels, has become a public concern in recent years. The use of solar energy (i.e., unlimited energy) to convert CO2 as fuels, such as formic acid and methanol, could address those concerns. The reactions for the generation of these fuels are based on the premise that dissolved CO2 can be reduced by accepting protons and electrons. Promoting the reduction reaction requires catalysts with high efficiency under favorable operation conditions. Two kinds of graphene-loaded TiO2, which were prepared from pure graphite and waste graphite, were used in this research to convert CO2 into fuels. This dissertation also focused on the mechanism of the reactions that are related to the characteristics of the catalysts, the selectivity of the final products, and the radicals involved in the reaction. In this research, the components of the catalysts were characterized via elemental analysis (EA), X-ray fluorescence (XRF), and energy-dispersive spectroscopy (EDS). The surface area was determined using an N2 adsorption/desorption isotherm analyzer (BET). X-ray diffraction (XRD) results confirmed that TiO2 had a mixed crystal phase of anatase and rutile. Functional groups that could affect the surface potential and polarity of the catalyst were determined via Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The presence of single- and multi-layered graphene was determined via morphological studies, specifically transmission electron microscopy (TEM). The optical characteristics and charge transfer ability of the catalysts were tested via UV-visible (UV-Vis) spectroscopy and photoelectrochemical analysis. To obtain the highest conversion efficiency, parameters such as graphene loading, catalyst loading, pH, and recycle times were analyzed. The maximum yield of the final products was obtained with 40% graphene loading and 0.4 g L-1 catalytic loading at neutral condition under visible light irradiation. Based on the results, the penetration efficiency of light was related to catalytic loading, which can inhibit the efficiency when the catalysts were loaded in excess. Furthermore, the efficiency of the reaction was also affected by graphene loading because of the charge separation ability of the different catalysts. Moreover, the relationships between the surface potential of the catalysts and the carbon species in the solution at varying pH were also found to be critical factors that affected CO2 reduction. The recycled catalysts exhibited stable reduction efficiency after two recycle times, thereby indicating the possibility for reuse. The final products and the radicals generated in the intermediate reaction step were identified to determine the plausible mechanism of the reaction. In this research, Gas Chromatography Mass Spectrometry (GC-MS) results showed that the final products were formic acid and methanol. Electron paramagnetic resonance (ESR), which can be used to analyze unpaired electrons, was utilized to determine and identify the radicals involved in the reaction. The ESR results indicate that carbon monoxide radicals were present, and these radicals can react with hydrogen ions and electrons to generate CH3OH. Combining the results of ESR and GC-MS, the possible reduction paths can be summarized as CO2→ HCOOH → CH2O → CH3OH and CO2 →∙CO2−→∙CO- → CH3OH. Two kinetic models were then developed based on the result of mechanism studies. First, the kinetic model for formic acid and methanol can be assumed to be a pseudo-first order model. Second, based on the possible pathway of CO2 reduction, the pseudo-steady-state hypothesis (PSSH) model was also utilized. This model was suit for the system with several intermediates of unknown concentration and was then utilized to investigate the process of CO2 reduction.