具結構缺陷之還原型二氧化鈦於太陽光催化水解產氫之生命週期評估

Abstract

目前全球之氫氣來源近九成皆來自各類化石燃料的重組或分解製得，而普遍認為較乾淨之水電解產氫僅佔5%左右。然而就水電解而言，乾淨與否仍然與電解使用之電力來源密切相關，若使用一般電網組合，即是相當於在化石燃料轉換到氫氣之間，額外加入一個電能的轉換過程，降低整體能量的轉換效率。因此，近年來許多基於再生能源進行產氫之程序受到大量關注，其中，太陽光催化水解產氫是一個直接將太陽光能轉換為氫氣之技術，不需額外的電能轉換。而在各類光觸媒中，二氧化鈦因其穩定的物化特性，為目前最為廣泛使用之光觸媒。然而眾所周知的是，二氧化鈦由於其能帶寬度使其為紫外光波段之光觸媒，對可見光利用效率較差。而近幾年間，為了延伸二氧化鈦之可見光利用性，許多有色二氧化鈦的研究不斷湧現，其中大多以高溫高壓的氫氣還原處理製備出黑色二氧化鈦。本研究則使用硼氫化鈉以相對較安全且快速之固相研磨方法，搭配低溫鍛燒(250-350℃)，製備還原型二氧化鈦（Reduced Titanium dioxide, R-TiO2），進行太陽光下之水解產氫。在材料分析方面，本研究使用XPS及EPR探究材料之電子特性，並分別證實材料中三價鈦與氧空缺之存在；以XRD、SEM、TEM及BET分析材料結晶性以及表面型態於還原改質後之變化情形，最後分別以UV-vis及PL光譜分析材料之光學吸收特性及載流子之宿命。在光催化產氫實驗中，根據本研究之鍛燒溫度及持溫時間組合製備之材料，分別進行1wt.%Pt負載，於模擬太陽光系統中進行產氫測試，結果顯示使用250℃鍛燒半小時之還原材料將有最佳之平衡產氫速率。且各材料經還原後，將可縮短照光至產氫速率提升之遲滯期。本研究最後則根據以上之還原型二氧化鈦製備程序進行放大化模擬，比較其與實驗室規模之環境衝擊差異。並以光催化產氫結果，比較使用還原型二氧化鈦以及原始之商用二氧化鈦進行產氫之環境友善性。由結果得知，製備過程經由放大化後，衝擊熱點由原先之電力消耗轉變為乙醇使用，且整體衝擊點數較實驗室規模小了兩個數量級。光催化產氫案例之評估則顯示兩案例之主要衝擊熱點皆為助催化劑Pt的使用，而最後之評估結果則顯示，使用還原型二氧化鈦進行產氫之單位氫氣環境衝擊低於直接使用商用二氧化鈦進行產氫。

Today, nearly 90% of the world's hydrogen production is generated from the reforming or decomposition processes of fossil fuels. It is generally considered that electrical energy is promisingly used to electrolyze water to hydrogen. However, in the case of water electrolysis, the cleanliness still depends on the source of electricity used. In general grid combination, it is equivalent to adding a power conversion process between the fossil fuel and hydrogen, which reduce the overall energy conversion efficiency. Therefore, there is an increased focus on renewable energy driven hydrogen production recently. Among many processes, solar photocatalytic hydrogen production directly converts solar energy into hydrogen without additional power conversion. Among many photocatalysts, titanium dioxide is the most widely used photocatalyst due to its stable physicochemical properties. However, it is well known that titanium dioxide has a wide bandgap so that it mainly utilizes UV light instead of visible light. In recent years, in order to extend the visible light activity, researches on colored titanium dioxide had emerged, which were prepared via high temperature and high pressure hydrogen reduction to produce black-TiO2. In this study, the Reduced Titanium dioxide(R-TiO2) was prepared using sodium borohydride in a relatively safe and fast solid phase grounding process with mild temperature calcination (250-350℃) for solar photocatalytic hydrogen production. The electronic properties and existence of Ti(III) and oxygen vacancies in the reduced TiO2 were identified by XPS and EPR; XRD, SEM, TEM and BET were used to analyze the changes of crystallinity and surface morphology after reduction process. Finally, the optical characteristics and the fate of carriers(h+/e-) were analyzed by UV-vis and PL spectroscopy, respectively. In the simulated solar photocatalytic hydrogen production test, a series of R-TiO2 prepared by the arrangement of the calcination temperature and the holding time were performed with 1wt.%Pt loading. The results show that materials calcining at 250 °C for 0.5 hr have the highest balanced hydrogen production rate. Additionally, after reduced, R-TiO¬2 show the shorter lag period from irradiation to the increase of hydrogen production rate. Finally, the simulation of the scale-up R-TiO2 preparation process had been carried out according to the lab scale, and Life Cycle Assessment(LCA) was used to identified the environmental impact difference with laboratory scale; besides, the environmental impact of hydrogen production by R-TiO2 and commercial TiO2 P25 was also conducted. The results reveal that the impact of lab scale process is two orders larger than large scale, and the impact hot spot changes from the power consumption to ethanol. The assessment of photocatalytic hydrogen production shows that the impact hot spots in both cases are the use of cocatalyst Pt, and that the total environmental impact of hydrogen production using R-TiO2 is lower than commercial TiO2 P25.