金屬以及金屬氧化物觸媒在乙醇氧化蒸氣重組反應的研究

Abstract

全球面臨能源短缺以及環境污染的問題，尋找替代性能源成為迫切的議題。其中，氫氣為重要的可再生能源之一，利用乙醇氧化蒸氣重組反應(Oxidatve Steam Reforming of Ethanol，OSRE)來產氫為其中一種可行的方式，反應機制包含全氧化反應、部分氧化反應、水煤氣反應以及其他副反應。本研究內容分為兩個部分，固溶體觸媒的探討、金屬鐵在不同載體的比較： 1.先前研究中利用溶膠-凝膠(sol-gel)法合成燒綠石結構的觸媒Y2Ce2-xRuxO7(x=0-0.4)，以氧化鋁作為載體，此觸媒對於OSRE反應具有不錯的催化活性。在本研究中，在Y2Ce1.6Ru0.4O7結構中摻雜鈧離子部分取代釔離子，提升觸媒催化活性。根據X光粉末繞射(PXRD)與晶格大小計算，ScxY2-xCe1.6Ru0.4O7(x=0-0.5)系列樣品的相寬為x=0-0.3。以X光電子能譜(XPS)和程序升溫還原系統(TPR)來分析，隨著鈧摻雜量增加，部分釕離子以高價態的Ru>4+存在於結構中，而三價與四價鈰離子相對濃度會隨之改變。利用實驗室自製的La2Zr2O7作為載體，以避免觸媒受高溫而改變結構，而影響催化活性。在ScxY2-xCe1.6Ru0.4O7(x=0-0.3)系列觸媒中，觸媒Sc0.2Y1.8Ce1.6Ru0.4O7在400°C的啟動溫度和C/O=0.6條件下進行OSRE反應，有優異的效率表現(乙醇轉換率~100%，氫氣選擇率~105%)，並且經過至少120小時的測試，觸媒效率維持穩定。反應後的觸媒經過PXRD以及拉曼光譜測試，觸媒結構穩定未改變，且無積碳生成，顯示觸媒在乙醇氧化蒸氣重組反應中具有潛力。 2. 研究不同載體吸附金屬鐵對於OSRE催化活性的實驗，利用同步輻射的X光粉末繞射及吸收光譜鑑定氫氣還原前後，觸媒中鐵的價態變化。在啟動溫度為500°C下，進行OSRE反應來比較三種觸媒的催化活性，Fe/Al2O3、Fe/ZrO2、Fe/La2Zr2O7。單一金屬鐵觸媒容易在OSRE反應中氧化產生鐵的氧化物而失去活性，導致乙醇轉換率下降，三種觸媒之氫氣選擇率皆小於30%。因此，金屬鐵的觸媒較不利於在OSRE反應產氫。進一步比較載體之間的差異，觸媒Fe/La2Zr2O7催化效率較觸媒Fe/Al2O3和Fe/ZrO2佳，發現La2Zr2O7載體助於水煤氣反應而增加氫氣的產量。根據PXRD與拉曼光譜的鑑定，在OSRE反應中碳氧比由0.4改變至0.7後，Fe/La2Zr2O7觸媒結構穩定，且無積碳生成。

The problems of energy shortages and environmental pollutions are global issues, and searching for alternative energy sources become an urgent goal. Hydorgen is one of promising renewable energy. Oxidative steam reforming of ethanol (OSRE) is a hydrogen production process that include total oxidation, partial oxidation, and water-gas shift reaction, etc. This thiesis is divided into two parts, the studies of solid solution and iron catalysts on different supports. 1. In our previous research, we utilized a sol-gel method to synthesize a series of Y2Ce2-xRuxO7(x=0-0.4) catalysts, a metal-doped pyrochlore phase. The catalysts supported on Al2O3 have great catalytic activity to OSRE. In this work, we try to enhance the catalytic activity by partially substituting scandium to the site of Y3+ in the structure of Y2Ce1.6Ru0.4O7. According to the powder X-ray diffraction (PXRD) analysis, the phase width of ScxY2-xCe1.6Ru0.4O7 is between 0-0.3. On the basis of X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR), oxidation state of substitued ruthenium ions are promoted to higher valence (Ru>4+) with the increase of the Sc3+ dopant amount, and also the relative concentration of Ce3+ and Ce4+ are related to the Sc3+ dopant amount. Use lab-made La2Zr2O7 as the support to preventing the decrease of catalytic activity due to the collapsed structure at high reaction temperature. The catalyst Sc0.2Y1.8Ce1.6Ru0.4O7 exhibits excellent catalytic activity (XEtOH~100%; SH2~105%) in ORSE under C/O=0.6 at 400°C. The time-on-stream OSRE test shows that the catalyst remains stable at least for 120 hours (XEtOH~100%; SH2≧96%). There is no detectable coke deposition for the used catalyst by Raman spectroscopy, and the structure of the catalyst remains stable. Therefore, the results indicate that the catalyst Sc0.2Y1.8Ce1.6Ru0.4O7 is a promising catalyst for hydrogen production in OSRE. 2. We compare the catalytic activity of iron-based catalysts that absorbed on different supports in OSRE. Utilzing synchotron PXRD and X-ray absorption spectroscopy (XAS) to analyze the valence change of the iron in catalysts before and after reduced by hydrogen. The catalysts, Fe/Al2O3, Fe/ZrO2, and Fe/La2Zr2O7, were carried out at 500°C, and the best catalytic activity is Fe/La2Zr2O7. However, catalysts with iron element are easily to oxide and lose the catalytic activity in OSRE, resulting in the decrease of ethanol conversions and the hydrogen selectivities. The hydrogen selectivities for all Fe-based catalysts are less than 30%. Therefore, iron-based catalysts are not good catalysts for OSRE reaction. Due to the support of La2Zr2O7, it could increase the yield of hydrogen by promoting the water-gas shift reaction. According to PXRD and Raman spectroscopy, the structure of the used Fe/La2Zr2O7 catalyst remains stable and coke deposition is undetectable, after changing carbon-to-oxygen ratio from 0.4 to 0.7.