利用具有燒綠石結構的觸媒經乙醇氧化蒸氣重組反應產氫

Abstract

溫室效應為目前相當受到重視的議題。使用氫氣作為未來的能源載體具有高質量密度(約120千焦/克)的優勢，並且在其使用的過程中不會排放任何對於環境具有破壞性的產物。透過乙醇氧化蒸氣重組反應來製備氫氣具有許多優點，該反應結合了乙醇的部分氧化以及水煤氣反應，在反應過程中可以利用乙醇的全氧化反應所放出的熱能來維持整個產氫過程需要的能耗，而水煤氣反應的參與則是能從水分子提取額外的氫氣。在本論文中，透過溶膠－凝膠法製備了一系列具有燒綠石結構的材料，其中包含了La2Ce2-xMxO7 (M=Fe and Ru) 與 La2Ce2-x-dMxO7-0.5x-d (M=Co, Ni, and Cu)；這些材料經過定性並用作為乙醇氧化蒸氣重組反應的觸媒。主體材料La2Ce2O7的活性首先經過了程溫還原系統以及催化反應的測試，實驗結果顯示La2Ce2O7與被廣泛研究的觸媒CeO2具有相近的反應性。據此結果，我們選用不同的活性金屬對La2Ce2O7進行摻雜，希望利用固溶體的概念製備具有高分散性的活性金屬的燒綠石材料，並藉此提升催化活性。所有被製備的材料都經過X光粉末繞射(PXRD)、感應耦合電漿原子發射光譜(ICP-AES)、程溫還原(TPR)以及X光光電子光譜(XPS)的定性分析。根據TPR和XPS的分析結果，材料中Ce4+離子的相對濃度對於晶格變化以及氧化還原能力有顯著的影響。對於La2Ce2-xRuxO7 (x=0-0.35)系列的觸媒，最佳化的觸媒是La2Ce1.8Ru0.2O7; 具有99(1) %的氫氣選擇率。至於La2Ce2-x-dNixO7-0.5x-d (x=0-0.45)系列的觸媒，最佳化的觸媒則為x=0.45的樣品，氫氣選擇率為82.60(5) %。對於這兩個系列的觸媒，乙醇都能完全被轉換並且穩定48小時；過程中可以發現，產物的組成會隨著時間有所轉變，主要的原因是觸媒本身會與氧化鋁載體產生反應，形成LaAlO3並進而改變了催化途徑。儘管如此，觸媒在使用前後，活性金屬在材料中都維持著良好的分散性。對於其餘的材料，La2Ce2-xFexO7 (x=0-0.2)觸媒在乙醇的氧化蒸氣重組反應中並不具有任何活性，而La2Ce2-x-dMxO7-0.5x-d (M=Co and Cu)觸媒則是促進了乙醇的脫水反應，乙烯的大量生成對於產氫過程將有很大的風險會造成嚴重的積碳問題。

Global warming, a critical topic, draws much attention. H2 is a prospective carrier of energy because of its content of energy per unit mass (i.e., 120.7 kJ/g) is larger than for other fuels; it burns cleanly without emitting pollutants to our environment. H2 produced by ethanol reforming is considered to be a sustainable process. An oxidative steam reforming reaction of ethanol (OSRE) combines the partial oxidation (POR) with steam reforming (SRE) and the water-gas-shift reaction (WGS) to take advantage of the heat generated by total oxidation (TOR). The extra H2 produced in the WGS reaction is supposed to obtain the ideally highest H2 yield. In this thesis, metal-doped pyrochlore phases La2Ce2-xMxO7 (M=Fe and Ru) and La2Ce2-x-dMxO7-0.5x-d (M=Co, Ni, and Cu), were prepared through sol-gel methods to assess their catalytic performance for OSRE. Catalytic activity of the host material, La2Ce2O7, is first confirmed by temperature-programmed reduction (TPR) and OSRE test; a compatible performance with those of CeO2 can be observed. Pyrochlores doped with catalytic active metal ions through the sol-gel process can produce materials with evenly distributed metal ions, which prevent metal aggregation and the formation of readily gasified metal oxide, such as RuO3 and RuO4. The as-prepared materials are characterized by powder X-ray diffraction (PXRD), an inductively coupled plasma atomic emission spectrometer (ICP-AES), TPR, and X-ray photoelectron spectra (XPS). According to the analyses of TPR and XPS, the relative concentration of Ce4+ is found to be a crucial factor that affects both the cell dimension and redox behavior of the as-obtained materials. For the series of La2Ce2-xRuxO7 (x=0-0.35), the optimized catalyst is La2Ce1.8Ru0.2O7 with H2 selectivity 99(1) %. As to the series of La2Ce2-x-dNixO7-0.5x-d (x=0-0.45) catalytic performance is affected by the Ni content and the H2 selectivity reaches to the highest value 82.60(5) % for sample of x=0.45. Although the activities of the catalysts remain stable for the time-on-stream test, the as-prepared catalysts were decomposed gradually due to the reaction between catalysts and Al2O3 supports. From the images analysis of scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the distributions of metal ions within the catalysts remain well dispersed before and after the OSRE reaction. For the other materials, the series of La2Ce2-xFexO7 (x=0-0.2) catalyst reveals no activity on OSRE reaction and an enhancement of dehydration of ethanol which may cause severe coke deposition is observed for La2Ce2-x-dMxO7-0.5x-d (M=Co and Cu) catalysts.