| 摘要: | 金一直以來都被視為不具活性且穩定的金屬物質，在其表面無法吸附反應物分子。直到日本Haruta教授提出奈米級金承載於擔體觸媒時具有很高的活性，能夠在低溫下催化CO氧化，而後才開始被重視並應用於各種反應。金觸媒重要的應用莫過於是在富含氫氣流下進行選擇性CO氧化反應；此反應之所以受到重視的原因不外乎由於氫能源可利用在燃料電池發電，然而以甲醇或汽油重組反應產生的氫氣來源，會含有大量副產物CO，若將其直接應用於燃料電池會毒化白金電極並降低電能轉化效率，使用金觸媒於產氫反應的尾氣處理，可有效降低一氧化碳濃度至5 ppm，避免白金電極毒化。故本研究目的即為發展金觸媒具備能降低一氧化碳濃度，同時不氧化氫燃料。金觸媒擔載在二氧化鈦上具有很高的CO轉化率但同時也使氫氣反應，為了改進CO選擇率過低的問題，本文中分別也討論了兩種氧化物如：氧化鈷及氧化鋅，作為促進劑，以改進單純使用Au/TiO2觸媒選擇率過低的問題。本研究以初濕含浸法製備複合性之金屬氧化物擔體，四氯化氫金為金的前驅物，於pH值為7且控制溫度在65 ℃，利用沉積沉澱法將奈米級金顆粒擔載於金屬氧化物上。經過180 ℃鍛燒四小時後，金觸媒具有高分散與熱穩定性，應用於燃料電池操作溫度範圍下，能有效的將一氧化碳去除。並以感應耦合電漿質譜分析儀，X光繞射分析儀、穿透式電子顯微鏡和高解析穿透式電子顯微鏡、X光電子能譜儀等儀器鑑定金觸媒的特性。反應則以固定床反應器填充0.1g觸媒，並以進料體積比CO: O2: H2: He = 1.33: 1.33: 65.33: 32.01，總流量控制在50 ml/min進行反應。根據文獻可知金擔載在二氧化鈦對一氧化碳的氧化有良好之催化效果，也是被研究與應用最多的擔體，但其在高於80 ℃時活性有大幅下降的趨勢，高溫轉化率皆在86 %以上，選擇率為43 %。由於氫氣與一氧化碳會進行競爭性氧化反應，如何有效的提高選擇率是主要關鍵。吾人使用氧化鈷及氧化鋅作為提升CO選擇率的促進劑，其中探討了添加劑鈷/鈦與鋅/鈦的莫耳比、製備時酸鹼值對反應活性的種種影響。研究中發現，在一系列Au/CoOx-TiO2觸媒中，最佳活性的觸媒取決於適當的添加量、對金顆粒大小的影響以及對金的化學狀態的影響，發現以鈷/鈦莫耳比5/95為最佳。而在Au/ZnO-TiO2觸媒中發現，在pH 6製備條件下之雙擔體金觸媒 (Zn/Ti為5/95)，其CO轉化率不管相較於Au/TiO2或Au/ZnO觸媒都來的高，加入氧化鋅並可提升CO選擇率。此研究結果證實，適當的促進劑搭配製備時酸鹼值調整，可成功獲得一系列高分散性奈米金觸媒，並具有高CO轉化率，同時避免氫氣氧化，應用於選擇性CO氧化反應。 Bulk gold had been known as an inactive catalyst due to the smooth surface of gold which inhibits chemisorption of reactant molecules. Since Haruta and co-workers reported that the nano-gold catalysts could achieve CO oxidation efficiently below ambient temperature, several applications for gold catalysts have been under attention recently. One of the important cases in these applications for gold catalysts is preferential CO oxidation in hydrogen-rich stream (PROX). Hydrogen has been recognized as a good energy carrier since the development of fuel cell. When hydrogen-rich fuel is produced from methanol or gasoline by partial oxidation and/or steam reforming combined with water gas shift reaction, the Pt anodes in fuel cell at these low temperatures are poisoned by CO, reducing the overall fuel cell performance. Gold catalyst has been confirmed as a catalyst to oxidize CO in hydrogen stream to reduce CO concentration less than 5 ppm. Develop a catalyst which has high CO conversion and low H2 conversion is the target of this study. Au/TiO2 has high conversion of CO and low selectivity of CO oxidation; Au/CoOx and Au/ZnO have high selectivity for CO oxidation and low conversion of CO. It was expected that by adding suitable amount of CoOx or ZnO into Au/TiO2, the catalyst may retain high CO conversion and suppress H2 conversion. In this study, multiple metallic oxides are prepared by impregnation method. Hydrochloro-auric acid is the gold precursor used to load on the support by deposition-precipitation method at pH 7 and 65 °C. The catalysts were calcined at 180 °C for 4 h. The gold particles have high dispersion and thermal stability. In the fuel cell operating temperature range (50–100 °C), gold catalysts can remove CO almost completely. The catalysts were characterized by inductively coupled plasma-mass spectrometry, X-ray diffraction, transmission electron microscope and high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy. The PROX reaction was carried out in a fixed bed reactor with a feed containing 65.33% H2, 32.01% He, 1.33% CO and 1.33% O2 (vol. %) at 30000 h-1 (GHSV). According to literature, TiO2 has been widely used in the synthesis of supported gold catalysts and active for selective CO oxidation. But its activity decreases obviously when the temperature reaches above 80 °C. 1 wt % Au/TiO2 has the CO conversion 86 % and CO selectivity 43% at temperatures around 80 °C. In thermodynamic aspect, hydrogen will compete with CO for oxygen at high temperature. The method to improve the CO selectivity is the key point. Two metal oxides, CoOx and ZnO have been regarded as additives into TiO2 support which was prepared by incipient-wetness impregnation method. The effects of pH in preparation and Co or Zn/Ti ratios of the catalyst on the catalytic properties of the catalysts were investigated. The high catalytic activity of Au/CoOx-TiO2 over Au/TiO2 has been attributed to the synergistic effect of gold particle size, optimum of combination metallic and electron-deficient gold species, as well as to Au-support interactions. Moreover, the results of this study have demonstrated that Au/ZnO-TiO2 (Zn/Ti = 5/95) catalyst prepared at pH 6 is a better catalyst than Au/ZnO and Au/TiO2 for PROX reaction. By adding suitable amount of promoters and by choosing suitable pH value during DP process to deposit gold, one is able to obtain a catalyst which has the best performance in PROX reaction. |
