| 摘要: | 由於全球面臨嚴重的能源短缺問題，各種替代能源的開發與研究日漸增加。目前備受矚目的燃料電池，主要利用甲醇蒸氣重組為氫氣來源，其中富含大量的一氧化碳，利用金觸媒對一氧化碳的高選擇性氧化特性，使其進料濃度低於5 ppm，避免毒化白金電極。 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X光繞射分析儀(XRD)、穿透式電子顯微鏡 (TEM)、X光電子能譜儀 (XPS)和感應耦合電漿質譜分析儀 (ICP-MS)等儀器鑑定金觸媒的特性。 根據文獻可知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；因此引入高溫下具高活性的二氧化鈰修飾擔體表面組成，其二氧化鈰/二氧化鈦最佳莫耳比例為1/9，高溫轉化率皆在80 %以上，選擇率為40~50 %。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。研究指出氧化鑭與氧化鈷具有提高擔體活性與穩定性之功效，而氧化銅/二氧化鈰可有效催化一氧化碳的氧化反應。選用此三種添加劑於金/二氧化鈰-二氧化鈦中，發現氧化銅在80 ℃時能大幅提高轉化率和選擇率。實驗數據顯示氧化銅有抑制氫氣氧化的能力，且同時具有類似貴金屬的活性，故以含浸法將氧化銅單獨添加於氧化鈦上。最佳觸媒為金/氧化銅-二氧化鈦(4.8:95.2)，其一氧化碳的轉化率在50~100℃下可達100%，高溫選擇率在60%之上。將觸媒隔絕光線照射儲存一個月後，其活性在高溫下差異不大；而暴露於光線照射一個月後，高溫活性約為90 %。值得注意的是本觸媒在80 ℃長時間測試下，以穿透式電子顯微鏡觀察其金顆粒仍然以粒徑2~3nm的半球狀均勻分散於擔體表面，金的表面電子組態也漸從氧化態還原成元素態，轉化率在經過49小時測試後，從100%略微下降到95%，選擇率維持在65 %以上。本研究透過擔體表面與金顆粒間的特殊作用力，有效的避免奈米金顆粒因反應或溫度影響所造成的聚集，維持長時間的活性及穩定性。 Recently, the whole world faces energy crisis such as oil shortages and natural resources shortages. Various development and study of alternative energy grow progressively. Increased attention now turns to development of fuel cell-powered system, because of their low environment impact. It is well known that 0.5–1 vol. % of CO usually present in the hydrogen fuel from the water-gas shift reactor will poison the Pt electrode. Supported gold catalysts, which can oxidize CO in H2-rich stream (PROX) effectively by applying its unique catalytic property to reduce CO concentration less than 5 ppm, is practicable. 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 X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS). 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. CeO2, which is active at high temperature were added to Au/TiO2 to modify its catalytic performance. 1 wt.%Au/CeO2-TiO2 (Ce/Ti = 1/9) has the CO conversion higher than 80% and CO selectivity around 40–50% at temperatures around 50–100 °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. La2O3 and CoOx can increase the catalytic activity and stability. CuOx-CeO2 has the ability to catalyze CO oxidation and increase the selectivity. Hence, they were chosen as additives to Au/CeO2-TiO2 for PROX reaction. The results indicated that CuOx can improve the catalytic performance at 80 °C surprisingly. It is suggested that CuOx may inhibit the oxidation and have the activity similar to noble metal (i.e., Au and Pt). When CuOx and TiO2 alone were used to prepare binary metal oxides support by impregnation method, 1wt%Au/CuOx-TiO2 (Cu/Ti = 4.8/95.2) is the best catalyst. The CO conversion can reach 100 % at 50–100 °C and the CO selectivity is 70 % at 80°C. In order to study the effect of storage, catalysts were stored without light (~25°C) for one month, the activity slightly decreased. However, those stored in the presence of light, the CO conversion was maintained about 90 %. It is worthy of attention that even after 80 °C for 49 h, Au/CuOx-TiO2 (Cu/Ti = 4.8/95.2) maintained high activity. From TEM analysis, it was observed that the gold particles still form hemispherical particles with the diameter of 2–3 nm, and well-dispersed on the support. The Au3+ becomes Au0 gradually after reaction, as analyzed from XPS results. The CO conversion decreased from 100 % to 95 % and CO selectivity was maintained above 60 %. It is due to the unique interaction between support and gold particles, and the prevention of aggregation caused by reaction and temperature. |
