| 摘要: | 本研究以氧化矽為擔體，利用沈澱固著法製備成氧化矽擔載銅觸媒，目的在發展氧化性甲醇蒸氣重組反應(CH3OH + 0.5H2O + 0.25O2 → 2.5H2 + CO2)產製氫氣的程序，利用感應耦合電漿質譜分析儀（ICP-MS）、熱重分析儀（TGA）、X射線繞射儀（XRD）、穿透式電子顯微鏡（TEM）、掃描式電子顯微鏡（SEM）及X射線光電子分析儀（XPS）、程式升溫還原(TPR)、N2O分解吸附(dissociative adsorption of nitrous oxide)等各項儀器與分析技術，分別對擔體及觸媒進行鑑定，藉以評估觸媒應用於質子交換膜燃料電池的可行性。從XRD結果看到銅的繞射峰不明顯，判斷為銅晶粒很小且結晶性不好。由TPR圖譜可看出，銅載量愈高愈難還原，不同煅燒溫度有相似的還原溫度。N2O分解吸附的結果，可看到當銅載量由4.7 wt%增加到24.4 wt%，其分散度會降低，而銅觸媒比表面積會變大，平均粒徑則由1.52增加至3.56 nm。從TEM的分析結果發現，以沈澱固著法製備出的銅觸媒，晶粒呈現圓球狀，晶粒會隨載量增加及煅燒溫度增加而變大，在OSRM反應後也有燒結現象。SEM則看出觸媒的表面結構為眾多網狀結構構成的板狀結構。由XPS的結果中發現，未煅燒過的觸媒中，Cu以三種狀態存在，金屬態（Cu0）、氧化態（Cu2+）以及Cu(OH)2；經過673 K煅燒、600 K還原，其狀態會由氧化銅還原成金屬銅，當經過OSRM反應後，大部分的金屬銅則被氧化，推斷金屬銅為主要活性部位，因此銅觸媒在OSRM反應中需先以氫氣在600 K下還原1小時。經過活性測試後發現，觸媒的活性與銅的顆粒大小及觸媒比表面積有絕對的關係，當銅觸媒比表面積愈大且顆粒又小時，氫氣選擇率越高，這也是17.3 wt%的Cu/SiO2化性反應較好的原因。另外進料中O2/CH3OH及H2O/CH3OH的莫耳比也是影響反應的因素，當O2/CH3OH 莫耳比= 0.25和H2O/CH3OH莫耳比=1時，甲醇轉化率可達到93%，氫氣產生速率可達到255 mmolkg-1s-1，而一氧化碳的選擇率則在2%以下。當增加反應溫度時，甲醇轉化率與氫氣選擇率都會同時增加。反應路徑假設為由連續的氧化性甲醇蒸氣重組、甲醇部分氧化、甲醇直接分解與甲醇蒸汽重組反應構成。 本研究所製備的氧化矽擔載銅觸媒在OSRM反應中有高度活性，能產生低一氧化碳的氫氣，是一種性能極佳的觸媒，此反應亦可適用於燃料電池的電動車。 Selective production of hydrogen by oxidative steam reforming of methanol (OSRM)(CH3OH + 0.5H2O + 0.25O2 → 2.5H2 + CO2) over Cu/SiO2 catalysts, prepared by deposition-precipitation method was studied. The catalysts were characterized by ICP-MS, TGA, TPR, Dissociative adsorption of nitrous oxide, XRD, SEM, TEM and XPS analyses. XRD patterns of Cu/SiO2 catalysts showed unclear peak or broad peak of metallic copper or copper oxide, it indicated that the copper species particles, which are amorphous or very small and highly dispersed within the matrix. From TPR, it was difficult to reduce for higher copper loading, had similar reduction temperature with different calcination temperature. Form dissociative adsorption of nitrous oxide, copper loading increases from 4.7 to 24.4 wt% with decrease in dispersion and increase in metallic surface area. TEM images show that copper crystallites are spherical in shape. The particles size of copper increases with increasing the copper loading and calcinaiton temperature. The catalysts were sintering after OSRM reaction. SEM observations show that plate-like structure of these catalysts is formed with great net work structure. XPS analyses demonstrate that in uncalcined catalysts copper existed in three different states i.e. metallic copper (Cu0), copper oxide (Cu2+) and copper hydroxide (Cu(OH)2). After calcination at 673 K and reduction at 600 K the copper states change from copper oxide to metallic copper. Upon reaction, the state reduce to copper oxide that proved metallic copper is the active site. The catalytic activity of Cu/SiO2 catalysts for the OSRM reaction significantly depended on the particle size and surface area of metallic copper. Increasing surface area and decreasing particle size raises the hydrogen production rate. In this study, the catalyst with 17.3 wt% loading and calcined at 673 K showed the highest activity for methanol conversion and hydrogen production. The appropriate molar ratios of O2/CH3OH and H2O/CH3OH for reaction were 0.25 and 1.0, respectively. Both hydrogen production rate and methanol conversion increased with increasing reaction temperature. The reaction pathway is suggested to consist of consecutive oxidative steam reforming of methanol, partial oxidation of methanol, methanol decomposition, and methanol steam reforming. The Cu/SiO2 catalysts showed high activity for OSRM that produce high hydrogen with low carbon monoxide. Therefore, the application of OSRM reaction over Cu/SiO2 in the fuel cells for electric-powered vehicles can be expected. |
