| 摘要: | 本研究以稻殼為起始原料，經過水洗、酸洗、熱解及碳燒等前處理程序，製成稻殼灰分(非晶型SiO2, 99 %以上)。使用沈澱固著法製備稻殼灰分擔體銅觸媒(簡稱為Cu/RHA觸媒)，再利用氧化性甲醇蒸氣重組反應(OSRM)產製氫氣的程序測試其活性。此反應有良好的氫氣產生速率，以及相當低的一氧化碳選擇率，可避免因一氧化碳濃度太高而毒化燃料電池中的鉑電極。本研究也利用感應耦合電漿原子放射光譜儀（ICP-AES）、熱重分析儀（TGA）、X射線繞射儀（XRD）、掃描式電子顯微鏡（SEM）、X射線光電子分析儀（XPS）、程式升溫還原（TPR）及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。 從XRD結果可看出銅的繞射峰不明顯，應為銅晶粒過小且結晶性不佳。由TPR圖譜可得知，銅載量愈高愈難還原，不同的煅燒溫度有相似的還原溫度。N2O分解吸附的結果，可知當銅載量由5.4 wt%增加到29.9 wt%，其分散度降低，平均粒徑則由1.10增加至3.56 nm，而銅觸媒表面積在載量20.4 wt%為最大。由SEM看出當載量、煅燒溫度增加時，觸媒顆粒皆變大。從XPS的結果中發現，銅觸媒中Cu0有利於OSRM反應。經過活性測試後發現，觸媒的活性與銅的顆粒大小及金屬銅表面積有關係。當銅觸媒表面積愈大且顆粒愈小時，氫氣產生速率愈高，故20.4 wt%的Cu/RHA活性反應較佳。而進料中O2/CH3OH及H2O/CH3OH的莫耳比也是影響反應的重要因素，當O2/CH3OH莫耳比為0.3、H2O/CH3OH莫耳比為1時，有較高的甲醇轉化率與氫氣產生速率，且一氧化碳選擇率可維持在2 %以下。而增加反應溫度，甲醇轉化率、氫氣產生速率與一氧化碳選擇率都會上升，523 K為最佳反應溫度。 In this work, rice husk ash (RHA) was used as a catalyst support. Selective production of hydrogen by oxidative steam reforming of methanol (OSRM) over Cu/RHA catalysts, prepared by deposition-precipitation method was studied. The catalysts were characterized by ICP-MS, TGA, TPR, Dissociative adsorption of nitrous oxide, XRD, SEM and XPS analyses. XRD patterns of Cu/RHA catalysts indicate 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 and higher calcination temperature. From dissociative adsorption of nitrous oxide, copper loading increases from 5.4 to 29.9 wt% with decrease in dispersion and increase in metallic surface area. When calcination temperature is at 673 K, it has the biggest metallic surface area and smallest particle size. SEM observations show that increasing copper loading, particle size become bigger and increasing calcination temperature, the catalyst is sintering. XPS analyses demonstrate that in calcination at 673 K and reduction at 573 K, the copper states change from copper oxide to metallic copper. The catalytic activity of Cu/RHA catalysts for the OSRM reaction significantly depends 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 20.4 wt% loading and calcined at 673 K shows the highest activity for methanol conversion and hydrogen production. The appropriate molar ratios of O2/CH3OH and H2O/CH3OH for reaction are 0.3 and 1.0, respectively. Both hydrogen production rate and methanol conversion increased with increasing reaction temperature. It has lower carbon monoxide selectivity at 523K. Comparing Cu/RHA with the Cu/SiO2 catalysts by OSRM in the same operating condition, we find that both two catalysts have the same hydrogen production rate and methanol conversion, but the Cu/RHA catalysts produce three times less carbon monoxide than the Cu/SiO2 catalysts. |
