| 摘要: | 本研究以沈澱固著法製備Au-Cu/TiO2與Au-Cu/TiO2-Fe2O3雙金屬觸媒應用於甲醇部份氧化反應產製氫氣(partial oxidation of methanol，POM，CH3OH + 0.5O2 2H2 + CO2，△H0298= -192.5 kJ/mol)。其中觸媒的物性分析包括感應耦合電漿原子發射光譜儀(ICP-AES)、熱重分析儀 (TGA)、X射線繞射分析儀 (XRD)、程式升溫還原 (TPR)、氨氣程溫脫附(NH3-TPD)、穿透式電子顯微鏡 (TEM and HR-TEM)、X-射線光電子能譜儀(XPS)等分析儀器及技術。觸媒的化性分析以甲醇部份氧化反應進行活性測試，藉由改變觸媒種類、擔體Ti/Fe比例、觸媒製備pH值、反應進料O2/CH3OH比例、煅燒溫度與反應溫度等的操作變數，將所得到的化性結果配合物性分析，以探討不同實驗參數對於觸媒活性的影響。並由實驗結果評估Au-Cu/TiO2與Au-Cu/TiO2-Fe2O3雙金屬觸媒催化甲醇部份氧化反應產製氫氣應用於燃料電池的可行性。 由實驗結果得知，銅在Au-Cu/TiO2雙金屬觸媒中扮演了阻止金晶粒粒徑增加的角色，此與金屬-金屬間作用力與電子由銅轉移到金進而提供更多的活性氧有著密切的關聯。由TEM的結果得知，經過煅燒的觸媒的金屬粒徑並沒有明顯增大，顯示出銅的存在使雙金屬觸媒有較強抗燒結的能力。由XPS的檢測結果發現Au-Cu/TiO2雙金屬觸媒在煅燒與POM反應後仍有氧化態金(Au?+)存在，銅與金共存可以穩定金屬粒徑並可以在煅燒程序中穩定氧化金的狀態，而此氧化金即是擔體金觸媒主要的活性基點。Au-Cu/TiO2雙金屬觸媒比Au/TiO2及Cu/TiO2雙觸媒有更佳的觸媒活性、氫氣選擇率與觸媒穩定性。反應進料比例在O2/CH3OH =0.3的反應時氫氣選擇率最高，達到92.4%。在O2/CH3OH =0.1，氧氣不足的情況下加速了甲醇進行直接分解反應，因此一氧化碳選擇率升高到11%左右。所以最適當的反應進料比例為O2/CH3OH= 0.3。觸媒的甲醇轉化率與氫氣選擇率隨著反應溫度升高有增加的趨勢，到523 K後所增加的幅度不大。反應溫度達到548 K時一氧化碳選擇率明顯的上升，這表示了在此溫度之上，甲醇分解反應與逆向水氣轉移反應同時進行，實驗結果發現523 K是最佳的反應溫度。 在Au-Cu/TiO2-Fe2O3觸媒中加入適量的氧化鐵會使甲醇轉化率增加，過多氧化鐵也可能阻擋活性金屬的活性基點而造成甲醇轉化率下降。在Ti/Fe=9/1的Au-Cu/TiO2-Fe2O3觸媒中，適量的氧化鐵與高分散性的金-銅晶粒之間增強的相互作用與電子轉移可使觸媒表面活性氧的濃度增加。在pH 7製備的觸媒，有著較小的金屬晶粒與較多活性基點，所以觸媒活性較高。Au-Cu/TiO2-Fe2O3觸媒在煅燒與反應後仍有活性成分(Au?+)存在，代表觸媒有著良好的穩定性。隨著反應溫度的升高，甲醇轉化率與氫氣產率隨之增加，其中氫氣產率在573 K時達到最高值288 mmol/kgcat-s。在448 K到473 K之間，氫氣選擇率約80%。這表示高度放熱反應的甲醇燃燒反應與甲醇部份氧化反應同時進行。在反應溫度提高到498 K時，氫氣選擇率上升到86.4%。當反應溫度在498到523 K時，氫氣選擇率持續的升高；另外，在498 K與548 K時所得到的高氫氣選擇率是因為甲醇蒸氣重組反應在此溫度範圍內也同時進行。在573 K時，有較明顯的一氧化碳選擇率的增加，這是因為在此反應溫度下甲醇直接分解反應與逆向水蒸氣轉移反應在反應系統中同時發生。本研究成果與其他關於鉑、銅與鈀觸媒應用在甲醇部份氧化反應的研究成果比較，發現Au-Cu/TiO2-Fe2O3 觸媒可以更有效率低溫催化甲醇部份氧化反應產製氫氣。 Part I：Au-Cu/TiO2 catalysts The catalytic activity of Au/TiO2 (2 wt.% Au), Cu/TiO2 (2 wt.% Cu) and Au-Cu/TiO2 (1 wt.% Au-1 wt.% Cu) catalysts were studied for partial oxidation of methanol (POM) to produce H2. The catalysts were characterized by ICP-AES, TGA, XRD, TEM, TPR and XPS analyses. The bimetallic Au-Cu/TiO2 catalysts are more active, stable and exhibit higher hydrogen selectively with smaller amount of CO compared to the Au/TiO2 and Cu/TiO2 catalysts. The enhanced activity, selectivity and stability of the bimetallic catalysts are due to Au-Cu interaction that creates smaller metal particles, which consequently stabilize the active component for POM to produce H2. The activity of Au-Cu/TiO2 catalysts at different pH during preparation, O2/CH3OH ratio, calcination temperature and reaction temperature were optimized. The Au-Cu/TiO2 catalysts prepared at pH 7 and dried at 373 K show higher activity. The catalytic performance at various reaction temperatures shows that the methanol conversion and H2 selectivity are increased with rise in temperature. The CO selectivity is increased beyond 548 K. Other possible reactions involved during POM are suggested as methanol combustion, steam reforming, decomposition, reverse water gas shift, water gas shift and CO oxidation. Part II：Au-Cu/TiO2-Fe2O3 catalysts Partial oxidation of methanol (POM) to produce H2 was investigated over Au-Cu/TiO2 and Au-Cu/TiO2-Fe2O3 catalysts. The catalysts were prepared by deposition-precipitation method and characterized by TGA, XRD, TEM, HR-TEM, ICP-AES, TPR, NH3-TPD and XPS analyses. Detail study on the Au-Cu/TiO2-Fe2O3 catalysts was performed to optimize Ti/Fe ratio, pH during preparation of the catalyst, O2/CH3OH ratio, calcination temperature and reaction temperature. The Au-Cu/TiO2-Fe2O3 catalyst with Ti/Fe= 9/1 atomic ratio is more active and exhibits higher methanol conversion compared to the Au-Cu/TiO2 catalyst. The higher activity of Fe-containing catalyst was attributed to the ability to supply reactive oxygen, thereby stabilize active gold species (Auδ+) in the catalyst. Studies on the optimization of pH during preparation of the Au-Cu/TiO2-Fe2O3 catalyst and calcination temperature shows that the catalyst prepared at pH 7 and dried at 373 K (uncalcined) exhibited higher activity. The catalytic performance at various reaction temperatures shows that both methanol conversion and hydrogen selectivity are increased with increasing the temperature. A small increase in CO selectivity was observed beyond 523 K, which is due to the decomposition of methanol and reverse water gas shift at high temperatures. |
