| 摘要: | 本研究以沈澱固著法製備稻殼灰分擔體銅觸媒(Cu/RHA)以及添加氧化鋅的稻殼灰分擔體銅觸媒(Cu/ZnO/RHA)進行甲醇部份氧化反應(POM)產製氫氣，分別針對不同銅載量、氧化鋅添加量、pH值、煅燒溫度、O2/CH3OH進料比例及反應溫度等參數進行討論，以得最佳製備與反應條件，最後再與最佳條件製備的商用二氧化矽擔體銅觸媒進行比較，以了解稻殼灰分取代商用二氧化矽作為擔體的可行性。研究中利用感應耦合電漿原子發射光譜儀(ICP-AES)、熱重分析儀(TGA)、X-ray繞射分析儀(XRD)、程式升溫還原(TPR)、N2O分解吸附(dissociative adsorption of nitrous oxide)與X射線光電子能譜儀(XPS)等分析技術，對觸媒進行物性分析鑑定；以甲醇部份氧化反應探討各項操作變因對於甲醇轉化率、氫氣選擇率及一氧化碳選擇率之影響，並由實驗結果評估稻殼灰分擔體銅觸媒應用在POM反應產氫的可行性。 由實驗結果得知，稻殼灰分為一高純度的二氧化矽。隨著銅載量增加，擔載於觸媒上之銅鹽類可完全均勻分散在擔體上，形成一種類孔雀石結構，但銅晶粒大小會隨載量增加而變大，因此需要較高的還原溫度；過高的煅燒溫度，會使觸媒有些許的燒結現象，使銅金屬表面積下降。10.2 wt.% Cu/RHA觸媒有最佳的銅金屬表面積、分散度，因此反應活性最佳。進料比O2/CH3OH=0.3時，有較好的產物分布，同時具有較高的甲醇轉化率、氫氣選擇率，以及較低的一氧化碳選擇率。523 K為最佳的反應溫度，因其同時具有高甲醇轉化率與氫氣選擇率，卻不會有大量的一氧化碳產生。由XPS結果可證明，Cu0為POM反應的活性點，因此若欲得到較好的催化活性，觸媒需要有較大的銅金屬表面積。 由XRD圖譜與N2O分解吸附結果得知，添加過多的氧化鋅，會降低銅金屬表面積，使得分散度降低，銅粒徑變大。利用pH=8製備的觸媒有最大的銅金屬表面積，與最好的分散度。POM測試的活性結果發現，添加1 wt.%氧化鋅、pH=8、煅燒溫度673 K的觸媒有最好的催化活性，對比物性結果發現，催化活性與銅金屬的表面積相關。反應溫度於523 K時觸媒有最好的活性表現，過高的溫度會導致副產物增加。不論有無添加氧化鋅促進劑，稻殼灰分擔體銅觸媒的反應活性皆比商用二氧化矽觸媒佳，因其孔洞特性為單一孔洞結構，而商用二氧化矽則為聯結孔洞型態，聯結型孔洞迂迴曲折，容易使銅金屬晶粒在熱活化階段或催化反應時，將孔洞出口阻塞，因此，商用二氧化矽擔體觸媒在POM反應過程中，觸媒表面活性點逐漸減少，使得失活速率加快。因此利用稻殼灰分取代商用二氧化矽是可行的。 Part I：Cu/RHA catalysts Copper catalysts supported on rice husk ash (Cu/RHA) were tested for partial oxidation of methanol (POM) to produce H2. The catalysts were prepared by deposition-precipitation and characterized by ICP-AES, TGA, XRD, TPR, XPS and N2O titration techniques. The Cu/RHA catalyst with 10.2 wt. % Cu loading and under 673 K calcination has higher copper dispersion and smaller Cu particle size. This catalyst exhibits higher activity and selectivity for POM to produce H2. The partial pressure of O2 plays an important role to determine the product distribution. Catalytic activity of the catalyst at different reaction temperatures shows that CH3OH conversion, H2 selectivity and CO selectivity increase with rise in temperature. At different temperatures along with POM, several reactions such as methanol combustion, steam reforming of methanol, methanol decomposition, water gas shift and oxidation of CO and H2 might be involved. Comparison of catalytic activity of Cu/RHA and Cu/SiO2 catalysts demonstrates that Cu/SiO2 catalysts deactivate with time with faster rate and have higher CO selectivity. This proves that Cu/RHA catalysts have good thermal stability and selectivity for POM to produce H2. Part II：Cu/ZnO/RHA catalysts ZnO-promoted copper catalysts supported on rice husk ash (Cu/ZnO/RHA) have been tested for partial oxidation of methanol (POM) to produce hydrogen. The catalyst was prepared by deposition-precipitation method and characterized by XRD, TPR, and N2O titration techniques. Detail study on the catalytic activity of the Cu/ZnO/RHA catalysts was performed to optimize the amount of ZnO promoter, pH, calcination temperature and reaction temperature. The results showed that with appropriate amount of ZnO promoter, CH3OH conversion increased significantly and the undesired by–product, CO was reduced. The improved activity and selectivity is due to the enhanced copper surface area and copper dispersion by the ZnO additon. The catalysts prepared with 1 wt.% ZnO promoter, pH 8, and calcined at 673 K showed the superior catalytic activity because of its high copper surface area under these experimental conditions. The CH3OH conversion and H2 selectivity are increased from 40% to 91.2% and 80.1% to 99.2%, respectively, when rising the temperature from 473 to 573 K. Beyond 523 K, the CO selectivity was significant. Comparison between Cu/ZnO/RHA and Cu/ZnO/SiO2 catalysts proves that Cu/ZnO/RHA is an active catalyst with good stability compared to the silica supported catalyst. |
