| 摘要: | 本研究分別以稻殼灰分(RHA)及稻殼灰分含浸硫酸鋁製成矽-鋁組合氧化物(RHA-Al2O3)做為觸媒擔體，並利用離子交換法製備擔載鎳觸媒。實驗中使用氮吸附法、感應偶合電漿原子發射光譜儀(ICP-AES)、掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)、X-射線繞射儀(XRD)、熱重分析(TGA)、氫氣程溫脫附(H2-TPD)、正-丁胺程溫脫附(n-C4H9NH2- TPD)及程溫還原(TPR)對觸媒進行特性分析，並在一大氣壓下進行二氧化碳氫化反應以探討各項製備及操作變數(如：金屬載量、鍛燒溫度與時間、還原溫度與時間、RHA-Al2O3組合氧化物中的氧化鋁含量及反應溫度)對觸媒活性的影響。最後經由以上的實驗結果分析建立RHA與RHA-Al2O3擔載鎳觸媒的模型。 在稻殼灰分擔載鎳觸媒(Ni/RHA)方面，實驗結果發現：在溶液pH值為8.5時，有最大的鎳離子吸附量。離子交換乾燥後所形成的觸媒先驅物為矽酸鎳，而矽酸鎳熱分解溫度至少要773K，並且形成非常難還原的氧化鎳，經還原得到的鎳金屬晶粒呈現圓球形，而且分佈均勻、大小均一。鎳的分散度隨鎳載量的增加漸次下降，但鎳金屬表面積隨鎳載量的增加而增加，直至16.7wt.% 以後卻隨之下降，而金屬粒徑亦隨著載量的增加而增加。觸媒的活性隨反應的進行受到積碳的影響而衰退，直至反應3h呈現穩定，而鍛燒處理不會影響觸媒的活性，但觸媒的活性會隨著反應溫度的增加而提升，直至773K時再漸次下降。並且，經由TPD的分析證明，稻殼灰分具有的擔體性質相較於一般常用的氧化矽膠，可使負載於表面上的鎳金屬有較高的金屬表面積， 在二氧化碳的氫化反應上有更好的活性。 在矽-鋁組合氧化物擔載鎳觸媒(Ni/RHA-Al2O3)方面，實驗結果發現：RHA-Al2O3組合氧化物擔體的BET比表面積隨著氧化鋁含量的增加漸次下降，而酸度卻隨著氧化鋁含量的增加而增加。離子交換法製備的Ni/RHA-Al2O3觸媒，在乾燥後所形成的觸媒先驅物為鋁酸鎳，而鍛燒鋁酸鎳形成氧化鎳所須的溫度約在773K，還原後得到的鎳觸媒其金屬分散度隨載量的增加漸次降低，鎳金屬的粒徑與表面積卻隨載量的增加而增加，而且粒徑比Ni/RHA觸媒大。觸媒的活性大致上不隨反應的進行而衰退，但是催化活性隨著反應溫度增加至某一值，而後增加反應溫度並不會增加觸媒的催化活性。在773K下進行鍛燒4h可得活性較佳的觸媒，還原溫度會影響催化活性，但不受還原時間的影響。RHA-Al2O3組合氧化物中的鋁含量增加會降低觸媒的活性。並且，經由4.44wt.% Ni/RHA-Al2O3-4與4.29 wt.%Ni/RHA的催化活性比較，反應溫度高於773K時4.44wt.% Ni/RHA-Al2O3-4觸媒有較佳的甲烷選擇率。因此，本實驗所製備的矽-鋁組合氧化物乃是一種具高促進效果的觸媒擔體。 Both the rice husk ash (RHA) and the RHA-Al2O3 composite oxides prepared by impregnation of RHA in aluminum sulfate were used as a catalyst support, respectively. Nickel catalysts supported on RHA and RHA-Al2O3 were prepared by the ion exchange technique. The catalysts were characterized by nitrogen adsorption method, inductively coupled plasma-atomic emission spectrometer(ICP-AES), scanning electron microscopy(SEM), transmission electron microscopy(TEM), X-ray diffraction(XRD), thermogravimetric analyzer(TGA), temperature-programmed desorption (TPD) of hydrogen and n-butylamine and temperature-programmed reduction (TPR). The catalytic activities of catalysts were tested by CO2 hydrogenation under normal atmospheric pressure. During the investigations of preparation and operation conditions, effects of nickel loading, calcination, reduction, alumina content of RHA-Al2O3 composite oxides and reaction temperature on catalytic performance were also examined. From the experimental results described and discussed above, models were developed to visualize the nature of the RHA-supported and RHA-Al2O3-supported nickel catalysts. For the RHA-supported nickel catalysts (Ni/RHA) system, the experimental results show that the pH value of 8.5 is the optimum for the preparation of nickel catalysts supported on RHA. That nickel silicate with a layer structure formed after drying step. The thermal decomposition of the layered nickel silicates starts above 773K and leads to the formation of NiO. Reduction of NiO from the thermal decomposition of the layered nickel silicates is found to be unusually difficult. After reduction, the nickel crystallites appear to be spherical in shape. They are homogeneously distributed over the support and exhibit a narrow size distribution. The dispersion of nickel gradually decreases with nickel loading. The nickel surface area increases with nickel loading up to 16.7 wt.% Ni and then decreases with further increase in nickel loading. Furthermore, the mean size of nickel crystallites increases with nickel loading. On the other hand, the results show that activity of catalyst decays as a function of reaction time until 3h due to coking. The activity of catalyst was found to be independent of calcination temperature and time. The catalytic activity was increased with an increasing reaction temperature up to 773K, but decreased with a further increase in the reaction temperature. Moreover, RHA supported nickel catalysts display both higher specific nickel surface area and activity than silica gel as revealed by the H2-TPD and the hydrogenation tests. In the RHA-Al2O3 composite oxides supported nickel catalysts (Ni/RHA-Al2O3) aspect, the results show that the BET specific surface area of support decreases with the increase in alumina content, while the acidity of support increases with the increase in alumina content. The nickel aluminate formed after the drying step. The decomposition temperature of nickel aluminate to nickel oxide started above 773K. Furthermore, increasing the metal loading decreases the metal dispersion. The crystallite size of nickel supported on RHA-Al2O3 is larger than that of nickel supported on RHA. Generally, the catalytic activity increased with the reaction temperature increases up to certain value and then remain constant. The calcination temperature of 773K and calcination time of 4h were the optimum conditions for the preparation of catalysts. The activity of catalysts was strongly influenced by the reduction temperature rather than by reduction duration. Increasing the alumina content in RHA-Al2O3 composite oxides decreases the catalytic activity of the catalyst. In comparing the catalytic activity of 4.44wt.% Ni/RHA-Al2O3-4 with that of 4.29 wt.%Ni/RHA, it is found that the former exhibits a higher selectivity when the reaction temperature is higher than 773K. |
