稻殼灰分擔載銅觸媒之製備與應用研究; Dehydrogenation of Etahnol over Copper Catalysts Supported on Rice Husk Ash

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题名:	稻殼灰分擔載銅觸媒之製備與應用研究;Dehydrogenation of Etahnol over Copper Catalysts Supported on Rice Husk Ash
作者:	郭文堯;Wen-Yao Kuo
贡献者:	化學工程與材料工程研究所
关键词:	稻殼灰分;擔載銅觸媒;乙醇脫氫;ethanol dehydrogenation;rice husk ash;supported copper catalyst
日期:	2005-06-28
上传时间:	2009-09-21 12:16:58 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	由廢棄稻殼製成稻殼灰分(RHA)，作為各種材料用途，近年來廣受矚目。本研究利用RHA作為擔體材料，以含浸法分別製備Cu/RHA與Cu/Cr/RHA兩種擔載銅觸媒，論文內容就以這兩個部分為主軸，探究銅金屬載量、煅燒溫度、氧化鉻促進劑等項因素對於觸媒表面特性與催化活性的影響。有關表面特性分析方面，採用氮吸附法(N2 sorption)、感應偶合電漿原子發射光譜儀(ICP-AES)、掃描式電子顯微鏡(SEM)、X-射線繞射儀(XRD)、熱重分析(TGA)、程溫還原(TPR)、N2O分解吸附(dissociative adsorption of nitrous oxide)等項目; 至於催化活性方面，則是利用乙醇在常壓下進行脫氫反應，加以分析比較。含浸法製備Cu/RHA觸媒的實驗結果顯示：經XRD圖譜與SEM影像的比對，證實較高銅金屬載量會引發銅金屬顆粒的聚集。經過煅燒的Cu/RHA前驅物，其TPR圖譜顯示氧化態的銅顆粒之中，CuO與Cu2+乃是同時存在的，其中Cu2+ 較CuO具備較強的「金屬-擔體效應」 (MSI)。銅金屬載量與脫氫反應的轉化率以及TOF似乎關聯性不大。隨著反應溫度的升高，乙醇脫氫反應的轉化率亦隨之提昇。此外，由乙醇脫氫反應的結果證實，擔載於稻殼灰分銅觸媒(Cu/RHA)的催化活性高於擔載於二氧化矽者(Cu/SiO2)。製備Cu/RHA觸媒的理想煅燒溫度大約723 K 左右，此一條件所製得的觸媒同時具備較高催化活性與較低活性衰退速率。含浸法製備Cu/Cr/RHA觸媒實驗結果顯示：隨著氧化鉻促進劑添加量增加至2 wt%為止，銅金屬的分散度亦隨之增加，並達到極大值; 但當氧化鉻添加量超過2 wt%時，銅金屬的分散度又逐漸隨著添加量而遞減。此外，2 wt%左右的氧化鉻添加量，不僅增進銅觸媒的催化活性，並可抑制觸媒活性衰退速率。銅觸媒的衰退主要由於銅金屬燒結所導致。RHA擔體的BET表面積雖然遠低於一般商用SiO2擔體，然而，RHA擔體表面孔洞大多屬於單一孔洞，而SiO2擔體表面大多屬於內部聯結的孔洞型態，相較之下，後者較易於製備過程中造成銅金屬顆粒阻塞孔洞，使得金屬分散度大幅降低。是故，RHA擔體優於一般商用SiO2擔體，更適合於擔載型銅觸媒的製造。 Rice husk ash-supported copper catalysts including both unpromoted (Cu/RHA) and Cr2O3 - promoted (Cu/Cr/RHA) ones, which were prepared by incipient wetness impregnation, have been investigated in two parts of this dissertation, respectively. Surface characterization by XRD(X-ray diffraction), TGA (thermogravimetric analysis), TPR (temperature-programmed reduction), SEM(scanning electronic microscopy), and H2-N2O titration, and catalytic activity by ethanol dehydrogenation have been examined extensively. The results derived from each part will be described as follows: 1. For the preparation of Cu/RHA catalysts: TGA results reveal that both the unsupported and RHA-supported precursors having been derived from copper nitrate trihydrate are completely converted to cupric oxide above ca. 600 K. The XRD patterns and SEM images show that higher copper loading leads to the agglomeration of CuO crystallites. The TPR profiles denote the probable existence of both CuO and Cu2+ in the calcined copper catalysts. CuO exhibits weak metal-support interaction (MSI) while Cu2+ in much lower content exhibits strong MSI. Ethanol conversion and turnover frequency (TOF) show little dependence on copper loading. Ethanol conversion increases with an increase in reaction temperature. Furthermore, copper catalysts supported on rice husk ash display higher catalytic activity than those supported on silica gel, as revealed by the test of ethanol dehydrogenation. Calcination temperature around 723 K, giving a high initial activity and a low deactivation rate, seems to be an optimal condition for manufacturing Cu/RHA catalyst precursors in this work. 2. For the preparation of Cu/Cr/RHA catalysts: Copper dispersion can be enhanced by the initial increase in Cr2O3 promoter content up to 2 wt%, while it then deteriorates gradually upon further increase in promoter content. It has been suggested that an optimal Cr content around 2 wt% not only enhances catalytic activity but also retards catalyst deactivation. Generally speaking, catalyst deactivation results predominantly from copper sintering. Despite of the lower BET surface area, RHA is superior to commercial silica gel as a candidate for catalyst support in this work, because the surface of the former may possess more unique pores, while the majority of surface pores on the latter are interconnected and clogged easily.
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