奈米金/氧化鈰觸媒之製備及在氧化反應之應用; PREPARATION OF CERIA-SUPPORTED NANO-GOLD CATALYSTS AND ITS APPLICATION IN OXIDATION REACTION

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題名:	奈米金/氧化鈰觸媒之製備及在氧化反應之應用;PREPARATION OF CERIA-SUPPORTED NANO-GOLD CATALYSTS AND ITS APPLICATION IN OXIDATION REACTION
作者:	董慕愷;Mu-Kai Tung
貢獻者:	化學工程與材料工程研究所
關鍵詞:	選擇性氧化;一氧化碳;氧化鈰;金;燃料電池;ceria oxide;CO oxidation;selective oxidation of CO in H2;fuel cell;nanoparticle;gold
日期:	2005-05-17
上傳時間:	2009-09-21 12:24:26 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	一直以來，金都被視為沒有活性的鈍態金屬物質，直到日本的Haruta博士於1989年發現當奈米級的金承載到擔體上時具有很高的活性，能夠在低溫環境催化一氧化碳氧化，金觸媒才開始被重視。雖然近年來對於金觸媒的研究很多，卻尚未有令人滿意的結果發表，顯示其仍有改善的空間。金觸媒的活性會隨製備方法、金的粒徑、形狀、反應環境及擔體而改變，沈澱固著法可將金粒子以半球型的方式承載於擔體上，進而增加金與擔體的接觸面積，且金粒子均勻散佈於擔體上，使反應活性提高。擔體需選擇可被部分還原的氧化金屬。本研究的主要目的，是製備出一種具有高活性的觸媒，能夠在室溫下完全氧化空氣中的一氧化碳，並在富氫氣環境中除去一氧化碳而不造成氫氣的損失。本研究將奈米級的金承載到一系列以不同方式製備氧化鈰擔體上，其中包含由Degussa公司取得之奈米氧化鈰(NanoCeria)以及由沈澱法製備之氧化鈰擔體。本研究中探討氧化鈰製備條件對於CO氧化反應之影響，包含酸鹼值(8到11)以及煆燒溫度(120到400oC)。承載型金觸媒是以沈澱固著法製備，以HAuCl4當作金的前驅物，並探討金的煆燒溫度(120以及180oC) 對反應活性的影響；在本研究中亦討論了加入錳於二氧化鈰中所製備之鈰錳複合擔體對於反應之影響。觸媒的物性鑑定使用包括Ｘ光繞射分析儀(XRD)、穿透式電子顯微鏡(TEM)、X光電子能譜儀(XPS)及氮吸附儀等儀器來量測。從XRD圖譜中可觀察到擔體皆為結晶良好的二氧化鈰，此外亦可發現XRD分析圖譜中偵測不到金的波峰，代表金的顆粒太小超過儀器的偵測限制(5奈米)，使其無法量測；TEM圖中則可以清楚看出奈米級金粒子完全地均勻散佈於擔體上，粒徑均小於4奈米，並可觀察到二氧化鈰擔體的顆粒大小約為10到20奈米。XPS量測結果顯示在180oC煆燒所製備的金觸媒具有較多的元素態。本研究以連續式固定床反應器來測試金承載於氧化鈰觸媒應用於一氧化碳氧化反應之活性。在所有的觸媒之中，金乘載於Degussa公司之二氧化鈰具有最高的CO氧化活性。比較金乘載於以不同製備條件之二氧化鈰擔體的活性，發現乘載於酸鹼值為10，400oC煆燒下製備之二氧化鈰擔體之金觸媒有最高的反應活性；此外，研究結果亦顯示當金在180oC煆燒時，對於室溫一氧化碳之氧化具有較高的活性。至於選擇性氧化方面，結果同樣顯示從Degussa公司所取得之氧化鈰具有最高的活性，這部分與室溫一氧化碳氧化反應之結果相同。若是針對金的煆燒溫度來探討時，卻發現當金以120oC煆燒，其對於50oC以上反應的活性表現是優於180oC的，此結果與室溫一氧化碳氧化反應是不同。金承載於鈰錳複合擔體的CO選擇性氧化結果顯示添加錳可增進觸媒於80oC反應的活性與選擇性。 Bulk gold has been regarded as a poorly active catalyst. A theoretical calculation has explained the smooth surface of Au is noble in the dissociation adsorption of hydrogen. However, when Au is deposited as nanoparticles on metal oxides, it exhibits surprisingly high catalytic activity for CO oxidation at a temperature as low as 200K. Removal of CO in H2 stream is an important subject in fuel cell performance. The aim of this study was to investigate the effect of preparation on the characteristics of CeO2. In addition, preferential oxidation of CO in H2 stream (PROX) over gold supported on CeO2 and MnO2-CeO2 mixed oxide were investigated. Several CeO2 supports were used in this study including NanoCeria from Degussa Company and a series of home-made CeO2 which were prepared by precipitation at constant pH using cerium nitrate and NH4OH as the starting materials. Several synthesis parameters, such as pH value (from 8 to 11) and heating temperature (from 120 to 400oC), have been varied. For the mixed oxide support, MnO2-CeO2, was prepared by impregnation method. Gold catalysts supported on CeO2 and MnO2-CeO2 were prepared by deposition-precipitation using HAuCl4 as the Au precursor. The effects of calcination temperature on the catalytic properties after loading gold were also studied. The supported gold catalysts were characterized by powder X-ray diffraction (XRD), N2 sorption, transmission electron microscopy (TEM), and X-ray Photoelectron Spectroscopy (XPS). The results show that CeO2 support prepared at pH 10 had the highest surface area (104 m2/g). The crystallinity of CeO2 increased with an increase of calcination temperature. TEM images showed that the particle sizes of CeO2 were about 20 nm. The XRD results showed that gold metal had a particle size under detection limit, which was less than 4 nm. TEM images confirmed that the particle sizes of gold for all the catalysts were less than 4 nm. The method applied in this study leaded to a fairly uniform dispersion of gold nanoparticles with diameter less than 4 nm and narrow size distribution. XPS spectra showed that metallic and oxidized gold species coexisted within nano gold particle. For the effects of calcined temperature after loading gold, XPS study showed that more metallic gold species were present in gold nanoparticles calcined at 180oC. Activities for CO oxidation on Au/CeO2 catalysts were measured using a fixed bed continuous flow reactor. Gold catalysts supported on NanoCeria from Degussa Company demonstrated the highest activity. For gold supported on home-made CeO2 catalyst, the CeO2 support prepared at pH value 10 and calcined at 400oC showed the highest activity. For the effects of calcination temperature after loading gold, the catalyst which calcined at 180oC after loading gold had the highest activity in CO oxidation than those calcined at 120oC. In contrast, gold catalyst which calcined at 120oC after loading gold showed a higher activity in preferential oxidation of CO in H2 stream at the reaction temperature above 50oC than those calcined at 180oC. Furthermore, the gold catalyst supported on MnO2-CeO2 mix oxide showed higher CO conversion and selectivity to CO oxidation by adding MnO2 on CeO2. By regulating the preparation and pretreatment procedure and the compositions of Au/MnO2-CeO2 catalyst, we have developed a catalyst which had high CO conversion and high selectivity to CO oxidation in PROX reaction.
顯示於類別:	[化學工程與材料工程研究所] 博碩士論文

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