奈米金/氧化鎂-氧化鈦在氫氣流中選擇性氧化一氧化碳之應用; Perferential Oxidation of CO in H2 stream on Au/MgO-TiO2

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Title:	奈米金/氧化鎂-氧化鈦在氫氣流中選擇性氧化一氧化碳之應用;Perferential Oxidation of CO in H2 stream on Au/MgO-TiO2
Authors:	陳致遠;Zhi-Yuan Chen
Contributors:	化學工程與材料工程研究所
Keywords:	氧化鐵;二氧化錳;氧化鎂;二氧化鈦;金;選擇性氧化;燃料電池;MnO2;TiO2;MgO;selective oxidation of CO;fuel cell;gold;Fe2O3
Date:	2007-07-03
Issue Date:	2009-09-21 12:28:02 (UTC+8)
Publisher:	國立中央大學圖書館
Abstract:	氫氣再近年來被認為是一種方便攜帶的能源。一氧化碳會由甲醇或其他燃料經部份氧化及結合氣液轉化反應來產生的副產物，一氧化碳可以在低溫的狀態下毒化燃料電池中的白金電極，使得電極的效率降低，甚至電極死亡，造成燃料電池無法運作。近年來，金觸媒已經使用在高濃度氫氣流去除一氧化碳，可以將一氧化碳濃度降低至50ppm。在此最主要的目的是找出觸媒可以對一氧化碳有高轉化率，但對氫氣有低的轉化性。我們發現金/二氧化鈦觸媒比起金/氧化鎂對一氧化碳有較高轉化率，但對一氧化碳選擇性較低於金/氧化鎂。我們希望加入適當的氧化鎂在金/二氧化鈦觸媒上，使其維持對一氧化碳的高轉化率及抑制對氫氣的轉化。本研究將硝酸鎂利用初濕含浸法，在573K煅燒4小時後製備在二氧化鈦上，成為氧化鎂/二氧化鈦之擔體。金觸媒是使用四氯酸金前驅物在338K及pH 8或9經由沉積沉澱法將金沉積在擔體上。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)及感應耦合電漿質譜分析儀(ICP-MS)。反應在固定床反應器中以每分鐘50毫升的流速進行，其進料氣體包含了65.33%氫氣、32.01%氦氣、1.33%一氧化碳及1.33%氧氣。所有的觸媒在一氧化碳及氧氣的比例為1.5時，都可以將氫氣流中的一氧化碳濃度減少至50ppm以下。但為了得到觸媒間不同的差異性，特意將一氧化碳及氧氣的比例保持在1及反應溫度不超過100℃的狀態下做比較。實驗結果中得到金/氧化鎂-二氧化鈦觸媒在353K及373K所顯現出對反應的特性都比金/二氧化鈦或金/氧化鎂的單一擔體觸媒好。可以透過穿透式電子顯微鏡(TEM)的顯現得知由沉積沉澱法製作在擔體上的金顆粒粒徑都在4nm以下。當擔體中鈦跟鎂的比例為4，製作pH值為8的觸媒，在反應溫度353K時有最佳的轉化率及選擇率。因此在加入適當的氧化鎂與二氧化鈦作為擔體的觸媒對反應有正向的幫助。依據文獻上的資料，二氧化錳及氧化鐵對金觸媒在反應上也有促進的效果，因此也使用了二氧化錳及氧化鐵做為促進劑。當氧化鎂/二氧化鈦擔體加入促進劑二氧化錳，可以使金在擔體上有好的分散度以及更小的顆粒，顆粒大小平均分布在1到2奈米之間。加入氧化鐵的觸媒會增加對反應的轉化率，原因可能是氧化鐵可以使擔體提供更多的氧。當觸媒加入9mol%或16.7mol%的促進劑氧化鐵，會造成一氧化碳的轉化率隨著溫度增加而降低。這可能是由於過多的氧化鐵無法提供有效的供氧機制，或者是在高溫反應主要是跟氫氣及氫氧基反應。金承載在加入促進劑二氧化錳或氧化鐵的擔體上，對增長反應的反應性並無太多的幫助。 Hydrogen has been recognized as a good energy carrier. When hydrogen rich fuel is produced from methanol or gasoline by partial oxidation and/or steam reforming combined with water gas shift reaction, the Pt anodes in fuel cell at these low temperatures are poisoned by CO, reducing the overall fuel cell performance. Gold catalyst has been used as a catalyst to oxidize CO in hydrogen to reduce CO concentration less than 50 ppm.1 Develop a catalyst which has high CO conversion and low H2 conversion is the target of this study. Au/TiO2 has high conversion of CO and low selectivity of CO oxidation; Au/MgO has high selectivity of CO oxidation and low conversion of CO. It was expected that by adding suitable amount of MgO on Au/TiO2, the catalyst may keep high CO conversion and suppress H2 conversion. In this study, varous MgO-TiO2 mixed oxides were prepared by incipient-wetness impregnation of aqueous solution of Mg(NO3)2. The materials were then calcined at 573 K for 4 h. Au catalysts were prepared by deposition-precipitation (DP) method using HAuCl4 as a precursor at pH 8-9 at 338 K. The catalysts were characterized by XRD, TEM, XPS and ICP-MS. The reaction was carried out at a fixed bed reactor with a feed containing 65.33%H2．32.01%He．1.33%CO and 1.33%O2 at 30000 h-1(GHSV). All the catalysts can decrease CO concentration less than 50 ppm providing that O2/CO ratio is 1.5. In order to magnify the difference for comparison , O2/CO ratio in the feed was kept at 1 and reaction temperature was kept below 373 K. Au/MgO-TiO2 has better performance than Au/TiO2 and Au/MgO, especially at 353 and 373 K. The size of gold particle in these catalysts was smaller than 4 nm as evidenced by TEM. The catalyst with Ti/Mg molar ratio of 4 and prepared at pH 8 had the highest CO conversion and selectivity at 353 K, the operation temperature of fuel cell. The results of this study demonstrated that adding suitable amount of MgO can suppress H2 oxidation without sacrificing CO oxidation. It is also in accordance with literature. MnO2 and Fe2O3 are promoter for gold catalysts. Fe3+ and Mn4+ were used as the promoters for 1% Au/MgO-TiO2(2:8) in this study. It was found very difficult to reproduce the preparation of small Au particles deposited on MnO2-MgO-TiO2 various ratio. Nevertheless, addition of MnO2 to MgO-TiO2 resulted in good reproducibility of manufacturing catalysts with a high dispersive Au phase with a narrow particle size distribution. The average of these gold catalysts is narrow at between 1 and 3 nm. The XRD patterns in figures have not appeared Au peaks. According the two phenomenon, the Au particles can deposited with small and well dispersed on MnO2-MgO-TiO2 various ratio. The addition of Fe2O3 promoter in the Au/MgO-TiO2 (2:8) prepared at pH 8 could increase the catalytic conversion at low temperatures with increasing oxygen supply. Nevertheless, when the catalyst with 9 mol% and 16.7 mol% Fe2O3 promoter has loaded on the support and subjected to PROX at high temperatures, CO conversion decreases with increasing temperature. It may be due to the non-availability of oxygen supply from Fe2O3, which would be inactive or it reacted with hydrogen and hydroxyl group at high temperatures. It may be due to the non-availability of oxygen supply from Fe2O3, which would be inactive or it reacted with hydrogen and hydroxyl group at high temperatures. The catalysts, Au/MgO-TiO2 (2:8) prepared by deposition-precipitation method at pH 8 reported high conversion and selectivity for PROX.
Appears in Collections:	[National Central University Department of Chemical & Materials Engineering] Electronic Thesis & Dissertation

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