金觸媒在選擇性一氧化碳氧化反應之應用; Preferential Oxidation of CO in H2 Stream over Supported Gold Catalysts

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题名:	金觸媒在選擇性一氧化碳氧化反應之應用;Preferential Oxidation of CO in H2 Stream over Supported Gold Catalysts
作者:	張立心;Li-Hsin Chang
贡献者:	化學工程與材料工程研究所
关键词:	選擇性一氧化碳氧化;二氧化錳;金;二氧化鈦;光沉積法;燃料電池;gold;photo-deposition;PROX;Fuel cell;Selective oxidation of CO;TiO2;MgO;MnO2
日期:	2008-06-04
上传时间:	2009-09-21 12:26:09 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	大顆粒的金一直以來都被視為穩定而不具觸媒活性的金屬，其無法應用的原因歸咎於金表面無法吸附反應物分子。然而，自從日本Haruta教授提出奈米級金擔載於氧化物擔體觸媒可在室溫下催化CO氧化的結果後，金觸媒才逐漸受到重視並應用於各種反應。近年來，金觸媒重要的應用莫過於是在富含氫氣流下進行選擇性CO氧化反應；此反應之所以受到重視的原因不外乎由於氫能源可利用在燃料電池發電，然而以甲醇或汽油重組反應產生的氫氣來源，會含有大量副產物CO，若將其直接應用於燃料電池會毒化白金電極並降低電能轉化效率，使用金觸媒於產氫反應的尾氣處理，可有效降低一氧化碳濃度至50 ppm，避免白金電極毒化。故本研究目的即為發展金觸媒具備能降低一氧化碳濃度，同時不氧化氫燃料。金觸媒擔載在二氧化鈦上具有很高的CO轉化率但同時也使氫氣反應，為了改進CO選擇率過低的問題，本文中分別也討論了兩種氧化物如：氧化鎂及氧化錳，作為促進劑，以改進單純使用Au/TiO2觸媒選擇率過低的問題。在本研究中，二氧化鈦擔體來自德國Degussa公司(P-25)，而混合物雙金屬氧化物擔體則使用初濕含浸法，將前趨物硝酸金屬溶液與二氧化鈦充分混合後煅燒製成；其後金以沉積沉澱法或光沉積法製備之。隨後金觸媒分別以感應耦合電漿質譜分析儀(ICP)，X光繞射分析儀(XRD)、穿透式電子顯微鏡(TEM)、氮-吸附(BET)、X光電子能譜儀(XPS)、紫外-可見光光譜儀(UV-vis)等儀器來鑑定其物理及化學性質。反應則以固定床反應器填充0.1g觸媒，並以進料CO/O2/H2/He體積比為1.33/1.33/65.33/32.01，總流量控制在50 ml/min進行反應。研究結果發現，在沉積沉澱法製備中，不論使用NaOH或NH4OH水溶液都可製備出具有奈米及金顆粒(< 4 nm)及高度分散性之Au/TiO2觸媒。其觸媒活性隨實際沉積之金含量遞減而遞減，其中，製備pH值由6提升至9時，沉積量遞減為pH 6的一半。最佳製備條件為pH 8 ，其觸媒具有最高CO轉化率及選擇率；然而選擇率隨反應溫度升高而降低，尤其以pH 6製備時觸媒為最明顯，其原因歸咎於氫氣與一氧化碳競爭氧化，另外，高沉積量的金觸媒容易受到反應溫度升高而聚集為大顆粒，然而對CO氧化而言金觸媒活性會明顯隨顆粒變大而下降，故導致選擇率明顯下降。此外，本研究發展光沉積法制備Au/TiO2觸媒，可成功獲得極小金顆粒約1.5奈米；其金顆粒大小決定於照光時間、光源強度及其他製備條件。在反應活性測試中，更可穩定維持活性達8小時，其中可能的反應機制也獲得證實。在添加劑的影響方面，吾人使用氧化錳及氧化鎂作為提升CO選擇率的促進劑，其中探討了添加劑錳/鈦與鎂/鈦的莫耳比、製備時酸鹼值對反應活性的種種影響。研究中發現，在一系列Au/MnO2-TiO2觸媒中，最佳活性的觸媒取決於適當的添加量、對金顆粒大小的影響以及對金的化學狀態的影響，發現以錳/鈦莫耳比2/98為最佳。而在Au/MgOx-TiO2觸媒中發現，在pH 9製備條件下之雙擔體金觸媒(Mg/Ti為2/8)，其CO轉化率不管相較於Au/TiO2或Au/MgO觸媒都來的高，加入氧化鎂並可提升CO選擇率。此研究結果證實，適當的促進劑搭配製備時酸鹼值調整，可成功獲得一系列高分散性奈米金觸媒，並具有高CO轉化率，同時避免氫氣氧化，應用於選擇性CO氧化反應。 Bulk gold had been regarded as an inactive catalyst due to the surface of gold which inhibits chemisorption of reactant molecules. Since Haruta and co-workers reported that the nano-gold catalysts could achieve CO oxidation efficiently below ambient temperature, several applications for gold catalysts have been under attention recently. One of the important cases in these applications for gold catalysts is preferential CO oxidation in hydrogen-rich stream (PROX). Hydrogen has been recognized as a good energy carrier since the development of fuel cell. 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 confirmed as a catalyst to oxidize CO in hydrogen stream to reduce CO concentration less than 50 ppm. 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 and Au/MnO2 have high selectivity for CO oxidation and low conversion of CO. It was expected that by adding suitable amount of MgO or MnO2 into Au/TiO2, the catalyst may retain high CO conversion and suppress H2 conversion. In this study, the promoter effect of various MOx-TiO2 mixed oxides was investigated and the support was prepared by incipient-wetness impregnation with aqueous solution of nitrate salt. TiO2 was supplied by Degussa Company (P-25). A series of preparation conditions of Au catalysts, prepared by deposition-precipitation (DP) method or photo-deposition (PD) method were discussed. The catalysts were characterized by inductively-coupled plasma-mass spectrometry (ICP-MS), X-ray diffraction (XRD), N2-sorption, transmission electron microscopy (TEM), high-resolution transmission microscopy (HRTEM), diffuse reflectance UV-vis spectroscopy (UV-vis DRS) and X-ray photoelectron spectroscopy (XPS). The PROX reaction was carried out in a fixed bed reactor with a feed containing 65.33% H2, 32.01% He, 1.33% CO and 1.33% O2 (vol. %) at 30000 h-1(GHSV). A high gold dispersion and narrow size distribution was obtained by DP method with different precipitating reagents, such as NaOH and NH4OH, for Au/TiO2 catalysts. The uptake of gold on TiO2 decreased as the pH value increased from 6 to 9. Au/TiO2 prepared at pH 8 has higher CO conversion and higher selectivity of CO oxidation than those prepared at other pH values. CO selectivity decreased with increasing reaction temperature on all catalysts. This phenomenon is more significant on Au/TiO2 prepared at pH 6 due to the uptake of more gold at low preparation pH value and hence the aggregation of gold particles during reaction. Au/TiO2 catalysts prepared by PD method had narrow particle size distribution of gold particles within a few nanometers. The catalysts prepared by this method could produce very small gold particles (1.5 nm) on the support. The size of gold nanoparticles deposited on the support depends on irradiation time and light source. We have successfully prepared very small gold particles with low power of UV light at suitable irradiation time and a possible mechanism of reaction has been proposed.
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