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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/3817


    Title: Au/FexOy 奈米材料之製備 及CO 氧化的應用;Preparation of Au/FexOy Catalyst and Its Application on CO oxidation
    Authors: 陳億玲;Yi-Ling Chen
    Contributors: 化學工程與材料工程研究所
    Keywords: 氧化鐵負載;CO氧化;奈米粒子;金觸媒;nanoparticle;selective oxidation of CO in H2;iron oxide support;fuel cell;CO oxidation;gold catalysts
    Date: 2004-05-10
    Issue Date: 2009-09-21 12:23:22 (UTC+8)
    Publisher: 國立中央大學圖書館
    Abstract: 摘要 自從日本的Haruta博士發現承載型金觸媒對低溫的CO氧化有特別高的活性,承載型金觸媒就已經是一個很熱門的研究題目,儘管已經投注這麼多的努力,但是在不同實驗室之間的報告,指出關於金的高活性仍然有很多的不確定因素及很廣泛的變異數,這些所有的結果都顯示觸媒的活性受到擔體的影響是淺而易見的。這個研究的主要目的,就是發展一個方法能製備具有高表面積和充足的OH基存在表面的氫氧化鐵擔體,當作金觸媒在CO氧化反應的擔體。 好幾個不同的參數被拿來研究氫氧化鐵的合成,像是鐵的鹽類(二價氯化鐵、三價氯化鐵、硝酸鐵)、pH值(從8到12)、煆燒溫度(從120℃到300℃)和進料的滴入速率等等,氫氧化鐵的物性鑑定使用XRD、TEM、XPS及氮吸附儀等儀器來量測,氫氧化鐵呈現出只有寬廣的XRD檢驗圖,沒有任何可定義的尖峰,顯示出氫氧化鐵可能是非晶相的物質或是粒子太小無法鑑定(<10 nm),從TEM的照片可以觀測到氫氧化鐵粒子的直徑小於20奈米,而XPS的圖譜顯現鐵氧化物介於Fe3O4和FeO兩個過渡相之間,氮吸附儀的分析則指出在適當的操作條件下可以使得氫氧化鐵的表面積>300 m2/g,甚至可達406 m2/g (控制在pH值11時以10ml/min的速率滴入最後再120℃煆燒),pH值和進料的滴入速率的控制是在得到高表面積的鐵氧化物的製備過程中扮演很重要的角色。氧化鐵的奈米粒子具有高活性被歸因於微小的粒子尺寸和高表面積及在表面存在高密度的同位不飽和活性位置,即使在室溫下,奈米級氧化鐵也展示出很高的活性,所以不但是高表面積的氧化鐵也具有很好的轉化率,很顯然的,奈米級氧化鐵不但是一個很好的觸媒也是一個很適當的金觸媒的擔體,就我所知,吾人所製備之氧化鐵的高表面積和室溫下就有的CO轉化率優於目前所有的文獻上的結果。 承載型金觸媒用析出沈澱法製備,以HAuCl4當作金的前驅物,使用上述合成方法製備的氧化鐵為擔體,另外也採用化學還原法製備把金的複合物還原成元素金,總共使用這兩種製備方法。金觸媒的物性鑑定使用氮吸附儀、XRD、TEM、TEM-EDS及XPS等儀器來量測,氮吸附儀分析得到金觸媒的表面積>250 m2/g,XRD的結果顯示金粒子尺寸小於偵測極限以致於無法測量,TEM的影像非常清楚顯現所有金的粒子直徑小於4奈米,由TEM-EDS可以知道金粒子的確存在,而金觸媒的Au 4f7/2的XPS圖譜皆呈現束縛能小於84.2 eV,因此金是以金屬元素態Au0存在,這份研究的製備方法將導致奈米級金粒子完全地均勻散佈且直徑小於4奈米和得到狹窄的尺寸分佈。 探討觸媒製備條件對金觸媒的低溫CO氧化性能的影響,觸媒活性的測試是用連續流動的固定床反應器。首先,我們使用還原法製備以氧化鐵為擔體的金觸媒在室溫下進行CO氧化反應,合成條件是保持pH值9再以120℃和180℃煆燒,吾人可觀察到獨特的重點就是擔體的表面積對觸媒活性的效應,這個研究明顯地展現出高表面積的鐵氧化物擔體可促使觸媒活性的提高,也指出增加鐵氧化物擔體的表面積將促進CO氧化並減緩觸媒的毒化。 再者用析出沈澱法製備Au/FexOy觸媒,控制pH值9或10.5再分別以120℃和180℃煆燒,並在室溫進行CO氧化反應,我們的重點在金觸媒對CO氧化反應受pH值的效應,且能夠很顯著的看到適當的pH值可以增加觸媒活性,很顯然地恰當的pH值是最重要的因素,而最佳的pH值是9,這也顯示選擇適當的pH值和煆燒溫度,所造成高觸媒活性對低溫CO氧化反應相較於高表面積的擔體是更有效的。當Au/FexOy觸媒製備時使用不同的pH值及120℃煆燒而擔體的表面積相同,析出沈澱法製備的Au/FexOy觸媒的活姓高於化學還原法,比較析出沈澱法和化學還原法,當採用高表面積的氧化鐵擔體以pH值9合成用180℃煆燒,所有的Au/FexOy觸媒都達成100 % CO轉化率,換句話說,若是維持pH值9及180℃煆燒時,其他的製備參數的影響將不明顯,在這份報告中,不論Au/FexOy觸媒的製備是用析出沈澱法或化學還原法,只要是承載在高表面積的擔體並以pH值9合成最後用180℃煆燒,都能表現出最佳的CO氧化性能,達到全部的轉化率在室溫下至少保持90分鐘。 Abstract Supported gold catalyst has been a subject of intense investigation since the report of its exceptionally high low-temperature CO oxidation activity by Haruta et al. In spite of these efforts, however, there is still great uncertainty of the cause of the high activity and there is a wide variation in the activities reported among different laboratories. Superficially, all results suggest that the catalytic activity depends on the support. The aim of this study was to develop a method to prepare iron hydroxide support which has a higher surface area and abundant hydroxyl groups on the surface. It was used as a support for gold in CO oxidation. Several parameters have been investigated for the synthesis of iron hydroxide, such as iron salt (FeCl3.6H2O, Fe(NO3)3.6H2O and FeCl2.4H2O), pH value (from 8 to 12), calcination temperature (from 120℃ to 300℃), feeding rate, etc. The iron hydroxide support was characterized by powder X-ray diffraction, TEM, XPS and N2 sorption. The XRD patterns of the iron hydroxide appeared only wide, without any definite XRD peaks, suggesting that the material was either amorphous or of a particle size too small (< 10 nm). TEM images show that the particle diameters were less than 20nm. XPS Fe 2p3/2 spectra showed the phase transition of iron oxide from Fe3O4 to FeO. The N2 sorption analysis indicated that the surface area of iron hydroxide was greater than 300 m2/g under suitable preparation conditions. The pH value and the feeding rates played the important roles to obtain iron oxide with high surface area. The higher activity of FexOy nanoparticles was attributed to a small particle size, high surface area and more densely populated surface coordination unsaturated sites. These nanosized iron oxide samples also demonstrated to have high activity even at room temperature. The higher the surface area of the iron oxide is, the higher the CO conversion is. Obviously, in this work, nanosized iron oxide was characterized both as a catalyst and as a support of Au catalysts. Supported gold catalysts were prepared by deposition-precipitation using HAuCl4 as the Au precursor. The as-synthesized iron hydroxide was used as the support. The in-situ chemical reduction method was applied to reduce gold compound to metallic state. The gold catalysts were characterized by N2 adsorption, XRD, TEM, TEM-EDS and XPS. The N2 adsorption analyses indicated that the surface areas of Au catalysts were greater than 250 m2/g. The XRD results demonstrated that gold metal had a particle size under detection limit, which is less than 4 nm. TEM images clearly showed that the particle diameters of gold for all the samples were less than 4 nm. TEM-EDS showed that gold is present on the Au catalysts in the form of metallic I particles. XPS spectra present the Au 4f7/2 peaks of the Au catalysts at binding energy below 84.2 eV. Therefore, Au is in metal state Au0. The method applied in this study leads to a fairly uniform dispersion of gold nanoparticles with diameter less than 4 nm and narrow size distribution. Effects of catalysts preparation conditions on the performance of gold catalysts for low-temperature CO oxidation was carried out. The catalytic activity was measured using a fixed bed continuous flow reactor. At first, for CO oxidation at room temperature, the iron oxide-supported gold catalysts were prepared by reduction process. The materials were synthesized at pH value of 9 and calcined at 120 ℃or 180.℃ A specific emphasis was on the effect of surface area of the support on the catalytic performance. This research clearly showed that higher surface area of FexOy support gave a higher catalytic activity. It should be pointed out that increasing the surface area of iron oxide support resulted in an enhancement over the CO oxidation and retard decay of catalyst. Secondly, CO oxidation of Au/FexOy catalysts prepared by deposition -precipitation method, synthesized at pH value of 9 and 10.5 and calcined at 120 ℃and 180℃ were tested. We focused on the effect of pH values on the CO oxidation over the Au/FxOy catalysts. It can be seen that the apparent increase in catalytic activity of the catalyst was due to the proper pH value. Obviously, the suitable pH value is an important factor, and the optimum value is 9. It also demonstrated that suitable selection of pH value and calcination temperature play a key role in creating high catalytic performance for low-temperature CO oxidation which are more effective than the high surface area of support. For the iron oxide-supported gold catalysts synthesized at different pH values and calcined at 120℃, the catalyst pretreated by in-situ reduction process was much less active than those by deposition- precipitation method, providing the same high surface area of support. Comparing the samples prepare by the deposition-precipitation and in-situ reduction, using high surface area support and calcined at 180℃ after preparation, all the samples demonstrated 100 % conversion of CO. In other words, if the pH value during deposition of Au was 9 and calcinations temperature was 180℃, there is no significant effect on the other preparation parameters. In this study, the Au/FxOy catalyst, prepared by either deposition-precipitation method or reduction process, supported on high surface area of iron oxide, synthesized at pH 9 and calcined at 180℃, shows the best performance of CO oxidation. Full conversion was kept at ambient temperature over 90 min on stream. II
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