| 摘要: | 長久以來，金觸媒是一門很熱門的研究題目並且被廣泛的討論。儘管在眾人的努力研究之下，金觸媒所擁有高活性的可能原因還是無法找出一個統一的解答，而且隨著實驗室的不同，也產生了分歧的見解。一般而言，所有的見解都提出擔體是影響金觸媒活性好壞的重要因素。而本篇研究主要是想發展出具有高度活性的金觸媒，而此一金觸媒是以氧化錳為擔體。在合成氧化錳的過程中，許多製備參數被改變，例如合成環境的pH值 (從8 ~ 12 )、煆燒溫度 (從120 ~ 400度)等等。除此之外，亦利用了好幾種方法來製備氧化錳，如固態反應法跟熱分解法等。此外，亦購入商業上所使用的氧化錳。這些氧化錳藉由XRD、XPS與氮吸附儀等儀器加以鑑定。由氮吸附儀的分析中得知由固態反應法所合成出的二氧化錳，在適當的合成環境下其表面積最高 (190 m2/g)。 金觸媒是藉由沈澱固著法來加以製備，以HAuCl4來當作金的前驅物，並以上段中所提到的氧化錳來當作擔體。而金觸媒亦由XRD、TEM 與XPS等儀器來加以鑑定。由於製備出的金粒子其粒徑約在4奈米之間因此超出XRD的偵測範圍，然而由TEM的鑑定結果可得知金的粒徑大小約在3~4奈米。而XPS亦顯示出金是以元素態存在。由上述鑑定結果可知本研究所使用的製備方法可製備出高度分散的奈米金粒子，其粒徑分佈約在3~4奈米之間。 對於低溫環境下的CO氧化反應方面，本研究是使用連續式固定床反應器。由反應結果顯示結晶性較好的氧化錳擔體具有較高的活性。此外，當金承載於四氧化三錳時，其反應性比承載在二氧化錳上差，這是因為二氧化錳是一種可還原性擔體。而可還原性擔體可提供CO氧化反應中所需的氧。 對在氫氣流下CO選擇性氧化反應方面，結晶程度高的二氧化錳擔體仍具有較高的觸媒活性。而承載在二氧化錳的金觸媒其反應活性依舊比承載在四氧化三錳者為高。而本篇研究中，氧化鈰以含浸法的方式添加進二氧化錳中。由於錳跟鈰彼此間交互作用的影響下，承載於其上的金觸媒在室溫一直到65度的反應環境下時，比單純承載在二氧化錳的金觸媒具有更高的活性。 Supported gold catalyst has been a subject of intense investigation. In spite of these efforts, 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 manganese oxide to be used as a support for gold to have a high activity. Several preparation parameters have been investigated for the synthesis of manganese oxide, such as pH value (from 8 to 12), calcination temperature (from 120℃ to 400℃), etc. Besides, several methods have been used to prepare MnO2, such as solid state reaction method and thermal decomposition. Some MnO2 was from commercial source. The manganese oxide was characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and N2 sorption. The N2 sorption analysis indicated that the surface area of MnO2 prepared in solid state reaction was very high (190 m2/g) under suitable preparation conditions. Supported gold catalysts were prepared by deposition-precipitation using HAuCl4 as the Au precursor. The as-synthesized manganese oxide was used as the support.The gold catalysts were characterized by XRD, TEM, and XPS. The XRD results demonstrated that gold metal had a particle size under detection limit, which was less than 4 nm. TEM images clearly showed that the particle diameters of gold for all the samples were 3-4 nm. XPS spectra presented the Au4f7/2 peaks of the Au catalysts at binding energy below 84.0 eV. Therefore, the Au was in metal state Au0. The method applied in this study leads to a fairly uniform dispersion of gold nanoparticles with diameter 3-4 nm and narrow size distribution. For low-temperature CO oxidation, the reaction was carried out. The catalytic activity was measured using a fixed bed continuous flow reactor. This research clearly showed that the well-crystallized MnO2 support gave a higher catalytic activity. In addition, in CO oxidation, the activity of 1 wt. % Au/Mn3O4 was lower than that of 1 wt. % Au/MnO2. This is because MnO2 is one of the reducible supports. The reducible support also can supply oxygen to the CO oxidation. For CO selective oxidation in hydrogen stream, the well-crystallized of MnO2 support also gave a higher catalytic activity. The activity of 1 wt. % Au/Mn3O4 was lower than that of 1 wt. % Au/MnO2. In this study, CeOx was added in MnO2 by impregnation method. There was some kind of interaction between Mn and Ce. This interaction promoted the activity for CO oxidation in H2 from room temperature to 65 oC. |
