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  • 學位論文

金觸媒在低溫水氣轉移反應上的應用

Gold Catalyst for Low Temperature Water-Gas Shift Reaction

指導教授 : 萬本儒

摘要


本研究藉由沉積沉澱程序將奈米金擔載於金屬氧化物CeO2與TiO2上,應用於低溫水氣轉移反應(200 oC),期望製備出具備高催化活性與穩定性之觸媒,探討的變因包含: (1) 製備程序的調整;(2) 擔體的選擇;(3) 前處理效應;(4) 水量對反應的影響。觸媒相關物化性質是由HRTEM、FTIR、XPS、TPR、AA與BET等儀器鑑定。 有關Au/CeO2研究,前處理效應顯示還原性氣體處理後的催化活性優於非還原性處理,且使用CO比H2更佳。由儀器鑑定可知還原性處理可使擔體表面傾向還原態、增加反應活性基;其中使用CO前處理可產生最多的氧空缺,似乎顯示Au/CeO2擔體上的氧空缺比金量更能影響催活性。另外,水量的多寡不僅會影響催化活性,同時也會影響穩定性;在使用可產生高純度氫氣的甲烷蒸汽進料時水量達20 mol%以上是較適之反應條件。探討沉積沉澱程序變因對製備觸媒影響時發現金溶液pH值( 6或 9)及CeO2是否經表面修飾,對金擔載量及Au/CeO2催化活性並無明顯影響。TPR研究結果顯示雖然CeO2經表面修飾會造成擔體的脫氧(被還原)能力明顯下降,但奈米金的存在能促使大部分CeO2在低溫脫氧,此時擔體表面缺陷造成的活性差異可被忽略。 有關Au/TiO2研究,前處理效應對催化活性不似Au/CeO2有明顯影響,但相同金含量下Au/TiO2的活性遠比Au/CeO2來的好。有關沉積沉澱程序變因對製備觸媒影響,增加金溶液pH值至9會減少金擔載量,但不會明顯改變單位金量之催化活性,因此就觸媒單位重量的催化活性而言,以pH6製備的Au/TiO2具高活性,是本研究中最佳的金觸媒。 本研究選用活性最佳的Au/TiO2(pH6)觸媒,在文獻反應條件下與各觸媒進行比較,發現其活性都比文獻結果來得高,且十分穩定、不受反應組成或溫度變化影響。因此,未來發展應以TiO2做為擔體,進行後續研究。

並列摘要


The main goal of this reseach is to prepare a gold catalyst with good activity and stability for low temperature water-gas shift reaction (WGS). The following effects on reaction activity were studied: (1) catalyst preparation methods; (2) choice of supports (CeO2 or TiO2); (3) catalyst pretreatment before WGS; (4) water amount during WGS. Moreover, the properties of the catalysts were investigated by using HRTEM, FTIR, XPS, TPR, AA and BET. For nano-gold supported on CeO2 (i.e., Au/CeO2), the catalyst pretreated with reductive gas possessed much higher WGS activity than that with non-reductive gas, among which activity under CO is better than that under H2. Reductive pretreatments lead to larger amounts of metallic gold on the catalysts and oxygen vacancies in CeO2, which can be the reason for better activity preformances. Research work concerning the effect of water amounts revealed that the discrapencies in water amounts would affect both the activity and stability for WGSR. When the reactant gas with highest H2/CO ratio, derived from steam-methane reforming, was introduced into the reactor, the amount of water should be higher than 20 mol% to prevent the oscillation of activity. For the study of variables in deposition-precipitation method to prepare Au/CeO2, pH value of gold solutions and a prior modification of CeO2 caused little differences on the activity of gold catalysts. Desorption of surface oxygen from CeO2 became more difficult after the prior modification, thus lowered the activity. However, the presence of nanogold causes CeO2 to be reduced under low temperature, which could compensate the poor performance of modified CeO2. For nano-gold supported on TiO2 (i.e., Au/TiO2), difference between pretreatments was less significant than that seen in Au/CeO2, but Au/TiO2 had much better activity than Au/CeO2 at 200 oC based on same gold amounts. The catalyst prepared at pH 9 had activity similar to that prepared at pH 6 per weight of gold, but possessed lower gold loading. Therefore, higher activity was obtained for Au/TiO2 prepared at pH 6 based on the same weight of total catalysts. Finally, by testing in different reaction conditions, it was found that Au/TiO2(pH6) possessed higher activity than other catalysts reported in the literature. Moreover, it was stable in long-term activity test. In summary, Au/TiO2(pH6) is the most suitable one for practical uses and future study on this catalyst is required for better activity performances.

參考文獻


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