不同的製備及塗佈銅錳鋅觸媒於微型甲醇重組器之研究

Different preparation and coating methods for Cu/Mn/ZnO catalysts in the compact micro-channel methanol reforming reactor

李紹淮(Shao-Huai Lee)

指導教授 ： 黃鈺軫

國立清華大學/原子科學院/生醫工程與環境科學系/碩士(2010年)

摘要

甲醇部分氧化(POM)反應製氫為一個放熱反應，不須額外添加熱源即可達到穩定狀態(steady-state)，有利於微型甲醇重組器系統微小化。微型重組器相較於固定式填充床，具有低壓降、不易爆炸、快速的熱傳與質傳等優點。本研究利用溶膠凝膠法(sol-gel)製備觸媒且以填滿且乾燥(Fill-and-dry)方法將觸媒沉積於以矽為基材之微型重組器進行測試。本實驗室已發展Cu/Mn/ZnO觸媒於甲醇部分氧化反應具有極佳之效能，故延用此觸媒使其沉積於微型甲醇重組器中。
本實驗嘗試以三種不同方法製備觸媒，一開始以檸檬酸複合法(citrate-complex method)製備觸媒，使觸媒於溶膠(sol)狀態時沉積於微流道中。由觸媒床發現觸媒活性隨著檸檬酸比例增加活性隨著下降。故選取添加檸檬酸與金屬比為一之觸媒以填滿且乾燥(Fill-and-dry)方法將觸媒沉積於重組器中，以觸媒溶液較濃之觸媒具有最佳活性，於225oC時17%甲醇轉換率與1.4*10-7 mol/min 氫氣產率。然而，觸媒均往流道兩側聚集觸媒分散不均勻。
為了解決觸媒分布不均之問題，利用檸檬酸凝膠法(citrate-gel or Pechini method)製備觸媒。藉由檸檬酸與乙二醇產生酯化反應與聚合作用使觸媒溶液黏度增加改善觸媒分佈。由SEM圖得知改善觸媒往兩側聚集之現象，但仍然具有許多裂痕，並導致微型甲醇重組器於甲醇部分氧化(POM)反應效能不佳。
事實上，具有良好之附著能力需觸媒與基材間產生化學鍵。先前之研究利用檸檬酸上氫氧基(-OH)與酸洗後矽基材上氫氧基(-OH)進行鍵結，但於鍛燒過程中檸檬酸在高溫下裂解導致觸媒附著能力下降。故選擇無機氧化鋁溶膠(boehmite/bentonite)做為黏著劑。於重量損失實驗中發現，隨著增加黏著劑比例增加對於觸媒與矽基材間極佳附著力也明顯增加。然而，隨著黏著劑比例增加反而導致觸媒活性下降。由於添加過量黏著劑導致觸媒活性位置遭受覆蓋以致觸媒活性下降。藉由調整觸媒漿料之固體含量、酸鹼值、顆粒大小、攪拌時間及黏著劑/觸媒比例，得到最佳觸媒漿料條件為固體濃度為5 wt%、礬土(bentonite)/軟鋁石(boehmite)為20 wt%、酸鹼值為七且攪拌時間為五小時。當此觸媒沉積於矽基材表面量為0.47 mg/cm2進行附著度測試，經由超音波震盪器震盪30分鐘重量損失為13%。微型甲醇重組器之甲醇部分氧化反應測試於225oC，甲醇轉換率為53%、氫氣選擇率為52%、氫氣產率為7.9*10-6 mol/min。

關鍵字

微型甲醇重組器 ； 觸媒塗佈 ； 氫能

並列摘要

POM is an exothermic reaction that be initiated and reach stable quickly without extra heat supply and suitable to be processed in the compact micro-channel reactor. The micro-channel reactor generally has advantages of low pressure drop, low probability of explosion, and fast in thermal and mass transfer. The heart of micro-channel reactors is active plates which were fabricated by fill-and-dry coating powders of active catalyst onto silicon-based micro-reactor and prepared catalysts by sol-gel method. Furthermore, based on the previous research on the laboratory, we developed the Cu/ZnO-based catalysts modified by manganese which had great performance under POM reaction.
The catalysts were prepared by three different procedures. First, we try to prepared catalysts by citrate-complex method. The catalytic activity of Cu/Mn/ZnO catalysts reduced with increasing ratio of citric acid in the POM reaction. In micro-reactor test, we selected the citric acid/all of metal ion ratio of 1 catalyst to load into micro-reactor by fill-and-dry method. At the 225 oC, the best activity of the catalyst had 17% conversion of methanol and 1.4*10-7 mol/min hydrogen yield. However, the catalytic layer had a serious problem that the catalyst was deposited on side of channel.
To solve this problem, we increased the viscosity of catalyst solution to enhance the adhesion to silicon substrate by citrate-gel method. SEM image showed that catalyst did not aggregate to side of channel because the viscosity of catalyst solution increased by esterification of ethylene glycol and citric acid. However, the catalytic layer still had many cracks. The result would lead to reduce performance of micro-reactor.
In fact, good adhesion was maybe attained by a formation of chemical bond with substrate. The previous study was based on the hydroxide group (-OH) of citric acid bonding with hydroxide group (-OH) of substrate. However, the catalytic layer was stripped adhesion due to the citric acid decomposed during calcination process. Thus, we use the inorganic binder of alumina sol (bentonite/boehmite) as binder. However, the catalytic activity was reduced with increasing binder/catalyst ratio. It was proposed that most of the catalyst particles are covered with the alumina sol and, therefore, the surface areas exposed to reactants is reduced. It would lead to reduce catalytic activity. Besides, the solid content, viscosity, pH, particle size and ratio of Binder/catalyst were important factors at coating catalyst. The optimum condition for preparation the slurry is the solid content (5wt.%), bentonite/boehmite (20wt.%), boehmite/catalyst (15wt.%), pH value (7) and stir time (5 hour) with ball mill technique. The weight loss was 13 wt.% when the density was 0.47 mg/cm2 on silicon substrate. At the 225 oC, the best activity of the catalyst had 53% conversion of methanol and 7.9*10-6 mol/min hydrogen yield.

並列關鍵字

micro-channel reactor ； catalyst coating ； hydrogen

參考文獻

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