鎳鈷鑭鈣鈦礦觸媒對低溫乙醇蒸氣重組產氫之研究

Abstract

在本研究中我們利用共沉澱氧化 (co-precipitation-oxidation) 法，並以具有節能、高效率的微波水熱法(microwave hydrothermal method)合成觸媒前驅物，再經空氣流600 °C 2小時煅燒得到觸媒，合成的觸媒分別以粉晶繞射(XRD)、傅立葉轉換紅外光譜(FT-IR)、紫外可見光譜(UV-Vis)、熱重分析(TGA)、穿透式電子顯微術(TEM)、氮氣等溫吸脫附(N2 isotherm adsorption)和層溫還原(TPR)等檢測。乙醇蒸氣重組(Stream Reforming of Ethanol; SRE)實驗是以流式固定床反應系統，在模擬生質乙醇的水醇比13(16 wt %)、進料速度0.6 mL/min、空間流速(WHSV)為22000 h-1的條件下進行反應，探討在反應溫度(200-400 °C)的反應活性及產物分布，並推論其反應機制。反應後的觸媒積碳(Coking)現象，分別以TEM、層溫氧化(TPO)及元素分析(EA)等分析。 以共沉澱氧化微波水熱法合成的初成品經XRD及TEM檢測為棒狀的La(OH)3，六方體狀的Ni(OH)2散布其間，煅燒後的觸媒經XRD檢測為Pervoskit結構的LaNiO3。在不同溫度的原位(in-situ)還原前處理下，分別比較觸媒活性差異，結果顯示400 °C還原前處理的觸媒活性最好，活性大小順序為：400 °C ＞ 500 °C ＞ 600 °C ＞ 300 °C；由TPR及TEM的特性分析顯示，經400 °C還原前處理的觸媒表面形成均勻分布的Ni金屬，應為其良好活性的主因，隨著還原溫度提高，Ni金屬顆粒增大，活性逐漸下降。 為比較不同合成方法觸媒對SRE反應的影響，另以傳統水熱法(hydrothermal method)製備LaNiO3觸媒，比較兩種水熱法經400 °C原位還原前處理溫度下的SRE活性表現，結果顯示兩種合成法的觸媒在反應溫度300 °C左右均有100 % 的乙醇轉化活性(XEtOH)，產物分布差異也相似，顯示前處理溫度對觸媒的活性影響遠大於合成法的不同。比較產物中氫氣選擇性(SH2)，傳統水熱合成法的觸媒之氫氣選擇性最高為75 %，而微波水熱法合成觸媒最高為73 %，兩種觸媒的氫氣選擇性在300 °C後均超過70 %。在積碳的研究中，經由XRD、TPO及EA的分析比較，水熱合成法製備的觸媒，由於具有少量La2O2CO3結構，其積碳量小於微波合成的觸媒。我們選擇傳統水熱合成法LaNiO3觸媒，作350 °C、80 小時的耐久實驗中，顯示乙醇轉化率維持100 %且氫氣的選擇性仍維持70 %，觸媒未因積碳而失活(Deactivation)，由XRD檢測顯示主要結構為La2O2CO3，具有消除積碳的的作用，能降低積碳生成，使觸媒不易失活，其積碳速率由TPO及EA實驗中計算得知為0.11 mgcarbon/gcatalyst•h。 為了降低LaNiO3觸媒在SRE反應中14 % 的甲烷選擇性，提升氫氣產量，嘗試在原有觸媒中添加入不同比例的Co，合成LaNi1-xCoxOx ( x = 0.25、0.5、0.75及1 ) 觸媒，由XRD檢測顯示煅燒後的觸媒均具Pervoskit結構。SRE的活性結果顯示乙醇完全轉換溫度隨著Co的添加量增加而升高，分別是350、400、400、425 oC；甲烷的選擇性隨著Co的添加量增加而降低，依序為4.5%、2.5 %、1.8 % 和1.4 %，氫氣的選擇性明顯提高，分別是77.1、76.7、76.9及71.9 %；Ni及Co的雙金屬觸媒，其氫氣的選擇性比原有純鎳或鈷觸媒為高；Co的引入確實能降低甲烷的量，但其乙醇完全轉換溫度卻逐漸由300 °C提高至425 °C。顯示Ni具有斷碳碳鍵及降低乙醇完全轉化溫度的功能，而引入Co對甲烷脫氫的能力，可由甲烷的減少與氫氣的提升得到證實，其中以LaNi0.75Co0.25Ox觸媒表現最佳。由本研究中得知適度的調配Co及Ni比例，製備LaNi1-xCoxO3觸媒應用於乙醇蒸氣重組產氫，可得到高的氫氣產量和二氧化碳及少量的甲烷和CO (低於0.1 %)，而La2O3消除積碳的能力，可使LaNi1-xCoxO3觸媒維持長效不易失活，具有成為未來重要產氫觸媒的雄厚潛力。

A LaNiO3 perovskite catalyst was prepared using coprecipitation-oxidation process using microwave-assisted hydrothermal method, followed by calcination in air flow at 600 °C for 2 h. The as-prepared sample was composed of La(OH)3 in nanorod structures and was covered with Ni(OH)2. The mixed metal hydroxides were converted into cubic LaNiO3 perovskite after calcination at 600 °C. A catalytic steam reforming of ethanol (SRE) reaction for hydrogen production was performed in a fixed-bed reactor. The gas hourly space velocity was maintained at 22,000 h-1. The activity evaluation was carried out stepwise as the temperature increased at a rate of 5 °C min-1 from 225 °C to 400 °C. The SRE reaction was performed using a water/ethanol molar ratio of 13 to mimic bioethanol obtained from biomass fermentation. Catalyst was in-situ activated in hydrogen flow at various temperatures (300, 400, 500 and 600 °C) prior to the reaction. Catalysts, before and after the SRE reaction, were examined by XRD, TGA, TEM, TPR and EA. Nearly 100 % ethanol conversion with 72 % H2 selectivity was achieved at 300 °C over the 400 °C in-situ reduced catalyst. The highly catalytic activity of the 400 °C reduced catalyst was due to the well-dispersion of Ni particles on the surface of active catalyst was formed in the in situ reduced catalyst. Based on TGA analysis, the amount of coke was 0.08 % after the SRE reaction. Another LaNiO3 perovskite oxide was prepared using the coprecipitation- oxidation hydrothermal (POH) method for comparing the activity of SRE reaction with microwave hydrothermal (POM) methods. The as-prepared sample was composed of La(OH)3 in nanorod structures but a few nanotubes and was covered with poorly crystalline Ni(OH)2. The XRD patterns showed a LaNiO3 perovskite structure for calcined catalyst. The catalyst was reduced in situ in hydrogen at 400 °C prior to the reaction. The ethanol conversion reached 100% at 300 °C with 70% hydrogen selectivity. The catalytic performance was sustained for the 80 h time-on-stream test at 350 °C with 100% ethanol conversion and 70% H2 selectivity. After an 80 h time-on-stream test at 350 °C, the used catalyst presented a La2O2CO3 component that was formed owing to the reaction of the CO2 product with La2O3. La2O2CO3 acted as a carbon reservoir to eliminate the deposited carbon and further stabilized the Ni particles on the La2O3 surface, which resulted in the highly catalytic activity during the entire reaction period. Based on TPO及EA analysis, the carbon deposition rate was 0.11 mgcarbon/gcatalyst•h after the 80 h test. LaNi1-xCoxO3 ( x = 0.25、0.5、0.75及1 ) perovskite oxides were prepared using coprecipitation-oxidation microwave hydrothermal (POM) methods. The replacement of Ni with Co was considering the capability of C-H rupture of Co for decreasing the 14% CH4 selectivity and increasing the 70% H2 selectivity over LaNiO3-POM catalyst on the SRE reaction. After calcination at 600 °C for 2 h, all the LaNi1-xCoxO3 catalysts showed perovskite structures. Based on the evaluation of SRE reaction, the temperature of ethanol complete conversion was increased as increasing the Co content. Moreover, the selectivity of H2 was increased to 77% and that of CH4 was decreased to 1.8%, as increasing the Co content. The LaNi0.75Co0.25O3 catalyst showed the best performance, 100 % ethanol conversion with 77 % H2 selectivity was achieved at 350 °C, on the SRE reaction. These results show that the moderate Co/Ni ratio of LaNi1-xCoxO3-POM catalyst can present the excellent performance, high H2 and low CH4 selectivities and a negligible amount CO (less than 0.1%), on SRE reaction. The LaNi1-xCoxO3 catalysts have good potential in commercialized catalysts for hydrogen production on SRE reaction in the near future.