A novel bimetallic Rh-Ni/BN catalyst for methane reforming with CO2

甲烷-二氧化碳重組反應可利用溫室氣體(CH4、CO2)生成具有高價值的氫氣與一氧化碳。工業上多使用鎳觸媒，但會因為積碳與金屬燒結的關係造成觸媒失活，於是利用加入第二金屬或將金屬負載在特殊載體上，進而改善觸媒失活並提高活性。 氮化硼(Boron Nitride，BN)和傳統氧化物載體，如氧化鋁等比較，具有許多特有的性質。根據之前的研究由於BN為惰性物質不易與金屬產生交互作用，特別容易有合金產生，有助提升活性。我們利用共臨界沾濕法製備不同比例的Rh-Ni/BN 觸媒，並另外製備Rh-Ni/γ-Al2O3作為比較。將所製備的觸媒分為未經鍛燒及經不同溫度鍛燒後的新鮮觸媒作為比較，並在不同溫度氫氣還原後進行實驗。所使用的觸媒檢測方法有氮氣比表面積測定(BET)、氫氣化學吸附測定、TPR、XRD、XPS及TEM。 由BET結果發現觸媒在還原前後並不會大幅影響表面積；由氫氣化學吸附結果發現Rh的加入可以增加觸媒上金屬的分散性;由TPR結果發現，加入第二金屬之後觸媒的還原溫度會移動至純鎳與純銠的還原溫度之間，推測也許有合金或是其他物種的產生，且金屬粒子在BN載體上所受到的作用力較γ-Al2O3小，使得金屬粒子在BN載體上容易自由移動而形成聚集體；由TEM中可以發現在BN上加入第二金屬後開始有非單獨金屬粒子的出現，推測是雙金屬彼此聚集形成聚集體或合金；由XPS分析結果中，發現負載於BN上的金屬在還原過後會變成金屬態，而負載於γ-Al2O3中的Ni金屬經過還原後的價態幾乎沒有改變，因此推測雙金屬負載於γ-Al2O3時兩種金屬之間並沒有發生作用。 反應結果顯示銠的加入有效地增加了Ni/BN的活性及觸媒壽命，在700℃的反應溫度下，甲烷與二氧化碳的轉化率可達到90%，比使用Rh-Ni/γ-Al2O3來的高，所得H2/CO的比例約為0.7且經過六小時的反應之後並沒有明顯的失活現象。而觸媒銠與鎳的比例會影響反應結果並存在一個最適值，當Rh/Ni為0.01時，甲烷的轉化率與產物的產率較其他比例高。Methane and CO2, which are greenhouse gases, can be consumed by dry reforming with CO2 to form valuable products in the form of CO and H2. Ni catalyst has been widely used in industry, however it results in coking and sintering, which deactivates the catalyst. By adding an second metal, or loading it on the support, sintering can be reduced resulting in catalyst activity increased. Boron nitride(BN) has many unique properties compared with traditional supports. Previous studies shows that BN is an inert material, it does not interact readily with other metal compounds, as a result alloys are formed which increase the catalytic activity. The incipient wetness method was used to prepare different ratios of Rh-Ni/BN catalyst. This catalyst was also compared with Rh-Ni/γ-Al2O3. The prepared fresh catalysts are examined under non-calcinated and different calcination temperature conditions. The reaction of catalysts was preformed at different temperatures under hydrogen flow. The catalyst was characterized using BET, hydrogen chemisorption, TPR, XRD, XPS and TEM. BET results showed that the catalyst surface area did not change significantly before and after the reaction. Hydrogen chemisorption experiments showed that the addition of Rh increased metal dispersion. It was found from TPR results that when a second metal was added, the reduction temperature would shift between that of pure Ni and pure Rh catalyst, this may be due to the formation of an alloy or other compounds. The interaction is smaller between the metal particle and BN than with γ-Al2O3. Therefore on the BN support, metal particles move freely and form clusters. TEM results show that when a second metal is added on the BN support, a second kind of particle can be seen, this could be a bi-metal cluster or alloy formation. XPS results showed that after H2 reduction, the metal compounds loaded on the BN support became metallic elements, however on the γ-Al2O3 support, Ni remained same oxidation state before and after reduction. This suggests that there is no interaction between the two metals on the γ-Al2O3 support. Experiments show that the addition of Rh, increases the activity of Ni/BN as well as the catalyst life. At a reaction temperature of 700℃, the conversion of methane and CO2 reaches 90%, this is higher than that using Rh-Ni/γ-Al2O3 catalyst. The ratio of H2/CO product is about 0.7 and does not show signs of deactivation after 6 hours of reaction time. The optimum ratio of Rh/Ni is 0.01, under this condition, the largest yield of products are formed and methane conversion is highest.摘要 I 英文摘要 III 目錄 V 圖目錄 IX 表目錄 XVIII 第一章緒論 1 第二章 文獻回顧 3 2.1重組反應 3 2.1.1 合成氣的來源 3 2.2 觸媒金屬效應 5 2.3 雙金屬觸媒 8 2.3.2 雙金屬觸媒在甲烷-二氧化碳重組反應上的研究 10 2.4 分散金屬觸媒 13 2.4.1 氧化鋁載體 14 2.4.2 BN載體 16 2.4.3 不同載體對甲烷-二氧化碳重組反應的影響 17 第三章 實驗方法 21 3.1 使用藥品 21 3.1.1 觸媒製備 21 3.2觸媒檢測 25 3.2.2 雷射光繞射法粒徑分析 26 3.2.3 氫氣學吸附測定 27 3.2.4 X光繞射（XRD） 28 3.2.5 X射線光電子分光儀（XPS） 30 3.2.6 穿透式電子顯微鏡（TEM） 31 3.2.7 程溫規劃還原（TPR） 32 3.3 反應裝置及操作步驟 33 3.4 標準校正線製作 36 第四章 觸媒特性分析與討論 42 4.1 載體檢測 42 4.1.1 N2吸附比表面積測定結果 42 4.1.2 雷射繞射法測粒徑分佈結果 43 4.1.3 XRD測定結果 44 4.2 氫氣化學吸附測定 47 4.3 XRD分析 48 4.4 X射線光電子分光儀（XPS） 53 4.5 穿透式電子顯微鏡（TEM） 59 4.6程溫規劃還原（TPR） 64 4.7 觸媒檢測綜合討論 67 第五章 反應結果與討論 69 5.1 甲烷-二氧化碳重組反應條件選擇 70 5.1.1 還原溫度 70 5.1.2 反應溫度之選擇 73 5.1.3 鍛燒溫度的影響 76 5.2 以BN支撐不同金屬組成觸媒的反應結果 79 5.2.1未鍛燒之BN支撐不同金屬組成觸媒的反應結果 79 5.2.2經鍛燒之BN支撐不同金屬組成觸媒的反應結果 83 5.3以γ-Al2O3支撐不同金屬組成觸媒的反應結果 86 5.3.1未鍛燒之γ-Al2O3支撐不同金屬組成觸媒的反應結果 86 5.3.2經鍛燒之γ-Al2O3支撐不同金屬組成觸媒的反應結果 89 5.4 觸媒再生測試 92 5.5 反應結果討論 95 第六章 結論 102 第七章 參考文獻 104 附錄 10