| 摘要: | 本研究之目的為發展低溫反應活性及耐久性良好的觸媒，並尋求最符合經濟效益應用於有機廢氣物之去除。處理有機廢氣物之觸媒主要分為(1)低活性但是價位便宜的金屬氧化物(CrO3、CuO)及(2)高活性，但是價位相對較高的貴重金屬(Pt、Rh、Pd、Au)。本研究選擇鈀觸媒是因為相較於其他貴重金屬(Pt、Au、Rh)，鈀金屬的價格遠低於金與白金，並且只需要極少的用量即具有很高的活性，因此鈀觸媒是一個好的金屬觸媒應用在有機廢氣物的去除，本研究使用甲苯做為有機廢氣反應指標物。 本研究探討了將奈米級的鈀承載於不同來源的氧化鈰擔體上對甲苯反應活性的影響，以初濕含浸法製備Pd/CeO2觸媒，其中擔體包含由Nikki公司取得之奈米氧化鈰以及由Evonik Degussa公司取得之奈米氧化鈰(NanoCeria)。其樣品結構鑑定方面，主要是以X光繞射儀(XRD)，氮氣吸附-脫附儀(N2-sorption)，穿透式電子顯微鏡(TEM)，高解析度穿透式電子顯微鏡(HRTEM)與X光電子能譜儀(XPS)，進行鑑定與分析。從XRD圖譜可觀察到擔體皆為結晶良好的二氧化鈰，此外亦可發現XRD分析圖譜中偵測不到鈀的波峰，代表鈀的顆粒太小超過儀器的偵測限制(4奈米)，使其無法量測。BET量測結果顯示氧化鈰(Nikki)比表面積約為氧化鈰(Degussa)的2倍。HRTEM圖中則可看出鈀粒子在氧化鈰(Nikki)擔體上，粒徑約為2奈米，而鈀粒子在氧化鈰(Degussa)擔體上，粒徑則約為5奈米。XPS量測結果顯示在氫氣還原300 ℃所製備的鈀觸媒具有較多的元素態。 本研究以連續式固定床反應器來測試鈀承載於氧化鈰觸媒應用於甲苯氧化反應之活性。在所有的觸媒之中，鈀承載於Nikki公司之二氧化鈰具有高的甲苯氧化活性，反應條件甲苯濃度為8.564 g/m3 (2085 ppm)，進料流速及空間流速分別為40 ml/min及10,000 h−1, 其完全氧化溫度為210℃，將此觸媒通氫氣還原於300 ℃ 2小時之後進行甲苯氧化反應，其甲苯完全氧化溫度降為170 ℃。本研究結果顯示以鈀承載於Nikki公司之二氧化鈰煅燒400 ℃ 8小時並通氫氣還原於300 ℃ 2小時之觸媒活性最佳，其原因為氧化鈰(Nikki)具有高表面積，鈀顆粒較小且分散性佳。適當的氫氣環原處理觸媒，可成功獲得一系列高分散性奈米鈀觸媒，並具有高甲苯轉化率，同時具有穩定的耐久測試，應用於甲苯氧化反應。 The purpose of this study was to develop a catalyst which had high activity and durability at the low-temperature for application in the volatile organic compounds removal. The most active catalysts belong to two categories: (1) low activity and low cost of metal oxide (Cr2O3, CuO) and (2) high activity and high cost of noble metals such as Pt, Rh, Pd and Au. In this study, compared to other noble metals (Pt, Au, Rh), Pd is better than Pt and Au because it is cheaper and only very small amount of Pd is needed. The palladium catalyst is a good metal catalyst used in the removal of volatile organic compounds. Toluene has been chosen as VOC probe molecule in this study. This study investigated the nano-palladium supported on the different sources of ceria from Evonik Degussa Company and Nikki Company. The catalysts were prepared by incipient-wetness impregnation. The supported palladium catalysts were characterized by powder X-ray diffraction (XRD), N2 sorption, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and X-ray Photoelectron Spectroscopy (XPS). XRD pattern showed the crystalline ceria. No distinct palladium peaks were observed, because the particle size of palladium was too small to detect. The BET surface area of CeO2 from Nikki was about as twice as that from Evonik Degussa. The HRTEM images clearly showed that the average particle size of Pd was ~2 nm on CeO2 (Nikki) support and the average particle size of Pd was ~5 nm on CeO2 (from Evonik Degussa) support. XPS spectra showed that the catalyst pretreated with hydrogen at 300 ℃ had more metallic palladium species. Activities for toluene oxidation on Pd/CeO2 catalysts were measured using a fixed bed continuous flow reactor. Palladium catalysts supported on ceria from Nikki demonstrated the higher activity. The reactant gas containing air/toluene (8.564 g/m3 (2085 ppm) of toluene) was fed into the reactor with a total flow rate of 40 ml/min (space velocity: 10,000 h−1), and the completely oxidation temperature of toluene was 210 ℃. For the catalyst pretreated with hydrogen at 300 ℃ for 2 h, the toluene completely oxidation temperature was 170 ℃. Palladium catalysts supported on ceria from Nikki Company calcined at 400 ℃ for 8 h and pretreated with hydrogen at 300 ℃ for 2 h demonstrated the highest activity, because ceria from Nikki had a high surface area and well dispersion of palladium particles. By regulating the preparation and pretreatment procedure and the compositions of Pd/CeO2 catalyst, we have developed a catalyst which had high toluene conversion and high stability for toluene oxidation reaction. |
