非貴金屬波洛斯凱特型觸媒應用於三元觸媒轉化之效能與氧化還原反應分析研究

Abstract

觸媒轉化器為於特定溫度下藉由觸媒之活性作用，以同時去除機動車輛引擎所排放之氮氧化物(NOx)、碳氫化合物(HCs)及一氧化碳(CO)，因此這些觸媒通稱為三元觸媒(Three-way catalysts, TWC)，希望能於於反應中分別將HCs及CO氧化並使NO還原，以轉換目標污染物達到淨化之功效，但此三種污染物是否同時互為氧化還原劑？以及彼此間之氧化還原反應趨勢關係為何？相關研究文獻探討依然有限。另一方面，鑒於結構穩定之波洛斯凱特型(Perovskite, ABO3)觸媒材料對污染物有較佳之熱穩定性，具有極佳的潛力應用於三元觸媒的反應中，然而商用貴金屬觸媒價格高昂，因此本研究選擇以非貴金屬組成，探討作為機動車輛三元觸媒之可能性，更同時釐清各金屬之特性與應用效益。 本研究利用Perovskite觸媒進行高溫熱劣化前後之三元觸媒效率測試，分別使用La、Ce、Sr作為A site金屬，而使用Mn、Fe、Co、Ni、Cu作為B site金屬，分別探討調整A site金屬其轉化過程之劣化情形與調整B site金屬之處理效能變化；效能測試反應溫度則為70-450℃，空間流速為60,000 h-1。綜合各項結果顯示，去除效率最佳者為CeMnO3新鮮觸媒，最佳NO轉換率為在215℃下可達42 %；去除50 % HC及CO (T50)之溫度點則分別為294℃與197℃；去除90 % HC及CO (T90)之溫度點分別為352℃與204℃。此外，本研究同時進行XRD、TEM、BET及NH3-TPD相關觸媒物化分析，以作為了解觸媒反應機制之參考依據。在氧化還原分析結果顯示，觸媒對NO之轉換效率較HC及CO在高溫下受氧氣氧化影響較為明顯，而加入CO還原劑則在低溫下可提升NO還原的能力。

For automotive emission control, CO and HCs would be oxidized to CO2 and H2O while NOx would be reduced to N2. The catalysts are referred as three-way catalysts (TWCs) since they can simultaneously remove three different types of air pollutants (CO, HCs and NOx). However, whether these three pollutants are interact at the same time or not, and also the relationship of redox reaction between these three pollutants were still not clear in the past research. On the other hand, due to the well-defined crystalline structure and high thermal stability, perovskite-type oxides (ABO3) have high potential for vehicle emission control catalysts. The commercial TWCs usually employ precious metals as the activated species. But in order to reduce the catalysts costs non-noble metals were selected and the characteristics of each metal were clarified in this study. In this study La, Ce or Sr were selected as the A site, while the B site was considered to be Mn, Fe, Co, Ni or Cu. Perovskite catalysts were prepared by precipitation method and the activities of fresh and thermal aged catalysts were tested in a wide temperature range of 70-450 oC under GHSV of 60,000 h-1. In addition, the physicochemical properties of the products were characterized by XRD, TEM, N2 physisorption measurement and NH3-TPD analysis. The results showed that CeMnO3 catalyst had the best overall performance among all tested catalysts. It has the best NO conversion efficiency of 42% at temperature of 215 oC. In addition, the T50 (temperature at 50% conversion) for C3H8 and CO were 294 oC and 197 oC, respectively; while the T90 for C3H8 and CO were 352 oC and 204 oC, respectively, for fresh CeMnO3 catalyst. Besides, the redox reaction analysis results showed that the increase of O2 concentration tends to increase the NO conversion at high temperature. And the addition of CO reductant can enhance the ability of NO reduction at low temperature.