以波洛斯凱特型及金屬改質沸石觸媒作為三元觸媒之研究

Abstract

汽機車配備之觸媒轉換器係利用選擇性觸媒還原法(Selective Catalytic Reduction, SCR)同時去除車輛主要排放之三種污染物，氮氧化物(NOx)、HC及CO，因此也稱作三元觸媒(Three-Way Catalysts)。其中以HC還原NO的反應又稱作碳氫化合物選擇性觸媒還原(HC-SCR)。汽機車三元觸媒須具備適應車輛排氣管劇烈溫度變化之熱穩定性，以及良好的催化活性；然而常見的三元觸媒主要以鉑(Pt)、鈀(Pd)、銠(Rh)為活性金屬組成，不僅價格昂貴，且Pt、Rh金屬易受高溫而燒結團聚，使觸媒催化活性下降。 本研究以波洛斯凱特型觸媒(Perovskite type catalysts)及金屬改質沸石觸媒分別進行熱劣化前後之三元觸媒效率測試。結果顯示整體效率最佳者為CeMn0.64Fe0.32Pd0.04O3，其去除50% NO、C3H8及CO之溫度(T50)分別為250、155、71℃，而經ICP分析計算其貴金屬使用量相較於商用觸媒減少約92.3%，效率也可與其匹敵。熱穩定性最佳者則為LaMn0.66Fe0.34O3，於熱劣化後其轉換率曲線僅位移約25℃。XRD及BET分析結果顯示金屬改質沸石的結構穩定；Perovskite則以LaMn0.66Fe0.34O3具有較佳之熱穩定性。NH3-TPD分析結果顯示觸媒酸性位基愈多有助於提升NO轉換率；然而會抑制觸媒對C3H8及CO的低溫氧化活性。XPS分析結果顯示Mn3+、Ce3+有助於提升觸媒效率，晶格氧比例的增加有助於提升觸媒之低溫氧化活性。研究結果雖顯示金屬改質沸石結構穩定，但三元觸媒效率因受限於製備方式而不及Perovskite；Perovskite能夠以不同活性金屬組成，因此具備不同金屬之特性，在製備參數調整上較為彈性，且研究結果顯示，選擇適當之金屬及比例組成，其三元觸媒效率能夠與商用觸媒相當。

In order to eliminate NOx, unburn-hydrocarbons (HC) and CO which emitted from vehicles, selective catalytic reduction (SCR) is widely applied. The redox reaction of HC and NO is also called HC-SCR. And because the catalysts are able to eliminate NOx, HC and CO simultaneously, they are also referred as Three-Way Catalysts (TWC). The temperature in the exhaust tube are highly deviated, thus the thermal stability and efficiency of the TWC need to be well controlled. The activated species of the commercial TWC are usually made of Pt, Pd and Rh. The efficiency of these precious metals are quite high, but their prices are high, too. Besides, the thermal stability of Pt and Rh are low, thus it is easy to be aggregated at high temperature which leads to loss of activity. In this study, zeolite based catalysts and Perovskite catalysts were prepared by ion exchange and co-precipitation, respectively. The efficiency of fresh and thermal aged catalysts will be tested. The results show that CeMn0.64Fe0.32Pd0.04O3 has the best efficiency, with 50 % conversion temperatures of NO, C3H8, CO at 250, 155, and 71℃, respectively. On the other hand, LaMn0.66Fe0.34O3 shows the best thermal stability; the efficiency curves only shift about 25℃ after thermal aging. The XRD and BET results reveal that the structure of zeolite based catalysts are stable, and LaMn0.66Fe0.34O3 has stable Perovskite structure. NH3-TPD results show that when the amount of acidic sites of the catalysts increase, NO conversion will be enhanced; but the low temperature activity of C3H8 and CO will be inhibited. The XPS results reveal that Mn3+ and Ce3+ can promote catalyst efficiency, and lattice oxygen can promote low temperature efficiency of the catalysts. Compare to the commercial TWC, CeMn0.64Fe0.32Pd0.04O3 reduced about 92.3 % noble metal used, and the efficiency is about the same. Although the structure of zeolite based catalysts is stable, but their efficiencies are not as well as Perovskite because of the limitation on catalyst preparation. However, Perovskite can be made by different metals, and the preparation conditions are flexible. The results also show the efficiency of Perovskite can be as good as the commercial TWC.