以La(1-x)Sr(x)Mn(1-y)Co(y)O3波洛斯凱特型觸媒用於低溫SCR處理NOx之研究

Abstract

選擇性觸媒還原法(Selective catalytic reduction，簡稱SCR)已經被長時間使用於固定汙染源中處理氮氧化物(NOx)。傳統上以MoO3/TiO2或V2O5/TiO2觸媒為主要的觸媒成分，然而其操作溫度過於狹窄(300-400℃)，限制了SCR在高NOx濃度低溫度排放 工業的應用。波洛斯凱特(Perovskite)型觸媒具有成本低和在1000℃高溫下結構穩定的特色，可被視為在工業應用上具潛力的SCR觸媒。 本研究中探討波洛斯凱特型觸媒在低溫SCR反應中的活性。一般而言，波洛斯凱特觸媒結構的通式可以寫成ABO3，其在不改變材料結構的狀態下，利用部分置換A或B位置的金屬陽離子來增加反應活性和改變原本A或B位置陽離子的氧化態。本研究中以共沉澱法製備La(1-x)Sr¬(x)Mn(1-y)Co(y)O3 perovskite-type oxides，並分為四大群組ABO3、AA'BO3、ABB'O3和AA'BB'O3共13種觸媒。於200ppm NO, 200ppm NH3, 空間速度70000h-1下探討各觸媒在低溫系統之NO轉化率，其中以LaCo0.2Mn0.8O3觸媒在系統溫度150℃下之95%NO轉化率為最佳，當系統溫度到達350℃，NO轉化率衰退到65%以下，在高空間流速100,000h-1下，NO轉換率下降至不到65%。在物性方面，根據BET分析結果顯示，反應溫度150℃下觸媒之NO轉化率與比表面積成正比，XRD分析結果指出晶相越不顯著者，NO轉化率越高;化性方面，由TPD分析結果顯示，NO脫附量越多，NO轉換效率越高，TPR分析中顯示還原波峰越早出現者，NO轉換效率越高。

The selective catalytic reduction (SCR) has been applied for several decades to reduce NOx emissions with ammonia from stationary sources. The MoO3 or V2O5 supported on TiO2 are the most commonly employed SCR catalysts. However, it is limited to a narrow and relatively higher temperature window (300~400℃). This limits the application of SCR to many other industrial processes such as the steel manufacture industry which has high NOx emissions at relatively lower temperatures of around 150℃. Perovskite-type oxides, with the characteristics of low-cost materials and high structural stability even at above 1000℃, could be the potential candidate for industrial NO removal catalysts at low temperatures. In this study, the reactivity of perovskite-type mixed oxides for low temperature SCR of NO with NH3 is investigated. The general formula of perovskite oxide is ABO3, the B-site cations could also be substituted by another one without changing the material structure and the oxidation state of B cations could be modified. A series of La(1-x)Sr¬(x)Mn(1-y)Co(y)O3 perovskite-type oxide catalysts were prepared by the co-precipitation method and they were divided into four groups of ABO3, AA'BO3, ABB'O3 and AA'BB'O3. The SCR tests were carried out under the typical reaction condition of 200 ppm NO, 200 ppm NH3, 150℃ and GHSV of 70,000 h-1. The experimental results showed that the highest NO conversion of 95% was observed by using LaCo0.2Mn0.8CoO3 when the reaction temperature was 150℃. The NO conversion declined to below 65% at 350℃. And as GHSV was 100,000 h-1, the NO conversion decreased to 64%. The BET results revealed that the specific surface area was highly correlated to the NO conversion. The XRD results showed that poor crystallinity correlated to a higher NO conversion. The TPD results revealed that the amount of adsorbed NO played an important role in the SCR reaction at 150℃. As the NO desorption amount was larger, the catalyst activity was higher. The TPR results indicated that as the reduction peak appeared at a lower temperature the NO conversion was higher, which is due to the lower energy was being consumed.