MnFe/TiO2低溫SCR觸媒之 NO轉換效率與SO2毒化影響研究

Study of NO reduction and SO2 poisoning effect for low temperature selective catalytic reduction on MnFe/TiO2 catalyst

李宗諭(Tsung-Yu Lee)

指導教授 ： 白曛綾

國立交通大學/工學院/環境工程系所/博士(2019年)

摘要

選擇性觸媒還原法 (Selective catalytic reduction, 簡稱SCR) 廣泛應用在處理工廠煙道廢氣中之氮氧化物(NOx)。傳統SCR觸媒之操作溫度需在300~400 oC之間，其將消耗額外之能源來加熱煙道尾氣，而低溫SCR觸媒因操作於較低之溫度區間內，可節約能源之消耗，因此，近年來低溫SCR觸媒廣泛的被研究。至今為止，低溫SCR觸媒最大之困難仍是SO2之毒化，但對於SO2之毒化機制、抗硫之因子以及活化方式仍未有全面之探討。因此，本研究將進行以下之研究『SO2毒化之反應機制與反應路徑分析』、『SO2毒化之主因與抗硫因子』、『改質擔體抑制SO2毒化』以及『活化毒化觸媒與毒化觸媒活性探討』，進而釐清毒化系統中之反應比例、毒化之主因以及毒化對於NO轉換效率之趨勢，進一步探索觸媒之改質以及毒化觸媒之活化方式。
在SO2毒化之反應機制與反應路徑分析中，經由計算可得知SCR系統中於不同條件下之反應百分比，在無毒化之SCR系統中，尾氣之N2O主要來自SCR反應產生而非NH3氧化產生。而當系統通入SO2時，SCR效率會受SO2毒化而逐漸下降，進而造成NH3之溢出，其在毒化6小時後系統中的N2O主要來自於溢出之NH3氧化產生而非SCR反應產生。當反應溫度為300oC，系統通入SO2時，其SCR效率將不受SO2毒化之影響，且隨著毒化時間增加其SCR產生N2O之比例也隨之降低。
此外，本研究更藉由不同之操作條件與分析，釐清了毒化之趨勢，於SCR低溫毒化中有四階段趨勢之變化。在無SO2暴露之下(階段I)：其系統中之氧化能力會隨反應溫度提高而升高，於200oC前皆是SCR反應主導，當進一步提升反應溫度至200oC後，其NH3與NO之氧化反應會逐漸與SCR反應競爭，進而導致NO轉換效率之下降；當SO2開始暴露於系統之初期(階段II)：其SO2會立即與NO競爭NH3，造成NO轉換效率之下降；效率停滯期(階段III)：此時，觸媒仍保有一定之反應活性，但因還原劑之不足使觸媒無法達到真正之效率，進而維持於系統所供給之還原劑劑量之效率。當SO2長時間暴露後(階段IV)：此時，系統之還原劑已開始破出，其表示NO轉換效率不受還原劑量之影響，僅受反應機制之影響。
在SO2毒化之主因與抗硫因子中，本研究利用僅含硫酸銨鹽、金屬硫酸鹽以及同時具有硫酸銨鹽與金屬硫酸鹽之觸媒比較不同硫化物對觸媒之影響，結果發現其金屬硫酸鹽之影響較為劇烈，其將造成(1)降低觸媒之比表面積；(2)降低觸媒Mn3O4以及 Fe3O4之結晶結構；(3)增加觸媒之之Lewis酸基使SO2將與NH3產生更強之鍵結，導致NO將更不易與NH3進行SCR反應；(4)降低Mn4+/Mn3+比例以及提升Oα/(Oα+Oβ)比例。
在找出抗硫因子後，本研究將進一步合成高比表面積之TNTs擔體來測試其抑制毒化之能力。在改質擔體抑制SO2毒化之研究中發現，比表面積與NO轉換效率有著直接之相關性。TNTs作為擔體之MnFe-TNTs觸媒，其比表面積相較於MnFe-TiO2增加2~4倍，其在經過5小時SO2毒化後，TNTs擔體之NO轉換效率亦為TiO2擔體之2~4倍。
而在活化毒化觸媒與毒化觸媒活性探討之探討中發現，使用1.2g、2.4g以及6.0g 總量之觸媒(空間流速50,000、25,000以及10,000h-1 )其完全毒化後之效率分別為17%、27%以及54%。其結果顯示完全毒化後之金屬硫酸鹽仍有少量之SCR活性。此外，比較水洗與熱處理之活化方式，其發現到水洗之方式能有效去除硫酸鹽類與金屬硫酸鹽並回覆NO轉換效率至初始效率。其11次水洗活化後(120小時之毒化時間)仍可維持80%以上之NO轉換效率。
本研究釐清了毒化系統中之反應比例、毒化之主因以及NO轉換效率之趨勢，綜上述研究可得知，SO2¬造成NO轉換效率降低之原因有二，其一為SO2會與NO競爭NH3，其將產生硫酸銨鹽造成NO轉換效率之下降，可藉由提供充足之還原劑亦或是提高反應溫度至250oC以上使SO2不與NH3反應來解決。其二為觸媒表面硫酸鹽類與金屬硫酸鹽之影響，其金屬硫酸鹽在高溫之系統中非但不受SO2毒化影響且會增加SCR之選擇性。然而在低溫之SCR系統中，金屬硫酸鹽是觸媒毒化之主因，其可藉由增加觸媒比表面積、水洗活化以及使用較多總量之毒化後金屬來維持其SCR之脫硝效率。

關鍵字

選擇性觸媒還原 ； 低溫觸媒 ； SO2毒化 ； 抗硫因子 ； 觸媒活化

並列摘要

Nitrogen oxides (NOx) produced from stationary sources are a major air pollutant, causing numerous environmental issues. The major technology for the removal of NOx is the selective catalytic reduction (SCR) process. However, the development of low temperature catalysts for selective catalytic reduction (SCR) of NOx with ammonia is still a challenge especially in the presence of SO2. Therefore, in this study, MnFe/TiO2 catalysts were used for low temperature SCR of NOx with ammonia to investigate reaction mechanisms, key factors for the inhibition of SO2 poisoning, and methods of regeneration after SO2 poisoning.
In this study, the products and byproducts of NH3 oxidation, NO oxidation and selective catalytic reduction (SCR) process of NO with NH3 with and without SO2 poisoning were analyzed. For SCR without the presence of SO2, it revealed that N2O was majorly from the SCR reaction instead of from NH3 oxidation reaction. Moreover, the byproducts of N2O and NO2 increased with increasing the reaction temperature, which cause decreased N2-selectivity of SCR reaction and decreased SCR efficiency, respectively. For SCR test with SO2 at 150 oC, there are two-step decays with different poisoning times. The first decay was due to that certain amount NH3 would be preferably reacted with SO2 instead of react with NO or O2. Then the catalysts were accumulated amount of metal sulfates and ammonium salts, which cause the second decay. For SCR test with SO2 at 300 oC, the SCR efficiency would not decreased with increasing poisoning time. Moreover, the metal sulfates may inhibit oxidation reactions, which cause concentrations of N2O had slightly decreased and increased N2-selectivity of SCR reaction.
Furthermore, the individual effect of manganese sulfate and ammonium sulfate over the MnFe/TiO2 catalyst for low-temperature SCR was investigated. The results showed that under the same exposure of SO2, the NOx conversion decreased more significantly over the MnFe/TiO2-M catalyst (where only metal sulfate was loaded) as compared to that of the MnFe/TiO2-AY catalyst (where only ammonium sulfate was loaded) and the MnFe/TiO2-P catalyst (where both metal sulfate and ammonium salts were observed). The results also demonstrated that under about the same poisoning amount of ammonium salts (1.2 wt.%), the NOx conversion efficiencies of MnFe/TiO2-A catalyst and MnFe/TiO2-P6 catalyst were significantly different (86% and 17%). In addition, the NOx conversion efficiency of MnFe/TiO2-P6 could be recovered from 17% back to 88% by water washing where both ammonium salts and manganese sulfate were removed. On the other hand, the thermal regeneration tends to remove ammonium salts only, it could not treat the manganese sulfate thus the NOx conversion could only be recovered back to 35%.
The analytical results of synchrotron-based XRD, BET, NH3-TPD, and XPS revealed that increasing the amount of manganese sulfate resulted in lower crystallinity, lower specific surface area, lower ratio of Mn4+/Mn3+, higher surface acidity and more chemisorbed oxygen, which then led to decreases in the SCR activity. Therefore, a series of iron–manganese oxide catalysts supported on TiO2 and titanium nanotubes (TNTs) were studied. The results demonstrated that higher Mn4+/Mn3+ ratios and larger specific surface areas were the main reasons for the excellent performance of MnFe-TNTs catalyst after SO2 poisoning. Moreover, the SO2 poisoning effect could be minimized by reducing the GHSV, increasing the reaction temperature, or increasing the [NH3]/[NO] molar ratio.

並列關鍵字

Selective Catalytic Reduction (SCR) ； Titanium nanotubes (TNTs) ； SO2 resistance ； Metal sulfates ； Ammonium sulfate ； Regeneration ； Low-temperature catalyst.

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