|摘要: ||本研究利用含浸硫酸亞鐵(ACS)、氯化鐵(ACC)、硝酸鐵(ACN)作為金屬活化劑製備碳觸媒，並將所製備成活性碳觸媒應用於氮氧化物之還原。以比表面積分析儀(BET)、掃描式電子顯微鏡與/能量分散分析儀(SEM/EDS)、X射線繞射儀(XRD)分析活性碳觸媒之表面結構、孔隙特性及活性金屬氧化物之形態，並利用TPD-NH3、TPD-NO、TPD-CH4分析觸媒表面對於還原性氣體之脫附。研究中將所研製之碳觸媒以選擇性觸媒還原法(SCR)與非選擇性觸媒還原法(NSCR)於不同溫度下，探討碳觸媒對於還原氮氧化物之活性。由三者表面及孔隙分析結果顯示，以氯化鐵含浸劑可製備得高比表面積碳觸媒(ACC10 BET=914(m2/g))，以硫酸亞鐵含浸劑可製備得高鐵含量碳觸媒(ACS50 Fe%=60%)。發現氮氧化物之還原效率與鐵金屬鹽含量多寡、鐵金屬分散性，比表面積大小及氧化鐵結晶型態(γ- Fe2O3)有明顯相關。
由選擇性觸媒還原反應(SCR)實驗結果發現，當實驗條件為: 氨1000ppm，氮氧化物(NO)1000ppmv，於550℃ 0.15M硝酸鐵觸媒及0.2M硫酸亞鐵觸媒對氮氧化物之轉化率均為95%以上。ACN15及ACS20具有鐵含量高達(50 Wt %)，適當BET大小(148-460m2/g)，及氧化鐵屬於γ- Fe2O3形態。因此顯示良好之選擇性觸媒還原反應效果取決於較大比表面積、較高活性金屬含量及適當活性金屬之形態，以硫酸亞鐵及硝酸鐵作為金屬活化劑，其含浸量提高雖可增加碳觸媒中鐵金屬含量，但卻急遽降低碳觸媒之比表面積，因此唯有金屬活化劑適量方能同時具備高BET、鐵含量、適當氧化物晶態，方能表現出最佳還原活性之觸媒。實驗結果發現以氯化鐵作為金屬活化劑，於SCR實驗條件為: 氨1000ppm，氮氧化物(NO)1000ppmv時，並無法表現良好之還原轉化率，其原因為以氯化鐵作為金屬活化劑所製備之碳觸媒具有低鐵含量，且γ- Fe2O3含量低，雖然有較大比表面積，但其高活性鐵金屬(γ- Fe2O3)含量低，因此無法有效促進還原反應之進行，以上之論述亦可由TPD-NH3、TPD-NO 具有較大脫附量但低還原活性加以印證。
觸媒進行非選擇性觸媒還原反應(SNCR)實驗結果發現，當實驗條件為: 甲烷4000ppm，氮氧化物(NO)1000ppmv，於600℃0.15M硝酸鐵觸媒及0.2M硫酸亞鐵觸媒氮氧化物之轉化率只有60%左右。硝酸鐵觸媒及硫酸亞鐵觸媒雖具有較高鐵含量，適當BET大小，且含γ- Fe2O3形態，但因γ- Fe2O3對於SNCR活性相對不高，因此無法顯現較高反應活性，至於ACC觸媒雖然具有適當BET大小，但其活性金屬於SNCR中不具活性，因此其含鐵量多寡對還原活性影響不大。
本研究結果顯示，以硫酸亞鐵、硝酸鐵作為金屬活化劑製備碳觸媒具有較高γ- Fe2O3含量及高比表面積，且於SCR及SNCR反應中具有催化活性，但氯化鐵作為金屬活化劑不具NO還原活性。研究結果得知，γ- Fe2O3顯著促進還原活性， 硫酸亞鐵觸媒、硝酸鐵觸媒燒結後具有高BET及γ- Fe2O3含量。
The purpose of this study is to prepare carbon supported catalyst via impregnating iron sulfate (ACS), iron chloride (ACC), and iron nitrate (ACN) and to apply for catalytic reduction of nitrogen oxide. The pore characteristics, morphology, and crystal form of iron oxides were analyzed by the BET, SEM/EDS, and XRD, respectively. The sorption/desorption properties were carried out by using the temperature programming desorption (TPD) measurement with ammonia, NO, and CH4. The reduction activity of catalyst in the selective catalytic reduction (SCR) and non selective catalytic reduction (NSCR) were also investigated. Base on the pore analysis, the iron chloride impregnated catalysts showed the superior surface area (over 900 m2/g) than the others. On the other hand, the iron nitrate and iron sulfate impregnated catalysts owned a higher iron content than the ACC with over 60% iron content in the catalyst. It was found that the reduction activity of catalysts strongly depended on the iron content, iron distribution, BET, and the crystal form of iron oxides.
Based on the results of SCR, it was found that the ACN15 and ACS20 owned over 95% conversion with 1000ppm ammonia and NO in feed at 550℃. The conclusion was that the activity of reduction strongly depended on the iron content, crystal form of iron oxide (γ- Fe2O3), and BET of the catalysts. It was also indicated that the higher BET, iron content, and the right crystal form of iron oxides dominated the reduction activity of carbon catalyst. The experiment indicated that the increasing in impregnating concentration significantly decreased the BET in the case of ACN and ACS catalysts. Therefore, the optimum dosage of impregnating iron in the preparation is necessary to obtain the highly active catalysts. On the other hand, it was also found that the ACC did not show the similar activity as ACS and ACN. The low activity was due to the low γ- Fe2O3 content in the ACC catalyst. It was also indicated that the iron oxide (γ- Fe2O3) play an important role in the SCR to converse the NO to nitrogen.
In the case of NSCR, it was found that the ACN and ACS catalysts owned an about 60% NO conversion rate with 1000ppm CH4 and NO in feed at 600℃. Base on the XRD analysis, it was indicated that γ- Fe2O3 did not show the superior activity. Though the iron content and BET were high in this case but the low conversions were also found with higher iron content and BET. It was indicated the activity of reduction in the case of NSCR was low than the SCR with the ACN and ACS. It is interesting to note that that ACC showed the less activity of reduction than the others. It was concluded that the high activity of ACS and ACN catalysts due to higher BET and γ- Fe2O3 content than the ACC catalysts. It was concluded that the reduction activity of carbon catalyst can be improved by increasing the BET and γ- Fe2O3 content in the matrix.
Keywords: iron sulfate (ACS), iron chloride (ACC), and iron nitrate (ACN), γ- Fe2O3, selective catalytic reduction (SCR), non selective catalytic reduction (NSCR)