汽車尾氣三元催化劑催化機理的量子化學研究
發(fā)布時間：2018-11-23 20:16
【摘要】：近年來,中國經(jīng)濟迅猛發(fā)展,汽車人均占有量不斷攀升,由此而引發(fā)的空氣污染問題也日益嚴重。汽車尾氣中含有多種有害物質(zhì),其中含量較高的為CH、CO和NO�，F(xiàn)有的較為常用的汽車尾氣處理技術大致可以歸納為三類:發(fā)動機內(nèi)部凈化技術、發(fā)動機外部凈化處理技術、燃料的改進和替換技術。目前最常用的是發(fā)動機外部凈化處理技術中的汽車三元催化技術,該技術的關鍵要素是催化劑的選用,而目前,對汽車三元催化技術的研究主要以實驗為主,對于反應的具體機理還沒有確切的結(jié)論。所以基于三元催化技術在處理汽車尾氣領域的重要作用,本文對部分催化反應的機理進行了詳細研究。首先,本文采用量子化學密度泛函理論對CH和CO直接還原NO的反應進行了計算研究,得到了直接反應的反應路徑和反應的活化能,并以此作為與催化還原反應對比的參照。之后,采用UB3LYP//SDD方法優(yōu)化得到負載在分子篩ZSM5不同原子附著位點(Si、Al、Ce)上的以Mn為代表的過渡金屬催化還原NO相關反應的反應物、過渡態(tài)、中間體及產(chǎn)物,分析得到反應路徑,并計算獲得了反應的活化能。最后,通過分析總結(jié),對反應機理進行了探討。計算結(jié)果表明:CH和CO直接還原NO的反應有多條反應路徑,CH直接還原NO的反應的活化能在150-180kJ/mol左右,CO直接還原NO的反應活化能大約為290-350kJ/mol,通過比較我們發(fā)現(xiàn),CH還原NO的反應活化能比CO均相還原NO反應的活化能低很多,說明CH還原NO的反應更容易進行,CH的還原性比CO的還原性強。使用UB3LYP//SDD方法計算優(yōu)化得到,在ZSM5-Mn催化劑存在的情況下CH還原NO的反應活化能為60-130kJ/mol左右,該催化劑存在時CO還原NO的反應活化能為120-220kJ/mol左右,所以,ZSM5-Mn具有明顯的催化效果;催化劑的附著中心原子不同,活化能也有差異,其中以Al作為催化附著中心原子反應的催化劑對兩個反應都具有較好的催化效果,這也間接反映了Al2O3作為載體的催化活性相對較好。參考相關文獻后推斷,溫度變化、氣體空速和載體等一系列因素都會對催化效果產(chǎn)生影響,所以,進行更深入的研究還需考慮更多因素。本文的量子化學計算揭示了ZSM5-Mn對汽車三元催化的催化機理,為進一步研究過渡金屬對三元催化反應的催化作用提供了理論參考。
[Abstract]:In recent years, China's economy has developed rapidly, the per capita share of cars has been rising, and the air pollution caused by it is becoming more and more serious. There are many harmful substances in automobile exhaust, including CH,CO and NO.. The existing automotive exhaust treatment technology can be classified into three categories: engine internal purification technology, engine external purification technology, fuel improvement and replacement technology. At present, the most commonly used technology is the automobile ternary catalytic technology in the engine external purification treatment technology. The key element of this technology is the selection of the catalyst. At present, the research on the automobile ternary catalytic technology is mainly based on experiments. There is no definite conclusion as to the specific mechanism of the reaction. Therefore, based on the important role of ternary catalytic technology in the field of automobile exhaust treatment, the mechanism of partial catalytic reaction is studied in detail in this paper. Firstly, the direct reduction of NO by CH and CO has been studied by using the density functional theory of quantum chemistry. The reaction path and activation energy of the direct reaction have been obtained and compared with the catalytic reduction reaction. After that, UB3LYP//SDD method was used to optimize the reactants, transition states, intermediates and products of transition metal catalyzed reduction of NO related reactions, which were supported on different atom attachment sites (Si,Al,Ce) of molecular sieve ZSM5, represented by Mn. The reaction path was analyzed and the activation energy of the reaction was calculated. Finally, through the analysis and summary, the reaction mechanism is discussed. The calculated results show that there are many reaction paths in the direct reduction of NO between CH and CO. The activation energy of the direct reduction of NO by CH is about 150-180kJ/mol, and that of NO by CO is about 290-350 KJ / mol. It is found by comparison that the activation energy of CH reduction NO is much lower than that of CO homogenous reduction NO reaction, which indicates that the CH reduction NO reaction is easier to carry out, and CH is more reductive than CO. UB3LYP//SDD method was used to calculate and optimize the results. The reaction activation energy of CH reduction NO was about 60-130kJ/mol in the presence of ZSM5-Mn catalyst, and the reaction activation energy of CO reduction NO was about 120-220kJ/mol in the presence of this catalyst. ZSM5-Mn has obvious catalytic effect. The activation energy of the catalyst is different with the attachment center atom, and Al is used as the catalyst to catalyze the attachment center atom reaction. This also indirectly reflects the relatively good catalytic activity of Al2O3 as the carrier. A series of factors, such as temperature change, gas space velocity and carrier, will affect the catalytic effect. Therefore, more factors need to be considered for further study. The quantum chemical calculation in this paper reveals the catalytic mechanism of ZSM5-Mn for automobile ternary catalysis, which provides a theoretical reference for the further study of the catalytic effect of transition metals on the ternary catalytic reaction.
【學位授予單位】：杭州電子科技大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：X734.2;O643.31
本文編號：2352580
[Abstract]:In recent years, China's economy has developed rapidly, the per capita share of cars has been rising, and the air pollution caused by it is becoming more and more serious. There are many harmful substances in automobile exhaust, including CH,CO and NO.. The existing automotive exhaust treatment technology can be classified into three categories: engine internal purification technology, engine external purification technology, fuel improvement and replacement technology. At present, the most commonly used technology is the automobile ternary catalytic technology in the engine external purification treatment technology. The key element of this technology is the selection of the catalyst. At present, the research on the automobile ternary catalytic technology is mainly based on experiments. There is no definite conclusion as to the specific mechanism of the reaction. Therefore, based on the important role of ternary catalytic technology in the field of automobile exhaust treatment, the mechanism of partial catalytic reaction is studied in detail in this paper. Firstly, the direct reduction of NO by CH and CO has been studied by using the density functional theory of quantum chemistry. The reaction path and activation energy of the direct reaction have been obtained and compared with the catalytic reduction reaction. After that, UB3LYP//SDD method was used to optimize the reactants, transition states, intermediates and products of transition metal catalyzed reduction of NO related reactions, which were supported on different atom attachment sites (Si,Al,Ce) of molecular sieve ZSM5, represented by Mn. The reaction path was analyzed and the activation energy of the reaction was calculated. Finally, through the analysis and summary, the reaction mechanism is discussed. The calculated results show that there are many reaction paths in the direct reduction of NO between CH and CO. The activation energy of the direct reduction of NO by CH is about 150-180kJ/mol, and that of NO by CO is about 290-350 KJ / mol. It is found by comparison that the activation energy of CH reduction NO is much lower than that of CO homogenous reduction NO reaction, which indicates that the CH reduction NO reaction is easier to carry out, and CH is more reductive than CO. UB3LYP//SDD method was used to calculate and optimize the results. The reaction activation energy of CH reduction NO was about 60-130kJ/mol in the presence of ZSM5-Mn catalyst, and the reaction activation energy of CO reduction NO was about 120-220kJ/mol in the presence of this catalyst. ZSM5-Mn has obvious catalytic effect. The activation energy of the catalyst is different with the attachment center atom, and Al is used as the catalyst to catalyze the attachment center atom reaction. This also indirectly reflects the relatively good catalytic activity of Al2O3 as the carrier. A series of factors, such as temperature change, gas space velocity and carrier, will affect the catalytic effect. Therefore, more factors need to be considered for further study. The quantum chemical calculation in this paper reveals the catalytic mechanism of ZSM5-Mn for automobile ternary catalysis, which provides a theoretical reference for the further study of the catalytic effect of transition metals on the ternary catalytic reaction.
【學位授予單位】：杭州電子科技大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：X734.2;O643.31
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