汽车尾气三元催化剂催化机理的量子化学研究

发布时间：2018-11-23 20:16

【摘要】：近年来,中国经济迅猛发展,汽车人均占有量不断攀升,由此而引发的空气污染问题也日益严重。汽车尾气中含有多种有害物质,其中含量较高的为CH、CO和NO。现有的较为常用的汽车尾气处理技术大致可以归纳为三类:发动机内部净化技术、发动机外部净化处理技术、燃料的改进和替换技术。目前最常用的是发动机外部净化处理技术中的汽车三元催化技术,该技术的关键要素是催化剂的选用,而目前,对汽车三元催化技术的研究主要以实验为主,对于反应的具体机理还没有确切的结论。所以基于三元催化技术在处理汽车尾气领域的重要作用,本文对部分催化反应的机理进行了详细研究。首先,本文采用量子化学密度泛函理论对CH和CO直接还原NO的反应进行了计算研究,得到了直接反应的反应路径和反应的活化能,并以此作为与催化还原反应对比的参照。之后,采用UB3LYP//SDD方法优化得到负载在分子筛ZSM5不同原子附着位点(Si、Al、Ce)上的以Mn为代表的过渡金属催化还原NO相关反应的反应物、过渡态、中间体及产物,分析得到反应路径,并计算获得了反应的活化能。最后,通过分析总结,对反应机理进行了探讨。计算结果表明:CH和CO直接还原NO的反应有多条反应路径,CH直接还原NO的反应的活化能在150-180kJ/mol左右,CO直接还原NO的反应活化能大约为290-350kJ/mol,通过比较我们发现,CH还原NO的反应活化能比CO均相还原NO反应的活化能低很多,说明CH还原NO的反应更容易进行,CH的还原性比CO的还原性强。使用UB3LYP//SDD方法计算优化得到,在ZSM5-Mn催化剂存在的情况下CH还原NO的反应活化能为60-130kJ/mol左右,该催化剂存在时CO还原NO的反应活化能为120-220kJ/mol左右,所以,ZSM5-Mn具有明显的催化效果;催化剂的附着中心原子不同,活化能也有差异,其中以Al作为催化附着中心原子反应的催化剂对两个反应都具有较好的催化效果,这也间接反映了Al2O3作为载体的催化活性相对较好。参考相关文献后推断,温度变化、气体空速和载体等一系列因素都会对催化效果产生影响,所以,进行更深入的研究还需考虑更多因素。本文的量子化学计算揭示了ZSM5-Mn对汽车三元催化的催化机理,为进一步研究过渡金属对三元催化反应的催化作用提供了理论参考。
[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.
【学位授予单位】：杭州电子科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：X734.2;O643.31

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