Fe-ZSM-5催化NO直接分解反应机理研究

发布时间：2018-04-08 23:12

本文选题：Fe-ZSM-5　切入点：NO　出处：《太原理工大学》2017年硕士论文

【摘要】：随着现代工业的迅速发展和机动车保有量的日益增加,工业废气和机动车尾气引起的环境问题日益严重,由此产生的NOx作为大气污染的主要成分,其日益增加的排放量人们的正常生活和身心健康都产生的严重的影响,引起的环境问题也严重阻碍经济的发展与人民的生活水平。目前工业上主要使用V_2O_5-MoO_3/TiO_2和V_2O_5-WO_3/TiO_2催化剂以NH_3为还原剂脱除NOx,但由于V_2O_5容易随尾气排放到大气中,其自身的生物毒性会威胁生态系统和人类健康。近年来,由于碱金属氧化物、过渡金属氧化物等活性中心分子筛催化剂的不断开发和革新,NOx直接分解反应的效率得到一定程度的提高并且成本大大降低,加之该技术工艺过程简单,没有二次污染物的副作用,经济性好,再次受到研究者的重点关注。基于上述思路,本文通过密度泛函理论,利用Materials Studio6.0软件的DMol3模块,对18T Fe-ZSM-5簇模型表面两个NO分子先后吸附和相应的直接分解机理进行研究。得到以下主要结论:(1)第一个NO分子在Fe-ZSM-5表面可以形成四种稳定的吸附构型,均放出热量,为稳定的化学吸附。稳定性顺序为:NO分子N端吸附于Fe位最稳定,NO分子的N原子和O原子同时吸附于Fe-ZSM-5次之,O端吸附于Fe-ZSM-5的稳定性最差。其中三种为NO分子在Fe原子的不同吸附方式,而另一种吸附实质上为NO与晶格氧反应生成NO_2并吸附于Fe原子。NO分子吸附后均被活化,且吸附反应活性与Fe-ZSM-5模型的Fukui函数预测相一致。(2)以第一个NO分子四种不同稳定吸附态分别作为起始反应点,第二个NO分子的吸附为反应初始步骤,以N_2O中间产物为关键反应中间体,N_2和O_2为目标产物,以穷举法对可能发生的催化分解过程进行研究,并通过过渡态搜索和频率验证分析方法进行确认,得到四种NO在Fe-ZSM-5模型表面直接分解机理。四种分解机理均可生成气态N_2和O_2,同时底物Fe-ZSM-5模型在反应后恢复初始状态可继续进行催化,从而完成催化剂的循环。在得到的四种反应路径中,当NO分子N原子和O原子同时吸附于Fe-ZSM-5后的反应路径能垒最低,且反应步骤较少。整个路径可描述为NO(g)→NO(ads)→NO(ads)+NO(ads)→OONN(ads)→O_2(ads)+N_2(ads)→O_2(ads)+N_2(g)→O_2(g)+N_2(g),反应热为-43.74 kcal/mol,控速步骤为OONN(ads)的生成即NO(ads)+NO(ads)→OONN(ads),其能垒为25.73kcal/mol。
[Abstract]:With the rapid development of modern industry and the increasing number of motor vehicles, the environmental problems caused by industrial exhaust gas and motor vehicle exhaust gas are becoming more and more serious. The resulting NOx is the main component of air pollution.Its increasing emissions of people's normal life and physical and mental health have a serious impact, resulting in environmental problems also seriously hinder the economic development and people's living standards.At present, V_2O_5-MoO_3/TiO_2 and V_2O_5-WO_3/TiO_2 catalysts are mainly used to remove no _ x with NH_3 as reducing agent in industry. However, because V_2O_5 is easy to be discharged into the atmosphere with tail gas, its own biological toxicity will threaten ecosystem and human health.In recent years, the efficiency of direct decomposition of no _ x has been greatly improved and the cost has been greatly reduced due to the continuous development and innovation of active center molecular sieve catalysts such as alkali metal oxide and transition metal oxide.In addition, the process of this technology is simple, no side effects of secondary pollutants, and good economy.Based on the above idea, the adsorption and direct decomposition mechanism of two no molecules on the surface of 18T Fe-ZSM-5 cluster model were studied by using density functional theory and DMol3 module of Materials Studio6.0 software.The main conclusions are as follows: (1) the first no molecule can form four stable adsorption configurations on the surface of Fe-ZSM-5, all of which give off heat and are chemically stable.The order of stability is as follows: the N and O atoms of N end adsorbed on Fe site and O atom simultaneously adsorbed on Fe-ZSM-5 and then O terminal adsorbed on Fe-ZSM-5 are the least stable.Three of them are different adsorption modes of no molecules at Fe atoms, while the other is that no reacts with lattice oxygen to form NO_2 and then adsorbs on Fe atoms. No molecules are activated after adsorption.The adsorption activity of the first no molecule is consistent with the Fukui function prediction of the Fe-ZSM-5 model, and the four different stable adsorption states of the first no molecule are taken as the starting reaction points respectively, and the adsorption of the second no molecule is the initial step of the reaction.Taking the intermediate products of Ns _ 2O as the key reaction intermediates, N _ S _ 2 and O _ s _ 2 as the target products, the possible catalytic decomposition process was studied by exhaustive method, and confirmed by transition state search and frequency verification analysis.The direct decomposition mechanism of four kinds of no on the surface of Fe-ZSM-5 model was obtained.All four decomposition mechanisms can produce gaseous Ns _ 2 and O _ 2s, while the substrate Fe-ZSM-5 model can continue to catalyze after the initial state is restored, thus completing the cycle of the catalyst.Among the four reaction pathways obtained, the lowest barrier of the reaction path was obtained when N and O atoms were adsorbed on Fe-ZSM-5 simultaneously, and the reaction steps were less.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：X701;O643.3

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