SCR催化剂失活机理及其分子设计研究

发布时间：2018-03-28 08:26

本文选题：SCR　切入点：V_2O_5　出处：《郑州大学》2016年硕士论文

【摘要】：大气中的主要污染物是氮氧化物(NO_x)。脱除NO_x技术中最成熟有效的是选择性催化还原技术(SCR)。它的原理是在合适催化剂的条件下通过合适的温度,利用还原剂把NO_x还原为N2和H2O。SCR催化剂是脱硝技术的核心,但是在工业生产中SCR催化剂会失活从而使脱硝的能力减弱,研究其失活机理从而设计出高活性的催化剂具有重要意义。本文建立了催化剂V_2O_5的晶体结构模型。研究了催化剂V_2O_5与NH_3的吸附机理,以及烟气中的碱金属K和重金属Pb使其失活的机理,最后结合化学反应时的分子轨道理论,设计出了高效的催化剂。得到的结论如下:(1)NH_3分子在催化剂表面的吸附机理探究。重点分析了NH_3分子与2×2×1V_2O_5(001)表面作用的两种机理模型:路易斯酸位的Top位和Bridge位以及布朗特酸位的吸附模型。NH_3在催化剂V_2O_5(001)表面路易斯酸位的Top位和Bridge位均发生化学吸附,在吸附过程中N原子上的电子转移到了相邻的H和衬底上,NH_3由中性分子分别变为带0.896和0.931个正电荷的共价态NH_3分子;NH_3分子与2×2×1 V_2O_5(001)表面的布朗特酸位吸附作用后催化剂表面V-O键的键长变长,H与NH_3之间的距离也接近NH_3分子计算得到的键长;NH_3由吸附前的电中性分子变为带正电的阳离子。进一步分析了两种吸附模型的原子态密度,说明衬底上的O原子以及H原子与吸附分子NH_3发生了较强的化学作用。(2)碱金属K、碱金属氯化物KCl和重金属Pb在催化剂表面的失活机理的探究。建立作用模型,利用密度泛函理论(DFT)研究了其在催化剂表面的失活机理。计算了吸附前后的吸附能、键长变化以及Mulliken电荷布局数。通过分析可知,K、KCl和Pb均在V_2O_5(001)表面形成较强的化学吸附,吸附能分别为-0.0985、-0.170、-0.136Ha较NH_3与催化剂的吸附能-0.063Ha更负,说明其吸附作用较强。这些物质会与还原性气体NH_3产生竞争吸附,影响V_2O_5催化剂的脱硝性能。(3)新型高效催化剂的设计。通过前线轨道理论设计出两大类六种可使V_2O_5催化活性增强的新型催化剂,一类是催化剂的活性位V原子被取代的催化剂模型:W@V-V_2O_5,Mo@V-V_2O_5,Nb@V-V_2O_5;另外一类是催化剂活性位的邻位的V原子被取代的催化剂模型:W@邻V-V_2O_5、Mo@邻V-V_2O_5和Nb@邻V-V_2O_5的催化剂。得出活性位被W和Mo取代的催化剂和活性位的邻位被Nb取代的催化剂可以作为高效催化剂。
[Abstract]:The main pollutant in the atmosphere is the nitrogen oxide. The most mature and effective way to remove the NO_x is the selective catalytic reduction. Its principle is to pass through the appropriate temperature under the suitable conditions of the catalyst. Reducing NO_x to N2 and H2O.SCR catalysts by reducing NO_x is the core of denitrification technology. However, in industrial production, SCR catalyst will deactivate and the ability of denitrification will be weakened. It is of great significance to study the deactivation mechanism of the catalyst and to design a catalyst with high activity. In this paper, the crystal structure model of catalyst V_2O_5 is established, and the adsorption mechanism between V_2O_5 and NH_3 is studied. And the mechanism of alkali metal K and heavy metal Pb inactivation in flue gas. Finally, combining with the molecular orbital theory of chemical reaction, A highly efficient catalyst was designed. The results are as follows: the adsorption mechanism of NH_3 molecule on the catalyst surface. Two mechanisms models of the interaction between NH_3 molecule and 2 脳 2 脳 1V2O5 / 001) surface are emphatically analyzed: the Top site and Bridge site of the Lewis acid site and the Bridge site of the Lewis acid site. Adsorption model of Bronte acid site. Chemical adsorption of Top site and Bridge site of Lewis acid site on catalyst V _ 2O _ 5T _ (001). During the adsorption process, the electrons on the N atom were transferred to the adjacent H and the substrates from neutral molecules to covalent NH_3 molecules with 0.896 and 0.931 positive charges, respectively, and the Brownitic acid sites adsorbed on the surface of 2 脳 2 脳 1 V _ 2O _ 5 / 01). The distance between the bond length of V-O bond and NH_3 on the surface of the chemical agent is also close to that of the bond length NH _ 3 calculated by NH_3 molecule, which changes from electrically neutral molecules before adsorption to positive cations. The atomic density of states of the two adsorption models is further analyzed. It is shown that O atom on substrate and H atom have strong chemical interaction with adsorbed molecule NH_3. The mechanism of deactivation of alkali metal K, alkali metal chloride KCl and heavy metal Pb on the surface of catalyst is studied. The mechanism of deactivation on catalyst surface was studied by density functional theory (DFT). The adsorption energy, bond length change and Mulliken charge distribution number before and after adsorption were calculated. It was found that both KCl and Pb formed strong chemisorption on V _ 2O _ 5T _ (001) surface. The adsorption energy is -0.0985- 0.170U -0.136Ha, respectively, which is more negative than that of NH_3 and catalyst (-0.063Ha), indicating that the adsorptive energy of these substances is stronger than that of the catalyst. These substances can compete with the reductive gas NH_3. The design of new high activity catalyst for V_2O_5. Two kinds of six kinds of new catalysts which can enhance the catalytic activity of V_2O_5 were designed by the frontier orbital theory. One is the catalyst model where the active V atom of the catalyst is replaced by the V atom of the catalyst, the one is the model of the V atom replaced by the active site of the catalyst: W @ W @ next to V-V _ 2O _ 5 / V-V_2O_5, and the other is the catalyst model: W @ neighbor V _ V _ 2O _ 5M @ next to V-V_2O_5 and Nb@ V-V_2O_5. The active sites are found to be the active sites of the catalyst by the catalyst model: W @ next door V atom replaced by the catalyst model: W @ o V _ 2O _ 5i _ Mo @ next to V-V_2O_5 and Nb@ @ neighbor V-V_2O_5. The W and Mo substituted catalysts and the NB substituted catalysts with active sites can be used as high efficient catalysts.
【学位授予单位】：郑州大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：O643.36

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