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题名: 银/氧化铝催化乙醇选择性还原NOx构效关系研究
作者: 邓华
学位类别: 博士
答辩日期: 2015-05
授予单位: 中国科学院研究生院
授予地点: 北京
导师: 贺泓
关键词: 选择性催化还原,Ag/Al2O3,密度泛函,构效关系,NOx去除,selective catalytic reduction, Ag/Al2O3, DFT, structure-activity relationship, NOx removal
其他题名: Structure-Activity Relationship of Selective Catalytic Reduction of NOx with ethanol over Ag/Al2O3 Catalysts
学位专业: 环境科学
中文摘要:     柴油车排放污染物是造成光化学烟雾、大气灰霾等大气污染现象的重要原因之一。控制柴油车氮氧化物(NOx)排放对保护人类赖以生存的大气环境意义重大。乙醇-银/氧化铝(Ag/Al2O3)被认为是碳氢还原剂选择性催化还原(HC-SCR)技术里极具应用前景的柴油车尾气NOx净化还原剂-催化剂组合体系,但其低温(<300oC)活性的改进和耐硫中毒的性能提升是其得以实用的前提。实验联合理论模拟,从原子/分子层面探讨催化剂结构与表面反应机制,实现催化剂微观活性结构的甄别,是设计和开发高效、稳定的Ag/Al2O3催化剂应用于柴油车尾气NOx净化的关键。
    (1)在Al2O3(Sigma-Aldrich)上负载0~4wt%的活性组分银,乙醇-SCR的活性评价、XPS、XANES、EXAFS和DFT量化模拟的结果表明:银的最佳负载量为1~2 wt%;在1 wt%负载量下,+1价银离子(Ag+)是银物种主要存在形态,是乙醇-SCR反应的活性物种;将负载量增加至2 wt%,Ag-Ag配位出现表明氧化银团簇(Agnδ+)生成,该结构的出现有助于提高乙醇-SCR低温活性;DFT模拟的催化剂银物种结构参数与实验结果高度吻合,活性最高的结构是Ag-O- Altetra:Ag+锚定于γ-Al2O3 (110)面AlIII位;态密度分析指出该结构体中Ag、O、Al三者之间的轨道融合作用显著降低了能隙,使得催化活性提高。
    (2)分别利用球磨(干法)、初湿球磨(半干法)和浸渍法(湿法)制备了Ag/Al2O3催化剂。采用初湿球磨和浸渍法均可制得活性较高的催化剂,而初湿球磨法可以免除旋蒸干燥,相对更加绿色和便捷。1H NMR表征说明Ag+可通过交换载体Al2O3上的羟基质子而最终被锚定。
    (3)选择AlOOH、Al(OH)3、Al2O3载体前驱体负载等量的银组分制备Ag/Al2O3催化剂,利用XPS、UV-vis DRS、XRD、BET、NMR和in situ DRIFTS表征了催化剂结构和HC-SCR反应过程。其中AlOOH是最优载体前驱体,最佳焙烧温度为600oC。在以AlOOH为前驱体制备的催化剂上,Ag+是主导银物种,通过与载体AlO4(Altetra)和AlO6(Alocta)位点的键合,形成了Ag-O-Altetra和Ag-O-Alocta 结构。乙醇-SCR反应中N2生成速率与Ag/Al2O3催化剂上Altetra含量呈现高度正相关,证实Ag-O-Altetra可能是乙醇-SCR反应的活性位。
    (4)通过DFT方法搭建了以Ag-O-Altetra和Ag-O-Alocta为典型结构的Ag/Al2O3催化剂模型,考察了HC-SCR过程中CH3CH2OH、CH2=CH-O-和-NCO物种在上述结构上的吸附几何构型及反应性能。-NCO物种中的N=C键能被Ag-O-Altetra显著活化,有利N2生成。其原因是Altetra能接受来自N=C σ成键电子,而Ag-O-Al杂化的轨道能反馈电子到N=C π反键轨道,最终使得N=C键被拉长活化。通过对比Ag-O-Altetra和Ag-O-Alocta上-NCO与NO2的反应路径及能量,发现Ag-O-Altetra上的-NCO与NO2反应不仅有热力学优势,且反应活化能垒更低,证实了Ag-O-Altetra是Ag/Al2O3催化乙醇选择性还原NOx的主要活性位,这为高活性催化剂设计提供了科学依据。
英文摘要:     The emission of pollutants from diesel engine exhausts leads to serious air pollution, such as photochemical smog and heavy haze, etc. Reducing the NOx emission from diesel engines is urgently needed to protect our living atmosphere. The combined system of ethanol-Ag/Al2O3 is considered one of most applicable technologies in the HC-SCR family for reducing NOx in diesel exhaust. However, improvements in the catalytic activity at low temperature (< 300 oC) and SO2 tolerance of Ag/Al2O3 are necessary for commercial application of this system. To this aim, particular attention should be paid to understanding the structural features of Ag/Al2O3 and the surface reaction pathway of the HC-SCR process at the atomic/molecular level. Identifying the true active centers and rational design of new and efficient Ag/Al2O3 materials are keys to the feasibility of HC-SCR technologies.
    In this study, four aspects of this task have been achieved. First, we investigated the nature of silver species on the Ag/Al2O3 catalyst. Second, the preparation method and operation parameters were optimized. Third, the support effect for Ag/Al2O3 catalysts was studied by comparing the effects of different alumina precursors. Finally, the electronic mechanism for the activation of intermediates during HC-SCR was revealed. The following results were obtained:
    (1) The nature of silver species on an Al2O3 support (Sigma-Aldrich) with different silver loadings (0~4 wt%) was studied by XPS, XANES, and EXAFS, combined with DFT calculations. It was found that the optimum silver content ranges from 1 wt% to 2 wt%. The supported silver species are predominated by +1 oxidation state ions under low silver loading of 1 wt%, which play a crucial role during the HC-SCR process. An Ag-Ag shell emerged clearly when silver loading was increased to 2 wt%, which confirmed the formation of Agnδ+, which was beneficial for the activity at low temperature. The theoretical models developed for Ag/Al2O3 were consistent with the experimental results. The most reactive silver ion seems to be anchored on a tri-coordinate AlIII site on the Al2O3(110) surface (finally Ag-O-Altetra).Density of states analysis revealed that the hybridization of Ag, O, and Al orbitals reduced the energy gap and promoted the catalytic activity.
    (2) Different methods were used to synthesize Ag/Al2O3. The wet impregnation method and semi-wet ball-milling method were able to produce the same highly efficient Ag/Al2O3 catalysts; but the semi-wet method is more green and convenient due to limited water usage. 1H NMR results suggested that Ag species are anchored on the support via proton exchange with hydroxyl groups on alumina.
    (3) The NOx reduction efficiency revealed that AlOOH is the best support precursor, compared to Al(OH)3 and Al2O3, with the optimum calcination temperature of 600 oC. The structural features of Ag/Al2O3 and the surface reaction pathway of the HC-SCR process were studied by XPS, UV-vis DRS, XRD, and BET, combined with in situ DRIFTS. Silver ions are predominant on the Ag/Al2O3 surface. Solid state 27Al NMR suggests that the silver ions might be anchored on Al tetrahedral and octahedral sites, forming Ag-O-Altetra and Ag-O-Alocta entities. A strongly positive correlation between the amount of Altetra structures and N2 production rate confirms the crucial role of Altetra in NOx reduction by ethanol.
    (4) Ag/Al2O3 models were constructed by means of density functional theory, including Ag-O-Altetra and Ag-O-Alocta entities. The adsorption states for CH3CH2OH, CH2=CH-O- and –NCO on Ag/Al2O3 were examined. The significant activation of the N=C bond in –NCO on Ag-O-Altetra aids the cleavage of –NCO to form N2, which can be attributed to electron forward donation from the N=C σ bond to the Altetra site and the electron back-donation from interaction between Ag and Al to the N=C π bond. In the reduction of NO2, –NCO species close to Ag-O-Altetra are more favorable than those on Ag-O-Alocta, both energetically and kinetically. Thus, Ag-O-Altetra rather than Ag-O-Alocta should be considered as the active site during the HC-SCR process. The above results provide us a strategy for rational design and synthesis of catalysts.
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Recommended Citation:
邓华. 银/氧化铝催化乙醇选择性还原NOx构效关系研究[D]. 北京. 中国科学院研究生院. 2015.
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