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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: 鄢勇
学位类别: 硕士
答辩日期: 2012
授予单位: 中国科学院研究生院
授予地点: 北京
导师: 余运波
关键词: 氮氧化物 ; NOx ; 选择性催化还原 ; selective catalytic reduction ; 银/氧化铝 ; Ag/Al2O3 ; 乙醇 ; ethanol ; 构效关系 ; structure-properties relationship
其他题名: Studies of Structure-properties Relationship in the Selective Catalytic Reduction of NOx by Ethanol over Ag/Al2O3
中文摘要:       银/氧化铝(Ag/Al2O3)催化含氧碳氢化合物如乙醇选择性还原(SCR)氮氧化物(NOx)是有望实际应用于柴油机尾气治理的技术之一。本论文从载体Al2O3前躯体的制备、活性组分银的存在形态表征入手,结合活性评价、动力学与反应机理研究的结果,探讨了Ag/Al2O3催化乙醇选择性还原NOx(乙醇-SCR)的构效关系、氨的形成机制及反应途径。 (1) 以水热法合成了拟薄水铝石纳米棒和纳米片,HRTEM表征表明,以此制备的催化剂载体为多晶,载体形貌对Ag/Al2O3催化乙醇选择性还原NOx的活性及耐硫性能的影响并不显著;介孔氧化铝较大的比表面积有利于活性组分Ag的分散,以此为载体的Ag/Al2O3具备了优异的乙醇-SCR活性,以一步法合成的介孔型Ag/Al2O3还原NOx的活性更高。 (2) 采用乙二胺为沉淀剂合成了诺三水铝石,经载银、焙烧后制备的并与催化剂比表面积可达300 m2/g以上,显示了较Al2O3纳米棒、纳米片及介孔Al2O3负载的银催化剂更优异的催化乙醇还原NOx的活性:在空速为50 000 h-1、600-750 K温度范围内,NOx的转化率接近100%,但反应中形成了大量的氨;通过与NH3-SCR或NH3-SCO催化剂联用,提高了Ag/Al2O3—乙醇组合体系还原NOx的N2选择性。 (3) 活性测试的结果表明,改变还原剂乙醇的用量及Ag/Al2O3催化剂上Ag的负载量,都无法完全避免Ag/Al2O3-乙醇组合体系还原NOx过程中NH3的产生;原位DRIFTS与气态产物分析实验证明,Ag/Al2O3催化乙醇选择性还原NOx过程中,关键中间体-NCO的水解是NH3产生的主要来源,反应气氛中共存的水蒸气及还原剂完全氧化产生的水都会引起NH3的产生;活性评价与原位红外的结果表明,源于-NCO水解产生的NH3可促进乙醇的部分氧化,并与表面烯醇式物种反应,重新生成关键中间体-NCO,并转化为最终产物N2,进而在一定程度上提高了NOx的还原效率,这有可能是Ag/Al2O3催化乙醇选择性还原NOx一条新途径。 (4) UV-vis分析表明,在负载量为2-8%的Ag/Al2O3催化剂上,Ag以氧化态(Ag+和Agnδ+)以及金属态的银簇(Agn0)存在,其中氧化态的Ag高于金属态;反应动力学、ICP-OES及H2-O2滴定的研究表明,高度分散的氧化态Ag在Ag/Al2O3催化乙醇选择性还原NOx过程中起到关键作用。 (5) 原位DRIFTS表征及量化模拟的结果首次表明,Ag的存在促进了Ag/Al2O3催化乙醇部分氧化过程中高活性烯醇式物种的形成,烯醇式物种倾向于吸附在高分散的Ag位以及Ag-Al界面上;这种与活性组分Ag紧密接触的烯醇式物种比吸附在Al位上的烯醇式物种具有更高的反应活性:能与NO+O2迅速反应形成关键中间体-NCO;原位DRIFTS-MS进一步验证了吸附于Ag位、Ag-Al位的烯醇式物种是乙醇-SCR反应的重要中间体,参与了最终产物N2的形成。
英文摘要:
       The selective catalytic reduction (SCR) of NOx by oxygenated hydrocarbons such as ethanol over Ag/Al2O3 is a promising candidate for NOx removal from diesel engine exhausts. Herein, different Al2O3 precursors were synthesized and employed to prepare Ag/Al2O3 catalysts. On the prepared catalysts, the states of silver species were carefully characterized. By combined with the activity tests, kinetic measurements and mechanism investigation, the structure-properties relationship in the SCR of NOx by ethanol (ethanol-SCR) over Ag/Al2O3 was set up. Also, the mechanism of NH3 formation and reaction pathway during the ethanol-SCR was proposed. The innovative results were obtained as follows: (a) Hydrothermal reactions were carried to synthesize boehmite nanorods and nanosheets. HRTEM images showed that the catalysts prepared from synthesized boehmite were polycrystal, with no specifically exposed crystalline planes. Thus, changing the morphology of supports has little influence on the activity and sulfur tolerance. The mesoporous Al2O3 supported Ag catalysts exhibited a relatively high surface area, improving the dispersion of silver. One-step synthesis by in situ loading of silver is more suitable for the preparation of mesoporous catalysts. (b) By simply precipitation with ethylenediamine anhydrous, nordstrandite was prepared. After Ag loading and calcination, the nordstrandite based Ag catalysts exhibited high surface area (above 300 m2/g). The catalysts with 2 wt% silver loading showed NOx conversions near to 100% within a wide temperature range of 600-750 K, while large amounts of NH3 were produced during the NOx reduction by ethanol. Combined with NH3-SCR or NH3-SCO catalysts, the selectivity to N2 enhanced significantly. (c) The production of ammonia during the ethanol-SCR is quiet common, which can be eliminated by changing the silver content on catalysts or the concentration of ethanol employed in the activity tests. During the ethanol-SCR over Ag/Al2O3, the hydrolysis of –NCO, as a key intermediate in NOx reduction, is the main source of NH3 yielding, as confirmed by DRIFTS and gaseous products analysis. Besides, DRIFTS results and activity tests revealed that NH3 reacted with enolic species to form –NCO, and then contributed to the formation of N2. (d) The results of UV-vis showed that Ag species were presented in both the oxidized state (including highly dispersed Ag+ ions (Ag+), oxidized silver clusters (Agnδ+)) and metallic silver clusters on Ag/Al2O3 (Agn0), of which the former was dominant on Ag/Al2O3 catalysts with 2-8 wt% silver content. The kinetic measurements, ICP-OES, H2-O2 titration characterizations confirmed that the Ag species in oxidized state played a key role in the reduction of NOx by ethanol. (e) In situ DRIFTS and DFT calculations showed for the first time that Ag doping promote the formation of enolic species, which prone to adsorbed on or close to silver site (RCH=CH-O-Ag, RCH=CH-O-Al-Ag). And this kind surface species are more active towards NO + O2 to form key intermediate of –NCO species, if compared to those enolic species absorbed merely on Al sites (RCH=CH-O-Al). By combining MS with DRIFTS, the important role of enolic species in the formation of N2 was confirmed.
内容类型: 学位论文
URI标识: http://ir.rcees.ac.cn/handle/311016/35169
Appears in Collections:大气污染控制中心_学位论文

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Recommended Citation:
鄢勇. 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. 2012.
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