催化学报

催化学报 ›› 2006, Vol. 27 ›› Issue (7): 636-646.

• 综述 • 上一篇    

NH3在选择性催化还原NO过程中的吸附与活化

刘清雅1,刘振宇2,李成岳1   

  1. 1 北京化工大学化工资源有效利用国家重点实验室, 北京 100029; 2 中国科学院山西煤炭化学研究所煤转化国家重点实验室, 山西太原 030001
  • 收稿日期:2006-07-25 出版日期:2006-07-25 发布日期:2006-07-25

Adsorption and Activation of NH3 during Selective Catalytic Reduction of NO by NH3

LIU Qingya1, LIU Zhenyu2*, LI Chengyue1   

  1. 1 State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China; 2 State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, The Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China
  • Received:2006-07-25 Online:2006-07-25 Published:2006-07-25

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1090

被引次数

367

摘要： 通过大量文献并结合自己的工作,以NH3在催化剂表面的阶段氧化脱氢为主线,分析归纳了选择性催化还原(SCR)反应机理和该体系中可能发生的NH3氧化副反应机理的联系和共性. 对于V2O5/TiO2催化剂,大部分学者认为SCR反应与Brnsted酸性位上的NH+4有关,中间体为NH+3(ads); 而少数学者认为SCR反应与Lewis酸性位上的NH3有关,中间体为NH2(ads). 对于其它SCR催化剂,普遍认可L酸性位上NH3活化脱氢形成的NH2(ads)既是SCR反应中间体,也是NH3氧化生成N2的中间体; NH3氧化生成N2O和NO的反应源于NH2(ads)的进一步脱氢. 尽管有关SCR反应中NH3的吸附位存在分歧,但从NH3吸附后活化的角度看, NH3无论吸附在L酸性位还是B酸性位,都先经过阶段氧化脱氢,然后再参与SCR反应. 由于反应中生成的H2O可能导致L酸向B酸转化,且该转化受反应温度影响,因此不同酸性位机理可能没有本质区别, SCR反应关键是NH3吸附位的氧化性. SCR活性取决于NH3在催化剂表面的吸附量和阶段氧化程度. 催化剂应能吸附足够的NH3, 这与其表面酸碱性有关; 吸附的NH3要能被活化脱氢且程度不宜太高,这与表面氧化还原性有关. 反应温度也会影响NH3的吸附量和活化程度,因此开发高效SCR脱硝催化剂的关键是根据反应温度调控其表面酸性和吸附位的氧化性.

关键词: 选择性催化还原, 氨, 一氧化氮, 吸附, 活化, 表面性质

Abstract: Selective catalytic reduction (SCR) of NO by NH3 is a well studied and demonstrated technology for effective NOx removal from flue gas. However, mechanisms involved in this reaction system including the SCR reaction and NH3 oxidation (side reactions of the SCR) are still not clear. For further advancement in SCR catalysis and catalyst design, systematic literature analyses on the SCR reaction and NH3 oxidation was carried out in this study. In the case of V2O5/TiO2 catalysts, most researchers believed that NH+4 on Bronsted acid sites is responsible for the SCR reaction and that NH+3 (ads) was the intermediate. A few researchers, however, suggested that the SCR reaction was related to NH3 on Lewis acid sites and that NH2(ads) was the intermediate. In the case of other SCR catalysts, it was thought that NH2(ads) from H-abstraction of coordinated NH3 on Lewis acid sites was the intermediate in both SCR reaction and NH3 oxidation to N2, and NH(ads) from H-abstraction of NH2(ads) was the intermediate in the oxidation of NH3 to N2O and NO. Despite the apparent differences, these two types of mechanisms share a common ground from the viewpoint of sequential H-abstraction (or oxidation) of NH3. The adsorbed NH3, either in the protonated form (NH+4) or in the coordinated form (NH3), is partially oxidized by the adsorption sites first before taking part in these reactions. Because the Lewis acid sites and Bronsted acid sites are exchangeable under the SCR conditions due to the presence of H2O, the differences in various SCR mechanisms due to the difference in acid sites are not fundamental. Based on this viewpoint, it was concluded that the SCR activity was determined by the amount of NH3 adsorbed on the surface and the H-abstraction extent of NH3(ads), which relate to the surface acidity and the redox properties. Temperature also affects the NH3 adsorption and H-abstraction ability of SCR catalyst. Regulations of the surface acidity and redox properties of a catalyst with respect to reaction temperature is of importance for the high SCR activity.

Key words: selective catalytic reduction, nitrate oxides, ammonia, adsorption, activation, surface property