Chinese Journal of Catalysis

Chinese Journal of Catalysis ›› 2006, Vol. 27 ›› Issue (7): 636-646.

• Articles • Previous Articles    

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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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