Selective Catalytic Reduction of NO with NH3 over Bimetallic Composite Catalysts Ce/Cu-SSZ-13

Selective catalytic reduction of NO with NH₃ over bimetallic composite catalysts Ce/Cu-SSZ-13

^1,3Ranran Zhang, ^1,2,3 Yonghong Li*

¹ Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.

² National Engineering Research Center for Distillation Technology, Tianjin 300072, China.

³ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China

* Corresponding author E-mail: yhli@tju.edu.cn

Nitrogen oxide from diesel exhaust is one of the important pollutants leading to hazy weather. Selective catalytic reduction with NH₃ is considered to be the most effective technology for the removal of NO_x. The one-pot synthesized Cu-SSZ-13 zeolite exhibites high hydrothermal stability, near 100% N₂ selectivity and excellent resistance to high space velocity between 150^oC and 550^oC, which becomes a promising catalyst for NH₃-SCR^[1]. However, it shows poor low-temperature activity, especially below 200^oC. In order to improve the low temperature (<200^oC) catalytic activity further in the startup process of diesel vehicle and widen the temperature window, ceria was considered as a preferred candidate for catalyst modification.

In this work, a series of Ce/Cu-SSZ-13 catalysts were prepared by Ce³⁺ ion exchange method based on the one-pot synthesized Cu-SSZ-13 and were employed to NH₃-SCR.

Fig. 1 shows the results of NO conversion and N₂ selectivity in the SCR reaction over the Ce/Cu-SSZ-13 and Cu/SSZ-13. Incorporation of cerium in Ce/Cu-SSZ-13 catalysts was found to improve the low-temperature (<175^oC) NO conversion significantly and widen the reaction temperature window. Ce/Cu-SSZ-13 exhibited higher NO conversion and less N₂O than Cu/SSZ-13, with more than 90% NO conversion, less than 10ppm undesirable N₂O and near 100% N₂ selectivity at 175-500^oC under 150,000h^-1. NH₃ and NO_x were readily adsorbed by ceria between 100^oC and 450^oC. NH₃ was oxidized to N₂  at low temperature. Zhang et al ^[2] reported that, the adsorbed NH₃ could be oxidized by ceria's surface oxygen at higher temperature, forming surface NO_x species, in turn, NH₃ and surface NO_x species reacted to form N₂ at high temperature.

Figure 2 shows XRD spectra of the Cu-SSZ-13 and Ce/Cu-SSZ-13 with characteristic peaks of SSZ-13 at 2¦È=9.5, 14.0, 16.1, 17.8, 20.7, 25.0 and 30.7^o, respectively, indicating that the crystal structure of Cu-SSZ-13 remained intact after Ce³⁺exchange. The SEM images of Cu-SSZ-13 and Ce/Cu-SSZ-13 catalysts are in Figure 3. It shows that Cu-SSZ-13 and Ce/Cu-SSZ-13 exhibits the same regular cube morphology, which seems to be intergrown and appeares as an assembly of agglomerated larger crystals. The SEM results are consistent with Wu's research results ^[3].

 

Figure1. NO conversion and N₂ selectivity on Ce/Cu-SSZ-13 and Cu-SSZ-13 catalysts under 150,000 h^-1

Figure2. XRD patterns of Cu-SSZ-13(a); Ce_0.015/Cu-SSZ-13(b); Ce_0.025/Cu-SSZ-13 (c);    Ce_0.05/Cu-SSZ-13(d); Ce_0.1/Cu-SSZ-13(e)

 

Figure3.SEM images of the one-pot synthesized

Cu-SSZ-13(a) and iron-exchanged Ce/Cu-SSZ-13(b)

References

[1]       L. Xie, F. Liu, L. Ren, X. Shi, F. Xiao, H. He, Excellent Performance of One-Pot Synthesized Cu-SSZ-13 Catalyst for the Selective Catalytic Reduction of NO_x with NH₃, Environ. Sci. Technol. 48(2014) 566-572

[2]       L. Zhang, J. Pierce, V. L. Leung, D. Wang, W.S. Epling, Characterization of Ceria's Interaction with NO_x and NH₃, J. Phys. Chem. C 117 (2013) 8282-8289

[3]       L.Wu, V. Degirmenci, P. C.M.M. Magusin, et al, Mesoporous SSZ-13 zeolite prepared by a dual-template method with improved performance in the methanol-to-olefins reaction, Journal of Catalysis, 298 (2013) 27-40