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: email@example.com
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 NOx. The one-pot synthesized Cu-SSZ-13 zeolite exhibites high hydrothermal stability, near 100% N2 selectivity and excellent resistance to high space velocity between 150oC and 550oC, which becomes a promising catalyst for NH3-SCR. However, it shows poor low-temperature activity, especially below 200oC. In order to improve the low temperature (<200oC) 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 (<175oC) 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 175-500oC under 150,000h-1. NH3 and NOx were readily adsorbed by ceria between 100oC and 450oC. NH3 was oxidized to N2 at low temperature. Zhang et al  reported that, the adsorbed NH3 could be oxidized by ceria's surface oxygen at higher temperature, forming surface NOx species, in turn, NH3 and surface NOx 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 30.7o, 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 .
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); Ce0.015/Cu-SSZ-13(b); Ce0.025/Cu-SSZ-13 (c); Ce0.05/Cu-SSZ-13(d); Ce0.1/Cu-SSZ-13(e)
Figure3.SEM images of the one-pot synthesized
Cu-SSZ-13(a) and iron-exchanged Ce/Cu-SSZ-13(b)
 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 NOx with NH3, Environ. Sci. Technol. 48(2014) 566-572
 L. Zhang, J. Pierce, V. L. Leung, D. Wang, W.S. Epling, Characterization of Ceria's Interaction with NOx and NH3, J. Phys. Chem. C 117 (2013) 8282-8289
 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