Low temperature performance and characterization of solid-state ion-exchanged Cu-CHA for ammonia SCR applications

Hanna  Härelind, Competence Centre for Catalysis, Chalmers University of Technology

The metal-exchanged small-pore zeolite Cu-SSZ-13 is a promising catalyst for selective catalytic reduction of nitrogen oxides (NOx) in diesel exhausts using ammonia as reductant (NH3-SCR)1. Conventionally, copper ions are introduced into the zeolite by aqueous ion-exchange (AIE)2. To avoid the use of water, copper is in the present study instead introduced to the micropores of the zeolite by solid-state ion-exchange (SSIE). The catalytic activity for NH3-SCR of Cu-SSZ-13 samples with a range of Cu loadings, is investigated, and the samples are characterized before and after the activity experiments in order to study the nature of the Cu species present in the micropores of the zeolite3.

The Cu-SSZ-13 samples were prepared by physically mixing different amounts of a Cu salt with the zeolite followed by heat treatment at 800°C. The activity for the standard NH3-SCR reaction was investigated in a flow reactor using powder samples of the catalyst at a GHSV of 205,000 h-1. The samples were characterized both directly after the synthesis, i.e. before the activity tests for NH3-SCR, as well as after the activity experiments. The characterization methods used were X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis, X-ray photoelectron (XPS) and diffuse reflection infrared Fourier transform (DRIFT) spectroscopy, ammonia temperature programmed desorption (NH3-TPD) and hydrogen temperature programmed reduction (H2-TPR).

The samples prepared by SSIE show high activity for NH3-SCR with maximum NOx conversions comparable to those reported for Cu-SSZ-13 synthesized by AIE4. Interestingly, significantly lower activities are observed for the first compared to the second and third consequent flow reactor experiment. In contrast to the results for Cu-SSZ-13 prepared by AIE reported in literature, the NOx conversions increased with increasing Cu loadings for the samples prepared by SSIE. Furthermore, UV-Vis spectroscopic, XPS and H2-TPR measurements suggest the existence of isolated Cu2+ species in the samples likely representing precursors for the active sites in the NH3-SCR reaction. Finally, DRIFT results show both Cu+ and Cu2+ species present in the zeolite structure. However, the presence of CuO outside the zeolite framework was observed for all samples.

 

In conclusion, we present a novel route for ion-exchange of Cu into the small-pore zeolite SSZ-13 utilizing solid-state ion-exchange. The results from the different characterization techniques suggest the existence of isolated Cu2+ species and, in addition, the DRIFT results indicate that Cu+ species are also present in the micropores of the zeolite. In contrast to findings for the corresponding samples ion-exchanged by AIE, however, also extra framework CuO species are observed to be present. This may explain the influence of the Cu loading on the NOx conversion in the NH3-SCR reaction.

 

References

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  2. W. Fickel, R.F. Lobo, J. Phys. Chem. 2010, 114, 1633.
  3. K.S Clemens, A. Shishkin, P.-A. Carlsson, M. Skoglund, F.J. Martinez-Casado, Z. Matej, O. Balmers, H. Härelind, ACS Catal. 2015, 5, 6209.
  4. H. Kwak, D. Tran, J. Szanyi, C.H.F. Peden, J.H. Lee, Catal. Lett. 2012, 142, 295.