Deactivation of Cu/SAPO-34 during low-temperature NH3-SCR

Kirsten  Leistner, Chalmers University of Technology

Deactivation of Cu/SAPO-34 during low-temperature NH3-SCR
Kirsten Leistner 1, Louise Olsson 1,*

1 Chemical Engineering, Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
* corresponding author: louise.olsson@chalmers.se

A major issue in catalyst development is catalyst stability and resistance to deactivation. Despite their excellent high-temperature stability, there have been some early reports of poor low-temperature stability in SAPO-n materials [1,2]. These studies found an often reversible loss of crystallinity and porosity upon low-temperature hydration of SAPO-34 powders and membranes. SAPO-34 is also known to be prone to hydrolysis when under-going aqueous ion exchange [3]. However, the low temperature hydrostability of copper-exchanged SAPO-34 in conditions relevant to NH3-SCR (smaller relative humidity, temperature between room temperature and 100 °C) has, to our knowledge, not been extensively studied. The objective of this study has therefore been to examine the effect of a low-temperature SCR atmosphere (including water vapour) on the performance of Cu/SAPO-34 with 1.27 and 2.60 wt.% Cu.

We observed that after a few hours of exposure to a feed containing water vapour at temperatures below 100 °C, the two catalysts experienced severe loss of activity. After only 3 h at 70 °C, the NOx conversion at 200 °C decreases from an initial 87% to 66%, after another 9 h to 6%. The deactivation was not reversible upon attempted regeneration in a dry atmosphere at 600 °C for 35 min.

This observation was accompanied by only a small loss of microporous volume and XRD showed that the crystalline structure of both catalysts is similar before and after deactivation. Thus, the dominating mechanism for the deactivation does not appear to be the breakage of the crystalline SAPO-34 framework. Further, ammonia storage was unaffected. There was a decrease in H2 consumption during TPR, indicating that fewer copper sites are available for the redox cycle, and this likely contributes to the deactivation. However, the magnitude of the decrease in TPR signals did not exceed 38%. It therefore seems likely that this is not the only reason for the deactivation, given its severity. We postulate that in addition a large number of the copper sites are transformed into an inactive form, and that this transformation is responsible for the loss of SCR activity.

[1] M. Briend, R. Vomscheid, M.J. Peltre, P.P. Man, D. Barthomeuf, J. Phys. Chem. 99 (1995) 8270–8276.
[2] J.C. Poshusta, R.D. Noble, J.L. Falconer, J. Membr. Sci. 186 (2001) 25–40.
[3] F. Gao, E.D. Walter, N.M. Washton, J. Szanyi, C.H.F. Peden, ACS Catal. 3 (2013) 2083–2093.
[4] K. Leistner, L. Olsson, Appl. Catal. B: Environmental 165 (2015) 192–199.

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