The abatement of environmentally harmful NOx compounds (NO, NO₂, and N₂O), especially when emitted from ‘lean-burn’ mobile or stationary power sources, remains a challenging task for the catalysis community. Selective catalytic reduction (SCR) with ammonia using metal-exchanged zeolites [1] is now being commercialized on diesel vehicles. In particular, recent patent literature indicates that the commercial formulations are based on zeolites with a chabazite (CHA) structure (see references 22 and 24 in [2]). We recently published the first open-literature paper describing the comparative performance of a Cu-CHA (SSZ-13) catalyst relative to Cu-beta and Cu-ZSM-5 for the SCR of NOx with NH₃, particularly focusing on the activity and N₂ selectivity [3]. In this presentation, we will describe results of more recent studies that address a number of additional aspects of these catalysts including the effects of NO/NO₂ ratio and hydrothermal aging on performance, as well as structural studies that account for the relative performance of these materials. The implications of these results for understanding the NH₃ SCR reaction mechanisms will also be discussed.

Experimental

ZSM-5 (CBV-3024, Si/Al₂ = 30), beta (CP-814C, Si/Al₂ = 38) and Y (CBV-100, Si/Al₂ = 5.2) zeolites were obtained from Zeolyst International Co.  The SSZ-13 zeolite was synthesized using the methods recently published by Fickel and Lobo [2], reported to give a material with a Si/Al₂ ratio of ~12. Copper ions were then exchanged into the zeolite in an aqueous ion-exchange process, using Cu(NO₃)₂ as precursor. After drying and calcination, intact zeolite structures were confirmed with XRD measurements. The SCR activities of these powder catalysts have been examined in a flow reactor system at various temperatures in a gas mixture containing 350 ppm NOx, 350 ppm NH₃, 14% O₂ and 10% H₂O with a balance of N₂. Catalyst characterization included synchrotron ²⁷Al NMR, XANES and EXAFS spectroscopies, XRD, TPR, and TPD, and with FTIR of probe molecules (e.g., CO and NO).

Results and Discussion

The effect of hydrothermal aging on selective catalytic reduction of NOx by NH₃ was examined under both “standard” (NOx = NO only) and “fast” (NO=NO₂) SCR conditions over several Cu-zeolites, with good performance observed for all fresh Cu/zeolite catalysts [4].  At higher temperatures, NO reduction activity was limited by the availability of ammonia as the ammonia oxidation reaction to NO_x becomes significant.  A considerable amount of N₂O was produced over Cu-Y [4] during “standard” SCR, and also over Cu-beta and Cu-ZSM-5 under “fast” SCR conditions.  In contrast, near 100% selectivity to N₂ for NO reduction (less than 5 ppm N₂O produced) was observed for Cu-SSZ-13 under all conditions studies.

Significant loss of “standard” NO reduction activity, in greatly varying extents, was observed for most of the Cu/zeolite catalysts after hydrothermal treatment in 10% H₂O in air at 800°C for 16 h.  While Cu-SSZ-13 was found to show essentially no change in NO reduction activity, Cu-Y lost its activity completely.  Both Cu-ZSM-5 and Cu-beta were found to lose NO reduction activity primarily at low catalyst bed temperatures, but maintain reasonable activity at high (>350°C) temperatures.  Similar sensitivities to hydrothermal aging were observed for “fast” SCR conditions, and undesirable N₂O selectivities were generally worse for all catalysts after aging except for Cu-SSZ-13 [4].

H₂-TPR spectra collected over fresh Cu/zeolites could be divided into two groups based on the reducibility of Cu ions.  For fresh Cu-ZSM-5, three reduction peaks were observed at 155°C (perhaps due to overexchanged Cu), 207°C and 315°C.  Similar to Cu-ZSM-5, two reduction peaks were observed at 200°C and 390°C for Cu-beta; these higher temperature features can be attributed to the reduction of Cu²⁺ to Cu⁺, and that of Cu⁺ to Cu⁰, respectively.  On the other hand, the two reduction peaks observed at 195°C and 310°C for Cu-Y catalysts are assigned only to the reduction of Cu²⁺ to Cu⁺ based on the literature, which suggested that Cu²⁺ ions inside supercages of faujasite (FAU) zeolites are reduced to Cu⁺ at 195°C, while reduction of Cu²⁺ ions to Cu⁺ inside sodalite cages occurs at 310°C [5].  Interestingly, only one H₂-TPR reduction peak at 230°C with a broad shoulder at ~300°C was obtained for Cu-SSZ-13.  During the TPR, Cu-SSZ-13 appeared white even at 700°C, which suggests the formation of Cu⁺, similar to Cu-Y.

Detailed characterization of these catalysts using ²⁷Al NMR, EPR, synchrotron XANES and EXAFS spectroscopies, XRD, and TPD and FTIR of probe molecules (e.g., CO and NO) will be presented to rationalize these results in the context of proposed NH₃ SCR reaction mechanisms (see, for example, [6,7]). References

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2.    D.W. Fichel, R. Lobo, J. Phys. Chem. C  114, 1633 (2010).

3.    J.H. Kwak, R.G. Tonkyn, D.H. Kim, J. Szanyi, C.H.F. Peden, J. Catal.  275, 187 (2010).

4.    J.H. Kwak, D. Tran, S.D. Burton, J. Szanyi, J.H. Lee, C.H.F. Peden, J. Catal. 287, 203 (2012).

5.    S. Kieger, G. Delahay, B. Coq, B. Neveu, J. Catal. 183, 267 (1999).

6.    J.H. Kwak, D. Tran, S.D. Burton, J. Szanyi, J.H. Lee, C.H.F. Peden, Catal. Lett. 142, 295 (2012).

7.    J.H. Kwak, H. Zhu, J.H. Lee, C.H.F. Peden, J. Szanyi, Chem. Comm. 48, 4758 (2012).