CLEERS Teleconference: Feng Gao, Pacific Northwest National Laboratory

Strategies for improving durability of Cu/chabazite selective catalytic reduction (SCR) catalysts

Feng Gao
Pacific Northwest National Laboratory

Abstract:

Small-pore zeolite-based SCR catalysts, i.e., Cu/SSZ-13 and Cu/SAPO-34, have been successfully commercialized for nearly 10 years for abating NOx generated by diesel-powered vehicles [1]. Both catalysts deactivate with time in use. Cu/SSZ-13 primarily deactivates by two causes: high-temperature hydrothermal aging and chemical poisoning. In addition to these two causes, Cu/SAPO-34 also experiences deactivation during low-temperature (< 100 °C) hydrothermal treatments. Creating new Cu/zeolite SCR catalysts with durability better than the current industrial catalysts is of obvious practical benefit. Its realization, however, requires detailed understanding of the catalyst structure, and how the zeolite support and the Cu active species evolve during hydrothermal treatments and chemical poisoning. This contribution deals primarily with rational design approaches that can be applied to improve Cu/chabazite stability on the basis of molecular-level understanding about the nature of the active sites. Two SCR active isolated Cu(II) sites are present in Cu-chabazite, i.e., Z2Cu and ZCuOH, where Z stands for a zeolite framework negative charge. Z2Cu has much higher hydrothermal stability than ZCuOH [2]; therefore, it is the “preferred” site for synthesizing hydrothermally stable catalysts. The population of Z2Cu, however, requires two negative charges to be present in close proximity. Therefore, high concentrations of Z2Cu only exist in catalysts with high framework charge densities. For SSZ-13, this means low framework Si/Al ratios; for SAPO-34, this means high framework Si densities. The disadvantage of increasing framework charge densities, however, is that the supports themselves become hydrothermally less stable: low framework Si/Al ratio zeolites tend to dealuminate excessively, and high Si SAPO-34 supports tend to desilicate excessively, rendering low durability of the SCR catalysts.

A few post-synthesis methods can also be used to enhance long-term stability of a Cu-chabazite catalyst. (1) For Cu/SSZ-13 catalysts that contain both ZCuOH and sufficient amount of residual Brønsted acid sites, a mild hydrothermal aging induces conversion of ZCuOH to Z2Cu by the following reaction: ZCuOH + ZH = Z2Cu + H2O [3]. This effectively increases catalyst stability due, again, to the fact that Z2Cu is hydrothermally more stable. (2) For Cu/SSZ-13 catalysts with low Si/Al ratios, to avoid excessive dealumination during aging, alkali co-cations, e.g., Na+ and K+ can be introduced to the catalysts to lower density of residual Brønsted acid sites. With optimized co-cation addition, the catalyst stability can be greatly improved [4]. The same method can be used to increase Cu/SAPO-34 durability. (3) For Cu/SSZ-13 and Cu/SAPO-34 catalysts with high Cu loadings, the addition of a CuO “passivator” to the catalyst can also effectively improve catalyst hydrothermal stability. This strategy follows since CuO, which typically generates during hydrothermal aging of high-Cu loaded catalysts, is highly detrimental to catalyst stability. The addition of a stabilizer that interacts with CuO at high temperatures can effectively lower its detrimental effects.

1. Beale, A.M.; Gao, F.; Lezcano-Gonzalez, I.; Peden, C.H.F.; Szanyi, J. Chem. Soc. Rev. 44, 7371 (2015).

2. Song, J.; Wang, Y.L.; Walter, E.D.; Washton, N.M.; Mei, D.; Kovarik, L.; Engelhard, M.H.; Prodinger, S.; Wang, Y.; Peden, C.H.F.; Gao, F. ACS Catal. 7, 8214 (2017).

3. Luo, J.; Gao, F.; Kamasamudram, K.; Currier, N.; Peden, C.H.F.; Yezerets, A.  J. Catal. 348, 291 (2017).

4. Cui, Y.; Wang, Y.L.; Mei, D.H.; Walter, E.D.; Washton, N.W.; Holladay, J.D.; Wang, Y.; Szanyi, J.; Peden, C.H.F.; Gao, F.  J. Catal. 378, 367 (2019).