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(216b) Insights into the Deactivation of Cu/SAPO-34 By Low- and High-Temperature Hydrothermal Treatment

Authors: 
Zheng, Y., Pacific Northwest National Laboratory
Wang, A., Pacific Northwest National Laboratory
Walter, E. D., Pacific Northwest National Laboratory
Gao, F., Pacific Northwest National Laboratory
Song, J., Pacific Northwest National Lab
Wang, Y., Pacific Northwest National Laboratory
Szanyi, J., Pacific Northwest National Laboratory
Peden, C. H. F., Pacific Northwest National Laboratory
Cu/Chabazite (CHA) catalysts, Cu/SSZ-13 and Cu/SAPO-34, have been successfully commercialized for selective catalytic reduction (SCR) of NOx in lean-burn engine exhausts due to their superior activity, N2 selectivity and durability. High-temperature (> 600 °C) hydrothermal aging treatments typically cause performance decrease for these catalysts, rationalized from partial structure degradation and loss of Cu-sites in active forms. However, recent studies have shown that the exposure of Cu/SAPO-34 to water vapor at low temperatures (< 100 °C) also result in an irreversible decline in NH3-SCR activity [1, 2]. This may be attributed to structure degradation since SAPO-34 is known to hydrolyze in the presence of water vapor at such low temperatures. However, transformation of active Cu-sites to less active forms cannot be ruled out.

To gain insights into the similarity and difference between low- and high-temperature deactivation mechanisms, model Cu/SAPO-34 catalysts with varying Cu loadings were prepared using a one-pot method. These catalysts were further hydrothermally treated at either 70 or 800 °C. Effects of hydrothermal treatments on catalytic performance were examined using standard SCR and non-selective NH3 oxidation kinetics. The zeolite framework integrity (crystallinity, porosity, coordination environments, Si-O-Al bonds, acidity) were characterized by XRD, N2 physisorption, solid state NMR, DRIFTS and NH3-TPD. A particular focus was given to changes in the local environments of Cu species during hydrothermal aging, examined using two-dimensional (2D) pulsed EPR. Finally, an attempt is made to elaborate the deactivation mechanisms that correlate the variation of NH3 storage sites (acid sites) and redox sites (Cu2+ sites) caused by low- and high-temperature hydrothermal treatments.

[1] Wang, J., Fan, D., Tie, Y., et al., J. Catal. 322:84-90, 2015

[2] Leistner, K., Olsson, L., Appl. Catal. B: Environ. 165: 192-199, 2015