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

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