Understanding the Hydrothermal Stability of Potential NH₃‑SCR Catalyst Cu-KFI Zeolite

Al-rich Cu-KFI was found to withstand hydrothermal aging (HTA) at 800 °C, while Cu-CHA with a similar Si/Al ratio was deactivated since a much higher energy barrier was required to transform Cu²⁺ ions to CuOx during HTA in KFI. Specifically, the rate-determining step is the hydrolysis of Cu²⁺ in KFI but the cross-cage migration of Cu(OH)₂ in CHA. Therefore, the inclusion of two Cu²⁺ ions in one repeat unit of KFI does not significantly promote CuOx production. However, the accumulated CuOx cannot be completely avoided under such severe hydrothermal aging. It may originate from Cu²⁺ in the same cage or adjacent cages. This shorter diffusion effectively mitigated the structural integrity deterioration of KFI in comparison to CHA. Nevertheless, HTA reduced the number as well as the reactivity of residual Cu²⁺ ions in Cu-KFI zeolite catalysts. Quantitative results determined that the activity and stability of the catalyst were maximized when the average lta cage contained about 1.4 Cu²⁺ ions.