Distinct NH3-SCR behavior over Cu-KFI zeolites synthesized from protonic and ammonic forms: Activity, hydrothermal stability and mechanism investigation

Title : Distinct NH3-SCR behavior over Cu-KFI zeolites synthesized from protonic and ammonic forms: Activity, hydrothermal stability and mechanism investigation

Abstract:

Nitrogen oxides (NOx) emitted from mobile and stationary sources are major atmospheric pollutants, necessitating efficient catalytic abatement technologies. Cu-exchanged low-silica KFI zeolites, synthesized without organic structure-directing agents, have recently emerged as promising catalysts for NH₃-SCR reactions. However, their hydrothermal stability and the structure–activity relationship between copper species and zeolite acidity remain insufficiently understood. In this study, Cu-KFI zeolites derived from ammonic and protonic precursors (denoted as Cu-N-F and Cu-H-F, respectively) were synthesized and systematically investigated. Comprehensive characterizations, including SEM, XRD, ²⁷Al NMR, H₂-TPR, NH₃-TPD, and in situ DRIFTS, were employed to elucidate the distribution of copper species and acid sites, as well as their roles in catalytic performance. The Cu-N-F catalyst exhibited superior low-temperature activity, achieving 90% NOx conversion at 178 °C, with a broad activity window of 178–614 °C, whereas Cu-H-F required higher temperatures (222–686 °C). A linear correlation between T90 and the ZCuOH/Cu²⁺ ratio revealed that a higher fraction of ZCuOH species significantly enhances low-temperature activity. After hydrothermal aging at 800 °C, Cu-N-F maintained excellent performance, while Cu-H-F suffered severe deactivation due to framework collapse and the formation of extra-framework aluminum. Furthermore, NH₃-TPD and in situ DRIFTS results demonstrated that Lewis acid sites dominate NH₃ adsorption, forming L-NH₃ species that actively participate in the reaction. The reaction predominantly proceeds via the Langmuir–Hinshelwood pathway, with contributions from the Eley–Rideal mechanism. However, excessive Lewis acidity promotes N₂O formation at elevated temperatures, indicating a trade-off between activity and N₂ selectivity. These findings provide new insights into the interplay between copper speciation and acid site distribution, offering guidance for the rational design of highly efficient and hydrothermally stable Cu-based zeolite catalysts for NH₃-SCR applications.

Biography:

Miao is a PhD candidate at Dalian Maritime University. Her research focuses on the development of zeolite-based catalysts for emission control, particularly in NH₃-SCR reactions. Her work emphasizes the role of active site distribution and reaction mechanisms in catalytic performance. She has published several SCI papers in top journals and is actively engaged in research on marine exhaust gas purification and greenhouse gas mitigation.