This study investigates concurrent reactions that compete with the selective catalytic reduction of nitrogen oxides by ammonia (NH₃-SCR), namely ammonia oxidation (AMO) and the formation of undesired by-products N₂O and NO₂, over Cu/SSZ-13 and Fe–Cu/SSZ-13 zeolite catalysts. Particular emphasis is placed on how the method of iron introduction and reaction conditions (dry vs. wet) influence catalytic activity, selectivity, and the nature of active sites.
Bimetallic Fe–Cu/SSZ-13 catalysts were prepared using a two-step approach in which copper was incorporated into the SSZ-13 framework via one-pot synthesis, followed by iron introduction either by impregnation or ion exchange. The experiments were performed in a quartz fixed-bed reactor coupled with a QMS detector (Hiden Analytical HPR-20) and FTIR spectrometer with a multiple reflection gas cell.
Figure 1: Hiden Analytical HPR-20 R&D.
In addition, density functional theory (DFT) combined with first-principles thermodynamics (FPT) modelling was applied to elucidate the speciation and stability of Cu–Cu, Fe–Fe, and mixed Cu–Fe active sites under relevant reaction conditions. Isotopic experiments using ¹⁴NH₃/¹⁵NH₃ enabled the separation and quantification of SCR and AMO pathways under steady-state conditions.
All catalysts retained the CHA structure of SSZ-13, but the preparation method strongly affected metal distribution and speciation. Ion exchange led to partial leaching of copper and the formation of segregated FeOx and CuOx species on the external surface, whereas impregnation preserved a higher intrazeolite copper content and resulted in a more favourable dispersion of iron. These structural differences translated directly into catalytic performance. Catalysts prepared by impregnation exhibited a broader operational temperature window, higher apparent turnover frequencies, and superior selectivity toward N₂ compared with their ion-exchanged counterparts.
The competition between SCR and AMO was shown to be highly sensitive to both temperature and catalyst composition. At low temperatures, SCR activity was dominated by isolated Cu²⁺ sites, while excessive NH₃ adsorption led to inhibition effects, particularly in Cu-depleted ion-exchanged samples. At high temperatures, ammonia oxidation became increasingly important, especially on segregated metal oxide species, causing a decline in NO conversion and enhanced N₂ formation unrelated to SCR. Importantly, Fe addition was found to mitigate these adverse effects when introduced by impregnation, significantly suppressing NH₃ oxidation and reducing N₂O formation by up to a factor of five.
Water vapor played a beneficial role by shifting the equilibrium of active sites from oxo to hydroxo forms, as confirmed by DFT/FPT modelling and operando IR spectroscopy. This transformation reduced nitrate and ammonium nitrate accumulation within the zeolite pores, thereby lowering N₂O formation and improving overall N₂ selectivity, particularly for Fe–Cu catalysts prepared by ion exchange.
Overall, the study demonstrates that careful control of metal speciation through synthesis strategy is crucial for optimizing SCR performance. Impregnation-derived Fe–Cu/SSZ-13 catalysts offer an effective approach to expanding the operational temperature window, suppressing parasitic ammonia oxidation, and minimizing N₂O emissions, providing valuable guidance for the design of next-generation NH₃-SCR catalysts.
Project Summary by: Monika Fedyna, Jagiellonian University, Kraków, Poland.
Paper Reference: Fedyna, M., Bartosz Mozgawa, Gryboś, J., Yin, C., Zhao, Z.,
Zasada, F., Pietrzyk, P. and Sojka, Z. (2025) ‘Concurrent processes of N2O/NO2 formation and NH3 oxidation competing with the main course of NH3-SCR over Cu/SSZ-13 and Fe-Cu/SSZ-13 catalysts.’ Research on Chemical Intermediates. Springer Science+Business Media, December. DOI: 10.1007/s11164-025-05867-z.
Hiden Product: HPR-20 R&D.
