Influence of framework Al density in chabazite zeolites on copper ion mobility and reactivity during NOx selective catalytic reduction with NH3

Cu zeolites catalyse low-temperature (<523 K) selective catalytic reduction (SCR) of nitrogen oxides (NOx) via a redox cycle involving dynamic interconversion between NH3-solvated mononuclear Cu-I and binuclear Cu-II complexes. Cu-I oxidation requires the pairing of two mobilized Cu-I(NH3)(2) complexes to form binuclear intermediates, implying that Cu-I oxidation kinetics should depend on framework Al density, given that Cu ions are ionically tethered to anionic charges at Al sites in zeolite lattices. Here we combine statistical simulations, steady-state kinetics and operando X-ray absorption spectroscopy to interrogate Cu-chabazite (Cu-CHA) zeolites of varying framework Al density (0.2-1.7 Al centres per cha cage). Increasing the Al density leads to systematic increases in both the fraction of Cu-I ions that are SCR active (that is, O-2 oxidizable) and Cu-I oxidation rate constants (per Cu), revealing insights into how anionic Al centres in zeolite frameworks regulate the mobility of ionically tethered Cu cations and their dynamic reactivity during low-temperature NOx SCR.

Automated summary

In this study, Cu-CHA zeolites with different aluminum densities (0.2-1.7 Al/CHA) were investigated using statistical simulations, steady-state kinetics, and operando X-ray absorption spectroscopy. The results show that increasing the aluminum density leads to an increase in the fraction of Cu-I ions that are SCR active and the Cu-I oxidation rate constants, providing insights into how anionic Al centers in zeolite frameworks regulate the mobility of ionically tethered Cu cations and their dynamic reactivity during low-temperature NOx SCR.