Hydrothermal Stability of Active Sites in Cu-Exchanged Small-Pore Zeolites for the Selective Catalytic Reduction of NO.

Combining operando X-ray absorption spectroscopy (XAS) and computational modelling shows unequivocally the distribution of active species in fresh and hydrothermally aged Cu-CHA and Cu-AEI zeolites during NH-assisted selective catalytic reduction of NO. Four principal species co-exist: (i) Cu cations coordinated to NH, (ii) Cu cations coordinated to the zeolite framework, (iii) solvated Cu cations, and (iv) framework-coordinated Cu species (Cu ) formed upon hydrothermal ageing of the zeolite sample. The Cu species were only observed in the hydrothermally aged zeolite samples and are formed upon the interaction of hydrated Cu cations with extra-framework Al (EFAl) generated during the hydrothermal treatment. These sites are inactive for NO reduction, leading to a decrease in the catalytic performance of the hydrothermally aged zeolites. Cu formation was higher in Cu-CHA (~46 %) than in Cu-AEI (~28 %). The better hydrothermal stability of Cu in the AEI framework is attributed to the tortuous channel structure of AEI that hinders the migration of hydrated Cu cations during hydrothermal ageing.