Interplay between copper redox and transfer and support acidity and topology in low temperature NH3-SCR

Low-temperature standard NH3-SCR over copper-exchanged zeolite catalysts occurs on NH3-solvated Cu-ion active sites in a quasi-homogeneous manner. As key kinetically relevant reaction steps, the reaction intermediate CuII(NH3)4ion hydrolyzes to CuII(OH)(NH3)3ion to gain redox activity. The CuII(OH)(NH3)3ion also transfers between neighboring zeolite cages to form highly reactive reaction intermediates. Viaoperandoelectron paramagnetic resonance spectroscopy and SCR kinetic measurements and density functional theory calculations, we demonstrate here that such kinetically relevant steps become energetically more difficult with lower support Brønsted acid strength and density. Consequently, Cu/LTA displays lower Cu atomic efficiency than Cu/CHA and Cu/AEI, which can also be rationalized by considering differences in their support topology. By carrying out hydrothermal aging to eliminate support Brønsted acid sites, both CuII(NH3)4ion hydrolysis and CuII(OH)(NH3)3ion migration are hindered, leading to a marked decrease in Cu atomic efficiency for all catalysts.