(602e) Effects of Framework Al Density in Chabazite Zeolites on Cu Ion Mobility and the Kinetics of NOx Selective Catalytic Reduction with NH3

The selective catalytic reduction (SCR) of NO_x with NH₃ using Cu-exchanged chabazite (CHA) zeolites occurs via a redox mechanism in which NO and NH₃ reduce Cu²⁺ and O₂ oxidizes Cu⁺. At low temperatures (e.g., 473 K), NH₃ solvates and mobilizes Cu cations, enabling Cu⁺ pairing and oxidation to form binuclear Cu²⁺ complexes [1]. NH₃-solvated Cu cations are electrostatically tethered to framework Al anions under such conditions, suggesting that their mobility depends on the density of framework Al.

Here, we use kinetic and operando X-ray spectroscopic measurements across widely varying O₂ pressures [2] to isolate the kinetics of Cu⁺ oxidation and Cu²⁺ reduction on Cu-CHA of varying framework Al density (0.2–1.7 Al/cage). Assuming Al are randomly distributed, statistical models show that the percentage of CHA unit cells that host multiple Al increases from 15% to 99% in the studied composition range (Figure 1a). Cu⁺ oxidation rate constants (per Cu) increase with Cu density at a given Al density (Figure 1b), reflecting kinetically relevant dual-site Cu⁺ oxidation [2]. Cu⁺ oxidation rate constants depend weakly on Al density (at fixed Cu density) between 0.2–0.8 Al/cage, implying that SCR turnovers do not require CHA cages that can permanently charge-balance multiple Cu cations. In contrast, Cu⁺ oxidation rate constants increase at high Al densities (> 1 Al/cage), implying changes to the free energy landscape for intercage Cu⁺ diffusion and pairing. These effects are assessed by molecular dynamics simulations of Cu ion mobility, and kinetic Monte Carlo simulations to capture non-mean-field behavior.

This work demonstrates that the anionic charge density in the zeolite framework impacts the mobility of Cu cations that interact to complete NO_x-SCR turnovers.

References.

1. Paolucci, C., et al. Science 2017, 357, 898.
2. Jones, C. B., et al. J. Catal. 2020, 389, 140–149