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

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

Authors 

Jones, C. B., Purdue University
Wang, Y., University of Notre Dame
Goswami, A., Dr. William F. Schneider
Schneider, W., University of Notre Dame
Gounder, R., Purdue University
Miller, J. T., Purdue University
Dean, D., Purdue University
The selective catalytic reduction (SCR) of NOx 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 NOx-SCR turnovers.

References.

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