(602e) Effects of Framework Al Density in Chabazite Zeolites on Cu Ion Mobility and the Kinetics of NOx Selective Catalytic Reduction with NH3
AIChE Annual Meeting
Thursday, November 11, 2021 - 1:42pm to 2:00pm
Here, we use kinetic and operando X-ray spectroscopic measurements across widely varying O2 pressures  to isolate the kinetics of Cu+ oxidation and Cu2+ reduction on Cu-CHA of varying framework Al density (0.2â1.7 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 . Cu+ oxidation rate constants depend weakly on Al density (at fixed Cu density) between 0.2â0.8 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.
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