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 (&lt;523 K), NH3 solvates and mobilizes Cu cations, enabling the reaction of two Cu+(NH3)2 complexes with O2 to form binuclear O2-bridged Cu2+ complexes in the SCR oxidation half-cycle. Given that NH3-solvated Cu cations are electrostatically tethered to framework Al anions under such conditions, SCR rates depend on Cu active site mobility and proximity. Here, we use steady-state kinetics and operando X-ray spectroscopy to quantify the effects of catalyst composition and temperature on SCR rates. Kinetic measurements across widely varying O2 pressures enable isolating rate constants for Cu+ oxidation on Cu-CHA of varying Cu and Al density. Cu+ oxidation rate constants (per Cu) increase with Cu density at a fixed Al density, reflecting the dual-site nature of Cu+ oxidation steps. Cu+ oxidation rate constants depend weakly on Al density (at fixed Cu density) between 0.2–0.8 Al/cage, but increase at higher Al densities (&gt;1 Al/cage) indicative of more facile Cu ion mobility. Apparent rate constants for Cu+ oxidation and Cu2+ reduction were measured at varying temperatures (448–488 K) to estimate apparent activation energies for each half-cycle. Apparent activation energies for Cu+ oxidation increase systematically with Cu density, reflecting changes to the energy landscape for diffusion and reaction of Cu+(NH3)2 complexes to form binuclear Cu structures as a function of Cu site proximity. These experimental observations of non-mean-field kinetic behavior are interrogated by statistical models of Cu and Al proximity, molecular dynamics simulations of Cu mobility, and kinetic Monte Carlo simulations that capture the distance-dependent reactivity of Cu sites. Taken together, this combined experimental and theoretical approach enables quantitatively describing the kinetic consequences of dynamic multi-site interactions between Cu active sites that occur during steady-state NOx-SCR catalysis.

Quantitative descriptions of non-mean-field kinetic behavior of Cu active sites in Chabazite zeolites during NOx selective catalytic reduction with NH3

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.