Kinetic Monte Carlo Analysis Reveals Non-mean-field Active Site Dynamics in Cu-Zeolite-Catalyzed NO Reduction.

Copper-exchanged chabazite (Cu-CHA) zeolites are the preferred catalysts for the selective catalytic reduction of NO with NH. The low temperature (473 K) SCR mechanism proceeds through a redox cycle between mobile and ammonia-solvated Cu(I) and Cu(II) complexes, as demonstrated by multiple experimental and computational investigations. The oxidation step requires two Cu(I) to migrate into the same cage to activate O and form a binuclear Cu(II)-di-oxo complex. Prior steady state and transient kinetic experiments find that the apparent rate constants for oxidation (per Cu ion) are sensitive to catalyst composition and follow nonmean-field kinetics. We develop a nonmean-field kinetic model for NO SCR that incorporates a composition-dependent Cu(I) volumetric footprint centered at anionic [AlO] tetrahedral sites on the CHA lattice. We use Bayesian optimization to parameterize a kinetic Monte Carlo model against available experimental composition-dependent SCR rates and Cu(II) fractions. We find that both rates and Cu(II) fractions of a majority of catalyst compositions can be captured by single oxidation and reduction rate constants combined with a composition-dependent Cu(I) cation footprint, highlighting the contributions of both Cu and Al densities to steady-state SCR performance of Cu-CHA. The work illustrates a pathway for extracting robust molecular insights from the kinetics of a dynamic catalytic system.