(677e) Influence of Active Site Density and Arrangement on the Kinetics of NOx Selective Catalytic Reduction with NH3 over Cu-CHA Zeolites

The selective catalytic reduction (SCR) of NO_x with NH₃ using Cu-CHA is a commercial technology for diesel emissions control, and proceeds via a redox mechanism in which NO and NH₃ reduce Cu(II) and O₂ oxidizes Cu(I). Under low-temperature SCR conditions (473 K, 0.03 kPa NH₃), NH₃ solvates and mobilizes isolated Cu ions. Cu(I) oxidation steps require formation of binuclear complexes via reaction of two NH₃-solvated Cu(I) ions, whose mobility is regulated by electrostatic interactions with anionic framework Al centers (Paolucci, C., et al. Science 2017, 357, 898). Thus, SCR rates depend on the spatial distribution of Cu and Al sites, but in complex ways because both dual-site Cu(I)-oxidation and single-site Cu(II)-reduction are kinetically-relevant at “standard” SCR conditions (~10 kPa O₂).

Here, SCR rates are measured over a wide range of O₂ pressures, which allows extracting rate constants for oxidation and reduction steps in low and high O₂ pressure regimes, respectively (Figure 1). Most abundant reactive intermediates are monitored in operando by X-ray absorption spectroscopy (XAS). On Cu-CHA zeolites of fixed Al content (Si/Al = 15), oxidation rate constants (per Cu)increase monotonically with Cu density, consistent with a dual-site oxidation mechanism. Reduction rate constantsshow a weaker positive dependence on Cu density, predominantly reflecting changes in the fraction of Cu sites that can pair and thus mediate SCR turnovers. These conclusions are consistent with transient XAS measurements showing that increasing Cu density does not systematically influence reduction rate constants, but increases the fraction of Cu(I) that can pair during O₂-assisted oxidation. These quantitative kinetic and active site descriptors are measured on CHA zeolites with a range of Cu densities and framework Al densities and arrangements, revealing how the mobility of electrostatically tethered Cu ions impacts both the fraction of sites that catalyze NO_x-SCR, and their turnover frequencies.