(762e) Influence of Zeolite Framework on Cu Ion Mobility and the Kinetics of Low Temperature NOX Selective Catalytic Reduction

Cu-exchanged chabazite (CHA) zeolites are used as catalysts in commercial diesel engine aftertreatment technologies for the selective catalytic reduction (SCR) of NO_X (x=1,2) with NH₃. During low temperature (<573 K) SCR catalysis, isolated Cu ions (Cu^II, Cu^I) become solvated by NH₃ to form cationic Cu-amine coordination complexes that are bonded ionically to anionic lattice oxygens near framework Al sites. The SCR reaction mechanism involves a Cu^II/Cu^I redox cycle in which NO and NH₃ are co-reductants of isolated NH₃-solvated Cu²⁺ complexes ( ([Cu^II(NH₃)₄]²⁺, [Cu^II(NH₃)₃(OH)]⁺) to form reduced [Cu^I(NH₃)₂]⁺ complexes. Diffusion of reduced NH₃-solvated Cu^I complexes through eight-membered ring (8-MR) CHA windows into adjacent CHA cages occurs with activation barriers less than that for kinetically-relevant reaction steps, providing a mechanism to form binuclear Cu^II‑oxo complexes from mononuclear Cu^I ions initially separated by greater than nanometer distances. The mobility of [Cu^I(NH₃)₂]⁺ species depends on the structural features of the zeolite support, including the framework topology, which is studied here using 8-MR zeolites with two-dimensional (FER) and three-dimensional (CHA) pore connectivity. Cu^II reduction rates measured using in operando and transient X-ray absorption spectroscopy (XAS) during stoichiometric reduction experiments (30 Pa NO, 30 Pa NH­₃, 473 K) show a first-order dependence in the concentration of Cu^II. Apparent first-order reduction rate constants are invariant with Cu density or zeolite topology, consistent with NO+NH_3-assisted single-site reduction of isolated Cu^II complexes. Cuⁱ oxidation rates in the presence of O₂, also measured using in operando and transient XAS show that oxidation of [Cu^I(NH₃)₂]⁺ in O₂­ (10 kPa O₂, 473 K) occurs with rates that are pseudo-second order in the concentration of Cu^I and that a larger fraction of Cu^I can be oxidized on CHA compared to FER at an equivalent Cu ion density. SCR rates (473 K, per Cu) measured under widely varying O₂ pressures (0-70 kPa) show a transition from a first-order to a zero-order dependence in O₂ pressure, which we describe using a simplified kinetic model that accounts for the single-site reduction of Cu^II that is zero-order in O₂, and the dual-site oxidation of Cu^I that is first-order in O₂. SCR rates measured in the high and low O₂ pressure limits allow for quantitative comparisons of catalytic behavior among zeolite samples in equivalent kinetic regimes. In the zero-order in O­₂ limit, SCR rates on FER are ~2x lower than on CHA of comparable Cu ion density, consistent with the lower fraction of Cu^I that can oxidize in the presence of O₂ on FER as measured by XAS. Insights from metadynamics simulations of [Cu^I(NH₃)₂]⁺ mobility in microporous voids of FER and CHA are used to rationalize the experimental observations. These results reveal how zeolite structural features influence Cu ion mobility and subsequently the steady-state kinetics of low-temperature NO_X SCR.