(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 NOX (x=1,2) with NH3. During low temperature (<573 K) SCR catalysis, isolated Cu ions (CuII, CuI) 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 CuII/CuI redox cycle in which NO and NH3 are co-reductants of isolated NH3-solvated Cu2+ complexes ( ([CuII(NH3)4]2+, [CuII(NH3)3(OH)]+) to form reduced [CuI(NH3)2]+ complexes. Diffusion of reduced NH3-solvated CuI 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 CuIIâoxo complexes from mononuclear CuI ions initially separated by greater than nanometer distances. The mobility of [CuI(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. CuII reduction rates measured using in operando and transient X-ray absorption spectroscopy (XAS) during stoichiometric reduction experiments (30 Pa NO, 30 Pa NHÂ3, 473 K) show a first-order dependence in the concentration of CuII. Apparent first-order reduction rate constants are invariant with Cu density or zeolite topology, consistent with NO+NH3-assisted single-site reduction of isolated CuII complexes. Cui oxidation rates in the presence of O2, also measured using in operando and transient XAS show that oxidation of [CuI(NH3)2]+ in O2 (10 kPa O2, 473 K) occurs with rates that are pseudo-second order in the concentration of CuI and that a larger fraction of CuI 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 CuII that is zero-order in O2, and the dual-site oxidation of CuI 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 CuI that can oxidize in the presence of O2 on FER as measured by XAS. Insights from metadynamics simulations of [CuI(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 NOX SCR.