(628a) Effects of Cu-Zeolite Pore Size, Shape, and Connectivity on the Selective Catalytic Reduction of NOx with NH3
Cu-exchanged chabazite (CHA) zeolites are used for the selective catalytic reduction (SCR) of NOx with NH3 in industrially relevant NOx abatement strategies for diesel and lean-burn engines. The SCR reaction over Cu-zeolites proceeds via a redox cycle of CuI/CuII ions. The CuI oxidation and CuII reduction half-cycles have different Cu site and density requirements in the low-temperature kinetic regime (<573 K), wherein Cu ions are solvated by NH3 and mobilized within zeolite void spaces. As a result, steady-state SCR rates and the number of active Cu sites on Cu-CHA are a function both of reaction conditions and sample composition. Here, we measure SCR rates (473 K, per Cu) over a wide range of O2 pressures (1-60 kPa) to enable quantifying rate constants in kinetic regimes that are first-order and zero-order in O2 pressure, which respectively describe rates of dual-site CuI oxidation and single-site CuII reduction events. These data reveal that the rates of both oxidation and reduction half-cycles depend on the spatial density of Cu ions, and their mobility with void spaces defined by the pore topology. Apparent zero-order rate constants are lower on FER (2D) than CHA (3D), while first-order rate constants are similar, indicating that lower pore dimensionality primarily reduces the effective volumetric footprint that mobile CuI(NH3)2 complexes can occupy during catalysis, consistent with metadynamics simulations. These findings are also consistent with in operando and transient CuI oxidation experiments, which show that the fraction of CuI that is oxidized by O2 (at 473 K) is lower on FER than CHA. We will also discuss data collected on zeolites of different pore size and dimensionality (1D, 2D, 3D) and topology (channel, cage-window), to understand how zeolite framework topology affects Cu ion mobility and the kinetics of low-temperature SCR.