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 NO_x with NH₃ in industrially relevant NO_x abatement strategies for diesel and lean-burn engines. The SCR reaction over Cu-zeolites proceeds via a redox cycle of Cu^I/Cu^II ions. The Cu^I oxidation and Cu^II 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 Cu^I oxidation and single-site Cu^II 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 Cu^I(NH₃)₂ complexes can occupy during catalysis, consistent with metadynamics simulations. These findings are also consistent with in operando and transient Cu^I oxidation experiments, which show that the fraction of Cu^I 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.