Effects of dioxygen pressure on rates of NOx selective catalytic reduction with NH3 on Cu-CHA zeolites

Abstract

At low temperatures (<523 K), the selective catalytic reduction (SCR) of NO with NH3 on Cu-exchanged zeolites occurs via elementary steps catalyzed by NH3-solvated Cu ions, which are reactive intermediates in a CuII/CuI redox cycle. SCR rates are typically measured under “standard” reaction conditions, in which O2 is the oxidant and present at partial pressures (~10 kPa O2) that cause both single-site CuII reduction and dual-site CuI oxidation to behave as kinetically relevant steps. As a result, “standard” SCR rates (per g) transition from a second-order to a first-order dependence on isolated Cu content (per g) among Cu-CHA zeolites with increasing Cu content and thus spatial density (Si/Al = 15, Cu/Al = 0.08–0.37), as dual-site CuI oxidation steps limit SCR rates to lesser extents. On a given Cu-CHA catalyst, SCR rates (per Cu, 473 K) show a Langmuirian dependence on dioxygen pressure when varied widely (0–60 kPa O2), enabling the isolation of first-order or zero-order kinetic regimes with respect to O2. These kinetic regimes respectively correspond to limiting conditions in which either CuI oxidation or CuII reduction becomes the dominant kinetically relevant step, consistent with in operando X-ray absorption spectra. First-order rate constants (per Cu) increase approximately linearly with Cu density, reflecting the dual-site requirement of O2-assisted CuI-oxidation steps. Zero-order rate constants (per Cu) increase more gradually with Cu density, in part reflecting the increasing fraction of isolated Cu ions that are able to form binuclear intermediates and thus participate in SCR turnovers. Combining steady-state and transient kinetic data with in operando spectra provides a methodology to quantitatively describe the rate dependences of SCR reduction and oxidation processes on Cu-zeolite properties such as Cu ion density as shown here, and others including the zeolite framework topology and its density and distribution of Al atoms.