Alternate pathway for standard SCR on Cu-zeolites with gas-phase ammonia

Abstract

Redox mechanisms have been theorized for the selective catalytic reduction (SCR) of NO_x over small-pore Cu-zeolites. These mechanisms generally rely on NH₃ solvation of active copper sites as the initial step in the reduction half cycle for standard SCR. In this work, we demonstrate that this pre-requisite for NH₃ solvation of active copper sites is inconsistent with experimental data for initial NO consumption under SCR conditions. Reactor data under standard SCR and reduction half cycle (no O₂) conditions shows instantaneous consumption of NO upon introduction of NO and NH₃. However, detailed and global kinetic models relying on a sequential adsorption–conversion mechanism predict an initial delay in NO consumption associated with the relatively slow macroscopic coverage dynamics. These model discrepancies at low NH₃ coverage are quantitatively resolved by relaxing the pre-requisite of NH₃ storage for NO_x conversion, via an alternate SCR pathway utilizing gas-phase NH₃. Addition of this standard SCR reaction via gas-phase NH₃ leads to predicted NH₃ reaction orders >1 at low NH₃ coverage, confirmed subsequently by experiments. The results shown here are consistent with an energetically feasible mechanistic pathway involving the reduction of copper sites by NO to generate mobile HONO intermediates, which can potentially react with gas-phase NH₃ in the zeolite cages.