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

Redox mechanisms have been theorized for the selective catalytic reduction (SCR) of NOx 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 NOx 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.