Passive-ammonia selective catalytic reduction (SCR): Understanding NH3 formation over close-coupled three way catalysts (TWC)

Abstract

NH3 formation was examined under steady-state and lean/rich cycling conditions over four commercial catalysts including: (1) a Pd-only, high precious metal loading (HPGM) three-way catalyst, (2) a Pd/Rh+CeO2, low precious metal loading (LPGM) three-way catalyst, (3) a combination of a HPGM and a LPGM (Dual-Zone) catalyst and (4) a lean NOX trap (LNT) catalyst. The goal of this work was to evaluate these catalysts for their potential use as the upstream component in a passive-NH3 SCR configuration. NH3 formation during steady-state operation was found to be dependent on the air-to-fuel ratio (AFR), temperature and catalytic formulation used. While all of the formulations produced significant amounts of NH3 when operated under sufficiently rich conditions, in general the steady-state NH3 yield decreased in the following order: HPGM≥Dual-Zone>>LPGM≈LNT. Under lean-rich cycling conditions that would be required for this mode of operation, lower air-to-fuel ratios were required to generate the same amount of NH3 as under steady-state conditions. Results obtained with the LNT catalyst demonstrated that at moderate temperatures (i.e., 275–500°C) NOX storage capacity significantly increased the amount of NH3 produced in relation to the amount of NOX slipped. Consequently, the addition of an “optimum” amount of NOX storage capacity in addition to well-controlled lean-rich timing, could significantly improve the performance of the three-way catalyst used as the upstream component in a passive-NH3 SCR configuration. When the CO, C3H6 and N2O concentrations in the effluent were considered in addition to the NH3 formation, an optimum temperature of 400–450°C was determined for the operation of these catalysts.