Effects of CO, H2 and C3H6 on Cu-SSZ-13 catalyzed NH3-SCR

Abstract

We investigated the steady-state and transient effects of reductants (CO, H2 and C3H6) on NO2 reduction, NH3-SCR (selective catalytic reduction), NH3 adsorption and oxidation, and N2O production on a Cu-SSZ-13 monolithic catalyst. The three reductants affect to different extents the standard SCR (NO + NH3 + O2), fast SCR (NO + NH3 + NO2), and slow SCR (NH3 + NO2). This study underscores the importance of accounting for the impact of reducing agents on conventional NH3-SCR reaction mechanism when SCR catalyst is subjected to either rich regeneration of integrated systems (LNT + SCR, SCR on DPF) or cold-start. Propylene is most effective in promoting NO2 reduction to NO by formation of organic intermediates. CO effectively reduces nitrates to nitrites that then react with NO2, releasing NO. H2 can follow a similar pathway as CO but is less effective. In addition, H2 can also enable a H2-based SCR pathway through the reduction of Cu cations to Cu0 which then catalyze the NOx reduction. This pathway is particularly evident at high temperatures and low O2 levels. As for NH3-SCR reactions, propylene competes with NH3 for adsorbed NO2, which generates NO and thus increases the NO/NOx ratio. This leads to the dominance of either fast or standard SCR for a slow SCR (NH3 + NO2) feed condition when C3H6 is present. CO has only a minor effect on both standard and fast SCR but a promoting effect on slow SCR. The ineffective reduction of NO2 to NO by H2 at low temperature (T < 250 °C) results in a negligible effect on slow SCR. In contrast to steady-state operation, lean/rich cycling enhances cycle-averaged NOx conversion for each of the NH3-SCR reactions when adding either C3H6 or a CO + H2 mixture in the rich phase. A decreased N2O generation rate from the slow SCR reaction is observed when any of the three reductants are present due in part to their reaction with ammonium nitrates.