NOx abatement for lean-burn engines under lean–rich atmosphere over mixed NSR-SCR catalysts: Influences of the addition of a SCR catalyst and of the operational conditions Original Research Article

Mixtures of equal amounts of a Pt–Rh/Ba/Al2O3 NOx storage reduction (NSR) model catalyst and Ag/Al2O3, Co/Al2O3 or Cu/ZSM-5 selective catalytic reduction (SCR) model catalyst were evaluated for the NOx removal activity under lean–rich atmosphere. NOx removal activity was increased by adding Co/Al2O3 or Cu/ZSM-5 to Pt–Rh/Ba/Al2O3 while adding Ag/Al2O3 had no significant influence. Experiments performed by using two catalytic beds (upstream Pt–Rh/Ba/Al2O3 and downstream Co/Al2O3 or Cu/ZSM-5) suggested that both SCR catalysts are able to reduce NOx with the NH3 produced during the rich step on Pt–Rh/Ba/Al2O3. Among the studied catalysts, the Pt–RhBa/Al2O3 + Cu/ZSM-5 physically mixed one showed the highest activity. This catalyst mixture presented an improved performance, as compared to the NSR catalyst, regardless of the reductant used (CO and/or H2) or of the reduction time (10, 5 or 2.5 s). The highest activity was obtained by using both CO and H2 as reductant during the rich pulse. The addition of water in the inlet gas led to a decrease of the NOx removal activity of the catalyst mixture. Nevertheless, the NOx removal activity of the mixed Pt–RhBa/Al2O3 + Cu/ZSM-5 catalyst was still significantly higher than that of Pt–RhBa/Al2O3.