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

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.

The NOx removal efficiency under lean/rich atmosphere can be greatly improved by the addition of a SCR catalyst (Cu/ZSM-5 or Co/Al2O3) to a model Pt–Rh/Ba/Al2O3 NSR catalyst. The improved performance is ascribed to NOx reduction on the SCR catalyst with the NH3 released from the NSR model catalyst, and it was observed regardless of the reductant used (CO or H2) and of the reduction time (10, 5 or 2.5 s).Figure optionsDownload as PowerPoint slide