Effect of aging on NOx reduction in coupled LNT–SCR systems

Abstract

The effect of simulated road aging on the NOx reduction performance of coupled Pt/Rh LNT and Cu-CHA SCR catalysts was studied using H2, CO and C3H6 as NOx reductants. Activity tests showed that the NOx reduction activity of the LNT was significantly decreased after aging, especially at low temperature. N2 physisorption, H2 chemisorption, elemental analysis and TEM results revealed two main changes which account for the degradation in LNT performance. First, residual sulfur in the aged LNT associated with the Ba phase was responsible for decreased NOx storage efficiency. Second, sintering of the precious metals in the washcoat occurred, resulting in decreased contact between the Pt and Ba, and hence in less efficient NOx storage and catalyst regeneration. The aged LNT showed higher NH3 selectivity than that of the fresh LNT in the temperature range 150–450°C, NH3 generated from the LNT being stored by the SCR catalyst which subsequently catalyzed its reaction with NOx slip from the LNT. Hence, the loss of NOx conversion over the LNT was partly compensated by the downstream SCR catalyst. N2O formation over the aged LNT was less than that over the fresh LNT and the N2O was partly mitigated by the downstream SCR catalyst. Microreactor tests further showed that in addition to NH3, the SCR catalyst was able to use slipped CO or C3H6 as NOx reductants. A comparison of the difference in NOx conversion for the aged LNT and LNT–SCR configurations for the three reductants used in this study showed that the greatest benefit was obtained when CO and C3H6 were the reductants. This is explained by the fact that the LNT catalyst was unable to utilize these reductants as efficiently as H2 due to their lower reactivity; hence, more NOx and reductant slip reached the SCR catalyst. The ability of the SCR catalyst to utilize these reductants therefore provides a significant advantage over the LNT-only configuration.