Effect of aging on NO_x reduction in coupled LNT-SCR systems

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• 作者：Jin Wang ; Yaying Ji ; Gary Jacobs ; Samantha Jones ; Dae Jung Kim ; Mark Crocker
• 关键词：LNT&ndash ; SCR ; Lean NOx trap ; NOx adsorber catalyst ; Selective catalytic reduction ; Catalyst aging
• 刊名：Applied Catalysis B: Environmental
• 出版年：27 April, 2014
• 年：2014
• 卷：148-149
• 期：Complete
• 页码：51-61
• 全文大小：1658 K

文摘

The effect of simulated road aging on the NO_x reduction performance of coupled Pt/Rh LNT and Cu-CHA SCR catalysts was studied using H₂, CO and C₃H₆ as NO_x reductants. Activity tests showed that the NO_x 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 NO_x 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 NO_x storage and catalyst regeneration. The aged LNT showed higher NH₃ selectivity than that of the fresh LNT in the temperature range 150-450 掳C, NH₃ generated from the LNT being stored by the SCR catalyst which subsequently catalyzed its reaction with NO_x slip from the LNT. Hence, the loss of NO_x 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 NO_x reductants. A comparison of the difference in NO_x 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 NO_x 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.