NOx conversion and selectivity in multi-layer and sequential DOC-LNT automotive exhaust catalysts: Influence of internal transport

Abstract

In order to pass strict emission standards, automobiles with internal combustion engines need to be equipped with complex exhaust gas aftertreatment systems. For example, Diesel engines are usually fitted with an oxidation catalyst (DOC), deNOx catalyst (either SCR or lean NOx trap – LNT) and a particulate filter. Integrating them into a single multi-functional device saves space, cost and weight while improving heat management during cold starts as well. This work investigates a compact DOC+LNT converter combining both catalyst types in a single device – porous layer of a DOC catalyst is added on top of a LNT washcoat. The performance of this dual-layer converter is compared with traditional single-layer DOC and LNT coated on two separate monoliths in series.Laboratory reactor experiments showed that the performance of the dual-layer sample was affected by transport limitations – mainly internal diffusion through the catalytic layers. An increased NOx slip was observed during NOx adsorption in the dual-layer sample. The lower amount of stored NOx was however more than compensated during NOx reduction at lower temperatures near the light-off, where the compact dual-layer monolith exhibited higher efficiency of NOx reduction and lower selectivity to N2O during rich regeneration periods. This was attributed to the fact that the NOx released from the bottom LNT layer has to pass through the top DOC layer where it can further react with the reducing agents from the rich feed. At higher temperatures, the two single-layer monoliths outperformed the compact dual-layer monolith.