Effect of diffusion limitation on the performance of multi-layer oxidation and lean NOx trap catalysts

Abstract Recent effort in emission control catalyst development aims for saving space and cost by combining different catalytic functions in a single multi-layered monolith reactor. However, the overall efficiency of the converter and particularly the bottom layer performance can be negatively affected by transport limitations. This work investigates transport limitation in double-layer systems consisting of oxidation (DOC) and lean NO x trap (LNT, NO x storage) catalysts. CO/HC oxidation and NO x storage/reduction experiments were carried out in a laboratory reactor. The LNT coating in the tested samples was overlaid with an additional layer that was either inert (γ-Al 2 O 3 ) or catalytically active (DOC). The inert layer served as a diffusion barrier with no catalytic activity to decouple the transport and reaction effects. The hindered transport affected both NO x storage (less efficient initial NO x uptake) and NO x reduction (changes in the selectivity, longer delay of NH 3 breakthrough, shift of N 2 O formation from primary to secondary peak). The presence of DOC layer on the top of LNT improved the catalytic performance at lower temperatures by increasing the effective NO x storage capacity (due to NO oxidation activity) and improved regeneration efficiency (due to water gas shift, steam reforming and NH 3 re-oxidation).