Spatiotemporal distribution of NOx storage and impact on NH3 and N2O selectivities during lean/rich cycling of a Ba-based lean NOx trap catalyst

We summarize results from an investigation of the spatiotemporal distribution of NOx storage and intermediate gas species in determining the performance of a fully formulated, Ba-based, lean NOx trap catalyst under lean/rich cycling conditions. By experimentally resolving spatiotemporal profiles of gas composition, we found that stored NOx was significantly redistributed along the monolith axis during the rich phase of the cycle by release and subsequent downstream re-adsorption. Sulfur poisoning of upstream NOx storage sites caused the active NOx-storage zone to be displaced downstream. This axial displacement in turn influenced rich-phase NOx release and re-adsorption. As sulfur poisoning increased, NH3 slip at the catalyst exit also increased due to its formation closer to the catalyst outlet and decreased exposure to downstream oxidation by surface oxygen. N2O formation was found to be associated with nitrate reduction rather than oxidation of NH3 by stored oxygen. We propose that the observed evolution of N2O selectivity with sulfation can be explained by changes in the spatiotemporal distribution of NOx storage resulting in either increased or decreased number of precious-metal sites surrounded by nitrates.