Spatiotemporal distribution of NO _x storage and impact on NH ₃ and N ₂O selectivities during lean/rich cycling of a Ba-based lean NO _x trap catalyst

We summarize results from an investigation of the spatiotemporal distribution of NO _x storage and intermediate gas species in determining the performance of a fully formulated, Ba-based, lean NO _x trap catalyst under lean/rich cycling conditions. By experimentally resolving spatiotemporal profiles of gas composition, we found that stored NO _x 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 NO _x storage sites caused the active NO _x-storage zone to be displaced downstream. This axial displacement in turn influenced rich-phase NO _x release and re-adsorption. As sulfur poisoning increased, NH ₃ 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. N ₂O formation was found to be associated with nitrate reduction rather than oxidation of NH ₃ by stored oxygen. We propose that the observed evolution of N ₂O selectivity with sulfation can be explained by changes in the spatiotemporal distribution of NO _x storage resulting in either increased or decreased number of precious-metal sites surrounded by nitrates.