Product Selectivity During Regeneration of Lean NOx Trap Catalysts

NO_x reduction product speciation during regeneration of a fully formulated lean NO_x trap catalyst has been investigated using a bench-scale flow reactor. NH₃ and N₂O were both observed during the regeneration phase of fast lean/rich cycles that simulated engine operation. Formation of both products increased with higher reductant concentrations and lower temperatures.

Steady flow experiments were used to decouple the regeneration reactions from the NO_x storage and release processes. This approach enabled a detailed investigation into the reactions that cause both formation and destruction of non-N₂ reduction products. Pseudo-steady state experiments with simultaneous flow of NO_x and reductant indicated that high concentrations of CO or H₂ drive the reduction reactions toward NH₃ formation, while mixtures that are stoichiometric for N₂ formation favor N₂. These experiments also showed that NH₃ is readily oxidized by both NO and O₂ over the LNT catalyst. These observations were incorporated into a schematic of the regeneration process that takes into account the spatial and temporal variations occurring within the catalyst.