(233e) Product Speciation during Regeneration of Lean NoX Traps

Formation of products other than N₂ during regeneration of a fully formulated lean NO_x trap (LNT) catalyst was investigated using a bench-scale flow reactor. Both NH₃ and N₂O were observed during the regeneration phase of fast lean/rich cycles that simulated engine operation. Formation of both byproducts increased with higher reductant concentrations and lower temperatures. Steady flow experiments were used to decouple the regeneration reactions from the NOx storage and release processes. This approach enabled a detailed investigation into the reactions that cause both formation and destruction of byproducts. 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. The steady state experiments were used to develop a detailed microkinetic mechanism for the regeneration process. Simulations performed with a model incorporating the regeneration mechanism showed good agreement with the steady flow experimental results.