The Effects of Aging Temperature and Air-Fuel Ratio on the NOx Storage Capacity of a Lean NOx Trap

This paper summarizes results from a study on the effects of aging temperature and A/F ratio on the NO_x storage capacity of a lean NO_x trap. When aged at stoichiometry at 700°C, the NO_x storage capacity of the NO_x trap dropped considerably during the first 200 hours of aging and then at a much slower rate beyond 200 hours. The NO_x storage capacity dropped more rapidly as the aging temperature increased, with the drop in capacity particularly evident between 900°C and 1000°C. The drop in NO_x capacity was significantly larger for samples aged with part-time lean operation and/or part-time rich operation than for samples aged continuously at stoichiometry. The detrimental effects of lean and rich operation increased as the temperature increased.

A Pt/Al₂O₃ model catalyst was exposed to reducing conditions at temperatures ranging from 670°C to 1041°C and then to oxidizing conditions over the same temperature range, and in-situ XRD was used to investigate Pt particle coarsening. No significant coarsening was observed after the exposure to reducing conditions, but Pt particle coarsening was observed after exposure to lean conditions beginning at 763°C and increasing as the temperature increased. XRD analysis was performed on a Pt/Ba/Al₂O₃ model catalyst after exposure to reducing, neutral, and oxidizing conditions at 900°C. The sample aged in an oxidizing environment exhibited more Pt particle coarsening than the sample aged in a neutral environment, while no significant coarsening was observed for the sample aged in a reducing environment. The sample aged in a reducing environment displayed more BaAl₂O₄ formation than the sample aged in an oxidizing environment, while the sample aged in a neutral environment had the lowest amount of BaAl₂O₄ formation.