Characterization of Lean NOx Trap Catalysts with In-Cylinder Regeneration Strategies

Lean NOx trap (LNT) catalysts with different formulations have been characterized on a light-duty diesel engine platform. Two in-cylinder regeneration strategies were used during the study. The reductant chemistry differed for both strategies with one strategy having high levels of CO and H₂ and the other strategy having a higher hydrocarbon component. The matrix of LNT catalysts that were characterized included LNTs with various sorbate loads and varying ceria content; the sorbate was Ba. Intra-catalyst measurements of exhaust gas composition were obtained at one quarter, one half, and three quarters of the length of the catalysts to better understand the affect of formulation on performance. Exhaust analysis with FTIR allowed measurement of NH₃ and thereby, a measurement of N₂ selectivity for the catalysts. Although overall NOx conversion increased with increasing sorbate load, the formation of NH₃ increased as well. Interestingly, the presence of ceria in the LNT allowed NH₃ to be oxidized to N₂ in the downstream half of the LNT, thereby greatly reducing the tailpipe NH₃ level. Despite different capacities for NOx storage, a similar pattern for NOx adsorption as a function of the length of the catalyst was observed for catalysts with Ba as the storage component while the addition of ceria changed the adsorption profile through the catalyst significantly by shifting activity downstream. Results from these engine based experiments will be discussed relative to the body of literature concerning fundamental and model LNT studies.