(547f) Modeling the Effect of Pt Dispersion and Temperature During Anaerobic Regeneration of a Lean NOx Trap Catalyst

A crystallite-scale model is incorporated into a reactor-scale model to study the effect of Pt dispersion and temperature during the regeneration of a lean NOx trap (LNT), based on a parallel experimental study [R.D. Clayton, M.P. Harold, V. Balakotaiah, C.Z. Wan, Appl. Catal., B 90 (2009) 662.]. It is shown that for a fixed Pt loading, an increase in the Pt dispersion results in an increase in the interfacial perimeter between Pt and Ba, where the reduction of NOx takes place. The rate determining process during the regeneration is found to be the diffusion of stored NOx within the Ba phase towards the Pt/Ba interface. The transient product distribution for three catalysts having varied Pt dispersions (3.2%, 8% and 50%) is explained by the localized stored NOx gradients in the Ba phase. Temperature-dependent NOx diffusivities in the Ba phase are used to predict the breakthrough profiles of H2, N2 and NH3 over a range of catalyst temperatures. Finite gradients in the stored NOx concentration are predicted in the Ba phase, thus showing that the nitrate ions are not extremely mobile. The model predicts that the highest amount of NH3 is produced by the low dispersion catalyst (3.2% dispersion) at high temperatures, by the high dispersion catalyst (50% dispersion) at low temperatures, and by the medium dispersion catalyst (8% dispersion) at intermediate temperatures, which is consistent with the experimental studies. The model considers the consumption of chemisorbed oxygen on Pt by H2, which is used to predict the low effluent N2 concentration for the 50% dispersion catalyst as compared to the 8% dispersion catalyst. Finally, a novel design is proposed to maximize the amount of NH3 in the effluent of a LNT, which can be used as a feed to a selective catalytic reduction (SCR) unit placed downstream of the LNT.