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) comprising a Pt/BaO catalyst. The current study is based on a recent experimental study [R.D. Clayton, M.P. Harold, V. Balakotaiah, C.Z. Wan, Appl. Catal. B 90 (2009) 662]. The model shows that an increase in the Pt dispersion for a fixed Pt loading increases the interfacial perimeter between Pt and Ba, and has a significant effect on the regeneration kinetics. The transient product distribution displayed by three catalysts having varied Pt dispersions (3.2%, 8% and 50%) is explained by the localized stored NOx gradients in the Ba phase. A rate determining process during the regeneration is found to be the diffusion of stored NOx within the Ba phase towards the Pt/Ba interface. 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 sufficiently mobile at lower temperatures for the low dispersion catalysts. 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 intermediate dispersion catalyst (8% dispersion) at intermediate temperatures, consistent with the experimental data. It is found that the net NH3 generation is favored under conditions when NOx transport to the Pt/Ba interface is the rate determining process. 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.