(69a) Modeling the Effects of Pt Dispersion During NOx Storage and Reduction on Pt/BaO/Al2O3

A crystallite-scale model is developed and incorporated into a reactor scale model to study the effects of Pt dispersion during the periodic lean-rich operation on a Pt/BaO/Al₂O₃ NOx storage and reduction (NSR) catalyst. The model is an extension of a previously developed regeneration model [D. Bhatia, M.P. Harold and V. Balakotaiah, Catalysis Today, 151 (2010) 314] and accounts for crystallite-scale diffusion limitations in the storage phase. The storage model is based on the concept of NO_x spillover from Pt to BaO and diffusion of stored NO_x in the barium phase. The model predicts the main features of NO_x storage, such as the increase in NOx breakthrough time for increasing Pt dispersion at fixed Pt loading. The predicted NOx storage increase with Pt dispersion is a result of the increase in exposed Pt area which increases the specific NO oxidation activity, and of the increased interfacial perimeter between Pt and BaO, which promotes the rate of spillover. A sensitivity analysis of the stored NO_x diffusivity reveals the importance of this process. The combined storage and regeneration model is used to simulate the entire lean rich cycles to study various cycled-averaged variables such as NO_x conversion and NH₃ selectivity. Simulations show the effectiveness of the catalyst to utilize the storage sites. The results reveal that the low dispersion catalyst (8%) is not able to utilize all of the available barium sites because NOx diffusion is too slow to access barium sites far from the Pt crystallites.  The dispersion also affects the extent of axial uniformity of stored NOx during a typical 1-2 minute lean phase. The axial distribution is much more uniform for the low dispersion catalyst whereas a rather sharp storage front is evident for the high dispersion catalyst.  Comparisons of the model predictions to experimental data reveal good agreement in the cycle-averaged trends.  The model is used to study various storage and regeneration timing protocol which shows that shorter storage time is required for lower dispersion catalysts to achieve higher cycle averaged NO_x conversion. The model is useful in elucidating the complex transient phenomena occurring in the lean NOx trap.

Keywords:  NO_x, Hydrogen, Platinum, Barium, NOx storage and reduction, Lean NOx trap, Dispersion, Spillover