Microkinetic modeling of lean NOx trap chemistry

A microkinetic chemical reaction mechanism capable of describing both the storage and regeneration processes in a fully formulated lean NOx trap is presented. The mechanism includes steps occurring on the precious metal, NOx storage, and oxygen storage sites of the catalyst. The complete reaction set is used with a transient plug flow reactor code (including boundary layer mass transfer) to simulate not only storage/regeneration cycles with a CO/H-2 reductant, but also steady flow temperature sweep experiments that were previously analyzed with just a precious metal mechanism and a simpler steady state code. The results imply that NOx storage was not negligible during some of the temperature ramps, necessitating a re-evaluation of the precious metal kinetic parameters. The parameters for the entire mechanism are inferred by finding the best overall fit to the complete set of experiments. Rigorous thermodynamic consistency is enforced for parallel reaction pathways and with respect to known data for all gas phase species. It is found that nearly all of the basic experimental observations can be reproduced with the transient simulations. (C) 2012 Elsevier B.V. All rights reserved.