Microkinetic modeling of lean NOx trap chemistry under reducing conditions

An elementary surface reaction mechanism describing the chemistry on the precious metal sites of a lean NOx trap is developed. Kinetic parameters for all of the reactions are found by fitting reactor simulations to an extensive experimental database. Each experiment involves the steady flow of a reactant mixture through a monolith core sample under conditions designed to minimize NOx storage; in each case, the temperature is slowly ramped over a wide range in order to provide a large amount of data. A reaction mechanism involving 28 elementary steps is able to reproduce the results for 21 separate experimental runs quite well. The thermodynamic consistency of the mechanism is assured through the imposition of constraints on a well-defined subset of the rate parameters. It is found that the mechanism can occasionally lead to multiple steady-state behavior due to the existence of parallel reduction pathways. DRIFTS experiments corroborate some of the key mechanistic steps.