Microkinetic Modeling of Lean NO_x Trap Sulfation and Desulfation

Abstract

A microkinetic reaction sub-mechanism designed to account for the sulfation and desulfation of a commercial lean NO x trap (LNT) is presented. This set of reactions is appended to a previously developed mechanism for the normal storage and regeneration processes in an LNT in order to provide a comprehensive modeling tool. The reactions describing the storage, release, and reduction of sulfur oxides are patterned after those involving NO x , but the number of reactions is kept to the minimum necessary to give an adequate simulation of the experimental observations. Values for the kinetic constants are estimated by fitting semi-quantitatively the somewhat limited experimental data, using a transient plug flow reactor code to model the processes occurring in a single monolith channel. Rigorous thermodynamic constraints are imposed in order to ensure that the overall mechanism is consistent both internally and with the known properties of all gas-phase species. The final mechanism is shown to be capable of reproducing the principal aspects of sulfation/desulfation behavior, most notably (a) the essentially complete trapping of SO 2 during normal cycling; (b) the preferential sulfation of NO x storage sites over oxygen storage sites and the consequent plug-like and diffuse sulfation profiles; (c) the degradation of NO x storage and reduction (NSR) capability with increasing sulfation level; and (d) the mix of H 2 S and SO 2 evolved during desulfation by temperature-programmed reduction. 4 ACKNOWLEDGMENTS