The NOx storage-reduction (NSR) process functions by adsorbing and storing NOx under lean conditions and then reducing the stored NOx to nitrogen during the periodic rich phase. The goal of this research is to develop a kinetic, and if necessary transport, model that can help understand the NOx storage mechanism on NSR catalytic systems and predict their performance. NOx storage experiments were carried out on a Pt/BaO/Al₂O₃ monolith catalyst with a 50 min storage and 5 min regeneration cycle at a space velocity of 30, 000 h^-1. A single channel of this plug flow, monolithic reactor was modeled using a transient, 1-dimensional approximation. External mass transfer was described by the film model. The resulting partial differential equations were solved by the finite element method using COMSOL^®. Model predictions of the outlet gas concentrations were fit to experimental data by coupling the COMSOL^® model with a nonlinear least-squares optimization function in MATLAB^®. Our current working model, based on a nitrite-nitrate pathway which discriminates barium sites close to the Pt phase and those away from the Pt phase, is being used to describe both NO and NO₂ adsorption on the catalyst: (1) NO+1/2O₂<->NO₂ on Pt sites, (2) 3NO₂+BaO ->Ba(NO₃)₂+NO on barium sites away from Pt, and (3.1) on barium sites close to the Pt phase, NO+NO₂+BaO->Ba(NO₂)₂, followed by (3.2) 2NO₂+Ba(NO₂)₂->Ba(NO₃)₂+2NO. In other words, we assume that, first, NO₂ disproportionation is the dominant reaction for NOx adsorption on both kinds of barium sites and is also the main reason for NO release during adsorption even when the feed gas has NO2 and oxygen. Second, NO adsorption results in nitrite formation with NO₂ as the oxygen source, and the nitrites are then oxidized to nitrates, again with NO₂ as the oxidant. Third, NO₂is a stronger oxidizing agent than O₂, so that even with percentage levels of O₂ in the feed, it is NO₂ that oxidizes nitrites to nitrates by releasing oxygen atoms at the Pt-Ba interface. Supporting evidence for this nitrite-nitrate mechanism comes from the observations that even though only small amounts of nitrites were observed at 300C, FT-IR data show that at 210C nitrite amounts are significant on the barium surface with NO+O₂ as the feed. The model can explain the experimental breakthrough curves of NO and NO2 for 300 ppm NO + 10% O₂ as the feed. It shows that after a long time of adsorption, when barium sites are almost saturated, NO and NO₂ breakthrough curves are governed by the NO oxidation reaction only. During the transient period, NO oxidation is the rate limiting step compared to NOx adsorption, resulting in a small amount of NO and NO₂ slip at the beginning. At 300C, the rate of nitrite oxidation is very fast compared to its formation, so that nitrite surface coverage is low and the majority of the surface is nitrate. This surface oxidation may be hindered at low temperature so that nitrites dominate at 210C and below. Application of this model, with the same parameter set as used for the NO + O₂ case, gives a qualitative fit to the NO2 + O2 experiment, indicating at least a partial validation of the model. The presence of CO₂ and H₂O in the feed does not change the proposed reaction pathway, but does alter the number of available sites and the reaction rate. Further refinements of the model are underway and will be discussed.

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