Comparison of different kinetic models for NO_X storage on a lean NO_X trap

Abstract

The kinetics of NOX storage on a lean NOX trap (LNT) were studied using NOX breakthrough curves measured with a laboratory reactor. A much wider range of temperatures (125–450°C) was used than in most studies to facilitate discrimination between potential models. The breakthrough curves were run until the LNT was saturated to (i) enable NO oxidation to NO2 to be studied in the absence of NOX storage, (ii) to enable the effective NOX capacity as a function of temperature to be determined, and (iii) to provide a more demanding test for potential models. Breakthrough curves for 125 and 450°C were very similar, while curves at intermediate temperatures initially followed that for 125°C before breaking away from this curve at a point that became earlier with increasing temperature. Thus, the breakthrough curves have a temperature independent initial portion, followed by a temperature dependant portion. This was explained by there being two types of site: fast sites and slow sites. Fast sites are responsible for the initial part of the curve; both the rate of storage on these sites and the available capacity are independent of temperature. The rate of storage on slow sites, on the other hand, is temperature dependant, as is the available storage capacity. Slow sites are more abundant than fast sites. The performance of three NOX storage models for these sites was compared. The amount of NOX stored as a function of temperature was bell‐shaped. The models naturally predicted that storage was limited by kinetics at lower temperatures and nitrate stability at higher temperatures.

Global Kinetic Modeling and Analysis of Lean NOx Traps (LNT) Catalysts

“…Mechanistic details of the NO x storage and reduction in LNT have been analyzed in some detail in the literature (see, e.g., refs − ). Urakawa et al reported on the dominance of Ba­(NO 3 ) 2 species and the absence of Ba­(NO 2 ) 2 in the storage phase, while Epling et al and Watling et al, proposed that NO x storage in LNT can be modeled using two types of storage sites (fast and slow storage). Mráček et al reported an indirect route to NH 3 and N 2 O formation, via hydrolysis or decomposition of an isocyanate intermediate when CO is used as a reducing agent.…”