Reductive Elimination as a Mechanism for Purging a Lean NOx Trap 2006-01-1067
The mechanism for the purging of a lean NOx trap has been investigated. For realistic purge times (e.g., 2 to 5 seconds), the stored NOx species do not decompose simply from equilibrium considerations (i.e., from the drop in O2 and NO concentrations during the rich purge). Instead, the decomposition of stored NOx is promoted by the reductants in the exhaust by a process referred to as reductive elimination. H2 is far more effective than CO or C3H6 for promoting this reductive elimination, particularly at low temperatures (e.g., 250°C). As long as H2 is available in the feedgas, H2O does not participate in the reductive elimination. However, if CO is the only reductant, H2O is needed to convert some of the CO to H2 through the water-gas-shift reaction. H2O is also important for the efficient storage of NOx during lean operation, possibly by enhancing the spillover of NO2 from a precious metal site to a NOx storage site. The kinetics of the reaction between H2 and the stored NOx have been studied with a model Ba/Pt/Al2O3 catalyst.