Reductive Elimination as a Mechanism for Purging a Lean NOx Trap

The mechanism for the purging of a lean NO_x trap has been investigated. For realistic purge times (e.g., 2 to 5 seconds), the stored NO_x 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 NO_x 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 NO_x during lean operation, possibly by enhancing the spillover of NO₂ from a precious metal site to a NO_x storage site. The kinetics of the reaction between H₂ and the stored NO_x have been studied with a model Ba/Pt/Al₂O₃ catalyst.