Role of NH3 oxidation in NOx reduction performance of dual-layer and mixed LNT-SCR configurations

A global kinetic model for NH3 oxidation is developed and incorporated into an existing LNT-SCR kinetic model to study its effect on the performance of dual-layer and mixed LNT-SCR configurations. The model predicts a decrease in the NOx conversion and an increase in the NOx storage with the inclusion of NH3-oxidation reactions for both the configurations. This is due to the selective oxidation of NH3 on the LNT catalyst instead of its consumption by the NOx reduction reactions on the SCR catalyst. The decrease in NOx conversion is negligible at low temperatures and is more prominent at intermediate temperatures (∼300 °C). The limited supply of stored NH3 for the oxidation reactions lowers the negative effect of NH3 oxidation at high temperatures. The fractional contribution of NH3-oxidation reactions towards the consumption of NH3 is calculated to be higher for the mixed configuration, which is further confirmed by the uniform axial profiles of stored NH3 in the mixed configuration as compared to the dual-layer configuration. This results in a more prominent decrease in NOx conversion for the mixed configuration as compared to the dual-layer configuration. The formation of NOx at high temperatures by the oxidation of NH3 in the combined configurations can result in a NOx conversion even lower than the standalone LNT catalyst. A higher NH3 slip is predicted for the mixed configuration as compared to the dual-layer configuration. It is highlighted that the composition of the LNT catalyst, its activity for NH3-oxidation and the selectivity of products formed by NH3-oxidation reactions could influence the inferences made by various groups on the comparison of LNT-SCR configurations, with the mixed configuration in particular.