A one-dimensional model of a NOx trap system was developed to describe NOx storage during the lean operation, and NOx release and subsequent reduction during the rich regeneration process. The development of a NOx trap model potentially enables the optimisation of catalyst volume, precious metal loading, substrate type and regeneration strategy for these complex systems.
To develop a fundamental description of catalytic activity, experiments were conducted to investigate the key processes involved in isolation (as far as possible), using a Pt/Rh/BaO/Al2O3 model catalyst.
A description of the storage capacity as a function of temperature was determined using NOx breakthrough curves and the storage portion of more dynamic lean-rich cycling experiments. NOx breakthrough curves were also used for determination of rate of NOx storage. Kinetics for NOx reduction, as well as CO and HC oxidation, were determined using steady state reactor experiments. Finally NOx release was modelled using data from lean-rich cycling of reactor experiments using CO, C3H6, or H2 as model reductants.
The NOx trap model was then tested using lean-rich cycling micro-reactor data with a more complete gas mixture. A good prediction of NOx storage and release was obtained at different temperatures and NOx concentrations. The model was then applied to simulate the NOx emissions under real engine test conditions. A good prediction was obtained for NOx emissions, as well as CO and HC, during the FTP test, demonstrating the accuracy and robustness of the model.