Global Kinetic Modeling of Lean NOx Traps

Lean NOx traps are an important concept for reducing NOx in lean exhausts from diesel engines and lean burn gasoline engines. A global kinetic model has been developed in this work that can describe the NOx emissions after the catalyst. The model is based on flow reactor experiments using a model Pt/Rh/BaO/Al2O3 catalyst. The objective with the model was to use global kinetics to facilitate rapid simulations and at the same time base the model on physical steps. A first step in NOx storage is to oxidize the NO to NO2, where the latter is stored on the barium to form barium nitrates. Therefore, a global reaction step was initially developed for describing NO oxidation on a Pt/Al2O3 catalyst. This model was used when developing the NOx storage model. The flow reactor experiments used for these simulations were cycling between lean and rich gas composition, using propene as a reducing agent. The experiments were conducted at three temperatures (320, 380 and 440°C) and using two inlet NO concentrations. To describe the mass-transfer in the catalyst a shrinking core model was used. The major path for the storage is in the model the formation of barium nitrates from NO2 and barium carbonate. In the lean period there is a large storage, but the NO oxidation is quite low. Therefore was it necessary to introduce a second reaction step where barium nitrites were formed initially from NO and barium carbonates. This is consistent with FTIR observations, which have shown that nitrites are first formed, but after prolonged exposures the nitrates are dominating on the catalyst. Reaction steps for the regeneration of the stored NOx with propene were added. The model was able to describe the NOx storage in the lean period, the reduction of NOx in the rich period and the breakthrough of NOx occurring in the initial part of the rich phase. The model was validated with separate experiments, not included in the model development. In these experiments the oxygen concentration was lowered to 4 %, compared to 8 % that were used in the other experiments. The model was able to adequately predict the results from these experiments, with the lowering of the NO2 formation and the storage. A second NOx storage model was developed for predicting the total NOx concentration after the lean NOx trap. The advantage with this model is that it only consists of one reaction for the storage and one reaction for the regeneration. Since this model contains less parameters, it is more rapid to tune in to other NOx adsorbers with different composition. In addition, experiments where only total NOx is measured (NO and NO2 not separated) can be used for fitting parameters in the model. Three experiments were in this case used for fitting the parameters. The model was validated with nine experiments performed at different temperatures and with varying the oxygen and NO concentration. The model was able to predict the experiments well.