Detailed kinetic modeling of NOxNOx storage and reduction with hydrogen as the reducing agent and in the presence of CO2 and H2O over a Pt/Ba/Al catalyst

A detailed kinetic model of NOxNOx storage and reduction, in the presence of H2O and CO2, with hydrogen as the reducing agent was developed and validated in this study. The mechanism was derived from flow reactor experiments conducted at 200–400 °C over a Pt/Ba/Al monolith sample. The detailed kinetic model is divided into four sub-models: (i) NO oxidation over Pt, (ii) NOxNOx storage, (iii) NOxNOx reduction over Pt, and (iv) NOxNOx regeneration. The sub-model for NOxNOx storage is based on our earlier work and is further developed in this study to include high concentrations of CO2 and H2O in the feed and also low temperature storage. In the model NOxNOx is allowed to be stored on two different types of storage sites: BaCO3 and a second storage site denoted S3. Based on experimental results many studies suggests multiple storage sites one these catalysts, and there are different explanations (i) alumina and barium sites (ii) bulk and surface barium, (iii) barium close and far from the noble metal, etc. The disproportionation route, where NO2 is stored over BaCO3, is included in the NOxNOx storage model. To account for the storage occurring at lower temperatures NO can be stored over both BaCO3 and S3 in the presence of O2. NO adsorbed over S3 can be further oxidized to NO2 by reacting with oxygen on neighboring Pt sites. The second storage site (S3) is important in order to explain NOxNOx storage at low temperatures and the disproportionation reaction is essential to describe the storage at high temperatures. The NOxNOx reduction sub-model used here was developed earlier over Pt/Si. Additional to the reduction of NOxNOx into N2, it describes the formation of NH3 over Pt. In the sub-model describing the regeneration of NOxNOx, adsorbed NOxNOx species react with hydrogen adsorbed on Pt sites. Ammonia oxidation over Pt and reactions between surface species of barium and NH3 according to ammonia selective catalytic reduction (SCR) chemistry are also incorporated in the regeneration sub-model. The full model can describe the complete uptake of NOxNOx in the beginning of the lean period, the NOxNOx breakthrough, and the slow NOxNOx storage in the end of the lean period very well as well as the following release and reduction. It can also predict the gradual decrease in the storage capacity occurring in lean/rich cycling experiments. Furthermore, the ammonia formation predicted by the model fits well with experimental data. The model was validated with short lean (60 s) and rich (15 s) cycles which were not included in the model development. The model could predict these experiments well for all three temperatures (200, 300 and 400 °C).