Fundamental Studies of Catalytic NOx Removal - Micro Kinetic Modelling, Monte Carlo Simulations and Flow Reactor Experiments

Doctoral thesis, 2002

It is essential to decrease the CO2 emissions from vehicles, since CO2 is a greenhouse gas. One way to improve fuel economy and decrease CO2 emissions, is to run the engine with oxygen excess, so-called lean-burn operation. However, a major problem here is the reduction of the NOx in a lean atmosphere. One solution to this problem may be the NOx storage concept, where NOx is stored in the catalyst during lean operation, followed by a regeneration of the catalyst during a short period of rich operation. The objective of this work was to investigate the removal of NOx from exhausts, focusing on the NOx storage concept. In the studies flow reactor experiments, micro kinetic modelling and Monte Carlo simulations have been used. Since NO oxidation is crucial for NOx storage, as well as important for other applications, we have separately investigated NO oxidation on Pt/Al2O3. A micro kinetic model was constructed, with the use of flow reactor experiments. Furthermore, it was observed that the NO oxidation rate increased when the size of the platinum particles increased. In addition, we found that the NO oxidation activity and NO2 dissociation activity on Pt/Al2O3 decreased over time. We suggest that this deactivation is due to formation of platinum oxides, which was supported by XPS measurements. The NOx storage on BaO/Al2O3, was investigated with both experiments and kinetic modelling. The kinetic model of NOx storage on Pt/BaO/Al2O3 was then constructed from the kinetic parameters obtained in the above mentioned models. To describe the increased NOx release for the Pt/BaO/Al2O3 case, compared to the BaO/Al2O3 case, we added one reversible spill-over step. The model was validated with additional experiments, which showed that NO promoted the release of NOx from the Pt/BaO/Al2O3 catalyst. Finally, reaction steps for propene regeneration were added to the model. We found a good agreement between these experiments and the model. Monte Carlo (MC) simulations were used to study the NO and CO reaction on a (111) metal surface, with the inclusion of step sites. The objective was to study the conditions at which step sites are important. Further, in the MC simulations the surface coverages were found to be non-uniform.