| The awareness of global warming problem in the world, as well as the need to conserve energy, is increasing. Lean burn engines offer lower fuel consumption and lower emissions of CO2 due to the more efficient combustion resulting from the lean operation. However, due to the substantial excess of oxygen, prombles arise for the reduction of NOx.An effective approach to NOx abatement for lean burn engine is a precious metal based catalytic NOx reduction syetem operated under periodic lean/rich conditions. The main objective of this work is to investigate the mechanism of NOx reduction over rhodium based catalysts. This is accomplished through microkinetic modeling. In particular, the NO reduction process when CO is used as the reducing agent is simulated in both transient and steady state. Additionally, a significant amount of N2O has been formed for NO reduction by CO, and the results show that the N2O formation is favored at low temperatures. Attention is also given to the influence of N2O on the performance of NO conversion, we note that the temperature for maximum N2O formation corresponds to the NO-CO reaction light-off temperature, which suggests that the N2O formation reaction steps may play an important role in initiating the NO-CO reaction. Then the CO oxidation reaction mechanism and the influence of coverage dependent kinetics on the catalyst behaviour are investigated. The experimental responses can be well described by this elementary step reaction mechanism and the measured average conversion of CO is appropriately predicted. The model could be applied in the design/optimization of catalytic exhaust after-treatment devices. Furthermore, this work potentially contributes to the development of applicable catalysts for vehicles equipped with diesel engines, lean-operated gasoline engines. |
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