Detailed kinetic modeling of NOx adsorption and NO oxidation over Cu-ZSM-5

Detailed kinetic modeling was used in combination with flow reactor experiments to investigate the NOx adsorption/desorption and NO oxidation over Cu-ZSM-5. NO oxidation is likely an important step for selective catalytic reduction (SCR) using urea and hydrocarbons, and thus was investigated separately. First the NO2 adsorption on Bronstedt acid sites in H-ZSM-5 was modeled using an NO2 temperature programmed desorption (TPD) experiment. These results, together with the results of the NO2 TPD and NO oxidation experiments, were used in developing the model for Cu-ZSM-5. A substantial amount of NO2 was adsorbed on the catalyst. However, the results from a corresponding NO TPD experiment showed that only very small amounts of NO were adsorbed on the catalyst and therefore this step was not included in the model. The model consists of reversible steps for NO2 and O-2 adsorption, O-2 dissociation, NO oxidation and two steps for nitrate formation. The first nitrate formation step was disproportionation of NO2 to form NO and nitrates. This step enabled us to describe the NO production during NO2 adsorption. Further, in the reverse step the NO reacts with the nitrates and decreased their stability. Without this step the nitrates blocked the surface resulting in to low NO oxidation activity. However, we observe that nitrates can be decomposed also without the presence of NO and in the second reversible step were the nitrates decomposed to form NO2 and oxygen on the copper. These steps enabled us to describe both the TPD and activity measurement results. NO oxidation was observed even at room temperature. Interestingly, the NO2 decreased when increasing the temperature up to 100 C and then increased as the temperature increased further. We suggest that this low-temperature NO oxidation occurs with species loosely bound on the surface and that is included in the detailed mechanism. An additional NO2 TPD at 30 degrees C was also modeled to describe the loosely bound NO2 on the surface. The detailed model correctly describes NO2 storage, NO oxidation and low-temperature NO oxidation. (C) 2008 Elsevier B.V. All rights reserved.