Global kinetic modeling of hydrothermal aging of NH3-SCR over Cu-zeolites
Journal article, 2015
In this study, a kinetic model describing the effect of hydrothermal aging (at 500, 600, 700, 800 and 900 degrees C) on Cu-zeolites is developed. The model accounts for the impact of hydrothermal aging on key reactions such as ammonia adsorption/desorption, NH3 oxidation, NO oxidation, standard SCR, rapid SCR, and NO2 SCR. In addition, a mechanism for the complex N2O formation were developed. The effect of aging on ammonia adsorption and desorption were established using micro-calorimeter data. Thereafter, an aging factor model was developed containing two aging factors, one related to over-exchanged copper sites and the other to under-exchanged copper sites. This approach worked well for ammonia and NO oxidation up to an aging temperature of 800 degrees C, whereas for the SCR reactions only to 700 degrees C. According to UV-vis, fresh and mildly aged catalysts are dominated by copper hydroxyls, while after aging at high temperature copper oxides are observed. We therefore introduce one SCR reaction associated with copper oxides, simultaneously with one SCR reaction associated with ion-exchanged Cu sites and the updated model could describe the experimental findings well. The results from the model also suggest that the standard SCR reaction is more deactivated during aging compared to SCR with NO2 present in the feed. After the 900 degrees C aging the BEA structure had collapsed, resulting in that several parameters in the model needed to be retuned. The results from this modeling study clearly show how complex the hydro thermal aging is over copper zeolites.