A kinetic model for ammonia selective catalytic reduction over Cu-ZSM-5

Kinetic modeling, in combination with flow reactor experiments, was used in this study for simulating NH3 selective catalytic reduction (SCR) of NOx over Cu-ZSM-5. First the mass-transfer in the wash-coat was examined experimentally, by using two monoliths: one with 11 wt.% wash-coat and the other sample with 23 wt.% wash-coat. When the ratio between the total flow rate and the wash-coat amount was kept constant similar results for NOx conversion and NH3 slip were obtained, indicating no significant mass-transfer limitations in the wash-coat layer. A broad range of experimental conditions was used when developing the model: ammonia temperature programmed desorption (TPD), NH3 oxidation, NO oxidation, and NH3 SCR experiments with different NO-to-NO2 ratios. 5% water was used in all experiments, since water affects the amount of ammonia stored and also the activity of the catalyst. The kinetic model contains seven reaction steps including these for: ammonia adsorption and desorption, NH3 oxidation, NO oxidation, standard SCR (NO + O2 + NH3), rapid SCR (NO + NO2 + NH3), NO2 SCR (NO2 + NH3) and N2O formation. The model describes all experiments well. The kinetic parameters and 95% linearized confidence regions are given in the paper. The model was validated with six experiments not included in the kinetic parameter estimation. The ammonia concentration was varied from 200 up to 800 ppm using NO only as a NOx source in the first experiment and 50% NO and 50% NO2 in the second experiment. The model was also validated with transient experiments at 175 and 350 °C where the NO and NH3 concentrations were varied stepwise with a duration of 2 min for each step. In addition, two short transient experiments were simulated where the NO2 and NO levels as well as NO2-to-NOx ratio were varied. The model could describe all validation experiments very well.