Model Development and Application over a Cu-CHA Catalyst

Michelle  Bendrich, Umicore AG & Co. KG

Authors: Michelle Bendrich, Alexander Scheuer, Robert E. Hayes, Martin Votsmeier

Mechanistic proposals for the different SCR subreactions are integrated into one surface reaction mechanism that describes the main SCR reactions (Standard SCR, Fast SCR, NO2 SCR), transient effects due to nitrate storage, as well as the production of the side product N2O over a copper chabazite catalyst. The mechanism is parameterised to steady state and transient experiments, and is shown to predict the behaviour of the catalyst during driving cycles, without any refitting of kinetic parameters.

Surprisingly, although a significant amount of inhibitive ammonium nitrate is modelled to form during low temperature Fast and NO2 SCR steady state experiments, almost no ammonium nitrate is predicted to form during the driving cycle, thus allowing for a higher reaction activity than predicted based on steady state data. To validate whether the minimal formation of ammonium nitrate during transient driving cycles holds true, an experimental method [2] and simulations are applied to compare the amount of ammonium nitrate stored during steady state and transient tests over the Cu-CHA catalyst. The results demonstrate that little ammonium nitrate accumulates on the catalyst surface during multiple cold WHTCs with a high cumulated NO2/NOx ratio owing to the slow accumulation of ammonium nitrate and the rapidly fluctuating inlet conditions. The NO2 emissions are much better predicted by a model that appropriately considers the ammonium nitrate storage.

Finally, the model is applied to optimize SCR dosing strategies during driving cycles, such that the NOx conversion is maximized while maintaining the ammonia slip below a defined level. The optimized dosing profile is applied at a dynamic laboratory test bench and confirms the increase in deNOx performance.

[1] M. Bendrich, A. Scheuer, R. E. Hayes, M. Votsmeier, „Unified Mechanistic Model for Standard SCR, Fast SCR, and NO2 SCR over a Copper Chabazite Catalyst,“ Appl. Catal. B 222 (2018) 76-87.

[2] M. P. Ruggeri, J. Luo, I. Nova, E. Tronconi, K. Kamasamudram, A. Yezerets, “Novel method of ammonium nitrate quantification in SCR catalysts,” Catal. Today 207 (2017) 48-54.