Development and Validation of a Two-Site Kinetic Model for NH₃-SCR over Cu-SSZ-13. Part 2. Full-Scale Model Validation, ASC Model Development, and SCR-ASC Model Application

We present herein the final part in the development and validation of a two-site kinetic model for NH₃-SCR over Cu-SSZ-13. To predict tailpipe emissions accurately, it was necessary to combine the kinetic model of the SCR catalyst developed in part I (Daya et al. 2018), with a reaction-diffusion model of the dual-layer ammonia slip catalyst (ASC). This dual-layer ASC model was developed following a three-step process, including development of the kinetic models of the individual layers, followed by parameterization of a parallel pore diffusion model of the dual-layer ASC. Reactor-scale validation of the dual-layer ASC model confirmed the kinetic model accuracy and highlighted the significance of intra-porous diffusion. Following this, the SCR model developed in part I of this paper was validated on an engine dynamometer through comprehensive steady-state experiments with inlet NH₃ to NO_x ratio (ANR) sweeps. The final SCR and ASC models were then evaluated on cold and hot heavy-duty transient (HDT) cycles, to examine the capability of predicting tailpipe NO_x, NH₃ slip as well as storage-based dynamics. Overall cycle-averaged NO_x conversion was predicted within 3% using these models. Validated models have significant application in model-based control as well as improving catalyst design through improved functional understanding. The present Cu-SSZ-13 SCR model was simulated using the four-step SCR protocol (Kamasamudram et al. Catal. Today 151(3):212–222) to calculate the intra-catalyst dynamic capacity. These numerical experiments showed that the dynamic capacity decreases upon hydrothermal aging but leads to higher NO_x conversion under standard and fast SCR conditions at 250 °C. This increase in NO_x conversion is due to more uniform NH₃ storage along the length of the catalyst, leading to higher NH₃ utilization near the rear of the aged catalyst. Similar numerical experiments on the dual-layer ASC model demonstrated intra-layer washcoat distributions causing NO slip during transient drive cycles for both hydrothermal aging conditions.