Development and Validation of a Two-Site Kinetic Model for NH3-SCR over Cu-SSZ-13. Part 1. Detailed Global Kinetics Development Based on Mechanistic Considerations

In this study, we present the first of two parts in the development and validation of a two-site detailed global kinetic model for NH 3 Selective Catalytic Reduction (SCR) over a well-characterized Cu-SSZ-13 catalyst. Based on fundamental literature studies and experimental data for two distinct hydrothermal aging conditions, it was observed that at least two distinct sites were necessary to describe the storage, oxidation, and SCR behavior for this catalyst. The site definitions were allocated based on analysis of simulated net desorption rates during the NH 3 -TPD experiment. The S1 sites were associated with extra framework copper ions stabilized by -OH ligands (ZCuOH), likely located near the eight-membered ring CHA cages, along with Brønsted acid sites, while S2 sites were associated with copper ions attached directly to the repeating units (Z 2 Cu) near the six-membered rings, along with physisorbed NH 3 sites and low temperature transient copper dimers. The selective NH 3 oxidation and NO oxidation reactions were only modeled over S1, in line with the expected catalytic sites for these reactions. Standard, fast, and NO 2 SCR reactions were modeled on both sites, with different activation energies. Finally, noticeable nitrate-based hysteresis effects were observed, in both N 2 O concentrations and fast SCR NO conversions. These were accounted for by explicitly modeling nitrate formation, titration by NO, and thermal decomposition to N 2 O. The developed SCR model was validated with additional reactor data at nominal inlet NH 3 -to-NO x ratios (ANRs) of 0.8 and 1.2. In general, the model showed good predictability in the temperature range of 150–550 °C for both hydrothermal ageing conditions and space velocities. Further full-scale engine dynamometer validation and development of a downstream NH 3 slip catalyst (ASC) reaction-diffusion model will be reported in the second part of the paper.