Dynamics and Reactivity of Cu-species in Cu-CHA for NH3-SCR

Ammonia assisted selective catalytic reduction (NH3-SCR) is currently the preferred method for abatement of NOx for lean burn engines. The copper exchanged chabazite is state-of-the-art catalyst for this reaction thanks to superior hydrothermal stability and good low-temperature activity. One challenge, however, is the sensitivity to sulfur compounds, present in the exhaust gas. Even small amounts of sulfur exposure can drastically deactivate the catalyst and shorten its operational lifetime. Therefore, it is critical to understand the mechanism behind the NH3-SCR activity and sulfur poisoning. During low-temperature NH3-SCR conditions, CuI ions are solvated by NH3, and present as [Cu(NH3)2]+ complexes. A critical step in the reaction is O2 adsorption, which requires the pairing of two [Cu(NH3)2]+ complexes and leads to the formation of CuII ions in a peroxo complex [Cu2O2(NH3)4]2+. In this thesis, various computational techniques and experiments are used to elucidate the pairing of [Cu(NH3)2]+ and SO2-deactivation of Cu-CHA. A machine learning force field (ML-FF) is developed including long-range interactions. Trained on density functional theory (DFT) data, the ML-FF enables molecular dynamics (MD) simulations of large systems over extended timescales with high accuracy. The results show that the pairing of [Cu(NH3)2]+ is promoted by increasing the Cu-loading and Al-content and that it is strongly influenced by counter-diffusion of nearby cations such as [Cu(NH3)2]+ or NH4+. DFT calculations are used to study the mechanism for SO2 poisoning during low-temperature NH3-SCR. The calculations suggest that SO2 reacts with the peroxo complex [Cu2O2(NH3)4]2+ forming NH4HSO4 species that accumulate inside the CHA cage. The accumulation destabilizes the pairing of [Cu(NH3)2]+ and, thus, O2 adsorption. Moreover, flow reactor experiments show that sulfation and regeneration depend critically on the temperature. Based on experimental data, a kinetic model is developed, which describes and rationalizes the dynamic behavior of SO2 poisoning and regeneration. The present work combines theoretical and experimental techniques to give a comprehensive understanding of the NH3-SCR reaction over Cu-CHA, and the deactivation caused by SO2 which is essential for guiding the development of more active and sulfur-resistant catalysts.