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

Doctoral thesis, 2025

Ammonia assisted selective catalytic reduction (NH₃-SCR) is currently the preferred method for abatement of NO_x 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, Cu^I 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 Cu^II 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.