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

Licentiate thesis, 2024

Copper exchanged chabazite (Cu-CHA) is a state-of-the-art catalyst for deNOx via ammonia assisted selective catalytic reduction (NH3-SCR) in lean burn engines, owing to its good low-temperature activity, and high hydrothermal stability. One challenge for Cu-CHA is, however, the sensitivity to sulfur species, which are present in the exhaust gas. Even at small concentrations, sulfur accumulates in the catalyst leading to a loss in activity and a reduction in the operational lifetime. A better understanding of the NH3-SCR activity and sulfur poisoning is important for the development of catalysts with high activity that are sulfur resistant.

In this thesis, density functional theory (DFT) calculations are used to study the mechanism for the sulfur poisoning of Cu-CHA during NH3-SCR conditions, with a focus on low-temperature deactivation. It is suggested that SO2 reacts with [Cu2(NH3)4O2]2+ resulting in accumulation of ammonium bisulfate species inside the chabazite cage. This hinders the pairing of [Cu(NH3)2]+ complexes, which is needed for adsorption of O2, leading to a loss in activity. At high temperatures, it is proposed that SO2 and SO3 primary react with ZCuOH and Z2CuOOCu complexes, forming stable copper sulfur species, with SO3 forming Cu sulfates with highest stability. The combination of DFT with micro-kinetic modeling has, moreover, been used to investigate H2 temperature programmed reduction (H2-TPR) profiles to aid the interpretation of experimental H2-TPR profiles.

Given the importance of [Cu(NH3)2]+ diffusion for the O2 adsorption and subsequently reduction of NO, a machine learning force field (ML-FF) has been constructed that is trained with DFT data. The use of ML-FF makes it possible to simulate system sizes and timescales inaccessible to conventional ab initio molecular dynamics (AIMD). The effect of zeolite composition on the mobility and pairing of [Cu(NH3)2]+ complexes is studied using different analysis tools. It is found that a high Cu/Al and low Si/Al ratio enhance the pairing of [Cu(NH3)2]+ complexes.