NH3-SCR over Cu-CHA catalysts: Mechanistic insights into hydrothermal aging, water inhibition, and sulfur poisoning
Doctoral thesis, 2025
Selective catalytic reduction using ammonia (NH3-SCR) is a critical technology for the control of NOx emissions from lean-burn engines. Cu-exchanged chabazite (Cu-CHA) materials are currently the preferred catalysts for NH3-SCR thanks to their excellent activity and selectivity at low temperatures. However, Cu-CHA suffers from deactivation due to water inhibition, hydrothermal aging, and sulfur poisoning. In this thesis, detailed mechanisms for the three deactivation paths are studied by density functional theory (DFT) based kinetic modeling and reactor experiments.
At low-temperature (200 °C), high partial pressures of H2O inhibits the NH3-SCR by competing with NO and NH3 for adsorption on active Cu sites. At high-temperature (>650 °C), H2O causes hydrothermal aging, primarily through dealumination, i.e., removal of framework Al thereby Brønsted acid sites (BAS). DFT calculations reveal similar fourstep hydrolysis pathways for dealumination in H-CHA and Cu-CHA, with extra-framework Al(OH)3(H2O) formed in H-CHA and Al(OH)3(H2O) together with Cu–Al species in Cu-CHA. The higher dealumination barriers in Cu-CHA and the thermodynamic stability of Cu–Al formation explain the experimentally observed enhanced stability of CHA in the presence of Cu. Dealumination of H-CHA is quantified by NH3-Temperature-Programmed Desorption (NH3-TPD) and 27Al Nuclear Magnetic Resonance (27Al-NMR) measurements. A quantitative comparison shows that NH3-TPD overestimates dealumination via a self-exchange reaction in which extra-framework Al(OH)3(H2O) accepts protons from neighboring BAS, leading to an apparent loss of acidity without framework Al removal.
Hydrothermal aging affects the kinetics of NH3-SCR at low-temperature (200 °C). Experiments show two different deactivation mechanisms attributed to loss of BAS and decreased formation of [Cu2(NH3)4O2]2+, which is a key intermediate for low-temperature SCR. Hydrothermal aging affects SO2 deactivation marginally, indicating that SO2 deactivation is largely independent of hydrothermal aging.
The proposed deactivation mechanism links loss in SCR performance to changes in Brønsted acid site and Cu speciation for each type of deactivation. The work provides mechanistic insights to improve catalyst durability during water-induced deactivation at
high temperatures, high pressures and SO2 exposure.
At low-temperature (200 °C), high partial pressures of H2O inhibits the NH3-SCR by competing with NO and NH3 for adsorption on active Cu sites. At high-temperature (>650 °C), H2O causes hydrothermal aging, primarily through dealumination, i.e., removal of framework Al thereby Brønsted acid sites (BAS). DFT calculations reveal similar fourstep hydrolysis pathways for dealumination in H-CHA and Cu-CHA, with extra-framework Al(OH)3(H2O) formed in H-CHA and Al(OH)3(H2O) together with Cu–Al species in Cu-CHA. The higher dealumination barriers in Cu-CHA and the thermodynamic stability of Cu–Al formation explain the experimentally observed enhanced stability of CHA in the presence of Cu. Dealumination of H-CHA is quantified by NH3-Temperature-Programmed Desorption (NH3-TPD) and 27Al Nuclear Magnetic Resonance (27Al-NMR) measurements. A quantitative comparison shows that NH3-TPD overestimates dealumination via a self-exchange reaction in which extra-framework Al(OH)3(H2O) accepts protons from neighboring BAS, leading to an apparent loss of acidity without framework Al removal.
Hydrothermal aging affects the kinetics of NH3-SCR at low-temperature (200 °C). Experiments show two different deactivation mechanisms attributed to loss of BAS and decreased formation of [Cu2(NH3)4O2]2+, which is a key intermediate for low-temperature SCR. Hydrothermal aging affects SO2 deactivation marginally, indicating that SO2 deactivation is largely independent of hydrothermal aging.
The proposed deactivation mechanism links loss in SCR performance to changes in Brønsted acid site and Cu speciation for each type of deactivation. The work provides mechanistic insights to improve catalyst durability during water-induced deactivation at
high temperatures, high pressures and SO2 exposure.
Cu-CHA
hydrothermal aging
water inhibitions.
dealumination
SO2 poisoning
NH3-SCR
Author
Shivangi Singh
Chalmers, Physics, Chemical Physics
Mechanism for Cu-enhanced hydrothermal stability of Cu-CHA for NH<inf>3</inf>-SCR
Catalysis Science and Technology,;Vol. 14(2024)p. 3407-3415
Journal article
Inhibition of NH<inf>3</inf>-SCR over Cu-CHA at high partial pressures of water: Measurements and DFT-based kinetic modeling
Journal of Catalysis,;Vol. 446(2025)
Journal article
S. Singh, T.V.W. Janssens, and H. Grönbeck, Rate of NH3-SCR over hydrothermally aged Cu-CHA
S. Radhakrishnan, S. Singh, C.V. Chandran, J.A. Martens, H. Grönbeck, T.V.W. Janssens, and E. Breynaert, Enhanced loss of Brønsted acid sites upon hydrothermal dealumination of chabazite zeolites by self-exchange with extra-framework aluminum
S. Singh, T.V.W. Janssens, and H. Grönbeck, Impact of hydrothermal aging on the SO2-induced deactivation of Cu-CHA catalyst for NH3-SCR
Emissions from heavy-duty diesel trucks are a major source of nitrogen oxides (NOx), which contribute to air pollution. Stringent environmental regulations have been introduced worldwide to limit NOx emissions from diesel engines. To meet increasingly stricter emission regulations, these vehicles rely on selective catalytic reduction (SCR), a technology that uses ammonia to convert harmful nitrogen oxides into harmless nitrogen and water.
Cu-CHA catalysts are widely implemented in SCR for heavy-duty engines because they provide high (NOx) conversion to nitrogen and robust performance under operating conditions. However, these catalysts must maintain reliable performance over long lifetimes in a harsh exhaust environment with high temperatures, water, and sulfur oxides, all of which gradually reduce their performance.
This thesis focuses on understanding the mechanisms of Cu-CHA catalyst deactivation under real-world operating conditions. By gaining deeper insight into catalyst deactivation by water under various conditions and sulfur poisoning, this work contributes to the development of more durable SCR catalysts with enhanced resistance to water-induced deactivation and sulfur poisoning for heavy-duty diesel vehicles. In the long term, such improvements will help ensure consistently low real-world emissions throughout the vehicle lifetime, thereby contributing to cleaner transportation and reduced health and environmental impacts.
Cu-CHA catalysts are widely implemented in SCR for heavy-duty engines because they provide high (NOx) conversion to nitrogen and robust performance under operating conditions. However, these catalysts must maintain reliable performance over long lifetimes in a harsh exhaust environment with high temperatures, water, and sulfur oxides, all of which gradually reduce their performance.
This thesis focuses on understanding the mechanisms of Cu-CHA catalyst deactivation under real-world operating conditions. By gaining deeper insight into catalyst deactivation by water under various conditions and sulfur poisoning, this work contributes to the development of more durable SCR catalysts with enhanced resistance to water-induced deactivation and sulfur poisoning for heavy-duty diesel vehicles. In the long term, such improvements will help ensure consistently low real-world emissions throughout the vehicle lifetime, thereby contributing to cleaner transportation and reduced health and environmental impacts.
KCK - Kompetenscentrum Katalys 2022-2026
Umicore Denmark ApS (KCK2022-2026), 2022-01-01 -- 2026-12-31.
Preem (KCK2022-2026), 2022-01-01 -- 2026-12-31.
Johnson Matthey (2500123383), 2022-01-01 -- 2026-12-31.
Volvo Group (PO:2435702-000), 2022-01-01 -- 2026-12-31.
Scania AB (Dnr:2021-036543Pnr:52689-1), 2022-01-01 -- 2026-12-31.
Cu-CHA zeolite-based catalysts for the selective catalytic reduction of NOx in exhaust diesel gas: addressing the issue of Sulfur Stability (CHASS)
European Commission (EC) (EC/H2020/955839), 2021-06-01 -- 2025-05-31.
Subject Categories (SSIF 2025)
Materials Chemistry
Theoretical Chemistry
Physical Chemistry
Infrastructure
C3SE (-2020, Chalmers Centre for Computational Science and Engineering)
DOI
10.63959/chalmers.dt/5802
ISBN
978-91-8103-345-8
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5802
Publisher
Chalmers
PJ-salen, Origohuset, Fysik, Chalmers.
Opponent: Prof. William S. Epling, Department of Chemical Engineering, University of Virginia, USA