Ammonia mobility in chabazite: insight into the diffusion component of the NH3-SCR process

Area of Science:

• Catalysis
• Materials Science
• Physical Chemistry

Background:

• Ammonia (NH3) selective catalytic reduction (SCR) is vital for controlling automotive emissions.
• Zeolite catalysts, particularly Cu-exchanged chabazite (Cu-CHA), are key in NH3-SCR systems.
• Understanding NH3 diffusion within these catalysts is essential for optimizing performance.

Purpose of the Study:

• To investigate the effect of copper counterions on ammonia diffusion in CHA zeolite.
• To compare ammonia mobility in Cu-CHA and H-CHA catalysts.
• To elucidate the mechanisms governing ammonia diffusion in these systems.

Main Methods:

• Quasielastic neutron scattering (QENS) experiments were performed to measure NH3 diffusion.
• Molecular dynamics (MD) simulations were used to model and compare NH3 diffusion.
• Both methods were applied to commercial Cu-CHA and H-CHA zeolite catalysts.

Main Results:

• QENS revealed jump diffusion of NH3 with a 3 Å jump distance in both Cu-CHA and H-CHA.
• Experimentally measured self-diffusion coefficients for NH3 were similar in both catalysts (5-10 × 10⁻¹⁰ m²/s).
• MD simulations showed good agreement with experimental diffusion coefficients and activation energies, indicating minimal impact of Cu(2+) on NH3 mobility.

Conclusions:

• Counterion presence, specifically Cu(2+), has a limited effect on the overall ammonia diffusivity in CHA zeolite catalysts on the timescale of QENS measurements.
• MD simulations suggest NH3 coordination with Cu(2+) shields the ion, allowing unhindered intercage diffusion through 8-ring windows.
• These findings contribute to the understanding of NH3-SCR catalyst mechanisms for improved emission control technologies.