Comparing ammonia diffusion in NH₃-SCR zeolite catalysts: a quasielastic neutron scattering and molecular dynamics...

Area of Science:

• Materials Science
• Catalysis
• Chemical Engineering

Background:

• Ammonia diffusion is critical for NH3-SCR catalysts used in NOx abatement.
• Levynite (LEV) is a potential NH3-SCR catalyst, while chabazite (CHA) is more established.
• Understanding intracrystalline diffusion dynamics is key for catalyst optimization.

Purpose of the Study:

• To measure and compare ammonia diffusion in LEV and CHA zeolites.
• To investigate the influence of zeolite framework topology on diffusion.
• To correlate pico- and nanoscale diffusion dynamics for catalyst design.

Main Methods:

• Quasielastic neutron scattering (QENS) experiments were performed at 273, 323, and 373 K.
• Molecular dynamics (MD) simulations were used to model diffusion on the nanoscale.
• Diffusion coefficients (Ds) were measured and compared between LEV and CHA.

Main Results:

• QENS indicated similar jump diffusion rates through 8-ring windows in both LEV and CHA.
• MD simulations revealed approximately twice the nanoscale diffusivity in CHA compared to LEV.
• The difference is attributed to CHA having more 8-ring windows per cage unit than LEV.

Conclusions:

• Ammonia mobility differs significantly between pico- and nanoscale in LEV and CHA.
• The number of diffusion pathways (8-ring windows) in the zeolite framework is crucial for overall diffusivity.
• Probing both timescales is essential for effective NH3-SCR catalyst design and porous material studies.