4.6 Article

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

Journal

PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 20, Issue 17, Pages 11976-11986

Publisher

ROYAL SOC CHEMISTRY
DOI: 10.1039/c8cp01022f

Keywords

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Funding

  1. Ramsay Trust
  2. Engineering and Physical Sciences Research Council (EPSRC) under Centres for Doctoral Training scheme [EP/G036675/1]
  3. EPSRC [EP/K009567/2, EP/K014706/1, EP/K014668/1, EP/K014854/1EP/K014714/1, EP/M013219/1]
  4. EPSRC [EP/K014714/1] Funding Source: UKRI
  5. NERC [NE/R009376/1] Funding Source: UKRI

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The diffusion of ammonia in the small pore zeolite and potential commercial NH3-SCR catalyst levynite (LEV) was measured and compared with its mobility in the chabazite (CHA) topology (more established in NOx abatement catalysis), using quasielastic neutron scattering (QENS) and molecular dynamics (MD) simulations at 273, 323 and 373 K. The QENS experiments suggest that mobility in LEV is dominated by jump diffusion through the 8-ring windows between cages (as previously observed in CHA) which takes place at very similar rates in the two zeolites, yielding similar experimental self-diffusion coefficients (D-s). After confirming that the same characteristic motions are observed between the MD simulations and the QENS experiments on the picosecond scale, the simulations suggest that on the nanoscale, the diffusivity is higher by a factor of similar to 2 in the CHA framework than in LEV. This difference between zeolites is primarily explained by the CHA cages having six 8-ring windows in the budding unit, compared to only three such windows in the LEV cage budding unit, thereby doubling the geometric opportunities to perform jump diffusion between cages (as characterised by the QENS experiments) leading to the corresponding increase in the MD calculated D-s. The techniques illustrate the importance of probing both pico- and nanoscale dynamics when studying intracrystaine diffusion in both NH3-SCR catalyst design, and in porous materials generally, where notable consistencies and differences may be found on either scale.

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