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Issue 17, 2018
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Comparing ammonia diffusion in NH3-SCR zeolite catalysts: a quasielastic neutron scattering and molecular dynamics simulation study

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Abstract

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 (Ds). 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 ∼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 building unit, compared to only three such windows in the LEV cage building 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 Ds. The techniques illustrate the importance of probing both pico- and nanoscale dynamics when studying intracrystalline diffusion in both NH3-SCR catalyst design, and in porous materials generally, where notable consistencies and differences may be found on either scale.

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

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Publication details

The article was received on 13 Feb 2018, accepted on 28 Mar 2018 and first published on 29 Mar 2018


Article type: Paper
DOI: 10.1039/C8CP01022F
Citation: Phys. Chem. Chem. Phys., 2018,20, 11976-11986
  • Open access: Creative Commons BY license
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    Comparing ammonia diffusion in NH3-SCR zeolite catalysts: a quasielastic neutron scattering and molecular dynamics simulation study

    A. J. O'Malley, M. Sarwar, J. Armstrong, C. R. A. Catlow, I. P. Silverwood, A. P. E. York and I. Hitchcock, Phys. Chem. Chem. Phys., 2018, 20, 11976
    DOI: 10.1039/C8CP01022F

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