Ammonia mobility in chabazite: insight into the diffusion component of the NH 3-SCR process

Alexander J. O'Malley, Iain Hitchcock, Misbah Sarwar, Ian P. Silverwood, Sheena Hindocha, C. Richard A. Catlow, Andrew P. E. York, P. J. Collier

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Abstract

The diffusion of ammonia in commercial NH3-SCR catalyst Cu-CHA was measured and compared with H-CHA using quasielastic neutron scattering (QENS) and molecular dynamics (MD) simulations to assess the effect of counterion presence on NH3 mobility in automotive emission control relevant zeolite catalysts. QENS experiments observed jump diffusion with a jump distance of 3 Å, giving similar self-diffusion coefficient measurements for both Cu- and H-CHA samples, in the range of ca. 5–10 × 10−10 m2 s−1 over the measured temperature range. Self-diffusivities calculated by MD were within a factor of 6 of those measured experimentally at each temperature. The activation energies of diffusion were also similar for both studied systems: 3.7 and 4.4 kJ mol−1 for the H- and Cu-chabazite respectively, suggesting that counterion presence has little impact on ammonia diffusivity on the timescale of the QENS experiment. An explanation is given by the MD simulations, which showed the strong coordination of NH3 with Cu2+ counterions in the centre of the chabazite cage, shielding other molecules from interaction with the ion, and allowing for intercage diffusion through the 8-ring windows (consistent with the experimentally observed jump length) to carry on unhindered
Original languageEnglish
Pages (from-to)17159-17168
Number of pages10
JournalPhysical Chemistry Chemical Physics
Volume18
Issue number26
Early online date1 Jun 2016
DOIs
Publication statusPublished - 14 Jul 2016

Cite this

O'Malley, A. J., Hitchcock, I., Sarwar, M., Silverwood, I. P., Hindocha, S., Catlow, C. R. A., York, A. P. E., & Collier, P. J. (2016). Ammonia mobility in chabazite: insight into the diffusion component of the NH 3-SCR process. Physical Chemistry Chemical Physics, 18(26), 17159-17168. https://doi.org/10.1039/C6CP01160H