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Issue 26, 2016
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Ammonia mobility in chabazite: insight into the diffusion component of the NH3-SCR process

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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.

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

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Article information


Submitted
20 Feb 2016
Accepted
31 May 2016
First published
01 Jun 2016

Phys. Chem. Chem. Phys., 2016,18, 17159-17168
Article type
Paper

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

A. J. O'Malley, I. Hitchcock, M. Sarwar, I. P. Silverwood, S. Hindocha, C. R. A. Catlow, A. P. E. York and P. J. Collier, Phys. Chem. Chem. Phys., 2016, 18, 17159
DOI: 10.1039/C6CP01160H

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