Ammonia mobility in chabazite: insight into the diffusion component of the NH3-SCR process.
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) m(2) 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 Cu(2+) 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.
University College London, Department of Chemistry, Materials Chemistry, Third Floor, Kathleen Lonsdale Building, Gower Street, London WC1E 6BT, UK. a.o'malley@ucl.ac.uk and UK Catalysis Hub, Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire OX11 0FA, UK.
Alexander J
AJ
O'Malley
Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, UK. paul.collier@matthey.com.
Iain
I
Hitchcock
Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, UK. paul.collier@matthey.com.
Misbah
M
Sarwar
ISIS Facility, STFC Rutherford Appleton Laboratory, Didcot, Oxon OX11 0QX, UK.
Ian P
IP
Silverwood
Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, UK. paul.collier@matthey.com.
Sheena
S
Hindocha
University College London, Department of Chemistry, Materials Chemistry, Third Floor, Kathleen Lonsdale Building, Gower Street, London WC1E 6BT, UK. a.o'malley@ucl.ac.uk and UK Catalysis Hub, Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire OX11 0FA, UK.
C Richard A
CR
Catlow
Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, UK. paul.collier@matthey.com.
Andrew P E
AP
York
UK Catalysis Hub, Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire OX11 0FA, UK and Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, UK. paul.collier@matthey.com.
P J
PJ
Collier