Cleaner Car Exhausts Using Ion-Exchanged Zeolites: Insights From Atomistic Simulations

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1 CLEERS conference, Ann Arbor, Michigan 20 th September 2018 Cleaner Car Exhausts Using Ion-Exchanged Zeolites: Insights From Atomistic Simulations A Combined Quasi Elastic Neutron Scattering (QENS) and Molecular Dynamics (MD) Study of Diffusion of NH 3 in Zeolites Alex O Malley 1,2,3, Ian Silverwood 4, Iain Hitchcock 5, Jeff Armstrong 4, Misbah Sarwar 5, Sheena Hindocha 5, Andrew York 5, Richard Catlow 1,2,3 Paul Collier 2,5 1. UCL, 2, Catalysis Hub, Harwell, 3, Cardiff University, 4,ISIS facility, Harwell, 5. JM Tech Centre, Sonning

2 Outline Molecular Modelling at Johnson Matthey Diffusion in zeolites NH 3 diffusion in CHA NH 3 diffusion in CHA compared to LEV Summary 2

3 Multiscale Modelling at Johnson Matthey Catalysis is a multi-scale problem years hours minutes Finite Elements Engineering Design Real World s ms ns ps fs Quantum Mechanics Molecular Mechanics Mesoscale Modelling FF-MD Electronic Structure Kinetics and microstructure/ Coarse Graining Å nm µm mm m 3

4 What do JM use Computational Chemistry for? Insight: how do our materials function? Mechanism Effect of reaction environment Effect of reaction conditions? Diffusion Screening: Scan phase space to predict new catalysts/materials fewer experiments/the right experiments Aid Characterisation: SS-NMR IR PDF XRD. 4

5 Ds (m2/s) Diffusion in Zeolites Focus on NH 3 -SCR: 4NO + 4NH 3 + O 2 4N 2 + 6H 2 O in Cu zeolites Empty Pore desorption adsorption Occupied Pore diffusion Catalytic Cycle of Zeolite Catalysis Occupied Active Site Occupied Pore diffusion How can we validate these results? 7E-09 6E-09 5E-09 4E-09 3E-09 2E-09 1E-09 0 A B C D E F G H I J Framework Adapted from Theoretical Heterogeneous Catalysis R.A. van Santen 5

6 Techniques to measure diffusion in zeolites Configurational Diffusion regime in zeolite micropores QENS and PFG-NMR techniques probe same length and time scales as MD simulations Adapted from Douglas M. Ruthven, Martin F.M. Post Introduction to Zeolite Science and Practice

7 S(Q, ω) Arb Units The Quasi Elastic Neutron Scattering (QENS) technique During scattering process neutrons transfer momentum to adsorbed molecules. Elastic scattering (sharp) type of diffusive motion (continuous, jump, rotations) Quasi-elastic scattering (broad): diffusion coefficients Energy Transfer mev Width (broadening) related to diffusion coefficient Can fit to models of diffusion for jump-diffusion e.g. Chudley Elliott 7

8 Case Study 1: Diffusion of NH 3 in CHA Does the presence of a counter-ion affect diffusion of NH 3 in CHA? Compare H-CHA with Cu-CHA Things to consider to make model and experiments comparable: Competitive adsorption/pre-adsorbed water Loading of NH 3 molecules Temperature Ion-location (next slide) H-CHA QENS Experiments Heat sample overnight to remove any pre-adsorbed water Loading =? Assume saturation (cell pressure =800bar) T=273K, 323K, 373K (0, 50, 100 C) Molecular Dynamics Simulations Only single component (NH 3 ) modelled Maximum loading : Monte Carlo simulation (90NH 3, 2x2x2 cell) 200ps equilibration, 1ns production, NVT ensemble, 1fs timestep Forcite, COMPASS forcefield T=273K, 323K, 373K Cu-CHA A.J. O Malley et al. PCCP, 2016, 18,

9 Where is the Cu active site? Jillian Collier, Loredana Mantarosie, Kerry Simmance, Dave Thompsett QM/MM approach Region of interest modelled using QM method (DFT DMol3, PW91, dnp) Rest of lattice modelled using interatomic potentials (Hill-Sauer) I Si O Al Cu IV II Cu-O: 1.95, 2.10, 2.12Å III Cu-O: 1.98, 2.11, 1.97Å Cu locations from literature: Site I = plane of 6R / below D6R Site II = cage below Site I Site III = middle of D6R Site IV = plane of 8R 9

10 IR with NO as a probe molecule Ion pulled out of 6R by interaction with NO : indicates some ion mobility IR shifts for NO in 8R higher than in 6R : can be used to distinguish different sites. Site E ads (kj/mol) CN (bare) CN (NO) IR (cm -1 ) 6R R Experimental IR shifts : cm -1 (1,2) Ion placed in 8R for MD simulations (1)Phys. Chem. Chem. Phys., 2013, 15, 2368 (2)ACS Catal. 2014, 4,

11 QENS results Ammonia seems to behave basically the same in both zeolites! Jump distance of 3 Å, similar residence times at each temperature (16-28 ps) How does NH 3 diffuse? Through 8-ring or 6-ring? Why is the copper ion not changing anything.? 11

12 Ds (m 2 /s) Ds (m 2 /s) Comparisons of D s between MD and QENS H-CHA Cu-CHA 6.00E E E E E E E T (K) QENS_H-CHA MD H-CHA 1.6E E E-09 1E-09 8E-10 6E-10 4E-10 2E T (K) QENS Cu-CHA MD Cu-CHA Good agreement between QENS and MD D s MD faster than QENS ideal crystal used in simulation; presence of defects in sample Values for Cu-CHA very close except at high T 12

13 Comparisons of E act between MD and QENS Calculate activation energy for molecule to jump from site to site from Arrhenius plot E act 3.5kJ/mol E act for H-CHA agrees well between methods E act 3.7 kj/mol E act 4.4 kj/mol MD activation energy for Cu-CHA significantly higher Why? E act 10.8kJ/mol 13

14 Comparison of D s from QENS and MD: H-CHA vs Cu-CHA MD QENS MD simulations predict that diffusion should be faster in H-CHA compared to Cu-CHA QENS experiment sees no difference! Why? 14

15 Insights from Molecular Dynamics Simulations How are the molecules diffusing? Diffusion exclusively via the 8R window. NH 3 : 2.6Å 6R: Å 8R: 3.8Å Pathway of NH 3 molecule NH 3 molecule is too big to fit through the 6R but can pass through the 8R s easily 15

16 g(r) Insights from Molecular Dynamics Simulations Clusters of NH 3 coordinated to the Cu 2+. Form a Shell which shields remaining NH 3 from the Cu 2+. Clusters are immobile during the course of the simulation Cu2+-N Al-Cu2+ The coordinated NH 3 move too slowly to be detected by the OSIRIS instrument (movements t> 200ps not detected) Only uncoordinated NH 3 s detected by QENS r (Ȧ) What if we ignore clusters and calculate D s for only the NH 3 s that move? D s Cu-CHA (m 2 /s) 3.69 x10-10 D s Cu-CHA (uncoord) (m 2 /s) 5.76x10-9 D s H-CHA (m 2 /s) 3.23x10-9 Ds Cu-CHA (uncoord)~ds H-CHA : better agreement with QENS E act (uncoord) = 10.1 kj/mol (lowered from 10.7 kj/mol) 16

17 Summary QENS experiments and MD simulations have been used to investigate diffusion of NH 3 in CHA Good agreement between diffusion coefficients between techniques Good agreement in E act for H-CHA Discrepancy in E act in Cu-CHA by QENS and MD due to presence of Cu-NH 3 clusters not detected by QENS 17

18 Case Study 2: Diffusion of NH 3 in LEV and comparisons with CHA Small pore zeolites have been shown to be beneficial for NH 3 -SCR Compare to another zeolite in the ABC-6 family CHA (3D) LEV(2D) Does dimensionality of the zeolite affect diffusion of NH 3? Compare diffusion of H-CHA with H-LEV by QENS and MD A.J. O Malley et al PCCP, 2018, 20,

19 Comparisons of D s between MD and QENS Loading = 90NH 3 for CHA and LEV Simulations extended to 5ns for both MD predicts that diffusion should be faster in CHA compared to LEV QENS sees no difference (within error) 19

20 QENS Spectra There is a significant elastic component at all temperatures Is this due to: Localised motion such as rotation? Or Translational diffusion with a component of NH 3 that is static? Or..a combination of both? Can characterise by computing the Elastic Incoherent Structure Factor (EISF) 20

21 Elastic Incoherent Structure Factor (EISF) Tried to fit to 3 rotational models All poor fit to EISF Best matching shape is model for diffusion confined to a sphere of r=3.5å Rotation combined with translation Shape of experimental EISF not replicated at 273K and 323K but is close at 373K Can discount significant contribution from rotational motion at lower T s but not at 373K 21

22 Dipole Autocorrelation Function from MD What kind of motion do we have in the simulation? Fast decay especially at 373K Suggests that the rotational relaxation is too fast to make a significant contribution to the spectra Elastic contribution caused by molecules that are static on the timescale probed by instrument Observing translational motion with an immobile fraction rather than NH 3 rotation 22

23 MSD (Ȧ 2 ) MSD (Å 2 ) Differences in D s : Mean Squared Displacement 4000 CHA LEV Time (ps) Time (ps) CHA: Diffusion in 3 directions LEV: Diffusion only in 2 directions. No diffusion in {001} (z). However even in {100} and {010} directions diffusion in LEV is slower than in CHA Why? 23

24 Comparisons of CHA and LEV cages CHA structure : 108 atoms : 80% 8R s LEV structure : 92 atoms : <50% 8R s R s which NH 3 can diffuse through 3 8R s which NH 3 can diffuse through For a small increase in size from CHA to LEV (108 vs 92) there is double the opportunity for NH 3 to leave the cage. 24

25 Diffusion pathway -trajectory plots Side Top Side Top CHA 6 pathways LEV 3 pathways Slower diffusion in {100} and {010} directions in LEV may be related to fewer pathways 25

26 Differences between QENS and MD techniques MD: Diffusion in CHA faster than LEV QENS: Diffusion ~ same QENS is probing localised motion at the picoscale MD simulations probe both the pico and nanoscale At the picoscale the diffusion of NH 3 in CHA and LEV are very similar. However at the nanoscale the differences become more marked. 26

27 Summary A comparison of D s in CHA and LEV was made by QENS and MD simulations MD simulations suggest that diffusion of NH 3 in CHA should be faster than in LEV but QENS results suggest diffusion is essentially the same. QENS experiments and MD simulations observe translational diffusion of NH 3. Contribution from rotational motion discounted. Differences in D s between techniques can be attributed to time-scale being probed. At the picoscale the diffusion between frameworks is similar At the nanoscale (MD) the diffusion is markedly different Differences in diffusion coefficients at the nanoscale can be related to number of 8R s in both structures Techniques are complimentary Future Work: effect of NH 3 loading implication of NH 3 clustering effect of hydrocarbon on NH 3 diffusion effect of other NH 3 -SCR species? NO, NO 2, H 2 O 27