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SSPC16, Grenoble, 09/12
Exploring the dynamics of ionic clathrate hydratesExploring the dynamics of ionic clathrate hydrates
A. Desmedt Groupe de Spectroscopie MoléculaireInstitut des Sciences Moléculaires (ISM - UMR5255) CNRS - Univ. Bordeaux I351 Cours de la Libération, 33405 Talence Cedex
L. Bedouret – ISM / Institut Laue LangevinJ. Ollivier, M.A. Gonzales, J. Combet, M. Johnson – ILL, GrenobleP. Judeinstein – ICMMO, Orsay / Laboratoire Léon Brillouin, SaclayF. Stallmach, J. Kärger - University of Leipzig, GermanyR.E. Lechner - HZB, Berlin, GermanyD. Cavagnat, F. Guillaume, J.C. Lassègues - ISM
Clathrates hydrates and scientific cases?Clathrates hydrates and scientific cases?
MethodologyMethodology
Observing the proton conductivityObserving the proton conductivity
Microscopic mechanisms of proton delocalizationMicroscopic mechanisms of proton delocalization
Concluding remarksConcluding remarks
OutlinesOutlines
SSPC16, Grenoble, 09/12
Clathrate hydrates and scientific cases?Clathrate hydrates and scientific cases?
SSPC16, Grenoble, 09/12
Clathrates hydrates ?Clathrates hydrates ?
Building host cages from H-bonded water molecules
Various clathrates structures
512 (7.8Å)
512 62 (8.7Å)
512 64 (9.5Å)
512 68 (11.4Å)43 56 63 (8.1Å)
46H2O
136H2O
34H2O
Guest molecules
Water molecules
Stable only with guests
SSPC16, Grenoble, 09/12
Scientific casesScientific cases
Energy sourceEnergy
Large quantities of natural
gas (methane) hydrates in
seafloor and permafrost
Environment
Methane, green-house gas
Sub-sea avalanches…
«Hydrate gun hypothesis »
Marine CO2 sequestration
Technological
Blocking the pipelines
Gas storage/transportation
Cool storage application
Fundamental research
Clathrates in the universe (Titan, Mars, comets, etc…)
Formation, decomposition and inhibition
“Glass-like” thermal conductivity, protonic conductivity
Chemical species isolated in “identical” environments
Understanding HOST-GUEST interactions
SSPC16, Grenoble, 09/12
Scientific casesScientific cases
DYNAMICS OF CLATHRATES HYDRATES
Ionic clathrates hydratesHost defects dynamics and protonic conductivity
Guest dynamics in clathrates hydratesProbing the potential surface of the cage
Binary clathrates hydratesHydrogen storage mechanisms
SSPC16, Grenoble, 09/12
+
SSPC16, Grenoble, 09/12
Ionic clathrate hydrates?Ionic clathrate hydrates?
Strong acid clathrate hydrates: anions (PF6-,ClO4-,BF4-…) within cationic cages (H2O and H3O+)
J.H. Cha et al, J. Phys. Chem C 112,13332 (2008) // D. Mootz et al, J. Am. Chem. Soc. (1987)
Structure SICubic (a ~12Å)
Pm3n2(512) + 6(51262)1Guest – 5.75H2O
Structure SVIICubic (a ~7.7Å)
Im3m2(4668)
1Guest - 6H2O
Phase transitions of HPF6 – xH2O
Protonic conductivity?Protonic conductivity?
Strong acid clathrates hydrates: HPF6 – 6H2O (structure VII) example
J.H. Cha et al, J. Phys. Chem C 112,13332 (2008)SSPC16, Grenoble, 09/12
elementary mechanismsof the proton conductivity?
MethodologyMethodology
Source: Forschung mit Neutronen - Status und Perspektiven, KFN
SSPC16, Grenoble, 09/12
MethodologyMethodology
0 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100Energy Transfer [eV]
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
Momentum Transfer [Å-1]
RAMAN
NEUTRONDIFFR
ACTION
Ab-initio or Classical Molecular Dynamics
Multi-technique approach
Molecular Dynamics simulations + Neutron Scattering experiments a common space
NMR
SSPC16, Grenoble, 09/12
MethodologyMethodology
Quasi-elastic Neutron Scattering: some basics
0 0,E kr
,S S
E kr
Qr
∂Ω
0SE Eω = −h
Energy transfer
Momentum transfer
0SQ k k= −r rr
h h h
),(**2
ωσω
σQSN Scat
r≈
∂Ω∂
∂
Measured Intensity Scattering cross section The scattering lawcontains the “Physics”
SSPC16, Grenoble, 09/12
sample
MethodologyMethodology
Quasi-elastic Neutron Scattering: some basics
Coherent scatteringCollective behavior (structure, phonons…)
Incoherent scatteringIndividual behavior (self-diffusion…)
( ) ( )( , ) ( , )
2 2
i Qr t i Qr tcoh incs
G r t e drdt G r t e drdtω ωσ σ
π π− −= +∫ ∫
r rr rr r r r
2 2 2
coh incd d d
σ σ σ
ω ω ω
∂ ∂ ∂= +
∂ Ω ∂ Ω ∂ Ω
Scattering cross sections σcoh , σinc Depend only on the sample isotopic composition Selective deuteration (incoh/coh “chemical switch”)
G(r,t), correlation function (van Hove formalism): self [Gs(r,t)] + distinct [G(r,t)] correlations
∫−= dtrdetrGQS
trQi rrr rr)(
),(2
1),(
ω
πω
D H
σσσσCoh
(barns) 5. 2.
σσσσInc
(barns) 2. 80.
SSPC16, Grenoble, 09/12
MethodologyMethodology
Quasi-elastic Neutron Scattering
Q [Å -1] ħω[meV]
Coherent scatteringChecking clathrate structure
Sexp(Q,ω) Incoherent scatteringGuest molecule dynamics
Elastic Incoherent Structure Factor (EISF) Geometry of motion (form factor)
Quasi-elastic broadening (HWHM) relaxation time
Sexp(Q,ω) = S(Q,ω) ⊗ R(Q,ω)
ħω [meV]
EISF
HWHM
At a given Q
SSPC16, Grenoble, 09/12
MethodologyMethodology
MD and QENS: a “simple” way for comparing experimental and theoretical results
Common observable
QENS Scattering law MD Atomic trajectories
Q
t
Intermediate scattering function:
I(Q,t)
Q
ħω
“Theoretical”MD-derived scattering law:
S(Q,ω)
Q
ħω
“Experimental”MD-derived scattering law:Sexp(Q,ω) = S(Q,ω) ⊗ R(Q,ω)
( )1( , ) ( , )
2
i Qr tS Q G r t e drdt
ωωπ
−= ∫r rr r r
( , )G r tr
SSPC16, Grenoble, 09/12
Structure, dynamics and conductivityStructure, dynamics and conductivity
Strong acid clathrates hydrates (protonic conduction): HClO4 – 5.5H2O (structure I)
SSPC16, Grenoble, 09/12
T (K)
Solid phase II
Solid phase I
Liquid phase
0.00
0.20
0.40
0.60
0.80
1.00
50 80 110 140 170 200 230 260 290
Conductivity*
Elastic intensity
Backscattering spectrometer IN10 @ILL, Q = 1.35Å-1, ∆E = 1µeV* T.-H. Huang et al, J. Phys. Chem. 92 (1988) 6874
Decomposing the QENS scattering law of ionic clathrate hydratesDecomposing the QENS scattering law of ionic clathrate hydrates
SSPC16, Grenoble, 09/12
0.0
2.0
4.0
-12 -8 -4 0 4 8 12
Série1
Série2
Série3
Série4
Energy transfer (meV)
S (Q
, ωω ωω) [a.u.]
Experimental points
Fitted spectrum
( , ) ( , ) ( , )LD LM
S Q S Q S Qω ω ω= ⊗r r r
Long-range proton diffusion(observation
of the proton conductivity)
Localized diffusive motions(microscopic mechanismsof the proton conductivity)
HClO4 – 5.5H2O @ T = 220K
Backscattering spectrometer IN10 @ILL, Q = 1.96Å-1, ∆E = 1µeV
∆E ≈ 1 µeV
Long range diffusion mechanismsLong range diffusion mechanisms
Translational HWHM of HClO4 clathrate hydrate
Isotropic jump diffusion (Chudlley-Elliot model) between oxygen sites
SSPC16, Grenoble, 09/12
1H PFG NMR: Self-Diffusion Coefficient Dt = 3.5 10-8 cm²/s at T = 220K
QENS: mean jump distance <d> = 2.77 Å and mean residence time <ττττ> = 3.7 ns
In clathrate hydrate (Type I): O...O = 2.68 - 2.93 Å
∆t(Q) (µeV)
0,00
0,05
0,10
0,15
0,20
0,25
0 1 2 3 4 5
HWHM2(µeV)
Ds.Q² (µeV)
HWHM (µeV)
∆E ≈ 1 µeVIN10@ILL
Low-Q limit: Dt(Q) ≈ Dt . Q² (Fick law)
Experimental points
Isotropic Translational Jump-Diffusion Model
Q² [Ų]
A. Desmedt et al, J. Chem. Phys. 121(23) (2004) 11916
Observing the proton conductivityObserving the proton conductivity
Strong acid clathrates hydrates (protonic conduction): anionic guest within cationic cages
* K. Shin et al, Chem. Asian J. 5, 22 (2010) A. Desmedt et al J. Chem. Phys., 121, 11916 (2004) // L. Bedouret, ILL-CNRS PhD Bordeaux 1
HPF6 – 6H2O (type VII)
Conductivity measurement*EA = 9.6 kJ.mol-1
HClO4 – 5.5H2O (type I)
Conductivity measurement*EA = 33.7 kJ.mol-1
SSPC16, Grenoble, 09/12
Microscopic mechanisms of proton Microscopic mechanisms of proton
delocalizationdelocalization
SSPC16, Grenoble, 09/12
0.0
2.0
4.0
-12 -8 -4 0 4 8 12
Série1
Série2
Série3
Série4
Microscopic mechanisms of proton Microscopic mechanisms of proton
delocalizationdelocalization
1 1 –– QENS investigation of QENS investigation of HClO4 clathrate hydrate
SSPC16, Grenoble, 09/12
0.0
2.0
4.0
-12 -8 -4 0 4 8 12
Série1
Série2
Série3
Série4
Various QENS componentsVarious QENS components
QENS spectra of the HClO4 – 5.5H2O clathrate hydrate (structure I) at 220K and Q = 1.96 Å-1
SSPC16, Grenoble, 09/12
Back-scattering spectrometer IN10@ILL∆∆∆∆E ≈≈≈≈ 1 µµµµeV
Tof spectrometer NEAT@HZB∆∆∆∆E ≈≈≈≈ 100 µµµµeV
Energy transfer [meV]
-1 -0.5 0 0.5 1
0,01
S (Q, w
) [a.u.]
Localized motionHWHM ~180µeV
0.0
2.0
4.0
-12 -8 -4 0 4 8 12
Série1
Série2
Série3
Série4
Localized motionHWHM ~ 2µeV
Several QENS components several relaxation processes
Energy transfer [µeV]
-1000 -500 0 500 1000 -12 -8 -4 0 4 8 12
Resolution dependent EISF of localized diffusive motionsResolution dependent EISF of localized diffusive motions
Strong acid clathrates hydrates (protonic conduction): HClO4 – 5.5H2O (structure I)
SSPC16, Grenoble, 09/12
Q = 1.0 Å-1
Q = 1.4 Å-1
Q = 1.8 Å-1
T = 220K
Tof Spectrometer (NEAT@HZB]
10-4
10-3
10-2
10-1
100
101
0.5
0.6
0.7
0.8
0.9
1.0
EIS
F
1/∆E (µeV-1)
Back-scattering spectrometer (IN10@ILL)
Two relaxation processes (at least) 75% of protons in H2O and 25% of protons in H3O+
H2O
H3O+
SSPC16, Grenoble, 09/12
Modeling the localized diffusive motionsModeling the localized diffusive motions
Hydronium ion: 3 protons / 8 sites
proton diffusion in H-bonds:
2 sites jump (ττττH-1)
reorientations:
C2, C3 or tetrahedral jump (ττττH3O-1)
( )( , ) ( , ) 1 ( , )LM water Hydronium
S Q f S Q f S Qω ω ω= + −r r r
Water molecule: 2 protons / 4 sites
reorientations:
C2, C3 or tetrahedral jump (ττττH2O-1)
C2 C3
ττττH-1
ττττH2O-1
ττττH3O-1
jump distance = 1.6Å
HH22O and HO and H33OO++ localized motions in localized motions in HClO4 – 5.5H2O at 220K
SSPC16, Grenoble, 09/12
-0.012 -0.008 -0.004 0 0.004 0.008 0.012
Energy Transfer (meV)
S (Q, ω)
∆∆∆∆E ≈≈≈≈ 1 µeVQ = 1.81Å-1
IN10@ILL
HWHM´s:
∆∆∆∆t(Q) + ∆∆∆∆H2O
∆t(Q) + ∆H3O+
∆t(Q) + ∆H3O+ + ∆H
∆t(Q) + ∆H
2 sites jump model
Water molecules reorientations
Jump rate τH2O-1 = 1.5 ns-1 (∆H2O = 2µeV)
Jump distance dH2O = 1.4 Å
HH22O and HO and H33OO++ localized motions in localized motions in HClO4 – 5.5H2O at 220K
SSPC16, Grenoble, 09/12
Energy Transfer (meV)
Hydronium ions reorientations
Jump rate τH3O-1 = 24 ns-1 (∆H3O = 63µeV)
Jump distance dH3O = 1.3 Å
tetrahedral jump model
S (Q, ω)
HWHM´s:
∆t(Q) + ∆H2O
∆∆∆∆t(Q) + ∆∆∆∆H3O+
∆t(Q) + ∆H3O++ ∆H
∆t(Q) + ∆H
∆∆∆∆E ≈≈≈≈ 50 µeVQ = 1.50 Å-1
NEAT@HZB
-1 -0.5 0 0.5 1 1.5 2
HH22O and HO and H33OO++ localized motions in localized motions in HClO4 – 5.5H2O at 220K
SSPC16, Grenoble, 09/12
Energy Transfer (meV)
Proton transfert within H-bond
Jump rate τH-1 = 0.7 ps-1 (∆H = 932µeV)
Jump distance dH = 0.9 Å
S (Q, ω)
HWHM´s:
∆t(Q) + ∆H2O
∆t(Q) + ∆H3O+
∆t(Q) + ∆H3O++ ∆H
∆∆∆∆t(Q) + ∆∆∆∆H
-2 -1 0 1 2 3
∆∆∆∆E ≈≈≈≈ 300 µeVQ = 2.00 Å-1
NEAT@HZB
2 sites jump model
Microscopic mechanisms of proton Microscopic mechanisms of proton
delocalizationdelocalization
2 2 –– abab--initio MD / QENS investigation of initio MD / QENS investigation of HPF6 clathrate hydrate
SSPC16, Grenoble, 09/12
0.0
2.0
4.0
-12 -8 -4 0 4 8 12
Série1
Série2
Série3
Série4
AbAb--initio MD of the HPFinitio MD of the HPF6 6 -- 6H6H220 clathrate hydrate0 clathrate hydrate
SSPC16, Grenoble, 09/12
Ab-initio MD (VASP): - 1 SVII unit cell (i.e 12 H2O + 2HPF6) with boundary conditions- DFT calculation with PAW - PBE functional in NVT ensemble- timestep of 1fs with 500ps MD
Proton transfer in H-bond (300K)
L. Bedouret, ILL-CNRS PhD Bordeaux 1
Characteristic time of proton transfer < 0.5ps
AbAb--initio MD of the HPFinitio MD of the HPF6 6 -- 6H6H220 clathrate hydrate0 clathrate hydrate
SSPC16, Grenoble, 09/12
Reorientations of H2O molecules and H3O+ ions at 300K (from ab-initio MD trajectories)
L. Bedouret, ILL-CNRS PhD Bordeaux 1
Water reorientations: ττττ ~ 30ps Hydronium reorientations: ττττ ~ 10ps
H3O+H2O
Q = 1.80 Å-1
MD-derived Intermediate Scattering Function Reorientational Structure Factor
H2OMD-derived EISF H3O+MD-derived EISFModels
2 sites (d=1.6Å)
3 sites (d=1.6Å) / tetrahedral jump (d=1.45Å)
tetrahedral jump (d=1.6Å)
17.4 1.5 kJ.mol -1
16.2 1.0 kJ.mol -1
11.4 0.7 kJ.mol-1
H3O+
H2O
Proton transfer in H-bond
QENS investigationQENS investigation
SSPC16, Grenoble, 09/12
Measured jump rates by means of QENS experiments
L. Bedouret, ILL-CNRS PhD Bordeaux 1
HPF6 – 7.67H2O (type VII) HClO4 – 5.5H2O (type I)
Slowest localized motion: water reorientations Reorientational jump distance < 1.6Å oxygen disorder
2 sites (d=0.9Å)
2 sites (d=1.45Å)
tetrahedral (d=1.6Å)
2 sites (d=1.0Å)
tetrahedral jump (d=1.3Å)
Proton transfer in HProton transfer in H--bond: relaxation or excitation?bond: relaxation or excitation?
SSPC16, Grenoble, 09/12
QENS spectra recorded on the HPF6-7.67H2O clathrate hydrate
L. Bedouret, ILL-CNRS PhD Bordeaux 1
∆∆∆∆E ≈≈≈≈ 100 µeV, Q = 2.23Å-1, T = 280K, IN5@ILL
Mode at ca. 1.3meV: proton rattling in H-bond?
H3O+
H2O
Concluding remarksConcluding remarks
Methodology:
Multi-technique approach required (broad timescale)
Appropriated experimental technique: neutron scattering, NMR, diffraction…
ab-initiomolecular dynamics:- a guide for interpreting the experimental data- large system simulations required
Summary:
Measurements of the long-range proton diffusion. Activation energies in agreement with conductivity measurements. Nanosecond timescale.
Quantitative experimental analysis of the elementary steps in ice-like systems.Picosecond timescale.
Organization of hydronium environment:Water molecules reorientation, the limiting step.Fluctuations of the oxygen-oxygen distance.
Observation of proton “rattling” in H-bond?
ab-initio MD: qualitative analysis in agreement with experimental results
SSPC16, Grenoble, 09/12
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