exploring the dynamics of ionic clathrate...

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


Clathrate hydrates and scientific cases?Clathrate hydrates and scientific cases?


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


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


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


+


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?


Source: Forschung mit Neutronen - Status und Perspektiven, KFN



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



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”


sample


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.



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



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


Structure, dynamics and conductivityStructure, dynamics and conductivity

Strong acid clathrates hydrates (protonic conduction): HClO4 – 5.5H2O (structure I)


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


0.0

2.0

4.0

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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


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 [Å²]

A. Desmedt et al, J. Chem. Phys. 121(23) (2004) 11916


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


Microscopic mechanisms of proton Microscopic mechanisms of proton

delocalizationdelocalization


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1 1 –– QENS investigation of QENS investigation of HClO4 clathrate hydrate


0.0

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Various QENS componentsVarious QENS components

QENS spectra of the HClO4 – 5.5H2O clathrate hydrate (structure I) at 220K and Q = 1.96 Å-1


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

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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)


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+


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


-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




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




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



2 2 –– abab--initio MD / QENS investigation of initio MD / QENS investigation of HPF6 clathrate hydrate


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AbAb--initio MD of the HPFinitio MD of the HPF6 6 -- 6H6H220 clathrate hydrate0 clathrate hydrate


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


Reorientations of H2O molecules and H3O+ ions at 300K (from ab-initio MD trajectories)


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


Measured jump rates by means of QENS experiments


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?


QENS spectra recorded on the HPF6-7.67H2O clathrate hydrate


∆∆∆∆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


Thank you for your attentionThank you for your attention


exploring the dynamics of ionic clathrate...

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