Wednesday 10 July 2013, Moorfoot & Kilsyth Rooms, 14:30-16:30
Hydrogen storage and other host-guest systems
Measurements and computation of the quantum dynamics of hydrogen molecules in simple and binary clathratehydrates
L Ulivi1, M Celli2, D Colognesi2, A J Ramirez-Cuesta3, M Xu4 and Z Bačić4
1
CNR-ISC, Italy, 2CNR-ISC, Italy, 3ISIS Facility, Rutherford Appleton Laboratory, UK, 4York University, USA
Hydrogen gas and water may form clathrates hydrates either with themselves (simple clathrates) or together with a
different help substance (binary clathrates). The confinement of the hydrogen molecules is substantially different in
the two cases: in binary clathrates one H2 molecule singly occupies the smaller dodecahedral cages, while in simple
clathrates also the larger cages (hexakaidecahedral) are filled with several (up to four) H2 molecules. The dynamics
of the two kind of molecules is qualitatively different. The Inelastic Neutron Scattering (INS) spectra of binary
clathrate samples exhibit sharp bands arising from both the rotational transitions and the rattling modes of the
guest hydrogen molecule. They agree well with the rigorous fully quantum simulations, which account for the effects
of the anisotropy, angular and radial, of the host cage on the microscopic hydrogen dynamics. On the contrary, the
INS spectrum of the H2 molecules in the large cages of simple clathrate samples, reported and discussed here for
the first time, presents a very different shape, without sharp bands. The theoretical calculation of this spectrum is
still an open problem. In this work we present data measured using two different neutron spectrometers, in order to
derive their dependence on the momentum transfer, and to discuss their temperature dependence, the effects of
the cage geometry, and the different features associated with the ortho-hydrogen and para-hydrogen species.
Dynamics of hydrogen molecules in clathrate hydrate
M Kofu1, T Okuchi2, H Mutka3, J Ollivier3, C Brown4, M Tyagi4 and O Yamamuro1
1
University of Tokyo, Japan, 2Okayama University, Japan, 3Institut Laue-Langevin, France, 4NIST Centre for Neutron
research, USA
Hydrogen hydrate is a sort of clathrate hydrate in which the lightest guest molecule, H2, is accommodated into the
host cages formed by water molecules. The stability of the clathrate hydrates is dominated by the van der Waals
contacts between the guest molecules and the cages. A hydrogen molecule is too small for any of clathrate cages
and so the hydrogen hydrate is stabilized only under pressure higher than 2 kbar. However, it was reported that the
stabilization pressure is much lowered (ca. 100 bar) by adding tetrahydrofuran (THF) as a helper gas, leading
extensive studies on hydrogen hydrates. The H2-THF hydrate has the typical clathrate sII structure and THF
molecules are accommodated in hexakaidecahedral cages, while H2 molecules are in pentagonal dodecahedral
cages. We aim at investigating the dynamics of the H2 molecules in the clathrate hydrate by quasielastic neutron
scattering (QENS) techniques. In order to selectively observe the motion of H2 molecules, THF and water molecules
were fully deuterated. In the QENS measurements on IN5 spectrometer (Eres = 25 micro-eV), we found that THF
molecules move in a wide temperature range between 40 K and 210 K. On the other hand, the motion of H2
molecules appeared above 150 K. The activation energies for the H2 and THF motions are 23 kJmol-1 and 3.9 kJmol1
, respectively. In the presentation, we will also show the data on HFBS spectrometer (Eres = 0.8 micro-eV) and
discuss the motion of the H2 molecules in a wide timescale.
ICNS 2013 International Conference on Neutron Scattering
Inelastic neutron scattering (INS) studies of hydrogen spillover on pure and pd decorated metal oxides
J Z Larese1, S Adak1, C Sumner2, L Daemen3, T Seydel4 and J Ollivier4
1
University of Tennessee, USA, 2Eastman Chemical, USA, 3Los Alamos National LAb, USA 4Institut Laue Langevin,
France
We will discuss our recent investigations using INS and QENS to examine the interaction of hydrogen with pure and
metal decorated metal oxide materials (MO), specifically Al2O3, ZnO and MgO. These MOs find widespread use as
energy materials especially as oxidation and hydrogenation catalysts. Our studies are geared toward illuminating
the microscopic details of the process(es) that underlie the phenomena often referred to as “hydrogen spillover”
(HS) in order to identify what, if any, role it plays in the catalytic cycle. We use HS when referring to the diffusion of
hydrogen from a surface capable of disassociating H2, onto the surface of an adjoining solid. HS from a metal to a
MO or carbon surface is of interest because most commercial catalysts consist of nanometer sized metal clusters
supported on either high surface area MOs or carbon, and many catalytic reactions involve hydrogen. Recent
experiments suggest HS offers a potentially useful route for improving the storage capacity of various metal
decorated high surface area carbon nano materials. Rather than clarifying the microscopic details of HS these
experiments add confusion concerning the presence and role of the HS process. We will present recent INS
observations that show that hydroxyl formation on these MO supports takes place only when the metal catalyst is
present even at low temperatures. Spectral signatures in both the rotational and vibrational portions of the INS
signals underscore this behavior. QENS data establishes that there is substantial translational diffusion at
temperatures well below 50K. Similarities and differences of the various support materials will be highlighted.
Water dynamics in anisotropic materials : coupling QENS and Molecular Dynamics
V Marry1, E Dubois1, N Malikova2, J Breu3, W Haussler4 and S Longeville2
1
Université Pierre et Marie Curie, France, 2Laboratoire Léon Brillouin, France, 3Universität Bayreuth,
Germany, 4Technische Universität München, Germany
The study of transport properties in materials such as clays, which are a main component of soils, is crucial for
environment and industry. Indeed they are used as barriers for the radioactive waste storage and for CO2 storage in
aquifers. Dynamics of water has been determined by Quasi Elastic Neutron Scattering (QENS) in several clay
systems in literature. However the anisotropy of the transport in these lamellar negatively charged alumino-silicates
is seldom considered in the analysis of the experimental results. We show here how to evidence this anisotropy from
the experiments on a hydrated well defined synthetic hectorite clay and also which model should be used for a
relevant interpretation of the experiments. The validity of the models is probed thanks to molecular dynamics
simulations which are essential to interpret the data, as they enable determination of information inaccessible to
experiments.
[1]
[2]
[3]
N. Malikova et al., Signature of low-dimensional diffusion in complex systems, P.R.L. 101 265901(2008)
V. Marry et al, Water diffusion in a synthetic hectorite by neutron scattering - beyond the isotropic
translational model, J. Phys. Cond. Mat. 20 104205 (2008).
V. Marry et al, Water dynamics in anisotropic materials : insights from molecular simulations as direct aid in
experimental analysis, submitted to J. Phys. Chem. C.
ICNS 2013 International Conference on Neutron Scattering
(invited) The structure of nanocarbons in solution by neutron scattering
N T Skipper1, EM Milner1, CA Howard1, P Lindner2, R Schweins2 and R K Heenan3
1
UCL, UK, 2Institut Laue-Langevin, France,3ISIS, STFC Rutherford Appleton Laboratory, UK
Nanostructured forms of carbon such as the fullerenes, nanotubes and graphene are relatively insoluble in common
organic and aqueous solvents. However, these materials can be reversibly reduced by the solvated electrons formed
in metal-ammonia solutions [1], and the resulting nanocarbon salts (fulleride, nanotubide and graphenide) are then
soluble in common aprotic solvents such as THF, DMF and NMP [2-4]. These solutions have great practical potential
as a means to purify and process nanotubes and graphene in particular. Neutron scattering in conjunction with
isotope labelling has proven to be a very powerful method for understanding the structure of both the host metalammonia solutions and also the dissolved nanocarbons. We will discuss the methodology and analysis, including
the use of small angle neutron scattering, necessary to study these complex fluids. In the case of single walled
nanotubes, in situ small-angle neutron scattering demonstrates the presence of dissolved, unbundled nanotubides
in solution, at concentrations reaching at least 2 mg/mL. Our ability to isolate individual nanotubes is confirmed by
atomic force microscopy. In the case of graphenes our experiments show that >95 vol % of the solute is present as
single-layer graphenide sheets. These charged sheets are flat over a length scale of >150 Å in solution and are
strongly solvated by a shell of solvent molecules. Atomic force microscopy on drop-coated thin films corroborated
the presence of monolayer graphene sheets. Our dissolution method thus offers a significant increase in the
monodispersity achievable in graphene solutions.
[1]
[2]
[3]
[4]
H Thompson, JC Wasse, NT Skipper, S Hayama, DT Bowron, AK Soper. Structural Studies of Ammonia and
Metallic Lithium-Ammonia Solutions. J. Am. Chem. Soc., 2003, 125, 2572–2581.
CA Howard, H Thompson, JC Wasse NT Skipper. Formation of Giant Solvation Shells around Fulleride
Anions in Liquid Ammonia. J. Am. Chem. Soc., 2004, 126, 13228–13229.
S Fogden, CA Howard, RK Heenan, NT Skipper, MSP Shaffer. Scalable Method for the Reductive
Dissolution, Purification, and Separation of Single-Walled Carbon Nanotubes. ACS Nano, 2012, 6, 54–62
EM Milner, NT Skipper, CA Howard, MSP Shaffer, DJ Buckley, KA Rahnejat, PL Cullen, RK Heenan, P
Lindner, R Schweins. Structure and Morphology of Charged Graphene Platelets in Solution by Small-Angle
Neutron Scattering. J. Am. Chem. Soc., 2012, 134, 8302–8305.+
Growth of highly oriented crystals in silicon nanochannels
T Hofmann and D Wallacher
Helmholtz-Zentrum Berlin, Germany
In recent years, nanostructured templates emerged as promising tools to tailor the microscopic structure and thus
the performance of functional materials. In this context, a comprehensive understanding of the principles that guide
the solidification of liquids or melts in confining spaces was identified as a basic prerequisite towards structure,
morphology and polymorphism control on nanometer-sized length scales. Confinement effects, critical nuclei sizes,
fast growing crystal directions, interfacial energies, and the microscopic structure of the patterned template itself
have been discussed to influence the solidification. Condensed gases inside porous substrates provide a tempting
and easily accessible route to study these effects and their interplay in detail. Here we present an elastic neutron
scattering study to elucidate the solidification of selected pore-condensates e.g. D2, Ne, Ar, Kr, and Xe. An array of
9nm wide tubular pores in anodized Si membranes was utilized to ascertain stable crystallographic phases and
preferred growth directions of crystals upon spatial confinement. Heterogeneous liquid phase epitaxy was identified
to guide the crystallization of hydrogen isotopes [1] and other gases. It triggers the growth of billions of nanocrystals
perfectly aligned with respect to each other and seems to favor an fcc-D2 phase over the hcp-bulk phase. [1] PRL
110: 065505 (2013), Hofmann et al.
ICNS 2013 International Conference on Neutron Scattering