Diffusive Motions of Molecular Hydrogen Confined in THF Clathrate Hydrate

Abstract

Clathrates hydrates are nanoporous crystalline materials made of water cages encapsulating guest molecules. By inserting H2 molecules with the help of a promoter (tetrahydrofuran, noted THF), systems relevant for hydrogen storage application are formed by using relatively soft pressure (of the order of 100 bar) near room temperature. Dynamic properties of hydrogen molecules confined in the small cages of the deuterated THF clathrate hydrate have been investigated at equilibrium by means of incoherent quasi-elastic neutron scattering (QENS). These QENS investigations provide direct experimental evidence about the fundamental aspect of translational diffusive motions of the hydrogen molecules. A comprehensive study of the hydrogen molecules dynamics above 100 K has been achieved through a quantitative analysis of the structure factors (i.e., the spatial extend of the H2 diffusive motion) as well as of the QENS broadening (i.e., the characteristic time of the diffusive motion). On the probed time scale, the H2 molecular translations occur within localized spherical area in the cage with low activation energy of 1.59 ± 0.06 kJ mol–1. The dynamical diameter of H2 molecules varies from 2.08 Å at 250 K to 1.64 Å at 100 K, and the diffusion constant ranges from 0.16 ± 0.03 rad ps–1 at 100 K to 0.49 ± 0.03 rad ps–1 at 250 K. These results indicate that no diffusion between the cages is observed in the picosecond time scale.

Intra-cage dynamics of molecular hydrogen confined in cages of two different dimensions of clathrate hydrates

“…The observed values of the mean square displacement range from 0.04 ± 0.07 Å at 10 K and 0.1 ± 0.07 Å at 50 K, which confirms strong localization of hydrogen in small cage. At higher temperatures the hydrogen molecules become more mobile and start to explore a small volume in the center of the cage with radii ranging from 0.5 Å at 90 K to 0.9 Å at 200 K 31 .…”

“…The clathrates samples were prepared using fine, 99.8% deuterated ice powder. The preparation was done following the procedures described before 31 39 . For the synthesis of the binary tetrahydrofuran clathrates we added deuterated tetrahydrofuran (99.5%) to deuterated water in stoichiometric proportion (17:1 mol).…”

Modeling the THF clathrate hydrate dynamics by combining molecular dynamics and quasi-elastic neutron scattering

“…The solubility of THF within water and the formation conditions closed to room temperature (277 K at 1 bar) provide clear advantages compared with other gas hydrates, making the THF clathrate hydrate commonly studied and used as analogue hydrate. THF hydrate is commonly studied as a substitute for natural gas hydrates [8][9][10][11], used as an additive to reduce the formation pressures for flow assurance [1], hydrogen storage [12,13], gas separation schemes [14,15] or refrigeration applications [16]. There have been numerous fundamental studies on the THF clathrate hydrate.…”

Fast methane diffusion at the interface of two clathrate structures

“…2 show a clear quasielastic signal, i.e., a broadening of the elastic line produced by interactions of the neutrons with diffusing atoms of the sample. Since the incoherent cross-section of hydrogen is almost two orders of magnitude larger than that of other atoms, the measured signal is essentially due to the dynamics of protons in the guest molecules 30 , 31 . We first applied the most common model used to fit quasielastic data, i.e., a Lorentzian function (whose half-width-half-maximum is noted Γ).…”