Abstract
Methane hydrates naturally form on Earth and in the interiors of some icy bodies of the Universe, and are also expected to play a paramount role in future energy and environmental technologies. Here we report experimental observation of an extremely fast methane diffusion at the interface of the two most common clathrate hydrate structures, namely clathrate structures I and II. Methane translational diffusion—measured by quasielastic neutron scattering at 0.8 GPa—is faster than that expected in pure supercritical methane at comparable pressure and temperature. This phenomenon could be an effect of strong confinement or of methane aggregation in the form of micro-nanobubbles at the interface of the two structures. Our results could have implications for understanding the replacement kinetics during sI–sII conversion in gas exchange experiments and for establishing the methane mobility in methane hydrates embedded in the cryosphere of large icy bodies in the Universe.
Highlights
Methane hydrates naturally form on Earth and in the interiors of some icy bodies of the Universe, and are expected to play a paramount role in future energy and environmental technologies
The topology of the water cages and the number of gas molecules trapped in these cages critically depend on the specific thermodynamic conditions of formation of the clathrate hydrate and on its formation kinetics[1, 5]
It is noteworthy that sI and sII are topologically incompatible without the intercalation of pentakaidecahedral (51263) cages;[8] the interplay between kinetic factors and thermodynamic stability during sI–sII cross-nucleation has been discussed in details[17]
Summary
Methane hydrates naturally form on Earth and in the interiors of some icy bodies of the Universe, and are expected to play a paramount role in future energy and environmental technologies. Methane translational diffusion—measured by quasielastic neutron scattering at 0.8 GPa—is faster than that expected in pure supercritical methane at comparable pressure and temperature This phenomenon could be an effect of strong confinement or of methane aggregation in the form of micro-nanobubbles at the interface of the two structures. Exchanging the guests in natural gas hydrate deposits with CO2 has been suggested as a two-in-one approach of energy recovery and concomitant CO2 mitigation[3] As they are believed to be the dominant methane-bearing phase in the nebula from which the outer planets and satellites are formed, the properties of methane hydrates are crucial to models of bodies in the outer solar system[4]. We probe the microscopic diffusion of methane in a methane hydrate (CH4–D2O) sample exhibiting coexistence of clathrate sI and sII by quasielastic neutron scattering (QENS) measurements. We measure methane hydrates in pure sI clathrate, in pure hexagonal clathrate structure H (space group P6/mmm)[26] and during transformation from sI to structure H (noted sH); the spectra of sI and sH do not exhibit any visible quasielastic signal, and the spectra of sI–sH show a very weak signal, orders of magnitude smaller than the signal from sI−sII
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