Low barriers for hydrogen diffusion in sII clathrate

Abstract

The transport of gas molecules in hydrates is presently poorly understood. In sII structured hydrates with hydrogen guests there is, for instance, a mismatch between experimental and computed values for diffusion constants. We provide an explanation for the experimentally observed diffusion rates, using DFT-based molecular dynamics simulations at 100 K. By considering the effect of cage occupancy, as well as the flexibility of the water lattice, we show that barriers for hydrogen diffusing between cages, can approach values as low as 5 kJ mol(-1), which is very close to experimental values.

Competing quantum effects in the free energy profiles and diffusion rates of hydrogen and deuterium molecules through clathrate hydrates

“…The freeenergy barriers for H 2 diffusion have been found to differ greatly for different H 2 occupancy values of the large cages involved, 35,36 implying that the diffusion rates, which have not yet been reported, will exhibit strong dependence on the cage occupancy as well. This means that in order to have a meaningful comparison with experiment, calculation of the diffusion rate of H 2 in bulk clathrate hydrate at a given temperature will require determining the quantum free-energy profiles for many possible combinations of H 2 occupancies of the adjacent large cages, using these to compute the rates of diffusion for each such combination, and finally averaging over these rates.…”

“…26 In both experiments, the clathrate crystal structure remained intact, demonstrating that the diffusive migration of H 2 must take place, 26 and the small cages always remained singly occupied by H 2 . [33][34][35][36] In two of these studies, 35,36 the free-energy barriers were computed for different H 2 occupancy of the neighboring large cages, and the barrier heights were found to decrease rather strongly with increasing H 2 occupancy.…”

Diffusion of gas mixtures in the sI hydrate structure

“…They propose that hydrogen could act as a promoter of the exchange, "attacking" the methane that is stuck in cages, and allowing the exchange to occur more rapidly. It is known that hydrogen moves with relative ease within sIIhydrates [20][21][22] . Molecular simulations 21,22 show that the multiple occupancy of cages helps reduce the free energy barrier for hydrogen hopping between cages.…”

“…It is known that hydrogen moves with relative ease within sIIhydrates [20][21][22] . Molecular simulations 21,22 show that the multiple occupancy of cages helps reduce the free energy barrier for hydrogen hopping between cages. It is possible that a similar effect can occur when hydrogen gas is present in the exchange between methane and carbon dioxide -even if it is clear that hydrogen does not remain enclathrated in the final hydrate structure.…”

Global-density fluctuations in methane clathrate hydrates in externally applied electromagnetic fields

“…23 Such an understanding of equilibrium dynamical properties and phonon scattering offered by molecular dynamics (MD) has indeed led to progress in recent years in enhancing our understanding of thermal-conduction processes in clathrate hydrates, [24][25][26][27][28] together with "hopping"-mediated guest-diffusion processes of interest of energy-and gas-storage applications. [29][30][31][32][33] In any event, the application of external electric and electromagnetic (e/m) fields to gas hydrates is of much technological interest. For instance, in the inhibition of hydrate formation in natural-gas pipelines, aside from the use of either thermodynamic or kinetic inhibitor additives injected into the gas stream, or localised application of higher temperature, 1,2 external electric and electromagnetic (e/m) fields may serve to disrupt already-formed hydrates (e.g., as pipeline plugs or possibly in situ "natural" hydrates).…”