Impact of the Confinement on the Intra-Cage Dynamics of Molecular Hydrogen in Clathrate Hydrates

Abstract:

We have studied the diffusive mobility of hydrogen molecules confined in different size cages in clathrate hydrates. In clathrate hydrate H₂ molecules are effectively stored by confinement in two different size cages of the nanoporous host structure with accessible volumes of about 0.50 and 0.67 nm diameters, respectively. For the processes of sorption and desorption of the stored hydrogen the diffusive mobility of the molecules plays a fundamental role. In the present study we have focused on the dynamics of the H₂ molecules inside the cages as one aspect of global guest molecule mobility across the crystalline host structure. We have found that for the two cage sizes different in diameter by only 34 % and in volume by about a factor of 2.4, the dimension can modify the diffusive mobility of confined hydrogen in both directions, i.e. reducing and surprisingly enhancing mobility compared to the bulk at the same temperature. In the smaller cages of clathrate hydrates hydrogen molecules are localized in the center of the cages even at temperatures >100 K. Confinement in the large cages leads to the onset already at T=10 K of jump diffusion between sorption sites separated from each other by about 2.9 Å at the 4 corners of a tetrahedron. At this temperature bulk hydrogen is frozen at ambient pressure and shows no molecular mobility on the same time scale. A particular feature of this diffusive mobility is the pronounced dynamic heterogeneity: only a temperature dependent fraction of the H₂ molecules was found mobile on the time scale covered by the neutron spectrometer used. The differences in microscopic dynamics inside the cages of two different sizes can help to explain the differences in the parameters of macroscopic mobility: trapping of hydrogen molecules in smaller pores matching the molecule size can to play a role in the higher desorption temperature for the small cages.

• Periodical: Materials Science Forum (Volume 879)
• Pages: 1294-1299
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.879.1294
• Online since: November 2016
• Authors: Margarita Russina*, Evout Kemner, Ferenc Mezei
• Keywords: Clathrates, Confinement, Diffusive Mobility, Hydrogen Storage, Neutron Spectroscopy

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