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

Margarita Russina,Ferenc Mezei,Ewout Kemner

doi:10.1038/srep27417

Abstract

In porous materials the molecular confinement is often realized by means of weak Van der Waals interactions between the molecule and the pore surface. The understanding of the mechanism of such interactions is important for a number of applications. In order to establish the role of the confinement size we have studied the microscopic dynamics of molecular hydrogen stored in the nanocages of clathrate hydrates of two different dimensions. We have found that by varying the size of the pore the diffusive mobility of confined hydrogen can be modified in both directions, i.e. reduced or enhanced compared to that in the bulk solid at the same temperatures. In the small cages with a mean crystallographic radius of 3.95 Å the confinement reduces diffusive mobility by orders of magnitude. In contrast, in large cages with a mean radius of 4.75 Å hydrogen molecules displays diffusive jump motion between different equilibrium sites inside the cages, visible at temperatures where bulk H2 is solid. The localization of H2 molecules observed in small cages can promote improved functional properties valuable for hydrogen storage applications.

Highlights

Cages are occupied by large molecules such as tetrahydrofuran[7] and the filling of the remaining small cages by H2 gas is realized already at a pressure of 60 bar of H2 at 260 K
In hydrogen filled clathrates the existence of quantum rattling at frequencies centered around 10 meV (80 cm−1) has been reported for the hydrogen enclosed in the small cage[18,19,20,21,25], while the existence of such modes in large cage was not confirmed experimentally yet
The feature measured in neutron spectroscopy is the dynamic structure factor S(Q, ω) that is the Fourier transform of Van Hove space-time correlation function, weighted by the scattering strength of the various atomic nuclei[26]

Summary

Introduction

Cages are occupied by large molecules such as tetrahydrofuran[7] and the filling of the remaining small cages by H2 gas is realized already at a pressure of 60 bar of H2 at 260 K. Neither does the Q-dependence of elastic intensities of hydrogen in small cages show any contribution of diffusive motion and can be described by a mean square displacement of the center of mass due to rotational and vibrational contributions.

Results

Conclusion