Russina, M. ; Kemner, E. ; Celli, M. ; Ulivi, L. ; Mezei, F.: Quantum confinement of hydrogen in ice-based clathrates. Journal of Physics : Conference Series 177 (2009), p. 012013/1-3
10.1088/1742-6596/177/1/012013
Abstract:
The importance of nanostructured materials is growing from day to day. Clathrate hydrates (water clathrates) with hydrogen are one example of this kind of new materials with important potential for hydrogen storage [1],[2]. In general clathrate hydrates are inclusion compounds, formed by a network of hydrogen-bonded water molecules that is stabilized by the presence of foreign (generally hydrophobic) molecules, hosted in cages of different forms present in the structure. Though topology of the water cages depends on the sizes and shapes of the guest molecules, the typical dimensions of the cages range between 4 - 15 Å. The confinement and small size of the cages strongly influence the behavior of the guest molecules and results in properties different from those of the bulk. The behavior of confined molecular hydrogen is of particular interest since in addition to the aspects of hydrogen storage it offers the opportunity to study the role and impact of the quantum effects in the confinement. We have studied the dynamics of the molecular hydrogen, confined into water clathrates of two different types by inelastic neutron scattering. In the first type, tetrahydrofuran clathrate one hydrogen molecule occupy the water cages of 5Å dimension, while in the second, fully hydrogenated water clathrate in addition larger cages of about 7 Å are also available to accept hydrogen, and two to four molecules can be placed there. The comparison of these two types of clathrates opens up the particular opportunity to distinguish between the behaviors of confined hydrogen in two cages of somewhat different sizes. We have found that confined hydrogen differs very strongly from other gases used as guest molecules and that the size of the cage has large impact on the hydrogen dynamics. In both cages we observe translational modes and rotational transitions of the hydrogen molecules. In the small cage, however, a new additional feature sets in around 2.4 THz. The analysis of the data complemented by molecular dynamic simulations leads to the conclusion that the nature of this feature is a new type of hybrid motion caused by strong coupling of the quantum rotational and translational degrees of freedom of the hydrogen molecules.