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

  • ️Fri Jan 01 2016

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

Margarita Russina et al. Sci Rep. 2016.

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.

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Figures

Figure 1
Figure 1. Structure of hydrogen clathrates.

Hydrogen molecules are indicated by yellow spheres, framework water molecules are shown by red and white lines. Magenta dashed lines indicate hydrogen bridges. The structure of clathrate type sII is cubic with a = 17.047 Å and is formed by 8 large cages of hexakaidecahedron (64512) and 16 small cages of dodecahedron (512) shapes with mean crystallographic radii of 4.73 Å and 3.95 Å, respectively. In the case of maximum H2 occupancy, the clathrate can be denoted as 48H2 × 136H2O with H2 storage capacity of up to 3.77 wt%. (In our samples we have replaced H2O by D2O).

Figure 2
Figure 2. Dynamic structure factors of molecular hydrogen.

(a) In bulk at ambient pressure, (b) Confined in small cages, (c) Confined in large cages. The solid line represents the instrumental resolution, which was measured independently with a standard elastic scatterer. Points show experimental data at Q = 1 Å−1 at different temperatures: ◼ T = 10 K, formula image T = 20 K, formula image T = 30 K and formula image T = 50 K. Dashed lines show fits by the model in Eqn. (4). Pronounced quasielastic signal of the hydrogen confined in the large cage can be observed even at T = 10 K indicating higher mobility of confined hydrogen compared to the bulk solid at the same temperature. The energy width of QENS intensity is about constant in the experimentally covered Q range revealing spatially confined diffusion.

Figure 3
Figure 3. Elastic incoherent structure factor (EISF) for confined H2 in large cage.

The momentum transfer Q dependence of elastic fraction A0 of the observed dynamic structure factor of molecular hydrogen confined in a large clathrate cage. Experimental data are represented by points, dashed lines show fits at different temperatures by the model of jump diffusion within the tetrahedral cluster of equilibrium positions and a mobile fraction increasing with temperature according to Eqn. (6), details are described in the text. A minimum around 1.3 Å−1 corresponds to jumps with length of about 3.45 Å between the equilibrium sites at the corners of a tetrahedron. With increasing temperature a small fraction of particles reversibly escapes the clathrate structure and cause the downwards shift towards Q = 0 Å−1 for T = 30 and 50 K.

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