On the thermodynamic stability of hydrogen clathrate hydrates

Abstract

The cage occupancy of hydrogen clathrate hydrate has been examined by grand canonical Monte Carlo ͑GCMC͒ simulations for wide ranges of temperature and pressure. The simulations are carried out with a fixed number of water molecules and a fixed chemical potential of the guest species so that hydrogen molecules can be created or annihilated in the clathrate. Two types of the GCMC simulations are performed; in one the volume of the clathrate is fixed and in the other it is allowed to adjust itself under a preset pressure so as to take account of compression by a hydrostatic pressure and expansion due to multiple cage occupancy. It is found that the smaller cage in structure II is practically incapable of accommodating more than a single guest molecule even at pressures as high as 500 MPa, which agrees with the recent experimental investigations. The larger cage is found to encapsulate at most 4 hydrogen molecules, but its occupancy is dependent significantly on the pressure of hydrogen.

Monte Carlo study of sII and sH argon hydrates with multiple occupancy of cages

“…Additional studies that consider the case of multiple occupancy include, among others, the experimental [24][25][26] and the theoretical [27][28][29] works for nitrogen, the experimental [30,31] and the theoretical [32][33][34][35][36][37] works for hydrogen, and the theoretical work for carbon dioxide [38].…”

Clathrate Hydrates

“…Katsumasa et al [46] examined the cage occupancy of hydrogen clathrate hydrates using grand canonical Monte Carlo (GCMC) simulations for wide ranges of temperature and hydrogen pressures. Their simulations were carried out with a fixed number of water molecules and a fixed chemical potential for the guest hydrogen molecules.…”

Reversible Hydrogen Storage in Hydrogel Clathrate Hydrates

“…Lee et al [6] reported a H 2 -storage capacity around 4 wt% at 12 MPa and 270 K, and the composition of the clathrate hydrates was tuned to optimize gas-storage capacity, [6,7] although subsequent studies suggest lower storage capacities under similar conditions. [8][9][10][11] Other clathrates capable of retaining a significant amount of H 2 at moderate pressures and temperatures are being investigated. [12][13][14][15] The extremely slow kinetics of H 2 encapsulation into clathrate hydrate cages, [6,8,16,17] resulting from mass diffusion in a bulk solid phase, may be a generic limitation for the applicability of clathrates in the storage of H 2 and other gases.…”

“…Previous studies have measured H 2 capacity in hydrates indirectly using Raman or NMR spectroscopy conducted at low temperatures, and this can lead to discrepancies [6,8,9] in the reported H 2 capacities since these are closely related to factors such as temperature, pressure, composition, and kinetic conditions. [11,23] In this work, volumetric release experiments [8,20] were carried out to further confirm the H 2 capacity as calculated from the observed pressure drop. The amount of H 2 evolved in each case was consistent with the calculated H 2 enclathrated in Figure 2a.…”