Mechanism for H 2 diffusion in sII hydrates by molecular dynamics simulations.

Among the many different types of molecules that form clathrate hydrates, H ₂ is unique as it can easily diffuse into and out of clathrate cages, a process that involves the physical-chemical interactions between guest (H ₂ ) and host (water) molecules, and is unlike any other molecular system. The dynamic and nano-scale process of H ₂ diffusion into binary structure II hydrates, where the large cages are occupied by larger molecules, was studied using molecular dynamics simulation. As the H ₂ molecules diffused from one cage to another, two types of diffusion processes were observed: (i) when moving between a pair of large cages, the H ₂ molecules pass through the central part of the hexagonal rings; (ii) however, when the H ₂ molecules move from a large cage to a small one, it requires one of the pentagonal rings to partially break, as this allows the H ₂ molecule to pass through the widened space. While the diffusion of H ₂ molecules between large cages was found to occur more frequently, the presence of SF ₆ molecules in the large cages was found to inhibit diffusion. Therefore, in order to attain higher H ₂ storage capacities in binary hydrates, it is suggested that there is an optimal number of large cages that should be occupied by SF ₆ molecules.