Hydrogen Inter-Cage Hopping and Cage Occupancies inside Hydrogen Hydrate: Molecular-Dynamics Analysis

Abstract

The inter-cage hopping in a type II clathrate hydrate with different numbers of H2 and D2 molecules, from 1 to 4 molecules per large cage, was studied using a classical molecular dynamics simulation at temperatures of 80 to 240 K. We present the results for the diffusion of these guest molecules (H2 or D2) at all of the different occupations and temperatures, and we also calculated the activation energy as the energy barrier for the diffusion using the Arrhenius equation. The average occupancy number over the simulation time showed that the structures with double and triple large-cage H2 occupancy appeared to be the most stable, while the small cages remained with only one guest molecule. A Markov model was also calculated based on the number of transitions between the different cage types.

Hydrogen and Deuterium Molecular Escape from Clathrate Hydrates: “Leaky” Microsecond-Molecular-Dynamics Predictions

“… 45 Therefore, more elevated concentrations need to overcome larger free-energy barriers to jump from one cage to another, with all large cages having high occupation. 9 , 41 , 42 , 46 , 47 …”

“…Each such trip has been labeled based on the starting and the secondary cage type, as large to large (LL), large to small (LS), small to large (SL), and small to small (SS); a schematic is shown in Figure 10 . 9 From the Markov model, the hydrogen travel of the large-to-large (LL) cage is more favorable at all temperatures, despite the larger-amplitude thermal activation for D2 at thigh temperature (cf. cage radii in Figures 8 and 9 —specifically high-temperature radius standard deviations).…”

“…For both H 2 and D 2 , the same force-field parameters were used in the previous MD investigation of sII hydrogen hydrates; the potentials are kept the same for both H 2 and D 2 , and the extra masses are added to the D 2 molecules; these replicated correctly the experimental gas-phase quadrupole moment of H 2 and D 2 . 9 MD simulations were carried out in the NVT ensemble using the 3D particle-mesh Ewald method to handle long-range interactions, 35 with a Nosé–Hoover relaxation time of 5 ps. Simulations were conducted for 1 μs, using the velocity Verlet algorithm; Lennard-Jones 12-6 and real-space Ewald interactions were subject to a 1.2 nm cut-off.…”

“…In our calculations, the sII-clathrate lattice 39 was used, and it is shown in Figure 1 . The movement inside the clathrate-hydrate structure of H 2 and D 2 molecules was analyzed and captured as the Markov-chain model, 9 , 40 and that facilitated by intercage hopping migration 41 − 43 was analyzed and captured as Markov-chain models. 9 , 44 It is necessary to remember that not all states can make a substantial contribution to the overall system’s configurational properties.…”

Controlling hydrogen release from remaining-intact Clathrate hydrates by electromagnetic fields: molecular engineering via microsecond non-equilibrium molecular dynamics

“…37) force-eld models, which have performed well for hydrogen-hydrate simulation. 7,9,23,24,34,37 We note that TIP4P/2005 has been used with success in many molecular-simulation studies on gas-hydrate structures, beyond of hydrogen hydrates also. 38,39 For modelling of intermolecular interactions, the Lennard-Jones potential (12-6 potential) and real-space Ewald interactions were subject to a 1 nm cut-off.…”