Dissociation mechanism of carbon dioxide hydrate by molecular dynamic simulation and ab initio calculation

Using molecular dynamic with consistent valence force field (CVFF), we simulated the dissociation behavior of carbon dioxide hydrate, along with methane and hydrogen hydrates for comparison. The detailed dissociation process is discussed in terms of structural snapshots, radial distribution functions, mean square displacements, and diffusion coefficient at different temperatures. With increasing temperature, the clathrate skeleton of water molecules is firstly distorted and damaged; then the encapsulated CO2 molecules escape and are distributed in the aqueous solution in the form of small CO2 bubbles; finally the clathrate skeleton of hydrate is completely destroyed and CO2 molecules aggregate into a large CO2 bubble. The dissociation of carbon dioxide hydrate is not only related to the overall occupancy, but also sensitive to the residence of specific cavities. Moreover, the diffusion barriers for guest molecules (CO2, CH4, H2) penetrating the water cages are obtained from ab initio calculations, which help illustrate the physical origin for the difference in the dissociation behavior of the corresponding hydrates, that is, CO2 and CH4 molecules have to diffuse after the water cages break while H2 can diffuse in the earlier stage due to low diffusion barrier.