Molecular Dynamics Simulation of CO ₂ Hydrate Nucleation in Coal-Based Media: Insight into Size Effect and Surface Hydroxylation

The sequestration of CO2 into coal-based media via the hydrate method contributes to achieving carbon neutrality; however, at the molecular scale, the mechanism underlying the influence of coal-based media on CO2 hydrate nucleation remains to be investigated. In this study, coal-based media were characterized using bilayer graphene. Taking the lattice size of sI-type CO2 hydrate as the basic unit of the simulation domain, two sets of models were constructed: the first set consists of five models with different lattice spacings in the z-direction, and the second set comprises three models with 10%, 20%, and 30% hydroxyl concentrations. By analyzing the synthesis trajectory, structural parameters, potential energy, and density of CO2 hydrates, the response characteristics of the size effect and hydrophilicity to CO2 hydrate nucleation were investigated. The results showed that the simulation domain formed a “CO2 adsorption layer-CO2 aqueous solution region-CO2 adsorption layer” structure, and hydrates (predominantly sI-type) only nucleated in the CO2 aqueous solution region. Under the coupled effect of temperature and pressure, a relatively low potential energy environment reduces the intensity of the violent motion of molecules within the system. CO2 bubbles dissolve in the aqueous phase, forming local low-potential energy regions, which in turn induce the regular arrangement of high-density water molecules to form initial cage-like structures. Nucleation is a cyclic process involving the formation and decomposition of cage-like structures, where water molecules continuously arrange themselves in an ordered manner around initial water cages, facilitating the formation of hydrate clusters and their gradual growth to the critical nucleus size. The z-direction spacing of the five CO2 hydrate unit cells was the critical size threshold, and a narrower spacing increased the system potential energy, thus inhibiting CO2 dissolution and nucleation. The hydroxyl groups on graphene form hydrogen bonds with water, reducing the free water and dissolved CO2 concentrations, thereby suppressing hydrate synthesis. This work provides molecular-level insights for optimizing coal-based CO2 hydrate storage via carbon capture and storage (CCS) technologies.