Local structure analysis of low-temperature neutron pair distribution function coupled with molecular dynamics simulations of CH4 and CO2 hydrates from 2 to 210 K

Abstract

CH4 hydrates occur naturally and are an abundant potential fuel source with a corresponding risk of potent greenhouse gas release, due to their stability conditions at low temperature and high pressure. Byproduct CO2 can be exchanged with CH4 in these natural deposits and can potentially stabilize the hydrates at higher temperatures. CH4, CO2, and mixed CH4-CO2 hydrates are studied with in situ neutron pair distribution function experiments from 2 to 210 K to investigate the impact of varying the CH4-CO2 guest composition in the gas hydrate structure. These experiments combined with Reverse Monte Carlo analysis allow for the characterization of intermolecular CH4 and CO2 interactions with the water molecules which form the hydrate lattice and how they impact the local structure of the lattice itself. Results indicate that when CH4 and CO2 co-occupy the hydrate, the host is more strongly distorted than in the pure CH4 and CO2 hydrates, but this becomes less defined with increasing temperature. The presence of CO2 in mixed hydrate increases the stability range and creates a barrier for CH4 to completely leave the structure.