Molecular Mechanisms of Gas Diffusion in CO₂ Hydrates

Abstract

Gas diffusion is considered a rate-limiting step in the formation of gas hydrates, yet its molecular mechanisms remain unclear. In this work, we present the molecular mechanisms of the CO2 cage-to-cage transport in gas hydrates, as directly observed from molecular dynamics simulations performed at elevated temperatures. We found that at least one water vacancy is required for the CO2 molecules to pass through five-membered water rings, while only the distortion of the local ring structure is required for the CO2 molecules to pass through the six-membered water rings. We used the transition-state theory to estimate the relevant kinetic parameters associated with the CO2 diffusion in gas hydrates. The calculated free energy of activation is about 44 ± 6 kJ/mol, and the diffusion coefficient is in the range of 1.0 × 10–16∼2.0 × 10–14 m2/s, for the CO2 diffusion at 270 K, in close agreement with previous experiments. This work suggests that the presence of empty cages is crucial for the CO2 cage-to-cage transport in gas hydrates.

Fast methane diffusion at the interface of two clathrate structures

“…Cage-to-cage hopping is a rare event that requires distortion of the host network 36 and the associated diffusion coefficient is of the order of 10 −11 to 10 −12 cm 2 s −1 at 250 K, as revealed by experimental 21 and computational 24 studies. Similar conclusions have been reported for the cage-to-cage hopping of other guest molecules 37 , 38 , including molecules forming clathrate sII 37 . It is also interesting to compare the present results to the translational diffusion coefficients of CH 4 in bulk water–methane mixtures and bulk pure methane.…”

Crystal phase content-dependent functionality of dual phase SnO2–WO3nanocomposite filmsviacosputtering crystal growth

“…The bandgap values of the synthesized SnO 2 and WO 3 thin lms were evaluated using the Tauc plots in Fig. 6(b) and (c), 24 respectively. The SnO 2 thin lm's bandgap was approximately 3.61 eV, whereas that of the WO 3 thin lm had a relatively small bandgap of 3.49 eV.…”

Surface crystal feature-dependent photoactivity of ZnO–ZnS composite rods via hydrothermal sulfidation

“…The development of oxide semiconductor photocatalysts for environmental remediation has been widely studied. [1][2][3] Zinc oxide (ZnO) has received global research interest for application as a highly efficient photocatalyst. However, low quantum efficiency, short-range disorder, and photocorrosion continue to affect the performance of ZnO in practical applications.…”