Effects of carbon nanotube on the nucleation and growth of CO2 hydrate: Insights from the molecular dynamics simulations

•A small number of carbon nanotubes had little effect on the formation of hydrate.•When the carbon nanotubes were abundant, multiple carbon nanotubes appeared to agglomerate due to hydrophobicity.•Only when the number of carbon nanotubes is appropriate can carbon nanotubes effectively promote the nucleation and growth of hydrate.•The length of carbon nanotubes plays an essential role in the stable growth of the hydrate cages. As an essential technology for achieving carbon neutralization, hydrate technology shows broad prospects in CO2 capture and storage. Carbon nanotubes have been used to promote gas hydrate formation as a material with good physical and chemical properties. In this work, molecular dynamics simulations were performed at a pressure of 40 MPa and a temperature of 250 K by adding carbon nanotubes with different quantities and lengths. The simulation results showed that adding one carbon nanotube had little effect on the formation of CO2 hydrate. As the cage structure formed, the hydrate moved away from the carbon nanotubes and repelled them into the critical aqueous phase. When three carbon nanotubes were added, they appeared to agglomerate due to hydrophobicity, which inhibited the formation of CO2 hydrate. Only when two carbon nanotubes were added can they effectively promote the nucleation and growth of CO2 hydrate. At the same time, shorter carbon nanotubes can maximize their excellent thermal conductivity, thus promoting nucleation and stable growth of hydrates. Therefore, the promotion or inhibition effect of CO2 hydrate by carbon nanotubes depends on their size and amount.