Growth kinetics of CO₂ hydrate just below melting point of ice

CO₂ gas hydrate formation was clearly observed through Raman spectroscopy, when CO₂ interacted with ice under high pressures of 1.6MPa. The CO₂ hydrate formation rate, which was measured by the change of the CO₂ gas volume consumed by the CO₂ hydrate formation, was found to be remarkably dependent on the temperature, especially at around the melting point of ice. At -10°C, the CO₂ hydrate formed gradually at reaction times longer than 1000min. On the other hand, at -1°C (just below the melting point of ice), the CO₂ hydrate formed rapidly until 70min, and the formation yield for the CO₂ hydrate becomes close to the stoichiometric value estimated through the number of water molecules. These results suggest the mechanisms for CO₂ gas hydrate formation depending on the temperature. At lower temperatures, the hydrate should be gradually growing only from the ice surface, because the delocalization of hydrate is hard to occur. On the other hand, at higher temperatures just below the melting point of ice, the delocalization of the hydrate in the ice is easy to occur, which generates the active surface for forming CO₂ hydrates and keeps the reaction rate high.