Neutron diffraction studies of CO2 clathrate hydrate: Formation from deuterated ice

The formation of CO2 clathrate hydrate was investigated by using time-of-fl ight neutron powder diffraction at temperatures ranging from 230 to 290 K w ith a CO2 gas pressure of 900 psi. CO2 elathrate hydrate was prepared in si tu from deuterated ice crystals at 230, 243, 253, and 263 K by pressurizing the system with CO2 gas to produce the hydrate in approximately 70% yield. Nearly complete conversion from the hexagonal ice to the sI type CO2 hydra te was observed as the temperature of the sample was slowly increased throu gh the melting point of D2O ice. The conversion of ice into hydrate is beli eved to be a two-stage process in which an initial fast conversion rate is followed by a slower, diffusion-limited rate. On the basis of a shrinking c ore diffusion model, an activation energy of 6.5 kcal/mol was obtained from the temperature dependence of the reaction. Our findings suggest that the formation of the hydrate is through a reaction between CO2 and water molecu les in the quasi-liquid layer (QLL). The CO2 hydrate remained stable follow ing removal of excess liquid CO2 and subsequent pressurization with helium, allowing for a low-temperature (14 K) structure analysis from powder diffr action data without the presence of solid CO2.