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The formation of CO 2 clathrate hydrate was investigated by using time-of-flight neutron powder diffraction at temperatures ranging from 230 to 290 K with a CO 2 gas pressure of 900 psi. CO 2 clathrate hydrate was prepared in situ from deuterated ice crystals at 230, 243, 253, and 263 K by pressurizing the system with CO 2 gas to produce the hydrate in approximately 70% yield. Nearly complete conversion from the hexagonal ice to the sI type CO 2 hydrate was observed as the temperature of the sample was slowly increased through the melting point of D 2 O ice. The conversion of ice into hydrate is believed 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 core 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 CO 2 and water molecules in the quasi-liquid layer (QLL). The CO 2 hydrate remained stable following removal of excess liquid CO 2 and subsequent pressurization with helium, allowing for a low-temperature (14 K) structure analysis from powder diffraction data without the presence of solid CO 2 .
Henning et al. (Thu,) studied this question.