Neutron Diffraction Studies of CO₂ Clathrate Hydrate:  Formation from Deuterated Ice

Abstract

The formation of CO2 clathrate hydrate was investigated by using time-of-flight neutron powder diffraction at temperatures ranging from 230 to 290 K with a CO2 gas pressure of 900 psi. CO2 clathrate hydrate was prepared in situ 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 hydrate was observed as the temperature of the sample was slowly increased through the melting point of D2O 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 CO2 and water molecules in the quasi-liquid layer (QLL). The CO2 hydrate remained stable following removal of excess liquid CO2 and subsequent pressurization with helium, allowing for a low-temperature (14 K) structure analysis from powder diffraction data without the presence of solid CO2.

Systematic kinetic studies on mixed gas hydrates by Raman spectroscopy and powder X-ray diffraction

“…The inward/outward diffusion of gas/water molecules is the rate-limiting second step of the formation process. This mechanism was confirmed by experimental data presented by Takeya et al [4] and Henning et al [5]. Henning et al [5] applied a simple shrinking core model that was originally developed to describe the hydration of cement grains to fit the diffusion-controlled stage.…”

“…This mechanism was confirmed by experimental data presented by Takeya et al [4] and Henning et al [5]. Henning et al [5] applied a simple shrinking core model that was originally developed to describe the hydration of cement grains to fit the diffusion-controlled stage. In 2002, Wang et al [6] extended this model to a third stage, which is controlled by the reaction of the guest molecules with residual ice.…”

Direct Space Methods for Powder X-ray Diffraction for Guest−Host Materials: Applications to Cage Occupancies and Guest Distributions in Clathrate Hydrates

“…26 Another earlier powder neutron diffraction study suggested that the CO 2 guest in each large cage lies in the equatorial plane of the cage at 14 K by assuming the carbon atom of the disordered CO 2 molecule to be at the center of the cages, and a CO 2 occupancy of >95% of the large cage and 60-80% of the small cage were reported. 27 The discrepancy between the results on the disordered model for CO 2 may well be caused by the assumption of the position of the carbon atom as the displacement parameter, which represents atomic motions and a possible static displacive disorder, and the cage occupancies are coupled.…”

Some current challenges in clathrate hydrate science: Nucleation, decomposition and the memory effect

“…This work describes a route to hydrate formation at low temperatures. Experimentally, hydrate formation from ice is a two-stage process with a relatively fast conversion of the ice surface [42][43][44]34] and a much slower process for bulk ice. Catalysts that interrupt surface hydrogen bonding can speed up this process by orders of magnitude [45].…”