High-Pressure Phase Equilibrium and Raman Microprobe Spectroscopic Studies on the CO₂ Hydrate System

Abstract

The three-phase coexistence curve of CO 2 hydrate + saturated water + saturated liquid CO 2 was investigated in the temperature range from 289 to 294 K and pressure up to 500 MPa. The temperature maximum point on the equilibrium curve was confirmed at 294 K and 328 MPa. The sign of (dp/dT) of the three-phase coexistence curve changes from positive to negative at this boundary point. The equilibrium curve continues up to the four-phase coexistence point, which lies at a slightly higher pressure than that of melting curve of CO 2 . The laser Raman microprobe spectroscopic analysis reveals that the stretching and bending vibration energies of CO 2 entrapped into the hydrate cage are lower than that of pure CO 2 at the same temperature and pressure. The Raman spectrum for the intermolecular vibration mode (O-O stretching) of water molecules was detected around 200 cm -1 in the hydrate phase. The pressure dependence of the Raman shift reveals that the hydrate lattice constructed of water molecules is shrunk by a pressure increase. The Raman spectroscopy suggests that the hydrate-cage-like structure is provided previously in the aqueous solution in equilibrium.

In situ apparatus for the study of clathrate hydrates relevant to solar system bodies using synchrotron X-ray diffraction and Raman spectroscopy

“…5 for ice, ice plus clathrate, and clathrate, for which the peak wavenumbers are 223.1 ± 0.5 cm −1 , 218.8 ± 0.3 cm −1 , and 216.2 ± 0.3 cm −1 , respectively. These values are consistent with this feature (unlike the above O−H stretch features) being sensitive to environmental changes (Nakano et al 1998): over the pressure range ∼100−5000 bar the Raman shift varies from ∼205 cm −1 to ∼224 cm −1 . Nakano et al also note that the 220 cm −1 O−O feature is specific to the CO 2 clathrate hydrate; this can be explained by the structural similarities between the hydrogen bonded H 2 O structures of sI clathrate and ice, in that both liquid water and sII clathrate show only weak broad features in this region.…”

“…4, top right) are the CO 2 Fermi diad ω 1 :2ω 2 resonance in CO 2 molecules trapped within the clathrate cages; we note that, in pure crystalline CO 2 at 6 K, these features appear at 1275.7 ± 0.1 cm and 1384.0 ± 0.1 cm (Ouillon et al 1985). Nakano et al (1998) found that there is little or no pressure-dependence of the Raman shift for the Fermi diad features, up to a pressure of ∼5000 bar. On the other hand, these features are sensitive to isotopic composition, at least in the fluid state (Irmer & Graupner 2002;Windisch et al 2012).…”

“…Firstly, as the melting point is approached, the ice forms a quasi liquid-like medium surrounding the clathrate (Mizuno & Hanafusa 1987;Henning et al 2000), allowing for increased inward diffusion (compared to diffusion through clathrate) of additional CO 2 from the environment. Secondly, since measured Raman shifts of the CO 2 molecule dissolved in the liquid phase are very close to those of CO 2 in the clathrate phase the structure of water around CO 2 in the liquid phase is likely to be similar to that of the hydrate cage (Nakano et al 1998), suggesting that hydrate formation is easier/faster in the quasi liquid-like phase. Molecular dynamics simulations (Jacobson et al 2010(Jacobson et al , 2011 of clathrate formation from the liquid phase suggest that crystalline clathrates result from a multi-step process.…”

“…4, bottom left) is due to the inter-molecular O−O vibration mode of the water molecules forming the clathrate structure (Nakano et al 1998). This feature is shown in greater detail in Fig.…”

On the Occupancy of Carbon Dioxide Clathrate Hydrates: Grandcanonical Monte Carlo Simulations

“…The occupancy seems to depend on the external condition, noticeably the temperature and the pressure of CO 2 . More interesting is its phase behavior, which appears in both high and low temperature region [6][7][8][9]. Thus, occupancy of CO 2 in CS-I deserves a close scrutiny by theoretical investigation.…”