Gas Hydrate Single-Crystal Structure Analyses

Kirchner, Michael T.; Boese, Roland; Billups, W. E.; Norman, Lewis R.

doi:10.1021/ja049247c

Cited by 234 publications

(210 citation statements)

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“…The difference between the lowest and highest point of this energy landscape is 22 meV, suggesting thermal activation of rotations at room temperature and quantifying an experimental assumption. 8 We have also studied the rotation of a methane molecule in a T cage and the results are very similar to the results presented in Fig. 2, with the difference that the maximal energy barrier is slightly smaller, i.e., 18 meV, which is not surprising, as the T cage is slightly larger and the methane molecule can rotate more easily.…”

supporting

confidence: 82%

“…Overall, our optimized lattice constants agree well with previous calculations 6 and experiment. 8 In Table I we further analyze the structure of the host materials by calculating the 153103-1 1098-0121/2011/84(15)/153103(4) …”

mentioning

confidence: 99%

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confidence: 99%

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Ab initioenergetics and kinetics study of H2and CH4in the SI clathrate hydrate

Kolb

Román-Pérez

et al. 2011

Phys. Rev. B

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We present ab initio results at the density functional theory level for the energetics and kinetics of H 2 and CH 4 in the SI clathrate hydrate. Our results complement a recent article by some of the authors [G. Román-Pérez et al., Phys. Rev. Lett. 105, 145901 (2010)] in that we show additional results of the energy landscape of H 2 and CH 4 in the various cages of the host material, as well as further results for energy barriers for all possible diffusion paths of H 2 and CH 4 through the water framework. We also report structural data of the low-pressure phase SI and the higher-pressure phases SII and SH. Clathrate hydrates are crystalline, ice-like structures formed out of water molecules.1 The water framework creates cavities in which gas molecules-typically O 2 , H 2 , CO 2 , CH 4 , Ar, Kr, Xe-can be trapped, which stabilize the framework. The existence of clathrates was first documented in 1810 by Sir Humphry Davy, and clathrates became the subject of intensive studies in the 1930s, when oil companies became aware that clathrates can block pipelines.2 Nowadays, clathrate hydrates are of particular interest for two reasons: (i) they are formed naturally at the bottom of the ocean, where they are often filled with CH 4 .3 These deposits mean a tremendous stock pile of energy, while-at the same time-representing a possible global warming catastrophe if released uncontrolled into the environment through melting; (ii) clathrate hydrates can be used to store H 2 in its cavities and can be a viable hydrogen-storage material (albeit with moderate hydrogen-storage density). 4 For both cases, an understanding of the interaction between the guest molecule and the host framework is crucial for their formation and melting processes, which are still poorly understood. 5 In this brief report, we present results that elucidate this crucial guest-molecule/hostframework interaction and complement a recent paper by some of the authors. 6 We show additional results of the energy landscape of H 2 and CH 4 in the various cages of the host material, and we show further results for energy barriers for all possible diffusion paths of H 2 and CH 4 through the water framework. We also report structural data of the phases SI, SII, and SH.At low pressure, the methane-filled clathrate forms the structure SI, consisting of two types of cages. Guest molecules such as H 2 and CH 4 in the cavities of the clathrate hydrates interact with the water framework through van der Waals forces. But even the water framework itself, i.e., the interaction of water molecules through hydrogen bonds, has a van der Waals component. 9 To capture these effects, we perform here density functional theory (DFT) calculations utilizing the truly nonlocal vdW-DF functional, which includes van der Waals interactions seamlessly into DFT. [10][11][12] We implemented vdW-DF using a very efficient FFT formulation 13 into the latest release of PWSCF, which is a part of the QUANTUM-ESPRESSO package.14 For our calculations we used ultrasoft pseudopotentials with a kinet...

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confidence: 82%

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confidence: 99%

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Ab initioenergetics and kinetics study of H2and CH4in the SI clathrate hydrate

Kolb

Román-Pérez

et al. 2011

Phys. Rev. B

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“…71 After filling the cages with the guest molecules, the geometrical parameters of the water cages will vary depending on the size and shape of the guest molecules. Every cage shrinks after encapsulating a methane molecule which is reflected by the average nearest O-O pair distance decreasing from 2.76 Å to 2.75 Å and the radius of each water cage shrinking by 0.01 Å or less.…”

Section: A Structures and Stabilities A) Position And Dangling H-bonmentioning

confidence: 99%

C–C Stretching Raman Spectra and Stabilities of Hydrocarbon Molecules in Natural Gas Hydrates: A Quantum Chemical Study

Liu

Ojamäe

2014

J. Phys. Chem. A

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Abstract:The presence of specific hydrocarbon gas molecules in various types of water cavities in natural gas hydrates (NGHs) are governed by the relative stabilities of these encapsulated guest molecule ─ water cavity combinations. Using molecular quantum-chemical dispersion-corrected hybrid density functional computations, the interaction energies (ΔEhost-guest) and cohesive energies (ΔEcoh), enthalpies and Gibbs free energies for the complexes of host water cages and hydrocarbon guest molecules are calculated at the ωB97X-D/6-311++G(2d,2p) level of theory. The zero point energy effect of ΔEhost-guest and ΔEcoh is found to be quite substantial. The energetically optimal host-guest combinations for seven hydrocarbon gas molecules (CH4, C2H6, C3H6, C3H8, C4H8, i-C4H10, and n-C4H10) and various water cavities (D, ID, T, P, H and I) in NGHs are found to be CH4@D, C2H6@T, C3H6@T, C3H8@T, C4H8@T/P/H, i-C4H10@H, and n-C4H10@H, as the largest cohesive energy magnitudes will be obtained with these host-guest combinations. The stabilities of various water cavities enclosing hydrocarbon molecules are evaluated from the computed cohesive Gibbs free energies: CH4 prefer to be trapped in a ID cage; C2H6 prefer T cages; C3H6 and C3H8 prefer T and H cages; C4H8 and i-C4H10 prefer H cages; and n-C4H10 prefer I cages. The vibrational frequencies and Raman intensities of the C-C stretching vibrational modes for these seven hydrocarbon molecules enclosed in each water cavity are computed. A blue shift results after the guest molecule is trapped from gas phase into various water cages due to the host-guest interactions between the water cage and hydrocarbon molecule. The frequency shifts to the red as the radius of water cages increases. The model calculations support the view that C-C stretching vibrations of hydrocarbon molecules in the water cavities can be used as a tool to identify the types of crystal phases and guest molecules in NGHs.2

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“…In this capacity, the method has been used on many systems including naturally formed air hydrate, [26] CH 4 hydrate, [27,28] CO 2 hydrate, [28,29] C 3 H 8 hydrate, [28] and CH 4 -C 3 H 8 mixed gas hydrate. [28] Using several techniques to cross-check cage-occupancy data would be helpful to reveal the complicated features of the formation kinetics in these mixed gas hydrate systems.…”

Section: Raman Spectroscopy Of Ch 4 -Co 2 Mixed Gas Hydrates With Varmentioning

confidence: 99%

Kinetics and Stability of CH₄–CO₂ Mixed Gas Hydrates during Formation and Long‐Term Storage

Uchida¹,

Ikeda²,

Takeya³

et al. 2005

ChemPhysChem

135

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The formation of CH4-CO2 mixed gas hydrates was observed by measuring the change of vapor-phase composition using gas chromatography and Raman spectroscopy. Preferential consumption of carbon dioxide molecules was found during hydrate formation, which agreed well with thermodynamic calculations. Both Raman spectroscopic analysis and the thermodynamic calculation indicated that the kinetics of this mixed gas hydrate system was controlled by the competition of both molecules to be enclathrated into the hydrate cages. However, the methane molecules were preferentially crystallized in the early stages of hydrate formation when the initial methane concentration was much less than that of carbon dioxide. According to the Roman spectra, pure methane hydrates first formed under this condition. This unique phenomenon suggested that methane molecules play important roles in the hydrate formation process. These mixed gas hydrates were stored at atmospheric pressure and 190 K for over two months to examine the stability of the encaged gases. During storage, CO2 was preferentially released. According to our thermodynamic analysis, this CO2 release was due to the instability of CO2 in the hydrate structure under the storage conditions.

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Gas Hydrate Single-Crystal Structure Analyses

Cited by 234 publications

References 13 publications

Ab initioenergetics and kinetics study of H2and CH4in the SI clathrate hydrate

Ab initioenergetics and kinetics study of H2and CH4in the SI clathrate hydrate

C–C Stretching Raman Spectra and Stabilities of Hydrocarbon Molecules in Natural Gas Hydrates: A Quantum Chemical Study

Kinetics and Stability of CH₄–CO₂ Mixed Gas Hydrates during Formation and Long‐Term Storage

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Gas Hydrate Single-Crystal Structure Analyses

Cited by 234 publications

References 13 publications

Ab initioenergetics and kinetics study of H2and CH4in the SI clathrate hydrate

Ab initioenergetics and kinetics study of H2and CH4in the SI clathrate hydrate

C–C Stretching Raman Spectra and Stabilities of Hydrocarbon Molecules in Natural Gas Hydrates: A Quantum Chemical Study

Kinetics and Stability of CH4–CO2 Mixed Gas Hydrates during Formation and Long‐Term Storage

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Kinetics and Stability of CH₄–CO₂ Mixed Gas Hydrates during Formation and Long‐Term Storage