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J. Phys. Chem. B 2002, 106, 30-33
In Situ Observations of High-Pressure Phase Transformations in a Synthetic Methane Hydrate Hiroyasu Shimizu,*,†,# Tatsuya Kumazaki,# Tetsuji Kume,† and Shigeo Sasaki†,# Department of Electronics, Gifu UniVersity, 1-1 Yanagido, Gifu 501-1193, Japan, and EnVironmental and Renewable Energy Systems, Graduate School of Engineering, Gifu UniVersity, 1-1 Yanagido, Gifu 501-1193, Japan ReceiVed: August 3, 2001
A methane hydrate (MH) single crystal was synthesized in a diamond anvil cell to investigate its intrinsic high-pressure properties. With increasing pressure, the cubic sI phase of MH changed to the MH-II phase at P ) 0.9 GPa and room temperature, and this phase remains stable up to P ) 1.9 GPa, which was visually observed by optical microscopy. In situ Raman spectra for CH4 molecules encaged in different cages of MH-II show two vibrational bands; the higher frequency band shows a remarkable increase in its frequency versus pressure (17.0 cm-1/GPa), and the lower band shows a progressive increase in frequency with pressure (6.3 cm-1/GPa). These results are interpreted on the basis of two different structures recently reported for MH-II. Above P ) 1.9 GPa, MH-II crystals visually decomposed and the O-H stretching Raman band of host cages became unobservable, indicating no more existence of the cage structure. Raman spectra of CH4 molecules in MH-III show almost the same behavior as those of pure solid methane up to at least 5.2 GPa, which may be consistent with the existence of a new type of MH.
Introduction Methane is an important energy source, and interest in it has recently heightened with the discovery of a large amount of methane hydrate (MH) near the sea floor in the deep ocean.1,2 In other respects, methane is a powerful greenhouse gas, and its outgassing from MH may contribute substantially the prospect of global warming.2 Therefore, the study of MH properties under high pressure is of fundamental importance in understanding the physical chemistry of clathrate hydrates of natural gases. MH is a hydrogen-bonded network of water molecules that forms host cages in which CH4 guest molecules are contained by van der Waals forces.3 There are three typical structures of sI, sII, and sH for gas hydrates:1,3 cubic sI, two small 512 and six large 51262 cages; cubic sII, sixteen small 512 and eight large 51264 cages; hexagonal sH, three small 512, two small 435663 and one large 51268 cages, where, for example, a 51262 cage is formed by 12 pentagons and 2 hexagons. Which of these structures is found depends mostly on the size of the guest molecules.1 Methane usually leads to the sI (ideally CH4‚ 5.75H2O). Recently, Hirai et al.4,5 have performed in situ X-ray diffraction measurements of a single-phase or fine-grained polycrystalline MH prepared in a diamond anvil cell (DAC) by using a powder sample of MH. They indicated that the sI phase of MH is stable up to P ) 2.3 GPa at room temperature, and above this pressure MH decomposes into ice VII and solid methane. Chou et al.6 have determined that MH shows phase * To whom correspondence should be addressed. E-mail: shimizu@ cc.gifu-u.ac.jp. Fax: +81-58-293-2681. † Department of Electronics. # Environmental and Renewable Energy Systems, Graduate School of Engineering.
transformations from sI to sII at P ) 0.1 GPa and from sII to sH at P ) 0.6 GPa at T ≈ 298 K by using X-ray diffraction and Raman spectroscopy with DAC. Very recently, Chou et al.7 have also found a new MH phase at approximately P ) 0.137 GPa and T ) 308 K by optical observations and Raman measurements. In contrast (after this work was completed), Loveday et al.8 published a study in which they found structural transitions at about P ) 1 and 2 GPa by neutron and X-ray measurements. They indicated that the phase between 1 and 2 GPa (MH-II) does not have the hexagonal clathrate structure (sH) but may be closely related to it and that a new clathrate phase above P ) 2 GPa (MH-III) remains stable to 10 GPa without further phase transformation. In this paper, we present the visual observations and in situ Raman measurements of an MH single-crystal grown in the DAC at pressures up to 5.2 GPa and T ) 296 K and indicate two phase transformations at P ) 0.9 and 1.9 GPa. These methods and the sample of MH single crystal are of crucial importance to investigate more clearly the intrinsic properties of MH under high pressure. The reasoning for this is that polycrystalline MH coexists with liquid water or ice, vapor phase, and sometimes contaminates indistinguishably. From the pressure dependence of Raman spectra and frequency shifts for CH4 molecules filling different cages of MH-II between P ) 0.9 and 1.9 GPa, we investigate and discuss the clathrate hydrate cages containing CH4 molecules on the basis of two different structures reported by Chou et al.6 and Loveday et al.8 In the MH-III phase above P ) 1.9 GPa, we indicate the breakdown of the cage structure by visual observations and in situ Raman spectra showing the disappearance of the O-H stretching band characteristic of host cages and present the spectra of CH4 molecules showing almost the same behavior as those of pure solid methane. These properties are investigated with a view of a new methane hydrate phase.8
10.1021/jp013010a CCC: $22.00 © 2002 American Chemical Society Published on Web 12/13/2001
High-Pressure Phase Transformations
J. Phys. Chem. B, Vol. 106, No. 1, 2002 31
Figure 1. Photomicrographs of methane hydrate (MH) in the sample chamber of DAC, about 0.3 mm in diameter, with corresponding Raman spectra of the C-H stretching mode for CH4 molecules in each phase. (a) MH single-crystal as grown at about P ) 0.02 GPa in sI phase, showing an octahedron shape surrounded by water. Two Raman peaks correspond to occupancy of two different cages. (b) sI single-crystal surrounded by water at P ) 0.86 GPa. Some pits are seen on its surface (see text). (c) A hexahedron-like single-crystal surrounded by water at P ) 0.90 GPa in MH-II phase. Raman spectrum changes to one broad band. (d) Coexistent state of MH-II phase, water, and ice VI at P ) 0.81 GPa. Pressure inside DAC decreases by the appearance of ice VI. (e) Ice VI surrounds a MH-II single-crystal at P ) 1.58 GPa. Raman spectrum shows two peaks separated. (f) A MH-II single-crystal disappears at P ) 1.92 GPa and the MH-III phase is surrounded by ice VI. The Raman spectrum changed to one Raman band relatively sharp.
Experimental Section In our high-pressure studies, we prepared successfully a pure MH single crystal (see Figure 1), which was grown by loading water and compressed CH4 gas into a small sample chamber (diameter of 0.3 mm, depth of 0.3 mm) of a DAC. The pressure was measured by the ruby-scale method. We observed, under a microscope, states and transformations of a MH single crystal and its surrounding water or ice with increasing pressure up to about P ) 5.2 GPa and made in situ Raman measurements9,10 by using a small spot size (5 µm φ) of an Ar ion laser. Results and Discussion Figure 1 shows photomicrographs of MH in a volume of water or ice with increasing pressure at 296 K. In Figure 1a, one can see a MH single crystal showing an octahedron surrounded by water at about P ) 0.02 GPa. Some pits on the surface occur usually from the existence of some droplets of compressed CH4 fluid on the growing processes of a singlecrystal MH. Everyone knows this crystal belongs to sI phase at P ) 0.02 GPa.1,4,6,8,11 The Raman spectrum of the symmetric C-H stretching mode for guest CH4 molecules is shown in Figure 1a and indicates this crystal is assigned to sI hydrate by its two characteristic Raman peaks6,12-14 with an intensity ratio of about 1:3, corresponding to the ratio of small to large cages in the unit cell. With increasing pressure, this sI phase remains stable up to P ) 0.9 GPa (see Figure 1), and Raman frequencies (wave-
Figure 2. Pressure dependence of Raman spectra for the symmetric C-H stretching mode of encaged CH4 molecules in MH-II phase. All Raman bands were systematically deconvoluted into two bands. These bands at low and high frequencies correspond to a large cage and small cage, respectively (see text). No other Raman signal exists around this frequency range.
numbers) of two peaks are plotted versus pressure in Figure 3. The Raman frequencies of C-H stretching vibration in the large cage show gradual increase, but their frequencies in the small
32 J. Phys. Chem. B, Vol. 106, No. 1, 2002
Figure 3. Pressure dependence of Raman frequencies (wavenumbers) for the symmetric C-H stretching mode of encaged CH4 molecules in sI and MH-II phases, and of CH4 molecules in MH-III phase without the cage structure. Vertical arrows indicate the sI f MH-II transition at P ) 0.9 GPa and the MH-II f MH-III transition at P ) 1.9 GPa.
cage increase steadily with pressure because of the effective compression of its host cage in which the small void space around a CH4 molecule yields the shortening of the C-H bond length. This behavior is almost the same as the result up to P ) 0.5 GPa by Nakano et al.13 The broad O-H stretching band of host cages formed by H2O molecules is observed around 3100 cm-1, which exists apart from the O-H stretching bands of water that appear around 3300 cm-1. This O-H band characteristic of host cages shows a frequency decrease versus pressure due to the nature of the hydrogen bond under compression. As seen in Figure 1 parts b and c, the octahedron of the sI phase changed its shape to a hexahedron-like crystal at P ) 0.9 GPa, accompanied with the drastic change in Raman spectra. During this transformation to the MH-II phase, the octahedron single-crystal melted and disappeared into the surrounding water and at the same time new crystals grew with increasing pressure. These observations of crystalline transformation seem to be important for understanding of hydrate formation processes. When the pressure was increased to 1.05 GPa, water began to freeze into ice VI, and as a result, the pressure in the DAC decreased to 0.81 GPa as shown in Figure 1d, which presents the coexistent state of three phases of MH, water, and ice VI. With a further increase of pressure, the hexahedron-like single crystal of the MH-II phase became completely surrounded by ice VI as seen in Figure 1e, and this MH-II phase visually remains stable up to P ) 1.9 GPa. Next, to investigate this MH-II phase, we present the pressure dependence of in situ Raman spectra in Figures 1 and 2. The one broad band gradually separated into two peaks with increasing pressure, clearly seen above P ) 1.4 GPa. By considering Raman spectra6,12,14 of CH4 molecules in the sII phase formed by the system of double hydrates, we can determine that this MH-II phase does not correspond to the sII hydrate. As shown in Figure 2, these Raman bands were deconvoluted into two bands. This procedure allowed us to determine the Raman frequencies as plotted against pressure in Figure 3. The higher frequency band shows a remarkable increase in Raman frequencies versus pressure (17.0 cm-1/GPa),
Shimizu et al. and the lower band around 2916 cm-1 shows a progressive increase in Raman frequencies with pressure (6.3 cm-1/GPa). At present, for this MH-II phase, there are two different structures reported by (1) Chou et al. 6 and (2) Loveday et al.,8 that is, (1) sH clathlate hydrate and (2) a hexagonal structure definitely not sH, respectively. Because we cannot make a decision between the two by our Raman data, it may be reasonable at present that we investigate and discuss our highpressure Raman spectra on the basis of their two structures. Chou et al.6 found an sH phase of pure MH at pressures above 0.6 GPa by high-pressure X-ray and Raman measurements and presented one broad Raman band around 2917 cm-1 at P ) 0.88 GPa. They assigned this band to overlapping Raman peaks due to CH4 in three cages of the sH and described that it would be necessary to put two or more CH4 molecules in the large cage to stabilize it and, at last, required further investigation of the cage occupancy of methane in the future.6 Although their results of high-pressure phase transformations are significantly different from our present studies (probably because of their repeated heating and cooling processes at low pressures6), our Raman band of Figure 1c at P ) 0.9 GPa seems to be almost the same as theirs at P ) 0.88 GPa and T ) 298 K. Unlike sI hydrates, which only require a single guest molecule, sH hydrates require normally two sizes of molecules to stabilize their structure (double hydrate), i.e., small molecules to fill the two small cages and a large molecule to occupy the large cage.12,15,16 Average radii of three cages found in sH gas hydrates are as follows:14,16 3.91 Å for three small 512(pseudospherical), 4.06 Å for two small 435663 (nonspherical), and 5.71 Å for one large 51268 (the most oblate) cages. Despite the three cages in sH hydrate, our Raman spectra show two bands. This may be because the two small cages have almost the same average radii noted above, and, as a result, CH4 molecules encaged inside them show almost the same frequencies. Namely, the CH4 molecules filling the two small cages may present one broad Raman band at higher frequency and the CH4 molecule in the larger cage shows also one Raman band at lower frequency as seen in Figures 2 and 3. To stabilize pure sH methane hydrates, the large cage should be occupied only by CH4 molecules, but the large cage is normally too large for one CH4 guest molecule.1,12 If its occupancy by one methane is assumed to be possible, its Raman frequency must be almost independent of pressure as seen in the sI phase (see Figure 3) because of the large void space inside the large cage. Therefore, the progressive increase in Raman frequencies with pressure (6.3 cm-1/GPa) for the lower band suggests the possibility of two or more CH4 molecules encaged inside one large cage. Finer experiments of X-ray and neutron measurements are needed to confirm this possibility, which is contrary to the well-accepted view. Loveday et al.8 stated that MH-II does not have the hexagonal clathrate structure (sH) but may be closely related to it. At present, the detailed structural data (for example, the type of cages, their number, and their size) have not been provided. From our observations of two Raman bands in MH-II, we can indicate the existence of two types of cages, that is, small and large host cages. By comparing the slope (dν/dP) of the lower frequency band in MH-II with that of lower band in the sI phase (see Figure 3), we can suspect that CH4 molecules in the large cage of MH-II can be more effectively compressed than those in the large cage of sI by optimizing the van der Waals contact between the guest CH4 molecule and the host-cage walls. Therefore, we can say that the structure and void space for large cages are different between MH-II and sI phases. Definitive structural data for the MH-II phase are highly anticipated.
High-Pressure Phase Transformations With further increase of pressure, the hexahedron-like single crystal of the MH-II decomposed at P ) 1.9 GPa as shown in Figure 1f. This higher pressure MH-III is surrounded by ice VI, which showed the phase transition to ice VII at about P ) 2.0 GPa. The O-H stretching band characteristic of host cages, which was confirmed in sI and MH-II phases, cannot be observed in the MH-III phase, which indicates no more existence of the cage structure above P ) 1.9 GPa. As seen in Figure 1 parts e and f and shown in Figure 3, C-H-stretching Raman bands changed to a single peak at the transition point. The halfwidth of this peak is narrower than that in the MH-II phase. The Raman frequency of this single peak is located at 2933 cm-1, which is 2-7 cm-1 lower than that of solid-phase I of pure methane around P ) 2 GPa.17-19 Its Raman frequency increases monotonically versus pressure to 5.2 GPa with a slope of about 10 cm-1/GPa (see Figure 3). This value is very close to the slope of solid methane.17-19 Therefore, the MH-III phase can be speculated to be (1) a new class of dense clathrate hydrate as observed in the H2-H2O system in which H2 molecules occupy voids in the open H2O frameworks,20,21 or (2) a decomposition state of solid CH4 and ice VI (or VII). The case of hypothesis 1 will correspond to the new clathrate hydrate (MH-III; body-centered orthorhombic) found by Loveday et al.8 Summary Synthetic methane hydrate (MH) single crystal was prepared in a DAC to reveal its intrinsic high-pressure properties. By the visual observations and in situ Raman measurements, we observed two phase transformations, sI f MH-II and MH-II f MH-III at P ) 0.9 and 1.9 GPa, respectively. These results are in good agreement with neutron and X-ray studies in very recent publication by Loveday et al.8 (after this work was completed, we learned of their study). The pressure dependence of Raman spectra in the MH-II phase was interpreted and discussed on the basis of two different structures reported by Chou et al.6 and Loveday et al.8 Above P ) 1.9 GPa, MH crystals visually decomposed with the breakdown of the cage
J. Phys. Chem. B, Vol. 106, No. 1, 2002 33 structure. Raman spectra of the MH-III phase show almost the same behavior as those of pure solid methane up to at least 5.2 GPa, which may support the new MH clathrate observed by Loveday et al.8 Acknowledgment. This work was partly supported by the Koshiyama Research Grant. References and Notes (1) Sloan, E. D. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker: New York, 1998. (2) Suess, E.; Bohrmann, G.; Greinert, J.; Lausch, E. Sci. Am. 1999, Nov., 52. (3) Gutt, C.; Asmussen, B.; Press, W.; Johnson, M. R.; Handa, Y. P.; Tse, J. S. J. Chem. Phys. 2000, 113, 4713. (4) Hirai, H. et al. J. Phys. Chem. B 2000, 104, 1429. (5) Hirai, H. et al. Chem. Phys. Lett. 2000, 325, 490. (6) Chou, I. M. et al. Proc. Nat. Acad. Sci. U.S.A. 2000, 97, 13484. (7) Chou, I. M. et al. J. Phys. Chem. A 2001, 105, 4664. (8) Loveday, J. S. et al. Nature 2001, 410, 661. (9) Shimizu, H.; Sasaki, S. Science 1992, 257, 514. (10) Shimizu, H.; Yamaguchi, H.; Sasaki, S. Phys. ReV. B 1995, 51, 9391. (11) Dyadin, Y. A.; Aladko, E. Y.; Larionov, E. G. MendeleeV Commun. 1997, 34. (12) Sum, A. K.; Burruss, R. C.; Sloan, E. D. J. Phys. Chem. B 1997, 101, 7371. (13) Nakano, S.; Moritoki, M.; Ohgaki, K. J. Chem. Eng. Data 1999, 44, 254. (14) Subramanian, S.; Kini, R. A.; Dec, S. F., Sloan, E. D. Chem. Eng. Sci. 2000, 55, 1981. (15) Ripmeester, J. A.; Tse, J. S.; Ratcliffe, C. I.; Powell, B. M. Nature 1987, 325, 132. (16) Pratt, R. M.; Mei, D.; Guo, T.; Sloan, E. D. J. Chem. Phys. 1997, 106, 4187. (17) Sharma, S. K.; Mao, H. K.; Bell, P. M. Carnegie Inst. Washington, Year Book 1980, 79, 351. (18) Hebert, P.; Polian, A.; Loubeyre, P.; Le Toullec, R. Phys. ReV. B 1987, 36, 9196. (19) Wu, Y. H.; Sasaki, S.; Shimizu, H. J. Raman Spectrosc. 1995, 26, 963. (20) Vos, W. L.; Finger, L. W.; Hemley, R. J.; Mao, H. K. Phys. ReV. Lett. 1993, 71, 3150. (21) Hemley, R. J. Annu. ReV. Phys. Chem. 2000, 51, 763.
