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Nondestructive Imaging of Anomalously Preserved Methane Clathrate Hydrate by Phase Contrast X-ray Imaging Satoshi Takeya,*,† Akio Yoneyama,‡ Kazuhiro Ueda,‡ Kazuyuki Hyodo,§ Tohoru Takeda,|| Hiroko Mimachi,# Masahiro Takahashi,# Toru Iwasaki,# Kenichi Sano,# Hiroshi Yamawaki,† and Yoshito Gotoh† †
National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan Hitachi Ltd., 2520 Akanuma, Hatoyama, Saitama 350-0395, Japan § High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan Kitasato University, 1-15-1 Kitasato, Sagamihara, Kanagawa 228-8555, Japan # Mitsui Engineering and Shipbuilding Co., Ltd., 1 Yawatakaigandori, Ichihara, Chiba 290-8531, Japan
)
‡
ABSTRACT: Nondestructive observation of anomalously preserved methane (CH4) hydrate stored outside its thermodynamically stable zone below the melting point of ice was performed at 193 K. By use of two different types of phase contrast X-ray imaging techniques, X-ray interferometric imaging and diffraction enhanced imaging, it was shown that CH4 hydrate stored under anomalous preservation conditions was completely enveloped and stabilized by a thin layer of ice. The thickness of the ice layer was inhomogeneous, with an average thickness of about 100 μm after storage for 24 h at 253 K. These results obtained were consistent with quantitative analysis by means of powder X-ray diffraction and Raman spectroscopy. It was revealed that the interior of CH4 hydrate retained a high CH4 gas storage capacity even after one month under thermodynamically unstable conditions.
’ INTRODUCTION The storage of gas in solids for energy applications is of paramount importance.1 Today, materials of ever-increasing complexity are being synthesized for methane (CH4) storage, including metal oxide frameworks,2 microporous organic polymers,3 and carbon nanotubes,4 because CH4 is a major component of natural gas and an important clean energy source, like hydrogen (H2). For practical application of these materials, their stability, adsorption, and desorption properties and storage capacity are important issues. Clathrate hydrates, also known as gas hydrates, are hydrate guesthost compounds and are crystalline materials consisting of water molecules that incorporate guest molecules inside hydrogen-bonded water cages.5 CH4 hydrate, now seen as a possible source of energy globally, is attracting attention for storage and transportation of CH4.69 Although the crystallographic properties of CH4 hydrate, such as cage occupancy and CH4 uptake of approximately 170 V (STP)/V, are well understood,10 controlling its high preservative property under atmospheric pressure and higher temperature conditions remain a challenge. It is known that some gas hydrates can be anomalously preserved at atmospheric pressure just below the melting point of ice (273.2 K), even though this is well outside the zone of thermodynamic stability of CH4 hydrate.11 This effect has been termed self-preservation12 and anomalous preservation.13 r 2011 American Chemical Society
The mechanisms of self-preservation of CH4 hydrate have been extensively studied by measuring that surface and mass fraction of dissociating gas hydrates1327 as well as theoretical calculations.2831 It has been concluded that the formation of an ice layer with different porosity or texture and morphology difference of ice crystals after hydrate dissociation and gas diffusion through the ice layer controls self-preservation of CH4 hydrate below about 240 K.14,15,17,19,22 However, the internal texture and role of the ice layer in anomalously preserved CH4 hydrate above 240 K remain unclear. It is important to develop a model to allow the highly stable CH4 hydrate to be formed on a macroscopic scale for practical applications. However, visualization of gas hydrate coexisting with ice using conventional techniques, such as optical microscopy, cryogenic scanning electron microscopy (cryo-SEM), X-ray absorption contrast imaging or conventional X-ray computed tomography (CT), and magnetic resonance imaging, is difficult because both materials are composed of water molecules, even though CH4 hydrate includes caged CH4 molecules. Phase contrast X-ray imaging is a unique technique used to identify gas hydrates coexisting with both ice32 and liquid Received: March 17, 2011 Revised: July 1, 2011 Published: July 22, 2011 16193
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The Journal of Physical Chemistry C water,33 while X-ray absorption-contrast imaging requires an additive to visualize gas hydrates coexisting with water, to enhance the difference in density between the hydrate and water.34 This is because an X-ray phase-shift cross section is more than a hundred times larger than that of X-ray absorption.35 The advantage of phase contrast imaging is that it is more pronounced in elements with low atomic numbers such as carbon and oxygen. In this respect, phase contrast X-ray imaging should be viable for the probing the structure of CH4 hydrate coexisting with ice. X-ray interferometric imaging (XII) and diffraction enhanced imaging (DEI) are usually used for twoand three-dimensional observation of large samples (several centimeters in diameter). XII is suitable for detecting gradual phase shifts and the absolute density of materials with high density resolution (∼0.0007 g/cm3),36 but the method has a limited dynamic range, and it is not applicable to see boundaries between phases with too different phase-shift as in the case of sample and air. On the other hand, DEI allows a wide dynamic range of densities to be probed, which enables observation of samples containing regions with large density gradients.36 The density resolution of DEI (∼0.01 g/cm3) is not as high as that of XII, but it is still applicable to systems containing gas bubbles. Visual observation of CH4 hydrate rather than alternative gas hydrates is necessary to assess the potential of CH4 hydrate as gas storage media. Herein, CH4 hydrate pellets stored outside their thermodynamically stable zone were used as an example of selfpreservation phenomena, and nondestructive, macroscopic observation of preserved CH4 hydrate was achieved using XII and DEI. In this study, macroscopic observation results coupled with quantitative analysis from powder X-ray diffraction (PXRD) and Raman spectroscopic measurements show the distribution of CH4 hydrate coexisting with ice. The high stability and potential of CH4 hydrate pellets for use in practical CH4 storage is revealed.
’ EXPERIMENTAL METHODS Pillow-shaped CH4 hydrate pellets formed as below were used in this study (Figure 1). CH4 hydrate slurry of about 10 wt % concentration of CH4 hydrate was formed in a high-pressure reactor by mixing liquid water and CH4 gas. The hydrate slurry formed was filtered for dewatering of unreacted water by gravity and pressure effects without using mechanical method, and the hydrate concentration was increased up to 3050 wt %. Then the remaining materials was supplied to a rotary drum type pelletizer for molding the semisolid state CH4 hydrate into the pillow-shaped pellets.37 All processes above were performed under hydrate formation conditions at 279 K with a temperature deviation of 1 K and 5.5 MPa of CH4 gas to avoid hydrate dissociation.38 After a rapid drop in pressure from 5.5 MPa to atmospheric pressure at 253 K, the pellet samples were stored at 253 K at atmospheric pressure under air and CH4 gas due to the hydrate dissociation. The pellets were stored at 253 K for between 10 min and about one month. Before use, each sample was cooled to 123 K to avoid further hydrate dissociation or formation of thermal cracks caused by excessive cooling. After one day, each sample was transferred under a N2 gas atmosphere at a temperature of less than 100 K before experiments were performed. The pellets were dense, with a porosity of less than 1%. Characterization of the CH4 hydrate sample was performed by PXRD. For PXRD measurements, a whole hydrate pellet was
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Figure 1. (a) CH4 hydrate pellets used in this study. (b) Size of a pellet.
ground into a finely powder under a N2 gas atmosphere at less than 100 K. The powder was loaded into the top of a specimen holder made of Cu and was covered with a Kapton film (Du Pont-Toray Co.) with a thickness of 7.5 μm. The sample was then placed in a cryogenic cell attached to an X-ray diffractometer. PXRD measurements were performed in the θ/2θ step scan mode with a step width of 0.02° over a 2θ range of 660° using Cu KR radiation and parallel beam optics (40 kV, 40 mA; Rigaku model Ultima III). Diffraction data were measured for 40 min on the CH4 hydrate from 93 to 153 K under a vacuum. The lattice constant of CH4 hydrate was determined with a fullpattern fitting method using the RIETAN-200039 program. Here, Pseudo-Voigt function and background parameters were fitted to observed PXRD profiles. Above 153 K, the lattice constant of CH4 hydrate was measured by monitoring the strongest Bragg reflection of structure I (321) for 1 min of measurements in 10 K increments up to 253 K. During each measurement above 153 K, the cryogenic cell was pressurized to 0.05 MPa with dry N2 gas to suppress hydrate dissociation,40 because powdered hydrate samples readily dissociate around or above their equilibrium temperature. Any sign of cubic ice formation for the pellet sample was not detected; two-phase analysis taking into account the structures of CH4 hydrate I (Pm3n) and hexagonal ice (P63/mmc) was performed by means of the Rietveld technique using the RIETAN-2000 program39 for quantitative analysis of the mass fraction of CH4 hydrate and ice within each sample. Here, the structure of deuterated methane hydrate41 and hexagonal ice42 determined by neutron diffraction were used as the structure model Tree different pellet sample for each storage time were measured to check the reproducibility of the diffraction pattern. Inclusion of CH4 within CH4 hydrate crystals was measured by Raman spectroscopy using a cryogenic cell at 90 K. An Ar ion laser operating at 488 nm was used for Raman excitation. The light scattered from the sample was collected in a 180° backscattering configuration and analyzed with a triple polychromator (SPEX1877). Spectra were recorded with a charge-coupled device (CCD) detector with a wavenumber range of 530 cm1 and resolution of 1.4 cm1 cooled with liquid nitrogen. Ne emission lines were used to calibrate the Raman spectrometer. For the Raman spectroscopic measurements, whole hydrate pellet was grounded into powder as in the case with PXRD measurements. For nondestructive imaging of CH4 hydrate pellets, phase contrast X-ray images were collected using a monochromatic synchrotron X-ray at a vertical wiggler beamline (BL-14C) at the Photon Factory in Tsukuba, Japan. The X-ray was monochromatized to 35 keV using a Si (220) double-crystal monochromator, and the beam was enlarged horizontally using an asymmetric Si (220) crystal with an asymmetric angle of 3.5°. For XII, an X-ray interferometer was used to detect phase shifts by converting phase shifts into interferograms. One of the two 16194
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Figure 3. PXRD pattern of CH4 hydrate, which was stored under atmospheric pressure at 253 K for 24 h, obtained at 93 K. The plus signs (+) denote the observed intensities; the red solid line was calculated from the best-fit model of the Rietveld refinement. The bottom curve shows the deviation between the observed and calculated intensities. The upper dashes represent the calculated peak positions for a structure I hydrate and the lower dashes represent those for hexagonal ice.
Figure 2. Schematic diagram showing the experimental setup for phase contrast X-ray-imaging. Top view of the setup of (a) XII and (b) DEI. Here, incident X-rays were enlarged horizontally by an asymmetric Si(220) crystal. (c) Schematic diagram of the sample chamber parallel to the direction of X-ray radiation.
generated interference patterns was detected using a CCD-based X-ray imager (detector dimensions 12.5 μm 12.5 μm pixel), while the other was detected using a feedback positioning system (as shown in Figure 2a).43 Three fringe scans were used to obtain a phase map, which is a spatial distribution of the phase shift. The field of view of the XII system was 25 mm 35 mm, with a spatial resolution of 40 μm. To obtain a phase contrast X-ray CT image, the methane hydrate sample was rotated 180° in 0.45° steps. Three fringe scans with an exposure time of 4 s were used to obtain each interference pattern, and the total measurement time was about 100 min. For DEI, an incident monochromatic X-ray beam enlarged horizontally by an asymmetric Si(220) crystal was used to irradiate the sample. The X-ray beam that passed through the sample was diffracted by a Si(220) analyzer crystal and then entered an X-ray imager (Figure 2b).44 A phase map was produced using two images obtained by scanning the analyzer
Figure 4. Raman spectra obtained for structure I CH4 hydrate samples. Samples were stored under atmospheric pressure at 253 K for 10 min, 6 h, and 24 h.
crystal at each position for 5 s, giving a total measurement time of about 60 min. The field of view of the DEI system was 25 mm 35 mm with a spatial resolution of 40 μm. To obtain threedimensional (3-D) phase contrast X-ray CT images, the pellet was rotated 180° in 0.72° steps. During each XII and DEI measurement, the samples were immersed in liquid methyl acetate (99.5%, Kishida Chemical Co.) maintained at 193 ( 1 K. The presence of liquid methyl acetate around the sample prevented undesirable outline contrasts from the outer surface of the sample, allowing internal observation. Gas bubbles were not formed caused by hydrate dissociation in the liquid methyl acetate at temperatures below 193 K. Even if some gas bubbles attached on the pellet surface being soaked into the methyl acetate, these bubbles were removed before XII measurements. 16195
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Figure 5. Mass fraction of CH4 hydrate in the CH4 hydrate pellets at 253 K as a function of time. The initial value was measured for a hydrate pellet stored for 1 h after synthesis. Solid squares and open squares represent data obtained for the change in weight of a CH4 hydrate pellet and from PXRD, respectively.
For density analysis using XII, a single crystal of hexagonal ice of 10 15 5 mm in size, which was grown by the Czochralski method, was measured by XII in liquid methyl acetate at 193 K as an external standard of density. PXRD measurements have calculated the density of such a crystal to be 0.927 g/cm3 at 193 K.45
’ RESULTS AND DISCUSSION To distinguish CH4 hydrate and ice using X-ray imaging techniques, the density of each crystal is very important. Although density is a fundamental physical property and essential for assessing gas hydrates as gas storage media, the density of CH4 hydrate as a function of temperature is not well-known. In this study, the density of the CH4 hydrate formed and stability of the CH4 hydrate pellets were assessed. PXRD allowed the crystal structure of CH4 hydrate to be identified as a regular cubic structure I hydrate, as shown in Figure 3. Diffraction peaks from hexagonal ice, which result from untransformed water into hydrate during formation process and hydrate dissociation, were also detected. In general, the coexistence of ice with CH4 hydrate makes it difficult to measure the density of CH4 hydrate by macroscopic methods. Raman spectroscopic measurements of the ν1 symmetric band of CH4 did not show any changes in the pellet samples over storage times from 10 min to 24 h at a temperature well above that recognized as the limit for thermodynamic stability, as shown in Figure 4. Therefore, the cage occupancies of the CH4 hydrate are likely to remain constant over the storage period investigated. The time dependences of the mass fraction of CH4 hydrate in pellets that were stored at 253 K under an air atmosphere, and that of CH4 gas released from the hydrate sample due to its dissociation, were evaluated by measuring the change in weight of the hydrate pellets caused by dissociation of CH4 hydrate. To estimate the mass fraction of CH4 hydrate in the pellets, cage occupancies of 97 and 89% for large and small cages of CH4 hydrate, respectively, which correspond to a hydration number of 6.05,10 were assumed to be constant over the storage period. The mass fractions of CH4 hydrate measured from the change in weight of the hydrate pellets agree with the values determined from Rietveld analysis of the PXRD data; 89% at 10 min, 89% at 6 h, and 85% at 24 h (Figure 5). More than 80% of the CH4 hydrate
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Figure 6. Temperature dependence of the lattice constant of CH4 hydrate measured by PXRD and corresponding densities calculated by assuming cage occupancies of 97 and 89% for large and small cages, respectively.10 The density of hexagonal ice (black line) is also shown for comparison.57 Open circles show the density of CH4 hydrate estimated by XII.
remained even after one month of the pellets being kept outside the zone of thermodynamic stability. Lattice constants of CH4 hydrate as a function of temperature are shown in Figure 6. As expected, thermal expansion of CH4 hydrate was larger than that of hexagonal ice. The density of CH4 hydrate as a function of temperature was estimated using the cage occupancy reported for CH4 hydrate.10 The densities of CH4 hydrate are higher than those of hexagonal ice (Figure 6), and the difference in density decreases as the temperature increases. Figure 7 shows nondestructive images obtained of three different CH4 hydrate pellets measured by XII at 193 K. Before performing XII measurements, the pellets were stored at 253 K for between 10 min and 28 days. Because the white regions within the hydrate pellets correspond to the regions with smaller density than surrounding, thus the white regions may correspond to ice formed by hydrate dissociation or freezing of untransformed water into hydrate during the hydrate formation process. Comparison of X-ray-phase-shift between the methyl acetate and the hexagonal ice and between the methyl acetate and the CH4 hydrate revealed that the density of the CH4 hydrate was 0.007 ( 0.003 g/cm3 higher than that of hexagonal ice (0.927 g/cm3) at 193 K. Therefore, the density of CH4 hydrate inside each pellet was estimated to be 0.934 ( 0.003 g/cm3 at 193 K. This density is equivalent to that obtained from PXRD analysis (Figure 6) and shows that it is possible to distinguish CH4 hydrate and hexagonal ice using XII. It is not known about grain size of the CH4 hydrate within the pellet sample formed from liquid water and CH4 gas, while it has been reported that mean grain size of laboratory produced CH4 hydrate from mixture of ice and CH4 gas were about 40 μm.46 Whether the grain size of CH4 hydrate is smaller or larger than the instrumental resolution, the consistency of the density of CH4 hydrate analyzed from PXRD and XII in this study suggests that effect of grain boundary or minute ice within is negligible for density analysis of the pellet samples done by XII in this study. In additional, the gray scale level of each image is almost homogeneous across the cross-section of each pellet within the instrumental resolution. This suggests that the density of preserved CH4 hydrate remained constant over the cross-section of the pellet. Therefore, it is expected that such pellets containing anomalously preserved CH4 hydrate exhibit a high capacity for the storage of CH4 gas even under thermodynamically unstable conditions. 16196
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Figure 7. XII images of CH4 hydrate pellets that had been stored at 253 K under atmospheric pressure. Cross section of the pellet sample stored for (a) 10 min, (b) 24 h, and (c) 28 days. An enlarged region of the outer surface of each pellet is also shown. Also, three-dimensional (3D) XII images of the pellet sample stored for (d) 10 min, (e) 24 h, and (f) 28 days depict internal structure. Here, red regions in the images indicate areas where the densities are lower than that of the surrounding CH4 hydrate, which correspond to ice. The inside of the CH4 hydrate pellet was made transparent to show the ice more clearly.
Next, the thickness of the ice layer that tightly enveloped the CH4 hydrate pellet was then analyzed. In Figure 7, curved edges correspond to the original outer surface of the pellet that was stored at 253 K, and straight edges correspond to sections cut at less than 100 K without hydrate dissociation for comparison. In the case of a CH4 hydrate pellet stored at 253 K for 10 min, we did not observe ice envelop within the instrumental resolution (Figure 7a). However, a CH4 hydrate pellet stored at 253 K for 24 h seems to be enveloped by a thin layer ice caused by hydrate dissociation, while cut sections were not (Figure 7b). The difference between the original outer surface and cut section is highlighted in parts a and b of Figure 8. These images suggest that the thickness of ice increased gradually over the initial 24 h of storage time, because the spatial resolution of XII was 40 μm, so an ice layer thinner than this may not be detected. The thickness of the ice layer increased slowly after the initial dissociation of CH4 hydrate. Afterward, the thickness of the ice layer was inhomogeneous, with a maximum value of about 200 μm and an average thickness of about 100 μm during the initial 24 h of storage (parts c and d of Figure 8). The initial inhomogeneous growth of ice may be enhanced by surface texture, roughness, or pores in the original hydrate pellets, which may act as a site for hydrate dissociation as is shown by the microscopic surface observations of CO2 hydrate done by cryo-SEM.47
Figure 8. Thickness and distribution of ice enveloping a CH4 hydrate pellet that had been stored at 253 K under atmospheric pressure for 24 h. (a) Cross section of the CH4 hydrate pellet measured by XII. (b) Gray scale level along the two lines indicated in (a) as AA0 and BB0 . Spike peak position on the line AA0 corresponds to ice. (c) Twodimensional distribution of an ice layer of different thicknesses enveloping a CH4 hydrate pellet; the plane is perpendicular to the plane shown in (a). The shading in the figure signals the variation in the thickness of the ice layer from 0 μm (black) to 255 μm (white). (d) Profile of the thickness of ice along the region indicated as an open arrow in (c). Each number in the figure represents a position shown in (c). Here, the thickness at position 2 was overestimated because of the shape of the pellet. 16197
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The Journal of Physical Chemistry C The greatest advantage of the phase-contrast X-ray imaging techniques is that it is possible to visualize the inside of materials to quantitatively analyze the distribution of CH4 hydrate and ice even though the spatial resolution is not as high as techniques such as SEM or synchrotron radiation X-ray cryo-tomographic microscopy (SRXCTM) with pixel size of 1.4 μm.48 Figures 7 show that there are some regions corresponding to ice inside of the CH4 hydrate pellet and that their size likely to remain constant over the storage period. Thus, the increase in the mass fraction of ice over the storage period can be attributed largely to the increase in the thickness of ice around the hydrate pellets, not growth of ice particles within the CH4 hydrate pellets, due to the hydrate dissociation. As described above, the mass fraction of ice increased 4% during the initial 24 h of storage (Figure 5), which corresponds to an increase in the thickness of ice of about 120 μm, assuming that the envelopment of ice around the pillow-shaped CH4 hydrate pellet is uniform. This estimated value is in good agreement with the mean value of the thickness of ice layer observed by XII. These results are also consistent with recent microscopic experimental results showing the formation of very thin ice layers (100 μm for CH4 hydrate27 and several μm for CO2 hydrate47) on the outer surface of gas hydrates, caused by its dissociation at temperatures just below the melting point of ice. Figure 9 shows a DEI image of a CH4 hydrate pellet taken at 193 K. The pellet had been stored at 253 K for 10 min, and then it was cooled to a temperature of less than 100 K under a N2 gas atmosphere before the measurement. Anomalous preservation of the pellet sample was expected due to its thermal history at 253 K, but this figure shows that gas bubbles formed due to hydrate dissociation at 198 K within few minutes in the liquid methyl acetate at around several hundreds μm inward of the outer surface of CH4 hydrate pellet as well as thermal cracks inside the pellet. It is reasonable to occur CH4 hydrate dissociation at 198 K because the experimental temperature was 3 K higher than the equilibrium temperature for CH4 hydrate under atmospheric pressure. Here, the point is that the pellet sample remained at 253 K was anomalously preserved (Figure 5), but then the pellet sample dissociated at 198 K. The phenomena observed in this study is likely to be in agreement with the earlier study that cooling from 268.2 K to 251 K increased dissociation rate of CH4 gas hydrate.16 We speculate that cooling of the pellet from 253 K may cause to form thermal cracks at the boundary between CH4 hydrate and ice layer due to differences in the thermal expansion of CH4 hydrate and ice (as shown in Figure 6). These cracks probably act as a nucleation site for hydrate dissociation and form CH4 gas bubbles on the cracks at 198 K. On the other hand, Figure 9 shows that hydrate dissociation did not occurred inside of the CH4 hydrate pellet and the interior of the pellet did not change at 198 K. Therefore, these results confirm the above experimental results that the interior of the hydrate sample retained a high storage capacity for CH4 gas. In this study, it was shown that a very thin ice layer prevented CH4 hydrate pellets from dissociating into ice even at 253 K under atmospheric pressure. The interior of the pellets retained a high CH4 gas storage capacity being expected under thermodynamically stable conditions. The advantage of CH4 hydrate is that it is easy to remove CH4 gas from it, because the dissociation rate of CH4 hydrate increases rapidly above the melting point of ice.13 A number of other challenges still remain for practical CH4 storage using CH4 hydrate.49 However, the possibility of new binary systems composed of CH4 and additional guest molecules,50,51 and the use of H2 hydrate for H2 storage are exciting prospects.52,53
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Figure 9. DEI images of a cross section of a CH4 hydrate pellet that had been stored at 253 K under atmospheric pressure for 10 min. An enlarged region of the outer surface of the pellet is also shown.
The use of dry water, which would increase CH4 uptake in CH4 hydrate without any mechanical mixing, would be interesting to investigate.54 However, these methods trade off gas storage capacity against hydrate stability or formation kinetics. In this respect, CH4 hydrate stabilized by its self-preservation phenomenon might one of the most favorable and environmentally friendly candidates for CH4 storage. Recently, it is reported that dissociation of gas hydrates strongly depend on type of gas molecules, gas composition, or ice formation due to hydrate dissociation.55,56 The X-ray imaging techniques reported herein are applicable for most of hydrate systems. Then further nondestructive observations may work for understanding of these phenomena.
’ SUMMARY Density of CH4 hydrate as a function of temperature was determined by using the PXRD method and reported value of cage occupancy for CH4 hydrate. The density of CH4 hydrate estimated by XII technique was in good agreement with that determined by the PXRD and Raman spectroscopic measurements. And it was shown that it is possible to distinguish CH4 hydrate and ice by means of XII. Nondestructive observation of anomalously preserved CH4 hydrate stored outside its thermodynamically stable zone below the melting point of ice were performed using XII revealed that CH4 hydrate stored at 253 K was tightly enveloped and stabilized by a layer of ice with an average thickness of 100 μm. On the other hand, DEI was performed for nondestructive observation of the pellet sample coexisting with bubble. DEI image of the CH4 hydrate sample show that CH4 hydrate started to dissociate within few minute in methyl acetate at 198 K without selfpreservation. Accordingly, it was revealed that the interior of the sample retained a high storage capacity for CH4 gas at 253 K, even after storage for one month. ’ AUTHOR INFORMATION Corresponding Author
*Phone: 81-29-861-4506. E-mail:
[email protected]. 16198
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’ ACKNOWLEDGMENT Part of this study was carried out under Proposal Nos. 2008G120, 2010G170, and 2009S2-006 and was approved by the High Energy Accelerator Research Organization. S.T. thanks Dr. A. Miyamoto of Hokkaido University for providing a single crystal of hexagonal ice. ’ REFERENCES (1) Morris, R. E.; Wheatley, P. S. Angew. Chem., Int. Ed. 2008, 47, 4966. (2) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469. (3) Wood, C. D.; Tan, B.; Trewin, A.; Su, F.; Rosseinsky, M. J.; Bradshaw, D.; Sun, Y.; Zhou, L.; Cooper, A. I. Adv. Mater. 2008, 20, 1916. (4) Maniwa, Y.; Matsuda, K.; Kyakuno, H.; Ogasawara, S.; Hibi, T.; Kadowaki, H.; Suzuki, S.; Achiba, Y.; Kataura, H. Nat. Mater. 2007, 6, 135. (5) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC, Taylor Francis: Boca Raton, FL, 2008. (6) Englezos, P. Ind. Eng. Chem. Res. 1993, 32, 1251. (7) Gudmundson, J.; Borrehaug, A. Proc. 2nd Intnl. Conf. on Natural Gas Hydrates; Monfort, J. P.; , Ed., Toulouse, 1996, 415 (8) Sloan, E. D. Nature 2003, 426, 353. (9) Thomas, S.; Dawe, R. E. Energy 2003, 28, 1461. (10) Ripmeester, J. A.; Ratcliffe, C. I. J. Phys. Chem. 1988, 92, 337. (11) Davidson, D. W.; Garg, S. K.; Gouugh, S. R.; Handa, Y. P.; Ratcliffe, C. I.; Ripmeester, J. A.; Tse, J. S. Geochim. Cosmochim. Acta 1986, 50, 619. (12) Yakushev, V. S.; Istomin, V. A. Physics and Chemistry of Ice; Hokkaido University Press: Sapporo, 1992, 136. (13) Stern, L. A.; Circone, S.; Kirby, S. H.; Durham, W. B. J. Phys. Chem. B. 2001, 105, 1756. (14) Takeya, S.; Shimada, W.; Kamata, Y.; Ebinuma, T.; Uchida, T.; Nagao, J.; Narita, H. J. Phys. Chem. A 2001, 105, 9756. (15) Takeya, S.; Ebinuma, T.; Uchida, T.; Nagao, J.; Narita, H. J. Cryst. Growth 2002, 237239, 379. (16) Stern, L. A.; Circone, S.; Kirby, S. H.; Durham, W. B. Can. J. Phys. 2003, 81, 271. (17) Komai, T.; Kang, S.; Yoon, J.; Yamamoto, Y.; Kawamura, T.; Ohtake, M. J. Phys. Chem. B 2004, 108, 8062. (18) Circone, S.; Stern, L. A.; Kirby, S. H. Am. Mineral. 2004, 89, 1192. (19) Kuhs, W. F.; Genov, G.; Satykova, D. K.; Hansen, T. Phys. Chem. Chem. Phys. 2004, 6, 4917. (20) Giavarini, C.; Maccioni, F. Ind. Eng. Chem. Res. 2004, 43, 6616–6621. (21) Kawamura, T.; Yamamoto, Y.; Yoon, J.-H.; Sakamoto Y.; Komai T.; Haneda, H.; Ohtake M.; Ohga, K. Proc. 14TH ISOPE, Toulon, France, 2004, 48-51. (22) Shimada, W.; Takeya, S.; Kamata, Y.; Uchida, T.; Nagao, J.; Ebinuma, T.; Narita, H. J. Phys. Chem. B 2005, 109, 5802. (23) Takeya, S.; Uchida, T.; Nagao, J.; Ohmura, R.; Shimada, W.; Kamata, Y.; Ebinuma, T.; Narita, H. Chem. Eng. Sci. 2005, 60, 1383. (24) Zhang, G.; Rogers, R. E. Chem. Eng. Sci. 2008, 63, 2066–2074. (25) Nagao, J.; Shimomura, N.; Ebinuma, T.; Narira, H. Proc. 6th Int. Conf. Gas Hydrates, Vancouver, Canada, 2008. (26) Melnikov, V. P.; Nestrov, A. N.; Reshetnikov, A. M.; Istomin, V. A.; Kwon, V. G. Chem. Eng. Sci. 2010, 65, 906. (27) Takeya, S.; Ripmeester, J. A. Chem. Phys. Chem. 2010, 11, 70. (28) Belosludov, V. R.; Subbotin, O. S.; Krupskii, D. S.; Ikeshoji, T.; Belosludov, R. V.; Kawazoe, Y.; Kudoh, J. J. Phys: Conf. Ser. 2006, 29, 198–205. (29) Subbotin, O. S.; Ikeshoji, T.; Belosludov, V. R.; Kudoh, J.; Belosludov, R. V.; Kawazoe, Y. J. Phys. Conf. Ser. 2006, 29, 206–209.
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