Article pubs.acs.org/JPCC
Investigating the Thermodynamic Stabilities of Hydrogen and Methane Binary Gas Hydrates Yuuki Matsumoto,†,‡ R. Gary Grim,† Naveed M. Khan,† Takeshi Sugahara,‡ Kazunari Ohgaki,‡ E. Dendy Sloan,† Carolyn A. Koh,† and Amadeu K. Sum*,† †
Center for Hydrate Research, Chemical Engineering Department, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401, United States ‡ Division of Chemical Engineering, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan S Supporting Information *
ABSTRACT: When hydrogen (H2) is mixed with small amounts of methane (CH4), the conditions required for clathrate hydrate formation can be significantly reduced when compared to that of simple H2 hydrate. With growing demand for CH4 as a commercially viable source of energy, H2 + CH4 binary hydrates may be more appealing than extensively studied H2 + tetrahydrofuran (THF) hydrates from an energy density standpoint. Using Raman spectroscopic and powder X-ray diffraction measurements, we show that hydrate structure and storage capacities of H2 + CH4 mixed hydrates are largely dependent on the composition of the initial gas mixture, total system pressure, and formation period. In some cases, H2 + CH4 hydrate kinetically forms structure I first, even though the thermodynamically stable phase is structure II.
■
INTRODUCTION Clathrate hydrates are a class of water-based inclusion compounds that form when water and various low molecular weight species are combined under high pressure and low temperature conditions. During the formation process, hydrogen-bonded water molecules surround and trap small guest molecules (e.g., hydrogen (H2), methane (CH4)); this creates many unique cagelike structures depending on guest size and thermodynamic conditions. Three of the most common cage types include the dodecahedron (512 cage), tetrakaidecahedron (51262 cage), and the hexakaidecahedron (51264 cage). Two 512 cages and six 51262 cages form a unit cell of the structure I (sI) hydrate. Similarly, 16 512 cages and 8 51264 cages form a unit cell of the structure II (sII) hydrate. As the amount of easily accessible petroleum-based energy resources continues to decline, the need for alternative energy materials is crucial. Of the recently proposed alternatives, two of the more promising include the use of H2 and natural gas (e.g., CH4). Examining the current technologies, many ways exist to store and transport these gases such as simple compression, liquefaction, metal hydrides, and metal−organic frameworks.1 Alternatively, with the ability to concentrate gases over 160 times their volume at normal conditions, clathrate hydrates, such as those occurring naturally in sediments under the ocean floor and in permafrost regions, have also been considered a promising energy storage medium.2−5 However, whereas the structure and thermodynamics natural gas hydrates © XXXX American Chemical Society
are relatively well understood, comparatively little is known about H2 + CH4 mixed hydrates. H2 was initially believed to be too small to stabilize hydrate cavities, and it was not until 1999 that the phase diagram of the H2 hydrate system was reported by Dyadin et al.6 Since this pioneering work, several other key discoveries have been made highlighting the structure, cage occupancies, and storage capacities of H2 hydrates.7−24 Despite encouraging initial results, the major consensus of much of the previous work has been that due to the extremely high pressures and/or low temperatures (e.g., p ≈ 150 MPa, T = 270 K) required for their stabilization,11 pure H2 hydrates may be impractical for application. As a means of easing the extreme thermodynamic requirements, several studies have investigated the possibility of adding small amounts of a second larger guest to better stabilize the hydrate structure.9,10,12−15,17,18,20,21,23,24 One of the more encouraging binary hydrate systems is H2 + tetrahydrofuran (THF) mixed hydrate. By simply adding 5.6 mol % THF, mixed hydrate can form at much more moderate conditions than simple H2 hydrate.13 Specifically, pure H2 hydrates form at approximately 150 MPa and 270 K,11 while H2 + THF mixed hydrates form at around 10 MPa and 280 K.9,13 However, the tradeoff in this scenario is that only a limited amount of H2 can Received: November 12, 2013 Revised: February 2, 2014
A
dx.doi.org/10.1021/jp411140z | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
■
Article
RESULTS AND DISCUSSION Hydrate Structural Dependence on Thermodynamic and Kinetic Conditions. Whereas the thermodynamics of binary hydrates including H2 and larger guest molecules such as THF have been well characterized,9,10,12−14,17,18,20,21,23 comparatively little is known about the thermodynamics of H2 + CH4 binary hydrate.15,22,24 From the available studies, the general agreement is that at modest mole fractions of CH4,22 sI is the preferred structure.15,24 However, it is expected that as the mole fraction of CH4 decreases, sII may become more favorable as pure H2 hydrates naturally form sII.7 In the following figures, we show that, like other known gas mixtures such as CH4 + C2H6,29−31 the hydrate structure of the H2 + CH4 mixed system is dependent on at least three variables: initial gas mole fraction (yCH4), pressure (p), and formation period (t). Figures 1 and 2 show Raman spectroscopic evidence of hydrate structural dependence on the initial composition of the
be enclathrated in the mixed hydrate because THF molecules occupy all of 51264 cages and H2 molecules can only occupy the 512 cages. For hydrates to become a practical storage medium, it is important to achieve a high energy density at relatively mild conditions. Addressing this issue, Lee et al. and Sugahara et al. reported a “tuning effect” for the H2 + THF hydrate system in 2005 and 2009, respectively.10,21 They claimed that by reducing the amount of THF added to below 5.6 mol %, H2 could occupy not only the 512 cage but also some of the 51264 cages. In principle, tuned H2 + THF hydrates could be an interesting energy storage medium. However, reproducing the tuning effect for the H2 + THF system has been a source of controversy.13,14,20,23 We suggest that, like THF, CH4 may also serve as a thermodynamic promoter, but unlike THF, CH4 is small enough that H2 may compete for occupancy in the larger cages, resulting in an overall increase in H 2 enclathration. Furthermore, H2 + CH4 mixed hydrates are expected to have the higher overall energy density than H2 + THF hydrates because CH4 serves as an additional source of energy, unlike THF. When compared to H2 + THF mixed hydrate, hydrates incorporating H2 and CH4 are comparatively underexplored,15,22,24 with some authors suggesting the H2 + CH4 system as incapable of forming binary hydrates.15 In this study, we confirm the formation of binary H2 + CH4 hydrates and report preliminary results on the thermodynamics of this system, including the effect of initial mole fraction of mixed gas, pressure, and formation period. Hydrate crystal structure and cage occupancies of guest species were obtained from powder X-ray diffractometry (PXRD) and Raman spectroscopy, respectively.
■
EXPERIMENTAL SECTION Hydrates studied in this work were synthesized by pressurizing ice powders incorporating particles less than 180 μm. About 1 g of the ice powder was loaded into a high-pressure cell of internal volume ∼4 mL followed by immersion in a glycol cooling bath held at 263 K. The cell containing the ice powder was allowed to equilibrate in the bath for more than an hour so that the inner cell temperature would rise from liquid N2 temperature to the bath temperature. The cell was then pressurized first with CH4 followed by H2 up to the respective desired pressures (p) for each experiment. The partial pressures of each component were calculated using the Peng−Robinson equation of state25,26 to satisfy the desired composition of the initial mixed gas (yCH4). As a means of confirming the initial gas composition, an empty cell was simultaneously pressurized in parallel with the same mixed gas, and the contents in the empty cell were later analyzed with GC (Hewlett-Packard 5890 Series II) with a thermal conductivity detector in order to determine any differences between the measured and estimated mole fractions. After pressurization, the cell was left in the glycol bath for a desired formation period (t). Then the cell was quenched in liquid N2 for 15 min and vented. The hydrate sample was then analyzed with PXRD (Siemens, Kristalloflex 805) with Cu X-ray (generation power: 45 kV, 40 mA) and Raman spectrometer (Horiba Jobin Yvon) with an argon ion laser (532.268 nm wavelength) at 0.1 MPa and 83 K. PXRD measurements were performed in the step-scan mode with a dwell time of 2 s and a step size of 0.02°. The PXRD pattern indexing and cell refinement were performed with PowderX27 and Chekcell28 programs.
Figure 1. Raman spectra indicating mole fraction (yCH4) dependence at p = 70 MPa and t = 72 h. (a) yCH4 = 0.10, (b) yCH4 = 0.07, (c) yCH4 = 0.05, (d) yCH4 = 0.03, and (e) yCH4 = 0.01.
gas mixture and total system pressure when held at constant temperature (T = 263 K) and formation period (t = 72 h). At a constant pressure of 70 MPa, samples prepared at yCH4 < 0.01 formed no hydrate, 0.03 ≤ yCH4 ≤ 0.05 formed sII, and yCH4 > 0.05 formed sI. At a fixed concentration (yCH4 = 0.05), the results indicate that sI hydrates are initially stable starting around 50 MPa, and it is only when the pressure exceeds ∼70 MPa does sII become the more favorable structure (Figure 2). Hydrate structure was confirmed directly by PXRD as well as indirectly by the Raman shift. In Figure 1 at yCH4 > 0.05, the C− H stretching region shows two peaks occurring at 2901 and 2913 cm−1 corresponding with CH4 occupancy of the 51262 and 512 cages, respectively, in sI.24 Furthermore, the approximate peak area ratio (AL/AS) of the two contributions is 3.26, which is similar to the stoichiometry of the sI unit cell (two 512 cages + six 51262 cages). Conversely, at yCH4 < 0.05, the approximate peak area ratio (AL/AS) of 0.55 confirms the hydrate structure had changed to sII (16 512 cages + 8 51264 cages). B
dx.doi.org/10.1021/jp411140z | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Figure 2. Raman spectra indicating initial pressure (p) dependence at yCH4 = 0.05 and t = 72 h. (a) p = 80 MPa, (b) p = 70 MPa, (c) p = 60 MPa, (d) p = 50 MPa, and (e) p = 40 MPa.
In contrast to the C−H stretching region, where only two peaks are observed (one per cage), in the H−H region, multiple peaks per cage are observed due to the multiple clusters and quantum spin of H2.19 H2 has two kinds of spin isomers, ortho and para,32 occurring in a ratio 3:1 a temperatures near the ice point. For sI hydrates, the peaks at 4121 and 4127 cm−1 are derived from H2 singly occupied in 512 cages, the peaks at 4126 and 4132 cm−1 are from H2 singly occupied in 51262 cages, and the peaks at 4144 and 4150 cm−1 indicate H2 doubly occupied in 51262 cages, as previously reported.24 In the case of sII hydrate, the peaks at 4121 and 4127 cm−1 are the same as sI hydrate, the peaks at 4130 and 4136 cm−1 are derived from doubly occupied 51264 cages, the peaks at 4138 and 4144 cm−1 indicate triply occupied 51264 cages, and the peaks at 4146 and 4152 cm−1 indicate quadruply occupied 51264 cages, in agreement with previous reported data.19 From the PXRD measurements in Figure 3, the space group and lattice constant of a typical H2 + CH4 sI hydrate sample was determined to be Pm3n with a = 1.187 ± 0.001 nm, and for a typical H2 + CH4 sII hydrate, Fd3m and a = 1.715 ± 0.003 nm. Interestingly, some of the sI and sII samples showed a significant amount of ice Ih in coexistence with the hydrate phase despite initially being well inside the estimated sII phase space (Figure 4). This appearance of ice is likely a result of isochoric procedure used in synthesizing the hydrates. Specifically, although the initial gas phase composition is within the thermodynamically stable region for sII, due to the larger molecular diameter of CH4, CH4 may preferentially become enclathrated, resulting in a depletion of available CH4 in the gas phase moving the system toward the ice−sII hydrate−vapor equilibrium conditions after a small fraction of conversion. From the viewpoint of H2 storage capacity, sII hydrate is expected to be more desirable than sI hydrate due to the ability of the large 51264 cages to enclathrate up to four H2 molecules compared to two H2 in the large cage (51262 cage) of sI.16 However, from Figures 1 and 2, sII hydrates were observed only in a limited range of conditions. Consequently, a series of
Figure 3. PXRD pattern of three hydrate samples obtained in the present study highlighting the characteristic peaks for sI, sII, and hydrate free samples (ice). (a) H2 + CH4 sI hydrate formed at yCH4 = 0.100, p = 40 MPa, (b) H2 + CH4 sII hydrate formed at yCH4 = 0.040, p = 80 MPa, and (c) ice remaining at yCH4 = 0.088, p = 10 MPa.
Figure 4. Experimentally observed hydrate structures over a range of pressures (p) and initial gas compositions (yCH4) in the H2 + CH4 + water system at t = 72 h and 263 K. Circles: H2 + CH4 sI hydrate; squares: H2 + CH4 sII hydrate; diamonds: simple CH4 sI hydrate; triangles: hydrate free.
additional experiments examining the hydrate structure a wider range of pressures and CH4 concentrations were performed to provide a better estimate of the phase space, as shown in Figure 4. Figure 4 shows that at 263 K four distinct regions are present in the H2 + CH4 system in the range of 10−80 MPa and 0−12 mol % CH4 initially in the gas phase; ice, sII, sI (binary), and sI pure CH4 hydrate. Some samples, for example measured at yCH4 = 0.10 and p = 70 MPa, showed the coexistence of sI and sII, this implying the system was not yet at equilibrium and may be a result of the isochoric method. These initial results reveal that C
dx.doi.org/10.1021/jp411140z | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
accurately capture the phase behavior of the H2 + CH4 hydrate system and overcome the experimental limitations associated with quantifying the relative hydrate occupancies of H2 and CH4, we applied the statistical thermodynamic approach for hydrate phase equilibria using the van der Waals and Platteeuw (vdWP) theory (see Supporting Information). Using the vdWP model and the pure component H2 and CH4 Kihara potential parameters,20 we first reproduced the experimental phase equilibria measurements performed by Pang et al.,22 as shown in Figure S1. From the data shown in the Supporting Information, there is good agreement between the predicted hydrate phase equilibria from the vdWP model and the available experimental data for mixtures containing H2, including binary systems with relatively high concentrations of CH4. Based on these results, it was desirable to see if the model could also be used to reproduce the observed trends in experimental diagram (Figure 4). Figure 6 shows the phase
from a practical standpoint, although sII may have a higher H2 density, the thermodynamic window for sII is small and the requirements necessary for formation, p > 50 MPa, remain extreme even after the addition of a CH4 coguest molecule. Furthermore, as the pressure decreases below ∼25 MPa, the driving force required to enclathrate H2 in either structure was too low, resulting in the formation of pure CH4 hydrate. In one additional study, the effect of formation time at a fixed mole fraction (yCH4 = 0.03) and pressure (p = 70 MPa) was studied (Figure 5). Interestingly, although sII was determined
Figure 5. Raman spectral change for each formation period (t) at yCH4 = 0.03 and p = 70 MPa. (a) t = 1 h, (b) t = 2 h, (c) t = 5 h, (d) t = 5 h, (e) t = 24 h, and (f) t = 168 h.
to be the thermodynamically preferred structure at these conditions (as shown above in Figures 1 and 4), a metastable sI phase was observed as the kinetically favorable structure in the early stages of formation. It was only at t = 5 h that the spectra indicate a change from sI to sII. To explain this structural change during the formation process, we propose that the ice initially surrounded with CH4 will quickly convert to a metastable CH4-rich sI hydrate seed. Then, the H2 + CH4 hydrate may then convert to the equilibrium sII structure with the spectra appearing mostly constant after 24 h. Thermodynamic Modeling. Figure 4 was generated from a series of ex-situ Raman and PXRD measurements at arbitrarily selected conditions to reflect the structural dependence on temperature, pressure, and composition. Consequently, Figure 4 should be considered only as a qualitative estimate of the actual phase space. Furthermore, although it is expected that sII hydrates will have an increased H2 storage capacity compared to sI, quantitative verification of the relative hydrate mole fractions could not be performed experimentally. Measurements on hydrate cage occupancies can be performed by dissociating hydrates postformation and subsequently examining the dissociated gas molecules with gas chromatography. However, because liquid N2 quenching was a necessary step in stabilizing hydrates prior to this type of analysis, we found that partial condensation and adsorption of the unreacted gas phase resulted in significant errors in the measurements and thus are not reported. In attempt to more
Figure 6. Phase equilibria of H2 + CH4 mixture at 263 K based on the vdWP model.
diagram at equivalent conditions as those shown in Figure 4, that is, T = 263 K, p = 10−360 MPa, and for yCH4 up to 0.10. The calculated phase diagram in Figure 6 clearly does not capture the regions shown in Figure 4. Specifically, from the experimental results, it was known that the binary system could form either sI or sII. To model this transition, the formation pressure was calculated at 263 K and over a range of compositions assuming first sI and then sII. Theoretically, whichever structure has the lowest formation pressure should be the thermodynamically preferred structure at a given set of conditions. However, the calculated formation pressures always predict sI as the most stable structure. These results emphasize that although the model was able to accurately reproduce the phase behavior at high concentrations of CH4 (yCH4 > 0.10) and a fixed structure (sI), there seems to be inherent limitations when it comes to modeling the phase transition. These results suggest that additional phase equilibria measurements may be required to better capture the structural transition observed experimentally. For the estimation of cage occupancies in each cage with vdWP modeling, H2 + CH4 sII hydrate prepared at yCH4 = 0.03, p = 70 MPa, and T = 263 K can achieve zCH4 = 0.83, the D
dx.doi.org/10.1021/jp411140z | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
■
amount of H2 is 0.31 wt % (2.70 mol %), and the amount of CH4 is 12.25 wt % (13.23 mol %) (θS,H2 = 0.017 68, θS,CH4 = 0.978 50, θL,1H2 = 0.120 58, θL,2H2 = 0.086 30, θL,3H2 = 0.054 90, θL,4H2 = 0.013 36, θL,CH4 = 0.718 00). Whereas, H2 + CH4 sI hydrate prepared at yCH4 = 0.07, p = 70 MPa, and T = 263 K can achieve zCH4 = 0.98, the amount of H2 is 0.02 wt % (0.25 mol %), and the amount of CH4 is 13.17 wt % (14.52 mol %) (θS,H2 = 0.005 64, θS,CH4 = 0.993 00, θL,1H2 = 0.020 91, θL,2H2 = 0.000 01, θL,CH4 = 0.975 80). As mentioned above, sII hydrate can store larger amounts of H2 because H2 can occupy a large sII cage with up to four molecules whereas H2 can occupy a large sI cage with up to two molecules, and it is easy for H2 to multiply occupy 51264 cages compared to 51262 cages.
CONCLUSIONS H2 + CH4 hydrates prepared at various conditions were analyzed by PXRD and Raman spectroscopy to reveal hydrate structure, cage occupancies of H2 and CH4, and mole fraction in hydrate phase. The structure of H2 + CH4 hydrate was determined to be strongly dependent on the mole fraction of mixed gas, total system pressure, and formation period. H2 and CH4 were observed to competitively occupy the 512, 51262, and 51264 cages. These hydrates can form at more moderate conditions compared to simple H2 hydrate and H2 + THF tuned hydrate. ASSOCIATED CONTENT
S Supporting Information *
Information on theory, method, and results of the thermodynamic modeling in the H2 + CH4 binary hydrate. This material is available free of charge via the Internet at http://pubs.acs.org.
■
REFERENCES
(1) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Hydrogen Storage in Microporous MetalOrganic Frameworks. Science 2003, 300, 1127−1129. (2) Kvenvolden, K. A. Methane Hydrate − A Major Reservoir of Carbon in the Shallow Geosphere. Chem. Geol. 1988, 71, 41−51. (3) Englezos, P. Clathrate Hydrates. Ind. Eng. Chem. Res. 1993, 32, 1251−1274. (4) Gudmundsson, J. S.; Parlaktuna, M.; Khokhar, A. A. Storing Natural-Gas as Frozen Hydrate. SPE Prod. Facil. 1994, 9, 69−73. (5) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008. (6) Dyadin, Y. A.; Larionov, E. G.; Manakov, A. Y.; Zhurko, F. V.; Evgeny, Y.; Aladko, E. Y.; Mikina, T. V.; Komarov, V. Y. Clathrate Hydrates of Hydrogen and Neon. Mendeleev Commun. 1999, 9, 209− 210. (7) Mao, W. L.; Mao, H.-k.; Goncharov, A. F.; Struzhkin, V. V.; Guo, Q.; Hu, J.; Shu, J.; Hemley, R. J.; Somayazulu, M.; Zhao, Y. Hydrogen Clusters in Clathrate Hydrates. Science 2002, 297, 2247−2249. (8) Lokshin, K. A.; Zhao, Y.; He, D.; Mao, W. L.; Mao, H.-K.; Hemley, R. J.; Lobanov, M. V.; Greenblatt, M. Structure and Dynamics of Hydrogen Molecules in the Novel Clathrate Hydrate by High Pressure Neutron Diffraction. Phys. Rev. Lett. 2004, 93, 122503. (9) Florusse, L. J.; Peters, C. J.; Schoonman, J.; Hester, K. C.; Koh, C. A.; Dec, S. F.; Marsh, K. N.; Sloan, E. D. Stable Low-Pressure Hydrogen Clusters Stored in a Binary Clathrate Hydrate. Science 2004, 306, 469−471. (10) Lee, H.; Lee, J.-W.; Kim, D. Y.; Park, J.; Seo, Y.-T.; Zeng, H.; Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. Tuning Clathrate Hydrates for Hydrogen Storage. Nature 2005, 434, 743−746. (11) Lokshin, K. A.; Zhao, Y. Fast Synthesis Method and Phase Diagram of Hydrogen Clathrate Hydrate. Appl. Phys. Lett. 2006, 88, 131909. (12) Strobel, T. A.; Taylor, C. J.; Hester, K. C.; Dec, S. F.; Koh, C. A.; Miller, K. T.; Sloan, E. D. Molecular Hydrogen Storage in Binary THF-H2 Clathrate Hydrates. J. Phys. Chem. B 2006, 110, 17121− 17125. (13) Hashimoto, S.; Murayama, S.; Sugahara, T.; Sato, H.; Ohgaki, K. Thermodynamic and Raman Spectroscopic Studies on H2 + Tetrahydrofuan Plus Water and H2 + Tetra-N-Butyl Ammonium Bromide Plus Water Mixtures Containing Gas Hydrates. Chem. Eng. Sci. 2006, 61, 7884−7888. (14) Anderson, R.; Chapoy, A.; Tohidi, B. Phase Relations and Binary Hydrate Formation in the System H2-THF-H2O. Langmuir 2007, 23, 3440−3444. (15) Skiba, S. S.; Larionov, E. G.; Manakov, A. Y.; Kolesov, B. A.; Kosyakov, V. I. Investigation of Hydrate Formation in the System H2CH4-H2O at a Pressure Up to 250 MPa. J. Phys. Chem. B 2007, 111, 11214−11220. (16) Strobel, T. A.; Koh, C. A.; Sloan, E. D. Hydrogen Storage Properties of Clathrate Hydrate Materials. Fluid Phase Equilib. 2007, 261, 382−389. (17) Yoon, J.-H.; Han, J.; Park, J.; Choi, S.; Yeon, S.-H.; Lee, H. Spectroscopic Identification, Thermodynamic Stability and Molecular Composition of Hydrogen and 1,4-Dioxane Binary Clathrate Hydrate. J. Phys. Chem. Solids 2008, 69, 1432−1435. (18) Duarte, A. R. C.; Shariati, A.; Rovetto, L. J.; Peters, C. J. Water Cavities of sH Clathrate Hydrate Stabilized by Molecular Hydrogen: Phase Equilibrium Measurements. J. Phys. Chem. B 2008, 112, 1888− 1889. (19) Strobel, T. A.; Sloan, E. D.; Koh, C. A. Raman Spectroscopic Studies of Hydrogen Clathrate Hydrates. J. Chem. Phys. 2009, 130, 014506. (20) Strobel, T. A.; Koh, C. A.; Sloan, E. D. Thermodynamic Predictions of Various Tetrahydrofuran and Hydrogen Clathrate Hydrates. Fluid Phase Equilib. 2009, 280, 61−67. (21) Sugahara, T.; Haag, J. C.; Prasad, P. S. R.; Warntjes, A. A.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Increasing Hydrogen Storage Capacity Using Tetrahydrofuran. J. Am. Chem. Soc. 2009, 131, 14616−14617.
■
■
Article
AUTHOR INFORMATION
Corresponding Author
*Tel (303) 273-3873; Fax (303) 273-3730; e-mail asum@ mines.edu (A.K.S.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Authors acknowledge the financial support of this work by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (DOE-BES Award DE-FG02-05ER46242, for R.G.G. and partial support for C.A.K. and Y.M.). Partial support for Y.M. is acknowledged from KAKENHI, Grant-in-Aid for JSPS Fellows (25·1430), and “Osaka University Scholarship for Short-Term Overseas Research Activities”.
■
NOMENCLATURE AL, Raman peak area corresponding CH4 occupying large cage [−]; AS, Raman peak area corresponding CH4 occupying small cage [−]; a, lattice constant [m]; p, initial pressure [Pa]; T, temperature [K]; t, formation time [h]; yCH4, mole fraction of CH4 in gas phase [−]; zCH4, mole fraction of CH4 in hydrate phase [−]; θL,ni, large cage occupancy of guest i as cluster of n molecule(s) [−]; θS,i, small cage occupancy of guest i [−]. E
dx.doi.org/10.1021/jp411140z | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
(22) Pang, J.; Ng, H.-J.; Zuo, J.; Zhang, D.; Ma, Q.; Chen, G. Hydrogen Gas Hydrate-Measurement and Predictions. Fluid Phase Equilib. 2012, 316, 6−10. (23) Grim, R. G.; Kerkar, P. B.; Sloan, E. D.; Koh, C. A.; Sum, A. K. Rapid Hydrogen Hydrate Growth from Non-Stoichiometric Tuning Mixtures during Liquid Nitrogen Quenching. J. Chem. Phys. 2012, 136, 234504. (24) Grim, R. G.; Kerkar, P. B.; Shebowich, M.; Arias, M.; Sloan, E. D.; Koh, C. A.; Sum, A. K. Synthesis and Characterization of sI Clathrate Hydrates Containing Hydrogen. J. Phys. Chem. C 2012, 116, 18557−18563. (25) Peng, D.-Y.; Robinson, D. B. A New Two-Constant Equation of State. Ind. Eng. Chem. Fundam. 1976, 15, 59−64. (26) Knapp, H.; Döring, R.; Oellrich, L.; Plöcker, U.; Prausnitz, J. M. Vapor-Liquid Equilibria for Mixtures of Low Boiling Substances; Chemistry Data Series Vol. VI; Dechema: Frankfurt, 1982. (27) Dong, C. PowderX: Windows-95-Based Program for Powder Xray Diffraction Data Processing. J. Appl. Crystallogr. 1999, 32, 838. (28) The Software Program Chekcell is available at http://www. ccp14.ac.uk. Developed by: Laugier, L.; Bochu, B. Laboratoire des Materiaux et du Genie Physique, EcoleSuperieure de Physique de Grenoble. (29) Subramanian, S.; Kini, R. A.; Dec, S. F.; Sloan, E. D. Evidence of Structure II Hydrate Formation From Methane Plus Ethane Mixtures. Chem. Eng. Sci. 2000, 55, 1981−1999. (30) Subramanian, S.; Ballard, A. L.; Kini, R. A.; Dec, S. F.; Sloan, E. D. Structure Transitions in Methane Plus Ethane Gas Hydrates − Part 1: Upper Transition Point and Applications. Chem. Eng. Sci. 2000, 55, 5763−5771. (31) Ballard, A. L.; Sloan, E. D. Structure Transitions in Methane Plus Ethane Gas Hydrates − Part II: Modeling Beyond Incipient Conditions. Chem. Eng. Sci. 2000, 55, 5773−5782. (32) Herzberg, G. Molecular Spectra and Molecular Structure I: Diatomic Molecules; Prentice-Hall: New York, 1939.
F
dx.doi.org/10.1021/jp411140z | J. Phys. Chem. C XXXX, XXX, XXX−XXX
