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2008, 112, 1885-1887 Published on Web 01/30/2008
Water Cavities of sH Clathrate Hydrate Stabilized by Molecular Hydrogen Timothy A. Strobel, Carolyn A. Koh, and E. Dendy Sloan* Colorado School of Mines, Chemical Engineering Department, Golden, Colorado 80401 ReceiVed: NoVember 20, 2007; In Final Form: December 17, 2007
X-ray diffraction and Raman spectroscopic measurements confirm that molecular hydrogen can be contained within the small water cavities of a binary sH clathrate hydrate using large guest molecules that stabilize the large cavity. The potential increase in hydrogen storage could be more than 40% when compared with binary sII hydrates. This work demonstrates the stabilization of hydrogen in a hydrate structure previously unknown for encapsulating molecular hydrogen, indicating the potential for other inclusion compound materials with even greater hydrogen storage capabilities.
1. Introduction Since the first experimental report of pure hydrogen clathrate hydrate in 1999,1 the encapsulation of hydrogen molecules within clathrate hydrate phases has provided grounds for both scientific curiosity and engineering utility. Initially, the hydrogen molecule was thought to be too small to contribute to the stability of clathrate hydrates.2,3 Although high pressures (or low temperatures) are required to stabilize pure hydrogen hydrate, unique features such as multiple cavity occupation,4 quantum effects,5-7 and molecular hydrogen separation distances smaller than solid hydrogen8 have stimulated interest in the general scientific community. Additionally, the intrinsic features of hydrogen hydrates have been proposed for utilization in both hydrogen storage materials9,10 and in gas separation technologies.11 Storage of hydrogen within clathrate cavities could potentially be advantageous relative to other hydrogen storage materials as the hydrogen is stored in molecular form (with a low binding energy and requiring no chemical reaction for release) and the primary byproduct is liquid water. To date, hydrogen has been shown to be enclathrated in only two types of clathrate hydrate structures: sII [8(51264)‚16(512)‚ 136H2O]4,10 and sI [6(51262)‚2(512)‚46H2O],12 where the values in parentheses indicate the number of sides in a cavity face, raised to the power of the number of such faces in a cavity. Recently, hydrogen has also been observed in a semi-clathrate with tetra-n-butyl ammonium bromide/fluoride.13 In the present study, we show that hydrogen can be stabilized within the clathrate hydrate structure H [1(51268)‚2(435663)‚3(512)‚34H2O] with the assistance of another large guest molecule. On the basis of available small cavities in the unit cell, the hydrogen storage capacity of sH hydrate could be favorable when compared with binary hydrates of other structures.14 Additionally, the ability to stabilize hydrogen within sH hydrate provides new insight into the molecular behavior of confined hydrogen (i.e., hydrogen can stabilize new types of clathrate cavities), which can aid in the design of future hydrogen storage materials. Moreover, the * To whom correspondence should be addressed. E-mail: esloan@ mines.edu.
10.1021/jp7110549 CCC: $40.75
ability of such a small molecule to stabilize sH hydrate has not been demonstrated until now. 2. Experimental Methods Hydrate samples were prepared by filling a high pressure stainless steel cell with crushed ice particles (400 MPa) sII pure hydrogen hydrate transforms into filled ice phases over a limited pressure-temperature (p-T) range,1,19 the findings of the present work may warrant additional research over a larger p-T range to investigate the possibility of forming a pure sH hydrogen hydrate, as it is known that many small molecules (Ar, N2) may form sH as single guest component hydrates.20 In this case, the hydrogen storage capacity could be as high as 5.6 wt %, approaching the 2010 U.S. DOE goal of 6.0 wt %. In order to investigate the hydrogen molecules within the hydrate phase, Raman spectroscopic measurements were performed on binary sH hydrogen hydrates, as well as the pure gaseous hydrogen phase and the well-defined sII tetrahydrofuran (THF)/H2 hydrate for comparison (Figure 2). At room temperature, the Raman spectrum of gaseous hydrogen exhibits four observable transitions in the Q branch [Q∆ν(J), where ν and J are the vibrational and rotational quantum numbers]: Q1(3), Q1(2), Q1(1), and Q1(0), where the intensity of the Q1(1) peak is dominated by the equilibrium ortho-para hydrogen ratio and rotational population distribution factor.21-23 As the temperature was decreased to the normal boiling point of liquid nitrogen (77 K), only the Q1(1)-4155 cm-1 and Q1(0)-4161 cm-1 peaks were observed, as the highest energy rotational states become depopulated with decreasing temperature (Figure 2). At 77 K, THF/H2 hydrate showed a distinct vibron peak at 4119 cm-1 with a well-defined shoulder at 4124 cm-1. At higher temperatures (∼280 K), these contributions are convoluted into one broad band.10 Because the large cavities are essentially completely filled with THF and because several studies have determined the
Figure 2. Vibron region in the Raman spectra for pure gaseous H2, sII THF/H2 hydrate, sH MTBE/H2 hydrate, sH MCH/H2 hydrate, and sH 2,2,3-TMB/H2 hydrate.
hydrogen occupancy in the small dodecahedral cavity to be no greater than unity (up to 350 MPa),8,24-27 we tentatively assign the THF/H2 hydrate vibron peaks at 4119 and 4124 cm-1 to the Q1(1) and Q1(0) transitions of single hydrogen molecules within the small dodecahedral cavities. This assignment is also supported by the separation between these peaks in the hydrate phase (∼5 cm-1) compared with that of the pure gaseous phase (∼6 cm-1) at the same conditions (cf. the difference in separation between the Q1(1) and Q1(0) peaks compared with the free gaseous phase for H2 adsorbed on carbon nanotubes28 or H2 within an argon matrix29). We note that other resolution limited bands may exist within these broad peaks. The Raman spectra for the binary sH hydrates were analogous to that of sII THF/H2 hydrate (Figure 2). As with the THF/H2 hydrate, the decreased frequency of the hydrogen vibron bands, when compared with the gaseous phase, is consistent with the enclathration of hydrogen molecules.4,10 For all of the sH hydrates studied, the most intense vibron peak was blue-shifted from that of the sII THF/H2 hydrate by about 1-2 cm-1. This increase in stretching frequency vibration could be caused by increased interactions with the host lattice induced by small differences in the cavity sizes between sII and sH lattices.30 In comparison to sII THF/H2 hydrate, sH contains small 512 cavities, but sH also comprises slightly larger 435663 cavities. To further examine the contributions from the sH cavities, the Raman spectra for the sH hydrates were artificially red-shifted so that the most intense points in sH and sII THF/H2 spectra were coincident. When the normalized intensities of both spectra are overlaid (Figure 3), it is apparent that some definition in the shoulder for the binary sH hydrates is lost when compared with the binary sII hydrate. Additionally, the sH vibron peaks appears broader than that of the sII hydrate peak. Because the average radius of the 435663 cavity is only about 0.1 Å larger than that of the 512 cavity,20 the Raman bands from the two cavities are expected to be heavily convoluted (cf. sH hydrate with CH431). The additional broadening and decreased shoulder definition observed in the binary sH hydrogen hydrates is consistent with another contribution to the total vibron peak area. We tentatively assign this contribution to that of hydrogen molecules in the 435663 cavities and, like the 512 cavities, expect that the Q1(1) and Q1(0) transitions are also present. However,
Letters
J. Phys. Chem. B, Vol. 112, No. 7, 2008 1887 hydrate guest. This opens a wide potential for hydrogen storage in other types of inclusion-based compounds that could potentially store significantly greater amounts of hydrogen than the common clathrate hydrate structures. Acknowledgment. The authors acknowledge support from the United States Department of Energy under contract DEFG02-05ER46242. References and Notes
Figure 3. Vibron region in the Raman spectra for several sH hydrates and sII THF/H2 hydrate. The sH hydrate spectra have been shifted by -1 to -2 cm-1, normalized, and overlaid with the THF/H2 spectrum for comparison.
higher resolution experiments are needed to confirm the individual cavity contributions. Generally, for symmetric stretching modes of most guest molecules within clathrate cavities, the larger the clathrate cavity, the lower the frequency of vibration.30 However, this is not the case for pure hydrogen hydrate where multiply occupied large cavities create a condition where a higher frequency vibration is observed for the large cavity in the Raman spectrum when compared with the small cavity.32 The separation between the large and small cavity contributions in the Raman spectrum of pure hydrogen hydrate is on the order of 20 cm-1. Noting the similar sizes of the 512 and 435663 cavities in sH, and no significant frequency change in the Raman spectrum when compared with sII THF/H2 hydrate (singly occupied 512 cavities24-27), it is unlikely that the 435663 cavities of sH contain more than one hydrogen molecule. 4. Conclusions With single occupancy of molecular hydrogen in the 512 and 435663 cavities, and complete occupancy of the 51268 cavity by the large guest molecule, the binary sH hydrate would show a 40% increase in the amount of hydrogen stored by weight compared with binary sII hydrates. Additionally, the volumetric storage capacity would increase, as a greater amount of hydrogen is stored within the smaller hexagonal lattice, and double occupancy of the slightly larger 435663 cavity may be possible at higher pressures. Moreover, now that hydrogen enclathration has been demonstrated in all three common clathrate hydrate structures, this shows that the stabilizing guest-host interactions of hydrogen clathrates are similar to those of the more common ones (e.g., CH4 clathrate). This is a stark contrast from the original reports that hydrogen is too small to act as a clathrate
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