Dynamics of hydrogen molecules in the channels of binary THF-H 2 clathrate hydrate and its physicochemical significance on hydrogen storage Yong Nam Choi a , J.M. Sungil Park a, *, Thierry Stra ¨ ssle b , Sun-Hwa Yeon c , Youngjune Park c , Huen Lee c, * a Neutron Science Division, Korea Atomic Energy Research Institute, Daejeon 305-351, Republic of Korea b Laboratory for Neutron Scattering, Paul Scherrer Institut, Villigen, Switzerland c Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea article info Article history: Received 24 December 2009 Accepted 13 April 2010 Available online xxx Keywords: Hydrogen storage Clathrate hydrate Inelastic neutron scattering Tetrahydrofuran abstract We have studied the dynamics of the H 2 loaded THF (tetrahydrofuran) clathrate hydrate by using an inelastic neutron scattering technique. At 2 K, a well defined single peak indica- tive of the ortho (J ¼ 1) to para (J ¼ 0) hydrogen transition is observed at 13.6 meV. The neutron spectrum undergoes two distinct changes as the temperature increases. First, additional peaks appear from splitting of the rotational transition above 10 K. Upon further heating, a quasi-elastic neutron scattering (QENS) signal due to hydrogen molecular diffusion through the THF hydrate crystalline lattice grows significantly above 65 K. These results indicate that H 2 molecules trapped inside the THF hydrate crystalline structure undergo two distinct transformations in their dynamical behavior. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. 1. Introduction In gas hydrates, water molecules form nanometer scale cages, inside which small guest molecules such as hydrogen or methane are trapped. To form such gas hydrates, extreme conditions such as the very high pressure that can be found at the bottom of the deep ocean are often required. Extraction of these guest molecules, on the contrary, can be as easy as melting the hydrate. Thanks to this useful property, gas hydrates possess many potential applications [1]. Among many gas hydrates, pure hydrogen hydrate has become a focus of attention lately. Mao et al. reported that clathrate hydrate of structure II (sII) can encapsulate up to e5 wt % hydrogen [2], opening the possibility of a massive and inexpensive hydrogen storage medium. In this sII structure, two and four hydrogen molecules were claimed to be trapped inside small (5 12 ) and large (5 12 6 4 ) cages made of water molecules, respectively. A recent neutron diffraction study disputes this claim [3]. Nevertheless, the very high pressure of 220 MPa required for its synthesis severely limits the practical applicability of this hydrate as hydrogen storage material. THF hydrate, on the other hand, requires much milder conditions to load hydrogen into the system [4]. By mixing 5.56 mol % THF with water, a stoichiometric (1THF:17H 2 O) THF hydrate is formed upon freezing the solution. Its structure was confirmed by using X-ray [4] and neutron powder diffraction [5] to be the same sII as the pure H2 hydrate. THF molecules, due to their large molecular size, are known to reside inside large cages while small cages are left empty. Hence hydrogen molecules, at a high enough pressure, may enter those small empty cages. According to an early report, an applied pressure * Corresponding authors. E-mail addresses: jmspark@kaeri.re.kr, h_lee@kaist.ac.kr (J.M. Sungil Park). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he ARTICLE IN PRESS international journal of hydrogen energy xxx (2010) 1 e5 Please cite this article in press as: Yong Nam Choi, et al., Dynamics of hydrogen molecules in the channels of binary THF-H 2 clathrate hydrate and its physicochemical significance on hydrogen storage, International Journal of Hydrogen Energy (2010), doi:10.1016/j.ijhydene.2010.04.073 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.04.073