Hydrogen clathrate
A hydrogen clathrate is a clathrate containing hydrogen in a water lattice. This substance is interesting due to its possible use to store hydrogen in a hydrogen economy.<ref name="hohs">Hirscher, Michael (4 August 2010). "Clathrate Hydrates". Chapter 3. Clathrate Hydrates. Wiley. pp. 63–79. doi:10.1002/9783527629800.ch3. ISBN 9783527629800. {{cite book}}: |work= ignored (help)</ref><ref name="chaap">Sabo, Dubravko; Sabo, Dubravko; Clawson, Jacalyn; Rempe, Susan; Greathouse, Jeffery; Martin, Marcus; Leung, Kevin; Varma, Sameer; Cygan, Randall; Alam, Todd (7 March 2007). "Hydrogen clathrate hydrates as a potential hydrogen storage material". MAR07 Meeting of the American Physical Society. 52 (1): S39.012. Bibcode:2007APS..MARS39012S. Retrieved 10 September 2011.</ref> A recent review that accounts the state-of-the-art and future prospects and challenges of hydrogen storage as clathrate hydrates is reported by Veluswamy et al. (2014).<ref>Veluswamy, Hari Prakash; Kumar, Rajnish; Linga, Praveen (2014). "Hydrogen storage in clathrate hydrates: Current state of the art and future directions". Applied Energy. 122: 112–132. doi:10.1016/j.apenergy.2014.01.063.</ref> Another unusual characteristic is that multiple hydrogen molecules can occur at each cage site in the ice, one of only a very few guest molecule that forms clathrates with this property. The maximum ratio of hydrogen to water is 6 H2 to 17 H2O.<ref name=lokshin2004>Lokshin, Konstantin A.; Yusheng Zhao; Duanwei He; Wendy L. Mao; Ho-Kwang Mao; Russell J. Hemley; Maxim V. Lobanov & Martha Greenblatt (14 September 2004). "Structure and Dynamics of Hydrogen Molecules in the Novel Clathrate Hydrate by High Pressure Neutron Diffraction". Physical Review Letters. 93 (12): 125503–1–125503–4. Bibcode:2004PhRvL..93l5503L. doi:10.1103/PhysRevLett.93.125503. PMID 15447276.</ref> It can be formed at 250K in a diamond anvil at a pressure of 300MPa (3000 Bars). It takes about 30 minutes to form, so this method is impractical for rapid manufacture.<ref>Mao, Wendy L.; Mao, A. F. Goncharov; V. V. Struzhkin; Q. Guo; J. Hu, J. Shu; R. J. Hemley; M Somayazulu & Y. Zhao (2002). "Hydrogen Clusters in Clathratehydrate". Science. 297 (5590): 2247–2249. Bibcode:2002Sci...297.2247M. doi:10.1126/science.1075394. PMID 12351785. S2CID 24168225.</ref> The percent of weight of hydrogen is 3.77%.<ref name=lokshin2004/> The cage compartments are hexakaidecahedral and hold from two to four molecules of hydrogen. At temperatures above 160K the molecules rotate around inside the cage. Below 120K the molecules stop racing around the cage, and below 50K are locked into a fixed position. This was determined with deuterium in a neutron scattering experiment.<ref name=lokshin2004/>
Under higher pressures a 1:1 ratio clathrate can form. It crystallises in a cubic structure, where H2 and H2O are both arranged in a diamond lattice. It is stable above 2.3 GPa.<ref>Vos, Willem L.; Finger, Larry W.; Hemley, Russell J.; Mao, Ho-kwang (August 1996). "Pressure dependence of hydrogen bonding in a novel H2O-H2 clathrate". Chemical Physics Letters. 257 (5–6): 524–530. Bibcode:1996CPL...257..524V. doi:10.1016/0009-2614(96)00583-0.</ref>
Under even higher pressures (over 38 GPa) there is a prediction of the existence of a clathrate with a cubic structure and a 1:2 ratio: 2H2•H2O.<ref>Qian, Guang-Rui; Lyakhov, Andriy O.; Zhu, Qiang; Oganov, Artem R.; Dong, Xiao (8 July 2014). "Novel Hydrogen Hydrate Structures under Pressure". Scientific Reports. 4: 5606. Bibcode:2014NatSR...4E5606Q. doi:10.1038/srep05606. PMC 4085642. PMID 25001502.</ref>
More complex clathrates can occur with hydrogen, water and other molecules such as methane,<ref name="hsimc"/> and tetrahydrofuran.<ref>Smirnov, G. S.; Stegailov, V. V. (17 December 2015). "Anomalous diffusion of guest molecules in hydrogen gas hydrates". High Temperature. 53 (6): 829–836. doi:10.1134/S0018151X15060188. S2CID 123843390.</ref>
Since hydrogen and water ice are common constituents of the universe, it is very likely that under the right circumstances natural hydrogen clathrates will be formed. This could occur in icy moons for example.<ref name="hsimc">Struzhkin, Viktor; Burkhard Militzer; Wendy L. Mao; Ho-kwang Mao & Russell J. Hemley (7 December 2006). "Hydrogen Storage in Molecular Clathrates" (PDF). Chem Rev. 107 (10): 4133–4151. doi:10.1021/cr050183d. PMID 17850164.</ref> Hydrogen clathrate was likely to be formed in the high pressure nebulae that formed the gas giants, but not to have formed in comets.<ref>Lunine, Jonathan I.; Stevenson, David J. (1985). "Thermodynamics of clathrate hydrate at low and high pressures with application to the outer solar system". The Astrophysical Journal Supplement Series. 58: 493. Bibcode:1985ApJS...58..493L. doi:10.1086/191050. Retrieved 10 September 2011.</ref>
References
<references/>
External links
- Vos, Willem L.; Larry W. Finger; Russel J. Hemley & Ho-kwang Mao (8 November 1993). "Novel H2-H2O Clathrates at High Pressure" (PDF). Physical Review Letters. 71 (19): 3150–3153. Bibcode:1993PhRvL..71.3150V. doi:10.1103/physrevlett.71.3150. PMID 10054870.
