: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.{{cite book|title=Handbook of Hydrogen Storage: New Materials for Future Energy Storage|isbn = 9783527629800|pages= 63–79 |chapter = Clathrate Hydrates|editor-last = Hirscher|editor-first = Michael|date= 24 March 2010|publisher=Wiley|doi=10.1002/9783527629800.ch3}}{{cite journal|url=http://meetings.aps.org/Meeting/MAR07/Event/61544|title=Hydrogen clathrate hydrates as a potential hydrogen storage material|volume=52|issue=1|pages = S39.012|last=Sabo|first=Dubravko|author2=Sabo, Dubravko |author3=Clawson, Jacalyn |author4=Rempe, Susan |author5=Greathouse, Jeffery |author6=Martin, Marcus |author7 = Leung, Kevin |author8=Varma, Sameer |author9=Cygan, Randall |author10= Alam, Todd |date=7 March 2007|journal=MAR07 Meeting of the American Physical Society|accessdate = 10 September 2011|bibcode=2007APS..MARS39012S}} 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).{{Cite journal|url=https://doi.org/10.1016/j.apenergy.2014.01.063|doi = 10.1016/j.apenergy.2014.01.063 |title = Hydrogen storage in clathrate hydrates: Current state of the art and future directions|year = 2014|last1 = Veluswamy|first1 = Hari Prakash|last2 = Kumar|first2 = Rajnish|last3 = Linga|first3 = Praveen|journal = Applied Energy|volume = 122|pages = 112–132| bibcode=2014ApEn..122..112V |url-access = subscription}} 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.{{cite journal|last=Lokshin|first=Konstantin A.|author2=Yusheng Zhao|author3=Duanwei He|author4=Wendy L. Mao|author5=Ho-Kwang Mao|author6=Russell J. Hemley|author7=Maxim V. Lobanov|author8=Martha Greenblatt|name-list-style=amp|date=14 September 2004|title=Structure and Dynamics of Hydrogen Molecules in the Novel Clathrate Hydrate by High Pressure Neutron Diffraction|journal=Physical Review Letters|volume=93|issue=12|pages=125503–1–125503–4|doi=10.1103/PhysRevLett.93.125503|bibcode=2004PhRvL..93l5503L|pmid=15447276}} 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.{{cite journal|last=Mao|first=Wendy L.|author-link=Wendy Mao|author2=Mao, A. F. Goncharov|author3=V. V. Struzhkin|author4=Q. Guo|author5=J. Hu, J. Shu|author6=R. J. Hemley|author7=M Somayazulu|author8=Y. Zhao|year=2002|title=Hydrogen Clusters in Clathratehydrate|journal=Science|volume=297|issue=5590|pages=2247–2249|bibcode=2002Sci...297.2247M|doi=10.1126/science.1075394|pmid=12351785|name-list-style=amp|s2cid=24168225}} The percent of weight of hydrogen is 3.77%. 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.

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.{{cite journal|last1=Vos|first1=Willem L.|last2=Finger|first2=Larry W.|last3=Hemley|first3=Russell J.|last4=Mao|first4=Ho-kwang|title=Pressure dependence of hydrogen bonding in a novel H2O-H2 clathrate|journal=Chemical Physics Letters|date=August 1996|volume=257|issue=5–6|pages=524–530|doi=10.1016/0009-2614(96)00583-0|bibcode=1996CPL...257..524V}}

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.{{cite journal|last1=Qian|first1=Guang-Rui|last2=Lyakhov|first2=Andriy O.|last3=Zhu|first3=Qiang|last4=Oganov|first4=Artem R.|last5=Dong|first5=Xiao|title=Novel Hydrogen Hydrate Structures under Pressure|journal=Scientific Reports|date=8 July 2014|volume=4|pages=5606|doi=10.1038/srep05606|pmid=25001502|pmc=4085642|arxiv=1310.1157 |bibcode=2014NatSR...4.5606Q}}

More complex clathrates can occur with hydrogen, water and other molecules such as methane, and tetrahydrofuran.{{cite journal|last1=Smirnov|first1=G. S.|last2=Stegailov|first2=V. V.|title=Anomalous diffusion of guest molecules in hydrogen gas hydrates|journal=High Temperature|date=17 December 2015|volume=53|issue=6|pages=829–836|doi=10.1134/S0018151X15060188|bibcode=2015HTemp..53..829S |s2cid=123843390}}

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.{{cite journal|last=Struzhkin|first=Viktor|author2=Burkhard Militzer|author3=Wendy L. Mao|author4=Ho-kwang Mao|author5=Russell J. Hemley|name-list-style=amp|date=7 December 2006|title=Hydrogen Storage in Molecular Clathrates|journal=Chem Rev|volume=107|issue=10|pages=4133–4151|url=http://www.lanl.gov/source/orgs/ees/ees14/clathrates/pdfs/StruzhinMaoEtAl.pdf|doi=10.1021/cr050183d|pmid=17850164}} Hydrogen clathrate was likely to be formed in the high pressure nebulae that formed the gas giants, but not to have formed in comets.{{cite journal |url=http://www.etde.org/etdeweb/details.jsp?query_id=1&page=0&osti_id=5363872 |title=Thermodynamics of clathrate hydrate at low and high pressures with application to the outer solar system |journal=The Astrophysical Journal Supplement Series |volume=58 |pages=493 |last=Lunine |first=Jonathan I. |author2=Stevenson, David J. |accessdate=10 September 2011 |bibcode=1985ApJS...58..493L |year=1985 |doi=10.1086/191050 }}