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Magnesium hydride

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Magnesium hydride

Magnesium hydride
Identifiers
CAS number  YesY
PubChem
ChemSpider  YesY
EC number
ChEBI  YesY
Jmol-3D images Image 1
Properties
Molecular formula MgH2
Molar mass 26.3209 g/mol
Appearance white crystals
Density 1.45 g/cm3
Melting point 285 °C (545 °F; 558 K) decomposes
Solubility in water decomposes
Solubility insoluble in ether
Structure
Crystal structure tetragonal
Thermochemistry
Specific
heat capacity
C
35.4 J/mol K
Std molar
entropy
So298
31.1 J/mol K
Std enthalpy of
formation
ΔfHo298
-75.2 kJ/mol
Gibbs free energy ΔG -35.9 kJ/mol
Hazards
EU Index Not listed
Main hazards pyrophoric[1]
Related compounds
Other cations Beryllium hydride
Calcium hydride
Strontium hydride
Barium hydride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY   YesY/N?)

Magnesium hydride is the chemical compound MgH2. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium.[2]

Preparation

In 1951 preparation from the elements was first reported involving direct hydrogenation of Mg metal at high pressure and temperature (200 atmospheres, 500 °C) with MgI2 catalyst:[3]

Mg + H2 → MgH2

Lower temperature production from Mg and H2 using nano crystalline Mg produced in ball mills has been investigated.[4] Other preparations include:

  • the hydrogenation of magnesium anthracene under mild conditions:[5]
Mg(anthracene) + H2 → MgH2

Structure and bonding

The room temperature form α-MgH2 has a rutile structure.[7] There are at least four high pressure forms: γ-MgH2 with α-PbO2 structure,[8] cubic β-MgH2 with Pa-3 space group,[9] orthorhombic HP1 with Pbc21 space group and orthorhombic HP2 with Pnma space group.[10] Additionally a non stoichiometric MgH(2-δ) has been characterised, but this appears to exist only for very small particles[11]
(bulk MgH2 is essentially stoichiometric, as it can only accommodate very low concentrations of H vacancies[12]).

The bonding in the rutile form is sometimes described as being covalent in nature rather than purely ionic;[13] charge density determination by synchrotron x-ray diffraction indicates that the magnesium atom is fully ionised and spherical in shape and the hydride ion is elongated.[14] Molecular forms of magnesium hydride, MgH, MgH2, Mg2H, Mg2H2, Mg2H3, and Mg2H4 molecules identified by their vibrational spectra have been found in matrix isolated samples at below 10 K, formed following laser ablation of magnesium in the presence of hydrogen.[15] The Mg2H4 molecule has a bridged structure analogous to dimeric aluminium hydride, Al2H6.[15]

Reactions

MgH2 readily reacts with water to form hydrogen gas:

MgH2 + 2 H2O → 2 H2 + Mg(OH)2

At 300 °C decomposes to produce H2 at 1 bar pressure, the high temperature required is seen as a limitation in the use of MgH2 as a reversible hydrogen storage medium:[16]

MgH2 → Mg + H2

Potential use for hydrogen storage

Its potential as a reversible "storage" medium for hydrogen has led to interest in improving the hydrogenation and dehydrogenation reaction kinetics.[16][17] This can be partially achieved by doping or by reducing the particle size using ball milling.[18][19][20] An alternative approach under investigation is the production of a pumpable slurry of MgH2 which is safe to handle and releases H2 by reaction with water, with reprocessing of the Mg(OH)2 into MgH2.[1] An application (yet to be examined) for a US Patent (US 2010/0163434 A1) [2] has been made in respect of a hydrogen energy storage system using laser excitation to assist desorption of hydrogen gas from magnesium hydride.

References

  1. ^ a b Synthesis of magnesium hydride by the reaction of phenylsilane and dibutylmagnesium , Michalczyk M.J., Organometallics; (1992); 11(6); 2307-2309. doi:10.1021/om00042a055
  2. ^ Catalytic Synthesis of Organolithium and Organomagnesium Compounds and of Lithium and Magnesium Hydrides - Applications in Organic Synthesis and Hydrogen Storage, Bogdanovic B., Angewandte Chemie International Edition in English, 24, 4, 262–73, doi:10.1002/anie.198502621
  3. ^ Wiberg, Goeltzer, Bauer, 1951, Z. Naturforsch. 6b, 394, (1951)
  4. ^ Nanocrystalline magnesium for hydrogen storage, A Zaluska, L Zaluski, JO Ström–Olsen, Journal of Alloys and Compounds, 288, 1-2, 1999, 217-225, doi:10.1016/S0925-8388(99)00073-0
  5. ^ Catalytic Synthesis of Magnesium Hydride under Mild Conditions, Bogdanovic B., Liao S-T, Schwickardi M, Sikorsky P., Spliethoff B., Angewandte Chemie International Edition in English, 19,(1980), 10, 818 – 819, doi:10.1002/anie.198008181
  6. ^ The Preparation of the Hydrides of Zinc, Cadmium, Beryllium, Magnesium and Lithium by the Use of Lithium Aluminum Hydride, Barbaras G.D., Dillard C., Finholt A. E., Wartik T, Wilzbach K. E., Schlesinger H. I., J. Am. Chem. Soc.; 1951; 73(10); 4585-4590, doi:10.1021/ja01154a025
  7. ^ Neutron diffraction study of magnesium deuteride, Zachariasen W.H., Holley C.E, Stamper J.F. Jnr, Acta Cryst. (1963) 16, 352-353, doi:10.1107/S0365110X63000967
  8. ^ Structure of the high pressure phase γ-MgH2 by neutron powder diffraction, Bortz M., Bertheville B., Böttger G., Yvon K., Journal of Alloys and Compounds, 287, 1-2, (1999), L4-L6, doi:10.1016/S0925-8388(99)00028-6
  9. ^ Structural stability and pressure-induced phase transitions in MgH2, Vajeeston P., Ravindran P., Hauback B.C., Fjellvåg H., Kjekshus A., Furuseth S., Hanfland M., Phys. Rev. B. (2006) 73, 224102 doi:10.1103/PhysRevB.73.224102
  10. ^ Structural Phase Transition of Rutile-Type MgH2 at High Pressures, Moriwaki T., Akahama Yu., Kawamura H., Nakano S., Takemura K., J. Phys. Soc. Jpn. (2006) 75(7) 074603, doi:10.1143/JPSJ.75.074603
  11. ^ Hydrogen Cycling of Niobium and Vanadium Catalyzed Nanostructured Magnesium,Schimmel, H. G.; Huot, J.; Chapon, L. C.; Tichelaar, F. D.; Mulder, F. M.,J. Am. Chem. Soc.; (Article); 2005; 127(41); 14348-14354. doi:10.1021/ja051508a
  12. ^ Grau-Crespo, R.; K. C. Smith; T. S. Fisher; N. H. de Leeuw; U. V. Waghmare (2009). from first-principles calculations and grand-canonical statistical mechanics"2"Thermodynamics of hydrogen vacancies in MgH. Physical Review B 80 (17): 174117.  
  13. ^  
  14. ^ Charge density measurement in MgH2 by synchrotron X-ray diffraction, T. Noritake, S. Towata, M. Aoki, Y. Seno, Y. Hirose, E. Nishibori, M. Takata and M. Sakata, Journal of Alloys and Compounds, 356-357, (2003), 84-86, doi:10.1016/S0925-8388(03)00104-X
  15. ^ a b Infrared Spectra of Magnesium Hydride Molecules, Complexes, and Solid Magnesium Dihydride, Xuefeng Wang and Lester Andrews, J. Phys. Chem. A, 108 (52), 11511 -11520, 2004. doi:10.1021/jp046410h
  16. ^ a b Hydrogen-storage materials for mobile applications, L. Schlapbach and A. Züttel, Nature 414, 353 (2001).doi:10.1038/35104634[3]
  17. ^ J Huot Hydrogen in Metals (2002) in New Trends in Intercalation Compounds for Energy Storage, Christian Julien, J. P. Pereira-Ramos, A. Momchilov, Springer, ISBN 1-4020-0594-6
  18. ^ Sakintuna, B.; F. Lamaridarkrim; M. Hirscher (2007). "Metal hydride materials for solid hydrogen storage: A review". International Journal of Hydrogen Energy 32: 1121–1140.  
  19. ^ Smith, Kyle; Fisher, Timothy; Waghmare, Umesh; Grau-Crespo, Ricardo (2010). "Dopant-vacancy binding effects in Li-doped magnesium hydride". Physical Review B 82 (13).  
  20. ^ Liang, G.; Huot, J.; Boily, S.; Van Neste, A.; Schulz, R. (1999). "Catalytic effect of transition metals on hydrogen sorption in nanocrystalline ball milled MgH2–Tm (Tm=Ti, V, Mn, Fe and Ni) systems". Journal of Alloys and Compounds 292 (1-2): 247–252.  
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