Preparation: Heat at 300-350°C, Ln + H₂ Æ LnH₂

Properties of LnH₂

• black, reactive, highly conducting
• fluorite structure

• most thermodynamically stable of all binary metal hydrides
• formulated as Ln³⁺(H^-)₂(e^-) with e^- delocalized in a metallic conduction band
• further H can often be accommodated in interstitial sites
  • Þ frequently non-stoichiometric
    • e.g. LuH_x where x = 1.83-2.23 & 2.78-3.00
• high pressure of H₂ Æ LnH₃
  • reduced conductivity: salt-like Ln³⁺(H^-)₃

    except for Eu and Yb (the most stable Ln^II)

The Hydrogen Storage Problem

see e.g. C.N.R. Rao & J. Gopalkrishnan, New Directions in Solid State Chemistry, CUP, 1986 p. 399-405

K. Kosuge, Chemistry of Non-Stoichiometric Compounds, OUP, 1994 p. 219-230

• The use of H₂ as a fuel is most attractive

Problem: Difficult to store/transport as a liquid

• low bpt & low density
• H₂ forms explosive mixtures with air Þ explosion risk on storage!

Solution: Store hydrogen as a solid compound (hydride) from which it can be re-extracted

• Metals show two common types of hydride-forming behaviour:-
  • Absorb Hydrogen reversibly but in small amounts

    e.g. Pd, V, Nb, Ta

  • Absorb large quantities of hydrogen, but, essentially irreversibly

    e.g. rare earths, alkaline earths, Ti, Zr

• Intermetallic Alloys between the two classes can be useful for hydrogen storage
  • e.g. LnNi₅ class of alloys

Possible Applications of Rare Earth Intermetallic Hydrides

1. Production of ultrapure hydrogen

2. Isotope Separation of deuterium and hydrogen

3. Source of fuel for motor vehicles

4. Electrodes in Protonic Batteries/Fuel Cells

5. Load Levelling in Power Stations

6. Chemical heat-pump systems

7. Useful hydrogenation agents in organic chemistry  

LaNi₅

• crystallize in the CaCu₅ structure
• provides 9 interstitial sites for H
• adsorption-desorption of hydrogen occurs topotactically
  • without drastic change in structure
  • however, lattice expands by ca. 25%

     

• At a certain pressure, uptake of H₂ begins
• A large amount of H₂ is adsorbed at nearly constant equilibrium pressure ~ the Plateau Pressure
• When the stoichiometry reaches LaNi₅H₆ further increase in pressure yields very little extra hydrogen-adsorption {at highest pressures LaNi₅H_8.35 is characterized}
• Plateau Pressure ca. 2.5 atm at 298 K
  • (ideal case ca. 1 atm!)
• Desorption is endothermic Þ faster at ca. 140°C
• Different LnNi₅ changes plateau pressures

  ~ log plateau pressure correlates linearly with unit cell volume of the LnNi₅ phase

  • Þ mixtures allow tuning of equilibrium pressure Æ `designer' compounds

Dependence of Hydrogen Plateau Pressure on

Unit Cell Volume for LaNi₅ compounds

{open circles LnCo₅, closed circles LnNi₅, open triangles LaCo_5-xNi_x}

Bibliography