Page 244 - Materials Chemistry, Second Edition

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3.5. Reversible Hydrogen Storage

There continues to be significant research efforts devoted to the design of suitable
catalysts that will improve the relatively slow kinetics associated with the reversible
H 2 storage of MAlH 4 compounds. A new high-pressure polymorph of the boron
analogue LiBH 4 has also been investigated, which appears to be more successful in
low-temperature release of hydrogen. [29] Intermetallic compounds and metal alumi-
num hydrides doped with Ti and Zr have been successfully used to improve the rate
of hydride formation/release in these compounds [30] ; typically, levels between 2 and
4 mol% Ti is sufficient to facilitate reversibility. The catalytic mechanism may be
rationalized by the donation of H 2 s-electron density to empty d orbitals on the Ti,
*
along with synergistic donation of electrons from filled Ti d orbitals to the s orbital
of H 2 . This two-way electron donation weakens the H–H bond, while strengthening
the H · Ti interaction (Figure 3.45).
Sometimes catalytic dopants do not alloy with the host metal; for example, the
addition of Nb and V to Mg form heterogeneous mixtures rather than intermetallic
compounds. A new generation of catalysts is being developed that deliver hydrogen
•
to the metal surface as H radicals rather than H 2 . [31] It may then be possible to
combine a high level of hydrogen storage with low T dec , both associated with
desirable adsorption/desorption kinetics.
It has recently been discovered that decreasing the particle size of metal alloy
particles through ball-milling processes will increase the adsorption kinetics by an
[32]
order of magnitude. This enhanced activity is due to the increased surface area of
the ground particulates, and decreased surface reaction path length. For LiAlH 4 , only
prolonged milling is of sufficient energy to desorb H 2 . When grinding is coupled with
catalytic dopants, the H 2 storage kinetics increases even further. Upon milling bulk

Mg 2 NiH 4 , the T dec decreases by ca.40 C. Such mechanical processing has also been
used to synthesize H 2 -storage compounds (e.g.,La 1.8 Ca 0.2 Mg 14 Ni 3 ,Li x Be y H x+2y ,
MAlH 4 ;M ¼ Mg, Ca, Sr) that consist of a metastable amorphous or nanocrystalline
structure.

IMPORTANT (AND CONTROVERSIAL!) MATERIALS
APPLICATIONS II: DEPLETED URANIUM

Due to the high radioactivity of the actinides, we would expect that their use for
materials applications would be limited. However, a relatively benign form of
uranium, known as depleted uranium (DU), has been widely used in applications
such as machinery ballast and counterweights, aircraft balancing/damping con-
trols, [33] radiation and penetration shielding, oil-well drilling equipment, and high-
impact weaponry.
Uranium is obtained from ores primarily located in New Mexico, Colorado, Wyom-
ing, Utah, and Arizona as well as many other locations throughout the world. There are
three isotopes of uranium: 99.28% of 238 U, 0.005% of 234 U, and 0.71% of 235 U. Only
the 235 Uand 234 U isotopes are used for nuclear power and weapon manufacturing.
When 235 Uand 234 U have been extracted from natural uranium, the remaining
“depleted uranium” is primarily 238 U. Although the 238 U isotope of uranium is 40%