Hydrogen Nanoreservoirs Made of Boron Nitride - Nanomaterials for Environmental Protection - page 462

automotive applications. Chemisorption binding energies on B nanotubes should be approximately 2.4-2.9 eV/H atom, similar
to the values obtained in C nanotubes. Finally, the energy barrier from molecular physisorption to dissociative chemisorption
of hydrogen is about 1.0 eV/molecule. Therefore, the calculations predict physisorption as the leading adsorption mechanism
of hydrogen at moderate temperatures and pressures. Wu et al. [69] investigated the feasibility of bare and metal-coated boron
buckyball B 80 with metals Li, na, K, Be, Mg, Ca, sc, Ti, and V for hydrogen storage using the dFT approach. Ca and sc were
found to be the best candidates for hydrogen storage with moderate adsorption energy of H 2 avoiding clustering of sc and Ca
on the B 80 surface. Further, it was observed that an isolated cluster Ca 12 B 80 (sc 12 B 80 ) can bind up to 66 (60) H 2 molecules with
an average binding energy of 0.096 (0.346) eV/H 2 , leading to a hydrogen storage capacity of 9.0 wt.% (7.9 wt.%). Two adsorp-
tion mechanisms, charge-induced dipole interaction and the dewar-Kubas interaction, were demonstrated and shown to be
responsible for high hydrogen storage capacity of Ca 12 B 80 and sc 12 B 80 . Most interestingly, the hydrogen-loaded B 80 sc 12 -48 H 2
complex can further adsorb 12 H 2 through charge-induced dipole interaction. In other words, these two mechanisms dominate
the adsorption of different parts of H 2 in the same cluster of B 80 sc 12 -60 H 2 . A comprehensive study was performed [70] on
hydrogen adsorption and storage in Ca-coated boron fullerenes and nanotubes by means of dFT computations. Ca strongly
binds to boron fullerene and nanotube surfaces due to the charge transfer between Ca and the B substrate. Accordingly, Ca
atoms do not cluster on the surface of the boron substrate, while transition metals (such as Ti and sc) persist in clustering on
the B 80 surface. B 80 fullerene coated with 12 Ca atoms can store up to 60 H 2 molecules with a binding energy of 0.12-0.40 eV/
H 2 , corresponding to a gravimetric density of 8.2 wt.%, while the hydrogen storage capacity in a (9, 0) B nanotube is 7.6 wt.%
with a binding energy of 0.10-0.30 eV/H 2 . The Ca-coated boron fullerenes and nanotubes proposed are favorable for reversible
adsorption and desorption of hydrogen at ambient conditions.
First-principles quantum chemical methods were used [71] to study ground-state energies and geometrical configurations of
boron-hydrogen chains. The ground-state energies were found to be comparable with those of pristine boron clusters. The
ground-state energies of the dimerized (BH) n chain were fitted into a model, in order to determine the corresponding parame-
ters. This dimerization induces a band gap of approximately 0.6 eV. Tam et al. [72] reported spectroscopic observations on B
atoms isolated in cryogenic parahydrogen p-H 2 , normal deuterium n-d 2 , and some noble-gas matrices. The 2 s 2 3 s ( 2 S )-2 s 2 2 p ( 2 P )
B atom rydberg absorption suffers large gas-to-matrix blue shifts, increasing in the sequence with n-d 2 < p-H 2 . Much smaller
shifts are observed for the 2 s 2 p 2 ( 2 D )-2 s 2 2 p ( 2 P ) B atom core-to-valence transition. Ultraviolet (UV) absorption spectra of B/p-H 2
solids showed two strong peaks at 216.6 and 208.9 nm, which were assigned to the matrix-perturbed 2 s 2 3 s ( 2 S )-2 s 2 2 p ( 2 P ) and
2 s 2 p 2 ( 2 D )-2 s 2 2 p ( 2 P ) B atom absorptions, respectively. This assignment is supported by the previous quantum path integral
simulations. Laser-induced fluorescence emission spectra of B/p-H 2 solids showed a single line at 249.6 nm, coincident with
the gas-phase wavelength of the 2 s 2 3 s ( 2 S )-2 s 2 2 p ( 2 P ) B atom emission. The UV laser irradiation results in photobleaching of
the B atom emission and absorptions, accompanied by the formation of B 2 H 6 . The review by grimes [73] cited U.s. Patents on
a widely used air bag propellant system for automobiles employing the dicesium salt of the B 12 H 12 2− ion as a burning accelerant
to ensure rapid but controlled inflation of the bag. On the one hand, hydrogen-terminated icosahedral B 12 H 12 2− , which has the
same structure as the unit in solids, is the most stable molecule among the various polyhedral boranes synthesized. On the other
hand, small boron clusters favor planar or nearly planar structures. The fact that the stable structure of boron clusters depends
on the hydrogen contents means that the structure is tunable bycontrolling the number of hydrogen atoms. Ohishi et al. [74-76]
reported the formation of icosahedral B 12 H 8 + through ion-molecule reactions of the decaborane ion B 10 H n + ( n = 6-14) with
decaborane B 10 H 14 and diborane B 2 H 6 molecules in an external quadrupole-static attraction ion trap. In the process of ionization,
a certain number of hydrogen and boron atoms are detached from decaborane ions by the energy caused by the charge transfer
from ambient gas ion to decaborane molecule. The hydrogen content n of B 12 H n + was determined by the analysis of the mass
spectrum. The result reveals that B 12 H 8 + is the main product. First-principles calculations indicated that B 12 H 8 + preferentially
forms an icosahedral structure rather than a quasiplanar structure. The energies of the formation reactions of B 12 H 14 + and B 12 H 12 +
between B 10 H x + ( x = 6 and 8) ions, which were considered to be involved in the formation of B 12 H n + , and a B 2 H 6 molecule were
calculated. The calculations of the detachment pathway of H 2 molecules and H atoms from the product ions B 12 H 14 + and B 12 H 12 +
indicate that the intermediate state has a relatively low energy, enabling the detachment reaction to proceed owing to the
sufficient reaction energy. This autodetachment of H 2 accounts for the experimental result that B 12 H 8 + is the most abundant
product, even though it does not have the lowest energy among B 12 H n + molecules. The hydrogen and boron contents of the
B 10− y H x + cluster were controlled by charge transfer from ambient gas (ne or He) ions. This gas leads to the generation of B 10− y H x +
clusters with x = 4-10 and y = 0-1 (with x = 2-10 and y = 0-2). The introduction of ambient gas also increases the production of
clusters. The dFT calculations were conducted to investigate the structure and the mechanism of formation of B 10− y H x + and
B 12 H n + clusters.
Among complex hydrides for hydrogen storage reviewed by Orimo et al. [77] are the boron-containing ones Li 4 Bn 3 H 10
amide, and LiBH 4 , naBH 4 , and some other tetrahydroborates. Hydrogen storage in metal hydrides is considered to be one of
the most attractive methods. Ozturk and demirbas [78] compared the hydrogen absorption-desorption behavior of the boron