Environmental Engineering Reference
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compounds with that of metal hydrides, which are considered as one of the most attractive hydrogen reservoirs. Boron com-
pounds have a very high energy density, much better than that of liquid hydrogen, and also a lot safer. LiBH 4 is a complex
hydride that consists of 18 wt.% of hydrogen. It has stability compared with other chemical hydrides and an easy conversion to
H 2 . Thus, there are good reasons that hydrogen storage materials for LiBH 4 will be used for power sources. Metallaboranes
constitute an attractive class of compounds intermediate between borane cages and transition metal clusters. While carbon
substitution into a borane cage is rather common and gives rise to an entire class of metallaheteroborane compounds, other
main-group atom substitution is rather rare. It was shown that the reaction excess of LiBH 4 , followed by thermolysis with
excess of BH 3 ∙ THF, leads to the formation of oxamolybdaborane clusters. These are notable examples of oxametallaborane
compounds where oxygen is contiguously bound to both cluster metals and boron atoms. similarly, the reaction with LiBH 4 is
followed by thermolysis with chalcogen powders of s or se. The insertion of O, s, and se atoms into the parent clusters shows
a shortening of metal-metal bond distances. Theoretical calculations of dFT type were carried out [79] to study the geometries,
energetics, and bonding properties in these metallaheteroborane compounds, aiming at completing their characterization and,
in particular, establishing the exact number of hydrogens in the structures.
Mechanically milling ammonia borane and lithium borohydride in equivalent molar ratio results [80] in the formation of a
complex LiBH 4 · nH 3 BH 3 . This complex was studied in terms of its decomposition behavior and reversible dehydrogenation
property. This study found that LiBH 4 · nH 3 BH 3 first disproportionates into (LiBH 4 ) 2 · nH 3 BH 3 and nH 3 BH 3 , and the resulting
mixture exhibits a three-step decomposition behavior upon heating to 450°C, totally yielding approximately 15.7wt.%
hydrogen. Metal borohydrides are of interest as hydrogen storage materials as BH 4 are 27% hydrogen by weight. When the
borohydride anion is paired with a light-weight cation such as Li + or na + , the compound has a hydrogen weight percentage that
is sufficiently high to be of potential practical importance for hydrogen storage. To optimize hydrogen release from materials
containing the BH 4 anion requires an understanding of the thermal decomposition mechanism. In particular, it is important to
identify any stable intermediates with a lower hydrogen-to-boron ratio that may form in the course of the decomposition of
borohydrides. One such intermediate is the B 12 H 12 2− anion, which was indentified in studies of borohydride decomposition.
Transmission Ir spectroscopy was used [81] to characterize the temperature dependence of the vibrational spectra of LiBH 4 ,
naBH 4 , KBH 4 , and K 2 B 12 H 12 . The tetrahedral BH 4 species has two Ir active fundamentals, the asymmetric B-H stretch and
the asymmetric BH 4 deformation, which are observed at 2292 and 1179 cm −1 , respectively, for KBH 4 . In addition, two other
peaks are observed in the B-H stretch region at 2225 and 2387 cm −1 in KBH 4 , due to the overtone of the asymmetric bend and
the combination band of the symmetric and asymmetric bending modes. Peak positions are at similar values for naBH 4 and
LiBH 4 . The high symmetry of the icosahedral B 12 H 12 2− anion leads to only three Ir active fundamentals, which were observed
at 716, 1076, and 2485 cm −1 for K 2 B 12 H 12 . Upon heating to 550 K, the Ir spectrum of KBH 4 shows transformation into a new
species with peaks at 716, 1074, and 2450 cm −1 . The changes in the spectrum provide good evidence for the formation of
B 12 H 12 2− as an intermediate in the decomposition of the BH 4 anion. For the first time, it was demonstrated [82] that hydrogen
can be released and reabsorbed from a promising storage material, overcoming a major hurdle to its use as an alternative fuel
source. nanoparticles of sodium borohydride were synthesized and encased inside nickel shells. Their unique “core-shell”
nanostructure showed remarkable hydrogen storage properties, including the release of energy at much lower temperatures than
previously observed: one can expect initial energy release at 50°C, and significant release at 350°C.
dFT calculation was performed [83] to investigate the electronic structures of cage B 12 H n for up to n ≤ 12 and AlB 12 H n for
up to n ≤ 13. Moreover, the computations were extended to the charged clusters of (B 12 H 12 ) q , (AlB 12 H 12 ) q , and (AlB 12 H 13 ) q , where
q = ±1 and ±2. Their energies were calculated and structural analysis was carried out. The cage form of B 12 remains stable
against hydrogen adsorptions. The binding energies of B 12 H n and AlB 12 H n are in a decreasing trend with n . The HOMO-LUMO
energy gaps show that B 12 H 11 has relatively higher chemical hardness. B 12 H 2 , B 12 H 4 , B 12 H 7 , and B 12 H 10 are energetically more
stable clusters. The AlB 12 H 3 , AlB 12 H 8 , AlB 12 H 10 , and AlB 12 H 12 clusters are also obtained as relatively more stable. In the charged
AlB 12 H n clusters, structural orientations are observed for n = 12 and 13. There is considerable interest in bare boron and metal-
boron clusters as they offer potential for materials suited for hydrogen storage. Böyükata and güvenç [84] presented studies on
Al-doped and hydrogenated B n cage AlB 12 H n clusters ( n = 1-14), as well as clusters and hydrogenated cage structures of MB 12 H n
complexes (for M = Ti, Cr, Fe, Co and n ≤ 13). For the computations of these systems dFT was utilized. Interaction energies of H,
2H, and H 2 were analyzed. One of the findings showed that the metal-coated cage structure of B 12 is stable against H adsorption.
TM atoms bound to B-doped fullerenes were proposed [85] as adsorbents for high-density, room-temperature, ambient-
pressure storage of hydrogen. C 48 B 12 disperses TMs by charge transfer interactions to produce stable organometallic buckyballs.
A particular sc can bind as many as 11 hydrogen atoms per TM, 10 of which are in the form of dihydrogen that can be adsorbed
and desorbed reversibly. In this case, the calculated binding energy is about 0.3 eV/H 2 , which is ideal for use onboard vehicles.
The  theoretical maximum retrievable H 2 storage density is approximately 9wt.%. In the work of Zhao et  al. [86], TM boride
and  carboride nanostructures were studied as model organometallic materials for hydrogen storage. The dispersed TM atoms'
function is H 2 sorption centered on the surface of the boron or carbon-boron substrate. The flexibility offered in the variety
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