Environmental Engineering Reference
In-Depth Information
27.4
Bn nanocoMpounds
A dFT study of small base molecules and tetrahedral and cubane-like group V clusters encapsulated in B
80
showed [55] that the
boron buckyball is a hard acid and prefers hard bases like nH
3
or n
2
H
4
to form stable off-centered complexes. The boron cap
atoms are electrophilic centers, and prefer mainly to react with electron-rich nucleophilic sites. The stability of the complexes
will be governed by the size and electron-donating character of the encapsulated clusters.
The adsorption of atomic and molecular hydrogen on armchair and zigzag boron carbonitride nanotubes was investigated
[56] with dFT calculations. The adsorption of atomic H on the BC
2
n nanotubes presents properties that are promising for nano-
electronic applications. depending on the adsorption site for H, the Fermi energy moves toward the bottom of the conduction
band or toward the top of the valence band, leading the system to exhibit donor or acceptor characteristics, respectively. The H
2
molecules are physisorbed on the BC
2
n surface for both chiralities. The binding energies for the H
2
molecules are slightly
dependent on the adsorption site, and they are close to the range that can work as a hydrogen storage medium. First-principles
calculations were carried out [57] to investigate Ti (sc)-decorated 2d boron-carbon-nitride BC
2
n sheets for their application as
hydrogen storage materials. The results showed that with four H
2
molecules attached to each metal atom, the Ti (sc)-decorated
BC
2
n can store up to 7.6 (7.8) wt.% of hydrogen in molecular form. The adsorption energy is within the range of 0.40-0.56
(0.13-0.27) eV/H
2
, which is suitable for ambient temperature hydrogen storage. An extensive first-principles investigation of
both exohedral and endohedral Ti-decorated BC
4
n nanotubes for hydrogen storage also was reported by Bhattacharya et al.
[58]. The results revealed that an endohedral capping of Ti is energetically favorable compared to an exohedral capping, albeit
marginally by approximately 0.1-0.4 eV/Ti atom. However, this endohedral insertion process is difficult since it requires over-
coming of a rather high energy barrier of approximately 4 eV/Ti atom, as obtained from nudge elastic band calculation of the
minimum-energy path. It was observed that each Ti
+
ion sitting on the hexagonal face of the exohedral Ti-BC
4
n can bind up to
four hydrogen molecules with successive energies of adsorption lying in the range of approximately 0.4-0.7 eV. Further, it was
predicted that at high Ti coverage, the system can absorb up to 5.6 wt.% of hydrogen. After establishing the adsorption of
hydrogen molecules on the Ti-BC
4
n nanotube, Mdin simulation was performed to understand the desorption behavior. It was
observed that at 300 K the system remains stable with all four H
2
molecules attached to Ti, while at 500 K hydrogen gets released
in molecular form from the Ti-BC
4
n nanotube without breaking the cage. Thus, the desorption temperature and kinetics are
quite favorable. This investigation underscores the potential of Ti-decorated BC
4
n nanotubes as a promising nanostructure can-
didate material for H
2
storage. When a planar sheet of carbon (graphene) or isoelectronic boron nitride is decorated by Ti atoms
for a high-capacity hydrogen storage material, due to charge transfer, the Ti becomes cationic and helps to adsorb hydrogen in
the molecular form. The principal bottleneck for using such a system for efficient H storage is the problem of metal clustering
on the surface. Using the first-principles dFT calculations, it was shown [59] that clustering of Ti atoms can be avoided by using
a composite BC
4
n planar sheet that is obtained through chemical modification of the graphene surface by systematically replac-
ing C atoms by B and n atoms. This Ti-BC
4
n system shows hydrogen storage capacity with a reasonably high gravimetric
efficiency (~8.4 wt.%). Further, using ab initio Mdin simulation, it was shown that this system is stable at 300 K, while desorp-
tion starts at approximately 500-600 K, which is lower compared to that of conventional graphene or Bn planar structures.
Motivated by successful fabrication of monolayer materials consisting of hybrid graphene and boron nitride domains, Liu
et al. [60] performed a first-principles study of hybrid graphene/boron nitride (C-Bn) nanoribbons with dihydrogenated edge(s).
The study suggested that hybrid C-Bn nanoribbons may possess semimetallicity with a certain range of widths for the graphene
and Bn sections. In general, the hybrid C-Bn nanoribbons can undergo the semiconductor-to-semimetal-to-metal transitions as
the width of both graphene and Bn nanoribbons increases. The calculated electronic structures of the hybrid C-Bn nanoribbons
suggest that dihydrogenation of the boron edge can induce localized edge states around the Fermi level, and the interaction
among the localized edge states can lead to the semiconductor-to-semimetal-to-metal transitions.
Boron nitride materials in the form of borazine polymers with high specific surface of approximately 50-700 m
2
/g (highly
macroporous Bn aerogels) were produced [61] and shown to adsorb H
2
and other gases. A series of porous Bn materials with
specific surface of approximately 440-710 m
2
/g were also produced [62] using polymeric precursors. As they can adsorb H
2
and
some other gases, these materials can serve as selective gas adsorbents utilizing the local polar characteristics of the individual
B-n bonds not present in the carbon structures. Ammonia borane (nH
3
-BH
3
, AB) is known as a lightweight material containing
a high density of hydrogen H
2
that can be readily liberated for use in fuel cell-powered applications. However, in the absence of a
straightforward and efficient method for regenerating AB compound from dehydrogenated polymeric spent fuel, its full potential
as a viable H
2
storage material will not be realized. It was demonstrated [63] that the spent fuel type derived from the removal of
more than two equivalents of H
2
per molecule of AB, that is, polyborazylene, can be converted back to AB nearly quantitatively
by 24 h treatment with hydrazine n
2
H
4
in liquid ammonia nH
3
at 40°C in a sealed pressure vessel. nanoscale h-Bn additive
for ammonia borane (AB) was shown [64] to decrease the onset temperature for hydrogen release. Both the nano-Bn and the
AB:nano-Bn samples were prepared by ball milling. The hydrogen release was measured by a volumetric gas burette system.
