Environmental Engineering Reference
In-Depth Information
of the nanofibers. A series of MgO/h-Bn composites with different mass ratios were synthesized by the impregnation method
and used as supports for ru catalysts in ammonia synthesis reaction [25]. The catalysts were characterized, in particular, by n
2
physical adsorption and temperature-programmed reduction of H
2
. The activity measurements of ammonia synthesis were
carried out in a reactor with a mixture of n
2
and H
2
atmosphere under a steady-state condition. The results showed that the rate
of ammonia formation was strongly influenced by the h-Bn content used in the catalysts' preparation process. At n
2
:H
2
= 1:3
atmosphere, the optimum activity was achieved when MgO:h-Bn = 8:2 was used as the catalytic support.
As is known, the c-Bn phase is an extremely promising multifunctional material. However, to exploit all possible applications,
large-area CVd of c-Bn films is required. For a successful CVd growth of high-quality c-Bn films, one must obtain a deeper
understanding of the structural and electronic properties of the dominant c-Bn growth surfaces under CVd conditions, that is,
the (100), (110), and (111) surfaces, and their modification in the presence of surface-stabilizing atomic hydrogen (H). In a study
by Karlsson and Larsson [26], the surface-stabilizing effect of H on the B- and n-terminated surfaces of c-Bn (100) was investi-
gated using dFT calculations. It was found that a 100% surface coverage of on-top H on some B-terminated c-Bn (100) surfaces
is not able to uphold an ideal bulk-like structure. For the n-terminated c-Bn (100) surfaces, opposite observations were made.
The process of H abstraction, with gaseous atomic H, was found to be significantly more favorable for a B-terminated c-Bn (100)
surface than for an n-terminated c-Bn (100) surface. It was also found that n radical sites are more stable toward radical surface
site collapse than B radical sites. In the work of Karlsson and Larsson [27], the ability of BH
x
and nH
x
species (
x
= 0, 1, 2, 3) to
act as growth species for the CVd of c-Bn, in an H-saturated gas phase, was investigated using dFT calculations. It was found
that the optimal growth species for CVd growth of c-Bn are B, BH, BH
2
, n, nH, and nH
2
, that is, decomposition of the incoming
BH
3
and nH
3
growth species is very crucial for CVd growth of c-Bn. It was also found that it would be most preferable to use a
CVd method where the incoming BH
3
and nH
3
growth species are separately introduced into the reactor, for example, by using
an atomic layer deposition type of method.
A systematic and comparative investigation of the number of different
IV
B-
VI
B transition metal carbides, nitrides, sulfides,
silicides, and borides as well as main group element ceramics including h-Bn for their electrocatalytic performance toward the
hydrogen evolution reaction was presented by Wirth et al. [28]. The performances of the electrocatalysts were compared with
Pt and ni benchmarks.
27.2
Bn nanotuBes
Like the hydrogenated Bn surfaces, Bn nanotubes have potential applications in hydrogen storage and were suggested as better
hydrogen storage media than C nanotubes. Physisorption of a hydrogen molecule together with chemisorption of atomic
hydrogen on Bn nanotubes was studied in detail. According to the theoretical results, chemisorption of hydrogen to boron is
exothermic. Chemisorption of hydrogen alters the electronic properties, indicating that the hydrogenated Bn nanotubes have
additional potential applications, such as nanoscale electronic devices.
Hydrogen uptake capacities of 1.8 and 2.6 wt.% were obtained [29] on Bn multiwalled and bamboo-like nanotubes, respec-
tively, under 10 MPa at room temperature. For Bn nanotubes synthesized by CVd over a wafer made by an Lani
5
/B mixture and
ni powder at 1473 K, which were straight with a diameter of 30-50 nm and a length of up to several microns, it was first verified
[30] that the Bn nanotubes can store hydrogen by means of an electrochemical method. The mechanism of the electrochemical
hydrogen storage process can be summarized as follows:
-
-
,
BN HO e NH OH
ads
+
+ ↔−
+
2
BN HBNH
ads
− ↔−
,
abs
2
BN
ad
− ↔+
H
2
BNH
.
2
Bn − H
ads
and Bn − H
abs
are denoted as adsorbed and absorbed hydrogen on Bn nanotubes, respectively. The hydrogen
desorption of nonelectrochemical recombination in cyclic voltammograms, which can be considered as the slow reaction in Bn
nanotubes, suggested the possible existence of strong chemisorption of hydrogen. The cathodic adsorption peak of hydrogen
was found in the cyclic voltammogram. The hydrogen desorption peak of nonelectrochemical recombination appeared at the
range between −0.78 and −0.85 V. It was concluded that the improvement of the electrocatalytic activity by surface modification
with metal or alloy would enhance the electrochemical hydrogen storage capacity of Bn nanotubes. The binding energy of
hydrogen atoms to a (10, 0) single-walled Bn nanotube was calculated [31] at 25, 50, 75, and 100% coverage using the dFT.
