Biomedical Engineering Reference
In-Depth Information
in the intergranular nanoregions in GNFs and nanostructured
graphite at 300 K and a hydrogen pressure in the range 1-10 MPa
is of the same order of magnitude of the carbohydride value (H
/
2
C
≈ 0.5. In other words, the hydrogen content tends to approach
the reference value reported in Refs. [1-3], making such materials
suitable for use as adsorbents for hydrogen-driven vehicles. This is in
agreement with the results reported in Refs. [5, 10, 19], processing the
sorption data from Refs. [12, 14, 54-56]. At the same time, for process
II (as well as for processes III and IV), the rate of diffusion discharge
of chemisorbed hydrogen from the material at room temperature is
low (basically because of the high value of
def
)
IIm
) and does not meet the
technical requirements [1-3] for hydrogen-driven vehicles.
Moreover, the indirect experimental value ∆
Q
II
H
= −560 ± 10
(12)II
kJ mol
(2H) is determined (including correction and matching of
all the involved quantities) either from the experimental values of
∆
−1
def
H
and ∆
H
(Eq. (2.16)) or from
Q
, ∆
H
, and
Q
(Eq. (2.18)).
(13)II
dis
II
dis
The value of ∆
obtained in this way is in a good agreement with
the theoretical value of −540 kJ mol
H
(12)II
-1
(2H) obtained with
ab initio
MO calculations of the chemisorption process IV, according to the
proposed model H, as sketched in Fig. 2.8 [36].
It is also comparable to the theoretical value of −476 kJ mol
-1
(2H), obtained [36] considering the model G sketched in Fig. 2.8.
2.2.2.3.b
Process I
Another process that closely resembles, in terms of energy
characteristics values, the nondissociative adsorption process II
described in Ref. [27], is the dissociative-associative or formally
nondissociative chemical adsorption of hydrogen from the initial
state H
gas
on the surface of isotropic graphite pellets [51, 53] (see Fig.
2.5, TPD peak I) and carbon nanostructures, single-wall nanotubes
[26, 63, 64], and multiwall nanotubes [62].
A chemisorption process I corresponds (similarly to process
II) to overall reaction (2.13) with stages (2.10)-(2.12) where the
surface areas of material, instead of defective or intergranular
regions of localization of sorption centers, should be considered. For
the process I, the chemisorption centers of two hydrogen atoms are,
apparently, the dangled C-C σ-bonds in the chair-like edge carbon
(monatomic) positions (model G in Fig. 2.8) and/or alternating
carbon (diatomic) positions in a graphene layer (model F in Fig. 2.8).
Reaction (2.12) is then applicable only to chair-like edge carbon
2
