Biomedical Engineering Reference
In-Depth Information
values obtained by DFT. This can be essentially attributed [38] to
the fact that a “collective stabilization” of chemisorbed hydrogen on
graphene surfaces has been ignored in the used models.
2.2.2.2.b
Process IV
The higher absolute values of the energy characteristics of sorption,
diffusion, and interaction of hydrogen with carbon materials,
including isotropic [51, 53] (see Fig. 2.5, TPD peak IV), pyrolytic
[60], and nanostructured [52, 53] (Fig. 2.7c, TPD peak IV) graphite,
manifest themselves in another process of dissociative chemisorption
of hydrogen (process IV, reactions (2.1)-(2.4)), which occurs in
defective regions of the graphitic structures. Chemisorption occurs
in carbon centers, or dangling C-C σ-bonds in “chairlike” and/or
“zigzaglike” edge positions (see Fig. 2.8, models C and/or D, sp
2
hybridization) located in defective regions of the graphite lattice
(C
l
).
The indirect experimental value of the enthalpy of C-H bonds
formation, linking hydrogen atoms to such chemisorption centers
(reaction (2.3)), is ∆
def
-1
(H). Process IV
(overall reaction (2.4)) is characterized by the experimental value
of the standard enthalpy of dissolution in defective regions of the
graphite lattice of one mole of hydrogen atoms ∆
H
= -364 ± 5 kJ mol
(3)IV
H
≈ ½∆
H
+
(4)IV
dis
−1
∆
(H) [60] and by the effective enthalpy
of the hydrogen-atom diffusion activation in the same regions of
the material,
H
= -140 ± 5 kJ mol
(3)IV
−1
(H) [52, 60]. The
characteristics of the process and the corresponding TPD peak IV
are described by equations similar to (2.5)-(2.9).
We note that the indirect experimental value of ∆
Q
≈ −∆
H
= 365 ± 50 kJ mol
IV
(3)IV
H
= −364 ±
(3)IV
−1
5 kJ mol
(H) is determined using the experimental values of ∆
H
(4)IV
l
and ∆
via Eqs. (2.7) and (2.9). Such a
value is in a satisfactorily agreement with the theoretical values of
−357 kJ mol
H
or the values of
Q
and
Q
dis
IV
MO
calculations corresponding to the assumed models (C and/or D in
Fig. 2.8) of chemisorption process IV [36].
(H) and -378 kJ mol
(H) obtained by
ab initio
−1
−1
2.2.2.3
Dissociative-Associative Chemisorption of Hydrogen:
A New Concept
2.2.2.3.a
Process II
One of the sorption processes similar to nondissociative adsorption
process II in Ref. [27] is the dissociative-associative, or formally
