Biomedical Engineering Reference
In-Depth Information
value, obtained from thermodynamic Eqs. (2.5)-(2.9)
and from the experimental values of ∆
The ∆
H
(3)III
[51, 58], can
be obviously interpreted as the indirect experimental value, which
is (in absolute value) approximately half of the C-H binding energy
in the methane molecule and equal to the H-H binding energy in
the hydrogen molecule (−∆
H
and ∆
H
(4)III
dis
value is close to
theoretical value of the interaction energy of hydrogen atoms and
the graphene (cylindrical) surface of various single-wall nanotubes
(50% filling) obtained by the density functional theory (DFT) [57].
This value is also close to the experimental value [59] of the energy
of C-H bond formation, binding hydrogen and carbon atoms in
fullerene C
H
) [58]. The ∆
H
dis
(3)III
(with H/C = 36/60 filling factor), i.e., in a quasigraphene
spherical layer.
The indirect experimental value of the binding enthalphy
60
∆
is significantly greater (by 8%, i.e., beyond the limits of
experimental error) than the dissociation enthalphy of half mole
H
H
(3)III
) [58]. It explains the experimentally observed fact
[51] that the process III (reaction (2.4)), a dissolution (dissociative
chemisorption) of hydrogen in a graphite lattice between graphene
layers, is exothermic. The desorption enthalpy ∆
(1/2 ∆
H
2
dis
is then close to
the energy value characteristic for the dissociative sorption process I
in Ref. [27] and represents approximately 8% of the C-H bond
rupture enthalphy (−∆
H
(4)III
); the remaining 92% of that energy is
provided by the enthalphy of association of one hydrogen atom mole
in a molecule (−1/2 ∆
H
(3)III
).
The thermodynamic characteristic −∆
H
dis
manifests itself
almost entirely in the effective energy of a bulk diffusion activation of
hydrogen atoms in the graphite lattice (
H
(3)III
) spent mainly
for the duffusant C-H bond breaking. The bulk diffusion of hydrogen
atoms in material is then accompanied by a reversible capture (local
equilibrium for reaction (2.3)) of the duffusant by chemisorption
carbon centers in graphene layers. It results in the thermodynamic
contribution −∆
Q
≈ −∆
H
III
(3)III
H
to
Q
, being much larger than the negligible
(3)III
III
l
kinetic contribution
.
Thus, a characteristic feature of the dissociative chemisorption
of hydrogen by carbon materials (process III) is the formation of
approximately 50% bonds weaker than the typical chemical C-H
bonds, e.g., in a methane molecule. Another interesting feature is the
desorption energy of 19 kJ mol
Q
−1
, close to the value of the energy
characteristic of process I, which the authors of Ref. [27] identify as
the rupture energy of the hypothetical weak chemical C-H bond.
