Biomedical Engineering Reference
In-Depth Information
def
def
A D
A D
,
II
II
D
(2.17)
II
K
K
K
(11)II
(12)II
(13)II
Q
Q
def
- (
D
H
+
D
H
)
D
-
H
(2.18)
II
(11)II
(12)II
(13)II
def
def
where
are the diffusivity and the difusion-
activation energy of hydrogen molecules in the intergranular
or defective regions of the carbon material in the absence of
chemisorption capture centers or at the maximum (carbohydride)
filling of these centers;
A
≈ const.;
D
and
Q
II
def
−1
Q
≈ 10 ± 5 kJ mol
(H
) (experimental
2
and theoretical estimates in Refs. [5, 10]);
K
and
K
are the
(11)II
(12)II
equilibrium constants for reactions (2.11) and (2.12); ∆
H
(10)II
≈ 0,
∆
H
≈ ∆
H
and ∆
H
are the standard enthalpies for reactions
,
(12)II
(11)II
dis
(2.10)-(2.12).
The desorption energy
-
∆
H
and the effective hydrogen-
(13)II
molecule diffusion activation energy
Q
represent only about 20%
II
of the chemical bond rupture energy
. The dominant part
(about 80%) of the energy of chemical bonds rupture, linking
hydrogen with chemisorption centers (
-
∆
H
(12)II
), comes from the
energy of hydrogen atoms association into molecules (
−
∆
H
(12)II
).
Therefore, TPD peak II (reaction (2.13)) corresponds to the
diffusion of hydrogen molecules in intergranular and/or defective
regions of the carbon material accompanied by a reversible
dissociation and diffusant capture on chemisorption C-2H centers (at
the local equilibrium for reactions (2.11) and (2.12)). The presence
of such mechanism in process II implies that only about 20% of the
chemical-bond rupture energy −∆
-
∆
H
dis
H
contributes to the energies
(12)II
characterizing hydrogen desorption (
-
∆
H
) and diffusion (
Q
).
(13)II
II
for process II, formally described
by the Henry-Langmuir isotherm for nondissociative adsorption
(Eqs. (2.14) and (2.14a)), are several times greater than the value of
the energy characteristics for process II (20-40 kJ mol
The values of −∆
H
and
Q
(13)II
II
), identified
with the rupture energy of hypothetical “superphysical” bonds [27].
The energy of chemical bond rupture per mole of hydrogen
atoms in process II is given by −∆
−1
−1
(H), which
corresponds to the intermediate value of the same parameter for
processes III and IV, equal to
H
/2 ≈ 280 kJ mol
(12)II
-1
-
∆
H
≈ 243 kJ mol
(H) and
-
∆
H
(3)III
(3)IV
−1
≈ 364 kJ mol
(H), respectively.
The calculations, based on Eqs. (2.14) and (2.15), suggest that
the equilibrium concentration of adsorbed hydrogen molecules
