Biomedical Engineering Reference
In-Depth Information
calculated without taking into account the effect of chemisorption
centers of the
diffusant capture into account.
The desorption energy (
−
∆
H
) and the effective difusion-
(13)I
activation energy (
) of hydrogen molecules for process I represent
only a small fraction of the rupture energy
Q
I
)
for bonds linking hydrogen to chemisorption centers. Great part
of the breaking energy comes from the energy (-∆
−
∆
H
(or
-
∆
H
(12)I
(12a)I
H
) due to the
dis
association of hydrogen atoms into molecules.
Therefore, process I is the diffusion of hydrogen molecules
in surface nanolayers of the carbon material accompanied by
the reversible dissociation and the diffusant capture at the
chemisorption centers, in local equilibrium for reactions (2.11),
(2.12), and (2.12a). Such a mechanism for the process I implies
that a small fraction of the chemical-bond breaking energy −∆
H
(12)I
(or
) manifests itself in the energy characteristics of the
hydrogen desorption and diffusion, -∆
-
∆
H
(12a)I
H
and
Q
,
respectively.
(13)I
I
2.2.2.4
Characteristics and Some Manifestations of
Chemisorptions Processes I-IV
Various characteristics and mechanisms of hydrogen chemisorption
in graphite and related carbon nanomaterials with sp
2
hybridization
for processes I-IV are listed in Table 2.1.
Processes II and III in carbon nanomaterials manifest themselves
in the IR spectra (process II), in the proton NMR spectra, in the nature of
neutron diffraction, in the X-ray absorption spectra, and also (process
III) in a substantial increase of the interplanar spacing between
graphene layers. Process II is also characterized by an accompanying
occurrence of a fairly small amount of hydrocarbons (CH
and others)
in thermal desorption spectra. The explanation of this phenomenon is
that the energy
4
for detachment of two hydrogen atoms from
a carbon atom of the sorption center is much higher than the energy
-
−
∆
H
(12)II
of detachment of a carbon atom from the two
nearest carbon neighbors (see Fig. 2.8, model H).
The value of the enthalpy of C-C σ-bond formation (sp
∆
H
≈ 485 kJ mol
−1
C-C
2
hybridization) in graphite (∆
H
) can be estimated by the well-
C-C
known formula ∆
H
≈ -(2/
z
) ∆
H
[68], where ∆
H
is the graphite
C-C
C
C
C
sublimation enthalpy [58] and
= 3 is the coordination number in
a graphene layer. Reasoning in a similar way, we can estimate the
enthalpy of the formation of a C-C σ-bond (sp
z
C
3
hybridization) in
-1
diamond as -357 kJ mol
at
z
= 4.
C
