Environmental Engineering Reference
In-Depth Information
While O
2
adsorbs molecularly only under specific circumstances or high coverages
[Parker et al., 1989], on most transition metals adsorption of oxygen is accompanied
by dissociation of the molecule, leading to atomic oxygen on the surface. In this case,
the last term of (5.6) may be replaced by N
O
m
O
(T, p
O
2
). Since the surface oxygens are
in contact with the O
2
atmosphere, the chemical potential m
O
is then determined by the
condition of thermodynamic equilibrium with the surrounding O
2
gas phase reservoir,
meaning that m
O
¼
2
m
O
2
(taking the O22O dissociation energy as an arbitrary con-
stant), and (5.6) becomes
g(T, p
O
2
, p
Pt
)
¼
1
(T, p
Pt
)
N
O
G(T, p
O
2
, p
Pt
, N
O
, N
Pt
)
N
Pt
g
bulk
2
m
O
2
(T, p
O
2
)
(5
:
7)
Pt
A
where, to be consistent with the existing literature, we have replaced the chemical
potential of Pt by the Gibbs free energy of bulk Pt, g
bulk
Pt
. In the case that the environ-
ment contains more than a single component, (5.7) contains additional terms similar to
the oxygen term on the right-hand side.
5.2.2.2 Solid Compound in Contact with a Single- or Multi-
Component Environment
Staying with the previous example of Pt in contact
with an oxygen environment, at very low temperatures or high oxygen partial
pressures, a bulk oxide (Pt
x
O
y
) will form. Since the Pt bulk oxide surface should be
in thermodynamic equilibrium with the surrounding, the following condition must
be fulfilled:
xg
bulk
Pt
þ
ym
O
¼
g
bulk
(5
:
8)
Pt
x
O
y
Insertion into (5.7) finally gives
g(T, p
O
2
, p
Pt
x
O
y
)
¼
1
G(T, p
O
2
, p
Pt
x
O
y
, N
O
, N
Pt
)
N
Pt
g
bulk
Pt
x
O
y
(T, p
Pt
x
O
y
)
A
x
m
O
2
(T, p
O
2
)
þ
1
2
y
N
Pt
x
N
O
(5
:
9)
5.2.3 Electric Double Layer
Compared with the non-electrochemical interface discussed in the previous section,
where a solid was in contact with a surrounding gaseous atmosphere, the electrode/
electrolyte interface is a multicomponent system and, besides temperature and partial
pressures/concentrations, it is also influenced by the electrode potential. This results
in greater complexity, which requires additional considerations prior to deriving an
expression for the interfacial stability.
In the literature, one often finds different expressions or definitions for the various
potentials relevant to the electrode/electrode interfaces. To provide a clear definition
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