Chemistry Reference
In-Depth Information
the equations. The only purpose was to obtain the simplest possible rate
equations, allowing verification if the theoretically predicted Tafel
slopes and reaction orders correspond to the experimentally obtained
values.
A possible explanation for the pseudo-limiting-current formation is as
follows: considering the global rate equations of stages 3 and 4 of mecha-
nism 1 (without restrictions, only anodic terms):
12
12
F
R
k
¢¢
k
¢¢
(
EE
-
)
-
12
ref
[4.48]
4
5
(
)
v
=
KK k
c
c
c
1
-
q
e
T
k
k
3
,a
1
2
3
-
-
O
-
OH
O
12
12
HO
2
¢¢
¢¢
2
-
4
-
5
F
R
k
k
(
)
(
)
05
.
EE
-
2
3
(
)
ref
[4.49]
v
=
KK
k
¢¢
c
c
1
-
q
1
-
q
e
T
4
,
a
1
2
4
-
-
-
-
OH
O
HO
O
2
2
-
3
Since stage 4 is the RDS, the (1 -q) term of O
2
-
is equal to 1. However,
this is not the case for the (1 -q
O
-
) term. What can be considered is that
k
-3
< k
4
≤ so that formed O
-
particles react only in a very restricted way back
into the OH
•
radical. In order to clarify the rate difference between stages
3 and 4, the rate relation can be examined, without the relation of the expo-
nents being simplified:
k
¢¢
12
F
R
(
)
EE
-
-
12
ref
5
c
O
e
T
2
k
¢¢
12
k
¢¢
12
v
v
3
,
a
-
4
-
5
=
[4.50]
12
k
k
¢¢
F
R
4
,
a
(
)
05
.
EE
-
ref
4
(
)
1
-
q
e
T
-
O
-
3
With a potential that becomes more positive, one expects from the poten-
tial dependency an increase in the rate relation. However, one should also
take into account the oxygen concentration and the (1 -q) term. Oxygen is
the reaction product of the hydrogen peroxide oxidation reaction which is
considered here. This means that the oxygen concentration in the vicinity
of the electrode surface increases with the potential becoming more posi-
tive, which will oppose the above-mentioned increase in rate of stage 3 and
hence also the increase in the O
-
surface coverage, with the result that the
value of the (1 -q
O
-
) term varies very little. This corresponds to the exper-
imental observation in the potential area of ca.
E
=-0.1 V to
E
= 0.2 V vs.
SCE. The fact that, with more positive potentials, the current bears off can
be explained by considering that the degree of saturation of dissolved
oxygen is attained in the vicinity of the electrode surface. The excess of
oxygen that is formed will turn into the gas phase (oxygen bubbles) and
will therefore not have any further influence on the dissolved oxygen con-
centration. This means that the oxygen concentration in the rate equation
of stage 3 becomes constant and no longer acts as a rate inhibitor. This will
result in the fact that the rate of stage 3 will increase much more strongly
