Chemistry Reference
In-Depth Information
than that of stage 4, and that the surface coverage of O
-
particles increases,
with a resultant decrease of the current.
Returning to Fig. 4.6, it can be seen that the potential range in which the
transfer coefficient applies amounts to ca.
E
= 0.3 V vs. SCE for low hydro-
gen peroxide concentrations. This can be explained on the basis of the
degree of saturation since, with lower hydrogen peroxide concentrations,
less oxygen is formed (lower current density), and so the degree of satura-
tion is attained only with more positive potentials. Analogously, the pseudo-
limiting-current can be explained for mechanism 2 by comparing the rate
of stage 3 (Equation 4.51) and 5 (Equation 4.52) on the basis of the relation
of the speed equations (Equation 4.53):
¢
¢¢
¢¢
F
R
k
k
k
k
(
EE
-
)
-
1
[4.51]
4
5
ref
(
)
v
¢
=
KK k
c
2
c
1
-
q
e
T
3,a
1
2
3
O
-
-
2
O
¢
HO
2
-
4
-
5
12
¢
F
R
k
k
k
k
(
)
(
)
05
.
EE
-
12
12
32
-
12
3
4
ref
¢¢
v
¢ =
KK
k
c
c
1
-
q
e
T
5,a
1
2
5
-
-
-
OH
12
HO
O
¢
2
2
-
3
-
4
[4.52]
12
12
12
F
R
KKK
k
(
)
EE
-
12
-
1
1
2
3
ref
c
c
e
T
O
-
2
¢¢
HO
v
v
¢
¢
2
3,a
-
5
[4.53]
=
F
R
1
(
)
(
)
05
.
EE
-
-
12
5,a
ref
c
1
-
q
e
T
-
-
OH
12
O
k
2
-
3
It can be concluded that the combination of sub-mechanism 1 with stage 4
as RDS and sub-mechanism 2 with stage 5 as RDS qualifies for a possible
global mechanism to explain the experimentally observed evidence.
In section 4.6.5, stage 5 of mechanism 1 as a possible RDS with poten-
tials more positive than ca.
E
= 0.2 V vs. SCE was not excluded. Based on
the above-mentioned arguments, this stage as RDS can be excluded. More-
over, the relation of the rate equations of stages 4 and 5 of mechanism 1
cannot explain that stage 5 would become slower than stage 4:
F
R
05
.
(
EE
-
)
2
ref
(
)
v
v
1
-
q
e
T
-
4,a
=
O
[4.54]
k
k
¢¢
¢¢
F
R
(
)
15
.
EE
-
5,a
5
ref
e
T
-
4
unless (1 -q
O
-
) ª 1 and k
-4
≤>>k
5
≤. However, this seems very unlikely since
both conditions predict much higher current densities than was expe-
rimentally observed. In the next stage, it should be verified whether
the current or rate equations which resulted from both postulated sub-
mechanisms can be combined, and, if so, if this combination can explain the
experimentally obtained data and connections.
