Chemistry Reference
In-Depth Information
For the fifth stage of the reaction sequence as RDS, the following rate
equation was calculated, making use of Equations 4.19, 4.32, 4.21, 4.22 and
4.42, and, under the additional condition k
-4
¢>>k
5
≤, the following applies:
32
c
F
R
F
R
(
)
(
)
-
05
.
EE
-
-
05
.
EE
-
12
12
12
¢
HO
ref
ref
[4.47]
v
¢ =
KKKKk
¢¢
e
-
k
¢¢
c
e
2
T
T
5
1
2
3
4
5
-
5
O
2
12
c
-
OH
When comparing the experimental Tafel slopes and the dependency of the
current of hydroxide ions and hydrogen peroxide concentration to what
Equation 4.47 theoretically predicts, mechanism 2 with stage 5 as RDS can
be considered as a correct sub-mechanism and this over the entire poten-
tial area. The predicted reaction orders amount to 3/2 and -1/2 for hydro-
gen peroxide and OH
-
, respectively. In section 4.9, it is verified whether a
combination of mechanism 1, where stage 4 or 5 is the RDS, with mecha-
nism 2, where stage 5 is the RDS, corresponds to the experimental data.
4.8
The pseudo-limiting-current
As discussed in sections 4.6 and 4.7, mechanism 1 with stage 4 as RDS and
mechanism 2 with stage 5 as RDS are taken into consideration as possible
mechanisms within the potential area limited by ca. -0.1 V and 0.2 V vs.
SCE. With potentials that are more positive than ca. 0.2 V vs. SCE, the
current increases less significantly than predicted by the anodic exponen-
tial term in Equations 4.38 and 4.47. The best known, most commonly oc-
curring cause of such a phenomenon is the depletion of the reacting
components at the electrode surface, in the case considered hydrogen per-
oxide and/or hydroxyl ions. This, however, can be left out of consideration
here because the pseudo-limiting-current of the prewave is influenced min-
imally by convection in the solution.
Another obvious possible explanation can be found in sub-stages that are
becoming rate-determining, sub-stages of which the rate of the anodic reac-
tion is independent of potential. In the postulated sub-mechanisms, the rate-
determining supposition of five stages in total (two in mechanism 1, see
Equations 4.31 and 4.34 and three in mechanism 2, see Equations 4.43, 4.44
and 4.46) appeared to lead to potential-independent reaction rates.
However, they should all be excluded, because the reaction orders pre-
dicted for hydrogen peroxide or hydroxyl ions are not compatible with the
experimental data.
It is clear that rate Equations 4.38, 4.40 and 4.47 do not appear to be able
to explain the deviation of the experimental current-potential relation
occurring with the more positive potentials in the prewave. However, these
relations apply only under restrictive conditions (k
-3
>> k
4
≤ and/or k
-2
>>
k
-4
≤ for Equation 4.38, k
-4
≤>>k
5
≤ for Equation 4.40 and k
-4
¢>>k
5
≤ for Equa-
tion 4.47), which have only been introduced to eliminate (1 -q) terms from
