Chemistry Reference
In-Depth Information
Ring-disc voltammetry (glassy-carbon disc/glassy-carbon ring) did not
provide a deeper insight either: except for oxygen, no electroactive reac-
tion products, generated in the disc reaction, could be detected on the ring.
We tried to detect intermediates by means of ring electrodes constructed
from precious metals. This provided no results either and, moreover, the
catalytic decomposition of hydrogen peroxide strongly interfered at some
of these materials (such as platinum).
A classical approach
72
to unravelling the mechanism of electrochemical
reactions and to identifying the rate-determining step (RDS) is based on
testing the validity of possible sequences of reaction stages according to the
elementary theory of electron transfer. As opposed to disc voltammetry,
one does not look for direct evidence such as the presence of intermediate
components. As a consequence, more reaction sequences appear to be
theoretically possible, which in the ideal case can be dismissed, all but one
on the basis of experimental evidence. It should be pointed out that neither
does the identification of intermediates by means of electrochemical or
non-electrochemical techniques automatically lead to the 'true' reaction
mechanism. It is only an aid in the sense that identified intermediates must
occur in a postulated reaction mechanism and that hence the number of
possible mechanisms can be reduced.
In order to apply the method, one needs to have a number of data, first
of all, the reacting component, the reaction product and the number of elec-
trons interfering in the global reaction. As to the global reaction in the
prewave, it is clear that the reaction product is molecular oxygen. The only
reaction product that could be detected with ring-disc voltammetry is pre-
cisely oxygen. Moreover, one can visually detect the formation of a gas in
the reaction. Considering the composition of the solution (H
2
O, H
2
O
2
and
NaOH), it is logical that, for an oxidation reaction, oxygen is the resulting
reaction product. Hence, in a sufficiently alkaline environment, the global
reaction is:
ยจ
[4.4]
HO
-
+
OH
-
O
+
H O
+
2
e
-
2
2
2
A key element in the analysis is the correspondence between the observed
and the theoretical dependence of the current on the potential. In other
words, one compares the theoretical Tafel slope for the postulated mecha-
nisms, or the transfer coefficients which are derived from these, to the
experimentally obtained values.
The experimental Tafel slope does not appear to deviate significantly
from 120 mV/decade, which corresponds to a transfer coefficient of 0.5, and
this over the complete investigated range of hydrogen peroxide concentra-
tion and pH (Fig. 4.6). This Tafel slope is found within a potential range
restricted from ca. -0.10 to 0.20 V vs. SCE. Above
E
= 0.20 V vs. SCE, the
inclination of the current-potential curve appears to decrease. In Fig. 4.7,
