Chemistry Reference
In-Depth Information
20
18
16
14
1
12
10
2
8
6
3
4
2
0
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
Co(II)TSPc (mol l
-1
)
7.5
Relationships between peak current and Co(II)TSPc concentration
of peak III
c
(curve 1) at scan number 1 (see Fig. 7.1) and peaks III
c
and IV
c
(see Fig. 7.1) for the scan number where the deposition
behaviour changes (curves 2 and 3, respectively), obtained at a
gold-disc electrode in a pH
=
12 buffer solution at
T
=
298.0 K.
(Reprinted from
Journal of Electroanalytical Chemistry
, Vol 567 No
2, De Wael et al, 'Study of the deposition of . . .', pp 167-73 (2004),
with permission from Elsevier.)
7.4
Electrocatalysis with modified gold electrodes
towards sodium dithionite
Common possible steps in an overall electrochemical reaction are electron
transfer, chemical reaction, transport and ad(de)sorption steps, the slowest
one being rate determining. It is known that depositing a catalyst at the
surface of an electrode can increase the reaction rate, in general by accel-
erating the charge transfer or chemical reaction kinetics. This means that if
improvement of the overall electrochemical reaction rate is the goal, such
a kinetic step should be rate determining at the unmodified electrode. This
also means that for redox systems behaving reversibly at the unmodified
electrode, electrocatalysis is not useful because, for reversible systems,
transport controls the overall rate of the reaction at all potentials.
Despite the fact that, for the oxidation of sodium dithionite at unmodi-
fied gold electrodes, well-defined voltammetric waves are obtained
12
,the
reaction still behaves irreversibly
12-14
.Therefore, it is in principle possible
to improve the oxidation rate of this reaction by using an appropriate cat-
alyst immobilised at the gold electrode surface. The modified electrode used
here was obtained by a 100-fold scanning experiment between -1.2 and
