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chemical reductants, such as ferrocenes, cobaltocene, or an organic macrocycle. Each
method has its own strengths and limitations, and studying the same catalyst by more
than one method, though rarely done, can yield mechanistic information that is not
attainable from any one method.
In the most common approach, a water-insoluble metalloporphyrin is deposited on
the surface of a rotating disk electrode (RDE) or on the disk of a rotating ring - disk
electrode (RRDE; Fig. 18.7a) as a film of poorly defined morphology, either by spon-
taneous adsorption from a solution of the catalyst in an organic solvent or by evapor-
ation of an aliquot of such a solution onto the electrode. It is impossible to know the
Figure 18.7 (a) Rotating ring - disk electrode (RRDE; left) and schematics of chemical
processes at an RRDE in studies of metalloporphyrin-catalyzed ORR: the water-insoluble cat-
alyst is deposited on the disk, and the electrode is immersed in aqueous electrolyte. Rotation of
the electrode generates flow of the electrolyte to the electrode surface, which delivers O
2
and H
รพ
to the catalyst. The products of the reduction (H
2
O and/or H
2
O
2
) diffuse and also are transported
hydrodynamically from the disk to the ring electrode, which is set at a potential at which H
2
O
2
is
rapidly oxidized to O
2
. The ratio of the ring and disk currents is proportional to the fraction of O
2
reduced to H
2
O
2
, i.e., selectivity of the catalyst to four-electron reduction. (b) Linear sweep
voltammograms at different frequencies of electrode rotation. When the catalytic currents are
proportional to bulk O
2
concentration and are limited by the rate of the catalytic reaction
rather than by mass transport, the dependence of the catalytic currents on the rotational fre-
quency of the electrode is described by the Koutecky - Levich equation. The slopes of the i
1
cat
versus v
21/2
graphs (Koutecky - Levich graphs) are proportional to n
av
and the intercept is pro-
portional to k.
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