Environmental Engineering Reference
In-Depth Information
residence time of H
2
O
2
in the film and the longer time H
2
O
2
spends coordinated to a
metalloporphyrin catalyst before escaping from the film. It was suggested that if two
criteria are satisfied—(i) independence of the i
r
/i
d
ratios from the electrode rotation
rate and (ii) the electrochemical reduction of H
2
O
2
being significantly slower than
that of O
2
by the same film—then the catalytic properties of the film reflect those of
an individual isolated molecule.
Neither, however, rules out the intermediacy of free H
2
O
2
by a film displaying
n
av
. 2. The first criterion does not take into account the fact that the H
2
O
2
flux reach-
ing the ring during O
2
reduction in a multilayer catalytic film may be determined
mainly by the residence time of H
2
O
2
in the film (which is independent of rotation
rate) rather than the time an H
2
O
2
molecule remains in the vicinity of the
disk - electrolyte interface (which is inversely proportional to the electrode rotation
rate). Under such conditions, the i
r
/i
d
ratio would be independent of rotation rate
even if free H
2
O
2
is generated and reduced further within the film. The second criterion
neglects the fact that the catalytic rates of H
2
O
2
reduction when H
2
O
2
is added to an
anaerobic electrolyte may be limited by the rate at which H
2
O
2
penetrates the film -
solution interface, rather than by the catalytic rate. In such a case, the magnitudes of
catalytic currents of H
2
O
2
reduction under anaerobic conditions do not reflect the
rate at which the catalyst reduces/disproportionates H
2
O
2
generated within the film
as a result of incomplete O
2
reduction by individual molecules.
Three new criteria were proposed [Collman et al., 2003a] to establish that the four-
electron reduction of O
2
by a catalytic film represents the selectivity of an individual
molecule of the catalyst: (i) independence of the i
r
/i
d
ratio on the catalyst surface cov-
erage; (ii) much higher stability of the catalyst in the ORR compared with reduction of
H
2
O
2
under conditions that reproduce the concentration of H
2
O
2
in the film that would
be generated if ORR proceeded by a step-wise mechanism; and (iii) the nature of the
turnover-determining step.
ORR catalysis by Fe or Co porphyrins in Nafion [Shi and Anson, 1990; Anson
et al., 1985; Buttry and Anson, 1984], polypyrrolidone [Wan et al., 1984], a surfactant
[Shi et al., 1995] or lipid films [Collman and Boulatov, 2002] on electrode surfaces
has been studied. The major advantages of diluting a metalloporphyrin in an inert
film include the ability to study the catalytic properties of isolated molecules and
the potentially higher surface loading of the catalyst without mass transport limit-
ations. Stability of catalysts may also improve upon incorporating them into a polymer.
However, this setup requires that the catalyst have a reasonable mobility in the matrix,
and/or that a mobile electron carrier be incorporated in the film [Andrieux and
Saveant, 1992]. The latter limits the accessible electrochemical potentials to that of
the electron carrier.
Relatively little work has been done on ORR catalysis by self-assembled mono-
layers (SAMs) of metalloporphyrins. The advantages of this approach include a
much better defined morphology, structure, and composition of the catalytic film,
and the surface coverage, and the capacity to control the rate at which the electrons
are transferred from the electrode to the catalysts [Collman et al., 2007b; Hutchison
et al., 1993]. These attributes are important for deriving the catalytic mechanism.
The use of optically transparent electrodes allows characterization of the chemical
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