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macrocycle or the peripheral subsituents of the same metalloporphyrin molecule. The
resulting organic radical probably decomposes rapidly and the product(s) are reduced
by the electrode or the chemical reductant that serves as the source of electrons for O
2
reduction. Therefore, generation of
OH does not directly contribute to lowering of n
av
;
however, it does lead to rapid degradation of the catalyst. Low stability of a
metalloporphyrin catalyst in ORR catalysis may be an indication of
OH generation.
This conclusion is strengthened if the incorporation of selective
OH scavengers in
the catalytic film increases the number of turnovers over which the film retains its cat-
alytic selectivity. Scavengers of O
2
2
/HO
2
and of
OH incorporated in a catalytic film
of a biomimetic Fe porphyrin (see Section 18.6) were used to determine that a very
small amount of partially reduced oxygen species produced by this catalyst during
O
2
reduction was mostly O
2
2
/HO
2
, which resulted from autoxidation of the cata-
lyst/O
2
complex [Boulatov et al., 2002].
18.4 ORR CATALYSIS BY SIMPLE METALLOPORPHYRINS
The “simple” porphyrin category includes macrocycles that are accessible syntheti-
cally in one or few steps and are often available commercially. In such metallopor-
phyrins, one or both axial coordination sites of the metal are occupied by ligands
whose identity is often unknown and cannot be controlled, which complicates
mechanistic interpretation of the electrocatalytic results. Metal complexes of simple
porphyrins and porphyrinoids ( phthalocyanines, corroles, etc.) have been studied
extensively as electrocatalysts for the ORR since the initial report by Jasinsky on cat-
alysis of O
2
reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964].
Complexes of all first-row transition metals and many from the second and third
rows have been examined for ORR catalysis. Of all simple metalloporphyrins,
Ir(OEP) (OEP ¼ octaethylporphyrin; Fig. 18.9) appears to be the best catalyst, but
it has been little studied and its catalytic behavior appears to be quite distinct from
that other metalloporphyrins [Collman et al., 1994]. Among the first-row transition
metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier
and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism,
the mechanism of ORR catalysis by Fe porphyrins is probably understood best among
all metalloporphyrin catalysts.
18.4.1 Simple Fe Porphyrins
Simple Fe porphyrins whose catalytic behavior in the ORR has been studied fairly
extensively are shown in Fig. 18.9. Literature reports disagree substantially in quanti-
tative characterization of the catalytic behavior (n
av
, overpotential, stability of the cat-
alysts, pH dependence, etc.). It seems plausible that in different studies the same Fe
porphyrin possesses different axial ligation, which depends on the electrolyte and
possibly specific residues on the electrode surface; the thicknesses and morphologies
of catalytic films may also differ among studies. All of these factors may contribute to
the variability of quantitative characteristics. The effect of the supporting surface on
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