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analogs, which in theory should lead to lower overpotential of ORR catalysis.
Unfortunately, the more oxidizing Co
III/II
potential also leads to lower affinity to O
2
(an exception being cofacial porphyrins; see Section 18.5). As a result, O
2
reduction
catalysis by simple Co porphyrins often starts at potentials 0.2 - 0.4 V more reducing
than that of the Co
III/II
couple [Song et al., 1998].
Simple Co porphyrins, whether adsorbed on an electrode, in a Nafion film, or in
solution are generally thought to be largely two-electron catalysts (reduction of O
2
to H
2
O
2
) [Fukuzumi et al., 2004; Song et al., 1998; Bianchini and Zoellner, 1997;
Anson et al., 1997; Postlethwaite et al., 1995; Hutchison et al., 1993]. A fair
number of examples have been reported [Deronzier and Moutet, 2003; Yuasa et al.,
1997, 2001; Song et al., 1998; Shi et al., 1997] of graphite-absorbed Co porphyrins,
including Co porphin (CoP; Fig. 18.12), manifesting voltammograms of the type
shown in Fig. 18.10a. The regime with n
av
. 2 is sometimes proposed to arise from
catalysis by face-to-face dimers that are formed spontaneously upon absorption of
the metalloporphyrin onto graphite. This conclusion is based on (i) the highly selective
four-electron reduction of O
2
by certain cofacial porphyrins (Section 18.5) and (ii) the
high propensity of simple neutral metalloporphyrins to p-stack. At present, however,
there is little direct evidence supporting the presence of such dimers in catalytic films,
and such dimers have never been shown to possess catalytic activity. An alternative
explanation, invoking p-donation from the peripheral substituents of the porphyrin
to Co, was proposed to explain some propensity of Co(TPHP) (Fig. 18.12) absorbed
on graphite to reduce O
2
to H
2
O [Anson et al., 1997]. The intermediacy of free H
2
O
2
with this catalyst has been suggested. Compared with their Fe analogs, simple Co
porphyrins that catalyze only two-electron reduction of O
2
to H
2
O
2
usually retain
their catalytic activity over a greater number of turnovers, which may be related to
the fact that Co porphyrins do not typically promote homolysis of the O - O bond in
free or metal-coordinated H
2
O
2
[Marusak and Mears, 1995].
The p-donation was also invoked to explain a remarkable catalytic activity of Co
porphyrins derivatized peripherally with Ru or Os ammine complexes (Fig. 18.12)
[Anson et al., 1997]. Since their invention by Anson in the early 1990s, many Co por-
phyrins derivatized peripherally with metal complexes have been reported [Araki and
Toma, 2006; Gadamsetti and Swavey, 2006]. Some of these catalysts manifest high
selectivity for the four-electron pathway, albeit within a limited pH range [Araki
and Toma, 2006]. In the original work, Co tetra-pyridylporphyrin was adsorbed on
a graphite electrode and this modified electrode was exposed to a solution of
[Ru(NH
3
)
5
(OH
2
)]
2
þ
, in which only the aqua ligand is labile and Ru
II
has a higher affi-
nity to pyridine than to H
2
O. Based on an increase in the voltammetric response of the
electrode after being exposed to a solution of [Ru(NH
3
)
5
(OH
2
)]
2
þ
, it was concluded
that between 3 and 4 [Ru(NH
3
)
5
]
2
þ
moieties coordinated per molecule of Co(TPyP),
presumably through the peripheral pyridyl groups. Using [Ru(NH
3
)
3
(H
2
O)
3
]
2
þ
instead of [Ru(NH
3
)
5
(H
2
O)]
2
þ
was thought to yield a coordination polymer with
one [Ru(NH
3
)
3
(H
2
O)]
2
þ
moiety linking two Co(TPyP) molecules. Such electrodes
catalyzed reduction of O
2
to a mixture of H
2
O (70 - 90%) and H
2
O
2
. The catalysis
started at potentials close to that of the Ru
III/II
couple (about 0.4 V vs. NHE).
Similar results were observed with Co complexes of trispyridylmonophenylporphyrin,
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