Environmental Engineering Reference
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in Fig. 18.15 were reported in a single publication and may have a larger uncertainty.
In other cases, the reports are contradictory: for example, reported values of n
av
for a
heterometallic complex, (DPA)CoFe, vary from 3.6 to 2, at the plateau of the first wave
(the complex exhibits a voltammogram as in Fig. 18.10a) [Ni et al., 1987; Guilard
et al., 1995]. As is the case for simple Fe and Co porphyrins, the catalytic performance
of cofacial metalloporphyrins (and probably other porphyrinoids, although at present
there are no data) appears to be quite sensitive to how the electrode surface was
prepared and the catalyst deposited.
The data in Fig. 18.15 are for metalloporphyrins adsorbed on a graphite electrode in
contact with an aqueous electrolyte with pH between 0 and 1, which are the conditions
that have been most commonly used for ORR catalysis by cofacial metalloporphyrins.
There is some controversy regarding the importance of pH and adsorption on graphite
for achieving four-electron catalysis. The selectivity of (FTF4)Co
2
was reported to
depend strongly on the pH of the aqueous electrolyte, with the predominantly
four-electron pathway observed only at pH ,3.5 [Collman et al., 1994]. In contract,
the selectivity of (DPA)MM
0
(M ¼ M
0
¼ Co, M ¼ M
0
¼ Fe, or M ¼ Co, M
0
¼ Fe)
was found to be only weakly pH-dependent [Liu et al., 1985]. In solution,
(FTF4)Co
2
was reported to be essentially a two-electron catalyst [Collman et al.,
1994]. A self-assembled monolayer of a (DPB)Co
2
analog, (DPB
S
)Co
2
(Fig. 18.13), was reported to catalyze O
2
reduction only to H
2
O
2
[Hutchison et al.,
1997]. In contrast, selectivities of (DPY)Co
2
(Y ¼ A, B, D, X) dissolved in benzoni-
trile containing HClO
4
were reported to be identical to those of graphite-adsorbed
catalysts [Fukuzumi et al., 2004].
At pH 0 the overpotential of ORR catalyzed by the best catalysts, (FTF4)Co
2
and(DPX)Co
2
(X ¼ A, B, D, P, X) is about 0.55 V. Under identical conditions,
most other bis-Co cofacial porphyrins manifest an overpotential of 0.6 - 1 V. In gen-
eral, the overpotential and n
av
do not correlate even for bis-Co derivatives. Replacing
even one Co ion in a bis-Co porphyrin invariably increased the overpotential: for
example, 0.6 V for (DPA)Co
2
versus 0.7 V for (DPA)Co (monometallic) and 0.9 V
for (DPA)Fe
2
. Likewise, replacing the porphyrin macrocycle with a porphyrinoid
increased the overpotential: for example, 0.8 V for (PCA)Co
2
versus 0.6 V for
(DPA)Co
2
(Fig. 18.13). As is the case for simple Fe and Co porphyrins, the stability
of the mostly four-electron catalysts is low.
Among cofacial porphyrins, the most selective ORR catalysts are (FTF4)Co
2
and (DPY)Co
2
(Y ¼ A, B, D, P, X), or the analogous porphyrin/corrole derivative,
(PCA)Co
2
, although the latter has not yet been studied extensively. It is not entirely
clear if (DPA)Fe
2
is a more selective catalyst than a simple Fe porphyrin (Section
18.3). Overall, we lack adequate atomistic understanding of the structure/activity
relationship in cofacial porphyrinoid catalysts, although many attempts have been
made to rationalize experimental observations. Most hypotheses postulate that some
interaction between the terminal O atom of the O
2
fragment bound to one metallopor-
phyrin fragment and the other porphyrin moiety (either with or without a metal) is
required to achieve high n
av
. Thus, the dramatic difference in the (FTFn)Co
2
series
(n ¼ 3 - 6) was attributed to the difficulty in forming intermediates containing
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