Environmental Engineering Reference
In-Depth Information
†
Making the Fe
III/II
potential more oxidizing—even at the expense of lowering
the O
2
affinity.
†
Decreasing the affinity of the catalyst in the Fe
II
redox state for H
2
O.
It appears that the heme/imidazole motif can be realized in metalloporphyrins
that are available in just two steps (Fig. 18.21b). However, it is not yet known
how to accomplish objectives 2 and 3. It is also important to understand the mechan-
ism of catalyst degradation during the ORR and to identify alternative functional
groups that may increase catalyst stability: to be useful in fuel cells, a metalloporphyrin
catalyst would probably have to retain its catalytic properties over at least 4
10
6
turnovers (about 1000 hours of operation at a turnover frequency of 1 s
21
), i.e.,
more than a hundred times longer than the most stable metalloporphyrin catalysts
reported to date.
18.7 SUMMARY AND CONCLUSIONS
Most of the O
2
consumed by aerobic organisms (including all mammals) is reduced to
H
2
O at a heme group of cytochrome c oxidase. Under physiological conditions, each
molecule of this enzyme catalyzes reduction of up to 50 molecules of O
2
per second
(turnover frequency of 50 s
21
), capturing over 80% of the free energy of oxygen
reduction to satisfy the energy needs of the organism. Although the O
2
-reducing
site of cytochrome c oxidase is bimetallic (in addition to an Fe porphyrin, it has a
Cu ion), four-electron oxygen reduction can be catalyzed efficiently by a 5-coordinate
heme, provided that outer-sphere electron transfer steps do not limit the turnover.
The prevalence of the heme in O
2
metabolism and the discovery in the 1960s that
metallophthalocyanines adsorbed on graphite catalyze four-electron reduction of O
2
have prompted intense interest in metalloporphyrins as molecular electrocatalysts for
the ORR. The technological motivation behind this work is the desire for a Pt-free
cathodic catalyst for low temperature fuel cells. To date, three types of metalloporphyrins
have attracted most attention: (i) simple porphyrins that are accessible within one or two
steps and are typically available commercially; (ii) cofacial porphyrins in which two
porphyrin macrocycles are confined in an approximately stacked (face-to-face) geome-
try; and (iii) biomimetic catalysts, which are highly elaborate porphyrins designed to
reproduce the stereoelectronic properties of the O
2
-reducing site of cytochrome oxidase.
Although simple porphyrins are attractive in terms of cost, they are generally poor
catalysts because they manifest one or more of (i) low selectivity for the four-electron
reduction; (ii) low turnover numbers (the number of molecules of the substrate that one
molecule of catalyst reduces before losing its catalytic activity); and (iii) high over-
potential. Simple Fe and Co porphyrins appear to be fairly inefficient in catalyzing
heterolysis of the O - O bond in peroxo-level intermediates. As a result, bond homo-
lyses, generating highly destructive hydroxyl radicals and/or release of free H
2
O
2
,are
important processes in ORR catalysis by these porphyrins.
A bis-Co cofacial porphyrin, reported in the early 1980s, is among the best molecu-
lar catalysts ever found for the ORR in acidic media in terms of overpotential (about
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