Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 677

0.55 V) and selectivity (n av 3.9). It appears fairly well accepted that in the very best
cofacial porphyrin catalysts, bimetallic cooperativity plays a critical role. The catalytic
mechanism remains to be adequately elucidated, complicating rational improvement
of these fascinating compounds. Cofacial porphyrins remain too unstable and prohi-
bitively expensive for practical applications.
Biomimetic metalloporphyrins provide examples of some of the best molecular
ORR catalysts in neutral ( pH 7 - 8) media, with a 0.55 V overpotential and n av . 3.9
being retained over .10 4 turnovers. Among all metalloporphyrin ORR catalysts,
ORR catalysis by biomimetic metalloporphyrins is the best understood mechanisti-
cally. In the context of fuel cell catalysis, further development of biomimetic catalysts
needs to focus on (i) simplifying the synthesis and identifying the stereoelectronic
moieties that are most critical to the catalysis; (ii) increasing the Fe III/II potential,
even at the expense of O 2 affinity; (iii) decreasing the affinity of the ferrous porphyrin
to H 2 O; and (iv) increasing the turnover number by at least 100-fold.
Reduction of O 2 at a metalloporphyrin site appears to proceed invariably through
partially reduced oxygen intermediates, such as bound peroxide. This mechanism,
however, does not necessitate large overpotentials or low selectivity, since coordi-
nation of peroxide to one or more metal ions can stabilize it both relative to O 2
(making the O 2 ! O 22 conversion nearly thermoneutral) and relative to free H 2 O 2
(making the two-electron reduction kinetically unfavorable). The efficiency of ORR
catalysis in monomeric metalloporphyrins seems to be controlled in large part by a
ligand trans to bound O 2 . Hence, understanding how to control this axial ligation in
a cost-effective manner may be a promising strategy to develop metalloporphyrins
that would be of use in low temperature fuel cells.
REFERENCES
Abramson J, Riistama S, Larsson G, Jasaitis A, Svensson-Ek M, Laakkonen L, Puustinen A,
Iwata S. 2000. The structure of the ubiquinol oxidase from Escherichia coli and its ubiqui-
none binding site. Nat Struct Biol 7: 910.
Abramson J, Svensson-Ek M, Byrne B, Iwata S. 2001. Structure of cytochrome c oxidase:
A comparison of the bacterial and mitochondrial enzymes. Biochim Biophys Acta 1544: 1.
Adzic R. 1998. Recent advances in the kinetics of oxygen reduction. In: Lipkowski J, Ross PN,
editors. Electrocatalysis. New York: Wiley. p. 197.
Alberts B, Johnson A, Lewis J, Raff M, Walter P. 2002. Molecular Biology of the Cell. 4th ed.
New York: Garland Science.
Andrieux CP, Saveant J-M. 1992. Catalysis at redox polymer coated electrodes. In: Murray RW,
editor. Molecular Design of Electrode Surfaces. New York: Wiley. p. 207.
Anson FC, Ni CL, Saveant JM. 1985. Electrocatalysis at redox polymer electrodes with separ-
ation of the catalytic and charge propagation roles. Reduction of dioxygen to hydrogen per-
oxide as catalyzed by cobalt(II) tetrakis(4-N-methylpyridyl)porphyrin. J Am Chem Soc 107:
3442.
Anson FC, Shi C, Steiger B. 1997. Novel multinuclear catalysts for the electroreduction of
dioxygen directly to water. Acc Chem Res 30: 437.