Environmental Engineering Reference
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CHAPTER 18
Metalloporphyrin Catalysts of
Oxygen Reduction
ROMAN BOULATOV
Department of Chemistry, University of Illinois, 600 South Mathews Avenue, Urbana, IL 61801, USA
18.1 INTRODUCTION
Sometime probably two billion years before humans became interested in efficient cat-
alysts for four-electron, four-proton reduction of O
2
to H
2
O, the so-called oxygen
reduction reaction (ORR),
O
2
þ
4H
þ
þ
4e
O
2H
2
O
E
o
¼
1
:
23 V
(18
:
1)
primitive organisms on Earth faced a similar problem [Knoll, 2003]. The appearance
of photosynthesis and the concomitant generation of O
2
produced evolutionary
pressure to deal with this new chemical, first as a detoxification strategy for anaerobes
and later as a new form of energy metabolism. The solution that life came up with is
quite different from that dominating current research into low temperature fuel cell
ORR catalysts: instead of nanoparticulate Pt or related bulk metals, biological O
2
reduction is catalyzed predominantly by hemes, Fe complexes of a planar macrocyclic
ligand, porphyrin (Fig. 18.1). Biological ORR catalysts have to meet a rather different
set of requirements than a fuel call catalyst, yet the predominance of heme in aerobic
metabolism over the past two billion years justifies a serious look at metalloporphyrins
as potential ORR catalysts for low temperature fuel cells.
In addition to their proven capacity to catalyze a highly efficient and rapid reduction
of O
2
under ambient conditions (e.g., cytochrome c oxidase, the enzyme that catalyzes
the reduction of .90% of O
2
consumed by a mammal, captures .80% of the free
energy of ORR at a turnover frequency of .50 O
2
molecules per second per site),
metalloporphyrins are attractive candidates for Pt-free cathodes. Probably the major
impetus for a search for Pt-free cathodic catalysts for low temperature fuel cells is
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