Environmental Engineering Reference
In-Depth Information
H
2
O and HO
2
ligands for ferric porphyrins at pH . 4) (see, e.g., Shigehara and
Anson [1982]). This coordinatively saturated complex must lose one water molecule
to bind O
2
[Reaction (18.9b) in Fig 18.11]. The equilibrium constant of this reaction
was reported for one biomimetic Fe porphyrin on the graphite surface, the equilibrium
being shifted strongly towards the 6-coordinate, aqua-ligated Fe
II
porphyrin, (K ,
10
22
M) [Boulatov et al., 2002]. It is, however, also possible that a minority species,
for example Fe
II
(por) axially ligated by a carboxylate or quinol residue of the graphite
electrode, is the catalytically active form.
Upon O
2
binding, the catalyst - O
2
adduct, which is formally a ferric - superoxo
complex [Taube, 1986], is probably reduced to a ferric - hydroperoxo intermediate,
whose subsequent reactions determine the product distribution of the catalysis. If
both the distal and proximal environments around the Fe porphyrin are appropriate,
as is in heme enzymes involved in O
2
metabolism, or in certain sophisticated biomi-
metic ORR catalysts (Section 18.6), then the O - O bond in the hydroperoxo intermedi-
ate invariably undergoes heterolysis. During O - O bond heterolysis, both oxygen
atoms become reduced to the 22 (“oxide”) redox state. If this heterolysis occurs in
concert with an electron transfer from a nonheme group, as is postulated for cyto-
chrome c oxidase (Section 18.2), then a ferryl intermediate is formed (Fig. 18.5). In
other heme enzymes, both electrons come from the heme, yielding a high valence
oxoferryl cation radical intermediate (Compound I) [Sono, 1996]. This reaction is
facilitated both by the proximal ligand (e.g., imidazole, thiolate, or phenoxide)
and the distal environment (see Fig. 18.4c for the definition of proximal and
distal sides).
Since simple Fe porphyrins lack appropriate distal/proximal structures, it seems
probable that O - O bond heterolysis in the ferric - hydroperoxo intermediate is inhib-
ited relative to the competing reactions, which include O - O bond homolysis
[Reaction (18.18) in Fig. 18.11], hydrolysis of intact H
2
O
2
(18.16), and reduction to
a ferrous - hydroperoxo intermediate (18.12b) under appropriately reducing potentials.
The propensity of simple Fe
III
porphyrins to induce O - O bond homolysis in H
2
O
2
is
well established from studies of stoichiometric reactions between H
2
O
2
and such por-
phyrins [Watanabe, 2000]. Predominance of this homolytic reduction pathway (18.18)
during catalytic O
2
reduction by electrode-confined Fe porphyrins would explain the
common observation that such catalysts lose their catalytic capacity only after a few
turnovers. (However, the uncertainty in the amount of catalytically active metallopor-
phyrin in the catalytic film makes any estimates of the turnover numbers only approxi-
mate.) In contrast, structurally analogous Co porphyrins, which catalyze reduction of
O
2
only to H
2
O
2
, retain their activity over a substantially larger number of turnovers.
This difference is consistent with the primary role of hydroxyl radicals, rather than
H
2
O
2
as sometimes suggested [Shigehara and Anson, 1982], in rapid degradation
of Fe porphyrin catalysts.
An increase in the fraction of the four-electron reduction pathway at more reducing
potentials (Fig. 18.10a, b) may be rationalized within at least two mechanisms. The
first is based on the kinetic competition between the release of H
2
O
2
from the
ferric - hydroperoxo intermediate [Reaction (18.16) in Fig. 18.11] and its (reversible)
reduction to a ferrous - hydroperoxo species, which undergoes rapid O - O bond
heterolysis (18.13b). Because H
2
O
2
and particularly HO
2
2
are more basic ligands
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