Environmental Engineering Reference
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Most of these results can be understood within a generic mechanistic scheme
(Fig. 18.11), which is based on numerous spectroscopic, computational, and electro-
chemical studies of reactions of O 2 or H 2 O 2 with Fe porphyrins under stoichiometric
conditions, and with heme enzymes under single-turnover conditions (see, e.g.,
Watanabe [2000]; Loew and Harris [2000]; Sono et al. [1996]). Indeed, such an O 2
reduction cycle at a heme site was characterized at atomic resolution by X-ray diffrac-
tion in two enzymes [Schlichting et al., 2000; Berglund et al., 2002]. Only ferrous por-
phyrins coordinated to an axial ligand (5-coordinate Fe II ) have any measurable affinity
for O 2 . Dioxygen is always bound end-on in such complexes. Without a nitrogenous
base trans to O 2 , dioxygen adducts of Fe porphyrins could only be prepared at 30 K in
O 2 matrices [Proniewicz et al., 1991]. Under ambient conditions, 4- and 6-coordinate
ferrous porphyrins do not bind O 2 , but may reduce it to O 2 2 by an outer-sphere mech-
anism, depending on the Fe III/II potential [Shikama, 1998]. All ferric porphyrins are
aerobically stable.
Anaerobic cyclic voltammetry suggests that simple Fe porphyrins deposited on an
electrode in contact with an aqueous buffer contain two axial water molecules (or an
Figure 18.11 Plausible catalytic cycle for the ORR by simple Fe porphyrins adsorbed on the
electrode surface and side Reactions (18.15) - (18.18). At pH , 3, the resting state of the catalyst
is assumed to be ferric - aqua.
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