Environmental Engineering Reference
In-Depth Information
The observed facile electroreduction and electro-oxidation of H
2
O
2
at potentials
.0.2 V very likely proceeded through the same ferric - hydroperoxo intermediate as
did ORR catalysis. Therefore, the turnover frequency of H
2
O
2
reduction placed the
lower limit on the rate of O - O bond heterolysis in the hydroperoxo - ferric intermedi-
ate (note that O - O bond homolysis was ruled out by the use of
OH scavengers and
the long (.10
4
turnovers) lifetime of the catalyst during the ORR). The resulting
oxoferryl/Cu
II
complex could be prepared independently by electrochemical
oxidation of the resting state of the catalyst at about 1 V (vs. NHE at pH 7). At the
rising part of the ORR catalytic wave, this oxoferryl/Cu
II
intermediate undergoes
rapid, essentially irreversible, proton-coupled reduction to a ferric - hydroxo/Cu
II
complex. The ferric - hydroxo and ferric - aqua complexes were found to be in a
rapid equilibrium (the pK
a
of the coordinated water was estimated to be 8.5), thus
regenerating the resting state of the catalyst.
The key feature of this mechanism is the generation of the ferric - hydroperoxo
intermediate in the TDS, which means that the (pseudo)-first-order rate constant for
the decay of this intermediate was .10-fold the pseudo-first-order rate constant of
the preceding protonation. Hence, this intermediate has a very short lifetime (or,
equivalently, is present in a very low molar fraction during steady-state ORR cataly-
sis), which minimizes the probability that it will release H
2
O
2
. Probably because of
distal Cu
I
, no evidence for O - O bond homolysis in the ORR or H
2
O
2
reduction
catalysis was observed (the Cu-free analog did generate
OH radicals in H
2
O
2
reduction, but not in the ORR). However, even the Fe-only (Cu-free) metallopor-
phyrins retained their catalytic activity over 10
2
-10
3
times as many turnovers as
simple Fe porphyrins, probably because the imidazole ligand trans to the peroxo
moiety lowers the activation barrier of the heterolytic pathway relative to O - O
bond homolysis [Watanabe, 2000; Sono et al., 1996].
The catalytic cycle in Fig. 18.20 also rationalizes the potential-dependent n
av
of
series 2 catalysts (Fig. 18.19). The primary partially reduced oxygen species was
determined to be superoxide, O
2
2
, by using O
2
2
scavengers incorporated in catalytic
films. Superoxide is produced by autoxidation, i.e., heterolysis of the Fe - O bond in
the ferric - superoxo intermediate [Shikama, 1998], probably induced by protonation
of the terminal O atom in bound O
2
. The hypothesis of protonation-assisted autoxida-
tion was supported by the observation that n
av
at the rising part of catalytic curves was
smaller in acidic media (more superoxide was produced), whereas no partially reduced
oxygen species were detected at any potentials in basic (pH . 8) electrolytes. The
autoxidation rate constant at pH 7 was estimated to be 0.03 s
21
(for the Fe-only
forms of series 2 catalysts) and ,0.01 s
21
for the FeCu forms.
Release of superoxide during ORR catalysis indicates that the ferric - superoxo
intermediate (Fig. 18.20) has a substantial residence time at 0.2 V (the potential of
the maximum production of superoxide), suggesting that the potential of the ferric -
superoxo/ferric - peroxo couple, E
per
(Fig. 18.20), is more reducing than 0.2 V. The
fraction of superoxide detected at potentials .0.2 V probably reflects the fact that
O
2
2
, which is a strong outer-sphere reductant [Huie and Neta, 1999], was oxidized
by the mostly ferric catalytic film before it could escape the film. There are two plaus-
ible explanations for the decrease in the fraction of superoxide byproduct released at
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