Environmental Engineering Reference
In-Depth Information
18.5.4 ORR Catalysis by Cofacial Diporphyrins: Plausible
Catalytic Cycles
Little is known about the mechanism of O
2
reduction even in the case of the most well-
studied catalysts, (FTF4)Co
2
and (DPY)Co
2
(Y ¼ A, B). Catalysis starts at potentials
between those of the [(dipor)Co
2
]
2
þ
/
þ
and [(dipor)Co
2
]
þ
/0
pairs observed under
anaerobic conditions [Collman et al., 1994]. In conjunction with the high O
2
affinity
of [(dipor)Co
2
]
þ
observed in nonaqueous media, the onset of O
2
reduction is com-
monly interpreted to indicate that [(dipor)Co
2
]
þ
is the catalytically active redox
state at potentials of the rising part of the O
2
reduction wave [Collman et al., 1994].
Certain polarographic studies of (FTF4)Co
2
were interpreted as indicative of O
2
bind-
ing primarily to the mixed-valence Co
III
Co
II
redox state [Collman et al., 1988, 1994].
The change of the dominant redox form of the catalyst from [(dipor)Co
2
]
þ
to
(dipor)Co
2
at about 0.5 V (vs. NHE at pH 0) was suggested to account for the decrease
in the catalytic selectivity of graphite-adsorbed (FTF4)Co
2
at potentials more reducing
than about 0.5 V (vs. NHE at pH 0), because the reduced catalyst was presumed to be
more prone to binding O
2
outside the cavity and thus reducing it noncooperatively.
Unfortunately, little is known about the O
2
affinities of the various redox forms of
(dipor)Co
2
in contact with an aqueous solution. For example, the presence of O
2
in
the aqueous electrolyte does not appear to perturb the position of the Co
III
Co
II
/Co
I
2
redox wave in graphite-adsorbed (FTF4)Co
2
[Collman et al., 2003a], suggesting
that the fraction of the oxygenated complexes, [(FTF4)Co
2
O
2
]
þ
/0
is quite low
during the redox cycling as a result of either thermodynamic or kinetic limitations.
In contrast, the positions of redox waves of (FTF4)Co
2
in anhydrous benzonitrile
solutions are strikingly different under anaerobic and aerobic conditions [LeMest
et al., 1995], indicative of rapidly attained highly favorable equilibrium: O
2
þð
FTF4
Þ
Co
2
O
½ð
FTF4
Þ
Co
2
O
2
þ
þ
e
. It seems plausible that competition between O
2
and
H
2
O for the Co sites may lower the O
2
affinity of graphite-adsorbed [(FTF4)Co
2
]
þ
in contact with an aqueous electrolyte.
Figure 18.16 Two plausible catalytic cycles of the ORR by cofacial bis-Co porphyrins that are
consistent with the 260 mV/pH dependence of the turnover frequency at the rising part of the
catalytic wave. Mechanisms A and B postulate that the mixed-valence [(dipor)Co
2
]
þ
and
the fully reduced [(dipor)Co
2
] redox states, respectively, are the catalytically active form of
the cofacial bis-Co porphyrin catalyst. The overall charges of, and the oxidation states of the
Co ions in, the intermediates are used only for electron-bookkeeping purposes, and are not
intended to imply a specific location of the hole or restrict the charge of any implicit ligand.
The binding mode of O
2
is unknown; Co ions may have additional ligands in some or all
redox states. ET and PT are electron and proton transfer, respectively. In mechanism B, both
irreversible (turnover-determining step, TDS) protonation and reversible protonation, followed
by an irreversible non-electron-transfer, non-proton-transfer (non-ET, non-PT) step would yield
the 260 mV/pH dependence. The simplest sequence is shown following the principle of
Occam's razor . Potential side reactions that account for the decrease in n
av
with more reducing
potentials are shown: they must include at least one electron transfer step to yield a potential-
dependent selectivity
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