Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 665

The half-wave potentials of (FTF4)Co 2 -mediated O 2 reduction at pH 0 - 3 shifts by
260 mV/pH [Durand et al., 1983], which indicates that the turnover-determining part
of the catalytic cycle contains a reversible electron transfer (ET) and a protonation, or
two reversible ETs and two protonation steps. In contrast, if an irreversible ET step
were present, the pH gradient would be 60/(n þ a)mV/pH, where n is the number
of electrons transferred in redox equilibria prior to the irreversible ET and a is the
transfer coefficient of the irreversible ET. The 260 mV/pH slope is identical to
that manifested by simple Fe porphyrins (see Section 18.4.1). The turnover rate of
ORR catalysis by (FTF4)Co 2 was reported to be proportional to the bulk O 2 concen-
tration [Collman et al., 1994], suggesting that the catalyst is not saturated with O 2 .
At least two catalytic cycles are consistent with these observations, depending on
whether the catalytically active redox state of the cofacial bis-Co porphyrin is mixed-
valence, [(dipor)Co 2 ] þ (Fig. 18.16, mechanism A) or fully reduced, [(dipor)Co 2 ]
(Fig. 18.16, mechanism B). Since the catalysis occurs at potentials about 0.1 V
more reducing than those of the [(dipor)Co 2 ] 2 þ / þ couple, the redox equilibrium
½ð dipor Þ Co 2 2 þ O ½ð dipor Þ Co 2 þ at the catalytic wave has a minimal impact on the
molar fraction of the catalytically active species, and therefore cannot contribute to
the observed pH dependence of the turnover rate. Hence, if the mixed-valence
redox state, [(dipor)Co 2 ] þ , is indeed the catalytically active form, as is usually
assumed [Collman et al., 1994], the catalytic cycle must contain another redox equili-
brium before the turnover-determining step (TDS). The sequence of reversible proto-
nation of the superoxo-level intermediate followed by its reversible one-electron
reduction is one possibility. Superoxo adducts of a number of cofacial bis-Co porphyr-
ins have been demonstrated to undergo reversible protonation in benzonitrile
[Fukuzumi et al., 2004; LeMest et al., 1997], but the protonated form is not known
to undergo reversible redox chemistry. Since only the solution chemistry of the proto-
nated form was studied, if the peroxo-level intermediate undergoes rapid intramolecu-
lar rearrangement, very fast sweep rates may be required in solution voltammetry to
observe redox equilibrium between [(dipor)Co 2 (O 2 H)] 2 þ and [(dipor)Co 2 (O 2 H)] þ
species. For graphite-adsorbed species, the redox equilibria are assumed to establish
very rapidly.
The alternative mechanism (Fig. 18.16, mechanism B) is based on the fully reduced
[(dipor)Co 2 ] state as the redox-active form of the catalyst. The redox equilibrium between
the mixed-valence and fully reduced forms is shifted toward the catalytically inactive
mixed-valence state, and hence controls the amount of catalytically active species in
the catalytic cycle and contributes to the 260 mV/pH dependence. The fully reduced
form is known to bind O 2 (probably reversibly) in organic solvents [LeMest et al.,
1997; Fukuzumi et al., 2004], and the resulting diamagnetic adducts are typically
viewed as a pair of Co III ions bridged by a peroxide, which are of course quite common
in the O 2 chemistry of nonporphyrin Co complexes. To obtain the 260 mV/pH
dependence of the catalytic turnover rate, a protonation step is required either prior
to the TDS or as the TDS. Mechanism B cannot be extended to monometallic cofacial
porphyrins or heterometallic porphyrins with a redox-inert ion, but there is no reason
to assume that the two classes of cofacial porphyrin catalysts, with rather different
catalytic performance (Fig. 18.15), must follow the same mechanism.