Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 651

than H 2 O, reduction of [X( por)Fe III ( 2 O 2 H)] would require more reducing potentials
than reduction of [X( por)Fe III (OH 2 )] þ (the latter corresponds to the onset of catalytic
O 2 reduction). Since O - O bond heterolysis in the ferrous - hydroperoxo intermediate
does not require oxidation of the porphyrin, it is more facile than O - O bond hetero-
lysis in the ferric analog. Hence, at potentials close to or more oxidizing than that of
the Fe III/II couple, simple Fe porphyrins catalyze mainly two-electron reduction of O 2
to H 2 O 2 , with possibly an important side reaction of O - O bond homolysis (18.18).
This reaction pathway dominates because the kinetics of O - O bond heterolysis
(18.12a) is unfavorable in simple Fe porphyrins relative to the two competing path-
ways (18.16) and (18.18). However, at potentials more reducing than that of the
Fe III/II couple under anaerobic conditions, one-electron reduction of the ferric -
hydroperoxo intermediate becomes sufficiently favorable that the sequence of steps
(18.12b) - (18.13b) becomes kinetically competitive with steps (18.16) and (18.18).
The flux of H 2 O 2 decreases, and the stability of the catalyst may increase.
The second mechanism often invoked to explain the increase in n av of simple
Fe porphyrins at potentials more reducing than that of the Fe III/II couple (under anaero-
bic conditions) is based on the fact that at such potentials the fraction of the catalyst
in the 5-coordinate ferrous state is maximal because (i) the equilibrium (18.9) is shifted
completely to the ferrous form and (ii) the concentration of O 2 in the catalytic film
is low owing to mass transport limitations. The higher the concentration of the 5-coor-
dinate ferrous porphyrin in the catalytic film, the greater the probability that any
released H 2 O 2 will re-enter the catalytic cycle by coordinating to a molecule of ferrous
porphyrin and decay according to (18.13b) instead of (18.17).
Both mechanisms can also rationalize an increase in n av due to the production of
superoxide/HO 2 (18.16), which appears to dominate the flux of partially reduced
oxygen species generated by certain biomimetic catalysts [Boulatov et al., 2002;
Boulatov, 2004]. It remains to be established if either of these two mechanisms
does indeed operate in simple Fe porphyrins, for example by carrying out single-turn-
over experiments similarly to the approach used to study ORR by cytochrome c
oxidase.
Within the mechanism in Fig. 18.11, it seems implausible that simple Fe porphy-
rins can be effective ORR catalysts, since large overpotentials are required to access
intermediates in which O - O bond heterolysis is facile. The only strategy discovered
so far to facilitate this O - O bond heterolysis in the ferric - hydroperoxo intermediate is
to control both the distal and the proximal environments of Fe porphyrins. In those
cases, the overpotential of ORR reduction appears to be controlled by the potential
of the ( por)Fe III/II couple (see Section 18.6).
18.4.2 Simple Co Porphyrins and Anson Catalysts
Like their Fe analogs, Co III porphyrins are aerobically stable. Co II , being a d 7 ion,
favors a square pyramidal coordination sphere, and simple Co II porphyrins typically
have fairly low affinity to axial ligands. As a result, many simple Co II porphyrins
are aerobically quite stable in the solid state, but oxidize slowly if dissolved in a coor-
dinating solvent. Simple Co III porphyrins are typically stronger oxidants than the Fe III