Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 648

which may proceed through disproportionation of H 2 O 2 and reduction of the
resultant O 2 .
† Depending on pH, increasing the acidity of the solution either makes the poten-
tial required to yield a fixed turnover frequency more oxidizing by 60 mV/pH or
does not affect it. This pH dependence is in most cases the same as that of the
Fe III/II couple in the absence of a substrate. These identical pH dependences
suggest a pre-equilibrium between the ferric and ferrous forms of the catalyst
followed, by a kinetically irreversible step that does not involve proton or
electron transfer (e.g., O 2 binding).
† The apparent redox stoichiometry of O 2 reduction catalysis [n av , Reaction (18.8)]
is pH-independent, but for many catalysts depends strongly on the applied poten-
tial (Fig. 18.10). The apparent selectivity of Fe porphyrins deposited on the elec-
trode surface typically increases with the amount of deposited catalyst.
† Usually, simple Fe porphyrins degrade rapidly during catalytic reduction of
O 2 or of H 2 O 2 .
Figure 18.10 Three types of polarization curves typically manifested by simple Fe and Co
porphyrins and cofacial metalloporphyrins (simulated voltammograms).