This presentation discusses selectivity in the reduction of O2 catalyzed by iron porphyrins. The oxygen reduction reaction is central to fuel cell technology, but the selectivity for H2O versus H2O2 production has remained difficult to study. In proposed mechanisms with iron porphyrins, the bifurcation between these two products is understood to occur from an iron-hydroperoxo intermediate, but occurs after the rate-determining steps, rendering it kinetically invisible. In this work, the relative rate laws of water versus peroxide formation from the key intermediate are probed by studying product ratios as a function of reaction conditions such as acid pKa and concentration. This kinetic analysis reveals that the porphyrin secondary structure determines the pathway for H2O2 formation. Three distinct mechanisms are proposed for the formation of H2O2, only one of which has been suggested in enzymatic heme systems. This work provides insight into the chemistry of reactive iron hydroperoxo intermediates, which are relevant in a wide variety of systems from bioinorganic chemistry to synthetic organic methodology. Furthermore, this kinetic analysis will be generally valuable to the study of selectivity in other electrocatalytic reactions.

Selectivity-determining steps in the oxygen reduction reaction catalyzed by iron porphyrins

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