Selectivity-Determining Steps in O2 Reduction Catalyzed by Iron(tetramesitylporphyrin)

The selectivity of the oxygen reduction reaction (ORR) for water over hydrogen peroxide is important for fuel cells and other technologies. Iron porphyrin catalysts have long been studied for the ORR, but the origins of their selectivity are not well understood because the selectivity-determining step(s) usually occur after the rate-determining step. We report here the effects of acid concentration, as well as other solution conditions such as of acid pKa, on the H2O2/H2O selectivity in electrocatalytic ORR by iron tetramesitylporphyrin Fe(TMP) in DMF. The results show that selectivity reflects a kinetic competition in which the dependence on [HX] is one order greater for the production of H2O than H2O2. Based on such experimental results and computational studies, we propose that the selectivity is governed by competition between protonation of the hydroperoxo intermediate, FeIII(TMP)(OOH), to produce water versus dissociation of the HOO– ligand to yield to H2O2. The data rule out a bifurcation based on ...