Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 673

Figure 18.20 A plausible ORR catalytic cycle by biomimetic catalysts 2 (Fig. 18.17). Cu is
ligated by three imidazoles (omitted for clarity) and potentially an exogenous ligand, whose
nature is not known. All intermediates other than ferric - peroxo and ferric - hydroperoxo
were prepared independently.
of a series 2 porphyrin is in the catalytically active form, 5-coordinate Fe II . The dioxy-
gen reactivity of 5-coordinate Fe II forms of series 2 porphyrins was studied under
stoichiometric conditions (see the previous section): the resulting O 2 adduct was
formulated as ferric - superoxo/Cu I ; the same isomer was assumed to be present on
the electrode during ORR catalysis. To give the observed 230 mV/pH dependence
of the turnover frequency, another redox equilibrium was proposed, between the
ferric - superoxo and the ferric - peroxo intermediates, followed by an irreversible
protonation (the TDS), presumably to yield a ferric - hydroperoxo species. A slow
protonation was the simplest way to account for both the 230 mV/pH dependence
and the fact that all other steps were too fast to give the overall measured turnover
frequency of 2 s 21 (at 150 mV). The structures of the peroxo-level intermediates
were not determined, and in these intermediates the terminal O atom of the peroxo
ligand may interact with distal Cu, forming a m-peroxo complex.