Surface-enhanced resonance Raman scattering and voltammetry study of the electrocatalytic reduction of oxygen by the μ-oxo dimer of iron(III) tetra-4-N-methylpyridylporphyrin

In this study, surface-enhanced resonance Raman scattering, SERRS, and voltammetric techniques (cyclic voltammetry, rotating disk electrochemistry, and rotating ring disk electrochemistry) are used to elucidate the mechanism of oxygen reduction on a silver electrode with iron(III) tetra-4-N-methyl-pyridylporphyrin. The results indicate that after the oxidation reduction cycle at pH 10 and pH 4, the iron porphyrin is adsorbed on the silver surface as a high-spin, five-coordinated mu-oxo-bridged dimer. On the forward voltage scan, this dimer undergoes two successive one-electron reductions at about -0.3 and -0.5 V to form a high-spin fully reduced Fe(II) mu-oxo-bridged dimer. SERRS spectra show that the first electron transfer forms a mixed-valence Fe(II)-O-Fe(III) mu-oxo-bridged dimer system. At pH 2, only the high-spin reduced Fe(II) monomer is adsorbed on the surface. At pH 4 and pH 10, a shift in the reduction of the oxygen overpotential by approximately +150 mV is observed and the active catalyst in this process is attributed to the partially reduced mu-oxo dimer. A value of n approximate to 4 was calculated for the number of electrons involved in this catalytic reaction, and rotating ring disk voltammetry showed that only 10% stable hydrogen peroxide species is produced on the dimer-modified surface. These results are used to formulate a mechanism for the catalytic reduction of oxygen.