Oxygen reduction at negatively charged iron porphyrins heat-treated and bridged by alkaline-earth metal ions

An ordered network of tetrasodium tetra(4-sulphonatophenyl)porphyrin iron(III) chloride (FeTPPS4Na4), which exhibited a higher catalytic activity for oxygen reduction than Co and Ir(CO)TPPS4Na4, was fabricated by complexation with alkaline-earth metal ions. Heat treatment of these complexes enhanced their catalytic activity with the highest performance observed with Ba2+–FeTPPS4. The onset potential for oxygen reduction (Eonset) was 720 mV vs. Ag|AgCl, which is almost the same as that for Pt-impregnated carbon black. The number of electrons, n, transferred during oxygen reduction at a Ba2+–FeTPPS4-coated electrode, as determined by rotating ring-disc experiments, was 3.9 and suggests that oxygen was reduced to water. A neutral solution of FeTPPS4Na4 was acidified by the addition of barium ion, and the elemental ratio Fe:S:Ba in the resulting Ba2+–FeTPPS4 complex was approximately 1:4:2.5. This suggests the formation of a highly ordered network with Fe sites bridged with barium ions in addition to the normal salt bridges between sulphonates. Catalytic oxidation reactions with nitric oxide and nitrite ion as the intermediate at the heat-treated Ba2+–FeTPPS4 indicate the formation of Fe(IV) and Fe(IV) π-cation radicals or Fe(V). The agreement between the potential of Fe(IV) π-cation radical formation and Eonset suggests that the redox cycle of the Fe(III)/Fe(IV) π-cation radical provides a sufficient driving force for the observed 4-electron reduction of oxygen at the heat-treated Ba2+–FeTPPS4 electrode.