Electrochemistry of various substituted aminophenyl iron porphyrins. Part II. catalytic reduction of dioxygen by electropolymerized films

Iron tetrakis(o-aminophenyl)porphyrin (Fe(o-NH₂)TPP), iron tetrakis(p-aminophenyl)porphyrin (Fe(p-NH₂)TPP) and iron tetrakis(p-N,N'-dimethylaminophenyl)porphyrin (Fe(p-NMe₂)TPP) were electropolymerized onto GC electrodes and were tested for catalysis of dioxygen reduction in aqueous solutions. They showed catalytic activity in the same potential ranges of the two reduction waves detected in deaerated solutions: +0.05 and -0.25 V for poly[Fe(o-NH₂)TPP], -0.04 and -0.23 V for poly[p-NH₂TPP] and -0.08 and -0.25 V for poly[Fe(p-NMe₂)TPP] at pH 3. The electrocatalysis was more effective in acidic solutions than in basic solutions: the overvoltage of O₂ reduction was reduced by about 450 mV in acid and by only 50 mV in base. Also the H₂O₂ yields reflected the different activities of the catalyst in the electrolyte solutions: 2.5% at pH 3 and 30% at pH 12 for O₂ reduction at a high surface coverage of a poly[Fe(o-NH₂)TPP] film. While the H₂O₂ yields for O₂ reduction at the other two iron porphyrin films at pH 3 were similar (2.8 and 3.1% for poly[Fe(p-NH₂)TPP] and poly[Fe(p)-NMe₂)TPP], respectively), they were considerably higher at pH 12 (41 and 58% for poly[Fe(p-NH₂)TPP] and poly[Fe(p-NMe₂)TPP], respectively). Unlike the poly[Fe(P-NH₂)TPP] and poly[p-NMe₂)TPP] films, poly[Fe(o-NH₂)TPP] was shown to reduce H₂O₂ catalytically at high pH, thus lowering the yield of H₂O₂ formed as an intermediate during O₂ reduction in basic solutions.