Electrochemistry of various substituted aminophenyl iron porphyrins

Abstract

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.