Redox and electrocatalytic properties towards dioxygen reduction of ruthenium tetra(ortho-aminophenyl)porphyrin complexes with various axial ligands

A. Bettelheim, D. Ozer, R. Harth, R. W. Murray

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17 Scopus citations

Abstract

The redox properties of ruthenium(II) tetra(ortho-aminophenyl)porphyrins (Ru(o-NH2)TPP) with various axial ligands (L = CO, pyridine, acetonitrile and tetrahydrofuran) have been studied after dissolving them in CH3CN (for ruthenium complexes with L= CO or pyridine) or HCL(g)-saturated CH3CN solutions (L = acetonitrile or tetrahydrofuran). The formal potentials for oxidation of the Ru(II)(o-NH2)TPP(CO) compound were +0.88 and +1.20 V. For the Ru(II)(o-NH2)TPP(L)2 com- plexes, two oxidation waves were observed in the 0 to +0.5 V range: E°' = +0.15, -0.38 V for the (pyridine)2 complex; +0.12, +0.28 for the (CH3CN)2 complex; and +0.10, +0.50 V for the (tetrahy- drofuran)2 complex. The carbonyl and pyridine porphyrin complexes, when adsorbed onto GC electrodes and tested for catalysis of dioxygen reduction in aqueous solutions, did not show any catalytic activity. In contrast, the adsorbed acetonitrile and tetrahydrofuran compounds catalysed the O2 reduction. The electrocatalysis was more effective in strong acid (0.5 M H2SO4) than in strong base (1 M NaOH): the overvoltage of O2 reduction was reduced by 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: 90% in base and insignificant ( < 0.5%) amounts of H2O2 in acid. Different reactivities were also found for the adsorbed Ru(o-NH2)TPP towards H2O2 in acid and base: while H2O2 is catalytically reduced in 0.5 M H2SO4, it is catalytically disproportionated in 1 M NaOH. Although the half-wave potential for O2 reduction in 0.5 M H2SO4 was less positive in the presence of adsorbed ruthenium porphyrin than that of the adsorbed cobalt analogue complex (Co(II)(o-NH2)TPP) (E 1 2=+0.05 and +0.18 V vs. Ag/AgCl, respectively), the use of a dual coating, Ru(o-NH2)TPP+Co(o-NH2)TPP, achieved a relatively high reduction potential in 0.5 M H2SO4 (+0.10 V vs. Ag/AgCl) as well as almost full reduction of dioxygen to water.

Original languageEnglish
Pages (from-to)139-154
Number of pages16
JournalJournal of Electroanalytical Chemistry
Volume246
Issue number1
DOIs
StatePublished - 10 May 1988
Externally publishedYes

ASJC Scopus subject areas

  • Analytical Chemistry
  • Chemical Engineering (all)
  • Electrochemistry

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