Dan Ozer

Catalysis of Oxygen Cathodic Reduction by Adsorbed Iron(111)‐Tetra (N,N,N‐Trimethylanilinium) Porphyrin on Glassy Carbon Electrodes

The adsorption of iron(111)‐tetra (N,N,N‐trimethylanilinium) porphyrin on glassy carbon was studied in aqueous solutions using cyclic voltammetry, differential pulse voltammetry, chronoamperometry, and the RRDE technique. The porphyrin was found to catalyze oxygen electroreduction; the overpotential is reduced by about 400 mV and the hydrogen peroxide yield is 5% compared to 25% in the absence of catalyst.

Dioxygen reduction and hydrogen peroxide dismutation using electropolymerized bilayers of cobalt + manganese tetrakis(o-aminophenyl)porphyrins

Bilayers of electropolymerized cobalt + manganese tetrakis(o-aminophenyl)porphyrins (poly[Co(o-NH2)TPP] + poly[Mn(o-NH2)TPP]) were used for the electrocatalytic reduction of dioxygen. The half-wave potential for O2 reduction at these bilayer films follows that of O2 reduction at single poly[Co(o-NH2)TPP] films in the entire pH range, indicating that this process is governed by the potential of the Co(III)/Co(II) couple in both cases (+0.2 and −0.2 V vs. Ag/AgCl at pHs 2 and 14, respectively). The Co/Mn bilayer porphyrin films are more efficient catalysts than the cobalt porphyrin films in basic solutions, as deduced from the lower peroxide yields (30 and 75% at pH 12 for the Co/Mn and Co porphyrin films, respectively). The addition of 3x10−3 M imidazole decreases the H2O2 yield obtained at the Co/Mn porphyrin bilayers (43 and 30% at pH 13, in the absence and presence of imidazole, respectively). This is attributed to a process in which H2O2 is dismutated by the imidazoleligated manganese porphyrin. The dismutation process is suggested to involve a μ-oxo-manganese(IV or V) porphyrin which is reduced by excess hydrogen peroxide to yield the original Mn(III) porphyrin. Increased stability of the Co/Mn porphyrin bilayer films was observed compared to that of the cobalt porphyrin films: the recorded current for O2 reduction at glassy carbon/film electrodes held for 20 h at −0.3 V vs. Ag/AgCl at pH 10 decreased by 62% and only by 25% at Co and Co/Mn porphyrin films, respectively.

Spectroscopic and electrochemical response to nitrogen monoxide of a cationic iron porphyrin immobilized in nafion-coated electrodes or membranes

Nafion membranes containing immobilized FeIII tetrakis(N-methyl-4-pyridyl)porphyrin show spectroscopic changes in the UV–VIS range after exposure to gaseous NO, indicating the formation of a nitrosyl complex; electroreduction of NO is mediated by this metalloporphyrin when immobilized in Nafion-coated glassy-carbon electrodes and the use of these film electrodes for the amperometric determination of NO in aqueous solutions is demonstrated.

Mediated electron transfer for the electrooxidation of glucose oxidase by manganese tetrakis(o-aminophenyl) porphyrin

However, manganese tetrakis(0-aminophenyl)porphyrin (Mn(0-NH 2 )TPP) which is the subject of the present study, was found to mediate direct electro-oxidation of glucose oxidase when both mediator and enzyme are immobilized in colloidal graphite and applied at glassy carbon electrodes

Electrocatalytic reduction of dioxygen by cobalt tetra(4-N N′N″-trimethylanilinium)porphyrin

Cobalt tetra(4-N N′N″-trimethylanilinium)porphyrin (CoTMAP) has been studied in aqueous solutions using cyclic voltammetry, differential pulse voltammetry as well as by RRDE and spectroelectrochemical methods. The CoIII/CoIITMAP couple dissolved in 0.05 mol dm–3 H2SO4 has a redox potential of +0.14 V. The λmax of the Soret band are 427 and 412 nm for CoIII and CoIITMAP, respectively. The rate constant for the reaction between CoIITMAP and dioxygen in 0.05 mol dm–3 H2SO4 was estimated to be 3 × 107 dm3 mol–1 s–1. The CoIIITMP has been found to absorb at monolayer levels on glassy carbon surfaces and to have a catalytic effect on dioxygen electroreduction in aqueous solutions. This catalytic effect is strongly dependent on electrolyte pH. The overvoltage of O2 reduction is reduced by 480 mV and insignificant amounts of H2O2 are produced at pH 1, while an overvoltage decrease of 300 mV and an H2O2 yield of 70% are observed at pH 8.

Electrochemical and spectroscopic properties of manganese tetra(4-NN′N″-trimethylanilinium)porphyrin

The electrochemistry of the manganese(III)/manganese(II) tetra(4-NN′N″-trimethylanilinium)-porphyrin couple (abbreviated as MnIII/MnIITMAP) has been investigated in aqueous solutions in the entire pH range by means of cyclic voltammetry and thin-layer coulometry. The spectroscopic characteristics of the oxidized and reduced porphyrin in the 350–700 nm range have been studied using a gold optical transparent electrode. A displacement reaction of Mn2+ from MnIITMAP by hydrogen ions was identified at pH < 3.6. The effect of adsorption of MnTMAP on the electrode process is described. The absorbed reactant is reduced at a potential close to that of the dissolved porphyrin; E1/2 is –0.375 V, independent of the pH of the solution. The catalytic aspects of MnIIITMAP for the reduction of molecular oxygen to hydrogen peroxide are also presented in terms of cyclic voltammetry. The overpotential is reduced by ca. 300 mV. The role of an oxygen adduct formed between molecular oxygen and the metal atom of the macrocycle is postulated to explain the difference between the redox potential for the MnIII/MnIITMAP couple and the potential at which the catalytic reduction of oxygen occurs.

Electrochemistry of the manganese(III)–manganese(IV) 5,10,15,20-tetrakis(p-trimethylammoniophenyl)porphyrinate couple

The redox behaviour of the [MnIII(tmap)]–[MnIV(tmap)] couple [tmap = 5,10,15,20-tetrakis(p-trimethylammoniophenyl)porphyrinate] has been studied in aqueous solutions using cyclic voltammetry, optical transparent electrode thin-layer spectroelectrochemistry, a rotating ring-disc electrode, and differential pulse voltammetry. The couple shows reversible behaviour at pH > 8.5 with a half-wave potential of +0.04 V at pH 13.4. The potential is shifted in a positive direction by 120 mV per decrease of pH unit. The [MnIV(tmap)] complex is quite stable at high pH values (half-life ca. 20 min at pH 13.4) and converts back to the MnIII state with a rate which gradually increases when the solution is acidified. The decomposition of water is described in terms of an electrochemical—chemical catalytic cycle which involves the [MnIV(tmap)] complex.

Dioxygen fixation by a cobalt(II)–ammoniacal complex and its electroreduction in a nafion coated solid-state three-electrode cell

The reduction potential for the CoII–NH3–O2 system at a glassy-carbon electrode in a solid-state Nafion-coated cell is about 800 mV more positive than obtained for a glassy-carbon–Nafion film electrode when immersed in an aqueous solution containing CoII, ammonia, and dioxygen (+0.3 and –0.5 V vs. Ag/AgCl, respectively); this is attributed to two different electrode reactions: reduction of [(NH3)5Co–O2–Co(NH3)5]4+ in the absence of a contacting liquid electrolyte solution and free O2 in aqueous CoII–ammoniacal solution.

Redox properties of copper tetra(4-N,N′,N″-trimethylanilinium)porphyrin. Electrochemical and spectral studies

The redox behaviour of the copper tetra(4-N,N′,N″-trimethylanilinium)porphyrin (abbreviated as CuTMAP) has been studied in acetonitrile and aqueous solutions using differential pulse polarography, cyclic voltammetry, a rotating ring–disc electrode and spectroelectrochemical methods. The reduction of CuII is characterized by a two-step one-electron reduction. The CuII/CuITMAP and CuII/CuIIITMAP couples show reversible redox behaviour in acetonitrile and have redox potentials of –0.18 and +0.45 V, respectively. In aqueous solutions and at low concentrations, the CuIITMAP complex is characterized by a Soret band at 412 nm and is reduced at (–0.56–0.057 pH) V, while a band in the 393–398 nm range and a reduction peak at (–0.79–0.057 pH) V predominate at high CuTMAP concentrations. E½ for the oxidation of CuIITMAP is +0.87 V in aqueous solutions and is independent of pH. The decomposition of water is described in terms of a catalytic cycle which involves the CuIIITMAP complex.