Abstract
Effects of tetrakis(4’-deсaoxyphenyl)porphyrin on the electrochemical reduction of oxygen on Pt electrode in dimethylformamide were studied by method of cyclic voltamperometry with rotating disk electrode. The oxygen electrochemical reduction is identified as a wide (from -0.75 to -1.5 V respect saturated calomel electrode) irreversible peak which disappears in oxygen free solutions. The current of electrochemical reduction of oxygen increases with the rotating rate of the electrode. The dependence of current on the square root of the working electrode rotation rate is a straight line, not extrapolated to zero of coordinate system. It allows us to conclude that electrochemical reduction of oxygen is limited by convective diffusion and kinetic processes. This condition can be analyzed using the KouteckyLevich equation. As it shown by calculation the addition of tetrakis(4'-decaoxyphenyl)porphyrin decreases both the diffusion and the kinetic limitations of electrode reactions. The decrease of the diffusion limitations can be explained if in the area near electrode the oxygen concentration are shifted due to catalyzed by porphyrin molecules decay of superoxide ions. The fact of kinetic current increasing can propose the following catalytic process of electrochemical reduction of oxygen on Pt electrode in the porphyrin presence: 1. The admixture of water is a source of protons in aprotic medium: Н 2 О → Н + + ОН 2. The negative potential of working electrode leads to a migration of protons and the acidification of the near electrode area of the solution. 3. The acidic media leads to the formation of the protonated form of porphyrin in the near electrode area of the solution: Н + + Н 2 Р ↔ Н 3 Р + 4. Protonated form of porphyrin forms an adduct with the dissolved oxygen: Н 3 Р + + О 2 → (О 2 …Н 3 Р + ) 5. Electrochemical reduction of the complex: (О 2 …Н 3 Р + ) +2e → О 2 Н 3 Р The decomposition of the reduced complex resulting in the reduced form of oxygen and initial porphyrin:
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