Reaction intermediates as a controlling factor in the kinetics and mechanism of oxygen reduction at platinum electrodes

A rotating platinum disk electrode was used to study oxygen reduction over the entire pH range and to compare the kinetics in acid and alkaline solutions. In both solutions two Tafel regions are confirmed. At low cd's ∂V/∂ln i = −RT/F and at high cd's it is −2RT/F. In the low cd region, the reaction order with respect to H3O+ is 32 in acid, and 12 in alkaline solutions. For the high cd region the reaction orders are 1 and 0 in acid and alkaline solutions, respectively. Transition of the kinetics from the low to the high cd region, as well as the fractional reaction orders at low cd's, are interpreted in terms of the first charge transfer step as rate determining in both cd regions but under Temkin adsorption conditions in the low, and Lagmuirian conditions in the high cd region. Coverages, θ, with oxygen intermediates are also determined as a function of potential and pH. At all pH's, substantial coverages are observed only at potentials in the low cd region. The usual Tafel slope of −120 mV for the first charge transfer step as rate determining is not observed at low cd's since it is modified by the dependence of activation energy on θ, and hence on V. At high cd, θ is low and ineffective and the usual slope for the first charge transfer step is observed. At all pH's the transition from low to high cd's occurs at potentials, VT, that decrease 60 mV as pH increases one unit. Dependence of VT and iT on pH is discussed. Fractional reaction orders at low cd's are also related to θ. Real reaction orders with respect to H3O+ are modified by the dependence of activation energy on θ, and hence on pH. The change of the kinetics from the acid to the alkaline region is discussed. The range of cd's accessible to measurements is limited at both high and low cd's. At high cd's it is limited by the supply of oxygen and/or H3O+ from the solutions. At low cd's, V—log i relationship extends only to a rest potential which decreases 60 mV as pH increases one unit. The limits of measurements and the nature of the rest potential are discussed.

[1]  E. Yeager,et al.  Oxygen Reduction on Platinum in 85% Orthophosphoric Acid , 1979 .

[2]  A. Damjanović,et al.  Electrode kinetics of oxygen reduction on oxide-free platinum electrodes☆ , 1967 .

[3]  P. Ross.  Structure Sensitivity in Electrocatalytic Properties of Pt: II . Oxygen Reduction on Low Index Single Crystals and the Role of Steps , 1979 .

[4]  A. Damjanović,et al.  Kinetics and mechanism of O2 reduction at Pt IN alkaline solutions , 1980 .

[5]  A. Damjanović,et al.  Adsorption and kinetics at platinum electrodes in the presence of oxygen at zero net current , 1967 .

[6]  A. Damjanović,et al.  Kinetics of oxygen evolution and dissolution on platinum electrodes , 1966 .

[7]  B. Conway,et al.  Kinetic theory of pseudo-capacitance and electrode reactions at appreciable surface coverage , 1962 .

[8]  A. Damjanović,et al.  OXYGEN ADSORPTION RELATED TO THE UNPAIRED d-ELECTRONS IN TRANSITION METALS , 1963 .

[9]  A. Damjanović,et al.  Oxide Growth at Platinum Anodes with Emphasis on the pH Dependence of Growth , 1979 .

[10]  U. Ebersbach,et al.  On the kinetics of the anodic passivation of iron, cobalt and nickel , 1967 .

[11]  James P. Hoare,et al.  Rest Potentials in the Platinum‐Oxygen‐Acid System , 1962 .

[12]  D. S. Gnanamuthu,et al.  A Generalized Expression for the Tafel Slope and the Kinetics of Oxygen Reduction on Noble Metals and Alloys , 1967 .

[13]  A. Damjanović,et al.  New evidence supports the proposed mechanism for O2 reduction at oxide free platinum electrodes , 1979 .

[14]  A. Damjanović,et al.  Growth of Oxide Films at Pt Anodes at Constant Current, Density in H 2 SO 4 , 1975 .

[15]  A. Damjanović,et al.  Dependence of the kinetics of O2 dissolution at Pt on the conditions for adsorption of reaction intermediates , 1970 .

[16]  J. O'm. Bockris,et al.  The Mechanism of Oxygen Reduction at Platinum in Alkaline Solutions with Special Reference to H 2 O 2 , 1967 .

[17]  L. Burke,et al.  Oxygen electrode reaction. Part 1.—Nature of the inhibition process , 1971 .

[18]  E. Gileadi,et al.  The Potential of Zero Charge of Platinum and Its pH Dependence , 1966 .

[19]  S. Schuldiner,et al.  The Pt/O2 Electrode in Sulfuric Acid Solution , 1963 .

[20]  B. Piersma,et al.  Potentiostatic current-potential measurements on a platinum electrode in a high-purity closed system. Interim report , 1967 .

[21]  A. Damjanović,et al.  The Role of Hydrogen Peroxide in Oxygen Reduction at Platinum in H 2 SO 4 Solution , 1967 .

[22]  M. Boudart.  Heterogeneity of Metal Surfaces , 1952 .