Kinetics and mechanism of anodic oxidation of chlorate ion to perchlorate ion on lead dioxide electrodes

The kinetics and mechanism of anodic oxidation of chlorate ion to perchlorate ion on titanium-substrate lead dioxide electrodes have been investigated experimentally and theoretically. It has been demonstrated that the ionic strength of the solution has a marked effect on the rate of perchlorate formation, whereas the pH of the solution does not influence the reaction rate. Experimental data have also been obtained on the dependence of the reaction rate on the concentration of chlorate ion in the solution at constant ionic strength. With these data, diagnostic kinetic criteria have been deduced and compared with corresponding quantities predicted for various possible mechanisms including double layer effects on electrode kinetics. It has thus been shown that the most probable mechanisms for anodic chlorate oxidation on lead dioxide anodes involve the discharge of a water molecule in a one-electron transfer step to give an adsorbed hydroxyl radical as the rate-determining step for the overall reaction.Nomenclature anodic energy transfer coefficient - ₂ potential of outer Helmholtz plane with respect to solution - _M potential of metal with respect to solution - dielectric constant of solution - ₂ permittivity of free space - faradaic efficiency for anodic chlorate oxidation - A adsorbed intermediate in Reaction 2 - B bulk species in Reaction 2 - c_A concentration of A at outer Helmholtz plane - c_B concentration of B at outer Helmholtz plane - c_B⁰ concentration of B in bulk - c_ClO3^–/0 concentration of ClO₃^– in bulk - c_ClO4^–/0 concentration of ClO₄^– in bulk - E electrode potential corrected for ohmic drop - E_a electrode potential as measured against reference electrode - E_s⁰ standard electrode potential of Reaction 2 - E_z potential of zero charge of the anode in test solution - F Faraday constant - f F/(RT) - I_t current at anode - I_OER current used for oxygen evolution reaction at anode - I current used for chlorate oxidation (=I_t–I_OER) at anode - i_t I_t/anode area - i_OER I_OER/anode area - i I/anode area - J total concentration of (uni-univalent) electrolytes in solution - K₂ integral capacitance of compact part of double layer - K_s standard rate constant for Reaction 2, corrected for double layer effects - n_s number of electrons involved in Reaction 2 - p ln(–i)/lnc_ClO3^–/0 - q_M charge density on metal surface - Q₁ quantity of electricity passed in given time interval - Q_OER quantity of electricity required for oxygen evolution reaction in given time interval - R ohmic resistance between anode and Luggin tip - R gas constant - r ln(–i)/lnJ - s ln(–i)/ pH - T absolute temperature - t ln(–i)/E - u (₂RT/2)^1/2 - V volume of gases evolved in given time interval - V_H volume of hydrogen evolved in given time interval - Z_B charge on species B