Work function, electronegativity, and electrochemical behaviour of metals IV. Simple electron exchange reactions. Fe2+ /Fe3+ redox couple

Abstract

Experimental determinations of the exchange current for the Fe 2+ /Fe 3+ redox couple in 1 M H 2 SO 4 , available in the literature for Pt, Pd, Ir, Au, Ni, Ti, have been extended to Ru and Ta. A linear dependence of log i o on Φ, the work function of the metal, first indicated by Bockris and his group for noble metals, is confirmed in this work also for low-Φ metals which usually form anodic oxide films. The slope of this relationship has been found to be opposite to that predicted from the analysis of the Frumkin effect in the equations of electrochemical kinetics carried out with φ 2 data calculated from capacity curves for Hg. A modified picture of ion-electrode electrostatic interactions for this system is proposed in order to explain in terms of double-layer effects the observed phenomena. The hypothesis that metals forming natural oxide films in the air could behave as bare metals in simple electron exchange reactions is discussed. This suggestion relies also on the observation that the activation energy at the equilibrium for the Fe 2+ /Fe 3+ redox reaction is consistently constant for all the metals examined. The numerical value of the activation energy is discussed in the light of the current theories for redox reactions in solution and at electrodes. The fact that the activation energy is independent of the nature of the electrode also in the case of oxide-forming metals seems to corroborate the hypothesis of direct tunnelling of electrons from the metal to the species in solution through the insulating layer. Some other experimental data supporting this idea are discussed. The suggestion by some workers that the nature of the metal can influence the exchange current for the redox reaction through the density of electron states is also examined.