Competitive adsorption and state of charge of halide ions in monolayer oxide film growth processes at Pt anodes

Abstract

The competitive electrodeposition of the monolayer of surface oxide on Pt electrodes is studied in the presence of co-adsorbed halide ions (X^–= Cl^–, Br^–, I^–), down to very low concentrations. These experiments give information essential to the understanding of the state of the surface of Pt anodes at which halogen evolution is occurring from aqueous halide ion solutions and hence provide a basis for interpretation of the kinetics of, and electrocatalysis in, anodic evolution of halogens, especially Cl₂. The isotherms for competitive electrochemical adsorption of OH and O species in the surface oxide film, relative to that of X^– ions, are evaluated quantitatively.

Cl^– selectively blocks the initially deposited OH monolayer at Pt while Br^– and I^– block the formation of the surface oxide non-selectively over a wide potential range, indicating a difference of the state of charge (‘electrosorption valency’) of adsorbed Br^– and I^–, in relation to that of adsorbed Cl^–. This is confirmed by the forms of the competitive adsorption isotherms: Cl^– gives linear-logarithmic behaviour in [Cl^–], indicating strong lateral interactions, while Br^– and I^– effects conform more to Langmuir behaviour.

A method for quantitatively treating the competitive adsorption, giving information on lateral interaction effects in the surface oxide film with co-adsorbed X^– ions, is given in terms of a ‘differentiated’ adsorption isotherm function.