Electrochemical reactions with adsorption of the reactants and electrosorption. Simple analytical solutions for a Henry isotherm

Abstract

A theoretical treatment of an electrochemical reaction O + ne − R with adsorption of the reactants is presented when the adsorption, at equilibrium, obeys a Henry isotherm. A simple analytical solution, valid for any degree of adsorption of O and or R, is established for a polarographic wave. In LSV, for slow sweep rates and weak adsorption, both the cathodic and anodic peaks have a ‘diffusional’ character; for high sweep rates and strong adsorption, they are both of a ‘surface’ nature. In the intermediate case, one peak has a ‘diffusional’ character (deposition process), while the other is of a ‘surface’ nature (redissolution process). Expressions of the peak potentials are given in all cases, for reversible or irreversible reactions. Study of their variations with log v allows some or all of the diverse constants (surface and heterogeneous rate constants, adsorption coefficients, transfer coefficients, etc.) to be determined. The case of normal pulse polarography is discussed briefly. Electrosorption is formally equivalent, as far as the diffusional problem is concerned, to the case where only one entity is adsorbed. Similarities and differences between the two cases are analyzed.