Fractional reaction order kinetics in electrochemical systems involving single-reactant, bimolecular desorption reactions

Electrochemical reactions are usually described by balance equations derived upon the assumption that whatever the interface particularities, reactants are perfectly mixed and every particle is able to interact with any other one (mean-field approximation, MFA). This paper investigates the limits of validity of this kind of approach in the case of electrochemical systems involving a single reactant, bimolecular desorption under conditions of hindered surface mobility of adsorbates. This is achieved by means of cellular automaton (CA) simulations of a multi-step electrochemical reaction on a bi-dimensional homogeneous lattice. Results show that local correlations between adsorbates lead to departures from the MFA predictions concerning the bimolecular desorption step rate. The desorption reaction seems to be better described by fractional order kinetics, which take into account non-homogeneous distribution of adsorbates over the surface. The dependence of this fractional reaction order on the kinetic parameters of the overall reaction, as well as the circumstances leading to a recovery of the standard second-order rate law, are fully discussed.