NUMERICAL-SIMULATION OF DESORPTION TRANSIENTS AT ELECTRODES ON THE BASIS OF NONLINEAR ADSORPTION-ISOTHERMS

Numerical simulation of desorption transients on the basis of the non- linear Frumkin isotherm is presented. The differential equations which describe the boundary value problem are solved by using the orthogona l collocation technique with expansion of the simulation space with re spect to time. The analysis is focused on the desorption process under gone by a modified electrode when it is transferred to a solution cont aining only electrolyte and a potential step is applied. Three differe nt cases for the kinetics of the desorption/adsorption step are consid ered: irreversible, quasi-reversible and reversible. In the irreversib le case it is shown that the shape of the chronoamperogram is governed by the difference between the interaction parameters involving adsorb ate with adsorbate, and activated complex with adsorbate. There is a c ritical value of this difference above which a maximum in the i-t curv e occurs. In the presence of the maximum a transformation of the chron oamperograms is proposed which allows one to define a unique curve reg ardless of the value of the kinetic and interaction parameters. In the quasi-reversible case, for certain values of the rate constants, an i nitial decay of the current before the appearance of the maximum is pr edicted. Under certain conditions for high coverage (theta approximate to 1) two maxima are observed. The effect of the potential on the log arithm of the maximum current and the time at which the maximum occurs is considered. At a more negative potential a limiting linear relatio nship between both quantities and electrode potential is predicted.