Kinetic Theory of Adsorption of Intermediates in Electrochemical Catalysis

Abstract

The behavior of adsorbed intermediates in consecutive electrochemical reactions is discussed in terms of the steady-state rate equations, and a comparison with results obtained using the quasiequilibrium assumption is made. The adsorption pseudocapacitance vs potential plot is shown to be symmetrical only when the quasiequilibrium assumption is made, namely when the ratio of specific rate constants for the desorption step (which is rate determining) and initial ion-discharge step, is exceedingly small. The C—V relationship is calculated for three types of mechanism: (i) discharge followed by atom—atom recombination, (ii) discharge followed by first-order decomposition, (iii) discharge followed by ion—atom desorption; cases for activated and nonactivated adsorption conditions are distinguished. Deviations from the symmetrical quasiequilibrium behavior are shown to depend on the kinetic pathway and, for a particular mechanism, are larger the larger is the value of the Temkin parameter r (defined by the rate of change of apparent standard free energy of adsorption of the intermediate with coverage). For a given value of r, these deviations are shown to be largest for the pathway when discharge is followed by ion—atom desorption, and smallest for the case when discharge is followed by a first-order decomposition step. Further, for any one mechanism, conditions of nonactivated adsorption lead to larger deviations than for the case when activated adsorption occurs.