Mechanism and kinetics of the electrosorption of sulphate anions on the (111) face of silver

The processes of adsorption of sulphate anions onto the (111) face of silver have been investigated by analysis of the cyclic voltammetry and potentiostatic pulse results. It was shown, by fitting the experimentally recorded q - E dependences, that the adsorption of anions follows Frumkin adsorption isotherm. The adsorbed structure was found to be less dense than expected for the dense packed anion monolayer and the process was characterized by high value of the interaction parameter and the Gibbs energy of adsorption (f = 16.5 and DG°ads,q->0 = - 64 kJ mol-1). From the slope of the Ep,a - log (c0) dependence it was concluded that complete charge transfer between the adsorbed anions and the silver substrate occurs. This is supported by the Ep,a - log v linear dependence in the region of high sweep rates (v > 10 V s-1) with rational value of line slope (~ 60 mV dec-1) which is possible only if the complete charge transfer reaction occurs. The electrosorption process of sulphate anions was found to be complex electrochemical reaction, taking place through three steps. It was proposed that the rate-determining step is the second step, i.e., the electrochemical formation of (SO4)ads species.