Reaction order and adsorption in the kinetics of organic electrode reactions. Applications to reduction of acetophenone

Abstract

In the study of organic electrode reactions, it is desirable to complement kinetic and polarographic information by direct adsorption studies in relation to data on reaction orders and Tafel slopes. In the present paper, the kinetics of reduction of acetophenone are considered in relation to (a) adsorption and orientation of reactant (and product) molecules; and (b) surface pressure behaviour as a function of potential and coverage. Adsorption studies have been made by a new electrocapillary procedure and the surface excess quantities and surface pressures have been obtained. Kinetic reaction orders R have been derived from potentiostatic current-voltage curves and examined in relation to the isotherms for reactant adsorption. Theoretical cases are considered for evaluation of R as a function of coverage and concentration for various types of isotherms. The observed experimental behaviour can be treated in terms of interaction effects (Frumkin/Temkin types of isotherm) arising from dipole repulsions. Primary H/D isotope effects have been examined in the kinetics in order to evaluate the role of proton transfer in the reduction mechanism. The importance of concurrent studies of reactant and product adsorption with evaluation of the kinetics of organic electrode reactions is stressed.