Oscillatory Kinetics in the Electrochemical Oxidation of Formate and Ethylene

Abstract

A number of electrocatalytic anodic oxidations of simple organic substances exhibit periodic effects in their kinetics. The anodic oxidation of formate and of ethylene has been studied at Pt and Pd under potentiostatic and constant-current conditions and the oscillatory behavior observed has been related to (a) mass transport effects; (b) coverage of the electrode by electrochemically formed surface-oxide species. In the formate case, the oscillations are associated with activation-controlled processes and are not eliminated by high-speed rotation of the electrode; in the case of ethylene, diffusion control is shown to play an important role. Kinetic conditions are examined for several mechanisms of formate oxidation; for certain circumstances, an autocatalytic surface-oxide removal step is consistent with the conditions required for oscillatory kinetics to arise. Such a process is supported by experimental observations on the periodic changes of oxide coverage which arise during the oscillatory behavior. For ethylene oxidation a more intuitive approach is employed in which it is shown that only imaginary solutions for the “steady-state” reaction flux can arise under certain conditions of adsorption and mass transfer in the partly diffusion-controlled reaction.