Electrochemical impedance spectroscopy study of a surface confined redox reaction: The reduction of azobenzene on mercury in the absence of diffusion

The kinetics of azobenzene reduction on mercury electrodes in the absence of diffussional mass transport is studied by electrochemical impedance spectroscopy (EIS) in acetic acid/acetate buffered solutions at different pH values. Cyclic voltammetry experiments confirm the absence of diffusion effects and provide the values of the surface equilibrium potential. The analysis of the impedance frequency spectrums at every potential within the faradaic region conforms well the model and provides the global rate constant of the process, kf. The potential dependence of kf suggests the existence of an EE mechanism, with two electron transfers controlling the overall rate. The kinetic parameters of every step are obtained and their pH dependences clarify the role played by the protonation steps.