In-situ studies of adsorption of organic compounds on platinum electrodes

Abstract

Studies of the electrosorption of organic compounds on polycrystalline platinum electrodes were carried out in 0.01 M HCl solution. In-situ techniques, i.e. radiotracer measurements, ellipsometry and Fourier transform infrared spectroscopy, were used in the measurements. Good agreement in results was obtained between these techniques. The electrosorption process was found to be slow, but a bell-shaped coverage θ versus potential V curve was generally obtained in the double-layer region with a submonolayer adsorption. The difference between the potential of zero charge and the maximum of adsorption is about ± 0.1 V. A water competition adsorption model, based on the statistical mechanics approach, was proposed. Both lateral interactions between adsorbed species and the effect of heterogeneity of the electrode surface were taken into account. The standard state for the adsorption was found by equating f(θ) to unity. A Temkin-type isotherm was obtained in all cases, indicating a strong effect of heterogeneity of the surface. Also, Elovich-type kinetics were found and the adsorption activation energy was calculated. The bonding strength between the adsorbate and the electrode surface was found to be around 20–40 kcal/mol which is comparable with that measured in the gas phase. The adsorbed species generally adopts a flat orientation. Electropolymerization for some phenyl species may occur at a potential more positive than 0.9 V vs. the standard hydrogen electrode.