Study by cyclic voltammetry and potential-modulated reflectance spectroscopy of the electroadsorption of methanol and ethanol on a rhodium electrode in acid and alkaline media

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The electroadsorption of methanol and ethanol on rhodium was studied by cyclic voltammetry, using a flow cell, and by potential-modulated reflectance spectroscopy (PMRS), a technique with which chemisorbed CO can be easily identified in situ. It was found that in alkaline medium and at an admission potential of 0.07 V, both methanol and ethanol physisorb on rhodium, a fact which apparently had not been reported before. The peak potential of electro-oxidation of these physisorbed alcohols coincides with that of hydrogen desorption, approximately 0.2 V vs. RHE. Dissociative chemisorption of methanol or ethanol on rhodium yields a chemisorbed species, probably CO, both in acid and alkaline media. The hydrogen coverage does not show the usual dependence on the chemisorbed residue upon increasing the admission potential of the alcohol: with increasing amounts of chemisorbed CO the hydrogen coverage does not decrease, or decreases less than expected. We attribute this behaviour to the production of molecular hydrogen at low admission potentials. The chemisorbed residue of the adsorption of methanol or ethanol on rhodium produces a maximum in the PMR spectrum, whose wavelength lies at 290–310 nm in acid, and at 283–328 nm in base, depending on the measurement potential and on previous potential excursions. These wavelength shifts could perhaps be due to differences in the CO coverage and/or the background PMR spectrum.