CYCLIC VOLTAMMETRIC AND POTENTIAL-MODULATED REFLECTANCE SPECTROSCOPIC STUDY OF THE ELECTROADSORPTION OF METHANOL AND ETHANOL ON A PLATINUM-ELECTRODE IN ACID AND ALKALINE MEDIA

The electroadsorption of methanol and ethanol on smooth Pt, both in 0.5 M HClO4 and 1 M NaOH, has been studied by cyclic voltammetry (CV) and potential-modulated reflectance spectroscopy (PMRS). It has been found by CV, apparently for the first time, that the two alcohols physisorb on Pt, both in acid and alkaline media, provided the admission potential (the potential at which the alcohol is first admitted into the cell) is lower than 0.1-0.2 V, and that there is alcohol in solution, which means that the physisorption is reversible. The amount of chemisorbed species was determined by CV in the absence of alcohol in solution, using a flow cell. In the four cases (methanol and ethanol in both acid and base) the amount of CO produced by dissociative chemisorption changes little over a range of admission potentials, decreasing rapidly at lower and higher potentials. It has also been found by CV that, in agreement with the general opinion, with methanol in alkaline medium the only chemisorbed species is CO. On the contrary, with methanol in acid medium, and with ethanol in both acid and alkaline media, other species besides CO become chemisorbed on Pt. The PMR spectra of both methanol and ethanol on Pt in acid medium show a maximum at 270 nm, which we attribute to linearly adsorbed (on-top) CO. In basic medium, methanol shows a PMRS maximum at 280 nm, which has been assigned to bridge-bonded CO, while ethanol shows no PMRS maximum, although the amount of chemisorbed CO, as estimated from the CV obtained after refluxing with base electrolyte, is about the same as with methanol.