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

The electroadsorption of CO, methanol and ethanol on palladium in 0.5 M HClO4 and in 1 M NaOH was studied by cyclic voltammetry and potential-modulated reflectance spectroscopy (PMRS). The voltammetric study allowed us to conclude that the oxide formed on palladium in base is a stronger inhibitor of dissolved CO electro-oxidation than the oxide formed in acid medium. A physisorbed species, apparently reported here for the first time, is present on palladium in base, provided the admission potential of CO gas is less than or equal to 0.37 V. The main residues of the dissociative chemisorption of methanol and ethanol on palladium in base are different: while methanol mainly yields chemisorbed CO, ethanol gives rise mainly to chemisorbed species different from CO, although a small amount of chemisorbed CO is also produced. The PMR spectra of CO chemisorbed on palladium in acid showed a maximum at 285-300 nm, attributed to CO chemisorbed in a bridged position, and a bipolar band at about 600 nm, whose transition energy increased linearly with increasing electrode potential, and which was assigned to an electronic transition from the palladium metal to an empty level in the chemisorbed CO molecule. In alkaline media the PMR spectra of palladium in the presence of CO, methanol or ethanol showed a maximum at 320-340 nm, which was tentatively assigned to CO in a bridged, or perhaps multiple, position. Only CO caused the appearance of a minimum at about 600 nm, whose energy increased linearly with increasing electrode potential, and which was correspondingly assigned to a transition from a filled palladium level to an unoccupied state in chemisorbed CO.