Studies of adsorbed unsaturated alcohols at well-defined Pt (111) electrode surfaces by cyclic voltammetry assisted by vibrational spectroscopy (EELS)

Surface vibrational states (electron energy-loss spectroscopy, EELS), molecular packing density (Auger spectroscopy) and adsorbed layer electrochemical reactivity (cyclic voltammetry) were investigated for the following unsaturated alcohols adsorbed from neat liquids and aqueous solutions at Pt (111): benzyl alcohol (BZA); 4-pyridylcarbinol (4PDC); 3-pyridylcarbinol (3PDC); allyl alcohol (AAL); propargyl alcohol (PGA); cis-2-butene-1,4-diol (CBED); and 2-butyne-1,4-diol (BYD). Chemisorption of the unsaturated centers of these alcohols is the principal mode of surface attachment (phenyl, pyridyl, CC or CC). Electrochemical oxidation of adsorbed BZA, AAL. PGA, CBED and BYD in aqueous fluoride electrolyte proceeds efficiently to CO₂, while adsorbed 4PDC and 3PDC are relatively inert. The adsorbed layer of each of these alcohols is stable in vacuum, such that surface spectroscopic methods in vacuum are directly applicable to the liquid/solid interfacial chemistry and electrochemistry of these compounds. 4PDC and 3PDC are adsorbed in a tilted vertical orientation with an angle near 70° between the pyridine ring and the Pt surface. The C-C bond-order for PGA and BYD is reduced dramatically to near that of a double bond as a result of the adsorption process. The aromatic or alkene moieties of BZA, 4PDC, 3PDC, AAL and CBED are retained and only small shifts in their vibrational frequencies are found. Ordered layers of these adsorbed alcohols were not found by LEED, although uniform molecular orientation is suggested by the packing density data and voltammetric behavior.