ELECTROCHEMICAL OXIDATION OF ADSORBED TERMINAL ALKENES AS A FUNCTION OF CHAIN-LENGTH AT PT(111) ELECTRODES - STUDIES BY CYCLIC VOLTAMMETRY, EELS, AND AUGER-SPECTROSCOPY

Studies are reported of the surface packing density, vibrational spectroscopy and electrochemical oxidation of a series of straight-chain terminal alkenes adsorbed from the vapor at a Pt (111) electrode surface. Compounds studied were: ethene (ETE), propene (PPE), 1-butene (BTE), 1-pentene (PTE), 1-hexene (HXE), 1-octene (OCE) and 1-decene (DCE). Vibrational spectra of the adsorbed layers were obtained by use of electron energy-loss spectroscopy (EELS). Molecular packing densities (nmol/cm2) in the adsorbed layer were measured by use of Auger spectroscopy. Electrochemical oxidation of each adsorbed layer in aqueous inert electrolyte (KF+ HF) was investigated by means of linear potential scan voltammetry. Attachment to the surface is through the CC bond. Based upon molecular packing densities, the aliphatic chains are pendant; regardless of its chain length, each chemisorbed alkene molecule occupies an area similar to that of chemisorbed PPE. The packing densities of ETE and PPE indicate an average orientation in which the CC moiety is parallel to the Pt (111) surface. EELS spectra indicate that the CC double bond is preserved in the adsorbed state of each 1-alkene studied. Measurement of the average number of electrons, nox, required to desorb an adsorbed 1-alkene molecule electro-oxidatively reveals that the catalytic electrochemical oxidation process involves primarily the CC double bond and one adjacent saturated carbon atom. © 1990.