Comparison of Vacuum-Annealed and Electrochemically Cycled Electrodes in Adsorption and Electrocatalysis: Aromatic Compounds at Platinum(111) and Polycrystalline Platnium

摘要

These studies compare adsorbed layer vibrational spectra and packing densities of various aromatic compounds at annealed Platinum (111) and polycrystalline Pt surfaces (Pt(poly)) with the behavior of surfaces pretreated by electrochemical cycling (the oxidation-reduction procedure commonly employed to pretreat electrodes prior to use). Surface structural changes produced by cycling exert a profound effect upon each of the properties studied. Absorbates studied represent various types of surface attachment: hydroquinone (HQ), which displays pi-bonding to Pt surfaces (horizontal orientation) when adsorbed from sufficiently dilute aqueous solutions; 2,2',5,5'-tetrahydroxybiphenyl (THBP), which adopts a mixture of horizontal and vertical orientations; 3-thiophenecarboxylic acid (3TCA), (3-pyridyl)-hydroquinone (3PHQ), and nicotinic acid (NA), which exhibit primarily sigma-bonding (tilted vertical orientation); and benzyl mercaptan (BM) and 2,5-dihydroxy-4-methylbenzyl mercaptan (DMBM), for which attachment occurs through a sulfur atom to form a benzyl pendant. Packing densities (moles adsorbed per unit area) were measured for each compound at each surface by Auger spectroscopy. Surface vibrational spectra were obtained by electron energy loss spectroscopy (EELS) and were assigned by comparison with the IR spectra of the pure compounds. Substrate surfaces were characterized by LEED> Cycling the Pt(111) surface affects the adsorbate packing density by up to 50%, while smaller effects are observed for Pt(poly). Cycling causes the LEED pattern of the Pt(111) substrate to become diffuse. Reprints. (AW)