The Surface Organometallic Chemistry of Pd Electrodes: Studies with Aromatic Compounds

To help establish the surface coordination and organometallic chemistry of palladium electrodes, the chemisorption properties (coverage, orientation, and reactivity) of selected aromatic compounds in aqueous solutions were investigated. Thin-layer electrochemistry was employed with atomically smooth polycrystalline electrodes to extract information on surface packing densities and adsorbate cross sections. Comparison of the latter with calculated values allowed the determination of the more plausible adsorbed-molecule orientations; in a few cases, verification was obtained via surface vibrational (high-resolution electron-energy loss) spectroscopy. Eleven aromatic compounds were studied: [hydroquinone (1), benzoquinone (2), methylhydroquinone (3), 2,3-dimethylhydroquione (4), 2,3,5-trimethylhydroquinone (5), 1,4-dihydroxynaphthalene (6), phenylhydroquinone (7), 2,3-dihydroxypyridine (8), 2,5-dihydroxythiophenol (9), 2-(8-mercaptooctyl)-1,4-benzenediol (10), and hydroquinone sulfonic acid (11)]. For the homoaromatic compounds, chemisorption was oxidative to form surface-coordinated quinones; flat orientations were observed at low concentrations, vertical orientations at much higher concentrations. The presence of substituents more surface-active than the aromatic moiety induced other orientations. Preferential chemisorption was found to increase in the order: phenyl ring < quinone ring < -SH.