| The chemisorption, from aqueous solutions, of hydroquinone (H2Q), benzoquinone (Q), hydroquinone sulfonate (HQS) and 2,5-dihydroxythiophenol (DHT) onto well-defined Pd(111) electrode surfaces has been studied by tandem electrochemistry, low energy electron diffraction, Auger electron spectroscopy, high-resolution electron energy loss spectroscopy and in situ scanning tunneling microscopy. The results indicate that H2Q is oxidatively chemisorbed to form Q that is oriented parallel to the surface but with a slight tilt. The same surface species is obtained when adsorption is from aqueous Q. Adsorption from aqueous HQS led to the formation of the quinone form that is oriented parallel to the surface but with a slight tilt enforced by the bulky sulfonate group. DHT is oxidatively chemisorbed through the sulfur atom in two discrete non-random orientations that depended upon the concentration of the adsorbate in solution. At high concentrations, DHT is attached exclusively through the sulfur atom in a vertical S-eta 1 orientation and displays reversible quinone/diphenol redox. At low concentrations, DHT is bond as a bidentate ligand, via the --SH and the diphenolic group, at this orientation, redox activity is hindered because the diphenolic group is no longer free (pendant). |
