The chemisorption and anodic oxidation of hydroquinone (H2Q) and
benzoquinone (BQ) at palladium electrode surfaces was studied by a combination of
electrochemistry (EC), Auger electron spectroscopy (AES), high-resolution electron-
energy loss spectroscopy (HREELS) and electrochemical-scanning tunneling
microscopy (EC-STM) on a smooth polycrystalline and well-defined (single-crystalline)
Pd(100) electrode surface. The results point to the following more critical conclusions:
(i) Chemisorption of H2Q from dilute (less than or equal to 0.1 mM) aqueous solutions forms surface-
coordinated BQ oriented parallel albeit with a slight tilt. (ii) At high concentrations (greater than or equal to 1mM), chemisorption yields an edge-vertical oriented diphenolic species. (iii) The extent of anodic oxidation of the chemisorbed organic strongly depends upon its initial
orientation; only the flat-adsorbed species are oxidized completely to carbon dioxide.
(iv) The rate of anodic oxidation is likewise dependent upon the initial adsorbate
orientation; the rate for vertically-oriented species is more than twice that of flat-
adsorbed species. (v) The chemisorbed species are not oxidized (to the same extent)
simultaneously; instead, oxidation occurs one molecule at a time. That is, molecules that
survive the anodic oxidation and remain on the surface retain their original identities