Chemisorption and anodic oxidation of aromatic molecules on Pd electrode surfaces: studies by UHV-EC-STM

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.