Chemisorption of Aromatic Compounds on Well-Defined Palladium Surfaces: Studies by Electron Spectroscopy and Electrochemistry

The chemisorption of aromatic compounds, derivatized with different functional groups, on well-defined Pd(111) surfaces was studied by a combination of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), high resolution electron energy loss spectroscopy (HREELS), and electrochemistry (EC). The results of this work led to the following trends and conclusions: (a) At low concentrations, 2,5-dihydroxythiophenol (DHT) chemisorbs on a Pd surface through both diphenolic ring and thiol group. At high concentrations, it chemisorbs only through the thiol group. (b) There is extensive intermolecular attraction between the co-adsorbed thiolated quinone and thiolated hydroquinone molecules. The interaction occurs through the Pd substrate and not through space. (c) The chemisorption properties of Nheteroaromatic compounds are pH-dependent. When the nitrogen heteroatom is protonated, it becomes very weakly surface-active. When the nitrogen heteroatom is deprotonated, surface activity stronger than the diphenolic ring is exhibited. (d) On a palladium surface, the binding strengths of ligands increase in the order: phenyl ring < quinonoid ring, < N-heteroatom < I < -SH.