Adsorption of Aromatic Compounds at Platinum Electrodes. A Comparative Study Illustrating the Deficiencies of Adsorption Measurements Based on Hydrogen Codeposition or Anodic Oxidation

摘要

The adsorption of aromatic compounds at smooth polycrystalline Pt electrodes in aqueous 1 M HCIO4 or H2SO4 has been investigated using thin-layer coulometry, hydrogen codeposition and anodic oxidation for comparative purposes. Three compounds were studied; hydroquinone, 1,4-dihydroxynaphthalene, and 2.2'5.5'-tetrahydroxybiphenyl; these compounds were previously shown by thin-layer coulometry to adsorb in specific orientational states which depend upon their solution concentrations. The hydrogen codeposition method does not give absolute packing densities, yields wrong fractional coverage data, and provides no indication of orientational transitions. These discrepancies point to severe deficiencies of the latter two methods for measurement of adsorption, and arise from the fact that conversion of hydrogen codeposition or anodic oxidation data to absolute packing densities or aedsorbed molecule cross-section requires assumptions which have been proved to be incorrect, at least for aromatic molecules.