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The role of charge transfer in the adsorption of aromatic molecules at metal electrodes

Electrochimica Acta
Publication Date
DOI: 10.1016/0013-4686(86)80014-7
  • Physics


Abstract Thermodynamic data for the electrosorption of aromatic molecules at metal electrodes (the electrosorption valency γ, surface concentration Γ, the double layer capacity C D , and the free adsorption enthalpy Δ G ad ) are analysed for phenol at mercury as an example. At small Γ the phenol molecule is adsorbed in a flat position, but at Γ > Γ ″ max this changes to perpendicular. γ is negative and λ ≈ 0.2 for flat orientation, but negligible for the perpendicular one. C D decreases with increasing coverage for both orientations, indicating a dominating thickening of the double layer. From Δ G ad values a small π-bond contribution Δ G π ≈ 50 kJ mol −1 is estimated. The energetic analysis shows that an anodic charge transfer takes place if the electrode potential approaches the oxidation potential ε ox and a cathodic charge transfer in the vicinity of ε red . Charge transfer data for other aromatic and heterocyclic compounds were obtained using the same thermodynamic analysis. In general, the charge transfer is positive, 0 < λ < 0.3. These data can be successfully correlated with the oxidation potential, the ionization potential and the electron density in the ring respectively.

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