Recent Trends in Experimental and Theoretical Investigations of Chemisorption on Metal-Electrolyte Interface. II. Contact Electric Resistance Method^1,2

V. A. Marichev

Department of Chemistry, University of Western Ontario, London, Ontario, Canada

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Abstract — New trends in experimental and theoretical investigations of chemisorption on electrodes are considered on examples of in situ spectroscopic studies and the density functional theory calculations. The partial charge transfer during ionic and molecular adsorption from aqueous solutions on coinage and platinum metals and the thermodynamic uncertainty regarding the direction of the charge transfer are discussed. The contact electric resistance technique (CER) was shown to offer a new approach to considering these problems. This technique provides information on the coefficient and direction of the charge transfer, the kinetics of the transformation of the electrode surface during the conversion of adsorbed ions to adatoms, and the role of competitive adsorption of water. It was examined how the charge transfer is related to the electrosorption valence of anions and what the role plays the tunneling of electrons in the generation of the CER signal. The CER was demonstrated to be dependent on the potential E, the coverage, and the residence charge of the adsorbate. For the adsorption of halide ions from aqueous solutions on 1B metals, the bell shaped CER vs. E dependences were obtained. The maxima in these dependences correspond to the onset of substantial charge transfer, their position and amplitudes being determined by the nature of the metal and anion and the concentration of the latter. At E< E_max, halide ions are adsorbed without appreciable charge transfer; within the range E_max (E_max 0.1 V), the charge transfer for silver and gold approaches 1, being far less for copper. For a given metal, E_max increases in the series < while for a given anion, E_max increases in the series Au < Ag Cu. Based on the measured CER-E dependences, a quantitative criterion of the substantial charge transfer during ionic and molecular adsorption was proposed. This criterion was demonstrated to be consistent with the published data on charge transfer in various adsorbate-adsorbent systems. In particular, it shows that the hydrophilicity and electronic properties of 1B metals differently influence charge transfer during ionic and molecular adsorption. New electrosorption equations have been proposed describing the partial charge transfer and screening of the adsorbate charge due to partial ionization of the surface atoms. An experimental approach to solve the “anion problem” in the density functional theory has been developed basing on the CER-measured values of E_max in corresponding electrochemical systems.