COADSORPTION OF METAL ATOMS AND ANIONS - CU UPD IN THE PRESENCE OF SO4(2-), CL- AND BR-

Chronocoulometry and the thermodynamics of the so-called perfectly polarized electrode were applied to determine Gibbs excesses for underpotentially deposited copper and coadsorbed SO42-, Cl- and Br- anions at the Au(lll) electrode surface. A complete description of the composition of the mixed copper-anion overlayers is given. It is shown that the coadsorption of a copper adatom and the anion has a cooperative character and that the anion affects the kinetics and energetics of the copper deposition reaction. Copper coadsorbed with sulfate forms a well ordered monolayer described well by a statistical mechanical model proposed by Blum and Huckaby. Copper coadsorbed with chloride and bromide forms a bilayer in which copper atoms are adsorbed on the gold surface and the halides are in the top layer. The adsorbed sulfate ion preserves the ionic character. In contrast, adsorbed Cl- or Br- are discharged so that they are present as neutral adatoms at the surface. The number of electrons flowing to the interface per one adsorbed copper atom is equal to two in the presence of Cl- and Br-. Somewhat lower values of the number of electrons per copper adatom, ranging between 1.8 and 1.4, were measured in the presence of sulfate. The results indicate that the chemisorption bond for Cu atoms coadsorbed with SO42- may have a polar character.