Abstract

This thesis reports studies of the surface structure of Au(111) electrodes modified to alter their reactivity. The electrodes have been modified by underpotential deposition of monolayer amounts of foreign metal adatoms which are known to catalyze the oxidation and reduction of small molecules. The surface coverages of these adatoms have been measured, and the causes of the open adlattices observed on the surface are determined.

The Bi underpotential deposition system has been found by chronocoulometry to have coverage of 0.25 monolayers (ML) in the potential region where it catalyzes hydrogen peroxide reduction. At more negative potentials where the catalytic activity ceases, the surface coverage of Bi is 0.67 ML. These coverages agree well with the coverages of the (2 x 2) and (p x $\surd3)$ Bi structures observed by SPM and SXS. In the potential region where the open (2 x 2) Bi adlayer is present on the surface, chronocoulometric measurements indicate that there is 0.17 ML of hydroxide coadsorbed with the Bi. Charge measurements indicate that the Bi adatoms are fully discharged. The presence of the coadsorbed hydroxide is used to explain the formation of the open Bi adlattice.

The coverage of Cd reaches a limiting value of 0.66 ML in the upd region. The charge passed per Cd adatom is 0.51 electrons, significantly less than the 2 electrons required for total discharge. The remaining positive charge on the Cd atoms is offered as an explanation for the banded structures observed in this upd system. The coverage of sulfate is 0.22 ML in the Cd upd region. Quartz crystal microbalance measurements indicate that there is considerably larger mass change at the surface than is expected based on the observed coverages of Cd and sulfate. The extra mass is explained in terms of additional sulfate required in the outer Helmholtz plane to balance the residual positive charge on the deposited Cd atoms.

The thallium upd system was also studied by chronocoulometry and quartz crystal microbalance. At anodic potentials, there is a phase characterized by a variable low coverage of partially discharged adatoms. At more cathodic potentials, the adlayer condenses to a close-packed, more fully discharged structure. Voltammetric data indicate that hydroxide anions are involved in stabilizing the low coverage structure. The system is compared to the Pb upd system and implications of these results for catalysis are discussed.

Finally, active molecular adsorbates are investigated. Cyclic voltammetric investigations of the $\alpha$-dodecatungstosilicate anion, which has potential applications in catalysis and corrosion inhibition, indicate that it binds to the Au(111) electrode throughout the double layer potential region. At positive potentials, it delays the onset of gold oxide formation. At extreme negative electrode potentials, the anion desorbs from the surface, confirming activity observed with SPM. Quartz crystal microbalance studies found a mass change smaller than that expected for a full monolayer of $\alpha$-dodecatungstosilicate anions, indicating that there is only a monolayer present on the surface and that it remains in the solution layer near the electrode surface after desorption. Finally, quartz crystal microbalance studies of uracil desorption from Au(111) in neutral solutions indicate that the desorbed mass changes linearly as a function of uracil concentration. This indicates that under these conditions uracil is actually adsorbed in a multilayer. (Abstract shortened by UMI.)