Abstract

This thesis contains studies related to the structure and electrocatalytic reduction of oxygen on modified electrode surfaces.

First, cyclic voltammetry, scanning tunneling microscopy (STM), and surface-enhanced Raman spectroscopy (SERS) are used to investigate the interactions between lacunary polyoxometalate, α-SiW₁₁O₃₉^8- and glassy carbon, Au(111), and Ag(111) surfaces. α-SiW ₁₁O₃₉^8- anions adsorb strongly only on Ag(111) and the adsorbed species retains its redox activity on the Ag surface. STM images reveal the formation of monolayers on Ag(111), which have hexagonal adlattice structures with a larger spacing than that observed for the complete Keggin ion. SERS measurements show that α-SiW₁₁0₃₉ ^8- is present on the Ag surface between 0.0 and -0.9 V.

Second, formation of multilayers from silicotungstic anion (STA), α-SiW ₁₂O₄₀^4-, and Ag⁺ on electrode surfaces are investigated. Surface X-ray scattering and STM measurements show that the STA stabilizes the Ag⁺ cation, which electrostatically assembles with the STA anion or the one-electron reduced species to form ordered multilayers. In contrast to other electrostatically assembled multilayers, those formed here exhibit considerable order.

Next, it is shown that poly(vinylpyridine) (PVP) coated glassy carbon surfaces containing Fe(CN)₆^3- exhibit catalytic activity toward electroreduction of H₂O₂. While Fe(CN) ₆^3- is catalytically inactive in solution phase, it exhibits catalytic activity upon incorporation into the PVP film. The catalytic activity could be stabilized by electrodeposition of Au particles. Characterization of the film using microscopy and spectroscopy shows that covalent attachment between the Au particle and the Fe-based catalyst is a likely mechanism for catalyst stabilization.

Finally, the mechanism of the electroreduction of oxygen on Au surfaces in basic media is examined using SERS and density functional theory calculations. The spectroscopy reveals superoxide species as a reduction intermediate, while no peroxide is detected. The spectroscopy also shows the presence of superoxide after the addition of hydrogen peroxide. The calculations show no effect of OH addition to the Au(100) surface with regard to O-O length. These results suggest that the four-electron reduction of O₂ on Au(100) in base arises from a disproportionation mechanism, which is enhanced on Au(100) relative to the other two low Miller index faces of Au.