Abstract

This thesis contains studies in two parts: (1) electroreduction of dioxygen on a series of bare and modified surfaces; (2) the orientation of molecule on the surface as a function of applied potentials.

First, electroreduction of H₂O₂ is examined on bare Au, Bi or Pb-modified Au, and Pt surfaces using Surface Enhanced Raman Scattering (SERS) along with density functional theory (DFT) calculations. On Bi/Au electrode, the spectroscopy shows the presence of Bi-OH and Bi-O species at potentials positive of that where peroxide is reduced. DFT calculations show that peroxide is unstable relative to Bi-OH at the Bi(2x2)/Au(111) surface. On Pb/Au, the spectroscopy strongly suggest the formation of several intermediates, PbOOH⁺, bimetallic dihydroxide AuPb(OH)₂ and superoxide during the electroreduction process. On bare Au, there is no variation for the ν(O-O) of H₂O₂. On Pt, the ν(O-O) of H ₂O₂ shifts to lower energy at negative potentials, indicating the bond elongation. Additionally, both SERS and DFT suggest the formation of hydroxide on Pt.

Second, O₂ electroreduction on a bare Au, Bi/Au, and Pt surfaces is studied. While O₂ reduction occurs in a 2-e pathway on bare Au and 4-e pathway on Bi/Au surface, superoxide is observed on both surfaces, indicating the importance of superoxide. Furthermore, the formation of Bi-OH is observed during the electroreduction process. On the Pt surface, only one mode associated with species containing O is observed and is assigned to the free O₂ in the solution before it binds to the Pt surface and be reduced subsequently.

Finally, thiocyanate (SCN) adsorption on Au electrode is examined. Both the calculations and the spectroscopy show that three different geometries are adopted by SCN in the potential region studied (0.0 V ≤ E ≤ 1.2 V vs. NHE). At low potential both N-bound and S-bound forms are found, at intermediate potentials around E_pzc the S-bound form dominates, and at high potentials SCN associates with the surface via a bridging geometry. The different Stark slopes observed for the C-N stretch in the end-on and bridging forms are attributed to the lowest unoccupied molecular orbital (LUMO).