| 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 nu(O-O) of H2O2. On Pt, the nu(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 Epzc 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). |
