MOLECULAR CHEMISTRY AT WELL-DEFINED SURFACES

Surface chemistry studies are described in which molecular layers chemisorbed at well-defined Pt or Ag electrode surfaces from aqueous solutions were characterized by means of ex-situ UHV surface analysis techniques such as Auger electron spectroscopy (AES), electron energy-loss spectroscopy (EELS), and low-energy electron diffraction (LEED), and in-situ electrode-solution interfacial characterization methods such as cyclic voltammetry (CV) and coulometry. Types of compounds studied include alcohols, ethers, aldehydes, ketones, carboxylic acids, amines, amino-acids, amides, cyanides, thiocyanides, halides, mercaptans, thiols, sulfones, sulfonic acids, benzenes, five- and six- membered heterocyclic aromatics, multinitrogen heteroaromatics, fused-ring aromatics, and deuterated molecules. Molecules chemisorbed at electrode surfaces are often not removed by rinsing facilitating investigation by electrochemical methods, and are stable also in vacuum, permitting identification and characterization of the adsorbed species by AES, EELS and LEED and related methods. Molecular stoichiometry (elemental composition) and packing density (surface concentration, molecules per unit area) are measured by means of AES. Surface molecular orientation can be deduced from molecular packing densities, supplemented with adsorbate vibrational spectra, electrochemical reactivity, LEED structure and other related observations. Studies of series of related adsorbates offer the advantage that additional insight can be gained from intercomparison of molecular surface behavior as a function of systematic structural and chemical differences. Electrochemical reactivity is found to be a function of molecular orientation: horizontally oriented adsorbates at Pt surfaces in most cases undergo complete electrochemical oxidation to CO2 and other oxides, while vertically oriented adsorbates undergo less extensive oxidation; reversible redox centers such as the hydroquinone and catechol moieties retain their electrochemical reactivity and reversibility at electrode surfaces only when the redox center is pendant, and direct attachment of a redox center at surfaces results in loss of redox electroactivity. Close similarity between adsorbed layer EELS spectra and the IR spectra of the unadsorbed liquid or vapor is usually found for vertically oriented adsorbates for which adsorption does not substantially perturb the main-frame molecular structure. Surface chemical phenomena involving molecular adsorbates at electrode surfaces, including molecular packing density, mode of attachment, surface orientation, long-range surface order, molecular structure, spectroscopic behavior, and chemical and electrochemical reactivity, are greatly influenced by factors such as adsorbate molecular structure, electrode material, crystallographic structure of electrode surface, electrode potential, solution pH, adsorbate concentration, and temperature.