ADSORPTION AND SURFACE STRUCTURAL CHEMISTRY OF THIOPHENOL, BENZYL MERCAPTAN, AND ALKYL MERCAPTANS - COMPARATIVE-STUDIES AT AG(111) AND PT(111) ELECTRODES BY MEANS OF AUGER-SPECTROSCOPY, ELECTRON-ENERGY LOSS SPECTROSCOPY, LOW-ENERGY ELECTRON-DIFFRACTION, AND ELECTROCHEMISTRY

Surface electrochemical studies are reported for a series of organic sulfur compounds, comparing their behavior at Ag(111) and Pt(111) electrode surfaces in aqueous solutions. Molecular packing density and stoichiometry were investigated by use of Auger spectroscopy. Molecular constitution and surface chemical bonding were characterized by electron energy loss spectroscopy (EELS) surface vibrational spectroscopy. Surface long-range order was examined by low-energy electron diffraction (LEED). Surface electrochemical behavior was explored by use of voltammetric methods. Compounds studied represent various types of aromatic thiols and alkyl mercaptans: thiophenol (TP); benzyl mercaptan (BM); 1-propanethiol (PT); 2-mercaptoethanol (ME); 2-aminoethanethiol (AET); and 2-mercaptoethanesulfonic acid sodium salt (MES). Also studied were pyridine and two other derivatives which chemisorb at Pt(111) electrodes but not at Ag(111): pyridine (PYR); pyridine-d5 (PYR-D5); and (3-pyridyl)hydroquinone (3PHQ). The results indicate that thiols and mercaptans are attached to the Pt(111) and Ag(111) surface primarily through the sulfur atom with the remainder of the molecule being present as a pendant. Dissolution of the sulfhydryl hydrogen accompanies adsorption. Molecular packing densities measured by means of Auger spectroscopy at Pt(111) and Ag(111) surfaces are similar and are in close agreement with theoretical limiting packing densities based upon molecular models. These adsorbates evidently retain their framework molecular structure in the adsorbed state based upon the close similarity between the EELS spectrum of each adsorbate and the IR spectrum of the unadsorbed compound. Thiols and mercaptans show a greater tendency to form monolayers having long-range order at Ag(111) than at Pt(111), as judged by LEED. For example, TP, BM, and MES form commensurate structures at Ag(111) but do not yield LEED patterns at Pt(111): Ag(111)(unroofed-radical 7 x unroofed-radical 31, 88-degrees) R40.9-degrees, TP; Ag(111)(2 unroofed-radical 3x2 unroofed-radical 3) R30-degrees, BM; and Ag(111) (3 x unroofed-radical 3, 90-degrees) R30-degrees, MES. Adsorbed layers of PT, ME, and AET were disordered at both surfaces.