SURFACE-CHEMISTRY OF MERCAPTOPYRIDINES AT AG(111) ELECTRODES STUDIED BY EELS, LEED, AUGER-SPECTROSCOPY AND ELECTROCHEMISTRY

Surface electrochemical studies of 2-pyridinethiol (2PyT, 2-mercaptopyridine) and 4-pyridinethiol (4PyT, 4-mercaptopyridine) at Ag(111) single-crystal electrode surfaces in aqueous fluoride solutions are reported. LEED patterns observed for 2PyT adsorbed at Ag(111) from a 1 mM aqueous solution containing 2 mM HF (pH 3) at potentials between -0.54 V and -0.64 V (vs. Ag/AgCl reference) reveal that an ordered layer is formed having a rectangular unit mesh, Ag(111) (2×0.53)R30°-2PyT, containing one molecule of 2PyT. Adsorption at potentials more positive than -0.54 V results in LEED patterns which are relatively diffuse and have not yet been identified. A weak but observable Ag(111) (30.53× 30.53)R30° LEED pattern is obtained for adsorption of 4PyT at -0.40 V from a 1 mM aqueous solution (pH 3). Molecular packing densities ( nmol cm2) measured by means of Auger spectroscopy indicate that 2PyT and 4PyT molecules are adsorbed with the pyridine ring perpendicular to the Ag(111) surface. The packing density of these adsorbates is virtually independent of adsorbate concentration from 0.01 mM to near saturation (about 0.2 M) at -0.60 V. Packing density of 2PyT increases slightly but abruptly when the electrode potential is increased above about -0.4 V. The Ag(111) (2×0.53)R30°-2PyT structure contains about 0.57 nmol cm2, slightly less than the saturation packing density, 0.67 nmol cm2, observed at more positive electrode potentials. EELS spectra of adsorbed 2PyT indicate that attachment to the Ag(111) surface at potentials less than -0.2 V occurs primarily through the sulfur atom, with dissociation of the sulfhydryl hydrogen atom. At potentials more positive than 0.0 V, a coupling reaction between an adsorbed 2PyT and a dissolved 2PyT occurs at the Ag(111) surface. The occurrence of this coupling reaction is more apparent for 4PyT at the Ag(111) surface. Amplitudes of the EELS bands due to CH bending and CS stretching depend upon the electrode potential at which adsorption is carried out. EELS spectra of 2PyT adsorbed at Ag(111) at negative potentials (E < -0.4 V) closely resemble the IR spectrum of the unadsorbed compound; the same is true for 4PyT. Apart from potential-dependent adsorption/desorption and coupling reactions, adsorbed 4PyT and 2PyT are relatively inert towards electrochemical oxidation and reduction. The 2PyT and 4PyT adsorbed layers are stable in UHV. © 1990.