Adsorption and electrochemistry of SCN −: Comparative studies at Ag(111) and Pt(111) electrodes by means of AES, CV, HREELS and LEED

Abstract

Surface electrochemical studies are reported of SCN − at Ag(111) and Pt(111) electrode surfaces in aqueous solutions. Adsorbate packing density and stoichiometry were investigated by use of Auger electron spectroscopy (AES). Adsorbate molecular constitution and surface chemical bonding were characterized by means of high-resolution electron energy-loss spectroscopy (HREELS), which yielded vibrational spectra spanning the entire IR frequency range. Surface electrochemical behavior was explored by cyclic voltammetry (CV). Packing densities of SCN − at Pt and Ag are essentially the saturation values at all practical concentrations, and at all electrode potentials for which the metal surface is stable. The SCN − ion remains intact during adsorption at Pt and Ag surfaces unless the potential is strongly oxidizing. When adsorbed onto Pt(111) at pH 3, SCN − is protonated (SCNH), while at pH 10 it is not protonated (SCN − K +). Protonation of Pt/SCN − leads to CH and NH vibrational modes in HREELS, suggesting that the adsorbed layer consists of at least two species (PtSCNH and PtNCHS). However, when adsorbed at Ag(111), SCN − is not protonated, even at pH 3. The C:S stoichiometric ratio (evaluated from AES) was essentially 1:1 under all conditions studied; the packing densities were about 0.5 SCN per surface Pt atom, or 0.25 SCN per surface Ag atom. The HREELS spectra suggest that the axis of the SCN moiety is approximately perpendicular to the Pt or Ag surface. Low energy electron diffraction (LEED) studies revealed a Pt(111)(1 × 2)-SCN structure (θ SCN -2~ 0.5), which was converted by heating (700°C) to Pt(111)(2 × 2)-S (θ s -2~ 0.25), and an Ag(111)(2 × 3√3, rectangular)-SCN structure (θ SCN -2 0.25). Adsorption of SCN − at Pt(111) or Ag(111) as a function of electrode potential revealed noticeable changes in the HREELS spectra, including a blue-shift of the CN stretch with increasing potential. Packing densities were virtually independent of potential, except at extremely positive potentials where a separate AgSCN phase is formed or where the Pt/SCN system undergoes extensive oxidation, and the surface becomes disordered as indicated by LEED.