Molecular Precursors to the Formation of H2 and the Reduction of O2 at an Ag Cathode. A Surface-Enhanced Raman Scattering Experiment

摘要

Surface-enhanced Raman scattering (SERS) has been used to probe the formation of several precursor adsorbates on a Ag cathode prior to and during the generation of gaseous H2. The chemical reactions occurring during a cathodic voltage sweep correspond to the sequential protonation of (Ag, Mn) O: (Ag, Mn) O yields (Ag, Mn)OH yields (Ag, Mn)OH2 Yields (Ag,Mn)HO-H-OH(Ag,Mn), where (Ag,Mn)O denotes a mixed metal oxide. The equilibrium concentration of each species is determined by the interfacial pH which in turn is dependent upon the cathode voltage relative to a saturated calomel electrode, VSCE. As VSCE becomes more negative, the interfacial pH decreases because of electrostatic attraction, and the amount of protonation increases. From a logical extension of this protonation model, the formation of the dioxonium complex, (Ag,Mn)HO-H-HO(Ag,Mn), is postulated from an analysis of the SERS spectra of the adsorbates observed at the onset of H2 gas generation. The addition of O2 gas to the electrolyte solution has the remarkable effect of reversing the protonation reaction sequence. Keywords: Surface enhanced Raman Scattering, Protonation, Metal oxide adsorbates, Silver, Manganese.