SERS investigation of the adsorption and electroreduction of 4-cyanopyridine on a silver electrode

Time-resolved surface-enhanced Raman scattering (TR-SERS) has been used to study the adsorption and reduction of 4-cyanopyridine (4-CNPY) in non-buffered 0.1 M KCl aqueous solutions on an Ag electrode. At electrode potentials of -0.1 to -0.6 V vs. SCE, well before the reduction process occurs, the nature of the adsorbed species depends on both the solution concentration and the time following an oxidation-reduction process. At 20.0 mM 4-CNPY a dimer or aggregated surface species which forms dynamically on the surface is deduced from the SERS spectra, whereas at 1.0 mM and below a monomer species oriented vertically to the surface and attached end-on through the ring nitrogen is deduced. A pump-probe TR-SERS method with an applied potential pump and a CW 488 nm optical probe was used to investigate the electrode potential region where the reduction process occurred, greater than or equal to -1.2 V vs. SCE. In the more dilute solutions, SERS spectra showed that pyridine and CN- are the products of the electrode reaction, whereas at concentrations of 10 mM and above the TR-SERS spectra indicated that a bimolecular reaction occurred with the most likely product being azo-4-methylpyridine. Rate constants for reaction steps in both the monomolecular and bimolecular processes are estimated. (C) 1997 Elsevier Science S.A.