ADSORPTION OF ACETYLENE ON RHODIUM-MODIFIED OR PLATINUM-MODIFIED SILVER AND GOLD ELECTRODES - A SURFACE-ENHANCED RAMAN-STUDY

Surface-enhanced Raman spectra (SERS) of acetylene adsorbed on rhodium- and platinum-modified silver and on rhodium-modified gold were determined at room temperature in both acidic and neutral supporting electrolytes. In acidic media the spectra exhibited intense bands in the vicinity of 1100 and 1500 cm-1; similar peaks were observed on unmodified gold, but not on unmodified silver. In a neutral environment the same bands appeared on rhodium-modified gold, and sometimes on unmodified gold; they were never detected on silver or on silver modified by transition metals. The 1100- and 1500-cm-1 bands were assigned to sp2 or sp3 rehybridized forms of acetylene, possibly produced by catalytic hydrogenation on the surface of the electrodes. Their intensities reversibly increased at negative potentials, suggesting a potential related formation and desorption of the new species. Weak bands, attributed to unhybridized acetylene molecules sigma-pi-bonded to the surfaces, were occasionally observed in the 1800-2300-cm-1 spectral region. No signals appeared on polished electrodes, even when these were modified by thin layers of rhodium or platinum, and the intensity of the peaks decreased gradually when increasing amounts of transition metal were deposited on the roughened substrates. The bands were therefore ascribed to surface enhancement phenomena rather than to bulk Raman scattering.