THE ELECTROOXIDATION OF CARBON-MONOXIDE ON NOBLE-METAL CATALYSTS AS REVISITED BY REAL-TIME SURFACE-ENHANCED RAMAN-SPECTROSCOPY

Sequences of real-time surface-enhanced (SER) spectra acquired during the linear sweep voltammetric oxidation of carbon monoxide on noble me tal catalysts in aqueous 0.1 M HClO4 are presented with the aim of ass essing the surface species present under reactive electrochemical cond itions. The three surfaces investigated, gold, and platinum and rhodiu m films on gold, provide distinctly different yet potent catalysts for CO electro-oxidation. The latter two surfaces constitute examples of a previously described tactic by which SERS activity can be imparted t o transition metals and other materials. Examination of the low-freque ncy (below 700 cm-1) spectral region, where metal-adsorbate vibrations are located, is shown to provide useful information on the interrelat ion between CO electro-oxidation and the occurrence of metal surface o xidation. The presence of adsorbed CO is seen to exert a marked influe nce on the latter processes. On gold, adsorbed oxygen species, as dete cted from a 500-550 cm-1 vibration, are induced to form at potentials substantially below those where surface oxide is observed spectroscopi cally and voltammetrically in the absence of CO. On platinum and espec ially rhodium, the presence of adsorbed CO is seen to influence the me tal oxidation by selectively attenuating bands at 350 and 290 cm-1 asc ribed to metal-oxygen bending vibrations, while leaving largely intact the higher-frequency metal-hydroxyl stretches (at 500-600 cm-1), also associated with metal surface oxidation. The possible roles of adsorb ed oxygen species in the CO electro-oxidations are discussed in the li ght of these findings.