The electro-oxidation 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 metal catalysts in aqueous 0.1 M HClO4 are presented with the aim of assessing the surface species present under reactive electrochemical conditions. The three surfaces investigated, gold, and platinum and rhodium 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 to transition metals and other materials. Examination of the low-frequency (below 700 cm−1) spectral region, where metal—adsorbate vibrations are located, is shown to provide useful information on the interrelation between CO electro-oxidation and the occurrence of metal surface oxidation. The presence of adsorbed CO is seen to exert a marked influence on the latter processes. On gold, adsorbed oxygen species, as detected from a 500–550 cm−1 vibration, are induced to form at potentials substantially below those where surface oxide is observed spectroscopically and voltammetrically in the absence of CO. On platinum and especially rhodium, the presence of adsorbed CO is seen to influence the metal oxidation by selectively attenuating bands at 350 and 290 cm−1 ascribed 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 adsorbed oxygen species in the CO electro-oxidations are discussed in the light of these findings.