Surface-enhanced Raman spectroscopy as a versatile in situ probe of chemisorption in catalytic electrochemical and gaseous environments

Some recent applications of surf ace-enhanced Raman spectroscopy (SERS) to the characterization of adsorbates at metal surfaces in electrochemical and ambient gaseous systems taken from studies in our laboratory are described with the objective of illustrating the virtues of SERS for examining catalytically relevant interfaces. Emphasis is placed on studies on Pt-group metal surfaces, prepared by ultrathin film deposition on to SERS-active gold substrates, reflecting their unique catalytic significance. The surface oxidation of Pt-group metals in gaseous oxygen and aqueous electrochemical environments is compared, as exemplified for rhodium. The occurrence of a more facile oxidation pathway with gaseous 02 in the presence of water at near-ambient temperatures is demonstrated, and shown to involve a catalytic electrochemical mechanism, as distinct from the thermal oxidation route in dry 02 which requires elevated temperatures. The central role of the surface potential in such oxidation pathways in both gaseous and conventional electrochemical systems is pointed out. The catalytic activity of such oxidized Pt-group surfaces in gaseous and electrochemical systems towards the oxidation of methanol and other small organic molecules is outlined on the basis of related SERS data. The application of time-dependent SERS as a means of diagnosing the presence of adsorbed reaction intermediates is exemplified for the case of carbon monoxide formed during formic acid electrooxidation on rhodium. The formation of reactive pi-bound ethylene, and also non-reactive ethylidyne, on Pt-group electrodes upon ethylene chemisorption is also briefly described. Copyright (C) 2002 John Wiley Sons, Ltd.