Surface-enhanced Raman spectroscopy at transition metal-gas interfaces: adsorption and reactions of sulfur dioxide on platinum-, rhodium-, and ruthenium-coated gold

Abstract

Surface-enhanced Raman (SER) spectra obtained using 647-nm excitation are reported for the adsorption and oxidation of sulfur dioxide in flowing argon-based streams at ambient pressures and at 300 K on electroelectrochemically roughened gold, and on gold surfaces modified by thin (2–3 monolayer) electrodeposited films of platinum, rhodium, and ruthenium. The spectra were recorded by using a spectrograph/charge-coupled device (CCD) detector system, enabling real-time spectral sequences to be obtained within 1–2 s. Dosing with sulfur dioxide yielded a band at 1135 cm −1 on platinum as well as on unmodified gold, assigned to a symmetric S-O stretch of molecularly adsorbed SO 2. This band is weak on ruthenium, and absent on rhodium. A feature at 300–320 cm −1 is also obtained on gold, platinum, rhodium, and ruthenium, and is identified as the metal-adsorbate stretch for sulfur formed by dissociative SO 2 adsorption. On platinum and gold, oxidation of the adsorbed SO 2 requires the presence of water and CO in addition to O 2 in the feed stream. Under these conditions, the 1135-cm −1 feature is replaced by bands at 1010 and 1020 cm −1 (Pt) or 1010 cm −1 (Au), ascribed to adsorbed sulfate. On ruthenium, including only water and O 2 in the feed yields SO 2 oxidation to sulfate, characterized by a band at 1030 cm −1. The stability of each species to temperature excursions up to 475 K is noted. Comparison is also made with potential-dependent SER spectra obtained for SO 2 at the corresponding metal-aqueous electrochemical interfaces. Similarly to the gas-phase systems, the surface oxidation as well as molecular adsorption of SO 2 can be monitored at these electrochemical interfaces with SERS. Dissociative chemisorption to yield adsorbed sulfur is strongly potential-dependent, as expected since the process is reductive. Some virtues of such parallel gas-phase and electrochemical surface spectroscopic studies are pointed out.