In situ FT–IRRAS study of SO2 adlayers formed on Pt(111) electrodes from open-circuit adsorption in acidic media

Abstract

The irreversible adsorption of SO2 on Pt(111) electrodes has been studied in acid media by means of cyclic voltammetry and in situ FT–IRRA Spectroscopy. The resulting adsorbed residue presents a characteristic vibrational band at 1220 cm−1 (0.65 V) either in 0.5 M HClO4 or 0.5 M HCl. In the potential region of constant coverage (0.65–1.0 V) the bandcentre frequency increases linearly with potential at a tuning rate of 65–75 cm−1 V−1. This shift may result from mixed contributions from Stark and/or backbonding effects. Below 0.65 V, the abrupt decrease in band frequency can be linked to the weakening of the dipole–dipole coupling effect resulting from the decrease in SO2 surface coverage. From the vibrational data for SO2–transition metal complexes and with the aid of the surface selection rule, the band at 1220–1240 cm−1 can be assigned to the asymmetric stretching mode of undissociated SO2, bound to the surface via S and one of the O atoms in a η2-bridging configuration.