INFRARED SPECTROSCOPIC ANALYSIS OF ANIONS ADSORBED FROM BISULFATE-CONTAINING SOLUTIONS ON PT(111) ELECTRODES

In situ FTIR spectroscopy has been used to study the potential depende nt changes in anion structure and composition at the surface of Pt(111 ) electrodes in HSO4--containing solutions. From the infrared differen tial normalized relative reflectance data, the maximum rate of intensi ty changes for three infrared bands can be obtained. Two modes associa ted with the adsorbed anion and one mode assigned to solution-phase bi sulfate all have extrema coinciding with the spike in the anomalous re gion of the cyclic voltammetry. The adsorbed species is not adequately described as either sulfate or bisulfate; the data are more consisten t with an adsorbed H3O+-SO42- ion pair, possibly with the three unprot onated sulfate oxygens interacting with Pt sites. A weak band at 940 c m(-1) correlates with the 1200-1250 cm(-1) band and is assigned to the S-O(H3O+) stretch of the ion pair. A band at 1040 cm(-1), which disap pears with increasingly more positive sample potentials, is assigned t o the symmetric stretching mode for solution-phase bisulfate ions.