In-situ FT-IR spectroscopic study of bisulfate and sulfate adsorption on platinum electrodes

The adsorption of bisulfate and sulfate ions on Pt in sulfuric acid has been studied by observing their asymmetric S-O stretching vibrations using potential difference Fourier transform infrared reflection absorption spectroscopy. It has been found that the two anions are coadsorbed over the potential range studied. A rather clear band of the bending mode of water was observed in the same spectra. This has been attributed to water molecules weakly bonded to the surface through H in the hydrogen region and O in the oxygen region. The band intensity ratio of sulfate against bisulfate ions is much larger than the calculated activity ratio in 0.5 M sulfuric acid solution, although the intensity ratio becomes closer to the calculated activity ratio in 0.05 M sulfuric acid. A large frequency shift with potential of 100 cm−1/V is observed for the adsorbed bisulfate ions in the double layer potential region, while the frequency shift is much smaller for sulfate ions. In the oxide formation region above 0.85 V (RHE), the sulfate ions appear to be replaced by bisulfate ions; this is interpreted as resulting from the formation of a hydrogen bond between HSO−4 and the negatively charged oxygen atoms which are present on the Pt surface.