In-situ FT-IR spectroscopic study of bisulfate and sulfate adsorption on a platinum electrode: Part 2. Mildly acid and alkaline sodium sulfate solutions

Abstract

Adsorption of sulfate/bisulfate ions and water molecules on a platinum electrode has been studied in a mildly acid solution (pH = 3.4) of 0.5 M Na 2SO 4 and H 2SO 4 and an alkaline solution (pH =11.5) of 0.5 M Na 2SO 4 and NaOH by potential difference FT-IRRAS (Fourier transform infrared reflection absorption spectroscopy). In the acidified sodium sulfate solution, sulfate ions and water molecules are adsorbed on top of the adsorbed hydrogen layer at 0.05 V/RHE. As the potential becomes more positive they are desorbed and replaced by bisulfate ions. The desorption of the sulfate ions is completed m the oxygen region. The number of sulfate ions and water molecules replaced by one bisulfate ion has been found to be constant throughout the entire potential range. In contrast to pure sulfuric acid solutions, the asymmetric S-O stretching frequency of the adsorbed bisulfate ions shows much smaller potential dependence. In the alkaline sodium sulfate solution, adsorption of sulfate ions increases continuously from 0.05 V/RHE to more positive potentials and saturates in the oxygen region, and no desorption of sulfate ions has been observed. The sulfate ion adsorption is accompanied by adsorption of water molecules. Only a very weak bisulfate band has been detected in the oxygen region.