Interconversion of a bisulfate anion into a sulfuric acid molecule on a Pt(111) electrode in a 0.5 M H2SO4 solution

The coadsorption of water and sulfur trioxide (SO3) molecules on Pt(111) was studied by thermal desorption spectroscopy, in-situ and non-situ infrared reflection absorption spectroscopy in order to reveal a double layer structure on a Pt(111) electrode surface in a 0.5 M H2SO4 acid solution. The vibrational frequencies of water, bisulfate and sulfuric acid molecules adsorbed on ultra-high vacuum (UHV) model surfaces at temperatures of 110–300 K were compared with those from a real double layer in a 0.5 M H2SO4 acid solution under active potentials. Model structures on Pt(111) under UHV obtained from surfaces after a lower- and a higher-temperature annealing agreed well with those from surfaces at a negative and a positive electrode potential, respectively. A sulfuric acid molecule on a Pt(111) electrode observed at 750–1100 mV was reproduced by dehydration from the coadsorbates of H3O+ and HSO4− at 200–300 K.