INTERCONVERSION OF A BISULFATE ANION INTO A SULFURIC-ACID MOLECULE ONA PT(111) ELECTRODE IN A 0.5 M H2SO4 SOLUTION

The coadsorption of water and sulfur trioxide (SO3) molecules on Pt(11 1) was studied by thermal desorption spectroscopy, in-situ and non-sit u infrared reflection absorption spectroscopy in order to reveal a dou ble layer structure on a Pt(111) electrode surface in a 0.5 M H(2)SO4 acid solution. The vibrational frequencies of water, bisulfate and sul furic acid molecules adsorbed on ultra-high vacuum (UHV) model surface s at temperatures of 110-300 K were compared with those from a real do uble layer in a 0.5 M H2SO4 acid solution under active potentials. Mod el structures on Pt(111) under UHV obtained from surfaces after a lowe r- and a higher-temperature annealing agreed well with those from surf aces 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.