The role of the superoxide radical in the OsO4-catalysed decomposition of hydrogen peroxide

Abstract

It is shown by direct e.s.r. spectrometry that the superoxide radical is formed in the OsO₄-catalysed decomposition of hydrogen peroxide. The initial rate of superoxide formation was found to be proportional to the 0.6 power of the concentration of hydrogen peroxide and the first power of the catalyst concentration. This is interpreted in terms of the formation of a peroxo-osmic acid, the anion of which decomposes into superoxide and osmium(VII) radicals. The latter is oxidized by hydrogen peroxide at pH < 9, but reduced by the peroxide anion at higher pH. Since the reoxidation of osmium(VI) by hydrogen peroxide slows down when the pH is increased, the decomposition of hydrogen peroxide goes through a maximum. As both superoxide and osmium(VII) were found to be oxidized by tetranitromethane, the decomposition of hydrogen peroxide is inhibited at pH < 9, but accelerated at pH > 9, for at higher pH the catalyst is transformed by this reagent into its more active [osmium(VIII)] form.