Reaction of Ozone with Hydrogen Peroxide (Peroxone Process): A Revision of Current Mechanistic Concepts Based on Thermokinetic and Quantum-Chemical Considerations

Abstract

The reaction of ozone with the anion of H(2)O(2) (peroxone process) gives rise to (*)OH radicals (Staehelin, J.; Hoigne, J. Environ. Sci. Technol. 1982, 16, 676-681). Thermokinetic considerations now suggest that the electron transfer originally assumed as the first step has to be replaced by the formation of an adduct, HO(2)(-) + O(3) --> HO(5)(-) (DeltaG degrees = -39.8 kJ mol(-1)). This decomposes into HO(2)(*) and O(3)(*-) (DeltaG(0) = 13.2 kJ mol(-1)). HO(2)(*) is in equilibrium with O(2)(*-) + H(+), and O(2)(*-) undergoes electron transfer to O(3) giving rise to further O(3)(*-). The decay of O(3)(*-) into (*)OH is now discussed on the basis of the equilibria O(3)(*-) right arrow over left arrow O(2) + O(*-) and O(*-) + H(2)O right arrow over left arrow (*)OH + OH(-), excluding HO(3)(*) as the intermediate originally assumed. To account for the observation of the peroxone process being only 50% efficient, the decay of HO(5)(-) into 2 O(2) + OH(-) (DeltaG(0) = -197 kJ mol(-1)) is proposed to compete with the decay into HO(2)(*) and O(3)(*-).