HO2 and O2- Radicals at Elevated Temperatures

Article abstract of DOI:10.1021/j100321a060

The spectra of HO2 and O2^- radicals were determined in the temperature range 20-300 deg C in aqueous solutions with 0.2-MPa oxygen and about 10-MPa hydrgen pressure.In this temperature range the wavelengths at maximum absorption and the half-widths of the spectra change very little.The relative temperature coefficient of ε_maxG is 0.15 and 0.25percent K^-1 for HO2 and O^-, respectively.In the temperature range 20-200 deg C the change in pK_a is small, but above 100 deg C pK_a increases with an increasing rate up to a value of 6.15 (molal unit) at 285 deg C.In acid solution, pH<*>2, the bimolecular decay rate constant for HO2 has an activation energy of 20.6 kJ mol^-1 (4.9 kcal mol^-1).At higher pH the decay is caused by the reaction between HO2 and O2^-.This reaction follows the Arrhenius parameters up to 100 deg C with an activation energy of 7.6 kJ mol^-1 (1.8 kcal mol^-1).Above 100 deg C the decay rate increases drastically.At higher temperatures (<*>200 deg C) a reaction with an activation energy of 80 +/- 12 kJ mol^-1 (19 +/- 3 kcal mol^-1) becomes rate determining. this reaction is tentatively ascribed to the equlibrium reaction O2^- + O2^- <*> O4^2- followed by H⁺ + O4^- -> HO2^- + O2.The mechanism may involve the unstable intermediate HO4^-.The high activation energy is ascribed to the forward reaction in the equilibrium which makes this reaction negligible at lower temperatures.