Hydroxyl Radical Self-Recombination Reaction and Absorption Spectrum in Water Up to 350 °C

Abstract

The rate constant for the self-recombination of hydroxyl radicals (^•OH) in aqueous solution giving H₂O₂ product has been measured from 150 to 350 °C by direct measurement of the ^•OH radical transient optical absorption at 250 nm. The values of the rate constant are smaller than previously predicted by extrapolation to the 200−350 °C range and show virtually no change in this range. In combining these measurements with previous results, the non-Arrhenius behavior can be well described in terms of the Noyes equation k_obs^-1 = k_act^-1+ k_diff^-1,using the diffusion-limited rate constant k_diff estimated from the Smoluchowski equation and an activated barrier rate k_act nearly equal to the gas-phase high-pressure limiting rate constant for this reaction. The aqueous ^•OH radical spectrum between 230 and 320 nm is reported up to 350 °C. A weak band at 310 nm grows in at higher temperature, while the stronger band at 230 nm decreases. An isosbestic point appears near 305 nm. We assign the 230 nm band to hydrogen-bonded ^•OH radical, and the 310 nm band is assigned to “free” ^•OH. On the basis of the spectrum change relative to room temperature, over half of the ^•OH radicals are in the latter form at 350 °C.