Reaction of OH* radicals with H2 in sub-critical water

The rate constant for the reaction of hydroxyl radicals (OH) with hydrogen in aqueous solutions has been measured from 200 to 350 °C by competition kinetics using nitrobenzene as a competing OH* scavenger. Measurements below 250 °C agree with previous results in other laboratories. At higher temperatures, the rate constant undershoots an extrapolation of the Arrhenius plot and actually decreases in value above 275 °C. At 350 °C, the measured rate constant is more than a factor of 5 below the Arrhenius extrapolation. We propose an explanation based largely on the hydrophobic solvation properties of the H2 molecule.