Kinetics of decay of hydroxyl radicals at low pressure

The kinetics of decay of OH radicals in excess He at 298 °K and about 1 torr pressure have been studied in a flow tube by ESR spectrometry and computer simulation. In agreement with previous work [Breen and Glass, J. Chem. Phys. 52, 1082 (1970); Mulcahy and Smith, J. Chem. Phys. 54, 5215 (1971)], a first order reaction of OH at the surface was found to accompany the gas-phase reactions OH + OH → H2O + O and OH + O → O2 + H. When [OH] is about 10−10 mole cm−3 or less, the rate of the heterogeneous reaction can be comparable with that of the homogeneous reaction. After allowing for the effects of diffusion resistance on the radial concentration profile, the nonsteady state concentration of O atoms, and the difficulties associated with accurate kinetic analysis of a system of simultaneous first and second order reactions, the rate constant for OH + OH → H2O + O was found to be k2 = (1.3 ± 0.3) × 1012cm3 mole−1 · sec−1.