Shock wave study of the unimolecular dissociation of H2O2 in its falloff range and of its secondary reactions

Abstract

The thermal decomposition of H₂O₂ was studied behind reflected shock waves using absorption spectroscopy at 215 nm with light from a laser source and at 215, 230, and 290 nm with light from a UV lamp. Due to the improved detection sensitivity for H₂O₂ and HO₂, rate constants for the reactions H₂O₂ (+Ar) → 2HO (+Ar) (1), HO₂ + HO₂ → H₂O₂ + O₂ (3) and HO + HO₂ → H₂O + O₂ (4) could be derived with much better precision than possible in earlier work. Rate constant minima for reactions (3) and (4) near 800 and 1100 K, respectively, were reconfirmed. Falloff curves of reaction (1) near to the low pressure limit were measured in experiments at about 1, 4, and 15 bar. Limiting low pressure rate constants for reaction (1) of about k_1,0 = [Ar]10^16.36±0.23exp[−21 962(±608) K/T] cm³ mol⁻¹ s⁻¹ were obtained over the temperature range 950–1250 K. The deviations from the low pressure limit at relatively low pressures may provide experimental evidence for non-exponential lifetime distributions of dissociative H₂O₂ such as recently observed in classical trajectory calculations on the ab initio potential of H₂O₂.