Shock wave study of the unimolecular dissociation of H2O2 in its falloff range and of its secondary reactions
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 k1,0 = [Ar]1016.36±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.