Relative rate study of the addition of HO2 radicals to C2H4 and C3H6
Abstract
Studies of the competitive oxidation of ethylene and propene in the presence of tetramethylbutane (TMB) as a source of HO2 radicals have been carried out between 400 and 500 °C using aged boric acid-coated vessels. From measurements of the relative yields of oxirane and methyloxirane, values of A9/A10= 2.2 ± 0.3 and E9–E10= 9.3 ± 0.8 kJ mol–1 have been obtained: HO2+ C2H4→ C2H4O + OH (9), HO2+ C3H6→ C3H6O + OH (10).
Evidence is presented to show that use of the TMB + O2 reaction as a source of HO2 radicals gives higher rate constants in KCl-coated vessels than in aged boric acid-coated vessels at temperatures > 450 °C. Computer analysis, however, shows that enhanced HO2 formation from the destruction of H2O2 at the surface is not the probable cause, as has been suggested elsewhere. Absolute values of A9= 109.45 ± 0.35 dm3 mol–1 s–1 and E9= 71.6 ± 5 kJ mol–1 are obtained by reference to the Arrhenius parameters for reaction (10) and it is argued that both sets of parameters refer to the actual addition step: [graphic omitted]. The implications of the relatively high activation energy of the addition step for recent suggestions concerning the mechanism of reaction (14) are discussed: C2H5+ O2→ C2H4+ HO2. (14)