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= 10^{9.45 ± 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)