Ethylene—Oxygen Reaction in Shock Waves

Abstract

The reaction of ethylene with oxygen in mixtures dilute in neon or argon was studied in shock waves by several methods. Observations of CH (A2Δ→X2π) chemiluminescence and of total chemi-ionization behind incident waves revealed induction periods followed by exponential rise of the signals. The former, for both phenomena, are representable by the equation log10(ti[O2])=−11.23±0.20+(23 110±1610)/4.58Tand the latter by log10(τ[O2])=−12.07±0.15+(23 950±1270)/4.58T.The units for both equations are seconds·moles liter−1. A time-of-flight mass spectrometer was used to measure the formation of neutral and chemi-ion product species behind reflected waves. The initial rate of formation of H2O was found to be exponential in time and the time constants were found to be consistent with the results obtained for chemiluminescence and chemi-ionization. Acetylene was found to be formed in large yields concurrently with H2O and CO and to be subsequently rapidly oxidized, except in mixtures deficient in oxygen. Chemi-ionization follows the formation of observable concentrations of acetylene. Chemi-ion mass spectra and yields are very similar to those observed in a previous study of the oxidation of acetylene. A tentative reaction scheme is proposed which involves a branching chain oxidation of ethylene, the rate determining step being C2H3+O2→C2H3O+O; the products are C2H2, H2O, CO, and H2. Chemi-ionization and CH chemiluminescence are attributed to the subsequent chain oxidation of acetylene.