Isotopic Exchange between Oxygen and Carbon Monoxide in Shock Waves

Abstract

The isotopic exchange reaction between ¹⁸O₂ and CO has been studied in a shock tube coupled to a time‐of‐flight mass spectrometer, over the temperature range 1700°–2600°K, using very dilute CO+O₂ mixtures in neon. The rate is found to increase with time. Auxiliary studies of the ¹⁶O₂–¹⁸O₂ exchange reaction and of the ¹⁸O₂–C¹⁶O exchange reaction induced by nitrous oxide pyrolysis have demonstrated that this exchange occurs via an atomic mechanism. The rates of O‐atom attack on O₂ and CO are found to be almost equal over the temperature range studied. Using the known rate of N₂O decomposition, at 1820°K the calculated rate constants for these two reactions are 11. × 10⁻¹² and 6.6 × 10⁻¹² cm³/molecule·sec, respectively, in excellent agreement with the extrapolations of the low‐temperature values in the literature. The reaction producing O atoms in the absence of N₂O is equal order in both O₂ and CO but the over‐all order is uncertain. The observed activation energy of the initiation process is less than that suggested in the literature. Oxygen‐atom concentrations which are a factor of 30 greater than calculated from the literature values of the rate coefficients of the reaction CO+O₂→CO₂+O indicate that some chain multiplication process, possibly involving trace impurities, is occuring.