Isotopic Exchange between Oxygen and Carbon Monoxide in Shock Waves

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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.