ISOTOPIC EXCHANGE BETWEEN OXYGEN AND CARBON MONOXIDE IN SHOCK WAVES

The isotopic exchange reaction between 18O2 and CO has been studied in a shock tube coupled to a timeof-flight mass spectrometer, over the temperature range 1700°-2600°K, using very dilute CO+O 2 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 O 2 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+O 2→CO2+O indicate that some chain multiplication process, possibly involving trace impurities, is occurring.