Mechanism of the Gas Phase, Thermal Decomposition of Ozone

Abstract

The pyrolysis of ozone has been reinvestigated experimentally, and it is now shown that most of the known data are quantitatively explained on the basis of the simple mechanism, M+O3[open phi] lim 21M+O2+OO+O3→ lim 32 O2,where M may be O2, O3, CO2, N2, He, etc. There is no evidence for a direct bimolecular reaction of ozone to produce O2 nor is there any evidence for important surface effects or energy chains. On the other hand, the results at very fast rates of decomposition indicate an acceleration which can be accounted for in terms of temperature gradients in the system. This region is usually very close to the thermal explosion limit. The values found for the rate constants are (for M equal to O3), k1=4.61±0.25×1012 exp(−24 000/RT) liter/mole−sec,k2=6.00±0.33×107exp(+600/RT) liter2/mole2−sec,k3=2.96±0.21×1010exp(−6000/RT) liter/mole−sec. The relative efficiencies of O2, N2, CO2, and He in activating O3 (compared to O3 itself) are, respectively, 0.44, 0.41, 1.06, and 0.34. As expected for energy transfer processes, k1 has an abnormally high pre-exponential factor, and k2 has correspondingly a negative energy of activation. It is further inferred that Reaction 3 which produces two O2 molecules with 99 kcal of excess energy between them does not produce more than one excited electronic state of O2 and that these hot O2 molecules are not very efficient in exciting O3 to decomposition. Calculations of the entropy of activation of Reaction 1 can be made and are shown to be in good agreement with this conclusion. From these findings it appears that the high quantum yields found in the photolysis of O3 at short wavelengths, if real, probably are attributable to metastable O atoms [1D] produced in the primary process which can generate chains in O3. It can be shown that the homogeneous production of O atoms from O2 at high temperatures proceeds through two different paths having different activation energies, the lower energy path involving O3 as an intermediate. Finally, possible mechanisms are considered for the chemical sensitization of the thermal explosion of O3 and its catalytic decomposition by H donors.