Reaction of O(1D) with methane

Abstract

The reaction of O(1D) with CH4 has been studied by photolysis (2537 Å) of O3-CH4 mixtures dissolved in liquid argon at 87°K. The quantum yields of O3 decomposition and the product yields based on O3 decomposed were measured. The products found were CH3OH, CH2O, and H2. Added O 2 was found to lower the quantum yields of O3 decomposition and to increase the product yields. Experiments with added 18O2 were carried out to trace the origin of the products. At 6000 Å, which can only give O (3P), no photo-reaction could be detected. The results indicate that the O(1D)-CH4 reaction has essentially zero activation energy and proceeds by three paths: O(1D)+CH4+M→CH3OH+M, O( 1D)+CH4→CH2O+H2, O( 1D)+CH4CH3+OH. The effect of added O 2 is attributed to scavenging of methyl radicals which otherwise react with O3. The reaction of CH3 with O3 has zero Ea and occurs at a rate approximately equal to the reaction of CH3 with O2. The magnitude of the scavenging effect shows that at least two O3 molecules are decomposed by each CH3. By contrast, OH radicals do not react with O3 in these experiments, indicating a nonzero activation energy for the reaction OH+O3→ HO2+O2.