Reaction Mechanism of Ozone with Methane Flame Soot: Langmuir–Hinshelwood or Unimolecular Decomposition?

The uptake of ozone on methane flame soot has been studied in a flow reactor with a mobile insert coupled to a beam-sampling mass spectrometer at T = 256 and 295 K in the range of [O3] = 1.1 × 1012–4.1 × 1013 cm–3. The uptake was found to be multistage. Both the fast and slow stages of the uptake occur by the reaction mechanism on the surface with an adsorbed particle. Based on the Langmuir concept of adsorption, the uptake coefficient can be represented as γ = γ0/(1 + KL[O3]) with the γ0 and KL parameters for the first stage of the uptake γ0(295 K) = (6.6 ± 0.4) × 10–4 and γ0(256 K) = (8.1 ± 2.6) × 10–4, KL(295 K) = (7 ± 1) × 10–14 and KL(256 K) = (22 ± 9) × 10–14 cm3. The same parameters for the second stage are γ0(295 K) = (1.3 ± 0.2) × 10–4 and γ0(256 K) = (1.2 ± 0.5) × 10–4, KL(295 K) = (11 ± 2) × 10–14 and KL(256 K) = (28 ± 13) × 10–14 cm3. The rate constant of the unimolecular decomposition of the surface complex with the adsorbed ozone molecule was evaluated from the dependence of γ on the exposure time of the soot coating to the О3 flow: kr(295 K) = (0.39 ± 0.01) s–1, kr(256 K) = (0.15 ± 0.01) × 10–2 s–1 for the fast stage and kr(295 K) = (2.8 ± 0.1) × 10–2 s–1, kr(256 K) = (1 ± 0.04) × 10–2 s–1 for the slow stage. In the limit of low concentrations, γ depends only weakly on [O3] and T due to the relation γ0 ∼ kr/kd and close values of the activation energy of the reaction rate constant kr and the heat of adsorption of the desorption rate constant kd.