Initial uptake of NO2 on methane flame soot

A thermostated flow reactor with a movable soot-coated insert coupled to a high-resolution mass spectrometer with low-energy electron ionization is used to study the uptake of NO2 reagent gas at [NO2] = 1 × 1012−2 × 1013 cm−3 and a humidity of [H2O] = 5 × 1012−1.8 × 1015 cm−3. The BET (Brunnauer-Emmett-Teller) method is used to determine the specific surface area of the soot coating: (40 ± 10 (2σ)) m2/g. A set of time-dependent uptake coefficients of NO2 on fresh soot coatings in this range of reactant gas concentrations is determined. An analysis of the experimental data shows that the uptake coefficient depends on the time as 1/γ(t) = 1/γ0 + c 1 t and yields the dependences of the parameters γ0 and c 1 on the NO2 concentration: 1/γ0 = c 2 + c 3[NO2] and c 1 = k[NO2] with constants c 2 = (6.5 ± 1.3) × 103, c 3 = (5.6 ± 1.3) × 10−10 cm3 molecule−1, and k = (2.4 ± 0.2) × 10−10 cm3 molecule−1 s−1. The gas-phase products of NO2 uptake on soot are NO and HONO, with the NO yield constituting ∼50% of the reacted NO2. It is experimentally demonstrated that an increase in the humidity causes no changes in the uptake coefficient and in the composition and ratio of the products. The initial stage of NO2 uptake on a methane soot coating is described using the Langmuir adsorption model, according to which the process of uptake consists of a sequence of elementary steps, such as reversible adsorption, surface complex formation, and its subsequent unimolecular decomposition to form products. Interpretation of the experimental dependence γ0 = f([NO2]) enables to estimate the Langmuir constant for the NO2-methane soot pair, K L = (8.6 ± 2.6) × 10−14 cm3 molecule−1, the rate constant for NO2 desorption from the soot coating, k d = (530 ± 160) s−1, and the rate constant for the monomolecular decomposition of the surface complex, k r = (8.2 ± 2.5) × 10−2 s−1.