Kinetics of N₂O₅ uptake on a methane soot coating

Abstract

The uptake of N₂O₅ on a soot coating at Т = 255 and 298 K was studied by low-voltage electron ionization using a thermostatted flow reactor with a mobile insert with soot deposited on it and a mass spectrometer while varying the N₂O₅ concentration in the range 1.3 × 10¹²–3.3 × 10¹³ cm^–3. A series of timedependent N₂O₅ uptake coefficients on fresh soot coatings were recorded in the indicated range of reactant gas concentrations. The uptake coefficient is described by the equation l/γ(t) = l/γ₀ + at. The dependences of the γ₀ and а parameters of this equation on the N₂O₅ concentration were determined: l/γ₀ = 1/γ ⁱⁿⁱ₀ (1 + K _L[N₂O₅]), a = k[N₂O₅] with the constants k, γ ⁱⁿⁱ₀ , and K _L equal to (0.8 ± 0.1) × 10^–10 cm³ s^–1, (4.2 ± 1.9) × 10^–4, and (2.3 ± 0.8) × 10^–13 cm³ (255 K) and (1.1 ± 0.1) × 10^–10 cm³ s^–1, (5.5 ± 0.2) × 10^–5, and (7.4 ± 1.4) × 10^–15 cm³ (298 K), respectively. The uptake is the result of the joint action of physical sorption and chemical reaction. NO was recorded as the only gas-phase product of uptake. The quantity of NO corresponds to ~60% of consumed N₂O₅. A description of the initial uptake of N₂O₅ was suggested based on the Langmuir concept of adsorption. It follows from the model description of the experimental dependences that K _L is the Langmuir constant. Other constants were evaluated: the rate constant of desorption k _d = 108 ± 17 (255 K) and 4030 ± 320 s^–1 (298 K) and its adsorption heat Q _ad = (52.4 ± 2.6) kJ mol^–1; the rate constant of the monomolecular heterogeneous reaction k _r = 0.2 ± 0.01 (255 K) and 0.8 ± 0.05 s^–1 (298 K) and its activation energy E _a = (21.9 ± 1) kJ mol^–1.