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/γ0ini (1 + KL[N2O5]), a = k[N2O5] with the constants k, γ0ini, and KL 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 KL is the Langmuir constant. Other constants were evaluated: the rate constant of desorption kd = 108 ± 17 (255 K) and 4030 ± 320 s–1 (298 K) and its adsorption heat Qad = (52.4 ± 2.6) kJ mol–1; the rate constant of the monomolecular heterogeneous reaction kr = 0.2 ± 0.01 (255 K) and 0.8 ± 0.05 s–1 (298 K) and its activation energy Ea = (21.9 ± 1) kJ mol–1.