Reactivity of NO2 and H2O on soot generated in the laboratory: a diffusion tube study at ambient temperature

Abstract

In this work we have obtained the kinetic parameters and the product spectrum of NO₂ and H₂O separately interacting with soot samples in terms of surface residence times τ_s of the interacting molecules and uptake rate constant expressed as uptake coefficients (γ). The NO₂ interaction on soot from either a decane or a toluene flame results in HONO as the main product in contrast to acetylene soot. NO was observed to be a minor product for all soot substrates investigated. Residence times up to 70 ms for NO₂ interacting with acetylene soot and up to 50 ms with decane and toluene soot were obtained as well as initial uptake coefficients γ₀^MC ranging from 0.26 for NO₂ on acetylene soot to an average value of 0.1 for NO₂ on decane and toluene soot. All experiments were subject to significant saturation at typical doses of 10¹⁵ molecules. A Monte-Carlo (MC) trajectory model was applied to extract τ_s and γ₀^MC values, as well as the average number of adsorption sites n_s. The interaction of NO₂ with toluene and decane soot was modeled using a simple MC-model using a single adsorption site, whereas acetylene soot required a two-site model. For the interaction of H₂O on soot, a residence time τ_s ranging from 0.1 ms on decane soot to 5 ms for acetylene soot has been determined using the single-reaction-site MC-model with τ_s values between 2 and 3 ms for toluene and diesel soot, respectively. For H₂O an upper limit for the uptake coefficient γ₀^MC of 2 × 10⁻³ has been measured for all soot samples.