Kinetics of NO3 uptake on a methane soot coating

Abstract

The dependence of the initial rate of NO₃ uptake on a methane soot coating at two temperatures (256 and 297 K) and NO₃ concentrations of 2.4 × 10¹² to 3.6 × 10¹³ cm^–3 is studied using a flow reactor with a movable insert and mass-spectrometric detection. It is found that, in this concentration range, the uptake of NO₃, unlike that of NO₂ and N₂O₅, occurs by the impact recombination mechanism. It is demonstrated that, before being deactivated, each active surface site destroys ~100 and ~150 NO₃ radicals at 297 K and 256 K, respectively. The uptake coefficient calculated per specific BET surface of the soot coating depends on the exposure time as γ(t) = γ₀exp(−t/τ), where γ₀ and τ are NO₃-concentration-dependent parameters. Based on the Langmuir concept of adsorption, the elementary parameters that control the uptake process are determined, such as the desorption rate constant k_d = ν_dexp(−Q_ad/RT) at ν_d = 3 × 10⁹ s^–1 and the heat of adsorption, Qad = 42.6 kJ mol^–1, as well as the rate constant of the bimolecular heterogeneous reaction of deactivation of active surface sites, k_r = A_rexp(–Ea/RT), with A_r = 1.1 × 10^–11 cm³ s^–1 and E_a = 9.5 kJ mol^–1.