Heterogeneous reactions of HNO3 with flame soot generated under different combustion conditions. Reaction mechanism and kinetics

Abstract

The reaction of HNO₃ with decane (C₁₀H₂₂) soot generated in the laboratory has been studied in a Knudsen flow reactor. Two different types of soot were produced: Soot originating from a rich flame of decane and air (“grey” soot) and soot generated from a leaner flame of decane and air (“black” soot). Both HNO₃ uptake and product release have been observed. Whereas soot from a rich decane flame leads to HONO as the main product, NO and small amounts of NO₂ are observed from soot generated in a lean flame. A reaction mechanism is proposed in which HNO₃ is reduced to HONO, which is released into the gas phase on grey soot, but undergoes decomposition reactions on black soot to NO and NO_2, the latter of which also reacts on black soot to produce additional NO. Experiments on black soot show that at [HNO₃] > 10¹¹ molecule cm⁻³ a higher order process is induced which is associated with an additional production of NO and NO₂. Further studies of the reaction of gaseous NO with adsorbed HNO₃ reveal a slow reaction leading to HONO and NO₂. Both initial uptake (γ₀) of HNO3 as well as steady state uptake probabilities (γ_ss) have been obtained: γ₀ = (4.6 ± 2.3) × 10⁻³ and γ_ss = (5.2 ± 1.3) × 10⁻⁴ for reaction on grey soot and γ₀ = (2.0 ± 0.1) × 10⁻² and γ_ss = (4.6 ± 1.6) × 10⁻³ for black soot, using the geometric surface area over which the samples are spread.