Chemical Kinetics of the Interaction of H₂O Vapor with Soot in the Range 190 K ≤ T ≤ 300 K:  A Diffusion Tube Study

The interaction of H 2 O vapor with flame soot has been investigated in the molecular flow regime with use of the molecular diffusion tube technique over a sizable temperature range. The primary real-time data consist of time-dependent mass spectrometric signals and enable the determination of the initial uptake coefficient γ 0 MC, the surface residence time τ s of adsorbed H 2 O, and the number n s of adsorption sites per square centimeter of soot substrate surface after applying a Monte Carlo trajectory model that accounts for surface saturation by the H 2 O pulse propagating across the tube. Typical values at 298 ± 2 K are γ 0 MC < 2 × 10 -3 and τ s < 5 ms for toluene, acetylene, and diesel soot whereas decane soot does not show any measurable interaction at 298 K. A detailed study of the interaction of H 2 O with well-characterized decane soot at lower temperature results in the following Arrhenius parameters for desorption of H 2 O from gray soot generated from a fuel-rich diffusion flame, log(1/τ s ) = (8.8 ± 0.5) − (7.0 ± 0.5)/ RT, and from black soot generated in a lean decane diffusion flame, log(1/τ s ) = (8.5 ± 0.5) − (9.0 ± 0.6)/ RT with R = 1.987 × 10 −3 kcal/(mol K). These expressions reveal τ s of 160 ms at 193 K and 400 ms at 243 K for gray and black decane soot, respectively. Spiking the fuel with thiophene (C 4 H 4 S) up to 500 ppm mass fraction (0.05%) does not lead to any change in the H 2 O adsorption behavior, and saturation experiments with H 2 O pulses reveal the limited number of H 2 O adsorption sites on soot accounting for a few percent of the surface carbon atoms. Some atmospheric implications are discussed.