Effect of Metal Oxides and Black Carbon (Soot) on SO₂/O₂/H₂O Reaction Systems

Abstract

Particulates of atmospheric interest, including soot, oxidized soot, and oxides of transition metals, “fellow travelers” in the combustion of solid or liquid fossil fuels, have been examined for their influence on the heterogeneous oxidation of S(IV) to S(VI), both with and without the presence of simulated solar radiation. Under all conditions, MnO₂ has been found to be the most active of the materials in this study. Effectiveness for the oxidation of SO₂ in the presence of oxygen and water vapor decreases in the order: MnO₂ « V₂O₅ > CuO > Fe₂O₃ > Al₂O₃ > Soot > TiO₂ ≈ SiO₂ (cab-o-sil) ≈ ZnO > ozonized soot ≈ blank. The oxidation of SO₂ also was carried out in the presence of soot-MnO₂, soot-V₂O₅, and soot-Fe₂O₃ mixtures. The respective sulfate yields were found to be significantly higher than those obtained under similar conditions for the individual substrates. Other combinations, such as MnO₂/V₂O₅ and MnO₂/Fe₂O₃, yielded sulfate at levels higher than those obtained by any of the transition metal oxides or other oxide mixtures used. The mixing ratio of 1:0.75 (wt/wt) in the MnO₂/V₂O₅ mixture was found to be optimum. With this mixture the sulfate yield was nearly twice the amount obtained if only MnO₂ was used and nearly four times the amount if V₂O₅ was used alone. Extensive analyses of the optimum mixture, to detect any unique identity, were inconclusive. The sulfate yield from the oxidation of SO₂ in the presence of a soot/MnO₂/V₂O₅ mixture was found to be higher than that obtained with any other combination in this study. The relative rates of depletion of SO₂ at part per million levels using MnO₂, MnO₂/soot, MnO₂/V₂O₅/soot, and soot were determined using a long optical path cell /Fourier transform infrared spectrometer (LOPC/FTIR). Oxidation pathways in the SO₂/MnO₂/H₂O/O₂ system have been proposed.