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

Open Access
Publisher Website
Google Scholar
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.