Heterogeneous Formation of Sulfur Species on Manganese Oxides: Effects of Particle Type and Moisture Condition

The heterogeneous reaction of SO2 on manganese oxides is poorly understood. By means of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), this study investigated the reaction kinetics and product fractions on four types of manganese oxides. Due to the positive and negative moisture impacts, 68% RH (relative humidity) becomes the most favorable condition for the uptake of SO2. Mn3O4 shows the greatest uptake capacity of SO2, followed by MnO2 and Mn2O3, with that of MnO being the weakest. Curve-fitting procedures were undertaken to further dissect the product spectra. Increased RH facilitates the physical adsorption of SO2 and generally weakens the oxidation capacity of manganese oxides. The oxidation ability is greatest for MnO, followed by MnO2 and Mn3O4, with that of Mn2O3 being the weakest. Additionally, the particle acidity (pH) was estimated by the ionization equilibrium of sulfurous acid and the relative contributions of S(IV) species. MnO and Mn3O4 become more acidic as RH increases while Mn2O3 and MnO2 are the most acidic at 50% RH, which can be attributed to the different uptake capacities of SO2 and H2O on each sample. Overall, for the heterogeneous reaction of SO2 on manganese oxides, both particle type and moisture condition influence the reaction kinetics and product fractions. This work improves the understanding of the heterogeneous process on atmospheric manganese-rich particles.