Adsorption of sulfur dioxide on hematite and goethite particle surfaces

The adsorption of sulfur dioxide (SO2) on iron oxide particle surfaces at 296 K has been investigated using X-ray photoelectron spectroscopy (XPS). A custom-designed XPS ultra-high vacuum chamber was coupled to an environmental reaction chamber so that the effects of adsorbed water and molecular oxygen on the reaction of SO2 with iron oxide surfaces could be followed at atmospherically relevant pressures. In the absence of H2O and O2, exposure of hematite (α-Fe2O3) and goethite (α-FeOOH) to SO2 resulted predominantly in the formation of adsorbed sulfite (SO32−), although evidence for adsorbed sulfate (SO42−) was also found. At saturation, the coverage of adsorbed sulfur species was the same on both α-Fe2O3 and α-FeOOH as determined from the S2p : Fe2p ratio. Equivalent saturation coverages and product ratios of sulfite to sulfate were observed on these oxide surfaces in the presence of water vapor at pressures between 6 and 18 Torr, corresponding to 28 to 85% relative humidity (RH), suggesting that water had no effect on the adsorption of SO2. In contrast, molecular oxygen substantially influenced the interactions of SO2 with iron oxide surfaces, albeit to a much larger extent on α-Fe2O3 relative to α-FeOOH. For α-Fe2O3, adsorption of SO2 in the presence of molecular oxygen resulted in the quantitative formation of SO42− with no detectable SO32−. Furthermore, molecular oxygen significantly enhanced the extent of SO2 uptake on α-Fe2O3, as indicated by the greater than two-fold increase in the S2p : Fe2p ratio. Although SO2 uptake is still enhanced on α-Fe2O3 in the presence of molecular oxygen and water, the enhancement factor decreases with increasing RH. In the case of α-FeOOH, there is an increase in the amount of SO42− in the presence of molecular oxygen, however, the predominant surface species remained SO32− and there is no enhancement in SO2 uptake as measured by the S2p : Fe2p ratio. A mechanism involving molecular oxygen activation on oxygen vacancy sites is proposed as a possible explanation for the non-photochemical oxidation of sulfur dioxide on iron oxide surfaces. The concentration of these sites depends on the exact environmental conditions of RH.