Adsorption of sulfur dioxide on hematite and goethite particle surfaces

Abstract

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 (SO₃ ²⁻), although evidence for adsorbed sulfate (SO₄ ²⁻) 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 SO₄ ²⁻ with no detectable SO₃ ²⁻. 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 SO₄ ²⁻ in the presence of molecular oxygen, however, the predominant surface species remained SO₃ ²⁻ 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.