Heterogeneous reactions of SO₂ on the hematite(0001) surface

Heterogeneous reactions at the surfaces of mineral dusts represent a key process in the formation of atmospheric aerosols. To quantify the rate of aerosol formation in climate modeling as well as combat hazardous aerosols, a deep understanding of the mechanisms of these reactions is essential. In the present work, density functional theory calculations, including a Hubbard-like +U correction, were employed to elucidate the reaction between SO₂ and the hematite(0001) surface. Three reaction conditions are considered: dry, wet, and aerobic. In the absence of water and oxygen, adsorption energies of SO₂ on the clean Fe-O₃-Fe-termination were found to be about -0.8 to -1.0 eV and resulted in the formation of an adsorbed SO₃-like species. The addition of water leads to surface hydroxylation and has little effect on promoting the SO₂ adsorption. Under such circumstances, an HSO₃-like species was formed with a smaller adsorption energy of about -0.5 eV. By contrast, the presence of molecular oxygen enhances the SO₂ adsorption significantly as the two species combine to form sulfate SO₄ ^2-, with adsorption energies of -1.31 to -1.64 eV. The calculated vibrational frequencies of the adsorbate species provide insight into the surface bonding and a useful spectral fingerprinting for experimental measurements. These results elucidate the atomistic mechanism of the reaction between SO₂ and hematite and highlight the important role of atmospheric O₂ in the formation of sulfates.