The adsorption and oxidation of SO2 on MgO surface: experimental and DFT calculation studies

Abstract

The heterogeneous oxidation of sulfur dioxide (SO₂) to sulfate on the surface of MgO particles was investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT). The in situ DRIFTS spectra show that the major products of SO₂ adsorption on MgO particles are sulfite, bisulfite and sulfate, while the coexisting NO₂ and O₂ promote the conversion of sulfite/bisulfite into sulfate. DFT calculations show that the main adsorption products of SO₂ on the perfect MgO (001) surface, the hydroxylated MgO (001) surface and the step sites of the MgO (001) surface are sulfite (SO_2,gas + O_lattice → SO_3,ads), bisulfite and sulfate, respectively. The oxidation of sulfite to sulfate was hardly promoted by the presence of O₂ at room temperature, because the process needs to overcome an energy barrier of 0.67 eV. The existence of NO₂ couldn't promote the formation of sulfate, because the direct oxidation of sulfite into sulfate by NO₂ is difficult (ΔE_a = 1.08 eV). However, coexisting NO₂ with O₂ can facilitate the oxidation of SO₂, which illustrates the micro-mechanism of the synergistic effect between SO₂ and NO₂ on MgO. In contrast to sulfite, surface bisulfite formed through the adsorption of SO₂ on surface OH could be oxidized by O₂ or NO₂via barrierless processes. This mechanism highlights the synergistic effects of O₂/NO₂ as well as surface hydroxyl on the heterogeneous oxidation of SO₂ on the MgO surface. Atmospheric implications of the heterogeneous oxidation of SO₂ on MgO under ambient atmosphere are discussed.