Synergistic formation of sulfate and ammonium resulting from reaction between SO2 and NH3 on typical mineral dust

Abstract

The heterogeneous reactions of SO₂ and NH₃ on typical mineral oxides were investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). A new sulfate formation pathway was proposed where NH₃ accelerated the formation of sulfate species. The results revealed that surface hydroxyls and oxygen played principal roles in the conversion of SO₂ to sulfate. It was proposed that NH₃ adsorbed onto Lewis acid sites, and hydroxyls and water molecules adsorbed on the surfaces of mineral dust. The enhancement of surface Lewis basicity by NH₃ induced more SO₂ molecules to adsorb on the surface, which were further oxidized to sulfate by interacting with surface hydroxyls and oxygen atoms. The formation of sulfate, in turn, contributed to the adsorption of NH₃, mainly as NH₄⁺ due to enhanced Brønsted acid sites. The IC results showed that the synergistic effect between SO₂ and NH₃ was more significant on acidic oxides like γ-Al₂O₃ and α-Fe₂O₃ compared to basic oxides like MgO.