Enhanced heterogenous hydration of SO₂ through immobilization of pyridinic-N on carbon materials

Carbon materials doped with nitrogen have long been used for SO₂ removal from flue gases for the benefits of the environment. The role of water is generally regarded as hydration of SO₃ which is formed through the oxidization of SO₂. However, the hydration of SO₂, especially on the surface of N-doped carbon materials, was almost ignored. In this study, the hydration of SO₂ was investigated in detail on the pyridinic nitrogen (PyN)-doped graphene (GP) surfaces. It is found that, compared with the homogeneous hydration of SO₂ assisted with NH₃ in gas phase, the heterogeneous hydration is much more thermodynamically and kinetically favourable. Specifically, when a single H₂O molecule is involved, the energy barrier for SO₂ hydration is as low as 0.15 eV, with 0.59 eV released, indicating the hydration of SO₂ can occur at rather low water concentration and temperature. Thermodynamic integration molecular dynamics results show the feasibility of the hydrogenated substrate recovery and the immobilized N acting as a catalytic site for SO₂ hydration. Our findings show that the heterogeneous hydration of SO₂ should be universal and potentially uncover the puzzling reaction mechanism for SO₂ catalytic oxidation at low temperature by N-doped carbon materials.