The physisorption mechanism of SO2 on graphitized carbon

Abstract

Sulfur dioxide (SO₂) in flue gases emitted from fossil fuel power plants dramatically reduces the CO₂ capture efficiency via adsorption, which is due to the potential reaction of SO₂ with basic functional groups on the adsorbent. Physisorption rather than chemisorption is preferred, because adsorbents can be more easily regenerated by either reducing the pressure or increasing the temperature. Carbon is a suitable adsorbent for SO₂ capture and widely used, and therefore it is important to study SO₂ adsorption onto carbon with the Monte Carlo simulation to provide microscopic details to demarcate the roles of the basal plane of the graphene layer and the functional groups in adsorption. SO₂ is a polar molecule like water, as they both carry partial charges, but they interact differently with functional groups. Instead of 3D-clusters in the case of water, SO₂ is localized around the functional groups and spreads over the basal plane to form 2D-molecular layers because of the strong dispersive interactions with graphite. The results indicate that the functional group has a negligible effect on the enhancement of adsorption and its role is to localize 2D-clusters of SO₂ molecules. For non-graphitized carbon, we have found that the greater loadings at low pressure compared to the highly graphitized carbon is due to the presence of defects (crevices) on the basal plane surface. Finally, to describe better the experimental data, we have found that the reduction in the interactions between adsorbed molecules in the first layer is because of the repulsion of their dipoles pointing normal to the surface, a phenomenon called surface mediation and is widely used in the description of gas adsorption on surfaces.