Modulation of CO2 adsorption thermodynamics and selectivity in alkali-carbonate activated N-rich porous carbons

Abstract

Here we analyze how changes in the charge density of activating alkali cations (lithium, sodium, and potassium) alters the synthesis and resulting physicochemical properties of N-rich activated carbons. In general, the synthesis reagents had significant influence on the total nitrogen content (2–24 at%), the chemical environment of the nitrogen species, the specific surface area (∼600–4300 m² g⁻¹), and the types of pores that formed in the activated materials. Each sample was screened for carbon dioxide (CO₂) and nitrogen (N₂) gas adsorption. From application of the ideal adsorbed solution theory, the predicted CO₂/N₂ selectivity spanned a large range from ∼8 to ∼150 at 15% CO₂ and was dependent on d-spacing, surface N content, and porosity. Finally, the materials were analyzed with a simplified temperature swing adsorption model to estimate the optimal working capacity and regeneration energy of the materials in a cyclic process. Overall, this study demonstrates that while the precursor nitrogen content drives significant changes in the isotherm shape, a careful choice of activating cation during synthesis of advanced porous carbons can strongly influence physicochemical properties and the resulting thermodynamics and selectivity of CO₂ adsorption.