Molecular simulation of the adsorption and diffusion of CO2, CH4, and N2 in alkali metal-doped low/medium-rank coal

Abstract

Coal-based adsorbents show promise for advancing Direct Air Capture (DAC) technology as support for carbon capture grows. Doping coal with alkali metals (Li, Na, K) enhances gas molecule interactions and improves adsorption capacity. Using Density Functional Theory (DFT) and Grand Canonical Monte Carlo (GCMC), the study investigated the adsorption and diffusion performances of CO₂, CH₄, and N₂ in modified coal. The results showed that binding energy increases with the increasing radius of the dopant atoms, with K-SM requiring the most energy (31.236 eV). The adsorption capacity followed the order: Na-coal > Li-coal > K-coal > Undoped-coal, with Na-coal achieving the highest CO₂ adsorption (5.934 mmol/g). After alkali metal doping, the adsorption amounts of CO₂, CH₄, and N₂ increased by 26.80 %, 43.29 %, and 56.60 %, respectively, with the Na-doped middle-rank coal showing an average increase of 32.33 % in CO₂ adsorption. The diffusion coefficient of Na-coal was lower than that of Li-coal and K-coal, with CO₂ having the smallest diffusion coefficient (1.627 × 10^-8 m²/s). Overall, alkali metal doping significantly enhances the adsorption and stability for CO₂, CH₄, and N₂, particularly with Na doping in middle-rank coal, providing insights for developing high-performance CO₂ adsorbent.