Specific alkali metal sites as CO2 traps in activated carbon with different pore size for CO2 selective adsorption: GCMC and DFT simulations

Abstract

Activated carbon is a kind of promising adsorbent for post-combustion CO2 adsorption and separation. However, due to the physical adsorption properties, the use of the reported activated carbon for CO2 adsorption capacity and CO2/N2 selectivity presents a huge challenge without deep cooling of flue gas to room temperature or even lower temperature and low CO2 partial pressure. To solve the above problem, the CO2 uptake in practical flue gas on M−doped (M = O, N, S and alkali metal) graphite surfaces (GS) with various pore widths (0.35–3 nm) were studied by density functional theory (DFT) and grand canonical Monte Carlo simulation (GCMC). The results show that the doping of oxygen, nitrogen, sulfur groups into graphite surface have no significant increase in CO2 uptake and CO2/N2 selectivity, while the doping of alkali metal into graphite surface can significantly improve the CO2 uptake and CO2/N2 selectivity in pores of less than 0.6 nm. Especially GS-Na shows the highest CO2 uptake and CO2/N2 selectivity, up to 20.7 mmol mL−1 (0.35 nm) and 1040 (0.4 nm), respectively. The above findings emphasize the importance of the combination of pore widths less than 0.6 nm and alkali metal groups in determining the CO2 uptake. By studying the influence of pore size and functional groups on CO2 adsorption, the internal enhancement mechanisms, including CO2 pore-filling adsorption and electrostatic interaction, are clarified. Meantime, the mechanism of functional groups and pore widths synergistically increasing the CO2 uptake was revealed. These results would provide theoretical guidance for the design and preparation of carbon-based adsorbents for CO2 uptake.