The increase of greenhouse gases (GHG), especially carbon dioxide (CO2, whose concentration in the air has reached an alarming 400 ppm volume [1]) has a serious impact on the global economy and ecosystem, such as sea-level rise, drought, which make the capture and storage of CO2 imperative. In recent years, governments are also aim to have CO2 emissions peak and achieve carbon neutrality. CO2 is mainly contributed by the flue gas from steel plants, power plants and petrochemical industries burning carbon-based fossil fuels. The flue gas contains mainly N2 and CO2, of which CO2 accounts for about 12–15% [2]. Although post-combustion capture technology (capturing CO2 from flue gas) is widely used, the conventional chemical absorption technology based on amine solution (such as mono-ethanolamine) requires 25–40% of the energy of power plant, and the consumption of regeneration process accounts for more than 60% of the total energy demand [3]. Therefore, new CO2 capture processes need to be not only technically feasible, but also cost-effective.
Among the technologies such as absorption, membrane separation, low-temperature distillation and adsorption, adsorption based on porous materials is of great interest because of its efficiency and economy. Typical adsorbents include Metal-Organic Framework (MOF) materials, zeolites, clays, and activated carbon (AC) [4–7], among which AC is competitive due to its large specific surface area, easy modification, and adjustable pore size (such as the mesoporous carbon synthesized by template method) [8, 9]. The adsorption and separation experiments of AC for CO2/N2 have been widely carried out. For examples, Shen et al. [10] demonstrated that bitumen-based AC beads have significant adsorption for CO2 than N2 and the latter diffuses faster at the same temperature. Similarly, Yi et al. [11] reported that microwave assisted AC has a high separation factor for CO2/N2.
However, due to the technical challenges of experimental measurements of multicomponent adsorption in AC [12], the selectivity of CO2 is usually obtained by the ideal adsorption solution theory (IAST), but the method has its own limitations [13]. What’s more, because of the complex internal structure of AC, the high adsorption capacity and selectivity of CO2 in the experiment are attributed to many factors [11]. It is difficult to investigate the effects of AC properties such as pore structure, surface curvature and surface chemistry, especially the first two, because they are difficult to be controlled and verified by experiments. It is here that molecular simulation is introduced to study the effects of various properties of AC on CO2/N2 adsorption and separation by explicitly considering the interaction between gas molecules and pore wall.
Generally, AC is established as slit pore structure in simulation, but experiments show that the pores are generally composed of curved surfaces or non-parallel graphene planes [14], which means that it is insufficient to study adsorption only through the classical slit model. Therefore, the wedge model [15] or disordered model [16] further improved on the slit model and the nanotube model with cylindrical structure [17] are of great significance to study the adsorption and separation of gases on AC. With the same idea, previous studies by Liu et al. [12] and Kumar et al. [18] showed that the pore structure of AC had a significant impact on the adsorption and separation of mixing gases. Specifically, the fluid-solid potential energy plays a decisive role in the selectivity of mixed gases. In the disordered structure, the nanotube structure and the foam like structure, nanotube structure with strong energy effect has the best separation performance for CH4/CO2 and CH4/H2. Their findings are very instructive, but the wedge pores are not considered, and other factors such as pore size are not constrained.
In addition, the evidences from TEM [19] and simulation [20] show that there are five membered rings and seven membered rings in non-graphitized porous carbon, and their existence leads to the generation of surface curvature. It is proved by ab initio quantum mechanics (QM) that the surface curvature will produce stronger binding energy for gas. Nguyen et al. [21] and Di et al. [22] obtained similar conclusions, and the AC with non-six membered rings has better adsorption capacity for CO2. The result is caused by the special hybridization of carbon between sp2 and sp3 [23], in other words, the AC with non-six membered ring produces stronger energetically adsorption sites than graphite. However, previous studies have often used C60 fullerenes and carbon nanotubes to explore the influence of surface curvature [24, 25], but it is not applicable to the disordered structure, because the surface curvature in this structure is caused by non-six membered rings [26]. In addition, researchers pay more attention to the change of adsorption energy and ignore its overall impact on adsorption and separation [24].
In this paper, it aims to study the effects of four representative structures of AC (slit pores, wedge pores, nanotube pores and disordered pores) and surface curvature on CO2/N2 adsorption and separation from a micro perspective. The changes of adsorption density, interaction energy and CO2 selectivity were discussed in detail by GCMC simulation.