## 3 Results and Discussion
### CO\({}_{2}\) Adsorption Performance  
The adsorption equilibrium data of CO\({}_{2}\) on the adsorbents under a pressure of less than 120 kPa at different temperatures (25, 40, or 55 \({}^{\circ}\)C) are shown in Figure 6a-c. The equilibrium time for each measured point was held to 45 s after achieving the respective pressure value. The highest CO\({}_{2}\) uptake of 2.74 mmole/g was observed on aACF at 25 \({}^{\circ}\)C and 1 atm, which was higher than those of ACF (2.33 mmole/g) and nACF (1.61 mmole/g). As the adsorption temperature increased, the adsorbed amounts of CO\({}_{2}\) decreased. The temperature dependence of CO\({}_{2}\) adsorption capacity was consistent with the behavior of an exothermic reaction. Figure 5: The profiles of (**a**) thermogravimetric analysis (TGA) and (**b**) differential thermogravimetry (DTG) of the samples. The nitrogen content, though, was limited; aACF exhibited the highest percentage of imine groups and pyridonic groups. The presence of these groups might lead to its higher CO\({}_{2}\) capacities. This implied that the CO\({}_{2}\) adsorption capacities might be subject to not only the nitrogen content but also the type and the location of N-functionalities [6; 21; 34]. According to the results of Q\({}_{\text{st}}\) (Figure 6d), estimated from the adsorption data, the CO\({}_{2}\) adsorption on aACF still behaved as a physical process. The calculated values of Q\({}_{\text{st}}\) for all adsorbents were under 40 kJ/mol, indicative of physical adsorption in the low CO\({}_{2}\) uptake range of 0.2-1.0 mmol/g. The values of Q\({}_{\text{st}}\) decreased with increasing CO\({}_{2}\) uptake, implying CO\({}_{2}\) adsorption occurred on an energetically heterogeneous surface [35]. A linear relationship between Q\({}_{\text{st}}\) and the CO\({}_{2}\) uptake was observed if the logarithmic scale of the amount of CO\({}_{2}\) adsorbed was used. The value of \(Q_{\text{st}}\) followed the order aACF \(>\) ACF \(>\) nACF, which indicated that the micropores in the adsorbents could have a more important role than the nitrogen functionalities. The adsorption model has a significant effect on the characteristic of Q\({}_{\text{st}}\)[21]. According to previous experience, the Freundlich model was selected to curve-fit the experimental adsorption data, and Figure 6a-c also portrays the fitted curves. The fitted results are summarized in Table 2. The values of R\({}^{2}\) were higher than 0.9993 and the ARE ranged from 1.0 to 2.4, indicating that the Freundlich equation provided a satisfactory fit for the adsorbents. The values of parameter \(n\) were greater than 1.3, implying the CO\({}_{2}\) molecules had an affinity for the surface of the adsorbents. As seen from the data, the values of \(K_{\text{F}}\) and \(n\) varied inversely with adsorption temperature, but took on a similar tendency with the CO\({}_{2}\) uptake. Figure 6: Adsorption isotherms of CO\({}_{2}\) of the samples: (**a**) as-received ACF; (**b**) KOH-activated ACF (aACF); (**c**) TPEA-modified ACF (nACF) and (**d**) the isosteric heat of adsorption (Q\({}_{\text{st}}\)) of CO\({}_{2}\). Further, in examining the correlation of CO\({}_{2}\) uptake at a lower pressure (0.15 atm) and at 25 \({}^{\circ}\)C with the physical and chemical properties of the adsorbents, it was obvious that the CO\({}_{2}\) adsorption capacity was highly associated with the micropores, especially the ultramicropore volume (Figure 7a-b), which indicated the importance of ultramicropores in CO\({}_{2}\) adsorption at low pressures. It is noteworthy that the CO\({}_{2}\) uptake changed positively with the surface oxygen content, but negatively with the surface nitrogen content (Figure 7c). In further analysis, for the N-functionalities, the percentage of NO\({}_{2}\) groups was the only significant factor influencing the CO\({}_{2}\) adsorption capacities (Figure 7d). However, for the O-functionalities, except for the \(-\)COOH group, all other O-groups were highly associated with the CO\({}_{2}\) uptake.