# Zinc-Decorated and Nitrogen-Functionalized Hierarchical Porous Carbons for Carbon Dioxide Capture
## 3 Results and Discussion
### CO\({}_{2}\) Adsorption
Therefore, the ability of adsorbents to separate gas molecules that are similar in size is highly dependent on their properties. Z8-650 and Z8-850 were utilized as examples to investigate the adsorption selectivity at 298, 313, and 328 K (Figure 11a). The adsorption selectivity decreased with increasing partial pressure of CO\({}_{2}\) or adsorption temperature. The adsorption selectivity of Z8-650 was higher than that of Z8-850, which can be ascribed to the higher Qst of Z8-650. It is noteworthy that the adsorption selectivity for CO\({}_{2}\) over N\({}_{2}\) of Z8-650 at 298, 313, and 328 K all intersected at a CO\({}_{2}\) partial pressure of 0.5 atm. This implied that at a partial pressure of CO\({}_{2}\) over 0.5 atm, if the interactions between CO\({}_{2}\) and the adsorbents are stronger, the adsorption selectivity for CO\({}_{2}\) over N\({}_{2}\) is higher at higher adsorption temperatures. Simulating the flue gas conditions under atmospheric pressure (15% CO\({}_{2}\) to 85% N\({}_{2}\) mixture) at 298, 313, and 328 K, the adsorption selectivity for CO\({}_{2}\) over N\({}_{2}\) was found to be 25.9, 20.0, and 19.2 for Z8-650 and 14.5, 12.2, and 11.7 for Z8-850, respectively (Figure 11b). The higher adsorption selectivity of Z8-650 for CO\({}_{2}\) over N\({}_{2}\) may be due to its higher V\({}_{<0.7\rm{nm}}\)/V\({}_{\rm{mi}}\) ratio and nitrogen and zinc contents. Under atmospheric pressure, all the carbonized ZIF-8 samples had superior CO\({}_{2}\) adsorption capacities compared to the as-prepared ZIF-8 polyhedrons. Under a lower pressure (e.g., 15 kPa), Z8-750 displayed the highest CO\({}_{2}\) adsorption capacity, while the CO\({}_{2}\) adsorption on Z8-650 was also unneglectable. These findings demonstrate that the specific surface area and total pore volume of nanomaterials may not be the most crucial factors for CO\({}_{2}\) adsorption. All the interactions between hierarchical porous structures and surface heteroatom functional groups determine the CO\({}_{2}\) adsorption performance. Figure 12 shows the relationships between CO\({}_{2}\) adsorption capacity and some key parameters of the ZDC samples. At 15 kPa (Figure 12a-c), besides the \(S_{\rm{mi}}\)/SSA ratio, C-C/C=C (%), C=O/O=C-N (%), and O=C-NH (at. %) also enhanced CO\({}_{2}\) adsorption at 298 K. At 313 K, the main parameters were the \(V_{<0.7\rm{nm}}\)/\(V_{\rm{t}}\) ratio, C=O (at. %), and COOH (%), while at 328 K, the \(S_{\rm mi}\)/SSA ratio, O=C-NH (at. %), C-C/C=C (%), C=O (%), and C=O/O=C-N (%) were the primary factors. At 101.3 kPa (Figure 12d-f), CO\({}_{2}\) adsorption at 298 K was highly correlated with the \(V_{\rm mi}/V_{\rm t}\) and \(V_{\rm<0.7 nm}\) to \(V_{\rm t}\) ratios, as well as the \(S_{\rm mi}\) and SSA. At 313 K, besides the aforementioned parameters (\(V_{\rm mi}/V_{\rm t}\) ratio, \(S_{\rm mi}\) and SSA), \(V_{\rm<0.7 nm}\) also affect the adsorption of CO\({}_{2}\). However, the \(V_{\rm<0.7 nm}\) to \(V_{\rm mi}\) ratio exhibited an inverse relationship, which implied that a higher volume ratio of the pores (between 0.7 and 2 nm) could facilitate CO\({}_{2}\) adsorption at 313 K. The same effect of the \(V_{\rm<0.7 nm}/V_{\rm mi}\) ratio was observed at 328 K; the \(S_{\rm mi}\) and the \(V_{\rm mi}/V_{\rm t}\) ratio also contributed to adsorption at 328 K. At 15 kPa, the micropore area ratio and surface functional groups primarily determined the CO\({}_{2}\) adsorption capacity. In addition, an appropriate metal Zn to Zn\({}^{2+}\) ratio may have had a positive effect on CO\({}_{2}\) adsorption. On the other hand, the ultramicropore volume ratio, micropore area, and SSA played more significant roles at 101.3 kPa. Table 3 summarizes the CO\({}_{2}\) adsorption capacities of ZIF-8-related nanomaterials from the literature and shows that the Z8-850 and Z8-750 materials developed in this study are promising CO\({}_{2}\) adsorbents.