# Zinc-Decorated and Nitrogen-Functionalized Hierarchical Porous Carbons for Carbon Dioxide Capture
## 1 Introduction
Hence, ZIF-8-derived carbons (ZDCs) have been investigated for use in adsorption [2] and electrochemistry applications [30]. Although the high-temperature pyrolysis of ZIF-8 can improve its porosity, it is accompanied by significant decreases in yield and nitrogen and zinc contents [2]. The pore size of ZDCs is determined by the size of the ZIF-8 polyhedrons [30]. It has been reported that a hierarchical pore structure is beneficial to high exposure of active sites and rapid mass transfer [31]. Porous nanocarbons obtained through the direct carbonization of ZIF-8 crystals have been reported to have large adsorption uptakes and fast sensor responses for toluene vapors [16]. The nitrogen functional groups within imidazole ligands play a significant role in the application of ZIF-8 [32]. It was found that as the treatment temperature increased, the nitrogen content of the samples gradually decreased [30]. Compared with the bond dissociation enthalpy of C=C and C=N bonds [33; 34], the breakage of C-N bonds occurs more easily. On the other hand, the methyl groups at the terminal end of the imidazole ligand are exposed. Therefore, once ZIF-8 is subject to heat treatment, the cleavage of methyl group from the imidazole ligand and the decomposition of C-N bond would probably take place first [32]. Nevertheless, the overall framework structure of carbonized ZIF-8 can be retained [33]. A systematic study [2] found that the decomposition of imidazole ligands and the evaporation of metal zinc were highly dependent on the carbonization temperature, reaction time, as well as heating rate. Once the processing temperature exceeds 1073 K, the formation of pores is accompanied by the loss of Zn and N atoms and a decrease in yield. The interactions between CO\({}_{2}\) molecules and ZIF-8 or ZDCs have significant effects on their adsorption of CO\({}_{2}\). The pyrrrolic N was found to be highly related to the CO\({}_{2}\) adsorption capacity and the interaction mechanisms between N functional groups and the CO\({}_{2}\) molecules, which could involve acid-base and hydrogen-bonding interactions [6]. ZIF-8 carbonized at 1273 K exhibits an outstanding adsorption capacity for methylene blue compared to those carbonized at 873 and 1473 K as well as ZIF-8 due to the change in the surface charge and pore size distribution [35]. The presence of N dopant on a graphite surface can enhance the chemisorption of O adsorbates due to an enhanced charge-transfer mechanism [36]. Studies have found that under atmospheric pressure, the adsorption of CO\({}_{2}\) on ZIF polyhedrons was restricted [1] and was only possible upon dehydration of the atmospheric gas [37]. Although several studies have tried to use ZIF-8 at high pressures [38; 39; 40], this approach is not economically viable. In this study, ZIF-8 polyhedrons were prepared using a simple and rapid colloidal chemistry route. Subsequently, a facile thermal treatment was utilized to prepare ZDCs. The thermodynamic parameters and isosteric heat of adsorption of CO\({}_{2}\), and the adsorption selectivity of the ZDCs for CO\({}_{2}\) over N\({}_{2}\) were evaluated. Moreover, the major factors influencing the CO\({}_{2}\) adsorption capacity under 101.3 kPa were identified. The N\({}_{2}\) adsorption isotherm at 77 K combined with the CO\({}_{2}\) adsorption isotherms at 273 K were used to determine the pore size distributions (PSDs) of the samples. The results showed that this benefits to confirm the existence of ultramicroores on the nanocarbons. The rhombic dodecahedron morphologies were well retained after ZIF-8 was subjected to the heat treatment. This study demonstrated that Zn-decorated and nitrogen-doped hierarchical porous carbons prepared via a facile synthesis route exhibited good CO\({}_{2}\) adsorption capacities under 101.3 kPa and at room temperature. In the literature, there is little discussion on the role of Zn and the changes in Zn functional groups after the thermal treatment of ZIF-8, but this was investigated in this study.