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Rational Design of Carbon-Based Porous Aerogels with Nitrogen Defects and Dedicated Interfacial Structures toward Highly Efficient CO2 Greenhouse Gas Capture and Separation

CO2 capture from flowing flue gases through adsorption technology is essential to reduce the emission of CO2 to the atmosphere. The rational design of highly efficient carbon-based absorbents with interfacial structures containing interconnected porous structures and abundant adsorption sites might be one of the promising strategies. Here, we report the synthesis of nitrogen-doped carbon aerogels (NCAs) via prepolymerized phenol–melamine–formaldehyde organic aerogels (PMF) by controlling the addition amount of ZnCl2 and the precursor M/P ratio. It has been revealed that NCAs with a higher specific surface area and interconnected porous structures contain a large amount of pyridinic nitrogen and pyrrolic nitrogen. These would act as the intrinsic adsorption sites for highly effective CO2 capture and further improve the CO2/N2 separation efficiencies. Among the prepared samples, NCA-1-2 with a high micropore surface area and high nitrogen content exhibits a high CO2 adsorption capacity (4.30 mmol g–1 at 0 °C and 1 bar) and CO2/N2 selectivity (36.5 at 25 °C, IAST). Under typical flue gas conditions (25 °C and 1.01 bar), equilibrium gas adsorption analysis and dynamic breakthrough measurement associated with a high adsorption capacity of 2.65 mmol g–1 at 25 °C and 1.01 bar and 0.81 mmol g–1 at 25 °C and 0.15 bar. This rationally designed N-doped carbon aerogel with specific interfacial structures and high CO2 adsorption capacity, high selectivity, and adsorption performance remained pretty stable after multiple uses.


INTRODUCTION
It has been well recognized that human emissions of carbon dioxide and other greenhouse gases are a primary driver of climate change, which is one of the world's most critical challenges. Hence, seeking a highly efficient fixing and capturing strategy of CO 2 using adsorption and conversions under mild conditions seems to be the most attractive option. 1−10 Generally, this requires the adsorbent to have marvelous adsorption kinetics, high adsorption capacity, good selectivity, and long-term stability under actual working conditions. 11−14 Currently, rationally designing carbon-based absorbents with interfacial structures, abundant activation sites, and specific adsorptions characteristics has attracted enormous scientific attention.
For the structure-tailing of carbon-based materials, scientists have developed several fabrication strategies to tune the crossing carbon frameworks and porous and interfacial structures with the aim of generating a bunch of activation and adsorptions sites toward efficient capturing of carbon dioxide. 15,16 Specifically, as one member of carbon-based porous materials, the carbon aerogels could be tuned into specific structures for a wide range of applications including adsorption, catalysis, and energy storage and conversions. 17 −26 They have been proven to be one of the most promising carbon-based adsorbents with outstanding performance, including superior adsorption characteristics. It is inferred from previous studies that the specific structural features of carbon aerogels are primarily attributed to their unique interfacial and hierarchical structures as derived from the cracking organic aerogel, which started from a liquid solution and culminated in the formation of a nanostructured material composed of a single piece. The creation and sequential formation of hierarchical pore structures usually depend on the growth and aggregated reaction of cross-linked polymer clusters by the appropriate monomeric precursor. 27,28 Scientists have devoted numerous efforts to modulating the aforementioned hierarchical pore structures via activations and sequential carbonization processes. Antonietti and co-workers reported a hypersaline template method for synthesizing phenol−formaldehyde resins based on an organic aerogel with an interconnected macroscopic structure and preparing carbon aerogels materials by using the organic aerogel as a precursor. 29 This method avoids the complicated preparation process of traditional carbon aerogels and regulates the porous structure by using ZnCl 2 as an activation agent. 29 The resultant carbon aerogels show promising potential in the adsorption/ separation of organic pollutants.
Other potential strategies, such as adulterating heteroatoms into the carbon matrix, also aroused great interest due to their features of increasing the number of adsorption sites, raising gas storage capacity, and eventually improving adsorption/ separation performance. Generally, while the characteristics of CO 2 molecules are considered, the critical interaction between porous carbon-based absorbents and the uptake of CO 2 molecules could be significantly enhanced by rationally constructing CO 2 -philic sites. 30,31 The introduction of nitrogen into carbon materials as the active base center has been well proven to increase the weak chemical interaction between weakly acidic CO 2 molecules and the carbon-based material. Hence, the adsorption capacity and selectivity of CO 2 can be accordingly improved remarkably. Extensive literature has been reported regarding the design and synthesis of nitrogendisturbed carbon-based materials by employing dedicated nitrogen-containing precursors containing biomass, waste papers, nitrogen-containing compounds, etc. 32−35 Most of these precursors only serve as nitrogen resources and may incur various nitrogen defects distributed within the surface and bulk of carbon materials. 36,37 It is inferred from previous studies that to reduce the structural shrinkage and nitrogen loss during pyrolysis and form a hierarchical porous structure with a high nitrogen residue, the key is to design nitrogencontaining polymers with rigid backbones. 38 The high nitrogen content usually can result in the formation of melamine species that possess the advantage of being the most competitive choice for carbon materials. Furthermore, melamine can accordingly generate melamine−formaldehyde resins with formaldehyde sections, and its synthetic methodology is very similar to that of phenol−formaldehyde resins. Interestingly, numerous investigations indicated that a proper combination strategy of cross-linking the phenol−formaldehyde resins and melamine−formaldehyde resins followed by producing 3D cross-linking networks structurally similar to organic polymers provides nearly perfect rigidity for the entire structure of the carbon materials. 39,40 Furthermore, the regulation of pores is also significant for tuning the amazing structures toward carbon dioxide adsorptions, which could be generally realized by controlling activation conditions, e.g., the activator amount, activation time, and working temperatures. 41,42 Therefore, it would be feasible to balance the adsorption capacity and selectivity toward CO 2 capture under working dynamic flow conditions by alternatively adjusting and tailoring the surficial and interfacial structures including nitrogen defects of the porous carbon materials.
Herein, a series of organogels homogeneously mixed with ZnCl 2 were prepared by using low-cost phenol, melamine, and formaldehyde as precursors and by controlling the amount of ZnCl 2 added and the M/P ratio of the precursor. The homogeneously dispersed ZnCl 2 in the organic aerogels can be used as a foaming agent and etching agent directly together with PMF to prepare nitrogen-doped carbon aerogels (NCAs) through the carbonization activation process. The effects of precursor component molecular proportions on the structure, morphology, physicochemical properties, and features of the nitrogen defects within carbon-based materials were accordingly investigated and summarized in terms of the research outcomes. The as-prepared porous carbon aerogel with high specific surface area, micropore volume (in particular, ultramicropore volume), and interfacial structures containing anionic nitrogen defects showed fascinating CO 2 adsorption capacity in various working conditions, including static adsorptions, dynamic adsorptions, and gas-mixture separations. It is believed that the obtained novel N-disturbed carbon aerogel (NCA) with plentiful surficial−interfacial structures and specific physical−chemical features including hierarchical porosity and rich adsorption site would find dedicated and promising utilization toward the highly efficient capture of CO 2 greenhouse gas species.

Chemicals.
The materials were used as received without further purification. Deionized water was used in all the experiments. All chemicals and solvents were purchased from Shanghai Chemical Reagent Co., Ltd.

Synthesis of Organic Aerogels.
The general synthesis process of organic aerogels (OAs) has been accordingly summarized as follows: Phenolic formaldehyde (PF) resins were obtained by the reaction of P and F. P is reactive at the ortho sites, allowing F to attach to the ring. In the process of synthesis, P and F first generate a monomer with a large number of hydroxymethyl groups through an addition reaction, then dehydrate and condense at a high temperature to form methylene and methyl ether bridges, and finally form a cross-linked polymer network. 37 The melamine−formaldehyde (MF) resin is also a common polymer that has been industrially produced, and its synthesis methodology is very similar to PF. ZnCl 2 was added to PF + MF solutions as a Lewis acid catalyst, and a white sticky sol was synthesized ( Figure S1a). To study the effect of M addition on the polymerization of organic aerogels and the nitrogen content in carbon aerogels, four different M/P molar ratios (PF/MF = 4:1, 2:1, 1:1, and 1:2) were selected. The molar ratios of F/P and F/M were fixed at 1:1 and 3:1, respectively, in all the cases. Solution A, which contained P and F, was stirred for 2 h at 40°C . Separately, solution B, which included M and F, was stirred for 10 min at 80°C. After that, solutions A and B were mixed and stirred at 40°C for 1 h. Then, 2 mL of the mixed solution (A + B) was obtained and slowly dropped into a glass vial that contained 8 g of ZnCl 2 . To avoid drastic heat release, ZnCl 2 was dissolved in an ice bath. The glass vials that contained the white sticky sol were sealed in a 50 mL Teflon-lined autoclave and heated at 160°C for 8 h. Then, the black monolith PMF-X (X = 4-1, 2-1, 1-1, 1-2) OAs were prepared (Figure S1b−d).

Synthesis of NCAs.
The obtained PMF-X (X = 4-1, 2-1, 1-1, 1-2) was heated under a flowing nitrogen atmosphere at 900°C without any drying process (heating rate: 3°C min −1 ) for 2 h. Next, the products were purified by HCl (1 M) with a magnetic stirrer for 12 h, followed by multiple rinsing with deionized water, and dried at 110°C overnight. The asresulted samples were denoted as NCAs (NCA-X). The synthesis of CA is similar to that of NCA-X, without adding solution B to the polymeric precursor solution. The synthesis strategy of NCA 0 -2-1 and NCA 4 -2-1 has been kept the same as that of NCA-2-1 (NCA 8 -2-1), in which specific amounts of ZnCl 2 (0, 4, and 8 g) were accordingly into the reactions systems under vigorous stirring followed by the process of formation.

Characterizations.
The morphologies of the PMF and NCA samples were analyzed using a field-emission scanning electron microscope (Nova Nano SEM450, USA). Transmission electron microscopy (TEM) was performed using a Tecnai F 20 instrument operated at 200 kV. The crystalline structure was analyzed via X-ray diffraction (XRD) using a diffractometer with Cu Kα radiation (D/max-2400, Japan), with the source light at a wavelength (λ) of 0.1541 nm. Elemental analyses (EAs) were evaluated with a Vario EL III elemental analyzer (Germany). The surface chemical compositions and functional groups of materials were further investigated via X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250XI, USA) using Al Kα radiation as the X-ray source. The binding energy of elements was calibrated using a C1s photoelectron peak at 284.6 eV. A Raman microscope (Thermo Fisher, USA) and a Fourier transform infrared (FTIR) spectroscope (VERTEX 70-FTIR, Bruker, Germany) were used to survey the surfaces of the PMF and NCA samples. Thermogravimetric (TG) analysis and differential scanning calorimetry were performed using a NETZSCH STA 449 F3 thermobalance at a heating rate of 10°C min −1 from 40 to 900°C under a nitrogen flow of 20 mL min −1 .
Nitrogen adsorption/desorption isotherms were measured at −196°C with a NOVA-4200e sorption analyzer (Quantachrome). Before measurements, samples (approximately 80 mg) were heated under a vacuum at 200°C for 2 h. Pore-size distributions were calculated by a quenched solid density functional theory (QSDFT) model. The total pore volume (V total ) was calculated from the amount adsorbed at a relative pressure of 0.99. The ultramicropore volume (V ultra , for pore sizes lower than 1 nm) was calculated via the t-plot method.

CO 2 Adsorption
Measurements. The static CO 2 and N 2 adsorption isotherms were measured with a NOVA-4200e sorption analyzer at vacuum pressure, 0°C, and 25°C. Before the measurement, 80 mg of the sample was degassed under a vacuum at 200°C for 2 h. After cooling down to room temperature, the CO 2 and N 2 adsorption capacity of the sample was analyzed using NOVA-4200e, and the adsorption isotherms of CO 2 and N 2 were assessed using the molar ratio of the adsorption capacities of CO 2 and N 2 ranging from 0 to 1.0 bar. The isosteric heat value was estimated from the CO 2 adsorption isotherms using the Clausius−Clapeyron equation (eq se1). 43 The selectivity for CO 2 from mixtures (CO 2 /N 2 = 15/85 v/v) was calculated by the IAST method (eq se2). 4 2.6. Dynamic Adsorption and Gas-Mixture Separation Measurements. CO 2 and N 2 uptakes in dynamic gasmixture flow by NCA-X were investigated. The breakthrough experiments of gas-mixture CO 2 /N 2 (15:85) of the sorbents were performed on a fixed-bed adsorber (quartz glass tube, with an inner diameter of 4 mm and length of 300 mm) at 25°C and 1.01 bar. The temperature of the adsorber was controlled by a thermostatic water bath. The adsorbent mass was 0.08 g, and all the samples were pretreated at 200°C for 12 h before dynamic adsorption experiments. Before the breakthrough experiments, the adsorbent sample was heated at 200°C for 2 h under argon at a flow rate of 80 mL min −1 . The experiment was performed by changing the gas flow from argon to gas-mixture CO 2 /N 2 (15:85) at a flow rate of 2 mL min −1 when the sample cooled down to room temperature. The effluent gas was monitored by an Agilent 7890A gas chromatograph (GC) using a thermal conductivity detector (TCD). The blank experiment was conducted in the same condition; the only change is to replace the adsorbent in the fixed bed with quartz sand.  Figure 1 and Figure S2, a condensation reaction by the groups of methylol between PF and MF forms methylene and methylene ether bridged polymers. These polymers can be copolymerized to form small clusters that can act as nucleation sites composed of branched polymers. Under hydrothermal conditions, they continuously react with many other unreacted particles to grow into microsphere-type co-condensed particles. This process is thought to be similar to Stober by monomer addition, in which nucleation is pretty rapid, followed by particle growth without further nucleation. 43 As polymerization goes on, the density and molecular weight of polymers increase as the M/P ratio increases. Steric retardation, chain elongation, and particularly chain branching dramatically reduce polymers' compatibility with water and consequently increase nucleation rates and decrease the time of growth stage. 37 Therefore, smaller microparticulates are formed at a higher M/P ratio. In Figure S3, the volume of PMF-X OAs decreased with a decrease in the M/P ratio. This phenomenon may be attributed to the faster cross-linking reaction of PFs under the catalysis of ZnCl 2 , which expands the macroscopic porous structure.

Characterization of Organic Aerogels. As shown in
The chemical composition of the PMF OAs was investigated by elemental analysis (EA). In Table S1, the EA data show the changes that occurred in the nitrogen content of PMF-X, which indicated that the aforementioned outcomes are in good agreement with the carbon/nitrogen atomic ratio in the precursors.
The successful incorporation of M into the PF network was further confirmed by the Fourier transform infrared (FTIR) spectra ( Figure S4), where all PMF samples averaged similar curves. However, a broad band that consisted of −OH and/or −NH molecules at approximately 3400 cm −1 and bands at 1547, 1480, and 1340 cm −1 attributed to the C−N stretching vibrations in the triazine ring and the carbon−nitrogen breathing mode have been calibrated and identified. All these results proved that the nitrogen atom was smoothly incorporated into the skeleton of PF.
The TG curve of MF shows a low residual weight (5%, Figure 2). This result suggests its poor thermal stability. MF resins have a relatively low thermostability with about 24% weight loss from room temperature to 375°C that further increases to 86% from 375 to 600°C. After the copolymerization of MF with PF, the residue was significantly increased. While the PF component increased from 33% (PMF-1-2) to 80% (PMF-4-1), the residual weight was as high as 20 to 47% of the entire sample at 900°C. This result suggested the successful copolymerization of PF with MF and the inhibition of the pyrolysis of the latter in the PMF precursor.

Characterizations of NCAs.
NCA-X was synthesized by using PMF-X-Z as the precursor. During the carbonization and activation processes, the PMF-X-Z monolith expanded because ZnCl 2 served as a dehydration and foaming agent. Herein, ZnCl 2 absorbed moisture from the PMF polymerization process and promoted the formation of PMF networks as a catalyst. Specifically, a large number of micropores were formed by ZnCl 2 on the CA obtained from the subsequent carbonization and activation processes. After the removal of ZnO (transformed from ZnCl 2 ) in the activated carbon sample by washing with dilute HCl and direct drying in the air under ambient conditions, a stable, lightweight black NCA-X was obtained.
The mass of ZnCl 2 strongly influences the morphology of the PMF-X-Z monolith, and only in a suitable mass range can a homogeneous PMF-X-Z monolith be obtained. Excessive amounts of ZnCl 2 will induce a mixed phase, not liquid, and when the ZnCl 2 content is too low, the mixed phase could be subsequently separated. The scanning electron microscopy (SEM) images of NCA with different ratios of ZnCl 2 are shown in Figure S5. NCA 0 -2-1 without ZnCl 2 in the prepolymerization and carbonization−activation steps with a poor microporous network and an extremely low specific area (70− 90 m 2 /g). Compared with NCA 0 -2-1, the NCA 4 -2-1 aerogel exhibits a more defined porosity (type I isotherm) and higher surface area. The NCA 8 -2-1 aerogel under hypersaline conditions exhibits the highest surface area, micropore volume, and interconnected porous structure (Table S2). The nanostructure of NCA 8 -2-1 was accordingly characterized via SEM. The results showed that the carbon material was composed of fine interconnected carbon frazzles of approximately 10 nm ( Figure S5c). By contrast, NCA 0 -2-1 without any ZnCl 2 resulted only in dense structures. This finding emphasizes the role of ZnCl 2 as a surface stabilizer and dispersant.
By referring to the outcomes of SEM and TEM characterizations, we determined that different M/P ratios exert a considerable impact on the microstructure of NCA-X ( Figure  3). With an increasing amount of M/P ratio, the small particles that comprise the PMF network exhibited a decrease in diameter. Carbons aerogels have a strongly developed internal surface, and they are usually characterized by a polydisperse capillary structure comprising pores of different sizes and shapes. As illustrated in Figure 4, NCA-2-1 possesses a randomly distributed worm-shaped slit pore structure. Figure 5a and Figure S6a show the N 2 adsorption− desorption isotherms, and the pore characteristics of the samples were quantified. The pore characteristics of the samples were quantified by measuring N 2 adsorption−− desorption isotherms and CO 2 adsorption isotherms. The structural development of aerogels depends on the composition and synthesis conditions of the precursors, which are critical to their gas adsorption capacity. 44 The nitrogen adsorption isotherm of NCA 0 -2-1 is like a type I adsorption isotherm, and the specific surface area (Table S2) is as low as 17 m 2 /g, indicating that the material without ZnCl 2 was nearly nonporous. The specific surface area of the NCA 4 -2-1 and NCA 8 -2-1 samples (933 and 1156 m 2 /g) is much higher than that of NCA 0 -2-1. The increased amount of ZnCl 2 in the

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http://pubs.acs.org/journal/acsodf Article precursor leads to a hierarchically porous structure of microand mesopores during the thermal decomposition process, thereby exhibiting high specific surface area and total pore volume ( Figure 5b and Table 1).
In contrast with NCA 0 -2-1, the NCA-X samples prepared with a high ZnCl 2 content exhibited typical type IV characteristics with a broad capillary condensation step in the relative pressure (P/P 0 ) range of 0.30−0.70, revealing high mesoporosity. This capillary condensation step shows that the samples exhibit disordered mesopores, 45 which are confirmed further by the PSD (Figure 5b) and the data obtained from the TEM analysis. These data represent that a moderate ZnCl 2 content could promote the specific surface area and facilitate the construction of the porous structure and total pore volume. The N-free carbon aerogel CA exhibits a higher specific surface area, and the average pore diameter than NCA-X reveals the presence of smaller mesopores (2−4 nm). For NCA-2-1, the prepolymerized OA shows a more clearly defined porosity (high surface area), suggesting a proper cross-linking of the precursors. For a higher M/P ratio (samples NCA-1-1 and NCA-1-2), the formation of the microporosity is more pronounced (Figure 5b and Table 1). Some authors interpreted similar results in terms of shorter C−N bond lengths that would induce a buckling of the basal plane and consequently a smaller porosity. 44 The increase in the number of micropores is conducive to the adsorption and separation of small molecules of CO 2.

1
The FTIR spectra of the prepared carbons are shown in Figure 6. A broad band at 3400 cm −1 attributed to either −OH and −NH moieties was detected, along with the bands at 1246 and 1600 cm −1 . All NCA-X samples showed a small peak at 1246 cm −1 , which can be assigned to the stretching vibrations of aromatic C−N bonds. The in-plane N−H deformation vibration at 1600 cm −1 was strengthened, which can be interpreted as the nitrogen-rich carbon framework and the activation of the ammonia released from the rupture of triazine rings. 44 In Figure 7a, a broad peak at 2θ is equal to the XRD pattern of 44°NCA-X, which corresponds to (100) diffracted amorphous carbon. Another extremely weak broad peak intensity was found at approximately 2θ = 22°, which can be indexed (002) by diffracted amorphous carbon. The XRD characterization results show that NCA-X exhibits amorphous characteristics and does not have long-term structural order, which is in good agreement with the TEM observations.   The structure of NCAs was characterized by Raman spectroscopy (Figure 7b). All samples show the broad bands of the carbon material (D and G bands) between 1000 and 2000 cm −1 . The variation of the I G /I D ratio can be used to measure ordered changes in the carbon network structure that cannot be detected by classical powder XRD. 45 The integral area values of the D and G peaks are summarized in Table S3 in the Supporting Information. CA without nitrogen exhibited an I G /I D value of 0.41, and a meaningful decrease of this ratio from 0.39 to 0.32 was detected when the M/P ratio increased from 1/4 to 2. This finding suggests that an increase in the ratio of conversion to MF and the incorporation of nitrogen induce the formation of more irregular structures.
The surface chemical states of NCA-X were probed through XPS analysis and are summarized in Figure 7c and Table S4. From the XPS results, the atomic percentages of nitrogen increased from 3.85 at. % in NCA-4-1 to 6.07 at. % in NCA-1-2. Nitrogen 1s spectra can be decomposed into four individual peaks: pyridinic nitrogen (398.2 eV), pyrrolic nitrogen (400.0 eV), quaternary nitrogen (401.2 eV), and oxidic nitrogen (402.5 eV). As the M/P ratio of NCA-X increases, a further increase in nitrogen content (Table S4) follows the expected decrease in the graphitic nitrogen composition and the consequent loss of structural order. Such results agree with the Raman spectroscopy data. Figure 8a,b shows the CO 2 adsorption isotherms of the five NCA-X samples at 0 and 25°C

CO 2 Adsorption.
. With the M/P ratio increase (from 1/4 to 2/1), the CO 2 adsorption capacities of the NCA-X sorbents are greatly improved, with similar trends at both temperatures. In the range of 0−1 bar, all samples show CO 2 uptake capacity, and no saturation was found. In 1 bar, the NCA-1-2 showed a better CO 2 uptake capacity of 4.30 mmol g −1 at 0°C and 2.65 mmol g −1 at 25°C ( Table S5). The amount of CO 2 uptake by NCA-1-2 (4.3 mmol g −1 ) is nearly double the amount of CO 2 adsorption by CA (2.39 mmol g −1 ) under the same conditions. This indicates that our modification of the material significantly improved its adsorption capacity for CO 2 .
For clarifying the relationship between CO 2 uptake capacity and samples' textural parameters, Figure 8c shows the relationship of CO 2 uptake capacity at 1 bar with the BET surface area and micropore surface area of NCA-X samples. As shown in the figure, the CO 2 uptake capacity is enhanced with the increase of micropore surface areas and V ultra , and it is inversely proportional to D p overall. In general, the specific surface area and total pore volume of an adsorbent are two of the most important parameters deciding its CO 2 adsorption capability. 46−48 However, in our work, we find that there is no direct linear relationship between the specific surface area/total pore volume of synthesized porous carbons and their CO 2 adsorption amounts. CA has the largest specific surface area, but the CO 2 adsorption capacity is lower than NCA-X. These results suggest that the specific surface area of the material is not the only factor affecting the adsorption capacity of CO 2 . Compared with CA (without N content), NCA-1-2 has a lower specific surface area and smaller pore volume but has a higher CO 2 uptake capacity. Notably, the CO 2 capture capacity increased with the increase of the micropore surface area (S mic ) (Figure 8a, red line) and ultramicroporous volume ratio (V ultra ) (Figure 8d, black line), while it was inversely proportional to S BET and D p in general. These results suggest a strong positive S BET : specific surface area calculated by the BET method. b V t : total pore volume at P/P 0 = 0.99. c V ultra : Ultramicropore volume (<1 nm) was obtained from the CO 2 adsorption branch at 273 K. d By XPS results: N at. %. e N wt % measured by elemental analysis results of precursors (PMF-X).  correlation between the CO 2 capture capacity and the S mic and V ultra of the synthetic porous carbon adsorbents. We assume that a certain number of mesoporous channels provide an ideal environment for the diffusion of CO 2 molecules, and then the ultramicropores around the mesoporous walls generate van der Waals forces for the interaction of carbon structures with CO 2 molecules, which enhances the adsorption of CO 2.

49
When comparing CA and NCA-4-1 in Figure 8c,d, CA shows a larger specific surface area and micropore specific surface area, a smaller average pore size, and an approximate ultramicroporous volume. However, the adsorption of CO 2 by CA is less than that of NCA-4-1. This is due to the introduction of N doping in NCAs. For carbon-based porous adsorbent materials, nitrogen doping can provide more abundant CO 2 adsorption sites, especially pyridine nitrogen and pyrrole nitrogen. The increase of adsorption sites can effectively improve the adsorption performance of the material for CO 2 (Figure 9a). This phenomenon can also be confirmed by the data on the isosteric heat of adsorption of the material CO 2 .
The isosteric heats of adsorption (Q st ) of NCA-X were calculated using the Clausius−Clapeyron equation based on the CO 2 adsorption isotherms at 0 and 25°C, and the results are shown in Figure 9b. Q st for NCA-X samples varies in the range of 23.5−33.6 kJ mol −1 , which is normally ascribed to physisorption. The rapid decrease to a plateau of the initial adsorption heat is driven by the more active nitrogen surface position. 50 CO 2 molecules are preferentially adsorbed on the stronger binding sites. As the number of adsorption increases, only the weaker binding sites remain available. The nitrogen contains groups that act as Lewis bases in the carbon framework, facilitating the interaction with acidic CO 2 molecules. The increase of active sites not only theoretically improves the adsorption efficiency of the material for CO 2 in

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http://pubs.acs.org/journal/acsodf Article the low-pressure section but may also be useful for CO 2 selectivity.

CO 2 Adsorption
Selectivity. The CO 2 /N 2 adsorption selectivity is an important factor to judge the adsorption separation performance of the material in simulated flue gases. In particular, the adsorption capacity of CO 2 at approximately 0.15 bar and 25°C can accurately reflect its application in real flue gases. 49 Figure 10a shows the adsorption isotherms of CO 2 and N 2 at 25°C of sample NCA-1-2. It can be seen that the adsorption capacity of the material for CO 2 is much higher than its capacity for N 2 . At 0.15 bar, the CO 2 adsorption of NCA-1-2 (0.81 mmol/g) was about 20 times higher than that of N 2 (0.04 mmol/g). To better visualize the adsorption selectivity of all samples, the CO 2 /N 2 selectivity was calculated according to the ideal adsorption solution theory (IAST). The results are shown in Figure 10b. Among all NCA-X samples, NCA-1-2 with an IAST CO 2 /N 2 selectivity of 36.5 is the material with the highest selectivity. Such excellent selectivity is mainly due to a large amount of pyridine nitrogen and pyrrole nitrogen (serving as Lewis basic sites) contained in NCA-2-1 (Table S4), which have an essential influence on CO 2 /N 2 selectivity. 51,52 The adsorption separation performance of the material was compared with other carbon materials at 25°C and 0.15 bar, and the selectivity was 36.5, which was similar to or even higher than those recently reported for many carbonbased adsorbents (Table S6).
Adsorption breakthrough experiments were carried out with CO 2 /N 2 = 15/85 at 1.01 bar and 25°C, and the results showed that NCA-1-2 exhibited excellent CO 2 separation performance and verified the conclusions of IAST. As shown in Figure 11a, N 2 (capacity: 0.16 mmol g −1 , 0.85 bar) was released from the adsorption column faster than CO 2 (capacity: 0.77 mmol g −1 , 0.15 bar), with a shorter breakthrough time. The sharp breakthrough curve shows a fast CO 2 adsorption kinetic process, and the adsorption capacity was almost equal to the equilibrium capacity of single component adsorption at 0.15 bar and 25°C. The N 2 breakthrough curve in Figure 11a showed an obvious ″roll-up″ during 4 to 17 min, which was due to CO 2 replacing the originally adsorbed N 2 by the adsorbent, and the N 2 concentration in the column was higher than the initial concentration. The results show that NCA-1-2 has excellent CO 2 capture and separation performance and can effectively remove CO 2 from simulated flue gas. The regeneration ability and stability of NCA-1-2 were investigated by eight consecutive adsorption−desorption cycles. As shown in Figure 11b, the almost the same cycle curve shows the excellent CO 2 adsorption performance of NCA-1-2, which can be used as a potential choice for practical industrial application needs.
As shown in Figure S7, the carbon aerogel material is considered to have good hydrophobic properties. We investigated the addition of 3% water vapor to the CO 2 /N 2 gas mixture to further observe the dynamic adsorption separation stability of NCA-1-2. The breakthrough experiments were carried out with CO 2 /H 2 O/N 2 = 15/3/82 at 1.01 bar and 25°C. The water vapor in the mixed gas was added by adding a gas circuit with a water saturator at the N 2 inlet to create the mixed gas with the required humidity. Before and after each experiment, the adsorbent was degassed and regenerated using pure helium (He). The sorbent tubes were  heated by a water bath to maintain a constant temperature of 25°C. The breakthrough curves are illustrated in Figure 12a. Cycle 0 is shown in the figure, representing the experiment with dry gas. After five cycles, the adsorption capacity of CO 2 slowly decreased from 0.81 to 0.69 mmol g −1 , still maintaining more than 85% of the adsorption capacity (Figure 12b). This indicates that the hydrophobic nitrogen-doped carbon aerogel material can still maintain a certain efficiency under the influence of 3% water vapor concentration.

CONCLUSIONS
In this work, phenol−melamine−formaldehyde resin organic aerogels (PMF) with small gel particles and high N content were obtained by controlling the addition amount of ZnCl 2 and the precursor M/P ratio. The homogeneously dispersed ZnCl 2 in the organic aerogels can be used as a foaming agent and etching agent directly together with PMF to prepare NCA through the carbonization activation process. Among the prepared samples, NCA-1-2 with a higher micropore specific surface area (903 m 2 g −1 ) and interconnected porous structures contains a large number of pyridinic nitrogen (3.31 at. %) and pyrrolic nitrogen(0.62 at. %). Under the combined effect of surface structure and adsorption sites, NCA-1-2 exhibits the maximum CO 2 adsorption capacity (4.30 mmol g −1 at 0°C) and higher CO 2 adsorption selectivity (36.5 at 25°C, IAST) toward selective separations of the CO 2 /N 2 . Specifically, the breakthrough research outcomes indicated great stability and cyclic regeneration performance by eight times CO 2 adsorption running. Under the influence of a 3% concentration of water vapor, NCA can still maintain more than 85% of CO 2 adsorption efficiency after five cycles.