From chitosan to urea-modified carbons: Tailoring the ultra-microporosity for enhanced CO2 adsorption

Carbon 159 (2020) 625e637

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From chitosan to urea-modified carbons: Tailoring the ultra-microporosity for enhanced CO2 adsorption Adeela Rehman , Soo-Jin Park * Department of Chemistry, Inha University, 100 Inharo, Incheon, 22212, South Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 November 2019 Received in revised form 19 December 2019 Accepted 26 December 2019 Available online 2 January 2020

In the past few years, nitrogen-enriched microporous carbons have garnered much attention as emerging CO2 adsorbents; however, the roles of ultra-microporosity and nitrogen content have rarely been studied. The present work focuses on designing a series of microporous carbons with different concentrations of nitrogen ranging from 0 to 11 at.%. With variation of the concentration of urea and KOH, carbons with a high surface area (368e2150 m2 g
1) and high micropore volume (0.2255 e1.3020 cm3 g
1) were attained. Tailoring the ultra-microporosity resulted in effective CO2 adsorption. The optimized material, CUK-112, exhibited the highest micropore volume (<1 nm) among the investigated materials and a maximum adsorption of 280 mg g
1 of CO2 at 273 K and 1 bar. A slight decrease in the ultra-micropore volume for pores smaller than 1 nm decreased the adsorption capacity to 259.2 mg g
1 of CO2. The comprehensive study of the CO2 isotherms within the framework of Dubinin’s theory further evidenced the role played by narrow micropores in CO2 adsorption. However, isosteric heats of adsorption revealed the importance of nitrogen content during the initial adsorption onto heteroatom-containing sites via quadrupole interaction with CO2. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Chitosan Ultra-micropores D-A theory CO2 adsorption CO2/N2 selectivity

1. Introduction Porous materials with exceptionally high gas adsorption properties, facile preparation and regeneration with the benign environmental impact are the few attractive features for drawing the attention of scientific community to innovate novel smart materials [1]. To circumvent the adverse effects of high concentration of CO2 in atmosphere, scientific community is focused to mitigate it by adsorption [2e4], reduction [5] and fixation [6,7]. More specifically, materials used in the past few decades for CO2 adsorption includes metaleorganic frameworks (MOFs) [8], conjugated polymers [9], zeolites [10], and microporous carbons [11]. To date, carbon-based porous adsorbents exhibit unprecedentedly high adsorption performance by micropore-volume filling via physisorption. Over the past several years, researchers are focused on incorporating heteroatoms in the carbon framework to enhance the interaction of CO2 molecules with the adsorbent surfaces. Albeit, it can enhance the adsorption performance yet textural factors such as micropore volume and pore size distribution also contribute significantly. To

* Corresponding author. E-mail address: [email protected] (S.-J. Park). https://doi.org/10.1016/j.carbon.2019.12.068 0008-6223/© 2020 Elsevier Ltd. All rights reserved.

generate high porosity, potassium salts are widely used as activating agents [12,13]. The mechanism proposed by Otowa et al. [14,15] for KOH activation includes several sequential reactions, some of which are shown in eqns (1)e(4):

2KOH / K2 O þ H2 O

(1)

C þ H2 O/CO þ H2

(2)

CO þ H2 O/CO2 þ H2

(3)

CO2 þ K2 O/ K2 CO3

(4)

Furthermore, LinareseSolano et al. [16] presented a stoichiometric reaction for KOH activation of anthracite (eqn (5)):

6KOH þ 2C/2K þ 3H2 þ 2K2 CO3

(5)

In the course of activation, K2CO3 formation begins at approximately 400  C and KOH is completely converted to K2CO3 at approximately 600  C. At high temperature, different potassium compounds are produced (eq (6)) which upon reaction with the carbon, generated from CO2 (eq (7)), reduced to metallic potassium as shown in eqns (8) and (9).

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K2 CO3 / K2 O þ CO2

(6)

work, we demonstrate the relevant insights for enhanced CO2 adsorption by nitrogen-doped microporous carbons.

CO2 þ C/2CO

(7)

2. Experimental

K2 CO3 þ 2C/2K þ 3CO

(8)

2.1. Materials

C þ K2 O/ 2K þ CO

(9)

Chitosan (99%), urea (99%), and KOH were acquired from SigmaAldrich.

In the view of aforementioned proposals, it is assumed that KOH activation can induce porosity by three different ways: (i) Chemical activation of potassium salts by the redox reaction with the carbon framework; (ii) Physical activation leading to the gasification of carbon; and (iii) the intercalation of resultant metallic potassium into the carbon matrix leading to the expansion of lattices. Therefore, chemical activation, physical activation and metallic potassium intercalation collaboratively enhance the surface area and porosity of KOH-activated carbons. Herein, biomass (chitosan) is used as a precursor for the production of microporous carbons offering an economically favorable and environmentally benign avenue. Present work focuses on tailoring the textural features of activated carbons by regulating the concentration of KOH and urea. From the literature, the role of KOH and urea for microporous nitrogen-doped carbon formation is wellevident. Shen et al. [17] reported samples prepared at different temperatures to generate carbon nanosheets. In his work, cellulose (C), urea(U) and KOH (K) were used to generate four different samples named as pristine carbon (C), carbon-urea (CU), carbonKOH (CeK) and carbon-KOH-urea (C-KU). The resultant yield of CeKeU was highest among them. Furthermore, upon decomposition of urea, exhausted gases can incorporate nitrogen into carbon matrix. Hence, urea plays a crucial role as nitrogen-dopant. Moreover, C-KU exhibit highest surface area (1854 m2/g) and pore volume (0.82 cm3 g
1) suggesting that the samples prepared in the presence of KOH and urea can successfully generate a nitrogendoped carbon nanosheets. Moreover, Zhou et al. [18] reported synergistic effect of KOH and urea in nitrogen-doped porous carbon structure formation. In his work, the sample prepared by KOH activation of sugar cane bagasse exhibits irregular agglomerates of carbon particles without sheet-like porous structure formation. Likewise, sample prepared with only urea-modification leads to a compact structure without well-defined porous texture. However, the optimized samples prepared in the presence of KOH and urea exhibited an interconnected porous structure by the dense layerstacking of nitrogen-doped carbon nanosheets. Henceforth, selecting an appropriate synthetic avenue in order to design high performing material is always a challenging task. Recently, various research approaches have been utilized to generate maximum porosity in the carbon adsorbents to attain maximum CO2 uptake at the laboratory scale [19e21]. However, the complexity of these approaches due to multi-step reactions, hazardous chemical activating agents and harmful residues hinders their utilization on industrial scale. In this regard, our work demonstrates a facile activation strategy for designing the sustainable carbons with the optimized performance/cost ratio. Enhancement in CO2 adsorption is achieved via fine tailoring of the ultra-microporosity of carbons. The low CO2 partial pressure conditions suggest that only adsorbents with a substantial amount of micropores can retain significant volume of CO2, while the effect of larger pores comes out to be negligible. From the micropore size distributions determination, numerous studies have evidenced linear relationship between the CO2 uptake at low pressures and pore volumes of a certain pore size. Nevertheless, different types of carbon produce diverse relationships suggesting the need of a more general description. From the detailed textural study of as prepared materials in the current

2.2. Synthetic route for microporous carbons As shown in Fig. 1, 5 g of chitosan was carbonized at a moderately high temperature, 650  C, for 1 h under a nitrogen atmosphere to produce black powder labeled as C-650. To 1 g of this powder, desired concentrations of urea and KOH were mixed thoroughly; the resulting mixture was then ground in agar mortar for 30 min. Subsequently, this homogeneous mixture was then transferred to an electrical furnace for carbonization at 800  C for 1 h under inert conditions. Black powder obtained after carbonization was allowed to cool at room temperature and then washed sequentially with 1 M HCl to remove the residual potassium impurities, and distilled water to maintain a neutral pH of the final product. Lastly, the final black powder was allowed to dry at 110  C for 20 h and labeled as CUK-XYZ, where X presents the chitosan mass ratio, Y denotes the urea mass ratio, and Z denotes the KOH mass ratio. 2.3. Characterization methods The characterization step utilizes Fourier transform infrared (FTIR) spectroscopy to investigate the functional groups over the frequency range of 500e4000 cm
1 by FTIR, Jasco PS-4000, thermogravimetryedifferential thermal analysis equipment (TGDTA, TG 209 F3, NETZSCH, Germany) for determining the thermal behavior of samples up to 800  C at a heating rate of 10  C min
1 under inert gas conditions. The elemental composition was obtained by X-ray photoelectron spectroscopy (XPS) from ESCA LAB MK-II spectrometer (VG Scientific Co.) and chemical elemental analysis by an EA1112 element analyzer. The morphological characteristics were studied by field-emission scanning electron microscopy (FESEM; SU8010, Hitachi Co., Ltd.) and field-emission transmission electron microscopy (FE-TEM; JEM-2100F). The graphitic-carbon nature was explored using Senterra R200-L dispersive Raman microscope (Bruker Optics Co. Ltd., Germany) for Raman spectroscopy and Bruker D2 PHASER X-ray diffractometer for X-ray diffraction (XRD). Moreover, for gas adsorption (N2 and CO2) studies, a Belsorp Max instrument (BEL Japan, Inc.) was used. 3. Results and discussion A two-step synthesisdcarbonization followed by activation and simultaneous nitrogen-dopingdwas used to synthesize CUK. Fig. 1 illustrates the synthesis route for the microporous carbon. The initial step was carbonization of chitosan under a nitrogen atmosphere at a moderately high temperature to produce a black carbonaceous powder (C-650). In the next step, KOH and urea were added to the C-650 as a porogen and nitrogen doping material, respectively. This mixture underwent a series of reactions during high-temperature carbonization to generate a well-defined porous system. The FT-IR spectrum is obtained to investigate the functional groups present in the nitrogen-doped microporous carbons (Fig. S1). The broad absorption region around 3400 cm
1 in all the prepared samples indicates the stretching vibrations of NeH and/or

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Fig. 1. Synthetic route for designing microporous carbons. (A colour version of this figure can be viewed online.)

OeH [22]. The peaks in the region of 1550e1590 cm
1 are attributed to in-plane deformation vibration of NeH or stretching vibrations of C]C in aromatic rings [23]. Moreover, CeN stretching vibrations are observed around 1250 cm
1 [17,24]. Furthermore, the region between 1200 and 1000 cm
1 is assigned to asymmetric stretching vibrations CeO. All the samples confirm the existence of CeN and NeH species. However, samples with high nitrogen content, CUK-111, CUK-121, and CUK-131 exhibit intense peaks at 1250 cm
1 and 1560 cm
1 attributed to NeH and CeN stretch, respectively. The TGeDTA curves of pristine chitosan and a mechanically ground mixture of chitosan, urea, and KOH are presented in Fig. 2. Two steps are observed in the TG curve of pristine chitosan: The first one occurs between 100 and 200  C, with 6.2% mass loss and an endothermic effect. This initial mass loss was attributed to the evaporation of adsorbed molecules from the polymer chain. The second mass loss, beginning at approximately 245  C and ending at 350  C, was associated with a 44.2% mass loss accompanied with the peak at 255  C in the DTA curve. This region was attributed to vaporization and combustion of volatile compounds produced from the thermal decomposition of the polymeric chain. The pyrolytic breakdown of chitosan chain involves homolytic bond scission (Fig. 2C) resulting in instantaneous new bond formation. Among various compounds formed during chitosan degradation, pyrazines and pyridine derivatives are the most important chemical species derived from the amino carbonyl intermediates. The sequential processes for decomposition of chitosan includes depolymerization of polymeric chains, degradation of pyranose rings by dehydration and deamination, and finally a ring-opening reaction [25e27]. Moving forward, the homogeneous mixture of chitosan, urea, and KOH in Fig. 2B exhibits the initial weight loss of approximately 22.3% as the temperature was increased to 300  C, ascribed to the vaporization of physically adsorbed moisture and endothermic evaporation of hydrogen-bonded water molecules. Meanwhile, preliminary deacetylation takes place accompanied by the crosslinking reactions while keeping the main chitosan chain intact. At this stage, the complex chemical reactions can weaken the

intramolecular forces and disintegrate molecular structures leading to the evolution of gases (hydrogen and ammonia). These bondbreaking and cross-linking reactions generate fused ring structures with nitrogen atoms located into the pentagonal or hexagonal sites of the cyclic carbon. These cyclic structures and fused-rings are thermostable and can likely tolerate high temperatures. Consequently, at a high-temperature range, 400e800  C, the TG curve shows no distinct weight-loss. This result suggests that the carbon residues are rigid and thermally stable because of the predominance of a p-conjugated structure in the polycyclic-type fused rings. The resultant yields of pristine chitosan and chitosan with KOHeurea was 32 and 66%, respectively. The difference in weight loss (~34%) between two samples proposes urea condensation with the chitosan polymer. Functionalization of a chitosan with urea is effective owing to the generation of strong interactions among pyranose units and amine groups leading to the crosslinked network formation. The initial rapid weight-loss (23%) of chitosan impregnated with KOH and urea before 300  C, indicates that the KOH promotes the decomposition of urea [28,29]. KOH is wellknown structural modifier through potassium metal intercalation by a series of reactions at high temperature during activation, as explained earlier. Here, urea act as nitrogen-dopant as well as structure directing agent to fabricate graphene-like structures. At a temperature around 500  C, urea is transformed into graphitic carbon-nitride like nanosheets, infiltered with the KOH and C-650, leading to an expanded structure. With rising activation temperature, the graphitization of C-650 begins followed by the decomposition of graphitic carbon-nitride like nanosheets to nitrogencontaining vapors (NH3, C2Nþ 2 and others with low thermal stability). Consequently, nitrogen-doping of graphene-like carbon sheets occurs. In summary, the mechanism for nitrogen-doped carbon materials from the C-650/urea/KOH mixture includes the following steps: (i) transformation of chitosan-derived carbon to graphenelike sheets followed by the successful nitrogen-doping from ureaderived carbon-nitride structure. (ii) Meanwhile, upon reaction of KOH with graphene-like sheets transformed them into threedimensional porous carbon-structure. Thus, the obtained material

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Fig. 2. TG-DTA curve for (A) pristine chitosan (B) CUK (C) possible mechanism for pyrolysis of chitosan to release volatile compounds. (A colour version of this figure can be viewed online.)

A. Rehman, S.-J. Park / Carbon 159 (2020) 625e637

possesses excellent textural properties as well as high nitrogen content [28,30]. Elemental analysis of all the prepared samples was performed by XPS and chemical elemental analysis, summarized in Table 1. The survey scans (Fig. S2) illustrate that all the samples are composed of predominantly carbon and, to lesser extents, nitrogen and oxygen. The high-carbon and moderate nitrogen contents determined by XPS are in accordance with the elemental analysis results. This is attributed to the fact that the concentration of these elements at surface of porous carbon is comparable to that in the bulk [31]. For the carbon precursor prepared at 650  C, C-650, elemental analysis revealed the presence of 9.3 wt % and 74.0 wt % of nitrogen and carbon, respectively. However, high temperature activation leads to evaporation of volatile nitrogen compounds. Thus, at 800  C, for sample prepared in the absence of urea, CUK101, exhibit a decreased nitrogen-content incorporated into the microporous carbon. This negligibly small amount of nitrogen content (0.6 wt %) is a remnant of nitrogen present in C-650. Nitrogen-content in the final carbon products increases with an increase in the urea ratio up to 3. Therefore, the total nitrogen content in CUK-131 increases to ~11.0%. However, while keeping the urea concentration fixed to 1 and increasing the KOH concentration to 3, the nitrogen content decreases to 0.62 wt % as a consequence of elimination of certain thermally unstable nitrogen species. Nitrogen-doping in the carbon lattice occurs when urea reacts with surface functionalities groups and undergo subsequent thermal degradations. For instance, during thermal treatment hydroxyl groups can react with the amino groups of urea, and as result nitrogen moieties are incorporated in the carbon lattice. Such incorporated nitrogen functionalities can be located on the edges of lattices in the form of amine, pyridinic, and pyrrolic nitrogen which upon high temperature can transformed to graphitic nitrogen. Distinct types of nitrogen in the prepared samples were studied by deconvolution of the N 1s spectra (Fig. 3). Four different nitrogen peaks observed are: pyridinic nitrogen at 398.3 eV, pyrrole/pyridine nitrogen at 400.1 eV, graphitic nitrogen at 401.5 eV, and oxidized nitrogen at 402.1 eV. Recent studies revealed that both the pyridine and pyrrolic groups are beneficial for CO2 uptake as they induce stronger hydrogen-bond interactions between the surrounding CeH bonds and CO2 molecules [32,33]. CUK-111, CUK-112, and CUK-113 are mainly composed of pyridinic and pyrrolic nitrogen, whereas graphitic and oxidized nitrogen are predominant in CUK-121 and CUK-131. Pyridinic nitrogen and pyrrolic nitrogen have been reported to be found in six-membered ring structures and in five-membered ring structures, respectively [34]. Pyridinic nitrogen donates one electron to aromatic p-system while pyrrolic-nitrogen offers two p-electrons to the p-system. Consequently, both these nitrogen forms exhibit Lewis basic character and are favorable for CO2 (a Lewis acid) capture. Contrarily, graphitic nitrogen is highly acidic, and it is present in greater concentrations in CUK-121 and CUK-131 than in the other samples. From the aforementioned results, CUK-112 is concluded to exhibit a

629

highly basic nature compared with the other samples. To demonstrate the role played by urea and KOH in enhancing and/or modifying the morphology, textural characteristics, and pore structure of the products, we observed all of the prepared samples by SEM. In Fig. 4, the as-prepared CUK-111, CUK-121, and CUK-131 samples exhibit dense layer-stacked nitrogen-doped carbon materials with somewhat hierarchical porous structure. CUK101, CUK-112, and CUK-113 samples prepared with a high KOH/ urea ratio exhibit a sponge-like morphology with an interconnected porous network [35e37]. At high KOH concentrations, the bulk morphology becomes spongier and numerous micropores are formed during the activation process. Furthermore, high-resolution TEM images (Fig. 5) for CUK-112 sample exhibit ultra-thin nanosheet-like morphology with evident worm-like porous structure. These carbon nanosheets show that micropores are highly distributed in the graphite-like structure [38]. The disordered arrangement of carbon pores and the subsequent pore overlap in the TEM imaging mode preclude the use of this technique for determining the pore size distribution. Hence, nitrogen sorption measurements at 77 K were conducted to precisely determine the specific surface area and pore size of the adsorbents prepared, which is explained in a detail in the next section. Moving forward, the crystallographic structure of microporous carbons was revealed by XRD and Raman spectroscopy. Fig. S3A shows the XRD patterns of all the porous carbon materials. Two diffraction peaks were observed in all the prepared samples at 2q z 24 and 43 , suggesting the graphitic nature of resultant carbon materials. The first broad peak at 2q z 24 is labeled as (002) reflection from the graphitic lattice [39,40]. The next weak diffraction band at ~43 correlate it to (100) and (101) reflections superposition characteristic of the graphitic-type lattice, signifying a limited degree of graphitization [41]. Fig. S3B shows the Raman spectra of as prepared materials. The specific nature of carbonbased materials is demonstrated by the first-order Raman peak located in the region of 1000e2000 cm
1 revealing the presence of disordered sp2 carbon. In our prepared samples, two broad peaks appeared at ~1349 and ~1576 cm
1 are labeled as D and G-band, respectively [42,43]. The E2g phonon of sp2 carbon atoms produce G-band, exhibiting a distinctive feature of the layered graphitic structure, and corresponds to the vibration of carbon atoms in a tangential position in a two-dimensional hexagonal lattice. The other band observed near 1350 cm
1 is considered as forbidden in a perfect graphitic structure and associated with the disorder induction or crystal defects generation of A1g symmetry. From the relative intensities of these two peaks, nature of graphitic material in terms of degree of graphitization can be determined [44]. In general, the degree of graphitization has an inverse relationship to the intensity ID/IG intensity ratio. Herein, the relative intensity ratio (ID/IG) is high, indicating the formation of numerous defects in carbon lattice, in accordance with the results obtained from XRD.

Table 1 Elemental composition determined by XPS and chemical elemental analysis. Samples Pyridinic-Na Pyrrolic-Na Graphitic-Na Oxidized-Na Total nitrogen content (at. %)b Carbon Total nitrogen content (wt. %)c Carbon Content (wt. %)c Content (at. %)b CUK-101 CUK-111 CUK-121 CUK-131 CUK-112 CUK-113 a b c

e 38.6 31.5 25.3 44.0 28.5

e 19.9 20.9 24.3 17.7 33.3

e 16.9 27.8 23.8 13.5 38.2

e 24.6 19.8 26.6 24.8 e

Relative % of total nitrogen. Elemental composition by XPS analysis. Elemental composition by chemical elemental analysis.

0.8 5.5 6.9 11.6 1.8 1.2

75.8 77.7 86.8 72.8 70.3 85.1

0.6 5.0 6.8 11.0 1.0 0.62

72.0 70.0 71.1 69.3 67.3 82.1

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Fig. 3. (A). XPS nitrogen 1s spectra (B) nitrogen content (at. %) of prepared microporous carbons. (A colour version of this figure can be viewed online.)

Fig. 4. FE-SEM images of all the prepared microporous carbon materials.

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Fig. 5. FE-TEM images of CUK-112 at magnifications of (A) 50 nm (B) 20 nm and (C) 10 nm.

The values of ID/IG are 0.957 (for CUK-111), 1.028 (for CUK-121), 1.086 (for CUK-131), 1.026 (for CUK-101), 1.032 (for CUK-112), and 1.082 (for CUK-113). These defects in the prepared materials may arise from nonplanar microstructure distortions or lattice defects. These lattice defects include lattice vacancies and edge dislocations. From the literature it is demonstrated that the ID/IG ratio is correlated with the adsorption capacity and porous network of prepared materials. Haghseresht et al. [45] demonstrated the relationship between volume of micropores/mesopores, specific surface area, gas adsorption capacity and ID/IG ratio. They presented that the gas adsorption capacity increases with increasing ID/IG ratio. They also showed that a higher proportion of disorganized carbon exhibit that lattice structure constitutes of predominantly aromatic compounds as compared to the aliphatic carbons leading to narrow space generation in between the aromatic structures. In our study, the ID/IG ratio for all of the samples was approximately 1.0. Thus, the samples exhibit a disordered structure with a highly porous nature, consistent with the SEM and TEM observations. However, though the intensity ratio of CUK-121 and CUK-131 is higher, their CO2 adsorption capacity is still lower. This can be attributed to their acidic nature, as demonstrated by XPS analysis. 3.1. Pore development and textural features The nitrogen adsorptionedesorption isotherms obtained at 77 K, pore size distributions, cumulative surface area, and cumulative pore volume, are shown in Fig. 6AeD, respectively; the complete data is listed in Table 2. Choice of appropriate precursors, initial concentrations and controlled synthetic conditions led to adsorbents with high surface area (368e2150 m2 g
1) and ultra-high micropore volume (0.2245e1.302 cm3 g
1). From Fig. 5A, it is obvious that all the samples exhibit type-I curves (IUPAC classification) [46,47]. A sharp rise in nitrogen adsorption at a low relative pressure (P/Po < 0.01) indicates a high concentration of micropores in the prepared samples [48]. Carbonization with a high KOH concentration results in the substantial rise in the nitrogen uptake indicating the formation of a well-developed porosity owing to the decomposition and evaporation of gaseous molecules from the porous structure. More specifically, with an escalation in the KOH/urea weight ratio from 0 to 3, the adsorption volume rapidly increases representing a highly microporous structure. Observation of the initial nitrogen adsorption (Fig. S4) at a very low relative pressure (<0.01) reveals a widening of the knee with increasing KOH concentration. This can be visualized in sample CUK-113, where KOH/urea weight ratio is 3 and knee of the isotherm exhibit a shift at a relative pressure (0.001e0.3), revealing pore widening to a somewhat mesoporous region [49]. A concomitant increasing positive slope at a relative pressure between 0.15 and 0.58 shows the existence of some mesopores in CUK-113. The literature well documents that chemical activation effect the prepared materials in two different ways:

primarily one is the micropore generation via chemicals injection, and secondary is the widening of existing pores through chemical reactions inside the opened pores [50,51]. At a high chemical activating agent concentration, pore widening begins for the material possessing high intensity of opened pores. For the other samples, a sharp rise occurs initially and the adsorption slope in the middle region is negligible, indicating an abundance of micropores. The pore size distribution was estimated using nonlocal density functional theory (NLDFT) method; the results are shown in Fig. 6B. Surprisingly, all of the prepared adsorbents exhibit a substantial proportion of micropores smaller than 1 nm, with maximum peaks observed in the region below 1 nm, predominantly centered at 0.68 nm and 0.9 nm. For CUK-112, a broad peak is observed in the whole micropore region, suggesting that a high concentration of pores lies in the microporous region, with the absence of any mesopores. However, in CUK-113 micropore widening began, resulting in mesopore formation by increasing KOH ratio to 3. Fig. 6 (C, D) presents cumulative surface area and cumulative pore volume against pore diameter. For samples prepared with the predominantly high KOH concentration, CUK-101, CUK-112, and CUK113, the cumulative surface area and pore volume increased rapidly in the pore size region below 0.68 nm. On the other hand, samples prepared from high urea concentration, CUK-121 and CUK131, exhibit negligible values below 0.68 nm. Moreover, CUK-113 exhibit a sharp rise in the surface area for larger microporous (larger than 0.8 nm) and in somewhat mesoporous region owing to the high volume of these pores. However, all of the other samples exhibit a negligible increase in cumulative pore volume and cumulative surface area in the mesoporous region, ideal characteristic for attaining high CO2 adsorption [19,20,52]. Consequently, CUK112 was optimized as the sample possessing the highest volume of micropores (<1 nm), a high surface area, and appropriate nitrogen concentration. 3.2. CO2 adsorption The finely tunable ultra-microporosity with a heteroatomdoped carbon framework encouraged us to investigate the prepared materials as CO2 adsorbents. Fig. 7 shows the CO2 isotherms recorded at 1 bar and 273, 283, and 298 K; the detailed data are presented in Table S1. Under all of the investigated experimental conditions, the isotherms show completely reversible behavior and efficient high uptake in the ranges 147.4e280.5 and 112.2e173.7 mg g
1 at 273 and 298 K, respectively. With increasing adsorption temperature, a substantial decrease in adsorption capacity is observed because of the high molecular kinetic energy of the gas molecules at higher temperatures. Compared with the nitrogen adsorption at 77 K, adsorption at 273 K of CO2 can be higher because the increase in CO2 kinetic energy leads to a reduced energy barrier for gas molecules to diffuse into the ultra-micropores. For this reason, the

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Fig. 6. (A) N2-adsorption desorption isotherms (B) pore size distributions (C) cumulative pore volume and (D) cumulative surface area of samples prepared. (A colour version of this figure can be viewed online.)

Table 2 Textural properties of the adsorbents prepared. Samples N2 adsorption at 77 K

CO2 adsorption at 273 K

SBETa (m2/g) V<1nmb (cm3/g) V<2nmc (cm3/g) V<5nmd (cm3/g) Dporee (<2 nm) Vof (cm3/g) Vg (cm3/g) Lh (nm) Eoi (kJ/mol) Vj (cm3/g) Lk (nm) Eol (kJ/mol) CUK-101 CUK-111 CUK-121 CUK-131 CUK-112 CUK-113 a b c d e f g h i j k l

1477 1014 795 368 1746 2150

0.56 0.56 0.39 0.19 0.61 0.431

0.27 0.06 0.05 0.02 0.43 0.87

0.02 0 0 0.01 0.06 0.49

0.68, 0.68, 0.73, 0.90 0.89 0.68,

1.29 0.97 0.90

1.29

0.267 0.24 0.27 0.19 0.45 0.34

0.04 0.04 e 0.035 0.07 0.03

0.53 1.17 e 1.17 0.57 0.46

31.6 20.6 e 20.4 30.1 34.8

0.22 0.19 0.27 0.16 0.38 0.31

1.88 2.01 0.89 1.79 1.26 1.36

12.7 11.9 23.5 13.4 19.0 17.6

Surface area calculated by BET method. Pore volume less than 1 nm determined at P/Po ¼ 0.98. Micropore volume. Mesopore volume. Micropore diameters at the maximum of pore size distribution determined by the NLDFT method. Total pore volume determined by D-A equation. Micropore volume by porous network I. Micropore diameters determined by D-A equation. Energy determined for porous network I. Micropore volume by porous network II. Micropore diameters determined by D-A equation. Energy determined for porous network II.


th-Kawazoe (HK) method was used to derive the pore size Horva distribution curves from the CO2 adsorption isotherms recorded at 273 K (Fig. 7D). These results compliment those obtained from NLDFT calculations for nitrogen adsorption because most of the micropores are smaller than 1.0 nm. At 1 bar and 273 K, linear fitting of the relation among CO2 uptake and the fraction of microporous volume (<0.68, <0.73, <0.84, and <1 nm) reveals that micropores with a volume fraction

smaller than 1 nm show a straight-line graph with a maximum correlation coefficient of 0.91 (Fig. 8). These results lead to the conclusion that, for high adsorption of gas molecules, micropore filling predominates and requires a high micropore volume (<1 nm) for efficient adsorption [53,54]. However, designing a structure with a narrow pore size distribution is a challenging task in carbons by chemical activation. Henceforth, some researchers prefer nanotemplating as a sophisticated procedure for generating

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Fig. 7. CO2 adsorption isotherms obtained at (A) 273 (B) 283 and (C) 298 K (D) micropore size distribution determined by HK method using adsorption-desorption isotherms attained at 273 K. (A colour version of this figure can be viewed online.)

controlled pore size, but it is economically unfavorable [55e57]. Moving forward, using CO2 sorption isotherms obtained at 273 K, the textural nature of prepared samples and micropore size were determined using the DubinineAstakhov (DeA) equation [58,59] eq. (10):

"



A V ¼ Vo exp
bEo

2 # (10)

In this equation keeping the temperature (T) and pressure (P/Po) fixed, V signifies the filled volume, Vo denotes the micropore volume, while A ¼ RT ln (Po/P), and Eo and b represent the characteristic energy and the affinity coefficient (b ¼ 0.35 for CO2), respectively. Using Eo from eq. (10), Stoeckli et al. deduce a simple equation (eq. (11)) to determine the average pore width (L) of prepared carbon materials [60]:

LðnmÞ ¼ 10:8 = ðEO
11:4Þ:

(11)

From the literature it is evident that vapor adsorption and immersion calorimetry studies demonstrated that value of n is 2 for the best fitting of DeA equation for microporous carbons. This exhibits an intermediary condition between a monodisperse system for which n ¼ 3 and a strongly heterogeneous microporous system (n < 2) [58]. CUK-121, CUK112, and CUK-113 exhibit a linear fitting of DeA equation at a wide pressure range, indicating the abundance of uniform micropores (Fig. 9). Moreover, the constant slope of these curves can be observed up to ~40 (kJ mol
1)2, revealing the homogenously generated micropores in the

carbonaceous materials. Below 40 (kJ mol
1)2, a slight upward deviation indicates the widening of micropores. By contrast, the deviation of the D
A plots from straight line in CUK-101, CUK-111, and CUK-131 (Fig. S5) corresponds to the heterogeneous microporosity. From the DeA equation two important parameters including micropore volume (Vo) used for CO2 retention and the characteristic energy Eo can be deduced (Table 1) [61]. Dubinin proposed the volume-filling mechanism for CO2 adsorption instead of surface coverage and a high characteristic energy values ~30 kJ mol
1 reveal the ultra-microporosity in the prepared materials. Presence of ultra-narrow pores can overlap the potential field of adjacent pore walls, hereby, increasing the values of Eo from standard range obtained for microporous carbons (29e22 kJ mol
1) [62]. From the theoretical simulations, it is well known in the literature that elevation in the adsorption energy is negligible for the pores with the size larger than around two times the diameter of CO2 molecule for slit-like pores or three times for cylindrical pores. Considering the slit-shaped micropores in activated carbons, the optimized pore size for maximum CO2 adsorption by volumefilling mechanism, should be less than 1 nm, considering the kinetic diameter of CO2 molecule z0.33 nm [63]. Present results demonstrate that at 1 bar CO2 adsorption primarily depends on narrow micropores smaller than ca. 1.0 nm. Therefore, the trend observed in our work for CO2 adsorption is CUK-112> CUK-101> CUK-111> CUK-113> CUK-121> CUK-131 similar to the trend observed for micropore volume below 1.0 nm. These results can be further validated with the lack of correlation between total pore volume, surface area, nitrogen-content and CO2 adsorption.

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Fig. 8. Linear fitting of the experimental data presenting a plot of CO2 sorption capacity vs. the fraction of micropore volume of pore size <0.68 nm, <0.73 nm, <0.84 nm, and <1 nm. (A colour version of this figure can be viewed online.)

Moving forward, to investigate the potential of prepared adsorbent as selective towards CO2 only, we carried out nitrogen adsorption measurements at 298 K and 1 bar. In Fig. S6 adsorption isotherms for CO2 and N2 are plotted together for the sake of comparison. The linear N2 adsorption isotherm suggests weak affinity of adsorbents for N2. The CO2/N2 selectivity values lie in the range of (12e25) for all of the prepared materials [64e68]. As the prepared materials exhibit high affinity for CO2, the nature of adsorption and strength of the interaction between gas molecules and the prepared adsorbents were further explored. ClausiuseClapeyron equation (eq. (12)) was used to determine the isosteric heat of adsorption (DHads) at two different temperatures (283 and 298 K) [69,70].

    P DHads 1 1 ln 2 ¼

T2 T1 P1 R

(12)

The plots of DHads vs. CO2 uptake for all the prepared adsorbents are illustrated in Fig. S7 and summarized in Table S1. At a low surface coverage, the initial DHads lies in the range of 26.0e36.2 kJ mol
1, typical for physisorption by carbon-based adsorbents. Notably, the DHads values for all of the carbons with high a nitrogen content are greater than those for the samples with a decreased nitrogen content, indicating that nitrogenfunctionalities play a crucial role in the initial interaction among CO2 molecules and the carbon surface. The higher non-carbon contents, especially the electron-rich nitrogen atom can exhibit a strong dipoleequadrupole interaction with CO2, result in a considerable rise in the adsorption enthalpy. However, high initial DHads can also be the consequence of extremely narrow pore size.

With increasing adsorption pressure, DHads decreases indicating a decrease in the strength of interaction between adsorbent and CO2 molecules. At a high CO2 coverage, sample CUK-112, with the highest CO2 uptake capacity, exhibits a moderate DHads of 22.6 kJ mol
1. However, the nitrogen-free CUK-101 sample, exhibits a DHads of 25.1 kJ mol
1, similar to the DHads for other samples. These results suggest that the adsorption primarily depends on the narrow pore size distribution (<1.0 nm) [71]. The detailed comparison of present work with recently reported literature is summarized in Table 3. Furthermore, to elucidate the role played by micropore volume and nitrogen-content, Fig. S8 presents a plot normalized values of pyridinic-nitrogen, micropore volume (<1.0 nm) and CO2 adsorption at 273 K/1 bar V samples prepared. In this plot, CO2 adsorption, pyridinic-nitrogen and micropore volume exhibit a similar shape of curve. However, at some points where micropore volume and CO2 adsorption curve exhibit a gap in between them (CUK-121 and CUK-131), pyridinic-nitrogen play a complementary role for efficient adsorption. Our data, therefore, demonstrate a secondary role played by nitrogen moieties, particularly pyridinic-nitrogen, for enhancing CO2 adsorption. Concluding, the adsorption potential for an adsorbent can be enhanced by increasing the population of subnanometer pores is critical [86]. Therefore, it can be declared that the adsorption capacity predominantly depends upon the intensity of the ultra-micropores instead of the surface area, total pore volume or nitrogen-content. 4. Conclusions In summary, a facile and environmental benign strategy has

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635

Fig. 9. DubinineAstakhov (DeA) plots of CUK-121, CUK-112 and CUK-113 obtained from CO2 adsorption at 273 K. (A colour version of this figure can be viewed online.)

Table 3 A detailed comparison table of present work with recent reported literature. Material

CUK-112 CUK-113 WSC-500-1 GN-650-1 CS-500-1.5 NPC-650 NHPCT-4-7 SA-2-700 SNMC-1-600 CSCN_9_C60 OTSS-2-450 NDC-1-550 NPCs-2-500 T-GU-700-6 NGC-700-4 LC-450-2

SaBET (m2/g)

Synthesis conditions

Temperature (oC)

Activating agent

800 800 500 650 500 650 700 700 600 900 450 550 500 700 700 450

KOH KOH NaNH2 KOH KOH KOH ZnCl2 KOH KOH CO2 NaNH2 ZnCl2 ZnCl2 KOH K2CO3 NaNH2

1746 2150 1416 1734 503 1561 1361 1759 1021 1708 774.0 924 1040 1032 2827 1651

Vb<2nm (cm3/g)

1.04 1.17 0.53 0.62 0.24 0.65 0.46 0.66 0.35 0.89 0.27 0.27 0.45 0.59 1.06 0.67

been innovated to design a series of nitrogen-doped porous carbon materials from chitosan precursor via a chemical activation method. Tailoring the ultra-microporosity by regulating KOH and urea concentration leads to a high surface area (as high as 2150 m2 g
1) and an ultra-microporous structure with a pore volume as high as 0.6149 cm3 g
1 (<1 nm). In particular, CUK-112 exhibits the excellent adsorption performance with 280.5 mg g
1

Total nitrogen content (wt. %)c

1.0 0.62 2.42 9.24 8.5 4.11 1.89 1.93 5.11 3.2 2.94 7.3 6.3 2.59 4.69 3.85

CO2 uptake (mmol/g) at 1 bar 273 K

298 K

6.37 5.47 6.04 6.70 3.51 5.26 5.53 6.82 5.82 6.09 4.40 3.73 4.0 3.24 6.05 5.18

3.91 3.06 4.5 4.26 2.68 3.10 3.55 3.77 3.98 3.72 2.94 2.40 2.50 2.40 3.61 3.33

References

Present work Present work [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85]

(6.36 mmol g
1) of CO2 uptake at 273 K and 1 bar. Besides, the moderately strong heats of adsorption (26.0e36.2 kJ mol
1) exhibit a high adsorption potential owing to the presence of narrow micropores. To validate our work, we correlated the CO2 uptake with the micropore volume for different pore sizes and found the best correlation with pore size (<1 nm). These findings revealed that the adsorption capacity preferably depend on the fine pore size

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distributions rather than the overall specific surface area, total pore volume, and total nitrogen-content. Therefore, our present research effort deduce the conclusion that an optimal nitrogen-content and narrow micropores (<1 nm) are crucial for efficient CO2 adsorption, with the effect of the narrow-micropores being predominant. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Adeela Rehman: Writing - original draft. Soo-Jin Park: Writing - review & editing. Acknowledgements This work was supported by the Technology Innovation Program (10080293, Development of carbon-based non-phenolic electrode materials with 3000 m2 g
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