Environmental remediation by microporous carbon: An efficient contender for CO2 and methylene blue adsorption

Journal of CO₂ Utilization 34 (2019) 656–667

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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou

Environmental remediation by microporous carbon: An efficient contender for CO2 and methylene blue adsorption Adeela Rehman, Soo-Jin Park

T

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Department of Chemistry, Inha University, 100 Inharo, Incheon, 22212, South Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: One-step condensation Ultra-microporous carbons CO2 adsorption Methylene blue adsorption Environmental remediation

Nitrogen-doped microporous carbons with extremely narrow pore size distribution were derived from sucrose and melamine with the molten LiCl/KCl salts acting as a template to generate pores. A high CO2 uptake of up to 197 mg/g at 273 K and 1 bar was observed, revealing the significance of the pore size (0.84 nm) for CO2 capture. Our findings demonstrate that the nitrogen-doped SMLK-1 has a large surface area (951 m2/g) and large micropore volume (0.42 cm3/g) with high nitrogen-content (11.2 at.%), but the adsorption capacity (180 mg/g at 273 K, 1 bar) is comparable to that of un-doped SMLK-0 (SBET = 520 m2/g, Vmicro = 0.26 cm3/g). The CO2 uptake capacity per pore volume and volume fraction of the pores (< 0.84 nm) in total pore volume exhibit a straight-line graph with a high correlation coefficient of 0.93 revealing that the adsorption capacity of the carbonaceous adsorbents may strongly rely upon the fine pore size distributions. Moreover, the prepared materials were also investigated as adsorbents for methylene blue from an aqueous solution. The sample with a melamine-to-sucrose ratio of 0 (SMLK-0) exhibited the excellent dye adsorption, 134.7 mg/g for an initial methylene blue concentration of 25 × 10−2 mM and 31.1 mg/g for an initial concentration of 5 × 10−2 mM. Therefore, the fabrication method presented here permits the fine-tuning of the pore network to exploit the prepared materials both as CO2 and dye adsorbents for environmental remediation.

1. Introduction Carbon materials comprised of predominantly sp2 bonds have astonishing structural diversity, ranging from buckyballs (C60) and nanotubes to graphene. Therefore, intense research efforts have been dedicated to finding facile synthetic routes to carbonaceous materials [1–4]. As one example, salt melts are nowadays employed as reaction media for several organic and inorganic reactions. As many salts dissolve in water; thus, after reaction, the molten salts can be easily separated from the solid products. Bojdys et al. studied the condensation reactions of dicyandiamide in a LiCl/KCl melt at 380–600 °C, and found that uniform nanoplates with crystalline hexagonal structure were generated at 580 °C [5]. Recently, Antonietti's group demonstrated a relatively facile method for generating well-defined porosity in heteroatom-doped carbon materials using a novel sustainable technique known as “salt templating.” In this technique, molten salts (for example, LiCl/ZnCl2 and LiCl/KCl) are used as both reaction medium and as a template for pores generation [6,7]. This novel molten salt synthesis strategy has several advantages, for example, molten salts are excellent solvents at high temperatures and allow wide operating temperature conditions. Additionally, molten salts can serve as efficient

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thermochemical media with high-thermal storage capability along with enhanced heat transfer ability, as well as act as templating agents for the formation of porous structures. Recently, carbonaceous materials derived from biomass have been prepared by effective and affordable carbonization and activation reactions using molten salts [8]. Because biomass is rich in carbon and readily available from both plant and animal sources at low cost, recent research efforts have focused on carbon materials derived from biomass using different molten salts as activating agents. For instances, molten Na2CO3/K2CO3 salts have been used at 850 °C to transform peanut shell, boiled coffee beans, firewood, and bamboo shell biomass into carbonaceous materials [9,10]. Some other researchers have reported ZnCl2 or ZnCl2/KCl melts at low temperatures as effective reaction media to generate nitrogen-functionalized porous carbons with excellent energy storage performance [11]. Moving forward, the increase in industrial growth has resulted in a decline in environmental quality. In particular, air and water pollution by organic and inorganic contaminants has become a significant environmental concern [12]. Among the different contaminants, organic dyes comprise a major class of water pollutants having carcinogenic and teratogenic effects on living beings. There are numerous methods to

Corresponding author. E-mail address: [email protected] (S.-J. Park).

https://doi.org/10.1016/j.jcou.2019.08.015 Received 25 June 2019; Received in revised form 14 August 2019; Accepted 20 August 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

Journal of CO₂ Utilization 34 (2019) 656–667

A. Rehman and S.-J. Park

2.3. Preparation of methylene blue solutions

remove these pollutants including ozone treatment [13], photocatalysis [14], adsorption [15–17], and ion exchange [18]. Among all, the most widely used method is adsorption, which can remove organic dyes and contaminants effectively without generating by-products using carbonbased adsorbents [19–24] and nanocomposites [25–38]. For example, poly(1-vinylimidazole) ionic liquid-doped graphene shows high adsorption capacity for methylene blue [39]. In addition to aquatic pollution, air pollution, such as the high atmospheric concentration of carbon dioxide, is a major problem and one that is driving climate change. To mitigate global climate change and increasing air pollution, effective new approaches are required to adsorb CO2 from large point sources and carbonaceous materials have drawn attention by virtue of their inexpensive and facile synthesis, excellent regeneration, and relative lack of sensitivity to moisture in contrast to other CO2-philic materials [40]. Recently, new-generation N-doped carbons have been found to be interesting materials because of their physical and chemical properties [41,42]. In the literature different preparation methods are used for designing nitrogen-doped microporous carbons including physical or chemical activation methods. Most of them focus potassium salts (KOH, K2CO3 etc) [40]. On the basis of literature, the aim of the current research work is to design highly porous nitrogen-functionalized carbon materials via the carbonization of sucrose and melamine in a eutectic salt mixture (LiCl–KCl) that can be easily washed away by deionized water. Sucrose, which can be prepared from renewable starch resources, contains many hydroxyl groups, and melamine contains abundant amino groups. Currently, a successful synthetic strategy for the doping of nitrogen moieties in the carbon framework via N-alkylation is based on the combination of melamine with hydroxyl groups. Generally conventional methods are two-steps procedures for porous framework generation followed by nitrogen-doping [41]. However, present synthetic strategy is facile and economically favorable as it is a single-step procedure. Furthermore, present work proclaimed molten salt as a porogen to fabricate materials with well-developed porous structures. Meanwhile, the adsorption performance of fabricated carbon materials declared them as an efficient candidate for removal of both an organic dye, methylene blue (MB), from aqueous solution and CO2 from the air. Thus, present synthetic strategy provides new avenues using single material for different pollutant adsorbent for effective environmental remediation.

Solutions with five different concentrations of methylene blue (5 × 10−2, 10 × 10−2, 15 × 10−2, 20 × 10−2, and 25 × 10−2 mM) were prepared by dissolving accurately weighed amount of dye in distilled water. 2.4. Material characterization The prepared carbon samples were characterized by several analytical techniques. Thermal stability of materials was investigated by Thermogravimetric thermal analysis (TGA; TG209F3) under nitrogen atmosphere. X-ray diffraction (XRD) patterns were recorded by using an X-ray diffractometer (D2 Phaser, Bruker). To explore the morphological structures, high-resolution scanning electron microscopy (HR-SEM; SU8010, Hitachi Co., Ltd.) and high-resolution transmission electron microscopy (HR-TEM; JEM-2100 F) were used. Further surface characterization was performed by X-ray photoelectron spectroscopy (XPS, Escalab MK-II, VG Scientific Co.). 2.5. Adsorption measurements The porous nature of the prepared materials was explored using N2 adsorption − desorption isotherms. The adsorption capacities for CO2 were evaluated at 273, 283, and 298 K at 1 bar. To explore the selective adsorption behavior of the prepared samples, nitrogen adsorption isotherms were recorded at 273 and 298 K. All adsorption isotherms were obtained from a Belsorp Max instrument (BEL Japan, Inc.). Prior to the adsorption measurements, all the samples were degassed at 120 °C for 6 h under vacuum to evaporate any moisture and gas molecules adsorbed in the pores. The methylene blue adsorption experiment was conducted by a spectrometer with a SA-13.1 diffuse reflector (S-3100, Scinco Co., Ltd.). 3. Results and discussion 3.1. Morphological and structural analyses The synthesis of microporous carbon materials used melamine, a molecule with six nitrogen atoms, together with sucrose as a carbon source. Unlike the conventional route, we opted for a salt template pyrolysis by a eutectic salt mixture of LiCl/KCl (45:55 wt.%), with a melting point of 353 °C, to synthesize microporous carbon with sheetlike structure. In order to investigate an appropriate heating mode for the preparation of the microporous carbon, TGA experiments on SMLK1 (a 1:1 mixture of sucrose and melamine), melamine, and sucrose were carried out. TGA and calculated differential thermal analysis (c-DTA) curves are shown in Fig. 1. Concerning the pyrolysis of SMLK-1, the TGA results reveal a total 30% weight loss up to 800 °C, where the first step exhibiting an exothermic weight loss in the c-DTA curve of 22% occurred at about 190 °C. This was attributed to the partial elimination of bounded water molecules and the carbonization of sucrose. An exothermic peak in the c-DTA curve and a further mass loss (ca. 13%) in the TGA curve depicts the next stage of degradation around ˜195 to 400 °C. This weight loss can be attributed to the evaporation of molten salt and carbonization of the carbon from sucrose by a number of chemical reactions, consequently breakdown of intramolecular forces and the breakup of the molecular structure takes place accompanied by the loss of small molecular gases, such as hydrogen and ammonia. During the third step of degradation, which occurred at a higher temperature ˜400 to 800 °C, no distinct weight loss is observed in the TGDTA curve, suggesting the thermal stability of the carbon residue owing to the rigid structure formation by the conjugated p-orbitals possessed by the polycyclic-type fused rings. In the melamine TGA curve, the calcination of melamine under a nitrogen atmosphere resulted in a sharp mass loss between 297 and 392 °C indicating the thermal

2. Experimental 2.1. Materials Melamine (99%), sucrose (99%), lithium chloride (95%), potassium chloride (95%), and methylene blue (98%) were used in the present work. All the chemical reagents were obtained from Sigma–Aldrich and used without any further purification. 2.2. Synthetic protocol Microporous carbon materials were designed from aromatic organic precursors, melamine and sucrose, via single-step polymerization followed by the in-situ activation. The precursors in mass ratio of 1:1 were grinded in agar mortar for 30 min. Afterward, inorganic salts LiCl/KCl were added in the reaction mixture with the mass ratio of 10:1 of activating agent to the organic precursors. The homogenous powder was then transferred to tubular furnace for carbonization and activation at 800 °C for 180 min at the ramp rate of 3 °C/min under nitrogen atmosphere. Subsequently, the obtained black powder was cooled to room temperature and washed several times with deionized water and HCl solution to remove left over salt impurities. Finally, the dark black powder was dried at 130 °C overnight in an oven and carbon samples were named as SMLK-X, where X represents the melamine-to-sucrose (M/S) mass ratio. 657

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Fig. 1. TG-DTA curve for SMLK, Melamine and Sucrose under nitrogen atmosphere.

evident that the morphologies of materials prepared via molten salt route are quite different from that of the sample prepared in the absence of inorganic salts. More specifically, the non-activated sample (SM) shows a typical non-porous structure. However, SMLK-0 exhibits a cheese-like morphology with irregular pits on the surface and interconnected pores. Sponge-like morphologies with micropores can be observed from the images of SMLK-0.5, SMLK-1, and SMLK-2. In the HR-TEM images (Fig. 3), a folded sheet like morphology with no apparent long-range order can been seen for the SMLK-1 sample, implying an amorphous structure in the as-prepared sample. Additionally, XRD patterns were recorded at the room-temperature to attain phase information for all the prepared samples. Fig. S1 displays XRD patterns of all the samples. All the samples containing melamine yielded two broad

condensation by melamine to generate carbon nitride and NH3. In the TG-DTA plot of pristine sucrose, a rapid weight loss initiated at 270 °C and dramatically raised up to 80% at 400 °C, inferring that the carbonization of sucrose took place between 270 and 400 °C. The initial weight loss until 100 °C was ascribed to the loss of adsorbed moisture from the sucrose surface [43]. Sucrose decomposes by the reaction shown in Eq. (1).

C12 H22 O11 → 12 C + 11H2 O

(1)

Interestingly, the mass of the residue of SMLK-1 is greater than either of pure sucrose or melamine, revealing the condensation reaction occurring between sucrose and melamine in the salt melt. Fig. 2 shows the HR-SEM images of the as-obtained samples. It is

Fig. 2. HR-SEM images of (A) SM (B) SMLK-0 (C) SMLK-0.5 (D) SMLK-1 (E) SMLK-2 at different magnifications. 658

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Fig. 3. HR-TEM images of SMLK-1 at different magnifications (A) 200 nm (B) 50 nm and (C) 10 nm.

presence of nitrogen species is necessary in an adsorbent for dye removal because nitrogen doping maximizes the electrostatic interactions between the sorbent and adsorbate at the surface, thus enhancing the sorption capacity. Recent research has demonstrated that nitrogen doping can generate electrostatic interactions in inert graphene-like carbon. However, the incorporation of nitrogen at different locations in the carbon structure has different effects. Generally, in comparison with the lower activity of graphitic nitrogen, pyrrolic nitrogen has a moderately high electron donating ability and high charge mobility and pyridinic nitrogen offers electron pair to conjugate with the π-electronic rings, which reinforces the carbon during electron-transfer reactions. Unsurprisingly, the nitrogen content of the SMLK samples strongly depended on the M/S ratio of the starting material and increased from 8.5 wt.% for SMLK-0.5 to 20.4 wt.%for SMLK-2. Presence of a small amount of nitrogen in SMLK-0 can be attributed to the nitrogen atmosphere used during calcination process. From the integrated areas, the nitrogen species distribution could be estimated for the SMLK-1 sample relative to total nitrogen: pyridinic nitrogen (31%), pyrrolic nitrogen(23%), and quaternary nitrogen (46%). As presented in Table S1, the quaternary nitrogen increased in the resultant SMLKs with an increase in M/S ratio up to 1. However,by further increasing the M/S ratio to 2, the quaternary N decreased to 15% in relative terms and up to 3.0% on an absolute scale. This indicates that selective removal of nitrogen took place during activation. Consequently, pyrolysis leads to generation of most stable pyridinic and quaternary nitrogen, whereas, under oxidizing environment such as during chemical activation, pyrrolic-nitrogen groups are expected to be form.

peaks located at 2θ values of around 25° and 43°, commonly labelled as (002) and (101) peaks, respectively, characteristics of hexagonal graphite structure, with the coke-like morphology and disordered carbonaceous interlayers [44]. The slightly sharper characteristic peak at 2θ of 43° probably arose from the (002) stacking of triazine units. The broad peaks propose poor crystallinity of the carbon materials [45]. For SM, 2θ peaks are intensified owing to shrinkage of the average carbonto-carbon distance, as a result of evaporation of volatiles during the pyrolysis reaction. No other inorganic salt impurities were observed in the XRD pattern. XPS was used to examine the nature of nitrogen species in the prepared materials. The XPS survey spectra (Fig. S2) contain three distinct peaks for carbon, nitrogen, and oxygen, with no K or Li signals,indicating the complete removal of inorganic salts during washing. Fig. 4 shows the deconvolution of the N1 s peaks, percentage fraction of distinct types of nitrogen, and location of nitrogen moieties in the carbon framework. The position of nitrogen in the porous carbons has been well‐known, and, generally, it exist in a highly coordinated "pyrrolic" nitrogen form together with the pyridinic nitrogen inoculated into the graphene layers [46]. Here, three peaks were appeared in the N 1s spectrum (Fig. 4A). The peaks located at 398.2, 399.4, and 401.1 eV can be ascribed as pyridinic, pyrrolic, and graphitic nitrogen [47,48]; the contents of the three types of nitrogen atoms are presented graphically in Fig.4B. SM prepared by pyrolysis contains the maximum fraction of graphitic nitrogen, which is considered to be the most stable form of nitrogen under pyrolysis conditions. On the other hand, for the activated carbons, the other two peaks representing pyridinic and pyrrolic nitrogen are of high percentages. These nitrogen‐functionalities act as Lewis basic sites for anchoring acidic CO2 molecules. Furthermore, the 659

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Fig. 4. (A) N 1s spectra (B) fraction % of distinct types of nitrogen (C) location of nitrogen moieties in carbon frame work of prepared samples.

activated carbon materials possess a significant portion of micropores below 1 nm. As evident, the surface area and pore volume for SMLK-0 and SMLK-1 increased abruptly for pores below 0.8 nm. In contrast, for SMLK-0.5, a sharp increase in the surface area was observed in the region of larger micropores (above 0.8 nm) owing to the larger volume of these pores. However, SMLK-2 showed a gradual rise in the surface area and pore volume over the whole range of micropores. Furthermore, to investigate the effect of the M/S ratio on the pore structures, we fixed the synthesis temperature at 800 °C. The Brunauer–Emmett–Teller surface area (SBET) value of sample SMLK-0 (no melamine) is 520 m2/g. With an increase in M/S ratio, the value of SBET increased from 520 m2/g to the maximum 951 m2/g, and the total pore volume (Vtot) value increased from 0.29 to 0.52 cm3/g. Nevertheless, further changing the M/S from 1.0 up to 2.0 leads to a reduced porosity. It is known that when pristine melamine is carbonized in LiCl/KCl at 800 °C, a negligible amount of carbon residue is left. Our results indicate that complicated polymerization and condensation reactions took place between melamine and sucrose in the molten salt medium. At high precursor concentrations, the monomers and intermediates

3.2. Pore development and textural features The porous nature and textural features of as-prepared materials were studied by low temperature nitrogen gas adsorption. Figs. 5(A–D) show the adsorption-desorption isotherms, pore size distributions, cumulative surface area, and cumulative pore volume, respectively, and the detailed data are summarized in Table 1. The non-activated carbon, SM, exhibit a decreased nitrogen uptake, demonstrating insufficient number of pores with a size greater than the kinetic diameter of N2 molecules (0.364 nm). The samples synthesized using inorganic salts showed type-I curves (IUPAC classification) with a sharp uptake at a low relative pressure (P/P0 < 0.01), which indicate that the samples mainly comprise of micropores. For SMLK-1 and SMLK-2, the broad "knees" in the isotherms indicate the presence of broad pore size distribution. At high relative pressure (P/P0 > 0.9), all the activated materials show a capillary condensation with a H1-type hysteresis loop, further confirming the microporous nature of the SMLKs [49,50]. In Figs. 5(B, C), the cumulative surface area and pore volume of the prepared samples are plotted vs. the pore width. Remarkably, all the 660

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Fig. 5. (A) Nitrogen adsorption-desorption isotherms, (B) cumulative surface area, (C) cumulative pore volume and (D) pore size distributions of the adsorbents prepared.

SMLK-1 reveals the presence of well-defined porous systems. These larger micropores serve as a route for adsorbate molecules to access ultra-micropores, thus enhancing adsorption.

exhibit a limited solubility in salt melts, which can give rise to premature phase separation. Additionally, the sublimation of excess melamine, consequently, decreases the actual precursor concentration during carbonization. In contrast, at a low M/S ratio, the excess salts can hinder the fabrication of continuous carbon networks. Therefore, the optimal melamine/sucrose precursor ratio for achieving a well-developed porosity seems to be 1.0 at 800 °C. Moreover, the sample obtained at 800 °C in the absence of molten salt has a BET surface area of only 45 m2 g−1 and total pore volume of 0.02 cm3 g−1, signifying the indispensable role played by the molten salt during pore development. The pore size distribution analyses (Fig. 5D) shows that the prepared samples mainly contained micropores. The ultra-micropores (< 0.7 nm) together with larger micropores (ca. 1.0 nm) in SMLK-0 and

3.3. Gas adsorption by microporous carbons The excellent textural features with well-defined porosity motivated us to explore the gas adsorption potential of the as-prepared materials. Fig. 6 illustrates the CO2 adsorption-desorption isotherms at three different temperatures (273, 283, and 298 K) and 1 bar. The isotherms for all the samples were completely reversible, except for that of SMLK-0, which exhibited a hysteresis, indicating that energy is required for the recycling of the adsorbents. Interestingly, the activated SMLK-0 sample

Table 1 Textural properties of the adsorbents prepared. Samples

N2 adsorption at 77 K SBET a(m2/g)

CO2 adsorption at 273 K

Vtotb(cm3/g)

V < 2nm c (cm3/g)

V2-5nm d(cm3/g)

SM SMLK-0

45 520

0.02 0.29

0.02 0.26

0.0001 0.03

SMLK-0.5 SMLK-1

410 951

0.28 0.52

0.23 0.47

0.05 0.05

SMLK-2

760

0.47

0.42

0.05

a

Dpore e(< 2 nm)

I

1.2 0.68, 1.20 0.97 0.58, 1.39 0.97

b

II

Vtotalf(cm3/g)

V g(cm3/g)

L h(nm)

V i(cm3/g)

L j(nm)

0.03 0.21

0.07

0.82

0.03 0.14

0.70 1.1

0.10 0.18

0.06

0.65

0.10 0.12

0.79 1.02

0.14

0.76

0.14 c

d

e

Surface area calculated by BET method. Total pore volume determined at P/Po = 0.98. Micropore volume. Mesopore volume. Micropore diameters at the maximum of pore size distribution determined by the NLDFT method. fTotal pore volume determined by D-A equation. gMicropore volume by porous network I. h Micropore diameters determined by D-A equation. iMicropore volume by porous network II. jMicropore diameters determined by D-A equation. 661

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Fig. 6. CO2 adsorption-desorption isotherms at three different temperatures and micropore size distributions of all the fabricated carbon materials determined at 273 K. Table 2 Gas and dye adsorption data of fabricated carbon materials. Materials

SM SMLK-0 SMLK-0.5 SMLK-1 SMLK-2

ΔHads for CO2 (kJ/mol)

CO2 uptakes(mg/g) 273 K 1 bar

283 K 1 bar

298K 1 bar

51 197 152 180 168

47 179 133 154 144

43 170 106 132 119

35.4 27.2 36.2 24.5 43.1

N2 uptake(mg/g)

Methylene blue adsorption

273 K 1 bar

298 K 1 bar

Quantity adsorbed (mg/g)

Removal efficiency (%)

4.48 15.69 14.96 12.40 10.03

2.31 8.35 9.16 6.62 5.41

29.5 31.1 29.3 31.0 31.3

92.3 97.3 91.9 97.0 98.0

2

exhibited an excellent CO2 uptake of 197 mg/g at 273 K and 1 bar, as shown in Table 2. At 1 bar, the carbon dioxide capturing capacity of the prepared materials lies in the range of 51–197 and 43–170 mg/g at 273 and 298 K, respectively. The adsorption capacity of prepared adsorbents is comparable to those reported earlier (Table S2). The CO2 uptake decreased significantly with an elevated adsorption temperature attributed to the increased molecular kinetic energy of CO2 at elevated temperature. Generally, at 77 K N2 adsorption is used to study textural features of activated carbons, but, at 273 K, an increased kinetic energy of the CO2 molecules (due to elevated adsorption temperature) leads to a small energetic barrier for the diffusion of CO2 into the narrow micropores [51,52]. Thus, the adsorption data obtained at 273 K were interpreted by the Horvath–Kawazoe (H–K) method to reveal the presence of ultramicropores (< 0.8 nm) in the prepared samples. In addition, the CO2 sorption isotherms were simulated using the Dubinin–Astakhov (D-A) equation [53,54], as shown in Eq. (2).

A ⎞⎤ ⎡ V = Vo exp ⎢−⎜⎛ ⎟ βEo ⎠ ⎥ ⎝ ⎣ ⎦

(2)

Here, at a particular temperature (T) and a relative pressure (P/Po) V denotes the volume filled, Vo represents the micropore volume, A = RT ln(Po/P), and Eo and β are characteristic energy and the affinity coefficient (β = 0.35 for CO2), respectively. The D-A plots deliver significant data regarding pore size in micro and ultra-micropore region [55]. The average pore width, L, can be determined by the empirical correlation given by Stoeckli et al. [56]:

L(nm) = 10.8/(EO − 11.4).

(3)

In the case of SMLK-0 and SMKL-1 (Fig.7), the characteristic curves indicate two different types of pores. At the region of highest adsorption potential, the curves deviate slightly downward revealing the presence of very narrow micropores. For all samples (Figs. 7 and S3), the characteristic curves have a constant slope in the region where adsorption takes place in the micropores. This region covers up to about 662

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micropores for efficient CO2 adsorption. Presser et al. [59] empirically correlate the amount of gas adsorbed and micropore volume of carbons and deduce that other pores with pore size larger than 0.8 nm have negligible effect on CO2 adsorption capacity at low pressure and temperature (i.e., T≈273–298 K, PCO2≤ 1 bar). Such results are in agreement with the lack of correlation between CO2 adsorption capacity of carbon and overall surface area and pore volume. To validate these studies, we correlated our observed CO2 uptake with the specific surface area or total micropore volume but found no direct relationship. For example, although the nitrogen-doped SMLK-1 has a large surface area (951 m2/g) and large micropore volume (0.42 cm3/g), but the adsorption capacity (180 mg/g at 273 K, 1 bar) is comparable to that of un-doped SMLK-0 (SBET = 520 m2/g, Vmicro = 0.26 cm3/g). These consequences reveal that the adsorption capacity of the carbonaceous adsorbents may strongly rely upon the fine pore size distributions instead of overall specific surface area, total pore volume, or heteroatom doping. Fig. 8 illustratethe relation between CO2 sorption capacity and the volume fraction of micropores with different sizes under similar sorption conditions. At low temperature (273 K), the CO2 uptake capacity per pore volume and volume fraction of the pores (< 0.84 nm) in total pore volume exhibit a straight-line graph with a high correlation coefficient of 0.93 (Fig. 8C), demonstrating that these narrow micropores primarily account for the higher CO2 uptake by carbons at 273 K. Moreover, micropore volumes determined by N2 (Vtot) and CO2 (D − R plot, Vtotal) reveal two interesting features. For the non-activated sample (SM), Vtotal > Vtot, which reflects the restricted diffusion offered by narrow micropores to N2 molecules. Contrarily, activated carbons exhibit Vtot > Vtotal owing to the restricted adsorption of CO2 molecules into micropores with diameters less than 0.8 nm. To probe the CO2/N2 adsorption selectivity, the nitrogen adsorption of the prepared samples was carried out at 273 and 298 K (Fig. S4). The CO2 adsorption capacity by all the prepared materials is far higher than that of N2, inferring that thestudied carbon materials exhibit excellent selectivity for CO2 uptake. Furthermore, to illustrate the critical role of small micropores in the CO2 uptake, the isosteric heat of adsorption (ΔHads) was calculated by applying Clausius–Clapeyron equation [60,61] to the CO2 adsorption isotherms at 273 and 283 K.

P ΔHads ⎛ 1 1 − ⎞ ln ⎛ 2 ⎞ = − T1 ⎠ R ⎝ T2 ⎝ P1 ⎠ ⎜

Fig. 7. Dubinin–Astakhov (D–A) plots of SMLK-0 and SMLK-1 obtained from CO2 adsorption at 273 K.

⎟

⎜

⎟

(4)

The ΔHads value reflects the strength of the interaction among CO2 molecule and adsorbents. The variation in ΔHads with CO2 sorption shows a decreasing trend (Fig. S5). In the initial stages of adsorption, a high ΔHads is observed, demonstrating that the pore surface of the carbon materials strongly interacts with CO2 molecules, which could be accredited to the strong adsorption potential of extremely narrow micropores. After the initial adsorption stage, the curves show a significant drop, which is almost constant with an increase in sorption pressure and indicates a significant decrease in the interaction strength between the carbon materials and CO2. Moving forward, nitrogen doping is generally considered beneficial for enhanced CO2 adsorption. However, the difference of CO2 adsorption between un-doped and nitrogen-doped carbons in many cases is marginal. In the present study, the trend for nitrogen content (at.%) is SMLK-2 (20.4 at.%) > SM (13.2 at.%) > SMLK-1 (11.2 at.%) > SMLK0.5 (8.5 at.%) > SMLK-0 (0.2 at.%). However, the CO2 adsorption trend at 273 K and 1 bar is SMLK-0 (197 mg/g) > SMLK-1 (180 mg/ g) > SMLK-2 (168 mg/g) > SMLK-0.5 (152 mg/g) > SM (51 mg/g). To determine the contribution of nitrogen in CO2 adsorption, the values of pyridinic, pyrrolic, graphitic and total nitrogen content were normalized, and plotted Vs samples prepared (Fig. S6 A). Total nitrogen and pyrrolic content exhibit a similar trend for all the prepared samples. However, pyridinic and graphitic nitrogen show difference in trend for samples prepared with high initial concentration of melamine. It is well-known that pyrrolic nitrogen contributes more for CO2 capture

40 (kJ/mol)2 indicating that the micropores were homogeneously generated in the prepared samples. From this point, the characteristic curves deviate slightly upward, revealing the presence of some wider micro/mesopores. Additionally, the adsorption mechanism for CO2 in these carbon materials is volume-filling instead of surface coverage and CO2 molecules filling these micropores are in a liquid-like state. Dubinin, demonstrated a detailed explanation of the volume-filling mechanism by suggesting a relationship between degree of micropore filling and the partial pressure of the adsorbate (D–R equation) [57,58]. It has been verified theoretically that the increase in the adsorption energy is insignificant for micropores with width more than around two times of the molecular diameter of CO2 for slit-shaped pores or three times for cylindrical-shaped pores. Considering the slit-like rather than cylinder shape of the micropores in carbonaceous materials, the size limit for optimum volume-filling can be estimated as ca. 0.7 − 0.8 nm for CO2 (kinetic diameter of CO2 molecule ≈ 0.33 nm). This outcome can easily relate the CO2 capture under conventional operating conditions (temperatures from 273 to 298 K and CO2 pressures from 0 to 1 bar). At relatively low pressures (P/P0 < 0.03), CO2 adsorption occurs via a volume-filling and predominantly depends on narrow micropores below ca. 0.8 nm. These findings conclude that non-functionalized carbon-materials mainly comprises of high volume of narrow

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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 with (A) presenting micropores < 0.68 nm, (B) < 0.73 nm, (C) < 0.84 nm, and (D) < 1 nm.

adsorbents under UV–vis light irradiation. The equilibrium adsorption values (qe, mg/g) after 50 min and the color removal efficiency are listed in Table 2, and the UV–vis spectra are presented in Fig. S7. From the data, it is evident that SMLK-0, SMLK-1, and SMLK-2 yield similar results. Hence, for a detailed study, SMLK-0 was selected and plots of the methylene blue adsorption under UV light for different periods and initial dye concentrations are presented in Figs. 9(A–D). According to Fig. 9C, the rapid initial adsorption stage is attributed to the dye molecules uptake by the vacant external surfaces of adsorbent. The subsequent step exhibits a slow adsorption owing to slow diffusion in porous structure as maximum available external sites are already occupied in the initial stages [64]. Optimal pore size (1.0–4.2 nm) of welldefined porous structure enable the facile diffusion of dye molecules with dimensional size is of 1.43 nm × 0.61 nm × 0.4 nm. With the passage of time, some of the residual vacant surface sites still remained unavailable for adsorption owing to steric hindrance between the adsorbed dye molecules and that in the solution phase. Furthermore, for different initial concentrations of MB, the highest qe value was obtained for SMLK-0, 134.7 mg/g at an initial MB concentration of 25 × 10−2mM. The concentration of the MB dye solution was reduced significantly based on the color removal. The surface area is not the only parameter regulating the adsorption performance of the carbon-based adsorbents [65,66]. It is obvious that SMLK-1 has a much larger surface area (951 m2/g) than SMLK-0 (520 m2/g), but they possess similar dye adsorption capacities. The excellent adsorption performance of the prepared carbon materials is attributed to the π-electron donor-acceptor interactions of the dye with the graphitic layers. MB contains doubly bonded CeC and π-electrons [67]. The interaction of π-electrons of MB with the π-electrons of graphitic carbon adsorbents generate strong π–π electronic forces and as a

[40]. Hence, in order to visualize the contribution of micropore volume (< 0.84 nm) and pyrrolic content for CO2 adsorption at 273 K and 1 bar, the values are normalized and plotted in Fig S6 B. Upon comparison, it is revealed that micropore volume is a major contributor towards CO2 adsorption as both have similar trend. However, one cannot deny the role of pyrrolic nitrogen content for SM, SMLK-0.5 and SMLK-2 where relatively big gap is observed between CO2 adsorption and micropore volume curves. At these specific points, pyrrolic nitrogen curve approaches gas adsorption curve, signifying a reimbursing role of pyrrolic nitrogen for the samples with moderate textural features. Hence, there has been no clear evidence for the primary role played by nitrogen atoms in CO2 adsorption, and the adsorption at lower pressures depends primarily on the fraction of the micropores (< 0.84 nm) [62]. Normally, strong van der Waals interactions drive the gas–solid adsorption potential. Particularly, the adsorption potential is maximum for the pores with the diameter in sub-nanometer range, where the decreasing pore size leads to overlapping of potentials, and as a consequence higher binding energy is generated by the formation of deep potential wells. Thus, it is crucial to increase the intensity of sub-nanometer pores [63]. The CO2 adsorption capacity of the SMLK-0 sample is, thus, due to the fine ultra-micropores. Thus, the adsorption capacity primarily depends on the population of the ultra-micropores instead of surface area or micropore volume. 3.4. Methylene blue adsorption To evaluate the effectiveness of the present work for environmental remediation under aqueous conditions, we designed an adsorption experiment with the organic dye methylene blue (MB). Using an initial dye concentration of 5 × 10−2 mM, all the samples were tested as 664

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Fig. 9. UV–vis spectra of SMLK-0 at different times, (B) plot of Ct/Co Vs time, (C) the adsorbed amount of methylene blue (qt) at different time intervals, and (D) the amount of methylene blue adsorbed at equilibrium (qe) at different initial concentrations.

Thus, it can be demonstrated that active sites for adsorption are homogeneously present on the surface of the adsorbent and MB adsorption follows Langmuir isotherm model.

result high adsorption of MB took place. On the other hand, the effectively high adsorption of MB by high nitrogen-containing SMLK-2 and SM is accredited to the cationic nature of MB generating a favorable electrostatic interaction with the basic nitrogen sites on the adsorbent surfaces. From the literature, it is evident that different adsorbents are used to remove organic dyes and pollutants from aqueous conditions [68–81]. The stepwise mechanism for MB adsorption involves the initial migration of dye molecules from solution to the adsorbent followed by the diffusion of these molecules on the adsorbent surface leading to their capture by the surface-active sites. Lastly, intra-particle diffusion transfers the dye molecules to the interior of porous network [82]. For kinetic modeling, linear plots of ln(qe− qt) vs. t, and t/qt vs. t, are shown in Figs. S8 (A, B), respectively. The determination coefficients, R2, for the pseudo-first-order and pseudo-second-order models comes out 0.78 and 0.99, respectively. The qe value determined from the pseudo-second-order model is in accordance with the experimental data. This shows that pseudo-second-order kinetics is followed during MB adsorption [83]. Besides, Langmuir and Freundlich isotherm models were used to further explore the adsorption isotherms. The Langmuir model proposes that adsorption occurs homogeneously on the surface of adsorbent as a monolayer only. Plotting Ce/qe Vs Ce leads to a straight line with a slope and intercept of 1/qm and 1/(KLqm), respectively (Fig. S9 A). On the other hand, the Freundlich isotherm suggest heterogeneous surfaces of adsorbent leads to multilayeradsorption. Plotting lnqe vs. lnCe results in a straight line with lnF and 1/nF as intercept and slope, respectively (Fig. S9 B). Upon comparison of both the isotherm models, it is revealed that the experimental data is best fitted by the Langmuir model than the Freundlich isotherm model.

4. Conclusions In summary, in this study, we focused on a single-step preparation and activation in the presence of molten salts, which acted as effective porogens, to produce carbon materials possessing high nitrogen content and well-defined porous structures. The maximum specific area obtained wasthat of the SMLK-1 sample (951 m2/g), with 0.52 cm3/g total pore volume.In contrast, the sample synthesized at 800 °C in the absence of molten salt exhibit a decreased surface area of only 45 m2 g−1 and 0.02 cm3 g−1 total pore volume, signifying the major contribution of the molten salt in order to design carbons with highly porous nature and increased surface area. Thus, molten salts can be used as effective porogens instead of sacrificial templates for generating well-defined porosity. At 1 bar, the carbon dioxide capturing capacities of the prepared materials were found to be in the range of 51–197 and 43–170 mg/g at 273 and 298 K, respectively. Present study reveals the indispensable role played by the number of ultra-micropores for improved CO2 adsorption capacity. This is because the ultra-micropores possess a high adsorption potential that leads to enhanced gas adsorption. Hence, a well-developed micropore network with the predominantly narrow ultra-micropore size distribution is a requisite for obtaining the optimal packing of the gas molecules at room temperature and low pressure. Furthermore, the prepared sorbents were investigated as adsorbents for organic dyes in an aqueous environment. 665

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The highest adsorption value by SMLK-0 was 134.7 mg/g at an initial methylene blue concentration of 25 × 10−2mM. Hence, the current strategy provides a route to prepare adsorbents with multiple uses in the area of environmental remediation.

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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.

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Acknowledgements

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This work was supported by the Technology Innovation Program (10080293, Development of carbon-based non-phenolic electrode materials with 3000 m2 g−1 grade surface area for energy storage device) funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea) and the Commercialization Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science and ICT (MSIT) [2018_RND_002_0064, Development of 800 mA h·g−1 pitch carbon coating materials].

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Appendix A. Supplementary data

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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2019.08.015.

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