Sulfur–Nitrogen Codoped Graphite Slit-Pore for Enhancing Selective

Aug 14, 2017 - Using Grand Canonical Monte Carlo calculations, sulfur and nitrogen codoped (S/N-) graphite slit-pores with different defects (Divacanc...
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Research Article pubs.acs.org/journal/ascecg

Sulfur−Nitrogen Codoped Graphite Slit-Pore for Enhancing Selective Carbon Dioxide Adsorption: Insights from Molecular Simulations Xiaofang Li,†,‡ Qingzhong Xue,*,†,‡ Daliang He,†,‡ Lei Zhu,†,‡ Yonggang Du,‡ Wei Xing,‡ and Teng Zhang§ †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, Shandong, People’s Republic of China ‡ College of Science, China University of Petroleum, Qingdao 266555, Shandong, People’s Republic of China § Nano Science and Technology Program, Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong S Supporting Information *

ABSTRACT: Using Grand Canonical Monte Carlo calculations, sulfur and nitrogen codoped (S/N-) graphite slit-pores with different defects (Divacancy 5-8-5, NS-1, NS-2, Stone−Wales (SW) 5577) are constructed to study their selective CO2 adsorption from CO2/H2, CO2/N2, CO2/CH4 and CO2/H2O mixtures. Among all the defective S/N-graphite slit-pores, it is found that the doped sites of S and N atoms affect slightly on CO2 uptake of graphite slit-pore. More importantly, the increasing ratio of S/N enhances the selective CO2 adsorption. For example, at 300 K and 1 bar, the full Ngraphite slit-pore with SW 5577 has a CO2 uptake of 79.77 mmol/mol with good CO2/H2 selectivity (∼356) whereas full S-graphite slit-pore with SW 5577 possesses a considerable CO2 uptake of 104.66 mmol/mol with excellent CO2/H2 selectivity (∼526). Furthermore, density functional theory calculations indicate this defective S/N-graphite slit-pore with higher ratio of S/N interacts more strongly with CO2 molecules compared with other gases, which demonstrates that S doping is a better choice than S/N codoping for impressive selective CO2 capture. KEYWORDS: Grand Canonical Monte Carlo, CO2 capture, Density functional theory, Defect, Adsorption energy

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INTRODUCTION Carbon dioxide (CO2), an impurity in natural gas, biogas, syngas, flue gas and many other gas streams, has caused global warming and environmental deterioration.1,2 Therefore, the capture of CO2 from emissions is a valid method to lower CO2 concentration in the atmosphere.3 Using cleaner energy is another effective method to reduce CO2 emissions. Natural gas mainly consisting of methane (CH4) is a kind of desirable fuel because it burns more cleanly and has a higher hydrogen to carbon ratio, which results in lower CO2 emissions.4 However, natural gas is degraded by the coexistence of CO2, which can also cause pipeline corrosion. Therefore, the separation of CO2 from natural gas is vital to obtain high-quality natural gas. Apart from natural gas, hydrogen (H2) is also an attractive and environmentally friendly fuel because it does not produce CO2 during combustion.5,6 Consequently, it is of great importance to separate CO2 from H2. Moreover, it should be noted that water and nitrogen (N2), which are also inevitable impurities in gas mixture, should also be considered because of their competitive adsorption with CO2.7−9 Carbon materials are regarded as better CO2 sorbents owing to their high surface and unique properties.10−12 However, the © 2017 American Chemical Society

CO2 uptake and selectivity of pristine carbon materials are extremely limited.13 As a consequence, recent studies are focusing on modifying carbon materials with heteroatoms and functional groups to enhance their CO2 adsorption capacity. Nitrogen (N)-containing functional groups are considered to enhance the CO2 adsorption capacity of porous materials.14−16 For example, Li et al.14 synthesized hierarchical porous nitrogen-doped carbons that showed a moderate CO2 uptake of 2.69 mmol g−1 at 25 °C and 3.82 mmol g−1 at 0 °C under 1 bar with an excellent CO2/N2 selectivity (∼134). Nandi et al.15 prepared a series of highly porous N-doped activated carbon monoliths that exhibited considerable CO2 uptake of 5.14 mmol g−1 at 300 K and 1 bar. The excellent CO2 uptakes of Ndoped carbons were ascribed to the abundant micropores (∼1 nm) and the presence of basic N-containing groups.16 Moreover, relative research has explained the enhancement mechanism of CO2 uptakes of N-doped materials using acid− base interaction,11 quadrupolar interaction17 and hydrogen bonds interaction.18 Received: May 22, 2017 Revised: July 31, 2017 Published: August 14, 2017 8815

DOI: 10.1021/acssuschemeng.7b01612 ACS Sustainable Chem. Eng. 2017, 5, 8815−8823

Research Article

ACS Sustainable Chemistry & Engineering Apart from N-containing groups, carbon materials with sulfur (S)-containing functional groups were also found to have considerable CO2 adsorption capacity.19−23 More importantly, the substitution of carbon atoms of a graphene structure with sulfur atoms has been successfully achieved by exposing it to H2S for 3 h at 1073 K.21 Xia et al.22 prepared a series of sulfurdoped, structurally well-ordered, zeolite-like microporous carbon materials by a nanocasting method using zeolite EMC-2 as hard template, which exhibited high CO2 adsorption energy of 59 kJ/mol. Seema et al.19 found that S-doped microporous carbon materials showed a large CO2 adsorption capacity of 4.5 mmol/g at 298 K and 1 bar. Besides, acid−base interaction and polar interactions were used to elucidate an enhancement mechanism of CO2 uptake for S-doped carbon materials.23 Very recently, sulfur and nitrogen codoped (S/N-) graphene24−29 has been successfully synthesized via different methods. Specially, Wei et al.26 succeeded in synthesizing sulfur and nitrogen dual-doped porous carbon nanosheets using the mixture of camellia petals and ammonium persulfate, which was found to have high BET surface of 1122 m2 g−1 and superior energy storage performance. As known, the graphite slit-pore, a kind of porous carbons, has been widely investigated for gas capture and separation.12,20,30,31 Huang et al. have studied the effects of the slit width, system temperature, concentration, and the constituent ratio of the mixture on adsorption of formaldehyde, oxocarbons (CH2O, CO2, CO), and water in graphitic slit-pores using molecular dymanics simulations. Besides, they found that self-diffusivity for gas mixtures was lower than the single adsorbate due to the interactions between the components. Importantly, water was found to react with CH2O and dissolve CO2, which could affect the cohesion between gas molecules and graphene surface.12 Additionally, they also found that the metal dopant in the graphitic micropores resulted in competitive adsorption because of the difference in the adsorption characteristics between the two distinct adsorbent materials.30 Here, we studied CO2 adsorption capacity of S/N-graphite slit-pores in the presence of N2, CH4, H2 and H2O through Grand Canonical Monte Carlo (GCMC) calculations. Furthermore, density functional theory (DFT) calculations were performed to examine the adsorption behavior of CO2 molecules and adsorption energies of CO2 molecules on S/Ngraphene surfaces in the presence of N2, CH4, H2 and H2O were also calculated to evaluate their CO2 adsorption capacity.

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Figure 1. Schematic representation of S/N-graphene with different defects, (a) S/N-1, (b) S/N-2, (c) Divacancy 5-8-5, (d) SW 5577 (Gray, blue, yellow and white atoms represent carbon, nitrogen, sulfur and hydrogen atoms, respectively).

Figure 2. Molecule model of S/N-graphite slit-pore used in this simulation. (DNP) was chosen as the basis set. Concerning the periodic boundary condition for all models, Brillouin zone was expressed using 6 × 6 × 1 Monkhorst−Pack meshes. Besides, the self-consistent field (SCF) calculations were carried out with a convergence criterion of 10−6 a.u. on the total energy with the purpose of high-quality results. A smearing point of 0.002 Ha was chosen in all calculations. The adsorption energies (Ead)33 of gases were calculated according to the following equation:

Ead = Egas + ES/N‐graphene − ES/N‐graphene + gas

(1)

where ES/N‑graphene+gas was the total energy of the S/N-graphene and gas system. ES/N‑graphene and Egas were the energies of isolated S/Ngraphene and gas, respectively. It should be noted that a more positive Ead indicated a stronger adsorption capacity. Grand Canonical Monte Carlo Calculations. GCMC34 calculations were performed to simulate CO2 storage of S/N-graphite slit-pore at a given temperature and pressure. In addition, the van der Waals (vdW) interactions were described using a Lennard-Jones (LJ) potential,35 applying Lorentz−Berthelotmixing rules to calculate interactions between different atom types. Electrostatic interactions were calculated by Ewald method. Moreover, the CO2 adsorption properties of S/N-graphite slit-pore were studied using Dreiding force field36 during these simulations, which was a purely diagonal force field with harmonic valence terms and a cosine-Fourier expansion torsion term. In this work, DFT calculations and GCMC simulations were performed using DMol3 module and SORPTION module in Materials Studio software, respectively. Adsorption Selectivity. To examine mixture adsorption behavior, multicomponent selectivity for gas mixture adsorption was calculated using ideal adsorbed solution theory (IAST),37 which was calculated using the following equation:

MODEL AND METHODS

Model. As shown in Figure 1, a 5 × 5 S/N-graphene was constructed with different defects (Divacancy 5-8-5, NS-1, NS-2, Stone−Wales (SW) 5577). Figure 2 shows a 3D periodic pore, named as S/N-graphite slit-pore, in which the upper and lower single-layer S/ N-graphenes served as the pore-wall. 6 × 6 unit cells of S/N-graphene were used during these simulations. Based on this model, GCMC calculations were performed to investigate the CO2 adsorption capacity of this S/N-graphite slit-pore. Besides, the pore size of S/ N-graphite slit-pore was 0.75 nm in this simulation according to previous literature.11,18,20 Density Functional Theory. DFT calculations32 were performed to investigate the CO2 adsorption behavior on S/N-graphene. We used generalized gradient approximation (GGA) with Perdew−Burke− Ernzerhof (PBE) function. In addition, a dispersion correction is considered for DFT calculations via Grimme’s method by adding a semiempirical dispersion potential for all calculations. An all electron double numerical atomic orbital augmented by polarization functions

S= 8816

q1/q2 p1 /p2

(2) DOI: 10.1021/acssuschemeng.7b01612 ACS Sustainable Chem. Eng. 2017, 5, 8815−8823

Research Article

ACS Sustainable Chemistry & Engineering Table 1. CO2 Uptakes of Different S/N-Graphite Slit-Pores at 1 bar and Different Temperatures Temperature (K)

SW 5577 (mmol/mol)

Divacancy 5-8-5 (mmol/mol)

NS-1 (mmol/mol)

NS-2 (mmol/mol)

273 300 350 400 450 500

153.15 114.85 78.81 52.66 32.69 18.56

149.43 106.15 71.74 48.16 28.37 15.87

146.24 105.37 71.71 48.09 28.22 15.56

146.01 104.89 71.69 47.60 28.01 15.43

where S was the selectivity, q1 and q2 represented the amount of adsorbed of component 1 and 2, and p1 and p2 represented the partial pressure of component 1 and 2, respectively.

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RESULTS AND DISCUSSIONS CO2 Adsorption Capacities of S/N-Graphite Slit-Pore. In this section, we investigate the CO2 adsorption capacities of S/N-graphite slit-pore with four kinds of defects (Divacancy 58-5, NS-1, NS-2, SW 5577) using GCMC calculations. Table 1 shows CO2 uptakes of different S/N-graphite slit-pores at 1 bar and different temperatures. It can be found that CO2 uptake of S/N-graphite slit-pore decreases with increasing adsorption temperature because the molecular kinetic energy of CO2 rises with increasing temperature. In addition, it is found that doped sites of S and N atoms have little effect on the CO2 uptake of S/N-graphite slit-pore (Table S1 and Figures S18 and S19) although S/N-graphite slit-pore with SW 5577 is found to exhibit a little higher CO2 uptake than other defective S/Ngraphite slit-pores. Furthermore, S/N-graphite slit-pore with SW 5577 shows an unprecedentedly high CO2 uptake of 18.56 mmol/mol (1.33 mmol/g) at 1 bar and 500 K. The isosteric heat of adsorption (Qst)38 of CO2, which is estimated using CO2 adsorption isotherms collected at certain temperatures, can be calculated according to the Clausius− Clapeyron equation, as shown in Table 2, indicating that doped Table 2. CO2 Isosteric Heats of Different S/N-Graphite SlitPores at 1 bar and Different Temperatures Temperature (K)

SW 5577 (kJ/mmol)

Divacancy 5-8-5 (kJ/mol)

NS-1 (kJ/mol)

NS-2 (kJ/mol)

273 300 350 400 450 500

41.10 39.98 39.17 38.12 37.53 37.08

37.09 36.72 36.42 35.93 34.61 33.02

36.93 36.81 36.70 36.59 34.48 33.05

36.85 36.71 36.46 36.12 34.84 33.07

sites of S and N atoms are found to have little influence on Qst of CO2 on S/N-graphite slit-pore. Therefore, it can be concluded that the similar CO2 uptakes for these defective carbon materials ascribe to the similar interactions between CO2 molecules and these defective S/N-graphite slit-pores. Adsorption Selectivity for CO2 over Other Gases in the S/N-Graphite Slit-Pore with SW 5577. Apart from the CO2 adsorption capacity, the selectivity is also considered to be a crucial property for CO2 sorbents. Therefore, in this section, GCMC calculations are performed to investigate the CO2 adsorption capacity of S/N-graphite slit-pore with SW 5577 in the presence of N2, CH4, H2 and H2O at 300 K. CO2/N2 (0.16:0.84) Selectivity. Figure 3a shows CO2 and N2 adsorption isotherms at 300 K. It can be found that CO2 uptakes of S/N-graphite slit-pore with SW 5577 increase with increasing adsorption pressure. Exactly, at 300 K and 2 bar, the

Figure 3. (a) CO2 and N2 adsorption isotherms of S/N-graphite slitpore with SW 5577 at 300 K, (b) isosteric heats of CO2 and N2 adsorption on S/N-graphite slit-pore with SW 5577 at 300 K, (c) CO2/N2 selectivity of S/N-graphite slit-pore with SW 5577 versus the adsorption pressure at 300 K.

CO2 uptake of S/N-graphite slit-pore with SW 5577 (80.76 mmol/mol) is about 8 times of N2 uptake. More importantly, at 300 K and 1 bar, the CO2 adsorption capacity of S/N-graphite slit-pore with SW 5577 is 71.63 mmol/mol, which is around 11 times of N2 uptake, indicating selective CO2 adsorption from CO2/N2 mixture decreases with increasing adsorption pressure. 8817

DOI: 10.1021/acssuschemeng.7b01612 ACS Sustainable Chem. Eng. 2017, 5, 8815−8823

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ACS Sustainable Chemistry & Engineering Besides, the IAST CO2/N2 selectivity is also calculated according to eq 2, as shown in Figure 3c, which is 43−110 at 0−2 bar and 300 K. The excellent CO2 uptake of S/N-graphite slit-pore with SW 5577 capture indicates that this defective carbon strongly interacts with CO2 molecules. Therefore, the Qst of CO2 and N2 is calculated, which is shown in Figure 3b. The Qst value of CO2 adsorption on S/N-graphite slit-pore with SW 5577 is 41−43 kJ/mol, which is higher than the Qst value (14−21 kJ/ mol) of N2, demonstrating that the interaction between S/Ngraphite slit-pore with SW 5577 and CO2 molecules is stronger than that between S/N-graphite slit-pore with SW 5577 and N2 molecules. Besides, it is well-known that the kinetic diameter of CO2 molecule (0.33 nm) is smaller than that of N2 molecule (0.38 nm),37 and that the CO2 molecule has a larger electric quadrupole moment resulting from the strong dipolar CO bond than N2 molecule. As known, the strength of interactions is affected by not only surface properties of sorbent but also characterics of targeted adsorbate molecule including polarizability, quadrupole moment.39 Thus, the outstanding CO2 uptake for CO2/N2 mixture is attributed to the large electric quadrupole moment of CO2 molecule and strong interaction between S/N-graphite slit-pore with SW 5577 and CO2 molecules. CO2/CH4 (0.43:0.57) Selectivity. Figure 4a shows CO2 and CH4 adsorption isotherms of S/N-graphite slit-pore with SW 5577 at 300 K. It shows that the CO2 and CH4 uptakes of this defective carbon are 86.31 and 7.32 mmol/mol at 300 K and 1 bar, respectively, indicating S/N-graphite slit-pore with SW 5577 interacts more strongly with CO2 molecules than CH4 molecules. In addition, Figure 4c shows its CO2/CH4 selectivity is 14−30 at 0−2 bar and 300 K. The Qst of CO2 and CH4 molecules is calculated to explain why its CO2 uptake is higher than its CH4 uptake. Figure 4b shows that Qst of CO2 is 38−46 kJ/mol and Qst of CH4 is 16− 26 kJ/mol, demonstrating that CO2 molecule is easier to adsorb on its pore walls compared with CH4 molecule due to the stronger interaction with the sorbent. CO2/H2 (0.50:0.50) Selectivity. As mentioned earlier, H2 is a clean fuel that can replace fossil fuel to lower CO2 concentration in the atmosphere.5,6 Therefore, we also study CO2 and H2 adsorption isotherms at 300 K, as shown in Figure 5a. It can be seen that CO2 uptake of S/N-graphite slit-pore with SW 5577 is much larger than its H2 uptake. For example, the CO2 uptake of S/N-graphite slit-pore with SW 5577 is 93.56 mmol/mol, which is around 406 times of H2 uptake (0.230 mmol/mol) at 300 K and 1 bar. Notably, at 300 K, its IAST CO2/H2 selectivity can reach up to 8615 at 0.01 bar and 412 at 1 bar, respectively (Figure 5c). The Qst of CO2 and H2 molecules are studied to interpret the higher CO2/H2 selectivity of S/N-graphite slit-pore with SW 5577 (Figure 5b). It can be found that the Qst values of CO2 molecules are 37−43 kJ/mol, whereas the Qst of H2 molecules is only about 2.5 kJ/mol, which manifests this defective carbon interacts more strongly with CO2 molecules than H2 molecules. Moreover, CO2 molecule has a larger electric quadrupole moment resulting from the strong dipolar CO bond than H2 molecule, which can affect the interactions between gas molecules and the sorbent,39 resulting in a stronger CO2 adsorption capacity than H2 of S/N-graphite slit-pore with SW 5577. CO2 Uptake S/N-Graphite Slit-Pore with SW 5577 in the Presence of Water. Water, which is an essential

Figure 4. (a) CO2 and CH4 adsorption isotherms of S/N-graphite slitpore with SW 5577 at 300 K, (b) isosteric heats of CO2 and CH4 adsorption on S/N-graphite slit-pore with SW 5577 at 300 K. (c) CO2/CH4 selectivity of S/N-graphite slit-pore with SW 5577 versus the adsorption pressure at 300 K.

composition of flue gas, has gradually attracted much attention because of its interactions with the adsorbents, which greatly affects CO2 adsorption.40−42 Therefore, we investigate the effect of moisture on the CO2 uptake of S/N-graphite slit-pore with SW 5577 at 300 K (Figure 6 and Figures S1 and S2). It is obvious that S/N-graphite slit-pore with SW 5577 can still keep high CO2 uptake in the presence of water. For example, for CO2/H2O mixture (0.90:0.10), its CO2 uptake is about 110.13 mmol/mol at 300 K and 1 bar, which is slightly lower than that (114.97 mmol/mol) without water. The Qst of CO2 on this defective carbon is calculated to understand why it can possess high CO2 uptake in the presence of water. Figure 6 indicates that water affects slightly the Qst value of CO2, which can be responsible for its high CO2 uptake in the presence of water. To be more exact, both the Qst value 8818

DOI: 10.1021/acssuschemeng.7b01612 ACS Sustainable Chem. Eng. 2017, 5, 8815−8823

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ACS Sustainable Chemistry & Engineering

Figure 6. CO2 adsorption isotherms and isosteric heats of CO2 adsorption for S/N-graphite slit-pore with SW 5577 in the presence of H2O at 300 K.

Effects of Ratio of S/N on CO2 Adsorption Capacity of S/N-Graphite Slit-Pore with SW 5577. According to the aforementioned discussions, S/N-graphite slit-pore with SW 5577 shows excellent selective CO2 capture. However, it should be noted that this defective carbon material consists of two kinds of heteroatoms (S and N). Therefore, it is significant to clarify the influence of ratio of S/N on its CO2 adsorption capacity. Figure 7 shows CO2 uptakes of S/N-graphite slit-pore

Figure 5. (a) CO2 and H2 adsorption isotherms of S/N-graphite slitpore with SW 5577 at 300 K, (b) isosteric heats of CO2 and H2 adsorption on S/N-graphite slit-pore with SW 5577 at 300 K. (c) CO2/H2 selectivity of S/N-graphite slit-pore with SW 5577 versus the adsorption pressure at 300 K.

Figure 7. CO2 uptakes of S/N-graphite slit-pore with SW 5577 and different ratios of S/N at 1 bar and different temperatures.

with SW 5577 with different ratios of S/N at 1 bar and different temperatures. It is clear that the CO2 uptake increases with increasing S content. For example, at 1 bar and 300 K, the full S-graphite slit-pore with SW 5577 possesses a respectable CO2 uptake of 133.50 mmol/mol whereas the S/N-graphite slit-pore with SW 5577 and S/N ratio of 1:2 has a CO2 uptake of 108.95 mmol/mol, indicating that the increasing S content contributes to increasing CO2 uptake. In addition, we also investigate how the ratio of S/N affects the selective adsorption of CO2/H2, CO2/CH4 and CO2/H2O mixtures on this defective carbon. Figure 8a and Table 3 demonstrate that the increasing S content results in an increasing CO2 uptake and CO2/H2 selectivity at 300 K and 1 bar. In terms of full N-graphite slit-pore with SW 5577, its CO2 uptake and H2 uptake are 79.77 and 0.22 mmol/mol with high CO2/H2 selectivity (∼356). For full S-graphite slit-pore with SW 5577, its CO2 uptake and H2 uptake are 104.66 and 0.20 mmol/mol with an increasing CO2/H2 selectivity (∼526).

of CO2 in the presence of water and Qst value of CO2 without water are 33−43 kJ/mol. Therefore, we conclude that S/Ngraphite slit-pore with SW 5577 would be a better sorbent for humid CO2 capture. We also investigate CO2 adsorption capacity of S/N-graphite slit-pore with SW 5577 in the presence of N2, CH4, H2 and H2O at 273 K (Figures S20−S23). It is obvious that the CO2 uptakes and selectivities of S/N-graphite slit-pore with SW 5577 at 273 K are larger than these at 300 K because the molecular kinetic energy of gas molecule increases with increasing temperature. For example, its CO2 and N2 uptakes are 71.62 and 6.53 mmol/mol with a moderate CO2/N2 selectivity of 50.78 at 300 K and 1 bar, whereas these are 85.02 and 7.12 mmol/mol with an increasing selectivity of 60.81 at 273 K and 1 bar, respectively. 8819

DOI: 10.1021/acssuschemeng.7b01612 ACS Sustainable Chem. Eng. 2017, 5, 8815−8823

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ACS Sustainable Chemistry & Engineering

Table 3. CO2/H2 Selectivity of S/N-Graphite Slit-Pores with SW 5577 and Different Ratios of S/N at 300 K and Different Pressures Pressure (bar)

Full N

S:N = 1:2

S:N = 1:1

S:N = 2:1

Full S

0.01 0.05 0.10 0.20 0.30 0.50 0.60 0.70 0.90 1.00 1.50 1.80

4484.07 3078.42 2150.56 1311.51 906.98 617.51 515.93 465.65 373.54 356.55 269.79 168.45

8130.73 3944.85 2488.08 1418.08 1003.02 658.76 565.31 504.83 432.02 411.12 321.16 284.39

10284.73 4450.75 2550.85 1482.59 1135.11 701.73 593.35 517.51 467.96 421.65 329.75 300.87

16491.03 5366.58 2709.76 1571.39 1189.85 770.88 674.02 606.39 514.29 480.66 390.81 363.08

21157.81 5613.88 2860.78 1656.67 1258.93 787.43 701.61 639.42 538.61 526.24 428.75 396.73

Table 4. CO2/CH4 Selectivity of S/N-Graphite Slit-Pores with SW 5577 and Different Ratios of S/N at 300 K and Different Pressures Pressure (bar)

Full N

S:N = 1:2

S:N = 1:1

S:N = 2:1

Full S

0.01 0.05 0.2 0.3 0.5 0.7 0.9 1 1.5 1.8 2

14.14379 19.3406 21.8018 20.29893 18.02301 17.32049 16.00804 15.01967 14.05999 13.21473 12.92868

19.91816 26.76906 25.40965 22.78613 21.19919 19.9373 17.52124 15.56697 14.88408 14.0246 13.01269

20.85736 32.78162 30.70244 24.8412 22.7266 20.97563 18.11039 16.18455 15.25279 14.58113 13.53305

31.81041 35.98632 34.6239 27.03349 24.25103 21.66197 19.71562 17.27604 16.24514 15.07546 14.86825

36.98884 36.39814 35.37772 28.02282 25.59162 22.77315 20.36363 18.07429 17.43239 16.42453 15.87113

129.71 mmol/mol whereas CO2 uptake of full N-graphite slitpore with SW 5577 and is 92.62 mmol/mol. In addition, we also investigate the relationship between CO2 uptake of S/N-graphite slit-pore with SW 5577 and the ratio of S/N, as shown in Figure 9. Notably, it is found that the CO2 uptakes for this defective carbon material are approximately linearly dependent on doped S content, which also demonstrates that S doping has positive effects on CO2 uptake compared with S/N codoping. Based on the above-mentioned discussions, it is concluded that S doping would be a better choice than S/N codoping on respectable selective CO2 adsorption of S/N-graphite slit-pore with SW 5577. Mechanism of CO2 Adsorption on S/N-Graphite SlitPore with SW 5577. DFT calculations are performed to study the CO2 adsorption behavior on S/N-graphene surface with SW 5577 in the presence of N2, CH4, H2 and H2O. Figure 10 shows the structural configurations of CO2 molecule adsorbed onto defective S/N-graphene surface in the presence of CH4, H2, N2 and H2O molecules. It is obvious that the distance between CO2 molecule and this defective carbon is shorter than that between CH4, H2 and N2 molecule and this defective carbon, indicating that the CO2 molecule more easily occupies the adsorption site of this defective carbon surface. However, for CO2/H2O mixture, the distance between CO2 molecule and this defective carbon surface is comparative to that between H2O molecule and this defective carbon surface.

Figure 8. CO2 adsorption isotherms of S/N-graphite slit-pore with SW 5577 with different ratios of S/N at 300 K. (a) CO2/H2 (0.50:0.50), (b) CO2/H2O (0.90:0.10), (c) CO2/CH4 (0.43:0.57).

Besides, it is also demonstrated that the increasing S content contributes to an increasing CO2 uptake and CO2/CH4 selectivity (Figure 8c and Table 4) whereas it affects slightly CH4 uptake at 300 K (Table S2). Exactly, at 300 K and 1 bar, the CO2 and CH4 uptakes of full N-graphite slit-pore with SW 5577 are 75.38 and 6.68 mmol/mol with a CO2/CH4 selectivity of 15.02 whereas the CO2 and CH4 uptakes are 98.67 and 7.11 mmol/mol with an increasing CO2/CH4 selectivity (∼18.07) for full S-graphite slit-pore with SW 5577, respectively. Intriguingly, we also find that the CO2 uptake in the presence of water increases with increasing S content, as shown in Figure 8b. For CO2/H2O mixture (0.90:0.10), at 300 K and 1 bar, the CO2 uptake of full S-graphite slit-pore with SW 5577 and is 8820

DOI: 10.1021/acssuschemeng.7b01612 ACS Sustainable Chem. Eng. 2017, 5, 8815−8823

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ACS Sustainable Chemistry & Engineering

Figure 9. CO2 uptakes of S/N-graphite slit-pore with SW 5577 and different ratios of S/N versus S content for (a) pure CO2 at 1 bar and different temperatures, (b) CO2/H2 mixture at 300 K and different pressures, (c) CO2/H2O mixture at 300 K and different pressures, (d) CO2/CH4 mixture at 300 K and different pressures.

Figure 10. Optimized structures for (a) CO2/CH4, (b) CO2/N2, (c) CO2/H2, (d) CO2/H2O adsorbed onto S/N-graphene surface with SW 5577. (Gray, blue, yellow, red and white atoms represent carbon, nitrogen, sulfur, oxygen and hydrogen atoms, respectively). Figure 11. Adsorption energies for CO2/CH4, CO2/N2, CO2/H2 and CO2/H2O mixtures on S/N-graphene surface with SW 5577.

The adsorption energies between different gas molecules and S/N-graphene surface with SW 5577 are calculated based on eq 1 to explain the adsorption behavior of gas molecules on this defective surface, as shown in Figure 11. For CO2/CH4 mixture, Ead for CO2 molecule is 0.48 eV whereas Ead for CH4 molecule is 0.27 eV. As for CO2/N2 mixture, Ead for CO2 molecule is 0.45 eV whereas Ead for N2 molecule is 0.25 eV. In terms of CO2/H2 mixture, Ead for CO2 molecule is 0.26 eV, which is about 2 times of Ead for H2 molecule. Therefore, this defective surface interacts more strongly with CO2 molecule compared with N2, CH4, H2 molecules. Notably, Ead for CO2 molecule is still high in the presence of water (0.35 eV). DFT calculations can well interpret why S/N-graphene slit-pore with SW 5577 possesses considerable CO2 uptake and selectivity. DFT calculations are also performed to interpret the impacts of the ratio of S/N on CO2 adsorption behavior on S/N-

graphene surface with SW 5577 in the presence of CH4, N2, H2 molecules (Figure 12). For CO2/H2 mixture, the Ead of CO2 molecule on full N defective surface is about 0.25 eV, which is comparable to the Ead of H2 molecule (0.20 eV). However, the Ead of CO2 molecule on the full S defective surface is about 0.36 eV, which is more than 2 times that of H2 molecule. Besides, the difference between the Ead of CO2 molecule and the Ead values of CH4 and N2 on the full S defective surface is larger than that on the full N defective surface. Moreover, with respect to CO2/H2O mixture, the Ead of CO2 molecule on the full N defective surface is about 0.33 eV, which is lower than the Ead (0.42 eV) of CO2 molecule on the full S defective surface. Our DFT results manifest that compared with N doping, S doping plays an important part in selective capturing CO2 molecule of 8821

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Figure 12. Adsorption energies for CO2/CH4, CO2/N2 and CO2/H2 mixtures on (a) full N-graphene surface with SW 5577 and (b) full S-graphene surface with SW 5577.

S/N-graphene slit-pore with SW 5577, which is consistent with the GCMC calculation results.

CO2 adsorption properties of S/N-graphite slit-pore with SW 5577 with different doping sites of S/N in the presence of CH4, H2 and H2O at 300 K; CO2 adsorption properties of S/N-graphite slit-pore with SW 5577 in the presence of CH4, H2, H2O and N2 at 273 and 300 K (PDF)

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CONCLUSIONS In summary, we investigate CO2 adsorption capacities of S/Ngraphite slit-pore with four kinds of defects (Divacancy 5-8-5, NS-1, NS-2, SW 5577) using GCMC and DFT calculations. It is found that the doped sites of S and N atoms affect slightly CO2 uptake of graphite slit-pore, which contributes to its practical applications. Besides, at 300 K, the CO2/H2 selectivity for S/N-graphite slit-pore with SW 5577 can reach about 8615 and 412 at 0.01 and 1 bar. Moreover, the increasing S content can enhance the selective CO2 adsorption of S/N-graphite slitpore with SW 5577. For example, at 300 K and 1 bar, full Ngraphite slit-pore with SW 5577 has a high CO2 uptake of 79.77 mmol/mol with good CO2/H2 selectivity (∼356), and full Sgraphite slit-pore with SW 5577 possesses an impressive CO2 uptake of 104.66 mmol/mol with excellent CO2/H2 selectivity (∼526), demonstrating that S atoms play a critical role in selective CO2 adsorption. The large interaction between CO2 molecule and S/Ngraphene with SW 5577 surface contributes to its outstanding CO2 uptake. Furthermore, the increasing of S content enlarges the difference between the adsorption energy of CO2 molecule and the adsorption energy of other gas molecules on S/Ngraphene with SW 5577 surface, leading to a remarkable selective CO2 adsorption. Our results demonstrate that S doping is a better choice than S/N codoping for impressive CO2 capture.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-532-86981169. E-mail: [email protected]. cn (Prof. Q. Xue). ORCID

Qingzhong Xue: 0000-0002-2873-8037 Wei Xing: 0000-0003-4098-4860 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (11374372, 41330313), Taishan Scholar Foundation (ts20130929), the Natural Science Foundation of Shandong Province (ZR2015AQ012), the Fundamental Research Funds for the Central Universities (16CX06024A).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01612. CO2 uptakes of different S/N-graphite slit-pores at 1 bar and different temperatures; CH4 uptakes (mmol/mol) of S/N-graphite slit-pores with SW 5577 and different ratios of S/N at 300 K and different pressures; CO2 adsorption properties of S/N-graphite slit-pore with SW 5577 in the presence of H2O at 300 K; CO2 adsorption properties of N-S-1 in the presence of CH4, H2, H2O and N2 at 300 K; CO2 adsorption properties of N-S-2 in the presence of CH4, H2, H2O and N2 at 300 K; CO2 adsorption properties of Divacancy 5-8-5 in the presence of CH4, H2, H2O and N2 at 300 K; schematic representation of S/N-graphene with SW 5577 for different doping sites; 8822

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