Superior Selective CO2 Adsorption of C3N Pores: GCMC and DFT

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Superior selective CO2 adsorption of C3N pores: GCMC and DFT simulations Xiaofang Li, Lei Zhu, Qingzhong Xue, Xiao Chang, Cuicui Ling, and Wei Xing ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09648 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

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ACS Applied Materials & Interfaces

Superior selective CO2 adsorption of C3N pores: GCMC and DFT simulations Xiaofang Li,†,‡,§ Lei Zhu,†,‡,§ Qingzhong Xue,*†,‡ Xiao Chang,†,‡ Cuicui Ling, ‡ Wei Xing, ‡

†

State Key Laboratory of Heavy Oil Processing, China University of Petroleum,

Qingdao 266555, Shandong, P. R. China ‡

College of Science, China University of Petroleum, Qingdao 266555, Shandong, P. R.

China §

These authors contributed equally

*Corresponding author: Tel: 86-532-86981169. E-mail: [email protected]; (Prof. Q. Z. Xue) 1

ACS Paragon Plus Environment


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Abstract The development of high performance sorbents is extremely significant for CO2 capture due to its increasing atmospheric concentration and impacts on environmental degradation. In this work, we develop a new model of C3N pores based on GCMC calculations to describe its CO2 adsorption capacity and selectivity. Remarkably, it exhibits outstanding CO2 adsorption capacity and selectivity. For example, at 0.15 bar, it shows exceptionally high CO2 uptakes of 3.99 mmol/g and 2.07 mmol/g with good CO2/CO, CO2/H2 and CO2/CH4 selectivity at 300 K and 350 K, separately. More importantly, this adsorbent also shows better water stability. Exactly, its CO2 uptakes are 3.80 mmol/g and 5.91 mmol/g for and 0.15 bar and 1 bar at 300 K with a higher water content. Furthermore, DFT calculations demonstrate that the strong interactions between C3N pores and CO2 molecules contribute to its impressive CO2 uptake and selectivity, indicating that C3N pores can be an extremely promising candidate for CO2 capture.

Keywords: Grand Canonical Monte Carlo calculations, water stability, Density functional theory, Adsorption energy, Isosteric heats of adsorption

2


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1. Introductions The increasing demand of fossil fuels has resulted in amounts of CO2 emissions in the atmosphere, which is a major contributor to environmental degradation.1 Besides, CO2 is also an impurity of natural gas (mainly including CH4) and in the production process of H2 and CO.2-4 However, CO2 is also a potentially valuable feedback if it can be converted into formic acid, CO or more highly reduced hydrocarbon products.5,6 Therefore, it is of significance to capture CO2 for its valuable conversion and utilization in order to curb its further proliferation. In recent decades, many significant efforts have been expended to develop novel adsorbent porous materials for excellent CO2 selective adsorption.7 Among these porous materials, carbon materials, such as porous activated carbons,8-13 carbon nanotubes,14,15 carbon fiber16 and so on, have received much attention due to versatile properties such as low regeneration energy, high CO2 adsorption capacity and good regenerability. Nitrogen-doped activated carbons were found to exhibit a high CO2 uptake of 1.6 mmol/g at 0.15 bar and 4.9 mmol/g at 1 bar and 300 K.9 Besides, Liu et al. synthesized nitrogen-enriched porous carbon sphere through a one-spot carbonization process that had a high CO2 uptake of 2.76 mmol/g at 1 bar and 348 K.10 In addition to nitrogen-containing functional groups, sulfur-containing functional groups were also found to enhance CO2 adsorption capacity of carbon materials.11-13 For example, Seema et al. found sulfur-doped microporous carbon materials displayed a high CO2 uptake of 4.5 mmol/g with good CO2 adsorption selectivity over N2, CH4 and H2 at 298 K and 1 bar.11 To further study the effect of S doping on CO2 uptake for porous carbons, Seredych and coworkers synthesized a series of nanoporous sulfur-doped carbons. Interestingly, at 273 K, its adsorption capacity were over 4 mmol/g at 1 bar and 8 mmol/g at 9 bar, respectively, which was interpreted by acid-base interactions.12 Importantly, our previous study demonstrated that the 33.12% sulfur doped graphite slit-pore showed a 44.34% rise in CO2 uptake compared with pristine graphite slit-pore in the presence of water at 300 K and 1 bar, indicating sulfur doping is a better choice for humid CO2 capture.13 3



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Apart from activated carbons, carbon nanscrolls (CNS) also showed great potential applications in gas adsorption because of its controllable interlayer space and high accessible surface area with a theoretical value of 2630 m2/g for individual sheets,17 which has been widely applied for CO2 capture.18-20 Peng et al.18 demonstrated that CO2 uptake of CNS with a interlayer space of 1.5 nm could reach 29.98 mmol/g at 298 K and 50 bar. Notably, our previous study demonstrated one-sided nitrogen-doped graphene could self-assemble into one-sided nitrogen-doped CNS at room temperature. Importantly, this carbon material possessed an ultra-high CO2 uptake of 8.7 mmol/g with impressive CO2/N2 selectivity (~163) at 300 K and 1 bar.20 Very recently, C3N, a hole-free two dimensional honeycomb lattice with a homogeneous and ordered distribution of nitrogen, has been successfully fabricated by polymerization of 2,3-diaminophenazine.21-23 The authors demonstrated that C3N sheet was more stable than graphene at room temperature, since it was found to be stable up to 1100 ℃ in Ar atmosphere and up to 550 ℃ in air. 21 As a member of graphene-like 2D materials, C3N inherited the structural characteristics and some excellent properties of graphene.21 As known, the graphite slit-pore that was one kind of porous carbons, has been commonly used to selective adsorption of CO2 and CH4.8, 13, 24

For example, Peng et al. found that the C60 intercalated graphite was a promising

material for CO2 purification for CO2/CH4 mixture by regulating the interlayer space at room temperature.8 Therefore, C3N pores may be promising candidates for CO2 capture. In this paper, we firstly used Grand canonical Monte Carlo (GCMC) simulations and density functional theory (DFT) calculations to investigate the performance of C3N in gas adsorption. Firstly, we investigated the influences of temperature and pressure on CO2 adsorption capacity for C3N pores. Then, we studied CO2/CO, CO2/H2, CO2/CH4 selectivity at different pressures and 300 K. Finally, DFT calculations were carried out to elucidate the simulation results.

2. Model and Methods 2.1 Model Figure 1a showed the structural properties of C3N supercell lattice that parts of 4


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carbon atoms in graphene were substituted by nitrogen atoms. The size of C3N is a = b = 14.76 Å, c = H + 27.30 Å, in which the pore size, H, was defined as the distance between the center of the top C3N plane and the center of the bottom C3N plane, and the unit cell had 6 carbon atoms and 2 nitrogen atoms, which was labeled with dashed parallelogram on the Figure 1a. In the C3N model, the length of carbon-carbon bonds presented was 1.412 Å and the length of carbon-nitrogen bonds was 1.418 Å, respectively, which was in accordance with literature results.23 As shown in Figure 1b, a 3D periodic pore model, defined as C3N pores, in which the top and bottom single C3N layer served as the pore-wall, was constructed to study its CO2 adsorption and selectivity using GCMC calculations.

Figure 1 (a) Optimized structure of C3N sheet; (b) Molecular model of C3N pores used in this simulation. Gray and blue atoms represent carbon and nitrogen atoms, respectively. 2.2 Grand canonical Monte Carlo calculations In this paper, all GCMC calculations,13, 20 which were performed by SORPTION code embedded in the Material Studio (MS) software, were carried out to study the CO2 adsorption capacity of C3N pores at given adsorption temperatures and pressures. A 3 × 3 supercell of C3N with periodic boundary conditions was employed. Besides, Dreiding force field, 25 which was a purely diagonal force field with harmonic valence terms and a cosine-Fourier expansion torsion term, was used to describe the interatomic interaction. Exactly, the van der Waals interactions with a cut-off of 12.5 Å were depicted by the Lennard-Jones potential and the electrostatic interactions were 5



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described via atomic monopoles and a screened (distance-dependent) Coulombic term. 2.3 Density functional theory calculations DFT26-28 calculations, which were carried out by DMOL3 code in MS software, were used to calculate the interaction energy between gas molecules and C3N surface. The generalized gradient approximation in Perdew-Burke-Ernzerhof functional form29 with all-electron double numerical with polarization function (DNP) was adopted because of its explicit description of the electronic structures of C3N monolayer. We chose a dispersion correction for DFT approach with Grimme’s30 van der Waals correction for all DFT simulations. In addition, the real-space global cutoff radius was set to 4.1 Å.28 The vacuum thickness was set to larger than 20 Å along the z direction to prevent the interactions between periodic images. For geometry optimizations, the convergence tolerances were 1 × 10-5 Ha for the total energy, 0.002 Ha/Å for atomic forces, 0.005 Å for maximum displacement, respectively. The Brillouin zone integration was implemented on a 6 × 6 × 1 Monkhorst-Pack k-point mesh for C3N. Hence, the adsorption energy (Ead)13,

27

of gas molecules on C3N surface was

calculated by the following equation, Ead = Egas + EC3N − EC3N+gas

(1)

where Egas, EC3N and EC3N+gas were the total energy of isolated gas molecules, isolated C3N surface and C3N surface with gas molecules, separately. Based on this equation, a more positive adsorption energy represented a much stronger interaction between gas molecules and the C3N surface.

3. Results and discussions 3.1 CO2 adsorption capacity of C3N pores In this section, we systematically investigate the CO2 adsorption capacities of C3N pores at different adsorption temperatures and pressures, as shown in Figure 2. Based on our previous work,20 the pore size during this simulation is 0.75 nm for CO2 capture. It is found that its CO2 uptake decreases with increasing temperature since the molecular kinetic energy of CO2 rises with increasing temperature. At 300 K, it 6


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shows an outstanding CO2 uptake of 3.99 mmol/g and 6.27 mmol/g at 0.15 bar and 1 bar, respectively. Intriguingly, at 0.15 bar and 350 K, its CO2 uptake can reach up to 2.07 mmol/g. Our simulation results demonstrate that C3N pores have an impressive CO2 uptake, which is higher than the literature results (Table 1).8-10, 31-34

Table 1 Comparison of CO2 uptake of C3N pores with other sorbents reported in the literature. Sorbent C60 Intercalated Graphite

Pristine Slit-pore

N-doped Slit-pore

Temperature (K)

Pressure (bar)

CO2 Uptakes (mmol/g)

0.15

1.56

1

4.96

0.15

−

298

1

4.4

0.15

−

1

4.6

0.15

−

1

2.07

0.15

-

10

4.00

0.15

1.38

1

4.30

0.15

-

1

1.86

0.15

3.99

1

6.27

0.15

2.07

1

4.24

298

308

SWNT (6, 6)

298

AC-750-0.5

298

AC-750-0.5

350

C3N pores

300

C3N pores

350

8

31

298

SWNT (9, 0)

Reference

31

32

33

34

34

This work

This work

In order to rationalize and interpret the influences of temperature on CO2 uptakes in C3N pores, when CO2 adsorption reaches equilibrium, the density fields of CO2 at 1 bar and different temperatures are shown in Figure 3. It is worth noting that the storage density of adsorbed CO2 in C3N pores is larger at 300 K than other higher temperatures, indicating a higher CO2 adsorption capacity at 300 K, which can well explain why CO2 uptake of this absorbent decreases with increasing temperature. 7



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Figure 2 CO2 adsorption isotherms of C3N pores at different temperatures and pressures.

Figure 3 Density fields of CO2 adsorption equilibrium in C3N pores at 1 bar and (a) 300 K, (b) 350 K, (c) 400 K, (d) 500 K. 3.2 Adsorption selectivity for CO2 over other gases in the C3N pores In addition to the CO2 adsorption capacity, the selectivity is also a crucial property for CO2 sorbents.11, 20, 24 Hence, GCMC calculations are carried out to investigate the CO2 adsorption capacity of C3N pores in the presence of CO, CH4 and H2 molecules. 8


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Here, we predict multi-component adsorption equilibrium using ideal adsorbed solution theory (IAST),35, 36 which is calculated using the following equation: q1 S

p1

q2

(2)

p2

in which S is the selectivity, q1 and q2 represent the amount of adsorbed of component 1 and 2, p1 and p2 represent the partial pressure of component 1 and 2, respectively. Besides, we investigate CO2 adsorption capacity of C3N pores with pore size of 0.65 nm and 0.75 nm in the presence of CO, CH4 and H2 molecules at 300 K (Figure S1). It is found that CO2/CO and CO2/CH4 selectivity at 0.65 nm is larger than that at 0.75 nm while CO2/H2 selectivity at 0.65 nm is similar to that at 0.75 nm. Therefore, in the next section, we make a detailed study of CO2/CO, CO2/H2 and CO2/CH4 selectivity of C3N pores with pore size of 0.65 nm. 3.2.1 CO2/CO (0.50:0.50) mixture Figure 4a and Figure S2 present the adsorption isotherms of CO2 and CO on C3N pores at 300 K. It is obvious that this carbon shows higher CO2 adsorption capacity in comparison with CO. Specially, at 300 K and 1 bar, its gas uptakes are 5.16 mmol/g and 0.53 mmol/g for CO2 and CO, respectively. More remarkably, at 300 K and 0.15 bar, its CO2 uptake is 3.10 mmol/g that is about 15 times of CO uptake (0.20 mmol/g), indicating that C3N pores possesses good CO2/CO selectivity. Besides, Figure 5 shows the IAST CO2/CO selectivity that is calculated according to eq. 2, which is 6-21 at 0-2 bar and 300 K. The isosteric heats of adsorption (Qst),37, 38 a crucial parameter to describe the interactions between gas molecules and sorbents, is calculated using the Clausius-Clapeyron equation to explain high CO2/CO selectivity of C3N pores, as shown in Figure 4b and Figure S3. The Qst value of CO2 adsorption on this adsorbent is about 34-38 KJ/mol which is higher than that (17-25 KJ/mol) of CO, demonstrating that it interacts more strongly with CO2 molecules than CO molecules. It is known that the kinetic diameter of CO molecule (~0.37 nm) is slightly larger than that of CO2 molecule (0.33 nm). Furthermore, the CO2 molecule has both larger values of 9



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polarizability (29. 11 × 1025 cm3) and quadrupole moment (4.3 × 1025 esc cm2) than these of CO molecule (19. 5 × 1025 cm3, 2.5 × 1025 esc cm2).39 As known, the strength of interactions is affected by both surface properties of sorbent and characterics of targeted adsorbate molecule including polarizability, quadrupole moment.39-41 Thus, the effective CO2 adsorption of C3N pores over CO is ascribed to its stronger interactions with CO2 molecule due to the larger polarizability and quadrupole moment of CO2 molecule.

Figure 4 (a) CO2 and CO adsorption isotherms of C3N pores at 300 K, (b) isosteric heats of CO2 and CO adsorption on C3N pores at 300 K.

Figure 5 CO2/CO selectivity of C3N pores versus the adsorption pressure at 300 K. 3.2.2 CO2/H2 (0.50:0.50) mixture As known, H2 is an attractive and environmentally-friendly fuel since it does not produce CO2 during combustion.42, 43 Consequently, it is an excellent alternative of fossil fuels to lower CO2 concentration in the atmosphere. Additionally, CO2 is also an 10


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impurity in the process of H2. Therefore, we estimate CO2 and H2 adsorption capacity of C3N pores at different pressures and 300 K (Figure 6a and Figure S4). At 300 K, the CO2 uptake of C3N pores increases with increasing pressure, and can reach up to 4.10 mmol/g at 1 bar while its H2 uptake is only 0.03 mmol/g under the same condition. Importantly, at 0.15 bar and 300 K, it possesses a reasonable CO2 uptake of 2.51 mmol/g that is about 597 times of its H2 uptake (0.004 mmol/g), demonstrating that C3N pores show an unusual CO2 selectivity over H2. In order to interpret its high CO2 adsorption capacity over H2, the IAST CO2/ H2 selectivity is calculated, as shown in Figure 7, which is 118-1570 at 0-2 bar and 300 K. To figure out the reason why C3N pores exhibit higher uptake of CO2 than H2, the Qst of CO2 and H2 in C3N pores is calculated using the Clausius-Clapeyron equation based on the adsorption isotherms measured at 300 K (Figure 6b and Figure S5). It can be found that Qst values are about 34-36 KJ/mol for CO2 molecules and only 2 KJ/mol for H2 molecules, which manifests that this carbon absorbent interacts more strongly with CO2 molecules than H2 molecules. Moreover, CO2 molecule not only has a larger electric quadrupole moment resulting from the strong dipolar C=O bond but also a larger polarizability than H2 molecule.39 Therefore, it can be concluded that C3N pores not only have a considerable CO2 adsorption capacity but also unusual CO2/H2 selectivity due to its strong interactions with CO2 molecules which are attributed to the polarizability and electric quadrupole moment of CO2 molecules.39-41

Figure 6 (a) CO2 and H2 adsorption isotherms of C3N pores at 300 K, (b) isosteric heats of CO2 and H2 adsorption on C3N pores at 300 K.

11



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Figure 7 CO2/ H2 selectivity of C3N pores versus the adsorption pressure at 300 K. 3.2.3 CO2/CH4 (0.43:0.57) mixture Apart from H2, natural gas (mainly including CH4) is also a desirable fuel due to its higher hydrogen to carbon ratio that can produce lower CO2 emissions during combustion.3, 44 However, CO2 is also an impurity that degrades natural gas. Hence, it is of extremely significance to separate CO2 from CH4. Thus, in this section, we investigate the CO2 adsorption capacity of C3N pores in the presence of CH4 (Figure 8a and Figure S6). It can be clearly seen that its CO2 uptake is much larger than its CH4 uptake although both of them increase with increasing pressure. For example, at 300 K and 1 bar, its CO2 uptake is 4.87 mmol/g while its CH4 uptake is only 0.30 mmol/g. Remarkably, it should be noted that this carbon material exhibits an outstanding CO2 uptake of 3.02 mmol/g and a lower CH4 uptake of 0.04 mmol/g at 300 K and 0.15 bar, respectively. Figure 9 shows the predications obtained using IAST for the coadsorption of CO2/CH4 at 300 K. It can be found that IAST CO2/CH4 selectivity decreases with increasing adsorption pressure, for example, it is 238 at 0.01 bar while it is 21 at 1 bar, respectively. In addition, it should be noteworthy that it also has a superior IAST CO2/CH4 selectivity of 87 at 0.15 bar. In order to interpret these results, it is necessary to calculate Qst of CO2 and CH4 that can reflect the interaction strength between gas molecules and C3N pores. Figure 8b and Figure S7 represents the Qst of CO2 and CH4 adsorption in C3N pores at 300 K. It is obvious that the Qst (34-38 KJ/mol) of CO2 is higher than that (~ 12 KJ/mol) of CH4, demonstrating that CO2 molecule is easier to adsorb on its pore walls compared 12


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with CH4 molecule due to the stronger interaction with the sorbent, leading to preferential adsorption of CO2 over CH4. As known, the kinetic diameter of CO2 molecule (0.33 nm) is smaller than that of CH4 molecule (~0.38 nm), and CO2 molecule has a larger electric quadrupole moment resulting from the strong dipolar C=O bond while CH4 displays no quadrupole moment.39 Based on the foregoing discussions, the respectable CO2/CH4 selectivity of C3N pores is attributed to its strong interactions with CO2 molecules that result from the electric quadrupole moment of CO2 molecules.39-41

Figure 8 (a) CO2 and CH4 adsorption isotherms of C3N pores at 300 K, (b) isosteric heats of CO2 and CH4 adsorption on C3N pores at 300 K.

Figure 9 CO2/CH4 selectivity of C3N pores versus the adsorption pressure at 300 K. 3.3 Water stability of C3N pores As noted, water, an inevitable composition of flue gas mixtures, can affect CO2 adsorption and selectivity of sorbents, since it not only competes with CO2 molecules for active sites but interacts with the atoms of absorbents that can cause the instability 13



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and even collapse of absorbents.3,

13, 45

Consequently, we perform simulations to

evaluate CO2 adsorption capacity of C3N pores in the presence of water. Figure 10a and Figure S8 show the CO2 and H2O uptakes versus adsorption pressures for C3N pores at 300 K. It is obvious discovered that this carbon sorbent still exhibits higher CO2 adsorption capacity in the presence of water. Exactly, at 300 K and 1 bar, its CO2 uptake can reach up to 6.07 mmol/g with a CO2:H2O ratio of 0.95:0.05. In addition, it shows a slight decrease CO2 uptake with increasing water content. Particularly, at 300 K and 0.15 bar, it has remarkable CO2 uptakes of 3.93 mmol/g and 3.80 mmol/g in the presence of water. To further illustrate the effect of water on CO2 uptake, we calculate Qst of CO2 at 300 K, as shown in Figure 10b. It can be obviously seen that Qst value (31-34 KJ/mol) of CO2 is still large in the presence of water and is affected slightly by water content. As known, a larger Qst value indicates stronger interactions between CO2 molecules and the sorbent. Therefore, we can conclude that the better water stability of C3N pores can promote its wide applications in CO2 capture from humid flue gas.

Figure 10 (a) CO2 adsorption isotherms of C3N pores in the presence of H2O at 300 K, (b) isosteric heats of CO2 adsorption on C3N pores in the presence of H2O at 300 K. 3.4 Mechanism of CO2 adsorption in the C3N pores in the presence of CO, H2, CH4 and H2O To elucidate the remarkable CO2 uptake for C3N pores over CO, H2 and CH4, we investigate the adsorption behavior of gas molecules on the C3N surface (Figure 11) and implement DFT calculations to compare the adsorption energy of gas molecules 14


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on C3N surface, as shown in Figure 12. Generally, Ead of CO2 molecule is larger than those of other gas molecules, particularly for CO2/H2 mixture. In the case of CO2/CO mixture, the calculated Ead are 0.25 eV and 0.21 eV for CO2 and CO, separately. Similarly, C3N surface also shows approximate Ead values of 0.26 eV and 0.23 eV for CO2 and CH4, respectively. By contrast, in terms of CO2/H2 mixture, Ead value of CO2 is about 0.30 eV that is approximately 2 times of that for H2 molecule. DFT simulation results well explain the remarkable CO2 uptake and selectivity of C3N pores.

Figure 11 Optimization structures for (a) CO2/CO, (b) CO2/H2, (c) CO2/CH4, adsorbed onto C3N surfaces. (Gray, red, blue and white atoms represent carbon, oxygen, nitrogen and hydrogen atoms, respectively)

Figure 12 Adsorption energies for CO2/CO, CO2/H2 and CO2/CH4 mixtures on C3N surface We also elaborate how H2O molecules affect CO2 adsorption capacity of C3N pores 15



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via DFT calculations. Figure 13 represents the most stable configurations for CO2 and H2O adsorption on a single side of C3N surface with periodic boundary condition. In addition, the calculated Ead value of CO2 is about 0.307 eV for C3N surface, which is larger than that (0.272 eV) for C3N surface with CO2/H2O mixture and that (0.232 eV) for C3N surface with CO2/3H2O mixture. Obviously, the distance between CO2 molecule and C3N surface is 2.91 Å that is less than those in the presence of one H2O molecule and three H2O molecules. In order to better understand the effects of H2O molecules on CO2 adsorption, we calculate the hirshfeld charge of parts of C3N sheet (Figure S9), as shown in Table 2d. For the C3N surface with one CO2 molecule without H2O molecules, the calculated charge is about -0.037 e. In the case of CO2/H2O mixture, the charge of C3N surface is -0.02 e. With respect to CO2/3H2O mixture, hirshfeld charge population analysis indicates that the charge of C3N surface is about -0.005 e. The above discussions obviously show that the electron transfer between C3N surface and CO2 molecule is weakened by the adsorbed H2O molecule, indicating that the interactions between C3N surface and CO2 molecule are weakened because of the existence of H2O molecules. However, it is obvious that C3N surface can still keep high interactions with CO2 molecule in the presence of H2O molecules. Thus, hirshfeld charge population analysis as well as adsorption energy can well explain why this carbon absorbent exhibits considerable CO2 adsorption capacity in the presence of water.

Figure 13 Optimization structures for CO2 molecules adsorbed onto C3N surfaces in the presence of (a) 0 H2O (b) 1 H2O (c) 3 H2O. (Gray, red, blue and white atoms 16


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represent carbon, oxygen, nitrogen and hydrogen atoms, respectively) Table 2 Hirshfeld chargea of parts of C3N surface.

a

Atoms

CO2

CO2/H2O

CO2/3H2O

C1 C2 C3 C4 C5 C6 C7 C8 C9 N1 N2 N3 N4 Total

0.012 0.012 0.012 0.011 0.014 0.012 0.013 0.012 0.013 -0.037 -0.038 -0.037 -0.036 -0.037

0.012 0.014 0.012 0.012 0.014 0.014 0.016 0.013 0.017 -0.036 -0.038 -0.035 -0.035 -0.020

0.015 0.014 0.013 0.014 0.016 0.015 0.016 0.013 0.016 -0.034 -0.035 -0.034 -0.034 -0.005

The unit of charge is electron

4. Conclusions In summary, we theoretically investigate CO2 adsorption capacity and selectivity of C3N pores. Interestingly, this carbon material shows unusual CO2 uptakes with outstanding CO2/CO, CO2/H2 and CO2/CH4 selectivity. Specially, at 0.15 bar, it shows outstanding CO2 uptakes of 3.99 mmol/g and 2.07 mmol/g at 300 and 350 K, separately. Besides, its CO2/CO, CO2/H2 and CO2/CH4 selectivity are 15, 579 and 87 at 300 K and 0.15 bar, respectively. Notably, this adsorbent also has better water stability, for example, its CO2 uptake is 3.80 mmol/g at 300 K and 0.15 bar in the presence of water. The remarkable CO2 adsorption capacity of C3N pores is attributed to its strong interactions with CO2 molecule. Additionally, because this carbon absorbent interacts more strongly with CO2 molecule owing to the larger polarizability and quadrupole moment of CO2 molecule, this carbon material exhibits excellent CO2/CO, CO2/H2 and CO2/CH4 selectivity. Moreover, our results manifest that C3N pores could be an extremely promising sorbent for CO2 capture. 17



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Supporting Information CO2/CO, CO2/H2 and CO2/CH4 selectivity of C3N pores with different pore sizes versus the adsorption pressure at 300 K; CO, H2, CH4 and H2O adsorption isotherms at 300 K; Isosteric heats of CO2, CO, H2 and CH4 adsorption on C3N pores at 300 K, 348 K and 373 K; Model of C3N sheet for calculated Hirshfeld charge; CH4 adsorption capacity of C3N pores in the presence of C2H4.

Acknowledgements This work was supported by the Natural Science Foundation of China (11374372, 41330313, 11604390), Taishan Scholar Foundation (ts20130929), the Fundamental Research Funds for the Central Universities (15CX08009A, 16CX06024A, 17CX02045).

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Graphical abstract


Superior Selective CO2 Adsorption of C3N Pores: GCMC and DFT

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