Effects of Sulfur Doping and Humidity on CO2 Capture by Graphite

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Effects of sulfur doping and humidity on CO2 capture of graphite split-pore: a theoretical study Xiaofang Li, Qingzhong Xue, Xiao Chang, Lei Zhu, Cuicui Ling, and Haixia Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14281 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 25, 2017

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ACS Applied Materials & Interfaces

Effects of sulfur doping and humidity on CO2 capture of graphite split-pore: a theoretical study Xiaofang Li,†, ‡ Qingzhong Xue,*, †, ‡ Xiao Chang,†, ‡ Lei Zhu,†, ‡ Cuicui Ling,§ Haixia Zheng†, ‡

†

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 §

Department of Civil and Environmental Engineering, University of California,

Berkeley, CA 94720, USA

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

ACS Paragon Plus Environment


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Abstract Using Grand Canonical Monte Carlo calculations, we study the effects of sulfur doping and humidity on the performance of graphite split-pore as adsorbents for CO2 capture. It is demonstrated that S doping can greatly enhance the pure CO2 uptake of graphite split-pore. For example, S-graphite split-pore with 33.12% sulfur shows a 39.85% rise in pure CO2 uptake (51.001 mmol/mol) compared with pristine graphite split-pore at 300 K and 1 bar. More importantly, it is found that S-graphite split-pore can still keep much higher CO2 uptake of than that of pristine graphite split-pore in the presence of water. Especially, the CO2 uptake of 33.12% sulfur doped S-graphite split-pore is 51.963 mmol/mol in the presence of water, which shows a 44.34% higher than that for pristine graphite split-pore at 300 K and 1 bar. In addition, CO2/N2 selectivity of S-graphite split-pore experiences an increase with increasing S content resulting from the stronger interactions between CO2 and S-graphite split-pore. Moreover, using density functional theory calculations, we demonstrate that S doping can enhance adsorption energy between CO2 molecules and S-graphene surface under different humidities and furthermore enhance the CO2 uptake of S-graphite split-pore. Our results indicate that S-graphite split-pore is a promising adsorbent material for humid CO2 capture.

Keywords: Grand Canonical Monte Carlo calculations, CO2 capture, Density functional theory, Adsorption energy, Sulfur doping.

2


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1. Introduction The accelerating global energy demand and consumption of carbon-based fuels cause an increasing carbon dioxide (CO2) concentration in the atmosphere,1 which has attracted considerable attentions due to the fact that it has resulted in a significant climate change, such as global warming and environmental deterioration.2 Therefore, great efforts have been devoted to developing new technologies to curb the proliferation of CO2 concentration in the atmosphere.3-5 To date, chemical absorption and cryogenic distillation have been developed for CO2

capture

and

separation.

However,

these

two

techniques

are

high

energy-consuming.6-8 Recently, physical absorption has been regarded as the most promising method and has attracted considerable research interest9-11 because of its easy cost-effective regeneration and lower energy cost of operation. Up to now, CO2 adsorption of various porous materials with high adsorption capacity and selectivity has been intensively studied,12,

13

such as zeolites,14-16

metal-organic frameworks (MOFs)17-19 and carbon materials.20-23 Among them, carbon materials have attracted extensive attention owing to their large surface area, pore volume and tunable pore size.24,

25

Graphite split-pore, one kind of carbon

materials, has been widely applied to CO2 capture.26-31 Atomistic simulations were performed to study CO2 adsorption on graphitic slit-pore with diverse surface heterogeneities, which indicated that oxygen-containing functional groups enhanced CO2 adsorption on microporous carbon materils.29 Using Grand canonical Monte Carlo (GCMC) simulations, Kumar et al.30 found that nitrogen doping could boost CO2 uptake and CO2/N2 selectivity of graphitic slit-pore. However, most of adsorbents have been only applied to pure CO2 capture, which makes it challenging to assess the best materials for capturing CO2 from a real flue gas, which has an essential composition of water. Water, taking up about 10% molar concentration in flue gas mixture, has attracted much attention since it has a significant influence on the CO2 adsorption capacity of absorbents.32-34 Moreover, water competes with gas molecules for the active sites or interacts with the atoms of absorbents,35, 36 resulting in the 3



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instability and even collapse of the structure of absorbents, which further reduces their gas storage capacity. Therefore, CO2 capture in real flue gas with the presence of water is particularly a great challenge.37 There are some valuable theoretical simulations and experiments for estimating the stability and CO2 capture performance of adsorbents in the presence of water.38-40 Billemont et al.41 demonstrated that CO2 uptake of nanoporous carbon decreased with the increasing number of water molecules using experiments and molecular simulations. Yu et al.42, 43 previously studied CO2 adsorption and CO2/N2 separation of HKUST-1 and Mg-MOF-74 under humid condition. They found that the CO2 adsorption capacity for HKUST-1 was strengthened while the CO2 adsorption capacity for Mg-MOF-74 was weakened in the presence of water. In addition, density functional theory (DFT) calculations demonstrated that water on Cu site in HKUST-1 strengthened binding affinity between CO2 and HKUST-1 while water on Mg site in Mg-MOF-74 lowered binding affinity between CO2 and Mg-MOF-74, which indicated that coordinatively unsaturated metal sites (CUMs) played an important role in estimating the impact of water on CO2 uptake of sorbents. Besides, the authors also investigated the response of various metals toward water and the effect of water on CO2 adsorption in an isostructural series M-HKUST-1 (M = Zn, Co, Ni and Mg). They found that water on CUMs mainly heightened CO2 uptake and its selectivity over N2 for Zn-, Co-, Ni-based frameworks while water lowered dramatically CO2 adsorption for Mg-based frameworks, which were beneficial to design appropriate absorbents for CO2 capture. 44 Very recently, 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.45 Moreover, relevant research46-48 indicated that sulfur doped porous materials contributed to strengthen CO2 adsorption on sorbents. Xia et al. 46 synthesized a series of sulfur-doped, structurally well-ordered, zeolite-like microporous carbon materials by nanocasting method using zeolite EMC-2 as hard template. These carbons were found to exhibit considerably high CO2 adsorption heat of 59 KJ/mol. Mirosław et al.47 found that sulfur doping had significant impacts on CO2 adsorption of 4


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nanoporous carbon, including adsorption energy and clustering patterns of CO2 in the pore system. Although sulfur doped porous materials have shown great potential in CO2 capture, corresponding experimental and theoretical researches on CO2 capture of sulfur doped porous materials in the presence of water are still deficient. In this paper, we systematically investigated the effect of sulfur doping on the CO2 adsorption capacity of sulfur doped (S-) graphite split-pore and pristine graphite split-pore without and with water using (GCMC) calculations. Furthermore, DFT calculations were performed to examine the adsorption behaviors of CO2 molecules on S-graphene surface and pristine graphene surface. Adsorption energies between CO2 molecules on S-graphene surface and pristine graphene were also calculated to evaluate their CO2 adsorption capacity.

2. Model and Methods 2.1 Model As shown in Figure 1, a 3D periodic pore, named as S-graphite split-pore, in which the upper and lower single-layer S-graphenes served as the pore-wall. 6 × 6 unit cells of S-graphene were used for molecular simulations. The pore size, H, was defined as the distance between the center of upper S-graphene plane and the center of lower S-graphene plane. Based on this model, GCMC calculations were implemented to investigate the adsorptions properties of CO2 molecules on this S-graphite split-pore in the presence of water. In this simulation, the pore size of S-graphite split-pore was in the range of 0.6 nm -1.1 nm.

Figure 1 Molecule model of S-graphite split-pore used in this simulation. (Gray, yellow and white atoms represent carbon, sulfur and hydrogen atoms, respectively.) 5



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2.2 Grand canonical Monte Carlo calculations GCMC49 calculations were performed to simulate CO2 storage of S-graphite split-pore and pristine graphite split-pore, which were in thermodynamics equilibrium with respect to absorbed CO2 molecules at a given temperature and pressure. The adsorption properties of CO2 molecules on the split-pore was studied using Dreiding force field,50 which was a purely diagonal force field with harmonic valence terms and a cosine-Fourier expansion torsion term. Electrostatic interactions were depicted by atomic monopoles and a screened (distance-dependent) Coulombic term. The Lennard-Jones potential was used to describe the van der Waals interactions. Meanwhile, hydrogen bonding was described by an explicit Lennard-Jones 12-10 potential 2.3 Density functional theory (DFT) DFT calculations51 were performed to evaluate the CO2 adsorption on S-graphite split-pore and pristine graphite split-pore. The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) function, which was adopted to depict the nonhomogeneity of the true electron density using the gradient of charge density, was implemented by the spin-unrestricted all-electron DFT calculations. The double numerical plus polarization (DNP) was employed to expand electronic wave functions. A dispersion correction was considered for DFT calculations by Grimme's method by adding a semi-empirical dispersion potential. The self-consistent field (SCF) calculations51, 52 were carried out with a convergence criterion of 10-6 a.u. on the total energy with the purpose of high-quality results. Furthermore, a smearing point of 0.002 Ha and a real-space global orbital cutoff radius of 4.5 Å were chosen in all calculations. The Brillouin zone was expressed using 6 × 6 × 1 Monkhorst-Pack meshes. DFT calculations and GCMC simulations were performed using DMol3 module and SORPTION module in Materials Studio software, respectively. 2.4 Adsorption Energy calculations The adsorption energy (Ead)52,53 is calculated by the following equations:

Ead1 = Es-graphene+CO2 –Es-graphene – ECO2

(1)

Ead2 = Egraphene+CO2 –Egraphene – ECO2

(2) 6


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where Es-graphene+CO2 and Egraphene+CO2 are the total energy of the S-graphene and CO2 system and the total energy of the pristine graphene and CO2 system, respectively.

Es-graphene and Egraphene are the energies of isolated S-graphene and graphene, respectively, ECO2 is the energy of isolated CO2 molecules. It should be noted that a more negative Ead indicates a stronger adsorption capacity.

3. Results and discussions 3.1 CO2 adsorption on pristine graphite split-pore and S-graphite split-pore The CO2 adsorption capacities of S-graphite split-pore and pristine graphite split-pore are estimated by GCMC simulations. Figure 2a shows the CO2 uptake of S-graphite split-pore and pristine graphite split-pore with different pore sizes (0.6 nm − 1.1 nm) at 300 K and 1 bar. For pristine graphite split-pore, its CO2 uptake increases with increasing pore size. When the pore size is about 0.70 nm, it reaches maximal CO2 uptake (~ 36.479 mmol/mol). However, when the pore size further increases, its CO2 uptake gradually decreases. Moreover, it also demonstrates that the CO2 uptake of graphite split-pore increases with the increasing sulfur content (Figure 2a and Figure 3). For example, the maximal CO2 uptake of S-graphite split-pore with 33.12 wt% sulfur can reach 51.001 mmol/mol, which shows a 39.85% increase compared with that of pristine graphite split-pore and a 23.63% growth compared with that of S-graphite split-pore with 7.94 wt% sulfur at 300 K and 1 bar. Based on the results, it can be concluded that sulfur doping can boost CO2 uptake of graphite split-pore and the optimum pore size of CO2 uptake occurs among 0.70 nm – 0.80 nm, which is in accordance with the existing results.54 The isosteric heat of adsorption (Qst)41 is a parameter that describes the average adsorption enthalpy for adsorbing gas molecules at a certain surface and is estimated using CO2 adsorption isotherms collected at certain temperatures. The Qst can be calculated by Clausis-Clapeyron equation.

(ln) = − R 1T + C

(3)

where T is the temperature, P is the pressure, N is the quantity of adsorbed CO2 7



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molecules, R is the universal constant and C is a constant. The Qst at each adsorption level is readily obtained from the slope of the plots of (ln P)N as a function of (1/T). Figure 2b shows the relationship between Qst value and pore size at 300 k and 1 bar. It can be found that Qst value for CO2 adsorption on S-graphite split-pore is much higher than that on pristine graphite split-pore under the same conditions, which manifests the interactions between S-graphite split-pore and CO2 molecules are stronger than those between pristine graphite split-pore and CO2 molecules. For example, Qst value of CO2 adsorption on S-graphite split-pore with 33.12% sulfur is 41.01 KJ/mol, which is a 45.12% rise than that on graphite split-pore. Moreover, it can be found that Qst value of CO2 adsorption on S-graphite split-pore shows a growing trend with the increasing sulfur content. The aforementioned discussions demonstrate that the strong interactions between S-graphite split-pore and CO2 molecules contributes to its excellent CO2 uptake.

Figure 2 (a) CO2 uptake curves of pristine graphite split-pore and S-graphite split-pore at 300 K and 1 bar, (b) Isosteric heat of adsorption as a function of pore size of pristine graphite split-pore and S-graphite split-pore at 300 K and 1 bar.

Figure 3 Snapshots showing CO2 adsorption patterns in the split-pore of 0.75 nm at 300 K and 1 bar for (a) pristine, (b) 7.94 % sulfur content, (c) 33.12% sulfur content. 8


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3.2 CO2 adsorption on pristine graphite split-pore and S-graphite split-pore in the presence of water When it comes to capturing CO2 from real postcombustion flue gas, water has an inevitable influence on CO2 uptake of sorbents.32-34 Therefore, at 300 K and 1 bar, we investigate CO2 uptakes of S-graphite split-pore and pristine graphite split-pore with different pores (0.6 nm - 1.1 nm) in the presence of water (Figure 4 and Figure S1-S5). It can be found that the maximal CO2 uptakes of S-graphite split-pore and pristine graphite split-pore occur among 0.7 nm ~ 0.8 nm, which indicates that water affects negligibly on the optimum pore size of CO2 uptake. Besides, it can be found that CO2 uptakes of both S-graphite split-pore and pristine graphite fall slightly with increasing amount of water, which indicates that water has a slight influence on CO2 uptakes of them. More importantly, the CO2 uptake of S-graphite split-pore still keeps higher than that of pristine graphite split-pore, which indicates that S-graphite split-pore has great potential for CO2 capture in the presence of water, especially for S-graphite split-pore with higher sulfur content. For example, the maximal CO2 uptake of S-graphite split-pore with 33.12% sulfur is 51.963 mmol/mol in the presence of water at 300 K and 1 bar, which shows a 44.34% growth compared with that of pristine graphite split-pore (Figure 3 and Table 1). According to the discussions, it can be concluded that S doping is an excellent method for higher CO2 uptake of graphite split-pore in the presence of water.

9



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Figure 4 CO2 uptake curves of pristine graphite split-pore and S-graphite split-pore at 300K and 1 bar with (a) CO2:H2O 0.99:0.01, (b) CO2:H2O 0.95:0.05, (c) CO2:H2O 0.90:0.10, (d) CO2:H2O 0.85:0.15. Table 1 Maximal CO2 uptakesa of S-graphite split-pore and pristine graphite split-pore with different humidities at 300K and 1 bar. Sulfur Pure CO2 content(%)

a b

CO2:H2O

CO2:H2O

CO2:H2O

CO2:H2O

0.99:0.01b

0.95:0.05b

0.90:0.10b

0.85:0.15b

0

36.479

36.001

35.001

34.301

33.537

7.94

41.253

40.374

38.866

38.203

37.512

18.48

44.234

44.005

42.795

42.101

41.505

24.72

46.551

46.293

45.001

44.604

43.301

30.44

49.471

49.584

48.037

46.981

45.969

33.12

51.001

51.963

49.501

48.766

47.901

CO2 uptake in mmol/mol Ratio of CO2 and H2O in this work is molar ratio. 10


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3.3 CO2/N2 (0.16:0.84) selectivity of S-graphite split-pore Apart from high CO2 uptake, high CO2 selectivity versus other gases is of equal significance to CO2 sorbents because there is always a competitive adsorption between CO2 and other gases in practical applications.23, 30 Therefore, we investigate CO2 adsorption capacity of S-graphite split-pore in the presence of N2. Figure 5a shows the effects of S content on the CO2 and N2 uptakes of S-graphite split-pore at 300 K. It is obvious that the increasing S content enhances its CO2 uptake while it has little influence on its N2 uptake. Exactly, at 300 K and 1 bar, the CO2 and N2 uptakes of S-graphite split-pore with 7.94 wt% sulfur are 19.82 mmol/mol and 5.37 mmol/mol, respectively, while the CO2 and N2 uptakes of S-graphite split-pore with 33.12 wt% sulfur are 27.46 mmol/mol and 5.33 mmol/mol, respectively. Interestingly, it is found that the increasing S content enhances CO2/N2 selectivity of S-graphite split-pore (Figure 6). It is noted that CO2/N2 selectivity of S-graphite split-pore is calculated according to ideal adsorbed solution theory,23 which is calculated based on the following equation:

q1 S=

q2

p1

(4)

p2

where, S is the selectivity, q1 and q2 represent the amount of adsorbed of component 1 and 2, and p1 and p2 represent the partial pressure of component 1 and 2, respectively. Qst of CO2 and N2 adsorption on S-graphite split-pore is calculated to interpret why S doping can enhance its CO2 uptake and CO2/N2 selectivity, as shown in Figure 5b. The Qst value of CO2 on S-graphite split-pore is 33-40 KJ/mol, which is much higher than that of N2 (8-17 KJ/mol). Notably, the increasing S content can increase Qst value of CO2 while it slightly affects Qst value of N2. In addition, it is known that the kinetic diameter of CO2 molecule (0.33 nm) is smaller than that of N2 molecule (0.38 nm),23 and that the CO2 molecule has a larger electric quadrupole moment because of the strong dipolar C=O bond than N2 molecule. The above mentioned discussions can explain well why the increasing S doping can enhance CO2 uptake and CO2/N2 selectivity of S-graphite split-pore. 11



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Figure 5 (a) CO2 and N2 adsorption isotherms of S-graphite split-pore with different S contents at 300 K, (b) isosteric heats of CO2 and N2 adsorption on S-graphite split-pore with different S contents at 300 K.

Figure 6 CO2/N2 selectivity of S-graphite split-pore with different S contents versus the adsorption pressure at 300 K. 3.4 Mechanism of CO2 adsorption on pristine graphite split-pore and S-graphite split-pore in the presence of water In this section, DFT calculations are used to calculate CO2 adsorption energy that is applied to evaluate influences of sulfur doping and water on the CO2 adsorption behaviors on the 5 × 5 × 1 supercell of pristine graphene and S-graphene surfaces. Figure 7 shows the most stable configurations for CO2 adsorption on a single side of pristine graphene and S-graphene surfaces with periodic boundary conditions. For the case of a little amount of CO2 molecules, it can be found that the CO2 molecules are parallel to the graphene surface while the absorbed CO2 molecules tend to tilt with the increasing number of CO2 molecules. However, the absorbed CO2 molecules on 12


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S-graphene surface tilt to the surface, which is irrelevant to the number of CO2 molecules.

Figure 7 Optimization structures for (a) 2 CO2, (b) 5 CO2, (c) 10 CO2, (d) 11 CO2 adsorbed onto S-graphene surface, (e) 2 CO2, (f) 5 CO2, (g) 6 CO2, (h) 10 CO2 adsorbed onto pristine graphene surface. (Gray, red and yellow atoms represent carbon, oxygen and sulfur atoms, respectively) Figure 8 shows the relationship between the average adsorption energy of CO2 (Ead per CO2) on pristine graphene and S-graphene surfaces and the numbers of CO2 molecules. When one CO2 molecule adsorbs on the surface, the adsorption energy of CO2 molecules on S-graphene surface is -0.317 eV, which is about a 29.8% increase compared with that on pristine graphene surface, which manifests that there are stronger interactions between CO2 molecules and S-graphene surface than that between CO2 molecules and pristine graphene surface. Besides, it can also be found that absolute value of Ead per CO2 shows a decreasing trend with the increasing number of CO2 molecules for both S-graphene surface and pristine graphene surface. Specially, when there are 5 CO2 molecules adsorbing on the pristine graphene surface 13



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(Figure 7f), Ead per CO2 is -0.212 eV. While 6 CO2 molecules adsorb on the pristine graphene surface (Figure 7g), Ead per CO2 is -0.203 eV, which is slightly lower than the physisorption energy (-0.210 eV) of CO2 molecules on graphene sheet.55 Therefore, it can be concluded that the maximal number of CO2 molecules pristine graphene surface is 5. In terms of the S-graphene surface, when there are 10 CO2 molecules (Figure 7c), Ead per CO2 is -0.211 eV. If the number of CO2 molecules further increases to 11 (Figure 7d), Ead per CO2 is -0.191 eV, which demonstrates that the maximal number of CO2 molecules on S-graphene surface is 10 that is two times of that on pristine graphene surface. In order to better understand CO2 adsorption behaviors on pristine graphene and S-graphene surfaces, the electron density distributions of two CO2 molecules on pristine graphene and S-graphene surfaces are shown in Figure 9. It can be found that there is no electron overlap between CO2 molecules and the pristine graphene surface, while a tangency between CO2 molecules and the S-graphene surface can be observed, resulting in relatively high adsorption energy between CO2 molecules and the S-graphene surface. According to the DFT results, it can well explain why CO2 uptake of S-graphite split-pore is higher than that of pristine graphene split-pore.

Figure 8 Adsorption energy per CO2 as a function of the number of CO2 molecules.

Figure 9 Electron-density distributions of CO2 adsorbed onto pristine graphene and S-graphene surfaces. (a) pristine graphene, (b) S-graphene surface. 14


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The most stable configurations for CO2 adsorption on a single side of pristine graphene and S-graphene surfaces in the presence of water are shown in Figure 10 and Table S1. Our simulation results show that water can affect the adsorption energy of CO2 molecules on pristine graphene and S-graphene surfaces. Figure 10a shows that there is one CO2 molecule absorbing on S-graphene surface with adsorption energy of -0.317 eV. When twelve H2O molecules as well as one CO2 molecule absorb on S-graphene surface, the adsorption energy (-0.315 eV) of CO2 molecule experiences a slight fall (Figure 10b). With respect to pristine graphene, Figure 10c shows that the adsorption energy of CO2 molecule is -0.245 eV while Figure 10d shows the adsorption energy of CO2 molecule in the presence of twelve H2O molecules is -0.220 eV, respectively. Exactly, an approximate 10% decrease is found in adsorption energy between CO2 molecules and pristine graphene because of the presence of water. Based on the discussions above, it can well interpret why CO2 uptake of S-graphite split-pore can still keep higher than that of pristine graphene in the presence of water.

Figure 10 Optimization structures for (a) 1 CO2, (b) 12 CO2 adsorbed onto S-graphene surface, and (c) 1 CO2, (d) 12 CO2 adsorbed onto pristine graphene. (Gray, red, yellow and white atoms represent carbon, oxygen, sulfur and hydrogen atoms, respectively)

4. Conclusions The influences of sulfur doping and water on the CO2 capture on S-graphite split-pore and pristine graphite split-pore have been systematically investigated using 15



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GCMC and DFT calculations. It is found that S doping can greatly enhance the pure CO2 uptake of graphite split-pore. For example, the pure CO2 uptake (51.001 mmol/mol) of S-graphite split-pore with 33.12 wt% sulfur shows a 39.85% rise compared with that of pristine graphite split-pore at 300 K and 1 bar. More importantly, S-graphite split-pore can still keep a higher CO2 uptake than that of pristine graphite split-pore in the presence of water. For example, the maximal CO2 uptake of S-graphite split-pore with 33.12% sulfur is 51.963 mmol/mol at 300 K and 1 bar in the presence of water, which is a 44.34% growth compared with that of pristine graphite split-pore under the same circumstances. In addition, CO2/N2 selectivity of S-graphite split-pore experiences an increase with increasing S content due to the stronger interactions between CO2 and S-graphite split-pore. The adsorption energy between CO2 molecules and S-graphene surface is discovered to be higher than that between CO2 molecules and pristine graphene surface, which results in excellent CO2 uptake of S-graphite split-pore. Moreover, it is found that water affects slightly the adsorption energy between CO2 molecules and S-graphene surface, which can well explain why S-graphite split-pore can keep a higher CO2 uptake in the presence of water. The simulation results are meaningful to understand the CO2 adsorption mechanism of graphite split-pore in the presence of water. Furthermore, our results demonstrate that S-graphite split-pore can be a promising adsorbent material for capturing CO2 from realistic flue gas. Supporting Information Table S1, properties of gases on S-graphene and pristine graphene; Figure S1, CO2 and H2O uptakes of S-graphite split-pores with 7.94% sulfur under different humidities; Figure S2, CO2 and H2O uptakes of S-graphite split-pores with 18.48% sulfur under different humidities; Figure S3, CO2 and H2O uptakes of S-graphite split-pores with 24.72% sulfur under different humidities; Figure S4, CO2 and H2O uptakes of S-graphite split-pores with 30.44% sulfur under different humidities; Figure S5, CO2 and H2O uptakes of S-graphite split-pores with 33.12% sulfur under different humidities. 16


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Acknowledgements This work was supported by the Natural Science Foundation of China (11374372, 41330313, 11604390), Taishan Scholar Foundation (ts20130929), the Natural Science Foundation of Shandong Province (ZR2014EMQ006), the Fundamental Research Funds for the Central Universities (15CX08009A, 16CX06024A).

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Graphic abstract


Effects of Sulfur Doping and Humidity on CO2 Capture by Graphite

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