Graphdiyne as a High-Efficiency Membrane for Separating Oxygen

Sep 27, 2016 - According to our first-principles calculations, the oxidation of the acetylenic bond in graphdiyne needs to surmount an energy barrier ...
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Graphdiyne as a high-efficiency membrane for separating oxygen from harmful gases: A first-principles study Zhaoshun Meng, Xirui Zhang, Yadong Zhang, Haiqi Gao, Yun-hui Wang, Qi Shi, Dewei Rao, Yuzhen Liu, Kaiming Deng, and Ruifeng Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08662 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 30, 2016

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Graphdiyne as a high-efficiency membrane for separating oxygen from harmful gases: A first-principles study Zhaoshun Meng,† Xirui Zhang,† Yadong Zhang,† Haiqi Gao,† Yunhui Wang,† Qi Shi,† Dewei Rao,*,‡ Yuzhen Liu,*,† Kaiming Deng† and Ruifeng Lu*,† †

Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, P. R. China. ‡

School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, P. R. China.

ABSTRACT: We theoretically explored the adsorption and diffusion properties of oxygen and several harmful gases penetrating the graphdiyne monolayer. According to our first-principles calculations, the oxidation of the acetylenic bond in graphdiyne needs to surmount an energy barrier of circa 1.97 eV, implying that graphdiyne remains unaffected under oxygen-rich conditions. In a broad temperature range, graphdiyne with well-defined nanosized pores exhibits a perfect performance for oxygen separation from typical noxious gases, which should be of great potential in medical treatment and industry.

KEYWORDS: monolayer membrane, graphdiyne, oxygen separation, adsorption, selectivity, permeance, first-principles calculations

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I.

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INTRODUCTION

Carbon, with various hybridized states (sp, sp2, sp3), can bond to other carbon atoms or different elements, to form numerous compounds. In particular, pure carbon materials contain a lot of allotropes: so-called zero-dimensional fullerene,1 one-dimensional

carbon

nanotube,2

two-dimensional

(2D)

graphene,3

three-dimensional (3D) diamond, etc. Up to date, 2D carbon materials have been paid great attention in many kinds of potential applications due to their extensive prospect. Since graphene, the most representative 2D single-layer structure, was first fabricated in experiments,4–8 scientists have made enormous effort to study almost all of its properties and found a series of possible applications except for gas permeation, which is not good even for the noble gas helium, as demonstrated both experimentally9 and theoretically.10 It is known that the permeability of a membrane is inversely proportional to its thickness,11 so the impermeable property of pure graphene (one-atom thick) is caused by its dense atom density. This prompts a lot of theoretical studies to design various pores with different size in graphene to increase its gas permeability.12,13 However, it is difficult to control the pore size precisely and uniformly in laboratory. Thus, seeking for a 2D carbon membrane with intrinsic pores of appropriate size will contribute to realizing gas purification or separation with high efficiency. Notably, Li et al. successfully fabricated large area graphdiyne films via a cross-coupling reaction on the surface of copper in 2010, and demonstrated that the porous graphdiyne film exhibits excellent semi-conducting properties.14 Subsequently, graphdiyne has been studied in extensive applications such as spintronics,15 lithium

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ion battery,16–19 photocatalysis,20,21 transistors,22 and metal-free catalysis.23,24 As a potential membrane for gas separation, graphdiyne is of one-atom thickness, and has good mechanical properties25–27 and natural uniform pores to guarantee the desired properties of gas permeation. Compared to other 2D nanoporous materials such as graphyne28 and polyphenylene,29 the size of the periodic pores in graphdiyne is bigger, which is suitable to make a more ideal separation membrane for the specific gases considering the larger energy barrier for target gas passing through smaller pores. In fact, by using density functional theory calculations, Jiao et al.30 explored the diffusion of hydrogen (H2), carbon oxide (CO), and methane (CH4) through graphdiyne and found that graphdiyne is a good hydrogen separation membrane. Full atomistic molecular dynamics helped Cranford et al.31 to draw a similar conclusion under ambient temperature and pressure. Apart from hydrogen separation, graphdiyne is thought as an efficient membrane for He3/He4 isotope separation.32 As is well known, oxygen with high purity is very important in medical treatment and industry. Under the situations of a fire accident or toxic gases leaking from chemical plants, the used gas masks require good performance in terms of free diffusion of oxygen while blocking harmful gases. Therefore, oxygen purification or separation is of vital value in many aspects. So far, almost all related studies have focused on oxygen/nitrogen separation and utilized various 3D materials including mixed

ionic-electronic

conducting

ceramic-based

membranes,33,34

polydimethylsiloxane/liquid crystal cross-linked membranes,35 composite phenoic resin-based carbon molecular sieve membranes,36 tetramethyl bisphenol F based

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polymeric membranes,37 and metal-organic frameworks.38,39 However, investigation on 2D membrane for oxygen separation from noxious gases has been scarcely reported. Although our previous study proposed a candidate membrane to this end by doping nitrogen in 2D polyphenylene network,40 such perfect N-doped polyphenylene is difficult to be synthesized in experiments. In this work, we select the pure graphdiyne membrane to study its properties with respect to gas permeation and expect it to be a promising monolayer membrane for oxygen separation from harmful gases. II. COMPUTATIONAL METHODS All the first-principles calculations using the plane-wave basis set and the projector augmented-wave method41 were performed by the Vienna Ab-initio Simulation Package.42 The energy cutoff was set to 400 eV and the Brillouin zone was sampled with 6×6×1 Monkhorst-Pack k-point grids, which ensure a high numerical accuracy. The structural relaxations without any symmetry constraint were carried out until the Hellmann-Feynman force on each atom is less than 10–2 eV Å–1, while the convergence criterion of energy for the electronic self-consistent calculation was 10–5 eV. To avoid the interaction between the periodic monolayers, we applied periodic boundary conditions with a vacuum space of 20 Å. The climbing image nudged elastic band (CI-NEB) method43–45 was employed to search the transition state involved in the gas diffusion. We adopted the spin-polarized calculations for the O2 diffusion while the non-spin-polarized calculations for the other gases. We carefully considered the van der Waals (vdW) dispersion46 by employing the D2 method of

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Grimme47

based

on

the

generalized

gradient

approximation

with

the

Perdew-Burke-Ernzerhof functional,48 so the obtained results for gas adsorption and diffusion are reasonable to make convincing conclusions. III. RESULTS AND DISCUSSION Graphdiyne consists of sp- and sp2-hybridized carbon atoms and belongs to the family of graphyne in which the number of acetylenic bonds differs from each other. As its name implies, graphdiyne has two acetylenic bonds between adjacent aromatic rings. The lattice parameters were optimized to be a = b = 9.44 Å, in good agreement with the previous data.49 Similar to graphyne, 2D graphdiyne network has two types of pores: one is the hexagonal pore from six-member carbon ring; the other is the quasi-triangular pore from acetylenic bond linkers.50 The former pore actually cannot allow any gas penetration at normal condition in accordance with the situation of perfect graphene,9,10 however, the larger quasi-triangular pore is appropriate for gas separation.30,31 The graphdiyne membrane has advantages of being one-atom thick, having a uniform pore distribution and mechanical robustness (the modulus bigger than 470 GPa and the ultimate strength bigger than 36 GPa),25 however, we must be aware of graphdiyne’s stability under O2-rich environment which is a prerequisite for our discussion on the O2 separation from harmful gases. There is one theoretical research on the topic of graphdiyne as a metal-free catalyst for low-temperature CO oxidation which considered the O2 binding on acetylenic bond of graphdiyne.23 However, the required energy barrier in the binding process of O2 molecule on graphdiyne still remains unknown. By adopting the CI-NEB method, we elaborately

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studied the oxidation reaction of the acetylenic bond in graphdiyne. Figure 1 displays the structures of the initial state (IS), the transition state (TS) and the final state (FS) as well as their corresponding energies. Apparently, although O2 can be chemically adsorbed on acetylenic bond of graphdiyne,23 the oxidation needs a very high energy of about 1.97 eV, indicating that graphdiyne structure will keep stable for O2 separation. We also optimized possible physisorption configurations on graphdiyne for all the studied gas molecules (O2, harmful diatomic Cl2 and HCl, triatomic HCN, CNCl, SO2, H2S, and tetratomic NH3 and CH2O). The adsorption energies collected in Table 1 are all less than 0.18 eV, which means that these gases will not gather around the pores of graphdiyne at certain temperature. Remarkably, the diffusion barrier for O2 (0.21 eV) passing through the quasi-triangular pore is the smallest one among the studied gases, intuitively predicting that O2 molecule can most easily penetrate graphdiyne membrane among these gases. In Figure 2, the IS, TS, and FS structures as well as their relative energies for O2 permeation through the graphdiyne membrane are displayed. The relative energy for O2 far away from graphdiyne is also presented in Figure 2, which equals to the absolute value of the O2 adsorption energy. To accurately compute the diffusion barrier, we choose the adsorption geometry as IS for all the studied gases. The red lines in the top view of the TS structure assist us to notice the tiny bend of the acetylenic bond carbon chains, which is similar to previous work.30 This can be understood that the overlapping electron densities around the pores reject the gas molecules passing through the membrane, as shown in Figure 3.

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Clearly, the overlap of electron densities between O2 and the pore rim of graphdiyne at TS is less pronounced than those for the harmful gases. This qualitatively explains the smallest energy barrier of O2 among all the studied gases, which is consistent with our work on polyphenylene membrane.40 The vdW surface of gas molecules and graphdiyne can also be used to make similar comparison with Figure 3, and electrostatic interaction together with vdW interaction are responsible for the energy barrier of the TS structures.51 To quantitatively evaluate O2 separation performance of graphdiyne from typical harmful gases, the selectivity of O2 over other gases and the permeance of the selected gases were calculated. As for the selectivity, the Arrhenius equation was adopted: A = A0 exp(–Ebarrier/kBT), where A is the diffusion rate, A0 is the diffusion prefactor, Ebarrier is the diffusion barrier, kB is the Boltzmann constant, and T is the temperature. Here we assume that the diffusion prefactors are identical for all gases under study.52 So we get the selectivity as: S = Aoxygen/Ax = exp(–(Eoxygen – Ex)/kBT), where Aoxygen (Eoxygen) and Ax (Ex) respectively stand for the diffusion rates (diffusion barriers) of oxygen and other gases. In Figure 4, each of the selectivities goes down along with the increase of temperature. It is obvious that the performance of O2 separation over H2S is the best. Even for the O2/CH2O separation, the selectivity reaches a considerably high value of 2×102 at room temperature (Table 1). Besides the selectivity, the other vital factor to describe the performance of a membrane is the permeance of the desired gas. The permeance can be computed by P = F/∆p, where F is the molar flux (mol m–2 s–1) of the gases, and ∆p is the pressure

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difference (Pa) between two sides of the membrane. The molar flux F is determined by F = N × f, in which N and f represent, respectively, the number of gas particles colliding with the wall and the probability for a particle to diffuse through the pore at a given velocity. Derived from the kinetic theory of gases, N = p/(A×(2πmkBT)1/2), with p, A, m, kB, and T denoting the pressure, Avogadro constant, the mass of the molecule, Boltzmann constant, and the temperature, respectively.53 The probability f ∞

is given by

f = ∫ f (v )dv vB

, where f(v) is the Maxwell velocity distribution and vB

represents the velocity corresponding to the diffusion energy barrier.54 This integral implies that gas molecules with kinetic energy big enough to overcome the corresponding diffusion barrier will be counted. We set the incoming pressure p to be 3×105 Pa and the pressure different ∆p to be 105 Pa,11 and the calculated permeances of the studied gases are plotted in Figure 5. As we can see, all the lines present upward trends when the temperature increases. This implies that the increasing of permeance results from the promotion of molecular thermal motions and indirectly ensures the rationality of the calculation of permeance. Obviously, the line for O2 is overall higher than those for all the harmful gases, and the values of the permeance of O2 exceed the industrially acceptable minimum (6.7×10–9 mol m–2 s–1 Pa–1) for O2/N2 separation in the whole range of temperatures.55 Comparatively speaking, the O2 permeance of 1×10–4 mol m–2 s–1 Pa–1 at 300 K (i.e., the O2 molar flux of 1×10–3 mol cm–2 s–1 with the pressure drop of 105 Pa), is 103 times larger than all reported values at higher temperatures (873–1800 K) of the mixed ionic-electronic conducting ceramic-based membranes.33 By taking both the permeance and selectivity into

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account, graphdiyne is regarded as an outstanding membrane for O2 separation from toxic gas (Cl2, HCl, HCN, CNCl, SO2, H2S, NH3 and CH2O). For a particular example, the optimal selectivity of O2/H2S separation using graphdiyne is as high as 6×1013 and the permeance of H2S is 5×10–18 mol m–2 s–1 Pa–1 at 300 K with 14 orders of magnitude lower than that of O2. In fact, we also calculated the permeation of various molecules with notable bigger size and the performance for separating O2 over them is conceivably excellent as expected. For instance, the carbonyl chloride (COCl2) needs 6.10 eV to penetrate the pore of graphdiyne, which corresponds to the striking selectivity of 7×1098 for O2 over COCl2 and the COCl2 permeance of 6×10–103 mol m–2 s–1 Pa–1 at 300 K (in other word, it is impermeable at all). IV. CONCLUSIONS In summary, via detailed first-principles calculations, we ascertained that graphdiyne is difficult to be oxidized (needs 1.97 eV to break the C≡C bond) and afterwards demonstrated graphdiyne to be a perfect membrane for O2 separation from Cl2, HCl, HCN, CNCl, SO2, H2S, NH3 and CH2O in terms of selectivity and permeance. Therefore, one-atom-thick graphdiyne with natural uniform pores and mechanical robustness is a very promising membrane for oxygen purification or oxygen separation from other gases, which will be of great interest and extensive use in scientific research, industrial and medical areas, and in the daily life of human beings. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected]

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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by NSF of China (21373113, 21403111, 11574151), Fundamental Research Funds for the Central Universities (30920140111008, 30916011105), the Natural Science Foundation of Jiangsu Province (BK2012394, BK20140526), China Postdoctoral Science Foundation Funded Project with Grant No. 2014M561576, and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Synthesis with Atomic Precision. Chem. Commun. 2009, 6919-6921. (30) Jiao, Y.; Du, A. J.; Hankel, M.; Zhu, Z. H.; Rudolph, V.; Smith, S. C. Graphdiyne: A Versatile Nanomaterial for Electronics and Hydrogen Purification. Chem. Commun. 2011, 47, 11843-11845. (31) Cranford, S. W.; Buehler, M. J. Selective Hydrogen Purification through Graphdiyne under Ambient Temperature and Pressure. Nanoscale 2012, 4, 4587-4593. (32) Bartolomei, M.; Carmona-Novillo, E.; Hernández, M. I.; Campos-Martínez, J.; Pirani, F.; Giorgi, G. Graphdiyne Pores: “Ad Hoc” Openings for Helium Separation Applications. J. Phys. Chem. C 2014, 118, 29966-29972. (33) Sunarso, J.; Baumann, S.; Serra, J. M.; Meulenberg, W. A.; Liu, S.; Lin, Y. S.; Diniz da Costa, J. C. Mixed Ionic–electronic Conducting (MIEC) Ceramic-Based Membranes for Oxygen Separation. J. Membr. Sci. 2008, 320, 13-41. (34) Geffroy, P. –M.; Fouletier, J.; Richet, N.; Chartier, T. Rational Selection of MIEC Materials in Energy Production Processes. Chem. Eng. Sci. 2013, 87, 408-433. (35) Rao, H. X.; Zhang, Z. Y.; Song, C.; Qiao, T. Polydimethylsiloxane/liquid Crystal Cross-Linked Membranes: Preparation, Characterization and Oxygen Transport Properties. React. Funct. Polym. 2011, 71, 537-543. (36) Teixeira, M.; Campo, M. C.; Pacheco Tanaka, D. A.; Llosa Tanco, M. A.; Magen, C.; Mendes, A. Composite Phenolic Resin-Based Carbon Molecular Sieve Membranes for Gas Separation. Carbon 2011, 49, 4348-4358. (37) Sundell, B. J.; Shaver, A. T.; Liu, Q.; Nebipasagil, A.; Pisipati, P.; Mecham, S. J.; Riffle, J. S.; Freeman, B. D.; McGrath, J. E. Synthesis, Oxidation and Crosslinking of Tetramethyl Bisphenol F (TMBPF)-Based Polymers for Oxygen/Nitrogen Gas Separations. Polymer 2014, 55, 5623-5634. (38) Murray, L. J.; Dinca, M.; Yano, J.; Chavan, S.; Bordiga, S.; Brown, C. M.; Long, J. R. Highly-Selective and Reversible O2 Binding in Cr3(1,3,5-Benzenetricarboxylate)2. J. Am. Chem. Soc. 2010, 132, 7856-7857. (39) Bloch, E. D.; Murray, L. J.; Queen, W. L.; Chavan, S.; Maximoff, S. N.; Bigi, J. P.; Krishna, R.; Peterson, V. K.; Grandjean, F.; Long, G. J.; Smit, B.; Bordiga, S.; Brown, C. M.; Long, J. R. Selective Binding of O2 over N2 in a Redox-Active Metal-Organic Framework with Open Iron(II) Coordination Sites. J. Am. Chem. Soc. 2011, 133, 14814-14822. (40) Lu, R. F.; Meng, Z. S.; Rao, D. W.; Wang, Y. H.; Shi, Q.; Zhang, Y. D.; Kan, E. J.; Xiao, C. Y.; Deng, K. M. A Promising Monolayer Membrane for Oxygen Separation from Harmful Gases: Nitrogen-Substituted Polyphenylene. Nanoscale 2014, 6, 9960-9964. (41) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. (42) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. (43) Mills, G.; Jónsson, H. Quantum and Thermal Effects in H2 Dissociative Adsorption: Evaluation of Free Energy Barriers in Multidimensional Quantum Systems. Phys. Rev. Lett. 1994, 72, 1124-1127. (44) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901-9904. (45) Henkelman, G.; Jónsson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978-9985. (46) Carrete, J.; Longo, R. C.; Gallego, L. J.; Vega, A.; Balbás, L. C. Al Enhances the H2 Storage Capacity of Graphene at Nanoribbon Borders but Not at Central Sites: A Study Using Nonlocal van der Waals Density Functionals. Phys. Rev. B 2012, 85, 125435.

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(47) Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787-1799. (48) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (49) Long, M. Q.; Tang, L.; Wang, D.; Li, Y. L.; Shuai, Z. G. Electronic Structure and Carrier Mobility in Graphdiyne Sheet and Nanoribbons: Theoretical Predictions. ACS Nano 2011, 5, 2593-2600. (50) Lu, R. F.; Rao, D. W.; Meng, Z. S.; Zhang, X. B.; Xu, G. J.; Liu, Y. Z.; Kan, E. J.; Xiao, C. Y.; Deng, K. M. Boron-Substituted Graphyne as a Versatile Material with High Storage Capacities of Li and H2: A Multiscale Theoretical Study. Phys. Chem. Chem. Phys. 2013, 15, 16120-16126. (51) Ji, Y. J.; Dong, H. L.; Lin, H. P.; Zhang, L. L.; Hou, T. J.; Li, Y. Y. Heptazine-Based Graphitic Carbon Nitride as an Effective Hydrogen Purification Membrane. RSC Adv. 2016, 6, 52377-52383. (52) Li, Y. F.; Zhou, Z.; Shen, P. W.; Chen, Z. F. Two-Dimensional Polyphenylene: Experimentally Available Porous Graphene as a Hydrogen Purification Membrane. Chem. Commun. 2010, 46, 3672-3674. (53) Hauser, A. W.; Schrier, J.; Schwerdtfeger, P. Helium Tunneling through Nitrogen-Functionalized Graphene Pores: Pressure- and Temperature-Driven Approaches to Isotope Separation. J. Phys. Chem. C 2012, 116, 10819-10827. (54) Blankenburg, S.; Bieri, M.; Fasel, R.; Müllen, K.; Pignedoli, C. A.; Passerone, D. Porous Graphene as an Atmospheric Nanofilter. Small 2010, 6, 2266-2271. (55) Zhu, Z. Q. Permeance Should be Used to Characterize the Productivity of a Polymeric Gas Separation Membrane. J. Membr. Sci. 2006, 281, 754-756.

Table 1 The adsorption energies (Ead, in eV)a of O2 and the studied harmful gases on graphdiyne, the diffusion barriers (Ebarrier, in eV)b of the gases, the selectivities (S, at T = 300 K) of O2 over the harmful gases (X) and the permeances (P, in mol m–2 s–1 Pa–1 at T = 300 K) of the gases when penetrating the pore of graphdiyne. Property Ead Ebarrier Soxygen/X P

O2 0.08 0.21 - 1×10–4

Cl2 0.12 0.65 2×107 7×10–12

HCl 0.14 0.83 3×1010 1×10–14

HCN 0.17 0.46 1×104 1×10–8

CNCl 0.13 0.59 3×106 7×10–11

SO2 0.10 0.54 4×105 4×10–10

a

H2 S 0.13 1.03 6×1013 5×10–18

NH3 0.16 0.38 6×102 3×10–7

CH2O 0.15 0.34 2×102 1×10–6

Ead = Ex + Eg – Ex@g, where Ex, Eg and Ex@g represent the energies of isolated gas molecule, the monolayer graphdiyne and the optimized IS structure for gas absorbed on graphdiyne, respectively. b Ebarrier = ETS – EIS, in which ETS and EIS stand for the energies of the optimized TS and IS structure, respectively.

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Figure 1. The relative energies of IS, TS and FS structures in the oxidation process of the acetylenic bond in graphdiyne. (C, green balls; O, red balls).

Figure 2. The schematic diagram of the process of oxygen passing through graphdiyne, in which the structures are respectively oxygen far away from graphdiyne, oxygen adsorbed on graphdiyne (IS with the energy set to be 0 eV), and the TS. (C, green balls; O, red balls).

Figure 3. The electron densities of the TS structures for O2, Cl2, HCl, HCN, CNCl, SO2, H2S, NH3, and CH2O penetrating the graphdiyne membrane. The isovalue is 0.075 a. u.

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Figure 4. Selectivities versus temperature for O2 over Cl2, HCl, HCN, CNCl, SO2, H2S, NH3 and CH2O passing through graphdiyne.

Figure 5. Permeances versus temperature for O2, Cl2, HCl, HCN, CNCl, SO2, H2S, NH3 and CH2O passing through graphdiyne.

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