Effect of N-Containing Functional Groups on CO2 ... - ACS Publications

Article pubs.acs.org/JPCC

Effect of N‑Containing Functional Groups on CO2 Adsorption of Carbonaceous Materials: A Density Functional Theory Approach Geunsik Lim,† Ki Bong Lee,*,† and Hyung Chul Ham*,‡ †

Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-713, Republic of Korea ‡ Fuel Cell Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil, Seongbuk-gu, Seoul 136-791, Republic of Korea S Supporting Information *

ABSTRACT: The amount of anthropogenic CO2 emission keeps increasing worldwide, and it urges the development of efficient CO2 capture technologies. Among various CO2 capture methods, adsorption is receiving more interest, and carbonaceous materials are considered good CO2 adsorbents. There have been many studies of N-containing carbon materials that have enhanced surface interaction with CO2; however, various N-containing functional groups existing in the carbon surface have not been investigated in detail. In this study, firstprinciple calculations were conducted for carbon models having various Nfunctional groups to distinguish N-containing heterogeneity and understand carbon surface chemistry for CO2 adsorption. Among N-functional groups tested, the highest adsorption energies of −0.224 and −0.218 eV were observed in pyridone and pyridine groups, respectively. Structural parameters including bond angle and length revealed an exceptional hydrogen-bonding interaction between CO2 and pyridone group. Charge accumulation on CO2 during interaction with pyridine-functionalized surface was confirmed by Bader charge analysis. Also, the peak shift of CO2 near Fermi level in the DOS calculation and the presence of HOMO on pyridinic-N in the frontier orbital calculation determined that the interaction of pyridinic-N is weak Lewis acid−base interaction by charge transfer. Furthermore, adsorption energies of N2 were calculated and compared to those of CO2 to find its selective adsorption ability. Our results suggest that pyridone and pyridine groups are most effective for enhancing the interaction with CO2 and have potential for selective CO2 adsorption.

1. INTRODUCTION

CO2 adsorption on porous carbon materials is based on interaction with porous solid surface, surface interaction. Therefore, enhancement of specific surface interaction to CO2 is critical for the development of adsorbents with high performance and selectivity. Recently, modification of surface chemistry by nitrogen heterogeneities characterizing basic nature is recognized as a promising method for increasing CO2 adsorption capacity.20−22 Kemp et al. synthesized a porous N-functionalized adsorbent through chemical activation and investigated its CO2 adsorption properties. The adsorbent showed well-preserved adsorption capacity, and the maximum capacity of 2.18 mmol/g was obtained with N content of 6.56 wt %.23 Sevilla et al. developed N-doped carbons from KOH activation of polypyrrole-based carbon source. As-synthesized N-doped carbons exhibited good adsorption capacity of 3.9 mmol/g at 25 °C and 1 bar.24 Although many experimental studies of carbon materials containing N-functional groups have been reported for CO2 adsorption, the effect of N-containing

The amount of anthropogenic carbon dioxide (CO2) emission has been increasing for decades. As a result, an atmospheric CO2 concentration over 400 ppm was recorded at Mauna Loa, and its gradual increase is considered as a major contributor of global warming and climate changes.1,2 To control and inhibit the increase of CO2 emission and its adverse effects, CO2 capture and sequestration (CCS) technologies are being studied and developed in various fields.3−6 Generally, absorption by liquid amine solvent, especially alkanolamines (primary and secondary), is the most conventional and matured technology to capture CO2.7−9 Although absorption is wellknown for its high sorption ability and reactivity,10,11 solvent degradation, byproducts, and high energy consumption are the challenges to overcome.11−13 In this respect, adsorption of CO2 by solid materials such as activated carbon, metal organic frameworks (MOFs), zeolites, silica, and alumina is considered an alternative method.14−17 Among them, carbonaceous adsorbents derived from various carbon sources have been highlighted due to availability of inexpensive carbon source, possibility of modifying physical and chemical properties, large surface area, and highly developed porosity.18,19 © XXXX American Chemical Society

Received: December 10, 2015 Revised: March 26, 2016

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DOI: 10.1021/acs.jpcc.5b12090 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Structures of N-functionalized carbonaceous material surface model. Atom colors: C = gray; H = white; O = red; N = blue.

interactions between core ions and valence electrons.30 Wave functions were expanded by plane wave basis set with a cutoff energy of 400 eV for a reasonable number of plane waves for description of wave functions. The advantages of plane wave basis set are freedom of basis set superposition error (BSSE) and independence of positions of atom.31,32 In terms of exchange-correlation functional, one proposed by Perdew, Burke, and Ernzerhof within the generalized gradient approximation (GGA-PBE) was selected.33 PAW pseudopotential that is more accurate than ultrasoft pseudopotentials was exploited for all atoms included in this study.34 Integration over the Brillouin zone was carried out with Monkhorst−Pack mesh of (2×2×1) k-points for geometry optimization and (8×8×1) for electronic structure calculation. For cutoff energy and kpoint grids, convergence is needed to be confirmed, and the result is summarized in the Supporting Information. Ionic relaxation was conducted until the force between two consecutive steps was smaller than 0.05 eV/Å, and the electronic self-consistent loop was broken after 10−4 eV/Å of change was reached. Dispersion force is of importance in the adsorption system to describe interactions precisely; however, it was impossible to reflect this van der Waals force in DFT approaches. To take this into consideration, the DFT-D2 method developed by Grimme was employed.35 2.2. Modeling of Adsorption System. To perform intensive calculation on the effect of nitrogen heterogeneity, especially nitrogen-containing functional groups introduced in carbonaceous material, a model of carbon material consisting of 32 C atoms, 16 terminating H atoms, and 9 aromatic rings with no branch was fully optimized (Figure 1a). The number of C and H atoms and the number of aromatic rings are varied depending on which functional group is introduced. The structural parameters of N-functionalized surface are summarized in Table S2. In terms of cell parameters, 30 Å for length and breadth and 12 Å for vacuum distance between layers were determined from the cell parameter optimization, and the result is explained in detail in the Supporting Information. This cell size was sufficient to minimize the effect of adjacent layer on CO2 adsorption. To ensure the validity of the model as carbonaceous material, some of the previous reports were referred. Carbon materials such as char, tar, and activated carbons have been modeled according to solid-state 13C NMR analysis, and the materials exhibit high aromaticity depending on treatment temperature and methods.36,37 These carbon materials mainly consist of carbon fragments of protonated aromatic carbon rings, having

functional groups on CO2 adsorption enhancement is not fully understood. To overcome the limitation of experimental results, various computational approaches along with molecular simulations for CO2 adsorption on carbon materials have been conducted. Madyal and Arora carried out calculations on tertiary aminefunctionalized adsorbents and suggested applicability of quantum chemical study to the design of desirable CO2 specific adsorbents.13 Mo et al. studied the adsorption of CO2 on N-containing molecular segment models of coal including 2-methylpyridine and C13H9N.25 They showed that the increase of π-system size enhances binding energy of CO2. Chen et al. calculated π−π interactions of CO2 with benzene, pyridine, and pyrrole by ab initio calculations and revealed that the π−π interaction involves the contribution of electron orbital interactions.26 In this study, the effect of N-containing functional groups in carbonaceous adsorbent on CO2 adsorption and the surface interaction with CO2 was investigated first in detail using computational calculation. Carbon material surfaces containing seven different N-functional groups were modeled for comparison, and the reliability of the adsorbent surface model was identified on the basis of both previous experimental and theoretical studies. Geometry optimization and binding energy calculations were carried out for each initial configuration. Additionally, charge transfer amount upon adsorption was calculated through Bader charge analysis. Also, several structural parameters such as bond angle and length, and the distance between molecule and surface, were measured. Further investigation and electrical properties were studied by density of states and HOMO of the surface. Additionally, adsorption strength of N2 was calculated and compared to that of CO2 to demonstrate the possibility of functional groups to selective and enhanced CO2 adsorption. Our objective is to provide an introduction of N-containing functional groups that are more specific to CO2 for designing optimal adsorbent.

2. COMPUTATIONAL DETAILS 2.1. Computational Methodology. All of the firstprinciples calculations were conducted on the basis of density functional theory (DFT) with the use of the Vienna ab initio simulation package (VASP, version 5.2.12).27−29 The projector augmented wave (PAW) method by Blöchl was employed to perform electronic structure calculations that enhance computational efficiency. The PAW method can describe B


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The Journal of Physical Chemistry C molecular weight of 200−400 and only a small amount of side chains.37 These experiment-based works formed the basis of our N-functionalized carbon surface model, an aromatic carbon fragment described in Figure 1. Similar carbon models have been used for CO2 adsorption in previous computational studies. Xing et al. adopted a carbon model with 8 sixmembered and 1 five-membered rings to account for interaction between modeled activated carbon and CO2.38 Espinal et al. also modeled char using several carbon models (zigzag, tip, and armchair) and applied for CO2 adsorption.39 A detailed explanation of the validity of our model is provided in the Supporting Information. Seven representative nitrogen-containing functional groups, including pyridone, pyridine, pyrrole, quaternary, pyridine-Noxide, cyanide, and amine groups (Figure 1b−h), were considered to compare the effect of different functional groups on CO2 adsorption. After geometry optimization of the functionalized carbon surface, CO2 adsorption on the surface was calculated. The adsorption energies were calculated from six different initial configurations for CO2 and N2 molecules as depicted in Figure 2. The configurations were considered with

Eads = Esurface + CO2 − (Esurface + ECO2)

where Eads, Esurface+CO2, Esurface, and ECO2 are adsorption energy and total energy of adsorbent−adsorbate complex, carbon surface, and isolated CO2, respectively. Because the equation includes subtraction of total energy of surface and CO2 from that of complex, a higher negative value indicates a more favorable and stronger interaction. For detailed analysis of CO2 adsorption system and comparison of various functional groups, structural parameters such as the distance between CO2 and adsorbent surface, change of CO bond length in CO2, change of OCO bond angle (θ) upon adsorption, and vibrational frequencies were investigated. Also, Bader charge analysis was performed to calculate the amount of charge transferred from (or to) CO2 and to identify the amount of electrons distributed during interaction with the surface.40,41 To elucidate the change of electronic properties and adsorption mechanism, density of states (DOS) were investigated for functionalized surfaces, adsorbed CO2, and nitrogen atoms with increased criteria for ionic and electronic relaxation. Finally, highest occupied molecular orbital (HOMO) of surfaces and lowest unoccupied molecular orbital (LUMO) of CO2 were analyzed to further investigate the functional groups showing the strongest interaction with CO2.

3. RESULTS AND DISCUSSION 3.1. Adsorption Energy Calculation. The most stable configuration for CO2 adsorption was determined in each functional group through binding energy calculation starting from six initial configurations as shown in Figure 2. Comparison of binding energies (Eads) calculated from different initial configurations provides the information on how the functional group interacts with the atom of CO2 and which configuration is most favorable upon adsorption. Figure 3 presents the optimized geometries, the most favorable CO2 adsorption configurations, and the corresponding adsorption energies in each functionalized surface. The binding energies of other configurations exhibit less stable and even unstable states. As shown in Figure 3a, without any functional group, Eads is less

Figure 2. Six binding configurations for adsorbate molecules (CO2 and N2). In the abbreviations, O, orthogonal; V, vertical; H, horizontal; A, above the group; N, next to the group.

respect to the relative position of the adsorbate to the functional group to figure out how strong adsorbent interacts with linear-shaped adsorbate molecule. The CO2 adsorption energy is calculated in the following manner:

Figure 3. Optimized geometry upon CO2 adsorption and Eads for the most stable CO2 adsorption configuration in each functionalized surface. C


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The Journal of Physical Chemistry C than −0.1 eV including weak van der Waals interaction. Considering initial configurations, parallel configurations (orthogonal and horizontal) give a stronger interaction with CO2. CO2 adsorption in this model is based on the assumption of having no curvature or a bowl-like structure and is comparable with other studies for graphene and two-dimensional aromatic molecule structure. Ghosh et al. studied the physisorption of CO2 on a pristine graphene layer with several adsorption configurations and found the highest binding configuration is a parallel configuration with both O atoms of CO2 pointing center of the six-membered ring.42 This corresponds to our result that the CO bond of CO2 is perpendicular to the C−H bond of the surface. Also, Lee et al. theoretically investigated the CO2 adsorption on graphene sheet with DFT and MP2 calculations.43 The C32 and C40 model was used without termination by hydrogen atoms, and it was shown that the parallel configuration is more favorable than the perpendicular configuration, having ∼1.0−1.2 kcal/mol (0.0434−0.056 eV) higher adsorption energy. Huang et al. studied the sensing ability of graphene nanoribbon (GNR) for several gas molecules including CO, CO2, NO, and NO2. Their study confirmed the bond formation of CO2 with C atom of 10AGNR (armchair GNR), which has an unpaired electron, implying that a dangling bond induces a strong chemical bond with gas molecule with sp2 hybridization and C−O−C angle of 127°.45 Unlike the study of Huang et al., all C atoms on the edge were terminated by hydrogen atom in our model to find the effect of the N-containing functional group instead of the dangling bond. Regardless of the type of N-containing functional group, functionalization enhances binding energies with CO2. Among the seven N-containing functional groups, the highest binding energy of CO2, −0.224 eV (21.58 kJ/mol), is obtained in the pyridone group (Figure 3b). Upon adsorption, CO2 locates closely to the hydroxyl group of the pyridone group due to hydrogen-bonding interaction. The distance from the nearest O atom of CO2 and H atom of hydroxyl group is 1.943 Å. This typical hydrogen bond (O−H···O) is due to high electronegativity of O of CO2. Xing et al. also studied the hydrogenbonding interaction in CO2 adsorption, and N-containing carbon surfaces exhibited an average binding energy in the range 5.65−11.59 kJ/mol.38 Being located orthogonal and next to the group (ON) configuration enables both hydrogen interaction and the interaction attributed to pyridone-N, which contribute the driving force of strong adsorption energy. To confirm the effect of the hydrogen-bonding interaction by hydroxyl group in pyridone group, pyridonic-N was replaced with −C−H group. Recalculation of Eads with replacement of pyridonic-N atom with −C−H exhibits a weaker interaction, resulting in Eads = −0.0889 eV. Therefore, it can be thought that the contribution of hydrogen-bonding interaction to the adsorption energy is slightly higher than the Lewis acid−base interaction in carbon surface containing pyridone group. In the case of a pyridine group-functionalized surface, Eads is almost identical to that of the pyridone group-functionalized surface. Even without a hydroxyl group, pyridinic-N is more suitable for CO2 adsorption due to its stronger electronegativity. CO2 at ON configuration is most favorable, and it can be thought that pyridinic-N prefers the electron-deficient C atom rather than the O atom in CO2 (Figure 3c). For the amine group-functionalized surface, −0.147 eV is the highest Eads among the considered initial configurations (Figure 3d).

Grafted amine group provides stronger basicity, causing stronger adsorption than pristine carbon.46 For other groups such as quaternary, pyridine-N-oxide, cyanide, and pyrrole, the difference of Eads from that on the pristine surface is less than 0.03 eV (Figure 3e−h), indicating that they are not sufficiently effective in enhancing CO2 adsorption strength. Note that vertical configuration is not preferred for CO2 in all surfaces. This is because of the preference of a quadrupole−quadrupole interaction between surface and molecule.44 When CO2 is adsorbed on a pyrrole group-functionalized surface, the horizontal and next to the group configuration (HN) has the highest Eads (Figure 3h), which is different from other cases and implies that hydrogen-bonding interaction (N− H···O) can occur. The distance between O atom of CO2 and H atom of pyrrole group is 2.135 Å, which is longer than that in the pyridone group and can be attributed to the difference in O−H and N−H groups. There is significant disparity in molecular distance and Eads between pyridone and pyrrole groups, but they are indistinguishable in experimental data such as XPS spectra.24 The different adsorption energetics and configurations of CO2 on functionalized carbon surfaces can be understood by examining their bonding nature with CO2. First, CO2 weakly interacts with pristine surface via van der Waals force where the π electron of carbon surface (having sp2 hybridization character) attracts the positively charged carbon atom of CO2. Next, for the pyridine, amine, quaternary, and cyanide group-functionalized surfaces, CO2 mainly interacts with the nitrogen atom of the functionalized surfaces. Here, pyridinic-N shows the highest adsorption energy (Eads = −0.218 eV), followed by amine (Eads = −0.147 eV), quaternary (Eads = −0.125 eV), and cyanide (Eads = −0.112 eV) groups. This difference is related to the different Lewis acid−base interaction where the lone pair electron of nitrogen atom (Lewis base) donates the electronic charge to the carbon atom of CO2 (Lewis acid) and steric hindrance of the interaction between CO2 and functionalized surfaces. The strength of the Lewis acid−base interaction may be indirectly understood by the transferred electronic charge to CO2 from functionalized surface. The higher is the charge transfer to CO2, the stronger is the Lewis acid−base interaction (the higher the basicity). Our Bader charge calculation shows that the order of the transferred electronic charge to CO2 from functionalized surface is pyridine (0.041 e) > amine (0.023 e) > cyanide (0.015 e) > quaternary (0.010 e) group, suggesting the highest basicity of nitrogen atom of pyridine surface and in turn the significantly enhanced binding strength of CO2 at the pyridinicN site. The reduced adsorption strength of CO2 with amine group is related to the decrease of basicity of the nitrogen atom and steric hindrance by the hydrogen atom of amine group.47 For the quaternary and cyanide groups, the weak interaction can arise from the lowest basicity of nitrogen atom. Notice the similar charge transfer to adsorbed CO2 from the quaternary and cyanide groups to the pristine case (0.015 e). In the pyridone group-functionalized surface, two types of interaction, the Lewis acid−base and hydrogen-bonding interactions, play an important role in determining adsorption behavior. As explained above, the hydrogen-bonding interaction between the oxygen atom of CO2 and the hydrogen atom of pyridone group is slightly stronger than the Lewis acid−base case. Note that the increase of electronic charge in adsorbed CO2 (0.011 e) is smaller than the Lewis acid/base-dominant D


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having specific hydrogen-bonding interaction show significant bond elongation (l1) and contraction (l2). Especially, CO bonds that are close to −O−H or −N−H group are extended, while the other bonds are shortened. The largest bond elongation by 3.6 × 10−3 Å and contraction by 4.0 × 10−3 Å occur upon interaction with pyridone group, and the strong interaction by hydroxyl group is responsible for this distinctive change. Bond angles are decreased by about 3.12° and 0.02° in the pyridone and pyrrole functional groups, respectively (Table 2). Interaction between CO2 and N-functionalized surface causes charge transfer and redistribution. The amount of charge transfer is summarized in Table 2. From Bader charge analysis, while 0.011 e of charge is transferred to CO2 in the pyridone group-functionalized surface, only 0.003 e is transferred to CO2 when pyridonic-N is replaced with −C−H group. This supports the contribution of pyridonic-N through Lewis acid−base interaction (charge transfer) with CO2. Electrophilic C atom of CO2 makes the molecule Lewis acidic and interacts with Ncontaining functional group through a weak electron acceptor− donor type interaction. This Lewis acid−base interaction becomes noticeable by pyridine group. Regarding the charge transferred to CO2, a relatively large amount of charge is transferred from the surface to CO2 through pyridinic-N. Because pyridinic-N is more electronegative than C atom and draws charge from adjacent C atoms, more charge can be transferred during CO2 adsorption. As a result, CO2 gains 0.041 e from the surface, and this interaction of Lewis acidic CO2 with basic pyridinic-N is the reason for higher Eads and the largest bond angle decrease of 6.09° with C atom pointing pyridinic-N atom. It is noticeable that both bonds are elongated in the pyridine group, while bond contraction is observed in all other groups. This is attributed to the nature of the functional group and less steric hindrance to interaction. Similar behavior of large amounts of charge transfer and higher Eads is observed in the amine functional group, and large amounts of charge transfer can be considered an index of strong adsorption energy. Weakly interacting functional groups donate less than 0.02 e to the acidic CO2 molecule, and the sum of bond lengths (l3) decreases (Table 1). 3.3. Density of States (DOS) Plots. To analyze electronic property of the surface functionalized by pyridone and pyridine groups, which show highest Eads for CO2 among the tested seven N-functional groups, density of states (DOS) were calculated as shown in Figure 4. Although Eads are almost the same for the two surfaces, type and magnitude of forces applied to CO2 are different as explained in the previous section. In DOS plots (Figure 4a and b), no significant change is observed for two adsorbent surfaces. This indicates that the interaction includes only weak forces such as Lewis acid−base interaction and van der Waals interaction (London dispersion force), and there is no chemical bond formation. Figure 4c shows the DOS of free and adsorbed CO2 on the surface containing pyridine group. Occupancy under E f increases after interaction, which corresponds to LUMO of CO2, and this also proves the charge transfer from surface to LUMO of CO2. This shift is recognizable as compared to that in Figure 4d for the surface containing pyridine group with the largest amount of charge transfer. Partial density of states (PDOS) of nitrogen atom in each functional group are separated from total density of states (TDOS) of adsorbent surface and displayed in Figure 4e and f.

interaction case (for example, 0.041 e for the pyridine case). To clearly understand the CO2−pyridone group interaction, we calculated the electrostatic potential for pyridone groupfunctionalized and pristine surfaces (see the Supporting Information for computational details). As shown in Figure S4a, the nitrogen and oxygen atoms in pyridone groupfunctionalized surface have the lowest electrostatic potential, indicating the relative abundance of electron at the nitrogen and oxygen sites. Here, the nitrogen atom can interact with the carbon atom of CO2 via the Lewis acid−base relationship. On the other hand, the electron-withdrawing inductive effect of the nitrogen and oxygen atoms gives rise to the highest electrostatic potential (the relative absence of electron) at a hydrogen atom in the β position for the pyridone group, which can play an important role in determining the hydrogen bonding with the oxygen atom of CO2. For the pyrrole group, CO2 interaction happens between the positively charged hydrogen atom of HN-functional and the oxygen atom of CO2, while for the pyridine-N-oxide group, the CO2 adsorption occurs through the interaction between the oxygen atom of functional group and the carbon atom of CO2. These bonding interactions (−NH···O and −NO···C) are weaker than the hydrogen bonding of pyridone group and the Lewis acid−base interaction of pyridine group, leading to the weaker adsorption energy of CO2. 3.2. Structural Parameters. Interactions between CO2 and the surface are further investigated by calculating several structural parameters. In the adsorption process, CO2 molecule undergoes deformation due to interactions. The deformation includes distortion of molecule (angle change of OCO bond), and bond lengthening and shortening (length change of CO bonds). The calculated average bond length of isolated CO2 molecule is 1.1767 Å, and it fairly corresponds with both experimental (1.1608 Å) and computational (1.174 Å from DFT-D2 method) data reported.48,49 The distance between CO2 and the functionalized surface and change of bond lengths are calculated and summarized in Table 1. In the CO2 adsorption, pyridone and pyrrole group-functionalized surfaces Table 1. Distance between CO2 and Functionalized Surface and Change of CO Bond Length during Adsorptiona d (Å) functional group pyridone (Pdo) pyridine (Pyd) amine (Ami) quaternary (Quat) pyridine-Noxide (POx) cyanide (Cyn) pyrrole (Pyr) pristine (Pris)

l (Å) CO2 (l2) × 103

l3 (=l1 + l2) × 103

3.6

−4.0

−0.4

2.68 (C--N)

1.3

1.7

3.0

2.86 (C--N) 3.13 (C--N)

0.6 ∼0

−0.1 −0.2

0.5 −0.2

0.3

−0.8

−0.5

0.2

−0.5

−0.3

1.5 0.1

−3.1 −0.5

−1.6 −0.4

shortest (d1)

C of CO2--N (d2)

1.94 (O--H)

3.01

2.77 (C--O)

4.05

2.84 (C--N) 2.13 (O--H) 3.21 (C--C)

4.33

CO1 (l1) × 103

a

d1 is the shortest distance between CO2 and surface. d2 is the distance between C of CO2 and N of functional group. l1 and l2 are the extended and/or contracted amounts of CO bond length. l3 is the bond length of CO2. E


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Table 2. Amount of Charge Transferred to CO2 Molecule (Each Atom and CO2 Molecule), Amount of Charge Transferred from N Atom, and the Change of Bond Angle after CO Adsorption (Angle Change in Parentheses)a amount of charge transferred (e)

a

functional group

O1

C

O2

CO2

N

pyridone (Pdo) pyridine (Pyd) amine (Ami) quaternary (Quat) pyridine-N-oxide (POx) cyanide (Cyn) pyrrole (Pyr) pristine (Pris)

0.498 0.457 0.474 0.452 0.476 0.454 0.463 0.451

−0.946 −0.903 −0.923 −0.890 −0.919 −0.917 −0.870 −0.918

0.459 0.487 0.472 0.448 0.452 0.479 0.463 0.482

0.011 0.041 0.023 0.010 0.008 0.015 −0.002 0.015

0.040 −0.122 −0.072 0.022 0.049 0.087 0.048 N/A

OCO bond angle (deg) 176.88 173.91 177.68 179.93 177.99 177.73 179.98 178.99

(3.12) (6.09) (2.32) (0.07) (2.01) (2.27) (0.02) (1.01)

The charge transfer is calculated by Bader charge analysis.

Figure 4. Total density of states (TDOS) of adsorbent and CO2 and partial density of states (PDOS) of nitrogen atom. (a) Surface containing pyridine group before and after CO2 adsorption, (b) surface containing pyridone group before and after CO2 adsorption, (c) adsorbed CO2 on surface containing pyridine group, (d) adsorbed CO2 on surface containing pyridone group, (e) S and P orbitals of pristine pyridinic nitrogen, and (f) S and P orbitals of pristine pyridonic nitrogen.

A peak of spin-up P orbital at Fermi level (Ef) is observed in pyridinic-N, and it is sharper than that of pyridonic-N at the

same location. In the way that such a peak represents the ability of interaction, the result is consistent with larger amounts of F


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Figure 5. HOMO of surface containing (a) pyridone group, (b) pyridine group, (c) pyrrole group, (d) quaternary group, and (e) pristine adsorbent surface. (f) LUMO of free CO2 molecule.

Figure 6. Examination of selective adsorption of CO2 over N2 by functionalized surface. (a) Eads of CO2 and N2 gas molecules, and (b) ratio and difference of adsorption energies.

On the other hand, in the CO2−pyridine interaction, the Lewis acid−base interaction plays an important role in determining the adsorption process of CO2 onto the surface of pyridine group. Here, the lone pair electron of the nitrogen atom in the pyridine group (Lewis base) may occupy HOMO, while the electron-deficient carbon of CO2 (Lewis acid) may consist of LUMO. We see that LUMO in CO2 molecule (Figure 5f) is mainly located in the carbon atom of CO2, implying that carbon atom may interact with the HOMO of carbon materials. Looking at the pyridine group case (Figure 5b), we see that HOMOs could be located in the nitrogen or carbon atoms. However, because CO2 preferentially binds with the nitrogen atom, the HOMO containing nitrogen atom may be responsible for the CO2−pyridine interaction. Our Bader charge calculation shows the highest charge transfer from pyridine group to adsorbed CO2 (0.041 e) among the functionalized carbon materials, leading to the substantial increase of CO2 adsorption strength as compared to the pristine carbon surface. 3.5. Comparison with N2 Adsorption. One of the requirements for good adsorbents is the ability to adsorb CO2 selectively over other gases, especially N2. In this regard, adsorption energies for N2 were calculated by the same method

charge transfer from surface and stronger adsorption energy in pyridinic-N and pyridonic-N than other N-functional groups. 3.4. HOMO−LUMO Calculations. To identify the possible surface sites of functionalized carbon materials for CO2 adsorption, we have presented the topology of the HOMO of the bare surface. This is based on the frontier molecular orbital theory for chemical reactivity where the occupied orbital of one molecule (HOMO) has a tendency to interact attractively with the unoccupied orbital of the other molecule (LUMO) and the strength of HOMO−LUMO interaction is related to the amount of transferred electronic charge between HOMO and LUMO.13 Figure 5 demonstrates HOMO distribution on adsorbent surfaces at the same level of isosurface and LUMO of free CO2 molecule. Looking at the pyridone group case (Figure 5a), HOMO is located in the carbon atom, rather than the nitrogen atom, implying that the contribution of the Lewis acid−base interaction to the CO2 adsorption process is small. Instead, as explained in the above section, the interaction (hydrogen bonding) of the positively charged hydrogen of hydroxyl group in pyridone group with the negatively charged oxygen atom of CO2 determines the CO2 adsorption behavior. G



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applied for CO2 as represented in Figure 2. The highest binding energies between each surface and adsorbate (CO2 and N2) are compared in Figure 6a. It can be noticed that the N2 adsorption is weaker than CO2 adsorption for all N-functionalized surfaces. The weaker adsorption energy of N2 is attributed to its characteristics of smaller quadrupole moment and polarizability than those of CO2. The difference of adsorption energies among different N-functionalized surfaces is within 0.05 eV for N2, while the difference is higher than 0.1 eV for CO2. Also, some functional groups exhibit weaker interaction with N2 than pristine surface, implying that N-containing functional groups are not effective in N2 adsorption. Selective adsorption of CO2 over N2 is based on different binding affinities on the adsorbent surface. Therefore, higher adsorption energy for CO2 and larger adsorption energy difference between CO2 and N2 can be indicators of selective adsorption. Figure 6b shows the ratio and difference of the highest adsorption energies for CO2 and N2 in each surface. Pyridine, pyridone, amine, and cyanide groups exhibit a relatively high ratio of adsorption energies of CO2 over N2. The highest ratio of 3.60 is obtained for the pyridine group. When the difference of adsorption energies for CO2 and N2 is considered, pyridone, pyridine, and amine groups appear to be suitable for selective adsorption and separation of CO2 over N2.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation (NRF) grant funded by the Korean government’s Ministry of Science, ICT and Future Planning, through the Basic Science Research Program (2015R1A1A1A05001363), the New & Renewable Energy Core Technology Program (20153030041170), and the Human Resources Development Program (20134010200600) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government’s Ministry of Trade, Industry and Energy.

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4. CONCLUSIONS The effect of N-containing functional groups introduced in carbonaceous surface was investigated for CO2 adsorption by first-principle calculations. Among the seven N-functional groups studied, pyridone and pyridine groups exhibit strong binding energies of −0.224 and −0.218 eV, respectively. The interaction between CO2 and surface mainly consists of weak Lewis acid−base interaction through electron donation and van der Waals interaction. From the calculation of structural parameters and Bader charge analysis, remarkable charge transfer to CO2 is observed from pyridinic-N, and this results in the largest decrease of bond angle. On the other hand, pyridone group shows exceptional hydrogen-bonding interaction that causes bond length stretch and compression. In DOS curves, a small peak shift of CO2 and a peak at Fermi level of pyridinic-N suggest weak interaction through charge transfer. Calculated HOMO of modeled surfaces and LUMO of CO2 also support the adsorption mechanism based on charge transfer from the presence of HOMO on each N atom. The surfaces containing N-functional group also show stronger adsorption energy of CO2 over N2, implying selective adsorption for CO2. The results in this study are expected to be helpful to reveal the CO2 adsorption mechanism and develop CO2 adsorbents based on carbonaceous materials containing N-functional groups.

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b12090. K-point optimization, cutoff energy, cell parameters (x and y), validation of adsorbent model, electrostatic potential plots, and charge density difference plot (PDF) H


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