Ambient Carbon Dioxide Capture Using Boron-Rich Porous Boron

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Ambient Carbon Dioxide Capture Using BoronRich Porous Boron Nitride: A Theoretical Study Lanlan Li, Yan Liu, Xiaojing Yang, Xiaofei Yu, Yi Fang, Qiaoling Li, Peng Jin, and Chengchun Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01106 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 12, 2017

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ACS Applied Materials & Interfaces

Ambient Carbon Dioxide Capture Using Boron-Rich Porous Boron Nitride: A Theoretical Study Lanlan Li*, Yan Liu, Xiaojing Yang, Xiaofei Yu, Yi Fang, Qiaoling Li, Peng Jin*, Chengchun Tang* Key Lab for Micro- and Nano-Scale Boron Nitride Materials in Hebei Province, School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China.

ABSTRACT: The development of highly efficient sorbent materials for CO2 capture at ambient conditions is of great importance for reducing the impact of CO2 on the environment and climate change. In this account, strong CO2 adsorption on boron antisite (BN) in boron-rich porous boron nitrides (p-BN) was developed and studied. The results indicated that the material achieved larger adsorption energies of 2.09 eV (201.66 kJ/mol, PBE-D). The electronic structure calculations suggested that the introduction of BN in p-BN induced defect electronic states in the energy gap region, which strongly impacted the adsorption properties of the material. The bonding between BN defect and CO2 molecule was clarified, and found that the electron donation first occurred from CO2 to BN double-acceptor state then followed by electron back-donation from BN to CO2 accompanied by the formation of a BN-C bond. The thermodynamic properties indicated that the adsorption of CO2 on BN defect to form anionic CO2δ- species was spontaneous at temperatures below 350 K. Both the large adsorption energies and the thermodynamic

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properties ensured that p-BN with BN defect could effectively capture CO2 at ambient conditions. Finally, to evaluate the energetic stability, the defect formation energies were estimated. The formation energy of BN defects was found to strongly depend on the chemical environment, and the selection of different reactants (B or N sources) would achieve the goal of reducing the formation energy. These findings provided a useful guidance for the design and fabrication of porous BN sorbent for CO2 capture.

Keywords: Porous boron nitrides, adsorption, carbon dioxide capture, boron antisite defect, density functional calculations.

1. INTRODUCTION Carbon-based fossil fuels, providing about 80% of the world’s current energy needs, are the main source of increased levels of carbon dioxide (CO2) in the atmosphere. CO2 is considered as a predominant contributor to the greenhouse effect believed to significantly affect the climate change, global warming,1-3 and ocean acidification.4-7 As measured by the Scripps Institute of Oceanography, the CO2 concentration increased from ca. 315 ppm in March 1958 to 391 ppm in January of 2011, and was close to 403 ppm in November 2016.8 The reduction of CO2 emissions is currently possible with the use of efficient technologies for Carbon Capture and Storage (CCS),9-12 which employ absorbents or solvents.13 On the other hand, the CO2 re-use into renewable feedstocks for the production of chemicals has been suggested as an appealing route to reduce climate change. However, this stills in its infancy and many questions over the system efficiency have been raised.13-15 Most of the utilized active catalysts in these processes are known to be scarce and precious metals often supported on high


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surface areas of porous oxides or sulfides.16,17 The effective and large-scale implementation of these strategies are, however, remain challenging.17-19 In the search for new active catalysts, numerous endeavors addressing the adsorption, activation, and subsequent catalytic conversion of CO2 on different systems have been proposed. It has to be kept in mind that relatively high adsorption energies are needed for CO2 to stick on the catalyst surface. But, this process is difficult due to the high stability of CO2 molecule. Moreover, CO2 adsorption normally occurs without significant activation, a process known to require charge transfer from the substrate resulting in a concomitant bending.20 The possible capture/activation of CO2 has theoretically been studied using the first principles calculations on metals,21-23 metal oxides,24,25 graphene-based materials,26 sulfides,27 zeolites,28 and metal-organic frameworks,29 among others. As the highly stable nature of carbon dioxide, the development of materials for efficient fixation and chemical activation of CO2 is an obvious point of focus. Porous boron nitride (p-BN) is a dielectric material similar to porous carbon with a wide band gap of around 4 eV.30,31 The material has unique physical and chemical properties, including high surface area per unit mass, superior total pore volume, elevated ultramicro-porosity, and numerous structural defects.32-35 Importantly, if compared to C–C bond, the ionic B–N bond may induce an extra dipole moment, which would increase the adsorption energy for gas and ionic molecules. These features render p-BN materials promising candidates for applications in various fields, especially those related to adsorptions of gaseous uptake, pollutants, and catalyst support under very harsh environments.36-42 However, pristine BN materials with large band gaps are almost inert towards closed-shell CO2. It is generally expected that very stable closed-shell molecules would only interact weakly with other materials. The introduction of defect sites43 and extra electrons at the negatively


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charged states44 to BN nanomaterials, improved the adsorption of CO2 molecules to become tightly bound. These interesting results made BN nanomaterials promising for potential applications as CO2 sorbents. Among various available BN materials, porous BN nanomaterials are the most potential candidates owing to their large surfaces although information about CO2 surface chemistry involving porous materials are rather limited and the mechanisms of interaction between the CO2 gas and BN materials are still unclear. Choi et al.43 reported that electron-deficient Lewis acidic B atom could have as tronger affinity for electron-rich O atoms in CO2. However, Sun et al.44 proposed that CO2 molecule is a Lewis acid which prefers to accept rather than donate electrons during reactions. Apparently, clarification of the interaction between CO2 molecule and BN would help in designing and developing better BN-based CO2 capture materials. In this paper, due to the excellent performances of p-BN in air and water purification, boron antisite (BN) defect in p-BN was investigated, along with its potential applications in CO2 capture and activation using the first-principles calculations. The calculations predicted that pBN with BN defects behaved as excellent absorbents for the capture and activation of CO2 gas with remarkably high adsorption energies (Eads) and appropriate thermodynamics properties. A particular focus was paid to how CO2 molecule reacted with adsorbent materials. 2. MODELS and METHODS Periodic DFT calculations were performed within the Generalized Gradient Approximation (GGA) using the DMol3 module in Materials Studio.45,46 To account for exchange-correlation effects, Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional model was used,47 either alone or in addition to D3 dispersion correction developed by Grimme (PBE+D).48 A double numerical plus polarization (DNP) basis set with a global orbital cut off of 4.6 Å was


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adopted.45 For core treatment, the most accurate all-electron relativistic method was employed,49 which considers that all core electrons have explicitly and introduced relativistic effects into the core. The self-consistent field (SCF) procedure was used with a convergence threshold of 10−6 au on the energy and electron density. The direct inversion of the iterative subspace technique developed by Pulay was employed, with a subspace size 6 to speed up SCF convergence on all systems.50 For structure relaxation and energy calculation, 9×9×1 k-meshes with Monkhorst– Pack scheme in Brillouin zone were set up to make sure that the total energy converged to around 1 meV. For calculations related to electronic densities of states (DOS), 21×21×1 kmeshes connecting specific points of Brillouin zone were adopted. A 3×3×2 dimension supercell of h-BN was employed to construct potential porous BN by introducing vacancies as previously reported,30, 31 with a 15 Å vacuum between sheets to prevent interactions in periodic images. The geometry optimizations were performed using a convergence threshold of 0.001 au/Å on gradients, 0.005 Å on displacements, and 10−6 au on energy. After full optimization, the h-BN with vacancies underwent a large reconstruction. As shown in Fig. S1, the initial six-ring structure was nearly destroyed to result in a new structure containing a giant central porous ring of 12 atoms. The lattice parameter of the porous BN structure was calculated as 6.83 Å. 3. RESULTS AND DISCUSSION 3.1 BN defect in porous BN


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Figure 1. (a) The geometric configurations and (b) phonon dispersions of p-BN with a BN defect. A single boron antisite (BN) defect was created by substituting one N atom with B in the p-BN supercell, forming a boron atom site at the original nitrogen site surrounded by three boron atoms. To ensure that the point defect was properly isolated from each other and preventing defect-defect interactions, a larger supercell with the dimension of 13.65 Å containing 96 atoms was used (Figure1a). The geometries were subsequently optimized with all relaxed atom coordinates and optimized geometry for BN defective structures (Figure1a). The calculated phonon dispersions for p-BN with a BN defect along the high-symmetry directions in the first Brillouin zone are shown in Figure 1b. No imaginary frequency was observed in the phonon dispersions of the optimized structure, implying that the structure was thermodynamically stable. The three B–BN bonds of the optimized structure were calculated as 1.665, 1.707 and 1.641 Å, respectively. These values were greater than the corresponding distances of 1.460, 1.502 and 1.405 Å of B–N bonds, measured for a perfect p-BN cell. The structural relaxation led to BN defect protruding outward from the p-BN surface by about 0.370 Å, as depicted in Figure 1a.The protruding geometry would facilitate the CO2 adsorption by reducing steric hindrance from the p-BN surface. 3.2 Adsorption of CO2 on porous BN


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Figure 2. Optimized geometry of CO2 physisorbed on p-BN(a), and the configurations I (b) and II (c) of adsorbed CO2 on top of BN defect. The distances were given in Å. First, the calculations of adsorbed CO2 on the pristine p-BN and p-BN sheets with a BN defect were performed, and the minimum energy configurations are shown in Figure 2. The adsorption energy (Eads) was calculated following Eq. (1).

Eads = E( BN ) + E(CO2 ) − E(CO2 _ BN )

(1)

where E(CO2_BN) and E(BN) represent the total energy of the cell with and without adsorbed CO2, and E(CO2) is the total energy of an isolated CO2 molecule. By definition, Eads> 0 corresponds to a favorable or exothermic adsorption of CO2 on p-BN. In order to further investigate the role of dispersive van der Waals forces, the study was completed at the PBE-D level. A full summary of the adsorption energies is gathered in Figure3.


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Figure 3. (A) Adsorption energy (Eads) of CO2 physisorbed on p_BN. Adsorption energy (Eads) of CO2 adsorbed on top of the BN defect for configurations I (B) and II (C). (D) Adsorption energy (Eads) of N2 adsorbed on p_BN with a BN defect. The configuration of CO2 on perfect p-BN sheet suggested physisorption, where the distance between the C atom of CO2 and B atom of p-BN was estimated to 3.588 Å (Figure 2a). The linear CO2 molecule was parallel to the p-BN cell, and the adsorbed CO2 molecule showed little structural changes when compared to a free CO2 molecule, with O−C−O angle of an absorbed CO2 of 179.9°. These results indicated the weak adsorption of CO2 on the pristine p-BN, with an adsorption energy of only 0.01 eV (0.96 kJ/mol, PBE-D). The latter was much smaller than the 4.46 kcal/mol (18.64 kJ/mol, PBE-D) reported for CO2 on the h-BN sheet.44 It should be mentioned that when compared to PBE Eads values, the PBE-D predicted more stable minima by 0.03 eV (2.89 kJ/mol). Thus, the main adsorption driving force could safely be attributed to the presence of van der Waals forces between the CO2 molecule and the BN sorbent. The CO2 adsorption on the pristine p-BN containingdifferent sized unit cells was also examined, and the results suggested that CO2 can only form weak interactions with perfect p-BN structures.


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For CO2 adsorption onto a BN defect of p-BN, two distinct stable adsorption configurations (I and II) were identified, as shown respectively in Figure 2b and 2c. In non-activated configuration I, the linear CO2 molecule was slantingly attached to BN using one O end located almost on top of the defect site. This case was similar to the physisorbed CO2 on Mg-MOF-7429 and B-rich BNNTs.43 The BN−O−C angle was estimated to 114.9°, and the BN−O distance was 1.800 Å, which was much shorter than the van der Waals distance of 3.520 Å. This indicated that configuration I was not controlled by typical dispersion forces, but rather enhanced by some types of chemical interactions. Due to these chemical interactions, the adsorbed CO2 molecule underwent small but noticeable structural changes. The lengths of two C−O bonds of CO2 in configuration I were estimated to 1.188 Å and 1.167 Å. The C−O bond close to the BN defect was elongated by 0.012 Å, but the other was shortened by 0.009 Å when compared to free CO2 (1.176 Å). The adsorption energy of configuration I was calculated as 0.32 eV (30.88 kJ/mol, PBE-D), which was much larger than that for CO2 physisorbed on perfect p-BN, but comparable to reported CO2 physisorption energy on B-rich BNNT.43 In configuration II, the adsorbed CO2 species were bent to a O−C−O angle of 139.4°, strongly proving the CO2 activation by charge transfer from the underlying BN defect. A C−O bond was significantly elongated to 1.302 Å on top of the BN defect, accompanying the double-bond breaking. The other C−O bond was also elongated to 1.205 Å. The BN−C and BN−O distances were recorded as 1.674 Å and 1.611 Å, respectively. This formed a C−BN−O triangle, which was similar to a 3-center-2-electron (3c-2e) bonding configuration.51 The triangle was contrasted with a 2+2 cycloaddition configuration of four-membered rings for chemisorbed CO2 on (8, 0) SiC nanotubes.52 In fact, many CO2 surface chemistry studies demonstrated the key role of these bent structures in further reactions.53-55


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The calculated adsorption energy notably increased to 2.09 eV (201.66 kJ/mol, PBE-D), indicating that the CO2 molecule was strongly chemisorbed on p-BN with a BN defect. The obtained Eads values were comparable to those reported for Ca-coated boron sheets,56 but many times larger than those published previously for MOFs,29 SiC nanotubes,52 carbon nanotubes,57 and B-rich BNNTs.43 Compared to PBE Eads values, the PBE-D predicted more stable minima, where Eads of chemisorbed configurations were estimated in this study to 1.15 eV (110.96 kJ/mol, PBE-D) at the PBE level. This proposed that the main adsorption driving force came from nondispersive interactions exerted between the surface and adsorbent of the chemisorption configurations. To demonstrate the superior selectivity of p-BN-containing a BN defect towards CO2 adsorption, the adsorption energies of N2 on BN defect were calculated and compared with respect to that of CO2, as N2 is the main component of air. Under the considered conditions as shown in Figure S3, the results indicated that the adsorption of N2 on BN defects was more physical than chemical in nature. The N···B distances for N2 adsorbed on BN defect (Figure S3a) in p-BN were estimated to 3.507 Å. On the other hand, the calculated adsorption energies of N2 molecules on BN defect was only 0.02 eV (1.93 kJ/mol, PBE-D), and the interactions were similar to that of CO2 molecules with the pristine p-BN. The energy difference between Eads(CO2) and Eads(N2) was estimated to 2.07 eV. This significant difference in energies suggested that the capture of CO2 by BN defect in p-BN was highly preferred over the N2 gas molecules. Thus, the defective p-BN could serve as relevant adsorbents for the separation between the two gases.

3.3 Electronic structure of CO2 adsorbed porous BN


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Figure 4: Total density of states (TDOS) for nonbonding (a), configuration I (b), and configuration II (c). Projected density of states (PDOS) for nonbonding (d), configuration I (e), and configuration II (f). The position of the Fermi level (EF) was marked in dashed line. To gain a better understanding of the microscopic origins of the enhanced CO2 adsorption on BN defect of p-BN, the electronic structure was estimated by calculating total and partial densities of states (DOS) for nonbonding, and adsorption configurations I and II (Figure 4a-f). The total DOS (TDOS) obtained for boron-rich p-BN clearly showed its large-gap semiconductor properties with defect-related gap states (Figure4a). The partial DOS (PDOS) plot presented in Figure 4d confirmed that BN defect induced one occupied level, one half-occupied level in the vicinity of the Fermi level and one unoccupied level, behaving as a triple acceptor of electrons. Figure 5a and 5b show that the half-occupied defect states had ppσ orbitals character and unoccupied defect state displayed ppπ or pz orbitals character. These orbitals were respectively named as σ(BN) and pz(BN). The BN molecule was well known for its polarity with an electron


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deficiency B site and an electron-rich N site. However, the large band gap doomed its chemistry inertness, meaning that perfect BN materials were not likely to both accept and donate electrons. For BN defective structures, the defective levels formed in the vicinity of the Fermi level rendered p-BN to accept and/or donate electrons. Moreover, the defect states formed around the Fermi level resulted in apparent electronic conductivity in the defective p-BN.

Figure 5. (a) Occupied defect states σ(BN) and (b) unoccupied pz-like double-acceptor state pz(BN) in p-BN with a BN defect. (c) The unoccupied π* like the CO2 state of CO2-physisorbed configuration. For the nonbonding configuration with the optimized geometry shown in Figure S3, the CO2 was 8.907 Å away from p-BN. Also, the highest occupied state (HOMO) of CO2 molecule were distributed at 4.1 eV below the Fermi level (EF) while the unoccupied pz(BN) was at 1.0 eV above EF (Figure 4d). As the CO2 was adsorbed in configuration I, no orbital mixing between CO2 and adsorbent appeared in the range of -5.0-0.0eV (Figure 4d). This indicated that interactions between CO2 and BN defects were consistently based on physical adsorptions. The occupied CO2 states were all down-shifted by 1.6 eV, and the unoccupied pz(BN) was up-shifted


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by 0.9 eV. This level repulsion corresponded to the so-called electron-donation mechanism because CO2 gave its electron to the triple-acceptor BN state. For CO2 adsorbed in configuration II, interestingly, the occupied states of CO2 (Figure 4f) were remarkably up-shifted. Newly occupied peaks of CO2 appeared in the range of -2.0-0.0 eV. The orbital mixing between the occupied states of CO2 molecules and orbitals of B atoms was observed at around the Fermi level, indicating the chemisorption of CO2 on the BN defect. Accordingly, the PDOS plot also revealed the overall increase in boron states at the valence band. This was attributed to level repulsion between one occupied σ(BN) states with the unoccupied CO2 state. This level repulsion corresponded to electron back-donation, namely, electron transfer from BN defect to CO2. The CO2 adsorbed in configuration I showed a LUMO comprising of an unoccupied π*-like CO2 state (Figure 5c), which could strongly couple with one σ(BN) state in the configuration II formation process. The electron back donation process led to the formation of BN–C bond, as well as elongation of BN–B and C–O bonds from1.717 Å to 1.819 Å in the nonbonding configuration and 1.176 Å to 1.302 Å of the free CO2, respectively.


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Figure 6. The total charge density of CO2 adsorbed on BN defect for (a) configuration I and (b) configuration II. Charge-density difference plot of CO2 adsorbed on BN defect for (c) configuration I and (d) configuration II. To gain a further understanding of the bonding between CO2 and BN defect, the electronic density along with the deformation density and Hirshfeld charge analysis58 were calculated, and the results are summarized in Figure6 and table 1.The deformation density could be obtained by subtracting the density of isolated atoms from the total electronic density (Figure 6c and d). The results revealed that Mulliken charges had few short comings, such as the elevated sensitivity to the basis set. Also, the Hirshfeld charge analysis was found more stable with respect to the basis set but seemed to generally underestimate the atomic charges. Thus, stable Hirshfeld chargewas selected to analyze the charge transfer between CO2 molecule and adsorbent. From the contours of total densities shown in Figure 6a and b, it can be seen that both CO2 adsorption configurations I and II depicted certain amounts of charge transfer between CO2 and BN defect, concomitant with the formation of BN–O and BN–C bonds. In configuration I, the deformation density showed charge depletion between BN and O related to charge-density enrichment in BN defect (Figure 6c). This revealed that small amounts of charge were transferred from CO2 to BN defect. As electrons were donated from CO2 to BN defect, boron atoms in the BN nanomaterials became less positively charged. Indeed, Hirshfeld charge analysis confirmed that 0.258 electrons were donated from the CO2 molecule to defect site in configuration I. In configuration II, the total charge density showed the formation of BN–C and BN–O bonds (Figure 6b). From the contours of deformation density depicted in Figure 6d, the charges located in BN defect were obviously smaller than that of configuration I. This was accompanied by an obvious increase in charge enrichment between B and C atoms, owing to the charge feedback


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from BN to CO2. The Hirshfeld charges analysis indicated that the charge attributed to CO2 was 0.214 for configuration II, with the formation of surface-bound anionic CO2δ- species. Thus, upon the chemical adsorption, the C–O bonds of CO2 obviously weakened. Table 1. Hirshfeld atomic charge of CO2 adsorption on BN defect inconfiguration I and II, as shown in b and c of Figure 2. Atom label

B1

B2

B3

B4

C

O1

O2

configurationI

-0.109 0.126

0.103

0.102

0.352

-0.015 -0.079

configurationII

-0.073 0.196

0.164

0.142

0.153

-0.142 -0.225

Charge/e

3.4 Thermodynamic properties of CO2 chemisorbed porous BN

Figure 7. Variation of the thermodynamic properties with temperatures (K), as when an isolated CO2 molecule is chemisorbed on p-BN with a BN defect for configuration II.


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To study the entropic and temperature effects on CO2 chemisorption in configuration II on pBN with BN defect, the variations of thermodynamic properties, such as changes in Gibbs free energy (∆G, kcal/mol), enthalpy (∆H, kcal/mol) and entropy (∆S, cal/(mol K)) as a function of temperature (K) were estimated. For finite temperature, the enthalpy (H) is computed by adding the enthalpy (H) correction to the total electronic energy at 0 K, and the correction is given by Eq. (2)59: H (T ) = Evib (T ) + E rot (T ) + Etrans (T ) + RT

(2)

where R is the ideal gas constant and Evib(T), Erot(T) and Etrans(T) stand for the vibrational, rotational and translational contributions, respectively. These correction components can be obtained by performing a vibrational analysis or Hessian evaluation expressed by Eqs. (3)-(6): Evib (T ) =

hν exp(− hν i / kT ) R1 R hν i + ∑ i ∑ k2 i k i [1 − exp(− hν i / kT ]

E rot (linear ) = RT E rot (nonlinear ) = Etrans =

3 RT 2

(3)

(4) 3 RT 2

(5)

(6)

where k is the Boltzmann's constant, h is Planck's constant, and νi are the individual vibrational frequencies. The entropy (S) of a perfect crystal at 0 K exactly equals zero and the entropy correction for finite temperature is given by Eqs. (7)-(11): S (T ) = S vib (T ) + S rot (T ) + S trans (T ) + RT S vib = R ∑ i

(7)

hν i / kT exp(− hν i / kT ) − R ∑ ln[1 − exp(− hν i / kT )] 1 − exp(− hν i / kT ) i


(8)

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 8π 2 IkT  S rot (linear ) = Rln  +R 2  σh  S rot (nonlinear ) =

S trans =

(9)

3 R  π 8π 2 cI A 8π 2 cI B 8π 2 cI C  kT   3 ln    + R h h h  hc   2 2  σ

5 3 R ln T + R ln w − R ln p − 2.31482 2 2

(10)

(11)

where w is the molecular weight, Ix(x=A, B and C) is the moment of inertia following the axis x, and σ is the symmetry number. The change in the enthalpy (∆H) and the entropy (∆S) for finite temperature can then be computed by the Eqs. (12) and (13): ∆H (T ) = H prod (T ) − H react (T ) ∆S (T ) = S prod (T ) − S react (T )

(12) (13)

Hprod and Hreact are the enthalpies for the product and reactant, respectively and Sprod and Sreact are the entropies for the product and reactant respectively. The changes in Gibbs energy (∆G) for finite temperature are calculated by the Gibbs–Helmholtz formula:

∆G (T ) = ∆H (T ) − T∆S (T )

(14)

The results of the calculated ∆H, ∆S and ∆G are gathered in Figure 7. It can be observed that ∆S decreased as temperature rose from 25 to 250 K, and ∆S was nearly constant above 250 K. On the other hand, the ∆H value was nearly constant from 25 to 1000 K, and the negative sign of ∆H suggested that adsorption of CO2 on p-BN with a BN defect was an exothermic process. The joint action of ∆S and ∆H resulted in a linear increase of ∆G as temperature increased. Furthermore, ∆G was negative at temperatures from 25 to 350 K, indicating that the adsorption of CO2 on p-BN with a BN defect to form chemisorbed configuration was a spontaneous process


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at temperatures below 350 K. This was a clear advantage of boron-rich p-BN over previously proposed adsorbents, namely the capture of CO2 at ambient conditions.

3.5 Stability of BN defect in porous BN In order to unveil the growth conditions of BN defect in p-BN and analyze the energetic stability of the structures, the defect formation energies (Eform) were estimated. The general procedure was based on fabrication of p-BN by creating defects through adjusting the growth conditions. This was mainly reflected in variations of the chemical potentials of B or N, such as µB and µN, through the formation energy expressed in Eq. (15).60, 61

E form = Etot − nB µ B − nN µ N

(15)

where Etot is the calculated total energy of boron nitride structure with a BN defect, and nB and nN correspond to the number of B and N atoms, respectively. µB and µN are the chemical potentials of B and N sources, respectively. The relationship between the chemical potentials of B and N species in fabrication of p-BN was determined by the thermodynamic constraint shown in Eq. (16).

µ N + µB = µ BN

(16)

where µBN is the chemical potential of a B−N pair or the unit cell of honeycomb structural p-BN. Two limiting growth conditions were thus possible under boron-rich (µB = µB,0 and µN = µBN-µB,0) and nitrogen-rich (µN = µN,0and µB = µBN-µN,0) conditions, where µB,0 and µN,0 represent the reference systems of B and N, respectively.


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The formation energies of BN defect under various chemical environments were estimated using Eq.(15), and the results are summarized in Table 2.

Table 2. The defect formation energies (Eform in eV) of BN antisite under various chemical environments. N-rich condition

B-rich condition

N2

NH3

B12

B2H6

B2H4

BH3

PBE

7.58

5.57

4.27

4.46

1.59

2.25

PBE+D

9.03

7.02

5.93

5.51

2.66

3.31

Table 2 clearly showed that the growth of BN defect was strongly associated with the chemical environment. It can be noticed that the growth of BN defect was much more probable under boron rich conditions if compared to the nitrogen-rich limit. The formation energies of BN defects also varied depending on the type of the boron source. When rhombohedral boron (RB12) was used as boron source, the BN formation energy was calculated as 4.27 eV. This value compared well with previously reported theoretical values for BNNT.43 When B2H6 gas was used, the resulting formation energy was similar (4.46 eV). However, when high-energy boron sources like B2H4 or BH3 gases were utilized, the BN formation energy decreased to 1.59 and 2.25 eV, respectively. Therefore, these findings illustrate that selecting high-energy boron sources as reactants and adopting extreme growth conditions far from the thermodynamic equilibrium were two effective means to reduce Eform, and thus facilitate the presence of BN defect.

4. CONCLUSIONS


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The CO2 capture ability of B rich p-BN materials was systematically evaluated using first principle calculations based-DFT. The study demonstrated that p-BN with BN defect was an excellent sorbent for CO2 capture at the ambient environment, resulting in large adsorption energy of 2.09 eV (201.66 kJ/mol, PBE-D) and appropriate thermodynamic properties. On basis of the density of states and in an effort to demonstrate the role of BN defect charge-density distribution, the deformation density, Hirshfeld charge population analysis, electronic structure and bonding characteristics of CO2 adsorbed on BN defect were estimated and analyzed. For practical applications, the growth conditions and energetic stability of BN defect in p-BN were examined based on the calculation of defect formation energy. Overall, these findings did not only suggest new and promising materials for the ambient capture of CO2 gas but also gave insight into the different types of interactions exerted between CO2 and adsorbents.

AUTHOR INFORMATION Corresponding Author *Email address: [email protected], [email protected], [email protected]. Tel/fax: +86-22-60204805.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21603052, 21103224, 51372066, and 51401074), the Science and Technology Innovation Fund for Outstanding Youth at the Hebei University of Technology (GrantNo. 2012007 and 2013006), the Program for Chang jiang Scholars and Innovative Research Team at the Hebei University of


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Technology (PCSIRT, Grant No. IRT13060), as well as the Natural Science Foundation of Hebei Province (No.E2014202131 and E2016202122) and Tianjin City (15JCYBJC47100) of China.

Supporting Information Available: The geometries of original structure, optimized structure and 2×2×1 supercell of porous BN (p-BN), the optimized geometry of nonbonding configuration for CO2 on p-BN with a BN defect, the geometric configurations of the N2 adsorption on p-BN with a BN defect. This material is available free of charge via the Internet at http://pubs.acs.org.

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Ambient Carbon Dioxide Capture Using Boron-Rich Porous Boron

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