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Unveiling Adsorption Mechanisms of Elemental Mercury on Defective Boron Nitride Monolayer: A Computational Study Xiaoping Gao, Yanan Zhou, Yujia Tan, Zhiwen Cheng, Qingli Tang, Jinping Jia, and Zhemin Shen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00062 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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Unveiling Adsorption Mechanisms of Elemental Mercury on Defective Boron Nitride Monolayer: A Computational Study Xiaoping Gao,† Yanan Zhou,‡ Yujia Tan,† Zhiwen Cheng,† Qingli Tang, † Jinping Jia,† Zhemin Shen, *,† †School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ‡School of Chemical Engineering, Sichuan University, Chengdu 610065, China

ABSTRACT: The control of mercury in flue gas is challenging, especially that of elemental mercury (Hg0). Recently, many researchers have focused on various mercury removal technologies. Here by performing density functional theory (DFT) calculations, we systematically studied the adsorption of Hg0 on several experimentally available hexagonal boron nitride (h-BN) nanosheets with defect-free, nitrogen vacancy (VN), boron vacancy (VB), and both nitride and boron vacancy (VN+B) as well as their structures and electronic properties. Our calculation results show that the presence of VN, VB, and VN+B vacancies enhances the adsorption energies of Hg0 by 9, 45, and 214 kJ/mol, respectively. Moreover, a more negative potential at the VB and VN+B vacancy sites results in the h-BN-VB and h-BN-VN+B surfaces more reactive than those of h-BN and h-BN-VN. The partial density of states (PDOS) analysis unveils

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that Hg atom interacts firmly with surface B or/and N atoms through the orbital hybridization. The tendency of equilibrium constant implies that adsorption of Hg0 on h-BN-VN+B surface is beneficial at low temperature. Our computational studies reveal that defective h-BN nanosheets with VB and VN+B have great potential to serve as novel sorbents for the efficient removal of mercury in flue gas.

Keywords: h-BN; defective; elemental mercury; adsorption mechanism; density functional theory 1. INTRODUCTION The emission of anthropogenic mercury (Hg) has attracted global attention in recent years.1, 2 The Minamata Convention on Mercury, aiming at protecting public health and ecosystem from the release and transport of mercury, entered into force on 16 August 2017.3 Thus, it is of great urgency to control mercury emissions, which mainly derives from coal-fired power plants.4 In coal combustion derived flue gas, mercury species mainly exist in the forms of elemental mercury (Hg0), oxidized mercury (Hg2+), and particle-bound (Hgp).5 However, Hg0 is the major mercury species and the most difficult to remove because of its high vapor pressure and low solubility.6 In order to satisfy the requirements of global mercury regulations, a variety of mercury removal technologies have been applied for controlling mercury releases from coal combustion. Among these technologies, the traditional activated carbon injection is regarded as an effective way to remove mercury species and has been commercialized.4 However, due to the low operating temperature, high operating cost, and limited mercury capture capacity of carbon materials, thus, some potential alternatives were studied. Metal surfaces with defects were found


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good materials for Hg0 adsorption.7 Wilcox et al. focused on experimental and density functional theory studies of Au(1 1 1).8, 9 Moreover, Pd and Ag also showed strong affinity for mercury species.10-13 However, the costs of these metals as sorbents are still very high. Hence, many reports also displayed that the low-cost transition metal oxides (such as Co3O4,14 ZnO,15 MnO2,16-20 Fe2O3,21 CuO,22 CoMnO3,23 and CaO24, 25) and spinel ferrite26 are suitable materials for the removal of Hg0. To date, novel sorbent injection is still considered to be a potential and cost-effective removal of mercury technology. A novel promising sorbent, hexagonal boron nitride (h-BN), known as two dimensional nanosheet, has attracted much attention in recent decades because of its motivating properties. The h-BN nanosheet has many inherent characteristics such as high chemical stability and superior oxidation resistance,27-29 especially, the much high thermal stability (up to 1000 K).30 Numerous publications show that h-BN is used as suitable adsorbent systems.31-35 Interestingly, some point vacancies (such as B-vacancy, N-vacancy, and (N+B)-vacancy) can be deliberately formed by electron beam irradiation36 or solvent exfoliation37 during the growth of h-BN nanosheet. Particularly, introducing these point vacancies enables h-BN nanosheet higher chemical activity,38, 39 which is beneficial for the adsorption of Hg0. However, to the best of our knowledge, few theoretical studies have been reported on defective h-BN nanosheet as Hg0 adsorption material, which is important to comprehend the adsorption mechanism of Hg0 on defective h-BN nanosheet. In this work, we perform a detailed DFT investigation of adsorption mechanisms of elemental mercury on defective boron nitride monolayer. The key question relates to what surface characteristics of defective h-BN are important to Hg0 adsorption. First, we outline our computational methods. Then, we display results on the structural properties of the four


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substrates and the adsorption systems, as well as adsorption energies of stoichiometric surfaces. Finally, the electronic properties of the adsorption systems at surfaces are analyzed with reference to Hg0 adsorption. 2. METHODS AND MODELS All of the spin-polarized DFT calculations were carried out with the DMol3 package in Materials Studio.40 The Perdew, Burke, and Ernzerhof (PBE)41 exchange-correlation functional on the basis of generalized gradient approximation (GGA)42 was employed. The TkatchenkoScheffler (TS) scheme was introduced for the accurate treatment of van der Waals interactions.43 The density functional semicore pseudopotential (DSPP) was used for the relativistic effects of Hg atom.44 The basis set for other elements were treated with double numerical plus polarization (DNP).45 The convergence criterions in total energy, maximum force, and maximum displacement were set at 1 × 10-5 Hartree, 2 × 10-3 Hartree/Å, and 5 × 10-3 Å, respectively. For the sake of high quality results, the real-space global orbital cutoff radius was chosen as high as 5.2 Å. To model pristine and defective h-BN monolayer, we first built a 5 × 5 periodic supercell in a vacuum space of 16 Å in the z-direction. Then, the vacancies were created through removing a boron or nitride atom from a pristine h-BN nanosheet, which was named as h-BN-VB or h-BNVN defect, respectively. Both a pair of boron and nitride atoms were removed to create vacancy to provide an anchoring site for element mercury, which was labeled as h-BN-VN+B defect. All the systems analyzed are neutral according to the Mulliken atomic changes analysis (Supporting information). The k-points sampling of the Brillioun zone was done using a 5 × 5 × 1 mesh for geometry optimizations and a 12 × 12 × 1 mesh for electronic properties calculations. The charge transfer was calculated through Hirshfeld charge analysis.46 The energy barrier of Hg


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transformation minimum energy path (MEP) was obtained by the linear synchronous transit/quadratic synchronous transit (LST/QST) method.47, 48 The adsorption energies (Eads ) of Hg0 on the four substrates were calculated by the following equation: Eads = EHg0 + substrate - EHg0 - Esubstrate

(1)

where EHg0 + substrate represents the total energy of the Hg0/substrate system, while EHg0 and Esubstrate are the total energy of the isolated Hg0 and the defective h-BN nanosheets at its equilibrium geometry, respectively. Adsorption energy is the energy released to form a stable system from two or more individual systems. A more negative Eads value corresponds to a more stable Hg0/substrate system. Besides, in order to study the exothermicity of the adsorption processes and to determine favorability of the spontaneous reaction as a function of temperature, the equilibrium constant (Keq)40 was calculated on the basis of the thermodynamic data from the frequency calculation. The general relationships were used for statistical thermodynamic partition functions (translational, rotational, and vibrational partition functions).49 We neglected the contributions from electronic motion because of the electronic ground state of the adsorption systems. The Keq is obtained according to the equation: ln (Keq ) = -

∆G RT

(2)

where ∆G is the change of Gibbs free energy of adsorption, R is the ideal gas constant, and T is the temperature. During the adsorption process, ∆G is given by Equation (3): ∆G ≈ ∆Eads + ∆E0 + T(∆Svib + ∆Strans,rot ) - kT ln (

P P0

)

(3)

where ∆Eads is the change of adsorption energy, ∆E0 represents the change of zero-point energy, ∆Svib and ∆Strans,rot are the changes of the vibrational and translational, rotational entropy during adsorption, respectively. k is Boltzmann’s constant, and the pressure terms are cancelled out in


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this constant pressure adsorption system. The ∆G and Keq for Hg0 adsorption are obtained form (1) to (3) equations in the 250–1000 K temperature range, which covers most experimental relevance for real adsorption systems. 3. RESULTS AND DISCUSSION 3.1. Structural properties of defective h-BN sheets with defect-free, VN, VB, and VN+B vacancies. The optimized lattice parameters of boron nitride are a = b = 2.504 Å and c = 6.661 Å, which are in good agreement with the experimental values of a = b = 2.524 ± 0.020 Å and c = 6.684 ± 0.020 Å.50 Moreover, Figure 1 shows the various optimized structures of defective h-BN sheets. For the optimized h-BN sheets with defect-free (Figure 1a), the calculated bond length of B-N is 1.446 Å, which is very close to the theoretical and experimental results.39, 51 Thus, the accuracy is reasonably satisfactory for the purpose of our work. By removing one B or/and N atom from h-BN nanosheet, the geometrical parameters around the formed vacancy could be changed. The average bond length of B-N around the vacancy site varies from 1.45 Å in pristine h-BN to 1.41 Å, 1.46 Å and 1.47 Å in h-BN-VB, h-BN-VN, and hBN-VN+B, respectively, which matches well with previous reports.39, 52 What’s more, the B or N atoms that nearest to the vacancy are connected by dotted lines which form a triangle. For the defected h-BN sheets with VN (h-BN-VN, Figure 1b), all distances between the exhibited B-B atoms are 2.28 Å, which is in line with the reported work.53 While for the B vacancy (h-BN-VB, Figure 1c), the three N atoms around the vacancy move away from each other resulting in a triangle with the larger length (2.61 Å) than that of the pristine one. This may be due to the reason that the inherent electronegativity of N atom could compensate the lost bonding by moving to other B atoms.39 In the case of the defected h-BN sheets with VN+B (h-BN-VN+B, Figure 1d), the length of B-B around vacancy is 1.96 Å, and the distance between N atoms


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around vacancy is 2.00 Å, which are less than that of pristine lattice structure. Moreover, the distance between B and N atoms nearest to the vacancy is 3.07 Å. This can be assigned to the repulsion by the localized electrons around N atoms and the attraction between B-B atoms.

Figure 1. Optimized structures of defective h-BN sheets with (a) defect-free, (b) VN, (c) VB, (d) VN+B vacancies. (B: pink; N: blue; Bond lengths are in angstrom.) The calculated electrostatic potential surfaces for different h-BN models are showed in Figure 2. The presence of VN, VB, and VN+B vacancies in the h-BN surface change the surfaces electrostatic potential in such a way that the potentials at the vacancy sites are more negative than at the other sites of the defective h-BN surfaces. Notably, compared with the pristine h-BN and h-BN-VN, more negative potential at the VB and VN+B vacancy sites are observed, which are beneficial in activating the outer-shell electrons of defective h-BN surfaces,54 thus, making the hBN-VB and h-BN-VN+B surfaces more reactive.


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Figure 2. Electrostatic potential of defective h-BN sheets with (a) defect-free, (b) VN, (c) VB, (d) VN+B. Red and blue colors represent positive and negative electrostatic potentials, respectively. The isosurface value is 0.05 au. 3.2. Adsorption of Hg0 on defective h-BN sheets with defect-free, VN, VB, and VN+B vacancies. . In consideration of several possible adsorption sites (hollow/top/bridge sites) on these h-BN sheets, a single Hg0 atom was initially placed on different adsorption sites. For brevity, the more energetically favorable structures are depicted in Figure 3, and the corresponding geometric properties and adsorption energies are listed in Table 1. As displayed in Table 1, the adsorption energies of all the systems are negative values, and vary from -23.8 to 237.8 kJ/mol, which suggest that the adsorption of Hg0 on these defective h-BN sheets are exothermic processes. Specifically, for the pristine h-BN sheet, the Hg0 atom adsorption on the B top site (Figure 3a) yields the lowest adsorption energy of -23.8 kJ/mol, and the corresponding location (Hg-N) is at 3.54 Å above the pristine h-BN sheet. Meanwhile, about 0.052e charges are transferred from Hg0 to the h-BN sheet. Thus, the adsorption of Hg0 on the pristine h-BN sheet can be assigned to


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physisorption. In addition, the similar adsorption is observed for the VN defects system. The obtained Hg0 adsorption energy on VN defects is -32.6 kJ/mol, and the distance between Hg atom and B atom is approximately 2.99 Å, as shown in configuration in Figure 3b. However, the Hg0 binds as a ‘substitutional’ atom with the adsorption energy of -68.3 kJ/mol, and occupies a “center” configuration at VB defects (Figure 3c), which is slightly outward from the h-BN sheet plane by 1.21 Å, due to the longer newly formed Hg-N bond (2.15 Å) in comparison with a B-N bond (1.45 Å). Additionally, about 0.512e charges of Hg0 are transferred to the h-BN-VB sheet. When the Hg0 atom is adsorbed at the VN+B vacancy, the Hg0 atom holds on a “cross” site and shows out of the sheet plane by 0.306 Å (Figure 3d). The corresponding calculated adsorption energy is -237.8 kJ/mol. From the kinetic, we calculated the diffusion barrier of the adsorbed Hg0 atom from the VN+B vacancy site to a neighboring hollow site. The result shows that there exists a high barrier (2.50 eV), as showed in Figure S1, thus this diffusion process could not possibly to occur. The strong interaction between the Hg0 atom and the defective h-BN-VN+B sheet can be attributed to the structural distortion of substrates,55 especially at the vacancy site. Therefore, it can be concluded that the adsorption types of Hg0 on the h-BNVB and h-BN-VN+B vacancies belong to chemisorption. The interactions of Hg0 adsorption on the h-BN-VB and h-BN-VN+B vacancies are stronger than that on pristine h-BN sheets and h-BN-VN.


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Figure 3. The optimized adsorption configurations of Hg0 on defective h-BN sheets with (a) defect-free, (b) VN, (c) VB, (d) VN+B vacancies. Table 1. The adsorption energies, geometric parameters, and Hirshfeld atomic charges for Hg0 adsorption on various substrates Eads (kJ/mol)

dB-Hg (Å)

dN-Hg (Å)

QHg (e)

h-BN

-23.8

3.54

-

0.052

h-BN-VN

-32.6

2.99

-

0.029

h-BN-VB

-68.3

-

2.15

0.512

h-BN-VN+B

-237.8

2.09

2.08

0.470


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3.3. Electronic properties of Hg0 binding on the defective h-BN sheets with defect-free, VN, VB, and VN+B vacancies. In an attempt to gain insights into the adsorption mechanism of Hg0 on these substrates, the electronic properties of each stable configuration was investigated. First, we calculated the electrostatic potential isosurfaces of Hg0 adsorbed h-BN, h-BN-VN, hBN-VB, and h-BN-VN+B systems, and the results are depicted in Figure 4. As shown in Figure 2 ，the negative potential around the VB and VN+B vacancy sites represent the more reactive activity. Thus, from the Figure 4, it is expected that the Hg0 is in the order of h-BN-VN+B > hBN-VB > h-BN-VN > h-BN for interaction with the surfaces. Furthermore, the positive potential of Hg0 on these substrates is also observed, which is well consistent with the charge transfer on Hg0 (from the Hg0 to the defective surfaces). According to this argument, a more negative potential at the VB or VN+B vacancy site makes the h-BN-VN+B and h-BN-VB surfaces more reactive than that of the h-BN-VN and h-BN surfaces to form interactions with Hg0. 38


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Figure 4. Electrostatic potential for Hg0 on defective h-BN sheets with (a) defect-free, (b) VN, (c) VB, (d) VN+B. Red and blue colors represent positive and negative electrostatic potentials, respectively. The isosurface value is 0.05 au. Then, to further gain a perspective on the number of states per interval of energy at each energy level, we investigated the partial density of states (PDOS) in detail. To make better comparison, the adsorption systems based on B or/and N sites of Hg0-h-BN, Hg0-h-BN-VN, Hg0h-BN-VB, and Hg0-h-BN-VN+B are illustrated in Figure 5, Figure 6, Figure 7, and Figure 8,


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respectively. For comparison, the PDOS of the individual Hg atom and the four substrates are also studied. For isolated Hg atom, as shown in Fig. 5a, s- and p-orbitals display single peaks at 12.07 and 6.95 eV, respectively. Additionally, Hg s-orbital centered at the Fermi level is also occupied. The peaks of d-orbital distributes at -3.05 and 15.22 eV. After adsorption of Hg0, all orbitals of Hg atom shift downward and lowered in energy because of the charge transfer from the Hg to the B or N atom of the substrates.

Figure 5. PDOS for surface system before and after adsorption of Hg0 on B sites of h-BN. PDOS of (a) Hg atom and (b) B atom before and after adsorption of Hg0. The Fermi level (Ef) is set to 0 eV. In terms of Hg atom in Figure 5a, the peaks of s- and d-orbital move to -0.76 and -3.82 eV after adsorption, respectively. Meanwhile, Hg s- and d-orbital locate at 12.07 and 15.22 eV disappears thoroughly after adsorption of Hg0, respectively. PDOS of both orbitals show small decrease trend. However, all orbitals of B atom display no significant changes except a peak of sorbital at 5.78 eV, compared with those before Hg0 adsorption (Figure 5b). From respect of PDOS overlap region, s- and d-orbitals of Hg atom at -0.76 and -3.82 eV are hybridized with sand p-orbitals of B between 0 and 5 eV, which substantially enhances the adsorption of Hg0-hBN system.


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Figure 6. PDOS for surface system before and after adsorption of Hg0 on vacancy sites of h-BNVN. PDOS of (a) Hg atom and (b) B atom before and after adsorption of Hg0. The Ef is set to 0 eV. In Figure 6a, compared with isolated Hg atom, the PDOS peaks of s- and d-orbital obviously shift to the energy of -3.13 and -6.34 eV after Hg0 adsorption, respectively. Moreover, the dorbital intensity of PDOS decreases by 2.73 electrons/eV. As for B atom in Figure 6b, the orbitals of B atom show a reverse tendency with that of Hg after adsorption. Peaks of PDOS exhibit increase trend, and the energies distribute in higher level. Specifically, the peak of B atom s- turns from -11.52 eV to -11.20 eV, and PDOS energies distribution of p-orbital move form -7.12 and -5.08 eV to -6.84 and -4.48 eV, respectively. This phenomenon demonstrates that B atom could accept certain amount of electrons leading to the increase of system energy. Considering the peaks’ overlapping of the orbital energy of related atoms, orbitals (s-, d-) of Hg atom and orbitals (s-, p-) of B atom are hybridized at -6.34 eV and -3.13 eV, which improves the interaction of Hg with B atom. This result is well in agreement with the results of adsorption energy and charge transfer of Hg.


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Figure 7. PDOS for surface system before and after adsorption of Hg0 on vacancy sites of hBN-VB. PDOS of (a) Hg atom and (b) N atom before and after adsorption of Hg0. The Ef is set to 0 eV. While in Figure 7a, for the adsorbed Hg atom on h-BN-VB, the PDOS peak of d-orbital shows the same shift as d-orbital of Hg atom in Figure 6a, except that a significant reduction about 13.88 electrons/eV of PDOS is obviously observed. Besides, the peak s-orbital of the adsorbed Hg0 turns to -4.24 eV with a value of 0.43 electrons/eV. For N atom (Figure 7b), no apparent changes are observed in orbitals before and after Hg adsorption. Similarly, in Figure 7 for Hg adsorption on the vacancy sites, the s- and d-orbitals of adsorbed Hg atom are strongly hybridized with p-orbital of N atom between 2.5 and 10 eV. The stronger orbital hybridization between s- and d-orbitals of Hg and p-orbital of N atom stabilizes the adsorption of Hg atom on the h-BN-VB surface, leading to the stronger interaction between Hg and h-BN-VB surface.


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Figure 8. PDOS for surface system before and after adsorption of Hg0 on vacancy sites of hBN-VN+B. PDOS of (a) Hg atom, (b) B atom, and (c) N atom before and after adsorption of Hg0. The Ef is set to 0 eV. In Figure 8a, for Hg0 adsorption on vacancy sites of h-BN-VN+B, the peaks of s-orbital show significant shift to -3.23 eV and peaks of d-orbital are located at -8.03 and -6.45 eV after Hg0 adsorption. Moreover, a small peak of p-orbital centered at -0.87 eV is discerned. In terms of B atom nearest to the vacancy of h-BN-VN+B after Hg0 adsorption (Figure 8b), the PDOS peaks of s-orbital mainly distribute at -0.97 and -8.83 eV with the PDOS slight increase and the PDOS peaks of p-orbital are at -3.99 and -0.97 eV. While the energy binds of N nearest to the vacancy of h-BN-VN+B are located at -7.95 and -3.34 eV for p-orbitals and -14.02 eV for s-orbitals (Figure 8c). Furthermore, s-orbital of Hg atom is hybridized with s- and p-orbitals of B atom and p-orbital of N atom at -3.23 eV. Meanwhile, the p-orbital of Hg atom at -0.87 eV has hybridization with s- and p-orbitals of B atom and p-orbital of N atom. Additionally, d-orbital of Hg atom also generates relative strong orbital hybridizations with s- and p-orbitals of B atom and p-orbital of N atom at -8.03 and -6.45 eV. Overlaps of s-, p-, and d-orbital of Hg and orbitals (s-, p-) of B atom and N atom between -10 and 0 eV illustrate that the multiple interactions between


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Hg atom and B atom, as well as between Hg atom and N atom are favorable for stabilization of Hg0 binding on the h-BN-VN+B surface. Therefore, our studies on electronic properties of these adsorbed complexes suggest that introducing defect makes significant change of the properties of h-BN surfaces and the design of h-BN surfaces with higher reactivity to Hg0, and hence the great potential for application in Hg0 removal technology. 3.4. Temperature effect on equilibrium constants of Hg0. The effect of temperature on the equilibrium constants for Hg0 adsorption on the h-BN-VN+B was also investigated, since the adsorption process can be significantly influenced by temperature. According to the thermodynamic data analysis obtained from the DFT, the computed ∆G and In(Keq) of Hg0 adsorption on the h-BN-VN+B in the temperature range of 250–1000 K are listed in Table S1. As shown in Table S1, the changes of the Gibbs free energy for Hg0 adsorption are negative, indicating that Hg0 adsorbed on h-BN-VN+B through spontaneous processes. Figure 9 depicts the relationship between the equilibrium constants for Hg0 adsorption and the temperature. It shows that Hg0 adsorption system has positive slope at all temperatures and the equilibrium constant decreases with increasing temperature. The same trend is found in the previous investigation of Hg0 adsorption on MnCl2(110),56 Co3O4(110),14 MnO2(001),17 and CaO(001)24 surfaces. The distinct slope during Hg0 adsorption process suggests that temperature has significant effect on the equilibrium constant for Hg0 adsorption. Specifically, the highest In(Keq) value reaches 102.2 at 250 K, and the minimum value is 26.5 at 1000 K. The In(Keq) reduces by 55.23% during the temperature increasing between 298 K and 675 K, which suggests that the adsorption efficiency of h-BN-VN+B sorbent may greatly decrease at temperature above 675 K. As for Hg0 adsorption on MnO2(001)17 and Co3O4(110)14 surfaces, the highest deduction rate is 91.71% and 57.73%,


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respectively. Comparing these three thermodynamic features, the same changing tendency for temperature influence on equilibrium constants of Hg0 is observed, and the adsorption of Hg0 is favorable at low temperature. While, h-BN-VN+B displays a higher equilibrium constant of Hg0 in the studied temperature range than that of Co3O4, suggesting that h-BN-VN+B is an effective material for Hg0 adsorption.

Figure 9. Equilibrium constants for adsorption of Hg0 on h-BN-VN+B as a function of temperature. 4. CONCLUSIONS The understanding of adsorption mechanisms of elemental mercury on defective hexagonal hBN monolayer (h-BN, h-BN-VN, h-BN-VB, and h-BN-VN+B) have been performed on the basis of DFT. Hg0 can be steadily adsorbed on defective hexagonal h-BN monolayer with adsorption energies of -23.8, -32.6, -68.3, and -237.8 kJ/mol for Hg0-h-BN, Hg0-h-BN-VN, Hg0-h-BN-VB, and Hg0-h-BN-VN+B, respectively. The differences of the adsorption energies imply that the presence of VN, VB, and VN+B vacancies in the h-BN monolayer are favorable for the trends of Hg0 to adsorb on the defective h-BN surfaces. Electrostatic potential analysis reveals that more


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negative potential at the VB and VN+B vacancy sites result in more reactivity of h-BN-VB and hBN-VN+B surfaces than that of h-BN and h-BN-VN. The PDOS shows that the orbitals of Hg atom are highly hybridized with the orbitals of B or/and N atoms on the surface. Hg atom can form multiple interactions with the B atom and N atom of the surface, which induces strong adsorption of Hg0 on the h-BN-VN+B surface. Besides, the tendency of equilibrium constant illustrates adsorption of Hg0 on h-BN-VN+B surface is beneficial at low temperature. We conclude that defective h-BN nanosheets with VB and VN+B have great potential to serve as efficient mercury capture materials for the removal of mercury in flue gas. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available on the file “supporting information”. The EMP for the diffusion of the adsorbed Hg0 on h-BN-VN+B from the defect binding site to a neighboring hollow site; the results of equilibrium constants for Hg adsorptions on h-BN sheets with VN+B within 250-1000 K (ln (Keq) and ∆G vs. 1000/T); Mulliken atomic charges of the systems; vibrational calculations of the more energetically favorable structures. AUTHOR INFORMATION Corresponding Author *Corresponding authors. Email: E-mail address: [email protected]. (Z. Shen). Tel: +86-2154745262, Fax: +86-21-54742863 ORCID Zhemin Shen: 0000-0002-8043-5228 Jinping Jia: 0000-0003-2409-338X


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Funding Sources The project was supported by the National Science Foundation of China (Project No. 21537002), and the program for New century Excellent Talents in Shanghai Jiao Tong University. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank Wenchao Ji for useful discussion and help.

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