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FeN3-embedded carbon as an efficient sorbent for mercury adsorption: A theoretical study
Chemical Engineering Journal 374 (2019) 1337–1343
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FeN3-embedded carbon as an efficient sorbent for mercury adsorption: A theoretical study Xiaoping Gaoa, Yanan Zhoub, Shiqiang Liua, Yujia Tana, Zhiwen Chenga, Zhemin Shena,c,
T
⁎
a
School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China School of Chemical Engineering, Sichuan University, Chengdu 610065, China c Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China b
H I GH L IG H T S
G R A P H I C A B S T R A C T
-C as an excellent sorbent for Hg • FeN adsorption is proposed by DFT study. The incorporated FeN in C endows • the carbon with magnetic and metallic
0
3
3
properties.
can be efficiently adsorbed • Hg FeN -C via the chemisorption me0
on
3
• •
chanism. The unoccupied Fe d-orbital accepts electron and hybridizes with Hg s-, dorbitals. Hg0 adsorption on FeN3-C is beneficial at low temperature.
A R T I C LE I N FO
A B S T R A C T
Keywords: Fe single atoms sorbent N-doped carbon Elemental mercury Adsorption mechanism Density functional theory
The control of mercury (Hg0) is one of the most important environmental problems but a long-standing challenge in environment. Current research efforts for mercury removal mainly focus on the development of economical, effective, and recyclable sorbents, while the carbon-based metal sorbents with magnetism have been rarely studied. Here, we proposed the FeN3-embedded carbon (FeN3-C) as the efficient sorbent for Hg0 adsorption from first-principles computations. Our calculation results show that the incorporated FeN3 in carbon endows the carbon with magnetic and metallic properties and that the gas phase Hg0 can be efficiently adsorbed on FeN3-C through the chemisorption mechanism with the adsorption energy of −66.27 kJ/mol. Moreover, the unoccupied d-orbital of Fe in FeN3-C can hybridize with the Hg s- and d-orbitals and accept electron density from the Hg sand d-orbitals, which is the “acceptance” process between Hg0 and FeN3-C. Additionally, according to the equilibrium constant, Hg0 adsorption on FeN3-C is beneficial at low temperature. Our theoretical study not only reveals that the nonprecious FeN3-embedded carbon possesses great promise to serve as an efficient sorbent for Hg0 adsorption, but also provides a coordination-engineered strategy for searching more efficient carbon-based metal sorbents.
1. Introduction The emission of anthropogenic mercury into the atmosphere has ⁎
caused global concerns in recent years [1,2]. To limit the mercury release from coal-fired power plants [3], the Minamata Convention on Mercury come into operation on August 16, 2017 [4]. Due to the high
Corresponding author at: School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail address: [email protected] (Z. Shen).
https://doi.org/10.1016/j.cej.2019.04.189 Received 6 March 2019; Received in revised form 26 April 2019; Accepted 27 April 2019 Available online 29 April 2019 1385-8947/ © 2019 Published by Elsevier B.V.
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vapor pressure and low solubility of elemental mercury (Hg0) as well as its closed electron structure of 5d106s2, it is very difficult to remove from the combustion flue gas [1,5]. In this regard, enormous amounts of research efforts to reduce mercury emission to meet the requirements of global mercury regulations therefore have focused on the development of economical, effective, and recyclable sorbents over the last decade [6]. A large number of promising sorbents/catalysts with excellent Hg0 removal performance have been developed, such as activated carbon [3,7], pure metal [8–11], metal oxides [12–17], metal sulfides [18,19], h-BN [20], g-C3N4 [21–23], and Mexenes [24]. However, most of sorbents cannot be recovered from fly ash and regenerated for reuse. Although the activated carbon is the most extensively studied including various modified carbons [25,26] and has been commercialized [3], its high operating cost and the inability to be regenerated of activated carbon inhibit its widespread utilization [24]. Therefore, some potential carbon-based sorbents decorating with inexpensive and recyclable metals should be further developed to conquer the above weak points. To the best of our knowledge, the magnetic materials, which can be recovered from fly ash for reuse by magnetic separation, have been regarded as promising, cost-effective, and recycling mercury removal sorbents [13,27,28]. Among these magnetic materials, transition-metal oxides containing nonprecious Fe have received great attention due to their prominent performance [28]. Therefore, there may be new forms of carbon-based metal sorbents incorporating the magnetism. In recent years, since the single-atomic-site (SAS) catalysts possess the maximum atom utilization efficiency and single atoms as the active sites, the SAS catalysts have become a new research frontier in catalysis [29–35]. One of the important advantages of SAS catalysts is the ordered and well-defined single atomic site, which can serve as perfect model systems for providing atomic-level insights into the catalytic reaction mechanism [36–38]. Currently, the typical class of atomic transition-metal nitrogen moieties embedded in carbon (TM–N–Cs) have been intensively investigated in the search for promising candidates for catalyzing the wide ranges of electrochemical processes [39–42]. For instance, Li et al., proposed nonprecious FeN3-embedded graphene as the catalyst for the conversion of di-nitrogen to ammonia from first-principles, and found that the spin moments of the Fe-graphene and FeN3-graphene are 0 and 3.16 μB, respectively [43]. Moreover, the FeN3-graphene has already been experimental available and studied in oxygen reduction reaction [44–46]. However, this newly emerged FeN3-graphene with magnetism is rarely applied in the mercury adsorption. Therefore, the ongoing application of the TM-N-Cs in mercury capture is of paramount importance. In this work, we report, by using density functional theory (DFT) calculations, the newly emerged nonprecious FeN3-embedded carbon serving as an economical, effective, and recyclable sorbent for Hg0 adsorption. First, we check the structural stability and related electronic properties of the formed FeN3-C. Then, we identify the most favorable active site for Hg0 adsorption on the FeN3-embedded carbon from our theoretical calculated adsorption energies. Finally, we reveal the electronic and geometric structure of the FeN3 active center and establish that the unoccupied d-orbital of Fe accepts electron density from the Hg s- and d-orbitals, thus improving Hg0 adsorption. These insights revealed in our calculated adsorption systems will help to understand other complex adsorption or catalytic processes.
while the double numerical plus polarization (DNP) was used for other elements [52]. The convergence thresholds 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. The global orbital cutoff radius was set as 5.2 Å. To model the nonprecious FeN3-embedded carbon, we constructed a 5 × 5 periodic supercell in a large vacuum space between two neighbouring surfaces of 20 Å in the z-direction to avoid inter-layer interaction. A single carbon vacancy in the graphene formed by doping three-nitrogen-atom (N3-C) was then created for confining the Fe atom site (FeN3-C). The k-points sampling of the Brillioun zone was done using a 5 × 5 × 1 grid for structural optimizations and a 7 × 7 × 1 grid for the electronic structure calculations. All structures were fully relaxed during structural optimizations. The charge transfer was calculated by Hirshfeld charge analysis [53]. The energy barrier of Fe transformation minimum energy path (MEP) was obtained by using the LST/QST method [54,55]. For comparing the relative stability of FeN3-C and Fe-C systems, the corresponding formation energies (Ef) were calculated by the equation of Ef = Etotal + nμC − (Egr + xμN + EFe); in which the Etotal, Egr, and EFe are the total energies of FeN3-C/Fe-C, pure graphene, and the isolated Fe atom, respectively. The μC and μN represent the chemical potential of a single C atom in graphene and a N atom defined as half of the N2 molecule energy, respectively. The x (=0, 3) is the number of N atoms in the FeN3-C/Fe-C systems and n (=x + 1) is the number of C atoms replaced by the Fe and N dopants. The adsorption energies (Eads ) of Hg0 on the sorbents were computed using the following expression:
Eads = EHg0+ sorbent- EHg0- Esorbent
(1)
where EHg0+ sorbent , EHg0 , and Esubstrate are the total energies of the Hg0/ sorbent system, the isolated Hg0, and the sorbents at their equilibrium geometries, respectively. In order to investigate the exothermicity of the adsorption processes and to evaluate favorability of the spontaneous reaction as a function of temperature, the equilibrium constant (Keq) [47] was thus computed via vibrational frequency calculation. The general relationships were used for statistical thermodynamic partition functions (translational, rotational, and vibrational partition functions) [56]. The Keq is given by the equation:
ln(K eq) =
-
ΔG RT
(2)
where ΔG represents the change of Gibbs free energy of adsorption, R is the ideal gas constant, and T is the temperature. For the adsorption process, ΔG is defined as follows:
P ΔG ≈ ΔEads + ΔE0 + T (ΔSvib + ΔStrans,rot ) - kT ln ⎛ ⎞ ⎝ P0 ⎠ ⎜
⎟
(3)
where ΔEads , ΔE0 , ΔSvib , and ΔStrans,rot are the changes of adsorption energy, zero-point energy, the vibrational, and translational, rotational entropy during adsorption, respectively. k is the Boltzmann’s constant, and the pressure terms are cancelled out in this constant pressure adsorption system. The ΔG and Keq for Hg0 adsorption are obtained from (1) to (3) equations in the temperature range of 250–1000 K, which covers most experimental relevance for real adsorption systems.
2. Computational methodology
3. Results and discussion
The spin-polarized DFT calculations were carried out by using the DMol3 module [47]. The exchange-correlation energy was treated with Perdew-Burke-Ernzerhof (PBE) functional based on the generalized gradient approximation (GGA) [48,49]. To treat the van der Waals interactions accurately, the Tkatchenko-Scheffler scheme was adopted [50]. The density functional semi-core pseudopotential (DSPP) was used to describe the relativistic effects of the Hg and Fe atoms [51],
3.1. Structures and stability of FeN3- and Fe-embedded carbon We first optimized the structure to embed a Fe atom in the single carbon vacancy formed in carbon by three-nitrogen-atom doping. As shown in Fig. 1a and b, we find that three equivalent Fe–N bonds are formed with the bond length of 1.88 Å (Fig. 1b) and that the adsorbed Fe atom is outward from the basal plane of the FeN3 structure by about 1338
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Fig. 1. Optimized structures of FeN3-C: (a) top and (b) side views, Fe-C: (c) top and (d) side views. Various adsorption sites on the FeN3-C sheets (T1, T2, T3, and T4 refer to four top sites; C1 and C2 refer to two center sites) involved in Hg0 adsorption. The unit of bond length is Å. Fig. 2. Band structures of (a) FeN3-C, and (b) Fe-C sheets. The Fermi level is set as zero in red dashed line.
1.01 Å due to the larger size of the Fe atom compared with that of the C atom. We also computed the Fe-embedded carbon (Fig. 1c and d) as a comparison. The bond lengths of Fe-C are about 1.77 Å (Fig. 1c), which are larger than the C–C bond length of 1.42 Å (Fig. 1d). These increased bond lengths of Fe-C force the Fe atom to protrude from the carbon plane by 0.91 Å. The bulged Fe atoms exposed in apical position above the carbon layer may be advantageous sites for Hg0 adsorption. In order to study the stabilities of the resultant FeN3-C and Fe-C, which are crucial challenges in the practical application and the stable single-atomic-site sorbent, we further calculated the binding energies of the Fe atoms on the FeN3-C and Fe-C sheets. The binding energy (Eb) of a Fe atom is calculated as the energy difference among the Fe-N3C/C systems, the isolated Fe atom, and the N3C/C complexes. The results show that the binding energies of the Fe atoms on the FeN3-C and Fe-C sheets are −4.45 and −7.48 eV, respectively, higher than the cohesive energies of the atomic Fe (−4.12 eV) [43], demonstrating that the Fe atoms can be tightly anchored in the single carbon vacancy. In addition, it is found that the formation energies of Fe-C and FeN3-C decrease from 0.42 to −1.00 eV with introducing the N dopants. The smaller Ef of FeN3-C (−1.00 eV) in comparison with the Fe-C systems means that the formation of FeN3-C is more preferential [57]. Moreover, we also found that the diffusion of the Fe atom on the N3-doped carbon from the FeN3 site to its neighboring hollow site is an endothermic process by 3.39 eV and the calculated energy barrier is 3.09 eV (Fig. S1 in supporting information), which vigorously excludes the Fe atom clustering problem. These results indicate that the resultant FeN3-C and Fe-C are highly stable and can serve as stable sorbents for mercury adsorption.
The computed results show that the FeN3-C sorbent exhibits metallic property, which will favor the electron transfer for Hg0 adsorption, while the Fe-C sorbent is a semiconductor with a band gap of 0.39 eV. This smaller band gap of FeN3-C than that of Fe-C can be ascribed to the incorporation of the N atoms. Due to the fact that a small band gap indicates low kinetic stability and makes the electrons more easily excited from the valence band to the conduction band [59], the chemical reactivity of the FeN3-embedded carbon will be higher than that of the Fe-embedded carbon.
3.3. Hg0 adsorption on FeN3- and Fe-embedded carbon From the previous studies, the chemisorption of elemental mercury onto the surface of the sorbents is usually helpful for an efficient Hg0 adsorption process. For certain transition-metal-based sorbents, their rare ability to bind Hg0 can be attributed to their advantageous combination of the unoccupied d-orbitals, which possess appropriate energy and symmetry to synergically accept electron density from Hg0 (Fig. 3a) [60]. Owning to the existence of the closed electron structure (5d106s2) of Hg0, the transition-metal should have unoccupied d-orbitals to accept this closed electrons. Therefore, this “acceptance” of electrons may be the nature of the interplay between the transition-metal and Hg0. In order to verify the above discussion, we move on to the evaluation of the adsorption performance of FeN3-C and Fe-C for Hg0 adsorption. The Hg0 atom was initially placed on several possible adsorption sites, including T1, T2, T3, T4, C1, and C2 sites (Fig. 1a) on the FeN3-C and Fe-C sheets. In addition, the adsorption of these pristine graphene (C), graphene with one C atom vacancy (V-C), and N3-doped graphene with single C vacancy (N3-VC) were also taken into consideration for comparison. These optimized stable configurations are presented in Fig. 3b-d, and Fig. S2. Table 1 summarizes the corresponding adsorption energies and geometric information. Apparently, as shown in Table 1, the adsorption energies of Hg0 on these sorbents are negative values and vary from −23.85 to −66.28 kJ/mol, which indicates that the adsorption processes of Hg0 on these carbon-based sorbents are exothermic. As expected, for the FeN3-C sorbent, the Hg0 atom prefers to interact with the Fe atom rather than the other atoms, and can be stabilized at the T1 and C1 sites (Fig. 3b and c), especially at the more energetically favorable T1 site, as confirmed by its higher binding energies of −66.28 kJ/mol. Importantly, the framework structure of FeN3-C can be well maintained after the adsorption of Hg0. Specifically, the equilibrium distances for Hg0 at the T1 and C1 sites on FeN3-C are 2.64 and 2.73 Å, respectively. These distances are significantly shorter than the
3.2. Electronic properties of FeN3- and Fe-embedded carbon The magnetism of the sorbents is important for Hg0 adsorption because it is related to whether the sorbents can be easily recycled or not. Consequently, we calculated the spin moments of FeN3- and Fe-embedded carbon. Our calculations display that when the Fe atom is embedded in single carbon vacancy of N3-C, this FeN3-C system is magnetic, which is in line with the previous study [43]. However, for the Fe atom embedding directly in single vacancy of carbon, the system is not magnetic, which coincides well with the previous report [58]. The spin moments of the Fe-C and FeN3-C are 0 and 3.48 μB, respectively (Table S1). This increased spin moment of FeN3-C is mainly attributed to the spin-polarized ground state induced by replacing three C atoms with N atoms, which endows the system with an odd number of electrons and half-occupied states [43]. Therefore, the FeN3-C sorbent is of comparative interest for its magnetism. Since the band gap has been widely used to evaluate the difficulty of electron transfer between different surfaces [59], we further calculated the band structure of FeN3-C, along with Fe-C for comparison (Fig. 2). 1339
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Fig. 3. (a) Simplified schematic of Hg0 bonding to transition metal Fe. Top and side views of the optimized adsorption configurations of Hg0 on FeN3embedded carbon at the (b) T1 site and (c) C1 site, and (d) on Fe-embedded carbon at the T1 site. The unit of bond length is Å.
Hg-C or Hg-N bond is observed in these adsorption configurations due to the longer equilibrium distances (3.83, 3.62, and 3.51 Å) than the sum of the radii of the Hg and C/N atoms. This phenomenon renders few charge transfer (only about 0.034e, 0.012e, and 0.020e) from Hg0 to the C, V-C, and N3-VC sheets, respectively. Therefore, the adsorption of Hg0 on the C, V-C, and N3-VC sorbents can be assigned to physisorption. Furthermore, by comparing the Hg0 adsorption on FeN3-C and Fe-C to that on C, V-C, and N3-VC, we can find that the adsorption energies are dramatically improved and that the amount of charge transfer is also increased after incorporating the transition-metal Fe into the carbon, which can strongly support our initial design concept.
Table 1 The adsorption energies, geometric parameters, and Hirshfeld atomic charges for Hg0 adsorption on various sorbents. Sorbents
Eads (kJ/mol)
dX-Hg (Å)a
QHg (e)
FeN3-C (T1) FeN3-C (C1) Fe-C C V-C N3-VC
−66.27 −39.58 −62.44 −23.85 −34.41 −26.76
2.64 2.73 2.68 3.83 3.62 3.51
0.219 0.135 0.173 0.034 0.012 0.020
a
X denotes the Fe, C, or N atoms.
sum of the radii of the Fe and Hg atoms (1.56 and 1.71 Å, respectively) [61], indicating the formation of the Hg-Fe bonds, as displayed in Fig. 3b and c. While the Hg0 atom cannot be stably adsorbed on the C or N sites by weak physisorption and therefore moves to the C1 site. The formation of the Hg-Fe bonds is similar to the formation of Hg-Fe, AuHg, Ag-Hg, and Pd-Hg amalgam in the literatures [10,62–64]. Moreover, these formed Hg-Fe bonds are in favor of electrons transfer from Hg0 (0.219 e and 0.135 e in Table 1) to the Fe atom and finally to the other atoms in the FeN3-C sheets. While for the Fe-C sorbent, Hg0 adsorption on the T1 site (Fig. 3d) yields the most stable configuration with the adsorption energy of −62.44 kJ/mol, which belongs to chemisorption. The corresponding location of Hg0 is on the top of the Fe atom with a balanced distance of 2.68 Å, which also favors the formation of the Hg-Fe bond. Thus, the electrons on Hg0 (0.173 e in Table 1) can transfer to the Fe atom. Meanwhile, we determined the stable adsorption configurations for Hg0 on the C, V-C, and N3-VC sorbents (Fig. S2). The adsorption energy is only −23.85 kJ/mol for the Hg0-C configuration, which is in agreement with the previous study [65]. While the adsorption energies increase to the range from −26.76 to −34.41 kJ/mol (Table 1) for the Hg0-V-C and Hg0-N3-VC configurations, respectively. Additionally, no
3.4. Electronic structures of Hg0 adsorption on the FeN3-embedded carbon To gain a deeper insight into the adsorption mechanism of Hg0 on FeN3-C and Fe-C, the electronic structures of the most stable configurations were investigated. We first calculated the electron density difference of Hg0-Fe-C and Hg0-FeN3-C configurations to display the charge redistribution caused by the chemisorption, and the results are shown in Fig. 4. For the Hg0 adsorption on the Fe-C sorbent (Fig. 4a and 4b), the charge depletion on the Hg atom can be clearly discerned and the formed Hg-Fe bond has a large charge accumulation region, which give rise to the electrons transfer from the adsorbed Hg atom to the Fe-C sorbent. Moreover, this charge accumulation around the Hg-Fe bond can be assigned to the delocalized electrons of the occupied Hg d- and Fe d-orbitals [62]. However, the electrons transfer only occurs between the interface of the Hg atom and the Fe-C sorbent. The electrons of the Hg atom cannot transfer to the other atoms on the Fe-C sorbent due to the presence of the band gap. Whereas for the Hg0-FeN3-C configuration, the similar charge accumulation and depletion can be easily observed (Fig. 4c and d). Interestingly, what the differences are that there are the much more significant charge accumulation between the 1340
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Fig. 4. Top and side views of the electron density difference of (a, b) Hg0-Fe-C and (c, d) Hg0-FeN3-C configurations, where the isosurface value is set to be ± 0.01 electrons/Å3 and the blue and yellow isosurfaces mean the charge accumulation and depletion region, respectively.
thermodynamic data analysis, are listed in Table S2. The negative ΔG values for Hg0 adsorption illustrate that the process of Hg0 adsorption on FeN3-C is spontaneous (Table S2). Meanwhile, the relationship between the Keq for Hg0 adsorption and the temperature is shown in Fig. 6. Clearly, the positive slope at all temperatures for the Hg0 adsorption process can be seen and the Keq increases with the temperature decrease. Interestingly, this trend is consistent with previous studies of Hg0 adsorption on Co3O4(1 1 0) [14] and MnO2(0 0 1) [15] surfaces. This distinct slope indicates that temperature has an important influence on the Keq during the Hg0 adsorption process. Specifically, the highest and minimum In(Keq) values are 32.26 at 250 K and 9.98 at 1000 K, respectively. Furthermore, when the temperature increases from 298 to 675 K, the In(Keq) value decreases by 51.02%, suggesting that the adsorption efficiency of the FeN3-C sorbent may obviously decrease at temperatures above 675 K and that the Hg0 adsorption is more beneficial at low temperature. However, such a change tendency can also be found in Co3O4(1 1 0) and MnO2(0 0 1), where the even more deduction rates are 57.73% and 91.71%, respectively [14]. Therefore, the relatively smaller influence of temperature on Keq for Hg0 adsorption on FeN3-C compared with that of Co3O4(1 1 0) and MnO2(0 0 1) in the studied temperature range further suggests that the FeN3-C is an efficient sorbent for Hg0 adsorption.
interface of the Hg atom and the FeN3-C sorbent (Fig. 4d), and that there are the larger region of electrons transfer on the FeN3-C sheets than that on the Fe-C sheets (Fig. 4c). Additionally, this much more significant charge accumulation may be due to the occupied Hg 6s and 5d orbitals as well as the occupied and unoccupied Fe 3d orbitals. The large region of electrons transfer on the FeN3-C sheets is attributed to its metallic properties, which is in line with the above discussion. Next, we further studied the partial density of states (PDOS) of the Hg and Fe atoms in the Hg0-FeN3-C configuration in detail to obtain a perspective on the number of states per interval of energy and on charge transfer, as shown in Fig. 5. As a comparison, the PDOS of the Fe atom in the FeN3-C sorbent and the individual Hg atom are also investigated. As illustrated in Fig. 5a, the Hg s- and d-orbitals show single peaks at the Fermi level (0 eV) and −3.02 eV, respectively. Both the sand d-orbitals peaks of Hg are dramatically shifted to the lower energy level after Hg0 adsorption on the FeN3-C sorbent (Fig. 5c) due to the charge transfer from the Hg atom to the Fe atom in the FeN3-C sorbent. However, no obvious changes on the Fe d-orbitals before and after adsorption of Hg0 on FeN3-C (Fig. 5b and d), which may be ascribed to the obtained charge on the Fe atom finally transferring to the other atoms in the FeN3-C sorbent, as the evidence depicted in Fig. 4c. In addition, after adsorption of Hg0, the Hg s- and d-orbitals are hybridized with the Fe d-orbital at −4.27 and −7.08 eV (Fig. 5c and d), respectively. These phenomena further confirm our initial design concept that the unoccupied d-orbital of Fe atom accepts electron density from the sand d-orbitals of Hg0. Therefore, our investigations results of the electronic structures of the Hg0-FeN3-C configuration, including the electron density difference and the PDOS, demonstrate that the FeN3-embedded carbon possesses excellent electrical conductivity and the unoccupied d-orbital, revealing the great potential as an efficient sorbent for Hg0 adsorption.
4. Conclusions In summary, we have proposed a new economical, effective, and recyclable sorbent, FeN3-embedded carbon, for Hg0 removal by performing density functional theory calculations. This FeN3-embedded carbon can greatly improve the magnetic property of the carbon sorbent, which makes it recyclable for Hg0 adsorption. Moreover, the FeN3 center shows metallic property and is responsible for the superior Hg0 adsorption activity. Hg0 can be chemically adsorbed on FeN3-C with the adsorption energy of −66.27 kJ/mol. In addition, the charge transfer evidenced by the electron density difference and PDOS analysis confirms the “acceptance” process between Hg0 and FeN3-C, which plays a vital role in the adsorption of Hg0. The PDOS shows that the unoccupied d-orbital of Fe can accept electron density from the Hg s- and d-orbitals and that the Hg s- and d-orbitals are highly hybridized with the Fe dorbitals. Besides, the trend of equilibrium constant indicates that Hg0 adsorption on FeN3-C is kinetically favorable at low temperature.
3.5. Effect of temperature on equilibrium constant of Hg0 adsorption on FeN3-C Since the process of Hg0 adsorption on FeN3-C can be dramatically influenced by temperature, we finally evaluated the influence of temperature on the equilibrium constant (Keq) for this process in detail. The computed ΔG and In(Keq) values for Hg0 adsorption on FeN3-C in the temperature range of 250–1000 K, which are obtained from the 1341
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Fig. 5. Partial density of states of the Hg and Fe atoms in surface systems before (a, b) and after (c, d) adsorption of Hg0 on FeN3-embedded carbon are shown. The Fermi level (Ef) is set to 0 eV (dashed line in figures).
Acknowledgement This work was supported by the National Science Foundation of China (Project No. NSFC key project 21537002) and National water pollution control key project 2017ZX07202005-005. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.04.189. References [1] A.A. Presto, E.J. Granite, Survey of catalysts for oxidation of mercury in flue gas, Environ. Sci. Technol. 40 (2006) 5601–5609. [2] J. Wilcox, E. Rupp, S.C. Ying, D.-H. Lim, A.S. Negreira, A. Kirchofer, F. Feng, K. Lee, Mercury adsorption and oxidation in coal combustion and gasification processes, Int. J. Coal Geol. 90–91 (2012) 4–20. [3] B. Padak, J. Wilcox, Understanding mercury binding on activated carbon, Carbon 47 (2009) 2855–2864. [4] Minamata Convention on Mercury, in Programme, United Nations Environment Programme, http://www.mercuryconvention.org/, 2017. [5] W. Lee, G.-N. Bae, Removal of elemental mercury (Hg(0)) by nanosized V2O5/TiO2 catalysts, Environ. Sci. Technol. 43 (2009) 1522–1527. [6] L. Ling, M. Fan, B. Wang, R. Zhang, Application of computational chemistry in understanding the mechanisms of mercury removal technologies: a review, Energy Environ. Sci. 8 (2015) 3109–3133. [7] Z. Liu, Y. Zhang, B. Wang, H. Cheng, X. Cheng, Z. Huang, DFT study on Al-doped defective graphene towards adsorption of elemental mercury, Appl. Surf. Sci. 427 (2018) 547–553. [8] L. Geng, L. Han, W. Cen, J. Wang, L. Chang, D. Kong, G. Feng, A first-principles study of Hg adsorption on Pd(111) and Pd/γ-Al2O3(110) surfaces, Appl. Surf. Sci. 321 (2014) 30–37. [9] D.H. Lim, J. Wilcox, Heterogeneous mercury oxidation on Au(111) from first principles, Environ. Sci. Technol. 47 (2013) 8515–8522. [10] D.H. Lim, S. Aboud, J. Wilcox, Investigation of adsorption behavior of mercury on
Fig. 6. Equilibrium constant for Hg0 adsorption on FeN3-C as a function of temperature.
Consequently, our theoretical study not only suggests that the nonprecious FeN3-embedded carbon holds great promise to serve as an efficient sorbent for Hg0 adsorption, but also provides a coordinationengineered strategy for searching more efficient carbon-based metal sorbents. 1342
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
FeN3-embedded carbon as an efficient sorbent for mercury adsorption: A theoretical study Xiaoping Gaoa, Yanan Zhoub, Shiqiang Liua, Yujia Tana, Zhiwen Chenga, Zhemin Shena,c,
T
⁎
a
School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China School of Chemical Engineering, Sichuan University, Chengdu 610065, China c Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China b
H I GH L IG H T S
G R A P H I C A B S T R A C T
-C as an excellent sorbent for Hg • FeN adsorption is proposed by DFT study. The incorporated FeN in C endows • the carbon with magnetic and metallic
0
3
3
properties.
can be efficiently adsorbed • Hg FeN -C via the chemisorption me0
on
3
• •
chanism. The unoccupied Fe d-orbital accepts electron and hybridizes with Hg s-, dorbitals. Hg0 adsorption on FeN3-C is beneficial at low temperature.
A R T I C LE I N FO
A B S T R A C T
Keywords: Fe single atoms sorbent N-doped carbon Elemental mercury Adsorption mechanism Density functional theory
The control of mercury (Hg0) is one of the most important environmental problems but a long-standing challenge in environment. Current research efforts for mercury removal mainly focus on the development of economical, effective, and recyclable sorbents, while the carbon-based metal sorbents with magnetism have been rarely studied. Here, we proposed the FeN3-embedded carbon (FeN3-C) as the efficient sorbent for Hg0 adsorption from first-principles computations. Our calculation results show that the incorporated FeN3 in carbon endows the carbon with magnetic and metallic properties and that the gas phase Hg0 can be efficiently adsorbed on FeN3-C through the chemisorption mechanism with the adsorption energy of −66.27 kJ/mol. Moreover, the unoccupied d-orbital of Fe in FeN3-C can hybridize with the Hg s- and d-orbitals and accept electron density from the Hg sand d-orbitals, which is the “acceptance” process between Hg0 and FeN3-C. Additionally, according to the equilibrium constant, Hg0 adsorption on FeN3-C is beneficial at low temperature. Our theoretical study not only reveals that the nonprecious FeN3-embedded carbon possesses great promise to serve as an efficient sorbent for Hg0 adsorption, but also provides a coordination-engineered strategy for searching more efficient carbon-based metal sorbents.
1. Introduction The emission of anthropogenic mercury into the atmosphere has ⁎
caused global concerns in recent years [1,2]. To limit the mercury release from coal-fired power plants [3], the Minamata Convention on Mercury come into operation on August 16, 2017 [4]. Due to the high
Corresponding author at: School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail address: [email protected] (Z. Shen).
https://doi.org/10.1016/j.cej.2019.04.189 Received 6 March 2019; Received in revised form 26 April 2019; Accepted 27 April 2019 Available online 29 April 2019 1385-8947/ © 2019 Published by Elsevier B.V.
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vapor pressure and low solubility of elemental mercury (Hg0) as well as its closed electron structure of 5d106s2, it is very difficult to remove from the combustion flue gas [1,5]. In this regard, enormous amounts of research efforts to reduce mercury emission to meet the requirements of global mercury regulations therefore have focused on the development of economical, effective, and recyclable sorbents over the last decade [6]. A large number of promising sorbents/catalysts with excellent Hg0 removal performance have been developed, such as activated carbon [3,7], pure metal [8–11], metal oxides [12–17], metal sulfides [18,19], h-BN [20], g-C3N4 [21–23], and Mexenes [24]. However, most of sorbents cannot be recovered from fly ash and regenerated for reuse. Although the activated carbon is the most extensively studied including various modified carbons [25,26] and has been commercialized [3], its high operating cost and the inability to be regenerated of activated carbon inhibit its widespread utilization [24]. Therefore, some potential carbon-based sorbents decorating with inexpensive and recyclable metals should be further developed to conquer the above weak points. To the best of our knowledge, the magnetic materials, which can be recovered from fly ash for reuse by magnetic separation, have been regarded as promising, cost-effective, and recycling mercury removal sorbents [13,27,28]. Among these magnetic materials, transition-metal oxides containing nonprecious Fe have received great attention due to their prominent performance [28]. Therefore, there may be new forms of carbon-based metal sorbents incorporating the magnetism. In recent years, since the single-atomic-site (SAS) catalysts possess the maximum atom utilization efficiency and single atoms as the active sites, the SAS catalysts have become a new research frontier in catalysis [29–35]. One of the important advantages of SAS catalysts is the ordered and well-defined single atomic site, which can serve as perfect model systems for providing atomic-level insights into the catalytic reaction mechanism [36–38]. Currently, the typical class of atomic transition-metal nitrogen moieties embedded in carbon (TM–N–Cs) have been intensively investigated in the search for promising candidates for catalyzing the wide ranges of electrochemical processes [39–42]. For instance, Li et al., proposed nonprecious FeN3-embedded graphene as the catalyst for the conversion of di-nitrogen to ammonia from first-principles, and found that the spin moments of the Fe-graphene and FeN3-graphene are 0 and 3.16 μB, respectively [43]. Moreover, the FeN3-graphene has already been experimental available and studied in oxygen reduction reaction [44–46]. However, this newly emerged FeN3-graphene with magnetism is rarely applied in the mercury adsorption. Therefore, the ongoing application of the TM-N-Cs in mercury capture is of paramount importance. In this work, we report, by using density functional theory (DFT) calculations, the newly emerged nonprecious FeN3-embedded carbon serving as an economical, effective, and recyclable sorbent for Hg0 adsorption. First, we check the structural stability and related electronic properties of the formed FeN3-C. Then, we identify the most favorable active site for Hg0 adsorption on the FeN3-embedded carbon from our theoretical calculated adsorption energies. Finally, we reveal the electronic and geometric structure of the FeN3 active center and establish that the unoccupied d-orbital of Fe accepts electron density from the Hg s- and d-orbitals, thus improving Hg0 adsorption. These insights revealed in our calculated adsorption systems will help to understand other complex adsorption or catalytic processes.
while the double numerical plus polarization (DNP) was used for other elements [52]. The convergence thresholds 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. The global orbital cutoff radius was set as 5.2 Å. To model the nonprecious FeN3-embedded carbon, we constructed a 5 × 5 periodic supercell in a large vacuum space between two neighbouring surfaces of 20 Å in the z-direction to avoid inter-layer interaction. A single carbon vacancy in the graphene formed by doping three-nitrogen-atom (N3-C) was then created for confining the Fe atom site (FeN3-C). The k-points sampling of the Brillioun zone was done using a 5 × 5 × 1 grid for structural optimizations and a 7 × 7 × 1 grid for the electronic structure calculations. All structures were fully relaxed during structural optimizations. The charge transfer was calculated by Hirshfeld charge analysis [53]. The energy barrier of Fe transformation minimum energy path (MEP) was obtained by using the LST/QST method [54,55]. For comparing the relative stability of FeN3-C and Fe-C systems, the corresponding formation energies (Ef) were calculated by the equation of Ef = Etotal + nμC − (Egr + xμN + EFe); in which the Etotal, Egr, and EFe are the total energies of FeN3-C/Fe-C, pure graphene, and the isolated Fe atom, respectively. The μC and μN represent the chemical potential of a single C atom in graphene and a N atom defined as half of the N2 molecule energy, respectively. The x (=0, 3) is the number of N atoms in the FeN3-C/Fe-C systems and n (=x + 1) is the number of C atoms replaced by the Fe and N dopants. The adsorption energies (Eads ) of Hg0 on the sorbents were computed using the following expression:
Eads = EHg0+ sorbent- EHg0- Esorbent
(1)
where EHg0+ sorbent , EHg0 , and Esubstrate are the total energies of the Hg0/ sorbent system, the isolated Hg0, and the sorbents at their equilibrium geometries, respectively. In order to investigate the exothermicity of the adsorption processes and to evaluate favorability of the spontaneous reaction as a function of temperature, the equilibrium constant (Keq) [47] was thus computed via vibrational frequency calculation. The general relationships were used for statistical thermodynamic partition functions (translational, rotational, and vibrational partition functions) [56]. The Keq is given by the equation:
ln(K eq) =
-
ΔG RT
(2)
where ΔG represents the change of Gibbs free energy of adsorption, R is the ideal gas constant, and T is the temperature. For the adsorption process, ΔG is defined as follows:
P ΔG ≈ ΔEads + ΔE0 + T (ΔSvib + ΔStrans,rot ) - kT ln ⎛ ⎞ ⎝ P0 ⎠ ⎜
⎟
(3)
where ΔEads , ΔE0 , ΔSvib , and ΔStrans,rot are the changes of adsorption energy, zero-point energy, the vibrational, and translational, rotational entropy during adsorption, respectively. k is the Boltzmann’s constant, and the pressure terms are cancelled out in this constant pressure adsorption system. The ΔG and Keq for Hg0 adsorption are obtained from (1) to (3) equations in the temperature range of 250–1000 K, which covers most experimental relevance for real adsorption systems.
2. Computational methodology
3. Results and discussion
The spin-polarized DFT calculations were carried out by using the DMol3 module [47]. The exchange-correlation energy was treated with Perdew-Burke-Ernzerhof (PBE) functional based on the generalized gradient approximation (GGA) [48,49]. To treat the van der Waals interactions accurately, the Tkatchenko-Scheffler scheme was adopted [50]. The density functional semi-core pseudopotential (DSPP) was used to describe the relativistic effects of the Hg and Fe atoms [51],
3.1. Structures and stability of FeN3- and Fe-embedded carbon We first optimized the structure to embed a Fe atom in the single carbon vacancy formed in carbon by three-nitrogen-atom doping. As shown in Fig. 1a and b, we find that three equivalent Fe–N bonds are formed with the bond length of 1.88 Å (Fig. 1b) and that the adsorbed Fe atom is outward from the basal plane of the FeN3 structure by about 1338
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Fig. 1. Optimized structures of FeN3-C: (a) top and (b) side views, Fe-C: (c) top and (d) side views. Various adsorption sites on the FeN3-C sheets (T1, T2, T3, and T4 refer to four top sites; C1 and C2 refer to two center sites) involved in Hg0 adsorption. The unit of bond length is Å. Fig. 2. Band structures of (a) FeN3-C, and (b) Fe-C sheets. The Fermi level is set as zero in red dashed line.
1.01 Å due to the larger size of the Fe atom compared with that of the C atom. We also computed the Fe-embedded carbon (Fig. 1c and d) as a comparison. The bond lengths of Fe-C are about 1.77 Å (Fig. 1c), which are larger than the C–C bond length of 1.42 Å (Fig. 1d). These increased bond lengths of Fe-C force the Fe atom to protrude from the carbon plane by 0.91 Å. The bulged Fe atoms exposed in apical position above the carbon layer may be advantageous sites for Hg0 adsorption. In order to study the stabilities of the resultant FeN3-C and Fe-C, which are crucial challenges in the practical application and the stable single-atomic-site sorbent, we further calculated the binding energies of the Fe atoms on the FeN3-C and Fe-C sheets. The binding energy (Eb) of a Fe atom is calculated as the energy difference among the Fe-N3C/C systems, the isolated Fe atom, and the N3C/C complexes. The results show that the binding energies of the Fe atoms on the FeN3-C and Fe-C sheets are −4.45 and −7.48 eV, respectively, higher than the cohesive energies of the atomic Fe (−4.12 eV) [43], demonstrating that the Fe atoms can be tightly anchored in the single carbon vacancy. In addition, it is found that the formation energies of Fe-C and FeN3-C decrease from 0.42 to −1.00 eV with introducing the N dopants. The smaller Ef of FeN3-C (−1.00 eV) in comparison with the Fe-C systems means that the formation of FeN3-C is more preferential [57]. Moreover, we also found that the diffusion of the Fe atom on the N3-doped carbon from the FeN3 site to its neighboring hollow site is an endothermic process by 3.39 eV and the calculated energy barrier is 3.09 eV (Fig. S1 in supporting information), which vigorously excludes the Fe atom clustering problem. These results indicate that the resultant FeN3-C and Fe-C are highly stable and can serve as stable sorbents for mercury adsorption.
The computed results show that the FeN3-C sorbent exhibits metallic property, which will favor the electron transfer for Hg0 adsorption, while the Fe-C sorbent is a semiconductor with a band gap of 0.39 eV. This smaller band gap of FeN3-C than that of Fe-C can be ascribed to the incorporation of the N atoms. Due to the fact that a small band gap indicates low kinetic stability and makes the electrons more easily excited from the valence band to the conduction band [59], the chemical reactivity of the FeN3-embedded carbon will be higher than that of the Fe-embedded carbon.
3.3. Hg0 adsorption on FeN3- and Fe-embedded carbon From the previous studies, the chemisorption of elemental mercury onto the surface of the sorbents is usually helpful for an efficient Hg0 adsorption process. For certain transition-metal-based sorbents, their rare ability to bind Hg0 can be attributed to their advantageous combination of the unoccupied d-orbitals, which possess appropriate energy and symmetry to synergically accept electron density from Hg0 (Fig. 3a) [60]. Owning to the existence of the closed electron structure (5d106s2) of Hg0, the transition-metal should have unoccupied d-orbitals to accept this closed electrons. Therefore, this “acceptance” of electrons may be the nature of the interplay between the transition-metal and Hg0. In order to verify the above discussion, we move on to the evaluation of the adsorption performance of FeN3-C and Fe-C for Hg0 adsorption. The Hg0 atom was initially placed on several possible adsorption sites, including T1, T2, T3, T4, C1, and C2 sites (Fig. 1a) on the FeN3-C and Fe-C sheets. In addition, the adsorption of these pristine graphene (C), graphene with one C atom vacancy (V-C), and N3-doped graphene with single C vacancy (N3-VC) were also taken into consideration for comparison. These optimized stable configurations are presented in Fig. 3b-d, and Fig. S2. Table 1 summarizes the corresponding adsorption energies and geometric information. Apparently, as shown in Table 1, the adsorption energies of Hg0 on these sorbents are negative values and vary from −23.85 to −66.28 kJ/mol, which indicates that the adsorption processes of Hg0 on these carbon-based sorbents are exothermic. As expected, for the FeN3-C sorbent, the Hg0 atom prefers to interact with the Fe atom rather than the other atoms, and can be stabilized at the T1 and C1 sites (Fig. 3b and c), especially at the more energetically favorable T1 site, as confirmed by its higher binding energies of −66.28 kJ/mol. Importantly, the framework structure of FeN3-C can be well maintained after the adsorption of Hg0. Specifically, the equilibrium distances for Hg0 at the T1 and C1 sites on FeN3-C are 2.64 and 2.73 Å, respectively. These distances are significantly shorter than the
3.2. Electronic properties of FeN3- and Fe-embedded carbon The magnetism of the sorbents is important for Hg0 adsorption because it is related to whether the sorbents can be easily recycled or not. Consequently, we calculated the spin moments of FeN3- and Fe-embedded carbon. Our calculations display that when the Fe atom is embedded in single carbon vacancy of N3-C, this FeN3-C system is magnetic, which is in line with the previous study [43]. However, for the Fe atom embedding directly in single vacancy of carbon, the system is not magnetic, which coincides well with the previous report [58]. The spin moments of the Fe-C and FeN3-C are 0 and 3.48 μB, respectively (Table S1). This increased spin moment of FeN3-C is mainly attributed to the spin-polarized ground state induced by replacing three C atoms with N atoms, which endows the system with an odd number of electrons and half-occupied states [43]. Therefore, the FeN3-C sorbent is of comparative interest for its magnetism. Since the band gap has been widely used to evaluate the difficulty of electron transfer between different surfaces [59], we further calculated the band structure of FeN3-C, along with Fe-C for comparison (Fig. 2). 1339
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Fig. 3. (a) Simplified schematic of Hg0 bonding to transition metal Fe. Top and side views of the optimized adsorption configurations of Hg0 on FeN3embedded carbon at the (b) T1 site and (c) C1 site, and (d) on Fe-embedded carbon at the T1 site. The unit of bond length is Å.
Hg-C or Hg-N bond is observed in these adsorption configurations due to the longer equilibrium distances (3.83, 3.62, and 3.51 Å) than the sum of the radii of the Hg and C/N atoms. This phenomenon renders few charge transfer (only about 0.034e, 0.012e, and 0.020e) from Hg0 to the C, V-C, and N3-VC sheets, respectively. Therefore, the adsorption of Hg0 on the C, V-C, and N3-VC sorbents can be assigned to physisorption. Furthermore, by comparing the Hg0 adsorption on FeN3-C and Fe-C to that on C, V-C, and N3-VC, we can find that the adsorption energies are dramatically improved and that the amount of charge transfer is also increased after incorporating the transition-metal Fe into the carbon, which can strongly support our initial design concept.
Table 1 The adsorption energies, geometric parameters, and Hirshfeld atomic charges for Hg0 adsorption on various sorbents. Sorbents
Eads (kJ/mol)
dX-Hg (Å)a
QHg (e)
FeN3-C (T1) FeN3-C (C1) Fe-C C V-C N3-VC
−66.27 −39.58 −62.44 −23.85 −34.41 −26.76
2.64 2.73 2.68 3.83 3.62 3.51
0.219 0.135 0.173 0.034 0.012 0.020
a
X denotes the Fe, C, or N atoms.
sum of the radii of the Fe and Hg atoms (1.56 and 1.71 Å, respectively) [61], indicating the formation of the Hg-Fe bonds, as displayed in Fig. 3b and c. While the Hg0 atom cannot be stably adsorbed on the C or N sites by weak physisorption and therefore moves to the C1 site. The formation of the Hg-Fe bonds is similar to the formation of Hg-Fe, AuHg, Ag-Hg, and Pd-Hg amalgam in the literatures [10,62–64]. Moreover, these formed Hg-Fe bonds are in favor of electrons transfer from Hg0 (0.219 e and 0.135 e in Table 1) to the Fe atom and finally to the other atoms in the FeN3-C sheets. While for the Fe-C sorbent, Hg0 adsorption on the T1 site (Fig. 3d) yields the most stable configuration with the adsorption energy of −62.44 kJ/mol, which belongs to chemisorption. The corresponding location of Hg0 is on the top of the Fe atom with a balanced distance of 2.68 Å, which also favors the formation of the Hg-Fe bond. Thus, the electrons on Hg0 (0.173 e in Table 1) can transfer to the Fe atom. Meanwhile, we determined the stable adsorption configurations for Hg0 on the C, V-C, and N3-VC sorbents (Fig. S2). The adsorption energy is only −23.85 kJ/mol for the Hg0-C configuration, which is in agreement with the previous study [65]. While the adsorption energies increase to the range from −26.76 to −34.41 kJ/mol (Table 1) for the Hg0-V-C and Hg0-N3-VC configurations, respectively. Additionally, no
3.4. Electronic structures of Hg0 adsorption on the FeN3-embedded carbon To gain a deeper insight into the adsorption mechanism of Hg0 on FeN3-C and Fe-C, the electronic structures of the most stable configurations were investigated. We first calculated the electron density difference of Hg0-Fe-C and Hg0-FeN3-C configurations to display the charge redistribution caused by the chemisorption, and the results are shown in Fig. 4. For the Hg0 adsorption on the Fe-C sorbent (Fig. 4a and 4b), the charge depletion on the Hg atom can be clearly discerned and the formed Hg-Fe bond has a large charge accumulation region, which give rise to the electrons transfer from the adsorbed Hg atom to the Fe-C sorbent. Moreover, this charge accumulation around the Hg-Fe bond can be assigned to the delocalized electrons of the occupied Hg d- and Fe d-orbitals [62]. However, the electrons transfer only occurs between the interface of the Hg atom and the Fe-C sorbent. The electrons of the Hg atom cannot transfer to the other atoms on the Fe-C sorbent due to the presence of the band gap. Whereas for the Hg0-FeN3-C configuration, the similar charge accumulation and depletion can be easily observed (Fig. 4c and d). Interestingly, what the differences are that there are the much more significant charge accumulation between the 1340
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Fig. 4. Top and side views of the electron density difference of (a, b) Hg0-Fe-C and (c, d) Hg0-FeN3-C configurations, where the isosurface value is set to be ± 0.01 electrons/Å3 and the blue and yellow isosurfaces mean the charge accumulation and depletion region, respectively.
thermodynamic data analysis, are listed in Table S2. The negative ΔG values for Hg0 adsorption illustrate that the process of Hg0 adsorption on FeN3-C is spontaneous (Table S2). Meanwhile, the relationship between the Keq for Hg0 adsorption and the temperature is shown in Fig. 6. Clearly, the positive slope at all temperatures for the Hg0 adsorption process can be seen and the Keq increases with the temperature decrease. Interestingly, this trend is consistent with previous studies of Hg0 adsorption on Co3O4(1 1 0) [14] and MnO2(0 0 1) [15] surfaces. This distinct slope indicates that temperature has an important influence on the Keq during the Hg0 adsorption process. Specifically, the highest and minimum In(Keq) values are 32.26 at 250 K and 9.98 at 1000 K, respectively. Furthermore, when the temperature increases from 298 to 675 K, the In(Keq) value decreases by 51.02%, suggesting that the adsorption efficiency of the FeN3-C sorbent may obviously decrease at temperatures above 675 K and that the Hg0 adsorption is more beneficial at low temperature. However, such a change tendency can also be found in Co3O4(1 1 0) and MnO2(0 0 1), where the even more deduction rates are 57.73% and 91.71%, respectively [14]. Therefore, the relatively smaller influence of temperature on Keq for Hg0 adsorption on FeN3-C compared with that of Co3O4(1 1 0) and MnO2(0 0 1) in the studied temperature range further suggests that the FeN3-C is an efficient sorbent for Hg0 adsorption.
interface of the Hg atom and the FeN3-C sorbent (Fig. 4d), and that there are the larger region of electrons transfer on the FeN3-C sheets than that on the Fe-C sheets (Fig. 4c). Additionally, this much more significant charge accumulation may be due to the occupied Hg 6s and 5d orbitals as well as the occupied and unoccupied Fe 3d orbitals. The large region of electrons transfer on the FeN3-C sheets is attributed to its metallic properties, which is in line with the above discussion. Next, we further studied the partial density of states (PDOS) of the Hg and Fe atoms in the Hg0-FeN3-C configuration in detail to obtain a perspective on the number of states per interval of energy and on charge transfer, as shown in Fig. 5. As a comparison, the PDOS of the Fe atom in the FeN3-C sorbent and the individual Hg atom are also investigated. As illustrated in Fig. 5a, the Hg s- and d-orbitals show single peaks at the Fermi level (0 eV) and −3.02 eV, respectively. Both the sand d-orbitals peaks of Hg are dramatically shifted to the lower energy level after Hg0 adsorption on the FeN3-C sorbent (Fig. 5c) due to the charge transfer from the Hg atom to the Fe atom in the FeN3-C sorbent. However, no obvious changes on the Fe d-orbitals before and after adsorption of Hg0 on FeN3-C (Fig. 5b and d), which may be ascribed to the obtained charge on the Fe atom finally transferring to the other atoms in the FeN3-C sorbent, as the evidence depicted in Fig. 4c. In addition, after adsorption of Hg0, the Hg s- and d-orbitals are hybridized with the Fe d-orbital at −4.27 and −7.08 eV (Fig. 5c and d), respectively. These phenomena further confirm our initial design concept that the unoccupied d-orbital of Fe atom accepts electron density from the sand d-orbitals of Hg0. Therefore, our investigations results of the electronic structures of the Hg0-FeN3-C configuration, including the electron density difference and the PDOS, demonstrate that the FeN3-embedded carbon possesses excellent electrical conductivity and the unoccupied d-orbital, revealing the great potential as an efficient sorbent for Hg0 adsorption.
4. Conclusions In summary, we have proposed a new economical, effective, and recyclable sorbent, FeN3-embedded carbon, for Hg0 removal by performing density functional theory calculations. This FeN3-embedded carbon can greatly improve the magnetic property of the carbon sorbent, which makes it recyclable for Hg0 adsorption. Moreover, the FeN3 center shows metallic property and is responsible for the superior Hg0 adsorption activity. Hg0 can be chemically adsorbed on FeN3-C with the adsorption energy of −66.27 kJ/mol. In addition, the charge transfer evidenced by the electron density difference and PDOS analysis confirms the “acceptance” process between Hg0 and FeN3-C, which plays a vital role in the adsorption of Hg0. The PDOS shows that the unoccupied d-orbital of Fe can accept electron density from the Hg s- and d-orbitals and that the Hg s- and d-orbitals are highly hybridized with the Fe dorbitals. Besides, the trend of equilibrium constant indicates that Hg0 adsorption on FeN3-C is kinetically favorable at low temperature.
3.5. Effect of temperature on equilibrium constant of Hg0 adsorption on FeN3-C Since the process of Hg0 adsorption on FeN3-C can be dramatically influenced by temperature, we finally evaluated the influence of temperature on the equilibrium constant (Keq) for this process in detail. The computed ΔG and In(Keq) values for Hg0 adsorption on FeN3-C in the temperature range of 250–1000 K, which are obtained from the 1341
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Fig. 5. Partial density of states of the Hg and Fe atoms in surface systems before (a, b) and after (c, d) adsorption of Hg0 on FeN3-embedded carbon are shown. The Fermi level (Ef) is set to 0 eV (dashed line in figures).
Acknowledgement This work was supported by the National Science Foundation of China (Project No. NSFC key project 21537002) and National water pollution control key project 2017ZX07202005-005. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.04.189. References [1] A.A. Presto, E.J. Granite, Survey of catalysts for oxidation of mercury in flue gas, Environ. Sci. Technol. 40 (2006) 5601–5609. [2] J. Wilcox, E. Rupp, S.C. Ying, D.-H. Lim, A.S. Negreira, A. Kirchofer, F. Feng, K. Lee, Mercury adsorption and oxidation in coal combustion and gasification processes, Int. J. Coal Geol. 90–91 (2012) 4–20. [3] B. Padak, J. Wilcox, Understanding mercury binding on activated carbon, Carbon 47 (2009) 2855–2864. [4] Minamata Convention on Mercury, in Programme, United Nations Environment Programme, http://www.mercuryconvention.org/, 2017. [5] W. Lee, G.-N. Bae, Removal of elemental mercury (Hg(0)) by nanosized V2O5/TiO2 catalysts, Environ. Sci. Technol. 43 (2009) 1522–1527. [6] L. Ling, M. Fan, B. Wang, R. Zhang, Application of computational chemistry in understanding the mechanisms of mercury removal technologies: a review, Energy Environ. Sci. 8 (2015) 3109–3133. [7] Z. Liu, Y. Zhang, B. Wang, H. Cheng, X. Cheng, Z. Huang, DFT study on Al-doped defective graphene towards adsorption of elemental mercury, Appl. Surf. Sci. 427 (2018) 547–553. [8] L. Geng, L. Han, W. Cen, J. Wang, L. Chang, D. Kong, G. Feng, A first-principles study of Hg adsorption on Pd(111) and Pd/γ-Al2O3(110) surfaces, Appl. Surf. Sci. 321 (2014) 30–37. [9] D.H. Lim, J. Wilcox, Heterogeneous mercury oxidation on Au(111) from first principles, Environ. Sci. Technol. 47 (2013) 8515–8522. [10] D.H. Lim, S. Aboud, J. Wilcox, Investigation of adsorption behavior of mercury on
Fig. 6. Equilibrium constant for Hg0 adsorption on FeN3-C as a function of temperature.
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