DFT insights to mercury species mechanism on pure and Mn doped Fe3O4(1 1 1) surfaces

Journal Pre-proofs Full Length Article DFT insights to mercury species mechanism on pure and Mn doped Fe3O4(111) surfaces Jiamin Chen, Wenxin Zhu, Xiong Chang, Ding Ding, Tingting Zhang, Changsong Zhou, Hao Wu, Hongmin Yang, Lushi Sun PII: DOI: Reference:

S0169-4332(20)30632-2 https://doi.org/10.1016/j.apsusc.2020.145876 APSUSC 145876

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

19 September 2019 11 January 2020 22 February 2020

Please cite this article as: J. Chen, W. Zhu, X. Chang, D. Ding, T. Zhang, C. Zhou, H. Wu, H. Yang, L. Sun, DFT insights to mercury species mechanism on pure and Mn doped Fe3O4(111) surfaces, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145876

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Title DFT insights to mercury species mechanism on pure and Mn doped Fe3O4(111) surfaces Author names and affiliations Jiamin Chena, Wenxin Zhua, Xiong Changa, Ding Dinga, Tingting Zhanga, Changsong Zhou*a, Hao Wua, Hongmin Yanga, Lushi Sunb a

Engineering Laboratory of Energy System Process Conversion and Emission Reduction Technology of Jiangsu

Province, School of Energy and Mechanical Engineering, Nanjing Normal University, 210042, Nanjing, China b

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 430074, Wuhan, China

Corresponding author Changsong Zhou Address: School of Energy & Mechanical Engineering, NNU, 210042, Nanjing, China Phone: (+86) 25-85481273; Fax: (+86) 25-85481273; E-mail: [email protected] (C. Zhou) Present/permanent address School of Energy and Mechanical Engineering, Nanjing Normal University, 210042, Nanjing, Jiangsu, China Declarations of interest: none

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Abstract The interaction between mercury species and Fe3O4(111), Mn doped Fe3O4(111) surfaces, with Feoct2- termination has been calculated by density functional theory (DFT). Different adsorption sites and placement of adsorbates, potential catalytic oxidation of Hg0 by the adsorbed Cl atoms on the Mn-doped Fe3O4 surface have been considered. The result revealed Hg0 physical and weak chemical adsorption on both pure surface and Mn-doped surface. Horizontally placed HgCl and HgCl2 were better chemisorbed on the surface. Although complete dissociation and partial dissociation occurred during the adsorption process of oxidation mercury, configurations of complete dissociation were more stable than that of partial dissociation. HgCl was chemically adsorbed on both Fe3O4 (111) and Mn doped Fe3O4 (111) surface through breaking into two atoms, and HgCl2 adsorbed through three ones. Both of the Cl atom and Hg atom tended to bind to the transition metals on the surface. When Mn and Fe atoms coexisted on the surface, Cl atoms preferred to bind to Mn atoms. Besides, all the decomposition reaction was exothermic process according to the negative values of adsorption energy.

Keywords: mercury; density functional theory; Mn-Fe spinel; dissociation; adsorption

1. Introduction Trace elements emissions such as mercury from coal combustion have enriched in organisms by participating in the ecological cycle on the earth, and seriously threaten the environment and human health [1-6]. Mercury mainly exists as gaseous element mercury (Hg0), monovalent and divalent mercury (Hg+ and Hg2+) and particulate mercury (Hgp) in coal-fired flue gas [7]. Compared with other mercury species, Hg0 is more difficult to remove. Because it is highly volatile and hardly solubilize in water [8,9]. When the temperature of flue gas decreased, some elemental mercury would be oxidized to HgCl2 for a large amount of chlorine in pulverized coal, and Meiji [10] found that Hg0 and HgCl2 2

co-exist in the flue gas. Carpi [11] believed that the different concentrations of HCl and other pollutants in coal and flue gas would affect the distribution of Hg0 and Hg2+, and higher concentrations of chlorine contributed to more Hg0 being oxidized to divalent mercury, which was mainly in the form of mercuric chloride [12-14]. Thus ，different mercury species were employed in our work to understand the adsorption mechanisms of them. Sorbent injection [15,16], catalytic oxidation [14,17,18] and many other methods have been applied for mercury control in the flue gas. Due to the high cost, secondary pollution and non-recycling of these methods, the search for more cost-effective mercury removal methods has never been stopped. Recently, many researchers have been devoted to removing gaseous pollutants including heavy metal ions by using spinel ferrites [19-22]. The spinel ferrites have advantages of thermal stability, low-cost, and corresponding magnetic properties [23,24]. Furthermore, the superior catalytic properties of the materials have attracted more and more researchers [25-30]. Active Fe3+-O species was formed by surface Fe2+ activating O2, manganese substitution (Fe3−xMnxO4) influenced the catalytic properties for CO at a higher temperature [25,26]. (Fe3xMnx)1-δO4 was

found as a kind of promising material to enhance the adsorption capability of Hg0 for Mn4+ and cationic

vacancies existing on the surface after numerous experimental studies [27]. A study of mercury and sulfur dioxide removal from flue gas before wet electrostatic precipitator (WESP) by taking advantage of Mn-Fe spinel catalyst with low-temperature oxidation was successfully performed. Moreover, the recycling of the Mn-Fe spinel catalyst was wonderful [22]. However, previous research has paid less attention on mechanism studies on the mercury removal by Mn doped Fe3O4(111) surface, which is equally important for the modification of catalysts. The emergence of density functional theory (DFT) makes the microscopic and complex quantum chemistry problems simple, and the study is effective and easy to implement [31-33]. It has been successfully applied to investigate the mechanisms in molecule interaction on pure magnetite surfaces or transition metal doped surfaces [34,35]. Zhou et al. [36-38] studied the co-interaction mechanism between Hg0 and H2O2 on Fe3O4(111), (110), and (001) surfaces by 3

DFT method. The co-interaction mechanism of carbon monoxide and H2O on Cu doped Fe3O4(111) surface with Fetettermination was also performed through DFT study [40,41]. Yang et al. [42] used first-principle calculations to explore the heterogeneous mechanisms of Hg0 oxidation on MnFe2O4(100) surfaces with the existence of HCl. Yang et al. [43] also explored Hg0 oxidation mechanisms by calculating Hg0 and HgO reaction on MnFe2O4(100) surfaces. Song et al. [44] revealed that Ti dopant played a key role in the oxidation process of nitric oxide by hydrogen peroxide on modified Fe3O4(111) surface with Feoct- termination. As far as we know, mercury species mechanism on pure and Mn doped Fe3O4(111) surface has rarely been reported. Transition metals including Co, Mn, Ni, Mo, and Zn doping on the Fe3O4(111) surfaces were calculated by Materials Studio based on density functional theory in our preliminary study. As shown in Table 1S, compared with their binding energies and geometric parameters, it was found that mercury preferred adsorption on the Mn-doped surface to that on other transition metals. Therefore, the Mn/Fe3O4 (111) surface was selected to explore the interaction between HgCl or HgCl2 with the doped surface. This study intends to reveal the reaction mechanism of different mercury species on Mn doped Fe3O4 (111) surface through different adsorption sites, adsorption energy, Mulliken charge, charge transfer, and density of states. The results are of great significance for understanding the adsorption mechanism and guiding the design of adsorbents.

2. Computational models and methods 2.1 Models The structure of magnetite is a complex ionic crystal, which is formed by Fe2+, Fe3+, and O2- through ionic bonds between them [45]. Different from all metal ions with +3 valence occupying octahedral sites in spinel, only 50% of the Fe3+ and all Fe2+ occupy octahedral interstitial sites in this structure [46]. The other half of the Fe3+ take up tetrahedral sites [47,48]. Fe3O4(111) was considered as the most reactive surface of the three main natural growth surfaces [49]. By 4

cutting along the direction of the stacking sequence of Fe3O4(111), a repetitive structure with six terminations on the surface of Fe3O4 (111) could be obtained [50]. The required energy of their five bonds breaking was higher than others’ two or four more bonds, as a result, the stability of Feoct2- and Fetet1- terminations was higher than other terminations [51]. Feoct2- termination has both Fetet and Feoct atoms, which brings about its higher activity, while Fetet1- only has Fetet on its first layer [44]. Thus, Feoct2- termination of Fe3O4(111) was employed in this paper. The lattice parameters of the optimized Fe3O4 unit cell were consistent with our previous research results [36-39]. In this paper, the octahedral and tetrahedral iron atoms substituted by Mnoct and Mntet on Fe3O4(111) surface were performed (as shown in Fig.1).

2.2 Methods The Cambridge Sequential Total Energy Program (CASTEP) was used to perform all calculations in this paper [52,53], using the exchange-correlation functional GGA-PBE to describe the exchange-correlation energy [54,55]. The cutoff energy was 380 eV and 3x3x1 k-points mesh was used to integrate the Brillouin zone. The spin polarization of the material was set for the magnetism of the material [37]. To find the most stable configurations, the method of minimizing total energy was adopted for atom and cell optimization, with a total energy difference of 2x10-5eV/atom, a maximum force tolerance of 0.05 eV/Å. The criteria of maximum stress and the maximum displacement were 0.1 GPa and 0.002 Å, respectively. The three outmost layers of all models were relaxed and other layers were fixed throughout the study. 12 Å vacuum region was employed in the z-direction to eliminate the influence of periodic structure on calculation. Before calculation, Hg, HgCl and HgCl2 were placed in individual cubic lattices of (10 Å)3 for geometric optimization. The atoms in Hg, HgCl and HgCl2 were fully relaxed upon the geometry optimization. Simultaneously, 1x1x1 k-points mesh was applied and the rest of the settings such as the formal spin of Fe atoms were the same as the previous [38]. Adsorption energy can be used to estimate the stability of the adsorbed surface, it can be calculated from: 5

E𝑏𝑖𝑛𝑑 = E（sub + ad） - E(sub) - E(ad) In the formula, E（sub + ad） represents the total energy of the substrate with adsorbates, and E(sub) represents the total energy of the substrate before calculation. Additionally, E(ad) is the ground state energy of adsorbates such as mercury. Moreover, all units of adsorption energy in this paper are converted from eV to kJ/mol. Linear synchronous transit/quadratic synchronous (LST/QST) method was employed to search the transition states during the process of Hg0 oxidation to HgCl2 [56] and the energy barriers (𝐸𝑏𝑎𝑟𝑟𝑖𝑒𝑟)were calculated from the next formula: 𝐸𝑏𝑎𝑟𝑟𝑖𝑒𝑟 = 𝐸𝑇𝑆 ― 𝐸𝐼𝑀 where (𝐸𝑇𝑟𝑎𝑛𝑠𝑖𝑡𝑖𝑜𝑛) and (𝐸𝐼𝑛𝑡𝑒𝑟𝑚𝑒𝑑𝑖𝑎𝑡𝑒) represent the total energies of the transition states and the intermediate states respectively. Moreover, the energy pathway shown in Fig. 1S in supporting information was obtained to describe the process of Hg0 oxidation from the adsorbed elemental state to oxidation one.

3. Calculation results and discussion 3.1 Hg0 interaction with pure and Mn doped Fe3O4(111) surfaces The p (1x1) and p (2x1) surface cells were used to investigate the interactions between Hg0 and pure surfaces, Mn doped Fe3O4(111) surfaces, respectively. A single adatom was allowed to approach Fe3O4 and Mn/Fe3O4 from different sites, including Feoct top, Fetet top, Mnoct top, Mntet top, O sites, and hollow sites. After calculation, all stable configurations are shown in Fig. 2 when the systems are in equilibrium; the optimized parameters of the configurations are given in Table 1. According to the configurations, Hg prefers to interact with Mn and Fe atoms on the surface. The adsorption energies suggest that Mnoct top is the most attractive sites to mercury, then Mntet top, and at last, Fetet top. Among the 6

obtained structures of p(1x1) cells, configuration 1G is most stable and its binding energy is -42.420 kJ/mol. The bond length of Hg-Mn is 2.770 Å. 0.03 e charges transferring from Hg0 to the substrate surface is shown by the bond population. In addition, with the increase of Mn atoms content, the binding energies are increasing, and the result corresponds with the experimental conclusion [27]. To reduce the computational complexity, the model of the substituted Mntet and Mnoct on the surface was used to clarify Hg0 adsorption mechanism on the doped p(2x1) surface. The binding energies of Hg0 adsorption are arranged in the following order: 1D’>1C’>1E’. Meanwhile, 0.03 e, 0.06 e, and 0.04 e charges transform to the Mn-doped surfaces and the bond populations are 0.22, 0.37, and 0.37, respectively. However, the adsorption energy ranges from -11.660 kJ/mol to -42.420 kJ/mol, suggesting that Hg0 adsorption on both pure surfaces and Mn-doped surfaces is physical and weak chemical behaviors. The possible catalytic oxidation process of Hg0 by the adsorbed Cl atoms on the Mn-doped Fe3O4 surface was also considered. As shown in Fig. 1S in supporting information, two energy barriers (262.4 kJ/mol and 306.8 kJ/mol) were found during Hg0 oxidation from the adsorbed elemental state to oxidation one. However, these energy barriers were relatively high, and the energy of the final state was 328.286 kJ/mol higher than that of the reactant state. It can be suggested that the oxidation process of Hg0 on the Mn-doped Fe3O4 surface was endothermic and hardly spontaneous due to the high energy barriers. Therefore, the removal mechanism of Hg0 by the Mn-doped Fe3O4 is mainly physical and weak chemical adsorption.

3.2 HgCl adsorption on pure and Mn doped Fe3O4(111) surfaces 3.2.1 HgCl binding on Fe3O4(111) surface HgCl was geometrically optimized before calculation. The bond length of HgCl is 2.471 Å. The number of Hg Mulliken charge is 0.37 e and that of Cl atom is −0.37 e, respectively. Three different configurations of the relative location between HgCl and Fe3O4 (111) sites were considered before calculation on the p(2x1) supercell surfaces. (1) Hg-Cl bond was set parallelly over the surface, and the HgCl was placed on the hollow site, Fetet, Feoct and O top sites; 7

(2) Hg-Cl bond was placed perpendicularly over the surface, and the Hg atom of HgCl was placed down toward the same adsorption sites as above; (3) Hg-Cl bond was placed perpendicularly over the surface, and the Cl atom of HgCl was placed down toward the same adsorption sites as above. After calculation, eight stable structures of HgCl binding on pure Fe3O4(111) surfaces are obtained. Fig. 3 shows the stable structures. The adsorption energy, Mulliken charge, bond population, and bond length are exhibited in Table 2. As shown in Table 2, the binding energy values ranging from -107.893kJ/mol to -314.199kJ/mol clarify that HgCl is chemisorbed on Fe3O4(111) surface with an exothermic process. Configuration 2A is the most stable model among the calculated results and its binding energy is -314.199 kJ/mol. The distance between Hg and Cl is extended to 3.665 Å. Mulliken charge population shows that 0.34 e charges transfer from the substrate to Hg and Cl. The bond population is 0.59. In Fig.5(a), s, p, and d orbitals for Cl adsorbed on pure Fe3O4(111) surface are analyzed to investigate the interaction between Cl and Feoct. Fig.5 (a) shows different superposition states of the p orbital of Cl and the d orbital of Feoct at -4.1, -1.9 and 0.5 eV. Thus, Hg and Cl have a strong interaction with Fetet and Feoct. The length of Hg-Cl bond in configuration 2B is elongated to 4.167 Å, which is much longer than that in original HgCl, and it contributes to the dissociation of HgCl. After the dissociation, Cl is strongly bound to the surface with Hg far from the surface. Meanwhile, 0.44 e charges transfer from the basement to the Cl atom. Contrary to 2B, the Hg atoms were put down toward the surface in configuration 2C and 2D. HgCl was adsorbed on the surface without dissociation. Configuration 2C shows that Hg interacts with Feoct and Fetet. 0.36 e charges transfer from the substrate to Hg and Cl. Different from 2A and 2B, initially, the Hg atom of HgCl was placed on O top in configuration 2D. After calculation, HgCl is adsorbed by Otet top yielding the energy of -107.893 kJ/mol. A new Hg-O bond is formed with a length of 2.265 Å in this structure. Simultaneously, the length of Hg-Cl bond is reduced to 2.418 Å, which is close to the original length of 2.471 Å. Thus, 2D is chemically adsorbed but not stable. 8

When the HgCl is adsorbed on the surface through breaking into two atoms, the adsorption energies are higher than the structures without HgCl dissociation. The result can be interpreted as chemisorption of HgCl binding to Feoct termination of the pure Fe3O4(111) through a dissociation adsorption reaction. In addition, it can be inferred that the decomposition reaction is an exothermic process according to the negative values of adsorption energy.

3.2.2 HgCl binding on Mn doped Fe3O4(111) surface P(2x1) supercells were employed to investigate interactions between HgCl and Mn doped Fe3O4(111) surface. The adsorption site referred to in this calculation is similar to that of HgCl adsorption on pure Fe3O4(111) surface. As shown in Fig.4, configuration 3A to 3H are obtained after geometric optimization when HgCl is placed horizontality. The order of the stability is 3D>3C>3E>3F>3A>3G>3H>3B. The bond of HgCl in these configurations is lengthened and breaks to Hg atom and Cl atom. In configuration 3D and 3F, Cl atoms are bound to Mnoct and Fetet on the surface with their respective bond length of 2.419/2.443 Å and 2.366/2.451 Å. 0.32 e and 0.33 e charges transfer from the basement to Hg and Cl, respectively. In configuration 3D, Hg is attached to the Feoct on the surface while Hg frees from the surface in configuration 3F. Cl atom strongly interacts with Mnoct-Fetet bridge site by forming two new bonds: Cl-Mnoct bond and Cl-Fetet bond. Cl, Mnoct and Fetet atoms are connected by the three bonds and the structure is in the shape of a triangle. The binding mechanism can be demonstrated by analyzing the PDOS of related atoms after adsorption. In Fig. 5, s, p, and d orbitals for Cl adsorbed on Mn doped Fe3O4(111) surface are analyzed to investigate the interaction between Cl and Mnoct /Fetet. Fig. 5 (b) shows different superposition states of the p orbital of Cl and the d orbital of Mn at -4.9 and -1.3 eV. Meanwhile, a multiple overlapping is shown, the p orbital of the Cl and the p, d orbitals of Fe overlap at -6.4 and -0.4eV. Strong orbital hybridization of Cl and Mnoct /Fetet result in enhancing the interaction between the HgCl and the substrates. The energy favorable for HgCl adsorption on the surface is -363.086 kJ/mol, which confirms the strong interaction. HgCl is adsorbed on Mn doped surfaces with a higher energy than that 9

on clean surfaces. In configuration 3A and 3B, Hg atoms and Cl atoms can be adsorbed by interacting with both Mntet and Mnoct. However, the interaction occurred on both Mnoct and Feoct surface in configuration 3C and 3E. Combined with the Mulliken charge analysis, a certain amount of electrons transfers to the substrate in configuration 3A, 3B, 3C and 3E. 0.33 e and 0.40 e charges transfer from the substrate to Hg and Cl, respectively. All parameters indicate that HgCl has a definite chemical interaction with Mn-doped surface. By placing HgCl on the doped surface perpendicularly, nine stable configurations (Configuration 3A’ to 3I’) are obtained after calculation. There is dissociation adsorption reaction in configuration 3D’, 3E’ and 3F’. The binding energy is among -278.887~-339.075 kJ/mol; all of them are strong chemical adsorption. Cl is bound to Mntet or Mnoct of the surface by strong chemical absorption in 3E’ and 3F’. The bond population of Cl and Mntet or Mnoct are 0.59 and 0.48 while the bond lengths of Cl and Hg are elongated by 1.213 and 1.907 Å, respectively. Therefore, Cl, in such configurations, interacts strongly with Mn atoms, weakening the interaction between Cl and Hg. Similar to configuration 3D and 3F, Cl is bound to the Fetet-Mnoct bridge site in configuration 3D’, but Hg frees from the surface. The binding energy of configuration 3D’ is a little lower than that of configuration 3D. When the structural system maintains its equilibrium, the structure of molecular HgCl does not change significantly in 3A’, 3B’, 3C’, 3G’, 3H’ and 3I’ with their bond length ranging from 2.417 to 2.472 Å. Therefore, Hg atom of HgCl prefers to attach to Mn top compared with other considered sites. In general, horizontally placed HgCl can be better adsorbed on the surface through comparative analysis. Once the dissociation of HgCl happens, the HgCl will be chemically adsorbed. Moreover, HgCl is most likely to dissociate and to be adsorbed on Mn doped Fe3O4(111) surface.

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3.3 HgCl2 adsorption on pure and Mn doped Fe3O4(111) surfaces Taking the large size of HgCl2 molecule and the mutual exclusion between the periodic adsorbates into account, only the p(2x1) supercell was employed for calculation. The initial placement of HgCl2 on the surface corresponds with the initial structures of HgCl adsorption on the above discussion. The specific parameters of these configurations are listed in Table 4. The optimized configurations after calculations are shown in Fig. 6. HgCl2 was optimized geometrically before calculation. Each Hg-Cl distance is 2.299 Å. The bond angle of Cl1-Hg-Cl2 is 180°. The Mulliken charge of Hg atom, Cl1, and Cl2 atom are 0.44 e, −0.23 e and −0.23 e, respectively. Configurations 4A and 4B depicted in Fig. 6 are stable structures of HgCl2 adsorbed on pure Fe3O4(111) with Feoct2- termination surface. The binding energy of configuration 4B is -74.100 kJ/mol, suggesting a weak relationship between HgCl2 and the basement. The Cl2 is adsorbed on the Feoct with a distance of 2.235 Å. However, the bond length of Cl2 and Hg is elongated to 0.452 Å. Thus, the HgCl2 in parallel position was weak chemisorption. The binding energy of configuration 4A is -306.720 kJ/mol, which is much higher than 4B. The molecular HgCl2 changes a lot with Cl1 and Cl2 adsorbed on two Fetet sites. Meanwhile, 0.18 e, 0.28 e and 0.18 e charges transfer from the substrate to the Cl1, Hg and Cl2 atoms, respectively. In Fig. 5 (d), s, p, and d orbitals for Cl atoms adsorbed on Mn doped Fe3O4(111) surface are analyzed to investigate the interaction between Cl and Fetet/Mnoct/Mntet. Different superposition states of the p orbital of Cl2 and the d orbital of Mnoct at -4.4, -3.4 and -2.1 eV were shown in the first figure. In the second figure, a multiple overlapping is shown, the p orbital of the Cl2 and the d orbital of Fetet overlap at -6.0, -3.6, -1.8 and 0.6 eV and the s orbital of Cl2 and the p orbital of Fetet overlap at -15.4 eV. As for Cl1, its p orbital and d orbital of Mntet overlap at -3.0 eV and its s orbital and p orbital of Mntet overlap at -15.1 eV. The last figure shows that the p orbital of Cl1 and d orbital of Mnoct overlap at -2.1 eV and the s orbital of Cl1 and the p orbital of Mnoct at -15.1 eV. Considered the bond lengths of Cl1-Fetet and Cl2-Fetet and PDOS of Cl and Fe atoms, the results clarify that HgCl2 is adsorbed chemically on the 11

substrate in configuration 4A. Simultaneously, the interaction between HgCl2 and Mn doped Fe3O4(111) with Feoct2- termination surface was investigated. All the obtained stable configurations are also demonstrated in Fig. 6. From the adsorption energy, it can be concluded that 4C’ is the most stable adsorption configuration. The structure of HgCl2 molecule greatly changes after adsorption in configurations 4A’-4C’. The two Hg-Cl bonds are broken with binding energies ranging from -342.890 to -371.185 kJ/mol. In configuration 4A’, Cl1 and Cl2 are adsorbed on the Mn and Fetet top sites, respectively. The angle of Cl1-Hg-Cl2 decreases to 98.5°. Meanwhile, 0.21 e, 0.29 e and 0.17 e charges transfer from the substrate to the Cl1, Hg and Cl2 atoms, respectively. The difference between configuration 4A’ and 4C’ is the free state of Hg in 4A’. Furthermore, the binding energy of 4C’ is a little higher than that of 4A’, which is predicted as the former state of 4C’. In Fig. 5 (d), s, p, and d orbitals for Cl atoms adsorbed on pure Fe3O4(111) surface are analyzed to investigate the interaction between Cl atoms and Fe atoms. The left one shows different superposition states of the p orbital of Cl2 and the d orbital of Fetet at -6.0, -5.0, -3.5, -1.9 and 0.5 eV. In the right figure, a multiple overlapping is shown, the p orbital of the Cl1 and the d orbital of Fe’tet overlap at -5.0, -1.8 and 0.5eV. The Hg, Cl1, and Cl2 atoms are all adsorbed on the 4A’ basement. However, adsorption sites are different from that in configuration 4C’. The structures of 4D’, 4E’, 4F’, 4G’, 4H’, and 4I’ are optimized when HgCl2 molecule is perpendicular to the surface. HgCl2 adsorption is through binding Cl atoms to Mn or Fe on the surface. In configurations 4E’, 4H’, and 4I’, Cl2 atom is adsorbed on the Mnoct-Fetet bridge site, the Mnoct-Mntet bridge site and the Feoct-Fetet bridge site, respectively. In configuration 4D’, 4F’, and 4G, HgCl2 breaks Hg–Cl2 bond with Cl1 atom adsorbed on the Mn top. The binding energy ranges from -54.916 to -86.171 kJ/mol, implying that HgCl2 is chemically bound to the substrate [57,58]. The interaction between entirely dissociative HgCl2 molecule is stronger than the interaction between partially dissociative HgCl2 molecule after doping Mn on Fe3O4(111) surface. Simultaneously, the overall adsorption effect of 12

vertical placement is far less than that of horizontal placement. Thus, completely dissociation adsorption of HgCl2 is most likely to occur on the surface and configurations 4A’-4C’ are more likely to be obtained.

4. Conclusions The mechanism of mercury species on pure and Mn doped Fe3O4(111) surface was carried out based on DFT. The most favorable configurations were revealed with the corresponding binding energies. Hg0 was relatively physically bound to the Mn Top site yielding adsorption energy of -42.420 kJ/mol while mercury atoms were weakly chemically adsorbed on both surfaces. The energy barriers of Hg0 potential oxidation on Mn doped Fe3O4(111) surface were 262.4 and 306.8 kJ/mol. Chemisorption was likely to clarify the adsorption mechanism of HgCl and HgCl2 yielding adsorption energies of -316.881, -363.086, -306.720 and -371.185 kJ/mol, respectively. Distances of Hg-Cl bonds of HgCl and HgCl2 molecules were lengthed and then broke into two or three atoms adsorption on the transition metals sites through an exothermic process. The high adsorption energies of HgCl and HgCl2 on both surfaces were likely caused by the formation of new Cl-Mn/Fe bonds. The superposition of Cl with the p orbitals and Fe/Mn with the p,d orbitals revealing a violent interaction between Cl and Fe/Mn. Moreover, Mn-Fe spinel was applied in the field of mercury and sulfur dioxide removal in coal-fired gas before wet electrostatic precipitator (WESP) in practical application. A further study will be investigated to explore the co-interaction mechanism over H2O and SO2 influence on mercury removal.

Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC) (Nos. 51906114, 51676101), the Natural Science Foundation of Jiangsu Province (Nos. BK20180731, BK20161558), China Postdoctoral Science Foundation (2018T621779), and the Foundation of State Key Laboratory of Coal Combustion (FSKLCCA2001).

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The list of figure captions: Fig.1 Models of Fe3O4(111) surface with the Feoct termination. (a) original Fe3O4(111) surface; (b) Mnoct doped Fe3O4(111) surface; (c) Mntet doped Fe3O4(111) surface; (d) Mnoct and Mntet doped Fe3O4(111) surface.

Fig.2 Structures of Hg0 adsorption on pure and Mn doped Fe3O4(1 1 1) surfaces: 1A, 1A’ and 1B’ for pure surfaces; 1B-1G and 1C’-1E’ for Mn doped surfaces; 1A-1G for p(1x1) surfaces; 1A’-1E’ for p(2x1) surfaces.

Fig.3 Structures of HgCl adsorption on pure Fe3O4(1 1 1) surfaces: 2A-2F on p(1x1) unit cell surface; 2A’-2D’ on p(2x1) super cell surface.

Fig.4 Structures of HgCl adsorption on Mn doped Fe3O4(111) surfaces. Fig.5 PDOS of Cl adsorbed on pure and Mn doped Fe3O4(111) surface in configuration 2A’, 3D, 4A and 4C’ (a) 2A’, 20

(b)3D, (c)4A and (d) 4C’ (the values of Fe and Mn were set to negative for analysis; red, s orbital; blue, p orbital; magenta, d orbital).

Fig.6 Structures of HgCl2 adsorption on clean surfaces and Mn doped surfaces: 4A and 4B for clean surfaces; 4A’-4I’ for Mn doped surfaces.

Fig.1. Models of Fe3O4(111) surface with the Feoct termination. (a) original Fe3O4(111) surface; (b) Mnoct doped Fe3O4(111) surface; (c) Mntet doped Fe3O4(111) surface; (d) Mnoct and Mntet doped Fe3O4(111) surface.

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Fig.2. Structures of Hg0 adsorption on pure and Mn doped Fe3O4(1 1 1) surfaces: 1A, 1A’ and 1B’ for pure surfaces; 1B-1G and 1C’-1E’ for Mn doped surfaces; 1A-1G for p(1x1) surfaces; 1A’-1E’ for p(2x1) surfaces.

22

Fig.3. Structures of HgCl adsorption on pure Fe3O4(1 1 1) surfaces: 2A- 2D on Mn doped surfaces.

23

Fig.4. Structures of HgCl adsorption on Mn doped Fe3O4(111) surfaces. 24

25

Fig.5 PDOS of Cl adsorbed on pure and Mn doped Fe3O4(111) surface in configuration 2A’, 3D, 4A and 4C’ (a) 2A’, (b)3D, (c)4A and (d) 4C’; the values of Fe and Mn were set to negative for analysis; red, s orbital; blue, p orbital; magenta, d orbital).

(a)

(b)

(c) 26

(d)

27

Fig.6. Structures of HgCl2 adsorption on clean surfaces and Mn doped surfaces: 4A and 4B for clean surfaces; 4A’-4I’ for Mn doped surfaces.

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The list of table captions: Table 1 The specific parameters of stable configurations for Hg0 adsorption on Fe3O4(111) surfaces p(1x1) surface cell

1A 1B 1C 1D 1E 1F 1G

p(2x1) surface cell

Ebind

RHg-X

MHg-X

QHg

-32.089 -32.526 -11.660 -29.235 -37.150 -26.819 -42.420

2.856 2.839 3.290 2.886 2.710 2.850 2.770

0.36 0.35 0.31 0.42 0.30 0.39

0.05 0.05 -0.06 0.03 0.02 0.00 0.03

1A’ 1B’ 1C’ 1D’ 1E’

Ebind

RHg-X

MHg-X

QHg

-31.913 -14.118 -39.865 -40.393 -31.086

2.853 2.896 2.808 2.918 2.826

0.36 0.31 0.37 0.22 0.37

0.06 -0.02 0.06 0.03 0.04

Notes: Ebind represents adsorption energy (kJ/mol), RHg-Mn and RHg-Fe represent the length of adsorbed Hg and Mn/Fe, respectively (Å), MHg-Mn and MHg-Fe represent the corresponding bond population between Hg and Mn/Fe, QHg represents Mulliken charges of Hg atom (e).

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Table 2 The specific parameters of stable configurations for HgCl adsorption on pure Fe3O4(111) surfaces

2x1 surface cell

Ebind

RHg-X

RCl-X

MHg-X

MCl-X

QHg

QCl

2A’

-314.199

2.887

2.202

0.34

0.59

0.07

-0.41

2B’

-271.182

-

2.210

-

0.54

-0.01

-0.43

2C’

-197.558 2.887/2.711

2.424

0.23/0.37

0.46

0.06

-0.42

2D’

-107.893

2.418

0.23

0.45

0.34

-0.34

2.265

Notes: Ebind represents adsorption energies (kJ/mol), RHg-X represents the length of adsorbed Hg and Fetet/Feoct/O, respectively (Å), RCl-X represents distances between the Cl atom and Fetet/Feoct/Hg, respectively (Å), MHg-X and MCl-X represent the corresponding bond population between Hg and Fetet/Feoct/O and Cl and Fetet/Feoct/Hg, respectively, QHg and QCl represent Mulliken charge of Hg atoms and Cl atoms, respectively (e).

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Table 3 The specific parameters of stable configurations for HgCl adsorption on Mn/ Fe3O4(1 1 1) surface Ebind

RHg-X

RCl-X

MHg-X

MCl-X

QHg

QCl

3A

-321.511

2.789

2.354

0.42

0.44

0.13

-0.46

3B

-301.121

2.876

2.276

0.34

0.53

0.09

-0.45

3C

-359.973

2.853

2.184

0.35

0.59

0.10

-0.44

3D

-363.086

2.835

2.419/2.443

0.38

0.34/0.31

0.09

-0.41

3E

-351.182

2.820

2.286

0.39

0.52

0.10

-0.45

3F

-350.526

-

2.366/2.451

-

0.33/0.32

0.06

-0.39

3G

-319.777

2.835

2.262

0.38

0.49

0.11

-0.44

3H

-319.153

2.829

2.262

0.34

0.52

0.06

-0.46

3A’

-198.876

2.692

2.444

0.51

0.44

0.01

-0.44

3B’

-216.403

2.628

2.430

0.61

0.43

0.10

-0.47

3C’

-216.707

2.571

2.417

0.61

0.43

0.11

-0.45

3D’

-339.075

-

2.400/2.406

-

0.34/0.32

0.02

-0.40

3E’

-300.843

-

2.232

-

0.59

-0.01

-0.45

3F’

-278.887

-

2.301

-

0.48

0.00

-0.47

3G’

-213.421 2.982/2.723

2.464

0.16/0.41

0.41

0.00

-0.45

3H’

-124.000

2.328

2.452

0.18

0.42

0.24

-0.37

3I’

-214.623

2.652

2.472

0.59

0.39

0.12

-0.49

Notes: Ebind represents adsorption energies (kJ/mol); RHg-X represents the length of adsorbed Hg and Fetet/Feoct/Mntet/Mnoct/O, respectively (Å); RCl-X represents distance between the Cl atom and Fetet/Feoct/Mntet/Mnoct/Hg, respectively (Å); MHg-X and MCl-X represent the corresponding bond population between Hg and Fetet/Feoct/Mntet/Mnoct/O and Cl and Fetet/Feoct/ Mntet/Mnoct/Hg, respectively; QHg and QCl represent Mulliken charge of Hg atoms and Cl atoms, respectively (e).

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Table 4 The specific parameters of stable configurations for HgCl2 adsorption on Fe3O4(1 1 1) surfaces Ebind

RHg-X

RCl1-X/ RCl2-X

MHg-X

MCl1-X/ MCl2-X

QHg

QCl1/Cl2

4A

-306.720

2.828

2.226/ 2.228

0.43

0.50/0.50

0.16

-0.41/-0.41

4B

-74.100

2.440

2.440/2.235

0.26

0.26/0.48

0.37

-0.39/-0.37

4A’

-342.890

-

2.231/2.437/ 2.443

-

0.50/0.33/0.31

0.07

-0.41/-0.39

4B’

-357.878

2.850

2.259/2.160

0.38

0.56/0.62

0.14

-0.43/-0.38

4C’

-371.185

2.834

2.481/2.488/ 2.543/2.338

0.40

0.31/0.31/ 0.27/0.38

0.15

-0.44/-0.40

4D’

-86.171

2.448

2.448/2.207

0.25

0.25/0.51

0.32

-0.39/-0.38

4E’

-107.799

2.504

2.504/2.384/2.397

0.23

0.23/0.33/0.34

0.21

-0.40/-0.38

4F’

-77.034

2.462

2.462/2.270

0.24

0.24/0.50

0.34

-0.39/-0.42

4G’

-54.916

2.475

2.335

0.24

0.24/0.43

0.28

-0.39/-0.43

4H’

-97.691

2.493

-/2.516/2.408

0.24

-/0.27/0.35

0.24

-0.38/-0.39

4I’

-72.695

2.492

2.492/2.425/2.428

0.24

0.24/0.29/0.33

0.20

-0.39/-0.36

Notes: Ebind represents adsorption energy (kJ/mol); RHg-X represents the length of adsorbed Hg and Feoct/Mntet, respectively (Å); RCl1-X represents distance between the Cl1 atom and Fetet/Mntet/Mnoct/Hg, respectively (Å); RCl2-X represents distance between the Cl2 atom and Fetet/ Feoct /Mntet/Mnoct/Hg, respectively (Å); MHg-X and MCl-X represent the corresponding bond population between Hg and Fetet/Feoct/Mntet/Mnoct/O and Cl and Fetet/Feoct/Mntet/Mnoct/Hg, respectively; QHg and QCl represent Mulliken charge of Hg and Cl, respectively (e).

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Dear Editor and Reviewers, This paper entitled ‘DFT insights to mercury species mechanism on pure and Mn doped Fe3O4(111) surfaces’ has these highlights: The adsorption of Hg0 on the original and doped surface is a physical process. HgCl and HgCl2 are dissociatively adsorbed during an exothermic process. Both of the Cl atom and Hg atom tend to bind to the transition metals on the surface.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Author Contributions Section Jiamin Chen: Conceptualization, Methodology, Formal analysis, Writing-Original draft preparation, WritingReviewing and Editing. Wenxin Zhu: Formal analysis, Writing- Original draft preparation. Xiong Chang: Formal analysis, Visualization, Investigation. Ding Ding: Investigation. Tingting Zhang: Validation. Changsong Zhou: Methodology, Software, Data Curation, Writing-Reviewing and Editing. Hao Wu: Resources, Project administration. Hongmin Yang: Resources, Supervision. Lushi Sun: Resources.

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