Mercury Species and Typical Flue Gas Components Binding on ZnS

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Mercury Species and Typical Flue Gas Components Binding on ZnS(110) Hailong Li, Shihao Feng, yang liu, and Kaimin Shih Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00213 • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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Mercury Species and Typical Flue Gas Components Binding on ZnS(110) Hailong Li1, Shihao Feng1, Yang Liu2*, Kaimin Shih3

1 School of Energy Science and Engineering, Central South University, Changsha 410083, China 2 Advanced Membranes and Porous Materials Center, 4700 King Abdullah University of Science and Technology, Thuwal 23955-6900, KSA 3 Department of Civil Engineering, The University of Hong Kong, Hong Kong SAR, China

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*To whom correspondence should be addressed: TEL: 86-18670016725 FAX: 86-731-88879863 Email: [email protected]

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ABSTRACT: Environmental benign zinc sulfide (ZnS) that entirely comprised of “active” sites has shown promising efficiency for capturing mercury from flue gas in recent experimental studies. In this work, the binding mechanism of Hg0 on ZnS(110) surface was investigated by the density functional theory (DFT) method. Meanwhile, the binding of two additional mercury forms, HgCl and HgCl2, and three essential flue gas compositions, H2O, SO2 and HCl, as well as their further effects on Hg0 binding strength on ZnS(110) surface were also evaluated. The results show that, in consistent with experimental observations, Hg0 can be chemisorbed on ZnS(110) surface with binding energies (BEs) as high as 87.80 kJ/mol. The enhanced electrostatic characteristics on the activated surface, especially the Zn sites, are beneficial to excite the outer-shell electrons of Hg0 and thereby enhance the binding strength of materials. HgCl and HgCl2 are chemisorbed with highest BEs of -174.33 kJ/mol and -132.79 kJ/mol, respectively. Moreover, H2O has positive effect on Hg0 binding, while SO2 and HCl have negligible effect. Our theoretical works demonstrate that ZnS has great potentials to serve as a novel sorbent for efficient removal of mercury from coal combustion flue gas. KEYWORDS: Mercury, Binding, ZnS, Density functional theory, Flue gas


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Introduction Mercury is considered as one of the most harmful chemicals due to its irreversible injuriousness to human body.1 By March 30, 2017, more than 120 countries, including China and United States, have signed the Minamata Convention on mercury to protect human health and global environment from anthropogenic mercury emission.2 Therefore, it is urgent to limit mercury emission from coal combustion, which was identified as the main source of anthropogenic mercury emission to the atmosphere. 3, 4 Various technologies have been developed to reduce the mercury emissions from coal combustion, the main source of mercury emissions to the atmosphere, to meet the goals of the global Minamata Convention and other reginal regulations. 5, 6 Among these technologies, only activated carbon injection (ACI) has been commercialized by far. ACI is identified to be the maximum achievable control technology (MACT) for mercury capture from coal-fired power plants,7 however, its efficiency needs to be further improved especially when flue gas temperature is relatively high. Although efforts, such as chemical emendations (mainly HS- or S2-) on activated carbons (ACs) to enhance the binding affinity between AC and mercury,8,9 have been extensively made to improve its efficiency, ACI technology still possesses significant drawbacks: (1) mercury adsorption rates drop significantly depending on coal quality and flue gas temperature;10 (2) sulfur impregnating on ACs needs toxic raw materials, such as carbon disulfide;11,12 (3) carbons introduced by ACI degrade the quality of concrete using fly ash as raw material, and hence prevent reuse of fly ashes.13 Moreover, fly ash containing spent activated carbon may need extra costs for disposal, because the chemical and biological stability of mercury on activated carbon is unknown.14, 15


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To overcome the above drawbacks, an alternative way is to develop sulfur contained non-carbon sorbents.14,

16-18

Although several such kinds of materials, such as zeolite, bimetallic iron-copper

nanoparticle and silica-based material, have shown good performance for mercury vapor removal,14, 19-21 the mercury capture efficiency of these materials is significantly affected by the surface coverage of sulfur,22 which restricts their application potentials. In this aspect, mineral sulfides, such as MoS2 and ZnS, could probably show advantages for gas-phase mercury removal because (1) the high ratio of sulfur in these mineral sulfides provides abundant ‘active’ sulfur sites for mercury adsorption ; 23, 24 (2) the fabrication of sulfide-based sorbents is eco-friendly.25 For example, MoS2 nanosheet exhibited excellent Hg0 removal performance at 50 °C, with a Hg0 adsorption capacity as high as 18.95 mg/g.26 We recently efficiently enlarged the surface area of ZnS using nanostructure particles synthesized by a liquid-phase precipitation method

27

. The ZnS with large surface area also showed far greater Hg0

adsorption capacity than the conventional bulk ZnS sorbent at typical flue gas temperature. Compared to commercial ACs, the ZnS was superior in binding capacity and binding rate, and therefore, is considered to be an advantageous alternative to ACs for Hg0 removal in power plants. Motivated by the promising experimental Hg0 capture performance of ZnS, we intend to understand the mechanism of such process using fundamental methods. In this following work, density functional theory (DFT) calculations were performed to investigate the binding of Hg0 on the most stable surface of ZnS, (110) face.28 In addition, the binding of various mercury species, such as Hg+ (HgCl) and Hg2+ (HgCl2), as well as other flue gas components, such as water vapor (H2O), sulfur dioxide (SO2), and hydrogen chloride (HCl), were also studied. We aim to explore the binding mechanism of Hg0 on ZnS and guide our future experiments regarding on the mercury capture performance of ZnS in real flue gas.


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Computational Details The most stable bulk ZnS form is sphalerite which is stable up to 1020 °C at ambient pressure.29 The (110) surface of sphalerite was reported to have the lowest surface energy (0.53 J/m2) among all surfaces,28 and thus was chosen as a representative model in this study. Figure 1(a) shows the zinc sulfide unit cell and Fig. 1(b) shows the cleaved (110) plane of ZnS based on 2×2×4 unit cells, which is used in all of the following calculations furtherly. During geometry optimizations, the atoms on the top two layers were allowed to relax, while those on the rest three layers were fixed in their bulk positions to save computational cost. Two distinguished binding sites were defined as A and B for the binding of each adsorbate specie, as shown in Fig. 1(c). Geometrically, site A locates at a triangle area formed by one Zn atom and two S atoms, while site B locates at a quadrangle area formed by two Zn atoms and two S atoms. The targeting molecule (Hg0, HgCl, HgCl2, H2O, SO2 or HCl) was initially placed on the above sites via different approaches (parallel or perpendicular), which was subsequently optimized to find the most stable configuration followed by energy calculation. The DFT-based geometry optimizations and energy calculations were conducted with DMol3 package.30 The PBE exchange-correlation functional with double numeric polarization (DNP)

31

basis

set was adopted in all calculations. Spin polarization correction was taken into account in all calculations. Special DFT effective core potential (ECP) was used to set the types of core treatment. A real-space orbital global cutoff of 4.1 Å was applied, and the convergence threshold parameters for the optimization were 1×10-5 (energy), 4×10-3 (gradient), and 5×10-3 (displacement), respectively. Binding energies (BEs) were calculated according to the following equation. BE = E(AB) − (E(A) + E(B))

(1)


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where, E(AB) represents the total energy of the adsorbate/substrate system, E(A) is the total energy of the isolated adsorbate at its equilibrium geometry, and E(B) is the total energy of the substrate. Binding energy is the energy released to assemble a new stable system from two or more separate systems (described as adsorbate and substrate system). A more negative BE value corresponds to a more stable adsorbate/substrate system. Results and Discussions

The analysis of ZnS(110) surface properties The surface properties of adsorbents or catalyst, especially electrostatic characteristics, provide fundamental understandings of their performance. As shown in Fig. 1(d) "I", "II" atoms, the crystal structure of ZnS is tetragonal symmetrical (space group F-43m) with each S anion coordinating with four Zn cations and vice versa, but on the top layer with each S anion coordinating with three Zn cations and vice versa as show in Fig.1(d) “III”, “IV” atoms. The ions on the top layer exposed to air are saturated by H- or OH- usually, whereas are activated upon heating, which is a necessary step before Hg0 capture step, to serve as “active” sites for Hg0 binding.27 As shown in Fig. 1, both of Zn and S ions on (110) surface are exposed to the atmosphere. The electrostatic potential of ZnS(110) surface (Fig. 2a) shows that the exposed Zn cations possess strong positive potentials, while the exposed S anions possess strong negative potentials. In addition, the Mulliken charges of the exposed Zn cations and S anions, obtained by Mulliken population analysis, are significantly enhanced comparing with unexposed ions, as shown in Fig. 2b. The Mulliken charge of exposed S anion is -0.24e, approximately 2.18 times higher than that of unexposed ones on lower layers; while for exposed Zn ions, the enhancement is 1.5 times. Even the Mulliken population value is relatively small

32, 33


(probably due to the difference 6

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among model sizes, computational functionals and specific settings), one can expect that these enhanced electrostatic characteristics are beneficial to excite the outer-shell electrons of Hg0 and thereby enhance the binding strength of materials. Hg0 binding on ZnS(110) surface First of all, the binding mechanism of Hg0 on Zn(110) surface were investigated. A Hg0 atom was initially placed on different binding sites of ZnS(110) surface, followed by geometry optimization to obtain the most stable configurations (1A and 1B), as shown in Fig. 3. Unexpectedly, Hg0 prefers to interact with Zn cations other than S anions, which is evidenced by the higher BE of the former (1A, 87.80 kJ/mol) compared to that of the latter (1B, -74.46 kJ/mol), as shown in Fig. 3. Specifically, the closest distance between Hg0 and the nearest Zn cation is approximately 2.97 Å, which is slightly lower than the summation of the radius of Hg atom and Zn atom (1.71 Å and 1.42 Å, respectively), suggesting the formation of Hg-Zn bond,34 as shown in configuration 1A in Fig. 3. This is in agreement with the common sense that mercury atoms can combine with zinc atoms to form zinc amalgam.35 Besides the Hg-Zn bond, the binding of Hg0 is further stabilized by the nearby S anions (1A, top view), which is in line with the literature that Hg0 atom can interact directly with the sulfur atom on a sorbent surface.36 However, no Hg-Zn bond was observed in 1B due to the steric hindrance by the bulgy S anions, as shown in Fig. 3. Hg0 is mainly stabilized by the multi-interactions with the closest S anions and Zn —

cations in 1B. Comparing with CaO(100),37 MnO2(110),38 α-Fe2O3(11 02),39 alloys,40 and activated carbons,41, 42 ZnS(110) shows much stronger or comparative binding strength over Hg0. Further insights into the mechanism of Hg0 binding on ZnS(110) surface were revealed by density of


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states (DOS) analysis which is a general method to describe the number of states per interval of energy at each energy level that are available to be occupied by electrons.43 Fig. 4 shows the partial density of states (PDOS) of Hg0, Zn, S1 and S2 for configuration 1A. As shown in Fig. 4a, the peak at approximately -4.5 eV corresponds to the binding interaction between the Hg d-orbital and Zn s-orbital, p-orbital, d-orbital, while the perks from -3 eV to 0 eV show the hybridization of the s-orbital and dorbital of Hg0 atom with the s-orbital, p-orbital and d-orbital of Zn cation. Besides, Hg0 and Zn also generate relative weak orbital hybridization at -6 eV and -11.5 eV. Similarly, strong hybridizations of Hg0 orbitals and S1 or S2 orbitals are also observed, as shown in Fig. 4b and 4c. Thus, the stabilization of Hg0 atom binding on ZnS(110) surface can be attributed to the multiple interactions between Hg0 and S anions as well as between Hg0 and Zn cations. One should note the significance of the Hg-Zn bond formed during Hg0 binding process for explaining the high experimental Hg0 capture efficiency of nano-ZnS sorbents at high temperatures (140-260 oC).27 In any mercury capture process, Hg0 is first stabilized on the surface of sorbents followed by the further oxidation (Hg0 → Hg2+) at a given temperature to overcome the energy barrier. The strength of Hg0 stabilizing on surface, or as-called binding strength in this work, determines the Hg0 capture efficiency of sorbents. For ZnS, Hg0 on the surface is stabilized by the formation of Hg-Zn bond and thereby is hypothesized to combine with S anion to form HgS on the surface at high temperature, as observed in experiments.27

HgCl binding on ZnS(110) surface HgCl is an intermediate in many mercury conversion processes over gas-solid interfaces in coal


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combustion flue gas,44, 45 thus it is necessary to investigate the binding of HgCl on ZnS surface. Fig. 5 shows the most stable binding configuration for HgCl on ZnS(110) surface. As shown in Fig. 5, Cl_HgCl and Hg_HgCl atoms multi-interact with two neighboring Zn cations simultaneously with distances of 2.30 Å and 2.86 Å, respectively, whereas such multi-interactions are absence in other stable configurations (shown in Fig. S1). The corresponding BE is -174.33 kJ/mol, suggesting a strong chemisorption. In addition, the Hg_Cl bond is enlarged from 2.51 Å to 2.96 Å, evidencing that the adsorbed HgCl tends to be dissociated on ZnS(110) surface upon binding. Thus we can conclude that: (1) HgCl intends to be binding on the Zn cation, instead of S anion. (2) Multi-interactions between HgCl and ZnS are helpful to further stabilize the binding of HgCl. More stable configurations of HgCl on ZnS(110) surface are available in the supporting materials.

HgCl2 binding on ZnS(110) surface In addition to HgCl, HgCl2 is considered as the main oxidized mercury species in coal combustion flue gas.46 The binding mechanism of HgCl2 on ZnS(110) surface were also studied. Table 1 summarizes the BEs and geometry information of HgCl2 binding on ZnS in various configurations. Fig. 6 shows the most stable configuration, while additional stable configurations are shown in Fig. S2. Apparently, the binding of HgCl2 is stabilized by the multi-interactions between HgCl2 and ZnS: the BE is only -74.18 kJ/mol for configuration 3B without multi-interactions, while the BE increases to be in the range of -93.36 to -132.79 kJ/mol for other configurations with multi-interactions, an enhancement of 25.86% to 79.01%, as shown in Table 1 and Fig. S2. In addition, HgCl2 prefers to be stabilized on the ZnS surface by forming bonds between Hg_HgCl2 and S anions as well as between Cl_HgCl2 and Zn cations, which induced a highest BE of -132.79 kJ/mol for HgCl2 binding on ZnS(110) surface, as ACS Paragon Plus Environment

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shown in Fig. 6. Thus ZnS can capture HgCl2 from flue gas efficiently.

H2O, SO2, and HCl binding on ZnS(110) surface The bulk flue gas contains a significant amount of water vapor, sulfur dioxide and chlorine hydride, which have been reported to affect the mercury capture performance for various adsorbents.44,

47-50

Theoretical investigations concerning the binding properties of these impurities on ZnS surface will provide useful information on evaluating the performance of ZnS for mercury capture. As polar molecule, H2O usually significantly affects the performance of adsorbents or catalysts in various applications.51 The effect of water vapor on mercury capture is complicated: on the one hand, increased humidity level may suppress Hg0 binding and result in significant reemission of the captured Hg0 from nano-sorbents;52 and on the other, water vapor may also facilitate mercury binding because of the competitive binding of water vapor over other harmful components on active sites.53 More importantly, water vapor may also affect the stability of ZnS. For example, sphalerite ZnS was found to be more stable after exposure to water vapor.54 Therefore, investigating the binding of water on ZnS(110) surface is meaningful. Fig. 7a shows the most stable configuration (4A) of H2O binding on ZnS(110) surface. The highest BE of H2O on ZnS surface is -55.08 kJ/mol, suggesting a weak chemical binding. Further calculations concerning Hg0 binding on H2O stabilized ZnS surface was also performed. Remarkably, the highest BE of Hg0 significantly increased from -87.80 kJ/mol (1A in Fig. 3) to -106.80 kJ/mol (4A’ in Fig. 7), approximately an improvement of 21.64%. Thus it is expected that the presence of water vapor has a positive effect on Hg0 capture, though this effect should be reconsidered carefully when regarding the oxidation of Hg0 to Hg2+. As an essential component of coal combustion flue gas, SO2 with polarity may impose different effect ACS Paragon Plus Environment

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on Hg0 binding.37, 49, 50 ZnS was reported to be chemically stable under acidic conditions.22 Table 2 summarizes the BEs of SO2 binding on ZnS surface in various configurations (Fig. S3). As shown in Table 2, SO2 prefers to be stabilized on ZnS surface in configuration 5A (Fig. 8) with a BE value of 63.83 kJ/mol, approximately 40% lower than that of Hg0. Geometrically, O_SO2 interact with two exposed Zn cations simultaneously in distance of 2.35 and 2.36 Å. When considering Hg0 binding on SO2 stabilized ZnS, the highest BE of Hg0 (-85.52 kJ/mol, 5A’ in Fig. 8) has almost no change compared to bare Hg0 binding (-87.80 kJ/mol, 1A in Fig. 3). Accompany with the much lower BE of SO2 than that of Hg0, such phenomenon clearly indicates that the binding of SO2 has a negligible effect on Hg0 binding on ZnS. We finally investigated the binding of HCl, the most important flue gas component affecting Hg0 removal,46 on ZnS surface. Fig. 9 shows the stable configurations of HCl on ZnS(110). HCl can be stabilized on the ZnS surface either by physisorption (BE = -35.05 kJ/mol, 6A) or by weak chemisorption (BE = -56.86 kJ/mol, 6B). In 6B, the distance between H_HCl and Cl_HCl increased from 1.29 Å (before binding) to 1.36 Å (after binding), suggesting HCl has dissociation trend after binding on ZnS(110) surface.55 While the BE of Hg0 on HCl stabilized ZnS surface (-86.66 kJ/mol, 6B’ in Fig. 9) is comparable to that of bare Hg0 binding (-87.80 kJ/mol, 1A in Fig. 3), indicating that HCl can hardly affect the binding of Hg0. Conclusion Hg0 is able to be chemisorbed on the activated ZnS(110) surface with a BE as high as -87.80 kJ/mol. The orbitals of Hg0 are highly hybridized by the orbitals of Zn cations on the surface. HgCl and HgCl2 can multi-interact with the S anions and Zn cations, which induce strong chemisorptions on ZnS


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surface. Although the impurities in flue gas, i.e. H2O, SO2, and HCl, can also be adsorbed on ZnS(110) surface, their binding strengths are significantly lower than that of mercury species. These impurities have either positive effect (H2O) or negligible effect (SO2 and HCl) on Hg0 binding. Our theoretical calculations support the previous experimental observations that ZnS can capture Hg0 efficiently from flue gas strongly. Considering the strong binding affinity and selectivity of Hg0 over gas impurities predicted in this work, we announce that ZnS has great potential to serve as efficient mercury capture material for coal-fired power plants. Our future works will focus on the oxidation mechanism of Hg0 on ZnS surface to HgS, which is another essential step for mercury capture. We will also consider various Zn-related materials including pure Zinc to deeply explore the role of sulfur on enhancing Hg0 binding. Acknowledgments The work was supported by the National Science Foundation of China (51476189), Hong Kong Scholarship Program (NO. XJ2014033) and General Research Fund (17257616) of the Research Grants Council of Hong Kong

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Zhao, H.; Yang, G.; Gao, X.; Pang, C. H.; Kingman, S. W.; Wu, T., Hg0 Capture over CoMoS/gamma-Al2O3 with MoS2 Nanosheets at Low Temperatures. Environ. Sci. Technol. 2016, 50, 1056-1064.


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Li, H.; Zhu, L.; Wang, J.; Li, L.; Shih, K. Development of nano-sulfide sorbent for efficient removal of elemental mercury from coal combustion fuel gas. Environ. Sci. Technol. 2016, 50, 9551-9557.

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Steele, H. M.; Wright, K.; Hillier, I. H. A quantum-mechanical study of the (110) surface of sphalerite (ZnS) and its interaction with Pb2+ species. Phys. Chem. Miner. 2003, 30, 69-75.

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Blake, M. P.; Kaltsoyannis, N.; Mountford, P. Synthesis, molecular and electronic structure, and reactions of a Zn-Hg-Zn bonded complex. Chem. Commun. 2015, 51, 5743-5746.

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Lim, D. H.; Aboud, S.; Wilcox, J. Investigation of adsorption behavior of mercury on Au(111) from first principles. Environ. Sci. Technol. 2012, 46, 7260-7266.


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Zhang, B.; Liu, J.; Zheng, C.; Chang, M. Theoretical study of mercury species adsorption mechanism on MnO2(110) surface. Chem. Eng. J. 2014, 256, 93-100.

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Jung, J. E.; Geatches, D.; Lee, K.; Aboud, S.; Brown, G. E.; Wilcox, J. First-principles investigation of mercury adsorption on the α-Fe2O3(11̅02) surface. J. Phys. Chem. C 2015, 119, 26512-26518.

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Aboud, S.; Sasmaz, E.; Wilcox, J. Mercury adsorption on PdAu, PdAg and PdCu alloys. Main Group Chem. 2008, 7, 205-215.

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Liu, J.; Cheney, M. A.; Wu, F.; Li, M. Effects of chemical functional groups on elemental mercury adsorption on carbonaceous surfaces. J. Hazard. Mater. 2011, 186, 108-113.

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Negreira, A. S.; Wilcox, J. DFT study of Hg oxidation across vanadia-titania SCR catalyst under flue gas conditions. J. Phys. Chem. C 2013, 117, 1761-1772.

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Liu, J.; He, M.; Zheng, C.; Chang, M. Density functional theory study of mercury adsorption on V2O5(001) surface with implications for oxidation. P. Combust. Inst. 2011, 33, 2771-2777.

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Liu, T.; Xue, L.; Guo, X.; Huang, Y.; Zheng, C. DFT and experimental study on the mechanism of elemental mercury capture in the presence of HCl on α-Fe2O3(001). Environ. Sci. Technol. 2016, 50, 4863-4868.

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Li, H.; Wu, C. Y.; Li, Y.; Li, L.; Zhao, Y.; Zhang, J. Role of flue gas components in mercury oxidation over TiO2 supported MnOx-CeO2 mixed-oxide at low temperature. J. Hazard. Mater. 2012, 243, 117-123.

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Ling, L. X.; Zhao, S. P.; Han, P. D.; Wang, B. J.; Zhang, R. G.; Fan, M. H. Toward predicting the mercury removal by chlorine on the ZnO surface. Chem. Eng. J. 2014, 244, 364-371.


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List of Tables Table 1. The BE (kJ/mol) and optimized parameters (Å) for HgCl2 binding on ZnS(110) surface. Table 2. The BE (kJ/mol) and optimized parameters (Å) for SO2 binding on ZnS(110) surface.


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Energy & Fuels

List of Figures Fig. 1. Slab models of ZnS(110) surface. (a) ZnS unit cell; (b) the establishment of the 2√2×4 crystal surface; (c) top view of ZnS(110) with two different binding sites; (d) side view of ZnS(110) surface. The yellow and gray spheres represent S and Zn atoms, respectively. Fig. 2. (a) Electrostatic potential mapped on the electron charge density isosurface of ZnS(110) surface. (b) Mulliken population analysis labeled on ZnS(110) surface. Fig. 3. Stable optimized geometries of Hg0 on ZnS(110) surface. (Atoms are represented as Hg, pink; S, yellow; Zn, gray). Fig. 4. 1A configuration PDOS for surface system after Hg0 adsorbed on ZnS(110) surface with (a) Zn site (b) S1 site (c) S2 site. Fig. 5. Most stable optimized geometries of HgCl on ZnS(110) surface. (Atoms are represented as Cl, green; Hg, pink; S, yellow; Zn, gray). Fig. 6. Most stable optimized geometries of HgCl2 on ZnS(110) surface. (Atoms are represented as Cl, green; Hg, pink; S, yellow; Zn, gray). Fig. 7. The stable optimized geometry of H2O on ZnS(110) surface and its effect on Hg0 adsorption. (Atoms are represented as O, red; H, white; S, yellow; Zn, gray; Hg, pink). Fig. 8. Most stable optimized geometries of SO2 on ZnS(110) surface and its effect on Hg0 adsorption. (Atoms are represented as O, red; S, yellow; Zn, gray; Hg, pink). Fig. 9. Stable optimized geometries of HCl on ZnS(110) surface and its effect on Hg0 adsorption. (Atoms are represented as Cl, green; H, white; S, yellow; Zn, gray; Hg, pink).


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Page 22 of 32

Table 1.The BE (kJ/mol) and optimized parameters (Å) for HgCl2 binding on ZnS(110) surface. BE

RHg-S

RZn-Cl

RHg-Cl

RS-Cl

3A

-132.79

2.48

2.52, 2.52

2.50, 2.50

_

3B

-74.18

_

3.57

2.32, 2.31

3.61

3C

-115.28

2.42

2.28

2.97, 2.34

_

3D

-109.22

2.78

2.56

2.37, 2.42

_

3E

-111.48

2.75

2.54

2.37, 2.43

_

3F

-113.39

2.88, 2.92

2.68, 2.72

2.46, 2.40

_

3G

-114.54

3.04, 3.06

3.66, 2.60

2.36, 2.40

_

3H

-93.36

2.98

3.80, 3.70

2.34, 2.35

3.50


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Energy & Fuels

Table 2.The BE (kJ/mol) and optimized parameters (Å) for SO2 binding on ZnS(110) surface. BE

RO-S

RO-Zn

RS-S

5A

-63.83

1.51, 1.51

2.35, 2.36

2.62

5B

-47.36

1.48, 1.52

2.30

2.78

5C

-37.72

_

2.34

3.20, 3.24

5D

-27.00

_

_

3.05

5E

-16.07

_

_

3.76, 3.76

5F

-26.15

_

2.43

_


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Fig. 1. Slab models of ZnS(110) surface. (a) ZnS unit cell; (b) the establishment of the 2√2×4 crystal surface; (c) top view of ZnS(110) with two different binding sites; (d) side view of ZnS(110) surface. The yellow and gray spheres represent S and Zn atoms, respectively.

4 unit cells

2 unit cells 2√2 unit cells 2 unit cells

(a)

(b)

A B

(c)

(d)


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Energy & Fuels

Fig. 2. (a) Electrostatic potential mapped on the electron charge density isosurface of ZnS(110) surface. (b) Mulliken population analysis labeled on ZnS(110) surface. (a)

(b)


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Fig. 3. Stable optimized geometries of Hg0 on ZnS(110) surface. (Atoms are represented as Hg, pink; S, yellow; Zn, gray). Top view

Side view

1A

1B


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Energy & Fuels

Fig. 4. 1A configuration PDOS for surface system after Hg0 adsorbed on ZnS(110) surface with (a) Zn site (b) S1 site (c) S2 site.

(a)

(b)

(c)


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Fig. 5. Most stable optimized geometries of HgCl on ZnS(110) surface. (Atoms are represented as Cl, green; Hg, pink; S, yellow; Zn, gray). Top view

Side view

2C


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Energy & Fuels

Fig. 6. Most stable optimized geometries of HgCl2 on ZnS(110) surface. (Atoms are represented as Cl, green; Hg, pink; S, yellow; Zn, gray). Top view

side view

3A


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Fig. 7. The stable optimized geometry of H2O on ZnS(110) surface and its effect on Hg0 adsorption. (Atoms are represented as O, red; H, white; S, yellow; Zn, gray; Hg, pink). Top view

Side view

4A

4A’


30

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Energy & Fuels

Fig. 8. Most stable optimized geometries of SO2 on ZnS(110) surface and its effect on Hg0 adsorption. (Atoms are represented as O, red; S, yellow; Zn, gray; Hg, pink). Top view

Side view

5A

5A’


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Fig. 9. Stable optimized geometries of HCl on ZnS(110) surface and its effect on Hg0 adsorption. (Atoms are represented as Cl, green; H, white; S, yellow; Zn, gray; Hg, pink). Top view

Side view

6A

6B

6B’


32

Mercury Species and Typical Flue Gas Components Binding on ZnS

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