Density Functional Theory Study of Mercury Adsorption on CuS Surface

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A DFT Study of Mercury Adsorption on CuS Surface: Effect of Typical Flue Gas Components Hailong Li, Shihao Feng, Zequn Yang, Jianping Yang, Suojiang Liu, Yingchao Hu, Lan Zhong, and Wenqi Qu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03585 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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Energy & Fuels

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A DFT Study of Mercury Adsorption on CuS Surface: Effect of Typical Flue Gas Components

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Hailong Lia, Shihao Fenga,d, Zequn Yangb, Jianping Yanga, Suojiang Liua, Yingchao Hua, Lan

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Zhongc, Wenqi Qua*

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a. School of Energy Science and Engineering, Central South University, Changsha, 410083, China

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b. Department of Civil Engineering, The University of Hong Kong, Hong Kong, Hong Kong SAR,

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China

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c. Deparment of Science and Technology, Hunan Institute of Metrology and Test, Changsha, 410083,

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China

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d. School of Metallurgy and Environment, Central South University, Changsha, 410083, China

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Submitted to

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Energy & Fuels

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13 14 15

*To whom correspondence should be addressed:

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TEL: (86) -731-88876554

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FAX: (86) -731-88876554

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Email:

[email protected]

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ABSTRACT: Copper sulfide (CuS) has been proved to be a potentially alternative to traditional

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sorbents for control of elemental mercury (Hg0) emissions downstream of the wet flue gas

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desulphurization (WFGD) systems. However, the detail reaction mechanisms involved in Hg0

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adsorption over CuS surface are still unclear. The density functional theory was applied to

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investigate Hg0 adsorption over CuS(001) surface. The results indicated that chemisorption

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mechanism was responsible for Hg0 adsorption over CuS(001) surface. The formation of Hg-S and

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Hg-Cu bonds was confirmed by depicting the projected densities of states profiles. The binding

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energies of Hg0 suggested that the crystal surface with two sulfur terminations (labeled

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CuS(001)-S-2) exhibited a better Hg0 adsorption activity than the crystal surface with copper and

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sulfur terminations (labeled CuS(001)-Cu/S). Moreover, the adsorption of the flue gas compontents

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downstream of WFGD (oxygen, sulfur dioxide and water vapor) were studied to understand the

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effect of the flue gas components on Hg0 adsorption over CuS surface. The slight competitive

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adsorption further identified the negligible influence of oxygen, sulfur dioxide and water vapor on

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Hg0 adsorption in previous experiments.

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KEYWORDS: Mercury, Copper sulfide, Adsorption, Density functional theory, Flue gas


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1. Introduction

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Although renewable and clean energy is playing a more and more important role in the energy

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structure and strategy in China, coal is still the main energy consumption which accounts for about

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64 percents.1 The emissions of numerous atmospheric pollutants during coal combustion have

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caused serious harm to both the environment and human health. Among the various hazardous trace

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elements, mercury is regarded as a global and highly toxic pollutant due to its strong volatility,

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persistence and bio-accumulation.2,3 According to the “Global Mercury Assessment Report”

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announced by the United Nations Environment Programme (UNEP) in 2013, coal-fired utilization in

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power plants is the main source of atmospheric mercury pollution.4 Subsequently, in August 2017,

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the Minamata Convention which has been generally accepted as the global legal binding force was

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signed to limit mercury emission by over 163 countries.5 Therefore, it is urgent to develop a highly

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efficient, economical and eco-friendly mercury-removing technology for coal-fired power plants.

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Elemental mercury (Hg0) in coal combustion flue gas is not only the main mercury species but

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also the most difficult to control.6-11 By far, adsorbents injection is the most mature technique for

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elemental mercury control. The applications of powdered activated carbon injected (ACI) prior to

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the particulate control devices has been demonstrated to be an effective,12,13 reliable option to meet

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emission standards from over 50 full-scale demonstrations.14 However, the ACI exhibited several

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shortcomings like low adsorption rate, high operating cost, and evidently negative effect of several

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flue gas components.15 Hence, impregnated activated carbons have been developed to improve the

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adsorption

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sulfur-impregnated activated carbons have shown to be an effective material mainly due to its high

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porosity with efficient micropores, and the strong affinity between mercury and sulfur.21,22

performance

of

activated

carbons

in

Hg0


adsorption.16-20

Among

them,

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Nevertheless, the problems related to carbon material such as the adverse effect on fly ash utilization

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still exist.

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On the contrary, metal sulfides can overcome most of the disadvantages of carbon-based

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adsorbents and exhibite large mercury adsorption capacity,11,22-28 for example, nano-structured CuS

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(Nano-CuS) has preferable mercury removal performance due to its Cu/S ratio is over 0.5.29 It was

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evidenced that the optimum temperature range for mercury removal by CuS was 25-125 °C and the

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adsorption capacity of Nano-CuS for Hg0 reached 122.40 mg Hg0/g.29 CuS was even positive in the

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subsequent utilization of fly ash since it can be used as a self-repairing additive for lime crusher in

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cement processing.30 Moreover, as a kind of mineral sulfides, the fabrication of CuS is simpler and

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environmentally benign. In our previous work, nanostructured CuS materials were applied to remove

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mercury from coal combustion flue gas between the WFGD and WESP systems, which warrants it

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free of the detrimental influence of the NOx.29,31 The Brunauer-Emmett-Teller (BET) surface area of

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the synthesized CuS materials was tested to characterize the number of active sites of the prepared

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adsorbents. Combined with Hg0 adsorption performance, it was concluded that the active sites

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played an important role in mercury adsorption over CuS. Compared with other mercury adsorbents,

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flue gas components such as SO2 and H2O slightly inhibited Hg0 adsorption over nano-CuS.29 Since

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the active sites can situate on different crystallo graphic positions associate with the characteristics

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of the CuS crystal surface, the binding affinity between mercury and CuS materials may vary from

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crystal structures. However, the Hg0 adsorption performance of CuS materials with different active

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sites is somewhat hard to confirm by experimental studies. Although S sites are expected to be the

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adsorption sites for Hg in some literatures, the sulfur sites of MoS2 and FeS2 are not active for Hg0

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on the surface.32,33 The responsible active sites for Hg0 bonding are still needs further exploration.

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More importantly, the Hg0 adsorption mechanisms over the Nano-CuS materials in flue gas and the 4 ACS Paragon Plus Environment

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detailed inherent correlations between Hg0 and flue gas components over CuS surface are still

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unclear.

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Nowadays, computational calculations have become more feasibly to predict active sites of

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conformational structures of different reactants.34-37 Specifically, quantum chemical methods based

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density functional theories can provide the parameters of reactants and products to understand the

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reaction process.38-48 Based on the molecular structure characteristics of CuS and micro-reaction

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mechanism, it can help in designing more reasonable and efficient CuS materials, and optimizing the

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operation condition of mercury removal from coal combustion flue gas. In this work, the periodic

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model of CuS was established, the bulk lattice constants and stability adsorption location were

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obtained by the geometry optimization. The projected density of states (PDOS), populations of the

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Mulliken charge and binding energies were analyzed to understand the adsorption of Hg0 over CuS

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surface. The impact of NOx and HCl are insignificant since the water-soluble NO2 and HCl are fully

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removed by the WFGD system. Therefore, the effects of other flue gas components (H2O, O2 and

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SO2) on Hg0 adsorption were discussed.

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2. Computational details

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2.1. Model development

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In our previous experimental section, CuS was synthesized based on the liquid-phase precipitation

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method.29 The composition and structure of synthesized CuS were investigated by X-ray diffraction

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(XRD) and X-ray photoelectron spectroscopy (XPS) spectra29 as shown in Figure 1. It can be

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concluded that CuS is the only product with the mainly exposed crystal surface (001, 101, 102, et

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al.), while no impurities such as Cu+ or Cu0 were detected. Hence, the hexagonal structure of CuS

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with space group P63/mmc (as shown in Figure 2a, a=b=3.796 Å, c=16.360 Å, α=β =90°, γ=120°)

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was adopted in the present study. 5 ACS Paragon Plus Environment

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The (001) surface of CuS has been increasing interest among all the direction due to its lower

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surface free energy.49,50 There are some kinds of crystal orientations with different chemical

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properties, particularly in their absorbability and heat stability. The S-S dimers along the (001)

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direction are the main crystallo graphic planes at the atmospheric pressure from room temperature to

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125°C. While the surface reconstruction was discovered as Cu/S termination when the temperature

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was over 125°C.49 The bulk structures of CuS(001)-Cu/S and CuS(001)-S-2 were established to

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simulate the two crystallo graphic planes at different temperature range. Figure 2 displays the top

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view and the side view of the two CuS(001) surface configurations. In Figure 2b, I, II, III, IV and V

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represent the Cu-top, S-top, hollow, Cu-bridge and S-bridge sites on CuS(001)-Cu/S surface

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respectively. In Figure 2c, the CuS(001)-S-2 surface includes six different types of surface

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adsorption sites, including Ssuf-top, Cusub-top, hollow, S-bridge (the site between Ssuf and Ssub),

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Ssub-bridge, and Cusub-bridge sites which are denoted as VI, VII, VIII, IX, X and XI respectively.

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An 8-layer slab with a (3×3) unit cell based on the a (3×3×1) primitive cell was used to model the

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structure of CuS(001) surface. In order to ignore the spurious interactions of the layers, the vacuum

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region between slabs is configured as 15 Å. The bottom three layers were fixed and the top five

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layers were relaxed during optimized calculating for geometry. Gas-phase species (Hg0, H2O, O2,

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and SO2) were also optimized as isolated molecules in a large crystal cell of 10×10×10 Å.

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2.2. Computational methodology

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All calculations were carried out in the framework of spin-polarized density functional theory by

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employing Dmol3 program package in Materials Studio 8.0. The exchange correlation potential was

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calculated using Perdew-Burke-Ernzerhoff (PBE) approximation in the place of the generalized

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gradient approximation (GGA) scheme.51 The interaction between valance electrons, inner electrons


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and atomic nucleus was performed with the double numerical basis sets plus polarization functional

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(DNP).

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The Monkhorst-Pack scheme k-points grid of 2×2×1 was used to simplify the Brillouin zone in the

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cell of CuS (001) and the real space basis set functions are truncated to 4.0 Å. The parameters

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criteria for the tolerances of energy, force, displacement, and SCF convergence criteria are 2×10-5

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Ha, 4×10-3 Ha/Å, 5×10-3 Å, and 10-5, respectively. A Methfessel-Paxton smearing of 0.005 Ha is

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used to improve calculation performance.

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The binding energies (BEs) of Hg0 and other flue gas components over CuS surface was calculated by the following expression: BE = E(AB) – [E(A) +E(B)]

(1)

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where the total energy of the adsorbate/substrate, the isolated adsorbate and the substrate are

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represented by E(AB), E(A) and E(B).

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In addition, the difference by subtracting the binding energy of Hg atom from the binding energies

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of gas phase species over CuS surface was evaluated to study the competitive adsorption of Hg0 and

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flue gas components, as the following expression:

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△E = BEHg-BEflue gas

(2)

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Under this definition, Hg0 is easier to be adsorbed on the activate sites of CuS if △E is a negative

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value. The molecular of flue gas components is easier to be adsorbed on the activate sites if △E is a

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positive value. The larger the negative △E, the higher influence on the adsorption of Hg0 over CuS

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surface.

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3. Results and discussion

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The structures of CuS(001)-Cu/S and CuS(001)-S-2 surface were optimized, and the bond lengths

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of Cu-S and S-S in the models were calculated to be 2.300 Å, 2.116 Å, respectively, The error is 7 ACS Paragon Plus Environment

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0.17% and 1.7% though comparing with the experimental values of 2.304 Å, 2.081 Å,52 which

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indicates the optimized results agree well with the experimental data. As shown in Figure 2(d), the

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Cu atoms on the top of CuS(001)-Cu/S move inwards from the surface. For CuS(001)-S-2, the Cu

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atoms at the fourth level move upwards (see Figure 2(e)), presumably these are caused by the

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decrease of coordinating atoms of Cu and S on the second and third layers.

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3.1. Hg0 adsorption over CuS(001)-Cu/S surface

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Hg0 adsorption on different active sites of CuS(001)-Cu/S surface was investigated, and the

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optimized bond lengths, Mulliken charge and binding energies are present in Table 1. The stable

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optimized configuration of adsorbates for Hg0 over CuS(001)-Cu/S surface are present in Figure 3.

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For model 1A in which the Hg0 atom was adsorbed on the site of Cu-top, the calculated binding

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energy is -57.30 kJ/mol. The distance between Hg0 and Cu is 3.01 Å, and the number of electrons

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transfer from Hg0 atom to CuS(001)-Cu/S surface is 0.04 e. Therefore, it can be concluded that the

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adsorption of Hg0 over Cu-top active sites of CuS(001)-Cu/S surface is weakly chemisorption. For

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model 1B in which the equilibrium distance between Hg and S atoms is 3.85 Å and the binding

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energy is -55.17 kJ/mol. Hg0 is adsorbed on the S-bridge sites with 0.02 electron transfer of Hg0. For

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model 1C, Hg0 was adsorbed on the hollow site of copper-sulfur six-membered ring with the binding

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energy -55.24 kJ/mol. The adsorption of Hg0 over every site of CuS(001)-Cu/S surface belong to

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chemisorption with the stability in the order of 1A>1C>1B.

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The projected densities of states (PDOS) profiles for Hg0 atom adsorbed CuS(001)-Cu/S surface

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was carried out to explore the Hg-Cu bonding nature. It can be seen from Figure 4 that Hg s orbitals

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with strongly mixed Cu d orbitals acted as the predominant contributor for the valence bond nearby

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-2 eV and -0.8 eV, indicating the formation of chemical bonds between the two atoms. In addition,

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The copper amalgam was detected in the temperature-dependent desorption experiment of copper 8 ACS Paragon Plus Environment

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sulfide adsorbed on mercury.29 Therefore, the calculation results accurately explain the experimental

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phenomenon.

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3.2. Hg0 adsorption over CuS(001)-S-2 surface

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There are four stable optimized configurations of adsorbates by investigating the interaction of

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Hg0 with different activate sites of CuS(001)-S-2 surface, as shown in Figure 5. The Mulliken charge

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of Hg absorbed over the top S of S-2 is 0.01, which is much smaller than other sites (in Table 2).

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While the Mulliken charge of Hg0 in 2A, 2B and 2C are from 0.15 to 0.19 (the corresponding

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binding energies is in the range of -93.20 ~ -99.39 kJ/mol), indicating a strong interaction between

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Hg0 and CuS. The stability of the adsorbates judged by binding energies are in the order of

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2A>2B>2C>2D, which is compatible with the Mulliken charge discussion. Therefore, the S-2 is the

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dominant active site. Compared with ZnS (-87.80 kJ/mol),11 CuS has the stronger binding energy,

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which indicates that the preferable adsorption capacity for elemental mercury. This conclusion is

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also consistent with the experimental result.29

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To further understand the interaction between Hg0 and S atom of CuS(001)-S-2 surface, the PDOS

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projections of 2A are plotted. It is observed from Figure 6 that there is a significant overlap between

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Hg d orbital and S1 p orbital nearby -6.6 eV. In addition, the overlap of Hg s and p orbitals with S1 p

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orbitals nearby 1eV are also observed. Therefore, there is a strong chemical bond between Hg0 and

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S1 atom. Similarly, two HgS were detected in the temperature-dependent desorption experiment of

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copper sulfide adsorbed on mercury.29

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Based on the analysis of Hg0 adsorption over CuS(001)-Cu/S and CuS(001)-S-2, it can be

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concluded that the Hg0 capture capacity of CuS(001)-S-2 is better than CuS(001)-Cu/S. The result

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has been verified by our previous experiments presented in Figure 7.29 As shown, the Hg0 adsorption

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performance of prepared CuS was similar in the range of 25-125°C. While the adsorption capacity 9 ACS Paragon Plus Environment

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decreased obviously at the temperature of 175°C. By the theoretical analysis in this paper, the

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experimental phenomena can be attributed to the changes of orientation from CuS(001)-S-2 surface

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to CuS(001)-Cu/S.

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3.3. Effect of flue gas components on Hg0 adsorption over CuS(001)-S-2 surface

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The adsorption of water vapor, oxygen and sulfur dioxide over CuS surface is discussed with the

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small molecular perpendicular and parallel to the surface. As shown in Figure 8, H2O interact with

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CuS(001)-S-2 surface by two hydrogen atoms and the equilibrium distance of H-S is 2.61 Å; O2 was

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adsorbed vertically on the surface of CuS(001)-S-2 with the oxygen on the bottom adsorbed on

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Ssub-bridge site; SO2 interacts with the sulfur atoms of the CuS(001)-S-2 surface sublayer by

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horizontal adsorption. The binding energies of H2O, O2, and SO2 over CuS(001)-S-2 surface were

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calculated to be -30.60 kJ/mol, -10 kJ/mol and -25.05 kJ/mol, respectively. The adsorption of these

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flue gas components over CuS surface is physical adsorption, which is remarkably different from

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that of other sorbents. Take carbonaceous materials for example, the adsorption of SO2 is strong

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chemisorption with the binding energy larger than -200.0 kJ/mol.53 The value of △E can be obtained

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to be -89.39 ~ -13.20 kJ/mol. As discussed in 2.2 section, it can be concluded that Hg0 is much easier

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to be adsorbed on the activate sites of CuS(001)-S-2 surface than H2O, O2, and SO2 molecular. The

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smallest △E is -13.20 kJ/mol from the adsorption of H2O, when Hg0 was absorbed on the activate

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site of Ssuf-top. Based on these calculated results, it can be concluded that the flue gas components

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can not occupy the main active sites of Hg0 adsorption. The competition adsorption between Hg0 and

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other flue gas components is not obvious over CuS surface.

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In the previous experimental results, the influence of oxygen and sulfur dioxide on Hg0 adsorption

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over CuS has been proved to be ignored which is consistent with our theoretic and computational

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conclusion. Although water vapor was shown to moderately suppress Hg0 capture efficiency which 10 ACS Paragon Plus Environment

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is controversy on our results, the adsorption capacity decline is account for more in the site

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prevention effects than recognized competitive adsorption. To further understand the effect of

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typical flue gas downstream the WFGD systems on Hg0 adsorption over CuS surface, the

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coadsorption calculation of H2O and Hg0, O2 and Hg0, SO2 and Hg0, all three components and Hg0

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were conducted over the CuS(001)-S-2 surface as shown in Figure S1. The Mulliken charge of Hg0

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is 0.22, 0.21, 0.21 and 0.28, respectively. It can be concluded that the charge transfer of Hg0 is

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promoted during the coadsorption of flue gas components and Hg0 over CuS(001)-S-2 surface.

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4. Conclusions

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DFT calculations were performed to compare the experimental studies of Hg0 adsorption over CuS

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surface. The crystallo graphic planes of CuS under different temperature ranges were established and

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optimized. The adsorption behavior of Hg0, H2O, O2 and SO2 over CuS surface was examined.

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Surface S atoms were identified as the main active site for Hg0 adsorption over CuS surface. H2O,

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O2 and SO2 were mainly adsorbed on the sub-layer S sites. The binding energies of Hg0 indicated

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that CuS(001)-S-2 would be a better Hg0 adsorption site than CuS(001)-Cu/S. H2O, O2 and SO2

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physically adsorb on CuS surface, and their competitive adsorption with Hg0 was slight. Further

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transition state studies by DFT method will be performed to examine the coadsorption reaction of

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flue gas components and Hg0 over CuS surfaces.

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Acknowledgements

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The research project is supported by the National Natural Science Foundation of China (No.

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51776227) and Xiang Jiang Scholars Planning Program (XJ2014033).

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Oxidation by Oxygen over Spinel-Type MnFe2O4. J. Hazard. Mater. 2017, 321, 154-161.

266

(12) Wilcox, J.; Sasmaz, E.; Kirchofer, A.; Lee, S.-S. Heterogeneous Mercury Reaction Chemistry

267 268 269 270 271

on Activated Carbon. J. Air Waste Manage. Assoc. 2011, 61, 418-426. (13) Padak, B.; Wilcox, J. Understanding Mercury Binding on Activated Carbon. Carbon 2009, 47, 2855-2864. (14) Sjostrom, S.; Durham, M.; Bustard, C. J.; Martin, C. Activated Carbon Injection for Mercury Control: Overview. Fuel 2010, 89, 1320-1322.

272

(15) Uddin, M. A.; Yamada, T.; Ochiai, R.; Sasaoka, E.; Wu, S. Role of SO2 for Elemental Mercury

273

Removal from Coal Combustion Flue Gas by Activated Carbon. Energy Fuels 2008, 22,

274

2284-2289.

275

(16) Zhong, L.; Li, W.; Zhang, Y.; Norris, P.; Cao, Y.; Pan, W. P. Kinetic Studies of Mercury

276

Adsorption in Activated Carbon Modified by Iodine Steam Vapor Deposition Method. Fuel

277

2017, 188, 343-351.

278

(17) Tong, L.; Yue, T.; Zuo, P.; Zhang, X.; Wang, C.; Gao, J.; Wang, K. Effect of Characteristics of

279

KI-Impregnated Activated Carbon and Flue Gas Components on Hg0 Removal. Fuel 2017, 197,

280

1-7.


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(18) Liu, W.; Vidic, R. D.; Brown, T. D. Impact of Flue Gas Conditions on Mercury Uptake by Sulfur-Impregnated Activated Carbon. Environ. Sci. Technol. 2000, 34, 154-159. (19) Yao, Y.; Velpari, V.; Economy, J. Design of Sulfur Treated Activated Carbon Fibers for Gas Phase Elemental Mercury Removal. Fuel 2014, 116, 560-565.

285

(20) Hsi, H. C.; Tsai, C. Y.; Lin, K. J. Impact of Surface Functional Groups, Water Vapor, and Flue

286

Gas Components on Mercury Adsorption and Oxidation by Sulfur-Impregnated Activated

287

Carbons. Energy Fuels 2014, 28, 3300-3309.

288 289 290 291

(21) Korpiel, J. A.; Vidic, R. D. Effect of Sulfur Impregnation Method on Activated Carbon Uptake of Gas-Phase Mercury. Environ. Sci. Technol. 1997, 31, 2319-2325. (22) Xu, W.; Hussain, A.; Liu, Y. A Review on Modification Methods of Adsorbents for Elemental Mercury from Flue Gas. Chem. Eng. J. 2018, 346, 692-711.

292

(23) Li, H.; Zhu, L.; Wang, J.; Li, L.; Shih, K. Development of Nano-Sulfide Sorbent for Efficient

293

Removal of Elemental Mercury from Coal Combustion Fuel Gas. Environ. Sci. Technol. 2016,

294

50, 9551-9557.

295 296 297 298

(24) Li, H.; Feng, S.; Liu, Y.; Shih, K. Binding of Mercury Species and Typical Flue Gas Components on ZnS(110). Energy Fuels 2017, 31, 5355-5362. (25) Wu, S.; Ozaki, M.; Uddin, M. A.; Sasaoka, E. Development of Iron-Based Sorbents for Hg0 Removal from Coal Derived Fuel Gas: Effect of Hydrogen Chloride. Fuel 2008, 87, 467-474.

299

(26) Zhao, H.; Yang, G.; Gao, X.; Pang, C. H.; Kingman, S. W.; Wu, T. Hg0 Capture over

300

CoMoS/γ-Al2O3 with MoS2 Nanosheets at Low Temperatures. Environ. Sci. Technol. 2016, 50,

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1056-1064.

302 303

(27) Li, H.; Feng, S.; Qu, W.; Yang, J.; Liu, S.; Liu, Y. Adsorption and Oxidation of Elemental Mercury on Chlorinated ZnS Surface. Energy Fuels 2018, 32, 7745-7751. 14 ACS Paragon Plus Environment

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Energy & Fuels

(28) Li, H.; Zhu, W.; Yang, J.; Zhang, M.; Zhao, J.; Qu, W. Sulfur Abundant S/FeS2 for Efficient Removal of Mercury from Coal-Fired Power Plants. Fuel 2018, 232, 476-484.

306

(29) Yang, Z.; Li, H.; Feng, S.; Li, P.; Liao, C.; Liu, X.; Zhao, J.; Yang, J.; Lee, P. H.; Shih, K.

307

Multiform Sulfur Adsorption Centers and Copper-Terminated Active Sites of Nano-Cus for

308

Efficient Elemental Mercury Capture from Coal Combustion Flue Gas. Langmuir 2018, 34,

309

8739-8749.

310 311

(30) Han, Z. Application of Nano Technology in Cement Field. China Powder Sci. Technol. 2006, 3, 30-34+44.

312

(31) Yang, Z.; Li, H.; Liao, C.; Zhao, J.; Feng, S.; Li, P.; Liu, X.; Yang, J.; Shih, K. Magnetic

313

Rattle-Type Fe3O4@CuS Nanoparticles as Recyclable Sorbents for Mercury Capture from

314

Coal Combustion Flue Gas. ACS Appl. Nano Mater. 2018, 1, 4726-4736.

315

(32) Xu, Z.; Lv, X.; Chen, J.; Jiang, L.; Lai, Y.; Li, J. First Principles Study of Adsorption and

316

Oxidation Mechanism of Elemental Mercury by HCl over MoS2(100) Surface. Chem. Eng. J.

317

2017, 308, 1225-1232.

318 319 320 321 322 323 324 325

(33) Yang, Y.; Liu, J.; Liu, F.; Wang, Z.; Miao, S. Molecular-Level Insights into Mercury Removal Mechanism by Pyrite. J. Hazard Mater. 2018, 344, 104-112. (34) Sasmaz, E.; Wilcox, J. Mercury Species and SO2 Adsorption on CaO(100). J. Phys. Chem. C 2008, 112, 16484-16490. (35) Suarez Negreira, A.; Wilcox, J. DFT Study of Hg Oxidation across Vanadia-Titania SCR Catalyst under Flue Gas Conditions. J. Phys. Chem. C 2013, 117, 1761-1772. (36) Sasmaz, E.; Aboud, S.; Wilcox, J. Hg Binding on Pd Binary Alloys and Overlays. J. Phys. Chem. C 2009, 113, 7813-7820.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(37) 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. (38) Suarez Negreira, A.; Wilcox, J. Role of WO3 in the Hg Oxidation across the V2O5-WO3-TiO2 SCR Catalyst: A DFT Study. J. Phys. Chem. C 2013, 117, 24397-24406.

330

(39) Jung, J. E.; Geatches, D.; Lee, K.; Aboud, S.; Brown, G. E.; Wilcox, J. First-Principles

331

Investigation of Mercury Adsorption on the α-Fe2O3(11̅02) Surface. J. Phys. Chem. C 2015,

332

119, 26512-26518.

333 334 335 336

(40) Zhao, L.; Wu, Y.; Han, J.; Lu, Q.; Yang, Y.; Zhang, L. Mechanism of Mercury Adsorption and Oxidation by Oxygen over the CeO2(111) Surface: A DFT Study. Materials 2018, 11, 485. (41) Lim, D. H.; Wilcox, J. Heterogeneous Mercury Oxidation on Au(111) from First Principles. Environ. Sci. Technol. 2013, 47, 8515-8522.

337

(42) Qu, W.; Yang, Y.; Shen, F.; Yang, J.; Feng, S.; Li, H. Theoretical Study on Hg0 Adsorption

338

and Oxidation Mechanisms over CuCl2-Impregnated Carbonaceous Material Surface. Energy

339

Fuels 2018, 32, 7125-7131.

340 341

(43) Liu, Y.; Li, H.; Liu, J. Theoretical Prediction the Removal of Mercury from Flue Gas by MOFs. Fuel 2016, 184, 474-480.

342

(44) Liu, J.; Qu, W.; Yuan, J.; Wang, S.; Qiu, J.; Zheng, C. Theoretical Studies of Properties and

343

Reactions Involving Mercury Species Present in Combustion Flue Gases. Energy Fuels 2010,

344

24, 117-122.

345

(45) Qu, W.; Liu, J.; Shen, F.; Wei, P.; Lei, Y. Mechanism of Mercury-Iodine Species Binding on

346

Carbonaceous Surface: Insight from Density Functional Theory Study. Chem. Eng. J. 2016,

347

306, 704-708.


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(46) Jung, J. E.; Liguori, S.; Jew, A. D.; Brown, G. E.; Wilcox, J. Theoretical and Experimental

349

Investigations of Mercury Adsorption on Hematite Surfaces. J. Air Waste Manage. Assoc.

350

2018, 68, 39-53.

351

(47) Gao, X.; Zhou, Y.; Tan, Y.; Cheng, Z.; Tang, Q.; Jia, J.; Shen, Z. Unveiling Adsorption

352

Mechanisms of Elemental Mercury on Defective Boron Nitride Monolayer: A Computational

353

Study. Energy Fuels 2018, 32, 5331-5337.

354 355

(48) Aboud, S.; Sasmaz, E.; Wilcox, J. Mercury Adsorption on PdAu, PdAg and PdCu Alloys. Main Group Chem. 2008, 7, 205-215.

356

(49) Morales-García, Á.; He, J.; Soares, A. L.; Duarte, H. A. Surfaces and Morphologies of

357

Covellite (CuS) Nanoparticles by Means of Ab Initio Atomistic Thermodynamics. Cryst. Eng.

358

Comm. 2017, 19, 3078-3084.

359 360 361 362 363 364 365 366

(50) Gaspari, R.; Manna, L.; Cavalli, A. A Theoretical Investigation of the (0001) Covellite Surfaces. J. Chem. Phys. 2014, 141, 044702. (51) Delley, B. From Molecules to Solids with the Dmol3 Approach. J. Chem. Phys. 2000, 113, 7756-7764. (52) Berry, L. G. The Crystal Structure of Covellite, Cuse and Klockmannite, Cuse. Am. Mineral. 1954, 39, 504-509. (53) Liu, J.; Qu, W.; Joo, S. W.; Zheng, C. Effect of SO2 on Mercury Binding on Carbonaceous Surfaces. Chem. Eng. J. 2012, 184, 163-167.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

368

List of Tables

369

Table 1. BEs (kJ/mol), optimized parameters (Å) and Mulliken charge (e) population analysis for

370

Hg0 binding on the CuS(001)-Cu/S surface.

371

Table 2. BEs (kJ/mol), optimized parameters (Å) and Mulliken charge (e) population analysis for

372

Hg0 binding on the CuS(001)-S-2 surface.


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Energy & Fuels

Table 1. BEs (kJ/mol), optimized parameters (Å) and Mulliken charge (e) population analysis for Hg0 binding on the CuS(001)-Cu/S surface. Configuration BEs(kJ/mol) RHg-Cu (Å)

RHg-S (Å)

Q(e)

1A

-57.30

3.01

-

0.04

1B

-55.17

3.81

3.85,3.85

0.02

1C

-55.24

4.04,4.06,4.06

3.83,3.82,3.82

0.02


Energy & Fuels 1 378 2 3 4 379 5 6 7 8 9 10 11 12 13 14 15 16 17 380 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 2. BEs (kJ/mol), optimized parameters (Å) and Mulliken charge (e) population analysis for Hg0 binding on the CuS(001)-S-2 surface. Configuration

BEs(kJ/mol)

RHg-Cu (Å)

RHg-S (Å)

Q(e)

2A

-99.39

-

2.58,2.67,2.71

0.19

2B

-98.98

-

2.47,2.46

0.15

2C

-93.20

2.92

2.61,2.73,2.76

0.19

2D

-43.80

-

3.64

0.01


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Energy & Fuels

382

List of Figures

383

Figure 1. XRD pattern of nano-CuS and XPS patterns of Cu 2p of fresh and spent nano-CuS 29.

384

Figure 2. Slab models of CuS(001) surface: (a) CuS unit cell; (b) top view of CuS(001)-Cu/S; (c)

385

top view of CuS(001)-S-2; (d) side view of CuS(001)-Cu/S; (e) side view of CuS(001)-S-2. Atoms

386

are represented as jacinth (Cu) and yellow (S) spheres, respectively. In Figure 2b, I, II, III, IV and V

387


388


389


390

Ssub-bridge, and Cusub-bridge sites which are denoted as VI, VII, VIII, IX, X and XI respectively. In

391

Figure 2(d), the Cu atoms on the top of CuS(001)-Cu/S move inwards from the surface after

392

geometry optimization. In Figure 2(e), for CuS(001)-S-2, the Cu atoms at the fourth level move

393

upwards after geometry optimization.

394

Figure 3. Stable optimized geometries of Hg0 on CuS(001)-Cu/S surface. Atoms are represented as

395

jacinth (Cu), yellow (S) and pink (Hg) spheres, respectively.

396

Figure 4. PDOS for Hg and Cu atoms in 1A configuration.

397

Figure 5. Stable optimized geometries of Hg0 on CuS(001)-S-2 surface. Atoms are represented as

398


399

Figure 6. PDOS for 2A configuration. (a) Hg and S1 atoms; (b) Hg and S2 atoms.

400

Figure 7. Effect of temperature on mercury removal efficiency of nano-CuS 29.

401

Figure 8. Stable optimized geometries of (a) H2O, (b) O2 and (c) SO2 on CuS(001)-S-2 surface.

402

Atoms are represented as jacinth (Cu), yellow (S), red (O) and white (H) spheres, respectively. 21 ACS Paragon Plus Environment

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Energy & Fuels

408

Figure 2. Slab models of CuS(001) surface: (a) CuS unit cell; (b) top view of CuS(001)-Cu/S; (c)

409

top view of CuS(001)-S-2; (d) side view of CuS(001)-Cu/S; (e) side view of CuS(001)-S-2. Atoms

410

are represented as jacinth (Cu) and yellow (S) spheres, respectively. In Figure 2b, I, II, III, IV and V

411


412


413


414


415


416


417


V

IV

VII III

I

VI

XI

IX

II VIII

X 418 419

(a)

(b)

(c)

420 421

(d)

(e) 23 ACS Paragon Plus Environment

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Page 24 of 38

423


424

jacinth (Cu), yellow (S) and pink (Hg) spheres, respectively. Configuration

Top view

Side view

3.01 Å 1A

3.85 Å

3.85 Å

1B 3.81 Å

1C 4.06 Å 4.04 Å

3.83 Å

3.82 Å 3.82 Å

4.06 Å

425

426

427

428

429


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431

Energy & Fuels


432 433

434


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436


437


Top view

2A

Side view 2.58 Å

2.71 Å

S1

2.67 Å

S2 2.46 Å 2.47 Å

2B

2C 2.76 Å

2.61 Å 2.92 Å

2.73 Å

3.64 Å 2D

438

439

440


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Energy & Fuels


(a)

(b)


Energy & Fuels 1 446 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 447 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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Page 29 of 38 1 449 2 3 4 450 5 6 7 8 9 10 11 12 13 14 15 16 17 18 451 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 8. Stable optimized geometries of (a) H2O, (b) O2 and (c) SO2 on CuS(001)-S-2 surface. Atoms are represented as jacinth (Cu), yellow (S), red (O) and white (H) spheres, respectively. (a)

(c)

(b)

2.61 Å 3.69 Å

3.72 Å


3.12 Å

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

List of Figures

2


3

Figure 2. Slab models of CuS(001) surface: (a) CuS unit cell; (b) top view of CuS(001)-Cu/S; (c) top

4

view of CuS(001)-S-2; (d) side view of CuS(001)-Cu/S; (e) side view of CuS(001)-S-2. Atoms are

5

represented as jacinth (Cu) and yellow (S) spheres, respectively. In Figure 2b, I, II, III, IV and V

6


7


8


9


10


11


12


13


14


15


16


17


18


19


20

Figure 8. Stable optimized geometries of (a) H2O, (b) O2 and (c) SO2 on CuS(001)-S-2 surface.

21

Atoms are represented as jacinth (Cu), yellow (S), red (O) and white (H) spheres, respectively.

22


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Energy & Fuels


24

25 26


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Page 32 of 38

27

Figure 2. Slab models of CuS(001) surface: (a) CuS unit cell; (b) top view of CuS(001)-Cu/S; (c) top

28

view of CuS(001)-S-2; (d) side view of CuS(001)-Cu/S; (e) side view of CuS(001)-S-2. Atoms are

29

represented as jacinth (Cu) and yellow (S) spheres, respectively. In Figure 2b, I, II, III, IV and V

30


31


32


33


34


35


36


V

IV

VII III

I

VI

XI

IX

II VIII

X 37 38

(a)

(b)

(c)

39 40 41

(d)

(e) 3 ACS Paragon Plus Environment

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Energy & Fuels

42


43


Top view

Side view

3.01 Å 1A

3.85 Å

3.85 Å

1B 3.81 Å

1C 4.06 Å 4.04 Å

3.83 Å

3.82 Å 3.82 Å

4.06 Å

44

45

46

47

48

49


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

50


51 52

53

54


Page 34 of 38

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Energy & Fuels

55


56


Top view

2A

Side view 2.58 Å

2.71 Å

S1

2.67 Å

S2

2.46 Å 2.47 Å 2B

2C 2.61 Å 2.76 Å

2.92 Å

2.73 Å

3.64 Å 2D

57

58

59

60


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61

Page 36 of 38


62 63

(a)

(b)

64


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Energy & Fuels



Energy & Fuels 1 68 2 3 4 69 5 6 7 8 9 10 11 12 13 14 15 16 17 18 70 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 38

Figure 8. Stable optimized geometries of (a) H2O, (b) O2 and (c) SO2 on CuS(001)-S-2 surface. Atoms are represented as jacinth (Cu), yellow (S), red (O) and white (H) spheres, respectively. (a)

(c)

(b)

2.61 Å 3.69 Å

3.72 Å


3.12 Å

Density Functional Theory Study of Mercury Adsorption on CuS Surface

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