Theoretical study on Hg0 adsorption and oxidation mechanisms over

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Theoretical study on Hg0 adsorption and oxidation mechanisms over CuCl2-impregnated carbonaceous materials surface Wenqi Qu, Yingju Yang, Fenghua Shen, Jianping Yang, Shihao Feng, and Hailong Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00712 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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Energy & Fuels

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Theoretical study on Hg0 adsorption and oxidation mechanisms over

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CuCl2-impregnated carbonaceous materials surface

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Wenqi Qu1, Yingju Yang2, Fenghua Shen2, Jianping Yang1, Shihao Feng1, Hailong Li1*

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

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2 State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong

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University of Science and Technology, Wuhan 430074, China

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Revision Submitted to Energy & Fuels

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*To whom correspondence should be addressed:

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Email: [email protected]

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ACS Paragon Plus Environment

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ABSTRACT: CuCl2-modified carbonaceous materials have been regarded as a kind of mercury

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sorbents, but the Hg0 reaction mechanism over CuCl2-impregnated sorbent surface is still unclear. In

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this work, the binding mechanism of Hg0 on CuCl2-impregnated carbonaceous materials surface was

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investigated using hybrid density functional theory (DFT). The results indicate that the dissociation

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mechanism is responsible for CuCl2 adsorption over carbonaceous materials sorbent surface. The

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active chlorine species generated from CuCl2 adsorption can significantly enhance mercury

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adsorption over carbonaceous materials sorbent surface. Hg0 adsorption over CuCl2-impregnated

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carbonaceous materials surface is dominated by chemisorption mechanism. Surface Cl and C atoms

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are identified as the active sites for Hg0 adsorption on CuCl2-impregnated carbonaceous materials

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surface. CuCl2 plays an important role in mercury adsorption on CuCl2-impregnated carbonaceous

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materials. CuCl2 includes the following roles: (1) CuCl2 can increase the reactivity of its neighbor

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adsorption sites on carbonaceous materials surface; (2) CuCl2 can provide additional active sites for

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Hg0 adsorption; (3) CuCl2 can provide Cl atoms for the oxidation of Hg0 into HgCl. Heterogeneous

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mercury oxidation over CuCl2-impregnated carbonaceous materials surface includes four steps: Hg0

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adsorption, Cl migration, HgCl2 formation and HgCl2 desorption. HgCl2 formation is identified as

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the rate-limiting step of Hg0 oxidation on CuCl2-impregnated carbonaceous materials surface.

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Keywords: Mercury; CuCl2-impregnated sorbent; Adsorption mechanism; Density functional theory

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Energy & Fuels

Introduction

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Mercury is a highly toxic element in the environment and has attracted wide attention due to its

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neurological health impacts.1 Among various anthropogenic mercury emission sources, coal

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combustion (especially coal-fired power plant) has been targeted as one of the major sources, and is

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estimated to contribute about 24% of the total global anthropogenic mercury emissions to the

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atmosphere.2 Minamata Convention on Mercury, an international treaty to protect humans and

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environment from the harm of mercury, entered into force on August 16, 2017.3 Consequently, it is

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urgent to limit mercury emission from coal combustion.

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Three main forms of mercury were detected in flue gas:4 elemental mercury (Hg0), oxidized

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mercury (Hg2+), and particle-associated mercury (Hgp). Among these, mercury with +2 oxidation

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state could be captured efficiently by wet flue gas desulfurization devices since its solubility in

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desulphurization wastewater. Particle-associated mercury could be removed by current dust control

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devices such as electrostatic precipitator 5 or fabric filter (FF).6 However, elemental mercury is the

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most dominant form of mercury species in flue gas, and is extremely difficult to be removal due to

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its inactive chemical properties and poor solubility in water.7 Therefore, a series of comprehensive

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methods have been developed to effectively prevent the discharging of Hg0 from coal combustion.

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To date, sorbent injection is the most mature and promising way to control the emission of Hg0

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in coal-fired power plants. The commercial sorbent for mercury removal is carbonaceous materials.

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However, the raw carbonaceous materials exhibit poor mercury removal efficiency, leading to a very

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high carbon-to-mercury mass ratio (3000-100000),8,

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content. Numerous researchers have focused on the enhancement of mercury capture capacity of raw

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carbonaceous materials.10-13 It was reported that CuCl2 could exhibit superior catalytic activity for

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Hg0 oxidation in flue gas.14-19 Thus, CuCl2 was widely used to modify the raw carbonaceous

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especially in the flue gas with low HCl

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materials.20-23 Compared with the raw carbonaceous materials, CuCl2-impregnated carbonaceous

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materials shows the excellent mercury capture capacity. The chlorine ions contained in CuCl2 may

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enhance the content of chlorine in flue gas and be beneficial to the oxidation of mercury24. Cu ions in

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CuCl2 may perform mercury oxidation capacity because copper is a kind of transition metal.14

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However, these studies mostly focused on the Hg0 oxidation and adsorption capacity of CuCl2

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promoted sorbents, while the involve reaction mechanisms are not well understood. Further studies

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of the role of CuCl2 played in mercury adsorption on CuCl2-impregnated carbonaceous materials

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need to be discussed.

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Lee et al. 23 have done an admirable job by using XAFS to characterize the mercury species on

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cupric chloride-impregnated sorbents. They investigated mercury oxidation process over

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CuCl2-impregnated carbonaceous materials, and found that the dominant mercury species is HgCl2

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rather than HgCl. The surface Cl atom of CuCl2-impregnated carbonaceous materials is identified as

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the active site for mercury oxidation. However, it was reported that different active sites (Cl site and

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Cu site) exist on the CuCl2-impregnated sorbent. For CuCl2 modified magnetospheres catalyst from

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fly ash, the binding energy of mercury on the Cu site is higher than that on the Cl site, suggesting

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that Cu site is the dominant active site.18 For CuCl2 modified carbonaceous materials, the role of Cu

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site is still unclear. It can be seen that there is no agreement on the active site of

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over CuCl2-impregnated carbonaceous materials. Also, Hg0 adsorption mechanism over

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CuCl2-impregnated carbonaceous materials need to be explored in depth. It was reported that the Hg0

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adsorption capacity decreased as CuCl2 loading on carbon increased.22 However, some experimental

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studies have proven that CuCl2 impregnation could enhance the physisorption of Hg0 on

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carbonaceous materials, while the amount of chloride on carbonaceous materials was found to be

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related to the oxidized Hg generated from the reactions between Hg0 and CuCl2-impregnated 4


Hg0 adsorption

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carbonaceous materials.17 Although a series of experiments were carried out to examine Hg0

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adsorption and oxidation on CuCl2-impregnated carbonaceous materials, the theoretical studies about

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the subject were extremely rare. Moreover, other chloride species like CaCl2 that have been used and

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were cheaper, the advantages of CuCl2-impregnated carbonaceous materials require further

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argumentations. Hg0 adsorption and oxidation mechanism over CuCl2-impregnated carbonaceous

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materials, especially on the atomic level, is still unclear.

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For the last several years, DFT calculation has been extensively used to understand the

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atomic-level mechanism of mercury adsorption over different sorbent/catalyst surfaces.25-31

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Therefore, in this work, DFT was performed to investigate surface reaction chemistry and the

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adsorption and oxidation of Hg0 on CuCl2-impregnated carbonaceous materials surface. The

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structure of CuCl2-impregnated carbonaceous materials was established, the mainly existence form

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of CuCl2 molecular on carbonaceous materials surface was examined with experimental data. The

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impacts of adsorbed CuCl2 on Hg adsorption and the binding characteristics of Hg0 on carbonaceous

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materials surface were investigated to probe into the process of mercury transformation. It is

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significant that exploring Hg0 adsorption mechanism on sorbents to synthesis high-efficiency

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sorbents for Hg0 removal in the flue gas.

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Model development and computational details

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Model development

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Carbonaceous materials possess the very complex structures. However, it was reported that the

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limited clusters of a single carbonaceous materials surface are the basic structure.32 The interaction

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properties and accuracy analysis of molecular systems highly rely on the local structure of the active

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site rather than the size of the carbonaceous materials surface. Thus, the cluster model with 4-7 fused

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benzene rings was widely applied to simulate graphite, active carbon and other carbonaceous 5


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materials for quantum chemical calculations.5, 33 In our previous studies,34 the impact of SO2 on

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mercury adsorption mechanism on active carbon was developed by a 6-fused benzene ring cluster. In

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addition, a finite cluster of five fused benzene rings model with zigzag edge sites was applied to

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investigate the adsorption of HgCl and HgCl2 on carbonaceous materials,35 the zigzag model has

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been proved to be very useful for simulating halogen-loaded carbonaceous materials surfaces by one

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halogen atom binding on the edge site of the surface.27 Recently, a cluster model with 7 benzene

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rings and zigzag edge shapes was used to simulate the carbonaceous materials surface, investigate

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the H2S adsorption mechanism over carbonaceous materials.36 Compared to armchair sites, the

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existence of unpaired electrons are found on zigzag edges. Therefore, zigzag sites show more

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reactive which can better describe the interaction of active ingredients and mercury on the

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carbonaceous materials surface than armchair sites.33 In addition, given in the occupation of CuCl2

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molecule and Hg0, a larger model is more needed. Therefore, the monolayer cluster model consists

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of 7-fused benzene rings with zigzag edge sites were selected in the study.

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Based on the above-mentioned studies, a monolayer cluster model with zigzag edge sites was

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used to imitate the surface of carbonaceous materials surface, as shown in Figure. 1. It can be seen

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that the surface model consists of 7-fused benzene rings. The bottom boundary was saturated with

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hydrogen atoms; the active sites were simulated by unsaturated edge atoms above the surface.

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Methodology

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Density functional theory was applied to study the adsorption mechanism of Hg0 on

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CuCl2-impregnated carbonaceous materials surface using Gaussian 03 program. B3PW91

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exchange-correlation functional was used to provide a better overall description of the electronic

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subsystem for the complexes involving strongly bound ionic hydrogen bonds. In our previous

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studies,27, 34, 35 B3PW91/RCEP28DVZ combination has been successfully provided accurate results 6


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for the systems involving carbonaceous materials, halogen atoms and heavy metals by comparing the

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theoretically determined geometries, frequencies and reaction enthalpies to experimental values

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found in the literature. Therefore, B3PW91/RCEP28DVZ was applied for the Hg atom in this

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research. and the 6-31G (d) basis was applied for other atoms (C, H, Cu, Cl).

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Geometric optimization results of the surface model show that both the optimized average bond

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lengths and average optimized angels of the structure are in accordance with the chemical test results

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(C-C: 1.42 Å, C-H: 1.07 Å, ∠C-C-C: 120, ∠C-C-H: 120)

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data.36 Therefore, the above mention surface model and the calculation methods are reliable.

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and other theoretical calculation

The adsorption energy (Eads) of adsorbate (Hg0 and CuCl2) on CuCl2-impregnated carbonaceous materials surface (substrate) was calculated according to the following equation38, 39: Eads = E(AB)－(E(A) +E(B))

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(1)

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E(A), E(B) and E(AB), represent the energy of adsorbate, substrate and adsorbate/substrate

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system in equilibrium state respectively. Under this definition, the adsorption progress is exothermic

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if Eads is negative value. The higher the negative adsorption energy, the more stable the

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corresponding adsorption configuration.

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Results and discussion

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CuCl2 adsorption on carbonaceous materials surface

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CuCl2 was loaded on carbonaceous materials surface to enhance the mercury capture ability of

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the raw carbonaceous materials. The interaction between CuCl2 and carbonaceous materials is a

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significant progress in mercury adsorption. Therefore, CuCl2 adsorption on carbonaceous materials

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surface was first investigated. The stable optimized structures of CuCl2 adsorption on carbonaceous

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materials surface are present in Figure. 2. The atomic bond population of different surface complexes

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is given in Table 1. It can be seen that there are three viable optimized structures (A1, A2, A3) for

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CuCl2 adsorption on carbonaceous materials surface.

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The atomic bond population was calculated to analyze the interaction as an index for predicting

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bond strength. A higher positive value corresponds to a stronger interaction, while a negative value

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represents anti-bonding molecular orbital. In A1, the bond population of Cu-Cl is about zero,

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suggesting that Cu-Cl bond is very weak. The CuCl2 molecular thoroughly dissociates into a Cu

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atom and two Cl atoms. The resulting Cu and Cl atoms are strongly binding on surface C atoms. As

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shown in Table 1, the bond population of Cl-C bonds is about 0.152 and 0.175, respectively. Padak

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et al. developed the adsorption behaviour of Cl atom on single carbonaceous materials layer and

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found that bond population of C-Cl is about 0.416. It is clear that the bond population of C-Cl in this

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work is much lower than that of C-Cl in Padak et al.’s work. This indicates that the presence of Cu

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atom can weaken the bond strength between C and Cl atoms. This weak strength is favourable for Cl

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releasing from carbonaceous materials surface. Thus, CuCl2 dissociation over carbonaceous

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materials surface can provide Cl atom for HgCl2 formation during mercury removal by CuCl2

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modified carbonaceous materials.

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In A2, CuCl2 is dissociatively adsorbed on carbonaceous materials surface, forming a CuCl

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molecule and a Cl atom. The formed CuCl molecule strongly interacts with C11 atom to a C-Cu

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bond. The bond population of C-Cu bond is 0.269. The resulting Cl atom binding on C9 atom,

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forming a C-Cl bond with the population of 0.237. In A3, the dissociation reaction of CuCl2 over

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carbonaceous materials sorbent surface forms a CuCl molecule and a Cl atom. The CuCl molecule is

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strongly adsorbed on carbonaceous materials surface with Cu atom toward C9 atom. The bond

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population of C-Cu bond is 0.252, as shown in Table 1. Another Cl atom strongly interacts with the

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C11 atom to form a C-Cl bond. The bond population of this C-Cl bond is 0.164. 8


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To further illustrate the chemical and physical properties, net charge distribution was calculated

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by Mulliken population analysis. Mulliken atomic charges for selected atoms of model A1, A2 and

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A3 were listed in Table 2. For carbonaceous materials surface, four unsaturated carbon atoms (C11,

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C9, C22 and C9 ) upper side the surface are the most active sites because of the excess negative

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charges. After CuCl2 adsorption on carbonaceous materials surface, the charge of these carbon atoms

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increases or decreases with varying degrees.

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In model A1, the charge of C9 atom changes from -0.081 e to -0.348 e after CuCl2 adsorption,

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which shows that CuCl2 in the carbonaceous materials surface enhance the number of electrons from

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the neighbouring active sites, making C9 atom more electronegative, and thus increases the

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propensity for Hg0 adsorption. Similarly, in model A2 and A3, the charge of C22 and C29 atoms

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changes from -0.081 e and -0.025 e to -0.100 e and -0.201 e respectively after CuCl2 adsorption. The

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improvement of the activity of carbon sites is attributed to the binging of Cu. The bond population of

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Cu-Cl bond suggests that CuCl2 is dissociatively adsorbed on carbonaceous materials surface. Based

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on the analysis, it was deduced that CuCl2 adsorption on carbonaceous materials surface belongs to

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dissociative process. This conclusion is supported by Li et al’s 23 experiments in which the Extended

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X-ray Absorption Fine Structure (EXAFS) spectroscopy found that some Cu-Cl bonds were broken

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after CuCl2 adsorption over carbonaceous materials surface. Moreover, the DFT calculation results

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are consistent with the X-ray diffraction(XRD) results

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appeared in the XRD patterns of CuCl2-impregnated carbonaceous materials.

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Hg0 adsorption on CuCl2-impregnated carbonaceous materials surface

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in founding that CuCl diffraction peak

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CuCl2 can enhance the mercury capture ability of the raw carbonaceous materials. The

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interaction between Hg0 and CuCl2 preadsorbed on carbonaceous materials surface plays a

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significant role in the enhanced mercury capture capacity. Thus, Hg0 adsorption on 9


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CuCl2-impregnated carbonaceous materials surface was investigated. Different active sites (Cu, Cl,

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bridge, C sites) was considered for Hg0 adsorption on CuCl2-impregnated carbonaceous materials.

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The stable optimized configuration is present in Figure. 3. The adsorption energies, bond populations,

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and Mulliken charge of different surface complexes are given in Table 3, Table 4, and Table 5

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respectively.

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The most stable structure of Hg0 adsorption on CuCl2-impregnated carbonaceous materials

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surface is B2 with the adsorption energy of -104.4 kJ/mol. In B2 structure, the CuCl2-impregnated

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carbonaceous materials surface is obtained from the A3 structure. During Hg0 adsorption process,

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gaseous Hg0 directly reacts with Cl atom of CuCl (see A3 structure) to form a HgCl molecule (see

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B2 structure). The formed HgCl molecule is chemically adsorbed on C active site with Hg atom

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toward surface C atom. The Hg-C29 bond is 0.324. As shown in Table 5, the Mulliken charge of Hg

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atom is 0.045 e, suggesting that some electrons are drawn from mercury atom to CuCl2-impregnated

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carbonaceous materials surface. This leads to the partial oxidation of Hg0 into HgCl. Meanwhile, the

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Cu-Cl bond is broken because of HgCl formation, as shown in B2 structure.

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Another stable structure of Hg0 adsorption on CuCl2-impregnated carbonaceous materials

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surface is B1. The CuCl2-impregnated carbonaceous materials surface of B1 configuration is

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obtained from the A2 structure. In B1 structure, Hg0 is simultaneously adsorbed on two C atoms. The

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corresponding adsorption energy is -98.5 kJ/mol. Moreover, the bond populations of Hg-C22 and

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Hg-C29 are 0.195 and 0.226, respectively. The Mulliken charge of Hg atom is 0.398 e, which is

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larger than that of mercury atom in B2 structure. As a result, the charge transfer at the interface of

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the B1 structure is much more than that at the interface of the B2 structure.

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Based on the above analysis, both Hg0 (B1 structure) and the formed HgCl molecule (B2

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structure) are adsorbed on the surface C atom. In B2 structure, gaseous Hg0 can be directly adsorbed 10


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Cl atom and react with Cl atom to form HgCl molecule. Therefore, the surface Cl and C atoms are

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identified as the active site for Hg0 adsorption on CuCl2-impregnated carbonaceous materials surface.

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The XAFS characterization

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Hg atom is mainly adsorbed on Cl atom. Therefore, the calculation results are in line with chemical

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test results.

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of mercury-loaded CuCl2 modified carbonaceous materials found that

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It was reported that the interaction with adsorption energy stronger than -50 kJ/mol could be

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considered as chemisorption.28, 41 In this study, the adsorption energy (-98.5 and -104.4kJ/mol) of

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Hg0 on CuCl2-impregnated carbonaceous materials surface is much higher -50 kJ/mol. Thus, it can

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be inferred that Hg0 adsorption on CuCl2-impregnated carbonaceous materials is closely associated

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with chemisorption mechanism. This is in good agreement with the experimental results

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found that chemisorption is the dominant mechanism for Hg0 adsorption on CuCl2-impregnated

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carbonaceous materials. The adsorption energies of Hg0 on raw carbonaceous materials are between

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-44.6 kJ/mol and -64.6 kJ/mol, which depends mainly on the local shape of carbonaceous materials

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surface.27,

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capacity of raw carbonaceous materials. This can be verified by various experiments.21, 22, 42

33, 34

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which

It indicates that CuCl2 modification can significantly enhance mercury capture

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As shown in Figure 3, mercury atom is adsorbed on C or Cl atoms. Therefore, CuCl2 plays an

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important role in mercury adsorption on CuCl2-impregnated carbonaceous materials. CuCl2 includes

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the following roles: (1) CuCl2 can increase the reactivity of its neighbor adsorption sites on

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carbonaceous materials surface; (2) CuCl2 can provide additional active sites for Hg0 adsorption; (3)

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CuCl2 can provide Cl atoms for the oxidation of Hg0 into HgCl (see B2 structure).

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Hg0 oxidation on CuCl2-impregnated carbonaceous materials surface

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It was reported that mercury species adsorbed on CuCl2-impregnated carbonaceous materials

237

surface mainly exists in the form of HgCl2.23 The adsorbed Hg0 can be oxidized into HgCl2 over 11


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CuCl2-impregnated carbonaceous materials surface. Hence, it is necessary to develop the mercury

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oxidation process over CuCl2-impregnated carbonaceous materials surface. According to the most

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stable configuration of Hg0 adsorption, the intermediate of mercury oxidation can be determined.

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Figure. 4 shows the energy profile of heterogeneous mercury oxidation over CuCl2-impregnated

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carbonaceous materials surface. The corresponding optimized configuration of intermediate,

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transition state and final state are also present in the Figure.

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Heterogeneous mercury oxidation over CuCl2-impregnated carbonaceous materials surface

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includes four steps: Hg0 adsorption, Cl migration, HgCl2 formation and HgCl2 desorption. As

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indication above, the most stable configuration of Hg0 adsorption on CuCl2-impregnated

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carbonaceous materials surface is B2. Therefore, B2 was used as the IM structure, as shown in

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Figure. 4. In the first step, gaseous Hg0 is adsorbed on CuCl2-impregnated carbonaceous materials

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surface, forming a HgCl molecule (see IM1 structure). The adsorption energy of Hg0 is -104.40

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kJ/mol. After Hg0 adsorption, the left Cl atom migrates from surface C atom to Cu atom, forming

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CuCl species (see IM2 structure). This migration step of Cl atom is endothermic indication with

252

activation energy barrier of 173.39 kJ/mol. Moreover, the energy change of Cl migration step is

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120.63 kJ/mol. Subsequently, HgCl molecule reacts with the left Cl atom to form a HgCl2 molecule

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(see IM3 structure). The distance between Hg atom and left Cl atom decreases gradually: 4.822 Å

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(IM2) → 3.091 Å (TS2) → 2.516 Å (IM3). This oxidation step of HgCl is activated by 254.81

256

kJ/mol, and endothermic by 232.55 kJ/mol. In the last step, HgCl2 desorbs from CuCl2-impregnated

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carbonaceous materials surface to form a gaseous HgCl2 molecule, leaving a Cu atom adsorbed on C

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site. This desorption step is an endothermic course with activation energy barrier of 71.02 kJ/mol and

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reaction heat of 68.56 kJ/mol.

12


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Compared to other three steps, HgCl2 formation presents the highest activation energy barrier of

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254.81 kJ/mol. It is well known that the reaction step with the highest activation energy barrier is

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regarded as the rate-limiting step of mercury oxidation.41,

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identified as the rate-limiting step of Hg0 oxidation over CuCl2-impregnated carbonaceous materials

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surface.

43-45

Therefore, HgCl2 formation is

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From the energy profile of heterogeneous mercury oxidation over CuCl2-impregnated

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carbonaceous materials surface, the formation of HgCl2 include the reaction of Cl from CuCl with

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HgCl and the desorption process of HgCl from carbonaceous materials surface. Although the

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activation energy of Cl migration is high(173.39 kJ/mol), the adsorption energy of Hg0 on the

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modified carbonaceous materials surface is large and belong to chemisorption. The calculation

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results indicated that the modification of carbonaceous materials surface is beneficial to the

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adsorption Hg0 and CuCl2-impregnated carbonaceous materials is one high efficient mercury

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adsorbent rather than catalyst, which was proved by many experimental results.22 Therefore, the

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results of DFT calculation are in line with the experimental results.

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Conclusions

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Hg0 adsorption and oxidation mechanisms over CuCl2-impregnated carbonaceous materials

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surface were systematically investigated by using quantum chemical methods based on DFT

277

analysis. The structures of CuCl2-impregnated carbonaceous materials surface were analyzed by

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atomic bond population and Mulliken atomic charge. CuCl2 binding on carbonaceous materials

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surface is a dissociative process which produces active Cl atoms for mercury oxidation. Surface Cl

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and C atoms are identified as the active site for Hg0 adsorption on CuCl2-impregnated carbonaceous

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materials surface. Although Cu site was proved as the dominant active site in CuCl2 modified

282

magnetospheres catalyst, Cu adsorbs Hg0 indirectly in CuCl2-impregnated carbonaceous materials 13


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

283

surface rather than directly according to the calculation results. The main role of Cu is reflected in

284

two aspects: (1) Cl dissociating facilitates the interaction between Hg0 and Cl more easily; (2) Cu

285

changes the activity of neighboring carbon active sites and promotes mercury adsorption. CuCl2

286

plays important roles in mercury adsorption on CuCl2-impregnated carbonaceous materials surface:

287

(1) CuCl2 can increase the reactivity of its neighbor C sites; (2) CuCl2 can provide additional active

288

sites for Hg0 adsorption; (3) CuCl2 can provide Cl atoms for the oxidation of Hg0 into HgCl.

289

Chemisorption mechanism is closely associated with Hg0 adsorption over CuCl2-impregnated

290

carbonaceous materials surface. Compared with other chloride modifications (such as CaCl2

291

modified carbonaceous materials), the role of metal ions is possibly different. The facilitation of Ca

292

ions for the ortho position is weaker than Cu ions based on the analysis of element properties. Hence,

293

this is the advantage of CuCl2 modification. Heterogeneous mercury oxidation over

294

CuCl2-impregnated carbonaceous materials surface is a four-step process, in which HgCl2 formation

295

is the rate-limiting step.

296 297

Reference

298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313

1. McNutt, M., Mercury and health. Science 2013, 341, (6153), 1430-1430. 2. Yang, Y.; Liu, J.; Shen, F.; Zhao, L.; Wang, Z.; Long, Y., Kinetic study of heterogeneous mercury oxidation by HCl on fly ash surface in coal-fired flue gas. Combustion and Flame 2016, 168, 1–9. 3. Wang, Z.; Liu, J.; Yang, Y.; Miao, S.; Shen, F., Effect of the Mechanism of H2S on Elemental Mercury Removal Using the MnO2 Sorbent during Coal Gasification. Energy & Fuels 2017. 4. Senior, C. L.; Sarofim, A. F.; Zeng, T.; Helble, J. J.; Mamani-Paco, R., Gas-phase transformations of mercury in coal-fired power plants. Fuel Processing Technology 2000, 63, (2), 197-213. 5. Espinal, J. F.; Montoya, A.; Mondragón, F.; Truong, T. N., A DFT Study of Interaction of Carbon Monoxide with Carbonaceous Materials. The Journal of Physical Chemistry B 2004, 108, (3), 1003-1008. 6. Yang, Y.; Liu, J.; Wang, Z.; Zhang, Z., Homogeneous and heterogeneous reaction mechanisms and kinetics of mercury oxidation in coal-fired flue gas with bromine addition. Proceedings of the Combustion Institute 2017, 36, 4039–4049. 7. Zhang, B.; Liu, J.; Dai, G.; Chang, M.; Zheng, C., Insights into the mechanism of heterogeneous 14


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Energy & Fuels

mercury oxidation by HCl over V2O5/TiO2 catalyst: Periodic density functional theory study. Proceedings of the Combustion Institute 2015, 35, (3), 2855-2865. 8. Lee, J.-Y.; Ju, Y.; Keener, T. C.; Varma, R. S., Development of Cost-Effective Noncarbon Sorbents for Hg0 Removal from Coal-Fired Power Plants. Environmental Science & Technology 2006, 40, (8), 2714-2720. 9. 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, (17), 9551-7. 10. Ghorishi, S. B.; Keeney, R. M.; Serre, S. D.; Gullett, B. K.; Jozewicz, W. S., Development of a Cl-Impregnated Activated Carbon for Entrained-Flow Capture of Elemental Mercury. Environmental Science & Technology 2002, 36, (20), 4454-4459. 11. Zhang, H.; Zhao, K.; Gao, Y.; Tian, Y.; Liang, P., Inhibitory effects of water vapor on elemental mercury removal performance over cerium-oxide-modified semi-coke. Chemical Engineering Journal 2017, 324, (Supplement C), 279-286. 12. Yang, J.; Zhao, Y.; Liang, S.; Zhang, S.; Ma, S.; Li, H.; Zhang, J.; Zheng, C., Magnetic iron–manganese binary oxide supported on carbon nanofiber (Fe3−xMnxO4/CNF) for efficient removal of Hg0 from coal combustion flue gas. Chemical Engineering Journal 2018, 334, (Supplement C), 216-224. 13. De, M.; Azargohar, R.; Dalai, A. K.; Shewchuk, S. R., Mercury removal by bio-char based modified activated carbons. Fuel 2013, 103, 570-578. 14. Li, X.; Liu, Z.; Kim, J.; Lee, J.-Y., Heterogeneous catalytic reaction of elemental mercury vapor over cupric chloride for mercury emissions control. Applied Catalysis B: Environmental 2013, 132-133, (Supplement C), 401-407. 15. Liu, Z.; Li, X.; Lee, J.-Y.; Bolin, T. B., Oxidation of elemental mercury vapor over γ-Al2O3 supported CuCl2 catalyst for mercury emissions control. Chemical Engineering Journal 2015, 275, (Supplement C), 1-7. 16. Zhou, X.; Xu, W.; Wang, H.; Tong, L.; Qi, H.; Zhu, T., The enhance effect of atomic Cl in CuCl2/TiO2 catalyst for Hg0 catalytic oxidation. Chemical Engineering Journal 2014, 254, (Supplement C), 82-87. 17. Yang, J.; Zhao, Y.; Zhang, J.; Zheng, C., Removal of elemental mercury from flue gas by recyclable CuCl 2 modified magnetospheres catalyst from fly ash. Part 1. Catalyst characterization and performance evaluation. Fuel 2016, 164, 419-428. 18. Yang, J.; Zhao, Y.; Zhang, J.; Zheng, C., Removal of elemental mercury from flue gas by recyclable CuCl2 modified magnetospheres catalyst from fly ash. Part 2. Identification of involved reaction mechanism. Fuel 2016, 167, (Supplement C), 366-374. 19. Lee, S.-S.; Wilcox, J., Behavior of mercury emitted from the combustion of coal and dried sewage sludge: The effect of unburned carbon, Cl, Cu and Fe. Fuel 2017, 203, 749-756. 20. Lee, J.-Y.; Ju, Y.; Lee, S.-S.; Keener, T. C.; Varma, R. S., Novel Mercury Oxidant and Sorbent for Mercury Emissions Control from Coal-fired Power Plants. Water, Air, & Soil Pollution: Focus 2008, 8, (3), 333-341. 21. Lee, S.-S.; Lee, J.-Y.; Keener, T. C., Bench-Scale Studies of In-Duct Mercury Capture Using Cupric Chloride-Impregnated Carbons. Environmental Science & Technology 2009, 43, (8), 2957-2962. 22. Lee, S.-S.; Lee, J.-Y.; Khang, S.-J.; Keener, T. C., Modeling of Mercury Oxidation and Adsorption by Cupric Chloride-Impregnated Carbon Sorbents. Industrial & Engineering Chemistry Research 2009, 48, (19), 9049-9053. 15


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360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405

23. Li, X.; Lee, J.-Y.; Heald, S., XAFS characterization of mercury captured on cupric chloride-impregnated sorbents. Fuel 2012, 93, (Supplement C), 618-624. 24. Wilcox, J., A Kinetic Investigation of High-Temperature Mercury Oxidation by Chlorine. The Journal of Physical Chemistry A 2009, 113, (24), 6633-6639. 25. Yang, Y.; Liu, J.; Zhang, B.; Liu, F., Mechanistic studies of mercury adsorption and oxidation by oxygen over spinel-type MnFe2O4. Journal of Hazardous Materials 2017, 321, 154–161. 26. Qu, W.; Liu, J.; Shen, F.; Wei, P.; Lei, Y., Mechanism of mercury-iodine species binding on carbonaceous surface: Insight from density functional theory study. Chemical Engineering Journal 2016, 306, 704–708. 27. Liu, J.; Qu, W.; Zheng, C., Theoretical studies of mercury–bromine species adsorption mechanism on carbonaceous surface. Proceedings of the Combustion Institute 2013, 34, (2), 2811-2819. 28. Yang, Y.; Liu, J.; Zhang, B.; Liu, F., Density functional theory study on the heterogeneous reaction between Hg0 and HCl over spinel-type MnFe2O4. Chemical Engineering Journal 2017, 308, 897–903. 29. Padak, B.; Brunetti, M.; Lewis, A.; Wilcox, J., Mercury binding on activated carbon. Environmental Progress 2006, 25, (4), 319-326. 30. Li, H.; Feng, S.; Liu, Y.; Shih, K., Binding of Mercury Species and Typical Flue Gas Components on ZnS(110). Energy & Fuels 2017, 31, (5), 5355-5362. 31. Rupp, E. C.; Wilcox, J., Mercury chemistry of brominated activated carbons – Packed-bed breakthrough experiments. Fuel 2014, 117, 351-353. 32. Montoya, A.; Truong, T.-T. T.; Mondragón, F.; Truong, T. N., CO Desorption from Oxygen Species on Carbonaceous Surface: 1. Effects of the Local Structure of the Active Site and the Surface Coverage. The Journal of Physical Chemistry A 2001, 105, (27), 6757-6764. 33. Padak, B.; Wilcox, J., Understanding mercury binding on activated carbon. Carbon 2009, 47, (12), 2855-2864. 34. Liu, J.; Qu, W.; Joo, S. W.; Zheng, C., Effect of SO2 on mercury binding on carbonaceous surfaces. Chemical Engineering Journal 2012, 184, 163-167. 35. Liu, J.; Cheney, M. A.; Wu, F.; Li, M., Effects of chemical functional groups on elemental mercury adsorption on carbonaceous surfaces. Journal of Hazardous Materials 2011, 186, (1), 108-113. 36. Shen, F.; Liu, J.; Zhang, Z.; Dong, Y.; Gu, C., Density functional study of hydrogen sulfide adsorption mechanism on activated carbon. Fuel Processing Technology 2018, 171, 258-264. 37. Chen, N.; Yang, R. T., Ab initio molecular orbital calculation on graphite: Selection of molecular system and model chemistry. Carbon 1998, 36, (7), 1061-1070. 38. Wilcox, J.; Robles, J.; Marsden, D. C. J.; Blowers, P., Theoretically Predicted Rate Constants for Mercury Oxidation by Hydrogen Chloride in Coal Combustion Flue Gases. Environmental Science & Technology 2003, 37, (18), 4199-4204. 39. Lim, D.-H.; Wilcox, J., DFT-Based Study on Oxygen Adsorption on Defective Graphene-Supported Pt Nanoparticles. The Journal of Physical Chemistry C 2011, 115, (46), 22742-22747. 40. Du, W.; Yin, L.; Zhuo, Y.; Xu, Q.; Zhang, L.; Chen, C., Catalytic Oxidation and Adsorption of Elemental Mercury over CuCl2-Impregnated Sorbents. Industrial & Engineering Chemistry Research 2014, 53, (2), 582-591. 41. Wang, Z.; Liu, J.; Zhang, B.; Yang, Y.; Zhang, Z.; Miao, S., Mechanism of Heterogeneous Mercury Oxidation by HBr over V2O5/TiO2 Catalyst. Environmental Science & Technology 2016, 16


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406 407 408 409 410 411 412 413 414 415 416 417 418

50, (10), 5398–5404. 42. Tsai, C. Y.; Chiu, C. H.; Chuang, M. W.; Hsi, H. C., Influences of Copper(II) Chloride Impregnation on Activated Carbon for Low-Concentration Elemental Mercury Adsorption from Simulated Coal Combustion Flue Gas. Aerosol & Air Quality Research 2017, 17, (6). 43. Yang, Y.; Liu, J.; Liu, F.; Wang, Z.; Miao, S., Molecular-level insights into mercury removal mechanism by pyrite. Journal of hazardous materials 2018, 344, 104-112. 44. Zhang, B.; Liu, J.; Yang, Y.; Chang, M., Oxidation mechanism of elemental mercury by HCl over MnO2 catalyst: Insights from first principles. Chemical Engineering Journal 2015, 280, 354-362. 45. Yang, Y.; Liu, J.; Zhang, B.; Zhao, Y.; Chen, X.; Shen, F., Experimental and theoretical studies of mercury oxidation over CeO2 -WO3/TiO2 catalysts in coal-fired flue gas. Chemical Engineering Journal 2017, 317, 758–765.

419

List of Tables

420

Table 1. Atomic bond population for CuCl2-impregnated carbonaceous materials surface.

421

Table 2. Mulliken total atomic charges in A1, A2 and A3.

422

Table 3. Adsorption energies of Hg0 on CuCl2-impregnated carbonaceous materials surface.

423

Table 4. Atomic bond population for Hg0 adsorption CuCl2-impregnated carbonaceous materials

424

surface.

425

Table 5. Mulliken charge of different atoms in B1 and B2.

426 427

17


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

428

List of Figures

429

Figure 1. Surface model of carbonaceous materials. a, b, c denote C2, C3, and bridge adsorption sites,

430

respectively. C2 and C3 denote two-fold and three-fold coordinated carbon atoms, respectively.

431

Figure 2. Stable optimized structures of CuCl2 adsorption on carbonaceous materials surface.

432

Figure 3. Stable optimized structures of Hg0 adsorption on CuCl2-impregnated carbonaceous

433

materials surface.

434

Figure 4. The energy profile of heterogeneous mercury oxidation over CuCl2-impregnated

435

carbonaceous materials surface.

436

18


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Page 19 of 32 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

437

Energy & Fuels

Table 1. Atomic bond population for CuCl2-impregnated carbonaceous materials surface. Bond

CS

CuCl2

A1

A2

A3

Cl(36)-Cu

-0.329

-0.008

0.285

0.312

Cl(37)-Cu

-0.329

0

-0.022

0.060

Cu-C(9)

0.089

Cu-C(11)

0.252 0.269

Cl(36)-C(11)

0.175

Cl(37)-C(9)

0.237

Cl(37)-C(11)

0.164

Cl(37)-C(22)

0.152

C(12)-C(11)

0.473

0.392

0.253

0.416

C(11)-C(8)

0.407

0.154

-0.115

0.248

C(8)-C(9)

0.334

-0.403

0.154

-0.040

C(9)-C(10)

0.341

-0.566

0.090

-0.022

C(10)-C(22)

0.341

-0.008

0.424

0.240

C(22) -C(25)

0.334

0.065

0.329

0.252

C(25) -C(29)

0.407

0.443

0.469

0.405

438 439

19


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

440

Page 20 of 32

Table 2. Mulliken total atomic charges in A1, A2 and A3. Bond

CS

CuCl2

A1

A2

A3

Cl(36)

-0.329

-0.020

-0.446

-0.389

Cl(37)

-0.329

0.006

0.132

0.072

Cu

0.657

0.446

0.365

0.626

C(11)

-0.025

-0.207

-0.216

-0.251

C(8)

0.071

0.147

0.120

0.185

C(9)

-0.081

-0.348

-0.258

-0.389

C(10)

0.098

0.189

0.141

-0.088

C(22)

-0.081

-0.307

-0.100

-0.077

C(25)

0.071

0.147

0.114

0.052

C(29)

-0.025

-0.046

-0.012

-0.201

441

20


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442

Energy & Fuels

Table 3. Adsorption energies of Hg0 on CuCl2-impregnated carbonaceous materials surface. Model

B1

B2

Eads (kJ/mol)

-98.5

-104.4

443 444

21


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

445

Table 4. Atomic bond population for Hg0 adsorption CuCl2-impregnated carbonaceous materials

446

surface. Bond

B1

Cl(36)-Cu

0.286

Cu-C(9) Cu-C(11)

0.185 0.273

Cu-C(22) Cl(37)-C(9)

0.040 0.244

Cl(37)-C(11)

0.240

Hg-C(22)

0.195

Hg-C(29)

0.226

Hg-Cl(36)

B2

0.324 0.160

447 448

22


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Page 23 of 32 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

449

Energy & Fuels

Table 5. Mulliken charge of different atoms in B1 and B2. Atoms

B1

B2

Cl(36)

-0.450

-0.313

Cl(37)

0.098

0.005

Cu

0.339

0.615

Hg

0.398

0.045

C(11)

-0.226

-0.184

C(9)

-0.259

-0.298

C(22)

-0.125

-0.299

C(29)

-0.018

-0.110

450 451

23


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 24 of 32

452

Figure 1. Surface model of carbonaceous materials. a, b, c denote C2, C3, and bridge adsorption sites,

453

respectively. C2 and C3 denote two-fold and three-fold coordinated carbon atoms, respectively. a

b

c

Benzene ring C 454 455 456

24


H

Page 25 of 32 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

457

Energy & Fuels

Figure 2. Stable optimized structures of CuCl2 adsorption on carbonaceous materials surface. 36

35

36 15

14

11 12

6

458

29 25

2

27

19

1

18

26

16

21

30

A1

28 24

20

32

15

33

14

31

23

7 3

5 17

22 10

4

13

35

37

9 8

36

34

37

11 12

9 8

6 16

32

19

27

18

26

21

30

12

28 33

24

11

15

31

23 20

2 1

29 25

7 3

5 17

22 10

4

13

35

37

34

A2

459 460

25


14

5

20

2

28 33

24

19

27

1

18

26

16

21

30

A3

32 31

23

7 3

6 17

29 25

10

4

13

22

9 8

34

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 26 of 32

461

Figure 3. Stable optimized structures of Hg0 adsorption on CuCl2-impregnated carbonaceous

462

materials surface.

463

B2

B1 Cl

464 465 466

26


Cu

Hg

C

H

Page 27 of 32

467

Figure 4. The energy profile of heterogeneous mercury oxidation over CuCl2-impregnated

468

carbonaceous materials surface.

4.960 Å 3.091 Å

400 TS3 319.80

Hg0

FS

TS2

300

271.04 2.497 Å

Relative energy (kJ/mol)

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

Energy & Fuels

317.34

71.02 IM3

7.101 Å

248.78 200 HgCl2

254.81

2.516 Å

TS1 68.99

100 Hg0+CuCl2-surface

IM2

0

16.23

173.39

0.00

CuCl

4.822 Å

HgCl 3.785 Å

-100

2.120 Å

IM1 -104.40 Cl

Cu

Hg

-200 Hg0 adsorption

469 470

HgCl2 formation

Cl migration

Reaction pathway

27


HgCl2 desorption

C

H

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

List of Figures Figure 1. Surface model of carbonaceous materials. a, b, c denote C2, C3, and bridge adsorption sites, respectively. C2 and C3 denote two-fold and three-fold coordinated carbon atoms, respectively. Figure 2. Stable optimized structures of CuCl2 adsorption on carbonaceous materials surface. Figure 3. Stable optimized structures of Hg0 adsorption on CuCl2-impregnated carbonaceous materials surface. Figure 4. The energy profile of heterogeneous mercury oxidation over CuCl2-impregnated carbonaceous materials surface.


Page 28 of 32

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Energy & Fuels

Figure 1. Surface model of carbonaceous materials. a, b, c denote C2, C3, and bridge adsorption sites, respectively. C2 and C3 denote two-fold and three-fold coordinated carbon atoms, respectively. a

b

c

Benzene ring C


H

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 30 of 32

Figure 2. Stable optimized structures of CuCl2 adsorption on carbonaceous materials surface. 36

35

36 15

14

11 12

6 16

28 24 27

19 18

26

21

30

A1

31

23 20

2 1

29 25

7 3

5 17

22 10

4

13

35

37

9 8

36

34

32

15

33

14

37

11 12

9 8

6

29 25

19

27

1

18

26

16

21

30

A2


12 33

34

11

15

28 24

20

2

32

31

23

7 3

5 17

22 10

4

13

35

37

14

5

31

20

2

19

27

18

26

16

21

30

A3

33

24

1

32

28

23

7 3

6

17

29 25

10

4

13

22

9 8

34

Page 31 of 32 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

Energy & Fuels

Figure 3. Stable optimized structures of Hg0 adsorption on CuCl2-impregnated carbonaceous materials surface.

B2

B1 Cl


Cu

Hg

C

H

Energy & Fuels

Figure 4. The energy profile of heterogeneous mercury oxidation over CuCl2-impregnated carbonaceous materials surface. 4.960 Å 3.091 Å

400 TS3 319.80

Hg0

TS2 271.04

300 2.497 Å

Relative energy (kJ/mol)

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 32 of 32

FS 317.34

71.02 IM3

7.101 Å

248.78 200 HgCl2

254.81

2.516 Å

TS1 68.99

100 Hg0+CuCl2-surface

IM2

0

16.23

173.39

0.00

CuCl

4.822 Å

HgCl 3.785 Å

-100

2.120 Å

IM1

-104.40 Cl

Cu

Hg

-200 Hg0 adsorption

Cl migration

HgCl2 formation Reaction pathway


HgCl2 desorption

C

H

Theoretical study on Hg0 adsorption and oxidation mechanisms over

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