Theoretical Study on Hg0 Adsorption and Oxidation Mechanisms over

Subscriber access provided by Miami University Libraries

Combustion

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 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

1

Theoretical study on Hg0 adsorption and oxidation mechanisms over

2

CuCl2-impregnated carbonaceous materials surface

3 4

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

5

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

6

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

7

University of Science and Technology, Wuhan 430074, China

8 9

Revision Submitted to Energy & Fuels

10 11

*To whom correspondence should be addressed:

12

Email: [email protected]

13

1

ACS Paragon Plus Environment

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

14

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

15

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

16

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

17

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

18

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

19

active chlorine species generated from CuCl2 adsorption can significantly enhance mercury

20

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

21

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

22

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

23

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

24

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

25

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

26

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

27

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

28

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

29

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

30

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

31

2


Page 2 of 32

Page 3 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

32

Energy & Fuels

Introduction

33

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

34

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

35

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

36

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

37

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

38

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

39

urgent to limit mercury emission from coal combustion.

40

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

41

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

42

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

43

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

44

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

45

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

46

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

47

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

48

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

49

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

50

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

51

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

52

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

53

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

54

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

9

especially in the flue gas with low HCl

3


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 4 of 32

55

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

56

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

57

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

58

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

59

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

60

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

61

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

62

need to be discussed.

63

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

64

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

65

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

66

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

67

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

68

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

69

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

70

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

71

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

72

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

73

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

74

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

75

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

76

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

77

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


Hg0 adsorption

Page 5 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

78

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

79

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

80

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

81

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

82

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

83

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

84

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

85

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

86

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

87

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

88

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

89

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

90

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

91

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

92

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

93

sorbents for Hg0 removal in the flue gas.

94

Model development and computational details

95

Model development

96

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

97

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

98

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

99

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

100

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


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

101

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

102

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

103

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

104

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

105

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

106

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

107

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

108

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

109

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

110

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

111

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

112

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

113

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

114

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

115

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

116

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

117

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

118

Methodology

119

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

120

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

121

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

122

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

123

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


Page 6 of 32

Page 7 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

124

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

125

theoretically determined geometries, frequencies and reaction enthalpies to experimental values

126

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

127

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

128

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

129

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

130

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

131

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

132 133

37

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))

134

(1)

135

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

136

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

137

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

138

corresponding adsorption configuration.

139

Results and discussion

140

CuCl2 adsorption on carbonaceous materials surface

141

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

142

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

143

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

144

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

145

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

7


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

146

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

147

CuCl2 adsorption on carbonaceous materials surface.

148

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

149

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

150

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

151

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

152

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

153

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

154

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

155

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

156

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

157

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

158

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

159

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

160

modified carbonaceous materials.

161

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

162

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

163

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

164

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

165

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

166

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

167

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

168

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


Page 8 of 32

Page 9 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

169

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

170

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

171

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

172

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

173

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

174

increases or decreases with varying degrees.

175

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

176

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

177

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

178

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

179

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

180

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

181

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

182

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

183

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

184

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

185

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

186

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

187

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

188

Hg0 adsorption on CuCl2-impregnated carbonaceous materials surface

40

in founding that CuCl diffraction peak

189

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

190

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

191

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


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

192

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

193

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

194

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

195

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

196

respectively.

197

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

198

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

199

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

200

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

201

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

202

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

203

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

204

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

205

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

206

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

207

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

208

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

209

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

210

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

211

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

212

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

213

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

214

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


Page 10 of 32

Page 11 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

215

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

216

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

217

The XAFS characterization

218

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

219

test results.

23

of mercury-loaded CuCl2 modified carbonaceous materials found that

220

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

221

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

222

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

223

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

224

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

225

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

226

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

227

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

228

surface.27,

229

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

33, 34

23

which

It indicates that CuCl2 modification can significantly enhance mercury capture

230

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

231

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

232

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

233

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

234

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

235

Hg0 oxidation on CuCl2-impregnated carbonaceous materials surface

236

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


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

238

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

239

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

240

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

241

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

242

carbonaceous materials surface. The corresponding optimized configuration of intermediate,

243

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

244

Heterogeneous mercury oxidation over CuCl2-impregnated carbonaceous materials surface

245

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

246

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

247

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

248

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

249

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

250

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

251

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

253

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

254

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

255

(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

257

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

258

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

259

reaction heat of 68.56 kJ/mol.

12


Page 12 of 32

Page 13 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

260

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

261

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

262

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

263

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

264

surface.

43-45

Therefore, HgCl2 formation is

265

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

266

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

267

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

268

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

269

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

270

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

271

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

272

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

273

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

274

Conclusions

275

Hg0 adsorption and oxidation mechanisms over CuCl2-impregnated carbonaceous materials

276

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

278

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

279

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

280

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

281

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


Page 14 of 32

Page 15 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

314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359

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


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

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


Page 16 of 32

Page 17 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

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


Page 18 of 32

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


Page 21 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

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


Page 22 of 32

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

Page 29 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 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

Recommend Documents