Mechanism of Heterogeneous Mercury Oxidation by HBr over V2O5

Subscriber access provided by University of Sussex Library

Article

Mechanism of Heterogeneous Mercury Oxidation by HBr over V2O5/TiO2 Catalyst Zhen Wang, Jing Liu, Bingkai Zhang, Yingju Yang, Zhen Zhang, and Sen Miao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00549 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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 free 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 accessible to all readers and 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.

Environmental Science & Technology 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 31

Environmental Science & Technology

1

Mechanism of Heterogeneous Mercury Oxidation by HBr

2

over V2O5/TiO2 Catalyst

3

Zhen Wang, Jing Liu,* Bingkai Zhang, Yingju Yang, Zhen Zhang and Sen Miao

4

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

5

430074, China

6

ABSTRACT:

7

(SCR) system is a promising method to reduce mercury emissions from coal-burning power plants. The

8

density functional theory (DFT) and periodic slab models were used to study the reaction mechanism of

9

Hg0 oxidation by HBr on V2O5/TiO2 SCR catalyst surface. The interaction mechanisms of Hg0, HBr,

10

HgBr, and HgBr2 on V2O5/TiO2(001) were investigated. The oxidation reaction energy profiles and the

11

corresponding geometries of the intermediates, final states and transition states were researched. The

12

results indicate that Hg0 and HgBr2 are weakly adsorbed on the oxygen sites of the V2O5/TiO2(001)

13

surface with physisorption. HgBr is chemically adsorbed on the surface. HBr is dissociatively adsorbed

14

on the surface with an energy barrier of 85.59 kJ/mol. The reaction of Hg0 oxidation by HBr follows the

15

Eley-Rideal mechanism: Hg0 interacts with a surface Br from HBr dissociation to form HgBr, and

16

surface HgBr further interacts with HBr to form HgBr2, lastly HgBr2 desorbs from the surface.

17

Comparing the energy pathway of Hg0 oxidation over V2O5/TiO2(001) surface by HBr to that of HCl, it

18

is found that the dissociation energy barrier of HBr is lower than that of HCl, the formation and

19

desorption energy barriers of HgBr2 are also lower than that of HgCl2, which explains why HBr is much

20

more effective than HCl in promoting Hg0 oxidation.

21

KEYWORDS: Hg0 oxidation mechanism; V2O5/TiO2; HBr; SCR catalyst

Catalytic oxidation of elemental mercury (Hg0) through selective catalytic reduction

ACS Paragon Plus Environment


Page 2 of 31

22

23

1. INTRODUCTION

24

Mercury has become a global concern pollution because of its high volatility, hypertoxicity,

25

bioaccumulation and durability.1 The largest anthropogenic sources of mercury emissions are the coal-

26

burning power plants.2 In July 2011, the State Environmental Protection Administration of China (SEPA)

27

released a new national standard (GB 13223–2011), in which the emission limit of mercury and its

28

compounds from coal-burning power plants is 0.03 mg/m3. In December 2011, the standard to limit

29

mercury, acid gases and other toxic pollution from power plants was declared by the U.S.

30

Environmental Protection Agency (EPA), which demands 91% of the mercury present in flue gas should

31

be captured for coal-burning power plants.3 In January 2013, the first international legally binding treaty

32

to limit mercury emissions, the Minamata Convention on Mercury was ratified by delegates from 140

33

countries. Therefore, effective technologies to control mercury emission from coal-burning power

34

plants require to be developed.

35

There are three forms of mercury in coal-burning flue gas: elemental mercury (Hg0), oxidized

36

mercury (Hg2+), and particulate mercury (Hgp), among which Hg2+ and Hgp are easy to be captured by

37

Air pollution control devices (APCDs).4,5 However, Hg0 is difficult to be controlled due to its water

38

insolubility and chemical inertness. The titania-supported vanadium oxides, V2O5/TiO2, is a generally

39

employed catalyst in commercial selective catalytic reduction (SCR) units.6 The active phase V2O5 can

40

not only catalyze NOx reduction but also promote Hg0 oxidation.7 Therefore, promoting Hg0 oxidation

41

through SCR units to Hg2+, which can be effectively captured in downstream APCDs,

42

is universally considered to be a cost-effective method to reduce mercury emissions from coal-burning ACS Paragon Plus Environment 2

Page 3 of 31


43

power plants.8 However, this method has a good effect for utilities when burning bituminous coals, but

44

does not work well for utilities when burning low-rank coals due to its lower chlorine content. Thus,

45

mercury removal with low-rank coals has faced more challenges.9

46

The experimental studies10,11 showed that the injection of oxidant, such as halogen species, upstream

47

of APCDs appears to be an efficient way to enhance mercury removal efficiency of low-rank coals. In

48

particular, HBr has been found to be one of the most efficient reagents in promoting Hg0 oxidation

49

through SCR system. The bromide injection has been tested in a full-scale system burning

50

sub-bituminous coal and has demonstrated good effects such as approximately 78% of the Hg0

51

oxidation through SCR at an injection rate of 3 ppm.12 Eswaran et al.13 have carried out experiments in

52

a lab-scale SCR reactor and found that HBr was shown to be a stronger mercury oxidant than HCl, more

53

than 85% of the Hg0 was oxidized even at a low HBr concentration of 2 ppm. The experimental studies

54

by Stolle et al.14 also observed that HBr was ten times more effective than HCl in facilitating the

55

oxidation of Hg0 over SCR catalyst. Vosteen et al.15 reported that the smaller production of Cl2 from

56

HCl than Br2 from HBr, which later oxidizes Hg0 via the Deacon process, may cause the different effect

57

of HBr and HCl on Hg0 oxidation, but the direct evidence is not provided. In addition, the previous

58

experimental studies indicated that the Deacon process was not the dominate pathway for Hg0

59

oxidation.16-18 To date, there is no general agreement on the reason for HBr is a much more effective

60

Hg0 oxidant than HCl over SCR catalyst. Therefore, the specific reaction mechanism of Hg0 oxidation

61

by HBr over V2O5/TiO2 catalyst, which is crucial for developing the reaction model and the kinetic

62

parameters, should be studied.

ACS Paragon Plus Environment 3


Page 4 of 31

63

Although there have been many experimental studies of Hg0 oxidation by HBr on V2O5/TiO2

64

catalyst, the theoretical studies on this subject are still very limited. Quantum chemistry methods, based

65

on density functional theory (DFT), have been proven effectively and increasingly used to study the

66

heterogeneous reaction mechanisms over catalysts.19-23 Negreira et al.24,25 have performed DFT

67

calculations to investigate the adsorption of Hg0, HgCl, HCl, and H2O on V2O5/TiO2 catalyst. They

68

found that the adsorbed H2O could increase the reactivity of the V2O5/TiO2(001) surface. In our

69

previous study, the mechanism of heterogeneous Hg0 oxidation by HCl over V2O5/TiO2 catalyst was

70

researched using DFT calculations.21,26 However, the detailed oxidation mechanisms of Hg0 by HBr

71

over V2O5/TiO2 catalysts have not been studied yet.

72

In this study, we focus on the adsorption of mercury-bromine species (Hg0, HgBr and HgBr2), the

73

adsorption and dissociation of HBr, and the reaction pathway of Hg0 oxidation by HBr over V2O5/TiO2

74

catalyst. Besides, the corresponding transition states and the energy barriers for the reaction pathway,

75

which are important for understanding the reaction mechanisms and building kinetic models, were also

76

investigated. As far as we know, none of the previously theoretical study involving Hg0 oxidation by

77

HBr on catalysts. The results of this study will provide insight into the detailed reaction mechanism of

78

Hg0 oxidation by HBr over V2O5/TiO2 catalyst and may be used to make clear why HBr is much more

79

effective in promoting Hg0 oxidation than HCl over SCR catalyst.

80

2. COMPUTATIONAL DETAILS

81

2.1. Catalyst Models

82

In order to understand the adsorption behavior of mercury-bromine species and oxidation pathway

83

of Hg0 on V2O5/TiO2 catalysts surface at the molecular levels, it is necessary to establish appropriate ACS Paragon Plus Environment 4

Page 5 of 31


84

models to simulate the V2O5/TiO2 structure. In this study, the periodic supported vanadium oxide

85

catalyst V2O5/TiO2(001) model, which was constructed on account of the reconstructed anatase

86

ad-molecule model proposed by Selloni et al.27,28, was applied because the practical SCR occur on the

87

titania-supported vanadium oxide catalysts. It has been broadly applied for different courses of

88

reaction.24-26,29,30 The configuration of V2O5/TiO2(001) surface and the considered active sites are

89

shown in Figure 1. The vanadia dimer is loaded on a (3 × 2)-reconstructed anatase TiO2(001) surface

90

with a superficial area of 0.85 nm2 (i.e. 7.552 Å × 11.328 Å) as shown in Figure 1b. In the vanadia

91

dimer, there are three types of oxygen sites: O(1), the single coordinated oxygen (V=O); O(2), the

92

bridge oxygen (V−O−V); and O(3), the anchor oxygen (V−O−Ti).

93

2.2. Computational Methods

94

The calculations were performed using the CASTEP program package31 based on the density

95

functional theory. The electron exchange correlation functionals were calculated using the generalized

96

gradient approximation (GGA)32 with the Perdew-Burke-Ernzerhoff (PBE).33 The electron-ion

97

interactions were described by Vanderbilt Ultrasoft pseudopotentials.34 The electronic wave functions

98

were expanded through plane waves with the kinetic energy cutoff of 400 eV. The grid independence

99

test certified that the proper grid size for this study is a 4 × 4 × 1 Monkhorst-Pack k-point grid with

100

consideration both the accuracy and the computational time. The edge conditions were periodic in three

101

dimensions. The vacuum space between slabs is 12 Å, which is verified to be large enough to prevent

102

the interactions between slabs of V2O5/TiO2(001) surface. The position of all atoms were allowed to

103

relax throughout the calculations. The convergence criteria for the structure optimization and energy

104

calculation were set as (a) an energy tolerance of 2.0 × 10−5 eV/atom; (b) a SCF tolerance of 2.0 × 10−6



Page 6 of 31

105

eV/atom; (c) a maximum displacement tolerance of 0.002 Å; (d) a maximum force tolerance of 0.05

106

eV/Å.

107

The optimal bulk lattice parameters (a = b = 3.776 Å, c = 9.486 Å) were less than +0.16% and

108

+0.17% error of the experimental determined lattice constants35, suggesting that the calculations are

109

credible. Gas-phase species (Hg0, HBr, HgBr and HgBr2) were optimized as isolated molecules in a

110

cubic crystal cell with constant of 10 Å. The examined bond distances of HgBr and HgBr2 are 2.612 Å

111

and 2.435 Å, respectively, which are in good agreement with the experimental values of 2.62 Å

112

(HgBr)36 and 2.37 Å (HgBr2)37. The adsorption energy (Eads) was calculated as follows:

113

Eads = E(adsorbate－substrate) – (Eadsorbate + Esubstrate)

(1)

114

where E(adsorbate－substrate), Eadsorbate and Esubstrate represent the total energies of the substrate with

115

adsorbate, the isolated adsorbate at its equilibrium geometry, and the substrate, respectively. A higher

116

negative Eads value corresponds to a stronger interaction. Normally, if the adsorption energy is less than

117

−30 kJ/mol, the interaction belongs to physisorption. If the adsorption energy is higher than −50 kJ/mol,

118

the interaction belongs to chemisorption.38

119

To research the minimum energy pathway for the Hg0 oxidation, Linear synchronous

120

transit/quadratic synchronous transit (LST/QST) tools, which have been well validated to search

121

transition state (TS) structure, were performed to search for all TSs along the reaction pathway.39 The

122

LST method was performed to obtain a maximum energy pathway between the reactant and product,

123

and TS approximation was constructed from the pathway by QST maximization. TSs were confirmed

124

by the amount of imaginary vibration with one negative frequency. The energy barrier was defined as

125

follow:


Page 7 of 31


126

Ebarrier = E(transition state) – Eintermediate

127

(2)

where E(transition state) and Eintermediate represent the total energy of transition state (TS) and intermediate

128

(IM), respectively.

129

3. RESULTS AND DISCUSSION

130

3.1. Adsorption of Hg0 on V2O5/TiO2(001) Surface

131

The first step of heterogeneous reaction over catalysts is the adsorption of gas phase reactants.

132

Consequently, before exploring the pathway of Hg0 oxidation reaction, the adsorption of Hg0, HgBr,

133

HgBr2 and HBr over V2O5/TiO2 catalyst surface were studied, respectively. Hg atom was placed on the

134

different adsorption sties of the surface: O(1) top, O(2) top, O(3) top and O(1)−O(2) bridge. The

135

adsorption energies of Hg0 is ranging from −10.61 to −27.93 kJ/mol, indicating that Hg0 adsorption on

136

V2O5/TiO2(001) is physisorption, which is in good agreement with previous experimental results.17,40,41

137

3.2. Adsorption of HgBr on V2O5/TiO2(001) Surface

138

HgBr could serves as an intermediate in the course of Hg0 oxidation. Therefore, investigating the

139

adsorption of HgBr on V2O5/TiO2(001) surface is helpful to better comprehend the global Hg0 oxidation

140

reaction and the surface reactivity. All potential adsorption orientations and adsorption sites were

141

considered. Figure 2 shows the stable configurations of HgBr on the different sites of the

142

V2O5/TiO2(001) surface, while the corresponding adsorption energies and geometric parameters are

143

given in Table 1. The most stable binding structure of HgBr is 1A with Hg-down direction upon

144

O(1)-top site. The adsorption energy is −138.30 kJ/mol and the Hg–Br bond distance is slightly

145

decreased to 2.423 Å. Besides, the Mulliken charge (i.e., Q in Table 1) analyses show that 0.48

146

electrons transport from HgBr to the surface, indicating a strong chemisorption. In the case of structure



Page 8 of 31

147

1B, the adsorption energy is −73.68 kJ/mol and the Mulliken charge of HgBr is 0.20 e, suggesting O(2)

148

site is also active for HgBr adsorption. In addition, for the Br-down orientation on the V-top site, the

149

adsorption energy of HgBr is −81.65 kJ/mol with Br−V bond length of 2.585 Å. Comparing the values

150

of adsorption energy and Mulliken charge, HgBr prefers interacting with O(1) atoms.

151

Density of state (DOS) analysis was carried out to get a deeper understanding of the interacting

152

mechanism of HgBr on the V2O5/TiO2(001) surface. The DOS of the system depicts the number of

153

states per interval of energy at each energy level that are usable to be occupied.42 Figure 3 displays the

154

partial density of states (PDOS) of Hg atom from HgBr and O(1) atom from V2O5/TiO2(001). Before

155

adsorption, the DOS of the Hg atom is represented by well-defined s- and d-orbitals near −4.8 and −2.3

156

eV, respectively, and an unfilled p-orbital near 5.6 eV. After adsorption, all orbitals of Hg atom shift

157

downward with the s- and d-orbitals broadened and decreased in energy due to the charge transfer from

158

the Hg to the O(1) atom of the surface. In particular, the p-orbital of O(1) atom are strongly hybridized

159

with the s- and d-orbitals of Hg atom at approximately −2.6 and −6.0 eV, respectively. The strong

160

orbital hybridization between Hg and O(1) atoms orbitals strengthens the binding of Hg atom on the

161

V2O5/TiO2(001) surface, leading to strong interaction between HgBr and V2O5/TiO2 surface, which

162

agrees well with the previous discussion.

163

3.3. Adsorption of HgBr2 on V2O5/TiO2(001) Surface

164

HgBr2 is the most important product in the Hg0 oxidation reaction by HBr addition.43 The stable

165

configuration for HgBr2 binding on the V2O5/TiO2(001) surface is shown in Figure 2 as 1F, and the

166

adsorption energies and geometric parameters are listed in Table 1. The results show that the most stable

167

structure (1F) for HgBr2 adsorption on the surface is on O(1)-top site with parallel orientation. The


Page 9 of 31


168

adsorption energy is −9.21 kJ/mol, and the distance between Hg and O(1) atom is 2.855 Å.

169

Consequently, HgBr2 is physically adsorbed on V2O5/TiO2(001) surface, and the desorption of HgBr2

170

from the surface will easily happen.

171

3.4. Interaction of HBr with V2O5/TiO2(001) Surface

172

HBr is one of the most efficient reactants in promoting Hg0 oxidation through SCR system. The

173

study on the interaction of HBr with V2O5/TiO2 catalyst will contribute to comprehend the activity of

174

the catalyst and the global Hg0 oxidation reaction. Figure 4 shows the stable binding structures of HBr

175

on the V2O5/TiO2(001) surface, and the corresponding energetics and geometries are provided in Table

176

2. The most stable binding structure of HBr on the V2O5/TiO2(001) surface is 2B with an adsorption

177

energy of −34.11 kJ/mol. In 2B, HBr interacts with O(1) atom, and H−Br bond distance is extended

178

from 1.435 Å (free HBr) to 1.452 Å. The Mulliken charge analyses show that 0.09 electrons transport

179

from HBr to the surface.

180

Heterogeneous catalytic reaction includes the adsorption, activation and reaction of reactants.

181

Therefore, after investigating the binding of HBr, the dissociation of HBr (i.e., activation of HBr) over

182

V2O5/TiO2 surface was further examined. The most stable binding structure of HBr on the

183

V2O5/TiO2(001) surface (2B in Figure 4) was acted as the original intermediate (IM1) for HBr

184

dissociation course. The potential dissociation of HBr on the V2O5/TiO2(001) surface was studied in the

185

following modes: (1) O(1)−Br and O(1)−H (where Br and H are separately placed on the O(1)-top sites);

186

(2) O(2)−Br and O(1)−H (where Br and H are placed on the O(2) and O(1) sites, respectively); and (3)

187

V−Br and O(1)−H (where Br and H are placed on the V and O(1) sites, respectively). The calculation

188

results indicate that the first two modes are not energetically favored because of the positive adsorption



Page 10 of 31

189

energy. The last one is energetically favored because of the negative adsorption energy, the

190

corresponding structure indicates that H bonded to the O(1) atom and Br bonded to the V atom forming

191

vanadium oxy-bromide complex. Therefore, HBr dissociates on the V2O5/TiO2(001) surface follows the

192

last manner.

193

The energy diagram and the corresponding configurations of intermediate (IM), transition state (TS),

194

and final state (FS) for HBr dissociation are shown in Figure 5. HBr dissociates on the surface via the

195

transition state TS1 to form the final state FS1. During this step, the distance between H and Br is

196

elongated from 1.452 Å in IM1 to 2.361 Å in TS1, suggesting the cracking of the H-Br bond. In the

197

meantime, the interval between O(1) and H lessens gradually: 1.904 Å in IM2 and 1.019 Å in TS1,

198

suggesting the H-O bond was formed. This process (IM1 → TS1 → FS1) is exothermic by 78.43 kJ/mol.

199

The energy barrier is 85.59 kJ/mol, which is lower than the energy barrier of HCl dissociation (101.53

200

kJ/mol) on V2O5/TiO2(001) surface obtained in our previous study.26 This indicates the dissociation of

201

HBr is easier than that of HCl, which is consistent well with the previous experimental studies.44 The

202

above analysis suggests that HBr could dissociates on V2O5/TiO2(001) surface and converts into surface

203

hydroxyl and vanadium oxy-bromide species, which is similar to previous theoretical and experimental

204

studies of HCl dissociation behaviors on V2O5/TiO2 catalyst.17,26 In addition, the relatively high energy

205

barrier (85.59 kJ/mol) indicates that the activation of HBr on V2O5/TiO2 surface is unlikely to take place

206

under low temperature, which is in good agreement with the experimental observation that V2O5/TiO2

207

catalysts display poor activity under 300 oC.45

208

3.5. Reaction Mechanism of Hg0 Oxidation by HBr on V2O5/TiO2 Surface


Page 11 of 31


209

Based on the above results, Hg0 is physically adsorbed on V2O5/TiO2(001) surface and HBr

210

dissociates on V2O5/TiO2(001) surface, it can be concluded that Hg0 oxidation by HBr via Eley-Rideal

211

mechanism, where Hg0 oxidation reaction begins with the dissociation of HBr to form vanadium

212

oxy-bromide compounds, then Hg0 interacts with the activated bromine atoms on V2O5/TiO2(001)

213

surface. Furthermore, the chemical adsorption of HgBr is stronger than the physical adsorption of

214

HgBr2, suggesting that HgBr-surface is a crucial intermediate and Hg0 oxidation tends to the pathway

215

Hg0 → HgBr → HgBr2 instead of pathway Hg0 → HgBr2.

216

Figure 6 lists the energy profiles and reaction pathways for Hg0 oxidation by HBr, and the

217

corresponding configurations of the intermediates, transition states, and final states in the pathways are

218

shown in Figure 7. The energies of the optimized configurations are relative to the reactants. In

219

consideration of the two stable adsorption configurations (Hg-down and Br-down) of HgBr on

220

V2O5/TiO2(001) surface, two paths for Hg0 oxidation by HBr were researched and shown in Figure 6.

221

Both paths undergo three processes: (1) Hg0 + HBr → HgBr, (2) HgBr + HBr → HgBr2, and (3) HgBr2

222

desorption. The energy barrier of Hg0 oxidation through the two pathways are given in Table 3.

223

In the case of pathway 1, the step one (Hg0 + HBr → HgBr), HgBr is formed via reactants → IM2

224

→ TS2 → IM3 on account of the above analysis of HgBr adsorption. It is exothermic by 67.01 kJ/mol

225

with an energy barrier of 23.52 kJ/mol. In IM3, HgBr is bonded to O(1) atom with O−Hg and Hg−Br

226

bond distances of 2.407 and 2.617 Å, respectively. The second step (HgBr + HBr → HgBr2) occurs via

227

IM3 → IM4 → TS3 → IM5. In this step, gaseous HBr is adsorbed on IM3 with an adsorption energy of

228

-54.90 kJ/mol to form IM4, and then HgBr2 forms on the surface via transition state TS3. In TS3, the Br

229

and H atoms of adsorbed HBr could be attracted by an adjacent Hg atom of HgBr and O(1) atom of the



Page 12 of 31

230

surface, respectively, which results in the crack of H−Br bond (elongated to 1.999 Å) and the formation

231

of Br−Hg…Br complex. In IM5, HgBr2 is formed with Hg−Br bond distance of 2.449 and 2.453 Å,

232

which are slightly larger than the bond distance of free HgBr2 molecule (2.435 Å) because of the

233

weakly interaction between HgBr2 and the surface. The energy barrier for this step is 76.60 kJ/mol

234

higher than that of the step one, indicating the rate-determining step of the pathway 1.The final step

235

(HgBr2 desorption) occurs via IM5 → TS4 → FS2 + HgBr2. This process is endothermic by 3.49 kJ/mol

236

with an energy barrier of 17.05 kJ/mol, indicating HgBr2 desorption readily occurs. After the transition

237

state TS4, the desorption of HgBr2 molecule is completed and two adsorbed H atoms are left at O(1) site.

238

The reaction energy for the whole pathway 1 is 144.58 kJ/mol, which is an exothermic reaction.

239

In the case of pathway 2, there is another potential reaction path for the first step Hg0 + HBr →

240

HgBr, where HgBr is directly formed through the adsorption of Hg0 on HBr pre-dissociated surface to

241

form IM2, and this process is spontaneous with an adsorption energy of 43.61 kJ/mol. In IM2, HgBr is

242

bonded to V atom with V−Br and Br−Hg bond distances of 2.505 and 2.741 Å, respectively. The step

243

two (HgBr + HBr → HgBr2) occurs via IM6→ TS5 → IM7, which is analogous to that of pathway 1.

244

This process is exothermic by 46.60 kJ/mol. The energy barrier is 66.97 kJ/mol which is smaller than

245

that of pathway 1. The final step is HgBr2 desorption through IM7 → TS6 → FS3 + HgBr2 with an

246

energy barrier of 19.55 kJ/mol. The whole pathway 2 is also an exothermal reaction with a reaction

247

energy of 144.26 kJ/mol.

248

Overall, comparing the two reaction pathways for Hg0 oxidation by HBr over V2O5/TiO2 catalyst,

249

pathway 2 is kinetically more beneficial than pathway 1. Nevertheless, it is noteworthy that the

250

superiority of pathway 2 is not remarkable, indicating that Hg0 oxidation reaction may occurs via both


Page 13 of 31


251

pathways, and pathway 2 is dominant because of its lower energy barriers. The rate-determining step is

252

the formation of HgBr2.

253

In addition, after Hg0 oxidation, two hydrogen atoms are left on V2O5/TiO2(001) surface to form

254

two surface hydroxyl groups as shown in FS2 and FS3 of Figure 7, and then water is formed via the

255

recombination of the two surface hydroxyl groups, and eventually desorbed from the surface. The

256

desorption of water will create O(1) vacant sites, which can be reoxidized by gas-phase O2 to form the

257

initial V2O5/TiO2 catalyst.

258

Furthermore, by comparing the formation and desorption energy barriers of HgBr2 to those of

259

HgCl226 on V2O5/TiO2(001) surface, it is found that the formation energy barrier of HgBr2 (66.97 kJ/mol)

260

is lower than that of HgCl2 (91.53 kJ/mol), meanwhile, the desorption energy barrier of HgBr2 (17.05

261

kJ/mol) is also lower than that of HgCl2 (21.03 kJ/mol), suggesting that the formation and desorption of

262

HgBr2 over V2O5/TiO2 catalyst are easier than those of HgCl2. This explains why HBr is a much more

263

effective Hg0 oxidant than HCl over V2O5/TiO2 catalyst.

264 265

■ AUTHOR INFORMATION

266

Corresponding Author

267

*Tel: +86 27 87545526; fax: +86 27 87545526; e-mail address: [email protected].

268

Notes

269

The authors declare no competing financial interest.

270

■ ACKNOWLEDGEMENTS



Page 14 of 31

271

This work was supported by National Natural Science Foundation of China (51376072), National

272

Basic Research Program of China (2014CB238904), Natural Science Foundation of Hubei Province

273

(2015CFA046) and Basic Research Project of Shenzhen (JCYJ20150831202633340).

274

■ REFERENCES

275

(1) Liu, J.; Abanades, S.; Gauthier, D.; Flamant, G.; Zheng, C.; Lu, J. Determination of kinetic law for

276

toxic metals release during thermal treatment of model waste in a fluid-bed reactor. Environ. Sci.

277

Technol. 2005, 39, (23), 9331–9336.

278

(2) Liu, J.; Qu, W.; Joo, S. W.; Zheng, C. Effect of SO2 on mercury binding on carbonaceous surfaces.

279

Chem. Eng. J. 2012, 184, 163–167.

280

(3) Kasey, P. EPA Issues Long-Expected Mercury Rule for Power Plants. State Journal (WV) 2011.

281

(4) Galbreath, K. C.; Zygarlicke, C. J. Mercury speciation in coal combustion and gasification flue

282

gases. Environ. Sci. Technol. 1996, 30, (8), 2421–2426.

283

(5) Liu, J.; Qu, W.; Zheng, C. Theoretical studies of mercury–bromine species adsorption mechanism

284

on carbonaceous surface. Proc. Combust. Inst. 2013, 34, (2), 2811–2819.

285

(6) Kamata, H.; Ueno, S.-I.; Naito, T.; Yamaguchi, A.; Ito, S. Mercury oxidation by hydrochloric acid

286

over a VOx/TiO2 catalyst. Catal. Commun. 2008, 9, (14), 2441–2444.

287

(7) Surnev, S.; Ramsey, M.; Netzer, F. Vanadium oxide surface studies. Prog. Surf. Sci. 2003, 73, (4),

288

117–165.

289

(8) Cao, Y.; Chen, B.; Wu, J.; Cui, H.; Smith, J.; Chen, C. K.; Chu, P.; Pan, W. P. Study of mercury

290

oxidation by a selective catalytic reduction catalyst in a pilot-scale slipstream reactor at a utility boiler

291

burning bituminous coal. Energy Fuels 2007, 21, (1), 145–156.


Page 15 of 31


292

(9)

Kilgroe, J. D.; Sedman, C. B.; Srivastava, R. K.; Ryan, J. V.; Lee, C.; Thorneloe, S. A.; Pages, E.

293

Control of mercury emissions from coal-fired electric utility boilers: interim report including errata

294

dated 3-21-02. Carbon 2001, 5, 33.

295

(10) Liu, S. H.; Yan, N. Q.; Liu, Z. R.; Qu, Z.; Wang, H. P.; Chang, S. G.; Miller, C. Using bromine gas

296

to enhance mercury removal from flue gas of coal-fired power plants. Environ. Sci. Technol. 2007, 41,

297

(4), 1405–1412.

298

(11) Cao, Y.; Wang, Q. H.; Li, J.; Cheng, J. C.; Chan, C. C.; Cohron, M.; Pan, W. P. Enhancement of

299

mercury capture by the simultaneous addition of hydrogen bromide (HBr) and fly ashes in a slipstream

300

facility. Environ. Sci. Technol. 2009, 43, (8), 2812–2817.

301

(12) Berry, M.; Dombrowski, K.; Richardson, C.; Chang, R.; Borders, E.; Vosteen, B. Mercury control

302

evaluation of calcium bromide injection into a PRB-fired furnace with an SCR, In Proceedings of the

303

Air Quality VI Conference, Arlington, VA, 2007.

304

(13) Eswaran, S.; Stenger, H. G. Effect of halogens on mercury conversion in SCR catalysts. Fuel

305

Process. Technol. 2008, 89, (11), 1153–1159.

306

(14) Stolle, R.; Koeser, H.; Gutberlet, H. Oxidation and reduction of mercury by SCR DeNOx catalysts

307

under flue gas conditions in coal fired power plants. Appl. Catal. B: Environ. 2014, 144, 486–497.

308

(15) Vosteen, B. W.; Kanefke, R.; Koser, H. Bromine-enhanced mercury abatement from combustion

309

flue gases-recent industrial applications and laboratory research. VGB powertech 2006, 86, (3), 70–75.

310

(16) Wang, J.; Anthony, E. J. An analysis of the reaction rate for mercury vapor and chlorine. Chem.

311

Eng. Technol. 2005, 28, (5), 569–573.

312

(17) He, S.; Zhou, J.; Zhu, Y.; Luo, Z.; Ni, M.; Cen, K. Mercury oxidation over a vanadia-based



Page 16 of 31

313

selective catalytic reduction catalyst. Energy Fuels 2008, 23, (1), 253–259.

314

(18) Niksa, S.; Fujiwara, N. A predictive mechanism for mercury oxidation on selective catalytic

315

reduction catalysts under coal-derived flue gas. J. Air Waste Manage. Assoc. 2005, 55, (12), 1866–1875.

316

(19) Zhang, B.; Liu, J.; Zhang, J.; Zheng, C.; Chang, M. Mercury oxidation mechanism on Pd(100)

317

surface from first-principles calculations. Chem. Eng. J. 2014, 237, 344–351.

318

(20) Zhang, B.; Liu, J.; Zheng, C.; Chang, M. Theoretical study of mercury species adsorption

319

mechanism on MnO2(110) surface. Chem. Eng. J. 2014, 256, 93–100.

320

(21) Liu, J.; He, M.; Zheng, C.; Chang, M. Density functional theory study of mercury adsorption on

321

V2O5(001) surface with implications for oxidation. Proc. Combust. Inst. 2011, 33, (2), 2771–2777.

322

(22) Zhang, B.; Liu, J.; Yang, Y.; Chang, M. Oxidation mechanism of elemental mercury by HCl over

323

MnO2 catalyst: Insights from first principles. Chem. Eng. J. 2015, 280, 354–362.

324

(23) Lim, D. H.; Wilcox, J. Heterogeneous mercury oxidation on Au(111) from first principles.

325

Environ. Sci. Technol. 2013, 47, (15), 8515-8522.

326

(24) Suarez Negreira, A.; Wilcox, J. DFT study of Hg oxidation across vanadia-titania SCR catalyst

327

under flue gas conditions. J. Phys. Chem. C. 2013, 117, (4), 1761–1772.

328

(25) Suarez Negreira, A.; Wilcox, J. Role of WO3 in the Hg oxidation across the V2O5–WO3–TiO2 SCR

329

catalyst: A DFT study. J. Phys. Chem. C. 2013, 117, (46), 24397-24406.

330

(26) Zhang, B.; Liu, J.; Dai, G.; Chang, M.; Zheng, C. Insights into the mechanism of heterogeneous

331

mercury oxidation by HCl over V2O5/TiO2 catalyst: Periodic density functional theory study. Proc.

332

Combust. Inst. 2015, 35, (3), 2855–2865.

333

(27) Lazzeri, M.; Selloni, A. Stress-Driven Reconstruction of an Oxide Surface: The Anatase TiO2


Page 17 of 31


334

(001)−(1× 4) Surface. Phys. Rev. Lett. 2001, 87, (26), 266105.

335

(28) Vittadini, A.; Selloni, A. Periodic density functional theory studies of vanadia-titania catalysts:

336

Structure and stability of the oxidized monolayer. J. Phys. Chem. B. 2004, 108, (22), 7337–7343.

337

(29) Avdeev, V. I.; Tapilin, V. M. Water Effect on the Electronic Structure of Active Sites of Supported

338

Vanadium Oxide Catalyst VOx/TiO2(001). J. Phys. Chem. C. 2010, 114, (8), 3609–3613.

339

(30) Avdeev, V. I.; Bedilo, A. F. Electronic Structure of Oxygen Radicals on the Surface of VOx/TiO2

340

Catalysts and Their Role in Oxygen Isotopic Exchange. J. Phys. Chem. C. 2013, 117, (28),

341

14701–14709.

342

(31) Segall, M.; Lindan, P. J.; Probert, M. a.; Pickard, C.; Hasnip, P.; Clark, S.; Payne, M.

343

First-principles simulation: ideas, illustrations and the CASTEP code. J. Phys.: Condens. Matter 2002,

344

14, (11), 2717.

345

(32) Perdew, J. P.; Burke, K.; Wang, Y. Erratum: Generalized gradient approximation for the

346

exchange-correlation hole of a many-electron system [Phys. Rev. B 54, 16 533 (1996)]. Phys. Rev. B

347

1998, 57, (23), 14999.

348

(33) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys.

349

Rev. Lett. 1996, 77, (18), 3865.

350

(34) Vanderbilt, D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys.

351

Rev. B 1990, 41, (11), 7892–7895.

352

(35) Burdett, J. K.; Hughbanks, T.; Miller, G. J.; Richardson Jr, J. W.; Smith, J. V. Structural-electronic

353

relationships in inorganic solids: powder neutron diffraction studies of the rutile and anatase

354

polymorphs of titanium dioxide at 15 and 295 K. J. Amer. Chem. Soc. 1987, 109, (12), 3639–3646.



Page 18 of 31

355

(36) Tellinghuisen, J.; Ashmore, J. G. The B → X transition in 200Hg 79Br. Appl. Phys. Lett. 1982, 40,

356

(10), 867–869.

357

(37) Deyanov, R.; Petrov, K.; Ugarov, V.; Shchedrin, B.; Rambidi, N. Automatic background

358

subtraction in gas electron diffraction: The covariance matrix in least-squares structure-parameter

359

analysis. J. Struct. Chem. 1985, 26, (5), 698–703.

360

(38) Xin, G.; Zhao, P.; Zheng, C. Theoretical study of different speciation of mercury adsorption on

361

CaO(001) surface. Proc. Combust. Inst. 2009, 32, (2), 2693–2699.

362

(39) Halgren, T. A.; Lipscomb, W. N. The synchronous-transit method for determining reaction

363

pathways and locating molecular transition states. Chem. Phys. Lett. 1977, 49, (2), 225–232.

364

(40) Eom, Y.; Jeon, S. H.; Ngo, T. A.; Kim, J.; Lee, T. G. Heterogeneous mercury reaction on a

365

selective catalytic reduction (SCR) catalyst. Catal. Lett. 2008, 121, (3-4), 219–225.

366

(41) Li, Y.; Murphy, P. D.; Wu, C. Y.; Powers, K. W.; Bonzongo, J.-C. J. Development of

367

silica/vanadia/titania catalysts for removal of elemental mercury from coal-combustion flue gas.

368

Environ. Sci. Technol. 2008, 42, (14), 5304–5309.

369

(42) Xiang, W.; Liu, J.; Chang, M.; Zheng, C. The adsorption mechanism of elemental mercury on

370

CuO(110) surface. Chem. Eng. J. 2012, 200, 91–96.

371

(43) Cao, Y.; Gao, Z.; Zhu, J.; Wang, Q.; Huang, Y.; Chiu, C.; Parker, B.; Chu, P.; Pan, W. P. Impacts of

372

halogen additions on mercury oxidation, in a slipstream selective catalyst reduction (SCR), reactor

373

when burning sub-bituminous coal. Environ. Sci. Technol. 2007, 42, (1), 256–261.

374

(44) Niksa, S.; Naik, C. V.; Berry, M. S.; Monroe, L. Interpreting enhanced Hg oxidation with Br

375

addition at Plant Miller. Fuel Process. Technol. 2009, 90, (11), 1372–1377.


Page 19 of 31


376

(45) Eswaran, S.; Stenger, H. G. Understanding mercury conversion in selective catalytic reduction

377

(SCR) catalysts. Energy fuels 2005, 19, (6), 2328–2334.

378



379

Table 1. Optimized parameters for HgBr and HgBr2 adsorption on V2O5/TiO2(001) surface. Eads (kJ/mol) RHg−O (Å)

RHg−Br (Å)

Q (e)

1A

−138.30

2.070

2.423

0.48

1B

−73.68

2.412

2.601

0.20

1C

−52.35

2.861

2.615

0.01

1D

−81.65

−

2.585

0.23

1E

−38.31

−

2.614

0.01

1F

−9.21

2.855

2.438/2.436

0.04

380 381


Page 20 of 31

Page 21 of 31

382


Table 2. Optimized parameters for HBr adsorption on V2O5/TiO2(001) surface. Eads (kJ/mol) RH−O (Å)

RH−Br (Å)

2A

−11.32

2.029

1.443

2B

−34.11

1.759

1.452

383 384



385

Page 22 of 31

Table 3. Energy barriers (in kJ/mol) for Hg0 oxidation pathways as shown in Fig. 6. Step 1 Hg + HBr → HgBr

Step 2 HgBr + HBr → HgBr2

Step 3 desorption of HgBr2

Pathway 1

23.52

76.60

17.05

Pathway 2

−

66.97

19.65

386 387


Page 23 of 31


388

List of Figures Captions

389

Figure 1. The configuration of V2O5/TiO2(001) surface and the active sites. (a) side view of

390

V2O5/TiO2(001) surface；(b) top view of V2O5/TiO2(001) surface.

391

Figure 2. Optimized configurations of HgBr and HgBr2 adsorption on V2O5/TiO2(001) surface.

392

Figure 3. PDOS of HgBr adsorption on V2O5/TiO2(001) surface before and after adsorption. (a) PDOS

393

of s, p, and d orbital for Hg atom of HgBr and (b) PDOS of s and p orbital for O(1) atom.

394

Figure 4. Optimized configurations of HBr adsorption on V2O5/TiO2(001) surface.

395

Figure 5. The energy profile of HBr adsorption and dissociation on V2O5/TiO2(001) surface and the

396

configurations of correlated intermediate, transition state, and final state.

397

Figure 6. The reaction pathways and energy profiles of Hg0 oxidation by HBr on V2O5/TiO2(001)

398

surface.

399

Figure 7. The configurations of intermediates, transition states, and final states in Hg0 oxidation

400

pathways over V2O5/TiO2(001) surface.

401



Page 24 of 31

Ti O 12 Å

V

Vacuum gap

O(1)

O(2)

O O(3)

VOx

Ti

~0.85 nm2

V

surface area

TiO2

402

(a)

(b)

403

Figure 1. The configuration of V2O5/TiO2(001) surface and the active sites. (a) side view of

404

V2O5/TiO2(001) surface；(b) top view of V2O5/TiO2(001) surface.

405


Page 25 of 31


Br Hg

2.861

2.070

1A

1C

1B

2.855

1D

1E Ti

406 407

O

V

1F Hg

Br

Figure 2. Optimized configurations of HgBr and HgBr2 adsorption on V2O5/TiO2(001) surface.

408



(b)

(a)

PDOS (electrons / eV)

Hg s state

409

O s state

Page 26 of 31

Before adsorption After adsorption

Hg p state O p state Hg d state

Energy (eV)

410

Figure 3. PDOS of HgBr adsorption on V2O5/TiO2(001) surface before and after adsorption. (a) PDOS

411

of s, p, and d orbital for Hg atom of HgBr and (b) PDOS of s and p orbital for O(1) atom.

412


Page 27 of 31


Br H

2.029

1.759

2A 413 414

2B Ti

O

V

H

Br

Figure 4. Optimized configurations of HBr adsorption on V2O5/TiO2(001) surface.

415



Page 28 of 31

Relative energy (kJ/mol)

80 TS1 51.48

40 VTi + HBr (g) 0

0.00 IM1

-40

-34.11 -78.43 FS1

-80 Reaction pathway Br H

1.452 1.759

IM1

2.361 1.019 2.264

TS1

FS1 Ti

416

O

V

H

Br

417

Figure 5. The energy profile of HBr adsorption and dissociation on V2O5/TiO2(001) surface and the

418

configurations of correlated intermediate, transition state, and final state.

419


Page 29 of 31


Pathway 1

VOx−HBr + Hg(g) + HBr (g) 0

Pathway 2 TS2 -41.79 TS5

Relative energy (kJ/mol)

-20.09 -40

IM2 -43.61

-67.01 IM3

TS3 -45.31

-80 -108.76 IM6 -120

IM4 -121.91

-160

Step 1: Hg + HBr → HgBr

-125.71 TS6 -145.36 IM7 TS4 -131.02 IM5 -148.07

-144.26 FS3 + HgBr2 FS2 + HgBr2 -144.58

Step 2: HgBr + HBr → HgBr2 Step 3: desorption of HgBr2

Reaction pathway

420 421

Figure 6. The reaction pathways and energy profiles of Hg0 oxidation by HBr on V2O5/TiO2(001)

422

surface.

423



Page 30 of 31

Hg H

2.617

Br

3.298

2.264

2.505

VOx−HBr

IM2

2.407

IM3

TS2

2.438

2.410

1.807

2.453

2.615

2.629 2.847

IM4

3.158

3.638

TS3

IM5

2.440

TS4

2.600

0.990

1.458 1.801

2.745

2.808

2.506

3.688

IM6

FS2

2.453

2.446

IM7

TS5

2.441 2.441

424

TS6

Ti

H

O

Br

V

Hg

FS3

425

Figure 7. The configurations of intermediates, transition states, and final states in Hg0 oxidation

426

pathways over V2O5/TiO2(001) surface.

427


Page 31 of 31

428


TOC/Abstract Art HgBr2 HBr

HBr

Hg Br

H2O

HgBr

H

429

V2O5/TiO2(001) Surface


Mechanism of Heterogeneous Mercury Oxidation by HBr over V2O5

Recommend Documents