Mechanisms of Elemental Mercury Transformation on Î±-Fe2O3(001

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Mechanisms of Elemental Mercury Transformation on #-Fe2O3(001) Surface from Experimental and Theoretical Study: Influences of HCl, O2 and SO2 Ting Liu, Lucheng Xue, Xin Guo, Jia Liu, Yu Huang, and Chuguang Zheng Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03406 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016

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Environmental Science & Technology

Mechanisms of Elemental Mercury Transformation on α-Fe2O3(001) Surface from Experimental and Theoretical Study: Influences of HCl, O2 and SO2 Ting Liu†, Lucheng Xue‡, Xin Guo*†, Jia Liu†, Yu Huang†, Chuguang Zheng† †

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

Huazhong University of Science and Technology, Wuhan 430074, China ‡

China United Engineering Corporation, Bin an Road, 1060, Binjiang District, Hangzhou

310052, China KEYWORDS. Mercury

α-Fe2O3

HCl O2

SO2

mechanism

1 Environment ACS Paragon Plus


1

ABSTRACT. The reaction mechanisms of a mixture gas of HCl, O2 and SO2 in Hg0

2

adsorption on α-Fe2O3(001) surface are clarified by a group of adsorption experiments and

3

theoretical calculations based on the density functional theory. The role of O2 in removing

4

Hg0 is greatly influenced by the reaction temperature, meanwhile, the O atom coverage could

5

affect the adsorption performance of Hg0. The dissociated O2 competes with the active sites

6

of Cl species on Fe surface at low temperature, however, at medium temperature HCl and O2

7

could simultaneously facilitate the Hg0 transformation. Combining with the theoretical

8

calculations, the role of SO2 and the probable pathways in removing Hg0 is discussed. Lower

9

concentration of SO2 as well as HCl could dissociate on α-Fe2O3(001) surface, and the

10

intermediates combines with the gaseous Hg0, forming mercury-sulfur, mercury-chlorine

11

compounds, etc. In addition, the different concentrations of SO2 are also discussed, and the

12

corresponding X-ray photoelectron spectroscopy analysis on contrasted samples is conducted

13

to research the morphological characterization, providing a reliable basis for judging the

14

probable pathways of Hg0 transformation.

15 16

17 18

INTRODUCTION

Mercury is a naturally occurring element throughout the world, leading to a global threat to human and environmental health. Various regulations

1, 2

has focused on the emission and


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19

transport of mercury, and set a goal to curb the global mercury emissions by developing an

20

international treaty. Due to the low solubility in water, high volatility in vapor phase and

21

thermodynamical stability at high temperature, the elemental mercury is difficult to remove 3, 4.

22

Consequently, decreasing the emission of mercury from coal-fired power plants has become a

23

major environmental concern.

24

One of the effective ways to remove Hg0 is by injecting sorbents into the flue gas, and 5-8

25

our previous studies

have proved that iron-based adsorbents could effectively remove the

26

amount of Hg0. It’s observed that the influence of HCl in removing Hg0 on Fe2O3 was

27

extremely different with the reaction temperature, in which both homogeneous and

28

heterogeneous mechanisms were presented, and accordingly a mixture of several gases

29

would have a significant impact on Hg0 adsorption and oxidation, which may be caused by

30

the alternation of adsorbent’s surface as well as interactions among α-Fe2O3 and gas

31

components, such as HCl, SOX, O2, and NOX 11-13.

5, 9, 10

32

Especially, HCl and SO2 are considered to be the critical conditions of affecting mercury

33

by metal oxides or carbonaceous sorbents , and as a co-beneficial removal, SO2 is an essential

34

factor being considered as well 14. Li et al. showed that SO2 itself over SiO2-TiO2-V2O5 (STV)

35

catalysts had no promotional effect on the mercury oxidation 15. Landalet al. asserted that SO2

36

could inhibit Hg0 oxidation 16, nevertheless Norton et al. made a contradicting observation 17,

37

reporting that it increased the extent of mercury oxidation. Furthermore, Ochiaiet al. found

38

that the presence of HCl and SO2 affected the characteristics of the mercury adsorption

39

species

18

. Smith et al. held that a comprehensive interaction existed with the simultaneous



40

HCl and SO2 19. Schofield 20, 21 proposed a mechanism for the oxidation of Hg0 to HgSO4, and

41

HCl reacted with the HgSO4 deposited on platinum surfaces.

42

In catalytic oxidation of Hg0 to Hg2+, the involved oxidants are mainly chlorine and

43

oxygen. It is also shown that the presence of O2 alone had no effect on Hg0 oxidation, on the

44

contrary, the Hg0 oxidation was enhanced by O2 in the presence of HCl, probably because of

45

its regenerate lattice oxygen

46

complexes was critical to Hg0 oxidation with both the HCl and O2. Many researchers

47

considered that the involved O2 was favorable 12, 18, 22-25 via the reaction of 2 HCl + 1/2 O2 →

48

H2O + Cl2 and Hg + Cl2 → HgCl2, or Hg0 + 1/2 O2 → HgO and HgO + x HCl →HgClx + x/2

49

H2O + (1/2 - x/4) O2, however, Granite et al. suggested that lattice oxygen of metal oxides

50

could serve as the oxidant of mercury by the reaction of Hg(ad) + MxOy → HgO(ad) + MxOy-1 24.

22, 23

. Gao et al.

22

held that the formation of chlorine-oxygen

51

Although there have been many researches on kinetics and experiments of the

52

laboratory-studied active carbons and SCR catalysts, investigations of α-Fe2O3 in these

53

aspects are lacking. In this study, the influences of combined gases in affecting Hg0

54

conversion are detailedly discussed based on the single species exploration. Our main aim is

55

to identify the adsorption mechanism of Hg0 on α-Fe2O3 with the coexistence of HCl, O2 and

56

SO2 in a designed gas environment, and the Hg0-adsorbed samples are analyzed by XPS tests

57

to study the morphological characterization. Combining with the contrasted experiments, DFT

58

calculations are adopted to explore the probable pathways of O2 and SO2 gases in Hg0

59

removal, offering a sufficient explanation on the Hg0 removal mechanism.

60

EXPERIMENTAL SECTION


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Characterization of Catalyst. Pure spherical α-Fe2O3 (Aladdin Chemistry Co. Ltd) with a

62

particle size of 30 nm and purity of 99.5% had been always used in our study 7.

63

Experimental Apparatus and Procedures. The experimental system diagram is pictured in

64

Figure S1, which consists of the gas distribution system, mercury generator, reaction

65

equipment and online mercury measuring instrument. An 50 mg of α-Fe2O3 was packed into

66

a tube reactor, and the inlet Hg0 concentration was around 40 ±1 µg/m3 with a total inlet gas

67

flow of 1.2 L/min, accompanying with a mixed gas of 50 ppm HCl, 4% O2, SO2

68

(100-1000ppm) and N2 (balance gas). The Hg0 concentration at the entrance of reaction

69

device is indicated as Cin while the one at exit Cout. In this paper, Hg0 removal efficiency is

70

defined as η, which is expressed as reaction (1):

71

η = (1-Cout/Cin) ×100%

(1)

72

Additionally, various experiments, which is illustrated in Table 1, were designed to 5

73

verify the probable pathways at different stages. A pre-treatment

of HCl or O2(or SO2) on

74

α-Fe2O3 was similarly applied to our experiments, in which the total gas flow of N2 and HCl

75

(or O2, SO2) flowed over the sample for two hours at certain temperature and then the sample

76

was flushed with nitrogen for half an hour. After pretreatment, the sample was designated as

77

HCl/α-Fe2O3 (or O2/α-Fe2O3, SO2/α-Fe2O3).

78

Contrastively, the sample reacted together with HCl (or O2) and Hg0 for one hour was

79

marked as HCl+α-Fe2O3+Hg0 (or O2+α-Fe2O3+Hg0, SO2+α-Fe2O3+Hg0). Simultaneously, the

80

homogeneous (gas phase) reaction, in which the gaseous chlorine species could be produced

81

by HCl on Fe2O3 surface, and then reacted with gaseous Hg0, was another mechanism to be



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82

considered. In the designed experiment, the gas flow of Hg0 was separately pumped at the tail

83

of the reactor through by-pass while the HCl and O2 through the samples in reactor, it is

84

marked as ‘HCl+O2, Hg0’ in III, Table 1.

85

Table 1 Designed Experimental Conditions at 80 and 380℃. Experiments

Samples

Gas compositions O2+Hg0

/ I

II

Temperature

α-Fe2O3

O2+Hg0

α-Fe2O3

HCl+Hg0

α-Fe2O3

HCl+O2+Hg0

HCl/α-Fe2O3

Hg0

(HCl+O2)/α-Fe2O3

Hg0

HCl /α-Fe2O3

I II

Hg0+O2 ; Hg0

80, 380 °C

80 °C

I HCl+Hg0 ; II HCl+O2+Hg0

α-Fe2O3

I II III

α-Fe2O3 III

O2+Hg0 ; HCl+Hg0; O2+Hg0

(HCl+O2)/α-Fe2O3

Hg

α-Fe2O3

I HCl+Hg0; II HCl+O2, Hg0; III HCl +O2+Hg0

α-Fe2O3

I II III

IV SO2/α-Fe2O3 (SO2+HCl)/α-Fe2O3

380 °C

0

SO2+Hg0 ; SO2+Hg0; SO2+HCl+Hg0

80 °C, 380 °C

Hg0

80 °C

0

80 °C

Hg

86

X-ray Photoelectron Spectroscopy (XPS) Study. In order to analyze the mercury speciation of

87

the used samples, a high resolution XPS spectrum(ESCALAB250Xi, Thermo Fisher) for the

88

key elements on the surface of sorbents before and after adsorbing Hg0 was conducted

89

For more details, please refer to Supporting Information.

90

Models and Computational Methods. All calculations in this paper are performed using the

91

CASTEP (Cambridge Sequential Total Energy Package) software package 6 Environment ACS Paragon Plus

26, 27

7, 10

.

, which is

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based on the DFT theory 7. For a detailed description of computational parameter selection

93

and the optimization of SO2 molecule on α-Fe2O3(001) surface, refer to Supporting

94

Information.

95

RESULTS AND DISCUSSION

96

Role of O2 on Hg0 Removal in the Presence of HCl. In our previous experiments with HCl 6,

97

the role of HCl was discussed typically in a wide temperature range (80-780℃). Based on the

98

gas-solid heterogeneous reaction, the Hg0 removal experiments with and without O2 are

99

respectively carried out at 80 and 380℃. Compared with the 20% amount of O2, Li and Yang

100

et al. had proved that 4% O2 was sufficient to exhibit its impact on Hg0 conversion on STV

101

sorbent and Co-MF catalyst

102

our Hg0 removal experiments.

15, 28

. So, we directly adopt the O2 concentration of 4% during

103

Figure 1 shows the Hg0 removal efficiency on α-Fe2O3 at 80 and 380℃. As shown in

104

Figure 1, the Hg0 removal efficiency in simultaneous reaction with O2 and Hg0 is respectively

105

7.3 and 7.6% at 80 and 380℃, and among O2, Hg0 and α-Fe2O3 the efficiency is 2.5 and 9.2%,

106

implying the impact of O2 turns out to be small. Nevertheless, the Hg0 oxidation efficiency

107

has been significantly changed with the emergence of HCl.

108



109

Figure 1. The Hg0 removal efficiency on contrasted samples on α-Fe2O3 at 80 and 380℃.

110

At 80℃, the Hg0 removal efficiency of HCl on α-Fe2O3 decreases from 62% to 42% as

111

O2 is added, displaying that O2 plays a hindering role in capturing Hg0 at low temperature.

112

However, something interesting is discovered at 380℃. Without O2 the Hg0 removal

113

efficiency is merely around 12%, in contrast, the addition of O2 significantly improves the

114

Hg0 removal efficiency, exceeding the former level by 5.5 times. Thus, it’s suggested that O2

115

positively promotes the Hg0 oxidation at medium temperature, and more details are discussed

116

as follows.

117

Mechanism of Hg0 Removal with HCl and O2 at Low Temperature. At 80℃, a series of

118

experiments are firstly designed to discover the pathway between HCl and O2 in removing

119

Hg0, which is shown in Figure 2. As described in Line 1, in the presence of O2 the ability for

120

capturing Hg0 on HCl/α-Fe2O3 sample is largely restricted, along with the outlet Hg0

121

concentration of 39 µg/m3. Once the O2 source is stopped, as shown in the second part of

122

Line 1, the outlet Hg0 concentration decreases to a stable level of 36.7µg/m3, implying that

123

the stopping O2 is favorable to Hg0 removal.

124 125 126

Figure 2. The outlet Hg0 concentration with different gas conditions on α-Fe2O3 at 80℃ By comparing with the Hg0 removal results of HCl/α-Fe2O3+O2+Hg0 in Line 1 and


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HCl/α-Fe2O3+Hg0, it can be seen that O2 suppresses the removal of Hg0. Comparatively, we

128

can observe that in Line 3, the (HCl+O2)/α-Fe2O3 sample has little effect on Hg0 removal,

129

revealing that the HCl and O2 dissociation on α-Fe2O3 is probably competitive.

130

In our calculation researches

7, 9, 13

, the density functional theory has been increasingly

131

used and is shown in Figure S2. Combining with the computational results of Cl and O atom

132

dissociative adsorption and experimental results with the coexistence of HCl and O2, HCl will

133

be dissociatively adsorbed on the surface of α-Fe2O3, leaving Cl atom bond with the surface

134

Fe. Then the gaseous Hg0 reacts with the Cl atom, meanwhile, because of the dissociation of

135

O2, the absorbed O surface atom competes and hinders the formation of Cl active sites at low

136

temperature, resulting in a negative effect in Hg0 removal.

137

Mechanism of Hg0 Removal with HCl and O2 at Medium Temperature. Subsequently, the

138

probable oxidation process at 380℃ is examined through the comparative experiments and

139

shown in Figure 3, in which the outlet Hg0 concentration under different gases conditions is

140

displayed. As observed in Figure 3a, the pure HCl contributes little to the Hg0 removal,

141

accompanying by an outlet concentration of 35.4 µg/m3, but after adding O2 the outlet

142

concentration is greatly reduced to 19.1 µg/m3. We conclude that O2 is beneficial to achieve a

143

high capacity for capturing Hg0 with α-Fe2O3 at 380℃, and it is very likely that at 380℃

144

some distinctive difference between O2 and HCl on α-Fe2O3 surface emerges and the higher

145

temperature strengths their reaction.



146

147

148 149

Figure 3. The outlet Hg0 concentration with different gas conditions on α-Fe2O3 at 380℃

150

As shown in Figure 3b, at the beginning, pure O2 have little capacity to oxidize Hg0, and

151

the switched HCl does a slight impact on the Hg0 removal. The outlet concentration of Hg0

152

decreases from 40.2 to 35.2 µg/m3, displaying that the prior O2 and following HCl is of no

153

use. However, when switching the gas from HCl to O2 once again, the outlet Hg0

154

concentration largely decreases to a minimum of 25.4 µg/m3, indicating that the prior HCl

155

adsorption turns out to be effective.

156

Hence, another group of experiments are carried on. As depicted in Figure 3c, the

157

(HCl+O2)/α-Fe2O3 sample exerts slight influence on Hg0 transition accompanying with the


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158

Hg0 removal efficiency of 5%. Though no obvious effectiveness is observed, the HCl and O2

159

could pre-adsorb on α-Fe2O3 slightly, benefiting to capture Hg0. To determine whether or not

160

generating favorable intermediate products in the presence of HCl and O2 gases on α-Fe2O3,

161

the last group of experiment Ⅲ(Table 1) is applied to verify the gaseous chlorine species on

162

Fe2O3 surface.

163

As can be seen in stage Ⅱ(Figure 3c), the gaseous Hg0 at the tail of by-pass is

164

controlled to join the simultaneous HCl and O2 gas, which have flowed through the α-Fe2O3

165

sample, and the outlet Hg0 concentration decreases to 34.8 µg/m3 because of the products

166

among HCl, O2 and α-Fe2O3. Accordingly, the Hg0 removal efficiency is merely about 13%,

167

revealing the Deacon process is applicable. At stage Ⅲ, when the Hg0 gas flow is switched

168

from the inlet of reactor to the continuous HCl and O2 gases, the outlet Hg0 concentration

169

sharply decreases to 24 µg/m3, implying that the successive intermediate species accumulate

170

the Hg0 transformation.

171

To verify the key elements on the contrasted sorbents before and after adsorbing Hg0, 10

172

XPS spectra is conducted to characterize the available functional groups

173

Figure S3, the Hg4f result reveals that Hg2+ appears in simultaneous Hg0 adsorption with HCl

174

and O2, implying that Hg0 could be oxidized with the addition of HCl and O2. Nevertheless, a

175

further research on HgO species is still needed. The Cl2p result demonstrates that chlorine

176

species is captured, demonstrating that HCl can dissolve on α-Fe2O3, which is consistent with

177

the experimental conclusions.

178

. As depicted in

In summary, at 380℃, the Hg0 removal mechanism with HCl and O2 on α-Fe2O3 could



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179

have the following two possible cases: (a), HCl and O2 simultaneously dissociates on α-Fe2O3

180

and the chlorinated surface of substrate improves the Hg0 removal efficiency. A probable

181

reaction is given by reaction (2) and (3).

182

HCl(g)+O2 (g)+Fe2 O3 (s) →Cl(s)

(2)

183

Hg0 (g)+Cl(s) →HgCl2 (s,g)

(3)

184

(b), on α-Fe2O3 surface, HCl and O2 forms intermediate products, which is effective to the

185

Hg0 oxidation. The possible pathway is summarized in reaction (4) and (5). Fe2 O3

186

HCl(g)+O2 (g) Cl2 (g)

(4)

187

Hg0 (g)+Cl2 (g) → HgCl2 (s,g)

(5)

188

Computational Research on the Adsorption of SO2 on α-Fe2O3(001) Surface. Theoretically,

189

for the role of SO2, it’s still unclear, and subsequently the calculations for Hg0 and

190

SO2 adsorption on α-Fe2O3(001) are conducted. Firstly, the structure of SO2 molecule is

191

optimized in a cubic crystal cell with a lattice of 1 nm, and the S-O bond length is 1.45 Å.

192

Comparing with the experimental value of 1.43Å, it’s proved that our calculations provide

193

reliable results.

194

Various adsorption sites for the adsorbed SO2 in parallel and perpendicular orientations

195

as well as the S atom up and down towards the α-Fe2O3(001) surface, are respectively

196

examined. Additionally, different adsorption sites including Fe top, O top and Hollow sites of

197

α-Fe2O3(001) surface are performed in each adsorption process. After geometry optimization

198

of configuration, three kinds of stable adsorption configurations are derived, and the

199

optimized structure and corresponding parameters are pictured in Figure 4 and Table S1

200

respectively.


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201 202

Figure 4. Three stable adsorption configuration of SO2 on α-Fe2O3(001) surface

203

As pictured in configuration A, B and C, there are three kinds of stable configurations in

204

SO2 adsorption process. In Configuration A, one O atom of the SO2 molecule binds with

205

surface Fe, forming Fe-O1 bond with the length of 1.95Å, and the adsorption energy of Fe-O1

206

bond is -87.8 kJ/mol, belonging to chemical adsorption. Besides, the S-O1 bond is elongated

207

to 1.55 Å and it’s inferred that a dissociation of SO2 happens, which is shown in reaction (6).

208

In Configuration B, two O atoms in SO2 molecule respectively bind with the surface Fe,

209

accompanying with the bond length of 2.06 and 2.02 Å, meanwhile, two S-O atoms in SO2

210

are elongated to 1.54 Å, yielding the chemisorptive energy of -118.7 kJ/mol. The second kind

211

of SO2 dissociative adsorption occurs, as shown in reaction (7).

212

SO2 (g) + Substrate → (O) Substrate + SO(g)

(6)

213

SO2 (g) + Substrate → (2O) Substrate + S(g)

(7)

214

The S-Fe bond in Configuration C is 2.51Å and the adsorption energy is -47.3 kJ/mol,

215

which is weaker than the other two configurations. In summary, there are three forms of SO2

216

adsorption on the α-Fe2O3(001) surface, one of which is that SO2 can attach to the

217

α-Fe2O3(001) surface by forming Fe-S bond. The other two is that the whole SO2 molecule

218

dissociatively adsorbs on α-Fe2O3(001) surface, and the O atom binds with Fe tom, leaving O



219

and SO atoms separated from the substrate.

220

Computational Research on the SO and S Adsorption on α-Fe2O3(001) Surface. Furthermore,

221

the optimization of SO on α-Fe2O3(001) is investigated. Similarly, the SO molecule is

222

optimized in a cubic crystal cell with a lattice of 1 nm, and the S-O bond length is 1.50 Å

223

after optimization. Then the SO molecule is placed on all the possible adsorption sites and

224

through optimization, two kinds of configurations, yielding the same adsorption energy of

225

-122.5 kJ/mol, are achieved. The optimized results are shown in Figure 5.

226 227

Figure 5. The stable adsorption configuration of SO and S on α-Fe2O3(001) surface

228

There are two different forms of SO adsorption behavior on surface Fe. In Configuration

229

D, a stable chemical bond of O-Fe forms and the S-O bond is elongated to 1.62 Å, indicating

230

that O atom remains on surface while S atom is desorbed. On the contrary, in Configuration E,

231

the S atom bonds with the surface Fe, forming a Fe-S bond length of 2.29 Å, yet the S-O bond

232

remains relatively constant. The possible reactions are listed as following reaction (8) and (9).

233

Additionally, in Configuration F the S atom strongly interacts with surface Fe atom, forming

234

Fe-S bond, which possesses the energy of -371.4 kJ/mol and bond length of 2.04 Å. The

235

probable reaction is as follows reaction (10).


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SO(g) + Substrate → O Substrate + S(g)

(8)

237

SO(g) + Substrate → OS Substrate

(9)

238

S(g) + Substrate → S Substrate

(10)

239

Unlike the dissociation of H2S 5, not only the surface S site forms on α-Fe2O3(001)

240

surface, but also the O site is simultaneously formed in SO2 dissociation, which probably

241

provides a synergistic promoting effect on Hg0 removal. In the following chapters the Hg

242

adsorption on these two substrates is discussed.

243

Computational Research on the Adsorption of Hg0 on S/α-Fe2O3(001) Surface. Based on the

244

configurations above, the O/α-Fe2O3(001) and S/α-Fe2O3(001) surface are finally obtained

245

and used as substrates. Then, adsorptions of Hg0 on them are calculated, which is pictured in

246

Figure S4. As listed in Configuration, the dissociated S surface has a high adsorption activity,

247

forming the S-Hg bond with a length of 2.48 Å. The adsorption energy of mercury on the

248

adsorbed S surface is -144.6 kJ/mol, as a contrast, on adsorbed O surface is -75.2 kJ/mol,

249

which is much lower. Therefore, we consider that in Hg0 transformation it’s mainly depended

250

on the formation of active S while the surface-adsorbed O atom had little contribution. More

251

importantly, the O atom probably restricted the active site of sulfur by competing adsorption

252

sites, which would be considered in the following experiments.

253

Role of SO2 on Hg0 Removal. According to the above calculations, we recognized the

254

probable pathways of SO2 in adsorbing Hg0, however, the influence of temperature couldn’t

255

be distinguished. Then the comparative experiments are performed to thoroughly explore the

256

role of SO2 at 80 and 380℃. In our experiments with SO2, even with different concentrations,

257

the Hg0 removal is hindered at 380℃, which is possibly because of the inability to form



258

effective and active materials at higher reaction temperature. The experiments at 80℃ are

259

mainly considered in the following sections.

260

Firstly, the concentration of single SO2 at 80℃ is considered, and from the Figure 6 we

261

can observe that its impact varies differently. As illustrated in Figure 6, the mercury removal

262

efficiency of SO2 varies from 31 to 37% at the SO2 concentrations ranging from 100 to

263

400ppm, however, the concentration of 1000ppm contribute little to it, displaying that lower

264

SO2 concentration is advantageous for removal of Hg0. Combining with the calculations it’s

265

possible that the SO2 dissociation on α-Fe2O3 could promote the removal at lower SO2

266

concentration, while at 1000 ppm SO2 the large amount of O atoms in SO2 dissociation

267

competed with the active sites with Hg0, hindering the performance for mercury removal.

268 269

Figure 6. The Hg0 removal efficiency with SO2 on α-Fe2O3 at 80℃.

270

Effect of SO2 on Hg0 Removal in the Presence of HCl. To clarify the effect of SO2,

271

interactions between SO2(400 ppm) and HCl at 80℃on three types of pre-treated samples,

272

containing SO2/α-Fe2O3, HCl/α-Fe2O3 and (SO2+HCl)/α-Fe2O3, are carried out. As listed in

273

Figure 7, the SO2/α-Fe2O3 sample achieves a maximum efficiency of 17% in removing Hg0,

274

working for a limited period time of twenty minutes, and the (SO2+HCl)/α-Fe2O3 has a

275

maximum efficiency of 40%. By contrast, the Hg0 removal efficiency in simultaneous

276

reaction with SO2 and SO2+HCl gases is 38 and 84% respectively. It’s possible that the


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277

amount of adsorbed SO2 on α-Fe2O3 is tiny but the reaction is continuous. Meanwhile, the

278

simultaneous reaction among SO2, HCl and Hg0 shows a longer Hg0 removal efficiency of

279

41%. It’s very likely that the HCl and SO2 synergistically facilitate the Hg0 oxidation.

280 281

Figure 7. The outlet Hg0 concentration on with different gases on α-Fe2O3 at 80℃.

282

From the results above, we conclude that SO2 and HCl could pre-adsorb on α-Fe2O3 at

283

80℃ and then Hg0 is effectively captured, revealing that the Eley-Rideal mechanism plays a

284

dominant role on α-Fe2O3 with both SO2 and HCl. Additionally, it’s discovered that the

285

breakthrough time of simultaneous mercury adsorption with SO2 as well as SO2+HCl on

286

α-Fe2O3 is merely about 40 minutes, showing a disadvantage of short effective period.

287

Perhaps because the accumulated O atoms hindered the Hg0 conversion and the dissociation

288

between SO2 and HCl competed against each other.

289

Furthermore, the XPS tests with contrasted samples at 80℃were performed by recording

290

the photoemission bands Hg4f, Cl2p, and S2p. As shown in Figure 8a, the S2p peak of

291

SO2+α-Fe2O3+Hg0 sample centers at 162.8 eV is assigned to S or S–, and the 168.1 and

292

169.1eV represents SO42–, according to the available spectrum peaks of FeS2, S, FeSO4,

293

HgSO4 and Fe2(SO4)3 at 162.8, 162.9, 168.0, 168.5 and 169.1eV. The formation of S and S– is

294

probably based on the dissociation of SO2 on α-Fe2O3 and the formation of mercury

295

compounds. In SO2/α-Fe2O3 sample, the S– species isn’t detected, probably because of the



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tiny amounts.

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Figure 8. XPS spectra of Hg4f, Cl2p and S2p for the contrastive samples at 80℃.

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As observed in Figure 8a, for SO2+α-Fe2O3+Hg0 sample, it’s observed that Hg2+ exists

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by the binding energy of 100.7eV, which is not far from the spectrum peaks of Hg2SO4 and

301

HgO with the binding energy of 101.0 and 100.8eV. As depicted in Figure 8b, the binding

302

energy of Hg4f peak at 99.8 and 101.3eV is marked, representing the generation of Hg2+, and

303

the Cl2p peak at 198.6eV represents the probable generation of Cl–. It’s worth pointing out

304

that in Figure 8b a peak at 163.2eV appears, and it indicates that some kinds of sulfur species

305

are discovered on α-Fe2O3 surface according to the binding energy of S, sulfur and S– at 163.1,

306

163.3 and 163.5eV in X-ray photoelectron spectrum. According to the results of calculations

307

and experiments, a probable pathway is separately presented as the reaction (11)-(12): Fe2 O3

308

Hg0 (g)+SO2 (g) Sx (s) + HgS(s)+ HgO(s)Hg(SO4 )x(s)

309

Hg(SO4 )x (s)+HCl(g) HgCl2 (s,g) +FeSO4 x (s)+H2 O(g)

Fe2 O3

(11) (12)

310

Thus, we confirm that at low temperature, SO2 plays an active role in capturing Hg0 and

311

synergistically facilitate the Hg0 oxidation in the presence of HCl. In conclusion, SO2 could

312

dissociate on α-Fe2O3 and the intermediates combine with gaseous Hg0, forming


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313

mercury-sulfur compounds, and at the same time HCl could simultaneously react with them

314

along with a formation of mercury-chlorine compounds.

315

ASSOCIATED CONTENT

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Supporting Information. The Supporting Information is available free of charge on the

317

Internet at http://pubs.acs.org. Information regarding experimental apparatus and procedures,

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X-ray photoelectron spectroscopy study, models and computational methods, mechanism of

319

Hg0 removal with HCl and O2 at low temperature, Table S1 and Figures S1-S4.

320

AUTHOR INFORMATION

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Corresponding Author

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*Xin Guo, Phone: 86-27-87545526. Fax: 86-27-87545526. E-mail:

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[email protected].

324

ACKNOWLEDGMENT

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Financial support was sponsored by the National Key Basic Research and Development

326

Program, the National Natural Science Foundation of China (NSFC) (Grant Nos.51176058),

327

and the Foundation of State Key Laboratory of Coal Combustion of China (FSKLCC1507).

328

REFERENCES

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Mechanisms of Elemental Mercury Transformation on Î±-Fe2O3(001

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