[MoS4] Cluster Bridges in Co-Fe Layered Double Hydroxides for

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[MoS4]2- Cluster Bridges in Co-Fe Layered Double Hydroxides for Mercury Uptake from S-Hg mixed flue gas Haomiao Xu, Yong Yuan, Yong Liao, Jiangkun Xie, Zan Qu, Wenfeng Shangguan, and Naiqiang Yan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02537 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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Environmental Science & Technology

1

[MoS4]2- Cluster Bridges in Co-Fe Layered Double Hydroxides

2

for Mercury Uptake from S-Hg mixed flue gas

3 4

Haomiao Xua,b, Yong Yuana, Yong Liaoa, Jiangkun Xiea, Zan Qua, Wenfeng

5

Shangguanb, Naiqiang Yana*

6

a. School of Environmental Science and Engineering, Shanghai Jiao Tong University,

7 8

Shanghai 200240, China b. Research Center for Combustion and Environment Technology, Shanghai Jiao

9

Tong University, Shanghai 200240, China

10 11

* Corresponding author: Naiqiang Yan

12

School of Environmental Science and Engineering, Shanghai Jiao Tong University,

13

800 Dongchuan RD. Minhang District, Shanghai, China

14

Tel: +86-021-54745591; Fax: +86-54745591

15

E-mail: [email protected].

16 17 18

TOC:

19 20

ACS Paragon Plus Environment


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ABSTRACT: [MoS4]2- clusters were bridged between CoFe layered double

22

hydroxide (LDH) layers using the ion-exchange method. [MoS4]2-/CoFe-LDH showed

23

excellent Hg0 removal performance under low and high concentrations of SO2,

24

highlighting the potential for such material in S-Hg mixed flue gas purification. The

25

maximum mercury capacity was as high as 16.39 mg/g. The structure and

26

physical-chemical properties of [MoS4]2-/CoFe-LDH composites were characterized

27

with FT-IR, XRD, TEM&SEM, XPS and H2-TPR. [MoS4]2- clusters intercalated into

28

the CoFe-LDH layered sheets; then, we enlarged the layer-to-layer spacing (from

29

0.622 nm to 0.880 nm) and enlarged the surface area (from 41.4 m2/g to 112.1 m2/g)

30

of the composite. During the adsorption process, the interlayer [MoS4]2- cluster was

31

the primary active site for mercury uptake. The adsorbed mercury existed as HgS on

32

the material surface. The absence of active oxygen results in a composite with high

33

sulfur resistance. Due to its high efficiency and SO2 resistance, [MoS4]2-/CoFe-LDH

34

is a promising adsorbent for mercury uptake from S-Hg mixed flue gas.

35 36

INTRODUCTION

37

Mercury is a highly toxic pollutant, discharging from industries into the atmosphere

38

and causing severe harmful environmental pollution.1-3 Mercury often stably exists in

39

coal and industrial ore and is present as HgS. However, after high-temperature

40

combustion from the above-mentioned industrial production process,4,

41

mercury is stripped from the raw materials and exists as gaseous elemental mercury

42

(Hg0) in high-temperature flue gas. Meanwhile, stable sulfur is also released after

43

thermal decomposition processes and primarily exists as SO2 in the flue gas, resulting

44

in S-Hg mixed flue gas. In addition, S-Hg mixed flue gases often exist in many types

45

of flue gases, especially in non-ferrous plants.6,

46

mercury and SO2 co-exist in smelting gas. The mixed gas is difficult to control using

47

traditional mercury control technologies.

7

5

stable

High concentrations of gaseous

48

Many technologies have been developed to remove Hg0 from S-Hg mixed flue

49

gases. In general, catalytic oxidation and adsorption methods have often been used to

50

stabilize mercury in liquid and solid materials.4, 5 The catalytic oxidation method


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converts gaseous Hg0 to an oxidized state (Hg2+, such as HgCl2, HgBr2 and HgO). The

52

oxidized mercury can be adsorbed by fly ash or slurry. The adsorption method

53

changes gaseous Hg0 or Hg2+ to its particle-bound state (Hgp). Various materials, such

54

as activated carbon, fly ash, noble metals, and transition metal oxides, have been

55

employed for the adsorption of metal ions.4, 5, 8 Carbon-based materials, such as active

56

carbon and fly ash, have mercury holding capacity due to the abundant porous

57

structure.9 To enhance the mercury capacity, some chemicals that have high affinity

58

performance, such as halogens10, sulfur8, 11 and transition metal oxides12, have been

59

used to modify carbon-based sorbents. Transition metal oxides, such as MnOx13, 14

60

and FeOx15, have high catalytic oxidation and adsorption performances for gaseous

61

mercury. After modification, Ce-MnOx and Fe-TiOx binary metal oxides often have

62

large mercury capacities. However, these selected catalysts or sorbents are often

63

poisoned due to the presence of SO216-18. Surface oxygen is the primary binding site

64

for mercury19. SO2 can cause surface sulfation and occupy surface oxygen sites,

65

resulting in low activity for Hg0 from S-Hg mixed flue gas. The mercury binding sites

66

are occupied by SO42-. In this process, SO2 is oxidized to SO3 by transition metal

67

oxides and reacted with surface oxygen to form SO42-, resulting in low mercury

68

capacity.

69

Sulfur is one element with affinity for mercury, and HgS stably exists in ore.

70

Sulfur is often used as a sorbent for chemical stabilization when capturing Hg0.

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However, the reaction between solid S and gaseous Hg0 is slow. Chalcogenides can

72

accelerate this reaction. Li et al. found that nano-ZnS with a high surface area has a

73

high mercury capacity and that mercury primarily exists as HgS on the material’s

74

surface.20 Yang et al. selected magnetic pyrrhotite, derived from the thermal treatment

75

of natural pyrite, to remove and recover Hg0 from flue gas. It has a fast reaction rate

76

and a high mercury adsorption capacity.21 Recent studies have found that polysulfides

77

are efficient mercury sorbents for Hg2+ or Hg0 capture. Transition metal complexes,

78

such as CoMoS/γ-Al2O3, were prepared as a sorbent for Hg0 capture.22 Chalcogels

79

showed promising catalytic and gas separation performances. Some polysulfide

80

chalcogels with ion-exchange properties participated in vapor sorption.23 The



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ion-exchangeable

82

selectivities for CO2 and C2H6 over H2 and CH4. This aerogel was also useful for

83

capturing iodine and mercury.24

molybdenum

sulfide

chalcogel

exhibited

high

adsorption

84

However, the main challenge could be that polysulfide should be supported by a

85

suitable material. Layered double hydroxides (LDHs), which are composed of mixed

86

metal oxides with the chemical formula [M2+1−xM3+x(OH)2]x+[(An−)x/n]x−·mH2O, have

87

excellent intercalation and ion-exchange properties25-27. Such structure is beneficial

88

for building a bridge structure. Previous studies have reported that polysulfide [Sx]2-

89

(x=2, 4) species were intercalated into a Mg-Al layered double hydroxide

90

(MgAl-LDH) by a [Sx]2-/NO3- anion-exchange reaction. Sx-MgAl-LDH materials

91

exhibit excellent performances for Cu2+, Ag+ and Hg2+ ion removal.25 In addition,

92

Sx/MgAl-LDH (x = 2, 4, 5) can efficiently capture large quantities of mercury (Hg0)

93

vapor.28 The [MoS4]2- cluster ions were intercalated into MgAl-NO3-LDH to produce

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MgAl-MoS4-LDH, which demonstrates highly selective binding and extremely

95

efficient removal of heavy metal ions, such as Cu2+, Pb2+, Ag+, and Hg2+.29 One

96

advantage is that this type of structure resulted in a porous material that was beneficial

97

for gas transfer and surface reaction. Another advantage is that the O binding atoms

98

were removed using anion-exchange methods. Sulfur clusters replaced O in the bridge.

99

This could be useful when SO2 is present in the simulated gases.

100

In this study, we report a series of [MoS4]2- cluster-modified LDH materials for

101

removing Hg0 from simulated flue gas. CoFe-NO3--LDH, NiAl-NO3--LDH,

102

ZnAl-NO3--LDH and CoAl-NO3--LDH were prepared as the supports for [MoS4]2-

103

clusters. The Hg0 removal performances over these composites were evaluated in a

104

fixed-bed adsorption system. The mechanism for Hg0 removal was discussed based on

105

the characterization results.

106 107

EXPERIMENTAL SECTION

108

Preparation of Materials

109

Synthesis of NO3--LDH materials: In a typical procedure, nitrate salts of the

110

metals were selected to prepared LDH materials. To synthesize CoFe-NO3--LDH, 20


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mmol of Co(NO3)2 and 10 mmol of Fe(NO3)2 were dissolved in 100 mL of deionized

112

water and stirred for 1 h. Afterwards, NaOH solution was used to adjust the pH to 10.

113

The product was then filtered, washed with deionized water and air-dried at 80 °C.

114

Similarly, NiAl-NO3--LDH, ZnAl-NO3--LDH and CoAl-NO3--LDH were prepared by

115

the same method. The ratios of Ni:Al, Zn:Al and Co:Al were 2:1.

116

Synthesis of [MoS4]2-/LDH materials: [MoS4]2-/LDH was prepared via an

117

anion-exchange reaction. For [MoS4]2-/CoFe-LDH, briefly, 0.4 g of as-prepared

118

CoFe-NO3--LDH was dissolved in deionized water under ultrasonic treatment for 30

119

min. Then, 0.4 g of (NH4)2MoS4 powder was dissolved in deionized water. The two

120

solutions were mixed together with stirring at ambient temperature for 24 h. The

121

resulting products were filtered and washed with deionized water. Finally, the samples

122

were air-dried at 80 °C. The [MoS4]2-/NiAl-LDH, [MoS4]2-/ZnAl-LDH and

123

[MoS4]2-/CoAl-LDH were prepared using the same method. For comparison,

124

S4/CoFe-LDH was also synthesized. Then, 1 mmol of Na2S and 3 mmol of elemental

125

sulfur were mixed together and stirred at 90 °C until the sulfur absolutely dissolved.

126

The mixed solution was added to the CoFe-NO3--LDH solution with 0.4 g of

127

CoFe-LDH. The product was also filtered, washed with deionized water, and air-dried

128

at 80 °C.

129

Characterization of Materials

130

X-ray diffraction (XRD) patterns were obtained using a Shimadzu XRD-6100

131

diffractometer with Cu-Kα radiation. The data were recorded at a scan rate of 10

132

deg·min−1 in the 2θ range from 10 to 80°. FT-IR spectroscopy was performed to

133

characterize the surface properties. The H2-TPR experiments were performed on a

134

Chemisorp TPx 290 instrument. The samples were degassed at 200 °C for 3 h under

135

an Ar atmosphere before the tests, and the reducing gas was 10% H2/Ar. X-ray

136

photoelectron spectroscopy (XPS) results were recorded with an Ultra DLD

137

(Shimadzu–Kratos) spectrometer with Al Kα as the excitation source, and the C 1s

138

line at 284.6 eV was used as a reference for the binding energy calibration. The

139

microstructures of the materials were characterized by field emission scanning



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140

electron microscopy (SEM, JSM-7001F) and transmission electron microscopy (TEM,

141

JEM-2100, operating at 200 kV). The BET (Brunauer-Emmett-Teller) surface areas of

142

the samples were determined using N2 adsorption at -196 °C using a quartz tube

143

(Quantachrome 2200e).

144

Measurement of gaseous mercury adsorption performances

145

A lab-scale fixed-bed adsorption system was assembled to evaluate the Hg0

146

removal efficiency by the as-prepared materials, as shown in the Supporting

147

Information (Figure S1). The system contains the gas distributing system, reaction

148

system, Hg0 detection system and tail gas purification system. In general, O2 and SO2

149

vapor was prepared with pure N2. The fixed-bed reactor was constructed to allow for

150

a total gas flow of 500 mL/min at temperatures from 50 to 150 °C. Then, 20 mg of

151

prepared material was used for each experiment, and it was placed in a quartz tube

152

with a diameter of 4 cm. In addition, active carbon and KMnO4 solution were used for

153

the off-gas cleaning. Active carbon can adsorb the mercury (Hg2+ and Hg0) in the flue

154

gas. KMnO4 was used for adsorbing the oxidized mercury.

155

In each test, the mercury inlet gas bypassed the sorbent bed and passed into the

156

analytical system until the desired inlet mercury concentration was established. A cold

157

vapor atomic absorption spectrometer mercury detector (CVASS) was used to detect

158

the concentration of off-gas, which was calibrated by Lumex RA 915+. Temperature

159

control devices were installed to control the mixed gas and reactor temperature. To

160

investigate the effect of temperature on the flue gas, the area under the breakthrough

161

curves, corresponding to Hg0 on the prepared sorbents during 180 min, was integrated.

162

To investigate the effects of various gas components, various SO2 concentrations were

163

chosen when needed. The Hg0 removal efficiency and mercury adsorption capacity

164

were calculated according to Equations (1) and (2):

165

166 167

r emoval ef f i ci ency=

Q=

Hgi0n - Hg0out Hgi0n

* 100%

1 t1 ( Hgi0n - Hg0out ）× f × dt m ∫t 2

(1)

(2)

where the Hgin0 is the inlet concentration of Hg0, and Hgout0 is the outlet concentration


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of Hg0. Q is the Hg0 adsorption capacity, m is the mass of the sorbent in the fixed-bed,

169

f denotes the flow rate of the influent, and t0 and t1 represent the initial and ending test

170

times of the breakthrough curves.

171

RESULTS & DISCUSSION

172

Elemental mercury removal performances

173

The performances of as-prepared [MoS4]2- clusters that modified various LDH

174

materials were evaluated in a fixed-bed adsorption system. As shown in Figure 1(a),

175

[MoS4]2- clusters that intercalated CoFe-LDH, NiAl-LDH, ZnAl-LDH and

176

CoAl-LDH materials were selected for Hg0 removal at 75 °C with 4% O2. These

177

composites exhibited different performances. [MoS4]2-/NiAl-LDH had the lowest

178

activity among the as-prepared samples, after 180 min of reaction, and it only had

179

approximately

180

[MoS4]2-/ZnAl-LDH materials enhanced the removal performances compared with

181

[MoS4]2-/NiAl-LDH, with removal efficiencies of 39% and 50%, respectively.

182

[MoS4]2-/CoFe-LDH had the highest Hg0 removal efficiency among these prepared

183

materials. It had greater than 90% Hg0 removal efficiency even after 180 min of

184

reaction. The reaction activities of the four as-prepared kinds of [MoS4]2- clusters

185

modified

186

[MoS4]2-/ZnAl-LDH > [MoS4]2-/CoAl-LDH > [MoS4]2-/NiAl-LDH.

LDH

31%

Hg0

materials

removal

follow

[MoS4]2-/CoAl-LDH

efficiency.

the

order

of

[MoS4]2-/CoFe-LDH

and

>

187

Furthermore, the Hg0 removal performances of CoFe-LDH and [S4]2-/LDH were

188

evaluated for comparison. As shown in Figure 1(b), CoFe-LDH had approximately 50%

189

Hg0 removal efficiency. With the modification of polysulfur, S4/CoFe-LDH increased

190

the Hg0 removal efficiency by approximately 10 percent. However, it was still lower

191

than that of [MoS4]2-/CoFe-LDH. Obviously, after [MoS4]2- cluster modification, the

192

Hg0 removal performance was significantly improved. The results indicated that

193

[MoS4]2- clusters may have a better mercury capture capacity than S4 poly-sulfur.

194

Figure 1(c) gives the mercury adsorption capacity for 180 min under various

195

temperatures over [MoS4]2-/CoFe-LDH. When the temperature was 50 °C, the

196

mercury capacity was 0.99 mg/g for 180 min of reaction. When the temperature

197

increased to 75 °C, it had the highest mercury capacity of 1.45 mg/g. However, the



198

Hg0 removal efficiencies and mercury capacities decreased when the temperature

199

continued to increase. [MoS4]2-/CoFe-LDH had mercury capacities of 1.39 and 1.15

200

mg/g at 100 and 125 °C, respectively. In addition, a higher temperature could result in

201

de-activity for Hg0. [MoS4]2-/CoFe-LDH only had a mercury capacity of 0.73 mg/g at

202

150 °C. A higher temperature was not favorable for Hg0 uptake over

203

[MoS4]2-/CoFe-LDH. The mercury adsorption breakthrough curve over the

204

[MoS4]2-/CoFe-LDH composite was evaluated (shown in Figure S2), and the mercury

205

adsorption capacity was as high as 16.39 mg/g.

206

The effect of SO2 on Hg0 removal was evaluated, and the results are presented in

207

Figure 2. As shown in Figure 2(a), [MoS4]2-/CoFe-LDH was initially tested under 4%

208

O2, and the Hg0 removal efficiency was approximately 90%. After 100 min of

209

reaction, 500 ppm SO2 was added to the simulated flue gas. The removal efficiency

210

was only slightly increased. SO2 did not have an obvious poison effect on Hg0

211

removal. To further indicate Hg0 removal performance under S-Hg mixed flue gas,

212

various concentrations of SO2 were added to the simulated flue gas. As shown in

213

Figure 2(b), SO2 does not have a significant influence on the mercury adsorption

214

capacity. Without SO2, the mercury capacity was 1.45 mg/g. When there was 500 ppm

215

SO2 in the simulated flue gas, the capacity was 1.398 mg/g. The mercury capacities

216

were 1.31 and 1.36 mg/g under 1000 ppm SO2 or 2000 ppm SO2, respectively.

217

Obviously, [MoS4]2-/CoFe-LDH can uptake Hg0 from S-Hg mixed flue gas under low

218

or high concentrations of SO2.

219

Characterization of as-prepared materials

220

The FT-IR spectroscopy shown in Figure 3(a) verified the formation of a [MoS4]2-

221

bridged compound. In the spectrum of CoFe-LDH, a strong band appearing at 1347

222

cm-1 corresponds to the NO3- of the NO3--LDH.29,

223

[MoS4]2- and S4 bridged in the NO3--LDH materials, suggesting that NO3- ions were

224

exchanged by [MoS4]2- and S4 ions. Peaks at 3377 cm-1 were assigned to surface

225

molecular water. After [MoS4]2- bridged into the CoFe-LDH layers, some peaks in the

226

range of 500 to 550 cm-1 were assigned to Mo-S stretching bands. Figure 3(b) shows

227

the XRD patterns of CoFe-LDH, S4/CoFe-LDH and [MoS4]2-/CoFe-LDH materials.

30

This band diminished after


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The XRD patterns of S4/CoFe-LDH was similar to that of CoFe-LDH. The basal

229

spacing of CoFe-LDH was 0.622 nm. After [MoS4]2- intercalated in CoFe-LDH, this

230

spacing was enlarged to 0.880 nm, indicating that [MoS4]2- bridged in the interlayer

231

space.29

232

[MoS4]2-/CoFe-LDH had a layered phase.27

Furthermore,

the

reflection

peaks

at

10-13°

indicated

that

233

As shown in Table 1, the BET surface areas, pore volume and pore diameter of

234

each material were tested. CoFe-LDH material has a surface area of 41.4 m2/g.

235

[MoS4]2-/CoFe-LDH has a surface area of 112.1 m2/g, which was three times larger

236

than that of CoFe-LDH. The average pore diameter of [MoS4]2-/CoFe-LDH decreased

237

and the pore volume increased, resulting in a large surface area. For comparison, the

238

BET surface area of S4/CoFe-LDH was only 21.1 m2/g, which was smaller than that

239

of CoFe-LDH.

240

Furthermore, the TEM and SEM images of [MoS4]2-/CoFe-LDH are shown in

241

Figure 4(a) - (f). Figure 4(a) shows the TEM image of CoFe-LDH material, which had

242

a layered morphology. The SEM image in Figure 4(d) further indicated the layered

243

morphology of CoFe-LDH. Figure 4(b) gives the TEM image of S4/CoFe-LDH, and

244

the material has a layered morphology. However, the layered sheets were not as clear

245

as those of CoFe-LDH from the SEM image shown in Figure 4(e). Some poly-sulfur

246

stacks on the surface of LDH sheets. As shown in Figure 4(c), the composite of

247

[MoS4]2-/CoFe-LDH had a layered morphology with benefits for gas molecule

248

transfer on its surface. Figure 4(f) gives the SEM image of [MoS4]2-/CoFe-LDH; the

249

layered sheets stack together and had a fluffy state when [MoS4]2- was added to the

250

CoFe-LDH materials.

251

Based on physical-characterization results, [MoS4]2- clusters were bridged in CoFe-

252

LDH layers using the ion-exchange method. With the addition of [MoS4]2- clusters

253

into CoFe-LDH materials, a porous structure was built which can benefit enlarging

254

the surface area of such composite. In addition, such structure was also beneficial for

255

Hg0 molecule uptake on its surface.

256

Mechanism study of Hg0 removal

257

The XPS analysis for the fresh samples is shown in Figure 5. As shown in Figure



258

5(a), for the O 1s of each sample, three peaks for CoFe-LDH, S4/CoFe-LDH and

259

[MoS4]2-/CoFe-LDH, centered at 530.9, 531.1 and 530.0 eV, were assigned to

260

hydroxyl oxygen.31 For Mo 3d in Figure 5(b), the peaks at 235.1 eV and 232.4 eV

261

were attributed to Mo 3d5/2 and Mo 3d3/2 XPS spectra for Mo6+, respectively.32 The

262

peaks at 229.6 and 227.3 eV can be assigned to Mo 3d5/2 XPS spectra for Mo-S

263

species. For S 2p in Figure 5(c), in the spectra of S4/CoFe-LDH, two peaks at 168.6

264

and 164.0 eV corresponded to poly-sulfur and surface sulfate, respectively.33 The ratio

265

of poly-sulfur to surface sulfate was 62.4 : 37.6. The higher poly-sulfur ratio indicated

266

that S existed in the -S-S-S-S- state. Two peaks were also detected in the spectra of

267

[MoS4]2-/CoFe-LDH; they were centered at similar positions and assigned to

268

poly-sulfur and surface sulfate. However, the ratio of poly-sulfur to surface sulfate

269

changed to 22.2 : 77.8, indicating that sulfur primarily existed in the Mo-S binding

270

state.

271

After reaction under 4% O2 and 350 µg/m3 Hg0 for 180 min, the spent

272

[MoS4]2-/CoFe-LDH samples were analyzed by XPS. As shown in Figure 5(d), the O

273

peak at 531.1 eV was assigned to hydroxyl oxygen. In addition, there were no other

274

peaks detected, indicating that oxygen was not the active site for mercury adsorption.

275

For Mo 3d in Figure 5(e), there were no obvious changes in its spectra. However, for

276

S 2p spectra in Figure 5(f), two peaks at 168.5 and 163.1 eV were assigned to

277

poly-sulfur and sulfate, respectively. The ratio of poly-sulfur was slightly increased,

278

indicating that part of the S was combined with mercury. As shown in Figure 5(g),

279

one peak at approximately 100.5 eV can correspond to surface HgS. Mercury

280

primarily existed as HgS on the surface of the [MoS4]2-/CoFe-LDH composite.

281

After reaction under 4% O2, 500 ppm SO2 and 350 µg/m3 Hg0 for 180 min, the

282

spent [MoS4]2-/CoFe-LDH samples were analyzed by XPS. The oxygen was also not

283

obviously changed in its spectra (Figure 5(h)). However, as shown in Figure 5(i), for

284

Mo 3d, the peaks at 229.3 and 227.1 eV, which correspond to Mo-S species, were

285

increased. This finding indicated that sulfate formed during the reaction under a SO2

286

atmosphere. As shown in Figure 5(j), two peaks at 163.5 and 168.4 eV corresponded

287

to surface sulfate and poly-sulfur, respectively. After reaction, two peaks appeared at


Page 10 of 25

Page 11 of 25


288

the same peak position, indicating the sulfur species did not change during the

289

reaction. However, the mass ratio of surface sulfate increased from 65.79% to 69.24%

290

compared to that in an atmosphere without SO2, suggesting that some sulfate was

291

generated during the reaction by adsorbing SO2 from the simulated flue gas. As the

292

Hg 4f spectra showed in Figure 5(k), there were no obvious peaks can be detected

293

which corresponded to surface HgS due to high intensity of Si which centered at

294

102.8 eV.

295

A H2-temperature programmed reduction (H2-TPR) measurement was performed.

296

Figure 6(a) shows the TPR profiles of as prepared [MoS4]2-/CoFe-LDH samples with

297

S4/CoFe-LDH and CoFe-LDH as reference samples. CoFe-LDH gave rise to three

298

peaks that were centered at 306, 438 and 544 °C. The first peak at a temperature of

299

306 °C was ascribed to the reduction of Co3O4 to CoO.34 A broad peak was observed

300

at the temperature range from 375 °C to 600 °C, which indicates the reduction of CoO

301

to Co, Fe2O3 to Fe3O4 and Fe3O4 to FeO. For the profile of S4/CoFe-LDH, there were

302

four peaks at 256, 322, 395 and 452 °C. However, it has a large peak centered at

303

395 °C. The primary reduction reaction was centered at 395 °C, indicating that

304

poly-S4 resulted in a reduction with a small temperature range. However, for the

305

profile of [MoS4]2-/CoFe-LDH, three peaks were located at 309, 379 and 476 °C. It is

306

difficult to define a clear boundary between each of the reduction peaks. The first

307

temperature reduction peak (309 °C) was higher than the first peak of CoFe-LDH.

308

The last peak was lower than that of CoFe-LDH, which indicated that [MoS4]2- was

309

beneficial for structural and thermal stability. During the reduction process, the

310

clusters were also beneficial for electron transfers among the CoFe-LDH layers,

311

resulting in reduction in a small range. The results suggest a strong synergistic effect

312

between [MoS4]2- clusters and CoFe-LDH, which has a beneficial influence on their

313

adsorption performance.

314

Furthermore, the Hg-temperature programed desorption (Hg-TPD) method was

315

used to detect the desorption performance. After 20 min of reaction, the spent material

316

was passed to pure N2 at a temperature increase from 75 °C to 600 °C. As shown in

317

Figure 6(b), mercury began to be released from the surface of spent



Page 12 of 25

318

[MoS4]2-/CoFe-LDH at approximately 180 °C and had a maximum desorption rate at

319

250 °C. After 60 min of reaction, almost all surface mercury was released from the

320

material surface. At such a desorption temperature range, mercury mainly existed in

321

the HgS state, which was identified with the result of XPS analysis.

322

Based

on

the

above

discussions,

gaseous

mercury

removal

over

323

[MoS4]2-/CoFe-LDH was a chemical adsorption process. As shown in Scheme 1,

324

gaseous mercury was first adsorbed on the material surface, and the special structure

325

of CoFe-LDH was favorable for mercury physical adsorption on its surface. [MoS4]2-

326

clusters enlarged the space of CoFe-LDH layers. Gaseous elemental mercury can

327

enter into this space and be captured by [MoS4]2- clusters. Second, the adsorbed

328

mercury bonded with S and formed HgS. The rich Mo-S network was favorable for

329

mercury capture during this reaction.

330 331 332

SUPPORTING INFORMATION A sketch of the Hg0 adsorption evaluation system (Figure S1) is illustrated in the

333

supporting information. The mercury adsorption breakthrough curve over

334

[MoS4]2-/CoFe-LDH is shown in Figure S2.

335 336

ACKNOWLEDGMENTS

337

This study was supported by the National Key R&D Program of China

338

(2017YFC0210502), Major State Basic Research Development Program of China

339

(973 Program, No. 2013CB430005) and the National Natural Science Foundation of

340

China (No. 51478261 and No. 21677096). This study was also supported by

341

postdoctoral innovative talent plans (No. BX201700151).

342 343


Page 13 of 25


344

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439 440 441



442 443 444

Page 18 of 25

Table 1. BET surface areas, pore volume and pore diameter Samples

BET surface areas (m2/g)

Pore Volume (m3/g)

Pore Diameter (nm)

CoFe-LDH

41.432

0.238

3.551

S4/CoFe-LDH

21.069

0.187

24.753

112.077

0.287

3.031

2-

[MoS4] /CoFe-LDH 445


Page 19 of 25


446 447 100

2-

(a)

[MoS4] /CoFe-LDH 2-

[MoS4] /NiAl-LDH

100

80

[MoS4] /ZnAl-LDH

80

CoFe-LDH S4/CoFe-LDH

2-

60

60

C/C0,%

C/C0,%

[MoS4] /CoAl-LDH

40

20

40

20

0

0

0

448

(b)

2-

[MoS4] /CoFe-LDH

2-

20

40

60

80

100

120

140

160

180

0

20

40

60

80

100

120

140

160

180

Time, min

Time, min 1.6

Mercury capacity, mg/g

1.4

o

50 C o 75 C o 100 C o 125 C o 150 C

(c)

1.2 1.0 0.8 0.6 0.4 0.2 0.0

449 450 451 452 453

Figure 1. Performances of gaseous mercury removal performances (a) over various [MoS4]2- intercalated LDH materials (75 °C); (b) CoFe-LDH, S4/CoFe-LDH and [MoS4]2-/CoFe-LDH materials (75 °C) and (c) under different temperatures over [MoS4]2-/CoFe-LDH materials (50-125 °C)

454

Reaction conditions: 4% O2 and 350 µg/m3 Hg0 with 500 mL/min flow rate

Different temperatures

455 456



457 458 1.6 100

without SO2

(a)

1.4

mercury breakthrough curve Mercury capacity, mg/g

80

C/C0, %

60

add 500 ppm SO2

40

20

(b)

500 ppm SO2 2000 ppm SO2 1000 ppm SO2

1.2 1.0 0.8 0.6 0.4 0.2

0

459 460 461 462 463

0

20

40

60

80

100

120

Time, min

140

160

180

200

0.0

Different concentrations of SO2

Figure 2. Effects of SO2 on Hg0 removal over [MoS4]2-/CoFe-LDH material, (a) mercury breakthrough curve when 500 ppm SO2 was added to the simulated flue gas and (b) under different concentrations of SO2. Reaction conditions: 4% O2 and 350 µg/m3 Hg0 with 500 mL/min flow rate

464


Page 20 of 25

Page 21 of 25


465

(a)

2-

[MoS4] /CoFe-LDH

(b) 2-

[MoS4] /CoFe-LDH

H2O

Mo-S S4/CoFe-LDH -

NO3

S4/CoFe-LDH

CoFe-LDH CoFe-LDH

466 467 468

3900 3600 3300 3000 2700 2400 2100 1800 1500 1200

900

600

10

15

20

25

30

-1

shift, cm

35

40

45

2 theta,

50

55

60

65

70

o

Figure 3. (a) FT-IR spectra and (b) XRD patterns of CoFe-LDH, S4/CoFe-LDH and [MoS4]2-/CoFe-LDH material

469



470

471 472 473 474

Figure 4. TEM images of (a) CoFe-LDH, (b) S4/CoFe-LDH and (c) [MoS4]2-/CoFe-LDH materials and SEM images of (d) CoFe-LDH, (e) S4/CoFe-LDH and (f) [MoS4]2-/CoFe-LDH materials

475


Page 22 of 25

Page 23 of 25


476 (b) Mo 3d

530.0

(a) O 1s

(c) S 2p

232.4

163.4 168.4

2-

[MoS4] /CoFe-LDH

531.1

2-

235.1

[MoS4] /CoFe-LDH

229.6 227.3

S4/CoFe-LDH

168.6

164.0

530.9

S4/CoFe-LDH

2-

[MoS4] /CoFe-LDH

CoFe-LDH 542

540

538

536

534

532

530

528

526

524

522

244

242

240

238

236

Binding energy, eV

(d) O 1s 0 after Hg adsortion

234

232

230

228

226

224

222

220

178

176

174

172

170

(e) Mo 3d 0 after Hg adsortion

531.1

168

166

164

162

160

158

156

158

156

Binding energy, eV

Binding energy, eV

(f) S 2p 0 after Hg adsortion

232.3

163.1

168.5 235.3

229.4 226.9 2-

[MoS4] /CoFe-LDH 2-

[MoS4] /CoFe-LDH

542

540

538

536

534

532

530

528

526

524

522

244

242

240

238

2-

[MoS4] /CoFe-LDH

236

Binding energy, eV

234

232

230

228

226

224

222

220

178

176

174

172

170

(g) Hg 4f 0 after Hg adsortion

102.8

(h) O 1s 0 after Hg adsorption under 500 ppm SO2

168

166

164

162

160

Binding energy, eV

Binding energy, eV

(i) Mo 3d 0 after Hg adsorption under 500 ppm SO2

531.3

232.2

235.2

107.1

229.3 237.1

100.5 2-

[MoS4] /CoFe-LDH 2-

2-

[MoS4] /CoFe-LDH

[MoS4] /CoFe-LDH 120 118 116 114 112 110 108 106 104 102 100

98

96

94

542

540

538

536

Binding energy, eV

2-


174

172

170

528

526

524

522

244

242

240

238

236

234

232

230

228

226

224

222

220

Binding energy, eV

102.8

2-


166

164

Binding energy, eV

477 478 479 480 481 482 483

530

108.0

168.4

178

532

(k) Hg 4f 0 after Hg adsorption under 500 ppm SO2

163.3

(j) S 2p 0 after Hg adsorption under 500 ppm SO2

534

Binding energy, eV

162

160

158

156

120 118 116 114 112 110 108 106 104 102 100

98

96

94

Binding energy, eV

Figure 5. XPS analysis for fresh samples: (a) O 1s of CoFe-LDH, S4/CoFe-LDH and [MoS4]2-/CoFe-LDH; (b) Mo 3d of [MoS4]2-/CoFe-LDH; (c) S 2p of S4/CoFe-LDH and [MoS4]2-/CoFe-LDH; after Hg0 adsorption samples of [MoS4]2-/CoFe-LDH for (d) O 1s, (e) Mo 3d, (f) S 2p and (g) Hg 4f; and after Hg0 adsorption under 500 ppm SO2 of [MoS4]2-/CoFe-LDH for (h) O 1s, (i) Mo 3d (j) S 2p and (k) Hg 4f.

484



(a)

476

(b) 500

2-

[MoS4] /CoFe-LDH

322

452 438

S4/CoFe-LDH 306

o

400

300

200

544

CoFe-LDH 100

485 486 487 488

150

200

Temperature, C

256

395

Mercury Signal

Intensity

600

379

309

Page 24 of 25

100 250

300

350

400

450 o

Temperature, C

500

550

600

650

700

0

10

20

30

40

50

60

70

80

90

100

Time, min

Figure 6. (a) H2-Temperature Programmed Reduction (H2-TPR) profiles of as-prepared samples and (b) Hg-Temperature Programmed Desorption (Hg-TPD) analysis for [MoS4]2-/CoFe-LDH materials

489


Page 25 of 25


490

491 492

Scheme 1. Hg0 removal mechanism over [MoS4]2-/CoFe-LDH composite

493 494


[MoS4] Cluster Bridges in Co-Fe Layered Double Hydroxides for

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