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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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[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*
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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:
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ABSTRACT: [MoS4]2- clusters were bridged between CoFe layered double
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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
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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
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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
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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.
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Sulfur is one element with affinity for mercury, and HgS stably exists in ore.
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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
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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
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chalcogels with ion-exchange properties participated in vapor sorption.23 The
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ion-exchangeable
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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,
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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
94
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,
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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
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Preparation of Materials
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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
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water and stirred for 1 h. Afterwards, NaOH solution was used to adjust the pH to 10.
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The product was then filtered, washed with deionized water and air-dried at 80 °C.
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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.
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Synthesis of [MoS4]2-/LDH materials: [MoS4]2-/LDH was prepared via an
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anion-exchange reaction. For [MoS4]2-/CoFe-LDH, briefly, 0.4 g of as-prepared
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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
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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
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[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
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sulfur were mixed together and stirred at 90 °C until the sulfur absolutely dissolved.
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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.
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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
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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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electron microscopy (SEM, JSM-7001F) and transmission electron microscopy (TEM,
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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
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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.
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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
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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,
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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
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[MoS4]2-/NiAl-LDH, with removal efficiencies of 39% and 50%, respectively.
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[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
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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.
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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
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removal. To further indicate Hg0 removal performance under S-Hg mixed flue gas,
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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
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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
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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
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[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
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REFERENCES:
345
1.
346
Lin, Z., Mercury flows in China and global drivers. Environmental Science & Technology 2017, 2017,
347
51 (1), 222–231.
348
2.
349
China. Environmental science & technology 2016, 50, (5), 2337-2344.
350
3.
351
emission inventory for 2000. Atmospheric Environment 2006, 40, (22), 4048-4063.
352
4.
353
mercury from coal-fired flue gas with selective catalytic reduction catalysts. Catalysis Science
354
& Technology 2015, 5, (7), 3459-3472.
355
5.
356
review on the heterogeneous catalytic oxidation of elemental mercury in flue gases.
357
Environmental science & technology 2013, 47, (19), 10813-10823.
358
6.
359
selenite on mercury re-emission in smelting flue gas scrubbing system. Fuel 2015, 168, 7-13.
360
7.
361
Investigation on mercury removal method from flue gas in the presence of sulfur dioxide.
362
Journal of Hazardous Materials 2014, 279, 289-295.
363
8.
364
sulfur-impregnated porous carbon – A review. Environmental Technology 2014, 35, (1), 18.
365
9.
Hui, M.; Wu, Q.; Wang, S.; Liang, S.; Zhang, L.; Wang, F.; Lenzen, M.; Wang, Y.; Xu, L.;
Lin, Y.; Wang, S.; Wu, Q.; Larssen, T., Material flow for the intentional use of mercury in
Pacyna, E. G.; Pacyna, J. M.; Steenhuisen, F.; Wilson, S., Global anthropogenic mercury
Zhao, L.; Li, C.; Zhang, X.; Zeng, G.; Zhang, J., A review on oxidation of elemental
Gao, Y.; Zhang, Z.; Wu, J.; Duan, L.; Umar, A.; Sun, L.; Guo, Z.; Wang, Q., A critical
Peng, B.; Liu, Z.; Chai, L.; Liu, H.; Yang, S.; Yang, B.; Xiang, K.; Liu, C., The effect of
Ma, Y.; Zan, Q.; Xu, H.; Wang, W.; Yan, N.; Ma, Y.; Xu, H.; Wang, W.; Yan, N.,
Reddy, K. S. K.; Shoaibi, A. A.; Srinivasakannan, C., Elemental mercury adsorption on
Suresh Kumar Reddy, K.; Al Shoaibi, A.; Srinivasakannan, C., Elemental mercury
ACS Paragon Plus Environment
Environmental Science & Technology
366
adsorption on sulfur-impregnated porous carbon–A review. Environmental technology 2014,
367
35, (1), 18-26.
368
10. Vidic, R. D.; Siler, D. P., Vapor-phase elemental mercury adsorption by activated carbon
369
impregnated with chloride and chelating agents. Carbon 2001, 39, (1), 3-14.
370
11. and, W. L.; Vidic, R. D.; Brown, T. D., Optimization of High Temperature Sulfur
371
Impregnation on Activated Carbon for Permanent Sequestration of Elemental Mercury Vapors.
372
Environmental Engineering Science 2000, 17, (6), 303-313.
373
12. Zhao, Z.; Wu, J.; Huang, T.; Sheng, P.; Zhang, J.; Tian, H.; Zhao, X.; Zhao, L.; He, P.;
374
Ren, J., Removal of elemental mercury by Ce-Mn co-modified activated carbon catalyst.
375
Catalysis Communications 2017, 93, 62-66.
376
13. Xu, H.; Qu, Z.; Zong, C.; Huang, W.; Quan, F.; Yan, N., MnOx/Graphene for the Catalytic
377
Oxidation and Adsorption of Elemental Mercury. Environmental Science & Technology 2015,
378
49, (11), 6823.
379
14. Xu, H.; Zan, Q.; Zong, C.; Quan, F.; Jian, M.; Yan, N., Catalytic oxidation and adsorption
380
of Hg 0 over low-temperature NH 3 -SCR LaMnO 3 perovskite oxide from flue gas. Applied
381
Catalysis B Environmental 2016, 186, 30-40.
382
15. Yang, S.; Guo, Y.; Yan, N.; Wu, D.; He, H.; Xie, J.; Qu, Z.; Yang, C.; Jia, J., A novel
383
multi-functional magnetic Fe-Ti-V spinel catalyst for elemental mercury capture and callback
384
from flue gas. Chemical Communications 2010, 46, (44), 8377.
385
16. Uddin, M. A.; Yamada, T.; Ochiai, R.; Sasaoka, E.; Wu, S., Role of SO 2 for Elemental
386
Mercury Removal from Coal Combustion Flue Gas by Activated Carbon. Energy & Fuels 2008,
387
22, (4), 13-9.
ACS Paragon Plus Environment
Page 14 of 25
Page 15 of 25
Environmental Science & Technology
388
17. Reddy, G. K.; He, J.; Thiel, S. W.; Pinto, N. G.; Smirniotis, P. G., Sulfur-Tolerant Mn-Ce-Ti
389
Sorbents for Elemental Mercury Removal from Flue Gas: Mechanistic Investigation by XPS.
390
Journal of Physical Chemistry C 2015, 119, (16), 8634–8644
391
18. Xu, H.; Xie, J.; Ma, Y.; Qu, Z.; Zhao, S.; Chen, W.; Huang, W.; Yan, N., The cooperation
392
of Fe Sn in a MnO x complex sorbent used for capturing elemental mercury. Fuel 2015, 140,
393
803-809.
394
19. Li, Y. H.; Lee, C. W.; Gullett, B. K., Importance of activated carbon's oxygen surface
395
functional groups on elemental mercury adsorption ☆ ☆. Fuel 2003, 82, (4), 451-457.
396
20. Li, H.; Zhu, L.; Wang, J.; Li, L.; Shih, K., Development of Nano-Sulfide Sorbent for
397
Efficient Removal of Elemental Mercury from Coal Combustion Fuel Gas. Environmental
398
Science & Technology 2016, 50, (17), 9551.
399
21. Liao, Y.; Chen, D.; Zou, S.; Xiong, S.; Xiao, X.; Dang, H.; Chen, T.; Yang, S., Recyclable
400
Naturally Derived Magnetic Pyrrhotite for Elemental Mercury Recovery from Flue Gas.
401
Environmental Science & Technology 2016, 50, (19), 10562-10569.
402
22. Zhao, H.; Gang, Y.; Xiang, G.; Pang, C. H.; Kingman, S. W.; Tao, W., Hg0 Capture over
403
CoMoS/γ-Al2O3 with MoS2 Nanosheets at Low Temperatures. Environmental Science &
404
Technology 2016, 50, (2), 1056.
405
23. Oh, Y.; Morris, C. D.; Kanatzidis, M. G., Polysulfide chalcogels with ion-exchange
406
properties and highly efficient mercury vapor sorption. Journal of the American Chemical
407
Society 2012, 134, (35), 14604-8.
408
24. Subrahmanyam, K. S.; Malliakas, C. D.; Sarma, D.; Armatas, G. S.; Wu, J.; Kanatzidis, M.
409
G., Ion-Exchangeable Molybdenum Sulfide Porous Chalcogel: Gas Adsorption and Capture of
ACS Paragon Plus Environment
Environmental Science & Technology
410
Iodine and Mercury. Journal of the American Chemical Society 2015, 137, (43), 13943-8.
411
25. Ma, S.; Chen, Q.; Li, H.; Wang, P.; Islam, S. M.; Gu, Q.; Yang, X.; Kanatzidis, M. G.,
412
Highly selective and efficient heavy metal capture with polysulfide intercalated layered double
413
hydroxides. Journal of Materials Chemistry A 2014, 2, (26), 10280-10289.
414
26. Shami, Z.; Amininasab, S. M.; Shakeri, P., Structure-Property Relationships of
415
Nano-Sheeted 3D Hierarchical Roughness MgAl-Layered Double Hydroxide Branched to
416
Electrospun Porous Nanomembrane: A Superior Oil-Removing Nanofabric. Acs Applied
417
Materials & Interfaces 2016, 8 (42), 28964–28973.
418
27. Wang, Q.; O’Hare, D., Recent Advances in the Synthesis and Application of Layered
419
Double Hydroxide (LDH) Nanosheets. Chemical Reviews 2012, 112, (7), 4124-55.
420
28. Ma, S.; Shim, Y.; Islam, S. M.; Subrahmanyam, K. S.; Wang, P.; Li, H.; Wang, S.; Yang,
421
X.; Kanatzidis, M. G., Efficient Hg Vapor Capture with Polysulfide Intercalated Layered Double
422
Hydroxides. Chemistry of Materials 2014, 26, (17), 5004-5011.
423
29. Ma, L.; Wang, Q.; Islam, S. M.; Liu, Y.; Ma, S.; Kanatzidis, M. G., Highly Selective and
424
Efficient Removal of Heavy Metals by Layered Double Hydroxide Intercalated with the MoS42-
425
ion. Journal of the American Chemical Society 2016, 138 (8), 2858–2866.
426
30. Ma, S.; Huang, L.; Ma, L.; Shim, Y.; Islam, S. M.; Wang, P.; Zhao, L.-D.; Wang, S.; Sun,
427
G.; Yang, X., Efficient uranium capture by polysulfide/layered double hydroxide composites.
428
Journal of the American Chemical Society 2015, 137, (10), 3670-3677.
429
31. Deng, L.; Shi, Z.; Peng, X., Adsorption of Cr (vi) onto a magnetic CoFe 2 O 4/MgAl-LDH
430
composite and mechanism study. RSC Advances 2015, 5, (61), 49791-49801.
431
32. Zhang, Z.; Liu, B.; Wang, F.; Zheng, S., High-temperature desulfurization of hot coal gas
ACS Paragon Plus Environment
Page 16 of 25
Page 17 of 25
Environmental Science & Technology
432
on Mo modified Mn/KIT-1 sorbents. Chemical Engineering Journal 2015, 272, 69-78.
433
33. Liao, Y.; Chen, D.; Zou, S.; Xiong, S.; Xiao, X.; Dang, H.; Chen, T.; Yang, S., A recyclable
434
naturally derived magnetic pyrrhotite for elemental mercury recovery from the flue gas.
435
Environmental Science & Technology 2016, 50, (19) 10562–10569.
436
34. Li, X.; Zhang, C.; He, H.; Teraoka, Y., Catalytic decomposition of N 2 O over CeO 2
437
promoted Co 3 O 4 spinel catalyst. Applied Catalysis B Environmental 2007, 75, (75), 167–
438
174.
439 440 441
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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
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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
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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
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(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
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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
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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-
[MoS4] /CoFe-LDH 176
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-
[MoS4] /CoFe-LDH 168
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.
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(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
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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
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491 492
Scheme 1. Hg0 removal mechanism over [MoS4]2-/CoFe-LDH composite
493 494
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