The adsorption and catalytic oxidation of the element mercury over cobalt modified Ce-ZrO2 catalyst

Article abstract of DOI:10.1039/c6ra19450h

Co was used as a modifier for Ce-ZrO₂ to improve the surface physicochemical properties of the catalysts. The obtained catalysts Co_x-Ce-ZrO₂ were investigated for Hg⁰ adsorption and oxidation in the temperature range of 100-350 °C. The results showed that Co modified catalysts had an excellent Hg⁰ removal efficiency. For Co_0.3-Ce-ZrO₂, the Hg⁰ adsorption efficiency was above 81.5% even after a 164.5 h adsorption test and the Hg⁰ oxidation efficiency reached 100% in the temperature range of 100-250 °C. XRD, BET and XPS results suggested that the Co modified catalysts had larger surface areas and more oxygen vacancies. H₂-TPR results showed that Co addition improved the redox performance of the catalyst. These results led to a better Hg⁰ removal efficiency by the Co modified catalysts. Furthermore, XPS results of Co_0.3-Ce-ZrO₂ before and after reaction showed that, during the Hg⁰ oxidation process, surface chemisorbed oxygen was dramatically consumed while some Co³⁺ and Ce⁴⁺ changed to Co²⁺ and Ce³⁺. This suggested that chemisorbed oxygen participated in the Hg⁰ oxidation process. A probable pathway of Hg⁰ oxidation was deduced based on these results. The effects of individual flue gas components (O₂, NO, H₂O, SO₂) on Hg⁰ oxidation efficiency were investigated and the results showed that O₂ and NO improved the Hg⁰ oxidation efficiency whereas H₂O and SO₂ inhibited it.

Full text of DOI:10.1039/c6ra19450h

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The adsorption and catalytic oxidation of the
element mercury over cobalt modified Ce–ZrO2
Cite this: RSC Adv., 2016, 6, 88332
catalyst
*
X. P. Zhang, Y. Z. Cui, B. J. Tan, J. X. Wang, Z. F. Li and G. H. He
Co was used as a modifier for Ce–ZrO2 to improve the surface physicochemical properties of the catalysts.
The obtained catalysts Cox–Ce–ZrO2 were investigated for Hg0 adsorption and oxidation in the
temperature range of 100–350 ꢀC. The results showed that Co modified catalysts had an excellent Hg0
removal efficiency. For Co0.3–Ce–ZrO2, the Hg0 adsorption efficiency was above 81.5% even after
a 164.5 h adsorption test and the Hg0 oxidation efficiency reached 100% in the temperature range of 100–
250 ꢀC. XRD, BET and XPS results suggested that the Co modified catalysts had larger surface areas and
more oxygen vacancies. H2-TPR results showed that Co addition improved the redox performance of the
catalyst. These results led to a better Hg0 removal efficiency by the Co modified catalysts. Furthermore,
XPS results of Co0.3–Ce–ZrO2 before and after reaction showed that, during the Hg0 oxidation process,
surface chemisorbed oxygen was dramatically consumed while some Co3+ and Ce4+ changed to Co2+ and
Ce3+. This suggested that chemisorbed oxygen participated in the Hg0 oxidation process. A probable
pathway of Hg0 oxidation was deduced based on these results. The effects of individual flue gas
components (O2, NO, H2O, SO2) on Hg0 oxidation efficiency were investigated and the results showed that
O2 and NO improved the Hg0 oxidation efficiency whereas H2O and SO2 inhibited it.
Received 1st August 2016
Accepted 25th August 2016
DOI: 10.1039/c6ra19450h
www.rsc.org/advances
Therefore catalytic oxidation method has received more atten-
tion. Transition metal oxides such as MnOx, VOx, WOx, CeOx,
ZrOx and CoOx have been widely used in the studies of Hg0
1. Introduction
Mercury is one of the major fatal pollutants due to its extreme
toxicity, high volatility, persistence and bioaccumulation in the
environment, which can cause serious damage to the human
central nervous system, digestive system and kidneys.1,2 Conse-
quently, increasing attention has been paid to mercury treatment.
The major anthropogenic source of mercury emission is coal
combustion, especially in traditional coal-red power plants.3
Mercury in coal combustion ue gas presents in three forms:
particle-bound mercury (Hgp), oxidized mercury (Hg2+) and
elemental mercury (Hg0).4,5 Most Hgp is easy to be enriched on
the surface of y ash and captured by particulate matter (PM)
control devices such as electrostatic precipitators (ESP) and
fabric lters (FF).6 Hg2+ can be easily removed by wet ue gas
desulfurization (WFGD).7,8 However, Hg0 is difficult to be
removed through traditional environmental protection equip-
ment due to its high volatility and insolubility in water.9,10
Therefore, the removal of Hg0 is the key and difficult point of
controlling mercury emission from coal-red power plants.
Adsorption method such as activated carbon injection is an
effective technique for removing Hg0, but the disadvantages of
high cost and low adsorption capacity limit its promotion.11,12
removal due to their various states, redox properties and rela-
tively low price. CeO2 with excellent oxygen storage capacity and
redox property has received widespread attention. Zhou et al.13
found that CeO2(0.2)/TiO2 showed a high Hg0 removal efficiency
above 80% under simulated syngas at a low temperature range
from 80 to 150 ꢀC. Li et al.14 reported that MnOx–CeO2 supported
on TiO2 performed high Hg0 oxidation efficiency over 90% in
simulated ue gas at 200 ꢀC. However, pure CeO2 was thermally
unstable and easily sintered at high temperature, thereby seri-
ously degrading its oxygen storage capacity, which could lead to
the deactivation of the catalyst.15–17 It was found that the intro-
duction of ZrO2 into the CeO2 lattice formed Ce–ZrO2 solid
solution could enhance the thermal stability and oxygen storage
capacity. Ce–ZrO2 as supporter, adsorbent and catalyst has been
applied to selective catalytic reduction of NO,18 methane oxida-
tion,19 naphthalene oxidation20 and Hg0 oxidation,21 and showed
high efficiency. Furthermore, cobalt with various valence states
and excellent redox ability has been introduced in the catalytic
system. Liu et al.22 used Co/TiO2 for Hg0 oxidation in presence of
HCl and O2, when the loading of cobalt was 7.5%, the catalyst
performed more than 90% oxidation efficiency within the
temperature range of 120–330 C. Zhao et al.23 investigated the
ꢀ
School of Petroleum and Chemical Engineering, Dalian University of Technology, 2
Dagong Road, Panjin, 124221, Liaoning, China. E-mail: hgaohong@dlut.edu.cn; Tel:
+86-427-2631916
Hg0 removal capacity over CoMoS/g-Al2O3 catalyst and it achieved
ꢀ
nearly 100% removal efficiency at low temperature 50 C. Yang
88332 | RSC Adv., 2016, 6, 88332–88339
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et al.24 studied Co–MF catalyst for Hg0 removal, its removal effi- iQ-C automated gas sorption system (Quantachrome Instru-
ciency reached 85.7% in the presence of O2 and enriched CO2 ments, USA). The pore size distribution was calculated using the
atmosphere. Based on the mentions above, cobalt modies desorption branches of the N2 adsorption isotherm and the
catalysts may perform excellent efficiency for Hg0 removal. Barrett–Joyner–Halenda (BJH) formula. All the samples were
Nevertheless, cobalt modies Ce–ZrO2 for removing Hg0 has not degassed at 300 C for 2 h prior to the measurements.
been investigated in our known. The powder X-ray diffraction (XRD) measurement were
ꢀ
In the present work, a series of cobalt modied Ce–ZrO2 carried out to determine the crystal structure distribution of the
catalysts were synthesized by a co-precipitation method for Hg0 samples with XRD-7000 S system using Cu Ka radiation (40 kV,
removal in simulated ue gas. The effects of the Co loading and 100 mA) (SHIMADZU Corporation, Japan). The samples were
individual ue gas components on Hg0 adsorption and oxida- scanned at 2q ranging from 8ꢀ to 80ꢀ with a scanning velocity of
tion were studied. XRD, BET, H2-TPR and XPS analyses were 5ꢀ minꢁ1 and a step size of 0.02ꢀ. The powder XRD phases
employed to characterize the physical and chemical properties present in the samples were identied with the help of JCPDS
of the catalysts to reveal the mechanism of Hg0 oxidation.
cards.
Hydrogen temperature-programmed reduction (H2-TPR) was
performed on a ChemBET Pulsar TPR/TPD (Quantachrome
Corporation, USA). Prior to H2-TPR experiment, 50 mg samples
2. Experimental
2.1. Catalyst preparation
ꢀ
were pretreated in He atmosphere at 300 C for 2 h and then
ꢀ
Ce–ZrO2, Co–Ce–ZrO2 were synthesized by co-precipitation
method. Ce(NO3)3$6H2O, ZrO(NO3)2 and Co(NO3)2$6H2O were
used as metal precursors. The precursors were dissolved in
deionized water with the desired molar ratios of Ce : Zr ¼ 1 : 1,
Co : Ce : Zr ¼ 0.10 : 0.45 : 0.45, Co : Ce : Zr ¼ 0.20 : 0.40 : 0.40,
cooled down to 50 C in the same atmosphere. Subsequently,
the samples were heated from 50 ꢀC to 900 ꢀC at the linear
heating rate of 10 ꢀC minꢁ1 under 10 vol% H2/Ar (30 ml minꢁ1).
The H2 consumption amount was measured by a thermal
conductivity detector (TCD).
Co : Ce : Zr
¼
0.30 : 0.35 : 0.35. Under stirring condition,
X-ray photoelectron spectroscopy (XPS) measurements were
carried out on an ESCALAB250 (ThermoScientic) using
monochromatic Al Ka source. The surface charging effects were
corrected with the C 1s binding energy value of 284.6 eV.
ammonia solution (25%) as a precipitator was added dropwise
into the solutions until the PH reached 10. The mixtures of
liquid and precipitator were stirred for 3 h at room temperature,
aged for 1 h and ltered in vacuo. The obtained precipitators
were dried at 80 ꢀC in a water bath for 12 h and nally calcined
at 500 ꢀC for 6 h to obtain the fresh catalysts Cox–Ce–ZrO2,
2.3. Catalytic activity test
Hg0 removal efficiency of catalysts was performed in a xed-bed
ow reactor (Fig. 1). 0.2 ml (about 0.7 g) catalyst was loaded into
the center of the quarter reactor. The components of simulated
ue gases included 5 vol% O2, 100 or 200 ppm NO (when used),
6 vol%, 8 vol% H2O (when used), 200, 400 or 800 ppm SO2 (when
used) and highly pure N2 as the balance gas. The total ux of gas
was 600 ml minꢁ1, and the gas hourly space velocity (GHSV) was
where x stands for the molar ratio of Co to catalyst. Ce0.50
–
Zr0.50O2, Co0.10–Ce0.45–Zr0.45O2, Co0.20–Ce0.40–Zr0.40O2 and
Co0.30–Ce0.35–Zr0.35O2 were denoted as Ce–ZrO2, Co0.1–Ce–ZrO2,
Co0.2–Ce–ZrO2 and Co0.3–Ce–ZrO2, respectively.
2.2. Catalyst characterization
The Brunauer–Emmett–Teller (BET) surface areas of the cata- about 180 000 hꢁ1. All the feed gases were mixed and preheated
lysts were gauged by N2 adsorption at ꢁ196 ꢀC on an Autosorb- in a gas mixing chamber before entering the reactor. The highly
Fig. 1 A schematic diagram of the fix-bed experiment system.
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pure N2 was divided into two branches: one branch converged 29.5ꢀ, 50.17ꢀ were corresponded to respective crystal planes
with the individual streams of O2, NO, SO2 and H2O formed the (111) and (220) of ZrO2 (PDF# 49-1642). With the addition of
main gas ow, and the other branch (200 ml minꢁ1) passed cobalt, the characteristic peaks of CeO2 became weaker and
through a mercury permeation device (VICI Metronics, U.S.) to broader, and that of ZrO2 disappeared. These results indicated
introduce Hg0 vapor into the system. In order to obtain that the doping of cobalt promoted the dispersion of CeO2 and
a constant permeation concentration of Hg0, the Hg0 perme- ZrO2 and then might lead to a higher surface area of the cata-
ation tube was placed in a U-shaped glass tube, which was lysts. Moreover, the diffraction peaks position of CeO2 shied to
immersed in a constant temperature (32 ꢀC) water bath. The a little higher aer cobalt addition due to an interaction
concentration of Hg0 in this system was controlled at between Co and Ce–ZrO2. To further investigate the effects of
a concentration of 50 mg mꢁ3
.
cobalt incorporation, the lattice parameters and crystallite sizes
An online mercury analyzer (QM201H, Qingan, China) was of the peaks of CeO2 at (111) were calculated by Cohen's method
employed to measure Hg0 concentrations of the inlet and outlet and Scherrer equation (summarized in Table 1). Compared with
of the reactor. The Hg0 of simulated gas from reactor and Ce–ZrO2, the lattice parameter decreases slightly aer cobalt
mercury analyzer was absorbed by active carbon before expel- doping, which implied that the Co3+ (0.61 A) with a smaller
˚
ling into the atmosphere. To avoid mercury condensation, ionic radius than Ce4+ (0.97 A) was inserted into the CeO2 lattice
˚
heater bands were used to keep the silicone pipelines at 100 ꢀC. resulted in forming Co–Ce–ZrO2 solid solution. Furthermore,
At the beginning of each test, the inlet gas was sampled to the average crystallite sizes of CeO2 of Co-containing catalysts
ensure a stable Hg0 feed concentration (Hg0in). For Hg0 oxidation were smaller than that of Ce–ZrO2 due to the interaction
test, the catalysts were kept in a 125 mg mꢁ3 Hg0 balance N2 between Co and Ce–ZrO2. When the mole ratio of Co to Co–Ce–
atmosphere for more than 80 h at 80 ꢀC to insure that the ZrO2 was 0.1, no Co phases were detected, indicating that Co
catalysts reached adsorption equilibrium, and then the inter- related species might be highly dispersed on the surface of
ference of Hg0 adsorption can be eliminated. And then the exit catalysts or incorporated into the Ce–ZrO2 solid solution to
Hg0 concentration of the reactor was described as Hgo0ut. The form a new Co–Ce–ZrO2 solid solution.25,26 With the cobalt mole
Hg0 oxidation efficiency (Eoxi) can be dened as follow:
Hg0in ꢁ Hg0out
ratio increased ($0.2) the diffraction peaks attributed to Co3O4
turned up and got intense. The crystalline phase of Co3O4 has
been reported to be benecial for Hg0 oxidation.22
Eoxi
¼
ꢂ 100%
Hg0in
3.1.2. BET analysis. The BET surface areas, pore volumes
and average pore sizes of fresh catalysts are listed in Table 1. It
can be seen that Ce–ZrO2 has the lowest BET surface area (77.50
m2 gꢁ1), highest pore volume (0.1083 cm3 gꢁ1) and pore size
(2.794 nm). Aer the addition of cobalt into Ce–ZrO2, the
specic surface area increased. It was consistent with the results
of XRD, which indicates that Co–Ce–ZrO2 solid solution formed
resulted in better dispersion of CeO2 and ZrO2. When the cobalt
mole ratio was 0.2, the catalyst Co0.2–Ce–ZrO2 showed the
largest surface area 101.18 m2 gꢁ1. However, further increasing
the Co loading led to a slight decrease of the BET surface area,
which means that excessive Co loading might block the internal
micropores of Ce–ZrO2.
3. Results and discussion
3.1. Characterization of catalysts
3.1.1. XRD analysis. The XRD patterns of Ce–ZrO2,
Co0.1–Ce–ZrO2, Co0.2–Ce–ZrO2 and Co0.3–Ce–ZrO2 are shown in
Fig. 2. The diffraction peaks of Ce–ZrO2 at 2q ¼ 28.56ꢀ, 33.19ꢀ,
47.64ꢀ, 56.53ꢀ, 59.28ꢀ, 69.61ꢀ, 77.05ꢀ and 79.5ꢀ can be indexed to
the (111), (200), (220), (311), (222), (400), (331), (420) crystal
planes of CeO2 (PDF# 34-0394), and the weak peaks at 2q ¼
3.1.3. H2-TPR analysis. H2-TPR experiments were per-
formed to investigate the reducibility of catalysts with different
cobalt loadings, and the results are shown in Fig. 3. The two
peaks of Ce–ZrO2 center at 573 ꢀC and 816 ꢀC were assigned to
the reduction of surface Ce4+ to Ce3+ and bulk CeO2, respec-
tively.15 The Co-containing catalysts exhibited three reduction
peaks around 300 ꢀC, 360 ꢀC and above 500 ꢀC, which were
attributed to the reduction of surface Co3O4 to CoO, the
reduction of CoO to Co0 (ref. 27 and 28) and the reduction of
Ce4+ to Ce3+ 29 respectively. It could be seen that the intensities
,
of these three peaks increased with an increase of the cobalt
loading, which implied the introduction of cobalt improved the
oxygen storage capacity of Ce–ZrO2. Meanwhile, the onset
temperature of reduction peaks of Co3O4 to CoO around 170 ꢀC
was lower than pure Co3O4,28,30 and the third peak above 500 ꢀC
shied to lower temperature with increasing cobalt contents. It
indicated cobalt addition enhanced the redox ability of
Ce–ZrO2.
Fig. 2 XRD patterns of fresh catalysts.
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Table 1 Structural parameters measured by N2 adsorption and calculated from XRD experiments
BET surface area (m2 gꢁ1
)
Pore volume (cm3 gꢁ1
)
Pore size (nm)
Lattice parameter (A)
Crystallite size (nm)
˚
Catalysts
Ce–ZrO2
77.50
94.54
101.18
90.00
0.1083
2.794
2.044
1.925
1.966
5.414
5.345
5.363
5.353
8.465
4.423
4.464
4.766
Co0.1–Ce–ZrO2
Co0.2–Ce–ZrO2
Co0.3–Ce–ZrO2
0.09661
0.09739
0.08846
the combination of cobalt oxide, ceria and zirconium increased
the concentration of surface chemisorbed oxygen. Aer Co
addition, the binding energy of O 1s decreased. It has been re-
ported that the lower binding energy of lattice oxygen could
make the surface oxygen more instability,33 moreover, Oa was
considered to be more active than Ob for its high mobility,
which was benecial for Hg0 removal.34,35
As shown in Fig. 4(b), the peaks labeled v, v2, v3, u, u2 and u3
were assigned to the 3d104f0 state of Ce4+, while the peaks
labeled v1 and u1 were assigned to the 3d104f1 state of Ce3+.36,37 It
indicated that both Ce4+ and Ce3+ existed on the surface of the
two samples, and the Ce4+ was the main state. The band energy
of fresh Ce–ZrO2 shied to lower value aer cobalt doping,
which could be due to the electron interaction between cobalt
and cerium. As shown in Table 2, compared to the fresh Ce–
ZrO2 (26.4%), the concentration of Ce3+ for fresh Co0.3–Ce–ZrO2
(28.4%) increased with the cobalt addition due to the interac-
tion between cobalt and cerium38 through CeO2 + CoO / Ce2O3
+ Co3O4.25 The presence of Ce3+ could generate a charge
imbalance resulted in more oxygen vacancies and unsaturated
chemical bonds on the catalyst surface, which was benecial for
Hg0 adsorption and oxidation.21,33,39
Fig. 3 H2-TPR profiles of fresh catalysts.
3.1.4. XPS analysis. XPS characterization was used to
identify the state of surface species over Ce–ZrO2 and Co0.3–Ce–
ZrO2. The XPS spectra of O 1s and Ce 3d are shown in Fig. 4 and
the surface atomic ratios are shown in Table 2.
As shown in Fig. 4(a), only one peak could be observed in
both of the two samples which could be splitted into two
overlapping peaks. The peak at lower binding energy (530–530.3
eV) could be attributed to the lattice oxygen (denoted as Ob) and
the peak at higher energy (531.5–531.9 eV) could be assigned to
the surface chemisorbed oxygen (denoted as Oa).31,32 As shown
in Table 2, the Oa ratio of fresh Co0.3–Ce–ZrO2 (63.7%) was
higher than that of fresh Ce–ZrO2 (53.1%), which indicated that
Table 2 XPS data of fresh Ce–ZrO2 and Co0.3–Ce–ZrO2 samples
Catalysts
Ce4+/(Ce4+ + Ce3+) (%)
Oa/(Oa + Ob) (%)
Fresh Ce–ZrO2
Fresh Co0.3–Ce–ZrO2
73.6
71.6
53.1
63.7
Fig. 4 O 1s and Ce 3d XPS spectras of fresh Ce–ZrO2 and Co0.3–Ce–ZrO2.
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be clearly observed that the Hg0 oxidation efficiency of Co-
containing catalysts rst increased with the increase of
temperature owing to high adsorption capacity and the activity
of catalysts components improved as the temperature
increased, and then the Hg0 oxidation efficiency decreased as
the temperature continuously rose since it restrained the
physisorption of the catalysts. When the mole ratio loading of
cobalt on Co–Ce–ZrO2 was 0.3, the Hg0 oxidꢀation efficiency was
100% at the temperature range of 100–250 C.
3.2. Performance of catalysts
3.2.1. Adsorption and oxidation of Hg0 catalysts. A break-
through experiment was introduced to investigate the adsorp-
tion capacity of the catalysts. As shown in Fig. 5, the Hg0
removal efficiency of Ce–ZrO2 was 37.9% at the beginning of the
test and decreased immediately to 0 aer 1.5 h reaction. Aer
cobalt addition, the activity was greatly improved. The Hg0
removal efficiency of Co0.1–Ce–ZrO2 maintained at 100% in the
initial 18 h and it decreased from 100% to 43.6% during 41 h
reaction. With the increase of Co loading amounts, the Hg0
removal efficiency was further improved. For Co0.3–Ce–ZrO2, the
Hg0 removal efficiency was nearly 100% during 120 h test and it
still maintained above 80% even 81.5% aer 164.5 h reaction.
Hg0 oxidation efficiency of the catalysts with the condition of
5% O2 and balanced N2 are shown in Fig. 6. It can be seen that
Ce–ZrO2 catalyst showed a poor Hg0 oxidation efficiency at the
The good performance of Co0.3–Ce–ZrO2 for Hg0 adsorption
and oxidation was related with the physical and chemical
properties of the catalyst. XRD and BET results showed that
cobalt modied catalysts had higher dispersion of CeO2, ZrO2
and larger surface areas which could give more surface active
sites. H2-TPR results showed that the addition of cobalt
enhanced the redox ability of Ce–ZrO2. It has been reported that
more reducible species and higher redox ability could enhance
the mobility of surface oxygen and were in favor of catalytic
activity.10,15,39 The results of XPS showed that the addition of
cobalt generated more oxygen vacancies and chemisorbed
oxygen which was benecial for Hg0 adsorption and oxidation.
All these results could lead to a good Hg0 removal efficiency.
3.2.2. Effect of individual ue gas components on Hg0
oxidation. Fig. 7 shows the effects of O2, NO, H2O and SO2 on
the Hg0 oxidation efficiency. Hg0 oxidation efficiency was
0 without O2 (not shown). When 2% O2 was introduced into the
ue gas, the Hg0 oxidation efficiency signicantly increased
from 0 to 84.5% and it increased from 84.5% to 97.3% when the
O2 concentration further increased to 5%. It indicated that O2
played an important role in the Hg0 oxidation process, due to
that gaseous O2 could generate lattice oxygen and replenish the
chemisorbed oxygen on the catalyst surface.35,40,41 However, no
obvious increase of Hg0 oxidation efficiency was observed when
the O2 concentration increased from 5% to 8%, which indicated
that 5% O2 was sufficient to maintain enough active oxygen
species for Hg0 oxidation.
ꢀ
temperature range of 100–350 C, which was only 5.4% at 150
ꢀC. When the cobalt was added, the Co-containing catalysts
presented signicantly higher Hg0 oxidation efficiency. It could
Fig. 5 Hg0 breakthrough curves of catalysts under pure N2 at 200 ꢀC.
Fig. 6 Hg0 oxidation efficiency of the catalysts (reaction condition: 5% Fig. 7 Effects of individual flue gas components on Hg0 oxidation
O2).
efficiency of Co0.3–Ce–ZrO2 at 250 ꢀC.
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The Hg0 oxidation efficiency increased to 100% when NO and the contents of chemisorbed oxygen on used Ce–ZrO2 and
(100 or 200 ppm) was introduced into simulate gas indicating Co0.3–Ce–ZrO2 decreased from 53.1% and 63.7% to 50.7% and
that NO could promote Hg0 oxidation with O2. The possible 38.1% (shown in Table 3), respectively. Then it can be inferred
reason might be CeO2 and Co3O4 could weakly adsorb NO, and that the chemisorbed oxygen took part in the reaction of Hg0
then NO was oxidized by lattice oxygen and/or chemisorbed oxidation. Furthermore, more surface chemisorbed oxygen of
oxygen of the catalyst resulted in forming new species, such as Co0.3–Ce–ZrO2 was consumed during Hg0 oxidation process.
NO2, NO+. It has been reported that NO2 could promote Hg0
oxidation.42,43
As shown in Fig. 8(b), compared with fresh samples of
Ce–ZrO2 and Co0.3–Ce–ZrO2, the intensities of the peaks repre-
H2O and SO2 intensively inhibit the Hg0 oxidation efficiency. sent Ce4+ decreased aer reaction. The ratio of Ce4+ decreased
When 6% or 8% H2O were introduced into ue gas, the Hg0 from 73.6%, 71.6% to 71.3%, 69.9% (seen in Table 3),
oxidation efficiency decreased from 97.3% to 41.2% and 39.5%, respectively.
respectively. When 200 ppm, 400 ppm and 800 ppm SO2 were
As shown in Fig. 8(c), there were two main peaks attributing
added, Hg0 oxidation efficiency decreased from 97.3% to 37%, to Co 2p3/2 and Co 2p1/2. By peaking-tting deconvolution, the
31.1% and 30.8%. According to previous studies, the inhibited peaks Co 2p3/2 and Co 2p1/2 could be divided into two peaks,
effects of H2O and SO2 could be attributed to the competitive respectively, the peaks position at 780.6–781.1 eV and 795.7–
adsorption between H2O and Hg0 and also SO2 and Hg0 on the 796.2 eV were assigned to Co3+, at 782.2–782.6 eV and 797.1–
surface of the catalyst.44–46 Furthermore, SO2 could react with 797.7 eV were attributed to Co2+ 38,47–49 It has been reported that
.
active components forming metal sulfate resulting in catalyst Co3+ species could generate more anionic defects resulting in
deactivation for Hg0 oxidation.25
more surface oxygen,50 which could promote Hg0 adsorption
and oxidation. Compared with fresh Co0.3–Ce–ZrO2, the surface
ratio of Co3+ on the used Co0.3–Ce–ZrO2 decreased from 53.7%
to 51.1% (as shown in Table 3) and Co2+ increased from 46.3%
to 48.9%, which indicated that Co3+ was reduced during Hg0
oxidation process. From the results of analyzing the XPS proles
of Co and Ce, it could be inferred that both Co3+ and Ce4+
participated in Hg0 oxidation process.
3.3. Identication of Hg0 removal mechanism
XPS characterization was used to reveal the removal mechanism
of Hg0 and surface changes of Ce–ZrO2 and Co0.3–Ce–ZrO2
before and aer reaction. The XPS proles of Ce 3d, Co 2p and O
1s, Hg 4f are shown in Fig. 8.
As shown in Fig. 8(a), compared with the fresh Ce–ZrO2 and
Co0.3–Ce–ZrO2, the intensities of the peaks represent surface
chemisorbed oxygen (denoted as Oa) decreased aer reaction
The Hg 4f spectra of the used Ce–ZrO2 and Co0.3–Ce–ZrO2 are
shown in Fig. 8(d). A peak at 101.4–101.6 eV corresponding to
Fig. 8 O 1s, Ce 3d, Co 2p and Hg 4f XPS spectras of fresh Ce–ZrO2, Co0.3–Ce–ZrO2 and Ce–ZrO2, Co0.3–Ce–ZrO2 used for Hg0 oxidation.
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1
2
Table 3 XPS data of Ce–ZrO2 and Co0.3–Ce–ZrO2 samples
Ce2O3 + O2 (g) / 2CeO2
Ce4+/(Ce4+ + Ce3+
(%)
)
Co3+/(Co3+ + Co2+
(%)
)
Oa/(Oa + Ob)
(%)
1
2
3CoO + O2 (g) / Co3O4
Catalysts
Fresh Ce–ZrO2 73.6
—
53.7
53.1
63.7
Fresh
71.6
4. Conclusions
Co0.3–Ce–ZrO2
Used Ce–ZrO2
Used
a
71.3
69.9
—
51.1
50.7
38.1
Ce–ZrO2 catalysts modied with different amount of Co were
used to remove Hg0 from simulate ue gas and various char-
acterization methods were performed to characterize the phys-
ical and chemical properties of the catalysts. It was found that
cobalt addition enhanced the dispersion of Ce–ZrO2, increased
the BET surface area, improved redox activity and increased the
amount of surface chemisorbed oxygen resulting in higher
a
Co0.3–Ce–ZrO2
a
The used catalysts were the catalysts used for Hg0 oxidation
experiment (in Fig. 6). The time used for Hg0 oxidation experiment
was about 22 h and the reaction temperature range was 100–350 ꢀC.
Hg2+ of Hg 4f7/2 was obtained.7 No peaks corresponding to Hg0 adsorption capacity and Hg0 oxidation efficiency. For Co0.3–Ce–
(around 99.9 eV) were observed. It might be due to the loss of ZrO2, the adsorption efficiency was still 81.5% even aer 164.5 h
Hg0 during the treatment for XPS analysis or the lower content adsorption test and the Hg0 oxidation efficiency could reach
of Hg0 on the surface than the XPS detection. It was worth 100% in the temperature range of 100–250 ꢀC. XPS results
noting that the peak of HgO on the used Co0.3–Ce–ZrO2 was showed that surface chemisorbed oxygen was consumed while
much stronger than that on the used Ce–ZrO2. It indicated that Co3+ and Ce4+ changed to Co2+ and Ce3+ during Hg0 oxidation
the higher adsorption capacity and more surface chemisorbed process. It suggested that O, Co and Ce participated in the Hg0
oxygen of Co0.3–Ce–ZrO2 could improve Hg0 oxidized into Hg2+. oxidation reaction. The surface chemisorbed oxygen could react
Hg0 adsorption and oxidation were the main process on the with adsorbed Hg0 forming HgO and the high mobility of
surface of the catalysts, and both lattice oxygen and chem- electron between different valence states of cerium and cobalt
isorbed oxygen participated in Hg0 oxidation process. Further- can give a redox reaction easily. The results of individual ue
more, Fig. 7 shows that Hg0 removal efficiency can be gas components effects on Hg0 oxidation efficiency showed that
dramatically improved aer O2 was introduced which indicate O2 and NO improved the Hg0 oxidation efficiency while H2O and
that O2 could generate lattice oxygen and replenish the chem- SO2 inhibited it.
isorbed oxygen on the catalyst surface, while electrons can
transfer between different valence states of cerium and cobalt,
Acknowledgements
respectively. According to catalytic performance and the results
of XPS, a probable mechanism of Hg0 oxidation could be
We gratefully acknowledge the nancial supports from the
National Natural Science Foundation of China (Grant No.
51408098), the Program for Changjiang Scholars, the Funda-
mental Research Funds for the Central Universities (Grant No.
inferred. Firstly, Hg0 was adsorbed on the active sites of the
Co0.3–Ce–ZrO2 to form Hg0 (ad). Then the Hg0 (ad) bonded with
lattice oxygen to form weakly bonded speciation Hg–O–M–Oxꢁ1
(M ¼ Ce or/and Co) and then formed HgO, or the Hg0 (ad)
DUT16RC(4)81).
directly reacted with chemisorbed oxygen to form HgO. Finally,
the consumed lattice oxygen and chemisorbed oxygen were
replenished by gaseous O2. Shis of cerium and cobalt between
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