High-yield synthesis of Ce modified Fe-Mn composite oxides benefitting from catalytic destruction of chlorobenzene

Article abstract of DOI:10.1039/c9ra10489e

Ce-Fe-Mn catalysts were prepared by an oxalic acid assisted co-precipitation method. The influence of Ce doping and calcination temperature on the catalytic oxidation of chlorobenzene (as a model VOC molecule) was investigated in a fixed bed reactor. The Mn₃O₄ phase was formed in Ce-Fe-Mn catalysts at low calcination temperatures (<400 °C), which introduced more chemisorbed oxygen, and enhanced the mobility of O atoms, resulting in an improvement of the reduction active of Mn₃O₄ and Fe₂O₃. Additionally, CeO₂ has strong redox properties, and Ce⁴⁺ would oxidize Mn^x+ and Fe^x+. Therefore, the interaction of Ce, Fe and Mn can improve the content of surface chemisorbed oxygen. As compared with Fe-Mn catalysts, the catalytic conversion of chlorobenzene over Ce(5%)-Fe-Mn-400 was about 99% at 250 °C, owing to high specific surface area, Mn₃O₄ phase, and Ce doping. However, with the increase in roasting temperature, the performance of the catalysts for the catalytic combustion of chlorobenzene was decreased, which probably accounts for the formation of the Mn₂O₃ phase in Ce-Fe-Mn-500 catalysts, leading to a decrease in the specific surface area and concentration of chemically adsorbed oxygen. As a result, it can be expected that the Ce-Fe-Mn catalysts are effective and promising catalysts for chlorobenzene destruction.

Full text of DOI:10.1039/c9ra10489e

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PAPER
High-yield synthesis of Ce modified Fe–Mn
composite oxides benefitting from catalytic
destruction of chlorobenzene†
Cite this: RSC Adv., 2020, 10, 10030
Anqi Li, Hongming Long,* Hongliang Zhang and Haijin Li *ac
b
ab
a
Ce–Fe–Mn catalysts were prepared by an oxalic acid assisted co-precipitation method. The influence of Ce
doping and calcination temperature on the catalytic oxidation of chlorobenzene (as a model VOC
3 4
molecule) was investigated in a fixed bed reactor. The Mn O phase was formed in Ce–Fe–Mn catalysts
ꢀ
at low calcination temperatures (<400 C), which introduced more chemisorbed oxygen, and enhanced
the mobility of O atoms, resulting in an improvement of the reduction active of Mn
O
3 4
2 3
and Fe O .
4+
x+
x+
Additionally, CeO
2
has strong redox properties, and Ce would oxidize Mn and Fe . Therefore, the
interaction of Ce, Fe and Mn can improve the content of surface chemisorbed oxygen. As compared
with Fe–Mn catalysts, the catalytic conversion of chlorobenzene over Ce(5%)–Fe–Mn-400 was about
ꢀ
9
9% at 250 C, owing to high specific surface area, Mn
3
O
4
phase, and Ce doping. However, with the
increase in roasting temperature, the performance of the catalysts for the catalytic combustion of
chlorobenzene was decreased, which probably accounts for the formation of the Mn phase in Ce–
Received 13th December 2019
Accepted 19th February 2020
2 3
O
Fe–Mn-500 catalysts, leading to a decrease in the specific surface area and concentration of chemically
adsorbed oxygen. As a result, it can be expected that the Ce–Fe–Mn catalysts are effective and
promising catalysts for chlorobenzene destruction.
DOI: 10.1039/c9ra10489e
rsc.li/rsc-advances
ꢀ
temperatures exceeding 850 C and leads to small amounts of
1
. Introduction
12
more toxic polychlorinated aromatic compounds.
By
Volatile organic compounds (VOCs) mainly originate from comparison, catalytic oxidation is regarded as one of the most
municipal burning and industrial manufacturing and are an promising techniques for elimination of chlorinated aromatic
ꢀ
important class of pollutants in the atmosphere. Among them, compounds at much lower temperatures (<500 C), due to its
2 2
chlorinated aromatic compounds have been always considered high efficiency, less harmful by-products (e.g., CO , HCl, Cl ),
1
–4
11,13–16
as the most harmful contaminants. The widespread emis- and low energy consumption.
sions of chlorinated aromatic compounds not only cause the
Catalysts based on noble metals, perovskites, and transition
pollution of the ecological environment but also damage metal oxides have been developed for the catalytic oxidation of
1
1,16,17
human health due to their acute toxicity and strong bio- chlorinated aromatic compounds.
Noble metals are
5,6
accumulation potential. Thus, the elimination of chlorinated expensive and always deactivate owing to chlorine (Cl)
aromatic compounds has received more and more attention. poisoning during catalytic oxidation processes, which limits
Currently, chlorinated aromatic compounds are usually their practical large-scale application. The temperature of
11
ꢀ
removed by absorption, thermal oxidation, and catalytic catalytic oxidation for perovskites is usually above 500 C, thus
7–10
oxidation.
The subsequent treatment of carbon materials they were not the ideal catalysts for low-temperature catalytic
18
adsorbed with a large amount of chlorinated aromatic degradation technology. Previous reports showed that transi-
compounds is very complex and can easily form secondary tion metal oxides as proper catalysts have the advantages of low-
19
11
pollutions. Direct thermal oxidation usually requires high temperature catalytic activity, thermal stability, and low cost.
Mn-based oxides, such as Mn , Mn , and MnO , are
capable of providing the mobile-electron environment required
3
O
4
2
O
3
2
a
Key Laboratory of Metallurgical Emission Reduction & Resource Recycling (Anhui
University of Technology), Ministry of Education, Ma’anshan, Anhui 243002, PR by redox catalysts, owing to polymorphism and polyvalence, and
China. E-mail: yahm@126.com; lihaijin@ahut.edu.cn
have exhibited remarkable activities in the removal of gaseous
b
20–24
School of Metallurgical Engineering, Anhui University of Technology, Ma’anshan,
pollutants (CO, NO
x
and VOCs).
The catalytic activity is in
Anhui 243002, China
the order Mn O > Mn O > MnO , which are benets from
3
4
2
3
2
c
School of Energy and Environment, Anhui University of Technology, Ma’anshan, Anhui
24
oxygen mobility in the catalyst. Nevertheless, the individual
2
†
1
43002, China
MnO catalysts are still some way from reaching a satisfactory
x
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra10489e
catalytic performance in terms of stability and activity to some
10030 | RSC Adv., 2020, 10, 10030–10037
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kinds of VOCs, for example, the poor activity to catalytically the mass ow controller. The content of the chlorobenzene in
oxidize chlorobenzene (CBz). Moreover, with the addition of the ue gas was controlled by the adjustment of the inlet
metal ions (such as Ce, Fe, Co, and Cu), the catalytic activity of concentration. The simulated gas was heated to a required
Mn-based oxides at low temperature could be obviously temperature by preheating for the reaction, and the catalysts
enhanced, due to possible complementary advantages of were in the reactor. The temperature of catalyst bed was
2
5–27
different metals in the catalytic activity.
Among the alter- controlled by an E-type thermocouple in the reactor. In this
was used as the equilibrium gas. The specic
The Mn–Fe–O operating conditions were shown in Table S1.†
catalysts exhibit high catalytic activity for toluene oxidation, Unless otherwise specied, in this experiment the weight of
arising from high amounts of Mn ions, and high concentra- catalysts was 0.2 g, which was prepared by oxalic acid co-
native transition metal oxides, Fe–Mn binary oxides are experiment, N
2
2
0,28
common choices for catalytic combustion.
4
+
1
1
tions of lattice defects and oxygen vacancies. However, Fe–Mn precipitation. The ratio of chlorobenzene conversion was
catalysts were not capable of removing chlorinated pollutants.
29
calculated as follows:
The active sites of Fe–Mn oxides were easily poisoned due to the
CBzin ꢁ CBzout
Chlorobenzene conversion ð%Þ ¼
ꢂ 100%
strong adsorption of Cl, which could be overcome by the addi-
CBzin
12
2
tion of other metals. Recently, CeO was used in the catalytic
where, the CBzin and CBzout were chlorobenzene concentration
of the inlet and outlet in the system at steady-state, respectively.
combustion of low concentration chlorobenzene, owing to its
high oxygen storage capacity, abundant oxygen vacancies and
strong redox property.
30
To meet these challenges and further improve low-
temperature performance for the catalytic destruction of chlo-
robenzene, herein, we prepare a series of Ce modied Fe–Mn
oxides by a co-precipitation process. The inuence of calcina-
tion temperature and the amount of the doped Ce on the
catalytic destruction of chlorobenzene were investigated. It was
3
. Results and discussion
The XRD patterns of Ce–Fe–Mn catalysts are shown in Fig. 1.
Pure CeO in the form of cerianite with a cubic uorite structure
exhibited diffraction peaks at 28.5 , 33.1 , 47.5 , and 56.3 ,
which correspond with reections of the (111), (200), (220), and
2
ꢀ
ꢀ
ꢀ
ꢀ
3 4 2 3
found that Ce–Fe–Mn oxides consisting of Mn O , Fe O , and
(311) crystallographic planes, respectively (JCPDS no. 34-0394).
CeO , exhibited high activity for the catalytic oxidation of
ꢀ
2
The XRD pattern of the Fe–Mn catalyst calcinated at 400 C is
chlorobenzene. The inner relationship between catalytic
behavior and structural properties was investigated by
a combination of performance and characterization studies
attributed to the combination of Mn
3
O
4
(JCPDS no. 24-0734,
q ¼ 18.0 , 28.9 , 32.3 , and 36.1 ) and Fe (JCPDS no. 39-
346, 2q ¼ 30.2 , 35.6 , 43.2 , and 57.3 ). The diffraction peaks
ꢀ
ꢀ
ꢀ
ꢀ
2
1
2 3
O
ꢀ
ꢀ
ꢀ
ꢀ
2
(XRD, BET, TEM, H -TPR, XPS, etc.).
2. Experimental
2.1 Catalysts preparation
The Ce–Fe–Mn catalysts were prepared by an oxalic acid assis-
ted co-precipitation method. For a typical synthesis, rstly,
a certain amount of FeSO
4
$4H
2
O, MnCl
2
2
$4H O (the molar ratio
of Fe : Mn of 1 : 4) and Ce(NO
3
)
3
$6H
2
O were dissolved in
deionized water, and stirred for 30 min in an ice water bath.
Then, the oxalic acid solution was added into the metal ion
solution to form a precipitate. Then the mixture was stirred in
an ice water bath for 1 h. The precipitate was collected by
suction ltration, and washed with distilled water and alcohol
three times. Finally, the obtained products were dried in
ꢀ
a vacuum freeze drier and calcined at 300 C (Ce–Fe–Mn-300),
ꢀ
ꢀ
4
00 C (Ce–Fe–Mn-400) and 500 C (Ce–Fe–Mn-500), respec-
tively. Aer grinding to 60 mesh, the catalysts were named as
Ce–Fe–Mn catalysts. As a contrast, CeO and Fe–Mn oxides were
2
prepared by the same process.
2.2 Catalytic performance test
The chlorobenzene oxidation experiments were performed in
a xed bed reactor as shown in Fig. S1.† The reaction consists of
a gas distribution system, a ue gas preheating system, a selec-
tive catalytic reduction reactor and a ue gas analysis system.
2
Fig. 1 XRD patterns of as-synthesied CeO , Fe–Mn, Ce–Fe–Mn
The ow rate of each gas in the mixing part was controlled by catalysts.
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Table 1 The BET surface area and pore properties of different catalysts
2
ꢁ1
3
ꢁ1
Catalysts
Surface area (S/m g
)
Pore volume (V/cm g
)
Pore diameter (D/nm)
Fe–Mn
33.4616
188.1677
190.8000
85.7452
73.7974
290.6160
47.0542
0.226514
0.321711
0.301823
0.208402
0.238484
0.319763
0.206230
266.9446
68.3881
61.1414
94.6542
126.6648
42.3521
178.4679
Ce(2%)–Fe–Mn-400
Ce(5%)–Fe–Mn-400
Ce(7.5%)–Fe–Mn-400
Ce(10%)–Fe–Mn-400
Ce(5%)–Fe–Mn-300
Ce(5%)–Fe–Mn-500
of 2% Ce modied Fe–Mn catalysts were not observed, owing to 300, Ce(5%)–Fe–Mn-400 and Ce(5%)–Fe–Mn-500 were esti-
low concentration of CeO in a highly dispersed phase. The low mated by the Scherrer equation to be 1.1, 8.1 and 21.4 nm,
2
ꢀ
intensity diffraction peak at 47.5 2q appeared in the high Ce respectively.
content (5, 7.5, and 10%) catalysts, coinciding with the face
centred cubic (fcc) uorite structure CeO . The XRD patterns of catalytic performance. The BET surface area, total pore volume
Ce(5%)–Fe–Mn-300 and Ce(5%)–Fe–Mn-400 were similar, which and pore size of the Ce–Fe–Mn catalysts are summarized in
It is well known that a high surface area is benecial to
2
exhibited the diffraction peaks of Mn
temperature of Ce–Fe–Mn was increased up to 500 C, the than the Fe–Mn catalyst. With increasing amounts of Ce
3
O
4
. When the calcination Table 1. All Ce–Fe–Mn catalysts exhibited higher surface areas
ꢀ
x 2 3
diffraction peaks of MnO were those of Mn O (JCPDS no. 41- doping, the specic surface area rstly increased and then
ꢀ
ꢀ
ꢀ
ꢀ
1
442, 2q ¼ 18.8 , 33.0 , 38.2 , and 55.2 ). The crystalline phase decreased at the same roasting temperature. 5% Ce modied
2
ꢁ1
transition of MnO was consistant with the TEM and H -TPR Fe–Mn catalysts had the largest surface area (190.8 m
g ),
x
2
results. In the meantime the particle sizes of Ce(5%)–Fe–Mn- which might be caused by the pore structure and good
Fig. 2 TEM images of Ce–Fe–Mn-400 (a)–(c), and HRTEM image of Ce–Fe–Mn-400 (d).
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dispersion of Ce. In addition, the sintering temperature had an consumed more H
2
than Ce–Fe–Mn-400 catalysts, demon-
obvious inuence on the BET surface area of the Ce–Fe–Mn strating the weaker Mn–O bonds of Fe–Mn and Ce–Fe–Mn-300
ꢀ
34
catalysts. When the roasting temperature was 300 C, Ce(5%)– catalysts, whereas, for the Ce–Fe–Mn-500 catalyst, the rst
2
ꢀ
ꢀ
Fe–Mn-300 catalysts had a wonderful surface area (290.6 m
reduction peak changed from 290 C to 325 C. The reduction
). With the increasing the calcination temperature to 500 C, temperature shiing to a higher temperature probably means
the BET surface area decreased signicantly, which is attributed a decrease in the lattice oxygen mobility on the Ce–Fe–Mn
ꢁ1
ꢀ
g
3
5
to the increase in the size of the particles, which agrees well with catalyst. Therefore, the catalytic activity of chlorobenzene
36
the sample morphology (Fig. 2 and S2†).
oxidation was correlated with the oxygen mobility. In other
The morphology of samples was characterized by TEM. As words, the higher the oxygen mobility, the higher the catalytic
shown in Fig. 2 and S2,† the Ce–Fe–Mn catalysts consisted of activity. According to previous reports, CeO samples showed
2
ꢀ
ꢀ
crumb-like nanoparticles with shaggy surface. In addition, it is two reduction peaks at 530 C and 800 C, which could be
4
+
found that the particle size of the samples obviously increased assigned to the reduction of surface and bulk Ce , respec-
37
with increasing the calcination temperature, which was tively. However, no obvious reduction peaks attributable to
consistent with the average crystallite size estimated from the CeO were observed for the Ce–Fe–Mn catalysts, implying that
Scherrer equation. Additionally, the spatial chemical composi- the interaction between the Ce, Fe and Mn in the catalysts was
tions of Ce–Fe–Mn catalysts were further identied by EDS large, which coincided with He’s observations.
spectroscopy (Fig. S3†), conrming the uniform distribution of On the basis of the XPS analysis, the surface chemical states
2
38
Ce, Fe, Mn and O elements on the catalysts. The high dispersion of Ce, Fe, Mn, and O species for the Ce–Fe–Mn catalysts were
of Mn, Fe, and Ce on the surface of the Ce–Fe–Mn-400 catalysts evaluated and are shown in Fig. 4, and the atomic concentra-
provided a large number of oxidative active sites, which tions on the surface of the catalysts are shown in Table 2. As
31
improved the oxidation efficiency of chlorobenzene.
In order to check the redox properties of Ce–Fe–Mn catalysts,
H
2
-TPR of the catalysts was carried out, and the results are
2
shown in Fig. 3. The H -TPR patterns strongly depend on the
oxidation state of Ce–Fe–Mn catalysts. As presented in Fig. 3, for
Fe–Mn and Ce–Fe–Mn catalysts, the reduction temperature
ꢀ
32
process of Fe O to Fe O occurred in the range 350–450 C.
2
3
3 4
Meanwhile, for Fe–Mn, Ce–Fe–Mn-300, and Ce–Fe–Mn-400
ꢀ
ꢀ
catalysts, two main reduction peaks (TTP: 290 C and 430 C)
were observed, which were ascribed to the reduction of
manganese from MnO
respectively. Compared with Ce–Fe–Mn-400 catalysts, the
reduction temperature 230 C appeared in Fe–Mn and Ce–Fe–
2 2 3 2 3 3 4
to Mn O and Mn O to Mn O ,
33
ꢀ
Mn-300 catalysts, which is consistent with the H -TPR results
2
24
previously reported. Therefore, Fe–Mn and Ce–Fe–Mn-300
catalysts were reduced at a lower temperature and they
Fig. 4 XPS spectra of the catalysts over the spectral regions of Mn 2p
Fig. 3 TPR profiles of Ce–Fe–Mn catalysts.
(A), Fe 2p (B), Ce 3d (C) and O 1s (D).
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ꢀ
shown in Fig. 4A, two main peaks of the Mn 2p spectra can be about 90% when temperature reached as high as 400 C. Upon
identied as Mn 2p3/2 and Mn 2p1/2. The deconvolution proce- addition of Ce to the Fe–Mn catalyst, the ratio of chlorobenzene
dure was applied for the Mn 3p3/2 signal, it can be reliably conversion remarkably increased up to a Ce content of 5%, and
separated into three contributing peaks 641.0–641.4 eV, 642.5– Ce(5%)–Fe–Mn was the most active among all catalysts achieving
42.9 eV and 644.1–646.1 eV, which may be assigned to Mn , ꢃ99% chlorobenzene conversion at 250 ꢀC. When the CeO
2+
6
2
3
+
4+
39
Mn and Mn , respectively. Because no crystalline MnO
2
or content continuously increased to 7.5–10%, the catalytic activity
Mn
calcined at 300 C and 400 C, MnO
may be well dispersed on the catalyst surface in an amorphous Ce–Fe–Mn catalysts to reach the largest chlorobenzene conver-
2 3
O species were detected by XRD on all the catalysts started to decrease, even worse than that of Ce(2%)–Fe–Mn,
ꢀ
ꢀ
2
2 3
or Mn O in the catalysts which indicated that the optimum concentration of Ce is 5% for
4
+
state. It can be seen from Table 2 that the concentration of Mn
sion. For Fe–Mn, the catalytic activity for chlorobenzene oxida-
ꢀ
in Ce(5%)–Fe–Mn-400 C was higher than that of other cata- tion was obviously lower than those of Ce–Fe–Mn, showing that
lysts, and this catalyst presented higher performance for the CeO addition offers signicant promotion over Fe–Mn.
2
catalysis of chlorobenzene. Accordingly, the higher ratio of
In order to further discuss the factors affecting the catalytic
4
+
Mn on Ce–Fe–Mn catalysts might lead to more active sites, efficiency of chlorobenzene, the catalytic performances of
thereby enhancing chlorobenzene oxidation. The Fe 2p XPS Ce(5%)–Fe–Mn catalysts with different calcination tempera-
spectra of the Ce–Fe–Mn catalysts is shown in Fig. 4B. Two main tures were investigated. From Fig. 6, it is evident that there is no
peaks of Fe 2p spectra correspond to Fe 2p3/2 and Fe 2p1/2. Peak signicant distinction in the ratio of chlorobenzene conversion

tting deconvolution of the Fe 2p peak indicates the presence between Ce(5%)–Fe–Mn-300 and Ce(5%)–Fe–Mn-400. This is
3/2
2
+
3+
of two different Fe species: Fe (710.2–710.7 eV) and Fe
because Ce(5%)–Fe–Mn-300 catalysts had a large BET surface
40
4+
(712.2–712.8 eV). The Ce 3dXPS spectra of the Ce–Fe–Mn area, while Ce(5%)–Fe–Mn-400 had more Mn content, result-
0
0
000
00
000
catalysts is shown in Fig. 4C. The peaks at u, u , u , v, v , and v ing in more chemisorption oxygen. Therefore, the ratio of
are attributed to Ce , and the peaks at u and v are considered chlorobenzene conversion with the two catalysts is basically the
to be Ce . As shown in Table 2, Ce was mainly in the +4 same aer the synergistic effects. When the calcination
valence state and only a small fraction of the +3 valence state
4
+
0
0
3+
41
existed in Ce–Fe–Mn catalysts, which might be due to the lower
content of lattice defect and oxygen vacancies.
The O 1s XPS spectra of the Ce–Fe–Mn catalysts is shown in
Fig. 4D. The O 1s spectra showed two peaks: the surface lattice
oxygen (denoted as O
oxygen species (denoted as O
species, the chemisorbed oxygen was more active than the
a
) at about 529.6 eV, and the chemisorbed
26
b
). Between the two oxygen
13
lattice oxygen. The surface chemisorbed oxygen has been re-
42
ported to be in favor of catalytic oxidation of chlorobenzene.
Therefore, O plays an important role in the Ce–Fe–Mn catalysts
b
during catalytic reaction. Table 2 shows that the ratio of
chemisorbed oxygen to lattice oxygen was higher for Ce(5%)–
Fe–Mn-400 than the others, which is consistent with the higher
catalytic performance against chlorobenzene.
The catalytic performances of the Fe–Mn and Ce–Fe–Mn
catalysts were evaluated for the oxidation of chlorobenzene. The
ratio of chlorobenzene conversion was investigated as a function
ꢀ
of the temperature, 100–400 C, and the results are shown in
Fig. 5. As can be observed, pure Fe–Mn catalyst performed the
lowest conversion with the ratio of chlorobenzene conversion at
Fig. 5 Catalytic performance of the Fe–Mn and Ce–Fe–Mn catalysts.
Reaction conditions: 250 ppm CBz, 80% N , 10% O , and GHSV of
2
2
ꢁ1
1
500 h , 0.2 g catalysts.
Table 2 XPS results of the Ce–Fe–Mn catalysts
Ce (at%)
Fe (at%)
Mn (at%)
O (at%)
3+
4+
2+
3+
2+
3+
4+
Catalysts
Ce
Ce
Fe
Fe
Mn
Mn
Mn
O
a
O
b
Ce(2%)–Fe–Mn-400
Ce(5%)–Fe–Mn-400
Ce(7.5%)–Fe–Mn-400
Ce(10%)–Fe–Mn-400
Ce(5%)–Fe–Mn-300
Ce(5%)–Fe–Mn-500
23.05
12.50
18.82
30.72
19.06
12.18
76.95
87.50
81.11
65.66
80.94
87.82
55.64
48.71
58.26
58.57
51.63
53.44
44.36
51.29
41.74
41.42
48.37
46.56
44.28
31.79
46.58
43.52
37.30
40.90
29.27
34.48
31.48
37.95
35.27
33.11
26.49
33.74
21.93
18.53
27.43
25.99
74.18
37.33
81.11
80.72
73.14
74.65
25.82
62.67
18.88
19.98
26.86
25.35
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Fig. 7 Synergistic mechanism of Ce–Fe–Mn catalysts.
catalysts
C
6
H
5
Cl þ O
2
ƒƒƒƒƒƒ! HCL=Cl
2
þ CO
2
þ H
2
O
(1)
Fig. 6 Catalytic performance of Ce–Fe–Mn catalysts with different
calcination temperatures.
Cl species need be removed as Cl or HCl from the surface of
catalysts so as to avoid deactivating the catalysts. In this study,
2
47
ꢀ
temperature rises to 500 C, the catalytic efficiency of chloro- the removal of Cl species was promoted by elevating the
ꢀ
benzene for Ce(5%)–Fe–Mn-500 obviously reduced, which was
temperature (e.g. to 250 C) and synergistic mechanisms on the
correlated with the production of Mn O , resulting in a decline
of the oxygen mobility.
The chlorobenzene oxidation reaction over Ce–Fe–Mn cata-
2
3
Ce–Fe–Mn catalysts. It may be deduced that the removal of Cl
species is a slow step so that it determines the rate of chloro-
benzene catalytic oxidation. According to a previous report, the
24
lysts is mainly determined by two critical factors, one is the reduction of Mn O and Fe O releases additional O atoms
3
4
2 3
catalyst’s ability to trap surface chemisorbed oxygen groups attached to the catalysts’ surface to become surface chemisorbed
43
41
which promotes oxygen transportability to enhance activity.
oxygen. The redox cycle of Fe–Mn catalysts is shown as eqn (2):
Based on the H -TPR results, it was shown that reaction
temperature of the catalysts was associated to the amount of
2
4
+
2+
3+
3+
Mn + Fe / Mn + Fe
(2)
lattice oxygen of the catalysts. The Mn O phase in Ce–Fe–Mn
3
4
Additionally, Ce plays a co-catalytic role in the Ce–Fe–Mn
catalysts was more reducible at lower temperatures and could
easily trap surface chemisorbed oxygen groups which increases
the mobility of O atoms compared to the other catalysts.
Another determining factor in catalytic oxidation reactions is
4
+
catalysts. Ce has strong oxidizing properties, which can
x+
x+
3+
oxidize Mn and Fe . At the same time, Ce is oxidized to
16
2 2
CeO by O in the air (eqn (3)–(5)).
44
the number of active surface sites. For Ce(5%)–Fe–Mn-400 and
Ce(5%)–Fe–Mn-300 catalysts, owing to a large surface area and
high dispersion of Mn, Fe, and Ce, a greater number of active
surface sites were present for the adsorption of chlorobenzene
molecules, resulting in the improvement of the catalytic activity.
Moreover, the decrease in specic surface area caused by
roasting is also one of the reasons why the activity decreases at
3
+
4+
2ꢁ
4+
3+
Mn + Ce + O / Mn + Ce
(3)
(4)
(5)
2
+
4+
2ꢁ
3+
3+
Fe + Ce + O / Fe + Ce
3+
Ce + O
4+
/ Ce + O
2ꢁ
2
Therefore, the interaction of Ce, Fe and Mn can accelerate
higher temperatures, because the active surface sites for the the conversion of O2 into surface chemisorbed oxygen and
adsorption of chlorobenzene molecules decreases.
improve the content of surface chemisorbed oxygen. Since the
According to the analysis above, the possible mechanism for adsorption of surface O atoms on the oxygen vacancies of
chlorobenzene catalytic oxidation by the Ce–Fe–Mn catalysts is
shown in Fig. 7. In the chlorobenzene catalytic combustion, C–
Cl bonds are weaker than C–H bonds, and hence, more prone to
catalysts is the rate determining step for the HCl oxidation
reaction, the surface chemisorbed oxygen is critical for the so-
called Deacon process (eqn (6)).
48
45
attack by nucleophiles. The rst step in the oxidation of
chlorobenzene is the C–Cl molecular bonds of chlorobenzene
2HCl + O / H
O + Cl
2
(6)
2
ꢁ
are destroyed, and the free Cl ions are adsorbed to the metal
Therefore, the higher the amount of surface chemisorbed
oxygen in Ce–Fe–Mn catalysts, the easier the removal of Cl from
the catalyst surface, resulting in the improvement of the reac-
tion efficiency. This implies that the synergistic mechanism
between Ce–Fe–Mn is combined to promote the chlorobenzene
oxidation mutually.
cations. On the other hand, during the catalytic oxidation,
chlorobenzene molecules are adsorbed on the surface of Ce–Fe–
Mn catalysts and oxidized by the active oxygen into H O and
2
46
ꢁ
CO2. The free Cl ions produce HCl, and react with chem-
isorbed oxygen to form Cl simultaneously. The chloroben-
38
2
zene oxidation reaction is shown eqn (1):
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6
B. K. Gullett, D. F. Natschke and K. R. Bruce, Thermal
Treatment of 1,2,3,4-Tetrachlorodibenzo-p-dioxin by
Reaction with Ca-Based Sorbents at 23–300 C, Environ. Sci.
4
Conclusions
ꢀ
In summary, Ce–Fe–Mn catalysts were synthesized by an oxalic
acid assisted co-precipitation method, and the inuence of
ceria addition to Fe–Mn with different calcination temperatures
upon the catalytic oxidation of chlorobenzene was examined. By
Technol., 1997, 31, 1855–1862.
7 A. Fadli, C. Briois, C. Baillet and J. P. Sawerysyn,
Experimental study on the thermal oxidation of
ꢀ
comparison, it was found that catalysts containing CeO
2
chlorobenzene at 575–825 C, Chemosphere, 1999, 38,
exhibited high surface area and amounts of surface-active
oxygen, leading to the enhancement of the catalytic activity of
chlorobenzene. In addition, calcination temperatures clearly
affected the activity of the prepared catalysts. When calcined at
temperatures of 300 C and 400 C, low reduction temperatures
of Ce–Fe–Mn catalysts could increase the mobility of oxygen.
Therefore, the synergistic mechanism between Ce–Fe–Mn was
further enhanced, more chemisorbed oxygen was produced,
2835–2848.
8 B. Higgins, M. J. Thomson, D. Lucas, C. P. Koshland and
R. F. Sawyer, An experimental and numerical study of the
thermal oxidation of chlorobenzene, Chemosphere, 2001,
42, 703–717.
9 R. Q. Long and R. T. Yang, Carbon nanotubes as superior
sorbent for dioxin removal, J. Am. Chem. Soc., 2001, 123,
2058–2059.
ꢀ
ꢀ
and so the catalytic activity of chlorobenzene was improved. The 10 M. B. Chang, K. H. Chi and G. P. Chang-Chien, Evaluation of
decrease in the activity of Ce–Fe–Mn catalysts calcined at a high
temperature can be attributed to the decrease in the oxygen
PCDD/F congener distributions in MWI ue gas treated with
SCR catalysts, Chemosphere, 2004, 55, 1457–1467.
mobility and surface area. For Ce(5%)–Fe–Mn-400 catalyst, the 11 C. Du, S. Lu, Q. Wang, A. G. Buekens, M. Ni and
higher surface area could lead to the increase in the number of
D. P. Debecker, review on catalytic oxidation of
oxygen vacancies, more of the Mn phase could produce
more chemisored oxygen, thus leading to a good catalytic
A
3
O
4
chloroaromatics from ue gas, Chem. Eng. J., 2018, 334,
519–544.
oxidation of chlorobenzene. This investigation may offer 12 C. He, Y. Yu, J. Shi, Q. Shen, J. Chen and H. Liu,
a promising strategy for designing efficient catalysts for the
Mesostructured
Cu–Mn–Ce–O
composites
with
destruction of chlorobenzene.
homogeneous bulk composition for chlorobenzene
removal: catalytic performance and microactivation course,
Mater. Chem. Phys., 2015, 157, 87–100.
Conflicts of interest
1
3 Z. Wang, G. Shen, J. Li, H. Liu, Q. Wang and Y. Chen,
There are no conicts to declare.
Catalytic removal of benzene over CeO
oxides prepared by hydrothermal method, Appl. Catal., B,
013, 138–139, 253–259.
4 P. Sun, W. Wang, X. Weng, X. Dai and Z. Wu, Alkali
Potassium Induced HCl/CO Selectivity Enhancement and
2 x
–MnO composite
2
Acknowledgements
1
This work was supported by the National Natural Science
Foundation of China (No. 51674002, 21673001, 21773114,
2
Chlorination Reaction Inhibition for Catalytic Oxidation of
21972065) and the Key Project of the National Nature Science
Chloroaromatics, Environ. Sci. Technol., 2018, 52, 6438–6447.
Foundation of China (U1660206).
15 P. Sun, W. Wang, X. Dai, X. Weng and Z. Wu, Mechanism
study on catalytic oxidation of chlorobenzene over
Mn
x
2
Ce1ꢁxO /H-ZSM5 catalysts under dry and humid
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