Catalytic combustion of toluene over CeO2-CoO: X composite aerogels

Article abstract of DOI:10.1039/d0nj00091d

A rational approach for the optimization of cobalt based catalysts suitable for VOC combustion is to modulate the pore structure and increase the dispersion of active species. In this study, we have prepared CeO2-CoOx composite aerogels with a continuous porous structure using an epoxide-initiated gelation process and evaluated them in the catalytic combustion of toluene. The influence of the Ce/Co ratio on the structural properties and catalytic activities of the CeO2-CoOx aerogels was investigated using various characterization methods. The surface area of the CeO2-CoOx aerogels showed a volcanic curve versus the Ce content. A higher specific surface area of 172 m2 g-1 was observed for the CeO2-CoOx-10 catalyst with a Ce/Co ratio of 1/10. In addition, the presence of Ce modified the redox properties and the surface chemical states of the CeO2-CoOx aerogels, such as the surface Co3+/Co2+ ratios, and surface absorbed oxygen species (O- and O2-), which played key roles in their catalytic activity. In comparison with different cobalt-based catalysts for toluene oxidation in the previous literature studies, the CeO2-CoOx-10 catalyst in this work exhibited better catalytic performance, reaching an optimal toluene conversion of 90% at 248 °C as well as the highest stability during the durability test under different temperature stages.

Full text of DOI:10.1039/d0nj00091d

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DOI: 10.1039/D0NJ00091D
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Catalytic combustion of toluene over CeO2–CoOx composite
aerogels
Received 00th January 20xx,
Accepted 00th January 20xx
Chao Miao a, Junjie Liu b, Jinxian Zhao a, Yanhong Quan a, Tao Li a, Yongli Pei a, Xiaoliang Li a and Jun
Ren a*
DOI: 10.1039/x0xx00000x
A rational approach for optimization of cobalt based catalysts suitable for VOCs combustion is to modulate the pore
structure and to increase the dispersion of active species. In this paper, we prepared CeO2–CoOx composite aerogels with a
continuous porous structure using an epoxide-initiated gelation process and evaluated in the catalytic combustion of
toluene. The influence of Ce/Co ratio on structural properties and catalytic activities of CeO2–CoOx aerogels were
investigated using various characterization methods. The surface area of CeO2–CoOx aerogels showed a volcanic curve
versus the Ce content. A higher specific surface area of 172 m2·g-1 was observed over CeO2–CoOx-10 catalyst with the
Ce/Co ratio of 1/10. In addition, the presence of Ce modified the redox properties and the surface chemical states of
CeO2–CoOx aerogels, such as the surface Co3+/Co2+ ratios, surface absorbed oxygen species (O–, O2 ), which played key
–
roles in catalytic activity. In comparison with the different cobalt-based catalysts for toluene oxidation in the previous
literatures, CeO2–CoOx-10 catalyst in this work exhibited better catalytic performance, reaching the optimal toluene
conversion of 90% at 248°C as well as the highest stability during durability test under the different temperature stages.
Many different methods have been developed for the
preparation of Co3O4 catalysts with high specific surface area. For
instance, Zhao et al.[18] prepared hollow Co3O4 polyhedron via a
1. Introduction
Volatile organic compounds (VOCs), as common pollutants in air,
promote the formation of fine particulates (PM 2.5), ozone (O3) and
other secondary pollutants, which cause serious environmental
problems and thus threaten humans’ health[1-3]. To date, numerous
effective methods, for instance, absorption, photocatalytic and
catalytic combustion, have been developed in the reduction of
VOCs[4-6]. Compared with other processing strategies, catalytic
combustion is regarded as a valid technique because of its
economical availability and practical effectiveness[7]. With respect
to noble-metal-based catalysts (including Pt[8], Pd[9] and Au[10]) with
template method with ZIF-67. Bai et al.[19] obtained porous Co3O4
nanowires via hydrothermal method in the presence of
a
polyethylene glycol. Fei et al.[20] fabricated Co3O4 nanotubes via a
precipitation method with oxalic acid. Although these Co3O4
nanomaterials all exhibited excellent catalytic activity in VOCs’
oxidation, most of these methods were limited by a relatively
complicated operation, high cost or low yield. Therefore, the
preparation of porous Co3O4 with high surface area is still a
challenge.
Cobalt-based aerogel, composed of interconnected nanometer-
level particles, is a particular open-cell foam, which exhibits low
mass density and high specific surface area[21]. On the one hand, the
high specific surface area of cobalt-based aerogels can supply
abundant surface-active sites. On the other hand, the continuous
porous structure can effectively inhibit sintering and aggregation of
active particles, thus it maintains excellent activity and stability for
application in the oxygen-reduction[22] and Fischer–Tropsch
reactions[23]. Recently, an epoxide-initiated gelation process has
been successfully used to prepare various metal oxide aerogels,
such as Zn2+-[24], Cu2+-[25] and Ni2+-based aerogels[26]. Compared with
the traditional sol–gel method, it utilizes stable inorganic metal
salts instead of metal alkoxides as precursors, resulting in a lower
synthesis cost and practical production routine. In addition, the
approach is flexible for the preparation of composite aerogels with
adjustable composition and structure.
high cost and poor antitoxic properties, noble-metal-free catalyst is
[11-14]
a preferable choice, such as MnO2, CeO2, CuO, and Co3O4
.
Among these metal oxides, cobalt oxide has aroused considerable
attention on account of its fairly good activity in the catalytic
combustion of CO, NO, soot, and VOCs due to its ease in liberation
of active oxygen species generated from the relatively weak Co–O
bond[15]. Furthermore, the specific surface area, as well as the
amount of surface-active oxygen species of Co3O4-based catalysts,
has a significant effect on the catalytic combustion of VOCs[16, 17]
.
a. Key Laboratory of Coal Science and Technology (Taiyuan University of
Technology), Ministry of Education and Shanxi Province, No. 79 Yingze West
Street, Taiyuan 030024, China.
b. Division of Nanoscale Measurement and Advanced Materials, National Institute
of Metrology, No. 18, Bei San Huan Dong Lu, Chaoyang Dist, Beijing 100029,
China.
* Corresponding author. Mailing address for correspondence: No. 79 Yingze West
Street, Taiyuan 030024, China. Tel/Fax: +86 351 6018598.
E-mail address: renjun@tyut.edu.cn (J. Ren).
It is well-known that the addition of foreign metals generally
modulates the structure of pristine metal oxide and facilitates the
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formation of surface-active oxygen species, which are conducive to morphologies of catalysts were investigated using a transmission
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the oxidation of VOCs[27]. CeO2 has been extensively employed as an electron microscope (TEM) (JEM-2010F, JEOL, Japan). The energy
DOI: 10.1039/D0NJ00091D
additive or active component in VOCs’ catalytic combustion due to dispersive spectrometer (EDS)-mapping images of catalysts were
its unique oxygen release/capture capacities combined with the captured using an EDS DX-4 test system.
particular redox activity of the Ce4+/Ce3+ pair[28, 29]. Lin et al.[30]
demonstrated the enhanced activity of flower-like Co3O4–CeO2 in
X-ray photoelectron spectroscopy (XPS) was tested using an
ESCALab 250Xi electron spectrometer with a target of Al Kα.
the catalytic combustion of 1, 2, 4-trichlorobenzene owing to the
H2 temperature-programmed reduction (H2-TPR) and oxygen
strong interaction between CeO2 and Co3O4. Based on these
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temperature-programmed desorption (O2-TPD) were performed on
foundations, we introduced ceria into cobalt aerogel to create a
a TP-5080 Automatic versatile chemisorption instrument with
plausible scenario to investigate the structure-activity relationship
thermal conductivity detector (TCD). For H2-TPR, 100 mg of sample
of CeO2–CoOx composite aerogels in toluene combustion.
was pretreated at 300°C for 60 min in N2 flow (27 ml·min-1) prior to
Attention in this work was paid to analyzing the impact of the
experiment. After cooling to room temperature, the gas was
Ce/Co ratio on the structural properties and catalytic performance
switched to 5 vol% H2/N2 (27 ml·min-1). Then, the sample was
of CeO2–CoOx composite aerogels. To this end, the physicochemical
heated from room temperature to 900°C with a heating rate of
properties of CeO2–CoOx aerogels with different Ce/Co ratios were
10°C·min-1. In the O2-TPD measurement, 50 mg of sample was first
characterized using several analytic techniques. Of particular
pretreated at 120°C for 60 min under He flow (30 ml·min-1) to
concern was the influence of oxide phase composition on textural
remove surface water and other volatile components. After that,
properties and surface-active oxygen species due to their critical
the sample was cooled to room temperature and then purged with
role in the catalytic performance. An attempt to relate the
5 vol% O2/He for 90 min. After treatment with pure He for 60 min,
interaction between CeO2 and CoOx with their catalytic properties
the sample was heated to 900°C at a ramping rate of 10 °C·min-1.
will be discussed.
2.3. Catalytic tests
Catalytic combustion of toluene was performed in a fixed bed
microreactor in the temperature range of 100–300°C under
2. Experimental
atmospheric pressure. In a typical evaluation, 100 mg catalyst and 1
g silica sand (40–60 mesh) were mixed to prevent hot spots and
then placed in the middle of a microreactor. The feeding gas,
toluene (1000 ppm), was introduced by flowing artificial air (20 vol%
O2/N2) past toluene in an ice–water bath, and the total flow was
130 ml·min-1, corresponding to a weight hourly space velocity
(WHSV) of 78,000 ml·g–1·h–1. The gaseous products were analyzed
online using a gas chromatograph (Haixin GC950) equipped with
flame ionization detector (FID) and thermal conductivity detector
(TCD). An SE-30 column and Carboxen 1000 column were used for
gas separation. The statistics of carbon balance between reactants
and products was within the limits of 100% ± 5%, indicating the CO2
was the only carbon-containing product and the toluene molecules
were completely oxidized in the current work. ( Calculation results
of carbon balance can be seen in Figure S1)[31]. The conversion of
toluene (X, %) was calculated using the following equation:
2.1. Catalyst preparation
CeO2–CoOx aerogels were synthesized using an epoxide-initiated
gelation process, combined with supercritical drying. In detail,
Co(NO3)2·6H2O (AR, Aladdin Chemical Co. Ltd.), Ce(NO3)3·6H2O (AR,
Kermel Chemical Reagent Co. Ltd.) and citric acid (abbreviated as
CA, AR, Kermel Chemical Reagent Co. Ltd.) were dissolved in
ethanol (AR, Kermel Chemical Reagent Co. Ltd.) to form
a
homogeneous solution, in which the molar ratio CA/(Ce + Co) was
1.25. Then, 1, 2-epoxypropane (named PO, AR, Sinopharm Chemical
Reagent Co. Ltd.) with molar ratio of PO/ (Ce + Co) of 10 was added
into the above solution with stirring. The obtained precursor
solution was aged for 36 h at room temperature. Noticeably, the gel
was required to be washed with ethanol every 12 h to remove
water and by-product during the aging process.
The wet gel was then dried in ethanol under supercritical
conditions (270°C, 8.0 MPa). The resultant aerogel was calcined at
300°C for 2 h with a heating rate of 1°C·min-1 in a muffle furnace. To
investigate the impact of Ce content on the physiochemical
properties of CeO2–CoOx aerogels, a series of aerogels have been
obtained by varying the Ce/Co ratios (0, 1/50, 1/20, 1/10 and 1/5) in
the precursor solutions, denoted as CoOx, CeO2–CoOx-50, CeO2–
CoOx-20, CeO2–CoOx-10 and CeO2–CoOx-5, respectively.
[Toluene]in [Toluene]out
X =
100%
,
[Toluene]in
where [Toluene]in, out were the concentration of toluene at inlet and
outlet of the reactor, respectively. T10, T50 and T90 represent the
temperatures corresponding to toluene conversion of 10%, 50%
and 90%, respectively.
2.2. Catalyst Characterization
X-ray diffraction (XRD) spectra was recorded over a range of 5–85°
at 5°/min using a D/max 2500 diffractometer (Rigaku, Japan) with
Cu Kα radiation (λ = 1.54056 Å, 40 kV and 100 mA).
3. Results and discussion
3.1. Catalyst characterization
3.1.1. Textural properties of CeO2–CoOx aerogels
N2 adsorption-desorption measurements were carried out on a
Micromeritics 3H-2000PS2 (BeiShiDe, China). The textural properties,
including the specific surface area, total pore volumes and pore size
distribution, were calculated from desorption isotherms using the
Barrett–Joyner–Halenda (BJH) method. The microscopic
N2 adsorption-desorption isotherms and BJH pore-diameter
distributions of the CeO2–CoOx aerogels are shown in Figure 1. The
isotherms for all the samples showed type IV isotherms with a
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The specific surface area, total pore volume, andVaievweArratigclee Opnolirnee
DOI: 10.1039/D0NJ00091D
diameter of the CeO2–CoOx aerogels are summarized in Table 1.
distinct H3-type hysteresis loop (Figure 1a), suggesting the
existence of irregular mesopores in the aerogels[32]. The pore-size
distribution curves in Figure 1b revealed an obvious bimodal
distribution centered at 3 and 24 nm, which were ascribed to the
condensation of the reactant molecules and the removal of organic
With the increase of Ce content, the specific surface area of CeO2–
CoOx aerogels gradually increased and achieved a maximum of 172
m2·g-1 at Ce/Co = 1/10, beyond which a distinct decrease in the
surface area was observed. The enhanced surface area of CeO2–
CoOx aerogels could be related to the formation of a more
matter during the preparation of aerogels[33]
.
(a)
continuous porous structure caused by introduction of Ce[34]
.
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CeO
2
-CoOx-5
As shown in Table 1, the pore volume of CeO2–CoOx aerogels
followed a similar tendency to Brunauer–Emmett–Teller (BET)
surface area. As the Ce/Co molar ratio rose from 0 to 1/10, the pore
volume of aerogels increased from 0.5 to 1.2 cm3·g–1. The increase
in pore volume could be due to the introduction of Ce, which
facilitated the formation of a stable network of interconnected
particles. With the increase of the Ce/Co ratio to 1/5, a significant
decrease in pore volume of the CeO2–CoOx aerogels was observed,
indicating that the porous structure of the aerogels may be
CeO
2
-CoOx-10
CeO
2
-CoOx-20
CeO -CoOx-50
2
destroyed or the mesopores may be blocked[35]
.
CoOx
3.1.2. XRD
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/Po)
 Co3O4  CeO
2
(b)
CeO2-CoOx-5
CeO2-CoOx-10
CeO2-CoOx-20
CeO2-CoOx-50
CoOx


CeO2-CoOx-5

0.04
CeO2-CoOx-10
CeO2-CoOx-20
0.03
0.02
0.01
0.00
CeO2-CoOx-50

CoOx








0
10
20
30
40
50
60
70
80
10
20
30
40
50
60
70
80
Pore diameter (nm)
2-Theta (degree)
Figure 1. N2 adsorption-desorption isotherms (a) and pore size
distributions (b) for different CeO2–CoOx aerogels.
Figure 2. XRD patterns of CeO2–CoOx aerogels with various Ce/Co
ratios.
Table 1 Structural properties of pure CoOx and CeO2–CoOx aerogels
a
b
SBET
Vtotal
Average pore diameterc
Crystallite sizes (nm)d
(nm)
Samples
(m2·g-1)
(cm3·g-1)
CoOx
62
0.5
24
22
24
17
13
9.7
9.2
8.4
5.0
9.9
CeO2–CoOx-50
68
0.5
CeO2–CoOx-20
81
0.8
CeO2–CoOx-10
172
134
1.2
CeO2–CoOx-5
a Calculated using BET method.
0.7
b Estimated from the N2 adsorption isotherms at P/P0 = 0.99.
c Calculated from the N2 desorption branches by BJH method.
d Calculated based on the XRD results according to the Scherrer equation using Co3O4(311) line broadening.
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Figure 2 shows XRD patterns of CeO2–CoOx aerogels with 47.5°, suggesting the occurrence of a phaseDsOepI:a1r0a.1t0io3n9/iDn0tNhJe0C0e0O912D–
different Ce/Co molar ratios. The diffraction peaks of the pristine CoOx-5 aerogels.
CoOx aerogel were located at 2θ values of 19.0°, 31.3°, 36.9°, 38.6°,
44.9°, 55.8°, 59.5°, 65.4° and 74.3°, corresponding to Co3O4 with 3.1.3 TEM
spinel structure (JCPDS No.74-1656). No obvious change was
Figure 3. displays the TEM images of the CeO2–CoOx
observed in the XRD pattern after addition of Ce when the Ce/Co
aerogels. In all cases, the samples exhibited a network-like
molar ratio was lower than 1/10. Interestingly, the full width at half-
structure that comprised near-spherical nanoparticles.
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maximum (FWHM) clearly increased after introduction of Ce,
However, the CeO2–CoOx-5 appeared to show a worse cross-
indicating the decrease in crystallite size of Co3O4, as shown in Table
link structure with more irregular colloidal nanoparticles,
1. However, for CeO2–CoOx-5, the apparent diffraction peaks of
which could be attributed to the formation of large crystalline
CeO2 (JCPDS No.43-1002) appeared at 2θ values of 28.5°, 33.1° and
CeO2 37]. It can be observed that with an increase in Ce
content, the particle size gradually decreased, in good
agreement with the decreased crystalline size calculated using
the Scherrer equation (Table 1). Compared with the other
samples (Figure 3), CeO2–CoOx-10 possessed abundant smaller
particles (~6 nm). When the Ce/Co molar ratio was lower than
1/5, ceria nanoparticles could not be observed in the TEM
images. However, in the high resolution TEM images of CeO2–
CoOx-5 (Figure 3), CeO2 particles could be distinguished by the
interplanar distance of 0.312 nm corresponding to the CeO2
(111) plane.
[36,
Figure 4 shows the EDS-mapping images of Ce, O and Co of
CeO2–CoOx-10, which confirmed the successful introduction of
Ce into cobalt-based aerogels. It was clear that the Ce and Co
species were uniformly mixed and had a high dispersion in the
CeO2–CoOx aerogels, which was in accordance with the XRD
results.
Figure 4. Elemental mapping of Co, Ce and O over CeO2–CoOx-10
catalyst.
3.1.4. H2-TPR
H2-TPR measurements were performed to elucidate the
reducibility of CeO2–CoOx aerogels and the results were shown
in Figure 5. For the pristine CoOx, there were two well-defined
reduction peaks labelled I and II ranging from about 200 to
550°C, corresponding to the gradual reduction of Co3O4 to
Figure 3. TEM patterns of CeO2–CoOx aerogels with various Ce/Co
ratios.
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metallic cobalt (Co3+ → Co2+, Co2+ → Co), respectively[38]. As
for the CeO2–CoOx aerogels, an additional reduction peak
CoOx-y (y = 10, 20, 50) due to the excess addition of Ce, which was
View Article Online
DOI: 10.1039/D0NJ00091D
in accordance with the TEM and XRD results.
3.1.5. O2-TPD
labelled III was observed in the range of 550
could be assigned to the reduction of Co2+ strongly interacted
with ceria[39, 40]
–750°C, which
O2-TPD profiles, shown in Figure 6, were used to characterize O2
adsorption/desorption behaviour[42]. The curve-fitting results of O2-
TPD profiles were listed in Table 2.
.
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Figure 5. H2-TPR spectra of CeO2–CoOx aerogels with various Ce/Co
ratios
Figure 6. O2-TPD profiles of CeO2–CoOx aerogels with various
Ce/Co ratios
In Figure 5, with the Ce/Co ratio increased to 1/5, peak I at first
increasingly decreased from 326 to 306°C, and then dramatically
rose to 329 °C in the CeO2–CoOx aerogels. Peak II followed a similar
tendency to peak I. The decrease in reduction temperature showed
that the addition of Ce enhanced the reducibility of the Co3+ species.
This fact could be associated with the strong interaction between
Typically, the adsorbed oxygen evolved in the following process:
–
O2 (ad) → O2 (ad) → O– (ad) → O2– (lattice)[43]. The pristine CoOx
displayed a weak and broad desorption peak in the range 50–700°C,
while the O2-TPD profiles for all the CeO2–CoOx aerogels exhibited
four distinct peaks divided according to desorption temperature.
The α-peak centered at around 110°C corresponded to the
desorption of adsorbed molecular oxygen, O2 (ad). The second one
(β, 230–260°C) and the γ-peak in the temperature range of 340–
390°C were attributed to the existence of surface absorbed oxygen
highly dispersed Co3O4 and CeO2, which weakens the Co–O bond[34]
.
With the increase of Ce addition, the area of peak II gradually
decreased but the area of peak III gradually increased at Ce/Co ratio
lower than 1/10. This also provided evidence for the enhancement
of the interaction between Co2+ ions and CeO2, especially for the
CeO2–CoOx-10[39]. Naturally, the stronger interaction restrained the
reduction of Co2+ via oxygen provided from ceria nearby or from the
interface, and led to an increase in the reduction temperature of
Co2+[41]. While the Ce/Co ratio increased to 1/5, peak II increased
and peak III decreased, suggesting that the distributions of Ce and
Co species in CeO2–CoOx-5 were quite different from the CeO2–
–
species (chemically adsorbed O2 (ad) and O– (ad))[44]. Compared
with CoOx, CeO2–CoOx aerogels exhibited much stronger β- and γ-
desorption peaks, suggesting a visible increment of O2 adsorption
after addition of Ce (Table 2)[45]. It was evident that CeO2 can
provide oxygen or share oxygen with neighbouring Co2+, creating
numerous oxygen vacancies at CeO2–CoOx interfaces, which can
facilitate O2 adsorption[46]
The last peak, δ (above 500°C), was associated with the lattice
.
Table 2 Gaussian fitting analysis of O2-TPD profiles of CeO2–CoOx aerogels with various Ce/Co ratios.
O2 desorbed
Samples
Peak of (β+γ)
Peak α
Peak β
Peak γ
Peak δ
CoOx
80.9
49.1
96.6
55.9
85.3
49.8
94.9
168.7
247.3
446.4
533.1
115.8
144.7
574.5
682.8
1281.2
648.2
CeO2–CoOx-50
CeO2–CoOx-20
CeO2–CoOx-10
CeO2–CoOx-5
155.6
127.7
397.6
300.8
418.9
555.1
883.6
347.4
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Table 3 Surface elemental analysis of pure CoOx and CeO2–CoOx aerogels
6
7
8
9
Composition (mol %)
Co
Sample
Co3+/Co2+
Osur/Olatt
Ce3+/Ce
Ce/Co
O
Ce
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CoOx
0.79
0.84
0.86
1.06
0.95
0.93
0.93
0.95
1.06
0.78
-
62.1
64.8
68.8
67.6
70.3
37.9
33.3
28.0
28.8
17.8
-
-
CeO2–CoOx-50
CeO2–CoOx-20
CeO2–CoOx-10
CeO2–CoOx-5
0.19
0.18
0.20
0.14
1.90
3.2
0.06
0.11
0.13
0.67
3.6
11.9
oxygen species O2– (lattice)[44]. At temperatures above 600°C, no
obvious peak was observed for CoOx, whereas CeO2–CoOx-10
exhibited a larger asymmetric peak δ, implying a strong interaction
between cobalt and cerium species, which was in good accordance
with H2-TPR results.
(a)
3.1.6. XPS
To understand better the influence of cerium on the surface
chemical state and composition of the catalysts, XPS
characterization was performed and the results were displayed in
Figure 7 and Table 3. Co 2p spectra can be deconvoluted into two
pairs of spin–orbit peaks denoted as D1 and D2 and three satellite
peaks (S1 to S3)[47], as shown in Figure 7a. The peak doublet D1
located at 779.2–779.5 and 794.3–794.6 eV with a separation of
15.1 eV were associated with Co3+, while the other at 780.5–780.9
and 796.0–796.3 eV with a separation of 15.5 eV were assigned to
Co2+ species. The satellite peaks S1 and S3 were ascribed to Co2+,
while S2 was assigned to Co3+[48]. As shown in Table 3, with
increasing cerium content, the ratio of Co3+/Co2+ in CeO2–CoOx first
increased and then dramatically decreased. The increase in
Co3+/Co2+ can be attributed to the electron transfer between Co and
(b)
Ce species via the reaction: Co2+ + Ce4+ → Co3+ + Ce3+[49, 50]
.
The O 1s spectra of CeO2–CoOx aerogels could be split into two
peaks, as exhibited in Figure 7b. The peak located at lower binding
energy of 529.4–529.8 eV was ascribed to the lattice oxygen, Olatt
(O2–), whereas the one at 530.9–531.2 eV was associated with
surface absorbed oxygen, Osur, such as O2– (ad) and O– (ad)[51]. With
the increase of cerium content, the Osur/Olatt ratios of CeO2–CoOx
aerogels followed a similar tendency as Co3+/Co2+ ratios, in good
agreement with the results reported previously[51, 52]. This further
confirmed that the progressive increase of the cerium content in
CeO2–CoOx aerogels was conducive to increasing the density of
Ce 3d3/2
Ce 3d5/2
(c)
V
3
V3
V
V
1
V2
V1
V
V2
C
C
eO2
-
C
C
oOx-
5
eO2
-
oOx-
10
C
eO2
-
C
oOx-
20
C
eO2
-
C
oOx-
50
920
910
900
890
880
surface-adsorbed oxygen[18]
.
Binding energy (eV)
Figure 7c shows Ce 3d XPS spectra for CeO2–CoOx aerogels.
Typically, the Ce 3d spectra could be deconvoluted into four spin-
orbit doublets corresponding to Ce 3d5/2 and 3d3/2[53]. Three pairs of
peaks labelled as V (881.9 and 900.4 eV), V2 (888.1 and 906.7 eV)
and V3 (897.7 eV and 916.1 eV) were assigned to Ce4+, while the
other pair labelled as V1 (884.3 and 902.4 eV) belonged to the
characteristic peaks of Ce3+[54]. As shown in Table 3, CeO2–CoOx-10
possessed the highest Ce3+/Ce ratio, which was favourable to the
Figure7. (a) Co 2p (b) O 1s and (c) Ce 3d XPS spectra of CeO2–CoOx
aerogels with various Ce/Co ratios
formation of oxygen vacancies[55]. Tang et al.[56] found that the
presence of oxygen vacancies greatly contribute to the capture of
oxygen molecules and the foramtion of surface activated oxygen
species. Furthermore, it also should be noted that the Ce/Co ratios
on the surface of the CeO2–CoOx aerogels were significantly higher
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Table 4 Catalytic activity of CeO2–CoOx aerogels in the oxidation of toluene at WHSV=78000 ml·g-1·h-1.
Reaction temperature (oC)
5
6
7
Samples
a
a
a
T10
T50
T90
8
9
CoOx
226.9
224.2
223.5
216.3
226.7
b
251.3
247.6
245.9
238.0
250.8
b
259.1
258.0
257.4
248.1
258.8
b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CeO2–CoOx-50
CeO2–CoOx-20
CeO2–CoOx-10
CeO2–CoOx-5
quartz
a T10, T50 and T90 refer to temperatures of 10%, 50% and 90% toluene conversion, respectively.
b Temperature is out of range.
than those in the precursors, implying that the cerium was
preferentially anchored on the surface due to its higher surface
energy than Co species[57]
.
3.2. Catalytic performance
The catalytic performances of CeO2–CoOx aerogels for toluene
combustion were evaluated in the temperature range 100–300°C
and the results were shown in Figure 8. The parameters T10, T50 and
T90 were listed in Table 4. A blank test was performed with quartz
sand (40–60 mesh) under the same reaction conditions. No toluene
conversion was observed in the blank test until the reaction
temperature reached 300°C. Apparently, the oxidation of toluene
started at 150°C and was boosted at 220°C over CoOx and CeO2–
CoOx aerogels. Pure CoOx aerogel displayed a noticeable activity
with T50 and T90 of 251.3 and 259.1°C, respectively. Both T50 and T90
over CeO2–CoOx gradually decreased with increasing Ce content.
The optimal T50 and T90 of 238.0 and 248.1°C were achieved when
the Ce/Co molar ratio was up to 1/10, beyond which both T50 and
T90 remarkably increased. According to this result, it could be
concluded that the addition of Ce had a strong influence on the
catalytic performance of CeO2–CoOx aerogels. In addition, the
catalytic activities of different cobalt-based catalysts for toluene
oxidation in the previous literatures were summarized in Table 5.
Obviously, the CeO2–CoOx-10 aerogel in this work exhibited better
catalytic performance than that of pristine CoOx aerogel as well as a
Figure 8 Catalytic performance of CeO2–CoOx aerogels for toluene
combustion. Reaction conditions: catalyst weight=100 mg,
reaction gas composition: 1000 ppm toluene, 20 vol% O2/N2, total
flow rate=130 ml·min-1 and GHSV=78000 ml·g-1·h-1
catalytic activity of CeO2–CoOx-5 was parallel to that of pristine
CoOx, although its surface area was more than twice that of CoOx. It
was further demonstrated that the surface area was not the only
decisive factor in the catalytic properties of CeO2–CoOx aerogels.
It is well-known that the oxidation of toluene over transition
metal oxides follows the Mars-van Krevelen (MVK) mechanism,
involving the participation of lattice oxygen (O2-) and surface
absorbed oxygen species (O–, O2–)[64]. In this work, the toluene
supported or doped cobalt-based catalyst[16, 18, 52, 58-61]
.
combustion over CeO2–CoOx aerogels may proceed as follows[65]
.
As aforementioned, both the textural and chemical properties of
CoOx aerogel were greatly affected by the second component
cerium due to the interaction between cobalt and cerium species.
The surface area of CoOx aerogel increased with the addition of
cerium and the CeO2–CoOx-10 aerogel showed the highest surface
area of 172 m2·g–1. As Aegerter et al.[62] and Wang et al.[63] reported,
the formation of a more stable gel network and the existence of
competitive growth between CeO2 and Co3O4 resulting from
addition of cerium could lead to the formation of smaller crystallites
of composite aerogel particles and continuous porous structure,
which greatly contributes to a larger surface area of CeO2–CoOx
aerogels. Consequently, the CeO2–CoOx aerogels with larger surface
area (Table 1) could supply more surface-active species, and
eventually result in enhanced catalytic activity[64]. However, the
The first stage is adsorption of Co ions and filling oxygen vacancy by
oxygen in gas phase. Then the absorbed toluene is oxidized by
surface reactive oxygen species including lattice/surface oxygen to
carbon dioxide and water, along with the reduction of Co3+ to Co2+
and the simultaneous creation of oxygen vacancies[66]. Finally, the
reduced Co2+ can be reoxidized by the reactive oxygen species
nearby or from the surrounding CeO2 lattice, creating plenty of
oxygen vacancies at the CeO2–CoOx interface[17]. The regenerated
oxygen vacancies in reaction (2) and (3) participate in the next
reaction cycle. Therefore, the interaction between CeO2 and Co3O4
had a great influence on the catalytic performance.
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Table 5 Catalytic performance of some cobalt based catalysts.
5
6
Surface area
T50
(oC)
T90
Samples
(m2·g-1)
Oxidation condition
Ref.
(oC)
7
8
9
CeO2–CoOx-10
La–Co EM 1:2
Co3O4-400(MOFs)
CoOx/g-C3N4
172
68
1000 ppm toluene, 130 ml·min-1, 78000 ml·g-1·h-1
266 ppm toluene, 500 ml·min-1, 600000 ml·g-1·h-1
12000 ppm toluene, 210 ml·min-1, 21000 ml·g-1·h-1
1000 ppm toluene, 130 ml·min-1, 78000 ml·g-1·h-1
1000 ppm toluene, 33.3 ml·min-1, 20000 ml·g-1·h-1
600 ppm toluene, 280 ml·min-1, 84000 ml/·g-1·h-1
1000ppm toluene, 100 ml·min-1, 60000 ml·g-1·h-1
1000 ppm toluene, 100 ml·min-1, 60000 ml·g-1·h-1
(1)
238
258
243
269
251
252
233
287
248
283
259
279
269
274
250
308
This work
[16]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
71.6
47
[18]
[52]
3Co3O4/3DOM-ESFO
CoMn CP
24.1
161
45
[58]
[59]
Co1Mn1 BHNCs
Co6Al2HT
[60]
123
[61]
Vo + O2 → [O]
Co3+ + [O] + toluene → Co2+ +Vo + oxidized product
Co2+ + CeO2 → Co3+ + CeO2–x + Vo
Co2+ + [O] → Co3+
(2)
(3)
(4)
As shown in Figure 6, owing to the significant interaction
between cobalt and cerium species, CeO2–CoOx aerogels showed
great superiority in O2 adsorption capacity compared with CoOx,
indicating the existence of more active oxygen species (O–, O2 ). XPS
data further confirmed the existence of abundant surface-adsorbed
oxygen species (such as O–, O2 ) in CeO2–CoOx aerogels, which
favoured the oxidation of toluene (reaction (2))[56]. In addition, as
shown in Figure 5, the redox properties of cobalt species had been
effectively modified after the addition of Ce. The intense interaction
between Co and Ce species resulted in the easier reduction of Co3+
and higher oxygen mobility, especially in CeO2–CoOx-10, which
facilitated the activation of toluene and the rapid redox cycle of
Co2+/Co3+[17]. Meanwhile, CeO2–CoOx-10 also possessed the highest
Co3+/Co2+ ratio (Table 3 and Figure 7), which resulted from the
electron transfer between Co and Ce species. Namely, higher
Co3+/Co2+ may increase the oxygen defects which favour the
creation of reactive oxygen species and the adsorption of toluene at
Co3+ sites[60]. As a result, CeO2–CoOx-10 exhibited better catalytic
activity in toluene combustion.
–
–
Figure 9. Long period test of catalytic oxidation of toluene over
CeO2–CoOx-10 at different temperature range. Reaction conditions:
catalyst weight =100 mg, reaction gas composition: 1000 ppm
toluene, 20 vol% O2/N2, total flow rate=130 ml·min-1 and
GHSV=78000 ml·g-1·h-1.
4. Conclusion
CoOx and CeO2–CoOx aerogels with various Ce/Co molar ratios
were prepared using an epoxide-initiated gelation process
combined with ethanol supercritical drying and were applied in the
catalytic combustion of toluene. The optimal activity was achieved
over CeO2–CoOx-10 catalyst with the Ce/Co molar ratio of 1/10
(T90=248°C for toluene oxidation). This was associated with the
strong interaction between cobalt and cerium species, which results
in the changes in physicochemical properties of catalysts. These
changes increased the surface Co3+/Co2+ ratios, surface reactive
Furthermore, a long-period test was carried out at different
temperatures (240, 250 and 300°C) to evaluate the catalyst stability.
As can be seen in Figure 9, no evident loss of catalytic activity can
be found during the test. Thus, the CeO2–CoOx-10 aerogel could be
considered as an excellent candidate for practical application in the
combustion of toluene.
–
oxygen species (O–, O2 ) as well as the easier reduction of Co3+,
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25. C.N. Sisk, L.J. Hope-Weeks, J. Mater. Chem., 2V0ie0w8,Ar1tic8le,2O6n0lin7e-
which were recognized as major factors for the superior catalytic
performance of CeO2–CoOx-10 catalyst. Furthermore, CeO2–CoOx-
10 catalyst exhibited a high stability at different temperature
stages, which could be of interest for practical application.
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Conflicts of interest
The authors declare that there is no conflict of interests.
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21776194, 21606159 and 21808154),
National Key R&D Program of China (2017YFF0205303) and Key
Research and Development Program of Shanxi Province
(201703D121022-1 and 201803D121039).
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DOI: 10.1039/D0NJ00091D
Graphical Abstract
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