Integral structured Co-Mn composite oxides grown on interconnected Ni foam for catalytic toluene oxidation

Article abstract of DOI:10.1039/c8ra10102g

Considering the three-dimensional ordered network of Ni foam-supported catalysts and the toxicity effects of volatile organic compounds (VOCs), the design of proper active materials for the highly efficient elimination of VOCs is of vital importance in the environmental field. In this study, a series of Co-Mn composite oxides with different Co/Mn molar ratios grown on interconnected Ni foam are prepared as monolithic catalysts for total toluene oxidation, in which Co_1.5Mn_1.5O₄ with a molar ratio of 1:1 achieves the highest catalytic activity with complete toluene oxidation at 270 °C. The Co-Mn monolithic catalysts are characterized by XRD, SEM, TEM, H₂-TPR and XPS. It is observed that a moderate ratio of Mn/Co plays significant effects on the textural properties and catalytic activities. From the XPS and H₂-TPR characterization results, the obtained Co_1.5Mn_1.5O₄ (Co/Mn = 1/1) favors the excellent lowerature reducibility, high concentration of surface Mn³⁺ and Co³⁺ species, and rich surface oxygen vacancies, resulting in superior oxidation performance due to the formation of a solid solution between the Co and Mn species. It is deduced that the existence of the synergistic effect between Co and Mn species results in a redox reaction: Co³⁺-Mn³⁺ ? Co²⁺-Mn⁴⁺, and enhances the catalytic activity for total toluene oxidation.

Full text of DOI:10.1039/c8ra10102g

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Integral structured Co–Mn composite oxides
grown on interconnected Ni foam for catalytic
toluene oxidation
Cite this: RSC Adv., 2019, 9, 6533
Xueding Jiang, * Weicheng Xu, * Shufeng Lai and Xin Chen
Considering the three-dimensional ordered network of Ni foam-supported catalysts and the toxicity effects of
volatile organic compounds (VOCs), the design of proper active materials for the highly efficient elimination of
VOCs is of vital importance in the environmental field. In this study, a series of Co–Mn composite oxides with
different Co/Mn molar ratios grown on interconnected Ni foam are prepared as monolithic catalysts for total
4
toluene oxidation, in which Co1.5Mn1.5O with a molar ratio of 1 : 1 achieves the highest catalytic activity with
ꢀ
complete toluene oxidation at 270 C. The Co–Mn monolithic catalysts are characterized by XRD, SEM, TEM,
H
2
-TPR and XPS. It is observed that a moderate ratio of Mn/Co plays significant effects on the textural
properties and catalytic activities. From the XPS and H -TPR characterization results, the obtained
(Co/Mn ¼ 1/1) favors the excellent low-temperature reducibility, high concentration of
2
Co1.5Mn1.5O
4
Received 9th December 2018
Accepted 18th February 2019
3+
3+
surface Mn
and Co
species, and rich surface oxygen vacancies, resulting in superior oxidation
performance due to the formation of a solid solution between the Co and Mn species. It is deduced that
DOI: 10.1039/c8ra10102g
3+
3+
the existence of the synergistic effect between Co and Mn species results in a redox reaction: Co –Mn
2+
4+
rsc.li/rsc-advances
4 Co –Mn , and enhances the catalytic activity for total toluene oxidation.
has been proven to excellent catalytic activity in numerous
reactions duo to its superior physical–chemical properties.
1
. Introduction
22–27
With increasing energy consumption and emission of air Additionally, extensive efforts have revealed that the synergistic
pollutants from industrial processes and transport vehicles, the effect of Co species and other transition metal oxides has
large amounts of volatile organic compounds (VOCs) in the dramatically enhanced the redox properties and catalytic
atmosphere are harmful to human health and the living envi- activities due to the formation of a solid solution, compared
1
–4
11,28–31
30
ronment. Catalytic oxidation is one of the low-cost and most with single oxides.
For example, Jiang et al. reported the
nanocubes with different Mn/Co
decades. In addition, the commercial catalysts are designed molar ratios derived from metal–organic frameworks. It is
by dispersing the active components into honeycomb ceramics shown that hierarchical Mn–Co–O mixed metal oxides exhibi-
2MgO$2Al $5SiO
). Unfortunately, this method always cau- ted the better redox properties, more exposed active sites and
ses inhomogeneous active components in impregnation superior oxidation performances of total toluene oxidation.
promising technologies for VOC degradation in recent preparation of Mn–Co–O
x
5–8
x
(
2
O
3
2
11
processes, which results in lower catalytic efficiency and limited Chen et al. synthesized mesoporous CoMnAl mixed metal
application. Meanwhile, some studies have been performed on oxides from layered double hydroxide (LDH) for total benzene
9
the development of metallic substrates or modular catalysts in oxidation. Results showed that CoMn
2
AlO-550 displayed rich
9–15
the environmental eld.
Among the metallic substrates, Ni oxygen vacancies and optimal catalytic activity due to the
foam has special advantages such as a low cost, high porosity, formation of a solid solution of cobalt–manganese oxides.
rich accessible electroactive sites, high thermal conductivity, Therefore, one effective strategy on introducing Mn species into
13,15,16
and mass transfer ability,
alternative to honeycomb ceramics.
Various transition metal oxides as active components in properties.
so may be considered as a better Co
3 4
O phase to develop multi-element mixed oxides, can be
successfully used for removal of VOCs and improved redox
reaction processes have been extensively studied to replace
Herein, a series of binary Co–Mn oxides with different Co/
noble metals over the past several years, because they have low Mn molar ratios embedded in interconnected Ni foam were
cost, unique structural morphology, adequate catalytic activity well prepared via a simple hydrothermal reaction, and their
3,17–21
and high thermal stability.
Co
3
O
4
, a transition metal oxide, catalytic performances were investigated in the total toluene
oxidation (a model reaction). Furthermore, integral structured
Co–Mn composite oxides grown on interconnected Ni foam
were characterized by numerous techniques, such as XRD, SEM,
School of Environment and Chemical Engineering, Foshan University, Foshan 528000,
China. E-mail: jgreatding@163.com; xwc1030@163.com
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ꢀ
TEM and XPS, to understand the correlation between physical– of 180–300 C under 1000 ppm toluene balanced with air at
ꢂ1
chemical properties and reactivity of Co–Mn composite oxides. a total ow rate of 120 mL min . Thus, a weight hourly space
ꢂ1 ꢂ1
This study is to provide guidelines for the rational fabrication of velocity (WHSV) of 30 000 mL g
h
or a gas hour space
ꢂ1
integral structured Co-based composite oxides grown on inter- velocity (GHSV) of 12 000 h was applied for the whole exper-
connected Ni foam for effectively catalytic toluene oxidation.
iment. The toluene conversion was persistently measured, and
was reached typically at the nal temperature for 1 h in each
testing temperature. The concentrations of toluene and prod-
2
. Experimental
ucts (CO or CO
line GC-2014 with two ame ionization detector (FID). The
An aqueous salt solution (with Co/Mn molar ratios of 3/0, 2/1, 1/ catalytic activities over the Co3ꢂxMn composite oxides were
2
) in the effluent gas was analyzed by using an on-
2.1 Preparation of catalysts
O
x 4
2
+
2+
1
, 1/2 and 0/3, Co + Mn ¼ 3 mmol) was prepared by dis- calculated as following equation:
3 2 2 3 2
solving Co(NO ) $6H O and Mn(NO ) into 40 mL of deionized
C
toluene;in ꢂ Ctoluene;out
h ¼
ꢁ 100%
(1)
water. The 12 mmol of solid urea was then added to the
homogeneous salt solution. The cleaned Ni foam (4 cm ꢁ 6 cm)
was immersed in the precursor solution, and were then trans-
C
toluene;in
where h, Ctoluene,in and Ctoluene,out are the total VOCs conversion,
toluene in the inlet and outlet gas, respectively.
ꢀ
ferred into in a 50.0 mL Teon-lined autoclave at 95 C for 12 h
The apparent activation energy (E
a
) values of were calculated
(2)
cat
in an electric oven. Aer cooling down to indoor temperature,
the covered Ni foams with array precursors were washed several
times with deionized water and ethanol. Finally, the covered Ni
by the equations:
N ꢁ h
ꢀ
g ¼
foams were heated at 400 C in air for 2 h to form the composite
W
oxide phase. These calcined Ni foams were designated as
ꢀ
ꢁ
ꢂꢃ
x
Co3ꢂxMn O
4
(x ¼ 0, 1, 1.5, 2 and 3), respectively.
E
a
g ¼ ꢂkc ¼ ꢂA exp ꢂ
c
(3)
RT
2.2 Catalyst characterization
The crystal structure of as-prepared structured catalysts was
characterized by X-ray diffraction (XRD) (Japan, D/max 2500)
ꢀ
3. Results and discussion
with Cu-Ka radiation (2q ¼ 5–90 ) at 40 kV and 30 mA.
The pore size distribution, pore volume and the Brunauer–
Emmett–Teller (BET) surface areas of as-prepared structured
catalysts were measured by using a Micromeritics ASAP2020 at
As shown in XRD patterns of as-synthesized Co–Mn composite
ꢀ
ꢀ
ꢀ
oxides, the strong peaks at 2q ¼ 44.8 , 51.5 and 76.3 could be
well indexed to nickel metal (JCPDS no. 04-0850). For Co-rich
ꢀ
ꢂ196 C. Before the tests, all structured catalysts were degassed
catalysts (Co
3
ꢀ
O
4
, Co
2
MnO
4
and Co1.5Mn1.5
O
4
), the diffraction
ꢀ
at 120 C for 2.5 h.
ꢀ
peaks at 36.3 and 64.8 can be attributed to the (311) and (440)
The surface morphology and microstructure of as-prepared
structured catalysts were carried out by using scanning elec-
tron microscopy (SEM, SU-8020) and transmission electron
microscope (TEM, JEOL 2100F), respectively.
3 4
planes of the spinel oxide phase (Co O , JCPDS 43-1003),
3
2
respectively. For Mn-rich catalysts (CoMn
2
O
4
and Mn
3
O
4
), the
ꢀ
ꢀ
characteristic peaks at around 30.9 and 32.4 were corresponds
3 4
to Mn O phase (JCPDS no. 89-4837). According to the XRD
results, it could be found that the Co1.5Mn1.5O catalyst with the
molar ratio of Co/Mn ¼ 1/1 has the lower intensity of the
diffraction peak, inferring that there is a low crystallinity which
will result in abundant crystal defects (Fig. 1).
The
2 2
H temperature programmed reduction (H -TPR)
4
measurements were carried out on an Automated Catalyst
Characterization System (Autochem 2920, MICROMERITICS).
Prior to H -TPR, the structured catalyst (1 cm ꢁ 3 cm) were
2
ꢂ1
heated under a gas ow of 5% O
temperature to 300 C. Aer cooling to room temperature, the
2
/He (25 mL min ) from indoor
The nitrogen adsorption/desorption isotherms and pore size
distributions of as-prepared Co–Mn composite oxides grown on
interconnected Ni foam are tested to further research the
porous structure, as shown in Fig. 2. With the increased Mn
species doped into Co O , the specic surface area and pore
ꢀ
2
structured catalyst was reduced under a gas ow of 10% H /Ar
ꢂ1
ꢀ
ꢂ1
(
30 mL min ) with at a heating rate of 10 C min
.
X-ray photoelectron spectroscopy (XPS) measurements were
3
4
recorded by using an XLESCALAB 250Xi electron spectrometer
from VG Scientic with monochromatic Al Ka (1486.6 eV)
radiation, and the peak positions were calibrated by the C 1s
peaks at 284.6 eV.
volume gradually decreases, as summarized in Table 1. There is
a type IV sorption isotherms with a type H2 hysteresis loop for
each of the samples (Fig. 2a), indicating the presence of
homogeneous mesopores. The average pore sizes of as-prepared
Co–Mn samples keep in a narrow size distribution (6.8–8.8 nm),
as described in Fig. 2b.
2.3 Catalytic performance test
The catalytic activities of total toluene oxidation over the
Aer hydrothermal reactions, the surface of Ni foams is
Co3 Mn O composite oxides were performed in a continuous- orderly covered via uniform coverage of vertical Co/Mn oxide
ꢂx
x 4

ow quartz tube (i.d. ¼ 10 mm) using about 0.24 g covered Ni arrays, as demonstrated in Fig. 3k. The morphologies of as-
foams (2 cm ꢁ 4 cm), the covered Ni foams were bent into the prepared Co–Mn arrays in Ni foam have changed with the
3 4
reaction tube. The test was carried out in the temperature range increased Mn species doping into Co arrays. The Co O
3
O
4
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Fig. 1 The XRD patterns of as-synthesized Co–Mn composite oxides
grown on interconnected Ni foam.
sample mainly exhibits a series of intertwined and hexagonal
nanosheets with the epitaxial nanowires in a parallel fashion, as
shown in Fig. 3a and b. When the atomic ratio of Co : Mn is
about 2 : 1, the nanowires grow signicantly at the edge of the
nanosheets in Fig. 3c and d. Compared with the Co
3 4
O sample,
the SEM images (Fig. 3e–h) on the Co1.5Mn1.5 and CoMn O
O
4
2 4
samples have obvious changes, showing that a large number of
nanowires are self-assembled into urchin shapes with a diam-
eter of 5–10 mm. For Mn O sample in Fig. 3i and j, it could be
3
4
observed that many hexagonal nanosheets with a diameter of 1–
mm aggregated on the margins of sample, and the morphology
of Mn sample is not completely uniform. In addition, EDX
mapping (Fig. 3l–o) shows that Co, Ni, Mn elements are
uniformly distributed on the surface of Ni foam.
2
3 4
O
Fig. 2 (a) Nitrogen adsorption/desorption isotherms and (b) pore size
distributions of as-prepared Co–Mn composite oxides grown on
interconnected Ni foam.
The micro-structure of as-prepared Co–Mn composite oxides
was investigated via TEM analysis. As shown in Fig. 4a and b,
a series of nanowires are uniformly distributed on the extremely
rough surface of nanosheets with tiny pores inside, which is
test. The H
2
-TPR curves of Co–Mn composite oxides are showed
in Fig. 5, there are mainly two to three reduction peaks over the
3
+
Co–Mn catalysts. The reduction step of Co O phase is Co
/
consistent with SEM results. A HRTEM image of Co
in Fig. 4b, the high-resolution lattice fringes calculated to be
3 4
.445 nm could be indexed to the (111) lattice plane of Co O
3
O
4
is shown
3 4
2
+
0
Co / Co , and the reduction step of manganese oxides is
4
+
3+
2+ 2,33
Mn / Mn / Mn .
3 4
For Co O sample with three
0
reduction peaks, the one peak in the low temperature range of
phase. TEM image in Fig. 4c further reveals that the morphology
of Co1.5Mn1.5O4 sample is composed of a large number of
nanowires with a diameter of 50–70 nm, the corresponding
HRTEM image in Fig. 4d shows that the lattice fringes with an
interplanar spacing of 0.286 nm is assigned to the (220) lattice
ꢀ
3+
2
00–300 C is associated with the reduction of surface Co into
2
+
Co , the overlapping peaks in the low temperature range of
3
into Co and metallic cobalt.
sample exhibits two separated reduction peaks at 303 C and
ꢀ
+
3+
00–400 C is attributed to the further reduction of bulk Co
2
19,34
The TPR proles of Mn O
3 4
ꢀ
plane of Co
are observed, indicating the formation of a solid solution
between Co and Mn species. The TEM image of Mn sample
3 4
O phase. No lattice streaks of manganese oxides
ꢀ
354 C, which are assigned to the following two reduction
4
+
3+
3+
2+
processes: Mn / Mn and Mn / Mn , respectively. In
addition, compared to single Co and Mn catalysts, the
rst reduction peaks of Co–Mn mixed phase catalysts are
3 4
O
O
4
3 4
O
in Fig. 4e exhibits hexagonal nanosheets with a diameter of 2
mm, some lattice fringes belong to the (101) lattice planes of
manganese oxides (lattice fringes ¼ 0.48 nm) can be observed.
The formation of a solid solution is favorable for low tempera-
ture reduction, electron transfer, abundant surface oxygen
vacancy and oxidation–reduction reaction, which will be further
conrmed via other characterization analysis.
3

gradually shied to low temperature regions, suggesting that
their low temperature reducibility is improved via the syner-
gistic effect of Co and Mn species. All the results reveal that the
Co1.5Mn1.5O is easier to be reduced in the low temperature
4
ꢀ
range of 200–300 C than other monolithic array catalysts,
meaning a facilitated redox process and a better catalytic
performance.
The reducibility of as-prepared Co–Mn composite oxides
grown on interconnected Ni foam was carried out by the H -TPR
2
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Table 1 BET data and catalytic activity of as-prepared Co–Mn composite oxides
BET surfaces
Pore diameter
(nm)
Total pore volume
2
ꢂ1
3
ꢂ1
ꢀ
ꢀ
ꢀ
a
ꢂ1 ꢂ1
Samples
area (m g
)
(cm g
)
T
10
C
T
50
C
T
90
C
g (mmol g
h )
Co
Co
3
O
MnO
4
26.22
25.76
14.20
13.99
13.95
0.056
0.061
0.033
0.036
0.029
7.8
8.1
8.8
8.5
6.8
255
245
240
250
240
273
264
263
275
255
277
268
267
287
280
0.32
1.34
1.34
0.44
1.10
2
4
Co1.5Mn1.5O
4
CoMn O
2 4
Mn O
3 4
a
ꢀ
The specic toluene reaction rates over all the catalysts were calculated at 270 C.
Fig. 6 shows the Co 2p, Mn 2p3/2, O 1s and Ni 2p XPS spectra shows four peaks at the binding energy of 856.4, 861.5, 873.1
of the ve samples. And the related results of chemical states and 880.2 eV attributed to Ni 2p3/2, satellite peak of Ni 2p3/2, Ni
35,36
are summarized in Table 2. The Ni 2p XPS spectra in Fig. 6d 2p1/2 and the satellite peak of Ni 2p1/2, respectively.
The Ni
3 4 2 4 4 2 4 3 4
Fig. 3 The SEM images of (a and b) Co O , (c and d) Co MnO , (e and f) Co1.5Mn1.5O , (g and h) CoMn O , (i and j) Mn O samples and (k)
covered Ni foam; (l–o) elemental mapping images of Ni, Co and Mn.
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Fig. 5
2
H -TPR curves of as-prepared Co–Mn composite oxides
grown on interconnected Ni foam, labels (a)–(e) correspond to the
Co , Co MnO , CoMn and Mn , respectively.
O
3 4
2
4
, Co1.5Mn1.5
O
4
2
O
4
3 4
O
3
+
between Co and Mn species to provide a redox reaction: Co –
3
+
2+
4+ 11
Mn 4 Co –Mn .
Fig. 6c shows that the O 1s XPS spectra are mainly decon-
voluted into three kinds of surface oxygen species. The peaks
located at the binding energy of 529.8, 531.2 and 533.2 eV could
be attributed to characteristic of the surface lattice oxygen (O ,
I
2
ꢂ
ꢂ
2ꢂ
ꢂ
O
), adsorbed oxygen (O , O , O2 or O ) and adsorbed
II 2
oxygen-containing hydrocarbons (O , OH, H O) species,
III
2
3 4
Fig. 4 TEM and HRTEM images of (a and b) Co O , (c and d)
38,39
respectively.
oxygen (O ) remarkably decreases with introducing Mn species
into Co phase, thereinto, the Co1.5Mn1.5 exhibits the
lowest surface lattice oxygen concentration. It is to be noted that
As shown in Table 1, the ratio of surface lattice
Co1.5Mn1.5O
4
3 4
and (e and f) Mn O samples (inset is the fast-Fourier-
transform pattern).
I
3
O
4
O
4
2p XPS spectra primarily come from Ni foams. As for Co 2p XPS
the electrophilic surface adsorbed oxygen species (Oads ¼ OII
+
spectra (Fig. 6a), the peaks at the binding energy of 779.9, 781.5
3
+
2+
O ) play vital roles in the deep VOCs oxidation, the formation
III
and 785.5 eV are related to surface Co , Co and satellite peak
1
7,18,31
of surface adsorbed oxygen species is due to the presence of
of Co species, respectively.
spectrum in Fig. 6b is deconvoluted into four peaks. According
to the H -TPR results, it could be observed that there is the
presence of Mn and Mn species. Thus, the three peaks of Mn
3/2 XPS spectrum at the binding energy of 641.2, 642.5, 644.3
In addition, the Mn 2p3/2 XPS
surface oxygen vacancies (V ). According to the Co 2p and Mn
O
2
p results, surface oxygen vacancies are benecial for main-
taining electrostatic balance and participating in the redox
reaction. Therefore, it is reasonable that the Co1.5Mn1.5 will
2
4
+
3+
O
4
2p
2
+
3+
4+
exhibit an excellent catalytic activity for total toluene oxidation.
The toluene catalytic activity of as-prepared Co–Mn
composite oxides as a function of temperature is shown in
Fig. 7A. It could be observed that the catalytic combustion of
and 646.1 eV are attributed to the surface Mn , Mn , Mn and
the satellite to the surface Mn species, respectively. According
3
+
4
3
+
2+
to the XPS results, the surface molar ratio of Co /Co follows
the sequence of Co1.5Mn1.5 (2.54) > Co MnO (1.96) >
CoMn (1.65) > Co (1.43), suggesting that the Co /Co
molar ratio of the Co1.5Mn1.5 is highest than those of other
Co–Mn composite oxides. Similarly, the surface molar ratio of
O
4
2
4
3
+
2+ toluene over catalysts increases with the increased temperature.
As presented, the Mn species doping into Co arrays has
2
O
4
3 4
O
3 4
O
O
4
a signicant inuence on the catalytic activity of toluene
oxidation. The optimal molar ratio of Co/Mn resulted in the
large increase of catalytic activity for toluene oxidation. The
reaction temperatures T10, T50 and T90 at which the toluene
3
+
Mn /Mntotal follows the sequence of Co1.5Mn1.5
4
O (0.471) >
CoMn O (0.456) > Co MnO (0.413) > Mn O (0.38). Surface
2
4
2
4
3 4
oxygen vacancies would be generated to maintain electrostatic
3
+
conversions of 10%, 50% and 90% is converted to CO , are
2
balance once a lower manganese state (Mn ) existed in the
37
summarized in Table 1, which are used to compare the catalytic
catalysts according to the following process:
activities for toluene oxidation. For Co O sample, T , T and
3
4
10
50
4
+
2ꢂ
4+
4+
3+
ꢀ
–
Mn –O –Mn – / –Mn –,–Mn – + 1/2O
2
T90 values are 255, 273 and 277 C, respectively, and toluene
could be completely converted into CO
2
and H
2
O at about
ꢀ
,
represents an oxygen vacancy in the catalysts.
280 C. The T an T50 of Mn O sample are superior to those of
1
0
3 4
3
+
The higher ratio of Mn /Mn
implies the presence of rich Co O sample, whose 10% and 50% toluene conversions could
total
3 4
ꢀ
oxygen vacancies. Moreover, there is the electronic transfer be obtained at 240 and 255 C, whereas the T90 value over
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Fig. 6 High-resolution XPS spectra of (a) Co 2p, (b) Mn 2p3/2, (c) O 1s and (d) Ni 2p over the as-prepared Co–Mn composite oxides.
Mn
3
O
4
obviously shis to high temperature region. It can be and Co1.5Mn1.5
O
h
4
catalysts exhibits higher reaction rate with
ꢂ1 ꢂ1
seen that adding manganese to cobalt improved the toluene 1.34 mmol g
conversions. According to the temperature values of complete rate of Co O catalyst with 0.32 mmol g
, which is four point two times the reaction
ꢂ1 ꢂ1
h . Furthermore, the
3
4
toluene oxidation (T ), the catalytic activity over the ve activation energies (E ) over the catalysts are calculated by the
9
9
a
ꢀ
samples follows the sequence of Co1.5Mn1.5
O
4
(270 C) z Arrhenius plots in Fig. 7d. The E
a
decreases from 154.78 to
in the sequence of Co , Mn and
, which is related to the catalytic performance for
concentration toluene oxidation.
is also detected during catalytic toluene reaction, and CO yield In addition, the stability of Co1.5Mn1.5
ꢀ
ꢀ
ꢀ
ꢂ1
Co
2
MnO
4
(270 C) > Co
3
O
4
(280 C) > CoMn
2
O
4
(290 C) > Mn
exhibits the highest Co1.5Mn1.5O
3
O
4
115.31 kJ mol
3
O
4
3 4
O
ꢀ
(300 C), suggesting the Co1.5Mn1.5
O
4
4
catalytic activity for total toluene oxidation. CO
2
2
4
O with the best
is shown in Fig. 7b. It can be concluded that toluene is catalytic activity is also tested by a long-term time at different
completely converted into CO . The specic toluene reaction temperatures, as shown in Fig. 8. Under the complete removal
2
ꢀ
ꢀ
rates are calculated at 270 C, as shown in Fig. 7c. The Co MnO
temperature (270
C), the toluene conversions with
2
4
Table 2 Surface chemical compositions of as-prepared Co–Mn composite oxides
3+
2+
3+
2+
4+
3+
2+
Samples
Co
Co
Co /Co
Mn
Mn
Mn
O
I
O
II
O
III
Co
Co
3
O
MnO
4
0.588
0.663
0.717
0.622
—
0.412
0.337
0.283
0.378
—
1.43
1.96
2.54
1.65
—
—
—
—
0.547
0.471
0.435
0.504
0.535
0.347
0.428
0.464
0.390
0.355
0.106
0.155
0.101
0.106
0.11
2
4
0.293
0.276
0.262
0.263
0.413
0.471
0.456
0.368
0.294
0.253
0.282
0.367
Co1.5Mn1.5O
4
CoMn
Mn O
2 4
O
3 4
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ꢀ
Fig. 7 (a) Toluene catalytic activity, (b) CO
2
yields and (c) reaction rate at 270 C of the as-prepared catalysts; (d) Arrhenius plots for total toluene
ꢂ1 ꢂ1
g .
oxidation over the Co
O
3 4
, Co1.5Mn1.5
O
4
and Mn
O
3 4
catalysts. Reaction conditions: Ctoluene ¼ 1000 ppm at WHSV ¼ 30 000 mL h
approximately 100% value are nearly unchangeable aer 30 h the increased Mn species into Co
3
O
4
, the Co-rich catalysts
ꢀ
test. When the reaction temperature is decreased to 265 C, the (Co
2
MnO
4
and Co1.5Mn1.5O ) exhibits superior catalytic
4
toluene conversion apparently decreased from 100% to 64%, performances for total toluene oxidation, indicating no direct
there are ve percent uctuation on toluene conversion. The relation between the specic surface area and the reactivity.
ꢀ
activity increased slightly aer 30 h reaction at 265 C, which is According to the H -TPR results, the Co1.5Mn1.5O owns low-
2
4
due to the adsorption of water molecules on the active sites in temperature reducibility than other monolithic array catalysts
begin and the exothermic reaction of toluene combustion. This due to the facilitated redox process from the synergistic effect of
result indicates that Co1.5Mn1.5
O
4
2
catalyst own outstanding Co and Mn species. The above H -TPR results are conrmed by
catalytic activity and sustainability.
XPS. There is the electronic transfer between Co and Mn species
Previous studies revealed that the reactivity of a catalyst for
VOCs oxidation can be greatly associated with physical–chem-
ical properties including the specic surface area, surface active
species, oxygen vacancies, reducibility and synergistic effect of
metal cation redox couple. The oxides with a formula of AB O
2
4
2
7
(
A
3
O
4
when A ¼ B), which is recognized as a spinel phase. In
2
+
the Co
to form a tetrahedral CoO
octahedral holes to obtain octahedral CoO
3 4
O structure, the Co cations occupy tetrahedral holes
3
+
4
sites and the Co cations occupy
sites. According to
the literature, the crystal structure of MnCo displays the
6
40
2 4
O
higher valence Mn and Co cations in the (110) plane, and the
Mn cations can substitute the octahedral site and octahedral
site of Co cations. The high valence Mn cations occupy partial
2
+
inactive Co sites, which promotes the reduction and syner-
gistic effect of cobalt and Mn species. In this work, introducing
Mn species into Co
3 4
O construction varied catalytic perfor-
mances and physical–chemical properties. Although the
specic surface area and pore volume gradually decreases with Fig. 8 Stability test of Co1.5Mn1.5
4
O catalyst at different temperatures.
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3
+
3+
2+
4+
to provide a redox reaction: Co –Mn 4 Co –Mn . In
addition, abundant surface oxygen vacancies are benecial for
maintaining electrostatic balance and participating in the redox
reaction. These results indicates the insignicant effect of low-
temperature reducibility, the synergistic effect of Co and Mn
species and surface oxygen vacancies on the catalytic activity for
total toluene oxidation.
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1
1
1
1
1
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Co1.5Mn1.5O with the molar ratios of 1 : 1 displayed the highest
131.
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the Co/Mn molar ratio played signicant inuence on the
textural properties and catalytic activities of obtained catalysts.
Pure Co O sample mainly exhibited a series of intertwined and
hexagonal nanosheets with the epitaxial nanowires, pure
Mn sample showed many hexagonal nanosheets with
a diameter of 1–2 mm, the Co–Mn composite oxides mainly
appeared urchin shapes self-assembled via a large number of
nanowires. Based on the temperature values of complete
toluene oxidation, the activity of toluene catalytic oxidation
follows Co1.5Mn1.5O z Co MnO > Co O > CoMn O > Mn O .
From the characterization results of XPS and H -TPR, intro-
ducing Mn element into Co
tion of a solid solution between Co and Mn species, improved
the low-temperature reducibility, concentration of surface Mn
and Co species, and surface oxygen vacancies. It is deduced
that he synergistic effect of Co and Mn species provided a redox
reaction: Co –Mn 4 Co –Mn and enhanced the catalytic
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4
2
4
3
4
2
4
3 4
2
1
3 4
O sample resulted in the forma-
3
+
3
+
2
2
2
2
2
3
+
3+
2+
4+
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the Natural Science Foundation of
Guangdong Province, China (Grant No. 2018A030313734).
2
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