Template-free Scalable Synthesis of Flower-like Co3-xMnxO4 Spinel Catalysts for Toluene Oxidation

Article abstract of DOI:10.1002/cctc.201800598

The rational design of low-cost transition metal catalysts that exhibit high activity and selectivity may be the most significantly investigated in heterogeneous catalysis. In this study, Co_3-xMn_xO₄ (x=0.75, 1.0, and 1.5) mixed metal oxides were successfully synthesized by a controlled template-free autoclave strategy and studied for toluene oxidation. It is found that the Co-rich sample showed markedly enhanced activity and the 3D dandelion-like Co_2.25Mn_0.75O₄ catalyst exhibited in the highest toluene oxidation rate (8.9 μmol/(g_cats)) and a 100 % toluene conversion at 239 °C. In situ DRIFTS study indicates that toluene was sequentially oxidized to benzyl radical, benzaldehyde, benzene, oxalic acid, and finally to CO₂ and H₂O. The interaction between Co and Mn, in conjunction with the high concentration of surface oxygen species and rich surface oxygen vacancies, reasonably explains the elevated catalytic activity and thermal stability for toluene oxidation over 3D flower-like Co_3-xMn_xO₄ spinel catalysts.

Full text of DOI:10.1002/cctc.201800598

Accepted Article
Title: Template-free Scalable Synthesis of Flower-like Co3-xMnxO4
Spinel Catalysts for Toluene Oxidation
Authors: Hamidreza Arandiyan, Yuan Wang, Yuxi Liu, Yijing Liang,
Yue Peng, Stuart Bartlett, Hongxing Dai, Sadegh Rostamnia,
and Junhua Li
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
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ChemCatChem
10.1002/cctc.201800598
FULL PAPER
Template-free Scalable Synthesis of Flower-like Co3-xMn
x
O
4
Spinel Catalysts for Toluene Oxidation
[
a]
[a],[b]*
[c]
[c]
[d]
[b]
Yuan Wang , Hamidreza Arandiyan
,Yuxi Liu , Yijing Liang , Yue Peng , Stuart Bartlett ,
[
c]
[e]
[d]
Hongxing Dai , Sadegh Rostamnia , and Junhua Li
Abstract: The rational design of low-cost transition metal catalysts
metal (e.g., Co and Mn) oxides for toluene oxidation. Normally,
transition metal oxide catalysts show a complete conversion of
trace toluene (> 99%) at temperatures above 250 °C,[
whereas typical supported noble metal-based catalysts can
achieve a complete conversion at temperatures below 200 °C.
Even so, precious metal catalysts have some inevitable
drawbacks, for instance, high cost, easy-poisoning tendency,
and limited resources. It is hence desirable to make efforts on
the development of efficient transition metal oxide catalysts with
low cost and high thermal stability to substitute noble metal
catalysts. Among the transition metal oxides, manganese and
cobalt are recognized as the most promising ones because of
their low price and adequate activities at relatively low
temperatures. Kim and Shim [ observed a catalytic activity
that exhibit high activity and selectivity may be the most significantly
1-2]
investigated in heterogeneous catalysis. In this study, Co3-xMn
x 4
O (x
=
0.75, 1.0, and 1.5) mixed metal oxides were successfully
synthesized by a controlled template-free autoclave strategy and
studied for toluene oxidation. It is found that the Co-rich sample
showed markedly enhanced activity and the 3D dandelion-like
Co2.25Mn0.75
O
4
catalyst exhibited in the highest toluene oxidation rate
(8.9 μmol/(gcats)) and a 100 % toluene conversion at 239 °C. In situ
DRIFTS study indicates that toluene was sequentially oxidized to
benzyl radical, benzaldehyde, benzene, oxalic acid, and finally to
2 2
CO and H O. The interaction between Co and Mn, in conjunction
with the high concentration of surface oxygen species and rich
surface oxygen vacancies, reasonably explains the elevated
catalytic activity and thermal stability for toluene oxidation over 3D
3]
3 4 2 3 2
order of Mn O > α-Mn O > β-MnO for toluene and benzene
flower-like Co3-xMn
x
O
4
spinel catalysts.
oxidation. Mn with a high valence is more active in total
oxidation. The further addition of alkali metal and alkaline earth
metal improves the catalytic performance. Moreover, Co oxide is
active due to the redox properties in toluene oxidation.
Apart from the catalyst composition, introducing porosity to a
base metal oxide can increase surface area of the catalyst and
improve its catalytic performance. This increase in surface area
can decrease the initial reaction temperature of VOC oxidation
and enhance the rate of the surface reaction. So it follows that
Introduction
Volatile organic compounds (VOCs) emitted from certain solids
or liquids in industrial processes and transport vehicles have
short- and long-term health risks, and become a notable
problem. In consideration of the environmental impact and
energy-saving, low-temperature catalytic oxidation, one of
frequently and widely distributed VOCs in the environment, is
one promising strategy for the removal of toluene.
the surface structure and properties of the catalyst are key to
[1-2, 4-8]
catalytic performance.
To improve the catalytic activity of
manganese and cobalt catalyst, many efforts have been made
to adjust the synthetic methods to achieve appropriate crystal
structures and morphologies. The impact of porosity in
mesoporous/macroporous manganese and cobalt oxides on
catalytic activity for toluene oxidation was investigated in our
previous works. For example, we synthesized three-
In the literature, precious metal catalysts (e.g., Pt, Pd, Ag, and
Au) demonstrated higher activity as compared with transition
[
a]
Miss Y. Wang, Dr. H. Arandiyan
2 3
dimensionally ordered macroporous (3DOM) Mn O -supported
AuPd alloy catalyst with AuPd particle sizes of 2–4 nm by the
PVA-protected reduction method.[9] Our recent research
Particles and Catalysis Research Group, School of Chemical Engineering,
The University of New South Wales, Sydney, NSW 2052, Australia
[
b]
Dr. H. Arandiyan, Dr. S. Bartlett
Laboratory of Advanced Catalysis for Sustainability, School of Chemistry,
The University of Sydney, Sydney 2006, Australia
reported a robust process for the preparation of noble metal-free
Ni/Co
3
O
4
catalyst with a hierarchically porous network that
hamid.arandiyan@sydney.edu.au (H. A.)
Dr. Y. Liu, Mr. Y. Liang, Prof H. Dai
exhibited excellent catalytic activity for CO
2
methanation.[
5]
[c]
Beijing Key Laboratory for Green Catalysis and Separation, and Laboratory
of Catalysis Chemistry and Nanoscience, College of Environmental
and Energy Engineering, Beijing University of Technology, Beijing
Herein, we demonstrate the rational design of low-cost transition
metal-based catalysts (flower-like spinel Co-Mn solid solutions)
derived from a template-free route for toluene oxidation, in which
the porous architecture of flower-like structure provided easy
access to the active sites, and allowed for the accessible
deposition of the active phases. Details on sample preparation
100124, China
[d]
Dr. Y. Peng, Prof J. Li
State Key Joint Laboratory of Environment Simulation and Pollution
Control, School of Environment, Tsinghua University, Beijing
100084, China
(
Fig. S1) were described in the Supporting Information. Toluene
[e]
Prof. Sadegh Rostamnia
Organic and Nano Group (ONG), Department of Chemistry, Faculty of
Science, University of Maragheh, P.O. Box. 55181-83111,
Maragheh, Iran
oxidation activity evaluation over the as-obtained samples was
performed according to the procedures depicted in the
Supporting Information. All of the catalysts were characterized
Supporting information for this article is given via a link at the end of
the document.
2
by the techniques, such as N adsorption-desorption (Brunauer–
Emmett–Teller, BET), inductively coupled plasma atomic
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10.1002/cctc.201800598
FULL PAPER
emission spectroscopy (ICP–AES), field emission scanning
electron microscopy (FESEM), field-emission high resolution
transmission electron microscopic (FE–HRTEM), energy-
oxidation. The morphology of MnCo-2 (higher Co/Mn molar
ratio) as shown in Fig. 1c and d was considerably different from
that of MnCo-1. The size of the 3D structure decreased
gradually and the morphology became a flower-like structure
formed by the network of nanoneedles.
dispersive spectroscopy (EDS), X-ray diffraction (XRD), H
temperature-programmed reduction (H -TPR), temperature-
programmed desorption of oxygen (O -TPD), in-situ diffuse
2
2
2
reflectance infrared Fourier transform spectroscopy (in-situ
DRIFTS). The detailed characterization procedures and catalytic
activity evaluation were described in the Supporting Information.
Results and Discussion
x 4
The Co3-xMn O (x = 0.75, 1.0, and 1.5) samples, which were
denoted as MnCo-3, MnCo-2, and MnCo-1, respectively, were
successfully fabricated by a controlled template-free autoclave
strategy. The results of ICP-AES investigations reveal that the
Co/Mn molar ratio was 1.09, 2.35, and 4.38, respectively, which
were close to the theoretical molar ratios of 1, 2, and 4 in the
samples, respectively (Table 1). The small discrepancy in
theoretical and measured Co/Mn molar ratios was due to the
instrumental errors during ICP-AES measurement processes.
The 3D flower-like Co-Mn oxide was generated through primary
configuration of nano-cores and successively anisotropic growth
through the direction followed by Ostwald ripening, as shown in
Fig. S2. The illustration of the step-wise process for the
formation of flower-like MnCo-3 was presented in Scheme 1. As
shown in reactions (Eqs. 1-4), the thermal decomposition of urea
occurred rapidly after being heated to 100ꢀ°C, followed by the
Scheme 1. Schematic illustration of the preparation process of the 3D flower-
like spinel catalysts.
When the molar ratio of Co and Mn increased to two (MnCo-3),
the morphology of the MnCo-3 sample remained similar to that
of MnCo-2, but the diameter of the particles increased to ca.
7
ꢀμm (Fig. 1e and f). The flower-like morphology gradually
became more clearly defined. As shown in Fig. 1f, the MnCo-3
particles had 3D dandelion-like structure. Numerous
nanoneedles radiated from the center of the spherical particles
Fig. S4).
release of NH
reacted with NH
3
and CO
2
. Metal cations (Mn and Co) slowly
/OH− in an aqueous solution. The
a
3
thermodynamically controlled protocol and reasonably slow
nucleation rate gave pointed, radial nuclei leading to the
observed anisotropic growth. Finally, with the growth of nuclei
and the extension in the Ostwald-ripening process, Mn and Co
nanoparticles would self-assemble into a porous structure, which
was composed of 3D flake-like (MnCo-1) or 3D flower-like
subunits (MnCo-2 and MnCo-3). The relevant chemical
reactions involved could be described by the following
equations:[10]
(
CO(NH
2
)
2
→ C
O → NH
3
H
6
N
6
+ 6NH
+ OH−
3
+ 3CO
2
Eq. 1
Eq. 2
Eq. 3
Eq. 4
+
NH + H
3
2
4
−
Mn + Co + 6OH → MnCo
2
(OH)
6
2
MnCo
2
(OH)
6
+ O
2
→ 2MnCo
2
O
4
2
+ 6H O
The morphologies and structures of the synthesized 3D Co3-
Mn samples were characterized by the FESEM and FE-
HRTEM techniques. It is noticed that the as-obtained Co3-
Mn samples with different Co/Mn molar ratios (Fig. 1)
followed by different growth mechanisms. SEM images in Fig.
a further reveal that the MnCo-1 particles (lower Co/Mn molar
x
x 4
O
x
x 4
O
1
ratio) had a flake-like framework (5-6 μm in diameter). The 3D
flake-like structures were composed of numerous two-
dimensional nanoplates (Fig. 1b). The high magnification
FESEM image of MnCo-1 shows the interconnected nanopetals
with an average thickness of 20 nm (Fig. S3). The multilayer
structure of flake-like MnCo-1 increased the surface area (32.2
Fig. 1 SEM and FESEM images of (a, b) MnCo-1, (c, d) MnCo-2, and (e, f)
2
MnCo-3.
m /g) of the composite, which was beneficial for toluene
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the Mn-rich (CoMn
2
4
O ) sample was likely to form a cubic spinel
structure (JCPDS PDF# 00-001-1130), as shown in Figs. 3b and
S6. The Co sample showed a cubic Co spinel and the
3
O
4
3 4
O
diffraction peaks corresponded to the standard sample (JCPDS
PDF# 00-074-2120). After the incorporation of Mn into the host
spinel matrix, the diffraction peaks of the MnCo samples were
broadened and the peak position was shifted to a lower degree
3 4
as compared with the pure Co O (Fig. S7), which might be
caused by the changes of the lattice parameters and stress from
lattice mismatching between Co and Mn. The crystallite size of
the MnCo sample reduced after Mn doping and such a behavior
was due to the restriction in the grain boundary movement in the
alien element-doped system with different ionic radii. The MnCo-
1 (Mn/Co molar ratio = 1) exhibited a homogeneous mixed
Fig. 2 (a, b) TEM and (c) FE-HRTEM images of the MnCo-3 sample, inset
EDS intensity line profiles extracted from the spectrum image along the line
drawn on image (d), and (d-h) EDS elemental maps for the MnCo-3 sample.
2 4 2 4
phase of tetragonal MnCo O spinel and cubic CoMn O at 2 =
To further demonstrate the composition of the as-obtained
MnCo-3 sample, the auxiliary-mapped via EDS was utilized to
detect the integrated intensity of Mn, Co, and O signals as a
function of the beam position when the FE-HRSEM/EDS was
operated in scanning mode (Fig. S5). To better characterize the
size and morphology of the 3D dandelion-like structure, TEM
and FE-HRTEM images of the MnCo-3 sample were obtained
18.2°, 32.9°, and 60.7°. Homogeneity of the sample could
improve the distribution of the Co-Mn mixed oxide, which was in
agreement with the EDS elemental mapping analysis (Fig. S5).
The reducibility and surface re-oxidizability of the as-obtained
Co-Mn oxide samples were studied using the H
TPD techniques, and the results are illustrated in Fig. 3c and d,
respectively. There were two peaks in the H -TPR profile of each
sample. The first reduction peak was in the range of 294-310 °C,
which was correlated with the stepwise reduction of Co
CoO and MnO → Mn
O4.[11] The second reduction peak in the
range of 366-460 °C was broad, which was associated with the
2 2
-TPR and O -
2
(
Fig. 2). The MnCo-3 structure consisted of nanoneedles
growing radially in the length of 500–800 nm with a hollow core
2-3 μm in diameter). The EDS element maps and line profile
3
4
O →
(
2
3
analysis of the MnCo-3 sample show that the three elements of
Mn, Co, and O were homogeneously distributed in the entire
particle (Fig. 2d-h).
Further evidence of the porous architecture can be seen from
2
the N adsorption-desorption isotherms (Fig. 3a), in which all of
0
[12]
3 4
subsequent reduction of CoO → Co and Mn O → MnO. It is
noticed that the small peak at 197-224ꢀ°C was caused by the
reduction of surface oxygen species formed at the oxygen
vacancies in the Co–Mn oxides. The result was in good
the catalysts exhibited a similar type II isotherm, suggesting the
presence of macropores and mesopores in the samples.
agreement with the results of the O
The similar results were also reported by Shi et al. who studied
the Mn
oxides for formaldehyde oxidation [12] and Jha et
2
-TPD as described below.
Hysteresis loops of types H3 in the relative pressure (p/p
of 0.8–1.0 was observed in the MnCo-3 sample, while the H2-
type hysteresis loops were seen in the p/p range of 0.4–0.8 for
0
) range
x
Co3−xO
4
al. who used the MnCo-MO catalyst for aerobic oxidation of
vanillyl alcohol.[
11]
0
the MnCo-1 and MnCo-2 samples. The result shown in MnCo-3
was related to the capillary condensation taking place in the
dandelion-like structure, affirming the formation of mesoporous
structure within the hollow core. An inset of Fig. 3a illustrates the
Barrett-Joyner-Halenda (BJH) pore-size distributions of the
samples. The results include one peak in the range of 3–5 nm,
indicating the existence of textural mesopores within the
structures, which was also confirmed by the FESEM and FE-
HRTEM results (Figs. 1 and 2). The BET surface areas, pore
volumes, and pore sizes of samples are summarized in Table 1.
The pore volumes of the as-obtained samples were in the range
2
As revealed in Fig. 3d of the O -TPD profiles, three oxygen
desorption peaks were observed: the one below 200 ꢀ°C, the one
below 400 °C, and the one above 650 °C, corresponding the
2
release of surface active oxygen (Oads, e.g., O
2
ˉ, O
2
ˉ or Oˉ),
surface lattice oxygen, and the thermal decomposition of the
bulk metal oxides (CoO , MnO and CoMnO ) (Olatt). As shown in
x
x
x
Fig. 3d, the Olatt decomposition of the MnCo-3 sample was much
larger and broader (in the range of 650-800 ꢀ°C) than that of the
other samples. Since the transfer of the bulk oxygen to the
surface sites happens first during the decomposition of lattice
oxygen in bulk oxides, the presence of surface oxygen
3
[13]
of 0.06−0.016 cm /g. The MnCo-3 sample, with a Co loading of
vacancies plays a critical role in bulk oxide decomposition.
A
x = 2.25 was found to have the a relatively large surface area of
shift to a lower temperature (298 °C) was observed in the Oads
peak for the MnCo-3 sample, demonstrating the richness of Oads
species on the surface of MnCo-3 and improved Olatt mobility
with of the rise in Co content. This strongly indicates that Olatt
mobility is significantly improved with the addition of cobalt to
manganese oxide. This would increase the O-vacancies, which
in turn increases the low-temperature adsorption. Overall
2
4
3
2
1.3 m /g. The MnCo-1 at x = 1.5 Co gave a surface area of
2
2.2 m /g and MnCo-2 (x = 1) gave the lowest surface area of
2
2.9 m /g. Hence the surface area decreases with increasing Co
loading. This might be caused by formation of the thinner
dandelion-like structure which contributed to the extra surface
area of the MnCo-3 sample.
The XRD patterns of the as-obtained samples are illustrated in
decreasing the activation of O
MnCo-3 sample. It is quite consistent with the results of the H
TPR investigations. Hence, lower onset temperatures of oxygen
2
molecules on the surface of the
Fig. 3b. The Co-rich (MnCo
O
2 4
) sample tended to form a
2
-
tetragonal spinel structure (JCPDS PDF# 00-077-0471), while
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desorption (Oads) and higher intensity of desorption peaks (Olatt
of the MnCo-3 sample resulted in better catalytic performance.
)
that at 230 °C (0.72 μmol/(gcats)) over MnCo-1. This result
suggests that Co was the active sites for the oxidation of toluene.
Among all of the samples, the MnCo-3 sample showed the
highest toluene oxidation rate (8.9 μmol/(gcats)), which was much
higher than that (3.3 μmol/(gcats)) over 3.4Au/3DOM
La0.6Sr0.4MnO
3
,[17] that (1.14 μmol/(gcats)) over 1.0Pt/Al
2 3
O –
that (5.51
[
18]
[19]
CeO
μmol/(gcats)) over 3.7Au/meso-Co
μmol/(gcats)) over 1.99AuPd/3DOM Co
2
,
that (4.54 μmol/(gcats)) over 0.3Pd/Al
2
O
3,
O4,[20] and that (5.70
O4.[21] Table S1 lists the
3
3
catalytic activities of various catalysts reported in the literature
for toluene oxidation. To probe the catalytic stability, an on-
stream reaction experiment was conducted over the CoMn-3
sample. The result showed that no obvious loss in catalytic
activity was detected within 70 h of reaction (Fig. S10).
Fig. 3 (a) N
distributions), (b) XRD patterns, (c) H
the MnCo-1, MnCo-2, and MnCo-3 samples.
2
adsorption-desorption isotherms (inset figure of BJH pore-size
2
-TPR profiles, and (d) O -TPD profiles of
2
Fig.
4 (a) Co 2p3/2 XPS spectra, and (b) toluene reaction rate versus
temperature over the MnCo-1, MnCo-2, and MnCo-3 samples.
XPS is an effective surface analysis technique to study the metal
oxidation states and adsorbed species of a solid solution sample. In this study, we used the in situ DRIFTS that is an efficient
Fig 4a illustrates the asymmetrical Co 2p3/2 XPS spectra of each
sample which could be decomposed into two components. The
component at binding energy (BE) of 780.4ꢀeV was due to the
surface Co3+ species, whereas the one at BEꢀ=ꢀ782.8ꢀeV
technology to monitor the transient species and states adsorbed
on the catalyst surface. The MnCo-3 sample with the highest
activity towards toluene oxidation was taken as an example to
study the reaction mechanism over the manganese-cobalt
catalyst. IR spectra of the MnCo-3 sample are shown in Fig.
5a.The absorption band at 3800-3600 cm-1 was ascribed to the
υ(M-OH) vibration and water molecules coordinated to the
surface metal cations M=Co and Mn, and the broad band
centered at ca.3400 cm-1 was assigned to the vibration band of
the H-bonding.[22] The bands in the two regions of 3200-2800
and 1600-1250 cm-1 were attributed to the features of toluene.
Four bands at 3068, 3028, 2925, and 2872 cm-1 in the 3200–
2800 cm-1 region were assigned to the υC–H stretching.[23] The
bands at 3068 and 3028 cm-1 were attributed to the C-H
stretching of the aromatic ring, and the bands at 2925 and 2872
cm-1 were assigned to the symmetric C-H stretching vibration of
methyl. In the low wavenumber range (Fig. 5b), the bands at
1596, 1538, and 1494 cm-1 were ascribed to the skeleton
stretching vibrations of the aromatic ring.[ With the rise in
temperature, the intensity of toluene bands decreased gradually,
and at the same time, some new IR bands appeared. The bands
at 1450 and 1415 cm-1 were caused by the asymmetric methyl
bending vibrations, and the ones at 1354 and 1304 cm-1 were
demonstrated the presence of the surface Co species.[14] The
2
+
3
+
2+
Co /Co molar ratio increased in the sequence of MnCo-
ꢀ<ꢀMnCo-2ꢀ<ꢀMnCo-3. It appears that the higher the amount of
1
3
+
exposed Co sites in the Co sample make oxidation more
favourable.[15-16] Hence, the MnCo-3 sample could show the
maximal oxidation activity.
The complete oxidation of toluene over the Co-Mn oxide
samples was carried out in a fixed-bed microreactor in the range
of 150300 °C. Under the conditions of toluene (concentration
2 2
1000 ppm) + O + N (balance) and the total flow rate of 33.4
mL/min, toluene conversion increased with the rise in
temperature. Evaluation of the catalytic performance of the
samples was conducted according to the procedure described in
2 2
Fig. S9. Toluene was fully oxidized to H O and CO over the Co-
24]
Mn oxide catalysts, and there were no products of incomplete
oxidation, as confirmed by the carbon balance (ca. 99.5%). In
order to compare catalytic activities of the samples, we used the
temperatures T10%, T50%, and T90% (corresponding to toluene
conversion =10, 50, and 90%). It can be observed that with a
rise in Co/Mn molar ratio from 1.0 (MnCo-1) to 4.0 (MnCo-3), the
caused by the bending vibrations of
(υ(CH
a
methylene group
)).[25-26] The bands at 1754, 1724, and 1672 cm were
-1
T90% decreased from 240 to 214 °C (Fig. S10). Toluene oxidation
2
rates at 230 °C normalized per gram of catalyst of the samples
are illustrated in Fig. 4b. Clearly, the catalytic performance was
improved with the rise in Co content. Toluene oxidation rate at
assigned to the υ(C=O) of an aldehyde group, together with the
-1
evidence of υ(C=C) (at 1649 and 1564 cm ) in the skeleton
vibrations of the aromatic ring, which demonstrated the
gneration of benzaldehyde.[24]
230 °C (3.55 μmol/(gcat s)) over MnCo-2 was much higher than
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Table 1. Weight percentages and Co/Mn molar ratio, BET surface areas, pore volumes, and pore sizes of MnCo-1, MnCo-2, and MnCo-3 samples.
Catalyst
Formula
Weight percentagea
Co/Mn molar ratio
Theoretical Measured
BET
Pore volume
Pore size
nm
3
surface area
cm /g
Co (wt%) Mn (wt%)
2
m /g
MnCo-1
MnCo-2
Co1.5Mn1.5
O
4
39.7
33.9
1
1.09
32.2
0.11
6.24
Co
2
Mn
1
O
4
52.3
60.0
20.7
12.8
2
4
2.35
4.38
22.9
41.3
0.06
0.16
6.15
6.75
MnCo-3
Co2.25Mn0.75
O
4
a
The data were obtained by the ICP-AES technique.
reaction, however, there were some obvious toluene peaks
remained, suggesting the strong adsorption of toluene on the
surface of the Mn-Co catalyst. The signal intensity of each peak
-1
2
except CO (at 2360 and 2341 cm ) in Fig. S12(B) did not
decrease obviously, indicating that the intermediate species had
a strong interaction with the catalyst surface. Combining the
results of DRIFTS characterization and previous works,[
24, 26, 28-29]
the oxidation of toluene could follow the pathways: benzyl
radical → benzaldehyde → benzene → oxalic acid, and finally to
CO and H O. Between the benzaldehyde and oxalic acid, there
2 2
were possible intermediate products, such as benzoic acids.
Conclusions
In summary, the low-cost transition metal-based Mn-Co
catalysts with a flower-like morphology were synthesized using a
simple template-free autoclave strategy. The Co2.25Mn0.75O
4
spinel sample with rich Co composition and well defined 3D
flower-like structure exhibited the best catalytic activity (100 %
toluene conversion at 239 °C) and stability (no obvious activity
drop after 70 h of on-stream reaction). The in situ DRIFTS
results have revealed the possible reaction pathway on the
synthetic Mn-Co catalyst, following the dissociation steps as
toluene to benzyl radical, benzaldehyde, benzene, oxalic acid,
Fig. 5 (a-c) In-situ DRIFT spectra of toluene oxidation over the MnCo-3
sample.
2 2
Moreover, similar bands (at 1760, 1725, 1690, 1660, 1645, 1558, and finally to CO and H O. The good activity and stability of
-
1
1
547 and 1512 cm ) have been reported for benzaldehyde
toluene oxidation on the resulted Mn-Co complex is raised from
the integrated effects by the large surface area, porous
morphology, rich surface oxygen vacancies and Mn-Co
interaction.
adsorption on TiO
2
and Degussa P25.[25-26] The results revealed
that weakly adsorbed benzaldehyde was formed during the
reaction.[26] Benzaldehyde molecules resulting from the partial
oxidation of toluene weakly interacted with the catalyst surface
and subsequently reacted with the hydroxyl groups on the
surface generated from the adsorbed water molecules. The
bands at 1709, 1691, and 1420 cm-1 were attributed to the C=O,
C-O, and C-C vibration of oxalic acid, which implies the
occurrence of the attack by the activated oxygen species or
hydroxyl radicals upon the carbon structures, confirming the
Acknowledgements: H. A. acknowledges financial support
through the USNW-Tsinghua Collaborative Research fund
(RG162774) and Research Fellowship (L0915/U2403) from The
University of Sydney.
x 4
Keywords: 3D flower-like catalyst; Co3-xMn O spinel; oxidation
of toluene; scalable synthesis; template-free strategy.
[
24, 27]
opening of the aromatic ring.
200-4000 cm spectra in Fig. 5c, the responses corresponding
From the whole range of
-1
1
-1
to CO
2
(at 2360 and 2341 cm ) were negative, which was due to
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FULL PAPER
FULL PAPER
[
a]
[a],[b],*
[c]
Yuan Wang , Hamidreza Arandiyan
,Yuxi Liu , Yijing
Flower-like spinel Co-Mn solid
solution are self-assembled using
the template-free strategy. The
low-cost transition metal-based
catalysts are active for toluene
oxidation. High adsorbed oxygen
[c]
[d]
[b]
[c]
Liang , Yue Peng , Stuart Bartlett , Hongxing Dai , Sadegh
Rostamnia[ and Junhua Li
e]
[d]
Page 1. – Page 6.
Template-free Scalable Synthesis of Flower-like Co3-xMn
Spinel Catalysts for Toluene Oxidation
x 4
O
species
enhance
catalytic
performance.
[
a]
Miss Y. Wang, Dr. H. Arandiyan
Particles and Catalysis Research Group, School of Chemical Engineering,
The University of New South Wales, Sydney, NSW 2052, Australia
[b]
Dr. H. Arandiyan, Dr. S. Bartlett
Laboratory of Advanced Catalysis for Sustainability, School of Chemistry,
The University of Sydney, Sydney 2006, Australia
hamid.arandiyan@sydney.edu.au (H. A.)
[
c]
Dr. Y. Liu, Mr. Y. Liang, Prof H. Dai
Beijing Key Laboratory for Green Catalysis and Separation, and Laboratory
of Catalysis Chemistry and Nanoscience, College of Environmental
and Energy Engineering, Beijing University of Technology, Beijing
100124, China
[d]
Dr. Y. Peng, Prof J. Li
State Key Joint Laboratory of Environment Simulation and Pollution
Control, School of Environment, Tsinghua University, Beijing
100084, China
[
e]
Prof. Sadegh Rostamnia
Organic and Nano Group (ONG), Department of Chemistry, Faculty of
Science, University of Maragheh, P.O. Box. 55181-83111,
Maragheh, Iran
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.