Rod-like CuMnO:X transformed from mixed oxide particles by alkaline hydrothermal treatment as a novel catalyst for catalytic combustion of toluene

Article abstract of DOI:10.1039/c6cp03433k

Rod-like copper manganese mixed oxides (CuMn_x-NR) have been synthesized from copper manganese mixed oxide particles by sodium hydroxide hydrothermal treatment, and a higher BET surface area of 221 m² g^-1 is obtained on the nanorod-like sample, which exhibits superior catalytic activity toward toluene combustion at 210 °C due to the increase in its oxygen mobility of the chemisorbed oxygen species as well as the increase in surface concentrations of higher valance cations, Cu²⁺, Mn³⁺ and Mn⁴⁺, in the samples.

Full text of DOI:10.1039/c6cp03433k

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Rod-like CuMnOx transformed from the mixed oxide particles by
alkaline hydrothermal treatment as a novel catalyst for catalytic
combustion of toluene
DOI: 10.1039/x0xx00000x
W. B. Li,*a,b Z. X. Liu, a,b R. F. Liu a,b , J. L. Chen a,b, B. Q. Xu a
www.rsc.org/
activities. Combustion method,12 hydrothermal method13 and sol-
The rod-like copper manganese mixed oxides (CuMnx-NR) have
been synthesized from the copper manganese mixed oxide
particles by the sodium hydroxide hydrothermal treatment, and
the higher BET surface area of 221 m2g-1 is obtained on the
nanorod-like sample, which exhibit superior catalytic activity
toward toluene combustion at 210 °C due to the increase in its
oxygen mobility of the chemisorbed oxygen species as well as the
increase in surface concentrations of the higher valance cations,
Cu2+, Mn3+ and Mn4+ in the samples.
gel method14 have also been applied to optimize the densities of
catalytic active sites. Nanomaterials due to high surface areas and
high densities of active site are considered to be excellent choices.15,
16 Nanofiber potassium-containing manganese oxide octahedral
(OMS-2) catalysts of ~10−30 nm in width and 200−350 nm in length
were synthesized by Genuino et al.17 To further enhance the low
temperature catalytic activity, Solsona et al.18 added Au particles to
CuMnOx nanowires and improved stability of catalyst, while the
addition of gold to CuMnOx did not lower the light-off temperature
significantly. In addition, Li et al.19 reported the higher oxidation
activity on the bimetallic Cu-Mn/MCM-41 supported catalyst, and
explained that the highly dispersed Cu-Mn mixed oxides on
mesoporous structures probably provide active sites for the
complete oxidation of toluene on these mesoporous catalysts. The
surface concentration of high valence cations (Cu2+, Mn3+ and Mn4+)
and chemisorbed oxygen species were essential for catalytic
performance.20 Suzuki et al reported that TiO2-derived nanotubes
could be prepared by hydrothermal treatment of TiO2 powder in
NaOH aqueous solution. 21 Particularly, the MnO2 nanorods22 and
MnO2 nanowires23 were synthesized by the hydrothermal method
and showed high activities for CO Oxidation22 and high-
Introduction
There have been extensive efforts to remove volatile organic
compounds (VOCs) pollutants such as toluene in many industry
plants because they are poisonous, smelly and carcinogenic.1
Catalytic combustion2 is one of the most effective approaches
among the various VOC purification technologies, where both
noble metal catalysts and non-noble metal catalysts have been
widely investigated. Noble metal catalysts such as Au, Pd and Pt
generally show good catalytic activities at low temperatures.3-8 For
example, Uchisawa et al.4 reported that 90% of toluene conversion
on their Pt catalysts at as low as 200 °C ; While transitional metal
oxides are regarded to have the lower cost, higher stability and the
better resistance to toxicity as compared to the noble metals, and
supercapacitor performances, 23 repectively. Herein, nanorod-like
copper manganese mixed oxides were synthesized for the first time
by an alkaline hydrothermal method and tested for the toluene
10
Cu-Ce oxide9 and CoOx catalysts, have also been investigated on
their catalytic performance towards toluene oxidation and their
catalytic performances at low temperatures still need to be combustion.
improved.
Copper manganese oxides have showed a promising catalytic
activity towards toluene oxidation at low temperatures. Li et al.11
synthesized CuMnOx catalyst, which was very close to the activity of
the widely investigated Pd catalyst, giving complete combustion of
toluene at about 220 °C. However, the metal oxides agglomeration
during the calcination restricts further improvement of catalytic
Experimental methods
Synthesis of CuMnOx
The CuMnOx powder was prepared using a coprecipitation
procedure. First, different Cu/Mn ratio solutions were prepared by
mixing aqueous solutions of Cu(CH3COO)2 (0.25 mol/L) and
Mn(CH3COO)2 (0.25 mol/L). the temperature of the pre-mixed
solution was maintained at 80 °C. Aqueous Na2CO3 (0.25 mol/l) was
added to the mixed solution with continuously stirring until pH
reached 8~10. The resulting precipitate was then left stirring for
30min and aged for 12 h at 80 °C. The precipitate was then filtered,
a.Department of Chemistry, Tsinghua University, Beijing 100084, China. E-mail:
wbli@tsinghua.edu.cn, wblichem@163.com,
b.Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China. †
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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washed six times with distilled water. The precipitate was dried at Mn2O3 peaks become weaker on the rod-like CuMn2-NR samples
View Article Online
95 °C for 10 h to give a material which was subsequently calcined at than the CuMn2 particle samples. The same DsiOtuI:a1t0io.1n03g9o/eCs6CfoPr0M34n33OKx
500 °C to obtain the CuMnOx powder. The obtained catalyst were and rod-like MnOx-NR, which implies that the hydrothermal
denoted as CuMn2 or MnOx.
treatment led to a decrease in the sample crystallinity. The
CuMn2O4 phase mainly provides surface excess oxygen, the surface
excess oxygen are more easily combine with toluene molecules，
which were adsorbed on the catalyst surface, and proceed
oxidation and reduction reaction 25. The π bond on the aromatic
Synthesis of CuMnOx nanorods
0.13 g of the CuMnOx or MnOx powder was mixed with 10 ml of
10 mol/L NaOH aqueous solution in a 20ml Teflon-lined autoclave,
followed by hydrothermal treatment at 110 °C for 20 h. The treated
powders were washed 4 times with distilled water followed by 0.1
mol/L HCl aqueous solution washed and ultrasound for 30mins.
Then the obtained materials were washed 2 times with distilled
water, dried at 80 °C for 12 h, and calcined at 250 °C for 1 h. The
obtained catalyst were denoted as CuMn2-NR or MnOx-NR.
ring could form complexation with Cu2+, the d electron of Cu2+ could
26
then be fed back to the π bond of the toluene molecule
.
Therefore, the existence of CuO could turn the toluene molecules
into active state.


CuMn O
Mn O

CuO
4
3
2
2
Characterization of the catalysts


Scanning electron microscopy (SEM) analysis was performed on a
HITACHI S4800 with pre-coating samples with gold. Transmission
electron microscopic (TEM) was carried out on a Hitachi model H-
800 transmission electron at an accelerating voltage of 100 kV. The
sample was ultrasonically dispersed in ethanol. The BET surface
area of the obtained materials was measured using nitrogen as
adsorption gas at 77 K on a Micrometrics ASAP 2020 instrument.
Powder XRD data of the samples were collected on Rigaku D/max
RB X-ray diffractometer using Cu Kα Rradiation. The TPR
experiments were collected in a conventional flow apparatus
equipped with a TC-detector. The sample was pretreated at 500 °C
for 1 h in 40 ml/min (STP) before reduction. A 40 ml/min (NTP) gas
flow of 5% H2 in N2 passed over 35 mg samples in a quartz reactor
to the detector. The reduction temperature was linearly raised at
10 °C /min from 50 to 500 °C, and then kept at 500 °C for 20 min. X-
ray photoelectron spectroscopy (XPS) was performed on a
performed with a PHI5300 XPS spectrometer with Al Kα radiation
Calibrated with the C 1s=284.6 eV.

CuMn -NR
2


















CuMn
2





MnO -NR
x









MnO
x



10
20
30
40
50
60
70
80
2θ (°)
Fig. 1 XRD patterns of the obtained samples
Catalysts Testing
The catalysts were tested for toluene combustion using a fixed-
bed 10 mm i.d. quartz tubular. The gas mixture used argon as
balance gas, consisting of toluene (0.35 vol.%), O2 (9 vol.%), at
controlled rates using mass flow controllers passed continuously
through a 0.2 g (40-60 mesh) catalyst sample bed with a total flow
rate of 60 ml/min. The outlet gas compositions were analyzed after
stepwise changes in the reaction temperatures by an on-line gas
chromatograph (GC-2010, Shimadzu) with a FID detector and a TCD
detector.
Results and discussion
Fig. 1 shows the XRD patterns of the obtained samples. Peaks at
2θ value of 18.43°, 30.32°, 35.72°, and 43.42° could be indexed to
(111), (220), (311) and (400) planes of CuMn2O4 (PDF 74-1921),
respectively. Peaks at 2θ value of 32.93°, 55.04°, and 65.62° could
be indexed to the (222), (440) and (622) planes of Mn2O3 (PDF 71-
0636), respectively. Peaks at 2θ value of 32.47°, 38.68°, and 68.00°
could be indexed to the (110), (111) and (220) planes of CuO (PDF：
80-1916), respectively.
Fig. 2 SEM images of CuMn2 (a) and CuMn2-NR (b); TEM images of CuMn2
(c) and CuMn2-NR (d)
Fig. 2(a,b) show SEM images of CuMn2 and their derivative
nanorod-like materials, with the lower and higher magnification
pictures. Obviously, the CuMn2 samples mainly present the particle
morphology with agglomeration particle diameters of 0.2~0.4 μm,
The catalyst calcined at 500 °C has a relatively crystallized
catalyst and is composed of the stoichiometric spinel phase
CuMn2O4, which contribute to the toluene combustion.24 The
2 | PCCP, 2016, 00, 1-3
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and these CuMn2 particles were obviously transformed into rod-like activities, which gives the lowest combustion temVpieewraAtruticrlee,Oni.lein.e,
structures after alkaline hydrothermal treatment. High 210 °C , for complete toluene combustionD, OwI:h1ic0h.10i3s9m/Cu6cChP0b3e4t3t3eKr
magnification TEM images demonstrate that each solid particle is than 225 °C on the CuMn2 mixed oxide particle sample.
composed of the nano-particles with a size ranging between 10-20
In order to understand the novel catalytic activity on rod-like
nm (Fig. 2c). Fig. 2d shows that the obtained rods on the CuMn2-NR catalysts as compared to the corresponding particle samples, XPS
sample have typical diameters of 10~20 nm and lengths up to about technique was used to analyze surface composition as well as the
400 nm. To the best of our knowledge, this is the first report for the chemical states of the metal cations and oxygen anions on these
preparation of nanorod-like copper manganese mixed oxides with samples. As shown in Fig. 4A, the Mn 2p3/2 binding energies for
a diameter of 10~20 nm via alkaline hydrothermal process. The CuMn2 and CuMn2-NR are 641.9 eV and 642.1 eV, respectively. The
samples transferred from solid particle structure to rod-like Mn 2p3/2 spectra can be separated into three peaks: Mn2+ peak,
morphology during alkaline hydrothermal treatment.
Mn3+ peak, and Mn4+ peak, with binding energy 640.3–640.7,
After the hydrothermal treatment by concentrated NaOH 641.8–642.0 and 643.2–644.5 eV, respectively.28 The relative
solution and rinsed by dilute HCl, rod-like morphology replaces amount of Mn2+, Mn3+, and Mn4+ in these catalysts could also be
most of the particle clusters. The BET surface area of CuMn2 is essential for catalytic performance.
almost twice that of MnOx, which may be due to the unique
The Mn 2p signal of the CuMn2 sample after the peak-fitting
reconstruction of CuMn2O4 species in CuMn2 particles, and the XRD deconvolution shows a relative amount of Mn2+, Mn3+, and Mn4+ as
results show the crystallinity of rod-like CuMn2-NR samples 37.05%, 42.61%, 20.33%, respectively. Correspondingly, after the
decreased simultaneously, which is probably closely related with alkaline hydrothermal process, the relative amount Mn2+ decrease
the catalytic activity as discussed hereafter. Generally, solid samples to 4.6% while Mn3+ increase to 55.0% and Mn4+ increase to 40.40%
with different morphology expose different crystal planes. The in the CuMn2-NR samples. While the Mn4+ and Mn3+ lead to a higher
adsorption and the reaction rate are also different when the activity by taking part in the oxidation and reduction reaction,
toluene molecules are absorbed on catalysts with different Cu2++Mn3+↔Cu++Mn4+.29 The hydrothermal process has increased
morphologies 27
.
high valance cationic content and promote the oxidation and
reduction reaction which are essential for catalytic performance.
The O 1s binding energies are nearly not changed, with 529.7
100
CuMn -NR
2
MnOx-NR
eV(CuMn2) and 529.8 eV(CuMn2-NR) as shown in Fig. 4B. The
chemisorbed oxygen species (donated as Oα) at high binding energy
(531.24 eV) and the lattice oxygen species (donated as Oβ) at low
binding energy (529.6–529.9 eV) are separated by peak-fitting
deconvolution of the O 1s XPS spectra.-30 Obviously, after the
hydrothermal process, the relative ratio of Oα on the CuMn2-NR
surface is increased 29.22% from 25.7% on the CuMn2 while the Oβ
has decreased from 74.30% to 70.80% . Chemisorbed oxygen
species are more active for the catalytic combustion than the lattice
oxygen species due to their higher mobility.
80
60
40
20
CuMn
2
MnOx
0
120
140
160
180
200
220
240
Temprature (℃)
Fig. 3 Catalytic activity curves of toluene combustion on various
catalysts
Fig. 3 shows the catalytic activity curves of toluene combustion
on CuMn2 and MnOx particle samples as well as rod-like CuMn2
and MnOx samples. The BET surface area of the obtained rod-like
CuMn2-NR sample is as high as 221 m2g-1 as compared to value of
73 m2g-1 on CuMn2 particle sample. The Cu:Mn ratios of the CuMn2
and CuMn2-NR samples are both with ratios of Cu:Mn =1:2.Toluene
conversion were recorded in
a
fixed-bed reactor when
consecutively increasing temperature at a program of 5 °C /min, and
the fixed reaction temperature for 20 min to measure the data to
ensure the data accuracy. The final products of the reaction were
CO2 and H2O, and no other by-products were detected. As shown in
Figure 3, the catalytic activity on the Cu-Mn mixed metal oxide is
higher than MnOx single metal oxide, and the samples after the
alkaline hydrothermal treatment , i.e., CuMn2-NR and MnOx-NR
samples are better than the corresponding particle samples. For
example, the rod-like CuMn2-NR sample exhibits the best catalytic
Fig.4 (A) Mn 2p XPS spectra; (B) O 1s XPS spectra; (C) Cu 2p XPS spectra; (D)
H2-TPR profiles of CuMn2 and CuMn2-NR
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Fig. 4C shows Cu 2p XPS spectra of CuMn2, CuMn2-NR samples. statement partially explains why the alkaline hydrothVeierwmAarlticpleroOcnelinses
DOI: 10.1039/C6CP03433K
The Cu 2p signal indicates the existence of two copper species, i.e., enhances the catalytic activity of the mixed oxide catalysts.
Cu+ at 930.6 eV and Cu2+ at 932.2 and 934.0 eV.31 The Atomic peak-
Fig. 4D shows the H2-TPR profiles of CuMn2 and CuMn2-NR. The
fitting deconvolution result shows that after the hydrothermal
process, the Cu+ cation has decreased from 4.23% to 0.71%.
reduction peak of CuMn2, with a maximum temperature at 350 °C ,
which can be assigned to the reduction of surface and bulk Mn3+ to
Mn2+,34 is shifted to a larger peak at 315 °C corresponding to
manganese species with CuMn2O4 on the rod-like CuMn2-NR.
Additionally the reduction peaks of CuMn2 at around 300 °C ,
assigned to the reduction of Cu2+ to Cu ,17 is shifted to a smaller
peak at 250 °C corresponding to the rod-like CuMn2-NR after the
alkaline hydrothermal process. Fig. 4D implies that the sample is
easily to be reduced and its redox properties of the CuMn2O4 phase
has increased after the sodium hydroxide hydrothermal process,16
which explains why the sample after the alkaline hydrothermal
treatment could show the higher catalytic activity and also indicates
that probably the CuMn2O4 phase playing a key role in oxidizing
toluene.
The oxidation and reduction reaction, Cu2++Mn3+↔Cu++Mn4+,
32
is considered as the critical source of superior catalytic activity
.
,
-
The reaction proceed on the catalyst surface: Oα+ Cu+↔ Cu2+ +Oad
could produce Oad which could react with toluene directly.33 The
alkaline hydrothermal process increased the activity of the catalyst
markedly by increasing the surface concentration of the higher
valence cations of Cu2+, Mn3+ and Mn4+, which facilitated the
catalytic combustion. After the hydrothermal process, the relative
-
-
ratio of Oα on surface is also increased. Then, Oad would also
increased due to the Oα+ Cu+↔ Cu2+ +Oad-.Thus, the above
Table 1 Comparison of catalytic activity for toluene combustion over various noble metal catalysts.
Catalyst
SBET /(m2·g -1)
GHSV(h−1)
VOCs (ppm)
T10a (℃)
T50a (℃)
T95a (℃)
Reference
CuMnOx
Pt/Fibrous Silica
Pt/Mesoporous Silica
Pd/Co3AlO
220.98
36000
27000
60000
30000
15000
20000
20000
40000
3500
200
1000
800
1800
1000
1000
1000
150
180
160
190
140
138
182
145
170
175
170
210
185
201
234
164
200
200
180
230
221
226
255
168
this work
4
3
7
8
6
690
93
18
32.7
318
36.2
Pd/LaFeO3
Au/LaMnO3
32
Au/SiO2
35
36
33
Au-Pd/Co3O4
a Temperatures at which 10%, 50%, and 95% conversion of VOCs
responsible for the increase in the catalytic activity on the
nanorod-like CuMn2-NR sample.
Table 1 shows the comparison of catalytic activity for toluene
combustion over various noble metal catalysts. It seems that the
obtained rod-like CuMn2-NR materials would be a potential
alternative to noble metal catalysts regarding catalytic combustion
of VOCs at lower temperature.
Acknowledgements
This work was funded by the National Key Basic Research
Program NO. 2013CB933103 funded by MOST and the
Program for the Fundamental Research Supported by
Shenzhen Science and Technology Innovations Council of
China (Grant No. JSF201006300047A, No. JC201105201126A,
and No. ZDSY20120619140933512).
Conclusions
In conclusion, using copper manganese mixed oxides as
precursor, rod-like CuMnOx samples with diameters of 10-
20nm are synthesized by a sodium hydroxide hydrothermal
method. It is shown that the BET surface area was increased
from 78 m2g-1 to 221 m2g-1 on the sample with molar ratio of
Cu/Mn=1:2 after being calcined at 500 °C . In addition, the
increase in the surface concentration of Cu2+, Mn3+ and Mn4+
with higher valence states in the nanorod-like sample after the
sodium hydroxide hydrothermal process could be responsible
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