A template-free method for preparation of MnO2 catalysts with high surface areas

Article abstract of DOI:10.1016/j.cattod.2016.12.044

High surface-area manganese oxides catalysts are widely employed in catalytic oxidation of volatile organic compounds (VOCs) because of their high efficiency and good stability. However, template agents are usually introduced to the synthesis process to create the high surface area (>150 m²/g). It is quite expensive and complicated, restricting the widespread applications. Herein, a template-free method was reported here to prepare high surface-area MnO₂ catalysts by selectively removing La elements from LaMnO₃ perovskites using acid treatment. The obtained MnO₂ catalysts possessed excellent catalytic activity in toluene oxidation. Besides, the reserved La elements after acid treatment could be recycled to repeat the preparation process. After three cyclic processes, the catalytic performances were improved gradually, which may provide the possibility for the practical application in VOCs elimination.

Full text of DOI:10.1016/j.cattod.2016.12.044

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A template-free method for preparation of MnO2 catalysts with high
surface areas
Yonghui Wei∗, Lei Ni, Minxia Li, Jili Zhao
College of Chemistry and Biology, Beihua University, Jilin 132013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
High surface-area manganese oxides catalysts are widely employed in catalytic oxidation of volatile
organic compounds (VOCs) because of their high efficiency and good stability. However, template agents
are usually introduced to the synthesis process to create the high surface area (>150 m2/g). It is quite
expensive and complicated, restricting the widespread applications. Herein, a template-free method was
reported here to prepare high surface-area MnO2 catalysts by selectively removing La elements from
LaMnO3 perovskites using acid treatment. The obtained MnO2 catalysts possessed excellent catalytic
activity in toluene oxidation. Besides, the reserved La elements after acid treatment could be recycled to
repeat the preparation process. After three cyclic processes, the catalytic performances were improved
gradually, which may provide the possibility for the practical application in VOCs elimination.
© 2016 Elsevier B.V. All rights reserved.
Received 9 November 2016
Received in revised form
12 December 2016
Accepted 27 December 2016
Available online xxx
Keywords:
MnO2 catalysts
Template-free method
Toluene oxidation
1. Introduction
to be recycled, which causes the waste of resources and restricts the
widespread applications. Design of template-free methods for the
Volatile organic compounds (VOCs) are severely harmful to the
human and environment due to their high biological toxicity and
extensive sources [1–3]. Catalytic oxidation is considered to be a
promising technology for the elimination of VOCs because of its
high efficiency and non-secondary pollution. And the key point on
this technique is the design of high-performance catalysts [4–8].
Noble metals catalysts, which show excellent catalytic activity for
VOCs oxidation, are limited to the practical applications owing to
their high-cost and low thermal stability [9–11]. Transition metal
oxides, especially for manganese oxides, are optimal candidates on
VOCs oxidation due to their tunable multiple valences, low price
and good stability [12,13]. A great deal of experiments has been
performed to improve the activity of manganese oxides on VOCs
oxidation, in which increasing the surface area is the most effec-
tive one [14–16]. The manganese oxides with high surface area
possess plenty of active lattice oxygen species (Olatt), which play
a key role in the process of VOCs oxidation and accelerate the cat-
alytic reaction (Mars–van Krevelen mechanism) [17,18]. However,
template agents are usually introduced to the synthesis process
of manganese oxides to create the high surface area (>150 m2/g)
[12,19–21]. This is so expensive and makes the synthesis process
much complicated. In addition, the worn-out template agent is hard
preparation of manganese oxides with high surface areas meet the
demand of industrial production and become increasing important
for the elimination of VOCs.
Herein, we report a facile template-free method to prepare
high surface-area MnO2 catalysts by selectively removing La ele-
ments from LaMnO3 (LMO) perovskites using acid treatment. The
obtained material exhibited excellent catalytic activity in toluene
oxidation. Furthermore, the reserved La elements could be recy-
cled to repeat the whole synthesis process (Scheme 1). After three
cyclic processes, the catalytic performances on toluene oxidation
were enhanced gradually. This technology provides a new route to
develop high surface-area transition metal oxides by template-free
synthesis.
2. Experimental
2.1. Preparation of LMO sample
LMO sample was prepared by a traditional sol-gel method and
calcined at 800 ◦C in air. The final black product was LMO sample.
The details are given in the supporting information.
2.2. Cyclic synthesis of MnO2 sample
∗
The MnO2 sample was prepared using a selective dissolution
method by immersing the LMO powder in dilute HNO3 solution
Corresponding author.
E-mail address: bhhxwyh@163.com (Y. Wei).
http://dx.doi.org/10.1016/j.cattod.2016.12.044
0920-5861/© 2016 Elsevier B.V. All rights reserved.
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Scheme 1. Schematic illustration for the cyclic synthesis process of MnO2 samples.
until no bubbles could be observed. The La3+ could be selectively
dissolved in acid, leaving behind MnO2. After filtered, the filtrate
containing La3+ was reserved, and the solid was washed by abun-
dant of deionized water. Then the MnO2 sample was dried at 60 ◦C
over night. The reserved La3+ solution was used to prepare LMO
sample by adding the same amount of Mn2+ agent as before. Then
the synthetic procedures for LMO and MnO2 were repeated as
mentioned above. After repeated for 3 times, the resulting MnO2
samples were denoted as MnO2-1, MnO2-2 and MnO2-3.
2.3. Characterization
The samples were characterized by X-ray diffraction (XRD),
N2 adsorption–desorption (Brunauer–Emmett–Teller, BET), X-
ray photoelectron spectroscopy (XPS), hydrogen temperature-
programmed reduction (H2-TPR) and oxygen temperature-
programmed desorption (O2-TPD). The detailed characterization
procedures are described in the Supporting Information.
Fig. 1. (a) Activity profiles and (b) Arrhenius plots of toluene oxidation over the
four samples. Reaction conditions: 2000 ppm toluene, 20% O2 and N2 (balance gas),
GHSV = 120000 mL/(g h).
2.4. Catalytic activity test
cations in the obtained MnO2 samples, which could be proved by
XPS results (Table 1). The amount of La elements in MnO2 sam-
ples decreased gradually with the increase of the cycle times. N2
adsorption–desorption isotherms were used to detect the changes
of surfaces areas of the four samples (Fig. S2). The surface areas
of the three MnO2 catalysts were 147–203 m2/g (Table 1), over
10 times higher than that of LMO, which suggested that the high
surface-area MnO2 catalysts could be obtained by such selective
removal method.
The catalytic performance for the toluene oxidation were inves-
tigated by a catalyst evaluation system using a continuous flow
fixed-bed quartz microreactor (i.d. = 6.0 mm). The gaseous toluene
was generated by passing N2 flow through a bottle containing
pure toluene in an ice-water isothermal bath. The gas mixture was
composed of 2000 ppm toluene, 20% O2 and N2 (balance gas). The
total flow rate was 100 mL/min, and the catalyst dosage was 0.05 g
(40–60 mesh). The concentrations of toluene were monitored by a
gas chromatograph (Agilent 7890A) equipped with a flame ioniza-
tion detector (FID) and a thermal conductivity detector (TCD).
3.2. Catalytic performance
3. Results and discussion
The catalytic performances of the four samples on toluene oxi-
dation are shown in Fig. 1(a). The activity profiles indicate that the
catalytic activity increased with the reaction temperature raised.
The LMO catalyst expressed a relatively low activity before 280 ◦C.
However, after the acid treatment, the catalytic performance of the
obtained MnO2 samples improved significantly. In order to com-
pare the catalytic activities of the samples, T50 and T90 (the reaction
temperature corresponding to the toluene conversion of 50% and
90%, respectively) were introduced here. For MnO2-1, MnO2-2 and
MnO2-3 samples, T50 are 247, 238, and 233 ◦C and T90 are 263, 259
and 256 ◦C, respectively (shown in Table 1). Therefore, it is sug-
gested that the activities of the three samples enhanced in the order
of MnO2-1 < MnO2-2 < MnO2-3. We used the apparent activation
3.1. Crystal structure and surface area
The crystal structures of the initial LMO sample and the MnO2
catalysts obtained by the three cycles (MnO2-1, MnO2-2 and MnO2-
3) were confirmed by XRD (Fig. S1). It could be found that the LMO
sample was single-phase and showed a highly crystalline charac-
ter, indexed as the perovskite phase [17]. After the acid treatment,
MnO2 species were formed and no LMO structure remained, which
proved that the dilute nitric acid could selectively attack La-O bonds
and remove La elements from LMO, leaving behind MnO2 materi-
als. It is worth noting that there still existed a small amount of La
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Table 1
Surface elements compositions, BET surface areas, toluene oxidation activity, and apparent activation energies (Ea) of the four samples.
XPS
BET surface area (m2/g)
toluene oxidation activity and
apparent activation energy
La/Mn molar ratio
Mn4+/Mn3+ molar ratio
Olatt/Oads molar ratio
T50 (◦C)
T90 (◦C)
Ea (kJ/mol)
LMO
1.38
0.13
0.11
0.07
0.37
0.55
0.84
1.56
2.00
2.08
4.01
5.26
11.0
–
–
71
58
57
55
MnO2-1
MnO2-2
MnO2-3
147.3
156.7
202.9
247
238
233
263
259
256
Fig. 2. H2-TPR profiles of the four samples.
energy (Ea) as a probe to clearly identify the differences among the
initial LMO sample and the MnO2 catalysts. The lower the Ea value
is, the easier the toluene can be oxidized. Fig. 1(b) illustrates the Ea
values of the four samples in toluene oxidation. It could be found
that the Ea values decreased from LMO to MnO2-3. The Ea values of
the MnO2 samples were obviously lower than that of LMO, indicat-
ing that the catalytic activity was significantly improved after the
acid treatment. Meanwhile, the Ea values of MnO2 samples were
decreased from MnO2-1 to MnO2-3 with the increase of cycle times,
which indicated that the catalytic activity in toluene oxidation was
not decreased after cyclic reaction, but increased gradually. This is
an infusive result and provides the possibility with the practical
application on VOCs removal.
3.3. H2-TPR experiment
Fig. 3. (a) Mn 2p3/2 XPS spectra, (b) O 1s XPS spectra of the four samples.
A series of experiments were carried out to explain the rea-
sons for the improvement on the catalytic activity. H2-TPR is an
ideal technique to investigate the reducibility of the catalysts. Fig. 2
shows the H2-TPR data of the four samples. It could be found that
the reduced peaks of LMO sample were extremely weak, exhibiting
poor reducibility. After the removal of La elements, the reduction
peaks shifted to lower temperature and the intensity of the peaks
were enhanced, which indicating the reducibility of the MnO2 sam-
ples was improved significantly. For all of the three MnO2 samples,
the two peaks can be assigned to the reduction of Mn4+ to Mn3+
(early one) and Mn3+ to Mn2+ (late one) in MnO2, respectively
[22]. The peaks shifted to low temperature with the increase of the
cycle times, which suggested that the reducibility became better
gradually. The MnO2 samples possessed excellent low-temperature
reducibility, which may result in the improvement on the catalytic
performance over the toluene oxidation.
3.4. XPS study
Fig. 3 illustrates Mn 2p3/2 and O 1s XPS spectra of the four sam-
ples and the element compositions data are listed in Table 1. It could
be found that the surface La/Mn atomic ratio for LMO (1.38) was
obviously higher than the theoretical value (1.00). This result sug-
gested that the native surface of LMO was preferentially occupied
by La cations, which was in accord with the literature [17,18]. After
selective removal, the surface La/Mn atomic ratio for MnO2-1 sam-
ple decreased to 0.13, indicating that the surface La elements were
indeed removed. The differences in the surface La/Mn atomic ratios
may bring varied performances to LMO perovskite and MnO2 sam-
ples on catalytic activity, because the catalytic reaction of toluene
at low temperatures usually occurred on the surface transition
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samples, the peak intensity of Olatt at around 530 ◦C became strong
and the Olatt peak after 800 ◦C moved to low temperature, which
suggests that the MnO2-3 sample possessed the largest amount of
Olatt and best Olatt mobility. It is universally acknowledged that the
oxidation of VOCs over transition metal oxides catalysts take place
on the basis of the Mars–van Krevelen type redox cycle, and the
nucleophilic attack of the Olatt result in the occurrence of this reac-
tion [25]. Thus the Olatt species play a vital role in catalytic reaction.
The large amount of Olatt and the low Olatt desorption temperature
may improve the catalytic activity for MnO2-3 sample in toluene
oxidation.
Based on the above results, the reason for the enhancement
on catalytic activity in toluene oxidation was proposed. LMO per-
ovskite was an unsatisfied catalyst in toluene oxidation. The general
explanation is that the perovskites usually possess low surface
areas, which restrict the contact between the VOCs molecules and
the surface of the catalysts [26,27]. In addition, it is well accepted
that the oxidation of toluene at low temperatures usually took place
on the surface transition metal ion-sites (such as Mn cations). The
native surfaces of perovskites are preferentially occupied by A-site
cations, which are not catalytically active [28–30]. Both of these
reasons restrict the catalytic performance of perovsikes on toluene
oxidation. Although the acid treatment method was reported in the
early literature, only surface A sites were partially removed [31]. In
this work, MnO2 catalysts were born with high surface areas and
exposed active Mn elements on the surface of the catalysts after the
thorough acid treatment, which brought better catalytic activities
of MnO2 catalysts than that of the LMO sample. Then the cycle reac-
tion was carried out and three manganese oxides (MnO2-1, MnO2-2
and MnO2-3) were obtained. The catalytic activity was enhanced
in the order of MnO2-1 < MnO2-2 < MnO2-3. With the increase of
cycle times, the amount of La elements decreased gradually from
MnO2-1 to MnO2-3, which resulted in the increase of the surface
areas. In addition, the Mn4+/Mn3+ ratios also increased with the
decrease of the amount of La elements in MnO2 samples, which
may take a great role in toluene oxidation. Meanwhile, the con-
centration and mobility of Olatt were improved with the increase
of the cycle times. This accelerated the process of the Mars–van
Krevelen type redox cycle and then enhanced the catalytic activ-
ity in toluene oxidation. The rise in surface areas, Mn4+/Mn3+ ratios
and active oxygen species concentration and mobility, especially in
Olatt concentration and mobility would be responsible for the high
performances on toluene oxidation.
Fig. 4. (a) O2-TPD profiles of the four samples, (b) normalization of O2-TPD peak
areas for the three MnO2 samples. (Oads: peaks at around 460 ◦C, Olatt-1: peaks at
around 530 ◦C, Olatt-2: peaks at around 830 ◦C, the area value of the three peaks for
MnO2-1 was considered as 1).
metal ion-sites (such as Mn cations) [23]. The surface Mn4+/Mn3+
and lattice oxygen/adsorbed oxygen (Olatt/Oads) molar ratios also
have an important influence on the catalytic performance of man-
ganese oxides. By making quantitative analysis on the XPS spectra,
it could be found that the surface Mn4+/Mn3+ molar ratio increased
from LMO to MnO2 after the acid treatment, which was owing to
In order to investigate the catalytic stability of the MnO2-3 sam-
ple, a 36h-uninterrupted reaction was performed (Fig. S3). Two
complete runs of catalytic tests (including heating process and cool-
ing process) were carried out initially, and then followed a 12 h
of on-stream reaction at reaction temperature of 252 ◦C, at last
another two complete runs of catalytic tests were carried out. The
activity of MnO2-3 decreased little during the total running time of
the test, which indicated that the catalyst possessed a good catalytic
stability.
the following reaction: 2Mn3+solid → Mn4+solid + Mn2+
[17,24].
liquid
This high surface Mn4+/Mn3+ ratio may improve the catalytic
activity in toluene oxidation. In addition, it was also found that
the Olatt/Oads molar ratio increased in the order of LMO < MnO2-
1 < MnO2-2 < MnO2-3, which means that the surface active oxygen
species concentration, especially the Olatt concentration, increased
after the selective removal. The better mobility of active oxygen
would improve the catalytic performance of MnO2 samples for the
total oxidation of toluene.
4. Conclusions
3.5. O2-TPD experiment
In summary, a template-free method was reported to prepare
a high surface-area MnO2 catalyst by selectively removing La ele-
ments from LMO perovskites with acid treatment. The obtained
MnO2 catalysts possessed much better performance on toluene
oxidation than LMO. Then the reserved La elements were recycled
to repeat the whole synthesis process. The catalytic activity was
improved gradually with the increase of the cycle time. What’s
more, the obtained MnO2 catalyst also exhibited a good stability
during a 36h-uninterrupted reaction, which provided a possibility
for the practical elimination of toluene.
The O2-TPD results could further confirm the changes of oxy-
gen species after the acid treatment. The oxygen desorption peak
at low temperature (<500 ◦C) is ascribed to Oads,and the peak at
high temperature (>500 ◦C) is ascribed to Olatt [22]. Fig. 4 shows
that the intensity of desorption peaks for MnO2 samples was obvi-
ously stronger than that for LMO sample. With the increase of cycle
times, the Oads shifted to lower temperatures, proving that the Oads
got easier to be released from MnO2-1 to MnO2-3. For the MnO2
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This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
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