Effect of a mixed precursor over monolith MnO: X /La-Al2O3catalyst for toluene oxidation

Article abstract of DOI:10.1039/d0nj01432j

Monolithic MnOx/La-Al2O3 catalysts were prepared using manganese nitrate, manganese acetate and their mixed precursor for the catalytic oxidation of toluene. The activity results indicated that the MNMA catalyst prepared with the mixed precursor exhibited the best catalytic activity and that complete conversion of toluene was achieved at 270 °C under a GHSV of 12 000 h-1, which was 35 °C lower than the conversion of the MN catalyst prepared by manganese nitrate. N2 adsorption-desorption, X-ray diffraction (XRD), Raman spectroscopy, high resolution trans-mission electron microscopy (HRTEM), H2 temperature-programmed reduction (H2-TPR), O2 temperature-programmed desorption (O2-TPD), X-ray photoelectron spectroscopy (XPS) and in situ infrared diffuse reflectance spectroscopy (in situ DRIFTS) techniques were employed for the characterization of the catalysts' properties. It was shown that the MNMA catalyst has better MnOx dispersion and more active α-MnO2 species as well as higher concentrations of Mn4+, surface adsorbed oxygen species and active Mn-O bonds. Therefore, this catalyst possessed better reducibility and the best oxygen mobility, which were conducive to the catalytic oxidation of toluene. In addition, the DRIFT spectra results showed that more benzoic acid intermediate was deeply oxidized to carbonate species at a lower temperature on the surface of MNMA catalyst, which improved the low temperature activity for toluene oxidation. This journal is

Full text of DOI:10.1039/d0nj01432j

NJC
View Article Online
View Journal
PAPER
Effect of a mixed precursor over monolith
MnO /La–Al O catalyst for toluene oxidation
x
2 3
Cite this:DOI: 10.1039/d0nj01432j
ab
b
b
a
b
Xiaoying Zhou, Yingnan Shang, Wei Wei, Tao Lin, * Jiecai Wang,
a
a
ac
Xiaoxiao Lai, Jianli Wang
and Yaoqiang Chen
x 2 3
Monolithic MnO /La–Al O catalysts were prepared using manganese nitrate, manganese acetate and
their mixed precursor for the catalytic oxidation of toluene. The activity results indicated that the MNMA
catalyst prepared with the mixed precursor exhibited the best catalytic activity and that complete
ꢀ1
conversion of toluene was achieved at 270 1C under a GHSV of 12 000 h , which was 35 1C lower than
the conversion of the MN catalyst prepared by manganese nitrate. N adsorption–desorption, X-ray
diffraction (XRD), Raman spectroscopy, high resolution trans-mission electron microscopy (HRTEM),
temperature-programmed reduction (H -TPR), O temperature-programmed desorption (O -TPD),
2
H
2
2
2
2
X-ray photoelectron spectroscopy (XPS) and in situ infrared diffuse reflectance spectroscopy (in situ
DRIFTS) techniques were employed for the characterization of the catalysts’ properties. It was shown
x 2
that the MNMA catalyst has better MnO dispersion and more active a-MnO species as well as higher
4
+
Received 23rd March 2020,
Accepted 29th May 2020
concentrations of Mn , surface adsorbed oxygen species and active Mn–O bonds. Therefore, this
catalyst possessed better reducibility and the best oxygen mobility, which were conducive to the catalytic
oxidation of toluene. In addition, the DRIFT spectra results showed that more benzoic acid intermediate was
deeply oxidized to carbonate species at a lower temperature on the surface of MNMA catalyst, which
improved the low temperature activity for toluene oxidation.
DOI: 10.1039/d0nj01432j
rsc.li/njc
Biochemical methods are effective for some VOCs’ purification,
but they are selective and more sensitive to the concentrations
1
. Introduction
With the development of industry and transportation, emissions of organic waste gas. Adsorption-based techniques are favour-
of volatile organic compounds (VOCs) have become a notable able for the treatment of diluted VOCs but simply transfer VOCs
4
problem due to their adverse effects on both the environment from the gas phase to a solid phase. Compared with these
1
and human health. Benzene compounds and halogenated techniques, catalytic oxidation is one of the most effective and
hydrocarbons are the largest organic waste gas emissions from economically feasible techniques for the oxidation of VOCs into
stationary sources. Benzene compounds are highly toxic and CO
photochemically reactive substances which greatly threaten much lower temperatures (250–500 1C).
2
, water, and other relatively less harmful compounds at
5
human health and the environment. Of these, toluene is often
The design of high-performance catalysts is the key point
present from various sources, such as printing, pressing, petro- of research on catalytic oxidation technology. Generally,
chemical industries, etc., and is commonly chosen as a repre- catalysts for VOC oxidation are mainly noble metals and
2
sentative for the aromatic VOCs.
transition metal oxides. Although the high efficiency of noble
Currently, a large number of methods (e.g., adsorption, metals has been proved in the catalytic elimination of
thermal incineration, catalytic oxidation and biological treatment) VOCs, their high cost, low thermal stability, easy sintering
3
are used to alleviate organic waste gas. Among them, thermal and tendency to poison have limited their industrial applica-
incineration is a conventional technology that requires tempera- tion. The transition metal oxides usually used for catalytic
tures exceeding 800–1200 1C and leads to the formation of oxidation of VOCs have been considered optimal candi-
undesirable by-products such as dioxins and nitrogen oxides. dates to replace noble metal catalysts because of their lower
6
,7
price and higher resistance against poisoning. Hence, it is
necessary and significant to study the properties of transition
metal oxides and to improve their low temperature catalytic
activity.
Among the transition metal oxides, manganese-based
oxide is a potential candidate to replace noble metals due to
a
College of Chemistry, Sichuan University, Chengdu 610064, China.
E-mail: lintaochem@scu.edu.cn
b
c
Chengdu Research Academy of Environmental Sciences, Chengdu 610000, China
College of New Energy and Low-Carbon Technology, Sichuan University,
Chengdu 610064, China
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020
New J. Chem.

View Article Online
Paper
NJC
its distinctive physical and chemical properties, such as its 2. Experimental
8
multivalent nature and nonstoichiometric composition. Also,
2
.1. Catalyst preparation
manganese oxides have been proved to be low cost, environ-
mentally friendly, highly active and durable and have been
applied as catalysts for the oxidation of volatile organic
The La–Al O support (the mass ratio of La/Al is 5/95) was
2
3
supplied by Rhodia Company. Mn(NO
ꢁ4H
Mn(Ac) O (CP grade, Sichuan, China) were used as the
precursors. Three MnO /La–Al catalysts with 20 wt% MnO
2
were prepared. First, the single Mn(NO and Mn(Ac) O
3 2
) (Sichuan, China) and
1
2
2
substances and hydrocarbons. As reported in the literature,
x
2
O
3
x
the precursor of manganese oxide has a high correlation to the
9
10
3
)
2
2
ꢁ4H
catalytic activity of the catalyst. Arena et al. studied the
influence of precursors on the structure and reduction pattern
of Mn-based catalysts and revealed that manganese nitrate was
more effective than potassium permanganate due to the
precursor solutions were prepared. Meanwhile, the uniform
mixed precursor solution of Mn(NO3)2 and Mn(Ac) ꢁ4H O
2
2
with a molar ratio of 1 : 1 was prepared. Then, the three
obtained precursor solutions were impregnated on the
good MnO dispersibility and the improved reducibility of the
x
1
1
La–Al
2 3
O support by wet impregnation method. Afterwards,
active phase. Li et al. researched the effects of precursors on
the surface MnO species over MnO /TiO catalysts for NO
reduction and concluded that manganese nitrate precursor
primarily resulted in MnO formation and manganese acetate
precursor resulted in Mn formation; the higher activity of
MnO /TiO with manganese acetate as precursor was attributed
to higher surface MnO concentration and good dispersion of
these powders were dried at 80 1C overnight and calcined at
x
x
2
500 1C in air for 3 h.
x 2 3
In order to obtain monolithic MnO /La–Al O catalysts, the
2
catalyst powders were ball-milled with a suitable amount of
distilled water into homogeneous coating slurries and coated
onto honeycomb cordierite (Corning, USA, 400 cells per inch ,
2 3
O
x
2
2
x
3
ꢀ1
1
2
2.2 cm ) with the loading of each catalyst being about 160 g L .
MnO on the catalyst surface. Kapteij et al. revealed that the
x
The coated catalysts were dried at 80 1C overnight and calcined
at 500 1C in air for 3 h. The as-prepared catalyst with the mixed
precursor was marked as MNMA. The term MN indicates that
only Mn(NO
that only Mn(Ac)
MnOx crystallites in the catalyst prepared with manganese
nitrate precursor are primarily Mn(III) and Mn(IV) manganese
oxide microcrystals deposited in the outer region of alumina,
while manganese acetate precursor produced a highly dispersed
surface Mn(III) manganese oxide homogeneously distributed
throughout the alumina particles.
3
)
2
was present in the precursor and MA indicates
ꢁ4H O was present in the precursor.
2
2
2.2. Catalyst characterization
On the other hand, the porous material g-Al O has been
2
3
widely applied as a carrier for catalysts because of its large The textural properties of the samples were examined by the N2
surface area, porous structure and low cost. It has been adsorption–desorption method at ꢀ196 1C using a Quanta-
2 3
reported that metal oxides such as La O could increase the chrome automated surface area and pore size analyzer
2 3 2 3
thermal stability of g-Al O . La–Al O is commonly used in (Autosorb SI). In order to remove impurities on the sample
industrial applications such as three-way catalysts, compressed surface, the samples were degassed under vacuum at 300 1C for
natural gas (CNG) purification catalysts and other automotive 3 h before testing. The specific surface area and pore volume
1
3
exhaust catalysts.
From the above analysis, it can be seen that various man-
were calculated by the BET and BJH methods, respectively.
The powder X-ray diffraction (XRD) analyses were performed
ganese precursors produce different MnO species and cause on a D/max-RA diffractometer (Rigaku, Japan) with Cu Ka
x
different dispersion performances. Manganese nitrate precur- radiation (l = 0.15406 nm) operated at 40 mA and 40 kV. The
sor mainly produces high-valence MnO
improve the redox performance of the catalyst. Manganese with an interval of 0.061.
x
crystallites, which can intensity data were acquired in the range of 201 r 2y r 801
acetate precursor can uniformly disperse the MnO
throughout the catalyst and thereby increase the surface MnO
x
species
Raman spectra of the samples were obtained on a LabRAM
HR Laser Raman spectrometer (HORIBA JOBIN YVON, France)
x
concentration. Research on a mixed manganese precursor has using a diode pumped YAG laser with a 532 nm excitation
rarely been found even though the manganese precursor wavelength and laser power of 73.5 mW.
has been much reported in literature. In this paper, a mixed
High resolution transmission electron microscopy (HRTEM)
precursor of Mn(NO3)2 and Mn(Ac) ꢁ4H O was attempted to characterization was performed on a Tecnai G2 F20 S-TWIN
2
2
prepare MnO
monolith catalyst possesses the advantages of low pressure 300 kV.
drop, uniform flow distribution, short diffusion distance and Hydrogen temperature-programmed reduction (H
better thermal shock and attrition resistance. Thus, monolithic carried out on an auto-adsorption apparatus equipped with a
MnO /La–Al catalysts were prepared in this paper for the thermal conductivity detector (TCD). Before measurement,
x 2 3
/La–Al O catalyst for toluene oxidation. The TEM (FEI Company, USA) with an accelerating voltage of
2
-TPR) was
x
2 3
O
purpose of potential real application. The prepared catalysts 100 mg granules were pretreated at 450 1C in a flow of pure
were characterized by N adsorption–desorption, XRD, Raman, N for 40 min and then cooled to room temperature. After that,
2
2
HRTEM, H -TPR, O -TPD and XPS methods. In addition, the the reduction progress was monitored in a flowing gas mixture
2
2
ꢀ
1
ꢀ1
in situ DRIFTS technique was used to study the processes of of 5 vol% H
toluene adsorption/desorption and oxidation on the surface of
the catalysts.
2
in N
temperature-programmed desorption (O
performed on an auto-adsorption apparatus. Before testing, the
2
(20 ml min ) at heating rate of 8 1C min
.
The O
2
2
-TPD) was
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020

View Article Online
NJC
Paper
samples (50 mg) were pretreated in He flow at 450 1C for 30 min 3. Results and discussion
2 2
and then cooled to 50 1C. Then, a mixture flow of 5% O /N was
3
.1. Catalytic activity results
introduced for 1 h. After purging the excess oxygen, the catalysts
ꢀ
1
The catalytic performance of all samples was evaluated for
toluene oxidation and the conversion curves are shown in
Fig. 1. In our experiments, toluene was nearly completely
were heated from 50 1C to 800 1C at a rate of 10 1C min in a
helium atmosphere. The concentration signals of the desorbed
O
2
were monitored by a TCD detector.
2 2
oxidized into CO and H O, as detected by FT-IR spectroscopy
The X-ray photoelectron spectroscopy (XPS) measurements
during the oxidation process. The reaction temperatures T50%
and T100% (corresponding to the temperatures at which the
toluene conversion reached 50% and 100%, respectively) of the
catalysts are summarized in Table 1.
were carried out on an AXIS Ultra DLD spectrometer (Kratos,
England) with Al Ka radiation. The binding energies were
calibrated on the basis of C 1s binding energy (BE) at 284.6 eV.
In situ DRIFTS analysis was performed on a Fourier trans-
form infrared spectrometer (Thermo Nicolet 6700) equipped
with a high temperature environmental cell fitted with a KBr
window and an MCT detector. Before measurement, the
As shown in Fig. 1, it can be seen that there was almost no
2 3
toluene conversion on La–Al O before 300 1C. The catalyst
activity was closely related to the manganese precursor; the
MNMA catalyst had better catalytic activity than the MN and
MA catalysts. The T50 and T100 of toluene conversion over
MNMA were as low as 255 1C and 270 1C, respectively. The
completed conversion temperature T100 of MNMA decreased
about 35 1C compared to that of MN, illustrating that the low
temperature activity for toluene oxidation was obviously
improved on MNMA. Compared with other literature, the
activity of MNMA is higher than those of Mn/Al
which oxidized toluene completely only above 300 1C.
Meanwhile, its activity is even higher than that of the CuO–
MnO /Al , MnO–CeO /Al and CuO–CeO /Al catalysts,
which completely oxidize toluene at temperatures around
samples in the IR cell were swept with N
2
for 30 min at
400 1C and then cooled to 50 1C. Then, the samples were
2
exposed to a flow of toluene/N (100 ppm) until the adsorption
was saturated. Following the removal of reversibly and physi-
cally bound toluene from the surface, the chamber was heated
directly in a flow of N (or air). During the progress, the total
2
ꢀ1
velocity of flow was kept at 100 ml min . The final infrared
spectrograms were obtained with the difference of background
and test spectra collected with increasing reaction temperature.
The final infrared spectrum as a function of temperature was
obtained by subtracting the background spectrum from the
reaction spectrum.
2 3
O catalysts,
1
4,15
x
2
O
3
2
2
O
3
2
2 3
O
1
6
280 1C under the present experimental conditions.
2.3. Catalytic activity evaluation
The catalytic activities were measured in a conventional quartz
tube fixed-bed flow reactor (the length was 860 mm and the
diameter was 50 mm). The monolith catalyst (the length of the
monolith was 25 mm and the diameter was 10.5 mm) was
placed in the constant temperature zone of the reactor. Gaseous
toluene was generated by flowing air through liquid toluene in
the toluene saturator, which was kept at 0 1C under an ice-water
bath. Air was used as a balance gas and all gases were regulated by
ꢀ
1
mass flow controllers. The total flow rate was about 440 ml min
,
ꢀ
1
yielding a GHSV of 12 000 h with an initial toluene concentration
of 1000 ppm. Before testing, the samples were pretreated in
flowing air at 450 1C for 1 h, followed by cooling to the reaction
temperature in reaction gases. The outlet gas was analyzed
on-line at a given temperature by a gas chromatograph
Fig. 1 Conversion of toluene as a function of temperature over the
(
GC-2000) equipped with an FID detector. In order to investigate
catalysts, with the initial concentration of toluene at 1000 ppm under a
the products of toluene oxidation, FT-IR spectroscopy (Antaris
IGS, Thermo Fisher Scientific) was used during the reaction
process. According to the literature and our previous research,
ꢀ1
GHSV of 12 000 h
.
8
CO and H O were the only two reaction products under the Table 1 Summary of textural properties of catalysts, conversion tem-
2
2
peratures of toluene and total hydrogen consumption
present experimental conditions. Therefore, the conversion of
toluene (X) could be calculated as:
Catalytic
activity
Total hydrogen
consumption
Textural properties
SSA Pore volume Pore
Cin ꢀ Cout
T50 T100 (mmol H2
size (nm) (1C) (1C) per mg Mn)
X ¼
ꢂ 100%
(1)
2
ꢀ1
3
ꢀ1
Sample (m g ) (cm g
)
Cin
MN
MNMA 169
MA
La–Al
173
0.51
0.52
0.49
0.59
3.86
3.28
3.88
3.27
285 305 9.48
255 270 5.77
266 285 5.41
where Cin stands for the inlet concentration of toluene and
out is the outlet concentration of toluene.
169
187
C
2
O
3
460
—
—
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020
New J. Chem.

View Article Online
Paper
NJC
3
.2. Textural properties of catalysts
In order to characterize the textural properties of the catalysts,
adsorption–desorption experiments were performed. Fig. 2
exhibits the N adsorption–desorption isotherms and pore size
distributions of the samples. It can be seen that all samples
have similar N adsorption–desorption isotherms and pore size
N
2
2
2
distribution curves. The N adsorption–desorption isotherms of
2
all samples exhibited the characteristics of IV-type adsorption
isotherm curves (based on the IUPAC classification) and
distributions of nano-pore sizes in the range of 2–10 nm.
Therefore, all samples are classified as mesoporous materials.
The hysteresis loops of all samples were similar, characteristic
of a mixture of cylindrical pores and ink-bottle-type pores, or
1
7
cylindrical-ink-bottle-type pores.
Fig. 3 XRD patterns of the catalysts.
Table 1 summarizes the textural property results of the
samples. As shown in Table 1, the specific surface areas and
pore volumes of all supported catalysts were lower than those of
18,19
consistent with the results reported in the literature.
Different from the MN catalyst, only a small amount of weak
2
ꢀ1
3
ꢀ1
the parent support (187 m g , 0.59 cm g ), which might
result from the occupation of parts of Mn compounds in the
pores of the support during the preparation process. The
average pore size basically increased after loading the active
component. This can be further observed from the pore size
distributions of the samples (Fig. 2(b)). After the addition of the
active component, the pore diameter distributions of the
samples moved in the larger pore direction to some extent.
However, in general, the Mn precursor has little effect on the
textural properties of the catalysts.
Mn
the XRD spectrum of the MA catalyst. It has been demonstrated
that MnO can be dispersed more uniformly on the surface of
3 4
O diffraction peaks (JCPDS, No. 80-0382) were observed in
x
the catalyst when the precursor solution only contains manga-
nese acetate. However, no obvious manganese oxide phase was
observed in the XRD spectrum of the MNMA catalyst, indicating
a higher dispersion of MnO
those of MN and MA.
x
on the MNMA surface than on
3.4. Raman spectroscopy
3.3. Structure properties of catalysts
Raman spectroscopy is sensitive to M–O bond arrangements
The XRD patterns of the catalysts are exhibited in Fig. 3. The and was carried out to characterize the structural features of
diffraction peaks at 37.61, 39.51, 45.81 and 66.81 in all samples these catalysts. As shown in Fig. 4, all catalysts exhibited a wide
are ascribed to the g-Al
2 3
O phase (JCPDS, No. 29-0063). No band in the Raman shift region from the wave numbers of
ꢀ1
ꢀ1
diffraction peak of La oxide was detected, which may be due to 500 cm to 650 cm . For the MN catalyst, a strong peak at
the fact that the content of La is too low to be detected. After 642 cm was observed, attributed to the Mn–O lattice vibra-
adding the active component, many strong and sharp MnO2 tions of b-MnO .
3 4
diffraction peaks (JCPDS, No. 24-0735) and a small amount of 648 cm can be attributed to Mn–O–Mn stretching in Mn O .
ꢀ
1
2
0,21
For the MA catalyst, the Raman band at
2
ꢀ1
22
ꢀ
1
weak Mn
detected in the MN catalyst, indicating that the MnO
was the main phase presented on the MN catalyst, which was of a-MnO
2
O
3
diffraction peaks (JCPDS, No. 76-0150) were For the MNMA catalyst, the band centered at 615 cm in MNMA
2
species belongs to the symmetric stretching vibration of the Mn–O bond
2
3
2
.
Compared with MN and MA, the Raman bands in
Fig. 2
2
N adsorption–desorption isotherms (a) and pore size distribution patterns (b) of samples.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020

View Article Online
NJC
Paper
2
not detected in the XRD results, indicating that a-MnO may be
highly dispersed on the MNMA catalyst surface. Additionally,
MNMA possessed the most active Mn–O bonds and the best
oxygen mobility, which are beneficial to the oxidation reaction.
3.5. HRTEM analyze
To further visualize the phase microstructure of the catalysts in
detail, HRTEM characterization was performed. As exhibited in
Fig. 5(a), some clear lattice fringes were observed on the MN
catalyst and the crystal face distance measured to be 0.311 nm
2
4
could be assigned to the (110) plane of b-MnO
2
.
The lattice
spacing of 0.198 nm observed on the MA catalyst in Fig. 5(c)
were ascribed to the (213) plane of Mn O (JCPDS #80-0382).
3
4
Meanwhile, the lattice fringe with the distance of 0.238 nm
x 2 3
Fig. 4 Raman spectra of the MnO /La–Al O catalysts.
8
corresponding to the (121) plane of a-MnO (JCPDS #72-1982)
2
was also observed, in addition to the (213) plane of Mn
the MNMA catalyst, as shown in Fig. 5(b). The states of
meaning weakening Mn–O bonds and higher lattice oxygen manganese species present on the three catalysts were basically
3 4
O on
ꢀ
1
MNMA were shifted to a lower wavenumber of 615 cm
,
1
,18
mobility existed in the catalyst.
The results of Raman spectroscopy were basically consistent experiment. It verified that the manganese nitrate and manga-
with the XRD result that b-MnO is the main MnO species on nese acetate precursors mainly led to the formation of
MN and Mn is the main phase on MA. On the other hand, MnO and Mn O , respectively. Both MnO and Mn O were
MnO on MNMA mainly exists in the state of a-MnO
consistent with the results of the XRD analysis and the Raman
2
x
3 4
O
2
3
4
2
3 4
x
2
but was mainly formed by the mixed precursor. Compared with the MN
Fig. 5 HRTEM images of (a) MN, (b) MNMA, and (c) MA catalysts.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020
New J. Chem.

View Article Online
Paper
NJC
catalyst, the manganese oxides on the MNMA and MA catalysts
were better dispersed and the lattice fringes were more blurred,
which could be presumed to indicate inferior crystallinity of the
2
5
Mn species.
3
.6. Results of H -TPR experiments
2
The reducibility of samples was evaluated by H -TPR experi-
2
ments and the results are shown in Fig. 6. Three overlapped
reduction peaks can be clearly observed centered at 420 1C,
5
15 1C and 679 1C in the MN catalyst, corresponding to the
4
+
3+
3+
2+
reductions of Mn - Mn , Mn - Mn and Mn species
interacting with Al
2
6,27
2
O
3
, respectively.
2
Unlike MN, the H -TPR
profiles of the other two catalysts had a continuous and broad
reduction peak between 200 1C and 550 1C. Moreover, the
reduction peaks of these catalysts shift to a lower temperature
in comparison with that of MN, illustrating a better reduction
performance for these catalysts. It has been shown in literature
Fig. 7
2
O -TPD profiles of the catalysts.
a
that a decreased reduction temperature is associated with 400 1C can be ascribed to surface adsorbed oxygen species (O )
2
8,29
higher oxygen reactivity and oxygen mobility.
which could be easily desorbed as they are weakly bound to the
32
Compared with the other catalysts, MN had a narrower surface. The desorption temperatures of 400–600 1C were clas-
reduction peak, which is characteristic of the reduction beha- sified as surface lattice oxygen species and desorption tempera-
3
0
viour of bulk manganese oxides. In addition, the total hydro- tures higher than 600 1C were attributed to lattice oxygen species.
gen consumption of these catalysts is exhibited in Table 1. By
As shown in Fig. 7, it can be seen that the desorption curve
further observing the total hydrogen consumption of these of the MN catalyst was different from the others at high
samples, it can be seen that MN had the largest hydrogen temperatures. There were two obvious desorption peaks
consumption and the total hydrogen consumption of these detected in MN but only one broad desorption peak observed
catalysts gradually decreased with the addition of manganese in the other catalysts above 400 1C. This may be caused by
acetate in the precursor. Combined with the XRD result analysis, desorption of surface and bulk lattice oxygen species of MnO
this may be ascribed to the presence of more aggregated MnO
crystals. Moreover, the desorption curves of the other catalysts
2
2
species in the MN catalyst. Therefore, it can be concluded that were broader due to higher dispersion of MnO
compared with the MN and MA catalysts, the MNMA catalyst with MN.
compared
x
prepared with the mixed precursor exhibited excellent low-
temperature reduction performance.
Generally, surface adsorbed oxygen species are more easily
activated to be reactive oxygen species, which can more quickly
3
3,34
participate in catalytic reactions.
desorption (T ) and the oxygen desorption amounts below
00 1C were summarized and the results are exhibited in
The peak temperatures of
3
.7. Results of O
-TPD measurements were performed to investigate the surface
and bulk oxygen species and the profiles are shown in Fig. 7.
2
-TPD experiments
d
4
O
2
Table 2. The MNMA catalyst had the lowest desorption tem-
perature of 228 1C and the maximum oxygen desorption below
31
According to the literature, desorption temperatures below
400 1C, indicating that it had the best oxygen mobility and can
provide more available oxygen species in the oxidation of
toluene.
3.8. Results of XPS surface analysis
To study the chemical states of the catalysts, XPS measure-
ments of Mn 2p and O 1s were conducted. The results are
presented in Fig. 8 and Table 2. As exhibited in Fig. 8, the Mn
2p
3/2 peaks of all samples can be divided into three peaks. The
peak with the binding energy of 640.5–641.0 eV was attributed
2
+
to Mn species, while the binding energies between 641.6–
3
+
4+
6
42.2 eV and 642.9–643.6 eV were attributed to Mn and Mn
8
species, respectively. Additionally, a very weak peak at about
47 eV, marked with an arrow in Fig. 8(a), can be attributed to
the presence of satellite Mn species with lower valence states
6
2
+ 35,36
(Mn ).
Moreover, in order to further study the surface
Fig. 6
H
2
-TPR profiles of the catalysts.
oxygen species types of the catalyst, the O 1s peak of the sample
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020

View Article Online
NJC
Paper
2
Table 2 Results of O -TPD and curve-fittings in XPS of catalysts
O
2
-TPD results
Oxygen desorption below 400 1C
Surface atomic concentration (%)
4+
Sample
T
d
(1C)
(mol O
2
per g catalyst)
Mn
O
Al
O
sur/Ototal
Mn /Mntotal (%)
MN
MNMA
MA
277
228
263
0.185
0.264
0.198
1.62
3.29
3.27
65.01
65.29
67.83
33.37
31.42
28.90
57.51
66.90
59.47
33.84
41.25
35.78
unchanged. Among the catalysts, the Mn atom content on the
surface of MNMA was the largest. MNMA also had the highest
4
+
proportions of Mn /Mntotal and Osur/Ototal, suggesting that
4
+
MNMA had the largest surface Mn and Osur concentrations,
consistent with the results of the O -TPD experiments. The
2
Osur/Ototal of MNMA was 66.9, which was higher than those of
MA and MN. In connection with the XRD results, it can be seen
that MnO
resulting in more active compounds exposed on the catalyst
surface. TEM tests exhibit that both a-MnO and Mn coexist
x
species are well-dispersed in the MNMA catalyst,
2
3 4
O
on MNMA, which might contribute to the formation of
oxygen vacancies. It has been reported that organic molecules
adsorbed on high-valence MnO are more easily activated and
x
Osur can rapidly participate in catalytic reactions as a reactive
3
7
4+
oxygen species. Therefore, both surface Mn and Osur can
promote the progress of the catalytic oxidation reaction.
3.9. Results of in situ DRIFTS tests
In situ infrared diffuse reflectance spectroscopy (in situ DRIFTS)
was performed to investigate the adsorption–desorption
and adsorption–oxidation processes of toluene on the MN,
MNMA and MA catalysts and the results are shown in Fig. 9
and 10, respectively. As shown in Fig. 9, at lower temperatures
(
o300 1C), all samples exhibited a broad absorption peak in the
ꢀ1
range of 3000–3600 cm which can be ascribed to the absorp-
tion of water. Two bands around 1630 cm and 1512 cm
on the MN and MNMA catalysts and two absorption peaks at
3
8
ꢀ1
ꢀ1
ꢀ
1
ꢀ1
1
632 cm and 1502 cm on the MA catalyst all belonged to
3
9
the typical aromatic ring vibration peak, indicating that
toluene was preadsorbed on the catalyst at low temperature.
Furthermore, the band at 1394 cm for MN and the peak
at 1352 cm for MA and MNMA were classified as carboxylate
species. In addition, two consecutive peaks were detected
ꢀ1
x 2 3
Fig. 8 XPS profiles of catalysts: (a) Mn 2p and (b) O 1s of MnO /La–Al O
ꢀ
1
catalysts. The arrow marks the location of satellite features that aid in
chemical state identification.
4
0
ꢀ
1
ꢀ1
around 1192 cm and 1124 cm and were attributed to the
vibrational peaks of the C–O bond in alkoxide species. The
was divided into two peaks. The peak with a binding energy of formation of C–O bonds in alkoxide species may be due to the
5
30–530.3 eV was classified as lattice oxygen species (Olatt) and adsorption of methyl groups in the toluene molecule through
3
9
6 5 2
the peak with a binding energy of 533.1–531.3 eV was classified the benzyloxy (C H –CH –O) form on the catalyst surface. For
as surface-adsorbed oxygen species (Osur). As shown in Table 2, MN and MNMA, the typical aromatic ring vibration peak
as the proportion of manganese acetate in the precursor disappeared when the temperature reached 150 1C. Meanwhile,
4
+
increased, the proportions of Mn /Mn
increased and then decreased.
and Osur/Ototal first the characteristic peak of the surface hydroxyl group appeared
total
ꢀ
1 41
at 3732 cm
and the peak intensity increased with the
In addition, by further observing the change in the surface increase of temperature. For MN, a small peak for carboxyl
3
9
ꢀ1
contents of various elements, it could be found that the content vibration appeared at 1542 cm when the temperature was
of Mn atoms on the surface of the MN catalyst was the smallest. increased to 300 1C. At the same time, the peak of an isolated
ꢀ
1 42
With the addition of manganese acetate in the precursor, the hydroxyl group was also observed at 3606 cm
,
indicating
Mn atom content first increased and then remained basically that a further reaction has occurred. For MNMA, the peak of
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020
New J. Chem.

View Article Online
Paper
NJC
2
Fig. 9 In situ DRIFT spectra of toluene desorption as a function of temperature in a flow of N for catalysts (a) MN, (b) MNMA, and (c) MA.
ꢀ1
carboxyl vibration was detected at a lower temperature (200 1C) of the surface hydroxyl group was detected at 3606 cm and its
and the peak of the isolated hydroxyl group was observed at peak intensity gradually increased with increasing temperature.
ꢀ1
around 3606 cm at 300 1C. It can be inferred that the bulk It could be inferred that the toluene adsorbed on MNMA could
of lattice oxygen species in MN and MNMA catalysts was undergo an oxidation reaction at a lower temperature than on MN
more active and participated in the deep reaction of toluene and the carbonate species was further decomposed and formed a
oxidation.
For the MA catalyst, only a small carboxyl vibration peak was
detected at 1539 cm and no peaks of a surface hydroxyl group was detected only at 1626 cm . As the temperature increased
new product at 300 1C.
For the MA catalyst, the aromatic ring vibration peak
ꢀ1
ꢀ1
were detected. This indicated that the reaction of toluene to 300 1C, the band of the carbonate species appeared at
ꢀ1
oxidation on MA was not more apparent than on the MN and 1378 cm and the peak intensity increased significantly as
MNMA catalysts. the temperature increased. When the temperature continued to
Fig. 10 exhibits the results of in situ DRIFTS in air atmo- rise to 350 1C, the band of the hydroxyl group was detected at
ꢀ
1
sphere. As shown in Fig. 10, when the temperature was lower 3520 cm and the peak intensity increased with the tempera-
than 300 1C, the bands of each sample were similar to those in ture. However, it can be seen that the adsorption intensity of
Fig. 9. For the MN catalyst, in addition to the bands of absorbed the carbonate species and hydroxyl group on MA were weaker
water, aromatic ring vibration and the vibration of the C–O than those on MNMA.
ꢀ
1
bond in the alkoxide species, a band of carboxylate at 1368 cm
was detected with a peak shift to a lower wave number as the on the three catalysts, it was found that the toluene adsorbed
temperature increased. This indicated that benzoic acid species on MA and MNMA could further react with O under air
formed at low temperatures. When the temperature was increased atmosphere, indicating that MA and MNMA had higher reac-
By comparing the adsorption–oxidation reactions of toluene
2
ꢀ
1
to 300 1C, a new band appeared at 1381 cm and the peak tivities than MN. Among these, MNMA oxidized toluene at the
intensity gradually increased as the temperature increased, lowest temperature to form more carbonate species and surface
43
indicating the accumulation of carbonate species.
hydroxyl group species, demonstrating that the MNMA catalyst
For the MNMA catalyst, the band of carbonate species had the best low temperature reactivity.
ꢀ
1
observed at 1381 cm
(
appeared at a lower temperature
Usually, the formation of intermediates during toluene oxida-
39
250 1C) compared to that of MN. In addition, at 300 1C, the peak tion essentially involves high oxygen mobility. Combining the
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020

View Article Online
NJC
Paper
Fig. 10 In situ DRIFT spectra of toluene desorption as a function of temperature in air flow for catalysts (a) MN, (b) MNMA and (c) MA.
Raman and O -TPD results, the MNMA catalyst possesses more exist on the MNMA catalyst surface, which benefits contact and
2
active Mn–O bonds and higher oxygen mobility as well as higher reaction with toluene molecules. In addition, it can be seen that
concentrations of surface oxygen species, which facilitate the a-MnO
benzoic acid species deeply oxidizing to carbonate species at consistent with the research of Liang et al., who prepared a-,
lower temperatures and further results in increased catalytic b-, g- and d-MnO nanorod catalysts for CO oxidation and
activity. found that the catalytic activity of a-MnO was better than that
of b-MnO
2 2
had higher catalytic reactivity than b-MnO , which was
4
4
2
2
2
.
Additionally, it is generally believed that oxidation of hydro-
carbons takes place via a redox mechanism in which the rate
determining step is oxygen removal from the metal oxide. Thus,
3
.10. Discussion
From the structural characterization of the catalysts, it is
concluded that the precursor has an important influence on
both the crystal phase of MnO
catalyst surface. The difference between the two precursors is
the acid ion. As seen in reported literature
of structural characterization, Mn nitrate precursor mainly
produces high-valence MnO
strong oxidizing properties. However, Mn acetate precursor
mainly results in highly dispersed surface manganese oxides,
the oxygen mobility and reducibility of MnO
x
appear to be
x
and its dispersion on the
critical parameters that affect catalytic performance for VOC
1
6
oxidation. Compared to the MN catalyst, the MNMA catalyst
had a more active Mn–O bond and improved reducibility
properties, which were conducive to the progress of the redox
cycle in catalytic toluene oxidation and thus beneficial for the
reaction activity.
1
0,11
and the results
ꢀ
2
microcrystals due to NO
3
having
ꢀ
because CH
3
COO , as an organic acid ion, can reduce the
surface tension of the solution and thereby promote the
dispersion of manganese oxides on the catalyst surface. There-
4. Stability test of the MNMA catalyst
fore, there are different well dispersed manganese oxides To examine the stability of the as-prepared MNMA catalyst,
ꢀ
ꢀ
produced in the MNMA catalyst when NO3 and CH COO
time-on-stream reaction experiments at a certain temperature
3
are both present in the precursor solution, which leads to more (270 1C, at which the toluene was completely converted) were
active components exposed on the catalyst surface. As the XPS conducted for 30 hours. As shown in Fig. 11, no signifi-
result show, greater concentrations of Mn and oxygen species cant drop in catalytic oxidation of toluene was observed in
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020
New J. Chem.

View Article Online
Paper
NJC
C. Hirschmann, T. Nevanper ¨a , M. Riihim ¨a ki, M. Piril ¨a and
R. L. Keiski, Top. Catal., 2011, 54, 1224–1256.
4
5
W. B. Li, J. X. Wang and H. Gonga, Catal. Today, 2009, 148,
81–87.
C. Gennequin, T. Barakat, H. L. Tidahy, R. Cousin, J. F.
Lamonier, A. Abouka ¨ı s and S. Siffert, Catal. Today, 2010,
157, 191–197.
6
7
8
9
Y. X. H. Huang, Q. Feng and D. Y. C. Leung, Catal. Sci.
Technol., 2015, 5, 2649–2669.
D. W. Lee and B. R. Yoo, J. Ind. Eng. Chem., 2014, 20,
3947–3959.
P. Liu, H. He, G. Wei, D. Liu, X. Liang, T. Chen, J. Zhu and
R. Zhu, Microporous Mesoporous Mater., 2017, 239, 101–110.
b. H. P ´e reza, P. Navarrob, J. Jos ´e Delgadoc and M. Montesb,
Appl. Catal., A, 2011, 400, 238–248.
Fig. 11 Stability test of the MNMA catalyst for toluene oxidation at 270 1C
for 30 hours (reaction conditions: GHSV = 12 000 h , toluene concen-
tration = 1000 ppm).
ꢀ1
1
1
1
0 a. T. T. Francesco Arena, A. Carmelo Raimondoa and
A. Parmalianac, Phys. Chem. Chem. Phys., 2001, 3, 1911–1917.
1 J. Li, J. Chen, R. Ke, C. Luo and J. Hao, Catal. Commun.,
the long-term test over 30 h. The result suggested that the
MNMA catalyst demonstrates good stability for the catalytic
combustion of toluene.
2007, 8, 1896–1900.
2 F. Kapteijn, A. D. Vanlangeveld, J. A. Moulijn, A. Andreini,
M. A. Vuurman, A. M. Turek, J. M. Jehng and I. E. Wachs,
J. Catal., 1994, 150, 94–104.
1
3 J.-L. Wang, J.-B. Zhong, M.-C. Gong, Z.-M. Liu, M. Zhao and
Y.-Q. Chen, J. Environ. Sci., 2007, 19, 644–646.
5. Conclusion
The monolithic catalyst MnO /La–Al O prepared with a mixed 14 S. C. Kim, J. Hazard. Mater., 2002, B91, 285–299.
x
2 3
precursor of manganese nitrate and manganese acetate was 15 A. S. Fabiola, N. Aguero, B. P. Barbero and L. E. Cad u´ s,
used for catalytic toluene oxidation. The MnO phase, MnO
Catal. Lett., 2009, 128, 268–280.
dispersion and redox performance of the catalyst were studied. 16 S. M. Saqer, D. I. Kondarides and X. E. Verykios, Appl. Catal.,
It was found that the MNMA catalyst prepared with the mixed
B, 2011, 103, 275–286.
precursor exhibited the best catalytic activity and a complete 17 J. Wang, J. Wen and M. Shen, J. Phys. Chem. C, 2008, 112,
conversion temperature of toluene as low as 270 1C, which
5113–5122.
was 35 1C lower than that of the MN catalyst prepared with 18 F. N. Aguero, A. Scian, B. P. Barbero and L. E. Cad u´ s, Catal.
manganese nitrate. Combined with the characterization
Today, 2008, 133-135, 493–501.
results, the excellent toluene catalytic activity of MNMA was 19 L. Wang, H. He, C. Zhang, Y. Wang and B. Zhang, Chem.
ascribed to the more active and well-dispersed a-MnO species,
Eng. J., 2016, 288, 406–413.
excellent reducing properties, abundant surface oxygen species 20 J. Chen, X. Chen, Z. Xu, W.-J. Xu, J.-J. Li, H.-P. Jia and
x
x
2
4
+
and Mn species on the surface of the catalyst.
J. Chen, ChemistrySelect, 2016, 1, 4052–4056.
21 M. M. C. Juliena, R. Baddour-Hadjean, S. Franger, S. Bach
and J. P. Pereira-Ramos, Solid State Ionics, 2003, 159,
Conflicts of interest
3
45–356.
2
2 Y. Han, L. Chen, K. Ramesh, E. Widjaja, S. Chilukoti,
I. Kesumawinatasurjami and J. Chen, J. Catal., 2008, 253,
There are no conflicts to declare.
261–268.
Acknowledgements
23 F. Buciuman, F. Patcas, R. Craciun and D. R. T. Zahn, Phys.
Chem. Chem. Phys., 1999, 1, 185–190.
We gratefully appreciate the National Key Research and Devel-
opment Program of China (Grant No. 2016YFC0204901).
2
2
2
4 N. Huang, Z. Qu, C. Dong, Y. Qin and X. Duan, Appl. Catal.,
A, 2018, 560, 195–205.
5 G. Zhu, J. Zhu, W. Jiang, Z. Zhang, J. Wang, Y. Zhu and
Q. Zhang, Appl. Catal., B, 2017, 209, 729–737.
6 M. Wu, X. Wang, Q. Dai, Y. Gu and D. Li, Catal. Today, 2010,
158, 336–342.
Notes and references
1
2
3
H. Huang, Y. Xu, Q. Feng and D. Y. C. Leung, Catal. Sci.
Technol., 2015, 5, 2649–2669.
S. C. Kim, Y.-K. Park and J. W. Nah, Powder Technol., 2014,
27 C. Zhang, C. Wang, S. Gil, A. Boreave, L. Retailleau, Y. Guo,
J. L. Valverde and A. Giroir-Fendler, Appl. Catal., B, 2017,
201, 552–560.
266, 292–298.
S. Ojala, S. Pitk ¨a aho, T. Laitinen, N. Niskala Koivikko, 28 K. Zhou, X. Wang, X. Sun, Q. Peng and Y. Li, J. Catal., 2005,
R. Brahmi, J. Ga ´a lov ´a , L. Matejova, A. Kucherov, S. P ¨a iv ¨a rinta,
229, 206–212.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020

View Article Online
NJC
Paper
2
3
3
3
3
3
3
3
9 N. Miniajluk, J. Trawczy n´ ski and M. Zawadzki, Appl. Catal., 37 Z. Hou, J. Feng, T. Lin, H. Zhang, X. Zhou and Y. Chen, Appl.
A, 2017, 531, 119–128. Surf. Sci., 2018, 434, 82–90.
0 A. L. Svatopluk Chytil, E. Vanhaecke and E. A. Blekkanb, 38 M. Kantcheva and I. Cayirtepe, Catal. Lett., 2007, 115,
Energy Procedia, 2012, 26, 98–106. 148–162.
1 J. Feng, Z.-Y. Hou, X.-Y. Zhou, H.-L. Zhang, T.-Q. Cheng, 39 Z. L. A. Hong Suna, S. Chenb and X. Quanb, Chem. Eng. J.,
T. Lin and Y.-Q. Chen, Chem. Pap., 2017, 72, 161–172. 2015, 58–65.
2 X. Lin, S. Li, H. He, Z. Wu, J. Wu, L. Chen, D. Ye and M. Fu, 40 E. L. S. Besselmann and M. Muhler, J. Mol. Catal. A: Chem.,
Appl. Catal., B, 2018, 223, 91–102. 2000, 162, 401–411.
3 A. T. Daniela Terribile, C. de Leitenburg, A. Primavera and 41 Y. Liao, X. Zhang, R. Peng, M. Zhao and D. Ye, Appl. Surf.
G. Dolcetti, Catal. Today, 1999, 47, 133–140. Sci., 2017, 405, 20–28.
4 H. Muroyama, H. Asajima, S. Hano, T. Matsui and K. Eguchi, 42 H. Lin, J. Long, Q. Gu, W. Zhang, R. Ruan, Z. Li and
Appl. Catal., A, 2015, 489, 235–240. X. Wang, Phys. Chem. Chem. Phys., 2012, 14, 9468.
5 B. S. A. S. D. Gardner, Surf. Interface Anal., 1998, 26, 43 W. Yang, Z. A. Su, Z. Xu, W. Yang, Y. Peng and J. Li, Appl.
035–1049. Catal., B, 2020, 260, 118150.
6 S. Azalim, R. Brahmi, M. Agunaou, A. Beaurain, J. M. Giraudon 44 S. Liang, F. Teng, G. Bulgan, R. Zong and Y. Zhu, J. Phys.
and J. F. Lamonier, Chem. Eng. J., 2013, 223, 536–546. Chem. C, 2008, 112, 5307–5315.
1
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020
New J. Chem.