Transient in-situ DRIFTS Investigation of Catalytic Oxidation of Toluene over α-, γ- and β-MnO2

Article abstract of DOI:10.1002/cctc.201901839

Manganese oxides with different crystal phases (i. e. α-MnO₂, γ-MnO₂ and β-MnO₂) were synthesized for catalytic oxidation of toluene. The catalytic activity was strongly influenced by the crystal phases of MnO₂. α-MnO₂ exhibited the highest catalytic activity with a 50 % toluene conversion (T₅₀) at 229 °C and a 90 % toluene conversion (T₉₀) at 238 °C, followed by γ-MnO₂ (T₉₀=252 °C) and β-MnO₂ (T₉₀=278 °C). The remarkable catalytic activity of α-MnO₂ was attributed to its superior redox property and lattice oxygen mobility. Different intermediates were detected on the surfaces of α-, γ- and β-MnO₂ by in situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS). Benzoate species were found to be the key intermediates on α-MnO₂ at 240 °C, while benzoyl oxide species and benzoate were the major intermediates on γ-MnO₂ at 260 °C. The surface of β-MnO₂ was mainly covered by carbonate and benzoate species after the adsorption of toluene at 280 °C.

Full text of DOI:10.1002/cctc.201901839

Accepted Article
Title: Transient in-situ DRIFTS investigation of catalytic oxidation of
toluene over α-, γ- and β-MnO2
Authors: Jie Fan, Quanming Ren, Shengpeng Mo, Yuhang Sun, Mingli
Fu, Junliang Wu, Limin Chen, Peirong Chen, and Daiqi Ye
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ChemCatChem
10.1002/cctc.201901839
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Transient in-situ DRIFTS investigation of catalytic oxidation of
toluene over α-, γ- and β-MnO
2
Jie Fan, [a] Quanming Ren, [a] Shengpeng Mo, [a] Yuhang Sun, [a] Mingli Fu, [a,b,c] Junliang Wu, [a,b,c] Limin
Chen, [a,b,c] Peirong Chen, [a,b,c] and Daiqi Ye*[a,b,c]
Abstract: Manganese oxides with different crystal phases (i.e. α-
MnO , γ-MnO and β-MnO ) were synthesized for catalytic oxidation
of toluene. The catalytic activity was strongly influenced by the crystal
phases of MnO . α-MnO exhibited the highest catalytic activity with a
0% toluene conversion (T50) at 229 °C and a 90% toluene conversion
90) at 238 °C, followed by γ-MnO (T90=252 °C) and β-MnO
90=278 °C). The remarkable catalytic activity of α-MnO was
temperatures [4], disadvantages such as high cost, easy sintering
and poisoning have restricted their application. Therefore, it is
highly desirable to develop economical and efficient low-
temperature catalysts.
In recent decades, non-noble metal catalysts including Ce,
Co, Fe and Mn-based catalysts have been studied to substitute
noble metal catalysts in toluene degradation [5]. Among them,
2
2
2
2
2
5
(T
2
2
(T
2
attributed to its superior redox property and lattice oxygen mobility.
x
MnO catalysts have been intensively investigated thanks to their
Different intermediates were detected on the surfaces of α-, γ- and β-
remarkable low-temperature redox property, which is essential for
the complete oxidation of toluene at low temperatures [
6]
.
MnO
spectroscopy (in-situ DRIFTS). Benzoate species were found to be
the key intermediates on α-MnO at 240 °C, while benzoyl oxide
species and benzoate were the major intermediates on γ-MnO at
60 °C. The surface of β-MnO was mainly covered by carbonate and
2
by in situ diffuse reflectance infrared Fourier transform
Furthermore, MnO
α-, β-, γ-, δ- and ε-MnO
arranging of [MnO ] basic units, and they usually exhibited
2
with various crystallographic forms, such as
2
2
, could be synthesized through the orderly
2
6
2
2
distinguished structures sensitive to different catalytic VOCs
oxidation reactions. Hu et al. [7] investigated the effect of crystal
benzoate species after the adsorption of toluene at 280 °C.
phase of MnO
combustion. The catalytic activities in benzene combustion over
various manganese oxides decreased in the order: γ-MnO > β-
. Zeng and Xing et al. [8] found that the
MnO > α-MnO >δ-MnO
higher catalytic activity in o-xylene was observed over ordered
2
on the catalytic performance of benzene
2
Introduction
2
2
2
Volatile organic compounds (VOCs), such as toluene,
emitted mainly from house-hold activities and industrial processes,
not only induce various atmospheric pollution problems (ozone
and photochemical smog), but also severely threat the health of
human beings [ . Currently, numerous technologies have been
investigated for VOCs control, such as adsorption, photocatalysis,
photothermal catalysis and thermal catalytic oxidation [2]. Among
mesoporous γ‑MnO
2
catalyst compared with mesoporous β-MnO
2
2
nanorods. Zhang et al. explored the
[9]
and nonporous α-MnO
catalytic activity of α-, β-, γ- and δ-type MnO
The δ-MnO catalyst exhibited the highest activity among the four
catalysts and achieved nearly a complete HCHO conversion at
0 °C. It was believed that the tunnel structure and active lattice
oxygen species were the main factors that determined the
. Li et al. [10] verified the important
excellent performance of δ-MnO
2
in HCHO oxidation.
2
1]
8
2 2
them, thermal catalytic oxidation of toluene into CO and H O has
2
been regarded as one of the most promising techniques due to its
high destructive efficiency and less harmful by-products [3]. The
key issue of this technique is the development of high-
performance catalysts. Although noble metal catalysts (Pt, Pd and
Au, etc.) usually exhibit high removal efficiency at low
promoting effect of oxygen vacancies for the remarkable low-
temperature catalytic activity in toluene oxidation through a
comparative study of δ- and β-MnO
oxygen vacancies presented a higher catalytic activity. Yet, effect
of crystal phase of MnO was mainly investigated in catalytic
2 2
. δ-MnO with abundant
2
oxidation of benzene, o-xylene, HCHO and toluene, and mainly
focused on the structure-performance relationship. Less attention
has been paid to the reactivity of adsorbed surface species on
manganese oxides with different crystal phases during the
catalytic oxidation reactions. Considering that the understanding
of the reaction mechanism is crucial for the targeted design of
high-performance catalysts for toluene oxidation, it is necessary
to explore the reactivity of adsorbed surface species during
toluene oxidation over manganese oxides with different crystal
phases.
In the present work, a series of manganese oxides with
2 2 2
different crystal phases, including α-MnO , γ-MnO and β-MnO ,
were successfully synthesized and applied in the catalytic
oxidation of toluene. The synthesized manganese oxides were
investigated by X-ray diffraction, Scanning electron microscopy ,
[
a]
J. Fan, Q. Ren, S. Mo, Y. Sun, M. Fu, J. Wu, L. Chen, P. Chen and
Prof. D. Ye
School of Environment and Energy
South China University of Technology
Guangzhou, 510006, China
E-mail: cedqye@scut.edu.com
M. Fu, J. Wu, L. Chen, P. Chen and Prof. D. Ye
Guangdong Provincial Key Laboratory of Atmospheric Environment
and Pollution Control (SCUT)
South China University of Technology
Guangzhou, 510006, China
M. Fu, J. Wu, L. Chen, P. Chen and Prof. D. Ye
National Engineering Laboratory for VOCs Pollution Control
Technology and Equipment
South China University of Technology
Guangzhou, 510006, China
[
b]
c]
[
2
Transmission electron microscopy, H temperature-programmed
Supporting information for this article is given via a link at the end of the
document.
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10.1002/cctc.201901839
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reduction, O
photoelectron spectroscopy. The reactivity of different adsorbed
surface species over α-MnO , γ-MnO and β-MnO were studied
2
temperature-programmed desorption and X-ray
3.2 Structural and morphological analysis
2
2
2
by in situ diffuse reflectance infrared Fourier transform
spectroscopy (in-situ DRIFTS).
Results and Discussion
3
.1 Catalytic performance
2 2 2
Figure 2. XRD patterns of α-MnO , γ-MnO and β-MnO
The XRD of α-MnO
Figure 2. Compared with the standard XRD patterns, α-MnO
β-MnO showed well-defined diffraction characteristics of
tetragonal cryptomelane type α-MnO (JCPDS NO 44-0141) and
pyrolusite β-MnO
(JCPDS NO 24-0735), respectively [11].
However, compared with that of α-MnO and β-MnO , diffraction
peaks for γ-MnO (JCPDS NO 14-0644) were greatly lower in
intensity and broader in width, implying the poor crystallinity of γ-
MnO . The results of XRD demonstrated the successful synthesis
of α-MnO , γ-MnO and β-MnO
2
, γ-MnO
2
and β-MnO
2
are depicted in
2
and
2
2
2
2
2
2
Figure 1. Conversion of toluene oxidation over α-MnO
2 2 2
, γ-MnO and β-MnO .
Reaction conditions: 0.1 g catalyst and 0.4 g quartz sands, 1000 ppm toluene,
2
-
1
-1
2
2 2
0% O /N and WHSV = 48,000 mL·g ·h .
2
2
2
.
2
The catalytic activity of manganese oxides (α, β and γ-MnO )
with different crystal phases was evaluated in toluene oxidation
and the results are shown in Figure 1. It could be found that
toluene could be completely decomposed into CO
all the samples below 290 °C. Notably, among these catalysts, α-
MnO exhibited the highest toluene conversion at the lowest
temperature with T50 = 229 °C and T90 = 238 °C. The catalytic
activity of other samples was followed by γ-MnO (T50=243 °C and
90=252 °C) and β-MnO (T50=266 °C and T90=278 °C). The T90 of
α-MnO was 40 °C lower than that of β-MnO , indicating the
2 2
and H O over
2
2
T
2
2
2
activity of above manganese oxides was strongly influenced by
2 2 2
Figure 3. SEM images of (a, d) α-MnO , (b, e) γ-MnO and (c, f) β-MnO .
their crystal phases. Particularly, the fastest light-off rate over α-
MnO
% to 100%, which further confirmed the significant influence of
crystal phases of MnO in toluene oxidation. The CO yield was
calculated to evaluate the CO selectivity of toluene oxidation over
α-, γ- and β-MnO catalysts. As presented in Figure S1, nearly
00% CO yield was achieved over the three samples at
temperatures with complete toluene conversion, indicating all the
catalysts exhibited an excellent CO selectivity.
2
could be achieved within 20 °C from toluene conversion of
9
The morphologies of above manganese oxides with different
crystal phases were characterized by SEM, as presented in
Figure 3. It could be clearly seen from Figure 3 a-c and the partial
enlarged images (Figure 3 d-f), that nanometer multi-rod linkages,
urchins self-assembled by nanorods, straight nanorods were
2
2
2
2
1
2
achieved over α-MnO
2
,
γ-MnO
2
and β-MnO
2
samples,
2
respectively. TEM and HRTEM were performed to further identify
the structure and exposed facets of these manganese oxides. As
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shown in Figure 4 a-c, similar morphologies as the SEM images
were observed in TEM images. From the HRTEM images (Figure
The N
2
adsorption-desorption isotherms of as-synthesized
catalysts are illustrated in Figure S3. According to the IUPAC
classification, all of the samples exhibited type-IV adsorption
4
d-f), well-resolved lattice fringes could be identified. The lattice
spacings of 0.308, 0.490 and 0.695 nm could be indexed to the
3
isotherms with an obvious type H hysteresis loops, indicating the
(
310), (200) and (110) planes, respectively, of standard α-MnO
JCPDS NO 44-0141). The lattice spacing of 0.243 nm was
(JCPDS NO 14-0644),
while the lattice spacing of 0.312 nm was ascribed to the (110)
plane of β-MnO (JCPDS NO 24-0735).
2
presence of irregular pore structure [14]. The different size of
hysteresis loops over various manganese oxides contributed to
the distinguishing textural properties. As summarized in Table 1,
(
assigned to the (131) plane of γ-MnO
2
2
-
the α-MnO
2
possessed the largest specific surface area (83 m ·g
1
3
-1
2
) and pore volume (0.53 m ·g ), which might facilitate the mass
transfer and provide adequate sites for toluene adsorption.
Besides, the specific surface area and pore volume of other
samples decreased in the order of γ-MnO
2
(63 m ·g and 0.25 m3
2
g ) > β-MnO (12 m ·g and 0.12 m ·g ). This is in line with the
2
-1
· -
1
2
-1
3
-1
sequence of decreasing activity. In order to exclude the effect of
BET, the specific reaction rate (rnorm, conversion normalized by
specific surface area) was calculated and the result was plotted
in Figure S4. It can be seen that a higher rnorm was obtained over
α-MnO
2
, indicating that the specific surface area was not the key
factor affecting catalytic activity.
3
2 2
.3 H -TPR and O -TPD
2 2 2
Figure 4. TEM and HRTEM of (a, d) α-MnO , (b, e) γ-MnO and (c, f) β-MnO .
2 2 2 2
Figure 5. H -TPR profiles of α-MnO , γ-MnO and β-MnO .
To further understand the subtle phase information of as-
synthesized catalysts, Raman spectroscopy analysis was
H
2
-TPR was employed to investigate the redox properties of
as-synthesized catalysts. As illustrated in Figure 5, two separated
consumption peaks were detected over γ-MnO (278 and
56 °C) and β-MnO (318 and 445 °C), belonging to the stepwise
2 2 2
conducted over α-MnO , γ-MnO and β-MnO . As displayed in
-1
Figure S2, the most intensive Raman band at 652 cm was
detected over all the samples, and two small Raman bands at 576
H
4
2
2
2
-
1
and 580 cm were observed on the α-MnO
2
and γ-MnO
2
samples,
respectively. The Raman band at 652 cm could be assigned to
the symmetric stretching vibrations (ν (Mn-O)) perpendicular to
the direction of the MnO octahedral double chains, indicating the
formation of III or IV manganese oxides [12]. The Raman peak at
80 cm-1 was the characteristic feature of MnO
for cryptomelane
reduction of Mn to Mn3+ and Mn to Mn , respectively [9, 15].
Different from other samples, only one strong H consumption
, indicating
4+
3+
2+
-1
2
2
peak centered at 287 °C was observed over α-MnO
2
6
that Mn4+ was completely reduced to Mn2+ below 350 °C [16]. The
mobility of oxygen plays an important role in the catalytic oxidation
of VOCs [17]. The easier complete reduction of Mn4+ at lower
temperatures reflected the excellent mobility of lattice oxygen
5
2
-1
2
type α-MnO and the Raman peak at 576 cm belonged to the
ramsdellite γ-MnO
[
11b,
13], which was in agreement with the
2
2
over α-MnO sample, resulting in the most outstanding activity for
results of XRD.
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toluene oxidation (Figure 1). In addition, the descending order of
H consumption below 290 °C (the temperature required for
2
complete conversion of toluene for all catalysts) was in good
agreement with the descending order of catalytic activity (as
(Figure 1), since active surface oxygen played an important role
in the catalytic oxidation reaction [20]. As for medium temperature
regions, the subsurface lattice oxygen desorption peaks centered
at 560 °C (with a shoulder peak at 468 °C), 495 °C and 553 °C for
shown in Figure 1), implying the superior redox ability of α-MnO
2
2 2 2
α-MnO , γ-MnO and β-MnO , respectively. Obviously, the
made for the highest catalytic activity. To further understand the
effect of redox property on the catalytic performance, the low-
temperature reducibility of these samples was compared by the
value of onset reduction temperature. It was worth noting that the
sequence of onset reduction temperature was followed as α-
maximum desorption peak area was found to result from
subsurface lattice oxygen on the three catalysts, indicating a high
availability of the subsurface lattice oxygen. Noticeably, there
were considerable amount of subsurface lattice oxygen desorbed
from α-MnO
relatively low oxygen desorption temperature at 468 °C on α-
MnO reflected a better mobility of subsurface lattice oxygen. This
superiority could be responsible (at least partially) for the high
catalytic activity of α-MnO in toluene oxidation.
2
at 468 °C. Compared with other samples, the
2 2 2
MnO (129 °C) < γ-MnO (172 °C) < β-MnO (209 °C). This order
was consistent with the sequence of the catalytic activity of
different manganese oxides in Figure 1. Consequently, the redox
properties of manganese oxides with different crystal phases
were significantly different, the higher the low-temperature
reducibility, the higher the catalytic activity at low temperatures.
2
2
3
.4 XPS results
2 2 2 2
Figure 6. O -TPD profiles of α-MnO , γ-MnO and β-MnO .
2
Figure 6 displays the O -TPD profiles of manganese oxides
with different crystal phases. The type and amount of oxygen
species on the catalysts could be distinguished according to the
O
2
desorption peaks in different temperature regions. Based on
the previous studies, the O desorption peak below 300 °C could
2
be attributed to the physisorbed molecular oxygen (<200 °C) or
active surface oxygen species, while the peaks at above 600 °C
resulted from the release of bulk lattice oxygen [18]. The desorption
peaks located in the medium temperature region (300-600 °C)
could be assigned to the subsurface lattice oxygen, which was
known to be relatively active and could notably affect the catalytic
activity [19]. By contrast, the bulk lattice oxygen received less
attention due to its weaker reactivity. As shown in Figure 6, both
the physisorbed molecular oxygen and active surface oxygen
species were detected over α-MnO
MnO ) in the low-temperature regions at 113-115 °C and 218-
38 °C, respectively. The absence of active surface oxygen
species at low temperatures over β-MnO might be responsible
for its inferior catalytic activity compared with the other samples
2 2
and γ-MnO (but not over β-
2
2 2 2
Figure 7. (a) Mn 2p3/2 and (b) O 1s XPS of α-MnO , γ-MnO and β-MnO .
2
2
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XPS was used to probe the surface chemical states and
atomic concentrations of different manganese oxides, and the
results are presented in Figure 7 and Table 1. As shown in Figure
of benzoate on the surface of α-MnO
2
, while the band at 1597 cm-
1
was attributed to v(C=C) of the skeletal vibrations of the aromatic
presented in Figure 8 (b), the adsorption
of toluene at 260 °C gave rise to two adsorption bands different
from those for α-MnO
, the band at 1165 cm-1 was assigned to C-
rings [24]. As for γ-MnO
2
7
(a), the Mn 2p3/2 XPS spectra of all the samples were split into
three banding energy peaks. The binding energy peaks at 640.7-
40.8, 642.1-642.2 and 643.4-643.6 eV could be ascribed to the
2
6
O bond of benzoyl oxide species (C
6
H -CH
5
2
-O), whereas the
2
+
3+
4+
-1
surface Mn , Mn and Mn species, respectively, revealing the
band at 1405 cm was ascribed to symmetrical vibration of COO-
co-existence of three kinds of Mn species on the surface of α-
[12b, 24b, 25], implying the formation of benzoate. In the case of β-
MnO2, γ-MnO
2
and β-MnO [
2
21]. The relative surface contents of
MnO
2
shown in Figure 8(c), the band at 1025 cm-1 was assigned
2
+
3+
(
Mn +Mn )/Mntotal based on the XPS peak areas were
to the C-H in-plane bending vibration of benzene rings, while the
bands at 1249 cm-1 belonged to the carbonate species,
respectively [24a, 24d, 26]. The band at 1412 cm could be attributed
to symmetrical vibration of COO-, implying the formation of
benzoate. Evidently, different chemisorbed modes of benzene
rings contained species were detected on the surfaces of α-, γ-
summarized in Table 1. It was found that the ratios of
2
+
3+
-1
(
Mn +Mn )/Mntotal varied with the change of crystal phase of
2
+
3+
manganese oxides. The highest (Mn +Mn )/Mntotal ratio of 0.58
was achieved on the surface of α-MnO , implying more surface
[10], which was beneficial for
2
oxygen vacancies existed in α-MnO
2
the catalytic oxidation of toluene.
Figure 7 (b) presents the O 1s XPS spectra of the three
catalysts, which were fitted into two distinct peaks, corresponding
and β-MnO
adsorbed surface species on α-MnO
oxide species and benzoate were the major adsorbed surface
species on γ- MnO at 260 °C. The surface of β-MnO was mainly
covered by carbonate and benzoate species after the adsorption
of toluene at 280 °C.
2
, respectively. The benzoate species were the main
2
at 240 °C, while benzoyl
to the surface chemisorbed oxygen O
coordination and lattice oxygen O (529.6-529.9 eV), respectively
[22].
The ratios of O /O were calculated and shown in Figure 7(b),
which was followed the sequence: α-MnO (3.5) > β-MnO (2.0) >
γ-MnO (1.4). It has been intensively reported that, as for
α
(531.3-531.4 eV) with low
2
2
β
β
α
2
2
2
manganese oxides catalysts, the concentration of lattice oxygen
could dominate the activity during the oxidation reactions, such as
CO and HCHO oxidation, which followed the Mars-van Krevelen
mechanism [ . Zhang et al. found that active lattice oxygen
23]
[9]
species were the main influential factor for the excellent
performance of HCHO oxidation over δ-MnO
could also be partially verified by the present work, namely, α-
MnO with the highest O concentration of 3.5 (in Figure 7b)
2
. This phenomenon
2
β
presented the highest toluene oxidation activity among the three
catalysts (in Figure 1). However, similar conclusions were not
valid for γ-MnO and β-MnO . Hence the reaction process
2 2
between toluene and oxygen species over manganese oxides
with different crystal phases should be further investigated.
3
3
.5 Transient in-situ DRIFTS studies
.5.1 Adsorption of toluene
In-situ DRIFTS is a powerful tool for studying reaction
mechanism, and can give insights into the transformation
processes of toluene and oxygen species . Thus, the α-MnO , γ-
MnO and β-MnO catalysts were further studied by in-situ
DRIFTS.
In-situ DRIFTS spectra of 500 ppm toluene adsorption over
α-MnO , γ-MnO and β-MnO samples were recorded to identify
the adsorbed surface species. As shown in Figure 8 (a), four
stable adsorption bands were detected over α-MnO at 1411,
535, 1550 and 1597 cm-1 after toluene adsorption at 240 °C for
20 min. The band at 1411 cm-1 could be attributed to the
2
2
2
2
2
2
2
1
1
symmetrical vibration of COO-, indicating the potential existence
of benzoate. The bands at 1535 and 1550 cm-1 were ascribed to
the asymmetric C-O stretching vibration, indicating the formation
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Figure 8. In-situ DRIFTS of adsorbed surface species over (a) α-MnO
MnO and (c) β-MnO samples exposed to 500 ppm toluene in N (50 mL/min)
at 240, 260 and 280 °C for 120 min. In-situ DRIFTS of dry air (80% N + 20%
) reacted with pre-adsorbed surface species over (d) α-MnO (e) γ-MnO and
f) β-MnO samples at 240, 260 and 280 °C, respectively. Pretreatment: 500
ppm toluene pre-adsorbed followed by N purging for 30 min.
2
(b) γ-
2
2
2
2
O
2
2
2
(
2
2
3
.5.2 Reaction between oxygen and pre-adsorbed
surface species
In order to investigate the reactivity of various adsorbed
surface species and distinguish the key intermediates, transient
in-situ DRIFTS experiments between dry air (80% N + 20% O )
2 2
and pre-adsorbed surface species were conducted. As shown in
Figure 8 (d), after pre-adsorption in 500 ppm toluene, the surface
2
of α-MnO was mainly covered by benzoate species. Followed by
switching to dry air, all the adsorption bands of adsorbed surface
species were consumed gradually, and the CO
2
adsorption bands
at 2335 and 2360 cm-1 were detected over α-MnO
all adsorbed surface species on α-MnO
intermediates and could be reacted with oxygen easily. As for γ-
[27]
. Obviously,
2
2
were the key
-1
MnO
benzoate (1405 and 1550 cm ) species were detected after
toluene adsorption. After switching to dry air, obvious CO
2
(Figure 8 e), benzoyl oxide species (1165 cm ) and
-1
2
adsorption bands could be observed in Figure 8 (e) with the
gradually depletion of benzoyl oxide species and benzoate
species, implying the reaction between oxygen and adsorbed
surface species. However, a small amount adsorbed surface
species could still be detected over γ-MnO after oxygen was
2
introduced into the in-situ reaction cell for 60 min, which might
attribute to the relatively surplus adsorbed surface species on γ-
MnO
of dry air, all the adsorbed surface species over β-MnO
disappeared within 10 min at 280 °C , indicating that benzoate and
2 2
. As for β-MnO presented in Figure 8 (f), after the admission
2
carbonate species were all the main intermediates over β-MnO
2
at 280 °C.
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3
.5.3 Adsorption and reaction of toluene in dry air (80%
2 2
N + 20% O )
Figure 9 presents the in-situ DRIFTS of adsorbed surface
species arising from α-MnO , γ-MnO and β-MnO treated with
00 ppm toluene in dry air (80% N + 20% O ) at 240, 260 and
80 °C , respectively. It could be seen from Figure 9 (a), only CO
throughout the whole test and the
increased with time, demonstrating that
intermediates on the surface of α-MnO exhibited high reactivity
2
2
2
5
2
2
2
2
was detected on α-MnO
intensity of CO
2
2
2
and could be quickly reacted with oxygen species. This
phenomenon could be attributed to the superior reducibility and
high mobility of lattice oxygen of α-MnO
more easier oxidation of intermediates by oxygen species on the
surface of α-MnO . As for γ-MnO , besides CO , adsorption bands
as in Figure 8 (b) were also detected on γ-MnO (Figure 9b) during
the co-adsorption of toluene and O . This observation not only
further confirmed that benzoyl oxide species and benzoate
species were easily adsorbed and accumulated on γ-MnO , but
also illustrated that benzoyl oxide species and benzoate species
were the major intermediates in toluene oxidation over γ-MnO
Similar to γ-MnO , the same adsorbed surface species shown in
Figure 8 (c) were observed over β-MnO (Figure 9c) and the
intensity of CO also increased with the time. Based on the above
discussion of Figure 8 and 9, it could be concluded that all the
adsorbed surface species on α-MnO , γ-MnO and β-MnO were
2
, which resulted in the
2
2
2
2
2
2
2
.
2
2
2
2
2
2
the active intermediates in toluene oxidation. Injecting toluene
and oxygen into the in-situ reaction cell simultaneously led to the
fastest consumption of intermediates on α-MnO catalyst.
2
Table 1. Specific surface area, Pore volume,
2
H consumption and
2+
3+
(Mn +Mn )/Mntotal of all the samples.
H [
a]
(Mn2++Mn3+
/Mntotal
)
Catalyst
Specific
surface
area
Pore
2
volume
consumption
mmol·g
3
-1
-1
(m ·g )
2
-1
(m ·g )
α-MnO
γ-MnO
β-MnO
2
83
63
12
0.53
0.25
0.12
8.06
4.07
2.42
0.58
0.53
0.42
2
2
[
2
a] H consumption below 290 °C .
Conclusions
A series of manganese oxides with different crystal phases
α-, γ- and β-MnO ) were successfully synthesized and were
employed in the deep oxidation of toluene. α-MnO exhibited the
(
2
2
highest catalytic activity with a T50 at 229 °C and a T90 at 238 °C.
The T90 of different manganese oxides was followed the
2
Figure 9. in-situ DRIFTS of adsorbed surface species over (a) α-MnO (b) γ-
sequence: α-MnO
activity of α-MnO
2
< γ-MnO
2 2
< β-MnO . The outstanding catalytic
MnO
2
and (c) β-MnO
2
samples arising from adsorption and reaction of 500 ppm
+ 20% O ) at 240, 260, 280 °C, respectively.
2
could be attributed to the excellent redox
toluene in dry air (80% N
2
2
properties and the high mobility of lattice oxygen. Transient in-situ
DRIFTS studies illustrated that all the adsorbed surface species
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ChemCatChem
10.1002/cctc.201901839
FULL PAPER
were active intermediates for toluene oxidation on α-MnO
2
, γ-
as the excitation source. The charging effects of all the samples were
compensated by calibrating all binding energy with C1s peak at 284.8 eV.
MnO
surface species and oxygen by in-situ DRIFTS verified that
benzoate species were the key intermediates on α-MnO , while
benzoyl oxide species and benzoate species were the major
2
intermediates of toluene oxidation over γ-MnO . For β-MnO ,
2 2
and β-MnO .Monitoring the reaction between pre-adsorbed
In situ diffuse reflection infrared Fourier transform spectrum (in-situ
2
DRIFTS) experiments were performed on
spectrometer. Prior to each experiment, the sample was pretreated at
00 °C in N (50 mL/min) for 1 h to remove trace impurities. The
background spectrum was collected in flowing N and automatically
subtracted from the sample spectrum at the corresponding temperature.
In the transient studies, the α-MnO , γ-MnO and β-MnO samples were
firstly pre-exposed to a flow of 500 ppm toluene in N (50 mL/min) at 240,
60 and 280 °C for 120 min, respectively. Then the sample was purged by
a Nicolet 6700 FTIR
2
3
2
carbonate and benzoate species were the main intermediates.
2
2
2
2
2
2
Experimental Section
N
2
(50 mL/min) for 30 min and subsequently switched the gas to dry air
+ 20% O , 50 mL/min) for a certain time. For the co-adsorption of
00 ppm toluene + 20% O , the α-MnO , γ-MnO and β-MnO samples
were exposed to mixed gases (50 mL/min) of 500 ppm toluene in dry air
80% N + 20% O ) at 240, 260 and 280 °C for 1 h and then the spectra
were recorded.
(80% N
2
2
Catalyst preparation
5
2
2
2
2
All chemicals used were analytical reagents purchased from
Shanghai Aladdin Co.Ltd. without further purification. The α-MnO , γ-MnO
and β-MnO samples were synthesized via hydrothermal method. For α-
MnO : 2.2 g KMnO and 2.4 g Mn(CH COO) ·4H O were dissolved in 32
mL deionized water under vigorous stirring. 0.5 mL HNO was added into
the above mixed solution and then reacted at 100 °C for 24 h in a Teflon-
lined stainless steel autoclave. The α-MnO was obtained by finally
calcined the sample at 400 °C for 4 h. For γ-MnO : 1.35 g MnSO ·H O and
.83 g (NH were dissolved in 32 mL deionized water, followed by
(
2
2
2
2
2
2
4
3
2
2
Catalytic performance measurements
3
The catalytic activity tests were carried out in a fixed-bed quartz reactor (6
mm i.d.) using 0.1 g (40-60 mesh) catalyst accompanied with 0.4 g quartz
sands to minimize the hot spots effect. The 80 mL/min total flow rate
2
2
4
2
1
4 2 2 8
) S O
(
2 2
containing 1000 ppm toluene, 20% O and N as balance) was employed
hydrothermal treatment at 90 °C for 24 h. The black powder was centrifugal
washing with deionized water several times and dried at 80 °C overnight.
For β-MnO : 8.11 g MnSO ·H O, 3.19 K S O and 2 mL HCl (37%) were
2 4 2 2 2 8
dissolved in 60 mL deionized water. Except the hydrothermal temperature
was 140 °C, the other processes were the same as the preparation of γ-
-
1
-1
to generate a weight hourly space velocity (WHSV) of 48,000 L·g ·h . The
concentrations of toluene were analyzed by an online gas chromatograph
(
GC-2014C, Shimadzu) equipped with two flame ionization detectors (FID).
The activity data were collected after the reaction reached the steady state
at each temperature for 1 h and evaluated using the T50 and T90, which
defined as the temperature for toluene conversions of 50% and 90%,
2
MnO .
2
respectively. The conversion of toluene (X), yield of CO , normalized
reaction rate rnorm (mol·m ·s ) were calculated according to the following
equations, respectively:
Catalyst characterization
-
2
-1
The crystalline structure of different samples was identified by the X-
ray diffraction (D8 advance diffractometer) with Cu Ka radiation (λ = 1.5418
Å) at 40 kV and 40 mA. The 2θ was scanned from 10° to 90° with a scan
퐶푡표푙푢푒푛푒,푖푛 − 퐶푡표푙푢푒푛푒,표푢푡
X =
× 100%
퐶푡표푙푢푒푛푒,푖푛
2
interval of 0.02 °. The N adsorption-desorption isotherms were obtained
at -196 °C on an ASAP 2020 automated gas sorption system. The specific
surface areas and total pore volumes were calculated from Brunauer-
Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods,
respectively. Raman spectra were collected using a LabRAM HR Evolution
Laser Raman Spectrometer with a CCD detector. The excitation line was
at 532 nm and the spectra range was 200-1000 cm . The Scanning
electron microscopy (SEM) images were obtained using a Zeiss Ultra at
an accelerating voltage of 3 kV. High resolution transmission electron
microscopy (HRTEM) and TEM images were recorded on JEM-2100HR
퐶ꢃꢄꢅ,표푢푡
퐶푂 푦ꢀꢁꢂ푑 =
× 100%
1
2
7
× 퐶푡표푙푢푒푛푒,푖푛
퐶푡표푙푢푒푛푒,푖푛 × 퐹
푟
푛표ꢆ푚
=
× ꢂꢈ (
ꢇ푐푎푡 × 푆퐵퐸푇 1 − 푋
)
-
1
where Ctoluene,out represented the outlet concentration of toluene at steady
state and Ctoluene,in was the inlet concentration of toluene,
represented the CO concentration in the outlet gas, F is the toluene flow
C
CO2,out
2
(JEOL, Japan). H
2
temperature-programmed reduction experiments (H
2
-
-
1
2
-1
rate (mol·s ), mcat is the mass of catalyst (g). SBET (m ·g ) is the specific
TPR) were performed on
a
chemisorption analyzer (Micromeritics
surface area.
Chemisorb 2920II). The sample (20 mg) was firstly pretreated at 300 °C
for 1 h under a Ar flow (30 mL/min), followed by cooled down the sample
to 60 °C in Ar. Afterwards, the sample was heated at a ramping rate of
1
2 2
0 °C /min from 60 to 800 °C in a flow of 5% H /Ar (30 mL/min). The H
Acknowledgements
consumption below 290 °C of all the catalysts was calculated by using CuO
as a calibration reference. Oxygen temperature-programmed desorption
This work was supported by the National Key Research and
Development Project (No. 2017YFC0212800)
2
(O -TPD) experiments were also carried out on the above-mentioned
equipment with similar pretreated progress, while changing the sample
mass to 100 mg and pretreated gas to He (30 mL/min), respectively. After
2
the sample was cooled down to 60 °C , 5% O /He was switched into the U-
shape quartz reactor for 1 h and then swept with He for 30 min. The
catalyst was heated from 60 to 800 °C in a He flow (30 mL/min) at
Keywords: VOCs • Toluene • Manganese oxides • DRIFTS •
Catalytic oxidation
1
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Entry for the Table of Contents (Please choose one layout)
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α-MnO
2
, γ-MnO
2
and β-MnO
2
were
Author(s), Corresponding Author(s)*
synthesized for catalytic oxidation of
toluene, and higher rnorm was obtained
over α-MnO2. Different intermediates
were detected on the surfaces of α-, γ-
Page No. – Page No.
Title
2
and β-MnO via in-situ DRIFTS.
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