Layered δ-MnO2 as an active catalyst for toluene catalytic combustion

Article abstract of DOI:10.1016/j.apcata.2020.117715

Layered δ-MnO₂ was successfully synthesized by redox reaction precipitation between KMnO₄ and n-butanol. Then, the as-synthesized layered δ-MnO₂ was used as catalyst for toluene catalytic combustion and showed excellent ca

Full text of DOI:10.1016/j.apcata.2020.117715

Applied Catalysis A, General 602 (2020) 117715
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Layered δ-MnO as an active catalyst for toluene catalytic combustion
2
a
a,
a
a
b
c
Renzhu Li , Long Zhang *, Simin Zhu , Shiyu Fu , Xiaoping Dong , Shintaro Ida ,
d,
a,
Lingxia Zhang *, Limin Guo *
a
b
c
d
School of Environmental Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, China
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China
Graduate School of Science and Technology, Kumamoto University, 2-39-1, Kurokami Chuo-ku, Kumamoto 8608555, Japan
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road,
Shanghai 200050, China
A R T I C L E I N F O
A B S T R A C T
Layered δ-MnO
Keywords:
2
was successfully synthesized by redox reaction precipitation between KMnO
4
and n-butanol.
δ-MnO
2
layered
Then, the as-synthesized layered δ-MnO was used as catalyst for toluene catalytic combustion and showed
2
Toluene
excellent catalytic performance. The T10 and T90 (the temperatures corresponded to the toluene conversion of 10
Catalytic combustion
%
and 90 %) were just 160 °C and 199 °C, respectively (1000 ppm toluene, GHSV = 60,000 mL/(g h, 20 vol.%
O
2
/N
catalysts were characterized by XRD, N
the referenced ε-MnO and γ-MnO
specific surface area, stronger low-temperature reducibility and richer oxygen vacancy. Finally, the toluene
2
). In addition, the layered δ-MnO
2
2
showed well catalytic stability and humid resistance. The as-prepared
sorption, SEM, TEM, XPS, H -TPR, O -TPD and so on. Compared with
was ascribed to its higher
2
2
2
2
, the higher catalytic activity of layered δ-MnO
2
catalytic oxidation mechanism over δ-MnO was also proposed based on the results of in-situ DRIFTs. And the key
2
successively intermediate species for toluene oxidation over δ-MnO
2
were benzyl alcohol, benzoic acid, maleic
anhydride and acetate.
1
. Introduction
factors. Wang et al. reported the MnO with high activity and stability
2
for toluene catalytic combustion and pointed out the catalytic perfor-
The anthropological emission of volatile organic compounds (VOCs)
mance was closely related to the average oxidation state of Mn species,
has gained much attention due to their harmful effects on human health
and contribution to photochemical smog and tropospheric ozone [1].
And many technologies were developed to abate VOCs emission, such
as incineration combustion, catalytic combustion, adsorption, photo-
catalytic oxidation and so on [2]. Among them, the catalytic combus-
tion is considered as one of the most promising technologies for VOCs
abatement due to its lower energy consumption and well selectivity
low-temperature reducibility of the MnO and the content of oxygen
x
species adsorbed on the catalyst surface [9]. Liao et al. reported that
hollow MnO showed the improved performance for toluene catalytic
x
combustion due to its high chemisorbed oxygen species concentration
and average Mn oxidation state [10]. In addition, the catalyst pre-
paration methods also showed very important influences on the cata-
lytic performances due to its great impacts on the physicochemical
towards the desired products, such as CO
2
and H
2
O [3]. And the cat-
properties of catalysts [11–15]. Wang et al. prepared the MnO based
x
alyst with high performance plays the key role among the whole pro-
cess of VOCs catalytic combustion. Now, the noble metal based cata-
lysts have already been widely used as catalysts for VOCs catalytic
combustion. However, the high cost and resources shortage limit their
practical application [4]. Then, transition metal oxides become alter-
native catalysts due to their low cost, resources abundance and accep-
catalysts with the various morphologies by the different synthesis
methods and found the rod-like α-MnO showed the higher toluene
2
catalytic combustion performance due to its higher chemisorbed
oxygen species concentration and better low-temperature reducibility
[16]. Tang et al. prepared the MnO catalysts by the different methods
x
and found the one prepared by template-free oxalic acid synthesis
method showed the highest BTEX (benzene, toluene, ethylbenzene and
xylene) catalytic performance due to its higher specific surface area,
smaller pore size, stronger low-temperature reducibility and more
abundant lattice oxygen [17]. Up to now, although many methods have
table catalytic performance [5–8]. Among them, the MnO based cat-
x
alysts with strong redox capacity have attracted much attention due to
their various valences, diverse crystal structure and morphology [3].
The catalytic performance of MnO
x
generally depends on various
⁎
Corresponding authors.
E-mail addresses: lzh131@gmail.com (L. Zhang), zhlingxia@mail.sic.ac.cn (L. Zhang), lmguo@hust.edu.cn (L. Guo).
https://doi.org/10.1016/j.apcata.2020.117715
Received 22 April 2020; Received in revised form 8 June 2020; Accepted 21 June 2020
Available online 22 June 2020
0926-860X/ © 2020 Elsevier B.V. All rights reserved.

R. Li, et al.
Applied Catalysis A, General 602 (2020) 117715
been developed to prepare MnO
x
based catalysts, the simple and easy
an on-line gas chromatograph (FULI 9790Ⅱ) equipped with a flame
ionization detector (FID) and a thermal conductivity detector (TCD).
0.05 g catalyst mixed with 0.1 g silica sand (40–60 mesh) was placed in
the fixed-bed rector. All the feed gases used in this work were of high-
scale up synthesis methodology for MnO based catalyst with high
x
performance is still lacked and highly favored for practical application
on VOCs catalytic combustion.
−1
In this report, the redox reaction between KMnO
4
and n-butanol was
purity grade (99.99 %). The total flow was 50 mL min
(20 vol.%O
2
developed to synthesize the layered δ-MnO
2
, which was used as catalyst
+ 80 vol.%N ) and the corresponding gas hourly space velocity (GHSV)
2
for toluene catalytic combustion and showed excellent catalytic ac-
tivity, high catalytic stability and humid resistance. A series of char-
acterization results demonstrated the high catalytic performance of as-
was 60,000 mL/(g h. The catalyst amount was adjusted in order to
change the GHSV from 90,000 mL/(g h to 30,000 mL/(g h (e.g., 0.1 g
-
1
catalyst, 50 mL min for 30,000 mL/(g h). The toluene concentration
was controlled to 500 ppm, 1000 ppm and 1500 ppm by changing the
injection speed using a syringe pump (KD Scientific 100).
The toluene conversion (Rtoluene, %) was calculated by the following
formula:
prepared δ-MnO was closely related to its high specific surface area,
2
strong low-temperature reducibility and rich oxygen vacancy. Finally,
the toluene catalytic oxidation mechanism was also proposed based on
the results of in-situ DRIFTS. Herein, the ε-MnO
2
and γ-MnO were also
2
prepared by redox reaction and precipitation method, respectively, as
Cinlet
Coutlet
Rtoluene (%) =
× 100%
the referenced catalysts.
Cinlet
(1)
2
. Experimental
Where Cinlet and Coutlet represent the concentration of toluene in inlet
and outlet gas, respectively.
All the chemical reagents for the catalyst preparation were A.R.
The reaction rates of toluene oxidation normalized by the specific
grade and were used without further purification. They were all pur-
chased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
surface area (R
S
) of the catalyst were calculated according to the fol-
lowing formula:
RtolueneQCf
MSBET
2
2.1. Catalysts preparation
RS (µmol/m h) =
(2)
(
a) δ-MnO was synthesized as follows
2
Where Q is the volumetric flow rate (mL/min), C
f
is the feed con-
KMnO (0.01 mol) was added to 480 mL deionized water under
4
centration of toluene (ppm), M is the weight of the catalysts (g) and
2
magnetic stirring for 30 min. n-butanol (0.1 mol) was dissolved into 20
S
BET is the BET surface area of the catalyst (m /g).
mL deionized water, and then poured into the above KMnO solution.
4
The mixed solution was then stirred for 24 h at room temperature. The
resulting precipitate was filtered and washed three times with deio-
nized water and once with ethanol. The obtained precipitate was dried
at 100 °C for 12 h and calcined at 300 °C in air for 4 h with a heating
2.4. In-situ DRIFTS
In-situ diffuse reflectance infrared Fourier transform spectroscopy
(in-situ DRIFTS) was conducted on an infrared spectrometer (Bruker
Tensor II, Germany) equipped with a mercury cadmium tellurium
(MCT) detector. The catalyst was pretreated in Ar stream (50 mL
−
1
rate of 1 °C min . The obtained sample was δ-MnO .
2
(
b) ε-MnO
KMnO (0.01 mol) was added to 350 mL deionized water under
magnetic stirring for 30 min. MnSO ·H O (0.015 mol) was added to 150
mL deionized water under magnetic stirring for 30 min, and then slowly
2
and γ-MnO
2
were synthesized as reported [18,19]
−1
min ) at 300 °C for 60 min and then a series of background spectrum
were collected at the corresponding temperature (100 °C, 150 °C, 200
°C, 220 °C or 250 °C). The toluene (1000 ppm) within a flow of Ar
4
4
2
−1
added to the above KMnO
4
solution. The mixed solution was then
stream (50 mL min ) was introduced into the reaction cell at 100 °C
stirred for 6 h at room temperature. The resulting precipitate was fil-
tered and washed three times with deionized water and once with
ethanol. The obtained precipitate was dried at 100 °C for 12 h and
for 30 min. Afterward, the Ar stream was changed to 20 vol.%O + 80
2
−1
vol.%Ar (50 mL min ), which was then introduced into the reaction
cell. Thereafter, the sample was successively heated to 150 °C, 200 °C,
220 °C and 250 °C, respectively. And the corresponding infrared spectra
were continuously collected for 30 min at each temperature, and the
background spectra were subtracted from each collected spectrum.
−1
calcined at 300 °C in air for 4 h with a heating rate of 1 °C min . The
obtained sample was ε-MnO
0 wt.% Mn(NO (0.02 mol) was added to 100 mL deionized
water under magnetic stirring for 20 min. NH HCO (0.048 mol) was
added to 100 mL deionized water under magnetic stirring for 20 min,
2
.
5
)
3 2
4
3
3. Results and discussions
and then slowly added to the above Mn(NO
3
2
) solution. The mixed
solution was then stirred for 4 h at room temperature. The resulting
precipitate was filtered and washed three times with deionized water
and once with ethanol. The obtained precipitate was dried at 100 °C for
3.1. Catalyst characterization
The crystal structure of as-prepared samples δ-MnO
MnO were characterized by XRD and the results were shown in Fig. 1.
For δ-MnO , the diffraction peaks at 12°, 25°, 37° and 66° correspond to
the (001), (002), (-111) and (-312) crystal faces of δ-MnO (JCPDS
80–1098), respectively [18]. For ε-MnO , the diffraction peaks at 37°
and 66.5° correspond to the (100) and (110) crystal faces of ε-MnO
(JCPDS 30−0820), respectively. For γ-MnO , the diffraction peaks at
25°, 31°, 37°, 42.5° and 56.5° correspond to the (201), (301), (400),
2
, ε-MnO
2
and γ-
1
2 h and calcined at 400 °C in air for 4 h with a heating rate of 2 °C
2
−
1
min . The obtained sample was γ-MnO
.2. Catalyst characterization
The catalysts were characterized by X-ray diffraction (XRD), N
2
.
2
2
2
2
2
2
2
sorption, Scanning electron microscope (SEM), Transmission electron
microscopy (TEM), X-ray photoelectron spectroscopy (XPS), H tem-
perature-programmed reduction (H -TPR), temperature-pro-
2
(202) and (402) crystal faces of γ-MnO (JCPDS 42–1316), respectively
2
2
O
2
[19]. All XRD patterns of as-prepared samples show the broad and weak
diffraction peaks, which indicate that the samples have low crystallinity
or tiny nanocrystal structure.
grammed desorption (O -TPD) and Thermogravimetric-Differential
2
Thermal Analysis (TG-DTA). The detailed information on the char-
acterizations were described in the Supplementary Materials.
The morphology and microstructure of the as-prepared samples δ-
MnO
2
, ε-MnO
2
and γ-MnO were characterized by SEM and TEM, and
2
2.3. Catalyst activity evaluation
the results were shown in Fig. 2. The δ-MnO
2
and ε-MnO show flake-
2
like structures (Fig. 2A and B) and both of them are curled due to the
surface tension and then further assemble into larger aggregates
Catalyst activity was evaluated in a tubular fixed-bed reactor with
2

R. Li, et al.
Applied Catalysis A, General 602 (2020) 117715
Table 1
Specific surface area, pore volume and pore diameter of δ-MnO
2
, ε-MnO
2
and γ-
MnO
Catalyst
δ-MnO
2
.
a
BET (m2 g−1
b
pore (cm3 g−1
c
S
)
V
)
Dpore (nm)
2
2
2
142
94
0.38
0.22
0.21
12.2
9.8
ε-MnO
γ-MnO
99
8.5
a
b
Surface area obtained by Brunauer-Emmett-Teller (BET) method.
Total pore volume was calculated by the single point adsorption total pore
volume at P/P
0
= 0.985.
c
Average pore diameter obtained by Barrett-Joyner-Halenda (BJH) method
from adsorption branch.
the aggregates of layered or plate-like MnO [20]. The corresponding
x
pore size distribution curves were obtained by the BJH calculation from
adsorption branch data and shown in Figure S1B. The detailed textural
properties of as-prepared samples were summarized in Table 1. Com-
Fig. 1. XRD patterns of as-prepared catalysts δ-MnO
2
, ε-MnO
2
and γ-MnO .
2
pared with the specific surface area of ε-MnO
2
and γ-MnO
2
, δ-MnO has
2
2
−1
(
Fig. 2D and E). δ-MnO
and D), while ε-MnO (Fig. 2B) and γ-MnO
spherically aggregated particles. According to the high resolution TEM
HRTEM) images of as-prepared samples, the well-identified lattice
fringes of 0.71 nm and 0.25 nm (Fig. 2G) correspond to the interplanar
2
shows irregularly shaped aggregate (Fig. 2A
the highest surface area (142 m
g
), which commonly benefits the
2
2
(Fig. 2C) aggregates are
high catalytic performance due to more active sites exposure and well
mass diffusion [11]. In addition, thermogravimetric analysis was used
to understand the thermal stability of as-prepared samples and the re-
sults were shown in Figure S2. At temperature below 300 °C, the weight
loss is ascribed to the adsorbed water. The weight loss above 500 °C is
(
distance of (001) and (-111) faces of δ-MnO
2
, respectively. ε-MnO
2
(
Fig. 2H) exhibits the lattice fringes of 0.24 nm, which well matches its
ascribed to the removal of lattice oxygen and the transition of MnO
2
interplanar distance of (100) faces. γ-MnO
2
(Fig. 2I) exhibits the lattice
crystal phase to Mn
2
O crystal phase [21,22]. There are no obvious
3
fringes of 0.25 nm and 0.48 nm, which well matches its interplanar
distance of (400) and (202) faces, respectively. The HRTEM results of
as-prepared samples are well consisted with the corresponding results
of XRD.
endothermic/exothermic peaks except the endothermic peak of water
desorption before 500 °C, which indicates that all the samples have
good thermal stability before 500 °C. Herein, the mass drop of γ-MnO
2
is ascribed to the incomplete decomposition of MnCO
ified by the CO -TPD curve of γ-MnO (Fig. S1D).
Hydrogen temperature-programmed reduction (H
was used to understand the reducibility of as-prepared samples, and the
results were shown in Fig. 3A. We can clearly observe δ-MnO has one
main reduction peak, while there are two main reduction peaks for ε-
3
, which is ver-
The N sorption technique was used to obtain the overall porosity of
2
2
2
the as-prepared samples and the results were shown in Figure S1. The
sorption isotherms of as-prepared samples (Fig. S1A) are type IV and
exhibit hysteresis loops with H3 type at high relative pressure (P/P
.45–1.0), which indicate the presence of mesopores originated from
2
-TPR) technique
N
2
0
=
2
0
Fig. 2. SEM and TEM images of as-prepared catalysts δ-MnO
2
(A, D, G), ε-MnO
2
(B, E, H) and γ-MnO (C, F, I).
2
3

R. Li, et al.
Applied Catalysis A, General 602 (2020) 117715
Fig. 4. Mn 2p (A) and O 1s (B) XPS spectra of as-prepared catalysts δ-MnO
MnO and γ-MnO
2
, ε-
2
2
.
−
Fig. 3. H
2
-TPR (A) and O
2
-TPD (B) curves of as-prepared catalysts δ-MnO
2
, ε-
usually correspond to the desorption of surface chemisorbed O
; the
2
MnO and γ-MnO
2
2
.
peaks between 400 °C and 600 °C are commonly ascribed to the deso-
−
rption of surface chemisorbed O and the lattice oxygen species des-
orption usually appear above 600 °C [24,25]. For the catalytic oxida-
tion of VOCs, the surface chemisorbed oxygen species are more active
than the lattice oxygen species and the oxygen species desorbed at low
temperature are considered to be the active species for toluene oxida-
MnO
2
and γ-MnO . The peak corresponding to the lower temperature
2
4+ 3+
can be attributed to the reduction of Mn
to Mn , and the peak
corresponding to the higher temperature is ascribed to the further re-
3
+
2+
duction of Mn to Mn . The shoulder peak for δ-MnO
2
around 250 °C
and the weak reduction
→MnO of
4
+
3+
tion [26]. The obvious desorption peak corresponding to surface che-
is ascribed to the reduction of Mn
to Mn
−
misorbed O
2
can be clearly observed for δ-MnO and no such peaks
2
peak around 430 °C may result from the reduction of Mn
3 4
O
can be observed for ε-MnO
2
and γ-MnO
2
, which implies δ-MnO may
2
larger particles [23]. Herein, the hydrogen consumption of standard
catalyst (CuO) was used as the reference, and the initial hydrogen
consumption of all samples were listed in Table 2. The initial hydrogen
consumption is commonly used to evaluate the reducibility of the
samples [17]. And the initial hydrogen consumption trend of as-pre-
have stronger oxidation ability for VOCs at lower temperatures.
X-ray photoelectron spectroscopy (XPS) was used to understand the
elemental chemical state of as-prepared samples and the results were
shown in Fig. 4A (Mn 2p) and 4B (O1 s). After the deconvolution for Mn
2p3/2 peak, the peaks at 644.2 eV, 642.5 eV and 641.0 eV were assigned
4+ 3+ 2+ 4+ 3+
pared samples is δ-MnO
2
> ε-MnO
2
> γ-MnO . The lower reduction
2
to Mn , Mn and Mn , respectively [27]. The ratio of Mn /Mn
for as-prepared samples was calculated based on the peak area asso-
temperature and higher initial hydrogen consumption for δ-MnO
2
de-
monstrate its better low-temperature reducibility.
4
+
3+
ciated to Mn and Mn and the results were summarized in Table 2,
To identify the surface active oxygen species of as-prepared sam-
4
+
3+
which show that the ratios of Mn /Mn
for as-prepared samples
ples, O -TPD analysis was conducted and the corresponding results
2
decreased as follows: δ-MnO < γ-MnO < ε-MnO
2
2
2
and δ-MnO
2
has the
were shown in Fig. 3B. Generally, the peaks between 200 °C and 300 °C
Table 2
H
2
consumptions, surface element composition and catalytic performance of as-prepared catalysts.
a
b
c
Catalyst
Peak 1
Peak 2
H
2
consumption
XPS
T
10
(°C)
T
90
(°C)
R
S
−
1
2
(
°C)
(°C)
(mmol g
)
μmol/(m
h
Mn4+/Mn3+
O
ads/Olatt
δ-MnO
2
2
225
308
307
333
357
428
1.90
1.86
1.79
0.55
0.69
0.56
0.57
0.42
0.78
160
197
210
199
222
234
8.72
1.79
0.25
ε-MnO
γ-MnO
2
a
b
c
The H
2
consumption was estimated by quantitatively analyzing the H -TPR profiles below 300 °C.
2
The value was calculated based on the peak area of XPS spectra.
The reaction rate per unit surface area was calculated on basis of catalytic performance data of as-prepared catalysts at 200 °C.
4

R. Li, et al.
Applied Catalysis A, General 602 (2020) 117715
2
(
1.79 μmol/(m2 h) and γ-MnO
2
(0.25 μmol/(m h). Table S1 summar-
ized the results of the toluene catalytic oxidation on manganese-based
catalysts reported in the literatures and we can clearly observe the
layered δ-MnO catalyst obtained in the study showed much improved
2
catalytic performance for toluene catalytic combustion. The Arrhenius
plots for the toluene catalytic combustion over as-prepared samples
were calculated and the results were shown in Fig. 5B. The toluene
conversion rates were calculated under differential conditions with
toluene conversion below 15 % in order to exclude the effect of mass
and heat transfer and the apparent activation energy (Ea) was obtained
from the slopes of the Arrhenius plots [31]. The Ea for δ-MnO
2
was
much lower than those of ε-MnO
2
and γ-MnO
2
and the value was
1
18.47 kJ/mol, which implies the easier toluene activation over δ-
MnO
2
. Furthermore, the toluene catalytic combustion performance over
δ-MnO
2
under different conditions were explored and the results were
shown in Fig. 6. The effect of GHSV on the catalytic performance of δ-
MnO was shown in Fig. 6A. It can be observed that the catalytic ac-
tivity of δ-MnO
2
2
decreased with the increasing of GHSV, which can be
ascribed to the decreasing of the contact time between the toluene
molecular and catalyst. Nevertheless, the total toluene catalytic con-
version can be still achieved below 240 °C even at 90,000 mL/(g∙h).
Fig. 6B showed the effect of toluene concentration on the catalytic
activity of δ-MnO for toluene catalytic combustion. It could be ob-
2
served that the catalytic activity of δ-MnO decreased with the in-
2
creasing of toluene concentration. And even the toluene concentration
was up to 1500 ppm, the temperature for the total toluene catalytic
conversion was only about 10 °C higher than that for 500 ppm, which
further showed the high catalytic activity of δ-MnO . In practical VOCs
2
exhaust, the moisture is unavoidably present and also has very im-
portant impact on the catalytic activity [32]. Fig. 6C showed the cat-
alytic activity for toluene combustion under different relative humidity
over δ-MnO . The catalytic activity curve for RH 50 % well overlapped
2
Fig. 5. Toluene conversion (A) and Arrhenius fitting curves (B) as functions of
reaction temperature over as-prepared catalysts δ-MnO
2
, ε-MnO
2
/N
and γ-MnO
).
2
with the catalytic activity curve for RH 0%. Even for RH 80 %, the
catalytic activity curve just slightly shifted to higher temperature.
Herein, the catalytic activity curves especially for the low temperature
range (140–180 °C) showed the slight decrease with the increasing of
the relative humidity, which was due to the competitive adsorption of
water vapor on the active sites of catalyst surface [3]. Nevertheless, the
(
1000 ppm toluene, GHSV = 60,000 mL/(g·h) and 20 vol.%O
2
2
3
+
highest surface Mn
content. According to the charge compensation
effect, δ-MnO will have the most surface oxygen vacancy content
2
among all the samples [14]. And more oxygen vacancies commonly
promote the oxygen activation on the catalyst surface, which can fur-
ther improve the oxidation ability of the catalyst [16]. Herein, the high
results demonstrated the well humid resistance of δ-MnO during the
2
toluene catalytic combustion. Fig. 6D showed the results of catalytic
2
+
stability testing at 195 °C and the toluene conversion over δ-MnO was
2
Mn
content on the surface of γ-MnO
2
is ascribed to the un-
well maintained at around 98 % during 12 h in the absence of water
moisture. After the introduction of water vapor (RH 80 %, inset of
Fig. 6D), the toluene conversion slightly decreased and remained
around 90 %. Once the introduction of water vapor was cut off, the
toluene conversion can be well recovered, which was ascribed to
competitive adsorption of water molecular on the active sites of catalyst
and demonstrated the inactivation was reversible.
decomposed MnCO
3
, which is also observed by the DTA analysis (Fig.
S2D). After the deconvolution for O 1s peak (Fig. 4B), the peaks cor-
responding to surface lattice oxygen (Olatt, 529.5–530.1 eV) and surface
adsorbed oxygen (Oads, 531.1–531.6 eV) can be well identified [28–30].
The ratio of Oads/Olatt for as-prepared samples was calculated based on
the peak area associated to Oads and Olatt and the results were sum-
marized in Table 2. The higher Oads/Olatt ratio for γ-MnO was ascribed
2
to the interference from the undecomposed MnCO , which is due to the
3
3
.3. In-situ DRIFTS study
binding energy of O in the CeO bond and CO] bond are near 533 eV
and 531 eV, respectively [43].
The surface change of δ-MnO was characterized by in-situ DRIFTS
2
at the simulated reaction condition in order to understand the toluene
3.2. Catalytic performance
catalytic oxidation mechanism over δ-MnO . The in-situ DRIFTS spectra
2
over δ-MnO were firstly recorded in 1000 ppm toluene/Ar flow at 100
2
The catalytic activity for toluene combustion over as-prepared
°C in Fig. 7A. When toluene and argon was introduced into the reaction
gas, the adsorption peaks appeared and gradually increased with time.
samples was investigated and the results were shown in Fig. 5A. The
toluene conversion over as-prepared samples exhibited S-shaped curves
and increased with the reaction temperature increase. Among them, δ-
−1
The peaks at 1595, 1493 and 1453 cm are attributed to the aromatic
−
1
ring deformation vibration; the absorption peak at 1025 cm
is at-
MnO
MnO
2
2
showed much better apparent activity compared with those of ε-
tributed to the alcohol species (the stretching vibration of the CeO
−1
and γ-MnO
2
. The corresponding T10 and T90 (the reaction tem-
bond) [33]; peaks at 1558, 1541 and 1393 cm
carboxylate [34,35] and the peak at 1365 cm
are attributed to the
−1
peratures at which the toluene conversion reached 10 % and 90 %) for
can be assigned to
δ-MnO
2
were just 160 °C and 199 °C, respectively. The reaction rates of
acetate species [36]. These results demonstrated the toluene molecules
toluene oxidation normalized by the specific surface area (R
S
) was
gradually accumulated on δ-MnO
oxygen species of δ-MnO even at 100 °C to form benzyl alcohol, ben-
zoic acid, and acetate species. To further clarify the adsorption species
2
and successively reacted with active
calculated and shown in Table 2. The toluene reaction rate over δ-MnO
2
2
2
is up to 8.72 μmol/(m2 h, which is much higher than those of ε-MnO
5

R. Li, et al.
Applied Catalysis A, General 602 (2020) 117715
Fig. 6. Toluene conversion as a function of reaction temperature over as-prepared δ-MnO
2
under different mass space velocities (A), different toluene concentrations
(
B) and different relative humidity (C); Toluene conversion as a function of on-stream reaction time over as-prepared δ-MnO (D) (1000 ppm toluene, GHSV =
2
6
0,000 mL/(g·h), RH = 0% or 80 % (inset), 195 °C and 20 vol.%O
2
/N ).
2
evolution, the in-situ DRIFTS spectra over δ-MnO
peratures after the catalyst was exposed to the toluene flow with O
were recorded and shown in Fig. 7B. Compared to the spectra at 100 °C
without O ), the spectrum dramatically changes with the temperature
elevation to 250 °C. The peaks at 1593, 1472 and 1069 cm
tributed to the aromatic ring, methylene (−CH ) and hydroxyl
2
at elevated tem-
We can well observe toluene (m/z = 91, 92), benzyl alcohol (m/z =
108, 109), benzoic acid (m/z = 122, 123), maleic anhydride (m/z =
98, 99) and acetic acid (m/z = 60) in outlet gas stream. On the basis of
the results of in-situ DRIFTS and on-line MS analysis, the possible to-
2
(
2
−
1
are at-
luene catalytic mechanism over δ-MnO
2
was proposed and illustrated in
are firstly oxidized by the
2
Fig. 7C. The toluene adsorbed on δ-MnO
2
(
CeOH), respectively [33,37,38]. The peaks at 1646, 1177 and 1023
active oxygen species on the catalyst and generate benzyl alcohol spe-
cies, thereafter the benzyl alcohol species are further oxidized to ben-
zoic acid species, and then the benzene ring skeleton is destroyed and
forms maleic anhydride species, thereafter maleic anhydride is oxidized
to acetate species, and finally is oxidized to carbon dioxide and water.
−
1
cm are attributed to the alcohol species (tensile vibration of the CeO
bond) [33,39]. All these peaks indicate that toluene adsorbed on δ-
MnO converted into benzyl alcohol species by breaking the CeH bond
−1
2
on methyl (CeH
3
). The peaks at 1561, 1536, 1446, and 1396 cm are
attributed to carboxylates [34,35] and the peaks at 1303 and 1367
−
1
cm
are attributed to acid anhydride and acetate, respectively
[
33,36,40,41], which indicate that benzoic acid, maleic anhydride and
4. Conclusions
acetate species were generated during the toluene catalytic oxidation.
However, no vibrational peaks of benzaldehyde species are observed at
all temperatures and the generated benzaldehyde species may be ra-
In summary, the layered δ-MnO
2
was successfully prepared by the
and n-butanol. And
simple redox reaction precipitation between KMnO
4
pidly converted into benzoic acid species on δ-MnO
2
[42]. All these
the as-prepared layered δ-MnO showed high catalytic activity, stability
2
peak intensity except for the peak of acid anhydride species gradually
increase with the elevated temperature below 220 °C and then decrease
and even disappear when the temperature is increased to 250 °C, which
and humid resistance for toluene catalytic combustion. The T10 and T90
were just 160 °C and 199 °C, respectively (1000 ppm toluene, RH 0%,
GHSV = 60,000 mL/(g h and 20 vol.%O
2
2
/N ). The high catalytic ac-
implies the enhanced oxidation reaction over δ-MnO
2
between 220 °C
tivity of layered δ-MnO
strong low-temperature reducibility and rich oxygen vacancy. The to-
luene catalytic oxidation mechanism over δ-MnO was proposed based
2
was related to its high specific surface area,
and 250 °C. Herein, the peaks corresponding to acid anhydride species
continuously increases up to 250 °C, implying the anhydride as the
important intermediate species and the oxidation of anhydride species
may be the rate-determining step (220–250 °C) for toluene catalytic
2
on the results of in-situ DRIFTS. And the key successively intermediate
species for toluene oxidation over as-prepared δ-MnO
alcohol, benzoic acid, maleic anhydride and acetate.
2
were benzyl
oxidation over δ-MnO . Figure S3 showed the results of on-line MS
2
spectrum for toluene catalytic oxidation (180 °C, 1000 ppm toluene, 20
−
1
−1
vol.% O
2
/Ar, GHSV = 60 000 mL g
h
and RH 0%) over δ-MnO .
2
6

R. Li, et al.
Applied Catalysis A, General 602 (2020) 117715
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2020.117715.
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Center of for analytical support.
7