Effect of pore size in mesoporous MnO2 prepared by KIT-6 aged at different temperatures on ethanol catalytic oxidation

Article abstract of DOI:10.1016/S1872-2067(18)63036-0

KIT-6 mesoporous silica aged at 40, 100, and 150 °C were used as hard templates to prepare different mesoporous MnO₂ catalysts, marked as Mn-40, Mn-100, and Mn-150, respectively. The catalytic activities of these catalysts and the effect of pore sizes on ethanol catalytic oxidation were investigated. Mn-40, Mn-100, and Mn-150 have triple, double, and single pore systems, respectively. On decreasing the aging temperature of KIT-6, the pore sizes of KIT-6 decrease and that of mesoporous MnO₂ catalysts increase. The pore sizes and catalytic activities increase in the order: Mn-40 >Mn-100 > Mn-150. Mn-40 catalyst has a higher TOF (0.11 s^?1 at 120 °C) and the best catalytic activity for ethanol oxidation because of a bigger pore size with three pore systems with maximum distribution at 1.9, 3.4, and 6.6 nm, decrease in symmetry and degree of order, more surface lattice oxygen species, oxygen vacancies resulting from more Mn³⁺ ions, and better low-temperature reducibility.

Full text of DOI:10.1016/S1872-2067(18)63036-0

Chinese Journal of Catalysis 39 (2018) 630–638
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article (Special Issue on Environmental and Energy Catalysis)
Effect of pore size in mesoporous MnO2 prepared by KIT‐6 aged at
different temperatures on ethanol catalytic oxidation
Bingyang Baia,b,*, Qi Qiao a,b, Yanping Li b, Yue Pengc, Junhua Li c,#
a State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
b Key Laboratory of Eco‐Industry of the Ministry of Ecological Environment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
c State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China
A R T I C L E I N F O
A B S T R A C T
Article history:
KIT‐6 mesoporous silica aged at 40, 100, and 150 °C were used as hard templates to prepare differ‐
ent mesoporous MnO2 catalysts, marked as Mn‐40, Mn‐100, and Mn‐150, respectively. The catalytic
activities of these catalysts and the effect of pore sizes on ethanol catalytic oxidation were investi‐
gated. Mn‐40, Mn‐100, and Mn‐150 have triple, double, and single pore systems, respectively. On
decreasing the aging temperature of KIT‐6, the pore sizes of KIT‐6 decrease and that of mesoporous
MnO2 catalysts increase. The pore sizes and catalytic activities increase in the order: Mn‐40 ＞
Mn‐100 ＞ Mn‐150. Mn‐40 catalyst has a higher TOF (0.11 s–1 at 120 °C) and the best catalytic
activity for ethanol oxidation because of a bigger pore size with three pore systems with maximum
distribution at 1.9, 3.4, and 6.6 nm, decrease in symmetry and degree of order, more surface lattice
oxygen species, oxygen vacancies resulting from more Mn3+ ions, and better low‐temperature re‐
ducibility.
Received 23 December 2017
Accepted 25 January 2018
Published 5 April 2018
Keywords:
Mesoporous MnO2
Pore channel
KIT‐6 mesoporous silica
Catalytic activity
Ethanol oxidation
© 2018, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
ic oxidation mainly contain metal oxides and noble metals, such
as Pt/Al2O3 [1], M/TiO2 (M = Pt, Pd, Ir, Rh, and Au) [5], Rh/Al2O3
Ethanol, a widely used solvent, is released into the atmos‐
phere through exhaust gases emitted from ethanol‐fueled vehi‐
cles. It is a type of volatile organic compounds (VOCs), and is
considered a substantial contributor to the air pollution and
photochemical pollution [1–4]. Catalytic oxidation is one of the
most promising technologies for the elimination of VOCs. This
technology has many advantages, such as high removal effi‐
ciency, a low light‐off temperature, a wide range of applica‐
tions, simple operational technique of the equipment, and no
secondary pollution effect. The key is to choose and prepare the
catalysts properly. At present, catalysts used in ethanol catalyt‐
[6], Ag/La0.6Sr0.4MnO3 [7], MnOx [8], MnO2 [3], OMS‐2 [4],
MnCuOx [9], MnOx–CeO2 [10], CuO/MnOx [11], CuO/Al2O3 [12],
CuO/Fe2O3 [13], LaCoO3 [14], Ce1‐xMnxO2 [15], MnCu/ZrO2 [16],
LaMnO3 [17], Mn0.6Ce0.4O2 [18], La0.8Sr0.2MnO3+x [19], and
CuMn2O4/Al2O3 [20].
Same materials with different morphologies, such as tubes,
sheets, flowers, rods, spheres, and pores, exhibit different cata‐
lytic activities for VOC oxidation [21–25]. MnO2 has different
types of morphology structures such as α, β, γ, and δ types, and
exhibits synthetic shapes such as tubes, wires, rods spheres,
and pores. It is widely used in catalysis and exhibits excellent
* Corresponding author. Tel: +86‐10‐84914902; Fax: +86‐10‐84914626; E‐mail: baibingyang@tsinghua.org.cn
# Corresponding author. Tel/Fax: +86‐10‐62771093; E‐mail: lijunhua@tsinghua.edu.cn
This work was supported by the National Key Research and Development Program Foundation of China (2016YFC0209203) and the National Natural
Science Foundation of China (21707130, 21325731).
DOI: 10.1016/S1872‐2067(18)63036‐0 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 39, No. 4, April 2018

Bingyang Bai et al. / Chinese Journal of Catalysis 39 (2018) 630–638
631
catalytic performance [22,31–33]. Mesoporous MnO2 has been
used as an effective environmentally friendly material for VOCs.
A previous literature reported that mesoporous MnO2, par‐
ticularly 3D‐MnO2, shows better activity for catalytic oxidation
[3]. Tüysüz et al. [34] reported that increasing the aging tem‐
perature of KIT‐6 could increase its pore size and decrease the
single pore size of nanocast Co3O4 materials prepared by KIT‐6
as hard templates, because of the change in the interconnectivi‐
ties between two channels of KIT‐6. Currently, the influence of
aging temperature of KIT‐6 on pore sizes in a 3D‐MnO2 materi‐
al with a double pore system has not been clarified; moreover,
the influence of pore size of 3D‐MnO2 on catalysis is also not
well understood. If the pore size in 3D‐MnO2 is changed by
adjusting the aging temperature of KIT‐6, there is a possibility
to improve the important factors influencing catalytic activity,
such as low‐temperature reducibility, oxygen species, and ac‐
tive sites. This would help further improve the catalytic per‐
formance for ethanol oxidation on 3D‐MnO2.
consumptions of reduction peaks were calculated by using a
standard CuO sample. X‐ray photoelectron spectroscopy (XPS)
patterns were tested on a PHI‐5300 spectrometer, utilizing Mg
Kα X‐ray source. The C 1s binding energy at 284.6 eV was ref‐
erenced as binding energy (BE) calibration of all elements.
2.3. Evaluation of catalytic activity
Ethanol oxidation was studied in a fixed‐bed quartz tube
reactor. The gas flow rate through the reactor with 0.2 g of cat‐
alyst (60–80 mesh) was maintained at 200 mL/min, which
included 500 ppm ethanol and 20% (vol) O2 and N2 equilibrium
gas. The effluent product was analyzed by an on‐line Agilent
7890A gas chromatograph (GC) equipped with a TCD detector
and a flame ionization detector (FID), using Porapak‐Q and
HP‐INNOWAX columns, respectively. Ethanol conversion was
calculated based on the export and import concentrations of
ethanol from the GC. The TOFs of the samples were defined as
the number of ethanol molecules converted per active site per
In this work, mesoporous MnO2 samples with different pore
sizes were prepared using KIT‐6 templates aged at different
temperatures as the hard template. The catalytic activities of
these samples on ethanol oxidation were investigated, and the
effect of pore sizes on catalytic activity was addressed.
hour.
Cethanol  Xethanol  Vgas
TOF =
(h1
)
n
where Cethanol: concentration of ethanol in gas mixture; Xethanol
:
ethanol conversion; Vgas: total molar flow rate; and n: the sum
of mole number of Mn4+ and Mn3+ ions calculated by H2‐TPR.
2. Experimental
3. Results and discussion
2.1. Catalyst preparation
3.1. Catalytic activity of ethanol oxidation
Mesoporous MnO2 with different pore sizes was prepared.
KIT‐6 mesoporous silica aged at different temperatures was
used as a hard template to prepare mesoporous MnO2 catalysts
with different pore sizes. Synthesis of KIT‐6 templates and
preparation of mesoporous 3D‐MnO2 were carried out follow‐
ing a method reported previously [35]. The KIT‐6 templates
aged at 40, 100, and 150 °C were marked as KIT‐6‐40,
KIT‐6‐100, and KIT‐6‐150, corresponding to the preparation of
Mn‐40, Mn‐100, and Mn‐150 samples, respectively.
The catalytic activities for ethanol oxidation using the cata‐
lysts are shown in Fig. 1. Ethanol conversions at 60 °C were
22% on Mn‐40, 20% on Mn‐100, and 8.3% on Mn‐150, and
complete conversion was observed at 150 °C on Mn‐40, at 160
°C on Mn‐100, and at 170 °C on Mn‐150, under the following
conditions: 500 ppm ethanol, 20 vol % O2, and GHSV = 60000
mL/(g·h) (Fig. 1(a)). The maximum acetaldehyde yields were
observed at 130 °C on Mn‐40, at 140 °C on Mn‐100, and at 140
°C on Mn‐150 (Fig. 1(b)). In this catalyst system, acetaldehyde
is an important intermediate species of ethanol oxidation. On
increasing the reaction temperature, the acetaldehyde species
are further oxidized to CO2 and H2O. Complete oxidation of
ethanol to CO2 was achieved at 160 °C on Mn‐40, at 170 °C on
Mn‐100, and at 180 °C on Mn‐150 (Fig. 1(c)). These results
show that Mn‐40 has a better catalytic activity for ethanol oxi‐
dation, because of the changes in pore sizes caused by tem‐
plates with different aging temperatures. Mn4+ and Mn3+ ions
provided efficient active sites for ethanol oxidation reaction,
because the oxidation reaction often occurs through redox re‐
cycles of high‐ and low‐valence state cations, like Co3+/Co2+
[23] and Ce4+/Ce3+ [36]. For comparison, TOFs of the samples
were estimated based on Mn4+ and Mn3+ ions as active sites
(Fig. 2). The TOF of Mn‐40 is 0.033 h–1 at 60 °C. With increasing
reaction temperature, the TOF rapidly soars to 0.11 h–1 at 120
°C. The TOF of Mn‐100 is 0.03 h–1 at 60 °C, and promptly in‐
creases up to 0.097 h–1 at 120 °C. The TOFs for Mn‐150 are
0.017 h–1 at 60 °C and 0.007 h–1 at 120 °C. This result further
2.2. Catalyst characterization
X‐ray diffraction (XRD) patterns were recorded on a TTR3
type X‐ray diffractometer using a 0.05°/min scanning speed of
Cu Kα radiation source (40 mA and 40 kV). N2 adsorp‐
tion‐desorption (BET) were tested on an Autosorb‐1MP in‐
strument. All samples were outgassed at 300 °C for 4 h under
nitrogen gas before the test. The surface areas and isotherms
were calculated by the Barrett‐Joyner‐Halenda (BJH) method.
Scanning electron microscopy (SEM) images were acquired on
a Hitachi S‐5500 instrument. All samples were dispersed on a
carbon film supported by a copper grid, using ultrasonic waves
in deionized water for 10 min. The samples were dried at 110
°C for 20 min. Hydrogen temperature programmed reduction
(H2‐TPR) was tested on a Chemisorb 2720 apparatus with a
TCD detector. All samples (0.05 g, 40–60 mesh) were pretreat‐
ed in flowing N2 at 300 °C for 1 h. All samples were reduced in
50 mL/min of gas flow with 5% H2‐95% Ar at 30–900 °C at a
heating rate of 10 °C/min after the pretreatment. Hydrogen

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Bingyang Baiet al. / Chinese Journal of Catalysis 39 (2018) 630–638
70
60
50
40
30
20
10
0
(b)
(c)
(a)
100
80
60
40
20
0
Mn-150
Mn-100
Mn-40
Mn-150
Mn-100
Mn-40
Mn-150
Mn-100
Mn-40
100
80
60
40
20
0
-10
60
80
100
120
140
160
180
60
80
100
120
140
160
180
90 100 110 120 130 140 150 160 170 180 190
Temperature (oC)
Temperature (oC)
Temperature (oC)
Fig. 1. Ethanol catalytic performance of the catalysts. (a) Ethanol conversion, (b) acetaldehyde yield during ethanol oxidation and (c) CO2 yield during
ethanol oxidation. Reaction conditions: 500 ppm ethanol, 20 vol % O2, N2 as balance gas, GHSV = 60000 mL/(g∙h).
confirms that Mn‐40 has the best catalytic activity. Although
Mn‐40 and Mn‐100 have an approximate quantity of active
sites from Table 2, the catalytic activity of Mn‐40 is still superi‐
or to that of Mn‐100 because of the change in the pore channel
structure and pore size.
56.7°, 59.4°, 64.8°, and 72.3°, which correspond to the (110),
(101), (111), (211), (220), (002), and (301) planes, respective‐
ly. The diffraction peaks indicate that the materials consist of a
β‐MnO2 crystalline phase, which corresponds to pyrolusite with
a rutile structure [37]. The Mn‐40 catalyst also exhibits diffrac‐
tion peaks at 33.0° and 55.2° (2θ), which correspond to the
(222) and (440) planes, respectively. It indicates that the
Mn‐40 catalyst has a small amount of Mn2O3 crystalline phase
(PDF#65‐1798), except a MnO2 crystalline phase, probably
because of the stable pore structure. After a repeated prepara‐
tion, Mn‐40 calcined at 400 °C still contains a little of Mn2O3
crystalline phase under the same preparation conditions, but
Mn‐100 and Mn‐150 do not, because of the change in the pore
channels.
3.2. Textural characterization
The wide‐angle XRD patterns of samples are shown in Fig. 3.
All samples exhibit diffraction peaks at 2θ = 28.7°, 37.3°, 42.8°,
Mn-150
12
Mn-100
Mn-40
10
Fig. 4 shows a low‐angle XRD patterns of the samples and
templates. Different low angles and peak intensity of the sam‐
ples can lead to different pore sizes and mesoporous charac‐
teristics. All samples have low‐angle diffraction peaks, indicat‐
ing that they contain ordered mesoporous characteristics.
Apart from the (211) peak at 1.24°, Mn‐40 possesses one (110)
diffraction peak at 0.62° because of the low interconnectivity of
KIT‐6 aged at 40 °C to reduce the symmetry of the sample [34].
In a nanocasting process of Mn‐40, it is impossible to com‐
pletely fill two relatively independent pore systems by the
manganese oxide phase. The manganese material is only col‐
lected in some regions of the two pore systems of the template
to form a metal oxide phase. If the internetwork between dou‐
ble channel systems of the KIT‐6‐40 template is plugged up, the
8
6
4
2
60
70
80
90
100
110
120
Temperature (C)
Fig. 2. TOFs for ethanol oxidation of the catalysts. Reaction conditions:
600 ppm ethanol, 20 vol% O2, N2 as balance gas, GHSV = 60000
mL/(g∙h).
211
Mn O crystalline phase

2
3
Mn-150
Mn-100
Mn-40
KIT-6-150
KIT-6-100
KIT-6-40
211
Mn-40

110

Mn-100
332
332
Mn-150
0.8
1.2
1.6
2.0
2
2.4
2.8
0.8
1.2
1.6
2.0
2
2.4
2.8
20
30
40
50
2
60
70
80


Fig. 4. Low‐angle XRD patterns of the samples and templates.
Fig. 3. Large‐angle XRD patterns of the samples.

Bingyang Bai et al. / Chinese Journal of Catalysis 39 (2018) 630–638
633
cating a weak mesoporous property because of the collapse of
the partial pore channels, in agreement with the low‐angle XRD
results. Mn‐40 lies somewhere in between the two. The inset of
the top left corner in Fig. 5(a) shows the pore size distribution
using the Barrett‐Joyner‐Halenda method. All the samples have
a pore size at 1.9 nm of maximum distributions, which is relat‐
ed to the density of the metal oxide phase in two channels or
one of the double channels of KIT‐6 [27,28,34]. The pore size at
1.9 nm of mesoporous MnO2 cannot be affected by the change
in the pore size of KIT‐6. Except for the pore size at 1.9 nm, the
other pore size of mesoporous MnO2 increases with decreasing
aging temperature of the templates.
Table 1
Physical parameters of the samples and templates.
Interplanar
spacing d211
(nm)
Pore
Surface area
A
Pore diameter
a
Sample
volume
Vp (cm3/g)
1.045
BET (m3/g)
DP b (nm)
KIT‐6‐150
KIT‐6‐100
KIT‐6‐40
Mn‐150
8.7
8.5
7.1
9.6
9.2
8.3
375
694
568
39
6.2
3.8/5.9
2.0/2.5
1.9/2.3
1.9/6.5
1.9/3.4/6.6
0.638
0.425
0.108
0.177
0.107
Mn‐100
Mn‐40
70
47
a Interplanar spacing of the samples calculated through Bragg equation
from the (211) diffraction peak in low‐angle XRD patterns. b Pore sizes
obtained from the N2 adsorption isotherms by the BJH method.
Mn‐100 shows two pore sizes with maximum distributions
at 1.9 and 6.5 nm, because it perfectly replicated the double
pore channels of the KIT‐6 template (Table 1). Interestingly, the
Mn‐40 catalyst has three pore sizes of maximum distribution at
1.9, 3.4, and 6.6 nm (Table 1), different from those of the
KIT‐6‐40 template with double pore systems (Fig. 5(b)). This
result is a reflection on the pore size of the symmetry and de‐
gree of order. In the nanocasting process of KIT‐6‐40, the man‐
ganese material will be aggregated in both relatively inde‐
pendent channel systems to form a manganese oxide phase.
The pore walls composed of bulk silica will form a bigger pore
channel after the template removal. KIT‐6‐40 has smaller pore
sizes (2.0/2.5 nm) (Table 1). Pores of size 1.9 and 6.6 nm form
because of the complete filling of the manganese oxide phase in
one relatively independent and bigger pore size (2.5 nm) of the
double channel systems of KIT‐6‐40. The formation of pores of
size at 1.9 and 3.4 nm results from the incomplete filling of
manganese oxide phase in one relatively independent and
smaller pore size (2.0 nm) of the double channel systems of
KIT‐6‐40 [27,34]. Therefore, Mn‐40 shows three pore systems
after the removal of the template. Mn‐150 almost shows small‐
er pore sizes of maximum distributions at around 1.9 nm re‐
lated to KIT‐6‐150. KIT‐6‐150 only has a single pore system of
maximum distributions at around 6.2 nm (Fig. 5(b)). A single
pore channel was formed after the template removal.
manganese material will be aggregated in both relatively inde‐
pendent channel systems. Then, the original symmetry of the
double pore systems will change after removing the template
by using NaOH solution. Mn‐100 shows stronger (211) and
(332) diffraction peaks at 2θ = 1.0° and 1.85°, respectively,
which are the same as those of the corresponding template
KIT‐6‐100, indicating that this sample is a perfect replica of the
used KIT‐6‐100 template with cubic symmetry (ia3d) [3]. The
Mn‐150 sample has a very weak (211) diffraction peak at 1.0°,
indicating a worse mesoporous regulation exists in this sample.
This may be attributed to a dense particle structure or collapse
of the partial mesoporous structure when removing the KIT‐6
template aged at 150 °C.
Fig. 5 shows the N2 adsorption‐desorption isotherms and
pore size distributions of the samples. The physical parameters
are displayed in Table 1. With increasing aging temperature,
d211 interplanar spacing of KIT‐6 decreases gradually, and the
amount of MnO2 materials prepared by KIT‐6 at different aging
temperatures also decreases (Table 1). It further indicates that
mesoporous MnO2 is a replica of the KIT‐6 templates. With
increasing aging temperature, the pore size of KIT‐6 gradually
increases. N2 adsorption‐desorption isotherms of the samples
have hysteresis rings and display type IV isotherms, confirming
that the samples have mesoporous characteristics (Fig. 5(a))
[38]. Mn‐100 has the largest surface area and hysteresis ring
(Table 1) because of the most perfect replica of KIT‐6‐100.
Mn‐150 has the smallest surface area and hysteresis ring, indi‐
Fig. 6(a)–(c) clearly shows that Mn‐40 has a 3D mesoporous
property, but the degree of order and symmetry of pore chan‐
nels decrease. These observations are consistent with the XRD
and N2 adsorption analysis results. The channels seem to be
140
(b)
KIT-6-150
700
(a)
Mn-150
Mn-100
Mn-40
KIT-6-100
120
100
KIT-6-40
600
500
400
80
60
2
3
4
5
6
7
8
9
10
Pore diameter (nm)
300
200
100
0
2
4
6
8
10
12
40
20
0
Pore diameter (nm)
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0
0.1 0.2
0.3
0.4
0.5 0.6
0.7
0.8
0.9
1.0
Relative pressure (P/P0)
Relative pressure (P/P0)
Fig. 5. N2 adsorption‐desorption isotherms and pore size distributions of samples (a) and corresponding KIT‐6 templates (b).

634
Bingyang Baiet al. / Chinese Journal of Catalysis 39 (2018) 630–638
sesses a 3D ordered mesoporous channel structure with excel‐
lent symmetry and regulation. In comparison with Mn‐40 and
Mn‐100, Mn‐150 has a rather dense pore structure (Fig. 6(g)
–(j)). In the nanocast process, manganese oxide can be easily
interpenetrated in the bigger channel system of KIT‐6‐150 to
form a dense composite containing MnO2 and silica phase.
Mn‐150 with a dense structure and smaller pore size will be
formed after template removal. SEM images showed that some
bulk MnO2 nanoparticles coexist with mesoporous MnO2 on
Mn‐150 because of the collapse of the partial pore structure
after removing the template.
3.3. Thermal reduction behavior of catalysts
The H2‐TPR patterns of the catalysts are shown in Fig. 7(a).
Mn‐40 exhibits four reduction peaks located at 242, 310, 323,
and 422 °C because of thereduction of Mn2O3 to MnO (peak 1),
MnO2 to Mn2O3 (peak 2), Mn2O3 to Mn3O4 (peak 3), and Mn3O4
to MnO (peak 4) [3], and correspond to H2 consumption
amounts of 1.62, 3.59, 1.97, and 3.25 mmol/g, respectively (Ta‐
ble 2). The reduction peaks of Mn‐100 were located at 310,
350, and 432 °C, and correspond to H2 consumptions of 3.24,
3.43, and 3.66 mmol/g, respectively (Table 2). Mn‐100 exhibits
three reduction peaks due to the reduction of MnO2 to Mn2O3
(peak 1), Mn2O3 to Mn3O4 (peak 2), and Mn3O4 to MnO (peak 3),
respectively. The reduction peaks of Mn‐150 were located at
341, and 422 °C because of the reduction of MnO2 to Mn2O3
(peak 1) and Mn2O3 to MnO (peak 2), and correspond to H2
consumptions of 4.99 and 2.78 mmol/g, respectively (Table 2).
Mn‐40 has peak 1 at a lower reduction temperature (242 °C),
attributing to the reduction of a little small amount of Mn2O3
crystalline phase in Mn‐40, as can be seen from large‐angle
XRD results. Peak 3 at 323 °C of Mn‐40 and peak 2 at 350 °C of
Mn‐100 occur because of a special disproportionation reaction
of Mn2O3 (III) to Mn3O4 (IV and II). The dismutation is related to
more abundant surface oxygen species. In this study, the total
H2 consumption of Mn‐40 and Mn‐100 is quite close to their
theoretical H2 consumption (11.5 mmol/g), indicating that a
substantial fraction of Mn4+ and Mn3+ ions in the catalysts have
been reduced. The total H2 consumption (7.8 mmol/g) of
Mn‐150 is the lowest, perhaps because Mn‐150 contains Si
impurity that decreases the reduction of mass because of the
Fig. 6. SEM/STEM images of Mn‐40 (a–d), Mn‐100 (e, f) and Mn‐150
(g–j).
disorganized and ruleless. Fig. 6(d) also clearly displays rela‐
tively free pore channels with reduced symmetry and degree of
order. Fig. 6(e) and (f) wonderfully shows that Mn‐100 pos‐
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Mn-40
Mn-100
Mn-150
(b)
(a)
341
422
350
Mn-150
310
432
422
323
Mn-100
Mn-40
242
235
1.675
1.700
1.725
1.750
1000/T (K1)
1.775
1.800
1.825
135
185
285
335
385
435
485
535
Temperature (oC)
Fig. 7. H2‐TPR patterns (a) and initial H2 consumption rates (b) of the samples.

Bingyang Bai et al. / Chinese Journal of Catalysis 39 (2018) 630–638
635
Table 2
Reduction temperatures, H2 consumptions and surface element composition of the catalysts.
Reduction temperaturea (°C)
H2 comsumptionb (mmol/g)
Active site number c Surface element molar ratio d
Sample
peak 1 peak 2 peak 3 peak 4
peak1 peak 2 peak 3 peak 4 total
N (mmol)
2.08
Mn3+/Mn4+
Olatt/Oads
1.3
Mn‐40
Mn‐100
242
310
341
310
350
422
323
432
−
422
−
−
1.62
3.24
4.99
3.59
3.43
2.78
1.97
3.66
−
3.25
−
−
10.4
10.3
7.8
1.8
2.06
0.7
1.0
Mn‐150
1.56
0.4
1.2
a Reduction temperatures corresponding to the maximum values of reduction peaks in the H2‐TPR result. b H2 consumptions calculated by reduction
peak areas. c Mole number of Mn4+ and Mn3+ ions as active sites were calculated by the H2‐TPR result. d Surface element molar ratio calculated by peak
areas of XPS.
collapse of the pore structure, as can be seen from SEM results.
H2 consumption of peak 1 from Mn4+ to Mn3+ for Mn‐150 can
indicate the number of Mn4+ ions. If the dismutation of Mn‐40
and Mn‐100 is taken into consideration, the sum of H2 con‐
sumptions from MnO2 to Mn2O3 and Mn3O4 to MnO can confirm
the number of Mn4+ ions. Mn‐40 has the lowest reduction tem‐
perature, followed by Mn‐100, indicating that Mn‐40 possesses
better low‐temperature reducibility (Table 2). From Fig. 7(b), it
can be deduced that Mn‐40 has the highest initial H2 consump‐
tion rate and exhibits a stronger reducibility. This indicates that
oxygen species adsorbed on the surface of Mn‐40 are more
easily desorbed and can combine with hydrogen.
defects and oxygen vacancies resulting from three pore sys‐
tems and Mn2O3 phase. Surface O2− and O− species of Mn‐40 are
directly involved in the reaction. Meanwhile, surface O2− spe‐
cies play a role in oxygen supply. The desorption peaks of
Mn‐40 appear at a lower temperature than those of Mn‐100
and Mn‐150. This indicates that the oxygen species of Mn‐40
are much easier to desorb at low temperatures and participate
in the oxidation reaction, owing to three pore channels and
more surface lattice oxygen vacancies resulting from lattice
distortion in channels. Lower beginning desorption tempera‐
tures can lead to better catalytic ability. Therefore, Mn‐40 has
the best catalytic activity.
3.4. Thermal desorption behavior of catalysts
3.5. Surface composition of the catalysts
Fig. 8 shows the O2–TPD patterns of the catalysts. The de‐
sorption peaks below 600 °C, between 600 and 700 °C, and
above 700 °C are attributed to the surface‐absorbed oxygen
Mn 2p2/3 of Mn‐100 and Mn‐150 shows two components,
indicating binding energies (BE) of 641.6 and 642.9 eV, because
of the surface Mn3+ and Mn4+ ions, respectively (Fig. 9(a)) [41].
Mn 2p2/3 of Mn‐40 has three components located at 641.6,
642.7, and 644.6 eV because of the presence of a small amount
of Mn2O3 crystalline phase, corresponding to the surface Mn2+,
Mn3+, and Mn4+ ions, respectively [42]. The surface Mn3+/Mn4+
molar ratios of Mn‐40, Mn‐100, and Mn‐150 are 1.8, 0.7, and
0.4, respectively (Table 2). This indicates that the surface of
Mn‐40 possesses more abundant surface Mn3+ species, owing
to the existence of a Mn2O3 crystalline phase. Fig. 9(b) shows
that the O 1s signals were observed at 529.3 and 531.3 eV, cor‐
responding to surface lattice oxygen (Olatt) and surface ad‐
sorbed oxygen (Oads), respectively [39]. The surface Olatt/Oads
molar ratios of Mn‐40, Mn‐100, and Mn‐150 are 1.3, 1.0, and
1.2, respectively (Table 2). These results indicate that the sur‐
face of Mn‐40 has the most abundant surface lattice oxygen
species because of some Mn3+ ions provided by the Mn2O3
phase, followed by Mn‐150, owing to a dense structure and
partial bulk MnO2 particles. Surface active oxygen species play
an important role in oxidation reactions [43]. Mn‐100 has more
surface adsorbed oxygen species than Mn‐150. This implies
that the catalytic performance of Mn‐100 is very probably bet‐
ter than that of Mn‐150. The surface lattice oxygen of Mn‐40
obviously increases, which most likely results in the formation
of oxygen vacancies, which are related to the coexistence of
Mn3+ and Mn4+ ions [22].
−
(O2 and O−), surface lattice oxygen (O2−), and bulk phase lattice
oxygen (O2−), respectively [39,40]. The surface‐active oxygen
species are easy to desorb from the metal oxide at low temper‐
atures. Mn‐100 has three desorption peaks at 483, 538, and
−
813 °C, which correspond to O2 , O−, and O2−, respectively.
Mn‐150 also has three desorption peaks at 562, 625, and 830
°C, belonging to O−, surface O2−, and bulk phase O2−, respective‐
ly. The occurrence of a desorption peak at 625 °C for Mn‐150
indicates that the amount of surface O2− species increase with a
decrease in the pore size. Mn‐40 has two desorption peaks at
510 and 810 °C. The peak at 510 °C belongs to the mixture of
surface O2 , O−, and O2− species, probably because of the lattice
−
562
625
538
830
Mn-150
Mn-100
483
813
510
810
Mn-40
KIT‐6 aged at different temperatures was used as the hard
template to prepare mesoporous MnO2 catalysts. The pore siz‐
es of MnO2 samples correspond to the pore walls of KIT‐6 tem‐
plates. With decreasing aging temperature of KIT‐6, the pore
235
335
435
535
635
735
835
935
Temperature (oC)
Fig. 8. O2‐TPD patterns of the samples.

636
Bingyang Baiet al. / Chinese Journal of Catalysis 39 (2018) 630–638
529.3
(b)
O 1s
(a)
Mn 2p3/2
Olatt
642.9
Mn4+
Mn3+
641.6
Oads
531.3
Mn-150
Mn-150
Mn-100
Mn-100
Mn3+
Mn4+
Mn2+
Mn-40
Mn-40
638 639 640 641 642 643 644 645 646 647 648
Binding energy (eV)
526
528
530
532
534
536
Binding energy (eV)
Fig. 9. XPS patterns (a, b) of the samples.
sizes of KIT‐6 decrease, increasing the pore sizes of mesopo‐
rous MnO2. Mn‐40, Mn‐100, and Mn‐150 have triple, double,
and single pore systems, respectively. The increase in the pore
sizes and pore channels affect the catalytic activity for ethanol
oxidation. The increase in the pore size is beneficial to the ad‐
sorption and diffusion of reactants and products. Three pore
systems with reduced degree of order and symmetry are fa‐
vorable to form oxygen vacancies or lattice defects. In this
study, active sites for mesoporous MnO2 catalysts were defined
as the combination of Mn3+ and Mn4+. The XPS results showed
that Mn‐40 had abundant surface Mn3+ ions because of the ex‐
istence of a little Mn2O3 crystalline phase, which can improve
the number of oxygen vacancies. The increase in the oxygen
vacancies of an oxide material is beneficial to the adsorption,
activation, and migration of oxygen species for the activation of
the ethanol molecule, which can enhance the catalytic activity.
Oxygen species of Mn‐40 are much easier to desorb at low
temperatures, as can be seen from the O2‐TPD results. Surface
4. Conclusions
KIT‐6 mesoporous silica aged at different temperatures was
used as a hard template to prepare mesoporous MnO2 catalysts.
All catalysts belong to mesoporous materials. With decreasing
aging temperature of KIT‐6, the pore sizes of KIT‐6 decrease
and that of mesoporous MnO2 catalysts increase. Mn‐40,
Mn‐100, and Mn‐150 have triple, double, and single pore sys‐
tems, respectively. The Mn‐40 catalyst has a decreased sym‐
metry and degree of order owing to lower interconnections
between the pore channels of KIT‐6 aged at 40 °C. With the
increase in the pore sizes and pore system numbers, the de‐
creased order of catalytic activities for ethanol oxidation is
Mn‐40 ＞ Mn‐100 ＞ Mn‐150. The Mn‐40 catalyst shows a
higher TOF (0.11 s–1 at 120 °C) and the best catalytic activity for
ethanol oxidation because of a bigger pore size with three pore
systems owing to a decrease in the symmetry and degree of
order, more surface lattice oxygen species, formation of oxygen
vacancies resulting from more Mn3+ ions, and better
low‐temperature reducibility.
O2 , O− species of Mn‐40 can participate in the oxidation reac‐
−
−
tion of ethanol at lower temperatures. When surface O2 , O−
species are quickly consumed, surface O2− species will provide
oxygen supply for the reaction through complex oxygen migra‐
tion. XPS results confirm that more surface O2− species exist in
the Mn‐40 sample, which are beneficial to the activation and
migration of oxygen species. In the entire reaction process,
Mn3+ ions and surface O2− species can mutually increase be‐
cause of the gain and loss of electrons. The H2‐TPR results show
that Mn‐40 exhibits the best low‐temperature reducibility,
which is favorable to enhance the catalytic oxidation reaction.
Some papers have reported that surface oxygen species, active
sites, low‐temperature reducibility, and oxygen vacancies are
directly connected to the catalytic ability [8,9,21–23,44]. The
order of pore sizes, pore systems, and catalytic activities is:
Mn‐40 ＞ Mn‐100 ＞ Mn‐150. The Mn‐40 catalyst, prepared
by KIT‐6 aged at 40 °C, has the best catalytic activity for ethanol
oxidation because of a bigger pore size with three pore systems
owing to a decrease in the symmetry and degree of order, more
surface lattice oxygen species, formation of oxygen vacancies
resulting from more Mn3+ ions, and better low‐temperature
reducibility.
Acknowledgment
This study was supported by State Key Laboratory of Envi‐
ronmental Criteria and Risk Assessment, Chinese Research
Academy of Environmental Science. This study was also sup‐
ported by State Environmental Protection Key Laboratory of
Sources and Control of Air Pollution Complex.
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Graphical Abstract
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Effect of pore size in mesoporous MnO2 prepared by KIT‐6 aged at different temperatures on ethanol catalytic oxidation
Bingyang Bai *, Qi Qiao, Yanping Li, Yue Peng, Junhua Li *
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不同KIT-6老化温度制备的介孔MnO2孔径对其催化氧化乙醇的影响
拜冰阳a,b,*, 乔 琦a,b, 李艳萍b, 彭 悦c, 李俊华c,#
a中国环境科学研究院环境基准与风险评估国家重点实验室, 北京100012
b中国环境科学研究院国家环境生态工业重点实验室, 北京100012
c清华大学环境学院环境模拟与污染控制国家重点联合实验室, 北京100084
摘要: 乙醇既是一种被广泛使用的溶剂, 也大量存在于乙醇燃料车尾气中. 它是一种挥发性有机化合物(VOCs), 能直接参
与光化学反应影响空气质量, 因此去除乙醇很有必要. 催化氧化法消除VOCs 是很有前景的技术, 其关键是催化剂的制备
和筛选. 目前, 用于乙醇催化氧化的催化剂主要是贵金属催化剂(Pt, Pd, Rh, Au, Ag)和金属氧化物催化剂(Cu, Mn, Co, Fe),
此外, 还有一些钙钛矿型催化剂. MnO2具有多种结构(α, β, γ和δ)和形貌(管状, 棒状, 球状和孔状等). 不同形貌和结构的
MnO2具有不同的VOCs 催化氧化性能. 我们已经报道了介孔MnO2, 特别是三维有序介孔MnO2, 具有良好的乙醇催化氧化
活性, 有一定的应用前景. 然而, KIT-6老化温度对介孔MnO2孔径的影响, 以及MnO2孔径对催化氧化乙醇活性的影响尚不
清楚. 如果通过调整KIT-6老化温度改变介孔MnO2的孔径, 很有可能改善催化剂低温还原性, 氧物种和活性位等, 进而提高
其催化性能. 本文以40, 100和150 °C 老化合成的KIT-6介孔硅为硬模板, 制备出不同的介孔MnO2催化剂, 分别记作Mn-40,
Mn-100和Mn-150, 用于乙醇氧化反应中, 讨论了催化剂孔径对其活性的影响. 采用X 射线粉末衍射(XRD), 氮气吸附-脱附
(BET), 扫描电子显微镜(SEM), 氢气程序升温还原(H2-TPR), 氧气程序升温脱附(O2-TPD), X 射线光电子能谱(XPS)等技术
对催化剂进行了表征.
XRD 广角结果表明, 各催化剂均具有软锰矿型MnO2晶相, 其中Mn-40催化剂存在少量Mn2O3晶相. XRD 小角和SEM
结果表明, 各催化剂均为介孔材料, Mn-100催化剂的有序度和对称性最好, KIT-6老化温度的改变使Mn-40和Mn-150的有序
度和对称性降低. BET 结果表明, Mn-40, Mn-100和Mn-150分别具有三孔, 双孔和单孔体系. 随着KIT-6老化温度的降低,
KIT-6的孔径降低, 而介孔MnO2催化剂的孔径增加. XPS 结果表明, Mn-40因少量Mn2O3晶相的存在而具有较多的Mn3+阳
离子和表面晶格氧物种, 能增加催化剂氧空位的数量, 有利于氧物种的吸附, 活化和迁移, 从而增强催化活性. TPR 和TPD
表明, Mn-40催化剂具有良好的低温还原性, 它的氧物种容易在低温下脱附并参与氧化反应. 催化剂活性测试结果表明, 随
着介孔MnO2催化剂的孔径增加, 其活性增加. 催化剂孔径和活性从大到小的顺序为Mn-40＞Mn-100＞Mn-150. 以老化温
度为40 oC的KIT-6模板制备的Mn-40催化剂, 具有较高的乙醇转化频率 (TOF), 120 oC的TOF 为0.11 s‒1. Mn-40催化剂具有
良好的乙醇氧化催化活性归因于较大孔径, 其孔径呈三孔体系分布, 最大孔径分布在1.9, 3.4和6.6 nm 处, 三孔体系的形成
是因为催化剂孔道的对称性和有序度降低. 此外, Mn-40催化剂具有良好的乙醇氧化催化活性也归因于由较多Mn3+阳离子
引起的较多表面晶格氧物种和氧空位以及较好的低温还原性.
关键词: 介孔MnO2; 孔通道; KIT-6介孔硅; 催化性能; 乙醇氧化
收稿日期: 2017-12-23. 接受日期: 2018-01-25. 出版日期: 2018-04-05.
*通讯联系人. 电话: (010)84914902; 传真: (010)84914626; 电子信箱: baibingyang@tsinghua.org.cn
#通讯联系人. 电话/传真: (010)62771093; 电子信箱: lijunhua@tsinghua.edu.cn
基金来源: 国家重点研发计划 (2016YFC0209203); 国家自然科学基金 (21707130, 21325731).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).