Synthesis of novel MnOx@TiO2 core-shell nanorod catalyst for low-temperature NH3-selective catalytic reduction of NOx with enhanced SO2 tolerance

Article abstract of DOI:10.1016/S1872-2067(18)63059-1

In this study, a MnO_x@TiO₂ core-shell catalyst prepared by a two-step method was used for the low-temperature selective catalytic reduction of NO_x with NH₃. The catalyst exhibits high activity, high stability, and excellent N₂ selectivity. Furthermore, it displays better SO₂ and H₂O tolerance than its MnO_x, TiO₂, and MnO_x/TiO₂ counterparts. The prepared catalyst was characterized systematically by transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, Raman, BET, X-ray photoelectron spectroscopy, NH₃ temperature-programmed desorption and H₂ temperature-programmed reduction analyses. The optimized MnO_x@TiO₂ catalyst exhibits an obvious core-shell structure, where the TiO₂ shell is evenly distributed over the MnO_x nanorod core. The catalyst also presents abundant mesopores, Lewis-acid sites, and high redox capability, all of which enhance its catalytic performance. According to the XPS results, the decrease in the number of Mn⁴⁺ active centers after SO₂ poisoning is significantly lower in MnO_x@TiO₂ than in MnO_x/TiO₂. The core-shell structure is hence able to protect the catalytic active sites from H₂O and SO₂ poisoning.

Full text of DOI:10.1016/S1872-2067(18)63059-1

Chinese Journal of Catalysis 39 (2018) 821–830
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)
Synthesis of novel MnOx@TiO2 core‐shell nanorod catalyst for
low‐temperature NH3‐selective catalytic reduction of NOx with
enhanced SO2 tolerance
Zhongyi Sheng a,c, Dingren Ma a, Danqing Yu b, Xiang Xiao c, Bingjie Huang a, Liu Yang a,*,
Sheng Wang d
a School of Environment, Nanjing Normal University, Nanjing 210023, Jiangsu, China
b School of Chemical Engineering and Technology, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China
c Suzhou Industrial Technology Research Institute of Zhejiang University, Suzhou 215163, Jiangsu, China
d State Power Environmental Protection Research Institute, Nanjing 210031, Jiangsu, China
A R T I C L E I N F O
A B S T R A C T
Article history:
In this study, a MnOx@TiO2 core‐shell catalyst prepared by a two‐step method was used for the
low‐temperature selective catalytic reduction of NOx with NH3. The catalyst exhibits high activity,
high stability, and excellent N2 selectivity. Furthermore, it displays better SO2 and H2O tolerance
than its MnOx, TiO2, and MnOx/TiO2 counterparts. The prepared catalyst was characterized system‐
atically by transmission electron microscopy, high‐resolution transmission electron microscopy,
X‐ray diffraction, Raman, BET, X‐ray photoelectron spectroscopy, NH3 temperature‐programmed
desorption and H2 temperature‐programmed reduction analyses. The optimized MnOx@TiO2 cata‐
lyst exhibits an obvious core‐shell structure, where the TiO2 shell is evenly distributed over the
MnOx nanorod core. The catalyst also presents abundant mesopores, Lewis‐acid sites, and high
redox capability, all of which enhance its catalytic performance. According to the XPS results, the
decrease in the number of Mn4+ active centers after SO2 poisoning is significantly lower in
MnOx@TiO2 than in MnOx/TiO2. The core‐shell structure is hence able to protect the catalytic active
sites from H2O and SO2 poisoning.
Received 19 February 2018
Accepted 12 March 2018
Published 5 April 2018
Keywords:
Low‐temperature selective catalytic
reduction
Core‐shell
Nanorod
SO2 resistance
MnOx
© 2018, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
selectivity at high temperatures [5–7]. Therefore, the develop‐
ment of novel catalysts for low‐temperature NH3‐SCR remains
Selective catalytic reduction (SCR) processes are the
state‐of‐the‐art for NOx abatement in coal‐fired power plants
[1–3]. However, the commercial catalytic (V2O5‐WO3
(MoO3)/TiO2) system exhibits some typical disadvantages [4],
such as the need for a high starting temperature, narrow reac‐
tive temperature window (280–350 °C), and the decay of the N2
a challenging task for industrial application. Recently, manga‐
nese oxide‐based catalysts have attracted much attention for
their superior activity in low‐temperature SCR reactions
[8–16]. However, they always suffer from severe SO2 poisoning.
Jin et al. [17] have reported that the NO conversion by Mn/TiO2
catalysts decreases rapidly in the presence of SO2, although an
* Corresponding author. Tel./Fax: +86‐25‐85891455; E‐mail: 15700079861@163.com
The work was supported by the National Natural Foundation of China (51508281, 41771498) and the Program of Natural Science Research of Jiangsu
Higher Education Institutions of China (16KJD610001).
DOI: 10.1016/S1872‐2067(18)63059‐1 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 39, No. 4, April 2018

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Zhongyi Sheng et al. / Chinese Journal of Catalysis 39 (2018) 821–830
enhancement of the SO2 tolerance during low‐temperature SCR
was achieved after Ce doping. Sheng et al. [18] have also pro‐
posed that the presence of high concentrations of SO2 in the
flue gas leads to prominent deactivation of a Mn‐Ce/TiO2 cata‐
to ambient temperature. The resulting solid product was fil‐
tered, washed carefully with deionized water, and finally dried
in an electric oven at 80 °C for 12 h [31]. The obtained powder
was used as the MnOx nanorod core. The TiO2 shell material
was prepared via a versatile kinetics‐controlled coating method
[32]. The MnOx nanorod (0.5 mmol) core material was dis‐
persed in ethanol (100 mL) and a concentrated ammonia solu‐
tion (0.30 mL, 28 wt%) by ultrasonication for 30 min. Then,
different molar amounts of tetrabutyl titanate were added
dropwise and the reactions were allowed to proceed for 24 h at
45 °C under continuous mechanical stirring. The resultant
products were separated, collected, and washed with deionized
water and ethanol for several times. Finally, the obtained pow‐
ders were dried in an electric oven at 100 °C for 12 h and cal‐
cined at 500 °C in air for 2 h [32]. The catalysts were denoted as
MnOx@TiO2(y), where y refers to the Mn:Ti molar ratio, which
were 1:4, 1:2, and 1:1 in this study.
lyst prepared by
a
co‐precipitation method for
low‐temperature SCR of NOx with NH3. Therefore, many re‐
searchers are working toward the development of superior
SCR catalysts with high activity and good SO2 tolerance
[19–22].
Core‐shell nanostructures have been widely studied for
their unique structure, which can effectively protect the active
components from migrating and sintering [23,24]. Therefore,
the design of a catalyst with a core‐shell structure seems a
promising approach to protect the active sites from SO2 poi‐
soning. It has been reported that ceria supported on titania
nanotubes shows high activity for intermediate‐ and
high‐temperature NH3‐SCR [25–27]. Pappas et al. [28] designed
a series of manganese oxide‐confined titania nanotube catalysts
with different morphological features and found that their re‐
markable low‐temperature SCR activity was due to the abun‐
dance of surface Mn4+ species. However, most of the literature
so far regarding core‐shell nanostructures for SCR reactions
have reported titania nanotubes or carbon nanotubes (CNTs)
as the shell [29,30], while granular metal oxide@TiO2 has sel‐
dom been used.
In this work, a novel MnOx@TiO2 core‐shell nanorod catalyst
was prepared by a two‐step method, as shown in Fig. 1. MnOx
nanorods were first prepared by a hydrothermal method [31],
and then a versatile kinetics‐controlled coating method [32]
was used to fabricate a series of MnOx@TiO2 nanorod catalysts,
one of which presented a core‐shell structure. The influence of
the presence of SO2 in the flue gas on the SCR activity of the
core‐shell catalyst was also investigated.
The counterpart MnOx/TiO2 nanomaterial was also pre‐
pared by
a hydrothermal method. KMnO4 (2 mmol),
MnSO4·H2O (3 mmol), and an appropriate amount of tetrabutyl
titanate were mixed in deionized water. Then, the mixture was
sealed in a Teflon‐lined stainless steel autoclave at 180 °C for 4
h after further magnetic stirring. The sediment was then cen‐
trifuged, washed with deionized water and ethanol, dried at
100 °C for 12 h, and finally calcined in air at 500 °C for 2 h. The
catalyst was denoted as MnOx/TiO2. MnOx nanorods were pre‐
pared using the first step of the MnOx@TiO2 synthesis, and TiO2
nanoparticles were prepared using a similar hydrothermal
method as that employed for the MnOx/TiO2 catalyst.
2.2. Catalyst characterization
The morphological features of all the prepared catalysts
were characterized by transmission electron microscopy
(TEM), high‐resolution TEM (HR‐TEM), and element mapping
using a FEI TECNAI G2 F20 instrument operated at 200 kV. The
crystalline structure of the samples was examined by X‐ray
diffraction (XRD) using a Bruker D8 Advance diffractometer.
Raman spectra were recorded on an SPEX‐1403 Laser Raman
Spectrometer at room temperature with a resolution of about
0.6 cm–1. An Ar‐ion laser with a wavelength of 514.5 nm in
backscattering configuration was used to excite the crystals.
The surface area and pore distribution of the catalysts were
calculated with the Brunauer‐Emmett‐Teller (BET) equation
using Micromeritics ASSP 2020 equipment at –196 °C by N2
physisorption. The surface atomic states of the catalysts were
analyzed by X‐ray photoelectron spectroscopy (XPS) on a
2. Experimental
2.1. Catalysts preparation
MnOx@TiO2 nanorod catalysts were prepared by a new
two‐step method. KMnO4 (2 mmol) and MnSO4·H2O (3 mmol)
were dissolved in deionized water (80 mL) at room tempera‐
ture. After stirring ceaselessly for 20 min, the solution was
transferred to a Teflon‐lined stainless steel autoclave. The au‐
toclave was maintained at 180 °C for 4 h and then cooled down
Thermo
Escalab
250Xi
instrument.
H2
tempera‐
ture‐programmed reduction (H2‐TPR) experiments were per‐
formed on a Tianjin XQ TP5080 auto adsorption apparatus.
Prior to the H2‐TPR measurements, 50 mg of the catalysts were
pretreated under N2 at a total flow rate of 30 mL·min–1 at 300
°C for 0.5 h, and then cooled down to room temperature under
N2 atmosphere. The temperature was then raised to 600 °C at a
constant heating rate of 10 °C·min–1 under H2 (5 vol%)/N2 flow
(30 mL·min–1). The H2 consumption during the experiment was
monitored by a thermal conductivity detector (TCD). NH3 tem‐
Fig. 1. Schematic illustration of the fabrication process of MnOx@TiO2
nanorods.

Zhongyi Sheng et al. / Chinese Journal of Catalysis 39 (2018) 821–830
823
perature‐programmed desorption (NH3‐TPD) was performed
on the same apparatus. Before the NH3‐TPD experiments, the
catalysts (150 mg) were purged at 300 °C under a N2 flow (30
mL·min–1) for 0.5 h and cooled to 100 °C under N2 flow. The
samples were then exposed to a flow of NH3 at 100 °C for 1 h,
followed by N2 purging for 0.5 h. Finally, the temperature was
raised to 800 °C under N2 flow at a constant heating rate of 10
°C·min–1.
seen that the MnOx nanorods are well formed, being thus good
precursors for core‐shell structures. From Fig. 2(b), it is clear
that a large number of homogeneous TiO2 nanoparticles were
successfully synthesized by the hydrothermal method, with a
particle diameter ranging from 5 to 15 nm. Fig. 2(c) shows that
the MnOx and TiO2 materials are present in MnOx/TiO2 as na‐
norods and nanoparticles, respectively, and that they are ran‐
domly agglomerated. Catalysts at different Mn:Ti molar ratios
(1:4, 1:2, and 1:1) were prepared to explore the effect of the Mn
loading on the formation of core‐shell nanorods and their TEM
images are shown in Fig. 2(d)–(f). As shown in Fig. 2(d), some
TiO2 nanoparticles are attached to the MnOx nanorods, alt‐
hough most of them appear aggregated. When the Mn:Ti ratio
was increased to 1:1, almost no TiO2 nanoparticles were ob‐
served adhered to the MnOx nanorods; both nanomaterials are
well‐shaped but show no interaction. However, an obvious
core‐shell structure is observed in the case of MnOx@TiO2(1:2)
(Fig. 2(e)). Abundant TiO2 nanoparticles are evenly distributed
on the surface of the MnOx nanorods. The HR‐TEM, selected
area electron diffraction, and element mapping results (Fig.
2(g) and (h)) also confirmed the core–shell nanorod structure.
It can be seen in the HR‐TEM image (Fig. 2(g)) that the spacing
distances between the lattice planes are 0.240, 0.515, and 0.352
nm, corresponding to the (101) and (200) crystal planes of
MnOx, and the (101) plane of anatase TiO2, respectively, in
agreement with the diffraction pattern (inset in Fig. 2(g)).
These results also indicate that the shell of the MnOx@TiO2(1:2)
catalyst was successfully fabricated in the form of anatase TiO2.
From Fig. 2(h), the Ti and Mn element mappings confirm that Ti
completely covers the Mn core. All these experimental results
support the formation of a core–shell structure.
2.3. Activity tests
The SCR activity was measured in a fixed‐bed reactor where
the samples (4 mL) were loaded. The temperature was raised
from 60 to 260 °C. The typical reactant gas consisted of 500
ppm NO, 500 ppm NH3, 200 ppm SO2 (if added), 10% H2O (if
added), 5 vol% O2, and balance N2 with a gas hourly space ve‐
locity (GHSV) of 24000 h–1. Water vapor was generated by
passing N2 through a heated bottle containing deionized water.
The NO, NO2, SO2, and O2 content was measured on a flue gas
analyzer (MRU Vario Plus, Germany), and N2O was measured
on a different gas analyzer (G200). The NOx conversion and N2
selectivity were calculated as follows:
in
NOx
out
NOx
C
C
NO
x
conversion (%) =
100
in
C
NOx
in
NOx
in
out
out
N2O
C
 C C  2C
NH3
in
NO2
in
N
2
selectivity (%) =
100
C
 C
NOx
NH3
3. Results and discussion
3.1. Microstructure and morphology
3.2. XRD analysis and Raman spectra of the catalysts
The morphology of MnOx, TiO2, MnOx/TiO2, and MnOx@TiO2
at different Mn:Ti molar ratios was investigated by TEM, and
the images are shown in Fig. 2(a)–(f). From Fig. 2(a), it can be
The XRD patterns of the catalysts with different structures
Fig. 2. TEM images of MnOx (a), TiO2 (b), MnOx/TiO2 (c), and MnOx@TiO2 at different molar ratios: Mn:Ti = 1:4 (d), Mn:Ti = 1:2 (e), and Mn:Ti = 1:1
(f); HR‐TEM image and selected area electron diffraction (g); element mapping for MnOx@TiO2(1:2) (h).

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Zhongyi Sheng et al. / Chinese Journal of Catalysis 39 (2018) 821–830

(b)
(a)
 Anatase
 MnO2
TiO2
anatase







TiO2









MnOx


MnOx/TiO2





MnOx/TiO2




-MnO2
MnOx@TiO2


MnOx@
TiO2





0
200
400
600
800
1000 1200 1400
10
20
30
40
50
2/(o)
60
70
80
90
Raman shift (cm1)
Fig. 3. XRD patterns (a) and Raman spectra (b) of the catalysts (MnOx@TiO2 refers to the catalyst with Mn:Ti = 1:2).
are shown in Fig. 3(a). The reflections of the TiO2 nanoparticle
catalyst correspond to the typical diffraction pattern for ana‐
tase TiO2. The MnOx nanorod catalyst was assigned as crypto‐
melane‐type α‐MnO2 (JCPDS 44‐0141, tetragonal, I4/m, a = b =
0.978 nm, c = 0.286 nm) [33,34]. The diffraction peaks of
MnOx/TiO2 were ascribed to anatase TiO2, while no obvious
signals for manganese oxides were found. This observation
suggests that the manganese oxides are well dispersed on the
TiO2 support [35]. MnOx@TiO2(1:2) presents peaks for both
anatase TiO2 and cryptomelane‐type α‐MnO2, consistent with
the electron diffraction image in Fig. 2(g), revealing the good
crystallinity of both anatase TiO2 and α‐MnO2. However, the
peak intensity of anatase TiO2 and α‐MnO2 has diminished,
possibly due to the excellent dispersion of anatase TiO2 on the
catalyst surface.
the catalysts. As illustrated in Fig. 4, the isotherm of
MnOx@TiO2(1:2) reveals a typical type‐IV curve, indicating the
presence of mesopores according to the definition of the Inter‐
national Union of Pure and Applied Chemistry. It can be seen
that MnOx@TiO2(1:2) and MnOx/TiO2 present H3‐type and
H2‐type hysteresis loops, respectively, also indicative of their
mesoporous nature. MnOx/TiO2 displays a higher absorption at
relatively high pressures compared to MnOx@TiO2(1:2), sug‐
gesting that its mean pore size is larger than that of
MnOx@TiO2(1:2). From the inset in Fig. 4, the pore size distri‐
bution of both MnOx@TiO2(1:2) and MnOx/TiO2 indicates a
pore size of around 10 nm, which means that these two cata‐
lysts are typically mesoporous. A rich mesoporous structure
provides channels for the reactant gases to contact the inner
surface of the catalyst.
Raman spectroscopy measurements were conducted to ob‐
tain more information on the crystalline structure of the cata‐
lysts [36]. As shown in Fig. 3(b), MnOx/TiO2 presents bands
located at 142, 395, 515, and 638 cm–1, which are typical of the
anatase TiO2 phase [37,38], in agreement with the XRD analysis
in Fig. 3(a). Many researchers have reported that the vibration‐
al features of MnO2 exhibit low Raman activity [39,40], and
only the stretching vibrations of [MnO6] octahedra have been
proven to appear in the region of 500–700 cm–1 [41].
MnOx@TiO2 presents lower‐intensity bands in the Raman
spectrum. The peaks at 577 and 675 cm–1 are assigned to the
stretching vibrations of Mn–O–Mn and Mn–O, respectively [42],
consistent with the bands for ‐MnO2. No obvious TiO2 phase is
observed, due to the porous TiO2 coverage of the MnOx nanorod
surface. This result is in good agreement with the previous XPS
results. The XPS and Raman results thus support the presence
of MnO2 in MnOx@TiO2(1:2), which is crucial for excellent SCR
performance.
3.4. Catalytic activity
The activity of the catalysts at different temperatures is
presented in Fig. 5(a). It can be seen that the NOx conversion
200
0.5
MnOx/TiO2
180
0.4
MnOx@TiO2
160
0.3
140
0.2
120
0.1
100
0.0
10
100
80
60
40
20
0
Pore diameter(nm)
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p0)
3.3. BET surface area and pore structure
Fig. 4. N2 adsorption‐desorption isotherms and pore size distribution
curves (inset) of the catalysts (MnOx@TiO2 refers to the catalyst with
Mn:Ti = 1:2).
N2 adsorption‐desorption measurements were performed
to determine the specific surface area and porous structure of

Zhongyi Sheng et al. / Chinese Journal of Catalysis 39 (2018) 821–830
825
101
(a)
(c)
100
80
60
40
20
0
(b)
100
100
MnOx
99
98
97
96
95
94
93
TiO2
80
60
40
20
0
MnOx@TiO2
MnOx/TiO2
MnOx@TiO2
MnOx/TiO2
MnOx@TiO2
60 80 100 120 140 160 180 200 220 240 260
Temperature (oC)
60 80 100 120 140 160 180 200 220 240 260
Temperature (oC)
0
100
200
300
400
500
600
Reaction time (min)
Fig. 5. (a) NH3‐SCR activity of all the catalysts; (b) N2 selectivity of MnOx@TiO2(1:2) and MnOx/TiO2; (c) stability test for MnOx@TiO2(1:2) at 150 °C
(MnOx@TiO2 refers to the catalyst with Mn:Ti = 1:2). Reaction conditions: 500 ppm NO, 500 ppm NH3, 5% O2, and balance N2, GHSV = 24,000 h–1.
over the different catalysts increased with the temperature.
MnOx/TiO2 and MnOx@TiO2(1:2) exhibit higher catalytic activ‐
ity than the other two catalysts for the SCR of NOx with NH3.
MnOx/TiO2 presents better catalytic activity at temperatures
below 90 °C, while the NOx conversion reached 100% rapidly
over MnOx@TiO2(1:2) at higher temperatures. The plot of the
N2 product selectivity over these two catalysts as a function of
the temperature is shown in Fig. 5(b). The N2 selectivity of
these two catalysts increased with the temperature, although
the N2 selectivity of MnOx@TiO2(1:2) was always higher than
that of MnOx/TiO2, reaching 100% at 130 °C. The stability of
MnOx@TiO2(1:2) at 150 °C is shown in Fig. 5(c). Full NOx con‐
version was reached rapidly and remained stable for more than
10 h. Therefore, it can be concluded that MnOx@TiO2(1:2) pre‐
sents good stability, high N2 selectivity, and high SCR activity.
induced by H2O may be due to competitive adsorption with the
reactant on the active sites of the catalyst surface [44]. Howev‐
er, when 200 ppm SO2 was introduced in the reaction system,
the NOx conversion over MnOx@TiO2(1:2) and MnOx/TiO2 de‐
creased from 100% to 37.3% and 14.0%, respectively, after 5 h
at 160 °C, confirming that SO2 has a poisoning effect on the SCR
activity at low temperatures. Upon removal of SO2, the NOx
conversion over MnOx@TiO2(1:2) gradually increased to 40.1%
within 2 h with relative stability, while the NOx conversion over
MnOx/TiO2 remained almost constant at about 14.0%. When
H2O and SO2 were introduced in the system together, the NOx
conversion over both catalysts decreased rapidly. After remov‐
al of the two poisoning agents, the NOx conversion over
MnOx@TiO2(1:2) was still maintained at 22.3%, while the
MnOx/TiO2 catalyst was completely deactivated, demonstrating
that MnOx@TiO2(1:2) has better resistance to SO2 than
MnOx/TiO2. Considering the similarities between the raw ma‐
terials, composition, and preparation method, the difference
between their SO2 tolerance must originate from the core‐shell
structure: the TiO2 shell protects the MnOx active core from SO2
poisoning.
3.5. Influence of H2O and SO2 on the catalyst performance
It has been reported that H2O and SO2 in exhaust fume can
induce the deactivation of catalysts [43]. The effects of H2O and
SO2 on the NOx conversion over the MnOx@TiO2(1:2) and
MnOx/TiO2 catalysts at 160 °C are illustrated in Fig. 6. The cat‐
alytic activity of both catalysts decreased in the presence of H2O
but recovered quickly the original value once H2O was removed
from the flue gas. This result indicates that the activity decay
3.6. XPS analysis
The atomic concentration and chemical state of each ele‐
(a)
(b)
100
80
60
40
20
0
100
H2O on
H2O on
SO2 on
80
SO2 on
H2O + SO2 on
H2O off
H2O + SO2 on
H2O off
SO2 off
SO2 off
H2O + SO2 off
60
H2O+SO2 off
40
10% H2O
10% H2O
200 ppm SO2
20
0
200 ppm SO2
10% H2O + 200 ppm SO2
10% H2O +200 ppm SO2
0
50 100 150 200 250 300 350 400 450
Time (min)
0
50 100 150 200 250 300 350 400 450
Time (min)
Fig. 6. H2O, SO2, and H2O + SO2 tolerance of MnOx@TiO2(1:2) (a) and MnOx/TiO2 (b) at 160 °C. Reaction conditions: 500 ppm NO, 500 ppm NH3, 5% O2,
and balance N2, with 200 ppm SO2 and/or 10% H2O, GHSV = 24,000 h–1.

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Zhongyi Sheng et al. / Chinese Journal of Catalysis 39 (2018) 821–830
Mn 2p1/2
Mn 2p
Mn 2p3/2
Ti 2p3/2
(a)
Ti 2p
Ti 2p1/2
(b)
Ti (+4)
Ti (+4)
Ti (+3)
Ti (+3)
(1)
(1)
(2)
(3)
(4)
(2)
(3)
(4)
660
655
650
645
640
635
466
464
462
460
458
456
454
452
Binding energy (eV)
Binding energy (eV)
O
S 2p
(d)
O
O 1s
(c)
(1)
(1)
(2)
(2)
(3)
(4)
(3)
(4)
536 535 534 533 532 531 530 529 528 527 526 525 174
Binding energy (eV)
172
170
168
166
164
162
160
Binding energy (eV)
Fig. 7. XPS high‐resolution scans of the Mn 2p (a), Ti 2p (b), O 1s (c), and S 2p (d) peaks for the catalysts. (1) MnOx/TiO2; (2) MnOx@TiO2(1:2); (3)
MnOx/TiO2 after SO2; (4) MnOx@TiO2(1:2) after SO2.
ment on the surface of MnOx/TiO2 and MnOx@TiO2(1:2) before
and after SO2 deactivation were investigated by XPS measure‐
ments. Fig. 7 shows the obtained XPS spectra for Mn 2p, Ti 2p, O
1s, and S 2p, and the corresponding surface atomic concentra‐
tions and relative concentration ratio of the different oxidation
states are summarized in Table 1.
MnOx@TiO2(1:2) mainly occurs on the TiO2 shell rather than on
the MnOx core active sites and that corrosion of the shell leads
to an increased Mn atomic content on the catalyst surface. The
protection of the MnOx core active sites by the TiO2 shell is the
main reason behind the superior SO2 resistance of the
MnOx@TiO2(1:2) catalyst.
As shown in Fig. 7(a), the Mn 2p3/2 spectra could be decon‐
voluted into two characteristic peaks attributed to Mn3+
(641.60 eV) and Mn4+ (643.40 eV) [45], while the Mn 2p1/2 peak
was centered at 653.44 eV (Mn4+) [5]. As listed in Table 1, the
Mn atomic content of MnOx/TiO2 is much higher than that of
MnOx@TiO2(1:2), which is ascribed to the core‐shell structure
of MnOx@TiO2(1:2). It is also well known that manganese spe‐
cies in higher oxidation state are more active for the SCR reac‐
tion over manganese‐based catalysts, since Mn4+ ions can pro‐
mote the oxidation of nitric oxide to NO2 [46]. Therefore, the
higher Mn4+/Mn3+ ratio in MnOx@TiO2(1:2) (59.62%) is more
suitable for low‐temperature SCR and may be one of the rea‐
sons behind its higher SCR activity compared to MnOx/TiO2.
Furthermore, the Mn atomic content of MnOx@TiO2(1:2)
slightly increased after SO2 poisoning, while in the case of
MnOx/TiO2, the Mn atomic content decreased dramatically to
less than half the original value, suggesting that sulfation of
The Ti 2p spectra shown in Fig. 7(b) reveals that the titani‐
um centers in MnOx@TiO2(1:2) are mostly in the highest oxida‐
tion state (IV) and that there is no obvious difference on the Ti
oxidation state after SO2 poisoning. However, it can be clearly
seen that the peaks attributed to Ti3+ (at around 643.0 eV [47]
and 457.4 eV [48]) are present in MnOx/TiO2 before and after
SO2 deactivation. Owing to the absence of Ti3+ on the surface of
MnOx@TiO2(1:2), the Ti 2p1/2 and Ti 2p3/2 peaks shift to lower
binding energies, suggesting that MnOx@TiO2(1:2) has better
stability than MnOx/TiO2. It can also be observed that the Ti
atomic percentage in MnOx@TiO2(1:2) is almost twice that in
MnOx/TiO2, possibly due to the good coating of the TiO2 shell
over the MnOx active center core, in agreement with the TEM
and XRD results.
The O 1s peak (Fig. 7(c)) was well fitted as a combination of
two kinds of surface oxygen. The peak at ~530.9 eV corre‐
sponds to chemisorbed oxygen or hydroxyl groups (Oα), as‐

Zhongyi Sheng et al. / Chinese Journal of Catalysis 39 (2018) 821–830
827
Table 1
Atomic surface composition of the catalysts as determined by XPS.
Atomic composition (%)
Relative atomic ratio (%)
Mn
O
Samples
Mn
Ti
O
S
Mn4+
57.83
59.62
53.50
57.72
Mn3+
42.17
40.38
46.50
42.28
O
O
MnOx/TiO2
18.13
8.17
8.75
9.10
11.42
22.25
17.98
18.28
55.91
56.98
56.94
56.87


2.40
2.26
62.54
15.80
14.46
29.81
37.46
84.20
85.54
70.19
MnOx@TiO2(1:2)
MnOx/TiO2 after SO2
MnOx@TiO2(1:2) after SO2
cribed to Ti–OH or adsorbed H2O on the catalyst surface, while
the peak at ~529.2 eV is assigned to lattice oxygen (Oβ), at‐
tributed to Ti–O in the TiO2 crystal lattice [49]. It can be clearly
seen that the binding energies of Oα and Oβ over
MnOx@TiO2(1:2) are shifted to higher values (Oα: 0.6 eV, Oβ: 0.6
eV) compared to those for the MnOx/TiO2 sample. A higher
binding energy indicates new stable covalent bonding [50].
Furthermore, it has been reported that a higher concentration
of Oα species is beneficial to the NH3‐SCR of NO [50]. The rela‐
tive concentration ratio of Oα species in MnOx/TiO2 is higher
than that in MnOx@TiO2(1:2), which may result in higher cata‐
lytic activity at lower temperatures. However, the concentra‐
tion of Oα species in MnOx@TiO2(1:2) is higher than in
MnOx/TiO2 after SO2 deactivation, which is one of the reasons
behind the higher SO2 tolerance of MnOx@TiO2(1:2).
Sulfur species are observed on both MnOx/TiO2 and
MnOx@TiO2(1:2) after SO2 poisoning, attributed to the sulfur
atoms in HSO4− (168.0 eV) and SO42− (169.5 eV) [51], suggest‐
ing that the components of both catalysts react with SO2 to
form sulfate species. The S atomic content of MnOx@TiO2(1:2)
is only slightly lower than that of MnOx/TiO2 after SO2 poison‐
ing, indicating that a similar amount of sulfates is formed on
both catalysts. However, the great difference between the vari‐
ation in the Mn content for both samples, which suggests that
the sulfates formed on the surface of MnOx/TiO2 and
MnOx@TiO2(1:2) are mainly Mn(SO4)x and Ti(SO4)y species,
respectively, further confirms that the core‐shell structure ef‐
fectively protects the active core of the catalyst.
MnOx@TiO2(1:2) can be ascribed to desorption from Brönsted
acid sites and the sharp peak at ~648 °C to desorption from
Lewis acid sites. The peaks for MnOx@TiO2(1:2) are compara‐
tively shifted to higher temperatures, suggesting the presence
of stronger Lewis acid sites in MnOx@TiO2(1:2) than in
MnOx/TiO2 [56]. It is well known that Lewis acid sites exert a
major influence in SCR systems, which might be key for the
higher SCR activity of MnOx@TiO2(1:2) compared to that of
MnOx/TiO2.
After SO2 poisoning, the peak associated with the Brönsted
acid sites of MnOx/TiO2 shifts to 377 °C and the peak attributed
to Lewis acid sites to 611 °C, suggesting that the adsorption on
Lewis acid sites becomes stronger after SO2 poisoning. The
possible reason behind this observation is that the original
Lewis acid sites are damaged by SO2, but the presence of SO2
enhances the adsorption on Lewis acid sites. In the case of
MnOx@TiO2(1:2), the peak corresponding to Brönsted acid
sites remains almost unchanged and the peak of Lewis acid
sites shifts to a lower temperature (640 °C), from which it can
be deduced that the Lewis acid sites are also damaged by SO2
and that SO2 adsorption is concentrated on the TiO2 shell, re‐
sulting in the higher tolerance of MnOx@TiO2(1:2) toward SO2.
3.8. H2‐TPR analysis
In order to investigate the redox capability of the catalysts,
hydrogen temperature‐programmed reduction experiments
MnOx/TiO2
3.7. NH3‐TPD analysis
MnOx@TiO2
592
543
710
MnOx/TiO2 after SO2
It has been proposed that the adsorption of NH3 on the sur‐
face of the catalyst plays a crucial role in the SCR reaction [52].
The adsorption of NH3 is associated with the surface acidity of
the material. Thus, ammonia temperature‐programmed de‐
sorption measurements were performed in order to investigate
the surface acidity of the catalysts before and after SO2 poison‐
ing. The results are shown in Fig. 8. One broad weak peak
ranging from 100 to 300 °C is observed for all samples due to
the desorption of NH3 from weak and medium acidic sites [53].
The strong peak observed at around 710 °C is probably due to
the desorption of N2 [54]. For MnOx/TiO2, the desorption peak
648
MnOx@TiO2 after SO2
342
680
380
377
611
640
391
+
at 342 °C corresponds to the release of NH4 from Brönsted
100
200
300
400
500
600
700
Temperature (oC)
acid sites and the peaks at 543 and 592 °C correspond to the
desorption of NH3 from Lewis acid sites [55]. In contrast, the
broad weak peak at ~380 °C in the desorption curve of
Fig. 8. NH3‐TPD profiles of the catalysts (MnOx@TiO2 refers to the cata‐
lyst with Mn:Ti = 1:2).

828
Zhongyi Sheng et al. / Chinese Journal of Catalysis 39 (2018) 821–830
MnO /TiO
x
2
340
MnO @TiO
x
2
MnO /TiO after SO
2
x
2
MnO @TiO after SO
2
x
2
480
513
410
380
345
538
590
382
420
530
607
668
Fig. 10. Mechanism of the SCR of NOx with NH3 over MnOx@TiO2(1:2) in
the presence of SO2.
100
200
300
400
500
600
700
800
900
Temperature (C)
The concentration of Oα species and Mn atomic content in
MnOx@TiO2(1:2) slightly increased after exposure to SO2. The
content of sulfur in MnOx@TiO2(1:2) was almost that in
MnOx/TiO2 after SO2 poisoning. All these results suggest that
sulfation of MnOx@TiO2(1:2) occurs mainly on the TiO2 shell
rather than on the MnOx core active sites, and that the corro‐
sion of the shell leads to an increase of the Mn atomic content
on the surface of the catalyst. According to the NH3‐TPD results,
the Lewis acid sites on MnOx@TiO2(1:2) are damaged by SO2,
but SO2 is effectively isolated on the TiO2 shell, affording the
observed higher tolerance of MnOx@TiO2(1:2) toward SO2. The
uniform distribution of the MnOx core and the protection of the
TiO2 shell minimize the poisoning of surface active sites by SO2,
resulting in high stability and improved SO2 tolerance.
Fig. 10 presents a schematic of the composition changes in
the MnOx@TiO2(1:2) catalyst in the presence of SO2. The pres‐
ence of a TiO2 shell inhibits the formation of MnSO4 and hinders
the reaction of SO2 with the active material, thus protecting the
Mn‐related active sites. This is an important factor to explain
the higher SO2 tolerance of MnOx@TiO2(1:2) compared to that
of catalysts lacking a core–shell structure.
Fig. 9. H2‐TPR profiles of the catalysts (MnOx@TiO2 refers to the catalyst
with Mn:Ti = 1:2).
were carried out. As illustrated in Fig. 9, MnOx/TiO2 shows
three distinctive peaks at 340, 480, and 513 °C, corresponding
to the stepwise reduction of MnO2 to Mn2O3, Mn2O3 to Mn3O4,
and Mn3O4 to MnO, respectively [30]. For MnOx@TiO2(1:2), the
corresponding temperature of the first peak shifts to 345 °C,
suggesting that the wrapping effect of TiO2 slightly weakens the
oxidation ability of MnO2. However, the latter two peaks shift to
lower temperatures (380 and 410 °C), indicating an enhance‐
ment of the oxidation capability of the surface Mn species
[57,58].
The three distinct reduction peaks for MnOx/TiO2 and
MnOx@TiO2(1:2) after SO2 poisoning are clearly seen in Fig. 9.
All these three peaks shift to higher temperatures after poi‐
soning, indicating that SO2 has reacted with MnOx and de‐
stroyed some active centers that could interact with NOx.
3.9. Discussion
A novel MnOx@TiO2 core‐shell nanorod catalyst with high
NH3‐SCR catalytic activity was successfully fabricated via a new
two‐step method (a hydrothermal process followed by a versa‐
tile kinetics‐controlled coating method), in which abundant
TiO2 nanoparticles are evenly distributed on the surface of
MnOx nanorods. At the same time, no core‐shell structure was
observed for the counterpart MnOx/TiO2 catalyst. Based on the
catalytic activity and XPS analyses, the lower catalytic activity
of MnOx@TiO2(1:2) below 90 °C may be attributed to fewer Oα
species, as mentioned in the discussion of Fig. 7(c). However,
the higher catalytic activity of MnOx@TiO2(1:2) compared to
that of MnOx/TiO2 with the increasing temperature (>90 °C) is
the result of a larger number of strong Lewis acid sites and
higher redox capacity, as proven by the NH3‐TPD and H2‐TPR
results.
4. Conclusions
In summary, a novel MnOx@TiO2 core–shell nanorod cata‐
lyst with high NH3‐SCR catalytic activity was successfully fabri‐
cated via a new two‐step method. The catalyst exhibits high
activity, excellent N2 selectivity, high stability, and enhanced
SO2 and H2O tolerance for the low‐temperature SCR of NOx with
NH3. The uniform distribution of the MnOx core and protection
of the TiO2 shell minimize the exposure of surface active sites to
SO2, reducing the formation of byproducts such as (NH4)2SO3
and NH4HSO4, which hence affords high stability and improved
SO2 tolerance. Our study provides a novel method to solve the
SO2 poisoning problem of low‐temperature SCR catalysts.
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MnOx/TiO2 may be the result of its unique core‐shell structure.
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Graphical Abstract
Chin. J. Catal., 2018, 39: 821–830 doi: 10.1016/S1872‐2067(18)63059‐1
Synthesis of novel MnOx@TiO2 core‐shell nanorod catalyst for low‐temperature NH3‐selective catalytic reduction of NOx with
enhanced SO2 tolerance
Zhongyi Sheng, Dingren Ma, Danqing Yu, Xiang Xiao, Bingjie Huang, Liu Yang *, Sheng Wang
Nanjing Normal University; Wuhan University of Science and Technology;
Suzhou Industrial Technology Research Institute of Zhejiang University;
State Power Environmental Protection Research Institute
This work reports a novel MnOx@TiO2 core–shell nanorod catalyst, which exhibits high activity and enhanced SO2 tolerance for the
low‐temperature selective catalytic reduction of NOx with NH3 due to the protective effect of the TiO2 shell.
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低温 NH3-SCR 新型 MnOx@TiO2 纳米棒核壳结构催化剂的合成及
抗 SO2 性能研究
盛重义 a,c, 麻丁仁 a, 俞丹青 b, 肖 香 c, 黄冰洁 a, 杨 柳 a,*, 王 圣 d
a南京师范大学环境学院, 江苏南京210023
b武汉科技大学化学工程与技术学院, 湖北武汉430081
c浙江大学苏州工业技术研究院, 江苏苏州215163
d国电环境保护研究院, 江苏南京210031
摘要: 氮氧化物 (NOx) 是造成大气污染的主要来源之一, 严重威胁人类的身体健康. 选择性催化还原 (SCR) 技术由于其独
特的优势而受到青睐. 但 SCR 催化剂容易受到 SO2 的侵蚀而失活, 使用寿命降低. 开发具有抗硫能力强的高活性催化剂
已成为脱硝技术研究的热点. 核壳结构催化剂由于其独特的外壳结构能有效地减少活性物质与 SO2 的接触, 减少催化剂
中毒, 延长催化剂的使用寿命. 而水热合成法能有效控制催化剂的外形结构, 动力学外裹法能将物质均匀分布在被包裹物
质的表面, 两者结合有利于核壳结构催化剂的合成.
本文采用两步法 (水热法 + 动力学外包裹法) 制备的 MnOx@TiO2 纳米棒核壳结构低温脱硝催化剂具有高活性、高稳
定性和优异的 N2 选择性. 此外, 与 MnOx, TiO2 和 MnOx/TiO2 等纳米材料相比, MnOx@TiO2 具有更好的 SO2 和 H2O 抗性,
表明 MnOx@TiO2 具备良好的运用前景. 采用 TEM, HR-TEM, XRD, Raman, BET, XPS, NH3-TPD 和 H2-TPR 等方法对催化
剂进行了系统的表征. TEM 和 HR-TEM 结果表明, MnOx@TiO2 表现出明显的核壳结构, 且 TiO2 颗粒均匀分布在 MnOx 纳
米棒上. XRD 和 Raman 结果发现 MnOx@TiO2 中含有 MnO2, 从而保证了 MnOx@TiO2 具有良好的脱硝活性. 同时, BET,
NH3-TPD 和 H2-TPR 等结果表明, MnOx@TiO2 具有丰富的中孔结构和路易斯酸位点, 以及强氧化还原能力, 因而其催化性
能提高. 根据 SO2 侵蚀后的 XPS 结果, MnOx@TiO2 与 MnOx/TiO2 相比, 两者中的 S 元素含量差异小, 但前者中的 Mn4+降
低量远少于后者. 表明 MnOx@TiO2 的核壳结构可以在 SO2 存在条件下有效地保护活性位点, 因而具有更强的 SO2 抗性.
关键词: 低温选择性催化还原; 核壳结构; 纳米棒; 二氧化硫抗性; 锰氧化物
收稿日期: 2018-02-19. 接受日期: 2018-03-12. 出版日期: 2018-04-05.
*通讯联系人. 电话/传真: (025)85891455; 电子邮箱: 15700079861@163.com
基金来源: 国家自然科学基金 (51508281, 41771498); 江苏省高校自然科学研究面上项目 (16KJD610001).
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).