Insight into the synergism between MnO2 and acid sites over Mn-SiO2@TiO2 nano-cups for low-Temperature selective catalytic reduction of NO with NH3

Article abstract of DOI:10.1039/c7ra11868f

The rational synthesis of low-Temperature catalysts with high catalytic activity and stability is highly desirable for the selective catalytic reduction of NO with NH₃. Here we synthesized a Mn-SiO₂/TiO₂ nano-cup catalyst via the coating of the mesoporous TiO₂ layers on SiO₂ spheres and subsequent inlay of MnO₂ nanoparticles in the narrow annulus. This catalyst exhibited superior catalytic SCR activities and stability for low-Temperature selective catalytic reduction of NO with NH₃, with NO conversion of ～100%, N₂ selectivity above 90% at a temperature ～140 °C. The characterization results, such as BET, XRD, H₂-TPR, O₂/NH₃-TPD and XPS, indicated that this nano-cup structure catalyst possesses high concentration and dispersion of Mn⁴⁺ active species, strong chemisorbed O^- or O₂^2- species and highly stable MnO_X active components over the annular structures of the TiO₂ shell and SiO₂ sphere, and thus enhanced the low-Temperature SCR performance.

Full text of DOI:10.1039/c7ra11868f

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Insight into the synergism between MnO2 and acid
sites over Mn–SiO2@TiO2 nano-cups for low-
temperature selective catalytic reduction of NO
with NH3†
Cite this: RSC Adv., 2018, 8, 1979
a
a
Siyi Zheng,a Lei Song,a Siyang Tang,
Changjun Liu,ab Hairong Yue
*
and Bin Liangab
The rational synthesis of low-temperature catalysts with high catalytic activity and stability is highly desirable
for the selective catalytic reduction of NO with NH3. Here we synthesized a Mn–SiO2/TiO2 nano-cup
catalyst via the coating of the mesoporous TiO2 layers on SiO2 spheres and subsequent inlay of MnO2
nanoparticles in the narrow annulus. This catalyst exhibited superior catalytic SCR activities and stability
for low-temperature selective catalytic reduction of NO with NH3, with NO conversion of ꢀ100%, N2
selectivity above 90% at a temperature ꢀ140 ꢁC. The characterization results, such as BET, XRD, H2-TPR,
O2/NH3-TPD and XPS, indicated that this nano-cup structure catalyst possesses high concentration and
Received 27th October 2017
Accepted 23rd December 2017
dispersion of Mn4+ active species, strong chemisorbed Oꢂ or O2
species and highly stable MnOX
2ꢂ
DOI: 10.1039/c7ra11868f
active components over the annular structures of the TiO2 shell and SiO2 sphere, and thus enhanced the
low-temperature SCR performance.
rsc.li/rsc-advances
The transitional mental oxides, including FeOX, CeO2, NiO,
CuO and MnOX have been extensively investigated for the SCR
reaction at the temperature below 300 ꢁC.8,14–16 The MnOX based
1. Introduction
The combustion of fossil fuels, i.e., coal, petroleum and natural
gas in stationary and mobile sources generates massive NOX
emissions in the world, which gives rise to serious environ-
mental problems such as the formation of acid rain, haze,
photochemical smog and the depletion of ozone.1–6 The selec-
tive catalytic reduction (SCR) of NOX with NH3 is a crucial
methodology for the reduction of NOX emissions from
stationary sources, such as coal-red power plants, because of
its low cost and high efficiency. The current technologies for the
industrial SCR processes primarily employ commercial V-based
catalysts and operate at temperatures of 300–400 ꢁC.7–9 There
are some key issues for the high-temperature SCR process, such
as obligatory placement of the catalysts in the upstream of the
electrostatic precipitator and desulfurizer, catalytic deactivation
by SO2 or y ash, and the generation of N2O at high tempera-
ture.10–13 In addition, the toxicity of V-based catalysts is detri-
mental to the environment and human-beings. Therefore, the
rational design and development of excellent and green cata-
lysts at low temperature is vital for the SCR of NOX with NH3.
catalysts possess excellent low-temperature activity and envi-
ronmentally benign property, and were regarded as excellent
candidates for the industrial application in SCR reaction.17–23
The strong redox property of Mn4+ as the dominant valence
species and the low crystallinity of MnOX nanopartilces in MnO2
catalysts is crucial to promote the oxidation of NO to NO2 and
lead to an excellent low-temperature NH3-SCR activity.24–27 Pena
et al. found that the MnOX catalysts with different valence states
and labile oxygen contributed to the excellent SCR performance
ꢁ
in a low temperature of 120 C. The presence of chemisorbed
oxygen, which is more active than the lattice oxygen, could also
facilitate the oxidation of NO for the formation of nitrates and
result in a high NO conversion.28–31
In addition, the catalyst support with Brønsted and/or Lewis
acid sites was veried to enhance the adsorption of NH3
through three hydrogen bonds, and thus facilitate the low-
temperature NH3-SCR reaction. Recently, Boningari et al. re-
ported the NH3-SCR reaction mechanism over Mn-based cata-
lysts via the correlation of activity with the acidity, manganese
oxide valence state and morphology.32 The results indicated that
the high dispersion of MnOX and appropriate surface acidity
play a synergistic effect on the SCR reaction. TiO2 is a widely
used catalyst support for its stable chemical properties and
environmental friendliness, especially the excellent resistance
of SO2 for the SCR reaction in the industrial ue gas.12,16,33,34 The
aMulti-phases Mass Transfer and Reaction Engineering Laboratory, School of Chemical
Engineering, Sichuan University, Chengdu 610065, China. E-mail: hryue@scu.edu.cn;
Fax: +86 22 85997677; Tel: +86 22 85997677
bInstitute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu
610207, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra11868f
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nature of TiO2, such as facet, shape and acid sites, also plays an several hours (i.e., 1.5, 2.5 and 3 h) to tune the size of SiO2 and
essential role in the catalytic activities and long-term the TiO2 layers. Then the samples were washed by ethanol for
stability.35,36 Deng et al. reported that MnOX/TiO2 catalyst with several times and dried at 80 C for 6 h. The obtained samples
ꢁ
a preferentially anatase TiO2 facet facilitated high NO conver- were denoted as the SiO2@TiO2, SiO2@TiO2-1.5, SiO2@TiO2-2.5
sion and low N2O selectivity at 80–280 ꢁC.37 Zhang et al. reported and SiO2@TiO2-3.
that the fabrication of core–shell nanomaterials could stabilize
Preparation of Mn–SiO2/TiO2 nano-cup catalysts. The cata-
the active species and enhance stability.38 Therefore, the lysts were prepared by ultrasonic impregnation method using
rational synthesis of TiO2 supported Mn catalysts with unique manganese nitrate as the precursor. Firstly, the SiO2 and
morphologies and surface chemistry for the high efficiency and SiO2@TiO2 spheres, and SiO2@TiO2 nanocups were dispersed
stability is vital for the catalytic SCR reaction.24,39–41
in water with precursor added and stirring for 0.5 h, and then
Herein, a novel Mn–SiO2@TiO2 nano-cup catalyst was under ultrasonic treatment (25 kHz and 400 W) for 2 h. The
synthesized via the coating of the mesoporous structured mixture was rst dried at 110 ꢁC for 12 h and calcined at 250 ꢁC
anatase TiO2 layers on the SiO2 spheres and subsequent inlay of for 3 h, respectively. The amount of Mn loaded on all the
MnO2 nanoparticles (NPs) in the narrow annulus. The chemical catalysts was 15 wt%. The obtained catalysts were denoted as
etching method was introduced to tune the structures of TiO2 the Mn/SiO2, Mn–SiO2@TiO2, Mn–SiO2@TiO2-1.5, Mn–SiO2@-
layers and the size of SiO2 nano-spheres, aiming at the fabri- TiO2-2.5 and Mn–SiO2@TiO2-3.
cation of highly dispersed MnO2 NPs and appropriate acid sites
for SCR reaction. The catalysts were systematically investigated
to understand surface chemistry as well as the evolution of
2.2 Characterizations
structures and active sites. The synergic mechanism between To observe the morphologies and the structures of the samples,
MnO2 and acid sites over the TiO2@SiO2 support for low- the eld emission scanning electron microscopy (SEM, JEM-
temperature SCR reaction was also proposed on the basis of 7500F, Japan JEOL) was introduced to obtain the microstruc-
the experimental results.
ture of the catalysts. The N2 adsorption isotherm of the catalysts
was obtained on ASAP 2460 (Micromeritics, USA) at ꢂ197 ꢁC.
The specic surface area was calculated based on the standard
Brunauer–Emmett–Teller (BET) theory, while the pore volume
and pore-size distribution using the Barrett–Joyner–Halenda
(BJH) method. The powder X-ray diffraction (XRD) pattern was
recorded on a Bruker D8 Advance diffractometer with CuKa
radiation to investigate the crystallographic phase of the
samples with a 2q range of 10–80ꢁ and a step size of 0.026ꢁ.
Inductively coupled plasma (ICP) method was used to deter-
mine the chemical composition of catalysts on a Varian Vista
Axial instrument. To obtain the chemical states of catalysts
elements on their surface, X-ray photoelectron spectroscopy
(XPS) measurement was applied on the Thermo Fisher Scientic
K-Alpha instrument with monochromatic Al Ka radiation.
The surface acidity of the catalysts was detected using the
temperature-programmed desorption of ammonia (NH3-TPD)
on a Quantachrome Autosorb-IQ-C chemisorption analyser. The
sample powders were rstly pre-treated in pure He at 300 ꢁC for
1 h and then cooled down to 80 ꢁC. The catalysts were constantly
saturated in anhydrous NH3 (10% in He) at a ow rate of 30
mL minꢂ1 for 30 min and the physical adsorption of NH3 was
removed by He for 30 min. Finally, the desorption was carried
2. Experimental section
2.1 Catalysis preparation
Preparation of SiO2/TiO2 nanocups. The formation process
of SiO2/TiO2 nano-cup was illustrated in Fig. 1. All reagents,
purchased from the Sinopharm Chemical Reagent Co., Ltd
(China), were of analytical standard and used without any
further purication. Firstly the mixture of tetraethyl orthosili-
cate (TEOS, 28%, 3.84 mL), deionized water (17.2 mL), dehy-
drated ethanol (99.8%, 92 mL) and ammonia solution (26%,
2.48 mL) was stirred for 4 h at room temperature. The spherical
SiO2 templates were separated from the mixture by centrifuga-
tion and washed with ethanol for 3 times. Then the spherical
SiO2 were dispersed in a mixture of hydroxypropyl cellulose
(HPC, 0.4 g), ethanol (80 mL), and deionized water (0.4 mL).
Aer stirring for 40 min, titanium tert-butoxide (TBOT, 98%, 4
mL) in 20 mL ethanol was added dropwise into the mixture with
stirring under reux conditions for 100 min until the suspen-
ꢁ
sion was heated to 85 C. The nal product was isolated using
ꢁ
centrifugation and calcinated in air at 500 C for 2 h (heating
rate of 2 ꢁC minꢂ1). The obtained SiO2@TiO2 samples in 40 mL
ꢁ
out by heating the samples from 80 to 650 C at a heat rate of
water were added with 8 mL NaOH (1.25 M) and treated for
10 ꢁC minꢂ1. The oxygen temperature-programmed desorption
(O2-TPD) and H2 temperature-programmed reduction (H2-TPR)
were also performed on the Quantachrome Autosorb-IQ-C
chemisorption analyser. For the O2-TPD experiment, 0.1 g
ꢁ
sample was rstly pre-treated in pure He at 250 C for 30 min
and then cooled down to 50 ꢁC. Then the samples were treated
under pure oxygen for 1 h and the oxygen desorption was
carried out by heating the sample from 50 ꢁC to 750 ꢁC. For H2-
TPR experiment, 0.05 g of catalyst sample was pre-treated at
200 ꢁC in pure Ar for 1 h and then cooled down to room
temperature. Then the tests were conducted under 10% H2/Ar at
Fig. 1 The formation process of SiO2@TiO2 and TiO2 nano-cups.
1980 | RSC Adv., 2018, 8, 1979–1986
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ꢂ1
ꢁ
ꢁ
a heat rate of 10 C min from room temperature to 800 C. surface of SiO2@TiO2 is rather rough, indicating the TiO2 was
The signal of H2, O2 and NH3 was recorded by the thermal perfectly clad around the SiO2 spheres. Aer 1.5 h etching
conductivity detector (TCD).
treatment with NaOH, the SiO2@TiO2 samples showed small
opening holes on the TiO2 outer layer encapsulated with SiO2
spheres. The increase of etching time enlarged the opening
holes and reduced the inside SiO2 core as nano-cup structures
at 2.5 h. Further increase of the etching time will lead to
disappear of inner SiO2 core at about 3 h. The formed well-
dened nanocups are with uniform diameter size of about
300 ꢅ 10 nm. The etched samples possess larger specic
surfaces (93.16 m2 gꢂ1 at 2.5 h) compared with the no etching
SiO2@TiO2 samples (40.25 m2 gꢂ1). The large pores and specic
surface of the nano-cups could ensure the MnO2 active species
loaded on the support with a high dispersion, and facilitate the
adsorption of NH3 on the surface for the SCR reaction. There-
fore, the MnO2 species were uniformly and well deposited on
the surface of the SiO2@TiO2 and TiO2 nano-cups, and no
obvious agglomerates was observed from the SEM images of the
MnO2 loaded samples in Fig. 2.
2.3 Activity test
The catalytic activity was tested in a xed-bed stainless steel ow
reactor (i.d. ¼ 8 mm) at atmospheric pressure. The reaction
conditions were set as follows: 0.21 g of catalyst, 1000 ppm NH3,
1000 ppm NO, 5 vol% O2, N2 as the balance, and the total gas
ow rate was 200 mL minꢂ1, and the gas hourly space velocity
(GHSV) was 30 000 hꢂ1. The concentrations of NO and NO2 were
measured by a Laoying 3022 Flue Gas Analyser, and the N2O was
monitored by a ThermoStar Mass Spectroscope Analyser.
All activity data was recorded aer 60 min when the reaction
condition was steady. The NOX conversion and N2 selectivity
were calculated by:42
½NOX ꢃ ꢂ ½NOX ꢃ
in
out
NOX conversion ð%Þ ¼
ꢄ 100% (1)
½NOX ꢃ
in
The physical properties of Mn-based catalysts were listed in
Table 1. The results showed a much small surface area for the
ꢀ
ꢁ
½N2Oꢃ
out
N2 selectivity ð%Þ ¼ 1 ꢂ
ꢄ 100% (2)
Mn/SiO2 (20.3 m2 gꢂ1) and Mn–SiO2@TiO2 (35.9 m2 gꢂ1
)
½NOX ꢃ ꢂ ½NOX ꢃ
in
out
samples, and small pore volume of Mn/SiO2 catalyst (0.072 cm3
gꢂ1). But the Mn–SiO2@TiO2 nano-cup catalysts possessed large
BET surface areas, with the largest BET surface of 89.98 m2 gꢂ1
and pore volume of 0.22 cm3 gꢂ1 for the Mn–SiO2@TiO2-2.5
catalyst. The further extension of the etching time led to
a decrease of specic surface area. The average pore diameter of
all the catalysts is about 10–14 nm. It was suggested that larger
surface area and appropriate pore diameter (e.g., >3 nm) were
benecial to the NH3 adsorption and the SCR performance.43
The XRD patterns of the catalysts were shown in Fig. 3. The
diffraction peak of the anatase TiO2 was detected in all the
samples. The increase of the etching time weaken the peaks of
anatase TiO2, suggesting that the etching treatment could result
in a lower crystallinity of TiO2 layer. In addition, the strong
peaks of MnO2 and Mn2O3 were detected in the Mn–SiO2, Mn–
SiO2@TiO2, Mn–SiO2@TiO2-1.5 catalysts. While in the Mn–
SiO2@TiO2-2.5 and Mn–SiO2@TiO2-3 catalysts, the intensities
3. Results and discussion
3.1 The formation and the morphologies of the catalysts
The formation of Mn–SiO2@TiO2 nano-cup catalysts involved
the preparation of the TiO2 or SiO2@TiO2 nano-cups and
subsequent inlay of MnO2 NPs in the narrow annulus using
ultrasonic impregnation. As shown in Fig. 1, the spherical SiO2
template was rst synthesized via the hydrolysis of TEOS, and
the TiO2 shell on the SiO2 spheres was formed by the hydrolysis
of tetrabutyl titanate. The amorphous TiO2 shell was trans-
formed into the anatase phase in the calcination process at
500 ꢁC. Then the SiO2@TiO2 nano-spheres were put into the
NaOH solution for different time to partly remove the SiO2 core
and TiO2 layer. The corresponding SEM images in Fig. 2 showed
that the surface of SiO2 spheres is pretty smooth, while the
Fig. 2 The SEM images of nano-sphere/nano-cup supports and MnO2-based catalysts.
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Table 1 The physical properties of the Mn-based nano-cup catalysts
Samples
SSAa (m2 gꢂ1
)
Pore volumea (cm3 gꢂ1
)
Pore diametera (nm)
Mnb (wt%)
Rate constantc
Mn/SiO2
20.34
35.91
69.86
89.98
77.05
0.072
0.118
0.190
0.223
0.214
14.17
13.18
11.43
10.18
11.22
13.45%
12.67%
13.85%
14.76%
14.22%
3.00 ꢄ 102
9.83 ꢄ 102
5.24 ꢄ 102
1.90 ꢄ 103
1.48 ꢄ 103
Mn–SiO2@TiO2
Mn–SiO2@TiO2-1.5
Mn–SiO2@TiO2-2.5
Mn–SiO2@TiO2-3
a
Specic surface area (SSA) and pore volume by N2-adsorption method. b Mn content of the catalysts quantied by ICP-OES. c Pseudo-rst order
rate constant at 140 ꢁC.
Mn species on the catalyst. According to previous report, the
redox process of Mn4+ species was of utter importance in low-
temperature SCR with NH3.26 High Mn4+ content could
promote the oxidation process of NO to NO2 and facilitate the
“fast SCR” reaction, and thus enhance the low-temperature
catalyst activity.47 The surface contents of Mn4+ species on
Mn/SiO2 and Mn–SiO2@TiO2-1.5 catalyst are 39.96% and
26.22%, respectively, which is much lower than the other
catalysts.
The O 1s XPS spectra of the catalysts were presented in
Fig. 4(b), which could be separated into two peaks aer a peak-
tting deconvolution. The peak at lower binding energy of
530.0 eV could be attributed to the lattice oxygen species Oa (Ti–
O), while the peak at higher binding energy of 533–533.5 eV
could be assigned to Si–O and chemisorbed oxygen Ob (mainly
Fig. 3 XRD patterns of the Mn-based nano-cup catalysts.
2ꢂ
Oꢂ or O2 belonging to defect-oxide or hydroxyl-like group).48–50
of MnO2 were much lower, indicating the highly dispersed
MnOX species as amorphous phase over the two catalysts. The
high concentration and high dispersion of Mn active species
with amorphous phase on the surface were also veried for
a superior SCR activity.44–46
The surface chemical states of the catalysts were also
examined and the results were listed in Table 2 and Fig. 4. The
results showed that the surface concentrations of Mn species
decreased with the increase of the chemical etching time. This
could be attributed to the enhanced surface permeability of the
support via chemical etching, which enabled more Mn species
into the inside of the spherical structures.
Because of the high mobility, it is evident that the chemisorbed
oxygen species are more active than the lattice oxygen species,
suggesting the high amount of chemisorbed oxygen species is
in favour of the oxidation of NO to NO2 and the subsequent “fast
SCR” reaction.51 However, Fig. 4(b) also showed the peak of Ob
overlapped with the peak of Si–O bond in Mn/SiO2, Mn–
SiO2@TiO2-1.5 and Mn–SiO2@TiO2-2.5 samples. Although the
trend of change in chemisorbed oxygen species could be
observed in the XPS spectra, the XPS analysis results are still not
adequate to determine the quantitative analysis of the chem-
isorbed oxygen species, further O2-TPD analysis was needed to
conrm the concentration of Ob over the catalysts.
The XPS spectra of Mn 2p3/2 in Fig. 4(a) clearly showed the
Mn 2p3/2 peaks at about 642.0 eV.25 The peak could be separated
into three overlapped peaks of Mn2+, Mn3+ and Mn4+, respec-
tively. The relative ratio of Mn4+ via deconvolution over the
catalysts was summarized in Table 2. The Mn4+ content over
Mn–SiO2@TiO2-2.5 catalyst is the highest among the other
catalysts, which could be attributed to the high dispersion of
3.2 O2-TPD of the catalysts
O2-TPD is an effective way to analyse the mobility and quantity
of oxygen species, and the results of the oxygen desorption
behaviour of the catalysts were shown in Fig. 5. At desorption
ꢁ
temperature below 200 C, the oxygen adsorption peaks could
Table 2 Chemical states of the Mn-based nano-cup catalysts
Samples
Mn (%)
O (%)
Mn4+ (%)
Si–O + Ob/(Oa + Ob) (%)
Mn4+ (%)
Mn/SiO2
5.03
10.8
6.85
6.52
5.46
66.77
65.58
66.88
65.85
66.91
39.96
51.57
26.22
60.64
50.90
87.32
39.26
54.97
61.38
51.43
2.00
5.56
1.80
3.97
2.78
Mn–SiO2@TiO2
Mn–SiO2@TiO2-1.5
Mn–SiO2@TiO2-2.5
Mn–SiO2@TiO2-3
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Fig. 4 (a) Mn 2p3/2 and (b) O 1s XPS spectra of the Mn-based nano-
cup catalysts.
Fig. 6 H2-TPR profiles of the Mn-based nano-cup catalysts.
be observed on the Mn–SiO2@TiO2-1.5, Mn–SiO2@TiO2-2.5 and
Mn–SiO2@TiO2-3 samples, while no signicant desorption
peaks on the Mn/SiO2 and Mn–SiO2@TiO2 catalysts. This low-
temperature desorption peaks may correspond to the physi-
cally adsorbed oxygen or weakly chemically adsorbed oxygen
species.52 With the increase of desorption temperature, all
samples showed oxygen desorption. The strong peak centred at
around 500 ꢁC was observed on all the samples, which could be
TPR analysis agree well with that of XPS results. It is noticeable
that the two reduction peaks in the prole of Mn–SiO2@TiO2-
2.5 sample moved to a higher temperature. The decrease of
reducibility over Mn–SiO2@TiO2-2.5 sample may be ascribed to
a much strong interaction of Mn and Ti and good dispersion of
surface active species over the catalyst, which will lead to a high
stable SCR reaction. And the enhancing interaction of Mn and
Ti over this sample could be explained by more Mn species
entering into the bulk facet of Mn–SiO2@TiO2-2.5 rather than
the surface facet of other catalysts. Moreover, the broader and
lager reduction peak of Mn–SiO2@TiO2-2.5 sample than other
samples suggested the higher oxygen storage ability and higher
Mn4+ content. On the other hand, the Mn–SiO2@TiO2-1.5
sample showed lower reduction peak areas and therefore, only
presented inferior SCR performance, especially in the lower
temperature range. The H2-TPR results are well in consistent
with the O2-TPD and activity results.
assigned to strongly chemically absorbed O2 or Oꢂ species.
2ꢂ
These oxygen species are derived from the interaction of MnOX
and O2. For theꢁMn–SiO2@TiO2-2.5 sample, an extra strong peak
at around 590 C suggested the strong interaction between the
Mn and Ti species. Moreover, the strongly chemically absorbed
oxygen peak at 500 ꢁC of the Mn–SiO2@TiO2-2.5 sample was
much higher and sharp than other catalysts, indicating the
excellent oxygen adsorption capacity, which may lead to
a favourable catalytic SCR reaction.53
3.3 H2-TPR of the catalysts
3.4 NH3-TPD of the catalysts
In order to analyse the redox behaviour of the catalysts, H2-TPR
was conducted and the results were presented in Fig. 6. For the
Mn/SiO2 and Mn–SiO2@TiO2 samples, the two strong reduction
peaks at around 295 ꢁC and 400 ꢁC could be assigned to the
reduction of MnO2/Mn2O3 to Mn3O4 and the reduction of
Mn3O4 to MnO, respectively.1,37 Consequently, the results of
The adsorption of NH3 species was regarded as a key step in
NH3-SCR reaction, which greatly relys on the surface acidity of
the catalysts.54 Therefore, the surface acidity of the catalysts
were analysed via NH3-TPD, and the results were shown in
Fig. 7. There were three main peaks over the Mn–SiO2@TiO2
Fig. 5 O2-TPD profiles of the Mn-based nano-cup catalysts.
Fig. 7 NH3-TPD profiles of the Mn-based nano-cup catalysts.
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catalysts exceptꢁfor the Mn/SiO2 sample. The rst peak centred catalyst, the high N2 selectivity (above 90%) was obtained
at around 150 C could be assigned to desorption of physical during the whole operation temperature range indicated that
+
adsorption NH3 and some NH4 on the weak Brønsted acid the product of SCR reaction is in favour of N2 and not N2O over
sites. The second peak located at around 280 ꢁC could be the Mn–SiO2@TiO2-2.5 catalyst.
+
assigned to NH4 on the strong Brønsted acid sites. And the
From the characterizations (such as XRD, XPS, O2/NH3-TPD
third strong peak was a broad NH3 desorption peak from 350– and H2-TPR) above, the Mn–SiO2@TiO2 nano-cup catalyst was
650 ꢁC, which represents the coordinated NH3 species strongly demonstrated with a large specic area, highly dispersed MnOX
adsorbed onto the Lewis acid sites.55 The calculated areas of as amorphous phase and high content of surface acidity. These
Lewis acid sites for Mn–SiO2@TiO2-1.5, Mn–SiO2@TiO2-2.5 and properties are crucial for the SCR reaction. In addition, the Mn–
Mn–SiO2@TiO2-3 catalysts were 1423, 1389 and 1272, respec- SiO2/TiO2 nano-cup catalyst possesses high concentration of
tively, suggesting the surface acidity of the these three samples Mn4+ species and oxygen vacancy, which facilitate the activation
were much higher than that of the Mn/SiO2 and Mn–SiO2@TiO2 of NH3 and oxygen chemisorption of NO, and thus enhanced
catalysts. The calculated areas of the peaks decreased gradually the low-temperature selective catalytic reduction of NO with
with the increase of the NaOH etching time, which makes sense NH3.
since the alkaline NaOH would inuence the surface acidity.
To testify the stability of Mn-based nano-cup catalysts, the
long-term stability test of the SCR reaction were performed over
Mn–SiO2@TiO2-2.5 catalyst and the results were illustrated in
Fig. 9. When the NO conversion reached up to 100% aer
reaction for 3 h over the Mn–SiO2@TiO2-2.5 catalyst, it showed
an excellent SCR stability with a NO conversion stable at a value
of 100% during the next 120 h test. The high stability could be
attributed to the high dispersion of the MnO2 active sites and
the unique annulus structure between mesoporous TiO2 layers
and SiO2 sphere for stabilizing the MnO2 nanoparticles. More-
over, we added the characterizations of the 120 h-stability-tested
Mn–SiO2@TiO2-2.5 sample using XRD, BET and SEM tech-
niques, respectively. The BET surface of the used catalyst is ca.
64 m2 gꢂ1, which showed a slight decrease compared with the
fresh sample. The XRD results (Fig. S1†) showed a low intensity
of MnO2 in the used sample, indicating that MnOX species still
maintained its high dispersity and stability during the reaction
process. Moreover, the SEM image (Fig. S2†) showed that the
used catalyst could maintain the morphologies aer the long-
time reaction. In addition, the impacts of H2O and SO2 on the
SCR activity of Mn–SiO2@TiO2-2.5 sample have also been
investigated and the results were summarized in Fig. S3.† The
results showed that the NO conversion over the Mn–SiO2@TiO2-
2.5 decreased from the initial 100% to 90% with the introduc-
tion of 5 vol% H2O. However the performance could be recov-
ered aer switching off H2O. With the addition of SO2
3.5 The catalytic activities and stabilities
The catalyst performance of the NO conversion and the N2
selectivity as the function of reaction temperature were pre-
sented in Fig. 8. The catalytic activity of Mn–SiO2@TiO2-1.5 and
Mn/SiO2 catalysts were very low in the temperature range of 80–
180 ꢁC compared with the other three catalysts, which also
showed distinct declination of NOX conversion above 200 ꢁC.
Nevertheless, the Mn–SiO2@TiO2, Mn–SiO2@TiO2-2.5 and Mn–
SiO2@TiO2-3 catalysts showed signicant NO conversion
(>80%) when the temperature at 140 ꢁC and exhibited excellent
activity in the temperature range of 140–240 ꢁC. The Mn–
SiO2@TiO2-2.5 sample achieved NO conversion of 45% at 80 ꢁC
and 100% in a wide operation temperature from 150 to 240 ꢁC at
the GHSV of 30 000 hꢂ1. However, the NO conversion of the Mn–
SiO2@TiO2-3 catalyst showed much lower than that of Mn–
ꢁ
SiO2@TiO2-2.5 catalyst in the low temperature (e.g., <160 C).
The N2 selectivities over the catalysts were illustrated in
Fig. 8(b). It is obvious that the N2 selectivity over Mn/SiO2
catalyst dropped sharply during the whole temperature range
due to the unselective catalytic oxidation of NH3 on Mn/SiO2
sample.56 The N2 selectivity of Mn–SiO2@TiO2, Mn–SiO2@TiO2-
1.5 and Mn–SiO2@TiO2-3 samples dropped to around 80% at
temperature of 180 ꢁC. However, for the Mn–SiO2@TiO2-2.5
Fig. 8 The NOX conversion (a), and N2 selectivity (b) of the Mn-based
nano-cup catalysts. Reaction conditions: [NO] ¼ [NH3] ¼ 1000 ppm,
[O2] ¼ 5%, balance N2, GHSV ¼ 30 000 hꢂ1
.
Fig. 9 Stability test profile of the Mn–SiO2@TiO2-2.5 catalyst.
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(200 ppm) for 4 h, the NO conversion declined to about 42% and higher oxygen storage ability. The inferior N2 selectivity of Mn/
could not be recovered aer switching off SO2. Since the SO3 SiO2 sample might result from the subsequent formation of
from the oxidation of SO2 reacts with NH3 to form NH4HSO4 surface nitrate groups since the NO2 could react rapidly with the
and (NH4)2SO4, they could physically block the active sites and active complex to generate N2O.
result in the drop of SCR activity. Therefore, the SO2 resistance
of manganese-based catalyst at low temperature still needed the
further improvement.
4. Conclusions
A novel Mn–SiO2@TiO2 nano-cup catalyst was synthesized via the
coating of the mesoporous TiO2 layers on SiO2 sphere and
subsequent inlay of MnO2 nanoparticles in the narrow annulus.
The Mn–SiO2@TiO2-2.5 catalyst exhibited superior activity and
stability for low-temperature selective catalytic reduction of NO
with NH3, with a NO conversion of ꢀ100%, N2 selectivity above
90% at the temperature ꢀ140 ꢁC, and a stable performance
during the 120 h stability test. The characterizations results
indicated that this nano-cup structure of the Mn–SiO2@TiO2-2.5
catalyst possesses high concentration and dispersion of Mn4+
active species, strong chemisorbed Oꢂ or O22ꢂ species and highly
stable MnOX active components over the annular structures of
TiO2 shell and SiO2 sphere, and thus are favourable to the low-
temperature selective catalytic reduction of NO with NH3.
3.6 The role of MnO2 species and acidic sites of Mn–SiO2/
TiO2 nano-cup catalyst for SCR reaction
On the basis of the characterizations and the catalytic perfor-
mance of the Mn–SiO2@TiO2 catalysts, both the MnOX species
and surface acid sites are playing an essential role in the low-
temperature selective catalytic reduction of NO with NH3.
Therefore, we proposed a synergism between MnO2 and acid
sites over Mn–SiO2@TiO2 nano-cups for low-Temperature SCR
reaction and the mechanism was illustrated in Fig. 10.
At rst, the NH3 molecule was absorbed onto the Lewis acid
center and the oxygen atoms of the MnO2 sites to form ami-
nooxy groups, while the NO absorbs onto the surface oxygen
vacancies as nitrosyls. Second, the absorbed NO would be
oxidized to gas-phase NO2 in the presence of gas-phase oxygen
and oxygen vacancies were recovered.57,58 The SCR reaction
happened between aminooxy groups and gas-phase NO2 to
produce N2 and H2O via the surface dehydroxylations and re-
oxidation of oxygen vacancies. At the same time, NH3 mole-
cule could be adsorbed onto the neighboring Brønsted acid
sites on the SiO2@TiO2 support. The adsorbed NH3 would react
with NO2 to form an active complex. This step may include NO
oxidation to NO2 or diffusion of NO2 to adsorbed NH3 if
signicant gas-phase NH3 is present. The oxidation of NO to
NO2 plays an important role in NH3-SCR reaction through “fast
SCR” pathway: NO + NO2 + 2NH3 / 2N2 + 3H2O.59 The NO
oxidation of Mn–SiO2@TiO2-2.5 was tested and demonstrated
in Fig. S5† and NO-TPD analysis was also performed to inves-
tigate the adsorption behavior of NO over the ve catalyst
samples and the results are shown in Fig. S6.† The formed
active complex will react rapidly with NO or NO2 to form N2/N2O
and H2O. Then the catalyst surface was recovered and the SCR
reaction process cycles. It is possible that the interactions with
other NH3 molecules will enhance the decomposition process
and facilitates the new SCR cycles. From the proposed syner-
getic mechanism, the superior NO conversion and N2 selectivity
over the Mn–SiO2@TiO2-2.5 catalyst could be attributed to its
higher concentration of Mn4+ and Lewis acid sites, as well as the
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors are grateful for the support from the Natural
Science Foundation of China (21236004, 21576169).
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