A γ-Fe2O3-modified nanoflower-MnO2/attapulgite catalyst for low temperature SCR of NOx with NH3

Article abstract of DOI:10.1039/c8nj04524k

A mesoporous γ-Fe₂O₃-modified nanoflower-MnO₂/attapulgite (γ-Fe₂O₃/nf-MnO₂-ATP) catalyst was fabricated through a facile hydrothermal method. The results of TEM and XRD indicated that γ-Fe₂O₃/nf-MnO₂-ATP presented a beautiful nanoflower shape and good dispersion. The BET results showed that the specific surface area and pore volume of γ-Fe₂O₃/nf-MnO₂-ATP significantly increased after surface modification with γ-Fe₂O₃. Also, the XPS and NH₃-TPD results illustrated that the γ-Fe₂O₃ coating could enhance the amounts of surface chemisorbed oxygen and acidic sites. SO₂ and H₂O tolerance tests and stability tests demonstrated that γ-Fe₂O₃/nf-MnO₂-ATP can effectively inhibit SO₂ and H₂O poisoning and improve the stability of the catalyst. The superior SCR performance can be attributed to the abundant acid sites, the strong adsorption of NO_x, and the formation of more oxygen vacancies as well as metal-metal interactions. Therefore, γ-Fe₂O₃/nf-MnO₂-ATP was proved to be an excellent catalyst for NH₃-SCR and may be of interest in other environmental catalysis applications.

Full text of DOI:10.1039/c8nj04524k

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A c-Fe O -modified nanoflower-MnO /attapulgite
2
3
2
catalyst for low temperature SCR of NO with NH
x
3
Cite this:DOI: 10.1039/c8nj04524k
a
b
a
a
a
Aijuan Xie,† Yiyang Tao,† Xiang Jin, Pengfei Gu, Xiaoyan Huang,
a
a
a
a
Xingmeng Zhou, Shiping Luo, * Chao Yao and Xiazhang Li
A mesoporous g-Fe
2
O
3
-modified nanoflower-MnO
2
/attapulgite (g-Fe
2
O
3
/nf-MnO
2
–ATP) catalyst was
fabricated through a facile hydrothermal method. The results of TEM and XRD indicated that g-Fe
nf-MnO –ATP presented a beautiful nanoflower shape and good dispersion. The BET results showed
that the specific surface area and pore volume of g-Fe /nf-MnO –ATP significantly increased after
surface modification with g-Fe . Also, the XPS and NH -TPD results illustrated that the g-Fe
coating could enhance the amounts of surface chemisorbed oxygen and acidic sites. SO and H
tolerance tests and stability tests demonstrated that g-Fe /nf-MnO –ATP can effectively inhibit SO
and H O poisoning and improve the stability of the catalyst. The superior SCR performance can be
attributed to the abundant acid sites, the strong adsorption of NO , and the formation of more oxygen
vacancies as well as metal–metal interactions. Therefore, g-Fe /nf-MnO –ATP was proved to be an
2 3
O /
2
2
O
3
2
2
O
3
3
2 3
O
2
2
O
Received 6th September 2018,
Accepted 30th December 2018
2
O
3
2
2
2
DOI: 10.1039/c8nj04524k
x
2
O
3
2
rsc.li/njc
excellent catalyst for NH
3
-SCR and may be of interest in other environmental catalysis applications.
resistance performance; therefore, they have become the main-
stream in the field of flue gas denitration catalysts. However,
Introduction
3
Over the past few decades, nitrogen oxide (NO
increased with the increases in energy consumption and in the mental protection requirements, the defects of traditional
number of cars each year. It is generally considered that NO is –WO /TiO catalysts became increasingly obvious, such as
the major cause of acid rain, haze, photochemical smog, ozone higher reaction temperature (300–450 1C), toxic vanadium spe-
depletion and greenhouse effects; it has gradually become one cies and the high cost of WO . The present stage of denitration
x
) emissions have with the development of denitration technology and environ-
x
V O
2 5
3
2
3
1
of the largest sources of environmental pollution. Therefore, it technology in general is to install desulphurization and dust
is very important to research and develop efficient, stable and removal devices before the denitration section in order to avoid
inexpensive denitration catalysts which can transform the NOx decreased activity of the denitration catalyst caused by particles
emitted from coal-fired power plants and motor vehicles into in flue gas and SO
2
, which can cause the temperature of the flue
gas in the denitration phase to decrease to 100–300 1C. The
-SCR) was the first denitration technology to be developed denitration efficiency of traditional V –WO /TiO catalysts is
and used in the 1960s in Japan. The main principle of SCR is low in this temperature range; thus, it cannot meet the require-
2
non-toxic N
NH
2 x 3
. Selective catalytic reduction of NO with NH
(
3
O
2 5
3
2
4
3
that NH or urea, as a reductant, reacts selectively with NO to ments of the present stage of denitration. Meanwhile, the
generate non-toxic N at low temperature under the action of a mechanical properties and molding performance of traditional
2
metal catalyst. Due to its high efficiency of denitration, this V O –WO /TiO catalysts are poor; the material is mainly coated on
2
5
3
2
process has become the mainstream flue gas denitration tech- cordierite, stainless steel plate, or a manufactured homogeneous
nology at home and abroad. Catalysts used in the current SCR extruded honeycomb catalyst which consists of titanium oxide,
denitration technology mainly employ V
active components and TiO as the carrier. These catalysts have alcohol as the binder in practical applications. Therefore, the
high denitration efficiency, high stability, and superior sulfur (SO
2 5 3
O –WO as the main tungsten oxide, vanadium pentoxide and glass fiber with polyvinyl
5
2
2
)
research and development of low temperature denitration catalysts
with excellent denitration performance between 100 1C and 300 1C
to replace traditional V O –WO /TiO catalysts has become a
2
5
3
2
a
School of Petrochemical Engineering, Changzhou University, Changzhou 213164,
P. R. China. E-mail: shiping_luo@163.com; Fax: +86 519 86330158;
Tel: +86 519 86330158
current research focus.
In recent years, Mn-based catalysts as the main active compo-
nent have been widely applied in this field; they exhibit excellent
denitration performance at low temperature (100–300 1C) due
b
School of Fine Arts, Nanjing Normal University, Nanjing, 210046, P. R. China
†
Both are equal contribution to this work and should be regarded as first joint
6
authors.
to their strong oxidation ability and ammonia activation.
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However, single Mn-based catalysts suffer from the disadvan-
7
tages of lower N
2
selectivity and lower SO
2
resistance. Therefore,
in order to address the defects of single Mn-based catalysts,
8
,9
10,11
12
13
many researchers have chosen Fe, Ce,
Cr, Ti and other
metal oxides as catalytic components to dope MnO to obtain
x
2 3
excellent denitration performance. Fe O has recently been
widely studied as a primary catalyst due to its low toxicity, high
1
4
thermal stability and excellent anti-SO
2
performance. France
et al. used manganese acetate and ferric nitrate as precursors
to form a sol and then obtained FeMnO catalysts by drying and
1
5
x
roasting. Their test results showed that the addition of Fe greatly
enhanced the activity and SO resistance of the catalysts; over
2
90% NO conversion could be achieved at the low temperature of
x
1
50 1C, and the SO
2
and H
2
O resistance of FeMnO
x
catalysts are
and
, respectively, through calcining at different temperatures
and compared the denitration performance of a-Fe and
g-Fe under the same conditions. It was found that NH and introduce –OH onto its surface. First, the acid-treated ATP was
NO were more easily adsorbed on the surface of g-Fe O relative dispersed in deionized water to form a dispersed slurry under
1
6
also superior. Furthermore, Liu et al. prepared a-Fe
g-Fe
2 3
O
2 3 2
Scheme 1 Schematic of the synthesis of g-Fe O /nf-MnO –ATP.
2 3
O
2 3
O
O
2 3
3
2
3
to a-Fe O ; also, g-Fe O showed superior denitration perfor- ultrasonic treatment; 2.0 g of KMnO was dissolved in the ATP
2
3
2
3
4
mance, N selectivity, and NO conversion rates.
suspension, and the mixed liquor was subjected to continued
2
x
Carrier-type catalysts are conducive to dispersion of active ultrasonic processing for 20 min to thoroughly mix the ATP
components; many reports have been published on different with KMnO . Then, the mixture was transferred into a 500 mL
Attapulgite three-necked flask, and 40 mL of glacial acetic acid was added
ATP) is a clay mineral that is rich in magnesium aluminosilicate to it to adjust the pH value of the solution to B3; the
4
1
7
18
2
carriers, such as Cu-SAPO-34 and meso-TiO .
(
and shows good plasticity. ATP is beneficial as a catalyst carrier temperature of the solution was maintained at 78 1C during
for catalyst formation. Also, its rich channels are advantageous the reaction for 4 h, followed by filtering and washing to
to gas adsorption, thus improving the performance of catalysts. neutral. The resulting filter cake was dried at 80 1C to obtain
1
9,20
Therefore, ATP is also widely used in the catalytic field.
Therefore, in this work, ATP was first purified to remove as
many surface impurities as possible. Then, the purified ATP as solved in deionized water and then moved into a three-necked
a carrier was mixed with KMnO solution; the pH value was flask to conduct a reflux reaction at 90 1C; air bubbles were
adjusted to B3 with acetic acid. KMnO was slowly decom- injected with an air pump during the progress of the reaction.
posed to form flaky MnO tightly coated on the surface of the After reacting for 2 h, the resulting liquid was cooled to room
nf-MnO
2
–ATP.
0.15 g of FeSO
4
ꢀ7H
2
O and 0.29 g of FeCl
3
ꢀ7H
2
O were dis-
4
4
2
ATP under low temperature hydrothermal conditions to synthe- temperature; then, the pH value was adjusted with ammonia
size nanoflower-MnO –ATP (nf-MnO –ATP). After that, the high- water to about 10, and the mixture continued to react for 5 h
2
2
performance g-Fe O3 nanoparticles were used to modify the under water bath reflux conditions to age the sedimentary
2
surface of nf-MnO
conditions to prepare the g-Fe
catalyst. The preparation conditions of the g-Fe
ATP catalyst were gentle, simple and feasible. The as-prepared adsorbed onto the surface of nf-MnO
catalyst exhibited excellent NH -SCR denitration performance at thus, g-Fe /nf-MnO –ATP was acquired. The whole prepara-
low temperature. The enhanced N selectivity and SO resistance tion process of g-Fe O /nf-MnO –ATP is illustrated in Scheme 1.
2
/ATP under low temperature hydrothermal grains. Next, the nf-MnO
/nf-MnO –ATP denitration and this system was maintained with stirring at room tempera-
/nf-MnO ture for 1 h so that the g-Fe nanoparticles were gradually
–ATP by charge effects;
2
–ATP suspension was slowly added,
O
2 3
2
O
2 3
2
–
2 3
O
2
3
2
O
3
2
2
2
2
3
2
are due to the excellent anti-SO performance derived from g-Fe O
2 3
2
Catalyst characterization
and the unique nanoflower morphology of g-Fe O /nf-MnO –ATP.
2
3
2
X-ray diffraction (XRD) analyses of the powered samples were
performed using an X-ray diffractometer with a Cu anode
(
D/Max 2500 PC, Rigaku Corporation, Japan) running at 60 kV
Experimental
Catalyst preparation
ꢁ
1
and 30 mA with a scan range from 101 to 801 at 31 min . The
transmission electron microscopic (TEM) images were obtained
Preparation of nf-MnO –ATP. ATP raw material was provided using a JEM-2100 electron microscope (JEOL, Japan) with an
2
by Nanda Zijin Technology Group Co, Ltd (Changzhou, China); accelerating voltage of 200 kV. The zeta potential values were
other chemicals were of analytical grade and were supplied by determined with a nano-laser particle size and zeta potential
Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). The analyzer (ZEN 3600, Malvern, UK). The specific surface areas
ATP raw material was pretreated with mixed acid (volume ratio were determined by the isothermal N
2
adsorption/desorption
of H SO /HNO was 3 : 1) to remove surface impurities and method at 77 K using a Micromeritics Adsorption instrument
2
4
3
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and were then calculated from the adsorbed nitrogen volumes
by an automatic volumetric apparatus according to standard
Brunauer–Emmett–Teller (BET) theory. The X-ray photoelectron
spectroscopy (XPS) data were obtained with an Escalab 250Xi
electron spectrometer from Thermo Fisher Scientific using
Al Ka radiation. The binding energy was corrected using the
C 1s spectrum at 284.8 eV.
Both the temperature-programmed desorption of NH
NH -TPD) and the temperature-programmed reduction of H
H -TPR) were conducted using a TP 5080 auto-adsorption
2
3
2
(
(
3
apparatus (XQ, Tianjin). About 100 mg of sample was pretreated
ꢁ1
at 250 1C for 30 min using high purity He (35 mL min ); then,
adsorption under NH or H /N (the mole fraction of H was
3
2
2
2
10%) was carried out over the catalyst. Finally, the sample was
heated from ambient temperature to 800 1C in N atmosphere at
2
ꢁ
1
a rate of 10 1C min . The desorption signal of the sample was Fig. 1 XRD patterns of the different catalysts.
recorded with the relevant detector.
SCR performance tests
2 3
respectively (JCPDS No. 39-1346). This proves that the Fe O
SCR activity measurements of the catalyst were performed in a prepared by the low temperature hydrothermal method mainly
2
1
fixed bed quartz reactor (i.d. 15 mm) supplied by Huasi Instru- exists in the form of g-Fe
2
O
3
.
2
For nf-MnO –ATP, four broad
ment Co., Ltd (Hunan, China). The reaction was carried out peaks at 12.51, 25.21, 37.31 and 65.41 belonging to birnessite
ꢁ
1
22
with a 2 L min total flow rate (under ambient conditions) of a MnO
mixture of 1000 ppm NO, 1000 ppm NH , 3 vol% O
, 200 ppm the strong peak at 2y = 8.421 derived from ATP significantly
SO (when used), 10% H O (when used) and N
as the balance weakened. The main reason for this is that KMnO is decomposed
gas in the inlet with a gas hourly space velocity (GHSV) of into MnO , which is adsorbed by the ATP and coated on its
ca. 45 000 h . Before testing, mixed gas (NO + NH + N + O ) surface under acidic conditions, resulting in a decrease of the
(d-MnO ) (JCPDS No. 80-1098) can be observed, while
2
2
3
2
2
2
2
4
2
ꢁ1
3
2
2
23
was continually aerated into the fixed bed quartz reactor for strength of the ATP diffraction peak. For g-Fe
2 3
O
2
/nf-MnO –ATP,
3
0 min to saturate the catalyst and thus avoid a concentration a set of peaks at 2y = 30.21, 35.61, 43.21, 53.71, 57.21, and 62.91 can
decrease of NO
x
arising from adsorption of the catalyst. 3 mL of still be clearly observed, which is in good agreement with the
. Also, the strong
was dosed via saturator. The inlet and outlet concentrations of peaks at around 2y = 8.421 and 12.51 and other weak peaks arising
NO were continually measured by an NO/NO /NO
analyzer from ATP and d-MnO are retained; however, they become thin
KM9106, Kane International Ltd). All the data were obtained relative to those of ATP and nf-MnO –ATP due to their interac-
after 30 min as the SCR reaction reached the steady state. NOx tions with each other, indicating that g-Fe nanoparticles
catalyst were used for each SCR activity test, and water vapor characteristic diffraction peaks of pure g-Fe O
2 3
x
2
x
2
(
2
2 3
O
conversion and N selectivity were respectively calculated using have been successfully loaded on the surface of nf-MnO
2
–ATP.
2
the expressions
TEM images
½
NOxꢂ ꢁ½NOxꢂ
in
out
NOx conversion ð%Þ ¼
ꢃ 100%:
(1)
The micro-morphologies of the different samples were
observed by TEM at different magnifications, and the results
are displayed in Fig. 2. Fig. 2a shows the typical rod-shaped ATP
morphology with a diameter of 20 to 30 nm. Fig. 2b presents
beautiful MnO2 nanoflowers, which are uniformly coated
on the ATP surface and combined closely with the ATP. The
½
NOxꢂ
in
½
NOꢂin þ ½NH3ꢂ ꢁ½NO2ꢂoutꢁ2½N2Oꢂ
in
N2 selectivity ð%Þ ¼
½
NOꢂ þ½NH3ꢂ
in
in
ꢃ 100%
(2)
where [NO
x
]in and [NO
x
]out represent the inlet and outlet coating thickness is in the range of 30 to 60 nm after the MnO2
concentrations of NO
x
in the steady-state status, respectively.
nano-flower coating. Moreover, after acidification treatment,
ATP presents good dispersibility; the main reason for this may
be that acidification can remove impurities on the surface of
ATP, decrease the aggregation of ATP bundles, and introduce
oxygen-containing functional groups on the surface, which
Results and discussion
Characterization analysis
2
4
XRD patterns. The crystal structures of ATP, g-Fe O , is beneficial to the dispersion and deposition of MnO2. The
2
3
nf-MnO –ATP, and g-Fe O /nf-MnO –ATP were analyzed by nano-flower-shaped MnO2 produced by the hydrothermal
2
2
3
2
XRD, and the results are presented in Fig. 1. The XRD pattern method has a larger gas contact area, which is more favorable
of g-Fe exhibits characteristic peaks at 2y = 30.21, 35.61, to the adsorption of NH and NO on the surface of the catalyst
3.21, 53.71, 57.21, and 62.91, which are assigned to the than the nano-particle MnO prepared by a conventional
220), (311), (400), (422), (511) and (400) planes of g-Fe precipitation method. Fig. 2c shows g-Fe
2
O
3
3
4
2
(
2
O
3
,
2
O
3
nanoparticles
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2 3 2
Table 1 Zeta potentials of g-Fe O and nf-MnO –ATP
Zeta potential (mV)
g-Fe
pH value
2
O
3
2
nf-MnO –ATP
1
3
5
7
9
17.5
20.2
23.2
33.3
ꢁ29.3
1.07
ꢁ4.62
ꢁ5.98
ꢁ18.3
ꢁ32.4
2 3 2
Fig. 2h presents EDX-mapping images for g-Fe O /nf-MnO –
ATP. Mn element is present in the nano-flowers coated on the
external surface of the ATP, while Fe element, which shows
graininess, is adsorbed on the surface of nf-MnO –ATP. There-
2
fore, it is proved that the active component of the catalyst in the
as-assembled g-Fe O /nf-MnO –ATP has good dispersion,
2
3
2
which is conducive to improvement of the catalyst activity in
NH -SCR denitration.
In addition, in order to further verify whether g-Fe
particles were successfully loaded on the surface of nf-MnO
ATP, the zeta potentials of the as-prepared g-Fe
MnO –ATP were investigated to test the surface charges under
3
2
O
3
2
–
2 3
O and nf-
Fig. 2 TEM images of ATP (a), nf-MnO
2
–ATP (b), g-Fe
2
O
3
(c, d, inset in c:
/nf-MnO –ATP
/nf-MnO
2
statistical analysis of particle size distribution), and g-Fe
e and f); HRTEM images (g) and EDX-mapping (h) of g-Fe
2
O
3
2
(
2
O
3
2
–ATP. different pH values. The pH value of each solution was adjusted
with acetic acid or ammonia, and the test data are shown in
Table 1. It can be seen from Table 1 that the zeta potentials of
prepared through the low temperature hydrothermal method. g-Fe
2
O
3
(33.3 mV) and nf-MnO
2
–ATP (ꢁ18.3 mV) have diametri-
As can be seen, g-Fe O presents a slight aggregation pheno- cally opposite charges at pH 7, which can generate attractive
2
3
menon; the dominant reason for this is that g-Fe O itself is a charges between them; thus, g-Fe
ferromagnetic material and thus can readily undergo aggrega- adsorbed on the surface of nf-MnO
2
O
3
can be successfully
2
–ATP. Compared to
2
3
tion due to magnetic attraction between particles. Moreover, mechanical mixing, the charge attraction between components
g-Fe
presents nanoparticle distribution; the large surface can strengthen the combination of g-Fe and nf-MnO –ATP,
energy between the nanoparticles can readily cause agglomera- thus decreasing aggregation and shedding of particles in the
2
O
3
2
O
3
2
2
5
28
2 3
tion of the particles. The average particle size of g-Fe O
is test process. Hence, in view of the XRD and TEM results, we
2 3 2
about 7.8 nm (inset of Fig. 2c). Fig. 2d shows a high resolution can further deduce that g-Fe O /nf-MnO –ATP has been suc-
transmission electron microscopy (HRTEM) image of g-Fe O ; cessfully fabricated.
2
3
the lattice fringe distribution of g-Fe O can be seen clearly
from the picture, and the widths of the grid lines are 0.29 and
2
3
BET analysis
0
.48 nm, respectively, corresponding to the (220) and (111) The N
2
adsorption–desorption isotherms of ATP, nf-MnO
2
–ATP
2
6
crystal planes of g-Fe
2
O
3
.
Fig. 2e and 2f show TEM images and g-Fe
2
O
3
/nf-MnO –ATP and the corresponding pore size
2
of g-Fe
after surface modification with g-Fe
morphology of nf-MnO –ATP shows almost no change. MnO
2
O
3
/nf-MnO
2
–ATP. It can be seen from Fig. 2e that distributions are displayed in Fig. 3. It can be seen that all the
, the flower-like surface materials have similar adsorption curves and obvious hysteresis
loops in the P/P range of 0.5 to 1.0. According to the IUPAC
2 3
O
2
2
0
with a nanometer morphology is still coated on the surface of classification, both samples show typical type II adsorption iso-
the ATP; however, tiny particles with a uniform distribution therms with type H hysteresis loops, suggesting the existence of
3
appear in the outer surface of the nf-MnO
be clearly observed from Fig. 2f, where the particles have catalysts. Meanwhile, on the basis of the BJH (Barret–Joyner–
been successfully embedded in the nf-MnO –ATP flake petals. Halenda) model, the pore size distributions of the samples were
This is beneficial to uniform dispersion of g-Fe and, thus, also calculated, and the results are shown in Fig. 3b; all the materials
2
–ATP, which can well-defined mesoporous structures on the surfaces of the two
29
2
2 3
O
avoiding catalyst aggregation in practical applications under show two different pore size distributions within 2 to 30 nm. The
high temperature conditions. Fig. 2g shows HRTEM images distribution around 3.8 nm mainly arises from the surface adsorp-
of g-Fe O /nf-MnO –ATP.
tion holes of the ATP clay, and the distribution around 21.2 nm
It can be found that two different lattice fringes appear on arises primarily from staggered stacking of ATP. After nf-MnO2
the surface of g-Fe /nf-MnO –ATP; the lattice fringe spacings surface loading, the pore size distribution of nf-MnO –ATP is
are 0.48 nm and 0.25 nm, respectively, which belong to weakened around 21 nm, while there is a significant increase at
the (111) crystal plane of g-Fe and the (200) crystal plane 3.88 nm. This may be due to the covering of flaky MnO on the ATP
of d-MnO , respectively, according to a literature report.
2
3
2
2
O
3
2
2
2
O
3
2
2
7
30
2
surface, which divides some larger pores into smaller ones.
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a larger specific surface area and pore volume are advantageous
to adsorption and contact of gas on a surface; these can provide
more reaction sites for catalytic reactions, which can improve
3
2
the NO conversion rate of the catalyst.
x
XPS analysis
XPS was employed to characterize the near surface atomic
compositions and the chemical states of nf-MnO –ATP and
g-Fe /nf-MnO –ATP. The wide survey spectra of nf-MnO –ATP
and g-Fe /nf-MnO –ATP reveal the presence of Mn, Fe, and O
2
O
2 3
2
2
2
O
3
2
3
3
elements, and the results are displayed in Fig. 4(a). It can be
seen that only Mn 2p and O 1s peaks are present in the survey
spectrum of nf-MnO –ATP; meanwhile, after modification by
2
g-Fe
in the survey spectrum of g-Fe
g-Fe was successfully combined with nf-MnO
2
2
O
3
, the Fe 2p peak at a binding energy near 710 eV appears
/nf-MnO –ATP, indicating that
–ATP.
O
2 3
2
O
2 3
Fig. 4b shows the Mn 2p spectra, where the two main peaks
of Mn 2p3/2 and Mn 2p1/2 can be observed at 641.9 eV and
653.9 eV, respectively. The Mn 2p3/2 spectra can be deconvolved
into two peaks that appear at 642.1 eV and 643.5 eV, corres-
3
+
4+
ponding to Mn (641.6 to 642.3 eV) and Mn (642.3 to
6
3
3
44.5 eV), respectively.
Table 3 displays the surface elements of the catalysts; the
4
+
4+
3+
percentages of Mn /(Mn + Mn ) are 36.9% for nf-MnO
and 39.5% for g-Fe /nf-MnO –ATP, implying that the Mn
content in nf-MnO –ATP increases after modification with
g-Fe O . The increase of Mn content with a higher oxidation
2
–ATP
4
+
2
O
3
2
2
4
+
2
3
state is helpful for redox reactions and can enhance the
conversion from NO to NO ; more NO is conducive to fast
2
2
Fig. 3
N
2
adsorption–desorption isotherms (a) and corresponding pore SCR reactions and also improves the activation of the catalyst
3
4
size distributions of ATP, nf-MnO
2
–ATP and g-Fe
2
O
3
/nf-MnO
2
–ATP (b).
during low temperature SCR.
In Fig. 4c, two distinct peaks appear at 711.02 eV and
24.7 eV that can be assigned to Fe 2p3/2 and Fe 2p1/2
respectively. The binding energy difference between Fe 2p3/2
and Fe 2p1/2 is about 13.68 eV. In addition, there are distinct
satellite (Sat.) peaks at around 719.5 eV and 739.5 eV; the
binding energy difference between the satellite peak at around
7
,
Furthermore, after surface modification by g-Fe
number of adsorption holes in the range of 3.7 to 3.9 nm
decreased slightly; this may be because g-Fe loading covered
some of the holes on the surface of the ATP.
2 3
O , the
2 3
O
Furthermore, to better reveal the porous structures of the
as-prepared catalysts, their specific surface areas, pore volumes
and pore diameters are listed in Table 2. Both catalysts show
larger specific surface areas due to the structural advantages of
ATP itself, which can not only provide a larger specific surface
area and porous structures, but can also be helpful for gas
adsorption and contact reactions. Additionally, the plasticity of
7
19.5 eV and that of Fe 2p3/2 is 8.48 eV. All the above evidence
further demonstrates that FeO in g-Fe /nf-MnO –ATP is in
, which is consistent with other literature
x
2
O
3
2
2 3
the form of g-Fe O
reports.
As shown in Fig. 4d, two distinct peaks can be separated in
the O 1s spectra by performing the same peak-fitting deconvo-
lution technique. The peak at a lower binding energy (529.6 to
35,36
3
1
ATP itself is beneficial to the catalyst molding. Compared
5
30.2 eV) is ascribed to lattice oxygen (marked as O ); the other
b
with nf-MnO –ATP, the specific surface area and pore volume of
2
peak at a higher binding energy (531.6 to 532.0 eV) is related
to chemisorbed oxygen (marked as O
O , corresponding to defect oxide or hydroxyl-like groups,
respectively.
2 3 2
g-Fe O /nf-MnO –ATP show significant increases, and the pore
2ꢁ
a
), which includes O
2
or
size also shows a certain increase. It is generally recognized that
ꢁ
3
7,38
It has been reported that surface-adsorbed
oxygen O presents higher mobility compared to lattice oxygen
a
39
Table 2 Specific surface areas, pore volumes and pore sizes of nf-MnO
2
–
O , which has more activity in oxidation reactions According to
b
2 3 2
ATP and g-Fe O /nf-MnO –ATP
the XPS results of O 1s, the ratios of O /(O + O ) in nf-MnO –ATP
a
a
b
2
and g-Fe O /nf-MnO –ATP are 55.6% and 61.2%, respectively,
Specific surface Pore volume Pore
2
3
2
2
ꢁ1
3
ꢁ1
Catalyst
area (m g
)
(cm g
)
diameter (nm) which demonstrates that the surface active oxygen O
compo-
2 3
nents obviously increase after modification with g-Fe O . A high
a
nf-MnO
2
–ATP
166.0
–ATP 199.7
0.294
0.359
7.09
7.29
g-Fe /nf-MnO
2
O
3
2
relative concentration ratio of O
a
/(O
a
+ O
b
) on the catalyst
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Table 3 Atomic concentrations on the surface of the catalysts deter-
mined by XPS
Atomic concen-
tration (at%)
Relative atomic ratio (at%)
4
+
3+
Catalysts
nf-MnO –ATP
Mn Fe
O
a a b
Mn /Mn Fe /Fe O /(O + O )
2
47.78 — 52.22 36.9
–ATP 34.86 11.6 53.54 39.5
—
55.6
61.2
g-Fe /nf-MnO
2
O
3
2
100
surface is beneficial to oxidation of NO and adsorption of
NH . It is generally believed that NH adsorption in the form
3
3
+
of NH4 can also be enhanced due to the production of larger
amounts of surface hydroxyl groups, which act as Brønsted acid
+
sites. The formed NH
4
can react with adsorbed NO
2
to produce
active intermediate species and then further react with gaseous
39
NO to produce N
2 2
and H O, thus improving the denitration rate.
NH -TPD analysis
3
It is commonly believed that NH molecules can be adsorbed
3
and activated by acid sites on the surfaces of catalysts; this is
viewed as a crucial step during the NH
the NH -TPD was measured to evaluate the strength distribu-
tions and the amounts of acidic sites on the catalyst surfaces.
2
As shown in Fig. 5, both nf-MnO –ATP and g-Fe /nf-MnO –
3
-SCR process. Hence,
3
2
2
O
3
ATP present three distinct desorption peaks. The desorption
peaks observed below 200 1C (denoted as a) and the desorption
peaks between 300 1C and 400 1C (denoted as b) can be
assigned to the NH desorbed by weak acid sites and medium
3
acid sites on the catalysts, respectively.
The peaks above 450 1C (denoted as g) can be attributed to
chemisorbed NH
sites. It is generally recognized that NH
3
molecules adsorbed by the strong acid
4
0
+
4
ions coordinated
to Brønsted acid sites are less thermally stable than NH
3
molecules bound to Lewis acid sites. The desorption peaks at
low temperature (below 350 1C) are associated with coordinated
2 2 3 2
Fig. 4 XPS survey spectra of nf-MnO –ATP and g-Fe O /nf-MnO –ATP
(a); high-resolution spectra of Mn 2p (b), Fe 2p (c), and O 1s (d).
surface is correlated with high SCR activity. Meanwhile, the
presence of more surface adsorbed oxygen on the catalyst Fig. 5 NH
3 2 2 3 2
-TPD profiles of nf-MnO –ATP and g-Fe O /nf-MnO –ATP.
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Table 4 NH
3
-TPD peak areas of the catalysts
Table 5
H
2
-TPR peak areas of the catalysts
Peak a
(mmol g
Peak b
(mmol g
Peak g
(mmol g
Peak a
(mmol g
Peak b
(mmol g
Peak g
(mmol g
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
Catalyst
)
)
)
Catalyst
)
)
)
nf-MnO
2
–ATP
174.6
205.7
65.7
100.8
579.2
1119.1
nf-MnO
g-Fe
2
–ATP
/nf-MnO
51.7
19.5
94.9
96.3
—
48.4
g-Fe /nf-MnO
2
O
3
2
–ATP
2
O
3
2
–ATP
+
NH4 ions derived from the Brønsted acid sites, whereas the Additionally, the H2 reduction peak of g-Fe O /nf-MnO –ATP
2
3
2
desorption peaks at high temperature (above 400 1C) are moves to
a
lower temperature, showing that g-Fe
–ATP has stronger oxidation performance and can
–ATP shifts oxidize H at low temperatures. A lower reduction temperature
obviously to the high-temperature side, indicating that the is considered to aid the catalytic reaction.
surface acidity of the catalyst is significantly enhanced after studied manganese oxide and found that the strong oxidation
2 3
O /
4
1,42
assigned to NH
3
molecules binding to Lewis acid sites.
/nf-MnO
nf-MnO
2
This NH desorption peak of g-Fe
3
2
O
3
2
2
4
7,48
49
Tang et al.
4
3
4+
modification with g-Fe O nanoparticles.
performance of MnO mainly arises from Mn . Based on the
2
2
3
4
+
Moreover, the integral areas of the peaks in nf-MnO2– XPS results, the g-Fe O /nf-MnO –ATP contains a rich Mn
2
3
2
ATP and g-Fe O /nf-MnO –ATP were calculated in order to ratio; this result is consistent with the H -TPR results. Usually,
2
3
2
2
compare the NH
g-Fe /nf-MnO
relative to nf-MnO
improvement of the catalyst acid strength and the increase of integration, and the results are listed in Table 5. It can be seen
surface NH adsorption will be beneficial to the NH -SCR reaction. from Table 5 that the surface of g-Fe /nf-MnO –ATP contains
3
desorption amounts. As shown in Table 4, the H
2
reduction peak area represents the amount of reduction
5
0
2
O
3
2
–ATP has a high NH
3
adsorption capacity species on the surface of a catalyst.
Therefore, the area of each reduction peak was estimated by
2
–ATP. Combined with the above analysis, the
3
3
2
O
3
2
more reducible species. This proves that surface modification
H -TPR measurements
2
with g-Fe O nanoparticles not only decreases the H reduction
2
3
2
H
2
-TPR measurements were also performed to investigate the temperature of nf-MnO
reducibility of the catalysts. In the profile of nf-MnO
Fig. 6), two obvious reduction peaks appeared at 278 1C conducive to enhance the catalytic reduction of the surface in
and 388 1C, respectively. These mainly arise from the gradual the process of NH -SCR and can improve the low temperature
reduction of surface manganese oxides. The first peak at 278 1C can performance of the catalyst.
be assigned to the reduction peak of MnO to Mn , and the
2
–ATP, but also increases the amounts
–ATP of reducible species on the surface of the catalyst, which is
2
(
3
2
2 3
O
O to MnO.
2 3
4
4,45 NH -SCR activity
3
second one at 388 1C may be due to reduction of Mn
In comparison to nf-MnO –ATP, there is an evident reduction In order to compare the low temperature performance of the
peak at 476 1C in the g-Fe /nf-MnO –ATP profile in addition to catalysts, the NO conversion rates of g-Fe , nf-MnO –ATP
peaks at 262 1C and 374 1C. According to the literature, /nf-MnO –ATP were investigated at 100 1C to
2
O
2 3
2
x
2
O
3
2
1
9,46
and g-Fe
2
O
3
2
the reduction peak around 380 1C is related to the reduction of 300 1C, and the results are shown in Fig. 7. It can be seen from
g-Fe O to Fe O and the other at 510 1C can be attributed to Fig. 7a that the catalytic activity of pristine g-Fe O3 nano-
2
3
3
4
2
the reduction of Fe O to FeO. Therefore, the reduction peak particles as a catalyst for NH -SCR denitration is relatively poor.
3
4
3
at 476 1C may be attributable to overlapping resulting from The NO conversion rate presents a slowly rising trend below
x
the reduction of g-Fe
O
2 3
to Fe O
3 4
and of Fe O
3 4
to FeO. 150 1C, increases more rapidly above 150 1C, and reaches
around 90% at about 230 1C. Comparing the NO conversion
rates of nf-MnO –ATP and g-Fe /nf-MnO –ATP, it can be
found that the two catalysts have similar increasing tendencies
at first. However, after 120 1C, the NO conversion rate of
g-Fe O /nf-MnO –ATP is obviously higher than that of
x
2
2
O
3
2
x
2
3
2
nf-MnO –ATP; it can reach more than 92% at about 150 1C
2
and does not decline within 300 1C. It is evident that the
g-Fe
denitration ability and a wide activity temperature range.
In addition, the N selectivities of the present catalysts were
investigated (Fig. 7b). For nf-MnO –ATP, the N selectivity
2 3 2
O /nf-MnO –ATP catalyst has excellent low-temperature
2
2
2
began to decrease at 150 1C; it reached about 60% when the
temperature increased to 300 1C. The main reason for this
is that non-selective oxidation of NH3 results in an increase
of N O byproduct. For g-Fe O /nf-MnO –ATP, there was no
2
2
3
2
significant decline, and the selectivity remained at around
8
5% in the whole temperature range; this proves that g-Fe
2
O
3
2
4
6
Fig. 6
H
2
-TPR profiles of the catalysts.
2
has excellent N selectivity. Yang et al. studied a Fe–Mn/TiO
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only about 1% of fluctuation was observed in the whole test
5
1
process. Zhang et al. found that the decreased catalyst stabi-
lity was mainly due to the aggregation of active group particles
on the surface at high temperature and during longtime reac-
tion. In the present work, the active components were coated
on the surface of ATP in the form of sheets or nano-flowers.
2 3
According to the TEM analysis, the modified g-Fe O is mainly
adsorbed in the centers of the nano-flakes; with the aid
of the physical isolation of the nanopetals, the aggregation
phenomenon can be effectively avoided, which can enable the
catalyst to retain higher stability in long-time tests.
SO
In the actual denitration process, flue gas contains small
amounts of SO gas and H O vapor, and the presence of these
substances has a certain effect on NH -SCR performance.
According to reports, manganese oxide has lower SO resis-
2 2
and H O resistance tests
2
2
3
5
2
2
tance ability due to fact that manganese oxide can readily react
with SO2 in flue gas to generate a large amount of sulfate,
resulting in a decrease of the amount of the active component
4
+
of Mn and thus leading to a decrease of the catalytic activity.
As an excellent denitration material, g-Fe nanoparticles
were modified on the surface of nf-MnO –ATP, which can
interact with nf-MnO –ATP to enhance the ratio of Mn on
the surface of the catalyst. On the other hand, this can play a
protective role and decrease the toxicity of SO gas to nf-MnO
2 3
O
2
4
+
2
2
2
–
ATP. Therefore, to evaluate the actual application capability of
the catalyst, 200 ppm SO was passed during the test process to
2
investigate the NH -SCR performance changes of the catalyst at
3
2
00 1C within 24 h; the results are presented in Fig. 8a. As
shown in Fig. 8a, both nf-MnO –ATP and the g-Fe /nf-MnO
ATP maintain high NO conversion rates of about 95% and
8%, respectively, in the absence of SO within the first 2 h.
Upon the addition of SO to the reaction system, the activities
2
2
O
3
2
–
x
9
2
2
of catalysts show evident declines. After reaction for 5 h, the
NO conversion rate of nf-MnO –ATP decreased to ca. 59%, and
x
2
with extension of the test time, the activity continued to slow
until it decreased to around 48%; this indicates that the active
component on the surface has experienced SO
turning off the SO gas, the catalyst activity was gradually
restored to ca. 55%. Meanwhile, the activity of g-Fe
nf-MnO –ATP also decreased after aeration with SO , but it
retained better activity of about 89% in the presence of SO2
denitration catalyst and found that the addition of g-Fe O to compared to nf-MnO –ATP. After stopping SO , the NO con-
2
poisoning. After
Fig. 7 NH
3
-SCR performance of g-Fe
selectivity (b) and stability (c) tests of nf-MnO
–ATP.
2
O
3
, nf-MnO
2
–ATP and g-Fe
2 3
O /
2
nf-MnO
and g-Fe
2
–ATP (a); N
/nf-MnO
2
2
2
–ATP
2 3
O /
2
O
3
2
2
2
3
2
2
x
4
+
the catalyst could decrease the adsorption of Mn to NH
thus avoid excessive activation of NH to NH and decreased
generation of N O. On the basis of this study, it is indicated components on the NH
that surface modification with g-Fe not only promotes the O vapor were passed through the reactor (Fig. 8b). After the
low-temperature NH -SCR activity of nf-MnO –ATP, but also addition of SO and H O vapor, the activity of the catalysts
increases the N selectivity of the catalyst.
appeared to decrease more significantly than in the presence of
Furthermore, the stabilities of the present catalysts were investi- SO alone. However, the g-Fe O /nf-MnO –ATP catalyst retained
3
and version rate of the catalyst was restored to 94% within 2 h.
In addition, in order to investigate the effects of mixed
-SCR activity, 200 ppm SO and 10%
3
2
3
2
2
O
3
H
2
3
2
2
2
2
2
2
3
2
gated using 200 1C as the reaction temperature under a mixed around 86% NOx conversion during the whole test process.
atmosphere; the results are shown in Fig. 7c. During a reaction After stopping the injection of SO and H O vapor, the activity
2
2
5
3
period of 30 h, both the nf-MnO
catalysts exhibited better stability. For nf-MnO
version rate decreased by about 3%, and for g-Fe
2
–ATP and g-Fe
–ATP, the NO
/nf-MnO
2
O
3
/nf-MnO
2
–ATP of the catalyst could be restored to around 90%. Xiong et al.
con- found that the effects of H O on catalyst activity arise mainly
–ATP, because the presence of H O vapor in the system will form
2
x
2
2
O
3
2
2
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Fig. 9 SEM images of the g-Fe
after (b) the H O and SO test.
2
O
3
/nf-MnO
2
–ATP catalyst before (a) and
2
2
the catalyst to produce the stretching vibrations of surface
sulfates (S–O) and metal sulfates (SQO). This is due to the
accumulation of sulphur ammonia species and blockage of the
5
5,56
pores, which deactivate the catalyst.
Based on the above results, the mechanism of the enhanced
performance of the g-Fe /nf-MnO –ATP catalyst can be
2
O
3
2
proposed as follows. First, It is known from the XRD results
that the surface Fe and Mn active materials have good crystal-
2 3
linity and Fe mainly exists in the form of g-Fe O , which can
4
+
decrease the adsorption of NH on Mn , thus avoiding excessive
3
activation of NH to NH, and can decrease the generation of N O;
3
2
this indicates that surface modification by g-Fe O can not only
2
3
promote the low-temperature NH
but can also increase the N selectivity of the catalyst. Second, it
is evident from the TEM images that g-MnO is uniformly coated
on the surface of ATP with a petal-like morphology, which proves
that g-Fe /nf-MnO –ATP has an excellent surface morphology
that is beneficial for uniform dispersion of g-Fe O and thus
3 2
-SCR activity of nf-MnO –ATP,
2
2
Fig. 8 NO
x
conversion over the nf-MnO /nf-MnO
2
–ATP and g-Fe
2
O
3
2
–
ATP catalysts in the presence of SO
0% H O (200 1C) (b) within 24 h.
2
(200 ppm) (a) and SO (200 ppm) +
2
O
2 3
2
1
2
2
3
avoids catalyst aggregation in practical applications under high
temperature conditions. The charge attraction between the
competitive adsorption with NH
resulting in a decrease of the adsorption quantity of NH
catalyst acidic sites and, thus, a decrease in the activity of
the catalyst. On the basis of the above NH -TPD analysis, the
3
on the surface of the catalyst,
on the
components can strengthen the combination of g-Fe
nf-MnO –ATP, thus decreasing aggregation and shedding of the
particles in the test process. Additionally, it is stated that the
larger specific surface area and pore volume of g-Fe /nf-MnO
2 3
O and
3
2
3
O
2 3
2
–
surface acid sites of the catalyst obviously increased after
ATP illustrated by the BET method are more advantageous to
adsorption and contact of gas on the surface, which can provide
more reaction sites for catalytic reaction; this will help improve
modification with g-Fe
adsorption of NH on the catalyst surface and thus decreases
the H O poisoning of the catalyst. This excellent performance
can enable the catalyst to maintain good denitration perfor-
mance in the presence of SO and H O vapor.
In addition, the effects of H O and SO on the catalytic
2 3
O , which is more advantageous to the
3
2
the NO conversion rate of the catalyst. In addition, XPS tests
x
show that surface modification with g-Fe
2
O
3
can increase the
contents of nf-MnO
and NO on the catalyst
-TPR results indicate that
–ATP is rich in reducing species on the surface
of nf-MnO –ATP, which is conducive to the reduction of NH -
2
2
4
+
Mn and surface adsorption oxygen O
ATP, which favours the oxidation of NH
surface. At the same time, the H
g-Fe /nf-MnO
a
2
–
2
2
3
performance were investigated via studying the morphology
changes of the catalysts. Fig. 9 shows SEM images of the
2
O
2 3
2
g-Fe
SO
component of the catalyst before the H
2
O
3
/nf-MnO
test. It can be observed from Fig. 9a that the active
O and SO test is evenly
2 2
–ATP catalyst before and after the H O and
2
3
2
SCR. It was found by NH -TPD that g-Fe O /nf-MnO –ATP has
3
2
3
2
2
2
more acidic sites and can adsorb more NH in the NH -SCR
3
3
coated on the surface of the ATP, and the flower-like morphology
is clearly visible. Meanwhile, the surface of the catalyst after the
H O and SO test shows abnormal attachments, and the flower-
reaction. Therefore, g-Fe
performance for low temperature SCR of NO
2
O
3
/nf-MnO
2
–ATP has good denitration
with NH
x
3
.
2
2
like structures are covered and almost invisible. According
5
4
to the literature, these surface attachments are classified as
Conclusions
ammonium sulfate species, such as NH
NH HSO ; these can form surface metal sulphates on the A g-Fe
catalyst, thus weakening the metal–oxygen bonds (M–O) in a facile hydrothermal method. The largest specific area of the
4 4 4 2 4
HSO , (NH ) SO , and
4
3
2 3 2
O /nf-MnO –ATP catalyst has been fabricated through
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2
ꢁ1
as-prepared g-Fe
2
O
3
/nf-MnO
2
–ATP was as high as 199.7 m g
,
14 R. M. Liu, Y. W. Jiang, Q. Chen, Q. Y. Lu, W. Du and F. Gao,
and the pore volume of g-Fe
2
O
3
/nf-MnO –ATP also showed a
2
RSC Adv., 2013, 3, 8261–8268.
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sites for the catalytic reaction and help improve the NO
conversion rate of the catalyst. Furthermore, in contrast to
nf-MnO –ATP, g-Fe /nf-MnO –ATP exhibited a higher NO
conversion of 92% at about 150 1C, superior N selectivity,
and better SO and H O resistance, as well as good stability.
x
16 C. X. Liu, S. J. Yang, L. Ma, Y. Peng, A. Hamidreza,
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2
O
3
2
x
2
2
2
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2
3
2
potential alternative applications in NH -SCR with a broad
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There are no conflicts to declare.
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