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ORIGINAL PAPER
Sol-gel preparation of mesoporous cerium-doped FeTinanocatalysts and its SCR activity of NOx with NH3
at low temperature
Sihui Zhan • Dandan Zhu • Shanshan Yang •
Mingying Qiu • Yi Li • Hongbing Yu •
Zhiqiang Shen
Received: 26 August 2014 / Accepted: 22 October 2014
� Springer Science+Business Media New York 2014
Abstract A series of cerium-doped mesoporous FeTi
nanocatalysts were synthesized through a sol-gel method,
and their performances for the selective catalytic reduction
(SCR) of NOx with NH3 were explored. The mesoporous
Ce(0.2) FeTi catalyst exhibited excellent low-temperature
catalytic activity and high resistance to sulfur poisoning.
A NOx conversion higher than 95 % was achieved at
200 �C over the Ce(0.2) FeTi catalyst. The strong inter-
actions between cerium, iron oxides and titania in the
Ce(0.2) FeTi catalyst resulted in a large amount of Ce3?,
more active chemisorbed oxygen and more Brønsted acid
sites, which contributed to the high catalytic activity for the
SCR of NOx in the low-temperature region. The enhanced
BET surface area and pore volume of its mesoporous
structure also played an important role in its catalytic
performance. Based on the DRIFTS analysis, an Eley–
Rideal mechanism was proposed for the SCR over the
Ce(0.2) FeTi mesoporous catalyst.
Keywords Low temperature � SCR � Mesoporous �DRIFT
1 Introduction
Nitrogen oxide (NOx) from automobile exhaust gases and
the industrial combustion of fossil fuels is considered one
of the major air pollution sources in China. It usually
causes acid rain, photochemical smog, ozone depletion and
greenhouse effects [1–3]. Recently, the selective catalytic
reduction (SCR) of NOx with ammonia using metal oxide
catalysts has been proved to effectively remove NOx from
the flue gas originating from stationary sources [4]. A good
many of new SCR catalysts have been prepared, such as
VWTi, MoFe, FeTi, CuTi, CrTi, MnTi, and MnCe catalysts
[5–11]. Among these, the VWTi catalyst has been com-
mercialized and is widely used for the removal of the NOx
emitted from stationary power plants. However, this cata-
lyst only shows catalytic activity within a narrow temper-
ature window from 300 to 400 �C. The high temperature
required by the catalyst makes it difficult to control the
formation of N2O and causes vanadium loss, which is
hazardous to the environment and human health. In addi-
tion, the catalytic reactor bed containing a high-tempera-
ture SCR catalyst is usually located upstream of
desulfurizers. The sulfur and dusts in the flue gas can cause
catalyst poisoning and deactivate the catalysts. Further-
more, the retrofitting of SCR devices into existing systems
causes more expenses due to the limited space and access
found in many power plants [12]. For these reasons, a
modification of the current catalysts or the development of
Electronic supplementary material The online version of thisarticle (doi:10.1007/s10971-014-3556-5) contains supplementarymaterial, which is available to authorized users.
S. Zhan (&) � D. Zhu � S. Yang � M. Qiu � H. Yu
College of Environmental Science and Engineering, Nankai
University, Tianjin 300071, People’s Republic of China
e-mail: sihuizhan@nankai.edu.cn; sihuizhan@gmail.com
S. Zhan
Department of Chemistry and Biochemistry, University of Notre
Dame, Notre Dame, IN 46556, USA
Y. Li (&)
Department of Chemistry, Tianjin University, Tianjin 300072,
People’s Republic of China
e-mail: liyitju@gmail.com
Z. Shen
Key Laboratory of Risk Assessment and Control for
Environment and Food Safety, Institute of Health and
Environmental Medicine, Tianjin 300050, China
123
J Sol-Gel Sci Technol
DOI 10.1007/s10971-014-3556-5
novel low-temperature catalysts is needed to reduce the
vanadium loadings and to ensure catalyst compatibility
with the gas outlet.
In recent years, iron oxides have attracted much atten-
tion because of their oxygen and redox properties. Efforts
have been made toward their application for the SCR of
NOx with NH3 [12]. He et al. synthesized a Fe2O3–TiO2
catalyst and found that the coexistence of iron and titanium
species favored the formation of a specific Fe–O–Ti crystal
structure, which was highly active for the SCR reaction
[13]. Li et al. synthesized an Fe2O3–TiO2 spinel catalyst
that showed excellent SCR activity, N2 selectivity, and SO2
durability in a temperature range of 300 to 400 �C [14].
Bruckner et al. prepared Fe–ZSM-5 catalysts and demon-
strated that accessible Fe3? species are the active sites. The
Fe3? species contributed to the formation of adsorbed
nitrato species and accelerated the reduction of NO [15].
These catalysts display various SCR catalyst activities
under different conditions. Among these catalysts, FeTi
catalysts exhibit the best catalytic activity at temperatures
higher than 250 �C but are more easily deactivated by SO2
poisoning. Metal-doped Fe2O3–TiO2 catalysts are a
promising type of catalyst that can increase the NOx con-
version and the resistance to SO2.
Cerium dioxide (CeO2) is a promising catalyst material
for the NH3-SCR of NOx because of its non-toxicity,
unique oxygen storage and good redox properties. Oxygen
can be stored and released when the redox shifts between
Ce4? and Ce3?. Thus, various catalysts with doped cerium
have been synthesized to enhance the oxidization of NO to
NO2 or to improve the SCR of NO with NH3 [16–20]. Shan
et al. synthesized a CeO2/TiO2 catalyst and found that the
synergistic effects between CeO2 and TiO2 can inhibit the
growth of anatase TiO2 crystallite, resulting in a high
surface-to-volume ratio, excellent NH3-SCR activity, high
N2 selectivity and a broad operating temperature window
[11]. Doped cerium can also significantly improve low-
temperature SCR efficiencies. Carja et al. [21] developed a
Mn–Ce/ZSM-5 catalyst in an aqueous phase that exhibited
an NO conversion of 75–100 % within a broad temperature
range of 240–500 �C.
Mesoporous materials have been widely used as catalyst
supports because of their highly ordered frameworks with
relatively large pore diameters and higher surface areas
[22]. Zhao et al. [19] prepared mesoporous F-doped V2O5/
TiO2 catalysts and demonstrated that the SRC of NOx over
the catalyst was remarkably improved compared with that
obtained with pure V2O5/TiO2. Ayari et al. synthesized
mesoporous Cr/Al2O3 catalysts by a sol-gel method and
found that all of the samples exhibited excellent NO
reduction activities at a temperature range of 100–400 �C
[23]. The mesopore channels existing in the mesoporous
catalyst enabled a dynamic balance between the formation
and decomposition of ammonium sulfate in the SCR
reactions [24].
In this work, both FeTi and Ce-doped FeTi mesoporous
catalysts were prepared, and their SCR catalytic perfor-
mances and SO2-poisoning resistances were tested in
detail. Compared with other catalysts, including commer-
cial VWTi catalysts, both catalysts showed higher catalytic
activity and resistance to SO2 poisoning. In addition, var-
ious analytical techniques, including BET, XRD, TEM,
TG, FTIR, XPS, H2-TPR, NH3-TPD, and DRIFT, were
used to explore the effects of Ce doping on the low-tem-
perature SCR activities of the catalysts. The results indi-
cated that the Ce-doped FeTi catalyst with a Ce/Ti molar
ratio of 0.2 showed highest SCR catalyst activity at a lower
temperature range of 50–300 �C. The surface properties of
various FeTi catalysts and the reaction mechanism of the
NH3-SCR reaction over the Ce(0.2) FeTi mesoporous
catalyst were further explored using in situ DRIFT
measurements.
2 Experiment
2.1 Catalyst preparation
All of the chemicals used in this work were of analytical
grade. Hydrochloric acid (36.0 wt%), cerium nitrate, ferric
trichloride, butyl titanate (Ti(OC4H9n)4), pluronic P123 and
anhydrous ethanol ([99.7 %) were purchased from Sin-
opharm Chemical Reagent Company (City, Country) and
used without further purification. Ce-doped mesoporous
FeTi catalysts with different CeO2/TiO2 molar ratios were
prepared by a sol-gel method. Briefly, 0.37 mmol of plu-
ronic P123 was dissolved in 0.1 mol of anhydrous ethanol,
and the mixture was stirred until the solution was clear
(solution A). In addition, 10.0 mmol of Ti(OC4H9n)4 and the
required amount of ferric trichloride were dissolved in
0.1 mol of anhydrous ethanol (solution B). Solution B was
poured into solution A, and 0.03 mol of concentrated HCl
was added immediately into the mixture to avoid the
hydrolysis of titanium alkoxide. The mixture was stirred
for 30 min at room temperature, and different amounts of
cerium nitrate were added during the stirring to obtain
catalysts with various Ce contents. The mixture was aged
at 50 �C for 12 h to yield a transparent gel. To completely
remove the organic solvents, the gel was sintered at 130 �C
for 8 h. The gel was then calcined by heating to a target
temperature of 500 �C at a heating rate of 1 �C min-1 and
was then air-cooled to room temperature. The prepared
mesoporous SCR catalysts were denoted Ce(x) FeTi, where
‘‘x’’ represents the CeTi molar ratio (x = 0, 0.1, 0.2, 0.3,
0.4, and 0.5). The FeTi molar ratio of all of the prepared
catalysts was fixed to 0.1. A commercial V2O5-WO3/TiO2
J Sol-Gel Sci Technol
123
catalyst was used as a reference and is denoted VWTi in
this paper. All of the catalysts were ground and sieved
through a 40–60 mesh for further tests.
2.2 DRIFT and XPS characterization
The in situ DRIFTS measurements were performed using a
Bruker VERTEX 70 FTIR spectrometer equipped with an
in situ diffuse reflectance pool and a high-sensitivity liquid
N2-cooled MCT detector. The catalyst was finely ground
and pressed into a self-supported wafer. Mass flow con-
trollers and a sample temperature controller were used to
simulate the SCR reaction conditions. Prior to each mea-
surement, the wafer was heated to 350 �C in N2 (99.999 %)
for 1 h and then cooled to the desired reaction temperature.
The background spectrum was collected with an N2 gas
flow and was automatically subtracted from the spectra of
the samples. The spectra were recorded by accumulating
100 scans with a resolution of 4 cm-1. X-ray photoelectron
spectroscopy (XPS) spectra were collected using an ES-
CALAB 250 multi-technique X-ray photoelectron spec-
trometer (Thermo Scientific, UK) with a monochromatic
AlKa X-ray source (hm = 1,486.6 eV). All of the XPS
spectra were recorded using an aperture slot of 300 9 700
microns. The survey spectra were recorded with a pass
energy of 160 eV, and the high-resolution spectra were
recorded with a pass energy of 40 eV.
2.3 Catalytic activity test
The catalytic activities of the prepared SCR catalysts were
evaluated using a bench-scale experimental system (Fig.
s1). One gram of catalyst was loaded into a temperature-
controlled fixed-bed quartz flow reactor (i.d. = 20 mm).
To simulate the flue gas, individual major component gases
were supplied from gas cylinders and mixed together. The
flow rates of each gas flow were precisely controlled by
mass flow controllers (MFC) to obtain a total flow rate of
300 mL min-1 (at 1 atm. and 298 K), corresponding to a
gas hourly space velocity (GHSV) of 40,000 h-1. The
typical composition of the initial reactant gas was 750 ppm
NO, 900 ppm NH3, 5 vol % O2, and N2 as the balance gas.
For the sulfur-poisoning resistance experiment, 200 ppm
SO2 was added to the initial reactant gas. The concentra-
tions of NO, NH3, NO2, N2O, O2 and SO2 at both the inlet
and outlet of the reactor were measured online using an
FTIR spectrometer (Gasmet FTIR DX4000, Finland). The
SCR reaction was maintained for 1 h at each reaction
temperature to ensure that a steady state was reached
before the measurement was performed [25–30]. The NOx
conversion and N2 selectivity were calculated as follows
(Eqs. 1 and 2):
NOX conversionð%Þ ¼ ½NOX�in � ½NOX�out
½NOX�in� 100 ð1Þ
N2 selectivityð%Þ ¼ ½NOX�in � ½NOX�out � ½N2O�out
½NOX�in � ½NOX�out
� 100
ð2Þ
0
20
40
60
80
100 a
NO
x
Tempreture ( C)
FeTiCe(0.1) FeTiCe(0.2) FeTiCe(0.3) FeTiCe(0.4) FeTiCe(0.5) FeTiVWTi
0
20
40
60
80
100
0
20
40
60
80
100b
FeTiCe(0.1) FeTiCe(0.2) FeTiCe(0.3) FeTiCe(0.4) FeTiCe(0.5) FeTi
N2O
(ppm
)
N2 se
lect
ivity
(%)
Tempreture ( C)
50 100 150 200 250 300
0
20
40
60
80
100
NO
x
Tempreture ( C)
Ce(0.2) FeTiCe(0.2) FeTi with SO2VWTiVWTi with SO2
c
Con
vers
ion
(%)
Con
vers
ion
(%)
50 100 150 200 250 300
50 100 150 200 250 300
Fig. 1 NOx conversion (a), N2 selectivity and N2O formation (b) of
the Ce(x) FeTi catalysts and VWTi catalyst. The NOx conversions of
the Ce(0.2) FeTi and VWTi catalysts in the presence of SO2 are
shown in c. Reaction conditions: [NO] = 750 ppm, [NH3] = 900
ppm, [O2] = 5 vol %, [SO2] = 200 ppm (when needed), N2 balance,
total flow rate = 300 mL/min, GHSV = 40,000 h-1
J Sol-Gel Sci Technol
123
3 Results
3.1 Catalytic activity
The FeTi catalysts with different Ce loadings and the
VWTi commercial catalyst were tested for the NH3-SCR of
NOx at a temperature range of 50–300 �C. As shown in
Fig. 1a, the FeTi catalyst without any doped cerium
showed a lower catalytic activity at the tested temperature
range and gave the highest NOx conversion of 40 % at
200 �C. The Ce-doped FeTi catalyst showed markedly
higher catalytic activity than the FeTi catalyst. A NOx
conversion higher than 95 % was achieved at 200 �C over
the Ce(0.2) FeTi catalyst. A further increase in the Ce
content resulted in unstable sols and markedly decreased
the catalytic activity. The commercial VWTi catalyst
showed the lowest performance and gave the highest NOx
conversion of 38 % at 200 �C. Figure 1b shows the N2
selectivity and N2O productivity obtained over all of the
prepared catalysts. The concentration of N2O formed over
all of the catalysts was \10 ppm, indicating the excellent
N2 selectivity of all of the FeTi catalysts. All of these
results show that the Ce content in the catalyst is a crucial
factor that affects the low-temperature SCR performance.
Among the catalysts, the Ce(0.2) FeTi catalyst presented
the best SCR performance with nearly 100 % NOx con-
version and excellent N2 selectivity. Therefore, Ce(0.2)
FeTi was selected for further investigation in the following
experiments.
The resistance to sulfur poisoning is another important
attribute of catalysts. As shown in Fig. 1c, the NOx con-
versions over the Ce(0.2) FeTi and VWTi catalysts
decreased in the presence of 200 ppm SO2 at the tested
temperature range, indicating a distinct poisoning effect on
the catalysts. However, the highest NOx conversion over
the Ce(0.2) FeTi catalyst remained as high as 80 % at
200 �C in the presence of 200 ppm SO2, whereas the
conversion over the VWTi catalyst was only 20 %, indi-
cating that the Ce(0.2) FeTi catalyst is resistant to sulfur
poisoning. Thus, Ce(0.2) FeTi has high catalytic activity
for the NH3-SCR reaction of NOx and is resistant to sulfur
poisoning at low temperatures.
3.2 TEM analysis
The microscopic morphologies of the Ce(0.2) FeTi catalyst
were characterized by HRTEM (Fig. 2). Highly ordered
mesoporous channels were observed in the catalyst
(Fig. 2a). The sizes of the channels were calculated by the
Scherrer formula to be approximately 6–7 nm with an
average particle size of approximately 11 nm, which is
inconsistent with the XRD analysis results (see Fig. s2)
[25]. As shown in Fig. 2b, clear lattice fringes were
observed on the surface of the Ce(0.2) FeTi catalyst. The
lattice fringes with an interplanar distance of 0.352 nm
correspond to the (101) plane of anatase TiO2 (PDF#
21-1272) [25–27]. The other two types of lattice fringes
with interplanar distances of 0.27 nm and 0.19 nm corre-
spond to the (104) plane of hematite Fe2O3 and the (101)
plane of cubic CeO2, respectively [27, 28].
3.3 H2-TPR analysis
The H2-TPR profiles of the FeTi catalysts with various Ce
loadings are shown in Fig. 3. For the FeTi catalyst, a broad
peak spanned the temperature range of 250–550 �C, and a
narrow peak spanned the temperature range from 700 to
900 �C, corresponding to the successive reduction steps of
Fe2O3–Fe3O4–FeO and FeO–Fe0, respectively [29, 30].
Similar peaks were also observed in the TPR profiles of the
Ce-modified catalysts. However, these peaks were much
broader and more intense than those of FeTi. In addition,
the starting temperatures of the peaks of the Ce-doped FeTi
catalysts were lower than those of FeTi, which indicates
that the introduction of cerium into the catalyst not only
improves its oxygen storage capacity but also enhances its
redox activity at low temperatures due to the interaction
between Fe and Ce species. The main reduction peak
shifted to a higher temperature range with an increase in
the Ce content, implying that the redox activity of the
catalyst was reduced by the addition of higher amounts of
Ce at a low temperature [18, 19]. This may be the main
reason why the catalytic activity decreased with a higher
Ce content (Ce/Ti[0.2). In addition, a weak peak appeared
at 800–900 �C on the TPR profiles for the Ce-modified
catalysts, and the peak intensity was enhanced by an
increase in the Ce content. This peak may be caused by the
reduction of oxygen species on the surface region of ceria
and iron oxide [12]. Therefore, the introduction of Ce into
the FeTi catalyst resulted in a new center for oxygen
storage and release. With a further increase in the Ce
content, the peak observed at the high-temperature range
gradually disappeared due to the interactions between Ce
and the other metal oxides [12].
3.4 XPS analysis
The surface characteristics were further analyzed using
XPS, as shown in Fig. 4 and Table s2. The binding ener-
gies of the Ti 2p photoelectron peaks at 458.6 and
464.5 eV were assigned to the Ti 2p3/2 and Ti 2p1/2 lines,
respectively, indicating that Ti exists as Ti4? with a
tetragonal structure. After Ce was introduced, the Ti 2p
peaks slightly shifted to a lower binding energy. This
binding energy shift was caused by oxygen vacancies
J Sol-Gel Sci Technol
123
induced by Ce loading and the breaking of Ti–O–Ti bonds
[7]. The two strong peaks at 725.8 and 711.5 eV shown in
Fig. 4b were assigned to Fe 2p1/2 and Fe 2p3/2, which are
characteristic of the Fe3? state in Fe2O3 samples [7, 19].
The intensities of the Fe 2p peaks gradually decreased as a
result of the reduced concentration of surface Fe species
and increased Ce contents. In addition, with an increase in
the cerium oxide content, the Fe 2p peaks shifted to a
higher binding energy, which indicates that cerium oxide
altered the chemical environment around the Fe species
and that an interaction occurred between them [19].
Figure 4c shows the complete Ce 3d XPS spectra of the
catalysts, and two multiplets, i.e., u and v, were found after
fitting. The bands labeled u1 (885.8 eV) and v1 (904.3 eV)
represent the 3d104f1 initial electronic state of Ce3?, whereas
the peaks at u (916.4 eV), u2 (907.4 eV), u3 (901.2 eV), v
(898.6 eV), v2 (889.2 eV), and v3 (882.6 eV) represent the
3d104f0 state of Ce4? ions [17, 27]. It is obviously observed
that the intensities of the Ce3? and Ce4? characteristic peaks
were different with increasing of the Ce content, and the
peaks of the Ce3? on the Ce(0.2) FeTi catalyst surface were
strongest compared with other catalysts, indicating the peaks
of Ce3? is mainly valence state on the Ce(0.2) FeTi catalyst
surface. It is reported that the presence of the Ce3? species
can create a charge imbalance and generate more oxygen
vacancies, which is favorable for the activation of surface
oxygen species in SCR. Therefore, the low-temperature SCR
catalyst activity of Ce(0.2) FeTi catalyst may be the highest,
which can be confirmed based on the following experiment.
The O1s peak of Ce(x)FeTi can be fitted by two peaks
corresponding to the lattice oxygen at 529.3–530.0 eV
(noted as Ob) and the chemisorbed oxygen at
531.3–531.9 eV (noted as Oa) (Fig. 4d). The relative ratio
of the Oa concentration was calculated as Oa/(Oa ? Ob).
All of the Ce(x) FeTi catalysts showed markedly higher
relative ratios of the Oa concentration than the FeTi cata-
lyst. Of the catalysts, Ce(0.2) FeTi presented the highest
relative ratio of 49.7 %. This finding suggests that the
chemisorbed oxygen contents on the Ce(x) FeTi catalysts
are high due to the addition of Ce. The charge imbalance,
vacancies and unsaturated chemical bonds caused by Ce
doping result in high amounts of surface chemisorbed
oxygen [20]. Surface chemisorbed oxygen has been
reported to be the most active oxygen and plays an
important role in oxidation reactions [29]. Therefore, the
Ce-modified FeTi catalysts exhibit better catalytic activity
for the oxidation of NO to NO2 than the FeTi catalyst itself.
This finding is consistent with the results of the activity
tests [18, 19].
3.5 DRIFTS studies
3.5.1 In situ DRIFTS of NH3/NO adsorption over the FeTi
and Ce(0.2) FeTi catalysts
The in situ DRIFTS spectra for the NH3 adsorption on the
FeTi and Ce(0.2) FeTi catalysts at the temperature range
Fig. 2 TEM (a) and HRTEM (b) images of the Ce(0.2) FeTi catalyst
100 200 300 400 500 600 700 800 900
Ce(0.3) FeTi
Ce(0.4) FeTi827
802
853
785
452
449
437Ce(0.2) FeTi
Ce(0.1) FeTi
FeTi
TC
D si
ngal
(a.u
.)
Temprerature ( C)
460
540
499Ce(0.5) FeTi
Fig. 3 H2-TPR profiles of the Ce(x) FeTi catalysts with different Ce
contents
J Sol-Gel Sci Technol
123
from 150 to 350 �C are shown in Fig. 5. Several bands in
the ranges of 1,100–1,700 and 3,100–3,400 cm-1 were
observed after the FeTi catalyst was exposed to NH3 at
various temperatures. The bands at 1,722 and 1,626 cm-1
are due to the symmetric bending vibrations of NH4?
chemisorbed on the Brønsted acid active sites, and the band
at 1,454 cm-1 is due to the asymmetric bending vibrations
of NH4?. The bands at 3,149, 3,255, and 3,332 cm-1 are
attributed to the N–H stretching vibration modes, and the
negative bands at approximately 3,650 cm-1 are due to
surface O–H stretching [31–33].
The bands at 1,230 and 1,159 cm-1 can be assigned to
the asymmetric and symmetric bending vibrations of the
N–H bonds in the NH3 that are coordinately linked to the
Lewis acid sites [34]. With an increase in the temperature,
the bands at 1,722, 1,626, 1,454, and 1,230 cm-1 became
weaker. The intensity of the band at 1,454 cm-1 decreased
markedly at high temperature, whereas the band at
1,159 cm-1 remained unchanged. These results indicate
that coordinate NH3 bonded to Lewis acid sites is more
stable than NH4? bonded to Brønsted acid sites. All of
these peaks were also detected on the Ce(0.2) FeTi catalyst
with the exception that the intensity of the band at
1,454 cm-1 due to Brønsted acid sites was much stronger
than that obtained on the FeTi catalyst. In addition, the
bands attributed to Lewis acid sites showed no obvious
changes with increasing temperature. It has been reported
that both ionic NH4? and coordinated NH3 can react with
NO2 species to form active intermediates during the SCR
process [34, 35]. Therefore, both Brønsted acid sites and
Lewis acid sites over the Ce(0.2) FeTi catalyst contribute
to its high SCR catalyst activity.
The adsorption of NOx species on FeTi catalysts at
various temperatures was investigated by FTIR spectros-
copy. Prior to NOx adsorption, the catalysts were treated at
350 �C in N2 for 1 h to remove any adsorbed species. After
the catalysts were cooled to room temperature, 750 ppm
NO and 5 % O2 were introduced into the IR cell, and the IR
spectra were recorded at different temperatures, as shown
in Fig. 5c, d. The FeTi catalyst surface was mainly covered
by monodentate nitrates (1,443 cm-1), bridging nitrates
(1,265 cm-1), bidentate nitrates (1,580 cm-1), NO2
(1,608 cm-1), and (NO3-)2 species (1,350 cm-1) (Fig. 5c)
[35–37]. Similar bands were found in the spectra of the
Fig. 4 XPS region of the FeTi and Ce(0.2) FeTi catalysts: a Ti 2p, b Fe 2p, c Ce 3d and d O 1s
J Sol-Gel Sci Technol
123
Ce(0.2) FeTi catalyst surface (Fig. 5d). However, more
nitrate species, including bridging nitrate (1,240 cm-1) and
monodentate nitrate species (1,290 cm-1), were absorbed
on Ce(0.2) FeTi [36]. These results indicate that the nitrate
species adsorption capacity of the catalyst was improved
by the addition of Ce. Under the SCR reaction conditions,
the adsorbed nitrate species can rapidly react with adjacent
adsorbed NH4? or NH3 to produce more reactive inter-
mediates, which can further react with gaseous NO to form
N2 and H2O and accelerate the SCR reaction.
3.5.2 In situ DRIFTS measurement of NH3 ? NO ? O2
adsorption over FeTi and Ce(0.2) FeTi catalysts
To identify all of the species present on the catalysts under
the SCR reaction conditions, the DRIFTS spectra of the
FeTi and Ce(0.2) FeTi catalysts in a flow of
NO ? NH3 ? O2 over a temperature range from 150 to
350 �C were collected, as shown in Fig. 6. The bands of
different nitrate species were observed at 1,240 and
1,540 cm-1 at 150 �C on the FeTi catalyst surface
(Fig. 6a). In addition, the bands attributed to the ionic
NH4? species on Brønsted acid sides were observed at
1,440 and 1,680 cm-1 [31]. The band at 1,610 cm-1 may
be caused by the overlap of the bands of NO2 and coor-
dinated NH3 on Lewis acid sides [33]. Several N–H
stretching bands were found at 3,699, 3,260 and
3,160 cm-1, indicating that both adsorbed NH3 and NOx
species may be involved in the SCR reaction at tempera-
tures \200 �C. With an increase in the reaction tempera-
ture, the intensity of the bands ascribed to the adsorbed
NOx species decreased significantly, whereas the bands
ascribed to the adsorbed NH3 species (1,680 cm-1)
remained unchanged [32]. Similar bands attributed to
various nitrate species (1,240 and 1,540 cm-1) and adsor-
bed NH3 (1,440 and 1,680 cm-1) were observed in the
DRIFT spectra of the Ce(0.2) FeTi catalyst at 150 �C
(Fig. 6b) [31–33]. A new band appeared at 1,176 cm-1,
which was attributed to coordinated NH3 [37]. Unlike
nitrate absorption on the FeTi catalyst, bands ascribed to
Abs
orba
nce
(a.u
.)
Wavenumber (cm-1)
350°C
300°C
250°C
150°C
200°C
17221626
14541230 11593255
31493332a
350°C
11401263
1454162517613142
3234
300°C
250°C
200°C
Abs
orba
nce
(a.u
.)
Wavenumber (cm-1)
150°C
3334
b
300°C
c 126513501608 15801443
250°C
200°CAbs
orba
nce
(a.u
.)
Wavenumber (cm-1)
150°C
350°C
4000 3500 3000 2500 2000 1500 1000 4000 3500 3000 2500 2000 1500 1000
2200 2000 1800 1600 1400 1200 1000 800 2200 2000 1800 1600 1400 1200 1000 800
d 124016081580 1290
1443
350°C
300°C
250°C
200°C
Wavenumber (cm-1)
Abs
orba
nce
(a.u
.)
150°C
1350
Fig. 5 In situ DRIFT spectra of different catalysts during NH3/NO adsorption experiments: a NH3 adsorption: FeTi, b NH3 adsorption: Ce(0.2)
FeTi, c NO ? O2 adsorption: FeTi, and d NO ? O2 adsorption: Ce(0.2) FeTi
J Sol-Gel Sci Technol
123
the nitrate species were still observed on Ce(0.2) FeTi at
high temperatures, even when the temperature was
increased to 350 �C. The competition between the
adsorptions of NOx and NH3 suggests that the surface
nitrate plays an important role in SCR reaction over Ce-
doped catalysts and has a favorable effect on their NH3-
SCR activity in the low-temperature region [35].
3.6 Mechanism of the SCR reaction on the Ce(0.2)
FeTi catalyst
As shown by the XPS analysis, the surface oxygen Oa is
more reactive in oxidation reactions than lattice oxygen Ob
due to its high mobility, which is beneficial to the oxidation
of NO to NO2 in the SCR reaction. Herein, the higher Oa/
Oa ? Ob ratio of the Ce(0.2) FeTi catalyst indicates the
presence of higher amounts of oxygen on its surface. The
existence of Ce3? species can generate more oxygen
vacancies, which may be one of important factors that
affect the mechanism of this catalyst. In addition, the Ce
and Fe species are interconnected in the form of Ce–O–Fe
through oxygen bridges, facilitating electron transfer [19,
20]. This favors NO oxidation and thus SCR activity.
The NOx adsorption behavior on Ce(0.2) FeTi is sim-
ilar to that on FeTi with the exception that it is more
stable. In addition, marked variation was observed in NH3
adsorption on the-cerium doped catalysts, as shown in
Fig. 5b. Strong Brønsted acid sites were detected on
Ce(0.2) FeTi, and these may arise from the unsaturated
coordination of Ce3? and Fe3? ions. The adsorbed
ammonia species on the FeTi catalyst were mainly
coordinated NH3 linked to Lewis acid sites [31]. There-
fore, the increased Brønsted acid sites may be caused by
the change in the Ce valence state. After the mixture of
NO and O2 was passed over the NH3-adsorbed sample,
the bands of the surface ammonia species disappeared
more rapidly than those on FeTi, indicating that all of the
absorbed NH3 species were active in the SCR reaction
[35]. When NH3 was introduced to Ce(0.2) FeTi pre-
adsorbed with NO and O2, stable bidentate nitrate species
were formed [37]. Bidentate nitrate species are different
from ammonia, and the SCR reaction cannot proceed in
this manner.
The in situ DRIFT analysis indicated that the catalyst
surface was mainly covered by adsorbed ammonia species
over the temperature range of 150–350 �C. This can be
ascribed to the fast reaction between NH3 species and the
weakly adsorbed NO. Brønsted acid sites serve as impor-
tant active sites. Because of the rapid decomposition of
NH2NO into N2 and H2O, no band due to NH2NO species
was observed on the catalyst surface under the SCR con-
ditions. In full, the NH3-SCR of NO over Ce(0.2) FeTi
mainly followed the Eley–Rideal mechanism, as shown in
Eqs. 3–6.
NH3ðgÞ�!Ce4þ
NH3ðaÞ ð3Þ
NH3ðaÞ �!Ce3þFe3þ
NHþ4 ðaÞ ð4Þ
NHþ4 ðaÞ ! NH2ðaÞ þ 2Hþ þ e� ð5Þ
NH2ðaÞ þ NOðgÞ ! NH2NOðaÞ ! N2 þ H2O ð6Þ
4 Conclusions
In this work, a series of Ce-doped FeTi mesoporous
nanocatalysts were prepared by a sol-gel method. The
addition of cerium improved the low-temperature catalytic
activity and SO2-poisoning resistance of the FeTi
1176b 1240
15401440
16803232
3284350°C
300°C
250°C
200°C
Abs
orbe
nce
(a.u
.)
Wavenumber (cm-1)
150°C
3699
4000 3500 3000 2500 2000 1500 1000
4000 3500 3000 2500 2000 1500 1000
a
1440
12401540168031603260
Abs
orbe
nce
(a.u
.)
Wavenumber (cm-1)
3699
350°C
300°C
250°C
200°C
150°C
Fig. 6 In situ DRIFT spectra of different catalysts during NH3 ?
NO ? 5 % O2 adsorption experiments: a FeTi and b Ce(0.2) FeTi
J Sol-Gel Sci Technol
123
mesoporous catalyst. The Ce(0.2) FeTi catalyst gave the
highest NOx conversion of 99.5 % and excellent selectivity
to N2 (as high as 98 %) with a simulated flue gas at a space
velocity of 40,000 h-1 at a temperature range of
50–300 �C. A NOx conversion higher than 80 % was
obtained with the Ce-modified FeTi catalyst in the presence
of 200 ppm SO2 at 300 �C. The structural characterization
revealed that the high catalytic activity of the Ce(x) FeTi
catalyst is attributed to its excellent dispersion, increased
redox properties, and the enrichment of Ce3? and chemi-
sorbed oxygen on its surface. In addition, the mesoporous
structure and large BET surface areas present in the sol-gel
catalyst significantly facilitate the SCR reactions. More-
over, the addition of Ce to the FeTi catalyst results in a
more active Brønsted acid, which causes NH4? to disap-
pear rapidly in the presence of NO and O2 and accelerates
the SCR reaction. The predomination of adsorbed ammonia
species and highly active Brønsted acid sites on the catalyst
surface caused the exclusion of many nitrogen oxide spe-
cies from the catalyst. Based on the DRIFTS analysis, an
Eley–Rideal mechanism was proposed for the SCR over
the Ce(0.2) FeTi catalyst.
Acknowledgments The authors gratefully acknowledge the finan-
cial support provided by the National Natural Science Foundation of
China (21377061, 21003094, and 81270041), the Asia Research
Center in Nankai University (AS1326), the Natural Science Foun-
dation of Tianjin (12JCQNJC05800), and the Key Technologies R&D
Program of Tianjin (13ZCZDSF00300), as well as the assistance
provided Dr. Raymond Seekell (University of Notre Dame) and
Professional Scientific English Language from Elsevier (50038) for
the manuscript preparation.
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