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.

References

1. Chen L, Li J, Ge M (2010) Environ Sci Tech 44:9590–9596

2. Han WL, Zhang P, Tang ZC, Pan X, Lu GX (2013) J Sol-Gel Sci

Technol 66:526–532

3. Camarillo MK, Stringfellow WT (2013) Appl Energy

106:328–336

4. Fenoll J, Sabater P, Navarro Vela GN, Perez-Lucas G, Navarro S

(2013) J Environ Manage 130:361–368

5. Li Q, Yang HS, Qiu FM, Zhang XB (2011) J Hazard Mater

192:915–921

6. Zhou CC, Zhang YP, Wang XL, Xu HT, Sun KQ, Shen K (2013)

J Colloid Interface Sci 392:319–324

7. Gao YS, Zhang Z, Wu JW, Duan LH, Umar A, Sun LY, Guo ZH,

Wang Q (2013) Environ Sci Technol 47:10813–10823

8. Mou XL, Zhang BS, Li Y, Yao LD, Wei XJ, Su DS, Shen WJ

(2012) Angew Chem Int Edit 51:2989–2993

9. Valverde JL, Lucas A, Sanchez P, Dorado F, Romero A (2003)

Appl Catal B Environ 43:43–56

10. Huang HF, Jin LL, Lu HF, Yu H, Chen YJ (2013) Catal Commun

34:1–4

11. Chang HZ, Chen XY, Li JH, Ma L, Wang CZ, Liu CX, Schwank

JW, Hao JM (2013) Environ Sci Technol 47:5294–5301

12. Yang SJ, Liu C, Chang X, Ma HZ, Qu LZ, Yan NQ, Wang CZ, Li

JH (2013) Ind Eng Chem Res 52:5601–5610

13. Yang SJ, Li JH, Wang CZ, Chen JH, Ma L, Chang HZ, Chen L,

Peng Y, Yan NQ (2012) Appl Catal B Environ 117:73–80

14. Liu FD, Asakura K, He H, Shan WP, Shi XY, Zhang CB (2011)

Appl Catal B Environ 103:369–377

15. Schwidder M, Grunert W, Bentrup U, Bruckner A (2006) J Catal

1:173–186

16. Liu J, Li XY, Zhao QD, Hao C, Zhang DK (2013) Environ Sci

Technol 47:4528–4535

17. Fang C, Zhang DS, Shi LY, Gao Rh, Li HR, Ye LP, Zhang JP

(2013) Catal Sci Technol 3:803–811

18. Heo I, Kim MK, Sung S, Nam I, Cho BK, Olson KL, Li W (2013)

Environ Sci Technol 47:3657–3664

19. Zhou JS, Hou H, Qi P, Gao X, Luo ZY, Cen KF (2013) Environ

Sci Technol 47:10056–10062

20. Zhao QS, Xiang J, Sun LS, Su S, Hu S (2009) Energy Fuels

23:1539–1544

21. Carja G, Kameshima Y, Okada K, Madhusoodana CD (2013)

Appl Catal B Environ 1–2:60–64

22. Qiu J, Zhuang K, Lu M, Xu BL, Fan YN (2013) Catal Commun

9:21–24

23. Zhao W, Zhong Q, Zhang TJ, Pan YX (2012) RSC Adv

20:7906–7914

24. Ayari F, Mhamdi M, Delahay G, Ghorbel A (2010) J Porous

Mater 3:265–274

25. Yu J, Guo F, Wu YL, Zhu JH, Liu YY, Su FB, Gao SQ, Xu GW

(2010) Appl Catal B Environ 95:160–168

26. Shu Y, Sun H, Quan X, Chen S (2012) J Phys Chem C

116:25319–25327

27. Gao RH, Zhang DS, Liu XG, Shi LY, Maitarad P, Li HR, Zhang

JP, Cao WG (2013) Catal Sci Technol 1:191–199

28. Zhou KB, Wang X, Sun XM, Peng Q, Li YD (2005) J Catal

229:206–212

29. Singh S, Srivastava P, Pal I, Kapoor S, Singh G (2013) J Therm

Anal Calorim 111:1073–1082

30. Deus RC, Cilense M, Foschini CR, Ramirez MA, Longo E, Sim

AZ (2013) J Alloy Compd 550:245–251

31. Wang XQ, Shi AJ, Duan YF, Wang J, Shen MQ (2012) Catal Sci

Technol 2:1386–1395

32. Long RQ, Yang RT (2002) J Catal 207:224–231

33. Karami A, Salehi V (2012) J Catal 292:32–43

34. Guan B, Lin H, Zhu L, Huang Z (2011) J Phys Chem C

115:12850–12863

35. Long RQ, Yang RT (2000) J Catal 190:22–31

36. Xu H, Zhang Y, Jiang Q, Reddy N, Yang Y (2013) J Environ

Manage 125:33–40

37. Si ZC, Wen D, Wu XD, Li J, Li G (2010) J Catal 271:43–51

J Sol-Gel Sci Technol

123