Nb-Modified CeAlOx Catalyst Used for the Selective Catalytic Reduction of NO by NH3: The Promoting Effect of Nb

Article abstract of DOI:10.1134/S0023158419020101

Abstract: A series of Nb modified CeAlO_x catalysts were prepared by hydrothermal method and tested in the selective catalytic reduction of NO with NH₃. The catalyst with an Nb/Al molar ratio of 1 : 1 exhibited the best catalytic activity and high resistance to sulfur poisoning. Nitrogen adsorption measurements, XRD, NH₃-TPD, H₂-TPR and XPS analyses as well as in situ DRIFTS technique were applied to characterize the catalysts. The characterization results showed that the introduction of Nb enhanced the redox power and increased surface acidity. Moreover, the modification enriched the catalyst surface with adsorbed O_α and Ce³⁺ species. In addition, in situ DRIFTS experiments demonstrated that doping with Nb increased catalyst adsorption capacity for NH₃ while decreased the adsorption capacity for NO + O₂. All of these factors played important roles in improving NH₃-SCR performance of the modified catalysts.

Full text of DOI:10.1134/S0023158419020101

ISSN 0023-1584, Kinetics and Catalysis, 2019, Vol. 60, No. 2, pp. 205–211. © Pleiades Publishing, Ltd., 2019.
Published in Russian in Kinetika i Kataliz, 2019, Vol. 60, No. 2, pp. 227–234.
Nb-Modified CeAlOx Catalyst Used for the Selective Catalytic
Reduction of NO by NH3: The Promoting Effect of Nb
R. Shia, b, c, *, Y. Zhangc, B. Yuanc, Z. Zhengc, L. Nic, R. Fenga, b, c, X. Lina, c, and L. Daia, b, c
aFujian Indoor Environmental Engineering Technology Research Center, Fuzhou, 350118 China
bChemical Safety Institute of Fujian University of Technology, Fuzhou, 350118 China
cCollege of Ecological Environmental and Urban Construction, Fujian University of Technology, Fuzhou, 350118 China
*e-mail: shironghui@fjut.edu.cn
Received July 24, 2018; revised October 14, 2018; accepted October 19, 2018
Abstract—A series of Nb modified CeAlOx catalysts were prepared by hydrothermal method and tested in the
selective catalytic reduction of NO with NH3. The catalyst with an Nb/Al molar ratio of 1 : 1 exhibited the
best catalytic activity and high resistance to sulfur poisoning. Nitrogen adsorption measurements, XRD,
NH3-TPD, H2-TPR and XPS analyses as well as in situ DRIFTS technique were applied to characterize the
catalysts. The characterization results showed that the introduction of Nb enhanced the redox power and
increased surface acidity. Moreover, the modification enriched the catalyst surface with adsorbed Oα and
Ce3+ species. In addition, in situ DRIFTS experiments demonstrated that doping with Nb increased catalyst
adsorption capacity for NH3 while decreased the adsorption capacity for NO + O2. All of these factors played
important roles in improving NH3-SCR performance of the modified catalysts.
Keywords: selective catalytic reduction, Nb modified, CeAlOx, NH3, NOx
DOI: 10.1134/S0023158419020101
INTRODUCTION
MnO2 [4], Ce–Cu–Zr mixed oxide [5] and Sb modi-
fied CeZrOx [6] showed high NH3-SCR activities
under conditions usually applied in test reactions. It
was reported that the introduction of Nb could consid-
erably improve the NH3-SCR activity and N2 selectivity.
The promoting effect of Nb on the NH3-SCR activities
was reported over CeO2–Nb2O5 [7], Cu/Ti–Nb [8],
Mn–Nb [9] and NbOx–CeO2–ZrO2 [10] catalysts. To
the best of our knowledge, no research about Nb
doped CeAlOx catalyst used in the NH3-SCR was
described.
In the present work, a series of Nb modified
CeAlOx catalysts prepared by hydrothermal method
was applied for the NH3-SCR. The catalysts were
characterized by means of N2-BET, XRD, H2-TPR,
NH3-TPD, XPS and in situ DRIFTS techniques.
Nitrogen oxides (NO and NO2), generated from
combustion of fossil fuels, have brought about serious
environmental problems, such as acid rain, photo-
chemical smog, ozone depletion and greenhouse
effects [1]. Selective catalytic reduction of NOx by
NH3 (NH3-SCR) was proved to be the best available
control technology to eliminate the nitrogen oxides from
coal-fired power plants. V2O5–WO3 (MoO3)/TiO2 cata-
lysts are widely used in this process due to their high
NOx removal efficiency and strong resistance to SO2
poisoning. However, V-based catalysts suffer certain
setbacks, including a narrow reaction temperature
window of 300–400°C, a low N2 selectivity at high
temperatures and the biological toxicity of V2O5 to
environment and human health [2]. Hence, the devel-
opment of environmentally benign catalysts is an
important challenge.
EXPERIMENTAL
Catalyst Preparations
A series of Nb modified CeAlOx catalysts with an
equimolar Ce/Al ratio and different Nb/Al molar
ratios were hydrothermally prepared. A certain amounts
Recently, Ce-based NH3-SCR catalysts attracted
research interest because of their super redox proper-
ties, high oxygen storage capacity and sulfur resistance
they provide in a variety of reactions. The reported Ce-
based catalysts including CeO2/TiO2 [3], Ce doped α-
205

206
SHI et al.
of Ce(NO3)3 ⋅ 6H2O, NbCl5 and Al(NO3)3 ⋅ 9H2O were NH3—500 ppm, 5 vol % O2, SO2—100 ppm (when
dissolved in deionized water and stirred for 1 h at room
temperature. Ammonia water solution was added into
the above mixed solution until pH value reached 10,
then the suspension was transferred into a Teflon
sealed autoclave and aged at 150°C for 24 h. Subse-
quently, the precipitate was collected by filtration,
washed with deionized water, dried at 110°C for 12 h
and then calcined at 500°C for 5 h.
tested), 5 vol % H2O (when tested) and N2 as balance gas.
The amount of catalyst was 0.6 mL catalyst was employed,
a total flow rate was maintained at 500 mL/min. The con-
centrations of effluent gas were analyzed continually
by an FTIR spectrometer (Nicolet Nexus 670). The
NOx conversion and N2 selectivity were calculated as
follows:
[NOx]in − [NO ]
NH3-SCR Activity Tests
The NH3-SCR activity was investigated in a fixed-bed
reactor under the following conditions: NO—500 ppm,
NO conversion =
x out ×100%,
[NOx]in
NO = NO + NO ,
[
]
[
]
[
]
x
2
N2 selectivity
[NOx]in + [NH3]in − [NOx]out − [NH3]out − 2[N2O]out
[NOx]in + [NH3]in − [NOx]out − [NH3]out
=
.
Here the subscript “in” denotes the inlet gas con- perature was increased to 600°C in flowing He at a
centration and the subscript “out” denotes the outlet heating rate of 10°C/min.
gas concentration.
The in situ DRIFTS experiments were performed
on an FTIR spectrometer (Nicolet Nexus 670)
equipped with a smart collector and a MCT detector
cooled by liquid nitrogen. Before each experiment, the
catalyst was pretreated in 20 vol % O2/N2 at 500°C for
1 h and then cooled to 200°C. The background spectrum
collected in flowing N2 was automatically subtracted
from the sample spectrum. The reaction conditions were
as follows: NH3—500 ppm, NO—500 ppm, 5 vol % O2,
N2 balance. All spectra were recorded by accumulating
Catalyst Characterization
The N2 adsorption–desorption isotherms over cat-
alysts were obtained at 77 K using a Micromeritics
Tristar-3000 system. Prior to the N2 adsorption, the
catalysts were evacuated at 300°C for 5 h. The specific
surface areas of the catalysts were determined by using
BET method under the 0.05–0.3 relative pressure
range. The total pore volume were measured from the
N2 desorption branches of the isotherms using the
BJH method.
Powder X-ray diffraction (XRD) measurements of
the catalysts were performed on a Rigaku D/max 2200
with CuKα radiation. The data of 2θ between 10° and
90° were collected at 5°/min with a step size of 0.07°.
100 scans with a resolution of 4 cm–1.
XPS measurements were conducted on an
ESCALab220i-XL electron spectrometer from VG
Scientific using 300 WMgKα radiation. Binding ener-
gies of Ce3d were calibrated using C1s peak (BE, BE =
284.8 eV) as standard.
The H2-TPR (hydrogen-temperature programmed
reduction) and NH3-TPD (NH3-temperature pro-
grammed desorption) experiments were conducted on
a Micromeritics Auto Chem II 2920 chemisorption
analyzer. For the H2-TPR, in each experiment a
100 mg sample was loaded into the quartz reactor and
pretreated in 20 vol % O2/He (50 mL/min) at 400°C
for 1 h and then cooled to room temperature under
flowing He. The temperature was ramped from 30 to
800°C in a gas mixture of 10% H2/He at 10°C/min.
RESULTS AND DISCUSSION
The NH3-SCR performances of CeAlOx and Nb
modified CeAlOx catalysts were tested and the corre-
sponding results were exhibited in Fig. 1. As showed in
Fig. 1a, the CeAlOx catalyst showed low NO conver-
sion with the maximum value which was only about
83% at 400°C. However, the NO conversions of the
other three catalysts doped with Nb remained above
90% in the temperature range of 200–400°C. As
Using a thermal conductivity detector the H2 con- exhibited in Fig. 1b, the CeAlOx catalyst showed poor
sumption amount was monitored. Prior to the NH3- N2 selectivity at the temperature above 300°C. With
TPD experiments, the 100 mg sample was pretreated the increase of Nb : Al molar ratio from 0.5 to 1.0, the
in a 20 vol % O2/He at 400°C for 1 h and cooled to high temperature N2 selectivity of the Nb modified
30°C, and then the catalysts were saturated with NH3 CeAlOx catalysts was improved markedly. However,
at a flow rate of 40 mL/min for 1 h. Finally, the tem- further increasing the Nb doping molar ratio to 1.5 did
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Nb-MODIFIED CeAlOx CATALYST USED
not lead to any further enhancement of N2 selectivity.
207
(а)
When the reaction temperature above 300°C, partial
oxidation of ammonia may occur on the catalyst sur-
face and then result in a decrease of N2 selectivity, the
similar phenomena has been reported by Lian et al.
[9]. Therefore, the CeNbAlOx catalyst presented an
optimal NH3-SCR performance with Nb:Al molar
ratio being 1 : 1.
NOx conversion, %
100
90
80
70
60
50
40
30
20
10
0
The presence of H2O and SO2 in the flue gas usu-
ally showed high poisoning effects on the NH3-SCR
activity. Therefore, it was worthwhile to investigate the
influences of H2O/SO2 on the NH3-SCR activity over
CeNbAlOx catalyst. As shown in Fig. 2, the effects of
H2O or/and SO2 on the activity of CeNbAlOx catalyst
were investigated as a function of time at 250°C. It
showed that the NO conversion slightly decreased
with introducing of H2O or SO2 individually. How-
ever, the NH3-SCR activity could restore to the origi-
nal level after turning off the H2O or SO2 entrance.
The coexistence of 100 ppm SO2 and 5 vol % H2O
induced a gradually decrease of NOx conversion at
250°C. After stopping the flows of SO2 and H2O, the
NO conversion was restored from 78.2 to 96.2%. It is
hinted that the loss of NH3-SCR activity might be
connected with the deposit of NH4HSO4 or
(NH4)2SO4 on the catalyst surface [6].
CeAlOx
CeNb0.5AlOx
CeNbAlOx
CeNb1.5AlOx
100 150 200 250 300 350 400 450 500
Temperature, °С
(b)
N2 selectivity, %
100
90
80
70
60
50
40
30
20
10
0
Table 1 showed the BET surface areas, pore vol-
umes and average pore diameters of the CeAlOx and
Nb modified CeAlOx catalysts. Compared with
CeAlOx, Nb modified samples possessed larger pore
volumes and average pore diameters. From the N2-
BET results, we can find that doping with Nb induced
the structural modification of the catalysts.
CeAlOx
CeNb0.5AlOx
CeNbAlOx
CeNb1.5AlOx
100 150 200 250 300 350 400 450 500
Figure 3 displayed the XRD patterns of CeAlOx
and Nb modified CeAlOx catalysts. It can be found
that some peaks appeared at 28.8°, 33.2°, 47.8°, 56.5°,
70.2°, 76.9°, 79.6° and 88.7°, respectively, corre-
sponding to the typical diffraction patterns for the
Cerianite CeO2 structure [11]. No diffraction peaks
attributed to Al or Nb species were observed in the
XRD patterns for all samples, which hinted that they
were in amorphous phase or the crystallite phase with
very small particle size. As the Nb/Al molar ratio
increased from 0.5 to 1.5, the intensities of the CeO2
Temperature, °С
Fig. 1. (a) NH -SCR activity and (b) N selectivity of
3
2
CeAlO and Nb modified CeAlO catalysts.
x
x
grain sizes of CeO2 on Nb modified samples were
smaller than that on the CeAlOx catalysts without Nb.
The adsorption capacity of NH3 on the CeAlOx and
CeNbAlOx catalysts were characterized by NH3-TPD
peaks became weaker gradually. It indicated that the in the range from 50 to 600°C and the corresponding
Table 1. BET surface area, pore volume and average pore size of CeAlOx and Nb doped CeAlOx catalysts
BET surface area, m2/g
Pore volume, cm3/g
Sample
Average pore size, nm
CeAlOx
155
132
110
104
0.15
0.21
0.29
0.24
4.2
5.4
7.8
6.2
CeNb0.5AlOx
CeNbAlOx
CeNb1.5AlOx
KINETICS AND CATALYSIS Vol. 60 No. 2 2019

208
SHI et al.
NOx conversion, %
1000
900
800
SO2
add
700
H2O + SO2
add
H2O
H2O
remove
600
add
SO2
remove
500
400
300
200
100
H2O + SO2
remove
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36
Time, h−1
Fig. 2. NH -SCR activity of CeNbAlO catalyst in the presence of 100 ppm SO or/and 5 vol % H O at 250°C.
3
x
2
2
curves were shown in Fig. 4. It can be seen from the were shifted to lower temperatures and the total acidity
increased to 612.4 μmol/g, which suggested that the
surface acidity of the CeAlOx catalyst had been
strengthened by doping with Nb.
figure, the desorption curve of the CeAlOx catalyst
showed two peaks at 142 and 365°C, respectively. The
peak at 142°C can be assigned to the weakly adsorbed
NH3, and the peak at 365°C can be attributed to the
strongly adsorbed NH3. The total acidity of the
CeAlOx catalyst was about 423.1 μmol/g. After modi-
fying with Nb, the positions of adsorbed NH3 peaks
The redox property of the catalyst also plays an
important role in the NH3-SCR reaction. The H2-TPR
experiments were performed to investigate the redox
ability of CeAlOx and CeNbAlOx catalysts and the
Intensity
Intensity
134
CeNb1.5AlOx
CeO2
357
CeNbAlOx
CeNbAlOx
142
365
CeNb0.5AlOx
CeAlOx
CeAlOx
10
20
30
40
50
60
70
80
90
100
200
300
400
500
600
2θ, deg
Temperature, °С
Fig. 3. XRD patterns of the CeAlO and Nb modified
Fig. 4. NH -TPD profiles of the CeAlO and CeNbAlO
3 x
catalysts.
x
x
CeAlO catalysts.
x
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Nb-MODIFIED CeAlOx CATALYST USED
Intensity
209
Intensity
(а)
498
Ce3+ ratio
691
CeAlOx
CeNbAlOx
527
U′′′
U
U′′
U′
27.2%
V′′′
V′′
V′
V
CeNbAlOx
CeAlOx
34.1%
100
200
300
400
500
600
700
800
920 915 910 905 900 895 890 885 880 875
Binding energy, eV
Temperature, °С
(b)
Fig. 5. H -TPR profiles of the CeAlO and CeNbAlO cat-
2
x
x
Intensity
alysts.
Oα molar ratio
results were shown in Fig. 5. For the CeAlOx catalyst,
the reduction peak at 527°C was attributed to the
reduction of surface Ce4+ to Ce3+ [12]. After doping
with Nb, two reduction peaks centered at about 498
and 691°C were detected on the CeNbAlOx catalyst.
The first peak at 498°C, which could be due to the
reduction of surface CeO2, exhibited lower reduction
temperature in comparison to CeAlOx catalyst. The
second peak at 691°C could be attributed to the reduc-
tion of bulk CeO2 [13]. The above results indicated
that redox ability of CeNbAlOx catalyst was improved
by the addition of Nb.
CeAlOx
44.6%
Oα
Oβ
CeNbAlOx
55.3%
536
534
532
530
528
526
The XPS analysis was also performed to reveal the
chemical state of the Ce and O on the catalyst surface.
Figure 6a displayed the Ce3d XPS spectra of the
CeAlOx and CeNbAlOx catalysts. The Ce3d peaks
were fitted by searching for the optimum combination
of Gaussian bands. The sub-bands labeled u' and v'
represent the 3d104f1 initial electronic state corre-
sponding to Ce3+, and the sub-bands labeled u, u'', u''',
v, v'', and v''' represent the 3d104f0 state of Ce4+ [6].
The ratio Ce3+/(Ce3+ + Ce4+) in CeNbAlOx (34.1%),
was higher than that in CeAlOx (27.2%), which
indicted the presence of more surface oxygen vacan-
cies on CeNbAlOx catalyst.
Binding energy, eV
Fig. 6. (a) Ce3d and (b) O1s XPS spectra of the CeAlO
x
and CeNbAlO catalysts.
x
that in CeAlOx (44.6%). The doping with Nb induced
the transformation of Ce4+ to Ce3+, which could create
oxygen vacancies and increase the surface-adsorbed
oxygen. The high Oα ratio is beneficial for the NO oxi-
dation to NO2 and therefore enhance the NH3-SCR
reaction.
In situ DRIFTS
Figure 6b also exhibited the O1s XPS spectra,
which could be fitted into two sub-bands by searching
for the optimum combination of Gaussian bands. The
sub-bands at 529.4–530.0 eV could be attributed to the
lattice oxygen (Oβ) and the sub-bands at 531.0–531.6 eV
are belonged to the surface adsorbed oxygen (Oα) [14].
The surface-adsorbed oxygen is more reactive than
lattice oxygen due to its higher mobility. The ratio
The in situ DRIFTS of NH3 and NO + O2 adsorp-
tion were measured to examine the changes of the sur-
face property on the CeAlOx and CeNbAlOx catalysts
at 200°C and the corresponding results were presented
in Fig. 7.
As shown in the Fig. 7a, after exposure to NH3, the
Oα/(Oα + Oβ) in CeNbAlOx (55.3%) was higher than CeAlOx catalyst was covered by several kinds of NH3
KINETICS AND CATALYSIS Vol. 60 No. 2 2019

210
SHI et al.
detected [20, 21]. After doping with Nb, the new band
(а)
at 1367 cm–1 assigned to the oxidized species of
adsorbed appeared obviously [22]. Meanwhile, the
bands at 1282 and 1620 cm–1 assigned to the Lewis
acid site and the bands at 1445 and 1676 cm–1 corre-
sponding to the Bronsted acid site were much stronger
than these on the CeAlOx catalyst [11, 23, 24]. In the
mean time, much stronger bands at 3178, 3266 and
3366 cm–1 attributed to N–H stretching vibration modes
were also detected on the CeNbAlOx catalyst [19].
Intensity
3256
3187
0.02
3330
1676
1620
1445
1367
1282
CeNbAlOx
3767
3610
Figure 7b presents the in situ DRIFTS results of
NO + O2 adsorption at 200°C. After NO + O2 adsorp-
tion and N2 purge, several bands attributed to
1307
1271
3282
3360
1455
1625
monodentate nitrate (1434 cm–1) [25], bidentate
nitrate (1280 and 1564 cm–1) [26, 27], bridging nitrate
(1617 cm–1) [28] and M–NO2 nitro species (1355 cm–1)
[29] appeared on the CeAlOx catalyst. After adding Nb
content, the bands assigned to the M–NO2 nitro and
monodentate nitrate species disappeared obviously.
The bands due to adsorbed nitrate species not only
showed an obvious decrease in intensity but shifted to
the new position (1379 and 1580 cm–1 bidentate
nitrate, 1604 and 1625 cm–1 bridging nitrate) [30–33].
It can be concluded that the addition of Nb limited the
adsorption of nitrate species significantly and changed
their forms as well. The above analysis indicated that
the introduction of Nb resulted in the formation of
more Lewis and Brønsted acid sites accompanied by
the reduction of basic sites, all of which were benefi-
cial to the NH3-SCR process.
3705
3660
CeAlOx
4000
3500
1800 1600 1400 1200 1000
Wavenumber, сm–1
(b)
Intensity
1604
1580
1625
0.02
1379
1564
CeNbAlOx
1280
1355
1617
1434
CONCLUSIONS
CeAlOx
Nb promoted CeAlOx catalyst prepared by hydro-
thermal method was used for NH3-SCR of NOx. The
CeNbAlOx catalyst with Nb/Al molar ratio of 1 : 1
exhibited high activity, N2 selectivity and SO2/H2O
durability under test conditions. The characterization
results demonstrated that Nb addition improved the
surface acidity, redox ability, surface adsorbed Oα and
2000
1800
1600
1400
1200
1000
Wavenumber, сm–1
Fig. 7. In situ DRIFTS spectra of (a) NH adsorption and
(b) NO + O adsorption on the CeAlO and CeNbAlO
catalysts at 200°C.
3
2
x
x
Ce3+ species ratios. The intensity of NH3 adsorption
species was enhanced and the NOx adsorption species
were limited by doping with Nb. All of the above fea-
tures are responsible for achieving the high NH3-SCR
reaction performance.
species. The band at 1271 cm–1 is assigned to the sym-
metric and asymmetric deformation vibration of NH3
adsorbed on Lewis acid site [15]. The band at 1307
cm–1 is due to the scissoring vibration mode of NH2
species [16]. The band at 1455 cm–1 is attributed to the
symmetric and asymmetric bending vibration ionic
ACKNOWLEDGMENTS
NH4+
We gratefully acknowledge the financial support of
Science Foundation of the Fujian Province (nos.
2017J01568, 2017J01673), the General Project of
Fujian Provincial Department of Education (no.
JAT160327, JAT170386) and the Scientific Research
Foundation of the Fujian University of Technology
species on Brønsted acid site [17]. The band at
1625 cm–1 is assigned to the asymmetric bending
vibrations of coordinated NH3 [18]. The bands at 3360
and 3282 cm–1 are attributed to N–H stretching vibration
modes [19]. Two negative bands at 3660 and 3705 cm–1
belonged to the surface O–H stretching were also (grant no. GY-Z14007).
KINETICS AND CATALYSIS
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Nb-MODIFIED CeAlOx CATALYST USED
211
16. Zhang, X., Wang, H., Wang, Z., and Qu, Z., Appl. Surf.
Sci., 2018, vol. 447, p. 40.
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