Iron-niobium composite oxides for selective catalytic reduction of NO with NH3

Article abstract of DOI:10.1016/j.catcom.2017.04.033

Iron-niobium composite oxides (Nb_aFeO_x, a represents the mass percent of Nb in Fe-Nb composite oxides) were studied for the selective catalytic reduction (SCR) of NO_x with NH₃. The Nb-doped Fe₂O₃ was found to be responsible for the improved activity. The doping of Nb into Fe₂O₃ resulted in the improvement of specific surface areas, redox property and acidic amount. The optimal Nb_30.3FeO_x sample exhibited nearly 100% NO_x conversion and N₂ selectivity from 250 °C to 400 °C, which would be a promising candidate for NH₃-SCR catalysts in the medium temperature ranges.

Full text of DOI:10.1016/j.catcom.2017.04.033

Catalysis Communications 97 (2017) 111–115
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Iron-niobium composite oxides for selective catalytic reduction of NO with
NH3
Nana Zhanga, Ying Xina, Xiao Wanga, Mingfen Shaoa, Qian Lia, Xicheng Mab, Yongxin Qic,
Lirong Zhengd, Zhaoliang Zhanga,⁎
a
School of Chemistry and Chemical Engineering, Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, University of Jinan, Jinan 250022,
China
b
School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China
School of Material Science and Technology, Shandong University, Jinan 250100, China
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
c
d
A R T I C L E I N F O
A B S T R A C T
Keywords:
Iron-niobium composite oxides
Nitrogen oxides
Selective catalytic reduction
Redox property
Acidity
Iron-niobium composite oxides (NbaFeOx, a represents the mass percent of Nb in Fe-Nb composite oxides) were
studied for the selective catalytic reduction (SCR) of NOx with NH3. The Nb-doped Fe2O3 was found to be
responsible for the improved activity. The doping of Nb into Fe2O3 resulted in the improvement of specific
surface areas, redox property and acidic amount. The optimal Nb30.3FeOx sample exhibited nearly 100% NOx
conversion and N2 selectivity from 250 °C to 400 °C, which would be a promising candidate for NH3-SCR
catalysts in the medium temperature ranges.
1. Introduction
the high acidity [17]. As early as in 1985, Okazaki et al. first reported
the promotion effect of Nb2O5 to Fe2O3 catalyst for NH3-SCR reaction
Nitrogen oxides (NOx) are main air pollutants causing photochemi-
cal smog, acid rain, ozone depletion, and greenhouse effects [1]. Among
various NOx control technologies, selective catalytic reduction of NOx
with NH3 (NH3-SCR) has been demonstrated as the most efficient one in
the after-treatment process. Nowadays, the most widely used NH3-SCR
catalyst is V2O5-WO3/TiO2 [2,3], however some unavoidable disad-
vantages still remained, such as the toxicity of vanadium species, the
poor activity at temperatures lower than 350 °C and the high oxidation
of NH3 at the high temperature [4,5]. Consequently, the exploration of
the novel eco-friendly SCR catalysts with high activity and N2 selectiv-
ity over wide-temperature ranges is never stopped.
It is well known that the excellent redox capability and acidity of
catalysts are favorable to the NH3-SCR reaction [6]. Thus, transition
metal oxides with prominent redox property are considered to be the
promising candidates for NH3-SCR catalysts [7,8]. Therein, iron oxides
(typically Fe2O3) have been received considerable attention because of
their non-toxicity and abundance [9–11]. Nevertheless, pure Fe2O3
often suffers from poor activity at the low temperature [12]. Therefore,
many researchers tried to introduce hetero atoms into Fe2O3 to improve
the properties and consequently the catalytic activity, for example, Fe-
Ti [13], Fe-Ce [14], and Fe-Mn composite oxides, etc. [15,16].
Generally, niobium oxides have gained attention as catalysts due to
[18]. However, the characteristic details were lost in the long history.
Afterwards, Vikulov et al. found similar results in Nb2O5-modified
V2O5/TiO2 [19]. Lian et al. showed that the addition of Nb to MnOx
enhanced the acidity, especially the Brønsted acidity, which brought
about the low temperature SCR activity [20]. While Ding et al. reported
that the introduction of Nb into CeZr2Ox allows for a high surface area,
the strong surface acidity and redox ability, all of which are beneficial
to the remarkable SCR performance [21].
Herein, a series of Fe-Nb composite oxides were synthesized and
characterized in depth. X-ray diffraction (XRD) showed the presence of
the Nb-doped Fe2O3 and FeNbO4 phases in Fe-Nb composite oxides.
However, only was the Nb-doped Fe2O3 found to be responsible for the
improved activity in NH3-SCR. The doping of Nb into Fe2O3 resulted in
the improvement of specific surface areas, redox property and acidic
amount, which gained the maximum at 30.3 wt% Nb. Nb30.3FeOx
exhibited nearly 100% NOx conversion and N2 selectivity from 250 °C
to 400 °C, demonstrating a promising candidate for the SCR catalysts in
the medium temperature range.
⁎
Corresponding author.
E-mail address: chm_zhangzl@ujn.edu.cn (Z. Zhang).
http://dx.doi.org/10.1016/j.catcom.2017.04.033
Received 9 January 2017; Received in revised form 7 March 2017; Accepted 17 April 2017
Available online 18 April 2017
1566-7367/ © 2017 Elsevier B.V. All rights reserved.

N. Zhang et al.
Catalysis Communications 97 (2017) 111–115
2. Experimental section
Nb2O5
2.1. Sample preparation
Nb35.4FeOx
The Fe-Nb composite oxides were prepared by a co-precipitation
method. In a typical process, a certain amount of NbCl5 was dissolved in
50 mL of deionized water with magnetic stirring at room temperature.
Similarly, FeSO4·7H2O and ascorbic acid (mole ratio is 1:1.1) were
dissolved in 100 mL of deionized water and stirred to obtain a clear
solution, and then it was dropwise added into the NbCl5 aqueous
solution with magnetic stirring for 10 min at room temperature. The
total amount of the metallic ions was 0.033 g/mL. Subsequently, the
excess urea solution (2 mol/L) was added into the mixed solution and
then heated under reflux aging at 90 °C over 12 h. The precipitate was
collected by filtration and washing with deionized water, followed by
drying at 100 °C overnight and calcinating at 500 °C for 5 h in air. The
obtained mixed oxides were denoted as NbaFeOx, where a represents
the mass percent (wt%) of Nb in Fe-Nb composite oxides based on
inductively coupled plasma-atomic emission spectrometry (ICP-AES).
For comparison, pure Fe2O3 and Nb2O5 were synthesized by the same
procedure, while FeNbO4 was prepared by the solid state reaction [22].
The mechanically mixed sample was obtained by grinding the mixture
of Fe2O3 and FeNbO4 nanoparticles with the Nb content according to
the quantitative XRD analysis of Nb30.3FeOx.
Nb30.3FeOx
Nb21.2FeOx
Nb12.0FeOx
Fe2O3
FeNbO4
10
20
30
40
50
60
70
80
90 32
33
34
Fig. 1. XRD patterns of Fe2O3, Nb2O5 and NbaFeOx.
determined by a chemiluminiscence NOx analyzer (42i-HL, Thermo), in
addition, NH3 and N2O were detected by using a quadrupole mass
spectrometer (MS, OmniStar 200, Balzers) at m/z = 44 for N2O, and 17
for NH3. The NOx conversion and N2 selectivity were calculated
according to the following equation:
[NOx]inlet−[NOx]outlet
NOx conversion =
× 100%
2.2. Characterization
[NOx]inlet
XRD patterns were recorded with a Rigaku D/max 2500PC diffract-
ometer using Cu Kα (=0.15405 nm) radiation and intensity data were
collected over a 2θ range of 10 to 90°. A Micromeritics ASAP2020M
instrument was used to measure the N2 adsorption isotherms of the
samples at liquid N2 temperature (−196 °C). Before the N2 physisorp-
tion, all the samples were degassed at 300 °C. ICP-AES experiments
were carried out on an IRIS Intrepid IIXSP instrument from Thermo
Elemental. High-resolution transmission electron microscopy (HRTEM)
was conducted on a JEOL JEM-2010 at an accelerating voltage of
200 kV. X-ray absorption fine structure (XAFS) measurements for the Fe
K-edge and Nb K-edge were performed in the transmission mode at
room temperature on the XAFS station of the 1W1B beamline of Beijing
synchrotron radiation facility (BSRF, Beijing, China). H2 temperature-
programmed reduction (H2-TPR) experiments were carried out on a
quartz reactor with a thermal conductivity detector (TCD) to monitor
H2 consumption. The samples (50 mg) in a quartz reactor were
pretreated at 500 °C for 30 min in O2 and cooled down to the room
temperature. Then a 50 mL/min gas flow of 5 vol% H2 in N2 was passed
over the samples with the rate of 10 °C/min up to 800 °C. NH3-
temperature programmed desorption (NH3-TPD) experiments were
performed in a quartz reactor using 50 mg catalyst. Prior to the
experiments, the samples (40–60 mesh) in a quartz reactor were
pretreated at 500 °C for 30 min in 10 vol% O2/He (50 mL/min) to
remove surface impurities and then cooled to the 30 °C. NH3 adsorption
was operated in 4000 ppm NH3/He (50 mL/min) until the concentra-
tion stabilized. Then the weak adsorbed ammonia was purged with
highly pure He. Finally, the samples were heated up to 700 °C at a
heating rate of 10 °C/min. NH3 was detected using quadrupole mass
spectrometer (OmniStar 200, Balzers).
N2 selectivity
[NOx]inlet + [NH3]inlet−[NOx]outlet−[NH3]outlet−2 × [N2O]outlet
[NOx]inlet + [NH3]inlet−[NOx]outlet−[NH3]outlet
=
× 100%
3. Results and discussion
3.1. XRD
Powder XRD was conducted to investigate the crystal structure of
NbaFeOx (Fig. 1). Pure iron and niobium oxides are present as Fe2O3
(JCPDS 33-0664) and Nb2O5 (JCPDS 30-0873), respectively. For
Nb12.0FeOx, the XRD peaks are indexed as a hematite phase of Fe2O3,
while no diffraction peaks of Nb-containing species was detected.
Increasing in the Nb content results in the emergence of a new phase
FeNbO4 (JCPDS 71-1849) (Fig. S1) as for Nb21.2FeOx, Nb30.3FeOx, and
Nb35.4FeOx. Furthermore, compared with pure Fe2O3, the peaks of
Fe2O3 in NbaFeOx shift to the higher angle (inset) deriving from the
lattice constriction. The decreased lattice parameter a (Table 1) for the
Fe2O3 phase correspond to the increasing Nb content in NbaFeOx, owing
to the substitution of Fe ions (0.0645 nm) by the smaller Nb ions
(0.064 nm) [23]. This suggests that a fraction of Nb atoms was doped
into the Fe2O3 lattice besides the formation of FeNbO4. The relative
amount of FeNbO4 in NbaFeOx can be obtained using the quantitative
XRD analysis by the reference intensity ratio (RIR) method (Table 1 and
Supporting information) [24]. In combination with the ICP data, the
doping amount of Nb into the Fe2O3 lattice was roughly calculated
(Table 1), which corresponded well with lattice parameter a. Further-
more, with the doping amount increasing, the crystallinity of hematite
is decreased, especially for Nb35.4FeOx.
2.3. Catalytic performance testing
3.2. N2 adsorption/desorption and TEM
All samples were tested for SCR activity in a fixed-bed quartz tube
reactor (6.0 mm i.d.) with a thermocouple placed inside the catalyst
bed and in the temperature range 150–450 °C. The experimental
conditions were controlled as follows: 500 ppm NO, 500 ppm NH3,
5.3% O2, 100 ppm SO2 (when used), 5% H2O (when used) and balance
He. The total flow rate was 300 mL/min and the gas hourly space
velocity (GHSV) was 50,000 h−1. Concentrations of NO and NO2 were
The surface areas and pore distribution were characterized by N2
adsorption/desorption (Table 1 and Fig. S2). As shown in Table 1, all
NbaFeOx samples possess much higher BET surface areas than those of
pure Fe2O3 and Nb2O5. The largest surface area of 84.7 m2/g was
obtained for Nb30.3FeOx, indicating the optimal doping amount for
specific surface areas. N2 adsorption/desorption isotherms exhibit
112

N. Zhang et al.
Catalysis Communications 97 (2017) 111–115
Table 1
ICP, XRD, surface area, H2-TPR and NH3-TPD data for Fe2O3, Nb2O5 and NbaFeOx.
Samples
Nb mass percent
(wt%)a
Nb in FeNbO4 (wt Nb doped into
Lattice parameter a
(Å)
Surface area
(m2/g)
Peak position of H2-
TPR (°C)
Total amount of NH3
desorption (mmol/g)
%)b
Fe2O3 (wt%)
Fe2O3
–
–
–
6.7
7.7
10.8
–
–
5.086
5.045
5.035
5.030
5.023
–
48.1
71.1
73.2
84.7
77.3
32.6
398
393
393
380
386
–
198.9
271.1
287.6
314.4
281.6
163.0
Nb12.0-Fe2O3 12.0
Nb21.2-Fe2O3 21.2
Nb30.3-Fe2O3 30.3
Nb35.4-Fe2O3 35.4
12.0
14.5
22.6
24.6
–
Nb2O5
–
a
ICP data.
b
Obtained from the quantitative XRD analysis.
typical IV curves and H1 type hysteresis loops (Fig. S2a), suggesting the
existence of mesopores and a small fraction of macropores (Fig. S2b),
which originate from the interstices between the particles.
The morphology and microstructures were further investigated by
TEM taking Nb30.3FeOx as an example (Fig. S3). The nanoparticles of
10–20 nm were aggregated together (Fig. S3a), which can be distin-
guished as the Nb-doped Fe2O3 and FeNbO4 (Fig. S3b), in good
agreement with XRD and pore distribution analysis for NbaFeOx.
Fig. 2a, Fe2O3 shows two characteristic peaks assigned to the Fe-O and
Fe-O-Fe shells together with an obvious shoulder peak, which was also
typical for α-Fe2O3 [13]. In the case of NbaFeOx, the coordination
environment of iron is similar to that of Fe2O3, while the intensity of the
shoulder peak is relatively stronger than that of the Fe-O-Fe shell,
which is caused by the co-existed FeNbO4. In Fig. 2b, FeNbO4 shows
two characteristic peaks, one belongs to Nb-O shell together with an
obvious shoulder and the other assigns to Nb-O-Fe shell. For NbaFeOx,
the position of the Nb-O-Fe peak shows a slight shift to higher R value
in comparison with FeNbO4, suggesting the contribution from the
doped Nb species in Fe2O3.
3.3. XAFS
XAFS can be used to determine the local environment around
specific atoms, which may influence the performance of catalysts.
Fig. 2 shows the radial structure function (RSF) curves of Fe and Nb
K-edge EXAFS spectra for Fe2O3, Nb2O5, FeNbO4 and NbaFeOx. In
3.4. H2-TPR
To study the redox property of the samples, H2-TPR was performed.
Fig. 3 shows the H2-TPR patterns of Fe2O3, Nb2O5, FeNbO4 and
NbaFeOx. The pristine Fe2O3 shows three reduction peaks at 396 °C,
638 °C and > 700 °C, which can be assigned to the reduction of Fe2O3
to Fe3O4 (396 °C), Fe3O4 to FeO (638 °C) and FeO to Fe (> 700 °C),
respectively [16,25]. Nb2O5 and FeNbO4 do not show obvious reduc-
tion peaks below 700 °C. Thus, the low-temperature redox peaks could
be attributed to the reduction of Fe species in the Nb-doped Fe2O3
phase. It is clear that the addition of Nb can result in the peak shift to
lower temperatures. The lowest reduction peak temperature was
reached at Nb30.3FeOx (Table 1), indicating the optimal doping amount
for redox ability. After that the reduction peak shifts to the higher
temperature.
a
Shoulder
Nb35.4FeOx
Nb30.3FeOx
Nb21.2FeOx
Nb12.0FeOx
Fe2O3
Fe-O-Fe
Fe-O
FeNbO4
5
3.5. NH3-TPD
0
1
2
3
R ( )
4
NH3-TPD was carried out to investigate the effect of Nb contents on
acidity of the samples. Fig. 4 shows the NH3-TPD spectra over Fe2O3,
Nb2O5 and NbaFeOx. The corresponding desorption amounts of NH3 are
b
FeNbO4
Nb35.4FeOx
Nb30.3FeOx
Nb35.4FeOx
Nb30.3FeOx
Nb-O
Nb21.2FeOx
Nb12.0FeOx
Nb21.2FeOx
Nb12.0FeOx
Nb-O-Fe
Shoulder
2
FeNbO4
Nb2O5
Fe2O3
Nb2O5
0
1
3
4
5
R ( )
200
300
400
500
600
700
800
Temperature (oC)
Fig. 2. The radial structure function (RSF) curves of Fe (a) and Nb (b) K-edge EXAFS
spectra for Fe2O3, Nb2O5, FeNbO4 and NbaFeOx.
Fig. 3. H2-TPR profiles of Fe2O3, Nb2O5, FeNbO4 and NbaFeOx.
113

N. Zhang et al.
Catalysis Communications 97 (2017) 111–115
250 °C (Fig. S5). Moreover, the resistance to H2O and SO2 over
Nb30.3FeOx at 250 °C is checked (Fig. S5). When 5% H2O and
100 ppm SO2 were added into the reaction gas, NOx conversion
decreased from 100% to ~80%. However, the initial activity can be
recovered after removing SO2 and heat-treatment of the catalyst at
500 °C.
Nb2O5
Fe2O3
Nb12.0FeOx
Nb21.2FeOx
Nb30.3FeOx
Nb35.4FeOx
4. Conclusions
Iron-niobium composite oxides were characterized and studied for
NH3-SCR. XRD, ICP-AES, HRTEM and XAFS show the presence of the
Nb-doped Fe2O3 and FeNbO4 phases in Fe-Nb composite oxides.
However, only is the former responsible for the activity. The doping
of Nb into Fe2O3 resulted in the improvement of specific surface areas,
redox property and acidic amount as detected by N2 adsorption/
desorption, H2-TPR and NH3-TPD, respectively. The optimal doping
amount was obtained at 30.3 wt% Nb due to the highest surface areas,
reducibility and acidic amount. Nb30.3FeOx exhibits nearly 100% NOx
conversion and N2 selectivity from 250 °C to 400 °C at a gas hourly
space velocity of 50,000 h−1, which demonstrated a promising candi-
date for the SCR catalysts in the medium temperature ranges.
100
200
300
400
500
600
700
Temperature (oC)
Fig. 4. NH3-TPD profiles of Fe2O3, Nb2O5 and NbaFeOx.
100
80
Nb12.0FeOx
Nb21.2FeOx
Nb30.3FeOx
Nb35.4FeOx
Acknowledgments
This work was supported by National Natural Science Foundation of
China (Nos. 21477046, 21277060, and 21547007), Key Technology
R & D Program of Shandong Province (No. 2016ZDJS11A03), and
Shandong Provincial Natural Science Foundation, China (No.
ZR2016BM32).
60
40
Nb2O5
Fe2O3
20
Appendix A. Supplementary data
Fe2O3+FeNbO4
FeNbO4
0
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2017.04.033.
150
200
250
300
350
400
450
Temperature (oC)
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