VOX supported on TiO2-Ce0.9Zr0.1O2 core-shell structure catalyst for NH3-SCR of NO

Article abstract of DOI:10.1039/c9ra05024h

In this experiment, a TiO₂-Ce_0.9Zr_0.1O₂ support with core-shell structure was successfully prepared by a precipitation method and VO_X/TiO₂-Ce_0.9Zr_0.1O₂ catalyst was prepared by an impregnation method, and the catalyst was used to catalyze the NH₃-SCR of NO. Based on the results of HRTEM, XRD, BET, H₂-TPR, NH₃-TPD, XPS, Py-IR, it was speculated that due to the interaction between TiO₂ and Ce_0.9Zr_0.1O₂, more oxygen vacancies and Ce³⁺ are generated, which are beneficial to the existence of low-valence V by electron transfer between high valence state V and Ce³⁺and increase the acidic sites on the catalyst surface. The catalytic activity (>97%) of the VO_X/TiO₂-Ce_0.9Zr_0.1O₂ catalyst is superior to the current commercial catalyst (V₂O₅-WO₃/TiO₂) and has a higher N₂ selectivity (>97.5%) at 40000 h^-1 GHSV and 250-400 °C.

Full text of DOI:10.1039/c9ra05024h

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VOX supported on TiO2–Ce0.9Zr0.1O2 core–shell
structure catalyst for NH3-SCR of NO
Cite this: RSC Adv., 2019, 9, 30340
a
Lvesheng Sun,
Guojun Dong
Shunxin Cao,a Yun Huang,a Yiming Zhang,a Youhong Xiao,b
a
c
*
*
and Yu Su
In this experiment, a TiO2–Ce0.9Zr0.1O2 support with core–shell structure was successfully prepared by
a precipitation method and VOX/TiO2–Ce0.9Zr0.1O2 catalyst was prepared by an impregnation method,
and the catalyst was used to catalyze the NH3-SCR of NO. Based on the results of HRTEM, XRD, BET,
H2-TPR, NH3-TPD, XPS, Py-IR, it was speculated that due to the interaction between TiO2 and
Ce0.9Zr0.1O2, more oxygen vacancies and Ce3+ are generated, which are beneficial to the existence of
low-valence V by electron transfer between high valence state V and Ce3+and increase the acidic sites
on the catalyst surface. The catalytic activity (>97%) of the VOX/TiO2–Ce0.9Zr0.1O2 catalyst is superior to
the current commercial catalyst (V2O5–WO3/TiO2) and has a higher N2 selectivity (>97.5%) at 40 000 hꢀ1
GHSV and 250–400 ꢁC.
Received 3rd July 2019
Accepted 13th September 2019
DOI: 10.1039/c9ra05024h
rsc.li/rsc-advances
a large number of modication studies have been performed.
Using the isovalent metal cations of smaller diameter to prepare
Introduction
solid solution by co-precipitation is one of the most common
methods. Studies show that these Ce-based solid solutions have
great potential for deNOX.10–14 Thanh Huyen Vuong and co-
workers have investigated the relations between structural
properties and catalytic performance in VOX/CeXTi1ꢀXO2 and
VOX/CeXZr1ꢀXO2 catalysts, respectively.15,16 The results indicates
that VOX sites effectively attach to the support surface and the
redox properties of the materials governs catalytic activity rather
than Lewis and Brønsted surface acidic sites in this catalytic
system. All above mentioned researches reveal the importance
of oxygen mobility in catalytic reactions. Li et al.17 prepared
CeO2–WO3/TiO2 by impregnation method and conrmed that
CeO2 is favorable to keep more acid sites and enhanced NO
reduction in NH3-SCR reaction compared with V2O5–WO3/TiO2.
Li et al.18 prepared a CeO2–MnOX catalyst with a core–shell
structure and it exhibits relatively good NO conversion in the
presence of SO2 on account of the fact that CeO2 shell prevents
the active sites of MnOX from being poisoned.
The SCR catalytic activity of V2O5–WO3(MoO3)/TiO2 mainly
stems from the adsorption of NH3 by acidic sites possessed by
highly dispersed V species on the TiO2 surface, forming
Bronsted-NH3 and further reacting with NO in gas phase.19,20
Differently, the Mars-van Krevelen mechanism of Ce-based
solid solution catalyst has been clearly claried.15,16 We expect
that adding this structure to traditional catalysts in another way
can bring different effects and further optimize this catalytic
system. In this work, the core–shell structure carrier of TiO2 and
Ce0.9Zr0.1O2 was prepared by precipitation method, and loaded
VOX on the carrier by impregnation method. Combined with the
results of XRD, BET, H2-TPR, NH3-TPD, Py-IR, HRTEM and XPS,
Nitrogen oxides (NOX) cause serious environmental pollution.
The technology of selective catalytic reduction (SCR) of NOX
with NH3 has been widely used to reduce pollution caused by
NOX. A lot of research work on SCR catalysts has been carried
out.1–4 But the most widely used catalyst is still V2O5–
WO3(MoO3)/TiO2 catalyst.5,6 The wide application of this cata-
lytic system benets from its reasonable cost and high NO
conversion. However, these catalysts still have some disadvan-
tages such as narrow temperature windows of activity (300–400
ꢁC) and poor catalytic activity in low temperature. Although
many known SCR catalysts have relatively high NO conversion
in a wider temperature range,7,8 their selectivity, stability, and
resistance to water and sulfur are not excellent. On the other
hand, the production costs of catalysts with the above advan-
tages may be greatly increased, making it difficult to achieve
industrial production.
For two decades, CeO2 has attracted more attention in the
catalytic eld due to its excellent ability to store and release
oxygen as a result of the efficient change of Ce valence between
+3 and +4.9 The earliest application of CeO2 in cleaning ue
gases is mainly to deal with automobile exhaust as the three-way
catalyst (TWC) in the presence of noble metals such as Rh, Pt
and Pd. In order to further expand the application of CeO2,
aCollege of Materials Science and Chemical Engineering, Key Laboratory of Superlight
Materials and Surface Technology of Education Ministry, Harbin Engineering
University, 150001 Harbin, Heilongjiang, P. R. China. E-mail: dgj1129@163.com
bCollege of Power and Energy Engineering of Harbin Engineering University, 150001
Harbin, Heilongjiang, P. R. China
cSchool of Chemistry, Chemical Engineering and Materials of Heilongjiang University,
150080, Harbin, Heilongjiang, P. R. China. E-mail: 2003086@hlju.edu.cn
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the reasons for the high activity and thermal stability of the Analysis System GSD320 analyzer (Pfeiffer Vacuum GmbH,
catalyst are explained.
Berlin, Germany). Activity data was recorded with a temperature
gradient of 50 ꢁC in the range of 100 ꢁC ꢀ450 ꢁC. The NO
conversion (CNO) and N2 selectivity (SN ) are calculated as
2
Experimental
Chemical reagents
following equations:
½NOꢂ ꢀ ½NOꢂ
in
out
CNO
¼
ꢃ 100%
Ce(NO3)3$6H2O (Analytical Reagent) was purchased from Tian-
jin Guangfu Fine Chemical Research Institute. Zr(NO3)4$5H2O
(Analytical Reagent) was purchased from Tianjin Fuchen
Chemical Reagents Factory. Nano titanium dioxide (Analytical
Reagent) was purchased from Tianjin Guangfo Materials Co.,
Ltd. Ammonium metavanadate (Analytical Reagent) was
purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd.
Citric acid (Analytical Reagent) was purchased from Tianjin
Dongli District Tianda Chemical Reagent Factory.
½NOꢂ
in
ꢀ
ꢁ
2½N2Oꢂ þ ½NO2ꢂ
out
out
SN
¼
1 ꢀ
ꢃ 100%
2
½NOꢂ ꢀ ½NOꢂ þ ½NH3ꢂ ꢀ ½NH3ꢂ
in
out
in
out
where [NO]in and [NH3]in represent the inlet concentration of
NO and NH3. The [NO]out, [NH3]out, [NO2]out and [N2O]out
represent the outlet concentration of NO, NH3, NO2 and N2O.
Catalysts preparation
Catalyst characterization
TiO2–Ce0.9Zr0.1O2 support were prepared by precipitation
method. 0.0225 M of cerium nitrate (Ce(NO3)3$6H2O) and
0.0025 M of zirconium nitrate (Zr(NO3)4$5H2O) were completely
dissolved in an appropriate amount of deionized water,
respectively. Then, the resulting solutions were mixed at room
temperature for 10 min. 0.025 M TiO2 powder was subsequently
suspended in the mixed solution under vigorous stirring and
the obtained mixture kept stirring for 1 h at room temperature,
X-ray diffraction (XRD) powder patterns were recorded by a D/
MAX-3A Auto X-ray diffractometer (Rigaku Corporation,
˚
Tokyo, Japan) with Cu Ka radiation (l ¼ 1.5418 A, 40 kV, 40 mA).
The diffraction patterns ꢀw1ere taken in the 2q range of 10–80ꢁ at
a scan speed of 10ꢁ min
.
HRTEM were recorded by FEI Talos F200X G2 electron
microscope (PHILIPS, Amsterdam, The Netherlands).
The pore structure and specic surface area (SSA) of the
catalyst were measured using a SSA-6000 pore and surface area
analyzer manufactured by Beijing Builder Electronic Instru-
ment Co., Ltd., and calculated by the Brunauer–Emmett–Teller
(BET) and the Barrett–Joyner–Halenda (BJH) method. The
sample was pretreated under vacuum at 200 ꢁC for 2 h, the
adsorption medium was N2 with He was carrier. The inlet
pressure of the system was 0.3 MPa.
In situ Py-IR was performed on a Fourier transform infrared
spectrometer (Nicolet 6700, Thermo Fisher Scientic, Waltham,
MA, America) equipped with a smart collector and MCT
detector cooled by liquid N2, collecting 32 scans with a resolu-
tion of 4 cmꢀ1. The catalysts were rstly pretreated in Ar at
ꢁ
then heated to 80 C to evaporate part of the water. When the
mixture was slightly viscous, the ammonia water was added
under stirring until pH ¼ 10, and metal ions were precipitated
on the surface of TiO2. Aer the mixture was stirred at room
temperature for 1 h, the obtained solution was aged at 70 ꢁC for
5 h in the blast drying oven. Then, the precipitate was ltered
and washed with deionized water. Finally, the obtained product
was dried at 120 ꢁC for 12 h and subsequently calcined in air at
ꢁ
500 C for 4 h.
VOX/TiO2–Ce0.9Zr0.1O2 (VOX/TiO2, VOX/Ce0.9Zr0.1O2) catalyst
with 3 wt% V2O5 loaded was prepared by wet impregnation.
0.195 g of the ammonium metavanadate (NH4VO3) was dis-
solved in 60 mL citric acid solution (10 wt% C6H8O7$H2O).
4.85 g of the TiO2–Ce0.9Zr0.1O2 (TiO2, Ce0.9Zr0.1O2) powder was
suspended in vanadium-containing solution and the obtained
mixture kept stirring for 1 h then heated to 100 ꢁC to evaporate
the excess water completely. The obtained product was further
dried at 120 ꢁC for 12 h and subsequently calcined in air at
ꢁ
ꢁ
250 C for 40 min, then cooled down to 20 C. Then, the back-
ground spectrum was recorded with the owing of Ar and
subtracted from the sample spectrum. Subsequently, the
gaseous pyridine were introduced to the catalyst for 30 min, and
then ushed with Ar for 10 min. The spectra were normally
collected at temperature ranging from room temperature to
ꢁ
400 C for 3 h.
ꢁ
350 C.
H2-TPR was performed by PCA-1200 chemical adsorption
NH3-SCR activity test
analyzer and 0.05 g sample was pretreated in Ar (30 mL minꢀ1
)
ꢁ
ꢁ
The SCR activity tests were conducted in a xed-bed quartz at 250 C for 40 min and then heated up to 800 C at a rate of
reactor with 250 mm length and 8 mm internal diameter 0.3 mL 10 C minꢀ1 under 5 vol% H2/Ar.
ꢁ
(400 mg) catalyst with 40–60 mesh size were used for all tests.
NH3-TPD was performed by PCA-1200 chemical adsorption
The typical reactant gas contained: 1000 ppm NO, 1000 ppm analyzer and 0.05 g sample was pretreated in Ar (30 mL minꢀ1
)
ꢁ
NH3, 3%O2 and balance Ar. The total gas ow rate was 200 at 250 C for 40 min and then the NH3 was adsorbed at room
mL minꢀ1 and regulated by mass ow controllers (Sevenstar temperature for 10 min. It was purged in Ar at 100 C for 1 h,
ꢁ
D08 series Flow Readout Boxed, Beijing Sevenstar Electronics then cooled down to the room temperature and nally heated to
ꢀ1
ꢁ
ꢁ
Co., Ltd., Beijing, China), corresponding to the gas hourly space 650 C at a rate of 10 C min under Ar.
velocity (GHSV) of about 40 000 hꢀ1. NO, N2, NH3, NO2 and N2O
X-ray photoelectron spectra were obtained with K-Alpha
outlet concentration were analyzed by the ThermoStar Gas spectrometer (Thermo Fisher Scientic, Waltham, MA,
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America) using Al Ka (1486.7 eV) radiation as the excitation
source with a precision of ꢄ0.3 eV. All binding energies were
referenced to the C 1s line at 284.8 eV.
XRD and BET analysis
Fig. 2 is the XRD comparison of the support and the VOX sup-
ported catalyst. The diffraction pattern for catalyst containing
TiO2 has six peaks at 25.28ꢁ, 37.80ꢁ, 48.05ꢁ, 53.89ꢁ, 55.06ꢁ and
62.69ꢁ corresponding to (101), (004), (200), (105), (211), and
(204) crystal faces of anatase TiO2 (PDF#21-1272), respectively.22
The diffraction peak at 28.55ꢁ, 33.08ꢁ, 47.48ꢁ, and 56.34ꢁ of the
catalyst containing Ce0.9Zr0.1O2 corresponds to (111), (200),
(220), (311) of cubic CeO2 (PDF#43-1002), respectively.23,24 It is
worth noting that in the XRD spectrum, no diffraction peaks
attributed to ZrO2 were observed, which indicates that the size
of Zr ion (0.084 nm) is signicantly smaller than that of Ce ion
(0.097 nm), the addition of Zr4+ does not destroy the lattice
structure of CeO2 itself meaning that the formation of a CeO2–
ZrO2 solid solution.25,26 This is consistent with the result that
the lattice fringes belonging to the (111) crystal plane having
a crystal face spacing of 0.312 nm in CeO2 were observed in
HRTEM.
For TiO2–Ce0.9Zr0.1O2 and VOX/TiO2–Ce0.9Zr0.1O2, although
TiO2 has a larger specic surface area (SSA), the diffraction
peaks attributed to Ce0.9Zr0.1O2 and TiO2 with a molar ratio of
1 : 1 indicate, that Ce0.9Zr0.1O2 is not simply dispersed on TiO2
surface.27 Instead, it crystallizes on the surface of TiO2 core to
form a shell structure or forms incomplete coating on the
surface of TiO2 core. Interestingly, due to the load of VOX, the
diffraction peak intensity of the VOX/TiO2–Ce0.9Zr0.1O2 catalyst
is enhanced, and the peak shape becomes sharper, which may
be that the loading of VOX weakens the interaction between
Ce0.9Zr0.1O2 and TiO2, and increases the degree of crystalliza-
tion of the TiO2–Ce0.9Zr0.1O2 core–shell structure. However,
compared with Ce0.9Zr0.1O2 and VOX/Ce0.9Zr0.1O2, the loaded
VOX has no signicant effect on Ce0.9Zr0.1O2. From the partial
comparison of XRD, we speculate that for TiO2–Ce0.9Zr0.1O2, the
loaded VOX complements the oxygen vacancies in Ce0.9Zr0.1O2,
and the cell of Ce0.9Zr0.1O2 is more complete, and the crystal-
linity of the sample is higher.28,29 For VOX/Ce0.9Zr0.1O2, the
oxygen vacancies cannot be replenished due to the lack of
interaction between TiO2 and Ce0.9Zr0.1O2, and the crystallinity
and unit cell distortion are not signicantly improved. For the
Results and discussion
The HRTEM analysis
High-resolution transmission electron microscopy (HRTEM)
was used to characterize the VOX/TiO2–Ce0.9Zr0.1O2 catalysts
(Fig. 1). The catalyst was well dispersed and a relatively obvious
core–shell structure was observed (Fig. 1a and b). The 3.5 nm
Ce0.9Zr0.1O2 shell is coated on the outside of the TiO2 core the
magnied portion of the core–shell structure in Fig. 1c was
shown in Fig. 1d. The lattice fringes of VOX/TiO2–Ce0.9Zr0.1O2
display inter planar spacings of 0.312 nm and 0.189 nm in the
particle, which match well with those of the (111) plane of cubic
Ce0.9Zr0.1O2 (CeO2) and (200) plane of anatase TiO2, respec-
tively.21 It needs to note that no lattice fringes attributed to VOX
were observed. In order to verify the distribution of the
elements, the catalyst was subjected to mapping (Fig. 1e), and
the distribution of the V element is shown in Fig. 1f. The V
element is evenly distributed on the support surface, also
indicating its existence.
Fig. 1 HRTEM images (a–d) and V element mapping of VOX/TiO2–
Ce0.9Zr0.1O2 (e and f).
Fig. 2 XRD spectrum of VOX supported catalysts and carriers.
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VOX-loaded catalyst, the VOX loading mass percentage is small,
the diffraction peak attributed to VOX cannot be observed,
indicating that the VOX is highly dispersed on the core–shell
structure carrier or in an amorphous form.30,31 This is consistent
with the fact that the lattice fringe attributed to VOX was not
observed in the HRTEM of VOX/TiO2–Ce0.9Zr0.1O2.
It is worth mentioned that TiO2 has the largest specic
surface area in all samples (Table 1), the loading of VOX or the
formation of core–shell structure with Ce0.9Zr0.1O2 will cause
the specic surface area to decrease. However, for Ce0.9Zr0.1O2,
the loading of VOX and the formation of a core–shell structure
with TiO2 will increase the specic surface area. This results in
a core–shell structure having a specic surface area between
Ce0.9Zr0.1O2 and TiO2. Combining the pore volume (PV) and the
pore size (PS) information, we can speculate that VOX and
Ce0.9Zr0.1O2 enter the pores of TiO2 during the formation
process of VOX/TiO2 and TiO2–Ce0.9Zr0.1O2, resulting in
a decrease in specic surface area, pore volume and pore size.
The cerium–zirconium solid solution constituting the core–
shell structure is different from pure Ce0.9Zr0.1O2 in terms of
specic surface, pore volume and pore size. It is a shell formed
by spreading on the surface of TiO2. The specic surface area is
relatively large compared with the simple Ce0.9Zr0.1O2, and this
can provide more acidic sites. Due to the interaction between
TiO2 and Ce0.9Zr0.1O2, the pores formed by Ce0.9Zr0.1O2 also
change. When VOX is loaded, it is arranged on the surface of the
pure Ce0.9Zr0.1O2 sample, and some special pores may be
formed to increase the specic surface area. For TiO2–
Ce0.9Zr0.1O2, VOX is not only arranged on the surface, but also
may enter certain special channels. Therefore, the specic
surface area, pore volume and pore size are reduced.
Fig. 3 H2-TPR curve of catalysts and carriers.
zirconium solid solution, the reduction temperature is slightly
higher, and the peak temperature is at 541 ꢁC. It illustrates that
there is a certain interaction between VOX and Ce0.9Zr0.1O2
inhibiting the conversion of V5+ to V4+ and a small amount of
V4+ to V3+, result in the shi to the higher temperature of the
reduction peak. The reduction peak area is increased compared
with Ce0.9Zr0.1O2, which can be attributed to the effect of
superposition of V5+, V4+ and Ce4+ reduction peaks.34 The
reduction peak of TiO2–Ce0.9Zr0.1O2 is higher at 544 ꢁC than that
of Ce0.9Zr0.1O2 alone, indicating the interaction between
Ce0.9Zr0.1O2 and TiO2. While the reduction peak of the catalyst
supported by TiO2–Ce0.9Zr0.1O2 is at 566 ꢁC, which is higher
than that of other catalysts and carriers. Similarly, the presence
of the Ce0.9Zr0.1O2 coating interacts strongly with the surface
VOX of the support, inhibiting the reduction process of VOX. The
larger reduction peak is the result of the superposition of Ce4+
to Ce3+, V5+ to V4+ and V4+ to V3+.
H2-TPR analysis
The redox performance of several catalysts was analyzed by H2-
TPR technique, and the spectrums are illustrated in Fig. 3.
According to Chen's report, the reduction ability of the anatase
TiO2 phase is weꢁak. There is no obvious reduction peak in the
range of 50–800 C, and the reduction performance of V2O5 is
strong, and there is obvious reduction peak in the range of 50–
800 ꢁC.32 All samples contained only one reduction peak at 400–
The hydrogen consumption of the samples are listed in
Table 2. Since only Ce4+ on the surface is reduced, Ce of the
internal bulk phase cannot be reduced in this temperature
range of 400–650 ꢁC, so the hydrogen consumption of
Ce0.9Zr0.1O2 is the least. For TiO2–Ce0.9Zr0.1O2, the specic
surface area of Ce0.9Zr0.1O2 is increased due to the formation of
the core–shell structure, and more Ce is exposed and reduced,
and the hydrogen consumption is signicantly increased. TiO2
ꢁ
650 C. The reduction peak of VOX/TiO2 at the temperature of
482 ꢁC corresponds to the transition of V5+ to V4+ and V4+ to V3+,
while in other samples, no reduction peak is observed at the
corresponding position.30 The reduction temperature of the
ꢁ
is not reduced in the range of 50–800 C, and all the hydrogen
consumption of VOX/TiO2 belongs to the reduction of VOX. By
comparing the hydrogen consumption between the VOX-loaded
ꢁ
cerium zirconium solid solution is 537 C, which is caused by
the conversion of Ce4+ to Ce3+.33 For the VOX-loaded cerium–
Table 2 The hydrogen consumption of samples
Table 1 Specific surface area, pore volume, pore size, of the catalysts
Hydrogen consumption
(mmol gꢀ1
)
Samples
SSA (cm2 gꢀ1
)
PV (cm3 gꢀ1
)
PS (nm)
Samples
TiO2
VOX/TiO2
Ce0.9Zr0.1O2
VOX/Ce0.9Zr0.1O2
TiO2–Ce0.9Zr0.1O2
VOX/TiO2–Ce0.9Zr0.1O2
114
72
56
57
74
65
0.42
0.04
0.03
0.26
0.04
0.03
9
1
1
9
1
1
VOX/TiO2–Ce0.9Zr0.1O2
TiO2–Ce0.9Zr0.1O2
VOX/Ce0.9Zr0.1O2
Ce0.9Zr0.1O2
163.5
87.8
136.5
50.0
VOX/TiO2
96.2
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Table 3 B/L and NH3 desorption of different samples
Amount of NH3 desorption (mmol gꢀ1
)
catalyst and the support, we can conrm our previous inference
about the superposition of the reduction peaks.
Samples
B/L
VOX/TiO2–Ce0.9Zr0.1O2 0.53 462.4
NH3-TPD and Py-IR Analysis
TiO2–Ce0.9Zr0.1O2
VOX/Ce0.9Zr0.1O2
Ce0.9Zr0.1O2
VOX/TiO2
0.86 244.5
0.47 122.7
0.88 148.3
0.76 401.3
In order to compare the number of surface acidic sites of several
samples, we analyzed them with NH3-TPD and showed the test
results in Fig. 4. The desorption peak of NH3 adsorbed physi-
cally on the surface of the catalyst is before 100 ꢁC, and the
desorption range at 100–200 ꢁC, 200–400 ꢁC and 400–500 ꢁC
correspond to weak acidic sites, medium strong acidic sites and
strong acidic sites on the catalyst surface, respectively. TiO2 and
VOX/TiO2 mainly provide weak acid sites and a small amount of
medium acid sites, Ce0.9Zr0.1O2, VOX/Ce0.9Zr0.1O2 and TiO2–
Ce0.9Zr0.1O2 can provide a small amount of strong acidic sites.
The VOX/TiO2–Ce0.9Zr0.1O2 catalyst has desorption peaks
throughout the temperature range, indicating that the catalyst
can provide three types of acidic sites.
VOX/TiO2–Ce0.9Zr0.1O2 has the largest amount of NH3
desorption (Table 3), indicating that the sample has the best
ability to adsorb NH3. This region can be thought of as VOX and
TiO2–Ce0.9Zr0.1O2 together to provide weak, medium and strong
acidic sites. The catalyst has a prominent ability to adsorb NH3
and has more medium and strong acidic sites, thus the catalyst
can exhibit the best catalytic activity and thermal stability.
Acid site type analysis was carried out at different tempera-
tures for different catalysts and carriers. The Py-IR spectrum is
shown in Fig. 5. The wave number corresponds to Lewis acid at
1437 cmꢀ1 and 1597 cmꢀ1, and 1580 cmꢀ1 and 1633 cmꢀ1
correspond to Brønsted acid and the ratio of Brønsted acid to
Lewis acid (B/L) is listed in Table 3.35–37 According to studies by
Li and Liu et al., anatase TiO2 can only provide L acidic sites.38,39
Compared with other samples containing TiO2, the peak area of
TiO2 is smaller and the wave number is slightly higher. It can be
considered that the peak of VOX/TiO2 is a result of superposition
of VOX and TiO2 peaks. Based on the special structure formed by
Ce0.9Zr0.1O2 and TiO2, the TiO2 core is mainly encapsulated in
the Ce0.9Zr0.1O2 shell. Although the medium acidic sites and
strong acidic sites on the surface of Ce0.9Zr0.1O2 are increased, it
does not provide too much weak acidic sites (Fig. 4). Therefore,
TiO2
—
350.4
the properties of providing weak acidic sites are not apparent in
the samples of the core–shell structure. The largest B/L ratio is
0.88 that belongs to Ce0.9Zr0.1O2. Ce0.9Zr0.1O2 forms a core–shell
structure with TiO2, it also has a large B/L ratio. However, due to
the loading of VOX, the ratio of B/L is signicantly reduced,
indicating that the loading of VOX can provide more L acid sites.
VOX/TiO2–Ce0.9Zr0.1O2 catalyst has the largest acid area. There is
interaction between the core–shell structure and the active
component, which increases the number of acidic sites on the
surface and is more conducive to the adsorption of NH3
resulting in better catalytic activity of the catalyst.
In order to specically explore the acid site composition of
the surface of VOX/TiO2–Ce0.9Zr0.1O2 catalyst, we performed Py-
IR desorption analysis at different temperatures, and the spec-
trum is shown in Fig. 6. With the increase of the temperature
desorption of weakly acidic sites combined with pyridine, and
the characteristic peak area of acidic sites is decreasing. Due to
the temperature rise, the increase of the vibration frequency of
the in-plane ring deformation of pyridine combined with the
medium acid sites and the strong acidic sites causes the char-
acteristic peak to continuously move toward the high wave-
number. When the temperature reaches 150 ꢁC, the adsorption
peak attributed to the B acidic site disappears completely, and
when the temperature reaches 300 ꢁC, the characteristic
adsorption peak of the L acidic site disappears, indicating that
the L acidic site is the main role in the reaction process. For
medium acid and strong acid sites, the temperature at which
they desorb pyridine is relatively high. The presence of
Fig. 4 NH3-TPD curves of different samples.
30344 | RSC Adv., 2019, 9, 30340–30349
Fig. 5 Py-IR spectra of different samples.
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The spectra of V 2p, O 1s, Ce 3d, Ti 2p and Zr 3d are given in
Fig. 7, and the relative contents of different valence elements
are listed in Table 5. Fig. 7a shows the V 2p photoelectron
spectroscopy of different catalyst surfaces. The vanadium
element is mainly present on the surface of the catalyst in the
valence state of V5+ (2p3/2) and V4+ (2p3/2), and the correspond-
ing binding energy is 517.3 eV and 516.3 eV, respectively.41 The
binding energy of V3+ (2p3/2) is 515.5 eV.42,43 The content of V3+
of Ce0.9Zr0.1O2 and TiO2 as carrier is small, while the content of
V3+ in TiO2–Ce0.9Zr0.1O2 carrier is increased, and the content of
V4+ is increased at the same time. Combined with the results of
the H2-TPR test, we speculate that this result is attributed to the
interaction between Ce0.9Zr0.1O2 and TiO2, which increases the
oxygen vacancies in the carrier, and the V element is supported
on the surface of the core–shell structure carrier in a lower
valence state. Due to the higher proportion of the V element on
the surface and the presence of more low valence states V
element, the catalyst has a higher catalytic activity.29,31
Fig.
6 Py-IR spectra of VOX/TiO2–Ce0.9Zr0.1O2 at different
temperatures.
a medium acid and a strong acid site is a major factor in the
relatively stable activity of the catalyst at higher temperatures.
The results of XPS spectrum tting of O 1s are shown in
Fig. 7b, where the peak corresponding to lattice oxygen (O2ꢀ
)
at 529.7 eV is expressed as Ob, and the binding energy is
531.4 eV corresponding to chemisorbed oxygen (O22ꢀ, Oꢀ),
expressed as Oa.44 For different carriers, the loading of VOX will
reduce the content of Oa, which may be because the loading of
VOX occupies the position of the support surface where the
oxygen can be chemically adsorbed, resulting in a decrease in
the Oa content of the catalyst surface aer the loading. It is
worth noted that Ce0.9Zr0.1O2 has the lowest Ob binding
energy, indicating that it has the best O migration ability.
When VOX is loaded or forms a core–shell structure with TiO2,
it will affect the migration ability of Ob. For TiO2–Ce0.9Zr0.1O2
and VOX/TiO2–Ce0.9Zr0.1O2, a peak of –OH is generated at
530.2 eV, which may be formed by the adsorption of O2 and
H2O by oxygen vacancies.45–47 Due to the loading of VOX,
a small amount of oxygen vacancies is supplemented by VOX,
and the peak belonging to the –OH and Oa were reduced in
VOX/TiO2–Ce0.9Zr0.1O2 catalyst.
Fig. 7c shows the XPS spectrum of Ce 3d. According to
previous studies,18,48 the XPS spectrum of Ce 3d can be tted to 8
peaks: where ‘v’ represents the 3d5/2 orbital and ‘u’ represents
the 3d3/2 orbital. The peaks of u1 and v1 can be assigned to Ce3+,
and the other peaks are related to Ce4+. The presence of Ce3+
causes charge imbalance and forms oxygen vacancies, which is
favorable for the formation of surface chemisorbed oxygen.
XPS analysis
To investigate the chemical states of the elements in the near
surface region of the catalyst samples, X-ray photoelectron
spectroscopy (XPS) analysis was performed. All binding energies
were referenced to the C 1s line at 284.8 eV. Table 4 is the
calculated value of bulk compositions and the surface atomic
ratios of different samples, for samples with a core–shell
structure, the shell structure is relatively thin, and/or the
incomplete coating, while the photoelectrons excite the elec-
trons of the Ce0.9Zr0.1O2 shell structure, some of the electrons of
Ti are also excited, the ratio of Ce and Zr to Ti is close to 1.5 : 1.
It is quite different from the calculated ratio in the core–shell
structure, indicating that the solid solution is not simply mixed
with TiO2. The Zr atom has a relatively stable atomic ratio,
which is consistent with the study by Yao et al.40 Although we
could not conrm the existence of V element in XRD and
HRTEM, the existence of V atom was found in XPS detection,
which indicates that V element is supported on the carrier. It
should be noted that for VOX-loaded catalysts, the atomic ratio
of the surface V element is much higher than that of the bulk
composition, because the V element is mainly supported on the
surface of the carrier, and the high content of the V element on
the surface is favorable for the NH3-SCR reaction of NO.
Table 4 The calculated value of bulk compositions and surface atomic ratio of catalysts
Calculated value of bulk compositions (%) Surface atomic ratio (%)
Catalysts
V
Ce
Ti
Zr
V
O
Ce
Ti
Zr
VOX/TiO2–Ce0.9Zr0.1O2
TiO2–Ce0.9Zr0.1O2
VOX/Ce0.9Zr0.1O2
Ce0.9Zr0.1O2
VOX/TiO2
TiO2
4.03
—
5.38
—
2.65
—
43.19
45.00
85.16
90.00
—
47.98
50.00
—
—
97.35
100
4.80
5.00
9.46
10.00
—
21.00
—
20.95
—
21.21
—
59.12
76.01
63.21
78.49
56.02
70.79
8.87
11.56
12.68
17.92
—
8.07
9.13
—
—
22.77
29.21
2.94
3.30
3.16
3.59
—
—
—
–
—
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Fig. 7 XPS spectra of catalysts ((a) V 2p, (b) O 1s, (c) Ce 3d, (d) Ti 2p, (e) Zr 3d).
By comparing the existence of Ce in different samples, it can be interaction between Ce0.9Zr0.1O2 and TiO2 and supplements the
found that the content of Ce3+ in VOX/TiO2–Ce0.9Zr0.1O2 catalyst is oxygen vacancies, resulting in a decrease in the content of Ce3+, but
reduced, and expect for v2, the binding energies of Ce 3d in VOX/ an increase in the content of V3+ and V4+.49
TiO2–Ce0.9Zr0.1O2 are higher than those of TiO2–Ce0.9Zr0.1O2.
Fig. 7d is the XPS spectrum of Ti 2p, where 464.4 eV is the
Combined with the change in the valence state of the V element, binding energy of Ti4+ 2p1/2 and 458.7 eV is the binding energy
we speculate that it may be that the loading VOX weaken the of Ti4+ 2p3/2. For TiO2–Ce0.9Zr0.1O2, the binding energy of Ti is
Table 5 Relative content of each element in different catalysts (%)
V 2p
O 1s
Oa
Ce 3d
O–OH
Catalysts
V3+ + V4+
V5+
Ob
Ce3+
Ce4+
VOX/TiO2–Ce0.9Zr0.1O2
TiO2–Ce0.9Zr0.1O2
VOX/Ce0.9Zr0.1O2
Ce0.9Zr0.1O2
VOX/TiO2
TiO2
54.61
—
31.28
—
43.69
—
45.39
—
68.72
—
56.31
—
20.47
27.24
27.47
33.52
20.49
26.32
56.09
45.13
72.53
66.48
79.51
73.68
23.44
27.63
—
—
—
18.25
21.18
21.72
21.46
—
81.75
78.82
78.28
78.54
—
—
—
—
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vacancies. The increase of oxygen vacancies has a great inu-
ence on the thermal stability of the catalyst carrier. The strong
interaction between the TiO2–Ce0.9Zr0.1O2 support with core–
shell structure increases the high temperature stability of the
composite supported catalyst, which also explains the higher
activity of the catalyst at higher temperatures.50
Combined with the results of the above characterization, we
can conrm that aer the preparation of TiO2–Ce0.9Zr0.1O2
core–shell structure, the electrons in Ce–O–Ti shi to Ce due to
the interaction between TiO2 and Ce0.9Zr0.1O2.49,51 The binding
energy of Ti4+ becomes higher and the electron density around
Ce increases. The electron cloud density of Ce increase, and the
migration ability of O in Ce0.9Zr0.1O2 is enhanced, which is
benecial to the generation of oxygen vacancies. The content of
Ce3+ and chemisorbed oxygen indirectly reects the amount of
oxygen vacancies.52 It is generally believed that there are three
types of oxygen vacancies (VOꢃ,VOꢅ andVOꢅꢅ ), which are inseparable
from the reactions (1)–(3).45,53–55 Oxygen vacancies adsorb
oxygen and dissociate O2 into oxygen atoms. Since oxygen
atoms have high activity, they can adsorb substances such as
water to form –OH. Due to the presence of oxygen vacancies, Ce
can be converted from the +4 valence to the +3 valence (reaction
(4)).
VꢃO4VOꢅ þ eꢀ
VꢃO4VOꢅꢅ þ 2eꢀ
VOꢅ 4VOꢅꢅ þ eꢀ
(1)
(2)
(3)
Ce4+ + eꢀ 4 Ce3+
(4)
When VOX is loaded, an interaction occurs between the
active component and the carrier, electron transfer occurs
between Ce3+ and VOX, and oxygen vacancies are replenished,
and the following reactions may occur during the preparation
process (reactions (5)–(8)):
V2O5 þ 2Ce3þ þ Vꢅꢅ /2VO2 þ 2Ce4þ
(5)
(6)
(7)
(8)
O
V2O5 þ 2Ce3þ þ 2Vꢅ /V2O3 þ 2Ce4þ
O
V2O5 þ VꢃO/2VO2
2VO2 þ VꢃO/V2O3
Fig. 8 NO conversion curve of CeXZr1ꢀXO2 (a), NO conversion and
selectivity of catalysts and carriers (b and c).
However, this reaction is difficult to accomplish in VOX/
Ce0.9Zr0.1O2 catalyst. The oxygen vacancies of Ce0.9Zr0.1O2
coated on the surface of TiO2 were supplemented. The intensity
of the diffraction peak in the XRD spectrum of the catalyst is
enhanced, and the peak shape is sharper the presence of more
low-valence V elements requires higher temperatures due to the
increased number of V4+ to V3+ conversions, resulting in a shi
in the peak temperature of the H2-TPR (Fig. 3). In summary, the
increased V4+ and V3+ content in VOX improves catalytic activity.
The interaction between the support and the active component
is responsible for the better thermal stability of the catalyst.
shied. The reason for this phenomenon is related to the
interaction between Ce0.9Zr0.1O2 and TiO2, which may cause
lattice distortion of TiO2 (it is reected in the XRD spectrum).41
It can provide more oxygen vacancies. Due to the load of VOX,
while supplementing the oxygen vacancies, the existence of
higher binding energy Ti is weakened as well. Therefore, the
higher binding energy of Ti in VOX/TiO2–Ce0.9Zr0.1O2 catalyst is
not obvious. However, the loaded VOX can provide more active
sites.
For the XPS of Zr (Fig. 7e), Zr mainly exists as +4 valence ion,
and the addition of Zr4+ is benecial to the generation of oxygen
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acidic sites, especially medium-strong acid sites. It is benecial
to the adsorption of the reaction gas NH3 on the catalyst surface
and increased thermal stability. Compared with the commercial
catalysts currently used on the market, both conversion rate and
selectivity are superior, as well as better thermal stability and
a wider temperature window, which provides ideas for the
development of more superior catalysts in the future.
Catalyst activity analysis
Fig. 8a is a graph showing the activity of solid solution of
different cerium–zirconium ratios. Ce0.9Zr0.1O2 exhibits better
catalytic activity, while the catalytic activity of Ce0.1Zr0.9O2 is the
worst among all solid solutions at the range of 100–400 ꢁC,
although it is better than others at 450 ꢁC. The above results
indicate that proper zirconium doping is benecial to improve
the activity and increase the thermal stability of the catalysts.
Based on the activity of the cerium–zirconium solid solution
catalyst, we selected Ce0.9Zr0.1O2 as the shell structure compo-
nent to prepare core–shell carrier.
Conflicts of interest
There are no conicts to declare.
Fig. 8b and c show NO conversion curves and selectivity
curves for different VOX supported catalysts and their supports.
Different colors are used to represent different samples, the line
chart represents N2 selectivity, and the histogram shows the
selectivity of N2O and NO2 in Fig. 8c. Under the condition of
GSHV ¼ 40 000 hꢀ1, Ce0.9Zr0.1O2 and VOX/Ce0.9Zr0.1O2 showed
higher activity before 250 ꢁC, which may be because the oxygen
vacancies and the redox of Ce3+ are the main catalytic sites at
low temperature. The VOX site is effectively attached to the
surface of the support and the redox properties of the catalysts
determine the catalytic activity, rather than the Lewis and
Brønsted surface acidic sites in the catalytic system.15,16 Aer
250 ꢁC, VOX/TiO2–Ce0.9Zr0.1O2 showed excellent catalytic activity
and maintained excellent selectivity and thermal stability with
NO conversion rate > 97%, N2 selectivity > 97.5% and temper-
ature window is 250–400 ꢁC, which is inseparable from the
existence of low-valence V and medium – strong acidic sites.
The former is benecial to increase the activity of the catalyst,
and the latter is advantageous for improving the thermal
stability of the catalyst. Compared with commercial catalysts,
VOX/TiO2–Ce0.9Zr0.1O2 has the higher NO conversion and N2
selectivity, wider temperature window and better thermal
stability.31 It is noteworthy that TiO2–Ce0.9Zr0.1O2 also exhibits
a high N2 selectivity, although its catalytic properties is not
particularly excellent compared to the commercial catalysts and
VOX/TiO2–Ce0.9Zr0.1. It indicates that the higher selectivity of N2
is derived from the TiO2–Ce0.9Zr0.1O2 core–shell structure
carrier.
Acknowledgements
This work was supported by National Key Research and Devel-
opment Plan (Grant No. 2017YFE0116200). We are grateful to
the teachers of the Harbin Engineering University Analytical
Testing Center for their help in testing.
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