Zr-doped CeO2 nanorods as versatile catalyst in the epoxidation of styrene with tert-butyl hydroperoxide as the oxidant

Article abstract of DOI:10.1016/j.apcata.2015.07.010

The CeO₂ nanorods with various Zr dopant concentrations were studied in terms of their activity in the selective oxidation of styrene to styrene oxide using tert-butyl hydroperoxide as the oxidant. Several techniques were used to characterize the synthesized Zr-doped CeO₂ nanorods. The results indicated that Zr ions were doped into the ceria lattice and formed fluorite-like solid solutions, resulting in an increase in oxygen vacancy concentration. It was also found that Zr content has a significant impact on intrinsic natures of the catalyst, namely, the specific surface area, oxygen vacancy concentration and the content of Ce³⁺ ions, which were found to determine the catalytic efficiency of the catalyst in the epoxidation of styrene. It is interesting that the remarkably increased catalytic performance of the ceria nanorods with 10 mol% zirconium is mainly attributed to the more oxygen vacancy and the higher percentage of Ce³⁺. Furthermore, recycling studies proved the heterogeneous zirconium-doped ceria nanorods did not lose the initial high catalytic activity after five successive recycles.

Full text of DOI:10.1016/j.apcata.2015.07.010

Applied Catalysis A: General 503 (2015) 117–123
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Zr-doped CeO2 nanorods as versatile catalyst in the epoxidation of
styrene with tert-butyl hydroperoxide as the oxidant
Xin Liua, Jing Dinga, Xi Lina, Ruihua Gaob,∗∗, Zhenhua Lia, Wei-Lin Daia,∗
a Department of Chemistry & Shanghai Key Laboratory of Molecular Catalysis and Innovative Material, Fudan University, Shanghai 200433, PR China
b State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The CeO2 nanorods with various Zr dopant concentrations were studied in terms of their activity in
the selective oxidation of styrene to styrene oxide using tert–butyl hydroperoxide as the oxidant. Several
techniques were used to characterize the synthesized Zr-doped CeO2 nanorods. The results indicated that
Zr ions were doped into the ceria lattice and formed fluorite-like solid solutions, resulting in an increase
in oxygen vacancy concentration. It was also found that Zr content has a significant impact on intrinsic
natures of the catalyst, namely, the specific surface area, oxygen vacancy concentration and the content
of Ce3+ ions, which were found to determine the catalytic efficiency of the catalyst in the epoxidation of
styrene. It is interesting that the remarkably increased catalytic performance of the ceria nanorods with
Received 27 March 2015
Received in revised form 15 June 2015
Accepted 4 July 2015
Available online 14 July 2015
Keywords:
Zr-doped CeO2 nanorods
Styrene
Oxygen vacancies
Selective oxidation
TBHP
10 mol% zirconium is mainly attributed to the more oxygen vacancy and the higher percentage of Ce3+
.
Furthermore, recycling studies proved the heterogeneous zirconium-doped ceria nanorods did not lose
the initial high catalytic activity after five successive recycles.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
of the thermo-stability, mobility of lattice oxygen and the density
of oxygen vacancies. Hence, pure ceria has gradually been replaced
Rare earth elements, especially ceria, have been widely inves-
tigated owing to their applications in the fields of industrial
heterogeneous catalyst, such as automotive three-way exhaust
(TWCs) [1], water-gas-shift (WGS) reactions [2], solid oxide fuel
cells anodes (SOFCs) [3], etc. These catalytic applications of ceria
are considered to originate from the redox properties and variable
valence states (+4 and +3), which was decided mainly by the mor-
phology, specific surface area, oxygen vacancies and preferential
exposure of reactive planes [4]. However, the poor thermal sta-
bility largely limits the applications of pristine CeO2. In order to
promote the physical and chemical properties of CeO2, many tran-
sition or rare earth metals, such as Zr [5,6], Ti [7], Co [8] and Y [9],
etc, are served as the alien metal to prepare ceria-derived oxides.
Nevertheless, ceria incorporated with Zr is of intensive interest and
great significance for researchers. Compared with pure CeO2, the
insertion of Zr into ceria lattice leads to structural distortions of
the cubic fluorite lattice, ultimately resulting in the enhancement
by CeO2–ZrO2 mixed oxides. Parghi’s group explored CeO2–ZrO2
as a bifunctional catalyst for the synthesis of ␤-amino alcohols
with good catalytic activity compared with pure CeO2 [10]. Si’s
group fabricated phosphate-containing Ce0.75Zr0.25O2 and discov-
ered that the plentiful oxygen vacancies, facilitating redox circle
between Ce3+ and Ce4+, greatly promoted the catalytic reaction rate
and desorption of product on the catalyst. Due to binding adsor-
bates more strongly than normal oxide sites and assisting in their
dissociation, the oxygen vacancies can act as specific reaction site
in heterogeneous catalysis [11,12]. Therefore, the introduction of
alien metal Zr is an effective method to improve the catalytic oxi-
dation activity of cerium oxide.
Epoxidation reaction of styrene plays an important role in indus-
trial processes. The products from epoxidation reaction of styrene
are versatile, which act as intermediates in fine chemicals for the
synthesis of plasticizers, perfumes, drugs, pharmaceutical, etc. [13].
To achieve high yield of styrene oxide, much long reaction time
is needed since styrene owns terminal olfinic group. Thus, tradi-
tionally, two methods are employed for the epoxidation of styrene.
One is halogenohydrin, which is not complicated but harmful to the
environment. The other is carried out by the oxidation of alkenes
with unsafe, corrosive, hazardous and expensive stoichiometric
peracids. Recently, a few studies have been reported on epoxida-
∗
Corresponding author. Fax: +86 5566 5572.
Corresponding author.
∗∗
E-mail addresses: ruihuagao@fudan.edu.cn (R. Gao), wldai@fudan.edu.cn
(W.-L. Dai).
http://dx.doi.org/10.1016/j.apcata.2015.07.010
0926-860X/© 2015 Elsevier B.V. All rights reserved.

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X. Liu et al. / Applied Catalysis A: General 503 (2015) 117–123
tion of styrene by easily separable solid catalysts, such as Mo [14], Fe
[15], Mn [16,17], Co [18], Cu and Ni [19] based catalysts. Meanwhile,
molecular sieve catalysts including TS-1 [20], Ti–SiO2 [21], Ti–HMS
[22], as well as ␥-Al2O3 [23], have been applied to the epoxidation
of styrene using TBHP, H2O2 and urea-H2O2 as oxidants. However,
the catalysts exhibited poor epoxide selectivity and/or low activity
or poor stability [20–23]. In addition, the deposition of nano-sized
gold on different metal oxides catalysts for styrene oxidation has
been reported by Patil and co-workers [24]. However, the high-
cost of noble metal hinders wide-spread applications of this kind
of catalysts. However, the literatures involved Zr-doped ceria in the
catalytic liquid-phase epoxidation of olefin is reported rarely.
The CeO2 nanorods with various Zr dopant concentrations are
prepared and investigated in terms of their activity in catalyzing
styrene oxidation. The structure of the Zr-doped CeO2 nanorods is
studied using various techniques in order to investigate the rela-
tionship between the structure and the activity. It is found that the
concentration of dopants has great effect on the specific surface
area, oxygen vacancy concentration and the content of Ce3+ ions,
finally leading to the variation of the catalytic activity in styrene
oxidation reaction.
source and a hemispherical electron energy analyzer to investigate
the surface chemical constitution and the oxidation state. The Mg
K˛ (1253.6 eV) anode was operated at 14 kV and 20 mA. The spec-
tra were recorded in the constant pass energy mode with a value of
46.95 eV, and all binding energies were calibrated referenced to the
carbonaceous C 1s at 284.6 eV. The actual composition of the cat-
alysts was determined with Optima 8000 ICP-OES spectrometer.
The laser Raman experiments were obtained by using a Jobin Yvon
Dilor Labram I Raman spectrometer equipped with a charge cou-
pled device detector, holographic notch filter, and the He–Ne laser
radiating at 632.8 nm. Specific surface areas of the catalysts were
measured by nitrogen adsorption–desorption at 77 K (Micromerit-
ics Tristar ASAP 3000) employing Brunauer–Emmett–Teller (BET)
method. The sample was pre-treated at 523 K in vacuum for 3 h
before test to desorb moisture adsorbed.
2.3. Activity test
The epoxidation of styrene was carried out at reflux oil bath
temperature of 353 K with magnetically stirred (400 r/min) in a
closed 25 mL regular glass reactor using 70 wt% aqueous t-butyl
hydroperoxide (TBHP) as the oxidant and acetonitrile as solvent.
In a typical experiment, 0.05 g of the catalyst, 10 mL of acetonitrile
and 8.7 mmol of 70 wt% aqueous TBHP were introduced into the
regular glass reactor at 353 K with magnetic stirring. The reaction
was started by adding 4.35 mmol of styrene into the above mixture
and maintained for a period time. The quantitative analysis of the
reaction products was carried out by gas chromatography (GC), and
the determination of different products in the reaction mixture was
performed by GC–mass spectroscopy (GC–MS). The conversion of
TBHP was determined by traditional iodometric analysis.
2. Experimental
2.1. Synthesis of Zr-doped CeO2 nanorods
All the materials were purchased from Sinopharm Chemical
Reagent Co., Ltd. China. All chemicals were used without further
purification. Deionized water was used throughout the experi-
ments.
Zr-doped
hydrothermal process [25], varying the mole ratios of
Zr(NO3)4·5H2O–Ce(NO3)3·6H2O. Typically, 0.5 mmol of
CeO2
nanorods
were
prepared
via
a
3. Results and discussion
Zr(NO3)4·5H2O and 4.5 mmol of Ce(NO3)3·6H2O were dissolved
in 5 mL deionized water under vigorous stirring until the corre-
sponding salts were completely dissolved. Meanwhile, 19.2 g of
NaOH was dissolved in 30 mL deionized water. Subsequently, the
two solutions was mixed together and kept stirring for 30 min,
and the obtained mixed slurry was transferred to a stainless
autoclave with 100 mL polytetrafluoroethylene liner and kept at
373 K for 24 h. Upon the stainless autoclave was cooled to room
temperature, the precipitates were separated by centrifugation,
washed with deionized water and ethanol until pH 7. After drying
at 353 K overnight, the products were calcined at 673 K for 5 h
in muffle oven with ramping rate of 2 K/min. The mole ratios of
Ce(NO3)3·6H2O to Zr(NO3)4·5H2O are 1:0 for CeO2; 0.95:0.05 for
Ce0.95 Zr0.05O2; 0.9: 0.1 for Ce0.9Zr0.1O2; 0.8:0.2 for Ce0.8Zr0.2O2,
respectively.
3.1. Characterization of the as-prepared material
The morphology and microstructure of the synthesized CeO2
nanorods and Zr-doped CeO2 nanorods are revealed by TEM images.
As presented in Fig. 1a, pure CeO2 nanorods are 120–200 nm in
length and uniform 11 3 nm in diameter. All the catalysts exhibit
rodlike morphology. Nevertheless, the doping of Zr in ceria results
in a decrease in particle size compared with pure ceria. Fig. 1b and
c shows the TEM images of the Zr-doped CeO2 nanorods with the
composition of Ce0.95Zr0.05O2 and Ce0.9Zr0.1O2, respectively. For the
Ce0.95Zr0.05O2 and Ce0.9Zr0.1O2, the measured size is approximate
with 30 ∼ 80 nm in length and 10 1 nm in diameter. When the
loading of Zr is up to 20 mol%, the morphology becomes irregular,
and some particles appear (Fig. 1d). Since the as-synthesized cat-
alysts with various zirconium contents show similar morphology,
the Ce0.9Zr0.1O2 is singled out as the representative for the further
characterization and discussion. Fig. 1(e,g) depicts a high revolution
TEM image of Ce0.9Zr0.1O2 nanorods with a fast Fourier transform
analysis (inset) and energy dispersive spectroscopy (EDS). Accord-
ing to the HRTEM image, a small number of (1 0 0) planes and the
dominating (1 1 1) planes are observed with an interplanar spacing
of 0.26 and 0.32 nm, being accorded well with the previous report
[26]. Through detailed observation of the HRTEM image in Fig. 1e,
there are some light points on the ceria nanorods, revealing the
existence of a number of surface defects [26]. Liu et al. [27] reported
2.2. Characterizations
The wide-angle XRD patterns were recorded on a Bruker D8
Advance X-ray diffractometer with nickel-filtered Cu K␣ radiation
(ꢀ = 0.15406 nm) in a scanning angle (2␪) range of 10–90◦ at a scan-
ning speed of 8◦ min−1, operated at 40 kV and 40 mA, respectively.
The lattice parameter a values was calculated using the Scherrer
equation from the (1 1 1) peak of the samples. The morphology
of the as-prepared samples was characterized through transmis-
sion electron micrographs (TEM) obtained on a JOEL JEM 2010
microscope. The samples for electron microscopy observation were
ground into powder and subsequent dispersed in ethanol by ultra-
sonication for 30 min. After that, a drop of very dilute suspension
was placed on the carbon-coated copper grids. The X-ray photoelec-
tron spectroscopy (XPS) were carried out with a RBD-147 upgraded
Perkin–Elmer PHI 5000C ESCA system equipped with a dual X-ray
that there are two types of oxygen vacancies, small neutral Ce3+
—
oxygen vacancy associated and larger sized clusters of these defects
in the ceria nanorods. They drew a conclusion that the ceria with the
(1 0 0) and (1 1 1) crystal planes predominantly exposed preferred
to form larger sized oxygen vacancy clusters. Hence, it can be spec-
ulated that the oxygen vacancy in this catalysts belonged to larger

X. Liu et al. / Applied Catalysis A: General 503 (2015) 117–123
119
Fig. 1. TEM images of pure ceria and Zr-doped CeO2 nanorods: (a) pure ceria, (b) Ce0.95Zr0.05O2, (c) Ce0.9Zr0.1O2, (d) Ce0.8Zr0.2O2 (e) HRTEM image of Ce0.9Zr0.1O2, (f) the
Ce0.9Zr0.1O2 nanorods after used for 5 times, (g) EDS pattern of the red circle in panel e.
sized oxygen vacancy clusters. The function of this kind of oxygen
vacancy will be discussed in detail below. Obviously, as shown in
Fig. 1g, the EDS spectra with the red circle region in Fig. 1e con-
firms the presence of elements Ce, Zr and O (the Cu element is from
the substrates), further indicated the homogeneity of the obtained
catalysts.
X-ray diffraction (XRD) patterns of the as-synthesized Zr-doped
CeO2 nanorods with various zirconium contents can be clearly seen
in Fig. 2. The diffraction peaks of all the catalysts assigned to the
(1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes are indexed to the
cubic fluorite structure of CeO2 crystals (JCPDS no. 34–0394). Sur-
prisingly, the peaks indexed to zirconium oxide are not detected in
all the samples even when the amount of Zr reaches up to 20 mol%,
which indicates Zr was successfully doped into the ceria lattice
structure. As shown in Fig. 2 inset, with the increase of Zr con-
centration, the (1 1 1) diffraction peaks shifts toward higher angle.
Simultaneously, a similar trend was observed by Balkus et al. for
the peaks variation of other metal doped CeO2 nanorods [28]. The
calculated lattice parameter a values for pure CeO2, Ce0.95Zr0.05O2,
Ce0.9Zr0.1O2, Ce0.8Zr0.2O2 are 5.448 Å, 5.413 Å, 5.395 Å, and 5.387 Å,
respectively. The diminishing trend of lattice parameter is caused
by the lattice contraction induced by the replacement of Ce4+
Fig. 2. XRD patterns of as-prepared doped-materials and a close-up of (111) plane
(inset).

120
X. Liu et al. / Applied Catalysis A: General 503 (2015) 117–123
Fig. 3. (a) Raman spectra of nanorods with various zirconium contents; (b) the
corresponding peak intensity ratio of I600/I460 of nanorods.
(0.97 Å) by slightly smaller radius of Zr4+ (0.84 Å). Additionally,
this trend may reflect an increase in oxygen vacancy concentration
with increasing Zr content, as the number of oxygen–ion vacancies
increases with a decrease in the effective anionic radius in fluo-
rite lattice [8,28]. Besides that, these results further confirm the
incorporation of Zr into the ceria lattice to form fluorite-like solid
solutions.
The existence of oxygen vacancies is demonstrated by Raman
spectroscopy as well (Fig. 3). The Raman spectra of Zr-doped CeO2
nanorods with various zirconium contents show one strong Raman
band at about 460 cm−1, which is assigned to Raman active vibra-
tional F2g as a symmetric breathing mode of the oxygen atoms
around the cerium ions [29,30]. With the increase of Zr concentra-
tion, the peak position blue-shifts. This blue-shift is related to the
variation of interatomic force in the ceria crystals which is influ-
enced by the variation of bond length as well as the lattice spacing
[31,32]. The relation between the shift of Raman band and lattice
constant can be described by the following equation:
ꢁw = −3␥␻0ꢁa/a0
Where ␻0 is Raman frequency of pure CeO2, a0 is lattice constant
of CeO2, ␥ is the Grüneisen constant, ꢁa is the variation of lattice
constant, and ꢁ␻ is the shift of Raman band [33]. Obviously, it can
be seen from the equation that the shift of Raman band varies with
the change of lattice constant, coincided with the (1 1 1) diffraction
peaks shifts in the XRD pattern.
Fig. 4. (a) Ce 3d, (b) O 1s XPS spectra of pure ceria and Zr-doped CeO2 nanorods.
ꢀ
␯ (∼881.9 eV), ␯ (∼884.7 eV), ␯” (∼888.5 eV), ␯”’ (∼897.6 eV), ␮
ꢀ
(∼900.4 eV), ␮ (∼902.8 eV), ␮” (∼907.2 eV) and ␮”ꢀ (∼916.2 eV).
The four ␮ bands represent the Ce 3d 3/2, and the other four ␯ bands
represent the Ce 3d 5/2 [38]. The 3d104f0 state of Ce4+ species are
labeled as ␮, ␮”, ␮”ꢀ, ␯, ␯”, and ␯”ꢀ, whereas the 3d104f1 state of Ce3+
are labeled as ␮’ and ␯’ [39]. From Fig. 4, the +4 oxidation state is
the main chemical valence of cerium on the surface characterized
by XPS, and a small amount of +3 oxidation state also co-existed.
+3 oxidation state of cerium was introduced by the spontaneous
transformation of Ce4+ (0.97 Å) to the larger Ce3+ (1.10 Å), compen-
sating for the lattice contraction induced by slightly smaller radius
Zr4+ (0.84 Å) [40]. The percentages of Ce3+ to the total Ce are 8.7%,
10.6%, 14.6%, and 9.5% for the CeO2, Ce0.95Zr0.05O2, Ce0.9Zr0.1O2 and
Ce0.8Zr0.2O2 catalysts, respectively. Obviously, the Ce3+ percent-
age of Ce0.9Zr0.1O2 reaches the maximum value. As the catalytic
reaction intermediates could interact with certain Ce3+ ions, oxy-
gen vacancy associated exposed Ce3+ ions on CeO2 are potentially
potent surface sites in the titled catalytic reaction [27].
Besides the band at 460 cm−1, there are still two bands at around
260 and 600 cm−1. The former is assigned to the transverse acous-
tic (TA) mode [34], and the latter is assigned to the nondegenerate
longitudinal optical (LO) mode [29], which is related to the oxy-
gen vacancies caused by the Ce3+ ion in the CeO2 lattice and the
small size effects [35]. The oxygen vacancies are very important
for the excitation of the oxygen in substrates in the catalytic reac-
tions. Remarkably, the I600/I460 values (Fig. 3b), which indicate
the concentration of oxygen vacancies [28,36], reveal a continuous
increase in the Zr doping range of 0 < x <0.2. Lu and coworkers [37]
calculated the O vacancy formation energy (EfV) as a function of Zr
content and calculated that Ef decreases when x changes from 0
V
to 0.5. Concluded from the theoretical calculation, the concentra-
tion of oxygen vacancy will increase with the Zr content, which is
in agreement with the result of Raman analysis.
To further investigate the surface composition and chemical
states, the Zr-doped CeO2 nanorods with various zirconium con-
tents were carried out by XPS technique. Ce 3d and O 1s spectra
of the catalyst and the relative element content of Zr-doped CeO2
determined are respectively shown in Fig. 4 and Table 1. The
Ce 3d spectra in Fig. 4a can be deconvoluted into eight peaks:
The O 1s spectra of Zr-doped CeO2 nanorods are shown in Fig. 4b.
The O 1s spectra can be deconvoluted into three peaks: lattice
oxygen (OL) at lower binding energy (∼529.5 eV); weakly bonded
oxygen species or/and chemisorbed oxygen (OC) (∼531.5 eV); sur-
face oxygen (OS) at the highest binding energy (∼533.5 eV) [41,42].
As shown in Table 1, the ratio of OL to OT (OT = OL + OC + OS) for all

X. Liu et al. / Applied Catalysis A: General 503 (2015) 117–123
121
Table 1
The surface area and atomic ratios of the different catalysts.
Samples
SBET (m2 g−1
)
Zr/(Zr + Ce)a(%)
Surface atomic ratiob
Zr/(Zr+Ce)(%)
Ce3+/(Ce3++Ce4+) (%)
OL/OC (%)
CeO2
79.1
93.6
103.9
70.5
0
0
8.7
67.9
62.0
58.7
61.7
Ce0.95Zr0.05O2
Ce0.9Zr0.1O2
Ce0.8Zr0.2O2
3.5
8.9
15.7
6.8
12.3
37.7
10.6
14.6
9.5
a
Determined by ICP.
Determined by XPS.
b
Table 2
Catalytic performance of styrene epoxidation over different catalystsa.
Entry
Catal
Conversion (%)
Styrene
Selectivity (%)
Styrene oxide
Yield of styrene oxide (%)
TBHP
Benzaldehyde
Othersb
1
2
3
4
Ce0.8Zr0.2O2
Ce0.9Zr0.1O2
Ce0.95Zr0.05O2
CeO2
65.3
78.1
70.2
32.2
32.7
32.2
55.5
5.4
49.0
56.2
45.6
22.0
18.0
21.2
33.3
3.0
77.9
79.3
66.1
83.1
82.6
82.2
80.3
81.0
17.6
16.0
31.3
10.5
15.2
11.2
16.7
19.0
4.5
4.7
2.6
6.4
2.2
6.6
3.0
0
50.9
61.9
46.4
26.8
27.0
26.5
44.6
4.4
5
ZrO2
6c
7d
8e
CeO2+ZrO2
Ce0.9Zr0.1O2
blank
a
Reaction conditions: 50 mg catalyst, 4.35 mmol styrene, 8.7 mmol TBHP (70% aqueous solution), solvent (CH3CN) = 5 mL, reaction temperature 353 K, reaction time 12 h.
The other products: benzoic acid and ␣,␤-dihydroxyethylbenzene.
The CeO2 + ZrO2 (mol ratio of Ce/Zr = 9) was fabricated by a physical mixing method.
The Ce0.9Zr0.1O2 prepared by traditional precipitate process.
b
c
d
e
Without catalyst.
samples was calculated. It is obvious that the incorporation of Zr
ity order of pure CeO2, Ce0.95Zr0.05O2 and Ce0.9Zr0.1O2 nanorods.
While a further increase in Zr content (i.e., to 20 mol%) resulted in a
decrease in surface area, concentration of oxygen vacancy and the
percentages of Ce3+. It may be attributed to the aggregation of Zr on
the surface of nanorod, finally results in the structure damage and
non-compliance with the trends of the 0%, 5%, and 10% samples. As
shown in Table 1, the surface Zr/(Zr + Ce) atomic ratios of all the cat-
alyst determined by XPS are slightly higher than that determined by
chemical analysis. However, for Ce0.8Zr0.2O2 sample, it is observed
that the percentage of Zr on the surface area is 37.7%, which is far
more than the bulk Zr concentration (15.7%). The aggregation of Zr
can further be verified by the TEM images (Fig. 1a–d). When the
loading of Zr is up to 20 mol%, the morphology becomes irregular,
and some particles appear.
Based on the discussion above and the literature results [27,43],
the possible catalytic mechanism of the Zr-doped CeO2 nanorods is
proposed. In the oxidation process for manufacturing styrene oxide,
highly reactive surface oxygen species are involved in the oxidation
reaction of the substances. The Ce3+ ions coupled with the large
sized oxygen vacancy clusters are surface active sites, promoting
the adsorption of styrene on the ceria surface. The highly reactive
oxygen species at the oxygen vacancy site react with styrene to gen-
erate products and leave oxygen vacancies. The oxygen vacancy
is then replenished by the TBHP, thus completing the reaction
cycle. Obviously, the Zr-doped CeO2 nanorods with the increased
ratio of Ce3+/(Ce3+ + Ce4+) show the enhanced catalytic activity. The
slightly smaller radius zirconium ions can induce lattice contrac-
tion, promotes the transformation of Ce4+ to the larger Ce3+ and the
production of oxidation vacancies, which are greatly beneficial for
the styrene oxidation.
can effectively decrease the lattice oxygen (OL) on the surface of the
CeO2 nanorods. Consummation of lattice oxygen in CeO2 indicates
the formation of oxygen vacancies as shown in Raman spectra.
3.2. Catalytic oxidation of styrene
3.2.1. Catalytic activity of the Zr-doped CeO2 nanorods
In the present work, the catalytic activity of different Zr-doped
CeO2 nanorods are evaluated in the oxidation of styrene to styrene
oxide using 70% TBHP as the oxidant and acetonitrile as the sol-
vent. All products are analyzed by GC–MS. Table 2 summarizes
the catalytic results of the conversion of styrene and correspond-
ing selectivities of the target product and other by-products.
5.4% styrene conversion is obtained when the reaction is car-
ried out in the absence of any catalysts, which is attributed to
the auto-oxidation of the styrene by THBP. However, pure CeO2
or ZrO2 shows a low styrene conversion of ∼32%. Meanwhile,
to certify the influence of preparation method, the CeO2 + ZrO2
(mole ratio of Ce/Zr = ∼9) composite prepared by physical mixing
method exhibits very low conversion of styrene (32%). Addition-
ally, ceria–zirconia materials prepared by traditional precipitation
show a low styrene conversion of ∼55%. Among the Zr-doped
CeO2 nanorods with various zirconium contents, the Ce0.9Zr0.1O2
exhibits the highest activity with 78 % styrene conversion and 79%
selectivity for styrene oxide with the highest TOF (1.44 h−1) as well.
This finding can be ascribed to the maximum specific surface area,
high oxygen vacancy concentration and a large percentage of Ce3+
,
which were supported by the analyses results of BET, Raman and
XPS. As known in Fig. 1e, the oxygen vacancies on (1 1 1) plane
in these catalysts belong to larger sized oxygen vacancy clusters.
And it has been reported that the Ce3+ ions coupled with the large
sized oxygen vacancy clusters can act as a potentially potent active
sites for catalysis [43], which efficient activate reaction substrate
and thus improve the catalytic performance. For the doped sys-
tems, the BET surface areas, concentration of oxygen vacancy and
the percentages of Ce3+ to the total Ce increased with increasing
Zr content in ceria up to 10 mol%, which are similar to the activ-
3.2.2. Influence of the reaction time
Fig. 5 illustrates the effect of reaction time on the conversion
of styrene and the selectivity to target products. The conversion
of styrene is 23% in the first 2 h and then increased to 78% with
the reaction prolonging for 12 h. A slight decrease in the selec-
tivity to styrene oxide and a slight increase in the selectivity to
benzaldehyde are observed when the reaction was kept from 12 h

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X. Liu et al. / Applied Catalysis A: General 503 (2015) 117–123
active components in the filtrate was verified by elemental anal-
ysis, further confirmed by the hot-filtration test. The structural
stability of the Ce0.9Zr0.1O2 catalyst was further confirmed by TEM
images (Fig. 1f) after five reaction recycle. Thus, the Zr-doped CeO2
nanorods are stable and efficient in the epoxidation of styrene, very
significant for its further industrial applications.
4. Conclusion
In summary, the Zr-doped CeO2 nanorods with various amounts
of dopant were synthesized via a facile simple hydrothermal
method. The Zr-doped CeO2 nanorods were found to possess supe-
rior catalytic performance for the selective oxidation of styrene to
styrene oxide to that of the undoped CeO2 nanorods. The role of
doping zirconium is confirmed to increase the specific surface area,
oxygen vacancy concentration and percentage of Ce3+. The catalytic
results indicated that the large sized oxygen vacancy clusters on
(1 1 1) plane and high percentage of Ce3+ associated with them can
efficiently improve the catalytic efficiency of styrene epoxidation.
Moreover, the heterogeneous catalyst can be easily separated from
the reaction mixture after the reaction with no obvious deactivation
even after five recycling runs, suggesting its super stability. Effects
of different crystal morphology of Zr-doped CeO2 on the structure
and catalytic performance are being underway.
Fig. 5. Time-dependence evolution of styrene conversion and main product selec-
tivity. Reaction conditions:50 mg Ce0.9Zr0.1O2 catalyst, 4.35 mmol styrene, 8.7 mmol
TBHP (70% aqueous solution), solvent (CH3CN) = 5 mL, reaction temperature 353 K.
Acknowledgments
We thank the financial support from the Major State Basic
Research Development Program (Grant No. 2012CB224804), NSFC
(Project 21373054, 21303023, 21173052), State Key Laboratory of
Catalytic Materials and Reaction Engineering (RIPP, SINOPEC) and
the Natural Science Foundation of Shanghai Science and Technol-
ogy Committee (08DZ2270500).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2015.07.
010
Fig. 6. Recyclability test of Ce0.9Zr0.1O2 nanorods for epoxidation of styrene. Reac-
tion conditions:50 mg Ce0.9Zr0.1O2 catalyst, 4.35 mmol styrene, 8.7 mmol TBHP (70%
aqueous solution), solvent (CH3CN) = 5 mL, reaction temperature 353 K.
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simultaneously conducted in the induction period of the reaction.
Meanwhile, during the reaction process, the epoxidation was the
dominant one, and the presence of epoxide suppresses the forma-
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