Materials Chemistry and Physics 236 (2019) 121767 [606109]

Materials Chemistry and Physics 236 (2019) 121767
Available online 20 June 2019
0254-0584/© 2019 Published by Elsevier B.V.Materials science communication
Different thermal behavior of nanostructured CeO 2-ZrO 2 based oxides with
varied Ce/Zr molar ratios
Jie Denga, Shanshan Lic, Lei Xiongb, Jianli Wangb,**, Shandong Yuana, Yaoqiang Chena,b,*
aInstitute of New Energy and Low-Carbon Technology, Chengdu, 610064, China
bKey Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Sichuan, Chengdu, 610064, China
cState Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Sichuan University, Chengdu, 610065, China
HIGHLIGHTS GRAPHICAL ABSTRACT
�The OSC value of Zr-rich CeO 2-ZrO 2
sample increased after thermal aging at
1000 �C.
�The OSC value of Ce-rich CeO 2-ZrO 2
sample declined after thermal aging at
1000 �C.
�XRD, Raman and XPS evidenced the
atom rearrangement during thermal
aging treatment.
ARTICLE INFO
Keywords:
Ceria-zirconia
Nanomaterials
Ce/Zr ratio
Thermal properties
Redox property ABSTRACT
Redox property is an important feature common to all three-way catalysts to eliminate harmful vehicle exhaust.
To understand the thermal behavior of redox ability for different ceria-zirconia based materials, five CeO 2-ZrO 2-
Y2O3-La2O3 quaternary mixed oxides (CZ) with varied ratio of Ce/Zr were synthesized. We investigated the
changes in redox properties and structures of the as-prepared CZ during thermal aging, together with their re-
lationships. It is found that the Ce-rich CZ samples (Ce/Zr F1) show a declined OSC after thermal aging,
however, an increased OSC is observed in the Zr-rich CZ samples (Ce/Zr �1). Based on XRD, Raman and XPS
analyses, we propose an atom rearrangement mechanism that Zr ions migrate into ceria lattices in the Zr-rich
samples during thermal aging accompanied by sintering, accounting for the enhanced OSC, while only sinter –
ing occurs in the Ce-rich samples, resulting in deteriorated OSC.
1.Introduction
Cerium based oxide has attracted intensive attention in recent years
for its broad-ranging applications in catalysts [1–4]. Particular attention
has generally been paid to its application in three-way catalysts (TWCs) as promoter and support, owing to its reversible oxygen storage/release
property [5]. It has been found that incorporation of Zr ions into ceria
lattices greatly enhances the thermal stability, reducibility and oxygen
storage capacity (OSC) of CeO 2 [6–8]. Thus cerium-zirconium based
oxides are used as key components in current new generation of TWCs.
*Corresponding author. College of Chemistry, Sichuan University, Wangjiang Road 29, Sichuan, Chengdu, 610064, China.
**Corresponding author.
E-mail addresses: [anonimizat] (J. Wang), [anonimizat] (Y. Chen).
Contents lists available at ScienceDirect
Materials Chemistry and Physics
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https://doi.org/10.1016/j.matchemphys.2019.121767
Received 23 April 2019; Received in revised form 14 June 2019; Accepted 18 June 2019

Materials Chemistry and Physics 236 (2019) 121767
2Generally, cerium-zirconium based oxide with different Ce/Zr ratio
exhibits distinct physic-chemical properties. For example, Ren et al. [9]
reported that the oxygen storage/release capacity of CexZr1-xO2
(1 �x �0.5) depended on the Zr content strongly. Zhang et al. [10]
reported that Zr-rich ceria-zirconia solid solution showed higher ther-
mal stability relative to Ce-rich samples, but was liable to change from
cubic to tetragonal phase. According to previous literature [6,11–15],
the ratio of Ce/Zr has a significant influence on the structure, texture
and oxygen storage capacity of cerium-zirconium based oxides. How-
ever, in the view of literature, thermal behavior of ceria-zirconia based
oxides with different Ce/Zr ratios has been poorly investigated, which is
of great significance for industrial applications.
According to the results of Zhao et al. [16,17], the OSC of
Ce0.67Zr0.33O2 or Ce0.67Zr0.33MO2-δ (M �La, Nd, Pr, Sm, and Y) declined
sharply after thermal aging at 1100 �C. The results of Wang et al. [18]
showed a significant improvement in OSC and reducibility of
Ce0.2Zr0.8LaO 2-δ after thermal treatment at 1100 �C, but no more definite
explanations could be achieved. Kim et al. [19] found that the reduction
temperature of Ce0.65Zr0.35O2 decreased after redox aging treatment.
Accordingly, ceria-zirconia based oxide with different Ce/Zr ratio pos-
sesses distinct thermal behavior. Therefore, the present work is aimed at
studying thermal behavior of redox property for CeO 2-ZrO 2 based oxides
with varied Ce/Zr ratios. Moreover, XRD, Raman and XPS are performed
to understand the relationship between redox behaviors and structural
changes during thermal aging treatment [20]. This might be of great
importance for rational designing the formulation of future new func-
tional catalysts.
2.Experimental section
CexZr0.9-xLa0.05Y0.05O1.95 (x �0.18, 0.3, 0.45, 0.6 and 0.72) were
prepared by co-precipitation method as described in our previous work
[21]. The fresh and aged materials, which were respectively calcined at
600 �C for 3 h and 1000 �C for 4 h, were denoted as CZ-yf and CZ-ya
(y �x/0.9-x �0.25, 0.5, 1, 2, 4) correspondingly.
Hydrogen-temperature programmed reduction (H2-TPR) measure –
ments were carried out using a thermal conductivity detector (TCD). The
sample was heated to 950 �C at a constant heating rate of 10 �C min1
under the gas flow of 5 vol% H2/N2 (20 ml min1). Oxygen storage ca-
pacity complete (OSCC) was measured by pulse injection method. A
given amount of O2 was pulsed at specified temperature (100, 200, 400
and 600 �C) every 2 min until no O2 consumption could be detected.
X-ray powder diffraction (XRD) spectra were recorded by a Rigaku
Ultima IV (Rigaku, Japan) diffractometer using Cu Kα (λ �0.15406 nm).
Raman spectra were carried out on a LabRAM HR laser Raman spec-
trometer (HORIBA JOBIN YVON, France). The excitation wavelength of 532 nm was selected. The collected frequency range was
100–1000 cm1. And X-ray photoelectron spectroscopy (XPS) mea-
surements were performed on a XSAM-800 electron spectrometer
(Kratos, Britain). The binding energies were calibrated with the C1s
level of adventitious carbon (284.6 eV) as the internal standard.
3.Results and discussion
As shown in Fig. 1, CZ0.25f and CZ0.25a exhibit the highest OSC
irrespective of the test temperature. This might be due to the largest
solid solubility of Zr ions in CeO 2 lattices, which accelerates lattice ox-
ygen mobility, consequently the utilization efficiency of Ce atoms is
elevated. By the way, the OSC tested at low temperature 100 �C is higher
than that tested at elevated temperatures (200, 400, 600 �C), especially
for CZ2f, CZ1a and CZ2a, the OSC of which decreases with increasing
test temperature. This might be because the oxygen storage process is
exothermic. After thermal aging at 1000 �C for 4 h, regardless of the test
temperature, OSC decreases with increasing Ce/Zr ratio, coinciding with
previous literature [6]. Particular attention should be paid to the OSC
value changes before and after thermal aging. Interestingly, the OSC of
Ce-rich samples (Ce/Zr F1) tested at any temperature declines after
thermal aging, while significant increase is found in Zr-rich samples
(Ce/Zr �1). This might be related to the atom rearrangement during
thermal aging, which will be described hereinafter.
Fig. 1.OSC values the fresh (yellow grid) and aged (blue) samples measured at different temperatures. (For interpretation of the references to colour in this figure
legend, the reader is referred to the Web version of this article.)
Fig. 2.H2-TPR profiles of the fresh (solid) and aged (dash) samples. J. Deng et al.

Materials Chemistry and Physics 236 (2019) 121767
3Reduction properties were evaluated by H2-TPR measurements
(Fig. 2). The reduction of all the samples occurs at a wide temperature
range of 350–650 �C, which is usually attributed to surface, sub-surface
and bulk reduction [10,17]. And the reduction peaks marked with ar-
rows at the highest temperature are contributed by bulk reduction. A
comparison of the fresh and aged samples finds that the reduction
temperatures of all samples shift towards higher temperatures. This is
owing to the sintering caused by thermal aging, leading to decreased
surface reducible species and increased resistance for bulk oxygen
migration. Much more noticeable is that a considerable increase in bulk
reduction temperature of about 60 �C is observed for Ce-rich samples,
however, only a slight higher temperature shift of about 20 �C occurs in
Zr-rich samples. Thus, we speculate that atom rearrangement occurs
during thermal aging, which restrains the degradation of reducibility
induced by sintering to some extent.
X-ray diffraction (Fig. 3) and Raman spectroscopy (Fig. 4) were
carried out to follow the crystal structure change during calcination. The
main peaks for the samples with Ce/Zr �0.5, which correspond to
(111), (200), (220), (311), (222), (400) and (331) planes, indicate the
formation of a cubic ceria-zirconia solid solution. The CZ0.25f and
CZ0.25a materials mainly show a tetragonal phase with diffraction
peaks of (101), (002), (112), (211), (202) and (220) planes. From the
enlarged view, it can be seen that the diffraction peaks shift towards
higher 2θ degree with increasing Zr content irrespective of the dried
precipitates, fresh or aged materials, owing to the lattice shrinkage induced by the insertion of smaller ions Zr4�(0.84 Å) into ceria lattices
(Ce4�(0.97 Å)) [22,23]. Moreover, the diffraction peaks of all the dried
precipitates shift to higher 2θ degree significantly after calcination at
600 �C, implying that considerable amount of Zr4�ions insert into ceria
lattice at moderate calcination process.
After calcination at 1000 �C, a further high angle shift is found for
CZ0.25a and CZ0.5a, which is consistent with the decreased lattice pa-
rameters of CZ0.25a, CZ0.5a and CZ1a (Table 1). This phenomenon is
not observed in the Ce-rich samples CZ2 and CZ4. Thus we propose that
some remaining Zr4�ions in the Zr-rich samples insert into ceria lattices
during thermal aging, namely, the aforementioned atom rearrangement.
This is responsible for the enhanced OSC and inhibited degradation of
reducibility. Further, the atom rearrangement is demonstrated by
Raman spectroscopy. It is reported that the Raman symmetric vibration
of Ce-O bond (F2g) within the cubic fluorite CeO 2 is located at around
462 cm1. The incorporation of Zr ions will shift this bond vibrational
frequency toward a higher wavenumber [24,25]. Also, the increase in
crystallite size could shift this vibration toward a higher wavenumber.
Thus high wavenumber shift is observed for all the materials after aging
treatment. However, this shift is more obvious in the Zr-rich samples,
which is contributed by the incorporation of Zr ions. The additional
peaks at 145, 252, 323 and 603 cm1 belong to tetragonal structure
[26]. The defective structure also features the peak at around 600 cm1.
The surface elemental composition was characterized by XPS. The
decomvoluted Ce 3d spectra are plotted in Fig. S1, from which the Ce3�/
Fig. 3.XRD patterns of the dried precipitates (orange), fresh (green) and aged (blue) materials. (For interpretation of the references to colour in this figure legend,
the reader is referred to the Web version of this article.) J. Deng et al.

Materials Chemistry and Physics 236 (2019) 121767
4Ce ratios are calculated. Fig. S2 displays O 1s spectra, in which the α and
β peaks are attributed to the lattice oxygen (O2) and defect oxygen
[27]. There might be also a contribution to the Oβ peak from hydroxyl or
carbonate species. In Table 1, the decreased surface O% after thermal
aging treatment might be related to the increased (Y �La)%. The Oβ/O
and Ce3�/Ce roughly increases with increasing Zr content, because the
formation of Ce3�(1.14 Å) could alleviate the lattice strain induced by
Zr4�(0.84 Å), and then oxygen vacancies are accompanied by Ce3�
[28]. What should be pointed out is that, the Ce/Zr ratio increases after
thermal aging. This evidences the atom rearrangement during thermal
aging further. Furthermore, the ΔCe/Zr value, which could characterize
the migration amount of Zr ions from surface to bulk, roughly decreases
with increasing Ce/Zr ratio, revealing that more Zr ions in Zr-rich
samples migrate from surface to bulk.
The most reasonable explanation we proposed is shown in Fig. 5. For
the Ce-rich samples, almost all available Zr ions incorporate into ceria
lattices during moderate calcination, owing to the limited amount of
total Zr ions, thus declined OSC and reducibility are observed after
thermal aging as a consequence of sintering. In contrast, the Zr ions
available in Zr-rich samples are too abundant to migrate into ceria lat-
tices completely during moderate calcination. And these remaining
available Zr ions insert into ceria lattices during thermal aging accom –
panied by sintering, which might accounts for the increased OSC and
suppressed degradation of reducibility to a certain extent.
4.Conclusion
In this study, changes in redox ability and structure of CeO 2-ZrO 2
based oxides with different Ce/Zr ratios during thermal aging process
were investigated. The Ce-rich CeO 2-ZrO 2 based oxide (Ce/Zr F1) ex-
hibits the declined OSC and deteriorated reducibility after thermal
aging. However, an increased OSC and weakly deteriorated reducibility
are observed in the Zr-rich samples (Ce/Zr �1). Combining the XRD,
Fig. 4.Raman spectra of the fresh and aged materials.
Table 1
Surface composition derived from XPS and lattice parameter calculated from XRD.
Samples Ce% Zr% (Y �La)% O% Oβ/O (%) Ce/Zr ΔCe/Zr Ce3�/Ce (%) Lattice parameter (Å)
CZ0.25f 5.60 21.54 2.67 70.18 34.24 0.26 – 26.92 3.6974/5.2153
CZ0.5f 9.61 16.94 2.80 70.65 33.70 0.61 – 24.53 5.2736
CZ1f 13.94 13.48 3.11 69.47 32.38 1.03 – 23.37 5.3077
CZ2f 20.04 8.10 2.62 69.24 31.59 2.47 – 19.81 5.3583
CZ4f 21.41 4.68 2.75 71.16 37.63 4.57 – 21.76 5.3655
CZ0.25a 6.67 20.16 3.44 69.73 33.47 0.33 26.92% 22.40 3.6756/5.2087
CZ0.5a 10.12 16.80 3.77 69.31 33.16 0.60 1.64% 21.94 5.2471
CZ1a 14.34 12.53 3.34 69.79 32.28 1.16 12.62% 17.59 5.3008
CZ2a 20.21 7.48 3.32 68.99 31.49 2.76 11.74% 17.22 5.3582
CZ4a 22.11 4.96 3.01 69.92 34.80 4.59 0.44% 12.09 5.3876
ΔCe/Zr: The ratio of the difference between the Ce/Zr ratios of fresh and aged materials to that of fresh material.
Fig. 5.The mechanism schematic of the Zr migration during heating treatment. J. Deng et al.

Materials Chemistry and Physics 236 (2019) 121767
5Raman and XPS analyses, we propose that the abundant available Zr
ions in Zr-rich samples do not incorporate into ceria lattices completely
during moderate calcination, and these remaining Zr ions migrate into
ceria lattices further during thermal aging accompanied by sintering.
This is considered as the most reasonable explanation for the enhanced
OSC of Zr-rich samples after thermal aging.
Acknowledgments
We gratefully acknowledge the National Key Research and Devel –
opment Program of China (2016YFC0204901) for the generous financial
support to our research.
Appendix A.Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.matchemphys.2019.121767 .
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