Low-temperature efficient degradation of ethyl acetate catalyzed by lattice-doped CeO2-CoOx nanocomposites

Article abstract of DOI:10.1016/j.catcom.2015.10.024

A series of CeO₂-CoO_x nanocatalysts have been synthesized by a facile surfactant-free hydrothermal method and investigated for the oxidative degradation of ethyl acetate (EA) at exceptionally low temperature. The combination of various techniques, such as H₂-TPR, XRD, XPS and HRTEM provides insights to the effect of various factors including redox properties, enhanced lattice oxygen and its mobility etc. on catalysis performance. The results demonstrate that the enriched lattice oxygen is the dominant factor for the low-temperature degradation of EA due to the interaction between lattice-mixed Ce and Co ions, which can be used to design the catalysts for removing VOCs with improved performance.

Full text of DOI:10.1016/j.catcom.2015.10.024

Catalysis Communications 73 (2016) 123–127
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Low-temperature efficient degradation of ethyl acetate catalyzed by
lattice-doped CeO2–CoOx nanocomposites
a
c
c
a,
Sadia Akram a,b,1, Zhen Wang a,1, Lan Chen a, Qi Wang a, , Genli Shen , Ning Han , Yunfa Chen , Guanglu Ge
⁎
⁎
a
CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Centre for Nanoscience and Technology, No. 11 Zhongguancun Beiyitiao, Beijing 100190, China
b
University of Chinese Academy of Science, No. 19A Yuquan Road, Beijing 100049, China
c
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 July 2015
Received in revised form 14 October 2015
Accepted 19 October 2015
Available online 20 October 2015
A series of CeO2–CoOx nanocatalysts have been synthesized by a facile surfactant-free hydrothermal method and
investigated for the oxidative degradation of ethyl acetate (EA) at exceptionally low temperature. The combina-
tion of various techniques, such as H2-TPR, XRD, XPS and HRTEM provides insights to the effect of various factors
including redox properties, enhanced lattice oxygen and its mobility etc. on catalysis performance. The results
demonstrate that the enriched lattice oxygen is the dominant factor for the low-temperature degradation of
EA due to the interaction between lattice-mixed Ce and Co ions, which can be used to design the catalysts for re-
moving VOCs with improved performance.
Keywords:
Ethyl acetate oxidation
Ce–Co composite oxides
Hydrothermal method
Redox properties
Lattice oxygen
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
of this work. Herein, the difference in catalytic activities by varying
molar ratios of Ce–Co is compared and evaluated, and the attention is
Volatile organic compounds (VOCs) are one of the most hazardous
classes of organic pollutants. They are emitted from various anthropo-
genic activities and cause many environmental problems such as the
formation of photochemical smog and the depletion of stratospheric
ozone [1–4]. Ethyl acetate (EA) is one of the most common, ascendant
and stable volatile organic solvents. Its long term exposure can cause se-
vere health problem to human beings. This hazardous nature of EA
urges the researchers to fabricate new materials and seek better ap-
proaches to its complete removal [5,6].
also paid to analyze the interaction between CeO2 and Co3O4 and the ef-
fect on its redox property, lattice oxygen storage and mobility.
2. Experimental
A facile surfactant-free hydrothermal method is used to prepare the
catalysts and the detailed procedure is provided in Supporting Informa-
tion (SI).
Catalytic combustion has been recognized as an efficient technology
for EA degradation, where both noble and transition metals have been
used as catalysts. However, transition metal oxides are more active for
the oxidation of oxygenated VOCs besides their thermal stability and
low cost, and are not prone to sintering and poisoning. Recently,
CeO2–CoOx composites have attracted special attention in catalytic re-
moval of numerous VOCs, which demonstrate higher catalytic activity
than their individual components [7–12].
Although many works have been done previously, to develop a sys-
tem to degrade EA at reduced temperature is still needed. Therefore, to
elucidate the low-temperature catalytic activity of the CeO2–Co3O4
composites towards the oxidative degradation of EA is the main aim
3. Results and discussion
3.1. Catalytic performance
Fig. 1a shows the degradation of EA over pure and composite oxides.
The catalytic activity decreases in the order Ce0.5Co0.5 N Ce0.7Co0.3
N
Ce0.3Co0.7 N CeO2 N Co3O4. The temperature of 50 and 90% of EA conver-
sion, T50 is 178, 184, 186, 189, and 218 °C while T90 is 195, 208, 211, 215
and 244 °C respectively. All other catalysts display higher T100 (total con-
version) for EA oxidation than Ce0.5Co0.5 (T100 = 200 °C). The activity fol-
lows the same trend for these catalysts even at lower temperature. In
addition, the superiority of Ce0.5Co0.5 among all the catalysts is confirmed
by calculating the reaction rate of the samples at 156 °C (EA conversion
b20%) as shown in Table S1, where Ce0.5Co0.5 shows the best catalytic per-
formance that may be attributed to the stronger interaction between
CeO2 and Co3O4 due to the doping of Co3O4 in an appropriate ratio into
⁎
Corresponding authors.
E-mail addresses: wangq@nanoctr.cn (Q. Wang), gegl@nanoctr.cn (G. Ge).
These authors contributed equally.
1
http://dx.doi.org/10.1016/j.catcom.2015.10.024
1566-7367/© 2015 Elsevier B.V. All rights reserved.

124
S. Akram et al. / Catalysis Communications 73 (2016) 123–127
Fig. 1. Ethyl acetate conversion as a function of temperature over CeO2, Co3O4 and Cex Co1−x catalyst (a) and effect of moisture on catalytic performance of Ce0.5Co0.5 catalyst at 200 °C (b).
CeO2 lattice. This assumption will be further discussed in the following
sections. In addition, the catalytic efficiency of the composite oxide is
compared with other groups' works for the combustion of EA under sim-
ilar conditions as shown in SI (Table S2).
or Co2+ (0.082 nm) is smaller than that of Ce4+ (0.111 nm), these find-
ings are in a good agreement with the Raman analysis (Fig. S2 and S3).
In addition, the characteristic peak of CeO2 (111) at 28.6° gradually shifts
towards the higher value as the cobalt content increases (Fig. 2b). The de-
creased lattice parameters of ceria are the evidence of solid solution for-
mation [13]. However, the incorporation of cobalt ions in ceria
framework leads to the formation of CeO2–Co3O4 solid solution and the
enhanced oxygen mobility due to increased oxygen vacancies [14] and
will be discussed in the following paragraphs.
Fig. 3a shows that pure CeO2 exhibits nanorod-like morphology
while Ce0.5Co0.5 composite oxide shows a mixture of nanorods and par-
ticles (Fig. 3b), where the nanoparticles are confirmed as Co3O4- and the
nanorods are CeO2-enriched phases as determined by HRTEM (Fig. 3e).
Pure Co3O4 has hollow donut-like morphology (Fig. 3c). Lattice fringes
(Fig. 3d) in HRTEM are 0.31 nm corresponding to the d(111) of CeO2
and 0.46 nm assigned to (111) of Co3O4 (Fig. 3f). The EDS spectra of
Ce0.5Co0.5 composite oxide are provided in SI (Fig. S4). Typical HAADF
images are displayed in Fig. 3g. Taking all these HAADF information
into account, it can observe that both Ce and Co are homogeneously dis-
persed across the whole Ce0.5Co0.5 catalyst, which is crucial for the en-
hanced oxygen storage and mobility, that in turn, affects the catalytic
activity.
Ce0.5Co0.5 is used to investigate the effect of the moisture and space
velocity on the oxidative degradation of EA due to its best performance
among all catalysts. Fig. 1b demonstrates that the moisture has a nega-
tive effect on the catalysis. However, in the absence of moisture the cat-
alytic performance recovers and remains constant for further 12 h. This
inhibitory effect of the moisture indicates that the water molecules may
compete with EA to adsorb on the surface of the catalyst during cataly-
sis. In addition, the increase of the gas flow rate (Fig. S1) appears to be
another negative effect on the catalytic performance which can be at-
tributed to the shortened time of interaction between the catalyst and
the VOC.
3.2. Catalyst characterization
The crystal phases are determined by XRD. Fig. 2a confirms that the
catalysts possess fluorite phase of CeO2 (JCPDS # 81–0792) and spinal
structure of Co3O4 (JCPDS # 74–2120) respectively. The addition of Co
to CeO2 causes the change in the structural (Table S3) and textural prop-
erties of the final products (Table S4). The decreased lattice parameter
suggests that Co3+ or Co2+ ions are incorporated into CeO2 lattice to
form a solid solution, since the effective ionic radius of Co3+ (0.065 nm)
The oxidation state of catalyst surface species is examined by XPS
analysis. Owing to the similarities in XPS patterns of all the samples,
(Fig. S5) the pattern of Ce0.5Co0.5 is chosen as the representative for
Fig. 2. XRD patterns of CeO2, Co3O4 and CexCo1−x composite oxides: (a) wide angle patterns, and (b) enlarged zone patterns. Crystalline phases detected: Co3O4 (*) and CeO2 (■).

S. Akram et al. / Catalysis Communications 73 (2016) 123–127
125
Fig. 3. TEM, HRTEM images of CeO2 (a, d), Ce0.5Co0.5 (b, e), Co3O4 (c, f) and High Angle Annular Dark Field mapping (HAADF) micrographs of Ce0.5Co0.5 (g).
the following discussion. Fig. 4a represents the XPS patterns for Ce 3d
core level of Ce0.5Co0.5. In these patterns, six peaks labeled as V0
(881.9 eV), V1 (888.5 eV), V2 (897.8 eV), V0′ (900.8 eV), V1′ (907.3 eV)
and V2′ (916.2 eV) can be assigned to the characteristic profile of Ce4+
3d final states [15,16]. The high binding energy (BE) doublet (V2/V2′) is
attributed to the final state of Ce (IV) 3d94f0O2p6, doublet V1/V1′ is orig-
inated from the state of Ce (IV) 3d94f1O2p5, and doublet V0/V0′ corre-
sponds to the state of Ce (IV) 3d94f2O2p4. The characteristic peaks of
Ce3+ are also observed at 903.3/884.6 eV and 897.9/879.2 eV labeled
as U1/U1′ and U0/U0′, respectively. The content of Ce4+ is estimated to
be 0.806 based on integrated peak area. Therefore, Ce species exist
mainly in tetravalent oxidation state [17].
Fig. 4b exhibits the Co 2p XPS pattern of Ce0.5Co0.5. It is quite difficult
to distinguish Co2+ and Co3+ due to the small difference in their BEs.
The spin–orbit splitting of the Co 2p peaks (ΔE) is found to be well cor-
related with the cobalt oxidation state [18] calculated by Formula (1).
The ΔE value is 16.0 eV for CoO [19], 15.0 eV for the cobaltic compounds
(Co2O3), and 15.2 eV for the mixed-valence Co3O4 [18]. All the catalysts
have ΔE = 15.2 eV, so Co3O4 is considered as the main phase in pure co-
balt oxide which conforms XRD and HRTEM analysis.
ꢀ
ꢁ
ꢀ
ꢁ
ΔE ¼ E Co 2p1 −E Co 2p3
ð1Þ
=
=
2
2
The O 1s spectra of Ce0.5Co0.5 (Fig. 4c) show the peaks at low
(529.0 eV) and medium (531.0 eV) BEs indicating lattice (O2−) and sur-
face adsorbed oxygen species (O2−, O−, O22−) respectively. Besides this,
a broad shoulder at high BE (533.5 eV) refers to the contribution of the
defective oxides or the presence of hydroxylated or carbonated oxygen
on the surface of the catalysts [20]. The relative ratio of Oα/Oβ for all cat-
alysts in terms of the lattice oxygen to surface species is summarized in
Table 1 and Fig. 4d, which shows that Ce0.5Co0.5 possesses the highest
ratio of Oα/Oβ. This property confirms the evidence of the enhanced lat-
tice oxygen storage and mobility incurred by the strong CeO2–Co3O4

126
S. Akram et al. / Catalysis Communications 73 (2016) 123–127
Fig. 4. XPS in the Ce 3d (a), Co 2p (b), O 1s (c) regions in Ce0.5Co0.5 composite oxide and the relative ratio of lattice O2 (d) in pure and CexCo1−x composite oxides respectively.
interaction which facilitates the oxidation of EA [11]. Moreover, lifetime
Positron annihilation study (Table S5) indicates that the oxygen vacan-
cies increase in Ce0.5Co0.5 compared with pure ceria due to Co ions dop-
ing into the crystal lattice of CeO2, which is consistent with the XRD and
Raman analysis. Taking all the cases into account, it can be concluded
that the increased lattice oxygen and oxygen vacancies are the determi-
nant factor responsible for the enhanced oxidative degradation of EA
over Ce0.5Co0.5. The synergistic effect of the transition metal doped
CeO2 catalysts on their catalytic activity has been evaluated [13,21],
where the replacement of Ce4+ by the secondary metal cations in
ceria lattice results in the change of the lattice oxygen defect and vacan-
cy concentration, and the improved catalytic performance.
Table 1 demonstrates the comparison between XPS and ICP-MS for
the elemental composition along with the dispersion of Ce ions on the
surface and in the bulk of the catalysts. In case of the composite oxides,
the atomic ratio of Ce/Co is lower in the bulk than that on the surface.
For Ce0.5Co0.5 catalyst, the value of the bulk Ce/Co atomic composition
is closest to that of the surface molar ratio of Ce/Co indicating that
CeO2 and Co3O4 are homogeneously distributed in either bulk or
surface. This implies the interaction between CeO2 and Co3O4 species
reaches to the maximum at this ratio and leads to the increased oxygen
storage and mobility. The result is in a good agreement with lattice ox-
ygen contents (Oα) calculated by XPS.
To determine the effect of cobalt doping on the redox nature of
CeO2–CoOx nanocomposites, H2-TPR analysis is carried out as shown
in Fig. 5. Pure CeO2 shows two reduction peaks at 477 and 725 °C. The
lower and higher temperature peaks are attributed to the reduction of
surface oxygen species and bulk reduction of CeO2 by the removal of an-
ionic oxygen from the lattice and formation of Ce2O3, respectively [23,
24]. Pure Co3O4 has an intense peak centered at 308 °C with a shoulder
around 270 °C [25]. This feature corresponds to the two step reduction
of Co3O4. Low-temperature peak refers to the reduction of Co3+ ions
to Co2+ accompanying the structural change from Co3O4 to CoO
(Eq. (2)), while high temperature peak indicates the CoO reduction to
metallic cobalt (Eq. (3)) [11].
Co3O4 þ H2 → 3CoO þ H2O
CoO þ H2 → Co þ H2O
ð2Þ
ð3Þ
For the composite oxides, TPR profile is more complex than the pris-
tine. The reduction start from 200 °C and is completed up to 500 °C. Two
overlapping peaks from 200 to 400 °C are ascribed to the step-wise re-
duction of Co3+ to Co2+ and then to Co0. Besides the reduction of cobalt
ions, reduction of surface Ce4+ to Ce3+ also occurs at the same time. As
a result, two important observations can be drawn: (1) all the compos-
ite oxides exhibit increased reducibility at lower temperature than pure
CeO2 as the TPR-H2 reduction peaks at higher temperature dramatically
shift towards the lower temperature with the addition of Co3O4, (2) the
composite oxides show higher oxygen mobility both on the surface and
within the bulk due to the addition of Co content. These observations
suggest that Co3O4 is able to enhance the reducibility of CeO2 owing to
Table 1
XPS results and Co/Ce atomic ratio of CexCo1−x samples.
Sample
BE (eV)
Ratio of Oα/Oβ
Atomic ratio of
Ce/Co
Oα
Oβ
ICP
XPSa
CeO2
529.0
529.1
529.0
529.2
529.6
531.5
531.3
531.2
531.3
531.6
1.64957
1.76923
2.59688
1.69129
1.14834
Ce0.7Co0.3
Ce0.5Co0.5
Ce0.3Co0.7
Co3O4
1.6
1.08
0.37
2.54
1.26
0.70
a
Refined by their Atomic Sensitivity Factors (ASF), 3.59 for Co 2p and 8.81 for Ce 3d [22].

S. Akram et al. / Catalysis Communications 73 (2016) 123–127
127
4. Conclusion
Catalytic oxidation of EA has been conducted over pure and compos-
ite Ce/Co oxides. The insertion of cobalt ions in CeO2 lattice results in the
loss of the charge equilibrium and increases the available lattice oxygen
in bulk. These properties play a vital role in the superior catalytic activity
of Ce–Co composite oxides, particularly Ce0.5Co0.5 towards catalytic ox-
idation. The oxidative degradation leads to a complete conversion of EA
to CO2 and H2O with higher efficiency at reduced temperature. Our re-
search opens an alternative way for the further application of Ce–Co
nanocomposites in the degradation of other VOCs.
Acknowledgments
The authors thank National Natural Science Foundation of China
(No. 51402061), Strategic pilot projects for science and technology of
CAS (No. XDB05050000), National Science & Technology Pillar Program
(No. 20114BAC021B00) for financial support. We also acknowledge
Open Research Fund of State Key Laboratory of Multiphase Complex
Systems (No. Y52501A12D).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2015.10.024.
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