Palladium catalyst supported on stair-like microstructural CeO2 provides enhanced activity and stability for low-concentration methane oxidation

Article abstract of DOI:10.1016/j.apcata.2016.06.040

Octahedral CeO₂ microparticles possessing a stair-like microstructure comprising self-assembled nano-sized rectangular blocks were synthesized by a hydrothermal synthetic route. The samples were characterized by X-ray diffraction, scanning/transmission electron microscopy, and nitrogen adsorption/desorption techniques. The structural functions of the as-synthesized CeO₂ as support for the Pd catalyst were investigated by the catalytic oxidation of low-concentration methane. The results showed that the unique stair-like structure of CeO₂ improved the stability of the Pd catalyst for continuous conversion of low-concentration methane, which suggests that the as-synthesized CeO₂ has potential application in the removal of low-concentration methane from ventilation air and other oxidations.

Full text of DOI:10.1016/j.apcata.2016.06.040

Applied Catalysis A: General 524 (2016) 237–242
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Palladium catalyst supported on stair-like microstructural CeO2
provides enhanced activity and stability for low-concentration
methane oxidation
a
b
b
a,∗
Tianyu Guo , Jianping Du , Jinting Wu , Jinping Li
a
Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan 030024, China
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Octahedral CeO2 microparticles possessing a stair-like microstructure comprising self-assembled nano-
sized rectangular blocks were synthesized by a hydrothermal synthetic route. The samples were
characterized by X-ray diffraction, scanning/transmission electron microscopy, and nitrogen adsorp-
tion/desorption techniques. The structural functions of the as-synthesized CeO2 as support for the Pd
catalyst were investigated by the catalytic oxidation of low-concentration methane. The results showed
that the unique stair-like structure of CeO2 improved the stability of the Pd catalyst for continuous
conversion of low-concentration methane, which suggests that the as-synthesized CeO2 has potential
application in the removal of low-concentration methane from ventilation air and other oxidations.
Received 30 March 2016
Received in revised form 29 June 2016
Accepted 30 June 2016
Available online 1 July 2016
Keywords:
Stair-like structural CeO2
Self-assembly
Methane oxidation
Catalytic properties
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
[8–10], fuel cells [11], optics [12], sensors [13], and sorbents [14].
Especially, CeO2 can be used as catalyst support materials [15–17].
Global warming has become a serious environmental problem,
Presently, CeO of various morphologies such as nanoparticles [18],
2
and greenhouse gases such as CO2 and CH4 are primarily responsi-
nanowires [19], nanotubes [20], nanorods [21], nanocubes [22],
and octahedrons [23] have been synthesized using different meth-
ods. The unique morphologies endow CeO2 with many functions.
For example, one-dimensional CeO2 nanotubes were used as a
photocatalyst for pollutant degradation [20], and CeO2 nanorods
suppressed TiO2 photocatalysis [24]. Obviously, the morphology-
dependent properties have attracted great interest [25]. Therefore,
it is essential to synthesize CeO2 possessing unique morphologies
and structures in order to improve the properties of Pd-based cat-
alysts for methane oxidation.
ble for it [1]. However, in comparison to CO , the warming potential
2
of methane is 21 times higher than that of carbon dioxide [2]. Obvi-
ously, tackling climate change requires us to reduce the emission
of methane. As we know, a large amount of methane comes from
coal mine ventilation air all over the world, especially in China.
Methane accounts for approximately 1vol.% of ventilation air (VA),
and massive amounts of VA is emitted into the atmosphere per
year at a very fast flow rate. Therefore, it is a challenge to reduce
or limit methane emission into the atmosphere. At present, the
feasible methods include adsorption separation and catalytic oxi-
dation [3,4]. It is confirmed that catalytic oxidation of methane is
an efficient method for capturing low-concentration methane [5,6].
However, the low stability of catalysts limits their effective applica-
tion in the catalytic oxidation of methane. It is therefore important
to develop a novel catalytic material.
Herein, octahedral CeO2 (OCO) possessing
microstructure constructed by the self-assembly of nano-
size rectangular blocks was synthesized by hydrothermal
a
stair-like
a
synthetic route and characterized by X-ray diffraction (XRD),
scanning/transmission electron microscopy (SEM), and N2 adsorp-
tion/desorption techniques. The properties of the Pd/OCO catalyst
were studied by catalytic oxidation of low-concentration methane
by simulating the components of ventilation air, and the results
showed that the as-synthesized CeO2 was an effective support
material.
Cerium oxide (CeO ) is one of the most important rare earth
2
oxides due to its strong storage–release oxygen capacity related
4
+
3+
to Ce /Ce redox cycles [7]. It has wide applications in catalysis
∗ Corresponding author.
E-mail address: jpli211@hotmail.com (J. Li).
http://dx.doi.org/10.1016/j.apcata.2016.06.040
0
926-860X/© 2016 Elsevier B.V. All rights reserved.

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T. Guo et al. / Applied Catalysis A: General 524 (2016) 237–242
microscopy (SU8000) and transmission electron microscopy
equipped with EDX (TEM, Hitachi H-800, Japan), respectively.
The Brunauer–Emmett–Teller (BET) surface area and pore-size
distribution were determined by N2 adsorption–desorption mea-
surements using a Micromeritics Tristar II 3020 analyzer. The
pore-size distribution was obtained from desorption isotherms by
the Barrett–Joyner–Halenda equation.
2.4. Catalyst test
The Pd/OCO catalysts were evaluated in a fixed-bed micro-
reactor (inner diameter of 6 mm) at atmospheric pressure. The
catalyst (150 mg, 20–40 mesh) was fixed in the center of the quartz
reactor. The composition of the feed gas was 1.0% CH , 20% O , and
4
2
1
−
7
9% N , and the total flow rate of gas was set to 40 mL min . The
2
Fig. 1. XRD pattern of the as-synthesized CeO2.
−1 −1
gas hourly space velocity was 16,000 mL g
h
. Prior to each reac-
−
1
tion, the catalyst was activated in N2 gas (30 mL min ) from room
◦
◦
−1
2
. Experimental
temperature to 200 C at a rate of 10 C min and maintained at
that temperature for 1 h, followed by reduction in H2 at 450 C for
2 h. The temperature was then reduced to 200 C and the mixed
◦
◦
2.1. Self-assembly of octahedral CeO2 with stair-like structures
gases were introduced into reactor. After CH4 was oxidized for an
hour, the conversion rates were recorded under steady-state con-
ditions. The temperature was then increased from 200 to 600 C
OCO consisting of nano-sized rectangular blocks was synthe-
◦
sized as follows. Cerium (III) nitrate hexahydrate (5.83 mmol) was
dissolved in a mixed solution of deionized water (48 mL) and
anhydrous ethanol (16 mL) under vigorous stirring. 2.74 mmol of
hexadecyltrimethylammonium bromide (CTAB) was then added,
and the solution was continually stirred for 30 min. Finally, the mix-
ture was transferred into a 100 mL Teflon-lined steel autoclave and
and data were collected by the same method at various tempera-
tures. The temperature was controlled with a K-type thermocouple
inserted at the center of the catalyst bed. The reaction tempera-
◦
◦
−1
ture was increased from 200 to 600 C at a rate of 10 C min in
◦
50 C increments. The effluent gases were analyzed online with a
◦
kept at 150 C for 24 h. The products were obtained by centrifugal
gas chromatograph (ZHONGKEHUIFEN GC-6890A) equipped with a
TDX–01 column and a thermal conductivity detector. The activities
were evaluated by temperatures (T10% and T90%), which represented
the temperatures at 10 and 90% methane conversions, respectively.
For comparison, commercial CeO2 and CeO2 nanowires were also
investigated.
separation.
2
.2. Preparation of the Pd/OCO catalysts
The as-synthesized CeO was used as a support material. The Pd
2
catalysts were prepared as follows, and Pd contents were 0.5, 1, and
.5 wt.%. First, the OCO samples were impregnated with an aque-
1
3. Results and discussion
ous solution of palladium nitrate for 24 h. Then, the products were
◦
dried naturally and calcined at 450 C for 2 h in a muffle furnace.
The XRD pattern of the as-synthesized CeO is shown in Fig. 1.
2
◦
◦
◦
◦
◦
◦
For comparison, Pd/CCO (CCO: commercial CeO ) and Pd/CONW
CONW: CeO2 nanowires; the synthesis process was described in
Typical diffraction peaks at 28.5 , 33.1 , 47.5 , 56.4 , 59.0 , 69.4 ,
2
◦
◦
(
76.7 , and 79.1 correspond to the diffraction of the (111), (200),
the Supporting information) catalysts were also prepared according
to the same process.
(220), (311), (222), (400), (331), and (420) planes of CeO , respec-
2
tively (JCPDS No. 34-0394), which indicates that the as-synthesized
CeO2 (OCO) had a cubic fluorite structure [26]. No other peaks are
observed, revealing the high purity of OCO.
2.3. Characterization of samples
The structure of OCO was further analyzed by the N2 adsorp-
tion/desorption measurements. Fig. 2 shows the isotherm and
The as-synthesized CeO2 was characterized by X-ray diffrac-
tometry using Cu K␣ radiation (␭ = 0.15418 nm). The mor-
the pore-size distribution of the as-synthesized CeO . A type IV
2
phologies and structures were analyzed by scanning electron
isotherm with an H3 hysteresis loop in the relative pressure (P/P0)
Fig. 2. (a) N2 adsorption/desorption isotherm and (b) pore-size distribution of as-synthesized CeO2.

T. Guo et al. / Applied Catalysis A: General 524 (2016) 237–242
239
synthesis of OCO, CeO2 was synthesized in the absence of CTAB
and its SEM images are shown in Fig. S1 in the Supporting infor-
mation. It was found that the obtained CeO2 was octahedral. The
images show accumulated irregular morphologies of non-uniform
size, implying that CTAB played a key role in the self-assembly of
the rectangular blocks, resulting in the stair-like microstructure.
The TEM images of octahedral CeO2 and the supported-Pd cat-
alyst are shown in Fig. 4. It is clear that octahedral CeO2 was
composed of nano-sized blocks (Fig. 4a) with regular edges, similar
to cubic blocks, and the lattice fringes are clearly visible (Fig. 4b).
The selected area was analyzed by EDX. Cerium and oxygen peaks
were observed, as well as copper peaks due to the Cu grid in the TEM
test (inset in Fig. 4a). The results indicate that pure cerium oxide
was synthesized. The stair-like microstructure of octahedral CeO2
may be effective in inhibiting the aggregation of metal nanoparti-
Scheme 1. Self-assembly of CeO2 and its function in the formation of stair-like
structures.
range of 0.2–1.0 was observed (Fig. 2a), indicating a mesoporous
cles, and so we prepared an octahedral CeO -supported Pd catalyst
structure characteristic of as-synthesized CeO . However, the low
2
2
(Pd/OCO). The TEM images show that the Pd nanoparticles were dis-
volume-adsorbed values showed that OCO had very few mesopores
or micropores, as evidenced by the low specific surface area (Table
S1, Supporting information). At relative pressures higher than 0.8,
the volume-adsorbed values increased quickly, implying that OCO
had macroporous characteristics, which may be ascribed to the
accumulated macropores. The pore-size distribution had a wide
range, and the pore diameter was more than 2 nm [Fig. 2b and Table
S1 (Supporting information)].
The SEM images show that the as-synthesized CeO2 had a
uniform size (approximately 2 ␮m) and dispersion (Fig. 3a). The
magnified image in Fig. 3b shows the octahedral morphology.
The red rectangular area was further magnified and is displayed
in Fig. 3c. The octahedral contour is clearly visible, and a sin-
gle particle indicates that the octahedral structure of CeO2 was
self-assembled by rectangular blocks 100–150 nm in size (Fig. 3d).
These blocks were piled up in an orderly manner, forming a stair-
like microstructure at different facets. During the growth process,
the CTAB surfactant acted as a structure-directing agent [27] and
induced growth of the CeO2 stair-like microstructure, which was
formed by the orderly arrangement of nano-sized blocks in certain
directions (Scheme 1). In order to elaborate the role of CTAB in the
persed on the different terraces of the stair-like structure (Fig. 4c
and d), corresponding to the gradient surfaces shown in Fig. 3d. The
average particle size of the Pd was approximately 10 nm. The fine
structures of Pd are also observed in the high-resolution TEM image
(inset in Fig. 4d). No diffraction peaks of metallic Pd were found in
the XRD patterns (Fig. S2a, Supporting information), implying there
were no large particles.
In all tests, only CO could be detected, and CO was not found in
2
the effluent gases, implying the complete conversion of methane to
carbon dioxide. The performance of the Pd/OCO catalysts (Pd load-
ings were 0.5, 1.0, and 1.5 wt.%) for methane oxidation is shown
in Fig. 5. It is clear that methane conversion over the Pd/OCO cat-
◦
alysts increased with increasing temperature from 200 to 600 C.
The conversion also increased with increasing Pd loading at con-
stant temperature, whereas there was low conversion of methane
over the pure OCO material (Fig. 5a). In particular, the catalyst
◦
maintained high stability from 400 to 600 C, suggesting that the
Pd nanoparticles anchored on the surfaces of the stair-like OCO
exposed more active sites and that the stair-like structures may
suppress migration of nanoparticles between different terraces.
Fig. 3. SEM images of as-synthesized CeO2 with stair-like structure.

2
40
T. Guo et al. / Applied Catalysis A: General 524 (2016) 237–242
Fig. 4. TEM images of (a, b) as-synthesized CeO2 and the (c, d) Pd/OCO catalyst.
◦
Fig. 5. Catalytic properties of the Pd/OCO catalysts in methane oxidation: (a) CH4 conversion over catalysts with various Pd loadings at temperatures from 200 to 600 C. (b)
◦
◦
Stabilities of the Pd/OCO catalysts for 0.5, 1.0, and 1.5 wt.% loadings at 600, 500, and 400 C, respectively. (c) Stabilities of the Pd/OCO catalysts at 600 C. (d) Activities of the
Pd/OCO catalysts at 200, 300, 400, 500, and 600 C.
◦

T. Guo et al. / Applied Catalysis A: General 524 (2016) 237–242
241
Fig. 6. (a) CH4 conversions over Pd/OCO, Pd/CCO, and Pd/CONW with the same Pd loading at various temperatures. (b) Stabilities of different catalysts with 0.5 wt.% loading
◦
at 600 C.
Fig. 5b shows the stabilities of the Pd/OCO catalysts with vari-
ous Pd loadings at different temperatures during the reaction over
at high temperatures. Obviously, the excellent performance of the
Pd/OCO catalyst is not attributed to the pore structure and BET
specific surface area, but rather to the unique stair-like structures
of OCO. The possible reason for these results is that the stair-like
microstructure formed by nano-sized blocks can inhibit the migra-
tion of nanoparticles on the gradient surface of CeO2 (Scheme 1),
which effectively prevents nanoparticle sintering at high temper-
atures, thus retaining more active sites on the Pd/OCO catalyst.
Therefore, octahedral CeO2 possessing a stair-like microstructure
will have potential application in catalysis.
2
0 h. The catalysts with 0.5, 1.0, and 1.5 wt.% Pd loading exhibited
◦
considerable stability at 600, 500, and 400 C, respectively. When
the temperature was further increased to 600 C, all the catalysts
◦
exhibited high stability over 20 h and the methane conversions
reached 100% (Fig. 5c), suggesting that the OCO material may sup-
press sintering of Pd nanoparticles at high temperatures, which was
confirmed by the XRD results of the Pd/OCO catalysts after the reac-
tion. Pd diffraction peaks were not found in the XRD patterns (Fig.
S2b, Supporting Information), implying that Pd nanoparticles were
not sintered on the OCO surface, which may have provided more
efficient active sites for methane conversion. From the turnover
frequency (TOF) results, it is clear that the activity decreased with
4. Conclusion
Octahedral CeO2 microparticles (OCO) possessing a stair-
◦
increasing Pd content from 0.5 to 1.5 wt.% between 400 and 600 C
like microstructure was self-assembed by nano-sized rectangular
blocks. The catalytic properties of the OCO-supported Pd cata-
lyst were studied by the catalytic oxidation of low-concentration
methane. The results showed that the stair-like microstructure of
OCO played a key role in the use of a Pd catalyst for the conversion
of low-concentration methane. As compared to commercial and
(
Fig. 5d). However, we found that the TOF values for the 1.5 wt.%
◦
loading were higher than for the 1.0 wt.% loading at 300 and 350 C,
and the corresponding values only decreased by 6%–35% at temper-
atures over 400 C (Table S2, Supporting information). The results
◦
also showed that the stair-like structure of ceria played a critical
role in inhibiting the migration of nanoparticles between different
terraces, especially for the catalysts with high loading.
nanosized CeO , OCO is a preferred support. The results showed
2
that the Pd/OCO catalyst exhibited enhanced activity and stability
for low-concentration methane oxidation, suggesting that OCO is
an effective support material, and thus it has potential application
in catalysis.
In order to further confirm the structural function of OCO, a com-
parison with commercial CeO (CCO) and CeO nanowires (CONW)
2
2
was performed. Commercial CeO2 consists of micrometer particles
Fig. S3a and b, Supporting information) and possesses mesoporous
(
structures (Fig. S4, Supporting information). CeO2 nanowires are
cage shaped (Fig. S3c and d, Supporting information), with micro-
porous structures and high BET surface areas (Fig. S5 and Table S1,
Supporting information). The properties of the Pd/OCO, Pd/CCO,
and Pd/CONW catalysts with 0.5 wt.% Pd contents are shown in
Fig. 6. The CH4 conversion over Pd/OCO is close to that over
Acknowledgement
The authors express thanks for partial financial support
by National Natural Science Foundation of China (21136007
and 51572185), Natural Science Foundation of Shanxi Province
(2014011016-4), and Coal-Based Scientific and Technological Key
Pd/CONW and higher than that over Pd/CCO at temperatures below
Project of Shanxi Province (MQ2014-10).
◦
3
50 C, whereas the CH4 conversion on the Pd/OCO catalyst was
higher than those over the Pd/CCO and Pd/CONW catalysts at tem-
Appendix A. Supplementary data
◦
peratures between 350 and 600 C (Fig. 6a). More importantly,
◦
the Pd/OCO catalyst maintained a higher stability at 600 C dur-
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2016.06.
ing methane oxidation as compared to the Pd/CCO and Pd/CONW
catalysts (Fig. 6b). The results showed that the Pd/CONW catalyst
exhibited low activity and the Pd/CCO catalyst exhibited low sta-
bility at high temperatures. In comparison, the OCO/Pd catalyst
exhibited high activity and high stability.
0
40.
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S1). However, the interesting results were that the Pd/OCO cata-
lyst exhibited good activity at low temperatures and high stability
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