Morphology-dependent performance of Co3O4 via facile and controllable synthesis for methane combustion

Article abstract of DOI:10.1016/j.apcata.2016.07.009

Spinel type cobalt oxide (Co₃O₄) nanocrystals were controllably synthesized with different morphologies (cubical, hexagonal and flower-like) via a facile hydrothermal method. The properties of the nanostructured Co₃O₄ were characterized by XRD, SEM, TEM, HRTEM and XPS techniques. The performance of the catalysts in methane combustion was evaluated under lean methane atmosphere. Superior catalytic activities for methane combustion were observed over these oxides. The performances seemed to depend on their morphologies. The catalytic activities of flower-like Co₃O₄ and hexagonal plate-like Co₃O₄ were comparable to those of noble metal catalysts due to the preferred exposure of more active {111} crystal plane. In case of cubical Co₃O₄ the {001} plane is the dominantly exposed crystal plane. The results may provide significant insights for the development of nanostructured metal oxide catalysts for the catalytic combustion of methane.

Full text of DOI:10.1016/j.apcata.2016.07.009

Applied Catalysis A: General 525 (2016) 94–102
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Morphology-dependent performance of Co3O4 via facile and
controllable synthesis for methane combustion
Zhiping Chena,b, Sheng Wanga,∗, Weigang Liua,b, Xiuhui Gaoa,b, Diannan Gaoa,
Mingzhe Wanga, Shudong Wanga,∗
a Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023 Liaoning, China
b University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 May 2016
Received in revised form 5 July 2016
Accepted 17 July 2016
Available online 18 July 2016
Spinel type cobalt oxide (Co3O4) nanocrystals were controllably synthesized with different morphologies
(cubical, hexagonal and flower-like) via a facile hydrothermal method. The properties of the nanostruc-
tured Co3O4 were characterized by XRD, SEM, TEM, HRTEM and XPS techniques. The performance of
the catalysts in methane combustion was evaluated under lean methane atmosphere. Superior catalytic
activities for methane combustion were observed over these oxides. The performances seemed to depend
on their morphologies. The catalytic activities of flower-like Co3O4 and hexagonal plate-like Co3O4 were
comparable to those of noble metal catalysts due to the preferred exposure of more active {111} crystal
plane. In case of cubical Co3O4 the {001} plane is the dominantly exposed crystal plane. The results may
provide significant insights for the development of nanostructured metal oxide catalysts for the catalytic
Keywords:
Morphology-dependent
Cobalt oxide (Co3O4)
Methane combustion
combustion of methane.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
mance [1,2]. However, the use of noble metals is limited to some
extent in commercial applications because of their high cost and
The known reserves of natural gas have doubled in the last
decade due to the increased usage of shale gas, coal bed methane,
and low-permeability gas found in sandstones (tight gas). It is
promising to substitute the petroleum-based transportation fuel
for methane. Compared to gasoline and diesel vehicles, natural
gas vehicles (NGVs) are more environmentally friendly. However,
unburned methane exhausted from NGVs has more significant
greenhouse effect. The global warming potential of methane is 21
times greater than that of carbon dioxide. Lean methane is vented
directly to the atmosphere during coal mining due to the lack of
proven technologies. Hence there is a potential emission reduction
if methane emitted can be converted to CO2. However, methane
is difficult to be oxidized due to the strongest C H bond among
hydrocarbons. The catalytic oxidation of methane has also been
used as a probe reaction to evaluate the performance of com-
bustion catalysts. So the studies on methane combustion catalyst
can contribute to promote the development of the relevant tech-
nologies. Supported noble metals such as Pd and Pt have been
widely studied in catalytic oxidation due to their superior perfor-
inferior anti-sintering. It is crucial to develop cost-effective and
active substitute catalysts. Transition and rare-earth metal oxide
catalysts have appealed considerable attention owing to their rela-
tively high catalytic activity and low cost [3–5]. Among these metal
oxides, cobalt oxide is known as the most active catalyst for CO
oxidation due to its weak oxygen bond strength and high turnover
frequency for redox reaction [6]. Studies have shown that its perfor-
mance in catalytic combustion is strongly morphology-dependent.
Currently, there is an explosive growth on the research on the
morphology-dependent nanocatalysis. Nanoparticles exhibit pecu-
liar characteristics due to the presence of numerous exposed corner
and edge atoms accessible for catalysis, while the nanocrystals
with various morphologies are enclosed by certain index facets.
They provide different abilities of electron transfer and amounts of
active sites responsible for the catalytic activity and selectivity. For
this reason, Co3O4 nanocrystals with different morphologies have
been synthesized such as sphere [7–9], nanorod [10–13], nanobelt
[14], nanocube [13,15,16], nanowire [17], nanoplate [13,18], hol-
low [19,20], hierarchical [21] and so forth. Studies showed that
their catalytic behaviors are strongly dependent on their morpholo-
gies and possibly attributed to the electron transfer properties of
different Co3O4 nanomaterials [13,22,23]. Additionally the {110}
crystal plane in Co3O4 nanorods is more active than the {111}
one over Co3O4 particles for CO oxidation [24,25]. More specifi-
∗
Corresponding authors.
E-mail addresses: wangsheng@dicp.ac.cn (S. Wang), wangsd@dicp.ac.cn
(S. Wang).
http://dx.doi.org/10.1016/j.apcata.2016.07.009
0926-860X/© 2016 Elsevier B.V. All rights reserved.

Z. Chen et al. / Applied Catalysis A: General 525 (2016) 94–102
95
cally, Co3+ is regarded as the active site for CO oxidation, whereas
the Co2+ is almost inactive [6,26,27]. The {001} and {111} planes
contain only Co2+ cations, while the Co3+ cations are dominant
on the {110} plane. Therefore the rod-shaped Co3O4 with active
sites of Co3+ is more active than Co3O4 particles containing only
Co2+ sites for the CO oxidation. Similar results were also found
by Li and et al. [14] that the activity in methane combustion was
dependent on the Co3O4 morphology. Co3O4 nanosheets exhibit
the highest activity due to the dominant presence of high index
crystal plane {112}. The morphology-dependent nanocatalysis was
also found over other metal oxides such as CeO2 [28]. According to
the studies, superior catalytic performance can be obtained by tun-
ing the morphology of Co3O4. However, most studies have been
focused on the CO catalytic oxidation. Only few pioneering studies
have been done to verify the morphology- or plane-dependence
in methane combustion over Co3O4 nanocrystals, especially under
lean fuel atmosphere. Furthermore, the synthesis schemes reported
previously for Co3O4 nanocrystals are very complicated and require
additional organic solubilizing agents [29–32]. In this study a facile
and “green” synthesis routine was developed. The Co3O4 syn-
thesized with specified morphologies (such as cubical, hexagonal
sheet-like, hexagonal plate-like, and flower-like) were evaluated
in methane combustion under lean-fuel conditions. Morphology-
dependent performance of Co3O4 is studied based on the structural
and morphological properties determined by BET, XRD, SEM, TEM,
HRTEM and XPS characterizations.
nitrogen adsorption at 77 K using a Quantachrome NOVA2200e
instrument. Prior to analysis, the samples were degassed under
vacuum at 300 ◦C for at least 2 h. The pore size distribution and
total pore volume were determined from the desorption branch of
the isotherms using the Barrett-Joyner-Halenda (BJH) desorption
method.
Phase of the samples was identified by Powder X-ray Diffrac-
tion (XRD) with a Rigaku RINTD/MAX-2500PC X-ray diffractometer
using Cu K␣ radiation at 200 mA and 40 kV. The samples were
scanned from 20◦ to 80◦ with a step size of 0.02◦. Phase identifi-
cation was done using the reference JCPD database.
X-ray photoelectron spectra (XPS) were measured using an
ESCALAB 250Xi spectrometer with an aluminum anode for K␣
(h␯ = 1484.6 eV) radiation. Detailed spectra were recorded for the
region of Co 2p, O1s photoelectrons with a 0.1 eV step at a pres-
sure 1 × 10−10 mBar. Analysis was performed by the XPSPEAK41
software, and charging effects were corrected by adjusting binding
energy (B.E.) of C1s to 284.5 eV.
CH4 temperature programmed reduction (CH4-TPSR) exper-
iments were performed on a flow system at a Quantachrome
CHEMBET3000 adsorption instrument equipped with a TCD detec-
tor and a MS detector. The moisture was removed from the TPR
effluent stream in a water trap before the TCD and MS detector. Dur-
ing each analysis, the as-prepared catalyst (ca. 20 mg) was placed
into a quartz reactor. First, the sample was pretreated in 3%vol.
O2/He mixture (20 mL/min) at 350 ◦C for 60 min, and cooled to
35 ◦C. Then the sample was heated up to 900 ◦C at a ramp rate of
10 ◦C/min in a flow of 20 mL/min of 10 vol.% CH4/Ar mixture.
The surface morphologies of sample were observed by an FEI
Quanta 200F scanning electron microscopy (SEM) equipped with
energy dispersive spectrometry (EDS).
2. Experimental section
2.1. Synthesis materials
The transmission electron microscopy (TEM) measurement was
carried out with an FEI Tecnai G2 Spirit equipment operated at an
accelerating voltage of 120 kV. The catalyst powder was ultrason-
ically dispersed in ethanol and dropped onto a copper grid coated
with amorphous carbon film, then dried in air.
All the reagents were analytic grade and used as received with-
out further purification: Cobalt nitrate (Co(NO3)2·6H2O), absolute
ethanol, polysorbate20, triethylamine, ethanolamine. All chemical
reagents are from Sinopharm Chemical Reagent Beijing Co., Ltd.
The high resolution transmission electron microscopy (HRTEM)
measurement was performed with an FEI Tecnai G2 F30 S-Twin
equipment at an accelerating voltage of 300 kV. The sample prepa-
ration procedure for HRTEM was identical to the one for TEM.
2.2. Catalyst synthesis
Flower-like Co3O4 (Co3O4-F) was synthesized by dissolving
20 mmol of cobalt nitrate in 100 mL of deionized water. Then 20 mL
of ethanolamine was dissolved in 50 mL of deionized water forming
a transparent solution. Two solutions obtained were mixed under
vigorous stirring for at least 30 min. Then the suspension was trans-
ferred into a Teflon-lined stainless steel autoclave, and aged 28 h
at 200 ◦C. Aged suspension was filtered and washed by deionized
water and absolute ethanol several times. The obtained solids were
dried under vacuum at 80 ◦C overnight. The samples were calcined
in air at 350 ◦C for 3 h. For preparation of hexagonal plate-like Co3O4
(Co3O4-P), synthesis conditions were identical to those of Co3O4-F
with two exceptions. Firstly, the transparent solution consisted of
2.5 mL of triethylamine, 2.25 g of polysorbate20 and 50 mL of deion-
ized water. Secondly, the suspension was aged 24 h at 160 ◦C. For
synthesis of hexagonal sheet-like Co3O4 (Co3O4-S) [33] and cubi-
cal Co3O4 (Co3O4-C) [16], synthesis conditions were identical to
those of Co3O4-P with only some exception. For Co3O4-C catalyst,
the 20 mmol of cobalt nitrate was dissolved in 100 mL of abso-
lute ethanol and 20 mL of triethylamine was dissolved in 50 mL
of absolute ethanol. While for the Co3O4-S catalyst, the 20 mmol of
cobalt nitrate was dissolved in 50 mL of deionized water and 5 mL
of triethylamine was dissolved in 100 mL of absolute ethanol.
2.4. Catalytic tests
The catalytic performance for CH4 combustion was tested in
a fixed-bed quartz reactor (i.d. 6 mm) packed with 0.2 g catalyst
(40–60 mesh). The reaction temperature was controlled by a PID
temperature regulator. The dry feed gas containing 0.2 vol.% CH4 in
air was supplied to the catalyst bed through a mass flow controller
at a gas hourly space velocity (GHSV) of 110,000 h−1. Additionally
the catalytic performance was also evaluated under the proxi-
mately real exhaust atmosphere but having higher water content.
The feed gas containing 0.4 vol.% CH4 and 10 vol.% H2O in air was
supplied to the catalyst bed at a GHSV of 80,000 h−1. The composi-
tions of the dry feed gas and the combustion flue gas were analyzed
by an on-line infrared gas analyzer from ONUEE Electronics Ltd.
The gas compositions were also analyzed by an Agilent 7890 gas
chromatograph equipped with FID and TCD detectors.
3. Results and discussion
3.1. X-ray diffraction study
2.3. Characterization
Fig. 1 shows the wide-angle XRD patterns of the precursors
of all Co3O4 samples. The precursor of Co3O4-C showed only the
peak of Co3O4 Fd-3m with lattice constant a = 8.0850 Å (JCPDS PDF
BET specific surface areas (SBET) of the catalysts were mea-
sured according to the Brunauer-Emmett-Teller (BET) method by

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Z. Chen et al. / Applied Catalysis A: General 525 (2016) 94–102
Fig. 1. Wide-angle XRD patterns of the precursors of (a) Co3O4-C, (b) Co3O4-S, (c)
Co3O4-P, (d) Co3O4-F. :Impurity ␤-Co(OH)2, ᭹:Impurity Co3O4.
∗
Fig. 3. Nitrogen adsorption/desorption isotherm curves for (a) Co3O4-C, (b) Co3O4-
S, (c) Co3O4-P, (d) Co3O4-F.
3.2. Specific surface areas and porosity by N2 adsorption
The nitrogen adsorption-desorption isotherms of the as-
prepared catalysts and their pore size distributions are plotted
in Figs. 3 and S-1. All the samples were synthesized with porous
structure [34]. All the isotherms exhibited the type IV isotherm
pattern with a H3 hysteresis loop according to IUPAC classification,
which indicated that capillary condensation occurred in the meso-
porous structure and the slit-shaped mesopores. However, there is
a significant difference in the pore structure and the pore size dis-
tribution of Co3O4-S and other three samples as seen in Table 1. The
BET surface areas for Co3O4-C, Co3O4-S, Co3O4-P and Co3O4-F are
28.86 m2/g, 48.83 m2/g, 31.26 m2/g and 32.21 m2/g, respectively.
The average pore radii are also divergent. The pore size distribu-
tion and porosity are dependent on the voids between the stacking
nanocrystals and the ones between the set of stacking nanocrystals.
Small mesopores correspond to the front one, whereas large pores
resulted from the latter one [35].
Fig. 2. Wide-angle XRD patterns of (a) Co3O4-C, (b) Co3O4-S, (c) Co3O4-P, (d) Co3O4-
F.
NO. 42-1467). In case of Co3O4-F, all the peaks can be indexed to
pure Co(OH)2 (JCPDS PDF 30-0443), which has a trigonal structure
(hexagonal lattice) with the P-3m1 space group (no. 164). Besides,
hP5 Co(OH)2 is dominant and accompanied by a small amount of
cF56 Co3O4 in the precursors of Co3O4-S and Co3O4-P. By compar-
ing the diffraction peaks of Co(OH)2 and Co3O4, it can be easily seen
that Co(OH)2 has larger crystallite size and higher degree of crys-
tallinity proven by sharper and narrower peaks in the XRD patterns
among all the precursors.
The XRD patterns of the as-prepared catalysts were given in
Fig. 2. For all the samples, the diffraction peaks of Co3O4 are
observed at 19.00◦, 31.27◦, 36.85◦, 38.54◦, 44.81◦, 55.66◦, 59.36◦and
65.24◦, and can be assigned to the (111), (220), (311), (222), (400),
(422), (511) and (440) planes of the Co3O4 phase (JCPDS NO.42-
1467) with a space group of Fd3m, respectively. No other peaks
for impurities were observed in the XRD patterns, illustrating the
complete transition of intermediate phases ␤-Co(OH)2 in precur-
sor of Co3O4-C, Co3O4-S and Co3O4-P into Co3O4. The high peak
intensities of all the as-prepared catalyst samples confirmed high
crystallization.
3.3. Microstructural characterizations
SEM images of the precursor of as-prepared catalysts are shown
in Fig. S-2–S-5. The SEM and TEM images of Co3O4 catalysts with
different morphologies are presented in Figs. 4 and 5. By com-
parison, it can be seen that the as-prepared Co3O4 samples keep
primarily their initial morphologies of precursors after calcina-
tion at 350 ◦C for 3 h. However, some discernable differences were
observed in Co3O4-S and Co3O4-P samples.During thermal treat-
ment, hydroxyl and/or carbonate species in the precursors were
removed. Therefore some gas transfer passages were formed, and

Z. Chen et al. / Applied Catalysis A: General 525 (2016) 94–102
97
Fig. 4. SEM images of as-prepared Co3O4catalysts with different morphologies. (A) Co3O4-C, (B) Co3O4-S, (C) Co3O4-P, (D) Co3O4-F, (E) locally magnified images of Co3O4-F.
much abundant pore structures were achieved in case of Co3O4-S
and Co3O4-P samples. The Co3O4-P exhibited superior porosity, as
shown in Table 1, while a large amount of nanoparticles (20–50 nm
in diameter) are attached to the surface of the Co3O4-S as seen in
Fig. 5(C) and (D). These particles are not spherical but close to cube.
Possibly some pores were blocked by their attachments, there is
a decrease in the pore radius and pore volume. For Co3O4-C, it is
dense and solid. So its pore structure is derived from the void of
the stacking of cubic nanocrystals. And it owns a large pore radius.
Additionally the size of as-prepared Co3O4-C is ca. 20–50 nm (see
Figs. 4 (A) and 5 (A)). The Co3O4-S shape is hexagonal, following its
crystallographic structure and the width of Co3O4-S samples varies
from 200 nm to 500 nm in Fig. 4(B). The shape of Co3O4-P is also
hexagonal with the width of ca. 100–200 nm and thickness of ca.
20–30 nm as seen in Figs. 4 (C) and 5 (F). Fig. 4(D) shows the over-
all morphology of the flower-like Co3O4. Obviously, Co3O4-F has a

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Z. Chen et al. / Applied Catalysis A: General 525 (2016) 94–102
Fig. 5. (A) TEM image of Co3O4-C; (B)HRTEM image and SAED pattern (insert) of
Co3O4-C; (C–D) TEM images of Co3O4-S; (E) HRTEM image and SAED pattern (insert)
of Co3O4-S; (F) TEM images of Co3O4-P; (G) HRTEM image and SAED pattern (insert)
of Co3O4-P; (H) TEM images of Co3O4-F; (I) HRTEM image and SAED pattern (insert)
of Co3O4-F.
hierarchical architecture assembled with numerous nanosheets as
seen in Fig. 4(D) and (E). The average thickness of the nanosheets is
about 20 nm. These nanosheets were interwoven together to form
the monodispersed microspheres with an average size of 8–12 ␮m
[36].
Fig. 6. (A) the Co2p XPS spectra of as-prepared catalysts; (B) the Co2p3/2 XPS spectra
of as-prepared catalysts; (C) the O1s XPS spectra of as-prepared catalysts; (a) Co3O4-
C, (b) Co3O4-S, (c) Co3O4-P, (d) Co3O4-F.
HRTEM images and SAED patterns of as-prepared Co3O4 sam-
ples are also shown in Fig. 5. As seen in Fig. 5(A) and (B), highly
single crystalline nature of Co3O4-C was formed. The measured

Z. Chen et al. / Applied Catalysis A: General 525 (2016) 94–102
99
Fig. 7. (A) the profiles of CH4-TPR of (a) Co3O4-C, (b) Co3O4-S, (c) Co3O4-P, (d) Co3O4-F.(B), (C), (D), (E) the profile of CH4-TPR and mass chromatograms(insert) of Co3O4-C,
Co3O4-S, Co3O4-P, Co3O4-F.
interplanar distance of 0.29 nm corresponds to the (220) crystal
plane. It is assumed that the predominantly exposed planes of
Co3O4-C are {001} because only they are normal to the set of (220)
planes [14]. According to the structure model and the interplanar
distance in Fig. 4, it can be deduced that dominant exposed planes
of Co3O4-S and Co3O4-P were both {111}. As seen in Fig. 5(I), the
interplanar distance and angle of Co3O4-F is almost similar to that
of Co3O4-P. Therefore the same planes of {111} are dominantly
exposed. Although as-prepared Co3O4-S, Co3O4-P, and Co3O4-F cat-
alysts exhibit different morphologies, they have the same exposure
plan {111}.

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Z. Chen et al. / Applied Catalysis A: General 525 (2016) 94–102
Table 1
Textural properties of the as-prepared catalysts with different morphologies.
Samples
Calculated density (g/cm3)
Pore radius (nm)
SBET (m2/g)
Pore volume (cm3/g)
Porosity (%)
Co3O4-F
Co3O4-P
Co3O4-S
Co3O4-C
3.9
3.9
3.9
3.9
12.53
9.58
17.06
87.34
32.21
31.26
48.83
28.86
0.16
0.18
0.11
0.16
38
41
30
38
Density was calculated from cell parameters given [35]. Porosity was calculated using the classical formula: Porosity: pore volume/(pore volume + 1/density).
3.4. Chemical structure characterization
XPS analysis was performed in order to determine the sur-
face compositions and chemical states of Co and O species in
as-synthesized Co3O4. Fig. 6(A)–(C) shows the XPS spectra of Co2p,
Co2P3/2 and O1s electronic levels of all Co3O4 samples, respec-
tively. Two sharp peaks at 795.1 eV and 780.0 eV correspond to
the Co2P1/2 and Co2P3/2 spin-orbit-split doublet peak of Co3O4
spinel, respectively [37,38]. There is an energy difference of approx-
imately 15.1 eV between them. The same chemical information can
be obtained by analyzing the Co2P1/2 and Co2P3/2 spectra [39].
Therefore only Co2P3/2 peaks in Fig. 6(B) were fitted and deconvo-
luted into two peaks at 780.8 eV and 779.5 eV, which are attributed
to Co2+ and Co3+, respectively. Fig. 6(C) shows the XPS spectra of the
O1 s electronic levels for as-synthesized catalysts. It can be seen that
the asymmetric O1s peak could be deconvoluted to two compo-
nents at 531.1 eV and 529.7 eV. The peak at 531.1 eV was attributed
to the surface adsorbed oxygen (Oads) species, while the peak at
529.7 eV was ascribed to the surface lattice oxygen (Olatt) species in
the Co3O4 [40,41]. The pioneering review pointed out that the redox
properties are strongly dependent on the activation of molecular
oxygen and the type of surface or lattice oxygen anion. Whereas the
variable oxidation state of the metal cation can tune the oxide ions
by providing surface lattice oxygen anions and pulling oxygen from
the gas phase. As a result, the performance of redox and oxidation
reaction is tuned [42]. Therefore the relationship between the mor-
phology and redox property was studied by detecting the chemical
state of the surface oxygen. In Table 2, results showed that the
lowest surface Oads/Olatt molar ratio (0.69) was achieved over the
Co3O4-C, while the Co3O4-P has the highest surface Oads/Olatt molar
ratio (0.81). The surface Oads/Olatt molar ratio decreased in the
order of Co3O4-P (0.81) ≈ Co3O4-F (0.79) > Co3O4-S (0.74) > Co3O4-
C (0.69), which is almost contrary to the sequence of temperature, in
which the larger consumption peaks of CH4 occur in the CH4-TPSR
profiles (shown in Section 3.5). However the trend is consistent
with their catalytic activities in methane combustion (shown in
Fig. 8. The catalytic activities of (a) Co3O4-C, (b) Co3O4-S, (c) Co3O4-P, (d) Co3O4-F for
methane combustion as the function of temperature. Reaction conditions: 0.2 vol.%
CH4 and air as balance gas, with a GHSV of 110,000 h−1
.
pronounced peak of CO2 occurs, which is probably assigned to the
dominant reduction of Co3O4 with CH4 from Co2+ species to Co0.
CO and H2 were formed by the reforming of CH4 with CO2 or H2O as
well as CH4 cracking over the formed Co0 species at high tempera-
ture [14]. As seen in Fig. 7(C)–(E), similar peaks are detected in the
profiles of CH4-TPSR for other three samples. For Co3O4-S, Co3O4-
P and Co3O4-F, the weak peaks of CO2 are centered at 592.32 ◦C,
586.88 ◦C and 545.27 ◦C, and the intense peaks of CO2 are located
at 731.32 ◦C, 724.88 ◦C and 683.10 ◦C, respectively. By comparison,
it can be seen that the reduction temperatures of Co3O4 with CH4
from Co3+ species to Co2+ as well as from Co2+ species to Co0 are
both in the order of Co3O4-C > Co3O4-S > Co3O4-P > Co3O4-F. The
more adsorbed oxygen exists, the more prone complete oxidation is
to occur and higher catalytic activity will be achieved. TPSR results
indicated that a large amount of surface adsorbed oxygen exists in
as-prepared catalysts. The redox abilities of as-synthesized Co3O4
catalysts are in the order of Co3O4-C < Co3O4-S < Co3O4-P < Co3O4-
F. The results were primarily consistent with the surface Oads/Olatt
molar ratios seen in Table 2 (in Section 3.6). Moreover, the adsorbed
oxygen is also associated with the catalytic performance. Gen-
erally, the surface lattice oxygen was the main species involved
in the hydrocarbon oxidation at high temperature whereas the
adsorbed surface oxygen was responsible for the hydrocarbon oxi-
dation at low temperature [44]. Owing to low reaction temperature
for methane combustion, the adsorbed surface oxygen is predomi-
nant [45,46] Maybe the case is different for CO or other hydrocarbon
oxidation.
Section 3.6). Generally, lattice oxygen is typically involved in selec-
2−
tiv−e oxidation reactions, while electrophilic species such as O2
,
O2 and O− participate preferentially in the complete oxidation
[43]. Thus, higher surface Oads/Olatt molar ratio corresponds to
higher redox properties for as-prepared Co3O4. Further analysis
will be given in combination with the CH4-TPSR results.
3.5. CH4-TPSR measurements
CH4-TPSR measurements were performed to investigate the
redox abilities of as-synthesized Co3O4. The profiles of CH4-TPSR
are shown as Fig. 7(A)–(E). In Fig. 7(A), the evident peaks are cen-
tered 755.47 ◦C, 731.32 ◦C, 724.88 ◦C and 683.10 ◦C for Co3O4-C,
Co3O4-S, Co3O4-P and Co3O4-F, respectively. The mass chro-
matograms inserted in Fig. 7(B)–(E) show that CO2, CO and H2 were
detected during the reaction of CH4-TPSR with all the samples. In
Fig. 7(B), a weak peak of CO2 generated is seen at 601.64 ◦C, and
no CO and H2 were detected. The peak is possibly attributed to
the dominant reduction of Co3O4 with CH4 from Co3+ species to
Co2+. CO and H2 appear at the temperature of 755.47 ◦C, and more
3.6. Catalytic methane combustion
Fig. 8 shows the activities of Co3O4 catalysts in methane com-
bustion. During the experiments of methane combustion, no CO

Z. Chen et al. / Applied Catalysis A: General 525 (2016) 94–102
101
Table 2
BET surface areas, surface Oads/Olatt molar ratio, and catalytic activity of different Co3O4 for CH4 methane under different condition.
Samples
Td (◦C)
Td (◦C)
Td (◦C)
Tw (◦C)
BET (m2/g)
surface Oads/Olatt molar ratio
10
50
90
50
Co3O4-F
Co3O4-P
Co3O4-S
Co3O4-C
1%Pt/␥-Al2O3 [50]
Pd0.65Pt0.35/Al2O3 [51]
253
254
267
287
346
347
354
390
360
395
428
428
439
466
375
375
387
394
32.21
31.26
48.83
28.86
0.79
0.81
0.74
0.69
Td referred to the temperature under the reaction condition: 0.2 vol.% CH4, air as balance gas, GHSV = 110,000 h−1. Tw referred to the temperature under the reaction condition:
0.4 vol.% CH4, 10 vol.% water vapor, air as balance gas, GHSV = 80,000 h−1
.
and H2 were detected. The selectivity of CH4 towards CO2 remained
always at 100% in the studied temperature range. The tempera-
tures at 10%, 50% and 90% of methane conversions (T10, T50 and
T90) are presented in Table 2. Co3O4-F and Co3O4-P exhibited the
same catalytic performances in methane combustion. The activi-
ties of Co3O4-F and Co3O4-P were higher than those of Co3O4-S
and Co3O4-C. In case of Co3O4-S and Co3O4-C the T50 values were
354 ◦C and 390 ◦C, increasing by 8 ◦C and 44 ◦C comparing to that
of Co3O4-F sample, respectively. Meanwhile the T90 also increased
by 11 ◦C and 38 ◦C, respectively. Obviously, the catalytic activ-
ity decreased in the following sequence: flower-like ≈ hexagonal
plate-like > hexagonal sheet-like > cubical. As previously stated, the
catalytic activity is dependent on the exposed crystal plane of the
as-synthesized Co3O4 with different morphologies. For Co3O4-S,
Co3O4-P and Co3O4-F, the dominantly exposed {111} crystal plane
is more active for methane combustion than the {100} one mainly
exposed in the Co3O4-C. The catalytic activity of Co3O4-S is a little
bit inferior to Co3O4-P and Co3O4-F even though the former has
much higher BET surface area than those of the latter, which is
partly because there is a small amount of cubical Co3O4 attached
on the Co3O4-S as seen in Fig. 5(D). Owing to the existence of
the less active species of cubical Co3O4 exposed dominantly {100}
plane, the activity of Co3O4-S is lower. It is further confirmed the
{111} crystal plane is more active than the {100} plane in methane
combustion. Previous studies [14,47,48] found that the density of
atoms in the surface is a crucial parameter for surface reactiv-
ity. More open surface and unsaturated bonds can promote the
reactivity of the surface. For example, atoms located at terraces,
edges, kinks, and vacancies are generally low-coordinated and have
been considered to be the active sites [48,49]. Because the {111}
plane is more open and has more unsaturated bonds than the
{100} plane, the {111} plane is more active in methane combus-
tion. Compared with the noble metal Pd/Pt/Pd-Pt-based catalysts
[50,51], the as-synthesized Co3O4 showed superior performance
in methane combustion. As shown in Table 2, the T50 values of
Co3O4-S, Co3O4-P and Co3O4-F catalysts are lower than the min-
imum temperatures reported for certain noble metal catalysts. The
Co3O4-C possesses the highest T50 value (390 ◦C) of all four Co3O4
samples, which is still lower than that of Pd0.65Pt0.35/Al2O3 catalyst
reported. It is promising that the as-prepared Co3O4 catalysts with
the specified morphologies could substitute for noble metal cat-
alysts in methane combustion. Besides, the as-synthesized Co3O4
catalysts were also evaluated under the reaction condition (2 vol.%
CH4 and air as balance gas, with a GHSV of 40,000 h−1) reported
in the literature [14]. Results showed that T10, T50 and T90 val-
ues of Co3O4-F are ca. 258 ◦C, 308 ◦C and 352 ◦C, respectively.
While for the Co3O4 sheets catalyst prepared by Li and cowork-
ers with the highest activity in methane combustion, its T10, T50
and T90 are 273 ◦C, 314 ◦C and 356 ◦C, respectively. Compared with
Co3O4 sheets, the as-prepared Co3O4-F in the present study exhibits
lower methane conversion temperature and the higher catalytic
activity in methane combustion. The results obtained from the
proximately real exhaust conditions are shown in Table 2 and
Fig. 9. As seen in Table 2, the prepared Co3O4 samples showed
Fig. 9. The catalytic activities of (a) Co3O4-C, (b) Co3O4-S, (c) Co3O4-P, (d) Co3O4-F
for methane combustion as the function of temperature with water vapor. Reac-
tion conditions: 0.4 vol.% CH4, 10 vol.% H2O, and air as balance gas, with a GHSV of
80,000 h−1
.
high reactivity even at more severe conditions. Generally, the
actual moisture content of vented methane/air mixture is ca. 4 vol.%
[52]. There is only a slight decrease in the T50 value compared to
the one obtained without water. Furthermore, it can be seen in
Fig. 9 that the sequence of catalytic activity (based on T50) of the
four samples in methane combustion was flower-like ≈ hexagonal
plate-like > hexagonal sheet-like > cubical, which is identical to that
of dry gas.
4. Conclusions
Flower-like, hexagonal plate-like, hexagonal sheet-like and
cubical Co3O4 were synthesized via a hydrothermal method. The
dominantly exposed plane was {111} in the flower-like, hexagonal
plate-like and hexagonal sheet-like Co3O4 samples whereas {100}
plane in the cubical Co3O4. The higher catalytic activity was found
with Co3O4 catalysts containing {111} planes which confirms the
effect of crystal planes on the catalytic performance. The flower-
like, hexagonal plate-like and hexagonal sheet-like Co3O4 catalysts
show very high catalytic activities, which even can be comparable
to certain noble catalysts and in addition relatively higher catalytic
activities than cubical Co3O4 catalyst with/without the presence of
water. The controllable synthesis of different shape of nanocrys-
tal catalysts exposing uniform crystal planes will provide broad
prospects in heterogeneous catalysis.
Acknowledgments
This research was supported by a grant from the National Nat-
ural Science Foundation of China (21276250) and National key

102
Z. Chen et al. / Applied Catalysis A: General 525 (2016) 94–102
research and development project (2016YFC0204305). The help-
ful suggestions for the revision of the manuscript provided by the
anonymous reviewers are gratefully acknowledged.
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