Exploring the Effect of Co3O4 Nanocatalysts with Different Dimensional Architectures on Methane Combustion

Article abstract of DOI:10.1002/cctc.201501056

We fabricated Co₃O₄ catalysts with different spatial structures, such as zero-dimensional (nanoparticles), one-dimensional (nanorods), two-dimensional (nanoplates), and three-dimensional (mesoporous and microporous) structures, for methane combustion. The Co₃O₄ catalysts with different dimensional architectures demonstrated different activities for the breaking of the C-H bond of methane. In particular, Co₃O₄ with 2 D structure gave rise to the highest activity among all the samples, in which methane could be initially ignited below 200 °C and completely converted to CO₂ at 375 °C. This activity is attributed to the collective contribution from all the exposed high-index planes of 2 D Co₃O₄ and to more surface-active species being formed on 2 D Co₃O₄. In this dimension: The effect of the structure of Co₃O₄ on methane combustion was explored, by studying Co₃O₄ nanoparticles (0 D), nanorods (1 D), nanoplates (2 D), and mesoporous and microporous structures (3 D). 2 D Co₃O₄ gave rise to the highest activity among all the samples, attributed to a collective contribution from the exposed high-index planes of 2 D Co₃O₄ and from more surface-active species formed on the 2 D Co₃O₄.

Full text of DOI:10.1002/cctc.201501056

DOI: 10.1002/cctc.201501056
Communications
Exploring the Effect of Co O Nanocatalysts with Different
3
4
Dimensional Architectures on Methane Combustion
[a, b]
[a, b]
[a]
[a]
[a]
Yongnan Sun,
Jingwei Liu,
Jianjun Song, Shuangshuang Huang, Nating Yang,
[a]
[a, c]
[a, c]
Jun Zhang, Yuhan Sun,*
and Yan Zhu*
We fabricated Co O catalysts with different spatial structures,
pursued. Many efforts have been made to obtain perovskite
3
4
[
4]
[5]
[2,6]
such as zero-dimensional (nanoparticles), one-dimensional
nanorods), two-dimensional (nanoplates), and three-dimen-
oxides, hexaaluminates, and transition metal oxides
for
(
methane combustion at low temperature. Co O is one of the
3
4
sional (mesoporous and microporous) structures, for methane
combustion. The Co O catalysts with different dimensional ar-
most intensively investigated transition metal oxides owing to
its excellent performance in various catalytic processes, for ex-
3
4
[
7]
[8]
chitectures demonstrated different activities for the breaking
ample, CO oxidation, NO reduction, and the oxygen reduc-
x
[
9]
of the CÀH bond of methane. In particular, Co O with 2D
tion reaction (ORR). The properties of as-synthesized Co O4
3
4
3
structure gave rise to the highest activity among all the sam-
mainly depend on its shape, surface structure, crystal phase,
crystal size, and dimensionality. It has been reported that, in
the catalytic combustion of methane, Co O nanosheets en-
ples, in which methane could be initially ignited below 2008C
and completely converted to CO at 3758C. This activity is at-
2
3
4
tributed to the collective contribution from all the exposed
high-index planes of 2D Co O and to more surface-active spe-
closed by {112} planes exhibit higher activity than the corre-
sponding nanobelts with exposed (011) facets or nanocubes
3
4
[
10]
cies being formed on 2D Co O .
with exposed (001) facets.
Co O nanotubes bounded by
3 4
3
4
{
112} planes show excellent activity and good stability com-
[
11]
pared with Co O nanorods with exposed (110) facets. So
3
4
Natural gas is widely used in power generation, chemicals, and
vehicle fuels. The main component of natural gas, methane, is
far, however, Co O as a heterogeneous catalyst with controlled
3 4
dimensionality has rarely been mentioned in this catalytic reac-
tion.
[
1]
a symmetrical molecule and a very stable hydrocarbon. Most
methane conversions are thermodynamically unfavorable at
low temperatures owing to the positive and large value of the
Gibbs free energy; thus, a high reaction temperature is gener-
ally required. The removal of methane from the environment is
highly challenging for catalyst suppliers, as the high-tempera-
ture combustion leads to undesirable surface reactions and the
In this work, to determine the spatial structure effect of
Co O on methane combustion, we set out to synthesize Co O
4
3
4
3
catalysts with fascinating architectures from 0D (dimensional)
to 3D, such as nanoparticles (0D), nanorods (1D), nanoplates
(2D), and mesoporous and microporous structures (3D). By
comparing their catalytic behaviors for methane combustion,
Co O with 2D structure (nanoplates) gave rise to the highest
[2]
emission of toxic nitrogen oxides (NO ) and CO. Therefore,
x
3
4
the development of active catalysts for low-temperature meth-
ane combustion is crucial to remove methane from the envi-
ronment and limit the emission of toxic gases. Although Pd is
catalytic activity among the five Co O catalysts, that is, meth-
3 4
ane could be initially ignited below 2008C and completely con-
verted at 3758C. This activity can be ascribed to the specifically
exposed (112) facets of the Co O nanoplates and the high re-
[3]
the best known methane combustion catalyst, owing to the
high cost of Pd and declining activity, alternative catalysts
based on non-noble metals or metal oxides are being actively
3
4
activity of the adsorbed oxygen species on the surface of the
Co O nanoplates. Furthermore, although the dominantly ex-
3
4
posed crystal planes of Co O4 catalysts with 3D structures
3
(
mesoporous and microporous) are the (110) and (111) orien-
[
a] Y. Sun, J. Liu, J. Song, S. Huang, N. Yang, Dr. J. Zhang, Prof. Y. Sun,
Prof. Y. Zhu
CAS Key Laboratory of Low-Carbon Conversion Science and Engineering
Shanghai Advanced Research Institute
Chinese Academy of Sciences
tation, respectively, which are similar to those of the 0D Co O4
3
and 1D Co O exposed (111) and (110) facets, there is a smaller
3
4
relative content of adsorbed oxygen species and less active Co
species on the surface of the 3D Co O catalysts, leading to
3
4
No. 99 Haike Road, Shanghai 201210 (P.R. China)
E-mail: zhuy@sari.ac.cn
the rather poor performance of 3D Co O for methane com-
3
4
bustion. The catalytic activity order for CH conversion of these
sunyh@sari.ac.cn
4
catalysts is 2D Co O >1D Co O >0D Co O >3D Co O .
[
b] Y. Sun, J. Liu
3
4
3
4
3
4
3
4
Department of Materials Science and Engineering
Shanghai University
No. 99 Shangda Road, Shanghai 200444 (P.R. China)
The representative morphologies of Co O from the 0D to
3 4
3
D structures are shown in Figure 1. Co O nanoparticles are
3 4
uniform in size, at about 20 nm (Figure 1a1), and the lattice
spacing is 0.281 nm, corresponding to the interplanar distance
of (220) (Figure 1a2). The corresponding fast Fourier transform
[
c] Prof. Y. Sun, Prof. Y. Zhu
School of Physical Science and Technology
Shanghai Tech University, 201210 (P.R. China)
(
FFT) image (inset of Figure 1a2) shows a set of bright spots,
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201501056.
which can be indexed as the [111] zone axis. Moreover, the ex-
ChemCatChem 2016, 8, 540 – 545
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Communications
Figure 1. TEM, HR-TEM and SEM images of Co
3
O
4
nanocrystals: nanoparticles (a1–a4), nanorods (b1–b4), nanoplates (c1–c4), mesoporous (d1–d3), and micro-
porous (e1–e3) materials. Insets in a2, c3, d2, and e2 are the corresponding FFT patterns and insets in a4, b4, c4, d3, and e3 are the corresponding structure
diagrams.
posed planes of the nanoparticles are (001), which are three
planes corresponding to the (31 1¯ ), (11 1¯ ), and (220) crystal
as obtained from transmission electron microscopy (TEM) and
high-resolution transmission electron microscopy (HR-TEM)
(Figure 1a1–a3).
planes with the lattice spacings of 0.242, 0.465, and 0.281 nm,
respectively. Therefore, the morphology of the nanoparticle
can be classified as a truncated octahedron, in which the dom-
inant exposed planes of Co O nanoparticle are {001} and
As shown in Figure 1b1, nanorod Co O is uniformly rod-
3
4
shaped and the average length and diameter of the rods are
40–60 nm and 10 nm, respectively. Figure 1b2 and b3 show
the HR-TEM images of two sections of a single Co O nanorod.
3
4
{
111}. Figure 1a4 shows the model diagram of nanoparticles
3
4
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In Figure 1b2, we can observe ( 1¯ 1 1¯ ) and (111) crystal planes
the {112} planes of 2D Co O with the rich coordinatively un-
3
4
2
+
along the [011] zone axis, which have the same lattice spacing
of 0.463 nm. However, the other section of the nanorod is
along the [001] zone axis, which is reflected by (040) and
saturated Co ion could provide more active sites for catalytic
reactions, which is also supported by the X-ray photoelectron
spectroscopy (XPS) studies below.
(
022) crystal planes with corresponding lattice spacings of
.210 and 0.283 nm. Therefore, by taking these TEM and HR-
TEM images into account (Figure 1b1–b3), the Co O nanorods
Figure 2 shows the N adsorption–desorption isotherms and
2
0
pore size distributions of the as-prepared Co O nanocrystals,
3
4
and their specific Brunauer–Emmett–Teller (BET) surface areas
3
4
are mainly rounded by {011} and {001} planes and can be re-
constructed by the simulation diagram in Figure 1b4.
Figure 1c1 presents the SEM (scanning electron microscopy)
images of a Co O microsphere with a diameter of about 3 mm,
3
4
and its internal structure is clearly observed through the
broken microsphere in Figure 1c2, which shows that the mi-
crosphere is assembled of nanoplates that serve as building
blocks. From the HR-TEM image shown in Figure 1c3, the
(
220) and (13 1¯ ) planes along the [112] zone axis are deter-
mined to have lattice spacings of 0.286 and 0.242 nm, respec-
tively. The fast Fourier transform analysis (inset of Figure 1c3)
exhibits a rhombic spot array corresponding well to the crystal
plane reflections of the Co O nanoplates. Thus, the dominant
3
4
exposed planes of the Co O nanoplates are of (112) orienta-
3
4
tion and its structure diagram can be seen in Figure 1c4.
As shown in Figure 1d1, mesoporous Co O synthesized by
3
4
Figure 2. a) N
tions of Co
mesoporous Co
2
adsorption–desorption isotherms and b) pore-size distribu-
nanoparticles (I), Co nanorods (II), Co nanoplates (III),
(IV), and microporous Co (V).
a nanocasting method using KIT-6 as a hard template can be
observed. The pore size of the mesoporous Co O is about 2–
3
O
4
3
O
4
3 4
O
3
4
3
O
4
3 4
O
3
0
0
nm, and a set of {111} planes with a lattice spacing of
.464 nm and a set of {222} planes with a lattice spacing of
.232 nm were observed for mesoporous Co O (Figure 1d2).
and pore properties are summarized in Table S1 (in the Sup-
porting Information). typical nitrogen isotherm with
a type H1 hysteresis loop was observed for mesoporous Co O ,
3
4
The inset in Figure 1d2 shows the FFT image from the meso-
A
porous Co O4 with [011] direction. Drawn from the lattice
3
3
4
fringes and FFT, it can be seen that the predominantly exposed
planes are {011} in the mesoporous Co O . Figure 1d3 shows
indicating that the sample has mesoporous structural features,
2
À1
and the BET surface area of this sample is 82.2 m g . The mi-
3
4
a schematic diagram of the KIT-6 hard template used to syn-
thesize mesoporous Co O .
croporous Co O4 sample exhibited a type I isotherm with
3
a type H3 hysteresis loop, suggesting the presence of a micro-
3
4
2
À1
Microporous Co O prepared by using NaY zeolite as a hard
porous structure, and its BET surface area is 126.8 m g . It can
be seen that the nitrogen adsorption–desorption data of the
mesoporous and microporous Co O are in good agreement
3
4
template (shown in Figure 1e3) is shown in Figure 1e1–e2,
and the SEM and TEM images of microporous Co O reveal
3
4
3
4
that the microporous Co O is constructed of compactly or-
with the TEM results. In addition, Co O nanoparticles, nano-
3 4
3
4
dered nanoparticles. The lattice spacings of 0.283 nm ((220)
planes) and 0.283 nm ((202) planes) with the included angle of
rods, and nanoplates showed typical type II isotherms with
type H3 hysteresis loops (IUPAC classification), and the mea-
sured specific BET surface areas are 112.6, 111.4, and
6
08 as well as the FFT image (inset of Figure 1e2) illustrate that
2
À1
the exposed facets in the microporous Co O are the (111)
45.5 m g , respectively (Figure 3 and Table S1).
3
4
facets.
The diffraction peaks of the five as-synthesized Co O sam-
To gain insight into the spatial structure effect of Co O cata-
3
4
lysts (from 0D to 3D) on methane combustion, we assessed
their catalytic properties as shown in Figure 3a. It is clearly
seen that the order of catalytic activity for methane conversion
with these catalysts is Co O nanoplates (2D)>Co O nanorods
3
4
ples match well with the cubic spinel Co O (JCPCD Card No.
3
4
4
3-1003, space group: Fd3m) with lattice constants a=8.084 Š.
No additional phases or impurities were detected, as shown in
Figure S1a (in the Supporting Information). Figure S1b pres-
ents a different atom arrangement of the face-centered cubic
3
4
3
4
(1D)>Co O4 nanoparticles (0D)>mesoporous Co O4 (3D)>
3
3
microporous Co O (3D). For comparison purposes, the reac-
3
4
(
{
fcc) Co O nanocrystals with exposed {011}, {011}, {111}, and
tion temperatures, including light-off (initial combustion), T50
(the temperature for 50% CH conversion), and T (the tem-
3
4
112} crystal planes. Given that the {112} planes are more
4
100
open than the other planes, the {112} planes are more reac-
perature for 100% CH conversion), were adopted to evaluate
4
[
10]
tive. In the normal-spinel structure, there are two kinds of
the catalytic performance of the Co O catalysts, as summar-
3
4
3
+
Co cations in Co O : Co in an octahedral coordination and
ized in Table 1. Clearly, plate-like Co O performed the best, de-
3 4
3
4
2
+
[7,12]
2
À1
Co in a tetrahedral coordination.
It has been reported
spite having the smallest surface area (45.5 m g ), giving the
2
+
o
that the amount of coordinatively unsaturated Co in {112}
planes is greater than that of the other three planes. Therefore,
Tlight-off, T , and T values of 175, 263, and 375 C, respectively;
50
100
whereas the microporous Co O gave the worst catalytic activi-
3
4
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Figure 3. a) Methane conversion as a function of reaction temperature on Co
3 4 3 4 3 4 3 4
O nanoparticles, Co O nanorods, Co O nanoplates, mesoporous Co O , and
microporous Co . b) Corresponding kinetic rate data for methane combustion.
3
O
4
Table 1. Surface element compositions, catalytic activities, and methane oxidation rate at 3258C.
Sample
Surface composition
Methane oxidation
Reaction rate
[mmolg s ]
T=3258C
[
a]
[b]
À1 À1
(
molar ratio)
activity
3
+
2+
[a]
Co /Co
Oads/Olatt
Tlight-off [8C]
T
50 [8C]
T100 [8C]
Co
Co
Co
3
3
3
O
O
O
4
4
4
nanoparticles
nanorods
nanoplates
0.85
0.74
0.61
0.86
1.11
1.12
1.49
1.64
0.92
0.85
200
175
175
200
200
285
274
263
340
412
425
400
375
500
600
504
860
1056
150
84
mesoporous Co
microporous Co
3 4
O
3
O
4
[
a] Oads and Olatt are the adsorbed oxygen species and the lattice oxygen, respectively. [b] Conditions: the reagent gas mixture (CH
4
/O
2
/N
2
=1:10:89) was
À1
led over 0.3 g of catalyst in the reactor at a flow rate of 50 mLmin
.
2
À1
ty despite its large surface area (126.8 m g ), for which the
ing the CÀH bond breaking of methane, ultimately resulting in
its complete oxidation to produce carbon dioxide and water.
The peaks below 3508C in Figure 4 are attributed to the de-
temperatures for CH conversion were 200, 412, and 6008C, re-
spectively for light-off, 50%, and 100%. As a result, the struc-
ture effect of Co O catalysts constructed with different dimen-
4
À
sorption of surface-adsorbed oxygen species (such as O2 and
3
4
À
sionality becomes unusually important, giving rise to a change
in catalytic activity towards methane combustion. Kinetic data
further confirmed that the as-prepared Co O catalysts dis-
O ), whereas the peaks beyond 3508C are assigned to the de-
[
9,13]
sorption of lattice oxygen species.
For a high-temperature
o
phase (above about 350 C), the larger is corresponding de-
sorption peak area of the surface oxygen species, the lower is
the catalytic activity for oxidation reaction. Thus, according to
the Figure 4, it can be seen that only the Co O nanoplates
3
4
played a strong structure-dependent activity (Figure 3b), and
the methane conversion rates at 3258C for the five Co O sam-
ples are summarized in Table 1. The specific reaction rate on
Co O nanoplates is the fastest compared with the other sam-
3
4
3
4
have a desorption peak at low temperature, whereas other
3
4
ples; it is about twice that with Co O nanoparticles, seven
3
4
times that with mesoporous Co O , and 12 times that with mi-
3
4
croporous Co O under identical experimental conditions.
3
4
Although the better catalytic performance of the nanoplates
can be mainly ascribed to the major exposed high-index crys-
tal planes such as {112}, this cannot completely account for
the better catalytic performance of the nanorods and nanopar-
ticles compared with the mesoporous and microporous cata-
lysts, as the exposed {011} planes of mesoporous Co O are
3
4
identical to those of the nanorods, whereas microporous Co O4
3
and nanoparticle Co O have the same exposed facets, such as
3
4
(
111). Therefore, besides surface structure, we need now to
consider the active species on the surface of these catalysts.
Oxygen species were studied by using O2 temperature-pro-
grammed desorption (O -TPD) and XPS. In general, the ad-
2
À
À
sorbed oxygen species (O , O ) are more active than the lat-
2
Figure 4. O -TPD profiles of Co O nanoparticles, Co O nanorods, Co O
2
3
4
3
4
3
4
2
À
tice oxygen (O ), in which the former is conducive to promot-
nanoplates, mesoporous Co
3
O
4
, and microporous Co
3
O
4
.
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Co O catalysts have no clear peaks in the low-temperature
mesopores>nanoparticles>nanorods>nanoplates, which is
in agreement with the experimental results obtained for meth-
ane combustion. Therefore, in addition to the exposed surface
structure of Co O catalysts, the active species such as surface-
3
4
region. The possible reason for this is that there is only a trace
content of active oxygen species, so the signal is not detected.
The trend for the desorption peak area of the surface oxygen
species in the high-temperature region over the Co O cata-
3
4
2
+
adsorbed oxygen and Co also play a pivotal role in the cata-
lytic performance for methane combustion.
3
4
lysts is: Co O4 nanorodsꢀCo O nanoparticles<mesoporous
3
3
4
Co O <microporous Co O . In view of the O -TPD results, it is
In summary, Co O with 0D (nanoparticles), 1D (nanorods),
3
4
3
4
2
3
4
worth pointing out that more reactive oxygen species are cru-
cial for methane combustion on Co O nanoplates, compared
2D (nanoplates), and 3D (mesoporous and microporous) struc-
tures were synthesized through precipitation, a hydrothermal
process, and a nanocasting method, respectively, and used as
catalysts to catalyze methane combustion. It has been found
that Co O with 2D structure gave rise to the highest activity
3
4
with the other Co O catalysts.
3
4
XPS is an effective technique to investigate the valence of
the metal oxide and the adsorbed species. In the O1s XPS
spectra in Figure 5a, an asymmetrical peak was recorded for all
the samples, which can be decomposed into three compo-
3
4
among all the samples, on which methane could be initially ig-
nited below 2008C and completely converted at 3758C. The in-
vestigation suggests that not only the exposed high-index
planes of Co O allow this improvement of the catalytic activi-
À
À
nents: superoxide ions O2 (531.8 eV), peroxide ions O
2À
(
530.6 eV), and lattice oxygen O (529.5 eV). It is well known
3
4
ty, but also the surface-active species such as surface-adsorbed
2
+
oxygen and Co are not to be neglected in the role of break-
ing CÀH bond of methane. Study of the catalytic mechanism
will aid in comprehending the improvement of oxide catalytic
activity. Activity control of catalytic reactions by changing the
dimensionality of catalysts should open up an approach to-
wards the design of advanced catalysts for molecule conver-
sion.
Acknowledgements
We are grateful for financial support from the National Natural
Science Foundation of China (21273151, 21301186), Hundred
Talent Program of CAS and Energy Technologies Institute LLP. We
thank Dr. Andrew Haslett from ETI and Prof. Graham J. Hutchings
from Cardiff University for helpful discussions.
Figure 5. a) O1s and b) Co2p XPS spectra of Co
nanorods, Co nanoplates, mesoporous Co
3
O
4
nanoparticles, Co
3
O
4
3
O
4
3
O
4
, and microporous Co
3
O
4
.
À
À
that the surface-adsorbed oxygen species (O2 and O ) are ef-
fective for the oxidation of hydrocarbons at low tempera-
Keywords: Co O · dimensionality · exposed facets · methane
combustion · nanocatalysis
3
4
[
11,14]
tures.
The ratio of the peak intensities of the surface-ad-
sorbed oxygen species to lattice oxygen is 1.12 for the nano-
particles, 1.49 for the nanorods, 1.64 for the nanoplates, 0.92
for the mesoporous, and 0.85 for the microporous materials,
as shown in Table 1. Therefore, taking into consideration these
above ratio values, high activity for methane combustion can
be obtained over nanoplates, which is consistent with the re-
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9
2
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[
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accordance with the XPS spectra. Co O nanoplates gave the
3
4
3
+
2+
lowest value of Co /Co among the samples, whereas micro-
3
+
2+
porous Co O gave the highest Co /Co value. The order for
3
4
2+
3
+
the Co /Co values of these catalysts follows micropores>
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Received: September 25, 2015
Revised: October 29, 2015
2
22.
Published online on December 22, 2015
ChemCatChem 2016, 8, 540 – 545
www.chemcatchem.org
545
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim