Highly dispersed CoOx in layered double oxides for oxidative dehydrogenation of propane: guest-host interactions

Article abstract of DOI:10.1039/c7ra01190c

Co-Al layered-double-oxides (LDOs) derived from inorganic anion (nitrate, carbonate, sulfate and phosphate) pillared Co-Al layered-double-hydroxides (LDHs) were synthesized. These LDO samples exhibited impressive catalytic performances in oxidative dehydr

Full text of DOI:10.1039/c7ra01190c

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Highly dispersed CoOx in layered double oxides for
oxidative dehydrogenation of propane: guest–host
interactions†
Cite this: RSC Adv., 2017, 7, 14846
*
Ming-Xiang Huang, Xin Wu, Xiao-Dong Yi, Guo-Bin Han, Wen-Sheng Xia
*
and Hui-Lin Wan
Co–Al layered-double-oxides (LDOs) derived from inorganic anion (nitrate, carbonate, sulfate and
phosphate) pillared Co–Al layered-double-hydroxides (LDHs) were synthesized. These LDO samples
exhibited impressive catalytic performances in oxidative dehydrogenation of propane (ODHP) at low
temperatures. Characterization techniques such as XRD, TEM, H2-TPR and O2-TPD were used to
measure the physicochemical properties of the samples. The results showed that the LDO samples
consisted of homogeneously mixed cobalt spinel oxides (Co2AlOx) containing both Co(III) and Co(II).
Co(III) instead of Co(II) was linked with the active sites for ODHP at low temperatures. The sulfate and
phosphate anions were preserved in the corresponding LDO samples, unlike the nitrate and carbonate
pillars. Sulfate (phosphate) around the cobalt decreased the size of the spinel oxide particles more
relative to nitrate and carbonate, and so the dispersion of CoOx in the LDO and the selectivity to
propylene in ODHP over these catalysts were improved. Both surface adsorbed oxygen species and
surface lattice oxygen species were significantly found over the phosphate-pillared LDO, but only lattice
oxygen species were available over the sulfate-pillared LDO. Different mechanisms of propane activation
were exhibited over these LDO samples for ODHP.
Received 26th January 2017
Accepted 28th February 2017
DOI: 10.1039/c7ra01190c
rsc.li/rsc-advances
temperatures, such as CO oxidation3 and partial oxidation of
light hydrocarbons.4 Performing the reactions under mild
1. Introduction
conditions can reduce the risk of extensive transformation of the
product, along with the amount of energy wasted, and thus
increase the product selectivity.
Oxidative dehydrogenation of propane (ODHP) into propylene is
an important reaction as an alternative route to the most largely
used pyrolysis processes. Pyrolysis suffers from some drawbacks,
such as thermodynamic limitations, rapid coking and high
reaction temperature.1 However, the target product, propylene, in
ODHP is more active than the reactant, and so is prone to being
further oxidized into products such as CO2 in the presence of
oxygen, resulting in low selectivity toward propylene. Thus the
development of catalysts for ODHP is a great challenge.
Among all the studied catalysts, vanadium and molybdenum-
based catalysts show relatively good performances for ODHP
because of their unique redox properties,2 but high operating
temperatures (above 500 ꢀC) are necessary, resulting in a signi-
cant probability of extensive transformation of the target product,
such as through C–C bond cleavage. Comparatively, cobalt oxides
have been used due to their ability to catalyze reactions at low
Furthermore, researchers have used active-site isolation to
improve the product selectivity for the selective oxidation of light
alkanes. For example, high yields of propylene in ODHP were
obtained over vanadium oxides supported on meso-cellulose
silica foams, due to the increased concentration of dispersed V
species.5 Isolated monomeric VOx species were also introduced
through the incorporation of Sb atoms, leading to enhanced
activity and selectivity.6 A similar strategy of isolating the active
sites was also achieved in cerium phosphate catalysts.7
On the other hand, we noted that hydrotalcite-like layered-
double-hydroxides (LDHs) contain positively charged brucite-like
layers and exchangeable anions in the interlayer, which are con-
ducive to the preparation of highly dispersed catalysts. The general
3+
molecular formula of an LDH is [M2+
M )x/n$mH2O,
x(OH)2](Anꢁ
1ꢁx
where M2+ and M3+ are di- and trivalent metal cations, Anꢁ is
a charge compensating inorganic or organic anion, and x is nor-
mally between 0.2 and 0.4. The M2+ and M3+ cations are uniformly
dispersed in an orderly manner within the layers in the LDH
lattice, and the derived metal oxides (layered-double-oxides, LDOs)
with improved stability and dispersion can be obtained through
thermal treatment of the LDH precursors. LDOs have been widely
State Key Laboratory of Physical Chemistry of Solid State Surfaces, National
Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters,
Fujian Province Key Laboratory of Theoretical and Computational Chemistry,
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian
361005, P. R. China. E-mail: wsxia@xmu.edu.cn; hlwan@xmu.edu.cn; Fax: +86-
592-2183047; Tel: +86-592-2181937
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra01190c
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used as supports or precursors in catalysis.8 Therefore, we here acceleration voltage of 15 kV. The samples were dispersed in
introduced cobalt into an LDO to combine a highly active ethanol and loaded onto a silicon wafer before the test.
component (Co) with a layer-structured material to synthesize
The content of Co, Al, S and P was determined by EDX
a catalyst exhibiting a good dispersion of active sites, i.e., Co–Al spectroscopy. The carbon and nitrogen content was determined
LDO. To the best of our knowledge, this is the rst report on the on an Elementar Vario EL III element analyzer.
use of Co–Al LDO catalysts in ODHP, especially those being pil-
N2 adsorption–desorption isotherms were recorded at ꢁ196 ꢀC
lared by different anions. In this work, the inorganic anion- on automated Micromeritics Tri-Star 3020 apparatus. Surface
pillared Co–Al LDO samples derived from LDHs featuring well areas were calculated using the multipoint BET equation. Pore
dispersed lamellar structures were investigated in ODHP at low size was calculated using the BJH method, using the desorption
temperatures. The textural, redox and structural properties of the branch of the isotherm. The samples were outgassed for 3 h under
catalysts were studied to account for their catalytic performances. vacuum at 300 ꢀC before the measurements were carried out.
Transmission electron microscopy (TEM) characterization of
the samples was performed on an FEI Tecnai-F30 FEG instru-
ment at an acceleration voltage of 300 kV.
2. Experimental section
2.1. Synthesis of the catalysts
Thermal gravimetric and differential thermal gravimetric
(TG-DTG) experiments were performed in air on NETZSCH TG
209 F1 apparatus. The LDH samples were heated from 30ꢁt1o
The Co–Al LDH pillared with nitrate was prepared using a modi-
ed co-precipitation method followed by hydrothermal treat-
ment.9 Typically, 0.08 mmol of Co(NO3)2$6H2O and 0.04 mmol of
Al(NO3)3$9H2O were dissolved into 120 mL of decarbonated
water, and the pH was adjusted to 10 by adding 2 M NaOH
aqueous solution. Care was taken to avoid CO2 contamination
during the synthesis. The suspension was magnetically stirred at
room temperature for 0.5 h, and aerwards was poured into
a Teon-lined autoclave, which was heated at 100 ꢀC in
a convection oven for 24 h. The precipitate was centrifuged,
washed several times with decarbonated water and then dried at
80 ꢀC for 12 h in a vacuum oven. The obtained sample was
800 ꢀC in a owing air stream at a heating rate of 10 ꢀC min
.
The ow rate of air was 20 mL minꢁ1
.
Laser Raman spectroscopy was performed using a Renishaw
UV-vis Raman 1000 system equipped with a CCD detector and
a Leica DMLM microscope. The 532 nm line of an Ar+ laser was
used for excitation.
The Fourier transform-infrared (FT-IR) spectra of the
samples in the form of KBr pellets were recorded at room
temperature on an FT-IR Nicolet Avatar 330 spectrometer.
H2 temperature-programmed reaction (H2-TPR) analysis of
the samples was carried out using ow apparatus with a 5% H2/
ꢁ
referred to as Co–AlNO3 LDH and denoted as CA-N LDH. A co-
Ar mixture owing at 20 mL minꢁ1. The heating rate was 10 C
ꢀ
precipitated sample (named CA-co) was also prepared without
the hydrothermal process and used as a reference sample.
For the preparation of the other anion-pillared Co–Al LDHs,
CA-N LDH in the form of a wet suspension was used, and anion
nitrate exchange was performed with the pertinent anions in
solution. A solution (100 mL) of the ammonium salt of the
relevant anion was added to the Co–Al LDH suspension, and the
solution was stirred for 24 h at room temperature. The precip-
itate was centrifuged, washed several times with decarbonated
minꢁ1. Hydrogen consumption was monitored with a thermal
conductivity detector (TCD) equipped to a Haixin GC-950 gas
chromatograph aer removing the formed water. All the samples
were pretreated in 10% O2/He at 400 ꢀC for 0.5 h before the
measurements.
X-ray photoelectron spectroscopy (XPS) experiments were
performed using an Omicron Sphera II hemispherical electron
energy analyzer at a base pressure of 5.0 ꢂ 10ꢁ9 mbar. The
instrument was equipped with a monochromatic Al Ka radia-
tion source operated at a power of 300 W. The spectra were
corrected using the C 1s signal located at 284.5 eV.
The electron paramagnetic resonance (EPR) spectra of the
samples (100 mg) were measured at ꢁ179 ꢀC on a Bruker EMX
X-band 10/12 spectrometer with a resonance frequency of 9.43
GHz and a modulation frequency of 100 kHz.
ꢀ
water and then dried at 80 C for 12 h in a vacuum oven. The
carbonate-, sulfate- and phosphate-pillared Co–Al LDH samples
were named as CA-C, CA-S and CA-P LDH, respectively.
LDO samples were obtained by calcination of the LDH
ꢀ
samples in air at 400 C for 4 h, and used as the catalysts for
ODHP. A pure Co3O4 catalyst was also prepared using a similar
procedure to that used for CA-co LDO but without the addition of
Al(NO3)3.
NH3 temperature-programmed desorption (NH3-TPD) and
O2 temperature-programmed desorption (O2-TPD) analyses of
the samples were performed with an AutoChem II 2920 equip-
2.2. Characterization of the catalysts
ped with a Balzers Omnistar mass analyzer. In the O2-TPD
ꢀ
X-ray diffraction (XRD) patterns were recorded on a Panalytical experiment, the LDO sample (0.3 g) was calcined at 400 C in
X'pert PRO powder diffractometer. The samples were ground 20% O2/He atmosphere for 1 h, and then cooled to 40 ꢀC. Aer
ꢀ
into a ne powder, loaded as a at layer onto a glass sheet with purging with He at 40 C for 1 h to make the baseline at, the
a square groove and scanned in the 2q range from 5 to 80ꢀ. Cu TPD plot was obtained from 50 to 800 C (heating rate: 10 C
ꢀ
ꢀ
Ka radiation (l ¼ 0.15406 nm) generated at 40 kV and 30 mA minꢁ1). In the NH3-TPD experiment, the LDO sample (0.2 g) was
was used as the X-ray source.
treated at 300 ꢀC in a He atmosphere for 1 h, cooled to 100 ꢀC,
Scanning electron microscopy (SEM) characterization of the and exposed toꢀpure NH3 for 1 h at 100 ꢀC. Aer purging with He
samples was performed on a Hitachi S-4800 FEGSEM microscope for 1 h at 100 C to make the baseline at, the TPD plot was
equipped with an energy dispersive X-ray (EDX) microprobe at an obtained from 100 to 800 C (heating rate: 10 C minꢁ1).
ꢀ
ꢀ
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CO2-TPD analyses of the samples were performed in a gas
The carbon balance was satisfactory in all runs with an error
ow system equipped with a HIDEN QIC-20 mass analyzer. The margin of ꢃ5%.
LDO sample (0.15 g) was calcined at 400 ꢀC in a He atmosphere
ꢀ
for 0.5 h, cooled to 50 C, and exposed to pure CO2 for 1 h at
3. Results and discussion
3.1. Textural properties and chemical composition of LDH
50 ꢀC. Aer purging with He for 1 h at 100 ꢀC to make the
baseline at, the TPD plot was obtained from 50 to 850 ꢀC
(heating rate: 10 C minꢁ1).
ꢀ
The successful preparation of the Co–Al LDH samples pillared
with different anions was conrmed by their XRD patterns as
shown in Fig. 1. A series of sharp and symmetric characteristic
reections are seen for all of the hydrothermally treated
samples (CA-N, CA-C, CA-S and CA-P LDH), indicating that
hexagonal Co–Al hydrotalcite-like structures are the only
phase.9 The peaks for CA-co LDH are relatively widened, indi-
cating that the structure of CA-co LDH is not as well dened as
for the other counterparts. From the S(T)EM images in Fig. S1 of
the ESI,† the CA-N, CA-C, CA-S and CA-P LDH samples are
further found to be hexagonal plate-like sheets with a diameter
of ꢄ150 nm, a thickness of ꢄ15 nm, and well-dened grain
boundaries, but the CA-co LDH sample contains stacked
amorphous particles with sizes in the range of 15–25 nm.
The lattice parameters (a and c) of the samples are presented
in Table 1. The a value is almost the same for all of these LDH
samples, while the c value is 2.33, 2.32, 2.43 and 2.36 nm for the
nitrate, carbonate, sulfate and phosphate intercalated LDH,
respectively. Furthermore, the interlayer spacing decreased in
the order of CA-S > CA-P > CA-N > CA-C, indicating that the
isolation of metal ions by the anion interlayer changed
depending on the pillared anion. These interlayer anions were
identied by FT-IR (Fig. S2, ESI†).10,11
C3H6-TPD and C3H8-TPD analyses of the samples were per-
formed in a gas ow system equipped with a HIDEN QIC-20
mass analyzer. The LDO sample (0.2 g) was degassed at
400 ꢀC in an Ar atmosphere for 0.5 h, cooled to 30 ꢀC, and
ꢀ
exposed to pure C3H6 or C3H8 for 0.5 h at 30 C. Aer purging
ꢀ
with Ar for 1 h at 30 C to make the baseline at, the TPD plot
was obtained from 30 to 800 C (heating rate: 10 C minꢁ1).
Surface acidity properties were determined by monitoring the
adsorption of pyridine in a Thermo Nicolet Nexus 470 FT-IR
spectrometer. The samples were degassed in situ in a cell with
ꢀ
ꢀ
ꢀ
CaF2 windows at 400 C for 0.5 h under vacuum. The pyridine
vapor was adsorbed at 100 C and the spectrum recorded aer
ꢀ
degassing.
2.3. Catalytic reaction
The catalytic performances of the samples were investigated with
a mixture of propane, oxygen and helium (C3H8 : O2 : He ¼
1 : 1 : 4, the molar ratio) as a reaction feed. The reaction was
carried out in a xed-bed quartz tubular reactor at atmospheric
pressure and at a total ow rate of 50 mL minꢁ1. The reactor was
packed with 100 mg of LDO sample diluted with 100 mg of quartz
sand (40–60 mesh size) between quartz wool plugs. The tempera-
ture was measured by a thin quartz wall thermocouple well xed in
the middle of the catalytic bed. The performances of the catalysts
were evaluated over the temperature range of 200–550 ꢀC with a gas
hourly space velocity (GHSV) of 30 000 mL gꢁ1 hꢁ1. The products
were analyzed by online gas chromatographs (Shimadzu GC-14C;
Shimadzu GC 2010), equipped with an alumina column and
a carbon sieve column (for the detection of CH4, O2/He, CO and
CO2) and an RT-PLOTQ column (for oxygenates). The conversion of
propane and oxygen (XC H and XO ), the selectivity for different
The chemical composition of the LDH samples was
measured by elemental chemical analysis. The value of the Co/
Al ratio for CA-co, CA-N, CA-C, CA-S and CA-P LDH is 1.91, 2.02,
1.97, 1.93 and 1.94, respectively, close to the expected value (Co/
Al ratio ¼ 2). The value of the Al/anion ratio for CA-N, CA-C, CA-S
and CA-P LDH is 1.16, 2.04, 2.17 and 2.90, respectively. Thus,
the chemical formula for the LDH samples could be derived (see
Table 1).
3
8
2
3.2. Physicochemical properties of the LDO catalysts
products (Si) and the yield of propylene (YC H ) were dened as:
3
6
3.2.1. Phase changes from LDH to LDO. In the thermo-
gravimetric measurements (Fig. S3, ESI†), it was observed that
the LDH samples transitioned to LDOs at 400 ꢀC. Furthermore,
n
C3H8;out
XC H ¼ 1 ꢁ
ꢂ 100%
3
8
n
C3H8;in
n
O2;out
XO ¼ 1 ꢁ
ꢂ 100%
2
n
O2;in
cini
X
Si ¼
ꢂ 100%
cini
Y
¼ X
ꢂ S
C3H6
C3H8
C3H6
where nC H and nO are the moles of propane and oxygen in the
3
8
2
reactants (in) and products (out), ci is the number of carbon
atoms in product i, and ni is the moles of product i.
Fig. 1 XRD patterns of the LDH samples: CA-co (1), CA-N (2), CA-C (3),
CA-S (4), and CA-P (5).
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Table 1 Lattice parameters (LP) and chemical composition of the LDH and LDO samples
Co/Al molar
ratioc
Al/anion molar
ratio
LP-LDOb/nm
LP-LDHb/nm
Sample
Formulaa
a
c
a
LDH
LDO
LDH
LDO
CA-co
CA-N
CA-C
CA-S
CA-P
[Co0.66Al0.34(OH)2](NO3)0.24(CO3)0.05$0.64H2O
[Co0.67Al0.33(OH)2](NO3)0.28(CO3)0.025$0.35H2O
[Co0.67Al0.33(OH)2](CO3)0.165$0.78H2O
[Co0.66Al0.34(OH)2](SO4)0.15(CO3)0.02$0.51H2O
[Co0.66Al0.34(OH)2](PO4)0.113$0.55H2O
0.304
0.307
0.307
0.306
0.306
2.270
2.330
2.320
2.430
2.360
0.795
0.797
0.799
0.797
0.798
1.91
2.02
1.97
1.93
1.94
1.93
2.00
2.01
2.05
2.03
1.39
1.16
2.04
2.17
2.90
—
—
—
2.25
2.96
a
b
The amount of H2O was determined by TG-DTG experiments. Determined by XRD, a ¼ 2d(110), c ¼ 3dspace, dspace ¼ (d(003) + 2d(006))/2.
c
Determined by EDX or elemental analyses.
Table 2 Textural properties of the LDH and LDO samples
Specic surface area/
Average pore
diameter/nm
Pore volume/
cm3 gꢁ1
Size of spinel oxide particles
for LDO/nm
m2 gꢁ1
a
Sample
LDH
LDO
LDH
LDO
LDH
LDO
Dav,XRD
Dav,TEM
CA-co
CA-N
CA-C
CA-S
CA-P
172.0
56.6
58.6
30.6
55.8
100.1
137.1
158.4
115.7
110.6
5.2
15.4
18.5
20.1
17.0
11.6
11.6
9.8
9.4
12.8
0.32
0.28
0.29
0.17
0.27
0.40
0.49
0.49
0.24
0.28
14.7
10.1
6.3
14.2 ꢃ 3.3
7.6 ꢃ 2.8
5.8 ꢃ 1.0
4.3 ꢃ 0.7
3.9 ꢃ 0.6
5.0
N.D.b
Calculated from the position of the reection of the (311) facet in Fig. 3 using the Scherrer formula. b The Scherrer formula is not applicable.
a
sulfate and phosphate decomposed above 600 ꢀC, while nitrate
and carbonate decomposed below 300 C. Thus, the interlayer
3.2.2. Structure and composition. Following dehydrox-
ylation, the LDH samples with [Co0.66–0.67Al0.33–0.34(OH)2]n+
ꢀ
anion (sulfate and phosphate) for the CA-S and CA-P samples
was not removed during the preparation of the LDO from the
LDH via calcination at 400 ꢀC. This was further conrmed by the
elemental chemical analysis of the LDO samples. The values of
the Co/Al ratio and the Al/anion ratio (for CA-S and CA-P) for the
LDOs are similar to those for the LDHs (Table 1).
cation sheets transitioned to LDOs, along with changes in their
structures. The pore diameter of the CA-co sample increased
from 5.2 to 11.6 nm following the transition of the LDH to an
LDO; and that of the other samples decreased from 15–20 to 9–
13 nm (the specic surface area and pore volume showed the
opposite trend) (Table 2). This suggested that the structures of
Fig. 2 TEM images of the LDO samples.
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a very homogeneous and stable spinel-like mixed oxide,
Co(Co,Al)2O4, in which Al isomorphically substitutes Co(III) of
the Co3O4 phase,12 as it is difficult to identify if the phase is
Co3O4, CoAl2O4, or Co2AlO4 because these phases exhibit
similar XRD reections. The Raman spectra of the LDO samples
in Fig. 4 show the characteristic peaks of F12g, Eg, F22g, F32g and A1g
vibration modes of Co3O4-like crystals (194, 480, 521, 617 and
684 cmꢁ1, respectively), with a blue shi compared to the
spectrum of Co3O4. This is signicantly different from the peaks
of CoAl2O4 (198, 412, 480, 519, 619, 690, and 753 cmꢁ1).13 Thus,
in combination with the chemical formula of the precursor LDH
and the fact that no peak related to amorphous Al2O3 was
observed in the Raman spectra,14 we think that the bulk phase
of the LDO samples consists of a homogeneous mixed cobalt
spinel, Co2AlOx, containing both Co(II) and Co(III) species.
Furthermore, the Co 2p XPS spectrum of the LDO samples is
shown in Fig. 5. The spin–orbital splitting peaks at 780.3 and
795.4 eV were assigned to Co 2p3/2 and Co 2p1/2, respectively,
along with an energy separation (Co 2p3/2–Co 2p1/2) of 15.1 eV.
These features are similar to the reported data for Co3O4,15 and
related to the Co(Co,Al)2O4 spinel phase.
Fig. 3 XRD patterns of the LDO samples: CA-co (1), CA-N (2), CA-C
(3), CA-S (4), CA-P (5) and Co3O4 (6).
The presence of sulfate and phosphate in the CA-S and CA-P
LDO samples was substantiated by the appearance of S 2p and P
2p peaks at the binding energies of 168.4 and 133.2 eV (middle
2ꢁ
part of Fig. 5), which was assigned to chemisorbed SO4 and
3ꢁ
Fig. 4 Raman spectra of the LDO samples: (1) CA-co, (2) CA-N, (3)
CA-C, (4) CA-S, (5) CA-P and (6) Co3O4.
PO4 , respectively.16,17 These anions caused the Co 2p3/2 and Co
2p1/2 peaks for the CA-S and CA-P LDO samples to be shied to
a higher binding energy relative to the CA-N and CA-C LDO
samples (le part of Fig. 5).
the samples displayed some shrinkage from the LDH to the
LDO, except for the CA-co sample. Moreover, the TEM images in
Fig. 2 showed that the CA-S and CA-P LDOs were plate-like
sheets similar to the corresponding LDHs, but the other LDO
samples were more or less converted to nanoparticles. Previous
in situ investigation of the thermal decomposition of Co–Al
hydrotalcite by Javier Perez-Ramirez showed that, during calci-
nation, the oxidation of Co(II) and diffusion of the Co(III) cation,
as well as the formation of O–M–O bonds between the layers, led
to the collapse of the layered structure.12 This process could be
hindered by the presence of anions.
By deconvoluting the Co 2p3/2 peaks of the LDO samples
according to reported data,18 we found that the ratio of Co(III)/
Co(II) on the surface of CA-co, CA-N, CA-C, CA-S and CA-P LDO
was 1.39, 1.10, 1.11, 1.12 and 1.23, respectively (right part of
Fig. 5). The highest amount of Co(III) was found on the surface
of CA-co LDO, because less Co(III) is isomorphically substituted
by Al in this sample. In the hydrothermally treated samples, CA-
N, CA-C, CA-S and CA-P LDO, the amount of Co(III) increased
gradually in the sequence of CA-N < CA-C < CA-S < CA-P. Thus,
host–guest interactions led to a relatively higher amount of
Co(III), especially for CA-P LDO.
The XRD patterns of the LDO samples are shown in Fig. 3,
where the characteristic peaks are assigned as a cubic cobalt
spinel phase. It seems that this phase can be assumed to be
Additionally, FT-IR spectra could provide some information
on the bonding state and the coordination around the metal ion
Fig. 5 XPS spectra of the LDO samples: CA-co (1), CA-N (2), CA-C (3), CA-S (4) and CA-P (5).
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3.2.3. Redox properties of the sample and its interaction
with oxygen. The pillared anions around the highly dispersed
spinel oxides would have a great effect on the redox properties
of the LDO samples. The H2-TPR proles of the LDO samples
are shown in Fig. 7. As seen in Fig. 7, the reduction of the LDO
samples occurred in two distinct steps in the temperature range
ꢀ
of 100–900 C. The rst consumption of H2 took place at 200–
450 ꢀC, and the second one occurred at temperatures higher
ꢀ
than 450 C, corresponding to the reduction of Co(III) to Co(II)
and Co(II) to Co, respectively.23 In particular, for the rst step,
the peak temperature was ꢄ410 ꢀC for CA-co and CA-N LDO,
Fig. 6 FT-IR spectra of the LDO samples: CA-co (1), CA-N (2), CA-C
(3), CA-S (4), CA-P (5) and Co3O4 (6).
ꢀ
ꢀ
and shied down to ꢄ355 C and ꢄ330 C for CA-C and CA-P
LDO, respectively; for the second step, the peak temperature
decreased in the order of CA-co (735 ꢀC) > CA-N (729 ꢀC) > CA-C
(721 ꢀC) > CA-P (700 ꢀC). The order was the same as that for the
particle size of the samples, i.e., the samples with a smaller
particle size were more easily reduced. Wang et al.24 obtained
similar results, showing that the reduction of small sized Co3O4
particles was easier than that of large ones.
in the LDO samples. As shown in Fig. 6, the bands at 669 and
566 cmꢁ1 are typical of a normal II–III spinel compound.19 The
former (669 cmꢁ1) was attributed to the M–O stretching vibra-
tion mode for the tetrahedrally coordinated divalent metal ions,
and the latter (566 cmꢁ1) was assigned to the octahedrally
coordinated trivalent metal ion. These two bands for CA-co LDO
appeared at 673 and 574 cmꢁ1, with a blue shi relative to the
other LDO samples. This was related to the surroundings of the
metal ions in CA-co LDO, which has a relatively lower Co/Al
ratio. Interestingly, the presence of sulfate and phosphate for
CA-S and CA-P LDO was supported by the broad bands at 1124
and 1069 cmꢁ1 assigned to n3 mode S–O and P–O stretching
For CA-S, a different H2-TPR process occurred. It seemed that
Co(III) ꢀwas reduced along with Co(II). Only a broad peak at
ꢄ535 C was situated between the Co(III) and Co(II) reduction
peaks, suggesting that Co(III) was more difficult to reduce in CA-
S LDO than in the other LDO samples. This could be associated
with its stronger host–guest interaction (spinel oxide–anion
interlayer) relative to the other LDO samples, as suggested by
the above XPS results. A similar case was found for the reduc-
tion of sulfate-modied Co/Al2O3 catalysts.25
2ꢁ
3ꢁ
vibration of tetrahedral SO4 and PO4 , respectively.20,21 The
M–O stretching vibration bands were weakened, but the peak
positions did not change. This can be explained by the lowered
dipole of M–O caused by the oxygen of the anions coordinated
to the metal.22 Thus, the divalent Co(II) and trivalent Co(III) or
Al3+ were located at the tetrahedral and octahedral sites,
respectively, and metal ions were connected with the oxygen of
the pillared anions (sulfate or phosphate) for CA-S and CA-P
LDO.
The interaction of the spinel oxides with oxygen was
measured with O2-TPD. As shown in Fig. 8, the oxygen desorp-
tion curves for the LDO samples were divided into two regions,
ꢀ
ꢀ
200–400 C and above 400 C, which corresponded to desorp-
tion of surface adsorbed oxygen species and surface lattice
oxygen, respectively.26 For the CA-co, CA-N, CA-C and CA-P LDO
samples, a small broad peak appeared at 200–400 ꢀC, assigned
to the surface adsorbed oxygen species; and a wide broad peak
was found at above 400 ꢀC, assigned to the surface lattice
oxygen. However, for the CA-S LDO sample, the two signicant
The metal was coordinated by the pillared anions, meaning
that the cobalt spinel oxides of the samples were isolated. The
size of the isolated cobalt spinel oxides should be relatively
small. As shown in Table 2, the size of the spinel oxide particles
for these LDO samples decreased from CA-co, to CA-N, CA-C,
CA-S and CA-P LDO (Dav,XRD: 14.7, 10.1, 6.3, 5.0, N.D. nm;
Dav,TEM: 14.2, 7.6, 5.8, 4.3, 3.9 nm, respectively), and the spinel
oxide for CA-P LDO displayed the smallest crystallite size. Both
the XRD and TEM measured values are consistent. Moreover,
the TEM images showed that the uniformity and dispersion of
the spinel oxides increased from CA-co, to CA-N, CA-C, CA-S and
CA-P LDO. The uniformity and size of the spinel oxides for CA-C
LDO were not much different from those of CA-S and CA-P LDO,
indicating that the carbonate anion also plays a signicant role
in improving the dispersion of the spinel oxide, though the
carbonate was not found in CA-C LDO, which was prepared by
calcination of CA-C LDH. However, Javier Perez-Ramirez12
found that the collapse of the layered structure occurred before
complete decarbonation during calcination, meaning that the
carbonate anion would physically obstruct the dispersion of the
spinel oxides. Thus, the dispersion of the spinel oxides for these
LDO samples is closely related to the pillared anions.
ꢀ
ꢀ
peaks appeared at 530 C and 820 C, ascribed to the surface
lattice oxygen and the decomposition of sulfate, respectively,
meaning that surface adsorbed oxygen species were observed
for the CA-S LDO sample. These results are in agreement with
a recent report that sulfate can hinder the formation of oxygen
Fig. 7 H2-TPR profiles of the LDO samples: CA-co (1), CA-N (2), CA-C
(3), CA-S (4) and CA-P (5).
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Co(III) and Co(II) for CA-S LDO emerged at around 500 ꢀC. Li et al.30
found that Co(III) species with spinel-like structures were
responsible for the ODHP activity. In our work, the ratio of Co(III)/
Co(II) was tuned by the pillared anions in the LDOs, and extra
Co(III) on the LDO (especially for CA-P LDO) could facilitate the
redox cycling of Co(III)/Co(II) and oxygen activation.
The ignition point of ODHP was 250, 260, 260, 250, 360, and
275 ꢀC over Co3O4, CA-co, CA-N, CA-C, CA-S and CA-P LDO,
respectively. This is related to the highly active CoOx over CA-co,
CA-N and CA-C LDO, which corresponded to the weakened
peaks at 225–275 ꢀC shown in the inset of Fig. 7. Over CA-P LDO,
the species that activate propane at low temperatures could be
surface adsorbed oxygen species, which were linked with the
peak located at 330 ꢀC in the O2-TPD measurements (Fig. 8). For
CA-S LDO, the reduction peak of Co(III) shied up to a higher
temperature (ꢄ500 ꢀC), and there were only lattice oxygen
species available. The oxidation activity of CA-S LDO is the
lowest among the investigated LDO samples, and so the igni-
tion point of ODHP over CA-S LDO is relatively high.
Fig. 8 O2-TPD profiles of the LDO samples: CA-co (1), CA-N (2), CA-C
(3), CA-S (4) and CA-P (5).
vacancies and reduce the mobility of surface oxygen species on
Pt/MnOx–CeO2 catalysts.27 As the peak area for the surface
lattice oxygen was far greater than that for the surface adsorbed
oxygen species, surface lattice oxygen was predominant in these
samples.
Comparing the catalytic performances over the CA-co, CA-N,
CA-C, CA-S and CA-P LDO samples, we found that the highest
yield of propylene at the investigated temperatures was 3.8, 4.4,
6.6, 8.3 and 10.0%, respectively, and the selectivity to propylene
increased in the same sequence. The structures of these samples,
in turn, became well dened, and the size of the cobalt spinel
oxide particles gradually decreased, and so the dispersion of
CoOx was increased. The propylene selectivity in ODHP seems to
be correlated with the particle size and the dispersion of cobalt
oxides.
Moreover, both the surface lattice oxygen and the surface
adsorbed oxygen species were more prominent over the CA-P
LDO sample than the other LDO samples, revealing that there
were more oxygen vacancies over the CA-P LDO sample than the
other LDO samples. This was conrmed further by EPR results
(Fig. S4, ESI†), where the signal with a g-factor of about 2.0 was
attributed to oxygen vacancies.28,29
3.3. Catalytic performances
The catalytic performances of the LDO samples in ODHP were
However, the size of the spinel oxides for CA-C LDO (5.8 nm)
tested. The optimized ratio of propane to oxygen in the feed wꢁas1 is close to that for CA-S and CA-P LDO (4.3 and 3.9 nm), and
ꢀ
also investigated at T ¼ 400 C and GHSV ¼ 30 000 mL gcat
propane conversion is not much different over these LDO
hꢁ1. It was found that propylene selectivity increased and samples, but the selectivity to propylene over CA-C is signi-
propane conversion decreased with the ratio (Table S1, ESI†). cantly less than that over CA-S and CA-P (22.1, 31.4 and 36.0% at
The highest yield of propylene was reached at C3H8 : O2 ¼ 1 : 1 400 ꢀC, respectively). Therefore, the particle size and the
for CA-N and CA-C LDO in ODHP, and at an even lower ratio of dispersion of oxides may not account completely for the
propane to oxygen for CA-S and CA-P LDO. As a result, we chose enhanced catalytic performances of these samples in ODHP,
a 1 : 1 ratio of propane to oxygen in the reactant feed for the implying that the pillared anions play a signicant role.
following comparison among the LDO samples in ODHP.
As the pillared anions (sulfate and phosphate) are repulsive
As shown in Table 3, the ODHP reaction over the LDO to the p electrons of propylene, the residual time for propylene
samples occurred at low temperatures (240–400 ꢀC) with is shorter on the surfaces of CA-S and CA-P LDO than CA-C and
a signicant conversion of propane and selectivity to propylene. CA-N, LDO, and so oxidation of the produced propylene in
The conversion of propane at 400 ꢀC is 41.1, 28.4, 24.2, 25.8, ODHP over CA-S and CA-P LDO is avoided more efficiently. This
24.5 and 27.2% over Co3O4, CA-co, CA-N, CA-C, CA-S and CA-P, is conrmed by the C3H6-TPD result that the desorption of
respectively. Propylene was not found at the selected tempera- propylene over CA-P occurred at a lower temperature than over
tures (240–400 C) over Co3O4, but the propylene selectivity at CA-C (580 vs. 625 C)31 (Fig. 9A). The large peak at 107 C was
ꢀ
ꢀ
ꢀ
400 C was 18.8, 21.5, 22.1, 31.4, and 36.0% over CA-co, CA-N, attributed to weakly bound propylene on acid sites,32 similar to
ꢀ
CA-C, CA-S and CA-P, respectively. That is to say, the LDO the case of C3H8-TPD (Fig. 9B), and the peak temperature for
samples with the segregation of Al on the surface displayed CA-P was slightly higher than that for CA-C. These peaks were
improved propylene selectivity in ODHP. These results also localized at ꢄ110 ꢀC, far lower than the ignition point of ODHP
reveal the good efficiency of the reaction at low temperatures.
over the LDO samples, and the effect of the species that these
The catalytic behavior of the LDO samples in ODHP at low peaks represent on the reactivity of the samples in ODHP can be
temperatures (400 ꢀC below) is actually derived from the reduction ignored. On the other hand, the interaction of cobalt with the
of Co(III) related species. As shown by H2-TPR measurements anions, especially sulfate, reduced the oxygen mobility of the
(Fig. 7), the reduction peaks of both Co(III) to Co(II) and Co(II) to LDOs where the presence of lattice oxygen species was
Co, for CA-co, CA-N, CA-C and CA-P LDO, appeared at below predominant, and the oxygen reactivity was lowered. For
400 ꢀC and above 700 ꢀC, respectively, and the reduction peak of instance, oxygen conversion in the initial activation of propane
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Table 3 Catalytic performances of various LDO catalysts for ODHPa
X/%
S/%
Sample
Co3O4
T/ꢀC
C3H8
O2
2.1
100
100
100
100
C3H6
CH4
C2H4
CO
0.0
CO2
Othersb
YC H /%
3
6
240
250
300
350
400
250
260
300
350
400
425
450
500
550
250
260
300
350
400
425
450
500
550
240
250
300
350
400
450
500
550
350
360
375
400
450
500
550
265
275
300
350
400
450
500
550
0.4
20.9
24.3
27.8
41.1
0.4
17.8
18.5
19.4
20.3
21.6
28.7
30.8
58.6
0.3
19.0
19.2
19.6
20.4
21.3
31.5
34.3
43.2
0.4
19.0
20.8
20.8
23.0
23.8
25.6
25.4
2.3
23.0
24.3
24.6
25.0
25.3
29.2
1.3
23.2
24.2
26.4
27.9
28.0
28.3
28.5
0.0
0.0
0.0
0.0
0.0
0.1
0.4
2.8
11.2
18.2
0.0
0.1
0.1
0.2
0.3
0.5
1.6
1.2
26.5
0.0
0.1
0.1
0.2
0.4
0.3
0.2
5.3
10.9
0.1
0.3
0.2
0.3
0.4
0.6
0.8
1.0
0.0
0.2
0.3
0.5
0.6
0.7
1.2
0.0
0.0
0.0
0.0
0.1
0.1
0.1
0.2
0.1
0.1
0.0
0.0
0.1
1.7
1.0
0.9
2.1
3.9
9.2
0.0
0.0
0.0
2.1
1.2
1.3
2.6
3.7
1.1
2.0
0.0
0.0
2.2
3.2
3.4
5.7
8.1
10.6
11.3
11.9
0.8
4.4
4.5
4.2
4.6
5.8
8.3
0.6
0.9
1.1
1.6
2.3
3.0
4.4
6.5
89.7
99.4
93.9
83.1
73.4
84.8
87.4
86.8
81.9
76.8
66.7
61.0
51.9
28.4
80.7
87.4
86.1
80.8
74.3
69.2
64.6
40.6
36.4
83.3
79.5
79.3
74.8
68.2
65.3
60.4
54.7
45.9
59.3
60.9
61.6
60.8
59.6
56.0
19.3
70.6
68.8
62.8
57.1
54.3
51.0
46.5
10.1
0.1
0.1
0.1
0.0
13
0.0
0.0
0.0
0.0
0.0
0.0
2.0
2.2
3.0
3.8
1.4
0.0
0.0
0.0
0.0
2.8
2.4
3.2
4.4
2.2
0.0
0.0
0.0
0.0
3.2
3.5
4.0
5.1
5.1
6.0
6.6
1.2
7.8
7.8
7.7
7.9
7.7
8.3
1.2
6.6
7.2
9.0
10.0
9.7
9.2
8.9
0.0
3.1
5.6
8.0
0.0
0.0
0.0
0.0
0.0
16.7
37.2
46.9
45.1
0.0
0.0
0.0
0.0
0.0
18.8
33.2
54.1
52.6
0.0
0.0
0.0
0.0
1.1
1.7
3.9
6.4
0.0
1.6
1.9
2.1
2.3
3.5
5.9
0.0
0.0
0.0
1.6
4.4
7.6
12.0
15.4
CA-co
CA-N
CA-C
1.9
0.5
100
100
100
100
100
100
100
100
11.3
11.9
15.6
18.8
6.7
0.0
0.0
0.0
1.2
11.2
12.3
16.1
21.5
10.5
0.0
0.0
0.0
1.3
16.9
17.0
19.1
22.1
21.5
23.5
25.9
51.6
34.1
32.3
31.4
31.6
30.3
28.4
77.4
28.4
30.0
33.9
36.0
34.8
32.4
31.2
0.2
0.2
0.1
0.2
0.3
0.1
0.1
0.0
15.7
0.1
0.1
0.2
0.1
0.2
0.1
0.0
0.0
13.1
0.1
0.1
0.1
0.1
0.2
0.1
0.1
1.7
0.3
0.2
0.2
0.2
0.2
0.2
2.6
0.1
0.1
0.1
0.1
0.1
0.1
0.1
1.6
100
100
100
100
100
100
100
100
1.7
100
100
100
100
100
100
100
13.5
88.8
97.0
CA-S
CA-P
100
100
100
100
6.7
100
100
100
100
100
100
100
a
The data were obtained aer 30 min on stream (reaction conditions: C3H8 : O2 : He ¼ 1 : 1 : 4; GHSV ¼ 30 000 mL gcatꢁ1 hꢁ1). b Others ¼ C3H4O +
CH3CH2CHO + CH3COCH3.
over the CA-S LDO sample occurred at higher temperatures LDO samples (20.3, 20.4, and 23.0%) at around 400 ꢀC. The
relative to the other samples (Table 3). The decreased reactivity increased selectivity to propylene induced by the anions around
of oxygen would reduce the probability of the extensive trans- cobalt was accompanied by a decrease in selectivity to CO2 (the
formation of the targeted products. Thus, the presence of the deep oxidation product), and the oxygen saved due to the
anions on the catalyst surfaces is benecial to an improvement decreased CO2 selectivity would transform more propane,
of the selectivity to propylene in ODHP.
provided that the amount of oxygen in the reactant is limited.
Although the oxygen mobility in CA-S LDO was lowered by the Thus, the conversion of propane was increased over CA-S and CA-
sulfate anion, propane conversion over the CA-S LDO sample P, though the oxygen mobility in CA-S was lower than in the CA-co,
(24.6%) was greater than that over the other CA-co, CA-N, and CA-C CA-N and CA-C samples.
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according to the Mars-Van Krevelen mechanism.33 For CA-co, CA-
N and CA-C LDO, the selectivity to propylene would be near zero if
the propane conversion in ODHP were extrapolated to zero,
indicating that the CO and CO2 formed over these LDO samples
resulted from the direct oxidation of propane. For CA-P LDO,
similar extrapolation showed that the primary products included
propylene, CO and CO2. As there were surface adsorbed/lattice
oxygen species over CA-co, CA-N, CA-C and CA-P LDO, and only
lattice oxygen species over CA-S LDO, the surface adsorbed oxygen
species (electrophilic oxygen species) were associated with the
deep oxidation products (COx), and the lattice oxygen species
(nucleophilic oxygen species) were linked with the formation of
propylene. This meant that the activation of propane over CA-co,
CA-N, CA-C and CA-P LDO was related to both the lattice oxygen
species and the surface adsorbed species.
The acidity and basicity of surfaces are oen used in tuning
the activity and selectivity of catalysts for selective oxidation
reactions.34 In this work, we noted that there were mainly weakly
acidic and basic sites over the surfaces of the LDO samples
(Fig. S5, ESI†). Desorption peaks in NH3- and CO2-TPD experi-
ꢀ
ꢀ
Fig. 9 (A) C3H6- and (B) C3H8-TPD profiles of the CA-C and CA-P
ments appeared at 150–180 C and 115 C, respectively. These
weakly acidic and basic sites would not have any inuence on
the desorption of the formed propylene from the surfaces, as
the ignition point of the reaction appeared at above 250 ꢀC, and
so they do not correspond to the improvement in propylene
selectivity over the catalysts for ODHP at low temperatures.
There were small amounts of moderate basic sites found for the
CA-C and CA-P LDO samples (CO2 desorption at 575 ꢀC,
Fig. S5†), which could facilitate propylene desorption and avoid
its extensive transformation, such as C–C bond cleavage.
The C–C bond cleavage in propane occurred at relatively high
temperatures. As shown in Table 3, a signicant amount of
methane in ODHP over the Co3O4, CA-co, CA-N, CA-C and CA-S
samples is seen at 300, 450, 500, 550 and above 550 ꢀC,
respectively, and almost no methane is found over CA-P at 270–
550 ꢀC. In other words, C–C bond cleavage occurred at
adequately high temperatures, and the temperature of C–C
cleavage in ODHP was increased over the anion pillared LDO
samples relative to the other counterparts.
Therefore, the pillared anions changed the properties of the
LDO samples such as the particle size, the reducibility and the
properties of surface oxygen species, and thus led to an
improvement in their ODHP performance. The phosphate-
pillared LDO sample displayed good catalytic performance for
ODHP: the conversion of propane and the selectivity to propylene
was 27.9% and 36.0% at 400 ꢀC, respectively (the propylene yield
reached to 10.0%). In comparison, the highest propylene yield of
10% was reached at a higher temperature (600 ꢀC) for CoMgAlO
mixed oxide catalysts,35 in which well-dispersed CoOx with tetra-
hedrally coordinated structures acted as the active sites; mont-
morillonitic clay pillared with Al3+ and/or Cr3+ also showed a high
activity at low temperatures (propylene yield at 450 ꢀC: 10.3%).36
LDOs.
The dependence of propane conversion on the selectivity to
propylene was further investigated over the LDO samples at
400 ꢀC by varying GHSV in the range of 30 000–300 000 mL gcat
ꢁ1
hꢁ1. As shown in Fig. 10, different selectivities to propylene were
observed over the LDO samples. CA-S and CA-P LDO exhibited
much higher selectivity to propylene than the other LDO samples,
illustrating that the pillared anions in the LDO samples were
benecial to improving the selectivity to propylene in ODHP. The
propylene selectivity over CA-S LDO decreased with propane
conversion, whereas the opposite trend was observed over the
other LDO samples (Fig. 10), implying that the mechanisms of
propane activation were different over these LDO samples.
Propylene would be the primary product over CA-S LDO if the
conversion of propane were extrapolated to zero, suggesting that
the formation of deep oxidation products (CO and CO2) was
derived from the extensive oxidation of propylene. This is
consistent with the above argument that the reduced (lattice)
oxygen mobility (oxygen reactivity) results in increased selectivity
to propylene. So the ODHP reaction over CA-S was performed
3.4. Stability of the catalysts in ODHP
Fig. 10 Correlation of propane conversion with propylene selectivity
in ODHP over LDOs (reaction conditions: C3H8 : O2 : He ¼ 1 : 1 : 4
(molar ratio); GHSV ¼ 30 000–300 000 mL gcatꢁ1 hꢁ1; T ¼ 400 ꢀC).
Fig. 11 shows the catalytic performances over the CA-P LDO
catalyst as the time-on-stream increased under conditions of T ¼
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such as reducibility, especially for the preserved anions (sulfate
and phosphate) in CA-S and CA-P LDO. Surface adsorbed/lattice
oxygen species were found over CA-P LDO, but only lattice
oxygen species were available over CA-S LDO. The sulfate
around cobalt made the reduction of Co(III) to Co(II) relatively
more difficult, and so the mobility of oxygen (lattice) in the LDO
was reduced; the phosphate around cobalt provided a signi-
cant amount of oxygen vacancies, and enhanced the mobility of
oxygen (surface adsorbed oxygen species). As both CA-S and CA-
P exhibited better catalytic performances in ODHP than the
other LDO samples, the mechanisms of propane activation over
CA-S and CA-P must be different. The mechanism over CA-S
depended on the mobility of lattice oxygen, and that over CA-
P was related to the participation of the surface adsorbed/
lattice oxygen in ODHP.
Fig. 11 The lifetime test of the CA-P LDO sample for ODHP (reaction
conditions: C3H8 : O2 : He ¼ 1 : 1 : 4 (molar ratio); GHSV ¼ 30 000 mL
gcat hꢁ1; T ¼ 400 ꢀC).
ꢁ1
ꢁ1
ꢀ
400 C, C3H8 : O2 : He ¼ 1 : 1 : 4, and GHSV ¼ 30 000 mL gcat
hꢁ1. Both the propane conversion (ꢄ26%) and propylene selec-
tivity (ꢄ39%) were observed to be almost unchanged over the
initial 12 hours of time-on-stream (propylene yield was 10%).
However, they were changed to ꢄ6% and ꢄ77%, respectively,
aer 50 hours of time-on-stream (propylene yield was ꢄ5%) and
the oxygen conversion decreased gradually from 100% to ꢄ25%.
These results suggest that the properties of the active sites of CA-P
LDO varied during a long period of ODHP reaction.
Acknowledgements
Support for this research from the National Natural Science
Foundation of China (21373169, 21373168), NFFTBS (No.
J1310024), and Program for Changjiang Scholars and Innovative
Research Team in the University (No. IRT1036) is gratefully
acknowledged.
The XRD pattern of the CA-P LDO catalyst aer the reaction
is shown in Fig. S6 (ESI†). The mixed spinel oxides were still the
main phase of CA-P LDO, but the characteristic peaks of the
oxides (Co2AlOx) became sharper aer 50 hours of reaction. The
peaks for CoO were signicant. A decreased amount of Co(III)
was also observed on the spent catalysts (Fig. S7, ESI†). This
indicated that the spinel oxide particles grew, and the Co
species was partially reduced aer a long period of reaction,
which reduced the ability of the catalyst to activate oxygen and
propane. Therefore, to improve the stability of the catalysts, we
could increase the thermal conductivity of the catalysts and the
concentration of the pillared anions or the interaction between
CoOx and the anions around it, which will help to prevent the
catalyst from being sintered and reduced.
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