Facet-Dependent Activity of Co3O4 Catalyst for C3H8 Combustion

Article abstract of DOI:10.1002/cctc.201901382

Clarifying the facet-dependent activity of Co₃O₄ towards C₃H₈ combustion is of great importance to have a deep understanding of reaction mechanism and develop highly active Co₃O₄ based catalysts for light hydrocarbon combustion. In this work, four different Co₃O₄ nanocatalysts which dominantly exposed {100}, {111}, {110} and {112} facets, respectively, were synthesized via hydrothermal method and further tested in C₃H₈ combustion. The catalytic activity of different crystal planes varied as the order of {111}>{100}>{110}>{112}. The Co₃O₄ catalyst with {111} facets exhibited the best catalytic performance with T₅₀ and T₉₀ as low as 209 and 239 °C, respectively. DRIFT and DFT calculations revealed that the facile activation of C?H bond on Co₃O₄ {111} planes is responsible for the high activity. Moreover, the higher activity of lattice oxygens and easy activation of gas phase oxygen could also contribute to the excellent catalytic performance.

Full text of DOI:10.1002/cctc.201901382

Accepted Article
Title: Facet-dependent activity of Co3O4 catalyst for C3H8 combustion
Authors: Junxuan Yao, Hui Shi, Dekui Sun, Huaiqian Lu, Bo Hou, Litao
Jia, Yong Xiao, and Debao Li
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
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ChemCatChem
10.1002/cctc.201901382
Facet-dependent activity of Co
O
3 4
catalyst for C
3
H combustion
8
a,b
a
a
a
a
a
*a
a
JunxuanYao , Hui Shi , Dekui Sun , Huaiqian Lu , Bo Hou , Litao Jia , Yong Xiao , Debao Li
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, The Chinese Academy
of Sciences, 030001 Taiyuan, PR China.
b University of Chinese Academy of Sciences, 100049 Beijing, PR China.
*
Corresponding author: Prof. Yong Xiao, E-mail: xiaoyong@sxicc.ac.cn
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ChemCatChem
10.1002/cctc.201901382
Graphic abstract
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ChemCatChem
10.1002/cctc.201901382
Highlights



Co
Co
3
3
O
4
4
exposed different facets are synthesized and employed in C
{111} exhibits the highest activity towards C combustion.
and better redox properties for the excellent performance.
3
H
8
combustion.
O
3
H
8
Facile activation of C
3
H
8
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ChemCatChem
10.1002/cctc.201901382
Abstract
Clarifying the facet-dependent activity of Co
3
O
4
towards C
3
H
8
combustion is of great
importance to have a deep understanding of reaction mechanism and develop highly active Co
based catalysts for light hydrocarbon combustion. In this work, four different Co nanocatalysts
which dominantly exposed {100}, {111}, {110} and {112} facets, respectively, were synthesized
via hydrothermal method and further tested in C combustion. The catalytic activity of different
crystal planes varied as the order of {111} > {100} > {110} > {112}. The Co
3
O
4
3
O
4
3
H
8
3
O
4
catalyst with
o
{
111} facets exhibited the best catalytic performance with T50 and T90 as low as 209 and 239 C,
respectively. DRIFT and DFT calculations revealed that the facile activation of C-H bond on
Co {111} planes is responsible for the high activity. Moreover, the higher activity of lattice
3
O
4
oxygens and easy activation of gas phase oxygen could also contribute to the excellent catalytic
performance.
Keywords: Co
3
O
4
; facet-dependent; C
3
H combustion; C-H activation.
8
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10.1002/cctc.201901382
1
. Introduction
Catalytic combustion is considered to be one of the most efficient means to eliminate the
atmospheric pollutions like CO [1, 2], methane [3, 4] and various VOCs [5-7]. Catalysts based on
precious metals such as Au [8], Pt [9] and Pd [10] are highly active towards catalytic combustion,
but the high cost and facile deactivation due to poisoning [11] or sintering [12] have limited their
large scale application.
Therefore, much attention has been paid to the non-noble metal oxide catalysts, for example
Co
19]. Among them, Co
Currently, many studies have focused on the catalytic performance of Co
morphologies including cubes [21, 22], octahedrons [21, 23], rods [24, 25], needles [26,27], belts
28,29], sheets [30,31] and plates [32,33]. The specific morphology possesses the dominantly
exposed facets, which have significant impact on catalytic performance of reactions such as water
splitting and catalytic combustion. Liu et al. [34] reported that the Co octahedrons enclosed by
111} facets exhibited the excellent activity in water splitting, because the {111} facets possessed
3
O
4
[13], MnO
x
[14], CeO
is one of the most active catalysts towards catalytic combustion [20].
with different
2
[15, 16] and mixed oxides such as spinels [17, 18] and perovskites
[
3
O
4
3
O
4
[
3
O
4
{
the biggest dangling bond density, highest surface energy for the facial adsorption of ionized
oxygen species and the smallest absolute value of Gibbs free energy for H adsorption. Xie et al.
o
[
35] fabricated the Co
3
O
4
nanorods which could oxidize CO at -77 C. The excellent performance
3
+
was due to the abundant active Co sites on the exposed {110} surface. Hu et al. [36] synthesized
the Co nanosheets with {112} facets, which exhibited superior activity in methane combustion.
3
O
4
The high activity was associated with the more open surface atoms arrangement of the {112}
planes. Therefore, it is meaningful to investigate the crystal plane effect on the catalytic activity
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10.1002/cctc.201901382
of different reactions.
, mainly comes from the LPG [37], is a kind of VOCs in atmosphere and should be
eliminated through appropriate methods. Due to the high stability of C , it usually acts as a
model reactant for catalytic combustion [38, 39]. Investigating the Co crystal plane effect on
combustion activity can help us to have a deep understanding of the reaction mechanism and
develop highly active Co catalysts. However, to the best of our knowledge, few studies care
about the influence of Co crystal plane effect on the catalytic performance of C combustion.
Therefore, we intend to fabricate the Co
the effect of different crystal planes on C
Herein, four different Co
through hydrothermal routes and further tested in C
C
3
H
8
3
H
8
3
O
4
C
3
H
8
3
O
4
3
O
4
3
H
8
3
O
4
catalysts with various exposed facets and investigate
combustion activity.
3
H
8
3
4
O catalysts with various shapes and crystal planes were synthesized
3
H
8
combustion. Co
3
O
4
-H with {111} planes
o
exhibited the best catalytic activity in terms of T50 and T90 as low as 209 and 239 C, respectively.
Through the combination of experimental results and DFT calculations, the mechanism of C-H
activation process on the surface of Co
111} planes was explored and the correlation between Co
activities was finally established.
3
O
4
as well as the reason for the superior activity of Co
3
O
4
{
3
O
4
crystal planes with intrinsic catalytic
2
. Experimental
2
.1.Preparation methods
All catalysts were synthesized through hydrothermal methods and roughly presented in
Scheme 1. The detailed processes are shown as follows:
Preparation of Co nanocubes (Co -C): 3 mmol of Co(OAc) ·4H O and 2.4 g PVP
3
O
4
3
O
4
2
2
(Mw=10000) were dissolved in 114 mL of water with stirring for 15 min. Then, 6 mL of 1 M
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10.1002/cctc.201901382
NaOH aqueous solution was added to the solution with further stirring for 20 min. After that, 0.2
g KClO
3
was added into the above solution with stirring for another 2 min. The mixture was
o
transferred to a Teflon-lined autoclave, sealed and maintained at 180 C for 24 h. After cooling
down to room temperature, the product was collected by centrifugation and washed by water for
o
o
several times. With further drying at 60 C and calcination at 350 C for 1 h, the catalyst was
obtained.
Preparation of Co
3
O
4
hexagonal nanoplates (Co
3
O
4
-H): 3 mmol of Co(OAc)
2
·4H O and 2.4
2
g PVP (Mw=10000) were dissolved in 114 of mL water under stirring for 15 min. Then, 6 mL of
1
.5 M NaOH aqueous solution was added to the solution with stirring for another 2 min. The
o
mixture was transferred to a Teflon-lined autoclave, sealed and maintained at 180 C for 24 h.
After cooling down to room temperature, the product was collected by centrifugation and washed
o
o
with water for several times. With further drying at 60 C and calcination at 350 C for 1 h, the
catalyst was obtained.
Preparation of Co
with 20 mL of ethylene glycol to form a transparent solution. Then, 1 mL of 1 M Na
solution and 5 mL of 1 M Co(NO ·6H O aqueous solution was added in turn into the above
3
O
4
nanorods (Co
3
O
4
-R): 12.5 mL of concentrated NH
3
·H
2
O was mixed
2
CO aqueous
3
3
)
2
2
mixture. After stirring for 20 min, the mixture was transferred to a Teflon-lined autoclave, sealed
o
and maintained at 140 C for 12 h. When cooling down to room temperature, the product was
collected and washed with water for several times. Finally, the sample was dried and calcined at
o
3
50 C for 1 h to obtain the catalyst.
Preparation of Co
3
O
4
nanosheets (Co
3
O
4
-S): 1.8 mmol of Co(NO
3
)
2
·6H O was dissolved in
2
4
0 mLof water, which was mixed with 40 mLof hexamethylenetetramine (HMT) aqueous solution
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10.1002/cctc.201901382
(containing 1.7 mmol of HMT). Then, 10 M NaOH aqueous solution was added drop wise into the
above solution and adjust the pH to 11. After stirring for 2 h, the mixture was transferred into a
o
Teflon-lined autoclave, sealed and maintained at 110 C for 24 h. When cooling down to room
temperature, the product was collected and washed with water for several times. Finally, the
o
sample was dried and calcined at 350 C for 1 h to obtain the catalyst.
2
.2.Characterization
Crystalline structure of catalysts were analyzed by X-ray powder diffraction (XRD)
(PANalytical Empyrean X'pert powder diffractometer) with Cu Kα radiation. The visible Raman
spectra was recorded on a LabRAM HR Evolution Raman spectrometer with the 532 nm laser as
excitation source. The specific surface area and the pore size distribution of catalysts were
measured by the N adsorption-desorption measurements on a Micromeritics ASAP 2460
2
equipment. The morphology and microstructures of the samples were characterized by
transmission electron microscope (TEM, JEM-2100F) with accelerating voltage of 200 kV. X-ray
photoelectron spectra was obtained by an AXIS ULTRA DLD X-ray photoelectron spectrometer,
the binding energies of all elements were corrected by the standard C 1s line at 284.6 eV.
H
2
-temperature programmed reduction (H
2
-TPR) measurements were conducted on a TP-
5
080 chemical adsorption apparatus with a TCD detector. 30 mg of sample was loaded in the
quartz tube. The feed gas (5 vol. % H
2
/ N
2
) flow was 30 ml/min. Each sample was heated from
o
o
room temperature to 800 C with the ramping rate of 10 C/min.
C
3
H
8
-temperature programmed reduction (C
3
8
H -TPSR) profiles were measured by an on-line
quadrupole mass spectrometer (MS, IPI GAM 200). 30 mg of sample was pretreated in the flow
o
o
of 10 vol. % O
2
/ Ar at 300 C for 1 h. After cooling down to 50 C, the sample was purged with 1
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10.1002/cctc.201901382
vol. % C
3
H
8
/ Ar for 1 h to eliminate the physical adsorbed O
2
. Following that, the sample was
o
o
o
heated with a ramping rate of 10 C/min from 50 C to 800 C. The effluent gas was monitored by
a mass spectrometer and the mass signal of CO (m/z=44) and C (m/z=29) were detected.
CO -temperature programmed reduction (CO -TPD) experiments were conducted on the same
2
3
H
8
2
2
on-line quadrupole mass spectrometer (MS, IPI GAM 200). Typically, 150 mg of sample was
o
pretreated in air flow at 300 C for 1 h. After cooling down to room temperature, the sample was
o
purged with 1 vol. % C
H
3 8
/ Ar to eliminate the physical adsorbed O
2
and then heated to 220 C
o
with a ramping rate of 10 C/min. When cooling down to room temperature, the sample was purged
o
o
with Ar for 1 h and heated to 900 C with the ramping rate of 10 C/min. The signal of CO
2
(m/z=44)
was detected.
In-situ Raman spectra were measured on the LabRAM HR Evolution Raman spectrometer
o
with excitation source of 532 nm. The sample was heated from room temperature to 300 C under
o
the oxygen atmosphere (0.5% vol. O
2
/Ar) with the ramping rate of 10 C/min. The spectra was
o
o
o
o
o
o
o
o
o
recorded at 50 C, 100 C, 150 C, 200 C, 220 C, 240 C, 260 C, 280 C and 300 C, respectively.
Before recording of every spectra, the sample should be stabilized at that temperature for 15 min.
Diffuse reflectance infrared Fourier transform (in-situ DRIFT) was conducted on a Nicolet
Magna 750 infrared spectrometer. First, the sample was loaded in the DRIFT cell and the spectra
was recorded as the background. Then, the gas flow of 1% vol. C
3
H
8
/Ar was purged into the cell
o
o
and the sample was heated from room temperature to 300 C with the ramping rate of 10 C/min.
o
o
o
o
o
o
The spectra was recorded at 50 C, 100 C, 150 C, 200 C, 250 C and 300 C, respectively. Before
recording, the sample should be maintained at that temperature for 15 min.
2
.3.Evaluation of catalysts
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The catalytic activity for C
3
H
8
combustion was tested with a continuous flow fixed-bed reactor.
Typically, 0.2 g catalyst mixed with 0.5 g quartz sands was loaded in the tubular reactor. The
catalyst was heated under the feed gas (1 vol% C and 10 vol% O balanced with N ) at flow
3
H
8
2
2
-1
-1
rate of 33.3 mL/min equivalent to the weight hourly space velocity (WHSV) of 10000 ml g h .
The feed and effluent gas was analyzed by gas chromatography equipped with a hydrogen flame
ionization detector (FID).
3
. Results and discussion
3
.1.Structural, textural and morphological characterization
The crystalline structures of catalysts were characterized by XRD. As depicted in Figure 1, all
o
o
o
o
o
samples show the similar diffraction patterns with intense peaks at 2θ=19.1 , 31.4 , 37 , 45 , 55.9 ,
o
o
5
9.6 and 65.5 , which are indexed to the spinel Co
attributed to CoO or any other impurities are detected, indicating the high purity and crystallinity
of the fabricated Co catalysts.
Visible Raman spectra was conducted to further investigate the sub-lattice structures of
3
O
4
(JCPDS No. 03-065-3103) and no peak
3
O
4
-1
catalysts. As seen in Figure 2, all samples possess five vibration modes located at 196 cm , 522
-
1
-1
-1
-1
cm , 620 cm , 482 cm and 693 cm , which are matched well with the F2g, E
of spinel Co [40]. It should be noted that the A1g symmetry is corresponding to the Co -O
vibration in octahedral sites (CoO ) of Co [41]. Any slight shift of A1g symmetry reflects the
change of Co-O bond, which can result in the lattice distortion in spinel structure [42]. As seen in
g
and A1g symmetry
3
+
2-
3
O
4
6
3
O
4
-1
-1
the insertion of Figure 2, an obvious red shift of A1g peak from 692 cm to 689 cm is observed
in Co -H, indicating lattice distortion in the catalyst. The lattice distortion can induce the
3
O
4
formation of defective structures such as oxygen vacancies, which may promote oxygen activation
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10.1002/cctc.201901382
-
1
-1
[
(
43, 44]. Moreover, as the peaks at 196 cm and 691 cm are the characteristic of tetrahedral sites
CoO
4
) and octahedral sites (CoO
6
) of Co
3
O , the intensity ratio of these two peaks may reflect the
4
2
+
3+
surface relative content of Co /Co [45]. As summarized in Table 1, the I196/I691 values follow
2
+
the order of Co
Co -H. The results can be further confirmed by XPS, which will be shown latter.
The textural properties in terms of BET surface area and pore volume are listed in Table 1. All
3
O
4
-H > Co
3
O
4
-C > Co
3
O
4
-P > Co
3
O
4
-R, indicating more Co on the surface of
3
O
4
2
-1
samples exhibit similar pore volume while the BET surface areas vary from 23.6 to 50 m g , the
difference may be due to the various morphologies of samples obtained by different synthetic
routes.
Figure 3 shows the TEM, HRTEM images of catalysts and the schematic diagrams of different
Co
3
O
4
facets. As seen in Figure 3a1, the Co
3
4
O -C sample exhibits uniform nanocubes with size of
about 20 nm. Two sets of mutually perpendicular (220) planes with lattice space of 0.28 nm can
be found and the corresponding FFT image spot array is well indexed as the [001] zone axis (Figure
3
a2), indicating the dominantly exposed facets of Co
3
O
4
-C are {100} planes. The Co
3
4
O -H sample
presents the hexagonal nanoplates with the average edge length of approximately 150 nm (Figure
o
3
b1). Three sets of (220) planes with lattice space of 0.28 nm and interfacial angle of 60 are
observed in Figure 3b2, the corresponding FFT image is well indexed as [1-11] zone axis,
demonstrating the preferentially exposed facets of Co -H are {111} planes. Figure 3c1 presents
the morphology of Co -R, the nanorods with average length of about 250 nm can be observed.
3
O
4
3
O
4
The HRTEM image in Figure 3c2 shows a set of (311) planes and a set of (222) planes with lattice
o
space of 0.24 nm and 0.23 nm and a 79 interfacial angle, which means the mainly exposed facets
of Co
3
O
4
-R are {110} planes. As seen in Figure 3d1, the Co
3
4
O -P exhibits the morphology of
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10.1002/cctc.201901382
irregular nanoplates with the average size of around 50 nm. HRTEM and the corresponding FFT
image show the dominantly exposed facets of Co -P are {112} planes, which are normal to both
the sets of (220) and (222) planes with lattice space of 0.28 nm and 0.23 nm.
.2.XPS
The cobalt valance state and surface oxygen species were analyzed by XPS. Figure 4a presents
the Co 2p spectra of samples. The intense peaks at around 795.0 eV and 779.8 eV are
corresponding to the Co 2p3/2 and Co 2p1/2 spin-orbital peaks of Co [33]. The Co 2p3/2 spectra
can be further deconvoluted into Co and Co component peaks, which located at around 779.9
O
3 4
3
3
O
4
3
+
2+
2
+
3+
eV and 781.2 eV, respectively [46]. As summarized in Table 1, the Co /Co values vary in the
2
+
order of Co
3
O
4
-H > Co
3
O
4
-C > Co
3
O
4
-P > Co
3
O -R, indicating higher concentration of Co on
4
the surface of Co
3
O
4
-H exposing {111} planes, which is consistent with the Raman results.
Figure 4b shows the O 1s spectra. It can be seen that all profiles can be fitted into four
component peaks at around 529.9, 530.5, 531.5 and 533.5 eV, corresponding to the surface lattice
oxygens (Olatt), surface adsorbed oxygen species (Oads), oxygens in surface adsorbed OH- and the
oxygens in surface adsorbed water of carbonates, respectively [47]. The deconvoluted results show
the Oads/Olatt values follow the order of Co
the higher ratio of adsorbed oxygen species on Co
active species in catalytic combustion and usually participate in the reaction through superficial
mechanism [48]. Hence, the more Oads on Co -H surface may be favored in C combustion.
.3.Redox properties
In order to investigate the redox properties of different Co catalysts, H -TPR
3
O
4
-H > Co
3
O
4
-C > Co
3
O
4
-R > Co
3
O
4
-P, which means
3
O
4
-H surface. It has been reported that Oads are
3
O
4
3
H
8
3
3
O
4
2
measurements were conducted. As illustrated in Figure 5a, all samples present two distinct
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ChemCatChem
10.1002/cctc.201901382
reduction peaks, which are attributed to the stepwise redution of Co
3
O
4
. Generally, the peak at
o
3+
2+
lower temperature (< 400 C) is assinged to the redution of Co to Co , while the peak at higher
o
2+
0
temperature (> 400 C) is associated with the redution of Co to Co [49]. The H
2
consumption is
calculated by integration of reduction peaks using the reduction peak area of CuO as calibration.
-
1
As seen in Table 1, all samples exhibit similar H
which is close to the theoretical H consumption of 16.6 mmol g . The results further confirm that
all samples are Co with high purity. As previously reported, the first reduction temperature is
a sign of activity and mobility of lattice oxygen in Co [50], which is closely related to the
2
consuption around 17 mmol g during H -TPR,
2
-1
2
3
O
4
3
O
4
catalytic performance in combustion reaction. It can be seen in Table 1, the first redution
o
temperature of Co
more active lattice oxygens in Co
Co
3
O
4
-H with {111} planes is 303 C, lower than other samples, indicating the
3
O
4
-H, which is favored in combustion reaction. While the
o
3
O
4
-P with {112} planes presents the highest reduction temperature of 344 C, which means the
lattice oxygens are more difficult to be activated and participate in the reaction.
-TPSR measurements were conducted to investigate the reaction of C
lattice oxygens of catalyst. As shown in Figura 5b, the MS signals of C (m/z = 29) and CO
m/z =44) were both detected during the experiments. The positive peaks in CO signals represent
the production of CO and the negtive peaks in C signals represent the consumption of C
It can be seen that the temperatures of the positive peaks in CO signals are in agreement with the
temperatures of the negtive peaks in C signals, suggesting the C can react with surface
lattice oxygens to form CO . It is reasonable that the peak temperatures and the peak areas in CO
C
3
H
8
3
8
H with surface
H
3 8
2
(
2
2
H
3 8
3
8
H .
2
3
H
8
3
H
8
2
2
signals can reflect the reactivity and amount of surface lattice oxygens. As seen in Table 1, the
o
Co
3
O
4
-H with {111} planes exhibits the lowest reaction temperature of 441 C, suggesting the
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10.1002/cctc.201901382
surface lattice oxygens of Co
3
O
4
-H are the most active among all samples. Co
3
4
O -C with {100}
planes shows the largest peak area, indicating the abundant active lattice oxygens on the catalyst
surface. These two catalytsts may exhibit higher activity towards C
Co
oxygens, which is possible to exhibit low activity in catalytic evaluation.
3
H
8
combustion. While the
3
O -P with {112} planes exhibits lower oxygen activity and fewer amount of surface lattice
4
3
.4.In-situ Raman
As mentioned above, C
H
3 8
can react with the surface lattice oxygens. According to the MvK
mechanism, the surface reaction is able to create oxygen vacancies which can be replenished
through the activation of gas phase oxygen to form the new surface lattice oxygens and further
participate in combustion reaction. The different oxygen activation abilities may have an impact
on the catalytic performance of catalysts. Therefore, we employed the in-situ Raman to investigate
the structural change of Co
oxygen activation abilities of different catalysts. Figure S1 shows the change of the dominant peaks
1g) for all samples with increasing temperature in the stream of 0.5% vol. O /Ar. It can be seen
that the peaks in all samples are weakened and shifted to lower wavenumbers with the temperature
3
O
4
catalysts during the oxygen activation process and deduce the
(A
2
o
o
raised from 50 C to 250 C, indicating activation of gas phase oxygen on catalyst surface, which
3
+
−
2+
3+
−
2+
may lead to the formation of weakly bound oxygen species (Co -O -Co or Co -O -Co ) and
2
affect A1g symmetry [51]. In order to determine the oxygen activation ability of different samples,
we analyze the change of peak intensity during the experiments. As seen in Figure 6, the peak
intensity decreases more rapidly in Co
molecules are activated more easily on the catalyst surface. This will be benefit to the redox change
of catalyst and result in the higher activity towards C combustion.
3
O
4
-H with {111} planes, which means the oxygen
3
H
8
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3
.5.In-situ DRIFT
In addition to the oxygen activation, the adsorption and activation of C
to the activity of C combustion. Many studies have demonstrated that C
catalysts surface through the C-H activation and C-H activation is the rate-determining step of
combustion reaction [20, 52]. Therefore, we employed the in-situ DRIFT to reveal the C
activation abilities of catalysts and investigate the possible transformation pathway of C after
adsorption on catalysts surface. Figure 7 shows the in-situ DRIFT spectra over the Co catalysts
/Ar with temperature increasing from 50 C to 300 C. The strong peaks
at 1332 cm assigned to δ(CH) is observed over Co -H with {111} planes and Co -C with
100} planes at high temperature, but relatively weak for Co -R with {110} planes and almost
absent for Co -P with {112} planes. As reported, C can be dissociatively adsorbed on
catalyst surface through the C-H activation, which can form the methylene species [53]. The strong
3
H
8
are also important
3
H
8
3
8
H can adsorb on
3
H
8
3
H
8
3
O
4
o
o
in the stream of 1% C
3
H
8
-1
3
O
4
3
O
4
{
3
O
4
3
O
4
3
H
8
-1
peaks at 1332 cm in the profiles indicates C
H and Co -C.
The adsorption peaks at 1440 cm , 1550 cm and 1390 cm , which are attributed to the
3
H
8
is activated more easily on the surface of Co
3
O -
4
3
O
4
-1
-1
-1
-1
-1
carboxylic species (1440 cm and 1550 cm ) and bicarbonates (1390 cm ) [54, 55], can be
observed for all samples. As the surface oxygen species may react with C to form carboxylic
and bicarbonate species, the growing peak intensities with increasing temperature indicate that the
H
3 8
-1
reaction is promoted under high temperature. However, a decrease of the peak at 1440 cm is
o
observed for Co
species in Co
3
O
4
-P at 300 C. As confirmed in the C
3
H
8
-TPSR, the insufficient active oxygen
3
O
4
-P hinder the reaction with C
H
3 8
, which may result in the less production of
-1
carboxylic species and lead to the decrease of the 1440 cm peak.
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ChemCatChem
10.1002/cctc.201901382
Figure S2 illustrates the dynamic change of carboxylic species and bicarbonates on Co
and Co -C by calculating the ratio of corresponding peak areas. There is an obvious
accumulation of surface carbonates on the surface of Co -C compared with Co -H. The
accumulation of surface carbonates may cover the active sites [56, 57] and hinder the reaction,
which has a negative effect on catalytic performance. The CO -TPD after surface reaction with
was conducted to further confirm the results. As seen in Figure S3, the desorption peak of
3
O
4
-H
3
O
4
3
O
4
3
O
4
2
C
3
H
8
CO
2
for Co
3
O
4
-C is much stronger than that of Co
-C after reaction with C
can dissociatively adsorb on catalyst surface and then react with
surface oxygens to form carboxylic species which can be further oxidized into surface carbonates
during C combustion [54]. Therefore, it can be inferred that C is dissociatively adsorbed on
the catalyst surface and form CH species. This process is more favored on the surface of Co -H
and Co -C. Then, the adsorbed C may react with surface oxygens and form carboxylic
species. This process is promoted with increasing temperature except for Co -P, which has
insufficient active surface oxygens to react with C . The carboxylic species can be further
3
O
4
-H, demonstrating more surface carbonates
on the Co
3
O
4
3
H .
8
It has been reported that C
3
H
8
3
H
8
3
H
8
3
O
4
3
O
4
3
H
8
3
O
4
3
H
8
oxidized into carbonates, which is more likely to be accumulated on the surface of Co
cover the active sites.
3
4
O -C and
3
.6.Catalytic activity towards C
The catalytic performance of C
The T50 and T90 are employed as the evaluating indicators to compare the activities of different
catalysts. It can be seen that the catalytic activity increases as the order of Co -P < Co -R <
Co -C < Co -H and Co -H with {111} planes is the most active catalyst achieving T50 and
3
H
8
combustion and DFT calculations
3
H
8
combustion over Co catalysts is shown in Figure 8a.
3
O
4
3
O
4
3
O
4
3
O
4
3
O
4
3
O
4
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ChemCatChem
10.1002/cctc.201901382
o
o
T
90 as low as 209 C and 239 C. Moreover, the apparent activation energies (Ea) were also
calculated from the Arrhenius plots presented in Figure 8b. It is noticed that the Co -H exhibits
-H < Co
-P, which is the inversed trend of catalytic activity, confirming the best
-H with {111} planes. In addition, the catalytic activity of Co -H was also
3
O
4
-1
the lowest E
C < Co -R < Co
performance of Co
a
value of 90 kJ mol and the variation of E
a
follows the order of Co
3
O
4
3
4
O -
3
O
4
3
O
4
3
O
4
3
O
4
compared with that of other reported catalysts listed in Table 2. Obviously, the catalytic activity of
Co -H is better than the reported catalysts based on the T50 and T90 values.
However, as the specific area has a significant impact on catalytic performance, it is necessary
to normalize the reaction rates to the specific surface areas of Co catalysts, which can reveal
the intrinsic activities of different crystal planes. As shown in Figure 8c, the specific reaction rate
decreases as the order of Co -H > Co -C > Co -R > Co -P, suggesting the activities of
combustion on different crystal planes follow the sequence of {111} > {100} > {110} >
112}.
It has been reported that there is a positive relationship between the adsorption strength of
dissociated C on different MnO facets and the catalytic performance towards C combustion
64]. Base on this study, the adsorption ability of dissociated C could be a descriptor for the
combustion. During the adsorption of C , C-H activation is of great importance. The C-H
3
O
4
3
O
4
3
O
4
3
O
4
3
O
4
3
O
4
C
3
H
8
{
3
H
8
2
3
H
8
[
3
H
8
C
3
H
8
3
H
8
activation can be proceeded through H abstraction by surface lattice oxygens which leads to the
formation of hydroxyl and alkoxy species [65, 66]. Therefore, in this work, the DFT calculations
were employed to analyze the adsorption behavior of dissociated C
on four different crystal planes of Co - {111} surface, {100}-O surface, {110}-B surface and
112} surface. Several possible adsorption sites involved with Co atoms or O atoms were studied.
3
H
8
molecule (CH
3
CHCH +H)
3
3
O
4
{
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ChemCatChem
10.1002/cctc.201901382
The most stable adsorption configurations on the four different crystal planes of Co
3
O
4
are shown
in Figure 9. The Co - {111} surface terminates to yield triply coordinated Co3c atoms and triply
3
O
4
coordinated oxygen atoms, which can be categorized into two types (seen in Figure S4a): O1st are
triply coordinated to the subsurface Co and O2nd are doubly coordinated to the subsurface Co but
also singly coordinated to the surface Co. In this case, the CH
3
CHCH and H species all favored
3
to bind to O2nd atoms (Figure 9a). On the {100}-O surface, the pentacoordinated Co5c atoms are
exposed. The four-fold coordinated Co4c atoms are located in the first sublayer. The surface oxygen
atoms includes two kinds of three-fold coordinated O atoms (denoted as O2-5c,1-4c and O3-5c in
Figure S4b). The O2-5c,1-4c atoms are linked to two Co5c atoms and to one Co4c atom, and the O3-5c
atoms are linked to three Co5c atoms. The CH
3
CHCH species was adsorbed on the O2-5c,1-4c atom,
3
while the H atom was adsorbed on the O3-5c atom (Figure 9b). The {110}-B surface has only one
type of four-fold coordinated Co4c atoms and two types of oxygen atoms: the two-fold coordinated
O
2c atoms and three-fold coordinated O3c atoms (seen in Figure S4c). On this surface, the
CH CHCH species preferred to bind to the O3c atom and the H atom was binding to the O2c atom
Figure 9c). The unusual {112} surface has one type of two-fold coordinated Co2c atoms and two
3
3
(
types of three-fold coordinated Co3c atoms. The surface oxygen atoms are categorized into one
type of two-fold coordinated O2c atoms and three types of three-fold coordinated O3c atoms (Figure
S4d). The CH
the O3c atom which is the nearest to the adsorbed CH
involved with O atoms on the four kinds of surfaces, which implies that the surface O atoms may
play important roles in the adsorption and activation of C molecule. As presented in Figure 9,
3
CHCH
3
species favored to bind to the O2c atom, while the H atom was adsorbed on
3
CHCH (Figure 9d). The adsorption is
3
3
H
8
the adsorption energies on {111} surface, {100}-O surface, {110}-B surface and {112} surface
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ChemCatChem
10.1002/cctc.201901382
are -4.10 eV, -3.16 eV, -2.96eV and -2.43 eV, respectively. The results indicate that the binding
strength between dissociated C molecule and the surface decreases in the order of {111} >
100}-O > {110}-B > {112}, which is consistent with the order of reaction activity in our
3
H
8
{
experiments. The results confirm that the activation of C
extent.
3
H
8
is crucial to C
3
H combustion to some
8
As mentioned above, the activation of C-H bond is the rate-determining step during C
combustion. Facile activation of C can increase the catalytic activity through the more efficient
conversion from C to the intermediates such as carboxylates and carbonates, which can be
further oxidized to CO . Moreover, the redox properties of catalysts can also influence catalytic
3
H
8
3
H
8
3
H
8
2
performance. Higher lattice oxygen activity can make the oxygen species more accessible in the
reaction and the facile activation of gas phase oxygen can promote replenish of oxygen vacancies
which are created by reaction between surface intermediates and lattice oxygens. As a result, the
catalytic activity is increased.
As confirmed by DRIFT and DFT calculations, the C-H activation is more readily achieved
on the Co
H with {111} planes. Moreover, the lattice oxygens in Co
as confirmed in H -TPR and C -TPSR, which enhances the interaction between the adsorbed
and the surface lattice oxygens and increase the catalytic activity. In addition, as illustrated
in in-situ Raman, the more facile activation of gas phase oxygen on Co -H surface can result in
the more efficient replenish of oxygen vacancies created by surface reactions, which is also
conducive to the catalytic activity. Therefore, the Co -H with {111} planes can achieve the best
catalytic performance towards C combustion. For Co -C and Co -R, the C-H activation
3
O
4
{111} planes. This may be the important reason for the high activity of the Co
3
4
O -
3
O
4
-H are more active than other samples
2
3
H
8
C
3
H
8
3
O
4
3
O
4
3
H
8
3
O
4
3
O
4
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ChemCatChem
10.1002/cctc.201901382
ability as well as the redox properties is inferior to Co
lower than Co -H. In terms of Co
surface active species both lead to the poor activity towards C
. Conclusion
Co nanocatalysts with different morphologies and different exposed facets were fabricated
through hydrothermal routes. The obtained catalysts mainly exposed {100}, {111}, {110} and
112} planes, respectively. The catalysts were employed in C combustion and the catalytic
activities of different facets varied as the order of {111} > {100} > {110} > {112}. The Co -H
3
O
4
-H. As a result, the activities are relatively
3
O
4
3
O
4
-P, the difficult activation of C-H bond and the lack of
combustion.
3
H
8
4
3
O
4
{
3
H
8
3
O
4
o
with {111} planes exhibited the best activity with T50 and T90 as low as 209 and 239 C,
respectively. DRIFT and DFT calculations confirmed that the C-H activation was more facile on
{
111} planes, which was of great importance for the high activity. Moreover, the higher lattice
oxygen activity and easy activation of gas phase oxygen also made the catalyst with {111} planes
more active towards C
3
8
H combustion.
Acknowledgements
This work was financially supported by a Project supported by the Shanxi Provincial Research
Foundation for Basic Research (Grant No. 201601D021044, 201701D121037, 201701D221056)
and the Coal-base Key Technologies R&D Program of Shanxi Province (Grant No. MH2014-13).
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ChemCatChem
10.1002/cctc.201901382
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Figure captions
Figure 1. Powder XRD patterns of investigated catalysts.
Figure 2. Visible Raman spectra of investigated catalysts.
Figure 3. TEM and HRTEM of investigated Co
c1-c2) Co -R; (d1-d2) Co -P. Insets in a1 to d1 are the schematic diagrams of the
corresponding catalysts. Insets in a2 to d2 show the FFTs of the HRTEM images. c3 to d3 present
present the surface structures of the Co (100), (111), (110) and (112) surfaces, respectively.
Figure 4. (a) Co 2p and (b) O 1s XPS of investigated catalysts.
Figure 5. (a) H -TPR and (b) C -TPSR profiles of investigated catalysts.
3
O
4
catalysts: (a1-a2) Co
3
O
4
-C; (b1-b2) Co
3
4
O -H;
(
3
O
4
3
O
4
3
O
4
2
3
H
8
Figure 6. The evolution of A1g peak intensity vs temperature during the in-situ Raman experiments
of investigated catalysts.
Figure 7. The in-situ DRIFT spectra over different Co
3
O
4
catalysts in the stream of 1% C
3
8
H /Ar
o
o
with temperature increasing from 50 C to 300 C.
Figure 8. (a) C combustion activity vs temperature profiles of investigated catalysts; (b)
Arrhenius plots of the invstigated samples; (c) Specific reaction rates of C combusiton over
different Co catalysts.
Figure 9. The most stable adsorption configurations of dissociated C
and adsorption energy (Eads) on the four different crystal planes of Co
100}-O surface, (c) {110}-B surface and (d) {112} surface
3
H
8
3
H
8
3
O
4
3
H
8
molecule (CH
3
CHCH +H)
3
3
O4. (a) {111} surface, (b)
{
Scheme 1. Schematic illustration of the synthetic routes for the investigated Co
3
4
O catalysts.
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ChemCatChem
10.1002/cctc.201901382
Figure 1
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ChemCatChem
10.1002/cctc.201901382
Figure 2
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ChemCatChem
10.1002/cctc.201901382
Figure 3
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