Synthesis, characterization and catalytic performance of nanocrystalline Co3O4 towards propane combustion: Effects of small molecular carboxylic acids

Article abstract of DOI:10.1016/j.jssc.2020.121712

A series of nanocrystalline Co₃O₄ catalysts were synthesized by low-temperature liquid phase complexation with different small molecular carboxylic acids (citric acid, oxalic acid, and tartaric acid). The effects of different carboxylic acids on the catalyst structure and surface microenvironment were discussed in detail, and the catalytic performance of propane combustion was investigated. The results showed that different carboxylic acids changed the distance between Co²⁺ ions in the catalyst precursor, which directly affected the crystallite size (12–16 ?nm) and specific surface area (10~31 ?m² ?g^?1) of nanocrystalline Co₃O₄. Among them, Co₃O₄ synthesized from the cobalt citrate precursor with the largest Co²⁺ ion distance has the best catalytic activity. The conversion rate of propane reached 90% at 250 ?°C and showed stable performance in 40 ?h continuous reactions. The excellent catalytic performance could be allocated to the smallest crystallite size, excellent texture properties, a high degree of lattice disorder, unique surface composition, and excellent reducibility.

Full text of DOI:10.1016/j.jssc.2020.121712

Journal of Solid State Chemistry 292 (2020) 121712
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Synthesis, characterization and catalytic performance of nanocrystalline
Co3O4 towards propane combustion: Effects of small molecular
carboxylic acids
,
,
,
,
,
Zhao Liu a b, Lijun Cheng a b, Jia Zeng a b, Xin Hu a b, Shiyun Zhangxue a b, Shanliang Yuan a,
Qifei Bo a, Biao Zhang a **, Yi Jiang a
,
,*
a National Engineering Laboratory for VOCs Pollution Control Material & Technology (Chengdu), Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences,
Chengdu, 610041, Sichuan, China
b University of Chinese Academy of Sciences, Beijing, 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Co3O4
Small molecular carboxylic acids
Complexing agent
Catalytic combustion
Propane
A series of nanocrystalline Co3O4 catalysts were synthesized by low-temperature liquid phase complexation with
different small molecular carboxylic acids (citric acid, oxalic acid, and tartaric acid). The effects of different
carboxylic acids on the catalyst structure and surface microenvironment were discussed in detail, and the catalytic
performance of propane combustion was investigated. The results showed that different carboxylic acids changed
the distance between Co2þ ions in the catalyst precursor, which directly affected the crystallite size (12–16 nm)
and specific surface area (10~31 m2 ꢀ1) of nanocrystalline Co3O4. Among them, Co3O4 synthesized from the
g
cobalt citrate precursor with the largest Co2þ ion distance has the best catalytic activity. The conversion rate of
propane reached 90% at 250 ꢁC and showed stable performance in 40 h continuous reactions. The excellent
catalytic performance could be allocated to the smallest crystallite size, excellent texture properties, a high degree
of lattice disorder, unique surface composition, and excellent reducibility.
1. Introduction
the development of inexpensive and highly active non-noble metal oxide
catalysts has practical significance. Recently, researchers have explored
Volatile organic compounds (VOCs) emitted by human production
activities have become the main pollutants in the atmosphere and
contribute significantly to the formation of photochemical smog, ozone,
and greenhouse gases [1,2]. Among them, short-chain alkanes from C2-4
are the most challenging species to eliminate due to their high activation
energy and stability of C-C and C-H bonds. Compared with other VOCs
treatment technologies, catalytic combustion has high VOCs removal
efficiency and harmless products H2O and CO2 at relatively low tem-
perature, which is extremely effective for VOCs control and emission
reduction [3].
The focus of the development of VOCs catalytic combustion tech-
nology is the design and preparation of high-performance catalysts. Re-
searchers have conducted extensive studies on noble metal-based
catalysts (Pt [4], Pd [5], Ru [6,7], etc.) and found that they are effective
catalysts for the complete oxidation of short-chain alkanes, but their
application is limited due to their high price and short service life. Hence,
non-noble metal oxide catalysts such as CoOx [8,9], MnOx [10,11], CeOx
[12], NiOx [13] and corresponding mixtures [14–17] for catalytic com-
bustion of short-chain alkanes, in which single nanocrystalline Co3O4 has
better catalytic activity [9,18,19]. The activity of Co3O4 depends on its
specific surface area, crystal size, reducibility, surface oxygen content,
morphology, and crystal plane [20–23]. Meanwhile, the preparation
method also makes a significant influence on the physical and chemical
properties of Co3O4. Several methods for preparing Co3O4 have been
suggested: precipitation [1,24], hydrothermal [21,25], hard template
[26,27], combustion [28], sol gel [29] and complex method [30]. Among
them, the complex method for preparing nano-oxides has the advantages
of high product purity, low heat treatment temperature, and short
preparation time.
The study found that complexing agents play a key role in controlling
the pore structure and morphology of metal oxides. For example, Yang
[31] et al. synthesized 3D micro/nanostructure CeO2 by changing the
* Corresponding author.
** Corresponding author.
E-mail address: yjiang@cioc.ac.cn (Y. Jiang).
https://doi.org/10.1016/j.jssc.2020.121712
Received 25 July 2020; Received in revised form 30 August 2020; Accepted 4 September 2020
Available online 9 September 2020
0022-4596/© 2020 Elsevier Inc. All rights reserved.

Z. Liu et al.
Journal of Solid State Chemistry 292 (2020) 121712
types of organic acids (acetic acid, propionic acid, n-butyric acid, and
oleic acid), and found that CeO2 synthesized with acetic acid displayed
hierarchical porosity and optimal toluene combustion active. Ren [32]
et al. used tartaric acid as the complexing agent and introduced a small
amount of nitric acid to adjust the gelation rate of LaMnO3 colloid, to
successfully construct LaMnO3 with 3DOM structure, showing good ac-
tivity in the catalytic oxidation of ethyl acetate. Besides, citric acid has
been widely used as a complexing agent to prepare metal oxides and
applied to the catalytic combustion of VOCs [19,24,33]. For example,
Zhang [34] et al. successfully prepared nanocrystalline CuMn2O4 oxide
with an abundant porous structure using citric acid, which exhibited
excellent catalytic performance in the catalytic oxidation of benzene.
However, due to the similar structure of many small molecule carboxylic
acids with citric acid, the effects of different carboxylic acids on the
composition, structure, morphology, and catalytic performance of metal
oxide catalysts are rarely mentioned. To our knowledge, the effect of
small molecular carboxylic acid on the catalytic performance of Co3O4 in
the catalytic combustion of short-chain alkanes has not been reported.
In this work, three different small molecular carboxylic acids, citric
acid (CA), oxalic acid (OA), and tartaric acid (TA), were used to assist the
synthesis of nanocrystalline Co3O4. The effects of carboxylic acids on the
physicochemical properties of nanocrystalline Co3O4 were studied by FT-
IR, XRD, BET, XPS, TEM/HRTEM, H2-TPR and O2-TPD, and their cata-
lytic performance in catalytic combustion of propane (a typical short-
chain alkane) was evaluated. The purpose of this work is to study the
effect of different small molecular carboxylic acid on the catalytic activity
of Co3O4 for propane combustion and systematically discuss the
structure-activity relationship between Co3O4 and propane conversion.
temperature-programmed unit and a thermal conductivity detector
(TCD). In the experiment, 15 mg of the catalyst was put into the constant
temperature section of the U-shaped quartz tube and pretreated at 300 ꢁC
for 1 h under high purity N2. After cooling to room temperature, the gas
was switched to a 5% H2–95% N2 mixture at a flow rate of 25 ml/min.
̊
The TCD signal was recorded at 10 C/min from room temperature to 600
ꢁC after the baseline was stable. The H2 consumption of all catalysts was
calibrated by the H2-TPR treatment of CuO powder. The oxygen tem-
perature program desorption (O2-TPD) device is the same as H2-TPR. In
each test, 100 mg of the catalyst was placed in a U-shaped quartz tube
and pretreated in high purity O2 at 300 ꢁC for 1 h. After cooling to room
temperature, the gas was switched to high purity He at a flow rate of 30
ml/min. The TCD signal was recorded at 10 ꢁC/min from room tem-
perature to 600 ꢁC after the baseline was stable. During the H2-TPR and
O2-TPD tests, the two ends of the U-shaped tube are connected to a dryer
equipped with a color-changing silica gel to eliminate the effect of water
vapor.
2.3. Catalytic tests
The catalytic combustion reaction of propane was carried out on a
CGK-5A fixed bed reactor with an inner diameter of 8 mm. The propane
concentration before and after the reaction was analyzed online using an
SC-8000 gas chromatograph equipped with a flame ionization detector
(FID), while the CO and CO2 concentrations from reaction tail gas were
measured using an SC-3000 gas chromatograph equipped with a thermal
capture detector (TCD). The composition of the raw gas was 0.3 vol %
propane balanced by air. The flow rate and catalyst loadings were 100
mL/min and 200 mg, respectively, corresponding to a gas hourly space
velocity (GHSV) of 30000 mlꢂgꢀ1ꢂhꢀ1. The conversion of propane is
based on equation (1):
2. Experimental
2.1. Catalyst preparation
YðC3H8Þ ¼ ð1 ꢀ ðC3H8Þout = ðC3H8ÞinÞ ꢃ 100%
(1)
The preparation process of the Co3O4 catalyst was similar to the
literature [24]. In a typical synthesis process, 0.0274 mol of CA, OA, and
TA were added to 25 ml of 1 mol/L Co(NO3)2ꢂ6H2O aqueous solution
under stirring, respectively. The obtained solution was continuously
stirred and evaporated at 80 ꢁC for 90 min and dried at 120 ꢁC for 6 h to
obtain the corresponding precursor. Then the precursor was calcined in
an air atmosphere at 300 ꢁC for 1 h at a heating rate of 2 ꢁC/min to
decompose the organic framework [35]. Finally, the corresponding cat-
alysts were obtained by heating to 400 ꢁC at the same heating rate for 2 h
and labeled as Co-CA, Co-OA, and Co-TA, respectively.
Where Y(C3H8) is the conversion of propane, (C3H8)in and (C3H8)out
represents the volume concentration of propane before and after the
reaction, respectively.
2.4. Reaction kinetics testing
Kinetic experiments are similar to reported in the literature [36].
Before the experiment, the effect of internal and external diffusion was
excluded. The conversion of propane is controlled below 15%, and the
reaction rate (r, mol/(s⋅g)) can be expressed by equation (2):
2.2. Catalyst characterization
r ¼ NðC3H8Þ ꢃ YðC3H8Þ=WðCatÞ
(2)
Fourier-transform infrared (FT-IR) spectrum was identified from 400
to 4000 cmꢀ1 on a Nicolet 6700 FT-IR spectrometer (ThermoFisher,
USA). The N2 adsorption-desorption analyses were measured on a NOVA
2200E nitrogen adsorption instrument (Quantachrome, USA) at ꢀ196 ꢁC.
Before testing, the catalysts were degassed under vacuum at 300 ꢁC for 3
h. The specific surface areas and pore size distributions were determined
by the Brunauer-Emmett-Teller (BET) and Barrett–Joyner–Halenda
(BJH) method, respectively. X-ray diffraction (XRD) was determined by a
Where N(C3H8) is the flow rate of propane (mol/s) and W(Cat) is the
mass of the catalyst (g). The process of propane oxidation is irreversible,
and the effects of reaction products H2O and CO2 on the reaction rate can
be ignored. Thus, the reaction rate equation can also be expressed by the
Arrhenius equation (3):
r ¼ Aexpð ꢀ Ea = RTÞPa POb
(3)
C3H8
2
diffractometer (Shimadzu, Japan) with Cu K
α radiation (λ ¼ 0.1541 nm)
Where T (K) is the reaction temperature, R is the ideal gas constant
(8.314 J/(mol⋅K)), Ea (kJ/mol) is the apparent activation energy, and A
is the pre-exponential factor. During the test, when the propane con-
version rate is less than 15%, the composition of the raw gas is approx-
imately constant. The relationship between ln r and RT can be obtained
by equation (4):
operated at 40 kV and 30 mA. The scan rate and step sizes were 4 ꢁC/min
and 0.02ꢁ, respectively. The transmission electron microscope (TEM/
HRTEM) was completed by the Tecnai G2 F20 electron microscope (FEI,
USA) at an acceleration voltage of 200 kV. X-ray photoelectron spec-
troscopy (XPS) was performed by the XSAM800 spectrometer (Kratos,
USA) using Al K
calibrated based on the C 1s spectrum (284.8 eV). The hydrogen
temperature-programmed reduction (H2-TPR) device consists of
α radiation (1253.6 eV). The electron binding energy was
lnr ¼ ꢀ Ea=RT þ C
(4)
a
2

Z. Liu et al.
Journal of Solid State Chemistry 292 (2020) 121712
form of bidentate. The strong peak at 1086.6 cmꢀ1 is attributed to δ(C–H)
deformation vibration as well as π(C–H) skeleton vibration mode.
FTIR shows that small molecular carboxylic acids and Co2þ in the
precursor have different binding modes. And the possible coordination
forms of different precursors are shown in Fig. 2. Among them, the hy-
droxyl group in CA and TA did not coordinate, while the CA, OA, and TA
coordinate with Co2þ in the form of monodentate, tetradentate, and
bidentate, respectively. Due to the coordination of CA with Co2þ in the
form of monodentate, the distance between Co2þ is relatively long.
Meanwhile, CA has the longest chain length among all carboxylic acids,
which will further expand the distance between Co2þ ions and effectively
dispersing Co2þ ions. OA coordinates with Co2þ in the form of tetra-
dentate, resulting in two five-membered rings between Co2þ, which is
slightly shorter than the distance between Co2þ in CA coordination.
However, TA coordinates with Co2þ in a bidentate form, resulting in only
a seven-membered ring between Co2þ, which has the shortest distance.
Therefore, the distance between Co2þ after coordination with carboxylic
acids is as follows: Co-CA precursor > Co-OA precursor > Co-TA pre-
cursor. The larger the distance between Co2þ, the more conducive to the
formation of high-dispersion and small crystalline size Co nanoparticles
after calcination, which is further confirmed by XRD, N2 adsorption-
desorption, and TEM. It can be inferred that the Co oxide prepared by
monodentate coordination with CA can fully contact with propane
molecules to enhance the catalytic activity.
Fig. 1. FT-IR spectrum of different Co3O4 precursor in the range of
400–4000 cmꢀ1
.
C stands for a constant. By calculating the r at a different temperature,
the Ea can be obtained by the slope of the ln r and 1/RT curves.
3.2. XRD results
3. Results and discussion
The XRD patterns of Co-CA, Co-OA, and Co-TA are shown in Fig. 3. All
the catalysts show diffraction peaks at 2θ ¼ 31.2ꢁ, 36.8ꢁ, 38.6ꢁ, 44.8ꢁ,
55.6ꢁ, 59.3ꢁ, 65.2ꢁ, corresponding to the (220), (311), (222), (400),
(422), (511) and (440) planes of spinel Co3O4 (JCPDS 43-1003). Mean-
while, no additional diffraction peaks appeared in all samples, indicating
that only pure Co3O4 phase exists. Also, the diffraction peak intensity of
catalysts is significantly different. Co-CA has the weak and broad
diffraction peak, which means small crystalline size and poor crystallinity
[42]. The average crystalline size of Co3O4 based on (311) plane is
calculated by Scherrer equation, and the corresponding results are shown
in Table 1. Compared to Co-OA and Co-TA, Co-CA has the smallest
crystalline size (11.8 nm), indicating that the addition of CA is conducive
to the formation of small crystalline size Co3O4.
3.1. FT-IR results
The molecular structure of the Co3O4 precursor was revealed by the
FT-IR spectrum. As shown in Fig. 1, the absorption peaks of Co-CA pre-
cursor, Co-OA precursor, and Co-TA precursor at 557.3 cmꢀ1, 494.1
cmꢀ1, and 548.8 cmꢀ1, respectively, belong to the
ν(Co–O) stretching
vibration, indicating that the corresponding carboxylic acid is coordi-
nated with Co2þ ion.
The absorption peak of the Co-CA precursor at 3395.9 cmꢀ1 is
attributed to the
ν(O-H) stretching vibration peak in the water molecule
or OH ligand. The peak at 1627.0 cmꢀ1 is ascribed to the
ν
as(C¼O)
antisymmetric stretching vibration of the carboxylate group in the co-
ordinated citrate. Moreover, absorption peaks at 1412.7 cmꢀ1 and
1384.6 cmꢀ1 can be attributed to the
νs(C¼O) symmetric stretching vi-
3.3. N2 adsorption-desorption results
bration of the carboxylate group. And the difference between the
wavenumber of the
The N2 adsorption-desorption isotherms and BJH pore size distribu-
tions of nanocrystalline Co3O4 are shown in Fig. 4. All catalysts exhibit
typical IV shape isotherms with H3-type hysteresis loops at P/Pa>0.75,
indicating the existence of mesoporous structure [43]. The relatively
broad hysteresis loop in the Co-CA isotherm indicates effective mass
transfer and less pore-blockage [44]. The pore size distribution of all
catalysts is mainly concentrated at 2~30 nm, having a center in the range
of 17 nm. The mesopores are disordered and may originate from the
extrusion between particles. The texture properties are summarized in
Table 1. Compared to Co-OA and Co-TA, Co-CA has the largest specific
surface area (31.4 m2 gꢀ1) and pore-volume (5.6*10ꢀ2 cm3 g), which can
effectively enhance the contact area between the catalyst surface and
gas-phase molecules [45].
ν
as(C¼O) and
ν
s(C¼O) is 214.3 cmꢀ1, indicating that
the carboxyl oxygen participates in the coordination in the form of
monodentate [37]. The absorption peak at 1312.3 cmꢀ1 is the δ(C-OH)
deformation vibration mode, indicating that the hydroxyl in CA does not
participate in the complexation of cobalt.
The strong peak of the Co-OA precursor at 3364.8 cmꢀ1 is allocated to
the stretching vibration of the (O-H) group, indicating the water of hy-
dration. The band at 1620.1 cmꢀ1 corresponding to asymmetric
ν
as(C–O)
and the closely spaced bands at 1360.2 cmꢀ1 and 1316.4 cmꢀ1 assigned
to symmetric s(C–O), respectively, clearly reveal the presence of
ν
bridging oxalates, with all four oxygen atoms coordinated to the cobalt
atoms [38,39]. The peak at 827.6 cmꢀ1 is assigned to the δ(O–C–O) band
appears [38].
For the Co-TA precursor, the strong and broad peak at 3418.9 cmꢀ1 is
attributed to the
water of hydration or OH ligand [40]. The sharp peaks at 1610.7 and
1414.7 cmꢀ1 are attributed to the
as(C¼O) asymmetric stretching vi-
bration and the
s(C¼O) symmetric stretching vibration of the carbonyl
groups. Close to 1414.7 cmꢀ1, the weak peak at 1384.2 cmꢀ1 is also
ν
(O-H) stretching mode, indicating the presence of
3.4. TEM results
ν
Fig. 5 shows the TEM images and particle size distributions of Co-CA,
Co-OA, and Co-TA. All Co3O4 are nanoparticles assembled from irregular
polyhedrons and have different degrees of aggregation. Among them, the
accumulation of small particles in Co-CA and Co-OA produces a large
number of mesopores, while the aggregation of larger particles in Co-TA
significantly reduces the number of pores, coinciding well with N2
adsorption-desorption results. And the average particle size of Co-CA
ν
caused by the
νs(C¼O) symmetric stretching. The absorption peak at
1240.6 cmꢀ1 is assigned to the out of plane bending vibration mode of
δ(C-OH), indicating that the alcohol hydroxyl is not dissociated [41].
These results indicate that TA and Co2þ are mainly coordinated in the
3

Z. Liu et al.
Journal of Solid State Chemistry 292 (2020) 121712
Fig. 2. Possible coordination forms of different Co3O4 precursors.
poor crystallinity and high lattice disorder, which can be attributed to
more crystal defects, whereas Co-OA and Co-TA have higher crystalli-
zation degree. Abundant surface defects may provide sufficient active
sites for deep oxidation reactions [46], which may contribute the much
improved activity in Co-CA over the Co-OA and Co-TA. The lattice fringe
spacing of Co-OA and Co-TA is 0.467 nm, corresponding to the (111)
crystal plane of Co3O4. However, Co-CA exposes lattice fringes with
spacing of 0.243 nm and 0.286 nm, which are assigned to (311) and
(220) crystal planes of Co3O4, respectively. According to the literature
[47–49], the (111) crystal plane of Co3O4 contains only Co2þ ions rather
than the active surface, while the (220) crystal plane of Co3O4 is mainly
composed of Co3þ ions, which can contribute enough active sites for
VOCs oxidation. Nanocrystalline Co3O4 with abundant surface defects
and exposed (220) crystal planes may exhibit better catalytic activity in
the propane combustion reaction.
3.6. XPS results
The surface composition and oxidation state of catalysts were inves-
tigated by XPS technology. Fig. 7(a) shows the Co 2p XPS spectrum of the
as-prepared nanocrystalline Co3O4. The electrons in the 2p orbital of Co
cause spin-orbital splitting, generating two main peaks of Co 2p3/2 and
Co 2p1/2 near BE of 780.0eV and 795.0eV, and two shake-up satellites
peaks near BE of 785.0eV and 803.0eV, which proves the presence of
Co3O4 [50]. By peak fitting deconvolution technology, the Co 2p3/2 peak
was decomposed into two peaks near BE of 779.5 eV and 781.0 eV, which
belongs to the surface Co3þ and Co2þ, respectively [50–52]. The XPS
parameters of catalysts are shown in Table 2. The order of the molar ratio
of Co3þ/Co2þ on the surface of Co3O4 from large to small is: Co-CA >
Co-OA > Co-TA, indicating more Co3þ ions on the surface of Co3O4
prepared with CA, while the Co3þ ions with lower Co3þ-O bond energy
are conducive to the release of reactive oxygen species, thus enhancing
the catalytic oxidation performance [53].
Fig. 3. XRD patterns of Co-CA, Co-OA and Co-TA.
Table 1
Texture properties and crystalline size of Co-CA, Co-OA and Co-TA.
Sample
SBET(m2⋅gꢀ1
)
Vp*10ꢀ2 (cm3 g)
DBJH(nm)
aCrystalline size (nm)
Co-CA
Co-OA
Co-TA
31.4
17.2
9.9
5.6
3.7
2.0
4.0
3.6
3.6
12.6
14.9
15.7
a
The crystalline size of Co3O4 was calculated based on the (311) plane.
(13.89 nm) is smaller than that of Co-OA (17.37 nm) and Co-TA (23.64
nm), which is in line with XRD results.
As shown in Fig. 7(b), the O1s peaks are also classified by peak fitting
deconvolution technique. The peaks at 528.9, 529.1 and 529.1eV belong
to the lattice oxygen species Olatt (O2ꢀ), the peaks at 530.5, 530.6 and
530.8eV belong to the surface-adsorbed oxygen species Oads (O22ꢀ、O2ꢀ、
Oꢀ), the peaks at 532.3, 532.4 and 532.3eV are attributed to molecular
3.5. HRTEM results
Fig. 6 presents the HRTEM images of different nanocrystalline Co3O4
catalysts. The lattice fringes of Co-CA are messy and unclear, showing
Fig. 4. (a) N2 adsorption-desorption isotherms; (b) Pore size distribution of Co-CA, Co-OA and Co-TA.
4

Z. Liu et al.
Journal of Solid State Chemistry 292 (2020) 121712
Fig. 5. TEM and particle size distributions of (a) Co-CA; (b) Co-OA; (c) Co-TA.
Fig. 6. HRTEM images of (a) Co-CA; (b) Co-OA; (c) Co-TA.
Fig. 7. (a) Co 2p and (b) O 1s XPS spectra of Co-CA, Co-OA and Co-TA.
Table 2
XPS parameters of Co-CA, Co-OA and Co-TA.
Catalyst
Binding energy (eV)
Surface element contents (mol. %)
Co3þ/Co2þ
Oads/Olatt
Co2þ
Co3þ
OꢀOH/CO3
Oads
Olatt
Co
O
2-
Co-CA
Co-OA
Co-TA
781.0
780.8
781.2
779.2
779.1
779.2
532.3
532.4
532.3
530.5
530.6
530.8
528.9
529.1
529.1
28.6
32.4
33.5
71.4
67.7
66.5
0.99
0.84
0.72
1.19
0.87
0.83
water (i.e., hydroxyl) or carbonate species [54,55]. It can be seen from
Table 2 that the mole ratio of Oabs/Olatt on the catalyst surface from high
to low is: Co-CA > Co-OA > Co-TA. Generally, the mobility of surface
adsorbed oxygen is higher than that of lattice oxygen, and thus more
active in oxidation reaction [46]. Therefore, the nanocrystalline Co3O4
prepared by CA complexation has more surface Co3þ ions and surface
adsorbed oxygen species, which will exhibit better activity in catalytic
combustion of propane.
3.7. H2-TPR results
The H2-TPR profiles of different nanocrystalline Co3O4 are shown in
Fig. 8. After peak fitting, two partially overlapping reduction peaks (
α
and β peak) in the center at approximately 340 ꢁC and 400 ꢁC are present,
which can be attributed to the reduction of Co3þ→Co2þ and Co2þ→Co in
Co3O4, respectively [42,56]. As for the Co-OA and Co-TA, the corre-
sponding reduction peaks shifted to a higher temperature compared with
that of the Co-CA, which indicates that the complexing agent strongly
5

Z. Liu et al.
Journal of Solid State Chemistry 292 (2020) 121712
Fig. 8. H2-TPR of Co-CA, Co-OA and Co-TA.
Fig. 9. O2-TPD of Co-CA, Co-OA and Co-TA.
Table 3
Table 4
Oxygen desorption peak area of catalyst.
H2-TPR parameters of the Co-CA, Co-OA and Co-TA.
The peak areas (<350 ꢁC)
The peak areas (350ꢁC–500 ꢁC)
Catalyst
H2-TPR
Catalyst
Temperature (ꢁC)
H2 consumption (mmol⋅gꢀ1
)
Co-CA
Co-OA
Co-TA
1578173
1095408
317700
331081
159011
52121
α
β
α
β
Co-CA
Co-OA
Co-TA
336
337
354
390
430
426
6.91
1.99
1.72
5.34
12.51
11.57
strong bond energy [43]. From Fig. 9, all catalysts produced several
distinct desorption peaks, corresponding to different oxygen species. The
desorption peaks in the regions below 350 ꢁC, between 350 ꢁC and 500
ꢁC and above 500 ꢁC are attributed to the desorption of surface adsorbed
oxygen, surface lattice oxygen, and bulk lattice oxygen, respectively [58].
The oxygen desorption peak areas of the catalysts are given in
Table 4. In the surface adsorbed oxygen desorption stage (below 350 ꢁC),
the oxygen desorption peak area of Co-CA is 1.44 and 4.96 times that of
Co-OA and Co-TA, respectively, which can provide more active oxygen in
the oxidation reaction species. Combined with N2 adsorption-desorption
and HRTEM analysis, Co-CA with a large specific surface and high lattice
disorder will form more heterogeneous, convex, and rough structures,
which is conducive to the generation of oxygen vacancies on the surface,
thus generating abundant adsorbed oxygen species and improving cata-
lytic oxidation performance. Besides, in the surface lattice oxygen
desorption stage (350ꢁC–500 ꢁC), Co-CA still has the largest oxygen
desorption peak area, which is 2.08 and 6.35 times that of Co-OA and Co-
TA, respectively, indicating its existence more surface lattice oxygen
species. When the surface adsorbed oxygen species are consumed, the
surface lattice oxygen starts to be activated and participates in the re-
action, and the generated oxygen vacancies will further capture gas phase
oxygen and form a redox cycle. Furthermore, the lattice oxygen
desorption peak temperature of Co-CA is lower than that of Co-OA,
indicating a better surface lattice oxygen mobility, which will effec-
tively promote the combination of oxygen species and reactant molecules
and the filling of oxygen vacancies, thereby improving catalytic oxida-
tion performance.
affected the reducibility of Co3O4. The reduction behavior of Co3O4 is
greatly affected by the particles size and dispersion state of cobalt [56,
57]. As described in XRD and TEM, Co-CA is composed of smaller
nanocrystals, which may result in more exposed areas and easier
reduction. The nanocrystalline Co3O4 prepared using citric acid as
complexing agent contained more easily reduced species than that of
Co-OA and Co-TA, which is promising for improved catalytic activity for
propane combustion. Table 3 summarizes the reduction peak tempera-
ture and H2 consumption of the catalyst. The total theoretical H2 con-
sumption for the reduction of well-defined Co3O4 is 16.6 mmol gꢀ1
.
However, the total experimental H2 consumption measured for the
different Co3O4 catalysts ranged from 12.3 to 14.5 mmol gꢀ1, which is
lower than the theoretical H2 consumption. This difference suggests a
proportion of Co2þ in the catalysts greater than that indicated by the
stoichiometric ratio. Whether it is
reduction peak temperature than Co-OA and Co-TA, indicating a weaker
Co-O bond strength and stronger oxidation performance. Besides, the
α peak or β peak, Co-CA has a lower
order of the H2 consumption of the
α peak is: Co-CA > Co-OA > Co-TA,
indicating a large number of easily dissociable Co3þ-O species on Co-CA
involved in the reduction of Co3þ→Co2þ, which is consistent with XPS
results. Combining HRTEM and XPS analysis shows that nanocrystalline
Co3O4 prepared by CA complexation has a high degree of lattice disorder,
which is conducive to the adsorption of oxygen species and participates
in the oxidation reaction as active oxygen, which may be the main reason
for the easier reduction of Co-CA.
3.8. O2-TPD results
3.9. Catalytic activity determination, kinetics study, and stability test
Fig. 9 shows the O2-TPD profiles of Co-CA, Co-OA and Co-TA. In
general, the desorption of oxygen species in inert atmosphere is closely
related to temperature [19]. Surface adsorbed oxygen species (Oꢀ2 , Oꢀ,
O22ꢀ) are more easily desorbed from transition metal oxides as a
surface-active oxygen species and participates in oxidation reactions, and
lattice oxygen (O2ꢀ) species are difficult to be desorbed due to their
The products of catalytic combustion of propane are shown in Fig. 10.
Only propane and CO2 were detected in the FID and TCD detection
systems, and no associated hydrocarbons and CO were detected, indi-
cating that propane was completely oxidized.
The catalytic combustion activity of propane under different nano-
crystalline Co3O4 is shown in Fig. 11(a). The light-off temperature (T10
)
6

Z. Liu et al.
Journal of Solid State Chemistry 292 (2020) 121712
Fig. 10. Composition of reaction products (a) Propane content; (b) CO2 content. Test conditions: The feed consisted of 0.3 vol% propane balanced by air, with a gas
hourly space velocity (GHSV) of 30000 mL/(h⋅g).
of all catalysts is about 180 ꢁC. As the temperature increases, the con-
version rate of propane increases rapidly. With a 90% propane conver-
sion rate (T90) as the activity evaluation criterion, the T90 of Co-CA is 250
ꢁC, showing a best low-temperature catalytic activity. However, the T90
of Co-OA and Co-TA is 262 ꢁC and 279 ꢁC respectively, which is 12 ꢁC and
29 ꢁC higher than that of Co-CA.
Fig. 11(b) presents the Arrhenius relationship curve between ln r and
1/RT, and the corresponding activation energy (Ea) is obtained from the
slope of the fitted straight line. The order of Ea is as follows: Co-TA > Co-
OA > Co-CA, where Co-CA has the lowest Ea of 43.9 kJ/mol. The lower
Ea has a lower reaction barrier, revealing that the catalytic combustion
reaction on Co-CA could be easier to initiate [59].
crystalline size. It can be clearly seen that the larger crystalline size
corresponds to higher T50 and T90 temperatures, while Co-CA has the
smallest crystalline size (12.6 nm), corresponding to the optimal catalytic
propane conversion activity. The small size effect of this nanocrystalline
Co3O4 is similar to the work of Rivas [60].
The catalytic performance with time on the line of the Co-CA is dis-
played in Fig. 11(d). After 40 h of operation at 250 ꢁC, the conversion rate
of propane is maintained at about 90%, indicating that Co-CA has good
stability.
The activity of the Co-CA was compared to that reported in the
literature, as shown in Table 5. In this work, the catalytic activity of Co-
CA can be comparable to other Co3O4, two-component transition metal
oxides, rare earth catalysts as well as noble metal catalysts, showing
relatively good catalytic activity.
Although all catalysts are composed of pure Co3O4 phase, Fig. 11(c)
shows that the activity of the catalyst has a certain dependence with the
Fig. 11. (a) Catalytic activity of propane combustion; (b) Arrhenius plots of the relationship between ln r and 1/RT; (c) Relationship between crystalline size and
catalytic activity (T50 and T90); (d) Stability test of Co-CA at 250 ꢁC. Test conditions: The feed consisted of 0.3 vol% propane balanced by air, with a gas hourly space
velocity (GHSV) of 30000 mL/(h⋅g).
7

Z. Liu et al.
Journal of Solid State Chemistry 292 (2020) 121712
order of catalytic activity is the same as the order of surface Oads/Olatt
ratio and Co3þ/Co2þ ratio, which can be attributed to the abundant Co3þ
and adsorbed oxygen species on the surface that produce enough active
oxygen species to participate in the oxidation reaction. Combined with
XPS, O2-TPD, and H2-TPR analysis, the surface of the Co-CA has more
adsorbed active oxygen species and faster lattice oxygen mobility,
resulting in stronger reducibility. These factors contribute to the stronger
catalytic oxidation performance of Co-CA than Co-OA and Co-TA.
Table 5
Comparison with catalysts reported in the literature.
Catalyst
C3H8 concentration
(ppm)
GHSV
T90
Reference
(mlꢂgꢀ1ꢂhꢀ1
)
(ꢁC)
Co-CA
Co3O4
Co3O4
Co3O4
NiCoO4
MnNiOx
LaMnOx
Pt/AlF3
3000
3000
5000
2500
10000
2000
2000
2000
30000
12000
30000
120000
45000
30000
30000
80000
250
280
270
241
425
242
255
250
This work
[61]
[62]
[63]
[64]
[65]
[36]
[66]
4. Conclusions
In this work, several nanocrystalline Co3O4 catalysts were synthesized
based on different small molecular carboxylic acids (citric acid, oxalic
acid, and tartaric acid). The effect of small molecular carboxylic acids on
the physicochemical properties of the catalyst was discussed in detail,
and the catalytic performance for the combustion of propane (a typical
short-chain alkane) was evaluated. The results showed that different
small molecular carboxylic acids changed the distance between Co2þ ions
in the catalyst precursor, thus strongly affecting the structure and surface
microenvironment of nanocrystalline Co3O4. Among them, the nano-
crystalline Co3O4 synthesized from the cobalt citrate precursor with the
largest Co2þ ion distance exhibited the best catalytic activity, reaching
90% propane conversion rate at 250 ꢁC, and showed stable performance
in 40h continuous reaction. Small crystallite size, high lattice disorder
degree, excellent texture properties, abundant surface Co3þ, and Oads and
strong reducibility are the main reasons for its excellent catalytic
performance.
CRediT authorship contribution statement
Fig. 12. Schematic of propane oxidation on cobalt oxide.
Zhao Liu: Formal analysis, conducted sample preparation and data
analysis. Lijun Cheng: Data curation, calculated the data. Jia Zeng: Data
curation, calculated the data. Xin Hu: Data curation, calculated the data.
Shiyun Zhangxue: Data curation, calculated the data. Shanliang Yuan:
characterized the samples. Qifei Bo: characterized the samples, All au-
thors read and contributed to the manuscript. No conflict of interest
exists in the submission of this manuscript, and the manuscript is
approved by all authors for publication. Biao Zhang: Formal analysis,
conducted sample preparation and data analysis. Yi Jiang: Formal
analysis, conducted sample preparation and data analysis.
In situ diffuse reflectance infrared spectroscopy (in situ DRIFT) ex-
periments have shown that the deep oxidation of propane on cobalt ox-
ides follows the Mars-van-Krevelen (MVK) mechanism [63,64]. The main
reaction process is shown in Fig. 12: In the first step, propane molecules
adsorb on the surface of cobalt oxides and react with oxygen atoms
attached to metal cations (Co2þ and Co3þ) to form H2O, CO2, and oxygen
vacancies. In the second step, the gas phase oxygen molecules are added
to the oxygen vacancies, and the metal or metal cations are oxidized. For
the Co-CA catalyst, due to the large number of adsorbed oxygen species
on its surface, the adsorbed oxygen can be used as active oxygen to
participate in the oxidation in the initial reaction. When the adsorbed
oxygen is consumed to a certain extent or cannot provide enough active
oxygen, the surface lattice oxygen begins to participate in the oxidation.
MVK mechanism shows that the lattice oxygen species on the catalyst
surface play an important role in the redox reaction. Likewise, this work
demonstrates that Co-CA has a distinct surface lattice oxygen desorption
peak and the lowest desorption temperature, thus exhibiting excellent
catalytic activity. Moreover, the disordered structure and rapid oxygen
mobility will generate high-frequency oxygen vacancies, which is
conducive to the cycle of the reaction, thereby maintaining stable cata-
lytic activity.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
This work was supported by the Sichuan Science and Technology
Program (2018GZ0314) and the “Light of the West” Fund of the Chinese
Academy of Sciences.
Overall, nanocrystalline Co3O4 prepared with CA, OA, and TA as
complexing agents has significantly different catalytic performance in
catalytic combustion of propane. Co-CA catalyst has the best catalytic
activity and excellent stability. FTIR showed that CA, OA, and TA in the
catalyst precursor coordinated with Co2þ in the form of monodentate,
tetradentate, and bidentate, which strongly affected the distance be-
tween Co2þ ions in catalyst precursor. XRD, N2 adsorption-desorption,
and TEM/HRTEM analysis further indicated that polyhedral nano-
crystalline Co3O4 were successfully synthesized using different small
molecular carboxylic acids, but the crystallite size, texture properties and
lattice disorder changed significantly. XPS results indicate that the type
of small molecular carboxylic acids has an important effect on the dis-
tribution of surface oxygen species and the oxidation state of Co ions. The
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://do
i.org/10.1016/j.jssc.2020.121712.
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