Decreasing Co3O4 Particle Sizes by Ammonia-Etching and Catalytic Oxidation of Propane

Article abstract of DOI:10.1007/s10562-016-1956-6

Abstract: Nanosized cobalt oxides were synthesized by ammonia-etching of cobalt hydroxide and subsequent calcination. The effects of ammonia etching on the structures and catalytic properties of cobalt oxide were investigated. The results suggested that the cobalt oxide is composed of cubic Co₃O₄having particle sizes in the range of 10–20?nm; smaller than the analogue prepared through the conventional precipitation method. Ammonia etching improved the reducibility of cobalt oxide, and facilitated the generation of surface oxygen vacancies and the production of surface-adsorbed active oxygen species. The activity of Co₃O₄in propane oxidation was greatly enhanced by ammonia etching, mainly due to the smaller particle size and better redox properties. The facile ammonia-etching synthesis method has potential for large-scale and low-cost production of cobalt oxide catalyst. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1956-6

Catal Lett
DOI 10.1007/s10562-016-1956-6
Decreasing Co3O4 Particle Sizes by Ammonia-Etching
and Catalytic Oxidation of Propane
Weidong Zhang1 · Lijuan Hu1 · Feng Wu1 · Jinjun Li1
Received: 21 September 2016 / Accepted: 15 December 2016
© Springer Science+Business Media New York 2016
Abstract Nanosized cobalt oxides were synthesized
by ammonia-etching of cobalt hydroxide and subsequent
calcination. The effects of ammonia etching on the struc-
tures and catalytic properties of cobalt oxide were investi-
gated. The results suggested that the cobalt oxide is com-
posed of cubic Co3O4 having particle sizes in the range
of 10–20 nm; smaller than the analogue prepared through
the conventional precipitation method. Ammonia etching
improved the reducibility of cobalt oxide, and facilitated
the generation of surface oxygen vacancies and the produc-
tion of surface-adsorbed active oxygen species. The activ-
ity of Co3O4 in propane oxidation was greatly enhanced by
ammonia etching, mainly due to the smaller particle size
and better redox properties. The facile ammonia-etching
synthesis method has potential for large-scale and low-cost
production of cobalt oxide catalyst.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1956-6) contains supplementary
material, which is available to authorized users.
* Jinjun Li
ljjcbacademy@163.com
1
School of Resources and Environmental Sciences, Wuhan
University, Wuhan 430079, China
Vol.:(0123456789)
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W. Zhang et al.
Graphical Abstract
synthesis methods have been developed, including com-
bustion [2], solid-phase reaction [10], sol–gel [11], pre-
cipitation [12], chemical vapor deposition [13], hydro-
thermal [14] and solvothermal treatment [15]. The main
difficulty in the synthesis of Co3O4 nanoparticles lies in
the prevention of excessive crystal agglomeration. Sev-
eral studies have overcome this problem by the addition
of an organic reactant, which acts as a dispersion agent
or template [16–18]. The addition of an organic reactant
inevitably increases the cost and environmental impact of
the process. Furthermore, in some cases, special equip-
ment or complex processes are required for the preven-
tion of excessive crystal agglomeration. For the use of
Co3O4 catalysts to be practical, the development of sim-
ple and low cost synthetic routes, in which no organic
surfactants are used, is still desirable.
Keywords Environmental catalysis · Alkanes ·
Nanoparticles · Total oxidation · Etching · Ammonia ·
Cobalt oxide
1 Introduction
Volatile organic compounds (VOCs) emitted from indus-
tries and vehicle exhausts can induce a series of environ-
mental problems, including photochemical smog, broken
ozonosphere, and environmental pollution [1, 2]. Moreo-
ver, many VOCs are highly toxic and harmful for human
health [3]. With increasingly strict laws and regulations
imposed on levels of VOCs emissions, removal of VOCs
from exhaust fumes is necessary. Catalytic oxidation is an
effective technique to abate low concentrations of VOCs
[4].
The aim of this study was to explore the synthesis of
nanosized cobalt oxide, using ammonia as a complexing
agent to partially etch cobalt hydroxide, and to evaluate
its activity in the total oxidation of propane. As the main
component in liquefied petroleum gas, propane is being
released in increasing amounts into the atmosphere [19].
Additionally, alkanes are believed to be among the most
inactive components of VOCs [20], increasing the difficulty
of their abatement; therefore, the development of a catalyst
that can oxidize propane at mild temperatures and which
Co3O4, a typical transition metal oxide, has been
extensively studied as a promising catalyst for oxidation
of VOCs [1, 3, 5–7]. It has been revealed that the activity
of Co3O4 is linked to several factors, such as particle size
[3], specific surface area [2], exposed lattice planes [8],
and redox properties [9]. The preparation method can, to
a great extent, influence the structure of the material and,
in turn, determine its redox properties and catalytic per-
formance. In order to obtain highly active Co3O4, various
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Decreasing Co3O4 Particle Sizes by Ammonia-Etching and Catalytic Oxidation of Propane
may be active for other VOCs as well [21], may represent a
significant advancement in the removal of VOCs.
H2 temperature-programmed reduction (H2-TPR) tests
were performed on a self-built setup and H2 consumption was
measured using a thermal conductivity detector. In each test,
the sample was firstly treated in O2 flow (20.8 vol% in N2, 40
mL/min) at 300°C for 30 min. After cooling down to room
temperature, the gas flow was switched to H2 (5.0 vol% in
N2, 50 mL/min), and the temperature was increased to 800°C
(rate 10°C/min), controlled by a temperature programmer.
O2 temperature-programmed desorption (O2-TPD) tests
were performed on the same setup as the H2-TPR tests
and desorbed O2 was measured using a thermal conduc-
tivity detector. Before each test, the sample was treated in
50 mL/min of synthetic air at 300°C for 30 min, cooled
down to room temperature to absorb O2 (20.8 vol% in N2,
40 mL/min) and maintained in this condition for 30 min,
and purged with 50 mL/min of high-purity N2 for 30 min.
Finally, the O2-TPD test was carried out under high-purity
N2 flow, from room temperature to 800°C with a ramp rate
of 10°C/min.
2 Experimental
2.1 Catalyst Synthesis
All reagents were purchased from Sinopharm Chemical
Reagent Company Limited, and were of analytical grade
and used as received without further purification. In a typi-
cal procedure, 80 mL of Co(NO3)2·6H2O (0.5 mol/L) and
80 mL of NaOH (1.0 mol/L) were mixed in a stirred 250
mL beaker; 15 mmol of ammonia (ca. 1 mL) were then
rapidly added to the resultant cobalt hydroxide slurry. This
mixture was further stirred for 3 h and separated by cen-
trifugation, washed with deionized water, and dried in air at
100°C overnight. Finally, the solid was calcined at 300°C
for 2 h, obtaining a sample labeled CoNP.
X-ray photoelectron spectroscopy (XPS) was conducted
on an ESCALAB 250Xi (Thermo Fisher, Massachusetts,
USA) spectrometer using Al Kα radiation (1253.6 eV). The
electron binding energy was calibrated by using the C 1s
(Eb = 284.6 eV) spectrum. The Co 2p3/2 and O 1s peaks
were fitted with Shirley background and 50:50 of Lorentz-
ian: Gaussian convolution product shapes.
To assess the influence of ammonia, some cobalt oxides
were prepared by precipitation without ammonia addition
and were labeled CoP.
2.2 Catalyst Characterization
The X-ray diffraction (XRD) patterns were recorded on an
X’Pert Pro diffractometer (PAN-alytical, Almelo, Neth-
erlands) using Cu Kα radiation at a generator voltage of
40 kV and a tube current of 40 mA.
2.3 Catalytic Activity Tests for Propane Oxidation
The activities of catalysts were tested for total propane oxi-
dation. In each test run, 0.05 g (ca. 0.25 mL) of catalyst
(40–80 mesh) was placed in a continuous-flow U-shaped
quartz reactor (internal diameter=4 mm) plugged with
quartz cotton on both sides. The catalyst temperature was
controlled by a K-type thermocouple placed inside the
catalyst bed. The concentration of propane was 1000 ppm,
balanced by air, and the total flow rate was controlled by a
mass flow controller at 100 mL/min, which corresponds to
a gas hourly space velocity of ca. 24,000 h−1. The concen-
trations of propane and carbon dioxide in the feed and efflu-
ent gases were measured by an online gas chromatograph,
equipped with a flame ionization detector and a methanizer.
At each sampling point, the catalyst temperature was kept
constant for 3 min prior to the determination of pollutant
concentration.
Infrared spectra (IR) were recorded with a NICOLET
5700 FTIR (Thermo Electron Corporation, Massachusetts,
USA) spectrometer using the Universal ATR Accessory
from 4000 to 400 cm−1 at 4 cm−1 resolution.
The texture properties were obtained using a V-Sorb
2800P (Application, Wuhan, China) surface area and
porosimetry analyzer. Before the measurements, sam-
ples were degassed under vacuum at 180°C for 3 h. The
Brumauer–Emmet–Teller (BET) specific surface areas
were calculated based on the linear part of the BET plot
(P/P0 =0.05–0.25). The Barrett–Joyner–Halenda (BJH)
pore distributions were calculated based on adsorption
branches and total pore volumes were calculated based on
the quantities of adsorbed nitrogen at the maximum relative
pressure (P/P0 =0.99).
Scanning electron microscopy (SEM) was performed
using an S-3400N (Hitachi Limited, Tokyo, Japan) scan-
ning electron microscope.
3 Results and Discussion
Transmission electron microscopy (TEM) images were
taken on a JEM-2100 (JEOL, Tokyo, Japan) transmission
electron microscope at an acceleration voltage of 200 kV.
Prior to observation, samples were crushed, dispersed in
ethanol, and deposited on a microgrid.
3.1 Ammonia Etching of Cobalt Oxide
Figure 1 presents the XRD patterns of the CoNP and CoP
catalysts. Both samples have diffraction peaks at 2θ of
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W. Zhang et al.
Table 1 Physicochemical properties of the CoNP and CoP catalysts
19.0°, 31.3°, 36.8°, 44.8°, 59.4°, 65.2°, which correspond
to the face centered cubic unit cell of Co3O4 with a spinel
structure (JCPDS PDF No.74-2120). CoNP shows weaker
diffraction intensity than CoP, indicating that the former
is worse crystallized than the latter. The average crystal-
line size of the CoNP catalyst calculated from the Scher-
rer equation based on (311) diffraction peak is smaller than
that of CoP (Table 1). Besides, the fingerprint IR absorp-
tion bands of Co3O4 at around 667 and 566 cm−1 (Fig.
S1) also confirm the phase structure of the catalysts [22].
We have also synthesized catalysts using various ammo-
nia amounts ranging from 15 to 60 mmol, and their XRD
patterns as well as crystallite sizes seems to be almost the
same (Fig. S2, Table S1), indicating that the ammonia con-
centration has little influence on the crystal structure of the
catalyst.
Catalysts SBET (m2/g) Vpore (cm3/g) Dxrda (nm) DHRTEMb (nm)
CoNP
CoP
48
32
0.47
0.38
15
19
14.6
20.0
aCrystallite sizes calculated based on the (311) lines in the XRD pat-
terns of Co3O4 using the Scherer equation
bParticle sizes evaluated based on the HRTEM images
smaller particle size generally ensures a greater extent of
exposed surfaces, as confirmed by the measured specific
surface areas (Table 1).
HRTEM images are shown in Fig. 4 (see more in Fig.
S4). Lattice fringes of Co3O4 can be clearly identified
in both catalysts. The insets indicate the corresponding
selected area electron diffraction (SAED). The SAED of
CoP shows a sharp diffraction ring due to well-crystal-
lized Co3O4, whereas that of CoNP appears to be weaker,
indicating its relatively lower crystallinity. Fig. S5 shows
the particle size distributions obtained from the HRTEM
images. CoNP possesses particle sizes in the range of
10–20 nm with an average value of 14.6 3.1 nm, while
CoP does in the range of 16–24 nm with an average value
of 20.0 2.7 nm (Table 1). This analysis also confirms the
XRD and SEM results, showing that the particle size of
CoNP is smaller than that one of CoP.
Figure 2 plots the N2 adsorption–desorption isotherms
and the pore size distributions for the two catalysts. Despite
the H3-type hysteresis loop, their adsorption–desorption
isotherms were close to the type III [23], and nitrogen
uptake mainly occurred at high relative pressure P/P0 >0.9,
suggesting a pore structure accumulated by flat and loose
particles. The pore size distribution curve shows that the
porosity of the two materials spans over a wide range, from
micropores to macropores. The presence of macropores is
probably due to the inter-granular porosity between parti-
cles [23]. The measured BET surface areas (SBET) and pore
volumes (Vpore) of the two catalysts are listed in Table 1.
CoNP not only has a larger pore volume but its specific sur-
face area is also 1.5 times higher than that of CoP.
To summarize, the ammonia treatment of cobalt
hydroxide led to evident changes in the textural proper-
ties, particle size, and morphology of the synthesized
cobalt oxide. When ammonia was added into the as-
prepared cobalt hydroxide slurry, partial cobalt ions
could coordinate with ammonia molecules to form com-
plex ions, which could cause etching of the bulk cobalt
Typical SEM images of the two catalysts are presented
in Fig. 3 (see more in Fig. S3). On both micrometer and
sub-micrometer scale, CoNP appear to be smaller than CoP
particles; additionally, CoNP shows a better dispersion. A
Fig. 2 a N2 adsorption–desorption isotherms and b pore size distri-
butions of the CoNP and CoP catalysts
Fig. 1 XRD patterns of the CoNP and CoP catalysts
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Decreasing Co3O4 Particle Sizes by Ammonia-Etching and Catalytic Oxidation of Propane
Fig. 3 SEM images of a, b CoP
and c, d CoNP catalysts
Fig. 4 HRTEM images of a
CoP and b CoNP catalysts
hydroxides and might break them into smaller aggre-
gates. Nie et al. [14] prepared N-doped graphene/Co3O4
hybrid via a one-pot hydrothermal reaction in the pres-
ence of NH3 and demonstrated that NH3 reduces Co3O4
particle size and enhances particle nucleation due to its
coordination with cobalt cations. Shi et al. [24] prepared
copper manganese oxides using a selective etching tech-
nique and found that ammonia can selectively remove
copper species from the copper manganese oxides. The
use of ammonia in this study is essential to reduce Co3O4
particle size due to its coordination with cobalt cations.
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W. Zhang et al.
3.2 Redox Properties of Cobalt Oxides
Figure 5 displays the H2-TPR profiles of the CoNP and CoP
catalysts. Both catalysts exhibit two obvious peaks, cen-
tered at different temperatures. The reduction process has
been reported to proceed in two steps, including the reduc-
tion of Co3+ to Co2+ and of Co2+ to metallic Co [6]. Obvi-
ously, CoNP shows better reducibility than CoP since the
reduction peaks of CoNP shifted towards much lower tem-
peratures after ammonia etching. Moreover, a small peak
centered at 107°C in the H2-TPR profile of CoNP, which
did not appear in that of CoP, may be evidence of very
active oxygen species. This can also be explained in terms
of the smaller particle size of CoNP. In a previous study,
it was suggested that small particles were beneficial to the
reduction of oxide species because they brought to a higher
exposed surface area, ensuring full access to H2 [25].
Fig. 6 O2-TPD profiles of the CoNP and CoP catalysts
Furthermore, the O2-TPD experiments were carried out,
and the O2-TPD profiles obtained for the two catalysts are
shown in Fig. 6. In general, the oxygen species of cobalt
oxide are desorbed following the sequence of steps: oxygen
To gain further information on surface composition of
the cobalt oxides, XPS analysis was carried out. Figure 7
shows the Co 2p and O 1s spectra of the samples. The two
major peaks, corresponding to the Co 2p1/2 and Co2p3/2
spin–orbit peaks centered at 794.7 eV and 779.6 eV, with
a peak separation of 15.1 eV, together with several shake-
up satellites, are characteristic of Co3O4 [2]. The two
peaks of Co 2p3/2 in the XPS spectra can be assigned to
Co2+ and Co3+ respectively, and the spectra of O 1s can
be divided into three peaks, corresponding to adsorbed
H2O molecules, surface adsorbed oxygen (Oads) species,
and surface lattice oxygen (Olatt) species [2, 28]. The
results of a quantitative analysis of the surface cobalt and
oxygen compositions are summarized in Table 2. The
Co3+/Co2+ molar ratio of CoNP was higher than that of
CoP, suggesting that CoNP possesses higher amounts of
Co3+. Abundant Co3+ ions are likely to increase the ani-
onic defect position, which provides sufficient active sites
for the absorption and activation of molecular oxygen
in the gas phase [6]. With respect to O 1s, the Oads/Olatt
molar ratio of CoNP was higher than that of CoP, indicat-
ing that CoNP possessed more surface-adsorbed oxygen
species than CoP [29]. Surface-active oxygen species are
of crucial importance in the oxidation reaction [30]. As
revealed by O2-TPD results, CoNP have more active sur-
face oxygen species, which are easily desorbed and may
leave oxygen vacancies to be filled by gas phase molecu-
lar oxygen, thus improving the oxidation capacity. There
is a direct correlation between the amounts of surface-
−
molecule (O2) → oxygen molecule anion (O2 ) → oxygen
anion (O−) → lattice oxygen (O2−) [2]. Among these oxy-
−
gen species, O2 and O− are believed to be surface-active
oxygen that may participate in catalytic oxidation reactions
[2, 6, 26, 27]. In the test temperature range, both catalysts
exhibit three main desorption peaks. According to litera-
ture [6, 26], the peaks appearing at ~100, ~300 and ~800°C
−
can be attributed to the O2 , O− and O2− species of cobalt
−
oxides, respectively. The O2 and O− desorption tempera-
ture relative to CoNP is clearly lower than that of CoP,
indicating that the former has more active oxygen species,
which may result from a larger specific surface area and
smaller particle sizes [3].
−
active oxygen species, observed in XPS, and the O2 and
O− species measured from O2-TPD [9].
Fig. 5 H2-TPR profiles of the CoNP and CoP catalysts
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Decreasing Co3O4 Particle Sizes by Ammonia-Etching and Catalytic Oxidation of Propane
Fig. 7 Co 2p and O 1s XPS spectra of the CoNP and CoP catalysts
Table 2 Surface element
Catalysts
O 1s
Co 2p3/2
compositions of the CoNP and
CoP catalysts
Olatt (eV)
Oads (eV)
Oads/Olatt ratios
Co2+ (eV)
Co3+ (eV)
Co3+
Co2+
/
ratios
CoNP
CoP
529.7
529.8
530.6
530.8
0.52
0.43
781.0
780.9
779.5
779.5
1.28
1.16
3.3 Catalytic Oxidation of Propane Over Cobalt Oxides
Figure 8 shows the conversion curves of propane oxidation
over the CoNP and CoP catalysts. Apparently, the light-
off temperature of propane oxidation over CoNP (184°C)
is much lower than that of CoP (248°C). CoP could only
achieve 28% propane oxidation, even when the temperature
was raised to 300°C. However, propane can be totally oxi-
dized into CO2 on CoNP at 270°C. Therefore, the activ-
ity of Co3O4 is greatly enhanced by the ammonia etching
process. As discussed previously, CoNP exhibits a smaller
particle size, which might provide more easily accessi-
ble active sites [3]. Meanwhile, CoNP appears to be more
reducible and possess more surface oxygen defects than
CoP. The higher surface oxygen vacancy density favors the
adsorption and activation of O2 molecules, hence enhanc-
ing catalytic activity of CoNP for propane oxidation. In
total oxidation of propane, the cracking of the C–H bond
is considered to be the rate-determining step [31–34]. Zou
et al. [33] inferred that the active surface oxygen vacancy
on transition metal oxides can extract H atoms from the
weakest C–H bonds of propane to generate surface hydrox-
ide ions and activate propane molecules. The activated pro-
pane could be further oxidized into intermediate bidentate
Fig. 8 Conversion curves for propane oxidation over the CoNP and
CoP catalysts
carbonate, which finally dissociates to produce CO2 [34].
According to Luo et al.’s report [31], propane oxidation
first takes place on the surface lattice oxygen (O2− and O−)
sites of catalysts. Compared with the bulk lattice oxygen
in Co3O4 crystallites, the presence of surface lattice oxy-
gen, relatively weakly bonded to the surface cobalt ions, is
1 3

W. Zhang et al.
Table 3 Comparison of catalyst
activity for propane oxidation
at 200°C
Catalysts Catalyst Flow rate Inlet C3H8
C3H8
(mL/min) concentration conver-
Activity Specific activ- Refs.
(µmol/
s/g)
mass
(g)
ity (µmol/s/
m2)
(%)
sion (%)
Co3O4
0.25
50
50
0.8
0.8
0.8
0.37
0.1
51
70
22
14
21
0.61
0.83
0.26
0.76
0.31
0.0050
0.0063
0.0056
0.0063
0.0065
[5]
Au/Co3O4 0.25
[35]
Co3O4
Co3O4
CoNP
0.25
0.05
0.05
50
[2]
98
[12]
100
This work
and the results are shown in the supplementary materials
(Table S2, Fig. S6 and Fig. S7). The used catalyst almost
maintained the same crystal structures, reducibility, parti-
cle diameters as well as textural properties as the fresh one,
which is also indicative of the good reusability.
4 Conclusions
By using ammonia as a complexing agent to break down
bulk cobalt hydroxide, cobalt oxide with decreased parti-
cle size and high reducibility was finally obtained. The
Co3O4 prepared by ammonia-etching has a particle size in
the range of 10–20 nm, whereas the analogue prepared by a
conventional precipitation method has a particle size in the
range of 16–24 nm. The former achieved 100% conversion
of propane below 275°C, whereas the latter only achieved
27% at 300°C.
Fig. 9 Reusability test over the CoNP catalyst
usually related to the catalytic activity. All in all, the larger
specific surface area, smaller particle size, and better redox
properties of cobalt oxide after ammonia etching led to its
improved performance in total propane oxidation.
Acknowledgements This work was supported by the National Nat-
ural Science Foundation of China (21477092) and the Natural Science
Foundation of Hubei Province (2014CFB182).
To further comfirm the excellent activity of CoNP, the
catalytic activity of our material was compared with the
catalysts reported in the literature [2, 5, 12, 35]. Taking dif-
ferent experimental conditions into consideration, we cal-
culated the activity defined by the reaction rate per catalyst
mass unit as well as the specific activity based on the spe-
cific surface area. From Table 3, one can conclude that the
specific activity of CoNP is comparable to the best cobalt-
based combustion catalysts reported thus far [2, 5, 12, 35].
It is noteworthy that the synthesis procedure of cobalt oxide
in this study is the simplest among these catalysts, so it may
show its superiority in industrial production.
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