Hydrogenation of CO2 to alcohol species over Co?Co3O4/C-N catalysts

Article abstract of DOI:10.1016/j.jcat.2019.09.018

Co/C-N materials were synthesized with calcination of ZIF-67 at a N₂ atmosphere. Co?Co₃O₄/C-N catalysts were prepared by partially oxidized of Co/C-N in air under different conditions. The catalysts were characterized with XRD, BET, TEM, TGA, H₂-TPD, ESR and XPS. The metallic Co in Co/C-N was the main activity site for the CO₂ hydrogenation as it supplied dissociative hydrogen on the surface. Partial oxidation of Co/C-N decreased the content of metallic Co, decreasing the amount of dissociative H₂ and thus the CO₂ conversion, while increasing the methanol selectivity. Oxygen defects in Co?Co₃O₄/C-N improved the dissociation of CO₂ and helped to produce desired alcohol species. The highest yield of MeOH, 2.0 mmol g^?1 h^?1, was obtained over Co?Co₃O₄/C-N (250, 2 h) at 220 °C.

Full text of DOI:10.1016/j.jcat.2019.09.018

Journal of Catalysis 379 (2019) 46–51
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Hydrogenation of CO2 to alcohol species over Co@Co3O4/C-N catalysts
⇑
⇑
Yun Lian, Tingfeng Fang, Yuhua Zhang, Bing Liu , jinlin Li
Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education & Hubei Key Laboratory of Catalysis and Materials Science, South-Central University
for Nationalities, Wuhan 430074, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Co/C-N materials were synthesized with calcination of ZIF-67 at a N2 atmosphere. Co@Co3O4/C-N cata-
lysts were prepared by partially oxidized of Co/C-N in air under different conditions. The catalysts were
characterized with XRD, BET, TEM, TGA, H2-TPD, ESR and XPS. The metallic Co in Co/C-N was the main
activity site for the CO2 hydrogenation as it supplied dissociative hydrogen on the surface. Partial oxida-
tion of Co/C-N decreased the content of metallic Co, decreasing the amount of dissociative H2 and thus
the CO2 conversion, while increasing the methanol selectivity. Oxygen defects in Co@Co3O4/C-N
improved the dissociation of CO2 and helped to produce desired alcohol species. The highest yield of
MeOH, 2.0 mmol gꢀ1 hꢀ1, was obtained over Co@Co3O4/C-N (250, 2 h) at 220 °C.
Received 7 August 2019
Revised 9 September 2019
Accepted 10 September 2019
Keywords:
CO2 hydrogenation
Alcohol synthesis
ZIF-67
Ó 2019 Elsevier Inc. All rights reserved.
Co@Co3O4/C-N catalysts
1. Introduction
and the enhancement of the electron transfer ability [10]. In CO2
hydrogenation reaction, the basic nitrogen improved the adsorp-
Carbon dioxide is the main greenhouse gas produced in many
industrial processes. The efficient utilization of CO2 to produce fuel
and high value chemicals has been drawing researchers’ attention
[1]. Alcohol species synthesis from CO2 hydrogenation
(CO2 + 3H2 ? CH3OH + H2O, 2CO2 + 6H2 ? C2H5OH + 3H2O), are
highly attractive products as they are widely used as intermediates
in many chemical industries, and are also transportable fuels to
replace petroleum [2]. For the CO2 hydrogenation, various catalytic
systems based on Cu [3], Co [4] Pd [5] have been studied. Methanol
synthesis by CO2 hydrogenation on Cu based catalysts was usually
accompanied with undesired CO formation via reverse water gas
shift reaction (CO2 + H2 ? CO + H2O, RWGS) [6]. Co based catalysts
have been extensively studied for the Fischer-Tropsch synthesis
reaction. However, methane was the main product in CO2 hydro-
genation process due to the strong hydrogenation ability of metal-
lic Co [7]. Somorjai et al. found cobalt oxides performed better than
fully reduced cobalt supported on TiO2 in CO2 hydrogenation, for
CoOx and partially encapsulated metallic Co formed some unique
active sites [8]. In our earlier research, the system formed with par-
tially reduced mesoporous Co3O4 and metallic Co nanoparticles
was shown to be the active site to produce alcohol’s by CO2 hydro-
genation [9].
tion of CO2, and enhanced the catalytic activity [11]. ZIF-67 (ZIF:
zeolitic imidazolate framework), which is composed of cobalt
cation’s and 2-methylimidazole ligand’s, would be a promising
precursor for the preparation of nitrogen-doped carbon supported
cobalt catalysts. Herein, nitrogen-doped carbon-supported cobalt
catalysts (Co/C-N) were prepared by the polymerization of ZIF-
67. Furthermore, the partially oxidized catalysts (Co@Co3O4/C-N)
were also prepared for comparison. To the best of our knowledge,
this application of Co@Co3O4/C-N in CO2 hydrogenation reaction is
unprecedented.
2. Experimental section
2.1. Catalyst preparation
Co/C-N catalysts were prepared by the calcination of ZIF-67.
Briefly, 2.16 g of Co(NO3)2ꢁ6H2O and 4.88 g of 2-methylimidazole
were dissolved in 150 mL methanol separately. The solution of 2-
methylimidazole was added dropwise into the solution of cobalt
nitrate. Then the mixture was stirred for 6 h at room temperature.
The obtained purple suspension was centrifuged and washed with
methanol for three times. The ZIF-67 was obtained by drying the
purple solid at 80 °C overnight, and then calcined at 600 or
800 °C for 6 h in N2 atmosphere. The as-prepared catalysts are
denoted as Co/C-N-600 and Co/C-N-800.
Nitrogen-doped carbon-supported metal catalysts have also
recently gained considerable attention. The incorporation of nitro-
gen in the carbon architecture played multiple roles to improve the
catalytic activity such as the stabilization of metal nanoparticles
Co@Co3O4/C-N catalysts were prepared by the oxidation of
Co/C-N-600 in air at 220 or 250 °C for 2 h or 6 h. The as-prepared
catalysts are denoted as Co@Co3O4/C-N (220, 2 h), Co@Co3O4/C-N
(250, 2 h) and Co@Co3O4/C-N (250, 6 h) respectively.
⇑
Corresponding authors.
E-mail addresses: liubing@mail.scuec.edu.cn (B. Liu), jinlinli@aliyun.com (j. Li).
https://doi.org/10.1016/j.jcat.2019.09.018
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

Y. Lian et al. / Journal of Catalysis 379 (2019) 46–51
47
2.2. Catalyst characterization
20 mL/min. The reaction for each catalyst was kept for 50 h at
ꢀ1
200 °C or 250 °C and GHSV = 6 L hꢀ1
g
cat
. The gas products were
The powder X-ray diffraction (XRD) patterns were recorded on
an automated powder X-ray diffractometer (40 kV, 40 mA, Bruker-
D8) using a Cu Ka radiation source (k = 1.54056 Å) in the range of
10–80° with step size of 2°/min.
The surface area and pore volume were measured with low-
temperature nitrogen adsorption/desorption (77 K, quantachrome
Autosorb-1-C-MS). The specific surface area was determined from
the linear portion of the standard Brunauer-Emmett-Teller (BET)
plot. The total pore volumes and the average pore sizes were calcu-
lated from the desorption branch of the adsorption isotherm using
the Barrett–Joyner–Halenda (BJH) formula. Before the adsorption
measurements, the sample was degassed at 350 °C under vacuum
for 3 h. The pressure range used for the BET measurements was
9.849–100.566 KPa.
The ability of CO2 adsorption for the catalyst was measured by
CO2-temperature program desorption (CO2-TPD) experiment. Prior
to adsorption of CO2, the sample (100 mg) was pretreated at 300 °C
for 1 h and cool 100 °C in flowing Ar. At this temperature, sufficient
CO2 (30 mL minꢀ1) was injected at least 30 min until adsorption
saturation, followed by purging with Ar (30 mL minꢀ1) for about
1 h. The temperature was then raised from 100 to 650 °C at a ramp
rate of 10 °C minꢀ1 to desorb CO2. The desorbed CO2 was detected
by on-line gas chromatography with a TCD detector.
analyzed on line by an Agilent GC3000 gas chromatography (ther-
mal conductivity and flame ionization detectors). The liquid prod-
ucts were collected in a cold trap and analyzed with a 4890gas
chromatography (flame ionization detectors) off line. The reaction
parameters of CO2 conversion, selectivity and space time yield
(STY) are defined as below (where MeOH refers to methanol)
nCO2;in ꢀ nCo2;out
nproduct;out ꢄ carbon number
Xco2
¼
;
selectivity ¼
nCO2;in
nCo2;out
FCO2; in ꢄ XCO2 ꢄ 60 ꢄ SMeOH
22:4 ꢄ Mcat
STYMeOH
¼
3. Results and discussion
The X-ray diffraction patterns of Co/C-N catalysts are shown in
Fig. 1. The XRD patterns of Co/C-N showed the peaks with 2h at
H2-temperature program desorption (H2-TPD) experiments
were carried out using AMI-200 from Zeton Altamira Company.
For each measurement, about 50 mg of the catalyst was placed in
a quartz tube reactor and reduced at 220 °C for 2 h using a flow
of high-purity hydrogen (>99.999%, 30 mL minꢀ1) and then cooled
from 220 °C to 50 °C in flowing hydrogen. Then hydrogen was
switched to high-purity Ar (>99.999%, 25 mL minꢀ1) for 30 min.
Subsequently, the sample was heated from 50 °C to 600 °C at a rate
of 10 °C minꢀ1 and kept at 600 °C for 2 h in a flow of Ar
(25 mL minꢀ1). The desorbed H2 was monitored by TCD and used
for quantification.
Thermogravimetric analysis (TGA) was carried out on
a
NETZSCH TG 209F3 instrument. The sample (ꢂ10 mg) was placed
in a ceramic crucible and heated from 30 to 700 °C with a ramp
rate of 10 °C minꢀ1 under air (flow rate 30 mL minꢀ1).
Fig. 1. XRD patterns of Co/C-N catalysts.
X-ray photoelectron spectroscopy (XPS) was conducted on a
Thermo VG scientific ESCA MultiLab-2000 spectrometer with a
monochromatized Al Ka source (1486.6 eV) at a constant analyzer
pass energy of 25 eV. The binding energy was estimated to be accu-
rate within 0.2 eV. Co 2p, N 1s and O 1s spectra were acquired, C 1s
was set at 284.6 eV and taken as a reference for binding energy
(BE) calibration. The software XPS PEAK was used to fit the XPS
spectra using a mixture of Gaussian and Lorentzian functions in a
40/60 ratio and a Shirley background.
Electron spin resonance (ESR) signals of radicals, trapped by a
spin-trap reagent DMPO (5,5-dimethyl-1-pyrroline-N-oxide) in
water, were examined on a Bruker model JEOL JES-FA200 spec-
trometer equipped with a Quanta-Ray Nd: YAG laser system as
the irradiation source (k ꢃ 420 nm). To minimize experimental
errors, the same type of quartz capillary tube was used for all
ESR measurements.
2.3. Typical procedure of CO2 hydrogenation
All catalytic tests were performed in a fixed-bed stainless steel
mini-reactor (length of 53 cm and inside diameter of 8 mm). A
mixed feed gas (22.5% CO2, 67.5% H2 and 10% N2) was used without
further purification. The pressure of the reactor was controlled
with
a backpressure regulator. For each experiment, the
catalyst to be tested (0.2 g) was mixed with 0.2 g of silica. The
system was pressurized to 2 MPa by feeding the mixed gas at
Fig. 2. TEM images and particle distribution of Co/C-N catalysts.

48
Y. Lian et al. / Journal of Catalysis 379 (2019) 46–51
Fig. 3. N2 adsorption–desorption isotherms and pore size distribution curves of Co/C-N catalysts.
44.2°, 51.5°, 75.9°, which were assigned to the (1 1 1), (2 0 0), and
(2 2 0) crystalline planes of metallic face-centered cubic (FCC) Co
(JCPDS Card No. 15-0806) respectively. The size of Co metallic par-
ticles was calculated from the XRD peak (44.2°) using the Scherrer
equation. The calculated crystallite sizes of Co were 11.5 nm and
17.8 nm for the Co/C-N-600 and Co/C-N-800, respectively.
The morphology of the Co/C-N was characterized with TEM. As
show in Fig. 2, the metallic Co was well dispersed on nitrogen dop-
ing carbon material. The average size of metallic Co was 12 and
18 nm for Co/C-N-600 and Co/C-N-800 according to the statistics
of more than 200 Co particles. This result was quite consistent with
the XRD calculations.
micropores in the samples [12]. At the tail of the isotherm (high
relative pressure), the absorbance increased quickly, suggesting a
large amount of mesopores. It confirmed the dual mesoporous
and microporous structure in Co/C-N materials [13]. With the
increase of calcination temperature from 600 to 800 °C, the surface
areas and micropore volumes of these two samples were found to
decrease, while the micropore diameter increased slightly
(Table 1).
Fig. 4 shows the TGA curves of Co/C-N catalysts. It could be seen
that Co/C-N catalysts undergoes a two-step degradation process.
The weight loss at about 100–250 °C should be ascribed as a result
of dehydration of water from the crystalline structure. The weight
loss at about 250–350 °C belonged to the oxidation of carbon and
nitrogen materials [14]. The Co contents calculated from the weight
loss were about 40% and 47% for Co/C-N-600 and Co/C-N-800.
The catalytic performance of Co/C-N was evaluated by the CO2
hydrogenation reaction at 200 and 220 °C successively. The pro-
duct selectivity and STY of methanol are shown in Fig. 5 and
Table 2, respectively. No deactivation of any catalysts, after 50 h
of reaction, was observed under the testing conditions.
It was noted that Co/C-N-600 had higher CO2 conversion than
Co/C-N-800 at both 200 °C and 220 °C, because Co/C-N-600 had
smaller Co particle size and thus more metallic Co active sites.
The STY of methanol for Co/C-N-600 was also higher than
The N2 absorption isotherms of Co/C-N catalysts are shown in
Fig. 3. The quantity of adsorbed N2 increased dramatically at a
low relative pressure (P/P0 < 0.08), indicating the abundance of
Table 1
The physical structure of Co/C-N catalysts.
Catalysts
Surface area
(m2/g)
Pore volume
(mL/g)
Pore diameter
(nm)
micro
meso
micro
meso
micro
meso
Co/C-N-600
Co/C-N-800
231
130
289
237
0.097
0.070
0.45
0.45
0.48
0.54
2.03
2.03
Fig. 5. Effect of temperature on CO2 conꢀv1ersion over the Co/C-N catalysts. Reaction
Fig. 4. TGA curves of Co/C-N catalysts.
conditions: P = 2 MPa, GHSV = 6 L hꢀ1 gcat, H2/CO2 = 3.

Y. Lian et al. / Journal of Catalysis 379 (2019) 46–51
49
Table 2
The activity and selectivity of Co/C-N catalyst.
Catalyst
Reaction temperature (°C)
CO2 conversion (%)
Selectivity (%)
STY
(mmol/gcat/h)
MeOH
CO
CH4
C2H6
MeOH
EtOH
Co/C-N-600
Co/C-N-600
Co/C-N-800
Co/C-N-800
200
220
200
220
10.6
19.4
7.3
0
0
0
0
70.4
85.7
74.4
80.5
0.6
0.6
0.7
0.7
27.8
12.6
21.4
15.3
1.2
1.1
3.5
3.5
1.8
1.5
0.9
1.5
16.6
Fig. 6. XRD patterns of Co/C-N and Co@Co3O4/C-N catalysts.
Co/C-N-800. Furthermore, as the reaction temperature increased
from 200 °C to 220 °C, CO2 conversion increased notably while
the selectivity of methanol decreased. Since the methanol synthe-
sis from CO2 hydrogenation is exothermic, low temperature’s
should thermodynamically favorable in the conversion of CO2.
However, the actual CO2 conversion was much lower than the the-
oretical equilibrium value. Thus, CO2 conversion was mainly
affected by dynamic factors. Although metallic Co led to superior
CO2 conversion, it produced methane predominately in the process
of CO2 hydrogenation. There was no CO produced in all Co/C-N cat-
alysts, while little ethane and ethanol, if any, was found in both gas
and liquid products.
Co/C-N-600 was oxidized in air at different conditions to obtain
different oxidation states. XRD patterns of Co/C-N, Co@Co3O4/C-N
were displayed in Fig. 6. Diffraction peaks of Co3O4 (JCPDS PDF
43-1004), as well as that of metallic Co, were observed in
Co@Co3O4 catalysts. As expected, the percentage of Co3O4
increased while that of metallic Co decreased with the increase
of oxidation temperature and time.
Fig. 7 shows the XPS spectra of Co 2p, O 1s and N 1s of the
Co@Co3O4/C-N catalysts. The Co 2p peak of Co/C-N-600 could be
deconvoluted into three peaks with binding energies (BEs) at
778.3, 780.3 and 782.2 eV, corresponding to metallic cobalt, Co3+
and Co2+ respectively [15]. The percentages of cobalt in oxidation
states increased with the increase of oxidation temperature and
time, and metallic Co almost disappeared in Co@Co3O4/C-N (250,
6 h). The O1s XPS spectrum could be fitted into three peaks at
BEs about 530.1, 531.7 and 533.5 eV, corresponding to lattice oxy-
gen, defect oxygen and adsorbed oxygen [16]. The oxygen defects
on the surface improved the dissociation of CO2 and tended to pro-
duce the desired alcohol species [17]. The N 1s XPS spectrum of Co/
N-C-600 could be fitted into three peaks with BEs at 398.5, 399.5
and 400.8 eV, corresponding to pyridinic N, pyrrolic N and graphi-
tic N, respectively [18]. Similarly, the N 1s of the Co@Co3O4/C-N
Fig. 7. XPS profiles of Co 2p, O1s and N1s of Co/C-N and Co@Co3O4/C-N catalysts.

50
Y. Lian et al. / Journal of Catalysis 379 (2019) 46–51
could also be fitted into the same three peaks yet with gradually
lower intensities than Co/N-C-600, suggesting that the nitrogen
content decreased with the increase of oxidation temperature
and time.
The morphology of the Co@Co3O4/C-N (250, 6 h) was character-
ized with TEM and HRTEM (Fig. 8). The lattice spaces of 0.28 and
0.46 nm corresponding to the (2 2 0) and (1 1 1) of Co3O4 con-
firmed the existence of oxidized Co species.
the Co/C-N catalyst showed two overlapping desorption peaks at
450 and 475 °C, which was ascribed to the chemisorbed CO2 on
the C-N support surface. Co@Co3O4/C-N catalyst showed two des-
orption at 400 °C and 500–550 °C, which were assigned to medium
and strong basic sites, respectively [19]. The larger amount of
adsorbed CO2 of Co@Co3O4/C-N than that of Co/C-N at high tem-
perature, indicative of the more basic sites on former catalyst sur-
face, which was consist with the N1s results in XPS profiles. High-
temperature desorption property also indicates the existence of
stronger metal–metal oxide (Co-Co3O4) interactions in Co@Co3O4/
C-N [20].
CO2-TPD was employed to investigate the adsorption ability for
CO2 on Co/C-N and Co@Co3O4/C-N catalysts. As depicted in Fig. 9,
All H2-TPD of the Co/C-N and Co@Co3O4/C-N catalysts (Fig. 10)
showed two major peaks: a low temperature peak (<200 °C) and
a high temperature peak (>400 °C), indicating different adsorbed
states of hydrogen. The low temperature peak was ascribed to che-
misorbed hydrogen on the metal Co surface, while the high tem-
perature peak located at about 520 °C was probably caused by
the desorption of spillover hydrogen from metallic Co to Co3O4
[21]. The intensity of low temperature peaks, as well as the desorp-
tion temperature of high temperature peaks, decreased with the
increase of oxidation temperature and time. It confirmed that the
oxidation treatment decreased the amount of dissociative H2 on
metallic Co and increased the interaction between metallic Co
and Co3O4, inducing H2 desorption to much higher temperature.
Fig. 8. TEM image and HRTEM image of Co@Co3O4/C-N (250, 6 h).
Fig. 9. CO2-TPD spectra of Co/C-N and Co@Co3O4/C-N catalysts.
Fig. 11. ESR spectra of Co/C-N and Co@Co3O4/C-N catalysts.
Fig. 12. Effect of temperature on CO2 conversion over Co/C-N and Co@Co3O4/C-N
catalysts. Reaction conditions: P = 2 MPa, T = 220 °C, GHSV = 6 L hꢀ1
g
, H2/CO2 = 3.
ꢀ1
Fig. 10. H2-TPD spectra of Co/C-N and Co@Co3O4/C-N catalysts.
cat

Y. Lian et al. / Journal of Catalysis 379 (2019) 46–51
51
Table 3
The activity and selectivity of Co/C-N and Co@Co3O4/C-N catalysts.
Catalyst
CO2 conversion (%)
Selectivity (%)
STY
(mmol/gcat/h)
MeOH
CO
CH4
C2H6
MeOH
EtOH
Co/C-N-600
19.4
18.6
13.5
4.4
0
0
0
1.3
85.7
79.5
79.7
55.1
0.6
0.7
0.8
0.8
12.5
18.3
18.7
39.8
1.2
1.2
0.8
3.0
1.5
2.0
1.5
1.1
Co@Co3O4/C-N (220, 2 h)
Co@Co3O4/C-N (250, 2 h)
Co@Co3O4/C-N (250, 6 h)
The formation of oxygen vacancy on the surface of Co@Co3O4/C-N
is confirmed by the electron paramagnetic resonance (EPR) spectra
of the samples, where the strong signal centering at g = 1.996
originates from the Overlapped electrons. The ESR signals of
Co@Co3O4/C-N catalysts increased with the increase of oxidation
temperature and time in Fig. 11. Since oxygen vacancy with
unpaired electrons is the main reason of the ESR response in these
materials, the higher intensity of ESR signal indicated the occurrence
of more defects formed in Co@Co3O4/C-N catalysts [22].
The values of CO2 conversion, product selectivity and STY of
methanol were shown in Fig. 12 and Table 3, respectively. Com-
pared with Co/C-N-600, CO2 conversion dropped, while selectivity
of methanol increased with the decrease of metallic Co. Especially
for Co@Co3O4/C-N (250, 6 h), CO2 conversion dropped from 19.4%
to 4.4%, however, the selectivity of methanol significantly
increased from 12.5% to 39.8%. CO2 conversion was directly related
to the content of available sites of metallic Co due to its supply of
dissociative hydrogen on the surface [7]. On the other hand, the
selectivity of methanol was related to the oxygen defects in
Co3O4, resulted in the partial oxidation of Co during heat treatment
in air, which was consistent with the H2-TPD and ESR
results. Balancing both factors, the highest STY of methanol
(2.0 mmol/gcat/h) was obtained on Co@Co3O4/C-N (220,2h).
Acknowledgments
This project was supported by the Fundamental Research Funds
for the Central University, China (CZT19007).
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Co/C-N materials were synthesized by calcination of ZIF-67 in
N2 atmosphere, while Co@Co3O4/C-N catalysts were prepared with
partially oxidized Co/C-N in air at different conditions. The two ser-
ies of catalysts were tested in the reaction of CO2 hydrogenation.
Characterization and catalytic evaluation showed that the metallic
Co was the main activity site for CO2 hydrogenation to methane
due to the supply of dissociative hydrogen on the surface. Partial
oxidation of Co/C-N decreased the content of metallic Co, weaken-
ing the strong ability to supply dissociative H2, and thus decreased
the CO2 conversion while increased methanol selectivity. The oxy-
gen defects in Co@Co3O4/C-N improved the dissociation of CO2 and
tended to produce desired alcohol species. The highest yield of
MeOH was obtained as 2.0 mmol gꢀ1 hꢀ1 over Co@Co3O4/C-N
(220, 2 h) at 220 °C.