Morphology effect of nanostructure ceria on the Cu/CeO2 catalysts for synthesis of methanol from CO2 hydrogenation

Article abstract of DOI:10.1016/j.catcom.2017.03.005

Three kinds of CeO₂ with different nanostructures were prepared by a controlled hydrolysis method, and Cu/CeO₂ catalysts were prepared by an incipient wetness impregnation method. The catalytic activity of Cu/CeO₂ catalysts was evaluated by the methanol synthesis from CO₂ hydrogenation. It was observed that the support morphology greatly affected the activity and selectivity of methanol synthesis from CO₂ hydrogenation. The nanorod CeO₂ supported Cu/CeO₂ catalyst by the exposure of (100) and (110) faces showed the strongest interaction between CuO and CeO₂, highest CuO dispersion and the highest catalytic activity with methanol yield of 1.9% at 240?°C and 2?MPa.

Full text of DOI:10.1016/j.catcom.2017.03.005

Accepted Manuscript
Morphology effect of nanostructure ceria on the Cu/CeO2
catalysts for synthesis of methanol from CO2 hydrogenation
Bi Ouyang, Weiling Tan, Bing Liu
PII:
DOI:
Reference:
S1566-7367(17)30091-2
doi: 10.1016/j.catcom.2017.03.005
CATCOM 4962
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Catalysis Communications
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Accepted date:
14 November 2016
18 February 2017
8 March 2017
Please cite this article as: Bi Ouyang, Weiling Tan, Bing Liu , Morphology effect of
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ACCEPTED MANUSCRIPT
Morphology effect of nanostructure ceria on the Cu/CeO2 catalysts for synthesis of
methanol from CO2 hydrogenation
Bi Ouyang, Weiling Tan, Bing Liu*
Key Laboratory of Catalysis and Materials Sciences of the State Ethnic Affairs
Commission & Ministry of Education, College of Chemistry and Material Science,
South-Central University for Nationalities, Wuhan 430074, PR China
*Corresponding
authors.
Tel./fax:
+86
27
67842572;
E-mail:
liubing@mail.scuec.edu.cn

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Abstract: Three kinds of CeO2 with different nanostructures were prepared by a
controlled hydrolysis method, and Cu/CeO2 catalysts were prepared by an incipient
wetness impregnation method. The catalytic activity of Cu/CeO2 catalysts was
evaluated by the methanol synthesis from CO2 hydrogenation. It was observed that the
support morphology greatly affected the activity and selectivity of methanol synthesis
from CO2 hydrogenation. The nanorod CeO2 supported Cu/CeO2 catalyst by the
exposure of (100) and (110) faces showed the strongest interaction between CuO and
CeO2, highest CuO dispersion and the highest catalytic activity with methanol yield of
1.9% at 240oC and 2MPa.
Key words: Morphology effect, CO2 Hydrogenation, Methanol, Cu/CeO2 catalyst

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1. Introduction
The conversion of CO2 into value-added chemicals has received considerable
attention because CO2 is both a major greenhouse gas and a cheap C1 resource [1].
Methanol is a key commodity chemical, which has been widely used for the
production of a number of chemical intermediates [2]. It would be attractive if the
distributed and produced in small-scale devices H2 reacted with CO2 to produced
methanol, the later can be easily stored and transported, and used for further
processing [3]. Currently, methanol is industrially synthesized from H2, CO2, and CO
at high pressure (50~100 atm) and high temperature (200~300oC) over Cu/ZnO/Al2O3
catalyst [4]. The methanol synthesis from CO2 hydrogenation (CO2+3H2 → CH3OH +
H2O) is a challenging task because of the chemical inertness of CO2 [5]. Due to the
inertness of CO2, the CO2 conversion was usually low. Except methanol, other
hydrogenated products such as higher alcohols and hydrocarbons were also produced
[6]. In addition, the relatively high concentration of CO2 and H2 also promoted the
reverse water gas shift reaction (CO2+H2→CO+H2O) to give CO [4, 5]. Therefore, the
effective synthesis of methanol is still a great challenge.
The catalysts for CO2 hydrogenation to methanol mainly focus on Cu, Pd [7, 8]
and other hybrid catalyst, such as Ni-Ga [9], Pt-Co [10]. Cu based catalysts have
received great interest for the synthesis of methanol from CO2 hydrogenation, as the
low cost of the Cu catalysts would reduce the industrial production cost. CeO2 is one
of the most important oxides in heterogeneous catalysis due to its high redox ability.
There are some literatures which use of CeO2 as a support in CO2 hydrogenation
process for methanol production [11-12]. Fan et.al reported the strong metal support
interaction (SMSI) between Pd and CeO2, which greatly improved the selectivity and

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lifetime for methanol synthesis form CO2 hydrogenation [12]. Graciani et al [4] found
that the Cu-ceria interface was active site for synthesis of methanol. The metal and
oxide interface afforded special path way for CO2 to methanol conversion.
CeO2 with various uniform and well defined morphologies has been successfully
prepared through controlled conditions. The effect of the morphology of the CeO2
support was also observed on catalytic performance over water-gas shift (WGS),
steam reforming of methanol (SRM) and other reaction [13-15]. In addition, the
support morphology also affects the activity and selectivity of methanol synthesis
from CO2 hydrogenation. Shik C. Tsang et al. [16] found the exposed polar (002) face
in platelike ZnO showed a much stronger material synergy with copper than other
crystal facets, which gave higher selectivity towards methanol from CO2
hydrogenation. However, there was no report on the morphology effect of CeO2 over
methanol synthesis from CO2 hydrogenation. In this paper, three kinds of CeO2 with
different nanostructures were prepared by a controlled hydrolysis method. The
Cu/CeO2 catalysts were prepared by an incipient wetness impregnation method and
applied in methanol synthesis from CO2 hydrogenation.
2. Experimental Section
2.1 Catalyst preparation
2.1.1 Preparation of CeO2 support with different shapes
Different shapes of nanoscale ceria (nanorods, nanocubes and nanoparticles) were
prepared via a controlled hydrolysis method according to the known procedures with
some modifications [17]. Typically, Ce(NO3)3.6H2O (4.34 g) was firstly dissolved in
deionized water (10 ml), then KOH (2.8 g) in 50 mL of deionized water was added
dropwise into Ce(NO3)3 solution at room temperature. After the complete addition, the
mixture was magnetically stirred for 2 h. Then the mixture was introduced into a

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o
o
stainless steel autoclave and kept at 100 C or 180 C for 24 h. The precipitants were
further separated, washed and dried at 100oC for 24 h. Finally, the as-made powder
o
was calcined in air at 400 C for 4 h to obtain CeO2 with nanorods with the
hydrothermal temperature of 100 oC and nanocubes with the hydrothermal
temperature of 180 oC. The procedures for the preparation of CeO2 nanoparticles were
almost the same as those for the preparation of nanorods CeO2 except the
crystallization temperature was changed to 100 oC and 0. 28 g of KOH was used.
2.1.2 Preparation of CuO/CeO2 catalysts
CuO/CeO2 catalysts were prepared by an incipient wetness impregnation (IMP)
method. Cu(NO3)2 was used as the precursor. The calculated Cu loading of 5wt.%.
Then the mixture was dried overnight at 100oC and calcined in air at 400 oC for 4 h to
get the CuO/CeO2 catalysts.
2.2 Catalysts characterization
The X-ray diffraction (XRD) patterns were determined using a Bruker-D8
diffractormeter with monochromatized Cu-Kα radiation (λ = 1.54056 Å) operated at
40 kV and 40 mA and collected by a Vantec-1 detector.
BET surface area of the CeO2 was determined by physisorption of N2 at 77 K by
using a quantachrome Autosorb-1-C-MS instrument. About 200 mg sample were
degassed in vacuum at 623 K for 3 h prior to the measurements to remove
pre-adsorbed components. The specific surface area was determined from the linear
portion of the Brunauer–Emmett–Teller (BET) plot.
Transmission electron microscopy (TEM) images of the catalyst samples were
obtained with a FEI Tecnai G20 instrument. The samples were prepared by directly
suspending the catalyst in ethanol with ultrasonic treatment. A copper microscope grid
covered with perforated carbon was dipped into the solution and then dried.

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H2-temperature processed reduction (H2-TPR) was experiments were carried out
using AMI-200 from Zeton Altamira Company. The sample (50 mg) in a quartz
reactor was purged with 30 ml/ min Ar while heating at a ramp rate of 10 oC /min to
150oC and maintaining that temperature for 1 h to remove traces of water. After
cooling to 50oC, the sample was reduced in a flow of 10 vol.% H2/Ar (30 ml/min)
while heating from 50oC to 800 oC at a rate of 10oC /min.
2.3 Typical procedure for CO2 hydrogenation reaction
All catalytic tests were performed in fixed-bed flow reactor (stainless steel having a
length of 47cm and inside diameter of 1/4 inch). The reactor system was equipped
with a programmable temperature controller. Typically, the Cu/CeO2 catalyst (0.1 g)
o
o
was first reduced at 400 C (1 C/min) for 2 h with H2 flow rate of 5 mL/min at
atmosphere pressure prior to methanol synthesis. The reactor was then cooled down to
100oC and the gas mixture (25% CO2+75% H2, 5 mL/min) was introduced
continuously for a certain time at different temperatures (240 ~300 oC) and 2MPa. The
GHSV = 3 L h-1gcat-1.The gas products were analyzed by Agilent GC 7890B on line
(thermal conductivity and flame ionization detectors). The liquid product was
collected in cold trap and analyzed by 4890 gas chromatography (flame ionization
detectors) off line. The conversion and selectivity reaction parameters are defined as:
nCO2,in − nCO2,out
XCO2
=
nCO2,in
n
product,out
Selectivity = n
−n
CO2,in
CO2,out
3. Results and Discussion
Firstly, CeO2 with different morphologies prepared with different conditions were
characterized by TEM technology, and the results are shown in Fig. S1. CeO2
nanorods were clearly observed in Fig. S1a, which had a uniform width of 10±3 nm

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with a length of 80–150 nm. High resolution TEM (HRTEM) images of Fig. S1b and
Fig. S1c revealed CeO2 nanorods exposed (100) and (110) faces. As shown in Fig. S1
o
d, CeO2 prepared at the hydrothermal temperature of 180 C showed nanocubes
structure with a side-size of 50-110 nm. HRTEM images of CeO2 nanocubes exposed
(100) face (Fig. S1e). The change of crystallization temperature and the concentration
of KOH produced CeO2 nanoparticles with a uniform diameter 21±3 nm (Fig. S1f),
exposing diverse faces in which the thermodynamically stable (111) faces are
predominant. TEM results indicated that the CeO2 shape greatly affected by the
preparative conditions, especially the crystallization temperature and substrate
concentration. Torrente-Murciano et al. [15] also found that the morphological phase
of CeO2 was related with the base concentration and temperature during the
hydrothermal synthesis. At low temperature and base concentration, the
dissolution/recrystallization rate was slow, resulting in the formation of ceria
nanoparticles. With the increasing of base concentration, the chemical potential
accelerated the anisotropic growth of the Ce(OH)3 nuclei, forming ceria nanorods.
Increasing the hydrothermal temperature further, the Ce(OH)3 nuclei are condensed to
ceria, nanocubes morphology formed.
The morphology of CuO/CeO2 catalysts was also characterized by TEM
technologies. As shown in Fig. S2, the morphologies of CeO2 were remained after
doping of CuO nanoparticles on the surface of CeO2. It was observed that CuO
nanoparticles were almost homogenously dispersed on the surface of the CeO2
supports without apparent aggregation.
H2-TPR profiles were performed to study the redox ability of the three kinds of
CeO2, and were shown in Fig.1a. All the three CeO2 had two reduction peaks. The
lower temperature reduction peak belongs to the reduction of surface oxygen in CeO2,

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and the higher reaction temperature reduction peak belongs to the reduction of bulk
oxygen [18]. The reduction temperature of surface oxygen increased in an order of
CeO2 nanoparticles, CeO2 nanocubes and CeO2 nanorods. The reduction temperature
and reduction peak area are summarized in Table 1. The H2 consumption of surface
oxygen for CeO2 nanorods was closed to that for CeO2 nanocubes, which were 110
and 102 μmol, respectively. The number was 83 μmol for CeO2 nanoparticles. The
BET surface area of the CeO2 was also shown in Table 1, the surface area was 56, 69,
77 m2/g for CeO2 nanorods, nanocubes and nanoparticles. The CeO2 nanorods had the
smallest surface area but the highest surface oxygen content. H2-TPR profiles of
o
CuO/CeO2 catalysts are show in Fig.1b. A main reduction peak at 179 C together
with a peak at 154oC were observed for 5% CuO/CeO2 nanoparticles, which were
attributed to the reduction of CuO and ceria on the interface of CuO-CeO2.[19]. The
o
reduction temperature increased to 251 oC and 309 C for 5% CuO/CeO2 nanocubes
and 5% CuO/CeO2 naorods catalysts, respectively. Compared to the bulk CeO2, the
reduction temperature of the catalyst decreased obviously due to the strong interaction
between metal and support (SMSI) effect, high calcination temperature will improve
this effect [20]. Cu can effectively activate the surface Ce-O bond and promoted the
surface reduction of CeO2. Araiza et al. [21] studied the CuO supported on different
morphology CeO2, and also found the CuO/CeO2-rod had the highest temperature of
main reduction peak. As CeO2 nanocubes has exposed (100) crystal planes and CeO2
nanorods has exposed (100) and (110) crystal planes, it is likely that the Cu precursor
preferentially interacts with the (110) crystal planes of CeO2 nanorods.

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100
80
60
40
20
0
80
CeO2 nanorods
b
a
CeO2 nanocubes
5% Cu/CeO2 nanorods
5% Cu/CeO2 nanoparticles
5% Cu/CeO2 nanocubes
CeO2 nanospheres
60
40
20
0
-20
100
200
300
400
500
600
700
800
100
200
300
400
500
600
700
Temperature(oC)
Temperature(oC)
Fig. 1 H2-TPR of CeO2 support and 5% CuO/CeO2 catalysts
Table 1. The surface area from BET experiment and reduction temperature, H2
consumption of CeO2 samples summarized in H2-TPR experiment
Surface oxygen reduction
Surface oxygen H2
surface area
(m2/g)ce)
temperature (oC)
consumption(μmol)
CeO2 nanorods
CeO2 nanocubes
CeO2 nanoparticles
472
399
391
110
102
83
56
69
77
XRD patterns of the three as-prepared CuO/CeO2 catalysts were shown in Fig. 2.
Diffraction peaks with 2θ at 28.6, 33.1, 47.5, 56.3, 59.1o are indexed to the (111),
(200), (220), (311) and (222) planes of face-centered cubic CeO2 with a fluorite
structure (PDF#34-0394). The CeO2 crystallite sizes were calculated from the XRD
peak widths using the Scherrer equation. The diameters were 16, 13 and 11 nm for
CeO2 nanocubes, nanoparticles and nanorods, respectively. The diffraction peaks with
2θ at 35.5 and 38.7 o were ascribed to (002) and (111) planes of CuO (PDF#45-0397)
in 5% CuO/CeO2 nanoparticles. However, there were no diffraction peaks of CuO in 5%
CuO/CeO2 nanorods and nanocubes catalysts, which indicated that small sized CuO
nanoparticles were homogeneously dispersed in 5% CuO/CeO2 nanorods and
naocubes catalysts.

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2500
Cu/CeO2 nanorods
(111)
(220)
Cu/CeO2 nanoparticles
2000
Cu/CeO2 nanocubes

CeO2
(200)
 CuO
1500




CeO2 16 nm
1000
500
0
CeO2 13nm


CeO2 11nm
70
10
20
30
40
50
60
80
2
Fig.2 XRD profiles of 5%CuO/CeO2 catalysts
The H2-TPR and XRD results show clearly that the tendency of reduction
temperature of CuO/CeO2 catalysts was the same as the CeO2 support (nanorods >
nanocubes > nanoparticles), but not consistent with the particle size of CuO
(CuO/CeO2 nanocubes > CuO/CeO2 nanoparticles >CuO/CeO2 nanorods). That means
the morphology effect of support was larger than the particle size effect of CuO in our
catalysts. H2-TPR experiments also indicated the interaction and reduction of the CuO
with the CeO2 support vary with the morphology and exposed crystal planes of the
CeO2 support. The CeO2 nanorods, which exposed (100) and (110) faces showed the
highest Cu dispersion and
interaction between CuO and
CeO2.
100
10
b
a
Cu/CeO2 nanocube
80
8
Cu/CeO2 nanorods
Cu/CeO2 nanocubes
Cu/CeO2 nanoparticles
Cu/CeO2 nanorods
Cu/CeO2 nanospheres
60
6
4
2
0
40
20
0
240
250
260
270
280
290
300
240
260
280
300
temperature(oC)
Temperature(oC)
Fig. 3. The temperature effect on CO2 conversion and methanol selectivity of
Cu/CeO2 catalysts
The effect of the reaction temperature on CO2 conversion was studied and the

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results are shown in Fig.3a. The catalytic activity of the three Cu/CeO2 catalysts
increased with the increase of the reaction temperature. However, the highest CO2
o
conversion was only 6.7% for Cu/CeO2 nanorod catalyst at 300 C The CO2
conversion was according the followed sequence, conv. Nanorods > conv. Nanocubes>
conv. Nanoparticles, which was consistant with the H2-TPR of Cu/CeO2 catalysts. The
Cu/CeO2 nanorods had the strongest interaction between CuO and CeO2, highest CuO
dispersion and largest CO2 conversion. Fig. 3b showed the methanol selectivity. The
methanol selectivity decreased sharply with the increase of the reaction temperature.
For instance, the methanol selectivity of Cu/CeO2 nanorod catalyst was 89.5% at
240oC, and that sharply decreased to 19.8% at 300oC. The highest methanol yield was
o
1.9% over Cu/CeO2 nanorods catalysts at 240 C and 2 MPa. The other by-product
were CH4 and CO. At lower temperature (240oC), the main side reaction was
methanation reaction (CO2+4H2 → CH4+2H2O), and at higher temperature (260, 280,
300oC), the reverse WGS reaction (CO2+H2 → CO+H2O) activity increased sharply
due to Cu based catalysts was also a good WGS or reversed WGS catalyst [22].
Conclusions
In conclusion, three nanostructure Cu/CeO2 catalysts with different morphology
were prepared and applied in CO2 hydrogenation to methanol. The Cu/CeO2 nanorods
catalyst showed the highest methanol yield of 1.9% at 240 oC and 2MPa. H2-TPR and
XRD results indicated CuO/CeO2 nanorods had the strongest interaction between
CuO and CeO2, highest CuO dispersion and largest CO2 conversion.
Acknowledgements
The Project was supported by National Natural Science Foundation of China (no.
21206200).
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Highlights
 Three Cu/CeO2 catalysts were applied in CO2 hydrogenation to methanol
reaction.

Support morphology greatly affected the catalyst activity of CO2
hydrogenation.
 The Cu/CeO2 nanorods gave the highest methanol yield of 1.9% at 240oC and
2MPa.