Influence of rare-earth elements (La, Ce, Nd and Pr) on the performance of Cu/Zn/Zr catalyst for CH3OH synthesis from CO2

Article abstract of DOI:10.1016/j.catcom.2014.05.014

A series of Cu/Zn/Zr (CZZ) catalysts with different rare earth elements (La, Ce, Nd and Pr) were prepared by co-precipitation method and tested for CO₂ hydrogenation to methanol. The influence of modifier (La, Ce, Nd and Pr) on the physicochemical properties of CZZ catalysts was studied. The results of catalytic test reveal that CO₂ conversion was correlated to the introduction of different promoters. The introduction of La and Ce favors the production of methanol, and Nd and Pr modified CZZ catalysts show relative low activity compared with the reference CZZ catalyst.

Full text of DOI:10.1016/j.catcom.2014.05.014

Influence of rare-earth elements (La, Ce, Nd and Pr) on the performance of
Cu/Zn/Zr catalyst for CH3OH synthesis from CO2
Hongyan Ban, Congming Li, Kenji Asami, Kaoru Fujimoto
PII:
DOI:
Reference:
S1566-7367(14)00195-2
doi: 10.1016/j.catcom.2014.05.014
CATCOM 3908
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Catalysis Communications
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Revised date:
Accepted date:
26 March 2014
18 May 2014
19 May 2014
Please cite this article as: Hongyan Ban, Congming Li, Kenji Asami, Kaoru Fuji-
moto, Influence of rare-earth elements (La, Ce, Nd and Pr) on the performance of
Cu/Zn/Zr catalyst for CH3OH synthesis from CO2, Catalysis Communications (2014),
doi: 10.1016/j.catcom.2014.05.014
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Influence of rare-earth elements (La, Ce, Nd and Pr) on the
performance of Cu/Zn/Zr catalyst for CH3OH synthesis from
CO2
Hongyan Ban1, Congming Li2,3*, Kenji Asami4*, Kaoru Fujimoto3
1 School of Chemical Engineering，University of Science and Technology
LiaoNing, Anshan 114051, China
2 Key Laboratory of Coal Science and Technology, Ministry of Education
and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024,
Shanxi, China
3
Japan Gas Synthesis Co. LTD, 5-20, Kaigan 1-Chome, Minato-Ku,
Tokyo, 105-8527, Japan
4
Department of Chemical Processes and Environment, Faculty of
Environmental Engineering, The University of Kitakyushu, 1-1, Hibikino,
Wakamatsu, Kitakyushu Fukuoka 808-0135, Japan
* Corresponding authors. Tel: 81-93-695-3387 (C.M. Li);
81-93-695-3284 (K. Asami);
E-mail:
licongming0523@163.com
(C.M.
Li);
asami@env.kitakyu-u.ac.jp (K. Asami).
_

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Abstract
A series of Cu/Zn/Zr (CZZ) catalysts with different rare earth elements
(La, Ce, Nd and Pr) were prepared by co-precipitation method and tested
for CO2 hydrogenation to methanol. The influence of modifier (La, Ce,
Nd and Pr) on the physicochemical properties of CZZ catalysts was
studied. The results of catalytic test reveal that CO2 conversion was
correlated to the introduction of different promoters. The introduction of
La and Ce favor the production of methanol, Nd and Pr modified CZZ
catalysts show relative low activity compared with the reference CZZ
catalyst.
Keywords: Rare-earth elements, CO2 hydrogenation, Cu/Zn/Zr catalyst

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1. Introduction
In recent years, chemical fixation of carbon dioxide has attracted
additional attention as a possible way to manufacture useful chemicals,
which is becoming one of the most promising ways to mitigate global
warming problems caused by greenhouse gas. Methanol is an important
chemical feedstock for several valuable chemicals and may be used as a
fuel additive or clean fuel [1-3]. It can also be converted to high-octane
gasoline by MTG, olefin by MTO, hydrocarbons by MTH [4]. Based on
the great significance of methanol synthesis from CO2 hydrogenation,
Olah et al proposed a new concept of “methanol economy” [1].
Cu/Zn/Al catalysts have been widely used for methanol synthesis from
CO2 containing syngas; however, they exhibited a relative poor catalytic
reaction performance for CO2 hydrogenation to methanol [5]. The reason
can be ascribed to the poisoning effect of water on the rate of methanol
formation and the strong hydrophilic characteristic of alumina [6].
Promoters such as Ga [7], La [8], Pd [9] on the Cu/ZnO based methanol
synthesis catalyst have been investigated. Among the various metal
oxides, ZrO2 is of special interest because of its mechanical and thermal
stability, its high specific surface and its semiconducting properties [10].
Zirconia-supported copper catalyst has been reported to have better
performance than conventional Cu/ZnO/Al2O3 catalyst [6, 11]. The
interaction of ZrO2 and metal Cu particles could lead to the stability of

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Cuδ+ sites and a “mix” of Cu0, Cuδ+ and oxide basic sites concurring to
the adsorption of H2, CO and CO2 [12]. Thus, zirconia-supported copper
catalysts have gained an increasing interest for its high activity in recent
years [6, 13-17]. And it was well established in literatures that the
addition of promoters, such as La2O3, Mn, Cr, B2O3 and Ga2O3, could
also enhance the catalytic performance of methanol synthesis [8, 18, 19].
Słoczyński et al. [20] reported that the addition of Mn and Mg promoters
could improve copper dispersion and the adsorptive properties of the
Cu/ZnO/ZrO2 catalysts for the synthesis of methanol from CO2
hydrogenation. They also reported the effect of B, Ga, In and Gd oxides
modified CZZ catalyst on the catalytic activity, and found the Ga2O3
additive was especially useful [21]. Moreover, some rare-earth elements
such as La, Ce and Y can also increase the Cu surface area, as well as
effectively increase the strength of the interaction and facilitated spillover
of H2 between Cu and ZrO2 and therefore stabilized the active center of
the catalyst. It could also improve the resistance to thermal sintering and
prevent the quick deactivation of Cu species in the presence of water and
CO2 [22-25].
It has been shown in the earlier studies that the catalytic performance
was influenced by the different promoter. Especially the incorporation of
rare earth element (La2O3) could enhance the interaction of reactant and
catalyst [8]. It was not reported in detail the effect of different rare earth

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elements on the properties of catalyst and reaction performance. The
objective was to study the different interaction between components and
to investigate the effect of rare earth element on Cu species, which were
responsible for the catalytic performance.
In the present study, the CZZX (X =La, Ce, Nd and Pr) catalysts were
prepared and characterized by XRD, BET, XPS and TPR techniques. The
aim of this work has been a systematic study of the influence of the
rare-earth elements on the structure and activity of CZZ catalyst in the
synthesis of methanol from CO2 hydrogenation.

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2. Experimental
2.1. Catalyst preparation
The investigated catalysts with weight ratio of CuO:ZnO:ZrO2=5:3:2
and CuO:ZnO:ZrO2:X=5:3:1.5:0.5 (X represents La, Ce, Nd and Pr
oxide) were prepared by a conventional co-precipitation method as
described in detail by our previous work [11, 26]. Typically, two aqueous
solutions, a metal salts solution of Cu(NO3)2·3H2O, Zn(NO3)2·6H2O,
ZrO(NO3)2·2H2O, and in necessary, La(NO3)3·6H2O, Ce(NO3)3·6H2O,
Nd(NO3)3·6H2O or Pr(NO3)3·6H2O (all Wako, total cation concentration
1 M) and a mixed solution of Na2CO3 precipitant, were added dropwise at
65℃to final pH of 7. The precipitate was then filtered, dried, and then
calcined in air at 400℃ for 5 h. Finally, the resultant powder was shaped
into granules and to 20-40 meshes for the reaction.
2.2. Catalyst characterization
XRD patterns of catalyst samples were measured under ambient
atmosphere with a RIGAKU X-ray diffract meter equipped with a Cu-Kα
radiation. The composition of every sample was determined by X-ray
fluorescence on HITACH ZSX101-e instrument. The specific surface area
of the catalysts was determined by BET method using a Micromeritics
ASAP 2010. X-ray photoelectron spectra (XPS) of the calcined catalysts
were recorded on an ESCALAB 250 spectrometer using Al Kα radiation
(1486.6 eV). The binding energies were calculated with respect to C1s

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peak at 284.6 eV.
TPR carried out with a BELCAT-B instrument. The sample (50mg) was
previously treated in He flow up to 350℃ and kept for 2 h followed by
cooling to 50℃. The 10% H2/He mixture was passed over samples at a
flow rate of 30 ml/min with a heating rate of 10℃/min up to 400℃. The
effluent gas was passed over a molecular sieve trap to remove the
generated water and then analyzed by GC equipped with TCD.
2.3. Procedures for catalytic reaction and product analysis
A pressurized flow type reaction apparatus with a fixed-bed reactor was
used for this study. The apparatus was equipped with an electronic
temperature controller for a furnace, a tubular reactor with an inner
diameter of 8 mm, thermal mass flow controllers for gas flows and a
back-pressure regulator. A thermocouple was set at the axial center of the
tubular reactor. One gram of catalyst was placed in the reactor with inert
quartz sands above and under the catalyst. All catalysts were reduced in
the flow of 5% H2 in nitrogen at 250℃ for 4 h before reaction. All the
products from the reactor were analyzed by on line gas chromatograph
(GC). The effluent gas component including CO, CO2 and CH4 were
analyzed by a GC equipped with the thermal conductivity detector (TCD)
and a column of activated charcoal, and methanol and other by-products
were analyzed by another GC equipped with the flame ionization detector
(FID) and a capillary column of Porapak-Q.

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3. Results
3.1. Characterizaiton
The data on BET surface area, pore volume and average diameter of the
calcined catalysts are shown in Table 1. It is found that the BET specific
surface area for the CZZX (X=La, Ce, Nd and Pr) samples are lower than
that for CZZ and increase in series: CZZCe < CZZNd < CZZLa <
CZZPr< CZZ. The chemical composition of catalysts after calcination
was determined by XRF as summarized in Table 1, which generally
agrees to the weight ratio of the starting precursor salts added.
Fig.1 shows the XRD patters of the samples after calcination. Some
broad peaks due to a poorly crystallized CuO phase (2θ=35.4o, 38.7o,
66.0o, 74.5o) are detectable for the all the samples. It is also observed that
some diffraction peaks appear at 2θ of 31.3o, 47.5o, 56.4o, 62.0o, 67.9o,
which was overlapping, indicating the present of crystalline ZnO [27].
But crystallization of ZrO2 is not obviously observed. For CZZCe and
CZZLa samples, the new phases of CeO2 and La2O3 (this part is enlarged
in the same figure) with weak intensity are segregated. No crystalline Pr
or Nd oxide phase is detected when Pr or Nd is added. This may be
attributed to the fact that these elements may be exist in the amorphous
state as low calcination temperature (400℃) does not favor the formation
of metal oxide crystalline.
TPR measurements were carried out to highlight the reduction patter of

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CZZ and CZZX (X=La, Ce, Nd and Pr) catalysts. The reduction profiles
of the studied catalysts are shown in Fig. 2. All of the systems display
reduction profiles characterized by a main peak between 150 and 190℃.
The reduction curve is not uniform and shows a shoulder at 164℃ for
CZZ. It was reported that the two reduction steps are due to the
consecutive reduction of Cu2+ first to Cu+ and then to Cu [28]. Whereas
the peak is much more symmetrical for CZZX, and the peak of CZZLa
and CZZCe is sharper than the reference catalyst CZZ, suggesting
relative strong interaction between La, Ce with CuO than other
investigated rare-earth elements.
Changes to surface bonding with rare-earth elements were monitored by
measuring the binding energies (BE) of Cu2p3 core electron, as seen in
Fig. 3. The BE of Cu2p3/2 gave value of 933.6eV, corresponding to CuO.
The presence of Cu2+ species can be readily identified by a strong satellite
peak at ~940 eV [29]. This satellite peak implies copper species on the
catalyst exist in the Cu2+ oxidation state. The lack of a peak at 932eV
indicates negligible Cu0 or Cu+ is present on the catalyst surface [30]. The
shift in BEs when comparing the different rare-earth element provides
evidence of an interaction between the copper and rare-earth element.
Inclusion of rare-earth elements into the system shifted the Cu2p3/2 peak
from 933.6 eV to 933.3 eV for La, 933.6 eV to 933.7 eV for Pr, no
obvious shift for Ce and Nd, respectively, demonstrating an interaction

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between CuO and rare-earth element oxide.
3.2. Catalytic performance
3.2.1. General features of CO2 hydrogenation over Cu/ZnO-based
catalysts
When a mixture of CO2/H2 was contacted with these catalysts at the
operating conditions (3MPa; 230℃; H2/CO2=3/1), the main products of
the reaction were methanol, CO and water. Methane was not detected as
the significant byproduct.
3.2.2. Reaction performance of CZZ and CZZX (X = La, Ce, Nd and Pr
oxide) catalysts
The catalytic performance of a CZZ and CZZX (X = La, Ce, Nd and
Pr) catalysts are listed in Table 2. Compared with the CZZ, the
conversion of CO2 increased by 4.6% for CZZLa and by 16.3% for
CZZCe, and the CH3OH selectivity was enhanced significantly with
introduction of La and Ce, achieving 49.8% for CZZLa and 53.0% for
CZZCe, which are 12.2% and 19.4% higher than that on the CZZ.
CZZNd and CZZPr catalysts exhibit relative lower activity in comparison
with that obtained over CZZ. The CH3OH yield is also accordance with
the order of CO2 conversion. A maximum CH3OH yield of 3.2 mol·h-1·
kg-1 with the CO2 conversion of 22.8% is obtained over the CZZCe at
230℃.
The reaction performance with regard to the conversion of CO2 and

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selectivity of methanol is obvious, indicating the influence of addition of
different rare earth element. Recently, Arena et al [17] investigated the
reaction of CO2 hydrogenation on CZZ and proposed the reaction
mechanism, the adsorption and dissociation of hydrogen occur over the
Cu site, while the adsorption of CO2 takes place on the ZrO2 site. Then,
the atomic hydrogen transports from the surface of Cu onto the surface of
ZrO2 via spillover and hydrogenates the adsorbed carbon containing
species to methanol. On this account, it is reasonable to draw a
conclusion that appropriate adsorption amount and adsorption strength of
CO2 are favorable for the hydrogenation of CO2. CeO2 was reported to
have an excellent oxygen storage capacity, which leads to a marked
increase in oxygen vacancies of CZZ, resulting in high activity [31-33]
among all investigated dopants. The XPS results also support this point,
that is, the addition of CeO2 changed the electron environment of Cu,
promoting the synergistic effect between CuO and the surface oxygen
vacancies of ceria, which was related to the activity and methanol
selectivity [34].
Similar to CZZCe, CZZLa also shows relative high activity compared
with CZZ, which could be related to its high ability of hydrogen
adsorption. The relatively low catalytic activity of CZZPr and CZZNd
was possibly attributed to the relative low ability of hydrogen adsorption.
3.2.3. The effect of reaction temperature

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Methanol synthesis over the CZZ and CZZX (X = La, Ce, Nd and Pr)
catalysts was studied in the temperature range from 210 to 250℃, and the
results are shown in Fig.4. The conversion of CO2 increases with the
increasing temperature and reach the maximum value when the
temperature arrives 230℃. However, the methanol selectivity decreased
with increasing temperature. Therefore, it can be deduced that the control
factor for methanol synthesis shifts from kinetics to thermodynamics.
From the results, it can be seen that the maximum conversion is obtained
at 230℃ under tested conditions.
3.2.4. The effect of space velocity
In order to clarify whether methanol is produced indirectly via the
intermediate formation of carbon monoxide and then hydrogenation or
directly by the carbon dioxide hydrogenation, the influence of the space
velocity on the methanol selectivity was investigated (as shown in Fig. 5).
It is well known that the reverse water gas shift reaction (RWGS) and
methanol synthesis reactions coexist in carbon dioxide hydrogenation
presented as
CO2(g) + 3H2 ↔ CH3OH (g) +H2O(g)
CO (g) + 2H2 ↔ CH3OH (g)
(1)
(2)
(3)
CO2(g) + H2 ↔ CO (g) + H2O (g)
It is clearly note that carbon monoxide was the main product at short
contact time for CZZX (X = La, Ce, Nd and Pr) catalysts and then the

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yield to methanol increases quickly and to carbon monoxide decrease
slowly with the increase of space velocity. At the same time, it is also
interesting to note that slight amount of methanol can be formed even at
short contact time. The results indicate the activation of carbon dioxide to
methanol by CZZA could pass through two parallel routes: one is the
direct process from CO2 and another is the one which pass through CO.
4. Conclusions
It is concluded that, the doped Ce in CZZ catalyst was the most active
giving CO2 conversion at 22.8% and methanol selectivity at 53.0%. The
methanol formation from CO2 over CZZr and CZZX (X = La, Ce, Nd and
Pr) were assumed to pass through two parallel routes: one is the direct
process from CO2 and another is the one which pass through CO.
Compared with CZZ, the presence of La and Ce was favorable for
improving the overall activity, the doping of Nd and Pr appeared to
decrease the activity.
Acknowledgements
The financial supports from Natural Science Foundation of Liaoning
Province of China (20102157) and New Energy and Industrial
Technology Development Organization (NEDO) of Japan are gratefully
acknowledged.
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Table 1
Physicochemical properties of the studied catalysts.
Sample
BET
(m2/g)
Pore Volume
(cm3/g)
Pore size
(nm)
Chemical composition
CuO
56.2
55.5
54.7
54.8
55.0
ZnO
26.7
26.5
25.7
26.0
26.1
ZrO2
17.1
13.0
14.0
14.2
13.6
X
CZZ
83.3
72.5
61.2
67.4
74.1
0.46
0.47
0.50
0.54
0.50
16.4
20.0
20.1
21.6
20.2
CZZLa
CZZCe
CZZNd
CZZ Pr
5.0
5.6
5.0
5.3

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Table 2
Reaction performance of the catalysts.
Cat
CO2
Conv.(%)
19.6
Methanol Productivity
Methanol Selectivity (%)
(mol h-1 kg-1)
CZZ
2.3
2.7
3.2
2.0
2.2
44.4
49.8
53.0
40.5
42.0
CZZLa
CZZCe
CZZNd
CZZPr
20.5
22.8
19.0
19.3
Reaction conditions: 3 MPa; 230 oC; H2/CO2=3/1, W/F=10 g·cat·h/mol.

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Fig. 1. XRD patters of CZZ and CZZX (X = La, Ce, Nd and Pr)

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CZZPr
CZZNd
CZZCe
CZZLa
CZZ
50
150
250
350
Temperature [℃]
Fig. 2. TPR profiles of CZZ and CZZX (X = La, Ce, Nd and Pr) catalysts.

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Cu2p1/2
Cu2p3/2
CZZPr
CZZNd
CZZCe
CZZLa
CZZ
970
960
950
940
930
Binding energy [eV]
Fig. 3. XPS spectra of CZZ and CZZX (X = La, Ce, Nd and Pr) catalysts.

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Fig.4. Effect of reaction temperature.
Test conditions: 3.0 MPa, 230 oC , H2/CO2=3.

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Fig.5. Effect of W/F.
Test conditions: 3.0 MPa, 230 oC, H2/CO2=5.

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Graphical abstract

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Highlights
 CZZCe was proved to be effective catalyst for CO2 hydrogenation.
 La and Ce was favorable for improving the overall activity.
 Methanol formation over CZZ and CZZX were assumed to pass
through parallel route.