Development of highly stable catalyst for methanol synthesis from carbon dioxide

Article abstract of DOI:10.1016/j.apcata.2013.10.010

Zr-doped Cu-Zn-Zr-Al (CZZA) catalyst showed excellent performances for the methanol synthesis from carbon dioxide and hydrogen such as activity, selectivity and especially stability under mild conditions (such as 230 C and 3.0 MPa). The catalyst showed excellent tolerance against water vapor. It was found that added alumina promoted the dispersion of Cu whereas it suppressed the reduction of copper oxide. On the other hand, added Zr promoted the catalytic activity of methanol synthesis from CO₂ and suppressed the inhibitive effect of water for the reaction as well as the catalyst deactivation. It was concluded that the methanol formation from CO₂ proceeds through two routes: one is the direct hydrogenation of CO₂ to methanol and another is the one which pass through the CO formation. The Zr-promoted catalyst gave methanol and CO at the selectivity ratio of 0.4 to 0.6, whereas the un-promoted catalyst gave only CO at the initial stage of the reaction. It was claimed that the doped Zr promote the in-situ reduction of oxidized Cu (which should be caused by the reaction with the co-product H₂O) by H ₂ to increase the content of reduced Cu (active site) and thus the catalyst activity. The promoted reductivity of the Zr-containing catalyst prevents the crystal growth of CuO_x which cause the irreversible deactivation of catalyst.

Full text of DOI:10.1016/j.apcata.2013.10.010

Applied Catalysis A: General 469 (2014) 306–311
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Development of highly stable catalyst for methanol synthesis from
carbon dioxide
Congming Li, Xingdong Yuan, Kaoru Fujimoto∗
Japan Gas Synthesis CO. LTD, 5-20, Kaigan 1-Chome, Minato-Ku, Tokyo, 105-8527, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 August 2013
Received in revised form 1 October 2013
Accepted 5 October 2013
Available online 15 October 2013
Zr-doped Cu-Zn-Zr-Al (CZZA) catalyst showed excellent performances for the methanol synthesis from
carbon dioxide and hydrogen such as activity, selectivity and especially stability under mild conditions
(such as 230 ◦C and 3.0 MPa). The catalyst showed excellent tolerance against water vapor. It was found
that added alumina promoted the dispersion of Cu whereas it suppressed the reduction of copper oxide.
On the other hand, added Zr promoted the catalytic activity of methanol synthesis from CO2 and sup-
pressed the inhibitive effect of water for the reaction as well as the catalyst deactivation. It was concluded
that the methanol formation from CO2 proceeds through two routes: one is the direct hydrogenation of
CO2 to methanol and another is the one which pass through the CO formation. The Zr-promoted catalyst
gave methanol and CO at the selectivity ratio of 0.4 to 0.6, whereas the un-promoted catalyst gave only CO
at the initial stage of the reaction. It was claimed that the doped Zr promote the in-situ reduction of oxi-
dized Cu (which should be caused by the reaction with the co-product H2O) by H2 to increase the content
of reduced Cu (active site) and thus the catalyst activity. The promoted reductivity of the Zr-containing
catalyst prevents the crystal growth of CuOx which cause the irreversible deactivation of catalyst.
© 2013 Published by Elsevier B.V.
Keywords:
Methanol synthesis
Zr-promoted catalyst
CO2
1. Introduction
Al2O3 has been used often as the promoter in order to increase the
stability and the activity. One of the present authors has shown that
Utilization of carbon dioxide has become an important global
issue due to the significant and continuous rise in CO2 con-
centrations, accelerated growth in the consumption of petrified
carbon-based energy in the worldwide, depletion of carbon-based
energy resources [1]. Catalytic hydrogenation of carbon dioxide
to produce various kinds of chemicals or fuels such as methanol
or hydrocarbons has been recently considered as one of the most
promising processes for the utilization of CO2 as well as the accep-
tor of hydrogen [2–7]. Generally, methanol has been produced
industrially from CO and H2 containing small amount of CO2. Dur-
ing the reaction, part of the CO2 is converted to CO and H2O. Thus,
the industrially made methanol contains H2O. However, it is not
clear whether CO2 is converted to methanol or to CO. It has been
pointed out that for CO hydrogenation zero valent copper (Cu0) is
the active component [8]. Al2O3 has been pointed out to work as
physical promoter to disperse Cu. Recently, many works concerned
with the methanol synthesis from CO2/H2 mixture have been pre-
sented. It has been shown that Cu/Zn-based catalysts are the most
useful systems for the catalytic hydrogenation of CO2 to methanol.
They are used under rather high temperature (above 250 ◦C) and
high pressure (more than 5 MPa) and show short life time [5,9,10].
a conventional Cu/Zn/Al (CZA) catalyst showed high activity and
stability for CO2 hydrogenation when it was mixed with Pd/SiO2,
and showed that the Pd/SiO2 acts to keep the content of Cu0 at
high level [11]. Gao et al. claims that the doped Zr promote the
basicity of CZA and the adsorption of CO2 [12]. Development of
active and stable catalyst under mild reaction conditions will attract
high attention from the point of the CO2 utilizing technology. For
the direct CO2 hydrogenation, the formation of water vapor is
inevitable (CO2 + 3H2 → CH3OH + H2O), which inhibit the reaction
strongly and leads to serious catalyst deactivation. However, little
attention has been paid to elucidate the influence or to suppress
its negative effects. In the present study, we will report our new
findings on the effects of promoters (Al and Zr) on the methanol
synthesis from CO2 hydrogenation. Also, the promotive effects of
Zr on the catalysis are analyzed from the relationship between
the structure and the reaction performances. The Zr oxide doped
Cu-Zn based catalyst was prepared by a simple co-precipitation
method.
2. Experimental
2.1. Catalyst preparation
∗
The investigated catalysts were prepared by
tional co-precipitation method. It was precipitated from
a conven-
Corresponding author. Tel.: +81 695 3202; fax: +81 695 3309.
E-mail address: fujimoto070@gmail.com (K. Fujimoto).
0926-860X/$ – see front matter © 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.apcata.2013.10.010

C. Li et al. / Applied Catalysis A: General 469 (2014) 306–311
307
an aqueous solution of Cu(NO3)2·3H2O, Zn(NO3)2·6H2O,
ZrO(NO3)2·2H2O, Al(NO3)3·9H2O (all Wako, total cation con-
centration 1 M) by dropwise addition of 1 M aqueous
a
solution of Na2CO3 at 65 ◦C to final pH of 7. The precipitate
was then filtered, dried, and then calcined in air at 400 ◦C
for 5 h. The resulting catalyst has the composition of CZA
(Cu/ZnO/Al2O3 = 4/3/3), CZZA (Cu/ZnO/ZrO2/Al2O3 = 4/3/1.5/1.5),
CZZ (Cu/ZnO/ZrO2 = 4/3/3) by weight. Finally, the resultant pow-
der was shaped into granules and to 20–40 meshes for the
reaction.
2.2. Catalyst characterization
XRD patterns of catalyst samples were measured under ambi-
ent atmosphere with a RIGAKU X-ray diffract meter equipped with
a Cu K␣ radiation. 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 K␣ radiation (1486.6 eV).
The binding energies were calculated with respect to C 1s peak at
284.6 eV.
TPRcarried out with a BELCAT-Binstrument. The sample (50 mg)
was previously treated in He flow up to 350 ◦C and kept for 2 h
followed by cooling to 50 ◦C. The 10% H2/He mixture was passed
over samples at a flow rate of 30 ml/min with a heating rate of
10 ◦C/min up to 400 ◦C. The effluent gas was passed over a molecular
sieve trap to remove the generated water and then analyzed by GC
equipped with TCD.
The surface of Cu metal number was measured by N2O titra-
tion method by using the same BELCAT-B instrument. Prior to
titration, the sample with about 50 mg was reduced at 250 ◦C
for 2 h with 10% H2/He flow, followed by purging and cooling
with He flow to 50 ◦C. The consumption of N2O as well as the
evolution of N2 on the metallic Cu sites (2Cu + N2O → Cu2O + N2)
was measured at 50 ◦C by a thermal conductivity detector (TCD).
The surface area of metallic Cu was calculated by assuming
1.46 × 1019 Cu atoms/m2 and N2O/Cu molar stoichiometry of 0.5
and the particle size of Cu was calculated with the equation of
6000/(8.92 × Cu metal surface area/Cu fraction in gram catalyst)
[13,14].
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 reac-
tor 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 cata-
lyst 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 ◦C for 4 h before reaction. All the products
from the reactor were analyzed by on line gas chromatograph
(GC).
Fig. 1. XRD patters of catalysts: (1) calcined; (2) reduced and (3) after 100 h reaction.
3. Results and discussion
the peaks at 43◦ and 50◦ were attributed to metallic Cu, which
performed as a role as active component for methanol synthesis
[16,17]. Using Debye–Scherrer Formula, the crystallite sizes of Cu
are calculated as 8.8 nm in CZA, 9.4 nm in CZZA and 12.5 nm in CZZ,
respectively.
The crystallite size growth of Cu occurred as well as CuO
during stability test, which could be estimated to be 11.4 nm
for CZZA and 13.8 nm for CZA, respectively, implying the
significant negative effect of crystal growth in the CO2 hydrogena-
tion.
3.1. Catalyst structure and their character
3.1.1. Structure
The XRD patterns of (1) calcined, (2) reduced and (3) used cat-
alysts are shown in Fig. 1. Results obtained suggest the presence
of a CuO phase (2Â = 35.5◦, 38.5◦) for calcined samples. In addi-
tion to these peaks, there also appeared peaks that were assigned
to ZnO at 31.8◦, 48◦, 56.6◦, 62.8◦, 68◦ for CZZ sample, which is
agreed well with that obtained by Wang [15]. After reduction,

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C. Li et al. / Applied Catalysis A: General 469 (2014) 306–311
Table 1
Characters of Cu/ZnO-based catalysts for methanol synthesis.
Cat
BET (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
C
CZ
CZA
CZZA
CZZ
9.8
29.8
78.3
106.3
90.9
0.11
0.40
0.35
0.54
0.44
55.8
29.5
16.6
16.6
3.1.2. Specific surface area and Cu surface
The data on BET surface area, pore volume and average diameter
of the calcined catalysts are shown in Table 1. It can be observed
that without the addition of alumina, even after Zn promotion, the
CZ (CuO/ZnO) catalyst shows very low surface area of 29.8 m2/g. It
is also found from Table 1 that the surface area of the CZZA cat-
alyst is significantly higher than the catalyst promoted on either
Al2O3 or ZrO2. It can be observed that the CZZA catalyst exhibits
the highest surface area of 106.3 m2/g, and CZZ (90.9 m2/g) and CZA
(78.3 m2/g) followed. Similar trends are also observed in the pore
volume data from BET. These results reiterate the role of Al2O3 as a
structure promoter in providing a highly dispersed CuO/ZnO struc-
ture. The increase in surface area of CZZA is possibly attributed to
further enhancement by the simultaneous addition of Zr, imply-
ing the same addition of Al2O3 and ZrO2 increases the total surface
area.
The Cu surface areas and Cu dispersions of the reduced sam-
ples, measured by N2O titration, are shown in Table 2. Interestingly,
the aluminum-containing catalyst (CZA) had the highest Cu surface
area. It is likely that the sintering of Cu in CZZA occurred during the
initial reduction, to decrease the Cu surface areas compared to that
in CZZ [18].
Fig. 2. TPR profiles of calcined catalysts.
3.1.3. Reducibility of CuO
The TPR profiles of the calcined samples, along with those
of pure CuO and CuO/ZnO prepared with a nominal composi-
tion of 40/30 (wt%) as reference catalysts, are given in Fig. 2.
The TPR profile of the calcined CuO sample (C) shows mainly
one reduction peak at around 250◦C, which was assigned to
the reduction of CuO → Cu or CuO → Cu2O → Cu. Although the
reduction of zinc oxide is thermodynamically feasible at high tem-
peratures, it was not reduced under the experimental conditions
described here [19]. The presence of one peak in the TPR pro-
file of CuO/ZnO (CZ) is in accordance with the literature reported
previously [20–22]. The reduction peak of the CuO/ZnO cata-
lyst was shifted by almost 50 ◦C toward lower temperature in
comparison to that of the CuO reference catalyst, implying that
the presence of zinc oxide enhances the reducibility of the cop-
per oxide phase, and this fact agrees with previous observations
[19].
with CZ catalyst. The results indicate that the presence of zinc oxide
enhances the reducibility of the copper oxide phase. As to the CZA,
the reduction peak obviously shifted to high temperature even
though the dispersion CuO on CZA is higher than that on CZZA,
which was ascribe to the strong interaction between Al2O3 and
CuO.
However, the reduction peak of CZZA is by about 75 ◦C lower
than that of CZA and even lower than that of CuO/ZnO. Also,
it should be noted that the addition of ZrO2 to CuO/ZnO lower
the reduction temperature by about 30 ◦C. All these phenomena
suggest that the added Al2O3 promotes the CuO dispersion and sup-
presses its reduction probably because of the small crystal, whereas
the added ZrO2 in CuO/ZnO-based catalyst promote the reduction
of CuO.
The addition of Al2O3 to CZ catalyst exhibited two broad reduc-
tion peaks, and the second peak centered at 250 ◦C made the shift
of reduction peak to higher temperature by about 50 ◦C compared
3.1.4. Electronic state
The chemical state of the calcined catalysts was evaluated by
XPS (Fig. 3). The XPS spectra of the calcined samples, CZA, CZZA
and CZZ show that the Cu 2p3/2 and Cu 2p1/2 peaks appeared
at 933.0, 952.7; 932.7, 952.7; 933.1, 952.7 eV, respectively, which
were the characteristics peaks of Cu2+ species [23,24]. Furthermore,
the presence of peak appearing at ca. 942.0 eV evidenced the pres-
ence of Cu2+ ions in the form of cupric oxide [25]. The binding
energies of Cu 2p3/2 and Cu 2p1/2 of the calcined catalysts were
different, revealing that the chemical state of CuO was influenced
by the composition of the catalyst, i.e., the electron density sur-
rounding Cu atom was changed with the addition of alumina and/or
zirconia.
Table 2
N2O chemisorption data for calcined catalyst.
Catalysts
Surface area of
copper (m2/g)
Metallic copper
Dm (%)b
particle size (nm)a
CZA
CZZA
CZZ
17.8
17.4
16.8
12.0
12.3
12.8
8.6
8.5
8.2
a
The particle size of Cu was calculated by using the equation of 6000/(8.92 × Cu
metal surface/Cu fraction in gram catalyst).
b
Copper dispersion: Dm = exposed copper atoms/total copper atoms.

C. Li et al. / Applied Catalysis A: General 469 (2014) 306–311
309
Fig. 3. XPS spectra of calcined and used catalysts (after stability test).
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 Cu/ZnO-based
catalysts at the operating conditions described above, 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 Cu/ZnO-based multicomponent
catalysts
The catalytic activity and selectivity of methanol on three cata-
lysts are listed in Table 3, which show results of methanol synthesis
reaction at 230 ◦C and 3 MPa for 20 h. Conversion of CO2 and
methanol yield were a little higher on CZZA compared to those
on CZA and CZZ. The selectivity of methanol on CZZA was by 17.3%
higher than that on CZA, 11.7% higher than that on CZZ. Obviously,
the copper surface area alone cannot explain the catalytic behavior
of the catalyst. These findings indicate that the promoting effect of
ZrO2 is exerted in the catalyst. First, it is probably due to the higher
dispersion of CuO particles in the Al2O3-ZrO2 promoted catalyst,
which provides a larger reactive surface area and, therefore, a high
activity. It can be seen that there is apparent correlation between
the surface area and the catalytic activity. It is noted that the con-
version of CO2 increases with increase of surface area as shown
in Tables 1 and 3. The Al2O3-ZrO2-promoted catalysts were found
to exhibit better catalytic properties than the catalyst supported
on either Al2O3 or ZrO2 alone. This phenomenon will be discussed
later.
Another excellent property is that the catalytic activity of
CZZA is much more stable than that of CZA. The advantage of
using Al2O3-ZrO2 as catalyst promoter was investigated by other
researchers [26–30]. These advantages include a high surface area,
basicity, and thermal stability, etc. by a combination of both the
supports in a mixed oxide. In Fig. 6 are shown the performance of
CZA, CZZ and CZZA for the CO2 hydrogenation as function of W/F.
It showed that at high W/F (long contact time), all of them show
Fig. 4. Influence of pre-addition of water.
similar CO2 conversion and product selectivity, which are close to
that equilibrium data. However, in the range of short contact time,
performances of these catalyst showed quite different features,
which will be discussed in the part of Section 3.
3.2.3. Influence of water
Taking into account that the equi-molar amount of water,
produced during the hydrogenation of CO2, it must affect the per-
formances of Cu-Zn based catalyst. The poisoning effect of added
water for CO2 hydrogenation was carried out. The curve with the
pre-addition of water was shown in Fig. 4. It was noted that when
water was added in the experiment, the conversion immediately
decreased to reach a steady state level depending on the amount of
water used. The activity was recovered slowly to the original level
when the water supply was stopped. The results suggested that
water temporally poison the active site of the catalyst and suppress
the methanol formation. It is also noted that the addition of water
have a promotion effect on the CO selectivity from reversed water
gas shift reaction even under the case of serious deactivation. Over
the investigated catalysts, CZZA is proved to be the most resistant
Table 3
Reaction performance of different methanol synthesis catalysts.
Cat.
CO2 Conv. (%)
Methanol productivity
Methanol
(mol/h/kg)
Selectivity (%)
CZA
CZZA
CZZ
18.7
23.2
19.3
2.15
3.75
2.51
43.0
60.3
48.6
Reaction conditions: 3 MPa; 230 ◦C; H2/CO2 = 3/1, W/F = 10 g cat h/mol.

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C. Li et al. / Applied Catalysis A: General 469 (2014) 306–311
Fig. 5. Stability of CZZA and CZA catalyst: CZZA: (ꢀ) CH3OH yield, (ꢁ) CO
yield; CZA: (ꢂ) CH3OH yield. (ꢀ) CO yield: test conditions: 3.0 MPa, 230 ◦C,
H2/CO2 = 3, W/F =10 g cat h/mol.
against water poisoning. Similar trends were also observed when
the concentration of water was increased. It should be noted that
the suppressive effect is most remarkably for CZA and lowest for
CZZA.
3.2.4. Stability
An important aspect of the catalyst is stability during the pro-
cess of reaction. As shown in Fig. 5, CZZA catalyst keep a constant
methanol synthesis activity and CO formation activity over almost
100 h, whereas CZA showed gradual decrease in methanol synthesis
activity from 8.1% at 1 h to 4.4% at 100 h while keeping CO forma-
tion almost constant. The marked effects of the ZrO2 to CZA catalyst
will be discussed, later. This suggested that water produced would
account for the poorer performance of Al2O3-promoted catalyst in
CO2 feedstock compared with ZrO2-Al2O3 promoted system, which
is in good agreement with the results obtained from the influence
of water as shown in Fig. 4.
The XRD data in Fig. 1 suggest that the size of crystal of Cu after
H2 reduction is smaller for CZA but become larger after 100 h reac-
tion compared to CZZA. The peak attributed to CuO was observed at
36.4◦ after reaction, suggesting part Cu was again oxidized during
the reaction. A small peak attributed to tetragonal ZrO2 appeared
at 30.9◦ in the patter for CZA and CZZA after stability test (Fig. 4(3))
[31]. The crystallite size of Cu for CZZA after 100 h was 11.4 nm and
significantly smaller than that for CZA.
Fig. 6. Effect of W/F: test conditions: 3.0 MPa, 230 ◦C, H2/CO2 = 5.
Fig. 3 also shows the XPS results of the used catalyst CZZA
and CZA after stability test. It was noted that the main Cu 2p3/2
peak of used CZA shifted toward higher binding energies compared
with used CZZA, which is attributed to Cu2+ in a CuO environment
and indicated that CZA is easy oxidized in the reaction leading to
larger crystallite size [19]. This result is good agreement with that
obtained by XRD and catalytic test.
shown in Fig. 8). While the initial product is only CO, and
the methanol formation start after CO formation for CZA and
CZZ.
During CO2 hydrogenation, equi-molar amount of water is
inevitably produced (CO2 + 3H2 → CH3OH + H2O, CO2 + H2 → CO +
H2O). It is well known that water is strong oxidant at high tem-
perature for metals such as copper (Cu + H2O → CuO + H2), Co for
Fischer–Tropsch reaction. Therefore, the methanol synthesis cat-
alyst is easy to lose its activity because of the oxidation of active
site. The oxidized (CuOx) can be reduced by hydrogen gas or CO
(CuOx + H2 → Cu + xH2O, CuOx + CO → Cu + CO2). However, when its
rate is not high enough, most of the active site is oxidized to lose
its activity. As shown in Fig. 1, the crystallite size of Cu0 of CZA
obviously increased after 100 h stability test. On the contrary, the
added Zr ion apparently promotes the reduction of catalyst by
hydrogen as indicated from the data in Fig. 2, suggesting that the
rate of reduction of CuO is higher for CZZA than that of CZA, which
are also well interpreted by the results of XRD. The phenomenon
could be the reason of the higher catalytic stability for CO2 conver-
sion to methanol.
3.2.5. Discussion on the promotional effect of ZrO2 on the CO2
hydrogenation
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 Figs. 6 and 7). It is interest-
ing to note that methanol and carbon monoxide can be formed
via simultaneously particularly at short contact time for CZZA
compared with that shown for CZZ and CZA. The results indi-
cate the activation of carbon dioxide to methanol by CZZA
pass through two parallel routes: one is the direct process
from CO2 and another is the one which pass through CO (as

C. Li et al. / Applied Catalysis A: General 469 (2014) 306–311
311
from CO2 over CZZA was assumed to pass through two routes:
one is the direct hydrogenation of CO2 to methanol, another is the
route which pass through CO formation, while the route which
pass through the CO formation is only route on CZA. The promoted
stability of CZZA in CO2 hydrogenation could be attributed to the
promoted reduction of copper oxide, by added zirconium, which
is formed through the oxidation by water vapor, and makes larger
CuO crystal.
Acknowledgments
This work was supported by New Energy and Industrial Tech-
nology Development Organization (NEDO).
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