Synthesis, characterization and activity pattern of Cu-ZnO/ZrO2 catalysts in the hydrogenation of carbon dioxide to methanol

Article abstract of DOI:10.1016/j.jcat.2007.04.003

A novel synthesis route based on reverse co-precipitation under ultrasound irradiation has led to Cu-ZnO/ZrO₂ catalysts (Zn_at/Cu_at, 0-3; ZrO₂, 42-44 wt%) with a remarkable development of total surface area (SA_BET, 120-180 m²/g) and very high dispersion (3-58%) and exposure (MSA 9-63 m²/g) of the active Cu phase. The activity pattern in the hydrogenation of CO₂ to CH₃OH (T_R, 433-533 K; P_R, 1.0-3.0 MPa) was addressed in comparison with a commercial Cu-ZnO/Al₂O₃ methanol synthesis catalyst. Volcano-shaped trends in total and metal surface area signal an optimum zinc loading (Zn/Cu, 0.3-0.7), ensuring higher concentration of active sites and methanol productivity values, whereas the basic relationships among dispersion, reducibility, and TOF indicate a structurally sensitive character of the title reaction and a superior reactivity of poorly dispersed Cu particles. Thermodynamic analysis of the reaction stream revealed that methanol formation proceeds along a parallel path, whereas a stronger water affinity accounts for the poorer performance of the conventional alumina-based catalyst compared with zirconia-based ones.

Full text of DOI:10.1016/j.jcat.2007.04.003

Journal of Catalysis 249 (2007) 185–194
www.elsevier.com/locate/jcat
Synthesis, characterization and activity pattern of Cu–ZnO/ZrO2 catalysts
in the hydrogenation of carbon dioxide to methanol
Francesco Arena a,b,∗, Katia Barbera a, Giuseppe Italiano a, Giuseppe Bonura b, Lorenzo Spadaro b,
Francesco Frusteri b
a
Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Università degli Studi di Messina, Salita Sperone 31 c.p. 29, I-98166 S. Agata (Messina), Italy
b
Istituto CNR-ITAE “Nicola Giordano,” Salita S. Lucia 5, I-98126 S. Lucia (Messina), Italy
Received 5 December 2006; revised 2 April 2007; accepted 2 April 2007
Available online 5 June 2007
Abstract
A novel synthesis route based on reverse co-precipitation under ultrasound irradiation has led to Cu–ZnO/ZrO catalysts (Zn /Cu , 0–3;
at
at
2
2
ZrO , 42–44 wt%) with a remarkable development of total surface area (SA
(MSA 9–63 m /g) of the active Cu phase. The activity pattern in the hydrogenation of CO to CH OH (T , 433–533 K; P , 1.0–3.0 MPa) was
, 120–180 m /g) and very high dispersion (3–58%) and exposure
BET
2
2
R
R
2
3
addressed in comparison with a commercial Cu–ZnO/Al O methanol synthesis catalyst. Volcano-shaped trends in total and metal surface area
2
3
signal an optimum zinc loading (Zn/Cu, 0.3–0.7), ensuring higher concentration of active sites and methanol productivity values, whereas the
basic relationships among dispersion, reducibility, and TOF indicate a structurally sensitive character of the title reaction and a superior reactivity
of poorly dispersed Cu particles. Thermodynamic analysis of the reaction stream revealed that methanol formation proceeds along a parallel path,
whereas a stronger “water affinity” accounts for the poorer performance of the conventional alumina-based catalyst compared with zirconia-based
ones.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Cu–ZnO/ZrO catalysts; CO hydrogenation; CH OH synthesis; Physicochemical characterization; Dispersion; Reducibility; Catalytic pattern;
2
2
3
Structure sensitivity
1. Introduction
novel process technologies based on “CO2 recycling,” are ac-
tually attracting the major scientific and technological interest
[3]. Indeed, the approach of using CO2 as “reagent” for pro-
ducing bulk chemicals like methanol and dimethylether (DME)
[4–8] appears to be particularly attractive, because these are
also potential substitutes for traditional oil-based fuels for au-
tomobiles, which may help reduce air pollution in metropoli-
tan areas [4]. Accomplishing the strategic goal of reducing the
Western nations’ dependence on oil, these energy carriers are
in fact easily accessible from natural gas (NG) via syngas. The
ability to exploit CO2-rich NG resources and the possibility of
decreasing the reforming temperature to reduce syngas genera-
tion costs [1–3] are other important advantages of the methanol
synthesis via CO2 hydrogenation.
Incessant technological progress over the past century aimed
at continuously improving lifestyle quality has produced a pro-
gressive deterioration of natural resources and rise in pollu-
tant levels in the atmosphere, responsible for global warming
(“greenhouse effect”) and, in turn, impressive climate changes
in recent years [1,2]. Pressed by the imperative to avoid fur-
ther irreversible damage to the environment with the resulting
catastrophic consequences for the future of mankind, many
countries worldwide are now pursuing a general reduction of
pollutant and carbon dioxide emissions, the major contributors
to the greenhouse effect [1]. Among the viable strategies to
reduce carbon dioxide emissions, mostly “sequestration” and
Currently, methanol synthesis is run mostly on Cu–ZnO/
Al2O3 catalysts at 493–573 K and a pressure of 5–10 MPa
with a syngas feed containing typically 5% CO–5% CO2 and
H2 [9–11]. However, recent studies have found that methanol
*
Corresponding author. Fax: +39 090391518.
E-mail address: Francesco.Arena@unime.it (F. Arena).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.04.003

186
F. Arena et al. / Journal of Catalysis 249 (2007) 185–194
Table 1
List of the studied catalysts and relative physicochemical properties
a
b
b
b
Code
Chemical composition (%)
Zn /Cu
at
SA
PV
APD
(Å)
MSA
2
D
d
Cu
at
BET
Cu
2
3
(%)
(nm)
(m /g
)
(cm /g
)
(m /g
)
cat
cat
CuO
ZnO
ZrO
2
cat
Cu(12)/ZrO (6)
Cu(11)ZnO(1)/ZrO (6)
Cu(10)ZnO(2)/ZrO (6)
Cu(9)ZnO(3)/ZrO (6)
Cu(8)ZnO(4)/ZrO (6)
Cu(7)ZnO(5)/ZrO (6)
Cu(3)ZnO(9)/ZrO (6)
58.6
51.9
45.0
41.2
36.2
31.6
15.1
0
0
4.6
41.4
43.5
43.2
44.0
44.2
44.4
43.1
43.9
0.0
0.1
0.3
0.4
0.5
0.7
2.7
–
118
128
163
174
162
159
147
103
105
0.33
0.24
0.59
0.56
0.54
0.36
0.35
0.31
0.23
74
91
118
106
110
85
73
121
70
8.7
17.4
63.0
60.8
59.7
59.3
44.7
–
3.3
6.2
32
17
4
4
3
3
2
–
5
2
2
11.8
14.8
19.6
24.0
41.8
56.1
33.1
27.6
29.1
32.5
37.0
57.9
–
2
2
2
2
2
ZnO(12)/ZrO (6)
2
c
Cu(4)ZnO(4)/Al O (6)
35.0
–
0.9
34.4
19.2
2
3
a
b
c
Number in parentheses refer to the atomic/molecular concentration.
From “single-pulse” (363 K) N O titration measurements (see Ref. [60]).
2
Commercial synthesis catalyst containing Al O (31.9 wt%) as carrier.
2
3
can be obtained with rates and carbon utilization factors supe-
rior than those of the traditional route by CO2 hydrogenation
[11,12]. Even if some details of the reaction mechanism remain
controversial, several authors have in fact claimed that under
typical industrial conditions, methanol is formed mostly via
CO2 hydrogenation, with CO serving as the CO2 source and as
a scavenger of oxygen atoms coming from water, which in turn
acts as an inhibitor of the active metal sites [9–11]. Just water
formation would account for the poorer performance of tra-
ditional alumina-supported catalysts in CO2-rich syngas feed-
stocks [13–15] compared with zirconia-based systems [16–22].
Moreover, despite being generally attributed to a synergism of
Cu0–Cu+ sites and the reactivity of ZnO-promoted catalysts
[6,14,23–28], the catalytic role of Cu0 [29,30] and/or Cu–Zn
alloy [31–37] also has been envisaged.
Many research groups have adopted unconventional prepara-
tion routes to improve the activity, selectivity, poison resistance,
or lifetime of Cu–ZnO systems [20–24,26,38–53]. For example,
Coteron and Hayhurst [54] found that Cu–Zn catalysts prepared
by “spark-erosion” feature high methanol selectivity, whereas
we ascribed higher TOF in the CO2 hydrogenation reaction
to a very “intimate” contact of the Cu/ZnO/ZrO2 phases en-
sured by the “combustion” route [55]. Köppel et al. [22] and
Sun and Sermon [56] adopted the sol–gel technique to en-
hance total and metal surface areas, whereas Jingfa et al. [57]
emphasized the superior activity of oxalate-coprecipitated cat-
alysts. Dopant addition [24,46–49] was found to improve ei-
ther dispersion or Cu–Zn(O) contact area [20–22,26,38–45],
whereas alternative active phases (e.g., Ag, Au) seem to ex-
hibit superior selectivity in the CO2 hydrogenation reaction
[23,50–53].
2. Experimental
2.1. Materials
A series of Cu–ZnO/ZrO2 catalysts with different Zn/Cu
atomic ratios (0–3) and a constant zirconia loading (≈44 wt%)
was prepared by the new reverse co-precipitation under ul-
trasound irradiation route [58] as follows. An aqueous solu-
tion (ca. 100 mL) of the Cu(NO3)2·3H2O, Zn(NO3)2·6H2O,
and ZrO(NO3)2·nH2O (Sigma Aldrich) precursors was added
dropwise under vigorous stirring to a 0.1 M NaHCO3 so-
lution (500 mL) kept in an ultrasound bath (Bransonic B-
120E1) operating at 47 kHz and with a power of 30 W (T =
310–320 K) [58]. The pH of the precipitating solution was
maintained in the range of 7.0–7.5 by the continuous addi-
tion of a 0.1 M NaHCO3 solution. After precipitation, the solid
was kept for another 30 min under stirring and ultrasound ir-
radiation, then aged at room temperature for 2 h, filtered, and
washed with hot distilled water. Thereafter the catalysts were
dried at 373 K for 16 h and further calcined in air at 623 K for
4 h. Powdered catalysts were pressed (40 MPa), crushed, and
sieved to the particle size fraction (40–70 mesh) used for both
characterization and testing measurements. A commercial Cu–
ZnO/Al2O3 methanol synthesis catalyst (G66A, Sud Chemie
AG) was used as a reference system. The catalysts’ relative
notation and main physicochemical properties are listed in Ta-
ble 1.
2.2. Physicochemical characterization
Surface area (SABET), pore volume (PV), and the pore size
distribution (PSD) were determined from the nitrogen adsorp-
tion/desorption isotherms at 77 K, using a fully automated
ASAP 2010 (Micromeritics) gas adsorption device. Before
analysis, all the samples were outgassed at 423 K under vac-
uum for 2 h. The isotherms were elaborated according to the
BET method for surface area calculation, with the Horwarth–
Kavazoe and BJH methods used for micropore and mesopore
evaluation, respectively.
Consequently, the present study was undertaken to assess
the efficiency of a new preparation method based on reverse
co-precipitation under ultrasound irradiation in improving the
physicochemical properties of the Cu–ZnO/ZrO2 system. Cat-
alytic tests in a wide range of experimental conditions and the
relative thermodynamic evaluations can provide fundamental
insight into process optimization and structural factors control-
ling the functionality of the Cu–ZnO system in the CO2 hydro-
genation reaction.
Temperature-programmed reduction (TPR) measurements at
273–1073 K were performed in a continuous-flow apparatus

F. Arena et al. / Journal of Catalysis 249 (2007) 185–194
187
using a linear quartz microreactor (4 mm i.d.) fed with a 6%
H2/Ar mixture flowing at 60 stp mL/min and heated at the
rate of 20 K/min. A ca. 15-mg catalyst sample was used, with
H2 consumption monitored by a TCD quantitatively calibrated
with a commercial CuO standard (Carlo Erba) [59].
Metal surface area (MSA) values were obtained with 5%
accuracy by single-pulse (1.0 mL) N2O titration (T = 363 K)
[60]. Before measurements, catalysts were reduced in situ at
573 K in a H2 flow (100 stp mL/min) for 1 h. After reduc-
tion, the samples were “flushed” in the He carrier flow at 583 K
for 15 min, and then cooled to 363 K. For the MSA calcula-
tions, a Cu:N2O = 2:1 chemisorption stoichiometry and a value
of 1.46 × 1019 Cu atom/m2 density were assumed, with the
volume surface average particle size (dCu) obtained from the
conventional formula
104
dCu (nm) =
D (%)
Fig. 1. TG and DSC profiles of the “dried” and “calcined” Cu(9)ZnO(3)/
ZrO (6) catalyst.
2
assuming a spherical shape of Cu particles [29,60].
X-ray diffraction (XRD) analysis of samples in the 2θ range
10–80◦ was performed using a Philips X-Pert diffractometer
operating with Ni β-filtered CuKα radiation at 40 kV and
30 mA. Thermogravimetric (TG-DSC) analysis of the dried and
calcined samples in the range 293–1073 K was performed us-
ing a Netzsch STA 409 C analyzer running at a heating rate of
12 K/min in air atmosphere.
and
ꢃ
SCH OH = CH3OHout
/
.
(bꢀ)
3
prod
Each data set was obtained, with an accuracy of 3%, from an
average of three independent measurements.
2.3. Catalyst testing
3. Results
Catalyst tests in the hydrogenation of CO2 were performed
at 433–533 K and 1.0–3.0 MPa using an Inconel microre-
actor (6 mm i.d.). A CO2–H2–N2 reaction mixture with a
molar ratio of 3/9/1 was fed at the rate of 40 stp mL/min
(GHSV = 4400 N L/(hkgcat)) or 80 stp mL/min (GHSV =
8800 N L/(hkgcat)) on a 0.5-g catalyst sample diluted with
0.5 g of same-sized SiC. The temperature was controlled by
a thermocouple in contact with the catalyst bed. Before each
test, the catalysts were reduced in situ at 573 K for 1 h in
H2 flow (100 stp mL/min) at atmospheric pressure. Then the
reactor was cooled to 433 K and the reaction mixture was ad-
mitted, raising the pressure to 1.0 or 3.0 MPa. The activity was
probed by performing heating–cooling cycles to rule out “hys-
teresis” phenomena. The reaction stream was analyzed by a gas
chromatograph equipped with a two-column analytical system
connected to a flame ionization detector (CH3OH, CH3OCH3)
and thermal conductivity detector (CO, N2, CO2, H2). Conver-
sion values were calculated by both internal standard (a) and
mass-balance (b) methods,
3.1. Physicochemical characterization
The systematic TG-DSC study of representative “dried” and
“calcined” catalyst samples was preliminarily carried out to
determine the minimum calcination temperature for attaining
complete decomposition of precursors. The TG-DSC pattern of
the dried Cu(9)ZnO(3)/ZrO2(6) catalyst (Fig. 1) exhibits con-
siderable (≈25%) weight loss in the range 300–620 K, with an
exothermal peak at ca. 600 K and only negligible weight vari-
ations thereafter up to 1073 K. Indeed, with a ca. 5 wt% loss
below 400 K due to dehydration, TGA confirms the substan-
tial thermal stability of the catalyst sample previously calcined
at 623 K for 4 h. Only a small exothermic peak at ca. 900 K,
analogous to that observed for the dried sample, is associated
with a comparably small weight loss (<1%). Based on this,
we systematically adopted a calcination temperature of 623 K
to ensure complete decomposition of catalyst precursor(s) and
minimize sintering phenomena.
Surface characterization data confirm that the new synthe-
sis route improves the total surface exposure to remarkably
high levels (Table 1), although the ZnO content has a signif-
icant influence on catalyst texture (Fig. 2A). Lower SABET
(120–130 m2/g) values are in fact recorded at low (ꢀ0.1)
and high (>0.7) Zn/Cu ratios, with the maximum value of
175 m2/g found for a Zn/Cu of 0.3–0.4 (Fig. 2A). It is note-
worthy that similar changes in the Cu surface exposure account
for a straight-line increase of MSA with SABET from 9 to
63 m2/gcat (Fig. 2B). Further, both pore volume (Fig. 2A) and
ꢀ
ꢁ
XCO = 1 − (CO2,out/CO2,in)·(N2,in/N2,out
)
(a)
2
and
ꢂ
ꢄ
ꢃ
XCO = CO2,out
/
+CO2,out
,
(b)
2
prod
with selectivity data obtained from standard formulas
SCH OH = CH3OHout/(1 − CO2,out
)
(aꢀ)
3

188
F. Arena et al. / Journal of Catalysis 249 (2007) 185–194
Fig. 2. (A) Effect of the Zn/Cu atomic ratio on the SA
and pore volume of
BET
the Cu–ZnO/ZrO catalysts. (B) Relationship between MSA and SA
of the
BET
2
Cu–ZnO/ZrO catalysts.
2
average pore diameter (Table 1) follow a trend analogous to
that of SABET due to marked changes in the PSD (Fig. 3).
The “unpromoted” Cu(12)/ZrO2(6) system (Fig. 3A) displays a
very “broad” and featureless PSD, accounting for a cumulative
volume of ca. 0.33 cm3/g. In contrast, the catalyst with inter-
mediate Zn/Cu ratio (0.4) is characterized by a prevalence of
70–120 Å mesopores (Fig. 3B), resulting in a much greater cu-
mulative volume (ca. 0.6 cm3/g). Although a pronounced max-
imum is centered at 90 Å (Fig. 3C), the Cu(3)ZnO(9)/ZrO2(6)
sample with the largest (2.7) Zn/Cu ratio has a pore volume
(0.3 cm3/g) comparable with that of low Zn-loaded samples
(Table 1).
The results of XRD measurements performed to obtain
information on the crystalline phases present both on dried
(Fig. 4A) and calcined (Fig. 4B) catalysts are collected in
Fig. 4. Apart from minor differences in the 2θ range of 15–30◦,
the rather featureless diffraction patterns indicate a significant
lack of “long-range” crystalline order in both dried and cal-
cined Cu(9)ZnO(3)/ZrO2(6) samples (Fig. 4A). However, sim-
ilar diffraction patterns signal that all of the calcined samples
(Fig. 4B) with Zn/Cu > 0.1 are characterized by a prevalently
amorphous architecture. Only at low (ꢀ0.1) Zn loading are the
typical reflexes of a crystalline tenorite (JCPDS 5-661) CuO
phase evident [59]. Applying Sherrer’s equation to the ꢁ002ꢂ
line produces mean CuO particle sizes of 12 nm for Cu(12)/
ZrO2(6) and 10 nm for Cu(11)ZnO(1)/ZrO2(6). Then the sig-
nificantly larger dCu value quoted for the former system from
N2O uptake measurements (Table 1) should indicate a major
resistance to sintering of the promoted Cu(11)ZnO(1)/ZrO2(6)
sample.
Fig. 3. Pore size distribution curves of representative Cu–ZnO/ZrO catalysts.
2
lar rise in the onset temperature of reduction with the Zn/Cu
ratio (Table 2). A shoulder on the left side of the maximum
is particularly evident at low Zn content [Cu(12)/ZrO2(6) and
Cu(11)ZnO(1)/ZrO2(6)], whereas the peak is sharper and much
more symmetrical for all of the other systems. The amount
of hydrogen consumption at 273–573 K is always close to
(although somewhat greater than) the stoichiometric amount
for CuO reduction (H2/Cu = 1.05–1.22). Moreover, a base-
line drift at 573–1073 K with a poorly resolved TM2 maximum
(830–890 K), the shape of which depends on the zinc loading,
TPR measurements were carried out to highlight the reduc-
tion pattern of the various catalysts and settle a proper activation
(reduction) protocol for testing. The reduction profiles of the
studied catalysts are shown in Fig. 5, and the onset (To,red
)
and maximum (TMi) temperatures and the values of hydrogen
consumption are summarized in Table 2. All of the systems
display reduction profiles characterized by a main peak with
a TM1 maximum between 486 and 511 K, well below that of
the standard bulk CuO (ca. 570 K), accounting for a regu-

F. Arena et al. / Journal of Catalysis 249 (2007) 185–194
189
Fig. 5. TPR profiles of the Cu–ZnO/ZrO and reference catalysts.
2
is associated with an ongoing reduction of the ZnO promoter
and/or the decomposition of a residual carbonate phase (Fig. 1).
The TPR profile of the reference catalyst is similar to that
of Cu–ZnO/ZrO2 catalysts, with a main peak centered at 521 K
and markedly skewed on the low-T side (To,red = 422 K), ac-
counting for a stoichiometric H2/Cu ratio (Table 2). Also at
T > 573 K, the trend in H2 consumption appears similar, even
though a poorly resolved maximum occurs at temperatures
above those for Cu–ZnO/ZrO2 catalysts (Table 2).
Fig. 4. XRD patterns of the “dried” and “calcined” Cu(9)ZnO(3)/ZrO (6) sam-
2
ples (A) and of the calcined catalysts with different Zn/Cu ratio (B).
Table 2
TPR data of Cu–ZnO/ZrO catalysts. Onset temperature of reduction (T
),
a
2
o,red
temperature of peak maxima (T ) and extent of hydrogen consumption
Mi
Catalyst
T
o,red
T
M1
T
M2
mmol H /g
H /Cu
2
cat
2tot
(K)
(K)
(K)
Cu(12)/ZrO (6)
Cu(11)ZnO(1)/ZrO (6) 390
Cu(10)ZnO(2)/ZrO (6) 395
Cu(9)ZnO(3)/ZrO (6)
Cu(8)ZnO(4)/ZrO (6)
Cu(7)ZnO(5)/ZrO (6)
Cu(3)ZnO(9)/ZrO (6)
ZnO(12)/ZrO (6)
2
388
501
511
486
488
497
501
506
–
831
870
831
845
826
845
862
891
962
–
9.2
7.8
8.4
8.2
6.7
6.1
2.7
1.4
6.1
12.6
1.22
1.15
1.06
1.13
1.05
1.07
1.11
–
2
3.2. Catalytic activity
2
2
404
428
431
448
–
2
Activity data of representative catalysts in the CO2 hy-
drogenation reaction (TR = 473–513 K; GHSV = 4400 N L/
(hkgcat); PR = 1.0; 3.0 MPa) are collected in Table 3 in terms
of CO2 conversion (XCO ) and CH3OH selectivity (SCH OH).
At lower pressure, all the systems feature a similar catalytic
pattern characterized by XCO values rising from 4 to 14%,
2
2
2
2
3
Cu(4)ZnO(4)/Al O (6) 422
CuO “bulk”
521
568
0.98
1.00
2
3
474
2
a
In the T range: 273–573 K.
counterbalanced by a drop in SCH OH from 63 to 17%, whereas
3
Table 3
Conversion–selectivity data (X
–S
CH OH
3
) of Cu–ZnO/ZrO and reference catalysts in the CO hydrogenation reaction at different temperature and pressure
2 2
CO
2
(GHSV, 4400 N L/(hkg ))
cat
Catalyst
P , 1.0 MPa
R
P , 3.0 MPa
R
T , 473 K
R
T , 493 K
R
T , 513 K
R
T , 473 K
R
T , 493 K
R
T , 513 K
R
Cu(12)/ZrO (6)
Cu(10)ZnO(2)/ZrO (6)
Cu(3)ZnO(9)/ZrO (6)
Cu(4)ZnO(4)/Al O (6)
3.9–63.0
5.8–55.2
4.1–59.5
5.5–53.8
7.1–41.1
10.0–35.5
8.6–34.6
10.2–30.9
11.4–25.2
14.1–18.0
13.3–16.9
14.2–18.6
7.2–70.2
6.2–66.9
6.5–70.9
6.4–72.0
12.6–57.4
10.8–49.9
12.6–55.2
11.5–56.0
17.6–48.8
16.5–45.0
17.5–48.4
15.9–48.4
2
2
2
2
3

190
F. Arena et al. / Journal of Catalysis 249 (2007) 185–194
at 3.0 MPa, a systematic increase of XCO from 6 to 18% im-
comparison, the productivity of the reference system is particu-
larly depressed at the highest GHSV (Fig. 7B).
2
plies a much smaller decrease in SCH OH from 73 to 45%. Over-
3
all, under such experimental conditions, a very slight affect of
the composition on reactivity of the Cu–ZnO system is found.
However, comparing the selectivity–conversion pattern of the
Cu(10)ZnO(2)/ZrO2(6) and reference catalyst at higher GHSV
(8800 N L/(hkgcat)) demonstrates the poorer performance of
the latter system at both 1.0 and 3.0 MPa, mainly in terms of
methanol selectivity (Fig. 6). Through a systematic decrease
Short-term stability tests on the reference and Cu(9)ZnO(3)/
ZrO2(6) catalysts at 473 K and 1.0 MPa (GHSV = 4400 N L/
(hkgcat)) were carried out to address the occurrence of poten-
tial deactivation phenomena. Both catalysts exhibited constant
conversion–selectivity values during 18 h of time on stream, in-
dicating substantial stability of the Cu–ZnO system irrespective
of the carrier.
with conversion, at 1.0 MPa an increase in XCO from 0.5 to
2
2.5% (reference catalyst) and to 3.5% [Cu(10)ZnO(2)/ZrO2(6)]
4. Discussion
causes a steep decay in SCH OH, from 100% to ca. 65%. At
3
3.0 MPa, a much more pronounced increase in XCO (from
ca. 2.5% to 7–7.5%) accounts for “smoother” decreases in the
4.1. Aims and strategy of the design of the new synthesis route
2
SCH OH from 100 to 50% on the reference catalyst and 75% on
Cu(10)ZnO(2)/ZrO2(6). Accordingly, productivity data (STY,
3
Addressing the effects of the preparation method (e.g., com-
bustion, coprecipitation), we previously ascertained a marked
influence of the textural properties on the CO2 hydrogenation
pattern of Cu–ZnO/ZrO2 catalysts [55]. However, the low de-
velopment of total and/or metal surface areas has prompted the
search for more effective synthesis routes to improve the tex-
ture and reactivity of the Cu–ZnO/ZrO2 system [55]. Due to
the different precipitation kinetics of each cation, the conven-
tional coprecipitation technique leads to mixtures of more or
less small “monophase” particles, with limited dispersion of the
active phase in the final catalyst [22,40,55–57,61]. To overcome
this drawback, we attempted to accomplish simultaneous pre-
cipitation of the three cations (Cu2+, Zn2+, and ZrO2+) through
a slow dropwise addition of the precursor solution to a large
volume of the precipitating agent (i.e., reverse coprecipitation),
keeping the pH between 7.0 and 7.5 to allow for coprecipi-
tation of precursors mostly as (hydroxi)carbonates. Moreover,
evaluating the influence of the ultrasound field on the textural
properties of the representative Cu(9)ZnO(3)/ZrO2(6) system,
we preliminarily found improvements in both SABET (≈10%)
and MSA (≈20%) due to irradiation during co-precipitation
[55,58]. Based on such evidence, we systematically adopted the
reverse co-precipitation under ultrasound irradiation technique
for catalyst preparation.
kgCH OH/(kgcat h)) as a function of the Zn/Cu ratio (Fig. 7) dis-
3
play an optimum Zn/Cu ratio of 0.3–0.5, ensuring higher STY
values at both low (Fig. 7A) and higher (Fig. 7B) GHSV’s. In
Fig. 6. Selectivity to methanol vs X
(GHSV, 8800 N L/(hkg )) for the
cat
CO
2
Cu(10)ZnO(2)/ZrO (6) and reference catalysts at 1.0 and 3.0 MPa in the range
2
473–513 K.
Fig. 7. STY of Cu–ZnO/ZrO catalysts vs Zn/Cu ratio at 453 and 473 K and 1.0 MPa. (A) GHSV, 4400 N L/(hkg ); (B) GHSV, 8800 N L/(hkg ).
2
cat
cat

F. Arena et al. / Journal of Catalysis 249 (2007) 185–194
191
4.2. Structural properties
can be assigned to the more difficult reduction of larger crys-
talline CuO particles [59,61]. In contrast, a narrower and more
symmetric peak shape of the catalysts with Zn/Cu > 0.1 de-
notes the simultaneous reduction of smaller CuO particles with
a much more uniform size distribution [59,61]. Such data in-
fer a fundamental role of the ZnO as a structural promoter of
the active Cu phase, which in turn affects the reducibility of
the system. In fact, an exponential increase in metal disper-
sion with the Zn/Cu ratio (Fig. 8A) parallels a straightforward
Surface area and, mostly, metal dispersion and MSA values
(Table 1) were much larger than those reported so far for bulky
Cu-based catalysts [22,40,55–57,61]. This finding provides the
best indication of the successful strategy of design of the re-
verse co-precipitation under ultrasound irradiation route [58].
Indeed, the improvement in textural properties depends on the
characteristics of the preparation method that allows precipi-
tation of very small oxide particles, also ensuring very good
mixing of the various precursors in the first stage of solid for-
mation [58]. Together, the evident CO2 release during aging,
the weight loss (Fig. 1) corresponding to ca. 70% decomposi-
tion of carbonates, and the peculiar diffraction pattern of the
dried Cu(9)ZnO(3)/ZrO2(6) sample (Fig. 4A) provide proba-
tory experimental evidence supporting the formation of a homo-
geneous hydroxycarbonate phase in the co-precipitation stage.
Its subsequent decomposition by the “soft” calcination treat-
ment (at 623 K) retains a disordered architecture with no sig-
nificant long-range crystalline order (Fig. 4B). Meanwhile, the
very fine texture of the oxide particles, the sintering of which
would be hindered by the release of great amounts of CO2
from (hydroxy)carbonate decomposition, accounts for a very
significant surface exposure. Then the stabilization of a preva-
lently amorphous structure for Zn/Cu > 0.1 (Fig. 4) matches
the greater development of total and metal surface area (Fig. 2).
Whereas at low ZnO concentration (Zn/Cu < 0.3), segrega-
tion of the crystalline CuO (tenorite) phase (Fig. 4) matches
the lower SABET, MSA, and D values of the Cu(12)/ZrO2(6)
and Cu(11)ZnO(1)/ZrO2(6) samples (Table 1). Therefore, the
shape of the reduction peak (Fig. 5) also should be related to
the morphology of the CuO phase, which is in turn affected
by the ZnO loading. In fact, a broad and “stepped” TPR peak
could signal the reduction of different-sized CuO particles in
the Cu(12)/ZrO2(6) and Cu(11)ZnO(1)/ZrO2(6) samples. Con-
sidering surface (Table 1) and structural (Fig. 4) characteriza-
tion data, and the higher TM1 value recorded for the bulk CuO
system (Table 2), the component at higher temperature (Fig. 5)
increase in To,red
with ZnO loading (Fig. 8B). This suggests
that a weaker Cu–O bond strength enables easier generation of
“nucleation” metal centers (e.g., To,red) on large CuO particles
characterized by only slight (if any) interaction with the ZnO
promoter, whereas the higher dispersion and stronger promoter
interaction of the CuO phase cause a shift of To,red to higher
temperatures at greater ZnO loadings. Nevertheless, a faster
reduction of smaller CuO particles in the bulk would explain
the lower TM1 values of the systems characterized by larger
Zn/Cu ratios [61]. Overall, higher TM1 and TM2 values signal
a more difficult reduction of both active phase and promoter on
the reference system, likely due to the stronger interaction with
the Al2O3 carrier [61]. Spanning a dispersion range (3–58%)
much larger than that attained so far for massive Cu-based cat-
alysts [22,40,55–57,61], the novel synthesis route indicates a
very effective CuO–ZnO/ZrO2 interaction affecting the texture,
morphology, and reactivity of Cu–ZnO/ZrO2 catalysts [61].
4.3. Structure–activity relationships
Although different physicochemical properties point to an
optimum Zn/Cu ratio ensuring the maximum development of
SABET and MSA (Fig. 2), catalyst productivity is not a direct
function of such parameters [31,61,62]. Actually, variations in
MSA of about 1 order of magnitude (9–63 m2/g) reflect in a
less than threefold rise in STY between minimum and maxi-
mum (Fig. 7). Then it can be argued that the different reactiv-
ities of active centers, a multicenter reaction path, and/or ther-
modynamic factors combine to “smooth” the potential of the
Fig. 8. (A) Effect of the Zn/Cu ratio on metal dispersion; (B) relationship between onset temperature of reduction (Table 2) and ZnO loading.

192
F. Arena et al. / Journal of Catalysis 249 (2007) 185–194
Fig. 9. (A) Methanol selectivity vs conversion data of the various catalysts in the range 453–513 K at 1.0 and 3.0 MPa. (B) STY of the various catalysts in the range
453-513 K. (- - -) Represent equilibrium data at 1.0 and 3.0 MPa.
Fig. 10. Influence of the metal dispersion (A) and average Cu particle diameter (B) on the turnover frequency of CO conversion and CH OH formation (T , 473 K;
R
2
3
P , 1.0 MPa; GHSV, 8800 N L/(hkg )).
R
cat
MSA exposure. This is confirmed by the fact that the catalytic
pattern of all Cu–ZnO/ZrO2 catalysts can be described by gen-
eral relationships between selectivity and conversion (Fig. 9A)
and between STY and temperature (Fig. 9B). Due to the marked
volume decrease of the synthesis reaction, these correlations are
strongly dependent on pressure. Overall, a much smaller de-
crease in methanol selectivity coupled with higher conversions
provide substantially larger STY values at 3.0 MPa (Fig. 9B),
with the maximum values (0.12–0.14 kgCH OH/(kgcat h)) equal
to or greater than those attained so far und3er even more favor-
able reaction conditions [22,29,35,41].
To highlight the slight influence of MSA on STY, it is help-
ful to evaluate the influence of Cu dispersion on the reactivity of
the metal phase. The turnover frequency (TOF, s−1) of CH3OH
formation and CO2 conversion (TR = 473 K; PR = 1.0 MPa;
GHSV = 8800 N L/(hkgcat)) as functions of Cu dispersion
(Fig. 10A) and particle size (Fig. 10B) are shown in Fig. 10.
Matching with the TOF values of methanol reforming reac-
tions under similar experimental conditions [61], such values
depict analogously decreasing curves (Fig. 10A), accounting
for the linear decrease in reactivity of metal sites with dCu
(Fig. 10B). Considering the close relationship between parti-
cle size and particle morphology, the latter trend may indicate
a different functionality of Cu sites located at different posi-
tions of the crystal structure, roughly mirroring the density of
planar sites with relative particle sizes [23,32,33,61]. However,
according to the direct relationship between To,red and ZnO
loading (Fig. 8B), a modification in the electronic properties
of smaller Cu particles could also produce a stronger bonding
of oxygen-containing intermediates (e.g., formate, water, hy-
droxyls), resulting in low TOF values [23,32,33,61,63]. The
possibility that high ZnO loadings can depress the stabiliza-
tion of Cu+ sites in the octahedral cavities of the zirconia car-
rier [6], perhaps due to a stronger CuO–ZnO interaction, must

F. Arena et al. / Journal of Catalysis 249 (2007) 185–194
193
Table 4
List of reactions considered for thermodynamic evaluations of catalytic activity data
a
Reaction
Stoichiometry
K
p
453 K
473 K
493 K
513 K
533 K
1
2
3
CO + 3H ꢁ CH OH + H O
1.81E−4
2.72E−3
1.50E+1
9.60E−5
4.26E−3
4.44E+1
5.33E−5
6.44E−3
1.21E+2
3.08E−5
9.41E−3
3.05E+2
1.84E−5
1.33E−2
7.25E+2
2
2
3
2
CO + H ꢁ CO + H O
2
2
2
CH OH ꢁ CO + 2H
3
2
a
Equilibrium pressure constant referred to pressure expressed in MPa.
be considered. This could enhance the negative effect of dis-
persion (>10%) on TOF in the case of a “multisite” reaction
path involving Cu0 and Cu+ centers for hydrogen and COx ad-
sorption/activation [6,14,23–28,40,55]. In the case of a reaction
path involving the adsorption/activation of CO2 on zirconia car-
rier [64], progressively increased coverage of the ZrO2 surface
by ZnO could occur to depress the TOF at high Zn/Cu ratios.
4.4. Thermodynamic analysis
A thermodynamic analysis of the reaction stream [65] was
carried out to address the kinetics of the various functionali-
ties and the influence of thermodynamics thereon. In particular,
CO2 hydrogenation can be described by a reaction network in-
volving (1) the synthesis of methanol, (2) the reverse water–gas
shift (RWGS), and (3) the methanol decomposition equilibri-
ums:
CO2 + 3H2 ꢁ CH3OH + H2O,
CO2 + H2 ꢁ CO + H2O,
(1)
(2)
and
CH3OH ꢁ CO + 2H2.
(3)
Here (3) represents a linear combination of (1) and (2), and
thus the thermodynamic evaluation depends on the analysis of
(1) and (2), although the (3) can shed light on the CO/CH3OH
distribution [e.g., K3 = {[(PCO) × (PH )2]/(PCH OH)}]. Sub-
Fig. 11. Thermodynamic analysis of the reaction stream. Effect of temperature
on the β values (e.g., β = K /K ) of the reactions listed in Table 4 at
x
exp
eq
2
3
1.0 (A) and 3.0 MPa (B). For reference, the average experimental values of
water vapor and hydrogen pressure are also shown.
stituting the Px values (from conversion–selectivity data) into
the expression of the equilibrium constants, we obtained exper-
imental Kexp values for the above reactions. Then the ratio (β)
between Kexp and the corresponding value of the equilibrium
constant Keq (Table 4) represents the distance from equilib-
rium as a measure of the relative kinetics of forward and reverse
functionalities. The values of the various βx at various tempera-
tures at 1.0 (A) and 3.0 MPa (B) are shown in Fig. 11. Such data
confirm that at the highest GHSV, all reactions below 473 K
proceed under a prevailing kinetic regime. Moreover, although
the trend of increasing β1 and β2 indicates a parallel reaction
network, a steeper increase in β1 leads to a steady decrease in
β3 at all pressures; that is, a β3 value larger than that recorded at
higher pressure (Fig. 11B) indicates that the Cu–ZnO/ZrO2 sys-
tem has a specific functionality for the hydrogenation of CO2
(reaction (1)) rather than that of CO (reaction (3)). But although
β2 values are affected only slightly (if at all) by a rise in pres-
sure (according to the negligible volume variation of the RWGS
reaction), the lower β1 values at 3.0 MPa indicate a decrease
in the relative rate of methanol formation. This finding proves
that the rate of the synthesis reaction does not increase with the
potential of the gas phase, due to the lower dependence of re-
action kinetics on pressure. This is in agreement with the fact
that the RDS is the surface decomposition of the formate in-
termediate and/or the abstraction of oxygen derived from its
decomposition [23,33,63]. Accordingly, the RDS would pro-
ceed at a lower rate at higher surface coverage (higher PR) and
on low-index sites of small Cu particles [23,32,33,63]. More-
over, such data also demonstrate the “negative” role of water
[see reactions (1) and (2)] on the methanol synthesis rate. Fa-
voring water desorption, a temperature increase and/or pressure
decrease mainly enhance(s) the rate of CH3OH formation, de-
spite the rise in PH O (Fig. 11). Then a much stronger chemical
2
affinity of alumina carrier for water, enabling more extensive
wetting of the catalyst surface, should account for the poorer
performance of the reference Cu(4)Zn(4)/Al2O3(6) catalyst in
the title reaction (Figs. 6 and 7).

194
F. Arena et al. / Journal of Catalysis 249 (2007) 185–194
5. Conclusions
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