Methanol synthesis over Cu/CeO2-ZrO2catalysts: The key role of multiple active components

Article abstract of DOI:10.1039/d0cy01762k

High surface area ceria-zirconia synthesized by a glycothermal approach was used as a support for copper nanoparticles. Cu-CeO2/ZrO2 catalysts containing 5-25 wt% copper demonstrate high carbon dioxide-to-methanol conversion rates (120-180 gMeOH kgcat-1 h-1) at 260 °C and 50 bar. The sample containing 5 wt% copper in the form of small nanoparticles (≤5 nm) demonstrates the highest activity normalized per mass of copper, while higher copper loading results in copper segregation and correspondingly lower activity. We attribute the high activity to a unique synergetic effect between the active components, copper, ceria and zirconia, where activation of hydrogen and carbon dioxide and subsequent methanol synthesis take place. The redox properties of the ceria-zirconia support and its ability to form oxygen vacancy sites play a crucial role in carbon dioxide activation. This journal is

Full text of DOI:10.1039/d0cy01762k

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Methanol synthesis over Cu/CeO –ZrO catalysts:
2
2
the key role of multiple active components†
Cite this: DOI: 10.1039/d0cy01762k
a
a
ab
and Jeroen A. van Bokhoven *ab
Maxim Zabilskiy, * Kaibo Ma, Arik Beck
High surface area ceria–zirconia synthesized by a glycothermal approach was used as a support for copper
nanoparticles. Cu–CeO /ZrO catalysts containing 5–25 wt% copper demonstrate high carbon dioxide-to-
methanol conversion rates (120–180 gMeOH kg
2
2
cat−
1
−1
h ) at 260 °C and 50 bar. The sample containing 5 wt%
copper in the form of small nanoparticles (≤5 nm) demonstrates the highest activity normalized per mass
of copper, while higher copper loading results in copper segregation and correspondingly lower activity.
We attribute the high activity to a unique synergetic effect between the active components, copper, ceria
and zirconia, where activation of hydrogen and carbon dioxide and subsequent methanol synthesis take
place. The redox properties of the ceria–zirconia support and its ability to form oxygen vacancy sites play a
crucial role in carbon dioxide activation.
Received 8th September 2020,
Accepted 2nd November 2020
DOI: 10.1039/d0cy01762k
rsc.li/catalysis
(
200–300 °C) to convert syngas (a mixture of carbon dioxide,
Introduction
carbon monoxide and hydrogen) over a ternary Cu/ZnO/Al O
2
3
1
8
An increasing amount of evidence during the previous
decades indicates that excessive carbon dioxide emission
causes detrimental effects on the global ecology system,
which requires urgent action. Significant efforts have been
focused on the mitigation of the atmospheric carbon dioxide
concentration by the advancement of highly efficient carbon
capture and separation technologies and the subsequent
processes of carbon dioxide conversion into valuable fuel and
catalyst to methanol. However, the catalyst is susceptible to
water poisoning produced during the reaction, complicating
the industrialization of Cu/ZnO-based catalysts for the direct
carbon dioxide conversion to methanol. Furthermore,
sintering and losing the copper–zinc oxide interface lead to a
dramatic decrease in the hydrogenation activity. Therefore,
new catalysts need to be explored by properly tuning the
activity and stability of such catalysts.
One of the strategies is to precisely engineer the metal–oxide
interface, which is assumed to be the active site for the
hydrogenation of carbon dioxide. Numerous reports indicate
that the synergetic interaction between copper and the adjacent
zinc oxide component in copper–zinc-based systems accounts
for the enhanced activity in the methanol synthesis reaction
even in the absence of a unified interpretation.
et al. reported CeO
in which the Cu/CeOx interface showed significant
improvement in the methanol synthesis rates compared to the
industrial Cu–ZnO-based systems. Wang et al. revealed the
important role played by the ZnO–ZrO interface and its ability
to form oxygen vacancies, which according to several studies are
claimed to be responsible for carbon dioxide activation.
1
–3
4
–7
chemicals or precursors thereof. Among these technologies,
methanol synthesis via catalytic carbon dioxide hydrogenation
has been studied intensively during the past several
8
–13
decades.
Recent developments in hydrogen production
from renewable energy sources and the necessity to store the
produced hydrogen in the form of dense chemicals further
stimulate the industrialization of methanol synthesis from
carbon dioxide and hydrogen. In this context, methanol
serves as a liquid hydrogen carrier and is an important
1
2,19–21
Graciani
(110) catalysts,
22
x
/Cu (111) and Cu/CeO
x
/TiO
2
1
4
chemical precursor for various industrial processes.
23
Methanol can be used as an energy carrier in direct methanol
fuel cells, as a precursor for chemicals and as a gasoline fuel
2
1
5–17
additive.
24–26
The current industrial process of methanol synthesis
requires elevated pressure (50–100 bar) and temperature
Furthermore, the ternary interaction between copper and ZnO/
ZrO2 enhances the amount of surface formate and methoxy
species, which were suggested to act as a crucial intermediate in
a
Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute, 5232
the methanol synthesis reaction and boost the catalyst
Villigen, Switzerland. E-mail: maxim.zabilskiy@psi.ch
27
performance.
Using a surface organometallic synthesis
b
Institute for Chemistry and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1,
IV
approach, Lam and Noh et al. proposed that isolated Ti and
Zr Lewis acid sites stabilize surface intermediates during
8093 Zürich, Switzerland. E-mail: jeroen.vanbokhoven@chem.ethz.ch
IV
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy01762k
methanol synthesis at the interface between copper
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nanoparticles and the support and facilitate carbon dioxide
facilitate nanoparticle precipitation. The dispersion was
centrifuged at 9000 rpm for 10 min and further dispersed in
deionized water. This procedure was repeated two times.
Finally, a gel containing ceria–zirconia nanoparticles was
obtained. The gel was dried overnight at 110 °C and calcined
at 400 °C in air. Pure ceria or zirconia supports were also
prepared by using only Ce(NO ) ·6H O or ZrO(NO ) ·6H O in
28–30
hydrogenation.
These reports highlight the advantages of
engineering the interface between different oxides in order to
create a highly active catalytic site.
Besides zinc oxide, zirconia is an often used oxide catalyst
3
1–33
component. Zirconia has weak hydrophobic properties,
which help to overcome the catalyst inhibition by water
produced during the reaction. Ceria has well-known redox
3
3
2
3 2
2
the first step.
Cu–CeO /ZrO
3
4
properties and easily forms oxygen vacancies.
The
2
2
catalysts containing different copper
aforementioned characteristics can be beneficial during
methanol synthesis from carbon dioxide. Despite the very
amounts were prepared using the deposition–precipitation
method. 500 mg of CeO /ZrO support were dispersed in 10
2
2
2
2
promising results obtained over Cu/CeO
2
model catalysts,
ml of deionized water. The appropriate amount of
where this material outperformed a Cu/ZnO system, the
combination of copper and ceria for catalytic methanol
synthesis from carbon dioxide is rarely investigated. A few
studies attempted to transfer the results obtained during
surface science studies to real catalysts. In most of these
studies, however, the observed activity and methanol
selectivity were significantly lower than those of benchmark
Cu(NO ) ·3H O (99% purity, Sigma-Aldrich) was added to the
3
2
2
abovementioned dispersion to reach the desired nominal
copper loading (5–25 wt%). Then 5 ml of a solution
containing 250 mg of sodium carbonate were injected
dropwise to ensure uniform copper hydroxycarbonate
precipitation on the ceria–zirconia support. The resulting
suspension was centrifuged, dried overnight and finally
calcined at 400 °C for 4 hours. The synthesized catalysts are
3
5–37
Cu/ZnO.
In this work, we combine both ceria and zirconia phases
and study this mixed oxide as a support for highly dispersed
copper nanoparticles for the catalytic carbon dioxide
hydrogenation to methanol. Common methods for the
preparation of ceria–zirconia nanoparticles, such as co-
precipitation, citrate combustion and nitrate decomposition,
2 2
labeled as X-Cu–CeO /ZrO (where X represents the net
copper loading in wt%).
Catalyst characterization
The analysis of specific surface area, total pore volume and
pore size distribution was performed at −196 °C using a 3Flex
Surface Characterization Analyzer (Micromeritics). Prior to
the measurement, all samples were activated according to the
following pretreatment protocol. First, the materials were
evacuated at 110 °C for 1 h, followed by 3 h dwelling at 300
3
8,39
usually yield a low BET specific surface area,
resulting in
low active phase dispersion and consequently reduced
interface between metal nanoparticles and support. To
overcome this limitation, we adapted the glycothermal
synthesis approach, which was initially developed for the
preparation of pure ceria nanoparticles with a high BET
−
1
°C (heating ramp
2 °C min ) using a Micromeritics
2
−1 40
surface area (up to 200 m g ). Cu–CeO
2
/ZrO
2
catalysts
SmartPrep degasser. The Brunauer–Emmett–Teller (BET)
method was applied to calculate the specific surface area of
the samples. The pore size distribution was derived from the
desorption branch of the isotherms employing the Barrett–
Joyner–Halenda (BJH) method. The total pore volume was
estimated at a relative pressure of 0.99.
prepared according to the abovementioned protocol show
superior catalytic activity in methanol synthesis.
Experimental
Catalyst preparation
X-ray diffraction measurements were performed at room
temperature on a Bruker D8 Advance AXS diffractometer
using Cu Kα1 radiation with λ = 0.15406 nm. Materials were
scanned in the 2θ range between 20° and 65° with 0.0215°
increments and 1 s acquisition time at each increment.
The redox properties of the synthesized materials were
studied by temperature-programmed reduction (TPR)
measurements using a gas mixture containing 10 vol%
hydrogen in argon. The experiments were performed at
ambient pressure using a Micromeritics AutoChem HP 2950
device. Typically, ∼100 mg of the Cu–CeO /ZrO sample were
CeO and CeO /ZrO supports were synthesized according to
2
2
2
a previously reported method for the preparation of CeO
mesoporous spheres. In a typical synthesis, 1.51 g of
2
4
0
Ce(NO ) ·6H O (99% purity, Sigma-Aldrich) and 0.79 g of
3
3
2
ZrO(NO ) ·6H O (99% purity, Sigma-Aldrich) were dissolved
3
2
2
in 2.6 ml of deionized water. Then, 61.4 ml of HOCH
2 2
CH OH
(
99.0% purity, Aldrich) and 2.4 ml of CH CH COOH (99%
3
2
purity, Sigma-Aldrich) were added under constant stirring.
The molar ratio of the prepared uniform solution was 0.6
Ce(NO ) : 0.4 ZrO(NO ) : 5.5 CH CH COOH : 190 HOCH CH -
3
3
3 2
3
2
2
2
2
2
OH. The homogeneous solution was put into a Teflon-lined
stainless steel autoclave after 30 min of stirring and aged in
an oven. The temperature was first kept at 230 °C for 60 min,
then decreased to 165 °C in 30 min and finally aged at 165
in situ pretreated at 300 °C in 10 vol% oxygen in helium
before doing the H -TPR test. During the TPR analysis, the
samples were heated from 20 to 300 °C with a heating ramp
2
−
1
of 5 °C min . The amount of consumed hydrogen was
monitored using a TCD detector. A liquid nitrogen–isopropyl
alcohol cold trap was used in order to condense the water
produced during the reduction processes. Following the H2-
°C for 90 min. After the nucleation process was finished, the
autoclave was cooled to room temperature and 30 ml of
acetone were added to the resulting suspension in order to
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TPR study, the samples were cooled in helium (6.0, 50 ml
min ) to −120 °C by using a CryoCooler (Micromeritics) and
an oxygen pulse-chemisorption experiment was performed at
(fraction 50–100 μm) diluted with 150 mg of silicon carbide
(Sigma-Aldrich) was loaded between two quartz wool beads
and positioned inside a stainless-steel tube reactor (6 mm
outer diameter and 4 mm inner diameter). The reactor was
−
1
−
120 °C in order to determine copper dispersion and particle
size. During this experiment, oxygen oxidized the partially
reduced ceria–zirconia support as well. Thus, in our
calculation we took into account this effect. As shown in
mounted inside a single-zone furnace (Carbolite). The
temperature was controlled with a Eurotherm 3508 controller
using a K-type thermocouple positioned inside the catalyst
bed. Before each catalytic run, the catalyst was pretreated
under ambient pressure in a flow of argon (5.0 quality, 50 ml
Table S1,† the amount of consumed hydrogen during H -TPR
2
experiments significantly exceeds the theoretical hydrogen
uptake required for complete reduction of copper oxide
present in the Cu–CeO /ZrO samples. As such, the amount
−
1
−1
min ) at 260 °C (heating rate 5 °C min ) for 2 h, followed
−
1
by a reduction in hydrogen (50 ml min ) at 260 °C and
ambient pressure for 1 hour. Finally, the total pressure was
increased to 50 bar by using a back pressure regulator
(Bronkhorst, EL-PRESS). The catalytic test of carbon dioxide
hydrogenation to methanol was started by switching the
reaction flow from hydrogen to a feed gas mixture containing
24 vol% carbon dioxide, 72 vol% hydrogen and 4 vol% argon
(all gases used are 5.0 qualities, Messer) at 260 °C and 50 bar
total pressure. Initially, the gas mixture flow rate was set to
2
2
of hydrogen consumed for support co-reduction was
calculated by subtracting the theoretical hydrogen uptake
from the amount of hydrogen consumed during H -TPR
2
experiments. The amount of oxygen required for the
complete re-oxidation of the ceria–zirconia support was
calculated taking into account the stoichiometry factor − 2
(ν(O2) = ν(H2)/2). This volume of oxygen was later subtracted
from the total amount of oxygen consumed during the pulse-
chemisorption experiment.
−
1
50 mL min
(controlled by a Bronkhorst mass flow
X-ray absorption fine structure (XAFS) measurements were
performed at the SuperXAS beamline, Swiss Light Source of
the Paul Scherrer Institute (Villigen, Switzerland). As-
synthesized samples were mixed with micronized boron
nitride powder (Sigma-Aldrich) and pressed in the form of
homogeneous pellets with a total absorption thickness of
around 2 above the Cu K-edge. XAFS data were collected in a
conventional transmission geometry using a fast, gridded ion
controller). After 24 hours on stream, the catalytic properties
of the material were further investigated under different
−
1
WHSV by varying the feed flow rate from 20 to 80 ml min .
After any flow rate changes, the catalyst was equilibrated for
1 hour. Then, catalytic data were acquired for 1 hour. The
analysis of outlet gases was performed by using a 3000 Micro
GC gas analyzer (Inficon) equipped with a 10 m Molsieve
column, an 8 m PlotU column and TCD detectors.
chamber and
a
quick scanning channel-cut Si(111)
4
1
monochromator (oscillation frequency of 1 Hz). A copper
foil standard was collected simultaneously for exact energy
calibration. In order to reduce radiation damage of the
samples, the beam was defocused to the size of 2500 microns
Results and discussion
Characterization of CeO /ZrO
2 2
Fig. 1a shows the diffraction patterns of ceria and ceria–
zirconia, both typical of the FCC fluorite structure.
Diffraction peaks at 29°, 33.6° and 48.4° in the pattern of
ceria–zirconia correspond to the formation of solid solution
and were shifted to higher angles compared to those of the
ceria phase. The calculated lattice cell parameter for the
ceria–zirconia sample equals 5.302(8) Å, which according to
Vegard's law corresponds to a solid solution where ∼41%
cerium atoms in ceria are substituted by zirconium. This
value is in good agreement with the theoretical ratio of
cerium and zirconium nitrate precursors (60 : 40) in the stock
solution before glycothermal aging, indicating that all
zirconia was successfully incorporated into the structure. The
average crystallite size of the synthesized ceria–zirconia
calculated from the (111) reflection (2θ = 29°) using the
Scherrer equation was 5 nm.
×
500 microns. Initial analysis and energy calibration were
4
2
performed using ProXAS v.2.34 software. To determine the
local environment of copper in the investigated samples (type
and average number of neighbors, bond distance and Debye–
Waller factor), quantitative EXAFS data analysis was
performed using the IFEFFIT program package Demeter. To
fit the experimental EXAFS spectra, we assumed a model of
the local environment of copper similar to that of the copper
4
3
4
4
2 2
oxide tenorite reference. The best fit for the 5-Cu–CeO /ZrO
sample was obtained assuming only two scattering paths:
oxygen in the first and copper in the second coordination
sphere. Fitting of the FT samples containing 10–25 wt%
copper requires including additional oxygen and copper
scattering paths.
Scanning
transmission
electron
microscopy
was
performed using an aberration-corrected JEOL JEM-ARM300F
transmission electron microscope (at 300 kV) at the Scientific
Center for Optical and Electron Microscopy (ScopeM, ETH
Zurich).
Catalytic methanol synthesis. The catalytic carbon dioxide
hydrogenation to methanol was investigated using a fixed-
bed stainless-steel reactor operating at up to 100 bar
pressure. In a typical experiment, 50 mg of the catalyst
Synthesis of ceria–zirconia using the glycothermal
2
−1
approach yielded a BET specific surface area of 150 m g .
Fig. S1† shows nitrogen adsorption–desorption isotherms
and pore size distributions for the synthesized ceria–zirconia
and pure ceria materials. Unlike pure ceria with two
hysteresis loops with p/p equal to 0.65 and 0.9, the ceria–
0
zirconia sample has only one at p/p0 of 0.65. Ceria
nanoparticles usually aggregate into nanospheres (60–80 nm
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in diameter), yielding two hysteresis loops corresponding to
(
i) voids between single nanoparticles and (ii) voids between
4
0
nanosphere agglomerates; therefore this material exhibits
bimodal pore size distribution (inset in Fig. S1†). In turn,
ceria–zirconia solids do not form nanospherical agglomerates
(
Fig. S2†), producing only one maximum in pore size
distribution centered at ∼5 nm due to voids between
nanoparticles.
Characterization of Cu–CeO
Fig. 1b shows XRD patterns of the as-prepared Cu–CeO2/
ZrO . The crystalline size calculated from the (111) reflection
2 2
/ZrO
2
of ceria–zirconia using the Scherrer equation was equal to 5.0
nm for all samples. For 5-Cu–CeO /ZrO and 10-Cu–CeO2/
2
2
2
ZrO containing 5 and 10 wt% copper, we did not observe
any reflection corresponding to a crystalline copper phase
and the CeO /ZrO support retained its structure after copper
2
2
2 2
deposition and calcination. 15-Cu–CeO /ZrO and 25-Cu–
CeO /ZrO showed reflections at 32.5°, 35.5°, 38.7° and 48.8°
2
2
typical of copper oxide. The copper oxide particle sizes for
these two samples were 21 and 25 nm (based on the Scherrer
equation).
Fig. 2 shows scanning transmission electron microscopy
(
STEM) images combined with energy-dispersive X-ray
spectroscopy (EDX) mapping of the lowest (5 wt%) and
highest (25 wt%) copper loaded samples. The STEM image
(
Fig. 2a) and corresponding EDX mapping (Fig. 2b) of as-
Fig. 1 XRD patterns of ceria and ceria–zirconia prepared using a
prepared 5-Cu–CeO /ZrO reveal a homogeneous elemental
distribution of copper, cerium and zirconium. Subnanometer
elemental mapping of the area containing the region
2
2
glycothermal approach (a) and XRD patterns of prepared Cu–CeO
ZrO catalysts and CuO ICSD reference (b).
2
/
2
Fig. 2 Scanning transmission electron microscopy and energy-dispersive X-ray elemental mapping: STEM micrographs of the fresh 5-Cu–CeO
ZrO catalyst (a–f) as well as the same material after pretreatment in hydrogen and catalytic carbon dioxide hydrogenation at 50 bar and 260 °C
for 24 hours (g–i). Micrographs (j and k) show heterogeneous copper distribution in the 25-Cu–CeO /ZrO fresh sample.
2
/
2
2
2
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enriched with copper (Fig. 2d–f) suggests the formation of
highly dispersed copper species which anchored to the CeO2/
and the peak right after the absorption edge (at ca. 9014 eV)
are less pronounced for the 5-Cu–CeO /ZrO and 10-Cu–
2
2
ZrO
2
crystallites. STEM–EDX mapping for spent 5-Cu–CeO
2
/
CeO
2
/ZrO
2
samples. This indicates the presence of a
4
5
ZrO2 was further acquired. Similar to the fresh solid, the
catalyst after catalytic carbon dioxide hydrogenation exhibits
disordered amorphous copper oxide phase, in agreement
with diffraction data, which showed the absence of any
crystalline copper oxide phase.
a
homogeneous distribution of cerium and zirconium
(
Fig. 2h). At the same time, the copper distribution besides
Fig. 4 and Table 1 show the Fourier transforms (FTs) of
the k -weighted Cu K-edge EXAFS and corresponding fitting
2
highly dispersed areas shows also the regions of denser
copper signal (Fig. 2i). This suggests the partial sintering of
the initially highly dispersed copper phase to small
nanoparticles of about 5 nm in size.
parameters. All the materials possess two distinct peaks in
the range of 1–3 Å. The peak around 1.5 Å is due to the Cu−O
scattering, and the other peak around 2.6 Å to Cu−Cu
scattering. These peaks resemble those of the tenorite
reference, confirming the formation of copper oxide
nanoparticles on the ceria–zirconia support. For the samples
with copper loading of more than 10 wt%, characteristic
Elemental mapping of the catalyst with the highest copper
loading
(25-Cu–CeO /ZrO )
shows
a
heterogeneous
2
2
distribution of copper already for fresh material before
catalysis. Segregated copper oxide nanoparticles with a size
of up to 50 nm were observed in addition to mixed copper
ceria–zirconia particles (Fig. 2k).
−
1
2
wiggles at 7–10 Å in the k -weighted Cu K-edge EXAFS
(Fig. 5) can be observed which result in distinct peaks of Cu–
Cu scattering between 4 and 6 Å in the Fourier transforms
(Fig. 4). This confirms the presence of well-ordered
Cu K-edge X-ray absorption near edge spectroscopy
XANES) and extended X-ray absorption fine structure
EXAFS) spectra of as-synthesized materials were collected in
(
(
nanoparticles of copper oxide in the 15-Cu–CeO
2 2
/ZrO and
transmission mode under ambient conditions at the
25-Cu–CeO /ZrO samples.
2
2
4
1
SuperXAS beamline (SLS, Switzerland).
The Cu K-edge
The first coordination shell in all cases consists of oxygen
atoms at a distance of 1.93–1.95 Å, like that in a typical
copper(II) oxide, tenorite. The coordination number of the
first Cu–O shell for the 5-Cu–CeO /ZrO and 10-Cu–CeO2/
XANES spectra of the catalyst are plotted along with that of
the copper oxide standard (Fig. 3). The energy positions of all
features in the spectra of Cu–CeO /ZrO coincide with those
2
2
2
2
in that of copper oxide, indicating the predominant presence
of the oxidic Cu state in these catalysts. The characteristic
2
ZrO samples was 3.4 and 3.6, respectively, while for the
other two samples this value was close to 4 as for a bulk
copper oxide. The contribution from the Cu scatters at 2.9 Å
2
+
shoulder located at 8985 eV (corresponding to 1s–4p
2
+
transition with shakedown contributions in tetragonal Cu )
2 2
gradually increases from 0.7 neighbors for 5-Cu–CeO /ZrO
2
Fig. 4 The Fourier transforms of k -weighted Cu K-edge EXAFS of
Fig. 3 Cu K-edge XANES spectra of as-prepared 5-Cu–CeO
black), 10-Cu–CeO /ZrO (red), 15-Cu–CeO /ZrO (blue), 25-Cu–
CeO /ZrO (green) and copper(II) oxide reference (black dashed line).
2
/ZrO
2
5-Cu–CeO
2
/ZrO
2
(black), 10-Cu–CeO
2 2 2
/ZrO (purple), 15-Cu–CeO /
(
2
2
2
2
ZrO (blue), 25-Cu–CeO
2
2
/ZrO (green) and CuO standard (gold). The
2
2
2
red dotted lines show the best theoretical fits (see Table 1 for details).
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Table 1 Structural and statistical parameters derived from the EXAFS
ZrO2 requires including additional oxygen at a distance of
spectra analysis of Cu–CeO
of copper. A best fit is obtained with the amplitude reduction factor S
2 2
/ZrO samples containing different amounts
2.76 Å and copper at a distance of 3.08 Å. These observations
2
0
=
indicate the presence of a more disordered or highly
dispersed copper oxide phase for the samples with copper
content below 10 wt%. Higher copper loading in 15-Cu–
0.885 determined by fitting the spectrum of the copper foil reference
a
b
σ2c [Å2]
d
Path
CN
/ZrO
R [Å]
o
ΔE [eV]/R-factor
5-Cu–CeO
2
2
2 2 2 2
CeO /ZrO and 25-Cu–CeO /ZrO samples results in the
Cu–O
Cu–Cu
3.4(2)
0.7(2)
1.93(1)
2.90(1)
0.004(1)
0.005(1)
−2 ± 1/0.0080
formation of a more developed tenorite phase, as can be seen
from a monotonous and significant increase of the overall
coordination number for Cu–O and Cu–Cu scattering paths.
Fig. 6 and Table S1† show the results of temperature-
programmed reduction (TPR) experiments conducted in 10
vol% hydrogen. A first reduction peak with maximum at ∼80
10-Cu–CeO
2
/ZrO
2
Cu–O
Cu–O
Cu–Cu
3.6(2)
0.4(4)
1.1(2)
1.95(1)
2.76(3)
2.90(1)
0.004(1)
0.004(1)
0.005(1)
−0.3 ± 0.6/0.0086
15-Cu–CeO
2
/ZrO
2
Cu–O
Cu–O
Cu–Cu
Cu–Cu
3.9(1)
1.95(1)
2.76(3)
2.90(1)
3.08(3)
0.004(1)
0.004(1)
0.005(1)
0.005(1)
−0.6 ± 0.4/0.0031
−0.8 ± 0.4/0.0011
°
C was observed only for 5-Cu–CeO /ZrO and 10-Cu–CeO /
2 2 2
0.9(3)
2.2(4)
1.6(3)
ZrO samples. At higher copper loadings, no low-temperature
2
reduction peak was revealed. The low-temperature reduction
of copper(II) is a well-known phenomenon for copper–ceria
materials. Usually, this peak is attributed to the reduction of
25-Cu–CeO
2
/ZrO
2
Cu–O
Cu–O
Cu–Cu
Cu–Cu
3.9(1)
1.95(1)
2.76(3)
2.90(1)
3.08(3)
0.004(1)
0.004(1)
0.005(1)
0.005(1)
1.0(3)
2.7(4)
2.4(3)
finely dispersed copper species strongly interacting with the
40,46
ceria support.
The next reduction peaks in the range
a
b
c
between 100 and 175 °C are usually associated with the
reduction of weak magnetic associates including several
copper(II) ions, reduction of small copper oxide clusters and
co-reduction of ceria support. For unambiguous assigning
of these peaks to the abovementioned reduction processes, a
thorough in situ XAS/TPR study is required, which is out of
the scope of this manuscript. The last peak presented in TPR
Coordination number.
Bond distance. Debye–Waller factor.
d
o
Potential shift (ΔE ) and fit agreement factor (R-factor).
4
7
2 2 2 2
profiles of 15-Cu–CeO /ZrO and 25-Cu–CeO /ZrO samples
with a maximum in the range between 175 and 200 °C is very
close to the reduction of bulk copper oxide. Previous studies
2
Fig. 5 The k -weighted Cu K-edge EXAFS of 5-Cu–CeO
2
/ZrO
(blue), 25-Cu–CeO
(green) and copper oxide standard (gold). The red dotted lines
show the best theoretical fits (see Table 1 for details). The blue shaded
2
(black),
1
0-Cu–CeO
2
/ZrO
2
(purple), 15-Cu–CeO
2
/ZrO
2
2
/
ZrO
2
−
1
2
area indicates the characteristic wiggles at 7–10 Å in the k -weighted
Cu K-edge EXAFS resulting in distinct peaks of the higher shell
structure between 4 and 6 Å in the Fourier transforms of 15-Cu–CeO
ZrO , 25-Cu–CeO /ZrO and copper oxide standard (Fig. 4).
2
/
2
2
2
Fig. 6 TPR profiles of 5-Cu–CeO
2
/ZrO
2
(black), 10-Cu–CeO
2 2
/ZrO
(
red), 15-Cu–CeO /ZrO (blue) and 25-Cu–CeO
2
2
2
/ZrO (green) during
2
sample to 2.7 for 25-Cu–CeO /ZrO material. Fitting of the FT
−1
2
2
reduction in 10 vol% H /Ar mixture (ramp: 10 °C min , ambient
2
samples containing 10–25 wt% copper unlike 5-Cu–CeO
2
/
pressure).
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attribute it to the reduction of segregated bulk copper oxide
4
0,48,49
phase.
Table S1† shows that consumption of hydrogen for all the
studied samples exceeds the theoretical values required for
complete reduction of all present copper oxide to metallic
copper that confirms co-reduction of the ceria–zirconia
support. Electronic interactions between copper and ceria
oxide phases promote the weakness of the metal–oxygen
bonds due to a “synergetic effect” and facilitate reduction of
not only copper oxide species but also the ceria support. The
5
-Cu–CeO /ZrO material stands out among other samples
2 2
and the amount of hydrogen consumed for ceria co-
reduction is almost 50% higher compared to all other
3
−1
3
−1
samples (15.3 cm
g
vs. 10.8 cm g ). We attribute this
phenomenon to the presence of highly dispersed copper
species in this sample (as confirmed by STEM and XAS
analysis), which results in an extremely developed copper–
ceria–zirconia interface and consequently a higher extent of
support reduction.
2 2
The reduction profiles of the 15-Cu–CeO /ZrO and 25-Cu–
CeO /ZrO samples look significantly different compared to
2
2
the samples with lower copper content. Fig. 6 clearly shows
the shift of the reduction maxima to higher temperature with
increasing copper content in the samples. Instead of a low-
temperature reduction peak presented in 5-Cu–CeO /ZrO2
2
and 10-Cu–CeO
2
/ZrO
2
, in the H
2 2
-TPR profiles of 15-Cu–CeO /
ZrO and 25-Cu–CeO
2
2
/ZrO we observed the appearance of
2
another peak in the high-temperature region attributed to
the reduction of bulk copper oxide. This indicates a gradual
increase of copper oxide particle size from finely dispersed
copper species strongly interacting with the ceria–zirconia
support (5-Cu–CeO
in 25-Cu–CeO /ZrO
2
/ZrO
2
) to crystalline bulk phase presented
2
2
.
Fig. 7 Methanol selectivity as a function of carbon dioxide conversion
Catalytic carbon dioxide hydrogenation
during catalytic methanol synthesis from carbon dioxide over different
2 2
Cu–CeO /ZrO materials at 260 °C and 50 bar (a). Methanol activity
Fig. 7, S3 and S4† show the catalytic carbon dioxide
normalized per catalysts mass and per mass of copper (b).
hydrogenation activity and selectivity data obtained during
methanol synthesis reaction over the investigated Cu–CeO /
2
ZrO materials at 260 °C and 50 bar. Methanol productivity
2
rates normalized per mass of the catalyst are independent of
2 2
Cu–CeO /ZrO samples measured at 260 °C and 50 bar total
copper loading (Fig. S3†). The absolute values of methanol
synthesis rate (120–180 gMeOH kgcat h ) are among the
pressure. Carbon monoxide was the only by-product. The
catalysts with different copper loading exhibit similar
selectivity-conversion trends, suggesting that carbon dioxide
hydrogenation occurs at the same active sites in each
catalyst.
In contrast, the activity normalized per copper loading
shows a clear trend, indicating that the higher the copper
content in the sample, the lower the activity normalized per
−
1
−1
highest compared to other catalyst formulations which do
9
not contain zinc oxide in their composition. An industrial
copper–zinc–alumina catalyst containing 63.5 wt% copper
under the catalytic conditions of this study (260 °C and 50
bar) over-performs the Cu–CeO /ZrO catalysts in terms of
2 2
carbon dioxide conversion (Table 2); however, the activity
normalized per mass of copper is only 25% higher compared
copper loading (Fig. 7b). The 5-Cu–CeO
2
/ZrO
2
catalyst
−
1
−1
to 5-Cu–CeO
2
/ZrO
2
(2.02 vs. 1.52 mmol gCu min ). The
possesses two to six times higher normalized activity
compared to the other catalysts. According to numerous
ceria–zirconia support does not produce methanol, and
catalysts prepared by deposition of copper on ceria and on
zirconia are less active (Table 2).
Fig. 7 shows methanol selectivity as a function of carbon
dioxide conversion during catalytic methanol synthesis over
3
0,50,51
studies,
the interface between copper and zirconia
phases plays a crucial role in catalytic carbon dioxide
hydrogenation to methanol. It was suggested that formate
species localized at this interface and stabilized by zirconium
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Table 2 Results of catalytic carbon dioxide hydrogenation to methanol over Cu–CeO
2
/ZrO
2
materials conducted in a fixed-bed reactor at 260 °C and
−1
50 bar absolute pressure (WHSV = 60 L gcat per hour)
Cu−
1
−1
Catalyst
Methanol selectivity, %
Carbon dioxide conversion, %
Normalized methanol productivity, mmol g
min
5
1
1
2
5
5
-Cu–CeO
2
/ZrO
2
42
41
39
46
25
32
41
1.7
2.0
2.1
1.2
0.5
1.52
0.85
0.60
0.24
0.25
1.03
2.02
0-Cu–CeO
5-Cu–CeO
5-Cu–CeO
2
2
2
/ZrO
/ZrO
/ZrO
2
2
2
-Cu–CeO
-Cu–ZrO
2
2
1.5
14.6
a,b
3
Cu/ZnO/Al
2
O
a
b
cat−1
Commercial methanol synthesis catalyst purchased from Alfa Aesar. Catalyst was investigated at WHSV = 120 L g
negative effect of water poisoning at higher conversion level.
per hour due to
5
0
surface sites are a key-observable reaction intermediate,
of surface copper atoms and shortening of perimeter
interfaces between copper and ceria–zirconia (schematically
shown in Fig. 8). This leads to a drastic decrease of methanol
productivity rate since the majority of deposited copper
became inactive and does not participate in the catalytic
cycle.
Finally, using ceria–zirconia material as a copper support
results in higher methanol productivity compared to using
pure ceria and zirconia oxides. We attribute this boost in
catalytic activity to the well-known redox properties of this
support and the ability to easily form oxygen vacancy sites
while copper nanoparticles activate and split hydrogen
required to hydrogenate formate species, completing the
catalytic cycle. Larmier et al. further revealed that the
most favorable adsorption mode of carbon dioxide is at the
interface between copper and zirconia. Here, the two oxygen
atoms of carbon dioxide interact with Zr Lewis acidic
centers and the carbon atom is bound to the surface of
copper nanoparticles. Furthermore, they found that
localization of formate species on zirconia in close vicinity to
the metallic copper nanoparticles facilitates hydrogen
transfer from copper and transformation of formate species
to methanol. Our results are consistent with the
abovementioned conclusions and the superior activity of
3
0
50
4
+
2
under reductive atmospheres, as was confirmed by H -TPR
experiments. Numerous theoretical and experimental reports
emphasize the exceptional role of oxygen vacancies in
methanol synthesis over the various catalytic systems and
ascribe such defects as sites needed for carbon dioxide
5
-Cu–CeO
2
/ZrO
2
sample illustrates the importance of
multiple components, well-dispersed copper species and
developed interface between copper and the supporting
oxides. A long-term stability test conducted over this material
Fig. S5†) reveals good catalytic stability and absence of
catalyst deactivation during the time frame of the
experiment.
1
0,24,52,53
activation.
These point-defects in the immediate
vicinity to copper clusters play a crucial role in the first step
of carbon dioxide adsorption and its activation prior to
hydrogen addition and transformation to formate species.
(
5
2 2
-Cu–CeO /ZrO possesses the smallest and well-dispersed
Conclusions
copper species. Higher copper loading results in copper
phase segregation which does not contribute to catalytic
By using
a glycothermal synthesis approach, we have
2 2
activity (Fig. S4†). 25-Cu–CeO /ZrO containing a five times
prepared ceria–zirconia nanoparticles having a high BET
2
−1
higher copper loading is less active in catalytic methanol
synthesis. Increase of copper particle size, as confirmed by
XRD, XAS and TEM investigations, results in lower number
specific surface area – 150 m g . Due to this, we were able
to stabilize copper nanoparticles supported on ceria–zirconia
and prevent them from further sintering during catalytic
methanol synthesis. We have shown that synthesized
materials can effectively convert carbon dioxide to methanol
at 260 °C and 50 bar. Methanol productivity values (120–180
−
1
−1
gMeOH kgcat h ) are among the highest compared to those
of other catalyst formulations which do not contain zinc
2 2
oxide in their composition. The 5-Cu–CeO /ZrO sample
possesses the highest normalized methanol productivity rate
in comparison to other Cu–CeO /ZrO materials with higher
2
2
copper content. Various physical–chemical characterization
techniques point out that with increasing copper loading the
growth and segregation of copper nanoparticles take places,
which results in the reduction of the interface between
Fig.
8 Importance of well-dispersed copper species and well-
metallic copper and the CeO /ZrO support. We attribute the
developed ceria–zirconia surface during catalytic carbon dioxide
2
2
hydrogenation to methanol.
high catalytic activity of Cu–CeO /ZrO materials to the
2 2
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enhanced redox properties of the ceria–zirconia support and
its ability to form oxygen vacancies under reductive
atmospheres. We believe that these sites located in proximity
to the metal–oxide interface participate in carbon dioxide
activation and hydrogenation to formate species and further
to methanol. Normalized methanol productivity for 5-Cu–
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