Study of Cu modified Zr and Al mixed oxides in ethanol conversion: The structure-catalytic activity relationship

Article abstract of DOI:10.1016/j.cattod.2021.02.015

Here, we study the influence of the Cu modified (Zr + Ce)O₂-Al₂O₃ systems composition and synthesis conditions on their catalytic properties in the ethanol conversion. First, we obtained varios ratios mixed Al-Zr supports at different synthesis temperatures using a sol-gel method. Then we modified the surface of the oxides by Cu, reduced in hydrogen flow. All obtained systems demonstrated а high alcohol conversion and selectivity to acetaldehyde. The surface area (S_BET), the pore volume, and the pore distribution were measured by the nitrogen adsorption method. The structure of the samples have been investigated by XRD and XAS-spectroscopy. A correlation between the synthesis temperature and contents of mixed oxide support and textural properties were observed. The results show that Al-Zr mixed oxide support structure plays a crucial role in forming a Cu active site for ethanol dehydrogenation to acetaldehyde.

Full text of DOI:10.1016/j.cattod.2021.02.015

Catalysis Today xxx (xxxx) xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Study of Cu modified Zr and Al mixed oxides in ethanol conversion: The
structure-catalytic activity relationship
A.I. Zhukova*, S.G. Chuklina, S.A. Maslenkova
Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya Street, Moscow, 117198, Russian Federation
A R T I C L E I N F O
A B S T R A C T
Keywords:
Here, we study the influence of the Cu modified (Zr + Ce)O2-Al2O3 systems composition and synthesis conditions
on their catalytic properties in the ethanol conversion. First, we obtained varios ratios mixed Al-Zr supports at
different synthesis temperatures using a sol-gel method. Then we modified the surface of the oxides by Cu,
reduced in hydrogen flow. All obtained systems demonstrated а high alcohol conversion and selectivity to
acetaldehyde. The surface area (SBET), the pore volume, and the pore distribution were measured by the nitrogen
adsorption method. The structure of the samples have been investigated by XRD and XAS-spectroscopy. A cor-
relation between the synthesis temperature and contents of mixed oxide support and textural properties were
observed. The results show that Al-Zr mixed oxide support structure plays a crucial role in forming a Cu active
site for ethanol dehydrogenation to acetaldehyde.
Alumina-zirconia mixed oxides
Ethanol dehydrogenation
Acetaldehyde
Copper catalysts
Selectivity
1. Introduction
in relation to acetaldehyde 50 % at 55 % of ethanol conversion at 300 ◦C
[13].
Ethanol derived from edible biomass can be selectively converted to
chemicals such as diethyl ether, acetaldehyde, etc. Bioethanol is a most
promising precursor for distributed hydrogen production from renew-
able biomass. The possibility of ethanol production from non-food
biomass feedstocks makes it a good alternative for the production of
hydrogen fuel and chemically valuable products with low by-product
carbon emissions [1–3].
There are many methods of purposeful change of properties of
copper-based catalysts, including variation of composition and structure
of the substrate, amount of copper applied, temperature treatments and
modifications [10,12,14]. In the works of Sato [6,9] on the example of
Cu/SiO2, Cu/ZrO2, Cu/SiO2-ZrO2 systems the dependence of selectivity
of alcohol transformation into aldehyde and ethyl acetate on the
chemical composition of support material [9] and on the phase
composition of zirconium oxide [6] was shown, while the selectivity of a
particular product was not achieved.
For this reason, bioethanol conversion catalysts have been inten-
sively developed in recent years. According to the literature, oxide
catalysts, the surface of which is promoted by metals, are the most
promising [1,4]. However, these catalysts have some disadvantages,
including high cost of precious metals used as the active phase, possible
decontamination of catalysts, low selectivity. It is known that the choice
of active metal and its quantity has a strong influence on the selectivity
and activity of ethanol conversion process [5,6].
Thus, the actual task is to select the composition of the supports and
its processing methods to obtain copper-containing catalytic systems
that allow the selective transformation of ethanol into valuable prod-
ucts, while maintaining a high conversion rate. It was found that the
Al2O3-ZrO2 systems exhibit better catalytic activity than catalysts pre-
pared based on Al2O3 or ZrO2 only, and such mixed systems are of
considerable interest in catalysis because of their successful use as
support materials [15–17].
Copper-based catalysts are actively studied in the process of alcohol
dehydrogenation [7–10]. However, to achieve optimal efficiency, the
existing copper systems still need to be improved [11]. In [12], catalytic
copper-containing systems based on hydrotalcites allow a dehydroge-
nation reaction with 60 % alcohol conversion to aldehyde with 99 %
selectivity, but after 18 h of operation the catalysts are decontaminated.
Such catalytic systems as CuO/Cr2O3/Al2O3 give the highest selectivity
In this study, Cu/(Zr + Ce)O2-Al2O3 systems are tested in the reac-
tion of gas-phase dehydrogenation of ethanol to obtain effective and
highly selective catalysts. According to the results of the work, the
dependence between the composition and structure of oxide supports
and the state of the active phase of copper, as well as the influence of
* Corresponding author.
E-mail addresses: pylinina-ai@rudn.ru (A.I. Zhukova), sofyaogan@gmail.com (S.G. Chuklina).
https://doi.org/10.1016/j.cattod.2021.02.015
Received 10 January 2020; Received in revised form 18 December 2020; Accepted 17 February 2021
Available online 6 March 2021
0920-5861/© 2021 Elsevier B.V. All rights reserved.
Please cite this article as: A.I. Zhukova, Catalysis Today, https://doi.org/10.1016/j.cattod.2021.02.015

A.I. Zhukova et al.
Catalysis Today xxx (xxxx) xxx
these parameters on the catalytic characteristics of dehydrogenation of
ethanol was investigated.
monochromatization of the X-ray beam Si(111) channel-cut mono-
tchromator was used, which provided an energy resolution ΔE/E ~
2*10ꢀ 4. Dumping of higher energy harmonics was achieved by distor-
tion of the monochromator geometry. Energy calibration was performed
by measuring XAS spectra of Cu foil. All experimental data were
collected in fluorescent mode, intensities of incidedent X-ray beam was
measured by ionization chambers filled with N2 and fluorescence in-
tensity was measured by Aptek SDD detector. In our measurements of
XAS spectra three ionization chambers were used providing simulta-
neous measurements of XAS spectra for sample and reference. In such
way the energy calibration of the monochromator could be checked and
corrected. Energy step in XANES region (from 40 eV before the edge to
80 eV above the edge) was 0.6 eV and in EXAFS region (from 80 eV to
800 eV above the edge) constant step 0.05 Å-1 in photoelectron wave
number were employed. At every energy point in XANES region signal
was integrated for 1 s, in EXAFS region integration time was set to 1 s at
the beginning of the region and increased to the 4 s at the end of the
spectra. For all the samples at least 3 experimental spectra were
collected and mearged using Demeter Athena software.
2. Experimental
2.1. Preparation of the nano-composite oxides
Samples of powders (Zr + Ce)O2/Al2O3 with 0, 5, 50, 75 and 100
mol.% of aluminum oxide (CZ, A5CZ, A50CZ, A75CZ, A100) were ob-
tained by the sol-gel method of 1 M solutions of ZrOCl2, Al(NO3)3, Ce
(NO3)3. Co-precipitation was carried out with aqueous ammonia solu-
tion at room temperature for 120 min. Since aluminum hydroxide is
partially soluble in high ammonia excesses, precipitation was carried
out at pH between 9.2 and 9.4. Gel-like precipitates (hydrogels) were
dried at 180 ◦C.
Further, precursor powders were modified according to the scheme
presented in Fig. 1. To estimate the impact of the support structure on
the activity of Cu containing oxide catalytic systems two series of sam-
ples were obtained: dried at T = 180 ◦C series samples denoted as Cu-CZ-
180, Cu-A5CZ-180, Cu-A50CZ-180, Cu-A75CZ-180, Cu-A100ꢀ 180 and
samples prepared using the same methodology preheated at T = 950 ◦C
in the air for 2 h denoted as Cu-CZ-950, Cu-A5CZ-950, Cu-A50CZ-950,
Cu-A75CZ-950, Cu-A100ꢀ 950. The synthesis method was developed
in the Institute of Metallurgy and Material Science RAS [18,19].
The introduction of copper on a surface of the mixed oxide was
carried out by impregnation with an aqueous Cu(NO3)2*2.5H2O solu-
tion. The amount of copper applied was 5 wt.%. The powders were dried
at room temperature overnight and reduced in hydrogen flow at 400 ◦C
for 1 h directly in the catalytic reactor.
The EXAFS (χexp(k)) data were analyzed using ARTEMIS program (a
part of IFEFFIT software package). Following standard procedures for
pre-edge subtraction and background removal, the structural parame-
ters - interatomic distances (Ri), coordination numbers (Ni), and
Debye–Waller factors (
σ
2i) - were determined via the non-linear fit of
theoretical spectra to the experimental ones with the equation
n
∑
ꢀ 2R
i
NiFi(k)
ꢀ 2σ2i k2
(k) = S02
e
e
sin(2kRi + φi(k) )
(1)
λ(k)
χ
R2i k
i=1
Theoretical spectra were simulated using photoelectron mean free
path length λ(k), amplitude Fi(k), and phase shift φ (k) parameters
i
calculated ab initio using the program FEFF6 [21]. For the FEFF6
calculation the crystal structure of metallic Cu, Cu2O and CuO was used.
S2o parameter was fixed at value 0.75. Fitting of the experimental EXAFS
spectra were performed in R space in a range from 1 to 3 Å using So2
parameter fixed at value 0.75.
2.2. Samples characterization
The phase composition of the prepared systems was determined by
X-ray diffraction. X-ray diffractograms were recorded at room temper-
ature on the XRD-6000 (Shimadzu) diffractometer with CuKα =1.54 Å
radiation in steps of 2θ = 0.02◦/s in the range of angles 2 from 10 to 70◦.
The surface area, the total pore volume and the pore size distribution
were determined from multipoint BET nitrogen adsorption isotherms at
77 K using the Micromeritics Tristar 3020 surface analyzer in the P/P0
relative pressure range from 0.0–1.0
2.3. Catalyst testing
Activity and selectivity measurements of the ethanol reaction, as
well as earlier in our studies [22,23], were carried out in a flow quartz
fixed-bed reactor, using 0.03 g of catalyst. Freshly prepared and reduced
catalysts were activated by heating at 400 ◦C in the inert gas flow for 1 h,
then cooled down to 200 ◦C before each catalytic experiment. The
analysis of the reaction mixture was carried out with the help of a gas
chromatograph Chromatech Crystal 5000 (FID, carrier gas velocity 20
mL/min) with the use of a packed column Porapak Q (temperature 135
◦C, length 1.5 m). Sampling was carried out by a thermo-controlled
dosing valve in the temperature range of 220ꢀ 360 ◦C every 20 ◦C.
Alcohol vapor in the helium flow (He 25 mL/min) was fed from the
barboters to the quartz reactor with the catalyst powder, which was
distributed in a thin layer on a porous filter. The catalytic experiment
was repeated over several days to check its stability. The ethanol con-
version, products selectivity and yield are defined by equations:
Conversion of ethanol defined as usual:
The Cu K-edge XANES and EXAFS spectra of six samples of impactites
as well as reference samples have been recorded at the Structural Ma-
terials Science beamline [20] using the equipment of Kurchatov Syn-
chrotron Radiation Source (Moscow, Russia). The storage ring with an
electron beam energy of 2.5 GeV and a current of 80–100 mA was used
as the source of radiation. All the spectra were collected in the trans-
mission mode using a Si (111) channel-cut monochromator. For the
Moles of reacted ethanol
Moles of ethanol in the feed
W (EtOH) =
× 100%
While selectivity S to product i is defined as follows:
Moles of each product
Moles of reacted ethanol
S (i) =
× 100%
Acetaldehyde yield N (AcH) per m2 is defined by equation:
w(EtOH)∗W(EtOH)∗S(AcH)
N (AcH) =
mcat∗SBET
Fig. 1. Samples 5%Cu/(Zr + Ce)O2/Al2O3 preparation scheme.
2

A.I. Zhukova et al.
Catalysis Today xxx (xxxx) xxx
γ-Al2O3 (JCPDS 10ꢀ 0425). Apparently, other phases can also be formed
at 950 ◦C but could not be indicated due to the intensive peaks of zir-
conia solid solutions. It was not possible to identify and quantify the
phases of copper on mixed oxide supports by XRD. A similar results have
been reported [7,24] (Fig. 2).
Table 1 shows the specific surface area data, pore size and pore
volume, calculated according to the BET theory for the materials pre-
pared for this study. As it was expected, catalysts with supports prepared
at 180 ◦C (ACZ-180) displayed a larger surface area than samples
calcined at 950 ◦C (ACZ-950). Moreover, the increase of aluminum
oxide in the sample composition leads to an increase in the SSA values
due to aluminum oxide texture characteristics, usually characterized by
a high specific surface area [25,26]. Nonetheless, the pore diameter of
samples decreases with more alumina molar ration for both series of
samples. It is also possible to notice with increase of synthesis temper-
ature we observed the increase of pore size due to complete formation of
zirconium oxide crystalline phases at 950 ◦C as described by XRD. When
the heating temperature was changed from 180 ◦C to 950 ◦C the pore
volume was increased for samples with more than 50 % of alumina in
composition, when the opposite trend was observed for samples with
less than 50 % aluminum oxide content. This fact experimentally con-
firms that the composites change their structure: A50-CZ and A75-CZ are
formed in the base phase of aluminum oxide, which is less susceptible to
sintering rather than ceria-zirconia solid solutions.
Fig. 2. XRD patterns for ACZ samples with various composition and synthesis
○
temperature. (•) – tetragonal zirconia, (◼) – cubic zirconia, ( ) – monoclinic
zirconia (□) – alumina.
Where w(EtOH) – represents the molar flow of ethanol fed to the reactor
(mol/h), W(EtOH) – total conversion of alcohol to reaction products (%),
S(AcH) – acetaldehyde selectivity (%)
It can be noted that the increase of alumina in sample composition
contributed to the less intensive formation of solid solution of cerium
and zirconium. The interaction of aluminum oxide and zirconium pre-
vents the phase transformation of zirconium and inhibits the crystalli-
zation of aluminum oxide [27].
(
)
m2
SBET – specific surface area
grams.
and mcat is the mass of catalyst in
gCAT
In order to determine the Cu charge state on the sample surface, we
performed Cu K-edge X-ray absorption spectroscopy. All spectra of the
catalyst samples and reference compounds were processed (a back-
ground was subtracted, and the normalization was performed) using the
Demeter Athena software. Fig. 3 (A, B) demonstrates XANES spectra for
all samples and reference ones. Different local environments of the Cu
species result in different pre-edge lines in the XANES spectra. The
simplest approach in the XANES data-processing is the linear combi-
nation fitting (LCF) method [28,29]. Having applied LCF spectrum is
modeled by the least-squares fitting using a linear combination of known
structures to fit an unknown spectrum. We performed linear combina-
tion fits of the obtained XANES spectra. The results are presented in
Fig. 3. Comparison of XANES decomposition results reveals certain
differences between low-temperature dried samples and supports
calcined at 950 ◦C (Table 2). Amorphous samples ACZ-180 contains
copper only in the form of Cu2+ on the initial surface. While calcined
catalysts show presence of Cu+ and Cu◦ species, especially in CZ-950 and
ACZ-950. Reliable determination of Cu+ and Cu◦ contribution is not
possible since the shapes and energy positions of Cu foil and Cu2O
XANES spectra are close to each other. Presence of Cu metallic phase
could be determined by EXAFS spectroscopy, which provides structural
information: coordination numbers (N), radii of coordination shells (R)
Experimental activation energy is calculated from Arrhenius de-
pendences of reaction rate presented in Supplementary data (Fig.S2).
3. Results and discussion
3.1. Sample characterization
XRD pattern for ACZ-180, which synthesis temperature did not
exceed 180 ◦C, doesn’t show formation of any crystalline phases. For all
samples calcined at 950 ◦C, peaks are observed at 30◦, 50◦ and 60◦ and
correspond to cerium and zirconium solid solution of Zr0.84Ce0.16O2
(JCPDS 38–1437), substitutional solution having a pseudocubic struc-
ture with tetragonal distortion. Same structure was observed for Cu-CZ-
950 sample without aluminum oxide in the composition. Note that for
Cu-CZ-950, we detected peaks at 2θ = 28◦ (JCPDS 65–1023) assigned to
ZrO2 monoclinic phase. Comparison of ZrO2 crystalline phase peaks
shows the highest intensity for Cu-A5CZ-950 sample. XRD patterns of
Cu-CZ-950, Cu-A5CZ-950 and Cu-A50CZ-950 show cubic zirconia
Zr0.4Ce0.6O2 solid solution phase with low peak intensity (JCPDS
◦
38–1439). At 950 C synthesis temperature aluminum oxide also can
form a crystalline structure. XRD patterns of Cu-A50CZ-950 and Cu-
A75CZ-950 show broad peaks at 66◦ that could be attributed to
and Debye–Waller factors (
σ
2). Each phase of Cu is characterized by
Table 1
Composition, SBET, pore diameter distributions (nm), pore volumes and synthesis temperature of the samples.
Sample
% Al2O3
Synthesis temperature, ◦
C
SBET m2/gCAT
DPOR nm
VPOR cm3/g
0.209
0.159
0.155
0.214
0.185
0.160
0.041
0.234
0.231
0.234
5Cu-CZ-180
0
180
180
180
180
180
950
950
950
950
950
104
120
134
158
236
28
8
5Cu-A5CZ-180
5Cu-A50CZ-180
5Cu-A75CZ-180
5Cu-A100-180
5Cu-CZ-950
5
5
50
75
100
0
5
5
3
23
15
21
11
11
5Cu-A5CZ-950
5Cu-A50CZ-950
5Cu-A75CZ-950
5Cu-A100-950
5
11
50
75
100
45
83
88
3

A.I. Zhukova et al.
Catalysis Today xxx (xxxx) xxx
Table 3
Structural parameters derived from EXAFS spectra fitting.
Sample
Atomic shell
N
R, Å
σ
, Å2
Cu foil
Cu2O
Cu
O
13.6 (1.4)
1.4 (0.2)
9.5 (5.2)
3.1 (2.6)
4.1 (0.3)
1.1 (1.5)
4.2 (0.3)
3.4 (2.3)
2 (fixed)
2 (fixed)
4.1 (0.6)
2.3 (1.5)
4.3 (0.8)
1.2 (0.9)
1.9 (1.1)
1.9 (1.8)
1.0 (1.1)
4.2 (1.0)
2.5 (1.4)
3.7 (0.7)
0.4 (0.5)
4.6 (3.9)
5.0 (1.7)
1.7 (1.6)
0.9 (0.6)
4.6 (1.2)
2.4 (0.6)
1.9 (0.7)
3.0 (1.5)
3.4 (0.7)
2.54 (0.01)
1.84 (0.01)
3.00 (0.02)
3.54 (0.05)
1.94 (0.003)
2.70 (0.07)
2.89 (0.01)
3.09 (0.02)
3.45 (0.02)
3.69 (0.02)
1.99 (0.01)
2.59 (0.02)
1.96 (0.01)
3.04 (0.02)
1.87 (0.07)
2.54 (0.03)
2.97 (0.07)
1.95 (0.02)
2.58 (0.02)
1.96 (0.02)
2.56 (0.03)
1.92 (0.05)
2.55 (0.02)
1.88 (0.07)
2.04 (0.13)
2.56 (0.02)
1.93 (0.02)
2.56 (0.01)
1.92 (0.04)
2.55 (0.03)
0.004 (0.001)
0.002 (0.001)
0.020 (0.005)
0.008 (0.009)
0.005 (0.001)
0.009 (0.02)
0.007 (0.001)
0.007 (0.001)
0.007 (0.001)
0.007 (0.001)
0.008 (0.002)
0.014 (0.007)
0.008 (0.002)
0.006 (0.005)
0.003 (0.002)
0.011 (0.002)
0.011 (0.002)
0.009 (0.003)
0.012 (0.005)
0.005 (0.002)
0.003 (0.010)
0.015 (0.012)
0.009 (0.003)
0.003 (0.009)
0.003 (0.009)
0.009 (0.002)
0.006 (0.003)
0.006 (0.002)
0.008 (0.007)
0.007 (0.004)
Cu
O
CuO
O
O
Cu
Cu
Cu
Cu
O
5Cu-CZ-180
Cu
O
5Cu-A5CZ-180
5Cu-A5CZ-180 spent
Cu
O
Cu
Cu
O
5Cu-A50-CZ-180
5Cu-A100-180
5Cu-CZ-950
Cu
O
Fig. 3. Normalized Cu K-edge XANES spectra of ACZ-180 and ACZ-950 sam-
ples. Reference copper compounds with different forms of Cu coordination
(CuO, Cu2O, Cu foil) are shown for comparison. The spectra are displaced
vertically for clarity.
Cu
O
Cu
O
5Cu-A5CZ-950
O
Cu
O
Table 2
5Cu-A50CZ-950
5Cu-A100-950
Copper reduction form (%) distribution due to reference.
Cu
O
Sample
Cu
Cu2O
CuO
Cu
5Cu-CZ-180
0.08
0.05
0.36
0.15
0.0
0.00
0.0
0.92
0.95
0.31
0.85
1.0
N—the number of neighbour atoms, R—distance,
σ —Debye-Waller factors.
5Cu-A5CZ-180
5Cu-A5CZ-180 spent
5Cu-A50CZ-180
5Cu-A100-180
5Cu-CZ-950
0.33
0.00
0.0
0.39
0.32
0.18
0.36
0.19
0.28
0.00
0.00
0.42
0.40
0.82
0.64
5Cu-A5-CZ-950
5Cu-A50-CZ-950
5Cu-A100-950
unique interatomic distances and coordination numbers. The first co-
ordination shell around Cu in Cu foil consists of 12 Cu atoms at 2.55 Å.
Local atomic geometry of Cu in CuO could be described by O atoms at
distances 1.95 Å, 1.96 Å, 2.78 Å, and Cu atoms at 2.90 Å, 3.08 Å, 3.17 Å.
Local atomic geometry of Cu in Cu2O could be described by 2 O at 1.85 Å
and 12 Cu at 3.02 Å. Detection the presence of O and Cu atoms and their
distances to absorbing Cu atom in XAS experiment could be used to
determine the chemical form of Cu atoms. For example, presence of Cu
in +1 charge state could be detected by the presence of Cu atoms at
distance 3.02 Å around absorbing Cu atom.
The usual starting point of EXAFS analysis fits of the spectra
measured for standard compounds Cu foil, Cu2O and CuO. The results of
such fitting are shown in Table 3 and demonstrate good agreement with
known structural parameters. The experimental spectra are compared
with the fit results in Fig. 4. EXAFS spectra for the samples dried at T =
180 ◦C were well fitted using two coordination shells: 4 oxygen atoms at
nearly 1.96 Å (contribution of CuO phase) and small contribution of
copper atoms at distance 2.55–2.59 Å, which corresponds to the metallic
phase of Cu. Oxygen coordination numbers, which are close to 4 indicate
a presence of well-structured CuO particles. Samples calcined at 950 ◦C
contain significant amount of metallic Cu particles according to EXAFS
results. These samples also contain a comparative number of Cu+ par-
ticles due to XANES results. EXAFS spectra for the samples A5-CZ-180
used and A5-CZ-950 demonstrate a significant contribution of oxygen
atoms at a distance 1.88 Å, which indicates the presence of a well-
structured Cu2O phase.
Fig. 4. Fourier transform of the χ(k) spectra shown in Fig. 3. Dotted black
line—experiment, solid red lines—fitted spectra (For interpretation of the ref-
erences to colour in this figure legend, the reader is referred to the web version
of this article).
spectrum and its first derivative peak with those for the reference
compounds in Fig. 5 (A,B) allow us to conclude that on fresh 5Cu-ACZ-
180 catalyst Cu2+ species is detected, in agreement with the studies
[30]. 5Cu-ACZ-180 after catalysis predominantly has Cu+ species.
Fourier transform magnitudes of the experimental EXAFS spectra
and respective fits for Cu metal and oxide patterns at Cu K-edge are
shown in Fig. 5.(C) for both samples, the FT spectra were well fitted by a
copper model structure in which the copper nearest neighbors and next-
nearest neighbors are oxygen and copper, respectively. The Cu–O
shoulder in the fitted spectra was more intense for fresh 5Cu-ACZ-180
than for spent one. According to EXAFS, sample 5Cu-ACZ-180 after
catalysis has a low amount of metallic copper, which opposes XANEX
After catalytic process, we show XAS data for as-prepared and used
in ethanol conversion reaction 5Cu-ACZ-180 sample to evaluate sample
surface condition after catalytic process. Comparisons of the XANES
4

A.I. Zhukova et al.
Catalysis Today xxx (xxxx) xxx
Fig. 5. Cu K-edge XAS spectra for samples and reference compounds for the fresh and spent A5CZ-180 sample: A, XANES spectra; B, first derivative of the XANES
spectra; C, fourier transform magnitudes of the k3-weighted Cu EXAFS spectra.
data and can be explained by its low concentration.
that activation energy magnitude in dehydrogenation of ethanol de-
pends on copper form [42]. Although the apparent Ea values were not
markedly different between the Cu-ACZ-180 and Cu-ACZ-950 samples,
there was a correlation between the pre-exponential factor (A)
(Table S1) and catalytic activity (Fig.S1, Table S2). The highest values
of a pre-exponential factor were obtained for Cu-ACZ-950 series when
supports were calcined at 950 ◦C, which could present more active
centers than the uncalcined supports due to distortions caused by sin-
tering or atom migration of support at higher temperatures. The
different state/amount of surface copper depends on the state of sup-
ports. These observations are in agreement with the results of [35]. As it
was shown, ethanol dehydrogenation on Cu catalysts is structure sen-
sitive [36]. However, there is no single opinion on the favorable copper
state for this process. Some authors [37] conclude that the most active
catalyst for acetaldehyde formation consists mostly of metal copper
nanoparticles, while reports by [5] suggest that ethanol dehydrogena-
tion reaction also requires the presence of a small amount of Cu+.
The importance of Cu+ particle in the catalyst is evident for 5Cu-
A5CZ-180 and 5Cu-A5CZ-950 catalyst (Fig. 6, Table S2), with the
highest selectivity to acetaldehyde among other samples with the same
synthesis temperature. In this case, there was clear evidence that cata-
lytic performance was improved by Cu+/Cu◦ species ratio (Table 2). The
repeated catalytic test also showed a large increase in ethanol conver-
sion and in selectivity to acetaldehyde (Fig. 7) caused by further
reduction of copper to Cu+/Cu◦ ratio (Table 2). Therefore, it could be
concluded that the intermediate Cu+ species or the mixture of Cu+/Cu◦
species are more active in ethanol dehydrogenation than fully reduced
Cu◦.
3.2. Catalytic activity
The temperature-programmable reaction of ethanol transformation
into the main reaction product acetaldehyde (AcH) was carried out for
◦
5Cu-ACZ-180 series and calcined at 950 C catalysts of 5Cu-ACZ-950
series. Ethanol conversion to acetaldehyde for all reduced samples and
a repeated experiment in the temperature 220ꢀ 360 ◦C range is available
in Supplementary data of this article Fig.S1. It should be noted that by-
products of ethanol reaction are also formed: diethyl ether (DEE) and
ethylene. The activity of all catalysts increases with increasing the re-
action temperature. The changes in catalytic activity and selectivity for
Cu-based samples depending on their composition and synthesis tem-
perature, are shown in Table S2. It was found that the addition of
aluminum oxide to zirconium-cerium oxide in the composition of the
supports shows the higher activity in the total ethanol conversion.
Simultaneously, the increase of aluminum oxide in the catalyst
composition leads to a decrease in selectivity in the dehydrogenation
reaction on ACZ samples. The selectivity towards ethanol trans-
formation products obtained during the reaction is summarized in Fig. 6
(Table S2). The product distribution was not sensitive to the synthesis
temperature but strongly influenced by support composition. The cata-
lysts with CZ, A5CZ, A50CZ and A75CZ produced significant amounts of
acetaldehyde (S > 80 %) related to ethanol dehydrogenation on copper
species [31,32]. An increase of the Al content in the catalysts support
promoted the formation of small amounts of ethyl ether and ethylene.
The dehydration products dominated for the Cu-A100 sample. The
selectivity of these products results from ethanol dehydration on
Brønsted and/or Lewis acid sites [33,34]. Notably, catalytic data is
comparable with those mentioned in the literature for Cu-based cata-
lysts in ethanol dehydrogenation to acetaldehyde (Table S3).
It is known that Cu+ is not a stable reduction form, due to high
electron exchange ability. Our results show that catalysts based on
mixed oxide support pre-heated at 950 ◦C could form this active dehy-
drogenation reaction site. Apparently, due to the presence of well-
structured metal particles on the mixed oxide surface, the Cu-O bond
on the surface of tested samples is similar to Cu2O bond, isolated and
stabilized (RCu-O and N number according to EXAFS parameters).
Therefore, Cu+/Cu◦ particles, located at the support-metal interface, is
responsible for dehydrogenation reaction and formed on calcined mixed
oxide support rather than non-binary or pure uncalcined oxides.
The excellent stability and activity of intermediate Cu + in the case
of the A5CZ-950 could be attributed to the formation of more reactive
surface copper species Cu+/Cu◦. The XRD analysis of sample indicates
The apparent activation energy of acetaldehyde formation was
calculated from the slop of Arrhenius plots Fig.S2 (Supplementary data).
Cu-CZ-180 and Cu-A100ꢀ 180 catalysts display the lowest Ea for the
acetaldehyde formation, which means a low dependence of the reaction
rate on the temperature (Table S.1). High specific surface area could
activated the formation of active sites with high heat of ethanol
adsorption (Brønsted–Evans–Polanyi principle). Low values of apparent
activation energy of these non-binary amorphous oxides did not show
the best activity and selectivity to acetaldehyde. Kinetic studies confirms
5

A.I. Zhukova et al.
Catalysis Today xxx (xxxx) xxx
Fig. 7. Temperature dependences of alcohol conversion on the sample A5CZ-
180 and selectivity of alcohol conversion into acetaldehyde.
properties of Cu modified alumina-zirconia catalysts obtained with
various support compositions at different temperatures were studied in
the ethanol dehydrogenation reaction. The results show that the Al-Zr-
supported copper species are in different reduction states, in the form
of Cu2+, Cu+ and Cu◦, and their distribution depends on the composition
support phase. Recently, these catalytic systems in heterogeneous
catalysis were named “cocktail” on the surface due to the number of
different active centers presented [40]. Catalytic activity tests and ki-
netic studies confirm that ethanol dehydrogenation reactions are
structure sensitive [41]. Structural characterization of the catalysts
indicated that the specific surface area of supports was responsible for
the reduction state of Cu species. It is preferable to use pre-heated at 950
◦C mixed oxide systems. They show higher selectivity to acetaldehyde:
calcination of catalytic supports based on stabilized zirconia promoted
with up to 5% Al2O3 improves the distribution of Cu on the surface. It
stabilizes Cu+/Cu◦ reduction state, which is necessary for the formation
of ethanol dehydrogenation reaction sites.
CRediT authorship contribution statement
A.I. Zhukova: Conceptualization, Supervision, Writing - review &
editing. S.G. Chuklina: Methodology, Investigation, Formal analysis,
Writing - review & editing. S.A. Maslenkova: Investigation, Writing -
original draft, Visualization.
Fig. 6. Selectivity to carbon-containing products from 320 to 360 ◦C for Cu-
containing ACZ-180 (a) and ACZ-950 (b) catalysts in the ethanol conversion.
Declaration of Competing Interest
predominant tetragonal zirconia solid solution which contributes to a
higher copper dispersion over the surface according to [6]. Highly
dispersed CuO species are stabilized over oxygen vacancies of m-ZrO2
[38]. Also known that tetragonal and cubic ceria stabilized zirconia solid
solutions has enhanced oxygen mobility of the catalysts [10], which
leads to high CuO dispersion and high activity.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
Simultaneously, non-binary Cu modified oxide (CZ-950), which ac-
cording to our XRD data also contains tetragonal zirconia, shows lower
catalytic activity due to not optimal balance between Cu+ sites and
metallic copper particles, detected by XANEX. Similar results [39] show
that increase in alcohol dehydrogenation activity lies in the synergetic
effect of Cu◦ and Cu+. This study’s main findings highlight the key role
of the textural features of support composition (alumina and zirconia
oxides) in the production of catalysts with Cu◦ and Cu+ species.
The catalytic research was funded by Ministry of Science and Higher
Education of the Russian Federation, grant number 0706-2020-0026.
This paper has been supported by the RUDN University Strategic
Academic Leadership Program.
The authors are grateful to the National Research Center "Kurchatov
Institute" and personally Alexander Trigub for XAS measurements.
Appendix A. Supplementary data
4. Conclusions
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.cattod.2021.02.015.
Relationships between catalytic activity, textural, and structural
6

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Catalysis Today xxx (xxxx) xxx
[22] S.G. Chuklina, S.A. Maslenkova, A.I. Pylinina, L.I. Podzorova, A.A. Ilyicheva, Effect
of composition and calcination temperature of ceria-zirconia-alumina mixed oxides
on catalytic performances of ethanol conversion, in: в: IOP Conf. Ser. Mater. Sci.
Eng, © Published Under Licence by IOP Publishing Ltd., 2017, https://doi.org/
10.1088/1757-899X/175/1/012031.
References
[1] J. Sun, Y. Wang, Recent advances in catalytic conversion of ethanol to chemicals,
ACS Catal. 4 (2014) 1078–1090, https://doi.org/10.1021/cs4011343.
[2] T. Hou, S. Zhang, Y. Chen, D. Wang, W. Cai, Hydrogen production from ethanol
reforming: catalysts and reaction mechanism, Renew. Sustain. Energy Rev. 44
(2015) 132–148, https://doi.org/10.1016/j.rser.2014.12.023.
[23] S.G. Chuklina, A.I. Pylinina, L.I. Podzorova, N.A. Mikhailina, I.I. Mikhalenko,
Ethanol dehydrogenation on copper catalysts with ytterbium stabilized tetragonal
ZrO2 support, Russ. J. Phys. Chem. A. 90 (2016) 2370–2376, https://doi.org/
10.1134/S0036024416120074.
[3] V.R. Surisetty, A.K. Dalai, J. Kozinski, Alcohols as alternative fuels: an overview,
Appl. Catal. A Gen. 404 (2011) 1–11, https://doi.org/10.1016/j.
[24] S. Tada, K. Oshima, Y. Noda, R. Kikuchi, M. Sohmiya, T. Honma, S. Satokawa,
Effects of Cu precursor types on the catalytic activity of Cu/ZrO2 toward methanol
synthesis via CO2 hydrogenation, Ind. Eng. Chem. Res. 58 (2019) 19434–19445,
https://doi.org/10.1021/acs.iecr.9b03627.
apcata.2011.07.021.
[4] E.A. Redina, A.A. Greish, I.V. Mishin, G.I. Kapustin, O.P. Tkachenko, O.
A. Kirichenko, L.M. Kustov, Selective oxidation of ethanol to acetaldehyde over Au-
Cu catalysts prepared by a redox method, Catal. Today 241 (2015) 246–254,
https://doi.org/10.1016/j.cattod.2013.11.065.
[25] H.A. Dabbagh, M. Zamani, Catalytic conversion of alcohols over alumina-zirconia
mixed oxides: reactivity and selectivity, Appl. Catal. A Gen. 404 (2011) 141–148,
https://doi.org/10.1016/j.apcata.2011.07.024.
[5] I.C. Freitas, S. Damyanova, D.C. Oliveira, C.M.P. Marques, J.M.C. Bueno, Effect of
Cu content on the surface and catalytic properties of Cu/ZrO 2 catalyst for ethanol
dehydrogenation, J. Mol. Catal. A Chem. 381 (2014) 26–37, https://doi.org/
10.1016/j.molcata.2013.09.038.
[26] M.R. Dumont, E.H.M. Nunes, W.L. Vasconcelos, Use of a design-of-experiments
approach for preparing ceria-zirconia-alumina samples by sol-gel process, Ceram.
Int. 42 (2016) 9488–9495, https://doi.org/10.1016/j.ceramint.2016.03.021.
[27] J. Wang, J. Wen, M. Shen, Effect of interaction between Ce0.7Zr0.3O2 and Al2O3
on structural characteristics, thermal stability, and oxygen storage capacity,
J. Phys. Chem. C. 112 (2008) 5113–5122, https://doi.org/10.1021/jp711331g.
[28] M.C. Hsiao, H.P. Wang, Y.W. Yang, EXAFS and XANES studies of copper in a
solidified fly ash, Environ. Sci. Technol. 35 (2001) 2532–2535, https://doi.org/
10.1021/es001374v.
[6] A.G. Sato, D.P. Volanti, D.M. Meira, S. Damyanova, E. Longo, J.M.C. Bueno, Effect
of the ZrO2 phase on the structure and behavior of supported Cu catalysts for
ethanol conversion, J. Catal. 307 (2013) 1–17, https://doi.org/10.1016/j.
jcat.2013.06.022.
[7] G. Vidya Sagar, P. Venkat, R. Rao, C.S. Srikanth, K.V.R. Chary, Dispersion and
reactivity of copper catalysts supported on Al2O3-ZrO2, J. Phys. Chem. B 110
(2006) 13881–13888, https://doi.org/10.1021/jp0575153.
[29] A. Gaur, D. Shrivastava, K. Joshi, Copper K-edge XANES of Cu(I) and Cu(II) oxide
mixtures, J. Phys, Conf. Ser. 190 (2009), https://doi.org/10.1088/1742-6596/
190/1/012084.
[8] H.M. Yang, M.K. Chan, Steam reforming of methanol over copper-yttria catalyst
supported on praseodymium-aluminum mixed oxides, Catal. Commun. 12 (2011)
1389–1395, https://doi.org/10.1016/j.catcom.2011.05.022.
[30] S.G. Sanches, J.H. Flores, M.I.P. da Silva, Cu/ZnO and Cu/ZnO/ZrO2 catalysts used
for methanol steam reforming, Mol. Catal. 454 (2018) 55–62, https://doi.org/
10.1016/j.mcat.2018.05.012.
[9] A.G. Sato, D.P. Volanti, I.C. De Freitas, E. Longo, J.M.C. Bueno, Site-selective
ethanol conversion over supported copper catalysts, Catal. Commun. 26 (2012)
122–126, https://doi.org/10.1016/j.catcom.2012.05.008.
[31] F.W. Chang, W.Y. Kuo, K.C. Lee, Dehydrogenation of ethanol over copper catalysts
on rice husk ash prepared by incipient wetness impregnation, Appl. Catal. A Gen.
246 (2003) 253–264, https://doi.org/10.1016/S0926-860X(03)00050-4.
[32] M.V. Morales, E. Asedegbega-Nieto, B. Bachiller-Baeza, A. Guerrero-Ruiz,
Bioethanol dehydrogenation over copper supported on functionalized graphene
materials and a high surface area graphite, Carbon 102 (2016) 426–436, https://
doi.org/10.1016/j.carbon.2016.02.089.
[10] D.W. Jeong, W.J. Jang, H.S. Na, J.O. Shim, A. Jha, H.S. Roh, Comparative study on
cubic and tetragonal Cu-CeO2-ZrO2 catalysts for water gas shift reaction, J. Ind.
Eng. Chem. 27 (2015) 35–39, https://doi.org/10.1016/j.jiec.2015.01.007.
[11] J. Shan, N. Janvelyan, H. Li, J. Liu, T.M. Egle, J. Ye, M.M. Biener, J. Biener, C.
M. Friend, M. Flytzani-Stephanopoulos, Selective non-oxidative dehydrogenation
of ethanol to acetaldehyde and hydrogen on highly dilute NiCu alloys, Appl. Catal.
B Environ. 205 (2017) 541–550, https://doi.org/10.1016/j.apcatb.2016.12.045.
[12] I.S.P. Campisano, C.B. Rodella, Z.S.B. Sousa, C.A. Henriques, V. Teixeira da Silva,
Influence of thermal treatment conditions on the characteristics of Cu-based metal
oxides derived from hydrotalcite-like compounds and their performance in bio-
ethanol dehydrogenation to acetaldehyde, Catal. Today 306 (2018) 111–120,
https://doi.org/10.1016/j.cattod.2017.03.017.
[33] H.R. Mahmoud, Highly dispersed Cr2O3-ZrO2 binary oxide nanomaterials as novel
catalysts for ethanol conversion, J. Mol. Catal. A Chem. 392 (2014) 216–222,
https://doi.org/10.1016/j.molcata.2014.05.021.
[34] G. Garbarino, R. Prasath Parameswari Vijayakumar, P. Riani, E. Finocchio, G.
Busca, Ethanol and diethyl ether catalytic conversion over commercial alumina
and lanthanum-doped alumina: reaction paths, catalyst structure and coking, Appl.
Catal. B Environ. 236 (2018) 490–500, https://doi.org/10.1016/j.
apcatb.2018.05.039.
¨
¨
[13] S. Tayrabekova, P. Maki-Arvela, M. Peurla, P. Paturi, K. Eranen, G.E. Ergazieva,
A. Aho, D.Y. Murzin, K. Dossumov, Catalytic dehydrogenation of ethanol into
acetaldehyde and isobutanol using mono- and multicomponent copper catalysts,
Comptes Rendus Chim. 21 (2018) 194–209, https://doi.org/10.1016/j.
crci.2017.05.005.
[35] W.H. Cassinelli, L. Martins, A.R. Passos, S.H. Pulcinelli, A. Rochet, V. Briois, C.
V. Santilli, Correlation between structural and catalytic properties of copper
supported on porous alumina for the ethanol dehydrogenation reaction,
ChemCatChem. 7 (2015) 1668–1677, https://doi.org/10.1002/cctc.201500112.
[36] K. Sun, M. Zhang, L. Wang, Effects of catalyst surface and hydrogen bond on
ethanol dehydrogenation to ethoxy on Cu catalysts, Chem. Phys. Lett. 585 (2013)
89–94, https://doi.org/10.1016/j.cplett.2013.09.005.
[14] M. He, M. Luo, P. Fang, Characterization of CuO species and thermal solid-solid
interaction in CuO/CeO2-Al2O3 catalyst by in-situ XRD, Raman spectroscopy and
TPR, J. Rare Earths. 24 (2006) 188–192, https://doi.org/10.1016/S1002-0721(06)
60091-4.
[15] P.S. Roy, J. Song, K. Kim, J.M. Kim, C.S. Park, A.S.K. Raju, Effects of
CeZrO2–Al2O3 support composition of metal-foam-coated Pd–Rh catalysts for the
steam-biogas reforming reaction, J. Ind. Eng. Chem. 62 (2018) 120–129, https://
doi.org/10.1016/j.jiec.2017.12.050.
[37] G. Garbarino, P. Riani, M. Villa García, E. Finocchio, V. Sanchez Escribano,
G. Busca, A study of ethanol dehydrogenation to acetaldehyde over copper/zinc
aluminate catalysts, Catal. Today (2019) 0–1, https://doi.org/10.1016/j.
cattod.2019.01.002.
[16] Q. Dong, S. Yin, C. Guo, T. Sato, Aluminum-doped ceria-zirconia solid solutions
with enhanced thermal stability and high oxygen storage capacity, Nanoscale Res.
Lett. 7 (2012) 542, https://doi.org/10.1186/1556-276X-7-542.
[38] V.P. Pakharukova, E.M. Moroz, D.A. Zyuzin, A.V. Ishchenko, L. Yu, P.E.
Strizhak Dolgikh, Structure of copper oxide species supported on monoclinic
zirconia, J. Phys. Chem. C. 119 (2015) 28828–28835, https://doi.org/10.1021/
acs.jpcc.5b06331.
[17] H.A. Dabbagh, M. Zamani, Catalytic conversion of alcohols over alumina–zirconia
mixed oxides: reactivity and selectivity, Appl. Catal. A Gen. 404 (2011) 141–148,
https://doi.org/10.1016/j.apcata.2011.07.024.
[39] Z. Xiao, X. Wang, J. Xiu, Y. Wang, C.T. Williams, C. Liang, Synergetic effect
between Cu0 and Cu+ in the Cu-Cr catalysts for hydrogenolysis of glycerol, Catal.
Today 234 (2014) 200–207, https://doi.org/10.1016/j.cattod.2014.02.025.
[40] D.B. Eremin, V.P. Ananikov, Understanding active species in catalytic
transformations: from molecular catalysis to nanoparticles, leaching, “Cocktails” of
catalysts and dynamic systems, Coord. Chem. Rev. 346 (2017) 2–19, https://doi.
org/10.1016/j.ccr.2016.12.021.
[18] L.I. Podzorova, S.G. Chuklina, A.A. Il’icheva, et al., Frame catalysts of
Al2O3–ZrO2–CeO2 system, Inorg. Mater. Appl. Res. 9 (2018) 960–964, https://doi.
org/10.1134/S2075113318050234.
[19] L.I. Podzorova, A.A. Il’icheva, N.A. Mikhailina, V.Y. Shevchenko, D.S. Bashlykov,
G.V. Rodicheva, L.I. Shvorneva, Effect of Synthesis Conditions on the Phase
Composition of ZrO2-CeO2-Al2O3 Sol-Gel Powders, Inorg. Mater. Appl. Res. 37
(2001) 51–57, https://doi.org/10.1023/A:1026737411203.
[41] D.Y. Murzin, On apparent activation energy of structure sensitive heterogeneous
catalytic reactions, Catal. Letters 149 (2019) 1455–1463, https://doi.org/10.1007/
s10562-019-02772-0.
[20] A.A. Chernyshov, A.A. Veligzhanin, Y.V. Zubavichus, Structural materials science
end-station at the Kurchatov Synchrotron Radiation Source: recent instrumentation
upgrades and experimental results, Nucl. Instruments Methods Phys. Res. Sect. A
Accel. Spectrometers, Detect. Assoc. Equip. 603 (2009) 95–98, https://doi.org/
10.1016/j.nima.2008.12.167.
[42] W. Chen, E.D. Cubuk, M.M. Montemore, C. Reece, R.J. Madix, C.M. Friend,
E. Kaxiras, A comparative ab initio study of anhydrous dehydrogenation of linear-
chain alcohols on Cu(110), J. Phys. Chem. C. 122 (2018) 7806–7815, https://doi.
org/10.1021/acs.jpcc.8b01698.
[21] S.I. Zabinsky, J.J. Rehr, A. Ankudinov, R.C. Albers, M.J. Eller, Multiple-scattering
calculations of x-ray-absorption spectra, Phys. Rev. B 52 (1995) 2995–3009,
https://doi.org/10.1103/PhysRevB.52.2995.
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