Selective oxidation of ethanol to acetaldehyde over Au-Cu catalysts prepared by a redox method

Article abstract of DOI:10.1016/j.cattod.2013.11.065

A series of the Au-Cu/SiO₂ catalysts including low-loaded samples with an overall metal content less than 0.5 wt% was synthesized by a redox method. The catalysts were tested in the selective oxidation of ethanol to acetaldehyde. The highest catalytic activity was observed for the catalysts having the lowest metal content due to the presence of nanosized Au particles and the synergetic effect of Au-Cu²⁺ interaction. Unusual reduction behavior was found for the catalysts with the 0.2 wt% content of Cu. XRD, TPR, EXAFS, and STEM techniques were applied for the catalysts characterization.

Full text of DOI:10.1016/j.cattod.2013.11.065

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Selective oxidation of ethanol to acetaldehyde over Au–Cu catalysts
prepared by a redox method
Elena A. Redina∗, Alexander A. Greish, Igor V. Mishin, Gennady I. Kapustin,
Olga P. Tkachenko, Olga A. Kirichenko, Leonid M. Kustov
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Prospect, Moscow 119991, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of the Au–Cu/SiO2 catalysts including low-loaded samples with an overall metal content less than
0.5 wt% was synthesized by a redox method. The catalysts were tested in the selective oxidation of ethanol
to acetaldehyde. The highest catalytic activity was observed for the catalysts having the lowest metal
content due to the presence of nanosized Au particles and the synergetic effect of Au–Cu2+ interaction.
Unusual reduction behavior was found for the catalysts with the 0.2 wt% content of Cu. XRD, TPR, EXAFS,
and STEM techniques were applied for the catalysts characterization.
Received 26 August 2013
Received in revised form 7 November 2013
Accepted 10 November 2013
Available online xxx
Keywords:
Low-loaded gold-copper catalysts
Redox method
© 2014 Elsevier B.V. All rights reserved.
Selective ethanol oxidation
Acetaldehyde
1. Introduction
catalyst [16]. The Au/CeO2 catalyst [17], Au supported on hydrotal-
cite [18,19] and Au supported on other metal oxides [20,21] were
Selective oxidation of ethanol to acetaldehyde by oxygen over
heterogeneous catalysts is of great interest as this process can
replace the conventional one based on the oxidation of ethanol
with very hazardous agents, such as chromate or permanganate [1].
Some essential results were obtained in the oxidative conversion
of ethanol over noble metal-based catalysts, but fast deactiva-
tion of the catalysts and their overoxidation were observed [2].
Also, acetaldehyde was selectively produced by dehydrogenation of
ethanol on the Cu containing catalysts [3] and by selective oxidation
of ethanol over such catalysts as V2O5/TiO2–SiO2 [4], FeSBA-15 [5],
supported manganese oxide [6] and graphite nanofibers [7]. How-
ever, in these cases, the catalysts revealed either a good selectivity
in the limited range of the ethanol conversions (not exceeding 50%)
or a low selectivity to acetaldehyde.
Supported gold catalysts can exhibit both high activity and
selectivity in the oxidation of various alcohols, such as polyols [8,9],
aliphatic [10,11], aromatic alcohols [12,13].
Rossi and Biella [14] studied the gas phase oxidation of primary
and secondary alcohols over the 1% Au/SiO2 catalyst, and in this case
the selectivity to corresponding aldehydes and ketones reached
around 100%. This catalyst was also active in the liquid phase
ethanol oxidation to acetic acid [15] as well as the Au/MgAl2O4
applied for the conversion of ethanol to carbonyl compounds. A
high selectivity to acetaldehyde was also obtained in ethanol oxi-
dation over the 1% Au/MoO3 catalyst [1] and Au/TiO2 with an Au
content 0.5–7 wt% [22,23].
Of particular interest are the catalysts on the basis of the Au–Cu
composition. For instance, the Au–Cu/SiO2 catalyst appeared to be
active in the benzyl alcohol oxidation to benzaldehyde providing
the 98% yield, the oxidation efficiency of the bimetallic catalysts
exceeded that of the monometallic systems, such as Au/SiO2 and
Cu/SiO2 [13]. But, there are only a few works concerning ethanol
oxidation over Au–Cu catalysts, in spite of the wide use of individu-
ally supported gold and copper in this reaction. A synergetic effect
of Au and CuOx constituents was revealed in the Au–Cu/SiO2 cata-
lyst which had a core-shell structure and was capable of catalyzing
partial oxidation of ethanol to acetaldehyde. In the authors’ opinion
[24], both the high oxidation activity and selectivity of this catalytic
system are caused by a close contact between the Au core and the
CuOx shell.
The present work is devoted to selective oxidation of ethanol to
acetaldehyde with air over the Au–Cu catalysts prepared by a redox
method. An intimate contact between two metal constituents in
the catalyst structure was provided by selective Au deposition on
the surface of Cu nanoparticles through the surface redox reactions
between the oxidized form of Au and metallic Cu [25,26]. Main
attention was paid to the preparation of the Au–Cu catalysts with
the total metal loading not exceeding 0.5 wt%
∗
Corresponding author. Tel.: +7 9152694149.
E-mail address: redinalena@yandex.ru (E.A. Redina).
0920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.11.065
Please cite this article in press as: E.A. Redina, et al., Selective oxidation of ethanol to acetaldehyde over Au–Cu catalysts prepared by a redox
method, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2013.11.065

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2. Experimental
(1.1 M) was added after 30 min a to the solid (nAu:nNH4OH = 1:55)
and the suspension was stirred for 10 min followed by addition of
NaBH4 (nAu:nNaBH4 = 1:6.26). Then the suspension was heated up to
60 ◦C and stirred for 10 min. The solid product was separated from
the parent solution by centrifugation and washed several times
with distilled water to remove Cl− ions from the catalyst surface.
During the procedure, the partial reduction of gold in the solution
took place. The Au and Au–Cu catalysts prepared by this method
were marked as x% Au/SiO2 and x% Au/y% SiO2-NaBH4. The 0.8%
Au/2% Cu/SiO2-NaBH4 catalyst was prepared as a reference to com-
pare the activities with the catalyst having the same metal content,
but prepared by a redox method.
2.1. Catalyst preparation
A commercial SiO2 (Russia) was used as a support for the cat-
alysts. Silica was chosen as a support to avoid undesirable side
reactions initiated by the carrier itself. Physical characterization of
the carrier was made using ASAP 2020 (Micromeritics). The surface
characteristics were as follows: the BET surface area was 218 m2/g;
the BJH pore volume was 0.81 cm3/g. The pore size distribution
was rather narrow; the average value was 13.3 nm. The fraction of
micropores did not exceed 1% of the total pore volume.
Metal precursors and other chemical reagents were of the ana-
lytical grade: Cu(NO3)2·3H2O (Acros Organics) and HAuCl4·3H2O
(Aurat), NaBH4 (Sigma Aldrich).
2.2. Catalyst testing
The reaction was carried out in the glass reactor (din = 5 mm)
with a fixed catalyst bed. The reactor was equipped with a tem-
perature controller; the end of the thermocouple was placed near
the bottom of the catalyst bed. The reaction was performed in the
50–450 ◦C temperature range at an atmospheric pressure. Before
the run, the catalyst (∼0.1 g) was treated at 300 ◦C for 1 h in an
air-He flow. The feed gas consisted of 1% C2H5OH (96%, Bryncalov
Ferein), 1.2% O2 (from air) and He. Ethanol was supplied by the
micropump. Ethanol was mixed at the inlet of the reactor with the
gas flow. The gas flow rate of 21 ml/min was maintained by a flow
controller, VGSH was 4800 h−1. The blank test confirmed that in the
absence of the catalyst no reaction occurred.
The analysis of the reaction products was performed by a
gas-liquid chromatograph equipped with a TCD detector. The
oxygen-containing compounds such as ethanol, acetaldehyde,
diethyl ether, and ethyl acetate were analyzed by the Chromatec
– Crystall 5000.2 chromatograph with the Carbowax 6000 stain-
less steel column (3 mm × 2.5 m). The gaseous products such as H2,
CO, CO2 and ethylene were analyzed by the LKhM-8D chromato-
graph equipped with a stainless steel column (3 mm × 6 m) packed
with Porapak Q.
2.1.1. Preparation of the Au/Cu/SiO2 catalyst by the redox
reaction (RR)
Preparation of the supported Au–Cu catalysts involved two
stages:
(i) Synthesis of the monometallic parent CuO/SiO2 catalyst was
performed by the incipient wetness impregnation of the sup-
port with an aqueous solution of Cu(NO3)2·3H2O (C = 0.174 M)
for 1 h followed by drying at RT and at 60 ◦C overnight, then
calcination at 400 ◦C for 4 h was performed. The wet impreg-
nation was used for the preparation of high-loaded samples. In
this case, a sample was dried under the vacuum using the rotary
evaporator at 80 ◦C until obtaining the dry residue and then it
was calcined at 400 ◦C for 4 h. The calcination temperature was
chosen on the basis of the TG-DTA data.
(ii) To obtain metallic copper supported on SiO2 the reduction of
the CuO/SiO2 catalyst was carried out in the hydrogen flow
(30 ml/min) at 400 ◦C for 2 h. At the end of this procedure
the reactor was cooled to RT and the required amount of the
HAuCl4·3H2O solution (7.68 × 10−3 M) was added to the solid,
while the contact with atmosphere was excluded. The suspen-
sion obtained was left overnight. Then a solution was separated
from the solid and the latter was washed several times with the
deionized water to remove adsorbed Cl− ions and molecular Cl2
from the catalyst surface. The amount of gold left in the solution
was measured by the reverse iodometric titration. In all cases,
the gold deposition on the catalyst surface was close to 99%.
Finally, the sample was dried at 60 ◦C overnight.
The carbon balance in all catalytic experiments was no less than
99%.
2.3. Catalyst characterization
2.3.1. XRD
The phase composition of the catalysts and the particle size of
the supported metal were examined by X-ray diffraction (XRD)
analysis. X-ray diffraction patterns were recorded using a DRON-
2 diffractometer with Ni-filtered Cu K␣ radiation (ꢀ = 0.1542 nm)
in a step scanning mode, with a step of 0.02◦ and a counting time
of 0.6 s per step in the range of 2Â = 20◦–80◦. Identification of the
phases was performed by comparison of the position and intensity
of the peaks with the data from the files of International Center for
Diffraction Data. The crystal size of nanoparticles was calculated
from X-ray peak broadening.
The Au–Cu catalyst samples prepared by the redox method dif-
fered by either the Au/Cu atomic ratios (0:1; 1:3; 1:8; 1:0) or
the weight percentage of the metals at the constant atomic ratio
(0.2–5 wt% for each metal). The metal loadings were calculated
using the following equations:
ω (Cu) = m (Cu) 100%/m (support)
ω (Au) = m (Au) 100%/m (support) + m(Cu)
The catalysts were marked as x% Au/y% Cu/SiO2 for the bimetallic
catalysts and y% Cu/SiO2 for the monometallic CuO/SiO2 sample,
where x% was the weight percentage of deposited Au and y% was
the weight percentage of deposited Cu.
2.3.2. TPR-H2
TPR measurements were performed in the lab-constructed flow
system. The Cu/SiO2 catalyst sample with a weight of 150 mg was
pretreated in an argon flow at 150 ◦C for 60 min. For the gold con-
taining catalysts, the pretreatment was carried out at 80 ◦C for
120 min. Then the catalyst sample was cooled in the Ar flow to
−50 ◦C prior to the TPR experiment. The heating from −50 to 850 ◦C
was carried out at the rate of 10 ◦C/min in the 4.6% H2 – Ar gas mix-
ture supplied with a space velocity of 30 ml/min. Then the sample
was kept at 850 ◦C until the hydrogen consumption ceased.
2.1.2. Preparation of the Au/SiO2 and Au/Cu/SiO2 catalysts by
incipient wetness impregnation followed by NaBH4 reduction
This method was described in [27]. SiO2 was impregnated with
an aqueous solution of HAuCl4·3H2O under stirring by a glass rod.
For the bimetallic Au–Cu catalyst, the co-precipitation with the use
of HAuCl4·3H2O and CuCl2·3H2O was applied. A solution of NH4OH
Please cite this article in press as: E.A. Redina, et al., Selective oxidation of ethanol to acetaldehyde over Au–Cu catalysts prepared by a redox
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2.3.3. STEM
The powder samples were placed on the fixed copper netting
with d = 3 mm. The microstructure of the samples was studied by
means of scanning transmission electron microscopy with a field
emission (FE-STEM) Hitachi SU8000 microscope. The experiment
was performed in the light field mode by registering transmitted
electrons under the accelerating voltage of 30 kV.
2.3.4. XAS
XAS (XANES + EXAFS) measurements were carried out in HASY-
LAB (DESY in Hamburg, Germany) on the beamline C1 (Au L3-edge,
11,919 eV, and Cu K-edge, 8979 eV) using a double-crystal Si(3 1 1)
monochromator, which was detuned to 50% of the maximum inten-
sity to exclude higher harmonics in the X-ray beam. The spectra
were recorded in the transmission mode at a low temperature
(T = 80 K) in order to decrease the Debye–Waller factors. All spectra
were measured simultaneously with the reference spectrum of Au
or Cu foil placed behind the second ionization chamber, so that the
absolute energy calibration was performed.
Fig. 1. XRD patterns of the 0.8% Au/SiO2 catalyst: fresh (a); used (b).
3. Results and discussion
3.1. Catalyst preparation by the redox method
A series of silica-supported Au–Cu catalysts was synthesized by
the redox method using HAuCl4 as the Au precursor. We varied
both the Au/Cu atomic ratio (1:0, 1:3, 1:8, 0:1) and the content
of each metal (0.2–5 wt%) while keeping the Au:Cu atomic ratio
unchanged. The redox method applied for Au deposition is based
on the surface redox reaction between metallic copper and oxidized
gold via semi-reactions 1–2 shown below [25,28]:
Cu − 2e− = Cu2+ (E◦ = +0.34 V)
(1)
(2)
AuCl4− + 3e− = Au + 4Cl− (E◦ = +0.99 V)
Fig. 2. XRD patterns of the 0.8% Au/0.8% Cu/SiO2 catalyst: fresh (a); used (b); after
As far as the standard redox potential of gold is higher than that
of copper, the Au oxidized form can be readily reduced by Cu0, while
Cu0 oxidizes to Cu2+ (CuO). The amount of Cu oxidized by Au3+
should be equal to the molar Au/Cu ratio. To study the reduction
conditions and behavior of supported copper oxide, the method of
temperature-programmed reduction with hydrogen was applied.
The data revealed that the reduction temperatures depended on the
copper content in the catalyst. A two-step reduction behavior of the
copper oxide species with high-temperature peaks (above 600 ◦C)
was observed (see below). The temperature of copper oxide reduc-
tion in the Cu containing samples was chosen as 400 ◦C in order to
avoid sintering of Cu nanoparticles. The TPR-H2 data allowed us to
suppose that copper oxide species are not fully reduced to Cu0, and
it is necessary to add the third possible semi-reaction, as follows:
reduction at 25–850 ◦C (c).
The average size of Au particles in the fresh sample was found to
be 8 nm, after using the catalyst in the reaction for three times and
more there was no increase in the Au crystalline size (Fig. 2b). Fur-
thermore, the reduction of this sample at 25–850 ◦C led to the shift
of the peak at 2Â = 38.2◦ to the position at 2Â = 40.5◦, which corre-
sponds to the AuCu3 alloy (Fig. 2c). The average size of the alloy
crystallites was found to be also 8 nm.
Cu+ − e− = Cu2+ (E◦ = +0.52 V)
(3)
3.2. Characterization of the catalysts by physico-chemical
methods
3.2.1. XRD
The XRD patterns of the catalysts are given in Figs. 1–3. One
can see that for all Au-containing samples the most intense line
is observed at 2Â = 38.2◦, and it can be attributed to the Au(1 1 1)
plane.
For the monometallic 0.8% Au/SiO2 catalyst (Fig. 1), one more
peak appears at 44.4◦, it is observed for both fresh and used sam-
ples. The X-ray line broadening analysis revealed the average gold
particles size to be about 8–10 nm.
The XRD pattern of the 0.8% Au/0.8% Cu/SiO2 sample (Fig. 2a)
Fig. 3. XRD patterns of the 5% Cu/SiO2 sample: fresh (a), used (b); and of the 5%
contains two lines at 2Â = 38.2◦ and 44.4◦ corresponding to Au0.
Au/5% Cu/SiO2 catalyst: fresh (c), used after reduction at 25–850 ◦C (d), used (e).
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Table 1
TPR study results of the Au–Cu samples prepared by a redox method.
Catalyst
Au:Cu
CuO content (mol 106)
H2 consumption (mol 106)
H2: CuO
Tmax (◦C)
0.2% Cu
0.2% Au/0.2% Cu
0.8% Cu
0.8% Au/0.8% Cu
5% Cu
5% Au/5% Cu
–
0.32
–
0.32
–
0.32
5.44
9.46
18.9
18.7
80.1
80.7
26.2
10.6
9.52
47.8
85.3
24.1
4.82
1.12
0.50
2.56
1.06
0.30
380, 609 (shoulder), 639, 662
268, 350, 510, 664
260, 674
280, 550, 677
248, 295, 644
239, 259, 302 (shoulder)
The XRD patterns obtained for the fresh and used 5% Cu/SiO2
catalysts are given in Fig. 3. The intense peaks at 2Â◦ 35.5◦ and 38.7◦
for the fresh sample (Fig. 3a) were ascribed to the (1 1 1) plane of
CuO. The XRD analysis of the used 5% Cu/SiO2 catalyst revealed
only the existence of the Cu2O phase, with an average particle size
of 18 nm (Fig. 3b). During the reaction, a partial reduction of Cu2+
to Cu+ is possible due to the coke formation. Such conversion of the
Cu species was estimated in [24] by the XPS method.
The XRD spectra of the fresh 5% Au/5% Cu/SiO2 sample reveal
resolved sharp peaks of Au0, but surprisingly, there were no signals
of CuO phase (Fig. 3c). Such phenomenon was also observed in [23].
An average size of the Au particles was found to be in the range of
9–10 nm. Employment of the catalyst in the oxidation reaction led
to some changes in the catalyst structure. The XRD pattern of the 5%
Au/5% Cu/SiO2 catalyst used in ethanol oxidation exhibits the peaks
assigned to Au0 and the CuO phase. The peaks attributed to Au and
CuO shifted by 0.5◦ to the higher values of 2Â◦, which pointed to
the alloy formation during ethanol oxidation. This effect was not
found for this sample which was preliminarily reduced before the
catalytic experiment.
of the crystallites (less than a detection limit) or low contents of
the mentioned phases in the samples. No signals assignable to Au0
were detected in the low-loaded samples; this could be caused by
the same reasons.
3.2.2. STEM
STEM images of the 0.8% Au/0.8% Cu/SiO2 sample show that
metal nanoparticles (NPs) are uniformly dispersed on the sup-
port surface (Fig. 4). The wide distribution of metal particles was
observed for both fresh and used samples. The catalytic reaction
caused changes in the distribution of the Au particle size. After
the reaction, a part of the metallic NPs fractions with d = 5–7 nm
and d = 13–15 nm decreased, while a part of the NP fraction with
d = 9–11 nm increased in about 1.5 times. At the same time, the for-
mation of agglomerates with d > 20 nm was observed too. Anyway,
after the reaction the average particle size did not change signifi-
cantly and the most abundant metal NPs size was in the range of
7–9 nm for both the fresh and used samples that was in a good
agreement with XRD data.
The peaks of the CuO phases were not detected in the XRD pat-
terns for the catalyst samples in which the Cu content was less
than 2 wt%, even after catalytic runs or high temperature treatment
under the TPR-H2 conditions. The reason was either a small size
3.2.3. TPR-H2 study
It was found that the reduction behavior of the catalyst samples
in the TPR-H2 study depended on the amount of Cu loaded in the
catalyst. The latter affected the reduction degree of the bimetallic
Fig. 4. STEM images and particle size distribution for the 0.8% Au/0.8% Cu/SiO2 catalyst: fresh (a), used (b).
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molar Au:Cu ratio, 0.32. This fact can be considered as an evidence
of the metallic Au deposition directly on the Cu species when using
the redox method. During the TPR procedure, CuO was reduced
(Fig. 5a), thus resulting in the double peak at lower temperatures,
at 240 ◦C and 263 ◦C, the appearance of which can be explained by
the same reduction processes described above for the 5% Cu/SiO2
catalyst and different Cu-NPs size. The existence of the peak shoul-
der at 310 ◦C and the disappearance of the peak at 640 ◦C obtained
while 5% Cu/SiO2 sample reduction point to more facile reduction
of Cu+ species before their interaction with SiO2 because of direct
Au–CuOx contact takes place.
Quite a different reduction behavior was observed for the sam-
ples characterized by a lower Cu content. The TPR profile of the
0.8% Cu/SiO2 sample (Fig. 5b) shows the two-step reduction. The
first peak at 260 ◦C can be ascribed to the reduction of CuOx clusters
to Cu0 and the partial reduction of the isolated Cu2+ species to Cu+.
The second peak at a significantly higher temperature, 674 ◦C, prob-
ably corresponds to the reduction of isolated Cu+ to Cu0 [30]. The
reduction of the supported copper oxide with 0.8 wt% Cu was not
complete and only 50% of the CuO phase was reducible (Table 1).
This could be accounted for by the presence of the isolated Cu2+
ions, which were more difficult to be reduced in comparison with
the small CuOx particles because of the stronger interaction with
the support [31]. As compared to the sample with a higher cop-
per content, the reduction profile peak of the 0.8% Cu/SiO2 sample
was slightly shifted to the higher temperatures due to the stronger
interaction between the Cu2+ ions and the support.
Two peaks were characteristic of the TPR curves of the 0.8%
Au/0.8% Cu/SiO2 sample (Fig. 5b). Deposition of gold (at the
Au:Cu = 1:3 atomic ratio) on the Cu/SiO2 catalyst led to unusual
excess of hydrogen consumption during the TPR run. The first peak
of hydrogen consumption was observed at 280 ◦C and related with
the reduction of the small CuOx clusters and the large CuO particles,
which occured in the sample, according to the STEM data. The sec-
ond high-temperature reduction peak was detected at 564 ◦C. This
can be ascribed to the peak observed for the 0.8% Cu/SiO2 sample at
674 ◦C shifted to lower temperatures for the Au–Cu catalyst indicat-
ing the facilitated reduction of copper oxide species. It was found
that the molar quantity of hydrogen consumed was twice higher
than the molar amount of the supported copper oxide.
Almost a fivefold excess in hydrogen consumption was detected
in the TPR study of 0.2% Cu/SiO2 (Table 1). Two types of the reduc-
tion peaks were also observed (Fig. 5c). The first peak (at 380 ◦C)
was caused by the reduction of isolated Cu2+ ions interacting with
the SiO2 support. As compared with the 0.8% Cu/SiO2 sample,
the first reduction peak was obtained at higher temperatures due
to stronger interaction between Cu2+ and SiO2. The complicated
reduction behavior was observed at temperatures above 600 ◦C.
Three peaks, at 609, 639, and 662 ◦C, were overlapped. These peaks
can be attributed to the reduction of isolated Cu+ ions to Cu0 as it
was mentioned above and/or to the reduction processes related to
the support itself.
Fig. 5. TPR profiles of the catalysts: (a) 5% Cu/SiO2 (1), 5% Au/5% Cu/SiO2 (2); (b) 0.8%
Cu/SiO2 (1), 0.8% Au/0.8% Cu/SiO2 (2); (c) 0.2% Cu/SiO2 (1), 0.2% Au/0.2% Cu/SiO2 (2).
Au–Cu catalyst after Au deposition. The TPR-H2 results are pre-
sented in Fig. 5.
The TPR-H2 profile of the 5% Cu/SiO2 sample (Fig. 5a) manifests
two intensive peaks: the main peak with a maximum at 250 ◦C and
a shoulder at 295 ◦C and a small peak with a maximum at 640 ◦C.
These peaks were ascribed to the reduction of the CuO nanopar-
ticles (250 ◦C) and to the stepwise reduction of Cu2+ ions to Cu+
(295 ◦C) and Cu+ to Cu0 (640 ◦C) [29]. The amount of the hydrogen
consumption coincides with the amount of copper oxide supported,
thereby indicating the complete reduction of CuO.
While comparing the 0.8% Au/0.8% Cu/SiO2 sample with 0.2%
Au/0.2% Cu/SiO2, in the latter case the Au deposition suppressed the
processes responsible for the additional hydrogen consumption.
The maxima of the peaks were shifted to lower temperatures as a
result of the easier reduction of the Cu2+ ions (Fig. 5c and Table 1).
Anyway, the total reduction behavior of the 0.2% Au/0.2% Cu/SiO2
sample was rather complex and it was difficult to assign all of the
observed peaks to the processes that could proceed on the catalyst
surface.
Noteworthy, these phenomena of the unusually high hydro-
gen uptake by the copper-containing supported systems (with low
loadings) have not been mentioned elsewhere, to the best of our
knowledge. The possible reactions are shown in Scheme 1. One
of the reasons of the effects observed in this study can be the
After deposition of 5 wt% Au on the surface of Cu0 particles in
5% Cu/SiO2 sample by the redox reaction, the hydrogen consump-
tion was observed also, although its value was smaller than for 5%
Cu/SiO2 (Table 1). Following the redox method, reduced copper was
oxidized to Cu2+ by Au3+ in the surface redox reaction. According to
the results, the molar H2: Cu ratio is 0.3, that is almost equal to the
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Fig. 7. FT Cu K EXAFS spectra of CuO (1) and 0.8% Au/0.8% Cu/SiO2 sample (2).
Scheme 1. Possible processes during reduction of CuO/SiO2 and Au–Cu/SiO2 cata-
lysts in TPR-H2 study.
˚
the first coordination shell containing 12 Au atoms at a 2.885 A real
distance from the central Au atom.
hydrogen spillover described for such systems in [32]. For the
bimetallic samples, the sequence of the reduction steps can be
as follows: the dissociative hydrogen adsorption or the hydrogen
spillover occurred on the surface of Au NPs with consequent reduc-
tion of the copper oxide species. Activated hydrogen adsorption
also can take place on the periphery of the Au–Cu contact site,
whereas the hydrogen spillover from Cu0 to silica is possible too.
Dissociative adsorption of hydrogen on the Au–Cu alloy nanopar-
ticles and the reduction of the support surface by the hydrogen
spillover were observed for the Au–Cu/TiO2 catalyst in [33]. In spite
of that, the hydrogen spillover effect is considered to be common
for “d-metal–reducible oxide” systems but not for the “metal-inert
support” such as silica; however, this effect was proved for the
Pt/Al2O3, Pt/SiO2 [34]. Copper is known to be active for the hydro-
gen spillover as well. Thus, as it was described above, after the
reduction of 0.8% Au/0.8% Cu/SiO2 the AuCu3 alloy formation was
observed. This indicated strong interaction between Au–Cu–SiO2
in the reduced sample and could lead to hydrogen spillover and
SiO2 reduction. Probably, in the 0.2% Au/0.2% Cu/SiO2 catalyst the
Cu2+–SiO2 interaction was much stronger than Au–Cu, but it was
weaker than in the 0.2% Cu/SiO2 sample and hydrogen consumption
was detected to be approximately equal to 1.
The FT EXAFS oscillations of the Au-containing sample resem-
bles the spectrum of the Au foil showing the metallic state of gold
in this sample and differs from those of the Au foil in the intensity
of the peaks. The Au spectrum of the sample may be readily fitted
in both r- and k-spaces with a single-shell model – the gold shell
around the central absorbing gold atom. The results of model fits
of the EXAFS spectra are given in Table 2.
The analysis of the EXAFS oscillations shows that the nearest
neighbors of the central Au atom in the samples are the Au atoms
´
˚
with an average coordination number (CN) about 10.8 at 2.85 A
real distances. With the assumption of a spherical shape, the aver-
˚
age maximal sizes in the range of ∼50 A are obtained for the Au
metal nanoparticles in this sample. Noteworthy, the presence of
the long-distance peaks in the EXAFS spectra of the sample under
study points out that the sample contains not only particles with
˚
the diameter 50 A but also larger particles.
3.2.4.2. Cu K-edge. The Fourier transformation of the EXAFS oscilla-
tions of the Cu-containing sample and the reference are presented
in Fig. 7. The spectrum of the sample resembles the spectrum of
CuO, therefore the Cu2+ state predominates in these samples. In the
spectrum of CuO, the first peak corresponds to two first coordina-
Silicide formation is to be considered after reduction of the 0.2%
Cu/SiO2 sample [35]. In addition, very strong interaction of Cu2+
with the support may lead to the reduction of silica and Cu2+ at the
same time. Au deposition could make this interaction weaker.
˚
tion shells containing four Cu atoms at 1.951–1.961 A real distances
from the central Cu atom.
The FT EXAFS oscillations of the Cu-containing sample are close
to that of the CuO reference in terms of the intensity and the posi-
tion of the first peaks. The Cu spectrum of samples may be readily
fitted in both r- and k-spaces with a single-shell model such as an
oxygen shell around the central absorbing Cu atom. The results of
model fits of the EXAFS spectra are collected in Table 2.
The analysis of the EXAFS oscillations shows that the nearest
neighbors of the central Cu atom in the samples consist of O atoms
3.2.4. EXAFS
3.2.4.1. Au L3-edge. The Fourier transformation of the EXAFS oscil-
lations of the Au sample and a reference are presented in Fig. 6. In
the spectrum of the Au foil, the first double peak corresponds to
´
˚
with an average CN of 3.4–3.5 at 1.96–1.98 A real distances. Note-
worthy, the EXAFS spectrum of the Cu sample differs from that of
CuO and this shows that copper species do not form an oxide phase.
Probably they are bound with the support via oxygen of silanol
groups of silica.
2
1
3.2.5. Catalysts testing in ethanol oxidation
The main product of ethanol oxidation over the prepared cat-
alysts was acetaldehyde (AA) and CO2 was obtained as the only
by-product. When the reaction was performed at temperatures
higher than 300 ◦C this led to complete EtOH oxidation to CO2 and
H2O. The maximum AA yields were observed in the temperature
range of 220–250 ◦C and the CO2 selectivity varied from 0% to 30%
depending on the catalysts composition. For all the gold-containing
0
2
4
6
8
Uncorrected distance, A
Fig. 6. FT Au L3 EXAFS spectra of Au foil (1) and 0.8% Au/0.8% Cu/SiO2 sample (2).
Please cite this article in press as: E.A. Redina, et al., Selective oxidation of ethanol to acetaldehyde over Au–Cu catalysts prepared by a redox
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Table 2
EXAFS data for the Au–Cu sample.
Catalyst
Path
r (Å)
CN
ꢁ2 × 10−3 (Å2)
ꢂE (eV)
DAu (Å)
0.8%
Au/0.8% Cu
Au–Au
Cu–O
2.85 0.01
1.97 0.01
10.8 0.2
3.4 0.1
8
6
1
1
6
15
1
1
42.6 8.8
–
100
80
60
40
20
0
2,5
2
0.2%Au
a
a
0.2%Au
0.2%Au/0.2%Cu
0.2%Au/0.2%Cu
(Au:Cu=1:3)
0.2%Au/0.2%Cu-
used
1,5
1
1
2
3
4
0.2%Au/0.5%Cu
(Au:Cu=1:8)
0.2%Au/0.5%Cu
0.2%Au/0.2%Cu-
used
1
2
3
4
0,5
0
100 150 200 250 300 350 400 450
100 150 200 250 300 350 400 450
Temperature, ºC
Temperature, оС
100
80
60
40
20
0
0.6
0.4
0.2
0
0.8%Au
0.8%Au
b
b
0.8%Au/0.8%Cu
5%Au/5%Cu
0.8%Au/0.8%Cu
(Au:Cu=1:3)
0.8%Au/2%Cu
(Au:Cu=1:8)
5%Au/5%Cu
(Au:Cu=1:3)
1
2
3
1
2 3
4
100 150 200 250 300 350 400 450
100 150 200 250 300 350 400 450
Temperature, ºC
Temperature, ºC
Fig. 8. a. Temperature dependence of the ethanol conversion over the catalysts:
0.2% Au/0.2% Cu/SiO2 (1); 0.2% Au/SiO2 (2); 0.2% Au/0.5% Cu/SiO2 (3); 0.2% Au/0.2%
Cu/SiO2 – used (4). (b) Temperature dependence of the ethanol conversion over the
catalysts: 0.8% Au/SiO2 (1); 0.8% Au/0.8% Cu/SiO2 (2); 5% Au/5% Cu/SiO2 (3).
Fig. 9. (a) Temperature dependence of the specific activity for the catalysts: 0.2%
Au/0.2% Cu/SiO2 (1); 0.2% Au/SiO2 (2); 0.2% Au/0.5% Cu/SiO2 (3); 0.2% Au/0.2%
Cu/SiO2 – used (4). (b) Temperature dependence of the specific activity of the cata-
lysts: 0.8% Au/0.8% Cu/SiO2 (1); 0.8% Au/SiO2 (2); 0.8% Au/0.2% Cu/SiO2 (3); 5% Au/5%
Cu/SiO2 (4).
catalysts, the 50% conversion was reached if the reaction tempera-
ture was below 225 ◦C (Fig. 8a and b), 100% selectivity to AA having
been observed. The lowest temperature for the 50% conversion was
characteristic of the 0.2% Au/0.2% Cu/SiO2 sample, namely 188 ◦C
(Fig. 8a). Noteworthy, no products were detected when the reaction
was carried out over the Au/SiO2 catalyst at temperatures below
250 ◦C under the oxygen-free conditions. This proved that the reac-
tion proceeded through the oxygen-assisted dehydrogenation of
EtOH discussed in detail in [36–38].
It follows from the data presented in Figs. 9 and 10 that the
catalysts characterized by the atomic ratio Au:Cu = 1:3 manifest
the higher values of the alcohol conversion and selectivity to AA
in comparison with the monometallic Au and Cu catalysts hav-
ing the same metal loadings. This proved the synergetic effect
In order to compare the efficiency of the prepared catalysts in the
ethanol oxidation, a specific catalytic activity (SCA) was calculated
as a quantity of AA (mol) produced per hour over the unit weight
of Au (g) in each catalyst sample (Fig. 9).
According to our data, the higher was the gold concentration
in the catalyst; the lower was the SCA of the gold nanoparticles
in the Au monometallic catalysts. This was caused by an increase
of the Au particle size detected by the XRD analysis. In the case of
the monometallic Cu catalysts, a higher Cu concentration led to the
higher values of the EtOH conversion but to the lower selectivity
because of the intensification of complete oxidation of EtOH with
formation of CO2. As for bimetallic Au–Cu catalysts, the higher was
the metal loading at the same Au:Cu atomic ratio the lower were
SCA values (Fig. 9 and Table 3). At the same time, an increase in
the copper content in the bimetallic catalysts at the same Au load-
ing (Au:Cu = 1:8) resulted in the drop of the EtOH conversion, the
selectivity to AA, and SCA as well (Figs. 9 and 10).
Fig. 10. Conversion and selectivity of the catalysts at 250 ◦C.
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E.A. Redina et al. / Catalysis Today xxx (2014) xxx–xxx
Table 3
Comparison of the catalysts.
Catalyst
t = 250 ◦
Specific activity,
mol (AA) g−1 Au
(Cu) h−1
C
d (Au/Cu), nm
(XRD data)
Phase
composition
Au/Cu (XRD,
TPR data)
0.2% Au
2.2
2.4
1.25
0.43
0.55
n.d.
n.d.
n.d.
n.d.
0.2% Au/0.2% Cu
0.2% Au/0.5% Cu
0.8% Au
Au0/Cu2+
n.d./CuO, Cu 2+
Au0
8 (15)a
8 (3–5)b/n.d.
0.8% Au/0.8% Cu
Au0/CuO,
Cu2+b, Cu0
Au0/CuO, Cu0
Au0/CuO
0.8% Au/2% Cu
0.8% Au/2% Cu-NaBH4
5% Au/5% Cu
0.37
0.30
0.08
8/n.d.
10/n.d.
10/n.d.
Au0/CuO, Cu2+
Cu0
,
0.2% Cu
2% Cu
5% Cu
0.2% Au/0.2% Cu-TPR
0.2% Au/0.2% Cu-used
0.8% Au/0.8% Cu-TPR
0.8% Au/0.8% Cu-used
0.3
n.d.
11
Cu2+
0.06
0.03
0.8
1
0.32
0.42
CuO, Cu2+
CuO
14 (18)a
n.d.
n.d.
n.d.
8
8/n.d.
Au0/Cu2+
AuCu3
Fig. 11. Stability of the catalysts expressed in terms of the conversion and selectivity
obtained at 250 ◦C.
Au0/CuO, Cu2+
a
After the catalytic test.
According to the EXAFS data.
b
The 0.8% Au/0.8% Cu/SiO2 catalyst appeared to be the most stable
system capable of maintaining its initial activity even after four
runs (Fig. 11). Taking into account the data obtained, it can be con-
cluded that metal nanoparticles with the average size of 8–10 nm
were resistant against sintering under the reaction conditions and
the particle growth proceeds until this value was reached.
Reduction in the hydrogen flow of the low-loaded samples
at 25–850 ◦C led to a decrease in of the SCA, EtOH conversion
and selectivity to AA (Fig. 11). The lower activity of 0.8% Au/0.8%
Cu/SiO2-TPR revealed after the high temperature reduction was
due to the AuCu3 alloy formation, which was proved by XRD anal-
ysis. An average Au and AuCu3 crystalline size in fresh and reduced
samples respectively was the same, about 8–10 nm. So, the dif-
ferences in the activity of the fresh and reduced samples can be
explained only by the different catalyst structure (Table 3). The
higher activity of the silica-supported Au–CuOx hybrid catalysts in
the selective oxidation of ethanol into acetaldehyde in compari-
son with the silica-supported Au–Cu alloy nanoparticles was also
observed in [24]. It should be noted that the same situation was
observed for the Au–Ir/SiO2 catalysts in gas phase ethanol oxida-
tion to acetaldehyde when intimate contact between Au and IrOx
occurred [39].
caused by Au0–Cun+ or Au0–CuOx interactions. One can conclude
from the values of SCA that if the reaction is performed in the
200–250 ◦C temperature range the highest activity was exhibited
by the bimetallic catalyst with the lowest metal loading and the
atomic ratio Au:Cu = 1:3, e.g. 0.2% Au/0.2% Cu/SiO2. This catalyst
provided the complete conversion of ethanol to AA with the 100%
selectivity and a very high value of SCA.
The drop of the initial activity of the bimetallic Au–Cu cata-
lysts with increasing metal loading at the constant Au:Cu atomic
ratio was likely caused by the growth of both metal particles and
transformation of the state of the copper species related with
the increase in the copper loading, which was confirmed by XRD
and STEM analysis. According to the XRD, TPR and EXAFS data, in
the samples with Cu content in the range of 0.2–0.8 wt%, copper
preferably existed in the form of the Cu2+ and Cu+ ions, while the
CuO phase predominated in the samples having the higher cop-
per loading. An increase in the atomic copper content led to the
intensification of the complete oxidation of ethanol and thus to the
lower values of the selectivity to AA and SCA. The obtained phase
composition of the catalysts is presented in the Table 3.
If we compare the 0.8% Au/2% Cu/SiO2 catalyst prepared by the
redox method with the catalyst of the same composition but pre-
pared by the NaBH4-reduction technique, we can notice that the
selectivity to AA and SCA for the latter catalyst is remarkably lower
compared to the former catalyst (Fig. 10 and Table 3).
4. Conclusions
The Au–Cu bimetallic catalysts with the different metal load-
ings were prepared by a very facile surface redox-reaction method.
Defined synthesis of the catalysts by redox method allowed us to
deposit Au NPs directly on the surface of copper oxide or Cu2+
species which was proved by TPR, EXAFS and STEM studies. The
obtained catalytic systems were active in the aerobic gas-phase
ethanol oxidation under the very mild conditions. The low-loaded
bimetallic Au–Cu catalysts did exhibit the highest catalytic activity
with almost full conversion and 100% selectivity to acetaldehyde
in ethanol oxidation. The most noticeable synergetic effect of the
Au–Cu interaction in comparison with the monometallic samples
having the same Au or Cu loadings was observed for the systems
with the Au:Cu atomic ratio of 1:3. Thus, the 0.2% Au/0.2% Cu/SiO2
sample produced about 120 g of AA per 1 g of supported gold per
hour with full conversion and 100% selectivity at the temperature
of 250 ◦C and under atmospheric pressure. Furthermore, the Au–Cu
bimetallic catalysts were very stable under the reaction conditions
in several consecutive runs. The highest stability was shown by 0.8%
Au/0.8% Cu/SiO2.
The most active sample, 0.2% Au/0.2% Cu/SiO2, lost its activity
after the run and the maximum value of SCA can be attained only at
higher temperatures. However, the value of SCA obtained at 250 ◦C
remained close to that of other Au–Cu samples or even higher, the
catalyst keeping this SCA level during further three runs (Fig. 11).
The decrease in the activity of the 0.2% Au/0.2% Cu/SiO2 catalyst can
be accounted for the sintering of the Au nanoparticles with a size
of 3–5 nm, the existence of which was confirmed by EXAFS even
for the sample with a higher Au content. STEM images of the 0.8%
Au/0.8% Cu/SiO2 sample indicated redistribution of the metal parti-
cle size after the catalytic reaction and a decrease of the amount of
the fraction with an average size of 7 nm related with the formation
of larger particles. Nevertheless, the most abundant particles were
still those with the size about 8–10 nm. In addition, interdiffusion
of Cu atoms in the SiO2 outermost atomic layer and disappearance
of the active sites located at the Au–CuOx interface could lead to
a drop in the catalytic activity of the low-loaded samples too [35].
Please cite this article in press as: E.A. Redina, et al., Selective oxidation of ethanol to acetaldehyde over Au–Cu catalysts prepared by a redox
method, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2013.11.065

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9
Elucidation of the catalysts structure by different physico-
chemical methods revealed that the high activity of the low-loaded
Au–Cu catalysts was due to small Au-NPs size and synergetic effect
of the strong Au0–Cu2+ interaction achieved by applying the redox
method for the catalyst preparation, where the Cu2+ species were
isolated ions but not the copper oxide phase. Strong interaction of
isolated Cu2+ ions with the support and Au-NPs caused unexpected
reduction behavior of the low-loaded Au–Cu catalysts.
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(DESY, Germany) for the possibility to perform XAS studies, project
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Please cite this article in press as: E.A. Redina, et al., Selective oxidation of ethanol to acetaldehyde over Au–Cu catalysts prepared by a redox
method, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2013.11.065