Selective oxidation of ethanol over vanadia-based catalysts: The influence of support material and reaction mechanism

Article abstract of DOI:10.1016/j.cattod.2016.04.042

The catalytic performance of vanadia supported on silica, alumina, zirconia, and titania was investigated in the selective oxidation of ethanol. It was shown that the activity and product distribution strongly depend on the support material, which determines the structure of supported vanadia species. On silica and alumina, low-active V₂O₅ crystallites were mainly formed regardless of the vanadium content. These catalysts demonstrated high selectivity toward only acetaldehyde. In contrast, monomeric surface vanadia species and polymeric surface vanadia species were mainly formed over TiO₂ when the vanadium content did not exceed what is necessary for the ideal monolayer. Over zirconia, both the surface vanadia species and the V₂O₅ crystallites existed regardless of the vanadium content. It was found that the surface vanadia species are more active in the selective oxidation of ethanol than the V₂O₅ crystallites. The highest activity was observed for the polymeric vanadia species and, correspondingly, the best catalytic performance was achieved on the monolayer V₂O₅/TiO₂ catalyst. At low temperatures between 110 and 150 °C, this catalyst demonstrated high activity in the oxidation of ethanol to acetaldehyde with the selectivity ranging between 80% and 100%. At temperature near 200 °C, the same catalyst was active in the oxidation of ethanol to acetic acid with the selectivity of approximately 65%. The surface intermediates and the catalyst state were also studied in situ by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It was shown that under reaction conditions near 100 °C, non-dissociatively adsorbed molecules of ethanol, ethoxide species, and adsorbed acetaldehyde exist on the catalyst surface, while at higher temperatures, V₂O₅/TiO₂ is mainly covered with acetate species. Titanium cations remained in the Ti⁴⁺ state, whereas V⁵⁺ cations underwent a reversible reduction under reaction conditions. On the basis of the in situ data complemented by the results of kinetic measurements, a reaction mechanism for the selective oxidation of ethanol to acetaldehyde and acetic acid over the monolayer catalysts was proposed.

Full text of DOI:10.1016/j.cattod.2016.04.042

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Selective oxidation of ethanol over vanadia-based catalysts: The
influence of support material and reaction mechanism
a
a,b,c,∗
, Yu.A. Chesalova,b,c, A.A. Saraeva,b,c
,
T.V. Andrushkevich , V.V. Kaichev
V.I. Buktiyarov
a
a,b
Boreskov Institute of Catalysis, Lavrentiev Ave. 5, 630090 Novosibirsk, Russia
Novosibirsk State University, Pirogova Str. 2, 630090 Novosibirsk, Russia
Research and Educational Center for Energy Efficient Catalysis in Novosibirsk State University, Pirogova Str. 2, 630090 Novosibirsk, Russia
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic performance of vanadia supported on silica, alumina, zirconia, and titania was investigated
in the selective oxidation of ethanol. It was shown that the activity and product distribution strongly
depend on the support material, which determines the structure of supported vanadia species. On silica
and alumina, low-active V2O5 crystallites were mainly formed regardless of the vanadium content. These
catalysts demonstrated high selectivity toward only acetaldehyde. In contrast, monomeric surface vana-
dia species and polymeric surface vanadia species were mainly formed over TiO2 when the vanadium
content did not exceed what is necessary for the ideal monolayer. Over zirconia, both the surface vanadia
species and the V2O5 crystallites existed regardless of the vanadium content. It was found that the sur-
face vanadia species are more active in the selective oxidation of ethanol than the V2O5 crystallites. The
highest activity was observed for the polymeric vanadia species and, correspondingly, the best catalytic
Received 8 January 2016
Received in revised form 25 April 2016
Accepted 27 April 2016
Available online xxx
Keywords:
Heterogeneous catalysis
Ethanol oxidation
Acetaldehyde
Acetic acid
Vanadia
performance was achieved on the monolayer V2O5/TiO2 catalyst. At low temperatures between 110 and
◦
1
50 C, this catalyst demonstrated high activity in the oxidation of ethanol to acetaldehyde with the selec-
◦
tivity ranging between 80% and 100%. At temperature near 200 C, the same catalyst was active in the
oxidation of ethanol to acetic acid with the selectivity of approximately 65%. The surface intermediates
and the catalyst state were also studied in situ by X-ray photoelectron spectroscopy and Fourier trans-
◦
form infrared spectroscopy. It was shown that under reaction conditions near 100 C, non-dissociatively
adsorbed molecules of ethanol, ethoxide species, and adsorbed acetaldehyde exist on the catalyst sur-
face, while at higher temperatures, V2O5/TiO2 is mainly covered with acetate species. Titanium cations
4
+
5+
remained in the Ti state, whereas V cations underwent a reversible reduction under reaction condi-
tions. On the basis of the in situ data complemented by the results of kinetic measurements, a reaction
mechanism for the selective oxidation of ethanol to acetaldehyde and acetic acid over the monolayer
catalysts was proposed.
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
also an industrially important solvent and an intermediate for the
synthesis of a wide range of organic compounds, such as pentaery-
thritol, crotonaldehyde, and pyridine derivatives. The global market
for acetaldehyde is forecast to reach 1.2 million tons by the year
2015. Currently, approximately 85% of acetaldehyde is produced
from ethylene via the Wacker-Hoechst process, 75% of acetic acid
for the chemical industry is produced by the catalytic carbonyla-
tion of methanol and 25% by classical fermentation [2]. All these
processes are liquid-phase, and the development of more effective
gas-phase technologies is one of the most important tasks of the
large-scale chemical industry.
Acetaldehyde and acetic acid are important reagents and indus-
trial chemicals. The global demand for acetic acid, which is mainly
used for the synthesis of vinyl acetate, acetic anhydride, and
acetates, as well as a solvent for the production of purified tereph-
thalic acid, is above 10 million tons per year [1]. Acetaldehyde is
∗ Corresponding author at: Boreskov Institute of Catalysis, Lavrentiev Ave. 5,
6
30090 Novosibirsk, Russia.
E-mail address: vvk@catalysis.ru (V.V. Kaichev).
http://dx.doi.org/10.1016/j.cattod.2016.04.042
920-5861/© 2016 Elsevier B.V. All rights reserved.
0
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2
To solve this task, the catalytic gas-phase oxidation of ethanol by
molecular oxygen (from air) is more attractive because it is an eco-
nomic and environmentally friendly process. Indeed, acetaldehyde,
which is more expensive than ethanol, can be produced with high
effectiveness by the gas-phase oxidative dehydrogenation (ODH)
over supported transitional metal catalysts or by oxidation with O2
over different vanadium and molybdenum based oxides [3–7]. A
number of heterogeneous catalysts have been also reported for the
formation of acetic acid by the oxidation of ethanol in the gas phase:
Mo0.61V0.31Nb0.08Ox/TiO [7], Ce-meso TiO [8], V O5/TiO [9], Mo-
temperature range. In addition, we carried out an in situ study of
the oxidation of ethanol over the most active monolayer V O5/TiO2
2
catalyst using Fourier transform infrared spectroscopy (FTIR) and
near ambient-pressure X-ray photoelectron spectroscopy (XPS).
2. Experimental
2.1. Catalyst preparation
2
2
2
2
Supported vanadia catalysts containing 2–22 wt% V O5 were
2
CeOx/SnO [10], VxM SbO (M = Fe, Al, Ga) [11], and Pt/Al O [12].
2
1-x
4
2
3
prepared by the impregnation of supports (SiO , ␥-Al O , ZrO ,
2
2
3
2
Definitely, the development of new catalytic technologies requires
the understanding of the mechanism for the selective oxidation of
ethanol on the atomic level.
and TiO ) with an aqueous solution of vanadyl oxalate synthe-
2
sized from V O5 (>99.6%, Reachim, Russia) and oxalic acid (>97%,
2
◦
◦
Reachim, Russia). The samples were dried in air at 110 C for 24 h
and then calcined in an air flow (50 ml/min) at 400 C for 4 h.
The oxide catalysts demonstrate high activity in the selec-
tive oxidation of ethanol to both acetaldehyde and acetic acid
under mild conditions. The high selectivity toward acetaldehyde
is usually observed at low temperatures ranging between 100 and
As support materials, we used commercial aerosil SiO2 (>99.6%,
2
Reachim, Russia) with the specific surface area SBET of 200 m /g
2
and TiO2 (anatase, AlfaAesar) with SBET of 350 m /g. ␥-Al O with
2
3
◦
◦
2
00 C, while at 200–250 C, because of the further oxidation of
2
SBET = 250 m /g was synthesized by the calcination of boehmite
acetaldehyde, the reaction shifts toward acetic acid. For example,
over Ce-meso TiO2 catalysts, the selectivity toward acetaldehyde
◦
AlOOH·nH O (n = 0.3–1.0) in air at 550 C for 4 h. ZrO was pre-
2
2
pared by the precipitation of Zr(OH) from a ZrOCl2 solution with
4
◦
achieves 93% at 150 C at the conversion of ethanol of 77% [8]. Sim-
◦
aqueous ammonia at 50 C; final pH was 8.5. The resulting zirco-
nium hydroxide was dried in air at 110 C for 12 h and then calcined
at 400 C for 4 h. Synthesized zirconia was a mixture of the mon-
ilar selectivity toward acetaldehyde was observed over MoOx/TiO2
◦
◦
at 200 C but at low conversions near 17% [7]. Over V0.7M0.3SbO4
◦
(
M = Fe, Al, Ga), the direct oxidation of ethanol to acetaldehyde with
oclinic (85%) and cubic (15%) phases. The specific surface area of
the selectivity above 80% was observed in a wide temperature range
ZrO , which was calculated by the Brunauer–Emmett–Teller (BET)
2
◦
from 150 to 230 C [11]. Multi-component metal oxides (Mo-V-Nb-
2
method, was 120 m /g.
Ox) can catalyze the direct oxidation of ethanol to acetic acid at
◦
2
40 C with the selectivity up to 95% at 100% ethanol conversion [7].
2.2. Catalyst characterization
A special attention has been paid to supported vanadia catalysts,
which demonstrate excellent catalytic performance in the selective
oxidation of ethanol [9]. Depending on reaction conditions, ethanol
can be transformed to acetaldehyde, acetic acid, diethyl ether, ethyl
acetate, ethyl formate, crotonaldehyde, ethylene, or carbon oxides
The catalysts were characterized by elemental analysis, Raman
spectroscopy, N2 adsorption and X-ray diffraction (XRD) tech-
niques. The elemental analysis was performed using an inductively
coupled plasma atomic emission spectrometer (Baird). Powder
XRD measurements were carried out using a Siemens D500 diffrac-
tometer using monochromatic CuK˛ radiation. The 2␪ scan covered
(
CO and CO ). The product distribution is also dependent on the
2
support material and vanadia surface density [10,13–17]; however,
the reason for this effect is still a topic of debate. Recently, we have
shown that the catalytic performance of supported vanadia cata-
lysts in the selective oxidation of methanol to dimethoxymethane
and methyl formate is mainly determined by the structure of vana-
dia species [18,19]. Herein, we demonstrate that this hypothesis is
also applicable for the selective oxidation of ethanol.
◦
a range of 10–70 . The specific surface area was calculated with the
BET method using nitrogen adsorption isotherms measured at liq-
uid nitrogen temperatures with an automatic Micromeritics ASAP
2
400 sorptometer. Raman spectra were obtained on a RFS 100/S
Raman spectrometer (Bruker) using a Nd:YAG laser as an excita-
tion source (␭ = 1064 nm, 100 mW). The laser radiation was focused
onto a spot with a diameter 50 ␮m. Before recording the spectra,
It is well known that vanadia can form different structures over
the surface of oxide supports: monomeric and polymeric species as
well as crystallites of vanadium oxide. As shown by Kilos et al. [14],
the acetaldehyde formation rate during the oxidation of ethanol
over VOx/Al O catalysts depends on the vanadia surface den-
◦
the samples were calcined in air at 400 C for 30 min.
2.3. Catalytic testing
2
3
sity. Polyvanadate surface species supported on alumina exhibit a
somewhat higher rate of ethanol ODH than monovanadate surface
species. In contrast, according to DFT calculations [20], vanadyl-
terminated monomers on CeO (111), that is, VO , are the most
active species. VOx components in the Mo0.61V0.31Nb0.08Ox/TiO2
catalyst [7] are responsible for the high reactivity of this material in
the selective oxidation of ethanol to acetaldehyde and acetic acid
while V-O-Ti linkages are responsible for the unselective oxidation
of acetaldehyde to COx. The contrary point of view was stated by
The steady-state activity of the catalysts was tested at atmo-
spheric pressure in a differential reactor with a flow-circulating
configuration [22]. The reactor was constructed from a Pyrex glass
tube with a 12-mm inner diameter and a 50-mm length. A coax-
ial thermocouple pocket with a 4-mm outer diameter was fitted
in the catalyst bed to control the temperature. The reactor was
placed inside an electric oven. The temperature was controlled
2
2
◦
within ± 0.5 C by a K-type thermocouple. The feed consisted of
Beck el al. [21] who suggested that the V
O
S linkages (where S is
ethanol, oxygen, and nitrogen in the molar ratios of 1:4:15 (5
a cation of a support) take part in the selective oxidation of ethanol
to acetaldehyde over vanadia supported on CeO , Al O , ZrO , and
vol.% C H5OH in air). The catalyst fraction 0.25–0.50 mm was used
2
in the experiments. Concentrations of the reactants and products
were determined with an on-line gas chromatograph equipped
with thermal conductivity and flame ionization detectors. Ethanol,
acetaldehyde (CH CHO), acetic acid (CH COOH), diethyl ether
2
2
3
2
TiO . The authors suggested that the alcohol first adsorbs dissocia-
2
tively, resulting in a breaking of the V
and S-OH species.
O S bond to form ethoxide
3
3
In order to elucidate these conflicting data and to develop the
mechanism for the selective oxidation of ethanol, we synthesized
a series of V O5/SiO , V O5/Al O , V O5/ZrO , and V O5/TiO cat-
((C H5) O), ethyl acetate (CH COO CH2 CH ), crotonaldehyde
2 2 3 3
(CH CH CHCHO), ethylene, water, and CO2 were analyzed with a
3
Porapak T column, while CO, oxygen, and nitrogen were analyzed
with a NaA molecular sieve column. All gas lines from the reactor to
2
2
2
2
3
2
2
2
2
alysts and tested their catalytic activity in a flow reactor in a wide
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◦
a sampling valve were maintained at 120 C to prevent the conden-
sation of reactants and products. Ethanol (A.C.S. reagent grade, 99%
purity) obtained from Aldrich was used in all the experiments. The
selectivity toward each product was calculated as the amount of the
detected product divided by the amount of converted ethanol using
corresponding stoichiometric coefficients. The carbon balance was
9
7 ± 2%.
2.4. In situ XPS and FTIR measurements
In situ XPS experiments were performed at the Innovative Sta-
tion for In Situ Spectroscopy (ISISS) beamline at the synchrotron
radiation facility BESSY II (Berlin, Germany) using the monolayer
V O5/TiO catalyst. The experimental station was described in
2
2
detail elsewhere [23]. In short, the station was equipped with
an electron energy analyzer PHOIBOS-150 (SPECS Surface Nano
Analysis GmbH), a gas cell, and a system of differential pump-
ing, which allowed us to obtain high-quality core-level spectra at
pressures up to 1 mbar. A powder sample was pressed into a thin
self-supporting pellet. The pellet was mounted on a sapphire sam-
ple holder between two stainless steel plates. The first plate had an
aperture of 5 mm in diameter for measuring the core-level spec-
tra of the catalyst surface. The second plate was used for heating
by a NIR semiconductor laser (␭ = 808 nm). The sample tempera-
ture was measured with a K-type thermocouple pressed directly
against the rear of the sample. The flows of ethanol and oxygen
into the gas cell were regulated separately with calibrated mass-
flow controllers (Bronkhorst). The flow rate of ethanol in all the
experiments was approximately 2 sccm. The total pressure in the
gas cell was of 0.25 and 0.5 mbar in the experiments with ethanol
and with an equimolar C H5OH/O mixture, respectively.
Fig. 1. XRD patterns of bulk oxides SiO2 and Al2O3 as well as V2O5/SiO2 and
V2O5/Al2O3 supported catalysts.
2
360 cm 1 (P branch of ꢀ(CO ) band) were used for the detection
−
2
of ethanol, acetaldehyde, acetic acid, CO, and CO , respectively. The
2
concentrations of surface intermediates and gas-phase products
were measured in separate experiments carried out under the same
conditions.
3. Results and discussion
3.1. Structure of catalysts
2
2
The synchrotron worked in the multi bunch hybrid mode that
provided a constant photon flux. The C1s, Ti2p3/2, V2p3/2, and O1s
core-level spectra were recorded at the photon energy of 720 eV.
The charge correction was performed by setting the Ti2p3/2 peak
at 459.0 eV [24]. The curve-fitting was done with the CasaXPS soft-
ware. The core-level spectra were resolved into their components
after a Shirley-type background subtraction. The line-shape of each
component was considered to be a product of Lorentzian and Gaus-
sian functions.
In full agreement with previous studies (see Ref. [24] and refs
therein), we have found that vanadia over SiO , Al O , ZrO , and
2
2
3
2
TiO2 can form the monomeric vanadia species VOx, the polymeric
vanadia species (VOx)n, and crystallites of vanadium oxide. The rel-
ative concentrations of these vanadia forms depend on the support
material and the vanadium content. The main characteristics of the
catalysts under study and their designations are summarized in
Table 1. Because of a weak vanadia-support interaction, the surfaces
of alumina and silica contain only V O5 crystallites, regardless of
2
To identify reaction intermediates involved in the oxidation
of ethanol, FTIR spectra were obtained in situ with a Cary 660
FTIR spectrometer (Agilent Technologies). The experiments were
performed using vanadia-titania catalysts that contained 10% and
the vanadium content [18,25]. The XRD patterns of these catalysts
exhibit well-defined reflections of the V O5 phase (Fig. 1), whereas
2
the core-level spectra of these catalysts demonstrate low V/Si and
V/Al atomic ratios even at the high vanadium content [18]. This
finding clearly indicates the low concentration of the surface vana-
dia species. Moreover, in the Raman spectra, only two sharp peaks
2
0% of V O5. The spectrometer was operated in the transmission
2
mode using a specially designed quartz cell-reactor with BaF win-
2
3
−1
dows. The volume of the cell-reactor was approximately 1.5 cm .
at 702 and 994 cm were observed (spectra not shown), which are
The catalyst powder (35–50 mg) was pressed into a thin self-
typical of crystalline V O5 [26]. The peaks were assigned to lattice
2
2
supporting pellet (∼15 mg/cm , 1 × 3 cm in size) and placed into
vibrations of the orthorhombic polymorph of V O5 [18].
2
the cell-reactor. The FTIR experiments were performed at atmo-
In contrast, in the V O5/TiO catalysts, mainly the surface vana-
2 2
spheric pressure using a feed of 1.5 vol.% C H5OH in air flowing at
dia species were observed. According to the literature [27–29], due
to strong vanadia-suport interaction, at the vanadia content under
10% of a monolayer (ML), unhydrous TiO2 contains only isolated
monomeric species with a tetrahedral coordination. Polymeric
structures such as chains and ribbons of VOx units with an octahe-
dral coordination appear at the vanadia concentration above 20% of
the monolayer [28]. When the vanadium content exceeds what is
2
5
0 sccm. Ethanol was dosed by bubbling air through a glass satu-
◦
rator filled with liquid ethanol at 0 C. As a result, the molar ratios
C H5OH:O :N were approximately 1:14:52. Before the exposure
2
2
2
to the reactant mixture, the sample was treated in a flow of air at
◦
2
50 C for 1 h. Subsequently, the cell-reactor and the catalyst sam-
ple were cooled to a desired temperature, and the air flow was
replaced with the flow of the ethanol/air mixture. The FTIR spec-
necessary for the ideal monolayer, V O5 crystallites are favorable.
2
−
1
tra were recorded in a range of 1100–4000 cm at a resolution of
These crystallites are characterized by weak bonding to TiO2 [29].
The monolayer coverage of polymerized vanadia species on differ-
−
1
4
6
cm
.
The spectra of the gas phase were also recorded using a Cary
60 FTIR. In these experiments, a special gas-cell with the optical
ent oxides was measured by Raman spectroscopy, and for TiO , it
2
was found to be approximately 7.9 vanadium atoms/nm2 [30]. In
path length of approximately 70 mm was connected to the out-
full agreement with this model, the V O5 phase was detected by
2
−1
let of the cell-reactor. Bands at 1065 cm
band), 1760 cm (P branch of ꢀ(C O) band), 1790 cm (P branch
(Q branch of ꢀ(C O)
XRD in the 20VTi catalyst, which contained approximately 1.5 ML
of vanadia, and only reflections of TiO2 (anatase) were observed in
the XRD patterns of the 2VTi and 10VTi catalysts in which the sur-
−1
−1
of ꢀ(C O) band), 2115 cm 1 (R branch of ꢀ(C O) band), and
−
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Table 1
Characteristics of catalysts under study.
2
Sample
V2O5 content, wt%
SBET, m /g
Surface concentration of
vanadium , atom V/nm
Phase composition
Relative concentration
of vanadia formsb
a
2
1
2
2
1
2
1
7
3
5VSi
0VAl
0VTi
0VTi
VTi
7VZr
VZr
.4VZr
15
22
20
10
2.0
17
7.1
3.4
134
146
111
153
135
96
10.3
9.9
11.7
4.2
0.9
11.9
3.9
V2O5, SiO2
100% V2O5
V2O5, ␥-Al2O3
V2O5,TiO2 (anatase)
TiO2 (anatase)
TiO2 (anatase)
V2O5, ZrO2
∼100% V2O5
∼30% V2O5∼70% (VOx)n
100% (VOx)n
100% VOx
∼80% V2O5∼20% (VOx)n
∼30% V2O5∼70% (VOx)n
100% VOx
120
122
V2O5, ZrO2
ZrO2
c
1.9
a
b
c
Refers to the specific surface area.
The data obtained from the results of Raman spectroscopy and XRD study.
In this catalyst, the V2O5 phase was removed selectively by washing in HNO3 [19].
Fig. 3. XRD patterns of bulk ZrO2 and V2O5/ZrO2 supported catalysts.
2
species on ZrO2 was found to be 6.8 vanadium atoms/nm . The
Fig. 2. Raman spectra of bulk oxides V2O5 (1) and TiO2 (2) as well as supported
catalysts 2VTi (3), 10VTi (4), and 20VTi (5).
Raman spectra of the 7VZr and 17VZr catalysts contain two peaks
at 617 and 632 cm due to the lattice vibrations of the monoclinic
−
1
−
1
polymorph of ZrO , and two peaks at 702 and 994 cm
due to
2
crystalline V O5. In addition, the Raman spectra of these catalysts
face concentration of vanadium was 0.11 and 0.53 ML, respectively
2
−
1
exhibit two peaks at 930 and 1030 cm
tion of V M (M = V or Zr) and a stretching vibration of vanadyl
O bonds in the surface vanadia species, respectively [18,19]. The
due to a bridge vibra-
[
18].
The presence of the surface vanadia species in the 2VTi and
0VTi catalysts was confirmed by Raman spectroscopy. The spectra
O
V
1
−
1
relative intensity of the peaks at 994 and 1030 cm allowed us
to estimate the relative concentration of different vanadia forms.
Correspondingly, only 20% of vanadia exists in the form of sur-
face vanadia species in 17VZr, whereas in 7VZr, the part of surface
vanadia is approximately 70% (Table 1). In addition, the 3.4VZr cat-
alyst, which contains only surface vanadia species, was prepared by
of the 2VTi, 10VTi, and 20VTi catalysts, as well as the spectra of bulk
oxides V O5 and TiO , are presented in Fig. 2. One can see that in
2
2
−
1
the range between 550 and 1100 cm , the spectrum of TiO2 con-
tains only a single sharp peak at 640 cm . The spectra of supported
catalysts contain an extra broad peak near 830 cm , which can
be attributed to the surface vanadia species. Indeed, broad peaks
near 840–940 cm , which were assigned to the monomeric and
polymeric vanadia species, have been observed in similar systems
−
1
−
1
−1
washing the 7VZr catalyst in an aqua solution of HNO . Earlier, we
3
have shown that such treatment removes V O5 crystallites, while
2
the surfaces vanadia species remain unchanged [24]. Indeed, no
[
9
31–34]. The spectrum of V O5 contains two sharp peaks at 700 and
2
−
1
reflections of the V O5 phase were observed in the XRD pattern
94 cm . The first peak corresponds to lattice vibrations localized
V bridge in the V O5 structure, while the second
2
after this treatment (Fig. 3).
within the V
O
2
peak corresponds to the stretching vibration of vanadyl V O bonds
[
7
35]. The spectrum of the 20VTi catalyst contains weak peaks at
00 and 994 cm , indicating the formation of bulk vanadium oxide
3.2. Catalytic performance
−1
over the titania surface at the high vanadia content.
The catalytic properties of the catalysts were tested at atmo-
spheric pressure in the temperature range between 130 and 250 C.
◦
Because of a specific interaction between vanadia and zirco-
nia, all the vanadia species mentioned above are formed over ZrO2
regardless of the vanadium content. This question was considered
It was found that, depending on the reaction temperature and the
contact time, ethanol can be transformed with different selectiv-
ity to acetaldehyde (AA), acetic acid (AcA), diethyl ether (DEE),
ethyl acetate (EA), crotonaldehyde (CA), ethylene (C H ), and car-
in detail elsewhere [18,19]. Indeed, the V O5 phase was detected by
2
XRD in both catalysts 7VZr and 17VZr in which the surface concen-
tration of vanadium was approximately 0.6 and 1.8 ML, respectively
2
4
bon oxides (COx). The results of the catalytic tests of the V O5/SiO2
2
(Fig. 3). These values were estimated on the basis of previous Raman
and V O5/Al O catalysts are presented in Fig. 4. Both catalysts
2
2
3
studies [30], where the monolayer coverage of surface vanadia
demonstrate high selectivity toward acetaldehyde even at low
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Fig. 4. Effect of reaction temperature on the conversion of ethanol and the selectivity to acetaldehyde, acetic acid, ethyl acetate, and carbon oxides over V2O5/SiO2 and
V2O5/Al2O3 catalysts.
Fig. 5. Effect of reaction temperature on the conversion of ethanol and the selectivity to acetaldehyde, acetic acid, ethyl acetate, and carbon oxides over V2O5/TiO2 and
V2O5/ZrO2 catalysts with high vanadia content (17VZr and 20VTi).
◦
temperatures. Over the V O5/SiO2 catalyst, the selectivity toward
acetaldehyde achieves 98% at 150 C. An increase in the reaction
Near 200 C, the main product is acetic acid. The maximal selec-
2
◦
tivity toward acetic acid is approximately 62% over the V O5/TiO
2
2
◦
temperature leads to an increase in the rate of the oxidation of
ethanol over both catalysts, which is accompanied with a decrease
in the selectivity toward acetaldehyde and results in the appear-
ance of the other products mentioned above (Table S1 in the
Supplementary data). The conversion of ethanol also increases with
the reaction temperature even at the same contact time.
catalysts at 220 C. At higher temperatures, the reaction shifts to
the nonselective oxidation of ethanol to carbon oxides. The selec-
tivity toward ethylene, diethyl ether, and crotonaldehyde does not
exceed 0.7%, 1.3%, and 7.2%, respectively (Table S2). The maximal
selectivity toward ethyl acetate achieves 14.5% over V O5/TiO at
2
2
◦
180 C.
The V O5/TiO2 and V O5/ZrO2 catalysts are more active in the
It should be noted that the V O5/SiO and V O5/Al O catalysts
2
2
2
2
3
2
2
demonstrate unsatisfactory activity in the selective oxidation of
ethanol to acetic acid. For example, in both cases, the selectivity
toward acetic acid does not exceed 5% at 200 C even at the conver-
selective oxidation of ethanol to acetaldehyde than V O5/SiO and
2 2
◦
V O5/Al O . At 150 C, the rate of oxidation of ethanol normal-
2
2
3
◦
ized to the number of supported vanadium atoms is 0.34 and
1.24 mmol/s over V O5/ZrO2 and V O5/TiO2 (Table S2), respec-
sion of ethanol above 70%. The maximal selectivity toward acetic
2
2
◦
acid achieves 22.6% over V O5/SiO at 250 C, which is even lower
tively, while over V O5/SiO2 and V O5/Al O the rate is distinctly
2
2
2 2 2 3
than the selectivity toward acetaldehyde. At higher temperatures,
the major products are carbon oxides with the total selectivity
above 50%. The V O5/Al O catalyst demonstrates similar catalytic
lower (Table S1). In comparison with the rate over the V O5/TiO2
2
catalyst, the rate over V O5/SiO2 and V O5/Al O is 10–20 times
2
2
2
3
lower. The catalytic activity increases with the reaction tempera-
2
2
3
performance apart of higher selectivity toward ethyl acetate, which
achieves approximately 18% at 250 C.
ture. Over the more active V O5/TiO2 catalyst, the normalized rate
2
◦
of oxidation of ethanol increases from 0.34 to 4.1 mmol/s when the
◦
The results of the catalytic tests of the V O5/TiO and V O5/ZrO
reaction temperature increases from 130 to 230 C. It should be
2
2
2
2
catalysts with high vanadia content are presented in Fig. 5. These
catalysts also demonstrate high selectivity toward acetaldehyde at
low temperatures, which achieves approximately 90% at 130 C.
An increase in the reaction temperature leads to an increase in the
conversion of ethanol and to changes in the product distribution.
noted that the rate of the selective oxidation of ethanol over bare
supports is lower by 2 orders of magnitude than that over vanadia
supported catalysts. Moreover, acetic acid is not detected among
the products of the oxidation of ethanol over bare supports in the
entire temperature range. It means that the selective oxidation of
◦
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Fig. 7. in situ FTIR spectra of the 10VTi catalyst obtained at different temperatures
under the flow of the following mixtures: 1.5 vol.% C2H5OH in air (a) and 1.5 vol.%
C2H5OH in He (b).
Table 2 shows the results of the catalytic tests of more active
sub-monolayer 2VTi and 10VTi catalysts, containing 0.11 and 0.53
ML of vanadia, respectively. On the one hand, these data illustrate
in more detail the data presented in Fig. 6 to confirm our conclu-
sion about the higher activity of the polymeric vanadia species in
the selective oxidation of ethanol in comparison with the isolated
monomeric species. On the other hand, one can see that the selec-
tivity toward acetaldehyde can reach 100% over the catalyst with
Fig. 6. Rate of the oxidation of ethanol vs. the surface concentration of vanadium
atoms over V2O5/ZrO2 (1) and V2O5/TiO2 (2) catalysts at the reaction temperature of
◦
◦
1
50 C and conversion of ethanol of 40% (a) and at 200 C and conversion of ethanol
of 70% (b).
ethanol to both acetaldehyde and acetic acid proceeds mainly on
supported vanadia species.
◦
To compare the activity of different vanadia forms, we tested
the V O5/TiO and V O5/ZrO2 catalysts with different vanadia
the sub-monolayer vanadia coverage at 110 C. An increase in the
reaction temperature leads to a decrease in the selectivity toward
acetaldehyde, which is accompanied with an increase in the con-
version of ethanol. Simultaneously, the selectivity toward acetic
2
2
2
contents in the selective oxidation of ethanol. The experiments
◦
were performed at 150 and 200 C, when the main products were
◦
acetaldehyde and acetic acid, respectively. The results are pre-
sented in Fig. 6. In these experiments, we used three types of
the catalysts with the coverage of vanadia species near 1–2, 4,
acid also increases, achieving 65% at 200 C. It means that the oxi-
dation of ethanol proceeds successively: first, ethanol oxidizes to
acetaldehyde and then acetaldehyde oxidizes to acetic acid. This
question is considered in detail elsewhere [13].
2
and 12 vanadium atoms per nm . In accordance with the XRD
and Raman spectroscopy results (Table 1), these catalysts contain
mainly the isolated monomeric species, the polymerized vanadia
species, and a mixture of the polymerized vanadia species and
Hence, the monolayer vanadia-titania catalysts are perspective
for the selective oxidation of ethanol to acetaldehyde at low tem-
peratures in the range of 100–150 C, and these catalysts can be
◦
the V O5 crystallites, respectively. One can see that the catalysts
with the coverage of vanadia species near 4 V atom/nm demon-
used for the selective oxidation of ethanol to acetic acid at higher
temperatures near 200 C.
2
2
◦
strate the maximal activity in the oxidation of ethanol to both
acetaldehyde and acetic acid (Fig. 6). It indicates that the surface
polymeric vanadia species have higher activity in the selective oxi-
dation of ethanol to acetaldehyde and acetic acid than the isolated
3.3. Mechanistic study
To understand clearly why different vanadia species have differ-
ent reactivity in the selective oxidation of ethanol, the elucidation
of the reaction mechanism is needed. The application of in situ
FTIR spectroscopy has allowed the identification of the key inter-
mediates formed on the catalyst surface during the reaction. Then,
we studied the catalyst state directly in pure ethanol and in the
monomeric species and the V O5 crystallites. This finding agrees
2
well with a conclusion by Kilos et al. [14], who showed that poly-
vanadate domains over ␥-Al O3 are more reactive in the oxidative
2
dehydrogenation of ethanol to acetaldehyde than monovanadate
structures.
Summarizing the catalytic data, we can conclude that the cat-
alytic performance of the vanadia-based catalysts in the selective
oxidation of ethanol is determined by the vanadia-support inter-
action. For alumina and silica, the vanadia-support interaction is
ethanol/O mixture using in situ X-ray photoelectron spectroscopy.
2
First, we performed an in situ FTIR study using the 10VTi and
20VTi catalysts. Fig. 7 shows the FTIR spectra for the 10VTi catalyst
◦
collected at 100, 150, and 230 C under the flow of two reactant
weak and, as a result, mainly the low-active V O5 crystallites are
mixtures. The reactant mixtures contained 1.5 vol.% C H5OH in air
2
2
formed on the catalyst surface regardless of the vanadium con-
tent. Definitely, the effective surface of vanadia in this case is also
low. In contrast, the chemical properties of the titania surface pro-
vide the strong vanadia-support interaction and, as a result, the
and 1.5 vol.% C H5OH in He. The spectrum from the catalyst before
the exposure to the reaction mixture and the spectrum from gas-
phase ethanol were subtracted from the raw spectra to identify the
2
contributions of the adsorbed surface species.
◦
V O5 crystallites start to form only after completion of the mono-
2
In the spectrum obtained at 100 C in the C H5OH/air flow
2
layer coverage. Zirconium oxide is characterized by intermediate
strength of the vanadia-support interaction, and thus its surface
(Fig. 7a), positive bands appear at 2976 (ꢀas(CH )), 2933 (ꢀas(CH )),
3
2
2876 (ꢀs(CH )), 1730 (ꢀ(C O)), 1678 (ꢀ(C O)), 1532 (ꢀas(COO)),
2
contains a mixture of the surface vanadia species and the V O5
crystallites regardless of the vanadium content.
1444 (ꢀs(COO)), 1383 (␦s(CH )), 1267 (␦(OH)), 1144 (␳(CH )), and
1090 (ꢀ(CCO)) cm . The assignment of the bands is based on the
2
3
3
−
1
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Table 2
Conversion of ethanol, selectivity toward main products, and rate of oxidation of ethanol over sub-monolayer V2O5/TiO2 catalysts.
◦
−1
−1 −1
T,
C
t,g s mL
X, %
Selectivity, %
AA
r, C2
Н
5OН mmol N
s
AcA
0
4.06
5.72
COx
EA
C2
Н
4
CA
DEE
2
1
1
2
VTi
50
80
00
1.02
1.02
1.02
35.7
65.1
73.3
73.3
55.7
31.6
9.47
18.8
56.3
8.17
10.4
4.02
0.21
0.28
0.51
8.51
10.5
1.74
0.37
0.26
0.18
3.30
5.48
8.23
1
1
1
1
1
1
1
2
0VTi
10
30
50
50
80
80
00
0.38
0.38
0.12
0.38
0.12
0.38
0.05
8.3
21.5
40.0
66.9
70.5
90.0
72.0
100
0
0
0
0
5.11
7.26
9.0
10.2
10.7
0
0
0.11
0.2
0.04
0.10
0.19
0.25
0
7.31
7.10
10.8
3.5
0
0
0.44
1.15
6.58
3.6
14.6
4.8
89.6
82.0
67.9
39.9
33.5
20.5
3.02
5.02
6.64
11.5
9.37
7.2
3.25
6.89
35.5
44.3
65.0
0.3
0.48
0.50
0.44
0.55
2.12
2.6
32.3
Fig. 8. Infrared-absorption intensities of ethanol and the main detected products − acetaldehyde, acetic acid, CO, and CO2 (a, b) as well as the main reaction intermediates
adsorbed on the catalyst surface measured during the oxidation of ethanol on the 10VTi catalyst as a function of temperature. The data (a, c) are obtained in the ethanol/air
mixture; the data (b, d) are obtained in the ethanol/He mixture.
literature data [36–40]. The ethoxide species (C Н5O V+n) are
−
acetaldehyde [36,37]. Both these bands decrease in intensity with
an increase in the reaction temperature. Other bands developed
with heating the catalyst could be attributed to adsorbed acetate
complexes and acetic acid. Bands at 1532 and 1444 cm could be
assigned respectively to ꢀas(COO) and ꢀs(COO) modes of surface
2
◦
the main adsorbed form of ethanol at 100 C. They are generated
as a result of the dissociative adsorption of ethanol. The bands
at 2976, 2933, 2876, 1383, 1144, and 1090 cm are assigned to
vibrational modes of these species. The weak band at 1267 cm is
−
1
−1
−
1
assigned to ı(OH) vibration of molecularly adsorbed ethanol. The
strong bands at 1730 and 1678 cm can be assigned to adsorbed
acetate complexes [36–38]. The bands of acetate species progres-
sively increase in intensity with heating to around 200 C and
−1
◦
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◦
then decrease upon heating between 200 and 250 C. Molecularly
−
1
adsorbed acetic acid is represented by the band at 1662 cm due to
the carbonyl stretching mode [37]. This band appears in the spec-
trum at 130 C and its intensity decreases with the temperature.
◦
The temperature dependence of the main bands is shown in Fig. 8c.
Fig. 7b presents in situ FTIR spectra obtained at the same man-
◦
ner in the C H5OH/He flow. The spectrum obtained at 100 C is
2
similar to the spectrum collected at the same temperature in the
presence of dioxygen (see Fig. 7a, spectrum 1). The only differ-
ence is the appearance of a rather narrow band with the maximum
−1
at approximately 1642 cm . The band could be assigned to the
C stretching mode of adsorbed crotonaldehyde [41]. It is well
C
known that the position of the ꢀ(C O) band in the infrared spec-
trum of this complex is likely to be very close to the band of
adsorbed acetic aldehyde [41]. Crotonaldehyde is the product of
acetic aldehyde condensation. Acetate complexes are the main sur-
face species formed under interaction of ethanol with the catalyst
◦
at 130–250 C. The surface concentration of acetates is much higher
in the absence of O2 than in its presence (Fig. 8a). Adsorbed acetic
Fig. 9. in situ FTIR spectra of the 20VTi catalyst obtained at different temperatures
under the flow of the mixture of 1.5 vol.% C2H5OH in air. The spectra in the region
of vanadyl groups are presented in the inset.
−
1
acid is not formed in the absence of O . A strong band at 1776 cm
2
−
1
and a weak band at 1850 cm are also observed in the spectra at
30–250 C. We suppose that these bands could be correspondingly
◦
1
◦
assigned to asymmetric and symmetric ꢀ(C O) modes of adsorbed
maleic anhydride, which is the product of crotonaldehyde oxida-
tion [41]. The temperature dependence of the intensities of main
bands is shown in Fig. 8d.
gaseous phase at 120 C. Its concentration achieves a maximum at
◦
200 C, while acetic acid and carbon oxides appear in the spectra at
180–220 C (Fig. 10b).
◦
Hence, the same surface intermediates are formed during the
oxidation of ethanol on both 10VTi and 20VTi catalysts. However,
these surface complexes exhibit a significantly different reac-
tivity. Indeed, on the surface of the monolayer 10VTi catalyst,
acetate complexes are generated at considerably lower tempera-
tures. Accordingly, this catalyst shows a significantly higher activity
in the formation of acetic acid as compared with a catalyst compris-
Fig. 8a and b demonstrate the temperature dependence of the
concentration of gaseous components (ethanol and products) at the
outlet of the IR cell with the 10VTi catalyst. Ethanol concentration
◦
decreases after the heating above 100 C, indicating that ethanol is
oxidized under these conditions. The intensity of the acetaldehyde
◦
signal is under the detection threshold below 90 C. At higher tem-
◦
peratures, it strongly increases, achieving a maximum at 150 C.
ing crystalline V O5.
2
At higher temperature, acetic acid, CO, and CO2 appear among the
products. The yield of acetic acid achieves a maximum near 200 C,
The in situ XPS study with the monolayer V O5/TiO catalyst was
performed under similar conditions: in pure ethanol, i.e. without
2
2
◦
while the yields of CO and CO2 increase with the reaction temper-
ature. Thus, the dynamics of transformation of surface complexes
and the changes in the concentration of gas-phase products indicate
the consecutive transformation of ethanol into acetaldehyde and
oxygen in the gas phase, and in the equimolar C H5OH/O2 mix-
2
ture. The results are presented in Figs. 11 and 12, respectively. In
both cases, before the exposure to the reactant mixture or ethanol,
◦
the catalyst was pretreated in 0.25 mbar of flowing O2 at 350 C
acetic acid. In the absence of O , acetaldehyde is the only detectable
for 30 min directly inside the XPS reaction gas cell. This pretreat-
ment led to the complete oxidation of vanadium. As a result, only
a narrow single peak at 517.7 eV which can be attributed to V
2
gaseous product of ethanol transformation (see Fig. 8b). It appears
◦
5+
at 90 C, and its concentration increases with the temperature ele-
vation. Acetic acid and carbon oxides are not formed in the absence
of O2 in the reactant mixture.
cations was observed in the V2p3/2 spectra. According to the litera-
ture data [24,42–47], bulk and supported V O5 are characterized by
2
The transformation of ethanol on the 20VTi catalyst, which con-
the V2p3/2 binding energy in the range of 517.0–517.7 eV, whereas
the V2p3/2 binding energies of V O and V O are within the ranges
tains the V O5 crystallites, was also investigated by in situ FTIR
2
2
4
2
3
spectroscopy. Fig. 9 demonstrates the FTIR spectra of this catalyst
collected at 100, 150, and 230 C under the flow of the mixture
of 516.0–516.5 and 515.8–515.9 eV, respectively. The treatment in
O led the complete burning of surface contaminations, and the C1s
2
◦
containing 1.5 vol.% C H5OH in air. Fig. 10 demonstrates the tem-
spectrum contains no peaks. This finding indicates high ability of
the surface polymeric vanadia species to activate dioxygen from
the gas phase.
2
perature dependence of the coverage of the catalyst by the main
surface species (a) and the concentration of the main gaseous com-
ponents at the outlet of the IR cell (b). The ethoxide species and
molecularly adsorbed ethanol are detected only in the spectrum
The treatment of the pretreated V O5/TiO2 catalyst in the
2
5+
4+
3+
ethanol flow led to the complete reduction of V to V and V
◦
recorded at 100 C. The formation of the ethoxide species is accom-
even at low temperatures. Indeed, the V2p3/2 spectra consist of two
peaks at 516.5–516.6 and 515.4–515.5 eV, which can be attributed
panied with a decrease in the intensity of the band assigned to the
−
1
4+
3+
3+
first overtone of ꢀ(V O) at 2036 cm (Fig. 9) and with the appear-
to V and V cations, respectively. The amount of the V cations
grows slightly with temperature. The subsequent treatment in oxy-
−
1
ance of a broad band at 3500–2900 cm due to the O H stretching
mode of H-bonded hydroxyl groups. Thus, we believe that in this
process, the proton from the alcohol hydroxyl group is transferred
◦
gen at 350 C led again to the complete oxidation of vanadium to
5+
V
. In contrast, no changes were detected in the Ti2p3/2 spectra
◦
to the vanadyl oxygen atom. When the temperature rises to 150 C,
under all used conditions: the spectra consist of only a narrow peak
at 459.0 eV, which is typical of bulk TiO2 [24]. It means that tita-
nium in the catalyst support remains in the Ti4+ state under reaction
conditions.
the concentration of ethoxide species decreases, and the bands due
to vibrational modes of adsorbed acetaldehyde and crotonaldehyde
appear in the spectra. With the further increase in the temper-
◦
ature to 230 C, the bands of acetate complexes dominate in the
The C1s spectra obtained in situ during the heating of the mono-
spectra. The bands of adsorbed maleic anhydride are also observed
in the spectrum collected at 230 C. Acetaldehyde appears in the
layer V O5/TiO2 catalyst in ethanol are well described with four
peaks at 284.5, 285.2, 286.4, and 289.1 eV (Fig. 11). Two strong
2
◦
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Fig. 10. Infrared-absorption intensities of ethanol and the main detected products − acetaldehyde, acetic acid, and CO (a) as well as the main reaction intermediates adsorbed
on the catalyst surface (b) measured during the oxidation of ethanol on the 20VTi catalyst as a function of temperature.
◦
Fig. 11. Normalized V2p3/2, Ti2p3/2, and C1s core-level spectra of the monolayer V2O5/TiO2 catalyst: spectra 1 are obtained in 0.25 mbar flowing O2 at 350 C; spectra 2–5
◦
are obtained in 0.25 mbar flowing ethanol at 110, 150, 200, and 250 C, respectively.
peaks at 285.2 and 286.4 eV could be assigned to two chemi-
cally distinct carbon atoms of molecularly adsorbed ethanol and
of the surface ethoxide species. At least at low temperatures, both
these species were detected by FTIR (Fig. 8c). The first peak cor-
responds to carbon atoms in the methyl groups, while the second
peak corresponds to carbon atoms bonded with oxygen atoms. This
assignment is based on an XPS study by Holroyd et al. [48], who
observed two similar C1s peaks at 285.0 and 286.0 eV for ethanol
adsorbed molecularly on Pd(110). The surface ethoxide species on
TiO2 is characterized by similar C1s peaks at 285.5 and 286.8 eV
peak decreases with temperature because of the decomposition of
acetate or carbonate species. The C1s peak at 284.5 eV is attributed
to different adsorbed CHx species (x = 0–3) produced by the C
C
bond breaking in the ethoxide species and by the dehydrogenation
◦
of methyl groups [51–53]. At low temperatures near 100 C, when
the rate of the C C bond breaking is small, the intensity of the C1s
peak at 284.5 eV is negligible. An increase in the reaction tempera-
ture leads to an increase in the rate of this process and, as a result,
the intensity of this C1s peak also increases.
The V2p3/2 spectra obtained in situ under the C H5OH/O2 mix-
2
[
49]. Definitely, other species, such as adsorbed CO, which are char-
ture contain two peaks at 517.6–517.7 and 516.4–516.5 eV that can
be attributed to V5+ and V , respectively (Fig. 12). An exception
4+
acterized by the C1s binding energy of approximately 285.2 eV,
may exist on the surface at low temperatures. The peak at 289.1 eV
could be assigned to surface acetate, which is characterized on TiO2,
for example, by the C1s peak near 290 eV [49]. The presence of
carbonate species, which are characterized by similar C1s bing-
ing energies cannot be excluded, as well [50]. The intensity of this
◦
is the spectrum obtained at 50 C (spectrum not shown), where an
3+
additional weak peak due to V is observed at 515.7 eV. These data
indicate that ethanol reduces V5+ to V and V ; however, in the
3+
4+
3+
4+
presence of O in the gas phase, the fast re-oxidation of V and V
2
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0
◦
Fig. 12. Normalized V2p3/2 and C1s core-level spectra of the monolayer V2O5/TiO2 catalyst: spectra 1 are obtained in 0.25 mbar flowing O2 at 350 C; spectra 2–5 are obtained
◦
under a flow of the equimolar C2H5OH/O2 mixture at 0.5 mbar during stepwise heating at 110, 150, 200, and 250 C, respectively.
cations occurs. Again, the Ti2p3/2 spectra consist of the single sharp
peak at 459.0 eV, which corresponds to Ti (spectra not shown).
The C1s spectra consist of five peaks at 284.5, 285.2, 286.1,
OH groups. Because the formation of the ethoxide species is accom-
panied with a decrease in the bands due to ꢀ(V O) and because the
IR spectra exhibit the strong band due to H-bonded hydroxyl groups
(Fig. 9), we believe that the chemisorption of ethanol is a heterolytic
process, during which the proton from the alcohol hydroxyl group
is transferred to the vanadyl oxygen atom and the oxidation state of
vanadium changes from V5+ to V (step 1). Acetaldehyde is formed
in a subsequent step via a transfer of a proton from the CH2 group
to the catalyst, which is accompanied with the partial reduction of
an adjacent vanadium atom (step 2). According to deuterium iso-
topic substitution experiments [14], ␣-C H-bond cleavage is the
rate-determining step in the oxidation of ethanol to acetaldehyde.
From this point of view, we believe that during this process, the
effect of a support must be significant and, by analogy with the
oxidation of methanol [19], the nascent hydroxyl group may be
bonded with titanium cations. The OH group ultimately recombines
4+
2
86.4, and 289.1 eV (Fig. 12). Two peaks at 285.2 and 286.4 eV
observed at low temperatures could be assigned to the ethoxide
species adsorbed on the partially reduced vanadia surface. The
peaks decrease in intensity with the temperature rise, which is in
good agreement with the FTIR data (Fig. 8d). The origin of the peaks
at 284.5 and 286.1 eV is not evident. Taking into account the FTIR
data, we can speculate that this doublet originates from molecularly
adsorbed ethanol. The slight shift of the C1s peaks to lower bind-
ing energy in comparison with the peaks observed under a flow
of ethanol (Fig. 11) may be determined by the different oxidation
state of vanadium on the catalyst surface. The peak at 284.5 eV also
can originate from the adsorbed CHx species at least at low temper-
atures. At higher temperatures, these carbonaceous species burn to
4+
CO and CO , which leads to a decrease in the intensity of this peak
with another OH to form H O and vanadyl oxygen species (step 3);
2
2
[
52,53]. The peak at 286.1 eV may originate from C O groups in
adsorbed acetaldehyde can desorb as a product. The catalytic cycle
is completed by a reoxidation step, the details of which have been
studied computationally [54].
the adsorbed products of acetic aldehyde condensation as well. In
good agreement with the FTIR data (Fig. 7), the acetate C1s peak
at 289.1 eV is observed only at high temperatures within the range
Hence, both the terminal V O bond and the bridge V-O-Ti bond
are involved in the oxidative dehydrogenation of ethanol through
the transfer of two electrons. This conclusion is confirmed by the
in situ data obtained during the oxidation of ethanol at low tem-
peratures: adsorbed ethanol, ethoxide and acetaldehyde species
◦
between 150 and 250 C.
The presented data agree well with the assumption that the
selective oxidation of ethanol over vanadium oxide catalysts pro-
ceeds via the redox mechanism involving oxygen species from the
surface lattice. It should be noted that no detailed mechanism for
the selective oxidation of ethanol over oxide catalysts has been pub-
lished elsewhere. Moreover, authors usually propose a sequence
of elementary steps without any details of the reductive-oxidative
process [7]. Here we develop a mechanism for the selective oxi-
dation of ethanol to acetaldehyde and acetic acid over monolayer
vanadia catalysts that includes the reduction and oxidation of vana-
dium cations.
5+
were detected by FTIR (Fig. 7a) and the partial reduction of V
4+
to V was detected by XPS (Fig. 12). It is more probable that
the selective oxidation of ethanol to acetaldehyde over the poly-
meric vanadia species supported on zirconia proceeds via a similar
mechanism. At the same time, the support may also provide sites
for binding substrate molecules, and, for instance, reducible sup-
ports such as CeO2 may be involved in the redox process as well
[55]. However, such processes are improbable on the monolayer
vanadia-titania catalysts. The oxidation of ethanol to acetic acid
proceeds via acetaldehyde intermediates that are converted further
to acetate species. Under certain conditions, the acetate species are
A proposed sequence of elementary steps for the oxidation of
ethanol to acetaldehyde is shown in Scheme 1. We suggest that
ethanol adsorbs intact on the acid-base sites of vanadia catalysts
and further can dissociate to form adsorbed ethoxide species and
Please cite this article in press as: T.V. Andrushkevich, et al., Selective oxidation of ethanol over vanadia-based catalysts: The influence
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11
Scheme 1. Mechanism for the selective oxidation of ethanol to acetaldehyde on the monolayer vanadia catalyst.
Scheme 2. Mechanism for the selective oxidation of ethanol to acetic acid on the monolayer vanadia catalyst.
decomposed to form acetic acid. The proposed mechanism for this
reaction is depicted in Scheme 2.
supported vanadium cations, which in turn determine the reactiv-
ity and the product distribution. Indeed, when the vanadia-support
We suppose that adsorbed acetaldehyde reacts with lattice oxy-
gen atoms to form adsorbed acetate species, which were detected
by FTIR during the oxidation of ethanol (Fig. 7). This process is
interaction is weak, low-active V O5 crystallites are mainly formed
on the support surface. Such effect is observed, for example, in the
2
V O5/SiO2 and V O5/Al O catalysts.
2
2
2
3
3+
accompanied with the further reduction of vanadium to V and
with the formation of oxygen vacancy. This hypothesis is confirmed
by the XPS data (Fig. 11), when the high concentrations of adsorbed
Increasing the vanadia-support interaction initiates the pre-
ferred formation of surface vanadia species that are more active in
the selective oxidation of alcohols. At the same time, the polymeric
vanadia species on the titania surface and on the zirconia surface
demonstrated different activity in the selective oxidation of ethanol
to both acetaldehyde and acetic acid (Fig. 6). In this case, the role of
the support goes far beyond a mere electronic polarization effect,
as captured by the electronegativity scale [55]. Definitely, the elec-
tronic polarization effect changes the ionicity of the V O bonds
to change the redox properties of the surface vanadia species. As
a result, the material support influences the reactivity. According
to a previous study by Wachs and Weckhuysen [56], the relative
3
+
acetate species and V cations were observed simultaneously in
◦
ethanol at 110 C. It is very important that no acetic acid and CO
2
were detected among the products in the absence of O , whereas
2
the surface concentration of acetate species was high (Figs. 7 and 8).
It means that the formation and desorption of acetic acid and CO2
does not occur on the reduced catalyst due to a high stability of
acetate species. At least the partial oxidation of the catalyst is
needed for the acetic acid formation. Indeed, in the presence of O2
in the gas phase, a weak signal of the acetate species is observed at
◦
3+
5+
2
00–250 C, despite no V cations were detected by XPS (Fig. 12).
Finally, we can conclude that the support effect in the selec-
extent of reduction of the V surface species during butane oxida-
tion follows the row of supports: TiO > CeO > ZrO > Al O > SiO .
2
2
2
2
3
2
tive oxidation of ethanol is a complex phenomenon. At least two
factors that affect the catalytic performance can be distinguished.
First, the support material determines the structure of supported
vanadia species, which have different activities in the target reac-
tion. Second, the support material affects the redox properties of
The same trend in the activity of supported vanadia catalysts was
observed for the selective oxidation of methanol to formaldehyde
[57]. This dependence directly reflects the redox properties of the
surface vanadia species and our catalytic data correlate well with
this row.
Please cite this article in press as: T.V. Andrushkevich, et al., Selective oxidation of ethanol over vanadia-based catalysts: The influence
of support material and reaction mechanism, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.042

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2
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4
. Conclusions
[12] L.V. Mattos, F.B. Noronha, J. Power Sources 145 (2005) 10–15.
[
[
13] V.V. Kaichev, Yu.A. Chesalov, A.A. Saraev, A. Yu. Klyushin, A. Knop-Gericke,
T.V. Andrushkevich, V.I. Bukhtiyarov, J. Catal. 338 (2016) 82–93.
14] B. Kilos, A.T. Bell, E. Iglesia, J. Phys. Chem. C 113 (2009) 2830–2836.
The influence of support material and the reaction mecha-
nism for the selective oxidation of ethanol were studied. It was
shown that the catalytic performance of vanadia supported on sil-
ica, alumina, zirconia, and titania differs and depends mainly on
the structure of supported vanadia species. The catalysts charac-
terized by a weak vanadia-support interaction, such as V O5/SiO
[15] Yu-Chuan Lin, Chih-Hung Chang, Chin-Chen Chen, Jih-Mirn Jehng,
Shin-Guang Shyu, Catal. Commun. 9 (2008) 675–679.
[
16] R. Tesser, V. Maradei, M. Di Serio, E. Santacesaria, Ind. Eng. Chem. Res. 43
2004) 1623–1633.
(
[17] N.E. Quaranta, J. Soria, V. Cortes Corberan, J.L.G. Fierro, J. Catal. 171 (1997)
1–13.
2
2
[
18] E.V. Danilevich, G.Ya. Popova, T.V. Andrushkevich, V.V. Kaichev, I.G. Danilova,
Yu.A. Chesalov, V.A. Rogov, V.I. Bukhtiyarov, V.N. Parmon, Appl. Catal. A 475
(2014) 98–108.
and V O5/Al O , contain mainly low-active V O5 crystallites. When
2
2
3
2
the vanadia-support interaction increases, surface vanadia species
are predominantly formed. The best catalytic performance in the
selective oxidation to both acetaldehyde and acetic acid has been
[19] V.V. Kaichev, G.Ya. Popova, Yu.A. Chesalov, A.A. Saraev, T.V. Andrushkevich,
V.I. Bukhtiyarov, Kinet. Catal. 57 (2016) 82–94.
[
20] C. Penschke, J. Parier, J. Sauer, J. Phys. Chem. C 117 (2013) 5274–5285.
shown by the monolayer V O5/TiO catalyst. The selective oxi-
2
2
[21] B. Beck, M. Harth, N.G. Hamilton, C. Carrero, J.J. Uhlrich, A. Trunschke, S.
Shaikhutdinov, H. Schubert, H.-J. Freund, R. Schlögl, J. Sauer, R. Schomäcker, J.
Catal. 296 (2012) 120–131.
dation of ethanol proceeds via the redox mechanism where the
oxidized catalyst surface oxidizes the reactant and is reoxidized
[
22] E.V. Ovchinnikova, T.V. Andrushkevich, G.Y. Popova, V.D. Meshcheryakov, V.A.
Chumachenko, Chem. Eng. J. 154 (2009) 60–68.
by gas phase oxygen. During the reaction on the V O5/TiO cata-
2
2
lyst, titanium cations remain in the Ti4+ state, whereas V cations
5+
[23] A. Knop-Gericke, E. Kleimenov, M. Hävecker, R. Blume, D. Teschner, S.
Zafeiratos, R. Schlögl, V.I. Bukhtiyarov, V.V. Kaichev, I.P. Prosvirin, A.I.
Nizovskii, H. Bluhm, A. Barinov, P. Dudin, M. Kiskinova, Adv. Catal. 52 (2009)
4+
undergo a reversible reduction under reaction conditions to V
and V3+. Ethanol dehydrates to acetaldehyde and then acetaldehyde
transforms to acetic acid through the adsorbed acetate species. The
formation and desorption of acetic acid occurs only in the presence
213–272.
[24] V.V. Kaichev, G. Ya. Popova, Yu.A. Chesalov, A.A. Saraev, D.Y. Zemlyanov, S.A.
Beloshapkin, A. Knop-Gericke, R. Schlögl, T.V. Andrushkevich, V.I.
Bukhtiyarov, J. Catal. 311 (2014) 59–70.
of O in the gas phase because the reduction of the catalyst stabilizes
2
[
25] E.V. Danilevich, G.Ya. Popova, T.V. Andrushkevich, Yu.A. Chesalov, V.V.
Kaichev, A.A. Saraev, L.M. Plyasova, Stud. Surf. Sci. Catal. 175 (2010) 463–466.
the surface acetate complexes. It was found that the activities of the
polymeric vanadia species on the titania surface and on the zirco-
nia surface are different, which is caused by the vanadia-support
interaction as well.
[26] C. Sanchez, J. Livage, G. Lucazeau, J. Raman. Spectrosc. 12 (1982) 68–72.
[
[
27] G.C. Bond, S.F. Tahir, Appl. Catal. 71 (1991) 1–31.
28] B. Grzybowska-Swierkosz, Appl. Catal. A 157 (1997) 263–310.
[29] G. Ya. Popova, T.V. Andrushkevich, E.V. Semionova, Yu.A. Chesalov, L.S.
Dovlitova, V.A. Rogov, V.N. Parmon, J. Mol. Catal. A 283 (2008) 146–152.
[
[
[
30] I.E. Wachs, Catal. Today 27 (1996) 437–455.
31] X. Gao, J.L.G. Fierro, I.E. Wachs, Langmuir 15 (1999) 3169–3178.
32] W.C. Vining, J. Strunk, A.T. Bell, J. Catal. 281 (2011) 222–230.
Acknowledgments
This work was partially supported by the Skolkovo Founda-
tion (Grant Agreement for Russian educational organizations no.
[33] Z. Wu, H.-S. Kim, P.C. Stair, S. Rugmini, S.D. Jackson, J. Phys. Chem. B 109
2005) 2793–2800.
34] D.A. Bulushev, L. Kiwi-Minsker, F. Rainone, A. Renken, J. Catal. 205 (2002)
15–122.
(
[
5
of 30.12.2015). The authors are grateful to G.Ya. Popova and
1
E.V. Danilevich for performing the catalytic tests and fruitful dis-
cussions. The authors thank A.Yu. Kluyshin, M. Hävecker, and A.
Knop-Gericke for their assistance in carrying out the XPS exper-
iments and L.M. Plyasova for performing the XRD measurements.
The authors are also grateful to the staff of BESSY-II for their support
during the beamtime.
[35] R. Baddour-Hadjean, M.B. Smirnov, K.S. Smirnov, V. Yu Kazimirov, J.M.
Gallardo-Amores, U. Amador, M.E. Arryo-de Dompablo, J.P. Pereira-Ramos,
Inorg. Chem. 51 (2012) 3194–3201.
[
36] J.M. Coronado, K. Kataoka, I. Tejedor-Tejedor, M.A. Anderson, J. Catal. 219
(2003) 219–230.
[
[
[
[
[
[
37] Z. Yu, S.S.C. Chuang, J. Catal. 246 (2007) 118–126.
38] M. Li, Z. Wu, S.H. Overbury, J. Catal. 306 (2013) 164–176.
39] R.G. Greenler, J. Chem. Phys. 37 (1962) 2094–2100.
40] A.M. Nadeem, G.I.N. Waterhouse, H. Idriss, Catal. Today 182 (2012) 16–24.
41] V. Lochar, H. Drobná, Appl. Catal. A 269 (2004) 27–31.
42] M. Sambi, G. Sangiovanni, G. Granozzi, F. Parmigiani, Phys. Rev. B 55 (1997)
Appendix A. Supplementary data
7
850–7858.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.04.
[43] M.E. Harlin, V.M. Niemi, A.O.I. Krause, J. Catal. 195 (2000) 67–78.
44] K.V.R. Chary, G. Kishan, C.P. Kumar, G.V. Sagar, J.W. Niemantsverdriet, Appl.
Catal. A 245 (2003) 303–316.
45] L.K. Boudali, A. Ghorbel, P. Grage, F. Figueras, Appl. Catal. B 59 (2005) 105–111.
46] Z. Wu, F. Dong, Y. Liu, H. Wang, Catal. Commun. 11 (2009) 82–86.
47] H. Zhao, S. Bennici, J. Cai, J. Shen, A. Auroux, Catal. Today 152 (2010) 70–77.
48] R.P. Holroyd, R.A. Bennett, I.Z. Jones, M. Bowker, J. Chem. Phys. 110 (1999)
[
042.
[
[
[
[
References
8
703–8713.
[
[
1] K. Sano, H. Uchida, S. Wakabayashi, Catal. Surv. Jpn. 3 (1999) 55–60.
2] N. Yoneda, S. Kusano, M. Yasui, P. Pujado, S. Wilcher, Appl. Catal. A 221 (2001)
[
[
49] P.M. Jayaweera, E.L. Quah, H. Idriss, J. Phys. Chem. C 111 (2007) 1764–1769.
50] A.A. Khassin, T.M. Yurieva, V.V. Kaichev, V.I. Bukhtiyarov, A.A. Budneva, E.A.
Paukshtis, V.N. Parmon, J. Mol. Catal. A 175 (2001) 189–204.
2
53–265.
[
[
[
3] T. Takey, N. Iguchi, M. Haruta, Catal. Surv. Asia 15 (2011) 80–88.
4] W. Song, P. Liu, E.J.M. Hensen, Catal. Sci. Technol. 4 (2011) 2997–3003.
5] J.C. Bauer, G.M. Veith, L.F. Allard, Y. Oyola, S.H. Overbury, S. Dai, ACS Catal. 2
[
[
[
51] V.V. Kaichev, V.I. Bukhtiyarov, G. Rupprechter, H.-J. Freund, Kinet. Catal. 46
(
2005) 269–281.
52] V.V. Kaichev, A.V. Miller, I.P. Prosvirin, V.I. Bukhtiyarov, Surf. Sci. 606 (2012)
20–425.
53] A.V. Miller, V.V. Kaichev, I.P. Prosvirin, V.I. Bukhtiyarov, J. Phys. Chem. C 117
2013) 8189–8197.
(
2012) 2537–2546.
4
[
[
[
6] T. Takei, N. Iguchi, M. Haruta, N. J. Chem. 35 (2011) 2227–2233.
7] X. Li, E. Iglesia, Chem. Eur. J. 13 (2007) 9324–9330.
8] Y. Eguchi, D. Abe, H. Yoshitake, Microporous Mesoporous Mater. 116 (2008)
(
[
[
[
[
54] X. Rozanska, E.V. Kondratenko, J. Sauer, J. Catal. 256 (2008) 84–94.
55] T. Kropp, J. Paier, J. Sauer, J. Am. Chem. Soc. 136 (2014) 14616–14625.
56] I.E. Wachs, B.M. Weckhuysen, Appl. Catal. A 157 (1997) 67–90.
57] G. Geo, I.E. Wachs, J. Catal. 146 (1994) 323–334.
4
4–50.
[
9] B. Jorgensen, S.B. Kristensen, A.J. Kunov-Kruse, R. Fehrmann, C.H. Christensen,
A. Riisager, Top. Catal. 52 (2009) 253–257.
[
[
10] F.M. Goncalves, P.R.S. Medeiros, L.G. Appel, Appl. Catal. A 208 (2001) 265–270.
11] B. Mehlomakulu, T.T.N. Nguyen, P. Delichere, E. van Steen, J.M.M. Millet, J.
Catal. 289 (2012) 1–10.
Please cite this article in press as: T.V. Andrushkevich, et al., Selective oxidation of ethanol over vanadia-based catalysts: The influence
of support material and reaction mechanism, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.042