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V.V. Kaichev et al. / Journal of Catalysis 338 (2016) 82–93
In the TPRS experiments the sample was heated at a constant
conversion graph: TOF monotonically increases with temperature
and attains approximately 5 Â 10À3 sÀ1 near 200 °C. The site time
yield (STY), defined as the number of molecules of acetaldehyde
or acetic acid formed per catalytic site and per unit time, was also
calculated, using corresponding values of selectivity. It should be
noted that the activity of the vanadia-based catalyst in the selec-
tive oxidation of ethanol is not high. For example, the TOF of the
dehydrogenation of ethanol to acetaldehyde (without oxygen) over
supported gold nanoparticles reaches 4–6 sÀ1 at 200 °C [9].
Fig. 2 demonstrates how the selectivity toward the main prod-
ucts depends on the conversion of ethanol at 130 and 180 °C. In
both cases, at low conversion, the selectivity toward acetaldehyde
is near 95%. However, at low temperature, only a small decrease of
the selectivity toward acetaldehyde is observed (from 95% to 85%)
when the conversion of ethanol increases from 20% to 90%. This is
accompanied by the formation of crotonaldehyde, ethyl acetate,
and carbon oxides (COx). The selectivity toward crotonaldehyde
reaches 7–8%; the selectivity toward ethyl acetate and COx reaches
only 4% and 2%, respectively. In contrast, at 180 °C the selectivity
toward acetaldehyde is decreased significantly with increasing
conversion of ethanol, which is accompanied with increasing selec-
tivity toward acetic acid. The selectivity toward acetic acid reaches
46% at 80% conversion of ethanol. A notable yield of carbon oxides
is also observed at a conversion of ethanol near 90%. Such depen-
dence of selectivity on the reaction temperature (Fig. 1) and on
the conversion of ethanol (Fig. 2) suggests a consecutive scheme
for the formation of the reaction products in the entire tempera-
ture range: ethanol ? acetaldehyde ? acetic acid ? COx.
rate from 50 to 350 °C in an equimolar mixture of ethanol and
O2. The heating rate was approximately 15 °C/min. The gas-phase
composition was monitored continuously with a Prizma QMS-
200 online quadrupole mass spectrometer (Pfeiffer Vacuum
GmbH) connected through a leak valve to the gas cell. Before the
experiments the mass spectrometer was calibrated with respect
to ethanol, oxygen, and the reaction products CO, CO2, H2, H2O,
and CH4. In the TPRS experiments, 11 MS signals with m/z ratios
of 2 (H2), 15 (CH4), 18 (H2O), 28 (CO), 29 (acetaldehyde), 32 (O2),
42 (CH2CO, ketene), 44 (CO2), 46 (ethanol), 60 (acetic acid), and
88 (ethyl acetate) were monitored simultaneously.
To identify the reaction intermediates involved in the oxidation
of ethanol, FTIR spectra were obtained in situ with a Cary 660 FTIR
spectrometer (Agilent Technologies) within a temperature range of
100–300 °C using the same catalyst. The spectrometer was oper-
ated in the transmission mode using a specially designed quartz
cell reactor with BaF2 windows. The volume of the cell reactor
was approximately 1.5 cm3. The catalyst powder (35–50 mg) was
pressed into a thin self-supporting pellet (15 mg/cm2, 1 Â 3 cm in
size) and placed in the cell reactor. The FTIR experiments were per-
formed at atmospheric pressure using a feed of 1.5 vol.% C2H5OH in
air flowing at 50 sccm. Ethanol was dosed by bubbling air through
a glass saturator filled with liquid ethanol at 0 °C. As a result, the
molar ratio C2H5OH:O2:N2 was approximately 1:14:52. Before
exposure to the reactant mixture, the sample was treated in a flow
of air at 250 °C for 1 h. Subsequently, the cell reactor and the cat-
alyst sample were cooled to the desired temperature, and the air
flow was replaced with the ethanol/air mixture flow. The FTIR
spectra were recorded in the range 1100–4000 cmÀ1 at a resolution
of 4 cmÀ1 during stepwise heating at 100, 130, 150, 180, 200, 230,
250, and 300 °C.
3.2. In situ FTIR
The spectra of the gas phase were also recorded using a Cary
660 FTIR. In these experiments a special gas cell with optical path
length approximately 70 mm was connected to the outlet of the
The formation of adsorbed species during the oxidation of etha-
nol was examined by infrared spectroscopy. Fig. 3 displays the FTIR
spectra obtained in situ in the temperature range 100–250 °C. In
this experiment a mixture of 1.5 vol.% C2H5OH in air was passed
through the IR cell reactor loaded with the monolayer V2O5/TiO2
catalyst. The spectrum of the catalyst before exposure to the reac-
tant mixture and the spectrum of gas-phase ethanol were sub-
tracted from the raw FTIR spectra to identify the contributions of
the adsorbed species. In the spectrum acquired at 100 °C, positive
bands appear at 2977, 2934, 2877, 1730, 1664, 1532, 1444, 1383,
1232, 1144, 1090, and 1040 cmÀ1. According to the literature
[24–28], most of these features can be assigned to the vibration
modes of molecularly adsorbed ethanol (CH3CH2OH) and ethoxide
(CH3ACH2OÀ) species, as outlined in Table 1. Both species are char-
acterized by similar bands of CAH stretching vibrations at 2977,
2934, and 2877 cmÀ1 and CAO stretching vibrations at 1144,
1090, and 1040 cmÀ1, as well as CH3 bending vibrations at 1444
and 1383 cmÀ1. The extra peak at 1232 cmÀ1 is certainly due to d
(OH) mode of molecularly adsorbed ethanol. This band disappears
completely at 130 °C, indicating dissociation or desorption of etha-
nol. In contrast, the bands at 2977, 2934, 2877, 1144, 1090, and
1040 cmÀ1 disappear only at 200 °C. The presence of two bands
at 1090 and 1040 cmÀ1 indicates that at least two kinds of
adsorbed ethoxide species are formed. The bands at 1144 and
1090 cmÀ1 may be characterized as monodentate ethoxide, while
the band at 1040 cmÀ1 is assigned to two bridging ethoxides
[26]. The negative signal in the hydroxyl region near 3650 cmÀ1
is due to removing OH groups. This process also stops at tempera-
ture above 200 °C.
cell reactor. Bands at 1065 cmÀ1 (Q branch of
m
(CAO) band),
(C@O) band), 1790 cmÀ1 (P branch of
(C@O) band), 2115 cmÀ1 (R branch of
(C@O) band), and
2360 cmÀ1 (P branch of
(CO2) band) were used for analysis of
1760 cmÀ1 (P branch of
m
m
m
m
ethanol, acetaldehyde, acetic acid, CO, and CO2, respectively.
3. Results and discussion
3.1. Catalytic results
The oxidation of ethanol was examined over the catalyst in the
temperature range 110–230 °C. Acetaldehyde (CH3CHO), acetic
acid (CH3COOH), diethyl ether ((C2H5)2O), ethyl acetate (CH3-
ACOOACH2ACH3), crotonaldehyde (CH3CH@CHCHO), ethylene
(C2H4), carbon oxides (CO and CO2), and water were detected as
products. The main results are shown in Fig. 1. One can see that
the conversion of ethanol increases monotonically with the reac-
tion temperature and attains 100% at 230 °C. At low temperatures,
acetaldehyde is the major product. Its selectivity is 100% at 110 °C.
The selectivity toward acetaldehyde decreases with temperature
and the reaction shifts toward acetic acid. The formation of small
amounts of ethyl acetate, ethylene, crotonaldehyde, CO, and CO2
was also observed. Between 180 and 230 °C, acetic acid becomes
the main reaction product. Its selectivity achieves approximately
60% at a conversion of ethanol near 95% at 200 °C. At temperatures
above 250 °C, the oxidation of ethanol to CO and CO2 predominates
(not shown).
It is important to note that the formation of the ethoxide spe-
cies is accompanied by a decrease in intensity of the band assigned
Fig. 1 also shows the temperature effect on the TOF (turnover
frequency, rate per surface vanadium atom) of ethanol conversion
over the monolayer V2O5/TiO2 catalyst. This curve repeats the
to the first overtone of m
(V@O) at 2029 cmÀ1. The corresponding
negative band is presented in the inset in Fig. 3. This means that
the terminal vanadyl groups are involved in the oxidation of