Effect of Modification of Supported V2O5/SiO2 Catalysts by Lanthanum on the State and Structural Peculiarities of Vanadium

Article abstract of DOI:10.1134/S0023158419040050

Abstract: Using the methods of diffuse reflectance electron spectroscopy, Raman spectroscopy and temperature-programmed reduction with hydrogen, the formation of surface phases in supported lanthanum-modified V₂O₅/SiO₂ catalysts was studied. It is shown that the order of depositing the active component has a significant effect on the phase composition and catalytic properties of the samples in the reaction of propane oxidative dehydrogenation.

Full text of DOI:10.1134/S0023158419040050

ISSN 0023-1584, Kinetics and Catalysis, 2019, Vol. 60, No. 4, pp. 465–473. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Kinetika i Kataliz, 2019, Vol. 60, No. 4, pp. 499–508.
Effect of Modification of Supported V O /SiO Catalysts
2
5
2
by Lanthanum on the State and Structural Peculiarities of Vanadium
a,
a
a
T. S. Kharlamova *, Kh. Kh. Urazov , and O. V. Vodyankina
a
Tomsk State University, Tomsk, 634050 Russia
*
e-mail: kharlamova83@gmail.ru
Received January 15, 2019; revised March 16, 2019; accepted March 20, 2019
Abstract—Using the methods of diffuse reflectance electron spectroscopy, Raman spectroscopy and tem-
perature-programmed reduction with hydrogen, the formation of surface phases in supported lanthanum-
modified V O /SiO catalysts was studied. It is shown that the order of depositing the active component has
2
5
2
a significant effect on the phase composition and catalytic properties of the samples in the reaction of pro-
pane oxidative dehydrogenation.
Keywords: supported vanadium catalysts, lanthanum oxide, oxidative dehydrogenation of propane, surface
two-dimensional phases
DOI: 10.1134/S0023158419040050
INTRODUCTION
15]. The indicated dependence of the reactivity of sup-
ported vanadium catalysts on the support nature
reflects the important role of the interaction of the
active component with the support and the strength of
the V–O bridging bond formed in the mechanism and
kinetics of the oxidative reactions occurring on them.
It is believed that the cations of the oxide support act
as ligands in the second coordination sphere of vana-
dium and change the redox potential of the surface
Supported vanadium oxide systems attract much
attention of researchers as catalysts for various redox
reactions, including oxidative dehydrogenation of
light hydrocarbons [1, 2], oxidation of methanol to
formaldehyde [3–5], formaldehyde to formic acid [6],
ethanol to acetaldehyde and acetic acid [6–8], and
selective reduction of NO with ammonia [9–11]. The
x
catalytic properties of such systems are determined by
the presence of two-dimensional or three-dimen-
sional vanadium oxide phases on the surface, the for-
VO phases [1, 2].
x
For reactions proceeding via a consecutive mecha-
mation of which depends on the amount of the sup- nism with the formation of a more reactive target
ported component, the nature of the support, and the product as an intermediate, such as oxidative dehydro-
nature of the vanadium precursor used for sample genation of propane, the influence of support on the
preparation [1, 3, 12–15].
redox potential of the surface vanadium oxide phases
The formation of two-dimensional VO phases on affects not only their reactivity with respect to the acti-
x
vation of the starting reagent molecule, but also subse-
quent deep oxidation of the target product [1, 2]. In
this regard, the most suitable supports for vanadium
catalysts of the oxidative dehydrogenation of propane
the surface, which most often attract the greatest
interest for catalysis, occurs when the active compo-
nent is supported within the monolayer [1, 2, 14, 15].
For most supports, the monolayer surface coverage of
are Al O and SiO , for which relatively high selectivities
the VO phases corresponds to a surface density of 8–9
2
3
2
x
2
are observed for propane conversion up to 10% [1, 16].
vanadium atoms per nm . An exception is silica,
which, due to the low surface reactivity, is character-
Amorphous silica is one of the widely used inex-
pensive inert supports, having a high specific surface
ized by the formation of a three-dimensional V O
2
5
phase already at a surface density of ~3 vanadium area S and a developed porous structure that is con-
sp
2
atoms per nm .
trolled over a wide range. However, the probability of
characterized by the
surface coverage by two-dimensional VO phases and low selectivity in the oxidative dehydrogenation of
The support affects both the maximum density of the formation of crystalline V O
2 5
x
their reactivity [1–3, 8, 14, 15]. For the majority of light alkanes is higher on its surface than for Al O ,
2 3
oxidative reactions on supported vanadium catalysts, a TiO , Nb O , or ZrO , which leads to a drop in the
2
2
5
2
decrease in the activity is observed in the series of sup- selectivity to the target product at relatively low con-
ports CeO > ZrO > TiO > Al O > SiO [1–3, 14, tent of supported vanadium. This requires the devel-
2
2
2
2
3
2
465

4
66
KHARLAMOVA et al.
opment of approaches to the formation of the active tion of ammonia followed by calcination at 900°C
and selective surface of V O /SiO catalysts with a high [30]. After each impregnation, the samples were cal-
2
5
2
vanadium content.
cined in air at 500°С for 2 h. Intermediate one-com-
ponent V O /SiO and La O /SiO were used as refer-
2
5
2
2
3
2
Grant et al. [17, 18] showed that supporting vana-
dium on silica modified by sodium makes it possible to
increase the coverage density of the support surface by
ence catalysts. The nominal weight content of vana-
dium oxide in the samples was 0.059 g V O per 1 g of
2
5
SiO , which corresponds to the density of the coating
two-dimensional VO phases without the formation of
2
x
of the surface of the support with vanadium of 0.25
crystalline vanadium oxide up to 9 vanadium atoms
2
2
monolayer or 2 vanadium atoms per nm of the sup-
per nm . It was noted that the addition of sodium does
port. The molar ratio of V : La was 1 : 1 in all two-com-
ponent samples. The use of coimpregnation for the
preparation of catalysts was difficult due to the insta-
bility of the mixed solution of the precursors (precipi-
tation and gas evolution were observed).
not lead to significant changes in the catalytic activity
and selectivity of catalysts at low sodium-to-vanadium
molar ratios (Na : V < 0.25). This is due to the influ-
ence of the additive only on the formation of two-
dimensional vanadium oxide phases on the surface of
silica and the absence of the effect of small amounts of
sodium on the electronic structure of VO [17]. An
x
Study of Physicochemical Properties of Samples
increase in the amount of sodium in the sample leads
to the formation of a crystalline sodium metavanadate
The physicochemical properties of the synthesized
characterized by low selectivity similar to V O [17, samples were studied by a complex of methods: low-
2
5
temperature nitrogen adsorption, X-ray diffraction
1
8]. At the same time, changes occurring in the first
(
XRD), UV–Viz diffuse reflectance spectroscopy,
and second coordination spheres of vanadium in the
surface vanadium oxide phases due to the formation of
V–O–modifier bonds may affect their redox potential
and catalytic properties [19–22]. In this regard, it is
interesting to study additives that affect both the for-
mation of two-dimensional vanadium oxide phases on
the surface of silicon oxide and their redox potential.
Raman spectroscopy, infrared (IR) spectroscopy and
temperature-programmed reduction with hydrogen
(TPR-H ).
2
The phase composition of the samples was studied
by XRD on an XRD6000 diffractometer (Shimadzu,
Japan) with monochromatic CuK radiation (λ =
α
Oxides of rare-earth elements, in particular cerium 1.54056 Å) in the range of angles 2θ = 10°–60°. The
and lanthanum oxides, are widely used as modifiers obtained diffraction patterns were analyzed using the
and promoters of various catalysts, including nickel PDF4 database.
oxide catalysts for the oxidative dehydrogenation of
UV–Viz diffuse reflectance spectra were obtained
light hydrocarbons [23–29]. The interaction of the
using an Evolution 600 spectrometer (ThermoScien-
modifier with the support and/or the active compo-
tific, USA). MgO was used as a standard. To record
nent plays a major role in improving the properties of
the spectra of dehydrated samples, they were kept in
the modified samples. The use of cerium and lantha-
special hermetically sealed reactors with a valve at
num oxides as modifiers of supported vanadium oxide
catalysts for the oxidative dehydrogenation of alkanes
remains poorly studied.
2
00°C for 2 h in air and then for 1.5 min in vacuum
(
10–2 Torr). The reflection spectra of the studied
samples were transferred to the absorption spectra
In this work, the effect of lanthanum oxide modifi- using the Kubelka–Munk function:
cation on supported V O /SiO catalysts with low sur-
2
5
2
2
face densities of supported vanadium on the formation of
surface 2D vanadium oxide phases in them, their struc-
tural features, and redox and catalytic properties in the
oxidative dehydrogenation of propane was studied.
(1 − R)
F(R) =
,
2R
where R is the reflection factor.
The Raman spectra of the synthesized samples
were taken on a Raman confocal microscope (Ren-
ishaw, United Kingdom) with a wavelength of 532 nm
and analyzed using the OMNIC program (Thermo
Nicolet Corporation, USA).
EXPERIMENTAL
Sample Preparation
A series of supported V O /SiO catalysts modified
2
5
2
The redox characteristics of the samples were stud-
with lanthanum oxide was prepared by consecutive
incipient-wetness impregnation of the support with
ied by the method of temperature programmed reduc-
tion by hydrogen (TPR-H ) on a Chemisorb 2750
2
aqueous solutions of precursors. VOSO (Reakhim,
4
chemisorption analyzer (Micromeritics, USA) with a
thermal conductivity detector. Before starting the
experiments, all samples were subjected to oxidative
9
9.0%) and La(NO ) · 6H O (Vekton, 99.0%) were
3
3
2
2
taken as precursors. KSKG silica (S = 190 m /g,
sp
3
Vpore = 0.61 cm /g) (Russia) hydrothermally treated to treatment in a flow of air (20 mL/min) at a heating
increase the pore size was used as a support. Hydro- rate of 10°C/min to 500°C. All TPR-H
experiments
2
thermal treatment was carried out in an aqueous solu- were performed in the temperature range from 25 to
KINETICS AND CATALYSIS
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No. 4
2019

EFFECT OF MODIFICATION OF SUPPORTED V O /SiO CATALYSTS
467
2
5
2
Table 1. Designation composition, and textural characteristics of samples
Composition, wt %
2
3
Sample
V O /SiO
S , m /g
Vpore, cm /g
Dpore, nm
sp
V2O5
La O
2
3
5.8
5.2
–
–
164
151
168
163
0.55
0.50
0.54
0.49
10.0
10.0
9.7
2
5
2
La O /V O –SiO
10.2
10.7
10.1
2
3
2
5
2
2
La O /SiO
2
2
3
V O /La O –SiO
5.6
9.3
2
5
2
3
Two-component catalysts La O /V O –SiO and V O /La O –SiO are obtained by impregnation of one-component samples
2
3
2
5
2
2
5
2
3
2
V O /SiO and La O /SiO , respectively. Dashes mean that the corresponding oxides are not available in the sample composition.
2
5
2
2
3
2
8
00°C. The gas was supplied at a rate of 20 mL/min,
The rate constants were determined using the first-
the heating rate was 10°C/min. The water formed order equation for a plug flow reactor:
during the reduction was frozen before a detector in a
special trap cooled by a mixture of liquid nitrogen and
isopropyl alcohol.
0
1
t
C
v
1
k = ln
=
ln
,
C
V
1 − x
0
where t is the contact time, s; С is the initial concen-
tration of the converted substance, vol %; С is the final
concentration of the converted samples, vol %; v is the
Study of Catalytic Properties of Samples
3
flow rate of the reaction mixture, cm /s; V is the cata-
The catalytic properties of the samples in the oxi-
dative dehydrogenation of propane were studied in a
flow-type reactor with a fixed catalyst bed in the tem-
perature range from 500 to 600°C and atmospheric
pressure. The analysis of products before and after the
reactor was carried out on-line on a Crystal 5000.2
chromatograph (Chromatec, Russia). The reactor was
a quartz tube with an inner diameter of 10 mm with a
coaxially located pocket for a thermocouple with an
outer diameter of 4 mm. The sample (0.20–0.25 g,
3
lyst volume, cm ; x is the conversion in fractions of
unity. When calculating the rate constant for the oxi-
dation of propylene, k , it was assumed that its initial
2
concentration is determined by the conversion of pro-
pane, and the final concentration is determined by the
selectivity of the oxidative dehydrogenation of propylene.
RESULTS AND DISCUSSION
Composition and Textural Characteristics of Samples
Table 1 shows the designations, composition, and
3
fraction 0.25–0.5 mm, density 0.5–0.6 g/cm ) diluted
3
with 1 cm of quartz crumb (fraction 0.5–1.0 mm) was
placed in the reactor. Before the experiment, the sam- textural characteristics of the synthesized samples.
ples were subjected to oxidative treatment in a flow of The content of supported components in the samples
air at 500°C for 30 min. A reaction mixture was 5 vol % varies in the ranges of 5.1–5.8 wt % for V O and 10.1–
2 5
С Н and 5 vol % O in nitrogen. The temperature in 10.7 wt % for La O . A decrease in the content of vana-
3
8
2
2
3
the reactor was controlled by a thermocouple placed in dium oxide in La O /V O –SiO compared with
2
3
2
5
2
the center of the catalyst bed.
V O /SiO and lanthanum oxide in V O /La O –SiO
2 5 2 2 5 2 3 2
compared with La O /SiO is due to a change in the
2
3
2
The conversion of propane Х(C H ) and selectivity
3
8
fraction of the corresponding component as a result
of the introduction of the second component. Differ-
ences in the content of oxides of lanthanum and
vanadium in two-component catalysts are associated
with experimental errors in the course of sample
preparation.
to propylene S(C H ) were calculated by formulas:
3
6
X(C H )
3
8
3
CC3H6 + 2CC2H4 + 2CC2H6 + CCO2 + CCO
CC3H8 + 3CC3H6 + 2CC2H4 + 2CC2H6 + CCO2 + CCO
100%,
=
3
Figure 1 shows the adsorption–desorption iso-
therms of nitrogen and the pore size distribution for
the synthesized samples and the initial support. The
adsorption–desorption isotherms of nitrogen for the
prepared catalysts are similar to the isotherm of the
initial support taking into account a change in the
weight of samples as a result of consecutive deposition
of lanthanum and vanadium oxides (Fig. 1a). For all
samples, the adsorption–desorption isotherm shows a
×
S(C H )
3
6
3
CC3H6
CC3H6 + 2CC2H4 + 2CC2H6 + CCO2 + CCO
100%,
=
3
×
whereCC3H8, C , C ,C ,C ,C (in vol %) are hysteresis loop of capillary condensation in the range
the concentrations of components at the reactor outlet.
C3H6
C2H6
C2H4
CO2
CO
of relative pressures of 0.7–0.95 with bending at ~0.86,
KINETICS AND CATALYSIS Vol. 60 No. 4 2019

4
68
KHARLAMOVA et al.
(
а)
X-ray diffraction pattern of initial SiO (not shown in
2
1
8
Fig. 2) and contains a wide halo in the range of 2θ =
SiO2
V O –SiO
La2O3/V2O5–SiO2
La O /SiO
1
5°–35° due to the presence of amorphous silica in the
16
2
5
2
sample. No reflection of the crystalline V O phase
2 5
14
was detected indicating the presence of vanadium in
the sample as surface vanadium oxide phases or highly
dispersed vanadium oxide, which is inaccessible for
detection by the XRD method. Modification of the
V O /SiO sample with lanthanum oxide leads to the
2
3
2
12
V O /La O –SiO
2
5
2
3
2
10
2
5
2
8
6
4
2
appearance of wide reflections on the pattern, which
are related to the LaVO phase with a monoclinic
4
structure (the corresponding bar graph is shown in
Fig. 2). The broadening of the LaVO phase reflec-
4
tions indicates its high dispersion and/or defective-
ness.
The XRD pattern of the La O /SiO sample is
2
3
2
0
0.2
0.4
0.6
0.8
1.0
characterized by the presence of wide halos: intense in
P/P0
the range 2θ = 15°–35° and weak in the range 2θ =
3
6°–55°. The reflections of crystalline phases of the
(b)
0
0
0
.14
.12
.10
La O modifier or the products of its interaction with
2 3
the support—lanthanum silicates—have not been
identified on the XRD pattern. However, the broad
peaks observed on the XRD pattern cannot be
explained only by the amorphous phase of silicon
oxide and are associated with the formation of amor-
phous lanthanum silicate on the support surface [31,
V O –SiO
2
2
5
La O /V O –SiO
2
3
2
5
2
La O /SiO
2
3
2
V O /La O –SiO
2
5
2
3
2
SiO2
0
.08
3
2]. The subsequent deposition of vanadium oxide
does not lead to significant changes in the XRD pat-
tern of the V O /La O –SiO sample, which indicates
0
.06
.04
2
5
2
3
2
the presence of vanadium in the sample in the form of
highly dispersed surface oxide phases.
0
0.02
Study of Samples by Raman Spectroscopy
0
1
0
100
Figure 3 shows the Raman spectra of hydrated
Pore size, nm
–1
samples at 123–1050 cm . No bands were observed in
the spectrum of the support (not presented) in the
specified range. The spectrum of the V O /SiO sample
Fig. 1. (a) Nitrogen adsorption–desorption isotherms and
b) the pore size distribution of the prepared samples and
the initial support.
2
5
2
(
–1
contains relatively narrow bands at 130 and 178 cm and
–
1
wide bands at 250–380, 410–740, and 750–980 cm .
The bands at 130 and 178 cm– are caused by lattice vibra-
1
–
1
which indicates the presence of mesopores in them. tions, and the bands at 250–380 and 450–740 cm
According to the data on the pore size distribution, the belong to vibrations of V–O–V bonds in hydrated VO
x
pore structure of the samples and the initial support is structures on the surface of silicon oxide [33]. The
–1
represented by pores with a size in the range of 4–20 nm. band at 750–980 cm corresponds to vibrations of the
The observed changes in the pore size distribution V–O–Si bonds in surface vanadium silicates [33].
indicate a uniform filling of pores in the support as the
The modification of the V O /SiO sample with
2
5
2
components are consecutively supported (Fig. 1b).
Thus, the presented results indicate that the textural
characteristics of the prepared catalysts are deter-
mined by the textural characteristics of the support.
All samples have a high specific surface area of 151–
lanthanum oxide leads to a significant change in the
shape of the spectrum. The spectrum of La O /V O –
2
3
2
5
SiO contains bands at 857, 837, 821, ~785, 440, 373,
2
–1
3
24, and 217 cm , related to lanthanum orthovana-
2
3
date LaVO [34, 35], and bands at 146, 480, 513 , 706,
4
1
68 m /g, a total pore volume of 0.49–0.55 cm /g, and
an average pore diameter of ~10 nm (Table 1).
–1
and 996 cm are characteristic of V O [36, 37]. The
2
5
broadening of bands indicates a high dispersion and
Figure 2 presents the XRD data. The X-ray diffrac- imperfection of these phases. Considering the XRD
tion pattern of the V O /SiO sample is similar to the data, it can be concluded that most of the vanadium is
2
5
2
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EFFECT OF MODIFICATION OF SUPPORTED V O /SiO CATALYSTS
469
2
5
2
4
3
2
4
3
1
15
20 25 30 35 40 45 50 55 60
La SiO PDF 040-0234
2
1
2
5
1
00 200 300 400 500 600 700 800 900 1000
m-LaVO PDF 22-0379
4
Wavelength, nm
Fig. 3. Raman spectra of hydrated samples: (1)
La2O3/SiO2; (2) V2O5/La2O3–SiO2; (3) V2O5/SiO2; and
La O PDF 05-0602
2
3
(
4) La O /V O –SiO .
2 3 2 5 2
V O PDF 41-1426
Study of Samples by UV–Viz Diffuse
Reflectance Spectroscopy
2
5
15
20 25 30 35 40 45 50 55 60
θ, deg
Figure 4 shows the electronic spectra of hydrated
and dehydrated samples at 200–900 nm. In the spec-
trum of La O /SiO , there is a weak band at 200–280 nm,
2
2
3
2
which does not make a significant contribution to the
spectra of vanadium-containing samples. In the spec-
tra of all vanadium-containing samples, the metal–
ligand V(V) charge transfer bands (CTB) were
detected at 200–600 nm [38, 39]. Low-intensity bands
at 700–900 nm in the spectra of V O /SiO and
Fig. 2. XRD patterns of (1) V O /SiO , (2) La O /V O –
2
5
2
2
3
2 5
SiO , (3) La O /SiO , (4) V O /La O –SiO and bar
2
2
3
2
2
5
2
3
2
5
graphs of expected crystalline phases V O , La O ,
2
2 3
LaVO , and La SiO .
4
2
5
2
5
2
present in the sample as lanthanum orthovanadate,
and vanadium oxide is the impurity phase.
La O /V O –SiO are caused by d–d transitions in
2
3
2
5
2
V(IV), which is present as impurity in samples as a
In the spectrum of the La O /SiO sample, only result of partial V(V) reduction. Bands caused by d–d
2
3
2
transitions in V(IV) are not observed in the spectrum
of V O /La O –SiO , which indicates the stabiliza-
the signals at the noise level are visible. No bands cor-
responding to La–O bond vibrations are observed, the
2
5
2
3
2
lanthanum oxide phase can thus be excluded. In the tion of vanadium(V) in it. The profile of CTB V(V),
which depends on the local environment of vanadium,
is different in the spectra of the samples and is deter-
mined by the method of catalyst preparation and the
conditions for obtaining spectra.
spectrum of V O /La O –SiO , characteristic bands
2
5
2
3
2
of the crystalline LaVO and V O were not detected
4
2
5
[
34–37]. Against the background of noise, wide bands
–1
at 400–980 cm are distinguishable and attributed to
vibrations of V–O–V bonds in hydrated surface VOx
The spectrum of the initial (hydrated) V O /SiO
2
2
5
structures on the surface of silicon oxide and V–O–Si sample is characterized by intense absorption at 200–
bonds in vanadium surface silicates [33]. However, the 600 nm with a pronounced band at ~400 nm associ-
intensity of the corresponding bands, especially at ated with vanadium in VO and/or VO (Fig. 4a).
6
5
–1
7
50–980 cm , is lower than in the spectrum of the Dehydration of the sample leads to a decrease in the
V O /SiO sample, which may be due to the presence intensity of this band, with the band at 270 nm, which is
2
5
2
of lanthanum in the sample. The presence of lantha- characteristic of vanadium in isolated VO tetrahedra,
4
num on the support surface can prevent the formation becoming pronounced. Absorption bands remain at
of V–O–Si bonds and contribute to the formation of 350–600 nm, which may be due to CTB in VO /VO
6
5
LaVO structures on the surface of silicon oxide.
in the surface structures present in the sample due to
x
KINETICS AND CATALYSIS Vol. 60 No. 4 2019

4
70
KHARLAMOVA et al.
(а)
(b)
(
(
(
(
1) V O /SiO
2 5 2
2) V O /La O –SiO
(1) V O –SiO
2
2
5
2
3
2
2
5
3) La O /V O –SiO
2
1
0
(2) V O /La O –SiO
2
3
2
5
2
2 5 2 3
2
2
2
1
4) La O /SiO
(3) La O /V O –SiO
2 3 2 5
2
3
2
1
1
2
2
3
3
4
0
2
00 300 400 500 600 700 800 900
Wavelength, nm
200 300 400 500 600 700 800 900
Wavelength, nm
(c)
(d)
3
2
1
3
2
1
0
2
0
00 300 400 500 600 700 800 900
Wavelength, nm
200 300 400 500 600 700 800 900
Wavelength, nm
Fig. 4. UV–Viz diffuse reflectance spectra of (a) hydrated and (b) dehydrated samples, and samples (c) V O /SiO and
2
5
2
(
d) V O /La O –SiO after dehydration (black spectrum) and further interaction with moist air (grey spectra). On parts (b) and
2 5 2 3 2
(d) of this figure, an arrow show an increase in the intensity of the band at 400 nm with time due to the interaction with moist air.
the rapid hydration during the study or in microcrys-
The modification of V O /SiO by lanthanum
2 5 2
talline V O . The absence of characteristic bands of oxide leads to a significant change in the UV–Viz dif-
2
5
V O in the Raman spectrum allows us to assign bands fuse reflectance spectra. The spectra of the hydrated
2
5
at 350–600 nm in the UV–Viz diffuse reflectance and dehydrated La O /V O –SiO sample are almost
2 3 2 5 2
spectrum of the dehydrated sample to CTB in identical and characterized by an intense band at
VO /VO in the surface structures. The subsequent 270 nm caused by V(V) in isolated VO tetrahedrons,
6
5
4
interaction of the sample with moist air leads to a and also by a relatively weak absorption at 350–550 nm
reverse increase in the intensity of the band in the associated with the CTB V(V) in VO /VO . The con-
6
5
region of 400 nm, which indicates that the hydration– stancy of the spectrum during dehydration–hydration
dehydration process is reversible (Fig. 4b). A change in indicates the presence of vanadium in it in stable crys-
the spectrum due to the hydration–dehydration of the talline phases, which agrees with the XRD and Raman
sample indicates the primarily formation of surface spectroscopy data confirming the formation of the
vanadium oxide structures accessible for interaction microcrystalline phases of lanthanum orthovanadate
with water in V O /SiO [38].
LaVO and vanadium oxide V O .
2
5
2
4
2
5
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2019

EFFECT OF MODIFICATION OF SUPPORTED V O /SiO CATALYSTS
471
2
5
2
The spectrum of the initial V O /La O –SiO sam-
2
5
2
3
2
(
1) V2O5/SiO2
ple is also characterized by an intense band at 270 nm,
caused by vanadium(V) in isolated VO tetrahedra,
(2) La2O3/V2O5–SiO2
(3) V2O5/La2O3–SiO2
4
with a shoulder at 400 nm associated with CTB V(V)
in VO . The dehydration of the sample leads to the
6
complete disappearance of the band at 400 nm, but,
during the subsequent interaction of the sample with
moist air, this band appears again in the spectrum
(
Fig. 4d), which indicates the presence of vanadium in
the dehydrated sample only in isolated VO tetrahe-
4
3
drons, part of which remains accessible for interaction
with water. The intense band at 270 nm in the spec-
trum of the hydrated sample indicates the partial sta-
2
1
bilization of vanadium in isolated VO tetrahedra,
4
which may be due to the presence of lanthanum in the
second coordination sphere of vanadium. However,
according to Raman spectroscopic data, the formation
3
00 400 500 600 700 800 900
Temperature, °С
of the lanthanum orthovanadate phase LaVO does
4
Fig. 5. TPR-Н profiles of samples.
2
not occur in the sample. Thus, a change in the local
structure of vanadium during the interaction with
water indicates the formation of some surface two-
that of the hydrogen consumption peak on the
V O /SiO profile. In this regard, this peak can be
dimensional LaVO structures rather than stable three-
x
2
5
2
dimensional phases in the V O /La O –SiO sample.
2
5
2
3
2
attributed to the reduction of vanadium from surface
LaVO structures available for hydration. In contrast
x
to V O /SiO , for the V O /La O –SiO sample, the
2
5
2
2
5
2
3
2
Study of Samples by TPR-Н2
shoulder profile at 550°C is not observed on the
Figure 5 shows the TPR-Н profiles of the studied
2
reduction profile, which indicates the absence of read-
catalysts. The profiles of all samples are characterized
ily reducible VO forms in it. The peak with the maxi-
x
by two peaks of hydrogen consumption at temperatures
mum at 717°C corresponds to the reduction of vana-
above 450°C. The TPR-Н profile of the V O /SiO
2
2
5
2
dium from surface LaVO structures with lanthanum
x
sample contains a relatively intense asymmetric peak
with a maximum at 613°C and a shoulder at 550°C, as
well as a small peak with a maximum at 886°C. The
low-temperature peak can be attributed to the reduc-
in the second coordination sphere, which are not
available for hydration. The amount of hydrogen con-
sumed in the reduction of V O /La O –SiO is higher
2
5
2
3
2
than in the reduction of V O –SiO and La O /V O –
SiO samples, which is consistent with the UV–Viz
2
5
2
2
3
2
5
tion of surface VO phases [39, 40], while the high-
x
2
temperature peak may be due to the reduction of vana-
dium from surface vanadium silicate according to the
Raman spectroscopy data. In general, the amount of
hydrogen consumed during the reduction of the
V O /SiO sample is low compared with other cata-
diffuse reflectance spectroscopic data on the absence
of vanadium(IV) in this sample and provides evidence
of further stabilization of vanadium(V) in the resulting
surface LaVO phases.
x
2
5
2
lysts, which, according to UV–Viz diffuse reflectance
spectroscopy, may be due to the presence of a rela-
tively large amount of vanadium(IV) in the sample.
Study of Catalytic Properties of Samples
Figure 6 presents the results of catalytic studies of
vanadium-containing samples in the reaction of oxi-
dative dehydrogenation of propane. According to the
data obtained in the presence of all catalysts, propane
conversion was low (less than 2%) in the temperature
range of 500–600°C, which is associated with a low
content of vanadium in them (Fig. 6a). V O /SiO is
The TPR-H profile of the La O /V O –SiO
2
2
2
3
2
5
sample exhibits an intense peak at 662°C and a less
intense peak at 770°C, which are determined by the
reduction of the dispersed crystalline phases of lantha-
num orthovanadate LaVO and V O [41–44]. In gen-
4
2
5
eral, the amount of hydrogen consumed during the
2
5
2
reduction of La O /V O –SiO is higher than that
2
3
2
5
2
the most active. The reaction rate for it at 500°C is
comparable to that for a similar sample in the works by
Grant et al. [17, 18]. Samples modified by lanthanum
oxide are characterized by the lower activity, while the
during the reduction of V O /SiO , which is probably
2
5
2
due to the stabilization of vanadium(V) in the crystal-
line phases of V O and LaVO .
2
5
4
The TPR-Н profile of the V O /La O –SiO
2
formation of crystalline phases in the La
2
O
3
/V
sample leads to a stronger decrease in the activity,
2
O –
2 5
2
2
5
2
3
sample is characterized by two peaks of hydrogen con- SiO
sumption with maxima at 613 and 717°С. The tem- which is consistent with the lower reactivity of the
perature of the maximum of the first peak is similar to samples in reduction by hydrogen. For all samples, an
KINETICS AND CATALYSIS Vol. 60 No. 4 2019

4
72
KHARLAMOVA et al.
(
а)
(b)
2
.0
.8
.6
.4
.2
.0
45
40
V2O5/SiO2
80
3.5
3.0
2.5
2.0
1
La2O3/V2O5–SiO2
V2O5/La2O3–SiO2
3
3
5
0
1
75
1
25
1
k2/k1
7
0
5
20
1
1
5
0.8
0.6
0.4
0.2
0
k2
6
10
1
.5
.0
.5
k1
60
55
0
.6
.4
1
0
0
V O /SiO
V O /La O –SiO
2 5 2 3 2
500
550
600
2
5
2
La O /V O –SiO
2
3
2
5
2
Temperature, °С
Sample
Fig. 6. Dependences of the reaction rate and propane conversion on temperature for various catalysts (a); the dependences of the
selectivity to propylene, the rate constants of the consecutive conversion of propane and their ratio of the composition of the cat-
alyst at 600°C (b).
increase in activity with increasing temperature is
observed.
CONCLUSIONS
The results of studying the modification of
Figure 6b shows the selectivity to propylene for the V O /SiO catalysts by lanthanum oxide at low surface
2
5
2
studied catalysts at 600°C. Due to the 1.3–1.8-times
densities of vanadium showed that the composition
differences in propane conversion for the samples, a and structure of the deposited phases formed in the
correct comparison of the obtained selectivity values is modified samples as well as their catalytic properties
not possible. To analyze the contribution of the non- are determined by the order of introduction of the sup-
selective propane conversion, the rate constants of ported components. The modification of the sup-
consecutive transformations were determined:
ported V
O /SiO catalyst contributes to the formation
2 5 2
of microcrystalline LaVO and V O phases in the
4
2
5
k1
k2
sample, which leads to a deterioration of the catalytic
properties (a noticeable decrease in the activity and
selectivity of the sample) compared with the unmodi-
fied catalyst. The deposition of vanadium oxide on the
C H ⎯ ⎯ ⎯→C H ⎯ ⎯ ⎯→CO
x
3
8
3
6
The contribution of direct oxidation of propane to the
products of deep oxidation was neglected [1]. The val-
ues of the corresponding rate constants and their ratios
k /k are presented in Fig. 6b. The value of k /k
preliminarily modified La O /SiO support contrib-
2
3
2
utes to the formation of surface LaVO phases in the
x
2
1
2
1
modified sample, which is accompanied by an
increase in the selectivity to propylene against the mod-
erate decrease in the activity compared to the unmodified
sample. Taking account of the relatively high La/V molar
ratio in the studied catalysts, we should expect further
improvement of the catalytic properties of the samples
decreases in the series La O /V O –SiO
@
2
3
2
5
2
V O /SiO > V O /La O –SiO , which corresponds
2
5
2
2
5
2
3
2
to an increase in the propylene selectivity. The com-
parison of the rate constants and k /k shows that the
2
1
formation of the crystalline phases LaVO and V O in
4
2
5
the sample La O /V O –SiO leads not only to a obtained by depositing vanadium oxide on the prelim-
2
3
2
5
2
noticeable decrease in its activity, but also to decrease inarily modified La O /SiO support with decreasing
2 3 2
in selectivity for propylene. The formation of the sur- the La/V molar ratio partly as a result of an increase in
the surface density of vanadium.
face LaVO phases in the V O /La O –SiO catalyst is
x
2
5
2
3
2
accompanied by an increase in the propylene selectiv-
ity against a moderate decrease in the activity as com-
pared to unmodified V O /SiO . This may be due to
Thus, it is shown that the introduction of lantha-
num into supported V O /SiO catalysts by modifying
2
5
2
2
5
2
the support surface can be considered as a promising
approach to the formation of active and selective vana-
dium oxide catalysts supported on silica characterized
by a high density of the support surface coating by two-
the absence of highly active, but not selective sites,
which are characterized by the lowest reduction tem-
peratures according to TPR-H , on the surface of the
2
V O /La O –SiO sample.
dimensional VO phases.
2
5
2
3
2
x
KINETICS AND CATALYSIS
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No. 4
2019

EFFECT OF MODIFICATION OF SUPPORTED V O /SiO CATALYSTS
473
2
5
2
ACKNOWLEDGMENTS
23. Brussinoa, P., Bortolozzia, J.P., Dalla Costab, B.,
Banúsa, E.D., and Ulla, M.A., Appl. Catal., A, 2019,
vol. 575, p. 1.
This work was supported by “The Tomsk State Univer-
sity Academic D.I. Mendeleev Fund Program” grant, proj-
ect 8.2.03.2018.
24. Wang, X., Kang, Y., Cui, D., Li, J., and Li, D., Catal.
Commun., 2019, vol. 118, p. 39.
2
5. Ismagilov, Z.R., Matus, E.V., Ismagilov, I.Z., Sukho-
va, O.B., Yashnik, S.A., Ushakov, V.A., and Kerzhen-
tsev, M.A., Catal. Today, 2019, vol. 323, p. 166.
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KINETICS AND CATALYSIS Vol. 60 No. 4 2019