Titanium dioxide doped with vanadium as effective catalyst for selective oxidation of diphenyl sulfide to diphenyl sulfonate
Journal of Thermal Analysis and Calorimetry
Titanium dioxide doped with vanadium as effective catalyst for selective oxidation of diphenyl sulfide to diphenyl sulfonate
Marcelina Radko 0 1 3 4 5
Andrzej Kowalczyk 0 1 3 4 5
Ewa Bidzin´ ska 0 1 3 4 5
Stefan Witkowski 0 1 3 4 5
Sylwia G o´recka 0 1 3 4 5
Dominik Wierzbicki 0 1 2 3 4 5
Katarzyna Pamin 0 1 3 4 5
Lucjan Chmielarz 0 1 3 4 5
0 Faculty of Chemistry, Jagiellonian University , Gronostajowa
1 & Lucjan Chmielarz
2 , 30-387 Krako ́w , Poland
3 Oxidation of Ph
4 Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences , Niezapominajek 8, 30-239 Krako ́w , Poland
5 Faculty of Energy and Fuels, AGH University of Science and Technology , Mickiewicza 30, 30-059 Krako ́w , Poland
Diphenyl sulfide was oxidized to sulfoxide and sulfone over V-doped TiO2 using a 30% solution of H2O2. The TiO2 samples with different intended content of vanadium (0.02, 0.05, 0.1 and 0.18 mass%) were prepared by incipient wetness impregnation. Physicochemical properties of the V-doped TiO2 were characterized by chemical analysis (ICP-OES), X-ray diffraction (XRD/in situ HT-XRD), UV-Vis diffuse reflectance spectrometry (UV-Vis DRS), N2-sorption measurements, electron paramagnetic resonance and cyclic voltammetry. Both vanadium oxide loading and calcination temperature influenced the structure of the V-TiO2 samples. Vanadium species deposited on TiO2 decreased temperatures required for anatase to rutile phase transformation. The V-TiO2 samples were found to be efficient catalysts for oxidation of sulfides to sulfones. The sample with the lowest vanadium content (0.02VTiO2) presented among the studied catalysts the best catalytic properties with respect to high conversion of diphenyl sulfide to diphenyl sulfonate. An increase in vanadium loading resulted in decrease in catalytic activity of the samples. Also non-modified TiO2 presented significantly lower catalytic activity in comparison with 0.02VTiO2. This interesting effect was related to the formation of highly dispersed vanadium species catalytically active in Ph2S oxidation in the case of the samples with lower V-content. An increase in vanadium loading results in the formation of more aggregated V-species inactive, or less active, in the process of diphenyl sulfide oxidation.
Titanium dioxide; Hydrogen peroxide
The interest in organic sulfoxides and sulfones production
continuously increases due to their importance as synthetic
intermediates for production of a broad range of chemically
and biologically active substances, which can be used as
therapeutic agents in many ailments including: antiulcer
(proton-pump inhibitor) [
], antibacterial, antifungal,
], antihypertensive [
] and cardiotonic
], as well as psychotronics [
] and vasodilators
]. Many various oxidants (e.g., KMnO4, HNO3, RuO4)
have been used for sulfide oxidation, but most of them are
not suitable as catalysts to a large-scale operation for
several reasons [
]. Most of them are characterized by high
costs of production, low content of effective oxygen and
formation unfavorable co-products during oxidation
Among various methods of organic sulfoxides and
sulfones synthesis direct catalytic oxidation of sulfides using
H2O2 as oxidizing agent seems to be one of the most
]. Hydrogen peroxide is not only effective
oxidant, but also environmental-friendly because does not
result in the formation of any harmful or toxic by-products.
(Only water is formed as by-product of H2O2
decomposition.) Moreover, an aqueous solution of H2O2 is
more effective from many other oxidants because of high
content of active oxygen (about 47%) [
] and relatively
low cost of its production [
] as well as safety in storage
and operation. Hydrogen peroxide due to above-mentioned
properties is often called as green oxidant. Moreover, H2O2
is very promising as oxidant in liquid-phase reactions
because of its high solubility in water and various organic
]. The selective oxidation of organic sulfides to
sulfoxides or sulfones using hydrogen peroxide as an
oxidant needs effective catalysts. There is still discussion
related to the role of catalysts and reaction mechanisms [
One of the most accepted mechanisms includes activation
of hydrogen peroxide by electrophilic catalysts and its
subsequent reaction with organic sulfides resulting in
oxidized products [
]. Recently, many various transition
metal (Ti, Fe, Mo, V, W, Mn, Re, Ru) compounds have
been studied as potential catalysts for Ph2S conversion
Titanium dioxide is often used as catalyst or catalytic
support due to its high chemical stability, low toxicity and
relatively low cost [
]. TiO2-supported vanadium
oxides are extensively studied as effective and selective
catalysts of organic compounds conversion [
example V-doped TiO2 was studied as catalysts for
selective oxidation of ethanol , degradation of aldehyde [
or photo-oxidation of methane in air [
V2O5–TiO2 oxide system is used as catalysts for the
selective reduction of NOx to N2 by ammonia in flue gases
emitted by power station and diesel cars [
Ti-modified zeolites and mesoporous silica materials were
tested as catalysts of the sulfoxidation reactions of aromatic
sulfur compounds with hydrogen peroxide [
catalytic activity of Ti-BEA and Ti-HSM was reported. On the
other hand, Chica et al. [
] studied oxidative
desulfurization of various organic S-containing compounds
(thiophene, 2-methylthiophene, benzothiophene,
4,6-dimethyldibenzothiophene) with tert-butyl hydroperoxide, as an oxidizing
agent, on different metal-containing molecular sieves. It
was shown that high catalytic efficiency of Ti-MCM-41 in
the studied processes was related to a relatively high
internal diffusion rate of reactants inside silica pores as
well as the resistance of Ti species incorporated into silica
wall to leaching.
In the presented studies, vanadium-doped commercial
TiO2 (P25) was tested as catalyst for oxidation of sulfides
to sulfoxides and sulfones by H2O2.
The following chemicals were used in the studies: titanium
(IV) oxide (Aeroxide P25, [ 99.5%, Acros Organics),
ammonium metavanadate (100%, Merck Millipore),
acetonitrile (99.8%, Aldrich), bromobenzene ([ 99.5%,
Aldrich), diphenyl sulfide (98%, Aldrich) and hydrogen
peroxide (30%, POCH).
Titania-supported vanadium catalysts were prepared by
incipient wetness impregnation. The samples of the
intended vanadium content of 0.02, 0.05, 0.1 and 0.18 mass%
were obtained by soaking of the commercial titanium
dioxide powder (P25) with an aqueous solution of
ammonium metavanadate of suitable concentrations. The volume
of the solutions used for impregnation was equal to water
sorption capacity of P25. Then, the samples were dried
overnight and finally calcined at 550 C for 6 h (an
increase of temperature from room temperature to 550 C
with the rate of 1 C min-1 and then isothermal
calcinations step at 550 C for 6 h).
The vanadium content in the samples was analyzed by
ICPOES method. Using Milestone mineralizer, 100 mg of the
sample was dissolved in a mixture of 8 cm3 HCl (30%),
2 cm3 HNO3 (67%) and 1 cm3 HF (50%) at 190 C. The
obtained solution was analyzed with respect to V-content
using ICP-OES instrument (iCAP 7400, Thermo Science).
The phases composition, crystal size and strain effects
of the samples were studied by X-ray diffraction method
(XRD) using a Bruker D2 diffractometer. The
measurements were taken using CuKa radiation in the 2-theta range
of 2–80 with a step of 0.02 . Moreover, the in situ
hightemperature XRD (HT-XRD) measurements were taken in
the temperature range of 25–900 C in air atmosphere
using a PANalytical Empyrean diffractometer (CuKa1/2,
1.54060A˚ ). HT-XRD method was used to study the in situ
phase changes as well as dynamics of crystal size changes
and strain effects. Refinement of the anatase/rutile content
was performed with the use of MAUD software for
Rietveld analysis based on RITA/RISTA algorithms [
Textural parameters of the samples were determined by
N2 adsorption at - 196 C using a 3Flex (Micromeritics)
automated gas adsorption system. Prior to the
measurement, the samples were outgassed under vacuum at 350 C
for 24 h.
The form and aggregation of vanadium species
deposited on TiO2 were analyzed by using UV–Vis DR
spectroscopy. The measurements were taken on an Evolution
600 (Thermo) spectrophotometer in the range of
190–900 nm with a resolution of 4 nm.
Electron paramagnetic resonance (EPR) spectra of the
studied samples were recorded at 25 C using an
ELEXSYS 500 Bruker spectrometer (Xband = 9.5 GHz) with the
following parameters: magnetic field in the range of 2000
G, modulation amplitude around 3 G and 10 mW of power.
Cyclic voltammograms of the calcined samples were
recorded in a three-electrode cell using a graphite paste
electrode as the working electrode, platinum coil as the
auxiliary electrode and Ag|AgCl as the reference electrode.
Composite paste was prepared by mixing synthetic
graphite (100–150 mg) with Nujol (0.05 cm3) and a small
amount of the sample (0.005–0.010 g). The measurements
were taken in acetate buffer (pH 4.0) as electrolyte at a
scan rate of 50 mV s-1. Before experiment, the solutions
were pretreated with argon to keep oxygen-free atmosphere
during the measurement.
The samples of vanadium-doped TiO2 were tested as
catalysts for oxidation of diphenyl sulfide (Ph2S) to diphenyl
sulfoxide (Ph2SO) and sulfone (Ph2SO2) using hydrogen
peroxide as an oxidation agent. The reaction was
performed in 100-mL round-bottom flask equipped with a
stirrer, dropping funnel and thermometer. The reaction
mixture consisted of 2 mL (0.4 mmol) of diphenyl sulfide,
20 mL of acetonitrile used as a solvent, 10 lL (0.1 mmol)
of bromobenzene used as internal standard and 25 mg of
catalyst. The obtained mixture was stirred (1000 rpm) at
25 C for 10 min, and then, 60 lL (2 mmol) of 30%
hydrogen peroxide was added. In order to avoid
photocatalytic reactions, the catalytic runs were performed in the
dark. The progress of the reaction was monitored by
analysis of the reaction mixture by HPLC method, using a
mixture of acetonitrile/water with the volume ratio of 80:20
as an eluent. The samples of the reaction mixture were
taken in regular intervals, filtered using 0.22-lm nylon
membrane filter and analyzed by PerkinElmer Flexar
chromatograph equipped with a COL-Analytical C18
column (150 mm 9 4.6 mm i.d., 5 lm pore size). The
column was maintained at 25 C throughout analysis, and UV
detector was set at 254 nm.
The leaching of vanadium species from P25 was studied
for the selected catalysts by chemical analysis of the
reaction solution after separation from the solid catalyst by
using ICP-OES method (iCAP 7400, Thermo Science).
Moreover, in order to check the catalytic activity of
V-species leached from the solid catalysts, into the solution
separated after catalytic test the fresh reactants (H2O2 and
Ph2S) were added and catalytic test was performed for the
next 4 h.
Results and discussion
Phase composition of the samples was analyzed by XRD
method. Diffractograms recorded for commercial TiO2
(P25) and its modifications with vanadium are presented in
Fig. 1. It should be mentioned that all these diffractograms
were recorded for the calcined samples, including also
nondoped TiO2 (550 C, 6 h). The reflections characteristic of
two TiO2 phases—anatase (A) and rutile (R)—were
identified in the studied samples [
]. It should be noted
that intensities of the reflections characteristic of rutile
increased, while intensities of the reflections characteristic
of anatase decreased after deposition of vanadium on the
TiO2 support. It has to be remained that all the samples,
including non-doped P25, were calcined at these same
conditions. Thus, it seems that vanadium species deposited
on TiO2 induced the phase transformation of anatase to
rutile. Similar results were recently reported by Shao et al.
, who suggested that this interesting effect is related to
many factors, duration and temperature of calcination,
synthesis method and type of dopant metal used. It is
suggested that the anatase to rutile phase transformation is
accelerated by incorporation of V4? cations into vacant
Ti4? positions in TiO2 (anatase) [
]. Such substitution
is possible because of similar ionic radius of Ti4? and V4?,
which is 0.061 and 0.058 nm, respectively . Moreover,
this same valency of both cations results in
electroneutrality of TiO2 anatase lattice. Consequently, V4?
incorporated into anatase lattice acts as initiation nuclei for
R AAR R R
Fig. 1 X-ray diffraction patterns of pure TiO2 (P25) and its
modifications doped with vanadium (A anatase, R rutile)
anatase to rutile phase transformation [
]. In our studies,
vanadium as V5? ions was introduced into TiO2 with using
a solution of NH4VO3. So the question is about possible
reduction of V5? to V4? during the V-TiO2 sample
synthesis. Recently, Kathun et al. [
] reported reduction of
V5? to V4? deposited on TiO2 during calcination
(450–750 C) in air. The V5?:V4? molar ratio, determined
by authors, was 67:33 in the case of the sample containing
3 mol.% of vanadium and 45:55 for the sample containing
6 mol.% of vanadium. Moreover, similar effect of V5? to
V4? reduction was observed by Banaras et al. [
] for the
V-TiO2 catalysts during the process of methane oxidation.
In order to have more insight into anatase to rutile
transformation in the presence of vanadium species an
additional analyses, the Rietveld refinement and in situ
HTXRD studies, were done. The example of the Rietveld
refinement of a theoretical line profile to the measured
XRD profile is presented in Fig. 2, while the phase
compositions of the samples determined by this method are
compared in Table 1. Moreover, the average size of the
anatase and rutile crystallites as well as the real vanadium
content is included in this table. First of all, it should be
mentioned that for all the samples the Rietveld refinement
matches well with the observed diffraction peaks (similarly
to the example presented in Fig. 2). Comparison of the
results of the Rietveld refinements clearly shows that in a
series of the samples calcined at 550 C a significantly
higher contribution of rutile phases is in the samples of
TiO2 doped with vanadium (56–57%) in comparison with
pure calcined TiO2 (13%). Moreover, it is shown that the
content of introduced vanadium does not influence the
anatase/rutile ratio in the samples. It should be also noted
that doping of TiO2 with vanadium resulted in an increase
of the size of anatase and rutile crystallites possibly during
calcination process. This effect is more significant for rutile
than for anatase. In Table 1, also textural parameters of the
studied samples are compared. It can be seen that after
introduction of vanadium the BET surface area of the
samples increases from 9 to 27–39 m2 g-1. This effect is
possibly related to partial opening of intercrystalline space
associated with the anatase to rutile phase transformation
and possibly also due to deposition of V-species on the
surface of P25 particles in highly dispersed form. The BET
surface area of the fresh (non-calcined) P25 sample was
about 40 m2 g-1 and after calcination decreased to
9 m2 g-1, possibly as a result of sintering of the TiO2
crystallites. It should be also noted that the real content of
vanadium deposited on the P25 surface is close to the
In situ HT-XRD studies, performed for non-calcined
pure TiO2 (P25) as well as its non-calcined modifications
with various vanadium lodgings—0.02VTiO2 and
0.05VTiO2 are presented in Fig. 3. The measurements
were taken at 25 C and then from 100 to 900 C with
steps of 100 C in air atmosphere with the linear
temperature increase of 5 C min-1. In the case of pure TiO2, a
very significant increase in the intensities of the reflection
characteristic of rutile and decrease in the intensities of the
reflection attributed to anatase occurred between 700 and
800 C. Such effect was also observed for the samples
doped with vanadium (examples of the results for
0.02VTiO2 and 0.05VTiO2 are presented in Fig. 3)
however, in these cases occurred at temperature lower by about
200 C (between 500 and 600 C). Thus, the results of
in situ HT-XRD analysis of the non-calcined samples are in
agreement with the results of XRD studies of the calcined
samples and support the hypothesis that vanadium
introduced into TiO2 (P25) catalyses thermally induced
transformation of anatase to rutile. The significant difference in
temperature of anatase to rutile phase transformations
determined in XRD studies of the calcined samples (Fig. 1)
and in situ HT-XRD measurements of the non-calcined
samples (Fig. 3) could be explained by the relatively long
duration (6 h) of the calcination process at 550 C and
relatively fast temperature increase (5 C min-1) in the
in situ HT-XRD studies of the non-calcined samples.
The nature of vanadium species deposited on TiO2 was
studied by two spectroscopic methods—UV–Vis DR and
EPR. Both vanadium and titanium species produce the
UV–Vis adsorption bands in similar range, and therefore,
the analysis of spectrum of the materials containing
simultaneously both these elements is difficult. In order to
overcome the problems with analysis of vanadium-doped
TiO2 the original spectrum of calcined P25 was subtracted
from the spectrum of the vanadium-modified P25. Figure 4
presents such differential spectra of the vanadium-doped
TiO2 samples calcined at 550 C and is related mainly to
the presence of vanadium species. The spectra of the
0.02VTiO2 sample are characterized by the presence of
only one intense adsorption band centered at 380 nm,
which is probably superposition of two sub-bands. The first
one, expected at about 350 nm, is related to rutile phases
(different contribution of rutile in calcined P25 and in the
samples doped with vanadium, see Table 1), while the
second sub-band, expected at 400 nm, is assigned to the
monomeric V5? cations in tetrahedral coordination
deposited on the TiO2 surface [
]. In spectra of the
samples with higher vanadium loadings (0.05VTiO2,
0.1VTiO2 and 0.18VTiO2), apart from the band at 380 nm
another band centered at about 450 nm can be found. The
intensity of this band increases with an increase in
vanadium loading. This band is related to octahedrally
coordinated V5? cations in small aggregates of V2O5 [
Thus, it could be suggested that small amounts of
vanadium introduced to TiO2 are deposited in the form of the
monomeric vanadium cations in tetrahedral coordination,
while an increase in vanadium content results in its
aggregation with the formation of V2O5. Banares et al.
 suggested that highly dispersed vanadium species are
stabilized by surface hydroxyl groups of titania. The
content of such surface hydroxyl groups is larger for anatase
than for rutile, and therefore, phase transformation from
anatase to rutile accelerates the formation of aggregated
V2O5 species, which are characterized by a weaker
interaction with titania support.
The EPR spectra of pure and V-modified TiO2 are
presented in Fig. 5. Compared with pure TiO2, the EPR
spectrum of a series of the VTiO2 samples is significantly
different. For undoped TiO2, the EPR spectra show small
peaks at around 3500–3550 Gauss, which are assigned to
the oxygen vacancy defects [
]. Such vacancy defects are
possibly a result of the formation of Ti3? ions with spectral
slitting characteristic for hyperfine interactions with 47,48Ti
]. In the case of V-doped TiO2, the peaks at
around 3500–3550 Gauss are still present, indicating the
paramagnetic features of the doped V-ions. The intensity of
these signals increases with an increase in loading of
vanadium introduced to the TiO2 support. In the case of the
samples with higher V-content, the signals were broadened
due to spin–spin interactions [
Moreover, the resolved hyperfine coupling, the same as
for the undoped and vanadia-modified samples, shows that
the dopant ions are dispersed in the crystal host. Tian et al.
] reported that the eight-component hyperfine structure
of the V-modified samples characterizes V4? ions, which
are incorporated into the crystal lattice of TiO2. (V5? ions
do not give signals in EPR.) The intensities of signals
raised sharply, when vanadium loading increased from 0.05
to 0.1%, while the differences in signals of 0.02VTiO2 and
0.05VTiO2 as well as between signals of 0.1VTiO2 and
0.18VTiO2 are less significant. This effect could be
explained by incorporation of V4? cations into vacant
positions in TiO2 lattice in the case of the samples with low
vanadium loadings. With increasing content of vanadium,
after filling of all available Ti vacancies by V4? cations,
vanadium is deposited in the form of extra-framework
species, which at higher vanadium content, as it was shown
by UV–Vis DRS studies (see Fig. 4), aggregate to small
crystallites of V2O5. The results of EPR studies support
presented earlier hypothesis related to the possible
oxidation of V4? to V5? on the surface of TiO2 as well as show
that V4? can be incorporated into Ti vacancies in TiO2
lattice and act as initiation nuclei for anatase to rutile phase
The electrochemical properties of the selected samples
were examined by cyclic voltammetry. Figure 6 shows the
cyclic voltammograms obtained for calcined TiO2 and its
modifications with vanadium—0.02VTiO2 and 0.18VTiO2.
The main oxidation wave for the TiO2 sample is visible at
Epa = 165 mV, while less intensive signals are located at
18, 370 and 617 mV. These signals are possibly related to
the oxidation of Ti3? cations present in the structure of
TiO2. As it can be seen stabilization of such Ti3? cations
against oxidation to Ti4? is different and possibly depends
on their surroundings. The reduction wave can be observed
at Epc = - 202 mV and could be assigned to the
reduction of Ti4? to Ti3?. The shape of the voltammogram
profiles shows that the redox process is irreversible and
oxidation dominates over reduction of the TiO2 sample.
Doping of TiO2 with very small amount of vanadium
(0.02VTiO2) resulted in a shift of oxidation waves from
370 and 617 mV to 357 and 613 mV, respectively. This
effect shows that doping of TiO2 with small amounts of
vanadium results in easier oxidation of stable Ti3? sites.
Also reduction process proceeded easier for the 0.2VTiO2
sample in comparison with TiO2 (shift in the position of
reduction wave from - 202 to - 191 mV). An increase in
vanadium content to 0.18 mass% (0.18VTiO2) resulted in a
shift of the main oxidative wave from 165 to 176 mV, what
is possibly related to stronger stabilization of Ti3? cations
in TiO2. However, a shift of small signal from 18 to
- 23 mV may suggest the existence of small fraction of
The samples of V-doped TiO2 were tested as catalysts
for oxidation diphenyl sulfide (Ph2S) by H2O2. The results
of the catalytic studies are presented in Fig. 7. The only
detected products of Ph2S oxidation are diphenyl sulfoxide
(Ph2SO) and diphenyl sulfone (Ph2SO2). It was reported by
many authors that transformation of sulfides to sulfones
proceeds via the sulfoxides intermediates [
]. Thus, the
progress in Ph2S oxidation can be determined by analysis
of the substrate conversion as well as the progress in
selectivity. The diphenyl sulfide oxidation in the presence
of TiO2 resulted in the Ph2S conversion of about 77%, with
67% selectivity to Ph2SO2, after 4 h of the catalytic test.
Introduction of small amounts of vanadium into TiO2,
resulting in the 0.02VTiO2 sample, caused dramatic
increase in the Ph2S conversion and selectivity to Ph2SO2.
The nearly complete conversion of diphenyl sulfide in the
reaction mixture with 100% selectivity to diphenyl sulfone
was obtained after 3 h of the catalytic tests. Thus, it can be
seen that modification of TiO2 with small amount of
vanadium resulted in a very significant activation of the
catalyst in oxidation process. An increase in vanadium
content deposited on TiO2 resulted in gradual decrease in
both Ph2S conversion and selectivity to Ph2SO2; however,
these parameters are still higher than in the case of pure
The leaching of active components deposited on the
support is a common problem of the heterogeneous
catalysis performed in a liquid phase. The analysis of the
solutions after catalytic runs shows that this problem is also
Fig. 7 The results of catalytic oxidation Ph2S by H2O2 over various
present in the case of our VTiO2 samples, especially those
with higher vanadium loadings, which lost about 10% of
active component. This effect was negligible for the
samples with the lowest V-content. It was also shown, by
additional test with the reaction mixture separated from the
solid catalyst, that V-species leached from the catalyst
were inactive in the process of Ph2S oxidation. Thus, the
V-species leached from the solid catalyst are inactive in the
studied process and interaction between such species and
TiO2 support is necessary for their catalytic activation.
Thus, it could be concluded that introduction of small
amounts of vanadium activated TiO2 much more
effectively than deposition of larger amounts of vanadium. As it
was shown by EPR and UV–Vis DRS studies disposition of
small amounts of vanadium on TiO2 results in
incorporation of V4? cations in vacant Ti positions of titania as well
as deposition of V5? cations (or small oligomeric
V-species) on the support surface. So, these forms of vanadium
should be taken into account as potential catalytically
active surface species. Banares et al. [
] reported that V4?
cations incorporated into Ti vacancies of rutile are very
stable and do not have any tendency to be oxidized. Thus,
no oxygen activation on such sites is expected. Probably
the role of catalytically active species play V5? cations
deposited on the TiO2 surface, which can be relatively easy
reduced to V4? or even to V3? and then re-oxidize to V5?
]. An increase in vanadium loading, as it was shown by
UV–Vis DRS, results in aggregation of highly dispersed
V-species to V2O5, which is probably inactive or less
active than dispersed V5? sites. Banares et al. [
suggested that aggregated V2O5 is characterized by weaker
interaction with titania support than highly dispersed
V-species. Similarly to a leak of catalytic activity of
V-species leached from the solid catalyst into solution,
possibly also in the case of V2O5 only weak interaction
with TiO2 support results in its lower catalytic activity in
the studied process. Thus, it seems that both the form of
V-species and their interaction with titania support
determine catalytic properties of the VTiO2 samples.
The V-doped TiO2 samples, with different concentrations
of vanadium, were obtained by wetness impregnation of
the commercial titanium dioxide (P25) powder with
aqueous solutions of ammonium metavanadate with
suitable concentrations. Afterward, the obtained samples were
characterized from the point of view of their structure,
morphology, thermal behavior and catalytic properties in
the process of Ph2S oxidation. Increasing V-content in
TiO2 did not influence in major extent the structural,
thermal and textural properties, but significantly change the
catalytic activity. The catalytic tests of the TiO2 samples
with different V-loading have shown that the introduction
of small amounts of vanadium into titanium support
(0.02VTiO2) resulted in more effective catalytic activation
of titania—the nearly complete Ph2S conversion and 100%
selectivity to final product. As the vanadium content
increases, both Ph2S conversion and selectivity to Ph2SO2
slightly decrease. However, for all the V-modified samples
the parameters characterizing the catalytic activity are still
It was suggested that V5? cations dispersed on TiO2,
which can be relatively easy reduced and re-oxidized, are
the active sites of the studed catalytic reaction. On the other
hand, V4? cations, identified by EPR, are located in vacant
Ti positions of TiO2 and are strongly stabilized against
oxidation. Therefore cannot play a role of oxygen
activation sites. An increase in vanadium loading results in
aggregation of surface V-species to vanadium oxide
(V2O5), which interact with titania significantly weaker
than dispersed vanadium ions in the samples with low
V-loadings and therefore catalysts with higher vanadium
loadings presented lower catalytic activity. Thus, it could
be concluded that both the form of V-species and their
interaction with TiO2 support determine activity of the
Acknowledgements Part of the research was done with equipment
purchased in the frame of European Regional Development Fund
(Polish Innovation Economy Operational Program—contract no.
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