Photocatalytic oxidation of diethyl sulfide vapor over Tio2-based composite photocatalysts

Article abstract of DOI:10.3390/molecules191221424

Composite TiO₂/activated carbon (TiO₂/AC) and TiO₂/SiO₂ photocatalysts with TiO₂ contents in the 10 to 80 wt. % range were synthesized by the TiOSO₄ thermal hydrolysis method and characterized by AES, BET, X-ray diffraction and FT-IR ATR methods. All TiO₂ samples were in the anatase form, with a primary crystallite size of about 11 nm. The photocatalytic activities of the TiO₂/AC and TiO₂/SiO₂ samples were tested in the gas-phase photocatalytic oxidation (PCO) reaction of diethyl sulfide (DES) vapor in a static reactor by the FT-IR in situ method. Acetaldehyde, formic acid, ethylene and SO₂ were registered as the intermediate products which finally were completely oxidized to the final oxidation products - H₂O, CO₂, CO and SO₂²ions. The influence of the support on the kinetics of DES PCO and on the TiO₂/AC and TiO₂/SiO₂ samples' stability during three long-term DES PCO cycles was investigated. The highest PCO rate was observed for TiO₂/SiO₂ photocatalysts. To evaluate the activity of photocatalysts the turnover frequency values (TOF) were calculated for three photocatalysts (TiO₂, TiO₂/AC and TiO₂/SiO₂ ) for the same amount of mineralized DES. It was demonstrated that the TOF value for composite TiO₂/SiO₂ photocatalysts was 3.5 times higher than for pure TiO₂.

Full text of DOI:10.3390/molecules191221424

Molecules 2014, 19, 21424-21441; doi:10.3390/molecules191221424
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Photocatalytic Oxidation of Diethyl Sulfide Vapor over
TiO -Based Composite Photocatalysts
2
Dmitry Selishchev 1,2,3,* and Denis Kozlov 1,2,3
1
Boreskov Institute of Catalysis, pr. Ak. Lavrentieva 5, Novosibirsk 630090, Russia
2
Novosibirsk State University, st. Pirogova 2, Novosibirsk 630090, Russia
3
Research and Educational Centre for Energoefficient Catalysis (NSU), st. Pirogova 2,
Novosibirsk 630090, Russia
*
Author to whom correspondence should be addressed; E-Mail: selishev@catalysis.ru;
Tel./Fax: +7-383-333-1617.
External Editor: Pierre Pichat
Received: 31 August 2014; in revised form: 4 December 2014 / Accepted: 12 December 2014 /
Published: 19 December 2014
Abstract: Composite TiO
TiO contents in the 10 to 80 wt. % range were synthesized by the TiOSO
method and characterized by AES, BET, X-ray diffraction and FT-IR ATR methods. All
TiO samples were in the anatase form, with a primary crystallite size of about 11 nm. The
photocatalytic activities of the TiO /AC and TiO /SiO samples were tested in the gas-phase
photocatalytic oxidation (PCO) reaction of diethyl sulfide (DES) vapor in a static reactor by
the FT-IR in situ method. Acetaldehyde, formic acid, ethylene and SO were registered as
the intermediate products which finally were completely oxidized to the final oxidation
2
/activated carbon (TiO
2
/AC) and TiO
2
/SiO
2
photocatalysts with
2
4
thermal hydrolysis
2
2
2
2
2
2
−
products – H
DES PCO and on the TiO
PCO cycles was investigated. The highest PCO rate was observed for TiO
photocatalysts. To evaluate the activity of photocatalysts the turnover frequency values
TOF) were calculated for three photocatalysts (TiO , TiO /AC and TiO /SiO ) for the same
amount of mineralized DES. It was demonstrated that the TOF value for composite
TiO /SiO photocatalysts was 3.5 times higher than for pure TiO
2
O, CO
2
, CO and SO
4
ions. The influence of the support on the kinetics of
/SiO samples’ stability during three long-term DES
/SiO
2
/AC and TiO
2
2
2
2
(
2
2
2
2
2
2
2
.

Molecules 2014, 19
21425
; activated carbon; silica; composite photocatalyst;
Keywords: photocatalysis; TiO
2
diethyl sulfide
1
. Introduction
Volatile organic compounds containing N, S, P or Cl heteroatoms are often highly toxic and very
dangerous for human health [1–3], and some of them could be used as chemical warfare agents
CWA) [4]. One of the best known CWAs is bis(2-chloroethyl) sulfide or mustard gas (HD). This species
(
is a highly toxic vesicant which causes destruction of cell membranes and nucleic acids. It binds with
nucleophilic groups like sulphur atoms in the SH-groups of proteins and nitrogen atoms in the nitrogen
bases of DNA [5]. The relative toxicity (LD50) for HD inhalation is about 1.5 mg·min/L and this value
is the highest among vesicants [6]. In this way the development of effective methods for HD
neutralization is an important task to ensure human safety.
The main chemical methods of HD detoxification include nucleophilic substitution or oxidation,
which result in the cleavage of C-S or C-Cl bonds and partial or even complete oxidation of the target
molecule [7]. HD can be destroyed by photolysis or oxidized by photogenerated ozone under UV light
irradiation [8]. At the same time the method of photocatalytic oxidation using TiO
is regarded as one of the promising methods of CWA disposal due to the high oxidative ability of TiO
under UV irradiation [9]. PCO makes it possible to destroy dangerous compounds completely with the
2
as the photocatalyst
2
−
3
2−
4
3−
4
−
formation of CO
2
, H
O, NO , SO , PO and Cl as final products [10].
2
In the view of the high toxicity of HD, researchers usually use for laboratory investigations simulants
such as 2-chloroethyl ethyl sulfide [11], 2-chloroethyl methyl sulfide [12], 2-phenethyl 2-chloroethyl
sulfide [13], diethyl sulfide (DES) [14] and dimethyl sulfide [15]. These simulants are safer due to their
lower toxicity, but at the same time they simulate well the chemical behavior of HD.
In the current work we focused on the investigation of DES vapor PCO. In our previous works we
demonstrated that DES can be easily decontaminated under the UV irradiation using TiO
photocatalyst with the formation of CO , H O and surface sulfate and carbonate species as the final PCO
products [14,16–18]. Acetaldehyde, ethanol, ethylene, SO and other trace products were detected as the
2
as the
2
2
2
gas-phase intermediates, while polysulfides, diethyl sulfone, diethyl sulfoxide were detected as the
surface intermediates. All intermediates were completely oxidized to the final products after long-term
irradiation [14,16]. Analysis of intermediates and products allowed the authors to propose the main
routes of the DES PCO, which include C-S bond cleavage, and oxidation of sulfur and carbon atoms.
To enhance the rate of air purification from DES vapor in a closed chamber a TiO aerosol generated
2
by a sonic method could be applied [19]. Aerosol spraying led to the fast adsorption of DES vapor and
its further photocatalytic oxidation under UV irradiation.
The main problem during the long-term DES oxidation is the deactivation of the TiO
An increase of the time required for the complete mineralization of DES was clearly seen from the kinetic
curves of CO accumulation during several oxidation cycles in a batch reactor [14]. FT-IR analysis
2
photocatalyst.
2
demonstrated that the accumulation of non-volatile organic intermediates like polysulfides, diethyl
sulfone, diethyl sulfoxide and sulfate species on the surface of photocatalyst are responsible for its

Molecules 2014, 19
21426
deactivation during the long-term experiments. The positive influence of using the composite
TiO /adsorbent photocatalysts was also discussed in our previously published paper devoted to the
computer simulation of the kinetics of photocatalytic reactions [20] where we demonstrated an increase
of the rate of substrate removal for TiO /adsorbent photocatalysts.
In recent years composite photocatalysts in which TiO is deposited onto the surface of a porous
2
2
2
support like activated carbon (AC), silica or zeolite were actively investigated in the PCO processes of
various pollutants, both in the gas and liquid phases [21–26]. In addition to the increase of the adsorption
capacity in some cases the increase of the PCO rate and the decrease of deactivation degree were
observed for such composite photocatalysts [27–29].
Concerning mustard gas, several research groups have studied PCO of HD simulants using composite
photocatalysts. Cr-modified TiO
DES vapor in a batch reactor under UV irradiation [30]. It was demonstrated that the TiO
the Cr-MCM-41 support increases the DES removal rate, but decreases the CO formation rate if
compares with the commercial Hombifine N TiO (Sachtleben Chemie GmbH, Duisburg, Germany).
Panayotov and co-workers investigated the PCO of 2-chloroethyl ethyl sulfide (CEES) and DES on
a mixed oxide TiO -SiO photocatalyst [31–33]. They revealed that the CEES adsorbs on the surface of
2
-loaded MCM-41 silica photocatalyst was studied in the oxidation of
2
deposition on
2
2
2
2
the composite photocatalyst through both the chlorine and sulfur atoms by bonding to isolated OH
groups. The authors also demonstrated that the presence of the Cl atom in the CEES molecule does not
significantly influence the PCO rate if compared with the DES molecule. Partially or fully oxidized
products were observed during the photooxidation of both tested molecules over the composite
photocatalyst. Partially oxidized products have been demonstrated to block OH groups on the surface of
photocatalysts and to prevent further adsorption of target molecules and to thus reduce the rate of
photooxidation. Unfortunately, no comparison between pure TiO
2
and composite TiO
2
-SiO
2
photocatalyst was done. The main drawback of the previous works is the absence of systematic
investigations of the behavior of composite photocatalysts during the long-term PCO of HD simulants.
In spite of the fact that AC is the most frequently used TiO
SiO material is also promising due to its higher hydrophilicity, transparency and quantity of OH-groups.
In this connection, the main objective of the current study was to investigate the PCO of DES in
the gas-phase over composite photocatalysts in which TiO was deposited onto AC or SiO surfaces. We
investigated the effect of the porous support on the kinetics of DES PCO and on the composite
photocatalyst activity in multiple long-term experiments. Finally, a comparison between pure TiO and
TiO /adsorbent photocatalysts was made.
2
support in composite photocatalysts, the
2
2
2
2
2
2
. Results and Discussion
2
.1. Characterization of the Synthesized Photocatalysts
Synthesis of TiO
method. This method has some advantages in comparison with the popular sol-gel method which utilizes
titanium alkoxides because titanyl sulfate is a cheaper precursor. Synthesized TiO samples were of
anatase crystal structure with a high surface area and good crystallinity. As a result the TiO photocatalyst
2
, TiO
2
/AC and TiO
2
/SiO
2
samples was performed by the TiOSO thermal hydrolysis
4
2
2

Molecules 2014, 19
21427
synthesized by this procedure usually have high photocatalytic activity in the oxidation of volatile
organic compounds [34].
In our previous work TiO
photocatalytic activity [22]. Therefore in this work we prepared TiO
contents equal to 65, 70 and 80 wt. %. The TiO /SiO photocatalysts have high photoactivity even at a
relatively low TiO content, so we prepared several TiO /SiO samples with estimated TiO contents in
the range from 10 to 80 wt. %. Varying the TiO content in the series of TiO /adsorbent samples allows
2
/AC samples with TiO
2
contents higher than 60 wt. % demonstrated high
2
/AC samples with estimated TiO
2
2
2
2
2
2
2
2
2
us to choose a photocatalyst with high adsorption capacity and at the same time with high photocatalytic
activity for further investigation of DES PCO. The results of AES and BET analysis are presented in the
Table 1.
Table 1. TiO
2
content and textural properties of the samples.
2
3
Series
Sample * TiO
2
Content, wt. % Surface Area, m /g Pore Volume, cm /g
AC
--
825
442
208
299
367
401
237
270
298
351
396
0.54
0.78
0.15
0.22
0.27
0.26
0.27
0.37
0.48
0.58
0.68
Supports
SiO
TiO
0-TC
70-TC
5-TC
2
--
TiO
2
2
100
77.5
68.5
62.5
76.5
57.5
39.7
22.2
12.3
8
TiO /AC
2
6
8
6
0-TS
0-TS
TiO
2
/SiO
2
40-TS
2
1
0-TS
0-TS
*
2
Number in the sample label indicates the estimated TiO content, wt. %.
2
It follows from the Table 1 that the synthesized TiO
2
sample has a high surface area (208 m /g) and
3
pore volume (0.15 cm /g). The corresponding values for composite TiO
2
/AC and TiO
2
/SiO
2
photocatalysts are higher because of the higher porosity of AC and SiO
2
. Moreover, the lower the TiO
2
content is the higher specific surface area and pore volume of the composite photocatalyst are
Figure 1). Figure 1 demonstrates that the specific surface area and pore volume of the composite
photocatalyst are slightly lower than the algebraic sum of the corresponding values of TiO and adsorbent
AC or SiO ). This means that a partial blocking of the support surface by TiO nanoparticles occurs.
XRD patterns for pure TiO and composite photocatalysts are presented in Figure 2. It can be seen
that the XRD patterns of TiO /AC catalyst only have anatase peaks while broad activated carbon peaks
are not detected due to the low AC content. Also a small amount of CaCO
(
2
(
2
2
2
2
3
admixture is observed in the
AC sample.
In the TiO
2
/SiO
2
samples, in addition to anatase peaks, there appears a broad silica peak at the 2θ
value equal to 20–30° indicating its amorphous structure. This silica peak overlaps with the 2θ = 25.3°
peak of anatase. By and large the anatase peaks are similar for pure TiO and composite TiO /AC and
TiO /SiO samples which indicates that TiO crystallites have the same size in all cases because the
value of the coherent-scattering domains size is about 11 nm for all samples.
2
2
2
2
2

Molecules 2014, 19
21428
content for the
Figure 1. Dependences of the specific surface area and pore volume on TiO
2
TiO /AC and TiO /SiO composite photocatalysts.
2
2
2
Figure 2. XRD patterns for the pure TiO
2
, AC, SiO and for the composite 70-TC and
2
4
0-TS photocatalysts.

Molecules 2014, 19
21429
Figure 3 shows the IR spectra of all synthesized photocatalysts, AC and SiO
2
supports measured by
the FT-IR ATR technique. Since all measurements were carried out under ambient conditions the water
−
1
δ
s
(H
absorption band in the 2800–3750 cm range corresponds to the stretching vibration of the surface
OH-groups and physically adsorbed H O molecules.
The 1055 and 1113 cm absorption bands in the spectrum of the pure TiO
2
O) absorption band at 1633 cm was recorded in all samples except for AC powder. A broad
−
1
2
−1
2
sample correspond to the
vibrations in sulfate complexes [35]. The presence of sulfur was additionally confirmed by the atomic
emission spectroscopy (AES) results, which revealed about 1.3 wt. % of S. This means that bonded
sulfate complexes remain on the catalyst surface even after thorough washing. The presence of sulfate
groups on the photocatalyst surface was also observed for the TiO
2
/AC sample. For the TiO
2
/SiO
2
catalysts identification of surface sulfate group was difficult because their signals overlapped with the
−
1
stretching vibration bands of Si-O-Si and Si-O-H bonds near the 1000 cm region.
Figure 3. ATR FT-IR spectra for pure TiO
2
, AC, SiO and for the composite photocatalysts.
2
2
.2. Kinetic Experiments
The main purpose of our work was to study the PCO of diethyl sulfide with the composite
photocatalysts and to investigate their stability in long-term experiments. In this connection in the
beginning we optimized the quantity of the photocatalyst. Then we chose the photocatalyst with
adsorptivity and photocatalytic activity in good proportions and finally we investigated its stability in
the DES PCO.

Molecules 2014, 19
.2.1. Effect of the Sample Quantity on the Photocatalytic Activity
21430
2
The photocatalytic activities of pure TiO
2
and composite samples were measured in a continuous flow
formation rate was used as a measure of the
reactor in the reaction of acetone vapor PCO. The CO
2
photocatalytic activity. Photocatalysts were uniformly deposited onto a 3 × 3 cm glass support and then
installed into the continuous flow reactor (see Experimental Section). The quantity of photocatalyst
2
deposited was measured in mg/cm units. For the quantity optimization experiments several glass
supports with different quantities were prepared for all photocatalysts. For the pure TiO
2
the quantities
2
were 0.25, 0.5, 1, 2 and 3 mg/cm . For the composite samples their quantities were adjusted in such a
2
way that the quantities of contained TiO
2
were in the 0.2–3 mg/cm range. For example the 40-TS
2
sample contains 39.7 wt. % of TiO
2
(Table 1) and to achieve the 0.25 mg/cm value a quantity of
2
0
.25/0.397 = 0.63 mg/cm of 40-TS sample was deposited onto the glass support.
Figure 4 demonstrates the dependencies of the steady-state rate of CO
oxidation over TiO , TiO /AC and TiO /SiO samples on the quantity of contained TiO
the contained TiO on the glass support, the thicker the photocatalyst layer is.
2
formation during acetone
2
2
2
2
2
. The higher is
2
Figure 4. Influence of the quantity of photocatalyst on its photocatalytic activity.
It could be seen that in all cases the PCO rate achieves the maximum value. It corresponds to the
situation when the incident light is completely absorbed by the photocatalyst and any further increase in
the quantity of photocatalyst leads to the formation of the bottom unirradiated photocatalyst layers which
do not work.
The quantity of photocatalyst which corresponds to the maximum PCO rate depends on the TiO
2
content and its dispersion. For example for the commercial Hombifine N TiO
2
(Sachtleben Chemie
2
2
GmbH, 100% anatase, SBET = 350 m /g) the maximum rate quantity is about 1 mg/cm , whereas for the
synthesized TiO
It should be noted that at low sample quantity the 40-TS sample is more active than pure TiO
follows from the Table 1 both photocatalysts have the same size of TiO crystallites—11 nm—therefore
sample it is about 2 mg/cm2.
2
2
. As it
2

Molecules 2014, 19
21431
the difference of activities could be explained by a higher dispersion of the TiO
2
particles deposited onto
the silica in the 40-TS sample than in the pure TiO
On the other hand activities of the TiO /AC samples are lower than for pure TiO
silica, AC absorbs UV irradiation. Due to low photocatalytic activity of TiO /AC photocatalysts we used
content. Two conclusions could be reached from the
2
.
2
2
because unlike
2
samples 80-TC and 70-TC with high TiO
2
above discussion:
(
1) Comparison of the photocatalysts’ activity should be done using high quantities when the
thickness of the photocatalyst layer is sufficient for complete light absorption (e.g., 2–3 mg/cm2).
We used this approach when choosing a photocatalyst with good adsorptivity and photocatalytic
activity (see Section 2.2.2);
(
2) Studies of long-term photocatalyst use should be done using a relatively low TiO
2
quantity (e.g.,
2
0
.5 mg/cm ) because in this case we can assume that the entire photocatalyst surface is irradiated
and is involved in the reaction process. This is the reason why we investigated the diethyl sulfide
2
oxidation with a 0.5 mg/cm quantity of TiO
2
.
2
.2.2. Effect of TiO
2
Content on the Photocatalytic Activity of the TiO
2
/SiO
2
Catalyst
Figure 5 demonstrates dependencies of the steady-state rate of CO
2
formation during acetone
oxidation over the TiO
2
/SiO
2
photocatalysts. The quantity of catalysts on the glass supports in these
2
experiments was 3 mg/cm in order to compare the highest possible photocatalytic activity of the samples.
Figure 5. Dependence of the CO
2
formation rate during acetone PCO on the TiO content
2
for TiO /SiO series.
2
2
All TiO
absorb UV light. The CO
.79 μmol/min and it was only 2.5 times lower than for a pure TiO
2
/SiO
2
samples demonstrate high activity, even at a low TiO
formation rate for the 10-TS sample which contains 12 wt. % of TiO
sample.
2
content, because silica does not
2
2
was
0
2

Molecules 2014, 19
21432
The oxidation rate increases with the increase of TiO
for the 40-TS sample with 40 wt. % TiO
activity and it means that at 40 wt. % TiO
the TiO particles. Therefore the following DES PCO experiments were conducted with the 40-TS
2
content and achieves almost the highest value
2
content. The 60-TS and 80-TS samples have slightly higher
content SiO particles are already completely covered with
2
2
2
sample which demonstrated high adsorption capacity due to its high content of porous support and at the
same time high photocatalytic activity.
2
.2.3. Kinetics of the DES PCO in a Static Reactor
The main objective of the experiments in the static reactor was to compare the kinetics of DES
oxidation over pure TiO
photocatalysts’ deactivation during three consecutive DES PCO cycles.
Composite 80-TC, 70-TC and 40-TS samples as well as pure TiO
2
and composite TiO
2
/AC and TiO
2
/SiO
2
photocatalysts and to compare the
2
photocatalyst were chosen for
these investigations. Sample quantities were correspondingly adjusted to 0.65, 0.73, 1.3, and 0.5 mg/cm2
2
for 80-TC, 70-TC, 40-TS and pure TiO
2
, so that the net TiO
2
quantity was equal to 0.5 mg/cm in all
cases. The same amount of active component (i.e., TiO
2
) placed in the reactor allowed us to carry out a
valid comparison of photocatalyst deactivation for the samples with different TiO
2
content and to
estimate the effect of the support.
Н
2
O, CO
2
and СO were detected as final gaseous oxidation products. The concentration of CO did
concentration which was
not exceed the 55 ppm level and it was which much lower than the final CO
2
equal to about 1400 ppm. The final surface products of DES PCO were sulfate complexes. The
accumulation of sulfates on the photocatalysts surface was confirmed by FT-IR analysis and it was the
reason of irreversible photocatalysts deactivation.
Acetaldehyde (CH
3
CHO), formic acid (HCOOH), ethylene (C
2
H
4
) and SO were detected in the gas
2
phase as intermediates of DES PCO. All intermediates were completely oxidized to final products during
the long-term irradiation. Noticeable concentrations were detected only for acetaldehyde and formic
acid, therefore their kinetic curves were discussed along with DES removal and CO
2
accumulation
kinetic curves.
SO
the 30-50 ppm level. Quantitative analysis of ethylene was not performed due to its low concentration.
Kinetic curves of DES, acetaldehyde, formic acid and CO during the first and the third cycle of
.5 μL DES PCO in the static reactor over TiO , 80-TC, 70-TC and 40-TS samples are presented in the
Figures 6 and 7, respectively.
2
was detected in the gas phase only during the first PCO cycle and its concentration did not exceed
2
0
2
Besides irreversible photocatalyst deactivation caused by attachment of sulfate ions to the photocatalyst
surface, temporal deactivation was also observed. In the kinetic curves this is well illustrated by the
intense accumulation of acetaldehyde in the gas phase and the increase of induction period for the CO
2
kinetic curves in the beginning of PCO run (compare the same samples in Figures 6 and 7).
The reason for this temporal deactivation is the formation of partial oxidation products like diethyl
sulfoxide, diethyl sulfone and others [14,16]. These non-volatile compounds accumulate on the
photocatalyst surface and hinder the PCO process. The continuous photocatalyst irradiation results in
the gradual oxidation of surface non-volatile species making the catalyst surface available for further
DES destruction. At this moment the fast removal of acetaldehyde from the gas phase and intensive

Molecules 2014, 19
21433
accumulation of CO
2
begin. Formation of formic acid in the gas phase during DES PCO could be
explained by its low adsorption on the photocatalyst surface due to its low molecular weight.
Figure 6. Kinetics of 0.5 μL DES PCO in the static reactor during the first cycle over the
pure TiO , 80-TC, 70-TC and 40-TS samples.
2
Figure 7. Kinetics of 0.5 μL DES PCO in the static reactor during the third cycle over the
pure TiO , 80-TC, 70-TC and 40-TS samples.
2

Molecules 2014, 19
21434
After the long-term irradiation complete oxidation of all intermediates was observed and the final
CO concentration reached the expected 1416 ppm value calculated from the mass balance.
2
The use of the composite photocatalyst increases the available surface. As a result the DES removal
rate and kinetics of photooxidation change. A decrease of the time needed for complete removal of DES
vapor from the gas phase was observed for the composite photocatalysts. For example, in the first
oxidation cycle the time values of DES removal were 73, 14, 9 and 22 min for TiO
2
, 80-TC, 70-TC and
4
0-TS samples, respectively. The increase of the rate of DES removal can be explained by reversible
transfer of non-volatile intermediates from the TiO
2
surface onto the support surface (AC or SiO ). As a
2
result active sites on the TiO surface remained free for further interaction with DES molecules. This
2
effect was discussed in our previous work [20].
The fast DES removal in the case of composite photocatalysts led to a decrease of the induction period
of CO
period in the first oxidation cycle was 18.9, 18.7, 14.7 and 33.2 ppm/min for TiO
0-TS samples, respectively. In contrast to the composite photocatalyst the CO accumulation rate over
pure TiO sample was declining strongly with the increase of reaction time. As in the case of acetone
PCO, the 40-TS demonstrated the highest activity in the DES PCO.
2
accumulation by about 2-fold (Figure 6). The initial rate of CO
2
accumulation after the induction
2
, 80-TC, 70-TC and
4
2
2
In each subsequent oxidation cycle over the same sample a decrease of DES PCO rate was observed.
As a result in the third cycle the times of complete DES removal were 600, 550, 315 and 220 min,
respectively, for the pure TiO , 80-TC, 70-TC and 40-TS samples (Figure 7). This means that a strong
2
deactivation of the samples occurs. To estimate the extent of photocatalyst deactivation the time of 90%
DES mineralization was calculated in each cycle. Calculated times for all samples in each oxidation
cycle are presented in Figure 8.
Figure 8. Time values of 90% DES conversion into CO
TiO and TiO /adsorbent composite photocatalysts.
2
in three oxidation cycles for pure
2
2

Molecules 2014, 19
21435
As it follows from Figure 8, the time of the complete mineralization of DES becomes higher for each
subsequent PCO cycle. For example, for the pure TiO sample in the first cycle the reaction was complete
after 290 min, but in the third cycle it took 1522 min. Photocatalyst deactivation is decreasing in the
following sequence: TiO > 80-TC > 70-TC > 40-TS. For example, the sum of mineralization times in
all three cycles for the pure TiO sample is equal to 2848 min but for the most active and stable composite
0-TS photocatalyst it is only 800 min.
The excellent behavior of the 40-TS sample in the DES PCO can be explained by its high
photocatalytic activity and large surface area which is available for the adsorption of intermediates.
TiO /AC samples are less active than the 40-TS sample, but are still better than pure TiO samples. It
2
2
2
4
2
2
should be noted that the 70-TC sample demonstrated lower deactivation than the 80-TC sample due to
its higher content of AC. In addition, the lowest concentration of gaseous intermediates, acetaldehyde
and formic acid among all synthesized samples, was also observed for the 70-TC sample.
Decrease of deactivation in the case of composite photocatalyst as well as the increase of DES
removal rates can be explained by reversible transfer of non-volatile intermediates, which are the reason
of deactivation, from TiO
activity of the composite photocatalysts is the possible transfer of OH radicals from TiO
support surface. Such a possibility was shown by Carretero-Genevrier et al. [36], who demonstrated that
OH radicals could migrate up to 10 nm distance from TiO surface into the SiO matrix.
Finally, turnover frequency (TOF) was calculated for all samples. The total amount of mineralized
2
particles onto the support [20]. Another possible explanation for the increased
2
onto the
2
2
1
8
DES for three consecutive runs was 3 × 0.5 = 1.5 μL or 8 × 10 molecules. For all samples the amount
of active component (i.e., TiO
2
) was the same—3.5 mg. To estimate the surface active sites concentration
1
4
2
we used the value of 5 × 10 a.s./cm proposed by Ollis in 1980 [37].
Based on these data it is possible to calculate the total number of active sites:
ꢀ
.ꢁ.
× 208 ꢈ × 0.0035g = 3.6 × 10 a.s.
ꢂꢃꢄꢅꢆꢇ
ꢆꢇ
× 1014
18
5
(
1)
The TiO specific surface area from Table 1 was used for estimation of the active surface area because
2
these experiments were performed at low sample quantity and we supposed that the entire photocatalyst
surface was irradiated. The total time of complete DES mineralization was calculated as the sum of
mineralization times in three consecutive PCO cycles presented in the Figure 8.
−5
−5
−5
−5 −1
The estimated TON values are 1.3 × 10 , 1.9 × 10 , 3.0 × 10 and 4.6 × 10 s for TiO
0-TC and 40-TS samples, respectively. The TOF value for 40-TS composite photocatalyst is 3.5 times
higher than for pure TiO sample. Fast purification of air from the DES vapor over composite TiO /SiO
2
, 80-TC,
7
2
2
2
photocatalyst and its low deactivation during the long-term oxidation can thus be used for the
development of purification methods against S-containing CWAs.
3. Experimental Section
3
.1. Materials
The following chemical reagents were used for the catalyst preparation and oxidation experiments:
titanyl sulfate (TiOSO
4
·2H
2
O, >98%, Vekton, St. Petersburg, Russia), sulfuric acid (H
2
SO , 93.5%–95.6%,
4

Molecules 2014, 19
21436
SC
PKF Ant, Russia), acetone (CH
3
COCH
3
, >99.8%, Mosreaktiv, Moscow, Russia), DES (C
2
H
5
2
H
5
,
>
98%, Fluka, Buchs, Switzerland). The reagents were used as supplied without further purification.
2
Activated carbon (AC) obtained by steam-gas activation of wood matter with SBET = 825 m /g and
2
silica with SBET = 440 m /g and particle size of 10–40 μm were used for immobilization of TiO
2
particles.
AC powder (Sorbent, Perm, Russia) was boiled before synthesis in distilled water during several hours
to remove ionic impurities and finally washed out thoroughly by deionized water. Silica powder was
supplied from Sigma-Aldrich (St. Louis, MO, USA) and used without any treatments. Titanyl sulfate
water solution with a concentration of approximately 10 wt. % was used for TiO
2
deposition by the
thermal hydrolysis method.
3.2. Synthesis of the Composite Photocatalyst
Composite photocatalysts were synthesized by thermal hydrolysis of TiOSO
4
according to the
procedure described in details previously [22]. Typically, a certain amount of activated carbon or silica
powder was suspended in a titanyl sulfate water solution (300 mL) and boiled for 5 hours under
constant mixing. The calculated TiO
AC-containing samples and 10–80 wt. % for SiO
by adding a certain amount of support to 300 mL of TiOSO
silica were marked as X-TC or X-TS correspondingly, where X was the TiO
reference TiO sample was synthesized by the same procedure without addition of support (AC or silica)
and marked as TiO
2
content in the sample was varied in the range of 65–80 wt. % for
-containing samples. The TiO content was adjusted
solution. The samples containing AC or
content (wt. %). The
2
2
4
2
2
2
.
3
.3. Characterization Method
The Ti content in the synthesized samples was determined by atomic emission spectroscopy using an
Optima 4300 DV spectrometer (PerkinElmer, Waltham, MA, USA). Content of TiO was recalculated
2
using these results according to the stoichiometric formula of oxides. The surface area and pore volume
of the samples were measured by nitrogen adsorption at 77 K using the ASAP 2020 instrument
(
Micromeritics, Norcross, GA, USA). The specific surface area was calculated using the BET analysis
and the pore volume was determined as total pore volume at P/P ~1. X-ray diffraction was applied to
determine crystal phase composition and size of crystalline particles. XRD patterns were recorded using
a D8 Advance (Bruker AXS GmbH, Karlsruhe, Germany) diffractometer with CuK radiation. The
0
α
calculation of coherent-scattering domains size was performed using the Scherrer equation:
ꢉꢊ
<
D> =
ꢋ(ꢌꢍ)ꢎꢏꢁꢍ
(
2)
with K equaled to 1.
The surface of samples and initial supports were investigated by FT-IR analysis using attenuated total
reflectance technique. IR spectra of sample surface were registered using a Varian 640-IR FT-IR
spectrometer (Varian Inc., Palo Alto, CA, USA) equipped by the ATR attachment. Samples were not
treated in any way before analysis.

Molecules 2014, 19
21437
3
.4. Kinetic Experiments
3
.4.1. Acetone Oxidation
Acetone oxidation was investigated in the continuous flow unit described in details previously [38].
The continuous flow unit was equipped by an IR long-path gas cell (Infrared Analysis Inc., Anaheim,
CA, USA) installed in a FT801 FT-IR spectrometer (Simex, Novosibirsk, Russia). Standard operational
parameters were the following: acetone concentration—20 ± 4 μmol/L, temperature—40 °C, relative
humidity—22% ± 2%, volumetric flow rate (U)—0.058 L/min. Detailed information about the effect of
acetone concentration on the oxidation rate in the continuous flow unit is presented in Figure S1 in the
Supplementary Information.
2
A certain amount of sample was uniformly deposited on a glass support of 9.1 cm surface area and
irradiated with UV light produced by a UV LED (Nichia, Tokushima, Japan) with λmax ~373 nm. The
2
sample irradiance in the 320–400 nm region was 9.7 mW/cm . The measurement of light intensity was
performed using a Spectrilight spectroradiometer (International Light Technologies, Peabody, MA,
USA). The emission spectrum of the UV LED is presented in Figure S2 in the Supplementary
Information section.
The concentration of acetone and CO in the reaction mixture was calculated from the FT-IR spectra
2
using the integral form of the Beer-Lambert law:
ꢓꢌ
ꢐ
ꢑ(ꢒ) ꢔꢒ = ε × l × C
(3)
ꢓꢂ

I (ω) 
0
where A(ω) = lg
—absorbance, ω and ω —limit of the corresponding absorption band


1
2
I(ω)


−1
−1
−2
(
cm ), ε —coefficient of extinction (L·μmol ·cm ), l —optical path length (cm), C—gas phase
concentration (μmol/L).
The rate of CO formation was used to evaluate photocatalytic activity and was calculated according
2
to the following formula:
ꢕꢖꢗꢇ = Δ ꢘꢖꢗꢇ × U
concentrations in the outlet and inlet air streams of the reactor and
(4)
where Δ ꢘꢖꢗꢇis the difference in CO
2
U is the volumetric flow rate.
3.4.2. DES Oxidation
Oxidation of the DES vapor was investigated in a 0.3 L static reactor installed in the cell compartment
of a Nicolet 380 FT-IR spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA). A detailed
description of the experimental setup was presented in our previous work [22].
2
The sample was uniformly deposited onto a 7.0 cm glass support which was placed in the reactor
and irradiated with UV light produced by the UV LED described above during several hours in order to
completely oxidize all organic species previously adsorbed on the catalyst surface during its storage.
The sample irradiance in the 320–400 nm region was 10.2 mW/cm2.
After sample training 0.5 μL of liquid DES was injected into the reactor and evaporated during
3
0 min to achieve adsorption-desorption equilibrium. Then the UV LED was turned on and IR spectra

Molecules 2014, 19
21438
were taken periodically. Concentrations of DES and other oxidation products in the gas phase were
calculated using the Beer-Lambert law described above. The details of the quantitative calculations using
IR spectra can be found in [39]. The IR spectra of individual substances which were detected in the gas
phase during the DES PCO and other information which was used for calculation of extinction
coefficients for each substance are presented in the Supplementary Information section in Figure S3 and
Table S1.
After complete mineralization of DES in the reactor (i.e., when the amount of accumulated CO
2
reached the expected level calculated from the stoichiometric equations) the reactor was swept with fresh
air and the next DES oxidation cycle was performed. Three oxidation cycles of the same amount of DES
were performed for each sample.
4
. Conclusions
Composite TiO
2
/adsorbent photocatalysts were synthesized by the TiOSO
and were tested in the photocatalytic oxidation of
acetone and DES vapor. The following conclusions were made:
4
thermal hydrolysis
method in the presence of activated carbon or SiO
2
(
1) The usage of composite photocatalyst results in up to an 8-fold decrease of DES removal time if
compared with pure unmodified TiO . This could be explained by an increase of the available
surface area in the case of composite photocatalyst and reversible transfer of non-volatile
intermediates from TiO surface to the support surface thus keeping the photocatalyst
2
2
surface available for further interaction with substrate. Additionally the removal of
intermediates—acetaldehyde and formic acid—occurs faster over composite photocatalyst;
2) The long-term oxidation of DES leads to a strong deactivation of the photocatalyst. The
(
deactivation decreases in the following sequence: TiO
and stable catalyst is the TiO /SiO one which contains 40 wt. % of TiO
number for this sample is 3.5 times higher than for pure TiO
2
>TiO
2
/AC>TiO
2
/SiO
2
. The most active
2
2
2
. The calculated TOF
2
.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/19/12/21424/s1.
Acknowledgments
The work was performed with support of the Skolkovo Foundation (Grant Agreement for Russian
educational organization №1 on 28.11.2013).
Author Contributions
Dmitry Selishchev synthesized the photocatalysts and performed experiments; Dmitry Selishchev and
Denis Kozlov analyzed the data and wrote the manuscript. All authors read and approved the final
version of the manuscript.

Molecules 2014, 19
21439
Conflicts of Interest
The authors declare no conflict of interest.
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(
Sample Availability: Samples are available from authors.
©
2014 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
http://creativecommons.org/licenses/by/4.0/).
(