Facile encapsulation of P25 (TiO2) in spherical silica with hierarchical porosity with enhanced photocatalytic properties for gas-phase propene oxidation

Article abstract of DOI:10.1016/j.apcata.2018.07.024

In this work, we have performed the encapsulation of a reference TiO₂ material (P25) in spherical silica with hierarchical porosity using a sol-gel methodology. The same P25 material has been encapsulated within a “classical” MCM-41 mesoporous silica and a precipitated silica. The materials synthesized in this work were characterized by ICP-OES, SEM, TEM, EDX, XRD, UV–VIS, TG, and nitrogen adsorption. It has been observed that the P25 samples encapsulated in silica present improved CO₂ production rates per mol of P25 in the photooxidation of propene, compared to P25 alone as well as the physical mixture of the two components. Moreover, the sample with a low content of P25 encapsulated in silica with hierarchical porosity presents the highest CO₂ production rates per mol of P25 with respect to the other P25/silica samples, due to a better accessibility of the titania phase and improved illumination of the active phase. Furthermore, the hierarchical porosity of the silica shell material favours mass transport and an increased concentration of reagents by adsorption near the titania phase. This improvement in photocatalytic activity is obtained by following a simple and reproducible synthesis methodology that employs an established silica preparation protocol. Thus, the choice of a silica with an adequate porosity for this application is proven to be a promising advancement in the development of efficient photocatalysts.

Full text of DOI:10.1016/j.apcata.2018.07.024

Applied Catalysis A, General 564 (2018) 123–132
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Facile encapsulation of P25 (TiO ) in spherical silica with hierarchical
2
porosity with enhanced photocatalytic properties for gas-phase propene
oxidation
⁎
J. Fernández-Catalá, D. Cazorla-Amorós, Á. Berenguer-Murcia
Materials Institute and Inorganic Chemistry Department, University of Alicante, Ap. 99, E-03080 Alicante, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords:
2
In this work, we have performed the encapsulation of a reference TiO material (P25) in spherical silica with
TiO
SiO
Composites
Photocatalytic activity
Propene
2
hierarchical porosity using a sol-gel methodology. The same P25 material has been encapsulated within a
classical” MCM-41 mesoporous silica and a precipitated silica. The materials synthesized in this work were
characterized by ICP-OES, SEM, TEM, EDX, XRD, UV–VIS, TG, and nitrogen adsorption. It has been observed that
the P25 samples encapsulated in silica present improved CO production rates per mol of P25 in the photo-
oxidation of propene, compared to P25 alone as well as the physical mixture of the two components. Moreover,
the sample with a low content of P25 encapsulated in silica with hierarchical porosity presents the highest CO
2
“
2
2
production rates per mol of P25 with respect to the other P25/silica samples, due to a better accessibility of the
titania phase and improved illumination of the active phase. Furthermore, the hierarchical porosity of the silica
shell material favours mass transport and an increased concentration of reagents by adsorption near the titania
phase. This improvement in photocatalytic activity is obtained by following a simple and reproducible synthesis
methodology that employs an established silica preparation protocol. Thus, the choice of a silica with an ade-
quate porosity for this application is proven to be a promising advancement in the development of efficient
photocatalysts.
1. Introduction
non-porous material) and low concentration of active surface groups,
such as hydroxyl groups, as main drawbacks [17].
One of the most extensively researched photocatalysts over the past
decades up to date is titanium dioxide (TiO ), due to its unique prop-
erties including high photocatalytic efficiency, physical and chemical
stability, and relatively low cost and toxicity [1–3]. Moreover, this
semiconductor is an important industrial product in many applications
such as inorganic pigment, photocatalysis, sunscreen and energy sto-
In the last years, many efforts have been made to increase the
porosity of TiO by synthesis of new nanoporous TiO materials
[18–20]. Many works have also focused on the fabrication of Ti/Ad-
sorbent composites or supported TiO on the surface of adsorbents to
improve the aforementioned factors, with the aim of enhancing the
photocatalytic activity of TiO . The most interesting materials used as
2
2
2
2
2
rage, among others [4–10]. An interesting commercially available TiO
powder is P25 (EVONIK), which consists approximately of 80% anatase
and 20% rutile titanium dioxide [11]. This commercial TiO is widely
used as photocatalyst in photochemical reactions due to its high activity
both in aqueous phase and in gas phase [12]. In many cases P25 is used
as benchmark material to compare the potential of different photo-
catalysts in both mediums [13,14].
2
adsorbents in these composites are carbon materials, zeolites, meso-
porous materials, and polymers, among others [17,21–23].
2
TiO
2
/SiO
2
materials have attracted a great deal of attention due to
as support or as component of composites, due
the advantages of SiO
2
to its tunable surface area and pore size, as well as interconnectivity of
the pore network facilitating mass transport, high surface acidity
(presence of Ti-O-Si) species, abundant surface hydroxyl groups and
transparency in a wide wavelength range in the UV/Vis region [17,24].
Some studies have pointed out that the photocatalytic activity of
2
Many factors influence the photocatalytic properties of TiO , such
as particle size, morphology, exposed lattice planes, and crystalline
phase [9,15,16]. Despite the fine-tuning through which P25 has already
undergone, this material presents low surface area (it is essentially a
TiO
2 2 2
/SiO composites is worse than that of the pure TiO due to a
blocking effect exerted by the silica on the active phase. For example,
⁎
Corresponding author.
E-mail address: a.berenguer@ua.es (Á. Berenguer-Murcia).
https://doi.org/10.1016/j.apcata.2018.07.024
Received 21 May 2018; Received in revised form 10 July 2018; Accepted 16 July 2018
Available online 17 July 2018
0926-860X/ © 2018 Elsevier B.V. All rights reserved.

J. Fernández-Catalá et al.
Applied Catalysis A, General 564 (2018) 123–132
K.J. Nakamura et al. showed a decreased rate of photocatalytic de-
work studies the effect of the percentage of P25 encapsulated in the
silica with hierarchical porous texture in order to test the illumination
efficiency and optimize the photocatalytic activity. This type of study,
has not been undertaken through this approach to the best of our
knowledge.
gradation of methylene blue when using TiO
25]. Another report by M. Nussbaum et al. demonstrated that as the
thickness of the SiO shell (measured in SiO layers prepared by che-
2 2
encapsulated in SiO
[
2
2
misorption-oxidation cycles according to the authors) around the TiO
photocatalyst decreased the activity of the active phase [26].
On the other hand, many studies show that the presence of SiO
2
The materials studied in this work will be tested in the elimination
of propene since this type of composites (TiO /SiO ) can improve the
2 2
2
with TiO
2
particles is not deleterious. In fact, several reports point out
[27–30]. In the last years,
photooxidation of VOCs as reported in the literature [17,21]. Moreover,
these compounds are known to be very harmful to the environment and
human health [34,35]. In particular, this work deals with the removal
of propene al low concentration because this molecule might be taken
as a representative example of low molecular weight VOCs [36]. These
results are compared to the benchmark P25 and other related materials
[37,38], in order to study the effect of the porous texture of the silica on
the photocatalytic performance.
that silica favours the photoactivity of TiO
2
many efforts have been made to synthesize a nanostructured photo-
catalyst with enhanced adsorption and molecular-sieving properties,
with tunable conformations, as well as improved mass transfer in order
to optimize the activity of the photocatalyst. In this aspect, S. Wang
et al. synthesised Core(TiO )@shell(SiO ) nanoparticles, with a void
2 2
interlayer [31]. Another approach was carried out by Y. Kuwahara et al.
where they developed yolk–shell nanostructured photocatalysts, con-
sisting of TiO
2
nanoparticles in the core of spherical hollow silica shells
/SiO and TiO /ZrO nano-
[
32]. Also, X. Chen et al. synthesised TiO
2
2
2
2
2. Experimental section
composites with hierarchical macro/mesopores [29]. In all cases the
authors intended the preparation of well-defined porous textures of
their composites in order to favour mass transport and the selective
concentration of reagents on the photoactive surface so as to optimize
the photocatalytic activity. Some authors used this type of composites
for the selective degradation of molecules due to the textural properties
that silica brings forth to the composite [31,33].
2.1. Materials
Titanium Tetramethyl orthosilicate (TMOS, 99%, Sigma-Aldrich),
Tetraethyl orthosilicate (TEOS, 99%, Sigma-Aldrich), glacial acetic acid
(HAc, 99%, Sigma-Aldrich), Pluronic F-127 (F-127, Sigma-Aldrich),
Cetyltrimethylammonium bromidedeionized (Sigma-Aldrich), urea
Considering the extensive background, this work approach consists
(99%, Merck), ammonium hydroxide (NH
ethanol (EtOH, 99.8%, Fisher Scientific), TiO
4
OH, 30%, Panreac), absolute
(P25, Rutile: Anatase/
of a facile and reproducible synthesis of TiO
2
/SiO
2
composites through
2
the encapsulation of commercial P25 in silica materials with markedly
different porous textures (combining micro, meso, and macro-porosity
in the best case scenario) in order to improve the photocatalytic activity
by enhancing the adsorption characteristics of the composites (see
Scheme 1). In this respect, the use of a silica shell displaying hier-
archical porosity favours the aspects commented previously. Also, this
85:15, 99.9%, 20 nm, Degussa) and deionized water were used in the
present work. All reactants were used as received, without further
purification. The ultrasonic probe equipment SONOPULS HD 2200
(BANDELIN electronic GmbH & Co. KG) was used to disperse the P25
powder in water.
Scheme 1. Schematic representation of the synthetic procedure followed for the preparation of silica samples with different porous textures.
124

J. Fernández-Catalá et al.
Applied Catalysis A, General 564 (2018) 123–132
2
.2. Preparation of P25 (TiO
2
) encapsulated in silica with hierarchical
samples was carried out by treating them with HF at room temperature.
Thermogravimetric analysis was done in a thermobalance (SDT
porosity
2
960 instrument, TA). In these analyses, the sample was heated up at
−
1
Commercial TiO
2
(Evonik P25) was encapsulated in spherical silica
900 °C in air (heating rate of 5 °C min ).
with hierarchical porosity following a straightforward sol-gel synthesis
adapted from previous reports [39].
As an illustrative example, the synthesis of the P25/spherical hier-
2
archical silica done to obtain a nominal loading of 20 wt.% TiO was
X-ray diffraction (XRD) analysis was carried out in a SEIFERT 2002
equipment. Cu Kα (1.54 Å) radiation was used. The scanning velocity
was 2°/min, and diffraction patterns were recorded in the angular 2θ
range of 6–80 º.
performed as follows: 0.32 g of P25 were dispersed in 10.1 g of water
using an ultrasound probe (Bandelin SONOPULS HD 2200) with a
power of 660 W operating at 30% power for 5 min. In order to prepare
the composite material, the P25 dispersion in water was mixed with
The UV-VIS/DR spectroscopy analysis was performed in an UV–vis
spectrophotometer (Jasco V-670), with an integrating sphere accessory
and powder sample holder. BaSO was used as the standard reference
4
and the reflectance signal was calibrated with a Spectralon standard
(Labsphere SRS-99-010, 99% reflectance). The absorption edge wave-
length was estimated from the intercept at zero absorbance of the high
slope portion of each individual spectrum in the range 200–800 nm
(absorbance method). Then, the band gap can be calculated [40] as:
0.9 g of urea, 0.81 g of F-127 and 5.8 μl of glacial HAc. This solution
was stirred for 80 min. The solution was cooled at 0 °C and 4 g of TMOS
were added dropwise under vigorous stirring. The resulting solution
was kept under stirring for 40 min at 0 °C. The solution was rapidly
transferred to a 40 ml autoclave and heated at 40 °C for 20 h to promote
gel formation. The temperature was later increased to 120 °C with a
dwelling time of 6 h to promote the decomposition of urea to generate
the mesoporosity. The resulting encapsulated P25 was calcined at
1
239.8
λ
Eg =
where Eg is the band gap energy (eV) and λ is the edge wavelength
(
nm).
500 °C for 6 h with a heating rate of 1 °C/ min in order to remove the
Nitrogen adsorption–desorption isotherms were performed in an
templates and any leftover reagents used in the synthesis.
Autosorb-6B apparatus from Quantachrome [37]. Prior to analysis all
samples were degassed at 250 °C for 4 h. BET surface area (SBET) and
total micropore volume (VN2) were determined by applying the Bru-
nauer–Emmett–Teller (BET) equation, and the Dubinin–Raduskevich
In this work, the amount of P25 in the synthesis was changed to
obtain composites with a nominal TiO
0.16 g and 0.64 g, respectively). All samples were calcined as described
above. The nomenclature of these samples is P25_10/SiO , P25_20/SiO
and P25_40/SiO for the samples containing 10, 20, and 40 wt.% of
nominal P25 loading, respectively.
2
loading of 10 and 40 wt.%
(
2
2
2
equation to the N adsorption data obtained at −196 °C, respectively.
2
Total pore volumes were determined by nitrogen adsorption volume at
a relative pressure of 0.95. Mesopore size distributions for all the
samples were obtained applying the Barrett-Joyner-Halenda (BJH)
2
.3. Preparation of P25 (TiO
2
) in a precipitated silica (M001) and a
2
equation to the N desorption branch data from the adsorption iso-
mesoporous silica (MCM-41)
therms at −196 °C, using the software provided by Quantachrome [41].
Transmission electron microscopy (TEM) images were taken using a
JEOL JEM-2010 equipment. Field-emission scanning electron micro-
scope (FE-SEM) images were taken using a ZEISS, Merlin VP Compact,
this equipment has incorporated a microanalysis system by Energy
Dispersive X-ray spectroscopy (EDX), BRUKER Quantax 400 for per-
forming elemental mapping of Si and Ti species present in the samples.
For comparison purposes, two more composites of P25 encapsulated
20 wt.% nominal loading) with precipitated silica [37] and a meso-
porous silica (MCM-41 type) [38] were also prepared. We used the
same methodology as that employed to encapsulate the P25 in hier-
archical silica.
The synthesis of the P25 encapsulated in a precipitated silica was
performed as follows: 0.4 g of P25 were dispersed in 20 g of water with
an ultrasound probe (Bandelin SONOPULS HD 2200) with a power of
(
2.5. Catalytic tests
6
60 W output operating at 30% power for 5 min. In order to prepare the
composite material, the P25 dispersion in water was mixed with 4 ml of
NH OH, 85 g of EtOH to favour the precipitation of the silica, 70 g of
The photocatalytic performance of the different materials was stu-
died using an experimental system designed in our laboratory. The
system consists of a vertical quartz reactor where the photocatalyst bed
is placed on a quartz wool bed. The reactor is 50 mm in height, its
diameter is 20 mm and the quartz wool support height is around
10 mm. A UV lamp is placed parallel to the quartz reactor, at a distance
around 1 cm. The UV lamp radiation peak appears at 365 nm. The
commercial reference of this lamp is TL 8 W/05 FAM (Philips, 1 W).
Finally, the coupled quartz reactor lamp is surrounded by a cylinder
covered with aluminum foil. A scheme of this system is depicted else-
where [36].
4
water and 8 ml of TEOS, at 380 rpm for 1 h. The mixture was filtered
and dried overnight at 323 K. The resulting encapsulated P25 was cal-
cined at 500 °C for 6 h with a heating rate of 1 °C/ min in order to
remove the templates and any leftover reagents used in the synthesis.
This sample was named P25/M001.
The synthesis of the P25 encapsulated in mesoporous silica was
performed as follows: 0.4 g of P25 were dispersed in 20 g of water with
an ultrasound probe (Bandelin SONOPULS HD 2200) with a power of
660 W operating at 30% power for 5 min. The composite material was
prepared by dissolving 2 g of the templating agent (cetyl-
trimethylammonium bromide) in 76 g of deionized water. The mixture
was stirred and heated gently until a clear solution was obtained. This
mixture was added to the P25 dispersion in water, together with 4 ml of
The photocatalysts synthesised in this work were used for the oxi-
dation of propene at 100 ppmv in air at room temperature under flow
conditions. The calibrated gas cylinder was supplied by Carburos
Metálicos, S.A.
NH
4
OH and 8 ml of TEOS. The resulting mixture was stirred at 380 rpm
The flow rate of the propene-containing stream was 30 (STP) ml/
min after purging the reactor with helium.
for 1 h. The suspension was filtered and dried overnight at 323 K. The
resulting encapsulated P25 was calcined as described above. This
sample was labelled as P25/MCM-41.
The weight of photocatalyst used in these experiments was 0.11 g.
However, in order to study the illumination efficiency and any possible
inter-particle mass transfer issues, tests at different flow rates of pro-
pene were done in which the flow of propene and the mass of P25 were
both changed in order to keep the space velocity constant (Vsp = flow
(ml/min)/ mass of photocatalyst (g)) at a value of 257 ml/g·min. The
2.4. Samples characterization
The percentages of TiO
2
2
and SiO present in the samples were
−
1
−1
−1
analyzed by inductively coupled plasma emission spectroscopy (ICP-
OES), in a Perkin-Elmer Optima 4300 system. Dissolution of the
ratio was 10 ml min /0.0367 g, 20 ml min /0.0735, 30 ml min
/
−
1
0.11 g and 40 ml min /0.147 g. This was done in order to properly
125

J. Fernández-Catalá et al.
Applied Catalysis A, General 564 (2018) 123–132
compare the samples prepared in this study. In addition, the sample
P25_40/SiO was measured with the same Vsp (257 ml/g·min) with
2
respect to the amount of P25. Additionally, blank tests were performed
under the same experimental conditions as the catalytic tests but in
absence of the photocatalysts, resulting in non detectable catalytic ac-
tivity.
To better study the combined effect between the silica and the P25,
the reactor was filled with both silica and P25 as separate powders in a
configuration in which the silica with hierarchical porosity was either
on top or under the titania. The resulting samples were named SiO
2
/
P25 and P25/SiO respectively. Furthermore, a physical mixture of P25
2
and silica (containing the appropriate amount of P25) was also pre-
pared with the same purpose, the nomenclature of this sample is PM.
The propene-containing stream was passed through the photo-
catalyst bed until the propene concentration was stable (after about
3
h). The lamp is then switched on and kept working until a constant
propene signal is achieved, that is, steady state conditions (usually after
h) and, afterwards the outlet gas is continuously analyzed by mass
3
spectrometry (Balzers, Thermostar GSD 301 01).
Propene conversion was calculated using the following expression:
CinitialC3H6 - Csteady stateC
3
H
6
Propene conversion (%)=
×100
CinitialC3H6
where Cinitial C3H6 is the initial propene concentration, 100 ppmv and
C
steady stateC3H6 is the propene concentration at steady state conditions
in the outlet gas when the UV light is switched on. Moreover the CO
2
production rate was calculated per mol of P25, taken as the active
phase, using the following expression (with the aim to normalize the
results with the amount of P25):
qgen
CO
2
production rate =
n
2 2
where qgen is the molar flow rate of CO generated (moles CO /s) and n
is the moles of catalyst (moles of P25).
3. Results and discussion
Fig. 1. (a) XRD patterns of all composites prepared with hierarchical porosity
3
.1. P25/SiO samples characterization
2
2
silica. The data for the SiO and P25 are showed for comparison. Key:
A = anatase; R = rutile (b) TG curves of the P25 encapsulated with hierarchical
porosity silica.
In this section, the results on the characterization for the P25
samples encapsulated in spherical silica with hierarchical porosity
SiO ) are presented, emphasizing the effect of the P25 loading in the
composites.
The results obtained by ICP-OES show the discrepancies between
the nominal and real loadings analysed by ICP-OES in the samples
Table 1). The sample with 10 wt.% nominal loading of TiO , results in
a 3.8 wt.% loading by ICP-OES analysis. The other samples en-
capsulated in hierarchical silica present the same trend as showed in
Table 1. As a result of the analysis of ICP-OES, the samples P25_10/
(
2
in the bottom of the vessel where the dispersion was prepared after
transferring this suspension.
2
The XRD pattern of the P25/SiO samples with different P25 loading
are showed in Fig. 1(a). In all cases, the particles of P25 encapsulated in
silica have the same crystalline phases as the benchmark P25 (rutile and
anatase) after calcination at 500 °C, indicating the presence of P25 in all
composites (P25_10/SiO , P25_20/SiO and P25_40/SiO ) without any
2 2 2
further modification. An elbow in the XRD pattern of all composites due
(
2
SiO
5.9 wt% of P25; these values will be used in the calculations in order
to obtain reliable figures. The discrepancies between the nominal
loading of TiO and the results obtained for the ICP-OES technique may
2 2 2
, P25_20/SiO , P25_40/SiO show a content of 3.8, 8.14 and
to the presence of amorphous silica is also observed, as showed in the
pattern for the naked SiO sample [39]. TG analyses show a small
2
1
weight loss of approximately 2 wt% in all samples (see Fig. 1(b)). This
suggests that all organic matter and any leftover reagents present in the
synthesis were eliminated in the calcination step. The low weight per-
centage losses observed might be due to adsorbed water or the possible
decomposition reactions of hydroxyl groups at higher temperatures
2
be due to a fraction of the P25 solid present in the dispersion which was
not transferred to the reaction medium. In this aspect, we observed the
presence of a significant amount of P25 solid adhered to the walls and
[
16].
The UV–vis spectra obtained for the P25/SiO
Table 1
2
with different P25
Percentage of titania of the sample prepared in this study.
loading samples are showed in Fig. 2. All hybrid photocatalysts with
Samples
% TiO
Nominal
2
% TiO
ICP-OES
2
different loading of P25 show a similar absorption band edge at
400 nm. This fact indicated that the loading of P25 in the composite
does not modify the absorption range and the band gap of the P25.
However the composites show a small discrepancy in the absorbance
range and in the band gap with respect to the naked P25 (Fig. 2)
P25_10/SiO
P25_20/SiO
P25_40/SiO
2
2
2
10
20
40
3.8 ± 0.9
8.1 ± 0.2
15.9 ± 0.3
126

J. Fernández-Catalá et al.
Applied Catalysis A, General 564 (2018) 123–132
Table 2
Textural properties of the samples prepared in this study. The data for SiO
P25 are showed for comparison purposes.
2
and
Samples
S
BET
2
T_SBET*
(m /g)
V
total,0.95
3
V
N2DR
3
Mean pore size (nm)
2
(m /g)
(cm /g)
(cm /g)
SiO
2
244
235
215
145
54
–
0.86
0.61
0.57
0.38
0.18
0.10
0.10
0.09
0.07
0.02
6.2
7.9
7.9
6.2
7.6
P25_10/SiO
P25_20/SiO
P25_40/SiO
P25
2
2
2
252
230
219
–
*
T_SBET represents the theoretical SBET of the composites synthesised in this
work (considering the composites as a physical mixture). This value was cal-
culated by the following expression: T_SBET= (SBET (Mesoporous silica) · (SiO
2
present in the composites (ICP-ES)) + (SBET (P25) · P25 present in the com-
posites (ICP-ES)).
P25_20/SiO
Nevertheless, the samples with the highest loading of P25 (P25_40/
SiO ) shows a much higher discrepancy (45%) between SBET and T_SBET
This difference indicates that high TiO loadings severely affect the
2
) show a discrepancy of only 7% between SBET and T_SBET.
2
.
Fig. 2. UV-VIS absorption spectra and band gap value (Eg) of the P25/SiO
with different P25 loading samples. The data for the benchmark material P25
2
2
are showed for comparison.
final morphology of the obtained composite, resulting not only in a
modification of the porous texture, but also in the shaping of the final
SiO
centrations of suspended TiO
position of the SiO -rich phase, resulting in a composite with an irre-
gular morphology. As a result, the mixing law fails to apply for this
sample.
2
/TiO
2
sample (vide infra). In this sense, it appears that high con-
indicating that the presence of the silica can slightly modify the band
gap of P25 from 3.2 to 3.3 eV.
The porous texture of the synthesized samples was investigated by
2 2
N adsorption measurements as showed in Fig. 3. The N physisorption
isotherm of P25 shows a typical type-II isotherm, indicative of a non-
porous solid. However, the sample does present a hysteresis in the high
relative pressure range due to interparticle adsorbate condensation. The
2
powder prevents the spinodal decom-
2
TEM images are showed in Fig. 4. The TiO
the hierarchical silica matrix (Fig. 4(a)). Moreover, the SiO
surrounding the P25 particles in the sample P25_20/SiO , which cor-
roborates the encapsulation of the TiO inside the SiO
shows the presence of P25 in the sample P25_20/SiO
2
crystals are encased in
2
is clearly
2
2
SiO prepared in our research group is a combination of type I and IV
2
2
matrix. Fig. 4b
as the particle
isotherms, typical of mesoporous materials with a certain degree of
microporosity. This silica has hierarchical porosity, presenting micro,
meso and macro-porosity [39]. As expected, the composites obtained in
2
shows the lattice fringes and interplanar distances characteristic of both
anatase and rutile present in P25 particles.
this work present similar isotherms to that of the pure hierarchical SiO
2
SEM images show that the samples with low P25 loadings (3.8% and
material, with a noticeable decrease in the adsorption uptake at low
relative pressures and the hysteresis loop as the P25 loading increases.
Concerning the textural properties showed in Table 2, the BET
surface area and Vtotal 0.95 decreases with increasing P25 loading, as
commented in the previous paragraph. However, the textural para-
meters cannot help in distinguishing whether the P25 is encapsulated or
8
.14 wt.%) (Fig. 5(a) and (b)) have a spherical morphology typical for
this silica (Fig. 5(d)) [39]. No particles with irregular morphology
representative of P25 NPs, Fig. 5(e)) are observed for these samples,
which indicates the encapsulation of P25 inside the SiO matrix, as
already hinted at by the TEM images. However the sample with the
highest P25 loading (P25_40/SiO ) presents both spherical and irre-
(
2
2
not in the SiO
T_SBET (obtained theoretically assuming that the composite is a physical
mixture), the samples with lower quantities of P25 (P25_10/SiO and
2
. Comparing the SBET (obtained from the isotherms) and
gular particles (Fig. 5(c)), in agreement with the comments made from
the adsorption isotherms results.
2
EDX mapping images are presented in Fig. 6(a)–(c). Sample P25_10/
SiO
the SiO
2
contains Ti in the composites indicating that P25 is probably inside
shell, owing to the composite presenting a spherical structure
2
as observed in the SEM images (Fig. 6(a)). This is a clear indication that
the spinodal decomposition process is not largely affected by the pre-
sence of the TiO powder. For sample P25_20/SiO a similar observa-
2 2
tion can be made although the presence of Ti is more noticeable as
showed in Fig. 6(b), allowing us to conclude that in both composites the
P25 particles are encapsulated within the SiO
2
matrix. Nevertheless, the
, Fig. 6(c)),
sample with the highest TiO loading (sample P25_40/SiO
2
2
presents a morphology distinct from spherical, which shows that the
spinodal decomposition is modified in the presence of large con-
centrations of photocatalyst. In turn, this encapsulated TiO sample
2
shows a significantly different morphology from the other two samples
as corroborated by the SEM images and the textural parameters of these
composites, in which the mixing law does not apply. In order to better
compare the results towards an understanding of the encapsulation
phenomenon Fig. 7(a) and (b) show EDX images mapping of sample
P25_20/SiO
2
and a physical mixture in which the two phases are clearly
and completely separated, with no observable encapsulation.
2
Fig. 3. N isotherms at 77 K for the different composites with hierarchical
porosity silica samples. The adsorption isotherm of SiO
for comparison purposes.
2
and P25 is also show
127

J. Fernández-Catalá et al.
Applied Catalysis A, General 564 (2018) 123–132
2 2
Fig. 4. TEM images of a representative sample prepared in this study: (a) P25_20/SiO , (b) Magnification of P25 NPs (sample P25_20/SiO ).
3
.2. P25/M001 and P25/MCM-41 samples characterization
absorption of the P25 encapsulated.
The N adsorption measurements at 77 K are showed in Fig. 10. The
2
N physisorption isotherm of sample M001 shows a type-II isotherm,
2
In this section, the results on the characterization of the P25 samples
encapsulated inside precipitated silica (M001) and a mesoporous silica
MCM-41) with a nominal TiO content of 20% are discussed. These
results are compared with P25 encapsulated in spherical hierarchical
silica (P25_20/SiO ) to better put the results in perspective.
indicative of a non-porous material, with some porosity arising from a
small hysteresis in the high relative pressure range due to interparticle
condensation. However, the composite P25/M001 presents a combi-
nation of type I and type IV isotherms. This fact is remarkable since this
(
2
2
The results obtained by ICP-OES (Table 3) show discrepancies be-
tween the samples encapsulated in different silicas where all samples
have a 20 wt.% nominal of P25. The sample with P25/M001 presents
2
composite with P25 (non-porous material) has higher N adsorption
capacity with respect to the naked M001, indicating the appearance of
additional porosity when the composite is formed probably due to in-
terparticle spacings with sizes in the below-nanometer region. The si-
lica MCM-41 shows a combination of types I and IV isotherms, typical
of mesoporous materials with a certain degree of microporosity as it has
been reported in the literature [37]. As expected, the composites ob-
tained with this silica present similar isotherms to that of the pure
MCM-41, with a noticeable decrease in the adsorption uptake at low
pressures and the hysteresis loop due the presence of a non-porous
material (P25).
The textural properties obtained from the adsorption isotherms are
showed in Table 4. The BET surface area and Vtotal 0.95 decrease with
the incorporation of P25 except in the case of the P25/M001 sample
where these values increase. The samples with MCM-41 present the
highest surface area and total pore volume.
TEM images show the possible encapsulation of P25 inside the
different silicas. (Fig. 11). SEM images show the morphology of the
2
samples P25/M001, P25/MCM-41 and P25_20/SiO (Fig. 12(a–c)). The
samples P25/M001 and P25/MCM-41 present an irregular morphology,
a feature common for both precipitated silica and P25. EDX mapping
images are presented in Fig. 12(d)–(f). All samples present Ti in the
composites indicating that P25 is inside the different silicas as observed
the lowest TiO
and P25_20/SiO
2
loading (3.2 wt.%). The other samples P25/MCM-41
present a similar TiO loading, 6.8 wt.% and 8.1 wt.%,
2
2
respectively, being the sample with the P25 encapsulated in hier-
archical silica the one with the highest loading. These values will be
used in the calculations in order to obtain reliable figures.
The XRD pattern for the P25 encapsulated in different silica samples
(
2
M001, MCM-41 and SiO ) are showed in Fig. 8. The particles of P25
encapsulated in the different silicas present the same crystalline phases
as the benchmark P25 (rutile and anatase) after calcination at 500 °C,
indicating the presence of P25 in the composites without any further
modification as indicated previously in Fig. 1 (a) for the composite
(
P25_20/SiO
P25/MCM-41 and P25/SiO
also observed, indicating the presence of the different silicas (M001,
MCM-41 and SiO ) in each composite used in this study.
The UV–vis spectra obtained for the P25 encapsulated with different
silica (M001, MCM-41 and SiO ) are showed in Fig. 9. All hybrid
2
). In these composites with different silicas (P25/M001,
2
) an elbow in each of the XRD patterns was
2
2
photocatalysts with different silica samples present a similar absorption
band edge at 400 nm and band gap. These values indicate that the use
of different silica (M001, MCM-41 and SiO ) does not modify the light
2
2 2 2 2
Fig. 5. SEM of the samples with hierarchical porosity silica: (a) P25_10/SiO , (b) P25_20/SiO , (c) P25_40/SiO . The SEM images of SiO (d) and P25 (e) are also
showed for comparison purposes.
128

J. Fernández-Catalá et al.
Applied Catalysis A, General 564 (2018) 123–132
2 2 2
Fig. 6. EDX images mapping of the samples (a) P25_10/SiO , (b) P25_20/SiO , (c) P25_40/SiO .
2
Fig. 7. EDX images mapping of the composite and the physical mixture with the same percentage of P25 (a) P25_20/SiO (composite), (b) PM (Physical mixture).
Table 3
Percentage of titania of the sample with different
silica samples.
Samples
% TiO
2
ICP-OES
P25/M001
P25/MCM41
P25_20/SiO
3.2 ± 0.1
6.8 ± 0.9
8.1 ± 0.2
2
Fig. 9. UV-VIS absorption spectra and the band gap (Eg) of the P25/SiO
different silica samples.
2
with
(1)
3 6 2 2 2
2C H + 9O ↔ 6CO + 6H O
It must be noted that in this study the mechanism of total photo-
oxidation of VOCs was not studied since it has been extensively studied
and reported in the literature [36,42]. Given that the samples used in
this study do not differ significantly from those studies, it is safe to
assume that the mechanism should be identical to that previously re-
ported.
The results of the effect of different P25 loadings in the photo-
catalytic activity of the samples with the hierarchically porosity silica
are showed in Fig. 13 (a). The naked P25 presents the best value of
propene conversion with respect to the composites where the P25 was
Fig. 8. XRD patterns of the composites with different silica. The data for the
different silica used in this work are showed for comparison Key: A = anatase;
R = rutile.
for sample P25_20/SiO
2
.
2
encapsulated in the spherical hierarchical silica (SiO ). In the case of
the composites, as the concentration of P25 (active phase) is increased
propene conversion increases. Nevertheless the samples with 8.1 wt.%
and 15.9 wt.% P25 loading present approximately the same propene
conversion. This might be due to sample P25_40/SiO presenting a
2
different morphology with respect to the other composites and this
factor affects the activity of the composite (Fig. 13(a)). When the results
3
.3. Photocatalytic activity
The photocatalytic activity of the materials prepared in this work
was evaluated by studying the mineralization of propene in the gas
phase at low concentration (100 ppmv in air) to CO at room tem-
perature, following the global reaction showed in Eq. (1) [36].
2
129

J. Fernández-Catalá et al.
Applied Catalysis A, General 564 (2018) 123–132
the sample was cycled in the photooxidation of propene as described in
the Catalytic Test for three cycles (the conversion values for the three
consecutive cycles was 14%). The performance of the sample did not
show a noticeable change in the propene conversion value at the end of
each cycle.
In order to deepen into the synergy between silica and P25 upon
encapsulation in these materials as described in literature [21,23,29],
the sample showing the highest TiO
pared in propene mineralization with composites in different config-
urations (PM, SiO /P25 and P25/SiO ) as described in the Catalytic
2 2
loading (P25_40/SiO ), is com-
2
2
Tests section. As showed above (Fig. 13(b)), the naked P25 presents the
best propene conversion value with respect to the other samples, due to
the composites having a significantly lower amount of P25. In the
composites and the mixtures (SiO and TiO ), the sample with the P25
2 2
encapsulated has better propene conversion than the other samples.
This result indicates a synergetic effect between the encapsulated P25
and the SiO ; it seems that hierarchical silica favours mass transport
2
and increases the concentration of reagents near the P25 particles.
Interestingly, P25 has the lowest photocatalytic activity when ex-
pressed as rate of CO formation per moles of P25, compared to the
2
composites (Fig. 13(b)) even when the P25 sample is not encapsulated.
In the case of the composites, the encapsulation of P25 (Fig. 13(a))
improved the activity per mol of P25. This is observed in the conversion
Fig. 10. N
P25/MCM-41, and P25_20/SiO
are also showed for comparison purposes.
2
isotherms at 77 K for the samples prepared in this study: P25/M001,
. The adsorption isotherms of the pure silicas
2
Table 4
Textural properties of the samples prepared in this study derived from the
analysis of the N
for comparison purposes.
2
of propene for the P25_xx/SiO samples, which hints at a synergy be-
2
adsorption isotherms. The data for SiO
2
and P25 are showed
tween the P25 encapsulated inside the hierarchical silica. In this re-
spect, the coexistence of a hierarchical porosity which favours mass
transfer of reagents and products, the possibility of this same porosity
acting as an adsorbent for reagents, and the intimate contact between
Samples
S
BET
2
V
total,0.95
3
V
N2DR
3
Mean pore size (nm)
(m /g)
(cm /g)
(cm /g)
2 2
the SiO and TiO phase results in an improved performance of the
encapsulated samples compared with all its physically mixed counter-
parts (Fig. 13(b)).
From our results showed in Table 2 and Fig. 14, it would appear that
there is a correlation between the BET surface area in the composites
M001
MCM-41
77
0.25
0.82
0.86
0.39
0.77
0.57
0.18
0.033
0.39
0.10
0.17
0.34
0.09
0.02
6.2
2.9
6.2
3.1
3.1
7.9
7.6
973
244
317
875
215
54
SiO
2
P25/M001
P25/MCM-41
P25_20/SiO
P25
2
and their CO
With this in mind, from the values showed in Table 4, the TiO
encapsulated in MCM-41 (SiO ) should outperform all samples. This
2
production rate in the propene mineralization reaction.
2
sample
2
of the photocatalytic activity are normalized per mol of active phase
P25) (CO production rate) expressed as moles CO /(moles P25 X s),
P25 has the lowest photoactivity compared to the composites due to the
silica favouring the activity per mol of P25 as showed in the literature
material should better concentrate the reagents favouring their oxida-
tion by the TiO2 phase, as explained in the literature [21,25,29]. In
order to probe into this effect in the activity of encapsulated P25, the
CO2 production rates for the samples with silica having different tex-
tural parameters (SBET) and morphologies are showed in Fig. 14. In-
terestingly, the composite with hierarchical silica presents the highest
CO2 production rate, despite having poorer textural parameters. It is
also observed that the sample encapsulated in mesoporous silica shows
better photooxidation of propene than that in microporous silica. This
result evidences the importance of using a silica not only with large
surface area but also with hierarchical porosity for optimizing the mass
transport in addition to an increased concentration of reagents due to
the hierarchical porosity interconnected which allows all the adsorbed
propene to arrive at the active phase (P25) even comparing these
samples with others showing a BET surface area four times larger which
lack this hierarchical structure as showed in Scheme 1. In the MCM-41
sample, the porous structure (tubular non-interconnected mesopores)
(
2
2
[
2
24]. With respect to CO production rate, the composites show a de-
crease in the conversion of propene upon increasing the percentage of
P25 in the composite which might be due to the change in porous
texture and morphology of the silica or to another important factor such
as the better illumination efficiency [43] of particles of P25 when the
composites have lower percentages of active phase as explained later.
Moreover in this section we evaluated the durability of the samples in
2
specific the P25_10/SiO as reference sample. The conditions used in
this test have been described in the Catalytic Test section but running
the propene abatement tests continuously for 14 h. In this sample the
propene conversion was stable after 14 h of reaction (the initial and
final propene conversion values were kept at 14%). In this sense, this
2
sample (P25_10/SiO ) was also recycled three times. For this purpose
2
Fig. 11. TEM of the composite prepared in this study: (a) P25/M001, (b) P25/MCM-41, (c) P25_20/SiO .
130

J. Fernández-Catalá et al.
Applied Catalysis A, General 564 (2018) 123–132
Fig. 12. SEM of the samples with different silica: (a) P25/M001, (b) P25/MCM-41, (c) P25_20/SiO
2
. Fig. 12(d–f) present the EDX images mapping (d) P25/M001, (e)
2
P25_20/MCM-41, (f) P25_20/SiO .
2
Fig. 14. Comparison of CO production rates of samples of P25 encapsulated in
different silica and its textural parameters (SBET).
Fig. 13. Comparison of propene conversion and CO
2
production rates for
samples with spherical hierarchical porosity silica and P25 for comparison
purposes: (a) Photoxidation varying the percentage of P25 in the samples, (b)
Comparison of the composite P25_40/SiO
amount of P25 is not encapsulated.
2
with samples where the same
Fig. 15. Comparison of propene conversion with different flow rates (The mass
of the samples is described in Experimental section) maintaining a constant
−1
−1
space velocity of 257 ml·g min
P25_40/SiO with the same mass of P25 (0.11 g) and space velocity is included
for comparison purposes.
(See Experimental section). The sample
does not guarantee a good access to the active phase via the mesopores,
since many of them may not be connected with the P25 particles. On
2
2
the other hand, in the silica with hierarchical porosity (SiO ) the in-
terconnected porosity allows a much higher accessibility to the active
sites, favouring the oxidation of propene on P25, as showed in Scheme
1.
With the purpose of studying the possible effect of the better
131

J. Fernández-Catalá et al.
Applied Catalysis A, General 564 (2018) 123–132
illumination efficiency in the encapsulated P25 particles as discussed
before, the conversion of propene was also obtained by varying the
propene flow and the mass of P25 (Fig. 15) keeping the space velocity
Another important factor is the election of a correct porous network for
the silica matrix. This fact, allows to synthesize new materials with an
improved mass transfer and catalytic efficiency for the abatement of
contaminants at low concentrations in gas phase.
(Vsp) constant with the purpose of comparing the naked P25 samples
with different mass as described in the Catalytic Test section. These
catalytic tests are usually done to detect diffusional limitations. The
results show an increase in propene conversion as the flow of reagents
and mass of catalysts are decreased. This is markedly different from the
expected results for classic catalysis, where the conversion remains
constant until it decreases due to diffusional problems [44]. This result
indicates some contribution of the photonic efficiency [43]. When the
weight of the sample (in this case only P25) is decreased (and conse-
quently the particles bed size), the propene conversion increases due to
a better illumination of the particles. This factor could partially explain
the increased photocatalytic activity of the composites compared to P25
as showed in Fig. 13(b). In fact, the results of the comparison of the
Acknowledgements
The authors thank MINECO (Project CTQ2015-66080-R, MINECO/
FEDER) for financial support. JFC thanks MINECO for a researcher
formation grant (BES-2016-078079).
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