Synthesis of nano-sized tungsten oxide particles encapsulated in a hollow silica sphere and their photocatalytic properties for decomposition of acetic acid using Pt as a co-catalyst

Article abstract of DOI:10.1039/d0ra01988g

Nano-sized tungsten oxide (WO₃) particles, each of which was encapsulated as a core in a hollow silica sphere (WO₃?SiO₂), were synthesized using calcium tungstate particles as the starting material. The calcium tungstate particles, each of which was covered with a silica shell, were converted to tungstic acid by nitric acid treatment and then to WO₃ by heat treatment to obtain WO₃?SiO₂. A hollow space was formed in WO₃?SiO₂ between the WO₃ core and the SiO₂ shell as a result of shrinkage of WO₃ during the heat treatment. The thus-obtained WO₃?SiO₂ was 40 nm in diameter, the WO₃ core was 10 nm in diameter, and the silica shell, which was permeable to gas and liquid, was 10 nm in thickness. WO₃?SiO₂ absorbed visible light to the wavelength of 454 nm, which enabled photocatalytic reaction under visible light; Pt was loaded on the WO₃ cores in the photocatalytic reactions. In contrast to Pt-loaded bulk WO₃ photocatalysts without an SiO₂ shell, Pt-loaded WO₃?SiO₂ showed continuous and complete decomposition of gaseous acetic acid in air under visible as well as UV irradiation.

Full text of DOI:10.1039/d0ra01988g

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PAPER
Synthesis of nano-sized tungsten oxide particles
encapsulated in a hollow silica sphere and their
photocatalytic properties for decomposition of
acetic acid using Pt as a co-catalyst†
Cite this: RSC Adv., 2020, 10, 15360
a
a
b
Takashi Harada, * En Yagi and Shigeru Ikeda
Nano-sized tungsten oxide (WO
sphere (WO @SiO ), were synthesized using calcium tungstate particles as the starting material. The
calcium tungstate particles, each of which was covered with a silica shell, were converted to tungstic
acid by nitric acid treatment and then to WO by heat treatment to obtain WO @SiO . A hollow space
was formed in WO @SiO between the WO core and the SiO shell as a result of shrinkage of WO
during the heat treatment. The thus-obtained WO @SiO was 40 nm in diameter, the WO core was
0 nm in diameter, and the silica shell, which was permeable to gas and liquid, was 10 nm in thickness.
@SiO absorbed visible light to the wavelength of 454 nm, which enabled photocatalytic reaction
under visible light; Pt was loaded on the WO cores in the photocatalytic reactions. In contrast to Pt-
loaded bulk WO photocatalysts without an SiO shell, Pt-loaded WO @SiO showed continuous and
complete decomposition of gaseous acetic acid in air under visible as well as UV irradiation.
3
) particles, each of which was encapsulated as a core in a hollow silica
3
2
3
3
2
3
2
3
2
3
3
2
3
1
Received 2nd March 2020
WO
3
2
Accepted 3rd April 2020
DOI: 10.1039/d0ra01988g
rsc.li/rsc-advances
3
3
2
3
2
developed a TiO
2
photocatalyst encapsulated in a hollow silica
), in which a gap space is formed between the
@SiO showed photo-
photocatalyst and has been used in various applications, such catalytic activity as high as that of an intrinsic TiO photo-
Introduction
shell (TiO @SiO
2
2
Titanium dioxide (TiO ) is well known as the most efficient core and the shell. As a result, the TiO
2
2
2
2
as production of renewable energy, decomposition of environ- catalyst, and showed size-selectivity for materials to be
mental pollutants and synthesis of useful organic compounds, decomposed, depending on the pore size of the permeable
5–8
because electrons and holes photogenerated in TiO
strong reduction and oxidation properties, respectively.
However, due to its strong oxidation property, TiO
indiscrim- temperature stability of core material and application for
inately decomposes organic compounds, which sometimes cascade reaction, have been reported.
leads to problems of decomposition of the support of TiO2
There have been many efforts to modify TiO
photocatalyts. To solve this problem, a core–shell structure in absorption of visible light; an intrinsic TiO
2
have shell. In addition, other features of core–hollow shell struc-
1
–3
ture, such as improvement adsorption properties, high
2
9–11
2
so as to enable
photocatalyst
2
which TiO
2
particles are covered with robust inorganic mate- shows low photocatalytic activity in normal environments
rials has been proposed. The shell is permeable to gas and because it shows photocatalytic activity only under UV light due
4
liquid, while it prevents contact between TiO
to suppression of degradation of supports made of organic include doping of TiO
materials. However, the structure had the disadvantage of ment of oxygen in TiO
lowered photocatalytic activity because a large part of the TiO2 approach is to use low band gap materials instead of TiO
surface, which is important for the photocatalyst, was covered Tungsten oxide (WO ) is a good candidate of these materials
with the shell material. To solve the problem, we have recently because of its narrow band gap (2.8 eV), high oxidation prop-
particles, leading to its large band gap (3.2 eV). Approaches to solve this problem
2
12–15
2
with transition metals
with sulfur or nitrogen.
and replace-
1
6–21
2
Another
2
.
3
22
erty, nontoxicity, and chemical stability. However, a pure WO3
photocatalyst shows low photocatalytic activity for decomposi-
tion of organic compounds because of the low energy level of its
conduction band (CB) edge, which is not high enough for the
a
Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama,
Toyonaka 560-8531, Japan. E-mail: harada@chem.es.osaka-u.ac.jp
b
Department of Chemistry, Faculty of Science and Engineering, Konan University,
2 2
electrons in the CB to reduce O or H O. Sayama et al. reported
Okamoto, Higashinada, Kobe 658-8501, Japan
that loading co-catalysts, such as CuO, Pd and Pt, on the surface
†
Electronic supplementary information (ESI) available: XRD patterns of reference
samples. TG-DTA curves time courses for CO evolution and acetic acid of WO
was effective for improving its photocatalytic activity.
3
23,24
2
decomposition and methanol production during photocatalytic reaction. See
Although the co-catalysts do not affect the energy level of the CB
DOI: 10.1039/d0ra01988g
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of WO
RSC Advances
3
, they kinetically enhance the electron transfer from WO
3
SiO
2
powder, which is synthesized by a slightly modied St ¨o ber
29
to O or H O and, as a result, increase photocatalytic activity of method. They were mechanically mixed and calcined at
2
2
ꢀ
WO . We expect that WO particles encapsulated in a hollow 700 C. The WO content of WO (B)/SiO was adjusted to be the
3
3
3
3
2
silica sphere (WO @SiO ) would be suitable as a photocatalyst same as that of WO @SiO .
3
2
3
2
used under visible light.
A number of synthesis procedures have been developed to
produce the core–hollow shell structure using, for example, the
Characterization
TEM images were taken on a Hitachi H-9000NAR instrument
operated at 300 kV. Powder X-ray diffraction (XRD) patterns
were recorded using a Rigaku MiniFlex X-ray diffractometer
25
26
27
Kirkendall effect, a hard template, and an etching process.
We previously reported the synthesis of TiO @SiO using
2
2
5–8
a carbon material as a hard template. The hard template
(
CuKa, Ni lter). Analyses of the surface area and pore structure
were carried out using a Quantachrome AUTOSORB-1 auto-
mated gas-sorption system with N as the adsorbate, aer
method is useful for obtaining well-dened core–hollow shell
5
–8,26
structures.
However, this method requires the formation
2
ꢀ
and removal of solid templates, which are troublesome proce-
dures, especially for practical use. Moreover, core–hollow shell
structures thus-obtained are oen large in size and irregular in
shape. Because these structures show low mechanical stability
due to the large hollow space, the structure needs to be small
and, ideally, spherical.
pretreatment of the sample at 200 C for 2 h under reduced
pressure. The BET surface area was calculated from the
adsorption branch of the isotherm. Ultraviolet and visible-light
diffuse reection (UV-vis DR) spectra were obtained with
a Hitachi U-4100 UV-vis-NIR spectrometer equipped with an
integrating sphere using BaSO as a reference. The reectance
spectra were converted to absorbance spectra by the Kubelka–
Munk method.
4
We recently developed a simple procedure for making nickel
28
nanoparticles encapsulated in a hollow silica sphere. In this
procedure, small crystals of a nickel hydrazine complex were
prepared in a reverse microemulsion system and were directly
covered with a silica layer. The nickel hydrazine crystals were
Photocatalytic decomposition of acetic acid in gaseous phase
converted to nickel nanoparticles by heat treatment. Because Photocatalytic decomposition of acetic acid was carried out over
the nickel nanoparticles were smaller than nickel hydrazine Pt-loaded WO photocatalysts (Pt/WO ) in gaseous phase. Pt-
crystals, a core–hollow shell structure was automatically ob- loading on WO photocatalysts was carried out by the photo-
tained during the heat treatment. Here, we applied this strategy deposition method sequentially from an aqueous methanol
3
3
3
30
to the synthesis of WO @SiO and evaluated its photocatalytic (10 vol%) solution of H
2
PtCl
6
$6H
2
O. The loading amount of Pt
3
2
activity for decomposition of acetic acid in gas phase, which was was xed at 0.5 wt% of WO
3
for all of the photocatalysts.
used as a model reaction.
Photocatalytic decomposition of acetic acid was performed
in a 225 cm Pyrex cylindrical reaction vessel (64 mm in diam-
3
eter) with a quartz top window. The photocatalyst, containing
Experimental
Synthesis of tungsten oxide photocatalysts
3
0 mg of WO
and set in the vessel. Aer closing the vessel, 1 ml of acetic acid
hollow silica sphere (17.5 mmol) was introduced into the vessel. Aer acetic acid had
) was synthesized as follows. To 34 g polyoxy- been fully vaporized and had reached an absorption equilib-
2
3
, was spread on the slide glass (38 mm ꢁ 26 mm)
Tungsten oxide encapsulated in
WO @SiO
a
(
3
3
ethylene cetyl ether (n ¼ 15) (30 mmol) dissolved in 75 cm of rium in the vessel, the sample was photoirradiated with a 300 W
3
cyclohexane were added 4.5 cm of 0.1 M aqueous Ca(NO )
(
2
Xe lamp though the quartz top window. CO was analyzed with
3
2
3
0.45 mmol) and 4.5 cm of 0.1 M aqueous Na WO (0.45 mmol). a Shimadzu GC-8A gas chromatograph equipped with a TCD
Aer the mixture had been stirred at 55 C for 1 h, 1.5 cm of detector and an activated carbon column, and organic
2
4
ꢀ
3
hydrazine monohydrate (30 mmol) was added and the mixture compounds were analyzed with a Shimadzu GC-2014 gas chro-
ꢀ
3
was stirred at 55 C for 1 h. Then 3 cm of tetraethylorthosilicate matograph equipped with a BID detector and a TC-FFAP
TEOS: 13.4 mmol) was added to the suspension, which was column. For the analyses, a portion of the gas phase was
(
ꢀ
3
stirred at 55 C for 2 h. Aer addition of 240 cm of 2-propanol sampled with an air-tight syringe at appropriate intervals.
to the suspension, the mixture was centrifuged (3000 rpm, 4
min) to recover the precipitate, which was washed with 2-
propanol three times. The precipitate was added to 2.5 M
Results and discussion
Synthesis and characterizations of tungsten oxide
ꢀ
aqueous HNO
3
at 80 C for 6 h and then washed with distilled
ꢀ
nanoparticles encapsulated in a hollow silica sphere
water until the effluent showed neutral pH. Aer drying at 80 C,
ꢀ
the powder was calcined at 700 C for 2 h in air to produce Synthesis of nano-sized WO
3
@SiO
oil (W/O) microemulsion system. In the rst step, an aqueous
Ca(NO solution was added to a cyclohexane solution con-
(B)) powder by the same procedure but without the use of taining polyoxyethylene cetyl ether (n ¼ 15) to form a W/O
the reverse micelle system and silica source. WO (B) physically microemulsion. Addition of an aqueous Na WO solution into
2
was performed in a water in
WO
As a reference photocatalyst, we synthesized bulk WO
WO
3 2
@SiO .
3
3 2
)
(
3
3
2
4
mixed with SiO2 (WO (B)/SiO ) was also used as a reference the W/O microemulsion solution induced a color change from
3
2
photocatalyst. This reference photocatalyst was prepared using transparent to white; it became turbid aer a while, indicating
WO , which is a precursor of WO (B), and about 30 nm of formation of solid components. By adding a hydrazine
H
2
4
3
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monohydrate solution and TEOS to this solution, poly-
condensation of the siliceous component was continuously
carried out. Aer 2 h agitation of the mixture, the solid part was
collected. Fig. 1a shows an XRD pattern of the thus-obtained
white powder. All of the peaks were assigned to CaWO as
4
referred from the JCPDS card (no. 41-1431). A TEM image of the
white powder showed spherical core–shell structure (Fig. 2a).
These results indicate that CaWO
4
particles were directly
). The diameter of the
covered with a silica shell (CaWO @SiO
4
2
CaWO core was 10–20 nm and the thickness of the silica shell
4
was about 10 nm.
The thus-obtained CaWO @SiO powder was treated with
4
2
ꢀ
aqueous HNO solution at 80 C for 6 h. During HNO treat-
3
3
ment, the white color of the turbid solution gradually changed
to yellow. The corresponding XRD pattern of the thus-obtained
yellow powder was assigned to H WO as referred from the
2 4
JCPDS card (no. 43-0679) (Fig. 1b). The TEM image showed that
the core–shell structure was not changed by the HNO treat- Fig.
2
TEM images of (a) CaWO
4 2 2 4 2
@SiO , (b) H WO @SiO , (c)
3
WO
3
@SiO and (d) WO (B). Scale bars are corresponding to 40 nm.
2
3
ment, as shown in Fig. 2b. These results indicate that the
Inset of (c) is HRTEM image of WO
sponding to 5 nm.
3 2
@SiO and scale bar is corre-
CaWO core particle of CaWO @SiO was converted to H WO
4
4
2
2
4
3
by the HNO treatment and that a core–shell material having an
H
2
WO
4
core and a silica shell (H
2
WO
4
@SiO
2
) was obtained.
ꢀ
Aer calcination of H
2
WO @SiO
4
2
at 700 C, the color
H WO @SiO , a hollow space was observed in this sample,
2
4
2
changed to light green. The XRD pattern of this sample showed
the formation of monoclinic WO (JCPDS card no. 43-1035)
Fig. 1c), which was obtained by dehydration of H WO . To
indicating the formation of WO encapsulated in a hollow silica
3
3
sphere (WO @SiO ). The hollow space was caused by shrinkage
3
2
(
2
4
of the volume as H
WO particles was about 10 nm in diameter, which is smaller
than that of the H WO core in H WO @SiO . The lattice
fringes of HRTEM image of WO particle are assigned to the
100) plane of monoclinic WO . This is agreement with the XRD
2 4 3
WO was converted to WO . The size of the
conrm dehydration of H WO , thermogravimetric-differential
2
4
3
thermal analysis (TG-DTA) was carried out. The TG-DTA curves
of H WO @SiO were shown in Fig. S2.† A weight loss with
2
4
2
4
2
2
4
2
ꢀ
ꢀ
3
endothermic peak was observed at around 100 C and 200 C.
These are due to desorption of physically adsorbed water
(
3
result. The silica shell thickness seems to be slightly decreased
molecule on H
molecules of H
peak was observed at 250–700 C, due to combustion of
2
WO
WO
4 2
@SiO particles and coordinated water
aer the heat treatment. A TEM image of WO (B) powder that
3
2
4
, respectively. A weight loss of exothermic
was prepared without using the reverse microemulsion system
and silica source showed that the particle size was large and
inhomogeneous (40–100 nm) as can be seen in Fig. 2d, although
WO (B) was obtained by the same route as WO @SiO (Fig. S1†).
3 3 2
These results suggest that the use of the reverse microemulsion
ꢀ
surfactant and siliceous compounds including removal of
31
residual coordinated water. A TEM image and high resolution
TEM (HRTEM) image of this sample are shown in Fig. 2c and
inset of Fig. 2c, respectively. In contrast to the TEM image of
system and the silica shell structure are effective for suppress-
ing the growth of WO particles during calcination.
3
Fig. 3 shows N adsorption–desorption isotherms of WO (B),
2
3
CaWO @SiO and WO @SiO samples. The isotherm of WO (B)
4
2
3
2
3
is classied as characteristic type III according to the IUPAC
2
ꢂ1
nomenclature and the BET surface area is 13 m g , indicating
a nonporous structure. The isotherm of CaWO @SiO is clas-
sied as characteristic type II and showed a slight increase in N
uptake at a relative pressure (P/P ) below 0.1, indicating the
existence of micropores in the silica shell. The BET surface area
4
2
2
0
2
ꢂ1
of CaWO @SiO was found to be 65 m g . The isotherm of
4
2
WO @SiO showed a characteristic type IV with a hysteresis
3
2
2
ꢂ1
loop and the BET surface area was 175 m g . Compared with
CaWO @SiO , WO @SiO showed a steep increase in N uptake
at P/P below 0.1, indicating the formation of a well-developed
4
2
3
2
2
0
micropore system probably due to removal of the surfactant
in the silica shell during high temperature calcination. More-
over, a hysteresis loop enclosed at a P/P0 of ca. 0.4–0.9 was
observed. These phenomena are attributed to the tensile
Fig. 1 XRD patterns of (a) CaWO
WO @SiO . The standard data of CaWO
JCPDS: 43-0679) and WO (JCPDS: 43-1035) are shown as
a reference.
4
@SiO
2
, (b) H
(JCPDS: 41-1431), H
2
WO
4
@SiO
2
and (c)
3
2
4
2
WO
4
(
3
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standard (BaSO
4
). The spectra indicate that the on-set values
of the absorption bands are located at 454 and 486 nm for
WO @SiO and WO (B), respectively. Considering that WO is
3
2
3
3
an indirect n-type semiconductor, its energy band gap (E ) can
g
be determined by optical absorption measurements using the
33
following relationship:
1
/2
(
F(R)hn) ¼ K(hn ꢂ E
g
),
(2)
where hn the incident photon energy and K is a constant. The
1
/2
inset in Fig. 4 shows plots of (F(R)hn) versus hn. By extrapo-
lating the plots linearly, the energy band gaps of WO @SiO and
WO (B) are estimated to be 2.65 and 2.63 eV, respectively. The
estimated energy band gaps are in good agreement with the
3
2
3
34
values (2.5–2.8 eV) reported in the literature. The reectance
and spectrum of the WO @SiO powder showed a slight blue shi
Fig. 3
N
2
sorption isotherms of (a) CaWO
4 2 3 2
@SiO , (b) WO @SiO
3
2
(
c) WO (B). Filled and open circles denote adsorption and desorption
3
compared with that of the bulk WO (B) powder, probably due to
3
isotherms, respectively.
35,36
3
the differences of WO particle size and/or crystallinity.
Photocatalytic decomposition of acetic acid
Photocatalytic decomposition of acetic acid in gaseous phase
was used as a measure of photocatalytic activity of WO having
different morphologies. Since WO photocatalysts show poor
catalytic activity for decomposition of organic compounds due
to their low reduction property, we loaded a Pt co-catalyst on
strength effect observed in materials having a hollow structure
encapsulated by a pore system of relatively small pore size.
These results are in agreement with the TEM images and indi-
cate the formation of a hollow structure with a well-developed
pore system in the silica shell.
32
3
3
The content of the WO
mated by the gravimetric method. The weight of the WO
component in WO @SiO was measured aer etching the SiO
3 3 2
component in WO @SiO was esti-
WO particles of all of the photocatalysts we examined; these
3
3
2
particles are denoted by Pt/WO3.
3
2
A comparison of time courses of CO2 evolution during
decomposition of acetic acid under full-arc irradiation from
component by diluted HF aqueous solution; we conrmed in
advance that WO was not dissolved in the dilute HF aqueous
3
a 300 W Xe lamp with various Pt-loaded WO
TiO photocatalyst made of commercially available TiO
Ishihara Sangyo Co. Ltd.) is shown in Fig. 5. In 100 min, the
TiO and Pt/WO @SiO photocatalysts produced CO continu-
3
photocatalysts and
solution. From the measurement, the WO content was found to
3
(ST-01,
be 60 wt% of WO
3
2
@SiO .
2
2
UV/Vis diffuse reectance spectra of WO
3 2 3
@SiO and WO (B)
powders are shown in Fig. 4. Reectance is converted to
absorbance by the Kubelka–Munk equation:
2
3
2
2
ously to the level of 35 mmol, which corresponds to the stoi-
chiometric amount of the introduced acetic acid (17.5 mmol),
(1) indicating complete decomposition of acetic acid. The initial
2
F(R) ¼ (1 ꢂ R) /2R,
high rate of CO
2 2
evolution with the TiO photocatalyst seems to
where F(R) denotes Kubelka–Munk function and R denotes
reectance of the sample relative to that of a non-absorbing
be due to its intrinsically high photocatalytic activity. However,
Fig.
decomposition of acetic acid over (a) Pt/WO
and Pt/WO (B)/SiO and (d) TiO (ST-01) under full-arc light irradiation
from a 300 W Xe lamp.
5
Time courses for CO
2
evolution during photocatalytic
3
@SiO , (b) Pt/WO (B), (c)
2
3
Fig. 4 UV-visible diffuse reflectance spectrum of (a) WO
b) WO (B).
3
@SiO
2
3
2
2
(
3
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the WO
3 3 2 3 2 2
photocatalyts (Pt/WO @SiO and Pt/WO (B)) showed with the fact that CO evolution continued even aer the CO
much higher activity than that of the TiO2 photocatalyst, as evolution rate had decreased over Pt/WO (B).
3
shown in Fig. 6, suggesting the usefulness of WO3 photo-
To conrm the effect of the intermediates, products in the
catalysts under visible light conditions. Of the WO3 photo- gas phase were analyzed during the photocatalytic reaction. As
catalyts,
decomposition of acetic acid, as shown in Fig. 5 and 6.
In Fig. 5, Pt/WO @SiO and Pt/WO (B)/SiO
slopes for CO evolution in the initial period. This unique formaldehyde is the major oxidation product from methanol.
Pt/WO
3
@SiO
2
was
effective
for
complete a result, methanol was observed in the gas phase together with
very small amounts of methyl formate and methyl acetate.
3
2
3
2
show concave However, formaldehyde was not observed. It is well known that
37
2
property of the photocatalyts seems to be related to the hollow Formaldehyde was not detected in this system probably because
shell structure. We consider that the release of CO from the formaldehyde was strongly adsorbed on the photocatalysts or
2
hollow space of the photocatalysts to the outer space is decomposed rapidly. The amount of methanol over Pt/
hindered to some extent by the porous SiO shell, leading to the WO @SiO was smaller than that over Pt/WO (B) (Fig. S4†). This
2
3
2
3
2
concave slope until the concentration of CO reaches a certain result is consistent with the hollow shell effect mentioned
level. For the same reason, we can expect that reactive products above.
produced in the hollow space have as higher chance of reaction In the case of Pt/WO
on the photocatalyst because their escape from the hollow space intermediates had been completely decomposed, the amount of
and supply of acetic acid to the hollow space are hindered by the CO was insufficient for a stoichiometric amount, indicating
3
(B), even aer methanol and other
2
silica shell. The concentration of the intermediate in the hollow that the formaldehyde produced is strongly adsorbed on Pt/
shell of Pt/WO @SiO can be lower than that of Pt/WO (B) in the WO (B). Formaldehyde is easily polymerized to a stable para-
3
2
3
3
gas phase because the intermediate competes with acetic acid formaldehyde at a high concentration as reported in the liter-
3
8,39
for the reaction sites (or photogenerated holes) on WO
3
. Such ature.
Since paraformaldehydes are relatively difficult to
evolution rate is drastically reduced if they
a function of the hollow shell (or hollow shell effect) will affect decompose, the CO
2
the activity of the photocatalyts as mentioned later.
Pt/WO @SiO and Pt/WO (B) showed almost the same
reaction rates for CO
period, as shown in Fig. 5. However, the evolution rate of CO2 proceeds efficiently due to the hollow shell effect, which
with Pt/WO (B) decreased drastically when the amount of CO maintains the concentration of formaldehyde at a low level. As
accumulate on the photocatalyst.
3
2
3
3 2
In the case of the hollow space of Pt/WO @SiO , however,
2
evolution during the initial reaction the sequential photo decomposition reaction of intermediates
3
2
reached half of the stoichiometric level, suggesting that meth- a result, formation of paraformaldehyde was probably sup-
anol or formaldehyde, both of which are the chief intermediates pressed and continuous complete decomposition of acetic acid
for decomposition of acetic acid, accumulates in the system. To was achieved. The catalytic activity of Pt/WO
conrm this assumption, we added acetic acid to the system physically mixed WO (B) with SiO , was almost the same as that
with Pt/WO (B) aer the CO evolution rate had decreased and of Pt/WO (B). These results indicate that the hollow structure,
we found that the CO evolution rate almost recovered to the not the SiO content, is important for the complete decompo-
3 2
(B)/SiO , which is
3
2
3
2
3
2
2
initial rate (Fig. S3†). This result strongly suggests that the sition of acetic acid. Although further studies are needed to
decrease in the CO evolution rate with Pt/WO (B) is not due to understand the detailed reaction mechanisms, the hollow
2
3
poisoning of the active sites of WO3 but is due to the slow structure is expected to be effective for the photodecomposition
decomposition rate of the intermediate. This is also consistent of organic compounds through stable intermediates.
Conclusions
3
We developed a procedure for the synthesis of WO nano-
particles encapsulated in a hollow silica sphere by effectively
using a reverse micelle system and volume change of tungsten
compounds. The obtained core–hollow shell structure was
composed of WO3 nanoparticles of 10 nm in diameter and
a porous SiO
micropores. The core–hollow shell structure of Pt-loaded
WO @SiO has great advantages in photocatalytic applica-
tions, as exemplied by the continuous and complete decom-
position of acetic acid, compared to other bulk WO
2
shell of 10 nm in thickness with highly developed
3
2
3
photocatalyts. The ndings will lead to further improvement in
the catalytic activity of WO @SiO by an optimized core–hollow
3
2
shell structure.
Fig.
decomposition of acetic acid over (a) Pt/WO
c) TiO (ST-01) under visible light irradiation (l > 400 nm).
6
Time courses for CO
2
evolution during photocatalytic Conflicts of interest
3 2 3
@SiO , (b) Pt/WO (B) and
(
2
There are no conicts to declare.
15364 | RSC Adv., 2020, 10, 15360–15365
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7 G. R. Torres, T. Lindgren, J. Lu, C.-G. Granqvist and
S.-E. Lindquist, J. Phys. Chem. B, 2004, 108, 5995.
Acknowledgements
TEM measurement was carried out by using a facility in the 18 H. Irie, Y. Watanabe and K. Hashimoto, J. Phys. Chem. B,
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Osaka University and TEM observation was supported by Dr 19 H. Irie, Y. Watanabe and K. Hashimoto, Chem. Lett., 2003,
Takao Sakata. This research was partially supported by Kansai
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Research Foundation for Technology Promotion and JSPS 20 T. Ohno, T. Tsubota, K. Nakajima and Z. Miyamoto, Chem.
KAKENHI Grant Numbers JP15K14127 and JP18K05273.
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