Fabrication of high-sensitivity palladium loaded tungsten trioxide photocatalyst by photodeposite method

Article abstract of DOI:10.1016/j.cattod.2014.07.044

Pd-supported tungsten oxide (Pd/WO3) was prepared by photodeposition by using black light. X-ray diffraction and X-ray photoelectron spectroscopy analyses of Pd/WO₃ confirmed the presence of Pd(0). Pd(0) was uniformly dispersed on the surface of WO₃. The optimum Pd amount for the degradation of aqueous methylene blue (MB) was 0.5 wt%, and the photocatalytic activity in this case was 27 times higher than that of pure WO₃. The optimum Pd amount for the decomposition of gaseous acetaldehyde was 0.1 wt%, and the photocatalytic activity using this amount of Pd was 5.9 times greater than that for pure WO₃. Pd as a support improved the charge separation efficiency. Further, hydrogen peroxide was produced on the Pd(0) side of the photocatalyst because of the movement of photoexcited electrons, and it contributed significantly to MB degradation. Moreover, electrons produced with MB moved to the Pd side, and contributed to hydrogen peroxide production. Only photocatalytic degradation contributed to acetaldehyde decomposition, while both photocatalytic and self-sensitized degradation contributed to MB degradation. The Pd/WO₃ sample containing the optimal amount of Pd acted as an effective photocatalyst, despite the difference between the optimal Pd amount required for acetaldehyde decomposition and that for MB degradation.

Full text of DOI:10.1016/j.cattod.2014.07.044

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Catalysis Today
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Fabrication of high-sensitivity palladium loaded tungsten trioxide
photocatalyst by photodeposite method
Yuka Sakai∗, Asami Shimanaka, Masahiko Shioi, Shigeru Kato, Shigeo Satokawa,
Toshinori Kojima, Akihiro Yamasaki
Department of Materials and Life Sciences, Faculty of Science and Engineering, Seikei University, 3-3-1 Kichijoji-kitamachi, Musashino-shi, Tokyo
180-8633, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Pd-supported tungsten oxide (Pd/WO3) was prepared by photodeposition by using black light. X-ray
diffraction and X-ray photoelectron spectroscopy analyses of Pd/WO3 confirmed the presence of Pd(0).
Pd(0) was uniformly dispersed on the surface of WO3. The optimum Pd amount for the degradation of
aqueous methylene blue (MB) was 0.5 wt%, and the photocatalytic activity in this case was 27 times
higher than that of pure WO3. The optimum Pd amount for the decomposition of gaseous acetaldehyde
was 0.1 wt%, and the photocatalytic activity using this amount of Pd was 5.9 times greater than that for
pure WO3. Pd as a support improved the charge separation efficiency. Further, hydrogen peroxide was
produced on the Pd(0) side of the photocatalyst because of the movement of photoexcited electrons, and
it contributed significantly to MB degradation. Moreover, electrons produced with MB moved to the Pd
side, and contributed to hydrogen peroxide production. Only photocatalytic degradation contributed to
acetaldehyde decomposition, while both photocatalytic and self-sensitized degradation contributed to
MB degradation. The Pd/WO3 sample containing the optimal amount of Pd acted as an effective photocat-
alyst, despite the difference between the optimal Pd amount required for acetaldehyde decomposition
and that for MB degradation.
Received 15 February 2014
Received in revised form 4 July 2014
Accepted 25 July 2014
Available online xxx
Keywords:
Pollution treatment
Tungsten oxide
Visible light
Fluorescent lamp
Photocatalyst
Photodeposition
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
the lower edge of its conduction band is more positive than
the reduction level of hydrogen. In addition, many photoexcited
Titanium dioxide (TiO2) is a well-known photocatalyst that is
widely used for the decomposition of volatile organic compounds
(VOCs) and the environmental remediation of wastewater [1–6].
Now, anatase-type TiO2 has high photocatalytic activity; how-
ever, its bandgap is 3.2 eV, as a result of which ultraviolet (UV) light
is required for its photoexcitation. To realize increased applications,
a photocatalyst that has good activity under visible light is required.
Therefore, some studies have attempted to modify TiO2 through
approaches such as metal ion implantation and anion doping to
realize improved properties [7–17]. Others have investigated the
bandgap structures of various semiconductor photocatalysts from
the viewpoint of finding an alternative to TiO2. Tungsten oxide
(WO3) has a bandgap of ∼2.8 eV [18–21], and it is known to absorb
light with a wavelength of 443 nm (visible light) or less. However,
electrons recombine without using electrons for the reductive
reaction. Therefore, the photoinduced redox reaction efficiency is
low although WO3 absorbs visible light.
Recently, many studies have focused on improving the photo-
catalytic activity of WO3 by preparing metal-supported WO3; in
such catalysts, an increased number of holes are available because
of the hydrogen peroxide generated on the support metal site and
the improved charge segregation efficiency of the excited electrons
[22–31].
The photocatalytic activity of platinum (Pt)-supported WO3 pre-
pared by the photodeposition method [23–26] using visible light
has been reported to be comparable to that of TiO2 (P25) for
the vapor-phase decomposition of VOC under Xe lamp irradiation
[23]. However, Pt is very expensive. Among studies attempting
to produce more inexpensive catalysts, Arai et al. reported that
WO3 powder that was mechanically mixed with palladium (Pd)
nanoparticles showed seven times the photocatalytic activity of
nitrogen-doped TiO2 under visible light irradiation [27]. In addition,
Liu et al. reported the use of Pd-supported WO3 powder (Pd/WO3)
∗
Corresponding author. Tel.: +81 422 37 3020; fax: +81 422 37 3871.
E-mail addresses: yukaji sakai1@yahoo.co.jp, sakai@ncassembly.jst.go.jp
(Y. Sakai).
http://dx.doi.org/10.1016/j.cattod.2014.07.044
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Please cite this article in press as: Y. Sakai, et al., Fabrication of high-sensitivity palladium loaded tungsten trioxide photocatalyst by
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produced by a similar method for malachite green (MG) degrada-
tion [30]. The interfacial area of Pd/WO3 obtained by this process
is quite small.
In this study, a high-performance visible-light-responsive pho-
tocatalyst, Pd-loaded WO3, was prepared by the photodeposition
method using black light. The photocatalytic activity of this pho-
tocatalyst, which is referred to as Pd/WO3, was investigated with
respect to the degradation of aqueous methylene blue (MB), and the
optimum Pd addition ratio for the highest activity was determined.
The photocatalytic activity of Pd/WO3 was further confirmed by
using it for the decomposition of gaseous acetaldehyde.
We also evaluated the relationship between the Pd addition
state and the photocatalytic activity of Pd/WO3, and investigated
the effectiveness of Pd/WO3 that was prepared by the photodepo-
sition method.
2. Experimental
2.1. Materials and sample preparation
WO3 powder was supplied by Kojundo Chemical Laboratory,
and Pd was obtained in the form of PdCl2 from Wako. Pd-supported
WO3 powder (Pd/WO3) was prepared by the photodeposition
method using black light irradiation. The powder sample was then
added to distilled water and mixed thoroughly. The resulting solu-
tion sample was dispersed by a supersonic wave and deaerated
using Ar gas. PdCl2 (wt.% vs. WO3) was added to this solution, and
Pd was deposited on the surface of WO3 by black light (Toshiba Co.,
FL20SBLB) irradiation for 5 h at room temperature with stirring.
Fig. 1. XPS (Pd-3d) binding energies of the Pd/WO3 photocatalysis prepared in dif-
ferent Pd addition rate.
produced during its decomposition were sampled, and their con-
centrations were measured using gas chromatography systems
equipped with a flame ionization detector (GC-FID) (Shimadzu,
GC-8A and GC-2014) and a methanizer (Shimadzu, MTN-1).
2.2. Characterization
3. Results and discussion
XRD spectra of the prepared composite and single-component
photocatalysts were measured in the measurement range of
20–60◦ by using a powder X-ray diffractometer (XRD, Rigaku Co.,
Ultima IV). UV spectra of the prepared samples were measured
in the measurement range of 250–800 nm by using a UV–Visible
(UV–Vis) spectrophotometer (Shimadzu Co., UV-3100 PC). The
Brunauer–Emmett–Teller (BET) specific surface area of the pre-
pared samples was measured by using a poresizer (Bel Japan Co.,
Belsorp-mini).
Using (ICP-AES), it was confirmed that the amount of Pd loaded
was the same as the amount of Pd fed (Thermo Fisher SCIENTIFIC
iCAP-6000 series).
The results of the X-ray photoelectron spectroscopy (XPS,
PERKIN ELMER) analysis showed that almost all of the loaded Pd
was in the zero-valence state (Fig. 1).
The X-ray diffractometer (XRD, Rigaku Co., Ultima IV) analysis
results of all the samples are shown in Fig. 2. The circle denotes
the monoclinic-phase peak attributable to WO3 [32,33]. A peak
attributable to the face-centered cubic (fcc) phase of Pd was noticed
for an addition ratio of 5 wt%. This result shows that Pd/WO3 con-
tained a considerable amount of Pd(0). However, when the addition
ratio was 1 wt% or lower, the peak attributable to Pd was not
noticed.
2.3. Dye degradation
Photocatalytic powder (1350 ppmw) was placed in a glass pail
with MB solution (Kanto Co., Japan; 10 ppm) and dispersed in an
ultrasound bath. It was then kept in the dark and agitated by using
a magnetic stirrer for 20 h to evaluate the MB adsorption capacity of
the photocatalyst. Then, it was irradiated using a fluorescent lamp
(Toshiba Co., FLR40S·D-ED-D65/M) for 8 h under the same stirring
conditions as those described above. During irradiation, 2 mL of the
sample solution was collected at regular intervals and diluted by
5 times using distilled water. The light absorption of the sample
solution was measured by using a UV–Vis spectrophotometer (Shi-
madzu Co., UV-1700) at the maximum absorption wavelength of
MB of 665 nm. Degradation rate of the MB concentration assumed
to the first-order reaction, and the photocatalytic activities were
compared based on the reaction rate constant.
Fig.
3 shows field-emission scanning electron microscope
(FE-SEM, JEOL Ltd., JSM-7001F) images and the Pd elemental con-
centration maps for Pd(0.5 wt%)/WO3 and Pd(5 wt%)/WO3 (energy
dispersive X-ray spectrometry: EDS, JED-2300F).
Pd(0) was dispersed uniformly on the surface of WO3; however,
the partial aggregation of Pd was also observed when the Pd addi-
tion ratio was high. In the case of the Pd/WO3 sample, obtained
by the photodeposition method, the interfacial area between Pd
and WO3 was higher. Furthermore, the black light used for the
photodeposition had an energy that greatly exceeded the bandgap
of WO3, which was 2.7–2.8 eV [18–21]. Therefore, it excited elec-
trons not only at the top of the valence band of the O2p orbital but
also at deeper levels. The fact that the excitation energy was far
greater than the bandgap of WO3 prevented the photoexcited elec-
trons from recombining with holes; these electrons resulted in the
reduction of Pd(II). Therefore, it can be assumed that most of the Pd
distributed on the surface of WO3 was reduced from a two-valence
state to a zero-valence state, as it was photodeposited using black
light.
2.4. Decomposition of gaseous acetaldehyde
The photocatalytic activity of Pd/WO3 with respect to the
decomposition of gaseous acetaldehyde was investigated using
a batch-type reactor and a 150 W Xe lamp equipped with a UV
(<400 nm) cut-off filter. The gaseous acetaldehyde and the CO2
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Fig. 2. XRD patterns of WO3 and Pd/WO3.
The UV–Visible absorption spectra of P25 (Degussa), WO3, and
the synthesized Pd/WO3, obtained from their diffuse reflections,
are shown in Fig. 4. The gray spectrum is attributable to P25 and
the black spectrum to unsupported WO3 [34,35]. Absorption in the
long-wavelength region increased with the Pd addition ratio. It is
believed that the absorption was owing to the surface plasmons
attributable to Pd black in the form of Pd(0) [36]. Visible-light
absorption increased markedly when the Pd addition ratio was
0.5 wt% or higher. This shows that the surface of WO3 covered by
Pd increased in area with an increase in the Pd addition ratio.
under visible-light irradiation. The first-order rate constant was
determined from the curves shown in Fig. 5; it was assumed that the
degradation reaction followed first-order kinetics after the sam-
ples had been irradiated. However, the MB concentration decreased
with an increase in the irradiation time as well. This overall reaction
is not a first-order one. Therefore, this calculation was performed
only for the initial reaction rate, for which there was a large enough
supply of MB.
The relationship between the Pd addition ratio and the first-
order rate constant is shown in Fig. 6. The highest value of
the first-order rate constant under visible-light irradiation was
0.59 h−1, which corresponded to a Pd addition ratio of 0.5 wt%; this
value was approximately 27 and 28 times higher than those for pure
WO3 and P25, respectively. When the Pd addition ratio was 0.5 wt%
or lower, the photocatalytic activity increased with an increase in
the ratio. We investigated MB degradation under a fluorescent lamp
including UV rays as well; the relation between the addition ratio
and the first-order rate constant was determined for this condition
Fig.
5 shows the relationship between the visible-light
(>417 nm) irradiation time and the absorbance during the degra-
dation of aqueous MB. No discoloration of the MB solution was
observed under these conditions when a photocatalyst was not
used. The results obtained in the case of unsupported WO3 and
P25 were used as references. For all the samples, the MB solu-
tion underwent slight discoloration in the dark, owing to surface
adsorption. The MB absorbance for each sample tended to decrease
Fig. 3. FE-SEM images and the Pd elemental concentration maps for (a) Pd(0.5 wt%)/WO3 and (b) Pd(5 wt%)/WO3.
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Fig. 4. UV–Visible absorption spectra of P25 (Degussa), WO3, and the synthesized Pd/WO3.
Fig. 5. Relationship between the visible-light (>417 nm) irradiation time and the
absorbance during the degradation of aqueous MB.
Fig. 7. Photocatalytic activities of the various samples during the decomposition of
gaseous acetaldehyde.
too. Under fluorescent-lamp irradiation, the first-order rate con-
stant for a Pd addition ratio of 0.1 wt% was approximately 7 times
that in the case of unsupported WO3. The highest value of the first-
order rate constant under fluorescent-light irradiation was 1.2 h−1
and corresponded to a Pd addition ratio of 0.5 wt%; this value was
approximately 23 and 10 times higher than those for pure WO3 and
P25, respectively.
Fig. 7 shows the photocatalytic activities of the various samples
during the decomposition of gaseous acetaldehyde.
The relationship between the addition ratio and the initial CO2
production rate is shown in Table 1 (the first 15 min of pho-
toinduced degradation test). The highest value of the initial CO2
production rate under visible-light irradiation was 22.8 ␮mol h−1
and corresponded to a Pd addition ratio of 0.1 wt%; this value was
approximately 5.9 times greater than that for pure WO3. The top
of the valence band of WO3 is approximately at 3.2 eV. Therefore,
the holes formed by photoexcitation have high oxidation power
and are effective in decomposing organic matter. However, because
the lower conductive area is approximately at +0.5 eV, it is thought
Fig. 6. Relationship between the Pd addition ratio and the first-order rate constant.
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Table 1
between the optimum amount of supported Pd and photocatalyst
revitalization is different.
Relationship between the Pd addition ratio and the initial CO2 production rate.
Photocatalyst sample
Initial CO2 production
The amount of CO2
at 6 h/␮mol
rate/␮mol h−1
4. Conclusion
WO3
3.85
8.15
22.8
8.89
10.9
12.5
20.7
10.4
Pd(0.01 wt%)/WO3
Pd(0.1 wt%)/WO3
Pd(0.5 wt%)/WO3
A
high-performance visible-light-responsive photocatalyst,
Pd/WO3, was prepared by photodeposition under black light. The
thus-synthesized Pd/WO3 was found to contain a large amount
of Pd(0). The results showed that Pd(0) can be distributed over
the entire surface of WO3 by ensuring that Pd(II) is reduced to a
sufficient degree during photodeposition using black light. Under
constant fluorescent-light or visible-light irradiation, the photocat-
alytic activity of WO3, when supported by Pd in amounts of 0.1 wt%
or greater, was higher those that of pure WO3 and P25. The opti-
mum addition ratio of Pd was determined to be 0.5 wt% for the
degradation of aqueous methylene blue (the photocatalytic activ-
ity in this case was 27 times higher than that of pure WO3) and
0.1 wt% for the decomposition of gaseous acetaldehyde (the photo-
catalytic activity in this case was 5.9 times higher than that of pure
WO3). Pd as a support served to improve the charge separation
efficiency; in addition, hydrogen peroxide was produced on the Pd
side of the photocatalyst. Further, MB discolored owing to excita-
tion by the light that it absorbed, resulting in the consumption of
the excitation electrons on the Pd/WO3 side. Therefore, because the
response mechanisms during MB resolution and acetaldehyde res-
olution were different, the relation between the optimum amount
of supported Pd and photocatalyst revitalization is different.
Finally, it was confirmed that the Pd/WO3 sample containing Pd
in the optimal amount acted as an effective photocatalyst, even
though there was a difference in the optimal amount of Pd for
acetaldehyde decomposition and that for MB degradation.
that, in the case of excited electrons, the reductive reaction does
not occur readily [24,28].
Moreover, in the case of the WO3-based photodecomposition
of gaseous acetaldehyde, the initial reaction rate is high; however,
WO3 is covered by the intermediate acetic acid, formaldehyde and
formic acid [28]. Therefore, it is highly likely that the reaction rate
became extremely low.
It is known that, when used as a support, Pd induces a mul-
tielectron reduction reaction [23]. In the case of Pd/WO3, the
photoexcited electrons migrate to the Pd site. For these electrons,
the degree of surface oxidation reduction is reduced, and, as a result,
hydrogen peroxide is generated. Therefore, in the case of the Pd
supporter, the efficiency of use of holes in Pd/WO3 improves.
When zero-valence Pd is present in a large amount and when
the Pd–WO3 interfacial area is high, photoexcited electrons in the
conduction band on the WO3 side are transferred efficiently to the
Pd side. It can be assumed that the increase in the charge separation
efficiency owing to the use of Pd as a support is attributable to the
increase in the efficiency of production of photoexcitation holes
[30,37]. In addition, on the Pd side, the transferred electrons are
likely to form superoxide anions on the surface, which results in the
formation of hydrogen peroxide [30]. On the basis of the fact that
hydrogen peroxide was shown to decompose MB under fluores-
cent light under the investigated test conditions, it can be assumed
that the additional photoreactions that occur owing to the created
hydrogen peroxide contribute to the fading. Moreover, holes with
very high oxidizing power are generated by optical excitation at the
oxygen sites, and these holes produce various activated species.
Because the light source used in this paper had an energy that
exceeds the WO3 energy gap, the following three reactions con-
tributed to the color degradation of MB (dye).
Acknowledgments
This work was supported by industry to support private uni-
versities building up their foundations of strategic research from
the Ministry of Education, Culture, Sports, Science and Technology
(MEXT) of the Japanese Government.
References
[i] MB discolored owing to excitation by the light that it
absorbed, resulting in the consumption of the excitation electrons
on the Pd/WO3 side; [ii] owing to the use of photoexcited elec-
trons on the WO3 side of the Pd side; and [iii] because of oxidation
resulting from the holes generated by optical excitation [30].
In case of the decomposition of MB, the amount of Pd that shows
high photocatalyst revitalization is the range of 0.5–5 wt%. This is
because the reaction of [ii] and [iii] does not progress readily. On
the other hand, it becomes easy for the reaction of [i] to progress
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The rate of generation of hole–electron pairs is determined by
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optimal balances. When the amount of Pd supported was 0.1 wt%,
acetaldehyde decomposition exhibited the highest revitalization.
As mentioned above, because the response mechanisms for MB
resolution and acetaldehyde resolution are different, the relation
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