Visible light induced decomposition of organic compounds on WO3 loaded PtPb co-catalysts

Article abstract of DOI:10.1016/j.catcom.2014.07.013

Tungsten oxide (WO₃)-supported ordered intermetallic PtPb nanoparticles (PtPb NPs/WO₃) were prepared through a co-reduction of Pt and Pb precursors with sodium borohydride in anhydrous methanol containing WO₃. The PtPb NPs/WO₃ were characterized based on the crystal structure obtained through powder X-ray diffraction (pXRD) as well as X-ray photoemission spectroscopy (XPS) and transmission electron microscopy (TEM). The formation of an ordered PtPb intermetallic phase on the WO ₃ surface was confirmed. The PtPb NPs/WO₃ were more efficient when decomposing acetic acid (AcOH) and acetaldehyde (AcH) with visible light irradiation compared to the same process over WO₃ loaded with Pt nanoparticles (Pt NPs/WO₃).

Full text of DOI:10.1016/j.catcom.2014.07.013

Visible Light Induced Decomposition of Organic Compounds on WO 3 Loaded
PtPb Co-catalysts
Takao Gunji, Takashi Tsuda, Arockiam John Jeevagan, Masanari Hashimoto,
Toyokazu Tanabe, Shingo Kaneko, Masahiro Miyauchi, Govindachetty
Saravanan, Hideki Abe, Futoshi Matsumoto
PII:
DOI:
Reference:
S1566-7367(14)00275-1
doi: 10.1016/j.catcom.2014.07.013
CATCOM 3970
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
30 May 2014
4 July 2014
5 July 2014
Please cite this article as: Takao Gunji, Takashi Tsuda, Arockiam John Jeevagan, Masa-
nari Hashimoto, Toyokazu Tanabe, Shingo Kaneko, Masahiro Miyauchi, Govindachetty
Saravanan, Hideki Abe, Futoshi Matsumoto, Visible Light Induced Decomposition of Or-
ganic Compounds on WO3 Loaded PtPb Co-catalysts, Catalysis Communications (2014),
doi: 10.1016/j.catcom.2014.07.013
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ACCEPTED MANUSCRIPT
Visible Light Induced Decomposition of Organic
Compounds on WO3 Loaded PtPb Co-catalysts
Takao Gunji, a Takashi Tsuda,a Arockiam John Jeevagan, a Masanari Hashimoto, a
Toyokazu Tanabe, a Shingo Kaneko,a Masahiro Miyauchi,b,c Govindachetty Saravanan,d
Hideki Abe,c,e Futoshi Matsumoto* a
aDepartment of Material and Life Chemistry, Kanagawa University,
3-27-1, Rokkakubashi, Kanagawa-ku, Yokohama, Kanagawa 221-8686, Japan.
bDepartment of Metallurgy and Ceramic Science, Graduate School of Science and
Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo,
152-8552, Japan
cPRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan.
dCSIR-National Environmental Engineering Research Institute (CSIR-NEERI)
Nehru Marg, Nagpur 440020, India.
eNational Institute for Materials Science (NIMS), Sengen 1-2-1 Sengen, Tsukuba,
Ibaraki 305-0047, Japan
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Abstract
Tungsten oxide (WO3)-supported ordered intermetallic PtPb nanoparticles (PtPb
NPs/WO3) were prepared through a co-reduction of Pt and Pb precursors with
sodium borohydride in anhydrous methanol containing WO3. The PtPb NPs/WO3
were characterized based on the crystal structure obtained through powder X-ray
diffraction (pXRD) as well as X-ray photoemission spectroscopy (XPS) and
transmission electron microscopy (TEM). The formation of an ordered PtPb
intermetallic phase on the WO3 surface was confirmed. The PtPb NPs/WO3 were
more efficient when decomposing acetic acid (AcOH) and acetaldehyde (AcH) with
visible light irradiation compared to the same process over WO3 loaded with Pt
nanoparticles (Pt NPs/WO3).
Keywords
Photocatalysis, Co-catalyst, Ordered Intermetallic Compound, Photo-decomposition.
Highlights
・ The WO3 supported PtPb catalysts was synthesized.
・ The formation of PtPb catalysts were characterized with TEM and XPS.
・ The photocatalytic activities of the PtPb/WO3 were investigated.
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Introduction
Semiconductor photocatalysts, such as titanium oxide (TiO2) and tungsten oxide
(WO3), have received significant attention for several decades because they are a
potential solution for the current energy and environmental problems. Semiconductor
photocatalysts are versatile candidates for environment remediation [1], water splitting
[2], CO2 reduction [3] and solar energy conversion [4]. Recently, numerous visible
light-driven semiconductor photocatalysts have been developed [5]. Metal oxides,
particularly WO3, have attracted attention because they act as ideal visible light
photocatalysts; this behavior is attributed due to their small band gap energies (2.4-2.8
eV) and the high oxidation power of their valence band holes [6]. However, WO3
exhibits a low photocatalytic activity due to the rapid recombination of its photoexcited
electrons and holes. To suppress this process and significantly increase the
photocatalytic efficiency, the consumption of photoexcited electrons and holes should
be enhanced using co-catalysts. Abe and co-workers have demonstrated that Pt-loaded
WO3 samples were highly efficient during the decomposition of organic compounds
under visible light irradiation with an oxygen reduction, which consumed the
photoexcited electrons [7]. Consequently, Pt nanoparticles (NPs) were used as co-
catalysts with WO3 to facilitate a multi-electron reduction of O2, allowing O2 to accept
electrons despite the insufficient reduction potential of the conduction band electrons (in
WO3) during its single-electron reduction; furthermore, photocatalytic activity can be
improved by selecting appropriate co-catalysts. Recently, a new approach that prevents the
inherent problems of disordered alloy catalysts has been proposed by Abruña et al. toward
highly active electrocatalysts for fuel cell applications [8]. In contrast with disordered alloys,
intermetallic compounds with definite compositions and structures, such as PtPb and PtBi,
exhibit excellent electrocatalytic performances toward FA oxidations in acidic solutions in
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terms of their onset potentials and current densities [9, 10]. After the report by Abruña, many
papers describing intermetallic NPs, such as PtBi, PtPb, Pt3Ti and carbon black (CB)-supported
intermetallic NPs, have been published for FA, MeOH and EtOH oxidations and oxygen
reduction reactions (ORR) [11-13]. In our previous study, the electrocatalytic activity of
the ordered intermetallic PtPb compounds exhibited higher electrocatalytic activity
toward formic acid, methanol (MeOH) and ethanol (EtOH) oxidation [14] compared to
conventional Pt-based alloys and Pt. In addition, we have recently reported that ordered
PtPb intermetallic NPs accelerated oxygen reduction reactions (ORR) in acidic
solutions [15]. Therefore, the ordered PtPb ordered intermetallic NPs should be
competent co-catalysts at the oxidation and reduction sites on WO3. We report a simple
one-pot approach toward WO3-supported ordered intermetallic PtPb NP catalysts.
Furthermore, we investigated the effects of using ordered intermetallic PtPb compounds
as co-catalysts with photocatalytic activity during the oxidative decomposition of acetic
acid (AcOH) and acetaldehyde (AcH).
Experimental
The WO3 particles were purchased from Kojundo Chemical Laboratory Co. Ltd.
(Type: WWO04PB). Fine particulate WO3 particles (50–200 nm) were separated from
the commercial WO3 powder, as reported in Ref. 7. The photodeposited Pt NPs/WO3
sample was synthesized as reported by Abe et al. [7]. The procedure used to prepare Pt
NPs/WO3 via chemical deposition with a Pt precursor and a reducing agent is described
in the ESI†. The synthesis of PtPb NPs/WO3 (PtPb loading: 1.0 wt%) proceeded as
follows. WO3 powder (0.5 g) was suspended in 15 mL of anhydrous methanol for 15
min in a two neck round bottomed flask. Afterwards, 0.013 mmole of chloroplatinic
acid (H2PtCl6·6H2O, Wako, Japan) and 0.026 mmole of lead(II) acetate (Pb(CH3COO)2,
Sigma-Aldrich) were dissolved in 15 mL of anhydrous methanol in a shielded vial under
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Ar before being transferred to the WO3 suspensions. The solution was stirred for 30 min
to yield a homogeneous yellow solution. Afterwards, 3 mmole of sodium borohydride
(NaBH4, Sigma-Aldrich) in methanol was injected into the suspension to reduce the
precursors. The product was centrifuged and washed with anhydrous methanol three
times before being dried under vacuum. The as-prepared PtPb NPs/WO3 were yellowish
green. The other experimental conditions are described in the Supplementary
Information.
Results and discussion
Fig. 1A shows the powder X-ray diffraction (pXRD) profiles of the WO3, Pt
NPs/WO3 and PtPb NPs/WO3 (Supplement information for the pXRD procedure). To
confirm the formation of the ordered PtPb intermetallic NPs on the WO3 through
pXRD, the Pt loading on WO3 was adjusted to 5 wt% on both the Pt NPs/WO3 and PtPb
NPs/WO3, while 0.1 or 1.0 wt% Pt and ordered intermetallic PtPb were used when
testing the photocatalytic activity. The simulated pXRD patterns for the Pt phase (face-
centered cubic (fcc), Fm-3m, a = 0.3925 nm, JCPDS 04-0802) and the ordered
intermetallic PtPb phase (NiAs structure, P63/mmc, a = b = 0.426 nm, c = 0.548 nm) are
denoted by the solid bars below the graph. The pXRD profile (Fig. 1A-(a)) for WO3
shows several major peaks, which are attributed to monoclinic WO3 (JCPDS 43-1035)
In addition, the pXRD profiles (Fig. 1A-(b, c)) for the Pt NPs/WO3 and the as-prepared
PtPb NPs/WO3 exhibit small characteristic peaks at 39.8° for Pt and at 41.0° and 42.4°
for the ordered intermetallic PtPb phases. The retention of the pXRD peaks at 41.0 and
42.4°, even after heat-treating the as-prepared PtPb NPs/WO3 (the reason for the heat-
treatment will be explained in Fig. 3), could be ascribed to the formation of stable,
ordered PtPb intermetallic NPs on the WO3 (Fig. S1).
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Figures 1 (B) and (C) show low- and high-resolution (HR)-TEM images of the
annealed 1.0 wt% PtPb NPs/WO3. Several low-resolution images were obtained from a
PtPb NPs/WO3 sample, as shown in Fig. S2, to show the particle size and the degree of
dispersion of the PtPb NPs on the WO3. The size distributions of the PtPb NPs were
evaluated based on the sizes of approximately 100 particles in the TEM images. The
average diameter of the PtPb NPs was 9.5 nm, and they exhibited a large particle-size
distribution. In particular, the PtPb NPs are different sizes within the observation area
even when the images were obtained from a single sample, as shown in Fig. S2.
However, the Pt NPs (average diameter: 5.4 nm) were well dispersed on the WO3
surface (the TEM images of the Pt NPs/WO3 prepared through chemical and
photodeposition are shown in Figs. S3 and S4). The HR-TEM image (C) of the interface
between the PtPb and WO3 phases shows clear lattice fringes, indicating that the PtPb
NPs have atomically ordered structures on the WO3 surface. Furthermore, the d-value is
clearly different from that of WO3. The interval of the lattice fringe (0.223 nm) on the
PtPb phase is consistent with the d-value for the {110} planes of the ordered PtPb
intermetallic phase (d = 0.213 nm, ICDD PDF File # 06-0374) but is not consistent with
any d-values for Pt phases (fcc structure). The fast Fourier-transformation (FFT) pattern
(Fig. S5) demonstrates that the atoms in the PtPb NPs are, as expected based on the
pXRD data, arranged in a NiAs-type structure. The compositional maps (Fig. 2) based
on the STEM images reveal that the Pt and Pb are uniformly distributed in the PtPb
NPs. STEM-energy dispersive spectroscopy (EDS) maps in Fig. 2 demonstrate that the
average molar ratios of Pt to Pb for the PtPb NPs (Pt:Pb = 56.2:43.8) were consistent
with the desired values for PtPb (Pt:Pb = 1:1). Inductively coupled plasma-mass
spectroscopy (ICP-MS) showed that the average molar ratio of Pt to Pb for the PtPb
NPs on WO3 was Pt:Pb = 49.8:50.2 for the as-prepared and annealed samples. The
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pXRD and TEM/STEM data clearly indicate that an ordered intermetallic PtPb phase
can be formed on a WO3 surface through a one-pot process.
Fig. 3 shows the diffuse reflectance UV-Vis absorption spectra for the Pt NPs/WO3
prepared from the Pt precursor on a WO3 support when using NaBH4 as the reducing
agent. To evaluate the influence of the reducing agent on the support after the Pt ions
were reduced in anhydrous methanol containing dispersed WO3, UV-Vis spectra
(Supplement information for the UV-Vis absorption spectroscopy procedure) were
collected, as shown in Fig. 3A. Non-treated, pure WO3 (a) exhibits an apparent
absorption edge at 465 nm, as reported previously [16]. The as-prepared Pt NPs/WO3
catalysts exhibit a visible absorption above 450 nm due to the reduction of the W6+ to
W5+ or W4+; the reducing agent reduced the Pt precursor in anhydrous methanol
containing dispersed WO3. A significant change in the visible absorption of the Pt
NPs/WO3 is induced through a heat treatment in air at 100 ºC over 1 h. Afterwards, the
absorption band exhibited no obvious change compared to the pure WO3, indicating that
the oxidation state on the Pt NPs/WO3 photocatalyst sample was restored to its original
state. The XPS results obtained with the as-prepared and annealed Pt NPs/WO3 also
support our hypothesis regarding the change in the oxidation state of the W ions
(Supplement information for the XPS procedure). The XPS spectra showing the W 4f
region of (a) non-treated WO3, (b) as-prepared Pt NPs/WO3 and (c) annealed (100 ºC)
Pt NPs/WO3 are shown in Fig. 3B. All of the binding energies in Fig. 3B are assigned
based on the literature values for WO3 [17–19]. The XPS spectra were curve-fitted,
and each curve was assigned to the corresponding oxidation state in the W 4f level. A
doublet was used to fit the W 4f level, as observed in Fig. 3B-(a). The peak for W 4f7/2
can be observed at 35.4 eV and is generated by the photoelectrons emitted from W+6
species (WO3). The spectrum (b) recorded with the as-prepared Pt NPs/WO3 can be
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assigned two doublets: the peak (W 4f7/2) of the first doublet is located at 35.4 eV and is
generated by photoelectrons emitted from W+6 species; the peak (W 4f7/2) for the second
doublet is located at 34.4 eV and can be attributed to the photoelectrons emitted from
tungsten atoms near the oxygen vacancies formed by the reduction reactions when the
oxidation state of W is below +6. As observed in (c), the second doublet was removed
after treating the Pt NPs/WO3 at 100 ºC, reforming the W6+. For the PtPb NPs/WO3, a
clear absorbance change could be observed between non-treated WO3 and as-prepared
PtPb NPs/WO3 (Fig. S6-A), but no drastic changes were observed in the oxidation state
of WO3 based on the XPS spectra (Fig. S6-B). The annealing process (in air at 100 ºC
for 1 h) was applied to the as-prepared PtPb NPs/WO3 samples to return the optical
properties of the WO3 in PtPb NPs/WO3 to that of the untreated WO3 supports. The UV-
Vis absorption spectra of the annealed Pt NPs/WO3 and annealed PtPb NPs/WO3 are
overlaid in Fig. S7 to compare their optical absorption properties. The Pt NPs/WO3
exhibit a higher visible absorption than the PtPb NPs/WO3.
The photocatalytic activities of the photodeposited Pt NPs/WO3, chemically
deposited Pt NPs/WO3 and annealed PtPb NPs/WO3 photocatalysts were examined
during a photocatalytic decomposition of AcOH in aerated aqueous suspensions
(Supplementary information contains the procedure used to evaluate the decomposition
rate of the AcOH). Fig. 4(A) plots the CO2 generated during the liquid-phase
decomposition of AcOH over the Pt NPs- and PtPb NPs-loaded WO3. The rates of CO2
generation over the Pt NPs/WO3 and PtPb NPs/WO3 photocatalysts were estimated
using the slope of the CO2 formation vs. time plot from 0-100 min. The CO2 generation
rate over the PtPb NPs/WO3 (115.3 mol·mg-Pt-1·h-1) was higher than that of the
-
photodeposited- (56.33 mol·mg-Pt-1·h-1) and chemically deposited (50.24 mol·mg-Pt
1·h-1) Pt NPs/WO3. The low activity of the annealed-chemically deposited Pt NPs/WO3
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versus the photodeposited Pt NPs/WO3 might be attributed to the reduced tungsten ions
in the WO3 in the annealed-chemically deposited Pt NPs/WO3. However, the XPS data
in Fig. 3B-c do not show any XPS peaks attributed to reduced tungsten ions (W5+ or
W4+) after annealing. Therefore, the contradictory data from the activity test and XPS
measurements should be examine. Although the low activity of the annealed-chemically
deposited Pt NPs/WO3 is attributed to a small change in the oxidation state of the
tungsten ions that cannot be detected through XPS, we propose that the differences in
the deposition sites of the Pt NPs on the WO3 of the annealed-chemically deposited Pt
NPs/WO3 and photodeposited Pt NPs/WO3 samples should also be examined. In the
annealed-chemically deposited Pt NPs/WO3 the Pt NPs were randomly deposited on the
WO3 surfaces. On the surfaces of the photodeposited Pt NPs/WO3 sample, however, the
Pt NPs were site-selectively deposited on the WO3. Because the Pt NPs were formed by
reductive deposition with photoexcited electrons, the Pt NPs were deposited on the site
(reduction site) where the photoexcited electrons appear on the surfaces of the WO3.
The co-catalysts deposited on the reduction site actively catalyze the reduction reaction
under visible light. Fig. 4(B) shows the AcH decomposition and CO2 generation profiles
over WO3 that was annealed after exposure to a reducing agent, photodeposited- and
chemically deposited Pt NPs/WO3, and annealed PtPb NPs/WO3 photocatalysts under
visible light irradiation (λ > 420 nm) (Supplementary information details the procedure
used to evaluate the decomposition rate of AcH). With visible light irradiation, the
amounts of AcH in the gas phase over the Pt NPs/WO3 and PtPb NPs/WO3 decreased
rapidly when the CO2 generation increased versus WO3. The complete decomposition of
AcH is proven by the molar yield of CO2 (ca. 530 ppm), which is twice that of the
molar amount of AcH injected (ca. 280 ppm). The Pt NPs/WO3 and PtPb NPs/WO3
photocatalysts completely decomposed AcH to form CO2 over different reaction times.
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In contrast, WO3 could not decompose AcH completely because the catalyst was
deactivated by the products that accumulated on its surface [20]. Of the examined
samples, PtPb NPs/WO3 perform the best by completely decomposing AcH to CO2
within 120 min.
Conclusion
Samples of WO3 loaded with Pt and PtPb NPs were used to photocatalyze the
decomposition of organic compounds in the liquid and gas phases with visible light
irradiation (λ > 420 nm). PtPb NPs/WO3 exhibit a higher photocatalytic activity than the
conventional Pt NPs/WO3 and the pure WO3. This enhanced photocatalytic activity
might be attributed to the efficient multi-electron ORR and the oxidative decomposition
of organic compounds, which induces charge separations in the PtPb. The particles of
the ordered PtPb intermetallic NPs on WO3 are much larger than the Pt NPs on WO3;
the Pt NPs on WO3 have much larger surface areas exposed to the AcOH solution and
to the gas phase containing AcH than the PtPb NPs fixed on WO3. Although the surface
area of the PtPb NPs is smaller than that of the reported catalysts utilized in this work,
the Pt NPs/WO3 exhibit a higher visible absorption than the PtPb NPs/WO3; PtPb
showed enhanced photocatalytic activity relative to the Pt NPs/WO3. Therefore, the
surfaces of the ordered intermetallic PtPb NPs are more efficient as co-catalysts on WO3.
Our strategy for preparing PtPb NPs on WO3 was successful, and the results
demonstrate highly efficient visible light-induced photocatalytic systems using ordered
intermetallic NP co-catalysts.
Acknowledgements
We thank Dr. Akihiro Yoshida and Prof. Shuichi Naito of Kanagawa University for
help measuring photocatalytic activity. This work was financially supported by the
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Strategic Research Base Development Program for Private Universities of the Ministry
of Education, Culture, Sports, Science and Technology of Japan.
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Figure captions
Fig. 1 (A) pXRD diffractograms of the WO3 (a), Pt NPs/WO3 (b) and PtPb NPs/WO3
(c) (for b and c, 5 wt% Pt on WO3). (B) TEM image of the annealed PtPb NPs/WO3
photocatalyst (1.0 wt% PtPb on WO3) and (C) a high-resolution (HR)-TEM image of
the interface between the PtPb and WO3 phases. The marks (* and #) in (A) correspond
to the peaks for Pt and PtPb, respectively.
Fig. 2 Scanning transmission electron microscopy (STEM) image of the annealed PtPb
NPs/WO3 (PtPb loading weight on WO3: 1.0 wt%) and the corresponding compositional
mapping images.
Fig. 3 (A) Diffuse reflectance UV-Vis absorption spectra for the (a) non-treated WO3,
(b) as-prepared Pt NPs/WO3 and (c) annealed (100º C) Pt NPs/WO3 (for b and c, 1.0
wt% Pt on WO3). (B) XPS spectra (red lines) and fitted curves (blue lines) in the W 4f
region for the (a) non-treated WO3, (b) as-prepared Pt NPs/WO3 and (c) annealed (100º
C) Pt NPs/WO3.
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Fig. 4 (A) The time courses of the CO2 evolution during the decomposition of AcOH
over the chemically deposited Pt NPs/WO3 (solid circle, 1 wt % Pt), photodeposited Pt
NPs/WO3 (solid triangle, 1 wt % Pt) and annealed PtPb NPs/WO3 (1 wt% PtPb, solid
square) photocatalysts suspended in aqueous AcOH in the presence of O2 with visible
light irradiation (λ > 420 nm). (B) The time trials for the decomposition of AcH (open
symbol) and the evolution of CO2 (solid symbol) in the presence of O2 over WO3
(cross), Pt/WO3 (triangle, Pt was deposited by photo deposition, 0.1 wt % Pt), Pt
NPs/WO3 (circle, Pt was deposited by chemical deposition, 0.1 wt % Pt) and annealed
PtPb NPs/WO3 (square, 0.1 wt% PtPb) photocatalysts with visible light irradiation (λ >
420 nm).
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Figures
Fig.1
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Fig. 2
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Fig. 3
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Fig. 4
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Graphical Abstract
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Highlights
・ The WO3 supported PtPb catalysts was synthesized
・ The formation of PtPb/WO3 catalysts were characterized with TEM and XPS
・ The photocatalytic activities of the PtPb/WO3 were investigated
・ PtPb/WO3 showed higher catalytic activity for decomposition of AcOH and
AcH than Pt/WO3
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