Visible-light-driven conversion of organic compounds over WO3-based microtubes with mesoporous-walled structure

Article abstract of DOI:10.1039/d0nj00196a

Pd-WO₃ microtubes with mesoporous-walled structure were prepared through a template-free urea assisted solvothermal method followed by reductive calcination, which demonstrated multifunctional photocatalytic performances depending on the pollutant target, (e.g. complete oxidation of dye molecules, and partial oxidation of propan-2-ol with high selectivity for acetone generation).

Full text of DOI:10.1039/d0nj00196a

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Visible-light-driven Conversion of Organic Compounds over WO3-
based Microtubes with Mesoporous-walled Structure
Received 00th January 20xx,
Accepted 00th January 20xx
Ying Lv,a b Zhanglian Xu,c and Lei Du*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Pd-WO3 microtubes with mesoporous-walled structure were doping method is proven effective. Abe et al. creatively used
prepared through a template-free urea assisted solvothermal platinum (Pt) as a co-catalyst which largely increased the
method followed by reductive calcination, which demonstrated photocatalytic activity of WO3,13 because the loaded Pt
multifunctional photocatalytic performances depending on the accelerated the multi-electron reduction of dioxygen. From
pollutant targets, (e.g. complete oxidation for dye molecules, and then on, the emerging studies of Pt loaded WO3 hybrid
partial oxidation of propan-2-ol with high selectivity of acetone materials
greatly
enriched
the
visible-light-driven
photocatalysts family.14-17 However, the following two
problems so far limit the practical applications of the Pt/WO3
system: (i) the noble metal Pt is too expensive, especially for
indoor application, and (ii) the synthesized WO3-based hybrid
materials are mostly irregular nanoparticles with random
shape, lacking a large variety of tailored morphologies. It has
been reported that deliberate control and comprehensive
tuning of the morphology and structure, size, dispersion state
and interface of this material system play a critical role in
miscellaneous applications. Therefore, there is still a great
potential to optimize the WO3-based hybrid material
associated with its architecture and interactions between
cocatalysts and WO3, because photocatalytic efficiency is
highly related to the surface area, particle size, morphology or
structure, and unique surface characteristics. In particular,
photocatalysts with mesoporous and/or nanotubular structure
favor to achieve significantly enhanced activities due to their
porous structure and large surface area. Regarding
mesoporous WO3 or tubular WO3, most of the studies are
associated with either amorphous, poorly crystallized
frameworks or low yields and purities18-20 owing to the
introduction of forming agents or templates.
In this study, we try to optimize the WO3-based hybrid
materials from both the rational choose of cocatalyst and the
deliberate design of WO3 microstructure. We paid attention to
the facile preparation of the Pd-loaded WO3 microtubes (Pd-
WO3 MTs) with mesoporous-walled structure through a
template-free urea assisted solvothermal method followed by
the heat treatment under reductive atmosphere. Pd has been
found to be one of the most effective cocatalysts besides Pt in
TiO2 and other photocatalysts system.21-23 More importantly,
the selected reason is that the cost of Pd is much lower than Pt.
On the other hand, researchers have noticed that urea can act
generation).
The
rapid
development
of
visible-light-driven
photocatalysts has attracted impressive attention in recent
years. These photocatalysts can directly utilize sunlight or
visible light for the degradation or conversion of organic
compounds.1-7 One of the most studied visible-light-driven
photocatalyst is the TiO2-based photocatalysts (e.g. sulfur or
nitrogen-doped TiO2).8-10 However, the inorganic element
doping leads to localized states in the bandgap of TiO2, and the
localized holes produced at the impurities level have a slower
mobility than those in the valence band (VB).8 So far, their
ability to completely converse organic compounds under
visible light has not been satisfactory. Therefore, it is
imperative and challenging to develop new and effective
visible-light-driven photocatalysts.
Tungsten trioxide (WO3) is regarded to be a good
candidate due to its many advantages including strong
adsorption of the visible light, stable physicochemical
properties and resilience to photocorrosion effect.11
Unfortunately, the conduction band (CB) level of WO3 is more
positive (+0.5 V vs NHE) than the reduction potentials of O2
due to the deeply positive level of VB,12 leading to the
inefficient oxidative decomposition of organic compounds. To
enhance the photocatalytic activity of WO3, noble metal
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as both hydrogen-bond donors through their two NH protons, and the transmission electron microscope (TEM). Fig. 1(a)
and acceptors through the lone pairs of the C=O group. The presents that a large quantity of the Pd-WO3 MTs were
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DOI: 10.1039/D0NJ00196A
robust hydrogen-bonded chains self-associated from the good obtained and well distributed after reductive heat treatment.
complementarity between the two groups can construct the These MTs have average inner diameters of 0.75 μm and
organic crystals24 and inorganic crystals25 into nanostructured average lengths of 17.5 μm, respectively. With clear
materials based on tubular or columnar architectures. Inspired observation of a single microtube (Fig. 1b and 1c), the tube
by this, we utilized urea to direct the Ostwald ripening growth wall was assembled by the crystalline nanoparticles with sizes
of the as-prepared nanoparticles from the alcoholysis of WCl6 of 20-100 nm, and showed a uniformly and thoroughly porous
and PdCl2. The hydrogen-bonded chains produced from the structure. Whereas the precursor MTs before calcination
urea react with the as-prepared nanoparticles through possessed a thoroughly solid tube wall (Fig. S1a and b). It thus
hydrogen-bonding, resulting in the tubular structure of the can infer that the nanoparticles with different sizes was
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final products.
rearranged and assembled together during the calcination of
The Pd-WO3 MTs were simply prepared through the the precursors, leading to the formation of the porous
solvothermal method by mixing WCl6 and PdCl2 in the urea- structure through the whole tube wall. It is worth noting that
assisted ethanol solution in an autoclave and subsequent heat the tubular structure and length were not destroyed during
treatment under CO-containing reductive atmosphere (see ESI the heat treatment, showing the robustness of the MTs. The
in detail). The procedure resulted in a relatively uniform tubular and porous structure was further evidenced by the
dispersion of Pd particles on the MTs surface. The results of the TEM observation (Fig. 1d and Fig. S2). Fig. 1e
photocatalytic conversion of organic compounds (methylene showed the high-magnification TEM image of the tube wall of
blue, acetaldehyde, and alcohol) over the Pd-WO3 MTs by the Pd-WO3 MTs. After heat treatment under reductive
utilizing visible light irradiation was investigated compared to atmosphere, Pd species with the particle sizes of 8-10 nm were
commercial WO3 particles and PdO-loaded WO3 microtubes observed to be well dispersed on the surface of the WO3
after air calcination (PdO-WO3 MTs). The Pd-WO3 MTs showed nanoparticles. The corresponding elemental mappings showed
an efficient photocatalytic activity under visible light the spatial distribution of W, O and Pd (Fig. S3), indicating Pd
irradiation
and
also
demonstrated
multifunctional nanoparticles were uniformly distributed at the whole tube. In
photocatalytic performance depending on the pollutant addition, the X-ray diffraction (XRD) spectrum of the Pd-WO3
targets.
MTs showed in Fig. 1f. It was identified as the monoclinic WO3
(JCPDS No. 43-1035) with two minor peaks in range of
39o<2θ<47o (the inset spectrum), which was indexed as Pd0
species. Meanwhile, the XRD patterns of the precursors before
calcination and after air calcination were confirmed as the
tungstic acid hydrate (Fig. S4a, JCPDS No. 48-0719) and
monoclinic WO3 (Fig. S4b), respectively. This result suggests
that the precursor with tungstic acid hydrate phase was totally
Fig. 1 SEM, TEM images (a-e) and XRD spectrum (f) of the Pd-
WO3 MTs (amount of Pd, 0.5 wt%) after heat treatment under
reductive atmosphere.
Fig. 2 XPS spectra of WO3 samples loaded with Pd species
under different heat treatment conditions. (amount of Pd, 0.5
wt %)
The structural information of the as-prepared samples
was investigated by the scanning electron microscope (SEM)
converted into pure WO3 without any impurities through
calcination and the reductive calcination did not affect the
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formation of the WO3 phase like in the case of air calcination. corresponding pore size distribution. The main poVrieewsAizrteicleoOf nPlidne-
The main characteristic peaks of PdO were not detected owing WO3 MTs was found to be 37 nm froDmOI:t1h0e.10B3J9H/D0pNoJr0e01s9i6zAe
to the overlap with that of WO3.
distribution (Fig 3b).
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The states of Pd species on the surface of MTs under
different heat treatment conditions were further confirmed by
XPS measurement. After calcination under CO-containing
atmosphere, the Pd species on the reduced product were
found to be the metallic Pd0 state, judging from the good
agreement with the peak position in reference Pd. In contrast
to the product obtained from air calcination, it was presumed
to be Pd oxide based on the reference PdO. Therefore, the
reductive heat treatment is considered to be an effective
method for the complete reduction of Pd species for WO3
system compared to photodeposition method by light system,
in which insufficient photons generally led to incomplete
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Fig. 4 Diffuse reflectance absorption spectra of (a) Pd-WO3 MTs
and (b) commercial WO3 and the square root of adsorption
coefficient α vs. photon energy (Pd-WO3 MTs, inset).
reduction of Pd species containing both Pd and Pd0 species.26
Ⅱ
(a)
The absorption range of light plays an important role in
the photocatalysis, especially for the photoconversion of
organic compounds. Fig. 4 shows the diffuse reflectance UV-vis
absorption spectra of catalysts. The Pd-WO3 MTs show
increased absorbance above 400 nm compared to the
commercial WO3 particles due to the light scattering by the Pd
particles.28 The bandgap energy of Pd-WO3 MTs was calculated
to be 2.5 eV (inset), suggesting the MTs can effectively absorb
visible light with a wavelength above 400 nm.
The activity of Pd-WO3 MTs for photodegradation of is
compared with that of PdO-WO3 MTs and the commercial WO3
particles in Fig. 5a. The bare commercial WO3 particles show
inactive for the photodegradation of MB. While the Pd-WO3
MTs almost completely decomposed the MB dye molecules in
1 h, which was twice faster than that over PdO-WO3 MTs. With
increase of irradiation time, the MB dye molecules were totally
decomposed by Pd-WO3 MTs. Maximum photocatalytic
degradation of MB achieved at 1.5 h under visible light
irradiation using Pd-WO3 MTs was 99.8%. In addition,
acetaldehyde (AcH), which is a very typical component of
volatile organic compounds, was also completely oxidized into
CO2 over Pd-WO3 MTs within 3 h (Fig. 5b). In contrast,
complete oxidation of acetaldehyde did not observe over the
PdO-WO3 MTs and the commercial WO3 particles. These
results of the complete decomposition of MB and
acetaldehyde over Pd-WO3 MTs are different from the report
from Matsuoka et al. where they showed the activity of a WO3
film coated with Pd by a sputtering method was almost as low
as that of a bare WO3 film in the photodegradation of organic
compounds.29 Pd can proceed multi-electron oxygen reduction
as well as Pt, it can perform the O2 reduction effectively by a
(b)
Fig. 3 (a) Nitrogen adsorption–desorption isotherms and (b)
the corresponding pore size distribution curves of Pd-WO3 MTs.
The surface area and the porous structure were further
investigated through the analysis of nitrogen adsorption-
desorption isotherm curves measured at 77 K using the
Brunauer–Emmett–Teller (BET) method. Fig. 3a reveals that
the Pd-WO3 MTs exhibit the type IV isotherm behavior with H3
hysteresis,27 implying that the as-obtained microtubes are
mesoporous with a Brunauer–Emmett–Teller(BET) surface
area of ca. 23 m2g-1. As a comparison, the specific surface area
of the commercial WO3 particles was calculated to be a low
SBET of 4.1 m2g-1 (Fig. S5). The high specific surface area of Pd-
WO3 MTs is attributed to their tubular structure and
mesopores in the tube wall, which were formed by the
assembly of annealed nanoparticles with different size.
Another reason may be the formed nanosized nanoparticles
after annealing. The result was further confirmed by the
multi-electron process (O2/H2O2=+0.68
V
vs. NHE:
O2/H2O=+1.23 V vs. NHE) as a catalyst and improved the WO3-
catalyzed photodegradation of organic substance totally.
Considering that the Pd-WO3 MTs showed much higher
photocatalytic activity than PdO-WO3 MTs and the commercial
WO3, we try to explain that mesoporous tubular structure
formed by the accumulation of the crystalline nanoparticles
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and doping of the Pd nanoparticles are two key factors that
improve the photocatalytic activity. As mentioned above, the compounds over Pt/WO3 system under visible light irradiation
photocatalytic efficiency is highly dependent on the surface as a candidate for a variety of environmentally benign
area, particle size, morphology or structure, and unique synthetic processes has been reported.33 However, the use of
surface characteristics. The unique mesoporous tubular the Pd-WO3 for photocatalytic conversion of organic
structure and the accumulated nanosized WO3 nanoparticles compounds with high selectivity is rarely reported. Herein, we
enhanced the specific surface area of the photocatalysts and investigated the oxidation of propan-2-ol over the as-prepared
thus increased the contact probability between the
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The photocatalytic conversion of the specific organic
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(a)
photocatalyst and the organic molecules which offered more
catalytic sites for photocatalytic reaction. Moreover, the
mesoporous MTs allowed the effective transportation of
reaction products. The mesoporous tubular structure can also
enhance the utilization of scattered light.30 When the
mesoporous MTs are irradiated by light, a considerable part of
the photons is not directly absorbed by MTs, but can be
trapped within the mesopores and then repeated reflection
until being completely absorbed. Another advantage of the
formation of the mesoporous-walled structure is conducive to
expose the reduced Pd nanoparticles on the surface of WO3 as
(b)
many as possible. It has been well studied that the low
conduction band level of WO3 (+0.5 V vs. NHE) is more positive
than the reduction potentials of O2 (O2/O2- = -0.56 V vs. NHE;
O2/HO2 = -0.13 V vs. NHE), and increases the recombination of
photogenerated electrons and holes, leading to ultralow
catalytic activity.31 In the presence of Pd, it can proceed
multielectron oxygen reduction (O2/H2O2 = +0.68 V vs. NHE;
O2/H2O = +1.23 V vs. NHE) as well as Pt,32 and largely improved
the catalytic activity of WO3 MTs. Thus, the complete
photodegradation capacity of MB and AcH over Pd-WO3 arises
(c)
from the synergy between the Pd nanoparticles and the
unique mesoporous tubular structure.
(a)
Fig.6 Time course of photocatalytic conversion of (a) propan-
2-ol in aerated aqueous solutions over different photocatalysts
under visible light irradiation and their corresponding (b)
acetone generation and (c) CO2 generation.
(b)
Pd-WO3 MTs under visible light. Fig 6 shows the time course of
oxidation of propan-2-ol (150 μmol in aerated water) over
WO3-based photocatalysts under visible light irradiation (λ>
420 nm). With the onset of light irradiation, the amount of
propan-2-ol decreased much faster in Pd-WO3 MTs than that
in PdO-WO3 MTs, while the commercial WO3 showed inactive
to the oxidation of propan-2-ol (Fig. 6a). In Pd-WO3 MTs, the
corresponding generation of acetone (shown in Fig. 6b)
proceeded with a fairly steady rate and was saturated after
Fig. 5 Time course of photocatalytic decomposition of (a)
irradiation of 3.5 h and the amount of acetone was almost
methylene blue (MB) and (b) AcH over different photocatalysts.
unchanged with even longer irradiation periods (up to 6 h) and
CO2 was negligible generated by the Pd/WO3 MTs during the
periods of irradiation, indicating peroxidation of acetone and
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reduction of O2 by photoexcited electrons through multi-electron
other intermediates (Fig. 6c). These results were different from
that in Pt/WO3 and Pt/TiO2 and Pd/TiO2 system, where the
produced acetone was peroxided into CO2 gas.26 The rate of
the conversion of propan-2-ol and the generation of acetone
was much lower in PdO-WO3 MTs. The generation of CO2 was
also negligible in PdO-WO3 MTs and the commercial WO3 (Fig.
6c). The selectivity for acetone, which was calculated on the
basis of the amount of consumed propan-2-ol, was 87% for the
Pd-WO3 photocatalyst in the initial period (2 h), with a low
conversion of propan-2-ol (57%). Even at high conversion (97%)
after a longer irradiation (6 h), the selectivity for acetone was
still as high as ca. 83%. it has been reported that the Pd/WO3
system showed high photocatalytic activity towards gaseous
organic pollutants, including aldehydes and ketones, resulting in the
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processes (O2 + 2H +2e → H2O2, E°(O2/H2O2) = +0.68 V vs NHE, O2
+ 4H+ + 4e− → 2H2O, E°(O2/H2O) = +1.23 V vs NHE) as well as Pt.32
In Pt/WO3 system, active Pt metal works as an electron magnet. The
produced H2O2 was quickly decomposed (2H2O2 → O2 + 2H2O),
because H2O2 acted as an efficient electron scavenger and thus
increased the amounts of photogenerated holes leading to direct
oxidation of organic compounds by holes on the surface of WO3.
However, Pd showed much lower activity than Pt. Therefore,
compared to the decomposition of H2O2 in Pt/WO3, the
accumulation of H2O2 generated from O2 on Pd-WO3 MTs seemed
to be reasonable due to the low activity of Pd. It is also well known
that H2O2 molecules can react with holes in some photocatalytic
reactions (H2O2 + 2 h+ → O2 + 2H+). Thus, with prolonging the
reaction time, a part of H2O2 molecules was preferentially
consumed by the photogenerated holes and decreased the rate of
•OH generation from water molecules, consequently keeping high
selectivity for acetone until the complete conversion of propan-2-ol.
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complete decomposition of the gaseous organic molecules into CO2
26
due to the activity of the Pd cocatalyst.
But the completely
decomposition ability was reduced in aqueous media. The
reason could be due to the result of the balance between the
active species produced by the photogenerated holes on the
photocatalyst surface and the active species produced by O2
reduction on the Pd surface. While in the case of PdO-WO3
MTs, it was calculated to be about 30% acetone selectivity at
63% propan-2-ol conversion in the 2 h and 64% acetone
selectivity at 30% propan-2-ol conversion after irradiation for 6
h. To further investigate the reusability and stability, Pd-WO3
microtubes were recovered and reused for photodegradation
of MB, AcH and propan-2-ol under the same conditions. As
shown in Fig. S6, the result shows that the as-prepared Pd-
WO3 microtubes are relatively stable although the
photocatalytic activity has a little decrease after 5 cycles of the
photocatalysis experiments. The decrease could be explained
for the residual adsorption of target molecules during the
recovery process and the amount loss of the catalyst during
the recovery experiment.
Therefore, we would like to conclude the photocatalytic
oxidation of propan-2-ol over Pd-WO3 MTs as following: In the
initiation of reaction, the photogenerated holes preferentially
oxidize water molecules to form •OH radicals due to the unique
reactivity of the WO3 surface. Due to the second-order kinetic
constant for the reaction between and •OH radical and propan-2-ol
much higher (average 1.9 ×109) than that with acetone (average 1.1
× 108).35 The •OH radicals generated preferentially reacted with 2-
propanol, resulting in a steady rate of acetone generation without
CO2 generation until the almost complete conversion of propan-2-ol.
Meanwhile, the photoexcited electrons are consumed for O2
reduction to H2O2 on Pd by multielectron reaction. However, along
with the amount of acetone reaching the maximum, some
accumulated H2O2 began to react with holes due to the low activity
of Pd. After reaching a maximum, the photogenerated holes
preferentially oxidized H2O2 into O2 whereas the photoexcited
electrons reacted with O2 to generate H2O2. Thus, such reaction
cycle between O2 and H2O2 largely suppressed the peroxidation of
acetone to CO2, resulting in acetone generation from propan-2-ol
with very high selectivity. Finally, we believe that the merits from
the unique mesoporous-walled tubular structure of Pd-WO3 MTs
that discussed in section of the degradation of MB and AcH also
contributed the high conversion rate of propan-2-ol and high
selectivity of acetone generation.
We try to explain the high selectivity for acetone production
from propan-2-ol over Pd-WO3 MTs under visible light irradiation. It
is generally understood that the photocatalytic oxidation of organic
compounds on semiconductors proceeds via direct oxidation by
photogenerated holes on the photocatalysts surface and indirect
oxidation by hydroxyl radicals generated from water molecules
(H2O + h+ → H+ + •OH) or partial contribution from the other active
-
radical species (•O2 ). Due to the higher affinity of TiO2 than WO3, a
Conclusions
great deal of the desired intermediate products from the raw
organic compounds absorb on the TiO2 surface and are directly
oxidized into CO2 by photoexcited holes,34 showing low selectivity
of the desired products. However, in the present case, WO3
We have successfully prepared the mesoporous-walled Pd-
WO3 MTs with high purity through a urea-assisted alcoholysis
of WCl6 and PdCl2, and subsequent heat treatment under
reductive atmosphere. The state of Pd species on WO3 MTs
after reduction is the metallic Pd0 state. The as-prepared
mesoporous-walled Pd-WO3 MTs have larger specific surface
area than the commercial WO3 and can absorb visible light
above 400 nm. The as-prepared Pd-WO3 MTs show
multifunctional photocatalytic performances depending on the
targets of organic compounds under visible light irradiation.
The complete decomposition of MB and AcH over Pd-WO3 MTs
may be attributed to the synergy between the advantages of
the unique mesoporous tubular structure (eg. larger specific
surface area, higher separation of charge carriers and more
absorbed photons) and the multi-electron oxygen reduction by
surfaces possessed
a much lower affinity toward organic
compounds including acetone. Such low affinity is expected to
decrease the probability of direct oxidation of acetone by
photoexcited holes. As evidenced in Fig. 6, the Pd-WO3 MTs
selectively generated acetone without releasing CO2 even when the
amounts of acetone were very high, indicating that the reaction
pathway involving •OH radicals formation was dominant in the
initiation of reaction. In addition, another possible reason for the
high selectivity of Pd-WO3 MTs for acetone could be the absence of
-
other active oxygen radical species (e. g. •O2 or HO2), which are
produced through one-electron reduction processes. It has been
previously demonstrated that the presence of a nano-particulate Pd
cocatalyst on WO3 photocatalysts effectively catalyzes the
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16 Z. Wen, W. Wu, Z. Liu, H. Zhang, J. Li and J. Chen, Phys. Chem.
Chem. Phys., 2013, 15, 6773; S. S. Ma,DKO.I:M10a.1e0d3Va9ie,/wDR0A.NrAtiJcb0lee0O1an9ln6indAe
the Pd nanoparticles. The as-prepared WO3 MTs also
demonstrated high selectivity in the photocatalytic partial
oxidation of propan-2-ol to acetone under visible light
irradiation. Such high selectivity arises from the unique
reactivity of the photogenerated holes on WO3 MTs based on
the balance of the low affinity of WO3 and the low activity of
Pd species.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This study is financially supported by the Clothing
Engineering Research Center of Zhejiang Province
(2019FZKKF01). Meanwhile, this work is also funded by the
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Excellent Doctor of Zhejiang top priority discipline of textile
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DOI: 10.1039/D0NJ00196A
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Pd-WO3 microtubes demonstrated complete oxidation for dye molecules and partial
oxidation of propan-2-ol with high selectivity of acetone generation.