Platinum nanoparticles supported on defective tungsten bronze-type KSr2Nb5O15 as a novel photocatalyst for efficient ethylene oxidation

Article abstract of DOI:10.1039/c7ta04611a

Efficient separation and extraction of photo-induced carriers are crucial aspects of efficient semiconductor photocatalytic systems employed for photocatalytic oxidation (PCO) technology. In this study, a Pt-loaded KSr₂Nb₅O₁₅ composite, as a novel photocatalyst, is reported for gaseous ethylene photo-oxidation, performing excellent PCO activity. Tungsten bronze-type KSr₂Nb₅O₁₅ (KSNO) with surface oxygen vacancies is prepared by a facile and low-cost molten salt synthesis method at 1200 °C for 6 h and a possible morphology evolution process was proposed. Pt was loaded on KSNO through a facile photodeposition method. The strong metal-support interaction (SMSI) between the Pt nanoparticles and the oxygen-deficient KSNO support is beneficial for superior photocatalytic efficiency and stability toward ethylene photo-oxidation. What's more, the structure and charge transfer path in the synthesized photocatalysts were investigated by X-ray photoelectron spectroscopy (XPS), high-resolution electron microscopy, and electron paramagnetic resonance (EPR).

Full text of DOI:10.1039/c7ta04611a

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Platinum nanoparticles supported on defective tungsten bronze-
type KSr2Nb5O15 as novel photocatalyst for efficient ethylene
oxidation†
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
b
a,c
Wei Zhang,a,b Xiaoyang Pan, Peiqing Long, Xitao Liu, Xia Long, Ying Yu, and Zhiguo Yi
*
*
DOI: 10.1039/x0xx00000x
Efficient separation and extraction of photo-induced carriers are crucial aspects of efficient semiconductor photocatalytic
systems employed for photocatalytic oxidation (PCO) technology. In this study, Pt-loaded KSr2Nb5O15 composite, as a novel
photocatalyst, is reported for gaseous ethylene photo-oxidation, performing excellent PCO activity. Tungsten bronze-type
KSr2Nb5O15 (KSNO) with surface oxygen vacancies is prepared by a facile and low-cost molten salt synthesis method at
1200 °C for 6 h and a possible morphology evolution process was proposed. Pt was loaded on KSNO through a facile
photodeposition method. The strong metal−support interaction (SMSI) between Pt nanoparticles and oxygen-deficient
KSNO support is beneficial for superior photocatalytic efficiency and stability toward ethylene photo-oxidation. What’s
more, the structure and charge transfer path in the synthesized photocatalysts were investigated by X-ray photoelectron
spectroscopy (XPS), high-resolution electron microscopy, and electron paramagnetic resonance (EPR).
www.rsc.org/
them move to catalytically active sites at semiconductor
surface. This means that the separation and extraction
efficiency of photo-induced carriers are both great challenges
1 Introduction
Ethylene (C2H4) as a volatile organic compound (VOC) has great
influence on plant growth and the storage life of food.1 Indeed,
a small amount of ethylene can induce harmful reactions for
the storage of fresh fruits and vegetables.1 Therefore, the
removal of trace amounts of ethylene is imperative.
Heterogeneous photocatalytic oxidation (PCO) using
semiconductors and solar energy is regarded as one of the
most promising approaches to remove VOCs.1-3 Under mild
to improve the PCO activity.
In order to promote the photo-induced carriers separation
efficiency, various methods, such as p–n junctions, polymorph
junctions and so on, have been proposed and put into practice,
but the preparation process is generally intricate and the
efficiency is still too low to separate electron-hole pairs
completely during photocatalysis.7-11 Recently, a new approach,
utilizing materials that contain internal electric field, is
proposed to promote the separation efficiency of photo-
induced charge carriers.7,8,12-17 It is well known that
ferroelectrics are typical materials that possess intrinsic built-
in electric field caused by spontaneous polarization.8
Considering the simple preparation method and inherent
internal electric field property of ferroelectrics, their
photocatalytic properties have been studied experimentally as
promising photocatalysts.7,18-24
Although the built-in electric field can separate photo-
induced carriers, a static electric field can be easily saturated
by free carries, and the enhancement of the photocatalysis,
thus, is halted.7 Therefore, in order to keep the photo-induced
carriers separation incessantly during photocatalysis process,
the extraction of photo-induced carriers, that is, making them
transfer to the active sites at semiconductor surface to initiate
the PCO processes, is equally important. Noble metals
(particularly Pt) can be the electron trapping centres through
the formation of a Schottky junction with the semiconductor
photocatalyst, thus make these electrons to involve in catalysis
more effectively.5,25-27 Therefore, in order to promote the
photo-induced carriers extraction efficiency, loading noble
conditions, the PCO can degrade
a broad range of
contaminants into innocuous products such as CO2 and H2O
without significant energy input.2,3 Despite the promising
future, this approach has faced a grand challenge in terms of
reaction activity.4
In general, PCO process involves three key steps, that is,
photo-induced carriers generation, transfer and consumption.5
However, most photo-induced electron-hole pairs tend to
recombine with each other before they diffuse to the active
sites at semiconductor surface to initiate the PCO process,
which causes the low PCO activity.4-6 Therefore, it is important
to develop strategies to separate electron-hole pairs and make
a.CAS Key Laboratory of Design and Assembly of Functional Nanostructures &
Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on
the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China.*E-
mail: zhiguo@fjirsm.ac.cn
b.Institute of Nanoscience and Nanotechnology, College of Physical Science and
Technology, Central China Normal University, Wuhan 430079, China.
c. University of Chinese Academy of Sciences, Beijing 100049, China
†Electronic Supplementary Information (ESI) available: Experimental details of EPR
test, TON calculation as well as additional FE-SEM, TEM, photocatalytic cyclic test,
UV-Vis, PL, and EPR data. See DOI: 10.1039/x0xx00000x
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metals is an efficient approach. However, most conventional ethanol to form a stable suspension (the volumeVireawtAiorticolefOHnl2inOe
DOI: 10.1039/C7TA04611A
support and metal nanoparticles (NPs) are structurally non- to ethanol equals 7:3). Then the suspension was stirred in dark
uniform and have weak interaction between them, which is for more than 6 h. Subsequently, the suspension was
not good for photo-induced carriers extraction.28 Inducing irradiated under simulated solar light for 3 h. After that, the
strong metal−support interactions (SMSI) state in the suspension was centrifugated, washed in ethanol and water
supported catalyst system will greatly facilitate the electron for several times. Finally, the product was dried in an oven at
transfer between metal NPs and the oxide support.25,29
60 °C. To confirm the practical content of loaded Pt, we did a
As a typical ferroelectric material, tungsten bronze-type series of ICP-OES analysis of (T-KSNO)/Pt samples. The actual
KSr2Nb5O15 (KSNO) has been widely investigated as loaded Pt contents of 0.25, 0.5, 0.75, and 1.0 wt % Pt/(T-KSNO)
ferroelectric and piezoelectric materials.30 What’s more, KSNO samples are 0.222, 0.506, 0.753, and 1.09 wt %, respectively,
is a unipolar ferroelectric with spontaneous polarization along which are largely consistent with the nominal contents.
the c axis,30 thus having built-in electric field along [001] 2.3 Physical characterization
direction and is a potential candidate for photocatalytic
The structure and crystallinity of the samples were
aplication.22 Herein, a molten salt synthesis method was used
investigated by X-ray diffraction (Rigaku Miniflex II) using Cu Kα
for preparing tungsten bronze-type KSNO with surface oxygen
vacancies, and Pt NPs were uniformly loaded on KSNO through
in situ photodeposition method. Strong metal−support
(λ = 0.514178 nm) radiation (30 kV, 15 mA). A scan rate of 5°
min-1 was applied to record the powder XRD patterns in the 2θ
range of 20°–70°. The particle size and morphology of the
interactions were found in the KSNO/Pt catalysts which
samples were characterized on a SU-8010 field emission
facilitates the photo-induced carriers extraction efficiency and
scanning electron microscope (FESEM) using the secondary
an outstanding photocatalytic activity for ethylene oxidation
electron imaging mode, and the accelerating voltage was 1 kV.
was achieved.
Images of transmission electron microscopy (TEM), high-
resolution transmission electron microscopy (HRTEM) were
obtained using a JEM 2010 EX instrument at an accelerating
voltage of 200 kV. The X-ray photoelectron spectroscopy (XPS)
2 Experimental sections
2.1 Preparation of KSr2Nb5O15 powder photocatalysts
measurements were performed on a Phi Quantum 2000
spectrophotometer with Al Kα radiation (1,486.6 eV). The
binding energies were calibrated using that of C1s (284.8 eV).
Photoluminescence spectra (PL) of the photocatalysts were
collected on a Varian Cary Eclipse spectrometer with an
excitation wavelength of 325 nm. Electron Paramagnetic
Resonance (EPR) spectra were collected on an ELEXSYS E500
spectrometer equipped with a nitrogen cryostat and the
scanning temperature was around 77 K. The diffuse
reflectance UV-visible spectra (UV-Vis) of the samples were
recorded at room temperature using a PerkinElmer Lambda
950 UV/VIS/NIR spectrometer that was equipped with an
integrating sphere covered with BaSO4 as the reference. The
practical Pt content in the KSNO/Pt photocatalysts was
analyzed by inductively coupled plasma optical emission
spectrometry (ICP-OES) technique, which was carried out on
Ultima2 plasma emission spectrometer from Jobin Yvon.
2.4 Photocatalytic oxidation of C2H4
The KSr2Nb5O15 (KSNO) powder crystals were fabricated by a
molten salt synthesis (MSS) method. All reagents were
analytically pure, commercially available and used without
further purification. Strontium carbonate (SrCO3), niobium (V)
oxide (Nb2O5, 4 N), and potassium chloride (KCl) were used as
the starting materials. In a typical preparation of tungsten
bronze-type KSNO (T-KSNO): 1.8075 g Nb2O5 and 0.7442 g
SrCO3 were mixed uniformly and milled using ethanol for 2 h
to obtain mixtures A. Then, 7.6551 g KCl and mixtures A were
milled using ethanol for 30 min to obtain mixtures B. Then, the
mixtures B were mixed by ball milling with zirconia balls as
grinding media in ethanol for 10 h. The well-mixed powders
were placed into an Al2O3 crucible, which was covered with a
flat Al2O3 lid to prevent KCl evaporation, and then the mixtures
B were calcined at high temperature for various time (e.g.
1200 °C for 6 h). After heat treatment, the products were
washed several times in hot deionized water in order to
remove any residual salts and then dried in air at 80 °C. Table 1
summarized the samples synthesized with different reaction
time and temperatures.
The photocatalytic oxidation of C2H4 was carried out in a
homemade fixed-bed pyrex reactor of 450 ml capacity (see Fig.
S1) and a flow-bed quartz reactor (28 mm×18 mm × 1 mm, see
Fig. S2), respectively. All experiments were conducted at
atmospheric pressure and room temperature. In a typical
fixed-bed reaction: 0.2 g of the samples was dispersed
uniformly on the bottom of a glass culture dish, which was
placed in the bottom of the reactor. 90 μL of C2H4 was injected
into the reactor by a micro syringe. Prior to photoirradiation,
2.2 Preparation of KSNO/Pt composites
The KSNO/Pt photocatalysts were prepared by the in-situ
photoreduction method, according to the following procedure:
first, the as-synthesized KSNO powder (0.3 g) and
chloroplatinic acid hexahydrate (H2PtCl6·6H2O) (0.1545 mol/L,
24.9, 50.0, 75.2 and 100.5 μL, respectively) were dispersed in
Table 1 A summary of KSNO samples prepared at different calcination temperatures or time
T-KSNO
1200
6
KSNO-1000
KSNO-1100
KSNO-1300
KSNO-1h
1200
1
KSNO-3h
1200
3
KSNO-12h
1200
Sample name
Temperature/°C
Time/h
1000
6
1100
6
1300
6
12
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the reaction system was placed in the dark for 2 h to attain the
adsorption/desorption equilibrium between C2H4 gas and the
surface of the catalyst in the reactor. At certain irradiation
time intervals, 4 mL of gas was sampled from the reactor and
analyzed by a gas chromatograph (GC 9720 Fuli) equipped with
a HP-plot/U capillary column, a molecular sieve 13X column, a
flame ionization detector (FID) and a thermal conductivity
detector (TCD).
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DOI: 10.1039/C7TA04611A
A typical flow-bed reaction proceeded was carried out as
follows: the sample was placed in a quartz reactor, and then
the mixed gas consisted of 78.9% N2, 21.1% O2 and small
quantity of C2H4 (approximately 200 ppm) was flowed through
the samples and analyzed directly by the gas chromatography
(GC9720 Fuli). Before illumination, the flowing carrier gas was
used to expel CO2 and other species that adsorbed on the
surface of the catalysts. During the reaction, a 300 W Xe lamp
was used to provide simulated solar light.
3 Results and discussion
3.1 Structure and morphology of KSr2Nb5O15 (KSNO)
photocatalysts
A series of comparative experiments were carried out to
investigate the growth behavior of KSNO. The crystal phases
and morphologies of the as-prepared KSNO samples were
established through X-ray diffraction (XRD) and field emission
scanning electron microscope (FESEM), respectively. The
results shown in Fig. 1 unambiguously demonstrate the
changes of crystal structures along with synthetic procedures.
First, in order to determine the optimum calcination
temperature, KSNO was prepared under different calcination
temperatures from 1000 to 1300 °C. As shown in Fig. 1a, pure
phase tungsten bronze-type KSNO (T-KSNO) was formed after
calcination at 1200 °C for 6 h, which is in good agreement with
the standard JCPDS card no. 34-0108. Clearly, T-KSNO samples
present a smooth surface and rod-like morphology with the
length up to a few microns (Fig. 2c). Moreover, the rod-like T-
KSNO samples with uniform length (ca. 20 μm) can be
observed in the low-magnification FESEM images (Fig. S3). We
did not obtain pure phase KSNO above or below 1200 °C (Fig.
1a). When calcined at 1000 °C, KSNO is the dominant products.
Impurities in the particles are SrNb2O6 (Fig. 1a KSNO-1000).
Besides, the shorter rods (ca. 10 μm) with some small
nanoparticles on the surface are observed in the FESEM
images of KSNO-1000, as shown in Fig. 2a, further indicating
the impurity in the samples. Unexpectedly, at the temperature
of 1100 °C, the KSNO-1100 samples present the shortest rod-
like morphology (ca. 5 μm) but with a smooth surface (Fig. 2b).
Through XRD analysis (Fig. 1a KSNO-1100), we find that the
KSNO-1100 samples include SrNb2O6 and Sr2Nb2O7 besides
KSNO. Increasing the calcination temperature to 1300 °C, the
formation rate of Sr5Nb5O15 impurity was accelerated (Fig. 1a
KSNO-1300), and we obtained more and bigger SrNb2O6
impurities indicating the calcination temperature plays an
important role in the reaction process. Fig. 2d shows that a
number of micrometer-scale irregular bulks (SrNb2O6) and
Fig.
1
XRD patterns of the KSNO samples prepared at different reaction
temperatures (a) and time (b) referencing Table 1.
larger rod-like morphology (KSNO) are clearly observed in the
KSNO-1300 sample. Thus, calcinated at 1200 °C for 6 h
represents an optimum condition to prepare the tungsten
bronze-type KSNO.
These results combined with literature helped us analyze
the reaction mechanism for the formation of KSNO. The
possible morphology evolution process of KSNO rods at
different reaction temperatures is shown in Fig. 3.
As previously reported,31-33 SrCO3 decomposes at low
temperature (550–650 °C), which results in the formation of
SrO (reaction 1), and SrNb2O6 is the first compound to be
formed (reaction 2) by the following reactions:
SrCO3 → SrO + CO2 ↑
(1)
SrO + Nb2O5 → SrNb2O6
(2)
Further reaction takes place between SrNb2O6 and KCl at
higher temperature (approximately 800 °C). KCl behaves as
both a molten salt liquid and the K+ source for KSNO formation.
KSNO possibly forms by reaction 3:
5SrNb2O6 + 2KCl → 2KSr2Nb5O15 + SrCl2
(3)
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Fig. 3 Schematic illustration for the morphology evolution process of the KSNO
rods.
respectively. The pure phase KSNO was obtained after 6 h of
reaction, as shown in the XRD patterns (Fig. 1b). As a by-
product, SrNb2O6 was detected after 1 and 3 h of reaction (Fig
1b), further indicating that reaction 1 and 2 are the first stage.
Fig. 2e-h show the representative FESEM images of the
samples prepared with different reaction time. As presented in
the images, all the KSNO are rod-like morphology.
Representative low-magnification FESEM images of the
samples are shown in Fig. S3. It could be clearly observed that
the length of KSNO samples grow longer with the increase of
reaction time.
Similarly, the possible morphology evolution process
under different reaction time at 1200 °C is shown in Fig. 3. For
1h and 3h, the reaction 3 doesn’t react completely, so we
obtained rod-like KSNO and SrNb2O6 nanopaticles. For 6h and
12h, pure rod-like KSNO was obtained. In addition, longer
KSNO samples were achieved with the increase of reaction
Fig.
2 FESEM images of the KSNO samples prepared at different reaction
temperature and time referencing Table 1: (a) KSNO-1000, (b) KSNO-1100, (c) T-
KSNO, (d) KSNO-1300, (e) KSNO-1h, (f) KSNO-3h, (g) T-KSNO, (h) KSNO-12h.
In addition, Sr2Nb2O7 would be formed resulting from the time, further indicating that the KSNO is growing along the
superabundant SrNb2O6, if reaction 3 didn’t consume SrNb2O6 certain direction.
timely. The following reactions may take place on the basis of 3.2 Photocatalytic performance of KSNO and KSNO/Pt
reaction 1 and 2:
2SrNb2O6 + 2SrCl2 + O2 → 2Sr2Nb2O7 + 2Cl2 ↑
photocatalysts
(4)
(5)
Photocatalytic ethylene (C2H4) oxidation activities of KSNO and
KSNO with Pt decoration (KSNO/Pt) samples were investigated
under simulated solar light irradiation with both fixed-bed and
flow-bed mode (Fig. S1 and S2). The original time-courses data
of the C2H4 photooxidation over the as synthesized samples
under the fixed-bed mode are shown in Fig. 4. To evaluate the
photoreactivity quantitatively, the reaction rate constants
were deduced from the Langmuir−Hinshelwood model, and
the results are presented in Fig. S4. KSNO without the
cocatalysts decoration exhibits very low photocatalytic activity
for ethylene oxidation, and for comparison purposes, the blank
experiment without any catalysts does not possess any activity
under simulated sunlight illumination (Fig. 4a). By strong
contrast, the photocatalytic activity of KSNO is enhanced
markedly by loading Pt. As shown in Fig. 4b, 0.75 wt % Pt
loading exhibits an extraordinarily high photocatalytic
performance, with the reaction rate constant of 1.21 min-1,
which is the highest in comparison with our previous studies.34-
SrNb2O6 + SrO → Sr2Nb2O7
At 1000 °C, reaction 4 and 5 are difficult to take place under
such condition. The rate of reaction 3 is not fast enough to
completely consume the SrNb2O6. Therefore, the products of
KSNO-1000 samples are rod-like KSNO and SrNb2O6
nanopaticles (Fig. 2a). At 1100 °C, the rate of reaction 1 and 2
become faster, this accelerates the reaction 4 and 5 and
facilitates generating bigger SrNb2O6. Consequently, we
obtained rod-like KSNO, SrNb2O6 and Sr2Nb2O7 (Fig. 2b). At
1200 °C, the rate of reaction 1 and 2 reaches equilibrium with
reaction 3, as a result, pure phase KSNO was obtained. When
further increasing temperature to 1300 °C, the rate of reaction
3 would be faster, then bigger KSNO was obtained (Fig. 2d).
Nevertheless, the loss of KCl is accelerated at higher
temperature. Meanwhile, the reaction 3 is limited with the
loss of KCl. Then the reaction balance is broken. As a result,
more micrometre-scale irregular bulks (SrNb2O6) and larger
rod-like morphology (KSNO) were obtained.
36
This could be largely attributed to the fact that intimately
Meanwhile, to investigate the process of KSNO crystal
growth, we fixed the calcination temperature at 1200 °C and
changed the reaction time to 1 h, 3 h, 6 h and 12 h,
attached Pt metal as a co-catalyst, not only can efficiently trap
the photogenerated electrons, but also provides effective
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the (T-KSNO)/Pt photocatalysts exhibit the best performance.
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Because the SrNb2O6 nanaoparticles on DtOheI: 1s0u.1r0f3a9c/eC7oTfA0K4S6N11OA
may not only become the recombination centers of
photocarriers, but also decrease photocatalytic active sites
provided by Pt.
To examine the mineralization ratio of ethylene oxidation,
we further conducted the ethylene oxidation reaction over the
(T-KSNO)/Pt sample with a flow mode (Figure S2). As shown in
Fig. 5, when the light was turned on, the concentration of C2H4
decreased rapidly to zero. At the same time, the concentration
of CO2 promptly increased to approximately 400 ppm. When
the light was turned off, the amount of CO2 rapidly decreased
to zero, and the amount of C2H4 returned to the constant
starting value (ca. 200 ppm). The generation of CO2 was
basically in line with the degradation of C2H4 and no other
carbon-containing species were detected. The mineralization
ratio of ethylene is determined to be ca. 100% in this reaction.
To further investigate the stability of (T-KSNO)/Pt, the test of
cycle reactions was examined. As shown in Fig. S7, the
photocatalytic activity is highly stable after ten cyclic C2H4
photo-oxidation reactions. The turnover number for ethylene
photo-oxidation is determined to be 2.216 (see ‘Turnover
number calculations’ in the ESI†), indicating that the reaction is
truly driven by a catalytic process.
Fig. 4 Time courses of C2H4 photo-oxidation: upon synthesized T-KSNO samples
and without catalyst (blank test) (a); upon T-KSNO samples with different Pt
loading (b); upon 0.75 wt% Pt loaded KSNO samples (c, d) that sintered at the
conditions referencing Table 1.
3.3 Structural analysis of (T-KSNO)/Pt and photocatalytic
mechanism
To confirm the distribution of platinum nanoparticles (Pt NPs)
on the surface of T-KSNO, as-prepared 0.75 wt % Pt/(T-KSNO)
was characterized by FESEM, transmission electron microscopy
(TEM) and high resolution TEM (HRTEM). Fig. S8 shows the
FESEM images of (T-KSNO)/Pt samples, indicating that the Pt
NPs are highly dispersed on the surface of T-KSNO, which is
consistent with the low-magnification TEM images, shown in
Fig. S9. We randomly chose two microrods of (T-KSNO)/Pt
samples, and the TEM images were shown in Fig. 6 and Fig. S9.
HRTEM images in Fig. 6a and b are taken from the red circle
area of Fig. S9b and d, respectively. HRTEM images further
verify that the Pt NPs (ca. 2-4nm) are intimately loaded on the
surface of T-KSNO. Notably, Pt NPs (Fig. 6a) or even clusters
(Fig. 6b) are observed on (T-KSNO)/Pt samples. The
characteristic lattice spacing of 0.396 nm corresponds to (001)
facet of KSNO and 0.223-0.225 nm corresponds to (111) plane
of cubic Pt, respectively.31,41-43 The results also indicate that
the KSNO particles are growing along the [001] direction,
which is consistent with the previous reports.31
Fig. 5 Photo-oxidation of C2H4 and CO2 generation on the 0.75 wt % Pt/(T-KNSO) in
a flow mode.
active sites, so that it can repel the photo-excited electrons
toward the bulk region, creating an electron depletion layer at
the surface.22,37,38 As seen from the photoluminescence (PL)
spectra shown in Fig. S5, the fluorescence intensity decreased
after loading Pt on the T-KSNO samples, which may be caused
by a lower recombination rate of photogenerated electron–
hole pairs in (T-KSNO)/Pt photocatalyst. The PL emission bands
at around 400 and 545 nm are observed, which can be
assigned to the emission of the band gap transition and charge
transfer transition of an oxygen vacancy trapped electron,
respectively.39 Furthermore, the Pt loading significantly
increases the visible light absorption of T-KSNO, as shown in
Fig. S6. However, the activity is hardly increased further when
the content of Pt is above 0.75 wt % (Fig. 4b), because excess
Pt metal may easily encounter delocalized holes at KSNO
semiconductors, resulting in inefficient separation of
photogenerated electrons and holes.40 The photocatalytic
activity of different KSNO samples loaded the same content Pt
(0.75 wt %) were also investigated (Fig. 4c and d). As expected,
To further examine the surface bonding state and the
composition of the as-prepared (T-KSNO)/Pt samples, X-ray
photoelectron spectroscopy (XPS) was performed (Fig. S10).
Fig. 7a and b show the XPS Pt 4f spectra of the (T-KSNO)/Pt
photocatalysts before and after the catalytic testing. The
spectra were deconvoluted using a standard peak-fitting
technique with
a few Gaussian–Lorentzian curves. The
theoretical intensity ratio was 1.33, and the peak separation of
44
3.3 eV was used for the spin-orbit doublet Pt4f7/2–Pt4f5/2
.
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Fig. 7 XPS spectra of the (T-KSNO)/Pt (0.75 wt %) sample for before (a, c, e)and
after (b, d, f) the photocatalytic reaction.
Fig. 6 HRTEM images of (T-KSNO)/Pt (0.75 wt %): (a) and (b) are taken from the
red circle area of Fig. S9b and d, respectively.
Before performing the catalytic test, the spectra of fresh
(T-KSNO)/Pt photocatalyst can be described by three Pt 4f
doublets with the Pt 4f7/2 binding energies near 70.0, 71.6 and
74.22 eV (Fig. 7a), further indicating the successful deposition
of trace Pt. It is important to note that, the first Pt 4f doublet,
with the binding energy near 70.0 eV (Fig. 7a), is shifted to
lower binding energy as compared to the most common value
measured for bulk metallic Pt.44-47 Such negative shifts can be
explained by the electron transfer from T-KSNO to Pt. It means
that the state of platinum may be described as a very electron-
rich state Ptδ−.44,47,48 This is attributed to the specific strong
interaction between Pt and T-KSNO, which is known as “strong
metal support interaction”.27,47 The second doublet located at
the Pt 4f7/2 binding energies of 71.6 eV, results most probably
from platinum in its oxidized state like Pt1+, which is in
agreement with the literatures.44,48 The third weak doublet,
with the Pt4f7/2 binding energy near 74.2 eV corresponds to
the Pt4+ state.49
Fig. 8 EPR spectra collected upon the (T-KSNO)/Pt (0.75 wt %) sample at 77 K
under various conditions: dark and air (a); under irradiation and air (b); under
irradiation and inject C2H4 (c).
formation of strong metal support interaction state in (T-
KSNO)/Pt photocatalyst. We suppose both the built-in electric
field in the T-KSNO and the excellent contact between T-KSNO
and Pt account for the formation of strong metal support
interaction herein.
Beyond that, the unaltered Nb 3d spectra (before and
after reaction) were shown in Fig. 7c and d. The theoretical
intensity ratio was 1.5, and the peak separation of 2.7 eV was
used for the spin-orbit doublet Nb3d5/2–Nb3d3/2. The doublet
located at 206.2 eV (Fig. 7c and d) is attributed to the Nb3d5/2
peak of Nb5+ in NbO6 octahedra.50,51 The lower binding energy
peak located at 205.4 eV is the Nb3d5/2 peak of Nb4+ in highly
distortional NbO6 octahedral, which may indicate the
formation of oxygen vacancies.52-54 This would be further
confirmed by the result of XPS O1s spectra and electron
paramagnetic resonance (EPR). As shown in O1s spectra (Fig.
A reasonable change of Pt 4f spectra can be observed in (T-
KSNO)/Pt photocatalyst after its catalytic testing (Fig. 7b). The
location of all peaks remains invariant, however, it should be
noted that the peak intensity of Ptδ− has been markedly
increased. This suggests the electron transfer from T-KSNO to
Pt during photocatalytic process and further proved the
6 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
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generated electrons to transfer toward Pt nanoparticles
View Article Online
DOI: 10.1039/C7TA04611A
instead of along the c-axis.
Conclusions
In summary, we have successfully synthesized tungsten
bronze-type KSNO with surface oxygen vacancies via molten
salt synthesis method. The growth behaviour of KSNO was
investigated by varying the calcination temperature and
synthesis duration, and then a possible morphology evolution
process was proposed. More importantly, we reported a novel
(T-KSNO)/Pt photocatalyst that synthesized by in situ
photodeposition method processes strong metal−support
interactions which significantly facilitates the photo-induced
Fig.
9 Schematic illustration of charge separation and extraction in the (T-
carriers extraction efficiency.
A
remarkably enhanced
KSNO)/Pt sample and associated photo-oxidation of C2H4. Psp is the polarization
and Esp is the internal electric field.
photocatalytic activity for ethylene oxidation over (T-KSNO)/Pt
has been demonstrated, which shows excellent stability as well.
Further in-depth studies of this material with different
potential applications in the field of photocatalysis are being
under way.
7e and f), the two peaks located at binding energies of 529.3
and 530.4 eV can be observed, which could be attributed to
lattice oxygen in the normal NbO6 octahedral, and highly
distortional NbO6 octahedral (Nb4+-O) leaded by oxygen
vacancy or adsorbed oxygen, respectively.55-57
Acknowledgements
In order to further confirm our hypothesis about electron
transfer pathway within this (T-KSNO)/Pt catalytic reaction
system, we performed electron paramagnetic resonance (EPR)
studies. The detailed experimental method and the schematic
illustration of EPR test process is shown in Fig. S11. EPR is a
powerful tool for investigating paramagnetic species with, at
least, one unpaired electron.58 Platinum with unpaired
electronic state possesses paramagnetism, which has been
detected through EPR in previous literature.59,60 Fig. 8 shows
the low temperature EPR spectra (at ca. 77K) collected on the
(T-KSNO)/Pt sample under various conditions. Under the dark
and air atmosphere, the fresh sample shows the signal that
corresponds to electrons delocalized at highly distorted
surface NbO6 octahedral sites, for the presence of Nb4+.61-63 It
can be clearly seen that, simulated solar light irradiation of this
sample creates distinctive signals (Fig. 8b), assigned to
paramagnetic species Pt (Ptδ-, g=2.082), which is consistent
with the previous reports.59,60 This result further indicates the
electrons transferred from T-KSNO to Pt, as a result, the
paramagnetic species Pt is formed. Understandably, after C2H4
was injected into the reactor, the signal of Ptδ- (g=2.082)
almost disappeared, as shown in Fig. 8c, indicating that the
PCO process consumed the transferred electrons. In contrast,
the EPR spectra of T-KSNO sample show almost no change
under the same experimental method (Fig. S12).
This work was financially supported by the National Key
Project on Basic Research (Grant No. 2013CB933203), the
Strategic Priority Research Program of the Chinese Academy of
Sciences (Grant No. XDB20000000), the Natural Science
Foundation of China (Grant No. 21373224, 21577143,
51502289 and 21607153), the Natural Science Foundation of
Fujian Province (2015J05044 and 2017J05031) and the Key
Project of Frontier Science of the Chinese Academy of Sciences
(QYZDB-SSW-JSC027).
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DOI: 10.1039/C7TA04611A
Platinum nanoparticles supported on defective tungsten bronze-type KSr2Nb5O15 was shown as a
novel photocatalyst for efficient oxidation of ethylene.