A New Defect Pyrochlore Oxide Sn1.06Nb2O5.59F0.97: Synthesis, Noble Metal Hybrids, and Photocatalytic Applications

Article abstract of DOI:10.1021/acs.inorgchem.8b00818

Noble metal nanoparticles have attracted considerable attention due to their useful capabilities as heterogeneous catalysts. However, they are usually prepared using various organic stabilizing agents that negatively affect their catalytic activities. Herein, we report a facile, clean, and effective method for synthesizing supported ultrafine noble metal nanoparticles by utilizing the reductive property of a new pyrochlore oxide: Sn_1.06Nb₂O_5.59F_0.97 (SnNbOF). Ultrafine Au, Pd, and Pt nanoparticles or clusters are homogeneously distributed on the SnNbOF surface. In addition, the atomic cavities and ion-exchange properties of pyrochlore-type SnNbOF can facilitate the synthesis of atomic Ag dispersed within the framework of SnNbOF. Noble metal-SnNbOF hybrids can be obtained in one step at room temperature, and no foreign reducing agents or stabilizing organics are required for the synthesis. We also show that the fabricated hybrids exhibit promising photocatalytic properties for ethylene oxidation and CO₂ reduction.

Full text of DOI:10.1021/acs.inorgchem.8b00818

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
A New Defect Pyrochlore Oxide Sn1.06Nb2O5.59F0.97: Synthesis, Noble
Metal Hybrids, and Photocatalytic Applications
Xiaoyang Pan,† Chao Li,‡ Jing Zheng,† Shijing Liang,§ Rong Huang,*,‡ and Zhiguo Yi*,†,∥
†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
‡Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai 200062, China
§Department of Environmental Science and Engineering, Fuzhou University, Fuzhou 350108, China
∥University of Chinese Academy of Sciences, Beijing 100049, China
S
* Supporting Information
ABSTRACT: Noble metal nanoparticles have attracted considerable attention
due to their useful capabilities as heterogeneous catalysts. However, they are
usually prepared using various organic stabilizing agents that negatively affect
their catalytic activities. Herein, we report a facile, clean, and effective method
for synthesizing supported ultrafine noble metal nanoparticles by utilizing the
reductive property of a new pyrochlore oxide: Sn1.06Nb2O5.59F0.97 (SnNbOF).
Ultrafine Au, Pd, and Pt nanoparticles or clusters are homogeneously
distributed on the SnNbOF surface. In addition, the atomic cavities and ion-
exchange properties of pyrochlore-type SnNbOF can facilitate the synthesis of
atomic Ag dispersed within the framework of SnNbOF. Noble metal−
SnNbOF hybrids can be obtained in one step at room temperature, and no
foreign reducing agents or stabilizing organics are required for the synthesis.
We also show that the fabricated hybrids exhibit promising photocatalytic
properties for ethylene oxidation and CO2 reduction.
presynthesized metal nanoparticles are stabilized by organic
agents and mixed with oxide supports, allowing precise control
over the size and morphology of the noble metal nano-
particles.16 However, the removal of the organic stabilizing
agent by thermal or oxidative treatments usually results in
significant changes in the sizes and shapes of the nano-
particles.23 Therefore, it remains an important challenge to
develop a simple and effective method to synthesize small and
homogeneously distributed oxide-supported noble metals.
Herein, a new defect pyrochlore oxide, Sn1.06Nb2O5.59F0.97
(SnNbOF), is synthesized and used as a reactive support for in
situ homogeneous deposition of ultrafine noble metal nano-
particles without organic reagents or additional reducing agents.
The redox reaction between the reductive Sn(II) ions in
SnNbOF and noble metal ions facilitates the in situ growth of
the noble metal (Au, Pd, Pt, or Ag) on SnNbOF at room
temperature. In the proposed technique, by utilizing the unique
properties of SnNbOF, ultrafine Au, Pd, and Pt nanoparticles or
atomically dispersed Ag can be readily prepared. The promising
photocatalytic properties of the fabricated catalysts for various
reactions are also demonstrated.
INTRODUCTION
■
Nanoparticles of noble metals such as Au, Pd, Pt, and Ag are of
great interest in the field of catalysis for diverse applica-
tions.1−12 The ability to control the size, shape, and distribution
of these nanoparticles is a primary goal of catalyst design.1,2,4,5
However, noble metal nanoparticles used in isolation suffer
from two serious problems in catalytic reactions. First, the
organic stabilizing agents on the metal surface may block the
active sites of the noble metal and attenuate its catalytic
activity.13,14 Second, the organic agents are not stable during
the catalytic process, resulting in aggregation or sintering of the
noble metal during catalytic reactions.15
To overcome these disadvantages, an effective method is to
anchor the noble metals to specific supports with less or no
organic agents. Of all the available support materials, metal
oxides are the most widely used.16 To prepare oxide-supported
noble metals, various strategies have been developed, including
coprecipitation and impregnation,16 photodeposition,17 chem-
ical vapor deposition (CVD),18 and colloidal methods.16 The
conventional coprecipitation,19 impregnation,20 and photo-
deposition21,22 methods are relatively simple but usually do
not provide precise control over the size and distribution of the
noble metal nanoparticles. The CVD method can be used to
produce ultrafine noble metal particles with good dispersion,
but this method requires expensive organic metal precursors
and complex synthetic procedures.18 In the colloidal method,
Received: March 26, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00818
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
RESULTS AND DISCUSSION
■
Synthesis and Characterization of SnNbOF. A new
pyrochlore-type oxide Sn1.06Nb2O5.59F0.97 (SnNbOF) was first
synthesized by a hydrothermal method. Figure S1 shows the X-
ray diffraction (XRD) patterns of the SnNbOF samples
obtained at various pH values. The XRD profiles varied with
the pH of the solution. The samples prepared at pH 8−10
exhibited sharp XRD peaks that can be attributed to the cubic
cell of the pyrochlore-type structure (space group Fd3m).
Furthermore, a pH of 8 resulted in the SnNbOF sample with
the best crystallinity. Therefore, this sample was chosen for
further investigation.
The composition of the sample of the pyrochlore-type
compound obtained at pH 8 and 353 K for 24 h was
investigated. The total amounts of Sn and Nb were determined
by inductively coupled plasma optical emission spectrometry
(ICP-OES) analysis. The amount of fluorine in the SnNbOF
was determined by measuring the concentration of residual F−
ions in the reaction solution following the SnNbOF synthesis
by ion chromatography. A negligible amount of nitrogen was
detected by elemental analysis. The molar ratio of Sn/Nb/F in
the sample was found to be 1.06:2:0.97. In addition, the valence
states of Sn, Nb, O, and F elements in SnNbOF are determined
to be +2, +5, −2, and −1, respectively (Figure S2a−d).
Therefore, the chemical formula of this material can be
expressed as Sn1.06Nb2O5.59F0.97, which is known as a defect
pyrochlore-type compound. The A, O, and O′ sites in
Sn1.06Nb2O5.59F0.97 are partially vacant, unlike those in typical
pyrochlore oxides, which have a general formula of A2B2O7 or
A2B2O6O′.24 The vacant sites in the defect pyrochlore endow it
with interesting ion-exchange properties because of the
mobility of the An+ ions.25
Figure 1. Physical characterization of the Sn1.06Nb2O5.59F0.97
(SnNbOF). (a) Observed and calculated powder XRD profiles. (b)
Crystal structure. (c) SEM image. (d) TEM image. (e) HR-TEM
image.
The results of the Rietveld refinement of Sn1.06Nb2O5.59F0.97
(SnNbOF) are shown in Figure 1a. All the observed peaks for
the sample can be attributed to a face-centered cubic crystal
(Tables S1). The crystal structure of the sample is a three-
dimensional framework with corner-shared NbO6 octahedra in
which the interstitial sites are filled with Sn2+ and F−/O2− ions
(Figure 1b and Table S2), consistent with the conclusion based
on the XRD patterns of the pyrochlore compounds such as
Pb1.2Ti0.4Ta1.6O6 (JCPDS No. 76-0177) and Sr0.4H1.2Nb2O6
(JCPDS No. 77-1165) shown in Figure S3.26
was first dispersed in water, and then metal salts were added to
this solution dropwise at room temperature. Once in contact
with SnNbOF, the noble metal salts were immediately reduced
by the Sn2+ ions (Figure S7) and subsequently nucleated on the
surface of SnNbOF, forming clusters and, eventually, nano-
particles.
Figure 2a shows the XRD patterns of SnNbOF and of the
SnNbOF composites loaded with 1 wt % noble metal (1 wt %
To understand the stability of the reductive Sn2+ ions of
SnNbOF in air atmosphere or under simulated sunlight
irradiation, XPS analysis is performed. The results show that
the Sn2+ ions are stable under these conditions (Figure S4a). In
addition, the crystal structure of SnNbOF is also not changed
(Figure S4b).
The scanning electron microscopy (SEM) images in Figure
S5 and Figure 1c show that SnNbOF is octahedral in shape
with the octahedron size ranging from 60 to 100 nm. The
transmission electron microscopy (TEM) image in Figure 1d
further confirms the presence of an octahedral morphology in
SnNbOF. The high-resolution transmission electron micros-
copy (HR-TEM) image shows a distinct lattice spacing of 0.602
nm for the (111) facet of the pyrochlore compound (Figure
1e).27 The Brunauer−Emmett−Teller (BET) surface area of
SnNbOF is determined to be 31.6 m2/g (Figure S6), while the
average pore size of SnNbOF is 13.6 nm (inset of Figure S6).
In Situ Growth of Noble Metals on SnNbOF. The
procedure used to synthesize noble metal−SnNbOF compo-
sites is shown in Scheme S1. Briefly, the as-obtained SnNbOF
Figure 2. XRD patterns (a) and UV−vis diffuse reflectance spectra (b)
of the 1 wt % noble metal loaded SnNbOF (1 wt % M-SnNbOF).
M-SnNbOF). The results show that the 1 wt % M-SnNbOF
samples with different loaded metals have XRD patterns that
are similar to those of SnNbOF. Notably, however, the XRD
peaks decreased in intensity after the noble metals were
deposited on the surface when compared to the XRD pattern of
as-prepared SnNbOF. This may be attributed to the oxidation
of the Sn(II) ions in SnNbOF. In addition, the characteristic
XRD peaks corresponding to Ag, Au, Pd, and Pt were not
B
DOI: 10.1021/acs.inorgchem.8b00818
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
observed in the 1 wt % M-SnNbOF samples. This is because of
the low weight ratio of the metal particles in 1 wt % M-
SnNbOF.28
indicates the presence of strong interactions between Au and
SnNbOF in 1 wt % Au-SnNbOF.29 The coexistence of Sn(II)
and Sn(IV) in 1 wt % Au-SnNbOF suggests that some of the
Sn(II) ions in SnNbOF were oxidized by the Au ions (Figure
S10a). That is, the redox reaction between Sn(II) and Au(III)
results in the formation of Au(0) nanoparticles dispersed on
the surface of SnNbOF as well as the partial oxidation of Sn(II)
in SnNbOF. In addition, XPS peak characteristic of O 1s, Nb
3d, and F were observed in the sample (Figure S10b−d).
We also synthesized 1 wt % Pd-SnNbOF, 1 wt % Pt-
SnNbOF, and 1 wt % Ag-SnNbOF nanocomposites using the
same method. Here, 1 wt % represents the nominal weight ratio
of noble metals. The actual weight ratios of Au, Pd, and Pt of 1
wt % M-SnNbOF are determined to be 1.05, 0.95, and 0.93 wt
%, respectively (Table S3). Figure 4a,b shows representative
The ultraviolet−visible diffuse reflectance spectra of the
samples are shown in Figure 2b. The spectra indicate that
SnNbOF exhibits strong absorption in the UV and visible light
regions and that the absorption edge is located at ∼530 nm.
The bandgap of SnNbOF is estimated to be ∼2.33 eV (Figure
S8a). A Mott−Schottky analysis revealed that the conduction
band minimum is −0.19 V versus normal hydrogen electrode
(NHE; Figure S8b). Thus, the corresponding valence band
maximum was calculated to be +2.14 V versus NHE.
The decoration of different noble metals has a significant
influence on the optical properties of 1 wt % M-SnNbOF. As
shown in Figure 2b, the addition of noble metals increases the
light absorption intensity in the range of 550−800 nm.
Specifically, a distinct absorption peak was observed in 1 wt
% Au-SnNbOF around 550 nm due to the surface plasmon
resonance of the metallic gold nanoparticles.
Figure 3a shows a TEM image of 1 wt % Au-SnNbOF (here,
1 wt % represents the nominal weight ratio of Au), revealing
Figure 3. Characterization of 1 wt % Au-SnNbOF. (a) TEM image.
(b) Size distribution of Au nanoparticles. (c) HR-TEM image. (d)
XPS spectrum of Au 4f.
that the SnNbOF sample retained its octahedral shape, whereas
the Au nanoparticles were homogeneously dispersed on the
surface and had an average particle size of 3.8 nm (Figure 3b).
The elemental mapping analysis also shows that the Au
nanoparticles are uniformly distributed onto the surface of 1 wt
% Au-SnNbOF (Figure S9a−f). The HR-TEM image in Figure
3c shows distinct lattice spacings of 0.235 and 0.202 nm, which
can be attributed to the (111) facet for Au and the (511) plane
for SnNbOF, respectively.29
The X-ray photoelectron spectroscopy (XPS) analysis
(Figure 3d) reveals that most of Au in 1 wt % Au-SnNbOF
is in the metallic state, whereas only a small amount of Au(+1)
can be detected. The shift in Au 4f7/2 in 1 wt % Au-SnNbOF
(83.5 eV) when compared with that in pure Au foil (84.0 eV)
Figure 4. TEM images and elemental mapping analyses of the 1 wt %
M-SnNbOF. (a−i) 1 wt % Pd-SnNbOF. (j−r) 1 wt % Pt-SnNbOF.
(s−x) 1 wt % Ag-SnNbOF. (y) Schematic illustration of the formation
of 1 wt % M-SnNbOF.
TEM and HR-TEM images of 1 wt % Pd-SnNbOF. These
images clearly show a uniform Pd distribution through the
SnNbOF support. The elemental mapping analysis (Figure 4c−
i) also confirmed the homogeneous dispersion of Pd on the
support. The XPS spectrum of 1 wt % Pd-SnNbOF shows
peaks corresponding to Pd with a Pd 3d5/2 apparent binding
energy of ∼335.8 eV, which is higher than that of bulk Pd (335
eV), as shown in Figure S11a.28 This phenomenon can be
C
DOI: 10.1021/acs.inorgchem.8b00818
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
attributed to the relatively small size of the metal particles,
which has been reported in previous studies.30 TEM and
elemental mapping analyses of 1 wt % Pt-SnNbOF show that
small nanoparticles were homogeneously deposited on the
surface of SnNbOF (Figure 4j−r). In addition, the Pt 4f7/2
binding energy (71.95 eV) was higher in 1 wt % Pt-SnNbOF
than in bulk Pt (71.2 eV), further supporting the small metal
particle size (Figure S11b).31
compared to the well-known narrow-band gap semiconductor
photocatalysts C3N4, WO3, and Ag3PO4 (Figure S14a). The
results show that the SnNbOF exhibits obviously enhanced
photoactivity than that of C3N4 and WO3 under simulated
sunlight irradiation (Figure S14b), while the photoactivity of
SnNbOF is lower than that of Ag3PO4. After noble metal
decoration, the photoactivity of SnNbOF is significantly
enhanced (Figure 5a). Furthermore, the photoactivities of
Compared to the 1 wt % Au−SnNbOF, no obvious noble
metal nanoparticles with distinct lattice spacing could be found
on the surface of 1 wt % Pd and Pt-SnNbOF nanocomposites,
while the elemental mapping analysis indicates the uniform
distribution of Pd and Pt elements. This result suggests that the
Pd and Pt elements may be doping into the lattice of SnNbOF.
To examine this, Ar+ sputtering is used to remove the surface
layer of 1 wt % M-SnNbOF (M = Pd or Pt), and the samples
were analyzed by XPS analysis. The results show that the Pd
and Pt elements are mainly distributed on the surface of 1 wt %
M-SnNbOF (Figure S12a,b). In addition, we also found that
the smooth surface of SnNbOF became rough after Pd and Pt
decoration. These results together suggest that Pd and Pt are in
the form of nanoclusters or atomically doped into the surface
layer of SnNbOF.
Unlike the SnNbOF hybrids with 1 wt % Au, Pd, and Pt, 1 wt
% Ag-SnNbOF was found to have a smooth surface without
obvious Ag nanoparticles or clusters (Figure 4s,t). However,
the elemental mapping analysis showed that Ag was
homogeneously dispersed within the SnNbOF support (Figure
4u,v,w,x). Furthermore, the XPS analysis indicated that only a
small amount (∼0.3 wt %) of Ag(0) was present on the surface
layer of SnNbOF (Figure S13a). The ICP-OES analysis showed
that the actual weight ratio of Ag in 1 wt % Ag-SnNbOF is 1.12
wt % (Table S3). To further investigate the distribution of Ag
in the 1 wt % Ag-SnNbOF sample, Ar+ sputtering was used to
remove the surface layer of the sample, and the sample was
then characterized by XPS analysis. The results showed that
Ag(0) was also presented in the lattice of SnNbOF (Figure
S13b).
Figure 5. Photocatalytic oxidation of C2H4. (a) Photo-oxidation of
C2H4 over SnNbOF and 1 wt % M-SnNbOF under simulated sunlight
irradiation. (b) Rate constants of photocatalytic C2H4 oxidation over
different catalysts and under different light sources. (c) Stability of 1 wt
% Pt-SnNbOF for C2H4 oxidation under simulated sunlight
irradiation. (d) Photocatalytic oxidation of C2H4 over 1 wt % Pt-
SnNbOF in a flow mode under simulated sunlight irradiation.
these samples were investigated under irradiation from different
light sources (Figure S15a−d, Figure S16a−d, and Figure
S17a,b). The results indicate that 1 wt % M-SnNbOF exhibits a
much higher photoactivity than SnNbOF under UV light,
visible light, and simulated sunlight irradiation (Figure 5b).
Specifically, 1 wt % Pt-SnNbOF exhibits the best photo-
activity of all of the samples tested under simulated sunlight
irradiation. The rate constant of 1 wt % Pt-SnNbOF (0.17
min−1) was found to be 73 times higher than that of the as-
prepared SnNbOF (0.0024 min−1) under simulated sunlight
irradiation and is stable during the recycled photocatalytic
reaction (Figure 5c). To further examine the rate of conversion
and the selectivity for ethylene oxidation, the performance of 1
wt % Pt-SnNbOF was investigated in a flow mode. As shown in
Figure 5d, before the light irradiation, the C2H4 concentration
was 200 ppm, and no CO2 was present. When the light was
turned on, the amount of C2H4 decreased rapidly to ∼5 ppm,
and the concentration of CO2 simultaneously increased rapidly
to ∼390 ppm. These results confirm that ethylene oxidation
was driven by a photocatalytic process. The mineralization ratio
of ethylene in this reaction was found to be ∼100%.
Together, these results indicate that metallic Ag was
dispersed within the framework of SnNbOF. This difference
in 1 wt % Ag-SnNbOF compared to the Au, Pd, and Pt
nanocomposite hybrids is mainly attributed to the differences in
the precursors that are used for noble metal preparation: the
Au, Pd, and Pt precursors were negatively charged MClyx−
,
whereas Ag+ ions were used as the precursor for Ag (Figure
4y). Because of the ion-exchange properties of SnNbOF, the
Ag+ ions would be exchanged with the Sn2+ ions and be
incorporated into the SnNbOF lattice. The exchanged Ag+ ions
would be simultaneously reduced by Sn2+ and therefore trapped
in the SnNbOF crystal lattice. In contrast, for 1 wt % Au, Pd,
and Pt-SnNbOF nanocomposites, noble metal elements are
mainly distributed on the surface of 1 wt % M-SnNbOF. This is
−
because the negatively charged noble metal precursors (AuCl4 ,
PdCl42−, and PtCl62−) could not exchange with the positively
charged Sn2+ ions.
Photocatalytic Properties. As a proof-of-concept for the
use of this technique for fabricating photocatalysts, we first
demonstrate the application of the SnNbOF and the 1 wt % M-
SnNbOF hybrids for photocatalytic C2H4 oxidation under
simulated sunlight irradiation, which is important for environ-
mental remediation due to the harmful effects of ethylene on
the fresh-keeping.32,33 Initially, the photoactivity of SnNbOF
(Eg = 2.33 eV) for ethylene oxidation was investigated and
To understand the effect of noble metal on the improvement
of photoactivities of 1 wt % M-SnNbOF, photoluminescence
analysis was first performed (Figure S18). The results show that
the noble metal decoration can obviously improve the charge
separation efficiency on SnNbOF and thus enhance the
photoactivity of it. We also noted that the blank noble metal
nanoparticles decorated on SiO2 (Figures S19a−c and S20)
also exhibit photoactivities for ethylene oxidation under
D
DOI: 10.1021/acs.inorgchem.8b00818
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
simulated sunlight irradiation (Figure S21). However, the
photoactivities are very weak, which may be ascribed to their
surface plasmonic resonance properties or photothermal effects.
The light irradiation would increase the temperature of the
reaction system, which could facilitate the thermcatalytic
oxidation of ethylene over noble metal (Figure S22). Note
that the enhanced light absorption properties of SnNbOF
(Figure 2b) caused by noble metal decoration also contributed
to the improved photoactivity of 1 wt % M-SnNbOF.
subsequently captured by molecular oxygen to form superoxide
radicals. These superoxide radicals oxidize C2H4 to generate
CO2 and H2O. Note that the photogenerated holes can also
participate in the C2H4 oxidation reaction. According to
previous findings, the reaction between photoexcited holes
and C2H4 could proceed through two reaction pathways:35−37
The photogenerated holes react with the water adsorbed on the
surface to form hydroxyl radicals, which are further oxidized by
C2H4 into CO2 or directly interact with C2H4 to break its C
C bond and facilitate its complete oxidation.38 In this case, the
C2H4 oxidation by photoexcited holes tends to proceed
through the latter pathway, because the photogenerated
holes, which have an anodic potential of 2.1 V versus NHE,
are unable to oxidize H2O to form hydroxyl radicals, which
have an anodic potential of 2.8 V versus NHE. Therefore, the
main reaction process of ethylene oxidation herein can be
described by the following steps:
To investigate the mechanism of the C2H4 oxidation over M-
SnNbOF, in situ electron spin resonance (ESR) analysis was
conducted. Figure 6 shows the ESR spectra obtained from the 1
SnNbOF + hv → SnNbOF(h+ + e−)
e− + O2 → O−2
(1)
·
(2)
C2H4 + 3·O−2 → 2CO2 + 2H2O + 3e−
(3)
C2H4 + 6O−O → 2CO2 + 2H2O + 6V−O
(4)
2V−O + O2 + 2e− → 2OO2−
(5)
Figure 6. Electron spin resonance spectra collected upon 1 wt % Pt-
SnNbOF sample at 100 K under various conditions. (a) Dark and air.
(b) Under irradiation and air. (c) Under irradiation and after injecting
C2H4.
It was also found that 1 wt % Pt-SnNbOF shows obvious
photoactivity for CO2 reduction, unlike SnNbOF, which
exhibits almost no activity for this reaction (Figure S23).
However, the reason for this behavior remains uncertain.
wt % Pt-SnNbOF sample under various conditions. Under dark
conditions and in an air atmosphere, the sample exhibited a
distinct signal at g = 2.005, which can be attributed to a single
electron trapped in an oxygen vacancy site (Figure 6a). Under
simulated sunlight irradiation (Figure 6b), an ESR signal that is
CONCLUSION
■
In conclusion, a novel and facile yet effective method has been
developed for the in situ growth of noble metals on a novel
pyrochlore oxide support, SnNbOF, under ambient conditions.
By utilizing the unique properties of SnNbOF, a variety of
noble metal−oxide hybrids with small metal particles were
uniformly dispersed on SnNbOF. The metal−oxide hybrids
exhibit promising properties for photocatalytic applications.
−
characteristic of O2 emerges (g1 = 2.032, g2 = 2.005, and g3 =
1.999).34 After C2H4 was injected into the reactor, the O2
−
signal almost disappeared, as shown in Figure 6c, indicating
−
O2 was consumed during the photocatalytic reaction.
On the basis of these results, a possible reaction mechanism
for the photooxidation of C2H4 is proposed. As shown in
Figure 7, under the simulated sunlight irradiation, SnNbOF is
excited, photogenerating electrons and forming holes. The
photoexcited electrons are trapped by the noble metal and
METHODS
■
Materials. Niobium(V) oxide (Nb2O5), hydrofluoric acid (HF),
ammonium hydroxide (NH4OH), tin(II) fluoride (SnF2), silicon
dioxide (SiO2), sodium dihydrogen phosphate (NaH2PO4), urea
(CH4N2O), poly(vinylpyrrolidone) (PVP), chloroauric acid tetrahy-
drate (AuCl3·HCl·4H2O), silver nitrate (AgNO3), hexachloroplatinic
(IV) acid hexahydrate (H2PtCl6·6H2O), and palladium(II) chloride
(PdCl2) were purchased from Sinopharm Chemical Regent Co., Ltd.
Deionized water was supplied from local sources. All of the materials
were used as received without further purification.
Synthesis. First, Nb2O5·nH2O was prepared as established in a
previous study.27 Then, Nb2O5·nH2O was mixed with SnF2 in a Nb5+/
Sn2+ molar ratio of 2:1. This mixture was dispersed into 70 mL of
deionized water and then transferred to a 100 mL Teflon-lined
stainless steel autoclave. The pH of the mixture was adjusted with
NH4OH while stirring vigorously. The autoclave was kept in an oven
at 473 K for 24 h. The product SnNbOF was filtered and washed with
distilled water several times and dried in an oven at 353 K overnight.
The Au, Ag, and Pt precursors were dissolved directly in deionized
water at 10 mM. Because PdCl2 is only slightly soluble in water,
hydrochloric acid was used to dissolve it such that the resulting
H2PdCl4 solution had the same 10 mM concentration as that of the
precursor solutions of Au, Ag, and Pt. In situ growth of noble metals
on the surface of SnNbOF was conducted as follows: 0.2 g of the as-
obtained SnNbOF was dispersed in 100 mL of deionized water with
Figure 7. Schematic illustration of C2H4 oxidation over noble metal
loaded SnNbOF (M-SnNbOF) under simulated sunlight irradiation.
E
DOI: 10.1021/acs.inorgchem.8b00818
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the aid of ultrasonication. Then, a given quantity of metal ions
(HAuCl4, AgNO3, H2PtCl6, or H2PdCl4) in an aqueous solution was
mixed with the SnNbOF suspension under strong stirring at room
temperature. After they were stirred for 48 h, the final products were
collected, washed with distilled water, and dried at 353 K in an oven.
C3N4, Ag3PO4, and WO3 were prepared according to our previous
report.39 Au, Pd, and Pt nanoparticles were prepared via the NaBH4
reduction method in the presence of PVP. Typically, 15 mL of noble
metal precursors (2 mM) were mixed with 21 mL of H2O and 0.0889
g of PVP, and the solution was stirred for 15 min at room temperature.
Then 2.4 mL of NaBH4 solution (10 mM) was added, and the solution
was stirred for 5 h. The resulting product is PVP-capped noble metal
solution. SiO2-supported noble metal composite was prepared by the
rotary evaporation of commercially available SiO2 with appropriated
amount of colloidal noble metal nanoparticles solution followed by a
sample drying at 373 K in an oven.
Characterization. The optical properties of the samples were
characterized by a Cary 500 UV−visible ultraviolet/visible diffuse
reflectance spectrophotometer (DRS), during which BaSO4 was
employed as the internal reflectance standard. Crystal structures of
the as-prepared samples were analyzed on a Rigaku Miniflex II X-ray
powder diffractometer using Cu Kα radiation. A field-emission
scanning electron microscope (FESEM, JSM-6700F) and transmission
electron microscope (JEM-2010, FEI, Tecnai G2 F20 FEG TEM) were
used to identify the morphology and microscopic structure of the as-
synthesized samples. X-ray photoelectron spectroscopy (XPS)
measurements were performed using a Thermo Scientific ESCA
Lab250 spectrometer consisting of monochromatic Al Kα as the X-ray
source, a hemispherical analyzer, and a sample stage with multiaxial
adjustability to obtain the composition on the surface of samples. All
the binding energies were calibrated by the C 1s peak of the surface
adventitious carbon at 284.6 eV. Inductively coupled plasma optical
emission spectrometry (ICP-OES) was performed on Ultima 2 to
determine the weight ratio of Sn and Nb. Ion chromatography analysis
(IC-2010) was performed to determine the content of residual F
element in the reaction solution for SnNbOF synthesis. Photo-
luminescence analysis was performed with a Varian Cary Eclipse
spectrometer at an excitation wavelength of 325 nm.
Photoelectrochemical Measurements. The photoelectrochem-
ical analysis was performed in a conventional three-electrode cell. Ag/
AgCl electrode was used as reference electrode, and Pt electrode acted
as the counter electrode. The work electrode was prepared according
to our previous report.40 Mott−Schottky analyses were performed in a
homemade three-electrode quartz cell with a CHI660D workstation.
The electrolyte was 0.2 M aqueous Na2SO4 solution (pH = 6.8)
without additive.
Catalytic Activities. Photocatalytic ethylene degradation was
performed in a custom-modified Pyrex reaction cell with a volume
of 450 mL under irradiation by a 300 W Xe lamp with different light
filters. First, 0.1 g of the photocatalyst was spread uniformly over the
bottom of the reactor. Then, the reactor was flushed with N2
repeatedly to remove the water, and CO2 adsorbed on the catalyst
and the interior walls of the reactor. Subsequently, 5 mL of O2 and 90
μL of C2H4 were injected into the reactor using a microsyringe; the
initial concentration of C2H4 was 200 ppm by volume. Prior to the
irradiation, the reactor was kept in the dark for 2 h to ensure that an
adsorption−desorption equilibrium between the photocatalyst and the
reactants was attained. Then, the reactor was irradiated by a 300 W Xe
lamp with various light filters. At fixed time intervals, 4 mL of the gas
was sampled from the reactor and analyzed by gas chromatography
(GC9720 Fuli) with an HP-Plot/U capillary column, a molecular sieve
13X column, a flame ionization detector, and a thermal conductivity
detector. The percentage of ethylene that had degraded at each
sampling was calculated as C/C0, where C is the concentration of
ethylene in the sample, and C0 is the initial concentration of ethylene
injected.
21.1% O2, and small quantity of C2H4 was flowed over the sample and
analyzed directly by gas chromatography (GC9720 Fuli). During the
reaction, a 300 W Xe lamp was used without filters to provide
simulated sunlight.
The photocatalytic reduction of CO2 was performed in a gas−solid
heterogeneous reaction mode under atmospheric pressure at ambient
temperature. A 40 mL Schlenk flask with a silicone rubber septum was
used as a reactor. The photocatalyst sample (20 mg) was loaded into
the reactor. The system was evacuated by a mechanical pump and
filled with pure CO2 gas; this evacuation/filling operation was repeated
three times. A bar of CO2 and 6 μL of liquid water were introduced
with a syringe via the septum. A 300 W commercial Xe lamp was used
as an irradiation source and placed vertically outside the reactor. The
temperature of the reactor was kept at 298 K by an electronic fan.
After 4 h of irradiation, 0.5 mL of the gas was taken from the reactor
with a syringe and analyzed by a GC-7890A gas chromatograph
equipped with a flame-ionized detector and a chromatographic column
(GASPRO).
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00818.
XRD, crystal data, TEM, XPS spectra, photocatalytic
activity, and photoluminescence analysis of the samples
(PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: rhuang@ee.ecnu.edu.cn. (R.H.)
*E-mail: zhiguo@fjirsm.ac.cn. (Z.Y.)
■
ORCID
Chao Li: 0000-0002-4212-3098
Shijing Liang: 0000-0001-9963-5935
Zhiguo Yi: 0000-0002-3828-1712
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Natural Science
Foundation of China (Grant Nos. 21607153, 21577143,
51702317, and 21677036), the Natural Science Foundation
of Fujian Province (Grant Nos. 2017J05031 and 2018I0021),
and the Key Research Program of Frontier Sciences of the
Chinese Academy of Sciences (Grant No. QYZDB-SSW-
JSC027).
REFERENCES
■
(1) Zeng, J.; Zhang, Q.; Chen, J.; Xia, Y. A Comparison Study of the
Catalytic Properties of Au-Based Nanocages, Nanoboxes, and
Nanoparticles. Nano Lett. 2010, 10, 30−35.
(2) Liu, K.; Wang, A.; Zhang, T. Recent Advances in Preferential
Oxidation of CO Reaction over Platinum Group Metal Catalysts. ACS
Catal. 2012, 2, 1165−1178.
(3) Guan, J.; Li, D.; Si, R.; Miao, S.; Zhang, F.; Li, C. Synthesis and
Demonstration of Subnanometric Iridium Oxide as Highly Efficient
and Robust Water Oxidation Catalyst. ACS Catal. 2017, 7, 5983−
5986.
(4) Zhang, S.; Chang, C.-R.; Huang, Z.-Q.; Li, J.; Wu, Z.; Ma, Y.;
Zhang, Z.; Wang, Y.; Qu, Y. High Catalytic Activity and Chemo-
selectivity of Sub-Nanometric Pd Clusters on Porous Nanorods of
CeO2 for Hydrogenation of Nitroarenes. J. Am. Chem. Soc. 2016, 138,
2629−2637.
The flow-mode experiments were conducted as follows: 1 g of the
metal particles was placed into a 28 × 18 × 1 mm quartz reactor, and
N2 was flowed to expel CO2 and other species that were adsorbed on
the surface of the catalyst. Then, a mixed gas comprising 78.9% N2,
F
DOI: 10.1021/acs.inorgchem.8b00818
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(5) Corma, A.; Garcia, H. Supported Gold Nanoparticles as Catalysts
for Organic Reactions. Chem. Soc. Rev. 2008, 37, 2096−2126.
(6) Hirakawa, T.; Kamat, P. V. Charge Separation and Catalytic
Activity of Ag@TiO2 Core−Shell Composite Clusters under UV−
Irradiation. J. Am. Chem. Soc. 2005, 127, 3928−3934.
(7) Fan, Z.; Zhang, H. Template Synthesis of Noble Metal
Nanocrystals with Unusual Crystal Structures and Their Catalytic
Applications. Acc. Chem. Res. 2016, 49, 2841−2850.
(8) Sekizawa, K.; Maeda, K.; Domen, K.; Koike, K.; Ishitani, O.
Artificial Z-Scheme Constructed with a Supramolecular Metal
Complex and Semiconductor for the Photocatalytic Reduction of
CO2. J. Am. Chem. Soc. 2013, 135, 4596−4599.
(9) Zhang, F.; Jiao, F.; Pan, X.; Gao, K.; Xiao, J.; Zhang, S.; Bao, X.
Tailoring the Oxidation Activity of Pt Nanoclusters Via Encapsulation.
ACS Catal. 2015, 5, 1381−1385.
(24) Jitta, R. R.; Gundeboina, R.; Veldurthi, N. K.; Guje, R.; Muga, V.
Defect Pyrochlore Oxides: As Photocatalyst Materials for Environ-
mental and Energy Applications - a Review. J. Chem. Technol.
Biotechnol. 2015, 90, 1937−1948.
(25) Uma, S.; Singh, J.; Thakral, V. Facile Room Temperature Ion-
Exchange Synthesis of Sn2+ Incorporated Pyrochlore-Type Oxides and
Their Photocatalytic Activities. Inorg. Chem. 2009, 48, 11624−11630.
(26) Xie, Y.; Yin, S.; Yamane, H.; Hashimoto, T.; Machida, H.; Sato,
T. Microwave Assisted Solvothermal Synthesis of a New Compound,
Pyrochlore-Type Sn1.24Ti1.94O3.66(OH)1.50F1.42. Chem. Mater. 2008, 20,
4931−4935.
(27) Liang, S.; Wu, L.; Bi, J.; Wang, W.; Gao, J.; Li, Z.; Fu, X. A
Novel Solution-Phase Approach to Nanocrystalline Niobates: Selective
Syntheses of Sr0.4H1.2Nb2O6.H2O Nanopolyhedrons and SrNb2O6
Nanorods Photocatalysts. Chem. Commun. 2010, 46, 1446−1448.
(28) Pan, X.; Xu, Y.-J. Defect-Mediated Growth of Noble-Metal (Ag,
Pt, and Pd) Nanoparticles on TiO2 with Oxygen Vacancies for
Photocatalytic Redox Reactions under Visible Light. J. Phys. Chem. C
2013, 117, 17996−18005.
(10) Cai, S.; Duan, H.; Rong, H.; Wang, D.; Li, L.; He, W.; Li, Y.
Highly Active and Selective Catalysis of Bimetallic Rh3Ni1 Nano-
particles in the Hydrogenation of Nitroarenes. ACS Catal. 2013, 3,
608−612.
(29) Pan, X.; Xu, Y.-J. Fast and Spontaneous Reduction of Gold Ions
over Oxygen-Vacancy-Rich TiO2: A Novel Strategy to Design Defect-
Based Composite Photocatalyst. Appl. Catal., A 2013, 459, 34−40.
(30) Tsyrul’nikov, P. G.; Afonasenko, T. N.; Koshcheev, S. V.;
Boronin, A. I. State of Palladium in Palladium-Aluminosilicate
Catalysts as Studied by XPS and the Catalytic Activity of the Catalysts
in the Deep Oxidation of Methane. Kinet. Catal. 2007, 48, 728−734.
(31) Xie, Y.; Ding, K.; Liu, Z.; Tao, R.; Sun, Z.; Zhang, H.; An, G. In
Situ Controllable Loading of Ultrafine Noble Metal Particles on
Titania. J. Am. Chem. Soc. 2009, 131, 6648−6649.
(32) Keller, N.; Ducamp, M.-N.; Robert, D.; Keller, V. Ethylene
Removal and Fresh Product Storage: A Challenge at the Frontiers of
Chemistry. Toward an Approach by Photocatalytic Oxidation. Chem.
Rev. 2013, 113, 5029−5070.
(33) Long, P.; Zhang, Y.; Chen, X.; Yi, Z. Fabrication of YxBi1‑xVO4
Solid Solutions for Efficient C2H4 Photodegradation. J. Mater. Chem. A
2015, 3, 4163−4169.
(34) Morazzoni, F.; Canevali, C.; Chiodini, N.; Mari, C.; Ruffo, R.;
Scotti, R.; Armelao, L.; Tondello, E.; Depero, L.; Bontempi, E. Surface
Reactivity of Nanostructured Tin Oxide and Pt-Doped Tin Oxide as
Studied by EPR and XPS Spectroscopies. Mater. Sci. Eng., C 2001, 15,
167−169.
(35) Yamazaki, S.; Tanaka, S.; Tsukamoto, H. Kinetic Studies of
Oxidation of Ethylene over a TiO2 Photocatalyst. J. Photochem.
Photobiol., A 1999, 121, 55−61.
(36) Hauchecorne, B.; Tytgat, T.; Verbruggen, S. W.; Hauchecorne,
D.; Terrens, D.; Smits, M.; Vinken, K.; Lenaerts, S. Photocatalytic
Degradation of Ethylene: An FTIR in Situ Study under Atmospheric
Conditions. Appl. Catal., B 2011, 105, 111−116.
(37) Long, P.; Zhang, Y.; Chen, X.; Yi, Z. Fabrication of YxBi1‑xVO4
Solid Solutions for Efficient C2H4 Photodegradation. J. Mater. Chem. A
2015, 3, 4163−4169.
(11) Zhang, T.; Zhao, H.; He, S.; Liu, K.; Liu, H.; Yin, Y.; Gao, C.
Unconventional Route to Encapsulated Ultrasmall Gold Nanoparticles
for High-Temperature Catalysis. ACS Nano 2014, 8, 7297−7304.
(12) Zhan, W.; He, Q.; Liu, X.; Guo, Y.; Wang, Y.; Wang, L.; Guo, Y.;
Borisevich, A. Y.; Zhang, J.; Lu, G.; Dai, S. A Sacrificial Coating
Strategy toward Enhancement of Metal−Support Interaction for
Ultrastable Au Nanocatalysts. J. Am. Chem. Soc. 2016, 138, 16130−
16139.
(13) Wang, Z.; Fu, H.; Han, D.; Gu, F. The Effects of Au Species and
Surfactant on the Catalytic Reduction of 4-Nitrophenol by Au@SiO2.
J. Mater. Chem. A 2014, 2, 20374−20381.
(14) Lopez-Sanchez, J. A.; Dimitratos, N.; Hammond, C.; Brett, G.
L.; Kesavan, L.; White, S.; Miedziak, P.; Tiruvalam, R.; Jenkins, R. L.;
Carley, A. F.; Knight, D.; Kiely, C. J.; Hutchings, G. J. Facile Removal
of Stabilizer-Ligands from Supported Gold Nanoparticles. Nat. Chem.
2011, 3, 551−556.
(15) Cozzoli, P. D.; Comparelli, R.; Fanizza, E.; Curri, M. L.;
Agostiano, A.; Laub, D. Photocatalytic Synthesis of Silver Nano-
particles Stabilized by TiO2 Nanorods: A Semiconductor/Metal
Nanocomposite in Homogeneous Nonpolar Solution. J. Am. Chem.
Soc. 2004, 126, 3868−3879.
(16) Munnik, P.; de Jongh, P. E.; de Jong, K. P. Recent
Developments in the Synthesis of Supported Catalysts. Chem. Rev.
2015, 115, 6687−6718.
(17) Zhang, W.; Pan, X.; Long, P.; Liu, X.; Long, X.; Yu, Y.; Yi, Z.
Platinum Nanoparticles Supported on Defective Tungsten Bronze-
Type KSr2Nb5O15 as a Novel Photocatalyst for Efficient Ethylene
Oxidation. J. Mater. Chem. A 2017, 5, 18998−19006.
(18) Okumura, M.; Nakamura, S.; Tsubota, S.; Nakamura, T.;
Azuma, M.; Haruta, M. Chemical Vapor Deposition of Gold on Al2O3,
SiO2, and TiO2 for the Oxidation of CO and of H2. Catal. Lett. 1998,
51, 53−58.
(38) Anpo, M.; Kubokawa, Y. Reaction of C2H4 with Photo-Formed
O− Hole Centers on Supported MoO3. J. Catal. 1982, 75, 204−206.
(39) Pan, X.; Chen, X.; Yi, Z. Photocatalytic Oxidation of Methane
over SrCO3 Decorated SrTiO3 Nanocatalysts Via a Synergistic Effect.
Phys. Chem. Chem. Phys. 2016, 18, 31400−31409.
(40) Pan, X.; Yi, Z. Graphene Oxide Regulated Tin Oxide
Nanostructures: Engineering Composition, Morphology, Band
Structure, and Photocatalytic Properties. ACS Appl. Mater. Interfaces
2015, 7, 27167−27175.
(19) Yan, W.; Mahurin, S. M.; Pan, Z.; Overbury, S. H.; Dai, S.
Ultrastable Au Nanocatalyst Supported on Surface-Modified TiO2
Nanocrystals. J. Am. Chem. Soc. 2005, 127, 10480−10481.
(20) Iizuka, K.; Wato, T.; Miseki, Y.; Saito, K.; Kudo, A.
Photocatalytic Reduction of Carbon Dioxide over Ag Cocatalyst-
Loaded AlA4Ti4O15 (A = Ca, Sr, and Ba) Using Water as a Reducing
Reagent. J. Am. Chem. Soc. 2011, 133, 20863−20868.
(21) Chen, J. I. L.; Loso, E.; Ebrahim, N.; Ozin, G. A. Synergy of
Slow Photon and Chemically Amplified Photochemistry in Platinum
Nanocluster-Loaded Inverse Titania Opals. J. Am. Chem. Soc. 2008,
130, 5420−5421.
(22) Pan, X.; Chen, X.; Yi, Z. Defective, Porous TiO2 Nanosheets
with Pt Decoration as an Efficient Photocatalyst for Ethylene
Oxidation Synthesized by a C3N4 Templating Method. ACS Appl.
Mater. Interfaces 2016, 8, 10104−10108.
(23) Xi, G.; Ye, J.; Ma, Q.; Su, N.; Bai, H.; Wang, C. In Situ Growth
of Metal Particles on 3D Urchin-Like WO3 Nanostructures. J. Am.
Chem. Soc. 2012, 134, 6508−6511.
G
DOI: 10.1021/acs.inorgchem.8b00818
Inorg. Chem. XXXX, XXX, XXX−XXX