Synergetic photo-epoxidation of propylene with molecular oxygen over bimetallic Au–Ag/TS-1 photocatalysts

Article abstract of DOI:10.1016/S1872-2067(17)62832-8

Au–Ag bimetallic nanoparticle-supported microporous titanium silicalite-1 catalysts were prepared via a hydrothermal-immersion method, and their structures were examined. These materials serve as efficient catalysts for the photosynthesis of propylene oxide via the epoxidation of propene. The Au/Ag mass ratio and reaction temperature were demonstrated to have significant effects on the catalytic activity and selectivity of propylene oxide. The optimal formation rate (68.3 μmol/g·h) and selectivity (52.3%) toward propylene oxide were achieved with an Au:Ag mass ratio of 4:1. Notably, the strong synergistic effect between Au and Ag resulted in superior photocatalysis of the bimetallic systems compared with those of the individual systems. A probable reaction mechanism was proposed based on the theoretical and experimental results.

Full text of DOI:10.1016/S1872-2067(17)62832-8

Chinese Journal of Catalysis 38 (2017) 831–844
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Synergetic photo‐epoxidation of propylene with molecular oxygen
over bimetallic Au–Ag/TS‐1 photocatalysts
Naixu Lia, Bin Yanga, Ming Liua, Yong Chena, Jiancheng Zhoua,b,c,
*
a School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, Jiangsu, China
b Department of Chemical and Pharmaceutical Engineering, Southeast University Chengxian College, Nanjing 210088, Jiangsu, China
c Jiangsu Province Hi‐Tech Key Laboratory for Bio‐medical Research, Southeast University, Nanjing 211189, Jiangsu, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Au–Ag bimetallic nanoparticle‐supported microporous titanium silicalite‐1 catalysts were prepared
via a hydrothermal‐immersion method, and their structures were examined. These materials serve
as efficient catalysts for the photosynthesis of propylene oxide via the epoxidation of propene. The
Au/Ag mass ratio and reaction temperature were demonstrated to have significant effects on the
catalytic activity and selectivity of propylene oxide. The optimal formation rate (68.3 μmol/g·h) and
selectivity (52.3%) toward propylene oxide were achieved with an Au:Ag mass ratio of 4:1. Notably,
the strong synergistic effect between Au and Ag resulted in superior photocatalysis of the bimetallic
systems compared with those of the individual systems. A probable reaction mechanism was pro‐
posed based on the theoretical and experimental results.
Received 2 January 2017
Accepted 1 April 2017
Published 5 May 2017
Keywords:
Gold
Silver
Titanium silicalite‐1
Photocatalysis
Propene epoxidation
Synergetic effect
© 2017, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
cient for the partial oxidation of propylene to PO because of the
excessively low propylene conversion (0.9%) and selectivity to
Propylene oxide (PO) is an increasingly important bulk
chemical for materials containing propylene glycol and polyu‐
rethanes, which are used to produce commercial products such
as adhesives, paints, and cosmetics[1]. The typical production
of PO through chlorohydrin and hydroperoxidation processes
is far from economically and environmentally benign [2]. Re‐
cently, numerous research activities on PO preparation over Au
species from propylene and molecular oxygen have resulted in
a significant advancement in the green and mild fabrication of
this compound.
PO (17.9%) during the initial 10 min of the reaction. Jin et al.
[4] achieved high selectivity toward PO (60.3%) on a
20%Ag–4%MoO3/ZrO2 catalyst at 673 K, 0.1 MPa, and GHSV =
‒
1
7500 h . However, the extra energy loss in the reaction, low
hydrogen efficiency, and safety concern that required H2O2
production facilities to be limited in size rendered these ap‐
proaches only temporary solutions. As a promising alternative,
the direct photocatalytic gas‐phase epoxidation of propylene
using oxygen, similar to the direct vapor‐phase epoxidation of
ethylene over Ag/α‐Al2O3 catalysts, has gained preference
among scholars. Recently, substantial efforts have been devot‐
ed to the engineering and tailoring of silica‐supported metal
oxides such as V2O5/SiO2 [5], Nb2O5/SiO2 [6], TiO2/SiO2 [7],
Suo et al. [3] reported Au‐supported SiO2 for the green con‐
version of propylene with dioxygen and dihydrogen to PO at
‒
1
598 K and GHSV = 6000 h . The catalytic system proved ineffi‐
* Corresponding author. Tel: +86‐25‐52090621; Fax: +86‐25‐52090620; E‐mail: jczhou@seu.edu.cn
This work was supported by the National Natural Science Foundation of China (21576050) and the Natural Science Foundation of Jiangsu Province
(BK20150604).
DOI: 10.1016/S1872‐2067(17)62832‐8 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 38, No. 5, May 2017

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Naixu Li et al. / Chinese Journal of Catalysis 38 (2017) 831–844
ZnO/SiO2 [8], MgO/SiO2 [9], and CuOx/SiO2 [10] to promote the
photo‐epoxidation of propylene in the presence of molecular
oxygen. However, studies on Ti‐based materials including tita‐
nium silicalite‐1, TS‐2, TS‐β, Ti‐MCM‐41, and Ti‐MCM‐48
[11–20] for Au chemical immobilization have demonstrated
that tetrahedrally coordinated Ti is favorable for the enhanced
activity and stability of Au species compared with the afore‐
mentioned materials. Both enhanced selectivity toward epox‐
ides and stability toward C3H6 conversion are accessible via Au
incorporation. Therefore, the catalytic materials have triggered
interest among scholars searching for superior materials for
the highly selective synthesis of PO via vapor‐phase C3H6 epox‐
idation.
The catalytic performance of Au‐based materials is strongly
dependent on the particle size; only particles with sizes below
5 nm exhibit high reactivity [21–23]. However, Au particles are
vulnerable to aggregation upon thermal treatment, leading to a
dramatic loss of activity. It is assumed that the addition of a
second metal would overcome the aggregation of the mono‐
metal [24,25] and would assist in the activation of oxygen
[26,27]. The physical and chemical properties of bimetallic
catalysts greatly vary depending on the composition and parti‐
cle size, and these catalysts show complementary advantages
and synergistic effects. Au‐containing bimetallic catalysts such
as Au–Cu [25,28], Au–Ag [29–34], Au–Pd [35], and Au–Pt [36]
can exhibit great catalytic performance in many reactions.
Among them, Ag has highly analogous cell parameters, exhibits
high affinity for Au atoms, and easily forms homogeneous bi‐
metallic Au–Ag alloys. Synergistic effects and improved activity
have been achieved when using Au–Ag bimetals for
low‐temperature CO oxidation [32,36]. Recently, Wittstock et
al. [37,38] claimed that a nanoporous Au catalyst that was
dealloyed by Au–Ag alloys exhibited high efficiency for oxida‐
tive coupling of methanol, and they confirmed that the remain‐
ing Ag could tune the availability of active oxygen and facilitate
the catalytic activity.
2.1. Preparation of photocatalysts
The TS‐1 support was synthesized according to the proce‐
dure presented in Ref. [39]. Typically, 2 g of polyoxyethylene
sorbitan monolaurate (Tween 20; s.d. KESHI) was dissolved in
32 g of distilled water (resistivity = 18.2 MΩ cm). This solution
was added to a tetrapropylammonium hydroxide (TPAOH;
25% in water) solution. To the above micellar solution, 36 g of
tetraethyl orthosilicate (TEOS; Aldrich) was introduced in a
dropwise manner under vigorous stirring. The stirring was
continued for 1 h. Next, a mixture of 1.808 g of tetra‐n‐butyl
titanate (TNBT; Aldrich) and 9.12 g of isopropyl alcohol (IPA;
s.d. SCR) was added to the above solution under intense agita‐
tion for 1 h. The gel solution was transferred to an autoclave
and heated to 433 K for 18 h under autogenous pressure. The
product was recovered by centrifugation, washed with distilled
water, and dried (383 K, 12 h). The starting feedstock had the
following molar composition: 0.03TiO2:SiO2: 0.12TPAOH:
0.009Tween20:0.88IPA:14.45H2O. The calcination was per‐
formed in a flow of air at 823 K for 6 h.
The Au–Ag bimetallic nanoparticles supported on mi‐
croporous TS‐1 (denoted Au–Ag/TS‐1) were prepared via an
immersion method [40]. TS‐1 (0.4 g) was suspended in water
(20 mL) containing the required amount of 4.168 mg/mL
HAuCl4·3H2O (purity 99.999% metal basis) and 1.25 mg/mL
AgNO3 (99.8%). The mixture was sonicated for 2 h and aged at
room temperature for 24 h in the dark. Then, the solution was
dried in a water bath at 353 K, calcined at 573 K for 4 h in air,
and subsequently reduced in H2 (99.999%) at 673 K for 10 h to
produce the final catalyst labeled xAu–yAg/TS‐1 (x and y define
the weight percentage loading of Au and Ag, respectively). TS‐1
samples modified with various mass ratios of Au to Ag were
prepared using the above protocol.
2.2. Characterization of photocatalysts
In this study, we prepared a series of Au–Ag bimetallic na‐
noparticles deposited on microporous TS‐1 to investigate the
effect of temperature and the bimetallic composition on the
catalytic activity. First, TS‐1 was hydrothermally treated at 433
K for 18 h. Second, noble‐metal‐decorated TS‐1 samples were
obtained via TS‐1 impregnation in precursor solutions. Subse‐
quently, the structure and component‐related information
were determined using nitrogen adsorption, X‐ray diffraction
(XRD), Fourier‐transform infrared (FTIR) spectroscopy, X‐ray
photoelectron spectroscopy (XPS), energy‐dispersive X‐ray
spectroscopy (EDX), ultraviolet‐visible (UV‐vis) spectroscopy,
photoluminescence (PL), scanning electron microscopy (SEM),
and transmission electron microscopy (TEM) analyses. A com‐
parative analysis of performance tests for an array of C3H6
photo‐epoxidation disclosed the unique synergistic effect of
these Au–Ag bimetallic catalysts. Finally, a possible reaction
mechanism was proposed based on the theoretical and exper‐
imental findings.
The N2 sorption isotherm was measured at 77 K using a
Quantachrome NovaWin analyzer. The samples were degassed
at 573 K for 3 h before the measurements. The specific surface
area was calculated using the Brunauer‐Emmett‐Teller (BET)
method. The average pore diameter was determined using the
Barrett‐Joyner‐Halenda method according to the desorption
isotherm branch. The total pore volume was dependent on the
absorbed N2 volume at a relative pressure of approximately
0.98. Powder XRD patterns were obtained using a SmartLab 3
equipped with a graphite monochromator and Cu Kβ radiation
(40 kV and 30 mA). The diffraction lines were identified by
matching them with reference patterns in the JCPDS database.
TEM and EDX measurements were obtained in scanning
TEM (STEM) mode using an H‐600‐4 electron microscope op‐
erated at an acceleration voltage of 200 kV. The catalyst pow‐
ders were lightly ground and ultrasonically dispersed in etha‐
nol at room temperature. The as‐obtained solution was then
dropped onto copper grids supported by holey carbon films.
XPS analysis was performed using an AXIS‐Ultra‐instrument
(Kratos Analytical) equipped with an Al Kα X‐ray radiation
source (hv = 1486.71 eV). A flood gun with variable electron
2. Experimental

Naixu Li et al. / Chinese Journal of Catalysis 38 (2017) 831–844
833
voltage (from 6–8 eV) was used for charge compensation. The
raw data were corrected for substrate charging with the bind‐
ing energy of the C 1s peak (284.6 eV). The measured spectra
were fitted using the least‐squares method to a product of
Gaussian‐Lorentzian functions after removing the background
noise. The concentration of each element was calculated from
the area of the corresponding peak and calibrated using the
Wagner sensitivity factor.
The UV‐vis diffuse reflectance spectroscopy of the
as‐reduced catalysts was performed on a UV‐vis‐near‐infrared
spectrophotometer (CARY5000 scan spectrophotometer), and
the reduced samples were directly used for the UV‐vis meas‐
urement. FT‐IR spectra were collected on a TENSOR27 PMA 50
FT‐IR spectrometer. The analysis of the recombination rate of
charges was performed using a F‐4600 FL spectrophotometer
with an excitation wavelength of 260 nm.
used to separate the propylene and oxidation products, includ‐
ing PO, propionaldehyde (PA), acrolein (AL), acetone (AC), ac‐
etaldehyde (AA), and ethanol (EtOH). However, a carbon mo‐
lecular sieve (60–80) was used to separate O2 and CO2. The
standard definition of conversion was not used in this study. In
our study, the conversions were low, in the range of
0.01–1.00%. To accurately quantify the conversions in the
photo‐epoxidation, we defined the C3H6 conversion as the sum
of all the products converted to C3. The PO formation rate, C3H6
conversion rate, and product selectivity were expressed as
follows [41]:
PO formation rate = Concentration of the product detected on
stream  flow rate/weight of the photocatalysts (1)
C3H6 conv. rate = ∑ Rate of all products converted to C3 (2)
Product selectivity = (Product converted to C3/all products
converted to C3)  100%
(3)
2.3. Photocatalytic activity
3. Results and discussion
Fig. 1 shows the apparatus used for the photo‐epoxidation
of propylene to PO. A total of 0.1 g of the photocatalysts was fed
into a photoreactor, and a 100‐W high‐pressure mercury lamp
(λ = 200–600 nm) was used as an irradiation source. The pho‐
toreactor was placed in a temperature control box in case
heating was required. The reactant gases consisted of a mixture
of C3H6 (99.99%), O2 (99.999%), and Ar (99.999%) with a
volume ratio of 1:1:19. A total gas flow of 21.2 mL/min was
applied with GHSV = 6000 h . The reactants and products
were analyzed online and periodically using a gas chromato‐
graph (GC‐9850‐5V) equipped with a flame ionization detector
and a thermal conductivity detector. A PLOT Q column was
3.1. Characterization of photocatalysts
3.1.1. Physical properties
Table 1 lists the BET surface areas, average pore sizes, and
pore volumes of the catalysts. The BET surface areas (470–528
m2/g) gradually decreased with increasing Ag content in the
bimetal loading, indicating that the pores in TS‐1 may have
been clogged by the metal. However, Au–Ag/TS‐1(4/1) had a
larger specific surface area than TS‐1, which suggests that the
noble metal was well dispersed on the support and that the
channels were not plugged. The bimetallic particle size also
showed the same increasing trend (from 2.1 to 4.2 nm) with

1
Fig. 1. Apparatus for gas‐phase photocatalytic epoxidation of propylene.
Table 1
Chemical composition and textural properties of catalysts and estimated metal particle sizes.
No. Photocatalyst (wt%)
Au/Ag mass ratioa
Pore size (nm) Pore volumeb (cm3/g) BET surface area (m2/g) Metal particle sizec (nm)
1
2
3
4
5
6
TS‐1
0.1Au/TS‐1
0.1Au‒0.025Ag/TS‐1
0.1Au‒0.1Ag/TS‐1
0.1Au‒0.15Ag/TS‐1
0.15Ag/TS‐1
‒
1:0
4.15:1
1.21:1
2.13:3.15
0:1
1.34
1.20
1.35
1.35
1.34
1.33
0.289
0.267
0.290
0.307
0.311
0.270
519
477
528
472
470
463
‒
2.1
2.4
2.9
3.2
4.2
a Au/Ag mass ratio determined by EDX.
b Total pore volume obtained from p/p0 = 0.98.
c Particle size measured by TEM.

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Naixu Li et al. / Chinese Journal of Catalysis 38 (2017) 831–844
increasing Ag content. In addition, the metal loading did not
have a substantial effect on the pore size or volume of TS‐1. The
Au/Ag nominal mass ratios were consistent with the composi‐
tions measured using EDX.
sively low loading (≤ 0.25 wt%) or high dispersion on the TS‐1
surface.
3.1.3. UV‐vis spectra
The UV‐vis spectra in Fig. 3(a) reveal one strong absorption
edge at 214 nm. This band is attributed to electronic transitions
3.1.2. XRD analysis

The structures of the as‐prepared catalytic materials were
confirmed by XRD. Fig. 2(a) shows diffraction lines at 2θ = 7.8°,
8.8°, 23.2°, 23.8°, and 24.5°, representing the indices of the
(101), (200), (501), (151), and (313) lattice planes, respective‐
ly. The perfect match of these peaks with the standard peaks of
the TS‐1 curve (JCPDS card no. 70‐4276) indicates that the or‐
thorhombic phase and MFI structure of the sample [42–44].
The incorporation of TiIV into the framework is indicated by the
change from monoclinic symmetry (S‐1) to orthorhombic
symmetry (TS‐1) by virtue of the disappearance of the peak
split at 2θ = 24.5° and 29.5° [44]. In addition, for the
Au–Ag/TS‐1 samples, the perfect coincidence of the shapes and
positions of the characteristic peaks with those associated with
TS‐1 indicates that the metal loading is independent of the lat‐
ter framework. Nevertheless, diffraction lines attributable to Au
and Ag are hardly observed in Fig. 2(b) because of their exces‐
between O2 and Ti4+ in the isolated framework TiIV centers in a
tetrahedral environment. The absence of an absorption band at
320–400 nm confirms that neither anatase nor extraframe‐
work Ti was present in these samples [45]. It is apparent that
the absorbance is closely connected with the metal loading. For
the monometallic loading, Ag/TS‐1 exhibits high intensity
compared with Au/TS‐1, which may be related to its physical
and chemical properties. Compared with Au, Ag shows a great‐
er tendency to conduct photogenerated charges [46]. For the
Au/Ag bimetallic catalysts, the absorbance deviated from that
of its counterpart with increasing Ag amount. This phenome‐
non is possibly connected to the bimetallic composition and
particle size [21–23]. Similar to the Au/Ag (4/1) bimetallic
catalyst, the particle size was small and well dispersed on the
support, which is favorable for the transfer of photogenerated
charges. Therefore, the small addition of Ag to Au is beneficial
(b)
111
(a)
Cubic Ag
311
200
220
0/1
0/1
2/3
1/1
4/1
2/3
1/1
4/1
1/0
1/0
TS-1
TS-1
22 23 24 25 26
Cubic Au
311
151
Orthorhombic TS-1
313
200
501
101
111
200
220
5
10 15 20 25 30 35 40 45 50
40
50
60
70
80
2 /(o)
2 /(o)
Fig. 2. XRD patterns of catalysts with different Au/Ag mass ratios. (a) 2θ = 5°–35°; (b) 2θ = 35°–80°.
TS-1
1/0
TS-1
1/0
(a)
(b)
4/1
4/1
2/3
2/3
1/1
1/1
0/1
0/1
4.52eV
4.78eV
200
300
400
500
600
700
800
3.5
4.0
4.5
5.0
hv eV)
5.5
6.0
Wavelength (nm)
Fig. 3. UV‐vis absorption spectra (a) and plot of (αhν)2 vs. hν (b) for the direct optical band gap of Au–Ag/TS‐1 with different Au/Ag mass ratios.

Naixu Li et al. / Chinese Journal of Catalysis 38 (2017) 831–844
835
for the photocatalytic reaction. Our results provide evidence of
TS-1
1/0
4/1
2/3
1/1
Excitation at 260nm
the close connection between the interfaces at the nanoscale
level, leading to the formation of a surface heterojunction,
which could affect its electron transfer. The band gap of the
catalysts can be calculated using the following empirical for‐
mula:
0/1
αhν = A(hν ‒ Eg)1/2
where α, ν, h, Eg, and A are the absorption coefficient, light fre‐
quency, Planck constant, bandgap energy, and proportionality
constant, respectively. The band gap energies of TS‐1 and
Au–Ag/TS‐1 are shown in the Fig. 3(b). Eg of pure TS‐1 was
estimated to be 4.78 eV, and after the bimetal loading, the ab‐
sorption edge exhibited a red shift, extending the Eg of
Au–Ag/TS‐1(4/1) to 4.52 eV. This result indicates that the ab‐
sorption intensity of TS‐1 in the region of UV light can be in‐
creased by bimetal loading, which may be explained by the
surface plasmon resonance effect of the metal.
350
400
450
500
Wavelength (nm)
Fig. 5. PL spectra of Au–Ag/TS‐1 with different Au/Ag mass ratios at an
excitation wavelength of 260 nm.
3.1.4. FTIR spectra
trons and holes, which translates into a high photocatalytic
activity. In our photocatalytic system, the fluorescence intensity
decreased with increasing metal loading, which may be ex‐
plained by the electron transfer from TS‐1 to the metal and the
recombination of photogenerated electrons and holes being
inhibited under UV‐light irradiation. For monometallic loading,
Ag/TS‐1 showed high intensity compared with Au/TS‐1. How‐
ever, with increasing Au content in the Au/Ag bimetallic cata‐
lysts, the fluorescence intensity gradually decreased, demon‐
strating that Au/Ag‐loaded on TS‐1 provided better ability to
deliver electrons and inhibited the recombination of photoin‐
duced charges and holes on the Au/Ag‐loaded TS‐1 surface.
Among the bimetallic catalysts, Au/Ag (4/1) exhibited the low‐
est intensity, which indicates it demonstrated the best separa‐
tion effect for the photocatalytic reaction. This phenomenon is
consistent with the catalytic activity findings.
To further understand the structure of TS‐1, FTIR spectra
were obtained. The major peaks at 1220, 1100, 800, 550, and
450 cm are consistent with those of the MFI topology struc‐
1
tures. Next, in all cases, the peak centered at 960 cm1 suggests
the incorporation of Ti into the framework [47–50], as ob‐
served in Fig. 4. Moreover, modification of TS‐1 with noble
metals did not result in variation of the shape or mode of the
FTIR curve, implying an untouched structure before and after
Au and Ag loading.
3.1.5. PL spectra
PL spectra serve as indicators of the recombination rate of
photogenerated excitons, which provides deeper insight into
photocatalytic processes. The PL spectra of pure TS‐1 and TS‐1
containing different Au/Ag mass ratios contained the same
fluorescence peaks with an excited wavelength of 260 nm (Fig.
5). It is assumed that a photocatalyst will generate electrons
and holes under light excitation and that the recombination of
electrons and holes will emit fluorescence. A low fluorescence
emission intensity indicates a low recombination rate of elec‐
3.1.6. SEM and TEM analyses
For propylene epoxidation catalyzed by metal nanoparticles,
the size of the nanoparticle is an important factor. The particle
size and distribution of the Au–Ag bimetallic catalysts were
determined using SEM and high‐resolution TEM. Figs. 6(a) and
7(a) display uniform, isolated, and smooth‐surface TS‐1 mi‐
crospheres with a narrow size distribution of 0.4–0.5 μm. Fig.
6(b) reveals the lattice spacing of 1.003 nm indexed as the
(200) planes of TS‐1. The metal nanoparticle size is of the first
magnitude for elevated catalytic behavior of propene oxidation.
High‐resolution TEM (HRTEM) images of TS‐1 modified with
different mass ratios of Au/Ag are presented in Fig. 9. The dif‐
fraction lines of Au–Ag bimetal are similar to those of mono‐
metallic nanoparticles because of their similar lattice constants
(0.408 vs. 0.409 nm) and the face‐centered‐cubic structures of
Au and Ag. Thus, the HRTEM image of Au–Ag/TS‐1(4/1) in Fig.
6(c) reveals lattice spacings of 0.236, 0.204, and 0.143 nm in‐
dexed as the (111), (200), and (220) planes of Au–Ag bimetal,
respectively. Fig. 6(d) presents an image and EDX pattern of
Au–Ag/TS‐1(4/1). The sample was a pink powder, with Ti, O,
and Si elements clearly detected in Fig. 6(d). The loadings of Au
and Ag were small; therefore, Au and Ag are not easily detected
968
1090 804
450
546
0/1
2/3
1/1
4/1
1/0
TS-1
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm–1)
Fig. 4. FTIR spectra of Au–Ag/TS‐1 with different Au/Ag mass ratios.

836
Naixu Li et al. / Chinese Journal of Catalysis 38 (2017) 831–844
(a)
((bb))
Overlap
(c)
(c)
Ti Kα1 (d)
Si Kα1
(b) 4 h
(c) 7 h
Fig. 6. SEM (a) and HRTEM (b) image of TS‐1; HRTEM image (c) and
EDX pattern (d) and image (inset) of Au–Ag/TS‐1(4/1).
(e)
(f)
Au Mα1
Ag Lα1
in Fig. 6(d). However, these metals can be clearly observed in
the elemental EDX mapping (Fig. 8). The metal particle size and
size distribution for the Au–Ag bimetallic catalysts are pre‐
sented in Fig. 7. The particle size of the Au–Ag bimetallic cata‐
lysts was much larger than that of Au/TS‐1. The samples with
Au/Ag mass ratios of 4:1, 1:1, 2:3, and 0:1 had average particle
sizes of approximately 2.4, 2.9, 3.2, and 4.2 nm, respectively.
With increasing Ag content, the particle size increased, which is
attributed to the interaction between Au and Ag. The nanoscale
Fig. 8. (a) STEM image of Au/Ag (4/1) catalyst, (b) overlapping ele‐
mental intensities, and (c–f) measured (c) Ti K, (d) Si K, (e) Au M, and (f)
Ag L intensities for the corresponding EDX mappings of Au/Ag (4/1)
catalyst in the region shown in (a).
(a)
(b)
(d)
(f)
d=2.1± 0.7 (nm)
d=2.1± 0.7 (nm)
25
20
15
10
5
25
20
15
10
5
elemental EDX mapping of the Au/Ag (4/1) catalyst (Fig. 8)
reveals that most of the Ag species were distributed in the Au
domain and were clearly distinguishable. The Si and Ti species
were equidistributed, and the Au and Ag species overlapped in
the Au–Ag/TS‐1 catalysts. The close integration of Au and Ag
may be the origin of the synergistic effect.
0
1
2
3
4
5
0
Particle size (nm)
1
2
3
4
5
Particle size (nm)
(c)
d=2.9± 0.6 (nm)
25
20
((aa))
(b)
20
16
12
8
d=2.4± 0.5 (nm)
d=2.9± 0.6 (nm)
25
20
15
10
5
15
10
5
(a) 2 h
(b) 4 h
(c) 7 h
0
4
1
2
3
4
5
Particle size (nm)
0
0
1
2
3
4
5
1
2
3
4
5
Particle size (nm)
Particle size (nm)
(e)
(d)
(d)
(c)
d=3.2± 0.7 (nm)
d=4.2± 1.5 (nm)
32
24
16
8
20
15
10
5
a) 2 h
(b) 4 h
(c) 7 h
0
0
1
2
3
4
5
2
4
6
8
10
Particle size (nm)
Particle size (nm)
Fig. 7. Particle size and size histograms for TS‐1 (a) and
Au/Ag‐modified TS‐1 for Au/Ag mass ratios of 1/0 (b), 4/1 (c), 1/1 (d),
2/3 (e), and 0/1 (f).
Fig. 9. HRTEM images of Au/Ag‐modified TS‐1 for the following Au/Ag
mass ratios. (a) 1/0; (b) 4/1; (c) 1/1; (d) 2/3.

Naixu Li et al. / Chinese Journal of Catalysis 38 (2017) 831–844
837
(b)
(a)
4/1
2p3/2
O 1s
C 1s
Si 2s
Si 2p
2p1/2
Ti 2p
0/1
4/1
1/0
Ag 3d
Au 4f
0/1
1/0
TS-1
TS-1
470 468 466 464 462 460 458 456
Binding energy (eV)
1200 1000 800
600 400
200
0
Binding energy (eV)
(d)
(c)
83.7 (4f7/2
)
87.3 (4f5/2
)
368.1 (3d5/2
)
374.0 (3d3/2
)
82
84
86
88
90
366
368 370
372 374
376
Binding energy (eV)
Binding energy (eV)
Fig. 10. XPS spectra. (a) Survey spectrum; (b) Ti 2p; (c) Au 4f: Au/Ag = 1/0; (d) Ag 3d: Au/Ag = 0/1.
photocatalytic propene epoxidation. Fig. 10 (c) shows the the‐
oretical binding energies of Au 4f7/2 and 4f5/2 for Au/TS‐1. The
theoretical binding energies of Ag 3d5/2 and 3d3/2 for Ag/TS‐1
are shown in Fig. 10(d). The peak intensities for Au and Ag
were near the background because of the small loadings of Au
and Ag. It is difficult to clarify the forms of Au and Ag because of
their small loadings. They may be assumed to be present in
metal form because Au and Ag are more easily reduced in bi‐
metallic catalysts under hydrogen [52].
3.1.7. XPS spectra
To further investigate the element state and interaction
between metal nanoparticles and TS‐1, XPS spectra were ob‐
tained, and the results are presented in Fig. 10. O, Ti, Si, C, Au,
and Ag were detected in TS‐1 and TS‐1 with different Au/Ag
mass ratios (1/0, 4/1, 0/1) catalysts, as shown in Fig. 10(a).
The binding energies of Ti 2p are presented in Fig. 10(b) for the
above catalysts, and the peaks at 460 and 465.8 eV were well
defined for TS‐1. The peak at 460 eV is usually attributed to Ti
in tetrahedral coordination [51]. With the metal loading, the
binding energy of Ti 2p3/2 shifted down by 0.4 and 0.5 eV for
Au/Ag mass ratios of 4/1 and 0/1, respectively, which is ex‐
plained by the greater tendency of Ag to conduct electrons be‐
tween a metal and support because of its low work function.
The work function of Ag (4.26 eV) is lower than that of the
support (4.78 eV); therefore, the electrons have the tendency to
aggregate on the support, which leads to the decrease of the
binding energy. For the Au–Ag bimetal, Au has a higher work
function (5.1 eV), which inhibits electron transfer; therefore,
the shift decreases compared with that of monometallic Ag.
This phenomenon indicates the generation of a new surface of
Ti metal, which promotes electron transfer and is favorable for
3.2. Photo‐epoxidation of propylene to PO
Typically, it takes 2–3 h to reach a steady state. To distin‐
guish the thermal effect from the photocatalytic process, an
epoxidation experiment was performed over TS‐1 under dif‐
ferent lighting conditions (with and without UV light). No activ‐
ity was observed with UV illumination, as demonstrated by the
results in Table 2. Therefore, the epoxidation over the catalyst
was clearly photocatalytic epoxidation. In addition, with metal
loading, PO and CO2 were not detected at the different temper‐
atures without UV illumination. Table 3 summarizes the results
of the photocatalytic epoxidation of propylene with oxygen
over various samples in a continuous packed‐bed reactor at

838
Naixu Li et al. / Chinese Journal of Catalysis 38 (2017) 831–844
Table 2
PO formation rates (μmol/(g·h)) over TS‐1 for various lighting conditions and temperatures.
Temperature (K)
323
37.5
ND
343
40.4
ND
363
42.3
ND
383
44.0
ND
403
47.9
ND
423
50.5
ND
443
52.5
ND
453
53.5
ND
463
52.7
ND
473
50.9
ND
UV‐light
No UV‐light
ND: not detected by gas chromatography.
various temperatures with UV light excitation. The formation
rate of PO ranged from 37.5 to 68.3 μmol/g·h and the PO selec‐
tivity ranged from 16.5% to 53.0%. In addition to PO, AA was
detected as major by‐product and AL, PA, AC, and EtOH were
detected as minor by‐products. Interestingly, Fig. 11(a) reveals
that the bimetallic catalyst with an Au/Ag mass ratio of 4:1
showed the best activity and demonstrated a collaborative
effect for propylene epoxidation compared with the catalyst
with monometallic loading. When the Au/Ag mass ratio was
increased from 4:1 to 1:0 or decreased from 4:1 to 1:1 or 2:3,
the activity decreased. This phenomenon may be attributed to
the metal particle size and bimetallic composition. For the bi‐
metals, 4/1(Au/Ag) had the smallest particle size and provided
the largest specific surface area, which are beneficial for reac‐
tant adsorption. In addition, with the incorporation of a small
amount of Ag, 4/1(Au/Ag) exhibited better electron transfer
and an improved separation effect for photogenerated elec‐
trons and holes. Monometallic Ag exhibited a larger particle
size (Table 1), which is unfavorable for this reaction. Although
Au particles are small, electron transfer from the metal to ad‐
sorbate is difficult because of the high work function of Au.
Detailed explanations are provided below.
3.2.1. Effect of reaction temperature on photo‐epoxidation
activity
Monotonic relationships were not observed for any of the
photocatalysts between the reaction temperature and the PO
formation rate or C3H6 conversion rate, as observed in Fig. 11.
Instead, the activity first increased with increasing temperature
(up to 443 K) and then decreased from 443–473 K. The photo‐
Table 3
Selectivities for photo‐epoxidation over Au–Ag/TS‐1 for photocatalysts with various Au/Ag mass ratios at various temperatures.
Selectivity (%)
Temperature
(K)
C3H6 conversion
rate (μmol/(g·h))
PO formation rate
(μmol/(g·h))
Entry
1
Catalyst
TS‐1
AA
EtOH
trace
20.3
16.7
14.2
12.7
11.8
11.3
10.7
10.7
10.5
trace
19.4
18.6
12.5
12.0
11.5
10.7
10.6
10.8
10.3
trace
20.1
15.9
11.3
10.6
10.1
9.9
PO
AL
PA
ND
AC
ND
323
343
363
383
403
423
443
453
463
473
323
343
363
383
403
423
443
453
463
473
323
343
363
383
403
423
443
453
463
473
72.1
168.3
215.3
249.1
266.7
279.4
296.6
295.7
290.3
288.7
71.9
170.3
220.2
264.0
291.7
304.8
310.5
306.3
304.4
302.5
74.1
37.5
40.4
42.3
44.0
47.9
50.5
52.5
53.5
52.7
50.9
37.8
40.9
48.4
53.6
59.8
62.6
64.9
62.8
61.2
59.0
38.8
41.4
50.1
60.4
64.9
67.8
68.3
66.1
63.5
60.4
48.0
22.8
20.8
27.8
32.4
33.4
34.2
34.9
34.7
33.7
47.4
23.7
28.2
31.6
32.6
32.8
32.6
32.4
31.5
31.3
47.7
23.2
21.8
33.2
33.7
34.3
34.3
34.3
33.5
32.7
52.0
24.0
20.6
18.4
18.3
18.1
17.7
17.5
17.2
16.7
52.6
24.7
22.0
20.3
20.5
20.5
20.9
20.5
20.1
19.5
52.3
24.0
21.1
20.5
20.6
20.7
20.5
18.9
18.6
17.8
trace
16.5
13.9
13.8
13.7
13.6
13.3
13.9
14.1
14.5
trace
16.8
16.1
13.9
13.8
13.8
14.3
14.8
15.1
15.4
trace
16.4
14.3
14.2
14.1
14.0
14.2
14.7
15.0
15.8
ND
16.3
14.6
12.3
10.8
10.1
9.9
13.4
13.5
12.1
13.0
13.6
13.7
13.9
15.4
ND
9.3
9.4
9.2
ND
2
Au/Ag (1/0)
ND
ND
15.4
15.1
10.5
9.7
11.1
11.4
11.9
12.4
12.6
13.6
14.8
ND
9.5
9.1
9.1
8.9
8.7
ND
3
Au/Ag (4/1)
172.5
237.6
294.6
315.0
327.5
333.1
349.7
341.3
339.3
ND
16.3
13.5
9.9
9.5
9.3
13.4
10.9
11.5
11.6
12.2
13.4
14.1
14.9
8.9
9.8
10.0
10.0
8.8
8.7
8.7
(To be continued)

Naixu Li et al. / Chinese Journal of Catalysis 38 (2017) 831–844
839
Table 3 (continued)
Temperature
(K)
C3H6 conversion
rate (μmol/(g·h))
PO formation rate
(μmol/(g·h))
Selectivity (%)
Entry
4
Catalyst
AA
EtOH
trace
19.4
15.1
13.3
13.0
12.4
12.2
12.1
12.1
12.2
trace
trace
trace
PO
AL
PA
trace
ND
AC
Au/Ag (2/3)
Au/Ag (1/1)
Au/Ag (0/1)
trace
23.2
23.2
29.6
30.6
31.0
30.6
30.4
30.0
29.7
47.0
28.5
30.1
trace
16.0
14.2
13.5
13.5
13.5
13.7
14.3
14.6
15.1
trace
21.3
16.9
trace
14.6
13.2
11.4
10.5
10.3
10.1
9.9
9.9
10.0
ND
19.4
14.0
323
343
363
383
403
423
443
453
463
473
323
343
363
383
403
423
443
453
463
473
323
343
363
383
403
423
443
453
71.2
151.8
210.2
245.6
263.2
272.5
273.8
270.1
267.6
270.9
71.3
131.4
196.1
260.3
283.4
301.1
297.5
298.5
293.4
289.8
71.5
135.3
205.6
238.1
261.3
272.2
271.4
269.9
266.0
268.9
37.6
40.7
44.5
52.3
54.5
56.7
57.5
57.2
56.2
54.1
37.8
40.5
45.7
52.6
56.4
59.6
60.4
59.8
58.7
55.1
37.6
40.6
44.0
48.1
52.8
55.0
56.2
55.2
54.0
51.9
52.8
26.8
21.1
20.5
20.7
20.8
21.0
21.1
21.0
20.5
53.0
30.8
23.3
19.6
19.9
19.8
20.3
20.3
20.0
19.0
52.6
30.0
21.4
20.2
20.2
20.2
20.7
20.3
20.0
19.3
13.2
11.7
11.7
12.0
12.4
12.2
12.4
12.5
ND
5
ND
15.7
31.0
31.4
33.0
33.1
32.7
32.2
12.6
12.7
11.4
11.0
11.0
11.0
14.6
13.7
13.5
13.7
14.0
14.3
11.4
12.2
12.7
12.4
12.5
13.2
10.8
10.6
9.6
9.5
9.4
9.3
31.4
47.4
29.0
29.4
11.5
trace
trace
trace
14.8
trace
21.1
17.3
13.6
ND
ND
9.7
ND
19.9
15.6
6
16.3
29.4
30.7
31.3
31.0
30.4
30.0
29.7
13.1
12.7
12.4
11.9
11.9
12.0
12.0
14.2
13.9
13.7
14.1
14.4
14.7
15.0
11.9
12.1
12.2
12.4
13.0
13.4
14.1
11.2
10.4
10.2
9.9
10.0
9.9
463
473
9.9
ND: not detected by gas chromatography.
AA: acetaldehyde; EtOH: ethanol; PO: propylene oxide; AL: acrolein; PA: propionaldehyde; AC: acetone.
reaction is often a light‐driven rather than a thermally induced
process. Kim et al. [53] reported that the true activation energy
is nil in a photoreaction. Therefore, the activation energy is the
apparent activation energy, which is very small. Hence, desorp‐
tion of the final product and reactant adsorption will become
important factors for this reaction. Desorption and adsorption
are endothermic and exothermic processes, respectively. In
other words, increasing the temperature favors desorption of
the photocatalytic reaction products, which enhances the reac‐
tion efficiency by releasing more active sites for reaction.
However, increasing the temperature may suppress propylene
adsorption on photocatalysts, which may reduce the reaction
efficiency. Therefore, the reaction temperature has competing
effects on the reaction rate in this study. During the
TS-1
TS-1
(b)
350
300
250
200
150
100
50
(a)
1/0
4/1
2/3
1/1
0/1
1/0
70
4/1
2/3
1/1
60
0/1
50
40
330 360
390 420
450 480
330 360 390 420 450 480
Reaction temperature (K)
Reaction temperature (K)
Fig. 11. Correlation between reaction temperature and PO formation rate (a) and C3H6 consumption rate (b) for photocatalysts with various Au/Ag
mass ratios.

840
Naixu Li et al. / Chinese Journal of Catalysis 38 (2017) 831–844
(b)
(a)
100
100
80
60
40
20
0
AC
PA
80
60
40
20
0
AC
PA
AL
AL
AA
EtOH
PO
AA
EtOH
PO
320 340 360 380 400 420 440 460
Reaction temperature (K)
320 340 360 380 400 420 440 460
Reaction temperature (K)
(c)
(d)
100
80
60
40
20
0
100
80
60
40
20
0
AC
AC
PA
PA
AL
AL
AA
EtOH
PO
AA
EtOH
PO
320 340 360 380 400 420 440 460
Reaction temperature (K)
320 340 360 380 400 420 440 460
Reaction temperature (K)
Fig. 12. Correlation between reaction temperature and product selectivity for TS‐1 (a) and TS‐1 modified with Au/Ag mass ratios of 1/0 (b), 0/1 (c),
and 4/1 (d).
low‐temperature period, propylene adsorption is dominant,
and the product desorption rate increases with increasing
temperature; therefore, the PO formation rate monotonically
increases. However, when the temperature increases above
412 K, product adsorption is the rate‐determining step, and
high temperature is unfavorable for propylene adsorption.
Therefore, the PO formation rate decreases with increasing
noted that heating may increase not only the desorption of
photocatalytic products but also the partial oxidation and/or
total oxidation of the intermediates. Therefore, the product
distribution would be rearranged.
3.2.2. Effect of Au/Ag loading on photo‐epoxidation
Fig. 11 reveals that loading with the monometal or bimetal
resulted in similar activity and selectivity as those of TS‐1 at
low temperature (from 323 to 343 K). Above 343 K, loading
with the monometal or bimetal greatly increased the PO for‐
mation rates. This phenomenon may be connected with reac‐
tant adsorption, product desorption, and photon utilization.
Low temperature is beneficial for reactant adsorption but un‐
favorable for product desorption, which inhibits the reaction
efficiency by occupying more active sites for the reaction [54].
Although the introduction of Ag to Au will affect its physical
and chemical properties, which is beneficial for pho‐
to‐epoxidation by improving the oxygen adsorption, activation,
and electron transfer [26,27,46], photogenerated electron uti‐
lization is suppressed because of the limited active sites.
Therefore, during the low‐temperature period, loading with the
temperature.
Fig.
11(a)
demonstrates
that
the
Au–Ag/TS‐1(4/1) catalyst at 443 K exhibits an optimal PO for‐
mation rate of 68.3 μmol/g·h. Fig. 12 shows the product distri‐
bution for various temperatures for TS‐1 and TS‐1 modified
with Au/Ag mass ratios of 1/0, 0/1, and 4/1. High temperature
led to low PO selectivity and high selectivity to the by‐products,
which may be attributed to the competition of multiple reac‐
tions toward different products. Nguyen et al. [54] reported
that high temperature inhibits the formation of hydroxyl radi‐
cal OH*(ads) intermediates, which are generated from water
molecules. Therefore, high temperature will restrain the con‐
version of PA into propionic acid (C2H5COOH)−, the intermedi‐
ate product of EtOH, resulting in an increase of PA and decrease
of EtOH with increasing temperature. These researchers also

Naixu Li et al. / Chinese Journal of Catalysis 38 (2017) 831–844
841
monometal or bimetal limits enhancement of the photoreaction
performance. With increasing temperature, the product de‐
sorption rate increases and more active sites are released.
Therefore, efficient electronic transmission and utilization are
crucial for this reaction. The addition of Ag to Au is beneficial
for electron transfer [46]; thus, bimetal loading should enhance
the photocatalytic activity compared with monometal loading
and TS‐1. In Fig. 11, the performance of the Au/Ag (4/1) bime‐
tallic catalyst is consistent with the above analysis. However,
with increasing Ag content in the bimetal, the photocatalytic
activity decreased, which can be explained by the chemical
properties and particle size of the bimetal. As previously men‐
tioned, the physical and chemical properties of the bimetal are
affected by the introduction of Ag to Au [26,27,46]. In addition,
the particle size is an important factor in the photocatalytic
reaction. Although the addition of Ag is beneficial for this reac‐
tion, the metal particle size (Table 1) greatly increased from 2.1
to 4.2 nm with increasing Ag content, which is adverse for the
reaction [21–23]. The catalytic activity has the trend
4/1(Au/Ag) > 1/0 > 1/1 > 2/3 > 0/1 > TS‐1. For an Au/Ag mass
ratio of 4/1, the highest reaction rate (68.3 μmol/g·h) is at‐
tained, which is higher than the rates previously reported for
Au/TiO2 and Au/TS‐1 [55]. The fact that the particle size be‐
comes large for all the photocatalysts containing Ag suggests
that aggregation occurred during the calcination process. Table
3 demonstrates that monometal or bimetal loading enhances
the selectivity to PO compared with that of pristine TS‐1. The
optimal selectivity to PO below 363 K was reached for Au/Ag =
1/1 and was higher than that achieved with monometallic
modification. Above 363 K, the selectivity of the bimetal was
equivalent to that of the monometal. In other words, the bimet‐
al plays an essential role in determining the formation rate,
selectivity of PO, and product distribution.
excitation and photo‐epoxidation processes are suitable for our
photocatalytic system.
Au binds oxygen very weakly, and oxygen activation is gen‐
erally thought to be a rate‐limiting step for most oxidation re‐
actions catalyzed by Au nanoparticles [59–61]. Kondarides et
al. [62] observed that Au and Ag bimetal facilitated oxygen ad‐
sorption by decreasing the activation energy for molecular
‒
•
oxygen adsorption. Wang et al. [46] also proposed that O2
formation via dioxygen activation on Au–Ag was more facile
than on Ag alone. Sandoval et al. [52] proposed that the bimetal
easily adsorbed and activated oxygen with the change of the
electronic and geometric structure of Au‐based catalysts with
the introduction of Ag atoms. In addition, the difficulty of elec‐
tron transfer on the Au(111) surface because of its high work
function [63] was alleviated by the modified electronic proper‐
ties of Au with the introduction of Ag as a powerful electron
donor. Therefore, a greater tendency for electron transfer be‐
tween the support and bimetal is achieved, as demonstrated by
the UV‐vis and XPS analyses. The PL spectra also confirmed
that the bimetal inhibits the recombination of photoinduced
electrons and holes. Therefore, the Au–Ag bimetals provide a
synergistic effect for this photoreaction. In addition, the epoxi‐
dation reaction is also connected to the reaction temperature.
For the low‐temperature period, product desorption is inhibit‐
ed, which occupies more active sites, leading to limited photo‐
generated electron utilization. Therefore, the synergetic effect
of bimetal catalysts is limited. With increasing temperature,
more active sites are released, and the bimetal results in an
obvious synergetic effect (Fig. 11), which is explained by the
improved reactant adsorption, activation, and separation of
photoinduced electrons and holes resulting from the introduc‐
tion of the bimetal. In our photocatalytic system, the surface
plasmon resonance effect of metals (Au, Ag) was easily induced
under light excitation (λ = 200–600 nm) and was favorable for
propylene epoxidation. The reaction mechanism could be ex‐
plained by the excited electrons of the metal reacting with O2
directly or transferring from the metal to TS‐1 and then partic‐
ipating in the reaction. Based on these findings, we propose a
mechanism for the photo‐epoxidation of propylene over
Au–Ag/TS‐1 photocatalysts, as illustrated in Fig. 13. Under
UV‐light irradiation, the generated electrons and holes are
3.3. Proposed reaction mechanism on Au/Ag‐TS‐1
photocatalysts
It is well‐documented that tetrahedrally coordinated Ti(IV)
within the framework of TS‐1 plays a critical role in the photo‐
catalytic epoxidation of propylene. As previously discussed, the
UV‐vis and XPS results suggest that Ti oxides were mainly dis‐
persed in the microporous materials as isolated tetrahedrally
coordinated metal oxides. The structures were reported to be
easily excited under UV‐light irradiation to reach the corre‐
sponding charge‐transfer excited state, which includes electron

transfer from O2 to Ti4+ species [56]:
‒

2
(Ti4+–OL ) + hv → (Ti –OL )
3+
*
Murata et al. [56] proposed that O2, rather than C3H6, was
‒
activated on the charge transferred (Ti3+–OL ) radical pair. They
concluded that the active oxygen species for PO production was
‒
an electrophilic O3 (O2 +…OL ) species generated on the pho‐
toformed hole center [56,57]. Because the photoinduced elec‐
tron in Ti3+ is consumed by molecular oxygen (Ti3+ + O2 → Ti4+
•
2
‒
•
+ O2 ), the oxidation state of Ti4+ is actually constant in the
‒
•
presence of oxygen. The generated active species (O2 ) and
(O3 ) were thought to react with C3H6 to generate PO and its
co‐products [58]. According to the above analysis, the UV‐light
‒
Fig. 13. Proposed mechanism for the synergetic photo‐epoxidation of
propylene over Au–Ag/TS‐1 photocatalysts.

842
Naixu Li et al. / Chinese Journal of Catalysis 38 (2017) 831–844
‒
•
‒
captured by O2 to produce O3 and O2 active species, which
react with C3H6 to generate PO and other by‐products. During
the photo‐epoxidation, the Au/Ag mass ratio and reaction
temperature greatly affect the catalytic activity and selectivity.
In this study, direct photo‐epoxidation of propylene with O2
was studied as a potential method for the green production of
PO. This is the first time that Au–Ag bimetal has been used for
the photo‐epoxidation of propylene. Although some improve‐
ment of the catalytic activity was achieved for photocatalytic
epoxidation, the results remain far from those needed for
large‐scale production. Therefore, the development of new
photocatalyst systems is needed to further improve the pho‐
to‐epoxidation.
Au/Ag mass ratio of 4:1.
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Graphical Abstract
Chin. J. Catal., 2017, 38: 831–844 doi: 10.1016/S1872‐2067(17)62832‐8
Synergetic photo‐epoxidation of propylene with molecular oxygen over bimetallic Au–Ag/TS‐1 photocatalysts
Naixu Li, Bin Yang, Ming Liu, Yong Chen, Jiancheng Zhou *
Southeast University; Southeast University Chengxian College
Au–Ag/TS‐1 bimetallic catalysts prepared using a hydrothermal‐immersion method showed a synergetic effect for photo‐epoxidation of
propylene using O2 as the oxidant.

Naixu Li et al. / Chinese Journal of Catalysis 38 (2017) 831–844
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