Controlling the selectivity of catalytic oxidation of styrene over nanocluster catalysts

Article abstract of DOI:10.1039/c6ra23014h

Atomically precise Au₂₅ and Ag₄₄ as well as alloy nanoclusters (Au_25-xAg_x and Au₁₂Ag₃₂) are investigated for catalytically selective oxidation of styrene. Distinct synergistic effects of Au-Ag bimetallic nanoclusters are identified. In addition, the addition of H₂O (a few vol%) to the reaction solvent is found to drastically affect the product selectivity. Based upon Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) measurements, a possible mechanism for the nanocluster-catalyzed styrene selective oxidation is proposed. Specifically, H₂O could change the surface composition of Au on the Au₂₅ nanocluster and the valence state of Ag on the Ag₄₄ nanocluster. Such effects are also present on the bimetallic Au_25-xAg_x, but not on the bimetallic Au₁₂Ag₃₂ nanocluster because for the latter Au exclusively acts as kernel atoms. This work provides insights into the different functional roles of water in the nanoclusters based upon their different atomic structures.

Full text of DOI:10.1039/c6ra23014h

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Controlling the selectivity of catalytic oxidation of
styrene over nanocluster catalysts†
Cite this: RSC Adv., 2016, 6, 111399
a
c
b
b
a
Jinsong Chai, Hanbao Chong, Shuxin Wang, Sha Yang, Mingzai Wu*
and Manzhou Zhu*b
Atomically precise Au25 and Ag44 as well as alloy nanoclusters (Au25ꢀxAg
catalytically selective oxidation of styrene. Distinct synergistic effects of Au–Ag bimetallic nanoclusters are
identified. In addition, the addition of H O (a few vol%) to the reaction solvent is found to drastically affect
the product selectivity. Based upon Fourier transform infrared (FT-IR) and X-ray photoelectron
spectroscopy (XPS) measurements, possible mechanism for the nanocluster-catalyzed styrene
selective oxidation is proposed. Specifically, H O could change the surface composition of Au on the
Au25 nanocluster and the valence state of Ag on the Ag44 nanocluster. Such effects are also present on
the bimetallic Au25ꢀxAg , but not on the bimetallic Au12Ag32 nanocluster because for the latter Au
x
and Au12Ag32) are investigated for
2
a
2
Received 15th September 2016
Accepted 11th November 2016
x
DOI: 10.1039/c6ra23014h
exclusively acts as kernel atoms. This work provides insights into the different functional roles of water in
www.rsc.org/advances
the nanoclusters based upon their different atomic structures.
17
oxidation. Tsukuda and co-workers found that single Pd
1
Introduction
doping signicantly boosts the performance of Au in aerobic
2
5
18
Alloys oen exhibit unique properties compared to the single alcohol oxidation.
1
–6
component systems. Interactions between different atoms
So far, Ag–Au bimetallic nanostructures represent one of the
can modify the alloy's electronic structure and surface compo- most extensively investigated categories because they can
sition, and lead to enhanced chemical and physical proper- possibly reduce the cost of the Au catalysts and enhance the
7
–11
12–14
15,16
26–30
ties.
Recently, fascinating optical,
electronic,
and catalytic performance.
Various gold and silver nanoparticles
17–19
catalytic
properties have been reported for bimetallic have been made and used as catalysts in recent years. For
nanostructures composed of noble metals such as gold (Au), example, Au–Ag bimetallic particles exhibit better catalytic
silver (Ag), platinum (Pt), and palladium (Pd). Such alloy performance than their monometallic counterparts in producing
nanostructures offer a wide range of applications, including
H
2
O
2
from an O
2
-saturated ethanol/water mixture under UV
irradiation. Recently, small mixed-metal clusters have been
In recent years, atomically synthesized by doping Ag into atomically precise Au nano-
Our group studied the catalytic activity of Ag–Au
nanomaterials and attracted signicant research interest due to bimetallic nanoclusters (e.g. Au24Ag46(SR)32 and Au12Ag32
20,21
22,23
31
surface-enhanced Raman scattering (SERS),
biosensors,
17,18
24,25
catalysis
and many others.
n
32–36
precise metal nanoclusters have emerged as a new type of clusters.
-
37
their well-determined structures which benet the deep mech- (SR)30) with different structures on styrene oxidation. Generally,
anistic understanding. According to the recent studies, substi- Ag gives higher selectivity for benzaldehyde and Au favours better
tution with heteroatoms can drastically affect the catalytic conversion, while in the case of Au24Ag46/CNT (where, CNT ¼
37
properties of alloy nanoclusters. For example, Pt
Ph)18 with single Pt atom doped into gold nanoclusters (NCs),
was reported by Jin and co-workers, and it exhibits largely the O
improved catalytic performance than Au (SR) for styrene oxidation, and propene epoxidation), water (typically in the
1
Au24(SC
2
H
4
-
carbon nanotubes) both advantages are amplied.
A well-known phenomenon in nanogold catalysis is that in
-involved reactions (such as CO oxidation, alcohol
2
2
5
18
form of vapour) was found to be signicant in determining the
38–47
conversion efficiency.
Therefore, deep understanding on the
a
School of Physics and Materials Science, Anhui University, Hefei, Anhui, 230601, P. R.
water effect in catalysis is of great importance. To this end, we
use styrene oxidation as a model reaction to explore the detailed
interactions between water and nanoclusters.
China. E-mail: mingzaiwu@gmail.com
b
Department of Chemistry and Centre for Atomic Engineering of Advanced Materials,
Anhui University, Hefei, Anhui, 230601, P. R. China. E-mail: zmz@ahu.edu.cn
c
Herein, the catalytic performance of structure-determined
homometal and alloyed nanoclusters were examined for the
Modern Experiment and Technology Center, Anhui University, Hefei, Anhui, 230601,
P. R. China
†
Electronic supplementary information (ESI) available: Detailed information reaction of styrene selective oxidation. tert-Butyl hydroper-
about the synthesis, optical absorption spectrum of nanoclusters, synthesis and
oxide (TBHP) is used as the oxygen source and toluene as the
catalytic test of nanoparticles, Table S1, Fig. S1–S8. See DOI: 10.1039/c6ra23014h
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2.3 Catalyst preparation and characterization
Preparation of catalysts. Au (SR) , Au
Ag (SR) ,
2
5
18
25ꢀx
x
18
Ag44(SR)30 and Au12Ag32(SR)30 were supported on commercial
carbon nanotubes (Beijing Bo Yu high-tech new material tech-
nology co., LTD). The CNTs were rst dispersed in toluene, and
nanoclusters were added to the suspension of CNTs under
vigorous magnetic stirring. The adsorption of nanoclusters was
allowed to proceed overnight. Then the product was separated
from the solution by centrifugation. The cluster/CNT composite
was dried in vacuum for 12 h. Calcination of the Au :SR/CNT,
2
5
Au25ꢀxAg
x
:SR/CNT, Ag44:SR/CNT, and Au12Ag32:SR/CNT was
ꢁ
performed in a quartz-tube oven under vacuum at 200 C.
Catalytic test. A 10 mL Schlenk bottle was charged with
0.5 mmol styrene and 1.5 mmol TBHP, 20 mg cluster/CNT cata-
lyst and 10 mg K
2
CO
3
, 2 mL solvent. Then the suspension was
ꢁ
stirred at 65 C for reaction. Aer the reaction, the suspension
was centrifuged to remove solid, and catalytic product was ana-
lysed by gas chromatography with internal standard.
x
Scheme 1 Catalytic selective oxidation of styrene over Au25ꢀxAg /
Characterization. The UV-vis absorption spectra were recor-
ded on UV-vis spectrophotometer (Agilent). The catalysts were
investigated by transmission electron microscopy (TEM) on
a JEM 2100 microscope. Time-conversion data were collected by
a GC 2010 plus (from Shimadzu) and a GC-MS saturn 2200
CNT (color labels: green ¼ Au, light blue ¼ Ag, red ¼ S, golden ¼ O and
magenta ¼ H).
solvent. The mixed solvents of toluene and water are then used
for comparison and probing the water effect. Interestingly,
H O is found to dominate the change of selectivity (Scheme 1).
2
The Fourier transform infrared (FT-IR) spectroscopic analysis
and X-ray photoelectron spectroscopy (XPS) measurements are
then carried out to investigate the major roles of water in the
catalytic reaction. Atomic level mechanistic insight into
the nanocluster-catalysed selective styrene oxidation is further
obtained.
(from Varian). To analyse whether the catalysts interacted with
water or not, we soaked the catalysts in water for 24 hours and
then dried the catalysts in an oven to remove the water adhered
to the surface of catalysts. The catalysts were analyzed Fourier
transform infrared (FT-IR) spectroscopy on a Bruker VER-
TEX80+HYPERION000. The X-ray photoelectron spectroscopy
(XPS) measurements were performed on an ESCALAB 250 high-
performance electron spectrometer with monochromated Al Ka
radiation as the excitation source.
2
Experimental section
3
Results and discussion
2.1 Chemicals
3.1 Characterization of the catalysts
4
8
All chemicals were purchased from commercial sources and
used without further treatments. All solvents were of chro-
The nanoclusters including Au (SR) , Au
1
Ag (SR) (x ¼
2
5
18
25ꢀx
x
18
32
36
–9), Ag (SR) , and Au Ag (SR) (ref. 36) were rst
44 30 12 32 30
matographic grade. Tetrachloroauric(III) acid (HAuCl $3H O,
4
2
prepared using the previously reported methods. Fig. 1a and c
show the crystal structures of homogold Au (SR) nano-
>
99.99% metals basis), AgNO , and sodium borohydride (>98%)
3
2
5
18
were purchased from ACROS Organic. Phenylethanethiol and
,4-diuorthiophenol ($97%) were from Aldrich. Toluene
HPLC grade, $99.9%, Aldrich), ethanol (HPLC grade, $99.9%)
and other solvents were from Aldrich. Pure water was ordered
from Wahaha Co LTD and OH (97% atom O) were from
2
Aldrich. Carbon nanotubes were from Beijing Bo Yu high-tech
new material technology co., LTD.
clusters and homosilver Ag44(SR)30 nanoclusters, respectively.
The crystal structure of surface-doped Ag–Au bimetallic nano-
clusters Au25ꢀxAg (SR)18 is shown in Fig. 1b and d shows the
x
3
(
crystal structure of the core–shell structured Au12Ag32(SR)30
nanocluster with an Au12 kernel and an Ag32 shell. The purity of
these nanoclusters was veried by comparing their UV-vis
1
8
18
32,36,48
absorption spectra with standard ones reported
(Fig. S1†).
Carbon nanotubes (CNT) were used as the support for the
nanoclusters since carbon does not strongly interact with gold
while transition metal oxide supports oen exhibit strong
interactions with gold nanoparticles. In addition, CNTs have an
2
.2 Synthesis of Au (SR) , Au
Ag (SR) , Ag (SR) and
25ꢀx x 18 44 30
2
5
18
Au Ag (SR)
30
1
2
32
49
We rst prepared the 25-atom Au (SR) and Au
Ag (SR)
25ꢀx 18
advantage of preventing diffusion of naked nanoclusters. Aer
2
5
18
x
(
x
¼
1–9) nanoclusters using our previously reported wet deposition of nanoclusters onto CNT, Au :SR/CNT,
25
32,48
methods.
The Ag (SR) and Au Ag (SR) nanoclusters Au
Ag :SR/CNT,
Ag :SR/CNT
and
Au Ag :SR/CNT
12 32
4
4
30
12 32
30
25ꢀx
x
44
36
were prepared using the procedures reported by Zheng et al.
The synthetic details are provided in the ESI.†
composites were collected by centrifugation. The supernatant
was colourless, indicating that the clusters were completely
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selectivity for epoxide is 63.9% over Au25/CNT and the homo-
silver Ag /CNT showed the highest selectivity for epoxide
4
4
(74.5%), despite its lower conversion of styrene (49.9%).
In this reaction, the bimetallic nanoclusters clearly show
synergistic effects in activity and/or selectivity. For the 25-atom
catalysts (Table 1, entries 1 and 3), the selectivity for epoxide was
increased from 63.9% over homogold Au25/CNT to 74.3% over
Au25ꢀxAg /CNT with a comparable activity. In contrast, for the 44-
x
atom catalysts, the bimetallic Au12Ag32/CNT nanocluster largely
improves the conversion of styrene relative to Ag /CNT (74.5%
4
4
vs. 49.9%) but the epoxide selectivity is less (60.0% vs. 74.5%).
An intriguing effect of water was discovered in our catalytic
reaction system when using a mixed solution of water and
toluene as the solvent (Table 1, entries 5–8). Similar to the
observations in entries 1–4 (without water), the homogold Au25
CNT had a higher conversion of styrene (78.3%) than the Ag44
/
/
CNT catalyst (45.1%), see Table 1 entries 5 and 6. Aer adding
water to the solvent, the selectivity for product was drastically
changed; benzaldehyde (instead of epoxide) becomes the
x
Fig. 1 Crystal structures of (a) Au25(SR)18; (b) Au25ꢀxAg (SR)18; (c)
Ag44(SR)30; (d) Au12Ag32(SR)30 (color labels: green ¼ Au, light blue ¼ Ag,
red ¼ S, gray stick ¼ C; all H atoms are not shown).
predominant product. Especially for Ag /CNT and Au Agx/
4
4
25ꢀx
CNT (entries 6 and 7), the selectivity is as high as 94.5% and
5.9%, respectively. Interestingly, the Au12Ag32/CNT catalyst
still retains the selectivity for epoxide (entry 8) which is
comparable to that of the case of no water. In addition, we also
have synthesized a series of nanoparticles of size 2–8 nm
9
adsorbed onto the CNTs. The hydrophobicity of clusters and
CNTs led to the efficient absorption of nanoclusters on CNTs.
The solids were dried in vacuum for 12 h. TEM images of
Au25(SR)18, Au25ꢀxAg
Fig. 2) showed that the clusters were adsorbed on the surface of
CNTs, and the size range is 0.8–1.6 nm.
x
(SR)18, Ag44(SR)30 and Au12Ag32(SR)30
(Fig. S8†) and used them as catalysts for selective oxidation of
(
styrene. The results (Table S1†) show no effect of water on the
selectivity of styrene oxidation, nor the conversion of styrene.
Therefore, the nanoclusters are unique in exhibiting the water
effect on controlling the selectivity of styrene oxidation.
To verify the aforementioned proposal, mixed solvents con-
taining different proportions of water and toluene were used.
3
.2 Selective oxidation of styrene
The Au25/CNT, Au25ꢀxAg /CNT, Ag44/CNT and Au12Ag32/CNT
catalysts were investigated for the selective oxidation of styrene
Table 1). TBHP was used as the oxygen source and toluene as
x
2
We added 0, 20, 40, 60, and 100 mL H O into 1.5 mL toluene,
(
respectively. As shown in Fig. 3, the selectivity to benzaldehyde
increases with the addition of water into the solvent (Fig. 3a),
while the selectivity toward epoxide decreases accordingly
the solvent. The results imply that epoxide is the predominant
product and benzaldehyde as the main side-product, while
50–52
other products are minor (Table 1, entries 1–4).
Among the
(
Fig. 3b). However, the catalytic performance of the bimetallic
core–shell structured Au12Ag32/CNT nanocluster catalyst is
distinctly different from the Au25ꢀxAg /CNT catalyst, as
catalysts, homogold Au25/CNT showed a higher conversion of
styrene (78.9%) than the homosilver Ag44/CNT 49.9%. The
x
Au12Ag32 maintains a nearly constant selectivity for epoxide in
different mixed solvents (Fig. 3, blue line).
3.3 Mechanism of styrene oxidation on catalysts
Taking together the above experimental results, we rationalize
that water induces the change of the catalytic selectivity via two
possible pathways: (1) water reacts with epoxide to form benz-
aldehyde; (2) water reacts with catalysts and leads to some
change of catalysts. To test the rst pathway, we performed the
hydrolysis reaction of epoxide using CNT (control), Au25/CNT,
x
Au25ꢀxAg /CNT, Ag44/CNT and Au12Ag32/CNT as catalysts,
respectively (Table 2). Only trace amounts of benzaldehyde (the
yields of benzaldehyde were <1%) were detected from the
hydrolysis reaction of epoxide, thus, the hydrolysis pathway is
irrelevant in our catalytic reaction, and instead the second
pathway should be the dominant pathway. The results of
selective oxidation reaction of styrene imply that the catalytic
Fig. 2 Typical TEM images and cluster size distributions of (a) Au25:SR/
CNT, (b) Au25ꢀxAg :SR/CNT, (c) Ag44:SR/CNT, (d) Au12Ag32:SR/CNT.
x
CNT ¼ carbon nanotubes.
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a
x
Table 1 The catalytic performance of Au25/CNT, Au25ꢀxAg /CNT, Ag44/CNT and Au12Ag32/CNT for the selective oxidation of styrene
c
Selectivity (%)
b
Entry
Catalytic
Solvent
Conversion (%)
Epoxide
Benzaldehyde
Other products
1
2
3
4
5
6
7
8
Au25/CNT
Ag44/CNT
Toluene
Toluene
Toluene
Toluene
Toluene and H
Toluene and H
Toluene and H
Toluene and H
78.9
49.9
77.4
74.5
78.3
45.1
76.5
74.4
63.9
74.5
74.3
60.0
19.3
5.1
29.6
17.4
23.7
35.2
73.9
94.5
95.9
41.2
6.5
8.1
<3
4.8
6.8
<3
Au25ꢀxAg
x
/CNT
Au12Ag32/CNT
Au25/CNT
2
2
2
2
O
O
O
O
Ag44/CNT
Au25ꢀxAg
x
/CNT
3.1
56.8
<3
<3
Au12Ag32/CNT
a
ꢁ
Reaction conditions: 20 mg catalyst, 2 wt% clusters loading, 57 mL (0.5 mmol) styrene, 144 mL (1.5 mmol) TBHP, 10 mg K
4 hours. Conversion ¼ (converted styrene)/(initial amount of styrene) ꢂ 100. Determined by gas chromatography with internal standard.
2
CO
3
, 2 mL solvent, 65 C,
b
c
2
a
Table 2 The hydrolysis reaction of epoxide
b
Entry
Catalytic
Solvent
Yield (%)
1
2
3
4
5
CNT
Ag44/CNT
Au25ꢀxAg /CNT
x
Au12Ag32/CNT
Au25/CNT
Toluene and H
Toluene and H
Toluene and H
Toluene and H
Toluene and H
2
O
2
O
2
O
2
O
2
O
<1
<1
<1
<1
<1
a
Reaction conditions: 20 mg catalyst, 2 wt% clusters loading, 57 mL (0.5
mmol) epoxide, 144 mL (1.5 mmol) TBHP, 1.5 mL toluene and 100 mL
ꢁ
b
H O, 10 mg K CO , 65 C, 24 h. Determined by gas chromatography.
2 2 3
Fig. 3 (a) The benzaldehyde selectivity as a function of the volume
percent of water. (b) The epoxide selectivity as a function of the vol% of
catalysts along with water-processed samples. The black line of
water (color labels: black line ¼ Au25, red line ¼ Au25ꢀxAg
x
, cyan line ¼ Fig. 5a gives the XPS signal of Au25/CNT without water, and the
Au 4f7/2 binding energy is 83.98 eV. The binding energy of Au25
Ag44, blue line ¼ Au12Ag32).
/
CNT soaked by water (Fig. 5a, red line) is almost identical to that
of Au25/CNT without water. Therefore, the water adsorption on
performance of the nanocluster catalysts correlate with their the surface of Au /CNT makes little inuence on the valence
2
5
atomic structures (i.e. the locations of Au and Ag atoms). The state of Au. Similarly, the water does not change the valence
interactions between the nanocluster and H O might be state of Au in Au2 Ag /CNT catalyst, either (Fig. 5c). By
distinctly different for the different atomic structures. The contrast, the major inuence of water on the valence state of Ag
Fourier transform infrared (FT-IR) spectroscopic analysis iden- in both Ag44 and Au
Ag can be clearly seen from Fig. 5b and
2
5ꢀx
x
25ꢀx
x
ꢀ
1
tied a band centred at 1630 cm (Fig. 4), which is assigned to d. In Fig. 5b, the binding energy of Ag 3d5/2 in Ag /CNT
44
53
ꢀ1
the dOH of co-adsorbed water. Note that the peak at 1630 cm
exists in all of these catalysts, indicating that water interacts (with water, red line). In Fig. 5d, the shi of the Ag 3d5/2 peak in
with the surfaces of all of them.
Au2 Ag /CNT changes from 367.88 eV to 368.48 eV. These data
increases from 367.78 eV (without water, black line) to 368.08 eV
5ꢀx
x
X-ray photoelectron spectroscopy (XPS) measurements were indicate the electron transfer from H O to Ag. Of note, the
2
then conducted to gure out the role of water. Fig. 5 displays the relationship between the binding energy positive shi and
54
x
XPS spectra of Au25/CNT, Ag44/CNT, and Au25ꢀxAg /CNT reduction of Ag is opposite to other elements.
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Fig. 6 XPS spectra of (a) Au 4f for Au12Ag32/CNT; (b) Ag 3d for
Au12Ag32/CNT.
x
Fig. 4 (a) FT-IR spectrum of the Au25ꢀxAg /CNT which had been
soaked with water. (b) FT-IR spectrum of soaked catalysts with water:
black line, Ag44/CNT; red line, Au25/CNT; green line, Au32Ag12/CNT.
ꢀ1
The peak at 1630 cm belongs to dOH of co-adsorbed water.
Scheme 2 Proposed mechanism for Au25 nanocluster catalyzed
styrene oxidation of (black line for dry Au25/CNT and blue line for
soaked) (color labels: green ¼ Au, red ¼ S).
catalyst (Scheme 2 black line). Thereaer, the interaction with
alkali (i.e. K CO , which was added to facilitate TBHP activa-
2
3
tion) and styrene enables the formation of the ternary ring
oxygen complex, from which the epoxide product can be
released. However, when H O is present in the solvent, FT-IR
2
spectrum of the Au25/CNT catalyst displays a peak at ꢃ1630
ꢀ
1
cm belonging to dOH of co-adsorbed water (as shown in
Fig. 4b). Comparing the Au 4f7/2 XPS spectra of these two types
Fig. 5 The XPS spectra of (a) Au 4f for Au25/CNT; (b) Ag 3d for Ag44
/
CNT; (c) Au 4f for Au25ꢀxAg /CNT; (d) Ag 3d for Au25ꢀxAg /CNT.
x
x
2
of Au25/CNT, we found that H O does not affect the valence state
of Au, so we hypothesize that H O changes the surface of Au
2
25
(
Scheme 2 blue line). Both H O and oxidant TBHP bonded with
2
catalyst Au25, forming a pentacyclic oxygen complex. When
styrene interacts with 2 , its a-C easily connects with dOH (due
to the steric hindrance of TBHP) and generates benzaldehyde as
In addition to the aforementioned analysis, the XPS
measurements were also performed on Au12Ag32/CNT samples
and water-soaked Au12Ag32/CNT samples. Similar to the obser-
vations in Fig. 5a and c, the water adsorption does not inuence
0
the primary product (Scheme 2 blue line). This mechanism is
18
supported by the isotope labelling experiment, in which the
O
the binding energy of Au 4f7/2. To our surprise, H O does not
2
benzaldehyde has been detected when the mixed solvent of
make an obvious impact on the valence state of Ag in Au Ag / 18
1
2
32
OH and toluene was used (with otherwise identical condi-
2
CNT, either (Fig. 6b).
tions as in Table 1 entry 5, see ESI† for the details of isotope
experiments).
Compared to Au25, the result of the styrene oxidation over
the Ag44 catalyst is easier to understand. Combined with the
From the XPS analysis, we proposed a possible mechanism
for Au25 catalysed styrene oxidation (Scheme 2). The reaction
starts with the binding of the oxidant TBHP onto the dry Au25
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4
Conclusion
This study presents the structure-determined catalytic results of
homometal (Au25/CNT and Ag44/CNT) and alloy nanoclusters
x
(Au25ꢀxAg /CNT and Au12Ag32/CNT) for styrene selective oxida-
tion by using TBHP as the oxygen source and toluene or mixed
toluene/water as the solvent. The catalytic activity of bimetallic
Au2 Ag nanoclusters outperforms the homogold and homo-
5ꢀx
x
silver (i.e., higher conversion than homosilver and higher
selectivity than homosilver). What is more important, it was
found that the selectivity of styrene has been discovered to be
2
controlled by an important factor – H O adsorbed on the
surface of nanoclusters. The possible route for nanocluster-
catalysed styrene selective oxidation with the action of water
has been proposed. According to the experimental measure-
Scheme 3 Schematic illustration showing Ag44/CNT (with or without
2
H O) nanocluster catalyzed styrene oxidation (color labels: light blue ¼
Ag, red ¼ S, golden ¼ O and magenta ¼ H).
ments, H O could change the surface composition of Au for
2
Au25 nanocluster and the valence state of Ag for Ag44 nano-
clusters. The advantages of both the silver and the gold have
experimental results and various characterization, the result of
the styrene oxidation catalysed by Ag44/CNT is illustrated in
Scheme 3. The water-free toluene was rst chosen as the
been well reected on the surface of doped bimetallic Au25ꢀx
-
Ag , while the core–shell structured bimetallic Au12Ag32 nano-
x
cluster gives no distinct result due to the inability of water in
affecting either the composition of Au or the charge state of Ag.
Overall, this study provides deep insight into the important of
water in the selective styrene oxidation using nanocluster
catalysts, which will be helpful for future mechanistic studies.
solvent. The binding energy for Ag 3d5/2 at 367.78 eV in Ag44
/
+
54
CNT is close to Ag , and thus we suggest that Ag44/CNT acts as
+
Ag in the styrene oxidation to epoxide. By contrast, the pres-
ence of water shis the Ag 3d peak of Ag /CNT from 367.88 eV
4
4
to 368.08 eV, and indicates the electron transfer from H O to Ag.
2
According to the XPS analysis, we suggest that the Ag of reduced
state (aer soaking in water) catalyses the selective oxidation of
styrene to benzaldehyde.
Conflict of interest
The authors declare no competing nancial interests.
Compared with the homometal nanoclusters, the bimetallic
x
Au25ꢀxAg /CNT catalysts could increase the selectivity and give
much better conversion than the homosilver nanocluster. The
Acknowledgements
FT-IR spectrum of the Au Ag /CNT also exhibits a band
centred at 1630 cm aer it is soaked by water (Fig. 4a), and
XPS spectra show an increase of 0.6 eV than the dry sample.
2
5ꢀx
x
We acknowledge nancial support from NSFC 21372006,
ꢀ1
11374013 & U1532141, the Ministry of Education, the Education
Department of Anhui Province, 211 Project of Anhui University.
Although the valence state of Au in the Au25ꢀxAg
affected by water, Au promotes the electron transfer from H
to Ag (binding energy shi, 0.3 eV vs. 0.6 eV). So the bimetallic
/CNT-catalysed styrene oxidation showed better
x
/CNT is not
2
O
Notes and references
Au25ꢀxAg
x
1
2
3
4
5
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nanoclusters.
From the aforementioned results and discussions, we
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1
2
aldehyde in styrene oxidation is caused by H O. In detail, water
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(
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