Synergistic effect in bimetallic copper-silver (CuxAg) nanoparticles enhances silicon conversion in Rochow reaction

Article abstract of DOI:10.1039/c5ra04575d

The oleylamine thermal reduction process was employed to prepare bimetallic copper-silver (Cu_xAg (0 ≤ x ≤ 50)) nanoparticles, such as Cu, Cu₅₀Ag, Cu₂₀Ag, Cu₁₀Ag, Cu₅Ag, CuAg, CuAg₂, and Ag, by using Cu(CH₃COO)₂ and AgNO₃ as the precursors. The samples were characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. The Cu_xAg hybrid nanostructure showed good particle dispersion, and Cu and Ag metals were well mixed. The catalytic properties of these bimetallic Cu_xAg nanoparticles as model catalysts for the Rochow reaction were explored. Compared to monometallic Cu and Ag nanoparticles, bimetallic Cu_xAg nanoparticles resulted in a much higher silicon conversion, which is attributed to the synergistic electronic effect between Cu and Ag metals. For example, the Cu atom was observed to have a lower electron density in the Cu_xAg bimetallic nanoparticle than that in monometallic Cu nanoparticles, which enhanced the formation of methylchlorosilanes on the silicon surface with chloromethane, demonstrating the significance of the Cu_xAg bimetallic catalysts in catalytic reactions during organosilane synthesis. The insights gained in this study should be conducive to the design of good Cu-based catalysts for the Rochow reaction.

Full text of DOI:10.1039/c5ra04575d

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Synergistic effect in bimetallic copper–silver
Cu Ag) nanoparticles enhances silicon conversion
(
x
Cite this: RSC Adv., 2015, 5, 54364
in Rochow reaction†
a
a
a
b
a
Zailei Zhang,* Yongjun Ji, Jing Li, Ziyi Zhong and Fabing Su*
The oleylamine thermal reduction process was employed to prepare bimetallic copper–silver (Cu
50)) nanoparticles, such as Cu, Cu50Ag, Cu20Ag, Cu10Ag, Cu Ag, CuAg, CuAg , and Ag, by using
Cu(CH COO) and AgNO as the precursors. The samples were characterized by X-ray diffraction,
transmission electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy.
The Cu Ag hybrid nanostructure showed good particle dispersion, and Cu and Ag metals were well
mixed. The catalytic properties of these bimetallic Cu Ag nanoparticles as model catalysts for the
Rochow reaction were explored. Compared to monometallic Cu and Ag nanoparticles, bimetallic Cu Ag
x
Ag (0 # x
#
5
2
3
2
3
x
x
x
nanoparticles resulted in a much higher silicon conversion, which is attributed to the synergistic
electronic effect between Cu and Ag metals. For example, the Cu atom was observed to have a lower
electron density in the Cu
enhanced the formation of methylchlorosilanes on the silicon surface with chloromethane,
demonstrating the significance of the Cu Ag bimetallic catalysts in catalytic reactions during
x
Ag bimetallic nanoparticle than that in monometallic Cu nanoparticles, which
Received 16th March 2015
Accepted 16th June 2015
x
DOI: 10.1039/c5ra04575d
organosilane synthesis. The insights gained in this study should be conducive to the design of good Cu-
based catalysts for the Rochow reaction.
www.rsc.org/advances
and CuRu lowered the activation energy for the hydrolytic
dehydrogenation of ammonia borane, etc.
1
. Introduction
23
Recently, the fabrication of bimetallic nanoparticles has been of
Rochow reaction was discovered in 1940s as the direct
great interest because of the ample opportunities provided to synthesis route to produce methylchlorosilanes via the reaction
24
modify and enhance their optical and electronic properties via of silicon (Si) with chloromethane on Cu-based catalysts. Over
engineering of the particle composition, structure and even the past few decades, many researches have indicated that the
1–8
geometric morphology. Many bimetallic nanoparticles, such Cu-based catalysts are the main catalysts applicable to Rochow
9
10
11
12
13
14
15
25–28
as AuCu, AgCu, RuCu, PdCu, PtCu, FeCu, NiCu,
reaction,
and Zn and ZnO are effective in promoting the Cu-
16
17
18
19
20
PdAu, PtPd, PdRu, PtCo, and PtNi have been applied in based catalyst by increasing the dimethyldichlorosilane selec-
9
10,20
29,30
aerobic oxidation, the oxygen reduction reaction,
hydroge- tivity and Si conversion.
the Sonogashira reaction, oxidation of formic are mixed with Zn or ZnO promoters both in the organosilane
hydrogen industry and in fundamental studies. In the past ve years, our
production, ethanol electro-oxidation, methanol electro- group has studied the Cu, Cu
In general, the Cu-based catalysts
1
1,16
12
nation,
acid,
1
3
14
degradation of organic contaminants,
15
17
31 32,33 34 35,36
2
O,
Cu@Cu
2
O, CuO,
Cu–
1
8
19
37,38
39
2
oxidation, and hydrogenolysis. In particular, several reports Cu O–CuO composites,
and CuCl as active components for
revealed that Cu-based bimetallic catalysts exhibited much this reaction, and found that the catalyst composition, struc-
higher activity than monometallic Cu particles in catalytic ture, size, and shape have signicant effect on it. However, the
1
reactions, e.g., CuAu improved the reduction of carbon dioxide,
above catalysts still suffer from low Si conversion, limiting their
CuPd enhanced the oxidation of cyclohexane by hydrogen practical application. Because of the presence of the synergistic
21
22
22,40
peroxide, CuAg enhanced the oxidation of methanol to CO
2
,
effect in some bimetallic catalysts,
and the vast opportuni-
ties for engineering the particle composition, size and shape,
etc., it is expected that the bimetallic catalysts may have high
State Key Laboratory of Multiphase Complex Systems, Institute of Process potentiality for Rochow reaction.
a
1,41
Engineering, Chinese Academy of Sciences, Beijing, China 100190. E-mail: zhangzl@
x
Herein, we synthesize the bimetallic Cu Ag (0 # x # 50)
nanoparticles via a oleylamine thermal reduction process using
ipe.ac.cn; su@ipe.ac.cn; Fax: +86-10-82544851; Tel: +86-10-82544850
b
Institute of Chemical Engineering and Sciences, A*star, 1 Pesek Road, Jurong Island,
Cu(CH COO) and AgNO as the precursors. The composite of
3
2
3
Singapore 627833
Cu Ag nanoparticles can be easily controlled by adjusting the
x
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra04575d
x
ratio of metal precursors. These bimetallic Cu Ag (such as
1
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Cu50Ag, Cu20Ag, Cu10Ag, Cu
5
Ag, CuAg, and CuAg
2
) nano- analysis was carried out on an ESCALAB 250Xi from Thermo
particles are used as model catalysts for Rochow reaction. Scientic Corporation using AlKa X-ray radiation. The amount of
Compared with the Si conversion for monometallic Cu (35.4% carbon deposited was measured using a CS-344 Infrared
for Cu) and Ag (0% for Ag) nanoparticles, these bimetallic Cu Ag Analyzer (Leco, US).
x
nanoparticles exhibit much enhanced Si conversion (65.4% for
5
Cu50Ag, 68.6% for Cu20Ag, 71.1% for Cu10Ag, 73.0% for Cu Ag,
2.3. Catalytic measurement
and 70.3% for CuAg), demonstrating the synergistic electronic
effect between Cu and Ag atoms in the AgCu bimetallic nano-
particles, which led to the lower electron density for Cu atoms,
in the Rochow reaction. This work provides useful clues for
improving the Cu-based catalysts to achieve higher Si conver-
sion in Rochow reaction.
The evaluation of catalysts was carried out with a typical lab
32

xed-bed reactor. The prepared Cu Ag catalysts were thermal
x
ꢀ
treatment at 350 C before the catalytic reaction. The Si powder
x
(20–50 mesh) and prepared Cu Ag catalyst together with Zn as a
promoter with a ratio of 200 : 10 : 1 were mixed homogeneously
to form a contact mass, which was loaded in the glass reactor.
The reactor system was purged with puried N
2
for 0.5 h fol-
ow rate of 25
ꢀ
2. Experimental
lowed by heating to 325 C within 1 h under a N
2
ꢁ
1
mL min . Subsequently, N was turned off, and CH Cl with a
2
3
x
2.1. Synthesis of Cu Ag (0 # x # 50) nanoparticles
ꢁ
1

ow rate of 25 mL min was introduced into the reactor to
A modied thermal decomposition process was used to
ꢀ
react with Si followed by heating to 325 C. Aer a given period
synthesize the Cu Ag bimetallic nanoparticles. AgNO (A.R.,
x
3
of 24 h, the reaction was stopped. The gas product was cooled
into a liquid phase with the circulator bath controlled at ꢁ5 C
Sinopharm Chemical Reagent Co., Ltd, China) was used as the
Ag precursor, Cu(CH COO) (A.R., Sinopharm Chemical
Reagent Co., Ltd, China) as the Cu precursor, and oleylamine
37N, Sigma-Aldrich) as the solvent, surfactant and
ꢀ
3
2
by a programmable thermal circulator (Ningbo Xinzhi biolog-
ical technology Co., LTD). The waste contact mass (solid residue
aer reaction), containing unreacted Si powder, Cu, Ag, and Zn
compounds were weighed for calculating Si conversion. The
products in the liquid solution were quantitative analyzed on an
Agilent Technologies (GC-7890A) gas chromatograph equipped
with KB-201 column and TCD detector. Gas chromatography
system was used for identication of the products, which was
18
(C H
reducing agent.
42,43
All chemicals were used as received without
further purication. In a typical synthesis for CuAg nano-
particles, 10 mmol of Cu(CH COO) and 10 mmol of AgNO
3
3
2
were added to 80 mL of oleylamine in a three neck ask, which
ꢀ
was then heated to 180 C for 8 h with stirring. The nano-
particles were washed with ethanol (A.R., Sinopharm Chemical
Reagent Co., Ltd, China) and hexamethylene (A.R., Sinopharm
Chemical Reagent Co., Ltd, China), and separated by several
rounds of centrifugation. The puried nanoparticles were then
mainly comprised of methyltrichlorosilane (CH
dimethyldichlorosilane ((CH SiCl M2), trimethylchlorosilane
(CH SiCl, M3), methyldichlorosilane (CH SiHCl , M1H),
dimethylchlorosilane ((CH ) SiHCl, M2H), low boiler (LB) and
high boiler (HB). The selectivity of the products was calculated
by the peak area ratio (in percentage). The Si conversion was
that the ratio of weight difference of contact mass (before and
aer reaction) and weight of Si (before reaction) (formula (1)).
3 3
SiCl , M1),
3
)
2
2
(
3
)
3
3
2
3
2
ꢀ
dried at 60 C for more than 24 h in a vacuum oven. Similarly,
Cu, Cu Ag, Cu Ag, Cu Ag, Cu Ag, CuAg , and Ag nano-
5
0
20
10
5
2
particles were prepared by varying the ratio of Cu and Ag
precursors. Meanwhile, Cu–Ag composites were prepared as a
comparison in the following way: 10 mmol of Cu(CH COO) was
3 2
Conversion of SiðCSiÞ ¼
added to 80 mL of oleylamine in a three-neck ask, which was
ꢀ
then heated to 180 C for 8 h with stirring. Subsequently, 10
weightcontact mass before reaction ꢁ weightcontact mass after reaction
ꢂ 100%
mmol of AgNO
3
was added to the three-neck ask and the
weightSi before reaction
solution was stirred for another 8 h. The nanoparticles were
(1)
washed with ethanol and hexamethylene via centrifugation, and
ꢀ
dried in a vacuum oven at 60 C for 24 h. In addition, Cu + Ag
composites were prepared by mixing the same weight of Cu 3. Results and discussion
nanoparticles and Ag nanoparticles.
3
.1. Catalyst characterization
.1.1. XRD analysis. Fig. 1 shows the XRD results of the
synthesized Cu Ag (0 # x # 50) nanoparticles obtained by
3
2.2. Materials characterization
x
3 2 3
X-ray diffraction patterns (XRD) were recorded on a PANalytical adding different ratio of Cu(CH COO) and AgNO precursors.
ꢀ
X'Pert PRO MPD using the Cu Ka radiation of (l ¼ 1.5418 A). For the sample Cu obtained aer only adding Cu(CH
3
COO)
2
ꢀ
The microscopic feature of the samples was characterized by precursor, diffraction peaks at 2q values of 43.5, 50.7, and 74.4

eld-emission scanning electron microscopy (FESEM) with an are observed, which are corresponded to the lattice planes of
43
energy-dispersive X-ray spectrometer (EDX) (JSM-7001F, JEOL, (111), (200), and (220) for Cu phase (JCPDS card no. 70-3039).
Tokyo, Japan) and transmission electron microscopy (TEM) When the AgNO3 precursor is added, besides the above Cu
with an EDX (JEM-2010F, JEOL, Tokyo, Japan) operated at 300 diffraction peaks, several other peaks at 2q values of 38.5, 44.5,
ꢀ
kV. Thermogravimetric (TG) analysis was carried out on an 64.7, 77.7, and 81.9 , which correspond to the lattice planes of
EXSTAR TG/DTA 6300 (Seiko Instruments, Japan) at a heating (111), (200), (220), (311), and (222) of Ag phase (JCPDS card no.
ꢀ
ꢁ1
44
rate of 2 C min in air. X-ray photoelectron spectroscopy (XPS) 04-0783), are also observed, suggesting that samples of
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Cu50Ag, Cu20Ag, Cu10Ag, Cu
5
Ag, CuAg, and CuAg
2
are composed attached on the surface of Cu nanoparticles for Cu50–Ag nano-
of both Cu and Ag phases. When the AgNO precursor is simply particles aer only adding 0.2 mmol AgNO precursor. When
3
3
added, the pure monometallic Ag nanoparticles are prepared. the AgNO precursor amount is increased to 0.5 mmol and 1.0
3
No other obvious crystalline impurities can be observed for mmol, small Ag nanoparticles are attached on the surface of Cu
these samples. Calculated over the (111) reection using the nanoparticles for Cu20–Ag (Fig. S1b†) and Cu10–Ag (Fig. S1c†).
classical Scherrer equation, the average crystallite diameters for With increase of the AgNO
Cu and Ag samples are 45.6 and 12.5 nm, respectively, sug- (Fig. S1d†), large Ag nanoparticles are attached on the surface of
gesting that the Ag nanoparticles have a smaller crystallite size Cu nanoparticles. With further increase of the AgNO precursor
3
precursor amount to 2.0 mmol
3
than Cu nanoparticles. In addition, the average crystallite amount to 10.0 mmol, the TEM image of Cu–Ag nanoparticles
diameters for Cu and Ag crystallites in Cu Ag, Cu Ag, Cu Ag, in Fig. 2e reveals that the larger Ag nanoparticles are highly
5
0
20
10
Cu Ag, CuAg, and CuAg bimetallic nanoparticles are 43.2 and dispersed on the surface of the large Cu nanoparticles. The
5
2
8
1
.5 nm, 38.6 and 8.9 nm, 29.5 and 9.6 nm, 21.7 and 10.2 nm, atomic ratio of Cu : Ag is also about 1 : 1 from the TEM-EDX
8.6 and 11.5 nm, 15.6 and 12.0 nm, respectively, indicating spectrum of Cu–Ag nanoparticles (Fig. 2f). In addition, the
that the average diameter of Cu crystallite is decreasing and Ag SEM image of Cu–Ag
crystallite is increasing with the rising amount of Ag. nanoparticles are aggregated together (not shown here). In the
.1.2. SEM and TEM characterization. Fig. 2a shows the preparation processes, the Cu nanoparticles act as a spacer
2
nanoparticles indicates Cu and Ag
3
transmission electron microscopy (TEM) image of CuAg nano- separating the Ag nanoparticles, and the size of Ag nano-
particles. The nanoparticles in the white box have a mean size of particles is increasing with the rising amount of AgNO3
1
2.2 nm and standard deviations of 2.1. The high-resolution precursor. In case with xed amount of Cu nanoparticles, it will
TEM (HRTEM) image of the CuAg nanoparticles in Fig. 2b have the xed adsorption sites for the given amounts of Cu
shows that Cu has a spacing distance of 0.21 nm, which nanoparticles. Aer adding less amount of AgNO precursor,
3
corresponds to the interplanar distance of the (111) plane of the the formed Ag nanoparticles with smaller in size are attached
Cu, while the lattice fringe spacing of 0.24 nm corresponds to on the xed adsorption sites of Cu nanoparticles. Upon further
the lattice fringe distance of the (111) plane in Ag. To further adding more AgNO precursor, the formed smaller Ag nano-
3
3
conrm the bimetallic structure and the composition of the particles can grow larger with the addition AgNO precursor.
CuAg nanoparticles, TEM-EDX image (Fig. 2c), line scan
(Fig. 2d), and TEM-EDX analysis (inset of Fig. 2c) are also carried
out. The line scan (Fig. 2d) along the direction denoted by the
white line in Fig. 2c shows that Cu and Ag are mixed well in the
nanoparticles. The TEM-EDX spectrum of CuAg nanoparticles
shown in inset of Fig. 2c demonstrates the presence of Cu and
Ag elements with an approximate atomic ratio of 1 : 1. TEM
image of Fig. S1a† shows that no obvious Ag nanoparticles are
Fig. 2 TEM (a), HRTEM (b), and TEM-EDX images (c) (inset is its EDX
Ag, spectrum), the line scan along the white line in (c) (d), TEM image (e)
and its TEM-EDX spectrum (f) of Cu–Ag nanoparticles.
Fig. 1 XRD patterns for samples Cu, Cu50Ag, Cu20Ag, Cu10Ag, Cu
CuAg, CuAg , and Ag nanoparticles.
5
2
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The TEM image of Fig. 4a shows that Cu nanoparticles with a
mean size of 73.6 nm and a standard deviation of 8.2 are
synthesized aer addition of Cu(CH COO) precursor only. The
3
2
HRTEM image in Fig. 4b indicates a spacing distance of 0.21 nm
for these Cu nanoparticles, which corresponds to the inter-
planar distance of the (111) plane. Aer addition of 0.2 mmol,
0
.5 mmol, and 1.0 mmol AgNO
obtained Cu50Ag (Fig. S1a†), Cu20Ag (Fig. S1b†), and Cu10Ag
Fig. S1c†) nanoparticles have the similar size of pure Cu
3
precursor together, the
(
nanoparticles (Fig. 4a). However, upon further increasing the
AgNO precursor amount to 5.0 mmol, 10.0 mmol, and 20.0
3
mmol, it is clearly seen that the size of Cu Ag (Fig. 3a), CuAg
5
2
(Fig. 2a), and CuAg (Fig. S1d†) nanoparticles are similar to the
pure Ag nanoparticles (Fig. 4c). Compared with Cu (Fig. 4a),
Cu50Ag (Fig. S1a†), Cu20Ag (Fig. S1b†), and Cu10Ag (Fig. S1c†)
nanoparticles, the prepared Cu
5
Ag (Fig. 3a), CuAg (Fig. 2a),
CuAg (Fig. S1d†), and Ag (Fig. 4c) nanoparticles are smaller in
2
size, consistent with the above XRD calculated results using the
classical Scherrer equation. Fig. 4d indicates that the lattice
fringe spacing of Ag is about 0.24 nm, corresponding to the
interplanar distance of the (111) plane. But we failed to obtain a
clear HRTEM image, probably because of the surface covering
Fig. 3 HRTEM image (a), TEM-EDX image (b), the line scan along the
direction denoted by the white line in (b) (c), and its TEM-EDX spec-
trum (d) of Cu Ag nanoparticles.
5
A similar morphology is observed for Cu Ag nanoparticles.
45
5
of the smaller Ag nanoparticles by oleylamine.
The HRTEM image of Cu Ag nanoparticles shown in Fig. 3a
5
3.1.3. XPS and TG analysis. We further characterized the
indicates that Cu and Ag have a spacing distance of about 0.21
nm and 0.24 nm, respectively, corresponding to the interplanar
distance of the (111) plane of the Cu and Ag phases. The line
scan (Fig. 3c) along the white line in Fig. 3b also shows that Cu
and Ag are mixed fairly in the nanoparticles. Fig. 3d reveals the
structure and composition of the Cu, Ag, and Cu Ag nano-
x
particles by Thermogravimetric (TG) and X-ray photoelectron
spectroscopy (XPS) analysis. Fig. 5a shows that, for CuAg
nanoparticles, the Cu 2p spectrum has one strong peak at 932.8
0
+
eV for Cu 2p3/2, which is close to that of Cu or Cu , but quite far
TEM-EDX spectrum of Cu Ag, indicating the presence of Cu and
2+
46
5
away from that of Cu (934.1 eV). However, two peaks at 932.6
eV and 934.6 eV are observed for pure Cu nanoparticles. These
Ag elements with an approximate atomic ratio of 5 : 1, consis-
tent with the initial atomic ratio of Cu(CH COO) and AgNO
3
3
2
0
+
2+
peaks can be attributed to Cu or Cu , and Cu , respectively,
suggesting that pure Cu nanoparticles are more easily oxidized
than Cu in CuAg nanoparticles without any deliberate oxidation
treatments. In the Ag 3d spectrum shown in Fig. 5b, the strong
peak at 368.2 eV is assigned to Ag 3d5/2 for CuAg nanoparticles,
precursors.
Fig. 4 TEM (a) and HRTEM (b) images of pure monometallic Cu
nanoparticles, and TEM (c) and HRTEM (d) images of pure mono- Fig. 5 XPS spectra for all the samples: Cu 2p (a), Ag 3d (b), and O 1s (c),
metallic Ag nanoparticles.
and TG curve for CuAg nanoparticles in air (d).
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ꢀ
and 368.0 eV to Ag 3d5/2 for pure Ag nanoparticles, indicating nanoparticles with a size range of about 1–5 nm at 180 C by
47
the metallic nature of Ag in CuAg nanoparticles, consistent shortening the reaction time to 1.5 h (Fig. S3†), in which, the Ag
with the above XRD analysis. and Cu particles were still not fused together. Furthermore, in
In addition, compared with the pure Cu and Ag nano- another control experiment, we synthesized Cu–Ag sample
particles, a peak shi is observed for bimetallic nanoparticles. (Fig. 2g) by adding and reducing Cu and Ag separately. Inter-
Ag has a higher redox potential than Cu, thus the former has estingly, the individual Cu and Ag particles were observed but
strong attraction than Cu for the electron in the Cu
tallic nanoparticles, leading to lower electron densities of Cu
atoms in the Cu Ag bimetallic nanoparticles than those in the nanoclusters and act as structure-directing agents to regulate
x
Ag bime- they formed loosely combined assemblies (Fig. 2e). Obviously,
18
C H37N can be absorbed on the surface of the Cu and Ag
x
pure Cu nanoparticles. Indeed, from Mulliken charge analysis, their surface state, inuence the nucleation and aggregation
the atomic charge of Cu in the Cu Ag bimetallic nanoparticle process of these nanoclusters, which are nally self-assembled
x
48
has a positive value. The peak shi in the XPS analysis clearly to the Cu Ag hybrid nanoparticles.
x
shows the lower electron density of Cu in the Cu
x
Ag bimetallic
nanoparticle than that of pure Cu nanoparticles, suggesting the
49
electronic interaction between the Ag and Cu. The increase of 3.3. Catalytic property
x
binding energy of Cu 2p3/2 and Cu 2p1/2 in Cu Ag bimetallic
Fig. 7 and Table 1 reveal the catalytic performance of Cu, Ag,
nanoparticles (Fig. 5a) indicates the electron transfer from Cu to
Ag, which may due to the size and the ratio of Cu : Ag in the
Cu Ag bimetallic nanoparticles. The peak increase for Ag 3d
Cu Ag bimetallic nanoparticles and commercial catalysts for
x
Rochow reaction. It can be seen that much higher Si conversion
are observed on Cu50Ag (65.4%), Cu20Ag (68.6%), Cu10Ag
x
5/2
and Ag 3d3/2 in bimetallic samples (Fig. 5b) is caused by the
strong attraction for electron by Ag than Cu. This can be
attributed to the synergistic electronic effect between Cu and
Ag. Fig. 5c displays the O1s spectra of the samples. There is one
peak for bimetallic nanoparticle, and a relative strong peak for
pure Cu nanoparticles at 531.7 eV, which can be ascribed to
copper oxide species due to the partial oxidation of the Cu
nanoparticles on the surface, further conrming that pure Cu
nanoparticles are more susceptible to oxidation than Cu in
(
5 2
71.1%), Cu Ag (73.0%), CuAg (70.3%), and CuAg (50.4%) than
that on the Cu nanoparticles (35.4%), Ag nanoparticles (0%)
and on the several reported Cu-based catalysts (Table 2), which
should be related to the synergistic electronic effect between the
Cu and Ag nanoparticles with electron transfer from Cu to Ag.
The Si conversion is rst increased with the increase of Ag
content (Cu/Ag from 1 : 0 to 5 : 1), but decreased again with
further increase of Ag content. It is understandable that the
addition of excess Ag will lead to the nal decrease of the Si
conversion, as it is Cu, not Ag, that acts as the active sites. This
can be evidenced in Table 1, the pure Ag has no activity in
conversion of Si while the pure Cu works well for it. In addition,
the top Si conversion are observed on Cu Ag (71.1%) and CuAg
x
Cu Ag nanoparticles. In addition, the atomic ratio of Cu and Ag
elements is also approximately 1 : 1 from XPS analysis. Fig. 5d
shows the TG curve for CuAg nanoparticles in air, the weight
loss of about 5.6 wt% may be derived from the trace amount of
organic residue on their surface. However, there is no further
weight increase for CuAg nanoparticles in air at higher
temperatures, probably because of formation of the CuAg alloy,
which is more resistant to oxidation even annealed them in air
at higher temperatures.
1
0
(
(
70.6%), which are obviously higher than that on Cu –Ag
10
60.6%), Cu–Ag (57.6%), and Cu + Ag (48.2%), possibly because
the Cu10Ag and CuAg nanoparticles have more proper syner-
gistic electronic interaction and local structure for Rochow
reaction than that of Cu10–Ag, Cu–Ag nanoparticles and Cu + Ag
composites, although the details are still not clear. Comparing
x
3.2. Formation process of Cu Ag (0 # x # 50) nanoparticles
The possible formation process of the Cu Ag (0 # x # 50)
x
nanoparticles is proposed according to the above XRD, TEM
and XPS analysis, and is illustrated in Fig. 6. In the rst stage,
2
+
2+
the Cu and Ag ions are reduced and nucleated to form Cu
and Ag nanoclusters under solvothermal conditions with
assistance of oleylamine (C18
surfactant as well as the reducing agent.
experiment, we also synthesized isolated Cu and Ag
H37N), which acts as the solvent,
42,43
In one control
Fig. 7 Selectivity of dimethyldichlorosilane (M2) and conversion of Si
for Cu, Ag, Cu
named Coca) for Rochow reaction. Error bars, standard deviation of
triplicated wells. The catalytic measurements were independently
x
Ag bimetallic nanoparticles and commercial catalysts
(
Fig. 6 The illustration of the formation process for Cu
x
Ag (0 # x # 50) repeated three times to obtain three M2 selectivity and Si conversion
nanoparticles.
for each catalyst.
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with pure Cu nanoparticles, the CuAg samples always have most valuable precursor in organosilane industry, and the
higher Si conversion but lower M2 selectivity. Similar phenom- bimetallic Cu Ag nanoparticles and commercial catalyst added
x
enon is observed for the commercial catalyst. Aer adding Ag with Ag nanoparticles always have lower selectivity of M2 than
nanoparticles, it shows a much higher Si conversion of 70.2%, a their counterparts without Ag. In the design of Rochow reaction
lower M2 of 72.6%, as compared with that of the commercial catalysts, it should carefully balance the Si conversion and M2
catalyst without adding Ag nanoparticles (42.3% and 82.3% selectivity by adjusting the ratio of Cu and Ag.
respectively), indicating the importance of the Ag in the Cu-based
catalyst. Since the high similarity in particle size in the prepared ited carbon aer reaction with Cu and CuAg catalyst were
Cu Ag and the commercial catalyst without Ag nanoparticles or further characterized by XRD, SEM, SEM-EDX. Fig. 8 displays
The compositions of contact masses including the depos-
x
added with Ag nanoparticles, the particle sizes of catalyst should XRD patterns of the contact masses aer reaction with Cu and
not be the main reason for the difference in Si conversion. In CuAg catalyst, in which the unreacted Si, Cu, and Ag phases
other words, the structural factors such as the synergistic elec- with CuAg catalyst, and the unreacted Si, and Cu phases with
x x
tronic effect of the Cu Ag catalysts (Ag has a higher redox Cu catalyst are detected. In Rochow reaction, Cu Si is normally
potential than Cu, and has strong attraction than Cu for the suggested as the key catalytic active species, from which M2 is
5
1,52
electron, leading to lower electron densities of Cu atoms in the produced.
x
Cu Si formed between Cu-based catalyst and Si
5
3
Cu Ag bimetallic nanoparticles) should have more important role interface is an indicator of the Cu-based catalyst activity.
x
for this catalytic reaction. As dimethyldichlorosilane (M2) is the When Cu-based catalyst and Si are brought together at
elevated temperatures, Cu Si is formed, from which methyl-
x
chlorosilanes can be produced. In addition of Cu, Ag, and Si
peaks, a small peak of Cu Si is observed in the XRD patterns of
the reacted contact masses with CuAg catalyst, suggesting that
the bimetallic CuAg nanoparticles are more active in gener-
x
x
Table 1 Catalytic performance of Cu, Ag, Cu Ag bimetallic nano-
particles and commercial catalysts (named Coca)
Product composition (%)
x
ating Cu Si active species. We also found there is a shorter
induction period for bimetallic CuAg nanoparticles (about 2 h)
as compared to the monometallic Cu nanoparticles (about
Sample
M1
M2
M3 M1H M2H LB
HB
CSi (%)
1
0 h), possibly because of the higher activity of the bimetallic
Cu
18.3 71.3 2.8
19.4 68.4 2.1
20.8 66.5 2.2
22.4 65.9 1.9
25.4 60.4 2.1
24.9 62.6 1.9
28.3 58.6 2.0
33.2 52.4 2.1
30.3 56.9 2.0
28.9 56.4 2.1
2.7
6.4
6.5
6.9
6.7
6.3
6.9
6.7
6.4
6.8
0
1.1
0.9
0.8
0.8
1.9
0.8
0.8
1.9
1.2
0.9
0
0.1 3.7
0.2 2.6
0.2 3.0
0.1 2.0
0.2 3.3
0.5 3.0
0.3 3.1
0.2 3.5
0.2 3.0
0.4 4.5
35.4
65.4
68.6
71.1
60.6
73.0
70.6
57.6
48.2
50.4
0
Cu50Ag
Cu20Ag
Cu10Ag
Cu10–Ag
Cu Ag
5
CuAg
Cu–Ag
Cu + Ag
CuAg
Ag
CuAg nanoparticles.
The scanning electron microscopy (SEM) and SEM-EDX
analysis results for the contact masses aer reaction with Cu
and CuAg catalysts are shown in Fig. 9. Fig. 9a reveals the SEM
image of the reacted contact mass with Cu catalyst, in which a
large number of deposited carbon bers are formed. The EDX
spectrum (Fig. 9b) demonstrates that the contact mass aer
the reaction mainly consists of C, O, Si, and Cu, of which C and
Si atoms are the most predominant. However, SEM image
2
0
0
0
0
0
Coca
12.2 82.3 2.4
0.1
0.1
0.2
0.2
0.4 2.4
2.5 2.3
42.3
70.2
Coca + Ag 19.7 72.6 2.6
Table 2 A comparison of the Si conversion for Cu
several reported Cu-based catalyst
x
Ag catalysts with
a
Sample
Cu Ag nanoparticles
CuAg nanoparticles
Porous Cu microparticles
C
Si (%)
Ref.
5
73.0
70.6
33.5
33.2
40.4
29.2
38.8
41.6
23.4
32.4
24.8
ꢃ12
ꢃ20
This work
This work
31
32
33
34
35
36
37
38
39
50
29
Mesoporous Cu
Hexahedron Cu
2
O microspheres
O microparticles
2
2
Core–shell Cu@Cu O microspheres
Flower-like CuO microparticles
Dandelion-like CuO microparticles
2
CuO–Cu O–Cu nanoparticles
Cu–Cu O–CuO microparticles
2
Tetrahedra CuCl microparticles
CuCl particles
CuCl particles
a
ꢀ
ꢀ
Ref. 29 Si : catalyst : Zn ¼ 200 : 10 : 1, temperature 300 C; ref. 31–39
Si : catalyst : Zn
¼
100 : 10 : 1, temperature 325
C; ref. 50
ꢀ
Si : catalyst : Zn ¼ 100 : 10 : 0.06, temperature 300 C; this work Fig. 8 XRD patterns of the contact masses after reaction with Cu and
ꢀ
Si : catalyst : Zn ¼ 200 : 10 : 1, temperature 325 C.
CuAg catalyst.
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Acknowledgements
The authors gratefully acknowledge the supports from the
National Natural Science Foundation of China (no. 51402299
and 51272252) and Hundred Talents Program of the Chinese
Academy of Sciences.
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