Heterojunctions generated in SnO2-CuO nanocatalysts for improved catalytic property in the Rochow reaction

Article abstract of DOI:10.1039/c5ra10996e

We report the improved catalytic performance of SnO₂-CuO hybrid nanocatalysts synthesized by rationally designing and controlling the local heterojunction structure. The SnO₂ nanoparticle (NP) decorated CuO nanorods (NRs) (SnO₂-CuO) with a mace-like structure and with various CuO:SnO₂ ratios were prepared via depositing pre-synthesized SnO₂ NPs on CuO NRs in the presence of polyvinylpyrrolidone molecules. The CuO NRs were obtained by a facile hydrothermal reaction using Cu(NO₃)₂·3H₂O as the precursor. The samples were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction analyses. The results indicated that in the SnO₂-CuO hybrid nanostructures, the heterojunctions were well generated as the SnO₂ NPs were well dispersed on the CuO NRs. Their catalytic performances were then explored via the Rochow reaction, in which solid silicon (Si) reacts with gaseous methyl chloride (MeCl) to produce dimethyldichlorosilane (M2). Compared to separate CuO and SnO₂ as well as their physical mixture, the SnO₂-CuO hybrids exhibit significantly enhanced M2 selectivity and Si conversion because of the enhanced synergistic interaction between SnO₂ and CuO due to the generated heterojunctions. This work demonstrates that the performance of heterogeneous catalysts can be improved by carefully designing and controlling their structures even when their composition remains unchanged.

Full text of DOI:10.1039/c5ra10996e

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Heterojunctions Generated in SnO -CuO Nanocatalysts for Improved
2
Catalytic Property in Rochow Reaction
a,b
b,*
b
b
Shanying Zou, Yongjun Ji, Guangna Wang, Yongxia Zhu,
b
a
a,*
c
b,*
Hezhi Liu, Lihua Jia, Xiangfeng Guo, Ziyi Zhong and Fabing Su
a
College of Chemistry and Chemical Engineering, Qiqihaer University, Qiqihaer 161006,
Heilongjiang Province, China.
b
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese
Academy of Sciences, Beijing 100190, China.
c
School of Chemical & Biomedical Engineering, Nanyang Technological University, 62 Nanyang
Drive, Singapore 637459.
*
To whom correspondence should be addressed. Eꢀmail address: yjji@ipe.ac.cn (Y. Ji),
xfguo@163.com (X. Guo), fbsu@ipe.ac.cn (F. Su), Tel.: +86ꢀ10ꢀ82544850, Fax: +86ꢀ10ꢀ82544851.
1

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Abstract
We report the improved catalytic performance of SnO
2
ꢀCuO hybrid nanocatalysts synthesized
by rationally designing and controlling the local heterojunctions structure. The SnO
NPs) decorated CuO nanorods (NRs) (SnO ꢀCuO) with a maceꢀlike structure and with various
CuO : SnO ratios were prepared via depositing preꢀsynthesized SnO NPs on CuO NRs in the
2
nanoparticles
(
2
2
2
presence of polyvinylpyrrolidone molecules. The CuO NRs were obtained by a facile hydrothermal
reaction using Cu(NO •3H O as the precursor. The samples were characterized by Xꢀray
3
)
2
2
diffraction, transmission electron microscopy, scanning electron microscopy, Xꢀray photoelectron
spectroscopy, and temperatureꢀprogrammed reduction analyses. The results indicated that in the
SnO ꢀCuO hybrid nanostructures, the heterojunctions were well generated as the SnO NPs were
2
2
well dispersed on the CuO NRs. Their catalytic performances were then explored via the Rochow
reaction, in which solid silicon (Si) reacts with gaseous methyl chloride (MeCl) to produce
2
dimethyldichlorosilane (M2). Compared to discrete CuO and SnO as well as their physical mixture,
the SnO ꢀCuO hybrids exhibit significantly enhanced M2 selectivity and Si conversion because of
2
the enhanced synergistic interaction between SnO and CuO due to the generated heterojunctions.
2
This work demonstrates that the performance of heterogeneous catalysts can be improved by
carefully designing and controlling its structures even maintaining the composition unchanged.
Keywords: CuO nanorods; SnO
2
nanoparticles; heterojunctions; catalytic property; Rochow
reaction.
2

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1
. Introduction
Because of the obvious growth of the silicone industry in the past few decades, there has been
evidenced a stable increase in the production of methylchlorosilanes (MCSs), which serve as the
primary monomeric intermediates for the manufacture of silicone products via the Rochow
1
,2
reaction. In this reaction, gaseous methyl chloride (MeCl) reacts with silicon (Si) in the presence
3
ꢀ5
of Cuꢀbased catalysts as following:
(
CH ) SiCl
3
3
(
CH ) SiCl
2
3
2
CH SiCl
3
3
copper catalyst
CH HSiCl
2
(1)
Si + MeCl
3
3
00ꢀ350oC
(
CH ) HSiCl
3 2
other low boilers
residue
Among these products, dimethyldichlorosilane ((CH
3
)
2
2
SiCl , M2) is the most important monomer
6
for the production of organosilane products in industry. Therefore, a high M2 yield is always
demanded. As reported, the catalysts used in the Rochow reaction are various Cuꢀbased catalysts,
7
8,9
10
11
12
13
including metallic Cu powders, CuCl, Cu
2
O, Cu, CuꢀSi alloy, and CuꢀCu
2
OꢀCuO, and the
5,14
promoters include Snꢀbased additives. In the past few years, porous cubic structures such as Cu
1
5
10
16
microparticles, mesoporous Cu
2
O microspheres, flowerꢀlike, and hierarchical dandelionꢀlike
1
7
CuO microspheres have been prepared in our group as model catalysts for the Rochow reaction,
18
and found that their catalytic performance mainly depends on the chemical composition, particles
19
16
size, and surface structure. However, the above catalysts still suffer from low M2 selectivity
and/or Si conversion, which seriously hinder their further application. Therefore, more efficient
Cuꢀbased catalysts should be developed.
Recently, CuOꢀbased heteroꢀstructured hybrid materials have caused strong attention owing to
their distinct properties and high diversity in structure and composition. For instance,
3

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Avgouropoulos et al. prepared a series of CuOꢀCeO
performance for the selective oxidation of CO than CuO alone. Li et al. observed that the
CuOꢀZnOꢀZrO catalysts synthesized by a surfactantꢀassisted coprecipitation method showed higher
catalytic activity than that of sole CuO as a result of more intimate contact at the interface between
2
catalysts which exhibited much better catalytic
2
0
2
21
Cu species and ZnO and/or ZrO . More recently, we have grown flowerꢀlike ZnO on urchinꢀlike
2
CuO microspheres using a simple solvothermal method, and the hierarchical structure displayed
better catalytic properties in M2 synthesis in comparison with the single CuO phase, probably
2
2
because of the enhanced synergistic effect between ZnO and CuO. However, the composite
particles were still in the micrometer scale with limited particle contact and weak synergistic effect.
On the other hand, to date there are few reports on preparing Cuꢀbased nanocatalysts for Rochow
reaction, which may show superior catalytic properties to their bulk counterparts. In addition,
lowering the particle size to nanoscale may create more intimate contacts at the interfaces which
will offer more possibilities to tune the catalytic property. This motivated us to design Cuꢀbased
nanoscale heterostructures for Rochow reaction. Furthermore, considering that SnO
promoter in Rochow reaction, we prepared CuO nanorods (NRs) and then rationally loaded
preꢀmade SnO nanoparticles (NPs) on them to construct a novel maceꢀlike hybrid as the model
catalyst. Compared with single CuO and SnO phases as well as their physical mixture, the
SnO ꢀCuO maceꢀlike hybrid nanocatalysts did show much higher M2 selectivity and Si conversion,
2
is an effective
2
2
2
obviously originated from the enhanced synergistic effect between CuO and SnO2. Also, the
relationship between the enhanced catalytic performance and the structure is studied.
2
2
. Experimental
.1. Material synthesis
4

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Copper nitrate trihydrate (Cu(NO
ammonia solution (NH
CH COCH , A.R., 99.5%) were purchased from Xilong Chemical Reagent Co. Ltd., China.
Stannic chloride pentahydrate (SnCl •5H O, A.R., 99.0%) and polyvinylpyrrolidone (PVP, MW 40
00) were obtained from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. All chemicals
3
)
2
•3H
2
O, A.R., 99.0%), ethanol (CH
3
CH
2
OH, A.R., 99.0%),
4
OH, A.R., 25.0ꢀ28.0%), sodium hydroxide (NaOH, A.R., 99.0%), acetone
(
3
3
4
2
0
were used as received without further purification.
The CuO NRs were prepared by a facile hydrothermal method. In a typical synthesis, 0.1 g of
3 2 2
Cu(NO ) •3H O and 0.003 g of PVP were dissolved into 3.0 mL of ethanol to form a
homogeneous solution, followed with addition of 10.0 mL of NaOH aqeous solution (5.0 M). The
obtained gel was transferred into a 100 mL of Teflonꢀlined stainlessꢀsteel autoclave, which was
o
sealed and maintained at 140 C for 24 h before it was cooled down to room temperature. Finally,
the resulting solid precipitate was collected by filtration, washed with deionized water and ethanol
o
for several times, and dried at 60 C for 8 h. On the other hand, the SnO
2
NPs were synthesized by
O as the precursor and NH OH as
the precipitating agent. Briefly, 8.8 g of SnCl •5H O was dissolved in 250.0 mL of deionized water
23
modifying the previously reported procedures, using SnCl
4
•5H
2
4
4
2
4
to get a clear solution with a Sn ion concentration of 0.1 M，and then NH OH solution was added
dropwise under vigorous magnetic stirring until the PH value reached ～7. After maintained for 1 h,
the precipitate was filtered, washed with deionized water and acetone for three times, and dried at
8
0 °C for 6 h. It was further calcined in air at 600 °C for 3 h to obtain the SnO
2
NPs.
SnO ꢀCuO hybrid maceꢀlike nanocatalysts were synthesized by adding a desirable amount of
2
preꢀmade SnO
2
NPs into a CuO NRs suspension in ethanol in the presence of PVP as shown in Fig.
1
. Typically, 0.1 g of CuO NRs were firstly dispersed in 100.0 mL of ethanol, followed with
addition of the desired amounts of SnO
2
and 0.005 g of PVP under continuous stirring. After
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mechanically stirred for 12 h at room temperature, the product was collected by filtration and
o
ethanol wash, and further dried at 60 C for 8 h. The synthesis was carried out by fixing the CuO
amount at 0.1 g while varying the amount of added SnO . The prepared samples are thus denoted as
2
0
.1 wt.% SnO
SnO ꢀCuO and 10.0 wt.% SnO
SnO was 0.1, 0.2, 0.5, 1, 5 and 10 mg, respectively. For control experiment, a physical mixture of
2
ꢀCuO, 0.2 wt.% SnO
2 2 2
ꢀCuO, 0.5 wt.% SnO ꢀCuO, 1.0 wt.% SnO ꢀCuO, 5.0 wt.%
2
2
ꢀCuO according to their measured compositions when the amount of
2
SnO
2
NPs and CuO NRs was prepared by mixing two samples at a mass ratio of 1:100. The sample
+CuO.
is denoted as 1.0 wt.% SnO
2
2
Fig. 1 Preparation process of the SnO ꢀCuO hybrid nanocatalysts.
2
.2. Characterization
XRD analysis was performed on a PANalytica X’Pert PRO MPD using the Cu Kα radiation
λ=1.5418 Å) at 40 kV and 40 mA. The size of the sample was calculated using the DebyeꢀScherrer
(
equation. The microscopic features of the samples were observed with field emission scanning
electron microscopy (SEM) with energy dispersive spectroscopy (EDS) (JSMꢀ6700F, JEOL, Tokyo,
Japan) and transmission electron microscopy (TEM) (JEMꢀ2010F, JEOL, Tokyo, Japan). The
HAADFꢀSTEM images were obtained by using 200 kV STEM (JEMꢀARM200F, JEOL, Tokyo,
6

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o
Japan). N
2
adsorption at ꢀ196 C was measured using a Quantachrome surface area and pore size
o
analyzer NOVA 3200e. Prior to the measurement, the sample was degassed at 300 C for 3 h under
vacuum. The specific surface area was determined according to the BrunauerꢀEmmettꢀTeller (BET)
2 2
method in the relative pressure range of 0.05ꢀ0.3. H temperatureꢀprogrammed reduction (H ꢀTPR)
was carried out on a Quantachrome automated chemisorption analyzer (ChemBET pulsar
TPR/TPD). Briefly, 0.2 g of sample was loaded in a quartz Uꢀtube and heated from room
ꢀ1
temperature to 200 °C at 10 °C min and maintained for 1 h in Ar flow. Then, the sample was
ꢀ1
cooled to room temperature and followed by heating to 800 °C at 10 °C min under a binary gas
ꢀ1
(
2
10 vol % H /Ar in TPR) with a gas flow of 30 mL min . Xꢀray photoelectron spectroscopy (XPS)
were taken on ESCALAB 250 (VG) using AlꢀKα Xꢀray source (hv = 1486.6 eV) radiation. The
elemental analysis was carried out with Inductively Coupled Plasma Optical Emission Spectrometer
(ICPꢀOES) (Optima 5300DV, perkin Elmer, American).
2
.3 Catalytic Measurement
The evaluation of catalytic performance was carried out with a typical MCS lab fixedꢀbed
11
reactor. 10.0 g of Si powder (20ꢀ50 mesh, provided by Jiangsu Hongda New Material Co., Ltd.)
was homogeneously mixed with 0.5 g obtained sample and 0.05 g of Zinc (Zn, A.R., Sinopharm
Chemical Reagent Co., Ltd.) as a promoter to form a contact mass, which was loaded in the glass
o
reactor. The reactor system was initially preheated for 1 h by heating to 310 C under N
2
gas flow
ꢀ1
ꢀ1
(
30 mL min ). Subsequently, N gas was turned off and MeCl gas with a flow rate of 25 mL min
2
o
was introduced into the reactor to react with Si at 295 C. After a given period of 24 h, the reaction
was terminated. The gas product was cooled down into liquid phase with a water circulating bath
o
controlled at ꢀ5 C by a programmable thermal circulator (GDH series, Ningbo xinzhi biological
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technology Co., LTD). The waste contact mass (residual solid after reaction) containing unreacted
Si powder, Cu compounds, and promoters was weighed to calculate Si conversion, while the
collected liquid was quantitatively analyzed on a gas chromatograph (GC) (Agilent Technologies
7
890A, Thermal conductivity detector, KBꢀ201 column). The products were identified by gas
chromatography mass spectrometry (GCꢀMS) (QP2010, SHIMADZU), which were mainly
comprised of methyltricholorsilane (CH SiCl , M1), 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
3
3
3 3
)
3
2
3 2
)
(
percentage). The Si conversion (CSi) was calculated according to the following formula:
weight contact mass before reaction ꢀ weight contact mass after reaction
×100%
(
2)
Conversion of Si (C ) =
Si
weight Si before reaction
3
3
. Results and discussion
.1. Characterization of synthesized materials
The SEM images in Fig. 2 show the morphology and size of the obtained products. As
indicated in Fig. 2a and Fig. S1 in Supporting Information, the asꢀprepared SnO NPs are nearly
2
spherical with a size range of 20ꢀ40 nm. Fig. 2b shows that the asꢀobtained CuO appears in the form
of rods with a length of approximately 0.7ꢀ1 ꢁm and a diameter of 20ꢀ40 nm, and the surface looks
2
quite smooth and distinct. After the addition of a small amount of SnO NPs into the CuO NRs
suspension of ethanol in the presence of PVP and after stirring at room temperature for 12 h, both
the morphology and the crystal size of CuO NRs are intact, but the surface becomes rougher due to
coating of SnO
2
NPs on the surface of the CuO NRs, suggesting the successful synthesis of
NPs, a series of SnO ꢀCuO
NPs, more
SnO ꢀCuO hybrids (Fig. 2c). By simply varying the amount of SnO
2
2
2
hybrids can be prepared using this method. Upon further increasing the amount of SnO
2
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dense SnO
S2 in Supporting Information). The elemental mapping images (Fig. 2eꢀh) show the elemental
distribution in 1.0 wt.% SnO ꢀCuO over large areas, confirming the presence of Cu, O and Sn and
2
NPs are loaded on CuO hybrids but still maintain the maceꢀlike shape (Fig. 2d and Fig.
2
their homegeneous distribution. The EDS analysis and lineꢀscanning results further verify the
presence of Cu, Sn and O elements (Fig. 2i and Fig. S3 in Supporting Information). Moreover, the
Sn : Cu atomic ratios determined by ICPꢀAES confirm that the compositions of the final products
are close to the initial feeding ratio of the components. The specific surface area of SnO
hybids, measured from N adsorption isotherms at 77 K, are decreased obviously compared to CuO
and SnO , as shown in Table 1.
2
ꢀCuO
2
2
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Fig. 2 SEM images of SnO
d), 1.0 wt.% SnO
SnO ꢀCuO, and corresponding EDS spectrum (i).
2
2 2
NPs (a), CuO NRs (b), 1.0 wt.% SnO ꢀCuO (c), 5.0 wt.% SnO ꢀCuO
(
2
ꢀCuO (e), elemental mapping images of Cu (f), O (g), Sn (h) of 1.0 wt.%
2
2 2
Fig. 3a shows the XRD patterns of CuO NRs, SnO NPs, and the SnO ꢀCuO hybrids with
various ratios. All samples have high diffraction intensity, indicating their good crystallinity. In the
o
case of pure CuO NRs (Fig. 3a), the observed diffraction peaks are located at 2θ values of 32.5 ,
o
o
o
o
o
o
o
o
o
o
3
5.6 , 38.7 , 48.8 , 53.4 , 58.2 , 61.6 , 66.3 , 68.0 , 72.3 and 75.0 , corresponding respectively to
the lattice planes of (110), (ꢀ111), (111), (ꢀ202), (020), (202), (ꢀ113), (ꢀ311), (220), (311) and (004)
10

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of monoclinic CuO (JCPDS No. 03ꢀ065ꢀ2309). For pure SnO
2
NRs, the diffraction peaks at 2θ
o
o
o
values of 26.2 , 33.9 and 51.8 can be indexed to the lattice planes of (110), (101) and (211) of
tetragonal SnO (JCPDS No. 01ꢀ077ꢀ0452). However, in addition to the above CuO diffraction
2
peaks, no peaks of SnO
in the hybrid that goes beyond the detection limit. With the increase of SnO
peak of tetragonal SnO becomes visible and progressively increased in intensity (5.0 wt.%
2
can be traced for 1.0 wt.% SnO
2
ꢀCuO due to the very small ratio of SnO
2
2
content, the diffraction
2
SnO
2
ꢀCuO hybrid to 10.0 wt.% SnO
2
ꢀCuO hybrid), as observed clearly by an enlarged view of the
o
XRD patterns in the 2θ range of 24ꢀ30 (Fig. 3b). The grain size of CuO and SnO
2
in each sample
o
o
was calculated by using the Scherer formula based on the 2θ values of 35.5 and 26.6 , respectively,
and these data are compiled in Table 1. It can be seen that the grain size of the CuO and SnO
2
in the
SnO ꢀCuO hybrids are in the same range as those of pure CuO, SnO , and their mechanical mixture.
2
2
Fig. 3 XRD patterns of CuO NRs, SnO NPs, 1.0 wt.% SnO ꢀCuO, 5.0 wt.% SnO ꢀCuO, and 10.0
2
2
2
o
wt.% SnO
2
ꢀCuO (a), enlarged view in the 2θ range of 24ꢀ30 of 5.0 wt.% SnO
2
ꢀCuO and 10.0 wt.%
SnO ꢀCuO (b).
2
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Table 1 The physical parameters of all the samples.
a
a
b
TPR
o
c
o
d
BET
Sample
d
CuO
d
SnO
2
T
( C)
T
M
( C)
S
2
ꢀ1
(
nm)
(nm)
ꢀ
(m g )
17.2
29.4
5.5
CuO
28.4
ꢀ
180ꢀ280
450ꢀ690
180ꢀ350
150ꢀ370
165ꢀ380
180ꢀ490
242
604
281
274
256
324
SnO
2
21.9
ꢀ
1
5
1
.0 wt.% SnO
2
ꢀCuO
30.7
30.4
31.2
28.4
.0 wt.% SnO ꢀCuO
20.1
21.5
21.9
7.4
2
0.0 wt.% SnO
2
ꢀCuO
8.6
1
.0 wt.% SnO +CuO
2
2.5
a
2
The crystal size of CuO and SnO calculated from the XRD patterns.
b
c
d
Reduction temperature range of the samples.
Temperatures at the peak maximum.
The BET surface area of the samples.
Fig. 4 shows the TEM images of the prepared samples, which are all highly crystallized. As
shown in Fig. 4a, the asꢀprepared CuO sample has rod morphology with a diameter of 200ꢀ400 nm,
in agreement with the above SEM results (Fig. 2b). Figure. 4b indicates that the asꢀobtained SnO
exhibits an almost spherical shape with an average size of about 25 nm. In the case of SnO ꢀCuO
2
2
2
hybrid, a large number of SnO NPs can be seen deposited on the surface of CuO NRs uniformly,
forming maceꢀlike structure (Fig. 4c and Fig. S4, S5 in Supporting Information). Representative
magnified highꢀresolution transmission electron microscope (HRTEM) images as shown in Fig. 4d
and 4e reveal that the SnO
spacing of about 0.25 nm and 0.33 nm, which correspond to (ꢀ111) and (110) planes of CuO and
SnO , respectively. Also, the interfacial regions between CuO and SnO indicated by white lines are
2
ꢀCuO hybrid sample has a high crystallinity, possessing clear lattice
2
2
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clearly observed.
Fig. 4 TEM images of CuO NRs (a), SnO
images of 1.0 wt.% SnO ꢀCuO (d, e).
2
NPs (b) and 1.0 wt.% SnO
2
ꢀCuO (c), and HRTEM
2
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A further characterization of the structure of the typical 1.0 wt.% SnO
2
ꢀCuO hybrid was
carried out by high angular annular dark field scanning transmission electron microscopy
HAADFꢀSTEM) (Fig. 5a, b), which displays obvious hybrid maceꢀlike nanostructures, consistent
with the above SEM and HRTEM observations. The EDS mapping results in Fig. 5(cꢀf)
demonstrates the actual distributions of Cu, Sn, and O elements separately in the single SnO ꢀCuO
(
2
hybrid. Clearly, all of them are evenly dispersed, matching well with the results of the above SEM
characterizations (Fig. 2fꢀh). In short, these characterizations support the successful synthesis of the
SnO
2
ꢀCuO maceꢀlike nanostructure with SnO
2
NPs loaded on the CuO NRs uniformly using our
strategy.
Fig. 5 TEM image (a), HAADFꢀSTEM image (b), and combined mapping image (c) of 1.0 wt.%
SnO
2
ꢀCuO, elemental mapping images of Cu (d), Sn (e), and O (f).
It should be pointed out that the presence of PVP is crucial for the successful preparation of the
ꢀCuO hybrids. In a control experiment without using PVP, plentiful preꢀmade SnO NPs were
SnO
2
2
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directly added to the CuO NRs suspension in ethanol and the mixture was further stirred for 24 h.
TEM images of the resulting product (Fig. S6 in Supporting Information) show that most of the
2
SnO NPs are not deposited on the surface of the CuO NRs and become severely agglomerated,
indicating that PVP attached on the CuO NRs provides a strong anchor on the surface for the SnO
species.
2
3
.2. Catalytic property
Table 2 shows the catalytic performance of all the samples for M2 synthesis via the Rochow
o
reaction at 295 C under atmospheric pressure for 24 h. As displayed, the SnO
2
NPs appeares to be
inactive. In contrast, the pure CuO NRs give a low M2 selectivity of 40.1% and Si conversion of
2.5% under the same reaction conditions. The catalytic performance for physically mixed CuO
NRs and SnO NPs is not improved, in which the Si conversion and M2 selectivity should be
contributed by the CuO NRs. Interesingly, the 1.0 wt.% SnO ꢀCuO hybrid nanocatalysts gives a
1
2
2
drastically enhanced M2 selectivity (78.9%) and Si conversion (56.3%) under identical conditions.
We have previously reported that the catalytic activities of CuO catalysts can be increased upon
22
forming composites with ZnO, which is attributed to the synergetic effects of CuO with ZnO.
Similarly, the enhanced activity of the hybrids in this work should be caused by the synergistic
interaction between CuO and SnO . However, the catalyst performance becomes poorer
dramatically again with the further increase of SnO /CuO ratio, possibly due to the excess SnO
would block the active sites of CuO. It is also found that the catalyst performances decreases when
the SnO /CuO ratios are less than 1%, indicating that the optimized ratio is 1.0 wt.%. Furthermore,
the catalytic property of the 1 wt.% SnO ꢀCuO hybrid is compared with that of previous reported
see Table S1). Although the test condition such as catalyst amount and reaction temperature is
2
2
2
2
2
(
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different among these catalysts, it is found that our catalyst, 1 wt.% SnO
2
ꢀCuO, exhibited excellent
M2 selectivity (90.1%) and yield (45.5%) even using much lower amount. It should be pointed out
here that the error bars for both conversion and selectivity are ±0.1% and all the catalytic data
obtained by at least three repeated experiments using the same silicon. These results demonstrate
that the prepared catalysts possessing nanoscale heterostructures with proper CuO and SnO ratio
2
have the best catalytic performance. In addition, byproducts such as M1, M3, M1H, M2H, LB, and
HB with a negligible difference over the various prepared catalysts are also detected except for
SnO
2
sample.
Table 2 Catalytic performance of all the catalysts for Rochow reaction.
Sample
CuO
Product selectivity (%)
CꢀSi (%)
M1
M2
40.1
ꢀ
M3
1.8
ꢀ
M1H
2.6
ꢀ
M2H
4.7
ꢀ
LBR
12.9
ꢀ
HBR
22.6
100
3.4
26.3
ꢀ
12.5
3.1
SnO
2
0
0
0
1
5
1
1
.1 wt.% SnO
2
ꢀCuO
18.5
17.2
11.9
11.2
13.3
15.4
16.2
70.9
76.5
78.1
78.9
25.4
5.6
1.9
0.7
1.4
1.2
1.6
2.7
1.7
0.4
0.2
0.2
0.6
0.6
0.3
5.2
2.9
1.6
1.9
2.3
2.9
1.1
2.7
2.0
1.1
1.7
1.6
3.5
4.6
16.9
18.9
22.3
32.1
56.3
10.2
7.9
.2 wt.% SnO ꢀCuO
2.7
2
.5 wt.% SnO
.0 wt.% SnO
.0 wt.% SnO
2
2
2
ꢀCuO
ꢀCuO
ꢀCuO
4.8
4.2
52.7
70.3
36.0
0.0 wt.% SnO
2
ꢀCuO
.0 wt.% SnO +CuO
21.3
5.9
2
Fig. 6a shows the XRD patterns of the waste contact masses (catalysts mixed with silicon after
o
reaction at 295 C). It can be seen that the waste contact masses are composed of Si and Cu, but
16

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lack of CuO and Cu
the lattice oxygen of the Cuꢀbased catalysts. An enlarged view of the XRD patterns in the range of
0ꢀ75° (Fig. 6b) for CuO, 1.0 wt.% SnO ꢀCuO hybrid and 1.0 wt.% SnO +CuO hybrid show the
presence of Cu Si species, suggesting the formation of alloyed Cu Si active intermediate during the
reaction. In the Rochow reaction, Cu Si is normally regarded as the key catalytic active specie,
2
O species. The formation of Cu may originate from the reaction of MeCl with
2
4
4
2
2
3
x
25
3
through which, MCSs are produced. When Cu and Si are brought together at elevated temperatures
2
6
in the presence of MeCl gas, Cu
the M2 selectivity and Si conversion. As noticed, the intensity of Cu
SnO ꢀCuO is much higher than that of the others, suggesting that 1.0 wt.% SnO
3
Si will be formed. The amount of Cu
3
Si can substantially affect
Si peaks for 1.0 wt.%
ꢀCuO hybrid is
11
3
2
2
more active in generating Cu Si than the others. This may be due to the proper proportion of SnO2
x
and CuO creates a stronger synergistic effect and more intimate contacts between the catalyst and
the solid Si, thus promoting the formation of active Cu
enhance the catalytic activity for M2 production.
3
Si phases, which is the key contribution to
To further study the synergetic effect of CuO and SnO
2
, the catalysts were investigated by
H ꢀTPR. Fig. 6c shows the H ꢀTPR curves of all the samples. The peak temperatures (T ) and
2
2
M
reduction temperature range of the samples are compiled in Table 1. As shown, for the pure CuO
o
and SnO
2
samples, the reduction peaks are clearly observed in the range of 180ꢀ280 C and 450ꢀ690
o
2
C, respectively. However, in the case of the 1.0 wt.% SnO ꢀCuO sample, there appear two
reduction peaks and the peak position shifts to the lower temperature range as compared with those
of the sole CuO and SnO , respectively. The above shifts should be attributed to synergistic
2
27ꢀ29
interaction between CuO and SnO
2
.
We further carried out the XPS analysis to elucidate the interaction between the two
18
components. As shown in Fig. 6d, the characteristic intense peaks at 934.3 and 953.7 eV are
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assigned to Cu2p3/2 and Cu2p1/2 of CuO NRs with binding energy calibrated with C1s = 284.8
30
10
eV. The peak around 943.8 eV is attributed to the shakeup line of CuO. However, the binding
energy of Cu2p3/2 and Cu2p1/2 in the SnO ꢀCuO hybrids are decreased by 0.35 and 0.33 eV
respectively as compared with that of CuO NRs, and the shakeup line at 943.8 eV is almost
2
31
disappeared, implying the change in the chemical state of Cu. In addition, for SnO NPs, the
2
binding energies of Sn 3d5/2 and Sn 3d3/2 are located at 495.4 and 487.0 eV, respectively, which
3
2
are similar to those for SnO
2
(Fig. 6e). But in the case of 1.0 wt.% SnO
2
ꢀCuO hybrid, the
indicates
the electronꢀdeficient state of Sn. Hence, the above observed peak shifts should be attributed to
observation of the higher, weaker and broader peaks compared to those of the single SnO
2
33
34
electron transfer from SnO to CuO, which leads to strong synergistic interaction in the SnO ꢀCuO
2
2
hybrid, thus enhancing the catalytic activities for the Rochow reaction. In addition, the O 1s peak
was located at 529.8 eV and 530.8 eV for the CuO and SnO
2
, respectively, as shown in Fig. 6f,
3
5
36
2
suggesting that the oxidation state of O is ꢀ 2 in CuO and SnO sample.
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Fig. 6 XRD patterns of waste contact masses (CuO (1), 1.0 wt.% SnO
2
ꢀCuO (2), 1.0 wt.%
o
SnO
2
+CuO (3) mixed with silicon after reaction at 295 C) (a), enlarged view in the 2θ range of
4
0ꢀ75° (b) of CuO (1), 1.0 wt.% SnO
2
ꢀCuO (2), 1.0 wt.% SnO
2
+CuO (3), H
2
ꢀTPR curves of the
CuO, SnO and 1.0 wt.% SnO ꢀCuO samples (c), Cu 2p spectrum of CuO and 1.0 wt.% SnO ꢀCuO
2
2
2
samples (d), Sn 3d spectrum of SnO
and SnO samples (f).
2
and 1.0 wt.% SnO
2
ꢀCuO samples (e), O 1s spectrum of CuO
2
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The SEM image of the contact masses before the reaction (Fig. 7a) shows that the Si surface is
smooth, and densely and uniformly distributed with catalyst particles (insert of Fig.7a). However
，
after the reaction, the Si surface became coarse or porous (Fig. 7b), suggesting the occurrence of
etching process during the reaction, consistent with the soꢀcalled anisotropic etching reaction
37
mechanis, and indicating that the 1.0 wt.% SnO ꢀCuO catalyst is very active for the Rochow
2
reaction. The element mapping images show the distribution of the elements Si (Fig. 7c) and Cu
(Fig. 7d) on the surface of the waste contact mass. As displayed, the Si mapping image clearly
shows the presence of the reacted (dark red) and unreacted (bright red) zones of Si particle, while
Cu (bright green) is distributed uniformly on the reacted or etched Si surface, indicating the
occurrence of catalytic reaction between Si particles and Cuꢀbased catalysts. Therefore, based on
the above results, we can schematically illustrate the Rochow reaction process as displayed in Fig.
7
e. In the process, a large quantity of Si is fully mixed with a small quantity of the SnO
hybrid before the reaction. With diffusion of SnO ꢀCuO hybrid into the Si matrix, the Cu Si alloy is
gradually formed at the interface between SnO ꢀCuO catalyst and the Si at the elevated
temperatures. In the reaction the bulk solid phase presented at reacting surface regions of Si
particles is Cu Si, which reacts with the gas MeCl to form MCSs. As the reaction proceeds, metallic
copper is continuously produced with the reduction of Cu Si alloy until the catalyst is deactivated.
2
ꢀCuO
2
x
2
x
x
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Fig. 7 SEM images of contact masses before (a) and after (b) reaction, elemental mapping images
of Si (c), Cu (d), and schematic illustration of the Rochow reaction process (e).
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4
. Conclusions
In summary, SnO
preꢀmade SnO NPs on CuO NRs in the presence of PVP. It is observed that SnO
2
ꢀCuO hybrid maceꢀlike nanocatalysts are prepared by simply depositing
NPs are
2
2
distributed on CuO NRs evenly generating a lot of heterojunctions. This process is facilitated by
PVP molecules. When applied to the Rochow reaction, the SnO ꢀCuO hybrid nanocatalysts exhibit
2
superior catalytic activities to CuO NRs, SnO
attributed to the enhanced synergistic interaction between CuO and SnO
nanocatalysts, as evidenced by the shifts of Cu2p and Sn3d in the XPS spectra, together with the
shifts of reduction peaks of CuO and SnO in the TPR profiles. This work provides conducive clues
2
NPs and their mixture. This activity enhancement is
2
in the designed
2
to design efficient nanoscale heteroꢀstructured catalysts for Rochow reaction.
Acknowledgements
The authors gratefully acknowledge the financial supports from the National Natural Science
Foundation of China (no. 51272252 and 21206172). Z. Z is grateful to the kind supports of both
Nanyang Technological University (NTU) and Institute of Chemical Engineering and Sciences
(ICES) under Agency for Science, Technology and Research (A*STAR).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version, at…
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