Flower-like ZnO grown on urchin-like CuO microspheres for catalytic synthesis of dimethyldichlorosilane

Article abstract of DOI:10.1039/c3ra00171g

We report the rational growth of flower-like ZnO on urchin-like CuO (f-ZnO@u-CuO) microspheres via a facile solvothermal method using copper nitrate and zinc nitrate as precursors in the presence of sodium nitrate and ethanol. A formation mechanism was proposed based on the observation of a series of reaction intermediates. The samples were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectrometer, and temperature-programmed reduction. It was found that the morphology of the samples was highly dependent on the synthesis conditions, particularly the reaction time and the ammonia amount added. As a copper-based catalyst for dimethyldichlorosilane synthesis via the Rochow reaction, f-ZnO@u-CuO microspheres show better catalytic performance than the Cu-based catalysts physically mixed with ZnO promoter, probably because of the well-developed p-n heterojunction structures at the CuO and ZnO interfaces that generate a much strong synergistic effect. The work provides a simple method to synthesize hierarchical CuO/ZnO composites and would be helpful for understanding the catalytic mechanism of the Rochow reaction.

Full text of DOI:10.1039/c3ra00171g

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Flower-like ZnO grown on urchin-like CuO microspheres
for catalytic synthesis of dimethyldichlorosilane3
Cite this: DOI: 10.1039/c3ra00171g
ab
b
b
b
b
a
Yongxia Zhu, Yingli Wang,* Lianying Song, Xin Chen, Wuyuan Liu, Jin Sun,
a
c
b
Xilin She,* Ziyi Zhong and Fabing Su*
We report the rational growth of flower-like ZnO on urchin-like CuO (f-ZnO@u-CuO) microspheres via a
facile solvothermal method using copper nitrate and zinc nitrate as precursors in the presence of sodium
nitrate and ethanol. A formation mechanism was proposed based on the observation of a series of
reaction intermediates. The samples were characterized by X-ray diffraction, transmission electron
microscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, X-ray photoelectron
spectroscopy, inductively coupled plasma optical emission spectrometer, and temperature-programmed
reduction. It was found that the morphology of the samples was highly dependent on the synthesis
conditions, particularly the reaction time and the ammonia amount added. As a copper-based catalyst for
dimethyldichlorosilane synthesis via the Rochow reaction, f-ZnO@u-CuO microspheres show better
catalytic performance than the Cu-based catalysts physically mixed with ZnO promoter, probably because
of the well-developed p-n heterojunction structures at the CuO and ZnO interfaces that generate a much
strong synergistic effect. The work provides a simple method to synthesize hierarchical CuO/ZnO
composites and would be helpful for understanding the catalytic mechanism of the Rochow reaction.
Received 13th January 2013,
Accepted 3rd April 2013
DOI: 10.1039/c3ra00171g
www.rsc.org/advances
2
decomposition, flower-like CuO-ZnO heterostructured nano-
1
. Introduction
3
wires by chemical and photochemical deposition, CuO-ZnO
hollow spheres by using highly uniformly monodispersed Cu-
embedded carbon spheres as templates under hydrothermal
conditions, corn-like CuO–ZnO structures by hydrothermal
and photo-deposition, Cu–ZnO nanocrystallites by aqueous
thermolysis, CuO/ZnO nanocorals by hydrothermal pro-
cess, ZnO/CuO nanotrees by hydrothermal method com-
bined with thermal oxidation on Cu substrates, and CuO–
ZnO wire-dots by photochemical precipitation. However, it
remains a great challenge to create hierarchically structured
CuO/ZnO with a regular size and morphology.
As we know, methylchlorosilanes (MCSs) are commercially
synthesized via the Rochow reaction discovered in 1940s, in
which, solid silicon (Si) reacts with gaseous methyl chloride
MeCl) in the presence of Cu-based catalysts physically mixed
As a p-type semiconductor, CuO has a narrow band gap (Eg =
1
–3
4
5
1.2 eV ) and is widely used as catalysts, sensors, and
6
superconductors. Meanwhile, ZnO is a well-known n-type
semiconductor (Eg = 3.37 eV at 300 K) with high electron
mobility and low recombination loss, and widely used in
optoelectronics, gas sensing,
number of methods such as chemical vapor deposition,
thermal evaporation,
thermal technique
nanostructured CuO and/or ZnO materials. Recently, it has
been reported that ZnO/CuO composites with p-n heterojunc-
tion structures exhibited improved performance in photocata-
7
10
7
13
8
9,10
11
12
catalysis, and solar cell.
A
20
1
2
18
1
1,13,14
15
template methods, and hydro-
have been developed to synthesize
3
1
6
21
3
,7
10
lysis,
chemical and humidity sensing,
environmental
1
7
18
pollution abatement, self-cleaning sensor, and selective
H S detection.
nanostructures have been fabricated, including CuO/ZnO
core/shell heterostructured nanowires prepared by thermal
(
1
9
As a result, many ZnO/CuO composite
21,22
2
with trace promoters
to produce MCSs including methyl-
SiCl M1), dimethyldichlorosilane
3 2 2 3 3
(CH ) SiCl , M2), trimethylchlorosilane ((CH ) SiCl, M3),
trichlorosilane (CH
3
3
,
(
2
3
and other trace Si-containing compounds. Among these
MCSs, M2 is the most desirable monomer used for the
production of silicone polymers in organosilane industry.
Generally, the M2 selectivity in the Rochow reaction is highly
a
College of Chemical and Environmental Engineering, Qingdao University, Qingdao,
China 266071. E-mail: xlshe@126.com; fbsu@mail.ipe.ac.cn;
Fax: +86-10-82544851; Tel: +86-10-82544850
2
4
b
State Key Laboratory of Multiphase Complex Systems, Institute of Process
2
5
Engineering, Chinese Academy of Sciences, Beijing, China 100190.
E-mail: wangyl@mail.ipe.ac.cn
dependent on the Cu-based catalysts, such as metallic Cu
26
27
28
and Cu compounds (Cu O, CuO, CuCl, and Cu–Cu O–
c
2
2
Institute of Chemical Engineering and Sciences, A*star, 1 Pesek Road, Jurong Island,
2
9
CuO composite ). However, the catalytic mechanism is not
Singapore 627833
3
0
fully understood. On the other hand, Zn or ZnO as Zn-based
promoter additives also play an important role in this
3
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3ra00171g
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1-33
reaction,
but their promotion mechanism is still not clear
Zn(NO
then 30.0 mL NH
g NaNO were added to form a solution. After stirred for 10
min, the solution was transferred into a stainless steel
autoclave with a Teflon-lined chamber, and the autoclave
was heated to 130 uC and maintained for 18 h. After cooled to
room temperature in air, the solid sample was filtrated,
washed with distilled water and ethanol several times, and
dried under vacuum at 60 uC for 6 h.
3
)
2
?6H
2
O were dissolved in 50.0 mL CH
3 2
CH OH, and
yet. The commercially used Cu catalysts always have dense
structures, irregular morphologies and are in micrometer
sizes. Thus, it is necessary to explore above complex gas-solid-
solid phase reaction using defined CuO–ZnO composites as
active Cu/Zn-based catalysts with specific morphology and
designed structures.
Herein, we present a simple solvothermal method to
rationally grow flower-like ZnO on urchin-like CuO (f-
ZnO@u-CuO) microspheres forming a novel hierarchical
composite as the model Cu/Zn-based catalysts for the
Rochow reaction. A possible formation mechanism is pro-
posed based on the observation of a series of reaction
intermediates. Compared to the Cu-based catalysts physically
mixed with ZnO promoter, the obtained f-ZnO@u-CuO micro-
spheres show better catalytic properties for M2 synthesis
because of the stronger synergistic effect between CuO and
ZnO.
4
OH, 10.0 mL NaOH solution (1.0 M) and 2.0
3
2.2. Characterization
X-ray diffraction (XRD) patterns were recorded on a PANalytica
X’Pert PRO MPD using the Cu–Ka radiation (l = 1.5418 Å). The
crystal size of the products was calculated using the Debye-
Scherrer equation. The microscopic features were observed by
field-emission scanning electron microscopy (SEM) (JSM-
6700F, JEOL, Tokyo, Japan) and transmission electron micro-
scopy (TEM) (JEM-2010F, JEOL, Tokyo, Japan). The composi-
tional analysis was performed with energy-dispersive X-ray
(
EDX) spectroscopy (Shimadzu). The chemical composition
was determined by X-ray photoelectron spectroscopy (XPS)
Model VG ESCALAB 250 spectrometer, Thermo Electron, U.K.)
2. Experimental
(
using a nonmonochromatized Al–Ka X-ray source (1486.6 eV).
The elemental analysis was carried out with Inductively
Coupled Plasma Optical Emission Spectrometer (ICP-OES)
2
.1. Material synthesis
All chemicals, including copper nitrate trihydrate
(Cu(NO
3 2 2
) ?3H O, A.R., 99%), zinc nitrate hexahydrate
(Optima 5300DV, perkin Elmer, American). The temperature-
(Zn(NO ) ?6H O, AR, 99%), ethanol (CH CH OH, A.R., 99%),
3
2
2
3
2
programmed reduction (TPR) measurements were carried out
on Automated Chemisorption Analyzer (chemBET pulsar TPR/
TPD, Quantachrome). 0.10 g sample loaded into a quartz
U-tube was first degassed at 300 uC for 60 min under helium.
ammonia solution (NH OH, A.R., 25–28%), sodium hydroxide
4
(NaOH, A.R., 99%), sodium nitrate (NaNO , A.R., 99%), copper
3
oxide (CuO, A.R., 99%, denoted c-CuO in this work), and zinc
oxide (ZnO, A.R., 99%, denoted c-ZnO) were purchased from
Sinopharm Chemical Reagent Co. Ltd., Shanghai, China, and
used without further purification.
After the temperature was cooled to 30 uC, the gas was changed
21
to 9.91% H –Ar (30 mL min ) and subsequently heated up to
1
2
2
1
100 uC at 10 uC min . The porous nature of the samples was
All the samples were synthesized by
a
solvothermal
investigated using physical adsorption of N at 2196 uC on an
2
2
7,34
method,
and the synthetic conditions such as hydrother-
automatic volumetric sorption analyzer (NOVA3200e,
Quantachrome). Prior to the measurement, the sample was
degassed at 200 uC for 5 h under vacuum. The specific surface
mal temperature, reaction time, the amount of chemicals used
in this work are compiled in Table 1. In a typical synthesis (e.g.
sample Z3 in Table 1), 0.5 g Cu(NO ) ?3H O and 0.6 g
3 2 2
Table 1 The synthesis conditions used in the sample preparation
Sample
Copper nitrate
g)
Zinc nitrate
(g)
Ethanol
(mL)
Ammonia water
(mL)
Sodium hydroxide
(mL)
Sodium nitrate
(g)
Temperature
(uC)
Time
(h)
(
Z0
Z1
Z2
Z3
Z4
Z5
Z6
Z7
1.0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0
50.0
50.0
50.0
50.0
50.0
50.0
50.0
50.0
50.0
50.0
50.0
50.0
50.0
50.0
50.0
50.0
50.0
50.0
30.0
30.0
30.0
30.0
30.0
30.0
30.0
30.0
30.0
30.0
30.0
30.0
5.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
13.5
13.5
13.5
13.5
13.5
13.5
5.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.0
3.0
4.0
2.0
2.0
2.0
2.0
2.0
2.0
180
80
18
18
18
18
18
0.4
0.5
2
30
18
18
18
18
18
18
18
18
18
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
100
130
150
130
130
130
130
130
130
130
130
130
130
130
130
130
Z8
Z9
Z10
Z11
Z12
Z13
Z14
Z15
Z16
Z17
10.0
15.0
20.0
25.0
30.0
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areas were determined according to the Brunauer–Emmett–
Teller (BET) method in the relative pressure range of 0.05–0.2.
denoted u-CuO), Z3 (flower-like ZnO grown on urchin-like
CuO, denoted f-ZnO@u-CuO), Z7 (flower-like ZnO partially
grown on urchin-like CuO, denoted f-ZnO/@u-CuO), and Z8
2.3. Catalytic measurement
(
flower-like ZnO mixed with urchin-like CuO, denoted f-ZnO/u-
CuO).
Fig. 1a shows the SEM image of u-CuO. It is seen that the
The evaluation of catalyst performance was carried out with a
3
5
typical MCS lab fixed-bed reactor. 5.0 g of Si powder (20–50
mesh, provided by Jiangsu Hongda New Material Co., Ltd. the
u-CuO microspheres in a size of 2–6 mm are assembled by
nanoplates in a size of several tens of nanometres (inset of
Fig. 1a). For f-ZnO/u-CuO shown in Fig. 1b, the u-CuO and
f-ZnO phases are separated, in which, the average diameter of
u-CuO microspheres is 6 mm, which are assembled by CuO
nanorods with an average cross-section diameter of 100 nm
(inset of Fig. 1b), while the f-ZnO flowers are also assembled by
nanorods with average cross-section diameter and length of
3
impurity level of other elements was listed in Table S1, ESI )
were homogeneously mixed with 0.5 g obtained sample as Cu-
based catalyst to form a contact mass. The reactor system was
initially purged with purified N
2
for 0.5 h followed by heating
2
1
to 325 uC within 1 h under N (12.5 mL min ). Subsequently,
2
2
1
N
2
was turned off and gas MeCl (12.5 mL min ) was
introduced into the reactor. After a given period reaction of
4 h at 325 uC, the reaction was terminated. The gas products
400 nm and 3.0 mm, respectively. Fig. 1c shows the SEM image
2
of f-ZnO/@u-CuO, in which, f-ZnO flowers are partially grown
on the u-CuO microspheres surface. For f-ZnO@u-CuO, its
SEM image (Fig. 1d) shows the f-ZnO flowers are grown out
from the surface of u-CuO microspheres. The size of this
archierarchical structure is about 3–8 mm. The yield of
f-ZnO@u-CuO is about 66.7 wt%. These results suggest that
the defined morphology and specific structure of CuO and/or
ZnO microspheres can be obtained under different synthesis
conditions. Fig. 1e shows the high magnification SEM image
of the f-ZnO@u-CuO sample. It is clearly seen that f-ZnO
covers on the surface of u-CuO, further confirmed by EDX
analysis shown in Fig. 1e-A and 1e-B, in which, the atomic
ratio of Cu to O is around 1.0 : 1.0, close to the stoichiometric
ratio of 1.0 : 1.0 for CuO in Fig. 1e-A, and the atomic ratio of
Zn to O is 0.9 : 1.0, close to the stoichiometric ratio of 1.0 : 1.0
for ZnO in Fig. 1e-B. The surface areas of these Cu/Zn-based
were cooled down to liquid phase using a water circulating
bath controlled at 5 uC by a programmable thermal circulator
(
GDH series, Ningbo xinzhi biological technology Co., LTD).
The products in the liquid solution were quantitatively
analyzed on Gas Chromatograph (GC) (Agilent
a
Technologies GC-7890A) equipped with KB-201 column (60
m) and TCD detector. The compositions of the reaction
products were analyzed by the peak area ratio (in percen-
3
6
tage). The products, which mainly contained M1, M2, M3,
methyldichlorosilane (CH SiHCl , M4), dimethylchlorosilane
(CH SiHCl, M5), low-boiling residue (M6), and high-boiling
residue (M7), were identified with gas chromatography-mass
3
2
(
3 2
)
3
7
spectrometry (GC-MS) (QP2010, Shimadzu). The Rochow
reaction is shown in the following formula 1:
2
21
catalysts compiled in Table 2 are 5.1 m g for c-ZnO + c-CuO,
2
21
2
21
1
2
2.4 m g for c-ZnO + u-CuO, 14.1 m g for f-ZnO/u-CuO,
2
21
2
21
0.6 m g for f-ZnO/@u-CuO, and 22.7 m g for f-ZnO@u-
CuO.
Fig. 2 shows the XPS spectra of the f-ZnO@u-CuO sample.
The wide XPS spectrum in Fig. 2a confirms the presence of Zn,
Cu, O, and C elements. Fig. 2b shows the Cu 2p spectrum, in
which, the characteristic intense peaks of Cu 2p are centered
at the binding energy (BE) of 934.3 eV (Cu 2p3/2) and 953.7 eV
(Cu 2p1/2), and the BE gap between Cu 2p3/2 and Cu 2p1/2 is
approximately 20 eV, suggesting that the divalent state of Cu in
The spent contact mass (residual solid after reaction)
containing unreacted Si powder, Cu compounds, and promo-
ters were weighed to calculate Si conversion (Csi) according to
3
7
the following formula:
3
8
the form of CuO on the surface of f-ZnO@u-CuO sample.
Fig. 2c shows the Zn 2p spectrum. It is seen that there are two
characteristic peaks located at BE of 1020.8 eV (Zn 2p3/2) and
3. Results and discussion
1043.9 eV (Zn 2p1/2). The BE gap between Zn 2p3/2 and Zn 2p1/2
3
.1. Characterization of synthesized materials
is about 23 eV, indicating that the oxidation state of Zn is + 2
3
9
in f-ZnO@u-CuO sample. Particularly, the first BE shows a
negative shift compared with the Zn 2p3/2 (1022.0 eV) of pure
ZnO, possibly ascribed to the interaction between u-CuO and
Science the morphology and structure of CuO and/or ZnO are
2
7,32
highly dependent on the synthesis conditions,
atically investigated the effect of the solvothermal temperature
Fig. S1, ESI ), time (Fig. S2, ESI ), amount of mineralization
agent (sodium nitrate) (Fig. S3, ESI ) and ammonia (Fig. S4,
ESI ) added. Several representative samples were selected for
we system-
4
0
f-ZnO. Moreover, the Cu : Zn atomic ratio derived from their
spectra is around 1.0 : 1.0, consistent with that of metal
precursors used, which is further confirmed by ICP-OES
analysis, suggesting the molar ratio of ZnO : CuO is 1.0 : 1.0
(
3
3
3
3
further characterization, including Z0 (urchin-like CuO,
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Fig. 1 SEM images of the samples: (a) u-CuO, (b) f-ZnO/u-CuO, (c) f-ZnO/@u-CuO, and (d and e) f-ZnO@u-CuO, together with (e-A) EDS spectra focusing on the A
zone in (e) and (e-B) EDS spectra focusing on the B zone in (e).
for the f-ZnO@u-CuO sample. For other samples of f-ZnO/u-
CuO and f-ZnO/@u-CuO, this ratio obtained by ICP-OES is also
about 1.0 : 1.0.
Fig. 3a shows the TEM image of f-ZnO@u-CuO, in which
f-ZnO flowers are grown on the surface of the u-CuO micro-
spheres. The high resolution TRM (HRTEM) image in Fig. 3b
presents the lattice structure of ZnO and CuO. The lattice
distance of the two parts (I and II) is 0.20 nm and 0.22 nm, in
good agreement with the (2111) lattice plane of monoclinic
CuO and the (101) lattice plane of hexagonal ZnO, respectively.
The TEM and HRTEM results demonstrate that the micro-
sphere is a composite containing both CuO and ZnO phases,
and f-ZnO is a single crystalline with [0001] as the preferential
growth direction. It is well known that composite structures
can form easily when the crystal planes of the two different
materials have a close inter-plane spacing and similar crystal
2
Table 2 The crystal sizes, H -TPR parameters, and surface areas of all the
catalysts
a
a
b
c
d
Sample
d
(
CuO
d
ZnO
T
(uC)
TPR
T
M
S
BET
2
21
41
nm)
(nm)
(uC)
(m g
)
structures. In the above microspheres, CuO has a monoclinic
structure and ZnO has a hexagonal structure, and the CuO
u-CuO
26.1
30.2
28.2
28.2
26.2
25.1
0.0
270–380
300–420
220–310
200–290
150–250
140–230
331
348
270
241
221
210
9.8
5.1
12.4
14.1
20.6
22.7
(
2111) and ZnO (101) planes have a similar inter-plane
c-ZnO + c-CuO
c-ZnO + u-CuO
f-ZnO/u-CuO
f-ZnO/@u-CuO
f-ZnO@u-CuO
50.9
50.9
43.5
42.5
41.4
spacing (0.20 vs. 0.22 nm), thus the composite CuO–ZnO
structure can form easily.
3.2. Formation mechanism of f-ZnO@u-CuO
a
The crystal size of CuO and ZnO calculated from the XRD patterns.
Reduction temperature range of samples. Temperatures at the
The possible reactions during the synthesis process are
expressed as following equations (Eq.):
b
c
d
peak maximum. The BET surface area of the catalysts.
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Fig. 2 XPS spectra of the sample f-ZnO@u-CuO: (a) wide spectrum, (b) Cu 2p spectrum, and (c) Zn 2p spectrum.
2
14
Cu(NO ) ?3H O
+ 4NH OH
A
(aq)
3
2
2
(aq)
4
(aq)
and 1.8 6 10 , as the precipitation pH value ranges of
Cu(OH) and Zn(OH) are 3.9–7.3 and 7.4–11.8 respectively.
After adding NaOH, [Cu(NH
NaOH to form intermediate Cu(OH)
2
+
2
[Cu(NH
3
)
4
]
(aq) + 7H
2
O + 2NO
3
(1)
(2)
2
2
2
+
3
)
4
]
species firstly react with
in autoclave at the high
Zn(NO
3
)
3
2
?6H
2
O
(aq) + 4NH
+ 10H O + 2NO
4
OH(aq)
A
(aq)
2
+
2
2
[Zn(NH ) ]
42
4
(aq)
2
3
temperature (eqn (3)). With the increase of reaction time (25
2
+
min), u-CuO crystal nucleus are generated via the decomposi-
[Cu(NH
3
)
4
]
(aq) + 2NaOH(aq)
+
A
4
3
tion of Cu(OH)2 (eqn (4)), and subsequently the u-CuO
microspheres (Fig. S2a, ESI ) are obtained via Ostwald
as shown in Fig. 4. Because of the high
Cu(OH)2(s) + 4NH3(g) + 2Na (aq)
(3)
(4)
3
4
4,45
ripening,
Cu(OH)2(s) A CuO Q + H O
(s)
2
2+
concentration of [Zn(NH ) ] in the solution, nucleation and
3
4
2
+
growth of ZnO will take place surrounding the created CuO
[
Zn(NH ) ] (aq) + 4NaOH(aq) + CuO(s)Q A
3 4
4
4,46
22 +
crystals via the Ostwald ripening,
owing to the lower
CuO?Zn(OH)
4
Q + 4NH3(g) + 4Na (aq)
(5)
(6)
activation energy barrier of heterogeneous nucleation. Thus,
2
2
CuO?Zn(OH)4 A CuO?ZnO Q + H O + 2OH
2
22
2+
CuO?Zn(OH)
species, which get rid of complexation on u-CuO microcrystal
eqn (5)). With the reaction time increasing to 30 min, f-ZnO
intermediate is obtained from [Zn(NH ) ]
(s)
2
4
3 4
(
The growth mechanism of f-ZnO@u-CuO is illustrated in
Fig. 4. Firstly, Cu(NO ?3H O and Zn(NO ?6H O are dis-
solved in ammonia solution to form [Cu(NH ) ]
nanorods are aligned perpendicularly to the u-CuO spherical
surface, but along the main crystallographic axes of CuO and
pointed to a common center, via the so-called ‘‘oriented
3
)
2
2
3
)
2
2
2
+
and
(eqn (1) and (2)), respectively. The reaction
3
4
2
+
10
[Zn(NH
3 4
) ]
4
7
2
20
3
attachment’’ process (Fig. S2b, ESI and eqn (6)), which is
constants of Cu(OH) and Zn(OH) at 25 uC are 4.8 6 10
2
2
shown in the Fig. 4. Finally, after the 18 h reaction, the growth
of f-ZnO on u-CuO is completed forming the f-ZnO@u-CuO
(Fig. S2d, ESI
3
).
3.3. Characterization of the Cu/Zn-based catalysts
To investigate the catalytic activity of these Cu/Zn-based
catalysts for Rochow reaction, we characterized some repre-
sentative samples with the same molar ratio of CuO and ZnO
(
1.0 : 1.0), including physically mixed commercial ZnO and
CuO (denoted c-ZnO + c-CuO) (their SEM images, XRD
patterns and H -TPR curves are shown in Fig. S5, ESI ),
2
3
physically mixed c-ZnO and u-CuO (denoted c-ZnO + u-CuO),
f-ZnO/u-CuO, f-ZnO/@u-CuO, and f-ZnO@u-CuO. Fig. 5a
shows their XRD patterns together with u-CuO as a compar-
Fig. 3 TEM (a) and HRTEM (b) images of f-ZnO@u-CuO.
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Fig. 4 Schematic illustration of the f-ZnO@u-CuO growth process.
ison. The diffraction peaks of all the samples can be readily
indexed to a monoclinic CuO (JCPDS: 65-2309) and a
hexagonal wurtzite ZnO (JCPDS: 36-1451). The crystal sizes of
CuO and ZnO are calculated based on the 2h values of 35.8u
and 36.4u respectively and shown in Table 2, which are in the
similar size level of 20–30 nm for CuO and 41–51 nm for ZnO.
Fig. 5b shows their enlarged view in the 2h angle range of 34–
Fig. 5c shows the H
2
-TPR profiles of all the catalysts. As a
comparison, the TPR curve of u-CuO is shown at the bottom of
the graph. All the Cu/Zn-based catalysts display one reduction
peak but they are localized at different temperature ranges.
M
The peak temperatures (T ) are compiled in Table 2. Although
the average CuO crystal size in all the catalysts is in a similar
level, the order of TM for all the Cu/Zn-based catalysts is c-ZnO
+ c-CuO (348 uC) > c-ZnO + u-CuO (270 uC) > f-ZnO/u-CuO (241
uC) > f-ZnO/@u-CuO (221 uC) > f-ZnO@u-CuO (210 uC),
suggesting that ZnO is able to promote the reduction of CuO
possibly because the regular morphology and small particle
size of CuO or ZnO have lower reaction activation energy than
37u, in which, a characteristic blue shift is observed for f-ZnO/
@
u-CuO and f-ZnO@u-CuO. Both the (2111) diffraction peak
of CuO and (101) diffraction peak of ZnO move to lower angle
region, implying the partial merging of the lattices of CuO
(
2111) and ZnO (101) due to the interaction of crystal
4
8
32
surface. As a result, the p-n heterojunctions are formed
between the lattices of CuO and ZnO.
the commercial CuO or ZnO. More importantly, f-ZnO@u-
CuO is formed by well-dispersed CuO and ZnO particles with
4
9
the intimate contact to each other and the well-developed
p-n heterojunction structure leading to a closer electrical
contact between ZnO and CuO particles with a consequent
coupling of their Fermi levels and probably promoting the
5
0
copper reduction. This idea has indeed been supported by
the XPS studies of these samples. Therefore, an enhanced
synergistic effect due to the stronger interaction of crystal
5
0
surfaces in f-ZnO@u-CuO is convincible, as evidenced by the
weakening of the Cu–O bond. Also, the peak area of these Cu/
Zn-based catalysts is about half of that of u-CuO because the
molar ratio of ZnO : CuO is approximately 1.0 : 1.0 in all Cu/
Zn-based catalysts.
Table 3 Catalytic performance of all the Cu/Zn-based catalysts for the Rochow
reaction
Product selectivity (%)
Sample
M1
M2
M3 M4
M5 M6 M7
C-Si (%)
4.8
c-ZnO + c-CuO 25.9 42.9 2.7
c-ZnO + u-CuO 14.2 46.4 2.2
f-ZnO/u-CuO
0.8 0.0 0.0 27.7
0.3 0.7 3.5 32.7 10.8
8.0 0.4 2.5 19.5 10.4
18.6 49.5 1.5
f-ZnO/@u-CuO 22.9 60.5 2.1 11.2 0.5 1.6
1.2 18.2
1.5 20.0
3.9 20.2
f-ZnO@u-CuOa 18.0 69.6 1.6
5.5 0.6 3.2
3.2 0.2 0.7
Fig. 5 XRD patterns (a), enlarged view in the 2h angle range of 34–37u (b), and
f-ZnO@u-CuO
21.7 68.6 1.7
2
H -TPR curves (c) of all the samples (A: u-CuO, B: c-ZnO + c-CuO, C: c-ZnO +
a
Repeated testing.
u-CuO, D: f-ZnO/u-CuO, E: f-ZnO/@u-CuO, and F: f-ZnO@u-CuO).
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.4 Catalytic property
Paper
3
after the reaction. It can be seen that, after the reaction, the
contact masses are composed of Si, Cu, and ZnO, but no CuO
species, probably due to the reaction of chlorosilane with the
lattice oxygen from CuO. As a promoter, ZnO remains
unchanged in the reaction. The enlarged view in Fig. 6b shows
the presence of Cu Si species in the two contact masses.
3
Although the intensity of Cu peaks for both contact masses are
very similar, a much higher intensity of Cu Si is observed in
f-ZnO@u-CuO + Si, implying that the f-ZnO@u-CuO catalyst is
Table 3 shows the catalytic performance of all the catalysts for
M2 synthesis via the Rochow reaction. On the physically mixed
c-ZnO + c-CuO catalyst, 42.9% of M2 selectivity and 4.8% of Si
conversion are obtained. For c-ZnO + u-CuO catalyst, the M2
selectivity and Si conversion rise up to 46.4% and 10.8%,
respectively, and those of f-ZnO/u-CuO are similar to those of
2
6
3
c-ZnO + u-CuO. For f-ZnO/@u-CuO catalyst, the observed M
2
selectivity (60.5%) and Si conversions (18.2%) are much higher
than those of c-ZnO + c-CuO, c-ZnO + u-CuO, and f-ZnO/u-
CuO. More importantly, the f-ZnO@u-CuO catalyst shows the
more active in creating Cu Si than c-ZnO + c-CuO. This may be
3
due to that the stronger synergistic effect existing in this
catalyst with a p-n heterojunction structure between the CuO
and ZnO in f-ZnO@u-CuO lowering the activation energy
although this synergistic effect is not fully understood yet at
the moment. The SEM images of the contact masses before the
reaction in Fig. 6c show that the Si surface was smooth and
dense with uniformly distributed f-ZnO@u-CuO (insert of
Fig. 6c). After the reaction, the Si surface became coarse or
porous (Fig. 6d), indicating that the f-ZnO@u-CuO catalyst
could ‘‘drills’’ the Si surface during the reaction and gradually
invaded into its interior. Therefore, the Cu-based catalyst
bonded tightly with the ZnO promoter should be significant
for the Rochow reaction.
highest M selectivity of 69.6% and the highest Si conversions
2
of 20.0% among all the catalysts, which is further confirmed
by the repeated testing.
As
organosilane industry, the Rochow reaction is carried out
industrially to maximize the selectivity for M . The above
results demonstrate that the f-ZnO@u-CuO shows much
higher M selectivity and Si conversion than the others. In
the Rochow reaction, the intermetallic phase of Cu Si is
2
M is the most valuable monomer precursor in
2
2
3
1,52
5
considered to be the major active intermediate phase.
believed that at the beginning of the reaction, Cu atoms, which
It is
2
2
are essentially mobile in the reaction system, firstly diffuse
on and into Si surface to form the active alloying Cu Si species
These species are the
Despite the fact that f-ZnO@u-CuO shows a relatively higher
M2 selectivity and Si conversion than the others, it should be
mentioned that the content of ZnO in f-ZnO@u-CuO is higher
than that of the commercially used (normally less than 10 wt%
3
2
3,53,54
at the interface of Cu and Si.
precondition of the synthesis reaction, which play an
important role in guiding the reactant-catalyst interaction
towards a higher specificity of the reaction.
promoter additives such as Zn, ZnO, and Sn are of importance
in improving the catalytic performance,
decrease the segregation energy of Si and promote the crystal
growth of Cu silicides. Fig. 6a shows the XRD patterns of the
contact masses (f-ZnO@u-CuO + Si and c-ZnO + c-CuO + Si)
58
of Cu content). However, our intension in this work is to
5
3,55
Meanwhile,
explore the importance of the synergistic effect between CuO
and ZnO in the Rochow reaction. It has been proved that, by
carefully design a Cu/Zn-based catalyst with a specific
morphology, the synergistic function between CuO and ZnO
can be significantly enhanced, thus improving its catalytic
performance. As we know, many Cu compounds such as
2
9,33,56,57
which could
2
5
26
27
28
metallic Cu and Cu compounds (Cu O, CuO, CuCl, and
2
2
9
Cu–Cu O–CuO composite ) are active for the Rochow reac-
2
tion, and thus along this way, the incorporation of promoters
3
1
32
such as Zn and ZnO within these Cu compounds to form
numerous novel Cu/Zn-based nanocomposites may pave a way
to find the highly efficient catalysts for M2 synthesis. In
addition, the nanostructured CuO/ZnO composites with a well-
developed p-n heterojunction structure at the CuO and ZnO
interfaces may be used as photocatalysts for the degradation of
1
7
2
dye contaminants,
2
photoelectrodes for H generation,
2
0
18
humidity sensor, superhydrophobic materials, and photo-
catalysts for H generation.
3
8
2
4. Conclusions
In summary, we have demonstrated the growth of the flower-
like ZnO on the urchin-like CuO (f-ZnO@u-CuO) micro-
spheres, and their improved catalytic function in dimethyldi-
chlorosilane synthesis. Compared to the physically mixed CuO
and ZnO catalysts, the f-ZnO@u-CuO catalyst exhibits higher
M2 selectivity and Si conversion. This is because the strong
synergistic effect of the p-n heterojunction formed at the
interfaces between CuO and ZnO. This works demonstrates
Fig. 6 XRD patterns of contact masses (f-ZnO@u-CuO + Si and c-ZnO + c-CuO +
Si) after reaction (a) and enlarged view in the 2h angle range of 40–50u (b), SEM
images of contact masses (f-ZnO@u-CuO + Si) before (c) and after (d) reaction.
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the possibility that a proper morphology variation is con-
ductive to the catalytic synthesis of dimethyldichlorosilane via
the Rochow reaction.
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The authors gratefully acknowledge the financial supports
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2
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