Yolk-Shell-Structured CuO?ZnO?In2O3 Trimetallic Oxide Mesocrystal Microspheres as an Efficient Catalyst for Trichlorosilane Production

Article abstract of DOI:10.1002/cctc.201902330

Trichlorosilane (TCS), the primary chemical feedstock for production of high-purity Si used in Si-based solar cells, is currently manufactured industrially via a non-catalytic hydrochlorination of metallurgical Si. This process generates a huge amount of undesirable silicon tetrachloride (STC) byproduct. Here we report the synthesis of yolk-shell-structured CuO?ZnO?In₂O₃ trimetallic oxide mesocrystal microspheres that can be employed as an efficient catalyst to produce TCS catalytically. The CuO?ZnO?In₂O₃ microspheres with multiple hetero-interfaces were prepared using a facile solvothermal reaction followed by calcination. We found that differing from a single CuO mesocrystal, the electronic density on Cu atoms in the CuO phase within CuO?ZnO and CuO?ZnO?In₂O₃ can be tuned by varying the composition. When used as a catalyst for Si hydrochlorination reaction to produce TCS, CuO?ZnO?In₂O₃ shows excellent catalytic performance with very high Si conversion and TCS selectivity. Under the same reaction conditions, the TCS yield increased 13 times relative to the catalyst-free process. This work demonstrates the possibility to decrease the amount of STC needed for the catalytic manufacture of TCS, and provides an approach to the facile synthesis of multi-component mesocrystal materials with a specific structure.

Full text of DOI:10.1002/cctc.201902330

Accepted Article
Title: Yolk-Shell-Structured CuO-ZnO-In2O3 trimetallic oxide
mesocrystal microspheres as an efficient catalyst for
trichlorosilane production
Authors: Fabing Su, Xin Li, Yongjun Ji, Jing Li, Yu Zhang, Hezhi Liu,
Qiongguang Li, Lihua Jia, Xiangfeng Guo, and Ziyi Zhong
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content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201902330
Link to VoR: http://dx.doi.org/10.1002/cctc.201902330
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ChemCatChem
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DOI: 10.1002/ ((please add manuscript number))
Article type: Communication
Title Yolk-Shell-Structured CuO-ZnO-In
2
O
3
trimetallic oxide mesocrystal microspheres as an
efficient catalyst for trichlorosilane production
Xin Li, Yongjun Ji,* Jing Li, Yu Zhang, Hezhi Liu, Qiongguang Li, Lihua Jia,* Xiangfeng Guo, Ziyi
Zhong, and Fabing Su*
X. Li, Prof. L. Jia
College of Chemistry and Chemical Engineering, Key Laboratory of Fine Chemicals of College of
Heilongjiang Province, Qiqihaer University, Heilongjiang Qiqihaer 161006, China
E-mail: jlh29@163.com
X. Li, Dr. Y. Ji, Dr. J. Li, Y. Zhang, Dr. H. Liu, Q. Li, Prof. F. Su
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese
Academy of Sciences, Beijing 100190, China
E-mail: yjji@ipe.ac.cn; fbsu@ipe.ac.cn
Dr. Y. Ji, Prof. F. Su
Zhongke Langfang Institute of Process Engineering, Fenghua Road No 1, Langfang Economic &
Technical Development Zone, Hebei Langfang 065001, China
Prof. X. Guo
College of Chemistry, Guangdong University of Petrochemical Technology, Guangdong Maoming,
5
25000, China
Prof. Z. Zhong
College of Engineering, Guangdong Technion Iarael Institute of Technology (GTIIT), 241 Daxue
Road, Shantou 515063, China; Technion–Israel Institute of Technology (IIT) Haifa, 32000, Israel
Prof. F. Su
Institute of Industrial Chemistry and Energy Technology, Shenyang University of Chemical
Technology, Liaoning Shenyang 110142, China
1
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ChemCatChem
10.1002/cctc.201902330
Abstract
Trichlorosilane (TCS), the primary chemical feedstock for production of high-purity Si used in Si-
based solar cells, is currently manufactured industrially via a non-catalytic hydrochlorination of
metallurgical Si. This process generates a huge amount of undesirable silicon tetrachloride (STC)
byproduct. Here we report the synthesis of yolk-shell-structured CuO-ZnO-In
mesocrystal microspheres that can be employed as an efficient catalyst to produce TCS catalytically.
The CuO-ZnO-In microspheres with multiple hetero-interfaces were prepared using a facile
2
3
O trimetallic oxide
2
O
3
solvothermal reaction followed by calcination. We found that differing from a single CuO
mesocrystal, the electronic density on Cu atoms in the CuO phase within CuO-ZnO and CuO-ZnO-
In
2
O
3
can be well-tuned via controlling the introduced composition. When used as a catalyst for Si
presents excellent catalytic performance
hydrochlorination reaction to produce TCS, CuO-ZnO-In
2
O
3
with much high Si conversion and TCS selectivity. Under the same reaction condition, the TCS yield
increased 13 times compared with the catalyst-free process. This work not only demonstrates the high
possibility to realize the catalytic manufacture of TCS with less STC but also provides an approach
to the facile synthesis of multi-component mesocrystal materials with a specific structure.
2
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ChemCatChem
10.1002/cctc.201902330
At present, the majority of the solar grade high-purity Si (hp-Si) is produced via the Siemens
process.[1] It involves non-catalytic hydrochlorination of metallurgical Si (m-Si) to produce
trichlorosilane (SiHCl
3
or TCS) with generation of a large amount of undesired byproduct silicon
tetrachloride (SiCl or STC) at the temperatures above 350 °C, and the selectivity of TCS and STC
4
in this process is around 85 and 15%, respectively. Therefore, further raising TCS selectivity or
lowering reaction temperature could result in a decrease of byproduct STC output and saving energy
consumption, and thus reducing TCS manufacture cost. Nowadays, the reactions of Si with CH
3
Cl or
ROH (R = CH , C ) (the so-called Rochow-Müller reaction) to generate organosilane monomers
3
2
H
5
such as methylchloro- or alkoxysilanes is well known. These reactions can proceed with the apparent
rate only in the presence of Cu-based catalysts. Previously, we investigated many Cu-based catalysts
for the Rochow-Müller reaction, such as defective Cu-O-Sn nanosheet,[2] hierarchical CuO-ZnO
[
3]
[4]
hollow microsphere, CuO-In
2
O
3
composites with P-N junctions, heterostructured ZnO/Cu O with
2
[5]
different exposed facets. These studies inspire us to explore the potential of the use of Cu-based
catalysts to catalyze the production of TCS. As we know, mesocrystals are a special class of crystalline
superstructures composed of highly ordered nanoparticles.[6-8] Comparing with the conventional
catalyst systems such as single crystals, polycrystals, and nanocrystals, mesocrystals have shown
[
9]
greatly improved properties. The unique structure enables mesocrystals new collective properties
emerging from the electronic and/or magnetic coupling between their constituent nanoparticles.
Recently, Bian et al.[10] organized CuO and ZnO into a binary nanocomposite mesocrystal, which
possesses a large number of interfaces between p- and n-type semiconductor nanoparticles. The
efficient charge transfer at the interfaces may significantly amplify their applications in catalysis,
energy conversion, and storage.[11,12]
Here we demonstrate the facile synthesis of yolk-shell-structured CuO-ZnO-In
2
3
O trimetallic
nanocomposite mesocrystals using a simple solvothermal method. The yolk-shell structure,[13]
a
unique core-shell structure with a gap between the core and shell, has shown distinct advantages in
3
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ChemCatChem
10.1002/cctc.201902330
catalytic applications: (a) the inner core can be in full contact with the reactant molecules, providing
more active surface for their adsorption and activation, (b) the cavity between the core and shell is
conducive to the enrichment of reactant molecules, and (c) the outer shell can provide adequate
protection for the inner core. We found that, when used as the catalyst in m-Si hydrochlorination
reaction to produce TCS, CuO-ZnO-In
2
3
O shows excellent catalytic property with much high Si
conversion and TCS selectivity. The development of such a mesocrystal catalyst may shift the non-
catalytic to catalytic production of TCS.
Yolk-shell-structured CuO-ZnO-In
Cu(NO ·3H O, Zn(NO ·6H O and In(NO
DMF) as the solvent, and acetic acid (CH COOH) as the complexing agent. The synthesis involves
2
O
3
mesocrystal microspheres were prepared by using
3
)
2
2
3
)
2
2
3
)
3
·4.5H O as precursor salts, N, N dimethylformamide
2
(
3
a facile template-free solvothermal process and subsequent calcination (details are shown in Table
S1). As shown in X-ray diffraction (XRD) pattern (Figure 1a), all of the diffraction peaks match well
with the standard data for monoclinic CuO (JCPDS No. 01-089-5898), and no signals of Zn and In
species are observed, probably because of their low contents or small particle sizes. By using the
Scherrer equation from the {111} peak, the average crystallite size of CuO is calculated to be about
1
50 nm (Table S2). The scanning electron microscopy (SEM) image (Figure 1b) shows that the
products exhibit a uniformly spherical morphology. These well-dispersed spheres have an average
size of 17 μm, and the surface of microspheres is rough (Figure 1c). Close observation of the surface
structure in Figure 1c reveals the existence of many regular stripes and boundaries (indicated by the
yellow circle). A broken microsphere (Figure 1d) further explains its yolk-shell structure, and the
shell thickness was estimated to be ca. 0.5 μm. Also, the porous core is self-assembled by a large
number of smaller nanoparticles with a size range of 100-150 nm. This material was sliced by an
ultramicrotome (detailed slicing process is shown in Figure S1). The highly porous internal structure
of CuO-ZnO-In
2
3
O mesocrystal microsphere can be clearly observed by the Focus ion-beam scanning
electron microscopy (FIB-SEM) image (Figure 1e and Figure S2), high-angle annular dark-field
4
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ChemCatChem
10.1002/cctc.201902330
scanning transmission electron microscopy (HAADF-STEM) image (Figure 1f) and TEM image
Figure 1g and Figure S3). Nitrogen-adsorption measurement (Figure S4) reveals that this sample has
(
2
-1
a mesoporous structure, and its surface area is about 3.9 m g . These mesopores with a mean size of
0nm are formed by the assembly of small nanoparticles, as evidenced by the magnified TEM image
Figure 1h), which also shows the ordered arrangement of nanoparticles. The selected area electron
2
(
diffraction (SAED) pattern (Figure 1i) reveals the quasi-single-crystalline structure of this material,
indicating that the assembly of the nanoparticle building units is oriented and aligned. Furthermore,
this result also suggests there is a small angle lattice mismatch between the boundaries of the
nanoparticles, which is a typical feature of the mesocrystal.[6-8] The contents of Cu, Zn, and In
determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Table S2)
analysis are consistent with the feeding ratios of the three precursors. These observations demonstrate
the successful synthesis of the trimetallic oxide nanocomposite mesocrystal microspheres with a
unique yolk-shell structure. For a comparison, yolk-shell structured CuO single metallic oxide
mesocrystal microspheres (Figure S5 and S6) and yolk-shell structured CuO-ZnO bimetallic oxide
mesocrystal microspheres with a Cu: Zn molar ratio of 100: 2 (Figure S7 and S8) were synthesized
using the same process.
5
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10.1002/cctc.201902330
Figure 1. (a) XRD pattern and (b-d) SEM images of yolk-shell structured CuO-ZnO-In
mesocrystal microspheres; (e) FIB-SEM image of the sliced material; (f) HAADF-STEM image and
g) TEM image obtained from the rim of the sliced material marked with the yellow square in image
2
O
3
(
e; (h) high-resolution cross-sectional TEM image obtained from the rim of the sliced material marked
with the yellow square in image g, and (i) the corresponding SAED pattern.
The bright-field STEM image of ultramicrotome (Figure 2a) also confirms the highly porous
characteristic of internal structure. The high-magnification energy-dispersive X-ray spectroscopy
(EDS) mapping analysis shows the simultaneous existence and homogeneous distribution of all the
elements of O (Figure 2b), Cu (Figure 2c) Zn (Figure 2d) and In (Figure 2e). This observation is
further supported by a broader range of EDX analysis (Figure S9). A closer examination of Figure 2f
reveals that the trace amount of Zn and In elements are highly dispersed in CuO. In the atomic-
resolution aberration-corrected HAADF-STEM image (Figure 2g) taken at the surface region, the
lattice spacings were measured to be 0.232, 0.25 and 0.248 nm, which correspond to the CuO(111),
In
2
O
3
(101) and ZnO(400) facets, respectively, indicating highly crystallized nature of CuO, ZnO, and
6
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In
nanoparticles in this sample. As seen from X-ray photoelectron spectroscopy (XPS) spectra (Figure
h and Table S3), the CuO-ZnO mesocrystal has a higher binding energy of Cu 2p3/2 peak relative to
CuO, indicating the electron deficiency of Cu atoms in it. However, for the CuO-ZnO-In
2
O
3
. It is clear that there generate intimate contact interfaces between CuO, In
2
O
3
, and ZnO
2
2
O
3
mesocrystal, the Cu 2p3/2 peak is shifted to lower binding energy in comparison with that of both CuO
and CuO-ZnOmesocrystals, which demonstrates that the electron density at the Cu atoms is increased
in the CuO-ZnO-In
CuO-ZnO-In mesocrystals are present in the form of ZnO (Figure 2i); furthermore, the binding
energy of the Zn 2p3/2 peak in CuO-ZnO-In mesocrystal also shifts to lower-energy side compared
2
O
3
mesocrystal. Moreover, the surface Zn species in both of the CuO-ZnO and
2
O
3
2
O
3
with that of CuO-ZnO mesocrystals (Figure 2i). In CuO-ZnO-In
2
3
O , the peak of In 3d5/2 at about
4
45.0 eV shown in Figure 2j is characteristic of In
2
O3.[14] In the O1s XPS spectra (Figure 2k, Figure
S10, and Table S3), following the order of CuO, CuO-ZnO, and CuO-ZnO-In
ratio between the chemisorbed oxygen (denoted as Oads) and lattice oxygen (denoted as Olatt
calculated by the peak area becomes larger gradually, indicating that introduction of ZnO and In
2
O , the value of molar
3
)
2
O
3
enhanced the surface enrichment of adsorbed oxygen species. The Raman analysis (Figure S11)
further shows that compared with those of CuO, the bands of the CuO-ZnO mesocryatal shift slightly
to high wavenumber; while for the CuO-ZnO-In
wavenumber, demonstrating that there is a different change in the Cu electronic structure when ZnO
or both of ZnO and In were introduced into the CuO mesocrystal This observation is consistent
2
O
3
mesocrystal, instead, the bands move to low
2
O
3
.
with the results of XPS. The surface Zn/Cu and In/Cu molar ratios measured by XPS are consistent
with those in bulk determined by ICP-OES (Table S3), suggesting that the distributions of ZnO and
In
above results clearly suggest: (1) both the nanocrystals of ZnO and In
CuO framework, leading to the formation of multiple interfaces in CuO-ZnO-In
2
O
3
nanocrystals are homogeneous in entire CuO-ZnO-In
2
O
3
mesocrystal. Taken together, the
are highly dispersed in the
; and (2) the Cu
2
O
3
2
O
3
7
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electronic states can be engineered by controlling the composition through interfacial charge transfer
between different oxide nanoparticles.
Figure 2. (a) High-resolution BF-STEM image of yolk-shell structured CuO-ZnO-In
2
3
O mesocrystal
microspheres and the corresponding elemental mappings of (b) O, (c) Cu, (d) Zn, (e) In and (f) the
merged image. (g) HRTEM image of the edges of the sectioned material. XPS spectra of (h) Cu 2p,
(i) Zn 2p, (j) In 3d and (k) O 1s
.
8
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3
In our synthesis strategy, the coexistence of CH COOH and DMF is the key to the formation of
uniform mesocrystal microspheres. As shown in (Figure S12), when DMF is replaced with other
solvents such as water, not the mesocrystals, but the irregular aggregates composed of nanoparticles
are formed. Without CH
S13). Similarly, if CH
particles are harvested (Figure S14). It is worth pointing out that replacing CH
cannot result in any precipitant, indicating the importance of the presence of oxygen-containing
ligand in the system. As the amount of CH COOH was gradually increased, the three-dimensional
ordered assembled microsphere gradually formed, confirming the effect of CH COOH on the
3
COOH in the system, we also obtain only disorganized solid spheres (Figure
COOH is substituted with other reagents such as H SO , only irregular
COOH with HCl
3
2
4
3
3
3
formation of oriented assembled structures (Figure S15). The time-dependent experiments are further
carried out to reveal the formation process. After 3h of solvothermal treatment (Figure 3a), solid
microspheres of ca. 3 µm size with many voids/defects on their surface are formed. A closer
examination of the image shows that the microspheres are composed of small nanoparticles with a
diameter of 50-80 nm (see the inset in Figure 3a). When the reaction time is increased to 4 h (Figure
3
b), these microspheres assemble into larger microspheres (about 17 μm size) with quite rough
surfaces. As shown in Figure 3c and Figure S16, with continuously increasing reaction time (6 h), the
surface of bigger microspheres gradually become smooth and compact; nevertheless, their interior is
still substantial (Figure 3c). XRD analysis reveals that the intermediate crystal after 6h of
solvothermal treatment mainly consists of Cu
2
O, without noticeable peaks of Zn and In species
observed (Figure S17, owing to their low content). Fourier transform infrared (FTIR) spectrum
(Figure S18) further shows the appearance of two distinct absorption peaks for this sample at about
-1
2
924 and 1624 cm , which correspond to the CH stretching vibrations and C-N modes, respectively,
3
indicating it was capped with DMF. After calcination in air, the solid microspheres finally evolve
into the unique yolk-shell structure (Figure 3d). Moreover, it is observed that as the reaction
progressed, the mixture experienced a noticeable color change from blue to green, to red-brown, and
9
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ChemCatChem
10.1002/cctc.201902330
finally to black (Figure S19), indicating the variation of the composition of the product. The XRD
patterns of the samples prepared at different times (Figure S20) show that the sample crystallinity
gradually increases with prolonging time, suggesting the simultaneous occurrence of crystallization
and particle assembly. Furthermore, as shown in Table S4, the change of the lattice parameters
verifies the formation of defects between nanoparticles, which can lead to different lattice strain.
Based on the evolution process at various stages, the formation mechanism is proposed and illustrated
in Figure 3e. First, a large number of Cu
2
O, ZnO and In
2
3
O primary nanoparticles capped by DMF
are generated at the initial stage due to the heterogeneous nucleation and growth via the coordination
2
+
2+
3+
reaction of Cu , Zn , In ions with oxygen atom in −COOH group of CH
in DMF, and followed by the reduction of Cu(CH COO) with the capped DMF and the
decomposition of Zn(CH COO) and In(CH COO) under solvothermal conditions. The relevant
3
COOH and nitrogen atom
3
2
3
2
3
3
chemical reactions are shown in the following Equation 1-6:
Then, as the reaction time is prolonged, these nanoparticles self-assemble in an oriented attachment
way into the secondary microspheres, which further attach orderly to form larger solid microsphere
intermediate that contains the ligands of DMF. In general, the main driving force of the oriented
attachment for the colloidal semiconductor system relies on nanoparticle interaction forces (van der
Waals force, hydrogen bonding, etc.) and a decrease in system free energy.[7,15,16] It is reported that
the -COOH group can act as a crystal growth modifier to aid in the oriented growth of nanocyrstal.[17]
In our study, the following factors are believed to contribute to the oriented aggregation: 1) It seems
reasonable that these small nanoparticles with high surface energies are temporarily stabilized by the
1
0
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ChemCatChem
10.1002/cctc.201902330
capping DMF, which can prevent their uncontrolled and random aggregation owing to the repulsive
interaction; 2) both of the -COOH group and dipole-dipole interaction between the nanoparticles
assist them to rotate continuously and then to orient each other and further to fuse across the interface
through an Oswald ripening into a crystallographic ordered aggregate. These two factors, therefore,
contribute to minimize the total system energy by decreasing the surface free energy and interface
energy. Finally, by oxidation at 400 °C in air, well-developed yolk-shell-structured CuO-ZnO-In
2
O
3
mesocrystal microspheres are obtained via diffusion-controlled growth due to the well-known
Kirkendall effect, that is, the outward diffusion rate of Cu atoms is faster than the inward diffusion
rate of O atoms, leading to a moderate increase in the size of the nanoparticle subunits because of
their further attachment. In summary, in our synthetic system, DMF serves as both the solvent and
the surface ligand, while CH
3
COOH serves as the crystal growth modifier to promote the formation
COOH has a vital role in complexing with metal
of mesocrystals. The unique property of CH
3
precursors and DMF to form nuclei and in assisting the oriented attachment of nanoparticles to form
nanoscale heterointerfaces.
1
1
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Figure 3. SEM images of the intermediate products of yolk-shell structured CuO-ZnO-In
mesocrystal microspheres with different solvothermal times before calcination for (a) 3 h, (b) 4 h and
c) 6 h, and (d) the sample after 6 h of solvothermal treatment was further calcined at 400 °C for 3 h.
Insets are the detailed inner structure of each sample. (e) Schematic illustration of the formation
mechanism of yolk-shell structured CuO-ZnO-In mesocrystal microspheres.
2
O
3
(
2
O
3
Table 1 summarizes the catalytic properties of various catalysts to produce TCS using metallurgical
Si powder reacted with gas HCl at 300 °C and 0.1 MPa for 6h in a fixed bed. The reaction equation
is shown in Scheme S1. For comparative purposes, some reference samples were also prepared,
including: (1) commercial CuO (denoted as C-CuO); (2) a physical mixture of yolk-shell structured
1
2
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10.1002/cctc.201902330
CuO mesocrystal microspheres, ZnO and In
2
O
3
at the molar ratio of 100: 2: 1 (denoted as P-CuO-
ZnO-In
denoted as NCs-CuO-ZnO-In
microspheres (SEM image shown in Figure S22); (5) yolk-shelled-structured CuO-ZnO-In
mesocrystal microspheres with different CuO: ZnO: In
S26); and (6) yolk-shelled-structured CuO-ZnO-In mesocrystal microspheres with lower amount
of CH COOH (0.3 g and 0.5 g, respectively, SEM image shown in Figure S7). As shown in Table 1,
in the absence of catalyst, the m-Si conversion and TCS selectivity is only 5.4% and 84.5%,
respectively (entry 1). Both the samples of ZnO (entry 2, SEM image in Figure S28) and In (entry
, SEM image in Figure S29) appear to be inert. C-CuO gives m-Si conversion of 10.8% and TCS
2
O
3
); (3) a composite of CuO, ZnO and In
2
O
3
nanocrystals with the molar ratio of 100: 2: 1
(
2
O
3
, SEM image shown in Figure S21); (4) solid CuO-ZnO-In
2
2
O
3
3
O
2
3
O ratios (SEM image shown in Figure S23-
2
O
3
3
2
O
3
3
selectivity of 88.8% (entry 4), suggesting that CuO is essentially active for this reaction. When using
yolk-shell-structured CuO mesocrystal microspheres as a catalyst, both values are respectively
improved to 20.5% and 90.5% (entry 5), confirming the advantage of this assembled structure.
Dramatically, yolk-shell-structured CuO-ZnO mesocrystal microspheres exhibit improved catalytic
properties: m-Si conversion is greatly increased to 41.2% with TCS selectivity of 91.5% (entry 6).
For the yolk-shell-structured CuO-ZnO-In
In molar ratio of 100: 2: 1, the m-Si conversion and TCS selectivity are further enhanced, reaching
6.1% and 92.8% (entry 7 and Table S5), respectively, which are much higher than those of P-CuO-
ZnO-In (entry 8), NCs-CuO-ZnO-In (entry 9) and solid CuO-ZnO-In mesocrystal
microspheres (entry 10). It is also observed that all the yolk-shell-structured CuO-ZnO-In
2
O
3
mesocrystal microspheres with the optimal CuO: ZnO:
2
O
3
6
2
O
3
2
O
3
2
O
3
2
O
3
mesocrystal microspheres show similar catalytic performances, further demonstrating the importance
of the assembled and the unique yolk-shell structure. More importantly, compared with that of the
current non-catalytic process, TCS yield on the CuO-ZnO-In
2
3
O mesocrystal microspheres achieves
more than a 13-fold increment. Furthermore, it exhibits superior long-term stability (Figure S30).
These results suggest that the selectivity and yield of TCS are remarkably boosted via the catalytic
1
3
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process using the yolk-shell structured CuO-ZnO-In
2
O
3
mesocrystal microsphere as the catalyst. Such
dramatic activity enhancement is closely related to the assembled and the unique yolk-shell structure,
as well as the formed nanoscale interfaces in CuO-ZnO-In
2
O
3.
Table 1. The catalytic properties of various catalysts.
Entry
Catalyst
No catalyst
ZnO
C
Si (%)a
5.4
S
TCS (%)b
84.5
84.9
84.7
88.8
90.5
91.5
92.8
89.9
86.4
88.8
S
STC (%)c
15.5
15.1
15.3
11.2
9.5
Y
TCS (%)d
4.6
1
2
3
4
5
6
7
8
9
5.8
4.9
In
2
O
3
5.6
4.7
C-CuO
10.8
20.5
41.2
66.1
32.2
35.7
45.1
9.6
CuO mesocrystal
CuO-ZnO mesocrystal
18.6
37.7
61.2
28.9
30.8
40.0
8.5
CuO-ZnO-In
P-CuO-ZnO-In
NCs-CuO-ZnO-In
CuO-ZnO-In
2
O
3
mesocrystal
7.2
2
O
3
10.1
13.6
11.2
2
O
3
1
0
2
O
3
microsphere
Reaction conditions: temperature, 300 °C; time, 6 h; catalyst loading, 0.15 g; Si, 10 g; HCl gas flow rate, 25 mL
-1
a
b
c
d
min . conversion of Si; selectivity of TCS; selectivity of STC; yield of TCS.
Previous studies have demonstrated that the Cu Si alloyed species formed during the reaction
x
process are the active catalytic phase in Si-involved Rochow reaction.[2-5, 18-21] In our work, the
measured XRD patterns (Figure S31) of the spent contact mass (unreacted residue) after a 10h
reaction show the formation of alloyed Cu
investigated, the peak intensity of Cu Si is the strongest in the case of the CuO-ZnO-In
mesocrystal, indicating its best ability to generate Cu Si with Si than the other samples. More
importantly, we found that both Si conversion and TCS selectivity increase continuously with
increasing Cu Si content (Figure S32). Moreover, increasing the reaction time can also enhance the
formation of Cu Si species and thus improve the TCS yield. These observations support that Cu Si
x
Si species during the reaction. Among all the catalysts
x
2
O
3
x
x
x
x
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ChemCatChem
10.1002/cctc.201902330
species generated under the reaction conditions are the real active phases, and the amounts of Cu
significantly affect the TCS selectivity and yield.
x
Si
The structural evolution of the Si surface observed by SEM images indicate the occurrence of Si
etching during the catalytic process (Figure S34). While for the CuO-ZnO-In mesocrystal, it
almost remains the intact structure after the reaction (Figure S35). Overall, the greatly enhanced
catalytic performance of the CuO-ZnO-In mesocrystal microspheres can be attributed to the
following factors: (1) the tuned electronic structure of Cu atoms in the CuO phase through controlling
the introduced component. As confirmed by the XPS and Raman analysis, in the CuO-ZnO-In
2
O
3
2
O
3
2
O
3
mesocrystal microspheres, efficient charge transfer can take place via multiple intimate interfaces,
which makes electron density of the Cu atoms higher, thereby promoting the adsorption of gaseous
HCl molecules; (2) the previous report[2] has revealed that relative to Olatt, Oads is more conducive to
the adsorption of Cl-containing species. In our work, a higher ratio of Oads in CuO-ZnO-In
2
3
O than
that of the CuO and CuO-ZnO mesocrystal microspheres was observed, which should enhance the
adsorption of HCl; and (3) the unique yolk-shell structure not only provides more contact surface for
adsorption of gaseous HCl molecules but also affords sufficient cavities to enrich gaseous HCl
molecules. All the factors can increase the in situ formation of Cu atom and accelerate the diffusion
of the formed Cu atoms to the Si surface, leading to the generation of more Cu Si active species and
x
thus high catalytic activity and selectivity. Moreover, the hierarchically oriented assembled structure
endows the microspheres with high structural stability, improving catalytic stability, although a more
detailed reaction mechanism needs further investigation. As we know, the solar Si is obtained from
TCS that is industrially manufactured via the non-catalytic process. Our work demonstrates that the
use of the catalytic process can lead to the tremendous increase of the TCS selectivity and yield as
well as less by-product of STC, thereby significantly reduce the cost of solar Si.
In summary, we have successfully synthesized the CuO-ZnO-In
2
O
3
trimetallic oxide mesocrystal
microspheres with the yolk-shell structure and uniform size using a facile solvothermal method
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ChemCatChem
10.1002/cctc.201902330
followed by calcination. This type of the microspheres is formed mainly based on the ordered
assembly driven by the employed CH COOH molecule and the intrinsic dipole field. This strategy
3
can generate well-defined nanoscale hetero-interfaces between different metal oxides that can
effectively facilitate the charge transfer. Furthermore, the change of electronic density (increase or
decrease) on Cu atoms in the CuO phase can be achieved by controlling the introduced component.
With these structural features, the CuO-ZnO-In
2
3
O mesocrystal microspheres exhibit excellent
catalytic performance in the production of TCS through hydrochlorination of Si. Also, this strategy
can apply to the synthesis of more complex mesocrystals as new catalysts.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (nos.
2
1878301, 21978299 and 21908224) and the Science Research Project of the Ministry of Education
of Heilongjiang Province of China (135309116). Z. Zhong thanks the kind support of GTIIT for the
collaboration.
Conflict of Interest
The authors declare no conflict of interest.
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
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This communication reports the synthesis of yolk-shell-structured CuO-ZnO-In
2
3
O trimetallic oxide
mesocrystal microspheres by a simple solvothermal reaction. The formation mechanism was well
investigated by tracking different stages of crystallization. The synthesized CuO-ZnO-In
2
O
3
mesocrystal with nanoscale hetero-interfaces and tuned Cu electronic density in the CuO phase
exhibited excellent catalytic property for silicon hydrochlorination to produce trichlorosilane.
Keyword: trimetallic oxide mesocrystals, yolk-shell structure, interfacial charge transfer,
superstructure, catalytic production of trichlorosilane
Xin Li, Yongjun Ji,* Jing Li, Yu Zhang, Hezhi Liu, Qiongguang Li, Lihua Jia,* Xiangfeng Guo, Ziyi
Zhong, and Fabing Su*
Title: Yolk-Shell-Structured CuO-ZnO-In
2
3
O trimetallic oxide mesocrystal microspheres as an
efficient catalyst for trichlorosilane production
TOC Figure
1
9
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