Rapid microwaves synthesis of CoSix/CNTs as novel catalytic materials for hydrogenation of phthalic anhydride

Article abstract of DOI:10.1016/j.jssc.2014.05.021

CoSi_x/CNTs catalysts with different CoSi_x phases (CoSi, CoSi₂) have been rapidly synthesized via a microwave-assisted route and applied for the liquid phase hydrogenation of phthalic anhydride. The synthesized catalysts were analyzed and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy-energy dispersive X-ray spectroscopy, thermogravimetric/derivative thermogravimetric analysis. The reaction progress of cobalt silicides and the ratio of Co:Si were monitored at different microwave irradiation times by XRD, giving insight into the formation mechanism. Compared to the Co/CNTs catalyst, all the prepared CoSi_x/CNTs catalysts exhibited excellent activity and good selectivity to phthalide under mild reaction conditions (180-220 °C and 4.0 MPa H₂). This novel methodology can be applied to the synthesis of other transition metal silicides such as FeSi, Ni₂Si, and Cu ₄Si.

Full text of DOI:10.1016/j.jssc.2014.05.021

Author's Accepted Manuscript
Rapid Microwaves synthesis of CoSix/CNTs as
novel catalytic Materials for hydrogenation of
phthalic anhydride
Liangliang Zhang, Xiao Chen, Shaohua Jin,
Jingchao Guan, Christopher T. Williams, Zhijian
Peng, Changhai Liang
www.elsevier.com/locate/jssc
PII:
S0022-4596(14)00229-1
http://dx.doi.org/10.1016/j.jssc.2014.05.021
YJSSC18485
DOI:
Reference:
To appear in: Journal of Solid State Chemistry
Received date: 27 February 2014
Revised date: 14 May 2014
Accepted date: 21 May 2014
Cite this article as: Liangliang Zhang, Xiao Chen, Shaohua Jin, Jingchao Guan,
Christopher T. Williams, Zhijian Peng, Changhai Liang, Rapid Microwaves
synthesis of CoSix/CNTs as novel catalytic Materials for hydrogenation of
phthalic anhydride, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.
jssc.2014.05.021
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Rapid Microwaves Synthesis of CoSix/CNTs as Novel Catalytic
Materials for Hydrogenation of Phthalic Anhydride
Liangliang Zhang1,2, Xiao Chen2, Shaohua Jin2, Jingchao Guan2, Christopher T.
Williams3, Zhijian Peng1,*, Changhai Liang2,*
1 School of Engineering and Technology, China University of Geosciences, Beijing 100083,
P.R. China
2 Laboratory of Advanced Materials and Catalytic Engineering, Dalian University of
Technology, Dalian 116024, P. R. China
3
Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208,
USA
*pengzhijian@cugb.edu.cn and changhai@dlut.edu.cn
Abstract
CoSix/CNTs catalysts with different CoSix phases (CoSi2, CoSi) have been rapidly
synthesized via a microwave-assisted route and applied for the liquid phase hydrogenation of
phthalic anhydride. The synthesized catalysts were analyzed and characterized by X-ray
diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy-energy
dispersive X-ray spectroscopy, thermogravimetric/derivative thermogravimetric analysis. The
reaction progress of cobalt silicides and the ratio of Co:Si were monitored at different
microwave irradiation times by XRD, giving insight into the formation mechanism.
1

Compared to the Co/CNTs catalyst, all the prepared CoSix/CNTs catalysts exhibited excellent
activity and good selectivity to phthalide under mild reaction conditions (180-220 °C and 4.0
MPa H2). This novel methodology can be applied to the synthesis of other transition metal
silicides such as Cu4Si, Ni2Si, and FeSi.
Keywords: Microwave-assisted preparation, Cobalt silicides, Hydrogenation, Phthalic
anhydride
1. Introduction
Metal silicides, the family of refractory intermetallic compounds between metals and
silicon, have various excellent physical and chemical properties, which are useful for the
applications including electronics and silicon-based devices [1]. Cobalt silicides have been
shown to be high-melting point materials with very low resistivity and good chemical stability,
offering the possibility for use as nanoscale electrical interconnections [2]. As novel catalytic
materials, only a few reports have been published on cobalt silicide. It has recently been
investigated for use as a solid catalyst for the formation of carbon nanotubes [3], and in
silicon carbide composites for structural application [4]. In our early research, CoSi/SiO2
synthesized by metal organic chemical vapor deposition of Co(SiCl3)(CO)4 showed high
catalytic activity and selectivity for naphthalene hydrogenation [5].
During the recent years, cobalt silicides have been synthesized by various methods, such
as spontaneous chemical vapour transport reaction, metal organic chemical vapour deposition,
and focused electron beam-induced deposition [2,6-10]. Although these methods are very
useful for preparing cobalt silicide samples, several shortcomings have restricted their
2

application in catalysis, including the high cost of devices used for synthesis, and the large
consumption of energy and time due to the large energy barrier for the diffusion of solids [11].
These drawbacks motivate the development of new synthesis methods that can overcome
these problems and facilitate the formation of well-dispersed metal silicides for catalytic
applications under controllable conditions.
Microwave-assisted solid-state synthesis is a simple, fast, energy-efficient, and
environmentally friendly technique. The fast synthesis is attributed to the nature of the
microwave volumetric heating and to the associated thermal runaway effect [12]. This method
has been reviewed extensively in the literature [13-15], and a number of intermetallic
compounds have been produced with the aid of microwaves [11]. The rapid synthesis may
lead to smaller grain size and to consequently better mechanical properties, and improve
product uniformity. This is very interesting from an application point of view. Vaidhyanathan
et al. prepared CoSi2 samples by using high purity cobalt (particle size, 270 μm) and silicon
(particle size, 37 μm) as starting materials, with high purity amorphous carbon powder as a
microwave absorbing material [16]. Jokisaari et al. put a mixture of elemental cobalt (99.9
wt.%, <5 μm) and silicon (99.5 wt.%, -325 mesh) into an industrial microwave furnace and
used SiC as the microwave absorbing material, synthesizing bulk cobalt silicides [17].
Compared with amorphous carbon and SiC, carbon nanotubes (CNTs) had been used as
catalyst supports and showed an enhancement effect on the catalytic activity due to their
unique mechanical, chemical and electrochemical properties [18-19]. In our previous research,
carbides have been synthesized by microwave-assisted solid-state reaction, which can be used
as efficient electrocatalyst supports for the oxygen reduction reaction [20-22]. Therefore,
3

microwave-assisted solid-state synthesis is a very useful technology to design cobalt silicides
as catalytic materials.
In the present study, cobalt silicides supported on CNTs (CoSix/CNTs) catalysts have
been synthesized through a microwave-assisted route, in which the CNTs act as both
absorbing material and support. The as-prepared cobalt silicides supported on CNTs have
been used in the hydrogenation of phthalic anhydride, and afford good activity and high
selectivity to phthalide.
2. Experimental
Reagents and Equipment
The applied reagents included powders of CNTs (purity: 97.0 wt.%, diameter: 20-40 nm,
producer: Shenzhen Nanotech Port Co. Ltd.), silicon (99.9 wt.%, -200 mesh), ferric chloride
(97.0 wt.%), cobalt chloride (98.0 wt.%), nickel chloride (98.0 wt.%) and cupric chloride
(99.0 wt.%). Silicon powder was directly used without further treatment. The CNTs powders
and metal chloride powders were dried at 130 oC to remove the adsorbed water before use. A
domestic microwave oven (WBFY-201) operated at 2.45 GHz with a power of 800 W was
used for synthesis.
Catalyst Preparation
The CoSix/CNTs catalysts were synthesized via microwave-assisted route. Typically, 0.3
g of CNTs, 0.0757 g of cobalt chloride (CoCl2), and a certain amount of silicon powder were
put in an agate mortar, and mixed for 20 min. The completely homogeneous precursor
mixture was put in a quartz-tube reactor and fluidized with argon for1 h to remove oxygen in
4

the reactor. Then, the reactor was placed in a domestic microwave oven operating at 2.45
GHz with a power of 800 W. Finally, the resulting products were cooled down naturally to
room temperature under argon. The microwave irradiation time varied from 10 to 30 min and
Co:Si ratios from 1:1 to 1:5 were investigated on the formation mechanism of the products.
Catalyst Characterization
Thermogravimetric/derivative thermogravimetric (TG/DTG) experiments were
performed on a Mettler Toledo TGA/SDTA851e thermogravimetric analyser. The samples
o
were placed in an atmosphere of Ar and heated at 5 C /min to the final temperature of 800
oC.
X-ray diffraction (XRD) analyses of the samples were carried out using a Rigaku
D/Max-RB diffractometer with Cu Kα monochromatized radiation source, operated at 40 kV
and 100 mA. The average size of cobalt silicide particles was evaluated by the Scherrer
formula as:
0.9λKα
1
L =
B(2θ ) cosθmax
(1)
where λKα1 is 1.54178 Å, and B(2θ) is the full width at half-maximum of the diffraction peak in
radian.
High-resolution transmission electron microscopy (HRTEM) and energy-dispersive
X-ray spectroscopy (EDX) were performed by using a T2-Tecnai F30 transmission electron
microscope, operated at 200 KV.
H2-temperature-programmed-reduction (H2-TPR) analysis was carried out using a
Quantachrome ChemBET TPR/TPD system. About 10 mg of sample was placed in a quartz
5

reactor and was reduced in a stream of Ar gas with a flow rate of 50 mL/min. The sample was
heated from room temperature to 900 °C at 10 °C /min. The amount of the hydrogen
consumption during the reduction was estimated with a thermal conductivity detector.
Surface compositions of the samples were characterized by X-ray photoelectron
spectroscopy (XPS, ESCALAB250, Thermo VG). The XPS measurements were performed
using an X-ray source of Al Kα (1486.6 eV) with a powder of 150 W. The individual
high-resolution spectra were recorded with a pass energy of 50 eV. Co 2p, Si 2p, and C1s
lines were monitored. All the core-level spectra were corrected by referencing to the binding
energy of the C1s neutral carbon peak at 284.6 eV, and were deconvoluted into Gaussian
component peaks.
Catalyst Activity Evaluation
The catalytic activity of cobalt silicides supported on CNTs has been evaluated in the
hydrogenation of phthalic anhydride. Before the reaction, the catalyst was activated in
hydrogen at 400 °C for 2 h. Liquid-phase hydrogenation of phthalic anhydride was carried out
in a 50 mL stainless steel autoclave. The catalyst (0.3 g) was mixed with 20 g
4-hydroxybutanoic acid lactone solution containing a 5 wt.% phthalic anhydride, followed by
H2 pressurization to 4 MPa. The reaction was carried out at 180~220 °C for 6 h under stirring.
The products of hydrogenation were analysed by gas chromatography using a FID with a
OV-101 capillary column. A known amount of cyclohexane was used as an internal standard
in the analysis.
3. Results and Discussion
The XRD patterns of CoSix/CNTs prepared by microwave pyrolysis with different Co:Si
6

ratios are shown in Figure 1a. The diffraction peaks at 26.5, 43.8, and 54.0° observed in the
patterns could be attributed to the hexagonal graphite structure of CNTs. During the
microwave-assisted pyrolysis process, the CNTs played roles as both absorbing material and
supporter. The XRD pattern of Co/CNTs sample showed three typical diffraction peaks at
44.2, 51.5, and 75.8°, which were assigned to the (111), (200) and (220) crystal faces of Co
(JCPDS 15-0806), respectively. After silicon was put in the precursor, the XRD patterns of
CoSix/CNTs showed no diffraction peaks that could be assigned to Co, indicating that cobalt
chloride has reacted with silicon thoroughly. The XRD pattern of the sample prepared with a
Co:Si ratio of 1:1 presented four typical diffraction peaks at 34.9, 40.5, 45.6 and 50.2°, which
were, respectively, assigned to the (111), (200), (210) and (211) crystal faces of CoSi (JCPDS
50-1337). The formation mechanism of the cobalt silicide nanoparticles could be expressed
by the following reaction equation.
2 CoCl2 (s) + (2x+1) Si (s) →2 CoSix (s) + SiCl4 (g)
With further decrease of the Co:Si ratio to 1:3, the (111), (220), (311) and (400) planes
appeared at 2θ=28.8, 47.9, 56.9 and 70.1°, respectively, indicating the presence of CoSi2
phase (JCPDS 38-1449). Upon decreasing the Co:Si ratio from 1:3 to 1:5, the diffraction
peaks of the CoSi phase became weaker, and those of CoSi2 phase became sharper. Since
CoSi is a metastable phase, it will further react with silicon powder to form the CoSi2 phase
when the Co:Si ratio decreased.
To further study the formation process of cobalt silicide, the XRD patterns of the
CoSix/CNTs prepared for different microwave times were recorded, as shown in Figure 1b.
When the microwave time was extended from 10 to 30 min, the phase of the cobalt silicide
7

also transformed from CoSi to CoSi2. Based on the (210) peak at 50.2°, the average crystallite
size of the CoSix samples were calculated to be ca. 37 nm using the Scherrer formula. The
particle size did not apparently change with the transformation of the cobalt silicide phase,
because the variation of peak width is inconspicuous.
Traditionally, the formation of metal silicides through the reaction between metal
o
chloride and silicon powder requires high temperature (1200 C) and long sintering time,
which leads to the aggregation of particles. To obtain a good understanding of the as-prepared
precursor reaction process in the synthesis of cobalt silicides, the thermal decomposition
process of the precursor was characterized by TG/DTG (Figure 2). The mass loss at about 100
o
oC is attributed to the elimination of molecular water, and those at about 550 and 700 C are
only owing to the elimination of molecular chlorine. No cobalt silicide nanoparticles were
synthesised, which further confirmed that the microwave-assisted thermolytic precursor
method was an efficient way for the synthesis of evenly dispersed cobalt silicides.
Solid-state microwave synthesis of cobalt silicide is not an isolated phenomenon for a
few selected substances. So the reaction between metal and the added material as an absorber
of the microwave radiation may be proceed to form alloys or intermetallic compounds.
According to this hypothesis, other transition metal silicides were attempted to be synthesized
using this novel methodology. As shown in Figure 3, the (110), (111), (200), (210) and (211)
planes appeared at 2θ=28.1, 34.6, 40.2, 45.1 and 49.7°, respectively, indicating the presence
of FeSi phase. The (211), (021), (220), (121) and (002) planes appeared at 2θ=39.4, 43.5, 44.3,
45.5 and 48.8°, respectively, indicating the presence of Ni2Si phase. And the planes appeared
at 2θ=43.0, 43.5, 46.0, and 47.8°, respectively, indicating the presence of Cu4Si phase.
8

Therefore, FeSi, Ni2Si and Cu4Si supported on CNTs can also be synthesized by these
microwave-assisted solid-state reactions.
The TEM image in Figure 4a shows uniformly and highly dispersed cobalt silicide
nanoparticles on the CNTs. The microwaves coupled with the absorption band of the
materials, which heated the materials uniformly in a short time, avoiding the aggregation of
cobalt silicide nanoparticles. The inset in Figure 4a displays a typical TEM image of the
nanoparticles decorated on the CNTs, indicating that its size was approximately 40 nm, which
is basically consistent with the calculated particle size value from XRD results by Scherrer
formula. The high-magnification TEM image (Figure 4b) could be used to further probe the
cobalt silicide nanoparticles. Based on two-dimensional fast Fourier transform (FFT), the
observed lattice spacing of 0.182 nm matched well with the reported value of the (211) plane
in the CoSi structure (JCPDS 50-1337) [7]. The EDX spectrum in Figure 4c from a single
cobalt silicide nanoparticle indicated that Co and Si atoms were the only elements presenting
in the nanoparticles with an atomic ratio of approximately 1:1, which was also in good
agreement with the value from XRD. This data confirms that the prepared cobalt silicide
particles have a composition of CoSi.
In order to investigate the catalytically active sites and surface chemical structure
relationships, the surfaces of CoSix/CNTs catalysts have been probed by XPS. Figure 5a
shows the XPS profiles, revealing that Co, C, Si, Cl and O elements were detected in the
samples. From the Co 2p spectra shown in Figure 5b, the peak at about 785 eV can be
attributed to the metallic cobalt phase for the Co/CNTs sample. With the decrease of the
Co:Si ratio to 1:2, one peak at 782.5 eV was observed, which can be assigned to the CoSi
9

phase. With further decrease of the Co:Si ratio to 1:3, the peak at 778.2 eV due to the CoSi2
phase can be observed in the Co-3Si/CNTs sample [23]. This result is consistent with the
XRD results. The analytical result on Si 2p signal is shown in Figure 5c. The peak in the Si 2p
region at 99 eV corresponding to silicon in cobalt silicide would shift to the higher binding
energy due to the electron shift from cobalt to silicon atoms with the increased content of Si,
which further confirmed the interaction between Co and Si [24]. Meanwhile, there is an extra
broad peak at 101-104 eV in the Si 2p spectra attributed to Si-Ox, which is owing to the
formation of passivated layers on the cobalt silicides [25].
TPR is a powerful tool for characterizing supported catalysts. Figure 6a shows the
H2-TPR profiles of cobalt silicide samples with different Co:Si ratios. Only one peak was
observed at about 500 oC for Co/CNTs, which could be tentatively assigned to the reaction of
cobalt oxide with H2. When silicon was mixed in the precursor, two peaks were observed at
about 470 and 530 °C, which could be tentatively assigned to the reaction of cobalt and cobalt
silicide with H2. In addition, the peak for the CoSix/CNTs is higher than that of Co/CNTs,
indicating that CoSix/CNTs catalysts have higher activation temperature. However, the peak
shifts to lower temperature for the samples with Co:Si ratio decreasing from 1:1 to 1:3, which
can be attributed to the phase of cobalt silicides transforming from CoSi to CoSi2. Because the
Si content in CoSi2 phase is significantly higher than that in CoSi, so more silicon oxides
passivation layer attached on the CoSi2/CNTs catalyst, which would lead to a lower activation
temperature of CoSi2 [5]. In addition, the silicides can react with H2 at relatively high
temperature (ca. 500 °C) as CoSix + 2xH2 → Co + xSiH4 [26]. As shown in Figure 5b, the Co
binding energy of CoSi2 phase was lower than that of CoSi phase. Therefore, the CoSi2 phase
10

can react with hydrogen easier than CoSi, indicating CoSi2 has a lower activation temperature
than CoSi.
The H2-TPR profiles of cobalt silicide samples with different microwave times were also
detected as shown in Figure 6b. For the samples prepared by increasing the microwave time
from 10 to 20 min, the TPR peak became sharper. However, when increasing the microwave
time to 30 min, the peak became weaker. The reason is also that the phase of cobalt silicides
transformed from CoSi to CoSi2 by increasing the microwave time to 30 min, which is in
good agreement with the result from XRD.
The scheme of hydrogenation of phthalic anhydride is shown in Scheme 1 [27-28]. The
results for hydrogenation of phthalic anhydride over Co/CNTs and CoSix/CNTs catalysts at
190 and 200 oC are shown in Table 1. It is clear that CoSix/CNTs catalyst could catalyze about
o
100% conversion of phthalic anhydride at 200 C, while Co/CNTs catalyst only catalyzed
about 80% conversion. Obviously, the addition of Si to Co/CNTs significantly promoted the
rate of hydrogenation of phthalic anhydride. It is possible that the interaction between Co and
Si atoms tuned the geometry and electronic structure, further effecting the catalytic activity
and selectivity to phthalide. However, at the same time, the Co/CNTs catalyst has about 80%
selectivity to phthalide. The as-prepared CoSix/CNTs catalysts have lower selectivity than the
Co/CNTs catalyst. The as-prepared CoSix/CNTs are more stable than the corresponding metal
catalyst in the acidic environment caused by the substrate, which is due to the higher chemical
and thermal stability of cobalt silicides. With increasing Si powder concentration from Co:Si
ratio 1:1 to 1:3, the phase of cobalt silicide samples transformed from CoSi to CoSi2, which
caused the conversion of phthalic anhydride to decrease from 98.8% to 44.4% at 190 oC. The
11

corresponding selectivity to phthalide increased from 72.7% to 77.7%. Increasing the
hydrogenation reaction temperature from 190 to 200 oC, the conversion of phthalic anhydride
increased.
Figure 7 shows the conversion of phthalic anhydride and selectivity to phthalide over
o
Co/CNTs and CoSix/CNTs catalysts at 190 C with different reaction times. The Co/CNTs
catalyst showed only 70% conversion of phthalic anhydride in 9 h reaction time, lower than
the conversion for the CoSix/CNTs catalysts, which is in good agreement with the result in
Table 1. When increasing the Si powder concentration from Co:Si ratio 1:1 to 1:3, the
prepared Co-3Si/CNTs had low conversion, because the phase of cobalt silicide transformed
from CoSi to CoSi2 phases. This result indicated that CoSi had higher catalytic activity than
CoSi2. When the catalytic reaction time is 7.5 h, the conversion of phthalic anhydride is about
100% by using the prepared Co-Si/CNTs as catalysts and the phthalide content is almost 80%.
With further increase in reaction time, a part of phthalide was further hydrogenated to benzoic
acid and o-toluic acid. So, it can be concluded that the catalyst activity of Co-Si/CNTs is
apparently higher than those of Co-2Si/CNTs and Co-3Si/CNTs.
To probe the effect of microwave time on catalytic activity, the catalyst with a Co:Si
ratio of 1:3 was treated at 10, 20 and 30 min. As shown in Figure 8, an optimum activity was
found by increasing the microwave time. When increasing the time from 10 to 20 min, the
conversion increased, but the selectivity to phthalide was almost invariant (about 75~80%).
When further increasing the microwave time from 20 to 30 min, the conversion and the
selectivity decreased. As shown in the results of XRD analysis (See in Figure 1b), cobalt
silicide phases transformed from CoSi to CoSi2 with increasing microwave time. However,
12

the particle size presented no significant change, keeping about 30-40 nm based on the
calculation by the Scherrer formula. Therefore, the change of catalytic activity is mainly
correlated to the phase composition, apparently indicating that CoSi phase have better
catalytic activity than CoSi2 phase.
4. Conclusions
Cobalt silicides with different phases (CoSi or CoSi2) supported on CNTs catalysts have
been successfully synthesized by a rapid microwave-assisted route. This novel methodology
can also be applied to synthesize other transition metal silicides, such as FeSi, Ni2Si and
Cu4Si. The as-prepared CoSix/CNTs can be employed in hydrogenation of phthalic anhydride.
Due to the improved chemical and thermal stability of cobalt silicide, CoSix/CNTs catalysts,
compared to their corresponding metal catalyst counterpart, presented high catalytic stability
in the hydrogenation of phthalic anhydride. Si atoms reside into the interstitial sites between
Co atoms and change the cobalt unit cell lattice, further effecting the catalytic activity and
selectivity to phthalide.
Acknowledgements
We gratefully acknowledge the financial support provided by the National Natural
Science Foundation of China (21373038), the China Postdoctoral Science Foundation
(2014M551068), and the Fundamental Research Funds for the Central Universities
(DUT12YQ03 and DUT14RC(3)007).
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Scheme and figure captions
Scheme 1 The scheme of hydrogenation of phthalic anhydride over CoSix/CNTs catalysts.
Figure 1 (a) XRD patterns of the CoSix/CNTs with 20 min microwave irradiation time and
different Co:Si ratios prepared by microwave pyrolysis, and the corresponding standard XRD
patterns (Co JCPDS 15-0806, CoSi JCPDS 38-1337, and CoSi2 JCPDS 38-1449). (b) XRD
patterns of the CoSix/CNTs with different microwave times prepared by microwave pyrolysis.
Figure 2 TG-DTG profile of the as-prepared CoSix/CNTs precursor.
Figure 3 XRD patterns of the metal silicide phases supported on CNTs: (a) FeSi, (b) CoSi, (c)
Ni2Si and (d) Cu4Si with 20 min microwave time prepared by microwave pyrolysis and the
corresponding standard XRD patterns (FeSi JCPDS 38-1397, CoSi JCPDS 38-1337, Ni2Si
JCPDS 50-0779, and Cu4Si JCPDS 03-0990).
Figure 4 TEM results of CoSix/CNTs prepared with Co:Si ratio 1:1: (a) low-magnification
TEM image of the CoSix/CNTs and typical image of the nanoparticles decorated on the CNTs
(the inset); (b) high-magnification TEM image and corresponding fast Fourier transforms of
the lattice-resolved images (the inset), and (c) TEM-EDX spectrum from Figure 4(b).
17

Figure 5 XPS profiles of (a) all elements in sample and regions: (b) Co 2p and (c) Si 2p of
CoSix/CNTs.
Figure 6 H2-TPR profiles of the cobalt silicides supported on CNTs (a) with different Co:Si
ratios and (b) with different microwave times.
Figure 7 Conversion of phthalic anhydride and selectivity to phthalide over Co/CNTs and
CoSix/CNTs with different Co:Si ratios and reaction times.
Figure 8 Conversion of phthalic anhydride and selectivity to phthalide over Co-3Si/CNTs
with different microwave times and temperatures.
Table 1. Conversion of phthalic anhydride and selectivity to phthalide over CoSix/CNTs with
the different Co/Si ratios
Catalyst (Co/Si
ratio)
Selectivity to phthalide
(%)
Temperature (oC) Conversion (%)
190
200
190
200
190
200
190
200
98.8
99.7
71.0
96.1
44.4
97.8
49.2
83.7
72.7
57.8
72.5
67.5
77.7
60.3
82.8
80.1
1:1
1:2
1:3
Co/CNTs
Graphical Legend
CoSix/CNTs catalysts with different CoSix phases (CoSi2, CoSi) have been rapidly
synthesized via microwave-assisted route, which involves the vaporization of CoCl2 and
subsequent reaction of CoCl2 with Si.
18

Scheme 1 The scheme of hydrogenation of phthalic anhydride over CoSix/CNTs catalysts.
(a
19

(b)
Figure 1 (a) XRD patterns of the CoSix/CNTs with 20 min microwave irradiation time and
different Co:Si ratios prepared by microwave pyrolysis, and the corresponding standard XRD
patterns (Co JCPDS 15-0806, CoSi JCPDS 38-1337, and CoSi2 JCPDS 38-1449). (b) XRD
patterns of the CoSix/CNTs with different microwave times prepared by microwave pyrolysis.
20

Figure 2
Figure 2 TG-DTG profile of the as-prepared CoSix/CNTs precursor.
21

Liangliang Zhang, et al., Journal of Solid State Chemistry, Figure 3
Figure 3 XRD patterns of the metal silicide phases supported on CNTs: (a) FeSi, (b) CoSi, (c)
Ni2Si and (d) Cu4Si with 20 min microwave time prepared by microwave pyrolysis and the
corresponding standard XRD patterns (FeSi JCPDS 38-1397, CoSi JCPDS 38-1337, Ni2Si
JCPDS 50-0779, and Cu4Si JCPDS 03-0990).
22

Liangliang Zhang, et al., Journal of Solid State Chemistry, Figure 4
(
(
b)
a)
(
c)
Figure 4 TEM results of CoSix/CNTs prepared with Co:Si ratio 1:1: (a) low-magnification
TEM image of the CoSix/CNTs and typical image of the nanoparticles decorated on the CNTs
(the inset); (b) high-magnification TEM image and corresponding fast Fourier transforms of
the lattice-resolved images (the inset), and (c) TEM-EDX spectrum from Figure 4(b).
23

Liangliang Zhang, et al., Journal of Solid State Chemistry, Figure 5
(a
(
(
Figure 5 XPS profiles of (a) all elements in sample and regions: (b) Co 2p and (c) Si 2p of
CoSix/CNTs.
24

Liangliang Zhang, et al., Journal of Solid State Chemistry, Figure 6
(a
(b
Figure 6 H2-TPR profiles of the cobalt silicides supported on CNTs (a) with different Co:Si
ratios and (b) with different microwave times.
25

Liangliang Zhang, et al., Journal of Solid State Chemistry, Figure 7
(b
(a
(c
(d
26

Figure 7 Conversion of phthalic anhydride and selectivity to phthalide over Co/CNTs and
CoSix/CNTs with different Co:Si ratios and reaction times.
27

Liangliang Zhang, et al., Journal of Solid State Chemistry, Figure 8
(a
(b
Figure 8 Conversion of phthalic anhydride and selectivity to phthalide over Co-3Si/CNTs
with different microwave times and temperatures.
28

Highlights
9 CoSix/CNTs catalysts have been rapid synthesized via microwave-assisted route.
9 The phases of CoSix were controlled by varying microwave time and Co:Si ratio.
9 FeSi, Ni2Si and Cu4Si were also synthesized via microwave-assisted route.
9 CoSix/CNTs catalysts can be applied in hydrogenation of phthalic anhydride.
29