Hydrodechlorination of silicon tetrachloride to trichlorosilane over ordered mesoporous carbon catalysts: Effect of pretreatment of oxygen and hydrochloric acid

Article abstract of DOI:10.1166/jnn.2016.11944

This paper reports on the catalytic reaction for the conversion of silicon tetrachloride (STC) to trichlorosilane (TCS) over pretreated ordered mesoporous carbon (OMC) catalysts by oxygen (denoted as OMC-O₂) and hydrochloric acid (denoted as OM

Full text of DOI:10.1166/jnn.2016.11944

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Hydrodechlorination of Silicon Tetrachloride to
Trichlorosilane Over Ordered Mesoporous Carbon
Catalysts: Effect of Pretreatment of Oxygen and
Hydrochloric Acid
Do-Hwan Kwak1, M. Shaheer Akhtar1ꢀ2, Ji Man Kim3, and O-Bong Yang1ꢀ2ꢀ∗
1School of Semiconductor and Chemical Engineering and Solar Energy Research Center, Chonbuk National University,
Jeonju, 561-756, Republic of Korea
2New and Renewable Energy Materials Development Center (NewREC), Chonbuk National University,
Jeonbuk, 579-841, Republic of Korea
3Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Republic of Korea
This paper reports on the catalytic reaction for the conversion of silicon tetrachloride (STC) to
trichlorosilane (TCS) over pretreated ordered mesoporous carbon (OMC) catalysts by oxygen
(denoted as OMC-O2) and hydrochloric acid (denoted as OMC-HCl) at 300 ꢀC under N2 atmo-
sphere. The OMC-O shows significantly improved the surface area (1341.2 m2/g) and pore volume
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2
(1.65 cm3/g), which resulted in the highest conversion rate of 7.3% as compared to bare OMC
IP: 50.44.163.227 On: Wed, 30 Dec 2015 08:04:37
Copyright: American Scientific Publishers
(4.3%) and OMC-HCl (5.7%). It is found that the conversion rate of STC to TCS is proportional
to the number of Si–O bond over OMC catalysts, which suggests that Si–O–C bond formation is
crucial to the reaction as active sites. The O2 pretreatment seems to promote the generation of
oxygenated species for the formation of Si–O–C.
Keywords: Ordered Mesoporous Carbon, Pretreatment, Trichlorosilane, Silicon Tetrachloride.
1. INTRODUCTION
silicon tetrachloride (STC, SiCl4) as a major byproduct,4
as represented in reaction (1).
Last few decades, silicon (Si) solar cells industries, R&D
groups, and researchers are devoted numerous efforts to
reduce the production price of solar grade polysilicon
(SoG-Si). Recent European economic crisis allows the cer-
tain entry of China in the photovoltaic market and visibly
observes the drastic reduction in the prices of SoG-Si i.e.,
up to 23.2$/Kg or less.1 A survey report reveals that the
SoG-Si production was globally increased approximately
of 227,000 tons in 2013.2 Until now the poly-Si produc-
tion based on trichlorosilane (TCS, HSiCl3)-Siemens pro-
cess is covering around 75% of total poly-Si worldwide.3
Recycling process and poly-Si from TCS are realized as
alternative way to reduce the poly-Si production cost.
The production of poly-Si through the chemical vapor
deposition (CVD) process of TCS (SiHCl3) is released the
4HSiCl3ꢀgꢁ → Siꢀsꢁ+3SiCl4ꢀgꢁ+2H2ꢀgꢁ
(1)
In the above process, 1 mole of Si is converted to
poly-Si, however, 3 to 4 moles of STC are formed by
the consuming large amounts of chlorine and valuable
silicon.5 Thus, to minimize the production cost of poly-
Si, the byproduct STC is needed to use as a reactant
for producing the TCS via an efficient process with low
cost. In general, TCS is prepared by hydrochlorination
of metallurgical grade Si (MG-Si) using the batch reac-
tor, which is displayed in the following reaction (2)
and (3);5ꢂ6
Siꢀsꢁ+3HCl = HSiCl3 +H2
Siꢀsꢁ+4HCl = SiCl4 +2H2
(2)
(3)
∗Author to whom correspondence should be addressed.
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J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 2
1533-4880/2016/16/1802/004
doi:10.1166/jnn.2016.11944

Kwak et al.
Hydrodechlorination of Silicon Tetrachloride to Trichlorosilane Over OMC Catalysts
Two processes such as clean recycling and dirty recy-
cling of STC to TCS are available.3 These processes are
expressed by the following reactions (4) and (5);
SiCl4ꢀgꢁ+H2ꢀgꢁ → SiHCl3ꢀgꢁ+HClꢀgꢁ
MG −Siꢀsꢁ+2H2ꢀgꢁ+SiCl4ꢀgꢁ → 4SiHCl3ꢀgꢁ
(4)
(5)
Both STC recycling processes involves a thermal reac-
tion at high temperatures exceeding 1000 ꢀC and high
pressures, which often have several disadvantages such as
the high energy consumption and undesirable chlorosilane
products.3 Therefore, it is realized to develop suitable pro-
cess such as catalytic process for the conversion of STC
to TCS at low temperature with less impurity.7 Recently,
the carbon and its composites have shown the good cat-
alytic activity towards the conversion of STC to TCS.7
Among various carbon materials, ordered mesoporous car-
bon (OMC) are presented the unique properties such as
high specific surface area, high electrical conductivity and
good stability in acid and alkaline media.8 In this work,
the effect of pretreatment of OMC on the catalytic activity
of STC conversion to TCS was studied. The oxygen pre-
treated OMC showed the high activity as compared to the
hydrochloric acid pretreated OMC, owing the generation
of oxygenated species for the formation of Si–O–C as an
active site.
Figure 1. Catalytic conversion of STC to TCS over bare OMC, OMC-
O2 and OMC-HCl at 700 ꢀC.
The conversion rate of STC to TCS was monitored by
the gas chromatograph (Model 7890 A, Agilent) in every
40 min.
3. RESULTS AND DISCUSSION
Figure 1 shows the conversion of STC to TCS with respect
to reaction time over the pretreated OMC catalysts. The
ꢀ
catalytic activity of OMC-O2300 C catalyst exhibits the
highest conversiꢀon rate of 7.27% for STC to TCS con-
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version at 700 C. Under the same condition, the bare
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2. EXPERIMENTAL DETAILS
2.1. Pretreatment on OMC Catalyst
ꢀ
OMC and OMC-HCl300 C show the lower conversion
Copyright: American Scientific Publishers
rates of 4.29% and 5.73%, respectively. However, the
conversion of STC to TCS without catalyst records the
Ordered mesoporous carbon (OMC) was synthesized by
employing the previously reported method.9 The oxygen
(O2) and hydrochloric acid (HCl) pretreatments were car-
ried out for improving the surface properties of prepared
OMC. The pretreatment was carried out in the U-tube reac-
tor under highly controlled high temperature reactor. First
OMC powder was packed into the U-tube and connected
the O2 gas line or HCl vapor line under N2 atmosphere.
ꢀ
O2 and HCl pretreatments were executed ꢀat 300 C for
2 h which were denoted as OMC-O2300 C and OMC-
HCl300 ꢀC, respectively. After completion of pretreatment,
the OMC was taken out after cooling under nitrogen gas
flow to room temperature and finally OMC materials were
ꢀ
reduced at 500 C with H2 under N2 Flow.
2.2. Catalytic Reaction Setup
The catalytic hydrodechlorination of STC to TCS was car-
ried out using the fixed bed reactor consisting of quartz
U tube with the dimensions (length: 450 mm, inner diam-
eter: 25 mm). In the beginning, OMC (200 mg) catalyst
was placed on a fritz plate positioned inside a quartz tube
and then introduced STC into the reactor tube from a STC
saturator by N2 gas and the temperature of STC saturator
ꢀ
was maintained at 25 C by heating bend. The flow rate
Figure 2. FESEM images of (a) bare OMC/before, (b) bare OMC/after
reaction, (c) OMC-O2/before, (d) OMC-O2/after reaction, (e) OMC-
HCl/before and (f) OMC-HCl/after reaction.
of H2 and STC, N2 gas mixtures was kept 463 ml/min and
H2/STC molar ratio was optimized to 4 for all reactions.
J. Nanosci. Nanotechnol. 16, 1802–1805, 2016
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Hydrodechlorination of Silicon Tetrachloride to Trichlorosilane Over OMC Catalysts
Kwak et al.
Table I. Surface properties and activity of OMC catalysts for the con-
version of STC to TCS at 700 ꢀC.
Surface properties
SBET
(m2 ·g−1
Micropore volume Pore size Conversion
Catalyst
OMC
)
(cm3 ·g−1
)
(Å)
(%)
1268.7
1.57
1.61
1.65
51.24
50.85
50.19
4.3
5.7
7.3
OMC-HC1300 ꢀC 1327.6
OMC-O2300 ꢀC
1341.2
negligible conversion rate (not shown here). It is clear that
O2 and HCl pretreatments on OMC catalyst are beneficial
for the effective dehydrochlorination of STC to TCS. The
enhancement in conversion rate could be interpreted by the
improvement in the catalyst properties such as morphol-
ogy, surface area, pore volume and surface groups, mainly
oxygen surface groups.
Figure 2 shows the field emission scanning electron
microscopic (FESEM, Model S4800, HITACHI) images of
OMC catalysts before and after catalytic reactions. The O2
pretreatment significantly improves the porous nature of
OMC, as visible in the Figure 2(c).
Figure 4. FTIR spectra of bare OMC, OMC-O2300 ꢀC and OMC-
HCl300 ꢀC after catalytic conversion of STC to TCS.
OMC catalysts due to the generation of surface defects by
oxygenated species or Cl− ions and structural changes.10
Figure 3 shows the correlation between the conversion
of STC to TCS and surface areas/pore volumes of OMC
catalysts. The conversion rates of STC to TCS increases
with the increase of surface area and pore volume of the
OMC catalysts. It is reported that the generation of active
sites on the carbon surface are dependent on the thermal
history of material such as heat treatment and heat pre-
treatment, which causes the surface changes in quantify
the carbon active sites.11 The O treatment on OMC might
The OMC particles are agglomerated and increase the
size of particles after catalytic conversion reaction. How-
ever, bare OMC and OMC-HCl catalysts display the less
porous and uniform OMC particles, which are also aggre-
gated after catalytic reactions. The agglomeration of OMC
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particles suggests the deactivation of catalytic surfaces
IP: 50.44.163.227 On: Wed,o3ri0giDnaetect2h0e1la5rg0e8n:0u4m:3b7er of oxygenated species, resulting
Copyright: American Scientific Publishers
after the reaction. Importantly, the pretreatment on OMC
in the high amount of active sites over the OMC. In sup-
port, the generation of oxygenated species might beneficial
for the adsorption of Si species (–Si–Cl of STC) over car-
bon surface. Therefore, OMC-O2 might create large num-
ber of active sites for the high conversion of STC to TCS
through the intermediate metal silicides on the carbon sur-
face. To understand the interaction of STC to carbon sur-
face, the structural characterization of OMC catalysts has
been done after catalytic conversion of STC to TCS.
Figure 4 shows FTIR spectra of OMC catalysts
after reaction. All catalysts display similar IR bands at
∼3520 cm−1, ∼1490 cm−1 and ∼1135 cm−1 cm−1, cor-
responding to O–H, C C and Si–O groups on the sur-
catalysts considerably enhances the surface area, porosity
and pore volume, which might increase the active sites
over the surface of OMC and conversion rate of STC
to TCS.
The surface properties of OMC catalysts are summa-
rized in the Table I. Bare OMC exhibits the BET sur-
face area of 1268.7 m2/g with pore volume of 1.57 cm3/g
and pore size of 51.2 Å. After pretreatment with O2 and
HCl, the surface area, pore volume and pore sizes are
increased. OMC-O2 catalyst obtains the highest surface
area of 1341.2 m2/g pore volume of 1.65 cm3/g and pore
size of 50.2 Å. This observation confirms that the pretreat-
ment of HCl and O2 is increment the surface properties of
face of OMC.12–15 The appearance of OH and C
C
Figure 3. Plots of conversion STC to TCS versus (a) surface area and (b) pore volume of OMC.
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Kwak et al.
Hydrodechlorination of Silicon Tetrachloride to Trichlorosilane Over OMC Catalysts
bonds confirms the nature of carbon. The origin of Si–O
bond clearly suggests the formation of metal silicides over
the surface of OMC catalyst. It is also noticed that the
extent of Si–O peak intensity in OMC-O2 is higher than
those of bare OMC and OMC-HCl catalysts. Thus, high
extend of Si–O bond in OMC-O2 may promote the large
number of active sites and increase the formation of sili-
cides on the surface of OMC-O2, resulting in the highest
conversion rate of 7.3%.
Resource Development program of “Advanced track for
Si-based solar cell materials and devices” (Project number:
20124030200080). This paper was supported by research
funds of Chonbuk National University in 2014.
References and Notes
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4. CONCLUSION
The surface analysis revealed that O2 pretreatment sig-
nificantly enhanced the surface area, and pore volume of
OMC as compared to bare OMC and OMC-HCl. The
OMC-O2 catalyst exhibited the highest conversion rate of
7.3% for the catalytic hydrodechlorination of STC to TCS
as compare to the bare OMC (4.3%) and OMC-HCl (5.7%)
catalysts. The conversion rate of STC to TCS is propor-
tional to the number of Si–O bond over OMC catalysts,
which suggests that Si–O–C bond formation is crucial to
the reaction as active sites. The O2 pretreatment seems to
promote the generation of oxygenated species for the for-
mation of Si–O–C. This result would offer the promising
pretreatment tool for the developing a stable and effective
catalyst for STC to TCS conversion.
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Acknowledgment: We gratefully acknowledge the
support from Korea Government Ministry of Trade, Indus-
try and Energy and Korea Institute of Energy Technol-
ogy Evaluation and Planning (KETEP) by the Human
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Received: 18 March 2015. Accepted: 9 April 2015.
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