Synthesis of mesoporous tungsten carbide by an impregnation-compaction route, and its NH3 decomposition catalytic activity

Article abstract of DOI:10.1039/b809923e

A mesoporous tungsten carbide (WC) with high surface area (138 m ² g^-1) has been synthesized at 700 °C using a modified template replicating route in which the pore entrances were sealed under external pressure after the W source impregnation. X-Ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray (EDX) spectroscopy and nitrogen sorption techniques were employed to characterize the replicated material. The prepared tungsten carbide with high surface area is well crystallized and has a partially ordered mesoporous structure. Temperature programmed reaction (TPR) and time-on-stream (TOS) results show that the prepared mesoporous tungsten carbide has high and stable catalytic activity for the decomposition reaction of NH₃, and the complete NH₃ decomposition temperature was obtained at 500 °C. This mesoporous WC appears to be a good catalyst for use for NH₃ decomposition.

Full text of DOI:10.1039/b809923e

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Synthesis of mesoporous tungsten carbide by an impregnation–compaction
route, and its NH3 decomposition catalytic activity
Xiangzhi Cui,a Hua Li,b Limin Guo,a Dannong He,c Hangrong Chena and Jianlin Shi*a
Received 11th June 2008, Accepted 28th July 2008
First published as an Advance Article on the web 8th October 2008
DOI: 10.1039/b809923e
A mesoporous tungsten carbide (WC) with high surface area (138 m2 g-1) has been synthesized at
700 ◦C using a modified template replicating route in which the pore entrances were sealed under
external pressure after the W source impregnation. X-Ray diffraction (XRD), transmission electron
microscopy (TEM), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray
(EDX) spectroscopy and nitrogen sorption techniques were employed to characterize the replicated
material. The prepared tungsten carbide with high surface area is well crystallized and has a partially
ordered mesoporous structure. Temperature programmed reaction (TPR) and time-on-stream (TOS)
results show that the prepared mesoporous tungsten carbide has high and stable catalytic activity for
the decomposition reaction of NH3, and the complete NH3 decomposition temperature was obtained at
500 ◦C. This mesoporous WC appears to be a good catalyst for use for NH3 decomposition.
gas mixture (H2/CO) prior to the NH3 decomposition reaction.
The NH3 decomposition on a commercial Fe ammonia synthesis
1. Introduction
catalyst (Amomax-10) with a surface area of 0.17 m2 g-1 reached
a conversion rate as low as 35% at 650 ◦C as reported by Sourabh
et al.13 As we know, the catalytic performances of the materials are
strongly dependent on the surface area and particular structures.
Nanoporous-structured tungsten carbides of high surface area
and appropriate pore size are highly desired for the applications
as catalysts.
Carbide materials, especially tungsten carbide (WC), have re-
ceived considerable attention in recent years because of their
high catalytic activities resembling metallic Pt in heterogeneous
catalysis, NH3 synthesis, ethylene hydrogenation and cyclohexane
dehydrogenation, methane reforming and CO hydrogenation.1–5
Moreover, their relatively low cost and especially high resistance
to chemical erosion and catalyst poisoning make them outstanding
candidates for catalytic applications among other catalysts.6
Unfortunately, WC materials prepared by conventional methods
usually have low surface area (5–30 m2 g-1) due to the difficulties of
synthesis.7–9 The temperature-programmed reaction method is the
major route to prepare tungsten carbides with high surface area,7
and J. M. Giraudon et al.8 explored the mechanism of reduction
and carburization of this route and obtained tungsten carbides
with a surface area as high as 30 m2 g-1, which is, however, still not
high enough if compared to other materials. Li et al.9 prepared
hollow spherical WC particles with mesoporosity and conducted
an electrocatalytic investigation, however, the surface area of the
prepared WC was only 4.9 m2 g-1.
Ammonia decomposition is an important reaction, and there
has been an increased interest in NH3 decomposition as a potential
COx-free source of on-site H2 generation for PEMFC.10 The reac-
tion of NH3 decomposition is also important in the cleaning-up
of syngas derived from coal11 and biomass gasification.12 Recently,
Sourabh et al.13 reported the catalytic activity of tungsten carbide
with a surface area of 1.5 m2 g-1 toward NH3 decomposition,
and the complete NH3 decomposition temperature on WC is
550 ◦C, however, the prepared WC needs to be treated with a
Mesoporous materials with ordered pore structures and high
surface areas are technologically important for a variety of appli-
cations such as in heterogeneous catalysis, adsorption, chemical
sensing, electrodes, transportation/storage of fluids and gases, and
also in some biological areas.14–18 More recently, Raman et al.19 and
Dong et al.20 synthesized mesoporous WC phase by a surfactant-
assisted route, however, a relatively high temperature calcination
◦
at 900 C in argon and hydrogen flow was needed, which led to
a rather low surface area of 76 m2 g-1 as compared to common
mesoporous materials by similar surfactant routes. Recently, the
replicating method using ordered mesoporous silica as hard
template and micro-reactor is regarded as an effective way to
synthesize novel ordered nanostructure or nanoarray mesoporous
materials, such as carbon, metal, transition metal oxides, sulfides,
and polymer.21–25 However, as far as we know there is no report
up to now about the synthesis of mesoporous WC by this method,
most probably due to the easy volatilization of W sources like
PW12 in the thermal process during replication. Herein, we present
a modified replication method to synthesize mesoporous WC with
high surface area of 138 m2 g-1, which is the highest up to now as
we could found in the literature, and crystallized framework using
the cubic Ia3d mesoporous silica as hard template and PW12 as the
W source. In this route the introduced PW12 was sealed in the pore
channels by compaction under an external pressure to prevent
the volatilization of W source during the following reactions with
carbon source at a certain temperature, and WC particles can
form only in pore channels which act as a closed micro-reactor.
Besides, the exaggerated growth of formed WC nanoparticles can
aState Key Laboratory of High Performance Ceramics and Superfine
Microstructures, Shanghai Institute of Ceramics, Chinese Academy of
Sciences, 1295 Dingxi Road, Shanghai, 200050, China. E-mail: jlshi@sunm.
shcnc.ac.cn; Fax: +86-21-52413122; Tel: +86-21-52412714
bSuzhou University, 50 Donghuan Road, Suzhou, 215021, China
cNational Engineering Research Center for Nanotechnology, 28 Jiangchuan
Road, Shanghai, China
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also be prevented by the confinement effect of silica template and
low reaction temperatures. This mesoporous material show high
catalytic activity for the decomposition of NH3 at relatively low
temperature.
from 650 ◦C to 700 ◦C at 3 ◦C min-1 and then held at 700 ◦C for 5 h
under ternary mixed gas flow of Ar, H2 and CO (1:0.05:1), finally
the compacts were cooled to room temperature under mixed gas
flow of Ar and H2 (1:0.05). The silica template was afterwards
removed with 2 M HF solution over night. The template-free
product was washed with distilled water thoroughly, collected by
centrifugation, dried in vacuum at 40 ◦C. This product was named
as WC-P. For comparison, the sample prepared according to the
above procedure but without compaction was named as WC-NP.
2. Experimental
A presentation of the modified template replicating route is
shown in Scheme 1. The tungsten precursor, 12-phosphotungsten
acid (PW12), was introduced into the pore channels of silica
template (KIT-6) by liquid impregnation method. After drying,
the PW@KIT-6 composites were pressed into compact discs
under 10 MPa so that the pore entrances of the mesoporous
silica template were blocked and sealed, and the originally open
pore structures were collapsed, forming a compact mixture of
PW@KIT-6. Then the PW12 was reduced and carbonized after
heat treatment under a certain atmosphere. Finally the removal
of silica template yielded the mesoporous WC replica. The
compaction process was employed to prevent the PW12 precursor
from volatilization and exaggerated growth of WC during heat
treatment.
2.2 Material characterization
X-Ray diffraction (XRD) patterns of prepared samples were
recorded on a Rigaku D/Max-2550V X-ray diffractometer with a
Cu-Ka radiation target (40 kV, 40 mA). The scanning rates were
0.6◦ per min and 6◦ per min for the low-angle and high-angle
XRD measurements, respectively. The N2 sorption measurement
was performed using Micromeritics Tristar 3000 at 77 K, and
specific surface area and the pore size distribution were calculated
using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-
Halenda (BJH) methods, respectively. Transmittance electron
microscopy (TEM) images were obtained on a JEOL 200CX
electron microscope operating at 160 kV. Energy-dispersive X-
ray (EDX) spectrum was collected from an attached Oxford
Link ISIS energy-dispersive spectrometer fixed on a JEM-2010
electron microscope operated at 200 kV. Field emission scanning
electron microscopy (FE-SEM) was conducted on JSM-6700F
field emission scanning electron microscopy.
The catalytic reactions for NH3 decomposition were carried out
under ambient pressure in a temperature-programmed, fixed-bed
quartz-glass reactor using a thermal conductivity detector (TPD)
of a gas chromatograph (HP 6890 series GC system). In a typical
reaction, 50 mg WC replica was loaded in a quartz tube reactor,
◦
and then thermally treated from room temperature to 450 C at
6
Scheme 1 The procedure for the modified replicating (impregnation—
compaction) route for the synthesis of mesoporous tungsten carbide by
sealing the pore channels of template.
◦C min-1 and from 450 ◦C to 650 ◦C at 1 C min-1. NH3 gas
was introduced and used as reaction gas under the carrying by He
(NH3:He = 1:54, volume ratio), and the total gas flow amounted
to 55 sccm.
◦
2.1 Material preparation
3. Results and discussion
The cubic Ia3d mesoporous silica KIT-6 was prepared according
to the method described in the literature.26 Typically, 6 g of
surfactant P123 and 6 g of n-butanol were dissolved in solution
of 217 g distilled water and 11.4 g conc. HCl (37 wt%). To this
homogeneous solution, 12.9 g of tetraethyl-orthosilicate (TEOS)
was added under stirring at 37 ◦C and continuously stirred at
37 ◦C for additional 24 h. After that, the mixture was transferred
to a Teflon-lined autoclave and heated at 100 ◦C for another 24 h.
The solid product was filtered, washed with distilled water for
several times and calcined at 550 ◦C for 5 h to remove the organic
template.
0.4 g KIT-6 powder was added into 5 mL of ethanol solution
containing 1.2 g PW12 at room temperature. After stirring for 3 h
at room temperature, the mixture was further stirred continuously
at 50 ◦C to evaporate the ethanol, the resultant dry powders
were pressed into compact discs under 10 MPa to seal PW12
precursor within the template. The compacts were heated from
room temperature to 650 ◦C at 10 ◦C min-1 and held at 650 ◦C for
3 h under a mixed gas flow of Ar and H2 (1:0.05), then heated again
3.1. Structural characteristics
Fig. 1 shows the XRD patterns of the prepared samples. The wide-
angle XRD (Fig. 1a) patterns reveal that both samples with and
without compaction are mainly composed of WC phase whose
three main peaks were seen clearly according to the JCPDS (25–
1047), indicating the well-crystallized framework of the prepared
products. Both samples WC-P and WC-NP show broadened
XRD diffraction peaks, suggesting that the prepared WC samples
should be nanosized. However, the diffraction peaks of WC-P are
even more broadened than those of WC-NP, most probably due
to the confinement effect of the sealed micro-reactor in WC-P
by compaction, which effectively prevented the volatilization of
tungsten source and the growing of WC particles. It is noted that
the sample WC-P shows a distinguishable small-angle reflection
peak (211) according to the small-angle XRD patterns in Fig. 1b,
which is corresponding to the main diffraction peak of the cubic
Ia3d symmetry of mesoporous silica KIT-6 template with a
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analysis. Some WC particles were aggregated together from the
selected areas in Fig. 2a due to the presence of WC component
not confined in the pore channels of the silica template during the
process of high temperature carbonization. The large aggregated
WC particles can be found to be composed of randomly and tightly
packed small crystallites without visible nanoporous structures
(Fig. 2b). From these images we can know that W source may
have volatilized, or formed WC particles which migrated out of
the pore structures of the silica template during the heat treatment
(reduction and formation of WC) due to the open pore structure
of WC-NP.
From the typical TEM images of sample WC-P in Fig. 3, it can
be clearly seen that the prepared WC retained ordered mesoporous
structure, though not as perfect as the template, in contrast to the
random structure of sample WC-NP. Such a mesoporous structure
should be attributed to the sealed micro-environment as a micro-
reactor for the formation of mesostructured WC of sample WC-P
derived from the compacted precursor mixture of PW@KIT-6.
TEM image in Fig. 3a is along the [100] direction, and those
in Fig. 3b and Fig. 3c are along the [110] direction, indicating
that the WC replica has basically inherited the cubic mesoporous
structure of the parent mesoporous silica after the removal of
silica template, though some disordered areas exist as shown in
Fig. 3b. This is consistent with the small-angle XRD result that
only one diffraction peak can be seen in sample WC-P. Fig. 3d
shows the high-resolution TEM image of a single WC nanorod
from the prepared mesoporous WC product, it can be seen that
the WC nanorod is composed of nanoparticles with a diameter of
about 6–7 nm, which is in accordance with the pore size of the silica
template (6.54 nm). The crystal stripes of mesoporous WC can also
be seen clearly, and the d-spacing value of 0.28 nm belongs to the
(001) reflection of WC phase (d = 0.2840 nm, JCPDS 25–1047).
Fig. 1 XRD patterns for the samples, A: small-angle XRD patterns and
B: wide-angle XRD patterns.
position shift towards a higher angle. This suggests that the replica
possesses a certain grade of pore structure ordering. While no
distinguishable small angle diffraction peak of sample WC-NP can
be identified, indicating that the replicated sample prepared from
the compacted precursor mixture has retained, at least partially,
the mesopore structure of the template, and also implying that W
precursor was reduced and carbonized within the pore structure of
the template during the heat treatment. Without compaction, the
open pore structure would lead to the volatilization of the W source
at high reaction temperature, and the reduced and carbonized WC
could migrate to the outside of the pore channels. In addition, the
formed WC particles out of pore structure may aggregate and/or
grow larger freely without the confinement of pore structure, thus
the ordered pore structure of the template cannot be inherited
into the replica, corresponding to the absence of any small angle
diffraction peaks of the sample WC-NP.
The TEM images of sample WC-NP prepared without com-
paction in Fig. 2 show the consistent results with the XRD
Fig. 3 TEM images of sample WC-P in the view of [100] (a) and [110] (b,
c) directions, respectively; High-resolution TEM image of the sample (d);
The inset in (d) is the selected area electron diffraction (SAED) pattern of
WC-P.
Fig. 2 TEM images of sample (WC-NP) without compaction.
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Table 1 The pore parameters of the samples calculated from the N2
The prepared mesoporous WC product is well crystallized and
has a polycrystalline character from the selected area electron
diffraction (SAED) pattern (inset in Fig. 3d).
sorption isotherms
Sample BET surface area/m2 g-1 Pore size/nm Pore volume/cm3 g-1
The representative FE-SEM images of the prepared sample WC-
P can be found in Fig. 4a and Fig. 4b. The overall morphology
of prepared mesoporous WC material is irregular from the low-
resolution FE-SEM image (Fig. 4a), however, many rod-like
particles can be seen in the high-resolution image (Fig. 4b) which
are about 100 nm in diameter and several hundred nanometers
in length. In addition, no Si signals can be detected from the
EDX spectrum (Fig. 4c) meaning that the silica template has
been removed completely for WC-P. A special advantage of the
present approach for the synthesis of mesoporous WC is that
the mesoporous WC structure can be formed at a relatively low
temperature of 700 ◦C, which prevents the WC nanoparticles from
over-growing and ensures the preservation of the mesoporous
structure.
WC-NP 138
4.8
10.8
6.51
0.28
0.23
1.26
WC-P
KIT-6
56.2
820
KIT-6: d211 = 9.52 nm, thickness of pore wall = 4.5 nm.
Fig. 5 A: Nitrogen sorption isotherms of the prepared WC samples
and the mesoporous silica (KIT-6) (inset); B: the corresponding pore size
distribution curves.
Fig. 4 FE-SEM images of the sample WC-P (a, b) and its EDX spectrum
(c).
average pore size of WC-P is 4.8 nm much smaller than that of
WC-NP of 10.8 nm. The pore size of sample WC-P matches well
with the wall thickness of silica template (4.5 nm) as shown in
Table 1, and the diameter of the WC nanorods (6–7 nm, Fig. 3d)
is also well corresponding with the pore size of the mesoporous
silica template (6.5 nm, inset of Fig. 5B). This indicates that the
WC-P prepared by the modified replicating route well inherits the
mesoporous structure of silica template.
Fig. 5 gives the N2 sorption isotherms of the prepared samples
and the corresponding pore size distribution curves. For sample
WC-P, the N2 sorption isotherm shows typical type IV character of
mesoporous solids,27 which is similar to that of mesoporous silica
(KIT-6) template in the inset of Fig. 5A. The adsorption volume
of WC-P is much higher than that of sample WC-NP according
to the sorption isotherms in Fig. 5A. The BET specific surface
areas and the calculated pore parameters of the two samples are
listed in Table 1. WC-P has a surface area up to 138 m2 g-1,
much higher than that of WC-NP (56.2 m2 g-1), also considerably
higher than the reported value of mesoporous WC (76 m2 g-1) by a
surfactant-directing route.19,20 In addition, the corresponding pore
size distribution of WC-P shows one sharp peak as compared to
the bimodal pore size distribution of the latter (Fig. 5B), and the
3.2. Catalytic activity for NH3 decomposition
The temperature programmed reaction (TPR) profiles for NH3
decomposition over prepared mesoporous WC products are
shown in Fig. 6. The as-synthesized tungsten carbide replica
was directly used as catalyst without any treatment. A control
experiment showed that NH3 does not thermally decompose
significantly in the temperature range up to 700 ◦C without using
the catalyst. NH3 decomposition on sample mesopoorous WC-P
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decomposition on mesoporous WC prepared by an impregnation–
compaction route.
4. Conclusion
Mesoporous tungsten carbides with a high surface area of 138 m2
g-1 have been synthesized at a relatively low temperature by a
modified template replicating route in which the W precursor
was sealed within the pore structure of the silica template by
compaction under an external pressure. The mesoporous WC
replica prepared by this route is well crystallized and has partially
ordered mesoporous structure. The mesoporous WC shows high
and stable catalytic activity for the decomposition reaction of
NH3, and the complete NH3 decomposition was achieved on this
mesoporous material at about 500 ◦C, considerably lower than that
of the non-mesostructured reference. Based on its similar catalytic
characteristic to Pt, this high surface area mesoporous WC should
be potentially applicable in various catalysis fields.
Fig. 6 TPR profiles for NH3 decomposition on prepared WC-P and
WC-NP products.
started at 340 ◦C and increased sharply at and above 450 ◦C. The
◦
comple◦te conversion was achieved at 500 C and maintained up
to 650 C. For comparison, the catalytic activity of sample WC-
NP was also studied. Compared to WC-P, NH3 decomposition
reaction on WC-NP took place at higher temperature of 430 ◦C,
and the complete decomposition temperature of NH3 is at about
580 ◦C. This shows that mesoporous WC-P with a higher surface
area and partially ordered pore structure has higher catalytic
activity for the NH3 decomposition reaction and considerably
lower reaction temperature as compared to the reference.
Acknowledgements
The authors gratefully acknowledge the financial support from
National Natural Science Foundation of China with Contract
(20633090), Qiming Star Project of Shanghai with Contract
05QMX1458, and Foundation of Shanghai nanotechnology
(0552nm030).
In order to investigate the stability and induction activity of
the mesoporous tungsten carbide (sample WC-P) during reaction,
the variations in % NH3 decomposition as a function of time-
on-stream (TOS) at selected temperatures were measured and the
results are shown in Fig. 7. The inlet concentration of NH3 was 1
sccm, with the balance being He. The reaction was characterized
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