A novel route to nanosized molybdenum boride and carbide and/or metallic molybdenum by thermo-synthesis method from MoO3, KBH4, and CCl4

Article abstract of DOI:10.1016/S0022-4596(02)00044-0

Nanosized molybdenum boride and carbide were synthesized from MoO₃, KBH₄, and CCl₄ by thermo-synthesis method at lower temperature. The relative content of Mo, Mo₂C, and molybdenum boride in the product was decided by the molar ratio between MoO₃, KBH₄, and CCl₄. Increasing the molar ratio of CCl₄ to MoO₃ was favorable to the production of Mo₂C. Increasing the molar ratio of KBH₄ to MoO₃ was favorable to the production of molybdenum boride. By carefully adjusting the reaction conditions and annealing in Ar at 900°C, a single phase of MoB could be obtained.

Full text of DOI:10.1016/S0022-4596(02)00044-0

Journal of Solid State Chemistry 170 (2003) 135–141
A novel route to nanosized molybdenum boride and carbide
and/or metallic molybdenum by thermo-synthesis method
from MoO3, KBH4, and CCl4
Yuanzhi Li,a,b Yining Fan,a,* and Yi Chena
a Laboratory of Mesoscopic Materials Science, Department of Chemistry, Nanjing University, Nanjing 210093, China
b Department of Chemistry, Three Gorges University, Yichang, Hubei 443000, China
Received 22 May 2002; received in revised form 4 September 2002; accepted 12 September 2002
Abstract
Nanosized molybdenum boride and carbide were synthesized from MoO3, KBH4, and CCl4 by thermo-synthesis method at lower
temperature. The relative content of Mo, Mo2C, and molybdenum boride in the product was decided by the molar ratio between
MoO3, KBH4, and CCl4. Increasing the molar ratio of CCl4 to MoO3 was favorable to the production of Mo2C. Increasing the
molar ratio of KBH4 to MoO3 was favorable to the production of molybdenum boride. By carefully adjusting the reaction
conditions and annealing in Ar at 9001C, a single phase of MoB could be obtained.
r 2002 Elsevier Science (USA). All rights reserved.
Keywords: Molybdenum boride; Molybdenum carbide; Thermo-synthesis
1. Introduction
tions [14], and photochemically promoted formation of
carbide from metallic molybdenum and graphite [15].
Several methods have been developed for synthesis of
molybdenum boride: solid-state reaction [16,17], me-
chanochemical synthesis [18,19], electrochemical synth-
esis [20–22], and multiphase diffusive reaction [23].
Usually, a much higher temperature condition (above
9001C) was required by almost all of the above-
mentioned methods. Moreover, the obtained molybde-
num borides usually are not nanosized powder.
Therefore, it is of interest and significance to develop a
new method of preparation of molybdenum boride at
lower temperature.
Molybdenum boride and carbide have long since
attracted considerable interest for technological applica-
tion (e.g., as hard coatings, in machine tools, high-
performance gear parts, golf shoe spikes, and snow tires)
because of their high melting point, chemical stability,
extremely high hardness, high strength, and excellent
resistance against mechanic and corrosive wear [1–3]. In
addition, molybdenum carbide showed catalytic proper-
ties similar to those of noble metals, which led to an
explosion of interest in the use of molybdenum carbide
as catalysts for a wide range of reactions, especially over
the past decade. They have been shown to be
particularly active for hydrogenation [4], dehydrogena-
tion [5–8], the Fischer–Tropsch reaction [9], hydrocar-
bon isomerization [10], and the oxyforming of methane
[11].
In this work, we present a novel route to the synthesis
of nanosized molybdenum boride and carbide by the
thermo-synthesis method at a lower temperature (3001C).
To our best knowledge, there have been no reports about
this new reaction of MoO3, KBH4, and CCl4.
Some of the methods developed for synthesized
molybdenum carbide include the temperature-pro-
grammed method developed by Boudart and co-workers
[12], pyrolysis of metal precursors [13], solution reac-
2. Experimental
Preparation: A mixture of MoO3 and KBH4 was
mixed, and comminuted in mortar according to a proper
molar ratio of MoO3 to KBH4. A certain mount of CCl4
*Corresponding author. Fax: +86-25-331-7761.
E-mail address: chem612@nju.edu.cn (Y. Fan).
0022-4596/03/$ - see front matter r 2002 Elsevier Science (USA). All rights reserved.
PII: S 0 0 2 2 - 4 5 9 6 ( 0 2 ) 0 0 0 4 4 - 0

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Y. Li et al. / Journal of Solid State Chemistry 170 (2003) 135–141
was added to the mixture of MoO3 and KBH4, and then,
the mixture of MoO3, KBH4, and CCl4 was transferred
into a 1 L autoclave. The autoclave was purged by Ar.
The autoclave was heated from room temperature to
3001C, and kept at 3001C for 4 h. In the process of
elevation of the temperature, the pressure in the
autoclave was observed. From the rate of change of
pressure in the autoclave, we could know whether the
reaction between MoO3, KBH4, and CCl4 proceeds
slowly or quickly as a lot of gases such as H2 and/or HCl
are produced when the reactants begin to react with
each other. After the reaction, the gases were let out,
and the autoclave was cooled to room temperature. The
product mixture was passivated by allowing a mixture of
1% O2/N2 to diffuse into the autoclave at a flow rate of
100 mL/min for 2 h to prevent the product from
violently oxidizing if exposed to air immediately
following the reaction. The final product is obtained
by dissolving the above-mentioned mixture in distilled
water, thoroughly washing, filtering, exchanging with
acetone for several times, and drying at room tempera-
ture.
Characterization of catalysts: The composition of as-
synthesized samples was determined according to the
procedure as follows. The content of C was measured on
Perkin-Elmer 240; the content of Mo and B was
determined by using induced coupled plasma (ICP) on
TJA1100. BET surface area of sample was measured on
a MICROMERITICS ASPAP-2000 adsorption analy-
zer using nitrogen as adsorbate. Powder X-ray diffrac-
tion analysis was performed with Ni-filtered CuKa
radiation with Shimadzu XD-3A X-ray diffractometer.
The working voltage of 35 kV and the electronic current
of 25 mA were employed. The morphology of the as-
synthesized samples was observed by transmission
electron microscopy on JEM-100CX.
molar ratio of KBH4 to MoO3 is 2:1, boron is not
detected in sample 1 and the content of C and Mo in
sample 1 is 1.43%, 98.57%. When the molar ratio of
KBH4 to MoO3 is 3:1, boron is detected in sample 2,
and the contents of C, B, Mo in sample 2 were 5.16%,
3.89%, and 90.95%. With further elevation of the molar
ratio of KBH4 to MoO3, the content of C in the sample
decreases gradually, and the content of B increases.
When the molar ratio of KBH4 to MoO3 is 4:1, the
product contains larger content of boron (7.43%) and
smaller content of carbon (1.65%). As the molar ratio of
KBH4 to MoO3 reaches 6:1, the content of B in sample 8
increases to 10.53%, and C could not be detected. As the
molar ratio of MoO3 to KBH4 is 5:1, the elevation of the
molar ratio of CCl4 to KBH4 from 0.52 to 1.27 leads to
the increasing of the content of C from 1.24% to 1.48%
and the decreasing of the content of B from 10.50% to
8.32% in the product.
Fig. 1 presents the patterns of XRD of as-synthesized
samples prepared under the condition of different molar
ratio of KBH4 to MoO3. It can be seen that the product
is a mixture of Mo2C and Mo when the molar ratio of
KBH4 to MoO3 is 2:1. When the molar ratio of KBH4 to
MoO3 increases to 3:1, molybdenum boride (MoB) with
tetragonal crystalline structure appears in the product
and the product is a mixture of Mo2C, Mo2B, and MoB.
When the molar ratio of KBH4 to MoO3 is 3.5:1, the
phase of Mo2B disappears, and the product comprises
of Mo2C and MoB. With the elevation of the molar
ratio of KBH4 to MoO3, the content of Mo2C decreases
gradually. As the molar ratio of KBH4 to MoO3 is 4:1,
the phase of MoB2 appears, and the product mainly
comprises of MoB with a small amount of Mo, Mo2C,
and MoB2. As the molar ratio of KBH4 to MoO3
reaches 5:1, the product is composed of Mo, MoB, and
MoB2 with only a small amount of Mo2C. Meanwhile, it
can also be seen that the as-synthesized sample is poorly
crystalline when the molar ratio of KBH4 to MoO3 is
above 4:1.
3. Results and discussion
The detailed preparation conditions and properties
are presented in Table 1. It can be seen that when the
In order to further determine the phase composition
of the as-synthesized samples prepared under the
Table 1
The properties of the products synthesized under the different conditions
No. Molar ratio
Content (wt%)
Average crystal size by XRD (nm)
Surface area (m2 gꢀ1
)
KBH4 and MoO3 CCl4 and KBH4 Mo
B
C
Mo
MoB2
Mo2C
MoB2
Mo2B
129.1
1
2
3
4
5
6
7
8
2
1.43
1.43
0.52
0.52
1.27
0.52
0
98.57
90.95
91.44
90.31
90.20
88.26
97.46
89.46
0
1.43
3.89
2.72
1.65
1.48
1.24
0
111.7
111.3
87.9
45.1
27.4
43.8
43.8
1.6
7.8
3
5.16
5.84
58.1
19.4
15.9
15.3
8
3.5
4
29.1
35.1
17.4
33.1
37.1
27.6
8.04
8.32
12
12.4
14.9
12.1
5
15.2
25
5
10.50
2.56
10.54
5
9.7
12.4
6
0.52
0
9.8
10.7
The average crystal sizes of Mo, Mo2C, MoB, MoB2, and Mo2B are determined from the broadening of corresponding X-ray spectral peaks (at 40.51,
39.51, 42.51, 45.31, and 40.991, respectively) by Scherrer formula: L ¼ 0:89l=b cos y:

Y. Li et al. / Journal of Solid State Chemistry 170 (2003) 135–141
137
Fig. 1. The patterns of XRD of as-synthesized samples prepared under the condition of different molar ratio of KBH4 to MoO3.
Fig. 2. The patterns of XRD of the samples annealed for 2 h at 9001C: (A) annealing sample 8 at 9001C; (B) annealing sample 6 at 9001C.
condition of higher molar ratio of KBH4 to MoO3,
samples 6 and 8 are annealed at 9001C for 2 h under
the atmosphere of Ar. Fig. 2 shows the patterns
of XRD of the annealed samples. Compared with
the pattern of sample 6, the phase of MoB2 dis-
appears, and the content of Mo decreases, and the
content of MoB increases after annealing, which is
indicative of the reaction between MoB2 and Mo at
9001C:
By annealing sample 8 at 9001C for 2 h under the
atmosphere of Ar, a single phase of tetragonal MoB is
obtained with disappearance of the phase of Mo and
MoB2, which shows that content of MoB2 in sample 8 is
equal to Mo.
Fig. 3 presents the patterns of XRD of as-synthesized
samples prepared under the condition of different molar
ratio of CCl4 and KBH4, and the same molar ratio of
KBH4 to MoO3. It can be seen that molybdenum metal
is obtained in the absence of CCl4. Although there is
2.14% of B in this molybdenum metal product, no
Mo þ MoB2-MoB:

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Y. Li et al. / Journal of Solid State Chemistry 170 (2003) 135–141
Fig. 3. The patterns of XRD of the samples prepared under the condition of different molar ratio of CCl4 to KBH4.
crystalline molybdenum boride and boron are detected
by XRD. In the presence of CCl4, a mixture of
molybdenum carbide and boride is obtained. With the
elevation of the content of CCl4 in the reactants, the
final product gets a better crystalline structure with
increased content of Mo2C and decreased content of
molybdenum boride. For example, when the molar ratio
of CCl4 to KBH4 was 0.52, the product has a poor
crystalline structure with a small amount of Mo2C. With
the elevation of the molar ratio of CCl4 to KBH4 from
0.52 to 1.27, the product has a better crystalline
structure with higher content of Mo2C. Meanwhile, a
new phase of orthorhombic MoB (b-MoB) appears in
the product (sample 5). When the molar ratio of KBH4
to MoO3 is 2, a mixture of Mo2C and Mo is obtained
from the reaction between CCl4, KBH4 and MoO3.
When the molar ratio of KBH4 to MoO3 is 3 or above 3,
the reaction between CCl4, KBH4, and MoO3 leads to a
mixture of Mo2C, molybdenum boride (MoB or Mo2B),
and/or metallic Mo. However, whatever the molar ratio
of KBH4 to MoO3 is 2, 3, 4, 5 or 6, the change of the
composition of the as-synthesized product with the
variation of the molar ratio of CCl4 to KBH4 has similar
tendency to one of the products prepared at the molar
ratio of KBH4 to MoO3 equal to 5: the higher the
content of the molar ratio of CCl4 to KBH4, the higher
the content of Mo2C in the as-synthesized product.
When the molar ratio of KBH4 to MoO3 is 2, the
product with a surface area of only 1.6 m2 gꢀ1 has an
average crystal size of more than 100 nm. As the molar
ratio of KBH4 to MoO3 reached 3, the nanosized
product with an average particle size of 87.9 nm (Mo2C),
58.1 nm (MoB), and 129.1 nm (Mo2B) is obtained. The
surface area increases to 7.8 m2 gꢀ1. With the elevation
of the molar ratio of KBH4 to MoO3, the surface area of
the product increases, and the average crystal size
decreases. However, when the molar ratio of KBH4 to
MoO3 is more than 4:1, a little change of the surface
area and average crystal size of product is observed.
Fig. 4 shows the morphology of as-synthesized samples.
As can be seen from Fig. 4, the particles produced under
the condition of molar ratio of KBH4 to MoO3 of more
than 3:1 are well dispersed, especially for samples 3 and
4. The particle sizes of sample 3 which mainly range
from 20 to 40 nm are obviously larger than one of the
samples produced under the condition of molar ratio of
KBH4 to MoO3 of more than 4:1. There are much more
particles with less than 20 nm observed in samples 4, 6,
and 8 than in sample 3. Compared with the morphology
of the product synthesized at a higher molar ratio of
CCl4 to KBH4 (sample 5), the product produced at a
lower molar ratio of CCl4 to KBH4 (sample 6) has
smaller particle size ranging from 5 to 30 nm. The
particle sizes are in agreement with the average crystal
sizes estimated by XRD. The more widely ranged
distribution of particle size is attributed to the different
average crystal sizes of several matters in the product.
As pure molybdenum boride or carbide has been
applied in technological fields (e.g., as hard coatings, in
machine tools, high-performance gear parts, golf shoe
spikes, and snow tires) [1–3], the as-synthesized mixture
of nanosized molybdenum boride and carbide could also
find application in these fields. The presence of metallic
molybdenum in some of molybdenum boride and
carbide could overcome the brittleness of these compo-
site ceramic materials, and make them more ductile and
more easily being processed because metals such as Cr,
Ni, Mo, and Fe, etc. as an adhesive usually have to be
added into metal boride or its composite to make
them more easy to process [24]. The application of

Y. Li et al. / Journal of Solid State Chemistry 170 (2003) 135–141
139
Fig. 4. The morphology of as-synthesized samples.
as-synthesized samples in catalytic selective hydrogena-
tion of alkadiene to alkene is in progress.
that a black fluffy solid matter composed of B and C
could be also produced in the reaction of KBH4 and
CCl4 at 3001C even in the absence of MoO3; the
effluents produced by this reaction are proved to be H2
and HCl. The result of XRD exhibits that KCl is
produced in this reaction.
From the above observation, it is reasonably inferred
that the mechanisms of the above-mentioned reactions
are as follows:
In order to determine the mechanism of the above-
mentioned reactions, the effluents produced by reaction
of MoO3, KBH4, and CCl4 are analyzed by absorbing
the effluent with distilled water. It is proved that the
effluents produced by the reaction of MoO3, KBH4, and
CCl4 are composed of HCl and H2, and the effluent
produced by the reaction of MoO3 and KBH4 only
comprises of H2. Fig. 5 presents the pattern of XRD of
the mixture of sample produced by the reaction of
MoO3, KBH4, and CCl4 with 1:3.5:1.82 of the molar
ratio between MoO3, KBH4, and CCl4. It can be seen
that the mixture contained KCl apart from molybdenum
carbide and boride. KCl was formed in all the above-
mentioned reactions of MoO3, KBH4, and CCl4. In all
the mentioned reactions of MoO3, KBH4, and CCl4, a
black fluffy solid matter attached to the stirrer of
autoclave was obtained except for the metallic molyb-
denum, molybdenum carbide and/or boride. The results
of chemical analysis show that this black solid is
composed of B and C of which the content depends
on reaction condition, e.g., the molar ratio between
MoO3, KBH4, and CCl4. The result of XRD shows that
this black matter is amorphous. Our experiment exhibits
KBH4 þ MoO3-KBO2 þ Mo þ H2;
ð1Þ
MoO3 þ KBH4 þ CCl4- HCl þ KCl þ Mo2C
þ CBy þ H2;
ð2Þ
MoO3 þ KBH4 þ CCl4- HCl þ KCl þ MoBx
þ CBy þ H2;
ð3Þ
ð4Þ
ð5Þ
KBH4 þ CCl4-HCl þ KCl þ CBy þ H2
KBO2 þ HCl-KCl þ HBO2:
It is discovered by our experiment that the reaction
between KBH4 and MoO3 (1) proceeds slowly in the
absence of CCl4; the reaction between KBH4 and CCl4
(4) also proceeds slowly in the absence of MoO3, and no

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Y. Li et al. / Journal of Solid State Chemistry 170 (2003) 135–141
Fig. 5. The pattern of XRD of the mixture of sample produced by the reaction of MoO3, KBH4, and CCl4 with 1:3.5:1.82 as the molar ratio between
MoO3, KBH4, and CCl4.
reaction is observed between MoO3 and CCl4 under our
experimental condition. But reactions (2) and (3)
proceed very rapidly so long as the temperature is
above 2601C or so. From these observations, it is
concluded that there is a cooperating role among
KBH4, MoO3 and CCl4 that could accelerate the
reactions.
4. Conclusion
Molybdenum carbide and boride are synthesized from
MoO3, KBH4, and CCl4 by the thermo-synthesis
method at lower temperature. The relative contents of
Mo, Mo2C, and molybdenum boride in the product are
decided by the molar ratio between MoO3, KBH4, and
CCl4. Increasing the molar ratio of CCl4 to MoO3 is
favorable to the production of Mo2C. Increasing the
molar ratio of KBH4 to MoO3 is favorable to the
production of molybdenum boride. There is a cooperat-
ing role among KBH4, MoO3, and CCl4 that could
accelerate the reaction of formation of molybdenum
carbide and boride.
From the above mechanisms, it can be concluded that
relative contents of Mo, Mo2C, and molybdenum boride
in the product is decided by the competitive rate of
reactions (1)–(3). To decrease the molar ratio of KBH4
to MoO3 and increase the molar ratio of CCl4 to KBH4
is favorable to reaction (2) as it may lead to more CCl4
molecules attacking the transient Mo species produced
by the reaction between KBH4 and MoO3 to form
molybdenum carbide. Increasing the molar ratio of
KBH4 to MoO3 is favorable to reaction (3) because it
leads to more KBH4 molecules attacking the transient
Mo species to form molybdenum boride. Under the
condition of lower molar ratio of KBH4 to MoO3, e.g.,
2:1, the final product is composed of Mo and Mo2C
without molybdenum boride, which indicates that
reactions (1) and (2) proceed more easily than reaction
(3). With the elevation of molar ratio of KBH4 to
MoO3, reaction (3) is accelerated, and moves towards
the left. The more the molar ratio of KBH4 to MoO3 is,
the higher the content of molybdenum boride in the
product.
Acknowledgments
The authors are grateful to Ministry of Science and
Technology of China(Grant No. 1999022400), Institute
of Jinling Petrochemical Co. Ltd., and Jiangsu Province
Hi-Tech Programme(Grant No. BG 2002016) for their
financial support.
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