A novel route to the synthesis of nanosized metallic molybdenum at moderate temperature and its catalytic properties

Article abstract of DOI:10.1016/j.materresbull.2003.10.020

Nanosized metallic molybdenum could be synthesized from MoO₃ and KBH₄ by solid-state reaction at moderate temperature. The crystallinity, morphology, surface properties of as-synthesized metallic molybdenum were investigated by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectral (XPS). The results of its catalytic activity test show that the as-synthesized nanosized metallic molybdenum is superior to the noble metal catalyst 0.3% Pd/Al₂O ₃ for the selective hydrogenation of alkadienes at higher temperature and pressure.

Full text of DOI:10.1016/j.materresbull.2003.10.020

Materials Research Bulletin 39 (2004) 195–203
A novel route to the synthesis of nanosized metallic molybdenum
at moderate temperature and its catalytic properties
Yuanzhi Lia,b,*, Guangfu Luoa, Yining Fanb,*, Yi Chenb
aDepartment of Chemistry, Three Gorges University, Hubei, Yichang 44300, PR China
bDepartment of Chemistry, Laboratory of Mesoscopic Materials and Molecular Engineering,
Nanjing University, Nanjing, Jiangsu 210093, PR China
Received 20 February 2003; received in revised form 13 June 2003; accepted 9 October 2003
Abstract
Nanosized metallic molybdenum could be synthesized from MoO3 and KBH4 by solid-state reaction at
moderate temperature. The crystallinity, morphology, surface properties of as-synthesized metallic molybdenum
were investigated by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray
photoelectron spectral (XPS). The results of its catalytic activity test show that the as-synthesized nanosized
metallic molybdenum is superior to the noble metal catalyst 0.3% Pd/Al2O3 for the selective hydrogenation of
alkadienes at higher temperature and pressure.
# 2003 Elsevier Ltd. All rights reserved.
Keywords: A. Metals; B. Chemical synthesis; C. X-ray diffraction; D. Catalytic properties
1. Introduction
Metallic molybdenum is widely used in alloy, electrode, metal particle toughened ceramic matrix
composites, and catalysts, etc. [1–10]. Nanosized materials have received much research and industrial
attention because of their unique physico-chemical properties compared with the properties of
corresponding materials with large grains. Preparation methods of metallic molybdenum include vapor-
state reduction of MoCl5 with hydrogen, H2 reduction of molybdenum oxide, thermo-decomposition of
carbonyl molybdenum, and plasma reduction [11]. Usually, a much higher temperature condition for
the synthesis of molybdenum was required by almost all of the above-mentioned methods. Moreover,
the obtained metallic molybdenum usually is not nanosized powder. To the best of our knowledge, there
have been a few reports on the preparation and physico-chemical properties of nanosized metallic
*
Corresponding author. Tel.: þ86-7176465546.
E-mail address: liyuanzhi66@263.net (Y. Li).
0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2003.10.020

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molybdenum so far. For example, Robert et al. [12] prepared nanocrystalline metallic alloy powders,
e.g. the nanocrystals that is made of an alloy of nickel and molybdenum, by high energy mechanical
alloying. They found the electrodes produced from these powders presented an excellent chemical,
electrochemical, and mechanical stability, and had an electrocatalytic activity for the hydrogen
evolution which is comparable or higher than the electrodes which are presently used in the
electrochemical industry. Zach et al. [13,14] prepared metallic molybdenum nanowire by a two-step
procedure of electro-deposition and reduction, and made investigation into the electronic and
mechanical properties. Cheng and Zhao [11] synthesized nanosized metallic molybdenum from
carbonyl molybdenum by microwave plasma. Therefore, it is of interest and significance to make
investigation into the preparation and physico-chemical properties of nanosized metallic molybdenum.
Herein, we presented a novel route to the synthesis of nanosized metallic molybdenum by thermo-
synthesis method from MoO3 and KBH4 at moderate temperature, and applied as-synthesized
nanosized metallic molybdenum in the selective hydrogenation of longer chain linear alkadienes, and
made a comparison with noble metal catalysts (Pd/Al2O3). It was discovered that the as-synthesized
metallic molybdenum exhibited better catalytic activity and selectivity for the selective hydrogenation
of longer chain linear alkadienes than noble metal catalysts (Pd/Al2O3). To our best knowledge, there
have been no similar reports up to now.
2. Experimental
MoO3 (7.2 g) was mixed with KBH4 according to 1:3 of molar ratio of MoO3 and KBH4, and
comminuted in mortar. Then, the mixture of MoO3 and KBH4 was transferred into a 1 l autoclave, and
Ar purged the autoclave. The autoclave was heated from room temperature to 300 8C, and kept at
300 8C for 2 h. After the reaction, the produced gases were let out, and the autoclave was cooled to
room temperature. The product mixture was passivated by allowing a mixture of 1% O2/Ar to diffuse
into the autoclave at flow rate of 100 ml/min for 4 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 temperature.
Powder X-ray diffraction analysis was performed with Ni-filtered Cu Ka radiation with Shimadzu
XD-3A X-ray diffractometer. The working voltage of 35 KV and the electronic current of 25 mA were
employed. Transmission electron microscopy (TEM) images were obtained on a JEM-200 CX
transmission electron microscope, using an accelerating voltage of 200 kV. The composition of as-
synthesized samples was determined by using induced coupled plasma (ICP) on TJA1100. The surface
species of the as-synthesized sample were analyzed by a ESCALab MK2 X-Ray Photoelectron
Spectrometer. Mg Ka radiation was selected as X-ray source. The sample was pressed into self-
supported wafer and fixed on a holder, then, the catalyst was out-gassed. The residual pressure in the
analysis chamber was maintained below 10À9 Torr. The spectra of sample were collected and corrected
by referencing the binding energy to carbon (C1s ¼ 284:6 eV).
To evaluate the catalytic properties of the as-synthesized nanosized metallic molybdenum catalyst, we
tested its catalytic activity for the liquid selective hydrogenation of longer chain alkadienes (C10–C13),
which is an important petrochemical industrial reaction for production of alkyl benzene. The
measurement of reaction of catalysts for selective hydrogenation of long chain alkadienes were

Y. Li et al. / Materials Research Bulletin 39 (2004) 195–203
197
accomplished on a homemade apparatus by using a technical process of hydrogenation of feedstock
saturated by hydrogen [15], which is similar to industrial process. The feedstock composed of 84.38%
alkanes, 10.40% alkenes, 1.26% alkadienes, and 3.96% aromatic hydrocarbons were firstly saturated by
hydrogen in a 2 l autoclave at 100 8C and 17 atm. Then the feedstock saturated by hydrogen was delivered
into a fixed-bed micro-reactor of 20 mm i:d: Â 400 mm stainless tube from the autoclave by a valve for
hydrogenation. Five grams of catalyst were loaded in this reactor. The as-synthesized catalyst was
pretreated in H2 at a rate of 80 ml (STP)/min under atmospheric pressure at 450 8C for 1 h, and 0.3% Pd/
Al2O3 was pre-reduced under same condition except for at lower temperature (200 8C). The feedstock and
products were analyzed by a Waters510 High Pressure Liquid Chromatograph equipped with water 410
differential detector, and with 3:9 Â 300 m PorasilTM and 3:9 Â 150 Nova-Pak1 silica column for
separations, using tri-methyl-pentane as flowing phase. The molar percent contents of alkadienes, alkenes,
alkanes, and aromatic hydrocarbons were calculated according to a normalization calculation method, the
molar corrected coefficients of alkadienes, alkenes, alkanes, and aromatic hydrocarbons were determined
by using corresponding standard samples provided by Institute of Jinling Petrochemical Co. Ltd.
3. Results and discussion
Fig. 1 presents the patterns of XRD of as-synthesized sample. All of the reflections of the XRD
pattern of the as-synthesized sample can be easily indexed to those of crystalline metallic molybdenum,
indicating the formation of single-phase metallic molybdenum with body-centered cubic [16]. In the
XRD pattern, the peaks were widened obviously due to crystalline size effect. The average crystal size
of as-synthesized metallic molybdenum, which is determined from the broadening of corresponding
X-ray spectral peaks (at 40.58) by Scherrer formula: L ¼ 0:89l/b cos y [17], is 20 nm.
110
211
200
20
30
40
50
60
70
80
90
2¦È(degree)
Fig. 1. XRD patterns of catalysts.

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Fig. 2. The morphology of as-synthesized sample.
Fig. 2 shows the morphology of as-synthesized sample. As can be seen in Fig. 2, the particles are
well dispersed. The particle sizes of sample ranged from 5 to 40 nm, and the size of most particles is
10–30 nm, which is in good agreement to the average crystal sizes estimated by XRD. It was
discovered that no obvious change of the average crystal size of as-synthesized metallic molybdenum
were observed with the increase of the molar ratio of KBH4 and MoO3 from 2.0 to 5.0.
ICP analysis result showed that the content of Mo and B in as-synthesized metallic molybdenum is
99.44 and 0.56 wt.%, respectively; no other element (e.g. K) was detected. This result showed that in
the as-synthesized metallic molybdenum, there might be a little amount of compound containing Mo
and B, such as molybdenum boride, which could not be detected by XRD.
The reaction of MoO3 and KBH4 is:
3KBH4 þ 2MoO3 ! 3KBO2 þ 2Mo þ 6H2
The reasons why the solid-state reaction between MoO3 and KBH4 leads to production of nanosized
metallic molybdenum, no large particle metallic molybdenum is mainly attributable to the following
two factors: firstly, the moderate reaction temperature of this reaction (ꢀ300 8C) could avoid sintering
of the produced nuclei of Mo; Secondly, in the solid-state reaction between KBH4 and MoO3, KBH4
molecules around MoO3 prevent the produced nuclei of Mo from aggregating to form the products with
larger crystal sizes.
The conversion and selectivity of as-synthesized catalysts were summarized in Table 1 (the data was
an average value of the results after the hydrogenation reached steady state). Palladium catalysts have
by far been proved to be the most active and selective for selective hydrogenation of alkadienes and
Table 1
Activity and selectivity of catalysts at 160 8C, 14 atm, H2/alkadienes, molar ratio ¼ 1:28, LSV ¼ 8:0 ml/g catalyst per hour
Catalyst
Conversion of alkadienes (%)
Selectivity of alkenes(%)
Yield of alkenes (%)
Mo (nano)
Mo (large particle)
0.3% Pd/Al2O3
74.2
6.8
79.4
90.9
58.9
6.1
53.8
À24.1
À13

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199
Fig. 3. Conversion and selectivity alkadienes hydrogenation on the as-synthesized catalysts at different reaction temperature,
14 atm, H2/alkadienes(mole ratio) ¼ 1.28, LSV¼8.0 ml/g catalyst per hour: 0.3% Pd/Al2O3; (2) Mo (nano).

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alkynes to alkenes. However, from Table 3, it can be seen that 0.3% Pd/Al2O3 catalyst has only 53.8%
conversion of alkadienes and À24.1% selectivity of alkenes at 160 8C, 14 atm, 8.0 ml/g catalyst per hour
and H2/alkadienes (molar ratio) ¼ 1.28. The negative selectivity of alkenes shows that there is a
net alkenes content loss after hydrogenation. The as-synthesized nanosized metallic molybdenum
has 74.2% conversion of alkadienes and 79.4% selectivity of alkenes under the same conditions. In order
to making a comparison of catalytic properties between nanosized and micron-sized metallic
molybdenum, we prepared micron-sized metallic molybdenum by annealing the as-synthesized
nanosized metallic molybdenum at 900 8C for 2 h under the atmosphere of Ar. It can be seen from
Table 1 that the micron-sized metallic molybdenum has only 6.8% conversion of alkadienes, which is
attributed that the micron-sized metallic molybdenum has much lower specific surface area than the
nanosized metallic molybdenum.
As a wide operation temperature range of the present industrial process of selective hydrogenation of
C10–C13 alkadienes is demanded, the effect of reaction temperature on the activity and selectivity of
longer chain alkadienes hydrogenation on as-synthesized catalysts were tested and the results are shown
in Fig. 3. For noble metal catalyst 0.3% Pd/Al2O3, with the elevation of reaction temperature from 100 to
140 8C, there is a little change of the conversion of alkadienes and selectivity of alkenes, but with the
further elevation of reaction temperature from 140 to 160 to 180 8C, the conversion increases from 49.2 to
53.8 to 59.2% and the selectivity of alkenes decreases rapidly from 22.2 to À24.1 to À33.0%, which
shows a considerable alkenes is hydrogenated to alkane. For the as-synthesized nanosized metallic
molybdenum catalyst, with the elevation of reaction temperature from 140 to 220 8C, the conversion of
alkadienes increases from 55.0 to 99.5%, but the selectivity of alkenes only decreases from 82.2 to 61.8%.
Obviously, the noble metal catalyst 0.3% Pd/Al2O3 with lower conversion and selectivity is much less
effective than the as-synthesized nanosized metallic molybdenum at higher temperature and pressure.
220 222 224 226 228 230 232 234 236 238 240 242 244 246
Binding energy(eV)
Fig. 4. Mo3d XPS spectra of as-synthesized metallic molybdenum.

Y. Li et al. / Materials Research Bulletin 39 (2004) 195–203
201
C1s
275
280
285
290
295
Binding energy(eV)
O1s
532.6
520
525
530
535
540
545
Binding energy(eV)
B1s
192.5
180
185
190
195
200
Binding energy(eV)
Fig. 5. C1s, O1s, and B1s XPS spectra of as-synthesized metallic molybdenum.

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Y. Li et al. / Materials Research Bulletin 39 (2004) 195–203
As the properties of catalyst are mainly determined by its surface properties, we use XPS to make an
investigation into the surface properties of the as-synthesized nanosized metallic molybdenum. X-ray
photoelectron spectral of Mo3d profiles of as-synthesized catalysts is shown in Fig. 4. The binding
6þ
energy of Mo3d5/20, Mo3d5/22þ, Mo3d5/23þ, Mo3d5/24þ, Mo3d5/25þ, and Mo3d5/2 are 227.8, 229.0,
229.9, 231.8, and 233.1 eV, respectively [18]. The spectral of XPS Mo3d for the as-synthesized catalyst
is de-convoluted by using a special software program. Thus, we obtained the surface composition of as-
synthesized nanosized metallic molybdenum: 31.8% Mo2þ, 7.4% Mo3þ, 11.2% Mo4þ, 22.4% Mo5þ,
and 27.1% Mo6þ. Although the sample has a metallic molybdenum bulk structure, no Mo0 species is
observed on this catalyst surface, which is attributed to the surface oxidation of the as-synthesized
metallic molybdenum by 1% O2/Ar to Mo species with higher oxidation state.
The C, O, and B were also detected by XPS, the X-ray photoelectron spectral of C1s, O1s, and
B1s profiles of as-synthesized catalysts are shown in Fig. 5. It can be seen in Fig. 5 that there is
only one form of carbon, which is assigned to contaminated carbon. There are two kinds of oxygen:
the main peak at 530.9 eV could be assigned to oxygen of molybdenum oxide; the shoulder peak
at 523.6 eV could be assigned to oxygen of KBO2. There also are two kinds of boron: the main
peak at 190.3 eV could be assigned to boron of molybdenum boride; the shoulder peak at 523.6 eV
could be assigned to boron of KBO2 [19]. These results further proved that there are several
Mo species with different oxidation state on the surface of as-synthesized nanosized metallic
molybdenum.
We know that the catalytic active species for the hydrogenation are Modþ(0 ꢀ d < 4) species on the
surface of molybdenum compound catalysts (e.g. molybdenum carbide and nitride), Mo species with
lower oxidation state has a higher activity than Mo species with higher oxidation state, therefore
decreasing of Mo0 content and improvement of Mo2þ and Mo3þ contents is favorable to the elevation
of selectivity of alkenes as the higher activity of Mo species with lower oxidation state inevitably
results in a lower selectivity of alkenes as the excessive hydrogenation of alkenes to alkanes is more
easily proceeding on the Mo species with higher activity than on the Mo species with lower activity
[15]. Therefore, the reason why the as-synthesized metallic molybdenum exhibits good activity and
selectivity for selective hydrogenation of alkdienes is attributable to the absence of Mo0 species and
higher content of Mo2þ and Mo3þ species on the surface of this catalyst.
4. Conclusion
Nanosized metallic molybdenum could be synthesized from MoO3 and KBH4 by thermo-synthesis
method at moderate temperature. The as-synthesized nanosized metallic molybdenum is superior to the
noble metal catalyst 0.3% Pd/Al2O3 for the selective hydrogenation of alkadienes at higher temperature
and pressure.
Acknowledgements
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 Program (Grant No. BG
2002016) for their financial support.

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