Influence of the composition of precursors and reduction conditions on the properties of magnesiothermic molybdenum powders

Article abstract of DOI:10.1134/S0020168517100119

We have investigated the preparation of molybdenum powders by reducing the oxide compounds MoO₃, MgMoO₄, and CaMoO₄ with magnesium vapor at residual argon pressures in the range 5–20 kPa and temperatures in the range 700–800°C. Using the MgMoO₄ and CaMoO₄ compounds as precursors, we have obtained molybdenum powders with specific surface areas of up to 20 m²/g. The powders have a mesoporous structure. The reduction of the molybdenum compounds under such conditions was accompanied by separation of the reaction products due to the removal of magnesium oxide from the reaction zone.

Full text of DOI:10.1134/S0020168517100119

ISSN 0020-1685, Inorganic Materials, 2017, Vol. 53, No. 10, pp. 1058–1063. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © V.N. Kolosov, M.N. Miroshnichenko, V.M. Orlov, 2017, published in Neorganicheskie Materialy, 2017, Vol. 53, No. 10, pp. 1081–1086.
Influence of the Composition of Precursors and Reduction Conditions
on the Properties of Magnesiothermic Molybdenum Powders
V. N. Kolosov*, M. N. Miroshnichenko, and V. M. Orlov
Tananaev Institute of Chemistry and Technology of Rare Elements and Minerals, Kola Scientific Center,
Russian Academy of Sciences, Akademgorodok 26a, Apatity, Murmansk oblast, 184209 Russia
*e-mail: tantal@chemy.kolasc.net.ru
Received February 1, 2017
Abstract⎯We have investigated the preparation of molybdenum powders by reducing the oxide compounds
MoO3, MgMoO4, and CaMoO4 with magnesium vapor at residual argon pressures in the range 5–20 kPa and
temperatures in the range 700–800°C. Using the MgMoO4 and CaMoO4 compounds as precursors, we have
obtained molybdenum powders with specific surface areas of up to 20 m2/g. The powders have a mesoporous
structure. The reduction of the molybdenum compounds under such conditions was accompanied by sepa-
ration of the reaction products due to the removal of magnesium oxide from the reaction zone.
Keywords: MoO3, MgMoO4, CaMoO4, magnesium vapor, reduction, molybdenum, powder, specific surface
area, porosity
DOI: 10.1134/S0020168517100119
INTRODUCTION
face areas. As shown previously, one promising pro-
cess for the preparation of refractive metal powders
with large specific surface areas is the magnesium
vapor reduction of binary oxide compounds contain-
ing refractory oxides [11–13]. The powders thus pre-
pared typically have a mesoporous structure because
the structure of the reduced particles contains magne-
sium oxide layers, which are subsequently removed by
leaching with solutions of mineral acids. The specific
surface area of powder with such structure of its parti-
cles is determined by the surface area of the pores
formed.
Molybdenum and its alloys are widely used in engi-
neering applications. The properties that make these
materials attractive include their high thermal and
electrical conductivities, small thermal expansion
coefficient, good high-temperature strength, and high
corrosion, abrasion, and wear resistances. To fabricate
compact products by powder metallurgy techniques,
starting materials in the form of fine, pure molybde-
num powders are needed [1]. Most often, such pow-
ders are prepared via hydrogen reduction of oxide
compounds. Ammonium paramolybdate or molybde-
num trioxide is first reduced to MoO2 in the tempera-
The purpose of this work was to study the influence
ture range 300–550°C [2]. The MoO3 → MoO2 trans- of precursor composition and reduction conditions on
the characteristics of molybdenum powders prepared
through magnesium vapor reduction of molybdenum
oxide compounds.
formation usually involves the formation of the
Mo4O11 oxide as a reaction intermediate. The molyb-
denum dioxide is then reduced to molybdenum metal
at a higher temperature using another apparatus [3].
The following methods for the preparation of molyb-
denum powder are worthy of special mention: self-
propagating high-temperature synthesis (SHS) [4, 5],
chemical vapor deposition [6], and electrochemical
EXPERIMENTAL
As precursors for the reduction process, we used the
molybdates MgMoO4 and CaMoO4 synthesized by sinter-
[7] and metallothermic [8–10] reduction of molybde- ing MoO3 (pure grade) and Mg5(СO3)4(ОН)2(Н2О)2
num compounds. The specific surface area of the
powders thus prepared is usually at the level of pow-
ders synthesized through hydrogen reduction and does
not exceed 10 m2/g.
(analytical grade) or MoO3 and CaCO3 (reagent
grade). To assess the effect of the refractory oxides
MgO and CaO on the characteristics of molybdenum
powders, MoO3 was used as a precursor for compari-
It is of interest to examine the possibility of pro- son. The apparatus and techniques used to prepare
ducing molybdenum powders with larger specific sur- and investigate molybdenum powders were similar to
1058

INFLUENCE OF THE COMPOSITION OF PRECURSORS
1059
those described previously [13]. The process was run
780
770
760
750
740
under an atmosphere of magnesium vapor and VCh
argon. A vessel containing magnesium (M95) was
mounted on the bottom of a reaction beaker. Weighed
amounts (5 g) of a precursor and molybdenum triox-
ide were loaded into metallic crucibles, which were
placed over the magnesium-containing vessel. A
shield was placed over the crucibles. The separation
between the surface of the precursor and the shield was
27–30 mm. The reaction beaker was covered with a
lid, which had an orifice in its center for the thermo-
couple sheath. The assembly was mounted in a stain-
less steel retort, which was hermetically sealed,
pumped down, filled with an inert gas, and heated to a
required temperature. The reaction beaker was kept
closed to avoid magnesium losses. The reduction pro-
cess was run in the temperature range 700–800°C at a
residual argon pressure in the range 5–20 kPa. To
determine the temperature directly in the reaction
zone, in a number of experiments a TP-A1 tungsten–
rhenium thermocouple junction was embedded in the
material being reduced. The reduction products were
treated with a 10% hydrochloric acid (reagent grade)
solution to remove the MgO and CaO. The molybde-
num powder was washed with distilled water until neu-
tral pH was reached and then dried in air at a tempera-
ture of 80°C.
1
2
3
4
0
50
100
Time, min
150
Fig. 1. Reaction mixture temperature as a function of
reduction time; precursors: (1) MgMoO , (2) CaMoO ,
4
4
(3) MoO ; (4) temperature maintained in the interior of
3
the reactor (pressure, 5 kPa).
In the working temperature range 700–800°C, the
starting materials, except magnesium and MoO3, and
the products of reactions (1)–(3) are characterized by
an extremely low vapor pressure, no higher than 10–18
to 10–14 Pa [14–16]. MgO3 vapor pressure is consider-
ably higher: 102 to 103 Pa in the temperature range in
question [15]. Because of this, at temperatures above
700°C considerable molybdenum trioxide sublimation
is possible, with a rate above 0.05 g/(cm2 s) at 750°C.
It is, therefore, reasonable to assume that, after reduc-
tion, some of the products of reaction (3) can be out-
side the crucible into which MoO3 was loaded. In
other cases, because of the low vapor pressure of both
the precursors and reaction products, the entire reac-
tion mixture should remain in the crucibles.
RESULTS AND DISCUSSION
The reactions underlying the magnesiothermic
reduction of tungsten oxide compounds,
At the same time, after the preparation of molybde-
MgMoO4 + 3Mg = Mo + 4MgO,
(1) num powders, dense deposits of a white substance
were found on the surface of the reaction products, on
the inner lateral surface of the crucibles above the pre-
ΔН2098 = –1006.0 kJ mol;
CaMoO4 + 3Mg = Mo + 3MgO + CaO,
ΔН2098 = – 898.8 kJ mol;
cursor load level, and on the shield, independent of
(2)
the chemical composition of the precursor and reduc-
tion conditions (Figs. 2, 3a). According to X-ray dif-
fraction data, the white substance was MgO (Fig. 3b).
MoO3 + 3Mg = Mo + 3MgO,
ΔН2098 = –1060.0 kJ mol;
(3)
Similar deposits of pure magnesium oxide were
formed in the case of magnesium vapor reduction of
tungsten oxide compounds: WO3 and MgWO4 [13].
are exothermic and are accompanied by the release of
a large amount of heat. The adiabatic temperatures of
reactions (1)–(3) are 3220, 3170, and 3813°C, respec-
tively. The temperature in the combustion zone of self-
propagating high-temperature synthesis by reaction
(3) reaches 3000°C, leading to the evaporation of con-
siderable amounts of reactants, and the process
becomes uncontrolled [5]. The magnesium vapor
reduction of oxide compounds makes it possible to
control the rate of metal reductant delivery to the reac-
tion zone, thereby considerably lowering the reaction
temperature and enabling control over it throughout
the reduction process (Fig. 1).
It is worth noting that MgO transport from the
reaction zone at a temperature considerably lower
than the melting point of MgO (tm = 2825°C) is a non-
trivial issue and is unrelated to chemical vapor trans-
port, like in the case of magnesium vapor reduction of
tungsten compounds [13]. Neither the reagents nor
the materials used in reactions (1)–(3) contain any
substances capable of forming volatile compounds
with magnesium oxide. On the other hand, the tem-
perature difference between the zone of possible for-
mation of a volatile compound and the zone of its
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1060
KOLOSOV et al.
(a)
(b)
(c)
(d)
(e)
(f)
20 mm
Fig. 2. Crucibles with reaction products (a–c) and corresponding shields (d–f) after reduction; precursors: MoO (a, left cruci-
3
ble), MgMoO (b, c, left crucibles), and CaMoO (a–c, right crucibles); pressure: (a, d) 5, (b, e) 10, and (c, f) 15 kPa; tempera-
4
4
ture, 740°C; time, 3 h.
decomposition is insignificant, especially in the case The amount of heat Q released per unit mass of a stoi-
of the formation of a thin, dense magnesium oxide chiometric starting mixture is 3911, 3293, and
layer directly on the surface of the reaction products 4887 kJ/kg for reactions (1), (2), and (3), respectively.
(Fig. 3a). The segregation of the reaction products also Because of this, in the case of the reduction of MoO3
cannot be explained by the volatility of the precursors,
because the molybdenum content of the MgO depos-
its does not exceed 0.1–0.2 wt %.
The present results demonstrate that the reduction
of MoO3 with magnesium vapor involves two stages.
First, molybdenum dioxide is formed:
directly to molybdenum metal, without an intermedi-
ate oxide, the maximum temperature of the reaction
system should be higher. However, the highest tem-
perature of the reaction system is reached in the case
of reaction (1), and the lowest temperature, in the case
of reaction (3) (Fig. 1). The reason for this is that reac-
tion (4) begins at low temperature, before the reactor
reaches steady-state conditions. For example, when
MoO3 is reduced with calcium vapor, reaction
MoO3 + Mg = MoO2 + MgO,
(4)
ΔН2098 = – 445.7 kJ mol.
MoO3 + Ca = MoO2 + CaO,
(6)
Next, the molybdenum dioxide is reduced to molyb-
denum metal:
ΔН2098 = – 436 kJ mol,
MoO2 + 2Mg = Mo + 2MgO,
(5)
has the highest rate in the range 550 to 650°C [9].
ΔН2098 = – 614.5 kJ mol.
Since the vapor pressure of MoO2 is considerably
lower than that of MoO3, and is comparable to those
of MgMoO4 and CaMoO4 [16], the MoO2 layer form-
ing during heating of the reactor prevents MoO3
The X-ray diffraction results in Fig. 4 confirm that
reactions (4) and (5) occur sequentially, which is con-
sistent with the observed variation in the temperature
of the reaction system during the reduction process. vaporization. Because of this, in the case of the reduc-
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INFLUENCE OF THE COMPOSITION OF PRECURSORS
(a)
1061
MoO2
MoO2
MoO2
MoO2
MoO2
MoO2
1
Mo
Mo
Mo
MoO2
MoO2
MoO2
Mo
MoO2
2
Mo
MgO
Mo
3
10
20
30
40
2θ, deg
50
60
70
80
10 mm
Fig. 4. Portions of X-ray diffraction patterns of washed
(b)
powders after reduction of MoO ; temperature, 730°C;
3
MgO
time, (1) 1, (2) 2, and (3) 4 h.
starting mixture is plotted against the residual pressure
in the reactor. It is seen that the weight of the reaction
products in the crucible exceeds the initial weight only
at a residual argon pressure of 5 kPa. At a higher resid-
ual pressure in the reactor, it can be smaller than the
initial weight on account of the removal of magnesium
oxide from the reaction zone. It follows from the data
presented in Fig. 7 that the amount of MgO removed
from the reaction zone increases with an increase in
both the residual pressure and the amount of heat Q
released per unit weight of the stoichiometric mixture.
MgO
MgO
MgO
MgO
10
20
30
40
2θ, deg
50
60
70
80
Fig. 3. (a) Dense layer of a white substance removed from
the surface of the reaction products and (b) portion of its
The specific surface area of the molybdenum pow-
ders is almost completely determined by the pore area,
and the specific surface area increases primarily on
account of an increase in the amount of small
nanopores (Fig. 8). The larger specific surface area of
the powders prepared by reducing CaMoO4 in com-
parison with MgMoO4 seems to be due to the removal
of a smaller amount of MgO from the reaction zone.
When calcium and magnesium molybdates are
reduced under similar external conditions, the weight
of the magnesium oxide deposit outside of the reac-
tion zone is larger in the case of the reduction of
MgMoO4 (Fig. 7) owing to the higher value of Q. The
removal of a larger amount of MgO from the structure
of the reduced particles of the oxide compound leads
to a decrease in the porosity of the powder and,
accordingly, to a decrease in its specific surface area.
This effect is responsible as well for the observed
X-ray diffraction pattern; MgMoO precursor; tempera-
4
ture, 730°C; time, 3 h; pressure, 10 kPa.
tion of molybdenum trioxide under the above condi-
tions, the percentage of molybdenum in the MgO
deposits approaches those obtained in the case of the
reduction of MgMoO4 and CaMoO4.
It can be seen from the data presented in Fig. 5 that,
in the case of the reduction of the molybdates, the spe-
cific surface area of the resultant molybdenum pow-
ders is four to six times greater than that of the powders
obtained by reducing MoO3 under similar conditions.
Analysis of porosity data indicates that the shape of the
adsorption curves for all synthesized powders corre-
sponds to type IV in the IUPAC classification, which
is characterized by a hysteresis loop and is typical of
materials with a mesoporous structure (Fig. 6).
In Fig. 7, the weight of the reaction products in the decrease in the specific surface area of the powders
crucible after reduction relative to the weight of the with increasing residual pressure in the reactor.
INORGANIC MATERIALS Vol. 53 No. 10 2017

1062
KOLOSOV et al.
surface area by magnesiothermic reduction. Note that the
(a)
specific surface area and nanopore structure of the
powders can be controlled by properly selecting the
starting oxide compound and reduction conditions.
20
15
10
5
3
2
CONCLUSIONS
We have studied in detail the reduction of the
molybdenum oxide compounds MoO3, MgMoO4,
and CaMoO4 by magnesium vapor at residual inert gas
pressures in the range 5–20 kPa and temperatures in
the range 700–800°C and the properties of the molyb-
denum powders thus obtained.
The example of the reduction of the binary oxide
compounds confirms the feasibility of increasing the
specific surface area of magnesiothermic molybde-
num powders by using precursors containing refrac-
tory oxides. The specific surface area of the molybde-
num powders prepared by reducing the MgMoO4 and
1
0
700 720 740 760 780 800
Temperature, °C
(b)
20
15
10
5
CaMoO4 compounds reaches 18–20 m2/g, which is a
factor of 3–6 higher than that of the powders prepared
by reducing MoO3 under similar conditions.
The shape of the adsorption isotherms of the
molybdenum powders corresponds o type IV in the
IUPAC classification, confirming that the powders
have a mesoporous structure.
3
2
In the case of the reduction of the molybdenum
oxide compounds, we observed dense MgO deposits
beyond the reaction zone. The weight of these deposits
has been shown to depend on the amount of heat
released per unit weight of the stoichiometric mixture
and to increase with increasing residual inert gas pres-
sure in the reactor.
1
0
5
10
15
20
Pressure, kPa
Fig. 5. Specific surface area of the molybdenum powders
as a function of (a) temperature at p = 5 kPa and as a func-
tion of (b) pressure at t = 740°C in the reactor; precursors:
(1) MoO , (2) MgMoO , and (3) CaMoO .
3
4
4
Thus, the present results demonstrate the feasibility
of utilizing calcium and magnesium molybdates for the
ACKNOWLEDGMENTS
This work was supported by the Russian Founda-
preparation of molybdenum powders with a large specific tion for Basic Research, grant no. 16-03-00620_a.
25
20
15
10
5
(a)
(b)
(c)
8
6
4
2
40
30
20
10
0
0.2 0.4 0.6 0.8 1.0
0
0.2 0.4 0.6 0.8 1.0 0
0.2 0.4 0.6 0.8 1.0
Relative pressure, p/p0
Fig. 6. Nitrogen adsorption–desorption isotherms of the molybdenum powders prepared by reducing various oxide compounds;
precursors: (a) MoO , (b) MgMoO , and (c) CaMoO ; temperature, 750°C; pressure, 5 kPa; time, 3 h.
3
4
4
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INFLUENCE OF THE COMPOSITION OF PRECURSORS
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3
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Average pore diameter, nm
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3
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4
4
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