Synthesis of Molybdenum Nitrides

Article abstract of DOI:10.1134/S0020168520110102

Abstract—: We have studied the effect of precursors on the synthesis conditions and characteristics of molybdenum nitrides. Mo, MoO₃, and MgMoO₄ powders were nitrided in flowing ammonia at temperatures in the range 500–800°C. The use of molybdenum nanopowder as a precursor has made it possible to reduce the synthesis temperature and time. We have demonstrated the possibility of direct ammonolysis of the double oxide MgMoO₄. Using this molybdate, we have obtained a material with a specific surface area up to 29 m²/g, which is a factor of 2 to 3 larger than that reached by nitriding MoO₃. In all cases, the synthesis products consisted of the γ- and β-phases of Mo₂N, with cubic and tetragonal lattices, respectively.

Full text of DOI:10.1134/S0020168520110102

ISSN 0020-1685, Inorganic Materials, 2020, Vol. 56, No. 11, pp. 1113–1121. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Neorganicheskie Materialy, 2020, Vol. 56, No. 11, pp. 1175–1182.
Synthesis of Molybdenum Nitrides
V. M. Orlova, *, R. N. Osaulenkob, and V. Ya. Kuznetsova
aTananaev Institute of Chemistry and Technology of Rare Elements and Minerals (separate subdivision), Kola Scientific Center
(Federal Research Center), Russian Academy of Sciences, Apatity, Murmansk oblast, 184209 Russia
bPetrozavodsk State University, Petrozavodsk, 185910 Karelia, Russia
*e-mail: v.orlov@ksc.ru
Received December 30, 2019; revised July 2, 2020; accepted July 2, 2020
Abstract—We have studied the effect of precursors on the synthesis conditions and characteristics of molyb-
denum nitrides. Mo, MoO3, and MgMoO4 powders were nitrided in flowing ammonia at temperatures in the
range 500–800°C. The use of molybdenum nanopowder as a precursor has made it possible to reduce the
synthesis temperature and time. We have demonstrated the possibility of direct ammonolysis of the double
oxide MgMoO4. Using this molybdate, we have obtained a material with a specific surface area up to 29 m2/g,
which is a factor of 2 to 3 larger than that reached by nitriding MoO3. In all cases, the synthesis products con-
sisted of the γ- and β-phases of Mo2N, with cubic and tetragonal lattices, respectively.
Keywords: nitridation, molybdenum, molybdenum trioxide, magnesium molybdate, molybdenum nitride,
ammonia
DOI: 10.1134/S0020168520110102
INTRODUCTION
of molybdenum pentachloride in dehydrated chloro-
form for 8 h. Next, the solvent was evaporated at a
temperature of 100°C in flowing dry ammonia. The
reaction product was heat-treated at temperatures
from 600 to 800°C in an ammonia atmosphere for 10
h. As the heat treatment temperature was raised from
600 to 800°C, x decreased from 1.321 to 0.769 and the
specific surface area of the material dropped from 49
to 16 m2/g.
Technological progress is impossible without using
novel functional materials, for example, such as tran-
sition metal nitrides [1]. One of them is molybdenum
nitride, Mo2N. It exists in two polymorphs: tetragonal
(β) and face-centered cubic (γ) [2]. Mo2N is thought
to be a promising catalyst for a variety of processes,
including ammonia synthesis, hydrodesulfurization
(HDS) of fuel, and oil hydrodenitrogenation (HDN)
[3–5].
There are several ways of preparing molybdenum
nitride powders. Verkhoglyadova et al. [6] examined
the feasibility of preparing nitrides via nitrogenation of
metallic molybdenum powder with an average particle
size in the range 2–7 μm. An attempt to prepare
nitrides in a nitrogen atmosphere at temperatures from
400 to 1000°C was unsuccessful. Nitrogenation in
flowing ammonia for 4 h at a temperature of 700°C
yielded the Mo2N nitride containing 6.8% nitrogen.
There are several processes for Mo2N synthesis with
the use of molybdenum pentachloride as a precursor
[7–9]. O’Loughlin et al. [7] prepared γ-Mo2N by
reacting MoCl5 with Ca3N2 at a pressure of 57 kbar.
The addition of 3 mol NH4Cl to the starting mixture
allowed a crystalline nitride to be obtained at normal
pressure. Nanocrystalline γ-Mo2N with an average
particle size of 30 nm was obtained by reacting metallic
sodium with anhydrous MoCl5 and NH4Cl in an auto-
clave at a temperature of 550°C [8]. Choi and Kumta
[9] synthesized MoNx nanoparticles via ammonolysis
Despite the diversity of approaches, the main pro-
cess for the synthesis of molybdenum nitrides is MoO3
ammonolysis because it is relatively easy to implement
[10–13]. Choi et al. [10, 11] used a temperature-pro-
grammed reaction, that is, heating at a constant rate.
MoO3 was heated in flowing ammonia from room
temperature to 623 K over a period of 0.5 h. The heat-
ing rate was 40 or 100 K/h from 623 to 732 K and 100
or 200 K/h at higher temperatures, up to 973 K. The
holding time at the final temperature was 1 h. The
resultant product was further heat-treated in flowing
hydrogen at a temperature of 673 K for 3 h. It consisted
of γ-Mo2N powder, occasionally with a small amount
of Mo or MoO2 impurities. Its specific surface area
varied from 4 to 116 m2/g, depending on ammonolysis
conditions. Mckaya et al. [12] prepared the molybde-
num nitrides γ-Mo2N and β-Mo2N0.78 using MoO3 as
a precursor. γ-Mo2N was synthesized in flowing NH3
at a heating rate of 5.6°C/min below 357°C,
0.5°C/min in the range 357–447°C, and 2.1°C/min at
higher temperatures, up to 785°C. The holding time at
1113

1114
ORLOV et al.
2
3
4
13
12
11
1
5
6
7
8
10
9
Fig. 1. Schematic of the experimental apparatus for nitridation: (1) ammonia cylinder, (2) pressure gage, (3) fine control valve,
(4) rotameter, (5) quartz reactor, (6) boat containing the material, (7) resistance furnace, (8) ammonia inlet tube, (9) shields,
(10) thermocouple, (11) Termodat-13K2, (12) flask, (13) water seal.
the final temperature was 5 h. The specific surface area
of the resultant γ-Mo2N was 85 m2/g. β-Mo2N0.78 was
synthesized by treating MoO3 in a flowing mixture of
H2 and N2 in the ratio 3 : 1 at a temperature of 700°C
for 2 h. The specific surface area of the nitride thus
EXPERIMENTAL
The starting chemicals used for nitridation were
molybdenum powder with a specific surface area of
4.8 m2/g (calculated particle size of 130 nm), prepared
by magnesiothermic reduction of calcium molybdate;
obtained was 9 m2/g. The Mo3N2 nitride in the form of the MgMoO4 molybdate synthesized via sintering
[17]; and reagent-grade MoO3 as a reference precur-
sor. The specific surface area of the MoO3 and
nanowire with a mesoporous structure and a specific
surface area of 45 m2/g was prepared via ammonolysis
of MoO3 nanowire in a tubular furnace at a tempera- MgMoO4 was 0.4 and 1.5 m2/g, respectively. We used
a reactor (Fig. 1) in the form of a quartz ampule
40 mm in diameter and 700 mm in length. Quartz
boats containing the material were placed in a uni-
formly heated zone of the furnace. The temperature
was measured by a Chromel–Alumel thermocouple,
whose hot junction was located over the boat contain-
ing the material. The temperature and heating rate
ture of 700°C and holding time of 3 h [13]. According
to Lee et al. [13], the mesoporous structure of the
nitride was due to the transformation of the structure
of the metal oxide to that of the nitride, which led to
pore formation in the material.
The use of metal nanopowders as precursors for the
synthesis of nitrides in flowing ammonia makes it pos- were maintained constant by a Termodat-13K2 tem-
perature controller. The experimental procedure was
described in detail previously [14].
sible to considerably lower the temperature of the pro-
cess and reduce its duration [14, 15]. Because of this,
it was of interest to study conditions for nitriding
molybdenum nanopowders prepared via magnesio-
thermic reduction of double molybdenum oxides [16]
and examine the feasibility of preparing nitrides with
an increased specific surface area via direct ammono-
lysis of a double molybdenum oxide.
During the nitridation process, the temperature
was raised to 500–800°C at a rate of 8 K/min. The
holding time at the highest temperature was varied
from 1 to 5 h. The ammonia flow rate was 150–
200 mL/min. After the heat treatment, the retort was
withdrawn from the furnace and the reaction products
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SYNTHESIS OF MOLYBDENUM NITRIDES
(а)
1115
(b)
6
5
4
3
2
1
10
8
6
4
2
0
0
500
550
600
650
700
750
800 500
550
600
650
700
750
800
Temperature, °C
Temperature, °C
Fig. 2. (a) Nitrogen content and (b) specific surface area as functions of molybdenum powder nitridation temperature (holding
for 1 h).
were cooled in flowing ammonia. Cooling to a tem-
Nitridation of molybdenum powder. Figure 2 shows
the percentage of nitrogen in the molybdenum powder
nitridation products and their specific surface area as
functions of process temperature. The nitrogen con-
tent of the powder was 3.9% after just 1 h of nitrogena-
tion at a temperature of 500°C and reached a maxi-
mum as the synthesis temperature was raised to 600°C
(Fig. 2a). The specific surface area of the nitrides syn-
thesized in the range 500–700°C was at the level of the
starting molybdenum powder and decreased slightly as
the temperature was raised to 800°C (Fig. 2b). Figure 3
shows X-ray diffraction patterns of the starting molyb-
denum powder and the synthesis products obtained at
different temperatures. Even after nitrogenation for
1 h at a temperature of 500°C, we observed weak,
broad peaks of new phases: tetragonal phase β-Mo2N
and face-centered cubic phase γ-Mo2N (Fig. 3, scan 2).
With increasing nitrogenation time and temperature,
the intensity of the reflections from the nitride phases
increased, and that of the reflections from Mo
decreased. After nitrogenation for 3 h at a temperature
of 800°C, no peaks corresponding to Mo were
detected (Fig. 3, scan 5).
Table 1 summarizes data on the structure and
phase composition of the Mo nitrogenation products
in relation to synthesis conditions. The absence of the
β-Mo2N phase in the sample obtained by nitrogena-
tion for 2 h at a temperature of 800°C can be
accounted for by the fact that it contains 9% nitrogen.
A second phase in it is the hexagonal nitride δ3-MoN.
Its structure is distinctive in that is contains ordered
defects in the nitrogen sublattice. The nitrogen atoms
occupy every other trigonal prismatic interstice built
from layers of metal atoms. The presence of such defects
is evidenced by the superstructure reflections from this
phase, marked by arrows, in scan 5 (Fig. 3) [18].
perature of 100°C took 15 min.
The residual MgO in the reaction products
obtained with the use of MgMoO4 as a precursor was
removed by double leaching with a 10% HCl solution
for 1.5 h in each step. The nitride powder was washed
with distilled water until neutral solution pH was
reached, and then dried at a temperature of 70°C for 8 h.
The phase composition of the reaction products
was determined by X-ray diffraction on a DRF-2 dif-
fractometer (CuKα radiation, graphite monochroma-
tor). The phases present were identified using the
JCPDS–ICDD PDF-2 database (release 2002).
Structural parameters were refined by the Rietveld
method using a DRON-4 automatic X-ray diffrac-
tometer and the Inorganic Crystal Structure Database
(ICSD) (Karlsruhe, release 2017). The specific surface
area and porosity parameters were determined by
static BET adsorption measurements and the Barrett–
Joyner–Halenda (BJH) method, respectively, with a
Micromeritics TriStar II 3020 analyzer. Oxygen and
nitrogen concentrations were determined by gas
adsorption chromatography in combination with
pulsed heating on a K-671 analyzer.
RESULTS AND DISCUSSION
Reactions of molybdenum powder and molybde-
num oxide compounds with ammonia can proceed
according to the following schemes:
8Мо + 4NH3 = 4Мо2N + 6H↑2,
(1)
2Мо + 2NH3 = 2МоN + 3H↑2,
(2)
4МоО3 + 8NH3 = 2Мо2N + 12H2O + 3N↑2,
(3)
Nitridation of oxide compounds. Table 2 presents
the percentages of nitrogen and oxygen in the samples
obtained by nitriding MoO3 and MgMoO4 and their
4MgMoO4 + 8NH3 = 2Mo2N
(4)
+ 4MgO + 12H2O + 3N↑2.
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ORLOV et al.
Mo
β-Mo2N
γ-Mo2N
δ3-MoN
5
4
3
2
1
36
49
62
75
88
101
114
2θ, deg
Fig. 3. X-ray diffraction patterns of the Mo powder (1) and the samples obtained by nitriding it at 500 (2, 3) and 800°C (4, 5) for
1 (2, 4) and 3 h (3, 5).
specific surface area (S) and phase composition in nitride from 28.7 to 34.5 at % (from 5.54 to 7.13 wt %)
relation to process conditions. Their X-ray diffraction led to a decrease in the temperature of the β-Mo2N ↔
patterns are shown in Figs. 4 and 5. The composition
of the samples obtained by nitriding MoO3 at a tem-
perature of 600°C suggests that the process involves
two steps. First, the hydrogen released as a result of
ammonia decomposition reduces MoO3 to MoO2,
and then nitrogen substitutes for the oxygen in this
oxide. Table 3 presents detailed data on the structure
and phase composition of the samples obtained by
MoO3 ammonolysis.
γ-Mo2N phase transition from 800 to 400°C. The
presence of the high-temperature phase γ-Mo2N can
be accounted for by two factors: the slow kinetics of
the transformation of the γ-phase to the β-phase and
the decrease in the degree of order in the nitrogen sub-
lattice with increasing nitrogen content. At 34.5 at % N,
the tetragonal lattice of the nitride transforms into a
cubic one [19].
We call attention to the high oxygen content, espe-
cially in the molybdate nitridation products. At the
Nitriding MgMoO4 at a temperature of 500°C for
2 h produced no changes in the phase composition of same time, no oxide phases were detected. It seems
the starting material (Table 2). At the same time, the likely that a considerable fraction of the oxygen
reaction products obtained by nitridation at a tem- detected by analysis was sorbed on the surface of the
perature of 600°C for just 1 h contained the Mo2N nitride powder. This is confirmed by the fact that the
lattice parameters of the nitrides are similar to those of
references. In addition, nitrogen sorption on the sur-
face is probably responsible for the fact that, despite
the presence of excess nitrogen with respect to the
Mo2N nitride in a number of samples, no MoN was
detected.
The pore structure of the synthesized samples
depends significantly on the precursor used. Figure 6
shows typical sorption curves. The curves of the sam-
ples obtained by nitriding Mo and MoO3 (Figs. 6b, 6c)
phase (Fig. 5a, scan 1). After the nitridation time was
increased to 2 h, all of the molybdenum in the compo-
sition of the compound formed the γ-Mo2N nitride
(Fig. 5a, scan 2). The X-ray diffraction patterns of the
samples obtained at a higher temperature are shown in
Fig. 5b. The Rietveld-refined phase composition and
lattice parameters of the samples obtained by nitriding
MgMoO4 are presented in Table 3.
Analysis of characteristics of the samples obtained
by nitriding all three precursors indicated that essen-
tially all of them consisted of a mixture of the β- and are similar in shape to type II in the IUPAC classifica-
γ-Mo2N phases. The high-temperature phase γ-Mo2N tion, characteristic of macroporous materials [20].
The hysteresis loop is similar in shape to type D, typi-
cal of materials containing V-shaped pores. The sam-
ples prepared by MgMoO4 ammonolysis have a radi-
is present even in the samples prepared by nitriding the
molybdenum powder at a temperature of 500°C. This
is consistent with data reported by Ettmayer [19], who
showed that increasing the nitrogen content of the cally different pore structure. Their sorption curves
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SYNTHESIS OF MOLYBDENUM NITRIDES
1117
Table 1. Phase composition and structural parameters of the molybdenum powder nitrogenation products
a, Å
c, Å
Phase
ICSD no.
Sp. gr.
wt %
100
ICSD
Starting Mo
3.14737 3.146
Nitrogenation at 500°С, 1 h
meas
ICSD
meas
Mo
Mo
52267
–
–
Im3m
52267
30593
172802
3.14737
4.2
3.1460(6)
4.2318(2)
4.1714(5)
71
16
13
Im3m
β-Mo2N
γ-Mo2N
8.0
7.9451(2)
7.9798(1)
8.0443(4)
5.6205(7)
I41 amd
Fm3m
4.16158
–
–
Nitrogenation at 500°С, 5 h
Mo
52267
30593
172802
3.14737
4.2
3.1424(8)
4.2261(8)
4.1887(9)
58
21
21
Im3m
β-Mo2N
γ-Mo2N
8.0
I41 amd
Fm3m
4.16158
–
–
Nitrogenation at 800°С, 1 h
Mo
52267
30593
172802
3.14737
4.2
3.1468(9)
4.2126(1)
4.1828(5)
3
29
68
Im3m
β-Mo2N
γ-Mo2N
8.0
I41 amd
Fm3m
4.16158
–
–
Nitrogenation at 80°С, 2 h
γ-Mo2N
δ3-MoN
172802
99452
4.16158
5.73659
4.1710(2)
5.7424(3)
70
30
Fm3m
P63mc
5.6188(4)
Table 2. Composition of the samples obtained by nitriding MoO3 and MgMoO4
S, m2/g
Precursor
t, °С
τ, h
wt % N
wt % O
Phases
MoO2
1
3
1
3
2
2
1
2
2
1
2
–
–
–
600
>12
2.6
6.6
MoO2, Mo2N
Mo2N, MoN
Mo2N, MoN
Mo2N, Mo
2.5
7.2
6.9
6.5
–
MoO3
–
–
700
–
2.1
11
800
500
600
600
700
800
800
MgMoO4*
–
–
–
MgMoO4*, MgO, Mo2N
–
–
5.2
8.1
3.4
5.1
Mo2N
Mo2N
Mo2N
Mo2N
4.2
7.7
8.1
5.6
–
MgMoO4
14.3
22.3
29
* Phase composition of the reaction products before leaching.
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ORLOV et al.
γ-Mo2N
Mo
MoO2
δ3-MoN
3
2
1
31
42
53
64
75
86
97
108
2θ, deg
Fig. 4. X-ray diffraction patterns of the samples obtained by nitriding MoO (1) at 600 °C for 3 h, (2) at 700°C for 1 h, and (3) at
3
800°C for 3 h.
(а)
γ-Mo2N
MgMoO4
2
1
30
40
50
60
70
2θ, deg
(b)
γ-Mo2N
β-Mo2N
2
1
40
50
60
70
80
90
100
110
2θ, deg
Fig. 5. X-ray diffraction patterns of the samples obtained by nitriding MgMoO at temperatures of (a) 600 and (b) 800°C for
4
(1) 1 and (2) 2 h.
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SYNTHESIS OF MOLYBDENUM NITRIDES
1119
Table 3. Phase composition and structural parameters of the MoO3 and MgMoO4 nitridation products
a, Å
c, Å
Phase
ICSD no.
Sp. gr.
wt %
ICSD
meas
ICSD
meas
Nitrogenation of МоО3, 700°С, 1 h
γ-Mo2N
172802
99452
4.16158
5.73659
4.16669
5.73652
–
91
Fm3m
P63mc
δ3-Mo2N
5.6188(4)
5.60233
9
Nitrogenation of МоО3, 800°С, 2 h
Mo
52267
3.14737
4.16158
3.14873
4.17926
–
–
3
Im3m
Fm3m
γ-Mo2N
172802
97
Nitrogenation of MgMoO4, 800°С, 1 h
4.16158 4.1768(5)
Nitrogenation of MgMoO4, 800°С, 3 h
γ-Mo2N
172802
–
–
100
Fm3m
β-Mo2N
γ-Mo2N
30593
4.2
4.2006(3)
4.1725(6)
8.0
8.0739(2)
35
65
I41 amd
Fm3m
172802
4.16158
(а)
(b)
Q, сm3/g
Q, сm3/g
11
10
9
8
7
14
12
10
8
Adsorption
Desorption
Adsorption
Desorption
6
5
4
6
4
3
2
1
2
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
р/р0
р/р0
(c)
(d)
Q, сm3/g
Q, сm3/g
70
60
50
40
30
20
10
14
Adsorption
Desorption
Adsorption
Desorption
12
10
8
6
4
2
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
р/р0
р/р0
Fig. 6. Sorption curves of the Mo powder (a) and the nitrides obtained under standard conditions using Mo (b), MoO (c), and
3
MgMoO (d) as precursors.
4
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ORLOV et al.
also correspond to type II, but there is negligible hys-
teresis (Fig. 6d). The pore volume ranges from 0.1 to
0.18 cm3/g. This is more than five times the pore vol-
ume in the samples prepared by nitriding Mo and
MoO3. The distinction is due to the additional macro-
pores produced by the leaching of the magnesium
oxide in the composition of the molybdate.
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The present results demonstrate for the first time
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nitriding the double oxide MgMoO4 in flowing
ammonia. Using this oxide as a precursor instead of
MoO3 has made it possible to increase the specific sur-
face area of the nitridation products by a factor of 2 to
3. The presence of additional magnesium oxide layers
in nitrided particles has led to changes in the pore
structure of the nitride after leaching, an effect
observed for the first time. The sorption curves of this
material also correspond to type II, but there is negli-
gible hysteresis.
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The phase composition of the materials obtained at
700 and 800°C is typically a combination of the low-
temperature tetragonal nitride β-Mo2N and the high-
temperature face-centered cubic phase γ-Mo2N. The
relative amounts of these phases depend on the syn-
thesis temperature and the percentage of nitrogen in
the nitride.
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Translated by O. Tsarev
INORGANIC MATERIALS Vol. 56 No. 11 2020