Co-precipitate precursor-based synthesis of new interstitial niobium molybdenum nitrides

Article abstract of DOI:10.1007/s11164-014-1749-8

A simple method of preparation of interstitial niobium molybdenum nitride solid solutions in the series Nb_1-x Mo _x N _y (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) has been developed. It is based on direct ammonolysis of precursors resulting from co-precipitation of aqueous solutions of the appropriate common metal salts. A study of the effect of method conditions on the outcome of the procedure is presented. Compounds in this series were prepared as single phases by nitridation at 1,173 K followed by rapid cooling of the samples. Similarly to the individual nitrides NbN and Mo₂N, all the Nb_1-x Mo _x N _y compounds in this series have the rock-salt crystal structure in which the metal atoms are in an fcc arrangement with N atoms occupying octahedral interstitial positions. The materials were characterized by X-ray powder diffraction, elemental analysis, scanning electron microscopy, and thermogravimetry under oxygen flow.

Full text of DOI:10.1007/s11164-014-1749-8

Res Chem Intermed
DOI 10.1007/s11164-014-1749-8
Co-precipitate precursor-based synthesis of new
interstitial niobium molybdenum nitrides
•
Abdelouahad El Himri Mamoune El Himri
•
•
´
´
Domingo Perez-Coll Pedro Nun˜ez
Received: 3 March 2014 / Accepted: 16 June 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract A simple method of preparation of interstitial niobium molybdenum
nitride solid solutions in the series Nb1-xMoxNy (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0)
has been developed. It is based on direct ammonolysis of precursors resulting from
co-precipitation of aqueous solutions of the appropriate common metal salts. A
study of the effect of method conditions on the outcome of the procedure is pre-
sented. Compounds in this series were prepared as single phases by nitridation at
1,173 K followed by rapid cooling of the samples. Similarly to the individual
nitrides NbN and Mo2N, all the Nb1-xMoxNy compounds in this series have the
rock-salt crystal structure in which the metal atoms are in an fcc arrangement with N
atoms occupying octahedral interstitial positions. The materials were characterized
by X-ray powder diffraction, elemental analysis, scanning electron microscopy, and
thermogravimetry under oxygen flow.
Keywords Ammonolysis Á Coprecipitation Á Niobium Á Molybdenum
A. El Himri (&)
Group of Materials Science, Energy and Environment, National School of Applied Sciences,
University of Mohammed I, BP 3 Ajdir, Al Hoceima, Morocco
e-mail: name2004@hotmail.com
M. El Himri
Laboratory of Applied Chemistry and Environment, Faculty of Sciences,
University of Mohammed I, BP 717 Oujda, Morocco
´
D. Perez-Coll
Instituto de Ceramica y Vidrio, CSIC, 28049 Cantoblanco, Madrid, Spain
´
´
P. Nun˜ez
Department of Inorganic Chemistry, University of La Laguna, Tenerife, Spain
123

A. El Himri et al.
Introduction
Transition metal nitrides have attracted attention mainly because of their
technologically important properties [1]. Some have been compared with noble
metals, on the basis of unusual features related to such properties as hardness,
thermal and electrical conductivity, or catalytic performance [2, 3]. Given that
catalysis is among emerging fields of application that have led to renewed interest in
transition metal nitrides [4, 5], their catalytic properties, which arise as a result of
the presence of small nitrogen atoms at interstitial sites of the metal lattice, have
been investigated.
Some studies of monometallic transition metal nitrides have demonstrated their
potential to replace noble-group-metal catalysts [6]. One possible way of improving
their catalytic performance is to introduce other elements into the material.
Molybdenum nitrides are examples of promising materials in this field, owing to
their activity and selectivity in reactions including ammonia synthesis [7–9], the
Fischer–Tropsh reaction [10–14], hydrogenation [15–18], oxidation [19, 20], and
hydrodenitrogenation and hydrodesulfurization [21–25].
Although monometallic nitrides have been widely investigated for a long time
[26], progress in nitride chemistry beyond polymetallic systems was limited until
approximately the middle of the 1980s, because of preparative difficulties [27].
Implementation of new synthetic methods in the 1980s resulted in acceleration of
systematic study of the polymetallic nitrides, and a useful diversity of properties and
applications of these materials has been reported [28].
The most significant problem in the synthesis of polymetallic nitrides is the large
energy of the bond of the N2 molecule (941 kJ mol). Although high temperatures
are required to activate the reaction between the gas and the starting materials, at
these high temperatures nitridation processes lead to the formation of materials with
low nitrogen content. A second difficulty is the high stability of the monometallic
phases. Normally, solid-state synthesis is limited by the low velocity of diffusion in
the solid. For synthesis of the nitrides, the velocity of diffusion of the gas inside the
metallic particles is slower than the equivalent diffusion velocity in the related
amorphous or crystalline precursor. Most of the materials, when reacting with
nitrogen or ammonia, form a coating of monometallic nitrides through which
diffusion of a solid or a gas is slow. In general, it is necessary to increase the
reaction temperature to increase the diffusion velocity. Increasing the temperature
limits the nitrogen content and leads to a separation of nitride phases.
A common feature of the new synthetic methods is the search for alternative
reaction paths by means of which cation diffusion distances can be reduced; this
would enable nitride preparation at relatively low temperatures [29]. It is, therefore,
important to design alternative methods of preparation to investigate different
metallic compositions in more complex systems [30–32]. Use of bimetallic mixed
oxide precursors [33–35], nitridation of metallorganic hydroxides [36, 37], and a
chemical solution method [38–40] have been reported, but have limitations which
have been discussed elsewhere. It is well known that the method of preparation has
a crucial effect on material properties, affecting the structure, morphology, grain
size, and surface area of the materials obtained.
123

New interstitial niobium molybdenum nitrides
Here we report the synthesis of bimetallic nitrides solid solutions in the series
Nb1-xMoxNy (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) via precipitation of polymetallic
precursors.
Experimental
Synthesis
The materials used as reagents in this investigation were NbCl5 (Aldrich, 99 %) and
MoCl5 (Merck, 98 %), which were dissolved in ethanol (CH3CH2OH, Panreac) to
prepare the starting Nb or Mo-containing solutions. The stoichiometric solutions
were combined to obtain Nb-Mo source solutions, with nominal molar compositions
Nb:Mo = 1 - x:x (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0). The resulting solutions were
evaporated to precipitate the precursors.
Niobium–molybdenum nitrides were synthesized by ammonolysis of the
appropriate amorphous or crystalline precursors. The gases used in the ammonolysis
were NH3 (99.9 %) and N2 (99.9995 %). A sample of the selected precursor (ca
0.5 g) was placed in an alumina crucible which was then inserted placed in a quartz
flow-through tube furnace. The gas input of the tube furnace was connected to the
gas line and the gas output was connected to an acetic acid trap. Before initiation of
the thermal treatment, the tube furnace was purged for 15 min with N2 and another
15 min with NH3. Several runs under different experimental conditions were also
performed to determine the appropriate conditions for preparation of pure samples.
The precursor powder was heated at 5 K min-1 to a final temperature (Tf) that
was held for a period of time (thold) under flowing ammonia (50 cm3 min-1). The
solid was then cooled at different rates (rc) in the same atmosphere, either by turning
off the oven and leaving the sample inside (slow cooling, ca 4 K min-1) or by
quenching at room temperature (fast cooling, ca 50 K min-1). After cooling, the
products were always passivated with flowing nitrogen for 15 min.
Characterization
Elemental analysis
Metal ratios in the solids were determined by energy dispersive of X-ray analysis
(EDAX), by use of a Jeol JSM 6300 scanning electron microscope. The equipment
included an Oxford detector with quantification performed by use of virtual
standards on associated Link–Isis software. The operating voltage was 20 kV and
the energy range of the analysis 0–20 keV. The nitrogen content of the nitrides was
evaluated by standard combustion analysis (Carlo Erba EA 1108); N2 and CO were
separated in a chromatographic column and measured by use of a thermal
conductivity detector. Oxygen content was determined by pyrolysis in an inert gas
by use of a Carlo Erba model 1500. Results from these analyses for the resulting
nitrides are summarized in Table 1.
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A. El Himri et al.
Table 1 Chemical composition, cell parameters, and crystallite size of niobium molybdenum nitrides
Nb1-xMoxNy
x
x (EDAX) Oxygen
(wt%)
Nitrogen
(wt%)
Proposed
stoichiometry
Cell
parameter
Particle size t (nm)
Scherrer
method
Rietveld
method
˚
a (A)
0.0
–
\1
9.2
14.4
14.1
14.5
13.9
13.4
6.8
NbN0.67
4.32432 (17)
33
30
32
26
27
26
13
29
27
28
23
23
22
11
0.2 0.18–0.22 \1
0.4 0.36–0.42 \1
Nb0.8Mo0.2N1.12 4.32304 (25)
Nb0.6Mo0.4N1.10 4.32219 (14)
Nb0.5Mo0.5N1.14 4.31957 (29)
Nb0.4Mo0.6N1.09 4.31910 (19)
Nb0.2Mo0.8N1.05 4.31904 (20)
0.5 0.47–0.5
\1
0.6 0.57–0.62 \1
0.8 0.76–0.81 \1
1.0
–
\1
MoN0.5
4.19378 (19)
X-ray diffraction
X-ray powder diffraction patterns were obtained by use of a Phillips X’pert
automated diffractometer using graphite-monochromated Cu-Ka radiation. Sample
powder was passed through a sieve on the holder surface. Routine patterns for phase
identification were collected in the angular range 2h = 30–70° with a scanning step
of 0.028 and a collection time of 1 s per step. The cell parameters for each product
were obtained by profile fitting of the patterns, by use of Le Bail’s method [41]
implemented in FULLPROF software [42]. For this purpose, the patterns were
collected over a wider angular range (2h = 25–85°), with a scanning step of 0.02°,
and a longer acquisition time (10 s per step), to improve the statistics. Fitting was
performed by use of a pseudo-Voigt peak-shape function. In the final runs, the usual
profile variables (scale factors, background coefficients, zero-points, half-width,
pseudo-Voigt and asymmetry parameters for the peak-shape) were refined. All
graphical representations relating to X-ray powder diffraction patterns were
performed by use of WinPLOTR software [43].
Microstructure characterization
The morphology of all the crystalline precursors and the resulting nitrides was
studied by use of a scanning electron microscope (Hitachi S-4100) operating at an
accelerating voltage of 30 kV. All the samples were coated with a thin film of gold
for better image definition.
Results and discussion
Nitrides with different phases can be prepared from parent nitride systems. In solid
solutions of the type a-MNx (M = Mo and Nb), c-Mo2N1?x and d-NbN1-x have
been shown to have cubic structures with the N soluble in the structure of the
metal [44, 45]. Other phases, for example c-MoN, g-NbN, d-NbN, and b-Nb2N, are
123

New interstitial niobium molybdenum nitrides
based on a hexagonal metal lattice, with N occupying interstitial holes [46, 47]. A
low-temperature phase, b-Mo2N1?x, derived from c-Mo2N1?x, in which ordered
transition of the N atoms results in a tetragonal structure and N-rich phases (Nb5N6,
Nb4N5) [48] has also been characterized.
The multiphasic character of these systems is a clear limitation to obtaining
polymetallic rock salt-type nitride solid solutions. To avoid phase segregation, it is
necessary to search for a given set of synthetic conditions under which the required
phase can be prepared.
We wondered about the possible existence of an isostructural Nb1-xMoxNy
(0 B x B 1) solid solution, which could be of interest in catalysis. At this point,
given that different molybdenum nitrides can result from ammonolysis, depending
on both the nature of the source and synthetic conditions [49, 50], it was necessary
to investigate the optimum conditions for obtaining cubic phases (i.e., the upper
limit of composition in the hypothetical solid solution) when starting from our co-
precipitated precursors.
We performed treatments for long times (12 h, to eliminate time as a variable)
and investigated in more detail the effects of temperature and the rate of cooling on
the nature of the resulting phases.
Figures 1 and 2 show the X-ray diffraction patterns of the products obtained from
the ammonolysis of precursors corresponding to the limiting metal compositions,
x = 0 and 1, when subjected to slow and fast cooling. It is apparent that the cubic
phase (d-NbN; JCPDS card 38-1155) is obtained from 873 to 1,073 K for samples
containing niobium only (x = 0), irrespective of the rate of cooling. At temperatures
higher than 1,073 K, it is apparent there is competition between the cubic (d-NbN;
JCPDS card 38-1155) and the hexagonal (g-NbN; JCPDS card 20-0801) phases.
The rate of cooling seems to affect samples containing molybdenum only
(x = 1). Thus, slow cooling of samples prepared at 873 to 1,073 K results in a
mixture of the cubic (c-Mo2N; JCPDS card 25-1366) and hexagonal (d-MoN;
JCPDS card 25-1367) phases, in which the proportion of the cubic phase increases
with temperature. This is indicative of competition between the cubic (c-Mo2N) and
the hexagonal (d-MoN) phases in Mo-containing samples. The hexagonal phase is
stabilized at low temperatures whereas the cubic phase should be stabilized at high
temperatures. Detection of the hexagonal phase in the high-temperature XRD
pattern might be a consequence of the process Mo2N ? 1/2N2 ? 2 MoN when the
rate of cooling is sufficiently slow (Fig. 2a).
For molybdenum nitride prepared by rapid cooling, the effective temperature
range is substantially reduced. At temperatures below 973 K, ammonolysis does not
occur, and only reduction to MoO2 (JCPDS card 32-0671) is observed. In contrast,
when the reaction is performed at 1,173 K metallic molybdenum (JCPDS card
04-0809) is the main crystalline product, with the cubic phase (Fig. 2b). By use of
similar conditions (reaction time, temperature, and rate of cooling) we prepared the
intermediate nitrides in the solid solution series, Nb1-xMoxNy (x = 0.0, 0.2, 0.4,
0.5, 0.6, 0.8, 1.0). The co-precipitated precursors were not appropriate for obtaining
single phases. In all cases, irrespective of the reaction conditions, their direct
ammonolysis did not lead to single phases, resulting in the systematic appearance of
segregation phases.
123

A. El Himri et al.
Fig. 1 X-ray diffraction patterns of the products resulting from ammonolysis of the co-precipitated
precursor containing niobium only, at different temperatures, with thermal treatment for 12 h and slow
(a) or fast (b) cooling
Fig. 2 X-ray diffraction patterns of the products resulting from ammonolysis of the co-precipitated
precursor containing molybdenum only, at different temperatures, with thermal treatment for 12 h and
slow (a) or fast (b) cooling
We therefore subjected the amorphous precursors to thermal treatment in air.
After treatment at 1,073 K for 12 h the amorphous precursors were transformed into
black solids with X-ray diffraction patterns indicating they were mixtures of Nb2O5
and MoO3. These solids, in different proportions (1-x)Nb2Mo3/xMoO3, were used
123

New interstitial niobium molybdenum nitrides
Nb:Mo (1.0:0.0)
Nb:Mo (0.8:0.2)
Nb:Mo (0.6:0.4)
Nb:Mo (0.5:0.5)
Nb:Mo (0.4:0.6)
Nb:Mo (0.2:0.8)
Nb:Mo (0.0:1.0)
25
30
35
40
45
50
55
60
65
70
75
80
85
2Theta (°)
Fig. 3 X-ray diffraction patterns of Nb1-xMoxNy materials prepared from crystalline precursors at
1,173 K, for 2 h, and rapid cooling
as crystalline precursors in the ammonolysis reaction. Ammonolysis performed at
1,173 K for 2 h with slow cooling led to single-phase products.
The optimized conditions for synthesis of cubic single phases of Nb1-xMoxNy are
ammonolysis of the crystalline precursors at 1,173 K, for 2 h, then slow cooling.
The X-ray diffraction patterns corresponding to samples of different composition in
the series Nb1-xMoxNy (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) are presented in Fig. 3.
All these compositions were prepared by ammonolysis of the respective
crystalline precursors under the conditions established as described above.
According to the X-ray diffraction patterns, all the compounds were obtained as
single phases, with rock salt structures (space group Fm3 m). The cell parameters
were calculated by profile fitting of the patterns, by use of LeBail’s method
implemented in FULLPROF software. The Rietveld refinement for one member of
the series Nb1-xMoxNy (x = 0.8) is shown in Fig. 4; the R factors are listed in the
figure.
Table 1 shows the results from chemical analysis of the resulting products. In all
cases, the Nb:Mo ratio is (within experimental error) equal to the nominal value for
the corresponding precursor. One aspect that deserves attention is the proposed
stoichiometry for nitrogen. Before this point we have systematically referred to the
NbN and Mo2N limiting compositions in the Nb1-xMoxNy solid solution. Both
extremes correspond, however, to crystalline phases which are tolerant of a specific
range of non-stoichiometry, NbN1-x (0 B xB 0.4 0.10) and c-Mo2N1?x
123

A. El Himri et al.
Fig. 4 Rietveld refinement for Nb1-xMoxNy (x = 0.8)
(-0.2 B x B 0.3). Accordingly, what might be expected for the solid solution is
that the N:metal ratio varies from ca 0.6 1.0 in the Nb-rich compositions to ca
0.40 0.65 in the Mo-rich compositions.
XRD can be used to evaluate the dependence of peak broadening on crystallite
size and lattice strain as a result of dislocation [51]. Crystallite size was determined
by the X-ray line broadening method, by use of the Scherrer equation and the
Rietveld method. In use of the Scherrer formula, average crystallite size is
calculated by fitting the peaks in XRD pattern by means of a Gaussian profile. The
crystallite sizes calculated by use of the Scherrer formula and the Rietveld method
are listed in Table 1. Crystallite size obtained by use of the Rietveld method is less
than that obtained by use of the Scherrer formula, because of correction for peak
broadening performed in the Rietveld method taking into account instrumental
factors not considered in the Scherrer method.
Figure 5 shows characteristic SEM images corresponding to representative
samples. The co-precipitated precursors comprise sheets with a typical width of
1.5 lm (Fig. 5a, b). The external appearance of the sheets remains practically
unchanged during ammonolysis, yielding Nb1-xMoxNy (Fig. 5c), although they now
have wrinkled surfaces. SEM images at high magnification (Fig. 5d) clearly reveal
that Nb1-xMoxNy grains are aggregates of nanometer-sized spherical particles with
typical diameters of approximately 20 nm.
Figure 6 shows characteristic TGA profiles for the individual nitrides (x = 0 and 1)
and an intermediate composition (x = 0.4). Oxidation always begins at relatively low
temperatures (approximately 480–550 K) and is complete between 700 and 740 K,
depending on the composition. The final products (800 K) are Nb2O5 for x = 0, and
MoO3, for x = 1. For all the remaining intermediate compositions, a mixture of
123

New interstitial niobium molybdenum nitrides
Fig. 5 SEM images showing the microstructure of the amorphous precursors (a, b) and the final nitride
(c, d) for the composition Nb1-xMoxNy (x = 0.5). Scale bars correspond to 5 lm, 500 nm, 2.5 lm, and
250 nm, respectively
135
MoN0.5
130
125
NbN0.67
120
115
Nb0.6Mo0.4N1.10
110
105
100
95
373
473
573
673
773
Temperature (K)
Fig. 6 Characteristic TGA profiles obtained from NbNy, Nb0.6Mo0.4Ny and MoNy
123

A. El Himri et al.
individual oxides is obtained. The low-temperature oxidation is consistent with the
pyrophoric character of these products, making passivation necessary.
Conclusions
We have prepared new interstitial niobium–molybdenum nitride solid solutions in
the series Nb1-xMoxNy (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0), by ammonolysis of the
corresponding coprecipitate precursors. This simple and inexpensive method of
synthesis of compounds of complex composition starts from aqueous solutions and
could be regarded as an alternative to relatively long processes using amide
precursors. The compounds are obtained as aggregates of nanometric spherical
particles with typical diameters of approximately 20 nm, which suggests the
materials have large surface areas. This method enables the preparation of samples
with high surface area by precise adjustment of the composition; both are crucial for
homogeneous distribution of metals, preparation of catalysts, etc.
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