Ammonolysis of Cobalt Molybdenum Oxides - In Situ XRD Study

Article abstract of DOI:10.1021/acs.inorgchem.8b00685

The reduction of cobalt molybdenum oxide under an ammonia atmosphere resulting in the formation of ternary interstitial nitride Co₃Mo₃N was studied. Intermediate phases were identified by an in situ powder X-ray diffraction using a reaction chamber. It was supplemented by a thermogravimetric analysis of the process. The presence of intermediate phases, CoMoO₄, Co₂Mo₃O₈, Mo₂N, metallic cobalt, and Co₂Mo₃N, was observed. A synthesis route of Co₃Mo₃N by an ammonolysis method was proposed.

Full text of DOI:10.1021/acs.inorgchem.8b00685

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Ammonolysis of Cobalt Molybdenum Oxides - In Situ XRD Study
́
Paweł Adamski, Dariusz Moszynski,* Agata Komorowska, Marlena Nadziejko, Adam Sarnecki,
and Aleksander Albrecht
West Pomeranian University of Technology, Szczecin, Faculty of Chemical Technology and Engineering, Institute of Inorganic
Chemical Technology and Environment Engineering, Pułaskiego 10, 70-322 Szczecin, Poland
S
* Supporting Information
ABSTRACT: The reduction of cobalt molybdenum oxide
under an ammonia atmosphere resulting in the formation of
ternary interstitial nitride Co3Mo3N was studied. Intermediate
phases were identified by an in situ powder X-ray diffraction
using a reaction chamber. It was supplemented by a
thermogravimetric analysis of the process. The presence of
intermediate phases, CoMoO4, Co2Mo3O8, Mo2N, metallic
cobalt, and Co2Mo3N, was observed. A synthesis route of
Co3Mo3N by an ammonolysis method was proposed.
admixture of potassium or chromium compounds to the
precursor of the material prominently changes the Co2Mo3N/
Co3Mo3N ratio. In general, potassium promotes the formation
of Co2Mo3N while chromium limits its occurrence.
INTRODUCTION
■
Ternary transition metal interstitial nitrides are a relatively new
group of materials which is recently extensively studied. Due to
their similarity with group VII noble metals, these materials are
promising candidates in different catalytic systems.1,2 The
systems containing cobalt molybdenum nitrides proved to have
proper catalytic properties in such chemical processes as
hydrodesulfurization,3 hydroprocessing of organic com-
pounds,4 hydrazine decomposition,5 or NO reduction.6,7
The greatest attention was attracted by an excellent catalytic
activity of cobalt molybdenum nitrides in ammonia synthesis.
Theoretical studies have indicated that a combination of cobalt
and molybdenum is the most active material in the ammonia
synthesis reaction.8 It was proven experimentally that the
catalysts based on Co3Mo3N promoted with cesium com-
pounds are more active than an industrial iron-based catalyst
for ammonia synthesis.9,10 The promotion with potassium
compounds also leads to the highly active ammonia synthesis
catalyst.10 There is a possibility to modify the structure of these
catalysts with other metals to improve their surface properties
and thermal stability.11 The Co-Mo-N catalysts are also
regarded as materials for ammonia decomposition.12
Recent reports have shown that the catalytic activity in the
ammonia process is associated with the Co2Mo3N/Co3Mo3N
ratio. The direct relation between Co2Mo3N concentration and
catalytic activity is difficult to assess since the influence of
promoters (potassium or chromium) must also be considered.
However, general behavior of the materials is such that the
higher content of Co2Mo3N leads to the higher catalytic
activity in ammonia synthesis.11 Therefore, the preparation
procedure leading to the formation of highly concentrated
Co2Mo3N can be a way to produce more active catalysts.
In this context, a preparation procedure of an active form of
a catalyst based on cobalt molybdenum nitride is crucial.
Usually, the active form is obtained in a two-stage process
consisting of the precipitation of a precursor in form of cobalt
molybdenum oxide and the reduction of the precursor under
an ammonia atmosphere.10,18,22 The latter stage especially has
a great influence on the final structure and phase composition
of the catalyst.
The information on the chemical and physical properties of
cobalt molybdenum nitrides is abundant. However, the
number of reports considering the reduction of the precursor
is limited. During this process, cobalt molybdenum oxides are
reduced and nitrides are formed under an ammonia
atmosphere. The development of the porous structure,
formation of crystallographic phases, and final composition
Furthermore, cobalt molybdenum nitrides have interesting
magnetic properties.13−15 Nanocrystalline Co3Mo3N is con-
sidered to be used as a material in the production of
supercapacitors16 and batteries.17
The most widely studied cobalt molybdenum nitride is the
Co3Mo3N phase.18 50% of the lattice nitrogen can be removed
from the structure of that phase under appropriate conditions,
yielding Co6Mo6N.19,20 Synthesis of CoMoN2 was also
reported.14 Cobalt-lean Co2Mo3N was observed, but only in
the mixtures with Co3Mo3N.21 It was shown that a small
Received: March 14, 2018
© XXXX American Chemical Society
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The chemical composition of the precursor as well as a final form of
the catalyst after nitriding was determined with an inductively coupled
plasma optical emission spectrometry (ICP-OES) instrument
(PerkinElmer Optima 5300 DV). The nitrogen concentration in the
cobalt molybdenum nitride was determined by the elemental analysis
with a LECO ONH 836 instrument.
The thermogravimetric observations were performed in a tubular
glass reactor equipped with the thermogravimetric measurement. An
inlet gas composition was controlled by means of a set of mass flow
controllers.
of the catalyst’s surface are key factors for a high activity of the
catalyst.23
The nitridation process of cobalt molybdenum nitrides is
usually performed at the temperature of around 700 °C, under
the ammonia flow.10 The evolution of phase composition was
monitored during the ammonolysis of a nonpromoted
precursor of Co3Mo3N catalyst as well as a Cs-promoted
one.24 At 700 °C, two intermediate phases were formed,
together with Co3Mo3N, Mo2N, and Co metal, respectively. In
the case of nonpromoted catalyst, these phases finally
transformed into Co3Mo3N. However, in Cs-promoted
Co3Mo3N catalyst, intermediate phases were observed even
after prolonged ammonia exposure. It means that addition of
alkali inhibits the formation of Co3Mo3N.
This paper is focused on the detailed analysis of the phase
transformations during ammonolysis of a cobalt molybdenum
nitride precursor. The reduction was performed under strictly
controlled conditions, and the sequence of phase trans-
formations was observed by in situ X-ray diffraction and
thermogravimetry.
RESULTS AND DISCUSSION
■
The compound following the formula CoMoO4 is reported as
a product of the preparation procedure used herein to obtain
the precursor of cobalt molybdenum nitrides. The chemical
analysis of the precursor obtained in the present experiments
and used as a reactant has shown that the molar ratio Co/Mo
is 1.07. It means that the material obtained after precipitation
contains some excess of cobalt atoms in comparison to the
stoichiometry of CoMoO4, where the molar ratio Co/Mo =
1.0. The phase analysis of the precursor was performed at
ambient conditions with X-ray powder diffraction. The
diffraction pattern acquired for the as-obtained sample is
shown in Figure 1. This pattern is in a good agreement with
EXPERIMENTAL SECTION
■
Preparation of the catalyst precursor was performed in a multistep
procedure consisting of a precipitation, filtration, and drying. An
aqueous solution of cobalt(II) nitrate Co(NO3)2·6H2O (7.46 M) was
added to an aqueous solution of ammonium heptamolybdate
(NH4)6Mo7O24·4H2O (0.32 M). The chemicals used are of an
analytical grade (Chempur Poland). The solution was heated to about
90 °C and simultaneously stirred with a mechanical stirrer. The purple
precipitate was isolated from the obtained suspension by vacuum
filtration and was rinsed twice with distilled water and once with
ethanol. The precursor was dried at 150 °C overnight and the
obtained powder was ground in an agate mortar.
The phase composition of the sample and its structural
transformation during nitriding was studied with the use of in situ
powder X-ray diffraction using a Philips X’pert MPD powder
diffractometer. The sample was placed in the closed rigid sample
holder inside the Anton Paar reactor chamber XRK 900 attached to
the diffractometer. The instrument worked in Bragg−Brentano
geometry, and a Cu Kα radiation source was used (λα1 = 0.154056
nm, λα2 = 0.154439 nm). Scans were collected with a solid-state
hybrid type PIXcel1D detector. In the incident beam path, a Ni filter
with 0.02 mm thickness was used, while, in the diffracted beam path, a
curved graphite crystal monochromator in the [002] position was
used. Diffraction data were recorded in diffraction angle ranges
between 20° and 60° 2θ, with 0.02 step size and collection time of
approximately 0.8 s per step.
The sample was exposed to the mixture of ammonia (99.98 vol %
(volume percent)) and nitrogen (99.999 vol %) or to the pure
nitrogen passing through reactor chamber. The cycle with ammonia/
nitrogen application was intended to perform a nitriding of the sample
material. The diffraction pattern of the sample was always collected
under the flow of pure nitrogen. During the nitriding cycle, the flow of
ammonia and nitrogen was 5 cm3/min and 45 cm3/min, respectively.
The experiments were conducted under ambient conditions as well as
during heating at 120 and 700 °C. The nitriding cycles were repeated
until when no differences were observed between two subsequent
diffraction patterns.
Figure 1. X-ray diffraction pattern of the precursor under ambient
conditions and of the sample after drying at 120 °C in nitrogen. Bars
above the diffraction pattern represent reflections corresponding to
the CoMoO4·3/4H2O phase.
the pattern for CoMoO4·3/4 H2O, reported by Eda et al.25
(ICDD No. 04-011-8282). No other crystallographic phase
was identified in the precursor. A medium degree of
crystallinity is supposed by the presence of broad scattering
peaks. Also, because of misfit in some reflection intensities in
the X-ray diffraction (XRD) pattern, texture may be present in
the material. Texture originates from the preferred orientation
(not random) of crystallites in a material and causes
anisotropic properties of the material.
The solid−gas process of cobalt molybdenum nitrides
synthesis is carried out at elevated temperature under an
ammonia atmosphere. The basic steps of this process are initial
drying during the first period of heating, possible phase
transformations taking place while the sample is heated to 700
°C, and finally, the chemical reactions and phase trans-
formations under ammonia exposure. The evolution of the
material during this procedure was observed with in situ XRD
experiments complemented with the thermogravimetric experi-
ment presented in Figure 2. The thermal program applied to
the thermogravimetric study as well as in situ XRD experiments
was alike. Initially, the temperature was increased to 120 °C
under nitrogen flow. A mass loss of 8 wt % (weight percent)
was observed. A corresponding diffraction pattern obtained by
Phase analysis and quantitative analysis were carried out with
PANalytical HighScore Plus v.3.0e software and the International
Centre for Diffraction Data (ICDD) PDF4+ database. The weight
fractions of identified crystallographic phases were calculated, based
on a Rietveld refinement. During this procedure, the following
parameters were varied: scale factor, unit cell and structural
parameters, peak profile shape, and full width at half-maximum
(FWHM).
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phases of cobalt molybdenum oxides: CoMoO4 (ICDD No.
04-017-6377)26 and Co2Mo3O8 (ICDD No. 04-001-9062).27
CoMoO4 is the expected product of CoMoO4·3/4 H2O
dehydration proceeded at elevated temperature. The other
cobalt molybdenum oxide phase, Co2Mo3O8, has a lower
oxygen to molybdenum ratio and is a product of CoMoO4
decomposition.28 According to the dissociation equation
2CoMoO → 2/3Co2Mo3O8 + 2/3CoO + O2
(1)
4
another solid product, CoO is expected. However, the XRD
peaks pertaining to CoO have not been found in the pattern.
The weight fractions calculated based on the Rietveld
refinement of the pattern given in Figure 3 are 70 wt % of
CoMoO4 and 30 wt % of Co2Mo3O8 (Table 1). CoMoO4 has
an identical molar ratio Co/Mo as an expected final product of
nitriding, Co3Mo3N.10 Co2Mo3O8 phase is a Co-lean
compound. Regarding only composition obtained by XRD
analysis, the molar ratio of Co/Mo equals to 0.87 and is lower
than that observed by chemical analysis of the precursor (Co/
Mo = 1.07). Considering eq 1, it is supposed that a part of
cobalt atoms is present in the material in an amorphous CoO,
which cannot be recorded by diffraction analysis.
Figure 2. Mass loss observed during the calcination and nitriding of
the precursor.
in situ XRD at 120 °C is shown in Figure 1. It is virtually
identical to the diffraction pattern collected for the initial
sample before heating. It shows that there is no structural
transformation at low temperature heating. There is only small
change in crystallinity of the sample, which can be observed in
diffraction peaks shape and intensities. Hence, the mass loss
observed by thermogravimetry is attributed to the desorption
of the contaminations, weakly bonded to the material surface,
supposedly mostly water.
After about 3 h of the calcination under nitrogen, the
material was exposed to the ammonia/nitrogen mixture (10
vol % NH3). Thermogravimetric analysis indicated intense
mass loss with the start of the NH3 exposure. It is attributed to
the reduction of cobalt and molybdenum oxides present in the
material. Due to the different construction of the reaction
systems, as well as different particle size of the powder used for
thermogravimetric and XRD analysis, the rate of reduction
observed in the thermobalance cannot be directly related to
the time dependences observed by XRD analysis. Therefore,
only the total mass loss is taken into consideration. After about
12 h of reduction, the stable mass was observed. The mass loss
occurring between the calcination stage and the end of the
process was 24 wt %. The total mass loss (excluding mass loss
observed at 120 °C) was 30 wt %.
Afterward, the temperature was increased to 700 °C while
the flow of pure nitrogen was kept. The mass loss of another 8
wt % had been observed by thermogravimetry before the
sample was exposed to the ammonia/nitrogen mixture (about
3 h from the beginning of the process). The phase composition
observed under the nitrogen atmosphere at 700 °C revealed by
XRD experiment (Figure 3) indicates the presence of two
The better insight into processes taking place during the
ammonolysis process is obtained by in situ XRD analysis. After
10 min of the ammonolysis carried out in the reaction chamber
of the diffractometer, CoMoO4 phase completely disappeared
from the material (Figure 4). The only oxide observed in the
sample was Co2Mo3O8. Instead, metallic cobalt was identified.
The observed reflections of cobalt are in agreement with the
pattern reported by Jarlborg et al.29 (ICDD No. 04-006-4263).
A very good fit was achieved by Rietveld refinement based
on the above phase composition. The molar ratio Co/Mo
calculated upon the weight fractions (see Table 1) is 1.03 for
Figure 3. X-ray diffraction pattern of the sample after calcination at
700 °C in nitrogen. Experimental data are indicated by the dotted line
and Rietveld refinement data by the solid line. Agreement indices for
the presented fit are Rexp = 4.58%, Rwp = 7.23%, Rp = 5.25%, GOF =
2.49.
Table 1. Concentration of the Crystalline Phase in the Sample during Ammonolysis
a
ambient conditions after calcination 10 min
30 min 50 min 70 min 80 min 100 min 110 min final product
crystalline phase
CoMoO4·3/4 H2O
CoMoO4
Co2Mo3O8
Co
Mo2N
Co2Mo3N
Co3Mo3N
concentration of crystalline phase in the sample [wt %]
100.0
70.0
30.0
89.8
10.2
51.0
28.8
20.2
44.5
17.7
15.0
17.2
5.6
20.2
16.9
19.9
43.0
8.6
16.0
27.0
48.4
8.1
8.3
2.8
4.2
30.9
52.7
30.6
62.4
100
a
Time presented in the table should be regarded as total time of ammonia flow through the reaction chamber; time needed to acquire the XRD
patterns is not included.
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which Co2+ cations occupy one-eighth of the available
tetrahedral (8a Wyckoff) sites and Co3+ cations occupy half
of octahedral (16d Wyckoff) sites.32 Nonstoichiometric
Co3−xO4 phase may be a result of cobalt atom vacancies in
octahedral sites, tetrahedral sites, or either of them.
Diffraction patterns of defected CoO and Co3O4 phases with
different vacancy degree were generated by an XRD pattern
simulation in the Highscore software. The best match with
experimental XRD pattern shown in Figure 5 was found for an
oxygen-rich Co3−xO4 phase. The unknown features in Figure 5
are best fitted with 60% occupancy of cobalt in octahedral sites
and 20% occupancy of cobalt in tetrahedral sites as presented
in Figure 6. In this figure, the dotted line is the experimental
XRD pattern from Figure 5 and the solid line represents the
simulated pattern of defected Co3O4 with cobalt vacancies.
Figure 4. X-ray diffraction pattern of the sample after 10 min of
ammonolysis. Experimental data are indicated by the dotted line and
Rietveld refinement data by the solid line. Agreement indices for the
presented fit are Rexp = 4.67%, Rwp = 9.48%, Rp = 6.14%, GOF = 4.11.
this experiment. The molar ratio of Co/Mo is higher than that
observed for the proceeding experiment presented in Figure 3
and relatively close to the value obtained by chemical analysis
of the precursor. It is supposed that, in the presence of
ammonia, a dissociation of CoMoO4 to Co2Mo3O8 proceeded.
CoO expected in the material, as mentioned before, is
supposedly transformed to metallic phase due to the exposure
to the reducing agent.
The next state of the sample was observed after 30 min of
ammonolysis by XRD analysis. The data shown in Figure 5
Figure 6. A simulated diffraction pattern of defected Co3O4 phase
(solid line) overlapped XRD experimental data from Figure 5 (dotted
line).
The Rietveld refinement was performed disregarding the
defected phase. Calculation of the weight fractions has been
performed with the exclusion of the misfitting reflections.
Therefore, the weight fractions presented for the sample after
30 min of reduction are only an estimation. The concentration
of metallic cobalt increased from approximately 10 to 30 wt %,
since cobalt-bearing phases are reduced to the metal. Mo2N
phase is observed as a first product of nitriding. Co2Mo3O8
remains the main constituent of the sample.
After 50 min of ammonolysis (Figure 7), aside from
Co2Mo3O8, Co, and Mo2N, two additional phases were
identified: cobalt molybdenum nitride Co2Mo3N and cobalt
molybdenum nitride Co3Mo3N. Reflections attributed to
Co2Mo3N are in a good agreement with the pattern reported
Figure 5. X-ray diffraction pattern of the sample after 30 min of
ammonolysis. Experimental data are indicated by the dotted line and
Rietveld refinement data by the solid line. Agreement indices for the
presented fit are Rexp = 4.81%, Rwp = 11.07%, Rp = 7.69%, GOF = 5.29.
pertain to this intermediate state of material during the
reduction process. Co2Mo3O8 and metallic cobalt were still
observed in the sample. However, the formation of
molybdenum nitride Mo2N was noticed (see Figure 5).
Observed reflections are in a good agreement with the
Mo2N pattern reported by Bull et al.30 (ICDD No. 04-013-
4024). Some misfits between the experimental pattern and
Rietveld refinement based on the phase composition
comprising Co2Mo3O8, metallic cobalt, and Mo2N are
observed. At reflection angles 2θ around 35.4° and 36.7°, a
peak broadening is observed. However, at the reflection angle
44.8°, a distinct additional peak can be noticed. No
crystallographic phase available in the ICDD database can be
assigned to unidentified peaks.
A plausible option to solve this problem is the existence of a
defected phase having a diffraction pattern different than an
ideal equivalent present in the ICDD database. Cobalt oxides
are well-known to have cation vacancies in their structure.31
Co3O4 is an ionic crystal with a normal spinel structure, in
Figure 7. X-ray diffraction pattern of the sample after 50 min of
ammonolysis. Experimental data are indicated by the dotted line and
Rietveld refinement data by the solid line. Agreement indices for the
presented fit are Rexp = 4.77%, Rwp = 7.55%, Rp = 5.68%, GOF = 2.51.
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by Prior et al.21 (ICDD No. 04-010-6426), while the pattern
reported by Jackson et al.18 (ICDD No. 04-008-1301) fits well
the reflections ascribed to Co3Mo3N.
that the material which initially occurs in the Co and Mo2N
crystallites is transferred and the formation of Co2Mo3N and
Co3Mo3N phases proceeds.
In that case, a deviation of the Rietveld refinement based on
the phase identification mentioned above is smaller than that
observed for the material after 30 min of reduction. The misfits
are found in the reflection angle regions mentioned for the
previous experiment. Therefore, it is assumed that they have an
identical origin. The Co/Mo molar ratio calculated on the
basis of the weight fractions equals 1.06, and it is virtually
identical to the one obtained by chemical analysis. It must be
noted that the concentration of Co2Mo3N in the sample after
50 min of ammonolysis is 3 times higher than the
concentration of Co3Mo3N. Under the ammonolysis con-
ditions, Co2Mo3N nitride is initially formed and then it
transforms into Co3Mo3N nitride.
After 400 min of exposure to the ammonia, only Co3Mo3N
phase is observed (Figure 8). All other phases observed during
this process are finally converted into Co3Mo3N. It proves that
Co2Mo3N is an intermediate phase in Co3Mo3N formation.
The general mechanism of the formation of cobalt
molybdenum nitrides from cobalt molybdenum oxides by
ammonolysis is illustrated in Figure 10. The equation of the
reduction of cobalt molybdenum oxide under an ammonia
atmosphere is given below:
3CoMoO + 8NH3 → Co3Mo3N + 7/2N2 + 12H2O
4
(2)
The XRD data collected for several further steps of
ammonolysis are not presented. However, the weight fractions
calculated upon these data are included in Table 1 and Figure
Figure 10. Synthesis route of cobalt molybdenum nitrides.
The present study shows that the process is complex and
several intermediate products occur. Since the precursor of the
nitrides is obtained by precipitation from a water solution, the
initial material is in the form of hydrate. It is identical with
CoMoO4·3/4 H2O and stable at the drying temperatures. At
700 °C, the water is released and CoMoO4 is formed. Under a
nitrogen atmosphere, this compound decomposes into
Co2Mo3O8. According to thermodynamic data regarding the
Co−Mo−O system, CoO phase is also expected.28 However,
the latter compound was not detected by XRD analysis. There
is indirect evidence that CoO is present in an amorphous form
or a highly disturbed crystalline phase.
Figure 8. X-ray diffraction pattern of the sample after ammonolysis.
Experimental data are indicated by the dotted line and Rietveld
refinement data by the solid line. Agreement indices for the presented
fit are Rexp = 6.00%, Rwp = 9.20%, Rp = 7.07%, GOF = 2.35.
Admission of ammonia into the reaction space instigates the
reduction of the oxides. Metallic Co is observed as the first
product of ammonolysis. This early occurrence of metallic Co
can be explained by the reduction of a fine amorphous CoO
mentioned above. Even though CoO is not observed by XRD,
its presence results in the formation of metallic cobalt which
further reconstructs into the crystalline phase observed in the
diffraction experiments. In the next step of ammonolysis,
Mo2N phase occurs in addition to metallic Co. It is a product
of the reduction of cobalt molybdenum oxides. These
observations indicate that, in the initial stages of ammonolysis,
bimetallic nitrides are not formed, but cobalt molybdenum
oxides decompose and they are reduced to metallic cobalt and
Mo2N. A relatively high concentration of a cobalt-lean cobalt
molybdenum nitride, Co2Mo3N, is identified in the stage when
cobalt molybdenum nitrides are observed first. At this stage,
there is still a high content of Co2Mo3O8, although Co and
Mo2N constitute more than 50 wt % of the sample. Cobalt
molybdenum nitrides are formed from the material of Co and
Mo2N crystallites. The process Co + Mo2N → Co2Mo3N
Figure 9. Dependence of the ammonolysis time on phase
composition in the sample.
9. A gradual disappearance of Co2Mo3O8 phase is observed. In
general, it is also the case for metallic cobalt and Mo2N for
which the weight fractions also gradually decrease. At first, the
concentration of Co2Mo3N is substantially higher than the
concentration of Co3Mo3N. However, starting from the data
set for 70 min of ammonolysis, the weight fraction ratio
Co3Mo3N/Co2Mo3N settles in the range between 1.7 and 2.0.
The disappearance of metallic Co phase and Mo2N phase is
accompanied by simultaneous increase of the combined
concentration of cobalt molybdenum nitrides. It is supposed
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requires intense mass diffusion between Co and Mo2N
crystallites. Therefore, it is hardly observed at lower temper-
atures. The reaction Co2Mo3N + Co → Co3Mo3N is supposed
to proceed consecutively. The final formation of Co3Mo3N
phase requires both prolonged exposition to ammonia and
elevated temperature of 700 °C.
Co2Mo3N and Co3Mo3N phases under apparently identical
conditions.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00685.
The required temperature for the formation of Co3Mo3N is
700 °C. At lower temperatures, even after prolonged exposure
to ammonia, the final products consist of Co and Mo2N only.
The influence of temperature ramp rates and dwell times has
been checked and no effect on formation of the final product
was observed. These parameters only influence the kinetics of
the ammonolysis process. The ammonolysis of cobalt
molybdate hydrate reported by Kojima et al.,10 Jackson et
al.,18 and Hunter et al.19 always has been conducted with
ammonia passing through the sample from the start of the
process. It is in opposition to the synthesis procedure used in
this work, in which the precursor had been first heated to 700
°C, before ammonia was introduced to the reaction chamber.
The mentioned difference possibly influences the composition
of a final product since the product reported in the mentioned
works10,18,19 consists only of Co3Mo3N. This discrepancy
encourages the authors’ for further studies. Influence of the
moment of ammonia introduction to the synthesis of
Co3Mo3N will be thoroughly examined in ongoing work.
Especially, the possibility of formation of Co3Mo3N without
Co2Mo3N as the intermediate phase will be tested.
Rietveld refinement data for all of the analyzed patterns
(PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: dmoszynski@zut.edu.pl. Tel.: +48 91 449 4132. Fax:
+48 91 449 4686.
■
ORCID
Paweł Adamski: 0000-0002-6889-2442
́
Dariusz Moszynski: 0000-0002-7722-7540
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.A. thanks the Polish Ministry of Science and Higher
Education for support through the project “Diamentowy
Grant” no. D12015 019445 funded 2016−2019.
On the other hand, the mechanism proposed in this work
can be used to elucidate the existence of Co2Mo3N/Co3Mo3N
mixtures observed in the experiments described else-
where.11,33−36 Since Co3Mo3N phase is a final product of the
nitridation of cobalt molybdenum oxides, each factor affecting
the reaction Co2Mo3N + Co → Co3Mo3N, namely, diffusion of
Co atoms, influences the final composition of products. An
obvious factor is temperature which increases the diffusion of
Co atoms and promotes the formation of Co3Mo3N. It was
formerly shown that the pretreatment in a 3:1 N2/H2 mixture
applied after the exposure to the ammonia instigates the
formation of well-crystallized Co3Mo3N.19 The process can be
also inhibited by the addition of some compounds such as
chromium or potassium salts into the precursor. Chromium
modification of the precursor results in the formation of stable
mixtures of Co2Mo3N and Co3Mo3N, with Co2Mo3N content
reaching up to 20 wt %.11,34 Introduction of alkali metal salt to
the precursor results in the Co2Mo3N/Co3Mo3N mixture,
where the concentration of Co2Mo3N reaches about 50 wt %.33
These mixtures were stable under prolonged heat treatments
under different gas compositions. It shows that the restriction
of Co metal atom diffusion is a very promising way to tailor the
final composition of Co2Mo3N/Co3Mo3N mixtures.
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■
The process of ammonolysis of cobalt molybdenum oxides
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