Ammonolysis of Cobalt Molybdenum Oxides - In Situ XRD Study

Jul 31, 2018 - Synopsis. This work provides experimental evidence for the existence of intermediate phases during a synthesis of Co3Mo3N ternary nitri...
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Ammonolysis of Cobalt Molybdenum Oxides - In Situ XRD Study Paweł Adamski, Dariusz Moszyński,* 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

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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.

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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 compounds,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 compounds 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 Furthermore, cobalt molybdenum nitrides have interesting magnetic properties.13−15 Nanocrystalline Co3Mo3N is considered 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 © XXXX American Chemical Society

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. 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 Received: March 14, 2018

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DOI: 10.1021/acs.inorgchem.8b00685 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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 transformations was observed by in situ X-ray diffraction and thermogravimetry.

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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. 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).

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 transformations under ammonia exposure. The evolution of the material during this procedure was observed with in situ XRD experiments complemented with the thermogravimetric experiment 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 B

DOI: 10.1021/acs.inorgchem.8b00685 Inorg. Chem. XXXX, XXX, XXX−XXX

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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 2CoMoO4 → 2/3Co2Mo3O8 + 2/3CoO + O2

(1)

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, Co3Mo3 N.10 Co2Mo3O 8 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. 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 %. 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 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. 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

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 ambient conditions crystalline phase CoMoO4·3/4 H2O CoMoO4 Co2Mo3O8 Co Mo2N Co2Mo3N Co3Mo3N

after calcination

10 mina

30 min

50 min

70 min

80 min

100 min

110 min

final product

8.1 8.3 30.9 52.7

2.8 4.2 30.6 62.4

100

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

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.

C

DOI: 10.1021/acs.inorgchem.8b00685 Inorg. Chem. XXXX, XXX, XXX−XXX

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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. 04-0134024). 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. D

DOI: 10.1021/acs.inorgchem.8b00685 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 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. 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 conditions, Co2Mo3N nitride is initially formed and then it transforms into Co3Mo3N nitride. 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

that the material which initially occurs in the Co and Mo2N crystallites is transferred and the formation of Co2Mo3N and Co3Mo3N phases proceeds. 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: 3CoMoO4 + 8NH3 → Co3Mo3N + 7/2N2 + 12H 2O (2)

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. 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 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.

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 E

DOI: 10.1021/acs.inorgchem.8b00685 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Co2Mo3N and Co3Mo3N phases under apparently identical conditions.

requires intense mass diffusion between Co and Mo2N crystallites. Therefore, it is hardly observed at lower temperatures. 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. 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. 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 elsewhere.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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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00685.

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Rietveld refinement data for all of the analyzed patterns (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +48 91 449 4132. Fax: +48 91 449 4686. ORCID

Paweł Adamski: 0000-0002-6889-2442 Dariusz Moszyński: 0000-0002-7722-7540 Notes

The authors declare no competing financial interest.

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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.

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REFERENCES

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CONCLUSIONS The process of ammonolysis of cobalt molybdenum oxides leading to the formation of cobalt molybdenum nitrides was observed by thermogravimetry and in situ X-ray diffraction experiments. The mechanism of the process was proposed. It comprises several stages including: water removal from the hydrates, decomposition of the CoMoO4 oxide, formation of Co metal, and Mo2N phases as intermediates to the formation of Co2Mo3N nitride. The latter compound transforms to final Co3Mo3N phase. The complex mechanism leaves space for a different route to prevail for different substrates and justifies the formerly reported formation of different mixtures of F

DOI: 10.1021/acs.inorgchem.8b00685 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.8b00685 Inorg. Chem. XXXX, XXX, XXX−XXX