Up today researchers work to obtain new catalytic systems for a next-generation ammonia synthesis process, which could reduce the energy costs and CO₂ emission. Cobalt molybdenum nitrides are potential candidates in this field, since they have higher activity in the synthesis of ammonia than the commercially used promoted iron catalyst [1]. The activity of the cobalt molybdenum catalyst can be further increased by addition of alkaline promoters such as caesium or potassium [2]. However, the addition of these promoters has a negative effect on the specific surface area of ​​the cobalt molybdenum catalyst. The development of an effective method to counteract these limitations is crucial to the potential application of cobalt molybdenum nitrides on a larger scale.

Only a few studies addressing the development of the cobalt molybdenum nitrides with a higher specific surface area were conducted. The change in the conditions of the activation process by additional temperature treatment is suggested [3]. The typical precursor consisting CoMoO₄ was changed to the Co(en)₃MoO₄, in which (en) denotes ethylenediamine molecule [4]. The addition of citric acid, which acts as a chelating agent, results in increased surface area and higher activity [5]. Double promotion with the use of potassium and chromium leads to simultaneously more active and more stable material, in which chromium acts as a structural promoter [6].

In this study, the problem of low thermal stability and tendency to sinter of cobalt molybdenum nitrides during ammonia synthesis was addressed by using catalyst support in form of active carbon. The supported catalysts were formed by the wet impregnation of the support with an aqueous solution of cobalt and molybdenum salts, followed by vacuum evaporation. The precursor was filtered, dried and subjected to a reduction under an ammonia atmosphere to obtain nitrides. Despite very high surface area, activated carbon in applications as catalyst carrier has a major disadvantage. It undergoes methanation under the conditions of ammonia synthesis, i.e. reacts with hydrogen to form methane [7].

To make insight into the activation process, in this study the ammonolysis of the precursors was examined via in-situ X-ray powder diffraction with the use of PANalytical X’pert Pro MPD diffractometer equipped with Anton Paar XRK 900 reaction chamber. Under the ammonolysis conditions, several structural transformations of the precursor were observed. Apart from broad hump peaks originated from activated carbon, several sharp peaks corresponding to intermediate phases were identified during ammonolysis process. Following phases were identified: MoC, metallic Co and Co₃Mo₃C. Cobalt molybdenum nitrides which are active in the ammonia synthesis were not present in the product.

[1] Kojima, R. & Aika, K. (2001). Appl. Catal. A 215, 149.

[2] Moszyński, D., Jędrzejewski, R., Ziebro, J. & Arabczyk, W. (2010). Appl. Surf. Sci. 256, 5581.

[3] Kojima, R. & Aika, K. (2001). Appl. Catal. A 219, 157.

[4] Duan, X., Ji, J., Yan, X., Qian, G., Chen, D. & Zhou, X.J.C. (2016). ChemCatChem 8, 938

[5] Podila, S., Zaman, S.F., Driss, H., Al-Zahrani, A.A., Daous, M.A. & Petrov, L. (2017). Int. J. Hydrog. Energy 42, 8006

[6] Moszyński, D., Adamski, P., Nadziejko, M., Komorowska, A. & Sarnecki, A. (2018). Chem. Pap. 72, 425.

[7] Chunhui, Z., Yifeng, Z. & Huazhang, L. (2010). J. Rare Earths 28, 552

Keywords: Cobalt molybdenum nitrides; ammonolysis; catalyst; XRPD in-situ

The scientific work was financed by The Polish National Centre for Research and Development, grant „Lider”, project No. LIDER/10/0039/L-10/18/NCBR/2019.

Wustyt is a non-stoichiometric form of iron(II) oxide with the general formula Fe_1-xO, which is currently used as a precursor of the iron catalyst for the synthesis of ammonia. The catalyst obtained from this precursor has higher activity compared to a traditional catalyst reduced from magnetite [1]. Promoters used in the magnetite precursor may have a different role in wustite catalysts. This is due to the different valence of iron in the structure of wustite and magnetite. As a result, some promoters are more or less likely to build in the catalyst grain [2]. In this study, the influence of magnesium oxide addition on the activity and thermal stability of the catalyst was investigated. It was suspected that as a result of similar valence, magnesium ions would be more likely to build into the grain of wustite, thus stabilize the active phase of the catalyst.

Iron catalyst precursors were obtained as a result of the melting of magnetite, aluminum oxides, calcium, magnesium, potassium nitrate, and metallic iron, which acts as a magnetite reducer. The XRPD method confirm the presence of wustite phase in each precursor. Chemical composition was determined by ICP-OES method. Evolution of the phase composition of obtained precursors during reduction with hydrogen were investigated by XRPD in-situ method, with the use of PANalytical X’pert Pro MPD diffractometer equipped with Anton Paar XRK 900 reaction chamber. Crystallite sizes of iron were calculated using Rietveld method. Activity tests in the ammonia synthesis reaction were carried under pressure 10 MPa in the temperature 450°C.

The promotion of wustite precursors with magnesium oxide contributes to the significantly increase of the crystallite sizes. The crystallite size increased by 47% comparing the catalysts with the lowest and the highest concentration of magnesium oxide. Similarly activity of obtained catalysts rate were also increased. The activity increased by over 40% comparing the catalysts with the lowest magnesium oxide concentration to the catalyst with the highest concentration of this promoter.

[1] Hua-Zhang, L., Xiao-Nian, L. & Zhang-Neng, H. (1996). Appl. Catal. A General 142, 209.

[2] Lendzion-Bieluń, Z. & Arabczyk, W. (2001). Appl. Catal. A General 207, 37.

Keywords: Ammonia synthesis; wustite; XRPD in-situ; crystallite size

The scientific work was financed by The Polish National Centre for Research and Development, grant „Lider”, project No. LIDER/10/0039/L-10/18/NCBR/2019.

Nitrides of transition metals are mostly associated with hardness and mechanical strength and tend to be thermally and electrically conductive. They exhibit the properties of both metals and ceramics [1]. On the other hand, they were proven to have high catalytic activity in various chemical reactions, e.g. ammonia synthesis, ammonia decomposition, hydrodesuplhurisation and NO reduction [2].

Particularly effective ammonia synthesis catalysts are nitrides of cobalt and molybdenum. These nitrides are usually obtained during the ammonolysis process of oxide precursors. Previous studies on cobalt molybdate reduction in ammonia confirmed the presence of multiple crystalline phases in the system, mainly: CoMoO₄∙nH₂O, NH₄H₃Co₂Mo₂O₁₀, Co₂Mo₃O₈, CoMoO₄, Co, Mo_2­N, Co₃Mo, Co₃Mo₃N and Co₂Mo₃N. Co₂Mo₃N phase is especially desirable due to its high activity in ammonia synthesis [3].

The crystalline structure of cobalt-molybdenum precursors can be modified by a change of pH value during their precipitation. Commonly the precipitation from the solution of cobalt(II) nitrate and ammonium heptamolybdate is conducted at pH between 5 and 6. Alkalisation of the reaction results in a different structure of the material obtained. Also, the course of the phase transformations observed for these materials by XRD analysis differs.

In the presented study, materials obtained in pH 5.5 and 7.5 are compared. The phase transformations during calcination and ammonolysis processes were studied in the reaction chamber attached to an X-ray diffractometer (Anton Paar XRK900, Philips X’Pert Pro MPD).

At first, two different precursor phases, CoMoO₄∙nH₂O and NH₄H₃Co₂Mo₂O₁₀, were obtained for pH 5.5 and pH 7.5, respectively. After 2 hours of calcination at 300°C under an inert atmosphere, both precursors transformed into the CoMoO₄ phase. At 500°C, besides the dominant CoMoO₄ phase, for precursor obtained in pH 7.5, Co₂Mo₃O₈ phase occurred. At 700°C, the CoMoO₄ phase gradually transforms into Co₂Mo₃O₈, Co₃Mo and metallic cobalt. After the ammonolysis, the concentration of main phases, Co₃Mo₃N and Co₂Mo₃N, for both samples was similar, but the width of the diffraction peaks and the content of trace phases were significantly different.

[1] Oyama, S. T. (1996). The Chemistry of Transition Metal Carbides and Nitrides. Blackie Academic and Professional: Glasgow.

[2] Gurram, V.R.B., Enumula, S.S., Chada, R.R., Koppadi, K.S., Burru, D. R. & Kamaraju, S.R.R. (2018). Catal. Surv. from Asia, 22, 166.

[3] Adamski, P., Moszyński, D., Komorowska, A., Nadziejko, M., Sarnecki, A. & Albrecht, A. (2018). Inorg. Chem. 57, 9844.

Acknowledgements: The scientific work was financed by The Polish National Centre for Research and Development, grant „Lider”, project No. LIDER/10/0039/L-10/18/NCBR/2019.

Ternary transition metals nitrides are a relatively new group of materials studied extensively nowadays. One of the most important properties of ternary transition metals nitrides is their tendency to form defected structures, with variable composition. Within these inorganic compounds, the Co-Mo-N system exhibits many promising properties. Among others, cobalt molybdenum nitrides could be used as catalysts, magnetic materials and electrodes. Cobalt molybdenum nitrides exhibit very high catalytic activity in ammonia synthesis, which makes them a plausible candidate to replace industrial iron catalyst [1-3].

The most widely used procedure to form cobalt molybdenum nitrides is a two-stage process consisting of a precursor preparation and a subsequent ammonolysis of mixed oxides. More insight into these stages, especially on structural and crystallographic transformations of precursors and intermediate compounds, is crucial for the enhancement of the material properties. The synthesis process is influenced by numerous parameters, i.e. composition and temperature of precipitation process, final precursor composition, ammonolysis temperature, composition and flow of a reducing agent. Consequently, the ammonolysis often results in the formation of mixtures of different crystallographic phases of transition metal nitrides with mismatched properties. Lack of reproducibility is a major disadvantage, which inhibits technology upscaling.

A synthesis method of cobalt molybdenum nitrides greatly affects its composition and properties. The stoichiometry alteration could be beneficial or detrimental to the cobalt molybdenum nitrides properties. For example, their catalytic activity in ammonia synthesis depends on Co₂Mo₃N to Co₃Mo₃N ratio [4]. Therefore, favourable is a synthesis method, which can affect stoichiometry in a controlled way. Such a procedure is the one used in this study. It bases on the mechanochemical formation of the mixture of cobalt and molybdenum salts with the controlled Co:Mo molar ratio, which is later reduced under the ammonia atmosphere.

To make insight into the activation process, the ammonolysis of the mixture of cobalt and molybdenum salts with the controlled Co:Mo molar ratio was examined via in-situ X-ray powder diffraction with the use of PANalytical X’pert Pro MPD diffractometer equipped with Anton Paar XRK 900 reaction chamber. A transformation of mixed cobalt molybdenum oxides into mixed cobalt molybdenum nitrides was observed. In the sample subjected to the temperature of 200°C the reflections corresponding to cobalt molybdate CoMoO₄ were identified. Instead of a separate step of precursor precipitation, a simple mechanochemical technique was implemented. As a result, the phase described as the precursor in the mentioned earlier synthesis methods was obtained. This result suggests that in the studied system the intermix of cobalt and molybdenum atoms, obtained via the mechanochemical method, allows the formation of a bimetallic phase at medium temperature. In the sample at 700°C under an ammonia atmosphere, the reflections corresponding to Co₂Mo₃N and Co₃Mo₃N were identified.

[1] Jacobsen, C.J.H. (2000). Chem. Commun. 1057.

[2] Kojima, R. & Aika, K. (2001). Appl. Catal. A 215, 149.

[3] Moszyński, D., Jędrzejewski, R., Ziebro, J. & Arabczyk, W. (2010). Appl. Surf. Sci. 256, 5581.

[4] Moszyński, D., Adamski, P., Nadziejko, M., Komorowska, A. & Sarnecki, A. (2018). Chemical Papers 72, 425.

Keywords: Cobalt molybdenum nitrides; ammonolysis; XRPD in-situ

Financed as a part of PROM Programme “International Scholarship Exchange of PhD Candidates and Academic Staff” co-financed by Polish National Agency For Academic Exchange and European Union through European Social Fund within the frame of Knowledge, Education, Development Operational Programme, project no. PPI/PRO/2019/1/00008/U/00001.

Tungstite (WO₃.H₂O), was successfully immobilized on the surface of mesoporous Silica SiO₂/WO₃ by in-situ reaction using poly (ethylene oxide) as polymeric template and Na2WO4 as precursor and immobilized tungsten trioxide SiO₂/WO₃-C was obtained by calcination of SiO₂/WO₃ at 350°C. The as-obtained materials were characterized by N₂ sorption, SEM, PXRD, FT-IR, UV-Visible and TGA.

Structural characterization of both materials indicates the succeed immobilization of tungstite and tungsten trioxide in amorphous silica. The diffraction picks in SiO₂/WO₃ are arising from two different phases corresponding to WO₃ and WO₃.H₂O, Rietveld refinement assume the orthorhombic crystal lattice for both compounds to with parameters value a=5.25 Å, b=10.72 Å, c=5.13 Å for WO₃ and a=5.25 Å, b=10.72 Å, c=5.13 Å for WO₃.H₂O. phases quantification assumes the presence of tungstite (WO₃.H₂O) as a majority phase by 75.3%, which allow us to investigate it crystallographic structure. The crystal structure of the immobilized tungstite is generally formed by layers of distorted octahedral building blocks of WO₆ in which one axial oxygen position is occupied by water molecule. After calcination at 330°C a phase transformation to the monoclinic structure is observed and water molecules are eliminated from the structure, lattice parameters obtained after Rietveld refinement are a=7.32 Å, b=7.54 Å, c=3.85 Å.

The as-prepared materials are highly efficient in the oxidative photo-degradation of sulfamethazine in water with an efficiency of 92.14% and 92.84% for SiO₂/WO₃and SiO₂/WO₃-C respectively, with different stability aspect. Indeed, SiO₂/WO₃-C show a poor stability when it reused for 6 times due to leaching problem. In the other hand SiO₂/WO₃ could be reused with a small loss of activity after 6 cycles of photocatalysis. The stability difference is due to crystallographic structure differences that is characterized by the presence of water molecules in SiO₂/WO₃ and its absence on SiO₂/WO₃-C. The good stability can be attributed to the strong van-der-walls interaction between the oxygen of silica network and the hydrogen of water molecule encapsulated in tungstite structure.

Palladium-based catalysts are extensively used in petrochemical industry for hydrogenation of unsaturated hydrocarbons. The catalytic process is associated with the formation of surface, subsurface and bulk palladium carbides and hydrides which affects the catalytic properties of materials. Here, combination of in situ and operando synchrotron-based X-ray absorption near edge structure (XANES) with extended X-ray absorption fine structure (EXAFS) spectroscopies with X-ray diffraction (XRD) provided the detailed information about the structure of supported palladium nanoparticles during their interaction with hydrogen and hydrocarbons.

The industrially relevant samples for investigation were provided by Chimet S.p.A. (Arezzo, Italy) and represented palladium nanoparticles with the average size of 2.6 nm and narrow size distribution supported on wood-based carbon. X-ray absorption and diffraction experiments were performed at BM31 beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The beamline allows fast switching (30 s) between absorption and diffraction setups allowing quasi-simultaneous measurements within a single experiment. The samples were loaded inside 1 mm quartz glass capillaries and connected to a remotely controlled gas line. Hydrogen, acetylene and ethylene were used as reactive gases; helium was uses as a carrier gas. The output of the capillary was monitored by a mass spectrometer (MS).

In the case of pure hydrogen was used, a combination of EXAFS and XRD analysis allowed highlighting the difference between the core and shell parts of the nanoparticle [1-2]. To provide an unambiguous proof of this hypothesis, we have performed more than 200 in situ measurements in the wide range of hydrogen partial pressures and temperatures. For the nanostructured samples, EXAFS data showed more smooth behavior of metal-hydride phase transition compared to XRD data, while for the reference bulk materials both methods provided identical results. The difference was explained by the contribution of the amorphous surface part of the nanoparticle to EXAFS, while only bulk region contributes to XRD. The latter was shown to behave similar to the bulk crystals having a sharp phase transition when forming the palladium hydride. Hypothesizing the ranges of the surface-to-bulk ratio for the given particle sized, we have discriminated the evolution Pd-Pd distances in the bulk and at the surface during the hydride phase formation.

In the following experiment we have introduced acetylene though the catalyst. In this case, XANES spectra allowed unambiguous differentiation between the hydride and carbide phases that can be formed in presence of both hydrogen and hydrocarbons [2-3]. Moreover, XANES region was shown to be sensitive to the adsorption and desorption of hydrocarbon molecules at the surface of the particles, which makes no effect on EXAFS and XRD data, therefore, allowing the detection of the relevant catalytic species [4-5]. During continuous operation of the catalyst in the mixture of ethylene and hydrogen, XANES spectroscopy demonstrated gradual and irreversible formation of palladium carbides even in the excess of hydrogen in the gas phase [5-6].

Thus, we have successfully shown that a combination of XRD, EXAFS and XANES techniques can highlight the difference of the structure in the bulk, subsurface and surface region of the nanoparticles. The above examples have shown, that such difference can occur under catalytic reaction conditions even for monometallic palladium particles due to the interaction with re reactive molecules.

References:

[1] A. L. Bugaev, A. A. Guda, K. A. Lomachenko, et al. J. Phys. Chem. C 121, 18202 (2017)

[2] A. L. Bugaev, O. A. Usoltsev, A. Lazzarini, et al. Faraday Discuss. 208, 187 (2018)

[3] A. L. Bugaev, A. A. Guda, A. Lazzarini, et al. Catal. Today 283, 144 (2017)

[4] A. L. Bugaev, O. A. Usoltsev, A. A. Guda, et al. J. Phys. Chem. C 122, 12029 (2018)

[5] A. L. Bugaev, A. A. Guda, I. A. Pankin, et al. Catal. Today 336, 40 (2019)

[6] A. L. Bugaev, O. A. Usoltsev, A. A. Guda, et al. Faraday Discuss. In press. DOI: 10.1039/C9FD00139E

Ceria supported nanoparticles (NPs) of noble metals are well-known catalysts for diverse hydrogenation and oxidation reactions [1, 2]. Their catalytic activity depends on the dispersion and shape of NPs, support, functionalization, etc. However, the use of high Z-support and small NPs limits their diagnostics especially in laboratory conditions [3]. In this study, we demonstrate a possibility of in situ monitoring the size and surface morphology of Pd/CeO₂ catalysts during the growth by using FTIR spectroscopy of adsorbed CO.

Ceria NPs used as support were synthesized according to the method described in [4] and impregnated by PdCl₂ [3]. Then, the material was put into the reaction chamber and heated in a flow of Ar up to 30, 150 or 300 °C (samples Pd-30, Pd-150, Pd-300) for 30 min. A mixture of H₂, CO and Ar (2.5, 1 and 46.5 mL/min) was passed through the sample for 1 hour to reduce Pd NPs.

XRPD didn’t allow distinguishing Pd NPs for all samples (Fig.1a). This fact could be explained by the small size of synthesized Pd NPs which caused broadening of peaks. Tests of catalytic (procedure described in [3]) shown that CO conversion was 25-70% for all samples at 150 °C even without calcination (in case of Pd-150 and Pd-300) and decreased in row Pd-30>Pd-150>Pd-300.

Figure 1. (a) XRPD patterns of all samples; (b) series of FTIR spectra during the synthesis of Pd-30.

The series of spectra collected in situ demonstrated that the reduction at high temperature (Pd-300 and Pd-150) was much faster than at low temperature (Pd-30). Also, it was observed that reduction was not complete for all samples: peaks of CO adsorbed on Pd²⁺ and Pd⁺ ions were observed (ca. 2160 and 2110 cm^–1). The last fact is explained by ceria support that prevented complete reduction. The process of reduction was observed in detail for the Pd-30 (Fig.1b) where the decrease of CO adsorbed on Pd ions was accompanied by the increase of peaks related to bridged carbonyls – evidence of appearing and growth of the extended surfaces. FTIR spectra allowed to determine the size of NPs which is proportional to the ratio of areas under peaks attributed to bridged (below 2000 cm^–1) and linear (2000-2100 cm^–1) carbonyls. Size decreased in row Pd-300>Pd-150>Pd-30. The dynamics of growth was clearly observed for Pd-30 and Pd-150 sample whereas for Pd-300 CO adsorbed only at 2- and 3-folded sites. Only carbonyls on Pd(111) faces were detected for Pd-150 and Pd-300 when both Pd(100) and Pd(111) facets were found for the Pd-30 sample.

While conventional techniques are limited by size of NPs (XRPD), poor contrast (TEM), require large scale facilities (XAS, SAXS), described laboratory technique allows determining the size and surface morphology in situ, at any desired moment of NPs growth.

[1] Zang W., Li G., Wang L., Zhang X. (2015) Catal. Sci. Technol., 5, 2532-2553.

[2] Liang Q., Liu J., Wei Y., Zhao Z., MacLachlan M. J. (2013). Chem. Comm., 49, 8928-8930.

[3] Tereshchenko A., Polyakov V., Guda A., Lastovina T., Pimonova Y., Bulgakov A., et al. (2019). Catalysts, 9, 385.

[4] Benmouhoub C., Kadri A., Benbrahim N., Hadji S. (2009). Materials Science Forum, 609, 189-194.

Keywords: palladium; ceria; nanoparticles; CO probing molecules; FTIR; adsorption;

The study was carried out with the financial support of the Russian Foundation for Basic Research (RFBR) in the framework of the scientific project №20-32-70227. Tereshcheno A. also acknowledge RFBR for funding according to the research project № 20-32-90048

An important frontier in materials science is to understand, characterize and quantize disorder in inorganic materials and its relation to their properties. This requires deep knowledge of both the average structure and the defects present in the samples. Spinel-type compounds form a family of industrially relevant materials¹ that potentially exhibit both atomic and/or magnetic disorder.^{2, 3} Spinel ferrites, AFe₂O₄, in particular have seen use in high-frequency applications due to their magnetism in conjunction with electrically insulating properties. The spinel structure consists of a distorted cubic closest packing of oxygen, in which 1/8 of the tetrahedral holes and 1/2 of the octahedral holes are occupied by cations. The general formula is AB₂O_4, where A are divalent and B trivalent cations.

Single crystals larger than 1 mm³ of ZnFe₂O₄ and NiFe₂O₄ have been grown using the flux method. These where chosen as model spinel ferrites exhibiting the normal and inverse configuration, respectively, with the possibility of magnetic disorder studies in ZnFe₂O₄.⁴ X-ray fluorescence measurements confirmed a low degree of flux inclusions in the crystals, on the order of 0.1 wt%.

Extensive diffraction data has been collected for initial benchmark structure determination, with synchrotron powder X-ray diffraction and single crystal X-ray diffraction being collected at SPring-8 in Japan, and single crystal neutron diffraction being collected at the Spallation Neutron Source at Oak Ridge National Laboratory.

Initial data modelling shows some systematic discrepancies between the structural parameters obtained from the different sets of diffraction data. Rietveld modelling of the powder data gives lower lattice parameters than either single crystal method, which has been attributed to abnormal peak asymmetry caused by a non-symmetric X-ray beam profile. The atomic displacement parameters obtained from the X-ray single crystal and powder data of ZnFe₂O₄ differs both in magnitude and temperature dependence, the cause of which has not been identified yet.

The powder patterns of NiFe₂O₄ reveal left shoulders at reflections with miller indices that are all multiples of four, which could be related to the compound’s magnetism.

The single crystal data show peak splitting indicating a degree of twinning on both spinel samples. Maximum entropy method analysis of the structure factors from the single crystal X-ray data showed no evidence of residual electron density at potential interstitial sites in the structure.

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2. S. Sommer, E. D. Bøjesen, N. Lock, H. Kasai, J. Skibsted, E. Nishibori and B. B. Iversen, Dalton Transactions, 2020, 49, 13449-13461.

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