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Introduction

Currently, a large number of works of modern research are devoted to the creation of nanocomposite structures, which are formed by mixing one or more heterogeneous materials at the nanoscale, which allows obtaining objects with improved properties. Nanocomposites demonstrate superhydrophobicity, superhydrophilicity, thermal energy transfer, electronic and ionic transport, resistance to abrasion and contamination, which makes them attractive for obtaining electrochemical capacitors [1-3], catalysts [4-6], sensors [7-9], adsorbents [10-13], etc.

Nanocomposites are interesting from the point of view of protecting human health and the normal functioning of electronic devices due to the increasing electromagnetic pollution of the environment [14,15]. We need materials, on the one hand, capable of converting electromagnetic energy into thermal energy, and on the other hand, absorbing microwave radiation and able to solve the problem of electromagnetic interference. e composite should consist of magnetic and dielectric components, since dielectric permittivity and magnetic permeability are complementary; have lightweight and strong absorption in a wide frequency band. Fe, Co and Ni combined with carbon have such properties.

The task of the process of obtaining such materials is to introduce metals into dispersed carbon, so that they would be uniformly distributed throughout the volume of carbon and in their dispersion would be close to it. For ex- ample, a method for the preparation of Co-Fe-C composites using an alginate r substrate was described, where sodi- um alginate and cobalt and iron chlorides were the starting products [16], impregnation of the r with cobalt and iron in a water-ethanol solution under the action of ultra- sound, washing, drying and carbonization of the sample in an argon atmosphere. In [17], electrodeposition method was used to obtain a similar Co-Fe-C composite. e au- thors [18] prepared FeCo-C s by electrospinn- ing from solutions of iron and cobalt acetylacetonates in dimethylformamide. In [19,20], metal nanoparticles embedded in porous carbon composites were synthesized by calcination of a mixture of organic carbon and metal ni- trates. y were uniformly dispersed in the porous carbon

The analysis of literature data shows that obtaining bimetallic composites is associated with a number of sequential operations aimed at bringing metal ions into contact with the organic framework, which must then be converted into carbon with simultaneous reduction of metal ions. If carbides are introduced into the organic phase, the synthesis of these carbides themselves is a complex challenge in its own right.

Previously we investigated the thermal decomposition of double complex compounds (DCCs). It was necessary to take into account the influence of the external gas atmosphere on the thermolysis process, so part of the work was the study of thermolysis in an inert atmosphere. It was found that some amount of carbon always remains in the solid residue from the calcination of most of the DCCs [23,24]. Consequently, such compounds can be used as precursors of metal-carbon compositions, which have not been previously considered in the literature. The method is characterized by simplicity, since it consists of two steps: synthesis of DCСs and thermolysis in an inert atmosphere. Ultimately, this leads to the formation of high-mogeneous materials and a decrease in the calcination temperature, i.e., a reduction in energy consumption compared to traditional methods of production.This paper describes the conditions of production and properties of new metal-carbon materials.

Experimental Section

The first step is the synthesis of DCS [24].Then, guided by TG and DTG curves obtained in argon atmosphere, we selected the required temperatures for thermolysis.

A sample of DCC was placed in a tubular quartz reactor inserted into a NaberthermRT 50-250/11 tube furnace (Nabertherm GmbH, Lilienthal, Germany), heated in an argon current at a rate of 10/min and kept at the desired temperature for one hour,after which it was cooled in an argon atmosphere to room temperature. In order to study the mechanism of thermolysis, the gaseous products of thermolysis were captured and tried to identify, and the solid products were subjected to physical and chemical investigation: elemental analysis, IR spectrometry, X-ray diffraction analysis,specific surface area measurement

Elemental analysis, X-ray diffraction (XRD), IR spectroscopy and electron microscopy were used to study the DCC and the products of their thermolysis. Analysis for carbon content was performed on an ELTRA CS-2000 automatic analyzer (Alpha Resources, LLC, and Stevensville, MI, USA). To determine the metal content, weighed portions of the complex or the products of its thermolysis were dissolved in a mixture of concentrated acids HNO3 and HCl. e resulting solutions were analyzed by the atomic absorption method on an Analyst 400 spectrometer (PerkinElmer, Inc. Waltham, MA, USA).diffraction patterns were obtained on a Shimadzu XRD 6000 diffractometer (Shimadzu Corp., Columbia, MD, USA) (using CuKα radiation (graphite monochromator, λ = 1.54Å) and compared with the data of the JCPDS-ICDD 2002 database.

The specific surface areas of thermolysis products were measured using Tristar 3020 and FlowSorb2300 devices from Micromeritics (Micromeritics Instrument Corp. Norcross, GA, USA).

Thermal analysis of the initial DCC was carried out in an argon atmosphere on a NETZSCH STA 409 PC / PG device (NETZSCH-Geratebau ¨GmbH, Selb, Germany), sample weighed amount 7-10 mg, heating rate 10°/min, temperature range 20-1000°C.

Scanning electron microphotographs were obtained using a SEM Leo 420 instrument (LEO, Assing, Italy).

Results and Discussion

Thermal decomposition in argon was carried out for the following compounds: [Co(NH3)6][Fe(C2O4)3]∙3H₂O [Ni(NH3)6]3[Fe(CN)6]2, [Co(en)3][Fe(C2O4)3], [Co(NH3)6] [Fe(CN)6], [Co(NH3)6]4[Fe(CN)6]3∙13H₂O, [Co(en)3] [Fe(CN)6], [Co(en)3]4[Fe(CN)6]3∙15H₂O, [Co(NH3)6]Cl [Cu(C7H4O3)2]∙3H₂O. Similar composites were also obtained from DCCs with aqua complexes in the cation: [Cuaq]3[Fe(-Co)(CN)6]2∙15H₂O and [Niaq]3[Fe(Co)(CN)6]2∙16H₂O.

The composition of residues from calcination (primary composites), their specific surfaces, temperature of obtaining, and morphology of particles are given in Table 1, the corresponding data for secondary composites - in Table 2.

In the course of studies, we found that [Co(NH₃)₆][Fe(C₂O₄)3]∙3H₂O does not form a composition with carbon during thermolysis in argon, that is, the oxalate complex completely decomposes with the release of carbon oxides. It seems that in the case of [Co(en)₃][Fe(C₂O₄)₃] the residual carbon arises at the expense of the cation. However, we further see a number of hexacyanoferrate and hexacyano-cobaltate DCCs whose cations do not contain carbon and which, however, form carbon-rich compositions. Consequently, the carbon source here is anions, more precisely, cyanogroup ligands or salicylate.

It is well seen (Table 1) that the main component of the primary composites are metal-complexing agents, the content of which can reach 80 wt%. The content of carbon is usually in the range of 20-30 wt. %, in some cases it reaches 35-40 wt. %. e ratio of elements indicates that the carbon content usually decreases gradually with increasing thermolysis temperature. The specifc surface area of the primary composites increases with increasing temperature, sometimes having a maximum, as for example in the case of [Co(en)₃][Fe(ox)₃]. If the initial DCS consists of relatively small ligands (NH3, CN-groups), the specific surface of thermolysis products does not exceed 80 m2/g. In the case of compounds with large ligands (ethylenediamine, oxalate, salicylate), the c surface area can reach 186 m2/g.This is also reflected in the morphology.Thermolysis of the most “compact” complexes [Co(NH₃ ) ₆ ][Fe(CN)₆ ] and [Co(en)₃][Fe(CN)₆] leads to the formation of rod-shaped oblong particles repeating the shape of the original crystals, which have the smallest surface area and probably the highest density among the studied composites. If the ratio of metals in the cation and anion is 4:3, we are initially dealing with less dense structures that are penetrated by channels with a large amount of crystallization water. This leads to the appearance of irregularly shaped spongy particles during thermolysis. Complex organic ligands, such as salicylate, produce a developed surface in the form of highly porous particles upon thermolysis in argon.This can be useful in those practical applications where c surface area plays an important role, such as in catalysis.

XRD patterns of metal-carbon primary composites obtained by thermolysis of DCCs in argon atmosphere are shown in Figure 1. For the products obtained at high temperatures we see characteristic peaks (2Θ = 43-45;65; 83°) corresponding to CoFe alloy and a very weak peak (2Θ = 23-26°) corresponding to carbon (Figure 1a, b). A similar XRD patterns were observed in [16,17] for CoFe/C composites. In the case of CoCu compounds the carbon refle- tions are absent (Figure 1d).

The primary composites were treated with 6 M hydrochloric acid to remove metals from them. It is not always possible to do this at room temperature and in a single step. In addition, residues containing copper had to be additionally treated with nitric acid. Table 2 shows the characteristics of samples separated from primary composites by acid treatment - secondary composites.

It can be seen that the extracted products still contain a t amount of metals. Equimolarity of metals is already broken here, the content of cobalt slightly exceeds the content of iron. The specific surface area value of the secondary composite is usually more than 2 times this value for the primary composite. It seems that the carbon envelops the metal particles and the acid treatment opens up these envelopes.The particle size was estimated using Scherrer's formula and S_sp.

When considering the morphology of carbon, it can be assumed that if carbon was absent in the cationic part of DCC, the secondary product is obtained as fibrous and if present, it is in the form of shapeless particles. It is also seen that treatment of primary composites with acid allows to regulate the content of metals in them, as well as their specific surface area

Figure 3 shows the thermal analysis curves of the secondary NiFe/C composite and pure carbon, It can be seen how well the TG curves of both samples match.The properties of this composite are very similar to expanded graphite. It has a highly developed surface (276 m²/g) and is stable in air when heated up to 450°C.

Conclusion

Thermal decomposition of double complex compounds in argon leads to formation of products with carbon content up to 40 wt. % and specifc surface from 20 to 190 m2 /g. Leaching with hydrochloric acid of a part of metals leads to a strong increase of specifc surface up to 470 m2/g with residual content of the sum of metals 10-20%. XRD patterns of the primary composites show almost no carbon reflections in the region 2Θ = 20-30 , whereas the secondary composites show them perfectly.Thermolysis of double complex compounds allows to obtain metal-carbon composites at temperatures 600-900°C, while alternative methods of obtaining products with similar properties require synthesis conditions at 1500-2000°C. Further studies are planned to expand the set of DCCs precursors, conductivity measurements of composites at different emission frequencies and optimize the conditions for obtaining the most promising samples.