Thermal and electrochemical reduction of nickel (II) ferrite under the influence of polymer stabilizers Nickel (II) ferrite samples have been prepared by co-precipitation of metal salts in an aqueous solution without and with the addition of polymer stabilizers (polyvinyl alcohol and polyvinylpyrrolidone) and further heat treatment at 500 °С, 700 °С and 900 °С. Their structural-phase changes before and after using as catalysts in the electrohydrogenation of acetophenone were revealed by X-ray phase analysis. Morphological features of the obtained NiFe2O4 and NiFe2O4 + polymer samples were studied by electron microscopy. It has been established that the addition of polymer stabilizer into the co-precipitation medium and its partial preservation in the composition of precursors (metal hydroxides and oxides) and in the formed nickel (II) ferrite have a noticeable effect on the structural-phase constitutions of the samples prepared. Differences in the phase constitutions are also depend on the heat treatment temperature of these samples. It was shown that, in contrast to nickel ferrite synthesized without stabilizers, NiFe2O4 + polymer samples are partially reduced during heat treatment and additionally in an electrochemical system with the formation of Ni-and Fe-Ni-containing composites. The reduced samples of nickel (II) ferrite were tested for their ability to exhibit electrocatalytic properties in the electrohydrogenation of acetophenone. Their low electrocatalytic activity in this process was determined, which is presumably conditioned by the structure of Fe-Ni-alloy particles with iron content predominant, as well as by the presence of magnetite inhibited the activity of nickel particles.
Copper ferrite (CuFe2O4) samples were prepared by the co-precipitation method without and in the presence of a polymer stabilizer (polyvinyl alcohol, PVA) followed by heat treatment at 500, 700, and 900°C and electrochemical reduction. The structure and morphological features of the samples were investigated by X-ray phase analysis and electron microscopy. It was established that copper (II) ferrite is reduced in an electrochemical system with the formation of Fe-Cu composites with different content of reduced metals, which is influenced by the heat treatment temperature. Copper ferrite prepared with the use of the polymer stabilizer is partially reduced during thermolysis, additionally in an electrochemical cell. The Fe-Cu composites were employed as electrocatalysts in the electrohydrogenation of p-nitrobenzoic acid and exhibited good activity in this process. To study the interaction in the CuFe2O4 + PVA complexes, the quantum-chemical calculations were performed using the density functional theory methods for the simplified metal-containing systems (atoms and ions of copper (II), iron (III), Cu2, Fe2 molecules) as well as dimers and trimers modeling the structure of polyvinyl alcohol.
The N-doped metal-carbon composites based on the carbonized melamine-formaldehyde polymer (MFP) with silver, silver-copper, and silver-cobalt particles have been synthesized. Mono- and bimetallic silver-containing composites were prepared by the reduction of metal nitrates with hydrazine hydrate in an aqueous ethanol medium in the presece of carbon black particles obtained by the heat treatment of MF-polymer at 500 °C. The structural-phase changes before and after the use of metal-carbon composites as electrocatalysts in the electrohydrogenation of p-nitroaniline (p-NA) were studied by X-ray diffraction analysis. The morphological features of the created MFBC + Ag, MFBC + (Ag+Cu) and MFBC + (Ag+Co) composites were studied by electron microscopy. It has been established that the synthesized composites mainly contain crystalline phases of the corresponding reduced metals or their alloys. The monometallic MFBC(1) + Ag(1) composite contains reduced silver crystallites of various shapes and localization on porous MF carbon black particles. A higher electrocatalytic activity of the synthesized Ag-containing composites in the p-NA electrohydrogenation was shown in comparison not only with the electrochemical reduction of p-NA, but also in the case of MFBC + (Ag+Cu) composites with electrocatalytic hydrogenation of p-NA on Ag + Cu particles (without MFBC). It was found that the use of the studied MFBC-composites increases the selectivity of formation of the main hydrogenation product (p-phenylenediamine) and decreases the yields of by-products
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