The solid-state combustion method was used to prepare nickel-based catalysts for CO2 hydrogenation from [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2. These complexes were synthesized by adding nickel nitrate and perchlorate to melted imidazole. The composition and structure of the obtained complexes was confirmed by ATR FTIR, powder XRD, and elemental analysis. The stages of thermal decomposition of the complexes and their kinetic parameters were established. It was found that incomplete gasification of more thermostable Ni(C3H4N2)6](ClO4)2 led to the formation of carbon, nitrogen, and chlorine impurities. According to powder XRD and XPS, the solid products of gasification of both complexes consist of NiO and Ni0 covered with nickel hydroxide and/or a carbonate layer. In the case of the sample prepared from [Ni(C3H4N2)6](ClO4)2, this layer was pronounced. Therefore, it limits the nickel reduction in the reaction medium of CO2 hydrogenation, even at 450 °C. The surface of the sample prepared from [Ni(C3H4N2)6](NO3)2 contains nickel oxide, which is easily reduced. So, the catalyst active phase is already formed at 250 °C in the presence of CO2 and efficiently catalyzes CO2 hydrogenation as the temperature increases. Therefore, [Ni(C3H4N2)6](NO3)2 is a promising precursor for the CO2 hydrogenation catalyst, and its solvent-free synthesis follows Green Chemistry principles.
The development of solvent-free techniques for nanoparticles synthesis is one of the challenges of Green chemistry. In this work, the principled opportunity to obtain copper-containing nanosized particles without use of any solvents was shown. The copper complexes were prepared as precursors by the melting-assisted solvent-free synthesis. The formation of tetra(imidazole)copper(II) nitrate complex was confirmed by XRD, elemental analysis, FTIR spectroscopy, and thermal analysis. It was noted that their thermal decomposition occurs in two stages: (I) the low-temperature step may be related to redox interaction between organic ligands and nitrate-anions; (II) the high-temperature step may be related to the oxidation of the products of incomplete imidazole decomposition. TEM and XRD studies of solid products of complex combustion have shown that they are oxides with particle size less than 40 nm. Thus, the combustion of [Cu(Im)4](NO3)2 complex under air can be considered as a new approach to prepare nanosized particles of copper oxides without the use of solvents.
This work describes the mathematical modeling of the thermal decomposition of the complex compound [Ni(En)3](ClO4)2 (En = C2H8N2 = ethylenediamine) in an inert atmosphere under non-isothermal conditions. This process is characterized by several simultaneous and intense stages: elimination of ethylenediamine from the nickel coordination sphere, decomposition of perchlorate anions, and explosive-like oxidation of free or bound ethylenediamine. These stages overlap and merge into a one step on the differential thermogravimetric curve. Typically, this curve is modeled as a one-stage process during kinetic analysis. In this paper, for the first time, the data from the dynamic mass-spectral thermal analysis and thermogravimetric analysis were modeled using the hybrid genetic algorithm, and the results were compared. A two-stage scheme of [Ni(En)3](ClO4)2 thermolysis was proposed and the kinetic parameters for each stage were obtained. It was shown that the decomposition of [Ni(En)3](ClO4)2 begins with the elimination of one molecule of ethylenediamine (stage A), then the perchlorate anions quickly decompose with the evolution of oxygen (stage B). We believe that the resulting ClO4−x− (x = 1–3), as stronger oxidizing agents, instantly start an explosive-like exothermic process of ethylenediamine oxidation (stage B).
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