Home – Home – Jornals – Chemistry for Sustainable Development 2018 number 5

A short review is given on the particular phenomena of abstracting lattice oxygen by conventional hydrocarbon under mechanical stressing. The process can be regarded as a reduction process similar to a traditional hydrometallurgy. However, the mechanochemical process can be regarded as a tool for introduc-ing oxygen vacancies and enables to acquire suboxides or the oxide with lower oxidation number, when we deal with transition metal oxides. Case studies on SiO₂ and V₂O₅, based on our recent experimental studies.

Processes of thermal decomposition of iron (III) oxalate placed into pores of structured mesoporous silica (SBA-15) were investigated. Synchronous thermal analysis of samples was carried out in inert (argon) and oxidizing (a mixture of argon and 20 % oxygen) media. The composition of gas thermolysis products was detected mass spectrometrically. The initial salt and solid-phase products of its decomposition were characterised by X-ray phase analysis and Mössbauer spectroscopy. It is well known that the process of thermolysis of iron (III) oxalate under inert atmosphere conditions proceeds in two stages: intermediate decomposition to anhydrous iron (II) oxalate and then the final one to Fe₃O₄ and α-Fe at 150–180 and 310–400 °C, respectively. Oxidative thermolysis occurs in a single step. In other words, iron (III) oxide is formed from the initial salt that is later crystallised into hematite phase. However, this step is complex. Instantaneous oxidation of anhydrous iron (II) oxalate formed as an intermediate explains high exothermicity of the process. It was found that the temperature of the initial decomposition step was reduced from 150 to 80 °C for a precursor placed into SBA-15 silica matrix pores in both oxidative and inert media. Herewith, the finishing temperature of the step for oxidative thermolysis is increased almost by 100 °C. Gas evolution spikes and extremes in the calorimetric curve of the second step of decomposition in argon are reduced by 30–40 °C. An increase in the reactivity of fine particles and the amorphous state of iron (III) oxalate explained a reduction in gas evolution peaks. In order to describe the behaviour of the precursor, upon oxidative thermolysis likely reactions determining the process were suggested.

The effect of conditions of mechanochemical activation and stoichiometric composition on generating products of mechanochemical reduction of β-germanium dioxide with magnesium was investigated by X-ray phase analysis. It was demonstrated that differently composed mechanochemical composites might be formed at certain steps during mechanical activation of germanium dioxide. They were comprised of germanium, magnesium oxide, germanium oxide, magnesium, and the GeMg₂ intermetallic compound. The process of mechanochemical reduction of germanium dioxide with magnesium comes to an end by 4 min of activation to form Ge/MgO composite. Conditions of separation of germanium powder from magnesium oxide in mechanochemical Ge/MgO composites were determined. As demonstrated by electron microscopic analysis, highly dispersed germanium powders consist of 50–100 nm primary particles similarly-shaped to spherical ones. The species are aggregated into secondary particles in 1 to 10 µm size range. The magnesium content in highly dispersed germanium powders is less than 2 %.

Mechanochemical preparation of nanoscale hafnium (IV) carbide was explored. It was demonstrated that the Hf/C mechanocomposite with a wide particle-size distribution was formed in the first step. Fused hafnium (IV) carbide was produced from nanoscale hafnium (IV) carbide and the hafnium/carbon mechanocomposite by treatment with high-intensity photon flux. According to research results, the mechanocomposite is more preferable and may be used as a precursor to make products of fused hafnium (IV) carbide using additive technologies.

The effects of a series of process parameters (the composition of external coagulant and the air-gap distance) on the morphology of oxygen-permeable microtubular (MT) membranes were explored. The latter were prepared from non-stoichiometric Ba_0.5Sr_0.5 Co_{0.8 – x}Mo_xFe_0.2 O_{3 – δ} perovskites (BSCFMх). It was demonstrated by scanning electron microscopy that the specific porous structure of MT membranes vanished when increasing the air gap between the draw hole and the bath with coagulant to 15 cm or using an ethanol solution. The oxygen permeability of MT membranes based on Ba_0.5Sr_0.5Co_{0.8 – x}Mo_xFe_0.2O_{3 – δ} (x = 0, 0.02, 0.05, 0.10) with the optimum morphology in air and under СО₂ was examined.

An opportunity for direct mechanochemical synthesis of double tin and alkaline earth metal hydroxides via two procedures was demonstrated. The latter were based on reactions of chloride (IV) pentahydrate and alkaline earth metal hydroxides or salts (in the latter case, upon simultaneous action of NaOH). It was possible to synthesize Mg/Sn layered double hydroxide using only the first procedure. Calcium, strontium, and barium compounds were synthesised by both procedures. The X-ray diffraction patterns of double hydroxides MSn(OH)₆ (M = Ca, Sr, Ba) are in good agreement with structural data acquired in early research works and presented in X-ray databases. The products contain trace carbonate impurities, which is related to their presence in the initial reagents.