Zara Cherkezova-Zheleva

Hallmarks of mechanochemistry: from nanoparticles to technology

The aim of this review article on recent developments of mechanochemistry (nowadays established as a part of chemistry) is to provide a comprehensive overview of advances achieved in the field of atomistic processes, phase transformations, simple and multicomponent nanosystems and peculiarities of mechanochemical reactions. Industrial aspects with successful penetration into fields like materials engineering, heterogeneous catalysis and extractive metallurgy are also reviewed. The hallmarks of mechanochemistry include influencing reactivity of solids by the presence of solid-state defects, interphases and relaxation phenomena, enabling processes to take place under non-equilibrium conditions, creating a well-crystallized core of nanoparticles with disordered near-surface shell regions and performing simple dry time-convenient one-step syntheses. Underlying these hallmarks are technological consequences like preparing new nanomaterials with the desired properties or producing these materials in a reproducible way with high yield and under simple and easy operating conditions. The last but not least hallmark is enabling work under environmentally friendly and essentially waste-free conditions (822 references).

Characterization of superparamagnetic MgxZn1−x Fe2O4 powders

Structural and magnetic properties of MgxZn1−xFe2O4 powders have been studied with respect to the application for thermal cancer therapy (magnetic hyperthermia). MgxZn1−xFe2O4 (x=0.1–0.5) powders with particle sizes between 5 and 8 nm were produced by citrate method. The X-ray diffraction patterns of the samples correspond to a spinel phase. The lattice constant and the volume of the elementary cell increase when x changes from 0.1 to 0.5. The FTIR-spectra ascertain the spinel phase formation. The Mossbauer studies reveal the presence of extremely small particles, which undergo superparamagnetic relaxation at room temperature. The core-shell model has been applied to explain quadruple doublets. The quadruple splitting at “shells” is bigger than those at “cores” whereas the isomer shifts remain close. Magnetic studies confirm the presence of extremely small particles that behave as superparamagnetic ones.

Biogenic iron compounds: XRD, Mossbauer and FTIR study

AbstractMaterials based on biogenic iron oxides, which are a product of the metabolic activities of the neutrophilic iron-oxidizing bacteria (NIOB) from Sphaerotilus-Leptothrix group, were investigated. Natural microbial probes were collected from freshwater flow from Vitosha Mountain (Bulgaria) and cultivated under laboratory conditions in respect to select suitable cultures and conditions (nutrition media) for biomaterial accumulation of biogenic oxides. Samples were studied by physicochemical methods: X-ray diffraction, Mossbauer spectroscopy and IR spectroscopy. Their phase composition and physicochemical properties were obtained. Presence of both amorphous and crystal phase (ultra- and highly dispersed particles) was proved. Iron-containing compound in the natural biomass consists of α-FeOOH. The cultivated materials have more complex composition with iron-containing ingredients as α-FeOOH, Γ-FeOOH, Γ-Fe2O3 and Fe3O4. The sample of natural biomass was tested in reaction of CO oxidation and it showed potential to be used as catalyst support.

Mechanochemical synthesis of ferroferriborate (vonsenite, Fe3BO5) and magnesium ferroferriborate (ludwigite, Fe2MgBO5)

Mossbauer spectroscopy and X-ray diffraction have been used to follow the evolution of the phase composition during mechanochemical synthesis of ferroferriborate (vonsenite) and magnesium ferroferriborate (ludwigite). The properties of the compounds obtained have been compared with those of samples prepared by a solid-phase thermal method. It has been established that the samples prepared by the two methods have similar compositions and cation distributions, but differ in crystallite size, specific surface area and crystal structure perfection.

Cobalt and iron modified activated carbon from coal tar pitch: preparation and application as catalysts for methanol decomposition

Activated carbons were prepared from waste coal treatment by-products by different preparation and activation procedures and applied as a host matrix of nanodispersed cobalt and iron species. Thus obtained composites were characterized by nitrogen physisorption, XRD, UV–Vis, FTIR, TPR and tested as catalysts in methanol decomposition to hydrogen and CO. It was established that the activated carbon, prepared from waste coal treatment by-products could be successfully used for the preparation of highly active catalysts for methanol decomposition. The facilitated effect of surface functionality decrease on the dispersion and catalytic activity of loaded metal species was demonstrated.

Study on the Composition of Biogenic Iron-Containing Materials Obtained Under Cultivation of the Leptothrix sp. on Different Media

The biogenic iron oxide/hydroxide materials possess useful combination of physicochemical properties and are considered for application in various areas. Their production does not require special investments because these compounds are formed during cultivation of neurophilic iron bacteria. Bacteria from genus Leptothrix develop intensively in the Sphaerotilus-Leptothrix group of bacteria isolation medium and feeding media of Fedorov and Lieske. These media are different in their composition which determined the present study as an attempt to clear up the reasons that define the differences in the composition of the laboratory-obtained biomasses and the natural biomass finds. FTIRS, Mössbauer spectroscopy, and XRD were used in the research. Comparative analysis showed that the biomass and control samples contain iron compounds (α-FeOOH, γ-FeOOH, β-FeOOH, γ-Fe2O3) in different ratios. The biomass samples were enriched in oxyhydroxides of higher dispersion. Organic residuals of bacterial origin, SO4, CO3, and PO4 groups were registered in the biogenic materials.

Magnetic properties of binary and ternary mixed metal oxides NiFe2O4 and Zn0.5Ni0.5Fe2O4 doped with rare earths by sol—gel synthesis

The spinel-type ferrites NiFe2O4 and Zn0.5Ni0.5Fe2O4 modified by lanthanide ions Eu3+ and Tb3+ were prepared by a sol—gel process with propylene oxide as a gelating agent. The phase homogeneity of the samples was tested by XRD and Mössbauer spectroscopy. Transmission electronic microscopy used for characterisation of the morphology of the samples revealed nanosized powdered samples with a narrow distribution of particle sizes. It was noted that the presence of Ln3+ ions influenced the magnetic properties of nanosized NiFe2O4 and Ni0.5Zn0.5Fe2O4 ferrites. The dependence of the magnetic properties of the samples on the rare-earth doping may be explained by the different grain sizes. The saturation magnetisation tends to decrease with increasing rare-earth doping and decreasing crystallite size. A similar trend was observed for the coercive field, with the exception of the Tb3+-doped Zn0.5Ni0.5Fe2O4 where it remained the same as in the pure ferrite.

A study of the dispersity of iron oxide and iron oxide-noble metal (Me = Pd, Pt) supported systems

Samples of one-(Fe) and two-component (Fe-Pd and Fe-Pt) catalysts were prepared by incipient wetness impregnation of four different supports: TiO2 (anatase), γ-Al2O3, activated carbon, and diatomite. The chosen synthesis conditions resulted in the formation of nanosized supported phases—iron oxide (in the one-component samples), or iron oxide-noble metal (in the two-component ones). Different agglomeration degrees of these phases were obtained as a result of thermal treatment. Ultradisperse size of the supported phase was maintained in some samples, while a process of partial agglomeration occurred in others, giving rise to nearly bidisperse (ultra-and highdisperse) supported particles. The different texture of the used supports and their chemical composition are the reasons for the different stability of the nanosized supported phases. The samples were tested as heterogeneous catalysts in total benzene oxidation reaction.

Characterization of double oxide system Cu-Cr-O supported on γ-Al2O3

Series of alumina supported chromium-copper catalysts were prepared by co-impregnation method. The samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and UV-visible diffuse reflectance spectroscopy. Dispersion and porosity was also obtained. The experimental and catalytic test results have drawn a conclusion that an interaction between copper and chromium ions takes place. This interaction is responsible for the enhanced catalytic activity of studied catalysts in reaction of total oxidation of industrial formaldehyde production exhaust gas, which contains CO, dimethyl ether and methanol as main components.

In Situ Mössbauer Study of Mixed-Valency Catalysts for Methanol Oxidation

Mixed-valency catalysts (the iron oxyborates ferroferriborate, Fe2+ 2Fe3+BO5, and the magnesium ferroferriborate, MgxFe2+ 2-xFe3+BO5) have been investigated in situ by Mossbauer spectroscopy during the oxidation of methanol. The ferroferriborate exhibits two kinds of thermally activated electron exchange: direct (3d6Fe2+↔3d5Fe3+) and indirect (3d6Fe2+↔O↔3d5Fe3+) In MgxFe2+ 2-xFe3+BO5 the direct exchange is blocked by Mg2+ ions and only indirect exchange (3d6Fe2+↔O↔3d5Fe3+) occurs.