Ivan Mitov

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).

Influence of the microscopic properties of the support on the catalytic activity of Au/ZnO, Au/ZrO2, Au/Fe2O3, Au/Fe2O3–ZnO, Au/Fe2O3–ZrO2 catalysts for the WGS reaction

It has been established that the gold catalysts on well crystallized supports, Au/Fe2O3 and Au/ZrO2, display higher catalytic activity in the water gas shift (WGS) reaction in comparison with the samples on amorphous and not well crystallized supports — Au/ZnO, Au/ZrO2, Au/Fe2O3–ZnO and Au/Fe2O3–ZrO2. It could be concluded that the catalytic activity of the gold/metal oxide catalysts depends strongly not only on the dispersion of the gold particles but also on the state and the structure of the supports.

Studies on the state of iron oxide nanoparticles in MCM-41 and MCM-48 silica materials

Phase transformations in and the reductive and catalytic properties of mesoporous MCM-41 and MCM-48 silica molecular sieves modified with iron oxide were studied by X-ray diffraction, nitrogen physisorption, Mossbauer spectroscopy, temperature-programmed reduction, and methanol decomposition as a catalytic test. Their behavior is compared to that of the related bulk materials. Various types of iron species with different properties were identified.

Toluene oxidation on titanium- and iron-modified MCM-41 materials

Iron- and titanium-modified MCM-41 materials, prepared by direct synthesis at ambient temperature or wet impregnation technique, were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction (TPR), UV-vis diffuse reflectance, Mössbauer and FT-IR spectroscopies. Their catalytic behavior was studied in total oxidation of toluene. Materials with high surface area and well-ordered pore structure were obtained. The increase of the titanium content (up to 50%) in the bisubstituted, iron and titanium containing materials leads to partial structure collapse of the silica matrix. Finely dispersed anatase particles were also formed during the impregnation procedure. The catalytic activity of the bisubstituted materials was influenced by the method of their preparation, but higher catalytic stability could be achieved, compared to iron monosubstituted one. The nature of the catalytic active sites is discussed.

Comparative study of the thermal decomposition of iron oxyhydroxides

Abstract The study is devoted to the thermal decomposition of the iron oxyhydroxides: γ-FeOOH (lepidocrocite), α-FeOOH (goethite) and Fe5HO8·4H2O (ferrihydrite). The changes in the crystal structure, in the phase composition and in the dispersion degree, occurring during thermal treatment have been followed by means of Mossbauer spectroscopy, X-ray diffraction, infrared spectra, transmission electron microscopy (TEM), DTA/TG analysis and by measuring the specific surface area. The schemes of thermal dissociation of oxyhydroxides have been outlined on the basis of the obtained results. Some relaxation effects of nano-sized particles of the intermediate products have been observed in the course of this investigation and described respectively. Depending on the precursor, from which α-Fe2O3 has been obtained, it possesses various morphological (needle-like or spherical shape) and dispersion (particle size and specific surface area) features. The obtained results enable forecasts with respect to the optimal final product to be used as initial material for preparing heterogeneous catalysts and magnetic materials.

Hydrogen sorption properties of an Mg-Ti-V-Fe nanocomposite obtained by mechanical alloying

Abstract The absorption–desorption characteristics with respect to hydrogen of a magnesium-based nanocomposite obtained by high-energy ball milling have been investigated. The composite contains 5 wt.% (∼3at.%) Ti, 10 wt.% (∼5.5 at.%) V and 10 wt.% (∼5 at.%) Fe, of which the former two transition metals only form a binary hydride. It has been shown that at 623 K the composite may be hydrided up to a very high absorption capacity whose values remain appropriate for practical purposes even at much lower hydriding temperatures. Part of the iron present in the composite has been found to interact with magnesium and hydrogen under the hydriding conditions, the ternary hydride Mg 2 FeH 6 being formed. Its presence in the composite-hydrogen system has been assumed to be responsible for the reduced rate of hydrogen desorption from the particle surfaces and for some peculiarities of the composite behaviour during hydriding.

Comparative study of the mechanochemical activation of magnetite (Fe3O4) and maghemite (γ-Fe2O3)

A comparative study on the kinetics and mechanism of mechanochemical activation (by high energy grinding) of magnetite (Fe 3 0 4 ) and maghemite (γ-Fe 2 O 3 ) has been carried out. It has been established that during the activation the crystal lattice is distorted and phase and structure transformations occur. The phase compositions of intermediate and final products are registered by Mossbauer spectroscopy and X-ray analysis. It was found that the end product of mechanochemical activation of Fe 3 O 4 and γ-Fe 2 O 3 is highly dispersed hematite. The kinetics of phase changes are determined for the two iron oxides which are characterized by different valency states and cation distributions in the spinel lattice. The different rates of phase transformations have been explained by a phonon mechanism of energy dissipation for maghemite and a mixed phonon-electron mechanism for magnetite.

Synthesis and characterization of CuO and Fe2O3 nanoparticles within mesoporous MCM-41/-48 silica

Two simple methods for the synthesis of pure siliceous MCM-41 and MCM-48 silica materials, modified with CuO or Fe2O3 nanoparticles, located almost exclusively within the mesopores are presented. The modified samples were characterized by powder X-ray diffraction, nitrogen physisorption, temperature programmed reduction, X-ray absorption spectroscopy, (XANES/EXAFS) or Mossbauer spectroscopy and methanol decomposition as a catalytic test reaction. The existence of small, slightly disordered metal oxide nanoparticles was proved. The redox and catalytic behavior of the modified samples depending on the metal oxide, the preparation method used and the type of the mesoporous support are studied and compared to the corresponding bulk oxide phases.

Influence of iron (II) on the transformation of ferrihydrite into goethite in acid medium

The transformation of ferrihydrite into goethite has been observed to be considerably facilitated under the conditions of oxidative hydrolysis of iron(II) sulphate. A probable scheme of the process mechanism has been put forward to explain the transformation that involves a step consisting of the formation of soluble complexes of ferrihydrite with ammonia.

Methanol decomposition on Fe2O3/MCM-48 silica catalyst

Fe2O3/MCM-48 silica samples are characterized by high catalytic activity and methane selectivity in methanol decomposition. The catalytically active phase is substantially changed by the reaction medium and/or hydrogen pretreatment.