Boris Kunev

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.

Copper-cobalt ferrites as catalysts for methanol decomposition

AbstractCopper-cobalt ferrites with composition Cu1−xCoxFe2O4, where x= 0.2 and 0.8 were prepared by thermal treatment of co-precipitated precursor. The obtained materials were characterized by TG-DSC, XRD, Transmission and Conversion Electron Mössbauer spectroscopy and temperature programmed reduction with hydrogen. The catalytic properties of ferrites were tested in methanol decomposition to CO and hydrogen.

Characterization of nanodimensional Ni-Zn ferrite prepared by mechanochemical and thermal methods

Nickel zinc ferrite nanoparticles, Ni1−xZnxFe2O4 (x = 0, 0.2, 0.5, 0.8, 1.0), with dimensions below 10 nm have been prepared by combining chemical precipitation with high-energy ball milling. For comparison, their analogues obtained by thermal synthesis have also been studied. Mossbauer spectroscopy, X-ray diffraction, and magnetic measurements are used for the characterization of the obtained materials. X-ray diffraction shows that after 3h of mechanical treatment ferrites containing zinc are formed, while 6h of treatment is needed to obtain NiFe2O4. The magnetic properties of the samples exhibit a strong dependence on the phase composition, particle size and preparation method.

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.

Mössbauer and magnetic study of CoxFe3--xO4 nanoparticles

Magnetic nanoparticles of cobalt ferrites CoxFe3−xO4 (x = 1 or 2) have been obtained either by mechanical milling or thermal treatment of pre-prepared layered double hydroxide carbonate x-LDH–CO3. Mechanical milling of the 1-LDH–CO3 leads to the large-scale preparation of nearly spherical nanoparticles of CoFe2O4, the size of which (5 to 20 nm) is controlled by the treatment time. Core-shell structure with surface spin-canting has been considered for the nanoparticles formed to explain the observed hysteresis loop shift (from ZFC–FC) in the magnetic properties. Annealing treatment of the 2-LDH–CO3 below 673 K results in the formation of nearly spherical pure Co2FeO4 nanoparticles. At 673 K and above, the LDH decomposition leads to the formation of a mixture of both spinels phases Co2FeO4 and CoFe2O4, the amount of the latter increases with annealing temperature. Unusually high magnetic hardness characterized by a 22 kOe coercive field at 1.8 K has been observed, which reflects the high intrinsic anisotropy for Co2FeO4.

MANGANESE-DOPED ZINC OXIDE NANOPOWDERS FOR PHOTOCATALYTIC DECOLORATION OF REACTIVE BLACK 5 TEXTILE DYE

Zinc oxide photocatalysts doped with manganese (0.04‐1 at.%) were synthesized via precipitation technique. Their characteristics were studied by Xray diraction (XRD) and BET analysis. The calculated mean size of the crystallites was in the range of 31‐45 nm. Manganese doping of ZnO samples results in slight decrease in crystalline size. Increasing the Mn content in ZnO powders results in a lower photocatalytic activity. The pure ZnO nanopowders decolorate Reactive Black 5 dye under ultraviolet light to almost 92% for 2 h.