Daniela Paneva

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.

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.

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.

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.

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.

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.

Iron modified mesoporous carbon and silica catalysts for methanol decomposition

Iron modified silica and carbon mesoporous materials with similar textural characteristics are compared in methanol decomposition to H2, CO and CH4. The influence of the support on the phase composition and reductive properties of the catalysts is studied by Mössbauer spectroscopy and TPR with hydrogen.

Transition metal modified activated carbons from biomass and coal treatment products as catalysts for methanol decomposition

Copper, iron and cobalt supported on activated carbon materials are compared as catalysts in methanol decomposition in view of their potential application as intelligent carriers of hydrogen. The activated carbon supports were obtained from renewable agriculture waste (peach shell) or coal treatment by-products (coal tar pitch). The parent activated carbons and their transition metal modifications were characterized by nitrogen physisorption, XRD, UV–Vis, FTIR, Mössbauer spectroscopy and TPR in hydrogen, and the surface functional groups were determined by the Böhm method. It was assumed that the formation of transition metal modified activated carbon catalysts is a complex process which proceeds during the preparation procedure with the activity of the support and also during the catalysis by the influence of the reaction medium. The decisive effect of carbon basal planes over the texture and surface functionality of the support on the formation of transition metals active phase was assumed. Among the studied materials, cobalt modifications exhibited excellent catalytic activity and selectivity in methanol decomposition to H2 and CO despite the nature of the activated carbons used.

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.

Effect of support pore size on the structural and catalytic properties of iron and cobalt oxides modified SBA-1, SBA-15, MCM-41 and MCM-48 silica materials

Abstract Structural, reductive and catalytic properties of iron and cobalt oxides, supported on different mesoporous silicas have been characterized and compared by N 2 physisorption, XRD, EXAFS/XANES, Moessbauer spectroscopy, TPR and methanol decomposition as catalytic test. Different effect of the support pore size on the dispersion and the catalytic behaviour of the loaded metal oxides have been established.