Elena B. Burgina

Catalytic synthesis of carbon nanostructures from polymer precursors

Abstract Carbon nanostructures were prepared by decomposition of polyethylene and polyvinyl alcohol using an iron catalyst at 600–750°C under a nitrogen flow. Heating a uniform distribution of catalyst particles in the polymer bulk to 600°C under flowing nitrogen led to the formation of amorphous carbon incorporating uniformly distributed catalyst particles. Subsequent heating of the samples to 750°C in a nitrogen flow led to the catalytic graphitization of the amorphous carbon matrix with the formation of different carbon nanostructures, i.e. carbon nanotubes. The mechanism of the formation of the obtained carbon nanostructures is discussed.

IR spectroscopic investigation of cation distribution in Zn–Co oxide catalysts with spinel type structure

Abstract The structure of Zn–Co spinel prepared by coprecipitation was studied by IR spectroscopy. The characteristic bands of extra OH− and H3O+ groups were shown to exist in IR spectra up to 700°C. Comparing IR spectra of a poorly crystallized low temperature catalyst with that of a perfect Zn–Co spinel, we have associated the noticeable splittings and shifts of the F1u bands with definite structural distortions. The presence of extra anions in the catalyst with a spinel-like structure in the temperature region 100–700°C is the factor that stabilized the unusual cation distribution in this catalyst. The most plausible cation distribution in such a spinel is proposed as follows: Co3+ and Zn2+ are in octahedral positions, and Co2+ and Zn2+ are in tetrahedral ones.

Ceria-Zirconia Nanoparticles Doped with La or Gd: Effect of the Doping Cation on the Real Structure

The real structure of nanocrystalline CeO2-ZrO2 (Ce:Zr=1:1) systems prepared via the polymerized polyester precursor (Pechini) route and doped with La3+ or Gd3+ cations, up to 30 at.%, was studied by X-ray powder diffraction, EXAFS and Raman spectroscopy and the surface features characterized by XPS and SIMS. Undoped CeO2-ZrO2 system revealed nanoscale heterogeneity, perhaps due to the co-existence of Zr- or Ce-enriched domains. With large La3+ dopant the system remains bi-phasic within the studied ranges of composition, incorporation of the smaller Gd3+ cation stabilizes the single-phase solid solution. For both systems, the increase of dopant content was accompanied by a decline of domain size and an increase of the average lattice parameter of fluorite-like phases. Depletion of the surface layer by smaller Zr4+ cations was observed, while the surface content of a doping cation is either, close to that in the bulk (La) or below it (Gd). Such a spatial distribution of components results in some ordering of cations within the lattice. It is reflected in different modes of rearrangement of oxygen coordination polyhedra with the Gd or La content (distances and coordination numbers by EXAFS), and specificity of XRD patterns not conforming to a simple model with statistical distribution of oxygen vacancies.

CATION/ANION MODIFIED CERIA-ZIRCONIA SOLID SOLUTIONS PROMOTED BY Pt AS CATALYSTS OF METHANE OXIDATION INTO SYNGAS BY WATER IN REVERSIBLE REDOX CYCLES

Ca and/or F-modified fluorite-like Ce-Zr-mixed oxides have been prepared by Pechinis method. The bulk structure of samples was characterized by XRD, EXAFS and FTIRS of the lattice modes. The surface properties were studied by SIMS and FTIRS of adsorbed CO and surface hydroxyls. The specific reactivity of the surface oxygen, its amount, coefficients of bulk and near-surface diffusion, as dependent upon the sample composition and temperature, were estimated using sample reduction by CO in the pulse/flow mode. Insertion of fluorine into the lattice results in decreasing the degree of oxygen polyhedra distortion, thus decreasing the amount of reactive oxygen and diffusion coefficients. Calcium and Pt addition counteracts this effect. At 500oC for Pt-supported Ce-Zr-O samples including those modified by Ca and F, the lattice oxygen is easily removed by methane generating CO and hydrogen with high selectivity. Reoxidation of reduced samples by water or carbon dioxide at the same temperature restores the oxygen capacity producing more hydrogen or carbon monoxide.

Modified Ceria-Zirconia Fluorite-Like Catalysts for the Combustion of Methane

Abstract For dispersed ceria-zirconia-based solid solutions prepared via the polymerized complex method and annealed at 700 °, effects of bulk doping by Ca, Mn, Co, Bi or Nb cations and surface modification by Mn and Pt on their structural features, surface/bulk oxygen reactivity and catalytic activity in methane combustion are considered. With up to 20 mol% doping, a structural type of homogeneous solid solutions of anion-deficient fluorite with disordered anion vacancies is formed. Doping by transition metal cations or Pt increases the mobility and reactivity of the surface/bulk oxygen. A broad variation in specific rates of methane combustion for the studied systems was observed, suggesting structural sensitivity of this reaction. In general, there is no universal relationship between the oxygen mobility, the reactivity and the catalytic activity in methane combustion, which is explained by the factor of specific methane activation on surface active sites. For the Pt-promoted samples, Pt efficiency in methane activation depends on the Pt-support interaction, and the most favorable ones being mixed Pt/MnOx and Pt/NbOx clusters on the surface of the supports that exhibit high lattice oxygen mobilities.

The novel acid catalysts - framework zirconium phosphates: the bulk and surface structure

Abstract Structural features and surface properties of framework zirconium phosphates synthesized via mechanochemical activation (MA) and sol–gel (SG) route were studied using XRD, TEM, 31P MAS NMR, IR and ESR spectroscopy and compared with their catalytic performance in the reaction of hexane isomerization. A high initial isomerization activity of crystalline zirconium phosphates prepared via MA can be determined by a larger share of the strong Lewis centers probed by TEMPON test molecule.

Investigation of the structure of protohematite : metastable phase of ferrum (III) oxide

Abstract A new modification of Fe(III) oxide — protohematite — was produced by thermal decomposition of crystalline hydroxides and salts and by plasma chemical method. Protohematite is a metastable phase which transforms into α-Fe 2 O 3 at 1100°C, under pressure or mechanic chemical activation. IR and Raman spectroscopy, X-ray powder diffraction, and transmission electron microscopy were used to study the structure of protohematite. It was concluded that protohematite has unique structural characteristics. The hypothesis concerning the structure of protohematite was suggested. Specific chemical activity of the protohematite was assumed to originate from the presence of some ferric cations in quasi tetrahedral (distorted octahedral) coordination.

Mobility and Reactivity of the Surface and Lattice Oxygen of Some Complex Oxides with Perovskite Structure

Mobility and reactivity of the surface and bulk oxygen of perovskite-like mixed oxides including lanthanum manganite (I) and ferrite (II) systems modified by Ca (I,II) and fluorine (I), as well as some Co, Fe-containing complex perovskites were considered. Combination of thermal analysis data, oxygen isotope exchange, O2 TPD, reduction by CO, H2 and CH4 TPR, were applied to characterize the accessible surface/bulk oxygen mobility and reactivity. Comparison of these results with earlier data on the real (defect) structure of these systems by TEM, EXAFS, XRD, FTIRS, SIMS allowed to elucidate factors determining the oxygen mobility and reactivity. A quantitative description of the experimental energetic spectrum of oxygen bound with regular and defect surface sites of perovskites was obtained by using the semiempirical Interacting Bonds method with a due regard for the surface face termination and relaxation. Pronounced effect of cation vacancies on the activation barrier for the oxygen migration in the perovskite lattice has been revealed.

Ammoxidation of methylpyrazine over vanadium-titanium catalysts modified by alkali additives

Vanadium-titanium catalysts modified with sodium or potassium additives (1-15 wt.% of Me2O) have been studied in methylpyrazine ammoxidation. Introduction of the additives results in a decrease in the activity and selectivity of the catalysts due to formation of low-active phase - bronzes (MeV6O15) and vanadates (α-NaVO3, KVO3 and K3V5O14). The active sites of the modified samples, similar to those in the V-Ti-O catalyst, are found to be V5+ cations strongly bound to TiO2 and located in a significantly distorted octahedral oxygen environment.

Nanoscale Structural Features of Ultra-fine Zirconia Powders Obtained Via Precipitation-hydrothermal Treatment Route

Genesis of the structure of zirconia fine particles prepared by precipitation of amorphous hydrated zirconia by ammonia from the ZrO(NO 3 ) 2 solution followed by a mild hydrothermal treatment (HTT) of precipitate, washing and calcination under air up to 1000 °C has been studied by HRTEM, X-ray diffraction, Raman and FTIRS. HTT rearranges the structure of amorphous zirconia, which helps to obtain nearly single-phase monoclinic nanozirconia (particle size 5-15 nm) after a mild calcination at 500 °C. Dehydroxilation and sintering of these nanoparticles at higher (600-650 °C) temperatures generate polysynthetic (001) twins. Modeling revealed that reappearance of the (111) “cubic” reflex in XRD patterns of samples calcined at 600-650 °C can be due to these extended defects. In their vicinity, the seven-fold Zr-O coordination sphere is retained, while packing of ZrO 7 polyhedra is varied towards more symmetric structures, thus causing disappearance of the Raman spectra.