Dmitrii I. Kochubey

Coprecipitated Ni-alumina and NiCu-alumina catalysts of methane decomposition and carbon deposition. II. Evolution of the catalysts in reaction

Using coprecipitated Ni-alumina and NiCu-alumina catalysts, up to 250 g/gcat of filamentous carbon was produced by methane decomposition at 823 K and a methane pressure of 100 kPa. The process evolved three general stages. In the induction period carbon was dissolved into the Ni particles as evidenced by in situ X-ray powder diffraction measurements. This promoted the formation of large metal particles pear-shaped in Ni catalysts and quasi-octahedral in NiCu ones. The steady-state growth of the filaments occurred on the second stage. Finally, the catalyst was deactivated producing porous granules composed of interlaced long carbon filaments. In deactivated Ni catalysts the nickel was found in metal state, while in NiCu samples about half of the nickel was atomically dispersed in carbon as revealed by extended X-ray absorption fine structure spectroscopy. The deactivation of the catalysts was suggested to include the fragmentation of the metal particles as well as an atomic erosion in NiCu samples. In addition, in both catalysts the filaments growth could also be limited when the close-packed structures of porous carbon were achieved.

Coprecipitated NiAl and NiCuAl catalysts for methane decomposition and carbon deposition I. Genesis of Calcined and reduced catalysts

Abstract The structure of coprecipitated NiAl and NiCuAl catalyst with high nickel content (60–90 wt.-% Ni-alumina) has been investigated by X-ray powder diffraction, extended X-ray absorption fine structure (EXAFS) spectroscopy and Auger electron spectroscopy of calcined and reduced samples. The alumina was shown to behave as a textural promoter. The model of the reduced NiAl catalyst consisted of highly dispersed nickel and alumina particles with a NiAl spinel phase at their interface. The NiAl spinel phase had mainly an inverse structure. In the reduced sample the paracrystallinity of nickel was evidenced by precise diffraction measurements using synchrotron radiation. The spinel structure was not observed when copper was introduced. As a result, the alumina particles were more dispersed and covered the surface of the NiCu particles enriched with copper.

Nickel catalysts supported on carbon nanofibers: structure and activity in methane decomposition

Structures of Ni catalysts supported on filamentous carbon (CFC) produced by methane decomposition over coprecipitated Ni and Ni-Cu/alumina catalysts were studied by EXAFS and TEM. Thermal pre-treatment in N2 at 350°C of samples impregnated by nickel nitrate precursor was found to produce either NiO or nickel carbide, Ni3C, phase. This was explained by different reducing properties of the carbon nanofibers which depend on the surface structure. High stability of the Ni/C catalysts in methane decomposition reaction at 550°C was found with those prepared from only nickel chloride precursor, due to the formation of large (30-70 nm) Ni particles further leading to new carbon filaments growth. Data implies a common mechanism of the filamentous carbon deposition in all Ni-based catalysts, independent of the support (silica, alumina, carbon) being used. However, accumulation of filamentous carbon is strongly influenced by morphology and texture of the support.

A new type of binuclear oximehydrazonate clathrochelates of iron(II): synthesis, spectra and structure

Abstract A series of macrobicyclic binuclear iron(II) oximehydrazonates has been synthesized encapsulation of preliminary obtained non-macrocyclic tris-complexes with boron- and tin-containing Lewis acids and direct template reactions on the Fe2+ ion. The structures of the synthesized compounds have been studied using IR, electronic, polynuclear (1H, 13C, 11B, 119Sn) NMR, Mossbauer (57Fe, 119Sn) and EXAFS spectra. The trigonal-prismatic and trigonal-antiprismatic geometry of the Fe2+ coordination polyhedron for boron- and tin-containg clathrochelates, respectively, has been established from Mossbauer (57Fe) parameters. The FeN distances ( ∼ 1.9 A ), typical for macrobicyclic azomethine iron(II) complexes, and intramolecular Fe…Fe ( ∼3.90 A ) and Fe…Sn ( ∼3.80 A ) distances have been obtaine EXAFS spectra. Cyclic voltammograms of synthesized binuclear compounds have demonstrated the formation of unstable mixed-valence Fe2+ −Fe2+ complexes by oxidation.

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.

Valency states of Bi in BaPb1−xBixO3−δ and Ba1−xKxBiO3−δ superconducting oxide

X-ray absorption measurements have been made above the L3-Bi(Pb) edges in Bi-based three-dimensional superconductive oxides: BaPb1−xBixO3−δ (x = 0, 0.25, 1; δ ≤ 0.2) and Ba1−xKxBiO3−δ (x = 0.4; δ ≤ 0.2). The Bi valence state as a crucial parameter for understanding superconductive properties of these oxides has been determined from XANES spectra analysis. It was found that in spite of similar Bi and Pb near-edge structures the oxygen deficiency leads to a change in the L3-Bi edge both for BaPbBiO and for BaKBiO while the Pb edge practically does not change for BaPbBiO. The character of the changes observed and the interpretation of the NES spectra carried out by calculation give us the opportunity to confirm that the Bi valence state differs from the Pb one. The disproportionality of Bi4+ on two different Bi valence states takes place for both compounds, but in addition to Bi3+ the second valence state is not exactly Bi5+, as it was considered earlier, but Bi3+Ln, n = 1–2. The results obtained give evidence of a local CDW increase with oxygen deficiency for both compounds.

Scanning tunneling microscopy study of porous carbon impregnated with palladium chloride

Mesoporous carbon “Sibunit” impregnated with aqueous solutions of H2PdCl4 has been studied with STM. The spontaneous reduction of Pd during the impregnation was confirmed by AES. The surface topography depends on the Pd surface coverage. When the Pd content on the surface is small, particles with an average diameter of ca. 2 nm are frequently observed, which are never imaged on the clean carbon support. When the Pd content is high, the surface still consists of small ca. 3–5 nm particles.

X-ray absorption spectroscopy of BaPb1−xBixO3−δ and Ba1−xKxBiO3−δ, superconducting oxides

Abstract X-ray absorption measurements have been made above the L3-Bi(Pb) edges of Bi-based 3D superconductive oxides: BaPb1−xBixO3−δ(x = 0,0.25, 1; δ ⩽ 0.2) and Ba1−xKxBiO3−δ (x = 0.4; δ ⩽ 0.2). The EXAFS data indicate that the Pb positions in BaPbO3 are equivalent but the Bi environment has two different BiO distances for BaBiO3. The Bi valence state has been determined from XANES spectra analysis. The character of changes observed under oxygen deficiency and the interpretation of NES spectra carried out by ab-initio MS calculation give us the opportunity to confirm that the Bi valence state differs from the Pb one. The disproportionation of Bi4+ in two different Bi valence states takes place for both compounds, but besides Bi3+ the second valence state is probably Bi +3 - L n , n = 1, 2.

The local structure transformation in Nd1.85Ce0.15CuO4 films irradiated by He+ ions: polarized EXAFS study

Abstract Copper K edge polarized EXAFS data of Nd1.85Ce0.15CuO4 films have indicated that He+ ion irradiation induces radiative defects in the Nd(Ce)-O(2) structures. Static disorder with typical displacement from the primary plane site up to 0.1 A for the O(2) environments was observed. A cumulant treatment has been applied for space-disorder analyses of the Nd(Ce) site in irradiated samples. We estimated the C2, C3, C4 expansion coefficients and found that the cumulant series of Cu-Nd(Ce) distance distribution is convergent at the range of photoelectron wave vector k=4−11 A −1 and a radiation fluence π=1016 cm−2 We have not found any local changes in the CuO2 plane structure after irradiation. The data obtained and superconductivity deterioration in the irradiated compounds point out the essential role of the ordering in the Nd(Ce)-O(2) complexes for superconductivity in the CuO2 planes.

A hierarchic structure in zirconium butoxide complexes in n-butanol solutions

The structure of particles in zirconium n-butoxide solutions in n-butyl alcohol is determined by means of EXAFS, SAXS, and molecular mechanics modeling. Zirconium atoms are found to be bonded to each other via the oxygen atom and to form large anisotropic particles in the solution. Primary particles have a shape close to spherical; their diameter together with the solvate shell is 28.9 Å. These particles then aggregate into anisotropic structures. During solution aging under normal conditions without contact with the atmosphere, the particle anisotropy increases because of the aggregation of complexes. When the solution concentration decreases, the particles are divided into primary spherical particles with a characteristic size of 28.9 Å. The described changes are confirmed by a decrease in the number of Zr-Zr distances of 4.8 Å and 5.1 Å, which according to the EXAFS data, correspond to the bonds between the primary particles. The characteristic maximum sizes of particles in solutions with concentrations from 0.1 g to 0.003 g ZrO2/ml are 160–80 Å.