Yu. V. Maksimov

Selective oxidation of CO in the presence of hydrogen on CuO/CeO2 catalysts modified with Fe and Ni oxides

The selective oxidation of CO in the presence of hydrogen on CuO/CeO2 systems containing Fe and Ni oxides as promoters was studied. The catalysts containing 1–5 wt % CuO and 1–2.5 wt % Fe2O3 supported on CeO2 and the CuO/CeO2 systems containing 1–2.5 wt % NiO were synthesized, and their catalytic activity as a function of temperature was determined. It was found that the additives of Fe and Ni oxides increased the activity of the CuO/CeO2 catalysts with a low concentration of CuO. In this case, the conversion of CO at 150°C approached 100%. At the same time, these additives had no effect on the activity of the CuO/CeO2 systems at a CuO concentration of 5 wt % or higher, which exhibited an initially high activity in the above temperature region. The forms of CO adsorption and the amounts of active sites for CO adsorption and oxidation were studied using temperature-programmed desorption. It was found that the introduction of Fe and Ni additives in a certain preparation procedure facilitated the formation of an additional amount of active centers associated with CuO. Data on the temperature-programmed reduction of samples (the amount of absorbed hydrogen and the maximum temperature of hydrogen absorption) suggested the interaction of all catalyst components, and the magnitude of this interaction depended on the sample preparation procedure. With the use of Mössbauer spectroscopy, it was found that the procedure of iron oxide introduction into the CuO/CeO2 system was responsible for the electron-ion interactions of catalyst components and the reaction mixture.

Study of cumene oxidation over zirconia-, titania- and alumina-based complex oxides obtained by sol-gel methods: activity-structure relationships

Abstract Activity-structure relationships have been studied in the low temperature liquid phase cumene oxidation over Fe O/ZrO 2 , Fe O/TiO 2 and Fe O/Al 2 O 3 complex oxides obtained by sol-gel methods. The study has been carried out by the measurements of overall kinetics, product distribution and by X-ray diffraction and Mossbauer spectroscopic methods. Kinetic study in the presence of homogeneous initiator (azo-bis-diazobutyronitrile) has led to the conclusion that the active surface of complex oxides participates in chain initiation most probably via R H bond rupture. Three groups of complex oxides are described: (1) two-phase polycrystalline zirconia-based catalysts showing high activity at low iron loading; (2) one-phase polycrystalline zirconia- and titania-based catalysts exhibiting either activity drop at some critical iron content (zirconia) or monotonic activity dependence (titania) on iron loading; (3) amorphous alumina-based catalysts containing γ-ferric oxide clusters and showing monotone activity dependence. Electronically excited terminal Fe O groups related to the surface FeO 6 polyhedra and iron-containing species located at the interface have been suggested as active centers acquiring the anion-radical behavior. When comparing thermodynamically stable titania (anatase) with metastable one-phase zirconia (cubic or tetragonal) as host matrices, the suggestion was introduced that greater activity of zirconia-based catalysts is due to more energy transfer from the host matrix to the particular active center.

Selectivity and mechanism of cumene liquid-phase oxidation in the presence of powdered mixed iron-aluminum oxides prepared by alkoxy method

Abstract Sol–gel chemistry routes using metallo-organic complexes as precursors were used to prepare iron–aluminum catalysts for low temperature liquid phase cumene oxidation. Catalytic and structural properties of Fe–O/Al 2 O 3 catalysts were studied by kinetic measurements combined with other techniques in order to obtain information about the nature of catalytic active centers and the selectivity of the cumene liquid-phase oxidation in the presence of single and mixed oxides. Our results allow us to expect a bifunctional role of the Fe–O/Al 2 O 3 surface as a combined participation of Lewis acid and terminal oxygen centers by a proposed mechanism which is known for homogeneous catalysis and is non-typical for heterogeneous catalytic oxidation.

Magnetic phase transitions in nanostructures with different cluster orderings

Nanostructures have been synthesized by (i) the micellar template method with the subsequent organization of Fe2O3 nanoclusters about 10 nm in size to a cluster crystal and (ii) by the aerosol method with the fixation of Fe2O3 nanoclusters about 10 nm in size in the NaCl matrix. The magnetic properties of the synthesized nanostructures have been studied. The Mössbauer spectroscopic examination of the cluster crystal revealed a magnetic phase transition of the first order, with the transition temperature being near the room temperature. This nanostructure is characterized by a steep magnetization curve, which is typical of an ordered structure with a weak intercluster interaction like a molecular crystal and a low energy of the magnetic anisotropy K = 3.5 × 104 J/m3. The disordered Fe2O3 nanostructure in the NaCl matrix is characterized by superparamagnetic behavior and smooth magnetization curves without saturation at 1 T and the magnetic anisotropy energy K = 2.5 × 104 J/m3, which is close to the corresponding value in bulk α- and γ-Fe2O3.

The formation and properties of one-dimensional FeHal2 (Hal = Cl, Br, I) nanocrystals in channels of single-walled carbon nanotubes

FeCl2@OCHT, FeBr2@OCHT, and FeI2@OCHT nanocomposites were obtained by capillary filling of the channels of carbon single-walled nanotubes (SWNTs) with melts of iron halogenides. The composites were studied by high resolution transmission electron microscopy (HRTEM), the capillary condensation of nitrogen at 77K, Raman spectroscopy, optical absorption spectroscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and Moessbauer spectroscopy. Substantial distinctions in the combination scattering spectra of SWNTs and nanocomposites in the region of radial modes and in the region of longitudinal and tangential oscillations were revealed. The presence of electron transfer between the nanocrystal and the SWNT wall was established in the nanocomposites. For the FeCl2@OCHT nanocomposite, two states of Fe+2 were found: the first is characterized by electron transfer from the nanotube to the nanocrystal, which leads to the electron structure of the SWNT and FeCl2 changing; the second corresponds to the strained intercalated state resulting from the mechanical effect of the small SWNT diameter on the FeCl2 nanocrystal.

Magnetic nanostructures based on nanoclusters of iron oxides

Using magnetization and Mössbauer spectroscopy, investigations of the magnetic properties of α-Fe2O3-SiO2 and γ-Fe2O3-SiO2 nanostructures, including α-Fe2O3 nanoclusters 2 nm in size and γ-Fe2O3 nanoclusters 3–4 nm in size in silica gel pores, have been conducted. For α-Fe2O3-SiO2 nanostructures, magnetic phase transitions of the first order are detected and examined with the transition temperature dependent on the nanocluster size and intercluster interactions. α-Fe2O3 nanoclusters up to 16 K remain in the noncompensated antiferromagnetic state (upper Morin point temperature). At low temperatures, α-Fe2O3 nanoclusters reveal quantum-size effects. For γ-Fe2O3-SiO2 nanostructures, supermagnetic behavior with a blocking point dependent on intercluster interactions is typical. At low temperatures, intercluster interactions lead to the appearance of a coercive force of 0.03 T.

Microstructure and gas-sensing properties of nanocrystalline NiFe2O4 prepared by spray pyrolysis

Nanocrystalline nickel ferrite with a crystallite size from 3 to 40 nm has been prepared by spray pyrolysis. The 57Fe Mössbauer spectrum of NiFe2O4 samples has been found to vary systematically with crystallite size. The sensing response of the nanocrystalline nickel ferrite to 50 ppm NH3 has been studied using in situ conductance measurements. NiFe2O4 offers a strong sensing response to ammonia at the level of its maximum concentration limit. The optimum nickel ferrite crystallite size and temperature for ammonia detection are determined.

Study of intracluster atomic mobility in ultrafine clusters of γ-ferric oxide by Mossbauer spectroscopy and thermodynamical analysis

The atomic dynamics of ultrafine clusters of γ-ferric oxide with diameters of 1–3 nm, obtained by thermal decomposition of ferric oxalate, has been studied by gamma-resonance (Mossbauer) spectroscopy and by thermodynamic analysis. The intracluster atomic mobility has been shown to increase by the action of surfactants. Thermodynamical and phenomenological cluster models describing the dynamic state of clusters have been developed and are compared with the experimental data. The conditions for formation of a specific solid-liquid state of clusters are discussed.

Mössbauer spectroscopic studies of passivated layers of iron-containing catalysts for ammonia synthesis

Combined conversion electron (CEMS) and transmission γ-ray (MS) Mössbauer spectroscopy was used to study the structure of passivated layers of promoted and unpromoted iron-containing catalysts. In both catalysts the oxide-coated films were found to consist of small “paramagnetic” (at 300 K) clusters of ferric oxide.AbstractСтруктура пассивационных плёнок промотированных и непромотированных катализаторов синтеза аммиака изучалась в мессбауэровской абсорбционной и конверсионной спектроскопии. В обоих типах катализаторов удалось наблюдать пассивационный слой вблизи поверхноти, состоящий из мелких парамагнитных (при 300 К) кластеров окиси железа.

Modification of a magnesium—Molybdenum olefin oxidation catalyst by alkali additives

Abstract The effect of alkali elements incorporated into a magnesium-molybdenum system containing iron and bismuth ions on the structure and catalytic activity in isobutylene oxidation to methylacrolein was studied. It was found by Mossbauer spectroscopy and EPR that potassium and cesium ions are capable of rearranging the iron molybdate structure to form a substituted garnet, whereas the lithium and sodium ions exhibit no such effect. The iron ions were found to interact in the modified system. Comparison of data on structure with the catalysis results revealed that the structural rearrangement of iron molybdate in a complex oxide catalyst increases the process selectivity, due to changes in the lattice oxygen state.