V. N. Kolomiichuk

Mechanochemical Synthesis and Catalytic Properties of the Calcium Ferrite Ca2Fe2O5

The formation of the real structure of calcium ferrite prepared by the calcination of a mechanochemically activated hydroxide mixture at 600–1100°C was studied by X-ray diffraction analysis, electron microscopy, thermal analysis, Moessbauer spectroscopy, IR spectroscopy, small-angle X-ray scattering, and secondary-ion mass spectrometry. It was found that low-temperature calcium ferrite is an anion-modified oxide, in which the ordering of oxygen vacancies was incomplete. Regions with a disordered structure were detected on the surface of crystallites. As the calcination temperature was increased, the brownmillerite crystal structure was improved and the intercrystalline boundaries were formed and then annealed. At the surface, these processes were accompanied by a change in the predominant form of adsorbed NO from nitrosyl to dinitrosyl species. An increase in the specific catalytic activity of samples with calcination temperature can be associated with the perfection of the brownmillerite structure and with a change in the state of adsorption centers.

Physicochemical and catalytic properties of La1-xCaxFeO3-0.5x perovskites prepared using mechanochemical activation

The phase composition of La1 – xCaxFeO3 – 0.5x perovskites synthesized from preactivated oxides was studied by powder X-ray diffraction analysis and differential dissolution. The system does not form a continuous series of homogeneous solid solutions. No intermediate samples from this series are monophasic. It was found that the synthesis under nonequilibrium conditions (mechanical activation + calcination at 900°С for 4 h) resulted in nonequilibrium microheterogeneous solid solutions with degrees of calcium substitution for lanthanum of no higher than 0.5. A longer calcination (for 16 h) or an increase in the calcination temperature of solutions up to 1100 °С decreased the calcium content of the samples down to x ∼ 0.2 because of the formation of a brownmillerite phase. The catalytic activity of the test samples in the oxidation of CO changed nonmonotonically with x, and it was maximum at x = 0.5–0.6, which correlates with the maximum density of interphase boundaries in these samples.

Scientific bases for the synthesis of highly dispersed framework zirconium phosphate catalysts for paraffin isomerization and selective oxidation

Results of the systematic study of the synthesis of highly dispersed framework zirconium phosphates stabilized by ammonium, lanthanum, aluminum, manganese, and cobalt cations are summarized. The synthesis involves the mechanochemical activation of a mixture of solid reactants (salts) or the sol–gel process each followed by the hydrothermal treatment (HTT) of obtained precursors in the presence of surfactants. The genesis of dispersed systems under investigation is studied by modern physical methods providing information on the state of the bulk and surface of the systems. It is found that the local structure of sol nanoparticles and zirconium phosphate crystalline nuclei arising from mechanochemical activation products depends on the nature of initial substances. This, in its turn, makes different crystallization mechanisms possible during the HTT process: the dissolution/precipitation mechanism or the mechanism of oriented mating of primary particles. The crystallization mechanism in HTT and the reaction system composition influence the nature of resulting complex zirconium phosphate phases, their thermal stability, dispersity, and porous structure parameters. The relationship between the bulk structure parameters of framework zirconium phosphates, which are controlled by varying the chemical composition and conditions of synthesis, and the surface characteristics of the systems (the strength and concentration of different Lewis and Br@nsted sites) is studied. It is shown that systems based on framework zirconium phosphates are promising catalysts for paraffin (pentane and hexane) isomerization, the selective oxidation of methane by oxygen into synthesis gas at short contact times, and the oxidative dehydrogenation of propane into propylene.

Structural sensitivity of the oxidation reactions catalyzed by dispersed transition metal oxides: role of defect structure

Publisher Summary Structural sensitivity manifestation for reactions of catalytic oxidation on transition metal oxides depends on the atomic structure of the surface planes, types and densities of the surface/bulk defects, and structure flexibility. Structural sensitivity of the catalytic reactions is one of the most important problems in heterogeneous catalysis. It has been rather thoroughly studied for metals, while for oxides, especially for dispersed ones, situation is far less clear, because of inherent complexity of studies of their bulk and surface atomic structure. In recent years, successful development of such methods as high resolution electron microscopy (HREM) and scanning tunneling microscope (STM) along with the infrared spectroscopy of test molecules have formed a sound bases for elucidating this problem in case of oxides. This chapter discusses the results of the systematic studies of the bulk/surface defect structure of the oxides of copper, iron, cobalt, chromium, manganese as related to structural sensitivity of the reactions of carbon monoxide and hydrocarbons oxidation. Surface coordinatively unsaturated centers have been studied by the infrared spectroscopy of adsorbed test molecules (CO, NO) and heats of oxygen adsorption and amounts of weakly bound oxygen have been determined. Catalytic properties in the reactions of carbon monoxide oxidation (all oxides) and butene oxidative dehydrogenation (iron oxides) were studied, using a microreactor with the vibrofluidized bed of catalysts and pulse/flow kinetic installation. Catalytic activities were characterized by the reaction rate W (molec. CO/m 2 s) in differential conditions and first-order rate constant K (dm 3 butene (STP)/m 2 .s.atm), respectively.

Porous Al2O3/Al metal ceramics prepared by the oxidation of aluminum powder under hydrothermal conditions followed by thermal dehydration: III. The reactivity of aluminum, the reaction mechanism of its oxidation with water vapor, and the microtexture of cermets

The macrokinetics of aluminum oxidation under hydrothermal conditions at 150–250°C and water vapor pressures of ∼0.5–4.5 MPa was studied. The apparent kinetic characteristics of the process were determined. The diffusion of water through the layer of a hydrated product was found to be a rate-limiting step. It was found that diffusion coefficients for various aluminum samples differed by almost two orders of magnitude; the main reaction steps were revealed. Changes in the texture of aluminum oxide in cermets in the course of the hydrothermal oxidation reaction were analyzed. This texture was found to depend on the ratio between the specific rate of the oxidation reaction and the rate of aging of the oxidation products under hydrothermal conditions.

Nanocomposites Based Upon Alumina and Zirconia Pillared Clays Loaded with Transition Metal Cations and Clusters of Precious Metals: Synthesis, Properties and Catalysis of NO x Selective Reduction by Hydrocarbons

Thermally stable alumina and zirconia pillared clays loaded with copper and cobalt cations and silver nanoparticles were synthesized. The structural and surface features of these nanosystems were studied and compared with those of bulk analogs -partially stabilized zirconias and γ-alumina loaded with the same active components. Specificity of the catalytic properties of nanocomposites in the reactions of nitrogen oxides reduction by propane, propylene and decane in the excess of oxygen appears to be determined both by the degree of interaction between pillars and active components and the type of reducing agent.

Methane selective oxidation into syngas by the lattice oxygen in ceria-based solid electrolytes promoted by Pt

Abstract Nanocomposites based upon ceria doped by Zr, Zr+La or Sm with supported Pt nanoparticles efficiently convert methane into syngas by their lattice oxygen. In red-ox cycles with pure methane as reagent, the surface carbon build-up is observed, which is lower for Sm -doped samples possessing a higher surface and lattice oxygen mobility. When stoichiometric amounts of CO 2 or H 2 O are present in the feed, the catalysts efficiently operate in methane steam and dry reforming at high space velocities without deactivation.

Reactivity of the Peroxo Complexes of Copper(II) Hydroxide

In the interaction with H2O2 in an alkaline medium, Cu(OH)2 forms terminal Cu–OOH and bridging peroxo complexes with the μ-1,1 and μ-η2:η2 structures. It was found that the terminal peroxide is active in the reactions of H2O2 decomposition, diphenol oxidation, and nitrile conversion into acid amides. The promoting effect of ammonia on these reactions was found. A possible mechanism is discussed.

Mechanism of the formation of porous silicate mesophases

The formation of mesoporous mesophase systems prepared by precipitation of soluble forms of SiO2 at the surface of micelles of cetyltrimethylammonium cations was examined. A molecular mechanism of the formation of a silicate coating and a macroscopic mechanism of the formation of a mesophase were found and discussed. A combination of these mechanisms describes the processes proceeding during the synthesis. It can also explain the observed changes in the structure and texture characteristics of the mesophase.

Synthesis of mesoporous complex framework zirconium phosphates via organic- inorganic nanocomposites: genesis of structure, adsorption and catalytic properties

This work presents the first results of synthesis of framework binary phosphates of zirconium and transition metal cations (Co, Cu, Ce) via nanocomposites of starting inorganic salts with citric acid and studies of their structure genesis. Nanoparticles of layered Zr phosphates with typical sizes in the range of 18-24 Ǻ are formed at the mixing stage. Less basic Cu and Co cations are mainly octa-coordinated with both phosphate groups of those nanoparticles and citric acid molecules. At subsequent thermal treatment, Cu and Co cations are incorporated within Zr phosphate nanoparticles acquiring a low coordination approaching a tetrahedral one while rearranging the nuclei structure into that of a framework type. Removal of citric acid by heating under air at 200—300 o C preserves the size of nanoparticles while their ordered stacking forms mesoporous structure with a narrow pore size distribution ~ 50 A and specific surface area up to 200 m 2 /g after calcination at 600 o C. The binary phosphates promoted by a small amount of Pt were found to be effective catalysts of NOx selective reduction by decane in the oxygen excess not subjected to coking with a high and stable performance at high space velocities in the presence of steam.