S. V. Tsybulya

Honeycomb catalysts for clean-up of diesel exhausts based upon the anodic-spark oxidized aluminum foil

Abstract The technology of the aluminum foil anodic spark oxidation in the water-based electrolytes has been applied to form strongly adhering protective alumina layer at the surface. Basic features of the oxidation were studied by SEM and XRD. Secondary supports, promoters and modifiers including rare-earth elements were used to increase the thermal stability of these composites up to 900°C. These materials were assembled as thin wall honeycomb supports loaded with a number of active components. The catalysts were tested in the reactions of CO and CHx oxidation, NOx selective reduction by hydrocarbons and demonstrated a high performance and a low pressure drop at high space velocities. City diesel buses field tests of converters equipped with those catalysts demonstrated their high and stable performance in clean-up of exhausts from SOF, CO, gaseous hydrocarbons and NOx.

Honeycomb-supported perovskite catalysts for high-temperature processes

Pechini route [US Patent No. 3,330,697 (1967)] was used for supporting perovskite-like systems on thin-wall corundum honeycomb support to prepare catalysts for high-temperature processes of methane combustion and selective oxidation into syngas. In this preparation, the surface of corundum monoliths walls was shown to be covered by strongly adhering porous perovskite layer formed by rounded crystals. At high temperatures when pore diffusion is expected to affect catalysts performance in fast reactions, this spatial distribution of the active component could be attractive. In the kinetically controlled region of methane oxidation, samples prepared via Pechini route possess activity comparable with that of samples made via support wet impregnation with mixed nitrate solutions, when an active component is uniformly distributed across the wall thickness. Corundum-supported lanthanum manganite and ferrite are the most active in the reaction of methane combustion, while its selective oxidation into syngas effectively proceeds on supported lanthanum cobaltite and nickelates. Corundum-supported perovskites are more thermally stable as compared with those on γ-alumina support.

Physicochemical and catalytic properties of La1-xCax FeO3-0.5x perovskites

The phase analysis of La1-xCaxFeO3-0.5x perovskites prepared by a ceramic process from oxides is studied by X-ray diffraction and differential dissolution methods. Atx < 0.5, the system does not form a continuous series of homogeneous solid solutions and does not consist of the members of a homological series. Atx < 0.5, the system contains two phases and calcium ferrite nanoparticles located on the surface of lanthanum ferrite. Atx > 0.5, the formation of the structures of a brownmillerite-based homological series is found. The catalytic activity of perovskites depends nonmonotonically on thex value and reaches the maximum atx = 0.6.

Real structure of metastable forms of aluminum oxide

Differences in the real structure of γ-Al2O3 samples obtained by the thermal decomposition of pseudoboehmite and boehmite prepared by the hydrothermal treatment of bayerite were found. The transformations of these structures during their conversion to δ-Al2O3 as the treatment temperature increased were studied. The rate of conversion of metastable alumina species into the stable α-Al2O3 phase significantly depends on the real structure of samples. The rate of this transformation is drastically retarded in the presence of extended defects in the oxides originated from boehmite, and the stability of metastable alumina species increased as the degree of surface dehydroxylation increased.

Real structure and catalytic activity of La1−xSrxCoO3 perovskites

Abstract Mechanoceramical synthesis of La 1− x Sr x CoO 3 (0≤ x ≤1) perovskites was made from simple oxides. Samples calcined at 900 and 1100°C for 4 h are nearly monophase and well crystallized. Sr adding was found to cause a structure rearrangement from the hexagonal (at x ≤0.4) to the cubic one (at 0.8> x >0.4) and back to the hexagonal at x >0.8. There are two maxima of the catalytic activity versus chemical composition: at x =0.3 and at x =0.8. TEM data for these samples were obtained and disordered surface layers were detected. There is a correlation between the catalytic activity and surface layers microstructure.

Monolith honeycomb mixed oxide catalysts for methane oxidation

Abstract Low cost and active monolith catalysts for methane oxidation were developed. The preparation procedure includes the mechanochemical activation of mixed La–Ce rare-earth oxides and transition metal oxides, followed by their annealing, kneading with the binder, extrusion of plastic pastes, drying and calcination. For disordered oxide composites with the perovskite- or fluorite-like structures intermixed with alumina, their specific catalytic activity in the methane oxidation weakly depends upon the nature of transition metal cations (Mn, Ni, Fe, Co). In the temperature range of the methanexa0+xa0air mixture ignition (∼500°C), the performance of optimized monolithic catalysts is mainly determined by their specific surface area controlled by the binder composition and preparation procedure.

Titania synthesized through regulated mineralization of cellulose and its photocatalytic activity

Cellulose used for thousands of years has been rediscovered recently as a novel smart material for various nanotechnological applications. Its insoluble fibrils are functionalized by using mineralization methods developed in nanochemistry. Here they are coated by titania synthesized in one stage by a new green approach. It consists of controlling the localization of very fast hydrolysis and condensation reactions. Cellulose fibrils are placed in ethylene glycol with such an amount of water that is absorbed entirely by the hygroscopic polysaccharide. This hydrating water works as a reaction centre when the precursor reaches it. Instant hydrolysis and following condensation reactions proceeding mainly on the fibrils provide their mineralization. Titania prepared at ambient conditions is in an amorphous state. It is transferred in crystalline forms under a variety of conditions including moderate temperature (80 °C), calcination in air and cellulose carbonization in an inert atmosphere. These treatments result in photocatalytic activity. Even cellulose treated at the moderate conditions demonstrates significant self-cleaning ability consisting of fast degradation of methylene blue under outdoor sunlight irradiation. Photocatalytic activity of carbon–titania hybrids includes a side reaction of the oxidation of the carbonized fibrils. Photocatalytic properties of some of the calcinated samples, not containing organics, were comparable with a commercial photocatalyst.

The microstructure and properties of framework zirconium phosphates based nanocomposites : catalysts of alkane isomerization

Abstractu2002Nanocomposites based upon framework zirconium phosphates with supported WO3, MoO3 and Pt nanoparticles were synthesized via the incipient wetness impregnation of high-surface-area mesoporous phosphate samples with water solutions of corresponding salts followed by drying and calcination. The structure and surface properties of nanocomposites were studied by using combination of structural and spectral methods. Due to a strong interaction between supports and supported species, the structure of the latter differs considerably from that of the bulk phases. Surface acid centers typical for zirconium phosphates disappear suggesting their participation in bonding nanoparticles of promoters. Instead, new types of strong acid sites associated with tungsten oxide clusters emerge. The effect of these promoters on performance of zirconium phosphates in the reaction of pentane and hexane isomerization is considered.

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.

Physicochemical features of the formation of siliceous porous mesophases: 1. General views on the mechanism

The formation of mesoporous mesophase materialsvia precipitation of soluble SiO2 forms on the surface of cationic cetyltrimethylammonium micelles was studied. The main physicochemical factors determining the formation of siliceous mesoporous materials and providing an explanation of the observed changes in the structural and textural characteristics of the materials were identified and discussed.