A. S. Ivanova

Physicochemical and catalytic properties of systems based on CeO2

The effect of synthesis conditions, the nature of components, and the ratio between the components on the phase composition, the texture, and the redox and catalytic properties of the Ce-Zr-O, Ce-Zr-M1-O (M1 = Mn, Ni, Cu, Y, La, Pr, or Nd), N/Ce-Zr-O (N = Rh, Pd, or Pt), and Pd/Ce-Zr-M2-O/Al2O3 (M2 = Mg, Ca, Sr, Ba, Y, La, Pr, Nd, or Sm) was considered. A cubic solid solution with the fluorite structure was formed on the introduction of <50 mol % zirconium into CeO2, and the stability of this solid solution depended on preparation procedure and treatment conditions. The presence of transition or rare earth elements in certain concentrations extended the range of compositions with the retained fluorite structure. The texture of the Ce-Zr-O system mainly depended on treatment temperature. An increase in this temperature resulted in a decrease in the specific surface area of the samples. The total pore volume varied over the range of 0.2–0.3 cm3/g and depended on the Ce/Zr ratio. The presence of transition or rare earth elements either increased the specific surface area of the system or made it more stable to thermal treatment. The introduction of the isovalent cation Zr4+ into CeO2 increased the number of lattice defects both on the surface and in the bulk to increase the mobility of oxygen and facilitate its diffusion in the Ce1 − xZrxO2 lattice. The catalytic properties of the Ce-Zr-M1-O or N/Ce-Zr-M2-O systems were due to the presence of anion vacancies and the easy transitions Ce4+ ai Ce3+, M12n+ ai M1n+, and Nδ+ → N0 in the case of noble metals.

Preparation and Microstructural and Textural Characterization of Single-Phase Aluminum Oxides

Conditions for the preparation of single-phase η-, θ-, and χ-aluminas were determined. The structures of η- and χ-aluminas were characterized. With the use of high-resolution electron microscopy, it was found that η-Al2O3 particles exhibited the most developed {111} face and consisted of coherently joined domains with a pronounced platelet shape. Planar defects in the (111) plane occurred in the η-Al2O3 particles. Microstructural differences between single-phase η-Al2O3 and γ-Al2O3 with a defect spinel structure were revealed. It was found that the η-Al2O3, χ-Al2O3, and θ-Al2O3 oxides are characterized by uniformly porous structures with average pore diameters of 47, 55, and 110 Å, respectively.

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.

Structure, Texture, and Acid-Base Properties of Alkaline Earth Oxides, Rare Earth Oxides, and Binary Oxide Systems

The effect of synthesis conditions on the formation of the phase composition, dispersity, pore structure, and acid-base properties of alkaline earth oxides, rare earth oxides, and the Mg-M-O (M = Y, La, or Ce) and Y(La)-M-O (M = Ca, Sr, or Ba) binary systems was studied. It was found that the nature of the system was responsible for the character of phase transformations: the Mg-M-O samples were a mixture of either MgO with Y2O3 or MgO with a solid solution based on rare earth oxides (LaMg)2O3 or (CeMg)O2); the Y(La)-M-O samples (M = Ca, Sr, or Ba) contained the M2Y2O5, MY2O4, and MLa2O4 compounds, which differ in chemical stability, in addition to La2O3 and Y2O3 phases. According to XPS data, the M/Mg atomic ratios were much higher than the bulk values; this is indicative of an enrichment of the surface of samples in the second component. An increase in the concentration of M2O3 from 5 to 25 mol % resulted in a decrease in the Ssp of the Mg-M-O samples from 220 ± 10 to 110 ± 10 m2/g; the Ssp of samples calcined at 750°C was lower by a factor of ∼1.5–2. The Ssp of the Y(La)-M-O samples was higher than the Ssp of individual La2O3 and Y2O3. The samples were characterized by a biporous texture. The concentrations and strength distributions of surface OH groups, Lewis acid sites, and Lewis base sites depend on the nature and concentration of rare earth elements in the binary samples. The activity of the Mg-M-O samples in the oxidative dehydrogenation reaction of propionitrile correlated with the acid-base surface sites. Among the Ru/Y(La)-M-O catalysts for ammonia synthesis, Ru/Y-Ba-O was the most active; this catalyst provided a higher yield of NH3 at 250–300°C, as compared with catalysts prepared with the use of other supports (Sibunit, KVU-1, and C/MgO).

Low-temperature oxidation of carbon monoxide on Pd(Pt)/CeO2 catalysts prepared from complex salts

Catalysts containing cerium oxide as a support and platinum and palladium as active components for the low-temperature oxidation of carbon monoxide were studied. The catalysts were synthesized in accordance with original procedures with the use of palladium and platinum complex salts. Regardless of preparation procedure, the samples prepared with the use of only platinum precursors did not exhibit activity at a low temperature because only metal and oxide (PtO, PtO2) nanoparticles were formed on the surface of CeO2. Unlike platinum, palladium can be dispersed on the surface of CeO2 to a maximum extent up to an almost an ionic (atomic) state, and it forms mixed surface phases with cerium oxide. In a mixed palladium-platinum catalyst, the ability of platinum to undergo dispersion under the action of palladium also increased; as a result, a combined surface phase with the formula PdxPtyCeO2 − δ, which exhibits catalytic activity at low temperatures, was formed.

Effect of preparation procedure on the properties of CeO2

The effect of preparation procedure on the physicochemical and catalytic properties of CeO2 was studied. Differences in the electronic and structural characteristics of CeO2 depending on preparation procedure and treatment temperature were found using X-ray diffraction analysis, transmission electron microscopy, UV-visible electronic spectroscopy, and X-ray photoelectron spectroscopy. With the use of the temperature-programmed reaction with CO, it was demonstrated that CeO2 samples with a high concentration of point defects—oxygen vacancies caused by the presence of Ce3+—were characterized by an increased mobility of bulk oxygen. The samples of CeO2 with a high concentration of structural defects—micropores of size 1–2 nm and stepwise vicinal faces in crystallites—exhibited a high catalytic activity in the reaction of CO oxidation.

Aluminum oxide and systems based on it: Properties and applications

The metastable forms of aluminum oxide that exist in the range of 300–800°C are characterized; differences in the microstructures of homogeneous γ-, η-, and χ-Al2O3 are demonstrated; and the acid-base properties of the above modifications are compared. The catalytic properties of aluminum oxide in ethanol dehydration and propionitrile ammonolysis were studied. It was found that an increased surface concentration of Lewis acid sites, including strong acid sites (ν(CO) = 2237 cm−1), is required for preparing an effective catalyst for the dehydration of ethanol, whereas the rate of propionitrile conversion increased proportionally to the surface concentration of Brønsted acid sites. γ-Aluminum oxide was used to prepare catalysts for carbon monoxide oxidation. It was found that the supporting of Pd on γ-Al2O3 did not change the support structure. Palladium on the surface of γ-Al2O3-550 (Tcalcin = 550°C, SBET = 300 m2/g) occurred as single particles (2–3 nm) and aggregates (∼100 nm). The single particles were almost completely covered with a layer of aluminum oxide to form core-shell structures. According to XPS data, they were in atypical states (BE(Pd 3d5/2) = 336.0 and 338.0 eV), which were not reduced by hydrogen in the range of 15–450°C and were resistant to the action of the reaction mixture. Palladium on the surface of γ-Al2O3-800 (SBET = 160 m2/g) was in the states Pd0 and PdO, which are typical of Pd/Al2O3, and the proportions of these states can change under the action of the reaction mixture. An increase in the Tcalcin of the Pd/Al2O3(800)-450 catalyst from 450 to 800 → 1000 → 1200°C led to the agglomeration of palladium particles and to an increase in the temperature of 50% CO conversion from 145 to 152 → 169 → 189°C, respectively. α-Aluminum oxide was used in the preparation of an effective Mn-Bi-O/α-Al2O3 supported catalyst for the synthesis of nitrous oxide by the oxidation of ammonia with oxygen: the NH3 conversion was 95–97% at 84.4% N2O selectivity.

Hydrocracking of vacuum gas oil in the presence of supported nickel-tungsten catalysts

The supports containing 70% Al2O3 and 30% β zeolite (AZ-1 and AZ-2), which differed in mixing procedures, and the Ni-W/AZ-1 and Ni-W/AZ-2 catalysts were characterized using an adsorption technique, high-resolution electron microscopy, IR spectroscopy, and X-ray photoelectron spectroscopy and tested in the hydrocracking reaction of vacuum gas oil (VGO). It was found that the supports differed in texture characteristics and surface Lewis acidity at the same composition and similar concentrations of Brønsted acid sites. The formation of Ni-W-S sulfide species on the surfaces of both of the supports occurred in different manners: multilayer Ni-W-S sulfide species were formed on AZ-1 (Ssp = 220 m2/g), whereas single-layer species were mainly formed on AZ-2 (Ssp = 380 m2/g). It was found that catalysts containing multilayer Ni-W-S sulfide species, which were characterized by a higher degree of sulfidation, provided a higher yield of diesel fuel upon the hydrocracking of VGO, whereas catalysts containing single-layer Ni-W-S sulfide species were more active in the reactions of VGO hydrodesulfurization and hydrodenitration.

Synthesis of aluminum oxides from the products of the rapid thermal decomposition of hydrargillite in a centrifugal flash reactor: II. Physicochemical properties of the products obtained by the centrifugal thermal activation of hydrargillite

A variety of physicochemical methods were used to characterize the product of the rapid thermal decomposition of hydrargillite in a centrifugal flash reactor under the following conditions: the average particle size of the reactant, 80–120 μm; the temperature of the solid heating surface (plate or cylinder), 300–700°C; hot-zone residence time, ∼1 s; transfer of the product to the cooled zone of the reactor. The composition of the product and the extent of decomposition of hydrargillite were determined as a function of the processing temperature. The centrifugal thermal activation (CTA) of hydrargillite affords an X-ray-amorphous, highly reactive product with a developed surface and a disordered and inhomogeneous porous structure. This structure is capable of forming different modifications of aluminum hydroxide and oxide. The properties of the CTA product are compared with the properties of the earlier reported hydrargillite rapid decomposition products obtained using a gaseous heat-transfer agent (thermochemical activation product) or a fluidized bed of a granular heat-transfer agent (thermal dispersion product).

Highly dispersed zirconium-containing oxide systems: Synthesis, properties, and applications

The formation of the phase composition, dispersion, pore structure, and surface state of binary zirconium-containing systems was examined depending on the nature and concentration of the second component, preparation conditions and procedures, and thermal treatment conditions. Various types of interactions were found, which are governed by the nature of the second component and the treatment temperature. The effects of surfactants on the physicochemical properties of precipitates depend on the conditions of synthesis: the surfactant amount retained by a precipitate at pH 3–4 is greater than that at pH 9 by an order of magnitude. The thermolysis of samples synthesized at acidic pH, along with dehydration and dehydroxylation, is accompanied by the decomposition and degradation of surfactants. Because of this, certain compounds and their fragments capable of reducing corresponding phases are removed stepwise. Highly dispersed compositions were obtained at 110–400°C with a minimum particle size of 2.5–16 nm; at 700°C, the particle size was no higher than 20 nm. The ratio between the ionic radii of added and main components is a factor determining the dispersion of formed phases. In this case, changes in the nature of surfactants and in the conditions of a particular synthesis make it possible to prepare highly dispersed systems with monomesoporous structures. The total volume and the average diameter of pores can be controlled over wide limits.