I. Yu. Molina

Dynamics of Structural Transformations in the Reduction of Copper Aluminate

The dynamics of structural transformations during copper aluminate reduction in the temperature range used for catalyst activation was studied by high-temperature X-ray analysis under controlled conditions (hydrogen, 2O–4OO‡C). The techniques of neutron diffraction analysis, IR spectroscopy, chemical phase analysis, and electron microscopy were also used at particular stages. In the course of reduction, copper metal is deposited onto the surface of spinel crystals from the bulk. Spinel becomes cation-deficient with respect to copper. An analysis of powder diffraction patterns demonstrated that copper is reduced and released from tetrahedral positions of the spinel structure at temperatures below ~300‡C and from octahedral positions only at temperatures above 300‡C. In this case, a redistribution of aluminum ions was observed simultaneously. It is likely that the electrical neutrality is attained by the formation of OH groups, the appearance of which in reduced samples was detected by IR spectroscopy and confirmed by neutron diffraction analysis. At a reduction temperature of 400‡C, the oxygen framework was partially disintegrated. The structures of reduced copper aluminates and chromites were compared.

Disperse electrolytic platinum and palladium deposits of submicron thickness on polycrystalline supports: An X-ray diffractometry and microscopy study

The structure of model electrode materials broadly used in electrocatalysis (platinized platinum, palladized platinum, platinized gold) is studied by x-ray diffractometry, scanning electron microscopy, and transmission electron microscopy. Disperse platinum and palladium coatings less than 1 μm thick are obtained by potentiostatic or galvanostatic deposition from solutions of complex chloride salts. Lattice parameters of disperse metals are shown to be considerably smaller than those of bulky crystals. Some new tendencies of variations in structural parameters with the deposition potential are revealed. Special attention is paid to a reliable determination of the size of regions of coherent scattering (ROCS) and microdistortion; in connection with this, results of approximation of reflections by various functions and a harmonic analysis of reflections are compared. The latter are used for constructing ROCS distributions by size. Good agreement is found with the STM data for platinum deposits obtained earlier. Conversely, in the case of palladium deposits, ROCS are smaller than the particle size in an outer layer of the deposit by the STM data. This result is interpreted in terms of strong concrescence of palladium nanoparticles. To determine the true surface areas of deposits and estimate continuousness of covering the support by the deposit and regularities of platinum aging on gold in conditions of potential cycling, a voltammetry method is applied.

Distribution of the cobalt-containing component in the pore space of HZSM-5 upon a postsynthetic modification of the zeolite with hydroxo compounds of Co2+

The distribution of the cobalt-containing modifying component in the pore space of zeolite HZSM-5 depending on the total cobalt content of the samples (0.5–5.0 wt %) was quantitatively studied for the first time. At cobalt concentrations to 3.0 wt %, the cobalt-containing modifying component mainly occurred as isolated CoOh2+ ions in the micropores (channels) of HZSM-5 at the ion-exchange positions of the zeolite and in one-dimensional CoO and CoAl2O4 nanoclusters. A further increase in the cobalt concentration to 5.0 wt % resulted, in addition to the filling of micropores, in the partial filling of the mesopore space of the zeolite with a small amount of three-dimensional CoO and CoAl2O4 nanoparticles. Using sorption data and electronic diffuse reflectance spectra, we were the first to find that the effective density of a cobalt-containing modifying component in the pore space of a zeolite matrix was lower than the density of a bulk CoO phase by a factor of 6.

Structural features of copper ferrite-chromites

The structural features and the distribution of cations over crystallographic sites of copperchromite, copper ferrite, and mixed chromite-ferrites with the spinel structure obtained by thermal decomposition at 600°C and 900°C of mixed hydroxy compounds of copper, iron, and chromium with the composition Cu2+/(Fe3++Cr3+) = 1/2 and different Fe3+/Cr3+ ratios are studied using a set of physicochemical methods. It is shown that the spinel-structure phases formed exist in two modifications: cubic and tetragonal, depending on the Fe3+/Cr3 ratio. The crystallographic relationship between the cubic and tetragonally distorted phases of spinel is analyzed. The distribution of cations over crystallographic sites, the character and degree of tetragonal distortion of Cu–Fe–Cr spinel depend on the Fe3+/Cr3+ ratio: when Fe3+/Cr3+ > 1, the ratio of unit cell parameters c*/а* > 1; when Fe3+/Cr3+ < 1, the ratio c*/а* < 1; when Fe3+/Cr3+ = 1, the spinel is cubic irrespective of the temperature of thermal treatment. The oxygen coordination of copper ions in the spinel structure has a significant effect on the catalytic properties of the samples in the low-temperature water gas shift reaction.

Distribution of copper- and nickel-containing modifier components in the pore space of HZSM-5 zeolite

The distribution of copper- and nickel-containing components in the pore space of HZSM-5 zeolite was quantitatively studied. It was found that the detailed distribution of a modifier in the micropore and mesopore volumes of the zeolite depends on both the chemical nature of the modifier and the conditions of supporting and the regime of M2+ polycondensation in the pore space of the zeolite. The experimental data on the low-temperature adsorption of nitrogen on Cu(n)ZSM-5 catalysts can be interpreted as the result of the partial filling of the zeolite micropore space (10 vol %) and the finest mesopores with D < 3 nm with the modifier. In the case of Ni(n)ZSM-5 catalysts, the penetration of the modifier into zeolite channels (micropores) in detectable amounts was not found, and it was arranged in mesopores on the surface of zeolite crystallites. The reason for differences between modifier distributions in the pore structure of the zeolite was explained from the standpoint of different structures of copper and nickel polyhydroxo complexes in impregnating solutions after polycondensation. It was found that, in the Cu(n)ZSM-5 and Ni(n)ZSM-5 catalysts, the modifier component contained copper and nickel only in a doubly charged state and mainly octahedral oxygen environments. In this case, three-dimensional nanoparticles or coarsely dispersed particles of CuO were not detected in the pore space of the support, whereas the presence of a small amount of sufficiently large NiO crystals with a coherent-scattering region of 80–100 nm was detected in Ni(n)ZSM-5, and these crystals occurred on the surface of zeolite crystals. It was found that the apparent density of a copper-or nickel-containing component arranged in the pore space of the zeolite was lower than the density of the bulk CuO and NiO phases by a factor of ∼3 and 4, respectively, because of the size effect.

Interaction of Hydrogen with Copper-Containing Oxide Catalysts: V. Structural Transformations in Copper Chromite during Reduction–Reoxidation

High-precision X-ray powder diffraction and differential dissolution methods are used to show that copper chromite with a structure of tetragonal spinel is recovered under certain conditions upon the reoxidation of the system of phases formed by the reduction of the initial copper chromite, cation-deficient spinel stabilized by hydrogen and Cu0 particles epitaxially bound to its surface at 250–350°C. However, because of destruction of the initial surface layer at the initial moment of reduction, equilibrium cannot be complete: reoxidized spinel is nonstoichiometric and contains some lattice oxygen-bound protons, and the copper oxide is formed on the surface in some amount. The difference in the phase transition of copper chromite during reduction–reoxidation from the reversible polymorphic transitions characteristic of copper chromite during temperature variations is analyzed.

Physicochemical investigation into the specific features of the phase transformations in electrodeposited nanocrystalline oxotungstate films

The structure of oxotungstate films (as-deposited and subjected to heat treatment at temperatures of up to 600°C) prepared through electrodeposition on platinum and gold polycrystalline substrates is investigated using different physicochemical methods. It is shown that the oxotungstate films consist of X-ray amorphous hydrated mixtures of isopoly compounds, predominantly in the form of paratungstates with [H2W12O42]10− anions. The structural transformation with an increase in the temperature in air is accompanied by the loss of water, the transformation of paratungstate anions into more oxidized forms, their destruction, and the crystallization of nonstoichiometric hydroxylated oxide phases.

New approach to the novel synthesis of boehmite (γ-AlOOH) by microwave irradiation of gibbsite: Kinetics of solid-phase reactions and dielectric properties of the reactants

Microwave irradiation of gibbsite is suggested as a method of obtaining crystalline boehmite. The kinetics of solid-state transformations of gibbsite under microwave radiation has been investigated, and the dielectric properties of the initial and microwave-activated gibbsite samples have been determined. Extending the gibbsite irradiation time leads to an increase in tanδ. This indicates that microwave-activated gibbsite has a stronger capacity for dissipating microwave energy owing to the formation of an amorphous component containing a variable amount of weakly bound molecular water. The general chemical formula of the amorphous component can be represented as Al2O3 · xH2O (0.5 < x < 3.0). The results of this study can provide a basis for developing new, low-waste, resource- and energy-saving methods for the synthesis of crystalline boehmite and for converting it into γ-Al2O3 with acid-base and textural properties that are atypical of the known low-temperature modifications of Al3+ oxides.

Bimetallic Pd-M (M = Co, Ni, Zn, Ag) nanoparticles containing transition metals: Synthesis, characterization, and catalytic performance

The reductive thermolysis of Pd(OOCMe)4M(OH2) (M = NiII, CoII, ZnII) and Pd(OOCMe)4Ag2(HOOCMe)4 molecular complexes results in the generation of bimetallic Pd-based Pd-M (M = Co, Ni, Zn, Ag) nanoparticles. The composition and morphology of nanoparticles and the electron state of metal atoms were characterized using electron microscopy, elemental ICP analysis, X-ray diffraction, and XAFS (XANES/EXAFS) techniques. The catalytic performance of nanoparticles was studied using the example of reactions of catalytic hydrazine decomposition and U(VI) reduction to U(IV) by hydrazine and formic acid. The catalytic performance of Pd-Ni nanoparticles is superior to that of the standard supported Pd/SiO2 catalyst containing a similar amount of Pd atoms, while Pd-Co, Pd-Zn, and Pd-Ag nanoparticles do not catalyze the studied reactions.

Effects of spatial ordering of quantum dot arrays from small-angle X-ray diffraction data under variation of growth parameters

The small angle X-ray diffraction method is applied to the system of germanium quantum dots in the silicon matrix prepared by molecular-beam epitaxy in self-organization mode using the Stranski-Krastanov technique. Depending on the growth mode, the specific distances between Ge quantum dots and deviations from their correlated distribution are determined. The results are in agreement with the scanning tunneling microscopy data.