T. P. Minyukova

Effect of the composition and structure of the precursor compound on the catalytic properties of cobalt-aluminum catalysts in the Fischer-Tropsch synthesis

The effect of preparation procedure on the anionic composition and structure of hydroxo compounds as precursors of Co-Al catalysts and on their catalytic properties in the Fischer-Tropsch synthesis was studied. The dynamics of changes in the composition and structure of the hydroxide precursors of Co-Al catalysts during thermal treatment and subsequent activation was studied by thermal analysis, IR spectroscopy, XRD analysis, and in situ XRD analysis with the use of synchrotron radiation. It was found that the precursor compounds prepared by deposition-precipitation of cobalt cations on γ- and δ-Al2O3 under urea hydrolysis conditions, which had a hydrotalcite-type structure and contained nitrate, carbonate, and hydroxyl groups, turtned into the oxide compounds Co3 − xAlxO4 (0 < x < 2) with the spinel structure in the course of thermal treatment in an inert atmosphere. The hydrogen activation of an oxide precursor led to the formation of cobalt metal particles through the intermediate formation of a cobalt(II)-aluminum oxide phase. The catalyst was characterized by high activity and selectivity for C5+ hydrocarbons in the Fischer-Tropsch synthesis.

Structure of the Active Component and Catalytic Properties of Catalysts Prepared by the Reduction of Layered Nickel Aluminosilicates

The reduction of Ni-Mg aluminosilicates with the amesite structure was studied using thermogravimetry, high-resolution electron microscopy, XPS, and XRD. It was found that the reduction with hydrogen at 920 K resulted in the formation of nickel particles coated with a difficult-to-reduce amorphous oxide shell. The reduced samples were incapable of chemisorbing oxygen; however, they exhibited a high adsorption capacity for hydrogen. The Ni0 core-oxide shell decorated particles were highly active in steam methane reforming and CO hydrogenation reactions. At the same time, they were inactive in the formation of graphite-like carbon in both methane decomposition and CO disproportionation.

Catalytic properties of copper chromite ferrites in water gas shift reaction and hydrogen oxidation

The catalytic properties of a series of copper chromite ferrite samples with the composition CuCr2–xFexO4 (where x = 0–2) and a spinel-type structure in reactions with reducing (water gas shift reaction, WGSR) and oxidizing (the oxidation of hydrogen) reaction atmospheres were studied. The samples were obtained by the thermal decomposition of mixed hydroxo compounds. The distribution of Cu2+ ions in the tetrahedral and octahedral crystallographic positions of spinel, which depends on the Cr3+/Fe3+ ratio, affects the apparent activation energy (Ea) in both of the reactions. In WGSR, Ea is ∼33 kJ/mol for CuCr2O4, in which Cu2+ ions mainly occupy tetrahedral positions, whereas Ea ≈ 100 kJ/mol for CuFe2O4, in which Cu2+ ions mainly occupy octahedral positions. In the reaction of hydrogen oxidation, Ea is ∼71 kJ/mol for CuCr2O4 or ∼42 kJ/mol for CuFe2O4. The value of Ea for the mixed chromite ferrites changes with the replacement of chromium ions by iron ions and, hence, with a ratio between the amounts of copper ions in the tetrahedral and octahedral oxygen positions of spinel.

Evolution of Cu-Zn-Si oxide catalysts in the course of reduction and reoxidation as studied by in situ X-ray diffraction analysis, transmission electron microscopy, and magnetic susceptibility methods

The reduced and reoxidized Cu-Zn-Si oxide catalysts as layered copper-zinc hydroxo silicates with the zincsilite structure were studied using in situ and ex situ X-ray diffraction analysis, transmission electron microscopy, and the temperature dependence of magnetic susceptibility. The catalysts were prepared by homogeneous deposition-precipitation. It was found that Cu0 particles were formed on the surface of a layered hydrosilicate with the zincsilite structure upon reduction with hydrogen. The reoxidation of the reduced samples with a mixture of oxygen and an inert gas, which contained no more than 0.05 vol % O2, resulted in the formation of individual Cu2O and CuO phases; copper ions did not return to the hydrosilicate structure. Catalytic tests of Cu-Zn-Si catalysts in methanol synthesis indicate that the specific catalytic activity of copper metal particles grows linearly with increasing zinc loading. This fact suggests that copper metal particles, which were obtained by the reduction of Cu2+ ions from the copper-zinc hydroxo silicate with the zincsilite structure, were responsible for activity in methanol synthesis. Consequently, the ability to return copper ions to a precursor compound in reoxidation with oxygen at low concentrations, which is known for reduced Cu/ZnO catalysts (these catalysts are highly active in methanol synthesis), is not related to the catalytic activity in methanol synthesis.

Cation distribution in Cu(Cr2–xAlx)O4 and Cu(Fe2–xAlx)O4 according to neutron-diffraction studies and their catalytic properties in the water-gas shift reaction

The neutron structural analysis of Cu(Cr2 − xAlx)O4 and Cu(Fe2 − xAlx)O4 (0 ≤ x ≤ 2) nanopowders is performed. The samples are prepared by the thermal decomposition of mixed hydroxy compounds at 900°C. Different spinel phases are shown to form in Cu(Cr2 − xAlx)O4: a tetragonally distorted phase when x ≤ 1.0, a cubic phase when x > 1.25, and a mixture of both phases when 1 < x < 1.25. As Al3+ ions substitute Cr3+ ions, a number of Cu2+ ions move from tetrahedral spinel sites to octahedral ones: the degree of inversion δ changes from 0 for CuCr2O4 to ≈0.4 for CuAl2O4. In the Cu(Fe2 − xAlx)O4 system, the cubic spinel forms at all x, except x = 0. The degree of inversion δ varies from 1 for CuFe2O4 to ≈0.4 for CuAl2O4 as Al3+ ions substitute Fe3+ ions. The change in the activation energy of the water-gas shift reaction correlates with the inversion of Cu-containing spinels.

New approaches to the preparation of highly efficient chromium-containing oxide catalysts for the water gas shift reaction

It was found experimentally that the solutions of Cr3+ nitrate and the nitrates of other metals that are the constituents of Cr-containing catalysts can be prepared by dissolving a corresponding metal (for example, cast iron and electrolytic copper) in a solution of chromic anhydride and nitric acid to reach the quantitative reduction of Cr6+ without the formation of nitrogen oxides. Analogously, the oxidation of Fe2+ cations to Fe3+ coupled with the reduction of hexavalent chromium can be performed. The precipitation of Fe3+, Cr3+, and Cu2+ ions at a ratio of Fe: Cr = 9 and a concentration of Cu2+ to 20 at % can result in the formation of a partially hydrated oxide with the hydrohematite structure—a dispersed and highly defective oxide structure with a high specific surface area more than 300 m2/g and a higher thermal stability, as compared with the goethite phase (α-FeOOH). The dehydration of hydrohematite occurred at a noticeable rate at temperatures higher than 400°C. Hydrohematite promoted with copper cations exhibited high activity below 400°C; this can decrease the starting temperature of the adiabatic high-temperature WGSR to 300°C or below.

Effect of the Cr/Fe ratio on the structure of Fe-Cr-Cu-containing oxide catalysts

The morphology and structural and electronic states of nanodispersed copper ferrite-chromites of with different Cr/Fe ratios calcined at 900°C in an atmosphere of air were studied using a set of physical methods. The morphology, the homogeneity of chemical composition, and the degree of perfection of spinels formed from mixed Fe-Cr-Cu hydroxo compounds upon heat treatment were studied by HRTEM. From an analysis of atomic radial distribution curves, it follows that copper ferrite-chromite with a partially inverted spinel structure was formed at the ratio Cr/Fe = 1. With the use of DRS, it was found that all of the Fe-Cr-Cu-containing oxide samples contained Cu2+ cations in tetrahedral and tetragonally distorted octahedral oxygen coordination. In this case, the DRS spectra exhibited a decrease in the absorption energy of Cu2+ cations as the Cr/Fe ratio was decreased from 2 to 1, which indicated a decrease in the tetragonal distortion of the octahedral oxygen environment of the Cu2+ cations. In general, the data on the distribution of cations in Fe-Cr-Cu oxides with different Cr/Fe ratios obtained by EXAFS and DRS are consistent with the results of crystallochemical analysis. It was confirmed that the distribution of Cu2+ cations significantly affected the activation energy of water gas shift reaction.

Study of the factors affecting the formation of copper–chromium/aluminum oxide compounds with a spinel structure

The effect of the Cr3+/Al3+ ratio on the crystallization temperature of mixed oxide compounds with a spinel structure and their structural features and morphological characteristics have been studied using a combination of physicochemical methods: thermal analysis, IR spectroscopy, X-ray powder diffraction, and electron microscopy. The role of temperature of synthesis and drying of Cu–Cr/Al hydroxy precursors in the formation of copper-containing spinels CuCrxAl2–xO4, where x = 0–2, has been elucidated. The results are of interest for selection of the optimal composition and conditions of synthesis and formation of copper-containing spinels for their practical use.

Dehydrogenation of methanol over copper-containing catalysts

A comparative study of copper-containing catalysts with different chemical and phase compositions is performed to determine conditions for the implementation of the vapor phase and highly selective dehydrogenation of methanol to methyl formate or syngas. A thermodynamic analysis of the reaction is also performed. It is shown that Cu0 nanoparticles formed in the course of reductive activation reveal different selectivities with respect to the formation of methyl formate from methanol or its dehydrogenation with formation of syngas. By correctly selecting the catalyst composition and process conditions, high (90–100%) selectivity with respect to either methyl formate or syngas can be attained. Catalysts based on Cu–Zn hydrosilicate of the zincsilite type and on CuAlZn aurichalcite are highly selective in the process of methyl formate formation. An estimation based on experimental data shows that the productivity of Cu/SiO2 catalyst, the one most effective in dehydrogenation to syngas, is as high as 20 m3/h of syngas at a methanol vapor pressure of 1 atm, a temperature of 200°C, and a contact time of 0.5 s.

Role of anionic impurities in the formation of the active state of catalysts based on transition metals

Experimental data concerning the structure of transition metal-based catalysts are analyzed. Anionic impurities in the oxide precursor markedly modify both its structure and the structure of the catalyst resulting from its reduction. The modifying anionic impurities exert a significant effect on the local environment of the transition metal cations (particularly on that of Jahn-Teller cations) and even on the very possibility of existence of a mixed oxide precursor. In some cases, the changes in the local environment of the cations under the action of modifying ions show themselves as radical changes in the catalyst reduction kinetics. The formation of epitaxial bonding between the particles of the active metal and the surface of the oxide support, as well as the decoration of the particles with an amorphous oxyhydroxide layer, can be favorable for the stabilization of the active metal particles in the reduced state. Presumably, the activation of hydrogen molecules and the substrate being hydrogenated (e.g., CO) occurs on the surface of metal particles completely covered by a thin layer of the amorphous oxyhydroxide. The experimentally observed high activity of these catalysts is unlikely to be solely due to the increase in the specific catalytic activity of the remaining uncovered surface of the metal. It should also be attributed to the high inherent catalytic activity of the metal particles decorated with the oxyhydroxide layer.