Adsorption of hydrogen on reduced copper chromite

Temperature programmed desorption and volumetric methods in static conditions were used to study hydrogen adsorption on the surface of metallic copper particles produced by the partial reduction of copper chromite CuCr₂O₄ with hydrogen. In the temperature range 300-573 K and in the range of medium surface coverages by hydrogen, the main state of adsorbed hydrogen reveals the heat of adsorption q= 78 kJ/mol and activation energy of adsorption E _a = 69 kJ/mol. In the temperature range 77-300 K, an adsorption state with lower heat and activation energy was found, indicating a non-uniformity of the copper surface within ca. 8% of the total number of surface sites. This revised version was published online in June 2006 with corrections to the Cover Date.     

Moderate-temperature reduction of copper chromite by hydrogen and hydrogen desorption from the surface of reduced chromite

The reduction of copper chromite CuCr₂O₄ by hydrogen at 300–573 K and a hydrogen pressure of 4×10⁴-8 × 10⁴ 10⁴ Pa and hydrogen desorption from the surface of a reduced sample are studied. The rate of copper chromite reduction becomes high at temperatures above 473 K. Hydrogen desorbs from the surface of metallic copper covering the surface of chromite during its reduction. The heat of hydrogen adsorption on the metallic copper is almost independent of the surface coverage, 70–80 kJ/mol.     

Interaction of hydrogen with copper chromite surface

Kinetics of the surface reduction of copper chromite, CuCr₂O₄, with molecular hydrogen has been studied. H₂S transformation proceeds with participation of copper reduced states, probably Cu⁰, formed as a result of surface reaction. Process kinetics has a topochemical character. Reaction rate as a function of surface reduction degree, x, reaches a maximum near x=0.5.     

Activation of nickel–chromium hydrogenation catalysts with hydrogen

The kinetics of the reduction of nickel cations in nickel oxide and nickel–chromium catalysts whose oxide precursors have different structures has been investigated by thermal analysis. The reduction of nickel oxide with a hydrogen-containing gas takes place at 250–330°C. The apparent activation energy of this reaction is about 88 kJ/mol. The introduction of up to 30 at % chromium cations into the nickel oxide structure shifts the reduction temperature of nickel in the oxide phase to 300–450°C and increases the apparent activation energy of the reduction of nickel cations to ～108 kJ/mol. The introduction of 67 at % chromium into nickel oxide results in the formation of an oxide precursor with a spinel structure. The apparent activation energy of the reduction of nickel cations in this spinel is about 163 kJ/mol. The results of this study can be used in optimizing the composition of Ni-containing hydrogenation catalysts and their activation and operation conditions.     

Anionic composition of precursors of the Co/Al2O3 catalysts for the Fischer-Tropsch synthesis

Cobalt and aluminum hydroxo compounds with the hydrotalcite-type structure containing nitrate, carbonate, and hydroxyl groups are formed due to the deposition by precipitation of cobalt cations from a nitrate solution on ??- and ??-Al₂O₃ under the conditions of urea hydrolysis. The influence of the method of preparation on the anionic composition of the hydroxo compounds (precursors of the Co-Al catalyst) was studied. A correlation between the anionic composition of the precursor compound and the catalytic properties of the catalysts in the Fischer-Tropsch synthesis was established.     

Effect of Synthesis Conditions on the Formation of the CuCrAlO<Subscript>4</Subscript> Spinel Structure

The effect of synthesis conditions of mixed hydroxy compounds (coprecipitation mode, residence and aging times, precipitator nature) on the structural characteristics of CuCrAlO₄ spinels is studied. Evolution of the spinel structure during the solid-phase synthesis is analyzed.     

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 CuCr _x Al_2–x O₄, 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.     

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 Co_{3 ? x} Al _x O₄ (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.     

Synthesis of an efficient catalyst based on nickel and chromium oxides for hydrogenation reactions

The dissolution of nickel metal in nitric acid in the presence of the dichromate anion as the oxidizer is reported. The formation of Ni(II) and Cr(III) nitrates takes place in two steps with the intermediate formation of nitrous acid. A new method to synthesize the nickel-chromium oxide catalyst from nickel and chromium nitrate solutions is suggested, in which the solutions are obtained by an environmentally friendly technology from nickel metal, chromium(VI) oxide, and nitric acid. The catalyst is highly active and selective in benzene hydrogenation and in CO preferential hydrogenation in the presence of CO₂.     

A comparative FMR study of the reduction of Co-containing catalysts for the Fischer–Tropsch process in hydrogen and supercritical isopropanol

An in situ comparative study of the reduction of Co-containing catalysts for the Fischer–Tropsch process in hydrogen and supercritical (SC) isopropanol is performed by ferromagnetic resonance (FMR) spectroscopy. According to the FMR data, the reduction of cobalt-containing oxide particles to metal in hydrogen starts at temperatures of ~360°C, which is substantially lower than a temperature of the formation of metal particles of the active phase according to powder X-ray diffraction and differential thermogravimetry data (Т ~ 450°C). In SC isopropanol, the reduction to Co metal occurs at lower temperatures (T ~ 245°C) as compared with the reduction temperature for these catalysts in hydrogen. It is shown that the reduction in SC isopropanol can lead to the formation of superparamagnetic Co nanoparticles with a narrow particle size distribution.