G. N. Kustova

Cobalt–aluminum co-precipitated catalysts and their performance in the Fischer–Tropsch synthesis

Cobalt–aluminum catalysts were prepared using either Co2+ precipitation onto freshly prepared Mg–Al or Zn–Al hydrotalcite (promoted samples) or co-precipitation of Co2+ and Al3+ (non-promoted samples). The evolution of initial hydrotalcite structure was monitored during its calcination and reductive treatment. It has been shown that, at moderate temperatures, hydrotalcites results decomposition yields a Co oxide phase supported by a highly defective inverted spinel-like structure. Cations Co2+ enter the support structure, and occupy both tetrahedral and octahedral positions. Octahedron coordinated Co species are reduced at 580–620°C. After the reduction at 470–480°C catalyst phase composition shows Co0 supported on inverted spinel-like structure, which contains Co2+ in the octahedral coordination. Further reduction at 600°C transforms the support to ‘ideal’ spinel, which contains no octahedron coordinated Co2+. Chemical properties of the Co–Al catalysts, including their performance in the Fischer–Tropsch synthesis (FTS), were found to depend on the catalyst reduction temperature, and thus on the support structure. Metal-support interaction is supposed to explain the observed properties of metallic cobalt.

IR spectroscopic investigation of cation distribution in Zn–Co oxide catalysts with spinel type structure

Abstract The structure of Zn–Co spinel prepared by coprecipitation was studied by IR spectroscopy. The characteristic bands of extra OH− and H3O+ groups were shown to exist in IR spectra up to 700°C. Comparing IR spectra of a poorly crystallized low temperature catalyst with that of a perfect Zn–Co spinel, we have associated the noticeable splittings and shifts of the F1u bands with definite structural distortions. The presence of extra anions in the catalyst with a spinel-like structure in the temperature region 100–700°C is the factor that stabilized the unusual cation distribution in this catalyst. The most plausible cation distribution in such a spinel is proposed as follows: Co3+ and Zn2+ are in octahedral positions, and Co2+ and Zn2+ are in tetrahedral ones.

Non-hydrothermal synthesis of copper-, zinc- and copper-zinc hydrosilicates

Abstract Cu/SiO2, Zn/SiO2 and Cu-Zn/SiO2 samples have been prepared by the homogeneous deposition-precipitation method. The samples were analyzed by thermal analysis, X-ray diffraction and infrared spectroscopy after various heat treatments and compared with data obtained for several minerals. It has been shown that interaction between the components occurs through formation of hydrosilicates. Copper-silica system at a Cu:Si ratio ≤ 1, gives rise to a hydrosilicate stable up to a calcination temperature of 930 K resembling the mineral Chrisocolla; at higher ratios a hydroxonitrate (gerhardite type) is also formed. Zinc-silica interaction produces two hydrosilicates such as a well crystallized Hemimorphite at Zn:Si = 2 and highly dispersed Zincsilite at Zn:Si ≤ 0.75, both stable up to 1073 K. The Zincsilite structure consists of three layered sheets (an octahedral layer sandwiched by two tetrahedral ones) like the Stevensite mineral group. For the copper-zinc-silica system no copper hydrosilicate is formed. Copper merely enters the Zincsilite structure independenly of the applied (Cu + Zn):Si ratio. Resulting layered copper-zinc hydrosilicate may be described by formulaZnx-yCuy(Zn3-x–zCuz–y▪x)[Si4O10](OH)2.nH2O,where Zn3-x-zCuz-y– ions are located in octahedral sites, Znx-yCuy–ions in the interlayer; ▪x are vacancies in the layers. Copper and zinc in excess of the Zincsilite ratio of Me:Si = 0.75, gives rise to copper and copper-zinc hydroxonitrates.

The state of absorbed hydrogen in the structure of reduced copper chromite from the vibration spectra

The reduction of copper chromite, CuCr(2)O(4), is followed by means of thermogravimetric analysis. The reduced state is studied by means of FT IR spectroscopy, Raman spectroscopy and inelastic neutron scattering. The reduction of copper occurs in two stages: absorption of hydrogen at 250-400 degrees C and dehydration of the reduced state at above 450 degrees C. The measured vibrational spectra prove that a considerable amount of hydrogen is absorbed by the oxide structure with absorbed protons stabilized in OH and HOH-groups (geminal protons). Three groups of vibration bands are observed in the INS spectra, which can be assigned to stretching, bending and libration vibrations. An increase in the reduction temperature of copper chromite results in softening of the stretching and hardening of the bending vibrations, what can be related to the strengthening of hydrogen bonding.

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.

Mechanistic features of reduction of copper chromite and state of absorbed hydrogen in the structure of reduced copper chromite

The review discusses the experimental data on the unusual mechanism of the reduction of copper cations from the copper chromite, CuCr2O4, structure. Treatment of copper chromite in hydrogen at 180–370°C is not accompanied by water formation but leads to absorption of hydrogen by the oxide structure with simultaneous formation of metallic copper as small flat particles which are epitaxially bound to the oxide. This process is due to the redox reaction Cu2+ + H2 → Cu0 + 2H+; the protons are stabilized in the oxide phase, which is confirmed by neutron diffraction studies. The reduced copper chromite which contains absorbed hydrogen in its oxidized state and the metallic copper particles epitaxially bound to the oxide phase structure exhibit catalytic activity in hydrogenation reactions.

Characterization of the nickel-amesite-chlorite-vermiculite system.

Synthetic TO (1 tetrahedral layer/1 octahedral layer) phylloaluminosilicates of Ni–Mg–Al with amesite (septechlorite) structure were synthesized and their evolution during calcination in inert and reducing media was studied. After treatment at 700–800 °C in hydrogen, the samples consisted of dispersed metallic nickel particles supported on TOT Mg-chlorite-vermiculite; none of the catalysts studied contained SiO2. The samples were stable in inert gas and hydrogen atmospheres at 850 °C, as well as in hydrogen plus steam at 20 bar and 650 °C. Thus, we consider Ni-containing amesite-like compounds to be suitable catalysts for the methane steam reforming process.

Physico-chemical study on the state of cobalt in a precipitated cobalt-aluminum oxide system

The evolution of Co–Al (1 ∶ 1) hydroxycarbonate with a hydrotalcite-like structure during its consequent calcination in an inert gas flow and reduction in H2 has been studied by means of X-ray diffraction, infrared spectroscopy, UV–VIS diffusive reflectance spectroscopy and magnetic susceptibility measurements. The oxidation of Co2+ species to Co3+ state during the calcination in the inert gas medium occurs simultaneously with the formation of a highly inverted anion-modified spinel-like structure. The samples reduced at 480°C and 620°C contain metallic cobalt particles. Ferromagnetic and paramagnetic contributions are distinguished and evaluated. The spontaneous magnetization of the sample reduced at 480°C increases monotonously with the temperature in the range of 78–300 K. The correlation of the results with earlier data on physico-chemical and catalytic properties has been discussed.

Evolution of the structure of Co stevensite during its treatment in the air, inert gas flow and flowing hydrogen

Abstract Synthetic TOT (2 tetrahedron layers,1 octahedron layer) trioctahedral hydrosilicates (stevensites) of Zn, Mg, Co and Co-Zn were prepared by the deposition–precipitation technique. The evolution of both the structure and spectral properties of the silicates were studied during their treatment in various media. The position of the ν(OH) absorption band and the temperature of crystallization of the anhydrous silicate were found to be useful indicators of the cationic composition of stevensites. The data obtained are used to analyze and to review the earlier data on Co/SiO2 catalysts. It is concluded that the formation of Co stevensite occurs in the majority of cases when the pH of the maternal solution during the preparation of a catalyst is above four.

Copper ions distribution in synthetic copper-zinc hydrosilicate

Abstract Copper ions distribution in the structure of synthetic copper-zinc hydrosilicate of zincsilite structure, obtained non-hydrothermal synthesis have been studied. Zinesilite is referred to the layered silicates of smectite group and is described by the formula Znx (Zn3-x▪x) [Si4O10](OH)2.nH2O, where Zn3-x – are the ions located in the octahedral positions of layers, formed by two sheets of [Si4O10] tetrahedrons; Znx are zinc ions in the interlayer; ▪x are the cation vacancies. Two types of copper ions were distinguished in accordance with the character of their interaction with hydrogen: (1) – substituting zinc ions in the octahedral positions of layers; (2) – substituting zinc ions in the interlayer. These two types of copper ions display the following properties when reacting with hydrogen: (1) – copper ions in octahedral positions start to be reduced at temperatures 553–573 K, and at 723 K reduction degree is 50% ; (2) – copper ions from interlayer start to be reduced at 503–533 K with a constant energy of activation, and their reduction may be complete at this temperature.