T. M. Yurieva

Metal-support interactions in cobalt-aluminum co-precipitated catalysts: XPS and CO adsorption studies

Cobalt-aluminum catalysts were prepared using either the precipitation of Co 2+ in the presence of freshly prepared Zn-Al hydrotalcite (the promoted sample) or the co-precipitation of Co 2+ and Al 3+ (the unpromoted samples). The evolution of the initial hydrotalcite-like structure was monitored during its calcination and the reductive treatment by means of XPS. It was shown that at 480 ◦ C the reduction of the calcined samples results in the formation of Co 0 species, the further reduction at 650 ◦ C results in an increase of the amount of the Co 0 species. The samples reduced at 650 ◦ C chemisorb readily carbon monoxide at 77 K, while the sample reduced at 480 ◦ C does not chemisorb CO at 77 K. At elevated temperatures, all reduced samples are found to be able to chemisorb CO. Terminal CO moieties as well as monodentate carbonates, formates and carboxyl species were detected at the surface of the reduced samples at their exposure to the CO medium at the elevated temperature. The intensity of the IR absorption bands of chemisorbed CO are found proportional to the surface fraction of the Co 0 species, measured by XPS. The apparent red shift of the IR absorption bands is observed for CO adsorbed on the samples reduced at 480 ◦ C. The obtained data correlate with the catalytic properties of the Co-Al samples in hydrogenation reactions. The conclusion on the existence of a strong metal–support interaction in the samples under the study is made.

Role of the Cu–Co alloy and cobalt carbide in higher alcohol synthesis

Abstract Formation and decomposition of the Cu–Co alloy and Co 2 C were studied using in situ X-ray diffraction (XRD), TG-DTA and TEM techniques. Cu–Co alloy with ratio Cu/Co=1:1 has been obtained under treatment of CuCoO 2 with hydrogen at 230–300°C. Co 2 C was formed from Cu–Co alloy at 280–310°C and decomposed at 390–400°C under CO. It was shown that the role of Cu–Co alloy consisted in formation of cobalt carbide was able to activate CO undissociatively that led to oxygenates synthesis.

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.

Mechanisms for hydrogenation of acetone to isopropanol and of carbon oxides to methanol over copper-containing oxide catalysts

Abstract Mechanisms for the synthesis of methanol from CO and CO2 and for the hydrogenation of acetone to isopropanol are discussed based on the recent experimental results obtained by the authors. The state of copper-containing compounds in a hydrogen medium at 200–400°C and the nature of their interaction with the reaction components were studied. Hydrogenation of carbon oxides and acetone was proposed to be the result of the ability of copper ions to reversible transformations to generate copper metal and protons. Activation of acetone and CO2 can be achieved through their interaction with Cu0, and activation of CO through its interaction with oxygen-containing sites of Cu+1OCu+1 type which are formed after oxidation of a portion of Cu0 with carbon dioxide.

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.

The nature of hydrogen stabilization in the reduced copper chromites

Abstract In situ powder X-ray and neutron diffraction showed that copper chromite retains its spinel structure (space group Fd 3 m , a = 8.348(3) A ) in hydrogen at 320°C. The spinel has a copper ion deficiency and is stabilized by the dissolved hydrogen. Two hydrogen states are observed in the spinel structure: as specific hydrogen species H (16c interstitial position) and as OH groups with covalent bounding (32e crystallographic position). The hydrogen species bonded to the lattice oxygen is formed as a result of the exchange interaction between hydrogen atoms and Cu 2+ ions. In this case a portion of copper ions are reduced to Cu 0 as flat particles onto the surface; another portion is reduced to Cu 1+ and transferred towards the 16c positions.

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.

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.

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.