A. A. Khassin

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.

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.

Dehydrogenation of methanol over copper-containing catalysts

Abstract The catalytic properties in methanol dehydrogenation of copper metal formed as a result of reduction by hydrogen of copper-containing oxides with different structure: copper chromite (tetragonally distorted spinel), copper hydroxysilicate (Chrysocolla), and copper-zinc hydroxysilicate (Zincsilite) have been studied. This process proceeds via successive reactions: (I) 2CH 3 OH=CH 3 OOCH+2H 2 and (II) CH 3 OOCH=2CO+2H 2 . The methyl formate selectivity for the catalysts studied was close to 1.0 at low methanol conversion, X ≤0.1, where the dehydrogenation process is represented by reaction (I), occurring far from its equilibrium. At 0.2≤ X ≤0.55, the selectivity decreases with increasing conversion, and the ratio of the activities in successive reactions may serve as a comparative characteristic for the catalysts. At high conversions, when reaction (I) is close to its equilibrium, selectivity is independent of the properties of studied catalysts and depends on the methanol conversion. Reaction (I) shows low sensitivity to the state of metal copper of reduced catalysts and, hence, low sensitivity to the composition and structure of oxides-precursors. The catalysts’ activity in reaction (II) greatly depends on the state of metal copper in the catalysts. It was assumed that the catalyst activity in methyl formate conversion to CO and H 2 and, hence, the selectivity of methanol dehydrogenation with respect to methyl formate in the region of moderate methanol conversion depends on the strength of interaction between metal copper particles and catalyst oxide surface, which is determined by the composition and structure of oxide-precursor.

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.

The impact of a dielectric barrier discharge on the catalytic oxidation of methane over Ni-containing catalyst

Methane oxidation with air was studied over a Ni-containing catalyst in a dielectric barrier discharge (DBD) at temperatures above 625 K. The DBD increases the methane conversion and shifts the process towards partial oxidation. This effect is related to a catalyst heating by the discharge.

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.

Cobalt-containing catalysts supported by synthetic Zn- and Mg-stevensites and their performance in the Fischer–Tropsch synthesis

Abstract Co-containing catalysts supported onto the synthetic stevensites of Zn and Mg were prepared using various methods. The evolution of the catalyst structure was monitored during the calcination and a reductive treatment. During the calcination, Co 2+ cations substitute the divalent cations of the support, that leads to the formation of the mixed Co–Me-stevensite. These Co species cannot be reduced at temperatures below 700°C. The phase composition of the catalyst after the reduction at 500°C contains Co 0 particles supported by the Co–Me mixed stevensites. Unexpectedly, the performance of the Co/stevensite catalysts in the Fischer–Tropsch synthesis was much worse, than that of Co/MgO. The supposition on the metal-support interaction, which causes the low electron-donor capacity of the metallic Co may explain the experimental data.

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.

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.