O. P. Krivoruchko

Study of multiwalled graphite nanotubes and filaments formation from carbonized products of polyvinyl alcohol via catalytic graphitization at 600–800°C in nitrogen atmosphere

Abstract The catalytic graphitization of amorphous carbon matrix was carried out at the temperature range of 600–800°C in nitrogen atmosphere. Amorphous carbon matrix with uniformly distributed Fe particles was obtained via catalytic carbonization of polyvinyl alcohol (PVA) at temperatures up to 600°C in nitrogen atmosphere. Using transmission electron microscopy (TEM), selected area diffraction (SAD), scanning electron microscopy (SEM), and X-ray diffractometry (XRD), graphite structures of three types were found in products of catalytic graphitization of amorphous carbon matrix: multiwalled graphite shells wrapping the catalyst particles, cockle-shelled graphite filaments (CSF), and multiwalled graphite nanotubes (MWNT). We suppose that the formation of CSF proceeds through the dissolution of amorphous carbon in the metal, transformation of the catalyst particles into a liquid state, and transfer of dissolved carbon via intermediates to growing filaments. Graphite nanotubes nucleate at the matrix surface and then grow in the porous space of the matrix.

Catalytic synthesis of carbon nanostructures from polymer precursors

Abstract Carbon nanostructures were prepared by decomposition of polyethylene and polyvinyl alcohol using an iron catalyst at 600–750°C under a nitrogen flow. Heating a uniform distribution of catalyst particles in the polymer bulk to 600°C under flowing nitrogen led to the formation of amorphous carbon incorporating uniformly distributed catalyst particles. Subsequent heating of the samples to 750°C in a nitrogen flow led to the catalytic graphitization of the amorphous carbon matrix with the formation of different carbon nanostructures, i.e. carbon nanotubes. The mechanism of the formation of the obtained carbon nanostructures is discussed.

Laser-induced luminescence of model Fe/Al2O3 and Cr/Al2O3 catalysts

The laser-induced luminescence of Cr3+ impurity ions in model Fe/Al2O3 and Cr/Al2O3 catalysts with different calcination temperatures was studied. It was found that an additional luminescence band at 770 nm appeared in the luminescence spectra of low-temperature samples as a result of the interaction of octahedrally coordinated Cr3+ ions with Fe3+ impurity ions. In the θ-Al2O3 phase with a concentration of Cr3+ ions higher than 0.1 wt %, the interaction of the Cr3+-Cr3+ ion pairs in the immediate surroundings resulted in the appearance of Nθ lines due to the splitting of Rθ lines. The differences of these lines from the Nα lines of α-Al2O3 were related to the individuality of the crystal lattice of the θ phase and the coordination of Cr3+ impurity ions in the immediate surroundings, which is different from that in the α phase. Based on the laser-induced luminescence spectroscopic data, it was found that regions with a local Cr3+ concentration higher than the average Cr3+ concentration in the bulk of a catalyst by one order of magnitude were formed in the α-Al2O3-Fe2O3 system with the bulk Fe and Cr concentrations of 2.5 and 0.04 wt %, respectively, which was calcined at 1220°C, as a result of the diffusion of chromium and iron ions.

Distribution of the cobalt-containing component in the pore space of HZSM-5 upon a postsynthetic modification of the zeolite with hydroxo compounds of Co2+

The distribution of the cobalt-containing modifying component in the pore space of zeolite HZSM-5 depending on the total cobalt content of the samples (0.5–5.0 wt %) was quantitatively studied for the first time. At cobalt concentrations to 3.0 wt %, the cobalt-containing modifying component mainly occurred as isolated CoOh2+ ions in the micropores (channels) of HZSM-5 at the ion-exchange positions of the zeolite and in one-dimensional CoO and CoAl2O4 nanoclusters. A further increase in the cobalt concentration to 5.0 wt % resulted, in addition to the filling of micropores, in the partial filling of the mesopore space of the zeolite with a small amount of three-dimensional CoO and CoAl2O4 nanoparticles. Using sorption data and electronic diffuse reflectance spectra, we were the first to find that the effective density of a cobalt-containing modifying component in the pore space of a zeolite matrix was lower than the density of a bulk CoO phase by a factor of 6.

Distribution of copper- and nickel-containing modifier components in the pore space of HZSM-5 zeolite

The distribution of copper- and nickel-containing components in the pore space of HZSM-5 zeolite was quantitatively studied. It was found that the detailed distribution of a modifier in the micropore and mesopore volumes of the zeolite depends on both the chemical nature of the modifier and the conditions of supporting and the regime of M2+ polycondensation in the pore space of the zeolite. The experimental data on the low-temperature adsorption of nitrogen on Cu(n)ZSM-5 catalysts can be interpreted as the result of the partial filling of the zeolite micropore space (10 vol %) and the finest mesopores with D < 3 nm with the modifier. In the case of Ni(n)ZSM-5 catalysts, the penetration of the modifier into zeolite channels (micropores) in detectable amounts was not found, and it was arranged in mesopores on the surface of zeolite crystallites. The reason for differences between modifier distributions in the pore structure of the zeolite was explained from the standpoint of different structures of copper and nickel polyhydroxo complexes in impregnating solutions after polycondensation. It was found that, in the Cu(n)ZSM-5 and Ni(n)ZSM-5 catalysts, the modifier component contained copper and nickel only in a doubly charged state and mainly octahedral oxygen environments. In this case, three-dimensional nanoparticles or coarsely dispersed particles of CuO were not detected in the pore space of the support, whereas the presence of a small amount of sufficiently large NiO crystals with a coherent-scattering region of 80–100 nm was detected in Ni(n)ZSM-5, and these crystals occurred on the surface of zeolite crystals. It was found that the apparent density of a copper-or nickel-containing component arranged in the pore space of the zeolite was lower than the density of the bulk CuO and NiO phases by a factor of ∼3 and 4, respectively, because of the size effect.

New approach to the novel synthesis of boehmite (γ-AlOOH) by microwave irradiation of gibbsite: Kinetics of solid-phase reactions and dielectric properties of the reactants

Microwave irradiation of gibbsite is suggested as a method of obtaining crystalline boehmite. The kinetics of solid-state transformations of gibbsite under microwave radiation has been investigated, and the dielectric properties of the initial and microwave-activated gibbsite samples have been determined. Extending the gibbsite irradiation time leads to an increase in tanδ. This indicates that microwave-activated gibbsite has a stronger capacity for dissipating microwave energy owing to the formation of an amorphous component containing a variable amount of weakly bound molecular water. The general chemical formula of the amorphous component can be represented as Al2O3 · xH2O (0.5 < x < 3.0). The results of this study can provide a basis for developing new, low-waste, resource- and energy-saving methods for the synthesis of crystalline boehmite and for converting it into γ-Al2O3 with acid-base and textural properties that are atypical of the known low-temperature modifications of Al3+ oxides.

Activity of cobalt sulfide catalysts in the hydrogenolysis of dimethyl disulfide to methanethiol: Effects of the nature of a support and the procedure of supporting a cobalt precursor

The conversion of dimethyl disulfide to methanethiol on various catalysts containing supported cobalt sulfide in an atmosphere of hydrogen was studied at atmospheric pressure and T = 190°C. On CoS introduced into the channels of zeolite HSZM-5, the process occurred at a high rate but with a low selectivity for methanethiol because the proton centers of the support participated in a side reaction with the formation of dimethyl sulfide and hydrogen sulfide. Under the action of sulfide catalysts supported onto a carbon support, aluminum oxide, silicon dioxide, and an amorphous aluminosilicate, the decomposition of dimethyl disulfide to methanethiol occurred with 95–100% selectivity. The CoS/Al2O3 catalysts were found to be most efficient. The specific activity of alumina-cobalt sulfide catalysts only slightly depended on the phase composition and specific surface area of Al2O3. The conditions of the thermal treatment and sulfurization of catalysts and, particularly, the procedure of supporting a cobalt precursor onto the support were of key importance. Catalysts prepared through the stage of supporting nanodispersed cobalt hydroxide were much more active than the catalysts based on supported cobalt salts.

New low waste and energy saving technology for supports and catalysts production: Electron beam activation of oxygen containing solid compounds

We present the know-how of fundamentally new low-waste and energy saving support and catalyst production. High-energy electron beam activation of initial substances forms the basis of this technology. We report results of studies on the catalytically important properties of activated hydrargillite and kaolinite. They can be used in the synthesis of supports and catalysts.

Regularities of Lewis site formation upon the microwave irradiation of gibbsite-γ-Al(OH)3

Spatially separated Lewis acidic and basic sites can be formed upon the microwave irradiation of gibbsite. This is demonstrated by IR spectroscopy and indicates the possibility of the selective activation of chemical bonds of different nature within the initial crystals of Al3+ hydroxide. A scheme for the formation of spatially separated Lewis acidic and basic sites is proposed. It is shown that water as a product of the micro-wave activation of gibbsite is adsorbed on these Lewis acidic sites in molecular form (without subsequent dissociation) and is desorbed at calcination temperatures no higher than 100–110°C. During standard (contact) thermal treatment of Al3+ hydroxides and oxides, dehydroxylation and water removal take place at 350–550°C to form the acid-base pair-Alδ+-Oδ−-. The microwave activation of gibbsite results in the formation of an amorphous component believed to consist of small -Al-O- complexes closely packed in the solid phase. It is established that the fraction of Al3+ atoms accessible for the low-temperature adsorption of CO in microwave-activated (for 10 min) gibbsite is 4.5 at % of the total number of Al3+ atoms present in the amorphous component of this material.

Synthesis and mesopore-micropore structure characterization of nanodisperse Fe3+ hydrogels (xerogels)

A preparation procedure was developed, and samples of nanodisperse Fe3+ hydroxide with a narrow particle-size distribution (2.5–3.5 nm) were synthesized. The occurrence of a substructure in the bulk of Fe3+ hydroxide nanoparticles was detected for the first time using light-field and dark-field transmission electron microscopy. It was found that structurally ordered regions with sizes of ∼1.0 nm, which were disoriented with respect to each other at angles of a few degrees, occurred in the bulk of the nanoparticles. The empirical formula of nanodisperse iron hydroxide was ∼Fe2O3 · 1.8H2O; the structure of this hydroxide contained crystal water, OH−, and O2−. The coordination number of Fe3+ cations with respect to oxygen was 6. It was found that both structural and nonstructural water can be removed almost completely from the bulk of nanoparticles in the course of sample heating to 150–250°C in a vacuum with the retention of their amorphous character and observed sizes. In the course of dehydration, the mutual mobility of nanoparticles within aggregates was retained in Fe3+ xerogels; this resulted in a decrease in the total pore volume, whereas the volume of mesopores with diameters of 3.4–3.5 nm progressively increased. The micropore structure of the samples of nanodisperse iron hydroxides was studied by the molecular probe method using the low-temperature (77 K) sorption of nitrogen and molecular hydrogen. It was found that, along with micropores of volume ∼0.02 cm3/g, which are accessible to both of the sorbates, the sorption of H2 exhibited an additional specific absorption of 1.0–1.7 cm3(STP)/g, which can be interpreted as an additional ultramicropore volume accessible to only hydrogen molecules.