S. A. Chernyak

CO2 hydrogenation over cobalt-containing catalysts

Carbon dioxide hydrogenation over catalysts consisting of 0.56–45 wt % cobalt supported on carbon nanotubes (CNTs), carbon nanofibers, few-layer graphite fragments, or CNTs–Al2O3 composites has been investigated. All of the Co/support catalytic systems have been characterized by temperature-programmed reduction, transmission electron microscopy, and scanning electron microscopy. Under the conditions of our catalytic experiment (1 atm, 180–500°C), the CO2 hydrogenation products are CH4 and/or CO and the activity of the catalysts depends on the size and phase state of the cobalt particles. The CNTs-supported materials containing less than 5 wt % Co are catalytically inactive because of the amorphism of the metal. They can be activated by cobalt crystallization by means of heat treatment. The size of the cobalt particles deposited on the carbon supports is about 4 nm. Methods of functionalizing the carbon nanomaterial surface for additional stabilization of metal nanoparticles are suggested.

Catalytic properties of CexZr1–xO2 prepared using a template in the oxidation of CO

The catalytic activity in CO oxidation of CexZr1–xO2 double oxides prepared using pine sawdust and cetyltrimethylammonium bromide (CTAB) as templates is compared. It is found by means of SEM and the low-temperature adsorption of N2 that biomorphic oxides reproduce the macropore structure of the template. It is shown via XRD and Raman spectroscopy that all samples contained mixed ceria-zirconia oxide. The double oxides form a cubic phase with a lattice of the fluorite type at a ratio of Ce: Zr = 4, regardless of the nature of the template; when Ce: Zr = 1, the biomorphic mixed oxide forms a tetragonal phase. According to Raman spectroscopy and XRD it was shown that the distortion of the oxygen sublattice is higher in biomorphic samples. Energy dispersive analysis shows that Ca impurities were present in the biomorphic samples, introducing additional distortions in the lattice of double oxide and leading to the formation of anionic vacancies. It is found that when Ce: Zr = 4, the conversion of CO on biomorphic oxide in the range of 100–350°C is higher than that observed for CexZr1–xO2 (CTAB); reducing the Ce: Zr ratio in the biomorphic sample to 1 results in a marked decrease in CO conversion at 100–200°C. It is concluded that these differences are due to changes in the mobility of the lattice oxygen.

Effect of cobalt weight content on the structure and catalytic properties of Co/CNT catalysts in the fischer–tropsch synthesis

Cobalt-based Fischer–Tropsch synthesis (FTS) catalysts containing 1 to 40 wt % cobalt supported on multi-walled carbon nanotubes (CNTs) have been investigated. The CNTs have been characterized by low-temperature nitrogen adsorption, scanning electron microscopy, and X-ray photoelectron spectroscopy. All catalysts have been prepared by impregnating, with an ethanolic solution of cobalt nitrate, the CNTs preoxidized with concentrated nitric acid and have been tested in the FTS at 220°C and atmospheric pressure. Correlations have been established between the cobalt weight content of the catalyst and the Co particle size determined by transmission electron microscopy and X-ray diffraction. The Co content and particle size have an effect on the activity and selectivity of the catalyst and on the target fraction (C5+) yield in the FTS. The highest CO conversion is observed for the catalyst containing 20 wt % Co; the highest selectivity and activity, for the catalyst containing 5 wt % Co; the highest C5+ yield, for the catalyst containing 10 wt % Co.

Promoting effect of potassium and calcium additives to cerium–zirconium oxide catalysts for the complete oxidation of carbon monoxide

The effect of potassium and calcium additives on the catalytic activity of the Ce0.8Zr0.2O2 system in the reaction of CO oxidation was studied. With the use of X-ray diffraction analysis, it was found that the Ce0.8Zr0.2O2 and Ce0.8Zr0.2O2–Ca,K samples contained a mixed oxide of cerium and zirconium; the presence of the independent phases of potassium and calcium compounds in the modified system was not detected. With the use of the low-temperature adsorption–desorption of nitrogen, X-ray photoelectron spectroscopy, and temperature-programmed reduction, it was established that the Ce0.8Zr0.2O2–Ca,K system (in spite of the fact that its specific surface area was lower than that of Ce0.8Zr0.2O2) contained more active oxygen on the surface; peroxide and superoxide complexes formed upon the chemisorption of O2 can act as active oxygen species. This can be the reason for a higher efficiency of the Ce0.8Zr0.2O2–Ca,K system in comparison with that of the unmodified oxide. The results obtained indicate that the ash impurities of Ca and K can increase the catalytic activity of the biomorphic mixed oxides Ce0.8Zr0.2O2 prepared with the use of sawdust as a template.

Catalytic activity of carbon nanotubes in the conversion of aliphatic alcohols

Carbon nanotubes (CNTs) obtained via the catalytic pyrolysis of hexane at 750°C were studied as the catalysts in conversion of C2–C4 alcohols. The efficiency of CNTs as catalysts in dehydration and dehydrogenation of ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and tert-butanol was studied by means of pulse microcatalysis. The surface and structural characteristics of CNTs are investigated via SEM, TEM, DTA, BET, and XPS. CNTs are shown to be effective catalysts in the conversion of alcohols and do not require additional oxidative treatment. The regularities of the conversion of aliphatic alcohols, related to the properties of the CNTs surface and the structure of the alcohols are identified.

Features of the oxidation of multiwalled carbon nanotubes

Features of the functionalization of multiwalled carbon nanotubes (MWCNTs) with a conical (Ni precursor) and cylindrical (Fe precursor) arrangement of graphene layers using various oxidizing agents are studied. The initial diameter of pyrolytically prepared tubes varies from 20 to 80 nm with a maximum at 40–45 nm and from 10 to 30 nm with a maximum at 18 nm in the first and second cases, respectively. Oxidative modification of the MWCNT surfaces is conducted using HNO3 and H2O2 with ultrasound activation, ozonation in a glow discharge plasma of oxygen, and treatment with liquid ozone. Thermal and elemental analyses and IR spectroscopy show that the highest content of functional groups is achieved in the samples treated with nitric acid, where the conical MWCNTs are subject to surface functionalization. It is concluded that in order to achieve a similar result, cylindrical tubes must be treated with liquid ozone.

3D Frameworks with Variable Magnetic and Electrical Features from Sintered Cobalt-Modified Carbon Nanotubes

3D frameworks of carbon nanotubes (CNTs) uniformly decorated by cobalt oxide or carbon-encapsulated cobalt nanoparticles were obtained by spark plasma sintering for the first time. The influence of the sintering temperature ( TS) and Co content on the morphology, structure, and electrical and magnetic properties of the obtained materials was investigated by Raman spectroscopy, electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and in situ magnetometry. It was shown that application of the SPS technique allowed simultaneous compaction of the material, formation of CNT framework, and Co oxide reduction. The appearance of the carbon shell around 4-10 nm Co particles was observed at TS > 600 °C. At higher TS, the Co particle size increased (up to 300 nm at 1400 °C), whereas the carbon shell ordered and thickened. The formation of large-size few-layers graphene sheets was observed at TS = 1400 °C. Electrical conductivity of the composites was found to be higher than that of sintered pristine CNTs and varied in the range of 500-12 500 Sm/m. Magnetic experiments demonstrated soft magnetization of the samples and the coercivity of 200-300 Oe. Thus, the obtained CNT-based material is simultaneously compact, formable, electroconductive, and ferromagnetic. Its properties can be tuned by variation of the sintering parameters. Synthesized cobalt-modified carbon 3D structures are promising for the application in magnetic separation, catalysis, fuel cells, and electromagnetic shielding.

Catalytic conversion of aliphatic alcohols on carbon nanomaterials: The roles of structure and surface functional groups

Carbon nanomaterials with the structure of graphene and different compositions of the surface groups are used as catalysts for the conversion of С2–С4 aliphatic alcohols. The conversions of ethanol, propanol- 1, propanol-2, butanol-1, butanol-2, and tert-butanol on carbon nanotubes, nanoflakes, and nanoflakes doped with nitrogen are investigated. Oxidized and nonoxidized multiwalled carbon nanotubes, nanoflakes, and nanoflakes doped with nitrogen are synthesized. X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning and transmission electronic microscopies, Brunauer–Emmett–Teller method, derivatographic analyses, and the pulsed microcatalytic method are used to characterize comprehensively the prepared catalysts. It was established that all of the investigated carbon nanomaterials (with the exception of nondoped carbon nanoflakes) are bifunctional catalysts for the conversion of aliphatic alcohols, and promote dehydration reactions with the formation of olefins and dehydrogenation reactions with the formation of aldehydes or ketones. Nanoflakes doped with nitrogen are inert with respect to secondary alcohols and tert-butanol. The role of oxygen-containing and nitrogen-containing surface groups, and of the geometrical structure of the carbon matrix of graphene nanocarbon materials in the catalytic conversion of aliphatic alcohols, is revealed. Characteristics of the conversion of aliphatic alcohols that are associated with their structure are identified.

Sawdust as an effective biotemplate for the synthesis of Ce0.8Zr0.2O2 and CuO–Ce0.8Zr0.2O2 catalysts for total CO oxidation

Wood sawdust (SD) was successfully used as a biotemplate to prepare precursors of Ce0.8Zr0.2O2 (CZ) and CuO–Ce0.8Zr0.2O2 (Cu–CZ, about 25% of CuO) catalysts, which were tested in total CO oxidation at 100–400 °C after calcination at 500 or 600 °C, and compared with analogous systems prepared using cetyltrimethylammonium bromide (CTAB). The improved specific surface area and oxygen mobility of the catalysts calcined at 500 °C resulted in an increased efficiency in CO oxidation. CZ (SD-500) demonstrated much higher catalytic efficiency than the CZ (CTAB-500) sample despite having a two times less SBET due to the promoting action of Ca and K inherited from sawdust and improved concentration of superoxide surface centers. Both facilitation of CO adsorption on peroxide and superoxide centers and increased oxygen mobility provide improvement in the high-temperature efficiency of biomorphic CZ in CO oxidation. The XRD, XPS and TPR data testify that Cu in modified systems partially exists as a separate CuO phase, in small amounts as Cu+, and partially is incorporated in the surface CuyCexZr1−x−yO2−z and/or CuyCe1−yO2−x phases. Modification with Cu enhances the low-temperature efficiency of CZ systems, prepared using both templates and calcined at 500 °C. Cu–CZ (SD-500) was a bit less active than Cu–CZ (CTAB-500) only at 100–150 °C, primarily due to the decreased CuO content, and the difference in CuO interaction with CZ support, found by TPR. Presumably CuO provides low-temperature CO oxidation through the Langmuir–Hinshelwood mechanism; the Cu+/Cu2+ pair can also participate as an additional redox pair in high-temperature CO oxidation by the Mars–van-Krevelen mechanism.

Synthesis of carbon nanotubes/alumina composites in supercritical media

Nanosized composites based on multiwall carbon nanotubes (CNTs) and Al2O3 have been obtained for the first time in supercritical (SC) media (water, hexane, and their mixture). For comparison, materials of the same net composition have been prepared by hydrothermal synthesis and sol–gel processing. The composites have been characterized by electron microscopy, X-ray diffraction, and thermal analysis. The structure of the materials synthesized in the SC media depends on the fluid composition. The most uniform composite containing alumina particles that are comparable in size to the CNT diameter and are stabilized on the carbon surface can be obtained in the SC mixture of hexane and water. When water and hexane are used separately, the formation of large alumina crystals on the CNT surface and contamination of the composite by the products of hexane pyrolysis and carbonization are, respectively, observed.