V. I. Zaikovskii

Coprecipitated Ni-alumina and NiCu-alumina catalysts of methane decomposition and carbon deposition. II. Evolution of the catalysts in reaction

Using coprecipitated Ni-alumina and NiCu-alumina catalysts, up to 250 g/gcat of filamentous carbon was produced by methane decomposition at 823 K and a methane pressure of 100 kPa. The process evolved three general stages. In the induction period carbon was dissolved into the Ni particles as evidenced by in situ X-ray powder diffraction measurements. This promoted the formation of large metal particles pear-shaped in Ni catalysts and quasi-octahedral in NiCu ones. The steady-state growth of the filaments occurred on the second stage. Finally, the catalyst was deactivated producing porous granules composed of interlaced long carbon filaments. In deactivated Ni catalysts the nickel was found in metal state, while in NiCu samples about half of the nickel was atomically dispersed in carbon as revealed by extended X-ray absorption fine structure spectroscopy. The deactivation of the catalysts was suggested to include the fragmentation of the metal particles as well as an atomic erosion in NiCu samples. In addition, in both catalysts the filaments growth could also be limited when the close-packed structures of porous carbon were achieved.

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.

CO monolayer oxidation at Pt nanoparticles supported on glassy carbon electrodes

CO monolayer oxidation on glassy carbon supported 1–2 nm Pt nanoparticles is studied using potential sweep and potential step methods. The CO stripping peak on the nanoparticles is significantly shifted to positive potentials vs. the corresponding feature at bulk polycrystalline Pt. Current transients at nanoparticulate electrodes are highly asymmetric with a steep rise, maximum at θCO≈0.8–0.9, and a slow decay following t−1/2. The experimental results are compared to the theoretical models of adsorbed CO oxidation described in the literature. A tentative model is suggested to account for the experimental observations, which comprises spatially confined formation of oxygen containing species at active sites, and slow diffusion of CO molecules to the active sites, where they are oxidized. The upper limit of the CO surface diffusion coefficient at Pt nanoparticles is estimated as approximately 4×10−15 cm2 s−1.

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.

On the influence of the metal loading on the structure of carbon-supported PtRu catalysts and their electrocatalytic activities in CO and methanol electrooxidation

PtRu (1:1) catalysts supported on low surface area carbon of the Sibunit family (S(BET) = 72 m(2) g(-1)) with a metal percentage ranging from 5 to 60% are prepared and tested in a CO monolayer and for methanol oxidation in H(2)SO(4) electrolyte. At low metal percentage small (<2 nm) alloy nanoparticles, uniformly distributed on the carbon surface, are formed. As the amount of metal per unit surface area of carbon increases, particles start coalescing and form first quasi two-dimensional, and then three-dimensional metal nanostructures. This results in a strong enhancement of specific catalytic activity in methanol oxidation and a decrease of the overpotential for CO monolayer oxidation. It is suggested that intergrain boundaries connecting crystalline domains in nanostructured PtRu catalysts produced at high metal-on-carbon loadings provide active sites for electrocatalytic processes.

New carbon-carbonaceous composites for catalysis and adsorption

Properties, structural features, and basic principles for the synthesis of some new carbon-carbonaceous composite materials (CCM) originating at the Boreskov Institute of Catalysis are discussed. The CCM are synthesized via chemical growth of carbon deposits in a pre-formed porous carbon matrix. Among the CCM are: Sibunit produced by supporting pyrocarbon (PC) on carbon black granules, carbon filaments (CFC) produced by decomposition of CH4 over Ni or Ni−Cu catalysts, CFC on Sibunit, CFC on ultradispersed diamond, and systems such as CFC/CFC, PC/CFC, etc. Basic mechanisms of structure and texture formation of CCM and some of their properties as adsorbents and catalyst supports are reported.

Effect of Pd/C dispersion on its catalytic properties in acetylene and vinylacetylene hydrogenation

In reactions of the selective hydrogenation of acetylene and vinylacetylene, the specific catalytic activity (turnover number, TON) and selectivity of Pd obtained via reduction of C3H5PdC5H5 anchored on active carbon, within a particle-size range of 30–110A, is roughly constant and close to the TON of Pd over oxide supports (SiO2, Al2O3). The binding energy of the Pd 3d5/2 level of a low-dispersed Pd does not depend significantly on its particle size and the nature of the support. The TON of Pd/C containing particles <30Ais several times lower than that of low-dispersed Pd.

Pt and Pd supported on glass fibers as effective combustion catalysts

Pd and Pt supported on glass fiber materials with developed porosity and high sp. surface areas were studied in total propane oxidn. The reaction was carried out in recycling reactor and the kinetic parameters were detd. under different reaction conditions in the temp. range 200-500 DegC. Pt catalysts were seen to be more active than Pd for the same metal loading on identical support. Catalytic activity was seen to depend on support compn. The highest activity was obsd. on Pt supported on glass fiber modified by titania, demonstrating the ignition temps. around 200 DegC. The catalyst surface morphol. and surface dispersion of active metal were characterized by high-resoln. electron microscopy. [on SciFinder (R)]

Peculiarities of filamentous carbon formation in methane decomposition on NI-containing catalysts

Abstract Experimental data on the effect of CH4 + H2 mixtures composition and temperature on catalytic filamentous carbon (CFC) formation in methane decomposition on Ni-containing catalyst are presented. Microscopy studies have shown that at the lowest methane concentration (2.5%) the amount of CFC produced per unit of catalyst mass is limited primarily by the surface effects imposed by the carbon growth centers (GC) rather than by volume ones. The shape of GC where carbon filament forms depends on the reaction medium composition and temperature. By varying the H2 concentration in the reaction medium, one can change the texture of the CFC produced.

Nickel catalysts supported on carbon nanofibers: structure and activity in methane decomposition

Structures of Ni catalysts supported on filamentous carbon (CFC) produced by methane decomposition over coprecipitated Ni and Ni-Cu/alumina catalysts were studied by EXAFS and TEM. Thermal pre-treatment in N2 at 350°C of samples impregnated by nickel nitrate precursor was found to produce either NiO or nickel carbide, Ni3C, phase. This was explained by different reducing properties of the carbon nanofibers which depend on the surface structure. High stability of the Ni/C catalysts in methane decomposition reaction at 550°C was found with those prepared from only nickel chloride precursor, due to the formation of large (30-70 nm) Ni particles further leading to new carbon filaments growth. Data implies a common mechanism of the filamentous carbon deposition in all Ni-based catalysts, independent of the support (silica, alumina, carbon) being used. However, accumulation of filamentous carbon is strongly influenced by morphology and texture of the support.