Formation Of Carbon Nanofilaments Research Articles

Catalytic Conversion of Hydrocarbons and Formation of Carbon Nanofilaments in Porous Pellets

Catalytic conversion of hydrocarbons occurring at metal nanoparticles in porous pellets is often accompanied by the formation of coke in the form of growing heterogeneous film-like aggregates or carbon nanofilaments. The latter processes result in deactivation of metal nanoparticles. The corresponding kinetic models imply the formation and growth of film-like coke aggregates. Herein, I present an alternative generic kinetic model focused on the formation and growth of carbon nanofilaments. These processes are considered to deactivate metal nanoparticles and reduce the rate of reactant diffusion in pores. In this framework, the kinetically limited reaction regime is described by simple analytical expressions. The diffusion-limited regime can be described as well but only numerically. The model presented can be used for interpretation of experimental results.Graphical

Microwave-assisted ethanol decomposition over pyrolysis residue of sewage sludge for hydrogen-rich gas production

Microwave-assisted ethanol decomposition over pyrolysis residue of sewage sludge (PRSS) was investigated in a multi-mode microwave (MW) oven, and was compared with ethanol decomposition over a commercial activated carbon (AC). The results indicated that PRSS showed higher MW heating performance than AC. Under the same microwave power (MP), the temperature of catalyst bed, hydrogen produc0oij9tion and ethanol conversion over PRSS were higher than that over AC. Both hydrogen production (per unit volume of ethanol) and ethanol conversion increased with the increase of MP, whereas decreased with the increase of the volumetric hourly space velocity of ethanol (VHSVeth). Additionally, comparison between ethanol decomposition by MW and conventional heating was also conducted. Results indicated that MW heating was much more beneficial for hydrogen production than conventional heating, and the energy efficiency of hydrogen production under MW heating was much higher than conventional heating. Finally, carbon nanofilaments formation over PRSS and AC were observed under MW heating. The size and structures of carbon nanofilaments over PRSS and AC were significantly different with each other, which may be strongly correlated with the formation of micro-plasmas during MW heating.

Formation of carbon nanofilaments from C6-C16 alkanes over nickel-containing catalysts

The properties of 85%Ni/Al2O3 and NiO/Sibunit series catalysts have been studied in the formation of carbon nanofilaments from n-hexane, n-undecane, and n-hexadecane. In the temperature range 450–600°C, the reactivity of these hydrocarbons decreases in the following order: n-hexane > n-undecane > n-hexadecane. The deposition rate of carbon nanofilaments from n-hexane or n-undecane over the 85%Ni/Al2O3 catalyst in the range 450–600°C slightly depends on the reaction temperature. The activation energy of the formation of carbon nanofilaments from n-hexane is 10 kcal/mol and that from n-undecane, 13.5 kcal/mol. The rate-limiting reaction is the interaction of a paraffin molecule with the nickel metal surface. As the reaction temperature increases to 700°C, the formation rates of carbon nanofilaments from the C6-C16 paraffin hydrocarbons are equalized, which is associated with the increase in the intensity of thermal pyrolysis reactions of alkanes. The resulting olefins, first of all, ethylene and propylene, become major contributors to carbon formation.

Effect of SF 6 incorporation in the cyclic process on the low temperature deposition of carbon nanofilaments

To enhance the carbon nanofilaments (CNFs) formation density, SF 6 was incorporated in the source gases (C 2H 2/H 2) during the initial deposition stage. The source gases and SF 6 were manipulated as the cyclic on/off modulation of C 2H 2/H 2/SF 6 flow in a thermal chemical vapor deposition system. The characteristics of the CNFs formation on the substrate were investigated according to the different cyclic modulation processes and the substrate temperatures. By the incorporation of SF 6 in the cyclic process CNFs could be formed at the substrate temperature as low as 350 °C. Among the different processes, mutual alternating cyclic modulation process of C 2H 2 and SF 6 could most highly enhance the CNFs formation density. The cause for the enhancement in the characteristics of as-grown CNFs by the incorporation of SF 6 was discussed in association with the slightly enhanced etching ability by the incorporation of SF 6.

C2H2/H2/SF6기체들의 싸이클릭 유량 변조를 통한 탄소 나노 필라멘트 직경크기 조절

탄소나노필라멘트의 직경크기를 조절하기 위하여 증착 반응초기에 <TEX>$SF_6$</TEX>를 증착원료기체(<TEX>$C_2H_2$</TEX>, <TEX>$H_2$</TEX>)에 주입하였다. 증착 원료 기체와 <TEX>$SF_6$</TEX>를 열화학기상증착시스템에서 시간에 따라 싸이클릭 유량 변조시켰다. 싸이클릭 유량 변조 프로세스와 기판의 온도에 따라 기판위에 증착된 탄소나노필라멘트들의 특성을 조사하였다. 싸이클릭 에칭기간에 <TEX>$SF_6$</TEX>를 투입하자 탄소나노필라멘트의 직경크기는 급격히 감소하였다. 이러한 탄소나노필라멘트 직경의 크기 감소 원인은 <TEX>$SF_6$</TEX> 기체의 주입에 따른 에칭능력 향상에 기인하는 것으로 이해되었다. To control the diameter size of the carbon nanofilaments (CNFs), SF6 was incorporated in the source gases (<TEX>$C_2H_2/H_2$</TEX>) during the initial deposition stage. The source gases and <TEX>$SF_6$</TEX> were manipulated as the cyclic on/off modulation of <TEX>$C_2H_2/H_2/SF_6$</TEX> flow in a thermal chemical vapor deposition system. The characteristics of the CNFs formation on the substrate were investigated according to the different cyclic modulation processes and the substrate temperatures. By <TEX>$SF_6\;+\;H_2$</TEX> flow injection during the cycling etching interval time, the diameter size of CNFs was extremely decreased. The cause for the decrease in the diameter size of the individual CNFs by the cyclic on/off modulation process of <TEX>$C_2H_2/H_2/SF_6$</TEX> flow was discussed in association with the slightly enhanced etching ability by the incorporation of <TEX>$SF_6$</TEX>.

Enhancement of Carbon Nanofilaments Formation Density and the Surface Electrical Conductivity by the Gas Phase Composition Cycling

For the enhancement of carbon nanofilaments (CNFs) formation density and the surface electrical properties, the source gas flow was intentionally manipulated as the cyclic on/off modulation of C2H2/H2 flow in a thermal chemical vapor deposition system. CNFs formation density on the substrate and surface electrical conductivity of as-deposited substrate were investigated as a function of the cyclic modulation interval time for one cycle, namely the time for H2 + C2H2 flow (C2H2 flow on) plus the time for H2 flow (C2H2 flow off). The optimal interval time for the enhancement of both CNFs formation density and the surface electrical conductivity was around 3.5 minutes. The cause for the variation in the characteristics of CNFs according to the interval time of the cyclic modulation process was discussed in association with the variation of the remaining time of pure hydrogen gas concentration.

Effect of the On/Off Cyclic Modulation Time Ratio of C2H2/H2 Flow on the Low Temperature Deposition of Carbon Nanofilaments

Low temperature (less than 600 degrees C) deposition of carbon nanofilaments (CNFs) could be achieved on the silicon oxide substrate by thermal chemical vapor deposition system. We used Fe(CO)5 as the catalyst precursor for CNFs formation. For the enhancement of CNFs formation density, the source gas flow was intentionally manipulated as the cyclic on/off modulation of C2H2/H2 flow during the initial deposition stage. The CNFs formation density on silicon oxide substrate could be much enhanced by the cyclic modulation process having the higher growing/etching time ratio (180/30 s). Furthermore, the lattice structures of CNFs developed into carbon nanotubes at the higher growing/etching time ratio (180/30 s) case. The solely hydrogen gas feeding (C2H2 flow off) time during the initial deposition stage seems to play an important role for the variation in the CNFs formation characteristics by the cyclic modulation process.

Formation of Carbon Nanofilaments by Metal Dusting

Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007

State of disperse alloy particles catalyzing hydrocarbon decomposition by the carbide cycle mechanism: TEM and EDX studies of the Cu-Ni/Al2O3 and Cu-Co/Al2O3 catalysts

The state of disperse bimetallic alloy particles in the Cu-Ni/Al2O3 and Cu-Co/Al2O3 catalysts during their carbonization in butadiene-1,3 is studied by high-resolution electron microscopy and energy-dispersive X-ray analysis. During the formation of carbon nanofilaments by the carbide cycle mechanism, the catalyst is in a dissipative state such that the bimetallic particles vary in composition and have an anomalous component distribution in their bulk. The extrapolation of this state provides insight into the processes occurring in the dissipative system.

Effect of zinc added to the Co/Al2O3 catalyst on the formation of carbon nanofilaments from methane and butadiene-1,3

The dynamics of carbon nanofilament growth from methane and butadiene-1,3 on Co-Zn/Al2O3 catalysts have been studied in the temperature range 500–750°C. The rate-limiting step in the growth of nanofilaments from butadiene is carbon atom diffusion through the bulk of the metal particle. In the case of methane, the process is controlled by one of the stages of hydrocarbon decomposition on the metal particle. The structure and morphology of the nanofilaments (composites) forming by the carbide cycle mechanism on fine zinc-promoted cobalt particles have been studied by electron microscopy and X-ray diffraction. The morphology and crystallographic properties of the nanofilaments depend on the ratio of the hydrocarbon decomposition rate (which is determined by the nature of the hydrocarbon) to the rate at which carbon atoms diffuse from their formation sites to the nanofilament formation sites (which is determined by the nature of the metal particle and by the carbon diffusion coefficient in the particle bulk). The properties of the resulting carbon nanofilaments can be controlled by varying the nature of the hydrocarbon to be decomposed and the reaction temperature and by introducing another metal into the cobalt particles.

Fullerenes as precursors of carbon-based materials

The interaction of fullerite with continuous and pulsed laser radiation is presented. In both cases cage-opening reactions and coalescence of fullerene fragments are observed, although the reaction mechanism involved and the final products are radically different. Under continuous high fluence (Ar +) laser irradiation, a photothermal oxidation reaction is responsible for the fullerene breaking and oxygen is present in large quantities in the resulting carbon phase. Surface etching is also observed. The irradiation of fullerite with Nd:Yag laser pulses induces strong luminescence associated with structural modifications, in particular the formation of carbon nano-filaments and bundles. We suggest that, in this case, the interaction of electronic photo-excitations is responsible for the breaking of C 60 units.