Growth Of Carbon Filaments Research Articles

Controlling carbon nanofibre morphology for improved composite reinforcement

Carbon nanofibres were grown at low temperatures on a novel fibre catalyst, palladium particles, using a novel method, growth in fuel rich combustion conditions. The growth of carbon filaments by this process was found to be remarkably rapid. Preliminary analysis of how changes in operating parameters, for example gas flow rate and reactant ratio, showed that fibre diameter and other factors such as helicity are very sensitive to the precise conditions employed, suggesting simple means to impact overall properties of the fibres. The ability to control diameter, helicity, and surface roughness are discussed with possible importance to advanced composites.

Growth of filamentous carbon by decomposition of ethanol on nickel foam: Influence of synthesis conditions and catalytic nanoparticles on growth yield and mechanism

This paper presents nano-sized carbon filaments grown by catalytic decomposition of ethanol on polycrystalline nickel foam. Our study is focused on the effects of synthesis conditions on the formation of filamentous carbon. The formation rate of filamentous carbon increased as the supply of liquid ethanol into the furnace and the synthesis time increased during growth process, whereas H 2 concentration in carrier gas had the opposite effect. The most favorable temperature for the highest formation rate was around 600 °C. We investigated the role of catalytic nanoparticles in the growth mechanism of filamentous carbon. Tip growth was the primary mechanism responsible for growth of the filamentous carbon; octopus-like carbon filaments were formed on large nanoparticles of about 100 nm in diameter. It was also discovered that surface break-up on the nickel foam precipitated the formation of filamentous carbon. We detected the presence of Ni 3C as synthesis conditions after a long exposure to ethanol.

Catalyst-free growth of needle-shaped carbon filaments at low temperature in a near-critical binary fluid

Ultraviolet laser irradiation into a mixture of supercritical carbon dioxide and benzene at 266 nm wavelength in an electric field of 8 kV/cm resulted in production of needle-shaped carbon filaments at low temperature, observed T min being 33.5 °C. Structures could only be produced under near- and supercritical conditions, and did not appear in sub-critical conditions, i.e., in the gas–liquid coexistence state. The filaments are straight, with maximum observed length approximately 20 μm, and are 0.5–1 μm in diameter at the base, and approximately 300 nm at the tip. They do not bundle together, but grow individually, aligned radially from the sharp edges of the silicon substrates attached to the electrodes. No catalyst particles were found inside, at the base or the tip of the filaments. We suggest that the primary facilitator in the filament growth is the local electric field strength, which corresponds to the substrate morphology, possibly further assisted by the unusual properties of the critical fluid phase, such as increased miscibility and high isothermal compressibility. The structures were observed by scanning electron microscopy, and further characterized by energy dispersive X-ray spectroscopy and visible and ultraviolet Raman spectroscopy.

Carbon deposited on Ni/Ce[sbnd]Zr[sbnd]O isooctane autothermal reforming catalysts

On-board reforming of liquid fuels is attractive for fuel-cell-powered auxiliary power unit (APU) applications in heavy-duty vehicles. However, a technology barrier is catalyst deactivation due to carbon deposition. In this work, we studied carbon identification and the effect of nickel loading on carbon growth during isooctane autothermal reforming (iC8-ATR) over Ni-supported CeZrO (Ni/CZO) catalysts. The crystallographic phase, type, deposition amount, and morphologies of deposited carbon were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Two types of carbon, coating and filamentous, were identified by SEM, with binding energies of 281.6 and 284.5 eV in C1s XPS spectra and oxidation temperatures of 392 and 572 °C in TGA curves, respectively. Both coating and filamentous carbon were X-ray amorphous for nickel loadings ⩽5%, even though deposited carbon reached up to 60.2%. Over the entire nickel loading range of 1–15%, with corresponding nickel crystal sizes of 30–103 nm, the coating carbon formation rates were low (0.04 g/m2Ni h) and independent of nickel particle size. The growth rates of filamentous carbon increased with increasing nickel particle size. Filamentous carbon growth may require not only a critical nickel particle size, but also a certain minimum number of nickel particles per unit support area. The results suggest a strategy for inhibiting the growth of filamentous carbon on Ni/CZO catalysts during autothermal reforming of liquid fuels.

A detailed model for the flame synthesis of carbon nanotubes and nanofibers

Many experimental investigations of CNT/CNF flame synthesis have been recently reported. However, there are as yet no comprehensive models regarding their formation, growth or structure. Herein, a CNT/CNF growth rate model is proposed that is applicable for any method of CNT/CNF production (although our particular interest lies in flame synthesis in ethylene/air flames). While it is usual for most existing models to consider only a single carbon-carrying gas that contributes towards carbon deposition, our extended model can consider a complex hydrocarbon mixture that can mimic a flame environment. The model shows that before carbon nucleation is initiated, the surface density of carbon atoms increases as they are added through diffusion from the leading face of the catalyst nanoparticle. Over time, the diffusion potential decreases due to a reduction in the carbon atom concentration gradient. However, with the onset of nucleation and growth, diffusion is reinstated as the major driving potential for carbon atom transport through the nanoparticle. Steady carbon deposition and filament growth occurs once there is a stable carbon cluster size due to nucleation. The results support our hypothesis that flame synthesis can be a much faster and higher throughput process than CVD. The CNT/CNF growth rate decreases with increasing height above the burner. Decreasing temperature at higher locations leads to slower catalyst deactivation, but the CO concentration, which is the major contributor to carbon deposition, also decreases with increasing height above the burner. One of the major findings in this work is the contribution of CO to CNT formation by flame synthesis. The concentration of hydrocarbons in the vicinity of the toroidal zone near, which most of the CNT growth is observed, is negligible compared to CO concentration. Our results provide a basis to conduct future multiscale simulations of CNT/CNF growth.

The dependence of the oriented growth of carbon filaments on the intensity of a magnetic field

Carbon filaments have been obtained by the thermal decomposition of mesitylene in the presence of a strong magnetic field. The shape dependence of the pristine carbon filaments on increasing the magnetic field is demonstrated. The proposed mechanism for filament growth involves the formation of a colloid-like system during dehydrogenation and dissociation of mesitylene. The imposed orientation of graphitic clusters (basic structural units) in the strong magnetic-field forces those located at the curved surfaces forming the tips of the filaments to expose their reactive edges, facilitating the observed preferential growth of filaments.

Implementation of an Electric Field (AC and DC) for the Growth of Carbon Filaments via Reaction under Autogenic Pressure at Elevated Temperatures of Mesitylene without Catalyst or Solvent

The electric-field-directed growth of carbon filaments by reactions under autogenic pressure at elevated temperatures (RAPET) of mesitylene without a catalyst, a support, or a solvent is demonstrat...

Relationship between the Growth of Carbon Nanofilaments and Metal Dusting Corrosion

Raman scattering, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were used to study the mechanism of the catalytic crystallization of carbon and metal dusting corrosion. A mechanism is proposed for both metal dusting and the growth of carbon fibers. Carbon cannot crystallize well by deposition from carburizing gases at low temperature without catalytic activation because of its strong C−C bonds and high melting temperature. To form good crystalline carbon, the carbon atoms must dissolve, diffuse through metal particles, and crystallize on an appropriate facet that can act as a template to help the epitaxial growth of carbon crystals. In this process, metal particles are liberated from the pure metal and alloys. This liberation leads to the metal dusting phenomenon. The catalytic growth of carbon filaments is due to the transportation of carbon from one facet of a metal or carbide particle that favors carbon deposition to another facet that favors carbon precipitation...

A kinetic model of CH 4 decomposition and filamentous carbon formation on supported Co catalysts

A kinetic model of CH 4 decomposition on supported Co catalysts, that describes the stable catalyst activity associated with filamentous carbon formation and the catalyst deactivation associated with the formation of encapsulating carbon, is presented. The rate of carbon nucleation at the tailing face of the supported metal particle, a precursor for the growth of filamentous carbon, is described by a cluster nucleation model. The deactivation or stable activity, observed after an initial rate increase that is associated with carbon nucleation, is explained by the competition between the rate of carbon diffusion through the metal particle and filamentous carbon growth, versus the rate of encapsulating carbon formation on the leading face of the particle. The site density profiles through the metal particle at different reaction times are also calculated, and the effect of metal particle size on the initial CH 4 decomposition rate is quantified.

Multifunctional Catalysts for Singlewall Carbon Nanotube Synthesis

Single-wall carbon nanotubes (SWNTs) are usually produced with the help of metal catalysts that first break down various carbon species such as graphite, carbon clusters, amorphous carbon, or hydrocarbons and then release them in the form of SWNTs. These catalysts therefore must perform several functions in order to produce high quality, high yield, long SWNTs, and the finesse of these catalysts is controlled by the balance between those functions. Such a production is critical to future applications such as telecommunication, molecular electronics, sensing, and mechanical devices. In general, multicomponent materials such as bimetallic nanoparticles (bi-MNPs) are much better catalysts than those made of single component materials because the two metals in the biMNPs can enhance certain functions by playing complementary catalytic roles. Furthermore, some functions can only be offered by bi-MNPs because the two metals are structurally arranged in such specific manners that the combined properties would otherwise be impossible to access by monometallic nanoparticles (mono-MNPs). Bimetallic catalysts have been proven superior with improved activity, selectivity, stability, and resistance to poisoning than their monometallic counterparts.1,2 For example, bi-MNPs are preferred heterogeneous catalysts in petroleum reforming processes.3 Bi-MNPs of large diameters (>50 nm) have also been used to catalyze the growth of carbon filaments.4 Those catalysts have also been used in ammonia synthesis

Isotopic and kinetic assessment of the mechanism of reactions of CH 4 with CO 2 or H 2O to form synthesis gas and carbon on nickel catalysts

Kinetic and isotopic measurements for catalysts and conditions that rigorously excluded transport and thermodynamic artifacts led to a common sequence of elementary steps for reactions of CH 4 with CO 2 or H 2O and for its stoichiometric decomposition on Ni/MgO catalysts. Turnover rates for forward reactions of CH 4/CO 2 and CH 4/H 2O mixtures were proportional to CH 4 pressure (5–450 kPa) and independent of the partial pressure of the CO 2 or H 2O coreactants (5–450 kPa). These turnover rates and their first-order rate constants and activation energies are also similar to those measured for CH 4 decomposition, indicating that these reactions are mechanistically equivalent and that CH bond activation is the sole kinetically relevant step in all three reactions. These conclusions were confirmed by identical CH 4/CD 4 kinetic isotope effects ( k H/ k D=1.62–1.71) for reforming and decomposition reactions and by undetectable H 2O/D 2O isotopic effects. The kinetic relevance of CH bond activation is consistent with the relative rates of chemical conversion and isotopic mixing in a CH 4/CD 4/CO 2 mixture and with the isotopic evidence for the quasi-equilibrated nature of coreactant activation and H 2 and H 2O desorption obtained from reactions of CH 4/CO 2/D 2 and 12CH 4/ 12CO 2/ 13CO mixtures. These quasi-equilibrated steps lead to equilibrated water–gas-shift reactions during CH 4 reforming, a finding confirmed by measurements of the effluent composition. These elementary steps provide also a predictive model for carbon filament growth and identify a rigorous dependence of the carbon thermodynamic activity on various kinetic and thermodynamic properties of elementary steps and on the prevalent concentrations of reactants and products, specifically given by P CH 4 P CO/ P CO 2 (or P CH 4 P H 2 / P H 2O ) ratios. These mechanistic features on Ni surfaces resemble those previously established for supported noble metal catalysts (Rh, Pt, Ir, Ru). These direct measurements of CH bond activation turnover rates allowed the first direct and rigorous comparison of the reactivity of Ni and noble metal catalysts for CH 4-reforming reactions, under conditions of strict kinetic control and relevant commercial practice and over a wide range of compositions and metal dispersions.

Metal–support interaction effects on the growth of filamentous carbon over Co/SiO 2 catalysts

The addition of BaO, La 2O 3 and ZrO 2 to the SiO 2 support of a 12 wt.% Co/SiO 2 catalyst modifies the reduction behavior of Co species and leads to changes in metal dispersion. These changes are due to a modified metal–support interaction (MSI) between cobalt species and the Me x O y /SiO 2 (Me x O y : BaO, ZrO 2 and La 2O 3) support. Catalyst characterization by temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) have been used to determine the relative strength of the MSI and the results suggest an increasing MSI in the order Co/SiO 2≈Co/BaO/SiO 2<Co/La 2O 3/SiO 2<Co/ZrO 2/SiO 2. The rate of catalyst deactivation during methane decomposition (CH 4⇔C+2H 2) is shown to increase with increasing MSI. Analysis of the used catalysts also shows that an increasing rate of deactivation correlates with an increasing amount of graphitic carbon versus carbidic carbon on the used catalyst. It is suggested that an increase in graphitic carbon is a consequence of a strong MSI that limits carbon removal from the metal surface by filament formation. Consequently, graphitic, encapsulating carbon is formed from the carbon deposited during methane decomposition, leading to deactivation of the catalyst.

Catalytic decomposition of methane over Ni-Al 2O 3 coprecipitated catalysts : Reaction and regeneration studies

The catalytic decomposition of methane over nickel catalysts is a potential alternative route to steam reforming or partial oxidation for the production of hydrogen from natural gas and other feedstocks. In the present paper, we report the results of characterization and catalytic behaviour of a Ni(30%)/Al 2O 3 catalyst during the catalytic decomposition of methane. The influence of the operating and reduction temperatures and feed composition on the methane conversion, hydrogen production and coking rate has been studied. The effects of the regeneration cycles with oxygen on activity and carbon formation are also investigated. It has been shown that H 2 inhibits both the carbon filament formation and the encapsulation of metallic particles by coke. An increase in the reaction temperature increases both the deactivation rate and the growth rate of filaments. However, at high reduction temperatures, there is a decrease in the number of filaments formed due to sintering of the Ni particles. A kinetic model has been developed for the prediction of H 2 production and of carbon, taking into account both stages of carbon formation, nucleation and filament growth.

Growth of Filamentous Carbon from the Surface of Ni/SiO2 Doped with Alkali Metal Bromides

The growth of ordered filamentous carbon, catalytically generated from the decomposition of ethylene, has been studied over the temperature range 673–898 K using an 11% w/w Ni/SiO2 catalyst doped to varying degrees (0.1–9.3% w/w) with a range of alkali metal bromides. The effect of these alkali metal/halogen adatoms in promoting/inhibiting carbon growth has been assessed and variations in the associated carbon structural characteristics have been examined. The introduction of Li consistently promoted filamentous carbon growth (where 723 K<T<823 K) while the presence of Na, K, Rb, or Cs resulted in an equivalent or lower carbon yield. The degree of carbon deposition was strongly dependent on the nature and loading of the alkali metal, the Ni/Br ratio in the activated catalyst, and reaction temperature; conditions for optimum carbon growth are identified. The response of carbon yield and structural order to alkali bromide doping is discussed in terms of Ni particle electronic structure and metal/support interaction(s). High-resolution transmission electron microscopy (HRTEM) has been used to probe the filamentous carbon structure and the dispersion/morphology/size of the supported Ni crystallites. Highly curved and helical filaments predominated over the doped (particularly CsBr) samples and this is attributed to a disruption in carbon diffusion through the Ni particle caused by a spreading/coating of the particle by the alkali adatom. Temperature-programmed oxidation studies have highlighted the changes in the graphitic nature of the carbon due to catalyst doping; the results are consistent with the TEM analysis.

A New Mechanism for the Catalytic Growth of Carbon Filaments and Metal Dusting Phenomenon

ABSTRACTRaman scattering and X-ray diffraction were used to study the mechanism of the catalytic crystallization of carbon and metal dusting. The following new mechanism is proposed for metal dusting and the growth of carbon filaments. Carbon cannot crystallize well by deposition from carburizing gases at low temperature without catalytic activation because of its strong C-C bonds and high melting temperature. The poorly crystalline carbon has higher free energy than that of good crystalline carbon. To form good crystalline carbon, carbon atoms have to dissolve, diffuse through metal particles, and crystallize on a proper lattice plane that can act as a template to help the epitaxial growth of carbon crystals. Metal particles are liberated from the pure metal and alloys in this process, which leads to metal dusting attack. The decrease of free energy from highly disordered to well crystalline carbon is the driving force for metal dusting and carbon filament growth through metal particles.

Filamentous Carbon Growth on Nickel/Silica: Potassium and Bromine as Catalyst Promotors

In a previous Communication, we reported the catalytic growth of highly ordered carbon filaments during the hydrodechlorination of chlorobenzene over nickel/silica at the remarkably low temperature of 553 K. This low temperature growth was attributed to the presence of bromine and potassium on the catalyst surface, which facilitated a restructuring of the active sites leading to a destructive chemisorption of the reactant that preceded the dissolution and precipitation of carbon in an ordered fashion. In this paper, we examine the effect of bromine and potassium built into (by impregnation with KOH, HBr and KBr) a Ni/SiO2 parent sample on filamentous carbon production through ethylene decomposition. The parent catalyst delivered a low yield of carbon (0.2 g(C)g(cat)(-1)) and promoted the competing hydrogenation (to ethane) step to the same degree; hydrogenolysis to methane represented the only other significant reaction. The nature of the carbon deposition has been characterised by high-resolution transmission electron microscopy (HRTEM) and temperature-programmed oxidation (TPO). Impregnation with KOH resulted in little change in carbon yield but the hydrogenolysis step was completely suppressed. A combination of CO chemisorption/temperature programmed desorption has been employed to probe the electronic structure of the nickel sites where the presence of potassium limited CO uptake in contrast to surface bromine which served, through electron withdrawal, to strengthen the CO-Ni interaction. Incorporation of bromine into the catalyst resulted in a marked increase in nickel particle size and a dramatic enhancement of carbon yield; the sample treated with KBr delivered the appreciably higher carbon yield of 7.6 g(C)g(cat)(-1)). While the presence of bromine on the surface served to enhance carbon deposition, the additional incorporation of potassium raised the degree of order in the carbon growth.

Growth of filamentous carbon from lithium and cesium bromide doped Ni/SiO2 catalysts

The catalytic growth of filamentous carbon, from the decomposition of ethylene, has been studied over the temperature range 673–898 K using Ni/SiO2 doped with LiBr and CsBr. Promotion/inhibition effects due to the presence of these adatoms have been assessed; an optimum alakali metal loading is reported for Li, while the presence of Cs inhibited the reaction at all temperatures. Transmission electron microscopy has been used to characterise the ordered nature of the filamentous carbon growth.

The Relationship between the State of Active Species in a Ni/Al2O3Catalyst and the Mechanism of Growth of Filamentous Carbon

The formation of active particles and their changes in the course of 1,3-butadiene decomposition on a Ni/Al2O3catalyst at temperatures from 400 to 800°C were studied by high-resolution electron microscopy. It was found that carbon filaments of different types were formed at 400–800°C. The growth of thin filaments (20–30 nm in diameter) takes place at 400–600°C on a conical Ni particle located at the growing end of the filament, whereas di-symmetrical filaments 50–100 nm in diameter grow on biconical metal particles. As the carbonization temperature was increased to 700–800°C, graphite nanotubes 5–20 nm in diameter were formed. It was found that the mechanism of formation and the structure of filaments are related to the state of catalytically active species, which consist of a solid solution of carbon in the metal. It is suggested that the metastable surface nickel carbide Ni3C1 – xis an intermediate compound in the catalytic formation of graphite filaments from 1,3-butadiene. Upon termination of the reaction, the metastable Ni3C1 – xmicrophase is decomposed with the formation of hexagonal nickel microinclusions. The role of epitaxy in the nucleation and growth of a graphite phase on the metal is discussed. Models are presented for the growth of structurally different carbon filaments depending on the formation of active metal species at various temperatures. Considerable changes in the structure of carbon and the formation of nanotubes at 700–800°C are related to the appearance of a viscous-flow state of metal–carbon particles.

Carbon Filament Growth on Fully Formulated Pd/Rh Automotive Catalysts

Abstract Automotive exhaust catalysts are typically operated under stoichiometric conditions to minimize nitrogen oxide, hydrocarbon, and carbon monoxide pollutants. These catalysts do not form carbon filaments under normal operating conditions. In development of catalysts, however, a stabilization procedure is sometimes utilized on used catalysts (dynamometer or vehicle) to purge the catalyst of sulfur prior to measuring the catalytic activity in sweep and light-off testing. The stabilization procedure consists of running the catalyst under rich (excess fuel) conditions for 0.5hr. This study documents the existence of carbon filaments due to the stabilization procedure and discusses the effect of filaments on subsequent testing. Two separate catalysts were used in this study. The first was a 50,000 mile vehicle aged catalyst that had also been through the stabilization procedure and then sweep and light-off tested. The other was only dynamometer aged for 120hr at 850°C (1560°F).

Symmetric twisted carbon filaments formed from butadiene-1,3 on Ni–Cu/MgO-catalyst: growth regularities and mechanism

High-resolution electron microscopy has been used to study the regularities of formation of symmetric helical carbon filaments from Ni–Cu/MgO-catalyzed butadiene-1,3 decomposition at 450°C. Dissymetrical spirally twisted carbon filaments have been shown to grow from a single Ni–Cu alloy particle. As the filaments grow, a microphase of metastable nickel carbide Ni 3C forms on the particles' frontal side, whereas the crystal particle of Ni–Cu alloy possesses a series of twinning planes (111) that is parallel to the dissymmetry plane of the crystal and separates the blocks of twins with a face-centered structure. The mechanism of symmetric helical filamentous carbon growth has been suggested.