Layer Of Carbon Nanofibers Research Articles

Silicon based microreactors for catalytic reduction in aqueous phase: Use of carbon nanofiber supported palladium catalyst

Carbon nanofiber layers containing palladium particles were synthesized inside the channels of a silicon based microreactor. Pd catalyst was prepared via dry impregnation using palladium acetylacetonate precursor solution in toluene. Several catalyst loadings were impregnated on five different CNF-layer thicknesses in the range 10–20μm: for all loadings a relatively large average Pd particle size of ∼8nm was obtained. The lateral and axial distribution of these metallic nanoparticles was uniform across the support for CNF-layer thicknesses up to 13μm. For larger thicknesses, the presence of a dense carbonaceous layer underneath the open CNF-layer negatively affects the accessibility for Pd nanoparticles, resulting in a less good lateral distribution. External and internal mass transfer properties were evaluated, by carrying out hydrogenation of nitrite ions in aqueous phase. For sufficiently thin CNF-layers (⩽13μm) hydrogenation was under kinetic control, indicating absence of mass transfer limitations and thus good accessibility and performance of the Pd containing CNF layers for reactant species.

Influence of reaction parameters on the attachment of a carbon nanofiber layer on Ni foils

Dense carbon (C) and entangled carbon nanofiber (CNF) layers were deposited on nickel foils by decomposition of ethylene in presence of different H2 concentrations at 450°C for different reaction times. Both C and CNF layer thicknesses increase with time, but samples pre-oxidized at 500°C normally lead to thinner CNF layers and thicker C layers, as compared to samples pre-oxidized and reduced at 700°C. The mechanical stability of CNFs decreases with growth time, especially for oxidized-reduced samples. The addition of H2 creates a maximum in the CNF thickness that coincides with a minimum in the C layer thickness, at 5% H2 for samples oxidized at 500°C and at 20% H2 for samples oxidized-reduced at 700°C. CNF layer stability increases with C layer thickness but decreases with CNF layer thickness. The ratio between the C layer thickness and the CNF thickness determines in the end the mechanical stability of the CNF layer.

Sorbitol Hydrogenolysis to Glycols over Carbon Nanofibers/Graphite-felt Composite-supported Ru Catalyst in a Trickle Bed Reactor

Carbon nanofibers were in-situ grown on graphite-felt, and then Ru was supported to synthesize Ru/CNFs/GF structured catalyst. The comparisons of pressure gradient and reaction results clearly indicated that the Ru/CNFs/GF structured catalyst was more proper than powdered Ru/CNFs catalyst for sorbitol hydrogenolysis in a trickle bed reactor. For the structured catalyst, increasing the linear velocities would increase the sorbitol conversion, but only slightly change the glycols selectivity, which was due to the intensified mass transfer. Nevertheless, increasing the thickness of the carbon nanofibers layer would significantly decrease the glycols selectivity, which was due to the increased diffusion distance for hydrogen and glycols in the mesopores of the carbon nanofibers layer.

Heterogeneous catalysis in a microchannel using a layer of carbon nanofibers on the channel wall

This paper presents the increase of the overall reaction rate of a heterogeneously catalyzed multi-phase reaction using a carbon nanofiber (CNF) based catalyst with a factor of 3.5–4 compared with an unsupported flat plate catalyst in a microreactor. This was done by quantifying the hydrogen reaction rates for three different CNF layers with different layer and fiber thicknesses and permeability attached to a microchannel wall at different flow regimes and comparing them with the reaction rate of an unsupported flat plate catalyst. The CNF layer with the most open structure and the largest CNF layer thickness shows an increase of the overall reaction rate with a factor of approximately 3.5–4 in comparison with the unsupported flat plate catalyst, whilst keeping the selectivity towards the intermediate product over 95%. Furthermore, the influence of the CNF layers on the gas hold-up and the flow regimes were investigated. The gas hold-up is linearly dependent on the relative gas velocity according to the Armand correlation for all the tested CNF layers. The CNF layers do not influence the flow patterns, and the Taylor/slug, slug/ring, ring and annular flow regimes are observed for all CNF layers.

Synthesis of carbon nanofiber/carbon-foam composite for catalyst support in gas-phase catalytic reactions

A carbon foam was prepared by self-bubbling using mesophase pitch as a precursor. To improve its specific surface area, the carbon foam was oxidized with 65 mass% HNO 3, and then a layer of carbon nanofibers (CNFs) was grown on the pore walls using chemical vapor deposition. The specific surface area and thermal conductivity of the carbon foam and CNF/carbon foam composite are 40 and 198 m 2/g and 107 and 125 W/mK, respectively. This CNF/carbon-foam composite can be used as a catalyst support for gas-phase catalytic reactions.

Wettability of carbon nanofiber layers on nickel foils

Carbon nanofiber (CNF) layers have been directly synthesized on nickel foils by chemical vapor deposition at 450°C using different H2 concentrations and reaction times. The addition of 5% H2 produces thicker, rougher and more porous CNF layers than when 1% H2 is used. The roughness and porosity increases with reaction time when 5%, 10% or 20% H2 are used; however, this effect is less pronounced when 1% H2 is used. CNFs are 50–55nm in diameter and have a fishbone type structure. We have studied the influence of CNF layer thickness, porosity and surface roughness on the interaction with water by measuring the contact angle. The water wetting properties of the samples are more significantly influenced by the CNF layer thickness than both surface roughness and porosity. When the CNF layer is thicker than ca. 20μm, the surface is hydrophobic and the contact angle increases with surface roughness and porosity. When the CNF layer is thinner than ca. 20μm, the surface is hydrophilic and the contact angle decreases with increasing surface roughness and porosity. This behavior is attributed to penetration of water, making contact with the hydrophilic C layer between the CNF layer and the foil.

Hydrodynamics and mass transfer in carbon-nanofiber/graphite-felt composite under two phase flow conditions

A carbon nanofiber (CNF)/graphite felt composite was synthesized by growing CNFs on the surface of graphite fibers and was used as the packing of a fixed bed reactor under two phase flow conditions. The pressure drop, axial dispersion and mass transfer in the liquid were studied by experiment and by piston dispersion exchange (PDE) model. It was shown that the pressure drop and total liquid up could be predicted by the slit model in an acceptable accuracy. The axial dispersion in the liquid phase in the composite and the mass transfer between the dynamic and static liquid are higher than in the packed bed of solid particles owing to the porous and fluffy CNF layer on the carbon felt fiber.

Fischer–Tropsch Synthesis on Hierarchically Structured Cobalt Nanoparticle/Carbon Nanofiber/Carbon Felt Composites

The hierarchically structured carbon nanofibers (CNFs)/carbon felt composites, in which CNFs were directly grown on the surface of microfibers in carbon felt, forming a CNF layer on a micrometer range that completely covers the microfiber surfaces, were tested as a novel support material for cobalt nanoparticles in the highly exothermic Fischer-Tropsch (F-T) synthesis. A compact, fixed-bed reactor, made of disks of such composite materials, offered the advantages of improved heat and mass transfer, relatively low pressure drop, and safe handling of immobilized CNFs. An efficient 3-D thermal conductive network in the composite provided a relatively uniform temperature profile, whereas the open structure of the CNF layer afforded an almost 100 % effectiveness of Co nanoparticles in the F-T synthesis in the fixed bed. The greatly improved mass and heat transport makes the compact reactor attractive for applications in the conversion of biomass, coal, and natural gas to liquids.

Synthesis of carbon foam covered with carbon nanofibers as catalyst support for gas phase catalytic reactions

Mesophase pitch based carbon foam (MPCF) covered with a layer of carbon nanofibers (CNFs) was prepared as catalyst support for gas phase catalytic reactions. Owing to the CNF layer, the specific surface area of MPCF increases from 40 to 198m2/g. This kind of catalyst support plays an important role in the enhancement of mass/heat transfer due to the large external surface area, high porosity and high thermal conductivity. When selective catalytic NO reduction was taken as a model reaction, more than 90% NO conversion could be achieved in a wide temperature range of 180–220°C over MnOx–CeO2/MPCF–CNF catalyst.

Ruthenium catalyst on carbon nanofiber support layers for use in silicon-based structured microreactors, Part I: Preparation and characterization

The preparation and characterization of ruthenium catalytic nanoparticles on carbon nanofiber (CNF) support layers via homogeneous deposition precipitation (HDP) and pulsed laser deposition (PLD) is presented. Prior to ruthenium deposition the CNF layers were functionalized via liquid phase oxidation treatment using nitric acid at 90°C. This acid treatment not only effectively removed accessible CNF-growth catalyst, but also resulted in the formation of oxygen containing functional groups on the external surface of CNFs. A variety of characterization techniques, viz. TEM, XRD, XRF, XPS, and point-of-zero-charge (PZC) measurements were used to analyze the influence of the oxidation pretreatment on physico-chemical properties of CNF layers qualitatively and quantitatively. HDP yielded a very sharp size distribution (∼85% of the particles had a diameter of 1.0–1.5nm), whereas PLD had a less narrow distribution (the diameter of ∼75% of the particles was 1–3nm). Both methods yielded a ruthenium loading of 2.3±0.1wt.%, and in particular HDP showed uniform anchoring of particles throughout the thickness of the CNF layer. Using optimal conditions, the space in a silicon-based microreactor channel was efficiently filled with open, entangled CNF layer, which were used as anchor points for Ru using HDP and PLD.

Synthesis of Platelet Carbon Nanofiber/Carbon Felt Composite on in Situ Generated Ni−Cu Nanoparticles

A platelet carbon nanofiber/carbon felt composite with a high surface area (>250 m2/g) is synthesized over a carbon felt supported Ni−Cu catalyst using ethane−hydrogen mixtures at 923 K. A uniform layer of carbon nanofibers is thus achieved via an in situ generation and dispersion of Ni−Cu nanoparticles induced by the surface reaction of hydrocarbon decomposition. This represents a cost-effective and flexible method without the requirement for presynthesis of uniformly distributed nanoparticles. Both thermodynamic analysis and experimental observation reveal that, in the initial growth period, formation of a molten phase of the nanosized Ni−Cu−C phase promotes the fragmentation of Ni−Cu parent particles of micrometer size. A unique octopus-like growth mode is observed during the formation of graphene layers from this liquid-like Ni−Cu−C system. A comparative study between Ni and Ni−Cu catalysts is performed. It is found that the addition of Cu in the Ni−Cu catalysts can enhance the fragmentation ability of the parent Ni−Cu particles when contacting with hydrocarbon. A treelike growth mode of carbon nanofibers grown on Ni−Cu catalysts is observed due to the further fragmentation of Ni−Cu particles during carbon nanofiber growth. In contrast to Ni−Cu catalysts, two different growth mechanisms on the Ni catalyst are found: a governing tip-growth mechanism by particles smaller than 50 nm and an octopus-like growth mechanism by larger particles (50−100 nm). The graphene sheet orientation in the carbon nanofibers depends on the composition of the metal particle; fishbone carbon nanofibers are obtained with carbon felt supported Ni catalysts, while platelet carbon nanofibers were obtained with Ni−Cu catalysts. The formation of platelet carbon nanofibers from the Ni−Cu catalysts is ascribed to a higher degree of interface wetting between the Ni−Cu particle and the produced graphene sheets.

Enhanced liquid–solid mass transfer in microchannels by a layer of carbon nanofibers

AbstractThis paper demonstrates that the observed rate of reaction of the liquid-phase selective hydrogenation of an alkyne is higher for an open and rough carbon nanofiber (CNF) layer positioned on a microchannel wall than for an unsupported flat plate catalyst or dense and smooth CNF layers. This enhancement can be contributed to two phenomena: (1) the kinetic reaction rate increases with a factor of 1.4, due to an increase of the open and freely accessible surface area of the CNF layer available for catalyst deposition; (2) the liquid–solid mass transfer increases with a factor of 2.8, due to an increase in the surface roughness of the CNF layer and the formation of large spherical islands of clustered CNFs. A thin layer of CNFs on a microchannel wall can therefore successfully be used to increase the overall rate of reaction per reactor volume compared to an unsupported flat plate catalyst.

Corundum impregnation conditions for preparing supported Ni catalysts for the synthesis of a uniform layer of carbon nanofibers

Impregnation techniques for corundum (SBET = 0.5 m2/g) as a support for Ni catalysts for C3–C4 alkane pyrolysis into catalytic filamentous carbon (CFC) are compared. The effects of the following factors on the uniformity of the active component (Ni) deposition on the inert support and on the CFC yield (g CFC)/(g Ni) are reported: (1) pH of the nickel nitrate solution, (2) presence of aluminum(III) nitrate in the solution, (3) addition of viscosifying agents (glycerol, glucose, sucrose) to the solution, (4) catalyst calcination conditions before pyrolysis, and (5) catalyst drying technique. The surface morphology of the Ni catalysts and of the carbon deposits resulting from the catalytic pyrolysis of C3–C4 alkanes in the presence of hydrogen has been investigated by scanning electron microscopy. The optimum way of preparing the supported Ni catalysts is by carrying out the incipient wetness impregnation of corundum with a nickel nitrate solution (0.05–0.1 mol/l) containing glycerol (20–25 vol %), drying the product in a microwave oven, and burning away the glycerol before alkane pyrolysis.

Carbon nanofiber growth on carbon paper for proton exchange membrane fuel cells

Homogeneous deposition precipitation (HDP) of nickel has been investigated for the growth of carbon nanofibers (CNFs) on carbon paper for use in proton exchange membrane fuel cells as a gas diffusion layer. Selective CNF growth on only one side of carbon paper is required to transfer the generated protons on platinum catalyst fast enough to avoid any proton mass transfer losses. For this purpose, a mask deposition holder was designed to deposit the nickel hydroxide which is reduced to nickel with hydrogen before catalyzing the CNF growth. The deposition time effect on the catalyst loading and the effect of the catalyst loading variation on the yield of CNF growth on carbon paper were investigated. Both effects had linear dependence in the region of interest. The HDP of nickel on the carbon paper ensured strong attachment of nickel crystals on the carbon paper fibers keeping the porosity at a promising level with an acceptable BET area of CNFs. The HDP method provides fine-tuning in the CNF layer thickness only on one side of the carbon paper with good reproducibility in the deposition of nickel.

Dual formation of carpets of large carbon nanofibers and thin crystalline carbon nanotubes from the same catalyst-underlayer system

A dense, micron-tall layer of carbon nanofibers (CNFs) was grown above a layer of carbon nanotubes (CNTs) during the same synthesis using a thick cobalt catalyst (15 nm). The CNFs had large diameters (100 nm) and were amorphous while the CNTs had small diameter (10–20 nm) and were crystalline. Base growth mechanism was at play for both the nanofibers and the nanotubes. High-resolution transmission electron microscopy characterization suggested that the main mechanisms leading to the growth of the two structures were based on the dewetting of the catalyst layer and its subsequent alloying with the Ta underlayer. We can extend these principles to grow diverse carbon nanostructures during the same synthesis using appropriate multilayer thin films for different applications, especially for electrochemical cells and supercapacitors.

The production of a homogeneous and well-attached layer of carbon nanofibers on metal foils

Carbon nanofibers (CNFs) were deposited on metal foils including nickel (Ni), iron (Fe), cobalt (Co), stainless steel (Fe:Ni; 70:11wt.%) and mumetal (Ni:Fe; 77:14wt.%) by the decomposition of C2H4 at 600°C. The effect of pretreatment and the addition of H2 on the rate of carbon formation, as well the morphology and attachment of the resulting carbon layer were explored. Ni and mumetal show higher carbon deposition rates than the other metals, with stainless steel and Fe the least active. Pretreatment including an oxidation step normally leads to higher deposition rates, especially for Ni and mumetal. Enhanced formation of small Ni particles by in situ reduction of NiO, compared to formation using a Ni carbide, is probably responsible for higher carbon deposition rates after oxidation pretreatment. The addition of H2 during the CNF growth leads to higher carbon deposition rates, especially for oxidized Ni and mumetal, thus enhancing the effect of the reduction of NiO. The diameters of CNFs grown on metal alloys are generally larger compared to those grown on pure metals. Homogenously deposited and well-attached layers of nanotubes are formed when the carbon deposition rate is as low as 0.1–1mgcm−2h−1, as mainly occurs on stainless steel.

Temperature-mediated control of the growth of an entangled carbon nanofiber layer on stainless steel micro-structured reactors

A well-adhered layer of carbon nanofibers (CNFs) was grown on stainless steel microreactors by decomposition of a hydrocarbon over microreactors previously coated with Ni dispersed on alumina. In a previous work, we reported that the growth temperature and hydrocarbon (methane or ethane) modulated the morphology and size of the grown carbon species. Using ethane, the carbon yield increased dramatically for temperatures exceeding 898K. At these temperatures some carbon protrusions arise, which plug microreactor channels and render the microreactor unsuitable for use. For methane at all the tested temperatures or for ethane at the lowest temperatures (853 and 873K), the microreactor channels were covered completely by a uniform mat of entangled CNFs ready for catalytic use. Here, we show that the growth temperature and hydrocarbon can also control the primary structure of CNFs, either rolled graphitic planes parallel to the axis (multi-wall carbon nanotubes) or graphitic planes forming an angle with respect to the axis (fishbone type).

Carbon nanofiber based catalyst supports to be used in microreactors: Synthesis and characterization

Carbon nanofiber (CNF) layers have been synthesized on flat fused silica and silicon substrates, as well as inside flow channels of silicon-technology based microreactors by thermal catalytic chemical vapor deposition of ethylene using nickel thin-film catalyst. These CNF layers are to be used as structured catalyst support. The influence of the ethylene concentration and addition of hydrogen to the carbon-containing gas on the morphology of CNF layers was studied. Very low amount of CNFs were produced at low ethylene concentrations (<25%) due to the restricted supply of carbon species. Addition of hydrogen during the CNF growth resulted in significant enhancement of the CNF-yield, producing thicker layers of CNFs and CNFs with smaller diameters. Channels containing silicon micropillars covered with these CNFs have a significantly enhanced surface-to-volume ratio compared to bare microreactor channels (3–4 orders of magnitude). Deposition of well-distributed platinum nanoparticles was carried out on these CNF layers, exemplifying their functionality as structured catalyst support to be used in microreactors.

Preparation of stainless steel microreactors coated with carbon nanofiber layer: Impact of hydrocarbon and temperature

We have coated stainless steel microreactors with a well-adhered layer of carbon nanofibers (CNFs). The CNFs have been grown by decomposition of a hydrocarbon over microreactors previously coated with Ni dispersed on alumina. As hydrocarbon we used both methane and ethane. Other parameters varied herein are the hydrocarbon:H 2 gas composition and the CNF growth temperature. For the same growth temperature, the carbon productivity is higher for ethane than for methane. Remarkably, using ethane as carbon source, the carbon yield increased abruptly for temperatures exceeding 898 K. This is coupled with a less defined morphology and size of nanocarbon materials when temperature increases. Some thicker CNFs with diameters up to 200 nm and some non-fibrous carbonaceous protrusions are present at the highest temperatures. Thus, the microreactor channels resulted plugged and with an uneven surface at growth temperatures higher than 898 K. With ethane at the lowest temperatures (853 and 873 K), the only carbon product present was CNF of diameter smaller than 50 nm. The characterisation of the carbon nanomaterials and growth catalyst shed some light about the relationship between the morphology of carbon species and growth temperature.

Influence of hydrogen on the formation of a thin layer of carbon nanofibers on Ni foam

Carbon nanofibers (CNFs) were catalytically grown on Ni foam by decomposing ethylene in the presence of hydrogen. Variation of hydrogen concentration during CNF growth resulted in significant manipulation of the properties of a thin layer of CNFs. Addition of hydrogen retards carbon deposition and increases the surface area of the CNF layer because of formation of thinner fibers. The thickness of CNF layer shows an optimum at intermediate hydrogen concentrations. These effects contribute to the competitive adsorption of hydrogen and ethylene, influencing the availability of carbon on the Ni surface, which is necessary for both the formation of small Ni particles by fragmentation of polycrystalline Ni, as well as for CNF growth after formation of small particles. Furthermore, decreasing the carbon supply via adding hydrogen also delays deactivation by encapsulation of Ni particles. The thickness of the micro-porous C-layer between the Ni surface and the CNF layer decreases with hydrogen addition, at the expense of a slight loss in the attachment of the CNFs to the foam, supporting the proposition that CNFs are attached by roots in the C-layer. The addition of hydrogen after the initial CNF formation in ethylene only causes fragmentation of the C-layer, inducing significant loss of CNFs.