Effect of Addition of Ni metal catalyst onto the Co and Fe supported catalysts for the formation of carbon nanotubes

Production of novel porous material is a major target in current material science research due to its wide applications. As carbon nanotube (CNTs) is a one dimensional hollow structure it is also one of the promising materials in applications ranging from electronics to hydrogen storage medium. Catalytic chemical vapor deposition (CCVD) is a method whereby CNTs can be produced in large amount. Thus, in this work, we have synthesized CNTs via pyrolysis of acetylene using various supported transition-metal catalysts in a fixed-bed reactor. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to investigate the CNTs structure. The structures of nanotubes formed by acetylene pyrolysis were dependent on the catalysts used. It was found that alumina supported Ni/Fe catalyst inhibited the formation of CNTs growth while alumina supported Ni/Co catalyst gave high density of CNTs. However, nanotubes grown over alumina supported Ni/Fe catalyst were less dense due to the deactivation of the catalyst at the early stage of the pyrolysis process.     

Synthesis of carbon nanotubes over nickel–iron catalysts supported on alumina under controlled conditions

Carbon nanotubes (CNTs) were synthesized by catalytic decomposition of acetylene over Fe, Ni and Fe–Ni catalysts supported on alumina. The growth of CNTs was carried out at various reaction conditions. The growth density and diameter of CNTs could be controlled by varying the catalyst composition and the growth parameters. The growth density of CNTs increased with increasing the activation time of catalysts in H₂ atmosphere and/or decreasing acetylene concentration. At 600°C, higher density of CNTs was observed at 60 min for higher Fe containing catalyst, whereas at 90 min for higher Ni containing catalyst. The growth density of CNTs highly increased with increasing reaction time from 30 to 60 min. For all the catalysts, the diameter of CNTs decreased with increasing growth time further mainly due to hydrogen etching. Bimetallic catalysts produced narrower diameter CNTs than single metal catalysts. The growth of CNTs followed the tip growth mode and the CNTs were multi-walled CNTs.     

Effects of bimetallic catalyst composition and growth parameters on the growth density and diameter of carbon nanotubes

Multi-wall carbon nanotubes (MWNTs) were synthesized by catalytic decomposition of acetylene over Fe, Ni and Fe-Ni bimetallic catalysts supported on alumina under various controlled conditions. The growth density and diameter of CNTs were markedly dependent on the activation time of catalysts in H₂ atmosphere, reaction time, reaction temperature, flow rate of acetylene, and catalyst composition. Bimetallic catalysts were apt to produce narrower diameter of CNTs than single metal catalysts. For the growth of CNTs at 600 ‡C under 10/100 seem flow of C₂H₂/H₂ mixture, the narrowest diameter about 20 nm was observed at the reaction time of 1 h for 20Fe : 20Ni : 60Al₂O₃ catalyst, but at that of 1.5 h for 10Fe : 30Ni : 60Al₂O₃ catalyst. It was considered that the diameter and density of CNTs decreased with the increase of the growth time mainly due to hydrogen etching. The growth of CNTs followed the tip growth mode.     

Low-Temperature Growth of Carbon Nanotubes on Bi- and Tri-metallic Catalyst Templates

Low temperature growth process of carbon nanotubes (CNTs) over bi-metallic (Co–Fe) and tri-metallic (Ni–Co–Fe) catalysts on Si/Al/Al₂O₃ substrates is carried out from acetylene precursor using hydrogen, ammonia or nitrogen as a carrier in a low pressure chemical vapor deposition system. Using the tri-metallic Ni–Co–Fe catalyst template, vertically aligned CNTs of ~700 nm length could be grown already at 450 °C within 10 min using ammonia as a carrier. Within the same period of time, on bi-metallic Co–Fe catalyst templates, ~250 nm long aligned nanotubes emerged already at 400 °C in nitrogen carrier. At low temperatures most of the catalyst materials were elevated from the support by the grown nanotubes indicating tip growth mechanism. The structure of catalyst layers and nanotube films was studied using scanning and transmission electron microscopy and atomic force microscopy.     

Aligned carbon nanotubes by catalytic decomposition of C2H2 over Ni–Cr alloy

High density, well-aligned carbon nanotubes (CNTs) were prepared by thermally decomposing acetylene at 700 °C with the help of Ni–Cr alloy as catalyst in a thermal chemical vapor deposition system. The density and alignment of CNTs were characterized by scanning electron microscope (SEM). It was found that the density of the CNTs could be remarkably increased and the alignment could be improved with the decrease of the thickness ratio of Ni:Cr. Also found in our experiment was that the catalyst encapsulated in CNT was single crystal Ni, which was confirmed by high-resolution transmission electron microscopy (HRTEM) and electron dispersion X-ray spectrum (EDX). Finally, the growth mode of CNTs was discussed based on the Ni–Cr alloy catalysts under our experimental conditions. The results are helpful in providing a better understanding of the acting of catalyst and the controlling of the desirable density and alignment of CNTs for various applications.     

以负载Fe的介孔分子筛为模板合成碳纳米管

以负载Fe的介孔分子筛Fe/MCM-41和Fe/ABW分别为催化剂,乙炔为碳源,采用化学气相沉积法对催化合成碳纳米管(CNTs)进行研究,讨论了反应温度、催化剂种类以及催化剂预处理对CNTs纯度和形貌的影响,通过场发射扫描电子显微镜、高分辨透射电子显微镜和X-射线衍射仪对产物的结构和形貌进行了表征和分析,并对CNTs的生长机理进行了推测.结果表明,在反应温度为700℃,两种不同的催化剂经H2还原后,催化生长出直径均匀(20nm～30nm)且晶化程度较好的CNTs.     

Synthesis of helical carbon nanotubes, worm-like carbon nanotubes and nanocoils at 450 °C and their magnetic properties

High purity (99.21 wt.%) helical carbon nanotubes (HCNTs) were synthesized in large quantity over Fe nanoparticles (fabricated using a coprecipitation/hydrogen reduction method) by acetylene decomposition at 450 °C. Field-emission and transmission electron microscope images reveal that the selectivity to HCNTs (with two or three coiled nanotubes connected to a catalyst nanoparticle) is up to ca. 93%. The yield of HCNTs (as defined by the equation: ) is ca. 7474% in a run of 6 h, higher than any of those reported in the literature. If hydrogen was introduced during acetylene decomposition for ca. 30 min, the HCNTs mainly consisted of two coiled tubes connected to a catalyst nanoparticle, and carbon nanocoils (CNCs) of different structures were generated. If hydrogen was present throughout acetylene decomposition, worm-like carbon nanotubes (CNTs) as well as CNCs were produced in large quantities. Because the HCNTs and worm-like CNTs are attached to Fe nanoparticles, the nanomaterials are high in magnetization.     

化学气相沉积法制备碳纳米管过程中NH3对其微观结构和石墨化程度的影响

以二茂铁为催化剂前驱体,C₂H₂为碳源,在NH₃和N₂气氛中在Al₂O₃多孔基体上化学气相沉积制备出了一致取向的碳纳米管阵列.采用拉曼散射、扫描电子显微镜、高分辨电子显微镜、X射线衍射和元素分析等技术对产物进行表征.实验结果表明,以NH₃为载气时,所制备的碳纳米管为竹节形结构,并且有一定量的N掺杂在其中,其质量含量大约在3.46%,碳纳米管的石墨层间距在0.384 nm左右,实验产物中有Fe₃C和Fe₂N或Fe₃N的生成,NH₃在催化剂表面的吸附导致催化剂的表面性质的改变以及N的掺杂使生成的碳纳米管结构有一些缺陷,石墨化程度有所降低.碳纳米管的管径随着反应温度的增加而增大.     

Agglomerated CNTs synthesized in a fluidized bed reactor: Agglomerate structure and formation mechanism

Agglomerated carbon nanotubes (CNTs) were synthesized by catalytic pyrolysis of propylene on Fe/Mo/Al₂O₃ catalysts in a nano-agglomerate fluidized-bed reactor (NAFBR) of 196 mm I.D. The macroscopic properties and microstructure of the CNTs and their evolution were systematically characterized by high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy. The CNTs from the NAFBR are sub-agglomerates entangled with each other. Their formation involves the initial fragmentation of the catalyst support, sub-agglomerate formation and expansion of the agglomerates due to CNT growth. When the structure of the catalysts is destroyed, the release of stress inside the catalyst particles will result in structural defects in the CNT shells. More perfect CNTs are obtained in fully developed agglomerates. A model is proposed to explain the process of agglomerate formation and based on its formation mechanism, an approach to control CNT quality in an NAFBR is proposed.     

Ni-based supported catalysts from layered double hydroxides: Tunable microstructure and controlled property for the synthesis of carbon nanotubes

The carbon nanotubes (CNTs) with straight and helical nanostructures have been synthesized by catalytic chemical vapor deposition of acetylene over a series of Ni-based supported catalysts, which were formed from Ni–Mg–Al layered double hydroxide precursors (LDHs) synthesized through homogenous decomposition of urea under hydrothermal conditions. The materials were characterized by power X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), temperature-programmed reduction experiments (TPR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results showed that the introduction of Mg into Ni-based supported catalysts could effectively improve the catalytic activities for the growth of CNTs, mainly proceeding from the inhibition effect of spinel phases formed in calcined LDHs on the agglomeration of metallic Ni particles. Furthermore, it is found interestingly that the addition of Mg also could induce the formation of helical structured CNTs with outer diameters of 20 nm and that the higher Mg content gave rise to the more helical nanotubes. The present work provides a simple and facile way to prepare metal-supported catalysts with a good dispersion of catalytically active metal particles for the growth of straight and helical CNTs.     

Highly dispersed Pt nanoparticles by pentagon defects introduced in bamboo-shaped carbon nanotube support and their enhanced catalytic activity on methanol oxidation

Bamboo-shaped carbon nanotubes (BCNTs), which were synthesized through chemical vapor deposition by using cresol as the carbon source, were explored as Pt catalyst support in comparison with conventional carbon nanotubes (CNTs) and Vulcan XC carbon blacks. The pyrolysis of cresol produced a large amount of pentagon defects introduced in the walls of BCNTs, which could possess higher chemical activity and stronger interaction with metal particles. After a mild purification, the BCNTs exhibited more oxygen-containing functional groups than CNTs, as shown by Fourier transform infrared spectra and cyclic voltammetry. The formed oxygen-containing functional groups as well as the pentagon defects could act as uniform active sites for metal particle loading. By ethylene glycol reduction, highly dispersed Pt nanoparticles with a narrow size distribution of 2-3 nm were easily supported on BCNTs, as shown by transmission electron microscope. The Pt/BCNT catalyst showed higher electro-catalytic activity on the methanol oxidation than the Pt/CNT and Pt/Vulcan XC catalyst, which could be largely ascribed to the highly dispersed Pt nanoparticles due to the introduced pentagon defects in the tube-walls (comparing with Pt/CNT) and the graphitic nanotube network that could provide good electron conduction (comparing with Pt/Vulcan XC).     

Effect of adding nickel to iron-alumina catalysts on the morphology of as-grown carbon nanotubes

The synthesis of carbon nanotubes (CNTs) from ethylene decomposition by Fe/Al₂O₃ and Fe/Ni/Al₂O₃ catalysts (Fe:Ni=10:1) is studied. A small amount of nickel introduced into the catalyst can significantly increase the yield of CNTs, but the nanotubes change from straight tubes with concentric parallel carbon sheets to helical tubes of the fish-bone type. Raman characterization of CNTs prepared at 823 and 1023 K and CNTs annealed at 2473 K shows that CNTs deposited on the Fe/Ni/Al₂O₃ catalyst have poor crystallinity, as compared with that on the Fe/Al₂O₃ catalyst. These differences are explained by a mechanism of formation of helical tubes of the fish bone type that takes into consideration the differences in the chemical nature of the catalyst with and without nickel.     

Formation and catalytic performance of supported ni nanoparticles via self‐reduction of hybrid NiAl‐LDH/C composites

In the present work, hybrid NiAl‐layered double hydroxide/carbon (LDH/C) composites with adjustable compositions were successfully assembled by crystallization of LDH in combination with carbonization of glucose under hydrothermal conditions, and further utilized as an integrated catalyst for the growth of carbon nanotubes (CNTs) in catalytic chemical vapor deposition (CCVD) of acetylene. The materials were characterized by X‐ray diffraction, Fourier transform infrared, elemental analysis, thermogravimetric and differential thermal analysis, SEM, transmission electron microscopy, X‐ray photoelectron spectra, and Raman spectroscopy. The results revealed that the supported Ni nanoparticles with the small crystallite size of about 10 nm could be obtained by in situ self‐reduction of as‐assembled hybrid LDH/C composites in the course of CCVD. The carbon in the hybrid structure as a reducing agent played a key role for the high dispersion of resulting Ni nanoparticles. Furthermore, the Ni nanoparticles obtained here exhibited excellent activity for catalytic growth of CNTs, which could be delicately tuned by varying the compositions of hybrid composites. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Synthesis of narrow-diameter carbon nanotubes from acetylene decomposition over an iron-nickel catalyst supported on alumina

Carbon nanotubes (CNTs) were synthesized by the catalytic decomposition of acetylene over 40Fe:60Al₂O₃, 40Ni:60Al₂O₃ and 20Fe:20Ni:60Al₂O₃ catalysts. High density CNTs of 20 nm diameter were grown over the 20Fe:20Ni:60Al₂O₃ catalyst, whereas low growth density CNTs of 40 and 50 nm diameter were found over 40Fe:60Al₂O₃ and 40Ni:60Al₂O₃ catalysts. Smaller catalyst particles enabled the synthesis of highly dense, long and narrow-diameter CNTs. It was found that a homogeneous dispersion of the catalyst was an essential factor in achieving high growth density. The carbon yield and the quality of CNTs increased with increasing temperature. For the 20Fe:20Ni:60Al₂O₃ catalyst, the carbon yield reached 121% after 90 min at 700 °C. The CNTs were grown according to the tip growth mode. Based on reports regarding hydrocarbon adsorption and decomposition over different faces of Ni and Fe, the growth mechanism of CNTs over the 20Fe:20Ni:60Al₂O₃ catalyst are discussed.     

The formation mechanisms of multi-wall carbon nanotubes over the Ni modified MCM-41 catalysts

Multi-wall carbon nanotubes (MWCNTs) were grown by thermal chemical vapor deposition (thermal CVD) of CH₄ by using Ni-MCM-41 as the catalyst. Methane pyrolysis has been performed in a quartz tube reactor over the catalyst surface to form carbon atoms via dehydrogenation process. The migration and rearrangement of the surface carbon atoms result in the formation of MWCNTs. Transmission electron microscope (TEM) and scanning electron microscope (SEM) were used to determine the morphologies and structures of CNTs, and Raman spectroscopy was exploited to analyze their purity with the relative intensity between the D-band (Disorder band) in the vicinity of 1,350 cm⁻¹ which is characteristic of the sp³ structure and G-band (Graphitic band) in vicinity of 1,580 cm⁻¹ which is characteristic of the sp² structure. In addition, the controlling factors of methane pyrolysis such as the catalyst composition; the reaction temperature, and the methane flow rate on the formation of MWCNTs were investigated to optimize the structure and yield of MWCNTs. SEM/TEM results indicate that the yield of the CNTs increases with increasing Ni concentration in the catalyst. The optimized reaction temperature to grow CNT is located between 640 and 670 °C. The uniform and narrow diameter MWCNTs form at lower flow rate of methane (∼30 sccm), and non-uniform in diameter and disorder structure of MWCNTs are observed at higher flow rate of methane. This is consistent with Raman analysis that the relative intensity of I _D/I _G increases with increasing methane flow rate. The formation mechanisms of the MWCNTs on the Ni-MCM-41 catalyst have been determined to be a Tip-Growth mode with a nanoscale catalyst particle capsulated in the tip of the CNT.     

Experimental and numerical investigation of the position-dependent growth of carbon nanotube–alumina microparticle hybrid structures in a horizontal CVD reactor

The hybrid structures of carbon nanotubes (CNTs) and alumina microparticles were produced in a horizontal chemical vapor deposition (CVD) reactor using ferrocene/xylene/acetylene mixture as the catalyst–carbon source. At a given temperature and hydrogen ratio, the CNT diameter, number density, growth rate and their hybrid structures varied greatly along the axial direction of the reactor. This non-uniform growth is attributed to the position-dependent chemical reaction kinetics inside the reactor, mainly along the gas flow direction. Mass spectrometry was used to identify and quantify the chemical species of the exhaust gas. A numerical simulation of the reacting gas flow in the reactor was conducted in parallel by taking into account the space-dependent pyrolysis kinetics of the catalyst and carbon sources, the reactor temperature gradient and the fluid dynamics. A good agreement existed between the modeling and the experimental results. A double-end-injection method was proposed based on the above results, and the uniform hybrid structures were synthesized in a larger zone inside the CVD reactor. A new kind of CNT structure containing iron crystal particles at their two extremities was obtained under certain conditions, which can be very useful for various CNT junction applications.     

Study of carbon nanotubes synthesis by spray pyrolysis and model of growth

Multi-walled carbon nanotubes (MWCNTs) were grown inside of quartz tubing by spray pyrolysis of ferrocene/benzene under argon flow. Carbon nanotubes (CNTs) with length of 200 μm were produced with reaction time of 10 min. The diameter of CNTs was influenced by the size of droplets formed in the nebulizer and the length was greatly influenced by ferrocene concentration and argon gas flow. It was found that temperature is a critical variable to produce CNTs at the experimental conditions used in this work. It was also found that CNTs only grew if ferrocene is added to gas flow, even if CNTs are previously seeded and formed on substrate, benzene cannot produce the CNTs without ferrocene. A model of CNTs formation and growth is proposed for spray pyrolysis of ferrocene/benzene, this mechanism consist of the formation of carbon/Fe nanoparticles during pyrolysis in the gas phase, these nanoparticles reach the walls of substrate, and the nanoparticles attach to substrate surface or to the nanotubes. Under proper conditions the displacement of Fe inside the graphitic structure induces the alignment of carbon walls, straightening this way the nanotubes.     

Over 99.6 wt%-pure,sub-millimeter-long carbon nanotubes realized by fluidized-bed with careful control of the catalyst and carbon feeds

To establish a method for sub-second conversion of acetylene to sub-millimeter-long carbon nanotubes (CNTs), we have proposed and developed an internal heat-exchange reactor for fluidized-bed chemical vapor deposition (FBCVD). This reactor enabled sufficient heating of the reaction gas and uniform heating of the bed of alumina beads at a space velocity as high as 3600 h⁻¹. The direct feeding of the catalyst vapors (aluminum isopropoxide for the alumina support layer and ferrocene for the iron particles) to the bed separately from the other gases, which were fed through the heat-exchange and preheating zone and the distributer, enabled the careful control of the catalyst particles deposited on the beads. By decreasing the acetylene feed concentration and preventing the deactivation of small Fe particles, we realized semi-continuous production of 99.6–99.8 wt%-pure, sub-millimeter-long, few-wall CNTs with an average diameter of 6.5 nm at a carbon yield of 42%. The FBCVD reactor with an internal heat-exchanger can be scaled-up for practical mass production with uniform and energy-saving heating.     

Electrical and thermal properties of carbon nanotubes/PMMA composites induced by low magnetic fields

Abstract

Ni particles supported on carbon nanotubes (CNTs) were dispersed in a polymethyl methacrylate (PMMA) matrix by solution blending and then cast onto an electrode to get composite films under low magnetic fields. The orientation of CNTs in the films was characterised by scanning electron microscope and optical microscope. Multimeter and high resistance meter were used to study the electrical behaviour of the nanocomposites. The glass transition temperature T _g of PMMA was determined by differential scanning calorimetry. The results show that the alignment of the CNTs dispersed in the PMMA was achieved under a low magnetic strength below 0·5 T. Because of the ferromagnetism of Ni particles, the magnetic alignment of CNTs susceptibly changed. The magnetic alignment units in this work were rod-like CNTs aggregates instead of single CNTs, which took part in the buildup of a specific CNTs network structure in PMMA matrix. The network structure played a key role in significantly improving electrical conductivity and T _g of the nanocomposites.     

Carbon Nanotube Growth on Calcium Carbonate Supported Molybdenum-Transition Bimetal Catalysts

A comparison of different catalyst systems (Fe–Mo, Co–Mo or Ni–Mo nanoparticles supported on calcium carbonate) has been performed in order to optimize the carbon nanotube (CNT) growth. The influences of the reaction temperature, metal loading and carbon source on the synthesis of CNTs were investigated. Dense CNT networks have been synthesized by thermal chemical vapor deposition (CVD) of acetylene at 720 °C using the Co–Mo/CaCO₃ catalyst. The dependence of the CNT growth on the most important parameters was discussed exemplarily on the Co catalyst system. Based on the experimental observations, a phenomenological growth model for CVD synthesis of CNTs was proposed. The synergy effect of Mo and active metals was also discussed.