Catalytic Chemical Vapor Decomposition Research Articles

Interface engineering and impedance matching strategy to develop core@shell urchin-like NiO/Ni@carbon nanotubes nanocomposites for microwave absorption

It is well recognized that interfacial effect and/or impedance matching play a great impact on microwave absorption. Herein, we proposed a facile strategy to take full advantage of interface engineering and impedance matching for boosting microwave absorption performance (MAPs). Three-dimensional (3D) hierarchical urchin-like core@shell structured NiO/Ni@CNTs multicomponent nanocomposites (MCNCs) were elaborately constructed and produced in high efficiency through a facile continuous chemical bath deposition, thermal treatment, and catalytic chemical vapor decomposition process. By controlling the pyrolysis time, the NiO/Ni@CNTs urchin-like MCNCs with different lengths and aggregation degrees of CNTs could be selectively synthesized. The obtained results revealed that the enhanced CNT contents provided abundant interfaces and effectively aggrandized their interfacial effects, which resulted in improved polarization loss, conductivity loss, and comprehensive MAPs. Impressively, the interfaces and impedance matching in the designed NiO/Ni@CNTs urchin-like MCNCs could be optimized by regulating the pyrolysis temperature, which further improved the comprehensive MAPs. And the designed NiO/Ni@CNTs urchin-like MCNCs could simultaneously display strong absorption capabilities, broad absorption bandwidths, and thin matching thicknesses. Therefore, our findings not only provided a simple and universal approach to produce core@shell structured magnetic carbon-based urchin-like MCNCs but also presented an interface engineering and impedance matching strategy to develop the tunable, strong absorption, broadband, lightweight high-efficiency microwave absorbers.

Low-temperature Synthesis of Carbon Nanofibers/graphite Felt Composites Under Contactless Induction Heating

In this report induction heating was used for the catalytic chemical vapor decomposition synthesis of carbon nanofibers (CNFs) on Ni-based catalyst. The CNFs were produced with a high yield at a relatively low temperature compared to that observed for conventional heating through convection and conduction. The process also occurred in the absence of secondary toxic organic compounds, formed through side reaction in the high temperature gas-phase or through decomposition on the hot wall of reactor as encountered with traditional Joule heating mode. The improved CNFs yield under induction heating was attributed to the high reaction temperature control thanks to the high temperature regulation rate provided by the induction heating coil. The local heating of the nickel nanoparticles by the electromagnetic field could also contribute to the improvement of the CNFs yield. The results obtained indicate that inductive heating mode could be of great interest for improving the heat transfer in catalytic processes and also to reduce the problem of gradient temperature occurring inside the catalyst bed during the operating of highly exothermic or endothermic processes. It is expected that such electricity-driven heating mode could have contributed in an efficient way toward the electrification of different catalytic processes in order to reduce the associated carbon footprint.

Preparation of nitrogen-containing carbon nanotubes and study of their performance as basic catalysts

Nitrogen-containing carbon nanotubes were prepared, characterized and used as catalysts for two reactions of interest in the biomass upgrading processes: acetone aldol and acetic acid decarboxylative condensations. N-CNT were synthesized in a fluidized bed reactor by catalytic chemical vapor decomposition of acetonitrile on iron-, cobalt- and nickel-supported catalysts. Ni-N-CNT catalyst exhibits the highest activity, attributed to the surface concentration of basic sites, particularly the pyridinic species detected by X-ray photoelectron spectra (XPS). Likewise, the selectivity toward the final products of the reaction pathway is related both to the strength of basic sites (Ni-N-CNT) and to the morphology and chemical environment of the active sites (Fe-N-CNT). Synthesized catalysts show high stability for acetone condensation, whereas for ketonic condensation the catalyst stability decreases, mainly due to acic–base interactions with the active sites.

Heat Conduction Characteristics of Vertically Aligned Single-Walled Carbon Nanotubes Measured by Raman Spectroscopy

In this work, the film thermal conductivity and the film-substrate interfacial thermal contact resistance of vertically-aligned single-walled carbon nanotube (VA-SWNT) films were measured with a proposed method utilizing temperature dependence of Raman spectrum. The proposed method harnesses the excitation laser power of the Raman spectroscopy to heat the VA-SWNT films synthesized on a silicon substrate by alcohol catalytic chemical vapor decomposition (ACCVD) method, and measures film temperature from the Raman spectrum. A relationship between the input laser heat and the measured temperature is modeled with a detailed heat conduction equation, and its numerical solutions were compared with the experimentally measured results to extract the film thermal conductivity and the film-substrate interfacial thermal contact resistance. The method found the thermal conductivity of the VA-SWNT film to be around 2 Wm-1K-1 and the film-substrate interfacial thermal contact resistance to be around 2×10-6 m2KW-1. The obtained film thermal conductivity corresponds to the thermal conductivity equivalent of an individual SWNT of several tens of Wm-1K-1. This value was more than an order of magnitude smaller than the values reported on individual SWNTs.

Fabrication and Characterization of Highly Crystalline and Stable Phase-Pure Rutile Nanowires

A homogeneous thin layer of TiO2 has been successfully coated on the surface of multiwalled carbon nanotubes (MWCNTs), which were produced by catalytic chemical vapor decomposition method, via an in situ sol-gel method. The obtained MWCNT-TiO2 composite materials were heat treated in air at high temperatures, attempting to produce highly crystalline pure rutile nanowires. Through comprehensive characterization obtained by scanning electron microscopy (SEM), transmission electron microscope (TEM), energy dispersive X-ray (EDX), and X-ray powder diffraction (XRD), the effect of heat treatment on crystallization and phase transformation was discussed, and the effect of absence of MWCNTs on the morphology of pure rutile nanowires was analyzed. Both anatase and rutile phases exist after heat treatment in 700 degrees C while only rutile phase exists after heat treatment in 800 degrees C. The crystal size of rutile is formed to be significantly larger than that of anatase, and the hollow tubular structure is found to be destroyed which resulted in nanowire structure.

Production of single-walled carbon nanotubes from methane over Co-Mo/MgO nanocatalyst: A comparative study of fixed and fluidized bed reactors

In this study, the performances of fixed and fluidized bed reactors in the production of single-walled carbon nanotubes (SWNTs) have been investigated. In both reactors, single-walled carbon nanotubes were grown by catalytic chemical vapor decomposition (CCVD) of methane over Co-Mo/MgO nanocatalyst under two different operating conditions. The synthesized samples were characterized by TEM, TGA and Raman spectroscopy. It is found that the performance of a fluidized bed in the synthesis of carbon nanotubes is much better than that of a fixed bed. The quality of carbon nanotubes obtained from the fluidized bed was significantly higher than that from the fixed bed and the former one with the I D /I G ratio of 0.11 while the latter one with the I D/ I G ratio of 0.71. Also, the yield of SWNTs in the fluidized bed was 92 wt%, while it was 78 wt% in the fixed bed. These advantages of fluidized bed reactors for the synthesis of carbon nanotubes can be attributed to more available space for the growth of carbon nanotubes and more uniform temperature and concentration profiles.

Non-Enzymatic Amperometric Glucose Biosensor from Zinc Oxide Nanoparticles Decorated Multi-Walled Carbon Nanotubes

The present work describes the development of novel ZnO dispersed multi-walled carbon nanotubes (MWNT) based non-enzymatic glucose biosensor with 1 M NaOH solution as the supporting electrolyte. For a comparison, the same material has been used for the fabrication of enzymatic biosensor and studied its electrochemical activity with phosphate buffer solution as the electrolyte. MWNT have been synthesized by catalytic chemical vapor decomposition (CCVD) and a simple sol-gel method was used for decorating crystalline ZnO nanoparticles on MWNT. Cyclic voltammetry and chronoamperometry were used to study and optimize the electrochemical performance of the resulting enzymatic and non-enzymatic ZnO/MWNT biosensors. The non enzymatic Nafion/ZnO/MWNT/GC electrode shows linearity in the range 700 nM to 31 mM with the detection limit of 500 nM. Similarly enzymatic biosensor fabricated using Nafion/GOD/ZnO/MWNT on glassy carbon electrode (GCE) shows a linearity from 1 microM to 22 mM. This excellent performance of non enzymatic Nafion/ZnO/MWNT/GC is due to high surface area, good electron transfer rate of ZnO/MWNT and the high electrochemical catalytic activity of ZnO in NaOH solution.

Growth of Carbon Nanotubes from Heterometallic Palladium and Copper Catalysts

Bamboo-structured carbon nanotubes (BCNTs) were synthesized with MgO-supported Pd and Cu catalysts, doped with either Mo or W, by the catalytic chemical vapor decomposition of methane. No nanotubes were observed to grow from the catalysts in the absence of the dopant metals. Additionally, the level of dopant in the catalysts was found to strongly affect the morphology of carbon produced. Amorphous carbon was generated on a 10 wt % Cu/5 wt % W (2:1) catalyst, while BCNTs were produced on 20 wt % Cu/5 wt % W (4:1) and a 30 wt % Cu/5 wt % W (6:1) catalysts. A pure Pd catalyst produced carbon nanofibres (CNFs), while BCNTs were able to grow from Pd/Mo catalysts. Density functional theory simulations show that the composite Cu/W and Pd/Mo bimetallic particles which generated BCNTs have similar binding energies to carbon, and comparable to metals such as Fe, Co, and Ni which are traditionally used to grow CNTs by chemical vapor deposition.

Fabrication, characterization and electrochemical performance of single strand carbon fiber prepared by catalytic chemical vapor decomposition method

Preparation, fabrication and voltammetric characterizations of a single strand of carbon fiber (SSCF) electrode and their potential applications for biosensor are presented. SSCFs of diameter ca. 10 ± 2 μm and few millimeters in length are prepared by catalytic chemical vapor decomposition (CCVD) method. Voltammetry with potassium ferricyanide, α-methylferrocene methanol and hexaammineruthenium(III) chloride on SSCF electrode are used as bench marks to validate the electrode properties. Quasi-steady state voltammograms obtained were fitted into a cylindrical diffusion model. From which, the standard rate constant ( k 0) and electron transfer coefficient ( α) are obtained. The use of SSCF electrode is demonstrated for the voltammetric detection of the micromolar quantity of dopamine in the presence of large excess (ca. 200 times) of ascorbic acid, without any fouling of electrode surface. The kinetics of electron transfer are investigated.

Electron field emission from magnetic nanomaterial encapsulated multi-walled carbon nanotubes

The present work describes the field emission characteristics of nanoscale magnetic nanomaterial encapsulated multi-walled carbon nanotubes (MWNTs) fabricated over flexible graphitized carbon cloth. Ni/MWNTs, NiFe/MWNTs and NiFeCo/MWNTs have been synthesized by catalytic chemical vapor decomposition of methane over Mischmetal (Mm)-based AB3 (MmNi3, MmFe1.5Ni1.5 and MmFeCoNi) alloy hydride catalysts. Metal-encapsulated MWNTs exhibited superior field emission performance than pure MWNT-based field emitters over the same substrate. The results indicate that a Ni-filled MWNT field emitter is a promising material for practical field emission application with a lowest turn-on field of 0.6 V/μm and a high emission current density of 0.3 mA/cm2 at 0.9 V/μm.

A Glucose Biosensor Based on Deposition of Glucose Oxidase onto Crystalline Gold Nanoparticle Modified Carbon Nanotube Electrode

A new amperometric biosensor, based on deposition of glucose oxidase (GOD) onto crystalline gold (Au) nanoparticle modified multiwalled carbon nanotube (MWNT) electrode, is presented. MWNTs have been synthesized by catalytic chemical vapor decomposition of acetylene over rare-earth-based AB2 (DyNi2) alloy hydride catalyst. Purified MWNTs have been decorated with nanocrystalline Au metal clusters using a simple chemical reduction method. The characterization of metal-decorated CNTs has been done using X-ray diffraction analysis, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, and energy-dispersive X-ray analysis. Amperometric biosensor fabricated by depositing GOD over Nafion-solubilized Au-MWNT electrode retains its biocatalytic activity and offers fast and sensitive glucose quantification. The performance of the biosensor has been studied using cyclic voltammetry, amperometry, and hydrodynamic voltammetry, and the results have been discussed. The fabricated glucose biosensor exhibits a linear response up to 22 mM glucose and a detection limit of 20 microM.

A multi-step strategy for cutting and purification of single-walled carbon nanotubes

A multi-step procedure has been developed to cut and purify single-walled carbon nanotubes (SWCNTs) from the pristine soot synthesized by catalytic chemical vapor decomposition, which contains significant amount of multiwalled carbon nanotube (MWCNT) impurities. The procedure involves refluxing in dilute HNO 3 solution followed by (NH 4) 2S 2O 8/H 2SO 4 ultrasonication and a novel ammonia heat treatment step. The metal impurities are removed in the HNO 3 refluxing step. In the (NH 4) 2S 2O 8/H 2SO 4 treatment step, the SWCNTs are cut uniformly into micrometer long, while the MWCNTs are selectively cut into very short curled pieces. These short MWCNT pieces and the amorphous carbon impurities are partly removed in the (NH 4) 2S 2O 8/H 2SO 4 treatment step, and most of them are finally removed by the final ammonia heat treatment. The ammonia heat treatment step also recovers the structures of SWCNTs by healing defects on the sidewall. The final SWCNT products have purity higher than 95%, a length distribution between 1 and 2 μm, and very few defects.

Carbon nanotube/felt composite electrodes without polymer binders

In this work we have investigated the suitability of composite electrodes consisting of cup-stacked and bamboo-like carbon nanotubes (CNT) synthesized directly onto a carbon felt for both lithium storage and double-layer capacitance applications. The CNT/felt composite electrode was prepared using catalytic chemical vapor decomposition on the carbon felt. The microstructure of the electrodes was characterized by scanning electron microscopy. Electrochemical characterization of the CNT/felt, either submitted or not to acid treatment for extraction of the catalytic particles used during the CNT growth, was carried out using 1 mol L −1 LiPF 6 in mixtures of ethylene carbonate, dimethyl carbonate, diethyl carbonate, and propylene carbonate. The carbon nanotubes loading and the type of CNT, whether open or closed, on the felt were the most significant factors regarding the electrochemical properties of the composite. With respect to the application of the composite to lithium storage, an anomalous behavior in the reversible specific capacity as a function of the current was detected. The capacity was found to be large at higher current values. The best reversible specific capacity was found for the open-CNT/felt (275 mAh g −1 at 0.16 A g −1, and 200 mAh g −1 at 0.82 A g −1), on an area of 0.634 mm 2. The double-layer capacitance of the CNT decreased with increasing current. In the case of the open-CNT with a CNT loading of 13.93 mg, the composite provided 40.3 μF cm 2 or about 12 F g −1 at 10 mA of polarization current using 1 mol L −1 LiPF 6 in mixtures of ethylene carbonate and dimethyl carbonate. For the closed-CNT with a CNT loading of 9.3 mg, the double-layer capacitance was 30 F g −1 at 20 mA in 1 M H 2SO 4.

Surface oxygen complexes as governors of neopentane sorption in multiwalled carbon nanotubes

Multiwalled carbon nanotube (MWCNT) samples were obtained by purification and ball-milling of a MWCNT sample synthesized by catalytic chemical vapor decomposition. These samples were oxidized with KMnO 4–H 2SO 4 solution. A heat treatment was employed to decompose the oxygenated groups created by the oxidation treatment. The samples were characterized by transmission electron microscopy (TEM), adsorption of nitrogen, pore size distribution, and by thermogravimetric analysis combined with mass spectroscopy (TG-MS). The dynamics of 2,2-dimethyl-propane (neopentane) sorption was studied by frequency-response (FR) spectroscopy. The FR spectra obtained clearly demonstrate the effect of (a) the ball-milling, (b) the oxidative treatment and (c) the heat treatment on the sorption uptake processes involved in these treated samples.

Monte carlo calculation of multiple scattering effects in thermal neutron scattering experiments: Modification to slab geometry

Nitrogen-containing carbon nanotubes were prepared, characterized and used as catalysts for two reactions of interest in the biomass upgrading processes: acetone aldol and acetic acid decarboxylative condensations. N-CNT were synthesized in a fluidized bed reactor by catalytic chemical vapor decomposition of acetonitrile on iron-, cobalt- and nickel-supported catalysts. Ni-N-CNT catalyst exhibits the highest activity, attributed to the surface concentration of basic sites, particularly the pyridinic species detected by X-ray photoelectron spectra (XPS). Likewise, the selectivity toward the final products of the reaction pathway is related both to the strength of basic sites (Ni-N-CNT) and to the morphology and chemical environment of the active sites (Fe-N-CNT). Synthesized catalysts show high stability for acetone condensation, whereas for ketonic condensation the catalyst stability decreases, mainly due to acic–base interactions with the active sites.

Monte Carlo calculation of multiple scattering effects in thermal neutron scattering experiments: Modification to horizontal cylinder geometry

Nitrogen-containing carbon nanotubes were prepared, characterized and used as catalysts for two reactions of interest in the biomass upgrading processes: acetone aldol and acetic acid decarboxylative condensations. N-CNT were synthesized in a fluidized bed reactor by catalytic chemical vapor decomposition of acetonitrile on iron-, cobalt- and nickel-supported catalysts. Ni-N-CNT catalyst exhibits the highest activity, attributed to the surface concentration of basic sites, particularly the pyridinic species detected by X-ray photoelectron spectra (XPS). Likewise, the selectivity toward the final products of the reaction pathway is related both to the strength of basic sites (Ni-N-CNT) and to the morphology and chemical environment of the active sites (Fe-N-CNT). Synthesized catalysts show high stability for acetone condensation, whereas for ketonic condensation the catalyst stability decreases, mainly due to acic–base interactions with the active sites.