Formation Of Multi-walled Carbon Nanotubes Research Articles

Promoting Effects of Copper and Iron on Ni/MSN Catalysts for Methane Decomposition

Copper and iron-based bimetallic nickel catalysts supported on Mesostructured Silica Nanoparticles (MSNs) with compositions of 50% Ni–5% Cu/MSN and 50% Ni–5% Fe/MSN were prepared using an impregnation method, and they were compared with a monometallic 50% Ni–MSN catalyst for their activity and stability in methane decomposition reaction. The influence of promoters, such as Cu and Fe, at different reaction temperatures (700 °C, 800 °C and 900 °C) was investigated. The results revealed that the Cu and Fe-promoted catalysts significantly increased the hydrogen yield in methane decomposition compared with the unpromoted catalyst. This could be attributed to the formation of Ni–Cu and Ni–Fe bimetallic alloys in the catalysts, respectively, and this favored the stability of the catalysts. With increasing reaction temperature, the hydrogen yield also increased. However, the hydrogen yield and the lifetime of the nickel catalyst were enhanced upon the addition of iron compared to copper at all the reaction temperatures. The analysis conducted over the spent catalysts validated the formation of multi-walled carbon nanotubes with a bamboo-like internal channel over the catalysts along with a high crystallinity and graphitization degree of the carbon produced.

Catalytic Design of Matrix-Isolated Ni-Polymer Composites for Methane Catalytic Decomposition.

Targeted synthesis of C/composite Ni-based material was carried out by the method of matrix isolation. The composite was formed with regard to the features of the reaction of catalytic decomposition of methane. The morphology and physicochemical properties of these materials have been characterized using a number of methods: elemental analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, temperature programmed reduction (TPR-H2), specific surface areas (SSA), thermogravimetric analysis, and differential scanning calorimetry (TGA/DSC). It was shown by FTIR spectroscopy that nickel ions are immobilized on the polymer molecule of polyvinyl alcohol, and during heat treatment, polycondensation sites are formed on the surface of the polymer molecule. By the method of Raman spectroscopy, it was shown that already at a temperature of 250 °C, a developed conjugation system with sp2-hybridized carbon atoms begins to form. The SSA method shows that the formation of the composite material resulted in a matrix with a developed specific surface area of 20 to 214 m2/g. The XRD method shows that nanoparticles are essentially characterized by Ni, NiO reflexes. The composite material was established by microscopy methods to be a layered structure with uniformly distributed nickel-containing particles 5-10 nm in size. The XPS method determined that metallic nickel was present on the surface of the material. A high specific activity was found in the process of catalytic decomposition of methane-from 0.9 to 1.4 gH2/gcat/h, XCH4, from 33 to 45% at a reaction temperature of 750 °C without the stage of catalyst preliminary activation. During the reaction, the formation of multi-walled carbon nanotubes occurs.

Morphology and Microwave-Absorbing Performances of Rubber Blends with Multi-Walled Carbon Nanotubes and Molybdenum Disulfide.

This study details microwave-absorbing materials made of natural rubber/nitrile butadiene rubber (NR/NBR) blends with multi-walled carbon nanotubes (MWCNTs) and molybdenum disulfide (MoS2). The mechanical blending method and the influences of fabrication on the morphology and microwave-absorbing performance of resulting compounds were logically investigated. It was found that interfacial differences between the fillers and matrix promote the formation of MWCNTs and MoS2 networks in NR/NBR blends, thus improving microwave-absorbing performance. Compared with direct compounding, masterbatch-based two-step blending is more conducive to forming interpenetrating networks of MWCNTs/MoS2, endowing the resulting composite with better microwave attenuation capacity. Composites with MWCNTs in NR and MoS2 in NBR demonstrate the best microwave-absorbing performance, with a minimum reflection loss of -44.54 dB and an effective absorption bandwidth of 3.60 GHz. Exploring the relationship between morphology and electromagnetic loss behavior denotes that such improvement results from the selective distribution of dual fillers, inducing networking and multi-component-derived interfacial polarization enhancement.

Optimizing barium promoter for nickel catalyst supported on yttria‐stabilized zirconia in dry reforming of methane

AbstractBarium doping effect on the activity and stability of nickel‐based catalysts, supported on yttria‐stabilized zirconia (Ni‐YZr), was investigated in dry reforming of methane. Catalysts were characterized by several techniques (nitrogen sorption, X‐ray diffraction [XRD], scanning electron microscopy with energy dispersive X‐ray, transmission electron microscopy [TEM], thermogravimetric analysis [TGA], temperature programmed oxidation, CO2‐TPD, H2‐TPR) and were tested in a fixed‐bed reactor at 800°C and 42,000 mL/h gcat. Barium played a crucial role in enhancing catalyst reducibility and CO2 adsorption at high temperatures, as indicated by the activity and stability of the Ni‐YZr catalyst. The addition of 4.0 wt% of barium appeared to be the optimal loading, allowing for CH4 conversion of 82%, which remained constant for 7 h of reaction, compared with 72% of barium‐unpromoted Ni‐YZr at 800°C. TEM images of the spent catalysts revealed the formation of multiwalled carbon nanotubes on all samples. The TGA analysis showed, however, that an increase in baria loading significantly reduced the coke formation amount, indicating the inhibition of coke formation and the enhancement of the catalytic activity. Such improvement in activity and stability was attributed to the incorporation of barium into YZr support, as revealed by XRD analysis, which inhibited the sintering of the catalysts support.

Insights into the formation of multiwall carbon nanotubes using simple flame pyrolysis method

Herein, we discuss the formation of multiwall carbon nanotubes (MWCNTs) during the simple and effective flame pyrolysis of ferrocene solution in ethanol with the help of alcohol lamp. The method is unique and simple one to prepare impure MWCNTs in the best possible way. Systematic investigations showed that the in-situ generated maghemite plays an important role in the formation and development of the MWCNTs. The growth of the maghemite impregnated MWCNTs were thoroughly studied using sophisticated instruments viz. XRD, BET, HR-SEM, and TEM analysis in details and on the basis of these, the growth mechanism is discussed.

Effect of Holmium Oxide Loading on Nickel Catalyst Supported on Yttria-Stabilized Zirconia in Methane Dry Reforming.

The carbon dioxide reforming of methane has attracted attention from researchers owing to its possibility of both mitigating the hazards of reactants and producing useful chemical intermediates. In this framework, the activity of the nickel-based catalysts, supported by yttria-stabilized zirconia and promoted with holmium oxide (Ho2O3), was assessed in carbon dioxide reforming of methane at 800 °C. The catalysts were characterized by N2-physisorption, H2 temperature-programmed reduction, temperature-programmed desorption of CO2, X-ray diffraction, scanning electron microscopy (SEM) together with energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) techniques. The effect of holmium oxide weight percent loading (0.0, 1.0, 2.0, 3,0, 4.0, and 5.0 wt %) was examined owing to its impact on the developed catalysts. The optimum loading of Ho2O3 was found to be 4.0 wt %, where the methane and carbon dioxide conversions were 85 and 91%, respectively. The nitrogen adsorption-desorption isotherms specified the mesoporous aspect of the catalysts, while the SEM images displayed a morphology of agglomerated, porous particles. The TEM images of the spent catalyst displayed the formation of multiwalled carbon nanotubes. TGA of the 4.0 wt % of Ho2O3 catalyst, experimented over 7-hour time-on-stream, displayed little weight loss (<14.0 wt %) owing to carbon formation, indicating the good resistance of the catalyst to carbon accumulation due to the enhancing ability of Ho2O3 and its adjustment of the support.

CO-promoted low-temperature conversion of CH4 to hydrogen and carbon nanotubes on Nanocrystalline Cr-doped ferrite catalyst

Catalytic decomposition of methane (CDM) to H2 and multiwalled carbon nanotubes (MWCNTs) was achieved by a nanocrystalline Cr-doped ferrite (FeCr) catalyst at 500 °C and atmospheric pressure with minor cofed CO. The exothermic Boudouard reaction increased the temperature and H2 from CDM at catalyst surface that induced Fe2+ reduction to Fe0. The Fe0 clusters along with the CO-originated surface oxygens enabled transfer of C and H to sustain the surface CDM and CO reactions. The metallic Fe-enabled C transfer led to the formation of MWCNTs. The Cr6+/3+ dopants facilitated the Fe redox cycles and maintained surface oxygens for high catalytic activity.

The Use of Diatomite as a Catalyst Carrier for the Synthesis of Carbon Nanotubes.

In this article, multiwalled carbon nanotubes (MWCNTs) have been synthesized on the surface of a diatomite mineral impregnated with transition metal salts using a propane-butane mixture in a chemical vapor deposition reactor at atmospheric pressure. The catalyst concentration and synthesis temperature have been varied in order to understand their effects on the formation of MWCNTs and their morphology. Diatomite was chosen as a catalyst carrier due to its elemental composition. It is mainly composed of amorphous silica, quartz and also contains such metals as Fe, K, Ca, Mn, Cr, Ti, and Zn, which makes it a promising material for use as a catalyst carrier when synthesizing carbon nanotubes (CNTs) by catalytic chemical vapor deposition (C-CVD). For the synthesis of carbon nanotubes by C-CVD on the surface of the diatomite, the following salts were used as a catalyst: CoCl2·6H2O; Ni(NO3)2·6H2O, and the concentrations of the solutions were 0.5; 1.0 and 1.5 M. Natural diatomite was characterized by X-ray diffraction analysis (XRD) and Scanning Electron Microscopy (SEM) analysis.

Synergistic Effect of Hexagonal Boron Nitride-Coated Separators and Multi-Walled Carbon Nanotube Anodes for Thermally Stable Lithium-Ion Batteries

In this work, we report the development of separators coated with hexagonal boron nitride (hBN) to improve the thermal stability of Li-ion batteries (LIBs). Aiming to achieve a synergistic effect of separators and anodes on thermal stability and electrochemical performance, multiwalled carbon nanotubes (MWCNTs) were prepared via plasma-enhanced chemical vapor deposition (PECVD) method and used as potential anode materials for LIBs. The grown MWCNTs were well characterized by using various techniques which confirmed the formation of MWCNTs. The prepared MWCNTs showed a crystalline structure and smooth surface with a diameter of ~9–12 nm and a length of ~10 μm, respectively. Raman spectra showed the characteristic peaks of MWCNTs and BN, and the sharpness of the peaks showed the highly crystalline nature of the grown MWCNTs. The electrochemical studies were performed on the fabricated coin cell with a MWCNT anode using a pristine and BN-coated separators. The results show that the cell with the BN-coated separator in a conventional organic carbonate-based electrolyte and MWCNTs as the anode resulted in a discharge capacity (at 65 °C) of ~567 mAhg−1 at a current density of 100 mAg−1 for the first cycle, and delivered a capacity of ~471 mAhg−1 for 200 cycles. The columbic efficiency was found to be higher (~84%), which showed excellent reversible charge–discharge behavior as compared with the pristine separator (69%) after 200 cycles. The improved thermal performance of the LIBs with the BN-coated separator and MWCNT anode might be due to the greater homogeneous thermal distribution resulting from the BN coating, and the additional electron pathway provided by the MWCNTs. Thus, the fabricated cell showed promising results in achieving the stable operation of the LIBs even at higher temperatures, which will open a pathway to solve the practical concerns over the use of LIBs at higher temperatures without compromising the performance.

Electrochemical sensor studies of dopamine using multiwalled carbon nanotubes by CVD technique

The synthesized Multiwall carbon nanotubes (MWCNTs) structural conformation studies were carried out using the powder X-ray diffraction. Plane 002 shows the crystallinity of graphite hexagonal carbon structure. The Fourier transform spectroscopy shows the presence of CC double bond which reveals the presence of MWCNTs. Morphology and Elemental analysis were studied by SEM-EDX analysis. HR-TEM and SAED pattern shows that the formation of MWCNTs and the same is matched with XRD pattern. In addition to that, the thermal stability of the MWCNTs was carried out using differential scanning calorimetry and thermo-gravimetric analysis. The UV–visible spectroscopy shows the strong absorption band at 357 nm and the corresponding energy band gap was found to be 3.34ev. In the present study the synthesized MWCNTs used as bare carbon paste electrode (BCPE) and modified carbon paste electrode (MCPE) to study the electrochemical biosensor behavior of dopamine (DA) using the cyclic voltammetric technique. DA represents the oxidation peak is 0.1009 V and 0.1049 V and also the reduction peak is 0.1470 V and 0.1522 V at the BCPE and MWCNT/MCPE. Hence the oxidation and reduction peak range of DA at the modified MWCNTs were significantly higher. The electron transfer reaction of DA is shown by the effect of scan rate and the concentration of dopamine. The modified electrode's stability was investigated with the outcomes indicating it is relatively stable. The developed sensor had a high level of sensitivity, stability, and reproducibility.

Preparation and characterization of carbon nanotubes by pulsed laser ablation in water for optoelectronic application

Abstract In the present work, preparation of multi-walled carbon nanotubes (MWCNTs) and carbon nanoparticles (CNPs) by using pulsed laser ablation of a graphite target in water without using catalyst was demonstrated. The effect of laser wavelength on the optical absorption and structural properties has been studied. X-ray diffraction (XRD) data shows the synthesized CNTs were polycrystalline and a peak related to the diamond structure was observed. Scanning electron microscope (SEM) investigation displays that the average diameter of CNTs synthesized with 532 nm was 20 nm and few micrometers in length, while the CNTs synthesized with 1064 nm have an average diameter of 75 nm and lengths of few sub-micrometers. Some of CNPs were noticed to deposit on the CNTs. Transmission electron microscope (TEM) was used to study the morphology of MWCNTs. Raman spectra confirm formation of MWCNTs during the presence of three peaks belonged to D-band, G-band and 2D-band. The intensity ratio of IG/ID is larger than unity for MWCTs prepared by two laser wavelengths. The colloidal MWCNTs prepared by 532 nm laser pulses showed higher absorption than that of MWCNTs prepared with 1064 nm. The current-voltage characteristics and responsivity of hybrid In/p-MWCNTs/n-Si heterojunction photodetectors prepared at different CNTs film thicknesses were investigated at room temperature. The responsivity of hybrid MWCNTs/n-Si photodetector has two peaks of response, the first peak was found at 650 nm and the second peak located at 850 nm. The maximum responsivity was 0.53 A/W at 532 nm was found for the photodetector fabricated with 532 nm laser. The energy band diagram of MWCNTs/Si heterojunction was constructed under illumination condition.

Effect of pre-treatment and calcination temperature on Al2O3-ZrO2 supported Ni-Co catalysts for dry reforming of methane

In this paper, the effect of pre-treatment and calcination temperature on a series of 5%Co/Al2O3-ZrO2, 5%Ni/Al2O3-ZrO2 and 2.5%Co-2.5%Ni/Al2O3-ZrO2 catalysts for dry reforming of methane was investigated. Main focus of our research was to improve the catalyst stability by proper pre-treatment and reaction conditions. The first approach aimed at the catalyst pre-treatment by using bimetallic systems and the second strategy at the in situ suppression of coke. The catalytic activity of bimetallic system was indeed higher compared to the monometallic in the temperature range of 500–800 °C (space velocity 18000 ml h−1·gcat−1, CH4/CO2 = 1). The bimetallic catalyst calcined at 800 °C showed highest CH4 conversion without deactivation and gave a H2/CO ratio of 91% and 0.96, respectively, and good stability with less coke deposition over 28 h at 800 °C reaction temperature. This improvement is assigned to the synergism between Co and Ni, their high dispersion according to interaction with support. It has been shown in our work that pretreatment temperatures and atmospheres have strong impact on stability of the catalyst. TEM, XRD and TPO investigations confirmed that the slight catalyst deactivation was related to the formation of multiwall carbon nanotubes with hollow inner tube structure. The addition of small amounts of steam or oxygen during DRM improved both the catalyst activity and stability as the bimetallic catalyst lost around 9.4% conversion in DRM, 5.4% in presence of water and only 3.2% in presence of O2.

Kinetic study of multi-walled carbon nanotube synthesis by thermocatalytic decomposition of methane using floating catalyst chemical vapour deposition

Multiwalled carbon nanotubes (MWCNTs) have been synthesized using methane in a floating catalyst chemical vapour deposition reactor at atmospheric pressure. The catalyst concentration, partial pressure of methane and synthesis temperature have been varied in order to understand their effects on the rate formation of MWCNTs, purity and size of MWCNTs. Reaction mechanism, rate controlling step and overall rate of reaction have been deduced using chemical reaction engineering approach. A systematic study was carried out in order to obtain the kinetic expression based on Langmuir-Hinshelwood type model. The kinetic model was based on the dissociative adsorption pathway of methane and subsequent removal of hydrogen from the adsorbed methyl group with two different active sites. The activation energy for the overall reaction was 46.1 kJ mol−1 in the temperature range of 1073–1273 K. The products have been characterized by transmission electron microscopy, thermogravimetry, BET and Raman spectroscopy. The BET surface area of purified MWCNTs were found to be 157 m2 g−1 with pore volume of 0.176 cc g−1.

Synthesis of Carbon Nanotubes on a Shungite Substrate and Their Use for Lithium–Sulfur Batteries

A shungite mineral has been used as a support material of catalyst particles to synthesize multiwalled carbon nanotubes (MWCNTs). Raman spectroscopy enabled us to follow the formation of MWCNTs. The morphology of synthesized MWCNTs was investigated by a scanning electron microscope and a transmission electron microscope. As a result of simple heat treatment at 300°C for 3 h in an inert atmosphere, a novel sulfur/multiwalled carbon nanotubes/polyacrylonitrile (S/MWCNT/PAN) composite was synthesized. These methods of obtaining MWCNTs and S/MWCNT/PAN composite based on heat treatment possess the advantages of simplicity and low cost. The introduction of MWCNTs into the composite gives a highly conductive and mechanically flexible framework with an enhanced electronic conductivity and the ability to absorb polysulfides between the Li anode and cathode, which leads to an enhanced cyclability and a higher coulombic efficiency. The cell with this S/MWCNT/PAN ternary composite cathode demonstrates a stable reversible specific discharge capacity of 800 mA·h·g–1 after 50 cycles at a battery C-rate of 0.2 C.

Optimization of supported bimetallic (Fe-Co/CaCO3) catalyst synthesis parameters for carbon nanotubes growth using factorial experimental design

The Fe–Co bimetallic catalyst supported on CaCO3 was prepared by a wet impregnation method. The interactive effects among the key synthesis parameters such as drying time, calcination temperature, stirring speed, mass of CaCO3 support on the yield and quality properties of the catalyst were investigated using 24 Factorial design of experiment. Additionally, the catalyst obtained under the optimal conditions was used to prepare Multi-walled carbon nanotubes (MWCNT) via chemical vapour deposition (CVD) of acetylene gas in a tubular horizontal reactor. The quality of the prepared materials was examined for their morphology, microstructure, elemental composition, surface chemical properties, thermal stability, surface area, and mineralogical phase by HRSEM, HRTEM, EDS, TGA, FTIR, BET and XRD. The results revealed that maximum catalyst yield of 99.6% obtained at the drying time of 12 h, calcination temperature of 100 °C, stirring speed of 1000 rpm and mass of CaCO3 support of 10 g favoured the formation of MWCNT at 700 °C and 60 min reaction time. The results of statistical analysis demonstrated a direct relationship and synergetic effect between stirring speed and mass of support and the two factors exerted highest impact on the yield of catalyst than other parameters. Microscopy analysis revealed successful dispersion of Fe and Co particles onto CaCO3 support while XRD patterns confirmed the presence of highly active crystalline mixed oxide (CoFe2O4) as the dominant phase. The HRSEM/XRD analysis displayed the formation of tubular networks of graphitic carbon materials with few traces of by-products. The study established the absorption and not diffusion mechanism of Fe and Co onto CaCO3 in the reaction sequence produced CoFe2O4 during the wet impregnation process employed for catalyst preparation.

Fabrication of open-cell thermoelectric polymer nanocomposites by template-assisted multi-walled carbon nanotubes coating

Abstract In this work, open-cell polyvinylidene fluoride (PVDF) templates were employed to assist the formation of multi-walled carbon nanotubes (MWCNT) network in polymer matrices. The continuous network of MWCNT coating and the macro-porosity helped to simultaneously promote the electrical conductivity and suppress the effective thermal conductivity of organic thermoelectric (TE) materials, and thereby enhance their TE conversion efficiencies. In-situ polymerization of polypyrrole during the template-assisted nanotube coating process also promoted the Seebeck coefficient of the polymer nanocomposites by two-fold, while suppressing their electrical conductivity. An optimum ZT value of 1.4 × 10−5 was achieved for PVDF/MWCNT nanocomposite foams with loaded 24.9 wt% MWCNT. A series of parametric experiments were performed in this study to investigate the effects of open-cell morphology, nanotubes content, and polypyrrole-nanotube interfaces on TE properties of organic materials.

Formation of Multiwall Carbon Nanotubes on Cylindrical Substrates in Synthesis by Metal-Organic Chemical Vapor Deposition

Processes of deposition of multiwall carbon nanotubes in the synthesis by metal-organic chemical vapor deposition on hollow cylindrical substrates and the effect of the substrate area on the yield of the target product were studied. Making larger the tubular deposition reactor and the area of the cylindrical substrate enabled a substantial increase in the yield of carbon nanotubes. The technological synthesis parameters of the carbon material were optimized. Methods were developed for purification of carbon nanotubes by annealing in air, and materials were obtained with purity of no less than 98%. Physicochemical analysis methods were used to examine the structure and properties of the materials.

Effect of carbon nanotubes irradiation by argon ions on the formation of SnO 2-x /MWCNTs composite

Abstract Methods of transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge structure (XANES) were used to investigate the influence of irradiation by argon ions of multi-walled carbon nanotubes (MWCNTs) on the composites SnO 2-x /MWCNTs formation. It has been shown that irradiation of the MWCNTs leads to the formation of structural defects and oxygen-containing groups in the walls of carbon nanotubes. The presence of the latest in MWCNTs walls leads to chemical interaction of the tin oxide with the surface of MWCNTs through formation of chemical Sn O C bonds at the interfaces “Tin oxide-MWCNTs” at synthesis of the composite by CVD method. It has been established that the crystalline structure of the tin oxide clusters forming on surface of MWCNTs depends essentially on the defectiveness in the structure of the outer walls of carbon nanotubes.

Effects of catalysts state on the synthesis of MWCNTs modified expanded graphite through microwave-assisted pyrolysis of ethanol

Multi-walled carbon nanotubes (MWCNTs) were prepared through microwave-assisted pyrolysis of ethanol to enhance the electron transfer of expanded graphite (EG). Three kinds of Fe-containing catalyst precursors (ferric nitrate, citric acid-enhanced ferric nitrate and ferrocene) were designated in the present work to investigate the influences of catalyst states on the formation of MWCNT. The cyclic voltammograms and electrochemical impedance of the prepared specimens were characterized. Furthermore, the microstructures of catalyst-loaded EG before and after microwave treatment were investigated by means of SEM and TEM. The results show that the growth of MWCNT under microwave treatment is determined by the state of catalyst and only nanoscaled Fe-containing catalysts favor the formation of MWCNT. The ferric nitrate tends to grow up into submicron Fe2O3 particles, which impedes the growth of MWCNT. In comparison, citric acid enhances the dispersion of ferric nitrate, allowing the growth of MWCNT. Ferrocene possesses dual functions, namely it assures well distribution of nanoscaled catalysts and is also responsible for the growth of first generation of MWCNT. With the growth of MWCNT on EG, the anodic peak current densities and charge transfer resistance of the MWCNT modified EG outperform those of pure EG remarkably.

Multiwalled carbon nanotube forests grown on the surface of synthetic diamond crystals

Abstract Forests of multiwalled carbon nanotubes (MWCNTs) were grown on the facets of synthetic diamond crystals by treating them in a rotary chemical vapor deposition reactor. Nickelocene was used as a source of both nickel catalyst and carbon for the formation of MWCNTs. The diamond crystals were not subjected to any surface treatment prior to the MWCNT growth. The MWCNTs were anchored in nickel particles adhered to the diamond facets and were partially filled with nickel. These morphological features of the diamond-MWCNT structures together with nanotube branching suggest the formation of MWCNTs by the combined growth mechanism.