Catalytic formation of carbon phases in metal modified, porous polymer derived SiCN ceramics

A novel method for the simultaneous formation of catalytic active metal nanoparticles, multiwall carbon nanotubes (MWCNTs) and/or turbostratic carbon and porous M@SiCN (M = Fe, Co, Pt, Cu, Ag, Au) ceramics during pyrolysis of metal modified polysilazanes and polyethylene (PE) particles as sacrificial filler is described. The thermal decomposition of the polyethylene leads not only to the generation of the porosity but also to an in situ reduction of the metal compounds to the metal nanoparticles, due to the reductive atmosphere. Depending on the metal, carbon nanotubes as well as turbostratic carbon were formed in different amounts, due to the chemical vapor deposition (CVD) like conditions. The resulting carbon phases, ceramics and metal nanoparticles were investigated using the combination of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) measurements, giving evidence for the presence of the carbon phases and the metal particles.     

Formation and Yield of Multi-Walled Carbon Nanotubes Synthesized via Chemical Vapour Deposition Routes Using Different Metal-Based Catalysts of FeCoNiAl,CoNiAl and FeNiAl-LDH

Multi-walled carbon nanotubes (MWCNTs) were prepared via chemical vapor deposition (CVD) using a series of different catalysts, derived from FeCoNiAl, CoNiAl and FeNiAl layered double hydroxides (LDHs). Catalyst-active particles were obtained by calcination of LDHs at 800 °C for 5 h. Nitrogen and hexane were used as the carrier gas and carbon source respectively, for preparation of MWCNTs using CVD methods at 800 °C. MWCNTs were allowed to grow for 30 min on the catalyst spread on an alumina boat in a quartz tube. The materials were subsequently characterized through X-ray diffraction, Fourier transform infrared spectroscopy, surface area analysis, field emission scanning electron microscopy and transmission electron microscopy. It was determined that size and yield of MWCNTs varied depending on the type of LDH catalyst precursor that is used during synthesis. MWCNTs obtained using CoNiAl-LDH as the catalyst precursor showed smaller diameter and higher yield compared to FeCoNiAl and FeNiAl LDHs.     

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.     

A novel approach to Co/CNTs catalyst via chemical vapor deposition of organometallic compounds

A novel approach for attaching well-dispersed cobalt nanoparticles homogeneously onto carbon nanotubes via metal organic chemical vapor deposition technique is reported. The obtained Co/CNTs catalysts feature a narrow size distribution of Co particles centering around 7.5 nm, and show high activity and regioselectivity for hydroformylation of 1-octene.     

Structure and magnetic properties of multi-walled carbon nanotubes modified with cobalt

The magnetic properties of multi-walled carbon nanotubes (MWCNTs) modified with cobalt nanoparticles were studied in the temperatures and magnetic field range of (4.2–290) K and (0.03–5) T, respectively. Nanoparticles of cobalt encapsulated inside MWCNTs were obtained by using the chemical vapor deposition technique. The low temperature SQUID magnetization measurements were supplemented with structural investigations by means of high-resolution transmission electron microscopy, scanning electron microscopy as well as thermogravimetric and X-ray diffraction analysis. X-ray diffraction revealed the presence of MWCNTs, f.c.c. Co and h.c.p. Co phases. The magnetic characterization provided the remanent magnetization value (M_R) of about 0.07 emu/g (∼40% of the saturation moment), while the coercive field (H_C) value amounts to 600 Oe. Both parameters M_R and H_C slightly decrease with the rise of temperature. The substantial magnetization increase observed at low temperatures suggests the existence of nano Co clusters (in the atomic scale size).     

Synthesis of boron nitride nanotubes by combining citrate-nitrate combustion reaction and catalytic chemical vapor deposition

High-quality boron nitride nanotubes were successfully synthesized via a novel two-step method, including citrate-nitrate combustion reaction and catalytic chemical vapor deposition. The composition, bonding features and microstructures of as-synthesized sample were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, transmission electron microscopy and selected area electron diffraction techniques. The results show that the as-synthesized boron nitride nanotubes with smooth surface are relatively pure. The diameter ranges between 20 and 80?nm, while the length is about dozens of micrometers. During the synthesis process of boron nitride nanotubes, citric acid chelates the cobalt ions and reacts with nitrate to form the cobalt oxide, depositing on the surface of boron powder homogeneously. The catalyst content and annealing temperature have a significant impact on the composition and microstructures of the final products. Based on the experimental results and thermodynamic analysis, the possible chemical reactions are listed, and vapor-liquid-solid mechanism is proposed to be dominant for the formation of boron nitride nanotubes.     

Synthesis of carbon nanotubes by pyrolysis of solid Ni(dmg)2

We describe the high yield synthesis of multi-walled carbon nanotubes (MWCNTs) and the determination of the optimum production conditions. The method involves the catalytic pyrolysis of solid Ni(dmg)₂ under an Ar atmosphere. The obtained materials were characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy and thermogravimetry analysis (TGA). The data revealed the formation of MWCNTs surrounded by a varying quantity of byproducts such as amorphous carbon and metallic particles, depending mainly on the reaction temperature. Pyrolysis of Ni(dmg)₂ at 900 °C results in the production of nanotube material with the highest degree of crystallinity.     

Synthesis of multi-walled carbon nanotubes over tungsten-doped cobalt-based catalyst derived from a layered double hydroxide precursor

Multi-walled carbon nanotubes (CNTs) were prepared over a series of W-doped Co-based catalysts derived from layered double hydroxide precursor by catalytic chemical vapor deposition (CCVD) of acetylene. The materials were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption–desorption experiments and Raman spectroscopy. The effect of the proportion of W in the Co-based catalysts on the carbon yield, diameter uniformity and quality of CNTs was investigated. The results demonstrated that with the increasing W/Co molar ratio from 0 to 1.0, both the mean number of walls and the average diameter of CNT produced over catalysts increased from about 8 to 28 nm and from about 12.1 to 23.7 nm, respectively. A small amount of tungsten added to the catalyst with the W/Co molar ratio of 0.3 could facilitate the dispersion of catalytically active Co species on the surface of support, and thus uniform and high-quality CNTs with a remarkably high yield of ca. 1600% were obtained.     

CO-free hydrogen from steam-reforming of bioethanol over ZnO-supported cobalt catalysts: Effect of the metallic precursor

The ethanol steam-reforming reaction was studied over ZnO-supported cobalt catalysts (10 wt.% Co). Catalysts were prepared by impregnation of nitrate and carbonyl cobalt precursors. Characterization was accomplished by transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), and in situ techniques: magnetic measurements, and diffuse reflectance infrared spectroscopy (DRIFT) coupled to mass spectrometry. The use of Co₂(CO)₈ as precursor produced a catalyst that was highly stable and selective for the production of CO-free hydrogen at reaction temperature as low as 623 K. The only by-product was methane and selectivity of 73% to H₂ and 25% to CO₂ was obtained. Under reaction conditions, the catalyst showed 92% of reduced cobalt, mainly as small particles.     

PtRu nanoparticles supported on nitrogen-doped multiwalled carbon nanotubes as catalyst for methanol electrooxidation

Nitrogen-doped carbon nanotubes (N-CNT) obtained by plasma treatment were compared to the conventional acid-treated carbon nanotubes (O-CNT) as catalyst support for platinum-ruthenium (PtRu) nanoparticles in the anodic oxidation of methanol in direct methanol fuel cells. PtRu catalysts were prepared by an impregnation-reduction method from chloride precursors with metal loadings of 20 wt.%, and were characterised by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and electrochemical methods. Voltammetry and chronoamperometry studies showed that the performance of PtRu/N-CNT was significantly higher compared to PtRu/O-CNT and also to the commercial E-TEK PtRu/C catalyst, indicating that N-CNT are an interesting support material for fuel cell electrocatalyst. Nitrogen plasma treatment produced pyridinic and pyrrollic species on the CNT surface, which acts as the anchoring sites for the deposition of PtRu particles. A mechanism for the deposition of PtRu on N-CNT is tentatively proposed and discussed.     

Highly active Fischer–Tropsch synthesis Co/SiO2 catalysts prepared from microwave irradiation

Studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) as well as energy-diffusive X-ray spectroscopy (EDX), a highly active Co/SiO₂ catalyst which was prepared through microwave irradiation showed more uniform and better dispersion of the Co particles within the catalyst pellets, compared to that prepared by the conventional heating method. The Fischer–Tropsch (FT) synthesis activity of the microwave-irradiated catalyst was greater than that of the conventional heating one. Not only did the catalytic activity depend on the particle distribution, but also the irradiative time. The longer irradiative time increased the catalytic activity. It was shown that the irradiative time of 14 min was an optimum. It was found that cobalt distributed uniformly inside the whole catalyst pellet prepared by the microwave irradiation. On the contrary, cobalt concentration at the outer surface, with agglomerated form, was greater than that at the center or inner part of the catalyst pellet made by the conventional heating method. Higher dispersion state of the supported cobalt, as also proved by XRD, realized at the microwave irradiated catalyst determined its improved FT activity.     

Synthesis of PtRu/carbon nanotube composites in supercritical fluid and their application as an electrocatalyst for direct methanol fuel cells

PtRu nanoparticles were decorated on multi-walled carbon nanotubes (MWCNTs) using H₂PtCl₆ and RuCl₃ as precursors with the aid of supercritical CO₂, and the resulting composites were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy. TEM observation showed that nanoparticles of size about 5 nm were distributed evenly on the MWCNTs, and XRD analysis showed that the particles had a face-centered cubic crystal structure. The loading content of the nanoparticles on the MWCNTs could be adjusted by manipulating the relative ratio of the precursor to MWCNTs. The as-prepared PtRu/MWCNT composites exhibited high activity for methanol electro-oxidation.     

Characterization of Polyacrylonitrile Nanocomposites by Reinforcement of Functionalized Single-Walled Carbon Nanotubes

Polyacrylonitrile/functionalized single-walled carbon nanotubes (PAN/f-SWCNTs) nanocomposites were synthesized by an emulsifier-free in situ polymerization process. Interaction of polyacrylonitrile with functionalized single-walled carbon nanotubes was evidenced by ultraviolet-visible and Fourier transforms infrared spectroscopy. The structure and morphology of nanocomposites were characterized by X-ray diffraction, field emission scanning electron microscopy and high resolution transmission electron microscopy. Electrical conductivity was found to be increased by addition of f-SWCNTs. Thermogravimetric analysis study of PAN/f-SWCNT nanocomposites show more thermal resistance compared to the virgin PAN. The oxygen barrier property of PAN/f-SWCNT nanocomposites was reduced by eight times with increasing f-SWCNTs proportions.     

The effect of carbon microfiber substrate pretreatment on the growth of carbon nanomaterials

The floating catalyst method was used to obtain carbon nanotubes, carbon submicrotubes and other carbon nanomaterials by changing the pretreatment conditions of carbon microfiber substrate. The morphology and microstructure of the obtained products were characterized by field emission scanning electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. The results showed that on untreated carbon microfiber surface, only some carbon particles and several carbon nanotubes were deposited. However, after carbon microfibers were boiled in the solution of H₂SO₄/HNO₃ and were immersed in the solutions of Fe(NO₃)₃/xylene, Fe(NO₃)₃/acetone, ferrocene/acetone and Fe(NO₃)₃/ferrocene/acetone, the obtained products were a high-density carbon nanotubes, carbon nanotubes with many carbon particles, carbon submicrotubes and a mixture of carbon nanomaterial, respectively. Thus the pretreatment of the carbon microfiber substrate greatly influenced the morphology and microstructure of the synthesized products.     

Characterization of epitaxial GaAs MOS capacitors using atomic layer-deposited TiO2/Al2O3 gate stack: study of Ge auto-doping and p-type Zn doping

Few-wall carbon nanotubes were synthesized by methane/acetylene decomposition over bimetallic Fe-Mo catalyst with MgO (1:8:40) support at the temperature of 900°C. No calcinations and reduction pretreatments were applied to the catalytic powder. The transmission electron microscopy investigation showed that the synthesized carbon nanotubes [CNTs] have high purity and narrow diameter distribution. Raman spectrum showed that the ratio of G to D band line intensities of I _G/I _D is approximately 10, and the peaks in the low frequency range were attributed to the radial breathing mode corresponding to the nanotubes of small diameters. Thermogravimetric analysis data indicated no amorphous carbon phases. Experiments conducted at higher gas pressures showed the increase of CNT yield up to 83%. M?ssbauer spectroscopy, magnetization measurements, X-ray diffraction, high-resolution transmission electron microscopy, and electron diffraction were employed to evaluate the nature of catalyst particles.     

Fabrication of graphene supported carbon‐coating cobalt and carbon nanoshells for adsorption of toxic gases and smoke

Graphene‐supported carbon‐coating cobalt and carbon nanoshells (Co/C‐GNS and CNS‐GNS) were fabricated and their applications in absorbing toxic gases and smoke have been investigated. Co₃O₄‐loaded reduced graphite oxide was first prepared via a coprecipitation process, then carbon coatings on cobalt nanoparticles were fabricated by a catalytic carbonization process. The obtained hybrids were characterized by X‐ray diffraction, transmission electron microscopy, Raman spectroscopy, N₂ absorption/desorption, and thermogravimetric analysis. Co/C core/shell structure and hollow carbon nanoshells in the size range of 15–22 nm were anchored onto the graphene surfaces. The resultant Co/C‐GNS and CNS‐GNS performed an important function in CO removal and smoke suppression during the combustion of acrylonitrile‐butadiene‐styrene. The good performance could be attributed to the combination effect of physical barrier of the GNS, porosity structure of the carbon nanoshells, and carbonization of the Co nanoparticles. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40457.     

Fischer–Tropsch synthesis over cobalt dispersed on carbon nanotubes-based supports and activated carbon

Carbon nanotubes (CNTs) and the ones grown on MgO and alumina are used as supports for cobalt catalyst in Fischer–Tropsch (FT) synthesis. Carbon nanotubes were synthesized by chemical vapor deposition of methane on 5.0 wt.% iron on MgO or alumina at 950 °C. The carbon nanotubes were characterized by SEM and TEM microscopy and Raman spectroscopy. Cobalt nitrate was impregnated onto the supports by impregnation, and the samples were dried and reduced in-situ at 400 °C for 12 h, and then FT synthesis was carried out in a fixed-bed reactor. The catalysts were characterized by BET surface area measurement, TPR and TPD. The effect of carbon nanotubes as cobalt support on CO conversion, product selectivity, and olefin to paraffin ratio of FT synthesis was investigated and compared with activated carbon as well as Al₂O₃, as a traditional support. The results revealed that the activity of the Co/CNT catalyst was improved by 22%, compared to the conventional Co/alumina catalysts. Also the cobalt supported on CNTs grown on MgO (Co/CNT–MgO) shows the highest selectivity to C₅₊ as the most desired FTS products. The C₅₊ selectivity enhancement was about 37, 34, 17, and 77% as compared to the Co/CNT, Co/alumina, Co/CNTs-alumina, and Co/activated carbon, respectively. Also the olefin/paraffin ratio on the Co/CNTs-MgO catalyst is about 7.7 times higher than the conventional cobalt catalysts. It seems that the degree of reduction of cobalt is higher when supported on CNTs than on alumina. This leads to higher FTS activity. Also, the particle size distribution of the cobalt is affected by the CNT–MgO support leading to higher C₅₊ selectivity.     

Carbon nanocapsules encapsulating cobalt nanoparticles by pulsed discharge plasma chemical vapor deposition

Silicon coated with a thin film of cobalt [Si/Co (10 nm)] is exposed to the plasma generated using CH₄–H₂ gas mixture by making a discharge between Si/Co substrates and Mo bent plate in pulsed discharge plasma chemical vapor deposition. At high plasma temperature and deposition pressure, carbon nanocapsules encapsulating Co nanoparticles are observed to form. They are investigated using high resolution transmission electron microscopy, scanning electron microscopy, visible Raman spectroscopy and X-ray diffraction. Present study indicates that the formation mechanism of carbon nanocapsules lie in the sputtering of Co thin film by the energetic ions from plasma at high deposition pressure which results in the formation of Co nanoparticles, on surface of which graphitic layers gets deposited at high plasma temperature. Present approach provides a novel strategy for the synthesis of high purity carbon nanocapsules encapsulating metal nanoparticles.     

Growth of few-wall carbon nanotubes with narrow diameter distribution over Fe-Mo-MgO catalyst by methane/acetylene catalytic decomposition

Few-wall carbon nanotubes were synthesized by methane/acetylene decomposition over bimetallic Fe-Mo catalyst with MgO (1:8:40) support at the temperature of 900°C. No calcinations and reduction pretreatments were applied to the catalytic powder. The transmission electron microscopy investigation showed that the synthesized carbon nanotubes [CNTs] have high purity and narrow diameter distribution. Raman spectrum showed that the ratio of G to D band line intensities of I_G/I_D is approximately 10, and the peaks in the low frequency range were attributed to the radial breathing mode corresponding to the nanotubes of small diameters. Thermogravimetric analysis data indicated no amorphous carbon phases. Experiments conducted at higher gas pressures showed the increase of CNT yield up to 83%. Mössbauer spectroscopy, magnetization measurements, X-ray diffraction, high-resolution transmission electron microscopy, and electron diffraction were employed to evaluate the nature of catalyst particles.     

Nanotubes and nanofilaments from carbon monoxide disproportionation over Co/MgO catalysts: I. Growth versus catalyst state

A catalyst prepared from the pyrolysis of Co and Mg nitrates and citric acid after their co-dissolution in water was used for carbon deposition from CO. Good yields of nanotubes or nanofilaments were obtained over catalysts which had been reduced by H₂ without preliminary treatment at high temperature. Nanotubes with 10 or more cylindrical carbon layers were obtained from pure CO or from CO+CO₂ mixtures. Nanofilaments with truncated conical layers were obtained from CO+H₂ mixtures in the 500–600 °C range. In both cases, high shape selectivity was obtained and almost all MgO could be eliminated by HCl treatment. The only significant impurities were embedded cobalt particles. This process is therefore suitable for preparing nanotubes or nanofilaments with good shape selectivity and 98 wt% purity. Lowering the Co content of the catalyst produces thinner nanotubes but reduces the yield.