Synthesizing carbon nanotubes and carbon nanofibers over supported-nickel oxide catalysts via catalytic decomposition of methane

Supported-NiO catalysts were tested in the synthesis of carbon nanotubes and carbon nanofibers by catalytic decomposition of methane at 550 °C and 700 °C. Catalytic activity was characterized by the conversion levels of methane and the amount of carbons accumulated on the catalysts. Selectivity of carbon nanotubes and carbon nanofiber formation were determined using transmission electron microscopy (TEM). The catalytic performance of the supported-NiO catalysts and the types of filamentous carbons produced were discussed based on the X-ray diffraction (XRD) results and the TEM images of the used catalysts. The experimental results show that the catalytic performance of supported-NiO catalysts decreased in the order of NiO/SiO₂ > NiO/HZSM-5 > NiO/CeO₂ > NiO/Al₂O₃ at both reaction temperatures. The structures of the carbons formed by decomposition of methane were dependent on the types of catalyst supports used and the reaction temperatures conducted. It was found that Al₂O₃ was crucial to the dispersion of smaller NiO crystallites, which gave rise to the formation of multi-walled carbon nanotubes at the reaction temperature of 550 °C and a mixture of multi-walled carbon nanotubes and single-walled carbon nanotubes at 700 °C. Other than NiO/Al₂O₃ catalyst, all the tested supported-NiO catalysts formed carbon nanofibers at 550 °C and multi-walled carbon nanotubes at 700 °C except for NiO/HZSM-5 catalyst, which grew carbon nanofibers at both 550 °C and 700 °C.     

Anti-coking and anti-carburizing behavior of amorphous AlPO4 coating

The aim of the present study was to examine the anti-coking and anti-carburizing behavior of amorphous AlPO₄ coating. So, aluminum phosphate composition was synthesized by sol-gel process and applied on the AISI 304 stainless steel by dip coating technique. Anti-coking performance was examined in a tube furnace at 1000 °C for 30 min under Ethane (C₂H₆) atmosphere. Carburizing test was performed in a sealed charcoal medium at 1100 °C for a total of 30 h exposure time. Phase composition of the samples was analyzed by X-Ray Diffraction (XRD) after coking and carburizing tests. Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS) were employed to study the morphology and elemental analysis of the samples after coke and carbon formation experiments. Microhardness indenter was applied on the cross section of the carbon-exposed specimens to plot the hardness profile through the carburizing zone. The results of the coking experiment revealed catalytic coke formed on the uncoated surface, while irregular spherical coke with no trace of catalytic coke was formed on the coated surface, indicating the great anti-coking performance of the amorphous AlPO₄ coating. The results of pack-carburizing test demonstrated that the thickness of the carbide layer formed on the bare surface was ～10 times greater than that of the coated sample. Hardness measurement for the amorphous AlPO₄ coated sample detected lower values compared to those for the uncoated one at all distances from the surface, indicating less carbon diffusion occurred beneath the coated surface. In overall, the results declared that the amorphous AlPO₄ coating could be a good candidate for surface protection of stainless steel against catalytic coke formation and carbon diffusion.     

Hydrogenation of catalytic carbons obtained by CO disproportionation or CH4 decomposition on nickel

The hydrogenation of catalytic carbons has been studied in the temperature range of their deposition (300–700°C) by CO disproportionation or by CH₄ decomposition on nickel powders. When obtained under non carbiding conditions, the catalytic carbons are very reactive between 350 and 600°C where uncatalysed carbons are inert. The reactivity does not depend on the temperature of deposition and on preliminary heat treatment, but depends on the degree of gasification. This reactivity is imputed to the quality of the metal carbon interface which allows a good deposition-gasification reversibility. When deposition occurs under carbiding conditions, both deposition and subsequent hydrogenation are poisoned by the carbon formed by thermal decomposition of the carbide.     

Preparation of nanostructured porous carbon composite fibers from ferrum alginate fibers

Nano‐microstructured porous carbon composite fibers (Fe₂O₃@C/FeO@C/Fe@C) were synthesized by the thermal decomposition of ferrum alginate fibers. The ferrum alginate fiber precursors were prepared by wet spinning, and calcined at 300–1000°C in high purity nitrogen. The resulting composite fibers consist of carbon coated Fe₂O₃/FeO/Fe nanoparticles and porous carbon fibers. All the prepared nanostructures were investigated using thermal gravimetry, X‐ray diffraction (XRD), Fourier transform infrared spectroscopy, transmission electron microscope (TEM), and nitrogen adsorption–desorption isotherm. The results show that there are five stages in the decomposition process of the ferrum alginate fibers. Transitions between the five stages are affected by the decomposition temperature. XRD results show that maghemite (Fe₂O₃), wüstite (FeO), martensite (Fe) nanoparticles were formed at 300–500°C, 600–700°C, 800–1000°C, respectively. Scanning electron microscopy and TEM results indicate that the composite fibers consist of nanoparticles and porous carbon. The diameter of the nanosized particles increased from 100 to 500 nm with increasing reaction temperature. The nitrogen adsorption–desorption results also show that the composite fibers have a micro‐ and mesoporous structure. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Coke deposition from acetylene,butadiene and benzene decompositions at 500–900°C on solid surfaces

Coke formation from decomposition of acetylene, butadiene, and benzene and decoking were investigated on Incoloy 800, aluminized Incoloy 800, and Vycor glass surfaces at 500–900°C. On Incoloy 800, the coke was greater in quantity and contained iron and nickel particles. On aluminized Incoloy 800, the coke contained a trace of aluminum, but on Vycor glass, no metal was in the coke. Coking-decoking sequences were highly corrosive on Incoloy 800 surfaces, but they had much less effect on the aluminized Incoloy 800 or Vycor glass. Filamenteous coke which is formed catalytically and contains nickel and iron was formed only on Incoloy 800 surfaces. A general mechanism for formation and deposition of coke is proposed. Filamenteous coke helps collect tar droplets formed by gas-phase reactions. Such droplets decompose on the surface to produce coke that contains no metal.     

Behaviors of oil-soluble molybdenum complexes to form very fine MoS2 particles in vacuum residue

Dispersion and thermal behaviors of oil-soluble Mo-dithiocarbamate (Mo-DTC) and Mo-dithiophosphate (Mo-DTP) as the MoS₂ catalyst precursors were studied in petroleum vacuum residue (VR) using FT-Far IR, XRD and TEM. FT-Far IR was proved to detect the Mo complexes and their derived MoS₂ in VR without the interference of the complicated organic matrix. Their transformation into MoS₂ was identified by detecting the changes in the ligand bonds and crystal structure. These complexes were found to be distributed in asphaltene (or maltene) by fractionation with hexane due to their solubility in the heavy oil, ruling out any chemical interaction of the complex with asphaltene. Mo-DTC was found to be decomposed at 350 °C to form definite MoS₂ in VR. Mo-DTP started its decomposition around 200 °C, and however, no definite formation of MoS₂ was confirmed by heating up to 500 °C. Dispersion of the complexes in VR and asphalthene was always good as indicated by TEM. Both complexes in VR at 380 °C under hydrogenation (HYD) conditions provided more or less MoS₂. H₂S and reactive sulfur species were assumed to accelerate the transformation of the Mo complex to MoS₂ during HYD reaction. Some difficulty of Mo-DTP to be transformed quantitatively into definite MoS₂ in VR may explain its poor activity for up-grading VR.     

Effect of carbon deposition over carbonaceous catalysts on CH4 decomposition and CH4-CO2 reforming

An investigation was made using a continuous fixed bed reactor to understand the influence of carbon deposition obtained under different conditions on CH₄-CO₂ reforming. Thermogravimetry (TG) and X-ray diffraction (XRD) were employed to study the characteristics of carbon deposition. It was found that the carbonaceous catalyst is an efficient catalyst in methane decomposition and CH₄-CO₂ reforming. The trend of methane decomposition at lower temperatures is similar to that at higher temperatures. The methane conversion is high during the initial of stage of the reaction, and then decays to a relatively fixed value after about 30 min. With temperature increase, the methane decomposition rate increases quickly. The reaction temperature has significant influence on methane decomposition, whereas the carbon deposition does not affect methane decomposition significantly. Different types of carbon deposition were formed at different methane decomposition reaction temperatures. The carbon deposition Type I generated at 900°C has a minor effect on CH₄-CO₂ reforming and it easily reacts with carbon dioxide, but the carbon deposition Type II generated at 1000°C and 1100°C clearly inhibits CH₄-CO₂ reforming and it is difficult to react with carbon dioxide. The results of XRD showed that some graphite structures were found in carbon deposition Type II.     

The accelerating and retarding effects of hydrogen on carbon deposition on metal surfaces

Carbon deposition of benzene on iron was studied at 550–700°C with 0–1 atm hydrogen in the carrier gas. At least three types of carbon are formed: amorphous, graphitic and carbidic (Fe₃C). The surface of Fe₃C is essentially inactive for benzene decomposition. In the presence of H₂, a metallic surface is maintained resulting in a high activity and hence an accelerating effect by H₂. In the reaction system five competing reactions are involved and the net rate of carbon deposition is the sum of the individual rates. Based on the results in this study, the retarding effects of H₂ on carbon deposition reported in the literature can also be explained. The methanation reaction of surface carbon by H₂ becomes important under conditions when the surface is relatively inactive for hydrocarbon decomposition, and under such conditions, H₂ has a retarding effect on carbon deposition.     

Synthesis and utilization of mesoporous alumina as a supporting material of Ce-promoted Ni-based catalysts in the methane reforming process

Due to the large applications of hydrogen as a feedstock of chemical industries and as an energy carrier, its production on large scales with low costs has attracted researchers. Steam reforming of methane (SRM) is the most common process for producing H₂-rich syngas over Ni/Al₂O₃ catalysts, which suffer from coke deposition and Ni particles agglomeration. For overcoming these issues, we have synthesized mesoporous alumina (MA) as a supporting material of Ni particles, structure, and activity, which were compared with the bulk alumina (BA) supported catalysts in the SRM process for the first time. Besides, cerium as an appropriate promoter for lowering deposited coke was added to all prepared catalysts. The reaction temperature (600–700°C), Ni loading (10–25 wt.%), and Ce loading (1–5 wt.%) were the parameters that were optimized for maximizing H₂ yield and CH₄ conversion. Prepared samples were characterized by various techniques before and/or after reaction. The results of TEM and XRD depicted the formation of nanocrystalline and mesoporous structure for Ni-MA catalysts compare to Ni-BA samples. The observations indicated that 20Ni-3Ce/MA had the highest catalytic performance, achieving a CH₄ conversion of 91.0% and H₂ yield of 92.8% at 700°C.     

Kinetics of the deposition of pyrolytic carbon on nickel

The kinetics of the deposition of laminar graphite on nickel by the pyrolysis of methane, ethane and ethylene at temperatures from 700–1000° has been studied. Two types of graphitic deposit are identified. Continuous films of laminar graphite are formed at higher temperatures, to a weight limit corresponding to the solubility of carbon in nickel at that temperature. It is concluded that such deposits are formed only as a result of a dissolution-precipitation mechanism. Deposits consisting of islands of graphite in a uniform graphite matrix are formed at lower temperatures. The kinetics of deposition are complex, but qualitative agreement is obtained with a model based on the surface aggregation of carbon atoms to account for island nucleation.     

Characterization of surface carbon formed during the conversion of methane to benzene over Mo/H-ZSM-5 catalysts

During the conversion of methane to benzene in the absence of oxygen over a 2 wt% Mo/H-ZSM-5 catalyst at 700°C, three different types of surface carbon have been observed by X-ray photoelectron spectroscopy: adventitious or graphitic-like C (284.6 eV), carbidic-like C (282.7 eV), and hydrogen-poor sp-type C (283.2 eV), where the C 1s binding energies for the respective forms of carbon are given in parentheses. Pretreatment of the catalyst at 700°C in CO also resulted in a strong signal at 283.2 eV; thus, the species responsible for this signal appears to be different from the usual aromatic-type coke. The coke with dominantly sp hybridization is concentrated on the external surface of the zeolite and is responsible for the gradual deactivation of the catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carbon deposition from acetone during oxidation of iron—the effects of chromium and nickel

Carbon deposition on iron, and its alloys with nickel and chromium, has been produced by catalytic decomposition of acetone in carbon dioxide at 700°C. Nickel has been identified as the major promoter of carbon deposition under these conditions, although iron is also active. When the conditions favoured the occurrence of carbon deposition, the morphology of the magnetite formed was changed from a crystalline structure to a type resembling “breakaway” oxide. Filamentary carbon was formed in all cases, the diameter of the filaments being dependent on whether chromium was present or absent in the substrate material. Initiation of filamentary growth on these materials is discussed.     

Pore structure alteration of porous carbon by catalytic coke deposition

The feasibility of preparing Carbon Molecular Sieve (CMS) by tailoring pore structure of activated carbon under catalytic cracking of benzene has been examined. In this method, benzene vapour was cracked over metal-impregnated activated carbon particles at 523–773 K. Among the metal catalysts tested, only cobalt exhibited significant cracking activity toward benzene. In this range of temperature coke was originated on the metal surface only, therefore an excessive coke deposition as indicated in non-catalytic process was not observed. The amount of coke and the site of deposition in the pore network were determined to some extent by the metal loading as well as the rate of benzene cracking. Raman spectra indicated that the coke produced was less amorphous than those produced in non-catalytic processes. Only a small loss in micropore volume and surface area was observed after the coke deposition process. The CMS produced was tested for its adsorption characteristics of carbon dioxide and methane. The improvement in the CO₂/CH₄ kinetic selectivity was observed.     

Kinetics of the reaction between steam and the basal plane of graphite

The gold decoration/transmission electron microscopy technique has been used to measure the rate of removal of carbon atoms from the basal plane of gra processes have been determined: removal of edge atoms surrounding the monolayer etch pits, and abstraction of carbon atoms within the basal plane which for edge carbon removal ranges from 0.02 at 600°C to 26 atom/atom active site/s at 900°C. The activation energy of 52 kcal/mole is close to the val of the saturated basal atoms (with 3 sp² bonds) may be expressed in terms of the probabilities of successful collisions of O atoms which are fo for both CO₂ and CH₂O reactions, are of the order of 10^?10 in the above temperature range. Comparison of the two reaction oxygen and water is the species responsible for basal atom abstraction.The gasification of graphite by steam is immensely anisotropic. At 23 torr H₂O the turnover frequency in the prismatic or edge directions, {10$\text{1}$0} and {11$\text{2}$0} faces, is higher than that on the basal {0001} plane by a factor of 4 × 10¹⁰ at 900°C and 4 × 10¹² at 700°C.     

Nickel catalysts supported on carbon nanofibers: structure and activity in methane decomposition

Structures of Ni catalysts supported on filamentous carbon (CFC) produced by methane decomposition over coprecipitated Ni and Ni-Cu/alumina catalysts were studied by EXAFS and TEM. Thermal pre-treatment in N₂ at 350°C of samples impregnated by nickel nitrate precursor was found to produce either NiO or nickel carbide, Ni₃C, phase. This was explained by different reducing properties of the carbon nanofibers which depend on the surface structure. High stability of the Ni/C catalysts in methane decomposition reaction at 550°C was found with those prepared from only nickel chloride precursor, due to the formation of large (30-70 nm) Ni particles further leading to new carbon filaments growth. Data implies a common mechanism of the filamentous carbon deposition in all Ni-based catalysts, independent of the support (silica, alumina, carbon) being used. However, accumulation of filamentous carbon is strongly influenced by morphology and texture of the support. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enhanced thermoelectric properties of 2H–MoS2 thin film by tuning post sulfurization temperature

Two-dimensional transition metal dichalcogenide semiconductors (TMDCs) like MoS₂ are becoming more popular as thermoelectric materials because they are abundant, nontoxic, and have good performance. In the study, the MoS₂ thin films have prepared by the sputtering and post-sulfurization process at various temperatures 450 °C, 550 °C, 650 °C, and 750 °C. The XRD data exhibits the formation of the 2H phase of MoS₂ thin film with (002), (004), and (006) planes. The Raman spectroscopy has confirmed the 2H–MoS₂ thin films with 2LA (M), A_1g, E_2g, and E_g vibrational modes. The SEM images have shown the thin MoS₂ flakes. The Seebeck and electrical conductivity data indicated an enhancement in Seebeck coefficient and electrical conductivity from 20 to 31 μV/^°C and 26–53 S/m, respectively, as the post sulfurization temperature increased from 450 °C to 750 °C. The enhancement of the Seebeck coefficient and electrical conductivity have been linked to the perfection of the 2H phase of MoS₂ film. The improved crystal structure has increased carrier mobility, leading to a high power factor of the 5.09 μWm^?1C^?2.     

Gas-Hydrate Formation and Phase Transformations of Adsorbed Water in Kuznetsk Basin Coal

Among the properties of coal that must be studied in order to optimize its preparation for coke production and deep processing are the interactions of adsorbed materials—in particular, water, methane, and carbon dioxide—with its surface. An important aspect of these interactions is phase transformation of the adsorbed materials in the internal porous structure to form hydrate or ordinary ice. In regular coal, a hydrate of carbon dioxide is formed at a CO₂ pressure of 2–4 MPa and temperatures below 10°C, when the moisture content of the coal exceeds the threshold value. However, at the same moisture content, no ice is formed at temperatures between +13°C and –13°C, while decomposition of the hydrate is observed close to the equilibrium curve. For gases that do not form hydrates (helium, nitrogen, argon) in the given temperature and pressure ranges, increasing the pressure to 12 MPa has no influence on the solidification of sorbed water and ice formation.     

Direct observation of carbon nanotube formation in Pd/H-ZSM-5 and MoO3/H-ZSM-5 based methane activation catalysts

The nature of carbonaceous species deposited upon MoO₃/H-ZSM-5 and Pd/H-ZSM-5 based catalysts during methane activation at 700 °C has been studied. TEM evidences the formation of open-ended multi-walled carbon nanotubes on MoO₃/H-ZSM-5 based dehydroaromatisation catalysts. Pd/H-ZSM-5 is more active, exclusively towards methane cracking and post-reaction analysis reveals the distribution of different carbonaceous species is more homogeneous which TEM demonstrates to be in the form of closed-end multi-walled carbon nanotubes.     

Steam reforming model compounds of biomass gasification tars: conversion at different operating conditions and tendency towards coke formation

The purification of biomass-derived syngas via tar abatement by catalytic steam reforming has been investigated using benzene, toluene, naphthalene, anthracene and pyrene as surrogated molecules. The effects of temperature and steam-to-carbon ratio on conversion, and the tendency towards coke formation were explored for each model compound. Two commercial nickel-based catalysts, the UCI G90-C and the ICI 46-1, were evaluated. The five tar model compounds had very different reaction rates. Naphthalene was the most difficult compound to steam reform, with conversions from 0.008 g_{org_conv}/g_cat min (790 °C) to 0.022 g_{org_conv}/g_cat min (890 °C) at an S/C ratio of 4.2. The most reactive compound was benzene, with a conversion of 1.1 g_{org_conv}/g_cat min at 780 °C and an S/C ratio of 4.3. The tendency towards coke formation grew as the molecular weight of the aromatic increased. The minimum S/C ratio for toluene was 2.5 at a catalyst temperature of 725 °C, and for pyrene at 790 °C ,it was 8.4. In general, catalyst temperatures and S/C ratios need to be higher than for naphtha in order to prevent the formation of coke on the catalyst.     

Effect of carbon deposition over carbonaceous catalysts on CH<Subscript>4</Subscript> decomposition and CH<Subscript>4</Subscript>-CO<Subscript>2</Subscript> reforming

An investigation was made using a continuous fixed bed reactor to understand the influence of carbon deposition obtained under different conditions on CH₄-CO₂ reforming. Thermogravimetry (TG) and X-ray diffraction (XRD) were employed to study the characteristics of carbon deposition. It was found that the carbonaceous catalyst is an efficient catalyst in methane decomposition and CH₄-CO₂ reforming. The trend of methane decomposition at lower temperatures is similar to that at higher temperatures. The methane conversion is high during the initial of stage of the reaction, and then decays to a relatively fixed value after about 30 min. With temperature increase, the methane decomposition rate increases quickly. The reaction temperature has significant influence on methane decomposition, whereas the carbon deposition does not affect methane decomposition significantly. Different types of carbon deposition were formed at different methane decomposition reaction temperatures. The carbon deposition Type I generated at 900°C has a minor effect on CH₄-CO₂ reforming and it easily reacts with carbon dioxide, but the carbon deposition Type II generated at 1000°C and 1100°C clearly inhibits CH₄-CO₂ reforming and it is difficult to react with carbon dioxide. The results of XRD showed that some graphite structures were found in carbon deposition Type II.