Decomposition of methane over Ni catalysts supported on carbon fibers formed from different hydrocarbons

In order to get pure hydrogen without CO and CO₂, the decomposition of methane into hydrogen and carbon fibers (CF) over Ni/carbon catalysts has been investigated. The reason for the use of carbon materials as supports is to avoid a costly elimination treatment of the catalyst from the formed CF. The Ni/carbon catalysts prepared by the impregnation of various carbon materials with Ni(NO₃)₂ dissolved in acetone, followed by reduction in hydrogen at 573 K, showed better catalytic performance in the decomposition of methane than those prepared by the impregnation with aqueous Ni(NO₃)₂. The Ni(40 wt%)/CF(from 1-C₄H₈) showed the highest catalytic performance giving a C/Ni value (moles of deposited carbon per mole of Ni on the catalyst) of 1920 until complete deactivation of the catalyst. SEM and TEM images of the CF formed from methane indicated their thickness to be ≈10-150 nm with the same size of Ni particles at their tips at the early stage of the decomposition of methane, but the thickness changed to ≈40-100 nm at the final stage of decomposition. An estimate of the average size of Ni crystallites from XRD measurement suggested that the carbons deposited from methane on various Ni/CF would modify the size of Ni crystallites during the reaction. It is suggested that ≈20 nm Ni crystallites are most active for the growth of carbon nanofibers.     

Methanol-mediated growth of carbon nanotubes

A new approach for the growth of carbon nanotubes (CNTs) is reported. In particular, the growth is mediated by adding methanol into n-hexane with dissolved ferrocene. The first mediating effect is that the thermal decomposition of n-hexane and consequent widespread formation of carbon particles can be suppressed. Thus, truly continuous production of CNTs can be realized at high temperature. The second mediating effect is that with increasing methanol addition the proportion of carbon which can be used for CNT growth is decreased. Therefore, CNTs with different diameters or different numbers of graphitic layers can be produced individually.     

Production of Carbon Nanotube by Ethylene Decomposition over Silica-Coated Metal Catalysts

Formation of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) through the decomposition of ethylene at 973 K was achieved using various metal catalysts covered with silica layers. CNFs of various diameters were formed by ethylene decomposition over a Co metal catalyst supported on the outer surface of the silica. In contrast, silica-coated Co catalysts formed CNTs with uniform diameters by ethylene decomposition. Silica-coated Ni/SiO₂ and Pt/carbon black also formed CNTs with uniform diameters, while CNFs and CNTs with various diameters were formed over Ni/SiO₂ and Pt/carbon black without a silica coating. These results indicate that silica layers that envelop metal particles prevent sintering of the metal particles during ethylene decomposition. This results in the preferential formation of CNTs with a uniform diameter.     

The Thermo-Catalytic Cracking of Hydrocarbons: Effect of Polymethylbenzenes Added to the n-Hexane Feed on the Reactivity of ZSM-5 Zeolite Containing Hybrid Catalyst

Hybrid catalyst containing an acidic ZSM-5 zeolite and a Ni bearing co-catalyst, when compared to its reference ZSM-5 catalyst, showed significant differences in terms of conversion and product yields in the thermo-catalytic cracking (TCC) of feeds comprising n-hexane and some 1,2,4-trimethylbenzene (TMB). Almost no difference was observed with the n-hexane feed containing bulkier pentamethylbenzene (PMB). The obtained results were interpreted using two physico-chemical properties of these aromatics and the catalysts used, i.e. (1) the accessibility of the zeolite internal surface to these molecules and (2) the compliance of these catalytic data with the hydrocarbon pool mechanism when hydrogen spillover species HSO phenomena were involved. TMB, when adsorbed by the ZSM-5 micropores, might induce some partial pore blocking, whereas PMB was completely excluded from the internal surface of the zeolite particles. At larger concentrations of TMB, the hydrogen spillover species showed some significant retarding effect on the coke formation while with a n-hexane feed containing PMB, this effect was much less visible because the adsorbed PMB was structurally much closer to the coke-precursor ion than the TMB, or because the less methylated benzene could undergo conversion in accordance with a newly hypothesized mechanism. Finally, the (HSO) species could affect the reaction intermediates only when the latter were formed on the external surface of the zeolite.     

Catalytic growth of macroscopic carbon nanofiber bodies with high bulk density and high mechanical strength

Carbon nanofibers (CNF) are non-microporous graphitic materials with a high surface area (100-200 m²/g), high purity and tunable surface chemistry. Therefore the material has a high potential for use as catalyst support. However, in some instances it is claimed that the low density and low mechanical strength of the macroscopic particles hamper their application. In this study we show that the bulk density and mechanical strength of CNF bodies can be tuned to values comparable to that of commercial fluid-bed and fixed-bed catalysts. The fibers were prepared by the chemical decomposition of CO/H₂ over Ni/SiO₂ catalysts. The resulting fibers bodies (1.2 μm) were replicates of the Ni/SiO₂ bodies (0.5 μm) from which they were grown. The bulk density of CNF bodies crucially depended on the metal loading in the growth catalyst. Over 5 wt% Ni/SiO₂ low density bodies (0.4 g/ml) are obtained while 20 wt% Ni/SiO₂ leads to bulk densities up to 0.9 g/ml with a bulk crushing strength of 1.2 MPa. The 20 wt% catalysts grow fibers with diameters of ∼22 nm, which grow irregularly in space, resulting in a higher entanglement and a concomitant higher density and strength as compared to the thinner fibers (∼12 nm) grown from 5 wt% Ni/SiO₂.     

Surface characteristics and surface behaviour of polymer carbons—IV: Adsorption isotherms of organic vapours

The adsorption isotherms of several organic vapours, such as methanol, ethanol, benzene, n-hexane, n-heptane and carbon tetrachloride, varying in size and polar character have been studied on five different polymer charcoals before and after activation. The shape of the isotherms is generally Type I and indicates that the charcoals are highly microporous with pores only a few molecular diameters in width. The adsorption of some vapours to varying extents and the little or no adsorption of others indicates that different charcoals have pores of different diameters. PF, UF and PVC charcoals adsorb only very small amounts of methanol, ethanol and benzene (only in PF charcoal) but do not adsorb any of the larger molecules such as n-hexane, n-heptane and carbon tetrachloride. These charcoals, therefore, have ultra fine micropores which are accessible only to smaller molecules. PVDC and Saran charcoals, on the other hand, have larger pores since they could also adsorb appreciable amounts of even larger molecules. The adsorption of these larger molecules, although appreciable, is smaller than those of the smaller molecules. The activation of Saran charcoal in steam enhances the adsorption capacity of the charcoal. The adsorption of polar molecules such as those of ethanol and methanol is also enhanced by the presence of associated oxygen, while the adsorption of benzene and others is enhanced by the elimination of the associated oxygen. The associated oxygen, which is evolved as CO₂ on evacuation, suppresses the adsorption of nonpolar benzene but its elimination and emergence of the CO-complex enhances the adsorption of benzene. The enhanced adsorption of benzene is attributed to the interaction of π electrons of the benzene ring with the partial positive charge on the carbonyl carbon atom. Thus while the presence of associated oxygen makes charcoals hydrophillic, its elimination renders charcoal organophillic in character.     

Characterization of dispersed hydroprocessing catalysts prepared in reversed micelles

Fe, Co and Ni particles were prepared in water/polyoxyethylene-4-laurylether/n-hexane and water/polyoxyethylene-4-laurylether/decahydronaphthalene microemulsions by reduction of metal nitrates dissolved in the water pools of the reversed micelles. The particles were mostly monodispersed with average diameters in the range 8 to 23 nm, as determined by dynamic light scattering (DLS). However, significantly smaller size estimates were obtained using transmission electron microscopy (TEM). The average DLS particle diameters increased with increased average diameter of the reversed micelles, and the diameter of the reversed micelles increased with an increase in the microemulsion water: surfactant ratio and metal ion concentration. The diameter of the reversed micelles was also dependent upon the metal dissolved in the water pool, increasing in the order Ni < Co < Fe. These trends are explained in terms of changes that occur to the microemulsion hydrophilic-lipophilic balance. Preparation of nickel and cobalt sulfides by sulfidation of the metal salt with H₂S at low temperature, yielded much larger diameter particles (average diameter 75 nm). Measurement of the activity of the sulfided catalysts showed that the Co catalyst was more active than the Ni catalyst for the hydrocracking of diphenylmethane, and Co was more effective than Fe in reducing coke yield during Cold Lake residue hydroprocessing.     

Multi-wall carbon nanotubes by catalytic decomposition of carbon monoxide on Ni/MgO

The redox behavior of the catalyst and the catalytic decomposition of carbon monoxide (CO) were investigated in the synthesis process of multi-wall carbon nanotubes (MWCNT) using Ni/MgO catalyst. The surface morphology of the heated Ni layer was observed by TEM to confirm the formation of NiO particles (50 nm or less) and NiO (222). The chemical reaction behavior of the catalyst in CO the atmosphere was displayed via TG-DSC analysis, and the reduction of NiO was revealed due to the mass decrease of 2.71 wt% and the exothermic peak at around 400°C. The deposition of carbon was identified with an increase in mass and the exothermic peak near 600°C. Ni (111) and carbon (002) facets was taken place in a diffraction pattern of carbon deposited catalyst, indicating the reduction in NiO and the graphitic carbon deposition. The crystallinity of the graphitic carbon was analyzed as the ratios of 0.998 for I_D/I_G and 0.26 for sp³/sp² in Raman and photoelectron spectra. The encapsulated Ni in MWCNT was observed through TEM-EDS, verifying the activation of the catalyst by CO.     

Transmission electron microscopy study of the microstructure of unidirectional C/C composites fabricated by catalytic chemical vapor infiltration

Unidirectional carbon/carbon (C/C) composites were fabricated by catalytic chemical vapor infiltration, using electroless Ni–P as catalyst. Transmission electron microscopy (TEM) investigations indicate that the catalyst particles (100–800 nm) in the pyrocarbon (PyC) matrix are composed of Ni₃P and Ni phases, but only the Ni₃P phase was observed in the tiny catalyst particles (<50 nm) in carbon fibers. The catalyst particles in the matrix were encapsulated by high-textured PyC shells, in which openings were observed. The thicknesses of the medium-textured PyC in the composites (720–850 nm) are greater than in conventional C/C composites (660–740 nm), but have no significant difference in texture degree. Catalysts were partially extruded out of the PyC shells and migrated into the carbon fibers, leading to the catalytic graphitization of the carbon fibers, and their structural homogeneity was destroyed. Based on the TEM observation, a dissolution/precipitation mechanism was proposed for the catalytic graphitization of carbon fibers, and a dissolution/precipitation/encapsulation/fracture/extrusion mechanism was proposed for the encapsulation of catalyst particles.     

Synthesis of carbon nanotubes by detonation of 2,4,6-triazido-1,3,5-triazine in the presence of transition metals

The formation of carbon nanotubes via explosive decomposition of an energetic precursor, 2,4,6-triazido-s-triazine C₃N₁₂, was investigated with respect to the presence of the transition metals Fe, Ni, Cu, Ti and the degree of confinement of the explosive charge. The carbonaceous detonation products were characterized by transmission and scanning electron microscopy, energy dispersive X-ray analysis and infrared absorption spectroscopy. Multi-walled carbon nanotubes with diameters from 50 to 150 nm and a variety of morphologies were found. Metal particles in the tubes and at some of the tube ends indicate a catalytic growth mechanism.     

TEM evidence for factors affecting the genesis of carbon species on bare and K-promoted Ni/MgO catalysts during the dry reforming of methane

The structure and morphology of carbon species generated under dry reforming of methane (DRM) at 650 and 800°C on ‘bare’ and ‘K-doped’ Ni/MgO catalysts have been comparatively investigated by Transmission Electron Microscopy (TEM) analyses of ‘used’ samples. K-addition (K_at/Ni_at, 0.125) strongly improves the resistance of the Ni/MgO catalyst to coking and sintering phenomena at any temperature. At 650°C, an extensive formation of filamentous (whisker carbon) carbon species on bare Ni/MgO catalyst causes the detachment of a large number of Ni particles from the support with a consequent destruction of the structure and remarkable sintering phenomena of the active phase. Considerably lower amounts of carbon deposits with a shell-like (encapsulating carbon) morphology, forming at 800°C on both catalysts, point to the Bouduard reaction as the main route of carbon deposition on Ni-based catalysts during DRM. The electronic effect induced by potassium on the active phase of the Ni/MgO system, timely monitored by a rise in E_app of DRM from 50 to 70 kJ/mol, markedly hinders the rate of coking also affecting the morphology of carbon whiskers, by inhibiting the processes of C diffusion and nucleation across Ni particles under steady-state conditions.     

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

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

KOH-浸渍-还原法在炭纤维表面制备不同金属纳米催化剂涂层及其应用

为了催化炭纤维原位生长纳米炭纤维／纳米碳管，研究纳米炭纤维／纳米碳管在炭／炭复合材料中的应用，采用KOH-浸渍-还原法在炭纤维上制备纳米催化剂颗粒。首先用KOH处理炭纤维改变其形貌，然后将炭纤维分别在硝酸钴和硝酸镍催化剂前驱体溶液中浸渍，干燥，再用H2气还原制得催化剂颗粒，最后催化热解CO在炭纤维上原位生长纳米炭纤维／纳米碳管。结果表明：KOH处理能使炭纤维表面变得凹凸不平，有效的阻止了催化剂前驱体液体的流动，使涂层均匀；浸渍-还原法能获得粒径小、均匀、适合纳米炭纤维生长的金属颗粒；与Co纳米颗粒相比，Ni分散效果和催化效果更好。     

CATALYTIC PRODUCTION OF GRAPHITIC NANORODS VIA THE GAS PHASE DECOMPOSITION OF ETHYLENE OVER SUPPORTED NICKEL

The catalytic growth of high aspect ratio carbon nanorods by ethylene decomposition over nickel/silica is presented as a viable low-cost selective route to a high-value product. This study focuses on the role of catalyst preparation in determining carbon yield and structural characteristics and considers the application of a 10% w/w Ni loading prepared by impregnation and precipitation/deposition. The latter is characterized by a narrower dispersion of smaller Ni particles (average diameter?=?2.4?nm) but lower carbon yields. Doping this catalyst with KBr resulted in a 40-fold increase in carbon production with >95% having rod diameters <10?nm; doping the impregnated catalyst had little effect on catalyst performance. High-resolution transmission electron microscopy and temperature programmed oxidation were employed to characterize the catalysts and the carbon product; the effect of temperature (673–873?K) on carbon yield is also reported.     

Preparation of carbon micro-coils involving the decomposition of hydrocarbons using PACT (plasma and catalyst technology) reactor

Carbon micro-coils as well as carbon fibers with various morphologies were prepared by the decomposition of hydrocarbons, such as acetylene, methane, propane, ethylene, etc., at 770°C using a PACT (plasma and catalyst technology) reactor. The preparation conditions, growth mechanism and morphology of the carbon micro-coils were examined. The Ni electrode of the PACT reactor was used as the catalyst as well as a plasma source electrode. It was found that hydrocarbons, such as methane, propane and ethylene, decomposed under the plasma and catalyst atmosphere to form acetylene as the main decomposition product, and then this acetylene was further decomposed to form carbon micro-coils. Using a Ni powder catalyst dispersed on the substrate, the carbon micro-coils with a double helix structure, in which two pieces of carbon coils entwine each other in the same coiling direction, grew among the single straight carbon fibers and paired straight fibers. On the other hand, the carbon micro-coils with a single helix structure and wide coil pitch were obtained by the indirect decomposition of acetylene using the N₂ plasma formed by the PACT reactor.     

Catalytic synthesized carbon nanostructures from methane using nanocrystalline Ni

Carbon nanostructures synthesized with nanocrystalline Ni catalyst from decomposition of methane are investigated by means of transmission electron microscopy (TEM). Two kinds of carbon nanostructures, carbon fibers and bamboo-shaped carbon nanotubes, are observed. The preferential growth direction of graphene sheets depends on the reaction conditions. The bamboo-shaped carbon nanotubes can be obtained only if the reaction temperature is higher than 1000 K, and carbon fibers can be obtained at lower temperatures. The role and state of the catalyst particles are also discussed.     

Synthesis of carbon nanocoils on substrates made of plant fibers

Carbon nanocoils (CNCs) with different shapes and coil diameters have been synthesized on three kinds of substrates made of plant fibers, i.e. tissue, cotton cloth and bamboo fiber cloth, using Fe₂ (SO₄)₃/SnCl₂ catalyst by a thermal chemical vapor deposition method. The average coil diameters of the CNCs on the tissue, cotton cloth and bamboo fiber cloth substrates are 560, 183, and 510 nm, respectively. It is found that the organization difference in the plant fiber substrates results in the difference in the aggregation states of catalyst particles on the fiber surfaces, which has a crucial effect on the morphology and production of the grown CNCs. The tight organization of the carbon fibers in the tissue and cotton cloth substrates can promote the catalyst aggregations to fabricate high yield CNCs. For the bamboo fiber cloth substrate, a relatively small number of catalyst particles are deposited on the surface and tend to be isolated, leading to the growth of a certain amount of the carbon nanofibers and carbon nanotubes. In addition, the catalyst adsorption ability of the bamboo fiber can be improved by coating calcium chloride particles to achieve high production of the regular CNCs.     

不同碳源制备气相生长炭纤维的研究

综述了几种不同的碳源,如低碳烃(甲烷、丙烯、乙炔和苯等)、脱油沥青、煤沥青,采用化学气相沉积(CVD)法,按不同转化过程制备出气相生长炭纤维(VGCFs)的研究现状.主要研究了以煤沥青为碳源、二茂铁为催化剂,借助CVD法制备气相生长炭纤维.经场发射扫描电子显微镜(FESEM)、高分辨透射电子显微镜(HRTEM)、X-射线衍射(XRD)及拉曼散射(Raman)分析,结果表明:产物主要为高纯度的VGCFs,直径分布均匀,外径大约为100 nm,长度数微米,并初步探讨了煤沥青制备气相生长炭纤维的生长机理.     

Growth of nitrogen-doped carbon nanotubes and fibers over a gold-on-alumina catalyst

Nitrogen-doped carbon nanotubes (N-CNTs) were synthesized by chemical vapor decomposition of a N,C-precursor over a supported Au catalyst. Large quantities of carbon nanotubes with a compartmentalized structure containing ca. 5 at.% N were produced over a 1.5% Au/δ-Al₂O₃ catalyst with a mean diameter of Au particles equal to 2.8 nm under continuous flow conditions at 800 °С and 1 bar of the reaction gas mixture containing pyridine (Py) vapor (5 vol.%), Н₂ (10–20 vol.%) and Ar (balance). The majority of the N-CNTs obtained after 10 min have outer diameters (ODs) of 13–45 nm and closed tips without any encapsulated gold particles of the catalyst which indicates the “base-growth” mechanism of N-CNT formation. Carbon deposits synthesized for 30–135 min contain carbon fibers with OD values up to several micrometers, formed by self-assembling of N-CNTs, and individual N-CNTs. X-ray photoelectron spectra provide evidence for various chemical states of nitrogen (pyridinic, “quaternary” (graphitic) and pyrrolic nitrogen) in the N-CNTs, and these are discussed.     

硫在螺旋炭纤维固相催化生长机理中的作用

通过电沉积法在石墨基体上电沉积含硫的光亮镍催化剂,采用化学气相沉积法直接生长出螺旋炭纤维,生长温度仅为650℃;螺旋炭纤维的螺旋直径为700nm,炭纤维直径为400nm,纯度高且质量好,螺旋炭纤维中由三股炭纤维相互规则缠绕成绳状结构炭纤维。催化剂不合硫元素时,炭纤维为无规则的弯曲形纤维;催化剂含硫量高时,炭纤维为规则的褶皱形纤维,催化剂硫含量影响炭纤维形貌生长。螺旋炭纤维生长机理为固态催化生长,硫元素通过影响镍催化剂表面的各向异性和内部催化活性来影响炭纤维的形貌生长。