Synthesis of composite materials of carbon nanofibres and ceramic monoliths with uniform and tuneable nanofibre layer thickness

Composite materials consisting of ceramic monoliths and carbon nanofibres (CNFs) have been synthesized by catalytic growth of CNFs on the γ-alumina washcoating layer covering the walls of a ceramic monolith. The composites possess a relatively uniform mesoporous layer of CNFs of relatively small diameter. The thin alumina washcoating (ca. 0.1 μm) prevents the CNFs from being trapped inside the alumina pores and hence the CNFs grow freely throughout the washcoating layer to form a uniform layer of CNFs that completely covers the surface of the monolith walls. The growth temperature is found to control the thickness of the CNF layer (2-4 μm), the growth rate of the nanofibres, and the mechanical strength of the resulting CNF-monolith composite. At ideal conditions, a complete adhesion of the CNF layer and higher mechanical strength than the original cordierite monolith can be obtained. The CNF layer has an average pore size of 17 nm with absence of microporosity which renders these monoliths promising candidates for the use as catalyst supports, especially for liquid phase reactions. The CNFs have small diameters (5-30 nm) due to the high dispersion of Ni particles in the growth catalyst and the CNFs exhibit an unusual branched structure.     

Controlling the yield and structure of carbon nanofibers grown on a nickel/activated carbon catalyst

Carbon nanofibers (CNFs) were grown via the chemical vapor deposition of C₂H₄ on an activated carbon (AC)-supported Ni catalyst. The texture of the CNF/AC composites can be tuned by varying the growth temperature and by treatment in reducing atmosphere prior to C₂H₄/H₂ exposure. The Ni-catalyzed gasification of the AC support increases the microporosity of the composite and shown to be dominant throughout the composite synthesis especially during reduction, subsequent treatment in reducing atmosphere, and CNF growth at low temperatures. N₂ isotherm and scanning electron microscope were used to characterize the texture and morphology of the composites. Subsequent treatment in reducing atmosphere were shown to increase the Ni catalyst activity to grow CNFs. High resolution transmission electron microscope however did not reveal any microstructural difference for Ni catalyst with and without the subsequent reduction treatment. We propose in this paper that the carbon dissolutions during treatment of the catalyst might have an implication on the CNF growth.     

Preparation of well‐controlled porous carbon nanofiber materials by varying the compatibility of polymer blends

The relationships between the compatibility in binary polymer blends and the pore sizes of carbon nanofibers (CNFs) prepared from the blends were investigated. Compatibility was determined by the difference between the solubility parameters of each polymer in the polymer blends. Porous CNFs were prepared by an electrospinning and carbonization process using binary polymer blends, consisting of polyacrylonitrile (PAN) as the carbonizing polymer and poly(acrylic acid) (PAA), poly(ethylene glycol), poly(methyl methacrylate) or polystyrene (PS) as the pyrolyzing polymer. The pore size of the CNFs increased with increasing difference in solubility parameter. The CNFs prepared using the PAN/PAA blend, which had the smallest solubility parameter difference, exhibited a pore size of 1.66 nm compared to 18.24 nm for the CNFs prepared using the PAN/PS blend. The prepared CNF webs with controlled meso‐sized pores showed a stable cycle performance in cyclic voltammetry measurements and improved impedance characteristics. This method focusing on the compatibility in polymer blends was simple to apply and effective for controlling the pore sizes and surface area of CNFs for application as electrode materials in energy storage systems. © 2013 Society of Chemical Industry     

Studies on catalytic gasification of coal chars. Part 1. Development of pores during gasification

Chars prepared from three coals were impregnated with nickel catalyst and gasified with hydrogen at 850°C and 10 atm. The surface area and porosity of gasified residues at several conversions were measured. Blair Athol char, which had a relatively large surface area, showed a unique gasification pattern, i.e., the rate increased with time until a maximum was attained. Catalytic gasification resulted in an increase in the volume of macropores of a given diameter range, which was characteristic of the catalyst system used. In non-catalytic gasification and in cases when activity of the catalyst was low, mainly pores smaller than 6 nm were enlarged. Possible explanations of the acceleration of the rate are discussed and some consequences of the porosity measurements for this explanation are indicated. As the macropores are enlarged during catalytic gasification, the kinetic pattern could be modified when intraparticle diffusion comes into play.     

K2CO3对兰炭催化气化特性的影响

在固定床中考察了不同K₂CO₃植入浓度和不同温度条件下兰炭催化气化特性。结果表明，5%的催化剂植入浓度主要起到填充孔隙的作用，当植入浓度增加到10%以后，催化剂发生堆积会使颗粒表面及内部形成较多孔隙。提高气化温度可提高兰炭转化率，超过750℃之后碳转化率增幅减缓，催化剂饱和装载浓度为10%。在颗粒表面和开放孔隙中的高浓度C(O)才具有较高的脱附速率，并提高CO生成速率。在非催化条件下，随着气化的进行CO/CO₂下降，而H₂/(2CO₂+CO)先增后减。在催化条件下，H₂/(2CO₂+CO)稳定在1.5～1.7。催化剂兰炭样品中出现了K₂Ca(CO₃)₂双金属碳酸盐、K₂O、KO₂等活性组分，并随催化剂植入浓度的增加而增加。催化剂植入浓度的增加会导致失活现象加重，但兰炭在750℃条件下气化1 h 催化剂没有完全失活。     

Growth of mechanically fixed and isolated vertically aligned carbon nanotubes and nanofibers by DC plasma-enhanced hot filament chemical vapor deposition

Vertically aligned, mechanically isolated, multiwalled carbon nanotubes (MWCNTs) and nanofibers (MWCNFs) were grown using an array of catalyst nickel nanowires embedded in an anodic aluminum oxide (AAO) nanopore template using DC plasma-enhanced hot filament chemical vapor deposition (HFCVD). The nickel nanowire array, prepared by electrodeposition of nickel into the pores of a commercially available AAO membrane, acts as a template for CNT and CNF growth. It also provides both a mechanical “fixed support” boundary condition and enforces sufficient spatial separation of the CNT/CNFs from each other to enable reliable and well-controlled mechanical testing of individual vertically aligned CNT/CNFs. In contrast with other AAO-templated growth methods, no post-growth etching of the AAO is required, since the CNTs/CNFs grow out of the pores and remain vertically aligned. A mixture of hydrogen and methane was used for the growth, with hydrogen acting as a dilution and source gas for the DC plasma, and methane as the carbon source. A negative bias was applied to the sample mount to generate the DC plasma. The filaments provided the necessary heat for dissociation of molecular species, and also heat the sample itself significantly. Both of these effects assist the CNT/CNF growth. Minimal heating came from the low-power plasma. However, the associated DC field was essential for the vertical alignment of the CNTs and CNFs. Scanning electron, transmission electron, and atomic force microscopy confirm that the CNT/CNFs are composed of graphitic layers, and form a vertically aligned, relatively uniform, and dense array across the AAO template. A significant number of the structures grown are indeed high quality nanotubes, as opposed to more defective nanofibers that are often predominant in other growth methods. This method has the advantage of being scalable and consuming less power than other techniques that grow vertically aligned CNTs/CNFs.     

Variation of the pore structure of coal chars during gasification

The variation of the pore structure of several coal chars during gasification in air and carbon dioxide was studied by argon adsorption at 87 K and CO₂ adsorption at 273 K. It is found that the surface area and volume of the small pores (<10 Å) do not change with carbon conversion when the coal char is gasified in air, while those of the larger pores (10-20 Å, 20-50 Å, 50-2500 Å) increase with increase of carbon conversion. However in CO₂ gasification, all the pores in different size ranges increase in surface area and volume with increase of carbon conversion. Simultaneously, the reaction rate normalized by the surface area of the pores >10 Å for air gasification is constant over a wide range of conversion (>20%), while for CO₂ gasification similar results are obtained using the total surface area. However, in the early stages of gasification (<20%) the normalized reaction rate is much higher than that in the later stage of gasification, due to existence of more inaccessible pores in the beginning of gasification. The inaccessibility of the micropores to adsorption at low and ambient temperatures is confirmed by the measurement of the helium density of the coal chars. The random pore model can fit the experimental data well and the fitted structural parameters match those obtained by physical gas adsorption for coal chars without closed pores.     

Pore development during gasification of South African inertinite-rich chars evaluated using small angle X-ray scattering

Pore development arising from steam and CO₂ gasification of a char, prepared from an inertinite-rich Witbank Seam 4 coal, was investigated using small angle X-ray scattering. The char, ∼75 μm, was gasified to specific conversions (10, 25, 35 and 50%) using two gasification reagents, CO₂ and steam. A novel ratio analysis technique was developed to study the pore development from experimental data. Differently sized pores grow at different rates with the difference not being simply due to gas accessibility. In particular, the pores between 1 and 40 nm in size showed more pore growth than larger or smaller sizes. Steam gasification created a more porous char with increased pore growth of pore sizes between 1 and 40 nm than CO₂ gasification. The pore growth rate of steam was up to a factor 7 times faster than CO₂, compared at the highest gasification temperatures. For the smaller pores, <1 nm, it was found that the rate of pore generation was slower compared to larger pores, though pore growth was still evident with the critical cross over pore size for CO₂ to be 1 nm compared to 0.6 nm for steam. This may be a direct consequence of CO₂'s greater kinetic diameter.     

Evaluation of Operational Cycles for Long‐Term Run of a Tar Removal Catalytic System

Biomass gasification experiments on pilot/demo scales have some issues related to the early deactivation of catalysts during the tar removal step. To avoid this problem, a method was developed in a bench‐scale micro activity unit using toluene as tar model compound in order to suppress this effect. The runs were performed with a commercial Pt catalyst supported on Ce‐Zr‐Al, alternating periods of regeneration and reactivation steps with steam, nitrogen, and hydrogen. The toluene steam reforming using operational cycles in order to reach a long‐term run provided useful information for pilot plant studies, mainly reactivation and regeneration procedures. The main concern on tar removal studies by steam reforming is the catalyst deactivation due to the presence of polyaromatic and olefinic compounds on the material pores, which is produced during biomass gasification.     

Mechanistic study of cobalt catalyzed growth of carbon nanofibers in a confined space by plasma-assisted chemical vapor deposition

Carbon nanofibers (CNFs) were grown in the porous anodic aluminum oxide (AAO) thin film grown on the Si wafer by electron cyclotron resonance chemical vapor deposition using cobalt as the catalyst. A larger Co particle electrodeposited in the AAO pore channel produced vertically aligned CNFs with a tube diameter in compliance with the pore size of the AAO template. On the other hand, a smaller Co particle resulted in CNF growth with a nonuniform distribution of the tube diameter and a sparse tube density. Amorphous carbon residue produced under the plasma-assisted CNF growth condition seemed to play an essential role leading to the observation. A growth mechanism is proposed to delineate the volume effect of the electrodeposited Co catalyst on the CNF growth confined in pore channels of the AAO template.     

Carbon nanofibers grown over graphite supported Ni catalyst: relationship between octopus-like growth mechanism and macro-shaping

Carbon nanofibers (CNFs) with a uniform diameter of ca. 30 nm have been grown via catalytic decomposition of C₂H₆/H₂ mixture over a nickel (1 wt.%) catalyst supported on graphite microfibers which constitutes the macroscopic shape of the final C/C composite. The productivity reached 50 g of CNFs / g of Ni / h on stream and is among the highest reported to date. The resulting composite consisting in a web-like network of CNFs covering the starting catalyst was characterized by SEM and TEM in order to get more insight on the relationship between the starting nickel catalyst particles and the as-grown CNFs. Apparently the CNFs growth proceeds from different mechanisms: base-growth mechanism involving especially the large nickel particles, tip-growth mechanism involving the smaller nickel particles and tip/octopus-growth mechanism, the most frequent involving all particles. The restructuration of the nickel particle from a globular to a more faceted structure seems to be the key step to produce an extremely large quantity of CNF with yields up to 100 wt.% after 2 h of synthesis.     

Effects of mixing on electrical properties of carbon nanofiber and polymer composites

We examined electrical properties of composites of carbon nanofibers (CNFs)/linear low‐density polyethylene (LLDPE) in the range of mixing time. An addition of CNFs into LLDPE led to a decrease of electrical resistivity, and the large amounts of CNFs were required to reach the electrical percolation threshold for the longer mixing time. For the research of these phenomena, we examined the effects of mixing time on the size (length) and spatial distributions of CNFs in the composites. SEM micrographs revealed the size reduction of CNFs in a series of mixing times, although the spatial dispersion of CNFs became more uniform at longer mixing time. To describe the reduction of CNFs theoretically, we hypothesize the size distribution of CNFs obeys a governing population balance kinetics based on an irreversible dissolution process. For the process, we proposed a size dependent breakage rate coefficient proportional to the size of CNFs. The model prediction for the time evolution of size distribution of CNFs has been validated with the experimental measurement showing good agreement. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Processing of organochlorine waste components on bulk metal catalysts

A method for destroying chloroorganic waste components on catalysts, particularly bulk metal nickel (99.99%), nichrome (80% Ni and 20% Cr), and chromel (90% Ni and 10% Cr) is proposed. The process is accompanied by the formation of carbon nanofibers (CNFs) with feathery morphology. Catalytic destruction of 1,2-dichloroethane on bulk nickel catalysts is characterized by a long induction period (～3 h) with spontaneous activation of the alloy’s surface. Preactivation of the catalyst with acids or by alternative treatment in oxidizing and reducing environments shortens the induction period by one order of magnitude. The state of the surface before and after activation is studied by SEM, TEM, and EDX. The activity of catalysts is determined for the decomposition of 1,2-dichloroethane at temperatures of 500 to 700°C. Nichrome exhibited the greatest activity (yield of CNFs, 400 g/g of catalyst); the yield of CNFs on catalysts prepared by coprecipitation and mechanical activation was considerably lower. The proposed approach combines organochlorine waste disposal with the production of a useful product (CNFs). The use of bulk metal catalysts is promising since it simplifies the technology for their preparation, and the absence of carriers makes it easy to cleanse CNFs of impurities of catalyst fragments.     

Oxygen reduction reaction properties of carbon nanofibers: Effect of metal purification

Carbon nanofibers (CNFs) are grown on metal catalysts and electrochemical treatment is used to remove the metal catalyst residuals from the as-grown CNFs. For comparison, the CNFs are also purified by a chemical method and a thermal method. The oxygen reduction reaction (ORR) properties of CNFs purified by these three methods are examined by cyclic voltammetry. CNFs treated by the electrochemical method have a more positive ORR onset reduction potential and peak potential compared with those treated by chemical and thermal methods, and this is because the microstructures of CNFs are less changed by electrochemical method. However, they have a lower electrochemical capacity and ORR peak current than those treated by the chemical method. Cyclic voltammetric measurements at different scan rates confirm that the oxygen reductions on CNFs treated by electrochemical and chemical methods are controlled by diffusion, while on CNFs treated by thermal method is partially influenced by diffusion.     

The CO2-gasification and kinetics of Shenmu maceral chars with and without catalyst

The CO₂ gasification of maceral chars was performed using CAHN TG-151 pressurized thermobalance under different conditions. The effect of mineral in macerals and catalyst on the gasification reactivity of maceral chars and the gasification kinetics were systematically investigated. The results showed that the apparent gasification rate of maceral chars depends on the temperature, pressure, BET surface area of chars and the gasification extent. With increasing temperature and pressure, the gasification rate of maceral chars all increase. After demineralization, the gasification reactivity of maceral chars all decrease. The gasification reactivity of maceral chars greatly increases with loading catalyst. And the loading method of catalyst has great effect on the gasification reactivity. The maceral chars loaded with catalyst by ultrasonic treatment have higher gasification reactivity than that by impregnation. The comparison of gasification reactivity of maceral charas demineralized maceral chars and maceral chars with and without catalyst showed that vitrinite chars always have higher gasification reactivity than inertinite chars. The kinetic results by distributed activation energy model showed that inertinite char has higher activation energy than vitrinite char, and the addition of catalyst greatly minimizes the activation energy and enhances the gasification rate.     

Application of nanocatalytic systems for deep processing of coal and heavy petroleum feedstock

The specific features of the catalysis of deep processing (hydrogenation and gasification) of coal and hydroconversion of heavy petroleum feedstock are considered. It has been shown that it is reasonable to use new catalyst forms and catalyst introduction methods for the processing of these raw materials in view of their specific composition and properties (large size of molecules, thermal instability, and the presence of inorganic compounds in the feedstock). In particular, a dispersion of nanosized spherical particles of MoS₂ in a liquid hydrocarbon medium is an effective catalyst for the coal hydrogenation and heavy-oil hydrocon-version processes. Gaseous alkali metal hydroxides have been proposed for the gasification of solid fuels. The mechanisms of the formation of catalyst systems and some of their properties are discussed.     

生物质炭水蒸气气化制取富氢合成气

以生物质炭为原料在上吸式固定床气化炉中进行水蒸气气化制备富氢合成气,考察了不同原料、粒径和催化剂对生物质炭水蒸气气化影响。结果表明,不同类型炭气化结果存在较大差异,其中木片炭气化结果最优,其次是玉米芯炭和稻壳炭,秸秆炭气化结果最差,木片炭产氢率最大为222.8g/kg。粒径的改变主要影响炭转化率,炭转化率随着粒径的增加呈增加趋势。通过炭吸收方式负载催化剂为有效的方法,其中在相同钾盐质量分数下,KOH催化能力较优于K₂CO₃,且气化速率为未加催化剂条件下的两倍。炭转化率随着碱液浓度的增加而增加,但浓度过高会增加灰分含量从而不利于产氢率,玉米芯炭催化气化最高产氢率为197.8g/kg,在碱质量分数为6%下获得。     

Formation of fine Fe-Ni particles for the non-supported catalytic synthesis of uniform carbon nanofibers

The morphological changes of Fe-Ni catalyst for the preparation of carbon nanofiber (CNF) were examined at 5 steps; (1) the precipitation of Fe-Ni carbonate from Fe-Ni nitrate solution, (2) the calcination of Fe-Ni carbonate into Fe-Ni oxide, (3) the reduction of Fe-Ni oxide, (4) the second reduction of Fe-Ni metal before the growth of CNF, and (5) the reaction with CO/H₂ for the growth of CNF. The Fe-Ni fine particle was formed from the Fe-Ni aggregate through the second reduction and successive CNF growth from CO/H₂. The temperature of these two steps is the most important factor which determines the size and shape of the Fe-Ni fine particle as a catalyst for CNF growth. The lower temperature of 580 °C provided hexagonal particles with very smooth surface sized around 100-200 nm which allowed the growth of platelet CNFs of the same diameter and cross-sectional shape of the formed catalyst particle. At the higher temperature of 630 °C, the Fe-Ni aggregate was found to give the very fine Fe-Ni particles by the two steps; the first step did the Fe-Ni particle sized around 100-500 nm which was successively degraded into smaller particles sized around 20-40 nm, thinner tubular CNFs growing with the contact of CO/H₂. Such smaller particles definitely originated from as-precipitated Fe-Ni carbonate through the steps. The metal particle on the top of CNF was almost exclusively composed of Fe although the catalyst particle before the growth of CNFs carried around 65% of iron and 35% of nickel. The preferential activity of Fe to CO gas may cause such the selectivity. The major role of Ni in the present reaction should be limited to provide the uniform particle of Fe. Controlling the size of the Fe-Ni particle through the reduction and reaction steps was proved to be a key factor to determine the dimension and structure of resultant CNF.     

A carbon activation model with application to longan seed char gasification

In this paper a new structural model is presented to describe the evolution of porosity of char during the gasification process. The model assumes the char structure to be composed of bundles of parallel graphite layers, and the reactivities of each layer with the gasification agent are assumed to be different to represent the different degree of heterogeneity of each layer (i.e. each layer will react with the gasification agent at a different rate). It is this difference in the reactivity that allows micropores to be created during the course of gasification. This simple structural model enables the evolution of pore volume, pore geometrical surface area and the pore size distribution to be described with respect to the extent of char burn-off. The model is tested against the experimental data of gasification of longan seed-derived char with carbon dioxide and it is found that the agreement between the model and the data is reasonably satisfactory, especially the evolution of surface area and pore volume with burn-off.     

无烟煤水煤浆燃烧特性和气化特性研究

为了提高水煤浆成浆浓度,选取一种无烟煤作为研究对象,分别利用常规研磨工艺和分级研磨工艺进行制浆实验考察其成浆性能。结果表明：利用分级研磨工艺优化粒度后,水煤浆浓度能达到67.3%,成浆性良好。选取2种工艺制得的样品进行了燃烧特性实验,发现采用分级研磨工艺制得的样品的燃点和燃烬温度有一定程度的降低,燃烬指数增大,加入助燃剂后该特性更加明显,这是由于分级研磨工艺所制水煤浆中细颗粒含量增加,反应活性变好。重点考察了不同催化剂加入量对煤样气化反应活性的影响。随着催化剂加入量的增加,气化反应活性有不同程度提高,实验样品的催化剂的最佳加入量为0.2%。