Preparation and characterization of trilobal activated carbon fibers

The objective of this research was to evaluate the effectiveness of several different methods for controlling the pore size and pore size distribution in activated carbon fibers. Variables studied included fiber shape, activation time, and the addition of small amounts of silver nitrate. Pure isotropic pitch and the same isotropic pitch containing 1 wt.% silver were melt spun to form fibers with round and trilobal cross sections. These fibers were then stabilized, carbonized, and activated in carbon dioxide. Field emission scanning electron microscopy (FE SEM), electron dispersive spectra (EDS), and wavelength dispersive spectra (WDS) were used to monitor the size and distribution of the silver particles in the fibers before and after activation. Each of these analyses showed that the distribution of silver particles was extremely uniform before and after activation. The fibers were also weighed before and after activation to determine the percent burn-off. The BET specific surface areas of the activated fibers were determined from N₂ adsorption isotherms measured at −196 °C. The results showed that round and trilobal fibers with equivalent cross-sectional areas yielded similar burn-off values and specific surface areas after activation. Also, activation rates were found to be independent of CO₂ flow rate. The porosity of the activated fibers depended on the total time of activation and the cross-sectional area of fibers. The N₂ adsorption measurements showed that the activated fibers had extremely high specific surface areas (greater than 3000 m²/g) and high degrees of meso- and macro-porosity. FE SEM was also used to investigate surface texture and size of pore openings on the surfaces of the activated fibers. The photos showed that silver particles generated surface macro- and mesopores, in agreement with the inferences from N₂ adsorption measurements.     

The effect of precursor chemistry and preparation conditions on the formation of pore structure in metal-containing carbon fibers

Activated carbon fibers (ACFs) prepared from petroleum isotropic pitch and the same containing silver and cobalt nitrate, cobalt and palladium acetylacetonates, as well as mixtures of two salts with a total metal or metal mixture content of 1 wt% have been studied. The processing parameters are summarized for activated carbon fibers containing individual metals and metal mixtures. The results suggest that the generation of metal-containing particles and the formation of pore structure depend on many different factors including the composition of the metal and pitch precursors, the interaction of the metal and pitch precursors during the fiber production process, and the interaction between the two metal precursors when more than one metal salt is used. The addition of silver and cobalt in the form of nitrate salts enlarges the micropores and generates small mesopores with a narrow range of sizes. The addition of palladium as an acetylacetonate salt leads to the formation of both small micropores and larger mesopores. The cobalt additive as an acetylacetonate salt catalyzes the activation process, creating large mesopores and macropores. Mixing of two different metal precursors affects the particle composition and size. This, in turn, controls the pore structure of the final activated fibers. During activation, the two metal precursors can act independently (Ag/Co mixture). However, in other cases their effect can be additive (Co/Pd mixture), or even synergistic (Ag/Pd mixture).     

Characterization of Silver Particles in Silver-Containing Activated Carbon Fibers Prepared from Liquefied Wood

Silver particles in silver-containing activated carbon fibers prepared from liquefied wood were characterized by X-ray diffraction, X-ray photoelectron spectrometer, scanning electron microscope, and nitrogen adsorption isotherms. Silver irons (Ag⁺) and metallic silver (Ag⁰) were detected in fibers, and the amount of Ag⁰ was much higher than that of Ag⁺. Ag⁰ were migrated and aggregated together to form silver particles with a wide size (0–5μm), which were distributed in micropores, mesopores, and surface of fibers. The mean size of silver particles on the surface was directly related to soaking concentration, while the larger silver particles were easier to peel off from the surface. Also, the increasing micropores and mesopores were blocked by silver particles at higher concentration, and some blocked mesopores were converted into micropores. When the washing treatment was carried out, the silver particles on the surface were removed significantly, resulting in an increase in mesopore quantity. However, most of the silver particles in micropores were firmly supported. The silver-containing activated carbon fibers showed the high and lasting antibacterial activity.     

The effect of molecular composition and structure on the development of porosity in pitch-based activated carbon fibers

Seven oligomeric fractions of well-defined composition and molecular weight distribution were generated via supercritical extraction from an isotropic petroleum pitch (M-50) and used as precursors for the production of activated carbon fibers. Both isotropic and mesophase-containing fractions were produced, so that the effects of molecular order and molecular weight could be separated. Carbonization weight loss was found to gradually decrease with increasing molecular weight (and oligomeric number), with mesophase content not being a significant factor. Similar behavior was observed for activation weight loss when the precursors were isotropic; however, even modest increases in molecular order significantly retarded the activation process and resulted in dramatic drops in specific pore volume. A 100% dimer precursor fraction with an average molecular weight of 480 Da produced activated carbon fibers with the highest (specific) pore volume, and the highest pore volume in the range desired for hydrogen adsorption (6–7 Å). Even for isotropic precursors, decreases in pore volume with increasing molecular weight were observed. The incremental pore size distribution generated from the nitrogen adsorption data consisted of discrete peaks, which is consistent with the formation of pores by the removal of short micrographene layers, with each layer being formed from an individual oligomeric molecule.     

Swelling of pitch-based carbon fibres during activation in carbon dioxide

Isotropic pitch-based carbon fibres are activated by carbon dioxide and by steam at a temperature ranging from 850 to 1000 °C. In both cases, a significant increase in micropore volume, as measured by physical adsorption of N₂ and CO₂, is found. Fibre diameter steadily decreases during gasification by steam. In contrast, an intermediate diameter increase is observed during activation by CO₂. The interval of burn-off corresponding to fibre swelling depends on the gasification temperature. Phenolic resin-based fibres do not show swelling which indicates that it is specific to pitch-based fibres activated by CO₂. Oxygen is taken up during swelling on the incompletely stabilized part of the isotropic pitch-based carbon fibres. The origin of swelling and its effect on microporosity are discussed in relationship to fibre stabilization during activation.     

Isotropic and anisotropic microporosity development upon chemical activation of carbon fibers, revealed by microbeam small-angle X-ray scattering

A sub-micrometer size beam (0.5 μm diameter) in a position-resolved small angle X-ray scattering set-up (μSAXS) has been used for the characterization of chemically activated carbon fibers (ACF). These materials have been prepared from isotropic carbon fibers (pitch carbon fibers) and anisotropic carbon fibers (PAN-based carbon fibers) by chemical activation with KOH and NaOH. The μSAXS experimental set-up made it possible to analyze different regions of a single fiber across its diameter and to distinguish the structural features already existing in the raw fibers or being created during the activation process. The results showed that depending on the precursor, the chemical activation process produces isotropic or anisotropic development of porosity. It was observed that chemically ACF prepared from isotropic carbon fibers present an isotropic development of the porosity and that a high micropore volume is developed not only in the external region of the fiber, but also in the core. On the other hand, in the case of anisotropic PAN-based carbon fibers the existence of two regions with different structure was detected by μSAXS measurements across the fiber diameter: an anisotropic external ring and a more isotropic fiber core. The results showed that these two regions remain after chemical activation and that the activating agents are reaching the fiber core. It seems that the more isotropic fiber core is activated easier by NaOH than KOH.     

Preparation of carbon fiber from isotropic pitch containing mesophase spheres

Naphthalene based synthetic isotropic pitch was found to give spinnable precursors which carried 10–100 μm anisotropic spheres of 45 and 50 vol% in the isotropic matrix, when the pitch was heat-treated at 375 and 380 °C for 20 and 23 hours, respectively. The spun fiber was stabilized and carbonized into a carbon fiber, which showed anisotropic belts running along the fiber axis, suggesting that both anisotropic spheres and isotropic matrix are deformed to align within the nozzle. The structure and mechanical properties of the carbon fibers, thus prepared, were examined under microscopes and by mechanical testing.     

Influence of boron on structure and oxidation behavior of graphite fiber, P120

P120 fibers, derived from mesophase pitch, were substitutionally doped with boron in the concentration ranges of 200-4600 ppm. An oxidation study was carried out in dry air at 973, 1023, and 1073 K at 95 kPa. Boron is preferentially positioned into the less disordered core region and in the external surface area (skin) of the fiber. Upon oxidation these regions are preferentially protected. Oxidation rates decreased by a factor up to 3, varying with boron concentration, burn-off level and oxidation temperature. The activation energy of oxidation increased from 151 kJ/mol for heat-treated P120 fibers to 180 kJ/mol for fibers with 3300 ppm B, then decreased to 122 kJ/mol for fibers containing 4600 ppm of boron. The observed decrease in oxidation rate is directly attributed to the location and concentration of boron. Boron doping inhibits oxidation by blocking specific active sites. It is proposed that 1000 ppm B as a threshold concentration at which the electronic, chemical, and physical (structural) behavior could be modified.     

Effect of activation on boron nitride coating on carbon fiber

Boron nitride (BN) thin coating has been formed on the surface of chemically activated polyacrylonitrile (PAN) carbon fibers by dip coating method. The chemical activation of PAN fibers was carried out by two different chemicals, i.e. nitric acid (HNO₃) and silver nitrate (AgNO₃) solution. The chemical activation changes the surface properties, e.g. surface area and surface microstructure of the carbon fibers. These surface modifications ultimately influence properties of boron nitride coating on carbon fibers. The boron nitride coating on carbon fibers showed better crystallinity, strength and oxidation resistance when carbon fibers were activated by HNO₃. This improvement in strength and oxidation resistance is attributed to better crystallinity of boron nitride coating on HNO₃ activated PAN fibers.     

Preparation of pitch-based antibacterial activated carbon fiber

Silver acetate dissolved in quinoline was mixed thoroughly with the spinnable isotropic petroleum pitch dissolved in the same solvent. After removing the quinoline, the resulting pitch was spun, stabilized, carbonized, and finally activated at 900°C under a stream of steam. The fiber with 33 wt% yield after activation contained 0.63 wt% of fine silver particles and showed N₂-BET specific surface area of 740 m²/g. It showed antibacterial activity against Staphylococcus aureus and Escherichia coli, before and after soaking in flowing tap water for 20 days. How to improve spinnability of the pitch containing silver acetate and how to increase the specific surface area effectively by the activation process are problems remaining to be solved.     

Structural characterization of N-containing activated carbon fibers prepared from a low softening point petroleum pitch and a melamine resin

The incorporation of heteroatoms like N in activated carbons is of interest to modify the surface chemistry of the materials and, then, to improve their behavior as catalyst or catalyst support. In this work, N-containing activated carbon fibers have been prepared using a petroleum pitch with a low softening point and an N-containing resin. The novelty of the preparation method is that it involves the steps used in the synthesis of activated carbon fibers, i.e. spinning, stabilization, carbonization and activation. The materials have been characterized with techniques such as XPS and UPS, which allows us to follow the changes in both the chemical state of N species and the valence band structure of the carbon samples during the preparation steps.     

Improvement of the reduction capacity of activated carbon fiber

The reduction property of activated carbon fibers (ACFs) enables them to be used in the recovery of noble metals from wastewater, or in the extraction of gold or silver from ore leaching solutions. In order to effectively recover or extract noble metals, it is important to enhance the reduction capacity of ACFs and to improve the particle size of the noble metal reduced and adsorbed on the surface of activated carbon fibers. In this paper, the effect of the preparation method and surface modification of ACFs on their reduction capacity was studied. The results show that the preparation methods of ACFs have significant influence on their reduction-adsorption capacities for silver ions in solution—those ACFs prepared with phosphoric acid or zinc chloride activation have much higher reduction capacities. Moreover, surface modification of ACFs with some inorganic oxidants such as nitric acid, potassium permanganate, or hydrogen peroxide, though resulting in a small decrease of specific surface area or pore volume, will enhance the reduction capacity of oxidized ACFs for silver ions. Furthermore, methylene blue, aniline, or p-nitrophenol present in solution or adsorbed on ACFs can also significantly increase the reduction capacities of sisal-based ACFs for silver ions.     

Properties of differently shaped activated carbon fibers

Differently shaped carbon fibers (R-, I-, C-, Y-, and X-type) were prepared from melt-spinning of reformed naphtha cracking bottom oil precursors through various shaped spinnerets. These carbon fibers were activated by steam and activation properties were compared. The decrease of hydraulic radius resulted in the extending of the external surface area of carbon fibers. Activation energy and rate of differently shaped carbon fibers were affected by external surface area. Especially, the activation rate of tetralobal carbon fibers (X-type) appeared much larger than other shaped carbon fibers due to the smallest hydraulic radius. Adsorption capacity of tetralobal activated carbon fibers was also larger than other shaped activated carbon fibers.     

Role of active sites in the steam activation of high unburned carbon fly ashes

Previous studies on carbon gasification have not included high unburned carbon content fly ashes, and therefore it remains unclear why not all fly ash carbon samples are equally suitable for activation. The concentration of active sites is well known to influence carbon gasification reactions. Therefore, the objective of this work was to investigate the effect of the concentration of active sites on the behavior of fly ash carbon samples upon steam activation. Six fly ash carbons were selected to produce activated carbons using steam at 850 °C. The concentration of active sites was determined by non-dispersive infrared analysis (NDIR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). XRD analyses were also conducted to determine the crystallite size. It was observed that the concentration of active sites played a more significant effect on the surface areas of activated carbons in the carbon burn-off zone of >60%. Statistical analysis was used to relate the surface areas of activated carbon variances with carbon burn-off levels.     

中间相和各向同性沥青基炭纤维的活化及其结果比较

研究了分别由中间相沥青和各向同性沥青为原料制备的两种活化纤维的结构与性能特征。通过低温N2气吸附等温线测定、扫描电镜（SEM）观察和X－射线衍射分析等方法,从微观结构上比较了两种沥青作为ACF制备原料的差异性。     

Characterization of pore distribution in activated carbon fibers by microbeam small angle X-ray scattering

In the present work, the results corresponding to the first experiments done with single activated carbon fibers (ACFs) at the microfocus beamline (ID13) in the ‘European Synchrotron Radiation Facility’ (Grenoble) are presented. The experiments done with CO₂ and steam ACFs have demonstrated the suitability of this technique to characterize a single ACF. The experiments show that scattering intensity increases with the burn-off degree, which agrees with SAXS experiments carried out using bigger amounts of fibers. Moreover, the two-dimensional scattering patterns show that, in this type of ACFs, the porosity development during the activation process is isotropic. In addition, it has been demonstrated that the use of an X-ray microbeam of 2 μm diameter allows the characterization of different regions of the same fiber with microscopic position resolution. The scans across the fiber diameter are the first direct proof for the previous results obtained by our research group. Thus, in the case of CO₂ ACFs, the scattering is high in different regions across the fiber diameter, confirming that CO₂ activation takes place within the fibers, generating a quite homogeneous development of porosity. On the other hand, in the case of steam ACFs, the scattering is much higher in the external zones of the fibers than in the bulk, which means that steam focuses the activation in the outer parts of the fibers.     

Biopitch-based general purpose carbon fibers: Processing and properties

Eucalyptus tar pitches are generated on a large scale in Brazil as by-products of the charcoal manufacturing industry. They present a macromolecular structure constituted mainly of phenolic, guaiacyl, and siringyl units common to lignin. The low aromaticity (60-70%), high O/C atomic ratios (0.20-0.27%), and large molar mass distribution are peculiar features which make biopitches behave far differently from fossil pitches. In the present work, eucalyptus tar pitches are evaluated as precursors of general purpose carbon fibers (GPCF) through a four-step process: pitch pre-treatment and melt spinning, and fiber stabilization and carbonization. Homogeneous isotropic fibers with a diameter of 27 μm were obtained. The fibers had an apparent density of 1.84 g/cm³, an electrical resistivity of 2 × 10⁻⁴ Ω m, a tensile strength of 130 MPa, and a tensile modulus of 14 GPa. Although the tensile properties advise against using the produced fibers as structural reinforcement, other properties give rise to different potential applications, as for example in the manufacture of activated carbon fibers or felts for electrical insulation.     

Liquid-phase adsorption of phenol by activated carbons prepared from bituminous coals with different oxygen contents

Activated carbons prepared from two bituminous coals were used to adsorb phenol in aqueous solutions. The major difference between the coal precursors is the oxygen content. The carbon preparation consisted of carbonization of the coals followed by activation in CO ₂ to various extents of burn-off. Experimental results show that the amount of phenol adsorbed generally increases with the BET surface area of the carbon. The carbons prepared from the coal with a higher oxygen content have larger surface areas, and, therefore, exhibit higher capacities for phenol. The surface area of the carbon increases with the extent of carbon burn-off, whereas the increase in the adsorptive capacity due to the increasing burn-off level does not show a linear relationship with the increase in area; the ratio of the capacity to BET surface area is not constant and decreases with the burn-off level. This has been attributed to the accessibility of phenol to the surface being affected by the length of diffusion path, which is an increasing function of the burn-off level or the particle size. The amount of phenol adsorbed decreases with the temperature for these carbons. It was found, according to the Langmuir model, that the adsorption process was significantly affected by the oxygen content in the coal precursors. © 1999 Society of Chemical Industry     

熔纺Kraft硬木木质素基活性炭纤维的活化工艺及性能研究

以熔融纺丝制备的Kraft硬木木质素纤维(HKL)为原料,经炭化得到木质素基炭纤维(HKL-CF),再采用水蒸气活化法制备了活性炭纤维(HKL-ACF),通过红外光谱仪和扫描电镜研究了水蒸气活化对活性炭纤维化学结构和表面形貌的影响,采用全自动物理吸附仪、X射线衍射仪和拉曼光谱仪等研究了活化时间、活化温度和活化水蒸气流量对所制备活性炭纤维的比表面积、孔结构和微晶结构的影响规律。研究表明,水蒸气活化处理提高了活性炭纤维中的C—O和C＝C结构含量;随着活化时间的延长,活性炭纤维的比表面积增大,且随活化温度和水蒸气流量的提高呈现出先增大后减小的趋势;晶粒尺寸随着活化时间和温度的提高,逐渐变小,纤维表面的石墨化程度随活化时间的增加,逐渐变大;活化温度800 ℃,活化时间4 h,水蒸气流量1 mL/min下制备的活性炭纤维的BET比表面积最高可达2 081.34 m²/g,总孔容最大为1.60 cm³/g。     

On the activation of Asturian anthracite following various pretreatments

An Asturian anthracite has been subjected to different pretreatments and subsequently activated by steam at 850°C to a total burn-off of 55 percent. The physical properties (micropore sizes and distributions, external surface areas, etc.) of the solids are compared with those of the carbon obtained by direct activation. Although the yield is generally low, it appears that better results are obtained by activation following preoxidation in air at 270°C for 3 days. In the case of pretreatments with a mixture of nitrogen, air, and water vapor at 450°C, the subsequent activation is less efficient. The micropore volumes and the pore size distributions are similar to those observed for soft precursors, but at a much lower degree of burn-off.