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.     

Microporosity and mesoporosity of PPTA-derived carbons. Effect of PPTA thermal pretreatment

Isothermal treatments of the polyaramid fiber, [poly(p-phenylene terephthalamide)] (PPTA) in an inert atmosphere below its decomposition temperature are known to induce an important increase in char yield and modify the chemical composition and some other properties of the resulting chars. The objective of this work was to study the effect of this isothermal stage on the porous texture of chars and activated carbon fibers (ACFs) produced from PPTA. To this end, chars and ACFs were prepared by PPTA pyrolysis to 850 °C followed by CO₂ activation at 800 °C to various burn-offs (BOs), introducing or not an intermediate isothermal pre-treatment under the conditions (500 °C, 200 min) known to lead to a maximum increase in char yield. The porosity characteristics of the resulting chars and ACFs were comparatively investigated by adsorption of CO₂ (0 °C), and N₂ (−196 °C). The isothermal stage led to a char with enhanced micropore volume and wider micropores. The ACFs prepared from this char exhibited larger amounts of wide micropores and mesopores than those prepared from PPTA pyrolyzed at a constant heating rate.     

The effect of the activating agent and temperature on the porosity development of physically activated coal chars

An experimental work on the influence of temperature and the activating agent on the development of surface area and porosity for activated carbons was carried out. Three coals from different regions of Colombia were activated with CO₂, steam and a CO₂–steam mixture. Coal from the Antioquia Region (La Capotera) was activated with a CO₂–steam mixture at 1073, 1123 and 1173 K and with CO₂ and steam at 1073 K. Other two coals from Antioquia and Cesar regions (La Grande and Borrego) regions were activated with a CO₂–steam mixture at 1073 K and these were compared with the La Capotera char for the same conditions. The content of ash was confirmed to affect the development of surface area: coals with lower amount of ash developed higher specific surface areas. Activation temperature also affected the development of surface area: the use of high temperature produced low surface areas. Results indicate that CO₂–steam produces larger surface areas than CO₂ and steam alone, and reactions with CO₂–steam and CO₂ develop a more uniform porosity than reaction with steam. The pore sizes are larger when steam is used and smaller when CO₂ is used.     

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.     

Activation of a spherical carbon for toluene adsorption at low concentration

This paper complements a previous one [1] about toluene adsorption on a commercial spherical activated carbon and on samples obtained from it by CO₂ or steam activation. The present paper deals with the activation of a commercial spherical carbon (SC) having low porosity and high bed density (0.85 g/cm³) using the same procedure. Our results show that SC can be well activated with CO₂ or steam. The increase in the burn-off percentage leads to an increase in the gravimetric adsorption capacity (more intensively for CO₂) and a decrease in bed density (more intensively for CO₂). However, for similar porosity developments similar bed densities are achieved for CO₂ and steam. Especial attention is paid to differences between both activating agents, comparing samples having similar or different activation rates, showing that CO₂ generates more narrow porosity and penetrates more inside the spherical particles than steam. Steam activates more from the outside to the interior of the spheres and hence produces larger spheres size reductions. With both activating agents and with a suitable combination of porosity development and bed density, quite high toluene volumetric adsorption capacities (up to 236 g toluene/L) can be obtained, even using a low toluene concentration (200 ppmv).     

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.     

Preparation of ultra-thin PAN-based activated carbon fibers with physical activation

This study elucidates the stabilization and activation in forming activated carbon fibers (ACFs) from ultra-thin polyacrylonitrile (PAN) fibers. The effect of stabilization time on the properties and structure of resultant stabilized fibers was investigated by thermal analysis, X-ray diffraction (XRD), elemental analysis, and scanning electron microscopy (SEM). Stabilization was optimized by the pyrolysis of ultra-thin PAN fibers in air atmosphere at 280°C for 15 min, and subsequent activation in steam at 1000°C for 0.75 to 15 min. Resultant ACFs were characterized by N₂ adsorption at 77 K to evaluate pore parameters, XRD to evaluate structure parameters, and field emission scanning electron microscopy (FESEM) to elucidate surface morphology. The produced ACFs had surface areas of 668–1408 m²/g and a micropore volume to total pore volume ratio from 78 to 88%. Experimental results demonstrate the surface area and micropore volume of 1408 m²/g and 0.687 cm³/g, respectively, following activation at 1000°C for 10 min. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Activation of ultra‐thin activated carbon fibers as electrodes for high performance electrochemical double layer capacitors

Activated carbon fibers (ACFs) contain pores with a weak resistance to electrolyte migration but with high electrical resistance between the fibers. The ACFs used herein were prepared from ultra‐thin polyacrylonitrile (PAN) fibers, to be used as electrodes in electrochemical double layer capacitors (EDLCs), by varying the activation temperatures and the holding times during steam activation. As the activation temperature and holding time were increased, the specific surface area increased along with the specific capacitance (F g^?1). A maximum specific capacitance as high as 283 F g^?1 can be obtained using the ultra‐thin ACFs fabricated at 1000°C for 10 min with a specific surface area of 1408 m² g^?1. This investigation demonstrates that the surface area, pore structure, and surface functional groups of ACFs were all significant factors in determining the capacitive characteristics of ACFs. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Surface analyses of polyacrylonitrile-based activated carbon fibers by X-ray photoelectron spectroscopy

In this work, polyacrylonitrile (PAN)-based activated carbon fibers (ACFs) were developed by the common processes of stabilization, carbonization, and steam activation. Those fibers were successively subjected to heat treatment in a vacuum at high temperature and reactivation in steam. The changes in specific surface area and surface chemical characteristics were studied. The vaccum treatment reduced the surface area dramatically, while the surface nitrogen decreased after activation in steam and the surface oxygen was lowered upon the vaccum treatment. By curve-fitting the X-ray photoelectron spectroscopy (XPS) of C1_s, O1_s, and N1_s, it was shown that the forms and compositions of the oxygen-containing functionalities seemed not to be affected significantly by the above-mentioned heat-treat-ment processes, whereas the nitrogen-containing functionalities showed some changes with these treatments. © 1996 John Wiley & Sons, Inc.     

Usefulness of CO2 adsorption at 273 K for the characterization of porous carbons

Characterization of microporous solids (activated carbons and carbon molecular sieves) has been carried out by N₂ (subatmospheric pressures) and CO₂ adsorption (at subatmospheric and high pressures) at 77 and 273 K, respectively. Because the relative fugacity range covered by our CO₂ study is similar to the relative pressure range covered with N₂, a suitable comparison of both adsorptives can be made. The results of such comparison show that both adsorptives give the same micropore size distribution (MPSD) for open porosity activated carbons. This observation confirms that the adsorption mechanism of both adsorptives is similar. However, carbon molecular sieves, with very narrow microporosity, cannot be characterized by N₂ at 77 K, due to the existence of diffusional problems. This is also extensive to many other carbon materials, such as carbon fibers and activated carbons with low degree of activation. As a consequence, in this type of samples, N₂ adsorption at 77 K is useless to determine neither the micropore volumes of the narrowest porosity nor their micropore size distributions (MPSD). In this work, the usefulness of CO₂ for the characterization of carbon molecular sieves and activated carbons with different activation degrees is demonstrated. In addition, examples of applications that cannot be explained from N₂ adsorption but yes by CO₂ are presented. As a result, we strongly encourage the use of CO₂ (i.e. at 273 K) as a complement to N₂ adsorption at 77 K.     

Structure of K2Co3-loaded activated carbon fiber and its deodorization ability against H2S gas

Coal tar pitch containing finely dispersed KOH was spun centrifugally, followed by stabilization through heating to 330°C under a (1:1) mixture of air and CO₂ and carbonization/activation by heating to 850°C under CO₂. The activated carbon fiber obtained possessed of a specific surface area of 491 m²g⁻¹ and contained ca. 2% of K as K₂CO₃ over the peripheral region of fiber. The fiber showed high deodorization ability against 30 ppm of H₂S gas in air at ambient temperature. H₂S gas did not diffuse to the most interior parts of the fiber and was oxidized around outer regions of the fiber. Elemental sulphur was deposited in the fiber after H₂S absorption. The deodorization mechanism was discussed. The role and action of the K₂CO₃ supported was explained.     

Soot surface area evolution during air oxidation as evaluated by small angle X-ray scattering and CO2 adsorption

The total surface area of two diesel engine produced soots, a high volatile content NIST standard (termed NIST) and a low volatile content soot (termed NEU), were determined with CO₂ adsorption and small angle X-ray scattering (SAXS), as a function of the extent of oxidation. During initial volatilization of condensables of the NIST and NEU soots in a thermogravimetric analyzer, in helium at 1073 K, their CO₂ surface areas increased sharply from 49 m²/g to 273 m²/g and from 96 m²/g to 367 m²/g, respectively. During oxidation, the CO₂ surface area increased by an additional 100-150 m²/g, until 50% conversion was reached. Thereafter, the CO₂ surface area was relatively constant with conversion for the NIST soot, but decreased to 150 m²/g for the NEU soot. Three porosity regimes were assumed for the calculation of SAXS areas; they were based on (a) constant density (shrinking core), (b) constant diameter, and (c) an observed (with a TEM) diameter variation. The best agreement between the CO₂ and SAXS surfaces area occurred for the constant density assumption, in contrast to the actual measured diameter variation. By applying fractal surface analysis to the SAXS data, this discrepancy is ascribed to the opening up of internal volume to reaction volatilization of condensables and oxidation.     

Understanding chemical reactions between carbons and NaOH and KOH: An insight into the chemical activation mechanism

Direct mixing of an anthracite with hydroxides (KOH or NaOH) and heat treatment up to 730 °C has shown to be a very good activation procedure to obtain activated carbons with very high surface areas and high micropore volumes. The reactions involved during the heat treatment of these hydroxide/anthracite mixtures have been analysed to deep into the fundamental of the knowledge of this chemical activation process, that has not been studied before. For this purpose, the present paper analyses the drying process, the atmosphere during the carbonisation, the chemical state of the activating agents (NaOH, KOH and Na₂CO₃) and the chemical reactions occurring during the heat treatment which have been followed by FTIR and TPD. The analysis of our results allows us to conclude that steam is a good atmosphere for the carbonisation process, alone or joined with nitrogen, but not as good as pure nitrogen. On the other hand, during the activation process, the presence of CO₂ should be avoided because it does not develop porosity. The reactions, and chemical changes, involved during this chemical process are discussed both from a thermodynamical point of view as well as identifying the reaction products (H₂ by TPD and Na₂CO₃ by FTIR). As a result, this paper helps to cover the present lack of understanding of the fundamentals of the reactions of an anthracite with hydroxides which are necessary to understand the activation of the material.     

不同原料基活性碳纤维的结构及吸附特征研究

采用Ｘ－射线衍射，ＳＥＭ和ＢＥＴ比表面分析仪，探讨了不同原料（聚丙烯脂，聚乙烯醇，粘胶，天然纤维和沥青碳纤维）基活性碳纤维的结构和吸附特征。实验表明，所制备的活性碳纤维都具有大致相同的乱层石墨结构，其微晶参数Ｌｃ，Ｌａ和ｄ值分别为１．０～１．２，３．９～５．０，０．３８～０．４０ｎｍ，微晶尺寸越小，活性碳纤维（ＡＣＦ）的比表面积越大。ＡＣＦｓ的表面形态都不相同且基本保持着原料纤维的形态，ＡＣＦｓ对有机物苯的吸附量与纤维的比表面积成正比，但对Ａｇ~＋的吸附则与比表面积关系不大，此外，吸附还原在ＡＣＦ上的Ａｇ粒形态与ＡＣＦ的表面形态有关，ＮＡＣＦ和ＡＡＣＦ上吸附的Ａｇ粒尺寸小于３００ｎｍ。     

Control of pore development during CO2 and steam activation of olive stones

Several series of activated carbons were prepared from olive stones by means of carbonization followed by activation with carbon dioxide, water steam and a mixture of them, under different experimental conditions. The changes in porosity of the original char during activation were studied by adsorption of N₂ at 77 K, CO₂ at 273 K and Hg porosimetry. The study was carried out covering a wide range of burn-off (19–83%) using activation times of 20–120 min, and temperatures between 650 and 950 °C. It is shown quantitatively how the individual factors influence the development of microporosity. It was found that in general terms, increasing activation produces a continuous increase in the volume of micropores and mesopores. However, this development occurs in a different proportion whether CO₂ or steam are used: while CO₂ produces narrow micropores on the carbons and widens them as time is increased, steam yields pores of all the sizes from the early stages of the process. The simultaneous use of these two activating agents resulted positive at times higher than 1 h, since it yielded carbons with higher volumes of 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.     

In situ polymerization and nano‐templating phenomenon in nylon fiber/PMMA composite laminates

Supercritical Carbon Dioxide (SC CO₂) is used as a reaction/processing medium in the fabrication of fiber‐reinforced composite materials. SC CO₂ allows resin (reactive monomer), to penetrate inside the fibers themselves, partitioning into the amorphous regions of the fiber. The crystal structure then templates polymerization of matrix within the fiber. This process produces a composite that exhibits ultralong‐range order from the nanoscale reinforcement of crystals to the macroscale fiber reinforcement of matrix. In addition, SC CO₂ lowers resin viscosity and aids in wetting out Nylon 6,6 fiber reinforcement in a process similar to reaction injection molding (RIM) or resin transfer molding (RTM). This article will discuss the fabrication technique in detail, including process parameters and the structure of resulting composites and morphology of modified fibers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1600–1607, 2003     

Effect of activation on the carbon fibers from phenol–formaldehyde resins for electrochemical supercapacitors

Activated carbon fibers (ACF) are prepared from phenol–formaldehyde resin fibers through chemical activation and physical activation methods. The chemical activation process consisted of KOH, whereas the physical activation was performed by activation in CO₂. The characteristics of the electrochemical supercapacitors with carbon fibers without activation (CF), carbon fibers activated by CO₂ (ACF-CO₂), and carbon fibers activated by KOH (ACF-KOH) have been compared. The activated carbon fibers from phenol–formaldehyde resins present a broader potential range in aqueous electrolytes than activated carbon and other carbon fibers. Activation does not produce any important change in the shape of starting fibers. However, activation leads to surface roughness and larger surface areas as well as an adapted pore size distribution. The higher surface areas of fibers treated by KOH exhibited higher specific capacitances (214 and 116 F g⁻¹ in aqueous and organic electrolytes, respectively) and good rate capability. Results of this study suggest that the activated carbon fiber prepared by chemical activation is a suitable electrode material for high performance electrochemical supercapacitors.     

Effect of iron enrichment with GIC or FeCl3 on the pore structure and reactivity of coking coal

The effect of a Lewis acid addition to a coking coal on the porosity and reactivity towards steam of the resulting iron enriched coal chars are studied. GIC (FeCl₃ graphite intercalation compound) or free FeCl₃ are used as iron containing additives. Coal iron enrichment was performed using either directly FeCl₃ in vapour phase, or by mixing of coal and additives in decaline or by common grinding of coal and additives under argon. Iron enriched coals were carbonized at 750°C (heating RATE = 5°C min) and activation made with pure steam at 800°C to a burn-off off of 50 wt%. The pore structures of coal chars before and after activation were evaluated on the basis of CO₂ and C₆H₆ sorption at 25°C. A significant development of the microporosity is observed in the iron enriched char before activation and its steam reactivity is also increased. After activation, BET surface area values are increased in presence of iron, and porosity is mainly microporous.     

Influence of pretreatment and activation conditions in the preparation of activated carbons from anthracite

The influence of pretreatment and activation conditions on anthracite activation was investigated. Separation of low ash coals by using dense media was conducted to obtain appropriate raw materials for activation. Activated carbons were produced from crushed and granule coals by physical activation (steam or CO₂) and physical activation with chemical pretreatment in mild and strong conditions. Microporous activated carbons having a surface area of 900 m₂/g were produced by steam activation from granules with 60% burn-off for 3 hrs of activation. Chemical pretreatment at the strong condition increased the surface area by 30% as compared with non-treated activated carbons. Chemical pretreatment, in general, affected activation degree, so pore volume increased by 20% and burn-off increased remarkably at the identical activation conditions. CO₂ activation was proven to be an effective method for producing microporous activated carbons having an average pore diameter of 20 å.