Effect of precursor composition on the activation of pitchbased carbon fibers

Pure and silver-containing carbon fibers were prepared from isotropic pitch precursors supplied by Conoco, Inc., and a Korean research team and activated in carbon dioxide to varying degrees of burn-off. The specific activation rates for the carbon fibers were measured as well as the nitrogen adsorption characteristics of the activated carbon fibers. Scanning electron microscopy was used to investigate the surface morphology and the behavior of silver particles during the activation process. Molecular composition of the two pitch precursors was determined using a gas chromatograph mass spectrometer and a MALDI TOF mass spectrometer. Results showed that specific surface area increased with the burn-off, and the trends were similar for the pure and silver-containing fibers formed from both isotropic pitch precursors. However, the catalytic behavior of silver during activation, the activation rate, and even the pore characteristics of the activated fiber were found to be dependent on the molecular composition of the precursor pitch.     

Preparation and characterization of activated carbons for SO<Subscript>2</Subscript> adsorption from Taixi anthracite by physical activation with steam

Taixi anthracite was used as a precursor to prepare activated carbons (AC) for SO₂ adsorption from flue gas. In this work the activated carbons were prepared by physical activation with steam. Specifically, the effects of activation temperature and burn-off degree on the physico-chemical properties of the resulting AC samples were comparatively studied. The different types of pore volumes, pore size distributions and surface chemistries of the activated carbons on the SO₂ adsorption were also analyzed. The results show that the increasing burn-off leads to samples with continuous evolution of all types of pores except ultramicropore. The ultramicropore volume increases to a maximum of 0.169 cm³/g at around 50% burn-off and then decreases for 850 °C activation. At higher activation temperature, the micropore volume decreases and the mesopore structure develops to a certain extent. For all the resulting AC samples, the quantities of the basic surface sites always appear much higher than the amount of the acidic sites. The activated carbon prepared with higher micropore volume, smaller median pore diameter and higher quantities of the basic surface sites represents better SO₂ sorption property.     

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.     

Comparative study of the textural characteristics of oil palm shell activated carbon produced by chemical and physical activation for methane adsorption

The objective of this study is to relate textural and surface characteristics of microporous activated carbon to their methane adsorption capacity. Oil palm shell was used as a raw material for the preparation of pore size controlled activated carbon adsorbents. The chemical treatment was followed by further physical activation with CO₂. Samples were treated with CO₂ flow at 850 °C by varying activation time to achieve different burn-off activated carbon. H₃PO₄ chemically activated samples under CO₂ blanket showed higher activation rates, surface area and micropore volume compared to other activation methods, though this sample did not present high methane adsorption. Moreover, it was shown that using small proportion of ZnCl₂ and H₃PO₄ creates an initial narrow microporosity. Further physical activation grantees better development of pore structure. In terms of pore size distribution the combined preparation method resulted in a better and more homogenous pore size distribution than the conventional physical activation method. Controlling the pore size of activated carbon by this combined activation technique can be utilized for tuning the pore size distribution. It was concluded that the high surface area and micropore volume of activated carbons do not unequivocally determine methane capacities.     

A new approach to quantify the microporosity of activated carbons by analysing the N2/77 K and CO2/273 K adsorption data by the simplex flexible method

A mathematical model has been applied to N₂/77 K and CO₂/273 K adsorption isotherms for a series of activated carbons prepared by carbonising olive stones in N₂ and then activating them in CO₂ to six different levels of burn-off in the range 8–80%. Narrow and wide micropore volumes of activated carbons were calculated from the Dubinin-Radushkevich and Dubinin-Astakhov equations considering one, two and three micropore size distributions in each sample, and allowing a variation of the micropore volume and characteristic energy of each distribution with the burn-off. The flexible simplex method was applied to obtain the parameters of each distribution in the mathematical model. Generally, it was found that increasing the number of micropore size distributions above two did not significantly improve fits. Each isotherm was fitted using six parameters at most. However, various constraints were imposed, and the parameters were estimated from each isotherm using non-linear, least-squares regression analysis. The results obtained confirm the valuable use of CO₂/273 K adsorption to quantify the narrow microporosity of activated carbons. Differences between N₂/77 K and CO₂/273 K adsorption in microporous activated carbons were due to the wide microporosity. An agreement between micropore volumes obtained from CO₂/273 K adsorption and that corresponding to one of the two distributions of micropores obtained from N₂/77 K adsorption was obtained. The Dubinin-Radushkevich equation was more successful than the Dubinin-Astakhov equation in the quantification of the microporosity with N₂/77 K and CO₂/273 K. On the other hand, the exponent n of the Dubinin-Astakhov equation was better correlated with the burn-off of the carbons than with the parameter B.     

Pore structures of activated carbon fibers from organometallics/pitch composites by nitrogen adsorption

A series of activated carbon fibers (ACFs) produced from organometallics compounds such as Y(acac)₃ or Al(acac)₃/pitch composites were characterized. The pore-size, surface state and graphitic crystallite size were examined by means of N₂ adsorption isotherm, X-ray photoelectron spectroscopy and X-ray diffraction (XRD) analysis. The presence of Y or Al increased the pore-size. Y(acac)₃/pitch and Al(acac)₃/pitch-based ACFs activated at 1148 and 1173 K, respectively, had an appreciable amount of mesopores. The size of the micrographites structure of the organometallics/pitch-based ACF composites by XRD was slightly greater than that of the pitch-based ACF. Ar ion etching using X-ray photoelectron spectroscopy showed that Y or Al atoms are coated with carbon layers.     

Characterization of activated carbon fibers using argon adsorption

In this paper, we present results of the internal structure (pore size and pore wall thickness distributions) of a series of activated carbon fibers with different degrees of burn-off, determined from interpretation of argon adsorption data at 87 K using infinite and finite wall thickness models. The latter approach has recently been developed in our laboratory. The results show that while the low bun-off samples have nearly uniform pore size (<0.6 nm), the pore size distribution of the high burn-off samples becomes broader, with a significant increase in proportion of larger pores. The results of pore wall thickness distribution are generally consistent with development of porosity with increasing degree of burn-off. Further they show good correspondence with X-ray diffraction.     

Porous properties of activated carbon produced from Eucalyptus and Wattle wood by carbon dioxide activation

This work focused on the preparation of activated carbon from eucalyptus and wattle wood by physical activation with CO₂. The preparation process consisted of carbonization of the wood samples under the flow of N₂ at 400°C and 60 min followed by activating the derived chars with CO₂. The activation temperature was varied from 600 to 900°C and activation time from 60 to 300 min, giving char burn-off in the range of 20/2-83%. The effect of CO₂ concentration during activation was also studied. The porous properties of the resultant activated carbons were characterized based on the analysis of N2 adsorption isotherms at −196°C. Experimental results showed that surface area, micropore volume and total pore volume of the activated carbon increased with the increase in activation time and temperature with temperature exerting the larger effect. The activated carbons produced from eucalyptus and wattle wood had the BET surface area ranging from 460 to 1,490 m²/g and 430 to 1,030 m²/g, respectively. The optimum activation conditions that gave the maximum in surface area and total pore volume occurred at 900°C and 60 min for eucalyptus and 800°C and 300 min for wattle wood. Under the conditions tested, the obtained activated carbons were dominated with micropore structure (∼80% of total pore volume).     

Morphology investigation of mechanically activated ZnO–SnO2 system

Powder mixtures of zinc oxide and tin oxide in the molar ratio, ZnO:SnO₂ = 2:1, were mechanically activated in a planetary ball mill in the time intervals of 0–160 min. The adsorption–desorption isotherms, specific surface area, pore volume and pore size distribution spectra of mechanically activated powder mixtures were established by N₂ adsorption at 77 K. Microstructure analysis was performed using scanning electron microscopy (SEM) and digital pattern recognition (DPR) microstructure quantity analysis. The phase composition of the mixed powders was determined by X-ray analysis. Mechanochemical activation of the ZnO–SnO₂ system resulted in fine grinding of the starting particles and generation of contacts between them, mass transfer at contacts zones and formation of Zn₂SnO₄ spinel, which was observed after 40 min of activation.     

Experimental and Kinetic Studies on Pore Development During CO2 Activation of Oil-Palm-Shell Char

The results of experimental and kinetic studies on pore development during CO₂ activation of char derived from oil-palm shell, an abundant solid waste in some tropical countries, were presented in this paper. CO₂ was used as an activating agent instead of air because the 21% oxygen content in air would cause severe burn-off of carbon contents, resulting in detrimental effects on pore development. In preparing the activated carbon from oil-palm shell by CO₂ activation, size of the starting material and CO₂ gas flow rate were identified to minimize the effects of gas diffusion. Under a kinetic-controlled condition, the effects of char characteristics and activation temperature on BET and micropore surface areas, porosity and pore size distribution were investigated. For the char prepared from oil-palm shell at a low carbonization temperature of 873 K, the activated carbon with a reasonably high pore surface area and predominant microporosity was obtained.Its applications are in gas-adsorbing processes such as air pollutant removal and gas separation. A random pore model was developed to describe pore development during the carbon-CO₂ reaction process. Model predictions were compared with data from thermogravimetric analyses. Kinetic study showed that the activation reaction rate was dependent on both the initial pore structure of the char and the transient pore structure which was developed progressively during the activation process.     

The effect of processing conditions on microstructure of Pd-containing activated carbon fibers

Palladium-doped activated carbon fibers are being evaluated as candidate materials for enhanced hydrogen storage at near ambient conditions. Pd-doped fibers were spun using a Pd salt mixed with an isotropic pitch precursor. Experimental techniques such as in situ X-ray analysis, thermogravimetric studies, scanning transmission electron microscopy and gas adsorption were employed to understand how processing conditions for the production of Pd-doped activated carbon fibers affect the microstructure, pore development, and dispersion of metal particles throughout the fibers. The results showed that PdO phase is present in the stabilized fibers and that this oxide phase is stable up to about 250 °C. The oxide phase transforms into Pd metal with increasing heat treatment temperature, going through the formation of an intermediate carbide phase. Sintering of Pd particles was observed with heat treatment at temperatures over 750 °C. It was also found that pore development during physical activation with CO₂ was not significantly affected by the presence of Pd particles within the fibers.     

Fractal characteristic of three Chinese coals

Experimental and theoretical investigation about coal/char structure is presented. Surface structures of parent coal and char with different burn-off ratios were analyzed. We introduced the fractal theory into Scanning Electron Microscopy image analysis and utilize the particle surface fractal dimension (D_ps) to quantitatively describe the surface character of coal/char particles. D_ps of three Chinese coals reach their maximum in the 35-45 wt% char burn-off interval and then decrease with increasing carbon burn-off ratio. The inner-pore information of coal/char particles was determined by N₂ isotherm adsorption/desorption. Using fractal BET model, internal surface fractal dimension (D_s) of coal/char particles was calculated. The D_s change trend of three Chinese coals is similar to their S_BET development. It means the D_s can quantitatively describe the inner pore structure character of coal/char particles.     

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.     

Production of activated carbons from coffee endocarp by CO2 and steam activation

In this work the use of coffee endocarp as precursor for the production of activated carbons by steam and CO₂ was studied. Activation by both methods produces activated carbons with small external areas and microporous structures having very similar mean pore widths. The activation produces mainly primary micropores and only a small volume of larger micropores. The CO₂ activation leads to samples with higher BET surface areas and pore volumes when compared with samples produced by steam activation and with similar burn-off value. All the activated carbons produced have basic characteristics with point of zero charge between 10 and 12. By FTIR it was possible to identify the formation on the activated carbon's surface of several functional groups, namely ether, quinones, lactones, ketones, hydroxyls (free and phenol); pyrones and Si–H bonds.     

The controlled reaction of active carbons with air at 350°C—I: Reactivity and changes in surface area

Two activated carbons prepared from almond shells and olive stones were reacted with air at 350°C to different percentages burn-off. The reactivity was studied in the temperature range 350–500°C where the reaction is relatively slow. The activated carbon from almond shells is more resistant to the reaction with air and the activation energy of that reaction is 101 kJ mol⁻¹. The adsorption of N₂ at 77 K has been used to characterize the adsorptive properties and surface area of all the obtained products, which have high surface areas (around or above 1000 m² g⁻¹). The gas adsorption results, together with mercury porosimetry have allowed a study of the variation of surface area and porosity as a function of the burn-off. In any case, the exposure of the active carbons to air at 350°C for several days does not considerably affect their adsorptive properties even for a weight loss up to 50%.     

Comparison of surface properties of carbons gasified in nitrous oxide and in oxygen

Surface structures and reactivities of carbons gasified in N₂O and in O₂ are compared in this study. The carbon employed was derived from pyrolysis of phenol-formaldehyde resins. It was found that greater surface area and pore volume are created for the carbon gasified in O₂ than that in N₂O. At high extents of burn-off the carbon gasified in N₂O shows a larger mesopore volume than that in O₂. The reactivity per unit surface area of the carbon in N₂O decreases with the extent of burn-off, while that in O₂ remains relatively constant at various extents of burn-off. Surfaces in carbon micropores do not appear to be fully utilized in the N₂O-carbon reaction. It is confirmed that standard N₂ or CO₂ surface area is not a proper characterization of the reactive surface area of carbon gasified in N₂O.     

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.     

Effects of activation time on the properties and structure of polyacrylonitrile‐based activated carbon hollow fiber

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate, then further oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of activation time of a precursor fiber on the microstructure, specific surface area, pore‐size distribution, and adsorption properties of PAN‐based activated carbon hollow fibers (PAN‐ACHF) were studied in this work. The BET surface area of PAN‐ACHF and surface area of mesopores gradually increase with activation time extending, and reach the maximum values, 780 and 180 m² g^?1, respectively, when fibers are activated at 800°C for 100 min. The adsorption ratio to creatinine changes little with activation time extending and all values over all activation time are above 90%. The adsorption ratio to VB₁₂ gradually increases with activation time extending before 60 min, and then becomes relatively constant from 60 to 100 min. The number of pores on the surface of PAN‐ACHF increases with activation time extending. The amount of mesopores in PAN‐ACHF made of fibers activated for different time increases with activation time extending and the dominant pore sizes of mesopores in PAN‐ACHF range from 2 to 5 nm. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 2565–2569, 2006     

An investigation of fractal characteristics of mesoporous carbon electrodes with various pore structures

Fractal characteristics of mesoporous carbon electrodes were investigated with various pore structures using the N₂ gas adsorption method and the transmission electron microscopy (TEM) image analysis method. The mesoporous carbons with various pore structures were prepared by imprinting mesophase pitch used as a carbonaceous precursor with different colloidal silica particles. All imprinted mesoporous carbons were composed of two groups of pores produced from the carbonisation of mesophase pitch and from the silica imprinting. The overall surface fractal dimensions of the carbon specimens were determined from the analyses of the N₂ gas adsorption isotherms. In order to distinguish the surface fractal dimension of the carbonisation-induced pore surface from that fractal dimension of the silica-imprinted pore surface, the individual surface fractal dimensions were determined from the image analyses of the TEM images. From the comparison of the overall surface fractal dimension with the individual surface fractal dimensions, it was recognised that the overall surface fractal dimension is crucially influenced by the individual surface fractal dimension of the silica-imprinted pore surface. Moreover, from the fact that the silica-imprinted pore surface with broad relative pore size distribution (PSD) gave lower value of the individual surface fractal dimension than that pore surface with narrow relative PSD, it is concluded that as the silica-imprinted pores comprising the carbon specimen agglomerate, the individual surface fractal dimension of that pore surface decreases.     

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