HEMP-derived activated carbon fibers by chemical activation with phosphoric acid

Activated carbon fibers were prepared by chemical activation of hemp fibers with phosphoric acid at different carbonization temperatures and impregnation ratios. Surface properties of the activated carbons fibers were significantly influenced by the activation temperature and the impregnation ratio. An increase of either of these parameters produced a high development of the porous structure of the fibers. Activated carbon fibers with apparent surface area of 1350 m²/g and mesopore volume of 1.25 cm³/g were obtained at 550 °C with an impregnation ratio of 3. The activated carbon fibers presented a high oxidation resistance, due to the presence of phosphorus compounds on the carbon surface. The oxidation resistance results suggest that C-O-PO₃ and mainly C-PO₃ and C-P groups act as a physical barrier, blocking the active carbon sites for the oxidation reaction.     

On the preparation and characterization of chars and activated carbons from orange skin

Activated carbons were obtained by carbonization of orange skin waste and partial gasification with CO₂. The orange skin contains a significant amount of inorganic matter mainly potassium, calcium and phosphorus. CO₂ gasification is catalyzed by potassium and calcium, resulting in carbons with a microporous structure. Thermal treatment up to 900 °C applied to orange skin-derived activated carbons yields carbons with a highly developed porous structure, and a significant contribution of mesopores, due to the activation effect of potassium compounds. This porous structure is initially blocked by the inorganic matter that is removed by a subsequent acid wash, opening the porous structure of the final carbon; an activated carbon with a very wide porous structure and a specific surface area of around 1200 m²/g was obtained. The activated carbon with high potassium content shows relatively high NO adsorption capacities in the presence of oxygen at 120 °C, probably due to the catalytic effect of potassium on the oxidation of NO. The breakthrough times of the NO adsorption in the presence of oxygen at 120 °C were predicted by the Bohart and Adams model with a relevant agreement between the calculated and the experimental times.     

Influence of HNO3 oxidation on the structure and adsorptive properties of corncob-based activated carbon

An investigation of the impact of strong oxidation with HNO₃ on the porosity and adsorption characteristics of char and activated carbons, derived from corncobs, is presented. Texture parameters, as obtained from N₂ adsorption at 77 K, showed a considerable decrease in surface area of the activated carbons with enhanced pore widening. The extent of porosity modification was found to depend on the scheme of activation of the precursor, simple carbonization, steam pyrolysis, steam gasification of the char, or chemical activation with H₃PO₄. Surface-chemical changes were detected by FTIR spectroscopy, where absorption bands assigned to carboxyl, carboxylate, carbonyl, and phenolic groups were observed. A SEM study demonstrated the erosive effect of HNO₃, detected by the presence of disintegration of the carbon grains, with the porous structure probably containing very large macropores. As a consequence of the oxidation process, elemental analysis showed high contents of O, H and N, and TG confirmed that the weight loss distribution in the thermogram becomes slower at higher temperatures. The removal of phenol decreased as a result of the formation of oxygen functionalities. Mono-nitrophenols were adsorbed in smaller amounts than phenol, and p-nitrophenol showed a relatively higher uptake than the other two mono-nitrophenols, whereas the uptake of Methylene Blue was improved. Removal of Pb²⁺ from aqueous non-buffered solution was considerably enhanced by chemical oxidation, which may be related to pore widening, increased cation-exchange capacity by oxygen groups, and the promoted hydrophilicity of the carbon surface.     

Spherical carbons: Synthesis,characterization and activation processes

Spherical carbons have been prepared through hydrothermal treatment of three carbohydrates (glucose, saccharose and cellulose). Preparation variables such as treatment time, treatment temperature and concentration of carbohydrate have been analyzed to obtain spherical carbons. These spherical carbons can be prepared with particle sizes larger than 10 μm, especially from saccharose, and have subsequently been activated using different activation processes (H₃PO₄, NaOH, KOH or physical activation with CO₂) to develop their textural properties. All these spherical carbons maintained their spherical morphology after the activation process, except when KOH/carbon ratios higher than 4/1 were used, which caused partial destruction of the spheres. The spherical activated carbons develop interesting textural properties with the four activating agents employed, reaching surface areas up to 3100 m²/g. Comparison of spherical activated carbons obtained with the different activating agents, taking into account the yields obtained after the activation process, shows that phosphoric acid activation produces spherical activated carbons with higher developed surface areas. Also, the spherical activated carbons present different oxygen groups’ content depending on the activating agent employed (higher surface oxygen groups content for chemical activation than for physical activation).     

The effect of activation method on the properties of pecan shell‐activated carbons

Pecan shell chars were activated using steam, carbon dioxide (CO₂), or phosphoric acid (H₃PO₄) to produce granular activated carbons (GACs). The GACs were characterized for select physical, chemical and adsorption properties. Air oxidation of the GACs was used to increase copper ion (Cu²⁺) adsorption. BET surface areas of pecan carbons were equal to or greater than commercial GACs. Carbon dioxide activation favored microporosity, while the other activations increased both mesoporosity and microporosity. Bulk densities and particle attrition of the pecan shell GACs were generally similar to the commercial carbons. Air oxidation of steam‐and CO₂‐activated GACs increased copper ion adsorption, although not to the same extent as GACs made by H₃PO₄ activation. Copper ion adsorption and the amount of titratable functional groups greatly exceeded the values for the commercial GACs. Steam‐and CO₂‐activated pecan shell carbons were similar to but in some cases exceeded the ability of commercial GACs to remove certain organic compounds from water. GACs from pecan shells showed considerable commercial potential to remove metal ions and organic contaminants from water. © 1999 Society of Chemical Industry     

Phosphoric acid activation of nutshells for metals and organic remediation: Process optimization

Almond, pecan, English walnut, black walnut and macadamia nut shells were chosen as hard, lignocellulosic precursors for the production of granular activated carbons (GACs). They were activated with H₃PO₄ under a variety of conditions. Following activation, a portion of each GACs was oxidized in air at 300°C for 4 h in order to create a carbon surface presumably with more oxygen functional groups for the adsorption of metal cations. Also investigated was a streamlining of the production techniques used. Several of the samples were subjected to a ‘Continuous’ process in which the carbon was activated and upon cooling to 300°C was oxidized with air. Beyond this activation, methods were developed wherein the activation process took place under air and without a discrete, separate oxidation step. These processes were designed to determine if any of the carbon's capabilities would be lost or enhanced in comparison to the more standard activate–cool–wash–oxidize method. The carbons produced from these various activation/oxidation methods consistently, and without regard to precursor composition, sequestered over 90% of available copper from a 3 mM copper chloride test solution and often adsorbed organic contaminants as effectively as some commercial carbons. © 1998 SCI     

Nitrogen in aramid-based activated carbon fibers by TPD, XPS and XANES

Activated carbon fibers were prepared from Nomex® [poly(m-phenylene isophthalamide)] by either H₃PO₄ activation, H₃PO₄-CO₂ activation, or simply CO₂ or steam activation. These treatments converted amide groups from the polymer precursor into complex and heterogeneously distributed nitrogen functionalities. TPD, XPS and XANES were used to study the effects of these treatments on the local bonding environment around nitrogen in the resulting carbons. These analytical techniques showed that nitrogen atoms are present in the 6-membered rings located at the edges of condensed polyaromatic systems as pyridine-like sp² nitrogen (N1 or N2) or in the interior, where nitrogen replaces one carbon atom and is bonded to three carbon neighbors (N3). The occurrence of a species (N2) hypothetically related to a pyridinic cycle bearing oxygen substituents or intracyclic oxygen atoms could be correlated with the degree of oxidation of the carbon surface. Assuming that a relative N3 increase is indicative of aromatization and that the reverse, correlated with a N2 increase, is indicative of surface oxidative denitrogenation, the ratio between these nitrogen species revealed that aromatization and oxidative denitrogenation processes occur sequentially or simultaneously to different extents according to the type of carbon activation and to the burn-off degree. Physical activation involves thermal aromatization reactions during the carbonization stage and the subsequent isothermal activation one. In this second activation stage, co-occurring thermal oxidation reactions lead to a less intense denitrogenation during CO₂ activation than during steam activation. H₃PO₄ activation induces the largest nitrogen retention in the final product in a double process of aromatization and “auto-activation” producing a moderate oxidative attack of nitrogen. However, an increase of the H₃PO₄ ratio fostered the oxidation of the carbon surface and consequently enhanced nitrogen gasification during the subsequent activation.     

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.     

Water vapour adsorption on lignin‐based activated carbons

Lignocellulosic wastes are interesting precursors for carbon products. The high amount of Na observed in kraft lignin makes it a promising precursor for the preparation of activated carbons for desiccant applications. Water adsorption capacity and kinetics of kraft lignin‐based chars and activated carbons with different burn‐off and inorganic matter content have been studied. CO₂ partial gasification of lignin char develops a wide porous structure. An increase of the micropore volume can be observed at low to medium burn‐offs. At degrees of higher activation the mesoporous structure develops. For very high burn‐off the porous structure is destroyed by coalescence of the pores and reduction of the carbon material. The carbons obtained show atomic surface concentrations of sodium from 7.6–15.4%, as revealed by XPS analysis. Water vapour adsorption isotherms have been obtained in a thermogravimetric system and have been fitted by a DS model, which properly represents the experimental data. The kinetics of water vapour adsorption follows a linear driving force mass transfer (LDF) model. The presence of sodium and oxygen surface groups on the carbon surface enhances water vapour adsorption at low relative pressure. Activated carbon produced at 41% burn‐off shows the highest water vapour adsorption at low relative pressures, as a consequence of the high sodium dispersion on its surface. The sodium dispersed over the carbon surface undergoes clustering as gasification proceeds, decreasing the number of active centres. For burn‐off higher than 41%, this behaviour produces a decrease in the water adsorbed at low relative pressures. Copyright © 2007 Society of Chemical Industry     

The effect of H2 treatment on the activity of activated carbon for the oxidation of organic contaminants in water and the H2O2 decomposition

In this work, hydrogen peroxide reactions, i.e. H₂O₂ decomposition and oxidation of organics in aqueous medium, were studied in the presence of activated carbon. It was observed that the carbon pre-treatment with H₂ at 300, 500, 700 and 800 °C resulted in an increase in activity for both reactions. The carbons were characterized by BET nitrogen adsorption, thermogravimetric analyses (TG), temperature programmed reduction (TPR), electron paramagnetic resonance (EPR), iodometric titration and determination of the acid/basic sites. TPR experiments showed that activated carbon reacts with H₂ at temperatures higher than 400 °C. The treatment produces a slight increase in the surface area. EPR analyses indicate the absence of unpaired electrons in the carbon. Iodometric titrations and TG analyses suggested that the treatment with H₂ generates reduction sites in the carbon structure, with concentration of approximately 0.33, 0.53, 0.59, 0.65 and 0.60 mmol/g for carbons treated at 25, 300, 500, 700 and 800 °C, respectively. It was also observed the appearance of basic sites which might be related to the reduction sites. It is proposed that these reducing sites in the carbon can activate H₂O₂ to generate HO^* radicals which can lead to two competitive reactions, i.e. the hydrogen peroxide decomposition or the oxidation of organics in water.     

The effect of activated carbon surface moisture on low temperature mercury adsorption

Experiments with elemental mercury (Hg⁰) adsorption by activated carbons were performed using a bench-scale fixed-bed reactor at room temperature (27°C) to determine the role of surface moisture in capturing Hg⁰. A bituminous-coal-based activated carbon (BPL) and an activated carbon fiber (ACN) were tested for Hg⁰ adsorption capacity. About 75-85% reduction in Hg⁰ adsorption was observed when both carbon samples’ moisture (∼2 wt.% as received) was removed by heating at 110°C prior to the Hg⁰ adsorption experiments. These observations strongly suggest that the moisture contained in activated carbons plays a critical role in retaining Hg⁰ under these conditions. The common effect of moisture on Hg⁰ adsorption was observed for both carbons, despite extreme differences in their ash contents. Temperature programmed desorption (TPD) experiments performed on the two carbons after adsorption indicated that chemisorption of Hg⁰ is a dominant process over physisorption for the moisture-containing samples. The nature of the mercury bonding on carbon surface was examined by X-ray absorption fine structure (XAFS) spectroscopy. XAFS results provide evidence that mercury bonding on the carbon surface was associated with oxygen. The results of this study suggest that surface oxygen complexes provide the active sites for mercury bonding. The adsorbed H₂O is closely associated with surface oxygen complexes and the removal of the H₂O from the carbon surface by low-temperature heat treatment reduces the number of active sites that can chemically bond Hg⁰ or eliminates the reactive surface conditions that favor Hg⁰ adsorption.     

Importance of activated carbon's oxygen surface functional groups on elemental mercury adsorption

The effect of varying physical and chemical properties of activated carbons on adsorption of elemental mercury (Hg⁰) was studied by treating two activated carbons to modify their surface functional groups and pore structures. Heat treatment (1200 K) in nitrogen (N₂), air oxidation (693 K), and nitric acid (6N HNO₃) treatment of two activated carbons (BPL, WPL) were conducted to vary their surface oxygen functional groups. Adsorption experiments of Hg⁰ by the activated carbons were conducted using a fixed-bed reactor at a temperature of 398 K and under N₂ atmosphere. The pore structures of the samples were characterized by N₂ and carbon dioxide (CO₂) adsorption. Temperature-programmed desorption (TPD) and base-acid titration experiments were conducted to determine the chemical characteristics of the carbon samples. Characterization of the physical and chemical properties of activated carbons in relation to their Hg⁰ adsorption capacity provides important mechanistic information on Hg⁰ adsorption. Results suggest that oxygen surface complexes, possibly lactone and carbonyl groups, are the active sites for Hg⁰ capture. The carbons that have a lower carbon monoxide (CO)/CO₂ ratio and a low phenol group concentration tend to have a higher Hg⁰ adsorption capacity, suggesting that phenol groups may inhibit Hg⁰ adsorption. The high Hg⁰ adsorption capacity of a carbon sample is also found to be associated with a low ratio of the phenol/carbonyl groups. A possible Hg⁰ adsorption mechanism, which is likely to involve an electron transfer process during Hg⁰ adsorption in which the carbon surfaces may act as an electrode for Hg⁰ oxidation, is also discussed.     

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.     

Textural Characterization of Activated Carbons Prepared from Oil-palm Stones Pre-treated with Various Impregnating Agents

Activated carbons with relatively high densities and well-developed porosities were prepared from oil-palm stones which were pre-treated with different types of impregnating agents (ZnCl₂, H₃PO₄ or KOH). The benefits derived from impregnation in terms of higher BET surface areas were generally in the following order: 20% ZnCl₂ > 40% H₃PO₄ > 10% KOH. The textural properties such as density and total porosity, overall yield, BET and micropore surface areas and pore size distributions of the activated carbon were related to the concentration of the impregnating solution and the activation conditions (activation temperature and hold time). For the highest BET surface area obtained in this study, the optimum conditions for CO₂ activation were found to be at an activation temperature of 750°C for 1 hour from oil-palm stones pre-treated with 20% ZnCl₂ for 24 hours. Pore size distribution suggests the application of oil-palm-stone activated carbons as gas-phase adsorbents for air pollution control.     

Preparation of active carbons from coal: Part III: Activation of char

Active carbons with a burn-off of 52% have been prepared from four coals of different rank and origin after preoxidation to different degrees at 543 and 473 K, and further carbonization at 1123 K. The activation has been carried out with CO₂ at 1123 K at two flow rates viz. 7 cm³ min⁻¹ and 500 cm³ min⁻¹. Active carbons have also been prepared from a preoxidized coal by activation to different degrees of burn-off between 10 and 80%. The effect of the degree of oxidation, the flow rate of the activating gas and the extent of burn-off on the porous structure development of active carbons has been examined. The active carbons prepared from unoxidized coal have poor textural characteristics (porosity, N₂ and CO₂ surface area). Nevertheless, the textural characteristics are enhanced as the degree of preoxidation of the coal is increased. The low flow rate of CO₂ (activating gas) produces active carbons with a better microporous character. The degree of activation (the extent of burn-off) of the carbon determines the porous structure of the active carbon. At low degrees of burn-off (less than 50%) the product is largely microporous. At higher degrees of burn-off between 35–65% the product has a mixed porous structure and contains all types of pores. Active carbons with a given textural character can be obtained by controlling the degree of oxidation of coal and the degree of activation of the carbonized material.     

Importance of carbon active sites in the gasification of coal chars

A demineralized lignite has been used in a fundamental study of the role of carbon active sites in coal char gasification. The chars were prepared in N₂ under a wide variety of conditions of heating rate (10 K min^?1 to 10⁴ K s^?1), temperature (975–1475 K) and residence time (0.3 s–1 h). Both pyrolysis residence time and temperature have a significant effect on the reactivity of chars in 0.1 MPa air, determined by isothermal thermogravimetric analysis. The chars were characterized in terms of their elemental composition, micropore volume, total and active surface area, and carbon crystallite size. Total surface area, calculated from C0₂ adsorption isotherms at 298 K, was found not to be a relevant reactivity normalization parameter. Oxygen chemisorption capacity at 375 K and 0.1 MPa air was found to be a valid index of char reactivity and, therefore, gives an indication, at least from a relative standpoint, of the concentration of carbon active sites in a char. The commonly observed deactivation of coal chars with increasing severity of pyrolysis conditions was correlated with their active surface areas. The importance of the concept of active sites in gasification reactions is illustrated for carbons of increasing purity and crystallinity including a Saran char, a graphitized carbon black and a spectroscopically pure natural graphite.     

Activated carbon from cherry stones by chemical activation: Influence of the impregnation method on porous structure

Cherry stones are utilized as a precursor for the preparation of activated carbons by chemical activation with phosphoric acid (H₃PO₄). The activation process typically consists of successive impregnation, carbonization, and washing stages. Here, several impregnation variables are comprehensively studied, including H₃PO₄ concentration, number of soaking steps, H₃PO₄ recycling, washing of the impregnated material, and previous semi-carbonization. The choice of a suitable impregnation methodology opens up additional possibilities for the preparation of a wide variety of activated carbons with high yields and tailored porous structures. Microporous activated carbons with specific surface areas of ～800 m² g^?1 are produced, in which > 60% of the total pore volume is due to micropores. High surface areas of ～1500 m² g^?1 can be also developed, with micropore volumes being a 26% of the total pore volume. Interestingly, using the same amount of H₃PO₄, either carbons with surface areas of 791 and 337 m² g^?1 or only one carbon with a surface area of 640 m² g^?1 can be prepared. The pore volumes range very widely between 0.07–0.55, 0.01–0.90, and 0.09–0.79 cm³ g^?1 for micropores, mesopores, and macropores, respectively.     

Possible errors in microporosity in chemically activated carbon deduced from immersion calorimetry

The microporosity of granular and disc-shaped activated carbons prepared by both ZnCl₂ and H₃PO₄ activation has been evaluated by adsorption of nitrogen at −196 °C and immersion calorimetry into liquids of different molecular dimensions (dichloromethane, benzene, 2,2-dimethylbutane, carbon tetrachloride and α-pinene). Experimental results show that immersion calorimetry into dichloromethane provides values of surface area more similar to nitrogen adsorption (BET equation) than benzene. No such effect is found for physically activated carbon. Some apparent anomalies have also been detected for the enthalpy of immersion of carbons activated with H₃PO₄ into α-pinene due to a small amount of phosphorous remaining in the carbon after washing. This is not the case for carbons activated with ZnCl₂, because the washing was more effective in the removal of the chemical.     

Physical and chemical properties of carbons synthesized from xylan, cellulose, and Kraft lignin by H3PO4 activation

Physical and chemical properties of activated carbons produced from commercial xylan, cellulose, and Kraft lignin by H₃PO₄ activation at various process conditions were studied. The results show that the more reactive the precursor under acidic conditions, the easier the porosity development, particularly mesoporosity. In addition, Boehm titration and Fourier Transform Infrared Spectroscopy (FTIR) characterization results demonstrated that the functional groups on the surfaces of these carbons consist of both temperature-sensitive and temperature-insensitive components. The temperature-sensitive component is primarily caused by the hydrolysis of raw materials under acidic conditions at low temperature, and the reaction between activation mixture and oxygen in the process of activation, particularly at low impregnation ratio. These surface groups decompose at high temperature. The temperature-insensitive contribution is mainly composed of phosphorus-containing groups arising from the reaction of H₃PO₄ (or pyro- and polyphosphoric acids) with precursor, and carbonyl-containing groups. This part of surface functional group is stable, even at high activation temperatures. This study also confirmed that the nature of precursor, impregnation ratio between H₃PO₄ and precursor, and activation temperature are important factors affecting the properties of final activated carbon products.     

Catalytic gasification of doped carbon — a microscopic study

Inorganic additives in carbons are often active, catalytically, during its gasification. The method of addition, particle size, chemical composition and subsequent heattreatment of the additives control their relative efficacies. There are difficulties in attempts to reproduce these efficacies. To overcome these problems, carbons have been prepared by carbonisation of the polymer prepared from furfuryl alcohol containing known concentrations of soluble additives. The resultant carbons contain atomic dispersions of metals in a manner analogous to doped semi-conductors. Such dispersions are catalytically active to gasification by CO₂, N₂O and O₂. Topographical changes produced on gasification are followed by scanning electron microscopy. Rarely, if ever, does the gasification proceed evenly over the entire surface. The resultant pitting can influence structural and adsorptive properties of commercial carbons, subject to gasification, i.e. parent materials for active carbons and cokes. Results indicate that diffusion of the catalytically active metal into carbon from catalytic material added in particulate form may be a significant stage in the mechanism of catalytic gasifications.