Synthetic carbons activated with phosphoric acid: I. Surface chemistry and ion binding properties

Synthetic activated carbons were prepared by phosphoric acid activation of a styrene-divinylbenzene copolymer at various temperatures in the 400-1000 °C range. The resulting carbons were characterized by elemental analysis, cation-exchange capacity measurement, infrared spectroscopy, potentiometric titration with calculation of proton affinity spectra, and copper adsorption from solution. The results indicate that the synthetic carbons obtained possess acidic character and show cation-exchange properties similar to those of oxidized carbons. However, the acidic compounds arising from treatment with phosphoric acid are tightly bound to the carbon lattice and are chemically and thermally more stable than those introduced by oxidative treatments. The largest amount of cation-exchange surface groups is introduced after activation at 800 °C. Infrared investigations showed that phosphorus compounds may be polyphosphates bound to the carbon lattice. Proton affinity distribution curves calculated from potentiometric titration experiments showed four types of surface groups on synthetic phosphoric acid activated carbons. Among them phosphorus-containing groups are the most important for the adsorption of heavy metal ions (copper) from acid solutions. Thus, carbons activated with phosphoric acid may be regarded as prospective cation-exchangers for the removal of heavy metals from water solutions.     

Synthetic carbons activated with phosphoric acid: II. Porous structure

Synthetic activated carbons were prepared by H₃PO₄ activation of a chloromethylated and sulfonated copolymer of styrene and divinylbenzene, using an impregnation weight ratio of 0.75 and carbonization temperatures in the 400-1000 °C range. Other impregnation ratios (0.93 and 1.11) were also used at a carbonization temperature of 800 °C. The porous texture of the resulting carbons was characterized by N₂ adsorption at −196 °C and CO₂ adsorption at 0 °C. All carbons exhibited a multimodal pore size distribution with maxima in the micropore and meso/macropore regions. Maxima in pore volume were attained at 900 °C for micropores and at 500 and 900 °C for mesopores. The mesopore volume was less sensitive than the micropore volume to changes in the impregnation ratio. It is concluded that the porous texture is not a prime factor in determining the outstanding cation exchange capacities of these carbons.     

Formation of oxygen complexes by ozonation of carbonaceous materials prepared from cherry stones: I. Thermal effects

In this study, the feasible use of ozone to form oxygen complexes in chars prepared from cherry stones (CS) is investigated. CS were charred at 450, 600 or 900°C for 2 h in nitrogen. Char samples were ozonated over the 25-250°C temperature range for 1 h. Elemental chemical analysis was effected for a few selected samples. The oxygen complexes were successfully analyzed by Fourier infrared spectroscopy (FT-IR) and by titration methods. Thermal decomposition of ozone in the gas stream was also studied and the mechanism of the ozonation process dealt with. The ozonation treatment of CS chars was found to yield products with a relatively high concentration of a number of oxygen complexes. These include phenolic hydroxyl, quinonic, carboxylic acid, and ether structures. The content of lactonic structures was very low in the ozonated samples. The type and quantity of oxygen complexes depended on the ozonation and charring temperatures. The formation of oxygen complexes was favored when the charring of CS was effected at 450°C and when the ozonation of the char prepared at 600°C was performed at 100°C. The ozone content in the gas stream was very sensitive to the temperature increase in the reactor. Several reaction routes have been proposed for the transformation of ether, aromatic, and olefinic structures present in CS chars into oxygen complexes.     

Kinetics of the oxidation of carbon black by NO2: Influence of the presence of water and oxygen

In a fixed bed reactor, the rate of carbon black oxidation by NO₂ is significant for temperatures above 300°C, leading to NO, CO and CO₂ formation. The presence of O₂ in the feed gas increases the rate of oxidation, as well as the presence of water. A cumulative effect is observed when both water and oxygen are present. An oxygen balance shows that oxygen atoms of water molecules are not consumed. Water acts as a catalyst for the C-NO₂ reaction. A kinetic mechanism in which intermediate nitro-oxygenated species are formed in the presence of NO₂ during an initial step is in agreement with all these observations. Oxygen and NO₂ are able to react with these species at 300°C. A parametric study of the effects of the temperature, NO₂, O₂ and H₂O concentrations was performed. With a one-dimensional model of NO₂ consumption along the thickness of the carbon black bed, kinetic constants were derived and a phenomenological law was proposed, accounting for the effect of the presence of oxygen and water.     

Catalytic filamentous carbon: Structural and textural properties

Catalytic filamentous carbon (CFC) synthesized by the decomposition of methane over iron subgroup metal catalysts (Ni, Co, Fe or their alloys) is a new family of mesoporous carbon materials possessing the unique structural and textural properties. Microstructural properties of CFC (arrangement of the graphite planes in filaments) are shown to depend on the nature of catalyst for methane decomposition. These properties widely vary for different catalysts: the angle between graphite planes and the filament axis can be 0° (Fe-Co-Al₂O₃), 15° (Co-Al₂O₃), 45° (Ni-Al₂O₃), 90° (Ni-Cu-Al₂O₃). The textural properties of CFC depend both on the catalyst nature and the conditions of methane decomposition (T, °C). The micropore volume in CFC is very low, 0.001-0.022 cm³ g⁻¹ at the total pore volume of 0.26-0.59 cm³ g⁻¹. Nevertheless, the BET surface area may reach 318 m² g⁻¹. Results of the TEM (HRTEM), XRD, Raman spectroscopic, SEM and adsorption studies of the structural and textural properties of CFC are discussed.     

Raman spectroscopy study of high-modulus carbon fibres: effect of plasma-treatment on the interfacial properties of single-fibre-epoxy composites: Part II: Characterisation of the fibre-matrix interface

High-modulus carbon fibres from different precursors were submitted to an oxygen plasma-treatment under similar conditions. Single-fibre epoxy composites were prepared from them, and fragmentation tests were performed in order to characterise fibre-matrix interfacial adhesion. Raman spectroscopy has been used in the present work to map the strain along the fibre during tensile loading of the matrix. The strain distributions obtained agreed well with the prediction of analytical models used conventionally to describe load transfer at interfaces. Interfacial shear stress distributions were then obtained from these distributions according to the conventional force-balance concept. The interfacial shear strength (IFSS) and frictional shear stress (τ_f) values were calculated to quantify the degree of fibre-matrix adhesion. It was found that both parameters increased dramatically after the surface treatment, confirming the ability of cold plasma oxidation to improve the adhesion of carbon fibre to epoxy matrices. A dependence of the IFSS on the degree of surface order, as given by the structural order parameter I_D/(I_D+I_G), calculated from the relative intensities of the D and G bands of Raman spectra, was found. This supports the role played by the graphitic structure in fibre-matrix adhesion.     

Estimation of activated carbon adsorption efficiency for organic vapours: I. A strategy for selecting test compounds

Strategies for developing quantitative structure-affinity relationships (QSAfR) for the prediction of break-through performance of 31 chlorinated hydrocarbons on activated carbon have been studied. Two different approaches for the selection of a limited set of compounds for modelling were evaluated through the predictive power of the resulting QSAfR models. When the model was based on a training-set selected without a rational strategy, the developed QSAfR model showed poor predictive performance. Accordingly, such models have a limited capability to produce information concerning the important adsorbate related parameters influencing adsorption. By using a strategy where multivariate data analytical techniques are used in conjunction with statistical experimental design to select a balanced set of compounds for break-through performance evaluation, it was possible to develop QSAfR models with high predictive capability.     

Formation of oxygen structures by ozonation of carbonaceous materials prepared from cherry stones: II. Kinetic study

In this second part, the kinetics of the ozonation process of a char prepared from cherry stones (CS) is investigated. The char was obtained by heat treatment of CS at 600°C for 2 h in nitrogen. The effects of reaction time, partial pressure of ozone, and mass transport phenomena on the formation of oxygen complexes are studied. The surface chemistry of the samples was examined by FT-IR spectroscopy and the elemental chemical analysis was also determined for some samples. Results showed that the ozonation of the char led to oxygen chemisorption and to carbon gasification. The amount of oxygen complexes formed in the chemisorption stage (i.e., OH groups, CO structures, and ether structures) was found to be very sensitive to the increase in the ozonation time. The type of oxygen complexes was also time dependent. Ozonated products with relatively high concentrations of CO groups and ether structures were prepared by applying high ozone doses, whereas the formation of OH groups was favored at low ozone contents. The particle size did not influence the surface chemistry of the ozonated products. Only when the gas flow rate was lower than 40 l h⁻¹, restrictions to ozone mass transport developed. For kinetics of the char ozonation process, a mechanism based on the Langmuir-Hinselwood adsorption-desorption model was proposed, and the intrinsic reaction rates were calculated as a function of ozonation temperature. The activation energy for the ozonation stage of the char was equal to 41.6 kJ mol⁻¹.     

Rapoport-Samoilenko test for cathode carbon materials: I. Experimental results and constitutive modelling

The sodium expansion of semigraphitic commercial cathode material used for aluminium production has been measured on solid cylinder samples. Experimental results have been obtained for three current density values using the Rapoport-Samoilenko-type apparatus. A constitutive model for cathode carbon materials which is able to reproduce the relationship between the sodium expansion and time during the Rapoport-Samoilenko-type test has been proposed. Parameters required in the proposed model such as the sodium expansion after infinite time and the diffusion coefficient have been calculated from experimental data by a least-square minimization process. The distribution of sodium concentration, radial stress, tangential stress, axial stress, first principal stress and von Mises equivalent stress in the solid semigraphitic cylinder with time have been obtained.     

Lignin-based carbon fibers: Oxidative thermostabilization of kraft lignin

The thermostabilization of lignin fibers used as precursors for carbon fibers was studied at temperatures up to 340 °C at various heating rates in the presence of air. The glass transition temperature (T_g) of the thermally treated lignin varied inversely with hydrogen content and was found to be independent of heating rate or oxidation temperature. A continuous heating transformation (CHT) diagram was constructed from kinetic data and used to predict the optimum heating rate for thermostabilization; a heating rate of 0.06 °C/min or lower was required in order to maintain T_g > T during thermostabilization. Elemental and mass analyses show that carbon and hydrogen content decrease during air oxidation at constant heating rates. The hydrogen loss is sigmoidal, which is consistent with autocatalytic processes. A net increase in oxygen occurs up to 200-250 °C; at higher temperatures, oxygen is lost. Spectroscopic analyses revealed the oxidation of susceptible groups within the lignin macromolecule to ketones, phenols and possibly carboxylic acids in the early stage of the reaction; the later stage involving the loss of CO₂ and water and the formation of anhydrides and possibly esters. Slower heating rates favored oxygen gain and, consequently, higher glass transition temperatures (T_g) as opposed to faster heating rates.     

Catalytic oxidation of Fe(II) by activated carbon in the presence of oxygen.: Effect of the surface oxidation degree on the catalytic activity

The catalytic oxidation of Fe(II) species in aqueous solution by activated carbons with different degrees of surface oxidation is described. The parent activated carbon was oxidized with aqueous solutions of nitric acid or hydrogen peroxide, and submitted to thermal treatment at 373, 523 and 773 K. The activated carbons prepared were characterized by N₂ adsorption and temperature-programmed desorption, and their catalytic behavior was determined by measuring the oxidation rate of Fe(II) to Fe(III) and the generation of hydrogen peroxide. Catalytic activity is a function of the nature of oxygen surface groups generated by oxidation.     

Graphite flake carbon composites with a ‘sinterable’ microbead matrix: I. Mechanical properties

In this paper we report a study of the effect of graphite flakes of different size and volume fraction on the mechanical properties of a fine-grained carbon produced by the ‘sinterable’ route. Mesophase microbeads have been used as a matrix and the volume percent and size of the graphite flakes have been varied. It is shown that the flakes significantly increase the work of fracture of the composite, the effect being dependent on both flake size and volume fraction. However, there is a corresponding decrease in the Young Modulus and flexural strength of the composites. The flakes are not bonded to the structure and effectively act as inherent ‘crack-like’ pores. Flakes aligned perpendicular to the surface form the flaws that control the fracture stress. However, they also contribute significantly to the bridging stresses in the wake of the crack, so enhancing the work of fracture. The results should be useful in understanding the role of graphite-flake inclusions in modifying the properties of carbon materials.     

Fluorination of carbon blacks: an X-ray photoelectron spectroscopy study: IV. Reactivity of different carbon blacks in CF4 radiofrequency plasma

The effect of CF₄-plasma enhanced fluorination on the surface modification of carbon blacks has been examined using XPS. Three different types of carbon blacks have been studied: a thermal black, a furnace black and a high electrical conducting black. The analysis of the XPS spectra of fluorinated carbon black samples indicates that all fluorine atoms, fixed at the surface and in a subsuperficial zone of the particles, are covalently linked to carbon atoms. The influence of the physicochemical properties and morphology of these three types of carbon blacks on the fluorination reaction has also been investigated. The proportion of different types of fluorinated carbon atoms, i.e. on one hand CF_x surface and border groups of graphitic bulk domains for which the planar configuration of the graphene layers is preserved together with the sp² character of C, i.e. structures of type I, on the other hand polyalicyclic perfluorinated structures in which sp³C form puckered layers similar to those of covalent fluorographites, i.e. structures of type II, and also the F/C ratio of the fluorinated groups are related to the surface morphology and depend on the microstructural organization of particles. When the microstructure ordering and graphitic character of the carbon increase, the size of the ordered graphitic domain also increases. At the same time the density, the size of defects and proportion of protonated sp³C entities bridging the graphene layers decrease. As a consequence, the proportion of carbon atoms, potentially able to form perfluorinated CF₂ and CF₃ groups, decreases. The relative contribution of those groups is appreciably higher in fluorinated compounds which are derived from carbon blacks with a lower structural order.     

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.     

Adsorption of cadmium(II) from aqueous solution by surface oxidized carbon nanotubes

Carbon nanotubes (CNTs) were oxidized with H₂O₂, KMnO₄ and HNO₃. Their physicochemical properties were investigated by BET N₂ adsorption, laser particle examination, Boehm’s titration, zeta potential measurement and cadmium(II) adsorption. The experimental results suggest that cadmium(II) adsorption capacities for three kinds of oxidized CNTs increase due to the functional groups introduced by oxidation compared with the as-grown CNTs. The cadmium(II) adsorption capacity of the as-grown CNTs is only 1.1 mg g⁻¹, while it reaches 2.6, 5.1 and 11.0 mg g⁻¹ for the H₂O₂, HNO₃ and KMnO₄ oxidized CNTs, respectively, at the cadmium(II) equilibrium concentration of 4 mg l⁻¹. Adsorption of cadmium(II) by CNTs was strongly pH-dependent and the increase of adsorption capacities for HNO₃ and KMnO₄ oxidized CNTs is more obvious than that of the as-grown and H₂O₂ oxidized CNTs at lower pH regions. The experiments of CNT dosage effect on the cadmium(II) adsorption show that the adsorption capacity for KMnO₄ oxidized CNTs has a sharper increase at the CNT dosage from 0.03 to 0.08 g per 100 ml than the as-grown, H₂O₂ and HNO₃ oxidized CNTs and its removal efficiency almost reaches 100% at CNT dosage of 0.08 g per 100 ml. Analysis revealed that the KMnO₄ oxidized CNTs hosted manganese residuals, and these surely contributed to cadmium sorption to a yet-undefined extent.     

Ab initio molecular orbital study of adsorption of atomic hydrogen on graphite:: Insight into hydrogen storage in carbon nanotubes

Ab initio molecular orbital (MO) calculations are performed to study the adsorption of H atoms on three faces of graphite: (0001) basal plane, (1010) zigzag edge and (1121) armchair edge. The relative energies of adsorption (or C-H bond energies) follow the order: zigzag edge>armchair edge>basal-plane edge, in agreement with previous semi-empirical MO results. However, it is found that adsorption on the basal plane sites is exothermic and stable, in contrast to previous semi-empirical results. On the edge sites, the C-H bond energy decreases by nearly 30 kcal/mol when two H atoms are adsorbed on the same site. On the basal plane, the C-H bond energy decreases from 46 kcal/mol when two H are adsorbed on alternating sites to 27 kcal/mol when they are adsorbed on two adjacent sites. Literature MO results of H adsorption on the exterior wall of SWNT are in fair agreement with that on the basal plane of graphite. The value 27 kcal/mol agrees well with experiment (23 kcal/mol) of TPD of hydrogen from MWNT. Three common features exist in the reported experiments on hydrogen storage in carbon nanotubes: slow uptake, irreversibly adsorbed species, and the presence of reduced transition metals (Fe, Co or Ni). Combined with the MO results, a mechanism that involves H₂ dissociation (on metal catalyst) followed by H spillover and adsorption (on nanotubes) is proposed for hydrogen storage in carbon nanotubes.     

Overcoming calcium catalysis during the thermal reactivation of granular activated carbon: Part II. Variation of steam-curing reactivation parameters

In order to overcome the deleterious effects of calcium catalysis during thermal reactivation, the authors have developed a methodology for first steam-curing spent GAC at 548-748 K, and then ramping the furnace temperature to 1023-1223 K while exposing the GAC to flowing N₂. In this article, the authors evaluated the influence of an array of parameters on the steam-curing plus ramped-temperature protocol that included curing time, curing temperature, ramped-to temperature and steam flow rate. Pore size distribution (PSD) measurements employed the density functional theory (DFT), and these revealed that the steam-curing time had the greatest influence on pore size distribution: increasing the steam-curing time from 15 to 60 min increased the <500 Å cumulative pore volume by ca. 10% and the 5.4 to 32 Å pore volume by ca. 12%. Several of the other process parameters exhibited only a slight effect on PSD. Furthermore, a spent GAC that first experienced the steam-curing and ramped-temperature protocol and then experienced acid washing had identical micropore volume as a spent GAC that first experienced acid washing and then experienced conventional reactivation. This confirmed that the steam-curing protocol overcame calcium catalysis and its destruction of microporosity.     

Fluorinated carbon blacks: influence of the morphology of the starting material on the fluorination mechanism

The effect of fluorination, using CF₄ r.f. plasmas, has been studied on three different types of carbon blacks: a thermal black, a furnace black and a high electrical conducting black. The influences of the morphology and structure of the three blacks on the fluorination mechanism have been investigated. In particular, the ratio Type I/Type II structures (i.e., surface (CF) and border groups of graphitic domains with sp²C/polyalicyclic perfluorinated structures with sp³C), has been correlated to the microstructural organisation. The transformation into Type II structures is more easily achieved in highly accessible XE2 blacks, whereas in materials with lower crystallinity (MT), the presence of numerous defects leads preferentially to surface (CF_x) perfluorinated groups.     

Simultaneous adsorption of MIB and NOM onto activated carbon: II. Competitive effects

The adsorption of an odour compound common in drinking water, 2-methylisoborneol (MIB), was studied on six activated carbons in the presence of six well-characterised natural organic matter (NOM) solutions. It was found that, although the carbons and the NOM solutions had a wide range of characteristics, the major competitive mechanism was the same in all cases. The low-molecular-weight NOM compounds were the most competitive, participating in direct competition with MIB for adsorption sites. Equivalent background compound calculations indicated a relatively low concentration of directly competing compounds in the NOM. Some evidence of pore blockage and/or restriction was also seen, with microporous carbons being the most affected by low-molecular-weight NOM and mesoporous carbons impacted by the higher-molecular-weight compounds.