Effect of air oxidation of Rayon-based activated carbon fibers on the adsorption behavior for formaldehyde

The tailoring of pore surface chemistry of activated carbon fibers is shown to be an effective method for improving the adsorption efficiency of various volatile chemical compounds (VOCs). An oxidation treatment with air resulted in a significant increase in the adsorption capacities and breakthrough time for Rayon-based activated carbon fibers (ACFs) in removal of formaldehyde. The porous structure parameters of Rayon-based ACFs were determined with standard nitrogen adsorption analysis. The pore surface chemistry of samples under study was analyzed by Fourier Transform Infrared spectra (FTIR). Thus to some extent, the relationship between the adsorption properties, porous structure and pore surface chemistry was revealed.     

The tensile behavior of carbon fibers at high temperatures up to 2400 °C

The tensile behavior of four different brands of carbon fibers (a rayon-based, a PAN-based, and 2 pitch-based fibers) has been investigated at various temperatures up to 2400 °C. The tests were carried out using an original fiber testing apparatus. Various mechanical properties including strength and Young's modulus, as well as Weibull statistical parameters were extracted from test data. Typical tensile behaviors were evidenced such as an essentially linear elastic behavior at room temperature and intermediate temperatures up to 1400-1800 °C, then a nonlinear elastic delayed response at higher temperatures and ultimately an inelastic response with permanent deformations at very high temperatures. Such unusual nonlinear responses for homogeneous materials were related to structure and texture features at the nanometer scale, that were described through an X-ray diffraction technique.     

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

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

Analysis of the microporosity shrinkage upon thermal post-treatment of H3PO4 activated carbons

Starting from a commercial pelletized phosphoric acid based activated carbon, with a typical opened and developed micro and mesoporosity, a post-heat-treatment in KOH, at different KOH/activated carbon ratios, has been studied. In all the cases, a pore size shrinkage has been observed. To find an explanation for the reason of this micropore size distribution shrinkage different factors have been studied, among them: (a) effect of the presence of impurities coming from the activation process with phosphoric acid; (b) effect of the KOH post-treatment temperature; (c) heat-treatment temperature of the precursor (without chemical agent); (d) effect of the reagent nature (NaOH, NaCl and KCl vs. KOH). The variable that produces the most intense shrinkage effect, and the disappearance of the mesoporosity, is the heat-treatment in presence of hydroxide, which affects even using a low hydroxide/activated carbon ratio. Such a low hydroxide/activated carbon ratio does not produce activation, nor porosity development of the starting activated carbon during the treatment. This shrinkage phenomenon, which seems to be independent of the method of preparation used to prepare the activated carbon, can be understood considering our previous studies about the reactions involved during chemical activation by hydroxides.     

Effect of oxygen functional groups on synthetic carbons on liquid phase oxidation of cyclohexanone

Synthetic carbons from phenolic resins were used as catalysts for the aqueous phase oxidation of cyclohexanone to C₄-C₆ dicarboxylic acids (adipic, glutaric and succinic acids) at 413 K under 50 bar total air pressure. The changes in microporous structure and surface chemistry, produced as a consequence of activation or heat treatment processes, were analyzed. Using CO₂ or air as activating agent increased significantly the surface area and the total pore volume responsible for the activity. The surface chemistry of the samples was also modified and was characterized by titration with bases of different strength and with HCl, by temperature programmed desorption, and by X-ray photoelectron spectroscopy. To determine the role of surface oxygen functionalities on the catalytic behavior of the carbons, heat treatments in nitrogen at different temperatures were used to selectively eliminate oxygenated groups. Thus, treatment at temperatures of 1173 K eliminating the carbonyl/quinone groups decreased the selectivity to adipic acid and dicarboxylic acids. Introducing quinone groups during the synthesis of the carbons also improved the selectivity to adipic acid, proving that the mechanism of oxidation involves the quinone type groups on the carbon surface.     

Oxidative dehydrogenation of ethylbenzene on activated carbon fibers

Activated carbon fibers from different precursors and with different degrees of activation were used as catalysts for the oxidative dehydrogenation of ethylbenzene. Within each group, the fibers exhibited similar surface chemistries, so that the observed catalytic performances could be interpreted exclusively in terms of their textural properties. Analysis of the catalytic results highlighted common trends. In particular, the fibers with an average micropore width larger than 1.2 nm were found to be the best catalysts for this reaction.     

Novel synthesis and characterization of activated carbon fiber and dye adsorption modeling

The main objective of this work is to prepare activated carbon fibers (ACF), analyze a synthesis mechanism of those fibers, and develop a new dye adsorption model. The surface chemical structures of the synthesized viscose rayon phosphates and ACF were analyzed using TOF-SIMS and ATR FT-IR. After steam-activation of carbon fiber at high temperature, the carboxyl group could not be observed due to the high temperature activation. Only the oxygen-contained carbon ring groups appeared. The adsorption mechanism of the developed model in this study, the bottle-neck model, was simple to understand and apply to the industrial adsorption plants. The model could predict theoretical concentration versus time or dye concentration in an ultra accurate manner in the medium and low concentration regions, which could not previously be attempted by other models.     

Oxidative stabilization of PAN/SWNT composite fiber

PAN/SWNT composite fibers have been spun with 0, 5, and 10 wt% single wall carbon nanotubes (SWNTs). Tensile fracture surfaces of polyacrylonitrile (PAN) fibers exhibited extensive fibrillation, while for PAN/SWNT composite fibers, tendency to fibrillate decreased with increasing SWNT content. The reinforcing effect of SWNTs on the oxidized polyacrylonitrile (PAN) fiber has been studied. At 10 wt% SWNTs, breaking strength, modulus, and strain to failure of the oxidized composite fiber increased by 100%, 160%, and 115%, respectively. Tensile fracture surfaces of thermally stabilized PAN and the PAN/SWNT fibers exhibited brittle behavior and well distributed SWNT ropes covered with the oxidized matrix can be observed in the tensile fracture surfaces of the fibers. No de-bonding has been observed between unoxidized or the oxidized PAN matrix and the nanotube ropes. Higher strain to failure of the oxidized composite fiber as compared to that of the oxidized control PAN fiber also suggests good adhesion/interaction between SWNT and the oxidized matrix. Thermal stresses generated on the composite fiber during the oxidation process were lower than those for the control fiber. The potential of PAN/SWNT composite fiber as the precursor material for the carbon fiber has been discussed.     

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.     

Structural modification of coal-tar pitch fractions during mild oxidation—relevance to carbonization behavior

Modified pitches with softening points of about 175 °C were prepared by air-blowing at 300 °C of coal-tar pitches from different commercial coke-oven tars. The modifications induced by the mild oxidation were monitored using solvent and extrographic fractionation, elemental analysis, ¹H NMR and HPLC. Optical microscopy was used to follow the effect of air-blowing on carbonization behaviour. Low molecular weight cata-condensed PAHs and those with basic nitrogen and hydroxylic functionalities present in extrographic fractions F2 and F4, respectively, are preferentially polymerized pitch constituents. In contrast, peri-condensed PAHs in extrographic fractions F2 and F3, are practically unreactive under the oxidation conditions used. The mild oxidation enhances the tendency of quinoline insoluble (QI) particles to form aggregates in an early stage of thermal treatment, modifying the mode of mesophase development and leading to a non-homogeneous optical texture. The enhanced propensity of QI to aggregation is discussed in terms of structural peculiarities of the parent pitch and possible oxidative polymerization reactions.     

The influence of oxidation on the texture and the number of oxygen-containing surface groups of carbon nanofibers

The effect of liquid-phase oxidation on the texture and surface properties of carbon nanofibers has been studied using XRD, TEM, SEM, N₂-physisorption, TGA-MS, XPS and acid-base titrations. Oxidation was performed by refluxing the nanofibers in HNO₃ and mixtures of HNO₃/H₂SO₄ for different times. The graphite-like structure of the treated fibers remained intact, however, the specific surface area and the pore volume increased with the severity of oxidation treatment. For the first time it is shown that the most predominant effect that gives rise to these textural modifications is the opening of the inner tubes of the fibers. Moreover, it is demonstrated that both the total oxygen content (O/C=0.02-0.07 at/at) as well as the number of acidic groups (1-3 nm⁻²) are a function of the type of oxidizing agent used and the treatment time. The total oxygen content of the oxidized samples turns out to be substantially higher than can be accommodated in the form of oxygen-containing groups at the exterior surface.     

Oxidation behavior of fine-grained SiC-B4C/C composites up to 1400 °C

Fine-grained B₄C-SiC/C composites were fabricated using a ball-milling dispersion process. The oxidation behaviors of both fine-grained B₄C-SiC/C composites and coarse-grained B₄C-SiC/C composites at temperatures of up to 1400 °C were analyzed by the differential thermal analysis technique, and the surface morphology of the composites after isothermal oxidation at 800, 1200 and 1400 °C was examined by scanning electron microscopy (SEM). The results indicated that fine-grained B₄C-SiC/C composites had excellent oxidation resistance with self-healing properties at 1400 °C. A general model and mechanism for self-protection against oxidation of carbon materials were proposed.     

Effect of pre-carbonization of petroleum cokes on chemical activation process with KOH

The influence of pre-carbonization of petroleum cokes on the properties of the activated carbon precursors and final carbons activated with KOH was investigated by using TG-DTG, FTIR, XRD, and BET techniques. TG-DTG study revealed the decomposition of volatile species and FTIR analysis identified the presence of C-O, C-O-C, C-O-H and some alkyl species on the surface of the precursors. XRD results indicated that a disorder occurred after the carbonization at 500 °C and an order happened gradually with increasing the temperature. The decreases in the BET surface area and N₂ adsorption capacity coincided with the reduction of the functional species on the precursor surface with the increase of the pre-carbonization temperature. These surface functional species were proposed to play a key role in the porosity development during the KOH activation. First, they reacted with KOH to form the intermediate C-O-K species, then the resultants further reacted with the carbon of the precursor, and finally developed the porous structure. As a result, the surface area and the pore size distribution of the activated carbon can be controlled by optimizing the conditions of the pre-carbonization and activation processes.     

Numerical modeling of the carbonization process in the manufacture of carbon/carbon composites

A method for the numerical simulation of the carbonization process is introduced. A general model for the transient analyses of heat and mass transfer together with stress and displacement predictions is constructed using two-dimensional FEM (finite element method) for arbitrary geometry. The established model is applied to the carbonization of a single-phase, homogeneous, isotropic phenolic foam, and an anisotropic, two-phase composite material. A damage model is introduced to account for the development of shrinkage cracks, and a CDM (continuum damage mechanics) model is implemented for the calculation of mechanical property degradation due to crack evolution. The established model is verified by comparison with experimental results, and is applied to various numerical examples.     

Co-adsorption of n-butane/water vapour mixtures on activated carbon fibre-based monoliths

Water-n-butane co-adsorption experiments on Activated Carbon Fibre based Monoliths (ACFMs) under equilibrium and dynamic conditions were performed at 30 °C. ACFMs proved to be better adsorbents than active carbon particles for the dry adsorption of n-butane at low concentration levels. The presence of water vapour in the gas stream has a negligible influence on n-butane adsorption for values of relative humidity (RH) equal to or below 25%. Higher water pressures cause a significant loss of adsorption capacity in ACFMs. Equilibrium water adsorption on ACFMs can be described by the Do and Do model [Carbon 38 (2000) 767] modified in order to account for both the variable size of the water aggregates on acid centres and the variable size of the clusters detached from the aggregates and inserted into the micropores. The model results show close agreement between the amounts of active sites for adsorption calculated by the model and the number expected from subjecting the ACFMs to different pre-treatments.The equilibrium of co-adsorption of water and n-butane can be described by a pore filling concept according to which the volume occupied by both gases adsorbed together from the gas mixture is approximately equal to the volume occupied by the pure component adsorbed separately to a greater extent.Pre-moistening of the monolith has little effect on its effective n-butane adsorption capacity (breakthrough time) during dynamic adsorption experiments. For values of relative humidity equal to or below 25%, the effective n-butane adsorption capacity of the monoliths was better than that of active carbon powder. Moreover, the monoliths produced a much lower pressure drop in the system. At these conditions, adsorption efficiency for the ACFMs never showed values below 85%.     

Stabilization and carbonization behavior of pitch-model compounds treated with iodine or bromine

Effects of iodine or bromine treatments on the stabilization and carbonization of pitches were investigated by using five kinds of pitch-model compounds with different molecular structures. Iodine treatment significantly modified the thermal properties of compounds with condensed aromatic-rings, which resulted in a high carbon yield. In iodine treatment remarkable reaction was more easily achieved for compounds in the liquid state than in the solid one. In contrast, bromine treatment had a different polymerization reaction from that for iodine, which was effective in increasing carbon yield on even the compounds without condensed rings. This result supports the hypothesis that bromine treatment is independent of the chemical structure of parent pitches.