The adsorption properties of surface modified activated carbon fibers for hydrogen storages

In this study, activated carbon fibers (ACFs) with high surface area and pore volume have been modified by Ni doping and fluorination. The surface modified ACFs were characterized by BET surface area, SEM/EDS, XRD, and Raman spectroscopy. The changes in pore structure and surface properties of these modified ACFs were correlated with hydrogen storage capabilities. After fluorination treatment, although the micropore volume of ACF was decreased, amounts of hydrogen storage were found to increase. Additionally, micropore volume on ACFs was found to be unchanged with Ni doping, hydrogen storage capacities were considerably increased due to the effect of catalytic activation of nickel. Though fluorination of ACFs increases hydrogen affinity, the effect of catalytic activation of nickel is more prominent, and thus led to better hydrogen storage. Hence, it was concluded that hydrogen storage capacity was related to micropore volumes, Pore size distribution (PSD) and surface properties of ACFs as well as specific surface areas.     

微波诱导活性炭纤维催化还原NO

采用微波辐照活性炭纤维(ACFs)催化还原一氧化氮(NO)进行脱氮,研究了气体流量、微波辐照功率对脱氮效率的影响,探讨了微波诱导催化反应的机理。结果表明:ACFs在微波场中升温迅速并最终保持最高温度;微波辐照功率对NO脱除率影响较大;在ACFs质量0.25g,辐照功率100W,NO流量50mL/ min条件下,99.87%的NO可被还原为氮气。     

A study on the effect of heat treatment on functional groups of pitch based activated carbon fiber using FTIR

Pitch based active carbon fibers (ACFs) were analyzed by a FTIR micro-ATR technique by introducing a very thin KBr layer on their surfaces. The ACFs were thermally treated at 600, 1100, and 1200 °C respectively, to investigate the change of their surface functionalities. As the heat treatment temperature increased, the amount of oxygen containing surface functional groups were reduced and the ACFs became more hydrophobic. When the ACFs were thermally treated, the decrease of carboxylic acid groups occurred first and ketone or quinone groups subsequently disappeared at higher heat treatment temperatures. The degree of graphitization of the ACF was increased significantly when it was treated at 1100 and 1200 °C and this was partially attributed to the release of the CO groups in the conjugated ketone or quinone structures.     

活性碳纤维对甲醛吸附转化性能的研究

用对胺基苯甲酸(PABA)浸渍粘胶基活性碳纤维(Rayon-ACF)和聚丙烯腈基活性碳纤维(PAN-ACF),考察了两者对甲醛吸附性能的差异。结果表明:PAN-ACF和Rayon-ACF经PABA溶液浸渍处理后,样品中的N含量都明显提高,但其比表面积、微孔容积和总孔容都有所下降;不过对甲醛溶液蒸汽的吸附容量都明显提高。这主要是归因于浸渍处理后样品中-NH2的增加,进而由于化学吸附而使得对甲醛吸附量增加。Rayon-ACF样品的甲醛吸附容量远高于PAN-ACF样品,这可能同Ray-on-ACF样品有较多表面含氧官能团以及较大的微孔容积有关。甲醛在ACF的微孔空间中易于形成高凝缩的三聚甲醛,从而其吸附容量相应提高。     

Improvement of the reduction capacity of activated carbon fiber

The reduction property of activated carbon fibers (ACFs) enables them to be used in the recovery of noble metals from wastewater, or in the extraction of gold or silver from ore leaching solutions. In order to effectively recover or extract noble metals, it is important to enhance the reduction capacity of ACFs and to improve the particle size of the noble metal reduced and adsorbed on the surface of activated carbon fibers. In this paper, the effect of the preparation method and surface modification of ACFs on their reduction capacity was studied. The results show that the preparation methods of ACFs have significant influence on their reduction-adsorption capacities for silver ions in solution—those ACFs prepared with phosphoric acid or zinc chloride activation have much higher reduction capacities. Moreover, surface modification of ACFs with some inorganic oxidants such as nitric acid, potassium permanganate, or hydrogen peroxide, though resulting in a small decrease of specific surface area or pore volume, will enhance the reduction capacity of oxidized ACFs for silver ions. Furthermore, methylene blue, aniline, or p-nitrophenol present in solution or adsorbed on ACFs can also significantly increase the reduction capacities of sisal-based ACFs for silver ions.     

Characterization of pore distribution in activated carbon fibers by microbeam small angle X-ray scattering

In the present work, the results corresponding to the first experiments done with single activated carbon fibers (ACFs) at the microfocus beamline (ID13) in the ‘European Synchrotron Radiation Facility’ (Grenoble) are presented. The experiments done with CO₂ and steam ACFs have demonstrated the suitability of this technique to characterize a single ACF. The experiments show that scattering intensity increases with the burn-off degree, which agrees with SAXS experiments carried out using bigger amounts of fibers. Moreover, the two-dimensional scattering patterns show that, in this type of ACFs, the porosity development during the activation process is isotropic. In addition, it has been demonstrated that the use of an X-ray microbeam of 2 μm diameter allows the characterization of different regions of the same fiber with microscopic position resolution. The scans across the fiber diameter are the first direct proof for the previous results obtained by our research group. Thus, in the case of CO₂ ACFs, the scattering is high in different regions across the fiber diameter, confirming that CO₂ activation takes place within the fibers, generating a quite homogeneous development of porosity. On the other hand, in the case of steam ACFs, the scattering is much higher in the external zones of the fibers than in the bulk, which means that steam focuses the activation in the outer parts of the fibers.     

Physical properties and biological effects of activated carbon fibers treated with the herbs

Activated carbon fibers (ACFs) were treated with various medicinal materials. The physical properties and biological effects of these ACFs were examined. In order to investigate the properties, adsorption isotherms, BET surface area and pore analysis for the herb material treated ACFs must be obtained. The pore size distribution of the samples was obtained by a regularization method on the basis of the non-local and smoothed density function theory approximation support these findings. To investigate the surface of treated ACFs, the surface morphology of the resulting samples by S.E.M. show that many of the treated herbs were distributed irregularly on the surface of the fiber. In results of biological effects in the treated herbs, most all of samples showed excellent antibacterial effects after 200-400 min.     

Defluorination-enhanced hydrogen adsorptivity of activated carbon fibers

Fluorinated activated carbon fibers (F-ACFs) were prepared by direct thermal fluorination of pristine activated carbon fibers. By the pyrolysis of F-ACFs at 1073 K under nitrogen gas flow, fluorine was subsequently eliminated and the sp²-bonded ACF structures were recovered. The micropore widths were 1.1 and 0.8 nm, and the isosteric heats of adsorption of nitrogen were 11.3 and 12.8 kJ/mol for pristine and defluorinated ACFs, respectively. These results strongly suggest that changes occurred in the structural properties of micropores in defluorinated ACFs. The hydrogen adsorption isotherms showed that the defluorinated ACFs adsorbed more hydrogen gas than pristine ACFs at 77 K, suggesting that the potential for interaction between hydrogen molecules and the defluorinated slit nanospaces was increased due to the changes in the pore structural properties and/or to the induced polarization of the pore walls making up the modified π-electron systems.     

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     

Influence of pore structure and surface chemistry on electric double layer capacitance in non-aqueous electrolyte

The performance as electric double layer capacitors (EDLC) in non-aqueous electrolyte of a series of alkaline agent-activated carbons with high surface area is presented in this work. The results obtained show that, in general, capacitance increases with surface area. However, the results obtained in this study confirm that capacitance not only depends on surface area, but also on two other parameters: pore size distribution and surface chemistry. It has been shown that capacitance is higher for a sample with wider micropore size distribution than for a sample with higher surface area but too narrow micropore size distribution. In addition, it has been observed that the sample with a very high amount of surface groups presents very high capacitance values. In the present study, a KOH-activated carbon with a capacitance as high as 220 F/g was prepared. Finally, the results obtained with a mesoporous sample have shown that the presence of mesopores in activated carbons with very high surface area (e.g. >2000 m²/g), do not seem to be effective for double layer capacitors.     

A novel method for production of activated carbon from waste tea by chemical activation with microwave energy

This study presents the production of activated carbon from waste tea. Activated carbons were prepared by phosphoric acid activation with and without microwave treatment and carbonisation of the waste tea under nitrogen atmosphere at various temperatures and different phosphoric acid/precursor impregnation ratios. The surface properties of the activated carbons were investigated by elemental analysis, BET surface area, SEM, FTIR. Prior to heat treatment conducted in a furnace, the mixture of the waste tea and H₃PO₄ was treated with microwave heating. The maximum BET surface area was 1157 m²/g for the sample treated with microwave energy and then carbonised at 350 °C. In case of application of conventional method, the BET surface area of the resultant material was 928.8 m²/g using the same precursor and conditions. According to the Dubinin–Radushkevich (DR) method the micropore surface area for the sample treated with microwave energy was higher than the sample obtained from the conventional method. Results show that microwave heating reasonably influenced the micropore surface area of the samples as well as the BET surface area.The samples activated were also characterised in terms of the cumulative pore and micropore volumes according to the BJH, DR and t-methods, respectively.     

Activated carbon fibers as redox mediators for the increased reduction of nitroaromatics

Activated carbon fibers (ACFs) were used as redox mediators in the anaerobic chemical reduction of two model nitroaromatic compounds: 4-nitrophenol and 3-chloronitrobenzene. The effect of ACF chemical properties on the reduction of nitroaromatic compounds was measured by chemical oxidation of ACFs with 8 M nitric acid and thermal treatment at 700 °C under an inert atmosphere of hydrogen:helium (1:99%), before their use in batch experiments. The results show that ACFs are necessary to transform the nitroaromatic compounds to their corresponding aromatic amines, indicating the function of ACFs as redox mediators. Furthermore, the oxidation and thermal modification of ACFs clearly changes the redox properties of these materials, with the former being the better option to increase the concentration of quinone groups (1.68 times), and therefore the redox mediating capacity of ACFs (up to 1.38 times).     

Characterization of microstructure and surface properties of heat-treated PAN-and rayon-based activated carbon fibers

Polyacrylonitrile- and rayon-based activated carbon fibers (ACFs), subject to heat treatment over 600–1,100°C under N₂ flow, were investigated using a number of surface analytical methods, including N₂ adsorption isotherm, elemental analysis, and X-ray photoelectron spectroscopy. The adsorption capacities of benzene, carbon tetrachloride, and water vapor on as-received and heat-treated ACFs were determined. Results show that the ACFs under study were highly microporous but heat-treated ACFs contained more mesopores in the range of 20–30 Å for PAN-based and 30–45 Å for rayon-based. It can be seen that the high-resolution α_s plot provided valuable information about structure properties. The pore size distributions of ACFs gave insight into the pore development with heat treatment temperature. Besides, phenolic groups were found to be the most abundant oxygen-containing functional groups on the surface of both ACFs. The vapor adsorptions on ACFs indicated that molecular size and polarity of vapors, as well as the microstructure and chemistry of ACFs profoundly influenced the adsorption performance.     

Preparation of fibrous porous materials by chemical activation: 1. ZnCl2 activation of polymer-coated fibers

Fibrous porous materials (FPMs) have been prepared by coating a glass fiber with a solution of polymer and ZnCl₂, followed by stabilization in air and heat treatment in N₂. The ZnCl₂ was then removed by washing with D.I. water and HCl. Four kinds of polymers, a phenolic resin, polyacrylonitrile, poly(vinyl alcohol) and cellulose, were used to prepare solutions with ZnCl₂. The results showed that ZnCl₂ acts as a dehydration agent to promote the thermal cross-linking of polymer at a much lower temperature, leading to FPMs having much higher char yields and very high surface areas. The porosity was created in part by dissolution of the ZnCl₂ left in the charred coating. The activation temperature and ZnCl₂ concentration play an important role in porosity development. In the early stage of heating, the specific surface area, micropore and mesopore volumes increased with increasing temperature. As the activation temperature increases above 450°C, ZnCl₂ begins to volatilize out of the coating, and further charring and aromatization of the coating results in a dimensional contraction leading to a decrease in the micropore and mesopore volumes. It was observed that the specific surface area, as well as micropore and mesopore volumes, increased with increasing ZnCl₂ concentration. Pore size analysis showed that the FPMs activated with ZnCl₂ were mainly microporous. For FPMs activated with concentrated ZnCl₂ (66 wt.%), there is a remarkable and large mesopore size distribution in addition to the typical micropore size distribution. In addition, such FPMs have very high specific surface area, more than 1600 for PAN-based and 2500 m²/g of coating for cellulose-based FPMs.     

Advances in the study of methane storage in porous carbonaceous materials

This paper presents an overview of the results of our research group in methane storage, in which the behaviour of different carbon materials in methane storage has been studied. These materials include physically activated carbon fibres (ACFs), chemically activated carbons (ACs) and activated carbon monoliths (ACMs), all of them prepared in our laboratories. These results have been compared with those corresponding to commercial ACFs, commercial activated carbon cloths and felts, and a commercial activated carbon.An in depth analysis (different raw materials, activating agent and preparation variables) has been done in order to obtain the carbon material with the best methane adsorption capacity by unit volume of adsorbent. The important effect of the micropore volume, micropore size distribution (MPSD) and packing density of the carbon materials in the methane adsorption capacity and delivery has been analysed. After this study, activated carbons with volumetric methane uptake as high as 166 v/v and delivery of 145 v/v have been prepared. In addition, ACM with methane uptake of 140 v/v and a delivery of 126 v/v has also been obtained.Moreover, the results corresponding to preliminary in situ small angle neutron scattering (SANS) study of CD₄ adsorption under pressure in different porous carbons and a zeolite are also included. These experiments have established SANS as a viable technique to investigate high-pressure methane adsorption. CD₄ adsorption at supercritical conditions produces changes in the SANS curves. The changes observed are in agreement with theoretical speculations that the density of the adsorbed phase depends upon the pore size.     

Electrochemically enhanced adsorption of aniline on activated carbon fibers

For adsorptive separation processes, the adsorption rate and capacity are two important factors affecting the costs. This study describes the anodic polarization of activated carbon fibers (ACFs), which can enhance the adsorption rate and capacity of aniline. The electrosorption kinetics and the affecting factors (bias potential, electrolyte, and pH) of isotherms for aniline on ACFs were investigated. The adsorption/electrosorption of aniline on ACFs follow pseudo-first-order adsorption kinetics, and the adsorption rate improves with increasing bias potential. The electrosorption isotherms, which exhibit a variety of responses depending on bias potential, electrolyte and pH, follow the two classical models of Langmuir and Freundlich. With electrosorption of aniline from aqueous solution, a two-fold enhancement of adsorption capacity is achievable. The initial and saturated ACFs were characterized using scanning electron micrograph (SEM) and Fourier transform infrared spectroscopy (FT-IR). The SEM micrographs show that the surface of ACFs is not oxidized, which is also verified by cyclic voltammetry results. The FT-IR spectroscopy suggests that the interaction between aniline and ACFs is main weak physisorption instead of chemisorption. These experimental results suggest that the electrochemical polarization of ACFs can effectively improve the adsorption rate and capacity of aniline, which may be due to the enhanced affinity between aniline and ACFs instead of the oxidation on the surface of ACFs or in the solution.     

Prediction of breakthrough curves for adsorption on activated carbon fibers in a fixed bed

A new model is proposed to describe the removal of volatile organic compounds (VOC) from a gas stream passing through a bed packed with activated carbon fibers (ACFs). Toluene was used as the test compound. Both pore diffusion and surface diffusion are considered in the model. The equilibrium behavior is shown to fit the Dubinin–Radushkevich isotherm with the values of parameters K and W₀ of 1.101 × 10⁻⁹ and 57.73 kg/m³, respectively. The experimental results show that this model can predict VOC breakthrough curve very well.     

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.     

Enhancement of electrosorption capacity of activated carbon fibers by grafting with carbon nanofibers

The composite films of activated carbon fibers (ACFs) and carbon nanofibers (CNFs) are prepared via chemical vapor deposition of CNFs onto ACFs in different times from 0.5 to 2 h and their electrosorption behaviors in NaCl solution are investigated. The morphology, structure, porous and electrochemical properties are characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, N₂ adsorption at 77 K, contact angle goniometer and electrochemical workstation, respectively. The results show that CNFs have been hierarchically grown on the surface of ACFs and the as grown ACF/CNF composite films have less defects, higher specific capacitances, more suitable mesoporous structure and more hydrophilic surface than the pristine ACFs, which is beneficial to their electrosorption performance. The ACFs/CNFs with CNFs deposited in 1 h exhibit an optimized NaCl removal ratio of 80%, 55% higher than that of ACFs and the NaCl electrosorption follows a Langmuir isotherm with a maximum electrosorption capacity of 17.19 mg/g.     

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

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