Activated Carbon Fibers ACF

2 USES OF VIRGIN CARBON FIBER 23.2.1 Activated carbon fibers (ACF) 

Rebouillat el al [4] and Suzuki [5] give good reviews of activated carbon fibers. Traditionally, activated carbon granules are made by the carbonization of a product such as coconut shells, which due to their physical granular form, tend to be difficult to handle and the development of an activated woven cloth by the British Chemical Defence Establishment at Porton Down [6,7] via the controlled heat treatment of a woven rayon cloth offers many advantages. The activated charcoal cloth (ACC) product was made under licence in 1977, by Charcoal Cloth Ltd. One such process used a 1.8 m wide fabric, reducing to about 1.0 m at the end of the process. To aid carbonization, the cloth was treated with a solution of chemicals to confer a measure of flame retardancy. As explained in Chapter 6, there are two forms of flame retardant—one where the flame retardant acts as a catalyst and promotes removal of the —OH groups and the other form, which actually reacts with the —OH 

After pre-treatment, the cloth is dried and carbonized up to about 850°C in an atmosphere of N2, using a heating rate of about 20°Cmin and at this stage, the fabric is extremely brittle and is unable to withstand applied tension or rubbing. At 850 1000°C, steam or CO2, is introduced to activate the fiber and sweep away the tars. The process of activation helps free the pores from occluded tars to give an apparent pore volume of about 0.5 cm g One problem associated with the use of chemicals is that they can leave a residue, which may be unacceptable for certain specific end uses. 

The oxidized fiber can then be treated with a chemical activation reagent such as ZnQ2, H3PO4 or HCl at 700-1000°C or, alternatively, gaseous activation can be undertaken in CO2, NH3 or steam in the presence of N2 from 700-1300°C. The product has a high N2 content (up to 15%) with a specific surface area of some 300-2000 m g and a fiber 

Toho introduced up to 0.3% P or B into the precursor [10] to aid processing. Various studies on the preparation of PAN based activated carbon fiber have been undertaken [11,12], including work with a hollow fiber [13]. The study of the activation stage has 

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Fig. 6. Breakthrough curves for aqueous acetone (10 mg 1" in feed) flowing through exnutshell granular active carbon, GAC, and PAN-based active carbon fibers, ACF, in a continuous flow reactor (see Fig. 5) at 10 ml min" and 293 K [64]. C/Cq is the outlet concentration relative to the feed concentration. Reprinted from Ind. Eng. Chem. Res., Volume 34, Lin, S. H. and Hsu, F. M., Liquid phase adsorption of organic compounds by granular activated carbon and activated carbon fibers, pp. 2110-2116, Copyright 1995, with permission from the American Chemical Society.

We have an excellent activated carbon of fiber morphology, so called activated carbon fiber ACF[3]. This ACF has considerably uniform slit-shaped micropores without mesopores, showing characteristic adsorption properties. The pore size distribution of ACF is very narrow compared with that of traditional granular activated carbon. Then, ACF has an aspect similar to the regular mesoporous silica in particular in carbon science. Consequently, we can understand more an unresolved problem such as adsorption of supercritical gas using ACF as an microporous adsorbent. 

In this chapter, we present in some detail gas adsorption techniques, by reviewing the adsorption theory and the analysis methods, and present examples of assessment of PSDs with different methods. Some examples will show the limitations of this technique. Moreover, we also focus on the use of SAXS technique for the characterization of porous solids, including examples of SAXS and microbeam small-angle x-ray scattering (pSAXS) applications to the characterization of activated carbon fibers (ACFs). We remark the importance of combining different techniques to get a complete characterization, especially when not accessible porosity exists. 

Interesting developments on activated carbon have been reported recently. They include chemical modification of the surfaces, activated carbon fibers (ACF), and CH4 and H2 storage. A brief discussion is given next. 

The high porosity that results from activation increases the area for adsorption. One gram of char can produce about 1000 m of adsorption area. After activation, the char is further processed into three types of finished product powdered form called powdered activated carbon (PAC), the granular form called granular activated carbon (GAC), and activated carbon fiber (ACF). PAC is normally less than 200 mesh GAC is normally greater than 0.1 mm in diameter. ACF is a fibrous form of activated carbon. Figure 8.7 shows a schematic of the transformation of raw carbon to activated carbon, indicating the increase in surface area. 

Now the research effort goes toward experimental verification of the elevation phenomena in the simplest geometry, a slit. Our main interest is in the range of a few to several nanomenters. Some experimental studies have already reported freezing point elevation in slit pores [8-10], but the materials used were activated carbon fibers (ACFs), which have only micropores less than 2nm. In such small pores the first layer adjacent to the attractive pore wall, which is known to form a frozen phase at a temperature well above the bulk freezing point, will occupy most of the pore spaces, and the freezing behavior in the interior of the pore space is difficult to be detected. Further, there may still remain some controversy if a liquid confined in a larger nanopore would exhibit elevation unless an experimental verification is made over such sizes. 

Recent research activities on nanoporous materials have stimulated fundamental studies on adsorption mechanism in micropores [1 5]. Both of the precise measurement of high resolution adsorption isotherms from the low P/Po region and molecular simulation showed the presence of monolayer adsorption on the micropore walls and further filling in the residual spaces after monolayer completion for supermicropores (0.7 nm < pore width w <2 nm) the contribution by the monolayer to the filling in the residual spaces is comparable to that by the pore walls [6-10]. Systematic researches on activated carbon fiber (ACF) having slit-shaped micropores[l 1,12] have contributed to elucidation of the mechanism of micropore filling to develop better adsorbents in adsorption and separation engineering. 

Usually the activated carbon granules are employed for chromium removal or for catalysts preparation. However in some previous reports the perspectives and advantages of activated carbon fibers (ACF) utilization for the same employment have been documented [2,4]. It seems from the analysis of the articles that the use of ACF or activated carbon cloth has a great potential. 

Electrosorption technique, which may use the electrical potential as the 3" driving force to the traditional adsorption and ion exchange mechanism, has reversible characteristics of purifying waste solution by adsorption and concentrating contaminants by desorption. Carbon materials satisfy the basic requirements for an efficient electrode material, and have good radiation and chemical-stability. Especially activated carbon fiber (ACF), which can be easily made into a variety of types (textures or sheet), has a high specific surfece area and electrical conductivity. 

In recent years, activated carbons fibers (ACFs) because of their high surface area, microporous character, and the chemical nature of their surface have been considered potential adsorbents for the removal of heavy metals from industrial wastewater [1 3]. The properties of ACFs are determined by their microstructure, it is therefore important to investigate the microstructure of ACFs in terms of specific surface area, micropore volume, pore size distributions, surface chemistry and so on. Also, the adsorption properties of carbonaceous adsorbents are dependent on not only the porous structure but also the surface chemistry [3,4]. 

On the other hands, it becomes obvious that SOx and NOx in flue gas can be removed at room temperature by using active carbon fibers (ACF) subjected to surface treatment such as heat treatment [1, 2], The flue gas treatment technology using ACF is a semidry oxidation type de-SOx method which is effective even around room temperature. In addition, this technology enables by-products such as sulfuric acid, sulfates, nitric acid, and various nitrates to be recovered, and is applicable in the field of flue gas treatment to which the conventional de-SOx method, such as the limestone gypsum method, could not be applied for economical reasons. 

In this study, activated carbon fibers (ACFs) deposited by copper metal were prepared by electroplating technique to remove nitric oxide (NO). The surface properties of ACFs were determined by FT-IR and XPS analyses. N2/77K adsorption isotherm characteristics, including the specific surface area, micropore volume were investigated by BET and t-plot methods respectively. And, NO removal efficiency was confirmed by gas chromatographic technique. From the experimental results, the copper metal supported on ACFs appeared to be an increase of the NO removal and a decrease of the NO adsorption efficiency reduction rate, in spite of decreasing the BET S specific surface area, micropore volume, and micro-porosity of the ACFs. Consequently, the Cu content in ACFs played an important role in improving the NO removal, which was probably due to the catalytic reactions of C-NO-Cu. 

Usually improvement combustion processing or after combustion treatment are used nowadays for NO reduction. However, they are some problems as like a complex, expensive setup, harmness gas emission, and corrosion metal. In recent years, to overcome these problems, some researchers have reported that NO is reduced more effectively use of the adsorption characteristics of activated carbons (ACs) and activated carbon fibers (ACFs) [6-8]. Also, some researchers are studying for NO reduction using metal supported ACs and ACFs by impregnation, metal plating, deposition, and so on [9-13]. However, metal supporting methods on ACs and ACFs in a second and their NO removal efficiency are not studied yet systematically. 

A relatively nonexpensive version starts from coal tar pitch. Activated carbon fibers (ACFs) are reviewed in [410]. Skundin has reported a felt texture to be an optimum for CM. 

There is growing interest in the development and application of activated carbon fibers (ACF), whose unusual properties can be advantageous in certain applica-... 

Activated carbon fibers (ACFs) offer a choice of other carbon forms for VOC removal. As discussed earlier, the narrow diameter of the fibers provides ready access of adsorptive species to the adsorbent surface. The incorporation of ACF into permeable forms such as felt, paper, and rigid monoliths helps to surmount the disadvantages of using loose fibers. Rigid ACF composites have been prepared at the University of Kentucky and examined for their potential for the removal of low concentrations of VOCs [31]. 

Radhakrishnan and co-workers [6] also studied the freezing of CCI4 in activated carbon fibers (ACF) of uniform nano-scale structures, using Monte Carlo simulation and differential scanning calorimetry (DSC), klicro-porous activated carbon fibers serve as highly attractive adsorbents for simple non-polar molecules. The DSC experiments verified the predictions about the increase in T/. and the molecular results were consistent with equation (1) for pore widths in the mesoporous range they also explained the deviation from the linear behavior in the case of micropores. 

The use of Positron Annihilation Lifetime Spectroscopy (PALS) technique to characterize porous carbon materials has been analyzed. Positron annihilation lifetimes have been measured in two series of petroleum pitch-based activated carbon fibers (ACF) prepared by CO2 and steam activation. Two lifetime components were found a short-lived component, Ti from 375 to 393 ps and a long-lived component, 1 2 from 1247 to 1898 ps. The results have been compared to those obtained by Small Angle X-Ray Scattering (SAXS) and N2 and CO2 adsorption at 77K and 273K respectively The correlation found demonstrates the usefulness of PALS to get complementary information on the porous structure of microporous carbons. 

The use of gas diffusion electrodes is another way to achieve high current densities. Such electrodes are used in the fuel-cell field and are typically made with porous materials. The electrocatalyst particles are highly dispersed inside the porous carbon electrode, and the reaction takes place at the gas/liquid/solid three-phase boundary. COj reduction proceeds on the catalyst particles and the gas produced returns to the gas compartment. We have used activated carbon fibers (ACF) as supports for metal catalysts, as they possess high porosity and additionally provide extremely narrow (several nm) slit-shaped pores, in which nano-space" effects can occur. In the present work, encouraging results have been obtained with these types of electrodes. Based on the nanospace effects, electroreduction under high pressure-like conditions is expected. In the present work, we have used two types of gas diffusion electrodes. In one case, we have used metal oxide-supported Cu electrocatalysts, while in the other case, we have used activated carbon (ACF)-supported Fe and Ni electrocatalysts. In both cases, high current densities were obtained. 

In the present work, CO2 electrochemical reduction was examined on higji area metal electrocatalysts supported on activated carbon fibers (ACF), which contain slit-shaped pores with widths on the order of nanometers. Such electrocatalysts were used in the form of gas difiusion electrodes (GDE), which are used in the fuel-cell field. The structure of this type of electrode is shown in Figure 1. The reaction takes places at the gas phase / electrolyte (liquid phase) / electrode interface, the so-called three-phase boimdary. 

To investigate the effect of micropores, we conducted electrolyses using the following catalysts, unmodified ACF, iron and nickel catalysts supported on non-activated carhon fibers (CF/Fe, CF/Ni), iron catalyst supported on activated carhon fibers (ACF/Fe) and two types of nickel catalysts supported on activated carbon fibers (ACF/Ni-1, ACF/Ni-2). Table 1 shows the reduction product distributions for the various catalysts at -1.8V vs. SCE. The ACF catalyst itself has very fittle activity for CO2 reduction, and hydrogen evolution was the principal reaction. The CF/Fe and CF/Ni catalysts showed very little activity as well. 

Recently activated carbons such as activated carbon fibers (ACFs) and superhigh surface area carbons have been developed. New activated carbons have more uniform micropore size distribution and greater surface area than traditional activated carbons. The carbon membranes for gas separation have been also developed lately[8]. The activation of the polyamide film leads to self-supported activated carbon film whose surface area is larger than 1100 m /g [9]. Thus various kinds of carbon adsorbents have been developed to find new applications. Scientific studies on activated carbon have been increasing according to development of these new carbon adsorbents with a special relevance to energy and environmental demands. In particular, controll of an adsorptive ability of activated carbon is requisite for new application. Consequently, basic principles for control of the micropore filling mechanism of activated carbons are shown here. 

Figure 3. High resolution Oj-plot of activated carbon fiber (ACF).

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