Activated carbon fibers surface chemical properties

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]. 

Using pitch-based ACFs, Mochida et al. [132] reported 87% conversion at room temperatnre in dry air. Lower conversions were obtained in the presence of water vapor. The anthors found that heat treatment at 1123 K enhanced the activity of the fibers. Such treatment removes oxygen functional groups from the surface of the ACFs the vacant sites created as a result of this treatment were thought to be the active sites for the reaction. On the other hand, the hydrophobic surface obtained after the heat treatment helps to decrease the amount of water adsorbed, which decreases NO conversion in humid air. An interesting point noted by Mochida et al. [131] is that PAN- and pitch-based ACFs exhibited the reverse order of activity for the oxidation of SO2 and NO. Thus, pitch fibers were best for NO oxidation, while PAN fibers were found to be more active for SO2 oxidation. No explanation was provided by the authors for this finding, which certainly reflects the different surface chemical properties of the two fiber types. A detailed kinetic study of this process was presented in a subsequent paper [133], while Guo et al. [134] compared the performances of different carbon fibers (PAN, pitch) and activated carbons. 

ACF and its characterization. Some of physical and chemical properties of two commercial pitch based active carbon fibers (ACF-1 and 2) used in the present study are listed in Table 1. After outgassing at 150°C for 4h, BET surface areas were measured by N2 adsorption at -196°C, using a Simazu ASAP 2000 apparatus. 

Carbon fibers, when used without surface treatments, produce composites with low mechanical properties. This has been attributed to weak adhesion and poor bonding between the fiber and matrix. Therefore, the carbon fibers are given surface treatments, the exact nature of which is a trade secret. This surface treatment increases the surface active sites which results in the improvements of the bonding between the fibers and the resin matrix. This tends to increase the wettability of the carbon fibers and enhances the mechanical properties [4-7]. Surface treatments may be classified into oxidative and nonoxidative treatments. An oxidative treatment involves gaseous oxidation, liquid-phase oxidation carried out chemically or electrochemically and catalytic oxidation. The nonoxidative surface treatments involves deposition of more active forms of carbon or metals such as whiskerization, pyrolytic coating, the grafting of the polymers, and metal deposition on the carbon fiber surfaces [8-11]. 

Activated carbons or carbon fibers are the most common materials nsed as adsorbents and catalysts. They are employed widely in both liquid and gaseous phases. This universality is due not only to their high surface area and high volume of pores, but also to the variety of chemical properties of their surfaces. Although for physical adsorption the porous structure is the most important feature, for reactive adsorption and catalysis the chemical environment plays an important role, provided that the structure is developed sufficiently for dispersion of active chemical species and for accommodation of molecules to be adsorbed or to undergo a targeted chemical reaction. 

This review provides an overview of the most preferable production methods of carbon fiber and nanofiber and activated form of them. Because of the extraordinary combination of physical and chemical properties exhibited by carbon nanofibers and activated form of it, which blends two properties that rarely coexist high surface area and high electrical conductivity, which are the result of the unique stacking and crystalline order present within the stracture, there are tremendous opportunities to exploit the potential of this form of carbon in a number of areas, some of which are discussed in this paper. 

Sepiolite is a fibrous silicate, Sii2MggOjo(OH)4(H20)4, made up of microporous channels parallel to the fiber axis. The chemical composition and stmcture of sepiolite are responsible for good adsorbent behavior towards polar molecules such as water, ammonia, amines and aldehydes in both gas and liquid phases because of its hydrophilic surfaces. Activated carbon is essentially microporous and hydrophobic, making it suitable for nonpolar molecules such as hydrocarbons. As these properties are complementary, a mixture of both could be useful in specific applications such as adsorption of mixtures of molecules. The preparation of discs or pellets is straightforward because in mixtures of carbon and sepiolite, the latter acts as a binder when adding small quantities of water. 

In this context, nanoporous carbons are extremely interesting materials which can be used either as electrodes of supercapacitors or hydrogen reservoir. They are commercially available at a low cost and under various forms (powder, fibers, foams, fabrics, composites) [3]. They can be obtained with well-developed and controlled porosity [4,5] and with a rich surface functionality [6,7], As far as electrochemistry applications are concerned, very important advantages of carbons are a high electrical conductivity, a good chemical stability in various electrolytic media and the possibility to control wettability by the nature of the surface functionality. When they are not playing the role of active material for the storage process, carbons may be also useful as additive in a composite to improve its physical properties. Particularly carbon nanotubes are able to improve the electrical conductivity and mechanical properties of electrodes [8],... 

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