Commercial production of carbon fibers is dated back to post World War II period. Union Carbide Corporation was the first to systematically synthesize carbon fibers from rayon and PAN based organic precursors by carbonization at high temperatures in early 50s. Many attractive properties of carbon fibers, including high mechanical strength, thermal stability, and high thermal conductivity values were immediately recognized. Soon after, carbon fibers found applications in aerospace and aeronautic industries, primarily as metal reinforcing agents. In the last decade, numerous research activities have further exploited the application of carbon fibers. One study shows that following metal, ceramic, and polymeric materials, carbon fiber will become the fourth generation essential industrial material.

Today's activated carbon fibers (ACF) are widely recognized as versatile materials having potential to act as a novel adsorbent, catalyst, as well as support to catalyst. This short write-up is intended to highlight those multi-facet roles of ACF in environmental application, in particular, air pollution emission control and describe research activities currently underway in our laboratory related to synthesis and characterization aspects of ACF.

We prepare ACF from the commercially obtained raw (non-carbonized and non-activated) carbonaceous fabrics based on viscose rayon and phenolic resin precursors. Following carbonization of the raw fibers, activation is carried out. The various surface characteristics of the prepared ACF samples are analyzed by X-ray diffraction (XRD), temperature programmed desorption (TPD), elemental analysis, and the Fourier transform infra-red (FTIR) spectroscopic techniques.

The unique characteristic of the prepared ACF that makes it distinct from its counterpart activated carbon granules is the structure of the pores and the pore size distribution. Figure below describes a typical pore size distribution of ACF.

In addition to its high BET area and uniform (meso) pore-size (3-10 nm), the internal pores in ACF are directly connected to the external surfaces. As a consequence, diffusion length from the surface to the internal micro pores is short resulting in the least intra pore diffusion resistance. The implications are significant in adsorption and separation processes. The process is adsorption rate controlled and adsorption breakthrough is suppressed for relatively much longer time. Inversely, the adsorbate saturated ACF is completely regenerated which renders it ready for the next adsorption cycle. It is fair to say that ACF has emerged as a high performance adsorbent, especially in the control of effluent gases such as SO 2 , NOx, and volatile organic compounds (VOC) vapors. A number of experiments carried out in the laboratory under varying operating conditions of gas flowrate, amount of material, and gas concentrations confirmed the superior performance of ACF in comparison to the other commercial adsorbents such as zeolites, silica gel and activated carbons.

ACF also exhibits catalytic activities. For example, ACF may be used as a catalyst in the oxidation of SO₂ and NO. The catalytic activities are attributed to the surface functional groups like hydroxyl, carboxylic and quinone that are incorporated during the activation stage. Depending upon the type of adsorbate (basic or acidic, anionic or cationic), the ACF surfaces may also be accordingly functionalized. In the preparation step, we quantitatively tailored activation routes for the optimal performance of ACF in the removal of the atmospheric gaseous pollutants. It was found that the surface groups containing electronegative oxygen atoms adversely affected the adsorption of VOCs. On the other hand, adsorption of SO₂ and NO was found to be primarily controlled by the extent of the surface oxygen functional groups. The results of surface characterization analysis in relation to those of breakthrough analysis showed that lesser was the extent of oxygen functional groups on the ACF surface, higher was the extent of adsorption or oxidation of SO₂ and NO.

Due to the large BET area and uniform pore size distribution, ACF may also be used as a catalyst support to various metal oxides in the catalytic oxidation or reduction of the gaseous species. The commercially obtained ACFs were impregnated with several transition metal salts in order to investigate the catalytic activity of the metal-dispersed ACFs in the oxidation of VOC and SO₂, and reduction of NO. The complete and continuous conversion of SO₂ for more than 50h was achieved by Cu (5% w/w) impregnated ACF for the reactor inlet concentrations of 3000 ppm SO₂, 50% O₂, and 20% H₂O. The figure given below sums up the varying roles of ACF in removal of gaseous species:

Based on the encouraging results we obtained from our works on ACF and metals impregnated ACF, we have recently synthesized a hierarchal micro-/nano-carbon web in which carbon nanofibers (CNF) are grown directly on the activated carbon micro-fibers (ACF) as substrate. Commonly practiced route to producing CNF through catalytic chemical vapor deposition (CVD) generally involves catalyst supports like metal oxides or zeolites, which subsequently need to be removed for use in certain end applications. The removal is often a tedious and expensive task which is desired to be omitted without reducing the applicability of CNF. It is therefore useful to grow carbon nanostructures on activated carbon fibers (ACF) directly, as it would allow them to be used without any further post-synthesis processing in applications such as electrodes for fuel cells or super-capacitors. In essence, the developed structure is a nanostructure (CNF)-microstructure (ACF) integrated material. As an example, we have shown this hierarchical structure to be highly efficient in reactions involving adsorption-reduction of NO. The structure comprising of ACF impregnated with metallic catalyst and decorated with CNF is thus found to be more efficient than untreated ACF and ACF impregnated with metallic catalyst. SEM images of grown CNF on ACF are produced below.

One can also synthesize carbon molecular sieves (CMS) from the ACF. Since the ACFs have slit-type pore structures and the micro-pores are directly connected to the external surfaces with minimum pore diffusion resistance, there is a strong potential of ACF to work as a precursor to the development of ultra-micro pores or pore size < 1nm to yield fibrous CMS suitable for the separation of a wide range of gaseous mixture including O₂/N₂, CO₂/N₂, and CH₄/N₂. In addition, unlike other molecular sieve materials such as zeolites, CMS has better thermal and chemical stability and high hydrophobicity. Schematic given below shows that by decreasing the size of the pore-mouth the meso-pores ACF may be used to prevent larger size molecules (shown in green, 0.2-0.3 nm) from entering into the pores (3-10 nm) of ACF, thus facilitating the separation of smaller size gas molecules.

Related References

• Singhal, R., Sharma, A., Verma, N. (2007), 'Activated carbon fibers as a substrate to grow carbon nanofibers' (communicated).
• Adapa, S., Gaur, V., Verma, N. (2006), 'Catalytic oxidation of NO by Activated Carbon Fiber (ACF)', Chem Eng. J. 1 16 (1), 25-37.
• Gaur, V., Sharma, A., Verma, N. (2006), 'Synthesis and characterization of activated carbon fiber for the control of BTX', Chem. Eng. Process, 45 (1), 1-13.

Dr. Nishith Verma
Department of Chemical Engineering
    Indian Institute of Technology Kanpur
    E-Mail: nishith@iitk.ac.in
    URL: http://www.iitk.ac.in/che/nv.htm