Micropore activated carbon fibers

R. Radhakrishnan, K.E. Gubbins, A. Watanabe and K. Kaneko, Freezing of Simple Fluids in Microporous Activated Carbon Fibers Comparison of Simulation and Experiment, J. Chem. Phys. Ill (1999) pp. 9058-9067... 

Detailed accounts of fibers and carbon-carbon composites can be found in several recently published books [1-5]. Here, details of novel carbon fibers and their composites are reported. The manufacture and applications of adsorbent carbon fibers are discussed in Chapter 3. Active carbon fibers are an attractive adsorbent because their small diameters (typically 6-20 pm) offer a kinetic advantage over granular activated carbons whose dimensions are typically 1-5 mm. Moreover, active carbon fibers contain a large volume of mesopores and micropores. Current and emerging applications of active carbon fibers are discussed. The manufacture, structure and properties of high performance fibers are reviewed in Chapter 4, whereas the manufacture and properties of vapor grown fibers and their composites are reported in Chapter 5. Low density (porous) carbon fiber composites have novel properties that make them uniquely suited for certain applications. The properties and applications of novel low density composites developed at Oak Ridge National Laboratory are reported in Chapter 6. 

Details about the porous texture properties of the studied materials can by found in our previous papers 4 18. In general, all activated carbons, activated carbon fibers and activated carbon monoliths are essentially microporous materials with a negligible contribution of meso- and macroporosity. 

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. 

Alcaniz-Monge, J., Linares-Solano, A., and Rand, B. Mechanism of adsorption of water in carbon micropores as revealed by a study of activated carbon fibers. J. Phys. Chem. B 106, 2002 3209-3216. 

Grebennikov SF and Udal tsova NN. Analysis of micropore volume size distributions of activated carbon fibers. Colloid J., 2006 68(5) 541-547. 

The study of methane adsorption on activated carbon fibers has demonstrated, as was previously explained, that these carbonaceous materials, because of their cylindrical morphology and smaller diameter, have higher packing density than activated carbons with similar micropore volumes [191]. Subsequently, the higher adsorption capacity for the powdered activated carbons against the higher packing density for the fibers helps both kinds of materials reach similar, maximum adsorption values [191]. 

Busofit is a universal adsorbent, which is efficient to adsorb different gases (H2, N2, 02, CH4, and NH3). Figure 2 shows the texture of the active carbon fiber filament. The carbon fiber refers to microporous sorbents with a developed surface and a complicated bimodal structure. The material can be performed as a loose fibers bed or felt or as monolithic blocks with binder to have a good thermal conductivity along the filament. 

The micropore distribution is performed mostly on the carbon filament surface. Nowadays a program was undertaken to examine the parameters of an active carbon fiber to optimize both the mass uptake of ammonia, methane and hydrogen and the carbon density. 

A test matrix of about 20 different carbon samples, including commercial carbon fibers and fiber composites, graphite nanofibers, carbon nanowebs and single walled carbon nanotubes was assembled. The sorbents were chosen to represent a large variation in surface areas and micropore volumes. Both non-porous materials, such as graphites, and microporous sorbents, such as activated carbons, were selected. Characterization via N2 adsorption at 77 K was conducted on the majority of the samples for this a Quantachrome Autosorb-1 system was used. The results of the N2 and H2 physisorption measurements are shown in Table 2. In the table CNF is used to designate carbon nanofibers, ACF is used for activated carbon fibers and AC for activated carbon. 

Martinez, A. A., et al. (1997). Microporous texture of activated carbon fibers prepared from aramid fiber pulp, Microporous Mater. 11, 5-6, 303-311. 

Figure 1. STM image of the microporous structure of activated carbon fiber prepared from Kevlar pulp. Reprinted with permission from ref. 15. Copyright 2001 American Chemical Society.

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. 

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]. 

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. 

S. J. Park and Y. S. Jang, Effect of micropore filling by silver and anti-bacterial activity of activated carbon fiber surfoces treated with AgN03, J. Korean Ind. Eng. Chem. 13 (2002) pp. 1-7. 

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