Recent Developments on Activated Carbon

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. 

Activated carbon fibers were a remarkable technological development. The ACFs have high mechanical strengths, high surface areas ( 1000 m2/g), and microporosity (8-10 A pore dimension) and can be formed into cloth. 

High-strength carbon fibers have been produced since 1950s, but ACFs have been available commercially only recently. An activation step is necessary starting from the carbon fibers. Excellent reviews of the development of and studies on ACFs have been made by Suzuki (1994) and Rouquerol, Rouquerol and Sing (1999). Many possible novel applications of the ACF s have been reported (Suzuki, 1994 Kaneko, 2000). 

Energy storage, i.e., storage of methane and hydrogen, has attracted much interest, particularly for onboard-vehicle applications. Activated carbon has been the most promising candidate as the sorbent for both methane and hydrogen storage. 

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Carbonylation of methanol to form acetic acid has been performed industrially using carbonyl complexes of cobalt ( ) or rhodium (2 ) and iodide promoter in the liquid phase. Recently, it has been claimed that nickel carbonyl or other nickel compounds are effective catalysts for the reaction at pressure as low as 30 atm (2/4), For the rhodium catalyst, the conditions are fairly mild (175 C and 28 atm) and the product selectivity is excellent (99% based on methanol). However, the process has the disadvantages that the proven reserves of rhodium are quite limited in both location and quantity and that the reaction medium is highly corrosive. It is highly desirable, therefore, to develop a vapor phase process, which is free from the corrosion problem, utilizing a base metal catalyst. The authors have already reported that nickel on activated carbon exhibits excellent catalytic activity for the carbonylation of... 

We will mention now two very recently published examples of STM on activated carbons. In one of them, Shi and Shiu [32] studied glassy carbon electrodes before and after electrochemical activation. They reported the development of pores following activation and a change in the electrochemical behavior of the sample, which was related to these structural changes. In the second example, Pfeifer et al. [33] examined a series of activated carbons which displayed an extended fractal network of channels. As expected from such structure, only sparse entrances ( 1.3 nm wide) were observed by STM on the surface of the samples. 

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. 

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. 

In a recent paper, Chiang et al. [22] reported values of the free energy, enthalpy, and entropy of adsorption of volatile organic compounds (exemplified by benzene and methylethylketone) on seven samples of activated carbon. The starting point for their development was Eqn (3.11) for the isosteric heat of adsorption. Linders et al. [23] determined adsorption heats from the adsorption equilibrium constant and found that these values agree quite well with those obtained from uptake experiments using the integrated form of Eqn (3.11). They analyzed the experimental data obtained for -butane adsorbed on two commercial activated carbons (Kureha and Sorbonorit B3) and for hexafluoropropylene adsorbed on activated carbon. 

More recently, Ustinov and coworkers [72, 73] developed a thermodynamic approach based on an equation of state to model the gas adsorption equilibrium over a wide range of pressure. Their model is based on the Bender equation of state, which is a virial-like equation with temperature dependent parameters based on the Benedict-Webb-Rubin equation of state [74]. They employed the model [75, 76] to describe supercritical gas adsorption on activated carbon (Norit Rl) at high temperature, and extended this treatment to subcritical fluid adsorption taking into account the phase transition in elements of the adsorption volume. They argued that parameters such as pore volume and skeleton density can be determined directly from adsorption measurements, while the conventional approach of He expansion at room temperature can lead to erroneous results due to the adsorption of He in narrow micropores of activated carbon. 

The recent developments on the metallation chemistry of oxazoles and benzoxazoles, isoxazoles and benzisoxazoles, pyrazoles and indazoles, thiazoles and benzo-thiazoles, and isothiazoles, benzo[c]isothiazoles, and benzoMisothiazoles have been reviewed. The two-decade history of catalytic carbon-carbon bond formation via direct borylation of alkane C-H bonds catalysed by transition metal complexes has been reported. The alkane functionalization via electrophilic activation has been underlined. " Recent advances of transition-metal-catalysed addition reactions of C-H bonds to polar C-X (X=N, O) multiple bonds have been highlighted and their mechanisms have been discussed. The development and applications of the transition metal-catalysed coupling reactions have been also reviewed. - ... 

Another approach that minimizes Ru content is to combine large surface area materials with RuOg. Supercapacitors based on activated carbon and ruthenium oxides were developed by Zheng et al. in 1996 [65] and recently optimized [66]. They demonstrated that the addition of carbon black with a high surface area (1300 m -g" ) increases electrode porosity, a benefit for the proton transfer within the electrode. In addition. 

The aim of the authors in writing this chapter on supercapacitors was to highlight the most recent advances in supercapacitor technology and in the basic studies of materials for supercapacitors. This review is also intended to elucidate the factors limiting the power of activated carbon DLSs, which are at present the most advanced version of supercapacitors already on the market with high-performance products. The development of activated carbons with optimized pore-size distribution as well as of electrolytes with higher conductivity and a higher decomposition potential window may lead to further DLS optimization. 

It is appropriate at this stage to consider active carbons generally, before leading on to introduce active carbon fibers, which are a relatively recent development of these materials. 

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