Activated carbons porosity

KOH-activated carbons, porosity characteristics, chemical structure, electrochemical capacitors. 

Py, X. A. Guillot, and B. Cagnorr, Activated carbon porosity tailoring by cychc sorp-tion/decomposition of molecular oxygen. Carbon. 2003,41(8), 1533-1543. 

Py X, Gullet A, Cagnon B. Nanomorphology of activated carbon porosity geometric models confronted to experimental facts. Carbon 2004 42(8-9) 1743-1754. 

Figure 3. Evolution of activated carbon porosity with the extent of parent coal preoxidation, when two different flow rates of activating gas were used.

Derbyshire, F., Jagtoyen, M. and Thwaites, M., Activated carbons - production and applications. In Porosity in Carbons, ed. J.W. Patrick, Halsted Press, New York, 1995, pp. 227 252. 

Activated carbon has the strongest physical adsorption forces or the highest volume of adsorbing porosity of any material known to mankind. 

Activated carbon filters remove a wide range of organic matter by adsorption onto the carbon bed. The bed may be derived from a number of different carbon sources, and the correct selection of bed type, capacity, and porosity is a specialized function. Activated carbon may be usefully employed in organic traps, complementing the resin bed, but its capacity and organic removal rate characteristics are flow-dependent. Excessive flows may compromise the rate of adsorption of organic matter. 

Table 1. Porosity parameters of the KOH activated carbons fA-C means ctivated carboni om coal Ceta ...

High porosity carbons ranging from typically microporous solids of narrow pore size distribution to materials with over 30% of mesopore contribution were produced by the treatment of various polymeric-type (coal) and carbonaceous (mesophase, semi-cokes, commercial active carbon) precursors with an excess of KOH. The effects related to parent material nature, KOH/precursor ratio and reaction temperature and time on the porosity characteristics and surface chemistry is described. The results are discussed in terms of suitability of produced carbons as an electrode material in electric double-layer capacitors. 

While keeping in mind all these implications, the primary requirement in an attempt to store a huge charge based on the electrostatic forces seems to be high surface area of an activated carbon used. Among different ways of porosity development in carbons, the treatment with an excess of potassium hydroxide is most efficient in terms of microporous texture generation. Porous materials with BET surface areas in excess of 3000 m2/g could be prepared using various polymeric and carbonaceous type precursors [5,6]. 

Varying KOH ratio in the mixture is a very effective way of controlling porosity development in resultant activated carbons. The trend in the pore volume and BET surface area increase seems to be similar for various precursors (Fig. la). It is interesting to note, however, a sharp widening of pores, resulting in clearly mesoporous texture, when a large excess of KOH is used in reaction with coal semi-coke (Fig. lb). Increase in the reaction temperature within 600-900°C results in a strong development... 

The treatment of semi-coke or mesophase (3 1 mixture) for 2 h at 600°C gives a material of pore volume VT about 0.7 cm3/g and surface area Sbet about 1700 m2/g which is typically microporous with rather narrow micropores (average size LD below 1.2 nm). Activated carbons produced within the temperature range of 700-800°C have fairly similar porosity characteristics, VT about 1 cm3/g and Sbet near 2500 m2/g. It is interesting to note that within the wide temperature range of 600-800°C the bum-off is at a reasonably low level of 20-23 wt%. 

Table 2. Porosity parameters ofKOH activated carbons producedfrom different precursors under the same reaction conditions (HOtf C, 5 hours, KOH/precursor ratio 4 1) ...

The choice of solid carriers spans a wide spectrum (Table 1) from materials most suitable for research purposes (sintered glass beads, laterite stone deposited on a gramophone disk) to industrial materials (pumice, activated carbon, etc.). Key properties that affect the performance of the carrier are porosity (from impervious to controlled-size pores), composition (from ceramics to activated carbon), and hydrophilic behavior. It is difficult to perform a direct comparison of different carriers. Colonization and biofilm growth depend strongly on the nature of bacteria and on their intrinsic propensity to adhere on hydrophilic vs. hydrophobic surfaces. 

When a solid particle of species B reacts with a gaseous species A to form only gaseous products, the solid can disappear by developing internal porosity, while maintaining its macroscopic shape. An example is the reaction of carbon with water vapor to produce activated carbon the intrinsic rate depends upon the development of sites for the reaction (see Section 9.3). Alternatively, the solid can disappear only from the surface so that the particle progressively shrinks as it reacts and eventually disappears on complete reaction (/B =1). An example is the combustion of carbon in air or oxygen (reaction (E) in Section 9.1.1). In this section, we consider this case, and use reaction 9.1-2 to represent the stoichiometry of a general reaction of this type. 

As observed in Figure 3, the results obtained for carbon nanofibres and nanotubes (closed symbols) fit in the tendencies obtained for activated carbons, showing that hydrogen adsorption depends on the porosity of the sample and does not depend on its structure. 

In general, the material density of porous solids decreases with the development of porosity, as it can be seen in Figure 4 for various powdered activated carbons, where tap and packing density values are plotted versus the micropore volume. 

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