Activated carbons pore texture

Activated carbons are widely used as adsorbents in either the gas or the liquid phase, and also as catalysts and catalyst support. In some cases their adsorption behaviour depends basically on their textural characteristics, i.e., porous structure and pore volume. As an example carbon molecular sieves (CMS) are a kind of activated carbons that make use of a narrow pore size distribution, of a few angstroms in diameter, to selectively separate gas mixtures [1], such as N2/O2, CO2/CH4, C3H6/C3H8 and some others. But also the surface chemistry can condition in many cases [2,3] the adsorption behaviour, as well as the activity as catalyst and catalyst support [4, 5]. This means that the adsorption properties of the activated carbons cannot be easily explained only on the basis of textural characteristics (surface area and pore size distribution) the nature of the chemical surface must also be taken into account. Therefore, for a complete characterization of the activated carbon surface, textural and chemical characteristics must be assessed. 

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... 

These macropores are not effective for adsorption of various molecules, but their presence before activation is preferable for creating micropores in the walls. The pore texture of most activated carbons is illustrated in Figure 2.17b, where macropores (>50nm width) and mesopores (2-50nm... 

Absorption of harmful organic compounds by activated carbons from gas and liquid media is of interest and importance for human and environmental protection purposes.1"21 The influence of the texture of carbon granules (size and volume of pores, specific surface area, granule size d, and carbon bed depth V), gas stream humidity and velocity, and amounts of pre-adsorbed water are investigated on adsorption of organics in different media.1 21... 

The porous textural characterization of activated carbons is a very important subject due to the growing interest in the preparation of materials with well-defined pore structures and high adsorption capacities. Porosity characterization is an essential task to foresee their behavior in a given use and requires a combination of different techniques. Gas adsorption techniques constitute the most common approach to the characterization of the pore structure of porous materials. However these techniques have some limitations. 

Ammonia adsorption was studied on several activated carbons with different textural and chemical characteristics by flow adsorption microcalorimeter. The textural and chemical nature of the samples was measured by N2 and CO2 adsorption and temperature programmed desorption (TPD-MS) respectively. The ammonia adsorption consists in reversible (related to physisorption) and irreversible (related to chemisorption on chemical groups) components. From the molar heats of adsorption it can be concluded that the samples have a wide distribution of acidic sites some of which are very strong. However, they are not always easily accessible to ammonia because constrictions in the pore-network hinder the access, forcing the adsorbed molecules to re-arrange. 

The activated carbons used in this work present a wide distribution of acid groups and some of which exceed in strength those existing on acidic zeolites and pillared clays, although they are not always accessible due to the presence of constrictions in the pore network. As a consequence of this, kinetic re-arrangements are produced. Thus, not only a large number of chemical surface groups is desirable, but also a proper textural structure that facilitates the access to them. 

The objective of the present work was to study and compare by scanning tunneling microscopy (STM) the microporosity and mesoporosity of several different carbon materials with various types and amounts of pores highly oriented pyrolytic graphite with artificially-generated model pores, activated carbon fibers, nonporous thermally treated carbon black and nonactivated carbon fibers with an ultramicroporous texture. 

Three carbon samples showing differences in pore structure are chosen to study the effect of porous texture on adsorption from liquid solutions. The benzene adsorption/desorption isotherms are applied to determine the properties of geometrical surface structure of investigated carbons. The liquid adsorption data are analyzed in terms of the theory of adsorption on heterogeneous solids. The relation between parameters of porous structure of the activated carbon samples and parameters of adsorption from the liquid phase is discussed. 

Moreno-CastiUa, C., Carrasco-Marin, F., Utrera-Hidalgo, E., and Rivera-UtiiUa, J. (1993). Activated carbons as adsorbents of sulfur dioxide in flowing air. Effect of their pore texture and surface basicity. Langmuir, 9, 1378—83. 

The variety of mechanisms that may be involved in the sorption process of metal ions onto activated carbon induces a great number of factors that control the adsorption the surface oxygen complex content, the pH of point of zero charge, the pore texture of carbon, the solution pH and its ionic strength, the adsorption temperature, the nature of the metal ion given by its speciation diagram, its solubility, and its size in adsorption conditions. The influence of these various conditions is detailed in Section 24.2.1.4. 

The recent work by Li and coworkers [18] provides a good illustration of the importance of the surface chemistry and pore texture of carbon materials on nonelectrolyte adsorption. They studied the adsorption of trichloroethene (TCE) and methyl ieri-butyl ether (MTBE) on different commercial activated carbons and activated carbon fibers with different porosity and surface chemistry. TCE is a relatively hydrophobic planar molecule. MTBE is tetrahedron-like and relatively hydrophilic. The results of the adsorption from aqueous solutions on the more hydrophobic carbons showed that TCE adsorption was controlled by a pore volume ranging from 0.7 to 1 nm width, as shown in Fig. 25.2. MTBE was primarily adsorbed in pores with widths between 0.8 and 1.1 nm. These micropore ranges were between 1.3 and 1.8 times the kinetic diameter of the adsorptives. 

In many liquid-phase applications, the bacterial colonization of activated carbons can occur quite readily [67]. This colonization [68] is considered to result from (i) the adsorptive properties of carbon, which produce an increase in the concentration of nutrients and oxygen as well as the removal of disinfectant compounds (ii) the pore texture of the carbon particles, which provides the bacteria with a protective environment (iii) the presence of a large variety of functional groups on the carbon surface, which enhances the adhesion of microorganisms and (iv) the nature of the mineral matter content of the carbon, which can favor bacteria adhesion. In general, bacteria attached to carbon particles are very resistant to disinfectants. 

The composition and pore texture are two key parameters to the carbon performances in the above-cited applications [1]. However, since the origin of the materials used for the preparation of activated charcoals varies constantly, it is difficult to keep these essential parameters unchanged. Indeed, the carbon final texture and composition strongly depend on the raw material chosen. In... 

These results show how it is possible to optimise catalyst selectivity and activity by tuning the textural parameters of the support in a realistic way. Diffusional limitations can be completely avoided by choosing an appropriate pore size range, which is made possible by the pore texture flexibility of carbon supports issued from evaporative drying and pyrolysis of resorcinol-formaldehyde aqueous gels. 

Iwasawa et al. [51] showed that polynaftoquinone, a carbonated material, was active in the ODE at low temperatures. Good results were also obtained with PPAN [52]. AUchazov et al. [53] were the first authors to test activated carbon. They observed that the ODE reaction could be performed at temperatures lower (623 to 673 K) than those normally used with oxide catalysts (723 to 823 K). Several reports on the use of activated carbon as catalyst for ODE appeared subsequently, but the results were interpreted mainly in terms of the textural properties of the catalysts (surface area/pore sizes). 

Similar conclusions concerning the textural requirements of the catalyst were reached by Kane et al. [58], who attributed the differences observed with a range of CMS to the effects of the pore structures on the coupled reaction and diffusion phenomena. They suggested that the pore structure should include substantial amounts of transport pores (meso- and macropores). Pereira et al. [59] used activated carbon fibers obtained from different precursors as catalysts in the ODE. They observed that fibers with an average micropore width lower than... 

A systematic study of the textural effects was published more recently [60], Starting with the same material, a Norit-activated carbon, two types of modification were carried out (1) the catalyst pores were enlarged by gasification and (2) the pores were narrowed by coke deposition. All the materials were subsequently treated under an inert atmosphere at 1173 K to remove surface functional groups. It was additionally confirmed that all the catalysts exhibited similar amounts of surface groups after reaction. Therefore, the kinetic results obtained could be correlated only with the textural properties of the activated carbons. The following conclusions were reached ... 

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