Porosity of carbon

Carbon is inert in nature and has a high surface area, making it highly suitable as a support for catalysts. The surface characteristics and porosity of carbon may be easily tailored for different applications. Acid treatment is often applied to modify its surface chemistry for specific applications. Typically, active metal species are immobilized on carbon for catalytic applications. 

Carbonate rocks consist mostly of calcite and dolomite with minor amounts of clay. The porosity of carbonate rocks ranges from 20 to 50%, but in contrast to sandstone, it tends to decrease with depth. Often, carbonate rocks are fractured, providing a permeability that is much greater than the primary one. In some cases, initial small-scale fractures in calcite and dolomite are enlarged by dissolution during groundwater flow, leading to an increase in rock permeability with time. 

Variation of Dm with Porosity of Carbon Rods. Before being able to calculate reactant concentrations through the rods at different reaction temperatures, it was necessary to determine experimentally values for Deii in the rods as a function of porosity. It has not been established that Dm is only a function of porosity for a given carbon material and independent of the temperature at which this porosity is produced, but for simplicity this has been assumed to be the case. Carbon rods in. in diameter and... 

Porosity of carbon blacks can be detected by the de Boer t-plot method [4.30]. The total surface area and the geometrical surface area outside the pores can be determined separately from the adsorption isotherm. Special attention must be paid to the selection of a suitable master t curve. Due to the small diameters of most carbon black primary particles, methods for the determination of mesopores are of no importance. 

Since the porosity of carbons is responsible for their adsorption properties, the analysis of the different types of pores (size and shape), as well as the PSD, is very important to foresee the behavior of these porous solids in final applications. We can state that the complete characterization of the porous carbons is complex and needs a combination of techniques, due to the heterogeneity in the chemistry and structure of these materials. There exist several techniques for the analysis of the porous texture, from which we can underline the physical adsorption of gases, mercury porosimetry, small angle scattering (SAS) (either neutrons—SANS or x-rays—SAXS), transmission and scanning electron microscopy (TEM and SEM), scanning tunnel microscopy, immersion calorimetry, etc. 

Physical adsorption of gases and vapors is a powerful tool for characterizing the porosity of carbon materials. Each system (adsorbate-adsorbent temperature) gives one unique isotherm, which reflects the porous texture of the adsorbent. Many different theories have been developed for obtaining information about the solid under study (pore volume, surface area, adsorbent-adsorbate interaction energy, PSD, etc.) from the adsorption isotherms. When these theories and methods are applied, it is necessary to know their fundamentals, assumptions, and applicability range in order to obtain the correct information. For example, the BET method was developed for type II isotherms therefore, if the BET equation is applied to other types of isotherms, it will not report the surface area but the apparent surface area. 

Moreover, it has been remarked that it is necessary to use more than one adsorbate for a correct characterization of the narrow porosity. Thus, in the case of CMSs and other carbon materials (i.e., highly activated carbons) with narrow micropores, N2 at 77 K is not a suitable adsorbate due to diffusion problems. Other adsorptives and conditions, like C02 at 273 or 298 K, avoid such problems. From all of these, it can be concluded that for a suitable characterization of the porosity of carbon materials by physical adsorption, the use of more than one adsorbate and the application of several theories and methods to the adsorption-desorption isotherms are recommended. 

Houseknecht, 1987). The porosity of carbonate sediments and rocks is reduced by cementation, recrystallization, and mechanical and chemical compaction (Mazzullo and Chilingarian, 1992). 

Surface area and porosity also change as the graphitization process pro-ceeds and physical adsorption of gases and liquids onto different carbons has been of major interest for many years. It is not the purpose of this article to discuss the wealth of data that exists except in so far as it pertains to the present topic. In this connection, it should be pointed out that surface area and porosity of carbons can be altered significantly by various treat-... 

In the following paragraphs, we discuss the use of immersion calorimetry for the assessment of the surface chemistry, wettability, surface area and porosity of carbons. 

The presence of various functional surface groups and the high conductivity and porosity of carbon material permit effective enzyme adsorption. Glucose oxidase has been irreversibly adsorbed to a graphite electrode by drying a concentrated enzyme solution on the surface (Ikeda et al., 1984). In the presence of p-benzoquinone an electrocatalytic current was observed at 500 mV vs SCE. The measuring signal was... 

There are also reports indicating that the surface area and porosity of carbons do not affect either the active-phase dispersion or the catalytic activity. A very important factor influencing active-phase dispersion is the precursor used to prepare it. Rodrfguez-Reinoso et al. [14] used two different iron precursors (iron nitrate in aqueous solution and iron pentacarbonyl in organic solution) to prepare iron catalysts supported on activated carbons with different pore size distributions. They obtained an increase in iron dispersion with the support surface area for the nitrate series, but a high and unaffected dispersion was found for the pentacarbonyl series. These catalysts were used in the CO hydrogenation reaction, where no important differences in catalytic behavior were found for catalysts in both series. 

Another reason, which affects the utilization factor, is the structure of the carbon particles themselves. Rao et al. [17] have demonstrated that the catalyst utilization factor may vary significantly depending on the porosity of carbon materials. They have prepared a series of Pt-Ru (1 1) catalysts supported on carbon materials from the Sibunit family with grossly different BET surface areas, ranging from 20 to 400 m /g, which were utilized as the anode catalysts in liquid-fed DMFC. To be able to distinguish between the influence on cell performance of the metal dispersion and the carbon support porosity, the metal dispersion was kept constant and close to 0.3. It was demonstrated that the catalyst utiUzation factor reached 100% for low-surface-area supports but dropped down to 10% for the high-surface-area Sibunit carbon. As a result, in methanol electrooxidation, both the mass activity (Ag Ru) and specific activity increased with a... 

Although direct relationship between the porosity of carbons and their hydrogen sulfide adsorption capacity was not established, the pore sizes should play a role in energetics of physical... 

CO2 concentration. The concentration of carbon dioxide in the atmosphere may vary from 0.03% in rural environments to more than 0.1% in urban environments. Comparatively high concentrations can be reached under specific exposure conditions, such as inside motor vehicle tunnels. As the CO2 content in the air increases, the carbonation rate increases. Accelerated tests carried out in the laboratory to compare the resistance to carbonation in different types of concrete show that, indicatively, one week of exposure to an atmosphere containing 4% CO2 will cause the same penetration of carbonation as a year of exposure to a normal atmosphere [8]. Some researchers suggest that with a high concentration of CO2 the porosity of carbonated concrete is higher than that obtained by exposure to a natural atmosphere, particularly if the concrete has been made with blended cement or has a high cement content However, this is controversial, since it was shown that even 100 % CO2 under increased pressure, produced the same microstructure as natural carbonation [9]. 

From a structural point of view, carbon materials, collectively, form a family of carbons. Yet, at the same time, each individual carbon possesses specific properties. Each carbon has a unique identity. This chapter considers this family of carbons in terms of related structures and how these affect porosity because the origins, extents and characteristics of porosity are intimate functions of structure within each carbon. Activated carbons are a major part of this family of carbons. The specific objective here is to present descriptions of structures of carbons and to assess how these structures control the characteristics of adsorption in the porosity of carbons. From this base of knowledge, the processes of adsorption and activation as described in Chapters 4-6 will be better understood. 

Gun ko VM, Mikhalovsky SV. Evaluation of slitlike porosity of carbon adsorbents. Carbon 2004 42(4) 843-849. 

Table 5.5. Characterization of micropore volumes (Vo(cm g )) of porosity of carbons from Eucalyptus Globulus (Bello et al., 2002).

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