Porosity and Surface Area

High surface area and a well-developed porosity are very important for achieving a high dispersion of the active phase in the catalyst dispersion is the fraction of metal atoms that are on the surface of the support in relation to the total metal loading). Carbon materials, especially activated carbon, exhibit surface areas much higher than those of other conventional catalyst supports (e.g., alumina, silica). However, a great part of this surface area may be contained in narrow micropores, in which case it may not be available to reactants. 

Many studies report the effect of porosity and surface area on metal dispersion and catalytic activity. Linares-Solano et al. [10] prepared platinum catalysts supported on a graphitized carbon black (V3G), which was subjected to various degrees of activation in air to increase the surface area. They observed that as the surface area of the parent sample increased from 62 m /g to 136 m /g, 

Some authors have found that the shape of pores in activated carbons can play an important role in the catalytic process when it is used as support, in opposition to other solids with pores that are not slit shaped. This is the case for Laine et al. 

In some cases, a high surface area of the carbon support may be detrimental if it is conhned in narrow micropores that are not accessible to the reactant molecules. This is especially important in processes where large molecules are involved, as in the treatment of petroleum feedstocks and in liquid-phase reactions in which diffusion of reactants and/or products may be hindered by the narrow porosity. 

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. 

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S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area, and Porosity, Academic, New York, 1982. 

Sing K S W, Everett D H, Haul RAW, Mosoul L, Pierotti R A, Rouguerol J and Siemieniewska T 1985 Reporting physisorption data to the determination of surface area and porosity Pure Appl. Chem. 57 603-19... 

For practical reasons, the application of the adsorption method to the study of surface area and porosity has to be limited to bodies which are either very finely divided or possess an extensive pore system. It is helpful to consider the case of finely divided bodies first. 

Finally, a number of useful definitions of quantities directly or indirectly involved in the study of the surface area and porosity of both particulate and massive solids are given in Table 1.6. 

In view of the widespread use of nitrogen and argon in surface area and porosity studies, data for the construction of the standard a,-curves for these adsorbates on hydroxylated silica, are given in Table 2.14 (p. 93) for nitrogen and in Table 2.15 for argon. From the arguments of Section 2.12, these should be adequate for other oxides such as alumina, if high accuracy is not called for. 

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