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Pore Structure of Solids

Abstract A small grain of iron catalyst, used in the synthesis of ammonia, has an external surface of about 1 cm2/g, but it has an internal surface of 100,000 cm2/g. Since catalysis takes place on the surface, obviously the surface in the pores is of vital importance. But not all pores are of equal importance; in very narrow pores the diffusion is slow; in large pores the surface is small. The sizes of pores have just as important roles in adsorbents, like silica gel and active carbon, and in other solids, like concrete. The determination of the distribution of the total pore surface and the total pore volume in pores of different widths is called pore structure analysis. The first methods for pore structure analysis were proposed in 1945; Ritter and Drake used the mercury porosimeter, and Wheeler used nitrogen adsorption isotherms. Subsequently others improved Wheeler's method. Recently we proposed two new approaches for pore structure analysis, both based on adsorption isotherms: one for the analysis of very narrow pores, the micropores, and one for the analysis of the wider pores. The first is the only method proposed for micropore analysis, the second is an improvement on the previous methods for the analysis of wider pores.