MIXED GEOMETRY CHARACTERIZATION OF ACTIVATED CARBONS

D. C. S. AZEVEDO; C. L. CAVALCANTE; RAÚL H. LÓPEZ; A. E. B. TORRES; J. P. TOSO; GIORGIO ZGRABLICH

Edinburgh

Simposio; International Symposium on the Characterization of Porous Solids (COPS-VII); 2008

The slit pore geometry is usually assumed for the characterization of activated carbons, especially in determining the Pore Size Distribution (PSD). However, observed high values of the heat of adsorption in activated carbons at very low pressure cannot be explained by slit micropores, even for ultra-small sizes. On the other hand, it is reasonable to assume that, at least in part of the material, carbon plates may accommodate in such a way as to form pores with a triangular section. This kind of geometry would provide adsorption regions of higher heat of adsorption near the corners of the triangle.             In the present work, pores with slit and triangular geometry are assumed to co-exist in undetermined proportions in activated carbons, and a method is proposed to obtain the corresponding PSD of such a material. By using a Grand Canonical Monte Carlo (GCMC) simulation method in the continuum space, families of N2 adsorption isotherms are generated both for slit and for triangular geometry corresponding to different pore sizes. These families of isotherms are then used to fit experimental adsorption data corresponding to a controlled set of samples of activated carbons, allowing the determination of the micropore volume, the proportion of slit and triangular pores and the PSD corresponding to the combined geometry. Experimental N2 isotherms were obtained for a set of microporous activated carbons prepared from coconut shells, using phosphoric acid as the activation agent, under different carbonization conditions. The same experimental data were fit using each of the “pure” geometries and combinations thereof so that conclusions were drawn about the reliability and convenience of the proposed characterization method.