Study of the importance of Molecular Shape in the Adsorption of Nitrogen in Nanopores of Different Geometries

DELGADO MONS, RODRIGO; CORNETTE, VALERIA; TOSO, JUAN PABLO; LOPEZ, RAÚL H.

Simposio; 2º Simposio Iberoamericano de Adsorción (IBA 2); 2015

A central problem in the characterization of activated carbons is the accuratedetermination of the Pore Size Distribution (PSD) from adsorption isotherms of aprobe molecule, usually N 2 at 77 K. Any method for the determination of the PSDbegins with the proposition of a model to represent the relevant geometric andstructural characteristics of the porous material. It is important to stress the factthat such a model is not intended to mimic the real porous structure, but it is ratheridealization intended to reproduce with a maximum degree of accuracy theadsorptive properties of the material.The slit model [1], which represents the material as a collection of slit geometrypores of different sizes, is usually assumed for the characterization of activatedcarbons and has been extensively used in determining their PSD [2?7]. However,the observed high values of the heat of adsorption in activated carbons, in therange between 4 and 6 kcal/mol at very low pressure [8?11], cannot be explainedsolely by slit micropores, even for ultra-small sizes, suggesting that an adsorbatemolecule may be under the influence of more than 2 graphitic plates at lowpressure. In previous works have been suggested the use of micropores with atriangular section [12]. This kind of geometry would provide adsorption regions ofhigher heats of adsorption, since adsorption in the center of a triangular pore isaffected by 3 graphitic plates instead of 2. On the other hand, the importance ofshape in the adsorption of nitrogen [13] (common molecular probes for solidcharacterization) in confined spaces is also investigated for its effects on theadsorption capacity and isosteric heat.In the present work we study both the importance of adsorbent's molecularshape (using pseudo-sphere models and multi-atom molecular models) as poregeometry in the adsorption of nitrogen in confined spaces. By using the GrandCanonical Monte Carlo (GCMC) simulation method in the continuum space,families of N 2 adsorption isotherms are generated both for slit and triangulargeometry corresponding to different pore sizes. The slit and triangular geometryfamilies of isotherms are finally used to the fit experimental N 2 adsorption datacorresponding to a family of activated carbons.