STUDY OF THE IMPORTANCE OF MOLECULAR SHAPE IN THE ADSORPTION OF NITROGEN IN NANOPORES OF DIFFERENT GEOMETRIES

RODRIGO DELGADO MONS.; VALERIA CORNETTE; J. P. TOSO; KARIM SAPAG; RAÚL H. LÓPEZ

Cartagena de Indias

Congreso; 2 Congreso Iberoamericano de Adsorción. IBA2015. Colombia; 2015

DEPARTAMENTO DE QUIMICA-UNIANDES

A central problem in the characterization of activated carbons is the accurate determination of the Pore Size Distribution (PSD) from adsorption isotherms of a probe molecule, usually N2 at 77 K. Any method for the determination of the PSD begins with the proposition of a model to represent the relevant geometric and structural characteristics of the porous material. It is important to stress the fact that such a model is not intended to mimic the real porous structure, but it is rather idealization intended to reproduce with a maximum degree of accuracy the adsorptive properties of the material. The slit model, which represents the material as a collection of slit geometry pores of different sizes, is usually assumed for the characterization of activated carbons and has been extensively used in determining their PSD. However, the observed high values of the heat of adsorption in activated carbons, in the range between 4 and 6 kcal/mol at very low pressure, cannot be explained solely by slit micropores, even for ultra-small sizes, suggesting that an adsorbate molecule may be under the influence of more than 2 graphitic plates at low pressure. In previous works have been suggested the use of micropores with a triangular section. This kind of geometry would provide adsorption regions of higher heats of adsorption, since adsorption in the center of a triangular pore is affected by 3 graphitic plates instead of 2. On the other hand, the importance of shape in the adsorption of nitrogen (common molecular probes for solid characterization) in confined spaces is also investigated for its effects on the adsorption capacity and isosteric heat. In the present work we study both the importance of adsorbent´s molecular shape (using pseudo-sphere models and multi-atom molecular models) as pore geometry in the adsorption of nitrogen in confined spaces. By using the Grand Canonical Monte Carlo (GCMC) simulation method in the continuum space, families of N2 adsorption isotherms are generated both for slit and triangular geometry corresponding to different pore sizes. The slit and triangular geometry families of isotherms are finally used to the fit experimental N2 adsorption data corresponding to a family of activated carbons.