Adsorption in micropores (nanopores): a computer appraisal of the Dubinin equations

Abstract

Monte Carlo computer simulation in simple pore models was used to investigate the pore filling mechanism proposed by Dubinin for adsorption in activated carbon. The first argument; that adsorption into micropores is primarily due to the potential field exerted by the solid is valid only for small micropores, and the second argument; that molecules fill the space and that the degree of filling depends on the adsorption potential (RT ln (p ₀/p)) is partly true. We check the validity of the characteristic curve where isotherms plotted as the logarithm of the density versus the square of the adsorption potential collapse onto a single straight line. We find that linearity is never observed for pores of uniform size, but a single curve is observed, as indicated by the Polanyi theory. For a given characteristic energy, the Dubinin equation corresponds to a solid with a specific pore size distribution. Plots of the adsorption heat versus loading follow a complex pattern: initially, they increase or decrease for sufficiently homogeneous or heterogeneous surfaces, respectively; at higher loadings, there is a sharp increase because of the increase of neighbouring molecules and then a sharp decrease because of the increasing importance of repulsion when the adsorbate is very dense.

Keywords:

• Dubinin equations
• adsorption
• micropores
• isosteric heat

Acknowledgements

Support from the Australian Research Council is appreciated.