Determination of pore accessibility in disordered nanoporous materials

Abstract

We propose a new algorithm based on application of cluster analysis to group adsorbate molecules of a highly dense adsorbed phase in the atomistic structural model of a disordered material into connected and disconnected clusters, through which pore network connectivity of the material is identified. Our proposed algorithm is then validated using a synthetic pore structure, as well as the reconstructed structure of a saccharose char obtained in our recent work using hybrid reverse Monte Carlo simulation. The algorithm also identifies kinetically closed pores in the latter structural model that are not accessed by adsorbate molecules at low temperature, at which their kinetic energy cannot overcome potential barriers at the mouths of pores that can otherwise accommodate them. The results are validated by transition state theory calculations for N₂ and Ar adsorption, showing that N₂ can equilibrate in narrow micropores at practical time scales at 300 K, but not at 77 K. Large differences between time scales for micropore entry and exit are predicted at low temperature for N₂, the latter being larger by over 3 orders of magnitude, suggesting hysteresis. Similar behavior is predicted for Ar in the same char at 87 K. The results explain several long standing issues such as the observed increase of adsorption of nitrogen with an increase in temperature in coals, hysteresis phenomena in microporous carbons, and underprediction of adsorption of supercritical gases using structural parameters extracted from subcritical adsorption of nitrogen. Finally, the determination of pore accessibility and connectivity in disordered porous carbons using our proposed model enables one to obtain correct adsorbed quantities as well as self-diffusivities and transport diffusivities using conventional grand canonical Monte Carlo and molecular dynamics simulations.