Characterization of microporous activated carbons using molecular probe method

Abstract

Accurate knowledge of an adsorbent’s porosity is fundamental for scientific and industrial applications of adsorption technology. Over the last decades many approaches have been established to assess porosity of adsorbent materials by analyzing their nitrogen uptake at 77K with volumetric measurement devices. Despite using highly sophisticated physical models, all approaches make assumptions on pore shape as well as on the interactions between adsorbent and adsorptive molecules. Subsequently, significant differences in pore size distributions are observed depending on which modeling parameters were used. The molecular probe method presented in this paper therefore restrains to a minimum of approximations by measuring isotherms of chemically similar substances of increasing molecular size. Differences in pore volume can be reduced to sterical limitations in micropores below the size of adsorptive, leading to a high-resolution pore size distribution below 0.7nm where only few comparable methods exist. The analytical procedure was customized to take account of the amorphous and heterogeneous pore structure of activated carbon. By measuring adsorption isotherms of N2, n-hexane, iso-octane and cyclohexane on various activated carbons, it is shown that differences in pore accessibility of tested adsorptives are specific for each adsorbent. Using molecular probe molecules hence appears to be a promising method for a complementary porosity analysis of activated carbons.