Description of chemically and thermally treated Multi-walled Carbon Nanotubes using sequential decomposition of adsorption isotherms

Abstract

The effect of wet acid oxidation by means of sulfuric/nitric mixtures, and high-temperature treatment of commercial arc-discharge synthesized multi-walled carbon nanotubes (MWCNTs) was studied. In order to analyze the adsorption capacities of differently trated MWCNTs, we employed a multistep method that considers separately different pressure ranges (zones) on the experimentally obtained isotherms. The method is based on simple gas isotherm measurements (N₂, CO₂, CH₄, etc.). Low pressure ranges can be described using Dubinins model, while high pressure regimes can be fitted using different models such as BET multilayer and Freundlich equations. This analysis allows elucidate how different substrate treatments (chemical and thermal) can affect the adsorbate-adsorbant interactions; moreover, theoretical description of adsorbate-adsorbate interactions can be improved if a combination of adsorption mechanisms are used instead of a unique model. The results hereby presented show also that, while MWCNTs are a promising material for storage applications, gas separation applications should carefully consider the effect of wide nanotube size distribution present on samples after activation procedures.