EPR characterization of graphitized and activated micro- and meso-porous carbons

Abstract

Porous carbons are widely used for adsorbing pollutants from gas or liquid phases, for chromatographic separation, and for catalysis. A computer aided electron paramagnetic resonance (EPR) analysis was performed on 11 commercially available carbons, at different activation and graphitization levels. A heating treatment of H 3PO 4-preactivated carbons dispersed into an inert glass fibres matrix was performed using a steam-N 2 flow directly in the EPR cavity, following the line-shape/line-width variations over temperature up to 673 K, at different heating speeds. The surface area and the pore size distribution (PSD) of the differently treated carbons were evaluated by nitrogen adsorption isotherms. Both PSD and transmission electron micrographs (TEM) indicated an heterogeneous porosities of some carbons. The variation of the EPR line shape as a function of the surface area allowed us to identify three main carbon structures at different porosities, classified as graphitized, activated and highly polar. In several cases, two or three structures were contemporaneously present and corresponded to different zones of heterogeneous carbons and to superimposing contributions in the EPR spectra. In these cases, the EPR analysis of the spectra allowed us to relatively quantify these three zones and to extrapolate the true area of each carbon region at different porosity.