Fluorination of Activated Carbon Fibers and the Interactions between Carbon and Fluorine Atoms

Abstract

Fluorinated active carbon fibers (F-ACF's) were prepared from active carbon fibers (ACF)by direct fluorination with elemental fluorine at low temperatures ranged from 20 to 200°C. With increasing temperature, the color of compounds varies from black through brown and yellow to white. The interaction between carbon and fluorine atoms in F-ACF's has been studied by ESCA, ¹⁹F-NMR and IR spectroscopies. Oxidizing ability of F-ACF's has also been measured by iodometry.
The nature of C-F bond in F-ACF's varies from physical adsorption to completely covalent bond depending on the reaction temperature as follows. ( 1 ) In F-ACF's prepared at temperature below 50°C, one third of fluorine atoms are liberated easily and oxidize iodide ions. Another two thirds of fluorine atoms form weak covalent bonds with carbon atoms including charge-transfer interaction in some extent. ( 2 ) In ACF's obtained at 100°C, the fraction of fluorine atoms which can oxidize iodide ions is less than 1% of all of the fluorine atoms. Almost fluorine atoms are covalently bound to carbon atoms, but the strength of these covalent bonds is widely distributed as in the case of F-ACF's obtained at temperatures below 50°C. ( 3 ) In F-ACF's prepared at 200°C, the C-F covalent bond is completed as well as that of graphite fluoride. F-ACF's obtained at this temperature are an admixture of like graphite fluorides, (CF)_n and (C₂F)_n.
These variations in the nature of C-F bond infer the color change of the compounds by fluorination temperature.