Preparation, texture and surface chemistry characterization of nanoporous-activated carbons co-doped with fluorine and chlorine

A new method is proposed for the simultaneous introduction of fluorine and chlorine into the surface layer of nanoporous-activated carbon (NAC). The method comprises treatment with dichlorodifluoromethane at 400–800 °C. Elemental analysis, nitrogen adsorption–desorption studies, electron microscopy, X-ray photoelectron (XPS) and solid-state 19F nuclear magnetic resonance (ssNMR) spectroscopies were exploited to analyze the composition, micro-, nanostructure, and functional surface coverage. The NAC modified at 400 °C has the lowest halogen content of 0.21 mmol g–1 of fluorine and 1.11 mmol g–1 of chlorine. Increasing the halogenation temperature to 500–700 °C significantly upsurges the efficiency of both fluorination and chlorination, up to 0.82–0.96 and 2.23–2.87 mmol g–1, respectively. Fluorine and chlorine are randomly but not evenly distributed in the porous structure of NAC. The outer carbon surface is enriched in fluorine, contrasting to the inner surface of the micropores. From XPS and 19F ssNMR measurements, mainly C–F, C–Cl, CClF2, CF2, and CF3 groups were identified as attached forms of halogens. High halogenation temperatures (up to 800 °C) cause a significant increase in the amount of "semi-ionic" fluorine, which is present as C–F and CF2 groups directly bonded to the π-electron system of the carbon matrix.