Fluorine and Bromine Dual-Doped Nanoporous Carbons: Preparation and Surface Chemistry Studies

A novel method for the concurrent introduction of fluorine and bromine into the surface of nanoporous activated carbon (NAC) is evaluated. According to the method, the preheated NAC was treated with 1,2-dibromotetrafluoroethane at elevated temperatures (400-800 °C). Potentiometric and elemental analysis, nitrogen adsorption-desorption, scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), and ¹⁹F solid-state NMR were used to study the NAC microstructure and changes in surface chemistry. The specific modification temperature was found to have a decisive influence on the resulting halogen content of the NAC surface. About 1.5 mmol g^-1 of bromine and only 0.5 mmol g^-1 of fluorine are chemisorbed on the NAC surface when dual-doped at 400 °C. The fluorination efficiency increases dramatically to 1.84-2.22 mmol g^-1 when the process temperature is increased to 500-700 °C. Under the same conditions, the bromination efficiency unexpectedly decreases to 0.66-1.32 mmol g^-1. Since halogen-containing groups undergo significant thermal decomposition around 800 °C, the overall halogenation efficiency decreases, accordingly. Both the volume and surface area of the micropores decrease moderately when halogen-containing groups are introduced into the carbon surface layer. Fluorine and bromine are unevenly distributed in the porous structure of the dual-doped NACs, and the outer surface is more halogen-rich than the inner surface of the micropores. XPS and ¹⁹F solid-state NMR revealed the selective formation of CF₂ groups in the NAC surface layer independent of the temperature. In contrast, the percentage of semi-ionic fluorine in the form of CF groups directly bonded to the π-electron system of the carbon matrix increases significantly with temperature.